Electrochemical Ammonia Synthesis from Dilute Gaseous Nitric Oxide Reduction at Ambient Conditions

Abstract

Converting gaseous nitric oxide (NO) to ammonia (NH₃) is important because of its environmental and industrial implications. The electrochemical transformation of nitrogen (N₂) to NH₃ faces several challenges, including a slow reaction rate and low Faradaic efficiency (FE). This study presents an innovative approach by integrating NO elimination and NH₃ production by electrochemical gaseous NO reduction reaction (NORR) under ambient conditions. Co and Mo-based catalysts were investigated for the continuous reduction of diluted NO gas (1%) to NH₃ within a proton exchange membrane (PEM) cell under ambient conditions. In electrochemical NORR tests conducted without a catholyte, CoMo-NC demonstrated notable NORR performance, achieving an NH₃ yield rate of 23.2 × 10⁻¹⁰ mol s⁻¹ cm⁻² at −2.2 V_cell and FE_NH3 of 94.6% at −1.6 V_cell, along with enhanced durability. Notably, this performance represents one of the highest FE_NH3 achievements for electrochemical gas-phase NO reduction at room temperature.

1. Introduction

In recent years, rapid industrial development and societal advancements have raised critical concerns about the growing impact of air pollution on the environment. As a result, global awareness and initiatives aimed at mitigating air pollution have steadily increased [1,2]. Among the primary pollutants, nitric oxide (NO), produced predominantly through the combustion of fossil fuels, poses significant environmental risks. These include ozone layer depletion, acid rain, and photochemical smog formation [3,4,5]. Therefore, reducing or converting NO emissions is crucial for environmental preservation. The selective catalytic reduction (SCR) process is currently the most effective method for NO control, converting NO into harmless N₂ and H₂O using NH₃ at temperatures exceeding 300 °C [6,7]. However, SCR’s sustainability is hindered by high operational costs, catalyst deactivation, and the risk of NH₃-slip, making it less viable in the long term [6,8]. As a more sustainable alternative, electrocatalytic NO reduction is being explored for converting NO into valuable nitrogen species, such as NH₃, using only water and electricity [9,10,11]. Notably, NH₃ is a key chemical in fertilizer production and has the potential to serve as a carbon-free energy carrier due to its high energy density [12].

Currently, the Haber-Bosch process remains the dominant method for synthesizing ammonia. However, it requires harsh reaction conditions (∼500 °C and 15–30 MPa), leading to high energy consumption and significant CO₂ emissions [7,13]. In response, various alternative ammonia synthesis technologies have emerged in recent years. Ambient electrochemical nitrogen fixation is a promising green approach for NH₃ production though it is hindered by the strong N≡N bond, competing hydrogen evolution reaction (HER), and the poor solubility of N₂ [12,14]. Electrochemical NO reduction to NH₃ (NORR) presents a viable alternative, offering a way to mitigate NO pollutants while facilitating NH₃ synthesis due to the weaker N=O bond and polar nature of NO compared to N₂ [3,15,16].

Several electrocatalysts, including transition metals, noble metals, single-atom catalysts, and metal sulfides, have been investigated for NORR applications [4,10,11,14,17,18,19,20,21]. Transition metals such as Fe, Ag, Ni, Mg, and Cu nanoparticles and foils have demonstrated encouraging results in aqueous electrolyte-based NH₃ synthesis from NO [4,14,22,23,24,25]. However, the corrosive nature of NO limits the long-term stability of these catalysts, as metal nanoparticles degrade and dissolve in the electrolyte during NORR, reducing their efficiency [26]. Different approaches have been studied to improve the chemical stability of electrocatalysts and increase NH₃ yield. For instance, using diluted NO gas has been shown to significantly boost NH₃ production, but long-term stability issues persist [26]. Additionally, introducing metal complexing agents in electrolysis can enhance NO solubility and slow catalyst degradation, though this can interfere with NH₃ yield measurements through colorimetric methods [11,14,25,27]. Other challenges, such as the need for an additional separation process of NH₃ from liquid and constant electrolyte replenishment to avoid NH₄⁺ accumulation, also remain unresolved. To address these limitations, gas-phase electrochemical NO reduction (NORR) presents a sustainable alternative for NH₃ production.

