The Oxford Battery Modelling Symposium was held in Oxford, UK, from 18th to 19th March 2019. The conference was specifically designed to gather mathematicians, chemists and engineers within the battery modelling community. It was very well received and brought together 170 participants with worldwide representation from academia, research organisations and industry involved in modelling at different scales (atomic length-scale, continuum and control-oriented modelling). This review will focus on eleven talks presented in the four sessions and organised as follows:

Electrochemical processes can be modelled using a continuum approach (that relies on material specific parameters, sometimes difficult to measure) or from first principles. As electrochemical processes are thermally activated, in ‘Connecting Electronic Structure to Phenomenological Continuum Models of Electrochemical Processes’ by Anton Van der Ven (University of California Santa Barbara, USA) it was shown that temperature and entropy play a key role for understanding the physics and the properties of materials. As such, a statistical mechanics approach is beneficial, although computationally very demanding. Van der Ven introduced the open-source software Cluster Approach to Statistical Mechanics (CASM) developed in his group and available from GitHub (1). In CASM, for a certain material, the thermodynamic and kinetic properties obtained from density functional theory can be fed into continuum models to realise fast and computationally undemanding first-principle multiscale simulations for dynamic processes such as electrochemical processes. This tool can be used to predict thermodynamic and kinetic properties of various classes of materials (such as layered, olivines, spinels and alloys), see Figure 1 (2).

Fig. 1.

In literature both accurate first-principle methods and continuum theories are available to predict the properties of materials and interfaces. However, rigorous ways to connect the two approaches are still lacking. In ‘Mind the Gap – Towards an Atomistic Understanding of Battery Materials Interfaces’, Denis Kramer (University of Southampton, UK) described strategies to build continuum models starting from first principle calculations and their application to crystallisation. The coverage effect in the size-stabilisation of nanocrystals during electrochemical processes and the crystallisation process of manganese(IV) oxide polymorphs have been discussed (3). Finally the effect of Li⁺ ions in the stabilisation of some MnO₂ polymorphs was described in this framework.

In ‘Modeling Porous Intercalation Electrodes with Continuum Thermodynamics and Multi-scale Asymptotics’ by Manuel Landstorfer (Weierstrass Institute for Applied Analysis and Stochastics, Germany), a description of the procedure for modelling porous cathodes was provided. For such electrodes three scales can be identified (the double layer scale, the macroscopic porous media scale and the microstructure scale). Landstorfer started by describing the metal-electrolyte interface and electron transfer in the double layer through a non-equilibrium thermodynamic continuum model (4). The model was scaled up to electrode particle scale and treated with a matched asymptotic expansion method. Finally, Landstorfer discussed a third scale: the macroscopic porous media scale. He introduced homogenisation techniques for the prediction of transport properties at porous scale. This multiscale methodology was applied to model thermodynamic properties, diffusion processes and the open circuit potentials of intercalation cathodes for Li-ion batteries with different chemical composition and porosity.

‘Electrochemical Energy Storage’ by John Newman (University of California Berkley, USA) was a keynote lecture on mathematical modelling approaches for the design of batteries. Newman introduced various methodologies (such as continuum modelling, (kinetic) Monte Carlo and molecular dynamics) and their application to the modelling of intercalation electrodes and electrolytes in Li-ion batteries. He showed how these tools can improve understanding of the electrochemical processes as well as of failure mechanisms taking place in battery materials, helping to design high power and high energy battery materials.

Bob McMeeking (University of California Santa Barbara, USA) presented a model for the redox kinetics at an interface between a solid electrolyte and a Li metal anode in ‘Redox Kinetics, Interface Roughening and Solid Electrolyte Cracking in Solid State Lithium-Ion Batteries’. This model was based on the extension of the Butler-Volmer equation through the inclusion of the effect of the mechanical stress across the anode-electrolyte interface. This method was applied to the investigation of the morphological stability of the interface between the Li anode and the solid electrolyte for various current densities and solid electrolyte resistivities. Moreover, the extended Butler-Volmer equation was used to model the evolution of cracking in ceramic solid electrolytes caused by Li insertion into pre-existing defects on the electrolyte surface. The model showed how the pressure generated by Li insertion into the flaw causes the propagation of cracking in the solid electrolyte and consequent Li dendrite growth. Finally, the extended Butler-Volmer equation was used to identify the maximum Li pressure and critical lengths (a₁) of defects within a series of ceramic electrolytes to avoid the propagation of Li dendrites (a₁ = 2 μm for Li₇La₃Zr₂O₁₂ (LLZO)) (5).

The kinetics and uniformity of Li insertion reactions at the solid-liquid interface govern the rate capability and lifetime of Li-ion batteries. Martin Bazant (Massachusetts Institute of Technology, USA) presented a model for the prediction of phase transformations of intercalation materials in ‘Control of Battery Phase Transformations by Electro-Autocatalysis’. The approach was based on a thermodynamic framework for chemical kinetics applied to charge transfer (namely, the Marcus and extended Butler-Volmer equations) (6).

Reaction-driven phase transformations are common in electrochemistry, when charge transfer is accompanied by ion intercalation or deposition in a solid phase. The model allows rationalisation of phase separation of Li-rich and Li-poor islands for low discharge rates that affect the stability and cyclability of Li-ion batteries. The model also proved that high discharge rates favour the formation of solid-solution phases through an electro-autocatalytic mechanism, later experimentally confirmed for lithium iron phosphate (LFP) (7). The model can be extended to the investigation of electrodeposition, corrosion, chemical intercalation, precipitation and cell biology.

