Kinetics and mechanism of oxygen reduction at hydrous oxide film-covered platinum electrode in alkaline solution

Abstract

This study uses rotating ring-disk electrode (RRDE) and linear sweep voltammetry (LSV) to characterize oxygen reduction kinetics in alkaline solution on platinum electrodes with various thickness of hydrous oxide (oxyhydroxy) film. Oxyhydroxy films are created on Pt electrodes by pretreatment in 1.0 mol dm −3 KOH at a constant voltage. The pretreatment voltage ranges from −1.2 to 1.0 V and is increased stepwise before each new experimental run to produce seven discreet films. LSV plots show oxyhydroxy film thickness strongly inhibits oxygen reduction and is inversely proportional to RRDE oxygen reduction current I D for LSV voltages E D from −0.1 to −0.46 V, but this trend reverses at E D more negative than −0.46 V so that the worst-performing electrode becomes the best. However, this improvement disappears at around −0.8 V, suggesting this change involves a negatively charged ion, possibly embedded into the metal in the top few atomic layers either interstitially or substitutionally. The 1.0 V-pretreated electrode in the E D range from −0.46 to −0.9 V of highest oxygen reduction current also exhibits the lowest hydrogen peroxide production, with zero H 2O 2 produced at −0.6 V, indicating the brief presence of the oxyhydroxy film on the Pt surface has strong lingering effects. The post-oxyhydroxy Pt surface is very different than the native Pt for oxygen reduction pathway and efficiency. Reaction order with respect to oxygen is close to 1. The rate constants of the direct O 2 to H 2O electroreduction reaction are increased with decreasing the potential from −0.2 to −0.6 V, but the O 2 to H 2O 2 electroreduction is contrary to this expectation. The rate constants of H 2O 2 decomposition on the oxyhydroxy film-covered Pt electrode are near constant around 1 × 10 −4 cm s −1 at E D > −0.5 V.