In this study, Co- and Mo-based carbon electrocatalysts (Co-NC, Mo-NC, and CoMo-NC) were developed for the electrochemical conversion of NO to NH₃. The goal was to investigate the reduction of dilute (1%) gaseous NO to NH₃ within a proton exchange membrane (PEM) cell under ambient conditions for potential practical applications. Various catalysts were prepared and analyzed, with CoMo-NC demonstrating superior performance, achieving an exceptional NH₃ Faradaic efficiency (FE_NH3) of 94.6% at −1.6 V_cell. This FE_NH3 is among the highest efficiencies reported for room-temperature electrochemical NO reduction to NH₃. The source of NH₃ generation was confirmed to be NO reduction through NO-Ar switching experiments.

2. Results and Discussion

2.1. Structural Characterizations

Three different types of metal-embedded nitrogen-doped carbon-based catalysts (Mo-NC, Co-NC, and CoMo-NC) were synthesized. To examine the morphologies and structures of these catalysts, transmission electron microscopy (TEM) and energy-dispersive spectroscopy (EDS) analyses were conducted, as shown in Figure 1 and Figures S1 and S2.

Table 1. List of catalysts used with abbreviations.

Abbreviations	Meaning
Co-VC	Cobalt doped Vulcan Carbon
Co-NC	Cobalt-embedded Nitrogen-doped Carbon
Mo-NC	Molybdenum-embedded Nitrogen-doped Carbon
CoMo-NC	Cobalt, Molybdenum-embedded Nitrogen-doped Carbon

provides a comprehensive list of all synthesized and commercial catalysts used in this study.

Figure 1a exhibits the FE-TEM image of CoMo-NC, which demonstrates a typical carbon-like structure. The high-resolution TEM (HR-TEM) image shown in Figure 1b revealed a clear lattice spacing of 0.34 nm, corresponding to the (002) planes characteristic of graphite. TEM images of Co-NC and Mo-NC, as shown in Figures S1 and S2, also demonstrate carbon structures with lattice spacings of 0.35 and 0.34 nm, respectively. The porous structure is more apparent in the scanning transmission electron microscopy (STEM) analysis. The combination of single atoms and clusters, uniformly dispersed on N₂-doped carbon support, can be seen in the HAADF-STEM images in Figure 1c,d as evidenced by bright areas. Numerous clusters with an average size of less than 1 nm are found homogeneously dispersed on the carbon support. This suggests the chemical coordination of some metallic species through single-atom-sized bonds with nitrogen. As determined by ICP-OES, the total Co and Mo content in CoMo-C was 0.03 and 0.0294 wt% respectively. The EDS mapping (Figure 1e,f) demonstrated a consistent distribution of Co, Mo, N, and C elements on the carbon substrate, confirming the well-dispersed presence of Co and Mo on carbon.

Similarly, morphological analyses of Co-NC (Figure S1) and Mo-NC (Figure S2) highlighted the well-dispersed Co and Mo on carbon, showing sub-nanoparticle clusters and nanoparticles in some areas. The larger size of the Co-NC nanoparticles (5–10 nm) compared to Mo-NC and CoMo-NC can be attributed to the differences in synthesis dynamics and the specific interactions of cobalt with the carbon and nitrogen framework. During synthesis, cobalt exhibits a greater tendency to aggregate, resulting in the formation of larger nanoparticles. In contrast, when molybdenum is combined with cobalt (as in CoMo-NC), it appears to promote a more uniform dispersion, leading to the formation of smaller particles [28]. The total content of Co in Co-C and Mo in Mo-C determined by ICP-OES was 0.16 and 0.075 wt%, respectively.

The XRD patterns for Mo-NC, Co-NC, and CoMo-NC are shown in Figure S3. Notably, the pattern shows two distinct reflections at 2θ = 23 and 43°. The broad peaks noted at approximately 23 and 43° correspond to the characteristic (002) and (100) planes of carbon, respectively [29]. The intensified (002) peak signifies stacked graphitic carbon, indicating a notably high degree of crystallinity associated with enhanced graphitization. Interestingly, Mo-NC, Co-NC, and CoMo-NC exhibit an absence of diffraction peaks linked to Mo and Co-related species, indicating the highly dispersed nature of the Mo and Co components.

