Single-Phase Spinel NiCo₂O₄ as Highly Active and Stable Electrocatalysts for Urea Oxidation Reaction in Urea Electrolysis

Exploring and designing a stable and active catalyst for the urea electro-oxidation reaction (UOR, CO-(NH₂)₂ + 6OH^- → CO₂ + N₂ + 5H₂O + 6e^-) is crucial for the long-term sustainability of ecological systems and clean energy production. We found that spinel NiCo₂O₄ is a stable and active electrocatalyst for UOR at a relatively low anodic potential without triggering the competing oxygen evolution reaction (OER). A urea electrolysis cell (CO-(NH₂)₂ + H₂O → CO₂ + N₂ + 2H₂) utilizing a spinel NiCo₂O₄ anode and a commercial Pt cathode was further characterized through galvanostatic polarization tests, demonstrating excellent structural stability at various current densities. Post-mortem analysis of long-term urea electrolysis measurements suggested that NiCo₂O₄ electrocatalysts maintained a stable spinel structure. However, redistribution of Ni³⁺ to Ni²⁺ valence on the catalyst surface was observed, in contrast to the intact Co valence, indicating that (i) Ni sites are active toward urea adsorption and sequential electro-oxidation; (ii) while urea oxidation proceeds primarily through the direct electro-oxidation mechanism, chemical reactions between the Ni³⁺ site and urea occur during long-term electrochemical UOR operation. Density functional theory (DFT) simulations were used to calculate the adsorption energies of urea molecules on NiO, Co₃O₄, and NiCo₂O₄, revealing the importance of regulating the configuration of adsorbed urea molecules on the NiCo₂O₄ surface.