Scattering investigations into the structures of polymer-electrolyte-fuel-cell catalyst layers exhibiting robust performance against varying water fractions of catalyst ink solvents

Catalyst layers in polymer electrolyte fuel cells have a performance-influencing hierarchical porous structure comprising Pt-loaded carbon (Pt/C) particles and ionomer molecules. Ionomer thin films are formed on Pt/C particle aggregates by wet-coating solvent-based dispersions of Pt/C and an ionomer. Although the solvent composition critically influences the coatability of the dispersion, its effects on the catalyst layer performance remain unknown owing to hurdles in the evaluation of the ionomer distribution on the particles. We investigate the catalyst layers formed using dispersions of a water–ethanol solvent with varying water fractions. Electrochemical analyses indicate that the different ionomer structures in the dispersion minimally affect the catalyst layer performance after solvent drying. Small-angle scattering and neutron reflectivity measurements reveal the origin of the robust performance of the catalyst layers. The solvent-related difference in the catalyst inks marginally changes the hierarchical structure of the ionomer, resulting in a robust performance with tunable dispersion structures.