Recent advances and challenges of nitrogen/nitrate electro catalytic reduction to ammonia synthesis

Abstract

The Haber-Bosch process is the most widely used synthetic ammonia technology at present. Since its invention, it has provided an important guarantee for global food security. However, the traditional Haber-Bosch ammonia synthesis process consumes a lot of energy and causes serious environmental pollution. Under the serious pressure of energy and environment, a green, clean, and sustainable ammonia synthesis route is urgently needed. Electrochemical synthesis of ammonia is a green and mild new method for preparing ammonia, which can directly convert nitrogen or nitrate into ammonia using electricity driven by solar, wind, or water energy, without greenhouse gas and toxic gas emissions. Herein, the basic mechanism of the nitrogen reduction reaction (NRR) to ammonia and nitrate reduction reaction ({\text{NO}}_3^{-} RR) to ammonia were discussed. The representative approaches and major technologies, such as lithium mediated electrolysis and solid oxide electrolysis cell (SOEC) electrolysis for NRR, high activity catalyst and advanced electrochemical device fabrication for {\text{NO}}_3^{-} RR and electrochemical ammonia synthesis were summarized. Based on the above discussion and analysis, the main challenges and development directions for electrochemical ammonia synthesis were further proposed.

Graphical abstract

Keywords

electrochemical ammonia synthesis / nitrogen / nitrate / nitrogen reduction reaction (NRR) to ammonia / nitrate reduction reaction (NO^–3 RR)

1 Introduction

Ammonia is one of major contributors and feedstocks to fertilizer, dynamite, chemical production and many other industries [1,2]. Meanwhile, with the rise of the concept of carbon neutrality, ammonia, as an ideal zero-carbon energy carrier, has attracted increasing attention in the field of chemical industry and energy [3,4]. Generally, ammonia is synthesized from nitrogen and hydrogen, and the rate determining step is opening the N−N triple bond [5]. The binding energy of nitrogen (941 kJ/mol) is not higher than some other less stable triple bonds like acetylene (962 kJ/mol) and carbon monoxide (1070 kJ/mol) [6–8]. However, N₂ is much less reactive than triple bonded molecules such as acetylene and carbon monoxide. There are several other reasons that stabilize the N−N bonds: the high binding energy of the first bond of those three (410 kJ/mol), much more than that of acetylene (222 kJ/mol), making it difficult to start breaking it. In addition, lacking a permanent dipole means that nitrogen molecules have a poor proton affinity (493.8 kJ/mol), in combination with the negative electron affinity (−1.90 eV), high ionization potential (15.84 eV) and large energy gap (10.82 eV) between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) [5,9,10]. These factors make nitrogen molecules hard to activate. Therefore, although the formation of ammonia from nitrogen and hydrogen is thermodynamically favored (\mathrm{\Delta }{H}_{\text{298 K}}^0 =-45.9\text{kJ/mol,}{K}_{\text{eq}}=750) [11], the kinetics of the reaction will be extremely slow even at a negative Gibbs free energy (\mathrm{\Delta }{G}_{\text{298 K}}^0= -16.4\text{kJ/mol}).

To overcome those difficulties, the Haber-Bosch process was introduced to make ammonia production commercially viable, and Fritz Haber and Carl Bosch won Nobel prize for this innovation [11]. In this process, synthesis of ammonia from nitrogen and hydrogen gas will take place at a high temperature (400‒600 °C) and high pressure (20‒40 MPa) with iron-based catalyst [12–14]. The reaction is exothermic, thus lower temperature should be favored, but thermal energy is required to break the nitrogen bonds. As a result, high pressure is necessary to push the reaction forward, or more than 99% of the ammonia will decompose under atmosphere pressure.

