(Invited) The More Elements the Better? Studying Structure-Function Relations in High Entropy Alloy Nanoparticles As Fuel Cell Electrocatalysts

Abstract

Low-temperature polymer electrolyte membrane fuel cells (PEMFCs), which convert chemical energy into electricity, are widely considered a suitable, sustainable technology to replace conventional combustion engines. However, fundamental issues such as fuel cell efficiency and durability still leave room for improvement, which could be achieved by finding more active and stable fuel cell electrocatalysts.[1] High entropy alloys (HEA) – solid solutions of five or more elements - are an emerging material class in the development of novel electrocatalysts. These multi-metallic compounds provide exciting new possibilities for catalyst design.[2] Recent theoretical analyses predict a great potential for HEAs as fuel cell catalysts due to the almost unlimited number of unique surface sites, which enables a wide distribution of adsorption energies. By changing the composition, the distribution of adsorption energies can be tailored toward the optimal binding energies of the catalytic intermediates.[3] The field of high entropy alloy catalysis enables a new, statistical approach to materials design. From an experimental viewpoint, however, HEAs come with new challenges regarding their synthesis and characterization.While HEA nanoparticles are essential for efficient catalysis to provide sufficiently large surfaces, their synthesis is only in its infancy. For a direct comparison between theoretically predicted properties and the performance of the prepared catalysts, single-phase HEA nanoparticles are essential.[4] Through studies of the synthesis pathways and in-depth structural characterization of the HEA nanoparticles with synchrotron X-ray techniques, it is possible to prepare model single-phase HEA nanoparticle catalysts. These well-mixed, multi-metallic materials allow us to study not only the different structure-activity properties of HEA nanoparticle fuel cell catalysts but to also explore the stability of these novel materials under reaction conditions. Studying the dissolution and degradation behaviour of model HEA catalysts in an acidic environment, the mechanisms that underly the degradation of multi-metallic PEMFC catalysts are explored. By relating structural characterization techniques with electrochemical analyses,[5] we can address the question of which role element mixing can play in the activation and stabilization of new fuel cell catalysts.[1] Schroeder, J.; Pittkowski, R. K.; Du, J.; Kirkensgaard, J. J. K.; Arenz, M. J. Electrochem. Soc. 2022, 169, 104504.[2] Löffler, T., Ludwig, A., Rossmeisl, J, Schuhmann, W. Angew. Chem. Int. Ed. 2021, 60, 2-12.[3] Batchelor, T. A. A, Pedersen, J. K., Winther, S.H., Castelli, I.E., Jacobsen, K. W., Rossmeisl, J. Joule 2019, 3, 834-845.[4] Pittkowski, R.; Clausen, C. M.; Chen, Q.; Stoian, D.; van Beek, W.; Bucher, J.; Welten, R. L.; Schlegel, N.; Mathiesen, J. K.; Nielsen, T. M.; Rosenkranz, A. W.; Bojesen, E. D.; Rossmeisl, J.; Jensen, K. M. Ø.; Arenz, M. ChemRxiv, 2022, https://doi.org/10.26434/chemrxiv-2022-khw4t.[5] Schroeder, J.; Pittkowski, R. K.; Martens, I.; Chattot, R.; Drnec, J.; Quinson, J.; Kirkensgaard, J. J. K.; Arenz, M. ACS Catalysis 2022, 12, 2077–2085