Optimizing local charge distribution of metal nodes in bimetallic metal–organic frameworks for efficient urea oxidation reaction

Abstract Electrocatalytic urea oxidation reaction (UOR) presents lower thermodynamic potential than oxygen evolution reaction (OER), thus exhibiting promising potential to enhance the efficiency of overall water splitting. However, due to the intrinsically sluggish six-electron transfer process, the efficiency of UOR is still not satisfied. In response, we synthesized a bimetallic NiFe-MIL-53-NH2 with superior activity toward UOR, which only needs a low potential of 1.398 V vs. RHE to obtain 50 mA cm-2 in 1.0 M KOH with 0.33 M urea, much lower than that in 1.0 M KOH (1.721 V vs. RHE). Furthermore, NiFe-MIL-53-NH2 presents significantly more excellent UOR activity compared to its monometallic counterparts. The TOF value in NiFe-MIL-53-NH2 (0.16 s-1) at 1.4 V vs. RHE is about 133- and 246-folds higher than that Ni-MIL-53-NH2 (1.2×10-3 s-1) and Fe-MIL-53-NH2 (6.5×10-4 s-1), respectively. XPS characterization discloses that the incorporation of Fe into Ni-MIL-53-NH2 framework optimizes the local charge distribution of metal nodes, leading to the formation of electrophilic Ni3+ and nucleophilic Fe3+ species, which can absorb electron-donating -NH2 and electron-withdrawing C=O groups in urea, respectively, and therefore result in excellent UOR. This simple strategy of regulating local charge distribution of active species by fabricating bimetallic MOFs provides a new strategy and direction to explore other highly efficient UOR electrocatalysts.