Energy-Efficient Cascade Synthesis of Ammonia from Air via Spark Plasma-Mediated Nitrite Generation and CuRu Nanoalloy Electrocatalysis

Abstract

Plasma–electrocatalytic cascade ammonia synthesis offers a promising route for decentralized nitrogen fixation, yet remains hindered by excessive NO₃ ^– formation and poor energy matching between the two stages. Here, we propose a strategy that breaks this bottleneck by selectively generating NO₂ ^– via spark-discharge plasma and efficiently reducing it over a CuRu₀.₅ nanoalloy. Systematic optimization of the plasma discharge and electrocatalytic interface enables synergistic energy matching, delivering a record-low specific energy consumption of 273.41 kWh/kg NH₃. Mechanistic studies reveal that Ru modulates the electronic structure of Cu, enhancing NO₂ ^– adsorption and deoxygenation while suppressing the hydrogen evolution reaction. This work establishes a generalized paradigm for low-energy, high-selectivity ammonia synthesis from air and water.

1. Introduction

Ammonia (NH₃), as an important fuel and chemical feedstock, is predominantly synthesized via the mature and efficient Haber–Bosch (H–B) process. − However, this process relies on high temperature and pressure, accompanied by significant energy consumption and carbon emissions, creating an urgent need to develop green, distributed synthesis routes based on renewable energy. , Therefore, finding an ammonia synthesis route that consumes low energy is urgent and beneficial to the energy crisis and environmental protection issues. The electrocatalytic nitrogen reduction reaction (eNRR) has attracted considerable attention due to its mild operating conditions, yet its development is hindered by the high inertness of N₂ and severe competition from the hydrogen evolution reaction (HER), resulting in low yield and efficiency. , In contrast, nitrogen oxyanions (NO _x ^–) offer a new pathway for efficient electrocatalytic ammonia synthesis, owing to their higher reactivity, superior solubility, and more favorable reduction potentials.

Among various nitrogen oxyanions, nitrite (NO₂ ^–) reduction is particularly advantageous over nitrate (NO₃ ^–) reduction for practical electrocatalytic ammonia synthesis. This preference stems from fundamental thermodynamic and kinetic differences,the reduction of NO₂ ^– to NH₃ requires only 6 electrons, whereas NO₃ ^– reduction necessitates 8 electrons, leading to an intrinsically lower theoretical energy input for NO₂ ^–. , Moreover, the N–O bond in NO₂ ^– (dissociation energy ∼204 kJ mol^–1) is weaker than that in NO₃ ^–, rendering NO₂ ^– more reactive and easier to activate catalytically. Critically, the standard reduction potential for NO₂ ^–/NH₃ is more positive than that for NO₃ ^–/NH₃, which thermodynamically favors the target ammonia formation and helps mitigate the competing hydrogen evolution reaction (HER) within a wider operational potential window. Traditional studies on electrocatalytic reduction of NO₂ ^– often treat NO₂ ^– as a readily available chemical reagent that can be purchased and used, while neglecting the process of how to obtain NO₂ ^– (which is usually a high-energy-consuming thermal catalytic oxidation or a mixture of NOx produced by N₂ oxidation, with high separation costs).

Given that 9 and 7 protons are involved in eNO₃ ^–RR and eNO₂ ^–RR, respectively, developing catalysts with high ammonia selectivity has become a focus of eNO _x ^–RR research. , Due to their favorable NH₃ selectivity, Cu-based catalysts are frequently employed in eNO _x ^–RR. − However, owing to complex electron-transfer processes, eNO _x ^–RR usually requires a considerable overpotential to achieve a dynamic equilibrium between the generation and timely consumption of adsorbed hydrogen atoms (H_ad). At such potentials, competitive adsorption of H₂O and NO _x ^– on Cu can deteriorate NH₃ selectivity by about 25%. Simultaneously, an excessively large overpotential leads to greater energy loss in eNO _x ^–RR, resulting in suboptimal energy efficiency. Hence, it is crucial yet challenging to enhance the supply efficiency of H_ad on the catalyst surface at low overpotentials to meet the demands of the hydrogenation process. − Doping appropriate metals into Cu-based catalysts can help reduce the reaction overpotential, thereby suppressing HER competition. −

In recent years, plasma-driven air oxidation coupled with electrochemical NO _x reduction (PAO-eNO _x RR) has emerged as a promising route for distributed green ammonia synthesis, offering rapid reaction kinetics, ambient operability, and a near-zero carbon footprint. − While earlier studies confirmed the conceptual feasibility of this cascade strategy, critical bottlenecks persist in energy efficiency and product selectivity, primarily due to the lack of coordination between plasma activation and electrocatalytic reduction. Initial efforts employing dielectric barrier discharge (DBD) plasma demonstrated the viability of activating N₂/O₂ to generate NO _x , which was subsequently reduced over Co-based catalysts. However, the high energy consumption of DBD (25.5 MJ/mol NN) and moderate NH₃ yields (49 nmol s^–1 cm^–2) highlighted the inherent inefficiency of nonoptimized discharge configurations.

Subsequent work shifted toward spark-discharge systems to improve energy utilization. Xu et al. utilized nanosecond pulse spark discharge coupled with a core–shell nickel boride catalyst, while Meng et al. adopted spark-activated N₂ with conventional Cu nanoparticles. Although spark discharge improved plasma efficiency, both studies reported limited NH₃ production rates (55.1 and 45.3 nmol s^–1 cm^–2, respectively). These results reveal a second critical gap: the absence of high-performance electrocatalysts specifically tailored for NO₂ ^– reduction that can deliver both high activity and near-unity Faradaic efficiency at low applied potentials.

Parallel to heterogeneous systems, transition metal complexes have emerged as a powerful platform for nitrogen reduction, offering atomic-level control over coordination environments. Recent advances highlight the potential of molecular catalysts: Smith et al. reported Fe-based macrocyclic complexes that modulate axial ligands to optimize N₂ binding; Brown et al. demonstrated controlled proton-coupled electron transfer in Mo and Fe dinitrogen complexes; and Lee et al. developed bioinspired Fe–S cluster mimics for sustainable ammonia synthesis. , Nevertheless, molecular catalysts often face challenges in long-term stability, catalyst recovery, and scalability for continuous, high-throughput ammonia production. In contrast, heterogeneous systems offer superior robustness and process compatibilityan advantage we seek to leverage in this work.

