From Depth to Durability: Investigating Gradient Effects in Pt/C Catalyst Degradation with X-ray Scattering

Long-term durability is a key area of improvement for Proton Exchange Membrane Fuel Cells (PEMFCs). Previous work on catalyst stability has indicated that degradation across the catalyst layer occurs heterogeneously. In the presented work, we investigated this phenomenon further by following the depth- and time-dependent degradation of two commercial Pt/C fuel cell catalysts with a combination of operando X-ray scattering techniques, i.e., X-ray total scattering with Pair Distribution Function (PDF) analysis, X-ray Diffraction (XRD), and Small-Angle X-ray Scattering (SAXS). The two Pt/C fuel cell catalysts with distinctly different average particle sizes were investigated in a newly developed operando cell, employing a Gas Diffusion Electrode (GDE) in a three-electrode configuration, which allows for high mass transport similar to membrane electrode assemblies. Through the combination of X-ray scattering techniques, in particular, with the help of a sophisticated Monte Carlo analysis of the SAXS data, we could establish size histograms and thus track changes in particle size over time and throughout the catalyst layer. With this approach, we identified specific, system-dependent degradation mechanisms such as particle agglomeration and coalescence, as well as Ostwald ripening. Our findings confirm that catalyst degradation is indeed depth-dependent, with the highest degradation occurring in the regions close to the catalyst/microporous transport layer interface. The results suggest that designing gradient catalyst layers might lead to an improvement in overall durability.