Pathways of the Electrochemical Nitrogen Reduction Reaction: From Ammonia Synthesis to Metal-N₂ Batteries

Abstract

Ammonia is considered as an alternative fuel resource for a sustainable green future. The production of ammonia involves the electrochemical nitrogen reduction reaction (NRR), which has gained considerable attention due to its eco-friendly resources and nonharmful byproducts. Even with the manifold works on NRR, the technique has not reached the industrial scale because of the impediments of NRR electrocatalysts, and in addition, state-of-the-art electrocatalysts have not yet been discovered. In this review, first, the mechanism of the NRR, key metrics, and operational procedures for NRR electrochemistry are presented. Then, the electrocatalyst designs for efficient NRR are briefly introduced, followed by a discussion on the influence of the electrolytes that enhance NRR performance. The counterion effects of electrolytes on NRR performance and strategies for suppressing the HER by electrolyte additives are also discussed. Later, the NRR mechanisms are upgraded, and a comprehensive review of metal-N₂ batteries is provided. This review summarizes the effective methods for performing the NRR and strategies to suppress the HER on various electrocatalysts by tuning electrolytes and their additives. The review concludes by discussing the prospects of metal-N₂ batteries.

Graphical Abstract

1 Introduction

The world’s population is expected to reach 11 billion by the end of this century (2100) from 7.7 billion people in 2019, according to a report launched by the United Nations (UN) [1]. Modern energy demands, due to this profusion, have put forward an essential threat to seeking renewable, sustainable, economical, and eco-friendly strategies for tackling the upcoming energy crisis. The development of electrocatalysts paved an efficient, economical path to obtain products, such as hydrogen (H₂), ammonia (NH₃), and hydrocarbons, and the production of value-added chemicals from earth-abundant materials, such as water, nitrogen (N₂), oxygen (O₂) and carbon dioxide (CO₂). Over decades, scientists have relentlessly designed electrocatalysts that target the nitrogen reduction reaction (NRR) [2], hydrogen evolution reaction (HER) [3], oxygen reduction reaction (ORR) [4], carbon dioxide reduction reaction (CO₂RR) [5], and many other [6] electrochemical reactions that facilitate the formation of fuel products and value-added chemicals.

Among the valuable fuel products, ammonia (NH₃) has stood out greatly, exhibiting better storage and transport facilities and more cost-efficient and eco-friendly production pathways than hydrogen, and ammonia is a long-term energy resource that serves as a low-cost hydrogen energy carrier [7]. Additionally, it serves as a source of fertilizers for agriculture and raw materials for explosives and coolants. The efficient production of ammonia, as shown in Eq. (1), has invariably been an important part of research interests for decades [8]. Currently, major ammonia production follows the polluting Haber–Bosch process. The Haber–Bosch technique uses high temperatures of approximately 350 °C and pressures ranging from 250 to 350 bar (1 bar = 100 000 Pa), and the minimum energy consumption is approximately 27.4–31.8 GJ ${\mathrm{t}}_{{\mathrm{NH}}_{3}}^{-1}$ with an efficiency of 65% [9]. This higher energy consumption in the Haber–Bosch process can be lessened by alternative routes, an enzymatic pathway and an electrochemical pathway. Along the enzymatic pathway, nitrogenase enzymes produce more hydrogen than the required ammonia, which shows the unreliability of the enzymatic pathway [10]. Ultimately, the choices are reduced to the electrochemical NRR, which is a promising candidate for green ammonia production.

$${\text{N}}_{{2}} + {\text{3H}}_{{2}} \to {\text{ 2NH}}_{{3}} \;\Delta_{{\text{}}} H_{f}^\circ = - {45}.{9}\;{\text{kJ}}\;{\text{mol}}^{{ - {1}}}$$

(1)

Electrochemical ammonia production involves NRR that adsorbs N₂ molecules on the catalyst surface. The electrochemical NRR uses electricity derived from renewable sources and N₂, along with protons from water dissociation.

Figure 1 represents the electrochemical ammonia production from water and renewable electricity and its application in various industrial and transportation sectors. Electrochemical NRR in an electrochemical cell is promoted by using an electrocatalyst that efficiently adsorbs N₂ gas onto the electrode surface and reduces it to ammonia. Electrocatalysts with a large surface area, highly porous structures with intrinsic catalytic activity, and excellent bulk conductivity exhibit better nitrogen adsorption energies and stability to catalyse the electrochemical reaction. However, this electrochemical NRR is suppressed by the slower kinetics of nitrogen adsorption onto the surface, and the splitting of the N₂ triple bond leads to reduced faradaic efficiency (FE) and lower ammonia production rates [11]. Studies show that the limiting potential of the HER is smaller than that of the NRR, which indicates that the HER competes with the NRR with smaller potentials for H₂ evolution in an electrocatalyst [12]. Additionally, from a kinetic point of view, protons and electrons undergo H₂ formation rather than NH₃ formation, which accounts for the extremely low faradaic efficiency (FE) and the reduction in ammonia yield of the electrocatalysts [13]. Thus, electrocatalysts are classified as noble metal-based, nonnoble metal-based, metal-free, and single-atom catalysts (SACs) that exhibit different orders of activity depending on their unique surface adsorption properties and affinity towards N₂ triple bond splitting.

Fig. 1

Schematic of electrochemical ammonia production and its application in various sectors

HER suppression is of utmost importance for effective NRR performance. The active sites of the electrocatalyst are heavily utilized by the protons from the electrolyte because of the electronic effects between the electrocatalyst and the H⁺ ions. This usage has a negative impact on the ammonia yield and the FE of the electrocatalyst, reducing the NRR performance. Hence, selectivity towards N₂ must be increased for electrocatalysts over the HER. Strategies such as tuning the d-bands of the electrocatalyst [14] and electrolyte modifications [15,16,17] have been followed and promising results are shown in reducing the HER. In later sections, we clearly discuss the important strategies that inhibit the HER for efficient NRR performance.

Upgrading the electrochemical NRR mechanisms into an energy storage device has proven to be the most needed plan for the current energy-demanding society. The importance of electrochemical energy storage (EES) devices has increased in the modern era of technology for electric vehicles and smart grid power storage [18, 19]. Various EES devices, such as rechargeable batteries, flow batteries, metal gas batteries, supercapacitors, and fuel cells, can store energy from smart grids and supply as needed. Among these devices, metal N₂ batteries have started attracting attention due to their readily available resources and the importance of the final product. Metal-N₂ batteries have importance due to ammonia production with simultaneous electricity generation. The history of metal-N₂ batteries dates back to 2017 when Zhang et al. [20,21,22,23,24,25] decided to fabricate a rechargeable Li-N₂ battery inspired by the high energy density of metal gas batteries, Al-CO₂, Li-CO₂, Li-SO₂, Na-CO_2, and Li-O₂. Later, several reports were published in response to the novelty of the work and prime expectations of metal-N₂ battery systems, as shown in Fig. 2 [26]. Soon after the first Zn-N₂ battery was reported in 2019 under an aqueous alkaline electrolyte, novel design strategies for the electrocatalyst were proposed and demonstrated as cathodes for the Zn-N₂ battery. The aprotic electrolytes in metal-N₂ batteries make their construction and maintenance tedious. Fortunately, alkaline metal-N₂ batteries have provided a safer, promising, and reliable approach due to the wide availability of resources and low maintenance costs. However, the limited NRR performance of alkaline metal-N₂ batteries shows the incubatory stage of these batteries for which various electrocatalysts have been constantly engineered to meet the current demands.

Fig. 2

© 2017, 2019, 2020, 2021 Elsevier Ltd. Adapted with permission from Ref. [27]. Copyrights © 2021, American Chemical Society. Adapted with permission from Refs. [31, 32, 47, 50]. Copyrights © 2019, 2020, 2021, Royal Society of Chemistry. Adapted with permission from Ref. [189]. Copyrights (c) 2019, John Wiley & Sons, Inc

Timeline representing the advancements in metal-N₂ batteries over the years. Adapted with permission from Ref. [20, 26, 29, 30, 48, 49]. Copyrights

Presently, various metal-N₂ batteries are available, each having its own advantage owing to its cathode reactivity and anode configuration. An active electrocatalyst as the cathode is the integral unit for the battery because it effectively increases the activity by offering minimum resistance for the charge‒discharge mechanisms. To date, only a few metal-N₂ batteries have been demonstrated (Li-N₂, Na-N₂, Al-N₂, Zn-N₂), and further upgrades to increase the faradaic efficiency (FE) and ammonia yield of electrocatalysts with a higher power density are foreseen. The real challenge lies in the activation of the strong triple bond of the nitrogen molecule, which prevents the battery system from reaching maturity.

Metal-N₂ batteries simultaneously undergo redox reactions at the cathode and metal anode to drive the charge storage mechanism. In the case of metal-N₂ batteries, N₂ reduction and oxidation reactions occur at the cathode for the discharge‒charge cycles, respectively [20, 27]. Therefore, simultaneous ammonia production and electricity generation can be manifested over metal-N₂ batteries, which show importance over conventional metal-air batteries [27, 28]. Among the reported metal-N₂ batteries, including Li-N₂ batteries [20], Na-N₂ batteries [29], and Al-N₂ batteries [26], Zn-N₂ batteries hold great prospects due to the stability of the Zn anode in an alkaline environment and the fabrication of the battery in an ambient atmosphere [30]. In addition, Zn is widely available in the Earth’s crust, is eco-friendly, has high capacity and stable discharge potential, and is a mature technology compared to other batteries [31, 32]. Therefore, Zn-N₂ batteries hold assuring commercial value and can be easily developed to provide industrial-scale applications.

The present review article focuses on the rational design strategies of an electrocatalyst from electrochemical NRR into metal-N₂ batteries. The review includes discussions on the novel electrocatalyst design strategies and the mechanisms of different electrocatalyst structures. The important role of electrolytes and the counterions in the electrolytes in driving the electrochemical NRR are reviewed. The advantage of suppressing the HER by electrolyte modifications is also explained. Moreover, incorporating NRR electrocatalysts into the metal-N₂ battery system and future aspects of metal-N₂ batteries are also discussed. Based on this discussion, the review develops a research orientation to incorporate an electrocatalyst as an active cathode for potential metal-N₂ battery applications.

2 Fundamentals of Electrochemical NRR and Metal-N₂ Batteries

The reduction of nitrogen to ammonia via the electrochemical NRR route is a feasible green method, and this approach is more energy efficient than the conventional Haber–Bosch process because of the low operating temperature. The electrochemical NRR process uses water, rather than natural gas, as a H₂ source [33]. NRR occurring at the electrocatalyst surface must be immune to hydrogen adsorption. The merits of the electrocatalyst include improving NRR selectivity and increasing ammonia yield and FE. NRR at the electrocatalyst surface occurs at more negative potentials, which can be compromised by HER. From a thermodynamic perspective, we can observe the HER being more favourable than the NRR due to the lower free energy of formation required for H₂ [34].

2.1 Theory of N₂ Adsorption

Nitrogen (N₂), a homonuclear nonpolar molecule, is a Lewis base surrounded by a lone pair of electrons. It does not possess a permanent dipole moment because of the even distribution of charges and equal sharing of electrons, as shown in Fig. 3a. Moreover, N₂ has a triple bond, with each atom holding 7 electrons with an electronic configuration of 1s²2s²2p³, which has pairs of electrons in the s orbitals with opposite spin and electrons in the p orbital with the same spin. N₂ forms a triple bond that exhibits excellent stability. However, ammonia production solely lies in N₂ bond cleavage and initial protonation. Further difficulties can be offered by the thermodynamic constraints from the intermediates during catalytic N₂ reduction [35, 36]. The adsorption of N₂ onto the electrocatalyst surface occurs at specific electron-deficient active sites. The active sites include transition metal sites with unoccupied d orbitals or heteroatoms with sp² orbitals that actively adsorb N₂.

Fig. 3

a Representation of the N₂ molecule—Lewis dot structure. Orbital interaction on the transition metal centre via b end-on adsorption and c side-on adsorption. d Heteroatom sp² orbital interaction of the N₂ molecule

As shown in Fig. 3b, c, the unoccupied d-orbitals receive an electron from N₂ to form a σ-bond and donate another occupied d-orbital electron back into the vacant π⃰ antibonding orbital, creating a π-back-donation effect, called the “donor–acceptor” mechanism. The donor–acceptor mechanism on N₂ activates the triple bond and forces it to remain adsorbed in the active site. Similarly, in the case of nonmetals, the vacant sites and the Lewis acid sites with an empty orbital accept an electron from N₂ to form a σ-bond and supply an electron from the neighbouring site into the π⃰ orbital of N_2, promoting the activation of the triple bond, as shown in Fig. 3d. This donor–acceptor mechanism of activating the N₂ triple bond has provided a general idea for designing electrocatalysts with transition metals and nonmetal substrates that exhibit higher affinity towards N₂ adsorption [36, 37].

2.2 Mechanism of N₂ Reduction

Ammonia can be produced by reducing N₂ in an electrochemical cell. Successful electrochemical N₂ reduction may follow an indirect pathway or a direct pathway. The indirect pathway is mediated through nonaqueous aprotic electrolytes and involves forming intermediates. These intermediates are usually the corresponding metal nitrides that can be hydrolysed to produce ammonia. Because of the instability, recyclability, and scalability of nonaqueous electrolytes, indirect ammonia synthesis does not compete with the current demands. However, because of the limited proton content, the indirect approach effectively suppresses the HER and side reactions, which produce higher ammonia yields with higher FE [38,39,40,41,42]. The Li-mediated ammonia synthesis (LiMEAS) pathway has been recognized by researchers and shows promising ammonia yields involving nitrides, but the process is energetically expensive and involves a two-step ammonia production method [40, 41, 43]. The direct pathway can be divided into two categories based on the nature of the electrolyte, viz., nonaqueous and aqueous. Ammonia production in nonaqueous electrolytes uses sacrificial proton donors, such as alcohols or cationic complexes, to protonate the adsorbed N₂ into ammonia [38, 39, 41, 42]. Ammonia production in aqueous electrolytes utilizes water as the proton source, which may be acidic, alkaline, or neutral in nature, for direct ammonia synthesis. The aqueous pathway is the most feasible approach to produce ammonia from atmospheric N₂, protons from water, and electricity from renewable sources.

