Adsorption of ionomer and ionic liquid on model Pt catalysts for polymer electrolyte fuel cells

Abstract

The adsorption of the perfluoro-sulfonic acid polymer of Nafion and ionic liquid (IL) of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide on the surface of Pt was investigated via voltammetric analyses, using stepped Pt single-crystal electrodes with (111) terraces and (110) steps, and surface-enhanced infrared absorption spectroscopy (SEIRAS) analyses using a Pt polycrystalline electrode. Sulfonate anion in Nafion was adsorbed on the stepped Pt single-crystal electrodes and suppressed the oxygen reduction reaction (ORR) activity by more than 50%, regardless of the terrace width. The IL molecules were preferentially adsorbed on the step sites through a simple IL coating procedure. The SEIRAS analysis indicated that the IL molecules were stable on the Pt surface throughout potential cycles, where the anionic moieties were in contact with the Pt surface and reoriented depending on the potential. The IL modification prior to Nafion coating mitigated ionomer adsorption on the Pt surface. However, the mitigation effect was not reflected in the ORR activity because water production led to IL desorption during the ORR activity measurement. Accordingly, IL modification is a promising method for improving the performance of Pt catalysts in polymer electrolyte fuel cells; however, further studies to prevent the leaching of IL are required for practical applications of this approach.