For further surface insights into the surface of the catalysts, X-ray photoelectron spectroscopy (XPS) studies were carried out on Co 2p (Figure 2a) and Mo 3d (Figure 2b). In the deconvoluted Co 2p curves, sub-peaks were assigned to Co²⁺ (approximately 783.42 and 798.62 eV) and two satellite peaks (around 787.88 and 805.53 eV), respectively. The formation of Co²⁺−N species is favorable for the single-atom, aligning well with the existing literature [30,31]. Similarly, the Co 2p peaks in the CoMo-C curve can be tentatively attributed to Co²⁺ with Co–N bonds. The appearance of a shake-up satellite peak at a binding energy of approximately 786.3 eV (about 3.0 eV higher than the Co²⁺ 2p₃/₂ peak at ∼783.4 eV) may be attributed to the presence of cobalt oxide phases (CoO/Co₃O₄) [32]. This suggests that a minor amount of cobalt oxide phases may coexist with the metallic cobalt phase. The detected oxide peaks are likely a result of the sample’s exposure to the atmosphere. For the Mo 3d curves of Mo-NC and CoMo-NC, they can be deconvoluted into sub-peaks corresponding to Mo⁴⁺ 3d₅/₂ (approximately 230.05 eV) and Mo⁴⁺ 3d₃/₂ (approximately 233.25 eV), respectively [30]. The presence of Mo⁴⁺ indicates the existence of Mo-N bonds. This analysis is consistent with XRD and TEM findings, indicating a uniform dispersion of Co and Mo with the metals bonded in a single form. Some of these can be seen in the cluster and sub-nanoparticles form in TEM images. Figure S4a in the Supporting Information presents the high-resolution C 1s spectra, revealing three distinct binding peaks at 284.5, 285.3, and 290 eV, corresponding to the presence of C–C, C–N, and O–C=O bonds, respectively. The identification of the C–N bond supports the occurrence of nitrogen doping within the carbon structure. Similarly, the high-resolution N 1s spectra were deconvoluted into three peaks at 398.8, 400.8, and 405.1 eV, corresponding to pyridinic-N, graphitic-N, and N–O bonds, respectively (Figure S4b). Figure S4c shows the high-resolution O 1s spectra, with two peaks at 529.5 and 531.1 eV, attributed to metal oxides. Additionally, comparable high-resolution XPS spectra were observed for the other catalysts, as illustrated in Figures S5 and S6, Supporting Information.

2.2. NORR Electrocatalytic Performance and Brief Discussion

Linear sweep voltammetry (LSV) was conducted to evaluate the electrochemical performance of the catalysts using a scan rate of 10 mV s⁻¹ for Ar, N₂, and NO atmospheres within an applied potential range of 0.5 to −2.5 V_cell. The LSV results highlighted distinctive behaviors due to differing mechanistic pathways observed in CoMo-NC during NORR compared to Ar and N₂ (Figure 3a). In particular, the CoMo-NC catalyst exhibited a lower onset potential (−0.65 V_cell) and higher current density for NO compared to Ar and N₂, indicating effective NO reduction to ammonia. However, an increase in current density beyond −1.2 V_cell was noted for N₂ and Ar, indicative of the background HER. Before reaching a current density limited by diffusion, the LSV analysis indicated the existence of two reduction waves. The polarization curve profile suggests diverse reaction pathways contingent on the applied potential, leading to multiple products [10]. The initial reduction peak aligns with the maximum diffusion-limited current for NH₃ formation, given that the electroreduction of NO to NH₃ is diffusion-controlled [3]. However, subsequent reduction potentials might relate to HER. The minute current observed at high potentials (>−0.6 V_cell) is attributed solely to gaseous products like N₂ and N₂O (E⁰ = 1.68 and 1.59 V_RHE, respectively) [3,33], as there are not any other viable NO products within this particular potential range. LSV plots were also evaluated for other electrocatalysts synthesized with various metals, namely Co-VC, Co-NC, Mo-NC, and CoMo-NC, under different humidity conditions and using various gases, specifically Ar, N₂, and NO, as shown in Figure S7. All four electrocatalysts show higher current densities and lower reduction potentials for NO and N₂ than in Ar, suggesting effective NO reduction. However, if we compare onset potentials for these four electrocatalysts, CoMo-NC shows the lowest of about −0.65 V_cell as compared to −1.0 V_cell (Co-NC), −1.12 V_cell (Mo-NC), and −1.6 V_cell (Co-VC). This indicates a better NO reduction ability of CoMo-NC among all four electrocatalysts.

