Studies of oxygen electrochemistry on spinel oxides

Abstract

Electrochemical studies of the spinels, Co3O4 and NiCo2O4, in alkaline media were conducted and show that the products and the oxygen reduction mechanisms vary. The 4e- reduction strongly predominates at NiCo2O4, a substantial amount of the 2e- reduction product (H2O2), 43%, is formed at the cobalt spinel. NiCo2O4 is a significantly better catalyst than Co3O4 in terms of both the overpotential for reduction and its limiting current density. The differences come from the enhanced rate of O – O bond cleavage early in the reduction sequence at the mixed spinel.

Based on the full physical, spectroscopic and electrochemical studies of a wide range of Mn content (MnxCo3-xO4, 0.0 ≤ x ≤ 2.0) in spinel cobalt oxide, the phase transition (from cubic to tetragonal), particle size, surface area, crystallinity and electrochemical activities towards the ORR can be tuned with Mn content in spinel cobalt oxide. The Mn ions are in oxidation state +3 and they have tendency to occupy tetrahedral sites rather than octahedral sites in the spinels. In terms of the highest limiting current and lowest onset potential for oxygen reduction, cubic phase MnCo2O4 (x = 1.0) possesses the highest catalytic activity amongst Mn doped spinels and follows the 4e- reduction mechanism with early cleavage of the O – O bond.

Three different synthesis methods for MnCo2O4 (co-precipitation, thermal decomposition and hydrothermal method) and the influence of conditions within hydrothermal method were investigated. The preparation conditions and methods were found to affect the morphology, phase, crystallinity, and ORR activity of the catalyst. Co-precipitation produced the catalyst with the highest surface area, smallest particle size, highest crystallinity and the highest ORR activity.

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ORCID for Andrea E. Russell: orcid.org/0000-0002-8382-6443

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