Electrocatalytic Performance of MOF-Derived Cu-Co Bimetallic Electrode for Nitrate Reduction

Abstract

The massive anthropogenic discharge of nitrate into aquatic environments disrupts the nitrogen cycle, posing threats to both ecosystems and human health. Electrocatalytic nitrate reduction to ammonia (ENRA) enables both nitrate removal and the production of high value-added NH₃. Developing highly efficient and stable electrode materials, along with overcoming the rate-limiting step of nitrite accumulation, is crucial for enhancing the catalytic reduction of nitrate. In this study, by employing Cu as the core metal, a MOF-derived Cu-Co bimetallic catalytic electrode was fabricated as the cathode for converting nitrate to ammonia, while simultaneously resolving the issue of nitrite accumulation. The preparation parameters of the catalytic electrode were optimized, and the effects of current, initial pH, metal loading sequence, and other factors on ENRA performance were investigated. Experimental results demonstrate that the MOF-derived Cu-Co catalytic electrode achieved NO₃⁻ removal and NH₄⁺ generation efficiencies of 94.26% and 90.58%, respectively, under a current density of 20 mA cm⁻² for 150 min, with a marked suppression of NO₂⁻ accumulation. This study provides experimental evidence and technical support for the Cu-catalyzed electroreduction of nitrate to ammonia.

1. Introduction

Nitrate is a ubiquitous inorganic contaminant in aquatic environments. It exhibits high water solubility and mobility, while being poorly immobilized in soil [1]. Excessive nitrate accumulation caused by anthropogenic emissions disrupts the nitrogen cycle, posing risks to both ecosystems and human health. Electrocatalytic reduction converts aqueous NO₃⁻ into N₂ or NH₄⁺ as final products via electron transfer in water. Notably, NH₃/NH₄⁺ serves as a critical chemical feedstock for fertilizer, pharmaceutical, and dye manufacturing, and it also plays a key role in energy storage and carbon-free energy transportation [2,3]. Therefore, electrocatalytic nitrate-to-ammonia conversion, featuring a simple process and low pollutant byproducts, offers a potential alternative route for both nitrate removal and ammonia synthesis.

Significant strides have been made in applying various electrocatalysts toward nitrate reduction [4,5,6]. The catalysts explored encompass single-metal, bimetallic, and metal-oxide types, commonly supported on nickel foam and nanocarbon [7,8,9]. Among these, transition metal catalysts, particularly those based on Cu, Fe, Co, Ni, and their alloys, have been extensively studied due to their availability and economic viability [10,11]. Various coordinating atoms around Cu atoms can effectively regulate the electronic polarity of the catalytic active center. In addition, Cu catalysts are low-cost, possess tunable electronic structures, and can effectively suppress the hydrogen evolution reaction while accelerating the formation of NO₂⁻ intermediates [1,12,13]. Based on these advantages, Cu catalysts are considered among the most promising catalysts for the electrocatalytic nitrate reduction to ammonia (ENRA). Cu single-atom, copper oxides, and Cu-based alloy catalysts have all been reported to be effective in nitrate reduction [14,15].

Transition metal Cu exhibits a strong affinity for nitrate and can effectively reduce nitrate to nitrite. However, this leads to the undesired accumulation of nitrite, which adversely affects the ammonia selectivity. Previous studies have shown [16] that the introduction of a second metal can effectively overcome the linear scaling relationship of single-type active sites, prevent strong adsorption from inhibiting product desorption, and avoid activity loss. The second type active site primarily serves to further reduce the nitrite generated at Cu sites into the final product ammonia. Co exhibits certain electrocatalytic activity, owing to its various oxidation states that offer abundant active centers and facilitate electron transfer during the reaction [17,18,19]. Despite exhibiting electrocatalytic activity for nitrate reduction and generating negligible byproducts during the reaction, Co suffers from relatively weak nitrate adsorption and activation [20]. Combining the advantages of Cu and Co in electrocatalytic nitrate reduction, free nitrate in solution is adsorbed and reduced to nitrite at Cu sites, while the nitrite is further reduced to ammonia at Co sites. The synergistic effect of the Cu-Co dual-active sites effectively reduces the accumulation of intermediate byproducts, enhancing the performance of ENRA [21,22].

