Electrochemical Nitrogen Fixation for Green Ammonia: Recent Progress and Challenges

Abstract

Ammonia, a key feedstock used in various industries, has been considered a sustainable fuel and energy storage option. However, NH₃ production via the conventional Haber–Bosch process is costly, energy‐intensive, and significantly contributing to a massive carbon footprint. An electrochemical synthetic pathway for nitrogen fixation has recently gained considerable attention as NH₃ can be produced through a green process without generating harmful pollutants. This review discusses the recent progress and challenges associated with the two relevant electrochemical pathways: direct and indirect nitrogen reduction reactions. The detailed mechanisms of these reactions and highlight the recent efforts to improve the catalytic performances are discussed. Finally, various promising research strategies and remaining tasks are presented to highlight future opportunities in the electrochemical nitrogen reduction reaction.

This review discusses recent progress and challenges related to the electrochemical nitrogen reduction reaction. In addition, this review highlights promising research strategies and remaining tasks for future opportunities in electrochemical nitrogen reduction catalysis.

1. Introduction

Artificial nitrogen fixation through the Haber–Bosch process, which was developed a century ago, is regarded as one of the most important technological pillars that support modern industrial society.^{[ 1} , ² , ³ , ⁴ , ⁵ , ⁶ , ^{7 ]} Thus far, ammonia (NH₃) has served as the key feedstock in various industries manufacturing fertilizers, pharmaceuticals, and textiles.^{[ 5} , ⁸ , ⁹ , ^{10 ]} NH₃ has recently emerged as a sustainable energy vector owing to facile liquefaction (−33.3 °C at atmospheric pressure or 7.5 bar at room temperature) and well‐established NH₃ storage and transportation infrastructures.^{[ 11} , ¹² , ¹³ , ^{14 ]} However, the Haber–Bosch process is an energy‐intensive and expensive process requiring conditions of 300–550 °C and 250 bar with iron oxide catalysts, resulting in a substantial carbon footprint from CO₂ production (Figure  1A).^{[ 15} , ¹⁶ , ¹⁷ , ¹⁸ , ^{19 ]} In nature, an enzyme called nitrogenase can facilitate nitrogen reduction under ambient conditions.^{[ 10} , ¹⁶ , ^{20 ]} Although biological nitrogenases in diazotrophic microorganisms have inspired the design of synthetic catalysts for nitrogen fixation,^{[ 21} , ²² , ²³ , ^{24 ]} no catalytic system developed thus far has demonstrated a high level of selectivity or efficiency of nitrogenases (Figure 1B).

Figure 1.

Nitrogen fixation. Three representative routes to produce ammonia from N₂: A) Thermochemical Haber–Bosch. B) biological N₂ fixation by nitrogenase enzyme and C) electrochemical nitrogen reduction system.

The electrochemical synthetic route for nitrogen fixation has received considerable attention as it can facilitate the green production of NH₃ without the formation of harmful pollutants while using electricity generated from sustainable sources (Figure 1C).^{[ 8} , ²⁵ , ²⁶ , ²⁷ , ²⁸ , ²⁹ , ^{30 ]} The electrochemical nitrogen reduction reaction (eNRR) proceeds as follows: 1) N₂ molecules approach the electrode surface; 2) electrical charges are exchanged and protons are injected to break bonds and hydrogenate N₂ (bond dissociation energy = 941 kJ mol⁻¹); 3) NH₃ is released.^{[ 26} , ^{31 ]} The net nitrogen reduction reactions in acidic and alkaline electrolytes follow different reaction pathways but have the same equilibrium potential of 0.56 V _SHE.^{[ 32} , ^{33 ]}

$${\mathrm{N}}_2+6{\mathrm{H}}^{+}+6{\mathrm{e}}^{-}\to 2{\mathrm{NH}}_3\left(\mathrm{acidic}\mathrm{electrolyte}\mathrm{condition}\right)$$ (1)

$$\begin{array}{ccc}\hfill {\mathrm{N}}_2+6{\mathrm{H}}_2\mathrm{O}+6{\mathrm{e}}^{-}& \to & 2{\mathrm{NH}}_3+6{\mathrm{OH}}^{-}\hfill \\ & & \left(\mathrm{alkaline}\mathrm{electrolyte}\mathrm{condition}\right)\hfill \end{array}$$ (2)

There are several review articles that have summarized studies of electrochemical nitrogen reduction (eNRR).^{[ 17} , ³⁴ , ³⁵ , ³⁶ , ³⁷ , ³⁸ , ³⁹ , ⁴⁰ , ⁴¹ , ⁴² , ⁴³ , ⁴⁴ , ⁴⁵ , ⁴⁶ , ⁴⁷ , ⁴⁸ , ⁴⁹ , ^{50 ]} In this review, we aim to classify recent eNRR studies into two distinct classes, and present the current mechanistic understanding of each pathway. Based on the reaction pathways (as shown in Figure  2 ), studies on eNRR can be divided into two categories: direct nitrogen reduction reaction (DNRR) and indirect nitrogen reduction reaction (INRR). The use of electrons is the most significant difference between DNRR and INRR. The term Indirect was chosen for Li‐mediated pathway because electron is used to reduce “Li⁺” to metal Li, whereas electron in DNRR pathway is used to break and activate the “nitrogen” triple bond. DNRR refers to direct electrochemical nitrogen activation on a heterogeneous catalyst surface. In this pathway, heterogeneous catalysts only provide the binding sites for dinitrogen molecules. In contrast, INRR involves reactive metal species, such as Li, with a strong reducing ability to break triple nitrogen bonds (N≡N). While the electrons from the electrode are utilized to “directly” activate dinitrogen in the DNRR pathway, electrons in INRR are solely consumed to form Li metal films, which “indirectly” convert dinitrogen into lithium nitride. In other words, instead of the sequential activation of nitrogen on the catalyst surface, INRR harnesses metal nitride intermediates to produce NH₃ from dinitrogen in a nonaqueous system. Although both selectivity (Faradaic efficiency (FE)) and reactivity (current density) in the INRR have been remarkably improved, the high energy input required to initiate Li plating on the electrode remains a considerable roadblock.

Figure 2.

Schematic illustration of DNRR and INRR pathways. DNRR route shows the stepwise hydrogenation of the adsorbed N₂ molecules on the electrode‐immobilized catalyst surface to produce ammonia. On the other hand, in INRR route, electroplating of lithium occurs on the electrode, followed by subsequent nitrogen splitting and hydrogenation on the Li surface.

This review highlights the most recent progress in the two eNRR methods, DNRR and INRR. We aim to provide useful guidelines for future research by identifying the most significant developments and critical challenges. Initially, we focus on the reaction mechanisms of DNRR and INRR, based on the findings from electrokinetic, in situ, and ex situ spectroscopic, and various computational studies for eNRR. The effects of the catalyst design, including the morphology and heterostructure, on the DNRR are presented, considering the binding affinity of dinitrogen molecules on the catalyst surface during catalysis. The engineering strategies for the electrochemical components of INRR are discussed, followed by an overview of recent advances in Li‐mediated nitrogen reduction. Finally, we point to the most salient challenges currently associated with DNRR and INRR, suggesting potential research avenues to prompt research advances in this fledgling field.

2. Electrochemical Mechanisms for DNRR and INRR

The sluggish nitrogen reduction kinetics are primarily attributed to the thermodynamically stable triple N─N bond in N₂, for which the bond (941 kJ mol⁻¹) and ionization (15.6 eV) energies are considerably high.^{[ 51} , ⁵² , ⁵³ , ^{54 ]} Moreover, N₂ has a negative electron affinity (−1.9 eV) and requires additional electrical energy for electron injection. For instance, the first protonation of *N₂ to *NNH in the DNRR is endothermic (∆H⁰ = +37.6 kJ mol⁻¹).^{[ 11} , ⁵⁵ , ^{56 ]} Furthermore, electron excitation in N₂ is significantly hindered by the large gap between the highest occupied and lowest unoccupied molecular orbitals: 10.82 eV (or 1044.0 kJ mol⁻¹). Finally, compared with other gases, N₂ has low solubility in aqueous solutions (0.6 mmol L⁻¹ in water at room temperature). Herein, we present the recent understanding of the mechanism of each pathway.

2.1. DNRR (Catalyst‐Mediated eNRR)

2.1.1. Proposed Mechanisms for DNRR

Three elementary steps are typically considered for DNRR (Figure  3A): 1) chemical adsorption of N₂ molecules and protons onto the cathode surface, 2) activation of N₂ and hydrogenation, and 3) rearrangement and desorption of the product NH₃ or other products (e.g., hydrazine [N₂H₄] and diazene [N₂H₂]) from the electrode surface and their migration into the electrolyte.^{[ 10} , ²⁶ , ⁵⁷ , ^{58 ]} The Haber–Bosch process produces NH₃ via a dissociation pathway because of high pressure and temperature operating conditions. In contrast, biological systems adopt an associative pathway to generate NH₃, which involves N₂ fixation via the enzymatic process of nitrogenases.^{[ 31} , ^{59 ]} For electrochemical nitrogen reduction, an associative path would be more appropriate because ambient reaction conditions are used in electrochemical systems, necessitating nitrogen adsorption and protonation as the first step for an efficient eNRR.^{[ 60 ]} The associative pathway can be further categorized into alternating and distal pathways based on the sequence of hydrogenation on the nitrogen atoms. Specifically, in the associative alternating pathway, hydrogenation occurs alternatively on the two nitrogen atoms, while in the associative distal pathway, it occurs on one nitrogen atom and then the other.

Figure 3.

