A study of oxygen reduction on platinum-dispersed porous carbon electrodes at room and elevated temperatures by using a.c. impedance spectroscopy

Oxygen reduction on unwetted and pre-wetted platinum-dispersed porous carbon electrodes (Pt/C) is investigated in 1 M H2SO4 solution at room temperature by using a.c. impedance spectroscopy in combination with a current-decay transient technique (chronoamperometry). From the appearance of an inductive arc in Nyquist plots from the unwetted Pt/C electrode specimen, it is suggested that oxygen reduction proceeds via formation of an intermediate state. The a.c. impedance spectra are also obtained from the Pt/C electrode in 85% H3PO4 solution at 140°C, with and without oxygen and air blowings. In both cases, an inductive arc in the low-frequency range of the Nyquist plots is observed. The change in impedance spectra with pre-wetting treatment of the Pt/C electrode specimen in 1 M H2SO4 solution and the alteration of spectra without oxygen and air blowings in 85% H3PO4 solution are discussed in terms of the Epelboin model. From the analysis, it is concluded that as depletion of the dissolved oxygen occurs markedly within the narrow pores of the Pt/C electrode, the inductive arc moves towards the capacitive arc in the low-frequency range. The results of current-decay transients strongly suggest the occurrence of depletion of the dissolved oxygen within the narrow pores.