1. Introduction
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 mgIr 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.
Summary of the reviewed wet-chemistry synthetic routes of state-of-the-art iridium catalysts for acidic OER
Table I
Summary of the State-of-the-Art Activities of Selected Catalysts Prepared by Wet-Chemistry Synthesis and Tested in a Rotating Disk Electrode
| Catalyst (method) and section vide infra |
Loading on the electrode, mgIr cm–2 |
Overpotential at 10 mA cm–2, mV |
Activity |
References |
| 3.2 Surfactant-assisted |
0.061 |
~290 |
100 A gIr–1 at 1.51 VRHE |
(8) |
| 3.3 Surfactant-free colloids |
0.0071 |
345 |
205 A gIr–1 at 1.5 VRHE |
(9) |
| 4. Supported on GCN |
0.07 |
278 |
580 A gIr–1 at 1.55 VRHE |
(10) |
| 4. Supported on TaTO |
0.02 |
~300 |
250 A gIr–1 at 1.51 VRHE |
(11) |
| 5.2 Selective leaching |
0.0277 |
N/A |
810 A gIr–1 at 1.51 VRHE, 3353 A gIr–1 at 1.55 VRHE |
(12) |
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.
2. Requirements to the Iridium Catalyst Performance and Structure: Activity vs. Stability
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 mgIr 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)):
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 OI–) 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)):
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.
OER and deactivation pathways in acidic OER, with green route being preferable at lower potentials, red route at higher potentials, while the blue route is potential independent. Reprinted with permission from (20), copyright John Wiley and Sons 2018
Catalyst featuring an electrochemically grown porous hydrous oxide layer, also known as oxy-hydroxide layer or amorphous IrOx catalysts, exhibit an OER mechanism that involves an electrophilic OI– 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):
where x is a vacancy in the porous hydrous oxide layer. Other references have attributed the increased activity to electrophilic OI– species (21). Ir-O-Ir would play a similar role to the proposed highly reactive, electrophilic oxygen OI– species (21, 24). The second step would have a similar function to the preliminary reaction proposed by Pfeifer et al. (24) (Equation (xi)):
where IrOx O represents the IrO2 matrix with an adsorbed oxygen. The pathway involving the electrophilic OI– species is also highlighted in Figure 2 in green, where HIrO2 would loosely represent the OI– intermediate.
The OER has been shown to be more active for the pathway involving the electrophilic OI– 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 OI– 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 IrOx 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 gIr–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.
Evaluation of IrO2/TiO2 catalyst stability in an RDE and membrane electrode assembly at 70 A gIr–1. Reprinted from (16) under the Creative Commons Attribution License, copyright 2019 The Authors
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 NafionTM (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. Synthesis of Unsupported Iridium/IrOx /Ir-OOH Catalysts
3.1 Adams’ Fusion Method
Adams’ method was originally developed to conquer the issues of irreproducible platinum catalyst synthesis; it was successfully scaled up and is used industrially for platinum (Adams’) catalyst production (50, 51) and thus is well positioned for potential scalability. It also appears to be one of the most used methods reported for the synthesis of IrO2 for OER, including supported and multimetallic composites. The method is based on the synthesis of iridium nitrate from an iridium molecular precursor heated in a solid mixture with sodium nitrate, followed by the iridium nitrate high-temperature decomposition to IrO2. The side products include poisonous nitrous oxides, which release must be appropriately managed. Synthesis parameters include the temperature and duration of the calcination, nature of the iridium precursor and its fraction in the mixture with NaNO3, all of which affect the crystallinity, oxidation state and surface area of the produced material and thus, the OER performance. The higher is the calcination