Electrocatalytic performance of different cobalt molybdate structures for water oxidation in alkaline media

Abstract

Cobalt molybdates with different crystalline structures, i.e., α, β, and hydrated (H)-CoMoO₄, were synthesized, and their electrocatalytic activities were thoroughly examined for catalyzing the oxygen evolution reaction (OER) in alkaline media. The material characteristics were associated with the electrocatalytic properties by evaluating the CoMoO₄ crystal structures (XRD and Raman), morphologies (TEM), and electrochemical features (electrochemically active surface area, roughness factor, electrochemical impedance, Tafel analysis, and controlled-current electrolysis). These combined findings revealed that the electrocatalytic performance is greatly influenced by the crystalline structures of CoMoO₄, following the order α-CoMoO₄ > H-CoMoO₄ > β-CoMoO₄. The H-CoMoO₄ catalysts crystallized in the triclinic space group, P (#2), with Z = 4. On the other hand, the α- and β-CoMoO₄ catalysts exhibited a monoclinic structure, C2/m (#12), with Z = 8. In the OER experiments, α-CoMoO₄ showed an overpotential of 0.43 ± 0.05 V compared to the 0.51 ± 0.05 V and 0.56 ± 0.04 V exhibited by the H-CoMoO₄ and β-CoMoO₄ catalysts, respectively, to achieve 10 mA cm⁻². All CoMoO₄ structures displayed stability for at least 6 h at a controlled current density of 10 mA cm⁻². Finally, computational simulations indicate that the coexistence of Co and Mo ions in edge-shared octahedral sites of α-CoMoO₄ may favor the interaction between the O atom of the water molecule and the metal adsorption sites due to its surface being electronically less dense than β- and H-CoMoO₄ surfaces, thus resulting in its higher performance for OER.