Cobalt nanoparticles coupled with polyoxometalate nanoclusters to boost electrocatalytic conversion of nitrite to ammonia at low potentials

Abstract

The electrochemical reduction of nitrite (NO₂⁻) to ammonia (NH₃) under mild conditions (NO₂RR) not only removes excess NO₂⁻ pollutants from groundwater but also enables sustainable recycling of nitrogen resources. The development and design of electrocatalysts that can efficiently produce NH₃ at relatively low potentials has become one of the bottleneck issues in this electrochemical conversion process. In light of this, this work innovatively coupled polyoxometalate nanoclusters with strong electron reservoir capacity and cobalt nanoparticles with good intrinsic NO₂RR activity to prepare a PMo₁₀V₂/Co@NC/CNT composite electrocatalyst. Herein, the cobalt nanoparticles serve as adsorption and activation sites for NO₂⁻, while the PMo₁₀V₂ clusters act as electron transfer promoters. The experimental results showed that at a relatively low potential of −0.3 V (vs. RHE), the faradaic efficiency of NH₃ could reach 97.09%, with a yield of up to 0.1342 mmol h⁻¹ mg_cat⁻¹. When assembled into a Zn–NO₂⁻ battery using PMo₁₀V₂/Co@NC/CNTs as the cathode, a power density of 4.1 mW cm⁻² was achieved. This study not only provides new insights into the design of high-efficiency cobalt-based NO₂RR electrocatalysts, but also offers a valuable reference for the application of nanomaterial–cluster composites in nitrogen cycle management and sustainable energy conversion.

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1. Introduction

Nitrite (NO₂⁻) represents a crucial intermediate species in the global nitrogen cycle.¹ Elevated NO₂⁻ concentrations have been recognized as a significant water pollutant, exerting substantial adverse effects on human and animal health.^2–5 Consequently, the conversion of NO₂⁻ into either harmless or value-added products carries profound implications for both public health and the remediation of nitrogen cycle imbalances. Ammonia (NH₃) serves as both a vital industrial feedstock and agricultural fertilizer, while also representing a high-energy-density hydrogen storage carrier.^6–10 However, the conventional Haber–Bosch process for NH₃ production is both energy-intensive and accompanied by substantial CO₂ emissions.^11–14 Therefore, the electrochemical reduction of NO₂⁻ in wastewater to value-added ammonia under ambient conditions presents an attractive alternative from both economic and ecological perspectives.^4,15

