Three dimensionally ordered mesoporous hydroxylated NixCo3-xO4 spinels for oxygen evolution reaction: on the hydroxyl-induced surface restructuring effect
Surface restructuration upon potential cycling of three dimensionally ordered NixCo3-xO4 spinels for oxygen evolution reaction (OER) in alkaline medium is studied using structural, spectroscopic and electrochemical techniques. It was shown that the intrinsic activity of the different catalysts depends on the incorporated amount of nickel and surprisingly correlates with the CoIII/CoIV peak potential. The electrochemical activity for OER is amazingly improved upon potential cycling. It was observed that potential cycling induces an increase of active sites up to 45% on the most effective electrocatalyst. This unexpected increase in activity is very pronounced and becomes stable after 30 voltammetric cycles. Such a phenomenon is explained by the formation of a layered mixed nickel/cobalt oxyhydroxyde active site whose oxidation potential is related to the nickel amount in the catalyst. The formation of this layer is promoted by the surface hydroxylation degree of non-cycled catalysts. In these catalysts, nickel modulates the electronic properties of the active site, which modifies adsorption energies of key oxygenated intermediates. The synthesis route proposed herein allows an efficient way for obtaining high specific surface areas as well as highly hydroxylated surfaces, the latter being the key factor in the enhancement of the electrocatalytic activity of nickel cobaltites.