Revealing the Nanoscale Morphology Changes of Polymer Electrolyte Membrane Fuel Cell Catalyst Layers due to Conditioning.

We applied ex situ and in situ scanning transmission X-ray microscopy (STXM) to reveal the structural degradation of polymer electrolyte membrane (PEM) fuel cell catalyst layers due to conditioning and thermal heating of the catalyst coated membrane (CCM). For electrochemically conditioned samples, a homogeneous ionomer-to-carbon-black weight ratio distribution, as quantified by STXM, was observed along the cathode catalyst layer, in contrast to a heterogeneous ionomer-to-carbon-black weight ratio distribution for the pristine cathode catalyst layer. The homogeneous ionomer-to-carbon-black weight ratio, as quantified by STXM, is attributed to the redistribution of ionomer and carbon black into spatially favored positions during operation, likely driven by the loss of structural cohesion within the catalyst layer. However, a heterogeneous porosity distribution was seen across conditioned catalyst layers, which we attributed to pore enlargement that resulted from the incipience of liquid water during conditioning. Furthermore, for the first time, we report real-time dynamic nanoscale contraction (up to 2.8%) of a conditioned PEM fuel cell membrane subjected to dry and heated conditions. The damage incurred by the catalyst layer from conditioning manifested via the susceptibility of the CCM to tearing during ultramicrotomy, and further worsening of this damage was observed when the membrane was subjected to drying and contraction.