Chemical and structural origin of lattice oxygen oxidation in Co–Zn oxyhydroxide oxygen evolution electrocatalysts

Abstract

The oxygen evolution reaction (OER) is a key process in electrochemical energy conversion devices. Understanding the origins of the lattice oxygen oxidation mechanism is crucial because OER catalysts operating via this mechanism could bypass certain limitations associated with those operating by the conventional adsorbate evolution mechanism. Transition metal oxyhydroxides are often considered to be the real catalytic species in a variety of OER catalysts and their low-dimensional layered structures readily allow direct formation of the O–O bond. Here, we incorporate catalytically inactive Zn2+ into CoOOH and suggest that the OER mechanism is dependent on the amount of Zn2+ in the catalyst. The inclusion of the Zn2+ ions gives rise to oxygen non-bonding states with different local configurations that depend on the quantity of Zn2+. We propose that the OER proceeds via the lattice oxygen oxidation mechanism pathway on the metal oxyhydroxides only if two neighbouring oxidized oxygens can hybridize their oxygen holes without sacrificing metal–oxygen hybridization significantly, finding that Zn0.2Co0.8OOH has the optimum activity. Oxygen evolution is one half of the overall water splitting reaction to produce hydrogen. Although this reaction is well studied, there remains debate over the particulars of the catalytic mechanism. Here, the authors investigate Co–Zn oxyhydroxide electrocatalysts, and suggest that the mechanism depends on the amount of Zn2+ they contain.