EIS tests were conducted to investigate electrode kinetics under different conditions, which are shown in Figure 3b. Comparing Nyquist plots of all electrocatalysts (Co-VC, Co-NC, Mo-NC, and CoMo-NC) drawn from EIS tests conducted at the same conditions at −2.2 V_cell, CoMo-NC shows lower ohmic and charge transfer resistances than other electrocatalysts. In addition, Nyquist plots revealed a reduced ohmic resistance (R_ohm) at lower negative potentials (−2.2 V_cell), showing a slight rise within the HER potential range. As the applied potential increased and the humidity of NO gas rose, the charge transfer (R_ct) decreased accordingly. This phenomenon was observed for each electrocatalyst (Co-VC, Co-NC, Mo-NC, and CoMo-NC) as shown in Figures S8–S11. Overall, CoMo-NC shows lower resistances among all four electrocatalysts, and each electrocatalyst shows lower charge transfer resistance at higher negative potential (−2.2 V_cell) and humid conditions.

To investigate the NH₃ yield rate and FE_NH3 at different potentials, chronoamperometry (CA) tests were conducted for Co-VC, Co-NC, Mo-NC, and CoMo-NC within a potential range of −1.6 to −2.2 V_cell for 1 h, as shown in Figures S12–S15. It can be seen that an increase of applied overpotentials results in larger cathodic current densities, indicating these electrocatalysts can catalyze the NO reduction at each given potential and cell humidity condition with high activity.

Post-electrolysis, an adequate electrolyte volume was sampled for product quantification. NH₃ concentration was determined through the indophenol blue method and an ion chromatography system (Figure S16). Other products, like N₂H₄, were also assessed using the corresponding colorimetric method (Figure S17). Throughout the NORR electrolysis, potential gaseous byproducts such as H₂, N₂, and N₂O were not considered to calculate. These are possible potential products other than NH₃ in NORR and especially H₂ from competing HER. NH₃ production commenced at −1.6 V_cell across all samples and increased with rising potential. The NH₃ yield rate peaked at −2.2 V_cell, while the maximum Faradaic efficiency was observed at −1.6 V_cell, shown in Figure 3c,d. CoMo-NC exhibited the highest yield rate among the samples at 23.2 × 10⁻¹⁰ mol s⁻¹ cm⁻², with a FE_NH3 of 94.6% (Figure 3e,f). Notably, N₂H₄ was not generated within the applied potential range, as no UV-vis absorption peak was generated at 455 nm (Figure S17). The electrocatalytic performance of all synthesized catalysts and Co-VC was assessed and compared for NH₃ yield and FE_NH3 (Figures S18 and S19), highlighting CoMo-NC with a notably high FE_NH3 at −1.6 V_cell. NH₃ yield rates and FE_NH3 were quantified by both IC and UV-vis simultaneously and compared, as shown in Figure S20. These results are almost similar in both cases, validating the accuracy of the NORR results. The optimum result of CoMo-NC compared with reported electrocatalysts for electrochemical NH₃ synthesis is listed in Table S1 [4,6,13,25,34,35,36,37,38].

The CoMo-NC and related catalysts exhibited their highest NH₃ yield at a more negative potential of −2.2 V_cell and the lowest yield at −1.6 V_cell. For example, CoMo-NC achieved an increased NH₃ yield rate of 23.2 × 10⁻¹⁰ mol s⁻¹ cm⁻² at −2.2 V_cell, compared to 8.71 × 10⁻¹⁰ mol s⁻¹ cm⁻² at −1.6 V_cell under wet conditions at 40 °C. This trend suggests that the higher availability of electrons at more negative potentials facilitates the conversion of nitrogen oxides to NH₃. Chronoamperometry tests support this assertion, indicating a heightened current density of −2.5 mA cm⁻² at −2.2 V_cell, in contrast to −0.29 mA cm⁻² at −1.6 V_cell.

