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

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 electro-chemical 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.

Automated summary

This study presents a combination of operando small- and wide-angle X-ray scattering to investigate the changes in mean particle sizes and phase fractions in fuel cell catalyst layers during accelerated stress tests. The results indicate that the degradation mechanism of the catalyst is classical Ostwald ripening via local diffusion of metal atoms on the support, rather than electrochemical Ostwald ripening. Furthermore, the study shows that the degradation of the catalyst layer is depth-dependent, suggesting the potential for future improvement of catalyst stability through a depth-dependent catalyst design.