OXYGEN REDUCTION ON PT IN AQUEOUS K2CO3 AND KOH

The electrolyte concentration dependences of platinum oxidation and oxygen reduction (OR) on Pt were investigated in KOH and K2CO3 electrolytes. Cyclic voltammetry was used to characterize the oxidation processes on the Pt electrode, and the rotating ring-disk electrode (RRDE) technique was used to evaluate the kinetic processes for OR in 0.1-6.9M KOH and 0.1-4.0M K2CO3, at room temperature. The charge due to Pt oxidation reached a maximum at a KOH concentration of about 2-3M for a range of electrode potentials. This is consistent with changes in the rates and the relative importances of the reversible and aging processes with changes in hydroxide-ion activity. No maxima were observed with carbonate electrolytes at comparable ionic strengths. At high overpotentials, in both electrolytes, current densities were limited by the rate of O2 adsorption onto Pt. The Tafel slope for this process was essentially infinite for the more-oxidized surfaces. At lower overpotentials, the observed Tafel slope decreased and the fraction of current passing through the 2-electron pathway increased with increasing hydroxide-ion activity. In carbonate electrolytes, the Tafel slope was indicative of the direct 4-electron pathway for OR with negligible amounts of detectable peroxide ion at all electrolyte concentrations. Comparisons of mass-transfer-corrected current densities for the two electrolytes at the same concentration showed higher currents in the KOH electrolytes, due to the much lower O2 solubility in K2CO3. When normalized to the same O2 concentration, the activity of Pt for OR is about two times greater in the carbonate electrolyte. © 1990, The Electrochemical Society, Inc. All rights reserved.