Due to the inherent complexity of the nitrite reduction reaction (NO₂RR), which involves multi-electron transfer processes and stability challenges of intermediate species, the development of high-efficiency catalysts represents one of the most critical scientific challenges in this field.^16–18 Cobalt-based electrocatalysts have demonstrated remarkable potential due to their effective activation ability toward NO₂⁻, exceptional conductivity, cost-effectiveness, and high stability across broad electrochemical windows.^19–28 In 2022, Sun et al. constructed Co@JDC by modifying cobalt nanoparticles on biomass-derived Juncus carbon materials.²⁹ Co@JDC demonstrated outstanding NH₃ production Faraday efficiency (FE_NH3) and NH₃ yield in alkaline medium at that time, reaching as high as 96.9 ± 2.1% and 2.8 ± 0.1 mol h⁻¹ g_Co⁻¹ at −1.0 V vs. the reversible hydrogen electrode (RHE). In the following text, unless otherwise specified, all the mentioned potentials are those relative to the RHE. Chen's group encapsulated cobalt–iron nanoalloys in a nitrogen-doped carbon framework (CoFe-NC), achieving a relatively high FE_NH3 of 94.5% at −0.7 V and a large NH₃ yield of 4050.6 mg h⁻¹ cm⁻² at −0.8 V.³⁰ Subsequently, Chen's group prepared a Co₃O_4−x/Co composite electrocatalyst by combining Co₃O_4−x with cobalt nanoparticles and investigated its NO₂RR activity (FE_NH3 up to 97% at −0.8 V and NH₃ yield up to 0.628 mmol h⁻¹ cm⁻² at −1.1 V).³¹ Yan's group synthesised a cobalt–tungsten alloy on the surface of self-supported cobalt foam (CoW/CF) and applied it as a NO₂RR electrocatalyst, achieving high FE_NH3 and NH₃ yield at −0.7 V (98.1%, 164.3 mg h⁻¹ cm⁻²).³² Lately, a cobalt porphyrin conjugated polymer was synthesized and explored as an effective NO₂RR electrocatalyst by Cao et al. (an NH₃ yield of 133.39 mg h⁻¹ mg_CoP⁻¹ at −1.0 V and a FE_NH3 of 98.0% at −0.8 V).³³ With the rapid development of single-atom catalysts, some cobalt single-atom NO₂RR electrocatalysts have also been reported successively, such as Co₁/C₃N₄ (an FE of 95.7% and an NH₃ yield of 403.2 μmol h⁻¹ cm⁻²)³⁴ and Co₁Ru (a FE_NH3 of 94.2% and an NH₃ yield of 476.8 μmol h⁻¹ cm⁻²).³⁵ Additionally, Luo et al. developed a series of cobalt–nitrogen–carbon (Co–N–C) electrocatalysts through thermal treatment of Co-MOF precursors at varying temperatures and applied them in the nitrate reduction reaction.³⁶ Thanks to the porous architecture of the precursor, the low-crystalline Co–N–C-500 contains highly active Co–N_x–C sites and a large specific surface area. This structure facilitates mass/charge transfer during the NO₂RR, achieving a FE_NH3 of 78.6% and an NH₃ yield of 1.14 mg h⁻¹ cm⁻² at −0.6 V. Although a number of Co-based electrocatalysts have demonstrated good activity in the NO₂RR process, we noticed that the NH₃ production activity still has room for improvement. Moreover, the optimal working potentials of most of the reported Co-based electrocatalysts are generally high, mostly above −0.6 V. However, the conversion of NO₂⁻ into NH₃ at high potentials not only increases the difficulty of inhibiting the hydrogen evolution side reaction, but also causes the problem of high energy consumption. From the perspective of economic feasibility, developing highly efficient Co-based NO₂RR electrocatalysts with relatively low working potentials is one of the key scientific issues for the application of such catalysts in practical industrial implementation.

Polyoxometalates (POMs), as an important category of inorganic metal oxygen clusters based on molybdenum, tungsten, vanadium, niobium, etc., can undergo gradual, rapid and reversible multi-electron transfer without changing the cluster structure.^37–42 It is precisely because of this unique and interesting property that POMs have been used as a redox mediator and electronic sponge in various electrocatalytic processes to enhance the reaction kinetics.^43–48 Li's research group creatively combined POM clusters with the active center of manganese-carbonyl (MnL); the introduction of POMs could significantly boost the selectivity and activity of MnL in the electrocatalytic reduction of CO₂ to produce CO.⁴⁹ Kan's research group investigated the role of POMs in boosting the electrocatalytic nitrogen reduction reaction (NRR) performance of FeCo-MOF.⁵⁰

Their research results show that the NRR activity of POM/FeCo-MOF increased by 3–4 times compared with pristine FeCo-MOF. This improvement originates from a direct electron channel between the POM cluster and the FeCo-MOF unit, which greatly promotes the electron transfer rate. Inspired by the above discussion, we intend to combine POM clusters with Co nanoparticles to construct a POM/Co NO₂RR electrocatalyst. The composite utilizes Co for NO₂⁻ adsorption/activation and POM clusters to boost electron transfer, improving NH₃ production while reducing the working potential of Co nanoparticles. To the best of our knowledge, research on composites of POM clusters and transition metal-based materials including Co nanoparticles in the electrocatalytic NO₂RR remains unexplored.

Considering the excellent redox activity and stability of Keggin-type H₅PMo₁₀V₂O₄₀·34.5H₂O (PMo₁₀V₂) over a wide pH range,^51,52 it was chosen as the electron transfer accelerator to combine with Co nanoparticles to construct the PMo₁₀V₂/Co composite and applied it as an electrocatalyst in the NO₂RR in this work. To expose more active sites and to enhance the conductivity of the material, we first prepared Co@NC/CNTs using cobalt-based ZIF-67 strung with carbon nanotubes (CNTs) as the precursor. Subsequently, PMo₁₀V₂ was anchored to the Co@NC/CNT surface through electrostatic interactions to obtain the final PMo₁₀V₂/Co@NC/CNT composite. Consistent with our expectations, such a PMo₁₀V₂/Co@NC/CNT composite indeed demonstrates outstanding electrocatalytic NO₂RR performance in neutral electrolyte; a FE_NH3 of 97.09% and an NH₃ yield of 0.1342 mmol h⁻¹ mg_cat⁻¹ could be obtained at a relatively low voltage of −0.3 V. It is worth mentioning that PMo₁₀V₂/Co@NC/CNTs can also be used as a cathode in Zn–NO₂⁻ batteries, where the removal of NO₂⁻, the production of NH₃ and the output of electricity (a high power density of 4.1 mW cm⁻²) can be achieved simultaneously. This work provides a certain reference for the future exploration of the composites of POM clusters and transition metal-based nanomaterials for boosting their electrocatalytic NO₂RR activity.

