Cu-loading-modulated NiFe-based composite catalysts for high-efficiency nitrate electroreduction to ammonia

Introduction: Electrocatalytic nitrate reduction reaction (NO3RR) is a sustainable route for ammonia (NH3) synthesis, yet it suffers from sluggish kinetics and low selectivity due to the eight-electron transfer process. Although Cu-based catalysts efficiently activate N O 3 − and mediate the deoxygenation step ( N O 3 − → N O 2 − ), while Ni/Fe-based hydroxides excel in water dissociation and N O 2 − hydrogenation, constructing a kinetically matched multimetallic tandem system remains challenging. Materials and methods: Here, a series of spherical Cu2O/NiFeCu(OH)x composites were synthesized via a one-pot hydrothermal method by tuning the Cu(NO3)2 loading (×0.17–×6 relative to the optimized recipe). The catalysts were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM)/high-resolution TEM (HRTEM), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma optical emission spectrometry (ICP-OES). Electrocatalytic performance was evaluated in 0.1 mol L−1 NaNO3/0.5 mol L−1 Na2SO4 using cyclic voltammetry (CV), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), and chronoamperometry, with NH3 quantified by the indophenol blue method and 1H nuclear magnetic resonance (NMR). Results: The optimized Cu2O/NiFeCu(OH)x (1× Cu loading) delivers an NH3 yield of 8.86 mg h−1 mgcat−1 and a Faradaic efficiency (FE) of 94% at −0.6 V vs. reversible hydrogen electrode (RHE), outperforming control samples (NiFe(OH)x, NiCu(OH)x, FeCu(OH)x) and many reported NO3RR catalysts. The catalyst maintains stable activity for 12 h at −0.4 V vs. RHE and ~90% FE over five cycles. Excess Cu loading accelerates ( N O 3 − → N O 2 − but causes N O 2 − accumulation owing to kinetic mismatch, whereas insufficient Cu limits the deoxygenation step. Electrochemical active surface area (ECSA) and charge-transfer resistance (Rct) analyses confirm that performance enhancement originates from composition modulation rather than surface-area variation. Conclusions: This work demonstrates that precise Cu-loading modulation in NiFe-based architectures achieves kinetic matching between Cu-mediated deoxygenation and Ni/Fe-mediated hydrogenation, enabling efficient tandem NO3RR to NH3. The strategy provides a mechanistic guideline for designing multimetallic LDH-type electrocatalysts for nitrogen-cycle applications.