In addition to the CO₂ conversion technologies described in Part I (1), there are several other approaches that are being investigated, although in general these are currently at a lower technology readiness level (TRL). With the sun being a desirable source of renewable power, the direct use of solar energy to produce chemicals from CO₂ akin to artificial photosynthesis is instinctively appealing. This is exploited in photochemistry using a photocatalytic (PC) reactor, which uses photons from the visible and ultraviolet spectrum to generate electrons from irradiating photosensitive semiconductors and transfers the electrons from a valence band to a conducting band in connection with a photocatalyst to promote the reduction of CO₂. Thus, an immediate advantage stems from not converting the solar energy to electricity first to supply the electrons, which brings in the associated upstream inefficiencies, as in the power-to-X type of conversion routes or electrochemical reduction. Commonly observed products from the photochemical reduction of CO₂ include CO, methanol, formic acid and methane (Figure 1). However, the process is complex to understand involving many mechanisms, especially for multiple proton and electron transfers for products such as methanol and methane with ‘CO₂ activation’ being a rate-determining step for many products. Much research effort is being concentrated on developing photocatalysts from earth-abundant, low-cost and less-toxic catalysts such as those based on iron, nickel, manganese and copper that are more scalable than those metals on the second and third row of the Periodic Table, such as rhenium, ruthenium and iridium (3). There has also been recent work looking at using metal-free photocatalysts such as graphene, nitrides and carbides (4). Using solar energy for CO₂ conversion does impart a degree of inflexibility regarding the energy source compared with for example an electrochemical route, with the energy conversion efficiency being key for cost and scalability. Although much promising research is being conducted in this area, the CO₂ conversion rates often observed in such systems under development appear impractically low for industrial implementation with some of the technical research challenges resting with developing photocatalysts that promote a high CO₂ reduction efficiency and product selectivity (5, 6).

Fig. 1

The attributes of photochemistry and electrochemistry can be combined in the approach known as photoelectrochemistry, conducted in a photoelectrochemical (PEC) cell, and has been investigated for CO₂ reduction to fuels and chemicals. A major benefit over a pure electrochemical approach is that a significant part of the energy necessary for the conversion is supplied by light, omitting or at least lowering the transmission and rectification losses associated with using solely an electrical supply (7). Fundamentally, PEC cells comprise an anode and cathode immersed in an electrolyte, with at least one of the electrodes being a photosensitive semiconductor that absorbs light and through photoexcitation provides some of the required electron flow (the current). The technology still faces challenges from complex reaction pathways, mass transfer conditions and large photovoltage requirements to generate sufficient product selectivities and solar-to-chemical efficiencies (8).

A process that would utilise renewable electricity for CO₂ conversion is non-thermal plasma (NTP) technology using a configured plasma reactor. Similar to a low-temperature electrochemical reactor, a plasma reactor has the ability to be switched on or off in response to the intermittency characteristic of the energy supply and requirements for grid-balancing. It also exhibits high reaction rates, attains steady-state quickly and can produce a range of hydrocarbon and oxygenated species, operating at near atmospheric temperature and pressure. The ability to enable thermodynamically uphill reactions to occur at ambient conditions arises from the generation of a low temperature ionised gas with electrons energised by applying an electric field to typically between 1 eV and 10 eV. This range is ideal for exciting molecular and atomic species and breaking chemical bonds, with the OC=O in CO₂ requiring 5.5 eV (9). Different types of plasma are being investigated for CO₂ conversion, involving various reactor configurations and how the electricity is supplied and its power rating, with the main three being dielectric barrier discharge (DBD), microwave (MW) plasmas and gliding arc (GA) plasmas, generating different performance for CO₂ conversion (10). The DBD configuration is probably the most widely investigated in this area. Some advantages and disadvantages of the main plasma technologies for CO₂ reduction are presented in Figure 2.

Fig. 2

One of the main CO₂ conversion reactions studied is CO₂ splitting and dissociation to CO and molecular oxygen, with CO being potentially used as a product itself or as a feedstock for fuels and chemicals. Other reactions involve a hydrogen source co-reactant such as methane (dry reforming of methane mainly to syngas but also forming hydrocarbons and oxygenates), hydrogen or water (hydrogenation to added-value fuels and chemicals). Akin to many processes the energy efficiency, reactant conversion and product selectivity are important parameters to evaluate from experiment and target achieving sufficient values (for example energy efficiency of >60% (10)) to merit further scale-up towards industrial commercialisation, with the specific energy input (SEI) being considered an important factor for influencing such performance for plasma technologies. NTP naturally involves radical-based chemistries, so improving selectivity of the desired product and lessening the burden on downstream separation costs is undoubtedly a challenge. The reported product yields and energy efficiencies for producing added-value hydrocarbons and oxygenates are generally lower than for CO and syngas type products, with the presence of molecular oxygen (from CO₂ dissociation) leading to the undesired oxidation of formed hydrocarbons (12). The use of heterogeneous catalysts such as metal oxides or zeolites to improve performance including selectivity is one such option, notably used in a packed-bed DBD configuration (13, 14). Any observation of high energy efficiencies appears to come at the expense of low conversions with augmenting both representing a long-term challenge involving further understanding of the complex plasma chemistry (11, 15). As with many novel CO₂ conversion reactors, the scale-up of plasma technology would lend itself to being modular, addressing the needs of small-scale, distributed production. When considering scale-up there will however be the need for hazard management regarding the in situ generation of molecular oxygen from the CO₂ together with flammable products from the CO₂ conversion, with potential ignition sources from the plasma generation equipment requiring the necessary safeguarding.

In general, the technologies described in this section are at lower TRLs than those mentioned in Part I of this review (1), especially when compared with the thermochemical routes. They each share some of the overall performance challenges of improving the energy efficiency and product selectivity to make them viable as contenders for future industrial scale-up. The use of realistic CO₂ feeds should be evaluated at the laboratory scale to ascertain the performance impact of inert or reactive gaseous components and seek to define tolerable catalyst poison levels, depending on the CO₂ source. Supporting studies such as technoeconomic assessment (TEA) and life cycle assessment (LCA) at the early stage are also recommended for these technologies to assist in understanding their viability for further development in the CO₂ utilisation space.

Microbial electrosynthesis (MES) is a combined biological and electrocatalytic process, where reducing energy cells in the form of electricity is provided to microbial for the synthesis of organic materials from CO₂. In MES, bioreactors fitted with electrodes transfer electrons to host organisms via hydrogen, a reduced mediator, or by direct transfer (Figure 3). This is thought to be an efficient means of providing reducing energy compared to the classic fermentative routes (16).

Fig. 3

MES was first demonstrated in 2010 with the production of acetate from CO₂ by a biofilm of the acetogenic Sporomusa ovata on a graphite cathode, which generated hydrogen (17). This was at a rate of 1.6 g acetate m^–2 day^–1 (relative to the cathode surface area). Since then, several examples using different wild type or engineered microbes converting CO₂ to chemical products have been published. With improvement in electrode design, microbial engineering and strain selection, improved yields of acetate from CO₂ have been achieved. Using a mixed culture biofilm and three-dimensional macroporous cathodes, acetate production at 685 g m^–2 day^–1 was demonstrated (18).

Apart from acetogens, the Knallgas bacterium Cupriavidus necator, which can use hydrogen or formate from the cathode as a source of reducing energy, has also been used in MES. Through genetic engineering and MES, C. necator has been used for the production of branched chain alcohols, alkanes and terpenes.

The use of MES for CO₂ conversion is still far from industrial use due to limitations in productivities and expense. Electron uptake by microbes is low, needing a wider cathode surface area and to be made with more biocompatible and cheaper materials. Limitations of the biological pathways themselves (slow carboxylation kinetics, high acetate products when using acetogens) also lead to low titres. How successfully these bottlenecks are addressed will determine whether MES will be more widely adopted as a means of CO₂ fixation in the future (16).

For the majority of C1 molecules, well developed routes exist for their manufacture from syngas which can be adapted to use CO₂ and renewable hydrogen to give a sustainable process. It is therefore hard for new processes to displace these existing technologies. Even in the case of a product such as formaldehyde, which is made commercially through highly efficient methanol oxidation processes (19), it is difficult to imagine a new process displacing the two well-established methods.

There are a couple of exceptions to this thinking. The first is the production of CO or syngas, which are very important intermediates in the production of a range of fuels and chemicals. The hydrogenation of CO₂ to CO is not a well-established technology: it runs at high temperatures and suffers from equilibrium limitations, problems with carbon laydown and formation of unwanted hydrocarbons. Hence there is an opportunity to use new processes such as electrochemical CO₂ reduction to drive these reactions.

Another molecule which could be of interest to develop new routes to is formic acid. Currently, formic acid is largely produced by hydrolysis of methyl formate, which is in turn made by carbonylation of methanol (20). Direct routes from CO₂ have been developed based on ruthenium homogeneous catalysts and on a range of heterogeneous catalysts (21) as well as electrochemical methods (22, 23). One challenge here is in the low current demand for formic acid. Its current uses are mainly as a preservative for animal feeds and silage, as well as in the leather and textile industries (20). Proposed future uses include as a fuel, a hydrogen storage and transport medium and also as a feedstock for biological processes to make higher carbon chain length products. Given the toxic and corrosive nature of formic acid (20), it remains to be seen to what extent these applications are realised.

The next classification of molecules to consider is those with a few carbon atoms. The key challenge here is typically selectivity to the chosen product. Unlike for C1 atoms, there are very few well-established catalytic processes for selectively converting C1 feeds such as syngas into C2–C4 products. The exception to this is dimethylether, which is produced in two steps from CO₂ by dehydration of methanol using acidic catalysts such as zeolites or alumina (24). Dimethylether is unusual as a C2 molecule which does not contain a C–C bond. It is proposed as a fuel of the future due to having similar combustion properties to diesel, although as a gas at room temperature it would require handling in a different way to current fuels.

Ethylene is an important intermediate in the production of chemicals, notably polyethylene but also a wide range of other molecules, and so a route from CO₂ to ethylene would be very useful to decarbonise ethylene production. There is no established direct heterogeneous or homogeneous catalytic conversion route from CO₂ to ethylene. By far the most progress has been made with electrochemical conversion, where in contrast to other molecules such as methanol, ethylene has been made with 70% Faradaic efficiency (FE) over copper catalysts (25, 26) at practical current densities of 500–1100 mA cm^–2. With further commercialisation, this could provide a viable pathway to a range of fuels and chemicals using ethylene as an intermediate. The main competing technology is a two-step method using the methanol-to-olefins process.

A range of other C2–C4 molecules have been prepared using electrochemistry, but only at laboratory scale and generally as part of a mixture of products. For example, Opus 12 reports having made eleven different compounds (27) in this range. Of other molecules formed electrochemically, ethanol is the best studied but is still only reported in modest selectivity. Ethanol is also reported as being produced by heterogeneous catalysis and biological approaches (see box).

Beyond C2, propanol has been reported as being made electrochemically from CO₂ on a molybdenum sulfide electrode (28) but only at <5% FE. At C4, biological systems can be very efficient in producing a number of molecules, particularly oxygenates such as butanediols or butyrates. Some systems are able to produce C3 molecules such as propionates, but generally even number chains are preferred due to the C2 coupling mechanism employed.

For larger molecules, direct synthesis from CO₂ becomes increasingly challenging. Biological systems offer perhaps the best route to making C6 molecules, but only a small number of examples have been published. However, the biological approach seems to offer a more selective direct conversion than is possible by other routes such as catalysis or electrochemistry. Interestingly, a number of the examples of C6 and even C8 production have been given through MES (29, 30).

As the product’s carbon chain becomes longer, using coupled processes becomes more relevant (Figure 5). It is important to note that not all molecules need to be accessed in a single step from CO₂, and in many cases it will be preferable to use two efficient steps rather than one less efficient one. Some examples of this are already well-known, for example reduction of CO₂ to CO which can then be converted to fuel range hydrocarbons by the Fischer-Tropsch (FT) process.

Fig. 5.

Efficient processes to make larger molecules tend to be based on a biological coupling of C1 or C2 molecules which can be prepared in different ways. Catalytic coupling processes such as Guerbet coupling of ethanol (31) or FT coupling of syngas tend to give a range of products, which is acceptable for end uses in fuels and in some cases solvents, but not for the majority of applications. Two-step processes with a second biological step can be based on a wide range of feedstocks, including formic acid (32), methanol (33) or syngas (34). An advantage of this approach is that all these molecules can be made readily by catalytic or electrochemical reduction of CO₂, enabling efficient processes. One possible concern is that the feedstock can be consumed by the microorganism and be converted back into CO₂. Approaches which have the potential to overcome this include MES, where feeding (renewable) electrons into the reaction supplies the microbes with energy without feedstock being consumed.

There are many viable routes to convert CO₂ using sustainable technologies, and many products are accessible, from C1 through to C10 and beyond. Different technological approaches suit different molecules (Figure 4) and have their own advantages and drawbacks. The use of two-step processes widens the range of molecules even further, and in some cases allows more energy-efficient syntheses to be developed. Key to making sustainable processes based on CO₂ is the supply of renewable energy as light, electricity or another source. In the case of electricity, intermittency is an important issue when power from solar panels or wind turbines is used. Chemical processes in general do not like to be switched on and off or turned up and down, although low temperature electrochemical and biological processes are more tolerant to this than most. However, low utilisation rates will stop processes being viable from a capital cost perspective.

Comparing Methods for Conversion of Carbon Dioxide into Ethanol

Anthropogenic CO₂ emissions from fossil fuels use and industry amounted to around 33 billion tonnes in 2019 (1, 2), mainly arising from the power generation sector. The use of carbon capture and storage (CCS) is considered a key contributor in the portfolio of options for achieving the CO₂ emission reduction targets agreed by the international community in 2015 during the 21st Conference of Parties on Climate Change (COP21). With the premise of keeping the Earth’s temperature rise below 2°C (and more ideally below 1.5°C) above the pre-industrial average (the ‘two degree scenario’), this will require that at least 94 billion tonnes of CO₂ are captured and stored until 2050 (3, 4). The extent of deployment will naturally depend on the economics together with government support and political will, but nevertheless a consequence is that CO₂ capture implementation will make large quantities of ‘pure’ CO₂ available within the next 10 years. Although the focus on CO₂ is being its contribution to climate change, value can be obtained from its use as a feedstock for a range of products and services. It can either be used directly (unconverted) such as for enhanced oil recovery (EOR) and for the food and beverage industry or indirectly through being converted to fuels, chemicals and building materials (Figure 1). At present, the annual consumption of CO₂ is approximately 230 million tonnes, with fertiliser production, namely urea, being the largest consumer (~130 million tonnes CO₂), followed by its use for EOR (~70–80 million tonnes CO₂) (5). With the growing interest for further development and investment in some of the newer (less technically mature) conversion routes then in the future these indirect CO₂ users could start contributing more towards the CO₂ consumption figures.

Fig. 1

The use of CO₂ as a raw material is viewed in the literature as becoming much more widespread than it is currently, possibly generating a ‘CO₂ economy’ (6). Carbon dioxide capture and utilisation (CCU) has even been proposed as an alternative to CCS as a climate change mitigation strategy, notably where CCS opportunities are limited, but the present scale of applications is too small to have a large impact in emission reductions, as pointed out by others (7). In the context of potential mitigation options this view is also relevant if considering a widespread electrochemical-based solution to CO₂ emissions (viz. CO₂ electroreduction) due to the seemingly inconceivable electricity demands and scale of marketplace that would be required for it to be considered a prime choice for addressing the climate change targets (8). However, directionally some benefit in emissions is obtained from using the waste CO₂ for the synthesis of fuels, powered by renewable electricity such as solar, wind, hydroelectric and geothermal (9). This approach comes under the umbrella of power-to-X schemes and has been reviewed in the literature (10–12). Many of the processes rely on water electrolysis to produce hydrogen, itself a gaseous fuel, followed by thermocatalytic reduction of captured CO₂ to produce synthesis gas (syngas), formic acid, methanol, methane (synthetic natural gas (SNG)) or higher hydrocarbons. Power-to-X schemes could produce carbon-based fuels as storage vectors at times when electricity production outweighs demand, which is a significant problem of the intermittent wind or solar power generation. One challenge in these schemes is the inevitable loss of efficiency when multiple processes are coupled together. From an electricity perspective it is more efficient to use electricity directly than to make chemicals; however, market demand for sustainable fuels and chemicals means that these concerns can be overcome, especially when electricity prices are low. It should be noted that compared with some of the more niche CO₂ uses, the market sector for fuels is vast and while this makes the opportunity for deriving them from CO₂ appealing it also brings about its own challenges. The potential scale of deployment is very large, with a choice of distributed or centralised production, which also needs to be supported by electrolyser manufacturing scale-up if water electrolysis is required. The processes are often energy intensive leading to high production costs to compete with their fossil fuel based equivalents. Regulatory requirements and technical specifications for some of the products (including whether product blends are acceptable rather than pure products), would also need to be met. Nevertheless, with process improvements gained from further development and project investment from governments with industrial support, it’s seen as an important area to target for CO₂ utilisation.

In addition to electrochemical and thermocatalytic routes other options include biological-based conversions that use CO₂ as the carbon source, and a co-reagent such as hydrogen for energy supply, with some technologies already reaching commercial status. Examples are LanzaTech’s gas fermentation technology platform for producing fuels and chemicals such as ethanol and 2,3-butanediol (13) and Electrochaea’s methanation process (14). Both the direct use of CO₂, or CO₂ conversion, should not be viewed as a mitigation strategy per se for reasons already mentioned, but instead as an enabler for increasing the value of a CO₂-capture project and helping the ‘business case’. By displacing the use of fossil fuels and promoting a more sustainable circular economy, CO₂ reuse can also help improve a country’s energy security. Furthermore, there is also the positive social-economic impact through creation of jobs around the construction and operation of such a facility.

From the outset, the conversion of CO₂ into fuels and chemicals by chemical reduction presents a thermodynamically uphill challenge, as is suggested firstly by its large C=O double‐bond energy (750 kJ mol^–1) and the higher, i.e. less negative, Gibbs free energies for some of the main carbon-based reduction products listed in Table I. The conversion chemistry is driven by the free energy differences between the reactants and products indicating their relative stabilities (16). With CO₂ being the most highly oxidised product in many carbon-based processes, including chemical and biological pathways, it exhibits the lowest energy state of all carbon-containing binary neutral species. The large energy required to reduce it is a significant obstacle and would be reflected in the energy consumption costs of any economic evaluation. This energy can either be supplied as physical energy, for example thermal or electrical, or indirectly via the use of reactive chemical species as reagents, such as hydrogen that exhibit a higher Gibbs free energy to promote the conversion of CO₂ from a thermodynamics perspective.

One result of the energetic challenges inherent in the CO₂ conversion chemistry is there being an exciting opportunity for catalyst development in this area, to generate the reaction rates and product selectivities required for an economic process (17).

A question that often arises is which fuels or chemicals should industry target for scale-up and production and which conversion technology should be used, with a realisation that some technologies are still in early-stage development such as direct low temperature CO₂ electroreduction exhibiting performance challenges with respect to product selectivity and energy conversion efficiency. The answer to this will often be dictated by the process economics and market demand for the product and a technoeconomic assessment (TEA) comparing technologies and target products will be an important decision-making exercise. The economics depend on not only the capital cost of the technology equipment items (including reactant purification, product separation duties and ancillary equipment) but also on the availability, intermittency and costs of energy such as renewable electricity and heat, which vary depending on location and can greatly influence the operating costs. Furthermore, this might be supplemented by any government incentives that might exist, for example the latest Renewable Energy Directive (RED II) for transport fuels in the European Union (EU) (18), which itself could be revised as a result of the European Green Deal initiative (19). As a result, a credible business case for justifying deployment of a particular technology to produce a target fuel or chemical product would be dependent on the policies within the host country (or shared policies between countries as in the EU) and also site-specific providing accessibility to the CO₂ feedstock and a suitable energy supply. Such a lined-up scenario is highlighted by the commercial renewable methanol facility that is owned and operated by Carbon Recycling International (CRI). The George Olah plant is located in Iceland’s Svartsengi geothermal field near Grindavík on the Reykjanes peninsula and exploits captured CO₂ from the nearby Svartsengi geothermal power station and renewable electricity from the Icelandic grid to produce electrolytic hydrogen for use in methanol synthesis via the direct hydrogenation of CO₂. The methanol is used in gasoline blends with the George Olah plant effectively recycling 5500 tonnes of CO₂ a year. The renewable methanol product from this facility is sold under the brand name of Vulcanol^TM and is displacing a small amount of fossil fuels in the transport sector (20, 21).

