Recent progress in ammonia production via plasma-electrocatalysis cascade system

INTRODUCTION

Ammonia (NH₃), an essential chemical product in modern society with a global demand of over 200 million tons annually, is a crucial feedstock in industries such as fertilizer, textile, and pharmaceutical, and is considered a carbon-neutral energy carrier due to its high energy density (4.25 kW·h/L)^[1-3]. Artificial NH₃ synthesis was not realized until Haber proposed a chemical method for NH₃ synthesis from atmospheric nitrogen (N₂) in the 20th century, which converts N₂ and H₂ into NH₃ at high temperatures (400-500 °C) and high pressures (100-200 kPa) by using iron [Figure 1A], where H₂ is generally obtained from steam methane reforming (SMR) process [Equation (1)].

Figure 1. General NH₃ synthesis processes. (A) H-B process: NH₃ is prepared from H₂ and N₂ under high temperature (500 °C) and high pressure (20-50 MPa) over a Fe-based catalyst^[18]; (B) Li-mediated three-step electrocatalytic cycling method for NH₃ synthesis^[11]; (C) Photocatalytic electron - excited N₂ to NH₃^[13]; (D) Plasma directly dissociates N₂ and then bonds with H⁺ to generate NH₃^[16]; (E) NH₃ synthesis by PECP pathway. H-B: Haber-Bosch; PECP: plasma-electrocatalysis cascade pathway.

However, the SMR process consumes fossil energy and produces a large amount of greenhouse gases, accounting for 2% of the world’s total carbon emissions^[4,5], accompanied by the energy consumption of 1.2 × 10¹⁹ joules per year^[6]. Due to the environmental unfriendliness of the Haber-Bosch (H-B) process, a large number of alternative methods have been developed, such as electrocatalysis, photocatalysis, and plasma catalysis.

The electrochemical route, such as N₂ electro-reduction, offers the potential to reduce carbon emissions which employs H₂O as a clean hydrogen source and ideally can be driven by renewable electricity [Figure 1B]^[7,8]. However, N₂ electro-reduction faces intense competition from the hydrogen evolution reaction (HER) side reaction due to the inertness of N₂. Several improvements (such as lithium-mediated N₂ electro-reduction) have been proposed, but they still exhibit low energy efficiency for NH₃ synthesis^[3,9-11]. Ruddlesden-Popper (RP) perovskite can also serve as an electrocatalyst for NH₃ synthesis under ambient pressure. However, the fundamental catalytic principles underlying its performance remain unclear^[12].

Another pathway is photocatalysis [Figure 1C]^[13]. Under light irradiation, the adsorbed N₂ and H₂O are catalyzed into NH₃ and O₂ by electrons and holes, respectively. Photocatalytic NH₃ synthesis can utilize solar energy directly, reduce energy consumption, and have no carbon pollution during the reaction. Yet, the photocatalytic reaction suffers from low NH₃ yield for large-scale production.

Plasma NH₃ synthesis could afford a higher reaction efficiency than the traditional thermal catalytic approach due to the readily activation of inert N≡N by high-temperature electrons [Figure 1D]^[14-16]. The N radical species can react with H⁺ to generate NH₃, N₂H₄, H₂, and other products^[17]. This path uses H₂ or H₂O as the hydrogen source and does not produce carbon pollutant gases. Nevertheless, the interconversion between NH₃ and N₂ leads to low NH₃ selectivity.

In recent years, inspired by natural lightning N₂ fixation and the rapid development of electrocatalytic NO_x electroreduction technology, plasma N₂ fixation coupled with electrocatalysis has gradually entered the public’s vision for NH₃ synthesis, i.e., plasma-electrocatalysis cascade pathway (PECP). The method integrates the advantages of both plasma thermal catalysis and electrocatalysis air and water as raw materials. N₂ and O₂ are readily activated into NO_x^- intermediates via plasma technology under ambient temperature and pressure. Subsequently, these intermediates are electro-reduced to produce NH₃. The entire process typically involves two steps: plasma activation and electrocatalytic reduction [Figure 1E].

The novelty of the PECP strategy lies in overcoming the challenges of low N₂ activation efficiency and poor yield in traditional electrocatalytic nitrogen reduction reactions, while also achieving continuous and efficient energy conversion through an integrated device design. Moreover, the technology operates under ambient conditions, avoiding the high temperature and pressure requirements of the traditional H-B process, thereby significantly reducing energy consumption and carbon emissions. Its compatibility with renewable energy sources and its potential to generate high-value byproducts further enhance its economic viability and environmental sustainability.

WORKING MECHANISM FOR PECP

Mechanism of nitrogen fixation by plasma

Plasma is considered to be the fourth state of matter, which consists of numerous positive ions, negative ions, electrons, and unionized neutral particles, but maintains overall electric neutrality^[15]. Plasma can be classified into high-temperature plasma and low-temperature plasma according to the system temperature.

The low-temperature plasma is more widely used because of its lower energy consumption working below 10⁴ K, which can be further divided into thermal plasma with an approximately balanced thermodynamic state and non-thermal plasma (or cold plasma) with an unbalanced thermodynamic state^[18]. The high-temperature plasma generally has a system temperature > 10⁴ K^[19], such as solar corona, magnetic confinement fusion, and inertial confinement fusion. Non-thermal plasmas include microwave discharge [Figure 2A]^[20], corona discharge [Figure 2B]^[21], glow discharge [Figure 2C]^[22], dielectric barrier discharge (DBD) [Figure 2D]^[23]. Common thermal plasmas include high-frequency discharge [Figure 2E]^[24] and electric arc discharge [Figure 2F]^[25]. There is also a gliding arc plasma [Figure 2G]^[26,27] between the equilibrium state and the non-equilibrium state. During the arc generation, it is in an equilibrium state. However, as the arc is pushed by the airflow and becomes longer, it will enter a non-equilibrium state. Liu et al. made improvements to the sliding arc plasma by using a magnetic field to drive the arc, which increased the length of the arc and enlarged the contact area with gas molecules, resulting in a better ionization effect^[28].

