Disordered spinel cobalt oxide electrocatalyst for highly enhanced HER activity in an alkaline medium

Abstract

Water splitting electrolysis is a promising sustainable methodology to produce clean and green hydrogen fuel, which is considered as a prominent replacement for conventional fossil fuels. Developing highly active, stable and low-cost electrocatalysts plays a critical role for achieving efficient production of hydrogen by electrolysis for practical applications. Herein, we report the synthesis of Co₃O₄ spinel oxides by simple calcination of commercially available cobalt salts at different temperatures (400, 600 and 800 °C) and the disordered structure induced highly enhanced hydrogen evolution reaction (HER) in an alkaline medium (1.0 M KOH). Powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) indicated the formation of a normal spinel Co₃O₄ structure at 400 (Co₃O₄-400) and 600 °C (Co₃O₄-600) calcination, whereas disordered Co₃O₄ at 800 °C (Co₃O₄-800). HR-TEM analysis revealed the formation of spherical crystalline particles in Co₃O₄-400 and featureless aggregated crystalline particles at higher temperatures. The d-spacing analysis using PXRD and the lattice fringe pattern showed a decrease of the spacing with increasing calcination temperature, which suggested higher strain for Co₃O₄-800. The electrocatalytic HER activity of Co₃O₄-400 and Co₃O₄-600 in an alkaline medium showed moderate activity. Co₃O₄-400 and Co₃O₄-600 required an overpotential of 293 and 240 mV, respectively, to achieve a geometric current density of 10 mA cm⁻². In contrast, disordered Co₃O₄-800 required a very low overpotential (93 mV) for producing the geometric current density of 10 mA cm⁻². Electrochemical impedance studies showed faster kinetics and charge transfer for disordered spinel Co₃O₄-800 and supported the enhanced HER activity. The electrocatalytic stability studies for Co₃O₄-800 were tested up to 48 h at 100 mA cm⁻² current density and confirmed the excellent stability. Thus, a low-cost earth abundant HER electrocatalyst was fabricated by a simple method.