Recent Advances in Electrochemical Nitrogen Reduction Reaction to Ammonia from the Catalyst to the System

Abstract

As energy-related issues increase significantly, interest in ammonia (NH₃) and its potential as a new eco-friendly fuel is increasing substantially. Accordingly, many studies have been conducted on electrochemical nitrogen reduction reaction (ENRR), which can produce ammonia in an environmentally friendly manner using nitrogen molecule (N₂) and water (H₂O) in mild conditions. However, research is still at a standstill, showing low performances in faradaic efficiency (FE) and NH₃ production rate due to the competitive reaction and insufficient three-phase boundary (TPB) of N₂(g)-catalyst(s)-H₂O(l). Therefore, this review comprehensively describes the main challenges related to the ENRR and examines the strategies of catalyst design and TPB engineering that affect performances. Finally, a direction to further develop ENRR through perspective is provided.

1. Introduction

Ammonia (NH₃) is an essential commodity for chemicals utilized in various industries, such as fertilizers, plastics, dyes, pharmaceuticals, and so on [1,2]. In particular, its use as a fertilizer made mass production of food possible and led to a population increase. Recently, NH₃ has been focused on as a sustainable fuel and a hydrogen storage medium with the promising properties of 5.52 kWh kg⁻¹ high energy density and 17.6% high hydrogen content [3]. According to the US Geological Survey, NH₃ production in 2021 has gradually increased to as much as 150 million tons [4].

Most NH₃ production mainly relies on the traditional Haber–Bosch process which converts hydrogen (H₂) and nitrogen (N₂) directly into NH₃ (N₂ + 3H₂ ⇌ 2NH₃) using Fe-based catalysts [5]. However, since the Haber–Bosch process requires extreme operating conditions—such as a temperature and pressure of over 400 °C and 20 MPa, respectively—to cleave the strong triple-bond energy (941 kJ mol⁻¹) of N₂ molecules, the process annually consumes a significant amount of energy accounting for around 1–3% of globally produced energy [6,7,8,9]. Furthermore, the requirement of the enormous H₂ supply generated by fossil fuels leads to significant carbon dioxide (CO₂) emissions, which amounted to 300 million tons [8]. When we convert it to an energy requirement per ton of NH₃ production, the theoretical energy requirement for the Haber–Bosch process is 22.2–28.8 GJ t_NH3⁻¹. Furthermore, CO₂ emission from the Haber–Bosch process is expected to be 1.673 t_CO2 t_NH3⁻¹ [10,11]. Considering the increasing need for NH₃ and the Haber–Bosch process’ economic and environmental issues, a novel environmentally friendly and economically efficient NH₃ production process is in high demand.

The electrochemical nitrogen reduction reaction (ENRR) is one of the most attractive NH₃ production processes. NH₃ can be produced using H₂O as a hydrogen source and N₂ gas at mild temperature and pressure under biased conditions. Theoretical minimum energy requirement for ENRR is expected to be only 19.9 GJ t_NH3⁻¹, as assuming ideal 1.17 V supplying for overall NH₃ production with 100% of faradaic efficiency. Furthermore, a negligible amount of CO₂ during ENRR is expected, if electricity supplied is from renewable energy such as wind and solar and the possibility of CO₂ emission from process engineering is excluded such as product separation, input gas compression, and so on [10,11]. Thus, ENRR can significantly reduce CO₂ emission and energy consumption compared to the Haber–Bosch process. However, ENRR has several critical challenges hindering its practical applications due to the insufficient performances recorded so far. For example, to apply the ENRR as a major NH₃ production pathway in industrial systems, an NH₃ production rate of at least 6120 μg h⁻¹ cm⁻² should be achieved [12,13]. However, despite great efforts for advancing the ENRR’s performance, the NH₃ production rate is still significantly low with 612 μg h⁻¹ cm⁻² and a Faradaic efficiency (FE) below 10% [14]. Thus, it is critical to understand the ENRR’s major limitations and the previous efforts of overcoming the challenges to propose the next generation of ENRR, which would be a practical available alternative to the Haber–Bosch pathway.

In this review, we will first discuss the essential ENRR thermodynamics fundamentals, kinetics related to reaction mechanisms, and other factors affecting the ENRR’s performance. Next, we review the widely studied catalyst design strategies and reaction environmental engineering. Moreover, we will thoroughly investigate each strategy’s effect on the ENRR’s performance (Scheme 1). Finally, we end this review by showing ENRR’s great prospects and the direction for further investigation to apply it in the future.

2. The Fundamentals

The ENRR is composed of multiple steps of proton-coupled electron transfers involving six protons and six electrons. The ENRR’s half-reactions in acidic (1) and basic solutions (2) with standard reduction potential against reversible hydrogen evolution (RHE) reaction are given, respectively [14]

N₂(g) + 6H⁺(aq) + 6e⁻ ⇌ 2NH₃(aq) (E⁰ = 0.092 V_RHE)

(1)

N₂(g) + 6H₂O(l) + 6e⁻ ⇌ 2NH₃ + 6OH⁻(aq) (E⁰ = 0.092 V_RHE)

(2)

The Pourbaix diagram for N₂-H₂O in Figure 1a represents thermodynamically favored species and phases depending on potential and the solution’s pH [3]. Thermodynamically, only the ENRR is available and thus FE for NH₃ must be 100% at the potential and pH within the yellow-colored area. This area is between the red solid line representing N₂ reduction to NH₄⁺ or NH₃ and the line ⓐ representing the hydrogen evolution’s reaction (HER, 2H⁺(aq) + 2e⁻ ⇌ H₂(aq), E⁰ = 0.000 V_RHE). Furthermore, ENRR can be more favorable than HER at the potential below 0 V_RHE. However, the ENRR’s performance is very poor in the yellow-colored area and the HER is more favorable than the ENRR at the potential below 0 V_RHE. The experimental observation can be supported by an additional challenge for the kinetic manner.