In contrast, Faradaic efficiency (FE) is maximized at the lower potential of −1.6 V_cell, likely due to the reduced current density, which was consistent across all tested catalysts (Co-NC, Mo-NC, and CoMo-NC). The performance and selectivity of these electrocatalysts in the nitric oxide reduction reaction (NORR) appeared to be influenced by both the applied potential and the humidity level of the NO feed gas. The effect of NO gas humidity on NH₃ yield rate and FE_NH3, as illustrated in Figures S18 and S19, follows a pyramid pattern, with optimal results for NO gas humidified at 40 °C. Lower yields and efficiencies with dry NO gas may result from the drying of the catalyst/ionomer interface, as adequate hydration of the MEA or ionomer layer likely plays a crucial role in achieving high FE toward ammonia.

The electrocatalytic activity of dual-metal catalysts, such as CoMo-NC, depends on the synergistic interaction between the individual metal atoms [30]. Complete NO conversion to NH₃ and H₂O involves five protonation stages, starting with the adsorption of NO onto the catalyst via various pathways, including N-end, O-end, or side-on configurations [3]. The N-end adsorption of NO is consistently more stable than the O-end configuration, with Mo demonstrating a preference for N-end adsorption. Once the NO molecule is adsorbed on Mo-C, the Mo atoms effectively bond with NO, increasing the N=O bond length from 11.96 nm to 12.73 nm, which indicates NO activation [39].

Charge differential analysis further supports Mo’s role, showing significant Mo–NO electronic coupling. This interaction leads to both positive and negative charge accumulations around the NO molecule, demonstrating Mo’s role in NO activation via an “acceptance-donation” mechanism [40,41,42]. Mo actively contributes electrons to activate the absorbed NO, facilitating the dissociation of the N=O bond, which is critical for the initial NO → NOH protonation step [41]. The active Mo sites stabilize NOH, demonstrating a strong NOH–Mo electronic coupling, which underscores the role of Mo-based catalysts in aiding NOH stabilization through subsequent protonation stages.

Complementing this, previous work by Xue et al. [43] demonstrated NO adsorption and activation on Co-based catalysts. Showing that NORR follows a pathway from NO* → NHO* → NHOH* → NH* → NH₂* → NH₃*, with the NO to NHO* hydrogenation step as the potential-limiting step (PLS). These insights validate the effectiveness of Co and Mo-based catalysts in electrochemically converting NO to NH₃, as indicated in Figures S18 and S19. Furthermore, the combined Co and Mo in CoMo-NC enhance electrochemical NORR activity, with Co serving as a proton donor to increase local proton availability, which supports the NO hydrogenation step near the active Mo sites [31].

2.3. Electrocatalyst Stability Tests

To evaluate the durability of the CoMo-NC electrocatalyst, an extended 20 h test was conducted at −2.2 V_cell within a humid NO environment (at 40 °C), as shown in Figure 4a. The Chronoamperometric test exhibited a consistently maintained current density (Average, 0.96 mA cm⁻²), demonstrating minimal fluctuations and emphasizing the enduring stability of CoMo-NC. After the 20 h test, both the ammonia yield rate (18.5 × 10⁻¹⁰ mol s⁻¹ cm⁻²) and Faradaic efficiency (51.35%) remained consistent, highlighting the sustained stability of CoMo-NC in comparison to the NH₃ yield (18.1 × 10⁻¹⁰ mol s⁻¹ cm⁻²) and Faradaic efficiency (49.6%) in 1 h test, as shown in Figure 4b. Furthermore, a set of 10 cyclic tests (Figure 4c) were executed to assess the cyclic stability of CoMo-NC, revealing uniform NH₃ yield rates and FE_NH3 in each cycle.

