Spin State Tuning of the Octahedral Sites in Ni–Co-Based Spinel toward Highly Efficient Urea Oxidation Reaction

Nickel–cobalt spinel oxide is an efficient electrocatalyst for urea oxidation reaction (UOR) due to its low cost and high activity. The eg orbital occupancy of transition metal ions in spinel oxides can determine the oxygen evolution reaction activity. However, the eg filling, the catalyzing active sites, and the related UOR mechanisms in spinel catalysts are still unknown. Here, from the perspective of electronic structure and metal redox capability, we explored the composition dependence of urchin-like NixCo3–xO4 (x = 0, 0.75, 0.86, 1, and 1.20) spinel in UOR to investigate the role of octahedral nickel atoms in the spinel structure. The results indicated that the UOR activity could be promoted by embedding more Ni ions into the octahedral sites, allowing the accumulation of a large number of nickel redox equivalents and an increase in the level of the low-spin state of Ni3+. Furthermore, a near-unity occupancy of the eg orbital of Ni ions could enhance the intrinsic UOR activity of spinel catalysts. Urea oxidation proceeds through the electrochemical oxidation mechanism and direct urea electro-oxidation reaction pathway, where NiOOH species act as active sites in catalyzing the process. This work offers a guideline for designing and synthesizing non-precious metal-coupled Ni-based catalysts with appropriate redox properties for urea oxidation applications.