The electrochemical nitrate reduction reaction (NITRR) offers a dual solution to environmental and energy challenges by converting nitrate pollutants into valuable ammonia (NH₃), a critical feedstock for fertilizers and a carbon-free energy carrier (Zhang et al., 2022; Zhou and Sun, 2022; Fu et al., 2023). Traditional NH₃ production via the Haber–Bosch process is energy-intensive, accounting for ∼1%–2% of global energy consumption and relying on fossil fuel-derived hydrogen (Cheng et al., 2018; Ma et al., 2020; Sun et al., 2021). In contrast, NITRR powered by renewable electricity operates under ambient conditions and utilizes nitrate—a widespread water contaminant from agricultural and industrial activities—as the nitrogen source (Atrashkevich et al., 2022; Xiong et al., 2024). However, the NITRR involves a complex eight-electron/nine-proton transfer process (NO₃ ⁻ + 9H⁺ + 8e⁻→ NH₃ + 3H₂O), which suffers from sluggish kinetics, a competing hydrogen evolution reaction (HER), and byproduct formation (e.g., NO₂ ⁻ and N₂), leading to low NH₃ selectivity and Faradaic efficiency (FE) (Yao et al., 2019; Fu, 2023; Yu et al., 2024). These challenges stem from the difficulty in synchronizing proton-coupled electron transfers and stabilizing multiple intermediates (Fan et al., 2022). Developing catalysts that balance nitrate adsorption, proton supply, and intermediate energetics is thus critical to advancing NITRR toward practical applications.

Copper-based catalysts are widely explored for the NITRR due to their strong affinity for nitrate adsorption and moderate activity (Ullah et al., 2023; Yin et al., 2024; Zhu et al., 2025). The d-orbital electrons of Cu facilitate nitrate binding, while its redox flexibility enables stepwise reduction (Xie et al., 2025). However, Cu faces the following intrinsic limitations: (1) excessive HER competition at high overpotentials due to insufficient *H coverage for NH₃ synthesis, (2) poor selectivity toward NH₃ as intermediates like NO₂ desorb prematurely, and (3) rapid deactivation from surface oxidation or poisoning (Zhou et al., 2023; Liu et al., 2024; Liu et al., 2025). To address these issues, strategies such as alloying, heteroatom doping, and heterostructure engineering have been employed (Liu et al., 2022; Geng and Ji, 2024; Li Y. et al., 2025). For instance, copper–palladium alloy/CuO heterostructure (CuPd/CuO) offers a more thermodynamically favorable NHO route than the isolated CuPd alloy, benefiting from the synergetic optimization of CuPd and CuO for water dissociation and the NITRR (Li J. et al., 2025). These approaches highlight the critical role of proton supply in an efficient NITRR process, which requires not only optimized active sites for nitrate activation but also a robust proton source to sustain the multi-step hydrogenation process. Integrating Cu with proton-rich co-catalysts to decouple HER competition and enrich active sites of N-intermediate is very promising strategy to boost nitrate-to-ammonia conversion efficiency.

Molybdenum phosphide (MoP), a highly active HER catalyst, may present a unique opportunity to enhance Cu-based NITRR. MoP exhibits strong water dissociation capability, generating abundant protons while suppressing H₂ evolution due to its near-optimal hydrogen binding energy, which holds great promise for the hydrogenation of NO₃ ⁻ to NH₃ (Gu et al., 2021; Zhang et al., 2021; Guo et al., 2024). Moreover, the semi-metallic nature of MoP is favorable for regulating the electronic structure of other materials upon the heterojunction construction. Different electronegativity values of counterparts in the MoP-based heterostructure can induce charge redistribution at the interface (Gu et al., 2021). For instance, Liu et al. (2020) revealed that coupling α-MoC_1-x with MoP can induce strong electronic interactions at the α-MoC_1-x–MoP interface, leading to moderate free energy of H adsorption and improved HER activity. Thus, a Cu/MoP heterostructure could synergistically address the limitations of standalone Cu catalysts by combining efficient nitrate reduction on Cu with proton generation and electronic modulation from MoP.