Although the Haber-Bosch process is still producing most of the ammonia in the world, the disadvantages are already concerned by various researchers: high temperature and high pressure means massive energy consumption, and nitrogen gas comesfrom cryogenic processes, which is almost equal to high carbon emission. Besides, 72% of the hydrogen gas for ammonia synthesis comes from steam reforming and water–gas shift reaction, which is another source of carbon dioxide [5,15]. Operating an exothermic reaction at a high temperature also means a low conversion rate (about 15%) and cycling back the reactants is necessary to achieve about 97% yield, which also wastes a lot of energy. Statistics show that the Haber-Bosch process consumes 1%‒2% of the global annual energy output and generates about 1.3% of global carbon dioxide emissions (1.8 t CO₂ for 1 t NH₃) [14,16]. Therefore, low carbon emission and energy efficient technologies for ammonia synthesis receive increasing attention. Electrocatalytic ammonia synthesis is a promising ammonia synthesis strategy, which can directly convert nitrogen or nitrate into ammonia using the electricity driven by solar, wind or water energy, without greenhouse gas and toxic gas emissions [17–20]. There are many detailed summaries of nitrogen reduction reaction (NRR) to ammonia [5,15] and nitrate reduction reaction ({\text{NO}}_3^{-}RR) to ammonia [21–25], respectively. However, there are few literatures that comprehensively discuss and analyze the two electrochemical ammonia synthesis pathways. In addition, some new electrochemical ammonia synthesis technologies, such as the solid oxide electrolysis cell (SOEC) technology, have not been systematically summarized and introduced. In this paper, the latest progress of electrochemical synthesis of ammonia in the recent three years is summarized. The new mechanism of ammonia electrosynthesis by nitrogen reduction and nitrate reduction are introduced, and new approaches and new technologies of ammonia electrosynthesis are discussed. Finally, the challenges and future perspectives in this field are also proposed.

2 Mechanism of ammonia electrosynthesis

2.1 Electrochemical NRR

A common strategy against stable molecules is electrolysis. Of course, it will be considered as a solution to the issues about ammonia production through nitrogen reduction. In electrochemical cells, N₂ is reduced to ammonia at the electrode surface [26–28]. Electrochemical NRR can be described simply as follow: First, nitrogen molecules are adsorbed and fixed on the cathode surface. Then, electrons are supplied from an external circuit to reduce the nitrogen molecules, while protons are added to form ammonia [5,15]. Protons can be supplied by H₂ or H₂O. Using H₂O as the hydrogen source for electrochemical ammonia synthesis can avoid the use of H₂ prepared from fossil fuels, which can effectively reduce carbon emissions [29–32]. The synthesis thermodynamics and mechanism of electrochemical ammonia from room temperature to 900 °C are studied [5,15,33–35]. The working temperature of the electrochemical ammonia synthesis cell is found to have a significant effect on the thermodynamics of electrochemical NRR. Fig.1(a) shows the thermodynamic reversible electrolytic voltages needed for electro ammonia synthesis and H₂O electrolysis at temperatures from 25 to 900 °C [5,32,36]. At an atmospheric pressure, 175 °C is a turning point for spontaneous and non-spontaneous NH₃ synthesis from N₂ and H₂ (red line). Below the temperature, the reaction is spontaneous. However, the reaction will require energy input above the temperature. On the contrary, the synthesis of ammonia from N₂ and H₂O (green line) is nonspontaneous. Therefore, the both half-reactions need to be promoted by energy input. The minimum voltage of NH₃ synthesis from N₂ and H₂O at 25 °C is 1.17 V, which is slightly lower than that of water decomposition (blue line).

The main challenge of the electrochemical NRR process is that it is difficult for the formation of reaction intermediates to perform thermodynamically, which limits the reduction of N₂ [37–40]. In theory, a decent negative potential applying on the electrode should allow the NRR to take place. However, transferring 6 electrons for each reduced N₂ molecule means that this is a multi-step reaction [38,41–43],

(1)\begin{array}{rl}& {\mathrm{N}}_2+2{\mathrm{H}}^{+}(\mathrm{a}\mathrm{q})+2{\mathrm{e}}^{-}\rightleftarrows {\mathrm{N}}_2{\mathrm{H}}_2(\mathrm{g})\\ & E^0=-1.19\mathrm{V}(\mathrm{v}\mathrm{s}.\mathrm{R}\mathrm{H}\mathrm{E})\end{array}

(2)\begin{array}{rl}& {\mathrm{N}}_2{\mathrm{H}}_2(\mathrm{g})+2{\mathrm{H}}^{+}(\mathrm{a}\mathrm{q})+2{\mathrm{e}}^{-}\rightleftarrows {\mathrm{N}}_2{\mathrm{H}}_4(\mathrm{a}\mathrm{q})\\ & E^0=0.529\mathrm{V}(\mathrm{v}\mathrm{s}.\mathrm{R}\mathrm{H}\mathrm{E})\end{array}