Recent progress in electrocatalytic nitrate/nitrite reduction further underscores the importance of rational catalyst design and cascade process optimization. − Nevertheless, a systematic strategy that integrates selective NO₂ ^– generation with energy-matched electrocatalytic reduction to minimize total energy input remains underdeveloped.

More importantly, previous studies have treated the plasma activation and electrocatalytic reduction as separate modules, with little attention paid to matching the output species (NO _x composition) with the catalyst’s specific activity. This systemic disconnect often leads to the generation of mixed NO _x species that are not ideally suited for the subsequent catalytic step, resulting in suboptimal overall energy consumption. Furthermore, the microscopic mechanisms of plasma-induced N₂ dissociation and the rational design of catalyst active sites to suppress the competing hydrogen evolution reaction (HER) during NO₂ ^– electroreduction remain largely unexplored.

In this work, we move beyond conventional plasma–electrocatalytic systems by establishing a selective NO₂ ^– generation–high-efficiency NO₂ ^– reduction cascade. By precisely controlling the plasma oxidation pathway to suppress NO₃ ^– formation and designing a Ru-modulated Cu-based electrocatalyst with optimized intermediate affinity, we achieve exceptional energy efficiency and product selectivity. This study provides a generalizable paradigm for the rational design of low-energy, high-selectivity nitrogen-fixation systems.

First, the nitrogen-fixation capability of the spark-discharge reactor was investigated. The microscopic mechanism of N₂ activation and dissociation to produce NO _x ^– was analyzed using optical emission spectroscopy, and the effects of key parameters (discharge power, gas flow rate, N₂/O₂ ratio, and electrolyte pH) on the reactor’s nitrogen-fixation performance were systematically examined. Furthermore, highly efficient CuRu nanoalloy catalysts were prepared via a simple and rapid chemical reduction method. By coordinating and matching the energy input between nonthermal plasma (NTP) and electrocatalytic reduction reaction (ERR), contour maps of the specific energy consumption for NTP and ERR under different energy-input scenarios were obtained, thereby avoiding resource waste due to mismatching and demonstrating significant energy-consumption advantages compared to other ammonia-synthesis processes. This work not only provides strategies for developing superior eNO _x ^–RR electrocatalysts but also offers insights into sustainable and distributed NH₃ synthesis using atmospheric nitrogen and renewable electricity.

2. Experimental Section

2.1. Plasma Device and Diagnosis

2.1.1. Plasma Device Construction

The experimental device used in this study is shown in Figure , which consists of a plasma power supply (CTP-2000K, China), a spark-discharge plasma reactor, and an electrical and optical diagnostic system. The display interface of the plasma power supply can directly read the input voltage and current, and the input voltage is controlled directly by the voltage regulator connected to the power supply. The low-voltage terminal of the reactor is connected to the low-voltage output terminal of the power supply corresponding to the internal current measurement sampling capacitor, with a capacitance value of 0.47 μF. The CH1 and CH2 terminals of the TBS 1102X oscilloscope are connected to the high-voltage output voltage and current detection interfaces of the plasma power supply, and the voltage and current signals of the plasma reactor are collected through the oscilloscope for calculating the output power of the plasma discharge. The main part of the spark-discharge plasma reactor is a 9 mm diameter quartz tube. A gas inlet is provided at the top of the quartz tube, and the gas flow rate is controlled by a mass flowmeter. A conductive metal rod of a certain diameter is fixed in the middle. The top of the metal rod is about 2 cm away from the quartz tube, and the bottom is about 2 mm away from the bottom of the quartz tube. There are four 0.5 mm diameter holes at the bottom of the quartz tube.

1.

Schematic diagram of plasma coupled CuRu nanoalloy electrocatalytic reduction of N₂/O₂ mixture ammonia synthesis equipment.

2.1.2. Plasma Emission Spectroscopy Diagnostics

Plasma emission spectroscopy (PES) detection is a method for studying the composition and properties of the plasma matter. When gas is excited, it forms a highly ionized plasma state and the electrons in molecules and atoms are excited and undergo energy level transitions. When electrons transition from a high-energy level to a lower-energy level, they can release energy in the form of emitted photons. Using an Avaspec 2018-USB2 multichannel fiber optic spectrometer, the emission spectrum corresponding to the electron transitions between different energy levels in N₂/O₂ plasma can be detected, thereby obtaining the emission spectrum in different wavelength regions. This is an in situ detection process, and during the detection process, it is necessary to ensure that the FC8-UVIR400 eight-channel fiber optic probe is at the same level as the gas–liquid mixed spark discharge and is about 2.5 cm away from the end of the high-voltage electrode. During the same group of data detection, the relative position of the probe and the high-voltage electrode must remain unchanged.

2.1.3. High-Definition Discharge Image Capture

In order to obtain clear images of the gas–liquid mixed spark discharge for the study of the plasma N₂/O₂ activation process, a high-definition camera was used to capture the clear images. The camera was fixed in position using a tripod at a distance of approximately 40 cm from the spark discharge, at the same level as the spark discharge, and the images were captured 3–5 min after the start of the reaction to ensure a stable discharge process. During the shooting, the external environment was kept dark, and the exposure time, ISO speed, and aperture value were adjusted to ensure clear images of the discharge. The high-definition camera used in this study was the EOS R10, with a resolution of 6000 × 4000 for the discharge image and a focal length of 150 mm.

2.2. Electrochemical System

2.2.1. Synthesis of CuRu _x NPs

The synthesis process of the CuRu _x NPs is shown in Figure a. First, certain amounts of CuCl₂, RuCl₃, and poly­(vinylpyrrolidone) (PVP) were added to a glass bottle and sonicated in an ice–water bath to dissolve. Then, a solution of NaBH₄ was slowly added to the precursor solutions of Cu and Ru. The reaction was carried out for 45 min in an ice–water bath, followed by centrifugation, washing, and drying to obtain the CuRu _x NPs. The molar ratios of different atomic proportions are given in Table S1. Compared with the common hydrothermal method, the chemical reduction method has a simpler operation process and shorter synthesis time and usually synthesizes metal nanoparticles with a diameter less than 50 nm. Although NaBH₄ was used as the reducing agent, elemental analysis (ICP-OES) and XPS confirmed the absence of boron in the final CuRu₀.₅ catalyst. This indicates that NaBH₄ serves only to facilitate metal reduction and does not contribute to the active composition.