As shown in Figure 1b, pathways for N₂ to NH₃ conversion using six transferred electron and proton pairs are mainly represented as dissociative or associative steps [21]. In the dissociative pathway, the dissociated N atom is strongly adsorbed to the catalyst surface by the perfect cleavage of triple bonds from the N₂ molecule. Then, NH₃ is produced by multiple hydrogenations. However, this dissociative process is much more relatable to the NH₃ production via the Haber–Bosch process performed under high temperature and pressure than the ENRR because the N₂ triple bonds cleavage requires enormous energy input. Meanwhile, the ENRR’s mechanism is well-followed by the associative pathway where the first hydrogenation occurs with the adsorbed N₂ (*N₂), forming an *N₂H intermediate [23]. The associative mechanism is further classified into two different pathways: the distal and alternating pathways. In the distal associative pathway, the N atom that is far from the catalyst’s adsorption site is first hydrogenated with the three transferred electrons and protons and then released as NH₃. The other N atom still adsorbed on the catalyst conducts further hydrogenation to form NH₃. On the other hand, in the alternating pathway, the protons are alternately bonded with both N atoms until the final N-N bond is cleaved [14]. Finally, two NH₃ molecules are released. The initial step of N₂ adsorption and activation by forming the *N₂H intermediate is a serious rate-determining step among the multiple steps for the ENRR. Furthermore, this multi-proton-coupled electron transfer process involves diverse intermediate formations, such as *NNH, *NH₂, and *NHNH with severe complexity. This results in sluggish kinetics relative to the HER, requiring a simple electron transfer with protons. Combining the thermodynamic and kinetic manner for the ENRR, a rational design of catalysts is critical for facilitating N₂ adsorption, activation, and selective NH₃ production with multiple byproduct evolution or competitive HER suppression.

The ENRR’s performance is evaluated mainly using two factors of FE that represent the utilization ratio of the passed coulomb in NH₃ production and the production rate that shows the amount of NH₃ produced using a unit area (1 cm²) of electrodes during an hour. The US Department of Energy (DOE) aims to achieve an FE of 50% for the ENRR and an NH₃ production rate of over 1700 μg h^—1 cm^—2 [24]. However, the FE and production rate typically trade off in most reports [25,26]. Therefore, the suitable three-phase boundary of N₂(g)-catalyst(s)-H₂O(l) with well-controlled mass transport of reactants and products, including catalyst engineering, is required to finally advance the NH₃ production rate [27]. The majority of ENRR has been performed in typical H-type cells, where the two compartments were separated with a membrane with a N₂ purging electrolyte as shown in Figure 1c [22,28,29]. However, since the N₂ gas solubility in H₂O is significantly low, reaching as low as 0.0126 mg·g⁻¹, the effective three-phase boundary representing active sites is insufficient, resulting in a low ENRR performance [20]. Thus, a strategy of adjusting the N₂ and H₂O concentrations in catalysts needs to be suggested. For example, a proposed representative is promoting the N₂ concentration near the catalyst by directly supplying N₂ gas across a gas diffusion electrode (GDE) as shown in Figure 1d [22]. Like this strategy, diverse reaction environmental engineering is required to enhance performances. Considering the discussed ENRR fundamentals, the strategies to design catalysts and systems and their effect on the ENRR’s performance will be reviewed in the next section.

3. Catalyst Engineering

Electrocatalysts for the ENRR have been developed using metals (Ag, Cu, W, Mo, Se, Fe, etc.), non-metals (Cl, B, F, P, S, O, N, etc.), metal oxides (TiO_2, Bi₄V₂O₁₁, etc.), metal sulfide (MoS₂), and carbon-based materials (graphdiyne, g-C₃N₄, N-doped carbon, etc.). In this section, we will review the representative strategies for ENRR improvement via facilitating N₂ activation and suppressing the competitive reaction.

3.1. Doping

Doping with atoms of different sizes and charge to the original electrocatalyst has been widely suggested in various electrocatalytic applications. The dopants can alter the electrocatalyst’s electronic structure and change the binding strength of the reactant, intermediate, or product at active sites. Thus, they govern the electrocatalytic performance [30,31,32,33].

Recently, Qing Jiang et al. prepared Ag-doped Cu nanosheets cultivated on carbon paper (Ag-Cu_ns/CP) by a simple electrochemical deposition [34]. The prepared Ag-Cu_ns/CP exhibited a highly advanced ENRR compared to undoped Cu_ns/CP. For example, at −0.4 V_RHE in 0.1 M of Na₂SO₄, the Ag-Cu_ns/CP catalyst showed an FE of 20.9% and an NH₃ production rate of 4.56 μg h⁻¹ cm⁻², whereas Cu_ns/CP showed an FE of only 3.14% and an NH₃ yield rate of 2.28 μg h⁻¹ cm⁻². As revealed by X-ray photoelectron spectroscopy (XPS) on the Ag-doped Cu nanosheets sample, as shown in Figure 2a, the binding energy of Cu⁰ positively shifted compared to the pristine Cu nanosheets and that of Ag⁰ was negatively shifted compared to bulk Ag particles. This indicates that electrons from Cu sheets were partially transferred into Ag dopants, resulting in the formation of electron-deficient Cu sites in Ag-Cu_ns/CP. The electron-deficient Cu sites prevent proton adsorption and simultaneously provide opportunities to accept lone-pair electrons of N₂ molecules due to the suitable overlapping of orbitals from N₂ and electron-deficient Cu. Consequently, the properties induced by Ag doping on the Cu nanosheets suppress the HER and facilitate ENRR.

Carbon-based metal-free catalysts have been receiving great attention due to their attractive properties, such as high surface area, excellent conductivity, easily controllable defects, and effective cost [35]. To utilize these characteristics, Lele Duan et al. suggested metal-free Cl-doped ultrathin graphdiyne (Cl-GDY) electrocatalysts prepared by annealing the pristine GDY precursors with Ar and Cl₂ gases [36]. At −0.45 V_RHE in 0.1 M HCl, the prepared Cl-GDY exhibited an FE of 8.7% and an NH₃ production rate of 10.7 μg h⁻¹ cm⁻², whereas pristine GDY exhibited an FE of 2.5% and a NH₃ production rate of 3.02 μg h⁻¹ cm⁻². Investigating the intensity ratio of the D to G band (I_D/I_G), representing the defect density and induced structural disordering of the carbon materials in Raman spectra revealed that the Cl-GDY exhibited a highly enhanced ratio compared with the pristine GDY. This implies that the Cl dopants increase defect sites, acting as active sites and structural distortions of GDY, resulting in improved ENRR performance.

Furthermore, the synergistic effect of dual heteroatom doping for ENRR was reported using citric acid-derived carbon electrocatalysts. Zhong-Yong Yuan et al. prepared dual heteroatom doped porous carbon (PC) by combining N dopants with B, F, P, or S dopants as shown in Figure 2b [37]. Depending on the dopant’s combination, their FEs were altered significantly as shown in Figure 2c. Among them, N, B-doped porous carbon (N,B-PC) exhibited the highest FE of 10.58% and an NH₃ production rate of 16.4 μg h⁻¹ cm⁻² at −0.2 V_RHE in 0.1 M HCl. The major contribution of dual dopants was investigated using Raman spectroscopy. Considering the (I_D/I_G), all dual-doped PCs, N,B-PC (1.025), N,F-PC (1.006), N,P-PC (0.978), and N,S-PC (0.976) demonstrated an increased structural disordering compared with the single N doped PC (0.962). Furthermore, N and B dual doping, pyridinic N ratio, and providing active sites for strong adsorption for N₂ largely increased compared to others in the XPS N 1s spectra (Figure 2d). Those two results indicate that dual heteroatom doping can effectively control the disordering and electronic structure of electrocatalysts affecting N₂ activation on active sites, resulting in ENRR improvement.