Greenhouse gas (GHG) emission reduction often plays an integral part in the justification of a CO₂ utilisation plant and a full and robust life cycle assessment (LCA) is required to confirm CO₂ mitigation benefits compared to the conventional (often fossil fuel) route, taking into account the whole value chain from CO₂ origin to the final use of the product: a ‘cradle-to-grave’ analysis. Any climate change benefits will depend on the source of the CO₂ feedstock (fossil fuel, biomass or directly from air), the carbon footprint of the conventional product or process route that is being displaced, the nature of the energy used for the conversion, the scale of the CO₂-utilising process and the lifetime of the CO₂ in the final product. The analysis would reflect that the CO₂ avoided is not the same as CO₂ used.

In terms of the origin of the CO₂ feedstock then in the first instance it would invariably be from point sources, for example power stations or industrial cement or steel works, with CO₂ clean-up and concentration being required for the majority of utilisation processes. However, there has been much interest lately in direct air capture (DAC) of CO₂ with differing capture technologies being developed and showcased (22, 23). Although from a fundamental analysis (24) the separation energy would conceivably be significantly higher than that from an industrial point-source (0.04 vol% CO₂ in atmospheric air vs. for example a power station flue‐gas containing 8–15% CO₂ depending on the fossil fuel) and hence also the separation cost, several companies are working on this and have already attained both demonstration and commercial operating levels. There have been projections made for the CO₂ capture cost from DAC to come down to approximately US$100 tonne^–1 within a decade (22, 23), which although encouraging is still high compared to cost targets for the demonstrated and in some cases commercialised liquid amine-based capture systems. However, as well as delivering a high-purity CO₂ stream and reducing the costs and emissions associated with any transport of the CO₂ feedstock, an obvious advantage of DAC is that it is not limited to locating the conversion or direct utilisation plant close to a CO₂ point-source. This attribute lends itself towards smaller-scale distributed production rather than necessarily building a large centralised facility that takes advantage of the economies of scale. The use of DAC CO₂ would close the carbon loop and likely provide improved carbon mitigation benefits from either carbon neutral or carbon negative emissions (depending on final product), to be quantified by an LCA. Furthermore, a DAC-based source of the CO₂ feedstock could qualify for government incentives for fuels and chemicals in the future. Currently the EU’s RED II allows for DAC-derived CO₂ for producing synthetic fuels (renewable fuels of non-biological origin (RFNBO)), but also accommodates the use of lower cost CO₂ from point-sources such as power station flue-gas, so not exclusively incentivising the deployment of DAC (23). Like for many developing technologies within the CCU space, government support through grants and tax credits and robust CO₂ accounting frameworks will ultimately be needed to help drive the technology through the metaphorical ‘valley of death’ and into commercialisation with an investable package.

This paper provides a perspective on some of the different CO₂ conversion technologies and their applicability to target products that are being developed by both academia and industry. It covers a range of approaches, which are mainly catalytic in nature including homogeneous and heterogeneous (thermo)catalytic, electrocatalytic and biological methods, each of which are at different technology readiness levels (TRL) and exhibiting different strengths and research challenges.

In nature, the bulk of CO₂ reduction is carried out by plants and autotrophic microorganisms, that are capable of converting CO₂ and an external energy source into biomass and side products. Central to autotrophic metabolism are carboxylase enzymes, that incorporate CO₂ (or in some cases, HCO₃^–) to specific organic molecules. Autotrophs also possess energy harvesting systems, that take reducing potential from light or inorganic electron donors for CO₂ reduction.

Recent advances in genetic engineering have led to the use of these biological tools (autotrophic microorganisms, a variety of carboxylases, autotrophic energy-harvesting systems) and others as modular units to create synthetic biological routes from CO₂ to virtually any product of interest. This review discusses three such synthetic biology approaches for CO₂ reduction.

The first approach is to use autotrophic microorganisms with genetic engineering and process optimisation to maximise yield and purity of their natural products (C1 to C4) or to add heterologous enzymatic pathways that lead to new products (Table II). A commercial example is LanzaTech’s acetogenic strain Clostridium autoethanogenum, that naturally converts CO, CO₂ and hydrogen to ethanol, acetate, 2,3-butanediol and other products. Strain selection and evolution combined with the development of the gas fermentation process have led to improved ethanol production. LanzaTech’s first commercial plant was established in China in 2018 and is located at a steel mill, using industrial waste gas as feedstock. Biomass and other organic waste can also be gasified, giving Clostridia and other acetogenic bacteria versatility in feedstock. The enzymatic pathway acetogens use, Wood-Ljungdahl, is the most energy-efficient natural autotrophic pathway. Depending on the species, acetate is either the main product or precursor and hydrogen is used as the source of reducing energy (25). Other species of Clostridia have been used to produce butanol (26).

Photoautotrophic microorganisms such as microalgae and cyanobacteria are another example, using light to split water to generate oxygen, as well as hydrogen and H⁺ gradient for energy. This is used for the Calvin cycle (also known as the Calvin-Benson-Bassham cycle or the reductive pentose pathway), an enzymatic pathway that requires a lot of energy to fix CO₂. Microalgae, typically grown in large scale in ponds and can use wastewater, convert CO₂ to biomass for animal feed and a range of lipids for biodiesel (27). Chemical production from CO₂ has seen recent advances due to the expansion of available genetic tools for cyanobacteria. Combined with improved photobioreactor design, this has enabled production of 1-butanol at 4.8 g l^–1 (302 mg day^–1) from Synechocystis, the highest production rate of 1-butanol from CO₂ (28). Phytonix Corporation is a producer of biobutanol and biooctanol using cyanobacteria (29).

The ‘Knallgas’ bacterium Cupriavidus necator also utilises the Calvin cycle, but takes reducing energy from hydrogen or formate. This bacterium naturally accumulates polyhydroxybutyrate, a bioplastic precursor. With genetic engineering, it has been exemplified to produce branched-chain alcohols and alkanes from CO₂ (25). C. necator has been used in microbial electrosynthesis (MES, discussed later) where formate or hydrogen is produced from the cathode.

The second approach is to take model heterotrophic (not able to grow on CO₂) microorganisms and introduce carboxylases or autotrophic pathways that enable CO₂ utilisation. These organisms, with well-established use in industrial fermentations, are capable of high growth rates (specific growth rates of 0.5 h^–1 to > 3 h^–1) and high productivities when grown on sugars or other carbon sources. In addition, their well-characterised metabolism and a wide range of available genetic tools make them ideal hosts for genetic engineering. Genetic modifications and the introduction of the Calvin cycle including ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) into bacterium Escherichia coli enabled the phospho-sugar synthesis from CO₂ (30). In yeast Saccharomyces cerevisiae, the same approach led to 10% increased ethanol production from glucose and galactose, at the expense of side product glycerol (31). More recently, the introduction of Calvin cycle components and the modification of a native metabolic pathway of the methylotropic yeast Pichia pastoris has enabled growth on CO₂ as a sole carbon source (specific growth rate of 0.018 h^–1). This is a significant demonstration of conversion of an industrial heterotroph to a synthetic autotroph. Pichia is widely used for the production of commercial enzymes and pharmaceutical chemicals and can use methanol as a carbon source. This work facilitates the use of Pichia for CO₂ conversion into commodity or specialty chemicals using methanol as an energy source (32).

The third approach is to integrate the first two: a simultaneous or sequential fermentation of autotroph (to reduce CO₂ to a C1 or C2 product, such as formate, ethanol or acetate) and heterotroph (to utilise the first product as a substrate for >C2). This combines the beneficial characteristics of both types of organisms. An example is the two-stage process using the acetogen Moorella thermoacetica to convert CO₂, CO and H₂ to acetate, which was then used in a separate bioreactor by the oleaginous yeast Yarrowia lipolytica to make triglycerides (33).

As genetic engineering and bioreactor designs continue to improve, certain bottlenecks become apparent. Limited energy and CO₂ uptake occurs due to inefficient microbial photosystems (in photosynthesis, only 1% of light energy is utilised for biomass and product conversion) and the low solubility of both CO₂ and hydrogen in aqueous solutions. CO₂ conversion is also rate-limited by the carboxylase enzymes in CO₂-fixation pathways, which evolved in environments where fast kinetics was not a requirement. These bottlenecks are being addressed through further engineering and targeted evolution of biological energy-harvesting systems and individual carboxylases and through studies of natural CO₂-concentrating mechanisms such as carboxysomes (subcellular compartments where RuBisCO is localised). Aided by advances in bioinformatics and high-throughput enzyme production, more efficient synthetic carboxylation pathways (featuring faster carboxylases and less energy-demanding steps) are also being designed, constructed and tested in either cell-free systems or microbial organisms.

3.1 C1 Molecules

3.2 Larger Molecules

Synthetic fuels from renewable energy sources (known as ‘e-fuels’) have become increasingly attractive to achieve GHG emission targets as discussed in the introduction. Increasingly abundant low-cost renewable electricity combined with site specific advantages and policies, has enabled electrochemical processes to compete with traditional thermocatalysis methods, for example the George Olah plant outlined in the introduction coupling hydrogen through electrolysis with thermocatalysis to produce methanol. Water electrolysis is a well-established technology with many reviews (66, 67), hence this section focuses on going beyond hydrogen to the direct electrochemical reduction of CO₂. This area, while much more embryonic, is pushing towards pilot scale with companies like Siemens and Evonik Industries (68), Avantium (69), Opus 12 (70), CERT Systems (71) and Skyre (72) developing commercial systems. High temperature electrolysis to produce CO and syngas (73, 74) using solid oxide electrolyser cell (SOEC) systems could be advantageous if coupled with thermochemical processes to reduce heating cycles. SOECs are not able to reduce CO₂ directly to other hydrocarbons and oxygenates, unlike low temperature electrolysis.

4.1 What are the Possible Products of Carbon Dioxide?

4.2 What are the State of the Art Systems?

4.3 Future Perspectives

Hydrogenations and hydrotreating reactions with molecular hydrogen are critical operations in chemical and related industries. Regardless of the nature of the hydrotreated reactants and products, as society walks away from fossil fuels, the need for hydrogen will remain indispensable. With the increasing access to surplus renewable electricity, the hydrogen production by water electrolysis may take its rightful place not only in transportation and energy storage, but also as a clean hydrogen supply for chemical and related industries (1), replacing greenhouse gas (GHG)-producing methane steam reforming. Among the commercialised and emerging technologies, acidic proton exchange membrane (PEM) electrolysers can operate at up to 20 A cm^–2 current density and deliver up to 700 bar hydrogen at high efficiencies (2). Active and durable electrocatalysts are required to reduce the power input, the bottleneck being the sluggish anodic OER. The acidic environment, however, demands corrosion-resistant materials at high potentials. The winner so far is the OER-active conductive and corrosion-resistant iridium at typical loadings of 1–2 mg cm^–2. Lower iridium requirements were demonstrated, for example, for a PEM electrolyser with the 3 M nanostructured thin film (NSTF) catalyst reaching 2 A cm^–2 current density at 1.86 V at 0.25 mg_Ir cm^–2 (3), which translates into ca. 100 tonnes iridium for the production of 1 TW hydrogen (4). It is obvious that with the current technologies, the terawatt-scale hydrogen production cannot be met with the annual supply of scarce iridium of less than 10 tonnes (4). The annual global demand of hydrogen was reported as 73.9 million tonnes in 2018; as it is almost entirely supplied from fossil fuels, its current production emits 830 million tonnes of carbon dioxide per year (5). The hydrogen production replacement in chemical industries with water electrolysis is relatively well-positioned as a target area for decarbonisation of the industrial sector.

Hundreds of research papers have been focussed on the development of active and durable iridium catalysts, deposition techniques and associated catalyst layer components, which may limit the performance of the most active iridium catalyst formulations. Recent reviews classified iridium-containing catalysts for acidic OER (6) and the variety of methods for the synthesis of iridium oxide (7). Commercial catalyst production methods must be scalable, preferably not require specialised equipment apart from what is available in the catalyst production industries, not produce significant waste and lack the need for large amounts of chemicals, especially those that are hazardous to the environment. With this in mind, the objective of the current minireview is to select a number of the most efficient state-of-the-art iridium catalysts for acidic OER within reported wet-chemistry synthesis methods, focussing on the practicality and scalability of the techniques. We address only wet-chemistry routes, as they are most frequently reported, being relatively accessible in a research environment. Figure 1 and Table I summarise the catalyst synthetic routes and selected catalysts, addressed in this review; this is not a comprehensive summary of all possible routes and catalysts, but rather a careful selection of studies demonstrating promising combination of activity and stability in acidic OER.

Fig. 1.

The catalyst layer preparation methods, such as deposition methods, are out of the scope of this work, although they significantly affect the catalyst performance. Gas-phase catalyst (layer) preparation techniques (13, 14) are omitted for the same reason, as they require specialised equipment and feature simultaneous catalyst formation and its deposition. The review is based only on published research works; we acknowledge that it may become obsolete due to the rapid developments in the field or may miss some critical proprietary information. The citations are chosen only to support our viewpoints; they cannot be considered as a comprehensive list of the relevant works. Herein, we aim to provide comprehensive insights in selected promising wet-synthesis methods. We hope that the review may help the reader in the selection of a state-of-art catalyst for benchmarking purposes, as well as to assist in further developments of potentially scalable synthesis of active and durable iridium catalysts.

Bernt (15, 16) estimated that, in order to decarbonise the transportation sector by transitioning to fuel cell vehicles fuelled by renewable hydrogen, the metal loadings on the anode of polymer electrolyte water electrolysers should be decreased to 0.05 mg_Ir cm^–2. This loading can meet the demand for approximately 150 GW year^–1 installed capacity while using only 50% of the annual iridium production. In order to meet this requirement and also use electrolysis for other needs, such as energy storage and chemical industry supply, the specific iridium activity must be increased substantially. An ideal OER catalyst would have negligible overpotential; a highly desirable catalyst would be one that requires 200–300 mV overpotential (1.43–1.53 V) at 10 mA cm^–2 current, with catalysts achieving this same current density at overpotentials of 300–400 mV (1.53–1.63 V) being acceptable (17). As a durability criterion, if the overpotential is maintained for 10 h, the catalyst may be suitable for real device fabrication, taking into consideration the catalyst nature and mass loading as well (17). The high activity would manifest itself in a low Tafel slope at real operating potentials. The catalyst specific activity (specific current for electrocatalysis) is the product of the following catalyst characteristics, assuming ideal kinetics with no transport limitations (Equation (i)):

(i)

This equation clarifies that it is not enough to develop highly dispersed iridium catalysts, which would be a relatively easy task to do by synthesising sub-2 nm iridium particles with >50% dispersion but that a proper type of iridium species must dominate the surface. Furthermore, the involved parameters must be stable over the device lifetime.

The activity of the catalyst is usually dependant on the OER pathway, which has been shown to depend on the treatment of the iridium catalyst. In general, two mechanisms have been proposed, with the main difference being the predominant involvement of electron-deficient electrophilic oxygen (denoted as O^I–) and as activated lattice oxygen in the reaction. The OER on rutile-type IrO₂ proceeds by means of the classical oxide, electrochemical oxide or electrochemical peroxide pathways involving M–O, M–OH and M–OOH intermediates (18, 19) (Equations (ii)–(vii)):

(ii)

(iii)

(iv)

(v)

(vi)

(vii)

where M represents the metal oxide IrO₂; the oxide pathway involves reactions (ii), (iii) and (v), the electrochemical pathway involves reactions (ii), (iv) and (v), and the peroxide pathway involves reactions (ii), (iv), (vi) and (vii). The peroxide pathway has recently been shown to provide trends that are in agreement with experimental observations by Schuler et al. (19). The electrochemical oxide path is highlighted in red in Figure 2.

Fig. 2.

Catalyst featuring an electrochemically grown porous hydrous oxide layer, also known as oxy-hydroxide layer or amorphous IrO_x catalysts, exhibit an OER mechanism that involves an electrophilic O^I– species (21) and an activated lattice oxygen pathway (22, 23). According to Geiger et al. (22), a simplified pathway highlighting the need for the outer layer of the catalyst to be involved in the reaction could be Equations (viii)–(x):

(viii)

(ix)

(x)

where x is a vacancy in the porous hydrous oxide layer. Other references have attributed the increased activity to electrophilic O^I– species (21). Ir-O-Ir would play a similar role to the proposed highly reactive, electrophilic oxygen O^I– species (21, 24). The second step would have a similar function to the preliminary reaction proposed by Pfeifer et al. (24) (Equation (xi)):

(xi)

where IrO_x O represents the IrO₂ matrix with an adsorbed oxygen. The pathway involving the electrophilic O^I– species is also highlighted in Figure 2 in green, where HIrO₂ would loosely represent the O^I– intermediate.

The OER has been shown to be more active for the pathway involving the electrophilic O^I– species and activated lattice oxygen. Unfortunately, this pathway tends to be deactivated due to the lattice oxygen evolution leading to iridium dissolution, and to the transformation of the oxo-hydroxide to less active anhydrous species (22, 25–27). Stabilisation of the iridium atoms in the pathway involving the electrophilic O^I– intermediate via enhanced crystallinity (26) or the use of mixed oxides could minimise stability issues. Crystalline IrO₂ has a lower intrinsic activity but is more stable due to strong Ir–O bonds between IrO₆ clusters, with only topmost layers of the rutile contributing to both processes (25). The superior stability of thermal IrO₂ is explained by slower kinetics of IrO₃ hydrolysis as compared to its decomposition (20). It has been suggested that both pathways occur during the OER with the activated lattice oxygen pathway being dominant at low potential, due to its high activity, and the classical pathway being dominant at higher potential (20). At potentials relevant to the OER, it is possible that the oxo-hydroxide layer slowly transforms to anhydrous oxide with a subsequent loss in activity and enhancement in stability as recently shown by atomic probe tomography (27).

Speaking of which iridium phase is to be synthesised and introduced into the electrochemical device, the literature features metallic iridium with a variety of predominant crystallographic orientations, amorphous hydrous IrO₂, crystalline rutile IrO₂, their mixtures, as well as multimetallic iridium composites. The metallic iridium may be oxidised by calcination in air before the catalyst layer assembly (28) or electrochemically in situ (26, 29). Thermal iridium oxidation to IrO₂ occurs between 200°C and 500°C (30), the higher the temperature, the higher the crystallinity and electrochemical stability, but the surface area and the activity decrease (31). The 400–500°C region was recommended to strike a balance between activity, stability and conductivity (28).

Many state-of-the-art catalysts, as shown below, use electrochemical in situ oxidation. The iridium (110) surface evolves into two chemically different iridium species, with an active accessible oxide-metal interface (32). The most dense (111) iridium surface is more resistant to the oxidation, and once the oxide is formed, the metallic interface is buried. Although the kinetics of oxide formation and redox properties of the two surfaces are different, their final reached OER activities are rather similar. The same work (32) recommends that for the formation of a porous hydrous IrO₂, the in situ Ir(0) activation should include oxidising-reducing cycles, instead of conventionally used electrooxidation, although another study argues that the repetitive electrochemical oxidation and reduction unavoidably leads to dissolution (33). The electrochemical oxidation proceeds via hydroxide to the irreversible Ir(IV) oxide formation in the nanoparticles, while bulk iridium preserves its metallic subsurface with porous Ir(IV) surface layers (34). Thus, one must be mindful of the iridium dissolution during electrochemical oxidation via hydrous IrO₂ growth (33). When 20 nm iridium films are used for the acidic OER, their lifetime is similar to the lifetime of the hydrous IrO₂ and is significantly lower than for crystalline IrO₂ (22).