(3)\begin{array}{rl}& {\mathrm{N}}_2{\mathrm{H}}_4(\mathrm{a}\mathrm{q})+2{\mathrm{H}}^{+}(\mathrm{a}\mathrm{q})+2{\mathrm{e}}^{-}\rightleftarrows 2\mathrm{N}{\mathrm{H}}_3(\mathrm{a}\mathrm{q})\\ & E^0=0.939\mathrm{V}(\mathrm{v}\mathrm{s}.\mathrm{R}\mathrm{H}\mathrm{E})\end{array}

As listed above, the E⁰ of Eq. (1) is a negative value, indicating that this reaction is difficult to occur thermodynamically. Besides, 2-electron hydrogen evolution reaction (HER) as a competitive reaction of reaction intermediates ({\text{N}}_{\text{2}}{\text{H}}_2) in NRR can reduce the Faradaic efficiency and reaction selectivity [11,44–47]. Therefore, promoting directional formation of reaction intermediates, increasing the faradaic efficiency and reaction selectivity of NRR become a major focus for electrochemical nitrogen NRR.

Reasonable catalyst design is one of the important methods to achieve efficient and high selective electrocatalytic NRR [40,48,49]. According to the difference of the N≡N bond breaking and hydrogenation order on the catalytical surface, the NRR reaction mechanism can be divided into two main types (in Fig.1(b) [15,38]) including dissociative reaction and associative reaction [39,40,50,51]. The dissociative pathway will break the nitrogen molecules into atoms, then attach them to the catalysts and hydrolyze them to from 2NH₃. Due to the high bond energy of N≡N, mild electrocatalytic systems are rarely achieved through the dissociative mechanism. It is possible to follow the dissociative mechanism only when the electrolytic conditions are set to use non-aqueous molten salt and molten alkali as electrolytes, and the experimental temperature is increased to 200‒300 °C and the pressure is increased to 101.325 kPa. If the triple bond break after attaching to the catalyst, and after forming −NH₃, it will be called an associative pathway, which can be further divided into three paths including the associative distal pathway, the associative alternating pathway, and the enzymatic pathway. When all the hydrogen attached to one nitrogen atom before the other, it is an associative distal pathway. On the country, in an associative alternating pathway, both atoms will receive hydrogen at the same time. As for the enzymatic pathway, it is similar to the associative alternating pathway, but the two N atoms in the dinitrogen molecule are coordinated with the catalyst surface. Due to the weak interaction, the low coverage rate, and the fast conversion rate of intermediates on the catalyst surface, it is very difficult to identify the actual reaction mechanism. With the development of catalyst design and research, the traditional dissociative and associative mechanisms have been challenged. Yao et al. [52] designed an oxygen-deficient In₂O₃ catalyst decorated with single atom CdO₅ and proposed a new surface hydrogenation-facilitated NRR on the catalyst, which was demonstrated by the density functional theory (DFT) calculation and experimental results. The mechanism indicated that a Volmer reaction ({\text{H}}_{\text{2}}\text{O +}{\text{e}}^{-}\to {}^{\ast }\text{H} +\text{O}{\text{H}}^{-}) preferred to occur instead of N₂ adsorption and activation first on the In₂O₃ surface. Then, N₂ was activated via electron transfer from *H, followed by *N₂H₂ formation. Finally, a NH₃ was produced by a further reaction with proton-electron pairs.

