Kinetic Analysis of Oxygen Reduction Reaction for Electrodeposition of ZnO Thin Films

Abstract

Oxygen reduction reaction (ORR) in aqueous zinc chloride solution results in an electrodeposition of highly crystallized zinc oxide (ZnO) thin films. Due to the mechanistic complexity of the ORR and the experimental difficulties associated with the precipitation of ZnO to change the electrode surface, kinetic analysis of the ORR during the electrodeposition of ZnO has not been achieved. In this study, we have employed a rotating ring-disk electrode (RRDE) and followed Nabae’s approach to tackle this goal. While full 4-electron ORR is predominant at the surface of ZnO, as long as the electrolyte solution is free of Zn²⁺, 2-electron ORR down to hydrogen peroxide (H₂O₂) becomes the main path during the electrodeposition of ZnO, although, even slowly, further reduction of H₂O₂ and direct 4-electron ORR do operate. Having pre-determined the kinetic parameters for the H₂O₂RR, we successfully came up with Koutecky-Levich expression of elemental steps of the ORR along with the determination of their kinetic parameters. Thus proposed comprehensive and universal model is able to describe quantitative contributions of individual steps of the ORR to the electrochemical precipitation of metal oxide thin films under given overpotential and diffusion flux.