To produce highly crystalline IrO₂, which is more stable but less active than hydrous IrO₂, preliminary annealing in air may be recommended, whenever possible. The exceptions, of course, include unsupported polymer-stabilised nanoparticles (26), where annealing would result in particle agglomeration, as well as metal carbides, where it would lead to oxidation and loss of conductivity (35). In such cases, the electrochemical oxidation procedure must be optimised as it affects the catalyst stability.

Fine tuning of the oxide crystallinity, crystallographic orientation, number of oxygen defects and length and strength of Ir–Ir and Ir–O bonds via thoughtful synthetic approaches may diminish the gap between the active but unstable and stable but less active phases. Some such examples, leading to state-of-the-art catalysts, are given below.

Thus, the treasure hunt for the most efficient iridium OER catalyst is a simultaneous optimisation of activity vs. stability. In addition, in order to achieve high iridium utilisation, the catalyst must be easy to integrate into a catalyst layer in order to yield a layer that has both low loadings, and excellent charge and mass transport. Charge transport should include both electronic and protonic in-plane and through-plane conductivities. Proton transport can be optimised by controlling the amount of electrolyte in the layer (36, 37), while improving electron conductivity can be achieved with the use of a conductive support and interconnected IrO_x network. Similarly, good in-plane conductivity could be maximised by using a porous transport layer (PTL) with a microporous layer (38).

The catalyst lifetime depends on its operating current density and overpotential. It may be estimated using the so-called activity-stability factor (39) or stability number (S-number) (22) i.e. the ratio of evolved oxygen to dissolved iridium at constant overpotential (39). Geiger et al. reported that IrO₂ powder features a one year lifetime at 200 A g_Ir^–1 vs. one month for hydrous oxide (22). The numbers are specific to the used electrochemical cell (Figure 3); for example, oxygen bubble accumulation, a common occurrence in rotating disk electrode (RDE), could make the stability studies not a reliable predictor for the catalyst lifetime in a PEM electrolyser long-term behaviour (16). In a PEM electrolyser, iridium dissolution might result in both iridium ions in the water feed, and migration and redeposition at the anode/membrane interface, membrane (40–42) and possible deposition on the cathode leading to platinum deactivation, especially at high current densities and overpotentials (42).

Fig. 3.

Last but not least, even if the most ideal active and stable iridium phase is developed, the benefits will only manifest if other occurring phenomena are not rate-limiting. The high intrinsic activity and stability may be masked by the limitations in the characterisation technique used, or by the electrode fabrication methodology (43, 44). For example, the commonly used RDE technique might be subject to inert backing passivation (44), and catalyst coated membrane (CCM) fabrication techniques, such as spray-coating, doctor blade and inkjet printing method, could result in very different electrode structures for full-cell testing. Inadequate RDE or CCM fabrication, or non-optimal electrolyte loading can result in excessive charge and mass transport limitations. Further, possible causes of loss of activity are poisoning of the catalyst surface by Nafion^TM (45), impurities in the electrolyte in RDE. In the presence of non-iridium components, cations place-exchange with sulfonic acid groups in the polymer electrolyte resulting in decreased proton conductivity (46). All these complicate not only the scale-up, but also the assessment of the intrinsic catalyst performance in laboratory-scale devices. It is possible that efficient catalysts have not been identified because of these limitations. Some benchmarking procedures for the OER evaluation have been proposed in RDE (26, 47) and alternative liquid-flow (44, 48, 49) or vapour-fed cells (19) and are urgently needed to be followed.

3.1 Adams’ Fusion Method

3.2 Iridium Nanoparticles Stabilised by a Capping Agent

3.3 Stabiliser-Free Wet Chemical Synthesis Methods

In heterogeneous catalysis, supports, typically with a high specific surface area, are used to stabilise highly dispersed active metal nanoparticles, both during catalyst synthesis and to ensure their stability against agglomeration during a reaction. In the case of electrolysers, the supports can also be used to enhance the electrode electrical conductivity as they will reduce the contact resistance between particles. The acidic OER environment dictates specific requirements to the type of support: it must be resistant to chemical and electrochemical dissolution, and preferably must have a high electronic conductivity. The latter need is mandatory if iridium loading is low; however, if IrO₂ covers most of the support, it may provide sufficient percolative transport for the electrons (3). The film, however, must be as thin as possible to provide advantages over unsupported IrO₂ nanoparticles. In this subsection, we focus on the wet synthesis of iridium catalysts on powdered supports, which could be mixed with a Nafion^TM solution for catalyst layer preparation.

In recent years, carbon and metal carbides have been receiving less and less attention because of carbon oxidation and volatilisation to CO₂ at high applied potentials (58). As a notable exception, one of the most active iridium catalysts reported so far features a carbon-based support (10). A 40 wt% iridium catalyst was prepared by impregnation of graphitic carbon nitride (GCN) nanosheets with the metal precursor followed by annealing in air at 350°C. Thus embedded IrO₂ possesses compressed Ir–Ir bonds and decreased coordination numbers of Ir–O and Ir–Ir, which was suggested to weaken the adsorption of oxygen intermediates leading to increased OER activity. The reported specific currents are 580 A g_Ir^–1 at 1.55 V and 1493 A g_Ir^–1 at 1.6 V. The catalyst required the overpotential of 278 mV at 10 mA cm^–2 at 0.07 mg_Ir cm^–2 loading (in RDE in 0.5 M H₂SO₄). Authors demonstrated only 35 mV potential increase at 20 mA cm^–2 for ca. 4 h in an RDE; and 78.5% current retention in a laboratory water splitting device after a 24 h operation at 1.6 V. The fate of GCN was assessed by holding 2.2 V_RHE for 2 h with intermediate cyclic voltammetry (CV) measurements between 0.4 V_RHE and 0.6 V_RHE; the double-layer capacitance decreased by 10% in the first 0.5 h and remained stable up to 2 h (10); apparently, the graphitic support nature with nitrogen heteroatom provides its stability in acidic electrolysis. Given the high catalyst activity and a rather easy and potentially scalable preparation of GCN and Ir/GCN, the studies of the catalyst durability and performance in a PEM electrolyser are warranted.

Among oxidation-resistant conductive metal oxide supports, antimony tin oxide (ATO), indium tin oxide (ITO) and fluorine tin oxide (FTO) have been the focus of the most research because of their relatively high conductivity. Unfortunately, the dopant’s corrosion brings down the conductivity, increasing ohmic losses and decreasing the energy efficiency (59). Dissolved cations may also ion-exchange with the membrane and lead to its degradation (46). A recent study observed neither activity nor stability benefits from the dopant addition (60). Although FTO possess the lowest conductivity, it was found to be the most stable material between –0.34 V_RHE and 2.7 V_RHE, followed by ITO and ATO. The stability is assigned to the oxygen atom exchange in SnO₂ with fluorine, instead of cation exchange in the case of ATO and ITO synthesis (59). ATO, in turn, was suggested to mitigate iridium dissolution by preserving it in lower oxidation states (61). Commercially available samples usually feature low surface areas; several synthetic techniques were suggested in literature for the preparation of mesoporous doped SnO₂ with relatively high areas (for example, between 125 m² g^–1 and 263 m² g^–1) (62). To deal with toxic NH₄F for the FTO synthesis, safety measures must be in place, as in any chemical and engineering process. A number of iridium catalysts supported on doped SnO₂ with high activities and low overpotentials at low iridium loadings were recently reported.

A popular method for the preparation of supported iridium OER catalysts is a colloidal precipitation of IrO₂ from an iridium molecular precursor by means of NaOH; the synthesis may proceed in ethylene glycol (29), which serves both as a solvent and a reducing agent, or, for example, in a hydrothermal microwave reactor (63). Small 2–3 nm particles may be obtained (61), or even smaller (1.5 ± 0.2 nm) if a stabiliser is added (64). The support may be added to the colloidal dispersion either during synthesis, or after the nanoparticle formation. The use of high-boiling ethylene glycol, though, complicates the potential process scale-up because solvent removal under vacuum is usually used (57), instead of centrifugation or filtration of highly diluted suspensions, as practiced in laboratories. As an example of such preparation method, when SnO₂ was doped with tantalum to produce tantalum tin oxide (TaTO) and used to deposit preformed 1.7 nm IrO_x nanoparticles at 11–18 wt% iridium loading, the OER activity of the fresh catalysts after electrochemical conditioning approached 250 A g_Ir^–1 at overpotentials of 280 mV and 370 mV at 0.020 mg_Ir cm^–2 loading (25°C, 0.05 M H₂SO₄) (11). Although the electronic conductivity of TaTO was two orders of magnitude lower than that of ATO, its use did not result in decreased activity, which was ascribed to the conducting role of well dispersed IrO_x nanoparticles. In accelerated ageing tests at 1.2–1.6 V potential steps, the IrO_x /TaTO catalysts demonstrated between 70% and 90% activity retention vs. 60% for the ATO-supported catalyst. The loss of the dopant was one or two orders of magnitude lower for tantalum as compared to antimony, while the loss of tin was not affected. The iridium dissolution was found dependent on the tantalum loading: the higher loading decreased iridium oxidation state contributing to its dissolution, while at lower loadings tantalum shell suppressed IrO_x nanoparticle detachment. This study (11) also demonstrates that the use of a support contributes to enhanced iridium leaching, as compared to commercial IrO₂. As a result, although the activity of unsupported IrO₂ is significantly lower than that of the developed catalysts, its stability to dissolution contributes to high S-numbers (ratio of evolved oxygen to dissolved iridium) (22). For example the S-number for IrO₂ was twice as high as that for selected IrO_x /TaTO catalysts at 1.6 V and similar at 1.5 V (11). When hydrous IrO_x was supported on ATO, its S-number was also lower than the one for unsupported IrO_x , but the calcined supported samples demonstrated up to two orders of magnitude higher S-number as compared to IrO₂ (60).

When the above-mentioned (Section 3.2) surfactant-stabilised iridium nanodendrites (39 m² g^–1 area) were deposited on high-surface-area ATO (235 m² g^–1), an initial overpotential of 260 mV at 10 mA cm^–2 at only 0.0102 mg_Ir cm^–2 loading (RDE, 0.05 M H₂SO₄) was observed while accelerated durability test showed a minor overpotential increase by 30 mV over 15 h compared to an abrupt increase for other tested catalysts at earlier times (29). The specific current at 280 mV overpotential was reported at 70 A g_Ir^{– 1} vs. 8 A g_Ir^–1 for Ir black. In a PEM electrolyser, the catalyst demonstrated the current density of 1.5 A cm^–2 at 1.8 V and 1 mg cm^–2 loading compared to 0.8 A cm^–2 for Ir black.

An atomically dispersed iridium on ITO with ultimate iridium dispersion was developed by grafting 0.86 wt% iridium as an organometallic iridium complex followed by calcination in air at 400°C (65). The specific current of 156 A g^–1 iridium was reached at 280 mV overpotential and 0.021 mg_Ir cm^–2 loading (0.1 M HClO₄). An overpotential of 350 ± 20 mV was required to drive the 10 mA cm^–2 current at such low metal loadings over the course of 2 h; some iridium agglomeration was observed in the used catalyst and its consequences on the long-term performance requires further analysis.

From the catalyst synthesis viewpoint, one must be mindful that the support’s chemical composition may change during the synthesis, affecting the electrochemical performance. For example, if Adams’ method involving high-temperature calcination is used to produce IrO₂ on a carbide support, the support oxidation leads to the loss of conductivity (for example, TaC lost its conductivity from 120 S cm^–1 to 10^-8 S cm^–1 at such circumstances) (35). A similar carbide oxidation to a less-conductive oxide was reported for iridium nanoparticle synthesis in the presence of a support by a polyol method (heating in a reducing ethylene glycol with precipitating NaOH) (58). Moreover, iridium, tin and indium oxides may form mixed oxides; the lattice vacancies are thus produced upon tin and indium in situ dissolution, improving the initial activity but jeopardising the durability (66) due to enhanced iridium dissolution.

Development of multimetallic iridium-containing catalysts has recently attracted considerable attention as a means of enhancing the OER catalyst performance, as had proven beneficial for catalyst development for fuel cells and alkaline water electrolysis. However, with iridium being the most corrosion resistant metal and still dissolving under the acidic OER conditions, any other metal would have an even higher dissolution rate. A rather popular combination of iridium and ruthenium features high activities due to the higher OER activity of ruthenium than that of iridium, but is not practical because of low corrosion resistance of ruthenium, which is also a scarce and expensive metal. The studies of the mixed oxide OER catalysts do typically address (and inevitably show) the dissolution of the catalyst components, but there is a lack of studies on the effect of the leached ions on the PEM system level. It is likely that the non-iridium cations may not only ion-exchange on Nafion^TM changing its properties (67), but may also travel to the cathode side and poison the platinum cathode as was shown for iridium (42). Membrane degradation may also occur due to the attack of HO^• and other radicals, whose formation is catalysed by transition metal cations (46). For example, iron and copper ions were shown to dramatically enhance membrane degradation (68).

Thus, a practical mixed oxide catalyst for an acidic OER application may be envisioned as one of the following composites (Figure 6): (a) an IrO_x shell fully covering the core with an earth-abundant metal increasing the iridium dispersion. In this case, electrolyte contamination with the second metal may be delayed as compared to the mixed alloys until the iridium shell atoms leach exposing the core atoms; (b) iridium nanostructures produced by the preliminary removal of a sacrificial second component from a bimetallic composite, either by potential cycling or chemically. The selective leaching of the second component leads to surface restructuring (69), porosity enhancement (39), formation of lattice vacancies (70) and ECSA increase (71). Lattice vacancies formation via secondary metal leaching is a unique opportunity to modify the electrophilicity and Ir–O bond length leading to the enhanced OER activity as compared to IrO_x synthesised only from an iridium precursor (70, 72). Some works report, though, that iridium leaching from the composites may even be increased due to the created lattice vacancies, as compared to monometallic iridium catalysts (39).

Fig. 6.

Below, we provide some examples for such catalyst synthesis and performance. We note that the direct deposition methods, such as reactive sputtering and physical vapour deposition, are very frequently used for the mixed oxide studies (39, 71–75), because they ensure structures with well-controlled composition and stoichiometry, thus, enabling the fundamental understanding of the composite’s behaviour. As this review does not cover the catalyst layer preparation methods, we only focus on the production of metal oxide powders or colloidal dispersions which were or could be mixed with the Nafion^TM solution.

5.1 Core-Shell Bimetallic Nanoparticles

5.2 Selective Leaching of the Sacrificial Component (Hard Templating)

The fast-growing pool of published studies on the iridium-catalysed acidic OER continually contributes to the mechanistic understanding of iridium activity and durability, the role of the second metal in alloys and metal-support interactions. As a myriad of more or less sophisticated methods and structures emerges, one must keep in mind the method practicality, safety, ease and scalability, and that it must use as little resources and produce as little waste as possible. This, most likely, precludes the use of surfactant-assisted routes of wet catalyst synthesis and might jeopardise stabiliser-free colloidal synthesis in vast amounts of organic solvents.

Despite the many advances on catalyst synthesis, and recently proposed metrics to assess catalyst activity and stability, such as the S-number and the activity-stability factor, a lack of consensus on the metrics used to report catalyst performance, as well as the electrochemical testing benchmarking procedures, make the comparison of different catalysts challenging. Activity studies reviewed for the fresh catalysts are not always accompanied by meaningful stability assessment, and high reported activities should be regarded with care, as the most active (hydrous) amorphous IrO_x is less stable than the less active crystalline IrO₂. With such considerations, it appears, to our subjective opinion, that Adams’ fusion method (or other thermal methods) producing relatively stable rutile, deserve closer attention and further modifications. To take advantage of the enhanced activity of electrophilic oxygen sites, as follows from the multioxide studies, modification of the electrophilicity and Ir–O bond length via secondary metal leaching is a possible way to bridge the gap between the activity and stability. The use of supports may increase the stability of the calcined catalysts, however, the addition of dopants to improve electrical conductivity must be used with care.

One must keep in mind that the catalyst layer fabrication can have a tremendous effect on the activity observed in a real electrode. The ionomer-to-catalyst ratio, particle size, the use of a support, catalyst-ionomer-solvent interactions in the dispersion, and deposition technique, can have a significant impact on the electrode electrochemical surface area, ion and electron conductivity and its ability to transport reactant and byproducts in and out of the cell. Gas-phase catalyst synthesis and deposition methods, for example (13), which were not reviewed here, may provide a promising alternative to the wet-chemistry techniques. Moving forward, not only the catalyst synthesis, but also development of common characterisation techniques and metrics to assess activity and stability in RDE, and optimal electrode fabrication methods must become paramount in our collective effort to develop the most efficient OER catalyst, and OER electrode.

Oils derived from oleaginous microbes, such as algae, yeast and bacteria (so-called single-cell oils (SCOs)) have long held promise as a biofuel precursor due to the high lipid content (>20%), high growth rate and the ability of these microbes to be cultivated on feedstocks or in areas that do not compete for terrestrial food production (1). Historically, the availability of microbial lipids has been quite limited, but technology developments in food and feed applications may begin to benefit lipid production for biofuels as well. Corbion produces an algae oil high in oleic acid for culinary use, and an algae-based omega-3 fatty-acid-enriched product for fish feed. Qualitas Health, DSM, Evonik and Veramaris also produce omega-3 enriched oils for food and feed applications, though to our knowledge only Qualitas Health is producing the algae photoautotrophically. While the target omega-3 fraction is not ideal for fuel production due to long carbon chains and numerous double bonds that would increase hydrogen consumption during hydroprocessing, the process to enrich omega-3 oils likely generates a byproduct of lighter and more saturated fatty acids that may be more suitable for fuel production than the omega-3 fraction. Additionally, although many oil majors and startup companies have pivoted away from algal biofuels, a partnership between Synthetic Genomics and ExxonMobil to improve algal biology for fuel production is still active. The increasing availability of cellulosic sugars may also pave the way for larger scale cultivation of heterotrophic oleaginous microbes.

While both the growth and characterisation of oleaginous microbes (1–13) and the conversion of vegetable oils to fuels (13–27) have been extensively studied, there has been considerably less research on converting SCOs to fuels, and especially on converting SCOs to so-called ‘drop-in’ hydrocarbon fuels that are compatible with existing fuel infrastructure (28–30). However, production of hydrocarbon fuels from SCOs is a growing area of research and hydroprocessing of these oils requires some considerations that are unique to the differing composition of SCOs. Enough recent studies have been published that a summary of findings and perspective on research directions is warranted, especially with respect to the differences between terrestrial oils and SCOs. In particular, lipid recovery, lipid cleanup and catalyst selection all appear to play a key role in hydroprocessing performance, and there are clear needs for deeper research in each of these areas.

In most cases, SCOs are produced intracellularly, and thus necessitate separation from the other cell components prior to upgrading. Additionally, the microbial biomass is typically recovered with a very high water content (≥80%) relative to terrestrial biomass. Thus, while vegetable oils can be effectively extracted by pressing, solvent extraction or a combination thereof, these operations are usually not effective when applied directly to microbial biomass. Furthermore, it is energy intensive to dehydrate microbial biomass and therefore strategies that can extract oil in high yields from wet biomass are required. The first step is commonly to rupture the microbial cell wall which allows access to the lipids.

Various cell wall lysis methods have been applied for this application, such as high-pressure homogenisation (HPH), bead milling, ultrasound, pulsed electric field (PEF), osmotic shock, microwave, subcritical water hydrolysis, enzymatic hydrolysis, autolysis and chemical hydrolysis (31). From the perspective of energy consumption, pretreatment methods such as HPH, subcritical water hydrolysis, enzymatic and chemical hydrolysis are more attractive due to the relatively lower energy requirements. Moreover, all these methods can be scaled up to industrial application (32). Researchers at the National Renewable Energy Laboratory (NREL) have shown that dilute acid pretreatment is an effective, low cost, energy efficient and scalable means to recover lipids from wet microbial biomass (31–34).