Dunmin Lin et al. also prepared dual-doped electrocatalysts, B,O-co-doped carbon microspheres (B,O-CMS) via a hydrothermal method [15]. The prepared B,O-CMS achieved a two-fold enhanced FE of 5.57% and an almost triple NH₃ production rate of 1.92 μg h⁻¹ cm⁻² compared with O-CMS at −0.25 V_RHE in 0.1 M HCl as shown in Figure 2e. The B,O-CMS showed an increased I_D/I_G ratio of 0.93 compared to the single doped O-CMS (0.88) in Raman spectra, indicating that the B,O-CMS has more defect sites acting as active sites than the O-CMS. Furthermore, as revealed by the Brunauer–Emmett–Teller (BET) measurements, the B,O-CMS provided more surface area than the O-CMS, in which each B,O-CMS and O-CMS showed a surface area of 488.7 and 373.6 m² g⁻¹, respectively.

These reports demonstrated that the doping strategy can adjust key factors for the ENRR by developing defect sites, providing or inducing neighbor’s active sites, ordering/disordering the shift, surface area, and so on.

3.2. Vacancy

Surface vacancy formation is one of the most used strategies for creating active sites for ENRR. Vacant sites can have abundant localized electrons to preserve charge neutrality. The enriched electrons can weaken the N₂ triple bond, resulting in facilitating the ENRR [38]. In this part, we will thoroughly review the progress on oxygen (O_v), nitrogen (N_v), and sulfur vacancies (S_v) derived from various electrocatalysts (TiO₂, C₃N₄, W₂N₃, and MoS₂).

The O_v formation has been the most developed strategy for ENRR, particularly for metal oxide-based electrocatalysts. Zhenyu sun et al. prepared O_v rich TiO₂ via a solvothermal method and then annealed at different temperatures of 700, 800, and 900 °C under Ar gas flow [39]. At −0.12 V_RHE in 0.1 M HCl, the O_v rich TiO₂ sample annealed at 800 °C achieved the highest FE of 6.5% and an NH₃ production rate of 3.6 μg h⁻¹ cm⁻² among others. For example, the pristine TiO₂ showed a low FE and NH₃ production rate (2.4% and 1.5 μg h⁻¹ cm⁻², respectively). They conducted O 1s XPS measurements to investigate atomic compositions and their chemical states on the catalyst’s surface. Each O_v rich TiO₂ sample, annealed at 700, 800, and 900 °C, was composed of O_v 18.4, 32.2, and 25.6%, respectively. Furthermore, the electron localized near the O_v sites managed to partially reduce the oxidation state of Ti⁴⁺, which served as an active site for ENRR. As the annealing temperature increased from 700 to 800 °C, the O_v amount also increased. However, when the annealing temperature increased from 800 to 900 °C, the O_v amount decreased. This was mainly due to the unintentional crystal structure’s change from original anatase to rutile over 800 °C. O_v formation is much more favored in the anatase phase relative to rutile [40]. As expected, the amount of reduced Ti⁴⁺ increased from 700 to 800 °C, but decreased from 800 to 900 °C in Ti 2p XPS. Those results reveal that reduced Ti⁴⁺ sites coupled with O_v were critical for the ENRR.

Jiantai Ma et al. prepared an N₂ vacant 2D C₃N₄ (2D C₃N₄-NV), showing good stability and a large specific surface area via annealing 2D C₃N₄ nanosheets at 600 °C under an N₂ gas flow for 15 min as shown in Figure 3a [41]. The prepared 2D C₃N₄-NV exhibited a highly advanced FE of 10.96% and an NH₃ production rate of 178.5 μg h⁻¹ cm⁻² at −0.30 V_RHE in 0.1 M HCl compared with pristine 2D C₃N₄ in Figure 3b. The dramatic performance advances by N_v formation were mainly due to the N₂ adsorption ability, which is regarded as the primary challenge for ENRR. Based on the N₂ adsorption–desorption isotherm measurements, the N₂ adsorption capacity of 2D C₃N₄-NV was 193.6 cm³ g⁻¹, which is almost 2 times and 1.4 times better than that of bulk C₃N₄ (91.5 cm³ g⁻¹) and 2D C₃N₄, respectively. Furthermore, density functional theory (DFT) calculations support that electron-rich N_v sites can facilitate N₂ adsorption via weakening the N₂ triple bonds, resulting in a superior ENRR performance for 2D C₃N₄-NV_.

Shi-zhang Qiao et al. prepared N_v on 2D layered W₂N₃ with excellent structural stability (NV-W₂N₃) by annealing Na₂W₄O₁₃ under an NH₃ gas flow [42]. The presence of N_v sites on W₂N₃ was revealed via XPS N 1s spectra, where N vacancy peaks at 400.2 eV could be observed in Figure 3c. Furthermore, in the extended X-ray absorption fine structure (EXAFS), NV-W₂N₃ showed a weakened W-N bonding, indicating a decreased W-N bonding number compared to pristine W₂N₃ in Figure 3d. The pristine W₂N₃ showed an FE of 3.5% and an NH₃ production rate of 0.94 μg h⁻¹ cm⁻² at −0.2 V_RHE in 0.1 M KOH. The ENRR performance was significantly improved by N_v formation, showing an FE of 11.67% and NH₃ production rate of 2.332 μg h⁻¹ cm⁻² at the same experimental condition as shown in Figure 3e. From the DFT calculation, it is revealed that electron-deficient W in W₂N₃ induced by N_v facilitates N₂ adsorption and lowers the thermodynamic limiting potential of ENRR.