2.4. Origin of NH₃ Yield

Multiple experiments were conducted under varying conditions to validate the origin of ammonia generated during NORR. All assessments were consistently performed at −2.2 V_cell in a humid gas setting (at 40 °C). As illustrated in Figure 4d, minimal ammonia was detected in the before-test, Ar, and NO scenarios at OCP. However, a notable amount of NH₃, i.e., 20.9 × 10⁻¹⁰ mol s⁻¹ cm⁻², was specifically produced under NO at −2.2 V_cell. In the electrolysis experiments involving alternating NO–Ar cycles (Figure 4e), the presence of NH₃ was evident exclusively during the NO cycles, while minimal NH₃ production was noted in the Ar cycles. These findings strongly support the conclusion that the ammonia produced solely originates from NORR.

2.5. Catalyst Loading and NO Flowrate Effect

The influence of the catalyst quantity was examined by altering the CoMo-NC loading within the range of 0.5 to 2 mg cm⁻². The impact on NH₃ yield rate and FE_NH3 at −2.2 V_cell is depicted in Figure 5a. The NH₃ yield rate and FE% increase with catalyst loading amount from 0.5 to 1.0 mg cm⁻² and then decrease for a higher amount of catalyst loading. For 1.0 mg cm⁻² loading, the NH₃ yield rate and FE% were 23.2 × 10⁻¹⁰ mol s⁻¹ cm⁻² and 66.35%, respectively. The reduction in NH₃ yield rate and FE_NH3 for CoMo-NC with increased loading could be linked to the catalyst layer’s thickness, potentially leading to higher charge transfer resistance [44].

NORR experiments were carried out at −2.2 V_cell with varying NO flow rates to eliminate the impact of NO flow rate on experimental outcomes. The results illustrated in Figure 5b exhibited insignificant variations in NH₃ production rate and FE, indicating that NO diffusion had a marginal impact on NORR performance.

3. Materials and Methods

3.1. Catalysts Preparation

Commercial electrocatalyst, 5 wt.% Co-Vulcan (Co-VC) was purchased and used in its as-received state. Co and Mo-based nitrogen-doped catalysts (Co-NC, Mo-NC, and CoMo-NC) were synthesized by adopting a method from work done by He et al. [45]. Co- and Mo-based Tetraphenylporphyrin (Co-TPP, Mo-TPP) were first prepared for use in catalyst preparation. The electrocatalyst synthesis procedures are explained briefly here.

3.1.1. Preparation of Co-TPP and Mo-TPP

Co-TPP was prepared under a N₂ atmosphere by dissolving TPP (1.0 mmol) and cobalt (II) acetate tetrahydrate (10.0 mmol) in 100 mL N, N-dimethylformamide (DMF) within a three-necked round-bottomed flask. The mixture underwent reflux at 150 °C for 3 h before naturally cooling to room temperature. Subsequently, DMF was removed via evaporation and further purified by silica gel column chromatography using CH₂Cl₂/hexanes/MeOH as eluents. After evaporating the eluents, the product was dried under a vacuum at 80 °C for 24 h. Following a similar procedure to Co-TPP, Mo-TPP was synthesized using molybdenum trioxide and phenol as the metal salt and solvent.

3.1.2. Preparation of Co-NC, Mo-NC, and CoMo-NC

A mixture of 0.04 mmol Co-TPP and 24 mmol anhydrous AlCl₃ were dissolved in 30 mL of dichloromethane and stirred under N₂ atmosphere at 80 °C for 24 h. After cooling, the obtained solid underwent filtration and successive washing with methanol, dichloromethane, tetrahydrofuran, DMF, and acetone. Further purification was achieved through 24 h of Soxhlet extractions using methanol and dichloromethane, and the obtained polymer was dried under vacuum at 80 °C for 24 h, then subjected to a horizontal tube furnace heated to 600 °C for 3 h under continuous nitrogen gas flow, followed by natural cooling to room temperature, yielding Co-NC. Mo-NC and CoMo-NC were prepared using the same method, adding 1.5 mmol TPP precursor with a molar ratio of 1:40 (Mo-TPP:TPP) and 1:1:40 (Co-TPP:Mo-TPP:TPP).