2.

Characterization of the structure and composition of CuRu _x . (a) Schematic diagram of the synthetic route for CuRu _x . (b) XRD images. (c, d) XPS spectra. (e, f) SEM image. (g) Distribution of Ru and Cu elements in CuRu_0.5 NPs.

2.2.2. Electrochemical Evaluation

All electrochemical evaluations were conducted using a custom-designed h-type electrochemical cell with CHI Potentiostat (CHI 760E). The anode compartment and the cathode compartment were separated by Nafion @ 117. Ag/AgCl was used as the reference electrode, a hydrophilic carbon paper coated with CuRu _x NPs (effective catalytic area of 1 cm²) was used as the working electrode, and a platinum foil electrode (10 × 10 × 0.1 mm³) was used as the counter electrode. The mixed solution containing a certain concentration of NO₂ ^– and NO₃ ^– was electrochemically reduced to ammonia for 30 min under the influence of plasma activation. The universal magnetic stirrer was used to promote the reaction at a speed of 450 rpm. Notably, no external nitrate or nitrite salts were added to the electrolyte; the NO _x ^– species for electrocatalytic reduction were exclusively generated in situ by the plasma reactor. (Unless otherwise noted, all electrochemical data reported in this work are the average of at least three independent experiments using separately synthesized catalyst batches. Error bars in all figures indicate the standard deviation (SD)).

2.3. Product Detection and Analysis

2.3.1. NH₃ Measurement

The content of ammonia in the electrolyte was measured by the indophenol blue method. Color developer A was obtained by mixing 2.891g of salicylic acid, 2.891g of sodium citrate, and 2.038 g of NaOH with 50 mL of deionized water and stirring thoroughly until the solid is completely dissolved. 0.4 wt % sodium hypochlorite aqueous solution was prepared as chromogenic agent B. Color developer C was obtained by dissolving 0.2025 g of sodium nitrosoferricyanide in 20 mL of deionized water. A solution was prepared with a known ammonia concentration of 0, 0.5, 1, 2, and 4 ug/mL. Take 2 mL of each and add 2 mL of color developer A, 1 mL of color developer B, and 0.2 mL of color developer C in turn. After leaving it in the dark for 2 h, measure the absorbance at 655 nm by a UV–vis spectrophotometer, as shown in Figure a. With absorbance as the vertical coordinate and ammonia concentration as the horizontal coordinate, a standard curve is drawn, as shown in Figure S1b. When measuring the concentration of ammonia in the electrolyte, the absorbance is measured first, and then the corresponding concentration is calculated from the standard curve.

2.3.2. NO₂ ^– and NO₃ ^– Measurement

The NO _x ^– concentration in the electrolyte was measured by the DIONX-ICS-1100 ion chromatograph. The supporting anion column is AS19. The eluent is KOH. The standard curve is drawn with known concentrations of NaNO₃ and NaNO₂. The curve is shown in Figure S1, where the horizontal coordinate is the peak area and the vertical coordinate is the corresponding ion concentration.

2.3.3. Characterization

X-ray diffraction (XRD) patterns were collected using an Empyrean 200895 diffractometer. X-ray photoelectron spectroscopy (XPS) studies were carried out on a Escalab 250Xi X-ray photoelectron spectrometer. The surface morphology of the catalyst was observed by a 120 kV transmission electron microscope (TEM) HT-7700. Spectral diagnosis of nitrogen plasma was carried out on a AVS-RACKMOUNT-USB2 multichannel high-resolution optical fiber spectrometer. Discharge parameters during the operation of the plasma generator were collected using TBS 1102X oscilloscope. High-definition images of the reaction area were captured using EOS R10 (18–150 mm) high-definition camera. Absorbance of the electrolyte was measured using a Cintra 303 ultraviolet–visible spectrophotometer. NO _x ^– concentration of electrolyte was measured using an ICS1100 ion chromatograph. The pH of the electrolyte was measured using an FE28 pH meter.

3. Results and Discussion

3.1. Synthesis and Characterization of CuRu _x Nanoalloys

The synthesis process of CuRu _x NPs is shown in Figure a. First, certain amounts of CuCl₂, RuCl₃, and poly­(vinylpyrrolidone) (PVP) were added to a glass bottle, which was then sonicated in an ice bath to dissolve the ingredients. Then, a solution of NaBH₄ was slowly added to the precursor solutions of Cu and Ru, and the reaction was carried out for 45 min in an ice bath. After centrifugation, washing, and drying, the CuRu _x NPs were obtained. The atomic ratios of different compounds are shown in Table S1. Compared with the common hydrothermal method, the chemical reduction method has a simpler operation process, shorter synthesis time, and usually produces metal nanoparticles with diameters of less than 50 nm. The SEM images and the size distribution of CuRu _x NPs measured using ImageJ software are shown in Figure S3, which shows that the CuRu alloys synthesized by chemical reduction are all approximately spherical nanoparticle morphologies. Furthermore, because of the control of surface-active agents and the reaction process on the growth of the nuclei, the particle sizes are basically all below 50 nm. It is worth noting that the size distribution of CuRu _x NPs is generally shifted to smaller particle sizes compared with that of Cu NPs and Ru NPs. As shown in Figure e, the average particle size of CuRu_0.5 was only 23.1 nm. Appropriate Ru doping helps to synthesize smaller nanoparticles, and smaller particle sizes mean higher specific surface area and potentially more exposed active sites (Figure S4).

Using the EDS attachment of the field emission scanning electron microscope to measure the Cu and Ru elements in the CuRu_0.5 nanocluster at a scale of 500 nm, as shown in Figure g, the uniform distribution of Ru and Cu elements in the nanocluster can be seen. At the same time, the Cu/Ru atomic ratio was detected and quantitatively analyzed using EDS for several different nanoclusters synthesized by chemical reduction method, as shown in Table S2. The actual detected Cu/Ru atomic ratio is basically consistent with the Cu/Ru mole ratio when the precursor was added, and the Ru atom content in the several nanometallic materials increases with the increase of Ru precursor. The XRD patterns obtained are shown in Figure b. For the Cu NPs, the main exposed crystal faces are observed at 36.4° and 43.3° corresponding to the Cu₂O­(111) and Cu(111) crystal faces, respectively, indicating the successful synthesis of Cuprous oxide nanoparticles. However, due to the inevitable processes of centrifugal washing and sample collection after drying, which directly expose the catalyst to air, some of the Cu is oxidized to Cu₂O. In addition, the diffraction peaks of the secondary exposed crystal faces of Cu(200), Cu(220), and Cu₂O­(220) at 50.4°, 74.1°, and 61.3°can also be observed. For the Ru nanoparticles, the diffraction peaks of the three main exposed crystal faces of Ru. For the CuRu alloy, we can see the coexistence of diffraction peaks corresponding to Cu, Cu₂O, and Ru, and the relative intensity of the Ru diffraction peak increases as the Ru content increases.