2.2 Electrochemical {\text{NO}}_3^{-} RR

Nitrate ({\text{NO}}_3^{-}) is abundant in nature, especially in environmental pollutants. The concentration of {\text{NO}}_3^{-} in water will cause water eutrophication, reduce the available oxygen of aquatic organisms, destroy aquatic ecosystems and thus affect human health [53–55]. Therefore, {\text{NO}}_3^{-} reduction to NH₃ under environmental conditions and in aqueous solution can not only provide a green and sustainable NH₃ synthesis technology, but also alleviate global energy and pollution problems [56–58]. The dissociation energy of the N=O bond (204 kJ/mol) is much lower than that of the N≡N bond in the N₂ molecule [59]. Therefore, electrocatalytic {\text{NO}}_3^{-} reduction to NH₃ is considered as a mild and sustainable green ammonia synthesis technology [60–64]. In general, the mechanism of {\text{NO}}_3^{-} reduction reaction ({\text{NO}}_3^{-} RR) can proceed in two pathways (Fig.2(a)), the electron transfer reduction (black arrow) in an acidic environment and the atomic hydrogen reduction (red arrow) in an alkaline environment. The reactions of two paths are shown in the following equations, respectively [65–67],

(4)\begin{array}{rl}& {\mathrm{N}\mathrm{O}}_3^{-}+9{\mathrm{H}}^{+}(\mathrm{a}\mathrm{q})+8{\mathrm{e}}^{-}\rightleftarrows \mathrm{N}{\mathrm{H}}_3(\mathrm{a}\mathrm{q})+3{\mathrm{H}}_2\mathrm{O}\\ & E^0=-0.12\mathrm{V}(\mathrm{v}\mathrm{s}.\mathrm{R}\mathrm{H}\mathrm{E})\end{array}

(5)\begin{array}{rl}& {\mathrm{N}\mathrm{O}}_3^{-}+6{\mathrm{H}}_2\mathrm{O}+8{\mathrm{e}}^{-}\rightleftarrows \mathrm{N}{\mathrm{H}}_3(\mathrm{a}\mathrm{q})+9\mathrm{O}{\mathrm{H}}^{-}\\ & E^0=0.69\mathrm{V}(\mathrm{v}\mathrm{s}.\mathrm{R}\mathrm{H}\mathrm{E})\end{array}

Electron transfer reduction in an acidic environment is the more common and discussed pathways. The {\text{NO}}_3^{-} RR involves an eight-electron transfer, leading to a complex reaction process, causing slow kinetics [68–70]. Meanwhile, a large number of nitrogen-containing products, such as {\text{NO}}_2^{-} , NO₂, NO, N₂O, N₂, NH₂OH and NH₃, can be generated. Therefore, a series of byproducts may be generated to reduce the selectivity and efficiency of {\text{NO}}_3^{-} RR to NH₃ [56,57,71,72]. The reactive pathway of {\text{NO}}_3^{-} RR is important for the final production [73]. Niu et al. [74] revealed five possible reactive pathways by DFT calculation, as shown in Fig.2(b), including O-end, O-side, N-end, and N-side pathways to NH₃, as well as the NO-dimer pathway to N₂. However, there is still a lack of clear experimental evidence to reveal the mechanism of {\text{NO}}_3^{-} RR. As a result, obtaining the efficient {\text{NO}}_3^{-} RR catalysts and proving their exact mechanisms still pose a major challenge and requires more efforts. Besides, nitrite and nitrogen oxides, such as {\text{NO}}_2^{-} , NO₂, NO, N₂O, and NH₂OH can also be used to electrochemical ammonia synthesis and should also be discussed in detail. However, these substances are all intermediates of {\text{NO}}_3^{-} RR, and the reduction process is similar to that of {\text{NO}}_3^{-} RR or even part of {\text{NO}}_3^{-} RR. Therefore, there are no separate electrochemical reduction processes for these substances in this study.

