Array-preserving in situ growth of CuMoO4 on CuO nanowires for built-in-field-enhanced nitrate reduction

Electrochemical nitrate reduction to ammonia (NO3RR) offers a sustainable route to couple NO3– remediation with decentralized NH3 production. However, achieving high activity, selectivity, and durability under practical conditions still remains challenging. Herein, we develop a self-supported CuMoO4/CuO nanowire-array heterostructure on Cu foam, where the work-function mismatch between CuMoO4 and CuO establishes a built-in electric field (BIEF) at the oxide–oxide interface. Benefiting from this interfacial field, the CuMoO4/CuO electrode delivers larger Faradaic efficiency (>95%), higher selectivity (~98.65%) and faster NH3 yield than CuO over a wide potential window with excellent cycling stability. Operando electrochemical impedance spectroscopy with distribution of relaxation times analysis reveals reduced charge-transfer and mass-transport resistances on CuMoO4/CuO, confirming that the BIEF accelerates key electrochemical steps. In situ attenuated total reflection infrared combined with density functional theory further show that the BIEF can lower the barrier of the *NO → *NOH rate-determining step, and suppress the competing hydrogen evolution reaction, leading to enhanced NO3RR activity. This work demonstrates that interfacial BIEF engineering is an effective strategy to boost NO3–-to-NH3 conversion, which can serve as a general design principle for advanced multi-electron electrocatalytic systems.