Dual-Site Synergy of Ag/FeOOH Boosts Electrocatalytic Reduction of Nitrate

Abstract

In nitrate electrochemical reduction reaction (NO₃RR), competing side reactions like hydrogen evolution often lead to poor selectivity and subpar kinetics, limiting practical use. Herein, using iron oxyhydroxide nanoarrays grown on a titanium mesh as the substrate, silver nanoparticles were introduced onto the tips of the iron oxyhydroxide nanowires via electrochemical deposition, thereby forming an Ag/FeOOH heterojunction electrocatalyst. At −0.85 V, Ag/FeOOH demonstrates excellent performance, with 97.56% ammonium selectivity, 92.45% nitrate conversion rate, and an ammonium yield of 3.21 mg h⁻¹ cm⁻². Furthermore, the Zn-NO₃⁻ battery exhibited a power density of 1.28 mW cm⁻². Ag/FeOOH’s structure enhances interfacial nitrate adsorption and reduces NO₃RR energy barriers, accelerating reaction kinetics. It promotes NO₃⁻-to-NO₂⁻ conversion via dual-site synergy, boosting NH₄⁺ yield and advancing electrocatalyst design.

1. Introduction

Ammonia (NH₃), as a vital chemical, finds extensive and indispensable applications across various industries, including agriculture, fertilizers, textiles, refrigeration, pharmaceuticals, and explosives [1,2,3]. However, current industrial-scale ammonia production primarily relies on the energy-intensive Haber–Bosch process [4,5]. Concurrently, the issue of nitrate pollution in water bodies is becoming increasingly severe [6,7]. Given the high solubility of nitrate ions (NO₃⁻) and the moderate dissociation energy (204 kJ mol⁻¹) of the N=O bond in their molecular structure, the production of ammonia through the electrochemical nitrate reduction reaction (NO₃RR) has emerged as a highly promising alternative pathway. This approach not only effectively mitigates nitrate pollution but also simultaneously generates valuable ammonia [8,9]. Despite its immense application potential, NO₃RR still faces a fundamental challenge in practical applications. Specifically, competitive side reactions such as the hydrogen evolution reaction (HER) and the formation of nitrite ions (NO₂⁻) occur during the NO₃RR process. The presence of these side reactions often leads to suboptimal selectivity and kinetic performance of NO₃RR [10]. Therefore, developing effective strategies to precisely regulate the local electronic environment at the active sites of catalysts, thereby synergistically enhancing catalytic activity, holds crucial significance for achieving high-performance NO₃RR.

The interaction between a metal and its support plays a pivotal role in regulating catalytic performance by modulating the electronic structure, geometric environment, and adsorption properties of active sites, thereby guiding the reaction pathway [11,12]. Introducing new catalytically active sites can create synergistic effects. For instance, anchoring metal atoms onto the surface of oxides or carbon materials enables atomic-level efficient utilization and introduces novel catalytically active sites [13,14,15]. These operations not only construct synergistic effects through strategies such as spatial separation and electronic modulation to enhance catalytic performance but also improve the corrosion resistance and cycle life of catalytic electrodes under harsh reaction conditions (e.g., high acidity and high potential) by regulating the surface structure, composition, and electronic properties of the catalysts. For example, Zhang et al. [16] incorporated Ta into RuO₂, significantly suppressing Ru dissolution while enhancing the activity of the oxygen evolution reaction (OER). The introduction of Ta stabilized the surface structure of RuO₂, reduced the corrosion rate, and rendered its stability in industrial water electrolysis close to that of IrO₂, with an overpotential reduction of 330 mV and a lifespan exceeding 2800 h. Gao et al. [17] constructed a rare-earth oxide (e.g., La₂O₃) coating on the surface of a Pt/γ-Mo₂N catalyst, effectively isolating the active sites from the corrosive environment and enabling the catalyst to achieve a lifespan exceeding 1000 h and a turnover number (TON) exceeding 15 million in methanol–water reforming for hydrogen production.