3 Technologies of electrochemical nitrogen NRR

3.1 Lithium mediated electrolysis

To optimize the ammonia production, various attempts were made with different methods. Electrochemical lithium mediated nitrogen reduction reaction (Li-NRR) is one of the vital current research directions. Fichter et al. [75] first investigated Li-NRR in an alcoholic solution of lithium halide in 1930. Then, the reaction conditions were optimized: the electrolyte was changed to tetrahydrofuran (THF) mixed with ethanol (EtOH) and the N₂ pressure was improved from atmospheric pressure to 506.625 kPa to enhance the current efficiency from 8% to 59% [76,77]. However, it is difficult to achieve high efficiency at atmospheric pressure, and due to the lack of in-depth understanding of the reaction mechanism of Li-NRR, it is difficult to rationally design the reaction process to improve the process current efficiency and yields. Subsequently, Lazouski et al. [78] proposed the reaction mechanism of Li-NRR by studying the Li-NRR in a mixed electrolyte system of THF and EtOH at normal temperature and pressure. The group illustrated that Li-NRR proceeds as shown in Fig.3(a): Li⁺ was reduced to active Li (Li_act) by capturing an electron. Then, two competing reactions occurred: lithium protolysis (the reaction between lithium and ethanol to form hydrogen) and lithium nitridation (the reaction between lithium and nitrogen to form Li₃N, which was the rate-determining step of NH₃ production, and ammonia was evolved by protonation of Li₃N). This mechanism reveals two facts: ① N₂ transport limitations of NRR at a high current density, ② organic solvent, i.e., high concentration ethanol, the most studied organic hydrogen donor in the field of Li-NRR [79], which supplies the NRR with protons. Optimally, the maximum Faradaic efficiency and production rate obtained were (18.5 ± 2.9)% and (7.9 ± 1.6) × 10⁻⁹ mol/(cm²·s), respectively, in this study.

Various rational designs were attempted to improve the efficiency and selectivity of Li-NRR. For instance, McEnaney et al. [80] designed a cyclic process to produce ammonia, as shown as Fig.3(b). Li is produced by LiOH electrolysis. Then it reacts with nitrogen gas to produce Li₃N which reacts with water to produce NH₃. This strategy physically and temporally separates the reduction of N₂ from the subsequent protonation to ammonia, avoiding the HER, thus the current efficiency can reach 88.5%. Li et al. [81] added a solid electrolyte interface (SEI) between the porous copper cathode and the solvent (Fig.3(c)) by the induction effect of lithium tetrafluoroborate electrolyte, which can suppress the unwished hydrogen evolution reaction as well as electrolyte decomposition process, thus the nitrogen reduction can be enhanced. As a result, ammonia production rate of (2.5 ± 0.1) μmol/(cm²·s) at a current density of −1 A/cm² with (71 ± 3)% Faradaic efficiency under 2000 kPa nitrogen were realized.

Fu et al. [82] pointed out that there are still two challenges to the lithium-mediated NRR route. The first one is the sacrifice of organic solvents, such as ethanol, which brings a high energy consumption. The high cell voltage leads to the oxidation of organic solvents to provide protons to ammonia electrosynthesis, which is an unsustainable method to provide hydrogen source for ammonia synthesis [83]. To address this challenge, Fu et al. [82] designed an anode-coupled hydrogen oxidation reaction strategy to provide hydrogen sources for the NRR, and developed a high-active PtAu anode catalyst to significantly reduce anode overpotential, thereby avoiding the oxidation caused by organic solvents.

The second challenge is the reactant mass transfer limitation and system stability. Most studies of Li-mediated ammonia synthesis are conducted in single-chamber batch reactors, which means that nitrogen have to be dissolved in the electrolyte to meet the NRR reaction. This process not only limits the mass transfer of the reactants, but also makes it difficult to scale up the intermittent ammonia production [83,84]. Fu et al. [82] designed a continuous flow cell with an effective area of 25 cm², as illustrated in Fig.3(d) and avoided the mass transfer limitations of the reactants by introducing a gas diffusion electrode. As a result, at atmospheric pressure, a Faradaic efficiency of up to (61 ± 1)% (for intermittent reactions, it is usually less than 10% at normal temperature and pressure) and an energy efficiency of (13 ± 1)% at a current density of −6 mA/cm² are achieved for Li-NRR to ammonia production.

3.2 SOEC

Another attempt of electrochemical NRR is to electrolyze nitrogen and water with SOEC. There are two types of SOEC, the proton conducting one (H-SOEC) and the oxide ion conducting one (O-SOEC). As shown in Fig.4(a) and 4(b), the mechanisms are different based on the two types of SOECs [85,86]. In H-SOEC, hydrogen will react at anode and produce H⁺, which will be sent to cathode to react with nitrogen and form ammonia [87]. In O-SOEC, nitrogen will react with water at cathode to produce ammonia and {\text{O}}_{}^{2-}, which will be sent to anode and form O₂ [88]. As for the NRR through SOEC, the selectivity and activity of catalysts are the two urgent issues to be addressed [89]. One possible direction of catalysts is perovskite materials that have a high proton and oxide ion conductivity [90].