2. Experimental

2.1. Synthesis of H₅PMo₁₀V₂O₄₀·34.5H₂O (PMo₁₀V₂)

The synthesis of H₅PMo₁₀V₂O₄₀·34.5H₂O was performed according to reported literature methods,⁵³ with detailed procedures provided in the SI. The elemental composition ratio of PMo₁₀V₂ was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). According to the results, the Mo/V elemental ratio in PMo₁₀V₂ was determined to be 4.66 : 1, which is close to the theoretical ratio of 5 : 1 (Table S1). In addition, the X-ray diffraction (XRD) pattern matched well with the theoretical diffraction data, confirming the successful synthesis of the PMo₁₀V₂ cluster (Fig. S1a).⁵⁴ In the following description, it will be abbreviated as PMo₁₀V₂.

2.2. Synthesis of (n-Bu₄N)₄H[P(Mo₁₀V₂)O₄₀]

Since PMo₁₀V₂ polyanions are soluble in water, an in situ substitution method was employed to prepare a (n-Bu₄N)₄H[P(Mo₁₀V₂)O₄₀] control sample for electrochemical testing. The detailed synthesis procedure can be found in the SI. Hereafter, it will be referred to as TBA-PMo₁₀V₂ for brevity. To evaluate the stability of the PMo₁₀V₂ polyanion under neutral conditions, we immersed TBA-PMo₁₀V₂ in 0.1 M PBS (pH = 7) containing 0.1 M NaNO₂ for 24 hours and subsequently conducted FT-IR analysis. The unchanged IR spectra before and after treatment confirm the structural integrity of PMo₁₀V₂, demonstrating its robust stability in neutral media (Fig. S1b).

2.3. Synthesis of ZIF-67, ZIF-67/CNTs and Co@NC/CNTs

The detailed synthesis process can be found in the SI.

2.4. Synthesis of PMo₁₀V₂/Co@NC/CNTs

The prepared PMo₁₀V₂ and Co@NC/CNTs were dispersed in 10 mL of deionized water at mass ratios of 1 : 20, 1 : 10 and 1 : 5, followed by ultrasonication for 10 minutes at room temperature. After ultrasonication, the solution was transferred to a three-neck flask and refluxed at 80 °C for 2 hours. The resulting products were washed three times with deionized water and then vacuum-dried at 60 °C for 12 hours to obtain the final products, denoted as PMo₁₀V₂/Co@NC/CNTs (1 : 20), PMo₁₀V₂/Co@NC/CNTs (1 : 10) and PMo₁₀V₂/Co@NC/CNTs (1 : 5), respectively.

Through further analysis of the feeding ratio, it was observed that when the PMo₁₀V₂ to Co@NC/CNT ratio was increased from 1 : 20 to 1 : 10, the concentrations of P, Mo, and V elements in the composite catalyst approximately doubled. However, further increasing the ratio to 1 : 5 did not result in the anticipated proportional enhancement of these elemental concentrations (Table S1). Therefore, considering raw material cost efficiency, the optimal feeding ratio was ultimately determined to be 1 : 10 (PMo₁₀V₂ : Co@NC/CNTs) for subsequent preparations. In subsequent descriptions, PMo₁₀V₂ : Co@NC/CNTs (1 : 10) will be abbreviated as PMo₁₀V₂/Co@NC/CNTs.

2.5. Synthesis of PMo₁₀V₂/Co@NC/CNTs-mix

5 mg TBA-PMo₁₀V₂ was evenly ground with 50 mg Co@NC/CNTs in a mortar, and the resulting mixture was named PMo₁₀V₂/Co@NC/CNTs-mix.