Reaction pathways of DNRR and INRR A) Scheme for three possible DNRR mechanisms: dissociative, associative alternating, and associative distal pathways. Reproduced with permission.^{[ 18 ]} Copyright 2016, Elsevier. B) Volcano plot for DNRR on metal electrodes depending on the mechanistic pathways. The volcano plot for the HER is overlaid for comparison. Reproduced with permission.^{[ 26 ]} Copyright 2012, RSC. C) SEIRAS spectra of an Rh film electrode in N₂‐saturated 0.1 m KOH. Reproduced with permission.^{[ 3 ]} Copyright 2020, Wiley VCH. D) Overall reaction schemes of Li‐mediated electrochemical nitrogen reduction on Cu electrode. Reproduced with permission.^{[ 78 ]} Copyright 2019, Elsevier. E) Stepwise Li‐mediated catalytic cycle for nitrogen reduction to ammonia. Reproduced with permission.^{[ 76 ]} Copyright 2020, Springer Nature. F) Heterogeneous nitrogen reduction on lithium metal surface, which is similar to DNRR pathway. Lithium (or LiH, Li₃N) layers are not involved in the cycle, instead, provide the binding sites to N₂ and nitrogen‐containing adsorbates. Reproduced with permission.^{[ 61 ]} Copyright 2020, Wiley VCH.

Because N₂ adsorption/activation is consistent with the formation of N₂H* species on the electrode, Medford et al. proposed that Brønsted–Evans–Polanyi (BEP) relationships result in a volcano plot, which helps to visualize the ability of metals to reduce N₂ electrochemically (Figure 3B).^{[ 25} , ²⁶ , ^{98 ]} The model focuses on the eNRR rate‐determining step (RDS) on catalyst materials, depending on the eNRR pathways (solid lines for the dissociative pathway and dotted lines for the associative pathway) and the surface atomic geometry (flat surfaces for black and stepped surfaces for red). For metals on the left side of the volcano plot, the hydrogenation process is considered the RDS in both the associative and dissociative pathways (*NH₂ + H⁺ + e⁻ → NH₃ on the stepped surfaces and *NH + H⁺ + e⁻ → *NH₂ for the flat surfaces). However, for metals on the right side of the plot, the RDSs are changed depending on the reaction pathways; the first protonation step of N₂ (N₂ → *N₂H) for the associative pathway and the N₂ splitting step (N₂ → *2N) for the dissociative pathway regardless of the surface atomic geometries. It is concluded that the flat surfaces of transition metals such as Sc, Y, Ti, and Zr effectively reduce N₂ to NH₃ through the dissociative mechanism with the applied potential of −1 V _NHE to −1.5 V _NHE, by comparing the free energies of *N (white area), *H (gray‐shaded area), and intermediates (those species are stable on the left side area from the vertical lines a) *NH, b) *NH₂, c) *N₂H, and d) *N₂H₂) on the catalyst surfaces. Furthermore, the slow rate of N₂ reduction observed on catalysts such as Pt, Pd, or Ru can be explained by free energy calculations. These calculations suggest that, under negative bias, these catalyst materials become saturated with hydrogen, hindering the adsorption of nitrogen and subsequently impeding NH₃ production. Also, N vacancy incorporated Mars‐van Krevelen (MvK) mechanism has been suggested by Yang et al. In the MvK mechanism, a lattice N on the surface of transition metal nitride catalysts is consumed to produce NH_3, leaving an N vacancy, which is reoccupied by adsorbed N₂.^{[ 62 ]} Despite the efforts to reveal the nature of DNRR process, eNRR mechanism, however, has remained elusive.^{[ 63} , ⁶⁴ , ⁶⁵ , ^{66 ]}

2.1.2. Spectroscopic Studies

Due to the low yields of NH₃ from DNRR, it is challenging to study the reaction intermediate species. A few reports have investigated the reaction intermediates using surface‐enhanced infrared spectroscopy (SEIRAS) and differential electrochemical mass spectrometry (DEMS) (Figure 3C).^{[ 67} , ⁶⁸ , ^{69 ]} Using the SEIRAS, Yao et al. discovered the potential‐dependent molecular characteristics on the Rh electrode surface. Figure 3C inset plots the band center (cm⁻¹) and intensity of each stretching mode of N = N & Rh‐H species versus applied voltages. The authors revealed that the extent of coverages of the N₂H _x and H species on the Rh electrode strongly depends on the applied potentials. Notably, maximum N₂H _x coverage was obtained between 0 and −0.2 V, and H deposition on the electrode surface increased below −0.2 V (inset of Figure 3C). Also, by integrating SEIRAS and DEMS, it was discovered that the final protonation of *N₂H₂ to NH₃ on the Rh electrodes is the RDS, which is consistent with the theoretical expectation for the associative eNRR mechanism, as suggested by Medford et al. N₂H _x (0 ≥ x ≥ 2) was detected using SEIRAS and DEMS, with an N═N stretching mode at ≈2020 cm⁻¹ and a signal at m/z = 29 (N₂H⁺), respectively. However, the N₂H₂ ⁺ species was not detected at m/z = 30. The same group also determined the DNRR mechanisms on the Ru, Au, and Pt electrode surfaces, but the N₂H₂ ⁺ species have not been detected thus far. These studies suggest an alternative mechanism via the facile formation of N₂H _x species on the catalysts and decomposition in the electrolytes.

2.2. INRR (Li‐Mediated eNRR)

2.2.1. Development of Li‐Mediated Nitrogen Reduction Methodology

The Li‐mediated eNRR started back in the 1990s. After it was first suggested by Fichter et al. in 1931,^{[ 70 ]} Tsuneto et al. reported Li‐mediated electrochemical N₂ fixation via organic solutions containing a small amount of proton source as an electrolyte to suppress the competing reaction, HER.^{[ 71} , ^{72 ]} The experiments were performed under 50 atm N₂ pressure with LiClO₄ (0.2 m) and ethanol (0.18 m) in a tetrahydrofuran (THF) electrolyte. The selectivity toward NH₃ was low, and it required a high nitrogen pressure.

McEnaney et al. proposed a strategy to separate N₂ reduction from the subsequent protonation to NH₃ in their molten‐salt system.^{[ 73 ]} The suggested phase diagram of Li─N─O─H incorporates the following Li cycle steps: 1) Reductive LiOH electrolysis generates Li in the hydrogen‐free environment. The phase diagram indicates that Li exists as a stable phase above −3.3 V versus the standard hydrogen electrode (SHE). 2) Introduction of 1 bar of nitrogen gas into the activated Li surface facilitates Li nitridation. Lithium nitride (Li₃N) is demonstrated to be more stable than Li for all potentials, implying that Li₃N formation is energetically favorable under mild conditions. 3) Li₃N is rapidly hydrolyzed to NH₃ upon adding water at 0 V versus SHE. Li₃N decomposes to stable LiOH and NH₃. Although McEnaney et al. successfully synthesized NH₃ with 88.5% FE at low N₂ pressure, their system had several practical limitations; for example, high operating conditions (> 400 °C) were required for providing a continuous liquid phase of molten salt electrolyte, which inevitably resulted in the formation of liquid Li metal (melting point = 180.5 °C). Thus, additional steps were necessary to separate liquid Li. Kwiyong et al. reported an alternative strategy using a Li‐ion conducting glass ceramic material (LISICON) system. Although elevated temperature (220 °C) and stepwise processes from different reaction chambers are required, LISICON allows Li⁺ ion migration and Li deposition at room temperature.^{[ 74 ]} Recently, several studies have been conducted on continuous Li‐mediated nitrogen reduction catalysis under mild conditions without any separation steps.^{[ 75} , ⁷⁶ , ^{77 ]} Protons from external proton donors or anodic reactions are fed into the catholyte, which directly reacts with the Li₃N layers to yield NH₃. This approach has received significant attention as it is a one‐pot synthesis without harsh operating conditions.

2.2.2. Proposed Mechanisms for INRR

The overall reaction of Li‐mediated electrochemical nitrogen reduction on the electrode is depicted in Figure 3D. This diagram well illustrates the key components of the reaction.^{[ 78 ]} The mechanism of continuous Li‐mediated nitrogen reduction typically involves three steps (Figure 3E): 1) a highly reactive Li layer is formed on the electrode surface, 2) Li₃N is formed when Li reacts with dissolved N₂, and 3) NH₃ is produced via sequential protonation, resulting in Li⁺ ions in the electrolyte that close the catalytic cycle. Based on this concept, the Li redox couple is “mediated” to complete mixed electrolytic and chemical reactions. The detailed mechanism can be subcategorized depending on whether each pathway is concerted or proceeds stepwise.

Cai et al. experimentally demonstrated three stepwise mechanisms, as suggested in Figure 3E.^{[ 79 ]} They observed that NH₃ continued to be produced from N₂ even when the voltage was no longer applied after a certain period of electrolysis. They claimed that the Li metal layers on the electrode remained sufficiently reactive to continue the N₂ splitting cycle until Li was completely consumed. Several control experiments were performed to test this hypothesis. They found that the reductive current values remained almost unchanged under N₂ and Ar environments or with and without ethanol. This implies that N₂ reduction or ethanol splitting is not involved in the electrochemical process, and the injected electrons are only consumed for Li plating.

For the concerted pathway of (1) and (3), the expulsion of NH₃ from Li₃N and Li redeposition was suggested. Andersen et al. suggested kinetic models wherein electrochemical Li deposition is accompanied by hydrogenation of the adsorbed N₂ molecules.^{[ 80 ]} Based on this mechanism, they reported that high‐energy electrons from highly negative potentials could be restored in the Li layer in the absence of an electric field and could be used to produce NH₃ from N₂. Additionally, Ma et al. suggested a (1) and (2) concerted model for N₂ fixation in the Li–N₂ battery cycle.^{[ 81 ]} During the discharging step, Li⁺ reacts with nitrogen to form Li₃N via an electrochemical process, while the Li anode provides Li⁺ ions.