Göran Lindbergh (KTH Royal Institute of Technology, Sweden) presented an extended physics-based porous electrode model accounting for particle surface stress that was used to describe ageing of nickel manganese cobalt oxide cathode (LiNi_x Mn_y Co_z O₂, NMC) with composition x = y = z = 0.33 (namely NMC111). In ‘An Extended Porous Electrode Model for NMC111 in Lithium-Ion Batteries’ the performances of NMC111 were experimentally investigated via a galvanostatic intermittent titration technique and two models were used to fit the experiments: (i) a standard pseudo-two-dimensional (P2D) model; and (ii) an extended surface stress P2D model that included a stress factor depending on the Li concentration gradient in the material. Model (ii) could accurately extract transport, kinetic, thermodynamic and stress properties for the whole spectrum of operative conditions (low and high charge-discharge rates, temperature and external pressure). Although the standard model works well for low potentials (less particle surface stress), the porous electrode stress model predicts the ageing of NMC at high potentials (high surface stress).

‘Physically-Informed Models for Improved Cell Design and Operation of Lithium-Sulphur Cells’ by Monica Marinescu (Imperial College London, UK) was a lecture about the necessity of using physically-informed models to predict the mechanisms and performance of batteries. The accumulated experience on physically-informed models for Li-ion was used as a starting point for engineering Li-S batteries. Modified equivalent circuit network models were used for Li-S batteries modelling in order to take into account phenomena like shuttling, dissolution and precipitation. Moreover, simple physics-derived zero-dimensional (0D) and one-dimensional (1D) continuum models for the prediction of open circuit voltage and of the effects of mass transport on discharge, degradation mechanisms and capacity fade for commercially-sized Li-S batteries were presented (8–10).

Gregory Plett (University of Colorado, Colorado Springs, USA) delivered a keynote lecture titled ‘Physics-Based Reduced-Order Models of Lithium-Ion Cells for Battery Management Systems’ about physically-informed control models. Plett reviewed the standard physics-based model particularly focusing on how this model could be converted to a physics-based reduced-order model (PBROM). Battery-management systems provide a continuous estimate of state-of-charge, state-of-health, available energy and available power of battery packs. Traditional computational methods rely on empirical equivalent circuit models of the batteries. These models are computationally fast and robust. However, although accurate for many tasks, they cannot predict the internal electrochemical state of the cell. On the other hand, physics-based models that provide good predictions of the internal electrochemical state are too complex to apply to battery-management systems, which are heavily parametrised and have robustness and convergence issues. PBROM is a method that, while reducing the computational requirement of physics-based models, retains their prediction accuracy and can be used for battery management systems.

In ‘Decoding the Electron Swelling for Advanced Battery Diagnostics’ Anna Stefanopoulou (University of Michigan, USA) presented a conjugated experimental and computational control model to account for battery degradation. Since standard control models do not account for swelling and ageing, during the talk Stefanopoulou introduced experimental apparatus to probe battery degradation and convert the observables (measured terminal voltage and surface temperatures) into parameters to implement control models to predict swelling and ageing (11). In particular, observations of the cell swelling during charging were used to estimate the loss of active material and loss of Li inventory in the anode, which is useful for avoiding Li-plating during fast charge.

The last talk was delivered by Scott Trimboli (University of Colorado, Colorado Springs, USA). The ‘Model Predictive Control using Physics-Based Models for Advanced Battery Management’ was a lecture on the model predictive control (MPC) developed in collaboration with Plett. Starting from the PBROM, Trimboli showed the mathematical implementation of MPC. MPC is an effective real-time control strategy that employs a ‘look-ahead’ approach to foresee dynamic behaviours in the battery pack before they happen. This approach can be coupled with the ability of PBROM to enforce hard constraints on internal electrochemical variables (precursors to degradation or unsafe operation conditions), making MPC appealing for advanced battery management, where safety, lifetime and improved performance are crucial (12).

The Oxford Battery Modelling Symposium aimed to bring together the battery modelling community. It was well attended and the 12 talks as well as the 25 posters were high quality. The four sessions of talks were successfully organised to provide a full overview of the current state of the art in Li-ion and next generation battery modelling, spanning from first-principle investigations to control-oriented approaches.

This Springer volume focuses on the design, characteristics and development potential of proton exchange membrane fuel cell (PEMFC) and solid oxide fuel cell (SOFC) technologies for both stationary and portable applications. The contents are organised into three themes: (a) energy policy and electrical power (Chapters 1–3); (b) optimisation of fuel cells (FCs) through design and synthesis of novel catalysts (Chapters 4–8); (c) optimisation of FCs through modelling and simulation (Chapters 9–18). The book forms a useful compendium of research activities across the globe that gives the reader a general overview rather than an in-depth treatment of any one area. The layout and presentation are of the usual Springer high standard with clearly visible graphs and illustrations; the book was compiled in 2018.

Chapter 1, ‘Fuel Cell Technology: Policy, Features, and Applications – A Mini-Review’ by S. Bashir (Texas A&M University-Kingsville, USA) et al., starts with a comparative analysis of the energy policies of presidents Eisenhower and Trump with lots of facts and figures regarding historical energy use and energy vector types. The text covering fuel cells is PEM-centric (no SOFC or phosphoric acid fuel cell (PAFC)) and battery vehicles are not included in the discussion.