It is also necessary to consider the compatibility of the cell lysis method with lipid extraction. For example, hexane is a commonly employed extraction solvent, but does not always maximise extraction yields. In many cases, modification of solvent polarity (for example employing a mix of hexane with ethanol or isopropanol) is necessary for high lipid recovery (35–39). However, incorporation of a polar solvent also often leads to more prevalent co-extraction of other polar materials, such as polar lipids, pigments, proteins and sterols, that require more severe cleanup of the lipid phase. In particular, extraction methods that incorporate chlorinated solvents, such as the well-known Folch and Bligh-Dyer extraction protocols, tend to extract significant amounts of polar lipids and other polar materials (36, 38). Combinations of hexane with alcohols, such as ethanol and isopropanol, have the potential to more selectively extract hydrophobic lipids while maintaining high extraction yields of these components (31, 39, 40). The optimal balance is thus a function of lipid speciation, content and speciation of other potential co-extractives, the lipid cleanup strategy and the hydroprocessing approach.

SCOs that are considered for biofuel production are mainly recovered from oleaginous algae, fungi (yeast and filamentous) and bacteria, with the number of known oleaginous species decreasing by family in the order listed. These microbes may be cultivated autotrophically (using only CO₂ as a carbon source), mixotrophically (using a mixture of CO₂ and organic carbon sources) or heterotrophically (using organic carbon sources, such as glucose and acetate). Several recent reviews have compiled oil content and broad composition of oleaginous microbes and their production of oils with an emphasis on wastes and lignocellulosic carbon sources (7–9, 11). A sampling of these compositions is reproduced in Table I, with comparison to common vegetable oils. It is important to note that analytical methods vary greatly across the references cited in Table I. The lipid content data, especially, should be taken with a grain of salt.

Both SCOs and terrestrial plant oils typically contain high proportions of palmitic (16:0), palmitoleic (16:1), stearic (18:0), oleic (18:1) and linoleic (18:2) acids. On the surface, the similar composition of both oils appears to make SCOs an attractive feedstock for catalytic upgrading. However, SCOs can contain a broader distribution of lipid classes than terrestrial plant oils, including higher proportions of polar lipids, free fatty acids (FFA), sterols, terpenes, carotenoids and chlorophyll, as well as unidentified compounds, that can be co-extracted with the desired monoacylglycerides (MAG), diacylglycerides (DAG), triacylglycerides (TAG) and FFA. Examples of these other lipid classes are shown in Figure 1. The proportions of each vary greatly depending on the organism, growth mode, growth condition, growth stage and postharvest processing conditions, including biomass and extracted oil storage and solvent extraction conditions (36, 60). For example, glycolipids and phospholipids can account for 17–90% of the total lipids in autotrophically-grown algae (61), compared to ~2% in soybean oil (62), and lipase enzymes present in the cells can convert acylglycerides to FFA during lipid or biomass storage (60).

Fig. 1.

TAGs and FFAs are usually considered as the favoured precursors for biodiesel or hydrocarbon-based biofuels such as renewable diesel or renewable jet fuel. Other components are often detrimental in catalytic hydroprocessing as they either increase hydrogen consumption due to a high degree of unsaturation (for example terpenes, carotenoids and chlorophyll), contain heteroatoms that can poison metal or acid catalysts (for example polar lipids and chlorophyll) or contribute to poor cold flow properties (for example sterol-derived cycloparaffins). For example, polar lipids mainly consist of phospholipids, glycolipids, lipoproteins and sulfolipids. A phospholipid molecule consists of a polar phosphorus-containing moiety (such as phosphate, phosphatidylethanolamine or phosphatidylcholine), whereas a glycolipid contains a polar carbohydrate moiety in place of the phosphor group. The sidechains of polar lipids (sugars, proteins or phosphorous-containing molecules) may also include other nitrogen, sulfur or phosphorus-containing moieties (21).

On the other hand, hydrotreating of some of these contaminants can be beneficial in that the high degree of branching (for example in terpenes and the phytol chain of chlorophyll) can potentially improve cold flow properties and increase the value of the renewable diesel fuel. Additionally, when a large proportion of the lipid fraction comprises nominally undesirable molecules (such as polar lipids), removal of the entire molecule in a cleanup strategy would result in low fuel yields. Thus, while some cleanup of the oils is likely necessary, the most desirable cleanup strategy is not universal across SCOs, and tradeoffs between overall yields and catalyst longevity must be considered.

Cleanup of SCOs targets many of the same impurities that are removed in refining of vegetable oils for human consumption, and thus cleanup approaches can be broadly categorised into the same four operations: bleaching to remove pigments and polar compounds, degumming to precipitate phospholipids, deodorisation to remove FFAs and distillation to remove additional FFAs and other volatile matter. When refining SCOs for hydrotreating, some modifications may be necessary. For example, FFAs are one of the preferred feeds for hydrotreating so deodorisation and distillation will not always be needed. However, in some cases it may be advantageous to hydrolyse the entire lipid stream to FFAs, distil these FFAs away from impurities, and route the distillate to hydrotreating. In these cases, bleaching and degumming may not be needed. Researchers have explored to some extent each of these steps except deodorisation. These techniques and their advantages and disadvantages are summarised in Table II.

Crude extracted oils from autotrophically-grown microbes are often high in chlorophyll content. The removal of chlorophyll is important in both hydroprocessing and biodiesel production as chlorophyll contamination can deactivate catalysts and degrade fuel quality (60, 63, 64). For example, the porphyrin head of the chlorophyll molecule is highly unsaturated and contains the catalyst-poisoning heteroatoms nitrogen and magnesium. The two most common approaches to remove chlorophyll are adsorption on a solid material and dealkylation via acid treatment.

Bleaching earths, activated carbon and silica are common adsorbents for oil refining. Adsorption of chlorophyll is initiated by ion exchange of the Mg²⁺ centre of the porphyrin head with a proton. The metal-free porphyrin is then protonated and adsorbed (65). Adsorbents are commonly pretreated with mineral acids to increase the number of exchangeable protons (66). Ultrasound-assisted adsorption has also been demonstrated to increase the rate of chlorophyll removal from oils (67). It is worth noting that the loadings of polar adsorbents to fully remove chlorophyll from SCOs may be much higher than typically used for edible oil bleaching due to the higher levels of chlorophyll (and other adsorbing contaminants). For instance, Chen et al. employed bleaching earth to adsorb chlorophyll from microalgal biodiesel at a mass loading of 16 wt% (60), whereas a typical edible oil bleaching process uses a mass loading of 1.5 wt% (68). Increasing adsorbent loading, while effective in reducing chlorophyll content, decreases yields due to co-adsorption of lipids. This is particularly true of bleaching earths, which can retain up to a third of their mass in oil (69), though even higher retentions have been claimed (70). Similarly, Santillan-Jimenez et al. removed chlorophyll from crude extracted algae oil via adsorption on K10 montmorillonite clay and activated carbon, recovering only 58 wt% and 46 wt% of the original extract mass, respectively (71). Gas chromatography-mass spectrometry (GCMS) analysis of the recovered materials indicated a high content of fatty acids, hydrocarbons and phytol isomers, while ultraviolet-visible (UV-vis) spectroscopic and inductively coupled plasma (ICP) analyses revealed complete chlorophyll, phosphorus and magnesium removal. Notably, the relative abundance of hydrocarbons in the activated carbon-bleached material was lower than in the K10-bleached material, likely due to the greater hydrophobicity of the activated carbon retaining more of the hydrocarbons. This observation is also consistent with the lower total recovered yield from the carbon adsorbent. Thus, adsorbents with higher polarity may be more selective for removal of the target compounds.

Phosphoric acid-catalysed dealkylation was originally developed by Diosady to refine Canadian canola oil, which has anomalously high chlorophyll content for a terrestrial oil due to weather conditions in the region (70). The acid bleaching approach employs anhydrous acids, such as phosphoric acid or sulfuric acid to cleave the phytol side chain from the porphyrin structure, the latter of which is not soluble in oil. Thus, this precipitable form of pheophorbide can be easily separated from the oil by filtration while the hydrophobic phytol is preserved. Retaining the phytol in the oil confers the additional benefit of improved biofuel yield and performance due to a high degree of branching. Dong et al. employed a modified version of Diosady’s method to remove 99.7% of chlorophyll from algae oil (63). In a direct comparison of adsorptive and dealkylative approaches, Kruger et al. found that phosphoric-acid-catalysed dealkylation was more effective than adsorption on silica for the removal of chlorophyll from algae oil (72). In the comparison, adsorption on silica gel removed 85% of the oil nitrogen content and still showed a detectable chlorophyll signal in UV-vis analysis, while acid bleaching removed 92% of the oil nitrogen and did not show a detectable chlorophyll UV-vis signal. Moreover, phosphoric acid bleaching is compatible with conventional vegetable oil refining practice, since phosphoric acid is routinely added for oil degumming. However, for the same reason and also because the acid must be washed out of the oil by adding water, phosphoric acid bleaching may suffer from yield losses and poor bleaching performance in oils with high levels of phospholipids.

Apart from adsorption and acid treatment, other novel methods of chlorophyll removal have been reported. Sathish developed a multi-step process to remove chlorophyll from microalgal crude oil, involving sequential aqueous acid, base and acid treatments to lyse cells, saponify lipids and precipitate chlorophyll, respectively (73). Though the extraction process was not optimised, roughly 20% of the lipids remained with the biomass solids and another 20% with the precipitated chlorophyll. However, the purified lipids showed no chlorophyll signal in UV-vis analysis. The process is advantageous in that it does not require anhydrous conditions, but the sequential use of concentrated acids and bases is likely to be expensive and generate highly saline wastewater. On the other hand, Li et al. removed chlorophyll from intact microalgae by saponification with sodium hydroxide (74).

Analogous to chlorophyll, SCOs can have higher phospholipid content than most terrestrial oils, and removal of these polar lipids is necessary to prevent catalyst fouling in downstream processes. Further complexity arises as a large portion of phospholipids in SCOs are non-hydratable, namely phosphatidic acid and phosphatidylethanolamine (75). These species complex with metal cations and cannot be removed by conventional water degumming processes that precipitate hydratable phospholipids. Other degumming methods have been developed to remove these non-hydratable phospholipids (76).

Acid degumming is the most common approach to removing non-hydratable phospholipids. An acid is added to decompose phospholipid salts to improve their hydratability. Phosphoric acid is routinely used in algae oil degumming given the co-benefit of chlorophyll reduction (32, 54, 60). A drawback of acid degumming, however, is that some decomposed phosphatic acid will remain in the oil phase (76). Subsequent addition of a small amount of diluted base – enough to neutralise phosphatic acid while avoiding saponification – has been shown to be effective in further reducing phosphorus content of acid-degummed oils (77). Other methods which target both hydratable and non-hydratable phospholipids, such as membrane or enzymatic degumming, are in nascent stages of development. While promising, the cost of these techniques are prohibitively expensive for SCO degumming in their current state (76).

Given the potentially high content of microbial polar lipids, a remarkable amount of microbial lipid may be lost in the degumming process (60). Nevertheless, in some SCOs degumming may be appropriate, and examples of both dilute acid and solvent degumming exist in the patent literature (78–80). For SCO streams containing large fractions of polar lipids, hydrolysis to a FFA stream is likely preferable for fuel production (34).

As an alternative to removing impurities from the lipid stream, approaches have also been developed to remove only the desired components from the lipid stream and leave everything else. Within this realm, hydrolysis and methanolysis are the primary approaches applied, generating FFA and FAME, respectively, both of which are distillable.

Hydrolysis can employ acids, bases or high temperature water to cleave fatty acid and other esters. Alkaline hydrolysis (i.e. saponification) is advantageous in that it requires mild temperatures and short reaction times (typically 80°C and 1 h, respectively). However, the stoichiometric consumption of alkali likely makes this approach too expensive for a fuel production scenario. Acid hydrolysis is favourable in that the acid is catalytic and mild temperatures are also typically employed (100°C or less), but reaction times are typically 8–24 h. Acid hydrolysis was the basis of the industrial Twitchell process, which was subsequently superseded by steam splitting. Steam splitting via the Colgate-Emery process is the current industrial standard, employing temperatures of 250–330°C and reaction times of 2–3 h (81). Higher temperatures can significantly shorten the reaction time (for example to ~10 min), but also significantly increase the operating pressure (for example to ~2500 psig) (82). Steam splitting is effective with both acylglycerides and with phospholipids (82). Lawal et al. demonstrated the effectiveness of hydrolysing algal lipids to FFAs and distilling the hydrolysed FFAs away from impurities (83). An alternative approach involves engineering microbes to produce and excrete FFAs directly into the growth medium, which could simplify the cell lysis and lipid extraction operations. This approach was recently demonstrated in the cyanobacteria Synechocystis sp. PCC6803 (45).

Alternatively, microbial lipids can be converted to methyl esters, which facilitates distillation without vacuum. The (trans)esterification to methyl esters is widely used on a commercial scale to produce biodiesel, and occurs under similarly mild conditions as saponification. Murzin and coworkers demonstrated the effectiveness of this cleanup approach for removing impurities in algae oil (84–86). However, this approach suffers from two significant drawbacks, namely that it requires a dry feedstock and that it consumes methanol stoichiometrically. Drying of microbial biomass is much more energy intensive than drying terrestrial crops, and may be economically untenable for fuel production. Process engineering may make it feasible to recycle the methanol (or valorise it, for example in a methanol-to-gasoline-type process) though this has not been explored to our knowledge.

Several types of SCOs have been upgraded to hydrocarbon fuels with and without prior cleanup. Among these, algae oils are by far the most common. Hydroprocessing approaches and cleanup approaches for use on algae oils have varied significantly, though many employed techniques to specifically remove chlorophyll. Hydroprocessing of yeast oils, which do not contain chlorophyll but may contain sterols, has also been reported by a few researchers. Among other microbes, to our knowledge only lipids from the methanotrophic bacterium Methylomicrobium buryatense have been upgraded to hydrocarbons. A summary of SCO hydroprocessing literature is provided in Table III.

Lawal and coworkers explored deoxygenation of algae oils from Nannochloropsis salina and Chlorella vulgaris over platinum, rhodium and nickel-molybdenum-based catalysts (83, 87, 88). The oils were obtained from Valicor, which employed a thermochemical pretreatment of the algal biomass to render the lipids extractable (83, 106). Notably, this pretreatment reduced the amount of phosphorus and iron in the Chlorella oil by nearly 100%, while sulfur was only reduced by 30%. For the N. salina oil, however, the sulfur and phosphorus content remained relatively high at 2033 ppm and 246 ppm, respectively. Additionally, the N. salina oil contained only 47% material that could be identified as acylglycerides or FFAs (83, 87). Analysis showed that the oil had up to 0.8% chlorophyll, 0.5% carotenoids, 5% sterols, 1–5% mannitol and a large fraction of unsaponifiable matter that was not identified (83). While most reaction conditions explored showed significant catalyst deactivation within 7 h time on stream (TOS), Zhou and Lawal were able to find a set of conditions for both a NiMo/Al₂O₃ and a Pt/Al₂O₃ catalyst that did not show significant deactivation within this timeframe, and obtained hydrocarbon yields above 60 wt% (87, 88). The optimal conditions for the platinum and nickel-molybdenum catalyst were significantly different, underscoring the need for reaction engineering studies for each combination of catalyst and oil.

Crocker and co-workers explored deoxygenation of algae oils from Scenedesmus acutus grown on power plant flue gas as a carbon source using nickel-based catalysts (71, 89–91). While the focus of these experiments was generally on model compounds, all of the catalysts employed with algae oils (Ni/Al₂O₃, Ni-Al layered double hydroxide and Ni-Cu/Al₂O₃) deactivated faster with the algae oils (within 4 h TOS) than with vegetable oils or model compounds such as triolein, despite the fact that the oil concentration was only 1.3 wt% in a dodecane solvent. The fast deactivation may have been in part due to the low temperature employed (260°C). In particular, the Ni-Al layered double hydroxide catalyst showed significantly less deactivation at 300°C. The faster deactivation with algae oils is also notable in light of the cleanup procedures employed: the lipids were purified by column chromatography using either K10 montmorillonite clay or activated carbon with silica gel, which was effective at removing chlorophyll and other pigments, magnesium and phosphorus. K10 montmorillonite appeared to be a more effective adsorbent than activated carbon, resulting in a greater fraction of products in the diesel range. The authors hypothesised that several unquantified components in the algae oil, including FFAs, polar lipids, sterols or fatty amides could be contributing to the deactivation (90). However, experiments with mixtures of model TAGs and FFAs suggested FFAs were not the culprit (90). A more detailed analysis of impurities also suggested that the oil cleanup quantitatively removed phosphorus and magnesium, though a small amount of nitrogen remained (71). Santillan-Jimenez et al. (71) also noted that the residual extraction solvent (namely chloroform) was likely the source of chloride detected on the catalyst in post-reaction analysis (71), underscoring the observation that the extraction protocol can impact downstream processes through routes other than influencing the types of lipids extracted.

Lercher and coworkers explored nickel-based catalysts for algae oil deoxygenation under similar conditions to those above (though also in batch mode), using commercial algae oil of an unspecified species (92–94). In these experiments, support acidity (HBEA zeolite or zirconia) was shown in mechanistic studies with model compounds to play a key role in the reaction kinetics, and the nickel particle size was also key to catalyst activity. The oil was not analysed for impurities (only fatty acid composition), but did not deactivate the catalysts for at least 72 h (for Ni/ZrO₂) (93) or 120 h (for Ni/HBEA) (92) TOS. Notably, some isoalkanes, which improve the cold flow properties of the diesel blendstock, were observed in the products due to the support acidity.

Nguyen et al. explored both nickel and molybdenum-based catalysts in deoxygenation of Chlorella algae oil extracted by supercritical hexane (95, 96). In these studies, the catalysts were rapidly deactivated (within 1 h) and produced less than 10% yield to hydrocarbons. In contrast, FFAs were the major product, with acylglycerides featured prominently as well. The authors noted that the crude extracted lipids comprised only around 50% fatty acids and 3–4% sterols, with the remainder unidentified (95). While low conversion of the FFA and acylglycerides was observed, no conversion of the sterols or steryl esters was found (96). Nguyen et al. also noted that while molybdenum nitride catalysts had much lower initial activity than nickel-based catalysts, the molybdenum nitride catalysts did not show as strong a deactivation as the nickel catalysts did (95). The low activity of the molybdenum nitride catalysts represents a significant challenge nonetheless. However, recovery of the lipids as methyl esters and purification over a bleaching clay produced a feedstock that could be readily converted over a Ni-HY zeolite catalyst (84–86). Interestingly, a Pd/C catalyst did not perform as well as the Ni-HY catalyst in deoxygenation of the purified methyl esters, possibly due to the lower acidity of the carbon support.

Researchers at NREL demonstrated both deoxygenation and hydroisomerisation of Scenedesmus algae oil recovered after acid pretreatment and hexane extraction (72, 97). Deoxygenation employed a Pd/C catalyst, while hydroisomerisation employed a Pt/SAPO-11 catalyst. Kruger et al. found that while relatively severe temperature and pressure (450°C and 1300 psig) and dilution of the algae oil in a hexane solvent were required for complete conversion and stable performance of the Pd/C deoxygenation catalyst, bleaching the oils with either silica gel or with phosphoric acid did not improve the hydroisomerisation performance, at least through 10 h TOS (72). The deoxygenation step also resulted in significant denitrogenation as well. The severe conditions required for deoxygenation led to significant cracking and lower yields to diesel range products, but hydroisomerisation produced renewable diesel blendstocks with cloud points below –10°C (72). The latter step is important to demonstrate given the non-zero levels of residual nitrogen in the deoxygenated feedstock that could poison acidic isomerisation catalysts.

Robota et al. also employed a Pd/C catalyst for deoxygenation of a neat algae oil, and then hydroisomerised the resulting alkanes with a Pt/USY zeolite catalyst (98). Despite the higher concentration of oil, less severe conditions were required for nearly complete conversion of the oil to alkanes than was observed by Kruger et al. (72), though a second pass and a polishing step to remove residual oxygenates were used to produce a clean alkane stream for hydroisomerisation. With the large-pore Pt/USY zeolite catalyst, isomerisation and cracking were competitive, resulting in a 40% or more mass loss to naphtha-range hydrocarbons. Though cloud points of the resulting products were not measured, solvent dewaxing yielded a product that remained liquid at –20°C.

