Optimization of Ascorbic Acid Contents in Preparation of Cobalt Oxide for Highest Oxygen Evolution Activity

Abstract

The activity of the cobalt oxide (Co3O4) material towards the oxygen evolution reaction is governed by cobalt oxidation states present at the surface. The fabrication methodology decides the concentration of Co+2 and Co+3 at the Co3O4 surface. We used ascorbic acid as a fuel (reductant) for cobalt oxide synthesis. On the variation of the precursor ratio (cobalt nitrate to ascorbic acid) 1:0.5, 1:1, 1:2, and 1:4, the Co+3 and Co+2 content at the surface increases and decreases, while the oxygen vacancy decreases with increasing the ascorbic acid content. The combined effect of oxygen vacancy with Co+2 and Co+3 contents shows the maxima activity for 1:1 sample. Further, low oxygen vacancy with moderate Co+2 and Co+3 contents show low activity compared to the 1:1 sample. Also, high oxygen vacancy with low Co+2 and Co+3 contents show low activity compared to the 1:1 sample. Thus, high Co+2 and Co+3 contents with moderate oxygen vacancy give the highest activity towards the oxygen evolution reaction. The Tafel analysis further supports the above with the help of the Tafel slope. The Tafel slope of the 1:1 sample shows 55.12 mV/dec. Finally, the impedance analysis supports the above by showing the lowest value for the adsorption resistance of the 1:1 sample. Thus, the above study showed that at the ascorbic acid's optimized value, the cobalt oxide showed the highest activity via the moderate oxygen vacancy and highest Co+2 and Co+3 content.