Engineering Catalysts for Anion Exchange Membrane Electrolyzer Anodes and Cathodes

Abstract

The development of catalyst for both Oxygen Evolution and Hydrogen Evolution reactions (OER and HER, respectively) in Anion-Exchange Membrane (AEM) electrolyzers is proceeding along with the development of better solids electrolytes, as well as electrode and system designs.AEM-based electrolyzers are now achieving operating lives exceeding 1000s of hours and 5.0 > A/cm2 at 1.8V [1] with increasing evidence that catalyst/ionomer interactions greatly effect both catalyst performance and ionomer stability[2]. Further, the promise of PGM-free electrolysis continues even though PGM-based catalysts seem to be most stable.Pajarito Powder is developing catalysts explicitly designed for AEM electrolysis and has been working with AEM materials and systems developers to measure and understand key performance issues for electrolysis catalysts. Together with partners like LANL, Nel, GT, and WUSTL Pajarito catalysts are now showing that for stable HER available PGM-free catalysts lag PGM catalyst by about 100 mV, compared to a near 250 mV lag for OER.The best OER catalysts demonstrated however by Nel, GT, and PP are based on PbRuOx [3], and have outperformed commercialized IrOx by >140 mV for up to 1000 h has been shown and outperform PGM-free catalysts by 250+ mV. However, the combination of Pb and the rarity of Ru have motivated alternative formulations. Leading stable PGM-free catalysts in turn include multi-site electrocatalysts such as barium and lanthanum oxide systems doped with strontium, samarium, and niobium are combined with Cr, Mn, Fe, Co, and Ni in perovskite, mullite, spinel, and delaffosite crystal structures. The activity and electronic conductivity of these systems have been achieved simultaneously through balancing the structural strain of the La and Ba doped with Sr, Sm, and Nb while the doping and exchange of the transition metal component refines the electronic structure and therefore bonding of the catalysts. These documented materials have some limitation in terms of conductivity and stability, but there are indications that improvements are possible through refining these types of catalysts to including use of mesoporous and conductive supports that both bolster catalyst system conductivity and increase the active surface area through dispersion of the catalyst phase.For stable HER the performance of Ni-alloys are rapidly approaching Pt-alloys for the HER, where the in-cell performance only lags by about 50-100 mV and they are stable for >1000 h. At the same time, improvements in PGM-free OER catalysts continue to be made, though challenging.The current status of electrocatalysts for AEM, prospects for further catalysts improvements, and challenges will be presented and discussed.[1] Li, D., Park, E.J., Zhu, W. et al. Highly quaternized polystyrene ionomers for high performance anion exchange membrane water electrolysers. Nat Energy 5, 378–385 (2020). https://doi.org/10.1038/s41560-020-0577-x and[2] Maurya S., Lee, AS., Li, D., Park, EJ., Leonard, DP., Noh, S. On the origin of permanent performance loss of anion exchange membrane fuel cells: Electrochemical oxidation of phenyl group Journal of Power Sources, Volume 436, 1 October 2019, 226866[3] Horowitz, H. S., Longo, J.M., and Horowitz, H. H. Oxygen Electrocatalysis on Some Oxide Pyrochlores J. Electrochem. Soc. 1983 volume 130, issue 9, 1851-1859 and Pyrochlore electrocatalysts for efficient alkaline water electrolysis, Parrondo, J., George, M., Capuano, C., Ayers, C., Ramani, V. J. Mater. Chem. A, 2015,3, 10819-10828.