Porous and defective NiCo2O4 spinel derived from bimetallic NiCo-based Prussian blue analogue for enhanced hydrogen production via the urea electro-oxidation reaction

The efficient hydrogen production via overall water splitting is seriously restrained by the slow kinetics of the oxygen evolution reaction (OER). The substantially low potential for the urea electro-oxidation reaction (UOR) can tackle this problem by replacing the OER at the electrolyzer anode. Herein, the NiCo₂O₄ spinel with rich oxygen vacancies (O_v) and embedded within mesoporous carbon network (O_v-NiCo₂O₄@mC) was derived from NiCo-based Prussian blue analogous (NiCo PBA) and exploited as the bifunctional electrocatalyst of the UOR and OER for boost the hydrogen production via the overall water splitting. Benefiting to the regular skeleton and abundant dual metal sites of NiCo PBA, the derived O_v-NiCo₂O₄@mC comprises abundant oxygen vacancies, lattice defects, and adjustable electron structure. These features afford O_v-NiCo₂O₄@mC the improved UOR ability, showing the potential of 1.33 V versus reversible hydrogen electrode (RHE) at the current density of 10 mA cm⁻², along with a small Tafel slopes of 31 mV dec⁻¹, remarkably lower than that of the OER (1.51 V versus RHE, Tafel slope = 85 mV dec⁻¹). The UOR performance of O_v-NiCo₂O₄@mC substantially outperforms to most of the reported Co and/or Ni-based electrocatalysts. Impressively, the assembled two-electrode urea-assisted overall water splitting device shows the small voltage for the hydrogen production (1.43 V versus RHE) and good long-term stability. The present work can provide a new alternative for the efficient hydrogen production using the MOFs-derivatives.