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

Oxygen reduction reaction (ORR) in aqueous zinc chloride solution results in an electrodeposition of highly crystallized zinc oxide (ZnO) thin fi lms. 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 fi lms under given overpotential and diffusion fl ux.