Boosting Electrocatalytic Reduction of Nitrate to Ammonia over Co₃O₄ Nanosheets with Oxygen Vacancies

Abstract

Electrocatalytic nitrate reduction into ammonia is promising for its restricted activity and selectivity in wastewater treatment, however, it remains challenging. In this work, Co₃O₄ nanosheet electrodes with rich oxygen vacancies (OVs) (Co₃O_4−x/NF) are prepared and then applied as efficient catalysts for selective electrocatalytic reduction of nitrate to ammonia. The resulting Co₃O_4−x/NF electrodes exhibit high NO₃⁻-N removal efficiency and NH₄⁺-N selectivity, at 93.7% and 85.4%, respectively. X-Ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance spectra (EPR) results clearly reveal the formation of OVs in Co₃O_4−x/NF. The electrochemical characterization results confirm that OVs can effectively improve electron transfer as well as the electrochemically active area. The Co²⁺/Co³⁺ ratio of Co₃O_4−x/NF increases after the electrocatalytic reduction of nitrate, highlighting the crucial role played by Co²⁺ in mediating ammonia production via the Co²⁺/Co³⁺ cycle. These findings offer valuable guidelines for the development of more efficient and sustainable approaches for nitrate-contaminated wastewater treatment and ammonia synthesis.

1. Introduction

Acid rain deposition of nitrogen oxides from the combustion of nitrogen-containing fuels, bacterial decomposition of nitrogen-containing fertilizers, and discharge of nitrate-containing domestic and industrial wastewater are all highly likely to lead to elevated nitrate concentrations in groundwater [1,2,3]. Nitrate (NO₃⁻) levels must be restricted to ensure a balanced ecosystem, otherwise, they can lead to environment pollution (e.g., eutrophication) and threaten the health of human beings (e.g., blue baby syndrome) [4,5,6]. Thus, significant efforts to remove NO₃⁻ have led to the development of a number of concepts and techniques, such as ion exchange, reverse osmosis, biological denitrification, electrodialysis, etc. [7,8,9,10,11,12,13,14]. Although these methods have proven to be efficient technologies for treating certain conditions, they may require significant capital investment. For instance, chemical or biological treatment processes may necessitate the use of reducing agents such as H₂ or acetic acid that serve as electron donors [15,16]. Recognizing the significance of nitrogen (N) is crucial, as it serves as an essential nutrient for plant growth and plays a key role in the production of industrial products [17,18,19]. As such, recovery or extraction of N from NO₃⁻ is greatly beneficial for both sustainable agriculture and industrial production. Unfortunately, current methods for treating waste water result in the conversion of NO₃⁻ to nitrogen gas, resulting in the loss of valuable nitrogen resources. Therefore, developing methods for recovery of N resources from NO₃⁻-contaminated water is critical for both sustainable development and environmental protection.

Currently, the electrocatalytic nitrate reduction reaction (NO₃RR) is generally considered an efficient method for remediating wastewater, with benefits including absence of chemical waste byproduct streams and lower capital costs; thus, it has attracted increasing attention. Importantly, high-value-added products such as ammonia (NH₃) can be the harvesting indication for NO₃⁻RR process [11,20,21]. It is well known that the Haber–Bosch process is the primary method of NH₃ production; this process, however, requires significant energy (~2% of the world’s energy) and emits a substantial amount of greenhouse gases (~1.5% of global carbon dioxide emissions) [22,23]. As a result, NO₃RR may be an alternative approach for producing NH₃ through a more sustainable and environmentally friendly manner while helping to solve water pollution challenges.