Figure 2. Schematic diagrams of some plasmas. (A) Microwave discharge plasma^[20]; (B) Corona discharge^[23]; (C) Glow discharge plasma^[21]; (D) DBD plasma^[22]; (E) High-frequency discharge plasma^[24]; (F) Electric arc discharge^[25]; (G) Gliding arc plasma^[26]. DBD: Dielectric barrier discharge.

In the study of N₂ fixation via plasma-generated nitrogen oxides, the reaction pathways of different discharge mechanisms show significant differences. These mechanisms can be broadly classified into three groups:

(1) High-energy electron-dominated discharge mechanisms: Microwave discharge generates high-energy electrons through the interaction of high-frequency electromagnetic wave energy with gas molecules. High-frequency discharge excites gas molecules via a high-frequency electromagnetic field. Arc discharge and gliding arc plasma achieve discharge through high current density and high-temperature environments.

(2) Discharge mechanisms involving synergistic effects of electric field and surface reactions: The reaction pathway of corona discharge begins with the ionization of gas molecules by electrons accelerated in the electric field near the electrode surface, producing free electrons and positive ions. DBD applies a high voltage between electrodes while restricting the discharge range with a dielectric barrier layer, forming microdischarge channels. Within these channels, high-energy electrons collide with gas molecules, triggering ionization and excitation reactions.

(3) Uniform discharge mechanisms at low pressure: In a glow discharge, high-energy electrons near the cathode generate free electrons and ions through collisional ionization. These electrons and ions then collide with N₂ and O₂ molecules to form excited-state gas molecules.

Despite differences in plasma discharge mechanisms, the N₂ fixation mechanisms share a high degree of similarity in their core reaction pathways, which can be primarily summarized as follows^[29]:

First, the generation of high-energy electrons: The essence of all plasma discharge mechanisms is the generation of high-energy electrons through various means. These high-energy electrons act to initiate subsequent chemical reactions. Then, the high-energy electrons collide with N₂ and O₂, causing ionization and excitation, and forming reactive intermediates.

(5) $$ \mathrm{e^-+O_2\leftrightarrow O+O+e^-} $$

(6) $$ \mathrm{e^-+N_2\leftrightarrow N+N+e^-} $$

Then, the next molecular bonding reaction depends on the vibrational energy of the molecule. The main chemical reaction for the generation of NO_x in atmospheric pressure air plasma is carried out through the vibration-enhanced Zeldovich mechanism^[30,31] which consists of the following reactions:

(7) $$ \mathrm{N+O_2/O_2(v)\leftrightarrow NO+O} $$

(8) $$ \mathrm{O+N_2/N_2(v)\leftrightarrow NO+N} $$

Last, the N atoms formed in Equation (7) can further react with the ground-state and vibrationally excited O₂ molecules to produce an NO molecule. The reaction also produces an additional O atom, which can further react with the ground-state and vibrationally excited N₂ molecules to form a tightly coupled reaction cycle. In addition, O₃ is formed as a byproduct and can be formed by the reaction of O atoms with O₂ molecules. The generated O₃ may be directly discharged or oxidized NO to generate NO₂^[29].

(9) $$ \mathrm{NO+O\leftrightarrow NO_2} $$

(10) $$ \mathrm{2NO+O_2\leftrightarrow NO_2} $$

(11) $$ \mathrm{O+O_2(g,v)\leftrightarrow O_3} $$

To understand the specific differences between these plasmas intuitively, some examples of plasma N₂ fixation are summarized in Table 1. The N₂ fixation efficiency, energy consumption, and their advantages and disadvantages are presented and compared.

Table 1

Comparison of N₂ fixation performance of different plasma devices

Discharge type	NOx volume fraction	Energy efficiency/(MJ·mol-1)	Advantages	Disadvantages	Ref.
Spark	10.9%	~9.82	High yield Low energy consumption	Easy to lead to fire and explosion accidents	[32]
DBD	0.3%	56-140	Easy to operate Easy to combine with the catalyst	Low yield High energy consumption	[33]
Magnetically confined microwave plasma	14%	0.28	High yield Low energy consumption	Harsh operating conditions Industrialize difficulty	[34]
Gliding arc plasma	1%-2%	2.8-4.8	Low energy consumption Good development prospect	The yield needs to be further improved	[14]
Propeller arc plasma	0.4%	4.2	No need for gas flow Discharge frequency can be controlled	The yield needs to be further improved	[33]
γ-Al2O3 catalyze DBD	0.5%	18	Low energy consumption Good development prospect	Need catalyst	[35]

DBD: Dielectric barrier discharge.

Table 1 shows that NO_x yields vary by a few percentage points among different discharge types, but energy consumption for N₂ fixation varies greatly, from 0.28 to 140 MJ/mol. The table indicates that microwave plasma is most effective for N₂ fixation at low pressures, with a 6% NO_x yield and 0.84 MJ/mol energy consumption for microwave plasma synergistic catalytic N₂ fixation. The lowest energy consumption of 0.28 MJ/mol corresponds to the highest NO_x yield of 14% for magnetically confined low-pressure microwave plasma. However, the need for vacuum equipment and reactor cooling at low pressures hinders the widespread industrial adoption of microwave plasma. Patil et al. currently reported the best results for gliding arc plasma N₂ fixation, achieving a NO_x yield of 2% and energy consumption of 2.8 MJ/mol^[34]. When DBD plasma was combined with catalysts, energy consumption was reduced from 140 to 18 MJ/mol, and the N₂ fixation efficiency was improved by 0.2%.

Overall, gliding arc plasma is considered suitable for N₂ fixation due to its ability to oscillate and excite N₂ molecules^[27,36]. Similarly, DBD is regarded as a promising candidate because it operates at atmospheric pressure and is compatible with catalyst integration^[37,38].

