Surface engineering and local electron structure modulation to accelerate electroreduction of low-concentration nitrate

Abstract

Electroreduction of low-concentration nitrate is constrained due to the competitive hydrogen evolution side reaction and sluggish reaction dynamics. Herein, we present a NiFe-layered double hydroxide featuring hydrophobicity and oxygen vacancies formed by in situ electrochemical intercalation of sodium dodecylbenzene sulphonate (V_O-NiFe-LDH/CF). Different from traditional intercalation by the ion-exchange method, V_O-NiFe-LDH/CF retains the hydrophobic properties of the alkyl chains in sodium dodecylbenzene sulphonate, while the sulfonic acid functional groups inhibit the formation of surface hydroxyl groups. The introduction of oxygen vacancies induces local electron redistribution and d-band center optimization, thereby enhancing nitrate adsorption capacity and reducing the energy barrier for the rate-determining step (*NO → *NOH). In situ FTIR measurement confirms the electrochemical hydrogenation route of nitrate. The optimized V_O-NiFe-LDH/CF can realize 96.8% nitrate removal efficiency and 96% ammonia selectivity at an initial NO₃⁻-N concentration of 50 mg L⁻¹. The NO₃⁻-N removal rate of V_O-NiFe-LDH/CF is 2.1-fold higher than that of NiFe-LDH/CF. Furthermore, V_O-NiFe-LDH/CF has excellent practicality for practical industrial nitrate-wastewater; the total nitrogen was reduced from 114.7 mg L⁻¹ to 7 mg L⁻¹. V_O-NiFe-LDH/CF shows outstanding stability and robust anti-interference performance during the long-term experiments.

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1. Introduction

The electroreduction of nitrate to ammonia is a promising approach for sustainable nitrogen management, which is a “turning waste into wealth” strategy for ammonia production and the removal of NO₃⁻-N contamination.^1,2 However, the electroreduction of low-concentration nitrate still suffers from competitive hydrogen evolution side reactions, particularly under acidic and neutral conditions. In addition, the process of nitrate conversion to ammonia is inherently constrained due to the requirement of multi-electron/proton transfer and the high energy barrier of the hydrogenation step. The design of durable and efficient electrodes is crucial for the electroreduction of low-concentration nitrate.

Layered double hydroxides (LDHs) are promising electrocatalysts for various electrocatalytic reactions due to their low cost, scalable synthesis and tunable composition.^3–6 LDHs are composed of positively charged brucite-like metal cation layers and interlayer galleries with charge compensating anions and water molecules. The metal cations are coordinated octahedrally by hydroxyl groups giving positively charged layers, and each hydroxyl group is oriented toward the interlayer region and may be hydrogen bonded to the interlayer anions and water molecules. Interestingly, due to the relatively weak interlayer bonding of LDHs, the controllable coordination environments of LDHs can be constructed by regulating the interlayer molecules. However, most of the previous research has focused on the modulation of interlayer spacing by various intercalations (e.g., polyoxometalates, biomolecules and metal complex anions).⁴ The NiFe LDH intercalated with dodecyl-sulfonate has exhibited stronger electrosynthesis activity for formaldoxime compared to that intercalated with Cl⁻, in which the enhanced catalytic ability is attributed to the enlarged interlayer spacing of LDHs achieved by using different interlayer expansion agents.⁷ The interlayer spacing of CoAl-LDH was enhanced from 0.76 to 1.33 nm by intercalation of sodium dodecyl sulfate, and the enlarged interlayer spacing enhanced the ion-diffusion channel.⁸ However, the mechanism of local electron redistribution by regulating the interlayer molecules of LDHs is still unclear.

The water splitting reaction in the region of the cathode has a significant impact on the electroreduction process of nitrate. The balance between the generation and consumption of atomic hydrogen (*H) is crucial for the electrocatalytic hydrogenation of nitrate to ammonia.^9,10 Electrode surface engineering plays a pivotal role in the design of catalysts, as it directly influences nitrate enrichment, proton–electron transfer kinetics and water splitting behavior on the electrode surface.^11,12 Previous studies found that the hydrophobic interface significantly reduces H⁺ concentration at the electrode surface compared to a hydrophilic interface, thereby effectively inhibiting the HER and utilizing *H for hydrogenation of nitrate.^13–16 Liu et al. reported that controllable interfacial wettability of a CNT electrode by altering the lengths of the alkyl chains in the surfactant allows fewer interfacial water molecules to reach the CNT surface, thereby significantly inhibiting the competitive HER and promoting NH₃ selectivity.¹⁷ Therefore, the development of LDH cathodes with a hydrophobic surface by changing the interlayer molecules is a promising strategy for electroreduction of low-concentration nitrate.