Further analysis of the charge distribution and chemical state of the CuRu alloy catalyst was conducted through XPS characterization. The Ru 3p XPS spectra of the CuRu0.5 NPs and Ru NPs are shown in Figure c. The characteristic peaks at 484 and 462 eV in the Ru⁰ 3p_1/2 and Ru⁰ 3p_3/2 of the two catalysts were assigned to Ru⁰ 3p_1/2 and Ru⁰ 3p_3/2, respectively. Compared with the Ru NPs, the binding energy of the Ru⁰ 3p_1/2 and Ru⁰ 3p_3/2 characteristic peaks of the CuRu_0.5 NPs shifted negatively (0.24–0.25 eV), indicating that the electron density around Ru increased, and Ru carried negative charge. The XPS spectra of CuRu_0.5 NPs and Cu NPs for Cu 2p are shown in Figure d. The characteristic peaks of Cu⁺/Cu⁰ 2p_3/2 and Cu⁺/Cu⁰ 2p_1/2 at 932.2 and 952.2 eV, respectively, can be detected, and the characteristic peaks of Cu²⁺ 2p_3/2 and Cu²⁺ 2p_1/2 at binding energy of 933.8 and 954.2 eV, respectively, can also be detected. It is worth noting that compared with Cu NPs, the characteristic peaks of Cu in CuRu_0.5 NPs are overall shifted to higher binding energy, the relative intensity of Cu²⁺ feature peak is significantly enhanced, indicating that the doping of Ru reduces the electron density around Cu, causing Cu to carry more positive charges. This phenomenon further indicates that the Cu atoms and Ru atoms in CuRu _x NPs may be adjacent to each other, rather than simply a collection of Cu nanoparticles and Ru nanoparticles, with strong interactions between the Cu atoms and Ru atoms, electron transfer from Cu to Ru, causing Cu to carry positive charges while the Ru atoms are in a state of having excess electrons.

3.2. Nitrogen-Fixation Effect of Spark Plasma

Diagnosis of N₂/O₂ plasma emission spectra. Under the condition of total gas flow rate of 200 mL/min and N₂:O₂ ratio of 1:1, the emission spectrum of N₂/O₂ gas–liquid mixed spark-discharge plasma was detected, as shown in Figure a, the 311–380 nm range corresponds to the second positive system (SPS) of nitrogen N₂(C³Π_u–B³Π_g) while the 390–410 nm range aligns with the first negative system (FNS) of nitrogen (N₂ ⁺(B²∑_u ⁺–X²∑_u ⁺)), , and the weak N atom peak at 700–900 nm range can be observed. Additionally, significant spectral peaks for OH (309.0 nm), H_α (656.3 nm), and O (777.5 nm) were also detected. A very strong O atom line was detected at 777.5 nm, and a weaker O atom line was detected at 844.7 nm.

3.

Emission spectra of (a) N₂/O₂ gas–liquid mixed spark plasma and (d) total feed gas flows of 50, 100, and 350 mL/min. Nitrite/nitrate concentration in the solution of (b) different oxygen ratios; (c) total feed gas flow; (e) different input voltages; and (f) different concentration of OH^– in the solution. (g) Gas–liquid mixed spark-discharge images.

There have been studies that show that both the vibrational and excited states of N₂ can react with O atoms to form NO, with the most important being the activation and conversion of vibrational N₂ molecules. , The different vibrational excited states of Active Nitrogen Clusters (ANCs) produced by high-energy electron initial excitation can increase the vibrational energy level of one N₂ molecule through a v–v pumping reaction (reaction ), thereby gradually increasing its vibrational energy level until it can react with O atoms , (reaction ). This reaction can greatly reduce the energy barrier that breaks the NN bond, allowing the N₂ molecule to undergo efficient oxidation in the ground state without the aid of a high temperature environment and a catalyst. The N atoms generated by this reaction can continue to react with the ground-state O₂ molecules and excited-state O atoms to produce NO (reactions and ).

$${\mathrm{N}}_2(X,v)+{\mathrm{N}}_2(X,w)\to {\mathrm{N}}_2(X,v+1)+{\mathrm{N}}_2(X,w-1)$$ 1

$$\mathrm{O}+{\mathrm{N}}_2(X,v>12)\to \mathrm{NO}+\mathrm{N},E_{\mathrm{a}}\approx 3\mathrm{eV}{\mathrm{molec}\mathrm{ules}}^{-1}$$ 2

$$\mathrm{N}+{\mathrm{O}}_2\to \mathrm{NO}+\mathrm{O}$$ 3

$$\mathrm{N}+\mathrm{O}\to \mathrm{NO}$$ 4

Therefore, in the NTP-ERR system described in this paper, a higher concentration of NO is generated near the gas outlet at the bottom of the high-voltage electrode. These NO molecules can easily react with O₂ at room temperature to form NO₂ (reaction ). When these gas molecules reach the gas–liquid contact surface, due to the presence of a high concentration of OH^– in the electrolyte, further reactions can occur, ultimately resulting in the fixation of NO₂ ^– and NO₃ ^– (reactions and ) in the electrolyte.

$$2\mathrm{NO}+{\mathrm{O}}_2\to 2{\mathrm{NO}}_2$$ 5

$$\mathrm{NO}+{\mathrm{NO}}_2+{2\mathrm{OH}}^{-}\to 2{\mathrm{NO}}_2^{-}+{\mathrm{H}}_2\mathrm{O}$$ 6

$$2{\mathrm{NO}}_2+{2\mathrm{OH}}^{-}\to {\mathrm{NO}}_2^{-}+{\mathrm{NO}}_3^{-}+{\mathrm{H}}_2\mathrm{O}$$ 7

