Heterostructural Coupling of Phase-Modulated NiMoO₄ with NiCo₂O₄ for Enhanced Urea Electro-Oxidation

Developing catalysts with high efficiency and low cost for the urea oxidation reaction (UOR) is attractive but challenging. Herein, relying on the high catalytic activity of Ni, low overpotential of Co, and superior antipoisoning resistance of Mo, a NiMoO₄-NiCo₂O₄ p-p heterojunction is constructed via a hydrothermal strategy followed by calcination. Interestingly, phase transformation of β-NiMoO₄ to α-NiMoO₄ occurs when a heterojunction is generated. The unique structure of NiMoO₄-NiCo₂O₄ enables faster charge transfer capability, greater active site availability, lower impedance, and reduced activation energy. Thus, a much better catalytic performance for the UOR is triggered when employing NiMoO₄-NiCo₂O₄ as a catalyst. A specific current density of 1306 mA cm^-2 mg^-1 (at 0.6 V vs Hg/HgO) is achieved for NiMoO₄-NiCo₂O₄, which is much larger than that for NiMoO₄ and NiCo₂O₄. Potential-dependent impedance analyses unveil that Ni³⁺ should be active sites and both indirect and direct urea oxidation paths should be accelerated on NiMoO₄-NiCo₂O₄. Phase transformation of β-NiMoO₄ to α-NiMoO₄ is vital. Making the energy bands of NiCo₂O₄ and NiMoO₄ match better, promoting Ni³⁺ formation, facilitating active sites exposure, and reducing alkalinity to enhance antipoisoning capacity all make sense. This work stresses the importance of crystal phases in developing heterojunctions with high catalytic performance.