3. Results and discussion

The preparation process of the PMo₁₀V₂/Co@NC/CNT nanocomposite is illustrated in Fig. 1a. First, the necklace-like ZIF-67/CNT precursor is obtained through in situ growth on the surface of CNTs.⁵⁵ Subsequently, the ZIF67/CNT precursor is further pyrolyzed in a furnace to obtain cobalt nanoparticle aggregates embedded in a nitrogen-doped carbon framework and connected in series with CNTs (Co@NC/CNTs).⁵⁶ Finally, PMo₁₀V₂ is loaded onto the surface of Co@NC/CNTs or confined within the CNTs through a low-temperature reflux method, resulting in the formation of PMo₁₀V₂/Co@NC/CNTs.

Fig. 1 (a) Synthesis process of PMo₁₀V₂/Co@NC/CNTs; (b) SEM image of PMo₁₀V₂/Co@NC/CNTs; (c and d) TEM images of PMo₁₀V₂/Co@NC/CNTs; (e) HRTEM image of PMo₁₀V₂/Co@NC/CNTs; (f) HAADF-STEM image of PMo₁₀V₂/Co@NC/CNTs; and (g–m) EDX maps of Co, N, C, P, Mo, V, and O elements in PMo₁₀V₂/Co@NC/CNTs.

The scanning electron microscopy (SEM) image of PMo₁₀V₂/Co@NC/CNTs is shown in Fig. 1b. In the pristine necklace-like ZIF-67/CNTs (Fig. S2), the ZIF-67 component displays a well-defined rhombic dodecahedral morphology. However, pyrolysis at elevated temperatures induced partial structural collapse of ZIF-67, resulting in the characteristic quasi-spherical morphology of Co@NC/CNTs (Fig. S3). As shown in Fig. 1b, the SEM image of PMo₁₀V₂/Co@NC/CNTs is similar to that of Co@NC/CNTs, indicating that the loading of PMo₁₀V₂ has almost no effect on the overall morphology of the composite material. From the transmission electron microscopy (TEM) images of PMo₁₀V₂/Co@NC/CNTs (Fig. 1c and d), it can be observed that each CNT is axially penetrated by multiple pyrolyzed ZIF-67 assemblies formed under high-temperature treatment. The high-resolution transmission electron microscopy (HRTEM) image in Fig. 1e displays lattice fringe spacings of 2.03 Å and 3.5 Å corresponding to Co (111) and CNTs (002) planes, respectively.^57,58 From high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) mapping, it can be found that Co, N, C, P, Mo, V and O elements are uniformly dispersed on the whole PMo₁₀V₂/Co@NC/CNT electrocatalyst, confirming the successful anchoring of PMo₁₀V₂ (Fig. 1f–m).

By comparing the XRD patterns of ZF-67 obtained from experiments and theoretical simulations, as well as the XRD patterns of CNTs, ZF-67 and ZF-67/CNTs, it can be found that the precursors of ZF-67 and ZF-67/CNTs have been successfully synthesized (Fig. S4). As shown in Fig. 2a, the XRD pattern of Co@NC/CNTs reflects that the high-temperature annealing process successfully formed cobalt nanoparticles, as evidenced by the good agreement with the standard reference card PDF#15-0806 (Co). Here, the peak located at 26° should be attributed to CNTs in Co@NC/CNTs.⁵⁹ In the XRD pattern of PMo₁₀V₂/Co@NC/CNTs, in addition to the peaks belonging to Co@NC/CNTs, peaks belonging to PMo₁₀V₂ at 26°, 29.76° and 36.37° are also observed, which once again confirms the successful combination of the two components. To further illustrate the presence of PMo₁₀V₂ in the PMo₁₀V₂/Co@NC/CNT composite, we studied the Fourier transform infrared (FT-IR) spectra of PMo₁₀V₂ and PMo₁₀V₂/Co@NC/CNTs, respectively. It can be seen from Fig. 2b that PMo₁₀V₂ exhibits four Keggin characteristic peaks at 600–1200 cm⁻¹, representing asymmetric stretching vibrations of P–O_a, Mo=O_d, Mo–O_b–Mo, and Mo–O_c–Mo from left to right, and this structural configuration remains unchanged after electrostatic self-assembly.⁶⁰

Fig. 2 (a) XRD patterns of Co@NC/CNTs, PMo₁₀V₂/Co@NC/CNTs and PMo₁₀V₂; (b) the FT-IR spectra of PMo₁₀V₂/Co@NC/CNTs and PMo₁₀V₂.