3.2. Catalysts Characterization

Field emission transmission electron microscopy (FE-TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and energy dispersive X-ray spectroscopy (EDS) were carried out using an F200 instrument (JEOL Ltd., Tokyo, Japan) and attached EDS analyzer (JED-2300T, JEOL Ltd.). All TEM images were captured using a CMOS camera (Rio 16, Gatan, Pleasanton, CA, USA). For the catalyst analysis, X-ray diffraction (XRD) diffraction peaks were obtained using a SmartLab High Resolution (Rigaku, Cedar Park, TX, USA) instrument fitted with a Cu Kα (1.5406 Å) radiation source (200 mA and 40 kV). X-ray photoelectron spectroscopy (XPS) analysis was conducted utilizing a K-alpha Plus spectrometer (Thermo Scientific, Waltham, MA, USA) with an Al Kα X-ray source (hν = 1486.7 eV). The correction of binding energy for the samples was achieved by aligning the adventitious carbon (C 1s) to 284.47 eV. Elemental composition analysis was carried out using CasaXPS 2.3.26 software, focusing on the Co 2p and Mo 3d signals. Peak deconvolution was performed using XPSpeak 4.1 software.

3.3. Electrochemical Characterization

Electrocatalytic tests for the reduction of NO gas in the gas phase were conducted using a multichannel potentiostat Multi Auto Lab/M101 (Metrohm, Utrecht, The Netherlands) and proton exchange membrane (PEM) electrochemical cell. The cell configuration included a Platinum-coated titanium mesh (9 cm²) serving as the anode and catalysts/CP (9 cm²) as the cathode. In the anodic compartment, a 3 mM H₂SO₄ solution was continuously circulated at a fixed flow rate, while the cathode side remained free of electrolyte. Constantly, a stream of 1% NO gas (balanced with N₂), both in dry and humidified forms, was supplied to the cathodic chamber at a rate of 30 sccm. The Nafion 117 membrane separated the two half-cells. Humidification of NO gas occurred by passing it through 3 mM H₂SO₄ solution, with the humidification level controlled by varying the temperature (RT, 40 °C, and 60 °C) of the solution. However, the cell temperature was maintained at room temperature. The resultant gas was collected in a 3 mM H₂SO₄ solution and analyzed for product quantification using ion chromatography and UV-vis systems. NORR electrolysis was conducted using a potentiostat in constant potential mode. Figure 1 provides a detailed depiction of the setup for gas-phase NO reduction. Before each electrochemical test, the ohmic resistance between electrodes was assessed via electrochemical impedance spectroscopy (EIS) spanning a frequency range of 100,000 to 1 Hz with a 2 mV amplitude. The resistance value was derived from the curve’s intersection with the Real (Ω) axis in the Nyquist plot. Current densities were computed based on the geometric area of the working electrode covered by the catalyst. All experiments were conducted under ambient conditions.

3.3.1. Electrode Fabrication

The GDE (3 × 3 cm²) was coated with the catalyst ink using a brush until the loading amount reached 2 mg cm⁻². The ink was prepared by mixing 35 mg of catalyst in a mixture of IPA (80 mg), DIW (150 mg), and Nafion solution (5 wt%, 150 mg). The resulting solution underwent 30 min of sonication to ensure the formation of a uniform and homogeneous catalyst ink.

3.3.2. MEA Preparation

The membrane electrode assembly (MEA) was prepared by combining a catalyst-loaded gas diffusion electrode (GDE) for the cathode, platinized titanium fiber for the anode, and Nafion 117 membrane. The membrane underwent an activation process involving specific steps: 1 h at 80 °C in a 3% H₂O₂ solution, followed by an hour at 80 °C in 1 M H₂SO₄, and then boiling for an additional hour in type-I ultrapure water. Finally, it was rinsed thoroughly with deionized water (DI water). Assembling the membrane between electrodes involved hot-pressing at 130 °C under a pressure of 0.25 metric tons for 3 min using a laboratory press. MEA was incorporated into an in-house-built PEM electrolysis cell of 9 cm² geometric area. Ensuring consistent compression across the cell area, the cell was securely fastened using eight stainless steel rods of 4 mm outer diameter, tightened with a 4 N·m set torque wrench.