The mechanism of NO_x reduction

There are two different pathways for the reduction of NO₃^-, including the indirect autocatalytic reduction pathway and the direct electrocatalytic reduction pathway^[39,40]. In high acidity (pH < 0) and high concentration of NO₃^- (> 1 mol·L^-1), there is a tendency for indirect autocatalytic reduction of NO₃^-, in which NO₃^- does not participate in electron transfer; the latter participates in the electron transfer process of NO₃^- at low concentrations (< 1 mol·L^-1), which is mainly discussed in this section.

Nitrate reduction reaction (NO₃RR) is a complex multi-electron transfer process with multiple possible reaction pathways^[41]. The mechanism and products of NO₃RR highly depend on the concentration of NO₃^-, the applied potential, and the pH of the electrolyte. Figure 3A shows the main reduction reactions and their thermodynamic potentials on the surface of the cathode electrode in alkaline or neutral solutions^[42]. Figure 3B shows that NH₃ and NH₄⁺ are the most thermally stable products under negative electrode potentials^[41]. The process of electrochemical conversion of NO₃^- to NH₃ involves the transfer of eight electrons coupled with nine protons [Figure 3C].

(12) $$ \mathrm{NO_3^-+6H_2O+8e^-\longrightarrow NH_3+9OH^-} $$

Figure 3. Pathways of electrocatalytic reduction of NO₃^- and Pourbaix diagram. (A) The thermodynamic potential of HER and nitric reduction^[42]; (B) Pourbaix diagram of N₂^[41]; (C) Reaction pathway of NO₃^- reduction^[41]. HER: Hydrogen evolution.

The direct electro-reduction of NO₃^- to NH₃ mainly includes two mechanisms. One is proton-coupled electron transfer (PCET), in which protons and electrons often appear in pairs to complete the reduction, and the other is hydrogen atom transfer (HAT), in which the reduction is carried out by directly adding active H^* from water reduction.

The specific path for the reduction of NO₂^- to NH₃ is as follows [Figure 4]: the NO₃^- in the bulk solution first adsorbs on the catalyst to form ^*NO₃^-, and further transforms ^*NO₂^- and ^*NO intermediates via PCET or HAT process. After the formation of ^*NO intermediate, there will be two different reaction paths toward NH₃. One is the N-side pathway: ^*H is first added to the N atom to form the ^*NHO intermediate, and then hydrogenated twice to form the hydroxylamine intermediate (^*NH₂OH). Moreover, it is noted that the ^*NH₂OH may be dissociated to NH₃ + ^*OH or ^*NH₂ + ^*OH, which subsequently undergo a hydrogenation process for the formation of NH₃. The other way is the O-side pathway, which is different from the previous path: ^*H first combines with the O atom, which leads to the formation of easily dissociated water molecules if the next hydrogenation still occurs at the O atom, leaving a single ^*N on the surface of the catalyst. ^*N is obtained by continuous hydrogenation to ^*NH₃, and then desorption produces free NH₃^[42-44].

Figure 4. Mechanism of the reaction path for the electro-reduction of NO₃^- to NH₃.

The mechanism of electrocatalytic NH₃ synthesis involves a multi-electron transfer process that reduces NO₃^- or NO₂^- to NH₃ via PCET and HAT. The complex reaction process and multiple intermediates pose significant challenges to efficient electrochemical NH₃ synthesis. In recent years, strategies such as catalyst design and interfacial engineering have significantly enhanced reaction efficiency and selectivity.

RESEARCH PROGRESS ON NH₃ SYNTHESIS USING PECP SYSTEMS

In parallel to the mechanism, experimental developments and system optimization of the PECP have played an important role in ambient NH₃ synthesis. Great efforts have led to significant progress in recent years [Table 2]. By comparing the NH₃ synthesis performance and energy consumption of different systems, a systematic reference can be provided for the design and performance optimization of PECP. Specifically, since plasma nitrogen fixation accounts for the majority of the total energy consumption in PECP, the following discussion on energy consumption will primarily focus on plasma N₂ fixation.

The pioneering study of NH₃ production by the PECP system was reported by Sun et al. in 2021. They developed a continuous and scalable non-thermal bubble column reactor electrochemical hybrid approach for NH₃ production and investigated the NO_x production efficiency from N₂ and O₂ by using different kinds of plasmas [Figure 5A]^[45]. Although this NH₃ synthesis strategy is emerging and promising, there is substantial room for improvement in terms of energy consumption (253 kWh·kg^-1 NH₃).

Figure 5. (A) The PECP model diagram was studied for the first time: comparison of N₂ fixation efficiency of various plasmas^[45]; (B) Best result research on PECP^[54]; (C) PECP model diagram of the complete production chain^[48]. PECP: Plasma-electrocatalysis cascade pathway.

It was mentioned that different plasmas have varying N₂ fixation efficiency in the previous chapter. Among them, the spark discharge is used in PECP by some researchers because of its high power and high N₂ fixation. For example, Ren et al. designed a spark plasma system with the same electrocatalyst for NH₃ synthesis^[52]. The NH₃ yield rate of ~40 nmol·s^-1·cm^-2 and 90% Faradaic efficiency (FE) was achieved. While spark discharge can efficiently generate NO_x, its high energy consumption (295 kWh·kg^-1 NH₃) cannot be ignored. Therefore, optimizing the N₂ fixation performance of plasma and reducing reaction energy consumption is key to achieving efficient NH₃ synthesis. Liu et al. further designed the PECP using Cu₁₀Fe₁ nanocatalyst^[51]. The yield of NH₃ (190.46 μmol·cm^-2·h^-1) and FE (93.74%) is improved together by using CuFe alloy compared with bare Cu due to the enhanced NO₃^- adsorption. In addition, Li et al. employed the improved Ni₃B@NiB_2.74 electrocatalyst^[49]. The system exhibits 100% FE of NH₃ with a yield of 3.37 mg·cm^-2·h^-1 owing to the enhanced H^* formation. Similarly, Liu et al. used Ni(OH)_x which could obtain much ^*H from H₂O dissociation to achieve a significantly improved NH₃ yield up to 3 mmol·cm^-2·h^-1 with 92% FE^[48]. Similarly, these studies all employed spark plasma for N₂ fixation. While these high-power plasmas can achieve significant performance improvements, further reductions in energy consumption are necessary. By adjusting discharge parameters and optimizing energy usage, it is possible to achieve both enhanced performance and energy savings.

