D.c. and a.c. analysis of Pt/H 2 microelectrodes

Abstract

With reference to the use of platinized carbon electrodes for fuel cells, the d.c. and a.c. behaviour of rotating Pt/H 2 microelectrodes in 0.1 – 2 N H 2SO 4 and 1 N KOH were reinvestigated. Current vs. potential curves of most active electrodes in H 2SO 4 are controlled by H 2 diffusion and in KOH by a mixture of H 2 diffusion and H 2-H + transfer. With inactive electrodes reaction control is found in both cases. An effective platinum activation procedure consists of repeated cycles of electrochemical oxidation-reduction to give a reproducible surface state as well as the highest H 2 limiting current. The predominant reason for the decay of activity with time is concluded to be recrystallization of the reduced PtO surface as suggested by previous low energy electron diffraction measurements. Poisoning with S 2- traces played an important role in KOH whereas other impurities did not. From this d.c. behaviour it is concluded that the only way to restore platinum activity in fuel cell H 2 electrodes is to carry out some kind of intermediate electrochemical oxidation. To obtain more information about the H 2 oxidation steps, impedance measurements were also carried out and were interpreted on the basis of loss tangent and capacitance spectra, which are closely related to relaxation models as postulated in earlier publications. Two types of relaxation effects are observed, one due to H +(K +) adsorption in the double layer and the other due to the H 2-H + transfer reaction. Because of the high exchange current in the latter process the effects are readily observable at the open-circuit potential without an H 2 atmosphere. Losses and capacitances depend strongly on the working potential, the ion concentration and the platinum activity. By varying these parameters the relaxation effects could be separated. Thus impedance analysis proved to be a valuable supplementary tool in observing reaction steps which are not rate controlling and are hence undetectable by d.c. methods. Via energy dissipation, impedance give information about the dynamics of double-layer charging and charge transfer at or near electrochemical equilibrium, as is the case in operating fuel cell electrodes.