Soni et al. (99) evaluated cobalt/natural clay catalysts for deoxygenation of a commercial algae oil, and achieved hydrocarbon yields around 85% after 8 h in a batch reactor at 580 psig hydrogen. This oil contained nearly 70% docosahexaenoic acid in its FFA profile, suggesting that the product alkanes would require some degree of cracking to serve as a diesel blendstock.

Fu et al. (100) demonstrated hydrogen-free decarboxylation of Chlorella oil at 330–370°C for 2 h in a batch reactor. The heating value and carbon content of the organic phase both increased and the organic product contained significantly isomerised compounds, though conversion, yield, oil characterisation and catalyst characterisation were not reported.

Kandel et al. (101) reported the conversion of an algae oil from Solix Biofuels into hydrocarbons using a nanoparticle iron catalyst supported on mesoporous silica. After 6 h at 290°C and 440 psig hydrogen, 67% of the algae oil had been converted to both saturated and unsaturated alcohols and hydrocarbons. The authors noted that the presence of unsaturated compounds suggests hydrogen may have been limiting.

Tang et al. (102) evaluated deoxygenation of a Scenedesmus algae oil in a batch reactor over a NiMo/Al₂O₃ catalyst. As the reaction temperature increased from 290°C to 350°C (and as catalyst loading increased from 0 wt% to 50 wt% of the oil), the product spectrum shifted from acids, esters and alcohols to paraffins, olefins, naphthenes and aromatics, with a small amount of isoparaffins also present. The authors used multiple solvent combinations to extract the lipids. Dichloromethane:methanol was the highest yielding solvent on a weight basis, but the primary components were sterols and tocopherols rather than acylglycerides and FFAs. Other solvents were lower yielding, but the extracts were also not analysed.

Poddar et al. deoxygenated Nannochloropsis lipids over a phosphorus-promoted CoMo/Al₂O₃ catalyst, achieving nearly complete conversion to alkanes and nearly 80% yield to liquid-phase products in 6 h at 375°C and 50 bar hydrogen (103). The authors conducted a detailed kinetic analysis but did not report detailed product analysis or post-reaction catalyst characterisation.

Wang et al. explored deoxygenation of a FFA stream derived from Dunaliella lipids (104). The lipids were hydrolysed in 250°C to release FFAs, which self-separated from the aqueous phase and were decarboxylated in fed-batch mode over a Pd/C catalyst. The catalyst maintained steady operation over 5 h of reaction, even with a hydrogen-poor headspace in the reactor.

Finally, Schulz et al. (105) deoxygenated Synechocystis (cyanobacteria) lipids over a Pd/C catalyst and isomerised the resulting alkanes over a Pt/CaY zeolite catalyst. The lipids were unique in that they were recovered in the form of excreted FFAs (mainly C12), adsorbed on and eluted from a resin. Once recovered, the lipids were also subjected to a number of cleanup protocols, including recrystallisation, activated carbon treatment, saponification and preparatory-scale liquid chromatography. These protocols had varying effectiveness in reducing sulfur and phosphorus contaminants in the oils (especially sulfur), which were reflected in varying performance of the Pd/C deoxygenation catalyst across FFA streams, giving 33–88% alkane yield depending on the stream. The two most promising streams gave relatively low conversion, but high isomerisation selectivity over the Pt/CaY catalyst.

Aside from algae lipids, yeast lipids have also been evaluated for deoxygenation and hydroisomerisation. Sànchez i Nogué et al. (10) employed the same conditions as those used by Kruger et al. (72) and Knoshaug et al. (97) to deoxgygenate lipids from Rhodosporidium toruloides yeast. The yeast lipids produced a renewable diesel blendstock of similar quality as that from the algae oils. Process optimisation to determine if the yeast lipids could be satisfactorily converted under less severe conditions due to the lack of impurities such as chlorophyll was not conducted. The lipids were found to contain a small amount of steryl esters and a significant amount of polar lipids as well.

Chuck and coworkers have also evaluated conversion of yeast lipids to fuels, though not through conventional hydroprocessing approaches (107, 108). In these studies, the yeast lipids were either catalytically cracked (107) or metathesised with ethylene using a Hoveyda-Grubbs catalyst (108). Although the Pd/C cracking catalyst and reaction temperatures were similar to those used for deoxygenation by other researchers, the gas atmosphere was quite different. Interestingly, however, the catalytic cracking experiments were performed with a yeast lipid from Metschkownia pulcherrima that included 9 wt% sterols, and the authors inferred that the sterols can act as hydrogen donors as they undergo aromatisation at 350–400°C in an argon atmosphere, which improved the yield to linear alkanes from the triglyceride fraction.

Among other oleaginous microbes, the only report of hydroprocessing to our knowledge is that of Dong et al. (34), who converted polar lipids from Methylomicrobium buryatense to linear alkanes over a Pd/SiO₂ catalyst. The bacterial biomass was hydrolysed by a sequential alkaline-acid method that both ruptured the cell wall (rendering the lipids extractable) and cleaved the polar head groups off the predominantly polar lipids, producing a clean FFA stream for deoxygenation. The high degree of polar head group cleavage was confirmed by the presence of only 4 ppm phosphorus in the oil, though the oil contained sulfur, nitrogen, sodium and halogens in higher amounts. With a relatively high catalyst loading and deoxygenation in batch mode, full conversion to linear alkanes was achieved. The authors also noted that a preliminary catalyst screening with nickel- and copper-based catalysts confirmed the hypothesis that these catalysts would be rapidly deactivated in the presence of phosphorus-containing feeds, while noble metals performed better.

Several interesting themes emerge in lipid composition, cleanup and hydroprocessing of microbially-derived oils. First, the studies reporting the longest and most stable catalytic performance have used refined algae oils, for example from Phycal or Solix. Although the details of the refining process are not readily available, these processes appear to sufficiently clean the oils of catalyst-deactivating impurities. In contrast, lipid streams extracted by researchers directly prior to catalytic processing have tended to deactivate catalysts more rapidly or perform more poorly, even when diluted by a factor of 100 into an inert solvent. Perhaps this is unsurprising given the differing descriptions of the oils’ appearances, for example bright orange transparent liquid vs. dark brown semisolid. The situation is additionally confounded by the variety of techniques and solvents used to extract the lipids, which may co-extract different undesirable components. Hexane is the simplest solvent employed, but can result in low extraction yields and form emulsions with wet biomass. Alcohols, such as methanol and ethanol can be added to increase the extraction yields and minimise emulsions, but can also increase co-extraction of impurities. Halogenated solvents can serve the same purpose as alcohols, but further confound the process by introducing additional heteroatoms (typically chloride from chloroform or dichloromethane) that can remain in the extract in trace amounts even after solvent removal.

Second, studies using noble metal catalysts have tended to report better performance than those using base metal catalysts. This is frequently true in other catalysis applications as well, wherein base metals such as nickel, molybdenum and copper are more prone to coking, oxidation by feedstock oxygen and poisoning by feedstock heteroatom impurities. Indeed, in the examples where microbial oils have been thoroughly characterised for heteroatom impurities, it is tempting to infer that supported noble metals may even be able to tolerate some degree of phosphorus, nitrogen and sulfur in the feedstock. However, times on stream in the literature reviewed here, less than 200 h TOS, have typically been too short to make a meaningful assessment. Nevertheless, while noble metals are frequently avoided due to the high cost, there may be environmental advantages (at least for metals such as ruthenium and platinum) (109), and some of the cost can be recouped by regeneration and recycling of the spent catalysts (110). It is worth noting that industrial processes producing green diesel fuels from vegetable oils, such as Honeywell-UOP’s Ecofining, Neste’s NExBTL, Axens’ Vegan and Renewable Energy Group’s Bio-Synfining processes employ nickel-molybdenum or cobalt-molybdenum sulfide catalysts similar to those used in petroleum refining (111). These catalysts tend to favour hydrodeoxygenation over decarbonylation or decarboxylation, which preserves carbon in the product, but also consumes more hydrogen in the process. At least one evaluation of the tradeoffs between carbon yield and hydrogen consumption concluded that decarboxylation would be economically favourable, provided that hydrogen consumption through subsequent methanation and reverse water-gas shift reactions is not significant (112). On the other hand, the deployment of large-scale wind and solar power has at times resulted in excess power production and low-cost electricity that can be used for water electrolysis to produce inexpensive hydrogen. Nickel-molybdenum catalysts are also frequently employed for hydrodesulfurisation and hydrodenitrogenation, which may indicate a higher level of robustness to heteroatoms than other types of catalysts. In particular, nickel-molybdenum catalysts typically require a co-fed source of sulfur, often hydrogen sulfide or dimethyl disulfide, to mitigate conversion of the active sulfide phase to a less-active oxide. Concomitantly, these catalysts tend to leach sulfur into the product (21), which may inhibit the ability of these catalysts to meet increasingly stringent sulfur limits in fuels. It is unknown whether the sulfur-containing species in microbial oils can satisfy this requirement of nickel-molybdenum catalysts.

Third, lipid stream cleanup by a number of approaches appears promising, but all of the techniques have some drawbacks and only a few can be reasonably expected to apply universally across microbial lipids. Saponification of the lipid stream to FFA followed by distillation is perhaps the most robust as it should be effective even with lipid feeds containing high levels of polar lipids, sterols, chlorophyll and metals. However, the distillation step is energy intensive compared to most of the other bleaching methods and the cost of the alkali is high. Methyl esterification and distillation should be similarly effective, but the methanol would need to be recovered for high material and economic efficiency, or else recovered as methane after hydrotreating. Degumming (precipitation of phospholipids) and phosphoric acid bleaching can work well for lipid streams that are low in polar lipids, but would suffer from significant yield losses otherwise, and the phosphoric acid must be thoroughly washed out of the cleaned lipid stream to avoid contamination from the bleaching agent. In lipid streams with high polar lipid content, hydrolysis to FFAs may prove more economical. Adsorption on a polar material such as silica, bleaching clay or activated carbon appear effective for some target impurities (for example chlorophyll and some metals), but similar to degumming, may result in unacceptable yield losses for lipid streams that contain large amounts of polar lipids. Additionally, post-adsorption oils have still tended to deactivate the catalysts. Interestingly, even when chlorophyll is removed to below detection limit, some nitrogen frequently remains in the oils, likely in the form of hydrophobic protein. There is thus a clear need for both fundamental science and process development on this topic.

Looking forward, converting microbial lipids to fuels is a promising approach to displace conventional fossil fuels, especially for diesel and jet fuels which share the same carbon number range as the microbial lipids. While previous research has focused primarily on algae lipids, the advent of third generation cellulosic sugars may spur further development of heterotrophic oleaginous microbes as well. Finally, to best allow for economically-viable biofuels production from SCOs, it is critical to design an integrated process, in which certain complementary units can be intensified to maximise the yield of high quality products for a given step. For example, the lipid hydrolysis step can be integrated with the biomass pretreatment or cell wall rupture step to simultaneously rupture the cell walls for bulk oil extraction and liberate FFA from polar lipids. In addition, the selectivity of the extraction solvent should be carefully considered to increase extraction of fatty acids while reducing the co-extraction of impurities. With these considerations, SCOs are poised to make a significant impact in sustainable fuel production.

Hydrogen is the first element of the Periodic Table and the most abundant element in the Universe. Currently, we are witnessing the emergence of hydrogen gas as an increasingly powerful energy vector for storing and delivering electricity (1). Firstly, besides having very low physical density, hydrogen has the highest gravimetric energy density of any known non-nuclear fuel (for example three times higher than gasoline and 150 times higher than a state-of-the-art lithium-ion battery), which sets the stage for hydrogen as an efficient energy-storage solution (2). Secondly, hydrogen is an environmentally benign fuel, since only energy and water are the end products of the reaction between hydrogen and oxygen, giving access to zero-emission electricity production when used in fuel cells. These benefits have made hydrogen a priority area within the “climate and resource frontrunners” of the European Green Deal.

Currently, most hydrogen (ca. 95%) is produced through energy-demanding steam reforming reaction of water with natural fossil fuels. Unfortunately, this translates into undesired generation of greenhouse gases. As an alternative, WE (Equation (i)) can be employed for hydrogen production, providing a completely carbon-neutral solution when working on renewable electricity from wind, solar, wave, tide, biomass or geothermal, thus eliminating carbon dioxide emissions.

(i)

Importantly, the resultant high-purity hydrogen can be stored in a compressed or liquefied form or in chemical compounds (metal hydrides and liquid organic hydrogen carriers), and then delivered as needed for fuelling small- and large-scale applications (3).

Although the era of WE began more than two centuries ago, its worldwide implementation is still limited to hydrogen production up to the megawatt range using liquid alkaline electrolysis (4). WE is a kinetically controlled process characterised by slow charge transfer and insufficient chemical reaction rates. A large overpotential, defined as the difference between the required and thermodynamic value of the WE voltage (E⁰ = 1.23 V vs. reference hydrogen electrode (RHE)), needs to be applied to drive this reaction. Therefore, catalysts, primarily based on pgms, are used to accelerate WE by reducing the value of the applied overpotential to conduct the cathodic HER and the anodic OER, which are the two half reactions of the WE (5).

The best-performing catalysts for WE are platinum for HER and iridium oxide/ruthenium oxide for OER, featuring topmost activity and long-term stability in both acidic and alkaline electrolytes. Unfortunately, these pgms are scarce and expensive, having the status of critical raw materials in the European Union (EU). Iridium and ruthenium are scarce even when compared to platinum because both are byproducts of platinum mining. Hence, the use of pgms has been recognised as one of the major bottlenecks for hydrogen production by WE on terawatt scale, and, therefore, the development of pgm-free catalysts is an economically sound strategy towards technological and industrial expansion of WE.

On the electrolyser side, the commercially available technologies for hydrogen generation are based on: (a) alkaline electrolysers (AELs); (b) solid oxide electrolysers (SOELs); and (c) (semi)solid polymer electrolysers (SPELs) (4, 6–9). The AEL technology has been commercially available for >50 years. It is characterised by long lifetime and low cost. Recent developments of AEL deal with the employment of high temperatures and pressures, targeting improvement of efficiency (7). The AEL electrodes are typically composed of abundant nickel with a catalytic coating. Shortcomings of the AEL are the use of corrosive liquid electrolytes (for example potassium hydroxide), low purity of the resultant hydrogen due to high cross-permeation through cathode and anode separator, and long start-up times not suitable for dynamic operation. The currently developing SOEL (also known as steam electrolysis) omits the need for corrosive liquid electrolyte, but operates at temperatures above 700°C, making the whole process energy demanding. Another issue is the instability and degradation of the electrode materials at the high operating temperatures. WE technology based on SPEL is well-suited to intermittent supply applications at scale (dynamic operation), offering high current densities compared to AEL, high-purity hydrogen at pressures up to 30 bar and efficiency of ca. 70%. Nevertheless, a number of obstacles, such as high capital costs, prevent the widespread use of SPEL (9).

Advantageously, SPELs operate at low temperatures (50–80°C), and the thin humidified polymer membranes feature high conductivity and low resistance, resulting in minimal current losses and low cross-permeation of hydrogen. There are three SPEL technologies: (a) proton-exchange membrane (PEM); (b) anion-exchange membrane (AEM); and (c) bipolar membrane (BPM).

PEM electrolysers are the present state-of-the-art SPEL technology. Relatively compact PEMWE systems are available from several small, medium and large companies, such as ITM Power, NEL, Giner Labs, Enapter and Siemens. Despite the fact that PEMWE has high technology readiness level (TRL), it is still mainly used for demonstration projects, largely subsidised by local or national governmental funding organisations. The main reasons for the limited commercial adoption are high capital expenditures (CAPEX) and operational expenditures (OPEX), making the produced hydrogen economically non-competitive compared to the traditional steam-reforming process.

More specifically, due to the extremely acidic protonic matrix of the ionomer and membrane (for example Nafion^TM, Aquivion^®, Celtec^®), the technology relies on scarce platinum/carbon and iridium oxide/ruthenium oxide catalysts, which exhibit high intrinsic activity and durability (10). Notably, only these materials can withstand the acidic PEMWE operation conditions over a 20 year electrolyser lifetime. Additionally, highly acidic and oxidative operation conditions dictate that the porous transport layers (PTLs) have to be made from corrosion-resistant but rather expensive titanium, which, combined with pgm catalysts, represents ca. two thirds of the cost of the PEMWE stack (several individual PEM cells stacked together to achieve higher hydrogen production).

The alternative AEMWE approach is rapidly growing from TRL2–3 to the level of TRL6–7 (11–13). AEM combines the advantages of AEL and SPEL, eliminating the need of corrosive liquid electrolyte and leveraging efficient membrane-based WE (14). Until recently, the main bottleneck in AEMWE was the lack of highly hydroxide conductive, stable and durable AEMs as compared to PEMs (15, 16). Fortunately, several promising AEMs have recently emerged on the market (17), such as those from Tokuyama, Fumatech, Ionomr Innovations, W7energy^®, Ecolectro, Dioxide Materials^TM and AGC. The non-aggressive AEM environment at pH ≈ 13 and the possibility to produce AEMs without the use of fluorine (regulated in several countries) makes AEM production more environmentally friendly and safer for workers and nearby inhabitants (11).

Importantly, AEMWE may occur efficiently on pgm-free catalysts based on inexpensive metals (such as iron, cobalt or nickel) that can also withstand prolonged operation in alkaline membrane environment. Therefore, the development of an electrolyser based on emerging AEMs and pgm-free catalysts should enable a significant reduction in both CAPEX and OPEX of electrolysis (18). Interesting work on AEMWE within the EU Horizon 2020 ANIONE project additionally illustrates these points very well. Notably, a further CAPEX reduction can be achieved by replacing the expensive titanium PTLs by simple stainless steel, which is stable under AEMWE operation. Nevertheless, the detailed technoeconomic analysis of AEMWE with pgm-free catalysts is complicated, and the projected cost of the as-produced hydrogen strongly depends on the selected model and input parameters (19, 20).

A third, significantly less explored, avenue for SPEL is the BPM (21). Here, a PEM and an AEM are connected in series, thus allowing the use of non-iridium or ruthenium catalysts on the anode side. Notably, BPM provides a thermodynamic advantage due to the pH gradient across the membrane, which makes it possible to conduct the electrolysis under applied potential below the standard potential of WE, thus requiring less current input. BPM technology is still in its infancy, and more research is required to improve its design, performance, stability and cost.

Our research is focused on the development of pgm-free catalysts for alkaline HER and OER that could be further translated into real-life application of AEMWE. Various classes of pgm-free materials, such as alloys, (oxy)hydroxides, borides, carbides, nitrides, phosphides, chalcogenides, oxides, spinels, perovskites, metal-organic frameworks (MOFs) and metal-free carbon-based compounds have been investigated as catalysts for alkaline WE. In this review, we will highlight several titled prospective pgm-free catalysts that have been studied in our laboratories.

In the simple scheme of a AEMWE cell (Figure 1), the OER and HER catalyst layers are directly coated on the respective sides of the AEM membrane, thus forming a membrane electrode assembly (MEA). Alternatively, the catalysts can be incorporated into the respective gas-diffusion layer (GDL), thus forming a gas-diffusion electrode (GDE). Using porous GDLs is preferential compared to planar ones, since the porosity accelerates the mass transfer of the reactant or product over the catalyst layer. The main role of the AEM is to ensure hydroxide transfer from the cathodic side of the cell to the anodic side. Normally, AEM electrolysers use a water feed with the addition of HCO₃^–/CO₃^2– or dilute potassium hydroxide electrolytes to achieve better performance.

Fig. 1.

AEMWE allows for the direct generation of hydrogen and oxygen through the following electrode reactions (Equations (ii)–(iv)):

(ii)

(iii)

(iv)

As shown in Figure 2(a), HER starts with electrochemical hydrogen adsorption according to Equation (v) (Volmer reaction), where H* designates a hydrogen atom chemically adsorbed on an active site of the electrode surface (M).

(v)

Fig. 2.

The initial adsorption can be followed by either electrochemical desorption according to Equation (vi), (Heyrovsky reaction), or chemical desorption in Equation (vii) (Tafel reaction). The exact mechanism and rate-determining step of the HER over certain catalysts can be established by analysis of the respective Tafel slope data (22).