Chapter 2, ‘Concept of Hydrogen Redox Electric Power and Hydrogen Energy Generators’ by K. Ono (Kyoto University, Japan), argues that particular bipolar electrode configurations and power supply arrangements in coupled electrolyser or FC systems can improve overall efficiencies markedly over existing setups. Ono maintains one can treat the system as a combination of electrostatic energy and electrical to chemical energy conversion. I found the reasoning difficult to follow, perhaps because there are considerable conceptual challenges faced in dealing with electrostatic terms in highly condensed phases (see e.g. (1)). Unfortunately, no experimental data are presented to support the claims by Professor Ono.

Chapter 3, ‘Evaluation of Cell Performance and Durability for Cathode Catalysts (Platinum Supported on Carbon Blacks or Conducting Ceramic Nanoparticles) During Simulated Fuel Cell Vehicle Operation: Start-Up/Shutdown Cycles and Load Cycles’ by M. Uchida (University of Yamanashi, Japan) et al., is a comprehensive work on the mechanistic details of degradation mechanisms and with proposed mitigation protocols. Different supports are looked at, not just carbon. We are reminded of the importance of understanding the practical challenges of stack design and operation in respect of membrane electrode assembly (MEA) degradation mechanisms. Degradation tests are based on voltammetric cycling to mimic start-up and shut-down automotive duty cycles. Perhaps not surprisingly, platinum dissolves and aggregates and carbon corrodes and different accelerated ageing protocols give different results. This is very important in the commercial world – you may not agree with the customers’ tests, but they are the ones your product will be judged by!

Chapter 4, ‘Metal Carbonyl Cluster Complexes as Electrocatalysts for PEM Fuel Cells’ by J. Uribe-Godinez (Centro Nacional de Metrologia, Mexico) offers a general introduction and good summary of work in the field on catalysis preparation for PEMFC systems. Carbonyl complexes can be heat treated to produce metallic-like clusters and here the author looks at rhodium, iridium and osmium species with a heat-treatment regime up to 500°C in either nitrogen or hydrogen or thermolysed by redox in a suitable solvent. Unfortunately, there are no mass or specific surface area activity-based data so although a comparison with a 30 wt% Pt on XC72R catalyst is made, it is difficult to assess specific-area based catalytic activity.

Chapter 5, ‘Non-Carbon Support Materials Used in Low-Temperature Fuel Cells’ is written by X. Cao (Soochow University, China) et al. Traditional carbon supports used in FCs are prone to degradation through oxidation and many attempts have been made to find substitutes that can offer competitive performance, durability and cost. The authors give us a survey of the state-of-the-art. However, it is clear that carbon is favoured as a support (for good reason) and is unlikely to be substituted in the near term for low-temperature FCs.

Chapter 6, ‘Noble Metal Electrocatalysts for Anode and Cathode in Polymer Electrolyte Fuel Cells’ by S. Sharma and C. M. Branco (University of Birmingham, UK) is potentially a vast subject to tackle and the chapter covers the basics of what is understood about the performance-morphology related aspects of precious metal catalysts for PEMFC electrocatalysis. There is a particular emphasis on the oxygen reduction reaction (ORR).

Chapter 7, ‘Nanomaterials in Proton Exchange Membrane Fuel Cells’ is written by Y. Zhang (Harbin Institute of Technology, China) et al. In this chapter the PEM emphasis is a little more on direct methanol oxidation than the previous chapter and there is a shift of emphasis towards zero-dimensional, one-dimensional and two-dimensional materials. Carbon features explicitly in the form of nanotubes and graphene.

Chapter 8, ‘Nanostructured Electrodes for High-Performing Solid Oxide Fuel Cells’ by H. Ding (Colorado School of Mines, USA), reviews solution-based, ion infiltration methods of catalysing surfaces in electrode structures. Solution impregnation is well-known in the catalysis industries in general and it is no surprise that it has been adopted with enthusiasm by the SOFC research and development community. Much of the know how has been developed through traditional empirical methods and this chapter reviews progress in the field for a wide variety of catalysts from base and precious metals to complex oxides.

Chapter 9, ‘Modelling Analysis for Species, Pressure, and Temperature Regulation in Proton Exchange Membrane Fuel Cells’ is written by Z. Wang (Texas A&M University-Kingsville, USA). The model emphasis is on understanding the controlling factors in flooding of the MEA under steady-state conditions. The construction of the conservation equations for momentum, mass, species, charge and energy are given in some detail.

In Chapter 10, ‘The Application of Computational Thermodynamics to the Cathode-Electrolyte in Solid Oxide Fuel Cells’ by S. Darvish and M. Asadikiya (Florida International University, USA), the authors use the calculation of phase diagrams (CALPHAD) modelling approach with an emphasis on perovskite and fluorite structural motifs. A comprehensive summary of the materials challenge for SOFC materials when used as electrolytes and cathodes is presented. Complexity is added wherein multiple phases can form due to reaction of the components with gaseous impurities either in the air supply or through, for example, carbon dioxide cross-over from the anode. The CALPHAD approach allows for a workable description of the important defect chemistry of the complex oxides to be predicted together with ionic and electronic conductivities.

In Chapter 11, ‘Application of DFT Methods to Investigate Activity and Stability of Oxygen Reduction Reaction Electrocatalysts’ by X. Chen (Southwest Petroleum University, China) et al., the authors describe the use of density functional theory (DFT) to model and understand the behaviour of PEMFCs at the catalyst level with a focus on the ORR. Not surprisingly, the oxygen binding energy to (pure) metal surfaces is an activity descriptor of choice and its simplest exposition is in the well-known volcano plot which has platinum and palladium close to the apex. More sophisticated approaches consider the energetics of binding of the key intermediates and a mapping of the associated potential energy surface. As well as metallic-type catalysts, some metal-centred, macrocyclic moieties are also investigated for activity. Finally, the stabilities of these various types of ORR catalysts are considered.