To elucidate the cycle with a concerted proton‐electron transfer model, Schwalbe et al. hypothesized that the long‐lived Li metal layer might provide heterogeneous active sites, which assist the coupled proton–electron transfer in the catalytic cycle.^{[ 61 ]} Figure 3F depicts the stepwise reaction pathways to yield NH₃ from N₂, analogous to the direct eNRR pathway. In this mechanism, Li⁺ is no longer participating in the catalytic cycle. Instead, N₂ splitting and its hydrogenation become voltage‐dependent processes on the Li metal layer. The authors suggested that the mixed Li _x N _y H _z species (e.g., lithium imide or lithium amide) formed on the electrode surface can act as catalytically active intermediates for the eNRR.

2.2.3. Electrokinetic Studies

Several research groups have endeavored to elucidate the complex kinetic nature of Li‐mediated nitrogen reduction.^{[ 82} , ^{83 ]} Lazouski et al. reported continuous Li‐NRR (0.2 m LiClO₄ and 0.2 m ethanol in THF) under ambient conditions. Furthermore, they attempted to analyze the overall kinetics of NRR through a series of electrokinetic studies.^{[ 75 ]} Figure  4A depicts two possible reaction routes in the Li‐NRR. After being plated on the electrode, Li can react with either N₂ to generate Li₃N or with a proton source (HA) to produce molecular hydrogen. Li₃N yields NH₃ by reacting with HA. The rate of each pathway can be expressed as follows

$$r_{{\mathrm{H}}_2}=k_1{\left[\mathrm{Li}\right]}^{\alpha }{\left[\mathrm{EtOH}\right]}^x$$ (3)

$$r_{{\mathrm{NH}}_3}=k_2{\left[\mathrm{Li}\right]}^{\beta }{\left[{\mathrm{N}}_2\right]}^1$$ (4)

Figure 4.

Electrokinetic studies of INRR. A) Two reaction pathways occur in the Li‐mediated NRR. When the lithium layer is formed by reduction, it is spontaneously consumed through either hydrogen evolution (upper) or ammonia formation (bottom) with the help of proton donor. Log–log plot of partial current density toward NH₃ with varying B) partial pressures of N₂ and C) ethanol concentration as proton donor. Reproduced with permission.^{[ 75 ]} Copyright 2019, Elsevier. Simulated Faradaic efficiency toward NH₃ depending on the proton donor concentration at different D) rate constant ($k_{{\mathrm{N}}_2})$ for nitrogen reduction and E) diffusivity of nitrogen (${\mathrm{D}}_{{\mathrm{N}}_2})$. Reproduced with permission.^{[ 77 ]} Copyright 2022, American Chemical Society.

The reaction order for nitrogen pressure was found to be first‐order (Figure 4B) from the dependence of the partial current density toward NH₃ ($i_{{\mathrm{NH}}_3}$) on the nitrogen partial pressure (Equation (2)). Similarly, Figure 4C displays the log–log plot of the $i_{{\mathrm{NH}}_3}$ versus the concentration of ethanol, a proton donor. The NH₃ partial current first increases with ethanol concentration up to 0.1 m, and then it monotonically decreases, exhibiting a −1.5 order dependence on ethanol concentrations. The authors claimed that the obtained negative reaction order was attributed to the contribution from the hydrogen evolution (Figure 4A, top), which depleted the available Li layer for nitridation.

The subsequent hydrogenation of Li₃N to NH₃ was assumed to be a rapid step because the Li₃N layer was scarcely observed in the cathode. Li₃N formation was considered the RDS. Because the partial current toward hydrogen was considerably higher than that toward NH₃, the authors further assumed that 1) the reaction rate should be $r_{{\mathrm{NH}}_3}\ll r_{{\mathrm{H}}_2}$, 2) Li was in a quasisteady state during NRR (the rate of change in Li concentration was ≈0), and 3) most of the applied current was used to generate the Li layer. The abovementioned experimental results and assumptions lead to the following relationships

$$\begin{array}{ccc}\hfill F\frac{d\left[\mathrm{Li}\right]}{dt}& =& I-k_1{\left[\mathrm{Li}\right]}^{\alpha }{\left[\mathrm{EtOH}\right]}^x-k_2{\left[\mathrm{Li}\right]}^{\beta }{\left[{\mathrm{N}}_2\right]}^1\hfill \\ & \approx & I-k_1{\left[\mathrm{Li}\right]}^{\alpha }{\left[\mathrm{EtOH}\right]}^x\approx 0\hfill \end{array}$$ (5)

$$\left[\mathrm{Li}\right]={\left(\frac{I}{k_1{\left[\mathrm{EtOH}\right]}^x}\right)}^{\frac{1}{\alpha }}$$ (6)

Subsequently, the obtained Li concentration (Equation (4)) was employed to correlate $F{E}_{{\mathrm{NH}}_3}$ and total current. $F{E}_{{\mathrm{NH}}_3}$ showed a one‐half‐order dependence on the total current, resulting in the following expression, which explains the dependence of $F{E}_{{\mathrm{NH}}_3}$ on the reacting species and current

$$\begin{array}{ccc}\hfill F{E}_{{\mathrm{NH}}_3}& =& \frac{k_2{\left[\mathrm{Li}\right]}^{\beta }{\left[{\mathrm{N}}_2\right]}^1}{I}\approx \frac{k_2{\left[\mathrm{Li}\right]}^{\beta }{\left[{\mathrm{N}}_2\right]}^1}{k_1{\left[\mathrm{Li}\right]}^{\alpha }{\left[\mathrm{EtOH}\right]}^x}\hfill \\ & =& \left(\frac{k_2}{k^{1+\frac{\beta -\alpha }{\alpha }}}\right)\frac{\left[{\mathrm{N}}_2\right]}{{\left[\mathrm{EtOH}\right]}^{x\left(1+\frac{\beta -\alpha }{\alpha }\right)}}{I}^{\frac{\beta -\alpha }{\alpha }}\hfill \end{array}$$ (7)

$$\frac{\beta -\alpha }{\alpha }=\frac{1}{2},-x\left(1+\frac{\beta -\alpha }{\alpha }\right)=-x\left(1+\frac{1}{2}\right)=-1.5$$ (8)

The ethanol reaction order x was deduced to be unity and the Li reaction order, α, and β, to 2 and 3, respectively.

In the subsequent study, Lazouski et al. further endeavored to describe the role of proton donors in terms of transport kinetics. They attempted to correlate the permeability of the solid electrolyte interface (SEI) and the types/concentrations of proton donors.^{[ 77 ]} They postulated that two different types of SEIs (permeable and impermeable) coexist in the system; the permeable SEI fraction was denoted by θ. The effective diffusivity of 1) nitrogen and 2) proton donors through the SEI was considered as a critical parameter of θ in their model

$$N_j=\left(D_{j,i}\left(1-\theta \right)+D_{j,p}\theta \right)\left(C_{j,\mathrm{bulk}}-C_{j,s}\right)$$ (9)

Where D_j is the diffusivity and N_j is the total diffusion flux of species j through the SEI; C_{j ,bulk} and C_j,s are the concentrations of species j at the SEI and at the reactive electrode surface beneath the SEI, respectively. At a steady state, the rate of the NRR should be equal to that of nitrogen diffusion through the SEI

$$\begin{array}{ccc}\hfill r_{{\mathrm{N}}_2}& =& k_{{\mathrm{N}}_2}{C}_{N_2,\mathrm{S}}{C}_{\mathrm{Li}}=\left(D_{{\mathrm{N}}_2,i}\left(1-\theta \right)+D_{{\mathrm{N}}_2,p}\theta \right)\left(C_{{\mathrm{N}}_2,\mathrm{bulk}}-C_{{\mathrm{N}}_2,\mathrm{s}}\right)\hfill \\ & =& N_{{\mathrm{N}}_2}\hfill \end{array}$$ (10)

The rate of all the reactions involving the proton donor can be expressed as follows. r _HA,red indicates the reduction rate toward hydrogen evolution, and r _HA,prot is protonation rate of dinitrogen molecule to ammonia

$$r_{\mathrm{HA},\mathrm{red}}=k_{\mathrm{HA}}{C}_{\mathrm{HA},\mathrm{S}}{C}_{\mathrm{Li}}$$ (11)

$$r_{\mathrm{HA},\mathrm{prot}}=6k_{{\mathrm{N}}_2}{C}_{{\mathrm{N}}_2,\mathrm{S}}{C}_{\mathrm{Li}}$$ (12)

$$\begin{array}{ccc}\hfill r_{\mathrm{HA}}& =& k_{\mathrm{HA}}{C}_{\mathrm{HA},\mathrm{S}}{C}_{\mathrm{Li}}+6k_{{\mathrm{N}}_2}{C}_{{\mathrm{N}}_2,\mathrm{S}}{C}_{\mathrm{Li}}\hfill \\ & =& \left(D_{\mathrm{HA},\mathrm{i}}\left(1-\theta \right)+D_{\mathrm{HA},\mathrm{p}}\theta \right)\left(C_{\mathrm{HA},\mathrm{bulk}}-C_{\mathrm{HA},\mathrm{S}}\right)=N_{\mathrm{HA}}\hfill \end{array}$$ (13)

