Elucidation of adsorption mechanisms of anionic moieties in polymers and their effect on the oxygen reduction reaction on a Pt surface using a Pt(111) single-crystal electrode and in-house synthesized ionomers

The behavior at the interface between the Pt catalyst and the ionomer, which functions as a reaction field determining the oxygen reduction reaction (ORR) activity in the cathode of polymer electrolyte fuel cells (PEFCs), was elucidated by combining the synthesis of new ionomers with precisely controlled model experiments. Four ionomers with different backbone rigidities and side-chain lengths were employed, and the effects of their molecular structural characteristics on the adsorptivity of sulfonate anions in the ionomer and on the ORR activity on the Pt surface were analyzed using a Pt(111) single-crystal electrode. As a result, it was revealed that the adsorptivity of sulfonate anions increases with increasing backbone flexibility and side-chain length. Furthermore, for ionomers with long side chains, the adsorption process was found to be governed by the kinetics of side-chain motion, whereas for those with short side chains, it was governed by the thermodynamics of backbone deformation — mechanisms that were clearly identified for ionomers with rigid backbones. For the ionomer with a flexible backbone and long side chains, the ORR activity decreased to 23% of that of the bare Pt(111) surface due to the adsorption of sulfonate anions. In contrast, by increasing the backbone rigidity and shortening the side chains, the activity was improved to 59% of that of the bare surface. The effect of backbone rigidity was particularly pronounced, providing a guideline for designing ionomer molecular structures with high ORR performance.