Cathodic reduction of oxygen on smooth platinum in acid solutions

Abstract

The mechanism of the cathodic reduction of oxygen on smooth platinum in 0·5 M H 2SO 4 solution has been studied on the basis of the current efficiency for H 2O 2 formation. H 2O 2 formation occurs at cathode potentials ≤680 mV (nhe), whereas the initial rest-potential lies at 960 mV. The current efficiencies for H 2O 2 formation depend on both cathode potential and duration of electrolysis, the maximum efficiency (99 per cent) being found at 450 mV with the shortest experiments (30 min). The proposed mechanism is at the highest potentials, the cathodic reduction occurs through the two reactions O 2 ads + H + + e → HO 2 ads (slow) and PtO + 2H + + 2 e → Pt + H 2O (fast). The first tends to inhibit the electrode surface progressively owing to the formation of adsorbed HO 2 radicals, and the second supports the greater part of the cathodic current: neither forms H 2O 2. At less noble potentials, the reaction O 2 + H + + 2 e → HO 2− begins, giving rise to H 2O 2 formation. At potentials ≤450 mV the process H 2O 2 + 2H ads → 2H 2O takes place, due to the intervention of adsorbed atomic hydrogen. The process O 2 + 2H ads → H 2O 2 can also overlap those just mentioned, and it may become the prevailing process when oxygen reduction proceeds in the presence of halide ions.