In this work, we found that NiFe-LDH can simultaneously obtain abundant oxygen vacancies and a relatively hydrophobic surface only through adding sodium dodecylbenzene sulphonate (SDBS) during the electro-deposition process (V_O-NiFe-LDH/CF). The alkyl-chains of SDBS contribute to the hydrophobic properties, and the sulfonic acid functional groups of SDBS couple with metal cations inhibiting the formation of surface hydroxyl groups, resulting in the formation of oxygen vacancies. The optimized V_O-NiFe-LDH/CF electrode achieved 96.8% nitrate removal efficiency and 96% ammonia selectivity at an initial NO₃⁻-N concentration of 50 mg L⁻¹. The nitrate removal rate of V_O-NiFe-LDH/CF is 2.1-fold higher than that of NiFe-LDH/CF without oxygen vacancies, while the hydrophobic electrode used in the traditional ion exchange method shows no significant improvement compared to NiFe-LDH/CF. This result indicated that the oxygen vacancies play an important role in the enhanced catalytic efficiency, and the hydrophobicity of the electrode reduces the accessibility of water molecules, thereby inhibiting the HER side reaction. The Bode phase plots and EPR spectra show that V_O-NiFe-LDH/CF maintains a good balance between generation and consumption of *H. The deoxygenation and hydrogenation process of electroreduction of nitrate was unveiled by in situ FTIR spectra. Density functional theory (DFT) calculations revealed that the introduction of oxygen vacancies resulted in an optimized d-band center, enhanced nitrate adsorption capacity and a reduced energy barrier of the rate-determining step (*NO to *NOH). Moreover, high N₂ selectivity (98.7%) was obtained in the presence of 1500 mg L⁻¹ Cl⁻. Furthermore, V_O-NiFe-LDH/CF has excellent practicality for treating industrial nitrate-wastewater; the total nitrogen was reduced from 114.7 mg L⁻¹ to 7 mg L⁻¹. Consequently, the modulation of hydrophobicity and optimization of the local electronic structure to enhance the conversion efficiency of nitrate to ammonia on the surface of the electrode are highly effective. The unique design of a hydrophobic interface with oxygen vacancy synergy provides novel insights into regulating the nitrate–water interface environment and strategically designing advanced electrocatalysts for low-concentration nitrate wastewater treatment.

2. Experimental section

2.1 Materials

All chemicals used in this work were bought from commercial suppliers and used without further treatment. The detailed information is described in Text S1.

2.2 Preparation of V_O-NiFe-LDH/CF

The NiFe-LDH with oxygen vacancies was prepared on the copper foam by the in situ electrochemical intercalation method and named V_O-NiFe-LDH/CF (Fig. 1a). Briefly, the electrodeposition electrolyte comprised 0.459 g FeSO₄·7H₂O, 0.97 g Ni(NO₃)₂·6H₂O, 0.213 g Na₂SO₄ and 0.03 g SDBS (C₁₈H₂₉NaO₃S) which were dissolved in 50 mL deionized water. The electrodeposition process was carried out at a current density of 7.5 mA cm⁻² for 15 min in a two-electrode system where commercial Ir–Ru/Ti and the CF were used as the counter electrode and working electrode, respectively. The prepared electrodes were repeatedly washed with deionized water and ethanol three times and vacuum dried for 12 h at 60 °C.

Fig. 1 (a) Schematic illustration of the V_O-NiFe-LDH/CF synthesis route. (b and c) SEM images of NiFe-LDH/CF and (d and e) V_O-NiFe-LDH/CF at different resolutions, (f and g) TEM image and HRTEM image, and (h) corresponding elemental mapping images of V_O-NiFe-LDH/CF.

For control experiments, NiFe-LDH/CF, V_O-Ni-LDH/CF and V_O-Fe-LDH/CF were prepared by following a similar procedure, except without the addition of SDBS, FeSO₄·7H₂O and Ni(NO₃)₂·6H₂O, respectively. NiFe-LDH/CF-SC and NiFe-LDH/CF-SDS were prepared by following a similar procedure, except for replacing SDBS with sodium citrate (SC) and sodium dodecyl sulfate (SDS), respectively.

2.3 The experimental procedure for electrochemical nitrate reduction

Nitrate reduction experiments were carried out in a two-electrode system powered by a direct current power supply. The reaction tank was a 100 mL integrated electrolytic cell. As-prepared electrodes (2 × 3 cm²) and commercial Ir–Ru/Ti (2 × 3 cm²) were employed as the cathode and anode, respectively. The distance of two electrodes was set at 1.5 cm and 70 mL electrolyte including 50 mM Na₂SO₄ and 50 mg L⁻¹ NO₃⁻-N was used. The reaction time was 3 h. If not specified, the applied current density is 10 mA cm⁻². Subsequently, 1.0 mL of electrolyte was collected at a 30-min interval and was quantitatively determined according to colorimetric methods through UV-vis absorption spectroscopy (Fig. S1) and the detailed measurement procedure is described in Text S2.

The NO₃⁻-N removal ratio, NH₄⁺-N generation ratio and selectivity (R_NO3⁻-N, G_NH4⁺-N and S_NH4⁺-N) were calculated using eqn (1)–(3):

where [NO₃⁻-N]₀ is the initial concentration (mg L⁻¹) of NO₃⁻-N. [NO₃⁻-N]_t, [NO₂⁻-N]_t, and [NH₄⁺-N]_t are the concentrations (mg L⁻¹) of NO₃⁻-N, NO₂⁻-N and NH₄⁺-N at time t, respectively. The other calculations are described in Text S3.