Chapter 12, ‘Hydrogen Fuel Cell as Range Extender in Electric Vehicle Powertrains: Fuel Optimization Strategies’ is written by R. Álvarez and S. Corbera (Universidad Nebrija, Spain). As the title suggests, the purpose described in this chapter is to optimise strategies for combining battery and FC power units for range extension. There is a useful summary of the current ‘competitive posturing’ between the various proponents of battery and FC-powered vehicles. A MATLAB^®/Simulink^® vehicle model, coupled with the use of genetic algorithm routines, has been developed to examine the interplay of the electrical and mechanical components of the system over selected drive cycles. Importantly, the FC in this case study is used to maintain the charge of the lithium ion battery rather than as an alternative power source for coupling to the drivetrain directly.

Chapter 13, ‘Totalized Hydrogen Energy Utilization System’ by H. Ito and A. Nakano (National Institute of Advanced Industrial Science and Technology (AIST), Japan) describes a hydrogen-based energy storage system utilising a reversible FC/electrolyser coupled with a metal hydride tank with fluctuating renewable electrical power inputs and heat and electrical power outputs (combined heat and power (CHP)). The heat flow from the system can be both positive and negative i.e. used for cooling or heating. The prototype demonstrator is a ten cell PEM-type stack with <1 kW output. The totalised hydrogen energy utilisation system (THEUS) was run continuously for three days on a fixed duty cycle and data collected and analysed.

Chapter 14, ‘Influence of Air Impurities on the Performance of Nanostructured PEMFC Catalysts’ by O. A. Baturina (Naval Research Laboratory, USA) et al., discusses the practical issues associated with using PEMFC units in the real world with different environments where air-borne pollutants or atmospheric conditions can pose a risk to the proper functioning and longevity of the PEMFC. An example shown is the dramatic and irreversible drop in cathode performance when exposed to low levels of compounds such as hydrogen chloride and bromomethane vapour. The various poisoning mechanisms are discussed together with possible mitigation strategies.

Chapter 15, ‘Solid-State Materials for Hydrogen Storage’ by R. Pedicini (Institute for Advanced Energy Technologies, Italy) et al., gives a general introductory review covering physisorption and chemisorption-based materials. The authors describe the often conflicting requirements that need to be met for a successful hydrogen storage material, such as the storage capacity, the kinetics of release and uptake and the resilience to mechanical degradation after many duty cycles. More novel, polymeric or inorganic hybrid materials are considered including polyether ether ketone-manganese dioxide (PEEK-MnO₂) composites and the use of more esoteric materials such as mixed metal oxides from volcanic ash.

In Chapter 16, ‘Stress Distribution in PEM Fuel Cells: Traditional Materials and New Trends’ by J. de la Cruz (CONACYT-INEEL, Mexico) et al., the authors remind us that as PEMFC stack technology advances, more attention needs to be focused on the mechanical and electrical engineering aspects of cell components such as the bipolar plates and membranes to optimise performance, manufacturability, durability and cost.

Chapter 17, ‘Recent Progress on the Utilization of Nanomaterials in Microtubular Solid Oxide Fuel Cell’ is written by M. H. Mohamed (Universiti Teknologi Malaysia) et al. Effective extension of the electrode-electrolyte-reactant interface in FCs presents material and electrode processing challenges. Micro-tubular SOFCs (MT-SOFCs) are a recent development for engineering porosity in the ceramic anode and cathode where the more traditional pore formers are substituted and supplemented using tailored hollow fibres. Inevitably, there is a compromise to be had in terms of ensuring good densification of materials to minimise ohmic drops while enabling reactant and product transport to function adequately at higher current densities. The authors present a brief review of progress in both medium and higher temperature SOFC systems.

Chapter 18, ‘Nanostructured Materials for Advanced Energy Conversion and Storage Devices: Safety Implications at End-of-Life Disposal’ is written by S. Bashir (Texas A&M University-Kingsville, USA) et al. It is increasingly important for manufacturers to demonstrate that they have considered and mitigated against environmental damage that may arise from the disposal of products at end of life. The conclusion from this work using iron oxide nanoparticles as a test probe of materials entering the environment is that best practice should use a combination of life cycle assessment (LCA) and risk assessment (RA) methodologies.

In summary then, this volume brings together an interesting collection of articles covering mainly hydrogen PEM and SOFC technologies that will help build a more balanced understanding of the commercialisation and technical challenges arising from catalyst behaviour through to stack design.

There has been significant interest in converting gas, in particular CH₄, into liquids (gas to liquid (GTL)) (1). Nowadays, two main GTL processes are used: (a) syngas production followed by Fischer-Tropsch (FT) synthesis (2, 3); and (b) liquified natural gas (LNG) (4, 5). However, these processes require huge investments and their economic viability generally requires them to be carried out at very large scale (6), preferably exceeding 1000 tonnes per year. Therefore, they are mainly employed when low priced natural gas is available, typically in large quantities. As of today, the interest in exploiting small reservoirs has increased significantly, particularly because the gas from such reservoirs is often simply burnt as no other conversion technologies are available or commercially viable. On the other hand, interest in biomass and waste conversion (7) is increasing and this will require processes that are economically viable at small scale.

