In Situ Derivation of Dual-Active Co/CoO Heterojunction Nanoarrays for Synergistic Catalytic NH₃ Synthesis

Electrocatalytic nitrate reduction reaction (NO3RR) is a promising alternative to the conventional Haber–Bosch NH3 synthesis process. To address the incompatibility between high yield and high selectivity for NH3 synthesis under alkaline conditions, we report an in situ-derived Co/CoO heterojunction nanoarray bifunctional catalyst (Co@CoO/NF). The dual-site catalytic configuration accelerates the H–OH bond cleavage of H2O without reducing NO3– active site utilization, providing sufficient protons for NO3– hydrogenation and suppressing competitive hydrogen evolution at high overpotentials. Density functional theory calculations together with kinetic analysis reveal that electron migration between heterogeneous interfaces enhances Co@CoO/NF’s targeted adsorption: CoO sites favor NO3– adsorption, while Co sites favor H2O adsorption. The synergistic effect of defect sites from annealing and the in situ-derived nanoarray structure from metal–organic frameworks improve electron transfer and mass transport efficiency, lowering the activation energy barrier for NO3RR and promoting intermediate migration along the interface. These benefits give Co@CoO/NF the improved NH3 yield rate of 129.83 mg h–1 cm–2, a Faradaic efficiency of 96.49%, and a current density of 1637.43 mA cm–2, along with long-term stability over a continuous 40 h cycle. This study presents a practical strategy for a dual active site catalyst electrode with enhanced NO3RR activity and selectivity, offering promising prospects for new energy applications.