2.4 Characterization and DFT calculations

The as-prepared electrodes were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Electron Paramagnetic Resonance (EPR). The electrochemical properties of electrodes were characterized using an electrochemical workstation (CHI 660E, Chenhua) with a three-electrode system with 50 mM NaSO₄ solution. The details are provided in SI Text S4.

All the computations were conducted based on the density functional theory (DFT) using the Vienna Ab Initio Simulation Package (VASP). The details are available in Text S5.

3. Results and discussion

3.1 Characterization of as-prepared cathodes

Fig. 1a illustrates the preparation process of V_O-NiFe-LDH on the Copper Foam (CF). All cathodes were prepared by a constant-current electrodeposition process with a fixed current density. Compared to the synthesis of NiFe-LDH/CF, SDBS was strategically introduced as an intercalating agent during the electrodeposition of V_O-NiFe-LDH/CF to simultaneously enhance hydrophobicity and construct oxygen vacancies. In previous study, SDBS was considered to affect the growth of crystals due to oriented aggregation based crystal growth.¹⁸ The interaction between metal cations and surface hydroxyl groups is weakened due to the influence of the sulfonic groups of SDBS, resulting in the introduction of O vacancies in the LDH structure. As shown in Fig. 1b and c, the SEM images of NiFe-LDH reveal a nano-flower-like structure composed of single or multiple nanosheets stacked at 100–300 nm (Fig. 1d and e). The effect of morphology on catalytic performance was minimized in this study, in contrast to the finding reported by Wang et al.⁸ In addition, the nanosheets of V_O-NiFe-LDH exhibit a smoother surface compared to those of NiFe-LDH. The results demonstrate that the introduction of SDBS enhances the coordination with metal ions, thereby facilitating the formation of nanosheets. The TEM images further confirm the hierarchical nano-flower-like structure composed of stacked nanosheets (Fig. 1f and S2). The high-resolution (HR-TEM) image (Fig. 1g) of V_O-NiFe-LDH/CF and the corresponding fast Fourier transform (FFT) pattern, inverse FFT (IFFT) image and line profile (Fig. S3a–c) confirm that the lattice spacing (0.25 nm) corresponds to the (012) crystal planes of hydrotalcite-like structure. In addition, lattice stripe fractures and discontinuities (marked by yellow dotted lines) can be clearly seen, which may be attributed to oxygen vacancies.¹⁹ Elemental mapping of V_O-NiFe-LDH/CF demonstrates the uniform distribution of Ni, Fe, S and O throughout the catalyst (Fig. 1h).

