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 behaviour 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 R CT under ORR conditions. The presence of TFMPA, PFEPA or H 3 PO 4 had significant effects on the cyclic voltammograms in the H upd 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 H 3 PO 4 , 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 H 3 PO 4 . . IntroductionSustainable energy sources are of growing interest due to environmental regulations and increasing energy consumption worldwide. To satisfy the need for clean energy, fuel cells, which convert chemical energy directly into electrical energy, are promising for both stationary and mobile applications, where high power densities are required.Compared with conventional proton exchange membrane fuel cells (PEM-FC) operated below 100 °C 1 , high-temperature operation (150 -180°C) holds certain advantages, including an increased overall efficiency, a reduced sensitivity to catalyst poisoning, and simplified water management 2 .Under such operating conditions, concentrated phosphoric acid is usually used as the common electrolyte because of its favorable physical and chemical properties at high temperatures. However, the slow ORR in phosphoric acid electrolyte causes a relatively high overpotential on the cathode side 3 , which limits the performance and practical applications of high temperature proton exchange membrane fuel cells (HTPEM-FC). The anions adsorbed at the electrode surface are known to strongly affect the ORR kinetics by occupying catalyst surface sites and hindering the dissociative adsorption of molecular oxygen 4-7 . Hence, they act as site-blocking spectator species. The anions also affect the ORR selectivity by changing the ORR pathway and promote the productio...

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