Similar to Equation (3), the accumulation rate of the Li layer can be expressed using the current density and consumption rate (Equation (12))

$$F\frac{d{C}_{\mathrm{Li}}}{dt}=I-F\left(k_{\mathrm{HA}}{C}_{\mathrm{HA},\mathrm{S}}{C}_{\mathrm{Li}}+6k_{{\mathrm{N}}_2}{C}_{{\mathrm{N}}_2,\mathrm{S}}{C}_{\mathrm{Li}}\right)\ge 0$$ (14)

The surface concentrations of N₂, HA, and Li species $(C_{{\mathrm{N}}_2,\mathrm{S}},C_{\mathrm{HA},\mathrm{S}}{C}_{\mathrm{Li}})$ can be obtained from Equation (12) as assuming steady‐state conditions and constraints from Equations (8) and (9). Considering the stoichiometry of the reacting species and the relationship between FE and the reaction rate, the following equations are derived

$$r_{{\mathrm{NH}}_3}=2·\min \left(r_{{\mathrm{N}}_2}\right)$$ (15)

$$F{E}_{{\mathrm{NH}}_3}=\frac{3F{r}_{{\mathrm{NH}}_3}}{I}$$ (16)

The rate of NH₃ evolution is limited by the diffusivity of nitrogen (Equation (8)). The FE for hydrogen is then derived by subtracting the NH₃ rate from the total HA‐involved reaction rate

$$F{E}_{{\mathrm{H}}_2}=\frac{F·(r_{\mathrm{HA}}-3r_{{\mathrm{NH}}_3})}{I}$$ (17)

Therefore, to minimize the $F{E}_{{\mathrm{H}}_2}$, the proton diffusion rate should be controlled to limit NH₃ formation (Equation (15)). Based on the kinetic diffusion model, the authors attempted to reveal the effects of diffusivity and rate constants. Figure 4D,E shows that the peak $F{E}_{{\mathrm{NH}}_3}$ is expected to be more strongly affected by the relative diffusivities of N₂ and H₂ than by the overall rate constant for the NRR. Because commonly used proton donors are sterically bulky, the nitrogen diffusivity in the SEI layer is typically higher than that of the proton donor. Thus, even if the rate constant for N₂ reduction is lower than for proton donor reduction, a high FE can still be achieved as diffusion of nitrogen is favored over proton in SEI. This implies that the SEI serves as a protective layer for selective NH₃ production by suppressing undesirable hydrogen evolution and exclusively promoting N₂ transport.

3. Strategies to Advance the eNRR Performance

3.1. Catalyst Design Strategies for Enhancing DNRR Performances

The practical applications of the eNRR are hindered by the slow reaction kinetics and competitive HER, resulting in low FE. N₂ is composed of two N atoms, each with five valence electrons in the 2s and 2p orbitals. The electrons occupy all bonding orbitals and one σ* orbital, making this chemically stable form. In terms of charge exchange, donating electrons from an occupied N₂ σ orbital to an empty metal d orbital, with the back donation from an occupied metal d orbital to an empty N₂ π* orbital, weakens the triple N─N bonds and promotes eNRR.^{[ 84 ]} Therefore, the electrocatalyst design can efficiently enhance eNRR performance because the charge exchange property is directly related to the chemical features of the catalyst surface.^{[ 3} , ^{85 ]} Direct electrochemical NH₃ synthesis from nitrogen and water is a simple alternative to the conventional Haber–Bosch method.^{[ 3 ]} Carbon‐based materials, metals, nonmetals, metal oxides, metal sulfides, metal nitrides, bimetallic and ternary nitrides, and single‐atom catalysts have been developed as catalysts for the eNRR in aqueous electrolytes. The intrinsic activity of the active sites is influenced by the electronic structure of the catalyst, which can be regulated through defect/strain engineering.^{[ 85 ]} In contrast, the apparent activity, which depends on the yield, is influenced by the structure and morphology of the catalyst. The apparent activity of the electrocatalyst can be enhanced through surface area modification, pore engineering, and hybridization. Herein, we discuss the effects of defect engineering, surface geometry/morphology, and heterostructure formation on the eNRR performance of electrocatalysts in aqueous electrolytes.

3.1.1. Defect Engineering

Defect engineering is an efficient strategy for regulating the electronic structure of electrocatalysts and consequently enhancing N₂ adsorption.^{[ 86} , ^{87 ]} Moreover, defect engineering alters the adsorption energy of the intermediates, thereby suppressing the HER and improving the selectivity of eNRR. The defects can primarily be induced by doping with heteroatoms or by forming vacancies. Doping induces charge redistribution and improves the charge transfer abilities of the electrocatalyst.^{[ 88 ]} Boron, nitrogen, and sulfur are commonly used as dopants in electrocatalysis. When graphene is doped with 6.2% boron, the NH₃ yield rate and FE of the doped graphene increase by 5 and 10 times, respectively, compared with those of the undoped graphene.^{[ 86 ]} This increase in performance is attributed to the difference in electronegativity between boron (2.04) and carbon (2.55). The electron‐deficient boron sites have an increased tendency to bind to N₂, thus enabling the formation of B─N bonds and the subsequent production of NH₃. Similarly, Wang et al. reported the role of N‐doping in improving the eNRR activity of NiO nanosheet arrays on carbon cloth (NiO/CC).^{[ 88 ]} Initially, a hydrothermal method was used to synthesize NiO/CC, which was subsequently doped with nitrogen (N–NiO/CC) by annealing in an Ar/NH₃ atmosphere. As deduced from density function theory (DFT) computations, N doping modified the electronic structure of NiO, thereby promoting the activation of N₂, improving the stabilization of the key intermediate *NNH, and decreasing the reaction energy barrier. Consequently, the eNRR activity was enhanced. N─NiO/CC resulted in an NH₃ production rate of 22.7 µg h⁻¹ mg⁻¹ and 7.3% FE in 0.1 m LiClO₄ aqueous electrolyte at −0.5 V versus the reversible hydrogen electrode (RHE). In contrast, vacancies create active sites with abundant localized electrons, which can weaken N≡N.^{[ 89 ]} Peng et al. reported the eNRR activity of carbon nitride with N_2C vacancies, which was grown on carbon fiber paper (CN/C) through chemical vapor‐assisted synthesis.^{[ 90 ]} CN with perfect planes is characterized by low catalytic activity, whereas CN with N_2C defects exhibits strong interactions with N₂. The growth conditions can control the number of vacancies on CN. CN/C₆₀₀ synthesized at 600 °C has more catalytic sites and a higher electrochemically active surface area (ECSA) than CN/C₅₀₀. The ratio of deconvoluted peaks (C═N─C/N─C₃) in the N 1s spectra of CN/C₆₀₀ and CN/C₅₀₀ (Figure  5A) indicates the presence of C═N─C N_2C vacancies in CN/C₆₀₀. In addition, the increased ratios of D/G peaks in the Raman spectrum of CN/C₆₀₀ (Figure 5B) indicate an increased number of defects compared with those in CN/C₅₀₀ and bare carbon paper. Based on the slope of the m_NH3–time plot, the NH₃ yield and FE for CN/C₆₀₀ at −0.3 V versus RHE were evaluated as 2.9 µg mg_cat ⁻¹ h⁻¹ and 16.8% FE, respectively (5.7 times higher than those for CN/C₅₀₀) (Figure 5C,D).

Figure 5.

Defect engineering of electrocatalysts. A) N 1s XPS spectra of CN/C₅₀₀ and CN/C₆₀₀, B) Raman spectra of CN/C₅₀₀, CN/C₆₀₀, and C paper, C) NH₃ yield rate, and D) Faradaic efficiency of CN/C₆₀₀ and CN/C₅₀₀ at different potentials. Reproduced with permission.^{[ 87 ]} Copyright 2020, American Chemical Society. E) TEM image, F) XRD pattern, G) EPR spectrum, and H) FE and NH₃ production rate of various C–Ti _x O _y /C hybrids. Reproduced with permission.^{[ 89 ]} Copyright 2019, Wiley.

Jin et al. reported the eNRR activity of W₂N₃ nanosheets with stable nitrogen vacancies.^{[ 91 ]} Long‐term electrolysis and postcharacterization techniques indicated the stability of nitrogen vacancies in the W₂N₃ nanosheets. DFT calculations predicted that the high stability of these vacancies could be attributed to the 2D confinement effect and the high valence state of the W atoms. Furthermore, active catalytic centers could be created by incorporating anions into metal oxides, thereby improving the eNRR performance.^{[ 92 ]} C─Ti _x O _y /C structures were obtained by annealing MIL‐125 (Ti) (a Ti‐based metal–organic framework) at different temperatures (800—1100 °C). The oxygen vacancies in TiO₂ were substituted with carbon atoms, thus forming Ti─C bonds in C─Ti _x O _y . The high‐resolution transmission electron microscopy (TEM) image of M‐1000 (MIL‐125 (Ti) annealed at 1000 °C) indicates the uniform dispersion of TiO₂ nanoparticles (NPs) ranging between 3 and 20 nm in an amorphous environment (Figure 5E). In the X‐ray diffraction (XRD) pattern (Figure 5F), peaks attributed to tetragonal rutile or anatase are observed. However, as the temperature increases from 800 to 1100 °C, peaks attributed to TiC are more dominant than rutile or anatase titania. The increased intensity of the peaks in the electron paramagnetic resonance (EPR) spectrum indicates a higher concentration of oxygen vacancies at higher temperatures (Figure 5G). The eNRR performance of C─Ti _x O _y /C is improved, with an NH₃ yield rate of 14.8 mg h⁻¹ mg_cat ⁻¹ and 17.8% FE (Figure 5H).