As shown in Fig. S4, the diffraction peaks of metallic Cu at 2θ = 43.3°, 50.4° and 74.1° correspond to the CF substrate (PDF#04-0836).²⁰ Additionally, XRD analysis of the sample scraped from the CF substrate confirms the formation of the NiFe-LDH phase (Fig. 2a). The diffraction peaks located at 11.79°, 23.77°, 34.74°, 60.28° and 61.66° are indexed to the (003), (006), (012), (110) and (113) reflections of the hydrotalcite-like structure of LDHs, respectively, demonstrating the successful synthesis of NiFe-LDH.²¹ The reduced crystallinity of V_O-NiFe-LDH/CF can be ascribed to the fact that the interaction between metal cations and sulfonic groups of SDBS inhibited the growth of the LDH. The (006) diffraction peaks at 23.77° of NiFe-LDH shifted to 22.6°, suggesting the intercalation of SDBS. According to Bragg's law,²² the d-spacing of A was calculated to be 0.75 nm and 0.79 nm, confirming the intercalation effect of SDBS. The characteristic peaks at approximately 548 and 456 cm⁻¹ in Raman spectra of NiFe-LDH/CF (Fig. 2b) correspond to the E_g bending mode and A_1g stretching mode of M-OH species in the NiFe-LDH.^23,24 The shifted characteristic peaks were found in V_O-NiFe-LDH/CF, which indicated the electron transfer between Ni and Fe. The FTIR spectra of as-prepared electrodes were collected in the range of 400 to 4000 cm⁻¹ as shown in Fig. 2c. The characteristic peaks located at 3292 and 1627 cm⁻¹ were identified as the stretching vibration of OH groups and the bending vibration of adsorbed water molecules, respectively.²⁵ Characteristic vibration bands at 2930 and 2856 cm⁻¹ were observed, which are ascribed to the stretching vibration of the alkyl group in the long carbon chain structure of SDBS ions.^26,27 The results further reveal the presence of SDBS intercalation anions in the interlayer space. XPS was employed to characterize the elemental composition and electronic states of the as-prepared electrodes. The survey spectrum (Fig. S5) of V_O-NiFe-LDH/CF displays distinct elemental signals for Cu 2p, Ni 2p, Fe 2p, O 1s, C 1s and S 2p, with the S 2p peak intensity notably higher than that of NiFe-LDH/CF. The characterization results confirm the successful growth of NiFe-LDH on the CF substrate, while the enhanced S 2p signal intensity further validates SDBS intercalation within the LDH interlayers.²⁸ The Ni 2p spectrum of NiFe-LDH/CF can be deconvoluted into peaks of Ni²⁺ (855.42 and 872.94 eV) and Ni³⁺ (857.07 and 874.39 eV) along with satellite peaks at 861.67 and 878.97 eV, as shown in Fig. 2d. In Fig. 2e, the high-resolution Fe 2p XPS spectrum of NiFe-LDH/CF reveals two spin-orbit splitting peaks corresponding to Fe 2p_3/2 and Fe 2p_1/2, along with satellite peaks at 718.46 and 732.39 eV.²³ The peaks at 710.89 and 723.89 eV are ascribed to Fe²⁺, while the peaks at 713.78 and 726.87 eV are assigned to Fe³⁺. The peaks of Ni 2p and Fe 2p of V_O-NiFe-LDH/CF shift towards lower binding energy compared with that of NiFe-LDH/CF. The negative shift in the binding energy of Ni 2p and Fe 2p implies changes in the electron distribution, which can be attributed to the delocalized electrons originating from oxygen vacancies.²⁹ As shown in Fig. 2f, the peaks at 529.8, 531.6 and 532.3 eV corresponded to the lattice oxygen (O_L), oxygen vacancies (O_V) with low coordination and hydroxy groups from surface-absorbed water molecules (O_A), respectively.³⁰ In the O 1s XPS spectrum, V_O-NiFe-LDH/CF demonstrated a comparatively lower proportion of O_L and a markedly higher intensity of O_V when compared to NiFe-LDH/CF. This observation suggests the generation of extra oxygen vacancies within the V_O-NiFe-LDH/CF electrode. Solid-state EPR was performed to further confirm the formation of oxygen vacancies. An obviously asymmetric signal at g = 2.003 was observed after the addition of SDBS, which can be attributed to the unpaired electrons trapped in the oxygen vacancies (Fig. 2g). This finding further demonstrates the presence of oxygen vacancies.³¹ Moreover, the S 2p spectrum of V_O-NiFe-LDH/CF is presented in Fig. S6. Furthermore, the specific surface area of as-prepared electrodes was investigated through N₂ adsorption–desorption experiments. The results indicated that V_O-NiFe-LDH/CF had a larger BET specific surface area compared to the other electrodes. In addition, the pore volume of the as-prepared electrodes was calculated as shown in Fig. S7. It was observed that there was an increased proportion of mesopores. The increased BET specific surface area and pore volume facilitate ion adsorption kinetics and enhance ion diffusion, thereby improving mass transfer efficiency.⁸ The hydrophilicity of the electrodes was characterized by water contact angle measurements (Fig. 2i). The NiFe-LDH/CF electrode exhibited a water contact angle of nearly 0°, indicating its superhydrophilic properties. After SDBS intercalation, the water contact angle of V_O-NiFe-LDH/CF increased to 137.8°, demonstrating a significant transition to hydrophobicity.

Fig. 2 (a) The XRD patterns, (b) Raman spectra, (c) FTIR spectra, (d–f) the Ni 2p, Fe 2p and O 1s XPS spectra, (g) EPR spectra, (h) N₂ adsorption–desorption isotherms of different electrodes and (i) water contact angle of NiFe-LDH/CF and V_O-NiFe-LDH/CF.