In addition to the type of active sites in catalysts, the dispersion of active sites is also a critical factor affecting the performance of catalysts. Metal–organic frameworks (MOFs) have attracted considerable attention due to their ordered porous structures and easily tunable pore sizes. Catalysts derived from MOFs can retain the morphological characteristics of MOF precursors, thereby preserving a large surface area that favors active site exposure. Furthermore, MOF materials readily enable the highly dispersed doping of heteroatoms, which can regulate the local electronic structure and thereby further boost the catalytic activity [23,24,25]. To improve the atomic utilization efficiency of metal catalysts, atomically dispersed metals derived from MOFs have also been employed in nitrate reduction to ammonia [26,27].

In this work, a MOF-derived dual-active-site catalytic electrode (CuCo-MOF-E) was constructed via anodic oxidation and cathodic electrodeposition, which was then employed to investigate its nitrate reduction performance in aqueous solutions. The effects of different current densities, initial pH values, and electrode fabrication conditions on the nitrate reduction performance of the prepared catalytic electrodes were investigated. This work strengthens the experimental foundation for the design of highly efficient electrocatalytic electrodes for the conversion of nitrate to ammonia.

2. Results and Discussion

2.1. Electrocatalytic Reduction Performance

MOF-derived CuCo catalytic electrodes were prepared via anodic oxidation and cathodic reduction methods. The Co(OH)₂/Cu electrode was obtained by electrodepositing Co onto a Cu mesh substrate. This electrode was then used as the anode to prepare a MOF-derived CuCo-MOF precursor via an anodic oxidation method [28]. Subsequently, the CuCo-MOF precursor was subjected to in situ electrochemical reconstruction [29] via a cathodic reduction process, forming the CuCo composite catalytic electrode, denoted as CuCo-MOF-E.

The SEM image of the CuCo-MOF-E electrode is shown in Figure 1a. The electrode surface exhibits a mesh-like structure with uniform Cu distribution. No obvious Co signal was detected by EDS scanning. The XRD patterns (Figure 1b) also indicate that the electrode presented a distinct Cu₂O signal and a relatively weak CoO signal. This is attributed to the use of Cu mesh as the electrode substrate and the low Co loading, where the intense Cu signal overshadows the Co signal. Similarly, no obvious Co signals were observed for the precursor catalyst powders CuCo-MOF. Since the electrode underwent electrochemical reconstruction via cathodic reduction, the electrode surface exists as Cu₂O/Cu, primarily in the form of low-valence Cu⁺. This serves as the catalytically active species for nitrate reduction, which is consistent with the XRD results. Further compositional analysis by ICP-MS was performed, and the results are shown in

Table 1. Concentration of Co in the CuCo-MOF-E catalytic electrode by ICP.

Element	Concentration (mg L−1)	Content (%)
Co	0.17 × 10−1	0.24

. The Co content in the CuCo-MOF-E electrode was quantitatively analyzed, confirming the successful fabrication of the CuCo-MOF-E bimetallic catalytic electrode. The low Co content (0.24%) may be attributed to the dissolution of Cu²⁺ from the Cu mesh during the anodic oxidation process for MOF precursor formation, which consequently limits the incorporation of Co.

To comparatively investigate the respective effects of Cu and Co on the reduction performance of the catalytic electrodes, CuCo-MOF-E, Cu-MOF-E (without Co), and Co(OH)₂/Cu (without Cu MOF structure) electrodes were prepared as working electrodes. Linear sweep voltammetry (LSV) was employed to investigate the nitrate reduction activity of the electrodes, as shown in Figure 2a. In a three-electrode system, the current densities of the LSV curves for Cu-MOF-E, Co(OH)₂/Cu, and CuCo-MOF-E catalytic electrodes significantly increased in the presence of NO₃⁻, indicating that these catalytic electrodes exhibit good reduction activity toward nitrate. Analysis of the electrochemical impedance spectroscopy (EIS) (Figure 2b) reveals that the charge transfer resistance of CuCo-MOF-E is significantly lower than those of Co(OH)₂/Cu and Cu-MOF-E. This suggests that CuCo-MOF-E has the fastest electron transfer efficiency in the nitrate reduction.

