Perfluoroalkyl-Phosphonic Acid Adsorption on Polycrystalline Platinum and Its Influence on the Oxygen Reduction Reaction

Abstract

Motivated by the need for a better understanding of the oxygen reduction reaction (ORR) in high temperature proton exchange membrane fuel cells (HTPEM-FC), we have investigated the impact of anion adsorption on the ORR activity and selectivity for small concentrations of perfluoroalkylated phosphorus-based model compounds. A polycrystalline Pt surface was used to evaluate the adsorption behavior of trifluoromethyl-phosphonic acid (TFMPA) and pentafluoroethyl-phosphonic acid (PFEPA) by means of cyclic voltammetry (CV), a rotating disc electrode (RDE), and a spectro-electrochemical flow cell. In situ Fourier transform infrared spectroscopy in the attenuated total reflection (ATR-FTIRS) configuration was performed to study the impact of the adsorbed species on the ORR activity and selectivity employing simultaneous detection of hydrogen peroxide formation during the ORR. Electrochemical impedance spectroscopy (EIS) was applied to determine the charge transfer resistance RCT under ORR conditions. The presence of TFMPA, PFEPA, or H3PO4 had significant effects on the cyclic voltammograms in the Hupd and oxide formation potential region. The ORR activity, measured under well-defined mass transport conditions, revealed a doubling of the kinetic current densities for TFMPA and PFEPA compared with H3PO4, suggesting a weaker adsorption tendency for these model compounds. The ORR selectivity was found to be dependent on the nature of the adsorbed species. These observations are consistent with ATR-FTIRS and EIS results, where the reduced interaction of perfluoroalkylated compounds with the Pt surface was observed compared to H3PO4.