Nitrogen and oxygen are converted into NO₂ ^– and NO₃ ^– by first being activated and then being absorbed. Therefore, the ratio of N₂/O₂ will have a significant impact on the concentration of NO _x ^– in the solution. As shown in Figure S5, there are two distinct ANCs spectral bands of N₂SPS and N₂ ⁺FNS in the plasma emission spectrum of pure nitrogen gas, but the intensity of the O atom spectral peak is relatively low, which is only derived from the dissociation of water molecules caused by the gas–liquid discharge. For the plasma discharge spectroscopy of pure oxygen, there are very strong O atom spectral peaks and weaker OH spectral peaks from water dissociation, but no ANCs spectral peaks. After 30 min of reaction, the NO _x ^– content in the detection solution was determined, as shown in Figure b. For the pure nitrogen discharge process, more ANCs could be produced, but the concentrations of NO₂ ^– and NO₃ ^– were both at a low level due to the lack of active O atoms. For the pure oxygen discharge process, no NO _x ^– was produced because there was no nitrogen element involved in the reaction system. Therefore, this study tested the yields of NO₂ ^– and NO₃ ^– under different nitrogen-to-oxygen ratios in the feed gas.

First, the research found that in the products obtained after activation, the concentration of NO₂ ^– was much higher than that of NO₃ ^–. This indicates that when there is a greater amount of NO in the activated gas mixture, reaction becomes the dominant reaction.

At the same time, it can be found that as the proportion of O₂ in the reaction gas increases, the proportion of NO₃ ^– in the generated NO _x ^– that contains more oxygen also gradually increases. It is worth noting that the experiment shows that the total amount of NO₂ ^– and NO₃ ^– generated in the reaction system is the largest when the ratio of N₂/O₂ is 1:1, which seems to contradict the 1:2 and 1:3 atomic ratios of NO₂ ^– and NO₃ ^–. In fact, the dissociation of water molecules in the electrolyte solution itself can provide a certain amount of oxygen atoms, and more importantly, due to the much smaller bond energy of OO bond (498 kJ/mol) than that of NN bond (941 kJ/mol), a small amount of oxygen supply in the reaction system can produce sufficient O atoms, so the high oxygen content is not needed in the NTP-ERR system.

The effects of different energy input and gas flow rates on the nitrogen fixation in the reactor were investigated. As shown in Figure c,d, too little or too much gas flow is not beneficial for generating more NO₂ ^– and NO₃ ^–. When the total gas flow rate is too low, the reactants decrease, and the ANCs collected during the reaction process also decrease, resulting in a decrease in the generation of NO₂ ^– and NO₃ ^–. When the flow rate is too high, although more reactants are introduced, by comparing the spectra corresponding to 350 mL/min and 200 mL/min, it can be found that the peak intensities of each active species also decrease. The excessive gas flow rate leads to a shortened gas residence time in the NTP region, causing some gas molecules to be unable to be excited to higher energy levels, thereby reducing the gas activation efficiency. Furthermore, as mentioned earlier, in the plasma discharge system used in this paper, the high-voltage end of the plasma discharge is a conductive metal bar, and the low-voltage end is an electrolyte. The electrode spacing in the system is the distance between the metal bar and the electrolyte. When the gas flow rate is too high, the bubbles produced when the small hole at the bottom of the quartz tube is vented become larger, and the gas is more concentrated near the hole. The distance between the high-voltage electrode and the electrolyte increases, which is equivalent to increasing the electrode spacing for discharge, making it more difficult for the gas to be ionized, and reducing the activation effect of the plasma on the reaction gas.

By adjusting the input voltage of the plasma power supply, the energy input to the NTP process can be changed. As shown in Figure e, the total concentration of NO₂ ^– and NO₃ ^– in the solution increases first and then decreases as the plasma energy input increases. It reaches its maximum value at 54 V. By comparing the emission spectra of the plasma under different voltages (Figure S6), it can be found that raising the voltage can improve the activation effect of N₂ and the peak intensity of ANCs is significantly increased. However, when the voltage is increased further, there is no obvious change in the peak intensity of ANCs, while the peak intensities of OH and Na become significantly stronger. First, spark discharge is a type of energy-concentrated discharge, and at a fixed input of reaction gas, an excessively high voltage cannot further enhance the activation strength of N₂, but may cause more nitrogen oxides to redissociate. Second, the presence of NaOH in the solution helps to directly observe the intensity contrast between the liquid-phase and gas-phase discharge under different voltages. As shown in Figure g, by observing the high-resolution discharge images under different voltages, we can see that the low-voltage spark is purple, which is the normal color of nitrogen plasma. However, when the voltage is increased to 90 V, the discharge intensity becomes stronger and the spark turns yellow, showing the sodium flame color. During this process, not only does more energy lose through light energy, heat energy, etc., increasing energy consumption, but the relative intensity of liquid-phase discharge is constantly increased under the excessive input of plasma energy, while the gas-phase discharge is weakened. Therefore, the input of plasma energy for nitrogen and oxygen activation and nitrogen fixation is not necessarily the better the better.

In the NTP-ERR system, NaOH solution, which serves as the absorbing solution for the NTP process and the electrolyte for the ERR process, is an important link between the two reaction processes. The concentration of OH^– also has an important impact on the content of NO₂ ^– and NO₃ ^– produced by the NTP process. As shown in Figure f, when the concentration of OH^– in the electrolyte is too low, the absorption effect of gaseous NO and NO₂ is poor. With the increase of OH^– concentration, the concentration of NO _x ^– accumulated in the solution first increases and then decreases, and reaches its maximum value at an OH^– concentration of 0.1 mol/L. As shown in Figure S7, the pH of the solution has decreased to different degrees after 30 min of reaction, indicating that NaOH has participated in the reaction and reacted with NO and NO₂ to form NO _x ^– (reactions –). Figure g shows the captured high-resolution spark-discharge images at different OH^– concentrations. It can be seen that with the increase of NaOH concentration in the electrolyte, the discharge spark intensity has been significantly improved. It was also observed during the experiment that as the OH^– concentration was increased from 0.025 to 0.5 M, the input current required by the plasma power supply at the same input voltage decreased significantly. This indicates that when the ion concentration in the electrolyte is too low, the conductivity at the low-voltage end is poor, and a higher input current is required for discharge, making the discharge more difficult. Appropriately increasing the NaOH concentration can improve the absorption effect of NO and NO₂ while promoting the discharge process. However, excessively high NaOH concentration will cause the liquid-phase discharge to gradually dominate, and the electric spark will gradually turn yellow at high NaOH concentration, reducing the gas activation effect.