3.1.2. Surface Geometry/Morphology Engineering

The apparent activity of the electrocatalyst can also be enhanced by tailoring the morphology of nanostructures and engineering the surface chemistry.^{[ 93} , ⁹⁴ , ^{95 ]} 3D bimetallic PdRu porous nanomaterials with interconnected voids and skeletons exhibit promising eNRR performance.^{[ 93 ]} These porous electrocatalysts provide numerous active sites for reducing N₂ to NH₃ and facilitate the mass transfer of N₂. Porous bimetallic PdRu provides a satisfactory NH₃ yield, which declines by only 8% after long‐term stability tests. Also, engineering the surface chemistry through appropriate electrolyte is important for enhancing eNRR performances. Wang et al. prepared a porous Au film electrodeposited on Ni foam (Au/NF) through micelle‐assisted deposition, and the Au/NF exhibited excellent eNRR performance. Compared to the total current density at −0.2 V _RHE in 0.1 m KOH, a lower total current density was obtained in 0.1 m Na₂SO₄ (≈0.25 mA cm⁻² in 0.1 m Na₂SO₄ and ≈3.3 mA cm⁻² in 0.1 m KOH), indicating the suppression of the HER in 0.1 m Na₂SO₄ due to the low charge and mass transfers. However, similar NH₃ yield rates were obtained in both electrolytes (9.42 µg h⁻¹ cm⁻² in 0.1 m Na₂SO₄ and 10.50 µg h⁻¹ cm⁻² in 0.1 m KOH), and the calculated FE was 9 times higher in 0.1 m Na₂SO₄ than in 0.1 m KOH (13.36% in 0.1 m Na₂SO₄ and 1.42% in 0.1 m KOH). In this case, Na₂SO₄ can be considered a promising electrolyte for the eNRR on the porous binder‐free Au/NF electrode.^{[ 96 ]}

Engineering surface features on an atomic scale has shown several insightful results for improving eNRR performance. For example, MoS₂ nanoflowers with disordered atomic arrangements and crystal defects are regarded as efficient catalysts for the eNRR.^{[ 97 ]} The disorder and defects improve the stability of the nanoflower by tuning the properties and activity of the reactive sites. Irregularly structured catalysts with high‐index facets and multiple edge/corner sites are favored because of their effective binding with the reactants.^{[ 98 ]} In contrast, Hao et al. showed that the eNRR performance of a Ni₃S₄ catalyst with low energy and a smooth surface was higher than that of those with distorted surfaces.^{[ 94 ]} Ni₃S₄ prepared using two different synthetic protocols resulted in extended flat facets and irregular surfaces, as shown in the scanning electron microscopy (SEM) images in Figure  6A (right image for the flat facet and left inset image for the irregular surface in the insets). The eNRR reaction products collected after 2 h were quantified using ¹H‐NMR (¹H nuclear magnetic resonance) spectroscopy under extended scan times, and spurious signals were eliminated based on control experiments under varied synthesis conditions. The catalytic activity of low‐energy Ni₃S₄ with flat facets was three times higher than that of Ni₃S₄ with a distorted surface (Figure 6B). An NH₃ production rate of 1.28 µg mg⁻¹ h⁻¹ and 6.8 ± 3.3% FE were achieved for Ni₃S₄ with smooth facets, whereas the FE of the distorted Ni₃S₄ was only 1.9 ± 3.6%. According to DFT calculations, the availability of multiple vacant Ni sites and the competing nitrogen and hydrogen adsorption influence the enhanced eNRR activity of the smooth facets (Figure 6C,D). Coordinatively vacant Ni sites in the flat low‐energy facets are readily available for N₂ binding, thus improving the eNRR performance.

Figure 6.

Engineering the surface/morphology of the electrocatalysts. SEM images of A) Ni₃S₄ powder with flat facets and the control Ni₃S₄ powder with irregular surfaces. B) Comparison of FEs for samples and blank controls used for subtracting the spurious signal. C) H adsorption energies on all facets referenced to 1/2H₂. D) N₂ adsorption energy for all Ni sites. Reproduced with permission.^{[ 94 ]} Copyright 2021, American Chemical Society. E) HAADF‐STEM image and corresponding elemental mapping analysis of Cu_1.81S hexagonal nanoprism. F) aspect ratio of the nanoprisms. G) NH₃ production yield and Faradaic efficiency. H) exposed facet densities (nm⁻¹) at basal (EFD‐B; red) and prismatic (EFD‐P; blue) planes. I) simulations of H* transfer configurations on Cu_1.81S(010) and Cu_1.81S(100). Reproduced with permission.^{[ 99 ]} Copyright 2022, American Chemical Society.

Accordingly, Jin et al. emphasized the importance of the coordination environment in catalytic activity and reported the influence of the atomic geometry of NPs on the eNRR kinetics.^{[ 99 ]} The dependence of eNRR activity on the surface geometry of copper sulfide (Cu_1.81S) nanocatalysts with exposed facets of (100) and (010) for smooth and zigzag planes, respectively, was investigated. High‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) images indicate the uniform distribution of Cu and S with the aspect ratio differences from 0.13 for CS‐10 to 4.37 for CS‐120 (x and y indicate zigzag and smooth planes, respectively) (Figure 6E,F). The NH₃ production rate and FE increase with the increasing length of the hexagonal nanoprisms (Figure 6G); the improvement in the eNRR performance is associated with the prismatic plane density (Figure 6H). Also, as the prismatic plane density increased, lower current density was obtained at −0.5 V _RHE under Ar saturated electrolyte. This result indicates that the zigzag surface geometry limits the competitive HER, which is consistent with the results of eNRR. DFT calculations reveal that the protruded atoms on the zigzag planes of Cu_1.81S nanoprisms have a reduced proton transfer distance from 2.81 Å on the (100) to 1.71 Å on the (010), and H efficiently migrates from H*@S to N₂*@Cu, thus improving the eNRR kinetics (Figure 6I).

The catalytic performance of single‐atom catalysts can be improved by engineering their geometry through metal atoms or coordination anions.^{[ 100} , ^{101 ]} For example, a tungsten single‐atom catalyst with oxygen and nitrogen coordination and a metal loading exceeding 10 wt% provides satisfactory eNRR performance.^{[ 101 ]} The O and N coordination on tungsten influences its adsorption behavior and selectivity, thus improving the performance. Li et al. reported a strategy of interfacial polarization to facilitate the breaking of N≡N bonds and enhance NH₃ production.^{[ 102 ]} The electric fields produced from the protruding Fe single‐atom catalysts immobilized on MoS₂ can polarize N₂ by attenuating the interfacial energy barrier. The resultant polarization fields between N₂ and Fe–MoS₂ drive additional electrons into the antibonding orbitals of N₂, thus improving the NH₃ production rate. Therefore, the morphology of the nanomaterial affects the electric field that is generated to polarize N≡N.

3.1.3. Heterostructure

Heterostructure engineering is a facile strategy to enhance the catalytic activity of nanomaterials. Owing to the synergy from individual components in multicomponent nanostructures (hybrid structures), their eNRR activity is higher than that of the individual components.^{[ 103} , ^{104 ]} The interface between two components in hybrid structures imparts unique properties to the individual components, promoting abundant electron channels and accelerating the electron transfer rate.^{[ 105 ]} Interface engineering primarily includes metal (derivative)–carbon, metal (derivative)–metal oxide, and intermetallic interfaces.^{[ 106 ]} SnO₂ quantum dots (QDs) distributed on reduced graphene oxide (SnO₂/RGO), synthesized via self‐propagating combustion, are demonstrated to be efficient catalysts for producing NH₃ at a rate of 25.6 µg h⁻¹ mg⁻¹ and 7.1% FE in 0.1 m Na₂SO₄ at −0.5 V (vs RHE).^{[ 107 ]} The RGO support improves the conductivity of the SnO₂ QDs and prevents their agglomeration, thereby increasing the surface area and number of active sites for N₂ adsorption. DFT calculations reveal that the highly conductive hybrid structure lowers the energy barrier of *N₂ → *N₂H and promotes eNRR activation. Wang et al. observed a similar phenomenon in a CuO/graphene nanocomposite.^{[ 104 ]} The interface engineering of 2D/2D materials enables face‐to‐face contact, facilitating strong electronic interactions at the interface.^{[ 108 ]} In the MoS₂/C₃N₄ heterostructure, electrons migrate from C₃N₄ to MoS₂ across the interface, thus creating a charge density difference. This electron redistribution significantly increases the conductivity of MoS₂/C₃N₄. The crucial intermediate *N₂H is stabilized on the Mo edge sites of MoS_2, and the reaction energy barrier decreases. Furthermore, *H is adsorbed on the S edge sites, thus protecting the NRR‐active Mo edge sites from the competing HER.

