Surface oxidation of cobalt carbonate and oxide nanowires by electrocatalytic oxygen evolution reaction in alkaline solution

Abstract

The electrocatalytic water electrolysis is the most eco-friendly technique for hydrogen generation, which is governed by the electrode reaction kinetics involving cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) in common alkaline electrolytes. Cobalt oxide (Co3O4) and related compounds are the most efficient OER catalysts, replacing the noble metals. In this work, the surface oxidations of the cobalt carbonate (Co(CO3)0.5OH·0.11H2O) and Co3O4 nanowires during the OER are carefully investigated by contrasting the polarization curves, Tafel plots, and x-ray photoelectron spectroscopy (XPS) spectra, before and after the 1000th cyclic voltammetry (CV) cycling in 1 M KOH alkaline solution. The overpotentials required to reach a current density (j) of 20 mA cm−2 (η 20) are estimated to be 313 mV for the 300 °C-calcined Co3O4, 350 mV for the 400 °C-calcined Co3O4, 365 mV for the 500 °C-calcined Co3O4, and 373 mV for the cobalt carbonate (Co(CO3)0.5OH·0.11H2O). The Tafel slope of cobalt carbonate (Co(CO3)0.5OH·0.11H2O) nanowires is the highest at 93 mV dec−1, while it is measured to be 57 mV dec−1 for the 300 °C-calcined Co3O4, 47 mV dec−1 for the 400 °C-calcined Co3O4, and 79 mV dec−1 for the 500 °C-calcined Co3O4. As a result, the oxidation from Co2+ to Co3+ within Co3O4 during the OER is detected, which improves the OER activity. On the other hand, the formation of cobalt hydoxide is found on the surface of the Co3O4 nanowires during the OER in alkaline solution, which decreases the OER activity. For the surface oxidation of the cobalt carbonate (Co(CO3)0.5OH·0.11H2O) nanowires, the increase in the amounts of Co3+ and oxygen vacancy and the formation of O-C-O and carbonates are found, which highly enhance the OER activity. These findings indicate that the surface redox kinetics during the electrocatalytic reactions should be considered important in order to enhance the electrocatalytic activity, and furthermore can provide insight into future challenges in designing advanced electrocatalysts.