3.2 Electrochemical nitrate reduction performance of as-prepared cathodes

The linear sweep voltammetry (LSV) curves of the V_O-NiFe-LDH/CF electrode are presented in Fig. 3a. A significantly increased current density and more positive onset potential were observed upon the addition of nitrate, demonstrating that nitrate electroreduction is preferentially promoted over the HER on the V_O-NiFe-LDH/CF electrode surface. The LSV curves of different electrodes were evaluated in a 50 mM Na₂SO₄ electrolyte with NO₃⁻-N, further confirming the enhanced current response of V_O-NiFe-LDH/CF compared to the other electrodes (Fig. S8). Additionally, the electrochemically active surface area (ECSA) analysis revealed that V_O-NiFe-LDH/CF possesses a higher double-layer capacitance (C_dl) of 43 mF cm⁻² than the other electrodes, indicating a higher density of active sites after the introduction of oxygen vacancies (Fig. 3b, S9 and S10). V_O-NiFe-LDH/CF exhibits a significantly lower Tafel slope than the other electrodes, indicating faster reaction kinetics (Fig. S11). As shown in Fig. 3c and S12, the electrochemical impedance spectroscopy (EIS) results reveal the smallest charge transfer resistance for V_O-NiFe-LDH/CF, demonstrating its superior charge transfer efficiency compared to the other electrodes. These electrochemical characterization studies demonstrate that V_O-NiFe-LDH/CF exhibits the highest intrinsic catalytic activity among all prepared electrodes, indicating that the intercalation of SBDS effectively enhances the electrochemical activity of the electrode. To further evaluate the nitrate reduction performance of the as-prepared electrodes, electrochemical nitrate reduction tests were conducted in a two-electrode system with a current density of 10 mA cm⁻², using an aqueous electrolyte containing 50 mM NaSO₄ and 50 mg L⁻¹ NO₃⁻-N. As shown in Fig. 3d–f, the CF substrate exhibits a negligible catalytic activity for NO₃⁻-N removal and NH₄⁺-N generation, indicating that the contribution of the substrate to nitrate-to-ammonia conversion is almost negligible in this reaction system. V_O-NiFe-LDH/CF exhibits a significantly higher NO₃⁻-N removal capacity (96.8% after 180 min of electrolysis) compared to the other electrodes. Furthermore, the NO₃⁻-N removal rate of V_O-NiFe-LDH/CF (0.01375 min⁻¹) is 14.3, 6.5 and 2.1 times higher than those of V_O-Fe-LDH/CF (0.00096 min⁻¹), V_O-Ni-LDH/CF (0.00213 min⁻¹) and NiFe-LDH/CF (0.00656 min⁻¹), respectively. V_O-NiFe-LDH/CF exhibits excellent selectivity for NH₄⁺-N (96%) and a lower generation ratio of NO₂⁻-N compared to the other electrodes. It is worth noting that the generation ratio of NO₂⁻-N in the V_O-NiFe-LDH/CF system remains within a relatively lower range (Fig. S13). To avoid measurement error of UV-vis detection, we further performed ion chromatography (IC) tests on the same samples. As shown in Fig. S14, the results obtained from UV-vis and IC measurements are in good agreement, with only minor discrepancies between the two methods. To further investigate the stability of V_O-NiFe-LDH/CF, continuous electrolysis experiments with the V_O-NiFe-LDH/CF cathode are presented in Fig. 3g, and the NO₃⁻-N removal efficiency shows no significant attenuation during the 20 cycling experiments. Additionally, Ni and Fe leaching concentrations remained below 0.01 mg L⁻¹ throughout the operation, further confirming the excellent long-term stability of the V_O-NiFe-LDH/CF cathode (Fig. S15). After long-time cycling experiments, V_O-NiFe-LDH/CF hardly changed and still retained a nano-flower-like structure (Fig. S16). Notably, the post-reaction XPS results show that the relative concentration of oxygen vacancies decreases from 56.5% (fresh electrode) to 48.8% (used electrode) (Fig. S17). This phenomenon can be attributed to the adsorption of reaction intermediates, which increases the proportion of O_A and thus reduces the relative content of O_v. This result is consistent with the reported literature.³² Furthermore, the XPS spectra of Fe 2p and Ni 2p show no obvious variation after long-term cycling experiments, further confirming the excellent structural stability of V_O-NiFe-LDH/CF. Additionally, compared with some reported electrodes synthesized by the in situ growth method, V_O-NiFe-LDH/CF exhibits a better performance in low-concentration nitrate removal at constant working current.^33–39 As shown in Fig. 3h, V_O-NiFe-LDH/CF has a stronger removal ability of NO₃⁻-N at relatively lower working current, which indicated that it may have promising industrial application prospects.

Fig. 3 (a) LSV curves of V_O-NiFe-LDH/CF with and without NO₃⁻-N, (b) linear fitting of the capacitive properties of current density vs. scan rate, (c) EIS Nyquist plots, (d–f) the performance (the removal ability and kinetic rate constant of NO₃⁻-N removal and NH₄⁺-N generation ratio) of electrochemical nitrate reduction of different electrodes, (g) the stability tests and (h) comparison of reported electrodes for low-concentration nitrate removal at constant working current.