The obtained samples were investigated by X-ray photoelectron spectroscopy (XPS) analysis. The survey scan of PMo₁₀V₂/Co@NC/CNTs further proves the presence of Co, N, C, P, Mo, and V elements in accordance with the XRD and HAADF-STEM mapping results (Fig. 3a). The upper part of Fig. 3b displays the high-resolution Co 2p spectrum of Co@NC/CNTs, with two pairs of 2p_3/2/2p_1/2 doublets belonging to Co⁰ (778.7/793.5 eV) and Co²⁺ (780.5/796.5 eV), respectively, while the binding energies of 786.03 and 802.1 eV correspond to their shake-up satellite peaks (Fig. 3b).^61,62 In addition, it should be noted that the generation of Co²⁺ may be attributed to the inevitable surface oxidation of Co⁰ in Co@NC/CNTs.⁶³ After PMo₁₀V₂ loading (lower part of Fig. 3b), it was observed that the binding energies of both Co⁰ and Co²⁺ slightly shifted towards the higher binding energy direction. Notably, in Fig. 3c, the binding energy of Mo 3d in the PMo₁₀V₂/Co@NC/CNT composite shifts to the lower binding energy region compared with pure PMo₁₀V₂ (for PMo₁₀V₂, the peaks at 236.3 and 233.2 eV are assigned to Mo 3d_3/2 and Mo 3d_5/2, respectively).⁶⁴ A similar phenomenon can be observed in the comparison of the V 2p and P 2p spectra of PMo₁₀V₂ and PMo₁₀V₂/Co@NC/CNTs (Fig. 3d and S5). These experimental results clearly indicate the existence of a strong interaction between PMo₁₀V₂ and Co@NC/CNTs in the PMo₁₀V₂/Co@NC/CNT composite.^64,65 The N 1s and C 1s spectra of PMo₁₀V₂/Co@NC/CNTs are displayed in Fig. S6.

Fig. 3 (a) XPS full-scan spectra of PMo₁₀V₂/Co@NC/CNTs. High-resolution (b) Co 2p spectra of Co@NC/CNTs and PMo₁₀V₂/Co@NC/CNTs. High-resolution (c) Mo 3d and (d) V 2p spectra of PMo₁₀V₂ and PMo₁₀V₂/Co@NC/CNTs.