In this work, a facile electrochemical reduction method was employed to successfully introduce Ag nanoparticles onto the surface of FeOOH. Experimental results demonstrate that this composite material exhibits outstanding performance in both the electrocatalytic NO₃⁻ reduction system and the Zn-NO₃⁻ battery system. FeOOH with introduced Ag nanoparticles offers significant advantages: Firstly, the introduced Ag catalytically active sites can accelerate the conversion rate of NO₃⁻ to NO₂⁻ while synergistically cooperating with the conversion process of NO₂⁻ to NH₄⁺ on FeOOH, thereby comprehensively enhancing the efficiency of the electrocatalytic NO₃RR (nitrate reduction reaction) of the catalytic material. Secondly, based on mixed electrolytes with different feed ratios of NO₃⁻ and NO₂⁻, by leveraging the differences in the conversion efficiencies of NO₃⁻ and NO₂⁻ at the dual active sites of Ag and FeOOH, the electrocatalytic NO₃RR process can be coordinated and optimized, ultimately achieving a superior ammonium yield. In conclusion, the Ag-FeOOH designed through the method of introducing active sites provides a novel approach for the design of electrocatalytic NO₃⁻ reduction catalysts. Moreover, the carefully designed metal interface modification and passivation treatment can induce an asymmetric charge distribution, facilitating electron transfer from Ag to FeOOH, accelerating the electron transfer process, and increasing ammonia production. This study focuses on the interface modification of metal particles and establishes the mutual synergy between metals and supports as a key principle for designing catalytic electric fields, significantly advancing the field of sustainable electrocatalysis for the nitrogen cycle.

2. Results

2.1. Characterization of Catalysts

Silver (Ag) sites were introduced onto FeOOH through electrochemical reduction, resulting in the synthesis of the catalytic electrode material Ag-FeOOH. As depicted in Figure 1a,b, dense and uniform FeOOH nanowires grew on the Ti mesh. Moreover, Ag nanoparticles grew at the tips of these FeOOH nanowires, enhancing both electrical conductivity and catalytically active sites. Additionally, the high-resolution transmission electron microscopy (HRTEM) results of Ag-FeOOH are shown in Figure 1c, revealing that smaller-sized Ag nanoparticles grew on the FeOOH nanowires. Furthermore, the results in Figure 1c indicate that the lattice fringes of 0.12 nm and 0.24 nm correspond to the (2 2 2) and (1 1 1) crystal planes of Ag, respectively, while the lattice fringe of 0.33 nm corresponds to the (2 1 1) [18] crystal plane of FeOOH. Finally, from the energy-dispersive spectroscopy (EDS) analysis of Ag-FeOOH, we observe that the distribution of the Ag element corresponds to the transmission image, confirming the successful introduction of Ag sites.

To gain insights into the electronic structure of the materials, we conducted X-ray photoelectron spectroscopy (XPS) tests on Ag, FeOOH, and Ag-FeOOH, as illustrated in Figure 2a–c. For Ag-FeOOH, the diffraction peak at a binding energy of 731.1 eV in the Fe 2p spectrum corresponds to Fe 2p₃/₂, whereas for FeOOH, the binding energy corresponding to Fe 2p₃/₂ is 731.6 eV. Additionally, in the Ag 3d spectrum of Ag-FeOOH, the binding energy of the Ag 3d₅/₂ diffraction peak is 368.5 eV, while for pure Ag, it is 368.1 eV. By comparing the Fe 2p₃/₂ binding energies of Ag-FeOOH and FeOOH, we observed a negative shift in the binding energy of Fe 2p₃/₂ after the introduction of Ag sites. Similarly, by comparing the Ag 3d₅/₂ binding energies of Ag-FeOOH and pure Ag, we found a positive shift in the binding energy of Ag 3d₅/₂ for Ag introduced onto the FeOOH surface. This indicates that the Ag introduced via electrochemical reduction on the FeOOH surface alters its original electronic structure. Specifically, electrons flow from FeOOH to Ag. The strong interaction between Fe and Ag modifies the electronic structure of the metal centers, thereby influencing the electrocatalytic activity of Ag-FeOOH. Furthermore, as shown in Figure 2e, the contact angles of Ti, Ag, FeOOH, and Ag-FeOOH are not identical. Ti and Ag are non-hydrophilic, whereas FeOOH is a hydrophilic material. Notably, the material obtained by introducing Ag onto the FeOOH surface via electrochemical reduction remains hydrophilic [19]. In studies on electrocatalytic nitrate reduction to ammonium, it has been found that the hydrophilicity of materials facilitates electrolyte transport and enhances the electrocatalytic rate. As depicted in Figure 2d, we performed X-ray diffraction (XRD) tests and analysis on the synthesized Ag-FeOOH. The results revealed diffraction peaks corresponding to both FeOOH and Ag, confirming the successful synthesis of our material.