3.3 Electroreduction mechanism of nitrate over V_O-NiFe-LDH/CF

In order to determine the role of oxygen vacancies and surface hydrophobicity in the electroreduction of nitrate, we have added two additional control electrodes. A hydrophilic LDH with a comparable oxygen vacancy concentration (NiFe-LDH/CF-SC) was synthesized using sodium citrate (SC) as the intercalating agent, while a hydrophobic LDH without oxygen vacancies (NiFe-LDH/CF-SDS) was prepared with sodium dodecyl sulfate (SDS) as the intercalating agent. SC induces oxygen vacancies mainly through the chelation of its hydroxyl and carboxyl groups with metal ions of LDH. This interaction disturbs the local coordination environment and causes lattice distortion, thereby generating unsaturated coordination sites and eventually leading to the formation of oxygen vacancies. By contrast, SDS undergoes directional adsorption and hydrophobic self-assembly via its long alkyl chains, solely endowing the electrode with surface hydrophobicity. Owing to the absence of strong electron-withdrawing groups that induce defects, SDS does not facilitate the formation of oxygen vacancies. The surface wettability and oxygen vacancy defects of the as-prepared electrodes were systematically characterized, as displayed in Fig. S18. As shown in Fig. S19, V_O-NiFe-LDH/CF exhibits a higher removal capacity of NO₃⁻-N than the other electrodes. Both NiFe-LDH/CF-SC and NiFe-LDH/CF-SDS also outperform NiFe-LDH/CF. Furthermore, NiFe-LDH/CF-SC exhibits better catalytic performance than NiFe-LDH/CF-SDS, indicating that oxygen vacancies play a more dominant role than surface hydrophobicity in the electrocatalytic nitrate reduction process. V_O-NiFe-LDH/CF exhibits a higher faradaic efficiency (44.2%) and NH₃ yield rate than the other control electrodes (Fig. S20), and the energy consumption for NH₃ is 2.17 kWh mol⁻¹. To further investigate the catalytic mechanism of V_O-NiFe-LDH/CF, operando EIS and DFT calculations were employed to study the reaction kinetics of water dissociation, nitrate activation and the reduction process.^40–42 Differential charge density analysis reveals stronger electron transfer at the V_O-NiFe-LDH and *NO₃ interface compared to NiFe-LDH (Fig. 4a), demonstrating that oxygen vacancies induce the local electron redistribution. The Bader charge analysis revealed that the V_O-NiFe-LDH transferred 0.39 electron equivalents to NO₃⁻, significantly higher than the values for NiFe-LDH (0.25). As shown in Fig. S21, the conversion efficiency of nitrate-to-ammonia is inhibited after the TBA addition. Control experiments were performed by replacing H₂O with deuterium oxide (D₂O) to clarify the role of *H, since *H originated from the electrochemical splitting of H₂O. The current densities of LSV curves decreased when D₂O was used instead of H₂O (Fig. S22a), and the kinetic isotope effect (KIE) value of V_O-NiFe-LDH/CF is 1.64 (Fig. S22b), demonstrating that *H plays a critical role in the electrochemical reduction of NO₃⁻-N. Furthermore, V_O-NiFe-LDH/CF exhibits stronger DMPO-*H adduct peaks than NiFe-LDH/CF, and the peak intensity decreases after the addition of nitrate, suggesting that V_O-NiFe-LDH/CF has an enhanced ability for *H generation and consumption (Fig. 4b). Fig. 4c presents the Bode phase plots of V_O-NiFe-LDH/CF at different working potentials. In the absence of nitrate, the Volmer step (water dissociation process) is the dominant reaction. A positive shift of peak position with increasing potential is observed, indicating enhanced water dissociation kinetics and increased *H generation. Upon application of a sufficiently large potential, the peak exhibits a distinct shift toward the middle frequency range, thereby accelerating the Heyrovsky step for the HER.⁴³ After nitrate addition, significant decreases in peak intensity and notable shifts in peak position toward higher frequency are observed, confirming that the suppressed Heyrovsky step and generated *H species are preferentially consumed for the nitrate reduction hydrogenation step compared to the HER. In Fig. S23a, V_O-NiFe-LDH/CF exhibits a significantly lower peak intensity than NiFe-LDH/CF, with peaks shifting to higher frequency at increasingly negative applied potential. These results indicate that the Volmer step has achieved dynamic equilibrium, and H* generated from water dissociation is preferentially consumed at the V_O-NiFe-LDH/CF surface during the nitrate reduction, thereby promoting the hydrogenation of nitrate to ammonia.⁴⁴ Additionally, the peak position of V_O-NiFe-LDH/CF shifts toward lower frequency in the middle frequency range (10⁰–10² Hz) compared with the other electrodes, further confirming that oxygen vacancy introduction and bimetallic collaboration can effectively suppress the Heyrovsky step, thereby inhibiting the HER side reaction (Fig. S23b). Furthermore, Fig. 4d presents the calculated projected density of states (PDOS) and corresponding d-band centers for V_O-NiFe-LDH (−2.29 eV) and NiFe-LDH (−3.08 eV), revealing distinct electronic structure differences induced by oxygen vacancies. A positive shift of the d-band center for V_O-NiFe-LDH is closer to the Fermi level compared to NiFe-LDH, which is beneficial for the adsorption of nitrate and the intermediates.⁴⁵ The calculations of *H free energy on the different electrocatalysts (Fig. 4e) demonstrate enhanced adsorption ability of *H, thereby promoting the hydrogenation steps in nitrate-to-ammonia conversion. In situ FTIR spectra were collected to monitor the intermediates adsorbed on the surface of the electrode (Fig. 4f and S24). The characteristic peak at around 1340 cm⁻¹ could be attributed to the vibration of *NO₃⁻.⁴² The characteristic peaks of the protonation intermediates were found, including –N–H (2850 and 2951 cm⁻¹),⁴⁶ *NH (1558 cm⁻¹), *NHOH (1525 cm⁻¹)^47,48 and the product NH₄⁺ (at around 1457 and 1507 cm⁻¹),⁴⁹ while the band at 1652 cm⁻¹ is attributed to *NO.⁵⁰ The relative peak intensity ratio of *NHOH versus *NO species gradually increased with increasing working potential, demonstrating the hydrogenation pathway of nitrate reduction. Moreover, the ratio over V_O-NiFe-LDH/CF was significantly higher than that over NiFe-LDH/CF at the same working potential, further revealing the promoted hydrogenation kinetics. In addition, the adsorption free energy of the NO₃⁻ → *NO₃ process is identified as the prerequisite rate-determining step in the overall nitrate reduction reaction. The adsorption energy of NO₃ decreases from 0.14 eV to −0.54 eV, indicating that the energy barrier of the prerequisite step is overcome and the reaction shifts toward a thermodynamically favorable spontaneous process. The enhanced nitrate adsorption ability of V_O-NiFe-LDH/CF was evaluated using electrochemical impedance-potential spectra at different nitrate concentrations (Fig. S25 and S26). The enhanced specific adsorption of NO₃⁻ on V_O-NiFe-LDH/CF was proved according to the Esin–Markov effect on the inner Helmholtz plane.⁵¹ The result is in favor of the successive deoxidation and hydrogenation steps. The Gibbs free energy profiles of the nitrate to ammonia on NiFe-LDH and V_O-NiFe-LDH are presented in Fig. 4g. The nitrate to ammonia process can be considered a direct reduction process in combination with electron transfer and N–H bonding. The nitrate reduction reaction proceeds through a series of intermediates with N valences ranging from +5 to −3: NO₃⁻ → *NO₃ → *NO₂ → *NO → *NOH → *NHOH → *NH₂OH → *NH₂ → *NH₃ → NH₃. DFT calculations reveal that the *NO → *NOH step is the rate-determining step for NiFe-LDH in the whole process. The presence of oxygen vacancies reduces the reaction energy barrier of *NO → *NOH (0.23 eV) compared to NiFe-LDH/CF (0.35 eV). In summary, to address the competitive HER and sluggish reaction dynamics associated with the removal of low-concentration nitrate, we proposed a strategy of adding SDBS during LDH electrode deposition to introduce oxygen vacancies. The strategy regulates the surface hydrophobicity and local electronic structure of the electrode, thereby enabling appropriate *H supply and reducing the energy barrier of the hydrogenation process (Fig. 4h).