3.4. Product Quantification

Potential gaseous byproducts, such as H₂, N₂, and N₂O, were not considered throughout the NORR electrolysis process. However, the remaining gas from the cell was directed into a cold trap (3 mM H₂SO₄) to track the existence of gaseous NH₃. The indophenol blue method and ion chromatography (IC) were used to quantify NH₃, while N₂H₄ was estimated using the colorimetric method. An Ion Chromatography system (DIONEX AQUION RFIC) paired with a Dionex™ AS-DV Autosampler (068907) (Thermo Fisher Scientific, Sunnyvale, CA, USA) was used for automated sampling, injection, and unsuppressed conductivity detection of cations. A 0.5 mL cold trap solution was used to quantify ammonium (NH₄⁺) ions and compared with prepared NH₄⁺ standard solutions in 3 mM H₂SO₄. Chromeleon 3.2 software facilitated data processing, visualization, and evaluation. For spectrophotometric NH₃ analysis, 1 mL of the anolyte was mixed with 1 mL of Solution A, 0.5 mL of Solution B, and 0.1 mL of Solution C. Solution A was prepared by combining NaOH (1M), salicylic acid (5 wt.%), and trisodium citrate dihydrate (5 wt.%). Solution B contained sodium hypochlorite (0.05 M), and Solution C comprised 1 wt% sodium nitroprusside. NH₃ yield was determined by measuring absorption at 655 nm after a 30 min incubation using a UV-vis spectrophotometer UV-1800 (Shimadzu, Kyoto, Japan). A calibration curve was constructed using standard solutions of NH₄⁺ in 3 mM H₂SO₄ of different concentrations.

Hydrazine (N₂H₄) was assessed using the Watt and Chrisp method. Initially, a color reagent was prepared by mixing para-(dimethylamino) benzaldehyde (5.99 g) with concentrated HCl (30 mL) and ethanol (300 mL). The anolyte (2 mL) was diluted with DI water, mixed with 1M KOH (1 mL), and 5 mL of the added color reagent. After a 10 min incubation, absorbance at 455 nm was measured to quantify the generated N₂H₄. Similarly, an absorption–concentration curve was established by mixing a standard solution of N₂H₄ hydrate with 3 mM H₂SO₄ solution.

3.5. Equations Used for Calculation

The average yield rate (Y_NH3) for ammonia formation was determined using the following formula:

$$Y_{NH3}={\displaystyle \frac{C_{NH3}\times V}{A\times t\times M_w}}$$

Here, Y_NH3 represents the ammonia formation rate (mol s⁻¹ cm⁻²); V, total electrolyte volume (mL); A, electrode area (cm²); t, time (s) for NORR; and M_w, product molar mass (g mol⁻¹).

FE_NH3 can be calculated using the following formula:

$$FE\left(\%\right)={\displaystyle \frac{n\times F\times Y_{NH3}\times t\times A\times {10}^{-6}}{\mathrm{Q}}}\times 100\%$$

Here, n shows the number of transferred electrons; F, Faraday constant (96,485 C mol⁻¹); Y_NH3, ammonia formation rate (mol s⁻¹ cm⁻²); t, time (s) for NORR; A, electrode area (cm²); and Q, consumption of charge during the reaction (C).

4. Conclusions

This study employed electrocatalysts based on Co and Mo for electrochemical reduction of dilute gas phase NO (1%) to NH₃ at room temperature. Through a series of control experiments, the CoMo-NC exhibited an impressive NH₃ Faradaic efficiency of 94.6% at −1.6 V_cell in a catholyte-free flow cell, highlighting its significant effectiveness. This investigation showcases the potential enhancement in electrochemical NO reduction by employing CoMo−NC composite catalysts. The CoMo composite serves as a heterogeneous mediator, facilitating electron transfer between the NO gas molecule and the electrode. Moreover, CoMo-NC demonstrated exceptional recycling stability over ten cycles and sustained durability for 20 h. Notably, NH₃ was solely derived from the reduction of NO gas during the NORR tests. The demonstrated performance of this electrocatalyst marks a significant advancement towards consistent electroconversion of NO into valuable chemicals. Specifically, the outstanding efficiency exhibited by the CoMo-NC catalyst for low-concentration NO gas (1%) at room temperature suggests its potential suitability for large-scale NH₃ generation.