Optimization of Ascorbic Acid in the Synthesis of Cobalt Oxide for Enhanced Electrocatalytic Activity toward the Oxygen Evolution Reaction

Abstract

In this work, we are reporting the combustion synthesis of Co3O4 nanoparticles (NPs) and investigation of the influence of the oxidizing agent [Co(NO3)2] and fuel content (ascorbic acid) on the microstructural and electrochemical characteristics of the NPs. Co3O4 NPs were prepared by varying the relative amount of cobalt nitrate to ascorbic acid in ratios of 1:0.5, 1:1, 1:2, and 1:4, and labeled as S1, S2, S3, and S4 respectively. X-ray diffraction data confirm the formation of the cubic phase of Co3O4, and transmission electron micrographs reveal the formation of fairly dispersed particles in the cases of samples S1 and S2 with average particle sizes of 24.5 ± 3.6 and 20.4 ± 1.2 nm and polydispersity indices of 14.7 and 6.5%, respectively. The X-ray photoelectron spectroscopy analysis reveals that the concentration of Co3+ ions at the surface of Co3O4 NPs increases from sample S1 to sample S2 and thereafter decreases in the case of samples S2 to S3, and S4. On the other hand, the oxygen vacancy decreases with an increase in the ascorbic acid content, that is, from sample S1 to sample S4, the oxygen vacancy decreases. Sample S1 having a high oxygen vacancy with low Co3+ possesses a low activity due to the decrease in conductivity caused by excess oxygen vacancies. Further, samples S3 and S4 showed low activity due to the low oxygen vacancy and low Co3+ content, having a low electrochemical surface area. Finally, sample S2 having the optimum oxygen vacancy with the highest Co3+ content showed the highest activity due to the high conductivity and the highest electrochemical surface area. The Tafel slope for sample S2 is found to be 54.46 mV/dec, denoting the highest activity. The electrochemical impedance analysis revealed that sample S2 exhibits the lowest polarization resistance during the oxygen evolution reaction test, showing enhanced kinetics for the oxygen evolution reaction.