temperature, the higher is the crystallinity of the produced oxide. However, the amount of active surface hydrous IrO2 decreases, as well as the catalyst surface area. Although the increase in the calcination temperature leads to the lower iridium dispersion (larger particle size) and lower turnover frequency (TOF) due the formation of less active crystalline IrO2, the latter is more stable towards dissolution. This indicates the existence of the optimal calcination temperature to achieve the activity-selectivity balance for the maximised iridium utilisation (31). Electrical conductivity is also improved with increased crystallinity at annealing (52). The highest reported surface area of an IrO2 produced by a modified Adams’ method is 350 m2 g–1, which was obtained from iridium acetylacetonate and calcined in air at 350°C for 30 min (31). The oxide consists of nanodisks with surface partially covered by active Ir(OOH), which however retained only 55% of its activity after 500 potential cycles due to mass loss and restructuring. When the original sample was further heated at 400°C for 1 h, the catalyst retained 70% of its activity after 500 potential cycles. Increased calcination temperature, however, led to a decrease in surface area to 250 m2 g–1. At the catalyst loading of 0.1 mgIrOx cm–2, the specific current of 26 A gIrOx –1 could be achieved at 295 mV overpotential before the stability tests (0.1 M HClO4). For both catalysts, the same activity loss (of 30%) occurred due to the partial oxidation of active sites, but due to the decreased leaching from the 400°C treated sample, the latter strikes the balance between the activity and stability (31). The increase in calcination temperature leads not only to the surface area decrease but also changes the particle morphology to rods with dominating {110} surface terminations (53).
Among other promising reported modifications of Adams’ method, the addition of cysteamine to the iridium precursor solution resulted in the formation of IrO2 nanoneedles of 2 nm diameter (6–8 layers of (110) plane, Figure 4) and 30 nm length after 450°C calcination (54). Although the needles possessed lower Brunauer–Emmett–Teller (BET) surface area than the catalyst formed without cysteamine (141 m2 g–1 vs. 197 m2 g–1), their electrical conductivity was six-fold higher. An overpotential of 313 mV was required to achieve 10 mA cm–2 at the catalyst loading of 0.21 mgIr cm–2 (1M H2SO4, 25°C) before and after a 2 h durability test. The needles were also tested in a PEM electrolyser and found more active and stable than the spherical IrO2 synthesised without cysteamine (54). Most likely, the less dense and well-connected structure of iridium needles contributed to the improved porosity and electrical conductivity. Thin needles with near-zero sphericity form packed beds with the highest near-100% porosity as opposed to 40% for the spherical particles.
Fig. 4.
(a) Ultrathin IrO2 nanoneedles; (b) ultrathin IrO2 nanoneedles consisting of eight (110) layers. Reproduced from (54) with permission from John Wiley and Sons. Copyright 2017
3.2 Iridium Nanoparticles Stabilised by a Capping Agent
If one has to produce monodisperse near-spherical nanoparticles with high dispersion (>50%, i.e., smaller than ca. 2 nm) to increase metal utilisation or form anisotropic nanostructures to increase the catalyst layer porosity or promote the formation of certain crystal terminations, colloidal synthesis in the presence of a capping agent is a popular method in academic research (55). Halogen-containing stabilisers, such as cetyltrimethylammonium bromide (CTAB), are known to act also as a growth-directing agent by the halogen selective adsorption on (100) surfaces resulting in rod-like structures. The produced structures are usually pre-washed from the excess chemicals, while the in situ electrochemical preconditioning removes the surfactant, for example, by 50 potential sweeps from 0.05 VRHE to 1.5 VRHE (RHE = reversible hydrogen electrode) (29). Since the metallic iridium is oxidised electrochemically, it is likely to possess a higher proportion of activity-relevant hydrous IrO2 on the surface, as opposed to calcined rutile IrO2.