The development of small GTL plants based on FT is already happening with the use of microreactors and improved catalysts (6). On the other hand, efforts in finding new ways of producing liquids from gas are continuing. An example is the direct production of ethylene from CH₄ by oxidative coupling (8–10). Ethylene is one of the largest-volume petrochemicals and the building block for a vast range of chemicals from plastics to antifreeze solutions and solvents (11). Ethylene is currently mainly obtained from energy-intensive steam cracking of a wide range of hydrocarbon feedstocks.

OCM was first investigated in the early 1980s by Keller and Bhasin (12). In 1985, Lunsford (2) showed that, starting from CH₄, lithium/magnesium oxide (Li/MgO) could give a 19% yield for C₂, with ethylene as the main C₂ species. Since then, many catalyst combinations have been investigated for OCM with the highest obtained C₂ yields in the range 25–30% (13, 14). The most common catalyst formulations have been oxides based on alkaline earth metals doped with alkali metals and rare earth metals doped with alkali or alkaline earth metals (3). Several studies have shown that the activity of the OCM catalyst is affected by catalyst structural properties, such as morphology (15, 16). Also basicity and oxygen ion conductivity, which have been identified as key parameters for this reaction, are influenced by catalyst structural properties, such as particle size (17–19).

In parallel to catalyst development and considering the challenges encountered in finding catalysts able to perform OCM economically, efforts have recently been directed to the reactor design. There have been a number of different approaches to novel reactor design, the common factor being the use of membranes. In particular, for OCM, the use of membranes has been of interest because of its perceived ability to control the oxygen concentration in the gas phase and, therefore, decrease the undesired over oxidation (20, 21). Johnson Matthey has been involved recently in four European projects working in OCM: CAtalytic membrane REactors based on New mAterials for C₁–C₄ valorization (CARENA), MEthane activation via integrated MEmbrane REactors (MEMERE), Adaptable Reactors for Resource- and Energy-Efficient Methane Valorisation (ADREM) and Oxidative Coupling of Methane followed by Oligomerization to Liquids (OCMOL). The first two, CARENA and MEMERE, have been dealing with the use of membranes for this reaction. CARENA, a four-year Seventh Framework Programme for Research and Technological Development (FP7) carried out between 2011 and 2015, aimed at investigating the use of relevant process intensification and catalytic membrane reactors to transform light alkanes (C₁–C₄) and CO₂ to added-value products. Currently running is MEMERE, a four-year EU Horizon 2020 project that started in 2015 aimed at the conversion of CH₄ to ethylene using a membrane reactor with integrated air separation. Additionally, the use of non-thermal plasma reactors is also being evaluated for OCM (22, 23). ADREM, a four-year EU Horizon 2020 project, is looking at using innovative reactor types for CH₄ activation processes including a plasma reactor for OCM.

OCMOL, a five-year FP7 project, aimed to integrate energetic coupling of OCM and CO₂ reforming in a heat exchange reactor that was used to recycle CO₂ produced by the OCM reaction. The integration of OCM with other well-known processes to produce fuels or chemicals is another interesting approach and, indeed, this integration seems to be more important than the actual catalyst performance. A new follow up project, Methane oxidative conversion and hydroformylation to propylene (C123), has recently started and will be looking at transforming CH₄ into C₃ products via OCM which will simultaneously provide an optimum ratio of ethylene, carbon monoxide and hydrogen for its further hydroformylation into propanal or propanol. Ultimately, propanol can be dehydrated into propylene; either by an integrated approach as part of the hydroformylation step or through a stand-alone approach. Siluria Technologies, Inc, USA is promoting a catalytic process that can transform natural or shale gas into transportation fuels and commodity chemicals in an efficient, cost effective, scalable manner using processes that can be seamlessly integrated into existing industry infrastructure. This process is based around two basic chemistries: OCM and ethylene to liquids. In particular, nanowires are used for the OCM reaction.

Although process integration and reactor design were key for the OCMOL project, understanding on how to improve the catalyst performance to obtain higher selectivity towards ethylene and higher CH₄ conversion is still relevant as this will positively impact the process economics. The activity of the materials for OCM is closely related to their properties and it is well known that different properties are expected when comparing nanoparticles against the bulk material (24). Indeed, preliminary work has shown that nanomaterials with different morphologies could enhance the OCM performance at low temperatures (19). The present paper presents a summary of the work carried out within OCMOL around process integration and more particularly elaborates on the systematic study of the use of differently sized La-based nanoparticles for OCM which was a main focus of the catalyst development work in OCMOL carried out by Johnson Matthey. Flame spray pyrolysis (FSP) and microemulsion were the two methods investigated to produce La-based nanoparticles and results from the latter method will be presented and compared to data previously reported on the materials prepared by FSP (25).

2.1. The Process

2.2. Oxidative Coupling of Methane – Lanthanum-Based Nanoparticles

3.1. Experimental

The use of different synthesis temperatures during stage two had no effect on the phase obtained, LaNO₃(OH)₂·H₂O (see Figure S1 in the Supplementary Information). This phase differs from the ones obtained using FSP, in which La₂O₃ and carbonates were formed. Therefore, the samples produced via microemulsion needed an extra thermal treatment to obtain the active phases (stage three). Although no change was observed with respect to the phase composition, the crystallinity of the samples was affected by the synthesis temperature as can be seen in Table I. The results are in accordance with the surface areas which decrease with the increase in synthesis temperature in the range from 25°C to 60°C. The temperature has been reported to exhibit an effect on the micelle formation, i.e., higher temperatures reduce the interfacial tension between the oil and water which enhances the diffusion of the water into the oil phase and increases the number of smaller sized droplets (52, 53). Therefore, when increasing the temperature, a decrease in particle size would be expected, however, the opposite effect was observed which can be attributed to the particles not being single crystals. The particles are constituted of multiple diffraction domains with different orientations and the temperature might help the growth of these domains (Figure 3).

