In situ ATR-FTIR study of oxygen reduction at the Pt/Nafion interface

We have developed a new half-membrane-electrode assembly (MEA)-type cell that allows us to conduct attenuated total reflectance-Fourier transform infrared (ATR-FTIR) measurements at the Pt/Nafion interface under humidified N-2/O-2 atmosphere. The cell consists of a gas-diffusion type anode placed on a carbon separator with a gas flow field, a Pt film cathode deposited chemically on an Si ATR prism and a Nafion NRE (R) 211 electrolyte sandwiched between them. The construction allows the control of the atmosphere at the cathode by those at the anode via the electrolyte of 20-mu m thickness. An infrared absorption band was observed at 1400-1403 cm(-1) under humidified oxygen atmosphere in close association with the appearance of ORR current. Its absence under N-2 atmosphere and insensitivity to the change from H2O to D2O humidification led us to ascribe the band to the O-O vibration of the adsorbed oxygen molecule O-2(ads). The band intensity increased with increasing ORR current but decreased significantly in the limiting current region. However, the stability of the species at potentials as high as 1.1 V vs. the reversible hydrogen electrode (RHE) led us to rule out the possibility that the band could be due to adsorbed superoxide O-2(-).