Analysis of oxygen reduction reaction pathways on Co3O4, NiCo2O4, Co3O4–Li2O, NiO, NiO–Li2O, Pt, and Au electrodes in alkaline medium

Abstract

The aim of present investigation is to theoretically identify parameters which are the root reason for differentiation of oxygen reduction reaction (ORR) on two different pathways with formation on oxide electrode materials either ОН− or НО2− ions. Theoretical analysis of the ORR data on cobalt and nickel oxides was performed. New original ORR mechanism was proposed. The mechanism is based on a concept of multistage electrochemical process with a slow chemical reaction stage. Triad of requirements describing properties of electrode materials which fully determine pathways of ORR was formulated. On a specific electrode material ORR proceeds with formation of ОН− ions when three following requirements are simultaneously fulfilled. First, oxide or hydroxide with atoms, which could change an effective positive charge as a result of electrochemical process, shall be present on electrode material surface at ORR range of potentials. Second, the electrode material shall have a surface crystal structure which allows formation of oxygen molecule “bridge” between two surface atoms with effective positive charge (the “bridged” chemical structure may be described as a surface binuclear oxide nanocluster). Third, electrochemical potential of transfer of oxide atoms with effective positive charge from oxidized to reduced state shall be more positive than the potential of formation of НО2− ions.Analysis of structure of Co3O4, NiCo2O4 and Co3O4–Li2O oxides indicated that on these oxides all three requirements are fulfilled. Therefore on Co3O4, Co3O4–Li2O and NiCo2O4 electrodes ORR proceeds via pathway with formation of ОН− ions. On NiO and NiO–Li2O oxides only first and second requirements are fulfilled. Therefore ORR proceeds with formation of НО2− ions.The triad of requirements formulated in a course of analysis of ORR on oxide electrodes was successfully used in cases with single-crystal Pt and Au electrodes. On Pt and Au (100) electrodes all three requirements are fulfilled, therefore ORR proceeds via pathway with formation of ОН− ions. On Au (111) and Au (110) electrodes the first condition is not fulfilled, therefore on such electrodes НО2− ions are formed.