In this study, a Cu/MoP composite was synthesized via one-pot calcination, forming intimate heterojunctions between Cu nanoparticles and MoP nanograins. Structural characterization confirms the successful construction of the abundant Cu and MoP interface, and the interface induces the formation of electron-deficient Cu and electron-rich MoP, which synergistically facilitate the NITRR process and active hydrogen generation. The optimized Cu/MoP composite achieves a record NH₃ FE of 98.9% and a yield rate of 30.72 mmol h⁻¹ cm⁻² at −0.5 V vs. RHE, outperforming isolated Cu or MoP. When deployed in a Zn-nitrate battery, the Cu/MoP cathode delivers a high open-circuit voltage of 1.35 V and a peak power density of 12.97 mW cm⁻², demonstrating its dual functionality in energy storage and environmental remediation.

The schematic illustration of the synthesis of the Cu/MoP composite is shown in Figure 1a. (NH₄)₆Mo₇O₂₄⋅4H₂O, NH₄H₂PO₄, and Cu(Ac)₂ were employed as Mo, P, and Ce sources, respectively. These precursors were thoroughly ground and then calcined at 800°C in an Ar/H₂ atmosphere to produce the Cu/MoP composite. In addition, PAN was particularly used to avoid grain growth and promote the formation of nanoparticles The isolated Cu or MoP samples were prepared by the same method, except for the former was synthesized without (NH₄)₆Mo₇O₂₄⋅4H₂O and NH₄H₂PO₄ and the latter without Cu(Ac)₂, in their respective synthesis processes.

Powder X-ray diffraction (XRD) characterization was utilized to study the phase composition and crystal structure of the as-obtained samples. As shown in Figures 1b,c, the as-prepared Cu/MoP composite has the typical crystal structure, and the corresponding diffraction peaks located at 27.86°, 32.02° and 42.98° can be, respectively, indexed to the diffraction patterns of (001), (100), and (101) planes of MoP phase (PDF#24-0771), while peaks at 43.24° and 74.30° were, respectively, assigned to the (111) and (220) planes of the Cu phase (PDF#85-1326), which suggests the presence of both Cu and MoP phases in the Cu/MoP composite (Kibsgaard and Jaramillo, 2014; Liu et al., 2023). Meanwhile, the XRD patterns proved that the isolated Cu or MoP with pure phase was successfully prepared. To investigate the effect of Cu content in the Cu/MoP composite on catalytic activity, two other samples, namely, Cu/MoP-5 and Cu/MoP-15, were also synthesized. As shown in Supplementary Figure S1, the same XRD patterns as the Cu/MoP sample were also detected in Cu/MoP-5 and Cu/MoP-15, indicating that all Cu/MoP samples contain Cu and MoP phases. Furthermore, normalized XRD patterns are shown in Supplementary Figure S1c. The content of the Cu phase increases in the following order: Cu/MoP-15 > Cu/MoP > Cu/MoP-5. Due to the different roles of Cu and MoP phases in electrocatalytic NITRR processes, increased Cu content may show significant effects on catalytic performance. Field-emission scanning electron microscopy (FE-SEM) and field-emission transmission electron microscopy (FE-TEM) were used to investigate the nanostructure and morphology of the as-prepared samples. As shown in Supplementary Figure S2; Figures 1d–f, these three samples, namely, Cu, MoP, and Cu/MoP, all exhibit aggregated nanoparticles, and the Cu/MoP composite displays a grain size similar to that of the isolated MoP sample and smaller than that of the isolated Cu sample, indicating that coupling MoP with Cu nanoparticles can effectively refine the grain size. This effect can be attributed to the different crystal structures and grain growth rates of Cu and MoP. High-resolution TEM (HR-TEM) images (Figures 1g–i) reveal that these three samples have a typical crystal structure. The observed interplanar spacing of lattice fringes is 0.274 nm for the isolated MoP and 0.208 nm for the bare Cu, which can be indexed to the (100) plane and (111) plane of MoP and Cu, respectively. For the Cu/MoP composite, both of the above-mentioned planes are observed, and abundant Cu and MoP heterojunction interfaces (indicated by blue dotted lines) are present, suggesting good coupling between Cu and MoP nanoparticles. Figure 1j shows the scanning transmission electron microscopy with high-angle annular dark-field (STEM-HAADF) images of Cu/MoP, and the corresponding elemental mapping images demonstrate the uniform distribution of Cu, Mo, and P, further confirming the uniform distribution of Cu and MoP nanograins in the Cu/MoP composite.

