Detailed Catalyst Layer Structure of Proton Exchange Membrane Fuel Cells from Contrast Variation Small-Angle Neutron Scattering

Despite the significant decrease in past decades, the cost of proton exchange membrane fuel cells, largely due to the rare and expensive electrocatalyst made of platinum, restrains their massive deployment. Therefore, reducing the platinum content in the electrode is the keystone of intense research efforts to increase the catalyst activity or utilization. The catalyst layer structure, especially the water and ionomer distributions, rules the active site availability for electrochemical reactions and thus catalyst utilization because of its influence on the transport of protons and oxygen. However, the rational design of more efficient electrodes faces the lack of accurate knowledge of their complex nanoporous structure. Specifically, ionomer and water distributions are very difficult to probe with conventional electron microscopy or X-ray techniques. This work provides quantitative information on the electrode structure, regarding ionomer and water distributions, thanks to an extensive analysis of small-angle neutron scattering profiles at different relative humidities and contrasts. A 2 to 3 nm-thin ionomer film spreads around the Pt/C catalyst particles, while a condensed water layer appears at the catalyst/ionomer interface depending on the type of carbon support.

Automated summary

Reducing the platinum content and improving catalyst activity are key research directions for increasing the efficiency of proton exchange membrane fuel cells.