The NO₃⁻ removal efficiency (RE), NO₂⁻, and NH₄⁺ generation efficiency (GE) of the three electrodes under 20 mA cm⁻² for 150 min are illustrated in Figure 2c,d. Cu-MOF-E benefits from the favorable enrichment and activation of NO₃⁻ by its porous framework, resulting in a higher NO₃⁻ RE than that of CuCo-MOF-E. However, its NH₄⁺ GE (85.90%) is lower than that of CuCo-MOF-E (90.58%), accompanied by relatively high NO₂⁻ residue (approximately 50%) during the reaction. This can be ascribed to the limitations of single-type active sites in Cu-MOF-E regarding the adsorption energy and hydrogenation kinetics toward NO₂⁻ intermediates, which cannot fully match the rapid preceding NO₃⁻ reduction step. In contrast, no NO₂⁻ accumulation was observed during the nitrate reduction process by using a Co(OH)₂/Cu electrode, indicating that the electrodeposition of Co(OH)₂/Cu on Cu mesh facilitates the conversion of NO₂⁻ to NH₄⁺. Therefore, upon introducing Co to construct CuCo-MOF-E, the bimetallic synergistic effect significantly optimizes the electronic structure. The incorporation of Co not only enhances the electrode’s adsorption capability toward nitrogen-containing intermediates and suppresses the desorption of NO₂⁻, but also accelerates the subsequent hydrogenation step. Thus, while maintaining high NO₃⁻ RE, the NH₄⁺ GE is also improved and the accumulation of intermediate NO₂⁻ is avoided. The nitrate reduction performance of the CuCo-MOF-E electrode prepared in this work, along with related materials reported in recent years, is presented in Table S1 in the Supplementary Materials.

The double-layer capacitance (C_dl) can reflect the number of active sites on the electrode surface to a certain extent. Figure 3 shows the CV curves and double-layer capacitances of CuCo-MOF-E, Cu-MOF-E, and Co(OH)₂/Cu catalytic electrodes obtained at scan rates of 20, 40, 60, 80, and 100 mV s⁻¹ in the non-Faradaic region. The C_dl of CuCo-MOF-E (3.59 mF cm⁻²) is significantly larger than those of Cu-MOF-E (1.46 mF cm⁻²) and Co(OH)₂/Cu (0.20 mF cm⁻²) (Figure 3d). A high electrochemically active surface area can increase the contact area between the catalyst and electrolyte, thereby enhancing the performance of electrocatalytic nitrate reduction.

To investigate the effect of current density on electrocatalytic nitrate reduction, experiments were conducted at current densities of 10, 20, 30, 40, and 50 mA cm⁻² with the electrode of CuCo-MOF-E, and the results obtained at a reaction time of 150 min are shown in Figure 4a. As the current density increased from 10 to 20 mA cm⁻², the NO₃⁻ RE increased from 74.58% to 94.26%, and the NH₄⁺ GE increased from 46.33% to 90.58%. As the current density further increased from 20 to 50 mA cm⁻², the NO₃⁻ RE continued to rise, while the NH₄⁺ GE gradually decreased to 81.36%. This is attributed to the occurrence of side reactions such as hydrogen evolution at excessively high current densities. As shown in Figure 4b, when the current density increased from 10 to 20 mA cm⁻², the current utilization efficiency exhibited an upward trend. At 10 mA cm⁻², the applied current was primarily consumed for the reduction of the catalyst precursor rather than the reduction of nitrate. When the current density increased from 20 to 50 mA cm⁻², the current utilization efficiency decreased from 73.29% to 28.78%, following the same trend as the NH₄⁺ GE. Considering the efficiency of nitrate conversion, ammonia generation, and current utilization, a current density of 20 mA cm⁻² was selected.

2.2. Optimization of CuCo-MOF-E Preparation Parameters

The content of the second active site Co on the CuCo-MOF-E electrode was optimized by controlling the electrodeposition time of Co(OH)₂. Using 25 mL of 20 mM Co(NO₃)₂·6H₂O as the electrolyte, with Cu mesh and Pt sheet as the cathode and anode, respectively, a constant current of 10 mA was applied for 100 s, 200 s, 300 s, and 400 s to obtain Co(OH)₂/Cu electrodes with different deposition times. A constant current experiment at 20 mA cm⁻² was then conducted in 25 mL of 100 mg L⁻¹ NO₃⁻-N aqueous solution (containing 0.05 M Na₂SO₄) for a reaction time of 150 min.