To investigate the NO₂RR activity of PMo₁₀V₂/Co@NC/CNTs, a standard three-electrode H-type electrolytic cell was employed for an electrochemical test under ambient temperature and pressure. The concentrations of possible products NH₃ and N₂H₄ were analyzed by UV-vis spectrophotometry (Fig. S7 and S8). Linear sweep voltammetry (LSV) was first conducted to study the activity of the PMo₁₀V₂/Co@NC/CNT electrocatalyst in the presence and absence of NO₂⁻. The test results are shown in Fig. 4a; the current density of PMo₁₀V₂/Co@NC/CNTs in the electrolyte containing 0.1 M NO₂⁻ is obviously enhanced compared with that without NO₂⁻, revealing that this composite electrocatalyst possesses a response to the NO₂RR. Additionally, the NO₂RR performance of Co@NC/CNT and PMo₁₀V₂ control electrocatalysts was also tested using LSV. The results demonstrate that the PMo₁₀V₂/Co@NC/CNT electrode exhibits higher current density than these two control samples over a wide negative potential range, confirming its superior electrocatalytic activity toward the NO₂RR (Fig. S9). Subsequently, we conducted a series of potentiostatic electrolysis experiments at various applied potentials and quantified the target product NH₃ using the indophenol blue method to evaluate the NO₂RR activity of the various electrocatalysts. PMo₁₀V₂/Co@NC/CNTs displays apparent NO₂RR activity with a record-high NH₃ production Faraday efficiency (FE_NH3) of 97.09% and an NH₃ yield of 0.1342 mmol h⁻¹ mg_cat⁻¹ at −0.3 V (Fig. S10), and their UV-vis curves and chronoamperometry (CA) curves are shown in Fig. S11. The electrochemical performance tests reveal that Co@NC/CNTs achieves optimal electrocatalytic performance at −0.4 V with a FE_NH3 of 77.4%, while pristine TBA-PMo₁₀V₂ exhibits an optimal FE_NH3 of 41.74% at −0.3 V, as shown in Fig. S12 and S13. By comparing the NO₂RR performance parameters of these three electrocatalysts (Fig. 4b and c and Tables S2 and S3), we found that both the FE_NH3 and NH₃ yields of PMo₁₀V₂/Co@NC/CNTs are significantly higher than those of Co@NC/CNTs and TBA-PMo₁₀V₂ throughout the electrochemical test window, indicating that anchoring PMo₁₀V₂ on the surface of Co@NC/CNTs could indeed enhance the NO₂RR activity of Co@NC/CNTs. Moreover, the composite PMo₁₀V₂/Co@NC/CNTs demonstrates not only significantly enhanced FE_NH3 and NH₃ yield, but also exhibits a positive shift of the optimal working potential from −0.4 V to −0.3 V compared to Co@NC/CNTs. This decrease of cathodic potential represents a crucial advancement for developing energy-efficient nitrite reduction electrocatalytic systems. As clearly illustrated in Fig. 4d and Table S4, the PMo₁₀V₂/Co@NC/CNT composite catalyst developed herein demonstrates superior NO₂RR performance over most reported systems, achieving 97.09% FE_NH3 at a low working potential of −0.3 V.^{29,31,34,66–74} Furthermore, to investigate the promoting effect of PMo₁₀V₂ reflux loading on electron transfer, we prepared a physical mixture of PMo₁₀V₂ and Co@NC/CNTs as a control sample (denoted as PMo₁₀V₂/Co@NC/CNTs-mix, Fig. S14). Electrochemical tests demonstrate that within the potential range of −0.1 V to −0.5 V, both the FE_NH3 and the NH₃ yield of PMo₁₀V₂/Co@NC/CNTs were significantly higher than those of PMo₁₀V₂/Co@NC/CNTs-mix, with its optimal reaction potential shifting positively (Fig. S15a and Tables S5 and S6). The corresponding LSV curves, CA curves and UV-vis absorption spectra are shown in Fig. S15b–S15d, respectively.

Fig. 4 (a) LSV curves with/without 0.1 M NO₂⁻ of PMo₁₀V₂/Co@NC/CNTs in 0.1 M PBS (PH = 7) electrolyte; (b) comparison of PMo₁₀V₂/Co@NC/CNTs, Co@NC/CNTs, and TBA-PMo₁₀V₂ in terms of FE_NH3 from −0.1 V to −0.5 V; (c) comparison of PMo₁₀V₂/Co@NC/CNTs, Co@NC/CNTs and TBA-PMo₁₀V₂ in terms of NH₃ yields from −0.1 V to −0.5 V; and (d) comparison of FE_NH3 of the NO₂RR on PMo₁₀V₂/Co@NC/CNTs and some reported NO₂RR electrocatalysts.

