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

We herein report a series of nanocrystalline Ni-doped Co3O4: Co3-xNixO4 (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 Co3O4 with no separated peaks related to Ni/NiOx 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 Co3O4: Co3-xNixO4 are active for oxygen evolution reaction (OER) and also shows that low amount of nickel doping in Co3O4 can remarkably enhance OER activity in neutral, alkaline and buffer (pH-7) electrolytes. Out of all compositions, 0.5% Ni-doped Co3O4 (CO2.985Ni0.015O4) seems to be more active than Co3O4 in terms of both current density and onset potential in K2SO4 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 Co3O4 and 0.5% Ni-doped Co3O4 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 Co3O4 (CO2.985Ni0.015O4) and hence gives the highest OER activity.