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
Pierre Toudret, Alexis Chennevière, J. Blachot, Gérard Gebel, Laure Guétaz, Marie Heitzmann, Arnaud Morin
Abstract
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.