Figure 5a shows the NO₃⁻ conversion efficiency and NH₄⁺ generation efficiency for the electrodeposition time ranging from 0 to 400 s. As the electrodeposition time increased to 300 s, the NH₄⁺ generation efficiency increased from 87.86% to 90.58%. When the deposition time was further increased to 400 s, both the NO₃⁻ conversion efficiency and NH₄⁺ generation efficiency exhibited a downward trend. This may be attributed to excessive Co(OH)₂ loading on the Cu mesh caused by prolonged electrodeposition time, leading to detachment issues during subsequent preparation processes. The variation trend of NO₂⁻ during the nitrate reduction is shown in Figure 5b. The NO₂⁻ accumulation for catalysts with electrodeposited Co(OH)₂ was significantly lower than that for Cu-MOF-E. Among these, the catalyst prepared under a deposition time of 300 s showed the least NO₂⁻ accumulation, with a maximum NO₂⁻ generation efficiency of only 12.44% at 60 min. Additionally, the ammonia selectivity exhibited the same trend as the NH₄⁺ generation efficiency (Figure 5c), reaching a maximum of 96.40% at a deposition time of 300 s. Therefore, the CuCo-MOF-E electrode with an electrodeposition time of 300 s demonstrated the best activity in this work, and 300 s Co(OH)₂ deposition was adopted for subsequent experiments.

The reduction time of CuCo-MOF was further optimized, as shown in Figure 6a. When the reduction time of the precursor CuCo-MOF increased from 0 min to 10 min, the NO₃⁻ RE increased from 50.89% to 94.26%, and the NH₄⁺ GE increased from 47.56% to 90.58%. Further extension of the reduction time resulted in essentially unchanged NO₃⁻ RE and NH₄⁺ GE. This indicates that the catalytic electrode reached a stable state after 10 min of reduction, and further increasing the reduction time had little effect on its nitrate reduction performance. Figure 6b shows the variation of NO₂⁻ GE during the reaction for catalytic electrodes prepared with different reduction times. The maximum NO₂⁻ GE were all below 20.0%, demonstrating that the prepared catalytic electrodes effectively suppress the accumulation of intermediate byproducts.

To investigate the effect of Cu-Co loading sequence on the catalytic reduction of nitrate, Cu-MOF precursors were first prepared, then Co(OH)₂ was electrodeposited onto the Cu-MOF precursors, and finally, reduction was carried out for 10 min to obtain the Co(OH)₂/Cu-MOF-E catalytic electrode. A constant current experiment at 20 mA cm⁻² was conducted in 25 mL of 100 mg L⁻¹ NO₃⁻-N aqueous solution (containing 0.05 M Na₂SO₄) for a reaction time of 150 min. The NO₃⁻ RE, NO₂⁻ GE, and NH₄⁺ GE are shown in Figure 7a,c. The NO₃⁻ RE (89.48%) and NH₄⁺ GE (84.94%) of Co(OH)₂/Cu-MOF-E were both lower than those of CuCo-MOF-E. This may be attributed to the detachment of some Co(OH)₂ caused by the shedding of organic matter from the electrode surface during the Co(OH)₂/Cu-MOF reduction process. In addition, NO₃⁻ in solution tends to adsorb on copper sites and undergo subsequent conversion, while the surface Co(OH)₂ layer partially hinders the mass transfer process of nitrate, thereby reducing its reduction efficiency. Furthermore, the trend of NO₂⁻ during the reaction process was similar to that of CuCo-MOF-E.

The effect of annealing the precursor on its catalytic activity was investigated. The CoCu-MOF was annealed in a tube furnace at 400 °C for 2 h with a heating rate of 5 °C min⁻¹. The resulting catalyst was subsequently reduced for 10 min to obtain the Co₃O₄/CuxO catalytic electrode. The comparison between Co₃O₄/CuxO and the CuCo-MOF-E electrode is shown in Figure 7b,d. The NO₃⁻ RE and NH₄⁺ GE of Co₃O₄/CuxO were significantly lower than those of CuCo-MOF-E, with the NH₄⁺ GE decreasing by approximately 50.00%. This may be attributed to the detachment of Co₃O₄ from the catalytic electrode surface along with carbonized organic matter during the annealing process, leading to the accumulation of NO₂⁻ during the reaction and thereby reducing the NH₄⁺ GE.

2.3. Electrochemical Stability

The effect of initial solution pH on ENRA was investigated, as shown in Figure 8a. When the initial pH increased from 3 to 7, the NO₃⁻ RE increased from 85.41% to 94.26%, and the NH₄⁺ GE increased from 81.03% to 90.58%. When the pH further increased to 11, the NO₃⁻ RE remained almost unchanged while the NH₄⁺ GE gradually decreased to 83.73%. Overall, the catalytic electrode exhibits optimal nitrate removal performance under neutral conditions. Both NO₃⁻ RE and NH₄⁺ GE maintained high levels above 80%, indicating that the catalytic electrode can sustain high efficiency over a wide pH range.

