Temperature Ramp Strategy for Regulating Oxygen Vacancies in NiCo2O4 Nanoneedles Towards Enhanced Electrocatalytic Water Splitting.

Abstract

The oxide-based systems often suffer from higher overpotentials compared to transition metal sulphides and phosphides for the electrochemical hydrogen evolution reaction. Interestingly, the generation of oxygen vacancy has been seen as strategy for further activating transition metal oxides (NiCo2O4 as a model system) for electrochemical watersplitting. Herein, we employ the temperature ramp strategy (ambient air calcination) for the generation of oxygen vacancies in NiCo2O4 (NCO) towards tuning of electrocatalytic enhancements. The NiCo2O4 synthesized at temperature ramp rates of 2 (NCO-2), 5 (NCO-5), and 10ºC/ min (NCO-10) depicts contrasting structural features and varying Ni:Co:O surface composition. The decrease in the crystallite size and converse trend in the particle strain were observed from NCO-2 to NCO-10. Interestingly, the surface Ni:Co:O ratios of 1:0.78:3.6, 1:0.81:3.3, and 1:0.69:2.8 for NCO-2, NCO-5, and NCO-10, respectively, were observed. The reduced relative oxygen ratio in the latter implies generation of ample amount of oxygen vacancy defects. HER performance depicts a consistent trend with enhanced oxygen defect concentration with the overpotential requirement of 700, 647, and 597 mV for NCO-2, NCO-5, and NCO-10, respectively, for the generation of a cathodic current of 25 mA cm-2. The same trend in an electrocatalytic enhancement is observed for other cathodic currents.