Recently, strategies for inducing sulfur vacancies (S_v) have been widely proposed, such as O_v and N_v formation. The molybdenum disulfide (MoS₂), which is a representative electrocatalyst of HER, has been proposed as a catalyst for ENRR by S_v engineering [43,44]. Deliang Chen et al. developed S_v-rich MoS₂ by molybdenite precursors annealing at 800 °C (MoS₂-800) in an Ar gas flow [45]. As a result, the prepared MoS₂-S_v achieved an FE of 17.9% and an NH₃ production rate of 9.352 μg h⁻¹ cm⁻² at −0.35 V_RHE in 0.1 M HCl, showing twice and 3.5-times higher FE and NH₃ production rate for pristine MoS₂, respectively. The major contribution of S_v was revealed in the XPS spectra of Mo 3d, where the peak attributed to Mo⁴⁺ was positively shifted for MoS₂-800 compared with pristine MoS₂. On the other hand, in the XPS spectra of S 2p, the peaks attributing to S^2- were negatively shifted for MoS₂-800 relative to MoS₂. The peak shift of Mo implies electron transferring from Mo to other defect sites, whereas the peak shift of S implies the existence of deficient electrons. Additionally, the N₂ temperature-programmed desorption (N₂-TPD) results clearly exhibit the critical effect of S_v as shown in Figure 3e. The MoS₂-800 represents a much stronger N₂ physisorption peak at 98 °C and chemisorption peak at 385 °C compared to the pristine MoS₂, indicating that S_v in MoS₂-800 enhanced the N₂ adsorption ability. The progresses discussed in this part shows that vacancy engineering can effectively alter oxidation states around vacancies, affecting N₂ adsorption by weakening the N₂ triple bonds and increasing N₂ adsorption capacities, consequently enhancing ENRR.

3.3. Single Atom Catalysts

Single atom catalyst (SAC) synthesis has been regarded as a promising technique for catalyst development. The prepared SACs show outstanding catalytic performances with an extremely high number of active sites and unique electronic properties affecting catalytic pathways [17]. Metal SACs—such as Au, Cu, Fe, and so on—have been proposed and exhibited excellent ENRR performances.

Yongwen Tan et al. prepared Au SACs on nanoporous MoSe₂ (Au_SA/np-MoSe₂) by chemical vapor deposition followed by chemical etching [46]. They compared their ENRR performances with Au nanoparticles on np-MoSe₂ (Au_NPs/np-MoSe₂) and np-MoSe₂ alone as shown in Figure 4a,b. At −0.3 V_RHE in 0.1 M Na₂SO₄, the Au_SA/np-MoSe₂ catalyst achieved an FE of 37.82% and an NH₃ production rate of 6.166 μg h⁻¹ cm⁻², whereas Au_NPs/np-MoSe₂ and np-MoSe₂ exhibited an FE and NH₃ production rate of 11.06% and 1.818 μg h⁻¹ cm⁻² and 11.78% and 0.69 μg h⁻¹ cm⁻², respectively. Since the Mo dichalcogenides including MoSe₂ provide excellent catalytic sites for HER [47], ENRR’s representative competitive reaction, the major contribution for NH₃ production enhancement was expected for Au particles. From the XPS spectra of Au 4f, Au_SA/np-MoSe₂ showed a positively shifted peak attributed to Au⁰ compared with Au_NPs/np-MoSe₂. This indicates that the Au single atom becomes electron-deficient due to a stronger electronic interaction between Au_SA and MoSe₂, unlike the interaction between Au_NPs and MoSe₂. Furthermore, Gibbs-free energies calculations for N₂ adsorption on Au additionally supported that the electron-deficient isolated Au single atoms on MoSe₂ efficiently inhibited the HER and facilitated N₂ adsorption with the following hydrogenation.

Stephen J. Pennycook et al. synthesized Cu single atoms in porous N-doped carbon (NC-Cu SA) using a surfactant-assisted method [48]. As shown in Figure 4c, the presence of a single Cu atom was revealed by EXAFS for Cu K-edge. The NC-Cu SA represents the Cu-N/C peak with a negligible Cu-Cu peak, while NC-Cu NP shows only the Cu-Cu peaks representing the Cu nanoparticle. Furthermore, the single Cu atom in NC-Cu SA represents a positively charged valence state close to Cu²⁺, unlike Cu particles representing a metallic Cu⁰ state in NC-Cu NP as shown in Figure 4d. The electron-deficient Cu single atoms can provide strong N₂ adsorption sites as the first step for the ENRR. The NC-Cu SA catalyst exhibited relatively excellent performance with an FE of 13.8% and an NH₃ production rate of 53.3 μg h⁻¹ cm⁻² at −0.35 V_RHE in 0.1 M KOH, whereas NC-Cu NP exhibited an FE of only 4.7% and an NH₃ production rate of 26.8 μg h⁻¹ cm⁻².

Naturally, NH₃ can be produced by the biological N₂ fixation over nitrogenase enzyme comprising Fe metal with ligands [49]. Xijun Liu et al. synthesized a Fe single atom on N-doped carbon frameworks (Fe_SA-NC) inspired by the nitrogenase using the hydrothermal method followed by carbonization [50]. The Fe_SA-NC exhibited an FE of 18.6% and an NH₃ production rate of 62.9 μg h⁻¹ cm⁻² at −0.4 V_RHE in 0.1 M phosphate buffer solution. The ENRR performance of Fe_SA-NC was highly advanced compared to that of Fe_NPs-NC. The presence of single Fe atoms on NC was revealed by EXAFS, where the Fe-N bond was shown with a negligible Fe-Fe bond, representing the Fe atom’s coordination with the nitrogen atoms but not between the Fe atoms. This result indicates that the Fe-N sites provide an active site for N₂ adsorption and that Fe single atoms contribute to weakening the triple-bond length of N₂ from 1.098 to 1.134 Å.

This subsection demonstrated that SACs have various properties compared to nanoparticle catalysts. For example, SACs change their oxidation state or those of surrounding materials, effectively adsorbing N₂ and weakening the triple bond, consequently enhancing the ENRR.

3.4. Amorphization

Surface amorphization is a promising catalyst engineering technique that partially or completely reduces the crystallinity of regular crystal structures on the catalyst’s surface. Compared to highly crystalline structured catalysts, catalysts with reduced crystallinity or an amorphous surface nature can form a lot of defect sites comprising uncoordinated dangling bonds. Furthermore, they can provide lots of sites for reactant adsorption, which is typically the first sluggish step for catalytic reactions [51,52,53].

Qing jiang et al. prepared amorphous Au NPs on CeO_x-reduced graphite oxide (a-Au/CeO_x-RGO) using the co-reduction method [54]. The CeO_x can reduce the crystallinity of Au and amorphized it. The RGO can provide substrates dispersing the Au NPs. The a-Au/CeO_x-RGO achieved an FE of 10.1% and an NH₃ production rate of 1.66 μg h⁻¹ cm⁻² at −0.2 V_RHE in 0.1 M HCl, whereas crystalline Au/RGO only exhibited an FE of 3.67% and a NH₃ production rate of 0.7 μg h⁻¹ cm⁻²_. In Au 4f XPS spectra, Au in both a-Au/CeO_x-RGO and c-Au/RGO shows a metallic state. However, in X-ray diffraction (XRD) patterns, unlike c-Au/RGO, a-Au/CeO_x-RGO shows no Au crystalline peak, indicating the amorphous state of Au/CeO_x-RGO. Thus, a-Au/CeO_x-RGO improves ENRR because its amorphous state has an abundance of unsaturated active sites.