(vi)

(vii)

The proposed alkaline OER (Figure 2(b)) commences with the electrochemical adsorption of hydroxide anion on the electrode active site (Equation (viii)), followed by the electrochemical formation of O- and OOH-bound intermediates (Equations (ix) and (x)). Finally, O₂ molecule is generated as a result of the last single-electron charge-transfer step according to Equation (xi).

(viii)

(ix)

(x)

(xi)

Notably, a scaling relation (23–26) between binding energies of the reaction intermediates results in a minimum theoretical excess potential, the so-called ‘overpotential wall’ of 0.37 V (27). This means that one would need to apply a minimum potential of 1.23 V + 0.37 V (i.e., 1.6 V vs. RHE) to conduct the OER reaction, which has found experimental confirmation (28). Breaking the OER scaling relation and thus reducing or eliminating the ‘overpotential wall’ still remains a challenge (29).

Table I summarises parameters typically used to present laboratory half-cell data for the evaluation of the catalyst performance and comparison between different catalysts with similar mass loading per geometric area. A number of these parameters will be used in the subsequent discussion. Readers interested in best practice for investigating novel HER and OER catalysts are referred to (30), while (31) provides useful guidance on analysing and presenting the catalytic data. Moreover, some critical aspects of electrochemical data evaluation are reported elsewhere (22, 32–34).

OER, as a kinetically slow 4e^– transfer reaction, governs the overall efficiency of WE. Since the largest overpotential stems from OER, catalyst development for this half reaction will offer the largest efficiency gains (26).

Interestingly, initial reports employing transition metal borides, carbides, nitrides, phosphides and chalcogenides (in general, ‘Xides’) as catalysts for alkaline OER putatively attributed the observed catalytic activity to the pristine materials. However, a careful consideration of following reasons indicated that all the aforementioned Xides should undergo oxidation during the OER into the respective oxides or (oxy)hydroxides (35):

Detailed characterisation studies, including diffraction, spectroscopy and microscopy analyses of the catalysts before and after alkaline OER, provided strong support for in situ oxidation. Accordingly, all transition metal Xides could be considered as precatalysts or ‘active’ supports for alkaline OER. The degree of oxidation could be different and largely controlled by the chemical nature of the Xide precatalysts. Moreover, the size and microstructure of the catalysts play an important role. If the catalyst nanoparticles (NPs) are ultrasmall or the active catalyst surface layer is very thin, the compounds will rapidly undergo full oxidation. In contrast, if the catalyst NPs or films are reasonably large or thick, then partial oxidation of the surface occurs, forming distinct core@shell NPs and nanoheterostructured films, preserving the bulk Xide structure underneath. In other words, in the case of poor catalysts, the oxidation of the bulk compound occurs forming non-conductive oxides or hydroxides, while for the good catalysts the oxide or hydroxide layer passivates the catalyst surface, preserving the bulk structure of the original compound.

The surface oxidation yielding the real catalyst is an unavoidable but crucial phenomenon to obtain highly active and stable OER catalysts. In the vast majority of studies, the resultant in situ formed catalysts show remarkably higher OER performance than their respective metal oxide or hydroxide counterparts. Full understanding of this performance enhancement is still lacking. Generally, there are several factors that should be pointed out: (a) the development of high surface area due to the formation of amorphous-like (oxy)hydroxide surface; (b) the high electrical conductivity of the non-oxide Xide precatalyst underneath the real catalyst; and (c) synergetic catalyst−precatalyst electronic interactions within the as-formed nanoheterointerface (35–37). These factors give rise to a larger number of catalytically active sites and faster charge transfer kinetics over the anode/electrolyte interface, thus beneficially boosting the OER performance.

Currently, there is a limited knowledge of the mechanism of surface transformation during OER because mainly ex situ studies are performed before and after OER. In situ surface-sensitive studies (such as spectroscopy, diffraction, imaging, electrochemical imaging, cyclic voltammetry, nanoimpacts and combination thereof), despite being extremely challenging, are crucial for the rational development of the OER precatalysts (38–42). Such knowledge will be essential to achieve control over the formation of the real catalyst, thus guiding future efforts towards active, stable and economic catalysts for alkaline OER (Figure 3).

Fig. 3.

To summarise, the structure of the bulk phase or core of NPs of a prominent OER catalyst is preserved during the reaction. The surface is, undoubtedly, oxidised due to the extreme oxidation potential applied and the presence of multiple active species, such as hydroxide radicals. The formed oxide layer passivates the surface, thus preventing further oxidation of the bulk of the precatalyst. The same oxide layer is an active catalyst. Thus, the formation of the oxide layer should not be avoided but rather judiciously directed to enhance stability and activity of the catalysts. A combination of computational and detailed in situ studies is a promising strategy in this research direction.

Thorough and comprehensive studies conducted by research groups around the globe have narrowed down the range of industrially-relevant pgm-free HER/OER catalysts for AEMWE to: (a) nickel, nickel alloys and intermetallic compounds as the most active and stable for the HER side of the MEA (43–51); and (b) base metal oxides, (oxy)hydroxides, spinels and perovskites for the OER side of the MEA (52–58).

In general, the catalysts for alkaline HER are similar to those used in AEM fuel cells (AEMFC) (59). Carbon-supported nickel, nickel-molybdenum, nickel-copper and other nickel-based catalysts have been synthesised and investigated (43–51). As a lighter analogue of pgms (especially being chemically similar to palladium), nickel plays the main role in HER, while the addition of other elements increases the cathode durability. Nickel-molybdenum catalysts have been intensively studied for both hydrogen oxidation reaction and HER under realistic AEMFC and AEMWE conditions, respectively (45, 48, 50). The nickel-molybdenum alloys enriched with nickel up to ≈87 at% were found to be not only active in alkaline HER, but also less prone to poisoning by functional groups of ionomers, which is a well-known shortcoming of pgm catalysts in AEMFC and AEMWE (45, 48). The properties of carbon supports, such as the surface area, level of graphitisation, bulk and surface chemical composition and hydrophobic or hydrophilic properties, significantly affect the AEMWE performance and should be carefully tuned as a function of the type, composition and weight loading of the HER catalysts (60).

Considering the OER catalysts, nickel (oxy)hydroxides or oxides with iron impurities in alkaline electrolyte, as well as nickel-iron (oxy)hydroxides, have become benchmark catalysts for AEMWE (61, 62). Although the exact nature of the active catalytic sites in such systems is not well understood (63), the nickel-iron-based materials for alkaline OER deserve a more careful study in the future, especially with respect to their low durability issues (64, 65).

Furthermore, a general direction in the development of catalysts for alkaline OER is to improve the electronic conductivity of already established active oxide catalysts, since the conductivity of the oxides is typically very low. Such an improvement is critical to reducing the applied overpotential during real AEMWE operation, where loading of OER catalysts can often be as high as 4–12 mg cm^–2 (53–55). Unfortunately, the oxidative operation conditions during OER at high potential of ≈1.9–2 V forbid the utilisation of traditional highly conductive carbon supports, which would simply oxidise and degrade during prolonged AEMWE. At the same time, using carbon as a supporting material for transition metal oxide catalysts is a common laboratory practice in short half-cell experiments for the initial assessment of intrinsic catalytic activity of the materials and performance losses due to insufficient electronic conductivity (52). For instance, highly graphitic carbon nanotubes or high surface area graphenes are widely used during the screening and selection of promising alkaline OER materials (12, 52). An alternative strategy is based on employing non-stoichiometric mixed oxides, perovskites, delafossites or spinels (58). Although these materials possess high conductivity (> 0.1 Ω^–1 m^–1) (58), specific synthetic methods should be developed to produce such conductive oxides on a large scale.

Notably, the commercial metal oxide products made on the multi-tonne scale are mainly silica, zeolites and titania. The protocols used to manufacture these materials are well-matured and based on the sol-gel, precipitation, hydrothermal, flame or spray pyrolysis. Nevertheless, these approaches have a limited ability to control physicochemical properties required for OER catalysts, including phase purity (required for high electronic conductivity), the surface area (required for higher density of active sites) and the primary particle size (required for the uniform distribution in the electrode structure).

For the past eight years Pajarito Powder redesigned and modified their proprietary manufacturing platform VariPore^TM for upscaling mixed oxide catalysts production. The schematic of the method is shown on Figure 4. The main idea of this method is to design material by a bottom-up approach with the ability to control all required properties of the final catalyst at every stage of the process. For example, selecting particle and pore forming agents (P&PAs) with different morphology will afford a final catalyst with an inverted structure of the respective P&PAs. The chemical nature of the precursors will allow selection of manufacturing conditions to obtain materials under low energy demanding regime, substantially decreasing the cost of final materials.

Fig. 4.

The method was successfully used for the preparation of phase-pure CuCo₂O₄ spinel catalyst for OER (Figure 5(a)). The P&PAs used in this design derived from high surface area spherical silica particles, which prevented NPs from agglomeration. The phase purity of the spinel was achieved by thermal treatment at just 550°C, allowing to preserve the unique spherical shape of agglomerates (Figure 5(b)). The latter is important for manufacturing dense electrodes for OER, allowing maximal catalyst utilisation on the GDEs. At the moment, this method is established as a robust, flexible and modular manufacturing platform for making different classes of HER/OER catalysts and practiced for commercial production at the kilogram scale per batch (54).

Fig. 5.

Among the large variety of pgm-free catalysts that are currently extensively explored, the interest of our groups have been focused on transition metal phosphides and borides, which belong to the larger family of non-oxide catalysts referred to as Xides. In the next two sections, we focus specifically on these two groups of Xides, highlighting some of our results along with important findings from other research groups. For a more general overview of pgm-free non-oxide electrocatalysts, the readers are referred to several recent reviews relevant to this topic (36, 66, 67).

6.1 Crystal and Electronic Structure

6.2 Hydrogen Evolution Reaction Catalysis

6.3 Alkaline Oxygen Evolution Reaction Catalysis

7.1 Crystal and Electronic Structure

7.2 Hydrogen Evolution Reaction Catalysis

7.3 Alkaline Oxygen Evolution Reaction Catalysis

Electrocatalysis is a rapidly evolving research area, and there are plenty of pgm-free catalysts reported in the literature, including nickel alloys, oxides, TMPs and TMBs, as detailed here. Recent findings have shown that, while HER most likely takes place on the surface of the pristine Xide catalyst, the in situ formation of oxide or oxohydroxide NPs as the true catalyst is clearly observed during OER as a result of surface oxidation of the Xides. Even though pgm-free nickel alloys, TMPs and TMBs do not outperform platinum, their promising HER activity can be of practical importance due to significantly lower costs. In contrast, many iron-, cobalt- and nickel-based catalysts outperform the standard rutile-type iridium/ruthenium oxides in OER activity, but little is known about the durability of these pgm-free materials. This is mainly due to the fact that the vast majority of the literature reports the results of laboratory half-cell measurements and the durability is only tested for a few weeks at most.

With respect to the synthesis, it is difficult to formulate the exact key requirements (for example chemical composition; promoters; crystallographic, electronic and surface structures; physical properties; morphology; fine microstructure; specific surface area; particle size distribution and supporting material) that should be fulfilled for the HER/OER catalysts to be economic, active, stable and durable for AEMWE. This challenge is mainly explained by the ever increasing complexity of the reported catalysts, their dynamic reconstruction during reactions and large scattering of the resultant half-cell testing data. The way to solve this shortcoming is to foster the international community working on WE to follow well-defined characterisation and testing protocols. Recent efforts in this direction (30, 31) provide hope that the aforementioned requirements will be established in the near-term, also involving the help of data mining and computation. It would be also interesting to see how Xides will compete and, more importantly, participate in the rapidly developing area of mass-efficient atomically dispersed catalysts for HER/OER (104, 105).

Considering the discovery of new, more efficient catalysts, solid-state chemistry offers hundreds of ternary and multinary phosphides, borides and other Xides, in addition to the relatively limited range of binary materials that have been studied thus far as potential HER/OER electrocatalysts. Such materials have unique crystal and electronic structures and may exhibit quite different catalytic properties as compared to their binary counterparts. Tuning the transition-metal d-orbital filling, local metal and non-metal (boron, carbon, nitrogen, silicon, phosphorus, sulfur, selenium, tellurium) coordination environments, and the density of states at the Fermi level is important to optimising the HER/OER catalytic performance. The large structural and compositional variety is a challenge as the synthesis and property characterisation protocols are tedious. Application of computational and machine learning approaches can substantially accelerate identification of the most promising candidates. Coupling the computational screening, which accounts for surface dynamics, together with experimental research should result in emergent catalysts with improved performance.

Assembling even a single AEMWE cell is a tedious process that requires expertise not commonly found in academic research laboratories. For instance, to the best of our knowledge, no full AEM electrolyser has been tested with promising TMP/TMB cathodes and anodes. Nevertheless, real AEMWE testing is quite important to demonstrate the applied potential of Xides as catalysts. Here, the early involvement of industry will be highly beneficial, allowing the real future prospects of the reported pgm-free catalytic systems to be understood.

Another issue is the lack of benchmarked AEMWE components (membrane, ionomer, GDL and PTL) and standard HER/OER catalysts accepted by the electrolyser community. This state of matters makes comparison of performances not only difficult, but in many cases meaningless. Substantial input from academia, national laboratories and industry into standardisation of materials and test protocols is needed. This work has been initiated in the international US–EU collaboration under the HydroGEN consortium and has already resulted in the harmonisation of testing protocols for PEMWE, which will be followed by expansion to the promising AEMWE field. Establishing such protocols will allow the electrolyser community to work at the device level to engineer AEMWE electrolysers with high performance and durability (16, 106).

The direct transformation of CO₂ to methanol requires the addition of three equivalents of hydrogen to generate methanol and water from CO₂. This can be performed at moderate temperatures and pressures (220–250°C and 10–30 bar) over heterogeneous metal oxide catalysts, particularly copper and zinc oxides with alumina (CZA, Cu/ZnO/Al₂O₃), which are based on catalysts that date back to the 1930s (8, 9). This process is now being carried out commercially by Carbon Recycling International, producing 4000 tonnes a year of sustainable methanol from Icelandic geological CO₂ and geothermal energy (10).

For this direct conversion, heterogeneous copper-based catalysts are the most extensively studied with catalyst performance thought to be generally dependent on the structure of the copper surface and also possibly the interface between the copper and the other transition metal components, most commonly zinc, zirconium or their oxides (11). Further weight is also given to the degree of dispersion of the copper within the catalyst structure, with increasing copper dispersion and thus copper surface area correlating directly with increased methanol yield (12).

Computational studies have suggested that CO₂ reactions on the stepped and close-packed Cu(211) representative surface gives the primary low-temperature reaction pathway via both formic acid and formaldehyde, the route via CO being an alternative and potentially competitive route (13). This CO, generated by the reverse water gas shift reaction (RWGS) can be problematic, particularly at higher temperatures, where excess carbon monoxide is generated from CO₂ and hydrogen (see Figure 2).

Fig. 2

Both the CO and the byproduct water can limit the selectivity to and yield of the desired methanol product, especially where carbon monoxide side production increases and the RWGS reaction dominates (14). While the water that is inevitably generated while the reaction proceeds can sinter and degrade the catalyst alone, high CO and H₂ concentrations from the RWGS reaction have also been shown to overly reduce these copper surfaces, making sintering happen more easily (15, 16). Additionally, the excess CO production typically promotes hydrocarbon and also higher-alcohol generation via well-understood Fischer-Tropsch chemistry, further increasing hydrogen consumption and complicating product purification (17, 18). However, as discussed in the later section of this article, the production of higher alcohols and other products directly from CO₂ by taking advantage of the greater propensity for CO to form new C–C bonds on these catalyst surfaces might be a way to generate CO₂ fuels more effectively than individual production of methanol and then carrying out subsequent dimethyl ether (DME), methanol to gasoline (MTG) or methanol to olefins (MTO) processes, thereby avoiding multiple individual processes (19).

One method used to avoid CO production and to generally promote milder reaction conditions is the use of precious metal catalysts such as palladium or platinum instead of, or in addition to, the traditional copper as these help to reduce the required temperature for the reaction and therefore increases reaction selectivity as the RWGS reaction is less favourable (20, 21). In addition, these catalysts can promote the hydrogenation of any CO to methanol, further reducing the CO:CO₂ ratio within the reaction mixture (22). However, these catalysts typically show poor CO₂ conversion, thought to be due to the lower strength of the bonding between the CO₂ and metal surface (23). Yet another approach is to use copper encapsulated in metal organic framework (MOF) catalysts which further promote the exclusion of water from the catalyst surface, increasing the catalyst turnover number. The frameworks can also increase the effective surface area of the active copper species by limiting the growth of the metal surface, a result that can also be achieved by using a hydrotalcite-like compound as the catalyst precursor (24).

For the indirect production of methanol, the RWGS reaction can alternatively be harnessed to generate CO as an intermediate that can then be used to create a sustainable synthesis gas (syngas). This syngas can then be used for methanol production separately. Using CO in this way has the benefit of very high selectivity (over 99.5%) and high yield, facilitated by the absence of water byproduction (which in this case would be removed during CO formation) and the fact that CO is a more reactive starting material than CO₂ (25). In some cases, when producing methanol from CO, the product methanol solution could even be used for further product generation without additional purification or drying (26). It is due to these advantages that general industrial (non-sustainable) methanol production typically uses this syngas route, and it supplies the overwhelming majority of the global methanol demand, which was 83 million tonnes in 2019 (27). The routes for creating the CO starting material from CO₂ are numerous, including classic hydrogenation (RWGS), disproportionation reactions with biochar (where elemental carbon reacts with carbon dioxide to form two equivalents of CO), electrochemical and even plasmolytic routes (28–31).

Compared with methanol, producing the four-carbon chain butanol from CO₂ is a far more challenging synthesis. However, butanol is a valuable potential ‘drop-in’ replacement for petrol as a liquid transport fuel and is compatible with existing fuel infrastructures. It can even be used alone as fuel for unmodified vehicles, with an octane number of 96 (32). It is hydrophobic enough to prevent water and salt corrosion in modern engines, which is a major drawback when using high ethanol content in road vehicle fuel in much of the world (33). Butanol in standard petrol engines has also been shown to have similar or even superior fuel economy than the petrol it replaces, despite having approximately 11% less energy density. This is due to its nature as a single-component fuel, rather than the wildly diverse mixture of compounds that make up fossil fuels, with the entirety of the fuel burning at the optimum rate. Unlike both methanol and ethanol, butanol can also be blended with aviation fuels in limited amounts and its corresponding diester, butyl butyrate, shows good compatibility with aviation kerosene (34). This may be crucial for decarbonisation of the aviation industry as major industrial nations move towards net zero CO₂ emissions in the coming decades.

While there are biological routes for the creation of sustainable butanol from carbon dioxide, particularly acetone–butanol–ethanol (ABE) fermentation, separation of low-concentration butanol from water is challenging due to the low volatility of the butanol and the fact that high concentrations of butanol are toxic to microorganisms, limiting the extent of the fermentation for butanol synthesis (35). One method to avoid these issues is to instead dimerise easily manufactured bioethanol via borrowed hydrogen or Guerbet chemistry (36) (see Figure 3).

Fig. 3

As with the biological routes above, most potential methods to synthesise butanol from CO₂ will similarly involve a multistep process. For example, the Guerbet route could also be appropriate for a CO₂ to fuels approach, by first carrying out the conversion of CO₂ to ethanol and then using the ethanol as a feedstock for the generation of butanol or other higher alcohols. This can be carried out using conventional transition metal catalysis, with reduction being combined with methylation of the absorbed and partially reduced CO₂ to generate the C₂ alcohol. The chemical stability of ethanol, a quirk of its structure, allows this process to be carried out under surprisingly mild conditions and with high selectivity. Catalysts that have shown good activity have included palladium-copper nanoparticles and cobalt-alumina catalysts, both have been able to produce ethanol at over 90% selectivity under 200°C, with the palladium-copper nanoparticles achieving production of over 100 mmol ethanol per gram catalyst per hour (37). Interestingly however, the cobalt-alumina catalysts, while less active for ethanol production, showed trace production of butanol directly, suggesting that a direct conversion of CO₂ to butanol may be possible with the development of the right catalyst and conditions (38).