2.2. NO₃RR Performance

To evaluate the impact of introducing Ag nanoparticles via electrochemical reduction on the electrocatalytic activity of FeOOH, we utilized a single-chamber electrolytic cell to assess the nitrogen reduction reaction (NO₃RR) activity of the catalysts. Initially, the double-layer capacitance (Cdl) and electrochemically active surface area (ECSA) of Ag, FeOOH, and Ag-FeOOH were evaluated through cyclic voltammetry curves obtained at different scan rates. As illustrated in Figure 3, the Cdl of Ag-FeOOH is 16.80 mF cm⁻², which is higher than that of Ag (13.60 mF cm⁻²) and FeOOH (8.96 mF cm⁻²). Consequently, the ECSA of Ag-FeOOH is greater than that of both Ag and FeOOH, initially indicating that Ag-FeOOH possesses superior NO₃RR performance.

Subsequently, to preliminarily investigate the current response of the catalysts to the nitrate reduction reaction (NO₃RR), linear sweep voltammetry (LSV) curves were measured for Ag, FeOOH, and Ag-FeOOH in solutions with and without the NO₃⁻-N electrolyte, as well as for Ag-FeOOH in a NO₃⁻-N-free electrolyte. As shown in Figure 4a, in the electrolyte solution containing NO₃⁻-N, Ag-FeOOH exhibited a stronger current response compared to both Ag and FeOOH, indicating that the Ag-FeOOH catalyst has a more pronounced response to NO₃RR. Specifically, LSV tests were conducted for Ag, FeOOH, and Ag-FeOOH in an NO₃⁻-free electrolyte (0 ppm). The results show that under NO₃⁻-free conditions, the current responses of all three catalysts are significantly lower than those in NO₃⁻-containing electrolytes. Moreover, the current increment of Ag-FeOOH upon introducing NO₃⁻ is markedly larger than that of Ag and FeOOH, indicating that the enhanced current is primarily attributable to NO₃RR rather than HER. Additionally, the contribution from NO₂⁻ to the total current was found to be negligible, further ruling out significant interference from intermediate product reduction. Furthermore, it is evident from the LSV curves that the current begins to deviate at −0.7 V vs. RHE, and this deviation becomes more pronounced with increasingly negative potential. This suggests that the catalyst starts to respond to the electrocatalytic reduction of NO₃⁻ at this potential. Therefore, we selected the voltage range of −0.7 V to −0.9 V vs. RHE as the window for subsequent performance tests. As depicted in Figure 4b,c, from −0.7 V vs. RHE to −0.9 V vs. RHE, the removal rate of NO₃⁻ and the ammonium yield gradually increased, reaching a peak at −0.85 V vs. RHE. At this potential, the NO₃⁻ removal rate, ammonium selectivity, Faradaic efficiency, and ammonium yield were 92.45%, 97.56%, 89.58%, and 3.21 mg h⁻¹ cm⁻², respectively. Subsequently, the changes in the concentrations of NO₃⁻-N, NO₂⁻-N, and NH₄⁺-N over time were evaluated, with the curves presented in Figure 4d. When the voltage was held constant at −0.85 V vs. RHE, NO₃⁻-N rapidly decreased while NH₄⁺-N rapidly increased, demonstrating the rapid conversion of nitrate to ammonium [20]. Meanwhile, the concentration of the NO₂⁻-N intermediate product initially rose and then declined during the reaction, with its concentration being almost negligible, indicating the excellent ammonium selectivity of the material. We conducted performance tests over 10 cycles on the same catalyst, as shown in Figure 4e. The results demonstrated that after multiple cycles of testing, the conversion rate of nitrate, ammonium selectivity, and ammonium yield did not exhibit significant degradation, confirming the excellent stability of the catalyst.

