One promising strategy for mitigating the environmental effects of CO2 emissions is to convert CO2 via electrochemical reduction into high value-added molecules. Such CO2 electrolysis systems can be broadly divided into two categories based on whether they operate at high or low temperatures, each with their own weaknesses and strengths. However, in order for either type of CO2 electrolyzer to make an impact on an industrially and societally relevant scale, various components of the electrolyzer still need to be optimized, one of which are the catalysts for the CO2 reduction reaction (CO2RR).
On March 20, 2020, CIFAR convened a virtual roundtable that brought together Fellows from CIFAR’s Bio-inspired Solar Energy research program with international experts in academia, industry and national government laboratories to discuss developments in catalysts for CO2 electrolyzers. Through short presentations and facilitated discussion, attendees shared their expertise on electrocatalysts in both high-temperature and low-temperature CO2RR, as well as the challenges, solutions and next steps for catalyst development. This is the second in a series of roundtables aiming to expand the societal impact of CIFAR’s Bio-inspired Solar Energy program, with the goal of working towards a roadmap for the research and development of commercial-scale CO2 electroreduction systems.
- One form of high-temperature CO2 electrolysis system that has been commercialized is the solid oxide electrolysis cell (SOEC). This system uses a solid electrolyte composed of a ceramic material such as yttria-stabilized zirconia (YSZ) that conducts O2- ions at the operating temperature of 700-800°C. The fuel electrode (cathode) is often a composite of the electrolyte material with Ni, which acts as the catalyst and electron conductor. The air electrode (anode) is most often a doped lanthanide perovskite such as Sr-doped LaMnO3 (LSM) or La(Fe,Co)O3 (LSCF). CO2 gas fed to the fuel electrode is thought to undergo reduction to CO at the triple phase boundary of the gas phase, the electron conductor (Ni catalyst) and the ion conductor (YSZ). The O2- then diffuses through the electrolyte to the air electrode, where it is oxidized into O2 at another triple phase boundary.
- By operating at high temperatures, SOECs can be more tightly integrated with processes of carbon capture from high-temperature CO2 sources such as flue gas, as well as downstream conversion of CO into other valuable products, e.g. via Fischer-Tropsch processes (although operating at the high pressures required for the latter remains a technical challenge for ceramic SOECs).
- SOECs are highly selective towards CO production, but there are currently no catalysts that can be used for high temperature CO2RR into C2 or C3 products. Certain catalysts may work at reaction temperatures brought down to around 400°C, but at these temperatures the ionic conductivity of the solid electrolyte is significantly decreased.
- Several mechanisms by which Ni catalysts in high-temperature SOEC systems may undergo degradation have been identified. One is that Ni is prone to S-poisoning - one study found that even 5 ppb S in inlet gases increase the degradation rate of the Ni electrode. This means that CO2 sources such as flue gas cannot be used without comprehensive purification. Another issue is that Ni is also a good catalyst for the formation of carbon nanotubes which become deposited on the fuel electrode and can deactivate the cell. Finally, the Ni catalyst has been observed to lose electric contact with the Ni network in the rest of the cell after prolonged operations.
- For low-temperature CO2 electrolysis systems, there are two common setups that share the feature of using aqueous electrolyte solutions. In one setup, based on an H cell configuration, the CO2 is dissolved in the aqueous electrolyte before reaching the cathode. A problem with this configuration is the low solubility of CO2, which limits the reaction rate at the cathode. An alternate flow cell setup, particularly important for scaled-up operations, uses a gas diffusion electrode (GDE) for its cathode, to which CO2 gas is fed directly. In this configuration, the catalyst is deposited onto the electrolyte-facing side of a hydrophobic support material (often based on polymers such as PTFE) that forms the GDE. The reduction reaction is thought to occur at the triple phase boundary of the gas phase reactant, the solid phase catalyst, and the liquid phase electrolyte (or the proton source from humidified CO2).
- A number of metal catalysts have been investigated for use in low-temperature CO2RR systems, including Au, Ag and Cu. Of these, Cu is of particular interest for its ability to catalyze reduction to multi-carbon products beyond CO (hydrocarbons and alcohols). This is likely due to the ability of Cu to properly bind CO as a reaction intermediate for the subsequent C-C coupling process, and also suggests the possibility of tuning the electronic structure of catalysts to alter reaction intermediates binding and hence product selectivity. Additionally, reaction activity and selectivity have also been shown to be influenced by catalyst morphology - factors such as particle shape and size, crystal facets, surface porosity and roughness, and the density of defects and grain boundaries.
- Currently, there is less research into catalyst degradation mechanisms in low-temperature CO2RR, as more focus is being placed on solving other issues such as cathode flooding due to loss of hydrophobicity of the GDE material. However, as these issues are resolved and the electrolysis systems are operated at more industrially-relevant current density or potential, catalyst stability will become a more prominent concern. While gas-phase impurities are known to lead to catalyst poisoning in high-temperature electrolyzers, the effect of impurities has not been studied extensively in low-temperature systems.
