A.C.-IMPEDANCE SPECTROSCOPY STUDY OF OXYGEN REDUCTION AT NAFION(R) COATED GAS-DIFFUSION ELECTRODES IN SULFURIC-ACID - TEFLON LOADING AND METHANOL CROSS-OVER EFFECTS

The kinetics of oxygen reduction have been studied at Nafion(R)-coated dual layer gas-diffusion electrodes at 60 degrees C in 2.5M H2SO4. A. c. impedance and steady state galvanostatic measurements have been car ried out on electrodes containing various PTFE loadings. A.c. impedanc e spectroscopy supplied information on charge transfer resistance, ser ies resistance and double layer capacitance of electrodes during the o xygen reduction process. These parameters, together with those derived by d.c. measurements (i.e. Tafel slopes, charge transfer coefficient, etc.), allowed a better identification of the features governing the electrode efficiency and the reduction mechanism. The best electrode w as found to be one containing 30% PTFE; this showed both the lowest ch arge transfer resistance and the lowest overpotential for oxygen reduc tion. This electrode also showed lower overpotential than the same unc oated electrode at low current densities. The poisoning effect by meth anol has been also investigated. Upon addition of methanol to the solu tion, no change was observed in the reaction mechanism at low overpote ntials. In contrast, a significant variation of the Tafel slope was ob served at high overpotentials. This behaviour is interpreted in terms of the variation of Pt-O bond strength as a function of electrode pote ntial.