Guihua Yu et al. prepared both amorphous and crystalline Bi₄V₂O₁₁/CeO₂ using electrospinning with a precursor solution containing different Ce:Bi ratios (1:2 for amorphous and 1:4 for crystalline) and followed calcination [55]. Since the CeO₂ adjusts heat transfer, affecting the neighbor metal’s oxidation during calcination, the Ce:Bi ratio can control the crystallinity of Bi₄V₂O₁₁. The amorphous Bi₄V₂O₁₁/CeO₂ produces an FE of 10.16% and an NH₃ production rate of 46.42 μg h⁻¹ cm⁻² at −0.2 V_RHE in 0.1 M HCl, which is about three-times higher FE and a 2.8-fold higher NH₃ production rate for crystalline Bi₄V₂O₁₁/CeO₂. The crystalline Bi₄V₂O₁₁/CeO₂ showed Bi₄V₂O₁₁ of orthorhombic type and CeO₂ cubic type in the XRD pattern, whereas amorphous Bi₄V₂O₁₁/CeO₂ showed no obvious Bi₄V₂O₁₁ peak. Interestingly, unlike the crystalline Bi₄V₂O₁₁/CeO₂ showing only Bi³⁺ and V⁵⁺, and dominant lattice O peaks in XPS spectra for Bi 4f, V 2p, and O 1s, respectively, the amorphous Bi₄V₂O₁₁/CeO₂ shows Bi³⁺ and Bi⁵⁺, V⁵⁺ and V⁴⁺, and an abundance of oxygen vacancy peaks. The additional Bi⁵⁺ and V⁴⁺ peaks with oxygen vacancies are observed only for the amorphous sample. This indicates that the amorphous Bi₄V₂O₁₁/CeO₂ have intrinsically localized electrons around the defect sites derived from amorphization. Furthermore, they can activate the N₂ adsorption and the first hydrogenation step using a transferred electron and proton.

Ke Chu et al. suggested amorphous FeB₂ nanosheets prepared by a facile reflux method as an electrocatalyst for ENRR [56]. This amorphous FeB₂ exhibited an FE of 16.7% at −0.2 V_RHE and an NH₃ production rate of 7.96 μg h⁻¹ cm⁻² at −0.3 V_RHE in 0.5 M LiClO₄, which are 2- and 3-times higher FE and production rates for crystalline FeB₂, respectively. This catalytic enhancement’s major contribution was revealed by electrochemical active surface area (ECSA) measurements. The amorphous FeB₂ exhibited 1.56-times higher ECSA than crystalline FeB₂, indicating that amorphous FeB₂ has a large active surface area than the crystalline one. Additionally, considering the results of N₂ TPD, the amorphous FeB₂ showed a stronger chemical desorption peak than crystalline FeB₂, implying an enhanced N₂ adsorption ability of amorphous FeB₂ due to the increase in uncoordinated and defective sites in amorphous FeB₂.

These studies support the idea that amorphization is a suitable strategy for increasing the active surface area, number of active sites, effectively providing the appropriate N₂ adsorption strength.

The performance for ENRR depending on strategies for catalyst modification are summarized in

Table 1. A brief summary of experimental studies on ENRR using various electrocatalytic engineering strategies.

Catalyst Engineering	Catalyst	Electrolyte	Potential (VRHE)	FE (%)	Production Rate (μg h−1 cm−2)	Refs.
Doping	B,O-CMS	0.1 M HCl	−0.25	5.57	1.92	[15]
	S-CNS	0.1 M Na2SO4	−0.7	7.47	1.907	[30]
	Mo-MnO2 NFS	0.1 M Na2SO4	−0.4	12.1	3.46	[33]
	Ag-Cuns/CP	0.1 M Na2SO4	−0.4	20.9	4.56	[34]
	Cl-GDY	0.1 M HCl	−0.45	8.7	10.7	[36]
	N,B-PC	0.1 M HCl	−0.2	10.58	16.4	[37]
	Co/NC_500	0.1 M KOH	−0.1	10.1	10.2	[57]
	PC/Sb/SbPO4	0.1 M HCl	−0.15	31	3.34	[58]
	PC/Sb/SbPO4	0.1 M Na2SO4	−0.1	34	2.7	[58]
	Fe,Mo-N/C	0.1 M Na2SO4	−0.1	14.2	25.4	[59]
Vacancy	CN/C600	0.1 M HCl	−0.3	16.8	1.93	[16]
	Ov rich TiO2	0.1 M HCl	−0.12	6.5	3.6	[39]
	2D C3N4-NV	0.1 M HCl	−0.3	10.96	178.5	[41]
	NV-W2N3	0.1 M KOH	−0.2	11.67	2.332	[42]
	MoS2-800	0.1 M HCl	−0.35	17.9	9.352	[45]
	C@CoFe2O4-X	0.1 M Na2SO4	−0.4	11.65	45.485	[60]
	Ov-TIO2@C/Cu	0.1 M Na2SO4	−0.55	6.04	4.025 @ −0.6 VRHE	[61]
	Li-TiO2(B)	0.5 M LiClO4	−0.4	18.2	0.87	[62]
	CoS1-x	0.05 M H2SO4	−0.15	16.5 ± 1.5%	12.1 ± 1.4	[63]
	VS -MoS2	0.1 M Na2SO4	−0.4	4.58	1.66 @ −0.5 VRHE	[64]
Single atom catalyst	AuSA/np-MoSe2	0.1 M Na2SO4	−0.3	37.82	6.166	[46]
	Cusa-NC	0.1 M KOH	−0.35	13.8	53.3	[48]
	Fesa-NC	0.1 M phosphate buffer	−0.4	18.6	62.9	[50]
	Ru SAs/N-C	0.05 M H2SO4	−0.2	29.6	30.8	[65]
	Y1/NC	0.1 M HCl	−0.1	12.1	23.2	[66]
	Sc1/NC	0.1 M HCl	−0.1	11.2	20.4	[66]
	SA Ru-Mo2CTX	0.5 M K2SO4	−0.3	25.77	12.17	[67]
	W-NO/NC	0.5 M LiClO4	−0.7	8.35	2.524	[68]
	Mn–N–C SAC	0.1 M NaOH	−0.45	32.02	5.36 @ −0.65 VRHE	[69]
	Pt SAs/WO3	0.1 M K2SO4	−0.2	31.1	171.2	[70]
	AuSAs-NDPCs	0.1 M HCl	−0.2	12.3	2.32	[71]
Amorphization	a-Au/CeOx-RGO	0.1 M HCl	−0.2	10.1	1.66	[54]
	amorphous Bi4V2O11/CeO2	0.1 M HCl	−0.2	10.16	46.42	[55]
	amorphous FeB2 nanosheets	0.5 M LiClO4	−0.3	16.7	7.96	[56]

.

4. Three-Phase Boundary Engineering

The formation of the three-phase boundary (TPB) of N₂(g)-catalyst(s)-H₂O(l) is critical for the actual ENRR [27,72]. The advanced TPB can effectively suppress HER, the representative competitive reaction, and develop the NH₃ production rate. In this section, strategies for engineering TPB from electrode surface modification to systematic reaction environmental engineering will be discussed in detail.

4.1. Electrode Surface Modification

In typical aqueous ENRR systems, the low N₂ solubility (0.0126 mg·g⁻¹) prevents the effective transfer of N₂, unlike the free accessibility of H₂O, causing a significantly lower N₂ concentration near catalysts than H₂O [20]. It provides an insufficient TPB condition, resulting in an ineffective FE and production rate for ENRR.

One strategy to advance the TPB is modifying the electrode’s surface which has a partial hydrophobic nature. Xing Yi Ling et al. suggested coating over the Ag-Au catalyst with the hydrophobic zeolitic imidazolate framework-71 (ZIF-71) (Ag-Au@ZIF-71) by a wet chemical deposition [73]. The distinct characteristics of ZIF-71, having a unique pore structure comprising metal centers and hydrophobic functional groups of dichloroimidazole linkers provides a superhydrophobic barrier, effectively suppressing the free access of H₂O to catalysts [74]. Furthermore, the N₂ molecules can freely diffuse through the ZIF-71 layer and thus be concentrated near the catalyst surface. The relatively reduced H₂O concentration and increased N₂ concentration on catalysts by surface modification with ZIF-71 develop the TPB, effectively resulting in an enhanced ENRR. Ag-Au@ZIF-71 electrode exhibited an advanced FE of 18 ± 4% and an NH₃ production rate of ~0.648 μg h⁻¹ cm⁻² at −2.9 V_Ag/AgCl in the solution of 0.2 M LiCF₃SO₃ added to ethanol containing dry tetrahydrofuran (TPB), whereas the uncoated Ag-Au electrode showed an FE of only 9% and an NH₃ production rate of 0.1296 μg h⁻¹ cm⁻². Furthermore, continuous local N₂ saturation led to long-term stability with improved accessibility of N₂ to the catalyst’s surface by ZIF-71. The Ag-Au@ZIF-71 represented constant CV features during 45 consecutive potential sweeps between −3 and −0.5 V_Ag/AgCl. On the other hand, the Ag-Au catalyst showed that the shape of CV was continuously deformed during the consecutive experiments, gradually flattening.

Miao Du et al. proposed a core–shell nanoporous gold (NPG) by encapsulating NPG with ZIF-8 (NPG@ZIF8) as shown in Figure 5a [19]. The 2-methylimidazole linkers comprising ZIF-8 provided a hydrophobic nature and suppressed H₂O accessibility toward NPG catalysts [74]. As shown in Figure 5b, NPG@ZIF-8 showed an FE of 44% and an NH₃ production rate of 22.0 ± 0.3 μg h⁻¹ cm⁻² at −0.6 V_RHE in 0.1 M Na₂SO₄, which is 7 times and a 10-fold higher FE and production rate than NPG alone, respectively. The improved ENRR performance mainly originated from ZIF-8 encapsulating NPG by effectively suppressing the competitive HER. NPG@ZIF-8 showed stability with very little change in FE and NH₃ production rate even after 10 consecutive experiments for 2 h each as shown in Figure 5c.

Even for photoelectrochemical applications, TPB engineering is important for producing NH₃ using N₂ and H₂O with generated photoexcited electrons. Shuangyin Wang et al. proposed a surface modification of a Si-based photocathode using a hydrophobic PTFE porous framework [75]. The photo-excited electrons generated by light absorption from Si were transferred to Au nanoparticles (Au NP), serving as active sites for ENRR. Since PTFE on Si photocathodes provides a hydrophobic environment originating from strong C-F bonds and resulting in low F polarization and low van der Waals force, H₂O access was dramatically suppressed with accelerating N₂ diffusion toward Au NPs. As a result, the PTFE modified Si photocathode exhibited an FE of 37.8% and an NH₃ production rate 18.9 μg h⁻¹ cm⁻² at −0.2 V_RHE in 0.05 M H₂SO₄ with 0.05 M Na₂SO₃ under 1 sun illumination. Its performance was 4- and 1.5-times higher than FE and production rate for pristine Si photocathode with Au NP, respectively.

Furthermore, Pei Kang Shen et al. suggested a way for accumulating N₂ molecules to increase the relative N₂ concentration, resulting in superior TPB for ENRR [76]. They prepared an aerobic-hydrophilic hetero-structured electrocatalyst using ultrathin Bi₅O₇I nanotubes (UP-BOIN) and carbon spheres. The UP-BOIN served as the active sites and showed a highly porous surface structure with a diameter of about 5 nm with a hollow tubular geometry. This morphological trait provides a super-aerophilic nature. Then, the UP-BOIN was combined with the hydrophilic carbon sphere, modified by immersing them in H₂O₂ and 70 wt % H₂SO₄ in series. The combined aerophilic UP-BOIN and hydrophilic carbon sphere can control N₂ accumulation and H₂O accessibility, affecting the superior TPB formation. For example, the 75% UP-BOIN with the 25% carbon sphere sample shows an FE of 6.10% and an NH₃ production rate of 2.286 μg h⁻¹ cm⁻² at −0.4 V_RHE in 0.1 M Na₂SO₄. On the other hand, the electrode of 0% UP-BOIN with a 100% carbon sphere shows an FE and a production rate close to 0, indicating that UP-BOIN is a key electrocatalyst providing active sites. Furthermore, the sole UP-BOIN showed a significantly lower 5.19% FE and 0.796 μg h⁻¹ cm⁻² NH₃ production rate compared to the carbon sphere combined UP-BOIN. This indicates that the effective control of reactant concentration, including H₂O, is critical for providing a sufficient TPB for the ENRR.