To investigate the performance of the catalytic material Ag-FeOOH under varying initial concentrations of NO₃⁻-N, we conducted electrolysis experiments at gradient potentials with initial concentrations of 50 ppm, 70 ppm, and 100 ppm, respectively. As illustrated in Figure 5, at an initial NO₃⁻-N concentration of 50 ppm, the removal rate and selectivity of NO₃⁻ by the Ag-FeOOH catalyst increased with rising potential, while the Faradaic efficiency exhibited a volcanic distribution, reaching its optimal overall performance at −0.84 V vs. RHE, with values of 87.61%, 92.56%, and 52.33% for removal rate, selectivity, and Faradaic efficiency, respectively. At an initial concentration of 70 ppm NO₃⁻-N, the removal rate and selectivity of NO₃⁻ also increased with the potential, but after reaching the optimal potential of −0.75 V vs. RHE, the Faradaic efficiency declined, showing a volcanic distribution and achieving its best overall performance at −0.85 V vs. RHE, with values of 80.11%, 95.32%, and 65.13%, respectively. At an initial concentration of 100 ppm NO₃⁻-N, the removal rate and selectivity of NO₃⁻ continued to increase with the potential, and after reaching the optimal potential of −0.85 V vs. RHE, the Faradaic efficiency also displayed a volcanic distribution, attaining its peak performance at the same potential, with values of 92.45%, 97.56%, and 89.58% (data adopted from Figure 5), respectively.

The results indicate that the catalytic material Ag-FeOOH demonstrates excellent removal rates of NO₃⁻ across various initial concentrations. Moreover, it exhibits superior selectivity and Faradaic efficiency at higher initial concentrations of NO₃⁻-N, suggesting that the reaction kinetics in the electrocatalytic NO₃RR process can be enhanced by appropriately increasing the initial concentration of the reactant. In summary, the catalytic material Ag-FeOOH showcases outstanding catalytic performance across a wide range of initial concentrations.

The source of ammonium was verified through blank control experiments and ¹⁵N isotope labeling experiments, while the ammonium yield was quantitatively analyzed using ultraviolet spectrophotometry and ¹H nuclear magnetic resonance (¹H NMR) spectroscopy [21]. As shown in Figure 6, the ¹H NMR spectra from our isotope labeling experiments revealed that similar ammonium yields were obtained regardless of whether Na¹⁴NO₃ or Na¹⁵NO₃ was used as the nitrogen source. Furthermore, after conducting electrolysis experiments with Na¹⁴NO₃ and Na¹⁵NO₃, the electrolytes were subjected to ¹H NMR testing. For more precise tracing, ¹H NMR technology with ¹⁵N labeling was employed for analysis. In the ¹H NMR spectra of standard electrolytes containing ¹⁴NH₄Cl and ¹⁵NH₄Cl, the ¹⁴NH₄⁺ ion exhibited a triplet peak at δ = 7.14, 7.05, and 6.96 ppm, whereas the ¹⁵NH₄⁺ ion displayed only a doublet peak at δ = 6.98 and 7.10 ppm. This further confirmed that the ammonium ions were produced through the electrocatalytic reduction of nitrate. The study further quantified the NH₄⁺ concentration using an external standard method (Figure 5d) and compared the results with those obtained by colorimetry, thereby validating the accuracy of the data [22].

2.3. Mechanism Analysis

To investigate the role and mechanism of Ag nanoparticles introduced via electrochemical reduction on the FeOOH surface in the electrocatalytic NO₃RR process, we designed mixed-feed experiments involving NO₃⁻ and NO₂⁻. We observed the linear sweep voltammetry (LSV) curves of Ag, FeOOH, and Ag-FeOOH under different mixed-feed ratios of NO₃⁻ and NO₂⁻ [23]. As illustrated in Figure 7a–c, the current responses of Ag varied under different ratios. Interestingly, as the NO₃⁻ content increased, the current response of the electrode gradually enhanced. Similarly, FeOOH exhibited different current responses under varying ratios, with its current response gradually increasing as the NO₂⁻ content rose. In contrast, Ag-FeOOH displayed irregular current responses under different feed ratios, with the mixed-feed ratio of 70:30 (NO₃⁻ to NO₂⁻) demonstrating the best current response. The optimal NO₃⁻:NO₂⁻ ratio of 70:30 likely arises from a balance between the two parallel reaction pathways. Although the two sites operate independently in terms of catalytic mechanism, the overall current response is still governed by the substrate availability at each site, which is concentration-dependent.