One of the most successful examples in this category is the 2.0 ± 0.4 nm iridium nanoparticles formed by IrCl3 reduction in ethanol with excess NaBH4 in the presence of CTAB (8). NaBH4 is a strong and fast reducing agent to produce metallic nanoparticles and is often used in the colloidal synthesis. In a protic solvent, borohydride decomposes to gaseous hydrogen, which, depending on the conditions, may proceed in a violent manner. To reach 10 mA cm–2 current, ca. 290 mV overpotential was required at only 0.061 mgIr cm–2 loading in an RDE (0.5 M H2SO4, 25°C). The catalyst demonstrated a similar Tafel slope of ca. 40 mV dec–1 as the calcined catalyst prepared by the Adams’ fusion method (31). However, the specific current at 1.51 V was an order of magnitude higher (100 A gIr–1). The nanoparticles formed a nanoporous structure with well-connected particles, which retained their metallic core but featured an active thin surface oxide layer (Figure 5(a) and 5(b)). Authors stressed the importance of complete IrCl3 removal by adding excess of reducing agent to prevent inhibition of electron transfer. Although the catalyst showed an order of magnitude higher specific current than Ir black (Umicore, Belgium) in an RDE, it required only 250 mV lower potential to reach 2 A cm–2 current (1.85–1.9 V) when tested in an unoptimised PEM electrolyser with 1 mgIr cm–2 loadings. This again indicates the effect of various factors in a PEM electrolyser.
Fig. 5.
(a)–(b) Stabilised interconnected iridium nanoparticles (8); (c) iridium nanodendrites (29) (reproduced from (8) and (29) under a Creative Commons Attribution 3.0 Unported License, published by The Royal Society of Chemistry); (d)–(f) highly-crystalline nanopompons (26) (reproduced from (26) under a Creative Commons License, published by Elsevier)
The use of tetradecyltrimethylammonium bromide (TTAB) during IrO2 precipitation from H2IrCl6 by NaOH, followed by reduction by NaBH4, resulted in the formation of 1.7 nm metallic iridium seeds that self-assembled into nanodendrites with 34% porosity at 39 m2 g–1 BET surface area (29). The high crystallinity favoured stability toward dissolution. The 10 mA cm–2 current was achieved at 410 mV overpotential (RDE, 0.05 M H2SO4) but at only 0.0102 mgIr cm–2 loading. At 1.51 V, the catalyst activity was 70 A gIr–1. Similarly to the CTAB-stabilised particles (8), the formed structure featured high porosity and well-connected individual particles (Figure 5(c)).
When a slow reducing agent is used for synthesis (such as glucose (26)), the CTAB suppressed the grain growth in (100) directions; the nanodendrites self-assembled into nanopompons (Figure 5(d)– (f)). Those highly crystalline structures with a high proportion of low-index crystal terminations were relatively resistant to dissolution but showed lower activity as compared to the hydrous IrO2 (26).
The stabiliser-assistant synthesis techniques are easy to implement in a wet laboratory without specialised equipment for academic research. This method of iridium synthesis is also used to preform iridium nanostructures prior to their deposition on a support. One must be mindful of a typically low metal concentration in the synthesis solution, the relatively large use of solvents, reductants, stabilisers and washing solutions, many of which are manufactured from fossil resources and expensive. Such synthesis methods are usually too cumbersome for industrial production, the improvements being feasible though for certain stabilisers (56).
3.3 Stabiliser-Free Wet Chemical Synthesis Methods
This category features one of the most active catalysts reported to date, although the electrodes were fabricated without NafionTM. Synthesis of 1.6 ± 0.3 nm iridium particles was performed without a stabiliser by heating a solution of IrCl3 in methanol, which reduces IrO2 precipitated by co-added NaOH; the resulting solution was used without purification (9, 57). At a loading of 0.0071 mg cm–2 achieved by drop casting of the native solution, the particles formed a uniform layer on a glassy carbon (GC) disk. In 0.1 M HClO4, in an RDE, the catalyst demonstrated an outstanding 205 A gIr–1 activity at 1.5 VRHE and 1130 A gIr–1 at 1.55 VRHE (57). The electrochemically active surface area (ECSA) was found to be 140 m2 g–1 (at a loading of 0.0071 mgIr cm–2). The overpotential to reach 10 mA cm–2 was 345 mV at 0.0071 mgIr cm–2 loading, or 325 mV at 0.0143 mgIr cm–2 loading (9). The iridium loading on the electrode was of vital importance: loading increase above 0.0071 mg cm–2 resulted in significant drop of the ECSA and thus specific current (9). The specific activity at the optimal loading surpasses the activity for the surfactant-mediated catalysts (8), and features a significantly easier, cheaper and scalable preparation. In addition to being surfactant-free and using a low-boiling easily recoverable solvent, the method is scalable to high metal concentrations (5 g l–1) (57).The fate of Na+ (10:1 Na:Ir) is to be investigated, as well as the catalyst durability and performance in a PEM electrolyser.
