Oxygen Overvoltage Measurements on Bright Platinum in Acid Solutions

Abstract

Oxygen overvoltage measurements were made on a Pt/O2 electrode in H202-stabilized acid solutions. The concentration of H202 was 0.1M. It was found that the rest potential did not reach the reversible O2/H202 potential (682 mv), and was independent of the H202 concentration and of the partial pressure of oxygen. These findings are explained in terms of a polye]ectrode model in which the rest potential is a mixed potential composed of the half reactions H202 "-> 02 ~-2H + ~ 2e and H202-~-2H + + 2e-> 2H20. From the polarization data, it is concluded that the mixed potential and the electrode kinetics are controlled by the reaction O2 + 2H + + 2e ~-H202 The system acts as though it were a reversible O2/H202 electrode with an Eo of 810 my. A value of 1.13 x 10-a mho is calculated for the rate of the electrode reaction. The rate-determining step both in the anodic and the cathodic directions involves the discharge of the first electron. In general, it is found that the presence of H202 may be detected in any system in which the reduction of oxygen takes place. Berl (1) found that oxygen is reversible to the HO2-ion in alkaline electrolytes at inert electrodes, and Yeager and co-workers (2) obtained reproducible results from their work on oxygen electrodes in alkaline solution when the system was "stabilized" by adding H202 to the system. For very dilute concentrations of peroxide in acid solutions, it was found (3) that the rest potential was shifted to less noble values with increasing H202 concentrations. For larger concentrations of H202, however, the rest potential did not reach the reversible value of 682 mv (4) quoted for the O2/H202 reaction but became independent of the H202 concentration and dependent on the pH, as observed by Bockris and Oldfield (5) and by Gerischer (6). Apparently H202 is always present in the reduction of oxygen because the O-O bond is so strong that a stable peroxide intermediate is formed. Tracer experiments carried out by Yeager and co-workers (7) in alkaline H202 solutions indicate that the O-O bond is never broken. The present report is concerned with oxygen over-voltage experiments carried out on Pt electrodes in acid solutions to which peroxide had been added. Experimental The cell, electrode, solution preparations, and experimental techniques are the same as those described before (3, 8). In most cases, the electrode was allowed to come to a steady state in oxygen saturated 2N H2SO4 solution. Then, the required amount of H202 was added to the solution with a hypodermic syringe after which polarization measurements were made. All potentials are referred to the normal hydrogen electrode (NHE). The solutions were stirred with purified oxygen at a rate of about 275 cc/min at which rate all observed potentials were independent of stirring. The work was carried out at a temperature of 25 ~ _ I~ Results A plot of the polarization curves at low current densities obtained on Pt, Au, and Rh electrodes in O2-saturated 2N H2SO~ solution which is 0.1M in H202 is presented in Fig. 1. The cathodic overvoltage curve, Fig. 2, and the anodic overvoltage curve, Fig. 3 [determined from the rest potential value rather than from the reversible oxygen potential (4) of 1229 mv], were obtained on bright Pt on O2-saturated 2N H2SO4 which is also 0.1M in H202. Figure 4 gives the partial pressure of oxygen (Po2) data. 90C-a. 600-APPARENT CURRENT DENSITY (,u,A/crn 2) Fig. 1. Polarization of Au, Rh, and Pt electrodes at low current densities in 0.1M H202 acid solution (2N H2SO4). All electrodes have the same rest potential (810 my) but different rates of reaction (-di/ dr)). r ~ [ (I__ d~-IOC-20~-6-5-4-3 LOG APPARENT CURRENT DENSITY (A/cm z) Fig. 2. Overvoltage measurements for the cathodic reduction of oxygen on bright Pt in 0.1M H202 acid solution (2N H2SO4). The overvoltage was determined from the rest potential instead of 1229 my. 608