Elucidating Fundamental Degradation Mechanisms in Polymer Electrolyte Membrane Water Electrolyzers

Abstract

Hydrogen is an attractive option for renewable energy applications due to its ability for large quantities to be stored over a long period of time, but current H2 production from fossil fuels is a major CO2 emitter; consequently, there is a need for efficient, renewable H2 production.1 Polymer electrolyte membrane water electrolysis (PEMWE) is a promising strategy, but major degradation issues and the slow kinetics of the anodic oxygen evolution reaction (OER) limit commercialization.2 Ir has been shown to be the most active metal towards the OER, while IrO2 is typically more stable in the harsh oxidizing conditions of the OER. High cost/limited availability of noble metals require a transition to lower loadings; however, low-loading Ir- and IrO2-based electrodes are not yet thoroughly understood,3 motivating further studies. The interfaces and interactions within the catalyst layer (CL) and between the neighboring membrane or diffusion media impact initial and long-term performance and need to be investigated in order to fully understand degradation mechanisms and propose potential pathways towards optimization.Commercial Ir and IrO2 black catalysts were obtained and studied as catalyst powders, fresh membrane electrode assemblies (MEAs), and tested MEAs. Electrochemical durability testing was performed on PEMWEs to replicate the start-up and shut-down process and an extended period of operation, following established protocols for catalysts4 and electrodes.3 Extensive physicochemical characterization was utilized in this study to correlate CL composition, structure, and morphology to activity and durability in order to deconvolute the impact of individual constituents within the CL on overall electrode structure and stability. Scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) combined with energy-dispersive x-ray spectroscopy (EDS) mapping were used to image powders, top-down MEAs, and cross-sectioned MEAs to elucidate morphological and structural information as well as elemental distribution as a function of electrochemical testing. Further chemical and structural information was obtained through surface-sensitive x-ray photoelectron spectroscopy (XPS) and bulk x-ray absorption spectroscopy (XAS). Finally, transmission x-ray microscopy (TXM) was used to visualize element-sensitive morphology information at a large scale. Preliminary results illuminate significant changes in the catalyst composition, catalyst-ionomer interface, and CL structure, as well as delamination of the CL from the membrane and overall CL collapse after cycling. These findings demonstrate a promising systematic characterization approach to isolate contributions of the catalyst, ionomer, and overall CL morphology, composition, and structure on the resulting electrode, in order to advance fundamental understanding of preparing uniform, stable CLs for PEMWE development.(1) Ayers, K. E. Curr. Opin. Electrochem. 2019, 18, 9–15.(2) Spöri, C.; Kwan, J. T. H.; Bonakdarpour, A.; Wilkinson, D. P.; Strasser, P. Angew. Chemie Int. Ed. 2017, 56 (22), 5994–6021.(3) Alia, S. M.; Stariha, S.; Borup, R. L. J. Electrochem. Soc. 2019, 166 (15), F1164–F1172.(4) Alia, S.M.; Anderson, G.C. J. Electrochem. Soc. 2019, 166 (4), F282-F294.