Nano-crystalline Co₃O₄ and a series of Ni-doped Co₃O₄; Co_3-xNi_xO₄ (0.0075 £ x £ 0.30) have been synthesized using solution combustion method. Complete characterization is carried out using XRD, TEM and BET surface area analysis. XRD shows pure nano-crystalline phase of Co₃O₄ with no peaks related to Ni or nickel oxide confirming that Ni is not present as a separated phase on the surface and has been substituted in the lattice. TEM studies for as prepared samples (400 ^oC) show that particles of 0.5% Ni doped Co₃O₄ (Co_2.985Ni_0.015O₄) are around 60 nm in size. Calcination of Co_2.985Ni_0.015O₄ at 600 °C for 10 hours results into well-defined hexagonal morphology which could be seen both by TEM and SEM. After calcination, particle size becomes 70 nm. Estimated BET surface area for Co_2.985Ni_0.015O₄ (calcined at 600°C) is 10m²/g.

Detailed electrochemical studies have been carried out in basic, neutral buffer and neutral electrolyte medium. Both Co₃O₄ and Co_3-xNi_xO₄ were found active for oxygen evolution reaction (OER) with an interesting behavior in doped compound. It is found that lower nickel doping significantly enhances the OER activity in all the mediums; basic, neutral and neutral buffer. Figure 1 depicts that just 0.5% Ni-doped Co₃O₄ (Co_2.985Ni_0.015O₄) is far more active than Co₃O₄ both in terms of current density and onset potential in K₂SO₄ medium. This enhancement nevertheless decreases once the doping concentration reaches beyond 0.5%. Figure 2 shows that OER activity (chronoamperomertic current) of Co₃O₄ increases after 0.25% Ni doping peaking at 0.5% and thereafter the reduction in the activity takes place. 1% Ni doped Co₃O₄ is less active than 0.25% Ni doped Co₃O₄. Once the doping concentration is 2.5% there is no enhancement in the activity compared to Co₃O₄. In the same way no effect is seen at 5 and 10% doping of Ni and the activity is similar to parent Co₃O₄. Same trend is observed in all other medium. Interestingly, in phosphate buffer solution (PBS) Co₃O₄ and 0.5% Ni-doped Co₃O₄ show OER at near thermodynamic potential i.e at a very low over potential. Similar effect was observed by Nocera and co-workers on cobalt oxide ^1-2. This conclusively states that Co₃O₄ and its doped variant form similar type of in situ active sites as noticed Nocera and co-workers.

References : 1. M.W. Kanan, D.G. Nocera, Science 321 (2008) 1072-1075. 2. Y. Surendranath, M.W. Kanan, D.G. Nocera, Journal of the American Chemical Society 132 (2010) 16501-16509