In this regard, both metal oxides and perovskite oxides were studied as electrochemical reduction catalysts for NRR. Klinsrisuk & Irvine [91] adopted a Pd/Ru-modified iron oxide as the cathode of H-SOEC to catalyze the NRR process. The highest ammonia formation rate of 4 × 10⁻⁹ mol/(cm²·s) was obtained from Pd-modified cell at 450 °C and the current efficiency of ammonia formation was in the range of 1%‒2.5%. Wang et al. [92] worked with Sr_0.9Ti_0.6Fe_0.4O_3−δ as cathode for H-SOEC and reached an ammonia production rate of 6.84 × 10⁻⁹ mol/(cm²·s) at 650 °C and 0.6 V, but the Faradaic efficiency is only about 2.79%. On the positive side, the cell performed a good stability over the 100 h test period. Based on their research, Wang et al. [93] used iron nanoparticles coated MXene as cathode and managed to reach a Faradaic efficiency of 8.4% and production rate of 8.24 × 10⁻⁹ mol/(cm²·s) at 650 °C and 1.6 V. The preparation and test configurations of the cell are shown in Fig.4(c). This research provides a possible direction for the future ammonia production technology.

Ferree et al. [94] managed to create a composite cathode consisting of La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) and Co₃Mo₃N shown in Fig.4(b) for O-SOEC to catalyze NRR. LSCF with a high ionic and electronic conducting property was used for catalyzing water electrolysis, and Co₃Mo₃N could catalyze the splitting of dinitrogen bond. However, from the ammonia formation rate of 4.0 × 10⁻¹¹ mol/(cm²·s) at 550 °C and 0.65 mA/cm², it is obvious observed that this technique is still far away from commercialization, and the precise mechanism of this reaction requires more efforts. Nevertheless, this result provides a direction for further research, and it means that ammonia production through SOEC is more than a theory, which is potentially promising in the future ammonia industry. Furthermore, the anode can participate in high value organism production, such as alkane assisted electrolysis of SOEC, to recover the production cost [95,96].

To improving the efficiency, selectivity, and practicability of NRR, a significant design and modification of electrochemical cells, including electrolyte, catalyst and electrode, is required. The preparation methods of highly active catalysts, such as catalyst nanization and composite catalyst fabrication, should be further explored. Continuous-flow electrolyzer designs should continue to move forward.

4 Technologies of electrochemical \mathbf{N}{\mathbf{O}}_{\mathbf{3}}^{\mathbf{-}}\mathbf{R}\mathbf{R}

4.1 High activity catalyst design

The process of {\text{NO}}_3^{-} RR synthesis is an eight-electron transfer reaction with a slow rate, and the unique D3h planar resonance structure of nitrate reduces the cathode adsorption and raises the energy barrier of the conversion process [65,97]. The development of efficient {\text{NO}}_3^{-} RR catalysts is challenging [63,98,99]. The design and fabrication methods of high-activity catalysts, such as catalyst nanization and single atom preparation, have been used for the creation of high activity {\text{NO}}_3^{-} RR catalysts. Jia et al. [100] reported a TiO₂ nanotube with rich oxygen vacancies (TiO_2−x, shown in Fig.5(a)) as the catalysts of electrochemical {\text{NO}}_3^{-} RR. The nano catalysts for ammonium synthesis from nitrate electroreduction demonstrates a high Faradaic efficiency of 85.0%, an excellent selectivity of 87.1%, and an outstanding conversion rate of 95.2% at the optimal potential of −1.6 V. Cu- and Co-based nano electrocatalysts have received extensive attention and research. Liu et al. [101] designed and fabricated NiCo₂O₄ nanowire array on carbon cloth (NiCo₂O₄/CC) as an electrocatalyst for {\text{NO}}_3^{-} RR. The existence of Ni in the crystal structure of the catalyst optimized the adsorption energy, thus exhibiting a superior catalytic activity of a high Faradic efficiency of 99.0% and a large NH₃ yield up to 973.2 µmol/(h·cm²). Fan et al. [102] presented a rational design of Co nanoparticles anchored on TiO₂ nanobelt array on titanium plate (Co@TiO₂/TP). The DFT calculation demonstrated that a built-in electric field was developed by the coupling metallic Co with semiconductor TiO₂ to accelerate{\text{NO}}_3^{-}adsorption and ensuring the selective conversion to NH₃. As a result, an excellent Faradaic efficiency of 96.7% and a high NH₃ yield of 800.0 µmol/(h·cm²) under neutral solution is attained. Zhang et al. [103] developed a diatomic catalyst of Fe/Cu anchored to the holey edge sites of nitrogen-doped graphene (HNG), and found that there was an appropriate interaction between Fe/Cu and {\text{NO}}_3^{-}, which promoted the adsorption and product release of {\text{NO}}_3^{-} (Fig.5(b)). The strong coupling of {\text{NO}}_3^{-} with the d orbital of the bimetal atom reduces the energy barrier of anion adsorption, and the diatom can further weaken the N−O bond, making the production of NH₃ have a low reaction energy barrier. As a result, the catalyst has a maximum ammonia Faradaic efficiency of 92.51% (−0.3 V(RHE)) and a high NH₃ yield rate of 1.08 mmol/(h·mg) (at −0.5 V(RHE)).