Consequently, we proposed the hypothesis of dual catalytically active sites. Subsequently, electrolysis experiments were conducted on Ag, FeOOH, and Ag-FeOOH in electrolytes with varying feed ratios at −0.85 V vs. RHE. As depicted in Figure 8a, the conversion rates of NO₃⁻ for the three different catalytic materials increased with the rising percentage of NO₃⁻, which aligns with kinetic theory. The results also revealed that FeOOH exhibited significantly lower NO₃⁻ conversion capabilities compared to Ag and Ag-FeOOH, suggesting that NO₃⁻ undergoes rapid conversion at Ag sites. Meanwhile, the concentrations of ammonium produced in the electrolyte after electrolysis experiments with electrode materials Ag, FeOOH, and Ag-FeOOH under different feed ratios are shown in Figure 8b. For FeOOH, the ammonium concentration decreased as the percentage of NO₃⁻ in the electrolyte increased. Conversely, for Ag, the ammonium concentration increased with the rising percentage of NO₃⁻, but a decline occurred when the NO₃⁻ percentage reached 100, compared to 70. This can be attributed to the rapid conversion of NO₃⁻ to NO₂⁻ on Ag, leading to the accumulation of a large amount of NO₂⁻ that occupied the active sites, thereby causing a slight decrease in ammonium yield. In contrast, Ag-FeOOH did not exhibit a linear relationship in performance across different electrolyte ratios, reaching its peak at a NO₃⁻ to NO₂⁻ ratio of 30:70. This indicates that during the electrocatalytic NO₃RR process on the Ag-FeOOH catalyst, different NO₃⁻ to NO₂⁻ ratios result in varying utilization efficiencies of catalytic sites, with a 30:70 ratio being optimal.

Simultaneously, as shown in Figure 8c, the NO₃⁻ removal rates and NO₂⁻ selectivities of Ag, FeOOH, and Ag-FeOOH in electrolytes without NO₂⁻ were examined. The results revealed that Ag and Ag-FeOOH exhibited higher NO₃⁻ removal rates compared to FeOOH, with Ag demonstrating the highest NO₂⁻ selectivity. Furthermore, electrolysis experiments were conducted on Ag and FeOOH in a 100 ppm NO₃⁻ electrolyte, and the changes in different substances over time are illustrated in Figure 8d,e. For Ag, during the electrocatalytic NO₃RR process, NO₃⁻ underwent rapid conversion, and NO₂⁻ continuously increased and accumulated within the first 80 min. In contrast, the conversion efficiency of NO₃⁻ for FeOOH was significantly lower than that of Ag. This implies that the conversion rate of NO₃⁻ to NO₂⁻ at Ag sites is extremely fast, and a substantial portion of NO₂⁻ fails to convert to NH₄⁺ due to its slower conversion rate compared to that of NO₃⁻ to NO₂⁻, resulting in the accumulation of a considerable amount of NO₂⁻. It also indicates that NO₃⁻ is more prone to conversion to NO₂⁻ at Ag sites.

Finally, as shown in Figure 8d, Ag, FeOOH, and Ag-FeOOH were employed as electrocatalysts for the nitrite reduction to ammonium reaction. The results demonstrated that FeOOH exhibited significantly higher NO₂⁻ removal capabilities compared to Ag, with Ag-FeOOH displaying the highest NO₂⁻ removal ability. This suggests that NO₂⁻ is more readily converted to ammonium at FeOOH sites. In summary, the Ag catalytic sites on Ag-FeOOH are primarily responsible for the conversion of NO₃⁻ to NO₂⁻, while the FeOOH sites are responsible for the conversion of NO₂⁻ to NH₃. Under the synergistic catalytic action of these dual sites, NO₃⁻ is rapidly converted to NH₃, achieving highly efficient electrocatalytic NO₃RR.