To characterize the surface electronic state and chemical composition of the as-prepared samples, X-ray photoelectron spectroscopy (XPS) was conducted. The survey spectrum of the Cu/MoP sample (Figure 2a) confirms the existence of Cu, Mo, and P in the composite, which are assigned to the Cu and MoP phases. However, the signals of Mo and P are not detected in the Cu sample, while the signal of Cu is not detected in the MoP sample, revealing the successful preparation of isolated Cu and MoP materials. High-resolution Cu 2p spectra of the isolated Cu and Cu/MoP samples are shown in Figure 2b. For the bare Cu sample, a pair of binding energies located at 932.40 and 952.31 eV are attributed to the 2p_3/2 and 2p_1/2 of Cu⁰, respectively, while two peaks positioned at 933.79 and 953.63 eV are assigned to Cu²⁺, respectively (Lin et al., 2024). However, the binding energies of Cu⁰ of the Cu/MoP composite are centered at 932.63 and 952.54 eV, showing a 0.23 eV positive shift compared to the isolated Cu nanoparticles. In high-resolution Mo 3d spectra of MoP nanoparticles (Figure 2c), the binding energies of Mo^δ+ 3d_5/2 and Mo^δ+ 3d_3/2 are located at 228.45 eV and 231.65 eV, which are ascribed to Mo-P species. The doublets at 233.60/236.30 eV and 229.25/232.95 eV are assigned to the Mo⁶⁺ (MoO₃) and Mo⁴⁺ (MoO₂) species, respectively (Wei et al., 2021). For the Cu/MoP composite, the doublet peaks of Mo^δ+ are positioned at 228.23 eV (Mo^δ+ 3d_5/2) and 231.43 eV (Mo^δ+ 3d_3/2), manifesting a 0.22 eV negative shift of binding energy compared to the isolated MoP nanoparticles. Furthermore, the high-resolution P 2p spectra (Figure 2d) can be deconvoluted into three peaks. In MoP, the P 2p_3/2 and P 2p_1/2 of Mo-P species, respectively, center at 129.50 eV and 130.40 eV, while the peak assigned to P₂O₅ is located at 134.00 eV. However, the binding energies of P 2p_3/2 and P 2p_1/2 in Cu/MoP negatively shift to 129.35 and 130.25 eV, respectively. These results suggest that the Cu and MoP interface has strong interactions, and electrons localized on the Cu surface in the Cu/MoP composite are transferred to the MoP surface, which produces a charge-polarized Cu^δ+ (0 < δ < 1) chemical environment and an electron-rich MoP surface. The electron-deficient Cu active sites may promote the production of active hydrogen species (*H) and facilitate the NITRR, and electron-rich MoP surface can strengthen H bonding, which may supply abundant *H for NITRR processes.