3.3. Performance of the NTP-ERR System in Synthesizing NH₃

The LSV curves and the initial potential are important indicators for evaluating the electrocatalytic performance of catalysts. As shown in Figure a, the LSV curves of CuRu_0.5 NPs in different electrolytes were tested. Under the condition of 0.1 M OH^– + 50 mL/min N₂ at −2 mA/cm² current density, the onset potential of CuRu_0.5 NPs in the electrolyte was −0.203 V. When 0.1 M of NO₂ ^– and NO₃ ^– was added to the electrolyte, the onset potential of CuRu_0.5 NPs moved forward to +0.161 V and +0.101 V, respectively. This indicates that the catalyst has good catalytic reduction effects for NO₂ ^– and NO₃ ^–. Meanwhile, the lower onset potential of NO₂ ^– indicates that the catalyst has better reduction performance for NO₃ ^–, which is beneficial for ERR in which the majority of NTP is NO₂ ^–. By optimizing the reaction parameters and operating conditions of NTP, the highest concentrations of NO₂ ^– and NO₃ ^– were 6.62 mmol/L and 0.53 mmol/L, respectively. Figure b compares the LSV curves of several different nanocatalysts in the electrolyte with the concentrations of NO₂ ^– and NO₃ ^– at the highest level. As the Ru doping content of CuRu _x NPs increases, its onset potential moves forward significantly. In particular, by comparing Cu NPs and CuRu_0.1 NPs, it can be found that a small amount of Ru doping can raise the onset potential nearly 0.12 V. This means that CuRu _x NPs will be beneficial for reducing NO₂ ^– and NO₃ ^– to synthesize ammonia at ultralow potential.

4.

Performance of the NTP-ERR system in synthesizing NH₃. (a, b) LSV curves. Ammonia production rate and Faraday efficiency of (c) Cu; (d) CuRu_0.1; (e) CuRu_0.5; (f) Ru. (g) Optimal potential of Cu, Ru, and their nanoalloys relative to RHE when the production rate of ammonia or the Faraday efficiency of ammonia is taken as the evaluation standard. (All data points represent the mean ± standard deviation (SD) of three independent experiments using different batches of catalysts. The maximum FE of 98.3% is the average of three measurements under identical conditions.)

To prove the above conclusion, and to provide a clearer demonstration of the reduction effect of the CuRu alloy on the mixture of NO₂ ^– and NO₃ ^– after NTP treatment, the synthesis of ammonia performance of different catalysts at different potentials was tested. The test results are shown in Figures c–f and S8–S10, where the blue markers represent the potential at which the ammonia Faraday efficiency of the catalyst is highest, and the yellow markers represent the potential at which the ammonia yield is highest. As shown in Figure d, the highest ammonia yield and ammonia Faraday efficiency of CuRu_0.1 NPs can be significantly shifted to −0.2 V and −0.15 V, respectively, by adding a small amount of Ru. A small amount of Ru doping can significantly enhance its ammonia yield and ammonia Faraday efficiency at low potentials. As the Ru doping content increases, the highest ammonia yield potential of CuRu_0.3 NPs, CuRu_0.5 NPs, CuRu_0.7 NPs, and Ru NPs are shifted to −0.15 V, −0.1 V, −0.05 V, and 0 V, respectively, and the best ammonia Faraday efficiency potential is shifted to −0.15 V, −0.1 V, −0.1 V, −0.05 V, and 0 V, respectively. This conclusion is well matched with the starting potential of several catalysts’ LSV curves and is intuitively demonstrated in Figure g.

However, although the optimal potential for Ru NPs is relatively the lowest, when comparing the highest ammonia yield and Faradaic efficiency among several catalysts in a lateral manner, nanoscale Ru is not the best performer. As shown in Figure e, the ammonia yield of CuRu_0.5 at −0.1 V (vs RHE) is 1.016 μg mg_cat ^–1 s^–1, and the Faradaic efficiency is as high as 98.3%, which are the highest values among several nanometallic materials at all tested potentials. It can also be found that compared with the highest Faradaic efficiency of 80.3% for pure Cu, the selectivity for ammonia synthesis from NTP-generated basic environment with low concentration of NO _x ^– is significantly improved after Ru doping, with Faradaic efficiency above 90% for all CuRu alloys. Therefore, for the reduction of low-concentration NO _x ^– in the basic environment generated by NTP, CuRu _x NPs can not only synthesize ammonia at ultralow potential and reduce the energy consumption of the ERR process but also adapt to different reduction potentials by adjusting the Cu/Ru ratio, thereby achieving high selectivity ammonia synthesis in a wider low-potential range.

The stability of synthesizing ammonia by a CuRu_0.5 nanocatalyst at −0.1 V in the corresponding concentration electrolyte after NTP treatment was tested. Each reduction reaction was set at 1h, and the test was repeated 12 times. The IT curves and Faraday efficiency are shown in Figure S11. The Faraday efficiency was above 90% and the current density was stable in all 12 tests, indicating that the synthesized CuRu alloy nanoparticles have good stability for ERR. To further confirm the reaction pathway, potential byproducts including H₂, N₂, N₂H₄, and NH₂OH were systematically quantified. GC analysis revealed negligible N₂ evolution and a low H₂ Faradaic efficiency (<3%). Colorimetric assays showed no detectable N₂H₄ or NH₂OH, confirming that NH₃ is the exclusive nitrogenous product with a selectivity exceeding 98%.