Fig. 3.

The samples produced by modifying the W:S ratio were also shown to be poorly crystalline materials when studied in stage two and also consisted of LaNO₃(OH)₂·H₂O (see Figure S2 in the Supplementary Information). The crystallite size increased with W:S, as determined by XRD (Table II). The surface area measured agreed with this observation. This effect can be logically explained because, when higher amounts of water are used (higher W:S ratio), bigger micelles are created. Indeed the effect of the W:S ratio has been shown in previous reported work to be one of the most defining synthesis variables for the particle size in microemulsion (48, 54). An example of this effect is described by Lisiecki et al. (55). These authors investigated the effect of the W:S ratio on the preparation of colloidal copper particles which were achieved using sodium bis(2-ethylhexyl)sulfosuccinate (AOT) as a surfactant, isooctane or cyclohexane as the solvents and an aqueous solution of hydrazine to reduce the Cu. The increase in particle size when increasing W:S ratio could be observed for the two different solvents.

The morphology of the materials obtained by microemulsion was very different from that obtained using FSP. The latter produced sphere like materials between 10–40 nm, while aggregates ranging from around 0.1 μm to 10 μm were observed for the samples prepared by microemulsion (Figure 4). These were shown to be flakes or needle like materials with different shapes which was confirmed by tilting the sample at different angles (Figure 5). This shape could be due to either aggregation after surfactant removal or it could be due to the colloidal nature of the synthesis mixture and the ability for these systems to form other shapes (56, 57).

Fig. 4.

Fig. 5.

To determine the origin of the flake or needle like morphology dynamic light scattering (DLS) analysis was performed on the microemulsions with and without the La precursor in the stage one at the W:S ratios used previously. The micelle size determined with DLS for the microemulsions without the La precursor in stage one followed the same trend observed previously for the materials obtained in stage two (Table II and Table III). A decrease of the W:S ratio resulted in an increase of micelle size. Unfortunately, the microemulsions containing the La precursor (stage one) could not be analysed with the Malvern Panalytical Zetasizer Nano ZS. This was due to the microemulsion becoming cloudy with the presence of the La precursor and not allowing the light to travel through the cuvette. The microemulsions were shown to be stable over time. Therefore, the flake or needle like morphology might be due to a non-spherical micelle shape. Although, the aggregation of spheres due to low stability of the nanoparticles once removed from the microemulsion cannot be eliminated.

As already mentioned, the samples prepared using microemulsion needed an extra thermal treatment to obtain the active phase for OCM, La₂O₃ and carbonate. Therefore, the effect of the thermal treatment on these samples to transform them into stage three materials was investigated using in situ XRD on the samples ME-T25 and ME-T60 (Figure 6 and Figure 7) as they represent the extremes of the crystallite sizes obtained for these materials. In this analysis, the evolution of phase and crystallite size was monitored during thermal treatment under two different atmospheres, air and N₂. The starting phase for both samples was different. While both contained LaNO₃(OH)₂^•H₂O, ME-T60 also contained La₂(OH)₃. The order of appearance of the phases was the same for the two samples, i.e., La(OH)₂NO₃, unassigned phase, Type Ia La₂O₂CO₃ and La₂O₃. The unassigned phase was determined to be constituted of a mixture of carbonates and nitrates as a loss of CO₂ and NO was observed at 340°C using mass spectrometry-thermogravimetric analysis (MS-TGA). The temperature at which each of these phases appeared depended on the starting sample and atmosphere. Not only the transition temperature between phases was different between the two samples, ME-T25 and ME-T60, but also the evolution of the crystallite size was different. These results showed that the systems are complex and further experiments should be done to understand the effect of the thermal treatment.

Fig. 6.

Fig. 7.

3.3. Oxidative Coupling of Methane Kinetics Performance

Synthesis of La-based nanomaterials using the microemulsion technique yielded flake like materials which contained nanocrystallites. The synthesis temperature has the most pronounced effect on the ultimate material properties. A minimum crystallite size was observed at 25°C, however this did not affect the final OCM activity. Other phenomena such as those occurring during the thermal treatment play an important role for the catalyst activity for these materials.

In situ XRD analysis demonstrated that the materials exhibit significant changes when submitted to thermal treatment to yield the final, catalytically active materials. The changes were difficult to control, rendering the microemulsion synthesis method a challenging one to produce La nanoparticles in a reproducible manner. Alternative techniques, such as FSP, seem much more promising in this respect. In terms of OCM activity, the presence of La carbonates in the materials used was crucial. This work has put in evidence the challenges encountered when trying to study the material properties, such as phase, particle size and morphology, independently from each other.

Optimisation of the OCM catalyst is a challenge and we believe the solution to achieve natural gas valorisation in an economically viable manner would be a process that integrates OCM. An example is the European project currently running, C123.

Carbon in its oxide and hydrocarbon forms is the cause of the energy and transport sectors’ biggest headache: climate change, so it is fitting that allotropes of this most versatile element look so promising to play a part in the modernisation of energy conversion. Naotoshi Nakashima (Kyushu University, Japan) brings together a collection of chapters showcasing some impressively creative nanoscience, predominantly from Japan, as part of Springer’s Nanostructure Science and Technology series. On reading, one is left with the impression that these fascinating materials will surely play some part in the coming decarbonisation of the economy.