The electrocatalytic NITRR performance of the as-obtained catalysts was evaluated in an H-type cell using a three-electrode system, where the cathode and anode compartments were separated using a commercial Nafion 117 membrane. First, the linear sweep voltammetry (LSV) tests were conducted to assess the catalytic activity of the as-prepared samples. Figure 3a shows the LSV curves of Cu, MoP, and Cu/MoP in 1.0 M KOH electrolyte without NaNO₃, and the corresponding current response is attributed to HER processes. These results show that MoP and Cu/MoP have certain catalytic HER activity in NaNO₃-free electrolyte, and the overpotential of MoP is 239 mV at a current density of 10 mA cm⁻², while that of the Cu/MoP composite is 265 mV, indicating that H₂ release occurs more readily on MoP nanoparticles than on the Cu/MoP composite. However, the isolated Cu nanoparticles are inert to HER, suggesting insufficient *H coverage. Interestingly, in the mixed electrolyte of 1.0 M KOH and 1.0 M NaNO₃, the current response of these three samples is significantly enhanced, which is ascribed to the NITRR processes. Under the applied voltage window from −0.1 V to −0.8 V, the polarization current density values of these three catalysts are in the following order: Cu/MoP > Cu > MoP, and the current density of Cu/MoP is up to 482 mA cm⁻² at −0.5 V, indicating its highest NITRR catalytic activity. According to the LSV curves of HER and NITRR, although the bare MoP has good catalytic capacity for water dissociation and active hydrogen generation, excessive H₂ evolution may lead to insufficient *H retention for the conversion of nitrate to ammonia. Although Cu is widely reported as an effective catalyst for the NITRR, the conversion efficiency suffers from insufficient active hydrogen (Ullah et al., 2023). Our results also prove this conclusion. The unsatisfied NITRR catalytic activity of isolated Cu is attributed to its poor catalytic ability for water dissociation, which is unfavorable for active hydrogen generation. Therefore, coupling Cu and MoP overcomes their respective shortcomings. At the Cu and MoP heterojunction, the Cu phase provides highly active sites for the NITRR, while the MoP phase offers abundant active hydrogens. Furthermore, strong electron transfer from Cu to MoP produces the electron-deficient state of Cu and the electron-rich state of MoP, which synergistically promote the transformation of nitrate and the generation of active hydrogen. To further evaluate the NITRR catalytic activity of the Cu/MoP composite, LSV curves at different NO₃ ⁻ concentrations were also recorded (Supplementary Figure S3), which reveals that the Cu/MoP composite exhibits excellent NITRR catalytic activity even at low NO₃ ⁻ concentrations in the electrolyte. In addition, to investigate the effects of different Cu contents of the Cu/MoP composite on the catalytic activity, Cu/MoP-5 and Cu/MoP-15 were also studied in NO₃ ⁻ and NO₃ ⁻-free electrolytes. As shown in Supplementary Figure S4a, the HER overpotentials of these three Cu/MoP composites are 233 mV (Cu/MoP-5), 265 mV (Cu/MoP), and 315 mV (Cu/MoP-15), respectively. The decreasing HER activity in the order of Cu/MoP-5 > Cu/MoP > Cu/MoP-15 indicates that MoP plays a key role in the water dissociation behavior of Cu/MoP, which produces active hydrogen. As presented in Supplementary Figure S4b, these three Cu/MoP composites have obviously enhanced current response in the mixed solution of 1.0 M KOH and 1.0 NaNO₃, and Cu/MoP has the best NITRR catalytic activity, while Cu/MoP-15 shows the worst catalytic activity, indicating the excessive copper content can lead to bad NITRR activity.

To further investigate the catalytic activity and selectivity of the NITRR on the as-prepared catalysts, the resultant solution after the NITRR at different applied potentials for 30 min was collected (Figure 3c), the indophenol blue method was utilized to assess the concentrations of NH₃. For the Cu/MoP composite, the product solution stained with the indophenol blue indicator is shown in the inset of Figure 3d, and the corresponding UV-visible absorption curves are presented in Figure 3d. With an increase in the given potential, the color of the product solution becomes darker and the corresponding light absorption intensity increases, indicating that the NH₃ concentration increases. According to the calibration of the NH₃ standard curve (Supplementary Figure S5), the highest Faraday efficiency of NH₃ conversion is 98.9% and is obtained at −0.5 V, and the corresponding NH₃ yield rate is 30.72 mmol h⁻¹ cm⁻², indicating the excellent NITRR catalytic activity of the Cu/MoP composite. In addition, the UV-visible absorption curves of NO₂ ⁻ and N₂H₄ in the product are shown in Supplementary Figure S8, where the Faraday efficiency of NO₂ ⁻ at −0.5 V voltage is 0.98% (Supplementary Figure S8c), according to the calibration of the NO₂ ⁻ standard curve (Supplementary Figure S6), while N₂H₄ can hardly be detected by the Watt–Chrisp method, indicating its low yield. The high NITRR catalytic activity and NH₃ FEs of the as-prepared Cu/MoP surpass those of most recently reported advanced catalyst for the NITRR (Supplementary Table S1), suggesting the great potential application of Cu/MoP for nitrate-to-ammonia conversion.