4.2 Advanced electrochemical device fabrication

The incompatibility of the pH or ionic composition of the anode/cathode electrolyte in the electrochemical device during continuous operation is one of the major challenges preventing the long-term stable operation of the {\text{NO}}_3^{-} RR under optimal reaction conditions. For instance, the oxygen evolution reaction (OER) at the anode generally tends to take place in an alkaline environment, while the reduction reaction at the cathode shows a kinetic advantage under acidic conditions [104,105]. To overcome the above issues, Xu et al. [106] adopted a stepwise strategy to construct a bipolar membrane (BM) with a stable C−C covalent interlock interface layer (CIBM, Fig.5(c)) as a diaphragm between the cathode electrolyte and the anode electrolyte. The physical bond strength and ion transport rate are enhanced exponentially by this design. In addition, the introduction of the CIBM can also successfully achieve an efficient and low-energy NH₃ synthesis process, with an ammonia yield of up to 70.9 mg/(cm²·h) under 2000 mg/L {\text{NO}}_3^{-} conditions, providing an innovative design principle for emerging efficient ammonia synthesis electrochemical devices.

5 Conclusions and perspective

Undoubtedly, due to the technological maturity, the Haber-Bosch process is still the most economically favored ammonia synthesis process. However, as the scale of renewable electricity continues to expand, electrolytic ammonia synthesis may be more economical than the Haber-Bosch process. Besides, there are many advantages for ammonia electrosynthesis, such as low temperature and pressure, low carbon emission and flexible configuration, which the Haber-Bosch process does not have. However, before the large-scale application of electrochemical ammonia synthesis technology can replace the Haber-Bosch process, there are still several challenges need to be overcome. First, the Faradaic efficiency and reaction rate of electrochemical ammonia synthesis are still relatively low, which is difficult to meet the requirements of industrialization. Giddey et al. [107] believed that the ammonia generation rate was close to 10⁻⁶ mol/(cm²·s), and the Faradaic efficiency was as high as 50% to meet the commercial needs of electrocatalytic ammonia synthesis system. However, no electrochemical method has met the above requirements. Therefore, creative design and modification of electrochemical cells, including electrolyte, catalyst and electrode, is required to enhance the yield and efficiency of ammonia synthesis. Next, most electrochemical cells for ammonia synthesis are liquid electrolytes, and the resulting ammonia often forms a mixture with the electrolyte, which makes ammonia separation difficult. The use of solid-state electrochemical cells, such as SOEC, which is relatively insignificant in current ammonia synthesis, is a viable solution to the ammonia separation problem. Finally, at present, in laboratory scale research, yield and efficiency are the main indicators of research on the electrochemical ammonia synthesis process, while the long-term operation stability and durability are ignored, which is very important for the industry in the future, to which more attention should be paid.

Obviously, ammonia electrosynthesis is one of the main research directions and is likely to play an important role in the future ammonia industry. Although there are still many challenges in this field, as the problems continue to be solved and the commercialization continues to advance, electrochemical ammonia synthesis will make a critical contribution to environmental governance and energy conservation.

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