(Invited) Catalyst Assessments and Device Incorporation in Low Temperature Electrolysis

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Although electricity feedstock currently dominates hydrogen production costs in commercial electrolysis, capital cost will become a significant factor as electrochemical water splitting is directly coupled with low-cost power sources. [1,2] To minimize device costs and reach cost targets, reducing catalyst loading in proton exchange membrane-based (PEM) electrolysis will be necessary and efforts to evaluate and improve upon catalyst performance and durability will become critical. Beyond PEM-based systems, anion exchange membrane-based (AEM) electrolyzers can also reduce capital cost, with the high pH enabling non-platinum group metal (PGM) catalysts and improving the durability of system components.In these areas, efforts at the National Renewable Energy Laboratory have included developing and evaluating electrolyzer catalysts, establishing baseline performance and durability, and linking ex- and in-situ testing for commercial nanoparticles and novel materials. In PEM electrolysis, ink compositions and coating methodologies were evaluated in rotating disk electrodes, establishing best practices for screening catalysts in the oxygen evolution reaction. [3] Test factors were considered, including working electrode dissolution and catalyst delamination, and electrochemical surface area measurements adapted to differentiate between site quantity and quality. While half- and single-cell performances and durabilities do not match, higher activities translated to kinetic improvements in membrane electrode assemblies and half-cell testing was a reasonable tool for assessing relative differences between material sets. Projecting differences at the device level, however, required segregating materials based on surface composition and oxide content, and some caution is needed when using rotating disk electrodes to assess catalysts for electrolysis applications. In membrane electrode assemblies, baseline durability was evaluated when accounting for low catalyst loading and intermittent load profiles. How catalyst layers are incorporated has a significant impact on device performance and durability, and efforts have been made to translate improvements from spray coating to roll to roll coating and manufacturing appropriate processes. Various catalyst and membrane combinations have been used to assess that ability of component development and system control to limit performance loss with extended operation.In AEM electrolysis, baselines and best practices have been established in rotating disk electrodes for catalysts in the oxygen and hydrogen evolution reactions. Test factors, including electrolyte purity, conditioning protocols, and counter electrode choices were found to significantly impact measurements, and some care is needed to avoid under- or over-estimating kinetic improvements. Novel catalysts were developed for the hydrogen and oxygen evolution reactions, including low- and non-PGMs, sulfides, and metal organic frameworks, where activity improvements ex-situ generally translated to membrane electrode assemblies. [4] In in-situ testing, ionomers were varied with standard catalysts to assess the role of catalyst-ionomer interactions and supporting electrolytes in device performance.[1] H2 at Scale: Deeply Decarbonizing our Energy System. Presented at Annual Merit Review, U.S. Department of Energy; Washington, DC, June 6−10, 2016. https://www.hydrogen.energy.gov/pdfs/review16/2016_amr_h2_at_scale.pdf.[2] Denholm, P.; O’Connell, M.; Brinkman, G.; Jorgenson, J. Overgeneration from Solar Energy in California: A Field Guide to the Duck Chart; Vol. NREL/TP-6A20-65023; National Renewable Energy Laboratory: Golden, CO, 2015. http://www.nrel.gov/docs/fy16osti/65023.pdf.[3] S. M. Alia and G. C. Anderson, J. Electrochem. Soc., 166, F282 (2019).[4] S. Ghoshal, S. Zaccarine, G. C. Anderson, M. B. Martinez, K. E. Hurst, S. Pylypenko, B. S. Pivovar and S. M. Alia, ACS Applied Energy Materials, 2, 5568 (2019).