Many studies feature a similar iridium nanoparticle synthesis without a surfactant with the use of a base (NaOH) in other reducing solvents, but typically such particles are deposited on a support, and are discussed in Section 4.
4. Wet Synthesis of Iridium Catalysts on Powdered Supports
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 NafionTM 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 gIr–1 at 1.55 V and 1493 A gIr–1 at 1.6 V. The catalyst required the overpotential of 278 mV at 10 mA cm–2 at 0.07 mgIr 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 VRHE for 2 h with intermediate cyclic voltammetry (CV) measurements between 0.4 VRHE and 0.6 VRHE; 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 VRHE and 2.7 VRHE, 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 IrOx nanoparticles at 11–18 wt% iridium loading, the OER activity of the fresh catalysts after electrochemical conditioning approached 250 A gIr–1 at overpotentials of 280 mV and 370 mV at 0.020 mgIr 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 IrOx nanoparticles. In accelerated ageing tests at 1.2–1.6 V potential steps, the IrOx /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 IrOx 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 IrOx /TaTO catalysts at 1.6 V and similar at 1.5 V (11). When hydrous IrOx was supported on ATO, its S-number was also lower than the one for unsupported IrOx , 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 mgIr 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 gIr– 1 vs. 8 A gIr–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 mgIr 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.
5. Mixed Metal Oxides
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 NafionTM 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 IrOx 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 IrOx 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.
Mixed metal oxides for the acidic OER catalyst preparation
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 NafionTM solution.
5.1 Core-Shell Bimetallic Nanoparticles
Multimetallic composites can be synthesised using all the techniques applicable for monometallic catalyst synthesis (see Section 3) with the addition of the second precursor, with the fine tuning of the reaction conditions. Very often, both precursors are added together during the synthesis. Simultaneous reduction of different ions with different redox potentials leads to the formation of either mixed alloy particles or core/shell nanoparticles (76). For the core-shell synthesis, methods like ionic substitution (galvanic replacement) can be used (77) where a precursor of the second metal is deposited onto the metallic nanoparticles of the other metal based on the standard electrochemical potential (78), or a hydrogen-sacrificial method (79) where a core metal is hydrogenated, followed by the second ion reduction by the surface hydrides and shell formation. It is important to understand that thermodynamically unstable bimetallic structures can be synthesised (in terms of metal distribution) but how fast they rearrange into the thermodynamically stable composites (for example, where a metal with a lower cohesive energy segregates to the surface) depends on temperature, chemical and electrical environment. Metals can even change their location in situ depending on the catalysed reaction or treatment conditions via so-called adsorbate-induced segregation (80–82). Under the OER conditions, the restructuring may be expected to be an ongoing process with transient equilibrium states due to the different rates of metal dissolution.