Gliding arc plasma could effectively use vibrationally excited N₂ particles to facilitate the N₂ fixation reaction at atmospheric pressure. Based on this, Wu et al. combined the gliding arc plasma and Co-SAC/N-C producing 3 mg·h^-1 NH₃ and 62% FE^[47]. The distance between the plasma and the electrolyte was optimized in this study to maximize the absorption of generated NO_x by the electrolyte, thereby further conserving energy (51.8 kWh·kg^-1 NH₃). Li et al. combined two plasma devices to improve the N₂ fixation efficiency using the gliding arc-microwave plasma and developed the Cu₂Pd/CBC electrocatalyst^[53]. The NH₃ yield reached 1,756.65 μg·h^-1·mg^-1 and the FE improved to 93.7%. Comparative studies have demonstrated that the synergistic action of plasma significantly enhances the yield of NO_x, though it also increases energy consumption (177.1 kWh·kg^-1 NH₃). This finding indicates that plasma-assisted N₂ fixation is both feasible and challenging. Guo et al. developed a perovskite oxide catalyst La_1.5Sr_0.5Ni_0.5Fe_0.5O₄ for NO_x reduction reaction (NO_xRR) under strongly acidic conditions with pH = 0^[54]. Synergies between many metals in the electrocatalyst combined with gliding arc plasma result in the best performance of PECP ever achieved. The FE of NH₃ is close to 100%, obtaining an NH₃ yield of 8.55 mmol·cm^-2·h^-1 [Figure 5B]. Owing to the low energy consumption characteristic of gliding arc plasma, the system’s energy consumption is correspondingly low (52.78 kWh·kg^-1 NH₃). This study can serve as a valuable reference for the future design and performance optimization of PECP.

There are many other kinds of plasma, such as pulsed plasma, radio frequency (RF) plasma, jet plasma, etc. They are also used for NO_x production in the PECP. Sun et al. developed a plasma bubble reactor driven by nanosecond pulses and used Cu nanoparticles to obtain an NH₃ yield of 30 nmol·s^-1·cm^-2 and 97% FE^[29]. The energy consumption of this process is 188.2 kWh·kg^-1 NH₃. When comparing the NO_x production rate and energy consumption, the system still has significant potential for further improvement. Zheng et al. designed the RF plasma couple with an N-doped molybdenum sulfide nanosheet (N-MoS2/VGs)^[50]. At -0.53 Vvs. reversible hydrogen electrode (RHE), an FE of 38.1% and an NH₃ yield of about 7.30 mg·cm^-2·h^-1 are obtained. Although the performance of this system lags significantly behind others, the localized ionization method used achieves lower energy consumption (39.2 kWh·kg^-1 NH₃). Further optimization of the electrolysis parameters may help maintain low energy consumption while enhancing NH₃ synthesis performance. Considering DBD plasma could be operated at atmospheric pressure and scalability, DBD plasma is frequently used to fix N₂. Liang et al. researched the system that coupled DBD with Co nanosheets for the first time and obtained 16.21 mg·h^-1 NH₃ yield and > 90% FE^[46]. The energy consumption of this process is 78.8 kWh·kg^-1 NH₃. Given its acceptable energy consumption and relatively favorable performance in electrochemical NH₃ synthesis, the system holds significant potential for future development. Different from previous work, Ding et al. established a small jet plasma air oxidation system^[55]. A nano-bubbler filled with γ-Al₂O₃ adsorbent is equipped at the end of the gas outlet, which can effectively increase the absorption of NO_x in the solution, leading to a greatly improved NH₃ yield of 34.96 mg·h^-1 with 98.21% FE by using PdNi alloying nanoparticles on N-doped carbon nanotubes (PdNi/N-CNTs) as the catalyst. Adding catalysts to plasma can enhance NO_x synthesis efficiency, but it may also hinder gas mass transfer, leading to increased energy consumption (190.1 kWh·kg^-1 NH₃). Therefore, developing porous plasma catalysts to facilitate mass transfer is highly necessary.

With the improvement of technology, PECP is no longer limited to small-scale production. In 2023, Gao et al. expanded the production scale. An enlarged self-made H-type electrolyzer improves the NH₃ yield up to 3.6 g·h^-1[56]. This represents a groundbreaking advancement, marking the first step toward scaling up electrocatalysis. Additionally, the designed MgNH₄PO₄ (MAP) synthesis from NH₃ guides its practical application. Further, Liu et al. put forward the idea of electrolyte circulation in the original process to reduce the cost of practical production^[48]. In addition, a relatively complete industrial chain, such as the downstream separation and purification process, was established, paving the way for the subsequent scale-up research [Figure 5C].

To date, PECP has been able to devise a relatively comprehensive system. However, achieving a complete large-scale production chain requires further work. In the field of plasma N₂ fixation, it is already understood that different plasmas or a combination of multiple plasmas can be employed for N₂ fixation, yet the low N₂ fixation efficiency and high energy consumption for N₂ fixation remain the primary challenges. Energy consumption is also a key factor in plasma N₂ fixation. Currently, gliding arc plasma demonstrates good performance in NO_x synthesis with relatively low energy consumption. In conclusion, there is significant potential in optimizing PECP by enhancing N₂ fixation efficiency. On the other hand, in the electrocatalytic synthesis of NH₃ from NO₃^-, researchers have primarily focused on catalysts, investigating the enhancement of NH₃ synthesis efficiency through doping, nanostructure engineering, and other catalyst modification strategies. Currently, FE has seen considerable progress, with the vast majority exceeding 90%, and there is still potential for further improvement in NH₃ yield. Catalyst modification and N₂ fixation efficiency improvement will become an important research method for enhancing the efficiency of PECP.

