  Wide proliferation of low temperature hydrogen fuel cell systems, a key part
of the hydrogen economy, is hindered by degradation of the platinum cathode
catalyst. Here, we provide a device level assessment of the molecular scale
catalyst degradation phenomena, using advanced operando X-ray scattering
tomography tailored for device-scale imaging. Each cell component, including
the catalyst, carbon support, polymer electrolyte, and liquid water can be
simultaneously mapped, allowing for deep correlative analysis. Chemical and
thermal gradients formed inside the operating fuel cell produce highly
heterogeneous degradation of the catalyst nanostructure, which can be linked to
the macroscale design of the flow field and water distribution in the cell
materials. Striking differences in catalyst degradation are observed between
operating fuel cell devices and the liquid cell routinely used for catalyst
stability studies, highlighting the rarely studied but crucial impact of the
complex operating environment on the catalyst degradation phenomena. This
degradation knowledge gap highlights the necessity of multimodal in situ
characterization of real devices when assessing the performance and durability
of electrocatalysts.