Fig. 4 (a) The charge density difference diagram and Bader charge of NO₃⁻ adsorption on the V_O-NiFe-LDH and NiFe-LDH. (b) EPR spectra of DMPO-*H during the electrocatalysis process. (c) The potential-dependent Bode plots of V_O-NiFe-LDH/CF. (d) The PDOS of d-bands, (e) the adsorption free energy of H and (f) in situ FTIR spectra of V_O-NiFe-LDH/CF. (g) The Gibbs free energy change of the NO₃RR over NiFe-LDH and V_O-NiFe-LDH (where * denotes the active site on the catalyst surface). (h) Schematic diagram of enhanced electroreduction ability of low-concentration nitrate over V_O-NiFe-LDH/CF.

3.4 The application potential of V_O-NiFe-LDH/CF

To further investigate the practicability of V_O-NiFe-LDH/CF, a series of electrochemical nitrate reduction experiments were conducted using various operational parameters, including SDBS dosage in the electrodeposition process, working current density, initial pH, initial NO₃⁻-N concentration and different interfering ions. As shown in Fig. 5a, b, S27 and S28, both NO₃⁻-N removal efficiency and NH₄⁺-N generation ratio increased significantly with increasing SDBS concentration and working current density. Then, the energy utilization efficiency of the nitrate-to-ammonia process on V_O-NiFe-LDH/CF at different working current densities was calculated.^52,53 As shown in Fig. S29, the energy utilization efficiency of the nitrate-to-ammonia process on V_O-NiFe-LDH/CF exhibited a gradual increase with increasing current density, reaching a maximum of 42.8% at 10 mA cm⁻². A decrease in energy utilization efficiency was observed at 20 mA cm⁻², which can be attributed to increased HER via Tafel or Heyrovsky mechanisms at higher current densities, resulting in a more negative cathodic potential and reduced faradaic efficiency for nitrate reduction.⁵⁴ Therefore, a working current density of 10 mA cm⁻² was identified for subsequent electrolysis experiments to balance energy efficiency and nitrate removal performance. In addition, both NO₃⁻-N removal efficiency and NH₄⁺-N generation ratio remain above 90% as the initial pH increases from 5 to 9, indicating the robust applicability of the nitrate-to-ammonia process on V_O-NiFe-LDH/CF across a broader initial pH range (Fig. 5c and S30). As shown in Fig. 5d and S31, electrolysis experiments conducted at varying initial NO₃⁻-N concentrations (25–100 mg L⁻¹) revealed that NO₃⁻-N removal capacity decreased from 96.8% to 58% with increasing initial NO₃⁻-N concentration. This observation can be attributed to the saturation of active sites on V_O-NiFe-LDH/CF, which limits nitrate adsorption and activation. These results indicate that the V_O-NiFe-LDH/CF cathode is preferentially effective for low-concentration nitrate-to-ammonia conversion in this system. Although the activity of V_O-NiFe-LDH/CF was suppressed at higher NO₃⁻-N concentrations, ammonia remained high as the dominant product, highlighting its potential for direct application in low-concentration nitrate wastewater treatment. Considering the effects of various constituents in real wastewater,⁵⁵ electrolysis tests were conducted at the V_O-NiFe-LDH/CF cathode with different interfering ions at various concentrations. These results demonstrate minimal impact on catalytic performance (efficiency and selectivity) in this system, indicating the excellent stability of V_O-NiFe-LDH/CF for nitrate removal and ammonia conversion (Fig. 5e–h).

Fig. 5 (a–h) The effect of different operating parameters and interfering ions on electrochemical nitrate-to-ammonia by V_O-NiFe-LDH/CF. (i) Total nitrogen (TN) removal from practical industrial wastewater obtained using V_O-NiFe-LDH/CF.

