Enhanced oxygen evolution activity of Co3−xNixO4 compared to Co3O4 by low Ni doping

We herein report a series of nanocrystalline Ni-doped Co<inf>3</inf>O<inf>4</inf>: Co<inf>3−x</inf>Ni<inf>x</inf>O<inf>4</inf> (0.0075 ≤ x ≤ 0.30) with a nickel doping percentage from 0.25 to 10 atomic percent synthesized using solution combustion method. These oxides are characterized by XRD and show pure nanocrystalline phase of Co<inf>3</inf>O<inf>4</inf> with no separated peaks related to Ni/NiO<inf>x</inf> and confirms that Ni has been substituted in the lattice. TEM results indicate that the morphology and size of all the compounds are similar. Electrochemical measurements indicate that Co<inf>3</inf>O<inf>4</inf> and Co<inf>3−x</inf>Ni<inf>x</inf>O<inf>4</inf> are active for oxygen evolution reaction (OER) and also shows that low amount of nickel doping in Co<inf>3</inf>O<inf>4</inf> can remarkably enhance OER activity in neutral, alkaline and buffer (pH-7) electrolytes. Out of all compositions, 0.5% Ni-doped Co<inf>3</inf>O<inf>4</inf> (Co<inf>2.985</inf>Ni<inf>0.015</inf>O<inf>4</inf>) seems to be more active than Co<inf>3</inf>O<inf>4</inf> in terms of both current density and onset potential in K<inf>2</inf>SO<inf>4</inf> medium. The enhancement in terms of OER activity, however, decreases until the doping concentration reaches beyond 0.5%. Phosphate buffer solution (PBS) studies reveal that Co<inf>3</inf>O<inf>4</inf> and 0.5% Ni-doped Co<inf>3</inf>O<inf>4</inf> show OER at near thermodynamic potential. Detailed x-ray photoelectron spectroscopy (XPS) studies have indicated that surface oxygen (lattice oxygen) concentration is an important factor in deciding the OER activity which is highest for 0.5% Ni doped Co<inf>3</inf>O<inf>4</inf> (Co<inf>2.985</inf>Ni<inf>0.015</inf>O<inf>4</inf>) and hence gives the highest OER activity.