Priorities and Next Steps
- The types of products that can be formed by CO2 electrolysis will have a significant impact on the economic viability of the approach. For CO2 electroreduction to be competitive in the market, the cost of generating a product from CO2RR must compare favourably with existing or alternative processes, such as using water electrolysis to generate H2 which is then thermochemically reacted with CO2. In the short term, it could be beneficial to focus on high-margin products in order to push CO2 electrolysis technologies forward.
- A major concern for scaling up any CO2RR system to an industrially-relevant scale is catalyst degradation (due to poisoning, ripening, depletion, etc.) after prolonged electrolysis. Tackling this issue while reducing the cost of catalyst materials requires either discovering catalysts that are stable for extended use (perhaps into the 20,000-25,000 hour range), or that the catalyst can be cost-effectively regenerated. In the latter case, the electrolyzer could, e.g., be flushed with a solvent to remove contaminants or to dissolve and re-form the catalyst particles, but the other electrolyzer components must then be able to withstand such treatment. Additionally, any shutdown of the electrolyzer system to flush and regenerate the catalysts would affect the system’s economic viability.
- Advanced in situ and in operando characterization methods, whether electrochemical, spectroscopic (e.g., X-ray photoelectron spectroscopy) or microscopy-based (e.g., electron microscopy, surface potential microscopy), are crucial for obtaining a better understanding of the surface processes involving catalysts in CO2 electrolyzers regardless of the operating temperature. For example, these techniques can be used to understand and monitor catalyst contamination and degradation, or to verify the existence of triple phase boundaries and understand whether/how the relevant reactions occur at these interfaces. Moreover, given that the reactions sometimes happen at minority reaction sites, such characterization methods will need to be complemented with better computational modelling and tools of chemical kinetics to obtain more experimental kinetic understanding of CO2RR in electrolysis systems.
- Curtis Berlinguette, Professor and Canada Research Chair in Energy Conversion, University of British Columbia / Fellow, Bio-inspired Solar Energy program, CIFAR
- Juergen Biener, Staff Scientist, Lawrence Livermore National Laboratory
- Amanda Brown, Berlinguette Lab Program Manager, University of British Columbia
- Christopher Capuano, Manager, Research and Development, Nel Hydrogen
- Phil De Luna, Program Director, National Research Council of Canada
- Shaffiq Jaffer, Vice-President, Corporate Science and Technology Projects, Total S.A. / Advisor, Bio-inspired Solar Energy program, CIFAR
- Feng Jiao, Associate Professor, University of Delaware
- Parisa Karimi, Research Scientist, Advanced Stack Technology, Hydrogenics
- Rainer Küngas, Principal Scientist, Haldor Topsoe A/S
- Tom Mallouk, Professor, University of Pennsylvania / Fellow, Bio-inspired Solar Energy program, CIFAR
- Benjamin Mowbray, Graduate Student, University of British Columbia
- Kenneth Neyerlin, Staff Scientist, National Renewable Energy Laboratory
- Claudie Roy, Research Officer, National Research Council of Canada
- Stafford Sheehan, Chief Technology Officer, The Air Company
- Andrej Singer, Assistant Professor, Cornell University
- Wilson Smith, Senior Scientist, National Renewable Energy Laboratory / Associate Professor, University of Colorado Boulder
- Yogesh Surendranath, Associate Professor, MIT / Azrieli Global Scholar, Bio-inspired Solar Energy program, CIFAR
- John Vohs, Professor, University of Pennsylvania
- Haotian Wang, Assistant Professor, Rice University / Azrieli Global Scholar, Bio-inspired Solar Energy program, CIFAR
- Jenny Yang, Assistant Professor, University of California, Irvine / Azrieli Global Scholar, Bio-inspired Solar Energy program, CIFAR
Optimizing catalysts for CO2 electroreduction (research brief)
Ion selective membranes in CO2 electrolysis (event brief)
Making fuel from water and CO2: The Importance of stabilizing highly reactive nickel catalysts in neutral pH solution (research brief)
Solar conversion of CO2 to CO using Earth-abundant electrocatalysts prepared by atomic layer modification of CuO (research brief)
Advances and Challenges in Understanding the Electrocatalytic Conversion of Carbon Dioxide to Fuels, by Yuvraj Birdja et al.
Gas-Diffusion Electrodes for Carbon Dioxide Reduction: A New Paradigm, by Drew Higgins et al.
Electrochemical CO2 Reduction for CO Production: Comparison of Low- and High-Temperature Electrolysis Technologies, by Rainer Küngas
Progress and Perspectives of Electrochemical CO2 Reduction on Copper in Aqueous Electrolyte, by Stephanie Nitopi et al.
Durable Cathodes and Electrolyzers for the Efficient Aqueous Electrochemical Reduction of CO2, by Uzoma Nwabara et al.
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