CO₂-to-ethanol has also been carried out using both atomically divided copper and traditional Cu/ZnO/Al₂O₃ catalysts, used for methanol synthesis, with electrochemical or plasma assistance. The former has recently been carried out with a Faradaic efficiency of over 90% at −0.7V, although both routes showed very low turnover rates (39, 40). Once again, the fact that butanol has also recently been generated directly from CO₂ by copper electrocatalysis in low yield is worthy of note: in this case, it is thought that the acetaldehyde intermediate has undergone in situ condensation, as found in Guerbet chemistry (41).

An alternative method to convert CO₂ to butanol, using Grignard chemistry, has been demonstrated by the authors (Figure 4). First methylmagnesium bromide is synthesised, which can be generated by the reaction of methyl bromide, produced from CO₂-derived methanol, with magnesium. This then can be reacted with dilute gas phase CO₂ at room temperature and atmospheric pressure to generate acetic acid, thus incorporating the CO₂ capture process itself into the fuel generation. This acetic acid was then dimerised via Claisen condensation and reduced using copper on zinc oxide and hydrogen to form a mixture of alcohols including butanol. In principle the magnesium halide byproduct could then be recycled by high efficiency electrolysis, also allowing for the creation of further methyl halide and an overall electrosynthesis-by-proxy route (42). The magnesium can be regenerated in existing electrolysis processes and returned to the process, an example of stoichiometric metal looping. As the electrolysis process is routinely used to produce magnesium from sea water, additional magnesium halide in the process will increase the efficiency of the metal production.

Fig. 4

As a nascent part of the CO₂-to-fuels research field, other routes could yet be discovered. Some of these routes could include a selective MTO process to produce either butenes or ethylene from CO₂-derived methanol. After dimerisation (if ethylene is used) the resulting butenes can be simply hydrated to produce (primarily) 2-butanol, which is the main industrial manufacturing method for 2-butanol, and also the preferred butanol isomer for transport fuels (43). Finally, butanol can be synthesised directly from sustainable synthesis gas by Cr/ZnO catalysts, although as might be anticipated, selectivity and yield to butanol by this route has so far been low (44).

Synthetic transport fuels will be in increasing demand as fossil-based fuels are phased out. The initial focus in this paper has been on methanol, however internal combustion engines need to be modified because of the corrosivity of high concentration methanol fuels (these are generally limited to below 18% of the fuel as a consequence) as discussed previously. Drop-in hydrocarbons such as synthetic diesel and kerosene have been developed, however these are expensive approaches and require considerable quantities of dihydrogen as a major byproduct of the reduction process is water. Recent developments have focused on DME, formed by the condensation of CO₂-derived methanol (45). Indeed, Volvo (46) and Ford (47) trucks in North America have developed DME vehicles that have been deployed in commercial environments. DME has a lower energy density than diesel but, unlike lower alcohols, is a direct drop-in fuel at 100% concentration. As it contains no C–C bonds it has much lower (approaching zero) particulate emissions than diesel (5) and as it is not fossil-derived it has no SO_x emissions. DME (CH₃OCH₃) has less hydrogen than diesel (C₁₈H₃₈) and so becomes a more economical proposition as less dihydrogen needs to be produced in order to reduce the CO₂. One major challenge will be to produce DME in a single-step process from CO₂ and dihydrogen, or preferably water. Photocatalysis may offer a viable route to achieve this sustainably.

In many countries, governments are proposing a transition to electric vehicles (EVs) as a means to defossilise road transport. However, while there are advantages such as zero tailpipe greenhouse gas emissions, there are also many problems. For a true picture of environmental impact, comparative ‘well to wheel’ and ‘weather to wheel’ technologies should be compared and the latter should be based solely on low carbon energy as a changing grid mix will lead to different carbon intensities (48) with the actual emissions being transferred to point source emitters; power stations. Furthermore, we will need fuels that are compatible with existing internal combustion (IC) engines for the foreseeable future if we wish to avoid the risk of creating a social transport underclass; restricting the use of older spark injection (SI) and compression injection (CI) vehicles.

Ford in Aachen, Germany announced the world’s first original equipment manufacturer (OEM)-built DME passenger vehicle, a Ford Mondeo, at the DME Sustainable Mobility Workshop at Landesvertretung NRW in Berlin in 2019 (49). Ford has claimed that the well-to-wheel CO₂ emissions for a DME powered CI engine could be as low as 5 g km⁻¹, compared to a conventional diesel fuel value of 116 g km⁻¹ (4). A technoeconomic analysis (TEA) of DME derived from CO₂ has shown that DME can be produced at a 740 tonnes per day scale with a minimum selling price of €2193 per tonne, which compares well with fossil-derived DME of around US$3000 per tonne (50).

Conventional DME synthesis relies on the formation of methanol from natural gas or syngas, the latter being an exothermic process as shown in Equation (iii). Direct hydrogenation (Equations (iv) and (v)) can also be achieved catalytically as discussed previously, with the enthalpy of reaction being less than that of CO.

(iii)

(iv)

(v)

Acid catalysed dehydration in the condensation of two molecules of methanol gives DME in an exothermic reaction (Equation (vi)). In order for this be considered a sustainable process, the methanol should be produced from captured CO₂ or from biogenic methanol. Both of these approaches have been applied to the commercial synthesis of DME by Oberon Fuels (51). The provenance of the supply has led to the DME gaining Renewable Fuel Standard approval from the US Environmental Protection Agency.

(vi)

Recently, considerable effort has been directed to the direct synthesis of DME from captured CO₂ and hydrogen (Equation (vii)). The advantages include a single process operation and a highly exothermic reaction. However, there is still an issue that of the six equivalents of hydrogen consumed, half end up in the water byproduct.

(vii)

It has been shown that DME can be produced by the direct reduction of CO₂, however a more exothermic process is the direct hydrogenation of CO in the form of syngas in a 1:1 stoichiometry (Equation (viii)) (52, 53).

(viii)

A benefit of this process is that all the hydrogen is retained in the DME product, however the byproduct is one equivalent of CO₂. There is of course the opportunity to separate the CO₂ and feed it into a second reactor to produce more DME using the processes described in the following discussions. This offers an opportunity to utilise waste gases from iron and steel industries which contain high concentrations of CO and CO₂ as well as hydrogen in the furnace off-gas (54).

Bifunctional catalysts, such as Cu-In-Zr-O with SAPO-34 zeolite (CIZO-SAPO) have been reported (55) for the direct conversion of CO₂ to DME at 250°C and 30 bar. While the selectivity to DME is good (64%) the conversion of CO₂ is disappointing at 4.2%.

Polierer et al. (56) have reported a direct synthesis of DME from CO₂ using a mixed catalyst system based on Cu/ZnO/ZrO₂ produced by continuous precipitation. The catalyst combines methanol synthesis with subsequent dehydration using CO/CO₂ mixtures at 230°C and 50 bar. DME synthesis was enhanced when CO₂-rich gas feeds were used.

The use of a hybrid Cu/ZnO/ZrO₂-ferrite (CZZ(C)-FER) catalyst to produce DME directly from CO₂ has been reported (57). A mechanism was proposed in which hydrogen is adsorbed onto the copper atom and CO₂ onto the Zn-Zr-FER surface. Formation of formate on the surface is achieved by hydrogen transfer which then undergoes dehydration through further hydrogen transfer to give the surface bound methoxide. Two methoxides then combine to give DME, completing the dehydration step.

Modak et al. (58) have recently reviewed the catalytic reduction of CO₂ to give amongst other things methanol and DME and have included the use of hydrides and carbon instead of hydrogen as the reducing agent. An optimised reactor has been used for the direct synthesis of DME from CO₂ (59) using CuO/ZnO/Al₂O₃ and γ-Al₂O₃ as the catalyst system.

In common with many reports, Kornas et al. (60) have reported the Cu-ZrO₂ system as an active catalyst for DME direct synthesis from CO₂, modified using a heteropolyacid, montmorillonite K10 as the acidic motif.

The energy density (MJ l⁻¹) of DME is only 54% that of diesel, and 60% of its specific energy (MJ kg⁻¹). However, it also contains significantly less carbon and so when it burns in air or oxygen it follows the following stoichiometric reaction, Equation (ix):

(ix)

By contrast, the combustion of diesel can be generalised as follows, Equation (x):

(x)

It has been shown that despite the lower energy density and specific energy, in a conventional compression engine, DME demonstrates lower well-to-wheel emissions when compared to fossil-oil derived diesel (61).

DME is a gas under ambient conditions so needs to be stored under slightly pressurised conditions (5.1 bar at 20°C), but considerably lower than the pressures required for hydrogen. In the vehicle, the DME must be pressurised to 12–30 bar to ensure the material is in the liquid phase (62). Oxymethylene ethers (OME_x ) may ultimately be a better option as those with more than three carbons (OME₁ and higher) are liquids that can be stored in conventional tanks. However, the emissions reductions decrease as the value of x increases as while the density becomes similar to diesel the energy density does not increase proportionately. Furthermore, as x increases so do the production costs.

A one-pot synthesis of OME₁ by hydrogenation of CO₂ in methanol using a 3% ruthenium catalyst over the high silica zeolite BEA has been reported (63) to give good selectivity at 150°C (Equation (xi)). A mechanistic study also suggests that the reaction proceeds via a bound formate species.

(xi)

TECNALIA developed a two-stage leaching and reduction process using deep eutectic solvents (DES), comprising choline chloride and oxalic acid (Figure 2). The solvent is first used to leach the pgms from the solid feedstock at a temperature <90ºC, followed by a reduction of the pgms to their metallic form, by heating the solution to >100ºC. Centrifugation was employed at this laboratory scale based on equipment availability, separately both for the removal of the depleted autocatalysts substrate and the pgm product. Recovered DESs can be recycled for the next leaching batch.

Fig. 2

The solvents used are readily prepared from renewable, non-toxic and naturally occurring chemicals when compared to traditional hydrometallurgical processes that employ strong acids (15, 31). Another advantage is that it is a one-pot leaching and reduction process where no additional reducing agent is required. An increase in the pgm purity is achieved from the selective reduction process with a highly pure (90–95%) pgm solid generated, a significant improvement from the low content feedstock. Therefore, it is a simple process for the concentration of pgms, with reduced costs and wastes generated, compared with the cementation process (31–33) and with a very low energy consumption compared to existing pyrometallurgical processes (15). Further separation steps are needed to separate the three pgms from each other. The DES must be kept close to the leaching operation temperature to avoid handling issues arising from DES solidification.

An advanced leaching process using microwave (MW) technology has been developed by VITO using a laboratory MW digester (flexiWAVE, Milestone Srl, Italy). The feedstock is contacted with 6 M HCl and H₂O₂, as an oxidising agent, before being heated using MW radiation (2.45 GHz) to the reaction temperature of 150ºC. MW heating allows short leaching times, consisting of 15 min heating with 10 min dwell time at the reaction temperature (34). The resultant leachate, containing the pgms, is filtered, to remove the undissolved catalyst substrate, leaving an aqueous solution containing the pgms in a chloride matrix.

The MW heating promotes fast, homogeneous volumetric heating enhancing leaching reproducibility and efficiency, with a positive impact on the energy efficiency. Optimisation of the leaching process has resulted in a significant reduction in the HCl acid concentration (i.e. 6 M HCl) required compared to previous results reported in literature (i.e. 12 M HCl) (34). The necessity of the H₂O₂ addition is dependent on the feedstock (Table I, data provided in presence of 10 v/v% H₂O₂ (31%)). In some circumstances its addition has been shown to have a significant impact on the pgm leachability. Through modelling and analyses of the reactor head-space gas, it was shown that H₂O₂ addition increased the formation of Cl₂ and H₂ gas, which must be considered in the process safety assessment.

Solvometallurgy is an alternative branch of metallurgy that uses non-aqueous solutions (35). KU Leuven investigated the oxidative dissolution of pgms from the feedstock using two different solvometallurgical approaches. In the first, an innovative method for the selective leaching of palladium from the feedstock was developed, avoiding the co-extraction of platinum and rhodium whose recovery can be achieved from the palladium depleted feedstock by varying the process conditions. Dilute and concentrated solutions of FeCl₃ in acetonitrile are used for the selective dissolution of palladium and the dissolution of platinum and rhodium, respectively. This solvometallurgical approach provides a preconcentration of the pgms after which further purification is needed to separate platinum and rhodium. In the second approach, highly concentrated solutions of AlCl₃·6H₂O and Al(NO₃)₃·9H₂O were used to dissolve palladium selectively from spent autocatalysts; 95% palladium was leached in only 15 min at 80ºC (36).

Ionic liquids (IL) are perceived as ‘green solvents’, mainly due to their low volatility, which signifies a reduction in the degree of negative environmental impact and health hazards, low toxicity and non-flammability (37). In metal extraction applications, they have provided dramatically higher extraction efficiencies than commonly used solvents. They can act either as solvents in the presence of an extracting agent or as selective extractants, since via modifications in their anion or cation their selectivity towards a certain metal can be tuned (38, 39).

VUT explored the properties of ILs for both leaching and separation processes (for separation see Section 2.5). Leaching of the pgms from the feedstock was performed using hydrophilic and low cost choline-based ILs. The choline-based IL leaching process was selected as the optimum due to its high extraction efficiency and selectivity. The process operates at mild conditions in the presence of an oxidising agent: <100ºC for 4 h in a sealed vessel with continuous stirring (Figure 3).

Fig. 3

This process leaches the pgms, but further separation steps are needed to separate the three pgms from each other. Depending on the feedstock, the loaded IL can be re-used for subsequent leaching batches until the IL capacity is reached. Of note is the mild process conditions used.

The major drawback is the high IL viscosity, making implementation on an industrial scale challenging. Nevertheless, dilution of the IL with water is feasible without compromising the pgm extraction efficiencies. Furthermore, larger scale system design and operation must ensure the violent exothermic reaction between the feedstock and oxidising agent is managed safely.

Due to the increased interest in ILs, a new concept surfaced: IL immobilisation on solid support materials, which is the deposition of a fine IL layer on a solid surface. Use of supported IL phases (SILPs) is an ideal strategy in order to make use of the benefits of IL and simultaneously avoid the inherent complications of IL-based separations such as mass-transport limitations and excessive usage of the IL (40).

VUT exploited the properties of hydrophobic ILs in both liquid and solid extraction processes; this was employed in conjunction with the VUT ionometallurgical leaching process (Section 2.4). In the liquid-liquid extraction process, a solution comprising 50 wt% phosphonium-based IL in n-heptane was used to extract palladium, platinum and rhodium from the loaded pgm hydrophilic leachate (generated from ionometallurgical leaching) to the hydrophobic IL phase. Quantitative extraction is obtained after continuous stirring for 2 h at room temperature. The pgm-free hydrophilic IL phase can be recycled to perform the next batch of IL leaching. No significant performance loss of the hydrophilic ILs was observed between the different cycles; only five cycles have been tested to date. This is particularly attractive to minimise environmental impact and operating costs.

The alternative approach, solid extraction, relied on SILPs using hydrophobic phosphonium-based ILs (Figure 4). The pgms are adsorbed onto the solid enabling their one-step separation from the main impurities of aluminium, iron and cerium due to their low retention on the solid material.

Fig. 4

The stripping is a two-step process: (a) an acidified thiourea solution strips the impurities retained on the SILP; (b) a more concentrated acidified thiourea solution strips palladium and platinum (rhodium is retained on the solid).

The use of SILPs allows fast and simple separation of the pgms from other metals with reduced chemical reagent consumption compared to liquid-based separations. There is the possibility to re-use the solid material for further separations without any loss in its retention and separation performance, to date one recyclability experiment has been conducted. An advantage of SILPs compared to most commercially available resins is no pre-equilibration is required; this could have significant impact on cycle times and effluent generation if re-equilibrium is necessary, as part of the load-elution cycle. A drawback of the process is the retention of rhodium alongside chromium on the SILP and the removal requires a suitable stripping agent not yet identified.

A split-anion solvent extraction process has been developed by KU Leuven for the separation of an aqueous mixture of pgms into their individual elements (Figure 5) (41). Generally, the split-anion extraction relates to the solvent extraction process where different anions are present in the aqueous and organic phases and the distribution of the IL anions strongly favours the IL phase. The pgms are extracted from the chloride leaching solution using the iodide form of the quaternary ammonium IL Aliquat^® 336, [A336][I], dissolved in p-cymene. The iodide anions, which have a strong affinity for the organic phase, coordinate with pgms to form stable iodo-complexes that can be extracted to the ionic liquid phase (42, 43). The split-anion extraction allows not only efficient extraction of pgms without changing from a traditional chloride feed solution, but also the selective recovery of the extracted metal complexes from the loaded organic phases. The organic is scrubbed and stripped of its pgms, as detailed in Figure 5.

Fig. 5

The ionic liquid-based split-anion extraction process is simple, selective and effective for the sustainable separation of pgms, using only one ionic liquid [A336][I] as the extractant, which can be regenerated for consecutive extraction-stripping cycles. The high viscosity of [A336][I] is a drawback, which has shown, during its pilot scale application in mixer-settlers, to slow the mass transfer. Some measures have been identified to reduce the IL viscosity, such as the use of water-saturated ionic liquid or use of green diluents (i.e. p-cymene), but the measures can only partially resolve the problem. To the best of our knowledge this is the first time that a process based on IL for the separation of pgms is tested in continuous mode using real pregnant leach solutions as feed. Other separations using IL have been developed and show good performance but have been only tested with synthetic solutions and not in continuous mode (44, 45).

Sorbents are an established method to selectively recover palladium and other noble metals for acidic aqueous streams such as leachates (16, 46–48). Many materials have been developed by different groups and their performance tested in powder form. However, to be applied in a continuous way, the shape has to be optimised to avoid clogging and pressure build-up in a column or the material has to be modified for easy recovery afterwards with for example a magnetic core (49, 50). Therefore, VITO has developed solid sorbents by first selecting or forming a suitable solid backbone before grafting active organic scavenging groups onto the support using a green aqueous synthesis method. Two types of regenerable three-dimensional (3D) structured metal oxide supports (Figure 6) have been investigated: monodisperse microspheres and 3D printed monoliths. These types of supports have the advantage of reducing mass diffusion limitations, optimising packing density and decreasing pressure drop while allowing fast adsorption and desorption cycling times.

Fig. 6

The functionalised microspheres show good selectivity for palladium over other pgms and impurities giving a method to remove palladium from acidic solutions. The adsorbed palladium can be easily recovered by stripping with concentrated HCl acid resulting in a concentrated acidic solution of palladium. The sorbents have a palladium capacity of 0.33 mmol g^–1 from acidic solutions (pH 2). When stripped with 3 M HCl, a seven-fold increase in the palladium concentration between the feed (100 mg l^–1 palladium) and stripping solution was observed. No significant performance loss of the sorbents was observed between the different cycles; only seven cycles have been tested to date.

The developed material is a hybrid (ceramic-organic) adsorbent selective to palladium over the rest of the pgms with good hydrolytic stability that can be regenerated and reused. If a degradation of the organic scavenging groups and hence, decreased performance is observed over a higher number of cycles, the metal oxide support can be recovered and refunctionalised. The biggest drawback is the limited sorbent capacity, but more developments to improve this are ongoing by increasing the specific surface of the supports and optimising the grafting conditions.

SINTEF investigated extracting pgms from the feedstock using pyrometallurgy, employing copper as the pgm collector, followed by a molten salt electrolysis process (Figure 7).

Fig. 7

The pyrometallurgical process was investigated at two laboratory scales (10 g and 5 kg). The pgm recovery rates, in the alloy phase, were close to 100%, and the copper-collector recovery rates were in the range 82–100%, when using the optimised parameters: 10 wt% copper-collector, 10–15 wt% calcium oxide, 1600–1650ºC and 1–1.5 h holding time. This pyrometallurgy step pre-concentrated the pgms by ~10 times from feedstock to the generated metallic phase.

In the electrolysis step, the copper-pgm alloy is used as anode in an electrorefining cell with the eutectic LiCl-KCl as electrolyte at 450ºC. The experiments demonstrated the selective extraction of the metal phase (copper), which was recovered at the cathode with a current efficiency of ca. 70%. Under these conditions, the pgms (and other impurities) remain in the anode residue giving a solid with >99.9% purity. Further separation steps are needed to separate the three pgms from each other.