The synergistic effect between Fe₃O₄ and Au in Au–Fe₃O₄ NPs results in a higher eNRR activity than that of Fe₃O₄, Au, and Au@Fe₃O₄ (core@shell) NPs.^{[ 103 ]} Au–Fe₃O₄ and Au@Fe₃O₄ NPs synthesized through a simple one‐pot wet‐chemical method have similar morphologies (Figure  7A). The Au 4f XPS spectra (Figure 7B) show that the ratio of Au³⁺ to Au⁰ in Au–Fe₃O₄ NPs is higher than that in the other catalysts. Additional Au─O bonds in the Au–Fe₃O₄ NPs facilitate their strong binding to the intermediates, thereby enhancing the eNRR. The Au–Fe₃O₄ NPs provide an NH₃ yield rate of 21.42 µg mg_cat ⁻¹ h⁻¹ and 10.54% FE at −0.2 V versus RHE (Figure 7C). The enhanced N₂ chemisorption peak of Au–Fe₃O₄ NPs in the N₂ temperature‐programmed desorption curves demonstrates the unique role of the heterojunction structure in the eNRR performance. Theoretical calculations indicate that the N₂ to *N₂H transition occurs on the Fe atoms, while Au optimizes the adsorption energy of NRR intermediates, making the NRR path on Au–Fe₃O₄ along an energetically favorable process. A metastable amorphous material with dangling bonds is known to be more catalytically active than its crystalline counterpart.^{[ 109 ]} Li et al. reported that the eNRR activity of amorphous Au NPs supported on bisubstrate CeO _x –RGO (a‐Au/CeO _x –RGO) was higher than that of their crystalline counterparts. This hybrid catalyst with a low Au loading (1.31 wt%) was prepared via coreduction. CeO _x facilitates the conversion from crystalline to amorphous Au, whereas RGO serves as a support for the uniform dispersion of the a‐Au NPs. The high concentration of unsaturated coordination sites in the metastable amorphous material consequently improves the chemical reactivity with N₂ molecules. Subsequently, Au sub‐nanoclusters (≈0.5 nm) embedded on TiO₂ with a loading of 1.542 wt% were demonstrated to be promising eNRR catalysts.^{[ 110 ]} The size of the Au clusters significantly influences the formation of NH₃ from N₂. Moreover, in an acidic electrolyte, the partially positively charged Au active centers, through the Au─O─Ti bond, preferentially adsorb electroneutral N₂ than H⁺ cations, resulting in an efficient eNRR.

Figure 7.

Heterostructure engineering of electrocatalysts. A) TEM images of Au–Fe₃O₄ (up) and Au@Fe₃O₄ (down) nanoparticles. B) XPS curves of Au 4f for different catalysts. C) yield and Faraday efficiency of different catalysts at −0.2 V versus RHE. Reproduced with permission.^{[ 103 ]} Copyright 2019, Wiley. HAADF‐STEM images of D) Ru/1T‐MoS₂ (left) and Ru/2H‐MoS (right). Schematics of the crystallographic difference between 2H and 1T‐MoS₂. E) NRR and F) HER activities of 2H‐MoS₂, 1T‐MoS₂, Ru/1T‐MoS₂, and Ru/2H‐MoS₂ in 10 mm aqueous HCl. Reproduced with permission.^{[ 111 ]} Copyright 2019, American Chemical Society.

To achieve high catalytic activity, Suryanto et al. reported a strategy for the polymorphic engineering of NPs.^{[ 111 ]} Ru clusters were decorated on the 1T and 2H polymorphs of MoS₂ to form Ru/1T‐MoS₂ and Ru/2H‐MoS₂ hybrids, respectively. Figure 7D depicts STEM images of the hybrid and crystal structures of 2H and 1T‐MoS₂. The catalytic activity and selectivity of Ru‐decorated semiconducting 2H‐MoS₂ toward eNRR are higher than that of metallic 1T‐MoS₂ (Figure 7E). Also, evaluation of HER performances revealed the crystal structure dependent eNRR/HER relationship. (Ru)/1T‐MoS₂ showed better HER performances than those of (Ru)/1H‐MoS₂ (Figure 7F). The eNRR performances show that Ru functions as N₂ active site and 1H‐MoS₂ provides protons to form NH₃ at the interface of Ru/1H‐MoS₂. Thus, a high eNRR activity can be obtained by tuning the HER kinetics of MoS₂ via polymorphic engineering. The synergistic effects between different metals in intermetallic compounds enable them to be potential candidates for the eNRR.^{[ 112} , ¹¹³ , ^{114 ]} Pd‐, Au‐, and Ru‐based alloys are commonly used as intermetallic compounds. According to Tong et al., body‐centered cubic PdCu NPs, obtained by annealing their face‐centered cubic (FCC) counterparts, are more favorable for the eNRR because the energy barrier is lower than that of FCC PdCu. The phase transformation of PdCu positively shifts its d‐band center and improves its ability to bind nitrogen molecules. DFT calculations reveal that the strong d–d coupling between Pd and Cu enhances the transfer activity, enabling an efficient eNRR.^{[ 115 ]}

3.2. Strategies for Enhancing INRR Performance

3.2.1. Engineering SEI Layers on the Li Cathode

As discussed in Section 2.2, the reaction is initiated after a highly reductive potential is applied to form a Li metal surface from Li⁺.^{[ 116 ]} The highly reductive environment can be detrimental to electrolytes, proton sources, or other electrolyte components, which electrochemically disintegrate into the SEI, and this has been extensively investigated in Li‐ion battery studies. The SEI is considered a “passivation layer” that exclusively allows Li cations to reach the electrode surface and acts as an ionic conducting separator to prevent further electrolyte decomposition or undesirable Li dendrite formation.^{[ 117} , ^{118 ]} The SEI promotes the stability and performance of the Li‐mediated NRR.^{[ 77} , ^{119 ]} Moreover, it can control the relative diffusion rates of Li⁺, H⁺, and N₂ ($r_{{\mathrm{Li}}^{+}}$, $r_{{\mathrm{H}}^{+}}$, $r_{{\mathrm{N}}_2}$), which are pivotal variables to affect the rate and selectivity. Despite considerable investigation, the formation mechanism and structure of the SEI in NRR remain unclear.

Several researchers have performed real‐time spectroscopic analyses to investigate the composition of the cathode surfaces during catalysis. Through the in situ XRD analysis of Au‐coated carbon fiber paper, Gao et al. confirmed that Au facilitates Li‐ion reduction (Figure  8A).^{[ 120 ]} They reported that the spontaneous initiation of nitrogen splitting to Li₃N and protonation to NH₃ occurred immediately after the rate‐determining Li‐ion reduction. A predominant peak of Li (101) was observed at ≈37°–38° only for Au on carbon paper (Au/CP) electrode, indicating that Au assists in rapid Li⁺ reduction kinetics. Only the Au/CP electrode exhibits a gradual increase in the Li₃N (100) peak, whereas the pristine CP electrode presents no signal.

Figure 8.

Investigation of SEI structure via spectroscopic analysis. A) Color map of operando‐XRD measurement during Li‐mediated NRR at Au/CP electrode to monitor the dynamic evolution of lithium‐containing intermediates. Reproduced with permission.^{[ 120 ]} Copyright 2020, Wiley VCH. B) XRD diffractograms of postelectrolysis electrode (Mo foil) with variation of O₂ mol%. Reproduced with permission.^{[ 118 ]} Copyright 2021, American Association for the Advancement of Science. C) Scattering‐length density profiles of before (Pre‐CP) and after (Post‐CP) NRR electrolysis on Mo electrodes demonstrating the formation of SEI layers and corresponding schemes of Pre‐CP and Post‐CP model, respectively. Reproduced with permission.^{[ 121 ]} Copyright 2022, American Chemical Society. D) Dept‐profiling XPS spectra of the Cu electrodes depending on the electrolyte materials, which shows the formation of different SEI layers. Reproduced with permission.^{[ 124 ]} Copyright 2022, Elsevier.

Although NRRs typically employ pure N₂ gas to maximize the N₂ content in the feed, practical applications may involve O₂ inputs from air or other sources. Li et al. discovered that adding small amounts of O₂ improves the efficiency and stability of the eNRR.^{[ 118 ]} To analyze these effects of O₂ on the SEI layer, the changes in the phase and surface composition were monitored using XRD and X‐ray photoelectron spectroscopy (XPS). The XRD data (Figure 8B) revealed that as the O₂ level increased to 3 mol%, various oxidized products were derived from the working electrode, owing to parasitic side reactions and oxygen reduction. Interestingly, the 0.8 mol% O₂ atmosphere only showed a distinct peak in N 1s XPS spectra, corresponding to the Li₃N layer, without forming any undesirable oxidized product. Through microkinetic modeling, the oxygen‐induced changes in the SEI layer were found to delay Li⁺ diffusion, while optimizing the availability of N₂ and H⁺ in the system.

Blair et al. investigated the dynamic behavior of the SEI structure and its composition via in situ neutron reflectometry.^{[ 121 ]} Scattering‐length density profiles of the cathode–electrolyte interface during Li‐NRR on Mo thin‐film electrodes were obtained before and after chronopotentiometry (CP). Figure 8C shows that only the Mo‐oxide surface layer is detected in the pre‐CP electrode, whereas additional components from the interfacial layer are observed in the post‐CP electrode. The secondary layer after CP was considered the decomposed electrolyte product (SEI layer).

Li et al. further attempted to reveal the effects of Li‐containing electrolyte salts through theoretical modeling and depth‐profiling XPS. Three different salts (LiBF₄, LiPF₆, and LiClO₄) were investigated using XPS spectra, based on the etching time (Figure 8D). SEI layer formation was confirmed after a short reductive electrolysis in all three cases. For example, a LiF‐enriched SEI layer was detected when using LiBF₄. They reported that a LiBF₄ electrolyte induced the formation of a compact and uniform SEI layer, resulting in homogeneous Li plating and enhanced NRR efficiency.^{[ 124 ]}

3.2.2. Electrode Engineering

Effective mass transport of N₂ to the electrode is crucial to facilitate the splitting of the dinitrogen bonds. However, owing to the low solubility of N₂ in nonaqueous solvents, the reactants are less accessible to the catalysts. To address the issue, Lazouski et al. designed an effective gas–liquid contacting gas diffusion electrode (GDE) in nonaqueous solvents.^{[ 76 ]} Figure  9A depicts the typical aqueous media/GDE system, which allows direct gas contact with the catalysts on hydrophobic carbon supports. On the other hand, with the nonaqueous solvent, the carbon supports are inevitably wetted, and the catalyst is flooded, which results in limited N₂ transport and a low mass‐transfer‐limited current. Lazouski et al. developed a catalyst‐deposited stainless‐steel cloth, and a pressure gradient was maintained across the cloth. As shown in Figure 9A, N₂ was able to freely approach the catalysts without passing through the liquid phase, significantly improving NRR reaction rates and selectivity.

