Enhancing Oxygen Reduction Reaction Activity on Pt/C Catalysts for PEFCs Using Phosphonium Ionic Liquid Layers: Effects of Proton Introduction and Physicochemical Properties

Abstract

Polymer electrolyte fuel cells (PEFCs) have attracted significant attention as clean energy conversion systems. Pt nanoparticles are commonly used as cathode catalysts in PEFCs due to their high activity for the oxygen reduction reaction (ORR). However, Pt is rare and expensive; therefore, its mass activity needs to be improved for the dissemination of fuel cells. Recently, ionic liquid (IL) layers have been used to enhance the ORR catalytic activity on Pt surfaces via the suppression of OH adsorption. In this paper, we discuss the optimum structure of phosphonium ILs, which have high thermal stability, favourable ionic conductivity, and hydrophobicity, for Pt/C-solid catalysts with ionic liquid layers (SCILLs). Using the microelectrode method, we found that the oxygen solubility depends on the molar volume of the IL, in accordance with regular solution theory, and that the diffusion coefficient approximately follows the Einstein–Stokes equation. This indicates that the resistance encountered by a solute O₂ molecule is affected more by the microscopic viscosity generated locally by the cationic structure rather than the macroscopic viscosity of the bulk IL. In addition to the reduction in OH poisoning achieved by hydrophobic ILs, some physicochemical properties that lead to high mass and specific activity for the ORR on Pt/C-SCILLs, such as the oxygen solubility, H⁺ activity, and conductivity, were clarified.