Hydrogen is an important chemical commodity that can be used to power fuel cells for transportation or stationary applications and can be stored for a long period of time. Polymer electrolyte membrane water electrolysis (PEMWE) produce hydrogen from water with only oxygen and heat as byproducts, but slow kinetics of the oxygen reduction reaction at the electrolyzer anode requires an expensive Ir-based catalyst. To meet cost targets at high-scale, a shift towards efficient, low-loading catalyst layers (CLs) is necessary. While it is known that significant degradation issues will occur in these electrolyzers, the extent and severity are not yet fully understood.<br/>Elucidation of the degradation mechanisms requires a comprehensive understanding of the changes in the chemical states and elemental distribution of the catalyst and ionomer, and morphology and structure of the catalyst layer at multiple scales. This requires a combination of lab-based characterization methods such as electron microscopy and X-ray photoelectron spectroscopy (XPS) with synchrotron X-ray based spectroscopy and tomography.<br/> In this work, catalyst layers were prepared using commercial Ir and IrO<sub>2</sub> catalysts. Catalyst powders, fresh catalyst-coated membranes (CCMs), and tested CCMs were studied to track catalyst and catalyst layer evolution after exposure to relevant testing protocols. Complementary electron and X-ray microscopy and spectroscopy methods were used to isolate surfaces, interfaces, and interactions within the catalyst layer. Scanning electron microscopy (SEM) and scanning transmission electron microscopy combined with energy-dispersive X-ray spectroscopy (STEM/EDS) were utilized to image the surface and cross-sections of the catalyst layers and assess changes in morphology and elemental distribution after testing. XPS and X-ray absorption spectroscopy (XAS) probed chemical changes and coordination environment at the CL surface (XPS) and throughout the layer (XAS). Finally, transmission X-ray microscopy (TXM) was employed to visualize the distribution, morphology, and chemistry of the Ir-based catalyst across a wider region of the CLs, as well as CL porosity. Results demonstrate the necessity for a systematic, multi-scale, multi-technique characterization approach to enable a deeper understanding of how observed degradation mechanisms are related, which is vital to drive the development of more efficient and stable electrodes.

Multitechnique characterization of degradation in polymer electrolyte membrane electrolyzers

ACS Fall 2021

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