By quantifying hydrazine (N₂H₄) generation during the NO₂RR, we systematically evaluated the reaction selectivity of PMo₁₀V₂/Co@NC/CNTs. Notably, we found that PMo₁₀V₂/Co@NC/CNTs did not produce N₂H₄ during the NO₂RR process, which confirms that PMo₁₀V₂/Co@NC/CNTs has high selectivity for NH₃ synthesis (Fig. S16). Alternating electrolysis was conducted at −0.3 V for 6 cycles, switching between solutions with and without 0.1 M NO₂⁻. Clearly, ammonia production occurred exclusively in the presence of NO₂⁻ (Fig. 5a). This result is also clearly observable from the CA curves and UV-vis spectra (Fig. S17). It can be further seen from Fig. 5b and S18 and 19 that no significant NH₃ generation (<0.1342 mmol h⁻¹ mg_cat⁻¹) was detected in all control experiments (open-circuit potential, blank solution, and 0.1 M PBS without NO₂⁻ after 1 h electrolysis),⁷⁵ which conclusively demonstrates that NH₃ production originates exclusively from NO₂⁻ reduction, eliminating electrolyte and equipment interferences. Additionally, at −0.3 V, we conducted tests at intervals of three hours (3 h, 6 h, 9 h and 12 h) and found that the FE_NH3 demonstrated a certain level of stability over prolonged testing (Fig. 5c). Moreover, after 12 hours of testing, the FE_NH3 reached 96.15% (Table S7). More importantly, after 120 hours of electrolysis at −0.3 V, the current density of PMo₁₀V₂/Co@NC/CNTs remained nearly unchanged, and its LSV curves before and after electrolysis showed minimal variation (Fig. 5d and S20). Furthermore, its faradaic efficiency and NH₃ yield remained stable over ten consecutive electrolysis cycles, further confirming the exceptional stability of this catalyst (Fig. 5e). Moreover, post-characterization SEM/TEM analyses confirm that the composite material maintains its original necklace-like architecture without observable agglomeration, demonstrating excellent structural stability throughout the testing process (Fig. S21). Besides, the characteristic diffraction peaks of PMo₁₀V₂/Co@NC/CNTs in the XRD patterns and FT-IR spectra before and after testing are clearly visible and show no significant changes, indicating that its structure remains intact (Fig. S22). ECSAs were evaluated using the double-layer capacitance (C_dl) technique, and nonpolarized CV curves of these catalysts were recorded within the potential range of 1.1 V to 0.9 V (Fig. S23 and S24). The PMo₁₀V₂/Co@NC/CNT catalyst shows the largest C_dl value of 17.39 mF cm⁻² compared to the other two samples, suggesting that the introduction of PMo₁₀V₂ can effectively increase the electrochemical surface area and further enhance NO₂RR performance. In addition, we conducted performance tests over a wide range of nitrite concentrations (0.1–1 M) to evaluate the feasibility of PMo₁₀V₂/Co@NC/CNTs for industrial or large-scale applications. The results demonstrated that the faradaic efficiency for ammonia production remained above 92% across the entire tested concentration range, while the ammonia production rate and yield showed an increasing trend (Fig. S25 and S26).

Fig. 5 (a) The NH₃ yields and FE_NH3 of PMo₁₀V₂/Co@NC/CNTs during alternating cycling tests at −0.3 V; (b) the NH₃ yields and FE_NH3 of PMo₁₀V₂/Co@NC/CNTs measured in different control experiments; (c) the NH₃ yields and FE_NH3 of PMo₁₀V₂/Co@NC/CNTs at −0.3 V under different testing durations (same electrode + electrolyte refresh for each test); (d) the CA curve of PMo₁₀V₂/Co@NC/CNTs during 120 hours of electrolysis at −0.3 V; and (e) the NH₃ yields and FE_NH3 of PMo₁₀V₂/Co@NC/CNTs during cycling tests.

To further rationalize the remarkable NO₂RR activity of PMo₁₀V₂/Co@NC/CNTs, we discussed the possible reaction mechanisms in the electrocatalytic process. From the electrochemical performance mentioned above (Tables S2 and S3), it can be found that the NH₃ yield and FE_NH3 of TBA-PMo₁₀V₂ are almost negligible, which indicates that the electrocatalytic active center should be located on the Co nanoparticles.⁷⁶ In fact, the reported studies have clearly demonstrated that Co nanoparticles are the sites for adsorbing NO₂⁻ ions, and the adsorbed NO₂⁻ can undergo a series of deoxygenation and hydrogenation processes on the Co nanoparticles to form the final target product NH₃.^77,78 Especially, the Co(111) plane has a more favorable energy barrier for the rate-determining step from *NH to *NH₂ compared to other crystal planes; that is, the Co(111) plane is the dominant crystal plane in the NO₂RR process.²⁹ Our experimental results are also consistent with previous reports, as Co@NC/CNTs does show good NH₃ yield and FE_NH3. When the PMo₁₀V₂ nanoclusters are loaded onto the surface of Co@NC/CNTs, the resulting PMo₁₀V₂/Co@NC/CNTs exhibits obviously improved NO₂RR performance at all potentials, and the optimal potential shows a slight positive shift. This improvement in electrocatalytic performance should be attributed to the electron storage and transfer characteristics of PMo₁₀V₂. Here, PMo₁₀V₂ serves as an electron reservoir and transfer station, continuously providing electrons to the active sites of Co nanoparticles through a unidirectional pathway. This process could promote the electron transfer from the electrodes to the active sites, thereby enhancing the overall electrochemical performance (Fig. 6). The electron reservoir function of POMs has also been confirmed by multiple literature studies.^79–82 Overall, in PMo₁₀V₂/Co@NC/CNTs, Co nanoparticles are the main adsorption and activation sites, and PMo₁₀V₂ nanoclusters act as electron transfer promoters, while NC and CNTs contribute to enhancing the overall conductivity of the electrocatalyst.