STRATEGIES TO IMPROVE PECP

In light of the two-step operation process of PECP, two paths for optimizing PECP can be given accordingly. This section will mainly explain the strategy of promoting NH₃ synthesis by improving the efficiency of N₂ fixation and electrocatalytic NH₃ synthesis from NO₃^-.

Optimization of nitrogen fixation efficiency

Improvement of plasma discharge device

Increasing the N₂ fixation efficiency can effectively increase the concentration of NO_x^- in the electrolyte, thus promoting the synthesis of NH₃. High energy consumption and low NO_x yield are the main problems faced by plasma N₂ fixation. The current energy consumption optimal value is 2.4 MJ/mol, which is still higher than the traditional H-B process and cannot meet the needs of industrial applications. To become fully competitive, it needs to be further reduced to 0.7 MJ/mol^[57]. In addition, improving the NO_x yield by improving the plasma discharge device is also an important means to improve the efficiency of N₂ fixation.

For example, Ren et al. researched the effect of spark discharge distance on NO_x generation^[52]. As shown in Figure 6A, as the spark distance increases (0.5-2.0 cm), the residence times of reactants in the discharge zone would augment, increasing the concentration of NO_x. However, further increasing the spark distance (2-2.5 cm), the electric breakdown of gaseous reactants would consume more energy and ultimately decrease the concentration of NO_x to some degree. In addition, Wu et al. discovered the distance between plasma and electrolytes was also a key parameter for the formed NO_x^[47]. The velocity field distribution was determined by plasma-electrolyte distance and gas flow rate, which in turn affected the air discharge and transportation of plasma-induced species [Figure 6B]. The closer the electrolyte solution is to the plasma, the higher the concentration of NO_x that accumulates, and as the distance increases, the concentration of NO_x decreases. Starting from nitrogen-fixing raw materials, Sun et al. adjusted the feed ratio of N₂ and O₂ and found that 36% was the best proportion of O₂^[58]. Meanwhile, it was found that changing the gas injection speed would also affect the yield of NO_x [Figure 6C].

Figure 6. (A) Different distances of spark discharge and the concentration of total NO_x^-[52]; (B) Velocity field and distribution of NO and NO₂^- concentration in different plasma-electrolyte distances^[47]; (C) NO_x yield at different flow rates and feed ratios^[58].

Spark distance, plasma-electrolyte distance, gas feed ratio, and injection speed all affect NO_x concentration. Optimizing these parameters can enhance nitrogen fixation efficiency and NO_x yield. Current research is predominantly focused on electrocatalysts for NH₃ electrosynthesis, yet there is substantial potential for enhancing the efficiency of plasma N₂ fixation. Future research should focus on optimizing discharge parameters and developing scalable reactor designs.

Catalyst promoted nitrogen fixation

In addition to improving the plasma device to optimize the nitrogen fixation efficiency, the collaborative nitrogen fixation of plasma and catalyst can also effectively improve the nitrogen fixation efficiency. This is mainly because the catalyst (TiO₂, Al₂O₃, Fe₂O₃, etc.) can adsorb NO, N₂O, and other substances, increase the reaction time, and increase the yield of NO_x.

Patil et al. introduced the direct synthesis of NO_x from N₂ and O₂ under atmospheric pressure and low-temperature conditions by non-thermal plasma^[59]. In a DBD reactor, the direct synthesis of NO_x was studied by filling different catalyst support materials [Figure 7A]. By changing the type and size of catalysts, the NO_x yield can be continuously optimized.

Figure 7. (A) NO_x concentration fixed in plasma devices filled with different catalysts^[59]; (B) NO_x^- yield at different GO ratios^[58]. GO: Graphene oxide.

Sun et al. also added different proportions of TiO₂/[graphene oxide (GO)] to the plasma reactor to explore the differences in N₂ fixation conversion performance^[58]. In Figure 7B, when the GO content in the TiO₂@GO catalyst is 2.5 wt% to 5 wt%, the generation rate of NO_x^- production and the reduction of energy consumption. However, a higher GO ratio (> 10 wt%) reduces the amount of catalyst available for the reaction, thus significantly negatively affecting catalytic performance. This indicates that an appropriate amount of GO doping is beneficial to the formation of NO_x.

Adding catalysts is an effective way to enhance N₂ fixation efficiency. However, the addition of a catalyst may affect the emission of NO_x because the catalyst has the potential to clog the gas outlet. The development of highly efficient and porous catalysts, which improve gas flow and facilitate plasma permeation, is likely to become a popular research focus in the future.

Improvement of NH₃ electrosynthesis efficiency

Optimizing the structure of the cathode catalyst is one of the powerful strategies to improve the electrocatalytic synthesis of NH₃. The reduction of NO₃^- is accelerated by increasing the active site on the catalyst. It can also improve the catalytic activity of the catalyst site and directly improve the efficiency of NH₃ synthesis. Thus, exploring suitable catalysts and catalyst structures is an important strategy to promote the efficiency of the electrocatalytic NH₃ synthesis from NO_x.

Catalyst nanostructure engineering

Electrode nanostructuring has been demonstrated as a promising way to realize high-performance NH₃ electrosynthesis. The high surface-to-volume ratios of nanostructures give large electrochemical surface areas with much more exposed atoms and hence improve the performance per unit electrode area and/or material mass^[60]. Shi et al. reported that a Cu nanowire electrocatalyst was prepared for NO₃^- reduction^[61]. As a result, the optimized Cu nanowire electrode showed a 3-fold increase in NO₃^- reduction activity with a 90% FE for NH₃ production at a low overpotential of -0.1 V vs. RHE. Among the copper nanowires (CuNW)-X (X = copper foam oxidation time) variants, CuNW-15 had the highest FE and current density [Figure 8A]. Increasing the number of active sites is a fundamental strategy for enhancing reaction performance. Another approach is to augment the catalytic strength of these sites.