The CuCo-MOF-E catalytic electrode was subjected to cycling tests for NO₃⁻ reduction at the optimal current density of 20 mA cm⁻². Before each cycle, a sample was taken for testing; after 180 min of reaction, another sample was taken, and the electrolyte was replaced with fresh solution for the next cycle. The results are shown in Figure 8b. Within five consecutive cycles, the NO₃⁻ RE and NH₄⁺ GE showed no significant decrease, demonstrating that the CuCo-MOF-E electrode has remarkable advantages in terms of both the stability and durability for ENRA.

3. Experimental Methods

3.1. CuCo-MOF-E Electrode Preparation

All chemical reagents used in this experiment were of analytical grade. The Cu mesh (100 mesh), serving as the substrate electrode material, was cut into 1 × 1 cm pieces and pretreated to remove surface oxides. It was ultrasonicated in acetone and 1 M H₂SO₄ for 10 min each, then ultrasonicated three times in deionized water and absolute ethanol to obtain clean Cu mesh.

The MOF-derived CuCo-MOF-E catalytic electrode was prepared via anodic oxidation and cathodic reduction methods [28]. First, in 25 mL of 20 mM Co(NO₃)₂·6H₂O electrolyte, the pre-treated copper mesh was used as the cathode and Pt sheet as the anode. A constant current of 10 mA was applied for 5 min to obtain the Co(OH)₂/Cu electrode. Then, the Co(OH)₂/Cu electrode was used as the anode and copper mesh as the cathode. The electrolyte consisted of 12.5 mL ethanol, 12.5 mL water, 1 g hexadecyl trimethyl ammonium bromide (CTAB, the surfactant), 10 mg 1,4,8,11-tetraazacyclotetradecane (TCD, the ligand), and 5 mg tetramethyl ammonium bromide (TMAB, the template agent). At 70 °C, a voltage difference of 10 V was applied between the anode and cathode for 5 min to form the CuCo-MOF electrode on the anode. The electrode was then removed and rinsed with deionized water. Before testing, the CuCo-MOF was used as the working electrode and Pt sheet as the counter electrode, with 100 mg L⁻¹ NO₃⁻-N and 0.05 M Na₂SO₄ as the electrolyte. The final CuCo-MOF-E electrode was obtained by reduction at a current density of 20 mA cm⁻² for 600 s, followed by rinsing with deionized water.

The chemical structural information of the catalytic electrodes was analyzed by X-ray diffractometer (SmartLab 9, Rigaku, Tokyo, Japan) with the Cu target as the X-ray source, a scanning rate of 5° min⁻¹, and a range of 5–90°. The morphological characteristics of the electrodes were analyzed by field emission scanning electron microscope (JSM-7800F Prime, JEOL, Tokyo, Japan) at an operating voltage of 5 kV.

3.2. Electrocatalytic Procedures

The electrocatalytic reduction reaction was performed with an Autolab electrochemical workstation (PGSTAT302N, Metrohm, Herisau, The Netherlands). The CuCo-MOF-E electrode was used as the cathode and Pt sheet as the anode. The cathode and anode were spaced 1 cm apart and immersed in 25 mL of 100 mg L⁻¹ NO₃⁻-N electrolyte (containing 0.05 M Na₂SO₄). A constant current was applied for the reduction experiments. Each test was conducted three times at room temperature under identical conditions. The electrochemical performance tests, including linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), and constant current tests (I-t), were all performed on the Autolab electrochemical workstation.

The concentrations of NO₃⁻, NO₂⁻, and NH₄⁺ in the reactants and products were determined by UV–visible spectrophotometry (HJ/T 346-2007, GB 7493-87, HJ 536-2009), respectively (detailed information is provided in the Supplementary Materials). The conversion efficiency of N-containing species, and the product selectivity of ammonia were calculated to evaluate the catalytic electrode performance. The calculation methods for the main parameters are listed as follows:

$$\mathrm{R}\mathrm{E}\left({{\mathrm{NO}}_3}^{-}\text{-}\mathrm{N}\right)={\displaystyle \frac{\mathrm{C}{\left({{\mathrm{NO}}_3}^{-}\text{-}\mathrm{N}\right)}_0-\mathrm{C}{\left({{\mathrm{NO}}_3}^{-}\text{-}\mathrm{N}\right)}_{\mathrm{t}}}{\mathrm{C}{\left({{\mathrm{NO}}_3}^{-}\text{-}\mathrm{N}\right)}_0}}\times \mathrm{100\%}$$

