Effect of the Side-Chain Structure of Perfluoro-Sulfonic Acid Ionomers on the Oxygen Reduction Reaction on the Surface of Pt

The effect of side-chain structures in perfluoro-sulfonic acid ionomers on the adsorption of the terminal sulfonate moiety on the surface of Pt is investigated with voltammetry and surface-enhanced infrared absorption spectroscopy (SEIRAS). Analyses with low-molecular-weight model anions with and without an ether group in the perfluoro-alkyl chain indicate that the anions are adsorbed on Pt through one or two oxygen atom(s) of the terminal sulfonate group and that the oxygen atom of the ether group also interacts with the Pt surface, leading to stronger adsorption of the anions with an ether group. On the basis of the results obtained with the model anions, the adsorption of the terminal sulfonate moieties in perfluorinated sulfonic acid ionomers and its effect on oxygen reduction reaction (ORR) is discussed. It is shown that the ionomers having longer side chains more strongly block ORR due to the flexibility of the side chains.