A small angle X-ray scattering approach for investigating fuel cell catalyst degradation for both ex situ and in operando analyses

Detailed multi-technique characterization of catalyst layer degradation is fundamental for improving catalyst stability and performances in Proton Exchange Membrane Fuel Cells (PEMFCs), and Small Angle X-Ray Scattering (SAXS) coupled to chemical and/or electrochemical analysis can provide important insights of processes involved in catalyst coarsening. In this extent, we present an approach to SAXS analysis able to describe all of the layers composing the Membrane Electrode Assembly (MEA): electrolyte, catalyst support, catalyst nanoparticles, and gas diffusion layers. This approach was used to compare morphological evolution of small clusters formed by catalyst nanoparticles in pristine and aged MEAs in both ex situ and in operando conditions, on a standard SAXS beamline, without exploiting the advantages of anomalous SAXS. Twin MEAs were aged with two different types of Accelerated Stress Tests (AST): one addressed to the catalyst support (s-AST) and one targeting the catalyst layer (c-AST). Limited growth of catalyst nanoparticle size was found when running s-AST, while remarkable evolution was revealed once applying c-AST. Such a difference was mainly reconducted to the disconnection of catalyst nanoparticles from the electrical paths, as supported by analysis of specific Electrochemically Active Surface Area (ECSA). In both cases, the small clusters were found becoming more compact after AST were run.