4.2. Reaction Environmental Engineering

As stated in previous sections, both catalyst modification and TPB advances can play key roles in ENRR development. Thus, it is significantly critical for developing superior TPB via controlling the local concentration of N₂ and H₂O near catalysts. In this section, we will further discuss reaction environmental engineering strategies to develop TPB, including modification of N₂ gas supply, additives in electrolytes, etc.

Kyoung-Shin Choi et al. revealed how local N₂ and H₂O concentrations influence ENRR by suggesting the simple cell modification for N₂ supplying [77]. As shown in Figure 6a, a BiSb alloy-based porous electrode—named the ‘regular electrode’—was placed in the typically used setup, where ENRR was performed using dissolved N₂ molecules in H₂O. Additionally, a pseudo-gas diffuse electrode (pseudo-GDE) was proposed by changing the N₂ purging position into the electrode instead of the electrolyte, resulting in a direct N₂ gas supply near the catalyst as shown in Figure 6b. Since they compared the ENRR’s performance depending on N₂ supply using perfectly consistent electrodes, the activity benefits from other possible activity-determining factors—such as electrode morphology, catalytic active sites, etc.—are negligible. As shown in Figure 6c, FEs from the pseudo-GDE were significantly altered against the N₂ flow rate because it could change the air pocket’s size inside the electrode corresponding to the N₂ concentration. With the increase in the N₂ flow rate up to 100 mL min⁻¹, the FEs increased, representing the N₂ local concentration increase’s positive influence on the ENRR. However, over the optimum rate, the efficiencies were decreased due to the significantly limited H₂O supply for catalysts, resulting in reduced TPB sites. At −0.6 V_RHE, with the optimal N₂ flow rate, the BiSb regular electrode showed an FE of 1.1% with an NH₃ production rate of 16.29 μg h⁻¹ cm⁻² in 0.5 M borate. On the other hand, the BiSb pseudo-GDE exhibited 5% of FE, a five-fold improvement compared to the regular electrode. Furthermore, the electrode exhibited a highly advanced NH₃ production rate of 66.90 μg h⁻¹ cm⁻², and the stability was shown through five experimental repetitions in which 5 C passed for each electrolysis.

Xiaofeng Feng et al. proposed a direct N₂ flow cell with a GDE which followed an additional hydrophobic layer modification [20]. Unlike the typically used H-cell which severely suffers from a low concentration of N₂ molecules near the catalyst due to low N₂ solubility (0.0126 mg·g⁻¹) in H₂O as shown in Figure 6d, the flow cell with the gas-diffusion layer in Figure 6e can supply sufficient local N₂ concentration via direct humidified N₂ gas fed towards the cathode. Additionally, for the most effective TPB engineering, the developed electrode can be assembled with GDE, catalyst nanoparticles layer deposited on porous and hydrophobic carbon fiber paper, and hydrophobic layer in series as shown in Figure 6e. If the design is experimentally applied, the N₂ gas directly flows and diffuses. Additionally, the protons adjusted by passing the hydrophobic layer are also transferred towards the catalyst layer, which is expected to form a suitable TPB for ENRR by effectively suppressing HER.

Another interesting way to adjust N₂ concentration near catalysts is modifying the composition of electrolytes affecting N₂ solubility. For example, the non-aqueous ionic liquid (IL) can intrinsically inhibit HER because of the significantly low concentration of protons. For example, N₂ solubility is about 20-times higher in trihexyl(tetradecyl) phosphonium tris(pentafluoroethyl) trifluorophosphate, [P_6,6,6,14][eFAP] (0.28 mg·g⁻¹), representative IL, than the solubility in H₂O (0.0126 mg·g⁻¹) at room temperature [78]. Therefore, electrolyte engineering can give an attractive approach for controlling TPB for ENRR.

Douglas R. MacFalance et al. prepared a Fe electrocatalyst on fluorine-doped tin oxide glass (FTO) substrate by electro-deposition and ENRR was performed using the electrode in the electrolyte of [P_6,6,6,14][eFAP] (Figure 6f) [81]. For comparison, ENRR was performed in another ionic liquid 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [HMIM][NTf₂] (Figure 6g) having a relatively low solubility of 0.017 mg·g⁻¹ than [P_6,6,6,14][eFAP], showing 0.28 mg·g⁻¹ [80]. The differing N₂ solubilities in each electrolyte are due to the interaction between N₂ and [eFAP]^- or [NTf₂]^-. According to the DFT calculation, the N₂ molecules can interact with [eFAP]^- in two different ways: N₂ binding with the C₂F₅ group or with F, showing a binding energy (Eb) of −0.53 and −0.42, respectively. On the other hand, N₂ molecules can make bonds with CF₃ groups of [NTf₂]⁻ only showing an E_b of −0.16. This indicates that [eFAP]^- provides a much stronger N₂ binding strength than [NTf₂]^-, resulting in a more favorable N₂ solubility. This distinct N₂ solubility difference in the two ILs proportionally affects the ENRR. The Fe electrode exhibited an FE of 60 ± 6% in [P_6,6,6,14][eFAP], whereas it showed an FE of 0.64% in [HMIM][NTf₂] at −0.8 V_NHE.

Zhongfang Chen et al. investigated polyethylene glycol (PEG) additive’s effect in 0.05 M H₂SO₄ electrolyte for ENRR using TiO₂ nanoarray electrodes [82]. Even if the majority of the electrolyte is H₂O, a molecular crowding effect formed by hydrogen bonding between H₂O and PEG can significantly suppress H₂O movement. In contrast, since the N₂ molecule is non-polar and its movement was not restricted by the effect, the N₂ molecule can freely move in the electrolyte to reach the active site. As a result, the TiO₂ electrode’s FE and NH₃ production rate in the electrolyte with PEG were recorded as 32.13% and 13.6 μg h⁻¹ cm⁻² for −0.3 V_RHE. On the other hand, it demonstrated an FE and production rate of only 0.8% and 5.1 μg h⁻¹ cm⁻² without PEG.

In case sufficient protons are present near the catalytic active site, H₂ can be easily adsorbed on electrode surfaces before N₂, leading to a poor ENRR performance. Meanwhile, local H₂O accessibility and its adsorption on active electrode surfaces can be controlled by developing the fundamentals of the electrical double layer (EDL) model [83,84]. For example, if we add additional alkali metal cations to the electrolyte, their hydration is promoted in the beginning. It then suppresses H₂O movement, finally reducing the local H₂O concentration. Furthermore, N₂ absorption strength can be enhanced via the direct adsorption of cations on the surface electrode [85].