3.4. Process and Mechanism Study

To investigate the promotion of Ru doping in CuRu alloy for the superlow potential electrochemical reduction of NO _x ^– to synthesize ammonia, and the reasons why the performance of CuRu_0.5 NPs is better than that of Cu NPs, Ru NPs, and other proportion nanocatalysts. First, the cyclic voltammetry curves of several nanometallic catalysts in the non-Faradaic range were tested, as shown in Figure S12. And the C _dl values were calculated, as shown in Figure S13. The increase in Ru doping in CuRu _x NPs enhances the electrochemical active area of the catalyst significantly, exposes more catalytic active sites, and improves the electrocatalytic reduction performance at lower potentials. This conclusion is corroborated by the LSV curves of several catalysts. However, although Ru NPs can shift the starting potential of the catalytic reaction to the lowest, its Tafel slope is lower than that of Cu and CuRu_0.5 NPs (Figure S14). The Tafel slope is an important indicator for evaluating the catalytic reaction kinetics, and it can be found that the increase of Cu content can result in a lower Tafel slope, indicating that the presence of Cu atoms in the catalyst is beneficial for enhancing the response strength of the electrochemical reaction rate to the potential change from the perspective of enhancing catalytic activity. From the perspective of reaction kinetics, it is more favorable for the electrocatalytic reduction process. Figure S15 shows the EIS curves of several catalysts, and it can be found that as the Ru content in CuRu _x NPs increases, the electrical impedance of the catalyst first significantly decreases and then gradually increases. CuRu_0.5 NPs obtained the lowest EIS value, indicating that CuRu_0.5 NPs have the smallest charge transfer resistance and the strongest charge transfer ability during the reaction process, thus being more conducive to promoting electron transfer and accelerating the reduction process. This phenomenon may be related to the electron donation effect between Cu atoms and Ru atoms. The electron-rich environment formed around Ru atoms promotes the adsorption of NO _x ^– and its electron acceptance and reduction reaction process.

Through the above research analysis, it can be found that Cu and Ru atoms play different roles in the CuRu alloy, and in the CuRu_0.5 NPs, the two metal atoms can further enhance their synergistic effect to improve the catalyst’s ability to reduce NO _x . It is hypothesized that the steps and mechanism of the plasma-fixed N₂/O₂ mixture coupled with the CuRu nanocatalyst for the synthesis of ammonia are shown in Figure c. It can be divided into two processes: NTP and ERR.

5.

(a) Free energy diagrams of NO₃ ^–/NO₂ ^–→ NH₃ on CuRu_0.5; (b) free energy diagrams of NO₃ → NH₃ on CuRu_0.5 and Cu; (c) mechanism of plasma-fixed N₂/O₂ coupled CuRu nanoalloy electrocatalytic synthesis of ammonia. (d) PDOS of Cu/Ru for CuRu_0.5 and Cu. (d) for Cu; (e) differential charge densities of the NO₃/NO₂ adsorption on CuRu_0.5 and Cu; (f) contour diagram of specific energy consumption corresponding to different energy inputs of NTP and ERR in this NTP-ERR system. (g) Specific energy consumption and ammonia synthesis performance of NTP-ERR system compared with the recent research progress in the fields of ENRR, Li-NRR, plasma NRR, and plasma-assisted ENRR for one-step NH₃ synthesis.

In the NTP process, nitrogen and oxygen molecules go through three steps to gradually convert into NO₂ ^– and NO₃ ^–. In the first step, nitrogen and oxygen molecules are activated by spark plasma discharge plasma and the nitrogen–nitrogen triple bond and oxygen–oxygen double bond are broken, forming ANCs and active O atoms, while liquid-phase discharge can also activate H₂O molecules and generate active O atoms to participate in the reaction. In the second step, a large number of ANCs can collide and react with each other to continuously increase their energy levels, and high-energy ANCs can react further with O atoms to generate NO and a small amount of NO₂. When these newly generated NO and NO₂ molecules move to the gas–liquid contact surface, the third step occurs, in which they react with the large amount of OH^– present in the solution and ultimately convert into NO₂ ^– and NO₃ ^–. Stable nitrogen is finally fixed in the electrolyte in a form that is more easily reactive and has a lower electrochemical reduction potential.

In the ERR process, there is a donor–acceptor effect between adjacent Cu and Ru atoms in CuRu_0.5 NPs, leading to charge transfer and making the Ru atoms around it present a rich electron state, enabling the stable adsorption of NO₂ ^– and NO₃ ^–, and providing electrons to these adsorption intermediates. After several deoxidation processes, *NO₂ and *NO₃ are converted to *N. At this time, due to the better performance of Ru in producing adsorbed hydrogen, the continuous hydrogenation steps can be carried out on the CuRu alloy with a lower energy barrier, ultimately generating ammonia. Meanwhile, the presence of Cu atoms can effectively prevent the self-coupling of *H, causing competition for HER.

As shown in Figure a–e, for the CuRu_0.5 alloy with optimal performance, modeling was conducted on the basis of its atomic ratio and experimentally determined crystal facets. The reduction reaction (NO _x RR) pathways on its surface were analyzed computationally. The results showed that NO₃ on this surface tends to undergo stepwise deoxygenation to form *N (adsorbed nitrogen atom), followed by stepwise hydrogenation to complete the reduction process. The rate-determining step (RDS) was identified as the final hydrogenation step. Notably, the adsorption energy of NO₂ on CuRu_0.5 was significantly higher than that of NO₃, indicating a stronger interaction between the surface and NO₂. This thermodynamic preference suggests that NO₂ reduction (NO₂RR) is more favorable than NO₃ reduction (NO₃RR) on this surface as the stronger adsorption facilitates subsequent electron transfer and protonation steps. Additionally, calculations of the adsorption capacity of reaction molecules on the pure Cu surface (Supporting Information) revealed that its adsorption ability is lower than that of CuRu_0.5. This indicates that the pure Cu surface is less supportive of subsequent reactions, hindering the following processes. Further analysis of the projected density of states (PDOS) for Cu before and after Ru doping demonstrated that the electronic states are primarily contributed to by the metal d orbitals. After Ru doping, the d orbitals of Cu were broadened, while the d orbitals of Ru were prominently distributed near the Fermi level. These electronic changes enhanced the adsorption capacity of the surface for the reaction molecules. Differential charge density analysis further corroborated these conclusions. It revealed stronger charge transfer for NO₂ than NO₃ on CuRu_0.5 and enhanced NO _x charge transfer on CuRu_0.5 compared to pure Cu. In summary, appropriate Ru doping modulates the electronic structure of the alloy surface, improving nitrate adsorption and activation and thereby facilitating the NO _x RR process.