Nanocarbons, as the name implies, are the allotropes of carbon that take the form of molecules of nanometre dimensions. The archetypal nanocarbon is graphene – a single layer of sp² hybridised atoms arranged in a two-dimensional hexagonal lattice. Nanotubes are essentially rolled up sheets of graphene, while fullerenes are essentially graphene sheets curled up into spheroids. Other nanoscale carbons, such as nanoporous carbon, carbon black and carbon foams are also discussed.

A large portion of the book is dedicated to the role of nanocarbons in fuel cells (FCs), so it is worth giving a brief overview of FCs and the associated functions of nanocarbons. FCs share features with both internal combustion engines and batteries. Like an internal combustion engine fuel is oxidised, producing exhaust gas, and like a battery chemical energy is converted to electrical energy. In the case of the popular proton exchange membrane fuel cells (PEMFCs) the fuel is commonly hydrogen, which is split into protons and electrons at the anode. Electrons are forced to flow through an external circuit because the electrodes are separated by an electrically insulating proton conducting polymer membrane such as Nafion^TM. Protons move through the membrane to the cathode where they react with oxygen and electrons that have travelled through the external circuit to form water. The overall reaction is the oxidation of H₂ to H₂O.

FC performance is largely determined by materials performance. Looking in more detail, at the anode side H₂ diffuses through the gas diffusion layer (GDL) to reach the catalyst. The GDL is often composed of carbon fibres which allow electrons to flow from the catalyst to the current collector and H₂ to diffuse between them to the catalyst. Platinum nanoparticles catalyse the splitting of H₂ into protons which are transported across the proton exchange membrane to the cathode and electrons which are transported along carbon fibres to the current collector.

Fig. 1.

Carbons are crucial to the performance and cost effectiveness of FCs, especially as catalyst supports where their high specific surface area enables a low Pt loading for a given power density and their conductivity provides a pathway for electrons to move between the Pt catalyst and the conducting fibres. FC performance can be greatly enhanced by improving the surface properties of nanocarbons for better gas accessibility and distribution of Pt nanoparticles. Their conductivity has a significant effect on power density and their chemical and thermal stability has a large influence on FC durability.

On the topic of FCs, some particularly interesting work on multiwalled carbon nanotubes (MWCNTs) as catalyst supports for H₂ PEMFCs by Naotoshi Nakashima and Tsuyohiko Fujigaya of Kyushu University, Japan, can be found in Chapter 1. The authors first address the difficulty of dispersing Pt nanoparticles onto nanotubes due to the lack of binding sites for deposition. Oxidation of the nanotubes is one method of introducing hydrophilic groups for binding, however this reduces the nanotubes’ electrochemical stability. To get around this problem the authors present a protocol for wrapping MWCNTs with conjugated polymers which bind to the nanotube surface through Π–Π interactions and on top of which the Pt catalyst can be more easily deposited. Chapter 1 also features durability tests of membrane electrode assemblies (MEAs) using polymer wrapped nanotubes as compared to polymer wrapped carbon black. Nanotube MEAs are found to maintain a significantly higher activity after several thousand cycles of durability tests compared to carbon blacks due to the inherent structural stability of the polymer wrapped pristine nanotubes.

Fig. 2.

Chapter 2, which is also written by Nakashima, follows on nicely from the first, exploring polymer wrapped nanotubes as catalyst supports for direct methanol FCs. A significant problem for direct methanol PEMFCs is methanol crossover where methanol diffuses through the Nafion^TM membrane to the cathode where it reacts with O₂, poisoning the Pt catalyst. While alternative transition metal cathode catalysts are more methanol resistant because they suppress its oxidation this is counterbalanced by lower oxygen reduction reaction (ORR) activities than Pt. Nakashima presents a solution to this problem by coating the polymer wrapped Pt decorated nanotubes with an outer layer of poly(vinylphosphonic acid) (PVPA) polymer, which increases methanol tolerance by the proposed mechanism of preferentially blocking diffusion of the larger molecule while only slightly reducing O₂ accessibility. The PVPA is also found to reduce carbon corrosion of nanotube and carbon black catalyst supports.

Another highlight on the topic of FCs is the review by Matsuhiko Nishizawa of Tohoku University, Japan, of carbon nanotube (CNT) based enzymatic biofuel cells in Chapter 15. Enzymatic biofuel cells are FCs in which an enzyme takes the place of Pt nanoparticles as the electrocatalyst. At the anode, the enzyme oxidises fuels such as fructose or glucose and generates electrons that are carried to the current collector by a conducting carbon support. At the cathode O₂ is reduced to H₂O by another enzyme. An advantage of biofuel cells is that the incredibly high selectivity of the enzyme means impure fuel feeds can be used and there is no need for a separator, making the overall design simply a pair of enzyme functionalised electrodes exposed to solutions containing fuel and O₂. The simplicity of the design makes them suitable for miniaturisation for use in implantable electronic devices. CNTs are presented as promising enzyme support materials due to their biocompatibility and high specific surface area. Previous attempts to immobilise enzymes onto nanotube electrode structures have created nanostructure films before enzyme modification. However, Nishizawa reports a method by which enzyme modification precedes film production so that the nanotubes pack ideally around the enzymes. This is achieved by adding an enzyme solution to a CNT forest which shrinks to a near hexagonal close packed structure on drying, entrapping the enzymes between the nanotubes and resulting in superior activity compared to previous production methods.