Figure 8. (A) Array: schematic diagram of the array; NH₃ yield of arrays formed at different oxidation times^[60]; (B) Heterojunction: schematic diagram of heterojunction; NH₃ yield rate of Fe SAC, Fe SAC/FeP@C, and FeSAC/FePO₄^[62]; (C) Schematic diagram of Ni₃N/Cu; NH₃ yield rate of NF/Ni₃N, NF/Ni₃N-Cu, NF/Ni₃N-Cu NP^[63]; (D) Tandem interaction of Cu(100) and Cu(111) facets^[64]; the FE of NO₂^- to NH₃ on Cu(111) at low concentration^[64]. NP: Nanoparticle; FE: Faradaic efficiency.

Different from the array, the heterogeneous interface formed by the heterojunction can enhance the catalyst interaction and increase the reaction activity. For instance, Song et al. demonstrated an unprecedented nano-single-atom heterointerface composed of Fe single-atoms and carbon-shell-coated FeP nanoparticles (Fe SAC/FeP@C) was an efficient electrocatalyst^[62]. The unique redistribution of charge at the nano-single-atom heterointerface and the regulation of the electronic structure of the Fe active center enabled Fe SAC/FeP@C to exhibit favorable adsorption of NO₃^- and enhanced activity of NO₃^- reduction [Figure 8B]. Similarly, Ouyang et al. proposed a Ni₃N nanosheet array decorated with Cu nanoclusters to enhance NO₃RR^[63]. By engineering the Ni₃N/Cu heterointerface, the reverse hydrogen spillover effect is strengthened, allowing ^*H generated from Ni₃N to rapidly transfer to the Cu site via the Ni–N–Cu bridge bond [Figure 8C]. This facilitates deoxygenation and hydrogenation steps, resulting in superior NO₃RR performance that reaches the state-of-the-art level.

The adsorption energy of different crystal faces for species is different, making it difficult to complete the step with a high energy barrier when reducing NO₃^-. By adjusting the crystal surface and improving the adsorption state, the reaction can be carried out more smoothly and the yield of synthetic NH₃ can be increased. Thus, Fu et al. reported a high-performance Cu nanosheet catalyst that has the tandem catalysis of Cu(100) and Cu(111) crystal planes^[64]. Among them, Cu(100) is more likely to adsorb NO₃^- and promote its conversion to NO₂^-. The generated NO₂^- then migrates to Cu(111) for further reduction, thereby promoting the generation of NH₃ [Figure 8D]. Electron transfer and reactant migration between crystal facets not only optimize reaction pathways but also significantly enhance reaction selectivity and efficiency. Analogously, Lu et al. prepared Co₃O₄ hexagonal nanosheets with exposed different facets. They discovered the (111) facet exhibited stronger adsorption of ^*H, facilitating the hydrogenation of NO_x nd NH_x intermediates and inhibiting the HER^[65]. Co₃O₄ (111) exhibited the highest NH₃ yield rate (5.73 mg·mg_cat.^-1·h^-1 at -0.9 V vs. RHE) and FE (> 99% at -0.7 V vs. RHE).

Catalyst nanostructure engineering focuses on the microscale regulation of catalysts. By exposing more active surface areas and modulating electron interactions, it enhances reactant adsorption, lowers reaction energy barriers, and promotes reaction progress. Additionally, grain boundary engineering enhances the catalytic activity and stability of noble metal catalysts by increasing the density of active sites.

Doping

Doping refers to mixing multiple substances, such as alloys, single atoms doped with heteroatoms, etc. This strategy can enhance the activity and stability of catalysts through interatomic interactions and synergistic catalysis.

Wu et al. reported the selective active NO₃^- reduction to NH₃ on an iron single-atom catalyst, which was mainly connected to the surface of the support by bonding with N, with an NH₃ yield as high as 0.46 mmol·h^-1·cm^-2 and a maximum FE of 75%^[66]. Independent exposure of active sites increases reaction site density. In addition to improving the utilization of atoms, the reaction rate is also increased. Moreover, compared to Fe nanoparticles in the N-doped carbon matrix (FeNP/NC), Fe-N₄ centers in Fe-SAC had a higher yield of NH₃ synthesis in Fe-SAC due to blocking the N-N coupling step [Figure 9A]. Alloys are the most common way of doping. Different from above, Zhang et al. prepared a Fe-Cu alloy catalyst, which produced more NH₃ than can be achieved using Fe or Cu alone [Figure 9B]^[67]. It was due to a relatively strong interaction between NO₃^- and Fe/Cu that promoted the adsorption and discharge of NO₃^- anions. N–O bonds were also shown to be weakened due to the existence of hetero-atomic dual sites which lowered the overall reaction barriers. In addition to the above, heteroatom doping is also a common and economical strategy for catalyst modification. Wang et al. discovered that B-doped Cu (BDCu) enhances water activation and weakens proton ^*H adsorption, facilitating ^*H migration to NO₃^- for NO₃^- reduction^[68]. Additionally, the electron-deficient nature of B facilitates electron transfer between B and Cu [Figure 9C]. This electron transfer from Cu to B in BDCu decreases the d-band center of Cu, modulating its electronic properties and altering the NO₃^- to NH₃ transition behavior on the Cu surface.

Figure 9. (A) Single atom: Schematic illustration of the synthesis of Fe SAC; NH₃ yield rate and partial current density of Fe SAC, FeNP/NC, and NC^[66]; (B) Alloy: Catalytic conversion steps from NO₃^- to NH₃ on Fe-Cu alloy; NH₃ yield rate of Fe/Cu-HNG, Fe-HNG, Cu-HNG^[67]; (C) Heteroatom: Schematic illustration of the synthesis of BDCu; Comparison of properties of BDCu and ZnCu_xO_1+x at different potentials^[68]. FeNP/NC: Fe nanoparticles in the N-doped carbon matrix; HNG: holey edge sites of nitrogen-dopedgraphene.