The electrocatalyst, an essential component of NO₃RR, directly impacts the rate of NO₃⁻ removal and selectivity of the NH₃ yield. Therefore, the rational design and development of high-performance electrocatalysts is crucial for achieving efficient and selective NO₃RR. Recently, electrocatalysts made of noble metals (e.g., Ru, Pd, and Pt) have been explored and found to exhibit high performance for NH₃ production by NO₃RR; however, undesirable capital output hinders their application [24,25,26]. Transition metal-based nanocatalysts represent an effective alternative to noble metal catalysts, and have gained significant attention for NH₃ production via NO₃RR owing to their low cost and abundance [27,28]. Among them, Co₃O₄ nanocomposite is considered a promising electrocatalyst due to its impressive catalytic properties [29]. Unfortunately, the widespread use of Co₃O₄ is limited by its poor electrical conductivity [30]. Recently, oxygen vacancies (OVs) have been demonstrated to modulate its electronic states, significantly altering catalytic activity [31]. For instance, Jia and co-workers found that the introduction of O-vacancies into TiO₂ nanotubes could significantly enhance the performance of electrodes used for synthesis of NH₃ from NO₃⁻ through electrochemical reduction [3]. Meanwhile, previous studies have demonstrated that the embedding of OVs affects the proportion of Co²⁺ and Co³⁺ in Co₃O₄ [32]. Notably, it has been reported that Co²⁺ can serve as an electron donor for NO₃⁻ reduction during the electrochemical reaction process. Therefore, it is reasonable to expect introduction of OVs into Co₃O₄ nanostructures with given facets to be a promising electrocatalyst for NH₃ production via NO₃RR.

In this work, OV-rich Co₃O₄ nanosheets grown on the surface of nickel foam (Co₃O_4−x/NF) were synthesized via NO₃⁻ electroreduction for use in NH₃ production. X-Ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance spectra (EPR) confirmed that abundant OVs were present in the Co₃O_4−x/NF. Benefiting from the improved electronic structure and Co²⁺/Co³⁺ ratio of Co₃O₄ after the introduction of OVs, the electrocatalyst exhibited high activity vs. Ag/AgCl at −1.2 V, with 93.7% NO₃⁻-N removal efficiency and 85.4% NH₄⁺-N selectivity. When comparing the states before and after the reaction, a significant change in the Co²⁺/Co³⁺ ratio was found, indicating that NH₃ production from NO₃RR was mediated by the Co²⁺/Co³⁺ cycle. This study is believed to be a feasible reference that can help to realize utilization of NO₃⁻ wastewater resources.

2. Materials and Methods

2.1. Chemicals and Reagents

Analytical-grade NaNO₃, NaNO₂, (NH₄)₂SO₄, and Na₂SO₄ were purchased from Kermel Chemical Reagent Co. Ltd. (Tianjin, China) and used to prepare the stock solution. All solutions were prepared using ultrapure water. Nickel foam (NF) purchased from Beijing Pengda Hengtai Technology Co., Ltd. (Beijing, China) was employed as the supporter for growing Co₃O₄ and Co₃O_4−x. In order to remove the impurities on the surface of the NF, it was rinsed several times with acetone and 1 M HCl, then again with ultrapure water.

2.2. Preparation of Electrodes

A piece of NF (3 cm × 2 cm × 0.1 cm) was first cleaned several times with 1 M HCl, acetone, ethanol, and H₂O to remove impurities. Next, Co₃O₄/NF was synthesized through a hydrothermal and annealing method. In the typical processes, 0.4202 g of CoSO₄·7H₂O and 0.36 g of urea were dissolved in 20 mL of H₂O and 20 mL of ethanol, forming a wine-red solution. Afterwards, this solution was transferred to a 100 mL Teflon-lined autoclave and the cleaned NF was simultaneously soaked in the mixture. After standing for 12 h without stirring, the Teflon-lined autoclave was heated at 90 °C for 8 h. A pink lump (2CoCO₃·3Co(OH)₂·H₂O/NF) was obtained and then calcined at 250 °C for 2 h under N₂ atmosphere to generate Co₃O₄ nanosheet arrays on the NF surface (Co₃O₄/NF). Finally, defect-engineered Co₃O_4−x/NF was prepared via immersing the Co₃O₄/NF in 1 M NaBH₄ solution for 1 h, then rinsed with deionized water and dried under vacuum at 60 °C for 8 h.

2.3. Electrocatalytic NO₃⁻ Reduction

Electrocatalytic NO₃⁻ reduction tests were performed in a three-electrode electrochemical cell with an effective volume of 100 mL controlled by an electrochemical workstation (CHI760E, Shanghai Chenhua, Shanghai, China). The resulting Co₃O_4−x/NF material, Ir-Ru/Ti, and saturated Ag/AgCl were used as the working electrode, counter-electrode, and reference electrode, respectively. The gap between each electrode was set to ~1.5 cm. Next, 80 mL of 50 mM Na₂SO₄ solution containing NO₃⁻ was electrolyzed and stirred at 600 rpm. The cathodic potential was controlled in the range of −0.8~−1.4 V (vs. Ag/AgCl). About 2 mL of reaction solution was taken regularly from the reactor (40 min) to detect the variations in NO₃⁻, NO₂⁻, and NH₄⁺.

2.4. Analytical Methods

An X-Ray photoelectron spectroscopy (XPS) survey of the spectra of Co 2p was carried out using an Escalab 250Xi (Thermol Scientific, Waltham, MA, USA) with an Al/Mg Kα line as the dual X-Ray source. The surface morphology was detected by Scanning Electron Microscopy (SEM, FEI Quanta 200 CZ, FEI, Eindhoven, The Netherlands). X-Ray diffraction (XRD, Ultima IV, Rigaku, Tokyo, Japan) was used to analyze the crystallinity of NF, Co₃O₄/NF, and Co₃O_4−x/NF. Materials Data, Inc. (MDI) Jade 5.0 software was used to analyze the diffraction peaks and crystalline phases using the Joint Committee on Powder Diffraction Standards (JCPDS) database as a reference. The electron paramagnetic resonance (EPR) was investigated on a Bruker A300 (Bruker Corporation, Karlsruhe, Germany). Electrochemical testing of the electrochemical impedance spectrum (EIS), linear sweep voltammetry (LSV), and cyclic voltammetry (CV) were carried out using an electrochemical workstation (CHI760E, Shanghai Chenhua, China) in a three-electrode system.

The concentrations of NO₃⁻ and nitrite (NO₂⁻) were measured by Dionex ion-exchange chromatography (ICS-900) with a 0.5 mL sample loop, conductivity suppressor (ASRS 300 × 4 mm), analytical column (AS23 4 × 250 mm), and precolumn (AG23 4 × 50 mm). The eluent (1.0 mL min⁻¹) was set to 9.4 mM sodium carbonate (Na₂CO₃) and1.8 mM sodium bicarbonate (NaHCO₃). The ammonia was analyzed by ion chromatography (Dionex ICS-900, Massachusetts, USA) with a 20 μL sample loop, cation self-regenerating suppressor (CSRS ULTRA Ⅱ, 4 mm), guard column (CS 12A 4 × 250 mm), and analytical column (CG12A 4 × 50 mm); 20 mM methanesulfonic acid was adopted as the eluent at the rate of 1.0 mL min⁻¹. The production of NO and N₂O from the electrocatalytic NO₃⁻ reduction for the Co₃O₄ electrode was negligible [4]. Herein, NO₃⁻ reduction ($R_{N{O}_3^{-}}(\%)$) and total nitrogen (in this study, the sum of NO₃⁻, NO₂⁻, and NH₄⁺) were respectively calculated according to the following equations:

$$R_{N{O}_3^{-}}(\%)=\frac{{({\mathrm{NO}}_3^{-})}_0-{({\mathrm{NO}}_3^{-})}_t}{{(N{O}_3^{-})}_0}$$

(1)

$$S_{N{H}_4^{+}}(\%)=\frac{{({\mathrm{NH}}_4^{+})}_t}{{({\mathrm{NO}}_3^{-})}_0-{({\mathrm{NO}}_3^{-})}_t}$$

(2)

where 0 and t stand for the initial and final states, respectively.

2.5. Calculation of Average Current Efficiency

Based on the amount of NO₃⁻ removed and the amount of nitrite (NO₂⁻), ammonia, and N₂ generated, the average current efficiency was calculated using the following equations:

$$\mathrm{Q}{\left({\mathrm{NO}}_2^{-}-\mathrm{N}\right)}_t=2\mathrm{F}\left(\frac{C{\left({\mathrm{NO}}_2^{-}\right)}_{t}V}{M}\right)$$

(3)

$$\mathrm{Q}{\left({\mathrm{NH}}_4^{+}-\mathrm{N}\right)}_t=8\mathrm{F}\left(\frac{C{\left({\mathrm{NH}}_4^{+}\right)}_{t}V}{M}\right)$$

(4)

$$\mathrm{Q}{\left({\mathrm{N}}_2-\mathrm{N}\right)}_t=5\mathrm{F}\left(\frac{\left(C{\left({\mathrm{NO}}_3^{-}\right)}_0-{\left({\mathrm{NO}}_3^{-}\right)}_t-\mathrm{C}{\left({\mathrm{NO}}_2^{-}\right)}_t-C{\left({\mathrm{NH}}_4^{+}\right)}_t\right)V}{M}\right)$$

(5)

$$Q_t={{\displaystyle \int }}_0^{t}I dt$$

(6)

$$\mathsf{\eta }=[\left(\mathrm{Q}{\left({\mathrm{NO}}_2^{-}\right)}_t+ \mathrm{Q}{\left({\mathrm{N}}_2\right)}_t+ \mathrm{Q}{\left({\mathrm{NH}}_4^{+}\right)}_t\right]/{\mathrm{Q}}_t\times 100\%$$

(7)

where η (%) stands for the average current efficiency, $Q_t$ is the total number of electrons at reaction time t (min), $\mathrm{Q}{\left({\mathrm{NO}}_2^{-}\right)}_t$, $\mathrm{Q}{\left({\mathrm{N}}_2\right)}_t$, and $\mathrm{Q}{\left({\mathrm{NH}}_4^{+}\right)}_t$ (C) are the electrons consumed during nitrate reduction to nitrite, N₂, and ammonia at time t, respectively, $C{\left({\mathrm{NO}}_3^{-}\right)}_0$ (mg N L⁻¹) is the initial concentration of nitrate, ${\left({\mathrm{NO}}_3^{-}\right)}_t,$ $\mathrm{C}{\left({\mathrm{NO}}_2^{-}\right)}_t$, and ${\left({\mathrm{NH}}_4^{+}\right)}_t$ (mg N L⁻¹) are the concentrations of nitrate, nitrite, and ammonia at time t, respectively, V is the volume of the solution (0.08 L), M is the molar mass of N (14,000 mg mol⁻¹), and F is the Faraday’s constant (96,500 C mol⁻¹).

2.6. Electrochemical Active Surface Area Measurement

The electrochemical active surface area (ECSA) was implemented in a three-electrode system at an electrochemical station (CHI760E, Shanghai, China). The working electrode was NF, Co₃O₄/NF, or Co₃O_4−x/NF, with an effective geometric surface area of 1 cm⁻². Platinum and Ag/AgCl served as the counter and reference electrodes, respectively. The ECSA was obtained according to Equations (8) and (9) [33]:

$$\mathrm{ECSA}={\mathrm{R}}_f\mathrm{S}$$

(8)

$$R_f=\frac{C_{dl}}{60 {\mathsf{\mu }\mathrm{F} \mathrm{cm}}^{-2}}$$

(9)

where R_f is the roughness factor, S was generally equal to the geometric area of the working electrode (1 cm⁻²), C_dl is the double-layer capacitance, and C_dl was estimated by plotting j (mA cm⁻²) at −0.02 V vs. Ag/AgCl against the scan rate, where j was acquired by Cyclic Voltammetry (CV) measurement under potential windows of −0.08~0 V vs. Ag/AgCl at a given scan rate (5, 10, 15, 20, 25, and 30 mV s⁻¹) (50 mM Na₂SO₄).

3. Results and Discussion

3.1. Characterization

In this work, pristine Co₃O₄ nanosheets supported on nickel foam (Co₃O₄/NF) were synthesized by hydrothermal and annealing methods. To introduce oxygen vacancies into Co₃O₄ NWs, the obtained Co₃O₄/NF was further treated by NaBH₄ at room temperature [32]. As shown in Figure 1a–c, as compared to pristine NF, the surface of Co₃O₄/NF was covered with dense nanosheets. It was worth noting that the nanosheet morphology of the as-prepared Co₃O_4−x/NF electrode exhibited a hexagonal shape. Meanwhile, Figure 1d shows that the thickness of the Co₃O₄ and Co₃O_4−x nanosheets was ~110 nm, indicating that the post-treatment process of NaBH₄ did not affect the morphology of Co₃O₄. The crystal structure of the as-prepared samples was analyzed by X-Ray diffraction (XRD). As shown in Figure 2a, the diffraction peaks of Co₃O₄/NF fully matched the hexagonal Co₃O₄ (PDF#43-1003). Meanwhile, the Co₃O_4−x/NF exhibited the same diffraction patterns, showing that our post-treatment process did not damage the phase structure. These results clearly reveal that Co₃O₄ and Co₃O_4−x nanosheets with hexagonal morphology were able to be densely grown on the surface of nickel foam. Usually, Co₃O₄ is partially reduced to CoO in the strongly reducing environment formed by NaBH₄ [32], which in turn confirms the advantage of our method of introducing defects. As shown in Figure 2b, the XPS spectra of the wide scan clearly show the constituent elements of Co₃O₄/NF and Co₃O_4−x/NF, namely, Ni 2p, Co 2p, and O 1s, which is in accordance with the above XRD results. In the XPS spectra of Co 2p (Figure 2c), the two fitting peaks at 780.5 and 782.1 eV are attributed to Co³⁺ and Co²⁺, respectively [4,34]. Noticeably, the Co²⁺/Co³⁺ surface ratio of Co₃O_4−x/NF was determined to be 1.04, which was higher than that of Co₃O₄/NF (0.78). The electron paramagnetic resonance (EPR) spectra result shows that the value of Co₃O_4−x/NF (in g) is 2.0038 (Figure 2d), which was due to the OVs in Co₃O₄ [35]. Therefore, nickel foam electrodes with surfaces covered by Co₃O₄ nanosheets with OVs were successfully prepared.

3.2. Electrocatalytic NO₃⁻ Reduction

The electrocatalytic activities of the electrodes towards NO₃⁻ reduction were analyzed by LSV, EIS, and C_dl. As shown in Figure 3a, the current density observed from −1.0 to −1.2 V vs. Ag/AgCl for Co₃O_4−x/NF was lower than that for Co₃O₄/NF, indicating that NO₃⁻ reduction was more kinetically favorable at the former electrode. EIS can reveal the electron transfer rate in electrochemical reactions based on the arc radius of the Nyquist plot [36]. After reduction by NaBH₄, the size of the arc radius decreased (Figure 3b), confirming that the introduction of OVs can reduce the interface impedance and facilitate electron transfer. Accordingly, Co₃O_4−x/NF showed higher conductivity and electron transfer properties, which are beneficial to electrochemical NO₃⁻ reduction through the charge-transfer and mass-transfer processes [37]. It is well known that a larger C_dl reflects a higher electrochemical active surface area (ECSA). Figure 3c and Figure S1 show that Co₃O_4−x/NF had a C_dl value of 6.20 mF cm⁻², which was 1.4 and 2.1 times higher than that of NF (4.37 mF cm⁻²) and Co₃O₄/NF (2.99 mF cm⁻²), respectively. This increased ECSA can result in enhancement of electrocatalytic active sites, thereby improving electrocatalytic activity for NO₃⁻ reduction.

The electrocatalytic reduction of NO₃⁻ was carried out with NF, Co₃O₄/NF, and Co₃O_4−x/NF electrodes. Figure 4a shows that 72.7% of NO₃⁻-N was reduced for Co₃O₄/NF after 280 min of electrolysis. Moreover, NO₃⁻-N removal efficiency reached up to 93.7% with the Co₃O_4−x/NF cathode. On the other hand, only 37.9% of NO₃⁻ was removed by bare NF, illustrating that the catalytic active sites for NO₃⁻ reduction were Co₃O₄ and Co₃O_4−x rather than NF. Noticeably, the electrocatalytic reduction of NO₃⁻ with different materials obeyed the pseudo-first-order kinetic model (Figure S2). The corresponding reaction rate constant of NO₃⁻ reduction with NF, Co₃O₄/NF, and Co₃O_4−x/NF was 0.0017, 0.004, and 0.01 min⁻¹, respectively. These results indicate that the Co₃O_4−x/NF cathode exhibited the highest electrocatalytic activity for NO₃⁻ reduction.

The products transformed via NO₃⁻ reduction on different electrodes were determined comparatively (Figure 4b–d). As shown in Figure 4b, generation of NO₂⁻ increased first and then decreased, suggesting that NO₂⁻ was an intermediate product. However, NH₄⁺ increased steadily with the reaction time, indicating that it was a final product (Figure 4c). Owing to the faster electron transfer rate and higher ECSA induced by OVs, Co₃O_4−x/NF achieved a remarkable 90% NH₄⁺-N selectivity in less than 40 min, compared to 0% and 50% for NF and Co₃O₄/NF, respectively. These results fully confirm that Co₃O_4−x/NF has outstanding electrocatalytic performance for NO₃RR into NH₃, making it highly suitable for practical applications.

3.3. Effect of Reaction Parameters on Electrocatalytic NO₃⁻ Reduction

Electrochemical redox reactions are heavily dependent on the electrode potential-. Several potentials (−1.0, −1.1, −1.2, −1.3, and −1.4 V vs. Ag/AgCl) were applied to disclose the effect mechanism of cathodic potential on NO₃RR. As shown in Figure 5a, NO₃⁻ removal efficiency was 35.8%, 80.7%, 93.7%, 99.6%, and 98.9% at −1.0, −1.1, −1.2, −1.3, and −1.4 V, respectively, indicating that reducing the potential was beneficial for NO₃⁻ removal, though the effect is not significant below −1.2 V. Meanwhile, the process conformed to the pseudo-first-order kinetic model with R² ≥0.95, and the value of k increased with decreasing voltage. Noticeably, η (

Table 1. Summary of electrochemical NO₃⁻ reduction with Co₃O_4−x/NF cathode.

	Cathodic Potential (V vs. Ag/AgCl) a					NO3− Concentration (mg N L−1) b
	−1.0	−1.1	−1.2	−1.3	−1.4	20 c	50	100 d	200 e
R(NO3−)	35.9	80.7	93.7	99.5	98.8	94.9	93.7	94.1	94.9
S(NH4+)	86.8	90.5	85.4	91.2	91.6	84.6	85.4	84.9	85.7
η f	11.5	24.6	26.9	24.5	22.8	12.3	26.9	36.5	45.9
k (min−1) g	0.0016	0.0055	0.0102	0.0170	0.0210	0.0128	0.0102	0.0093	0.0069
R2	0.98	0.97	0.99	0.98	0.95	0.98	0.99	0.98	0.97

^a Initial NO₃⁻ concentration = 50 mg N L⁻¹, Initial pH = 7.0, Stirring rate = 600 rpm, Solution volume = 80 mL, Reaction time = 280 min; ^b Cathodic potential = −1.2 V (vs. Ag/AgCl), Initial pH = 7.0, Stirring rate = 600 rpm, Solution volume = 80 mL, Reaction time = 280 min; ^c Reaction time = 240 min; ^d Reaction time = 320 min; ^e Reaction time = 400 min; ^f η is the average current efficiency; ^g k is the pseudo-first-order kinetic rate constant.

) showed a volcanic pattern with decreasing potential, with a maximum value of 26.9% at −1.2 V. The products were shown to be NH₄⁺-dominated, and the selectivity was 86.8%, 90.5%, 85.4%, 91.2%, and 91.6 at −1.0, −1.1, −1.2, −1.3, and −1.4 V, respectively. As is well known, OVs can weaken N-O bonding, hindering the formation of byproducts [3]. Generally, the optimal applied potential of Co₃O_4−x/NF for NO₃RR was −1.2 V vs. Ag/AgCl.

Initial concentration is an important parameter for evaluating the application prospects of catalysts, and was accordingly investigated. Electrolysis reactions were performed under several initial NO₃⁻ concentration (20–200 mg N L⁻¹). Figure 5c shows that the NO₃⁻-N removal efficiency was 94.9%, 93.7%, 94.1%, and 94.9% for 20, 50, 100, and 200 mg N L⁻¹, respectively, at different reaction times, indicating that the Co₃O_4−x/NF electrode can be used for treating different NO₃⁻ concentrations in water. As mentioned in Table 1, η values increased with increasing NO₃⁻ concentrations; more specifically, the value of η was 12.3, 26.9, 36.5, and 45.9 at 20, 50, 100, and 200 mg N L⁻¹, which is due to the weakened competition in HER at relatively high nitrate concentrations [13]. NH₄⁺ selectivity was consistently maintained ~85% at different NO₃⁻ concentrations, indicating that the Co₃O_4−x/NF electrode has outstanding potential for ammonia production via NO₃RR. Notably, as shown in

Table 2. Comparison of NO₃RR performance between Co₃O_4−x/NF and reported catalysts.