For comparison, the products of Cu and MoP were also evaluated using the same method. As shown in Supplementary Figure S9, the highest Faraday efficiency of ammonia production of the isolated Cu nanoparticles is 76.8% at a voltage of −0.3 V, but its yield is only 4.30 mmol h⁻¹ cm⁻², and it also shows a high Faraday efficiency of NO₂ ⁻ production at this voltage. Meanwhile, NH₃ faraday efficiency of the isolated Cu sample is 52.56% at −0.5 V (vs. RHE), while the corresponding NO₂ ⁻ faraday efficiency is 0.98% and 22.2% at the same potential. Therefore, a dramatic increase in NH₃ selectivity and a significant decrease in byproduct NO₂ ⁻ of the Cu/MoP composite reveal high-efficiency conversion of NO₃ ⁻ to NH₃, which requires effective assistance of sufficient active hydrogen and efficient N-intermediate conversion. In addition, the Faraday efficiency of ammonia close to 100% indicates that hydrogen evolution on the Cu/MoP complex is inhibited, and the generated active hydrogen mainly participates in nitrate-to-ammonia conversion. For MoP, the highest Faraday efficiency of ammonia production is 60.34% at −0.3 V, but its yield is only 1.44 mmol h⁻¹ cm⁻² (Supplementary Figure S9). Therefore, both Cu and MoP exhibit lower NITRR ammonia production efficiency and yield, while Cu nanoparticles coupled with MoP exhibit enhanced NITRR catalytic activity and high ammonia production selectivity. According to the electrochemical HER and NITRR results, MoP nanoparticles can effectively promote water decomposition to produce protons, which are easily released in the form of H₂ on the isolated MoP nanoparticles. Cu nanoparticles can provide abundant active sites for the NITRR process, but the conversion efficiency from NO₃ ⁻ to NH₃ is limited due to the lack of active hydrogen. For the Cu/MoP composite, protons produced by the MoP phase in Cu/MoP may be more inclined to participate in the reduction conversion from NO₃ ⁻ to NH₃ on the surface of Cu nanoparticles. Therefore, the synergic catalysis of Cu and MoP can significantly improve the NITRR catalytic activity. To confirm that NH₃ produced originates from the NITRR rather than nitrogen-containing contaminants, a¹⁵N isotope labeling experiment was performed for the Cu/MoP (Figure 3f). With ¹⁴NO₃ ⁻ as the N source, the ¹H-NMR spectrum presents a triple peak with the coupling constant of J_N-H = 52.4 Hz, which is ascribed to the standard ¹⁴NH₄ ⁺. When fed with ¹⁵NO₃ ⁻, a doublet with a coupling constant of J_N-H = 72 Hz is detected (corresponding to the characteristic peak of ¹⁵NH₄ ⁺) (Zhang et al., 2022). This result confirms that the generation of NH₃ is from the NITRR.