(1)

$$\mathrm{G}\mathrm{E}\left({{\mathrm{NO}}_2}^{-}\text{-}\mathrm{N},{{\mathrm{NH}}_4}^{+}\text{-}\mathrm{N}\right)={\displaystyle \frac{\mathrm{C}{\left(\mathrm{N}\right)}_{\mathrm{t}}}{\mathrm{C}{\left({{\mathrm{NO}}_3}^{-}\text{-}\mathrm{N}\right)}_0}}\times \mathrm{100\%}$$

(2)

$$\mathrm{S}\left({{\mathrm{NH}}_4}^{+}\text{-}\mathrm{N}\right)={\displaystyle \frac{\mathrm{C}{\left({{\mathrm{NH}}_4}^{+}\text{-}\mathrm{N}\right)}_{\mathrm{t}}}{\mathrm{C}{\left({{\mathrm{NO}}_3}^{-}\text{-}\mathrm{N}\right)}_0-\mathrm{C}{\left({{\mathrm{NO}}_3}^{-}\text{-}\mathrm{N}\right)}_{\mathrm{t}}}}\times \mathrm{100\%}$$

(3)

$$\mathrm{\eta }={\displaystyle \frac{\mathrm{Q}{\left(\mathrm{N}\text{-}\mathrm{products}\right)}_{\mathrm{t}}}{\mathrm{It}/1000}}\times \mathrm{100\%}$$

(4)

$$\mathrm{Q}{\left(\mathrm{N}\text{-}\mathrm{products}\right)}_{\mathrm{t}}={\displaystyle \frac{\mathrm{n}\times \mathrm{C}{\left(\mathrm{N}\right)}_{\mathrm{t}}\times \mathrm{V}\times \mathrm{F}}{{\mathrm{M}}_{\mathrm{N}}}}\times \mathrm{100\%}$$

(5)

where RE(NO₃⁻-N) is the nitrate removal efficiency; GE(NO₂⁻-N, NH₄⁺-N) is the generation efficiency of nitrite and ammonia, respectively; S(NH₄⁺-N) is the ammonia selectivity; η is the current utilization efficiency; C(NO₃⁻-N)₀ is the initial concentration of NO₃⁻N, mg L⁻¹; C(N)_t is the concentration of N-containing species (NO₃⁻, NO₂⁻, and NH₄⁺) at time t, mg L⁻¹. Q(N-products)_t is the total amount of electricity required to convert NO₃⁻-N to N-containing products including NO₂⁻-N and NH₄⁺-N at time t, C; It/1000 is the total amount of electricity provided during time t, C; I is the current, mA; t is the time, s; n is the number of electron transfers; F is the Faraday constant, 96,487 C mol⁻¹; V is the volume of the solution, L; M_N is the molar mass, 14,000 mg mol⁻¹.

4. Conclusions

In this work, a CuCo-MOF-E catalytic electrode was constructed on a conductive Cu mesh substrate through anodic oxidation and cathodic electrodeposition strategies. The MOF structure in the precursor provided a high specific surface area and a three-dimensional hierarchical porous structure, with periodically arranged organic ligands uniformly dispersing Cu and Co active sites while maintaining close spatial proximity. Through ligand-bridged electronic coupling, the coupled tandem catalytic pairs formed by Cu sites and Co sites were strengthened. This dual-site synergistic mechanism resolved the selectivity issue arising from the mismatch between intermediate generation rates and hydrogenation rates on single Cu sites. Under reaction conditions of 20 mA cm⁻² for 150 min, CuCo-MOF-E achieved a NO₃⁻ removal efficiency of 94.26% and an NH₄⁺ generation efficiency of 90.58%, with NO₂⁻ accumulation effectively suppressed. This result not only reveals the unique advantages of MOF structure in bimetallic tandem catalysis, but also provides clear structure–function relationship insights for designing efficient electrocatalysts based on MOF platforms. The high activity and selectivity exhibited by this electrode at moderate current densities make it suitable for decentralized treatment of low-concentration nitrate wastewater and ammonia resource recovery. For this system, systematic investigation of the regulatory effects of MOF ligand types and defect engineering on the microenvironmental acidity/alkalinity and proton transfer rates remains to be addressed; overcoming these challenges will further advance this system toward better industrial applications.