A proton
exchange ionomer is one of the most important components
in membrane electrode assemblies (MEAs) of polymer electrolyte membrane
fuel cells (PEMFCs). It acts as both a proton conductor and a binder
for nanocatalysts and carbon supports. The structure and the wetting
conditions of the MEAs have a great impact on the microenvironment
at the three-phase interphases in the MEAs, which can significantly
influence the electrode kinetics such as the oxygen reduction reaction
(ORR) at the cathode. Herein, by using the Pt(111)|X ionomer interface
as a model system (X = Nafion, Aciplex, D72), we find that higher
drying temperature lowers the onset potential for sulfonate adsorption
and reduces apparent ORR current, while the current wave for OHad formation drops and shifts positively. Surprisingly, the
intrinsic ORR activity is higher after properly correcting the blocking
effect of Pt active sites by sulfonate adsorption and the poly(tetrafluoroethylene)
(PTFE) skeleton. These results are well explained by the reduced water
activity at the interfaces induced by the ionomer/PTFE, according
to the mixed potential effect. Implications for how to prepare MEAs
with improved ORR activity are provided.