Figure 9.

Electrode engineering strategies in INRR. A) Schematic illustration of structures of gas diffusion electrode in aqueous/porous carbon, nonaqueous/porous carbon (flooded), and nonaqueous/steel cloth. Proposed overall eNRR schemes with NRR at cathode and HOR on anode. Reproduced with permission.^{[ 76 ]} Copyright 2020, Springer Nature. B) SEM images of pristine Ni foam and HBT‐Cu/Ni foam. Reproduced with permission.^{[ 122 ]} Copyright 2021, American Chemical Society. C) Cross‐sectional SEM images of porous Cu electrode and its chronopotentiometry profile with LiBF₄ electrolyte. Reproduced with permission.^{[ 124 ]} Copyright 2022, Elsevier. D) Cyclic voltammetry curves of various metal anode materials in H₂ saturated 0.1 m LiNTf₂/THF solution with varying humidity conditions, Pd/C [H₂O] = 9 mm (semi dry) and 0.6 mm (dry), Ru/C [H₂O] = 5 mm (semidry) and 0.4 mm (dry), respectively. Reproduced with permission.^{[ 125 ]} Copyright 2022, American Chemical Society.

In the Li‐NRR scheme, the working electrode provides active and stable sites for Li deposition. Tsuneto et al. tested several metals as working electrodes; Mo and Cu electrodes have typically been used in Li‐mediated NH₃ synthesis because they do not form alloys with Li,^{[ 123 ]} thus ensuring the reversible formation of the Li layer on the electrode. Figure 9B displays the SEM images of pristine Ni foam (left) and Cu electrodeposited on Ni foam using the hydrogen bubble template (HBT) method (right).^{[ 122 ]} Li et al. reported that a high‐surface‐area Cu electrode fabricated via the HBT method successively demonstrated a current density of −100 mA cm_geo ⁻² at 20 bar N₂. Owing to the porous Ni foam and the HBT method, a high ECSA of 66.5 ± 7.1 cm² was obtained with a geometric surface of 0.5 cm². The HBT–Cu electrode exhibited a NH₃ yield rate (46.0 ± 6.8 nmol s⁻¹ cm_geo ⁻²) superior to that of the pristine Cu electrode. Norskov & Chorkendorff group further attempted to use stainless steel mesh to prepare porous Cu via the HBT method.^{[ 124 ]} Figure 9C shows cross‐sectional SEM images of the porous Cu electrode on the stainless steel mesh. The three porous Cu samples prepared with different deposition times (15 s, 1 min, and 5 min) exhibited different specific capacitances and ECSA. The 5 min Cu sample exhibited the highest performance (15.4 mF cm_geo ⁻²; 308 cm²), resulting in 95% ± 3% FE at a current density of −100 mA cm_geo ⁻² under 20 bar N₂. Additionally, stable ammonia production was achieved on the porous Cu/steel electrode with the use of LiBF₄ as the electrolyte condition.

In the typical Li‐NRR scheme, the anodic reaction has not been thoroughly investigated compared to the cathodic reaction. Recently, several studies have been conducted to replace the anodic reaction with a hydrogen oxidation reaction (HOR) by introducing an external hydrogen feed.^{[ 125} , ^{126 ]} In this scheme, the anodic reaction can contribute to the efficiency of the Li‐NRR by providing protons to the catholyte. To determine the catalytic properties of the HOR, Hodgetts et al. investigated the poisoning on the catalyst surface by varying the water content.^{[ 125 ]} The authors hypothesized that poisoning of the surface from the adsorption of the electrolyte species and organic solvent oxidation products can be overcome by choosing proper electrode materials and water contents. Figure 9D displays the cyclic voltammetry curves of Pd/C and Ru/C catalysts under semidry and dry conditions (semidry: [H₂O] = ≈4–10 mm/dry: [H₂O] = ≈0.4–0.6 mm). The HOR current densities of Ru/C at 0.4 V versus Fc^0/+ are higher than those of Pt/C under dry conditions. Both catalysts exhibit favorable HOR activities, but are plagued by low stability owing to water poisoning. Accordingly, bimetallic PtRu/C HOR catalysts were suggested with optimal catalytic activity and water tolerance under HOR.

3.2.3. Electrolyte Engineering

The recent efforts for engineering SEI compositions and electrode materials have been discussed thus far. Other approaches have been also suggested for enhancing the Li‐NRR performance.^{[ 127 ]} Manthiram et al. examined diverse proton donors to reveal the effect of the proton donor structure on the transport kinetics in Li‐NRR.^{[ 77 ]} Figure  10A shows that slight changes in the proton donor structure significantly impact the selectivity toward NH₃. The electrolytes that form similar SEI species to that derived from THF can result in higher FEs owing to the enhanced SEI permeability, which directly affects the diffusion rates of N₂ and the proton donor to the electrode surface. Here, n‐butanol exhibits the highest NH₃ FE and forms an SEI similar to that obtained from THF.

Figure 10.

Electrolyte component engineering in INRR. A) Faradaic efficiency toward NH₃ with the variation of proton donor species and concentration. Reproduced with permission.^{[ 77 ]} Copyright 2022, American Chemical Society. B) NH₃ yield rate and Faradaic efficiency versus conductivity of various electrolyte species. Reproduced with permission.^{[ 128 ]} Copyright 2022, Springer Nature. C) ³¹P NMR spectra of phosphonium proton carrier during deprotonation and reprotonation cycle. [P_6,6,6,14]⁺ cation peak at 32.9 ppm diminished after the reaction with Li₃N and a new peak at 9.3 ppm emerged, which was assigned to the deprotonated ylide form. The acetic acid treatment restored the peak at 32.9 ppm. Reproduced with permission.^{[ 129 ]} Copyright 2021, American Association for the Advancement of Science.

Apart from adjusting the proton donors, the Li‐containing electrolyte, another key component in Li‐NRR, is being investigated. Recently, Du et al. reported almost 100% FE with 2 m LiNTF₂.^{[ 128 ]} According to the plot of FE and NH₃ yield rates against the conductivity of various Li compounds (Figure 10B), LiNTF₂ has the highest NH₃ yield rate and FE. The dense ionic assembly formed at the electrode–electrolyte interface suppresses electrolyte decomposition and provides high Li⁺ transference numbers.

In INRR scheme, the “proton shuttle” can play an important role to deliver the protons generated from the anode to the lithium cathode for the hydrogenation of lithium nitride. The proton shuttle can reversibly and reproducibly operate the catalytic cycle without participating in the side reaction during catalysis. In this vein, Suryanto et al. suggested reproducible phosphonium cation trihexyltetradecylphosphonium ([P_6,6,6,14]⁺) as a proton shuttle.^{[ 129 ]} The authors demonstrated the reversibility of the salts with ³¹P NMR (Figure 10C) analysis. When the [P_6,6,6,14][eFAP] (([P_6,6,6,14]‐phosphonium tris(pentafluoroethyl)trifluorophosphate) solution was treated with excess Li₃N, [P_6,6,6,14]⁺ cation peak at 32.9 ppm was diminished and a new peak at 9.3 ppm emerged, corresponding to the deprotonated ylide form. Recovery of the proton shuttle was achieved by adding a proton source.

4. Summary and Outlook

In this review, we have highlighted the recent progress in electrochemical nitrogen reduction catalysis for NH₃ production. We defined two approaches in terms of the reaction mechanisms: DNRR and INRR. DNRR follows a conventional electrocatalysis scheme and thus the development of an effective catalyst governs the overall performance of the system. We also discussed the currently adopted mechanistic pathways of nitrogen reduction and introduced several strategies for developing efficient catalysts. The DNRR requires considerable overpotentials from −0.7 to −0.2 V based on the electrolyte conditions. Among the reported catalysts, Mo‐based chalcogenide catalysts showed good DNRR performances, as shown in Figure  11A; and Table S1 (Supporting Information). Although Bi‐based catalysts showed a decent performance,^{[ 130 ]} the FE and NH₃ yields of most electrocatalysts are still low for practical ammonia production due to the competing HER and the low solubility of N₂.

Figure 11.

Summary and challenges of eNRR. Plots of ammonia yield and Faraday efficiency of previously reported studies in DNRR, and INRR. Each reactivity point is further categorized based on engineering strategies. A) Catalyst design strategies for DNRR; Engineering of the defect (blue), morphology (red), heterostructure (green), and Mo‐based chalcogenide catalysts (yellow). B) Strategies for INRR; Engineering of the electrode (blue), SEI (red), electrolyte (green), the integrated strategy of electrode and SEI layers (purple). C) Remaining tasks in DNRR; Increasing N₂ affinity, suppressing H₂ evolution, and avoiding N _x H _y poisoning. Remaining tasks in INRR. D) achieving the uniformity of the SEI layer. E) Minimizing resistive losses, and F) optimizing the solvent‐electrolyte composition.

On the other hand, the performance of the Li‐mediated INRR is superior in terms of FE and partial current. Notably, the FE approached 100% in recent studies (purple circle in Figure 11B; and Table S2, Supporting Information).^{[ 124 ]} It demonstrated a higher possibility of realizing the electrochemical Haber–Bosch process. However, INRR also has several limitations to be addressed. The Li/Li⁺ redox couple requires a high reductive potential, which results in energy inefficiency and undesirable side reactions such as solvent/electrolyte decomposition and SEI layer formation.^{[ 131 ]} In addition, the use of the scarce and expensive Li metal may limit the applicability of this method. Therefore, considering the current achievement and remaining issues, we provide the key challenges to be addressed and future research directions, as follows.