Direct ammonia synthesis occurs under various pathways, of which dissociative and associative pathways are primarily followed. The dissociative and associative mechanisms are differentiated by either direct dissociation of the N₂ triple bond or by protonation of the adsorbed N₂ molecule, respectively. Furthermore, the associative mechanism is classified into two pathways: associative alternating and associative distal, as shown in Fig. 4. The dissociative pathway involves a large kinetic barrier that consumes high input energy for N₂ triple bond dissociation (∆H = 941 kJ mol⁻¹) [44]. The hydrogenation of the dissociative adsorbed N usually occurs with the H₂ produced from natural gas, producing CO₂ as the byproduct.

Fig. 4

Reaction scheme representing various electrochemical NRR mechanisms: a dissociative, b alternating associative, c distal associative, d and MvK mechanism

The associative pathway reduces the nitrogen molecule into ammonia in six proton-coupled electron transfer (PCET) steps. In the associative pathway, the N‒N bond remains intact until all or some of the hydrogenation process is under the influence of end-on adsorption of dinitrogen to the catalyst surface. Moreover, this associative mechanism is further classified into alternating and distal pathways based on the sequential addition of hydrogen to the activated dinitrogen. In the associative alternating pathway, hydrogenation occurs simultaneously on both nitrogen atoms, and the ammonia molecule is released in the subsequent steps, as shown in Fig. 4b. In the associative distal pathway, the hydrogenation series occurs only on the first nitrogen atom, and only after the release of the first ammonia molecule does protonation of the other nitrogen occur, as shown in Fig. 4c [35].

Apart from these mechanisms, another pathway preferentially occurs on the surface of transition metal nitrides (TMNs) called the Mars–van Krevelen (MvK) mechanism, as shown in Fig. 4d [45]. In this pathway, first, the nitrogen atoms on the surface of the catalyst are protonated to reduce to ammonia (NH₃), leaving behind a vacancy. Later, the vacancy is replenished with externally supplied nitrogen gas and is reduced to ammonia following the distal pathway [46]. In particular cases, the nitrogen atoms from the bulk are transferred to the N-vacant sites and are consecutively reduced to ammonia. This bulk migration of the N atoms to the surface transforms the electrocatalyst, hindering its stability [46].

2.3 Metal-N₂ Battery

Metal-air batteries have had a large impact on the scope of efficient energy storage devices. Inspired by the dual role of metal-CO₂ batteries in gas conversion with simultaneous electricity generation, researchers envision the metal-N₂ battery system for greater utilization of the conversion of atmospheric N₂. Similar to metal-air batteries, the metal-N₂ battery is constructed with a metal anode and NRR electrocatalyst-coated electrode as the cathode [20, 32, 47]. The electrode terms cathode and anode for the electrocatalyst-coated electrode and metal electrode remain the same, irrespective of the charge‒discharge cycles. The mechanism of the metal-N₂ battery with the electrode terms is explained below for further clarification. The redox reaction in a metal-N₂ battery varies with the nature of the electrolyte, viz., nonaqueous and aqueous electrolytes. In a nonaqueous metal-N₂ battery, during discharge, N₂ reduction at the cathode by the corresponding metal ions forms nitrides (Fig. 5a), which can later be hydrolysed to ammonia [20, 29, 47]. During charging, under an external bias, the nitrides at the cathode dissociate into metal ions and gaseous nitrogen, as shown in Fig. 5b [20, 29, 47]. The cathode serves as a conductive substrate as well as a nucleation site for anchoring the discharge products in nonaqueous metal-N₂ batteries. The common nonaqueous batteries fabricated thus far are the Li-N₂ battery, Na-N₂ battery, and Al-N₂ battery [20, 29, 47].

Fig. 5

Schematic of the metal-N₂ battery and its discharge‒charge mechanisms in a, b aprotic electrolytes and c, d alkaline electrolytes

Because of the economic, operational, and maintenance difficulties of using aprotic electrolytes, research has been targeted to design aqueous metal-N₂ batteries [26, 27, 30,31,32, 48, 49]. In aqueous metal-N₂ batteries, the cathode proceeds through the nitrogen reduction reaction (NRR) and oxygen evolution reaction (OER) during discharging and charging, respectively [32]. During discharging, the cathodic reaction is the NRR, which draws electrons and protons from the oxidized metal anode and electrolyte, respectively, forming ammonia (Fig. 5c) [26, 31]. Thus, simultaneous ammonia production and electricity generation can be manifested [49, 50].

During charging, the external bias reduces the oxidized metal, and the direction of electron flow is reversed (Fig. 5d) [32]. Accordingly, the anodic reaction over the cathode evolves oxygen as a result of the OER. The oxidation reaction at the electrocatalyst incorporates electrons into the cathode from the electrolytic hydroxide anions. Thus, in an alkaline metal-N₂ battery, the discharge process at the cathode occurs with the NRR, resulting in ammonia formation, and the charging process at the cathode occurs with the OER.

The NRR during discharge usually follows the associative pathway or the MvK mechanism, as stated previously, depending on the nature of the electrocatalyst. The mechanism of the OER following a 4-electron pathway during charging is stated below [51, 52].

$$^{*} \, + {\text{ OH}}^{ - } \to^{*} {\text{OH}} + {\text{e}}^{ - }$$

(2)

$$^{*} {\text{OH }} + {\text{ OH}}^{ - } \to^{*} {\text{O}} + {\text{e}}^{ - } + {\text{H}}_{{2}} {\text{O}}$$

(3)

$$^{*} {\text{O}} + {\text{OH}}^{ - } \to^{*} {\text{OOH}} + {\text{e}}^{ - }$$

(4)

$$^{*} {\text{OOH}} + {\text{OH}}^{ - } \to^{*} {\text{O}}_{{2}} + {\text{e}}^{ - } + {\text{H}}_{{2}} {\text{O}}$$

(5)

where * represents the surface adsorption site of the bifunctional electrocatalyst at the cathode. Initially, upon charging, the first hydroxide ion is adsorbed on the electrode active site by one electron transfer (Eq. 2). Then, the second and third hydroxide ions react with the adsorbed hydroxide ion to form oxide and oxyhydroxides, which involve the second and third electron transfer steps (Eqs. 3, 4), respectively. Finally, the fourth OH¯ ion reacts with the oxyhydroxide to release water and oxygen molecules into the electrolyte (Eq. 5).

2.4 Key Metrics in Determining NRR Performance

The key parameters for determining the rate of the NRR are ammonia yield and faradaic efficiency (FE). Graphs are drawn for the applied potential (V) vs. the obtained production rate of ammonia and FE, respectively. These parameters determine the activity of the electrocatalyst towards the NRR and its selectivity over the HER.

2.4.1 Calculation of Ammonia (NH₃) Yield

The NH₃ yield rate indicates the amount of ammonia produced per catalyst loading mass/area and unit time, which directly relates to the adsorption (N₂)/desorption (NH₃) properties of the electrocatalyst. Experimentally, the ammonia yield is measured by using chronoamperometry analysis for a period of time and evaluated through Eq. (6).

$${r}_{{\mathrm{NH}}_{3}}=\frac{[\mathrm{N}{{\mathrm{H}}_{4}}^{+}] \times V}{t \times A}$$

(6)

where [NH₄⁺] is the measured concentration of ammonia from UV‒vis spectrometry and the calibration curves of the standard ammonia solution, V is the volume of the electrolyte, t is the reaction time, and A is the electrode surface area [53].

2.4.2 Calculation of Faradaic Efficiency (FE)

Faradaic efficiency (FE) is given by the ratio of the faradaic current used for the NRR to the total current supplied to the electrochemical system. This parameter determines the selectivity of the electrochemical process of the NRR over the competing HER. The FE of any catalyst surface can be obtained using Eq. (7).

$$\mathrm{FE }\left / (\mathrm{\%}\right)= \frac{3F \times {r}_{{\mathrm{NH}}_{3}}}{j}$$

(7)

where 3 is the number of electrons for producing one NH₃ molecule, F is the Faraday constant (F = 96 485.33), and j is the current density measured for the corresponding ammonia yield ${r}_{{\mathrm{NH}}_{3}}$ [53].

2.4.3 Determination of Cell Voltage

Experimentally, the cell potential is calculated by determining the potential difference between the two electrodes (cathode and anode) and is expressed in volts (V), as shown in Eq. (8) [54].

$$E^\circ = E^\circ_{{{\text{cathode}}}} - E^\circ_{{{\text{anode}}}}$$

(8)

2.5 Determination and Analysis Techniques

Detecting and quantifying the produced ammonia, postsynthesis, is essential for reviewing the NRR activity of the electrocatalyst. The detection techniques are performed on the sampled electrolyte, and the methods for ammonia detection must be extremely delicate, dependable, precise, and reproducible. Ammonia quantification must be accurate because of various factors that contribute to overestimating ammonia, such as the time of quantification, the pH of the solution, foreign agents that produce ammonia-containing products other than the catalyst, and the nature of the electrolyte. Initially, ammonia detection is performed on the sampled electrolyte by the UV‒vis spectrophotometric method, involving the colouring agents of –(N=N)–. Later, the quantification of the produced ammonia using ¹⁵N₂ feeding gas was observed to verify the source of nitrogen used in ammonia synthesis [55, 56].

2.5.1 Spectrophotometry Method

The spectrophotometry method, also known as the colourimetric method, is based on the Berthelot reaction, which involves the indophenol blue method to form a dye product with produced ammonia. This product can be observed under UV‒vis spectroscopy to record the absorption spectrum at certain wavelengths corresponding to the abovementioned methods. The indophenol blue method, because of its lower detection limit (0–0.6 mg NH₃–N L⁻¹), better reproducibility, and consistent colour development, is preferred. In this method, the sampled electrolyte is treated with a colouring solution of NaOH containing salicylic acid and sodium citrate. To this solution, a catalyst solution of sodium nitroprusside in DI water and oxidizing solution made of sodium hypochlorite (NaOCl) in NaOH is added [7, 57, 58]. The resulting solution is left undisturbed for an hour, and the UV‒vis absorption spectra are recorded. The spectra reveal an absorption band at approximately 680 nm corresponding to the blue region, indicating the presence of ammonia. The higher the intensity of the absorption peak is, the higher the concentration of ammonia in the sample is. To calculate the concentration of the produced ammonia, concentration–calibration graphs are drawn for the standard solution of ammonia, and peak fitting gives the obtained ammonia concentration.

The peak fitting of the concentration–absorbance graph at the absorption peak maximum gives a linear equation y = mx + c. The straight-line equation is the linear relation between absorbance and the concentration of the standard ammonia solution, as shown in Eq. (9).

$$A = \varepsilon C + b$$

(9)

where A is the absorbance intensity, ε is the molar extinction coefficient (the slope) obtained from the concentration–calibration graph, C is the concentration of the ammonia in the sample, and b is the y-intercept of the concentration–calibration graph. The absorbance values for a particular voltage for an electrocatalyst are applied to the linear equation to calculate the concentration of the produced ammonia in the sample. Irrespective of the method used for ammonia determination, a second procedure for quantifying ammonia is also needed to account for the source of produced ammonia.

2.5.2 Isotope Labelling Method

The isotope labelling procedure is followed to quantify the source of the produced ammonia, which is either N from the substrate or the N₂ feeding gas. The isotope labelling method for ammonia quantification uses ¹⁵N₂ as the feeding gas, whose chemical properties do not greatly differ from those of ¹⁴N₂. NRR experiments under ¹⁵N₂ feeding gas can be used to quantify the source of the produced ammonia. The sampled electrolyte after NRR experiments with ¹⁵N₂ feeding gas must contain only ¹⁵NH₄⁺ ions; therefore, any other contaminants from the catalyst or impurities from the electrolyte can be easily neglected to determine the actual ammonia yield rate due to the NRR. For the ¹⁵N₂ feeding gas experiment, the amount of detected ¹⁵NH₄⁺ must be greater than that of ¹⁴NH₄⁺, and the total ammonia yields for the ¹⁵N₂ and ¹⁴N₂ feeding gases must be similar. The similar ammonia yield results corroborate that no external impurities or degradation of the catalyst contributed to the final ammonia yield [59]. Thus, determining and quantifying the produced ammonia are essential parameters of the NRR, and purified gas must be provided to avoid any contamination and false determination.

3 Electrocatalysts for Ammonia Production

Electrochemical NRR is a possible route for synthesizing ammonia under ambient conditions. The stable nonpolar bonds of N₂ make the molecule chemically inert, which inhibits N₂ bond cleavage in an ambient atmosphere. This problem outlines the need for an electrocatalyst with high selectivity towards N₂ adsorption and sufficient energy to activate the triple bonds. The electrocatalysts must offer a few amenities that promote N₂ adsorption and NH₃ desorption, thereby limiting the HER, which can be fabricated as an electrode in a metal-N₂ battery. The Sabatier principle states that “the binding energy between the catalyst and the reactant must be neither too strong nor too weak.” This rule is the key aspect of designing any electrocatalyst and is of major concern when screening electrocatalytic materials that exhibit sustainability over cost [60].

Activity, selectivity, and stability are the three parameters that determine the performance of an electrocatalyst. For efficient activity, the catalyst must have many available active sites and sufficient pore sizes for the reactants to access the active sites. In selectivity, the material must have surface adsorption potentials that favour the electrochemical NRR by inhibiting the inimical HER. Finally, the catalyst must have a stable structure and tolerate corrosion to sustain long-term operation that eventually reduces the maintenance cost and achieves the efficient use of materials for designing electrocatalysts.