The underlying mechanism of remarkable NITRR activity of Cu/MoP was further explored by assessing electrochemical impedance spectroscopy (EIS) and electrochemical active surface area (ECSA). Figure 3g shows the corresponding Nyquist plots, and the similar shape of these three samples suggests that all samples undergo similar charge transfer processes. Furthermore, an equivalent circuit (inset of Figure 3g) was used to simulate the Nyquist plots, which consist of solution resistance (R _s), charge transfer resistance (R _ct), and constant phase element (CPE). According to the fitting results, Cu/MoP exhibits a smaller R _ct value (7.8 Ω) than Cu (17.4 Ω) and MoP (20.2 Ω), indicating that coupling Cu with MoP is favorable for reducing charge-transport impedance and accelerating the electron transport rate. As shown in Supplementary Figure S11, the R _ct value of Cu/MoP is also smaller than that of Cu/MoP-5 (10.8 Ω) and Cu/MoP-15 (12.5 Ω), which may be attributed to more Cu and MoP interfaces in the Cu/MoP composite. The cyclic voltammetry (CV) curves of the as-obtained samples were recorded at various scan speeds between 10 and 110 mV s⁻¹ within the non-faradic range (Supplementary Figure S12), which were used to calculate the double-layer capacitance (C _dl) and assess the ECSA. As illustrated in Supplementary Figure S12f, the C _dl value of Cu/MoP is 27.01 mF cm⁻², larger than that of MoP (0.60 mF cm⁻²), Cu (1.07 mF cm⁻²), Cu/MoP-5 (23.79 mF cm⁻²), and Cu/MoP-15 (19.47 mF cm⁻²), suggesting that the Cu/MoP composite has more potential active sites for the NITRR. Fast charge transport rate and abundant active sites result from enriched Cu and MoP interfaces and their induced modulation for electronic structures in the Cu/MoP composite. A long-term NITRR stability test was performed to evaluate the catalytic robustness of Cu/MoP under a high NH₃ production rate. Figure 3h shows the chronoamperometry curve of Cu/MoP in the mixed solution of 1.0 M KOH and 1.0 M NaNO₃ at −0.5 V. As observed, the initial current density is recorded at 406 mA cm^–2. The reduction current density steadily decreases as the reaction time extends to 8 h, but it returns to the initial current density value upon adding fresh electrolyte, suggesting that the decrease is due to the rapid depletion of NO₃ ⁻. This result indicates that the Cu/MoP composite exhibits robust catalytic activity. NH₃ FE during the long-term operation was also evaluated by assessing the NH₃ concentration of the product solution at 1, 2, 4, 8, and 10 h. According to the UV-Vis absorption spectrum and the calibration curve (Supplementary Figure S13), the NH₃ FE keeps remains nearly unchanged, implying good durability of the Cu/MoP catalyst for the NITRR.

Considering the excellent electrochemical NITRR performance of the Cu/MoP composite, we further assembled a zinc-nitrate (Zn-NO₃ ⁻) battery using Cu/MoP as the cathode and a Zn plate as the anode, and the cathode electrolyte consisted of a mixed solution of 1.0 M KOH and 1.0 M NaNO₃ aqueous solution, while 1.0 M KOH was employed as the anode electrolyte. The two compartments were separated by a Nafion membrane. The schematic diagram of the battery device is shown in Figure 4a. A “killing three birds with one stone” capability can be achieved by this Zn-NO₃ ⁻ battery, enabling simultaneous energy supply, ammonia production, and nitrate pollutant removal. As shown in Figure 4b, this Zn-NO₃ ⁻ battery delivers an open-circuit voltage of 1.35 V, and the output voltage remains stable during more than 25 h of testing. The battery performance was also evaluated usinf power density. The discharge curve was recorded at a scan rate of 5 mVs⁻¹ (Figure 4c). The corresponding power density was calculated using the equation P = I × V (Zhou and Sun, 2022), and the peak power density of this Cu/MoP-based battery reaches 12.97 mW cm⁻², which exceeds that of recently reported noble metal and non-noble metal-based NITRR catalysts. Furthermore, the discharge performance was also assessed at different current densities. As presented in Figure 4d, the voltages at different discharge current densities from 5 to 20 mA cm⁻² remain stable, indicating remarkable rate stability. These results indicate that Cu/MoP-based Zn-NO₃ ⁻ battery delivers excellent battery performance, showing good potential for practical application.

This study presents the synthesis and application of a Cu/MoP composite catalyst for enhanced electrochemical NITRR and excellent zinc-nitrate battery performance. The integration of Cu and MoP in the composite optimizes electronic structure and produces the electron-deficient state of Cu and the electron-rich state of MoP. Thus, the NITRR process and the generation of active hydrogen were synergistically optimized. The Cu/MoP composite, prepared via one-pot calcination, exhibits superior NITRR activity, achieving a high NH₃ Faradaic efficiency of 98.9%. Employed as a cathode catalyst for the zinc-nitrate battery, this composite shows robust stability and high power density. This work reveals the potential of Cu/MoP composites for sustainable ammonia production and energy storage applications, offering an efficient solution to current challenges in the electrochemical NITRR of Cu-based catalysts.