Polymer electrolyte membrane water electrolyzers (PEMWEs) are devices of paramount importance, enabling the large-scale storage of hydrogen from intermittent renewable energy sources such as wind and solar. But a transition towards lower noble metal catalyst loadings and intermittent operation is needed for the widespread utilization of this technology. Although kinetic losses tend to dominate in membrane electrode assembly (MEA) results, it has been suggested that morphological changes and interfaces between the catalyst, ionomer, and membrane will also contribute to overall degradation. Moreover, the combination of degradation to the catalyst layer (CL) constituents will further lead to structural changes that have not been widely explored. The multitude and complexity of degradation mechanisms, which likely occur simultaneously, require a characterization approach that can explore surfaces and interfaces at a range of length-scales to probe chemical, morphological, and structural changes of constituents within the catalyst later. This paper presents a comprehensive characterization approach that features scanning electron microscopy (SEM), scanning transmission electron microscopy with energy-dispersive X-Ray spectroscopy (STEM/EDS), X-Ray photoelectron spectroscopy (XPS), X-Ray absorption spectroscopy (XAS), and transmission X-Ray microscopy (TXM) with X-Ray absorption near-edge structure (XANES) chemical mapping to study degradation of the catalyst layer with a focus on MEAs after intermittent and steady-state operation. Catalyst changes including dissolution, oxidation, and agglomeration were observed, as well as redistribution and dissociation of the ionomer. These smaller-scale changes were found to have a large influence on overall stability of the electrodes: they caused the formation of voids and segregation of constituents within regions of the film. Delamination and collapse of the overall catalyst layer were observed in some instances. Greater changes were observed after an extended 2 V hold compared to IV cycling, but similar degradation mechanisms were detected, which suggests the larger issues would likely also be experienced during intermittent PEMWE operation. These findings would not be possible without such a systematic, multi-scale, multi-technique characterization approach, which highlights the critical importance of detailed analysis of catalyst layer degradation to propose mitigation strategies and improve long-term PEM water electrolyzer performance.
Bibliographical noteFunding Information:
This material is based in part upon work supported by the U.S. Department of Energy, Office of Science, Office of Workforce Development for Teachers and Scientists, Office of Science Graduate Student Research (SCGSR) program. The SCGSR program is administered by the Oak Ridge Institute for Science and Education for the DOE under contract number DE‐SC0014664. S.F.Z. and S.P. would like to thank Mines and the U.S. Department of Energy H2NEW consortium for financial support. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02–76SF00515. The authors would like to thank Dr. Ryan Davis for assistance with XAS data collection and analysis; Dr. Bryan Pivovar, Dr. Shaun Alia, and Dr. Guido Bender for useful discussions; and Samantha Medina and Sara Kim for assistance with electron microscopy sample preparation and discussion. The authors also acknowledge Mines’ Shared Instrumentation Facility, specifically the microscopy resources, and use of the E-XPS system at Mines, which was supported by the National Science Foundation under Grant No. 1626619. The authors would like to thank to Daniel Holtz and Stefanie Fischer for the experimental support. Access to the infrastructure at the Ernst Ruska-Centre, Forschungszentrum Jülich is gratefully acknowledged.
© 2022 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited