Co-Carbonization of Straw and ZIF-67 to the Co/Biomass Carbon for Electrocatalytic Nitrate Reduction

Abstract

Electrocatalytic nitrate reduction enables the recovery of nitrate from water under mild conditions and generates ammonia for nitrogen fertilizer feedstock in an economical and green means. In this paper, Co/biomass carbon (Co/BC) composite catalysts were prepared by co-carbonization of straw and metal–organic framework material ZIF-67 for electrocatalytic nitrate reduction using hydrothermal and annealing methods. The metal–organic framework structure disperses the catalyst components well and provides a wider specific surface area, which is conducive to the adsorption of nitrate and the provision of more reactive active sites. The introduction of biomass carbon additionally enhances the electrical conductivity of the catalyst and facilitates electron transport. After electrochemical testing, Co/BC-100 exhibited the best performance in electrocatalytic nitrate reduction to ammonia, with an ammonia yield of 3588.92 mmol g_cat.⁻¹ h⁻¹ and faradaic efficiency of 97.01% at −0.5 V vs. RHE potential. This study provides a promising approach for the construction of other efficient cobalt-based electrocatalysts.

1. Introduction

The development of urbanization, industrialization, and agricultural modernization has led to severe nitrate (NO₃⁻) pollution in water bodies and eutrophication, posing threats to human health, particularly that of infants [1,2,3,4,5,6]. The United States Environmental Protection Agency (EPA) and the World Health Organization (WHO) have established that the maximum concentration of NO₃⁻ in drinking water should be below 10.0 mg L⁻¹ [7,8,9]. However, the concentration of NO₃⁻ in industrial and agricultural wastewater seriously exceeds this threshold. NO₃⁻ is a non-ligand oxygen anion with high solubility and mobility in water [10,11,12,13]. While NO₃⁻ naturally exists in geological structures, its pollution is primarily caused by human activities such as agricultural runoff and industrial wastewater discharge [14,15,16,17,18]. With the growing awareness of environmental protection, tackling NO₃⁻ pollution has emerged as a major challenge for humanity [19,20,21]. Currently, the main methods used for NO₃⁻ removal include biochemical and physical–chemical methods such as reverse osmosis, ion-exchange resins, and hydrogen reduction. These traditional processes not only require sufficient organic carbon sources but also produce carbon dioxide (CO₂) [22,23,24]. Due to the recently increasing emphasis on carbon neutrality and peak carbon emissions, there is an urgent need for a strategy that is more compatible with the green concepts of energy saving, emission reduction, and resource recycling simultaneously [25,26].

Electrochemical reduction is a wastewater treatment method that combines electrochemistry and catalytic chemistry [27,28,29]. Electrochemical NO₃⁻ reduction reaction (NO₃RR) has garnered significant attention due to its controllable reaction rates and selectivity without the need for chemical additives, along with low costs. Self-degradation is achieved by electrochemical reduction of NO₃⁻ to ammonia (NH₃) in the cathode chamber. The reduction of NO₃⁻ to NH₃ is considerably easier than the reduction of nitrogen (N₂) to NH₃ since the bond energy of the N≡N bond (941 KJ mol⁻¹) is much higher than that of the N=O bond (204 KJ mol⁻¹), and nitrogen (0.02 g L⁻¹) is less soluble than nitrate (880 g L⁻¹) in water [30,31,32,33]. Therefore, the activation of N₂ molecules during electrocatalytic nitrogen reduction requires a large overpotential and is accompanied by complex gas–liquid–solid interfacial interactions [34,35]. Additionally, NH₃ serves as a nitrogen source for the production of ammonium fertilizers at a relatively low cost [36,37]. Current industrial-scale production is still dominated by the Haber-Bosch method, with high energy consumption and harsh process conditions, and the large amount of CO₂ emitted during production exacerbates environmental problems [14,38]. Therefore, electrochemical production will become a competitive production method due to its low cost, energy saving, and high efficiency.

However, the electrochemical reduction of NO₃⁻ to NH₃ involves a complex process of eight electron transfer reactions and exhibits slow reaction kinetics. Therefore, the development of efficient and selective catalysts is essential for enhancing the nitrate reduction reaction’s performance. Noble metal catalysts exhibit high activity in NO₃RR, but their prohibitive cost limits their large-scale application [39]. The transition metals have attracted much attention for their low cost and variety among the non-precious metals [40,41]. Recent studies have reported that cobalt (Co) has lower adsorption energy for NO₃⁻ than other transition metals, such as manganese (Mn), copper (Cu), and iron (Fe), suggesting that Co is more favorable for NO₃⁻ adsorption. Niu et al. developed a Cu-Co₃O₄ catalyst for NO₃RR, but its poor conductivity prevented it from achieving both high selectivity and high yield [42]. Li et al. developed a single-atom catalyst Co-CNP, but its limited active sites resulted in low yield at low concentrations [43]. Kuznetsova et al. designed a bimetallic electrocatalyst based on iron and cobalt nanoparticles, but both yield and selectivity need to be improved [44]. However, the recently reported CoNiO₂ and Co₃O₄-Cu₂₊₁O/CF electrocatalysts both exhibit high catalytic activity and selectivity, which are meaningful for subsequent catalyst designs [45,46].

In this work, we synthesized a Co/biomass carbon (Co/BC) composite catalyst with favorable electrical conductivity and abundant active sites through hydrothermal and annealing using straw and ZIF-67 as precursors. Biomass carbon exhibits promising electrical conductivity, while ZIF features a stable three-dimensional structure that provides abundant metal active sites [47,48,49,50], making it an excellent catalyst for electrochemical NO₃RR under mild conditions. The test results demonstrated that Co/BC-100 achieves the highest electrochemical NO₃RR performance, with a yield up to 3588.9 mmol g_cat.⁻¹ h⁻¹ at −0.5 V vs. RHE and a corresponding faradaic efficiency of 97.01%. The faradaic efficiency could be maintained above 88.02% after five cycles of stability tests, exhibiting good stability.

2. Results and Discussion

Co/BC composites were prepared as shown in Figure 1. Co/BC composites were synthesized by annealing at 800 °C for 1 h in a nitrogen atmosphere, with the addition of hydrothermal carbon at concentrations of 0 mg, 50 mg, and 200 mg. The morphology, composition, and structure of the catalyst can be characterized by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images. The TEM image of Co/BC-100 shows Co nanoparticles embedded in the carbon layer, and the lattice stripes are distinctly exhibited in the HRTEM image (Figure 2a–c). specifically, the lattice spacing of 0.46 nm corresponds to the (3 1 1) crystalline surface of Co. Additionally, the elemental mapping analysis also indicates the uniform distribution of Co, C, O, and N elements (Figure 2d–h).

To further investigate the crystal structures of the obtained BC, Co/BC-0, Co/BC-50, Co/BC-100, and Co/BC-200, the materials were characterized using X-ray powder diffraction (XRD). As shown in the XRD patterns from Figure 3a, three distinct diffraction peaks exist for Co/BC-0, Co/BC-50, Co/BC-100, and Co/BC-200. Specifically, the characteristic peaks at 44.2°, 51.5°, and 75.8° can be attributed to the (1 1 1), (2 2 2), and (2 2 0) crystal planes of Co, respectively. In addition, a broad peak at 22° can be observed for BC, Co/BC-50, Co/BC-100, and Co/BC-200, which is the (0 0 2) crystallographic plane of disordered carbon. Subsequently, Raman spectroscopic characterization of Co/BC-100 was carried out to obtain information about the vibrational modes of the material and thus assist in determining the composition and structure of the material (Figure 3b). Two characteristic peaks (D-band and G-band) are visible at 1350 cm⁻¹ and 1600 cm⁻¹ and are attributed to the sp³ and sp² hybridization of carbon [51]. The broad peaks in this band from 600 to 900 cm⁻¹ may be related to the D and G bands or related low-frequency modes (e.g., the D’ peak). The D’ peak is a low-frequency mode related to the D and G peaks, and it usually appears near the G peak. In the case of highly aggregated defects or at high frequencies, the D’ peak may combine with the intensity of the D and G peaks to affect the Raman spectrum, resulting in broad peaks in the range of 600–900 cm⁻¹. In addition, the contact angle of Co/BC-100 was tested to examine its hydrophilicity. The red line represents the baseline of the liquid or the level of the solid surface, and the blue line shows the outline of the droplet, demonstrating the contact angle between the droplet and the solid surface. The contact angle of Co/BC-100 is 11.4°, as revealed in Figure 3c. The narrow contact angle may be attributed to the capillary phenomenon produced by the presence of numerous micropores on the BC, which results in a better affinity for water and this facilitates the adsorption of NO₃⁻ from the electrolyte by the catalyst.

The elemental composition and surface chemical properties of the obtained materials were studied using X-ray photoelectron spectroscopy (XPS). As shown in Figure S1, the XPS spectrum reveals the presence of four elements (Co, C, N, and O) in Co/BC-100. The high-resolution C 1s spectrum suggests that the peak at 284.8 eV is assigned to the C-C bond, and the characteristic peaks at 286 eV and 288.2 eV are attributed to C-O and C=O, respectively (Figure 4a). The elemental N content is relatively low and mainly originates from biomass carbon. In the N 1s spectrum (Figure 4b), the peak located at 399.9 eV belongs to pyridine nitrogen, and the peak at 401.2 eV corresponds to graphitic nitrogen. The high-resolution Co 2p spectrum demonstrates the presence of three species, Co⁰, Co²⁺, and Co³⁺, with Co²⁺ and Co³⁺ possibly arising from the oxidation of the catalyst surface (Figure 4c). Therein, Co⁰ is located at 779.7 eV, and Co²⁺ and Co³⁺ are located at 779 eV and 780 eV, respectively. Co 2p_3/2 and Co 2p_1/2 exhibit a separation of 14.99 eV, which is in accordance with the previous results [52,53]. Two characteristic peaks can be observed in the high-resolution O 1s spectra, with C=O located at 532.2 eV and Co-O at 529 eV (Figure 4d).

The Co/BC and BC samples were loaded on carbon paper as working electrodes separately to test their NO₃RR performance using a three-electrode system. The cyclic voltammetry (CV) tests were performed on the working electrodes at a sweep rate of 50 mV s⁻¹ at the potential ranging from −0.73 to 0.27 V vs. RHE before carrying out the electrocatalytic NO₃RR aiming at complete activation of the working electrodes (Figure 5a). Subsequently, linear scanning voltammetry (LSV) tests were performed on Co/BC-0, Co/BC-50, Co/BC-100, and Co/BC-200 at the potential ranging from −0.73 to 0.27 V vs. RHE with a sweep rate of 10 mV s⁻¹ to compare their current densities as well as onset potentials. As shown in Figure 5b, the starting potential of Co/BC catalysts is relatively lower and the current density of Co/BC-100 at the same potential is larger than that of Co/BC-0, Co/BC-50, and Co/BC-200, indicating that Co/BC-100 has a better response to electrocatalytic NO₃RR.

Additionally, the activity, selectivity, and economics of the prepared catalysts can be further reflected by quantitative analysis of the products. The amperometric tests for nitrate reduction were firstly carried out at the five different potentials of −0.2, −0.3, −0.4, −0.5, and −0.6 V vs. RHE, and the duration of the tests was 1 h for each potential. The i-t curves at different potentials are shown in Figure 5c, where the current densities steadily increase with increasing potential. Subsequently, the electrolyte from the cathode cell was collected, and the concentration of NH₃ in the electrolyte was determined using Nessler’s reagent spectrophotometric method. The UV absorption spectra are presented in Figure 5d, and the NH₃ yield and faradaic efficiency (FE) can be calculated from the UV absorption value at 420 nm (Figure 5e). The FE reaches the maximum level (97.01%) at the potential of −0.5 V vs. RHE, and the NH₃ yield is up to 3588.92 mmol g_cat.⁻¹ h⁻¹, exceeding many of the values reported in the literature (Table S1). The NH₃ yield reaches 4369.97 mmol g_cat.⁻¹ h⁻¹ at the potential of −0.6 V vs. RHE, and the FE decreases to 94.78%, which is attributed to the competition of hydrogen evolution reaction at high potentials.

Five i-t tests were performed on Co/BC-100 (Figure 6a), and the corresponding UV absorption spectra are shown in Figure 6b. After five cyclic stability tests at a potential of −0.4 V vs. RHE, the NH₃ yields could still be maintained above 3267.66 mmol g_cat.⁻¹ h⁻¹, and the FEs remained above 87.9% (Figure 6c). Moreover, the NO₂⁻ yields and FEs arising from the five cyclic stability tests were also examined (Figure 6e), and their UV absorption spectra are shown in Figure 6d. The NO₂⁻ yields and FEs increase with the cycling times, but they still remain at a relatively low level, with little influence on the selectivity of NH₃. After a long stability test of 12 h (Figure 6f), the current density was virtually unchanged from the beginning of the test, indicating the excellent stability of the Co/BC-100.

To compare the electrocatalytic performance of different catalysts in nitrate reduction reaction, the electrocatalytic NO₃RR performance tests were conducted on Co/BC-0, Co/BC-50, and Co/BC-200 within the potential range of −0.6 to −0.2 V vs. RHE (Figures S2–S4). Co/BC-0 achieved the highest FE of 54.76% at −0.4 V vs. RHE, with a corresponding yield of 959.86 mmol g_cat.⁻¹ h⁻¹ (Figure S5). Co/BC-50 reached the highest FE of 68.14% at −0.3 V vs. RHE, with a yield of 1104.29 mmol g_cat.⁻¹ h⁻¹ (Figure S6). Co/BC-200 also achieved its highest FE of 83.79% at −0.3 V vs. RHE, with a yield of 1077.76 mmol g_cat.⁻¹ h⁻¹ (Figure S7). At the same potential of −0.5 V vs. RHE, the yield of Co/BC-100 was 3.1, 3.4, and 2.3 times that of Co/BC-0, Co/BC-50, and Co/BC-200, and the FE was 2.05, 2.02, and 2.26 times that of Co/BC-0, Co/BC-50, and Co/BC-200, respectively. Therefore, excessive biomass carbon can cause the active sites of the catalyst to be covered, thus reducing the effective catalytic reaction area and lowering the reaction rate. In addition, the surface nature of the catalyst in contact with the reactants is an important factor affecting the catalytic efficiency. Excessive addition of biomass carbon may result in the formation of a more compact structure, limiting the diffusion and penetration of reactants, which in turn reduces the catalytic performance.

A series of control experiments was conducted (using empty carbon paper and electrolytes without NO₃⁻) to study the role of Co/BC-100 in the electrocatalytic reduction of NO₃⁻ (NO₃RR). As shown in Figure 7a, the linear sweep voltammetry (LSV) curves were tested under the conditions of empty carbon paper and electrolytes without NO₃⁻, revealing that the initial current density was significantly lower than that measured with Co/BC-100. This indicates that the catalytic activity is quite low under these conditions. In Figure 7b, it is shown that the amount of NH₃ produced by Co/BC-100 is 816.9 mmol in the electrolyte containing NO₃⁻, whereas nearly no NH₃ was generated in the electrolyte without NO₃⁻. There was also almost no NH₃ generated in the electrolyte after testing with empty carbon paper as the working electrode. The results demonstrate that the nitrogen source for the electrocatalytic NO₃RR synthesis of NH₃ is derived from KNO₃ in the electrolyte rather than from the catalyst or atmospheric nitrogen and that Co/BC-100 contributes predominantly to the catalytic reaction. Subsequently, isotope labeling experiments were performed to confirm whether the nitrogen in the generated NH₃ came from the electrolyte. K¹⁴NO₃ was replaced with K¹⁵NO₃ in the electrolyte, and the resulting electrolyte was collected for 1H NMR analysis. It can be observed that the 1H NMR spectrum of the electrolyte using K¹⁵NO₃ as the reactant exhibits characteristic double peaks of ¹⁵N with a coupling constant J = 72 Hz, whereas the spectrum with K¹⁴NO₃ shows characteristic triplets of ¹⁴N with a coupling constant J = 50 Hz (Figure 7c). The 1H NMR analysis clearly indicates that the nitrogen source in the produced NH₃ derives from the applied nitrogen source in the electrolyte rather than from other contaminants [54].

To investigate the reasons behind the excellent performance of Co/BC-100 in electrocatalytic NO₃RR reduction, the as-prepared catalysts were subjected to Capacitance of Double Layer (Cdl) testing. The electrochemically active surface area was studied through comparative analysis. As shown in Figure 8a–e, the CV curves of Co/BC-0, Co/BC-50, Co/BC-100, and Co/BC-200 were measured at different scan rates (20, 40, 60, 80, and 100 mV s⁻¹). The calculated Cdl for Co/BC-100 is 18.6 mF cm⁻², which is 2.62 times, 1.15 times, and 93 times that of Co/BC-0, Co/BC-50, and Co/BC-200, respectively. This indicates that Co/BC-100 possesses more reactive sites that facilitate the electrocatalytic NO₃RR reaction. Furthermore, the electrochemical impedance spectroscopy (EIS) results show that Co/BC-200 exhibits low electrochemical impedance, suggesting good conductivity (Figure 8f). The interface effect between Co and BC enhances electron transfer, resulting in a faster electron transfer rate and better reaction kinetics, which are favorable for the electrocatalytic NO₃RR process.

The intermediates were analyzed through in situ infrared technique to gain insight into the reaction process and mechanism of Co/BC-100 electrocatalytic reduction of nitrate to ammonia. The variations of reactants, products, and intermediates were monitored to understand the activity and selectivity of the catalyst. Surface-enhanced in situ FT-IR taken in 0.1 M KOH and 0.1 M KNO₃ electrolytes from a potential ranging from 0 to −1 V vs. RHE are shown in Figure 9a. Positive and negative bands in the in situ FT-IR spectra represent the production and depletion of active intermediates, respectively. Specifically, the upward bands at 1140 cm⁻¹ and 1550 cm⁻¹ can be attributed to NH₂OH and *NO species, respectively, and the gradual enhancement of the upward band intensities with decreasing potentials attests to the production of NH₂OH and *NO intermediates. The broad and strong upward band at 1660 cm⁻¹ corresponds to the dissociation of water, and the reactive hydrogen produced during the dissociation of hydrolysis can be involved in the stepwise hydrogenation of *NO_x in solution. The upward band at 1460 cm⁻¹ strengthens as the potential decreases, indicating the gradual production of NH₄⁺, whereas the downward band at 1345 cm⁻¹ proves the depletion of NO₃⁻. From the above results, it can be deduced that the nitrate reduction reaction follows the following path: *NO₃ → *NO₂ → *NO → *NOH → *NH₂O → *NH₂OH → *NH → *NH₂ → NH₃ (Figure 9b) [38,55,56,57].

Based on the above analysis, we speculate that the performance of catalysts in enhancing nitrate reduction to ammonia can be described as the coordinated action of each component in the catalyst. ZIF features a stable three-dimensional structure, which allows Co/BC-100 to display large specific surface area, which is conducive to the adsorption of nitrate and the provision of more reactive active sites. Biomass carbon exhibits excellent electrical conductivity, which promotes the rate of charge transfer in the material. The interfacial effect of Co and BC enhances the electron transfer and facilitates the conversion of nitrate to ammonia, ultimately achieving a highly active and selective nitrate reduction electrocatalyst.

3. Materials and Methods

3.1. Materials and Characterizations

All materials, such as 2-methylimidazole (C₄H₆N₂), methanol (CH₃OH), cobalt dinitrate hexahydrate (Co(NO₃)₂·6H₂O), potassium hydroxide (KOH), potassium nitrate (KNO₃), and ethanol (C₂H₆O), were analytical-grade reagents and purchased from Sigma Aldrich (St. Louis, MO, USA) and Sinopharm (Beijing, China). All reagents and chemicals were used as received without further purification.

The morphology and microstructure of the samples were observed in a scanning electron microscope (SEM, JEOF-JSM-7800F, Japan Electron Optical Laboratory, Tokyo, Japan) and a transmission electron microscope (TEM, FEI-Talos-F200s, Thermo Fisher Scientific, Waltham, MA, USA) equipped with an energy dispersive spectroscopy (EDS) detector. X-ray diffraction (XRD, D2 Phaser, Bruker, Ettlingen, Germany) data were obtained with a wide-angle diffractometer with Cu Kα radiation (λ = 1.5418 Å) at a generator voltage of 30 kV and current of 10 mA. X-ray photoelectron spectroscopy (XPS) analysis was collected on a Thermo Scientific K-Alpha+ X-ray photoelectron spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) using Al Kα radiation. In situ ATR-FTIR spectra were obtained by Thermo Scientific Nicolet iS50 (Thermo Fisher Scientific, Waltham, MA, USA).

3.2. Synthesis of BC

Corn stover powder was added to 50 mL of deionized water, stirred well, and then transferred to an autoclave reactor to be hydrothermalized at 180 °C for 12 h. After drying, it was added to 1 M KOH to be ultrasonicated for 30 min and impregnated for 24 h. Subsequently, it was transferred to a quartz boat and calcined in a tube furnace under an argon atmosphere at 800 °C for 120 min and then dried to obtain hydrothermal carbon powder. The defect level is an important indicator of the quality of biomass carbon materials. In the lower temperature range, increasing the carbonization temperature can significantly increase the defects in the material. However, this increase does not always take place, and the material becomes saturated with defects at temperatures above 700 °C [58]. Therefore, the carbonization temperature of 800 °C that we chose is an appropriate temperature in combination with experience and practice.

3.3. Synthesis of Co/BC

Next, 209.03 mg of (Co(NO₃)₂·6H₂O) was added to 40 mL of methanol and stirred for 5 min; then, 100 mg of hydrothermal charcoal was added, with continued stirring for 10 min. Subsequently, 0.6568 g of 2-methylimidazole was dissolved in 40 mL of methanol and stirred for 5 min and then slowly added dropwise to cobalt nitrate solution and stirred for 24 h. It was washed with methanol and deionized water, respectively, and then annealed at 800 °C under N₂ atmosphere for 1 h to obtain Co/BC-100. Co/BC-0, Co/BC-50, and Co/BC-200 were prepared in the same way as described above, except that the masses of added hydrothermal carbon were 0 mg, 50 mg, and 200 mg, respectively.

3.4. In Situ FT-IR Testing

A layer of 100 nm Au film was deposited onto the reflecting plane of a Si prism by vacuum evaporation using a thermal evaporator (PuDi vacuum PD-400, Wuhan Puti Vacuum Technology Co., Wuhan, China). Before the Au deposition, the Si prism was polished with 0.05 μm Al₂O₃ suspension and cleaned by sonication sequentially in baths of acetone and deionized water. The working electrode was made by airbrushing the catalyst ink onto the above-prepared Au film. The in situ FT-IR measurements were conducted in a two-compartment spectro–electrochemical cell comprising three electrodes including the working electrode, a platinum-wire as the counter electrode, and a standard Ag/AgCl electrode as the reference. All the in situ FT-IR spectra were acquired using a Fourier Transform Infrared Spectrophotometer (FT-IR, Nicolet iS50, Thermo Fischer scientific, Waltham, MA, USA) equipped with a mercury cadmium telluride (MCT) detector. In a typical test, the working electrode was subjected to an initial activation by running CV cycles between 0 and −0.8 V vs. RHE at a scan rate of 0.05 V S⁻¹ until the system was stabilized. The spectrum under open-circuit voltage was then collected as the background. The cathode potential was then swept from 0 V to −0.8 V vs. RHE, with each potential lasting 2 min for spectra acquisition. All measurements were performed at a spectral resolution of 4 cm⁻¹ and presented in transmission units after subtracting the background.

3.5. Electrochemical Measurement

All the electrochemical measurements were carried out on a CHI 660E electrochemical workstation (CH Instrument, Shanghai, China) at room temperature (20 °C). The H-type electrochemical cell was used as a measuring device and Co/BC, Pt sheet (1 cm × 2 cm, 1 mm in thickness) and HgCl₂/Hg (saturated KCl) as cathode, anode, and reference electrode, respectively. The anion exchange membrane (FAB-PK-130, FuMATech, Bietigheimer Schloss, Germany) was used to separate the cathode and anode of the H-cell. The working electrode area was controlled at 1 cm², and 2.5 mg catalyst was dispersed into 500 μL isopropanol/water mixture (V:V = 3:1) containing 50 μL of 5% Nafion and kept ultrasonic for 1 h. Subsequently, 50 μL of the catalyst ink was loaded on the CP and dried at room temperature (catalyst loading: 0.25 mg·cm⁻²). The equation of E (vs. RHE) = E (vs. SCE) + 0.244 V + 0.0591 × PH was used to convert electrode potentials to the reversible hydrogen electrode (RHE). The electrolyte in all electrochemical tests was 1 M KOH presence of 0.1 M KNO₃. And electrolyte volume was 35.0 mL in each test. The linear sweep voltammetry was scanned at a rate of 20 mV s⁻¹. The nitrate electroreduction performance of all catalysts was evaluated by potentiostatic electrolysis for 1 h in the electrolyte at ambient conditions. The linear sweep voltammetry (LSV) curves were measured at a scan rate 5 mV s⁻¹.

