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.