3.5. Techno-Economic Analysis and Outlook

Beyond catalyst design, system-level energy matching is decisive for the overall efficiency of plasma–electrocatalytic cascade ammonia synthesis. In multistep processes involving heterogeneous energy inputs (electrical discharge vs electrocatalysis), mismatched performance between stages leads to unnecessary energy dissipation and suboptimal output. Inspired by critical-point analysis in process system engineering, we systematically mapped the unit energy consumption as a function of plasma power and electrocatalytic current density (Figure f). This approach identifies the global energy optimum where the outputs of the plasma stage are quantitatively matched to the reduction capacity of the electrocatalyst, thereby avoiding energy waste from overoxidation or insufficient reduction. Through this integrated optimization, a record-low specific energy consumption of 273.41 kWh/kg NH₃ is achieved, representing a leapfrog improvement in system-level energy efficiency.

For the NTP-ERR reaction system described in this paper, by using an air–liquid mixing spark plasma discharge plasma reaction device, more N can be stored in an easily reducible form by fixing nitrogen to the electrolyte first and then electrocatalytically reducing it, with the same plasma energy input. This can significantly reduce the specific energy consumption of the NTP process. At the same time, by optimizing the design of the CuRu alloy to enhance its electrocatalytic reduction of NO₂ ^– and NO₃ ^– to synthesize ammonia at ultralow potential, the specific energy consumption of the ERR process can also be effectively reduced. As shown in Figure S20a, the specific energy consumption of the ERR process takes two lower values respectively when the ERR energy input is low (0.05 V) and when the ammonia production rate is high (−0.05 V). The specific energy consumption of the NTP process is shown in Figure S20b. Similarly, the NTP process also achieves two lower values when the energy input is low (18 V) and when the ammonia production rate is the highest (54 V). With the optimization of energy consumption in these two processes, the lowest specific energy consumption of the NTP-ERR system is only 273.41 kWh/kgNH₃. Our work has been compared with the latest advances in the field in terms of SEC and ammonia synthesis performance (as shown in Figure f and detailed in Table S7). Our NTP-ERR system has an NPR that is 3–4 orders of magnitude higher than direct ENRR. The NH₃ production efficiency of our NTP-ERR system exceeds that of other electrochemical NH₃ synthesis alternatives, such as eNRR, lithium-mediated eNRR, and other reported NTP-ERR systems.

To more clearly illustrate the impact of the ERR process and the NTP process on the energy efficiency, the importance of matching the energy input of the ERR process and the NTP process was explained, and a Contour diagram of specific energy consumption was drawn. As shown in Figure g, it can be found that the energy efficiency of the NTP-ERR system is not linearly related to the increase of either the NTP or ERR energy input, but the total energy efficiency rises rapidly when the NTP energy input is too high. A trough zone of the energy efficiency can be seen at the plasma power input voltage of 50–60 V, indicating that appropriately increasing the NTP energy input can help improve the ammonia yield without increasing the energy efficiency. For ERR, it can be found that lower potentials are beneficial for achieving lower energy efficiency, which is more obvious in the region where the NTP input voltage is below 70 V. This indicates that the excellent performance of the CuRu alloy in synthesizing ammonia at ultralow potentials is of great significance for further reducing the energy efficiency of the system.

As summarized in Figure g, the specific energy consumption achieved in this work (273.41 kWh/kg NH₃) surpasses most reported plasma–electrocatalytic systems and is comparable to or better than traditional Haber–Bosch routes. Crucially, the advantage of this work lies not merely in catalyst activity, but in the system-level design philosophy, which differs fundamentally from previous studies:

Li et al. established the plasma–electrocatalysis cascade but relied on nonselective plasma oxidation, producing a mixture of NO₂ ^– and NO₃ ^–. The excess NO₃ ^– required additional reduction energy, resulting in higher system-level energy consumption. While achieving good energy efficiency, Liang et al. focused on optimizing individual components rather than the energy matching between stages. Consequently, the plasma power and electrocatalytic current density were not systematically coupled, leaving room for energy waste.

In contrast, our work introduces selective NO₂ ^– generation combined with critical-point energy matching, ensuring that the output of the plasma stage is quantitatively aligned with the reduction capacity of the CuRu₀.₅ catalyst. This systemic synergy, rather than incremental material improvement, is the origin of the record-low energy consumption reported here.

4. Conclusions

In summary, this study designed a gas–liquid mixed spark plasma atmospheric pressure (NTP) and enhanced reactor (ERR) coupled system for the fixed N₂/O₂ mixture and CuRu Nanocatalyst to synthesize ammonia through a cascade reaction. The successful conversion of N₂/O₂ to NO₂ ^– and NO₃ ^–, and then to ammonia was achieved. The activation and immobilization processes of N₂ and O₂ were analyzed in detail by combining the discharge images and plasma emission spectra. The ammonia yield reached 1.016 μg mg_cat ^–1 s^–1 at a low potential of −0.1 V (vs RHE) and the Faraday efficiency was as high as 98.3%. After the energy consumption and economic analysis, the detailed analysis of the energy input ratio of NTP and ERR on the energy consumption was carried out, and the lowest specific energy consumption of the optimized NTP-ERR system was calculated to be only 273.41 kWh/kgNH₃, which has significant energy consumption or yield advantage compared with other ammonia synthesis processes. This work demonstrates that system-level energy matching, rather than isolated component optimization, is the key to unlocking ultralow-energy distributed ammonia synthesis. This study provides a promising alternative solution for synthesizing ammonia using N₂/O₂ mixture as raw material, aqueous solution as hydrogen source, and using renewable energy under mild conditions.

While the current laboratory-scale system achieves a record-low energy consumption of 273.41 kWh/kg NH₃, several challenges remain to be addressed for practical, large-scale deployment. Future research should focus on (i) the scale-up design of plasma reactors to maintain selective NO₂ ^– generation under high gas throughput; (ii) the long-term durability of both the plasma electrodes and the CuRu₀.₅ catalyst under continuous operation; and (iii) the development of low-energy ammonia recovery processes to efficiently separate NH₃ from alkaline absorption liquids without additional thermal or chemical input. Addressing these engineering barriers will be crucial for translating the current plasma–electrocatalytic cascade strategy into viable decentralized ammonia production technologies.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/cbe.6c00044.

• Additional experimental details, materials, and methods including XRD, XPS, in situ DRIFTS measurement details, and DFT calculation details (PDF)

The authors declare no competing financial interest.