Evidence of Well-Dispersed Ionomer in the Cathode Catalyst Layer of a PEMFC by Small Angle Scattering and the Effect of its Content on Performance

The cathode catalyst layer plays a crucial role in the performance of proton exchange membrane fuel cells (PEMFCs). It consists of a catalyst (platinum nanoparticles deposited on carbon nanoparticles) and an ionomer. The I/C ratio (ionomer mass/carbon mass) is a key parameter in optimizing catalyst layers. Here, catalyst layers containing a catalyst made of platinum nanoparticles deposited on high-surface-area carbon and Nafion D2020 as ionomer, with different I/C ratios, were structurally and electrochemically characterized. An I/C of 1.1 was found to be optimal in terms of performance and protonic resistance. Structural characterization of the ionomer in the catalyst layer was conducted using transmission electron microscopy, small-angle X-ray scattering, and small-angle neutron scattering. An innovative data processing method enabled small-angle X-ray scattering to provide the same information into the ionomer structure as small-angle neutron scattering. All characterizations showed that the ionomer was mostly well dispersed for all the tested I/C ratios. Increasing the I/C led to a more widespread ionomer network, which explains the observed improvement in performance and reduction in protonic resistance. However, scanning electron microscopy revealed that at an I/C of 1.3 or higher, porosity sharply decreased, partially explaining the reduced performance at higher I/C ratios.