Catalyst	${\mathbf{NO}}_3^{-}$—N Concentration (ppm)	${\mathbf{NO}}_3^{-}$—N Conversion (%)	${\mathbf{NH}}_4^{+}$—N Selectivity (%)	Ref.
Co3O4−x/NF	20	94.9	84.6	This work
	50	93.7	85.4
	100	94.1	84.9
	200	94.9	85.7
Cu MNC-7	100	94.9	81.8	[38]
TiO2−x/Ti foil	50	95.2	87.1	[3]
Fe2O3 NRs/CC	1400	<1	75.2	[26]
Co3O4@NiO	200	46	62.3	[39]
Fe SAC	7000	~7	69	[40]

, the Co₃O_4−x/NF electrocatalyst showed competitive NO₃⁻-N conversion and high NH₄⁺ selectivity. The conversion of nitrate is a key aspect of wastewater treatment, with a high conversion rate indicating more effective purification. In our study, the Co₃O_4−x/NF electrode consistently achieved an NO₃⁻-N conversion rate over 93.7% across a range of nitrate concentrations, demonstrating its great potential for practical applications. The selectivity of NH₄⁺-N is an important factor for the resourceful disposal of NO₃⁻ wastewater, as higher values indicate greater potential for useful applications. Our study found that the Co₃O_4−x/NF electrode consistently maintained an NH₄⁺-N selectivity of approximately 85% across various nitrate concentrations, which exceeds the values reported in many previous studies [38,39,40]. To sum up, our study highlights the substantial practical application value of Co₃O_4−x/NF electrodes in the management of NO₃⁻-containing wastewater. Their consistently high NO₃⁻-N conversion rates and superior NH₄⁺-N selectivity make them a promising candidate for resourceful disposal of these types of effluents.

3.4. Proposed Reaction Mechanism

To gain insights into the selective synthesis of NH₃ over Co₃O_4−x/NF, we conducted an XPS analysis to investigate the valence change of Co. In fact, the electrocatalytic activity of cobalt oxide is closely related to its electronic states [30], and oxygen defect engineering as an effective mean for regulating the electronic state of materials has been generally confirmed by previous studies [31]. As shown in Figure 4, the introduction of OVs can significantly enhance both NH₃ production performance as well as the kinetics of the electrochemical reduction of NO₃⁻. In addition, it is notable that the XPS results clearly show the Co²⁺/Co³⁺ ratio increasing from 0.78 to 1.04 after the introduction of OVs. Therefore, it is reasonable to speculate that Co²⁺ plays a crucial role in NO₃⁻ reduction and NH₄⁺ production. As shown in Figure 6a,b, the ratio of Co²⁺/Co³⁺ in Co₃O_4−x/NF decreased to 0.85 after electrocatalytic reduction. Such changes not only indicate that the electrocatalytic reduction of NO₃⁻ is mediated by the Co²⁺/Co³⁺ cycle, they show that Co²⁺ acts as the active site for transferring electrons to NO₃⁻ toward ammonia production. As a result, the OVs can regulate the conversion of Co³⁺ to Co²⁺, which improves the poor conductivity of Co₃O₄ and provides more active sites, thereby enhancing the catalytic performance of the electrocatalytic reduction of NO₃⁻ into NH₃.

4. Conclusions

In summary, Co₃O₄ nanosheets with OVs supported on nickel foam were successfully synthesized and demonstrated to be an efficient catalyst for NH₃ synthesis from NO₃⁻ electroreduction. At −1.2 V, 93.7% nitrate removal and 85.4% NH₃ production selectivity were obtained vs. Ag/AgCl. Compared with Co₃O₄/NF, the EIS and C_dl results confirmed that the introduction of OVs enhanced the electron transfer capability and ECSA of the electrode. In addition, the presence of OVs influenced the Co²⁺/Co³⁺ ratio due to the reduction of Co³⁺ into Co²⁺. The Co²⁺/Co³⁺ cycle was identified as a key mediator in the electrocatalytic reduction of NO₃⁻ for NH₃ production. These results can shed light on the importance of the Co²⁺/Co³⁺ ratio in the electrocatalytic synthesis of ammonia, and highlight the potential of oxygen-defective catalysts for developing more efficient and sustainable electrocatalytic processes. The findings of our study suggest that electrochemical denitrification may hold promise for ammonia production; however, further research is necessary in order to maximize the utilization of NH₄⁺ and enable its in situ recovery.