According to the Hume-Rothery substitution rule, in order to form a continuous solid solution, it is imperative that the difference between the atomic radii of the solvent and the solute not exceed 15% of each other and they should possess similar crystal structures (83). One of the most common metals alloyed with iridium is ruthenium, but regardless of the efforts being made to inhibit ruthenium leaching, literature report continuous ruthenium dissolution, irrespective of methods of synthesis and structure of the mixed metals oxides. Ruban et al. reported a comprehensive table of surface segregation energies in transition-metal alloys, which can help predicting the final dealloyed structure of iridium composites (84). In bimetallic alloys with iridium, metals such as copper, zirconium, rhodium, palladium, silver, hafnium, platinum and gold would segregate to the surface, while iridium would surface-segregate from alloys with titanium, vanadium, chromium, manganese, iron, cobalt, nickel, niobium, molybdenum, technetium, ruthenium, tantalum, tungsten, rhenium and osmium (84). Indeed, for example, in ruthenium-iridium alloys iridium formed a protective shell, offering an extended stability to ruthenium (85).
Another example, conforming to the iridium surface segregation prediction, relates to the iridium-nickel composite. Bimetallic 7 nm IrNi3.2 alloy nanoparticles were prepared by a simultaneous reduction of iridium and nickel precursors in the presence of a stabiliser (70). The followed potential cycling from 0.05 VRHE to 1.5 VRHE for 50 cycles resulted in partial nickel leaching, dealloying and oxidation with the formation of a metallic IrNi alloy(core)-IrOx (shell) nanostructure. The IrOx shells are doped with holes (originated from nickel leaching); they feature shorter Ir–O bonds and are more electrophilic than conventional IrO2, which affects the rate of O–O bond formation during OER and enhanced intrinsic activity per iridium site. A specific current of 676 A gIr–1 was reported at 300 mV overpotential with 0.0102 mgIr cm–2 loading, which is one of the highest in the above presented examples (70). This example shows an extraordinary combination of increased iridium utilisation due to its preferential location in the nanoparticle shell, as well as its beneficial electronic and thus catalytic activity modification upon dealloying.
5.2 Selective Leaching of the Sacrificial Component (Hard Templating)
When a secondary metal in bimetallic composites with iridium is selectively removed, the process results in the formation of porous iridium nanostructures with high accessible active site density and modified IrOx electronic and geometric properties, which cannot be achieved via a monometallic iridium catalyst synthesis. Some examples of a sacrificial metal are nickel (12, 86), cobalt (86) and osmium (39). Among different leaching methods, the most common are acid leaching (12) and potential cycling (39, 71). For example, iridium was deposited on nickel nanowires via galvanic displacement, followed by the nanowires (‘hard template’) removal by acid leaching (12). The residual composite with 90.5 wt% iridium demonstrated a specific current of 1650 A gIr–1 at 0.0306 mg cm–2 loading at 300 mV overpotential in RDE in 0.1 M HClO4. A parent mixed metal structure can be prepared as stabilised nanoparticles in a colloidal solution, such as an iridium-osmium oxide (39). The potential cycling resulted in dealloying and fast osmium dissolution, leaving behind a nanoporous architecture of iridium metal core and IrOx shell with an optimised stability and conductivity (39). An important observation was that a high amount of Os–Ir bonds in the parent alloy led to the maximum iridium dissolution upon fast osmium leaching.
This category features some of the most architecturally-sophisticated porous nanostructures, such as hollow nanocrystals synthesised via formation of a iridium-cobalt-nickel solid nanoparticles, followed by cobalt and nickel etching with Fe3+ (86), or double-layered nanoframes produced in a solution via reduction of nickel, copper and two types of iridium precursors with different reduction kinetics followed by acid leaching (87). Theoretically, this approach may produce highly porous connected iridium-only nanostructures, but its practical implementation is complicated by the nuances in the size and structure control, as well as secondary component complete leaching without the structure collapse.
It appears that the main achievement of the selective etching is not in the improvement of iridium surface area and dispersion, as the thus-synthesised catalysts do not feature areas above 100 m2 g–1 as compared with monometallic IrO2 synthesised by Adams’ method with 250 m2 g–1 (31). Instead, the etching allows for modification of the IrOx electronic structure and vacancies, affecting its OER activity and stability.
6. Conclusions
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 IrOx 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.
