Synchrotron X-Ray Tomography Investigation of Fundamental Degradation Mechanisms in Polymer Electrolyte Membrane Water Electrolyzers

Abstract

Electrochemical energy conversion devices such as fuel cells and electrolyzers provide the potential to meet global energy demands, particularly as a renewable alternative to fossil fuels or as part of a more environmentally friendly combination. In both polymer electrolyte membrane (PEM) fuel cells (PEMFCs) and water electrolyzers (PEMWEs), degradation studies are extremely important. Degradation mechanisms will become even more problematic as electrolyzers transition towards intermittent operation; additionally, there is a need to reduce noble metal loadings, used to combat slow kinetics of the anodic oxygen evolution reaction (OER), to meet cost goals at scale.1,2 A shift towards scalable, efficient low-loading catalyst layers operated with intermittent power will lead to significant degradation concerns, but the extent and severity are not yet fully understood.2–4 Advanced physicochemical characterization techniques are required to interpret electrochemical data and fully understand how the device components degrade. Synchrotron X-ray based tomography and spectroscopy techniques in particular can provide sensitive analysis of chemical states, elemental distribution, and visual information to elucidate large-scale changes in catalyst layer (CL) chemistry, morphology, and structure after relevant testing protocols. These methods are highly complementary to lab-based characterization methods, such as electron microscopy and X-ray spectroscopy.In this study, commercial Ir and IrO2 black catalysts were obtained and studied as catalyst powders, fresh catalyst-coated membranes (CCMs), and tested CCMs. Electrochemical durability testing was performed on PEMWEs to replicate either the start-up/shut-down process (intermittent operation) or an extended hold (steady-state operation). Transmission X-ray microscopy (TXM) was used to visualize distribution and morphology of iridium throughout the CL, as well as CL porosity. By collecting microscopy images while scanning the X-ray energy across the Ir absorption edge, X-ray absorption near-edge structure (XANES) chemical maps were generated to track iridium oxidation state to compare Ir and IrO2 CLs as well as the impact of testing. These synchrotron methods were supplemented with electron microscopy of the CCM surface as well as cross-sectional analysis to investigate changes in morphology, structure, and elemental distribution, and surface-sensitive X-ray photoelectron spectroscopy (XPS) to track relevant surface species after testing. Together, these techniques provide an unparalleled look into degradation of PEMWE catalyst layers at multiple scales to isolate the impact of individual CL constituents on overall electrode structure and stability.(1) Ayers, K. E. Curr. Opin. Electrochem. 2019, 18, 9–15.(2) Alia, S. M.; Stariha, S.; Borup, R. L. J. Electrochem. Soc. 2019, 166 (15), F1164–F1172.(3) Ayers, K. E.; Danilovic, N.; Ouimet, R.; Carmo, M.; Pivovar, B. S. Annu. Rev. Chem. Biomol. Eng. 2019, 10, 219–239.(4) Ayers, K. E.; Renner, J. N.; Danilovic, N.; Wang, J. X.; Zhang, Y.; Maric, R.; Yu, H.