In conclusion, the stability of the catalyst is enhanced by the coordination ability of heteroatom-doped single atoms, and its utilization efficiency is greatly improved by the single-atom structure. The alloy enhances the adsorption and reactivity of the substance through synergistic effects and electronic structure changes, improving NH₃ synthesis efficiency.

Relay catalysis

Due to the adsorption of different substances by various catalysts, the reaction activity is different, and the complex process of eight electrons and nine protons experienced by NO₃RR is difficult to effectively reduce by a single catalyst. A relatively high overpotential is needed to overcome the kinetic barrier of the reaction, for the NO₃^--to-NO₂^- step which is often considered to be the rate-determining step^[69-71]. The use of different catalysts for relay catalysis is also one of the measures to promote NH₃ synthesis.

NO₂^- is a main side product that lowers the FE for NH₃ production, while the intermediately produced NO₂^- can be further reduced to NH₃ on the electrode, suggesting a unique dual role of NO₂^- in the NO₃RR catalysis. Thus, Ren et al. proposed a tandem catalytic mechanism involving the reduction of NO₃^- on Cu to form NO₂^-. NO₂^- is transferred through the solution, and then further reduced to NH₃ at the Co base site [Figure 10A]^[72]. Cu/Co(OH)₂ catalysts showed higher NO₃RR activity and NH₃ selectivity than Cu, Co(OH)₂ and Cu-Co alloys alone. Using a flow cell, the CuNW/Co(OH)₂ electrode achieved a complete single-pass conversion of NO₃^- to NH₃ at -0.1 V vs. RHE and maintained a nearly 100% conversion. Similarly, He et al. designed the Cu–Co binary metal sulfides^[1]. In Figure 10B, it is obvious to discover that CuCoSP has better performance than CuSP and CoSP on FE and the yield of NH₃. This is due to the high selectivity of Co-based materials and complexes for NH₃ generation during the NO₃RR and the specific NO₃^--to-NH₃ conversion in the previous report^[73]. Lv et al. designed a Cu-SrRuO₃ (Cu-SRO) that synergistically triggers tandem catalysis^[74]. The ^*NO₂^- generated on Cu active sites is subsequently hydrogenated on the Ru sites. Figure 10C results indicate that O-Cu-SRO exhibits higher catalytic activity than L-Cu-SRO (lower Cu) and H-Cu-SRO (High Cu). Combined with theoretical studies, it is confirmed that the tandem catalyst significantly reduces the energy barriers of the rate-determining step (^*NO to ^*NOH), thereby enhancing NH₃ synthesis efficiency.

Figure 10. (A) Cu/Co(OH)₂ relay catalyst: the faradaic efficiency for the NO₃RR on the Cu foam, Co(OH)₂ sample, Cu–Co alloy, and Cu/Co(OH)₂ tandem catalyst at selected potentials in H-cell^[72]; (B) CuCoSP relay catalyst: the faradaic efficiency and the yield of NH₃ on the CuSP, CoSP, CuCoSP, and CuCo hybrids^[73]; (C) Cu-SrRuO₃ relay catalyst: the faradaic efficiency and the yield of NH₃ on the O-Cu-SRO on diffierent voltage^[74]. NO₃RR: Nitrate reduction reaction.

Leveraging the catalytic activities of different catalysts towards distinct reactive species, relay catalysis - employing multiple catalysts in sequence - emerges as an efficient strategy for complex catalytic processes. During relay catalysis, intermediates are promptly captured and converted into substrates for subsequent reaction steps, minimizing the likelihood of side reactions involving these intermediates. This significantly enhances overall reaction efficiency.

Pulse voltage

At present, most of the strategies to improve the efficiency of NH₃ electrosynthesis are based on the design of catalysts and reactors, while the influence of electrolytic methods is usually ignored. Recently, pulse voltage has been used in electrocatalytic experiments because it can reconstruct the catalyst structure, change the local species concentration, and strengthen the mass transfer process^[75-77].

Li et al. employed Cu single-atom gel (SAG) under pulsed electrolysis to achieve highly efficient NO₃^- to NH₃ conversion^[78]. At low potentials, the primary process was NO₂^- reduction to NO₂^-. At higher potentials, accumulated NO₂^- facilitated NH₃ formation while suppressing the HER [Figure 11A]. Compared to constant potential electrolysis, this approach enhanced the FE and NH₃ yield while conserving electrical energy.

Figure 11. (A) Cu-sag led catalytic reaction at different potentials; (B) Oulse adjusts the catalyst structure: the pulsed electrolysis (upper panel); evolution of Cu valence state under pulse reaction conditions (lower panel)^[79]; (C) Enrichment of NO₃^- by pulsed voltage: Comparison of NO₃^- concentration under constant potential and pulse potential; Comparison of FE and yield under constant potential and pulse potential^[75]. FE: Faradaic efficiency.

Through the repeated application of positive and negative potentials, the catalyst can improve its structure and improve the efficiency of NH₃ synthesis. Bu et al. reported a pulsed electrolysis strategy to achieve a reliable Cu/Cu₂O structure. The Cu/Cu₂O heterojunction exhibits high reactivity due to the strong electronic interactions at the Cu/Cu₂O interface^[76]. Moreover, the pulse voltage will not completely reduce Cu_xO, and the Cu/Cu₂O structure is more stable^[79]. Alloying with Ni further modulates hydrogen adsorption, promoting NH₃ formation with a high NO₃^--to-NH₃ FE (88.0% ± 1.6%, pH 12) and NH₃ yield rate (583.6 ± 2.4 μmol·cm^-2·h^-1) under optimal pulsed conditions [Figure 11B].

