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-microm 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 H(2)O to D(2)O 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)(-).