2.4. Zn-NO₃⁻ Battery Application of Ag-FeOOH

Motivated by the excellent NO₃RR performance of Ag-FeOOH, a Zn–NO₃⁻ battery was assembled using Ag-FeOOH as the cathode and a polished zinc plate as the anode. The following presents the preliminary exploratory results. As illustrated in Figure 9a,b, the battery tested on an electrochemical workstation exhibited a constant open-circuit voltage (1.722 V vs. Zn/Zn²⁺), which was largely consistent with the open-circuit voltage measured using a multimeter. We evaluated the discharge and power density curves of the battery, as shown in Figure 9c. From the discharge curve, it can be observed that as the cathode potential decreased, the output current density exhibited an opposite trend. The power density curve displayed a volcanic shape, with a peak power density of 1.28 mW cm⁻². Additionally, we conducted discharge tests at various current densities, as depicted in Figure 9c. At a current density of 0.5 mA cm⁻², the discharge voltage of the Zn-NO₃⁻ battery tended to stabilize at 0.73 V. Similar stability was maintained at other current densities, indicating that the Zn-NO₃⁻ battery possesses excellent discharge stability. To comprehensively assess the repeatability of the Zn-NO₃⁻ battery, we performed charge–discharge cycle tests and presented the charge–discharge curves in Figure 9e. The results demonstrated that after multiple charge–discharge cycles, the battery’s output voltage remained within a specific range without significant deactivation, fully confirming the charge–discharge stability of the Zn-NO₃⁻ battery [24].

3. Materials and Methods

3.1. Material Preparation

Using a titanium mesh as the substrate, the Ag-FeOOH material was prepared via a two-step process combining hydrothermal synthesis and electroplating. The detailed steps are as follows.

(1)

Hydrothermal Synthesis of FeOOH Catalyst

Place the pre-treated titanium mesh into a 50 mL mixed solution containing 0.03 M FeCl₃·6H₂O and 0.03 M Na₂SO₄. Conduct a hydrothermal reaction at 60 °C under normal pressure for 12 h to obtain the FeOOH catalyst. Rinse the catalyst three times with deionized water and then dry it at 60 °C for subsequent use.

(2)

Electroplating of Ag onto FeOOH

Utilize the as-prepared FeOOH catalyst as the working electrode in a three-electrode system for electroplating. The plating solution is a 50 mL mixed solution containing 3.55 g Na₂SO₄, 0.32 g Na₃C₆H₅O₇, and 0.024 g AgNO₃. Perform electroplating at a potential of −0.35 V vs. RHE using an I-t test for 30 s. By varying the concentration of AgNO₃, electrode materials with different Ag contents can be synthesized. After electroplating, rinse the electrode three times with deionized water and dry it at 60 °C for further use.

3.2. Product Detection

All products were detected using a UV-Vis spectrophotometer [25]. Subsequently, the concentrations of the products in the electrolyte were calibrated using the corresponding standard curves.

(1)

Determination of NO₃⁻-N Concentration: An appropriate amount of the electrolyte sample was pipetted into a centrifuge tube and diluted to 5 mL with deionized water. Then, 200 μL of NH₃SO₃ reagent (5 wt%) was added. The mixture was placed on a laboratory bench and allowed to react at room temperature in the dark for 15 min. After the color reaction was complete, the absorbance values at dual wavelengths of 220 nm and 275 nm were measured using a quartz cuvette in a UV-Vis spectrophotometer. During data processing, the dual-wavelength correction method was used to calculate the net absorbance value (A = A₂₂₀ nm − 2 × A₂₇₅ nm). The standard curve of NO₃⁻-N (Figure 6a,b) was obtained by fitting the absorbance values corresponding to different concentrations of NO₃⁻-N solutions [26].

(2)

Determination of NO₂⁻-N Concentration: First, 25 mL of deionized water was placed in a beaker, followed by the sequential addition of 2 g of C₆H₈N₂O₂S reagent and 0.1 g of C₁₂H₁₄N₂·2HCl while stirring. Then, 5 mL of H₃PO₄ was slowly added dropwise to form a transparent solution. An appropriate amount of the electrolyte sample to be tested was pipetted and diluted to 5 mL, mixed well, and then 100 μL of a freshly prepared color reagent was added. The mixture was placed in a dark box and allowed to stand in the dark for 20 min. The absorbance value of the colored solution was measured at the characteristic absorption wavelength of 540 nm using a UV-Vis spectrophotometer. The standard curve of NO₂⁻-N (Figure 2 and Figure 3b) was obtained by fitting the absorbance values corresponding to different concentrations of NO₂⁻-N solutions [27].

