Tracking the Catalyst Layer Depth-Dependent Electrochemical Degradation of a Bimodal Pt/C Fuel Cell Catalyst: A Combined Operando Small- and Wide-Angle X-ray Scattering Study
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A combination of operando small- and wide-angle X-ray scattering is here presented to provide insights into the changes in mean particle sizes and phase fractions in fuel cell catalyst layers during accelerated stress tests (ASTs). As a fuel cell catalyst, a bimodal Pt/C catalyst was chosen that consists of two distinguishable particle size populations. The presence of the two different sizes should favor and uncover electrochemical Ostwald ripening as a degradation mechanism, that is, the growth of larger particles in the Pt/C catalyst at the expense of the smaller particles via the formation of ionic metal species. However, instead of electrochemical Ostwald ripening, the results point toward classical Ostwald ripening via the local diffusion of metal atoms on the support. Furthermore, the grazing incidence mode provides insights into the catalyst layer depth-dependent degradation. Although the larger particles show the same particle size changes close to the electrolyte-catalyst interface and within the catalyst layer, the smaller Pt nanoparticles exhibit a slightly decreased size at the electrolyte-catalyst interface. During the AST, both size populations increase in size, independent of the depth. Their phase fraction, that is, the ratio of smaller- to larger-size population, however, exhibits a depth-dependent behavior. Although at the electrolyte-catalyst interface, the phase fraction of the smaller-size population decreases, it increases in the inner catalyst layer. The results of a depth-dependent degradation suggest that employing a depth-dependent catalyst design can be used for future improvement of catalyst stability.
|Publication status||Published - 2022|
This work was supported by the Swiss National Science Foundation (SNSF) via the project no. 200021_184742 and the Danish National Research Foundation Center for High Entropy Alloy Catalysis (CHEAC) DNRF-149. S. B. Simonsen and L. Theil Kuhn, Technical University of Denmark, are thanked for access to the transmission electron micsroscope. The authors also thank ESRF for beamtime at the ID31 beamline and H. Isern and F. Russelo for technical support.
- accelerated stress test (AST), bimodal Pt/C catalyst, fuel cell catalyst degradation, small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS)