Perfectly Hexagonal Sponge-Like NiO-NiCo2O4 with Rich Electromicrostructural Physiognomies for High-Efficiency Electrocatalytic Urea Oxidation

In order to design high-efficiency electrocatalysts for the development of urea oxidation reaction (UOR)-based energy conversion and storage systems, herein, a very facile and kinetically controlled material growth strategy has been strategized to prepare extremely uniform and perfectly hexagonal sponge-like NiO-NiCo₂O₄ with high BET surface area (126 m² g⁻¹), monomodal distribution of mesopores (~3.9 nm), hierarchical surface as well as matrix porosity, mixed-phase lattice structure, thorough atomic non-stoichiometry and multiple valency of Ni and Co (i. e. Ni²⁺, Ni³⁺, Co²⁺ and Co³⁺). The potential of NiO-NiCo₂O₄ is thoroughly explored for electrocatalytic UOR in alkaline electrolyte medium. The in-depth electrochemical analyses demonstrate rich redox reversibility, high UOR current density, very-low charge transfer and series resistance, and typical Warburg response indicative of facilitated diffusion of electrolyte ions during electrocatalytic UOR. Furthermore, the NiO-NiCo₂O₄ requires lower overpotential for effective UOR and exhibits minimal current loss during electrocatalytic UOR for prolonged duration. Proposedly, the multiple oxidation states of Ni and Co in NiO-NiCo₂O₄, combined with its rich physicoelectrochemical physiognomies, offer lowly-impeded electrolyte ion intercalation-deintercalation, good electronic conductivity, higher number of accessible redox active sites, facile adsorption of urea on the electrocatalytic sites and inhibition in the blockage of active sites by side products to augment the overall UOR kinetics. The optimized approach presented in this study is poised to advance the catalyst systems for UOR, which will lead to the development of high-efficiency urea-based energy conversion and storage systems for prospective integration in contemporary electronic architectures.