Nanocrystalline Co3-XNixO4 with Low Ni Concentration: Enhanced Oxygen Evolution Activity Compared to Co3O4

Monday, 29 May 2017: 14:20
Grand Salon D - Section 19 (Hilton New Orleans Riverside)
A. Singhal (School of Engineering and Applied Science, Ahmedabad Uni.) and A. Bisth (IIT Gandhinagar)
Nano-crystalline Co3O4 and a series of Ni-doped Co3O4; Co3-xNixO4 (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 (Co2.985Ni0.015O4) are around 60 nm in size. Calcination of Co2.985Ni0.015O4 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 Co2.985Ni0.015O4 (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 Co3-xNixO4 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 (Co2.985Ni0.015O4) 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