Revealing the Impact of Cell Conditioning on PEMWE Catalyst Layer Morphology

Abstract

Polymer electrolyte membrane (PEM) water electrolysis is a promising green hydrogen generation technology to mitigate the effects of anthropogenic climate change. To achieve stable cell voltages, a conditioning procedure is typically required, where current or voltage cycling is applied until stable performance is achieved. Previous studies have demonstrated that a decrease in voltage occurs within the first few hours of operation [1], but this conditioning period has not been extensively studied for PEM electrolyzers, despite a variety of standard conditioning protocols available for PEM fuel cells in the literature. Previous works additionally suggest that changes in performance may be induced due to structural changes in the anode catalyst layer (CL) caused by large gas bubble evolution during the first few hours of operation [2]. However, to establish and optimize a conditioning procedure for PEMWEs, the specific structural changes caused by conditioning on the anode CL must be elucidated and correlated with the electrochemical performance parameters.In this work, we applied scanning transmission X-ray microscopy (STXM) to PEMWE catalyst layers to reveal pore and ionomer distributions in pristine and conditioned commercial CL samples. Prior to imaging, CL samples were embedded in epoxy and cut to 50 nm thick slices using an ultramicrotome. Using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, Carbon 1s and Fluorine 1s absorption edges were probed to reveal pore and ionomer distributions, respectively. After image acquisition, the image stacks were processed to obtain the ionomer and epoxy spectra to enable spectral fitting and thresholding. Through this analysis, we quantified various morphological properties of catalyst layers (including CL thickness, porosity, pore size distribution, agglomerate distribution, and ionomer content) and compared these properties to those of pristine CLs to explain changes in cell performance. The insights gained from this work will aid in the development of efficient conditioning protocols and identify optimal post-conditioned anode CL morphology to inform the design of next-generation catalyst materials. A. Weiß et al., J Electrochem Soc, 166, F487–F497 (2019).O. Panchenko et al., Mater Today Energy, 16, 100394 (2020).