3.1 Screening Metals for NRR Electrocatalysis

An indispensable aspect of electrocatalytic material is selectivity for the valuable NRR over the hindering HER. Theoretically, the best electrocatalysts are transition metals, which were analysed by Skúlason et al. [35] for efficient NRR electrolysis and derived from a volcano plot. The plot sorts out the possibility of using d-metals as catalysts for their preferred activity and selectivity towards the NRR, as shown in Fig. 6a. The top of the diagram indicates that Rh, Ru, and Ir have the highest H adsorption energies, while Pd and Pt are at more negative energies, which indicate prime H adsorption. Moreover, transition metals such as Zr, Sc, Y, and Ti show more negative N adsorption energies, as observed from theoretical studies. Later, researchers used screening procedures to compile and engineer materials suitable for the requirements using simulations and experimental procedures. In addition, Dražević et al. [61] studied the N₂ binding energy parameters from experimental data and the limiting potentials from DFT studies. The study includes the results from Trasatti et al. [62], who calculated the possible HER reactivity of metals and derived a plot to demonstrate the binding energies of the M‒H bond over various metals (Fig. 6b).

Fig. 6

Copyright © 2011, The Royal Society of Chemistry. b Volcano plots for the exchange current density of HER vs. MH bond strengths for different metals; 1 kcal mol⁻¹ = 4 185.85 J mol⁻¹. c Volcano plots for 31 metals for the estimated limiting potentials of the first and fifth PCET vs. E_cell (MN) values. d Extended inset plot of (c) for the elements on the right leg of the volcano plot. Reprinted with permission from Ref. [61]. Copyright © 2020, Elsevier Ltd

a Volcano plot for the reduction of nitrogen on transition metal surfaces through dissociative and associative mechanisms. Reprinted with permission from Ref. [35].

The result of the study of M‒N binding energy was a volcano plot, as shown in Fig. 6c, d, which revealed Mn, Ga, and In as potential NRR candidates and unidentified elements on top of the plot. Combining the experimental HER exchange current densities and the Pourbaix diagrams, the author confirmed the preferable activity of Mn, Ga, and In towards the NRR in water over the HER. However, contrary to previous studies, Fe, Au, Cu, Bi, and Pd required smaller potentials to initiate the HER than the NRR. Similar studies have been performed to evaluate the selectivity for N₂ adsorption for efficient HER suppression and preferential NRR pathways over modified transition metal-incorporated carbon-based electrocatalysts [63, 64]. Thus, an effective screening of various metals helps experimentalists to design an electrocatalyst preferring the NRR over the HER with an appropriate choice of metals.

The electrocatalysts are primarily classified into three categories based on the materials used for their construction, viz., noble metal-based, nonnoble metal-based, and metal-free catalysts. Researchers have developed technologies for screening efficient metal and nonmetal surfaces by theoretical studies that indicate the redox potentials available on the electrocatalyst surface to estimate the catalytic activity for the noxious HER and the expected NRR [46]. To date, a large amount of work has concentrated on designing electrocatalysts to perform in aqueous electrolytes. Nonaqueous electrocatalysts, mostly gas diffusion electrodes, are made of Cu electrodes, stainless steel cloth, Mo foil, bare carbon cloth, and Au-coated carbon fibrous paper (Au/CP) [40, 43, 65,66,67]. They primarily target the adsorption of N_2, and the absence of protons in the electrolyte facilitates increased ammonia yield and higher FE. However, because of the unreliable, non-eco-friendly nature of nonaqueous systems, aqueous-based ammonia production techniques have gained increased attention.

3.2 Noble Metal-Based Catalysts

Various electrocatalysts have been reported for the NRR, including noble metal-based electrocatalysts, such as Pt [68], Pd [69], Au [70], Ru [71], and Rh [72]. Noble-metal SAC electrocatalysts exhibit excellent NRR activity with high NH₃ yield and considerable faradaic efficiency (FE), but they are susceptible to corrosion, and their lack of availability is also a detriment to efficient catalysis [73, 74]. Researchers have utilized noble metals to their fullest potential via strategies that enhance accessibility to metal active sites. Strategies such as designing a firm substrate for metal loadings, doping noble metals with transition metals, single-atom catalysts (SACs) and phase interface structures are followed to minimize the catalyst loading and extract the highest performance from the limited metal availability. To utilize noble metal atoms effectively, Yu et al. [75] adopted the SAC method. They developed a mesoporous Rh film with B- and S-doping on Ni foam (B, S-mRh/NF) to show the efficient NRR catalytic activity of noble metals and the effective utilization of scarce metals. B- and S-doping greatly enhances the catalytic activity by tuning the electronic density of Rh, which increases the N₂ adsorption ability, and S-doping maintains the selective hydrogenation of N₂ to NH₃. The synergistic effects of the noble metal with the heteroatoms for a selective function of the electrochemical NRR and an effective choice of a substrate directly influence the electrocatalytic activity.

Another strategy for the effective use of noble metal catalysts is through alloy engineering of either two noble metal atoms or a transition metal and a noble metal atom (Fig. 7b) [76]. On the basis of the alloy structures, Zhao et al. designed a bimetallic nanoalloy (NA) made of Ru and Rh, forming an fcc crystal lattice (Rh_xRu_1−x NA) [7]. The coupled effects of the Rh-Ru metal atoms induce electronic interactions between metals and the substrate with an ammonia yield of 57.75 μg h⁻¹ mg_cat⁻¹ and an FE of approximately 3.39%.

Fig. 7

Copyright © 2021, Elsevier Ltd. b General strategies for alloy engineering of two noble metals. c Comparison of the ammonia yields and FE of Pd/α-MnO₂, Pd/β-MnO₂ and Pd/γ-MnO₂. Reprinted with permission from Ref. [77]. Copyright © 2020, MDPI. d LSV curves for WO₃ and single atoms and nanoparticle Pt atoms on WO₃ and Pt nanoparticles, respectively, in an Ar-saturated 0.1 M K₂SO₄ (1 M = 1 mol L⁻¹) electrolyte. e Comparison of the ammonia yield and FE of Pt SA/WO₃ and Pt NP/WO₃ for various applied voltages in 0.1 M K₂SO₄. Reprinted with permission from Ref. [68]. Copyright © 2020, John Wiley & Sons, Inc

a Schematic of the electrochemical cell with BS-mRh/NF as the cathode. Reprinted with permission from Ref. [75].

The interfacial activity of noble metals with modified transition metal substrates exhibits considerable NRR performance. Sun et al. [77] reported that the phase interface of Pd/γ-MnO₂ was active for nitrogen reduction to ammonia. Additionally, this study suggested an increased activity of the γ-MnO₂ substrate compared with the α-MnO₂ and β-MnO₂ substrates. This increased activity can be attributed to the combined planar and pyramidal oxygen atoms on the γ-MnO₂ substrate (Fig. 7c) [78]. The Mn atoms initially adsorb N₂ on the surface because of their strong N₂ adsorption ability. Pd atoms then protonate the adsorbed N₂ by the Grotthuss-like proton hopping mechanism. Thus, utilizing the individual properties of different phases can enhance ammonia production because of the modified interfacial properties. Although Pt is a state-of-the-art electrocatalyst for the HER, single Pt atoms have been effectively implanted on suitable substrates to improve NRR performance and suppress the HER. Hao et al. [68] deposited isolated Pt atoms on a WO₃ nanoplate (Pt SAs/WO₃) substrate, which suppressed the HER and improved the NRR activity to deliver higher ammonia yields and FE. The observed NRR performances reveal that the single-atom Pt catalysts (Pt SA/WO₃) exhibit better NRR activity than the nanoparticle Pt catalysts (Pt NP/WO₃), as shown in Fig. 7d, e. Thus, the single-atom catalyst strategy is a worthy candidate for expressing higher yield and FE by noble metal-based NRR electrocatalysts. Thus, the appropriate use of a noble metal and a better substrate choice can enhance the electrocatalytic activity of the metal loadings and the doped heteroatoms, and despite its availability and cost, it delivers outstanding activities.

3.3 Transition Metal-Based Catalysts

Although noble metals are efficient electrocatalysts for the electrochemical NRR, their lack of availability in the Earth’s crust and their high cost are detrimental to the cost-efficient NRR process. Transition metals are available in abundance and facilitate active sites for N₂ adsorption with their partially filled d-orbitals [79]. Transition metal derivatives such as oxides [53], nitrides [80], carbides [81], dichalcogenides [82,83,84], bimetallic alloys [85, 86], MXenes, and MBenes [87, 88], as well as metal–organic frameworks (MOFs) [70] and single-atom catalysts [89], provide a platform for incorporating more accessible active sites, improved reaction kinetics, selectivity for the NRR, and stability. Transition metal-based electrocatalysts adopt various design strategies and morphologies that substantially enhance the NRR.

3.3.1 Metal–Organic Framework Electrocatalysts

Researchers have been obsessed with designing an efficient structure capable of providing large pore sizes for mass transport, as well as an effective conducting path for electrons. In this context, Cong and his group synthesized porphyrin-based single-site MOFs with Fe (Fe-TCPP) nodal sites as the catalytic centres for artificial N₂ fixation (Fig. 8a) [90]. Among the Fe-, Co, and Zn-based TCPP MOFs, the Fe-TCPP MOFs play an important role in N₂ adsorption and conversion. The high adsorption activity of Fe-TCPP can be attributed to the longer Wiberg bond index (WBI), which can more readily activate N₂ than Co-TCPP and Zn-TCPP. The Fe-TCPP electrocatalyst exhibits an FE of nearly 16.23% and a maximum ammonia yield of approximately 44.77 μg h⁻¹ mg⁻¹, as shown in Fig. 8b, c, respectively. This electrocatalyst exhibits a higher FE, which indicates selectivity towards the NRR over the HER on the Fe–N–C active sites.

Fig. 8

Copyright © 2011, The Royal Society of Chemistry. d Illustration of the synthesis scheme of COF_BTC. e Comparison of the ammonia yields and FEs of Ti-COF, Cu-COF, Co-COF, and pristine COF. f PDOS after N₂ adsorption for Ti(3d) and *N₂ (2p) of Ti-COF. Reprinted with permission from Ref. [91]. Copyright © 2022, American Chemical Society

a Schematic of the synthesis route of the Fe-TCPP MOF. b Comparison of the FEs of Fe-TCPP, Co-TCPP, and Zn-TCPP. c Comparison of the ammonia yields of Fe-TCPP, Co-TCPP, and Zn-TCPP. Reprinted with permission from Ref. [90].

Based on the literature suggestions, NRR mediation along with HER suppression can be effective over metal-coordinated N centres (M-N_x) [34]. Jiang et al. [91] reported a pyrolysis-free template strategy for constructing a 2D M-covalent organic framework (COF) using benzene-1,2,4,5-tetracarbonitrile (BTC) and metal chlorides (TiCl₄, CuCl₂, CoCl₂) (Fig. 8d). The N atoms form a linker bond with the metal sites that exhibit increased N₂ adsorption, thereby suppressing the HER. The COF_BTC electrocatalysts exhibit an enhanced NRR performance with higher ammonia yields (Fig. 8e) and FE compared to other framework electrocatalysts.

The partial density of states (PDOS) of Ti-COF after N₂ adsorption show the overlapping orbitals of Ti(3d) and *N₂(2p), indicating increased antibonding states above the Fermi level facilitating the adsorption of N₂ (Fig. 8f). Although a handful of theoretical works implement a COF into electrocatalysts, experimental insights into the same structures are most expected. These structures can possibly meet the requirements of electrode materials for air-battery applications, as well as flexible, rechargeable battery applications [92, 93]. The prominent nonnoble metal-based electrocatalysts are derivatives of transition metal complexes. They are made to tune the intrinsic electronic properties, and they are planted onto a substrate that enhances the electronic conductivity of the electrocatalysts.

3.3.2 Single-Atom Electrocatalysts

Single-atom catalysts as metallic electrocatalysts offer an efficient design strategy for metal species that are well distinguished by their mononuclear active sites.

The SAC strategy is adopted to effectively utilize the metal species and increase the specific activity of the electrocatalyst. SACs anchored on substrates exhibit higher exposure to electrolytic ions, which enhance adsorption towards N₂. Similar to noble metal SACs, transition metals such as Mo [94], Mn [95], and Fe [96] were reported as SACs with mostly M–N–C coordinated sites, which expose more active sites for selective N₂ adsorption.

The M–N–C active sites can efficiently perform heterogeneous catalysis with their isolated single atoms dispersed along the substrate. For instance, Han et al. [94] reported single-atom catalysts (SACs) and nanoclusters (NCs) of Mo atoms embedded on nitrogen-doped porous carbon (SA-Mo/NPC) for the NRR (Fig. 9a-c). When Mo atoms form clusters, the activity is obvious, and their active site density is reduced compared to that of a single Mo atom. The improved activity of a SAC can be discerned through comparison with the NRR performance of its nanocluster counterpart. Additionally, the authors synthesized SA-Co/NPC, and their nanocluster assemblies exhibit superior activity for single-atom Co/NPC but not higher than SA-Mo/NPC.

Fig. 9

Copyright © 2019, John Wiley & Sons, Inc. d Scheme representing the active sites for N adsorption of the Mn–N–C SAC electrocatalyst. Comparison of the e ammonia yields and f faradaic efficiencies of the Mn–N–C single-atom catalyst and NC-nanosheets. Reprinted with permission from Ref. [95]. Copyright © 2021, Springer Nature Ltd

a Illustration of SA-Mo/NPC and the respective atomic structure model. b EDS mapping of SA-Mo/NPC. c Ammonia yield for various Mo loading in SA-Mo/NPC Reprinted with permission from Ref. [94].

Similarly, Wang et al. [95] developed a 2D Mn–N–C SAC via an acid self-assembly strategy that exposes Mn atoms for selective N₂ adsorption and higher active sites (Fig. 9d). The well-exposed Mn–N–C coordinated sites on the 2D matrix reveal a high ammonia yield of approximately 21.43 μg h⁻¹ mg⁻¹_cat and an FE of nearly 32.02% (Fig. 9 e, f). The Mn–N–C coordinated sites show excellent selectivity towards the NRR over the HER, which contributes to the higher ammonia yield and FE. Therefore, studies have concluded the superior activity of SACs in the NRR. Thus, SACs are bound to exhibit excellent NRR electrocatalytic activity with their large active site density and maximized metal utilization [97].

3.3.3 Transition Metal Oxides

Transition metal oxides have ruled the electrocatalytic field for decades and have proven their superiority in almost all redox reactions owing to their defect modulation ability and electron affinity behaviour, which actively tunes the intrinsic electronic configuration of the electrocatalysts [98, 99]. Xiao et al. [100] synthesized nanoporous NiO/MoO₃ by incorporating abundant oxygen vacancies (V_O) into the catalyst for efficient NRR behaviour (Fig. 10a). These bimetal sites with V_O tune the electronic structure for the π-back-donation process, which reduces the hydrogenation character and promotes the NRR (Fig. 10b). This result is due to the improved conductivity, active N₂ adsorption and intrinsic performance of the metals. Thus, incorporating oxygen vacancies into bimetallic complexes can improve the overall NRR performance.

Fig. 10

Copyright © 2021, American Chemical Society c Material characterization studies of the Bi₄V₂O₁₁-CeO₂ interface and elemental distribution. d Scheme representing the interface of Bi₄V₂O₁₁-CeO₂ for the NRR and the electron transfer between Bi₄V₂O₁₁ and CeO₂ structures. Reprinted with permission from Ref. [103]. Copyright © 2018, John Wiley & Sons, Inc. e Illustration of the oxygen vacant sites and anion vacancies of La_xFeO_3−δ perovskite oxides, which are active for the NRR. Reprinted with permission from Ref. [104]. Copyright © 2021, John Wiley & Sons, Inc

a Scheme representing the NRR performance in oxygen-vacant nanoporous NiO/MoO_3. b Illustration of the donor–acceptor mechanism for N₂ adsorption on the nanoporous NiO/MoO₃ electrocatalyst. Reprinted with permission from Ref. [100].

Similarly, Wen et al. [101] synthesized oxygen vacancy-rich Zn-doped Co₃O₄ electrocatalysts derived from ZIF 67 and ZIF 8. The rich oxygen vacancies and the synergistic effects of Zn-doped Co₃O₄ reveal a higher NRR performance compared to pristine Co₃O₄. Additionally, considering the morphology engineering strategy, metal oxides can be incorporated into nanosheets [102], spinel structures [53], etc., to use the active sites. Apart from most crystalline and ordered materials that facilitate the NRR, Yu et al. [103] synthesized an amorphous–crystalline hybrid electrocatalyst that intrinsically includes large oxygen vacancies that act as active sites for nitrogen adsorption. The Bi₄V₂O₁₁-CeO₂ catalyst in the amorphous phase (BVC-A) has a high oxygen content compared to the crystalline counterpart (BVC-C). TEM images and the XRD patterns of BVC-A revealed a definite nanorod morphology with uniform distributions and the presence of partially amorphous structures, respectively (Fig. 10c). The HRTEM and SAED analyses showed the coexistence of the amorphous Bi₄V₂O₁₁ phase and the crystalline CeO₂ phase in the BVC-A samples. Because of the high valence of the Bi⁵⁺ defect, V_O and V⁴⁺ proceed through the π-back-donation process. This activity has not been observed in the crystalline form (BVC-C) and pristine (BVC) conditions. The improved activity indicates that the amorphous form of BVC facilitates more oxygen vacancies, which tunes the band alignment that increases the number of active sites in the material and leads to superior electron conductivity (Fig. 10d).

In addition, Chu et al. [104] reported deficiency-induced oxygen vacancies on La_xFeO_3−δ perovskite oxides with tunable oxygen vacancies, which are proportional to the deficiencies at the La site (Fig. 10e). For L_xF (x = 1, 0.95, 0.9), L_0.9F exhibits improved NRR activities due to the La deficiency and oxygen vacancy. Additionally, L_0.9F exhibited lower electronic resistance (R_ct), and the La d-orbital strongly adsorbed N₂ and the intermediates. L_0.9F eventually exhibits an NH₃ yield rate of 22.1 µg h⁻¹ mg_cat⁻¹ and an FE of 25.6%, which are very high compared to pristine LF. The L_0.9F active sites show limited selectivity towards H⁺ adsorption, which is attributed to the high ammonia yield and FE. Thus, enabling stable structures and increased oxygen vacancies enhances the rate of nitrogen adsorption and the protonation of the adsorbed N₂ molecule, which eventually produces more ammonia with better faradaic efficiency. Therefore, oxygen-vacant electrocatalysts are actively imparted into Zn-N₂ battery applications, where a stable architecture combined with a large active site density could be of actual use in the system [98, 105, 106].

3.3.4 Transition Metal Nitrides

The transition metal nitride, on the other hand, behaves as the unique structure of all the electrocatalysts, in which the catalyst itself participates in the reduction reaction via the MvK mechanism [87, 107]. Nitride surfaces guarantee their inactivity for HER in the MvK mechanism, yet using the TMN catalyst has a few drawbacks [108]. First, the experimental proof for NRR is laborious because the nitrogen atoms from the catalyst surface are reduced to form ammonia, where a quantitative study of the ¹⁵NH₃ formation is mandatory using the ¹⁵N₂ feeding gas. Second, the long-term stability of the nitride catalyst, which can be compromised by surface vacancy replenishment with feeding N₂ gas, occurs only when N vacancies are stable at the surface [46, 79].

Du et al. [109] observed the degradation of tetragonal Nb₄N_5, which produces ammonia simultaneously by reducing the surface nitrogen atoms, an irreversible process. However, the author proves the presence of small amounts of nitrogen in the electrocatalyst and the contribution of the contaminants in the previously reported VN electrocatalysts that show ammonia production with low turnover numbers (Fig. 11a) [110, 111].

Fig. 11

Copyright © 2021, Elsevier Ltd. c Comparison of the NRR activity of the nitrogen vacancy in the W₂N₃ electrocatalyst. d Charge density difference profiles for the NV-W₂N₃ electrocatalyst. e UV absorption profiles over a controlled environment for the NV-W₂N₃ electrocatalyst. f ¹H NMR results of NV-W₂N₃-catalysed ammonia production under ¹⁴N₂ and ¹⁵N₂ feeding gases. Reprinted with permission from Ref. [80]. Copyright © 2019, John Wiley & Sons, Inc. g Scheme representing the one-step synthesis route for γ-Mo₂N/h-BN sheets. h UV absorption profiles for γ-Mo₂N/h-BN control samples. Reprinted with permission from Ref. [112]. Copyright © 2021, Elsevier Ltd

a Schematic of the MvK mechanism on the VN surfaces. b Representation of the NRR activity on the surfaces of bulk VN and VN@NSC. Reprinted with permission from Ref. [49].

In addition, Lv et al. [49] reported a VN nanodot supported on an ultrathin N, S codoped carbon substrate pyrolyzed at 900 °C (VN@NSC), which follows an MvK mechanism that exhibits high activity and offers pH independence for NRR (Fig. 11b). The real-time NRR mechanisms were observed by using in situ FTIR and revealed that the carbon substrate could also participate in NRR by an associative alternating pathway. However, ¹H NMR shows both the doublet (¹⁵NH₄⁺) and triplet peaks (¹⁴NH₄⁺) for the ¹⁵N₂ feeding gas, and the catalyst exhibits cycling stability for over six consecutive cycles for 24 h. The detection of the doublet and triplet peaks of NH₄⁺ indicates the presence of surface-N protonated ammonia and the adsorbed N₂ transformed ammonia.

However, in contrast to the degradation of the electrocatalyst by the MvK mechanism, few reports have suggested the outstanding NRR performance of the electrocatalyst. Moreover, certain M-N_x coordinated active sites reveal lower selectivity for H⁺ adsorption, resulting in a high FE and maximum ammonia yield. Therefore, fabricating metal nitrides is highly anticipated to achieve higher ammonia yields and suppressed HER. Consequently, Jin et al. [80] reported a nitrogen vacancy-rich W₂N₃ (NV-W₂N₃) electrocatalyst, which shows enhanced ammonia production performance by the associative distal pathway. Figure 11c reveals the lower limiting potential for NRR, resulting in a high ammonia yield and FE. In addition, the charge density difference calculations reveal the electronic structures of the NV-W₂N₃ electrocatalyst, which shows that the active adsorption sites of the electrocatalyst are electron-deficient nitrogen vacant sites (Fig. 11d). The UV‒vis spectroscopic studies over the control samples exhibit no absorption peaks other than for the N₂-saturated electrolyte (Fig. 11e). The theoretical studies and the ¹H NMR results clearly explain the stability of the electrocatalyst over ammonia production by the electrocatalytic process and not by surface reconstruction (Fig. 11f). In addition, Yesudoss et al. [112] reported the one-step synthesis of γ-Mo₂N anchored on 2D h-BN sheets (γ-Mo₂N/h-BN), which exhibited a considerable FE of approximately 61.5% (Fig. 11g). The improved activity can be attributed to the interfacial engineering of the cubic γ-Mo₂N on the hexagonal BN sheets incorporated with numerous nitrogen vacancies (Fig. 11h). Thus, tuning the electronic structures of the electrocatalyst by vacancy engineering can modify the electrocatalytic properties that improve the NRR and effectively suppress the HER [113].

3.3.5 Transition Metal Carbides

The design for electrocatalysts is generally based on altering the d-bands of the electrocatalytic materials to increase the adsorption behaviour of the analyte [114,115,116]. Thus, altering the d-bands increases the density of states (DOS) near the Fermi level, which enhances electrocatalytic activities, as in noble metals [114]. Interestingly, transition metal carbides (TMCs) are observed to have similar electronic structures and electrocatalytic activities as noble metal catalysts [115]. Hence, TMCs are engineered as efficient electrocatalysts for NRR. As a result, Zhang et al. [81] reported vanadium carbides (V₈C₇) as NRR electrocatalysts, which reveal anionic vacancies as active sites for efficient N₂ adsorption (Fig. 12a). The tuned electronic structure due to the carbide vacancies enhanced the N₂ reduction performance, with a relatively high ammonia yield of 23.2 μg h⁻¹ mg⁻¹ and an FE of 18.3% (Fig. 12b, c).

Fig. 12

Copyright © 2020, Elsevier Ltd. d Scheme representing the phase change of MoSe₂ from 2H to 1T. e LSV polarization curves of the MoSe₂ electrocatalyst under N₂ and Ar saturation. f Comparison of the FE and ammonia yields of various prepared electrocatalysts. Reprinted with permission from Ref. [83]. Copyright © 2020, The Royal Society of Chemistry. g Illustration of the Bi-Ni bimetallic alloys for N₂ chemisorption. h Bi 4f XPS spectra (the inset: scheme representing ligand effects between Bi and Ni). i Comparison of the ammonia yields of the Bi-Ni prepared electrocatalysts. Reprinted with permission from Ref. [125]. Copyright © 2021, John Wiley & Sons, Inc

a Illustration of the carbide vacancies in the V₈C₇ electrocatalyst. b Ammonia yield and c FE of the V₈C₇ electrocatalysts. Reprinted with permission from Ref. [81].

Similarly, Ramaiyan et al. [117] prepared origami-like molybdenum carbides (Mo₂C) as NRR electrocatalysts. The catalyst revealed a low FE and a considerable ammonia yield due to the surface oxidation of the Mo₂C electrocatalysts. However, Mo₂C exhibited robust stability for 50 h of continuous electrolysis operation with negligible change. Hence, the carbides were reported to exhibit enhanced NRR performances due to the tunable electronic structures and show better electrochemical stability. Interestingly, several reports have used Fe₃C@C [118], Mo₂C/C nanodots [119], and TiC/C [120] as structurally modified NRR electrocatalysts to achieve better NRR performances with robust stability.

3.3.6 Transition Metal Dichalcogenides

Transition metal dichalcogenides (TMDCs), such as sulfides, selenides, and tellurides, have been explored as efficient electrocatalysts for NRR because of their unique electronic structures and high specific surface areas [121]. Several strategies, such as defect engineering and morphological engineering, were followed to increase NRR activity [83, 122]. Transition metal sulfides are the most studied candidates for dichalcogenide materials, which show tunable electronic structures and active sites for improved NRR performance [122]. In addition to sulfides, selenides and tellurides have also been studied as low-cost NRR electrocatalysts that provide considerable NRR performance [121]. As a first report for TMDC as an NRR electrocatalyst, MoS₂ was analysed, which revealed a low FE and ammonia yield due to its inactive edge and basal planes [82]. Notably, the 2D TMDC exhibits poor electrocatalytic performance due to the inactive basal planes and edge sites [122]. Hence, Zhao et al. [123] designed VS₂ electrocatalysts by shearing the sulfur edges by a defect engineering strategy, which increased the N₂ adsorption towards V metal sites. This strategy has modified the electronic structure, which enhances the DOS near the Fermi level of VS_2, improving ammonia productivity. Morphology engineering strategies have an important role in enhancing the active sites and tuning the active phase for redox reactions in an electrocatalyst. For instance, Zhang et al. [83] reported MoSe₂ quantum dots with an effective ultrathin morphology to achieve better NRR performance after a phase change from the 2H to 1T phase (Fig. 12d). The 1T-MoSe₂ electrocatalyst exhibits better N₂ reduction performance than the 2H phase and other prepared electrocatalysts (Fig. 12e, f). The ultrasmall layers of MoSe₂ QDs exhibit higher electrocatalytic active sites with abundant defects and edges, which tune the electronic structure for selective N₂ adsorption. Therefore, based on previous reports and studies, TMDCs can be effectively tuned by morphology, defect, and dopant engineering strategies to alter their electronic structure and achieve higher NRR performances.

3.3.7 Heterogeneous Alloy Electrocatalysts

Recently, heterogeneous alloys have been reported to achieve higher NRR performances due to the synergistic activity of transition metals. Heterogeneous alloys can enhance electrocatalytic activities by tailoring structural, compositional, and electronic effects [124]. Recently, Fang et al. [125] reported that amorphous Bi-Ni bimetallic alloys promote N₂ adsorption and activation by synergistic effects between p-block Bi and d-block Ni elements (Fig. 12g). The ligand effect between Ni and Bi imposes lower limiting potentials for N₂ chemisorption and ammonia formation (Fig. 12h). Moreover, Bi-Ni bimetallic alloys in the amorphous phase exhibit a relatively high ammonia yield rate and high surface area for electrocatalytic reactions compared to the other prepared electrocatalysts (Fig. 12i). Similarly, Li et al. [126] reported Co₃Fe-MOFs as efficient NRR electrocatalysts because of the coupling effects of Fe and Co atoms. In addition, bimetallic alloys were also supported on N-doped carbon substrates, which enhanced the NRR electrocatalytic activity and conductivity [86]. Therefore, heterogeneous alloys with tunable structural, compositional, and electronic properties are inspired as efficient NRR electrocatalysts.

3.3.8 Main Group Metal-Based Electrocatalysts

Other than the abovementioned electrocatalysts, researchers have explored the main group metals that hold potential abilities to suppress the HER and enhance NRR performance. The main group metals, viz., s block (Li, K) and p block (Al, Ga, Sn, In, Bi, Sb)-based electrocatalysts, are effective in suppressing the HER due to the limited availability of electrons in their valence orbitals [127], unlike transition metals. For instance, Sebastian et al. [128] reported K₃Ti₈O₁₇ nanorods as NRR electrocatalysts, which achieved a higher ammonia yield and considerable FE (Fig. 13a). The repulsive effect of K⁺ ions on H⁺ ions reduces proton adsorption over the electrocatalyst, which suppresses the HER. Similar studies involving the Li incorporation strategy in electrocatalysts have shown relatively higher ammonia selectivity over the HER [129, 130].

Fig. 13

Copyright © 2022, The Royal Society of Chemistry. b Schematic of N₂ adsorption over 2D mosaic Bi NS electrocatalysts. c Comparison of the ammonia yield and FE of the prepared Bi NS and Bi NP electrocatalysts. Reprinted with permission from Ref. [132]. Copyright © 2019, American Chemical Society. d Comparison of the ammonia production of Ti₃C₂-MNS and Ti₃C₂-MNR. Reprinted with permission from Ref. [134]. Copyright © 2020, John Wiley & Sons, Inc. e Comparison of the ammonia yields of the MnO₂-supported Ti₃C₂T_x and pristine samples. f Distributed charge density diagram of the MnO₂-Ti₃C₂T_x heterostructure adsorbing a N₂ molecule. Reprinted with permission from Ref. [135]. Copyright © 2019, The Royal Society of Chemistry. g Schematic of the MBene structures over various transition metals for the electrocatalytic NRR. Reprinted with permission from Ref. [137]. Copyright © 2020, The Royal Society of Chemistry

a Comparison of the ammonia yields of prepared K₃Ti₈O₁₇ electrocatalysts. Reprinted with permission from Ref. [128].

Among the p-block elements, Bi exhibits poor HER performance, which reveals the efficient electrocatalytic property of Bi-based NRR electrocatalysts [131]. Li et al. [132] fabricated 2D mosaic Bi nanosheets with a high surface area and optimum electrocatalytic activities (Fig. 13b). The reduced Bi-Bi interlayer distance and the electronic structure of 2D mosaic nanosheets along with the delocalized electrons in the p-orbital of Bi atoms near the edge sites impose abundant active sites for N₂ adsorption and activation. The 2D Bi-NS reveal a high ammonia yield and FE compared to the Bi nanoparticles (Fig. 13c).

In addition, a comparison study of the metal tellurides MeTe_x (Me = Sb, Co, Cd, Bi) revealed SbTe₃ hexagonal plates as efficient NRR electrocatalysts that exhibit a high ammonia yield rate of approximately 13.8 μg h⁻¹ cm⁻² and 27.7% FE [84]. The higher NRR performance can be ascribed to the selectivity of the electrocatalyst for N₂ adsorption over HER and a higher DOS near the Fermi level. Theoretical studies reveal lower H binding energies for Ga, Sn, In, and Bi, which can be potential elements for NRR electrocatalysts [61]. Several main group element-based electrocatalysts have been widely studied, revealing promising ammonia yields and FEs by inhibiting the inimical HER.

3.3.9 MXenes and MBenes

On the other hand, 2D MXenes are widely studied as a part of transition metal nitrides and carbides, as their structure is represented as M_n+1X_nT_x (n = 1, 2, 3). M shows the n + 1 layers of transition metals, X represents the n-interleaved carbon (C) or nitrogen (N) elements with transition metals, and T_x represents the terminal molecules on the MX surface (O, OH, F and/or Cl) that are bonded to other M layers [133]. Wei et al. [134] synthesized Ti₃C₂ MXene nanosheets (Ti₃C₂-MNS) and nanoribbons (Ti₃C₂-MNR) and observed a skyrocketing ammonia yield for Ti₃C₂-MNR, which is 5 times that of Ti₃C₂-MNS (Fig. 13d). The FE was also observed to increase by 2%, and a stable current density was observed for all the samples. Thus, a structural transformation from a 2D MXene to a 3D MXene has been proven to increase the catalytic active sites that eventually improve the NRR. In addition, Kong et al. [135] reported a sandwich-like structure that separates the Ti₃C₃T_x layers and forms a hybrid heterostructure material for NRR. Moreover, the heterostructure formation of MXene with MnO₂ (MnO₂-Ti₃C₂T_x) is found to exhibit enhanced ammonia yield performance. With Mn being the adsorption site for N₂ adsorption and MXenes being the electron conductive region, ammonia production can be substantially increased. The NRR activity of the MnO₂-Ti₃C₂T_x hybrid structures can be verified by control experiments (Fig. 13e) and deformed charge density plots (Fig. 13f).

MBenes are the boride forms of MXenes that possess the same monolayer-heterolayer structures that form stable architectures with high thermal and electrical conductivities. They are already used in the field of energy storage as anodes for Li/Na ions [136]. However, experimental methods for MBenes have yet to be explored in the case of NRR. Theoretical studies have been satisfying for the application of NRR, where Yang et al. [137] considered 10 compositions of MBenes and computed to report the excellent stability of the catalysts in aqueous solutions that show high NRR preference over the inimical HER (Fig. 13 g). Fu et al. [138] studied MoAlB as an NRR electrocatalyst, which revealed a high selectivity for N₂ adsorption due to the stronger bonding interactions between the p-orbitals of Al and nitrogen absorbers. The partially occupied p-orbitals, the ability of the surface B sites to adapt electrons and the N₂ adsorption properties make MBenes a viable choice for the NRR electrocatalyst. Thus, TMB can be considered an efficient option for NRR electrolysis that can be modulated for intrinsic electronic properties and stable structure formations that can be used in energy storage and conversion applications.

3.3.10 Other Transition Metal-Based Electrocatalysts

Other transition metal-derived NRR electrocatalysts include transition metal fluorides and transition metal phosphides. Transition metal fluorides (TMFs) have been studied as electrocatalysts that are observed to alter the d-bands of transition metals to achieve better performances in specified applications [139]. Therefore, Li et al. [140] studied lanthanum fluoride (LaF₃) as an NRR electrocatalyst that inhibits the HER compared to La₂O₃. The 2D platelet-like structures reveal a high surface area for N₂ adsorption and activation, with a high ammonia yield of approximately 55.9 μg h⁻¹ mg_cat⁻¹ in 0.5 M LiClO₄ with a considerable 16% FE. The fluorides are studied to express excellent durability and cycling performances. In addition, transition metal phosphides (TMP) are also studied as electrocatalysts for NRR because of their high conductivity, controllable elements, and unique physicochemical properties. P bonding to transition metals regulates the internal electronic structure of the active sites to improve NRR catalytic activity [141]. Guo et al. [142] studied cobalt phosphide hollow nanocages (CoP HNC) as NRR electrocatalysts. The CoP HNC exhibits a lower kinetic barrier for the HER but shows higher selectivity for ammonia, inhibiting hydrazine formation.

3.4 Metal-Free Electrocatalysts

The design of electrocatalysts based on metal-free materials has reduced the production cost of electrocatalysts, supplemented tunable pore sizes, and enhanced surface adsorption properties. Metal-free electrocatalysts increase selectivity for NRR [143], improve mass transport [144] and electronic conductivity [145, 146]. Several design strategies, including heteroatom dopant engineering and vacancy engineering methods, and utilizing various metal-free materials, such as graphitic-carbon nitride (g-C₃N₄), graphene, boron nitride (BN), black phosphorous (BP), and organic frameworks (MOF/COFs), have been successful in achieving efficient NRR electrocatalysts. Dopant engineering involves the doping of heteroatoms (B, N, S, P) into carbonaceous materials, which modulate the electronic structures of the materials to achieve higher NRR performances [147,148,149,150]. Boron-based electrocatalysts have been found to exhibit enhanced NRR performance due to their electron deficiency nature and inactivity towards the HER [151].

As an example of a doping strategy, Yu et al. [148] reported boron-doped graphene (BC₃) as an NRR electrocatalyst that exhibits excellent NRR properties at the B site, enhancing the N₂ binding capability (Fig. 14a). The HOMO–LUMO plots and the charge density difference diagrams reveal the charge-deficient regions for possible N adsorption sites (Fig. 14b, c). Hence, this work highlights the potential ability of the boron-doped metal-free matrix for efficient NRR. The carbon framework provides structural stability and enhanced electron conductivity, as well as mass transport, which provides the added advantage of choosing a framework for electrocatalytic applications. Similarly, Song et al. [149] engineered N, P-codoped MOF-5 carbon (NP-C-MOF-5), exhibiting an ammonia yield of nearly 1.08 μg h⁻¹ mg_cat⁻¹ with considerable evolution of N₂H₄. Likewise, Tian et al. [150] reported N, S codoped graphene (NSG) as a durable NRR electrocatalyst that exhibited a high ammonia yield of nearly 7.7 μg h⁻¹ mg⁻¹ with a high selectivity towards NRR. Heteroatom doping in carbonaceous materials has been studied to generate charge distributions in the carbon matrix that enhance electron conductivity [146], tune the selectivity towards NRR [152], promote N₂ adsorption [153], and create more defective sites for effective NRR performance.

Fig. 14

Copyright © 2018, Elsevier Ltd. d Representation of the S dopant position in the graphene sheets for NRR. Reprinted with permission from Ref. [164]. Copyright © 2020, Elsevier Ltd. e Illustration of exfoliated black phosphorous sheets with different charge densities around various positions. Reprinted with permission from Ref. [155]. Copyright © 2019, John Wiley & Sons, Inc. f Representation of boron-doped black phosphorous nanosheets showing the NRR mechanism. Reprinted with permission from Ref. [151]. Copyright © 2019, Royal Society of Chemistry. g Illustration of the synthesis route for Eex-COF in a N₂ atmosphere. h Modified charge distribution on the COF substrate after electrochemical excitation. Reprinted with permission from Ref. [145]. Copyright © 2019, Springer Nature Ltd

a Illustration of NRR on boron-doped carbon graphene sheets with N₂ adsorption activity. b Diagram representing the HOMO–LUMO of BC₃ sheets. c Modified charge density distribution on the BC₃ nanosheets. Reprinted with permission from Ref. [148].

As metal-free noncarbonaceous NRR electrocatalysts, boron nitride (BN) has been anticipated to achieve better selectivity towards NRR, and its layered structure provides highly accessible active sites. For instance, Zhao et al. [154] designed mesoporous boron nitride (MBN) electrocatalysts that show a large surface area and highly accessible active sites due to their mesoporous structure. Compared to the bulk BN, a high ammonia yield can be observed in the MBN electrocatalysts. In addition, black phosphorous has also been observed to exhibit excellent NRR activity due to its anisotropic structure and abundant edge sites that enhance ammonia productivity. Zhang et al. [155] reported exfoliated black phosphorous (FL-BP) nanosheets that were observed to produce high ammonia yields and FE on the zigzag edges of FL-BP (Fig. 14e, f).

Similar to MOFs, Liu et al. [145] reported a facile synthesis procedure for 2D boron-rich COF materials that are electrochemically excited (Eex-COF/NC) to exhibit better NRR performances and suppress HER over the B-active sites (Fig. 14g, h). The COF structures linked by boroxine rings (B₃O₃), boronate esters (C₂O₂B), amide and imide connecting structures are addressed to be Lewis acid sites that can efficiently adsorb a weak Lewis base, such as the N₂ molecule. Importantly, these framework structures exhibit poor electron conductivities despite the boron rings that actuate electrochemical NRR. To overcome these difficulties, the authors embedded these COFs on a nitrogen-doped carbon template to offer external electron conductivity and a stable substrate. Thus, metal-free electrocatalysts play a crucial role in NRR electrocatalysis by inhibiting the HER by electronic modifications in a carbon matrix and incorporating more accessible active sites, increasing the specific activity.

3.5 Bifunctional Electrocatalysts for NRR and OER

In the case of aqueous metal-N₂ batteries, the bifunctional NRR and OER performances during discharging and charging reactions become an essential aspect [32]. The electrocatalyst for NRR must exhibit a high ammonia yield rate and suppressed HER for a higher FE. Similarly, for the OER, the electrocatalyst must exhibit a low overpotential and faster reaction kinetics for effective performance [116]. Previous studies show better OER performances with low overpotentials and lower Tafel slope values and reveal improved NRR performances of the electrocatalyst. For instance, Li et al. [126] synthesized Co_xFe-MOF as an effective NRR and OER electrocatalyst because of the presence of abundant active sites and the positive synergistic effect of Co-Fe metals. Co_xFe-MOF exhibits a very low Tafel slope of 38 mV dec⁻¹ and a lower overpotential of 280 mV @ 10 mA cm⁻² for the OER. The electrocatalysts also reveal a high ammonia yield and FE of 25.64% at a very low potential (− 0.2 V vs. RHE), which hold prospects for a metal-N₂ battery. Similarly, Sun et al. [156] designed 2D NiFe nanomesh arrays on nickel foam, which imposes the combined effects of Ni–Fe and the hierarchical porosity from arranged nanolayers for higher OER/NRR performance. The electrocatalyst exhibits an even lower OER overpotential of approximately 191 mV and a considerably lower Tafel slope of approximately 43 mV dec⁻¹. The electrocatalyst delivers better NRR performance with a considerable yield and an FE of approximately 9.8%. Therefore, the electrocatalysts must hold efficient electrocatalytic activity for NRR and OER, which reduces the required charging potential and increases the discharge cell voltage, respectively (Table 1).

Table 1 Comparison of the NRR performances of the previously reported electrocatalysts

Thus, electrocatalysts can be designed with various strategies that effectively promote the NRR and suppress the HER. Electrocatalysts for nonaqueous-based systems have not attracted interest because of the high maintenance and instability of electrocatalysts in nonaqueous media. However, research on electrocatalysts has been targeted for aqueous-based systems for which the kinetics has been sluggish but shows promising results. Moreover, the bifunctional performances of the electrocatalysts are highly anticipated to achieve efficient rechargeability of aqueous metal-N₂ batteries. Metal-N₂ batteries can be fabricated with electrocatalysts having a stable architecture, increased active site density, large surface areas, selective adsorption towards N₂, and prominent HER mitigation.

4 Electrolytes for NRR

Electrocatalysts are the most studied entity in the NRR process. However, electrolytes form the environment for efficient NRR electrocatalysis, which plays a major role in determining the better performance of the electrocatalyst and the metal-N₂ battery. They are found to actively inhibit the progress of the HER and supply protons to the surface of the electrocatalyst [14]. Designing the appropriate electrolyte for better NRR performance is as essential as designing the electrocatalyst because the kinetic regulation of the protons and nitrogen is processed by the electrolytes. One important property of the electrolyte is the solubility of N₂ for feasible transport to the active site [169]. The pH of the electrolyte, the nature of the electrolyte and the counterions determine nitrogen and proton migration to the surface of the electrode. Researchers have presented various electrolytic strategies to overcome the HER through the electrolyte configuration by limiting proton availability and proton migration and creating a swift pathway for the N₂ species. From the electrolyte perspective, N₂ electroreduction can be classified into two major categories: the direct pathway and the mediated pathway. The direct pathway producing ammonia directly from NRR experiments is again classified as nonaqueous and aqueous pathways. The mediated pathway involves intermediate formation and is obviously nonaqueous. The intermediates are usually nitrides, which can be further hydrolyzed to produce ammonia.

Aqueous electrolytes are generally termed water-based electrolytes that contain deionized H₂O as the solvent and are applied to electrochemical cells for redox reactions. They are further classified as acidic (0.1 M HCl, 0.1 M H₂SO₄), alkaline (0.1 M KOH, 0.1 M NaOH) and neutral (PBS, 0.1 M Na₂SO₄) electrolytes that exhibit the activity of their own kind. The deviation of the pH values will be larger near the electrode surface than in the bulk of the electrolyte because of the protonation of N₂ that affects the yield rate and FE. NRR in acidic electrolytes is slightly sluggish even with the best electrocatalyst, and HER occurs feasibly and rapidly in the presence of hydronium (H₃O⁺) ions, which are abundant in aqueous acidic electrolytes [170]. The DFT studies suggest that Mo atoms in MoS₂ hold a more positive charge that provides sites for nitrogen adsorption and are more susceptible to NRR, but they exhibit a low ammonia yield and a minimum faradaic efficiency of 0.096% in 0.1 M HCl [82]. This result is ascribed to the strong hydrogen production in the acidic medium that is inferior to that of the 0.1 M Na₂SO₄ electrolyte. However, very few reports suggest increased ammonia production over the HER in acidic electrolytes. For example, Liang et al. [171] improvised MoS₂ sheets through F-doped strain engineering, which suppressed the HER and enhanced the NRR selectivity (Fig. 15a). The smaller size and great electronegativity of the F atom create strain in the MoS₂ lattice, making it defective, exhibiting an FE of approximately 20.6% in 0.05 M H₂SO₄ electrolyte. However, acidic electrolytes with lower pH values, i.e., increased H⁺ concentrations, surprisingly facilitate NRR over HER.

Fig. 15

Copyright © 2019, Royal Society of Chemistry. b Comparison of the ammonia production of the C-ZIF-1000–1 h catalyst under various electrolytes. c Ammonia production performance in acidic and alkaline electrolytes at various temperatures. Reprinted with permission from Ref. [172]. Copyright © 2020, Elsevier Ltd

a Scheme representing strain engineering by F doping of the MoS₂ structure for higher NRR performance in acidic electrolytes. Reprinted with permission from Ref. [171].

Despite acidic electrolytes being the prime proton donors for the HER in aqueous solution, they can catalyse ammonia production in a controlled environment. However, the long-term stability of electrocatalysts in an acidic electrolyte poses a serious threat. Additionally, the evolution of hydrogen from the excess protons can lead to a higher HER than the expected NRR. Alkaline electrolytes tackling all the abovementioned issues of proton migration are found to suppress the intrinsic HER.

Alkaline electrolytes are the most preferred medium for most nitrogen reduction experiments. They are inexpensive, easy to handle, do not corrode the electrochemical cell and provide sufficient H⁺ ions for protonation. Mukherjee et al. [172] studied ZIF-8 frameworks for electrocatalytic NRR for various pyrolysis temperatures, varied concentrations of Fe doping and different electrolytes (0.1 M KOH, 0.1 M NaOH and 0.1 M HCl). For the C-ZIF-1000–1 h catalyst, the electrolytes KOH and NaOH with varied cations are examined, which indicates a higher ammonia yield (Fig. 15b, c). Including the concentration effects, it is observed that the NaOH electrolyte is always one order behind the KOH electrolyte, indicating feasible K⁺ ion transport in the electrolyte, suggesting favourable NRR kinetics in the KOH electrolyte.

In addition to acidic and alkaline electrolytes, neutral electrolytes play an efficient role in electrochemical N₂ reduction. Notably, phosphate buffer saline (PBS) electrolytes have found importance in the NRR by suppressing the HER and exhibiting better catalytic activity than NaOH and H₂SO₄ electrolytes. For example, Wei et al. [15] extensively studied the flow reactor for ammonia synthesis and electrolytes of varied pH (H₂SO₄, K₂SO₄, PBS and KOH) (Fig. 16a). The study finds effective HER suppression in the neutral electrolyte initially, followed by the alkaline electrolyte (PBS > K₂SO₄ > KOH > H₂SO₄) (Fig. 16b, c). The buffer effects of H₂SO₄/PBS, where PBS is a neutral medium, effectively reduce the proton concentration, which increases the pH of the electrolyte. Hence, the electrocatalytic surfaces are limited to adsorbing protons, and relatively high N₂ species are adsorbed on the active sites. However, protonation of the adsorbed nitrogen is essential for forming ammonia, where the limited proton availability enhances the ammonia production rate and suppresses the HER. Therefore, increasing the pH of the electrolyte suppresses the HER, and the limited proton availability is facilitated to enhance the NRR. Additionally, the PTFE (polytetrafluoroethylene)-coated gas diffusion layer (GDL) prevents the electrode from becoming wettable, which increases N₂ diffusion and inhibits hydrogen production in the flow cell (Fig. 16d, e).

Fig. 16

Copyright © 2021, American Chemical Society

a Scheme representing ammonia production on the GDL embedded with Ru NP under a flow reactor. b Comparison of the cell potential of the flow reactor for NRR under various electrolytes. c Comparison of the ammonia yields and FE under various electrolytes. d Illustration of the synthesis route of Ru/CB NPs and their fabrication onto the GDL. e Scheme representing ammonia production in the GDL embedded with Ru NPs in a flow reactor. Reprinted with permission from Ref. [15].

In addition to the reported electrolytes, a noteworthy strategy is used to suppress the HER by limiting proton migration and improving feasible N₂ movement into the electrolyte. This novel scenario involves using an additive in the electrolyte that effectively limits proton migration into the electrolyte.

The molecular crowding effect observed in living cells restricts water activity by hydrogen bonding interactions between water molecules and long-chain macromolecules (Fig. 17a, b) [173,174,175,176]. Inspired by this mechanism of restricting water molecule mobility, Guo et al. [16] utilized PEG400 (polyethylene glycol), a long-chain polymer, to effectively suppress the ion flow in the aqueous electrolyte by hydrogen bonds. PEG additives increase the electrochemical stability window for water electrolysis inhibiting the HER. Because of the suppression in ion flow, the electrolyte supplies sufficient protons to protonate the adsorbed N_2, which effectively inhibits the HER, as represented in Fig. 17c. Consequently, an increase in the solution resistance (Fig. 17d) and a decrease in the ionic conductivity (Fig. 17e) of the electrolyte were observed with increasing PEG400 concentration. Therefore, an optimum 20 wt% (wt% means the weight percentage) PEG400 concentration was adopted to achieve a higher FE of approximately 32.13% in acidic electrolytes, indicating excellent HER suppression performance. Similar effects of lower conductivity and high solution resistance with an increase in concentration and longer chains of PEG are observed in previous studies [177]. In addition, the neutral and basic solutions reveal similar activity under PEG400 additive concentrations. Hence, the presence of additives in the electrolyte to suppress HER decreases the conductivity of the electrolyte but manages to improve the NRR performance.

Fig. 17

Copyright © 2021, John Wiley & Sons, Inc

a Representation of the hydronium ions in the acidic electrolyte. b PEG-400 showing the interactive sites for hydronium ion interaction through hydrogen bonding. c Scheme showing the water molecule interaction with PEG-400 under acidic and alkaline electrolytes and free N₂ mobility. d Nyquist plots for the PEG400-containing electrolytes. e Plots depicting the variation in conductivity and Tafel slopes with increasing PEG400 concentration in the electrolyte. Reprinted with permission from Ref. [16].

One important effect that determines the electrolytic effect in NRR is the effect of counter ions in the medium. For instance, Song et al. [178] examined the effects of counter ions (Li⁺) in the electrolyte with the corresponding perchlorate solutions (LiClO₄) diluted in deionized water. The ions form a Stern layer across the electrode that effectively mediates N₂ adsorption and repels H⁺ from the electrode. The study included other perchlorate solutions as a control experiment with Na⁺ and K⁺ ions from their perchlorate solutions (NaClO₄ and KClO₄). Higher ammonia production rates and faradaic efficiency can be observed from the Li⁺ ion-mediated electrolyte due to the steric effects between the counter ions and N₂ (Fig. 18a, b). Additionally, the rates are found to decrease with an increasing ionic radius (Li⁺ < Na⁺ < K⁺). Thus, Li⁺ counterions in the electrolyte enrich the electrodes electrostatically, which increases the N₂ concentration on the electrode surface compared to that in the electrolyte medium.

Fig. 18

Copyright © 2018, American Association for the Advancement of Science. c Schematic of LiMEAS on Cu electrode surfaces and the mechanism of Li nitridation. d Diagram showing the Li cycling for LiMEAS in the electrolyte mixture of LiBF₄, THF and EtOH. Reprinted with permission from Ref. [43]. Copyright © 2019, Elsevier Ltd

NRR performances under the influence of Li⁺, Na⁺_, and K⁺ counterions. a ammonia yields and b faradaic efficiency. Reprinted with permission from Ref. [178].

Nonaqueous electrolytes can be employed to effectively suppress the HER and increase N₂ solubility in an electrolytic medium. Recently, organic and ionic liquids have attracted interest owing to the steric effects of the ions and the N₂ gas in the electrolyte. A novel strategy for suppressing HER with higher ammonia production is by adopting the Li-mediated electrochemical ammonia synthesis (LiMEAS) technique under organic electrolytes with proton sources for ammonia production [40, 41, 179]. Lazouski et al. [43] reported the LiMEAS procedure with lithium tetrafluoroborate (LiBF₄) mixed with sieve-dried THF and varying EtOH concentrations as the proton source (Fig. 18c, d). With Li counterions in the electrolyte, faster reaction kinetics and feasible product (Li₃N) formation are observed, which upon protonation with sufficient ethanol concentrations is converted to ammonia. In addition, the author calculated the effects of nitrogen and proton carrier concentration on the formation of ammonia. It is also found that even smaller amounts of water in the electrolyte drastically affect ammonia formation and produce hydrogen at a very fast rate. Thus, an organic liquid (THF) is used as the electrolyte, and its stability in LiMEAS must be analysed. Hence, Sažinas et al. [180] analysed three organic liquids (DG, THF, DME) and reported the compatibility of THF, which is applicable to the novel LiMEAS approach. In nonaqueous conditions, proton sources are necessary to protonate the adsorbed N species for ammonia production. Because of the feasible H-donating property of ethanol, it is widely used as a sacrificial proton source in LiMEAS to form LiOEt [43]. Other than ethanol, ammonia production in LiMEAS is facilitated by using tetraalkyl phosphonium salt, which undergoes cycles of deprotonation–reprotonation, additionally improving the ionic conductivity [181]. Further, the hydrogen oxidation reaction (HOR) is used as the proton source to produce ammonia from the LiMEAS procedure with high yield and FE [182]. Therefore, sacrificial proton sources and H₂ oxidation reactions are adopted to protonate the adsorbed N_2, resulting in ammonia formation through LiMEAS. However, the stability of the ionic electrolytes, controlled environment and frequent maintenance pose a serious threat and involve high cost and labour to stabilize the system for practical use. Thus, metal-N₂ batteries can be envisioned to work more efficiently under low maintenance in aqueous electrolytes than under ionic electrolytes.

5 Summary of the Strategies for Improving Ammonia Productivity

The major aspects to achieve higher ammonia productivity are (i) increasing the available number of active sites, (ii) suppressing the HER, and (iii) improving the electrochemical characteristics of the electrocatalysts. A schematic representation of the strategies for improving NRR performance in aqueous environments by electrocatalyst and electrolyte engineering is presented in Fig. 19.

Fig. 19

Schematic representation of the general strategies for increasing NRR performance in aqueous environments

First, improving the number of active sites is achieved through electrocatalyst engineering by tailoring the structural, morphological, and electronic effects of various metals. Modifications include defect engineering [81], morphological engineering [161], dopant engineering [75], strain engineering [171], heterostructure formation [183], alloy engineering [125], and crystal facet regulation [184], which improve the structural and electronic effects in electrocatalysts, achieving superior performances. Structural and morphological engineering incorporates highly accessible active sites and the stability of the structures. Electronic modifications alter the conduction and valence bands of metals near the Fermi level, resulting in higher activity and selectivity towards NRR [185]. Studies have shown promising improvement in ammonia yield by tuning the electrocatalyst for a higher number of active sites.

Second, competitive side reactions such as the HER must be preferably suppressed to increase the selectivity for N₂ adsorption, resulting in higher ammonia yields and FE. HER can be effectively suppressed by electrocatalyst modification by appropriately tuning the electronic structures of the active sites [37], adopting early transition metal-based electrocatalysts [35], and main group metal-based electrocatalysts [127]. Additionally, the HER can be suppressed by a reliable electrolyte engineering strategy. Reducing the proton concentration [15], resisting proton migration [16], and increasing the potential window for electrolysis [17] are proven to be effective strategies for inhibiting the HER in aqueous electrolytes. Adopting nonaqueous electrolytes with various proton sources is also an effective route to generate better NRR performance and inhibit the HER [43].

Third, the intrinsic electrochemical characteristics, such as conductivity and electrochemical active surface area (ECSA), of the electrocatalyst are improved [186, 187]. The electrocatalyst must exhibit lower charge transfer resistances and electrode–electrolyte resistances to achieve faster electronic conductivity and faster reaction kinetics. The electrocatalysts are incorporated on a N-doped carbon matrix, which can be designed by several methods to achieve faster conductivity due to the highly polarized electronically tuned substrates [188]. Moreover, increasing the ECSA of the electrocatalyst incorporates higher exposure of active sites for N₂ adsorption and electrolytic ions. Hence, the abovementioned strategies can be followed to achieve higher ammonia yield and FE and to exhibit better electrochemical performances for the electrocatalyst.

Nevertheless, there is a high risk of overestimating or underestimating the ammonia yield due to the low sensitivity of detection methods, the nitrogenous contents of the electrode and the presence of impurities in the electrolyte, membrane/separators and analyte gas contents [59]. Proper purification procedures and careful observation of the contents of electrocatalysts and electrolytes are mandatory while performing NRR studies.

6 Development of Metal-N₂ Batteries

The abovementioned, recently reported electrocatalysts are observed to exhibit outstanding activity, selective adsorption towards N₂ and considerable stability towards NRR. These features of electrocatalysts with different morphologies and varied spans of activity can be incorporated into a metal N₂ battery system to provide overall battery performance. Different metal-N₂ battery systems with various anode materials (Li, Na, Al, Zn) have been reported, and their performances are analysed. Metal N₂ batteries can be differentiated based on their mechanisms in nonaqueous and aqueous electrolytes. The Li-N₂ battery, Na-N₂ battery and Al-N₂ battery have been demonstrated under nonaqueous electrolytes, which form nitrides at the cathode. The Zn-N₂ battery and Al-N₂ battery have been demonstrated under aqueous alkaline electrolytes, which follow the NRR as the cathodic reaction during discharge. Various metal-N₂ batteries and their reaction schemes in nonaqueous and aqueous environments are explained in the following sections.

6.1 Nonaqueous Metal-N₂ Batteries

Nonaqueous metal-N₂ batteries perform under aprotic electrolytes, which form nitrides as intermediates at the cathode. Ammonia production involves a two-step process in aprotic metal-N₂ batteries. Li-based batteries are the first reported metal-N₂ batteries that perform in aprotic electrolytes (Fig. 20a) [20]. During discharge, the adsorbed N₂ on the cathode is reduced by the positive Li ions from the electrolyte to form a Li₃N intermediate on the cathode. During charging, the intermediate on the cathode is oxidized to release N₂ gas and the Li ions solvate to the metal electrode for deposition. The Li-N₂ battery, as reported by Zhang et al. [20], has the FE of approximately 59% in ether-based organic electrolytes. This Li-N₂ battery with bare carbon cloth (CC) as the GDE exhibited maximum FE at − 1.08 V, additionally, with a charging FE of nearly 51%. Cyclic voltammetry curves for the Li-N₂ battery under Ar- and N₂-saturated electrolytes exhibit Li intercalation and N₂ fixation on the cathode, respectively (Fig. 20b). In the case of the charging cycle, almost 87% of the Li₃N product decomposed back to N₂ and was quantified by colourimetric assays that show the charge‒discharge cycle of Li-N_2, proving its efficiency (Fig. 20c).

Fig. 20

Copyright © 2018, Elsevier Ltd. d Schematic of the Na-N₂ battery with the MnO₂/CC cathode in an organic electrolyte. e Comparison of the first charge‒discharge curves of the CC/α-MnO₂ samples at various current densities. f Cycling performances of NAA batteries at 50 mA g⁻¹. Reprinted with permission from Ref. [29]. Copyright © 2019, Elsevier Ltd. g Illustration of the Al-N₂ battery with the graphene/Pd/CC cathode in an ionic electrolyte. h Cycling performances of the Al-N₂ battery at 0.05 mA cm⁻² with graphene/Pd/CC as the air cathode. Reprinted with permission from Ref. [47]. Copyright © 2020, Royal Society of Chemistry

a Schematic of the Li-N₂ battery with the CC cathode in an organic electrolyte. b CV curves for the Li-N₂ battery under Ar and N₂ atmospheres. c Charge‒discharge curves for various cycles for the Li-N₂ battery at 0.05 mA cm⁻². Reprinted with permission from Ref. [20].

Later, Zhou et al. [189] studied the Li₃N intermediate for stability and its water-absorbing tendency after testing for 30 days in TEGDME, which showed considerable stability. Later, Na-N₂ batteries were reported under a similar organic electrolyte, and the N₂ fixation performance was analysed with CC/α-MnO₂ as the cathode (Fig. 20d) [29]. The charge‒discharge mechanisms were similar to those of Li-N₂ batteries with nitride formation at the cathode. The Na-N₂ battery with the CC/α-MnO₂ cathode exhibited an FE of approximately 26%, which indicated poor catalytic activity. The first charge‒discharge curves of the Na-N₂ battery reveal a constant potential density for higher specific capacity values of the battery under various current densities (Fig. 20e). However, the Na-N₂ battery was demonstrated in simulated actual air conditions (NAA-80% N₂ and 20% O₂), which show that the Na-N₂ batteries can deliver a specific capacity that retains at least 10 mAh g⁻¹ at 50 mA g⁻¹ even after 70 cycles. The NAA experiments reveal the promising activity of the Na-N₂ battery that can be cycled at ambient atmospheres in the future (Fig. 20f).

Following the Na-N₂ battery, the Al-N₂ battery was fabricated with the ionic electrolyte and exhibited a maximum FE of approximately 51.2% in an open circuit voltage of 1.55 V (Fig. 20g) [47]. The deposited discharge product was further hydrolysed to produce ammonia or recycled during the charging process. The Al-N₂ battery fabricated with graphene/Pd/CC as the air cathode reveals a modest change in the charge‒discharge curves for 120 h at 0.05 mA cm⁻², which reveals the promising activities of this battery (Fig. 20h). One major advantage of the aprotic electrolyte is the suppression of proton migration in the electrolyte, which increases the efficiency of the metal-N₂ battery and ensures nitride formation at the cathode. Moreover, the nitrides formed during discharge can be hydrolysed to produce ammonia [20, 29]. However, the controlled environment, availability and cost of the electrolytes, instability, and high maintenance pose a major drawback in designing a metal-N₂ battery for scalable applications in nonaqueous electrolytes.

6.2 Aqueous Metal-N₂ Batteries

Using aprotic liquids as electrolytes has disadvantages with the sustainability of the battery and is obviously not environmentally friendly. To overcome the limitations of ionic electrolytes, aqueous metal-N₂ batteries were fabricated. The first reported alkaline metal-N₂ battery was fabricated by Du et al. [50], as shown in Fig. 21a.

Fig. 21

Copyright © 2019, Royal Society of Chemistry. d Polarization and power density plots for the Zn-N₂ battery fabricated with NbS₂ NS as the air cathode. e Potentials at various discharging current densities for the NbS₂-fabricated Zn-N₂ battery. f Optical photograph of the coupled Zn-N₂ battery with the NbS₂ cathode. Reprinted with permission from Ref. [30]. Copyright © 2020, Elsevier Ltd. g Discharge curves of the Zn-N₂ battery with the Fe-HTN cathode (the inset: hollow FeHTN catalysts mimicking the human alveolus). h Polarization curves and power density plots for the FeHTN-fabricated Zn-N₂ battery. i Comparison of the ammonia yield for various samples from the Zn-N₂ battery. Reprinted with permission from Ref. [31]. Copyright © 2021, Royal Society of Chemistry

a Illustration of the Zn-N₂ battery at the discharge cycle. b Polarization curves and power density plots of the aqueous Zn-N₂ battery with the Cu/CP cathode. c Cycling tests of the Zn-N₂ battery with the Cu/CP cathode. Reprinted with permission from Ref. [50].

The reported Zn-N₂ battery was constructed with Cu nanoparticle-deposited carbon paper (Cu/CP) as the cathode, Zn metal as the anode and 1.0 M KOH as the electrolyte. The Zn-N₂ battery delivered a maximum power density of approximately 10.1 μW cm⁻² and a striking FE of nearly 59% (Fig. 21b, c). In aqueous metal-N₂ batteries, discharge occurs with NRR, as in Eq. (10), and charging occurs with the OER, as in Eq. (11). The work widened the scope for aqueous metal-N₂ batteries and then started to amass several aqueous-based Zn-N₂ battery systems with different cathode designs.

The general discharge‒charge mechanism for aqueous alkaline electrolytes occurring at the cathode is as follows.

$${\text{Discharging}}\;\;{\text{(NRR)}}: {\text{6H}}_{{2}} {\text{O}} + {\text{N}}_{{2}} + {\text{6e}}^{ - } \to {\text{2NH}}_{{3}} + {\text{6OH}}^{ - }$$

(10)

$${\text{Charging}}\;{\text{(OER)}}:{\text{4OH}}^{ - } \to {\text{O}}_{{2}} + {\text{H}}_{{2}} {\text{O}} + {\text{4e}}^{ - }$$

(11)

After addressing the drawbacks of nonaqueous Li-N₂, Na-N₂ and Al-N₂ batteries, Wang et al. [30] designed a Zn-N₂ battery with exfoliated niobium disulfide nanosheets (NbS₂ NSs), which showed a promising peak power density of 0.31 mW cm⁻² in 1.0 M KOH electrolyte (Fig. 21d). In addition, the Zn-N₂ battery with the NbS₂ cathode delivered a stable current density at various potentials, which shows the reliability of the battery performance (Fig. 21e). The coupled Zn-N₂ battery was demonstrated to light up a red LED under a constant N₂ supply, which displays prospects for potential applications (Fig. 21f). Following exfoliated metal dichalcogenides, a novel electrocatalyst design with iron-doped hollow titanium oxide nanospheres (FeHTN) was synthesized as the cathode material for a Zn-N₂ battery [31]. The mechanism of the FeHTN electrocatalyst was similar to that of the human alveolus because the negative pressure of the produced ammonia sucks in the nitrogen gas for further reduction (Fig. 21g). The nanospheres increase the time of stay of the nitrogen molecules in the electrocatalyst, which increases the ammonia yield. The FeHTN-fabricated Zn-N₂ battery exhibited a maximum power density of approximately 27.65 μW cm⁻² in a 0.1 M KOH electrolyte (Fig. 21h). After coupling three Zn-N₂ batteries, it could light up a white LED, which shows the potential applicability of FeHTN for Zn-N₂ batteries (Fig. 21i).

Incorporating a heterostructure strategy to combine the specific activity of two phases is highly anticipated to achieve boosted electrocatalytic activity. Because of the importance of the heterostructure strategy, Lv et al. [49] synthesized a vanadium nitride nanodot-anchored ultrathin N, S-doped carbon matrix (VN@NSC) and employed it as a cathode for a Zn-N₂ battery. The VN@NSC electrocatalyst performs as a dual-engine mechanism where VN acts as the “capturer” and NSC with defect sites acts as the “converter” for ammonia synthesis. The VN@NSC-fabricated Zn-N₂ batteries exhibited a power density of approximately 16.42 μW cm⁻² in a 0.1 M KOH electrolyte (Fig. 22a, b).

Fig. 22

Copyright © 2020, Elsevier Ltd. c Polarization and power density plots for the Zn-N₂ battery with the CoPi/HSNPC cathode. d Polarization curves for the Zn-N₂ battery with the CoPi/HSNPC cathode under different atmospheres. Reprinted with permission from Ref. [32]. Copyright © 2021, Royal Society of Chemistry. e Charge‒discharge curves and power density plots for the Zn-N₂ battery with the CoPi/NPCS cathode. f Cycling performance of the Zn-N₂ battery with the CoPi/NPCS cathode. Reprinted with permission from Ref. [27]. Copyright © 2021, American Chemical Society

a Polarization curves and power density plots for the VN@NSC-incorporated Zn-N₂ battery. b Cycling performance of the Zn-N₂ battery with the VN@NSC cathode. Reprinted with permission from Ref. [49].

Consecutively, the group investigated the performance of cobalt phosphate nanospheres (CoPi/HSNPC), which capture N₂ inside their spheres, increasing the chances of reduction [32]. The Zn-N₂ battery with CoPi/HSNPC exhibits a maximum power density of approximately 0.15 mW cm⁻² and a cell potential of approximately 0.6 V (Fig. 22c, d).CoPi/HSNPC was also evaluated for OER performance, which shows promising applications in the rechargeability of the electrocatalyst. The activity is obviously due to the Co²⁺ redox site that selectively adsorbs N₂ and the phosphorous atom that enhances the electronic conductivity of the electrocatalyst. The spheres efficiently support the mass transfer of the ions that form intermediates during the discharge cycle.

In addition, cobalt phosphate anchored on N-doped carbon nanosheets (CoPi/NPCS) achieves a higher surface area for the interaction of N₂ with the active sites [27]. The CoPi/NPCS-incorporated Zn-N₂ battery in the flow configuration achieved a maximum power density of approximately 0.49 mW cm⁻² and a cell potential of nearly 0.6 V (Fig. 22e). The battery can withstand a stable current density for longer periods (Fig. 22f). However, the battery under an O₂ atmosphere is found to reduce O₂ more preferably than N₂. The OER performance highlights the potential of the Zn-N₂ battery for rechargeable applications.

To analyse the performance of 2D structures as electrocatalysts, Lv et al. [48] studied 2D porous exfoliated FePS₃ nanosheets with rich sulfur vacancies (V_s-FePS₃ NS), which exhibit a higher NRR performance. The 2D structure imparts a high surface area for catalytic activity. The Zn-N₂ battery fabricated with V_s-FePS₃ NSs exhibits a power density of approximately 2.6 mW cm⁻² in a 1.0 M KOH electrolyte (Fig. 23a, b).

Fig. 23

Copyright © 2020, Elsevier Ltd. d Schematic of the aqueous Al-N₂ battery. e Polarization curves and power density plots for the Al-N₂ battery with the MoPi/HSNPC cathode. f Discharge polarization curves for the Al-N₂ battery with scrap Al metal. Reprinted with permission from Ref. [26]. Copyright © 2021, Elsevier Ltd

a Charge‒discharge curves of the Zn-N₂ battery with the FePS₃ electrocatalyst. b Polarization and power density plots for the Zn-N₂ battery with the FePS₃ catalyst. c Optical photograph of the coupled FePS₃-fabricated Zn-N₂ battery powering a red LED. Reprinted with permission from Ref. [48].

The Zn-N₂ battery delivers a constant current density for a longer period of time with insignificant changes. The coupled Zn-N₂ battery was used to light a red LED, exhibiting the potential prospects of the Zn-N₂ battery with the FePS₃ cathode (Fig. 23c). Following aqueous alkaline Zn-N₂ batteries with various cathode materials, Ren et al. [26] fabricated an alkaline Al-N₂ battery with molybdenum phosphide spheres (MoPi/HSNPC) as the cathode in a 1.0 M KOH electrolyte (Fig. 23d). Previous reports suggest Al corrosion during discharge in alkaline electrolytes, which was overcome by the corrosion-inhibiting anolyte additives of 2 mM Na₂SnO₃, 0.1 mM In(OH)₃ and 1.5 mM ZnO. The Al-N₂ battery uses the flow reactor model with a gas diffusion electrode forming a three-phase interface for N₂ reduction. The Al-N₂ battery delivers a peak power density of approximately 2.37 mW cm⁻² and an FE of approximately 5.06% (Fig. 23e). The Al-N₂ battery finds marvellous applications due to the large availability and lightweight property of the metal. This study also includes an Al-N₂ battery fabricated with anode scrap Al metal, which shows the promising sustainability of the Al-N₂ battery for the future. The battery performance was almost similar, with a current density of nearly 8.46 mA cm⁻² at 0 V vs. Al(OH)₄¯/Al for scrap Al metal (Fig. 23f).

Thus, after serious consideration of various metal-N₂ batteries, Zn-N₂ batteries were considered a reliable and rechargeable aqueous metal-N₂ battery choice. Research was then focused on improving the efficiency of Zn-N₂ batteries by incorporating various electrocatalyst designs as cathode materials, with the electrolyte commonly being alkaline KOH. The Zn-N₂ battery can be fabricated and maintained at ambient temperatures and pressures with very little maintenance and low operational costs. Zn metal is the fourth most abundant element in the Earth’s crust and can sufficiently meet the large metal demands for anodes and the prospects of Zn-N₂ batteries in alkaline environments. However, the battery efficiency and stability rely on the type of electrocatalyst used for the Zn-N₂ battery. Moreover, the rechargeability of metal-N₂ batteries has been targeted and shows promising applications in alkaline Zn-N₂ batteries. However, metal-N₂ batteries in aprotic electrolytes exhibit a higher FE than the reported alkaline metal-N₂ batteries, but their scalable application poses a major risk. Al-N₂ batteries exhibit better performance under alkaline environments, but their corrosion under alkaline environments is inevitable and requires corrosion-inhibiting additives [190, 191]. The Zn-N₂ batteries reveal insignificant changes in the charge‒discharge cycling performances for nearly 80 h [32] and exhibit stable discharge potential over 2 h [30,31,32, 48]. Zn-N₂ batteries exhibit relatively low power densities, yet they show potential prospects for development [49, 50]. Considering the performances of metal-N₂ batteries, promising applications of rechargeable aqueous Zn-N₂ batteries for a sustainable future are proposed. Crafting a suitable electrocatalyst with exceptional NRR performance and increased stability can improve Zn-N₂ battery efficiency in the future (Table 2).

Table 2 Comparison table for various metal-N₂ battery systems. Data are from different referenced resources

7 Conclusion and Perspectives

Developing green ammonia synthesis has a considerable impact on sustainable future energy technologies. Green ammonia synthesis from the electrochemical NRR has convincing applications in various fields, particularly in the future hydrogen economy. Ammonia production by aqueous electrochemical NRR uses water and electricity from renewable resources that are facilitated by electrocatalysts. With the multitude of works relating to the electrochemical NRR, there have not been any state-of-the-art electrocatalysts that set a goal for industrial ammonia synthesis. However, the performance of electrocatalysts is limited by their activity, selectivity, and stability towards efficient electrochemical NRR. Many electrocatalysts have been observed to prefer HER over NRR, and the adsorption of N₂ on some electrocatalysts has been sluggish. Some electrocatalysts even produce N₂H₄, an intermediate product, which sabotages ammonia production.

Research has adopted various strategies by modifying various parameters of the electrochemical NRR cell that are expected to produce ammonia efficiently. The important strategies for increasing ammonia production are (i) increasing the available number of active sites, (ii) suppressing the HER, and (iii) enhancing the conductivity of electrocatalysts. These strategies can be incorporated by engineering the electrocatalysts, modifying the nature of the electrolytes and adopting the LiMEAS procedures to achieve high commercial yields. Recently, novel metal-N₂ batteries have been demonstrated and have found promising applications in the field. However, stability, activity and the controlled environment have been considerable problems to be addressed duly. It is desirable to make great efforts to increase the activity of electrocatalysts to achieve sustainable and commercial ammonia synthesis and metal-N₂ batteries under ambient conditions. In this review, we summarize some of the current challenges and future directions for the commercialization of NRR and metal-N₂ batteries.

• 1. Improving Electrocatalyst Design. The electrocatalyst in the first case must express long-term stability that provides unperturbed activity and nondegradation of the electrode for long battery cycles. Additionally, the intermediate intercalation or settlements on the surface of the electrocatalysts must be considered, as they may block the active sites for N≡N activation and resist ion flow across the electrode. Modifying the electrocatalysts with vacancies, heterostructure formations, core–shell structures, alloying metals, tuning the morphology and size effects, and electronic modifications by heteroatom dopants are potential strategies for increasing the activity of the electrocatalysts. The electrocatalyst must be sensitive to N₂ adsorption and provide a remarkable activity with maximum FE under NRR electrocatalysis. The electrocatalysts should impose high porosity, increased electronic conductivity, and a nonreactant to the electrolyte that can be incorporated as an electrode to provide faster kinetics and notable efficiency. In addition, the electrocatalysts must be highly conductive to make the active sites readily available for electrons, which may influence the efficient charge‒discharge profiles of the metal-N₂ battery later. Thus, high-performance electrocatalysts can be fabricated as bifunctional electrodes in a metal-N₂ battery. The electrocatalyst must be compatible with a longer cycle life and bifunctional activity for rechargeability.

• 2. Electrolyte effect. The electrolytes and the intermediate formation must also be considered to push the working efficiency of the metal-N₂ battery system beyond limits. Tactful control of the N₂ selectivity over protons by modifying the nature of the electrolyte has been a potential route to increasing ammonia yield. The molecular crowding effect, inspired by living cells, has been employed to suppress water molecule migration to the electrode surface. The novel molecular crowding approach is compatible with alkaline, acidic, and neutral solutions. The metal-N₂ battery fabricated with organic/ionic electrolytes has shown promising higher FE and charge‒discharge performance. However, because of the operational costs, instability of the electrolytes, and impractical scalability, nonaqueous metal-N₂ batteries are still in their infancy. Metal-N₂ batteries operating in aqueous electrolytes have been the desired application for the current energy-demanding society. Zn-N₂ batteries and Al-N₂ batteries have been fabricated under aqueous electrolytes, which show promising applications for a sustainable future. However, Al metal is corrosive under alkaline environments, which requires corrosion-inhibiting additives to retain the activity. Nevertheless, the Zn-N₂ battery has been the most studied and provides promising applications in the future of rechargeable metal-N₂ batteries under alkaline environments. Metal-N₂ batteries have been found to produce lower ammonia yields, which can be investigated further by electrolyte engineering strategies to enable sustained ammonia synthesis with higher charge‒discharge efficiencies.

• 3. Metal-N₂ batteries. Metal-N₂ batteries are still in their infancy and need further improvements for commercial-level battery fabrication. The battery type and its mechanism vary based on the nature of the electrolytes used. Li-N₂, Na-N_2, and Al-N₂ batteries have been reported under aprotic electrolytes and exhibited a higher FE, but the maintenance and operational costs make them less favourable for sustainable application. However, alkaline metal-N₂ batteries in Zn-N₂ and Al-N₂ have shown promising efficiency and considerable power density, which can be further improved to achieve commercialized metal-N₂ batteries. Moreover, Na-N₂ batteries have been used under ambient conditions (NAA performance), which shows promising results for futuristic battery fabrication technology. Detailed studies on Zn-N₂ batteries show better charge‒discharge performances with a stable discharge voltage in an aqueous environment. Moreover, Li metal and Na metal cannot be handled in an ambient atmosphere, which shows the tedious fabrication process of the corresponding metal-N₂ batteries. Therefore, it can be inferred that Zn-N₂ batteries with an appropriate electrocatalyst hold more prospects for industrial-scale applications than other candidates.

• 4. Rechargeability. The amount of importance given to the electrocatalyst for NRR during discharge must also be given to the rechargeability of the metal-N₂ batteries. The charging process in an aqueous metal-N₂ battery evolves O₂ following OER by the adsorption of four OH¯ ions at the cathode surface. Hence, the counterreaction must also be equally assisted in providing efficient rechargeability of the battery and maintaining long-term cycling. Additionally, the intermediate stability of the electrocatalyst in nonaqueous metal-N₂ batteries and electrolyte media must also be evaluated for long-term storage applications. In addition, aqueous Zn-N₂ batteries show better rechargeable performances than Al-N₂ batteries because of the higher oxidation behaviour of Al (Al³⁺/Al =  − 1.66 V) compared to Zn metal (Zn²⁺/Zn =  − 0.76 V). Hence, the Zn-N₂ battery shows promising advantages over other candidates for rechargeability in ambient aqueous conditions and minimum maintenance, elucidating the versatility of the Zn-N₂ battery in energy storage.

• 5. Future study developments. It is of utmost importance to analyse the catalytic reactions and the side reactions that pose unfavourable conditions in the electrochemical cell. In situ electrochemical and characterization studies and theoretical/simulation studies of the metal-N₂ battery can provide a better understanding of the kinetics of the electrochemical reactions and the transformation of the electrocatalyst surface with respect to time. This understanding can provide a better formulation strategy for the metal-N₂ battery system. Aqueous metal-N₂ batteries have gained attention due to their easy fabrication, corrosion-free environment, and extended battery life. With metal-air batteries gaining substantial updates, it is envisioned that metal-N₂ batteries will be commercialized. To date, metal-N₂ batteries have been tested under a controlled N₂ environment, and future studies must fabricate electrocatalysts tuned to uptake N₂ conversion over O₂ in the ambient atmosphere.