3.6. Isotope Labeling Experiments

1H NMR was used to perform isotope tracing experiments on a H-nuclear magnetic resonance (H-NMR) technic (Bruker AVANCEⅢ400 MHz) as well as for the quantifying the content of NH₃ in liquid products.

3.7. Determination of NH₃

Nessler’s reagent and potassium sodium tartrate solution (ρ = 500 g/L) were used as color reagents for NH₃-N. First, 0.1 mL potassium sodium tartrate solution and 0.1 mL Nessler’s reagent was added to 5 mL diluted electrolyte. Then, standing for 20 min, the absorbance was measured by UV–vis spectrophotometry at a wavelength of 420 nm. The calibration curve was obtained by using a series of standard NH₄Cl solutions. The faradaic efficiency (FE) was measured by the following formula:

$$\mathrm{F}\mathrm{E}={\displaystyle \frac{8\mathrm{F}\times {\mathrm{c}}_{{\mathrm{N}\mathrm{H}}_3}\times \mathrm{V}}{\mathrm{Q}}}\times 100\%$$

where c is the concentration of _NH3 in mmol L⁻¹; V is the volume of electrolyte solution added in mL; m is the mass of catalyst loaded on the CP in mg; F is Faraday’s constant of 96,485 C mol⁻¹; and Q is the total charge consumed in C.

3.8. Determination of NO₂⁻

To prepare the color reagents, 0.4 g p-aminobenzenesulfonamide and 0.02 g N-(1-naphthyl) ethylenediamine dihydrochloride were dissolved in a mixed solution containing 5 mL deionized water and 1 mL phosphoric acid (ρ = 1.70 g/mL). Next, 0.1 mL color reagent was added to the 5 mL diluted electrolyte. The solution was allowed to stand for 20 min before testing the absorbance by UV–vis spectrophotometry at a wavelength of 540 nm. The calibration curve was obtained by using a series of standard NaNO₂ solutions. The faradaic efficiency (FE) was measured by the following formula:

$$\mathrm{F}\mathrm{E}={\displaystyle \frac{2\mathrm{F}\times {\mathrm{c}}_{{{\mathrm{N}\mathrm{O}}_2}^{-}}\times \mathrm{V}}{\mathrm{Q}}}\times 100\%$$

where c is the concentration of _NH3 in mmol L⁻¹; V is the volume of electrolyte solution added in mL; m is the mass of catalyst loaded on the CP in mg; F is Faraday’s constant of 96,485 C mol⁻¹; and Q is the total charge consumed in C.

3.9. Contact Angle Testing

The Contact Angle Measuring Instrument (SZ-CAMD3, Shanghai Sunzern Instrument Co., Shanghai, China) was used to test the contact angle of materials. Specifically, the angle between the liquid droplet and the solid surface was measured, thus reflecting the hydrophilicity or hydrophobicity of the solid surface to the liquid.

4. Conclusions

In summary, Co/BC-100 composites were successfully synthesized for efficient electrocatalytic nitrate reduction to ammonia by in situ carbonization using ZIF-67/biomass carbon as the precursor. Under mild reaction conditions, the yield of Co/BC-100 reached up to 3588.9 mmol g_cat.⁻¹ h⁻¹ at −0.5 V vs. RHE potential, corresponding to a faradaic efficiency of 97.01%. Numerous studies have demonstrated that biomass carbon displays outstanding electrical conductivity, which enhances the charge transfer rate within the material. ZIF features a stable three-dimensional structure, which allows Co/BC-100 to display a large specific surface area and rich exposed metal active sites. The interfacial effect of Co and BC enhances the electron transfer and facilitates the conversion of nitrate to ammonia. Additionally, the intermediate products of the reaction were analyzed through in situ characterization, and the reaction pathway for electrocatalytic nitrate reduction to ammonia was determined. This paper provides a feasible approach for the design of Co-based composites for electrocatalytic nitrate reduction to ammonia.