(3)

Determination of NH₄⁺-N Concentration: The concentration of NH₄⁺-N was detected using Nessler’s reagent colorimetric method. First, Nessler’s reagent was prepared by dissolving 0.8 g of NaOH in 5 mL of deionized water and allowing the solution to cool to room temperature. Then, 0.35 g of KI and 0.5 g of HgI₂ were added, and the mixture was stirred until fully dissolved. Subsequently, an appropriate amount of the electrolyte sample was pipetted into a centrifuge tube and diluted to 5 mL with deionized water. After that, 100 µL of NaKC₄H₄O₆ solution (0.5 g mL⁻¹) and 100 µL of Nessler’s reagent were added sequentially. The mixture was placed in a dark box and allowed to stand in the dark for 30 min. The absorbance value of the colored solution was measured at the characteristic absorption wavelength of 420 nm using a UV-Vis spectrophotometer. The standard curve of NH₄⁺-N (There is a typo in the original text; it should be NH₄⁺-N instead of NO₃⁻-N and NO₂⁻-N here, Figure 2 and Figure 3c) was obtained by fitting the absorbance values corresponding to different concentrations of NH₄⁺-N solutions [28]. Gaseous products (N₂, N₂O) were not quantitatively detected. The ammonia selectivity was calculated based on UV-Vis and ¹H NMR results and may be subject to a certain degree of overestimation, which is an inherent limitation of the conventional methods in this field.

3.3. Characterization

X-ray diffraction (XRD) patterns were recorded on a Smart Lab X-ray diffractometer (Beijing Purxi General Instrument Co., Ltd., Beijing, China) operated at 40 KV and 40 mA with Cu Kα radiation in the 2θ range of 10–90°. Scanning electron microscope (SEM) images were obtained by a scanning electron microscope (S-4800, Hitachi High-Technologies Corporation, Tokyo, Japan), which was equipped with an energy-dispersive spectroscopy (EDS, Extreme) detector. Information on the microstructural features and lattice fringes was obtained via transmission electron microscope (TEM) and high-angle annular dark-field scanning TEM (JEM-F200, JEOL Ltd., Tokyo, Japan), while elemental distribution maps were acquired with the attached EDS detector. X-ray photoelectron spectroscopy (XPS) analysis was carried out using a scanning X-ray microprobe (ESCALAB 250Xi Thermo Fisher Scientific, Waltham, MA, USA). Ultraviolet–visible absorption spectra were recorded on a Tianmei UV2600 spectrophotometer (Techcomp Instrument Co., Ltd., Shanghai, China). ¹H NMR was recorded on a JNM-ECZ600R NMR instrument (JEOL Ltd., Tokyo, Japan).

4. Conclusions

In summary, to boost FeOOH’s catalytic activity, we employed electrochemical reduction to deposit Ag nanoparticles on its surface, creating dual catalytic sites. This setup enabled a synergistic catalytic environment for nitrate reduction to ammonium (NO₃RR), optimizing the sequential steps of NO₃⁻ → NO₂⁻ and NO₂⁻ → NH₄⁺, thus enhancing Ag-FeOOH’s electrocatalytic efficiency. Tests using electrolytes with varied NO₃⁻/NO₂⁻ ratios confirmed distinct roles: Ag sites mainly drove NO₃⁻ to NO₂⁻ conversion, while FeOOH sites facilitated NO₂⁻ to NH₄⁺ transformation. Under dual-site synergy, Ag-FeOOH significantly optimized NO₃RR efficiency. The modified FeOOH showed exceptional performance across a wide NO₃⁻ concentration range. In a 100 ppm NO₃⁻-N electrolyte, at −0.85 V vs. RHE, 97.56% ammonium selectivity, 92.45% nitrate conversion, and 3.21 mg h⁻¹ cm⁻² ammonium yield were achieved. The Zn-NO₃⁻ battery with Ag-FeOOH cathode also exhibited excellent electrical performance and stability. This work, by introducing Ag nanoparticles to create dual active sites, effectively coordinated and optimized NO₃RR processes under synergistic catalysis, offering a new strategy for catalyst design and application in electrocatalytic NO₃RR.