A fantastic piece of FC material characterisation work is presented in Chapter 5 by Somaye Rasouli and Paulo J. Ferreira working at the University of Texas at Austin, USA. They describe the technique of identical location transmission electron microscopy (TEM) as a way to understand the mechanism of Pt nanoparticle growth on CNTs in PEMFCs. One of the main causes of performance decline of PEMFCs is the instability and coarsening of Pt nanoparticles on carbon supports, which reduces the total surface area of active catalyst. The authors proposed four possible mechanisms of coarsening: Ostwald ripening, particle migration on the carbon support and coalescence, particle detachment and particle dissolution and reprecipitation. While the ideal way to investigate the mechanism would be to do in situ TEM and concurrent voltage cycling on a MEA, the release of moisture into the vacuum chamber precludes this. Furthermore, accelerated stress tests on MEAs make it difficult to tease apart the contribution to performance decline from different components. In order to study specifically Pt nanoparticle instability Arenz et al. first deposited the nanotube supported catalyst onto a gold TEM grid, initially observed the foil to define an area of interest, cycled the grid in a three-electrode electrochemical cell and then re-characterised the identical area of interest by TEM. It was found that carbon corrosion of nanotubes in voltage cycling in which carbon is lost as carbon dioxide, converting the hexagonal lattice to heptagon and pentagon rings, causes the Pt nanoparticles to move across the nanotube surface, possibly to reduce interfacial energy. As the nanoparticles move on the nanotubes they make contact with each other before coalescing to form larger particles.

Aptly, the complementary theme of the role of nanocarbons in H₂ production is explored in Chapters 9 and 19. In Chapter 9 Yutaka Takaguchi and Tomoyuki Tajima of Okayama University, Japan and Hideaki Miyake of Yamaguchi University, Japan, describe a new category of H₂ evolving photocatalysts based on semiconducting single walled carbon nanotubes (s-SWCNTs). Nanotubes can be metallic or semiconducting, i.e. have or not have a band gap, depending on the rolling angle between the axis of the tube and the crystallographic directions of the rolled graphene sheet. The productivity of H₂ from photocatalysts can be improved by expanding the range of active wavelengths from ultraviolet to near infrared (IR). The new category of nanotube based photocatalyst reported promisingly shows H₂ evolution under near IR radiation. However, s-SWCNTs are seldom used for this application due to the high exciton (electron-hole pair) dissociation energy, the fact that nanotubes form bundles that allow excitons to be transferred between tubes and also because they are difficult to disperse in H₂O. These problems are addressed rather ingeniously through the fabrication of a coaxial cable composed of an s-SWCNT covered with a layer of C₆₀ fullerenes, which are themselves functionalised with a hydrophilic dendron moiety which readily complexes with Pt, the cocatalyst for H₂ evolution. The cable is made simply by sonicating the nanotubes in a H₂O solution of the amphiphilic fullerodendron, which self assembles around the nanotubes due to Π–Π interactions. This hydrophilic dendron moiety makes the nanotubes more easily dispersible in H₂O, while the nanotube-C₆₀ heterojunction formed improves exciton dissociation. Furthermore, the problem of bundle formation is resolved by the isolation of the nanotubes from each other.

The FC’s biggest competitor in the race to decarbonise transport – the lithium-ion battery – may also benefit from the use of nanocarbons in future. Of particular interest is the review of nanocarbons as alternatives to graphite in anode materials written by Seok-Kyu Cho and Sang-Young Lee of Ulsan National Institute of Science and Technology (UNIST), South Korea and JongTae Yoo of Korea Institute of S&T Evaluation and Planning (KISTEP), South Korea in Chapter 18. The main advantage of these materials is that they could potentially have significantly higher Li capacities than graphite. For example, the capacity of C₆₀ fullerenes was shown by Armand et al. to be 12 Li atoms per fullerene. However, this was only realised once the problem of reduced C₆₀ dissolving in liquid electrolyte was worked around by substituting for a polyethylene oxide-based gel polymer electrolyte. Theoretical calculations of the Li storage potential of nanotubes show them to have capacities much greater than graphite, however these have yet to be realised experimentally. The authors conclude that the gap between theory and experiment motivates more work to better understand the lithiation mechanism of nanotubes.

The book covers the potential roles of nanocarbons in several aspects of the future decarbonised economy, covering power generation from H₂ and production of H₂. Chapter 20 covers the applications of nanotubes in grid energy generation from solar cells with an emphasis on CNT-silicon solar cells and CNT based perovskite solar cells. Feijiu Wang and Kazunari Matsuda of Kyoto University, Japan and Nagoya University, Japan provide a clear overview of the general principles of solar cells, which nicely sets the context for the chapter. Perhaps most intriguing is the ability of semiconducting SWCNTs to generate multiple excitons from one photon, meaning they may be able to surpass the Shockley-Queisser limit on the maximum efficiency of single junction solar cells, which assumes one exciton per photon. While there are many applications for nanotubes, from hole transport layers to transparent conducting electrodes, the authors point to a significant barrier to commercialisation: the presence of metallic nanotubes in the mixtures used for studies, which increase contact resistance due to their difference in work function and band gap.

This book covers a very broad range of applications for nanocarbons and while much of the underlying chemistry and materials science transfers between chapters, it is unlikely that any single reader would be familiar with all the concepts covered. The reviewer would recommend this book for any researchers working with carbon nanomaterials, particularly nanotubes, as well as researchers working with PEMFCs. Overall, an interesting read that reminds the reader of the impressive versatility and seemingly endless applications of carbon nanomaterials.