Indeed, ammonia recovery at low concentrations may lack economic feasibility compared to high-concentration recovery processes.⁵⁶ The treatment strategies for nitrate wastewater across different concentration ranges require in-depth consideration of “waste-to-wealth” and “waste-to-health” principles. Therefore, the subsequent conversion of generated NH₄⁺-N to harmless N₂ assisted by chloride ions in wastewater aligns with human health and ecological sustainability goals.⁵⁷ The electrocatalytic performance of V_O-NiFe-LDH/CF was evaluated at different Cl⁻ concentrations, as presented in Fig. S32. The Cl⁻ addition significantly improves N₂ selectivity, which reaches 98.7% at a Cl⁻ concentration of 1.5 g L⁻¹ under optimal conditions. In addition, the reaction was conducted under weakly alkaline conditions (pH 8–9), which has been demonstrated to effectively suppress the formation of highly toxic chloramines and favor their complete oxidation to N₂ through breakpoint chlorination.^58,59 Furthermore, the treatment ability of V_O-NiFe-LDH/CF for practical industrial nitrate-wastewater was evaluated at an initial total nitrogen (TN) concentration of 114.7 mg L⁻¹ (Fig. 5i), and the water quality parameters are provided in Table S1. The TN was reduced to blow 7 mg L⁻¹ through the synergistic effect of nitrate-to-ammonia conversion at the cathode and active chlorine species oxidation at the anode, achieving a TN concentration below the World Health Organization (WHO) emission limit standard.⁶⁰ Moreover, Ca²⁺, Mg²⁺ and chemical oxygen demand (COD) were efficiently removed during nitrate elimination, accompanied by significant chromaticity reduction in treated wastewater compared to the original sample (Fig. S33). Notably, V_O-NiFe-LDH/CF demonstrates a remarkable reduction in energy consumption for nitrate treatment (2.08 kWh mol⁻¹) compared with the other electrodes, as shown in Table S2.

4. Conclusion

In summary, this work presents a hydrophobic NiFe-LDH electrode formed by in situ electrochemical intercalation of SDBS. Different from traditional intercalation by the ion-exchange method, V_O-NiFe-LDH/CF not only retains the hydrophobic properties of SDBS but also induces rich oxygen vacancies. Structural characterization confirmed the successful synthesis of oxygen vacancy-rich, moderately hydrophobic LDH nanoflowers self-grown on copper foam. Electrochemical tests revealed that V_O-NiFe-LDH/CF accelerates the charge transfer kinetics. The V_O-NiFe-LDH/CF electrode exhibited a NO₃⁻-N removal efficiency of 96.8% and an ammonia selectivity of 96% at an initial NO₃⁻-N concentration of 50 mg L⁻¹, which is 2.1-fold higher than that of NiFe-LDH/CF. The enhanced nitrate electroreduction performance can be attributed to the optimized local electron structure and suppressed HER under low-concentration nitrate wastewater conditions. The hydrophobic surface suppresses the HER side reaction through repelling water molecules, while the introduction of oxygen vacancies modulates the local charge distribution of NiFe-LDH and reduced the energy barrier of hydrogenation. The Bode phase plots and EPR spectra show that V_O-NiFe-LDH/CF retains a good balance between generation and consumption of *H. Density functional theory (DFT) calculations revealed that the introduction of oxygen vacancies results in an optimized d-band center, enhanced nitrate adsorption and a reduced energy barrier for the rate-determining step (*NO to *NOH). Furthermore, the V_O-NiFe-LDH/CF electrode demonstrated outstanding long-term stability and robust anti-interference performance in cyclic durability tests and practical industrial wastewater treatment. This study provides a novel avenue to synthesize electrodes to realize the efficient treatment of low-concentration nitrate wastewater.

Author contributions

Zhifeng Gao: formal analysis, investigation, methodology, writing – original draft. Jianxing Liang: formal analysis, investigation. Mingdi Sun: formal analysis, investigation. Xu Yin: investigation, methodology, supervision. Haojie Ling, methodology. Jiaxiu Wang, methodology. Kan Li: funding acquisition, writing – review and editing. Kajia Wei: funding acquisition, supervision, writing – review and editing. Weiqing Han: funding acquisition, supervision.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Data are available from the corresponding author upon reasonable request.

Supplementary information (SI): additional experimental details including electrochemical measurements, DFT calculations, and catalyst characterization studies including SEM, XPS, etc. See DOI: https://doi.org/10.1039/d6sc02057g.

Acknowledgements

This research was financed by the National Natural Science Foundation of China (No. 52070098, 52570084 & 52000104), the Jiangsu Province Carbon Peak Carbon Neutral Technology Innovation special fund project (BE2022861), the Youth Science and Technology Talent Lifting Project of Jiangsu Province (TJ-2023-099), the Natural Science Foundation of Shanghai (Grant No. 25ZR1402228), the China Postdoctoral Science Foundation under Grant Number 2025M771245, and the Postdoctoral Fellowship Program of CPSF under Grant Number GZC20241003.

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