The efficient reduction of NO₃^- can be achieved at a low concentration of NO_x^- by applying the pulse voltage of alternating positive and negative transformation. When a negative voltage is applied to the cathode, the reduction reaction proceeds normally. When converted to a positive voltage, NH₄⁺ near the electrode is more likely to diffuse into the electrolyte, while negatively charged NO_x^- is attracted by the positive charge and accumulates around the electrode, preparing for subsequent reduction at negative potential. Constant potential reduction cannot achieve this effect. Furthermore, Huang et al. verified pulse voltage is beneficial to the enrichment of NO₃^-[75]. In particular, at low concentrations, the FE and yield of NH₃ increased significantly due to the continuous supplementation of NO₃^- [Figure 11C].

In summary, the pulsed electrochemical process primarily serves two main functions. One is to enhance mass transfer: in pulsed electrochemistry, periodic voltage changes promote the enrichment of reactants and the desorption of products through electrostatic interactions. The other is catalyst reorganization: under constant current conditions, catalysts are prone to oxidation or reduction, losing their original structure and thereby reducing their performance. The pulsed electrochemical process, through parameter optimization, can maintain the catalyst structure or induce reorganization via alternating potentials, thereby enhancing catalytic efficiency.

SUMMARY AND OUTLOOK

The PECP is an emerging green technology. Compared with the traditional H-B process, it offers several advantages, including mild reaction conditions, low energy consumption, and no greenhouse gas emissions. To date, PECP has developed a relatively comprehensive system. The subsequent development and optimization of the PECP primarily focus on enhancing N₂ fixation and electrocatalytic efficiency.

For N₂ fixation efficiency, plasmas from spark discharge, gliding arc discharge, and DBD discharge have been explored. System parameters, such as discharge conditions and residence time, are crucial for developing efficient reactors and achieving cleaner NO_x production with lower energy input. DBD and gliding arc plasmas are major future research directions for N₂ fixation. Optimizing electrocatalytic efficiency primarily focuses on catalyst design. Providing an adequate supply of H^* and suppressing the HER are key factors for designing efficient catalysts. Strategies such as nanostructural engineering and relay catalysis are emerging as major directions for future electrochemical NH₃ synthesis. Additionally, modifying the electrolysis method, for example, using pulsed electrolysis, is a popular strategy for enhancing NH₃ synthesis efficiency.

Despite some breakthroughs in this field, several key challenges and limitations must be overcome before industrial application can be realized:

(1) Energy consumption issues. Generating plasma requires significant electrical energy, particularly to maintain stable plasma conditions. Further optimization of overall energy consumption is needed; (2) Technical and equipment limitations. Most studies currently separate plasma discharge and electrocatalytic reduction into distinct steps, limiting continuous operation and restricting industrial-scale application; (3) Product collection and separation challenges. During NH₃ synthesis, the low concentration of NH₃ necessitates additional separation and purification steps.

Future research must address existing technological bottlenecks to advance this technology toward higher efficiency, sustainability, and industrial feasibility. The following section outlines potential research directions:

(I) Optimization of the PECP
(i) N₂ fixation. Improving plasma discharge parameters, such as adjusting the discharge distance of the plasma, and controlling the gas flow rate, can not only enhance the efficiency of plasma N₂ fixation but also reduce energy consumption. Additionally, employing catalysts in conjunction with plasma N₂ fixation is another means to improve fixation efficiency. Developing efficient and breathable catalysts can be considered a future research direction.
(ii) NH₃ electrosynthesis efficiency. Improving the efficiency of NH₃ electrosynthesis involves not only the design of catalysts but also the alteration of electrolysis methods, such as pulsed and alternating current electrolysis. In the design of catalysts, many efficient catalysts have already been developed. Future research directions may focus on developing more economical and environmentally friendly catalysts. Strategies such as heteroatom doping and relay catalysis using different metals may become the main approaches to replace precious metals in catalysts.
(iii) Design integrated reactors. To enhance the efficiency of NH₃ electrosynthesis, a device can be inserted between the plasma and electrocatalysis processes, allowing NO_x generated by the plasma to be directly used for electro-reduction to produce NH₃. For instance, Liu et al. designed an absorption pool that can absorb the generated NO_x on one side and transfer the absorption liquid to the electrolysis pool for electrolysis on the other side, basically achieving the goal of integrating PECP^[48].

(II) Industrialization and subsequent NH₃ processes
(i) Industrial-relevant current density. High-current electrolysis is essential for industrialization. Utilizing membrane electrode assemblies, which feature efficient proton transport, allows the electrolysis current to reach the ampere level. For instance, Guo et al. designed a stacked membrane electrode assembly that integrates the catalyst, which can increase the current by nearly ten times^[54].
(ii) Subsequent NH₃ separation and purification processes. NH₃ has a high solubility but is unstable. NH₃ in the electrolyte can be blown out and absorbed, or the solution pH can be adjusted to weakly alkaline, slightly heated, and then NH₃ is released and absorbed by the acid solution to evaporate and form ammonium chloride crystals. Su et al. achieved an NH₃ recovery rate of over 85% using this method^[80]. Developing advanced membrane separation technology or adsorbent materials can also efficiently separate low-concentration NH₃. In addition to the recovery and utilization of NH₃, the synthesis of subsequent chemical products based on NH₃ may also become a future research direction, such as nitrogen fertilizers and MAP. The synthesis of these chemical products may bring greater economic and practical application value.

(III) Techno-economic assessments
Future research endeavors are suggested to incorporate comprehensive techno-economic assessments to evaluate the industrial feasibility of emerging technologies. This involves a detailed analysis of various cost components, such as material costs, the energy consumption of plasma and electrocatalyst reaction, the service life of a catalyst, equipment maintenance, and NH₃ separation costs. By conducting such assessments, researchers can identify the most cost-effective and sustainable approaches, ensuring that the technology is not only scientifically sound but also economically viable for large-scale industrial applications.

In summary, the PECP holds significant importance in green chemistry and sustainable development. Future research will focus on addressing current challenges and advancing the industrial application of this technology.