In Situ ATR-FTIR Analysis of the Structure of Nafion–Pt/C and Nafion–Pt₃Co/C Interfaces in Fuel Cell

To obtain clues to how to improve the effectiveness of cathode nanocatalysts in fuel cells, the interfacial structure of Nafion–Pt/C and Nafion–Pt 3 Co/C in fully humidified N 2 at room temperature was, for the first time, investigated by in situ attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy by use of a membrane-electrode assembly (MEA) type cell without any supporting electrolyte solution. We identified four types of adsorbed species at the interface with changing the electrode potential, that is, sulfonate anions (SO 3 – ) with ether groups (C–O–C) in the Nafion side chain, water molecules (adsorbed H 2 O ad and/or hydrating SO 3 – groups), and hydrated protons [H + (H 2 O) n ]. For a Pt skin layer formed on Pt 3 Co/C, it was clearly observed that the band intensities assigned to the water bending mode δ(HOH) of both H + (H 2 O) n (1800 and 1720 cm –1 ) and H 2 O ad (1610 cm –1 ) decreased with increasing potential more positive than 0.2 V vs RHE, accompanied by increases in those of ν s (SO 3 – ) (1050 cm –1 ) and ν(C–O–C) (965 cm –1 ). This indicates that the SO 3 – groups with an ether side chain in Nafion adsorbed specifically on the catalyst surface, while expelling H + (H 2 O) n and H 2 O ad originally adsorbed at less positive potentials. We also found that the onset potential for the specific adsorption of the SO 3 – group in the Nafion side chain on the Pt skin layer formed on Pt 3 Co was less positive by ca . 0.2 V than that on Pt, which is a tendency similar to that found for the specific adsorption of ClO 4 – anions on these electrodes in our recent results obtained by use of the electrochemical quartz crystal microbalance (EQCM).