An-Xiang Yin et al. investigated the K⁺ cation’s effect in acidic potassium sulfate for ENRR using Bi catalysts supported on carbon black [85]. They found FEs for ENRR from 9.8% to 66% at −0.6 V_RHE depending on the concentration of the K⁺ from 0.2 to 1.0 M. Additionally, the NH₃ production rate increased from about 230 to 884 μg h⁻¹ cm⁻². With the increase in K⁺ concentration, the amount of hydrated cations increases. Thus, they can be concentrated in the diffusion layer near the surface at a low potential. The advanced performance is mainly due to proton movement suppression by the hydrated K⁺ cations. Therefore, N₂ is first adsorbed to the electrocatalyst’s surface and activated. Meanwhile, K⁺ directly adsorbed on the electrode can change the Bi electrode’s electronic structure, thereby reducing the Gibbs free energy difference for N₂ activation ($\Delta$G_*NNH). Thus, by adding the K⁺ cation in electrolytes, direct proton adsorption was effectively suppressed and N₂ adsorption and activation was facilitated.

The performances for ENRR depending on strategies for environmental engineering are summarized in

Table 2. A brief summary of experimental studies on ENRR using various TPB engineering strategies.

TPB Engineering	Catalyst	Electrolyte	Potential (VRHE)	FE (%)	Production Rate (μg h−1 cm−2)	Refs.
Electrode surface	NPG@ZIF-8	0.1 M Na2SO4	−0.6	44	22.0 ± 0.3	[19]
	Ag-Au@ZIF-71	0.2 M LiCF3SO3 added to ethanol containing dry TPB	−2.9 VAg/AgCl	18 ± 4	0.648	[73]
	AuNp@PTFE	0.05 M H2SO4 with 0.05 M Na2SO3	−0.2	37.8	18.9	[75]
	UP-BOIN	0.1 M Na2SO4	−0.4	6.1	2.286	[76]
Environmental engineering	BiSb(pseudo GDE)	0.5 M borate	−0.6	5	66.9	[77]
	Fe on FTO	[P6,6,6,14][eFAP]	−0.8 VNHE	60 ± 6	0.25	[81]
	TiO2 nanoarray	PEG in 0.05 M H2SO4	−0.3	32.13	13.6	[82]
	nitrogen-doped CNS	0.25 M solution of LiClO4	−1.2	11.5	96	[83]
	Cu	5 M LiClO4	−0.6	12.1 ± 0.8	12.06 ± 2.39	[84]
	Bi on CB	acidic potassiumsulfate (pH 3.5, 1.0 mol L−1 of K+)	−0.6	66	884	[85]
	Pt	gel polymer electrolyte and polyacrylic acid polymer with 6 M KOH	0.5 V (two-electrode system)	0.01%	2.448	[86]
	Nano-Fe/CFC	1 M K3PO4	1.0 V~0.0 V	16.68	4.83	[87]

.

5. Summary and Perspective

NH₃ is an essential precursor for chemicals that are widely used in a variety of industries. Furthermore, it is an important sustainable fuel and media for hydrogen storage. Electrochemical N₂ reduction for NH₃ production (ENRR) has been considered a greatly promising process alternative to the traditional Haber–Bosch process that requires a high-energy input and which emits enormous CO₂ gas. Since the ENRR is performed at an ambient temperature and pressure using N₂ gas and H₂O as a hydrogen resource, it is economically effective and environmentally friendly. Considering those ideal potentials, the scientific community has devoted considerable effort to advancing the performance metrics, including the FE and NH₃ production rate. This review presented ENRR’s governing fundamentals, such as thermodynamics, mechanisms, and mass transport. We then introduced strategies for improving its performance.

To design robust electrocatalysts that could provide superior active sites for N₂ adsorption and activation while suppressing HER, defect engineering via heteroatom doping and vacancy formation have been suggested. Furthermore, effective new types of catalysts have been proposed, such as catalysts comprising a single atom grown on a supporter or having an amorphous phase on the surface. These strategies could mainly control electronic structures promoting N₂ adsorption, *N₂H, the first intermediate, formation, and further hydrogenation with inhibiting proton adsorption. Meanwhile, to advance the ENRR’s performance, not only catalyst engineering but also promising TPB engineering have proved to be critical. Electrode modification with functional layers and reaction environmental modification using direct N₂ gas supply, ionic liquids, and additional cations have been proposed. The desired concentration of N₂ and H₂O near catalysts can be suggested from the modifications, overcoming the major challenges of N₂ mass transport caused by its low solubility in aqueous solutions, and relatively free H₂O accessibility. Recently, the research has also been proposed to improve performance by forming a compact and uniform solid electrolyte interphase (SEI) [88]. Via multiple engineering modifications, the Bi catalysts reported the highest ENRR performance with an FE of 66% and an NH₃ production rate of 884 μg h⁻¹ cm⁻² at −0.6 V_RHE in acidic potassium sulfate (pH 3.5, 1.0 mol L^–1 of K⁺).

However, although strategies discussed in this review significantly advanced the ENRR, the performance is still inadequate for the ultimate technical target of practical NH₃ production; namely an FE of 50% and a NH₃ production rate of at least 6120 μg h⁻¹ cm⁻² [12,13]. Therefore, a combination of strategies for catalysts and reaction engineering is necessary for developing ENRR with ultra-high performance. For example, electrochemical CO₂ reductions have similar challenges to ENRR, such as the facile competitive reaction of HER, sluggish first active intermediate formation using transferred electrons and protons, limited TPB caused by imbalanced CO₂, and robust H₂O concentration due to low CO₂ solubility [89,90,91]. However, their performance was significantly advanced, making them economically feasible by combining robust catalyst developments, environmental reactions, and environmental engineering, including system and process modification. Benchmarking the signs of progress of CO₂ reduction proves that ENRR could be further improved.

To turn ENRR into a practically available pathway alternative to the Haber–Bosch process, the desired electrocatalysts, environmental conditions, and the other cell components must be sustainable for at least several months. However, those stability issues are barely investigated, even for catalysts tested only for a few hours. Electrocatalysts may decompose due to poisoning, dissolution, and deactivation of active sites [14]. Furthermore, proper reaction environmental conditions can gradually differ compared to the initial state due to varying reactant and product concentrations. Furthermore, the cell components may be decomposed due to unintentional corrosion, pressure changes, etc. Therefore, prolonged stability test periods are highly recommended because they are useful for figuring out the imperfect durability’s major origins. If the remaining challenges are successfully solved using the new frontiers suggested above, the ENRR can boost the NH₃ production process, significantly reducing energy consumption and CO₂ emission by replacing the traditional Haber–Bosch process.