Fig. 6 Proposed electrocatalytic processes of PMo₁₀V₂/Co@NC/CNTs for the NO₂RR.

We further assessed the electrochemical performance of PMo₁₀V₂/Co@NC/CNTs, which also displayed excellent NO₂RR activity in a proof-of-concept Zn–NO₂⁻ battery. Based on the previous experimental results that PMo₁₀V₂/Co@NC/CNTs has been verified as a high-efficiency NO₂RR electrocatalyst toward NH₃ synthesis, we thus assembled a PMo₁₀V₂/Co@NC/CNT-based Zn–NO₂⁻ battery (Fig. 7a). The assembled battery demonstrates a high open circuit voltage (OCV) of approximately 1.81 V vs. Zn (Fig. 7b). Notably, the constructed Zn–NO₂⁻ battery with the PMo₁₀V₂/Co@NC/CNT cathode achieved a power density of 4.1 mW cm⁻², and it surpasses the performance of most reported NO₂RR electrocatalysts (Fig. 7c and d and Table S8). As expected, the battery shows a FE_NH3 of 94.32% when the current density is up to 9 mA cm⁻² with an NH₃ yield of 0.1312 mmol h⁻¹ mg_cat⁻¹ (Fig. 7e and Table S9). Fig. 7f shows the time-dependent discharging curves of the PMo₁₀V₂/Co@NC/CNT-based Zn–NO₂⁻ battery with different discharging current densities, showing good long-term electrochemical stability.

Fig. 7 (a) Schematic diagram of the PMo₁₀V₂/Co@NC/CNT-based Zn–NO₂⁻ battery; (b) OCV of the PMo₁₀V₂/Co@NC/CNT-based Zn–NO₂⁻ battery. The inset is an optical photograph of the assembled Zn–NO₂⁻ battery; (c) polarization curves and power density of the Zn–NO₂⁻ battery with the PMo₁₀V₂/Co@NC/CNT cathode; (d) comparison of power density between the present assembled Zn–NO₂⁻ battery and some reported Zn–NO₂⁻ batteries; (e) NH₃ yields and FE_NH3 of the Zn–NO₂⁻ battery at different current densities; and (f) discharging tests at various current densities.

4. Conclusions

This study successfully constructed a composite electrocatalyst based on PMo₁₀V₂ and cobalt nanoparticles (PMo₁₀V₂/Co@NC/CNTs) for highly efficient electrocatalytic nitrite reduction to ammonia. The experiments demonstrated that the introduction of PMo₁₀V₂ significantly enhanced the electron transfer capability of the catalyst, enabling the reaction to achieve a FE_NH3 of 97.09% and an NH₃ yield of 0.1342 mmol h⁻¹ mg_cat⁻¹ at a low potential of −0.3 V, surpassing most reported cobalt-based NO₂RR catalysts. More notably, when this PMo₁₀V₂/Co@NC/CNT electrocatalyst was applied to a Zn–NO₂⁻ battery system, it demonstrated exceptional power density and FE_NH3, confirming its great potential for large-scale electrochemical conversion and energy storage systems. In summary, this study investigated the potential synergistic effects between PMo₁₀V₂ and transition metals, not only providing valuable insights into the relevant catalytic mechanisms but also offering new strategies for developing highly efficient and energy-saving NO₂RR catalysts. These findings hold significant implications for environmental pollution control and sustainable nitrogen resource utilization.

Author contributions

The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. The authors declare no competing financial interest.

Conflicts of interest

There are no conflicts of interest to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI), and comprise, but are not limited to, XRD, TEM, SEM, and electrochemical testing results. Supplementary information is available. See DOI: https://doi.org/10.1039/d5qi01527h.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22201244, 22374125, 21971221 and 21773203), the Yangzhou University Interdisciplinary Research Foundation for Chemistry Discipline of Targeted Support (yzuxk202010), the High-Level Entrepreneurial and Innovative Talents Program of Jiangsu, the ‘Qing Lan Project’ in Colleges and Universities of Jiangsu Province, the Lvyangjinfeng Talent Program of Yangzhou, and the China Post-doctoral Science Foundation (2022M722688).

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