One significant advantage of pyrometallurgical over hydrometallurgical processes is the lower reagent use in relation to the feedstock pgm content (kilograms of reagent per kilogram of pgm) (51). As such, it provides attractive conditions for preconcentrating pgms from very dilute wastes, such as the PLATIRUS feedstocks; the optimised conditions of the SINTEF process showcase that. High recovery rates of the copper collector at the electrolysis cathode have been achieved and it can be recycled for the next pyrometallurgical step.

The electrolysis process allows the extraction of copper from the pgm-containing copper anode in a molten salt electrolyte with better selectivity and kinetics as well as lower energy consumption than in state-of-art copper-refining processes using aqueous solutions (52). Cu(I) species are stable in the molten salt electrolyte, thus the voltage (and energy) needed in the electrorefining process is lower than in an analogous aqueous solution process where Cu(II) are the solely stable species. In general, the kinetics of the electrode charge-transfer reaction in molten salts are considered faster due to the high operational temperature (53).

The challenges of operating the SINTEF pyrometallurgical step are the same as those found in industry. The separation of pgm microparticles from the molten slag phase is impacted by the slag viscosity and metal-to-slag interfacial tension and in turn affect the pgm extraction efficiencies. The energy consumption was ca. 5.5 kWh kg^–1 copper recovered in the pyrometallurgical step and ca. 7 kWh kg^–1 pgm recovered in the electrolysis step (equal to 0.3 kWh kg^–1 copper refined) under optimised experimental conditions. Energy consumption can be challenging to extrapolate from laboratory to industrial scale. Due to the energy intensiveness of pyrometallurgical processes, energy consumption and associated cost must be evaluated at larger scale.

Molten chloride mixtures can be used as a reaction media in the chlorination of oxide mixtures, ores or industrial byproducts. The dissolution reaction generates chloride compounds at much lower temperatures (ca. 450ºC) than those needed in solid-gas chlorination reactions (ca. 1000ºC). This is due to significant solvation effects of the dissolved metal cation with the chloride ions of the molten chloride media.

SINTEF investigated the selective recovery of the pgms using molten salts and chlorine gas as an oxidiser and chlorination agent, followed by an electrolysis process (Figure 8). LiCl-KCl eutectic mixture was chosen as the best candidate. The feedstock was fed into the reactor at 450ºC without any pretreatment or up-concentration steps, resulting in a single step pgm extraction process. Silicon, magnesium and aluminium were not dissolved thus remain as a solid sludge at the bottom of the reactor. The analysis of the residue shows that, after 3 h, the total chlorination and therefore dissolution of the pgms is close to 50%. The dissolved pgms are recovered in an electrolysis process as a metallic-pgm alloy at the cathode resulting in chlorine evolution at the anode. Only 80% of the dissolved pgms can be accounted for in the molten chloride as pgm-chlorocomplexes; it is believed unaccounted pgm mass corresponds to formed volatile pgm-chlorocomplexes that could be recovered from the off-gas system by condensation; estimates calculated from a mass balance suggested a feed volatilisation of ca. 10% palladium, 10% platinum and 20% rhodium.

Fig. 8

This chlorination process presents clear advantages in terms of rate, conversion and selectivity when compared with traditional gas-solid reaction systems. In the latter, the rate and conversion are limited by the contact of the gaseous chlorinating agent and the material to be chlorinated, and the reaction occurs non-selectively as, at the much higher temperature, chlorination of all other elements contained in the material occurs, not only pgms.

Though faster than gas-solid reactions, the kinetics are still slow when using chlorine gas and the use of other gaseous chlorination agents, such as HCl, should be tested. Further optimisation of the process is required to achieve competitive pgm recovery rates, including recovery of the volatile pgm-chlorocomplexes.

GDEx is defined as the reactive precipitation between metal precursors in solution and intermediates from the reduction of gases at a gas-diffusion electrode. When the gas is air or O₂, the O₂ reduction reaction leads to hydroxyl ions and hydrogen peroxide being formed in the pgm solution, which react, forming oxides or hydroxides (Figure 9). Alternatively, the process can run with other gases. The GDEx process is enabled by VITOCoRE^® multi-layered gas-diffusion electrodes (54).

Fig. 9

GDEx uses the cleanest possible reagent, the electron, and it is highly versatile, as it can be used to recover many different metals. The process uses an inexpensive reactor. As it operates in a flow-cell configuration, it is easily up-scalable by stacking multiple individual cells, without a reduction in performance. The process is highly reproducible, involves mild operation conditions (room temperature and atmospheric pressure). The process has a low energy consumption, for example ~2-6 kWh kg^–1 of materials recovered, when compared with electrowinning platinum from chloride media reported at 21 kWh kg^–1 platinum (55). Finally, it is efficient, i.e. 50% current efficiency for the formation of the reactive intermediates that fully react with the metal precursors to achieve the targeted recovery.

Notably, the recovery of dilute metals and simultaneous synthesis of nanostructures with GDEx is fast, with rates approaching ~3–15 kg per day, using a single, inexpensive, electrochemical reactor under flow regime.

The best results were achieved with dilute metal concentrations, and significant optimisation is required for solutions with metal concentrations above 10 g l^–1. Especially with high metal concentrations, it is expected that, after a period of operation, the electrode would become clogged and require an acid treatment to regenerate the electrode porosity. The recovery of the precipitated materials is impacted by the unoptimised downstream separation and drying processes. Some reagents, such as sulfur-based compounds, are known to interfere with the process under defined processing conditions, coprecipitating with the pgms.

A laboratory scale MW system (flexiWAVE) equipped with a spinning carousel holding 15 pressure-sealed Teflon-lined reactors was used to process ~1.3 kg autocatalyst (Figure 13).

Fig. 13

The leaching temperature is measured by a thermowell contained optic fibre, placed in one of the 15 reactors. Each of the 15 reactors was loaded with 5 g of the feedstock and 50 ml 6 M HCl solution. The MW-assisted reaction took place at 150ºC for 10 min, with a heating time to the set temperature of 15 min. Subsequently, the reactor was cooled, opened and the leachate was vacuum filtered. It is noteworthy that this leaching process did not require addition of H₂O₂ as an oxidation agent and thus the formation of hazardous head space gas mixtures (containing H₂ and Cl₂ gas) was avoided. During vacuum filtration, the leach residues were washed with 6 M HCl at room temperature. Both leachate and washing liquids were collected.

Overall, ~1.3 kg of material was leached by performing 18 MW-leaching runs. Three batches of material with different grain sizes and total masses were processed, hence three leachates (L1, L2, L3) and washing waters (W1, W2, W3) samples were obtained (Table III).

The leachate containing L1, L2 and L3 (Table III), as prepared by VITO, was processed using the solvent extraction flowsheet developed at KU Leuven (Figure 5). Due to the low pgm concentration of the leaching residue washings, these solutions were not processed. A continuous solvent extraction demonstration using multi-stage mixer-settlers was undertaken to process the leachate (Figure 14).

Fig. 14

First, Pd(II) and Pt(IV) were quantitatively extracted in two countercurrent stages with [A336][I] in p-cymene at O/A = 1/3, leaving Rh(III) in the raffinate. The impurities (mainly aluminium, barium, cerium, iron and tin) in the loaded organic phase were removed with NaCl solution. Then selective stripping of palladium was achieved by equilibrating the scrubbed organic phase with aqueous ammonia in NaCl solution in four-stage mixer-settlers at O/A = 3/1. Followed by the recovery of platinum from the palladium-free loaded organic phase was performed using acidic thiourea solution [CS(NH₂)₂/HCl] in four countercurrent stages (O/A = 2/1). After being washed with diluted HCl, the IL requires regeneration by contact with KI to replace the chloride for an iodide anion reforming [A336] [I].

The work at KU Leuven demonstrated that continuous solvent extraction is feasible for not only the preparation of [A336][I] but also the selective recovery of individual pgms, with recoveries of 81 ± 3% Pd(II), 62 ± 2% Pt(IV) and 54 ± 5% Rh(III), from the feedstock leachate. The mass loss was mainly as a result of the samples taken for analysis and the mixer-settlers start-up and finish steps. These losses are not representative of industrial scale operation in which mixer settlers would operate continuously.

The rhodium raffinate and platinum strip were further processed using conventional chemical transformations to convert the pgms to the nitrate form. The palladium strip was sent for further processing by the GDEx process.

GDEx attained a recovery of ~70 ± 1% of palladium from the KU Leuven strip sample (Figure 15). Palladium was recovered with a purity of 91–93%, with platinum, rhodium and aluminium as the major impurities, and additional minor impurities of barium, iron, magnesium, cobalt and copper. Only two batches were processed in comparison to the eighteen leaching batches performed thanks to the up concentration in the solvent extraction process. The selectivity of the recovery can be optimised by further investigation into the effect of the different GDEx operational variables, such as the influent concentration, hydraulic retention time, applied potential or current.

Fig. 15

The palladium sponge generated from the GDEx technology was further processed using conventional chemical transformations to convert it to palladium nitrate.

The platinum group metal iridium is perhaps the most puzzling metal on Earth due to its property of being cleavable and a plastic solid simultaneously (1). This refractory face-centred cubic (fcc) metal (T_melt = 2446°C) serves as the structural material for applications under extremely hard conditions (2, 3) such as containers for fuel sources in radioisotope generators for deep space missions (4), or crucibles for growing oxide crystals for power lasers (5). Industrial technology for refining and processing iridium, based on traditional chemical refining methods (2, 6, 7), has been developed over the past 60 years (8–10). Based on these achievements, it has been shown that polycrystalline iridium exhibits limited plasticity due to intergranular fracture at room temperature, but its plasticity increases considerably under elevated temperatures (9–12). The segregation of non-metallic impurities on the grain boundaries was considered the cause of poor workability of polycrystalline iridium (12). This type of deformation behaviour agrees with empirical knowledge on deformation and fracture of metals (13, 14). On the other hand, single crystalline iridium behaved unusually: it cleaved under tension after considerable elongation (13, 15), but never failed under compression (16–18). At room temperature the fracture mode of iridium single crystals was attested as BTF (1, 15), while brittle intergranular fracture (BIF) was the fracture mode for polycrystalline iridium (11, 19). Analysis of the causes of cleavage in iridium has shown that it satisfies some empirical cleavage criteria (18–20) due to features in the elastic moduli in comparison with other fcc metals (18, 21). This fact leads to the conclusion that the inclination to cleavage is an intrinsic property of iridium, whereas impurities only reinforce it (18–20, 22, 23). However, the analysis of interatomic bonding in iridium has shown that BIF may also be considered as the intrinsic fracture mode of polycrystalline iridium (24).

The pyrometallurgical scheme for the refining of iridium, including: (a) oxidation induction melting; (b) electron beam melting; and (c) growing massive single crystalline workpieces by electron beam, became an alternative technology to manufacture ‘plastic’ iridium (25–27). Pyrometallurgical iridium demonstrated considerable plasticity prior to failure under tension in both the single crystalline state (28) and the polycrystalline state (29), while its elastic properties were the same as findings obtained earlier (30). It was confirmed that the intrinsic fracture mode of this ‘plastic’ iridium is BTF, while BIF is induced by harmful non-metallic impurities such as carbon and oxygen (31, 32). Indeed, the portion of BTF on the fracture surface of plastic iridium is considerably higher than BIF (33, 34). Deformation mechanisms and, hence, behaviour of pyrometallurgical iridium were the same as a normal fcc metal excepting the special fracture mode (35–38). Recent studies of deformation and fracture behaviour of iridium have shown that new participants achieved the technological level that allows ‘plastic’ iridium to be manufactured (39, 40), including iridium single crystals (41). Also, the old problem concerning the intrinsic fracture mode of polycrystalline iridium or the competition between BTF and BIF in iridium remains (42). Therefore, in the present paper, the deformation and fracture behaviour of iridium wires under tension at room temperature are considered in light of the discussion on this problem.

Pyrometallurgical iridium was used in this work. It was high purity metal, free of non-metallic contaminants such as carbon and oxygen. The refining procedure and, hence, impurities content were the same as the metal used in earlier work by our group: non-metallic elements <0.1 ppm; tungsten, molybdenum, niobium, iron, zirconium, copper, gadolinium, yttrium, gallium, nickel, palladium, zinc, magnesium, calcium –0.1–1 ppm; platinum, rhodium ~10 ppm (26, 27, 29, 38). Experience has shown that pyrometallurgical refining could be limited by the first and second procedures without loss of quality of the metal. Therefore, the operation of the growth of the massive single-crystalline iridium workpieces was not carried out in this work. The mechanical treatment of the pyrometallurgical iridium included: (a) forging the ingot into sheet at 1500–2000°C in air; and (b) rolling the sheet at ~800°C in air. The resulting metal could be processed like platinum. The cold drawing iridium wire, whose diameter varied from 2.7 mm to 0.5 mm, was prepared from this plastic metal. No long-term recrystallisation annealing of this wire was carried out because this procedure leads to embrittlement and failure of iridium wire due to BIF. Tensile testing was carried out with the help of an Autograph AG-X 50N tensile/compression tester (Shimadzu Corporation, Japan) (traverse rate of 1 mm min^–1) at room temperature. The lengths of working parts of iridium wire samples were 100 mm. The structure of the samples before and after testing was examined by conventional X-ray diffraction (XRD) technique on the D8 Advance diffractometer (Bruker Corporation, USA) with copper kα irradiation. Back surfaces of each sample before and after testing were documented on a light metallographic microscope. The fracture surfaces of samples were studied on the scanning electron microscope JSM-6390 (JEOL Ltd, Japan).

The first set of iridium samples consisted of 10 pieces taken from a commercial parcel of cold drawing thin iridium wire produced by UralInTech (Russia) having a diameter of 0.5 mm. The microstructure of this wire had a strongly deformed lamellar morphology, where the grains of the polycrystalline matrix practically disappeared (Figure 1). The main feature of this lamellar structure is the narrow highly elongated grains collected in a bunch like a rope. As a result, deformation tracks, such as slip bands or twin lamellae, could not be revealed on the surfaces of the samples after deformation. An XRD spectrum taken from the cold drawing iridium wire prior to testing is shown in Figure 2. There are two high narrow peaks ((200) and (220)) in the middle angles of the spectrum taken from the sample. No visible changes in the spectrum were revealed after tensile testing of the sample. It may be concluded that a stable drawing texture is formed in the iridium wire in comparison with an annealed polycrystalline sheet, which does not depend on further tensile deformation.

Fig. 1.

Fig. 2.

The second set of iridium samples consisted of 10 cold drawing wires with a diameter of 2.7 mm taken from the workpiece that was used to manufacture the thin plastic iridium wire. The Vickers microhardness of these samples in the undeformed state was about 7 GPa. The third set of iridium samples contained 10 cold drawing wires with a diameter of 2 mm. In contrast with the second set, these samples were annealed at 1000–1200°C for 20 min in a low vacuum and, as a result, their Vickers microhardness dropped up to 5 GPa. This operation is also used in the technological process for the manufacture of plastic iridium wire.

The stress-strain curves of the cold drawing iridium wires are shown in Figure 3 and some of their mechanical characteristics are collected in Table I. The back surfaces of the deformed samples are shown in Figure 4, while their fracture surfaces are given in Figure 5. It is clearly visible that the deformation behaviour of the samples from the first set (Figure 3, curve A) is similar to the behaviour of annealed copper wire (Figure 3, curve B). The long stage of plastic flow takes place after the short stage of material strengthening (Figure 3, curves A and B, respectively). Indeed, the total elongation of both materials may be estimated as considerable for a polycrystalline wire sample (30% for iridium and 43% for copper). In addition, there is a clearly visible advanced necking region on the back surfaces of the deformed samples (thinning of 20% for iridium and 55% for copper) (Figures 4(a) and 4(b) and Table I). However, in contrast with copper, iridium exhibits much higher yield stress and ultimate tensile strength (Table I). In spite of the features that are inherent to the ductile deformation behaviour, the fracture mode of the iridium samples from the first set is attested as BTF in the strongly deformed lamellar structure (Figure 5(a)). The same findings were obtained for cold drawing iridium wire with a diameter of 0.3 mm in the temperature range 20–800°C in (29).

Fig. 3.

Fig. 4.

Fig. 5.

The deformation behaviour of thick cold drawing iridium wire (Figure 3, curve C) can be attested as brittle: its stress-strain curve has an almost rectilinear profile, the yield stress is similar to the ultimate tensile strength, while the deformation prior to failure is small in comparison with the previous case. No necking was observed on the back surfaces of the deformed cold drawing thick iridium wires (Figure 4(c)). The fracture mode of the samples agrees with their brittle behaviour, it is BTF (Figure 5(b)). The short-term vacuum annealing of the thick cold drawing iridium wire at a temperature close to the point of recrystallisation of iridium leads to a change in mechanical behaviour from brittle to ductile. Indeed, the behaviour of the stress-strain curve becomes similar to annealed copper (Figure 3, curve D) when after a short stage of strengthening follows the plastic flow stage, while the yield stress and the tensile strength drop considerably (Table I). However, its deformation prior to failure is very small (Table I) for a plastic material and the neck is absent in the deformed samples (Figure 4(d)). The fracture mode does not change from brittle to ductile: it is attested as a mixture of BTF and BIF (Figure 5(c)).

It was shown that the deformation behaviour of cold drawing iridium wire under tension at room temperature depends on its structural state. Wire with grains of 50–100 μm behaves as a brittle material and exhibits BTF as the fracture mode. Vacuum annealing at 1000–1200°C causes a drop of yield stress of the iridium wire, but does not lead to significant increase of plasticity, while its fracture mode continues to be brittle. On the other hand, thin cold drawing iridium wire having a lamellar structure demonstrates considerable elongation prior to failure and clearly visible necking, despite BTF as its fracture mode. This finding gives the basis for the conclusion that such lamellar structure is the correct morphology for plastic polycrystalline iridium. It is important to note that this morphology is formed in the iridium workpiece under the cold drawing process, while a few short terms annealing at 1000–1200°C are included in the procedure after some rolling passes (29).

Earlier, it was shown that BIF is the impurities induced fracture mode of high purity polycrystalline iridium (31, 32). Indeed such non-metallic elements as carbon and oxygen contained in a low vacuum (10^–2 MPa) induce grain boundaries brittleness, but the kinetics of the process depends on the working temperature and its duration (31). For example, under annealing of 20 min at 1200°C, the portion of BIF on the fracture surface is considerably less than the portion of BTF, while after 24 h annealing BIF covers the whole fracture surface. It means that the regime of annealing of the cold drawing iridium wire used in this work is optimal because the hardness and the yield stress of the iridium workpiece are decreased, but the cohesion strength of grain boundaries does not drop.

The low plasticity of the thick cold drawing iridium wire, which was strongly hardened during preliminary processing, may be explained by the supposition that its resource of plasticity is finally exhausted under tension as it takes place in iridium single crystals under the same experimental conditions (26, 37). As a result, the cleavage crack can appear on any dangerous macroscopic surface defect and, hence, such wire is prone to separation by the brittle route without necking. Following this logic, the thin cold drawing iridium wire should behave the same way; however, it exhibits ductile mechanical behaviour, except its fracture mode. One cause of this puzzling effect may be the special configuration of the defect structure of iridium, whose feature on the microscopic level is a lamellar morphology. Indeed, high purity polycrystalline iridium is able to undergo severe deformation under high-pressure torsion at room temperature when the nanocrystalline structure is forming in the material (38). It is puzzling, but in this case, the surface defects play the role of the initiation of cleavage in the neck region only (29). Indeed, iridium meets some empirical cleavage criteria (18–20). However, this effect should be considered as an artefact because in contrast with other cleavable solids iridium is a plastic material in both the single crystalline and polycrystalline states and its inclination to cleavage depends on the structural state.

The lamellar structure that forms in iridium wire during the cold drawing process provides the excellent mechanical properties of polycrystalline iridium under tension: it behaves like a ductile fcc-metal excepting the brittle fracture mode. It was shown that the inclination of iridium to cleavage depends on its structural state.