4.1. Direct Nitrogen Reduction Reaction

4.1.1. Enhancing the Availability of N₂

Because DNRR is a surface‐confined reaction, enhancing N₂ affinity via surface modifications such as atomic configuration (or functionalization) and maximization of electrode surface area would be a useful strategy (Figure 11C). Hybrid structures, such as metal–organic frameworks composed of the catalyst and functional agent, may effectively capture N₂ on the catalyst surface and accelerate N₂ dissolution based on Le Chatelier's principle.^{[ 132 ]} Also, since eNRR in the practical membrane electrode assembly (MEA) device is operated with humidified N₂ gas due to the activation of membrane electrolyte, fine‐tuning the pore structures of the catalyst layer like in the case of the fuel cell would be an effective approach to enhancing the eNRR performances by expediting gas phase reactant supply and product removal in the MEA.

4.1.2. Efficiently Suppressing the HER

As discussed in Section 3, most reports introduced enhanced DNRR performance, but most faradaic currents predominantly came from the HER.^{[ 48} , ^{58 ]} As protons are obtained from water in aqueous electrolytes, modifying the wettability of the catalyst surface can effectively control proton availability. Generally, in electrochemistry, hydrophilic carbon (on hydrophobic gas diffusion layers) is used as a supporting material. The thing is that the carbon electrode typically shows marginal HER performance at the eNRR potentials or promotes the HER by adsorbing water molecules on their surfaces. Therefore, tuning the surface wettability should be considered when designing DNRR catalysts.^{[ 133} , ^{134 ]} Furthermore, DNRR in nonaqueous electrolytes should be considered for enhancing the DNRR currents by stoichiometrically matching the number of protons for DNRR and minimizing HER currents.

4.1.3. Preventing Surface Poisoning from Hydrazine or NH₃

Similar to CO and CO₂ poisoning in electrochemical hydrogen and methanol oxidation reactions, surface poisoning from species such as CO, NH₃, and N₂H₂ would considerably affect the DNRR performance because surface passivation may induce catalyst deactivation and degradation. For example, a graphene layer coating on the catalyst surface may effectively minimize the poisoning by controlling the surface adsorption behaviors of these species (for example, CO poisoning during hydrogen oxidation).^{[ 135 ]}

4.1.4. In Situ and Operando Studies to Gain Mechanistic Insights

To gain a deeper understanding of reaction mechanisms and to elucidate the dynamic behavior that occurs at the reaction interface, it is essential to use a combination of electrokinetic, in‐situ, ex‐situ spectroscopy, and computational studies. In recent years, in situ techniques such as differential electrochemical mass spectrometry (DEMS), surface‐enhanced infrared absorption spectroscopy (SEIRAS), surface‐enhanced Raman spectroscopy (SERS), and attenuated total reflection infrared spectroscopy (ATR‐IR) have been utilized in the study of ammonia oxidation, the reverse reaction of nitrogen fixation. These techniques have enabled the identification of N‐containing intermediate species formed on active sites.^{[ 136} , ¹³⁷ , ^{138 ]} By using a combination of DEMS and cyclic voltammetry, as well as DEMS and SEIRAS, it is possible to obtain information on adsorbed intermediates during the reaction. Combining these in situ instruments can provide valuable insights for the advance of catalyst designs.

4.2. Indirect Nitrogen Reduction Reaction

4.2.1. Engineering the Architecture of the SEI Layer

Engineering the architecture of the SEI layer: As discussed previously, Li plating frequently induces the growth of an undesired SEI layer owing to solvent/electrolyte decomposition (Figure 11D, left). The permeability of the SEI layer is a crucial factor in the Li‐mediated NRR, because the diffusion of N₂ and proton donor molecules to the electrode can be significantly affected by the structure of the SEI layer. Modifying the electrolyte composition and concentration can influence the formation kinetics of the SEI layer; an optimized current density might prevent undesirable dendritic Li growth and result in a uniform layer. Constructing an artificial SEI layer from electrolyte additives might suppress the randomized growth of the Li and SEI layers. According to previous studies on Li batteries, adding a fluorinated polysulfonamide electrolyte or SnF₂ stabilizes the SEI composition and inhibits dendrite formation. Similar strategies may also be used for optimizing the SEI.

4.2.2. Overcoming the Inferior Energy Efficiency

Li⁺ reduction to Li metal requires a high negative potential accompanied by solvent degradation and oxidation at the anode, which results in extremely high electrolyte resistance (typically 1000–1200 ohm); therefore, the energy efficiency is low (below 3%) for NH₃ production. Decreasing the solution resistance can possibly increase energy efficiency. Another promising strategy involves coupling the INRR with hydrogen oxidation or other sacrificial anodic reactions to protect the solvent molecules. Furthermore, modifying the electrolyte composition can decrease unwanted energy loss. Other factors that need to be addressed include FE or NH₃ loss during synthesis and limited cell lifetime (Figure 11E).

4.2.3. Ideal Electrolyte Composition for eNRR

Most studies involving Li‐NRR have typically used THF because of its broad negative potential window. Figure 11F shows several candidate solvents that could be stable at highly negative potentials. Ionic liquid‐based electrolytes could be alternative candidates as stable N₂‐soluble solvents.^{[ 139 ]} In Section 3.2.3, we discussed that the structures of the solvent and proton donor determine the diffusivity of the reacting species. Therefore, various electrolyte/solvent/HA components must be investigated to determine the optimal combination for an efficient NRR.

4.2.4. Effective Proton Delivery

For continuous ammonia production, effective proton transport to the lithium nitride layer is particularly crucial. High‐performance continuous synthesis for commercialization necessitates a consistent stream of proton sources during the reaction. So far, hydrogen oxidation or anodic oxidation of proton donors have served as proton sources.^{[ 76} , ⁷⁷ , ^{125 ]} Facile accessibility of proton to cathode has been tried by several groups^{[ 129} , ^{140 ]} introducing proton shuttle concepts. Additionally, in order to facilitate proton transport, it is important to regulate the solvation of protons in nonaqueous media. In this vein, Kamlet–Taft solvent parameters can provide valuable guidelines to find out the optimal composition of the electrochemical system.^{[ 141 ]} Polarizability (π*) and hydrogen bond acceptor ability (β) of the solvents and proton donors would have a strong correlation for the transport of proton to SEI layers. Controlling SEI structure and proton transport will be the key to maximizing the overall efficiency of the Li‐mediated NRR.

In conclusion, we believe that eNRR would provide numerous possibilities to expand the applications of heterogeneous electrocatalysts and also to achieve global carbon neutrality. At present, the eNRR is technically at the initial stage, and distinct merits and drawbacks have been suggested for the two eNRR routes. In particular, the Li‐mediated INRR reaches ampere‐scale currents of NH₃ production, which potentially can be comparable to the commercialized electrolysis system, in near future. We hope that, with this review, researchers in the fields of nanomaterials and electrochemistry gain significant insights into the development of eNRR‐related materials and technologies.

Conflict of Interest

The authors declare no conflict of interest.

Author Contributions

H.J., S.S.K., and S.V. contributed equally to this work. Conceptualization, H.J., S.S.K., and K.J.; Writing—original draft, H.J., S.S.K., S.V., and K.J.; Figure sets—J.L.; Writing—review & editing, H.J., K.L., and K.J.; funding acquisition, K.J. and K.L.; supervision; K.J and K.L.

Supporting information

Supporting Information

Biographies

Haneul Jin is an assistant professor of energy and materials engineering at Dongguk University‐Seoul. He received his Ph.D. in inorganic chemistry from Korea University in South Korea in 2019. He then worked as a postdoctoral researcher at the Korea Institute of Science and Technology (KIST) from 2019 to 2022. His research interests include understanding surface/interface chemistry in energy‐related electrocatalysis through nanocatalyst design.

Suyeon S. Kim is a master course student under the supervision of Prof. Kyoungsuk Jin in the department of chemistry at Korea University. She received her B.S. degree (2021) from Sookmyung Women's University. Her research focuses on the fundamental understanding of electrocatalytic oxygen‐atom functionalization reactions.

Sandhya Venkateshalu received her Master's degree in Nanotechnology from Visvesvaraya Technological University, India, in 2015. Afterward, she worked as an assistant professor for a year at Dr. T. Thimmaiah Institute of Technology, India. She received her Ph.D. in Nanotechnology from the Vellore Institute of Technology, India, in 2021. She is currently a research professor at the Korea University, Republic of Korea. Her research interest includes the synthesis and characterization of nanomaterials, 2D nanomaterials, MXenes, energy storage, and conversion devices.

Kwangyeol Lee obtained his Ph.D. degree (1997) in Chemistry from the University of Illinois at Urbana‐Champaign. After fulfilling his military obligation, he joined Korea University in 2003 as a chemistry faculty member, before being appointed as a professor. His current interests include the development of synthetic methodologies for nanoscale materials and the development of nanotechnologies to support the environment by creating sustainable energy.

Kyoungsuk Jin is an assistant professor in the department of chemistry at Korea University. He received his Ph.D. degree (2016) in Material Science from Seoul National University. During his postdoctoral research at the Massachusetts Institute of Technology (MIT), he studied the electrochemical oxygen functionalization reactions. He joined the department of chemistry at Korea University to begin his independent career in 2020. His research focuses on the fundamental understanding of electrochemical reactions and organic electrosynthesis including various oxygen‐atom functionalization catalysis, asymmetric synthesis, and biochemical reactions.

Jin H., Kim S. S., Venkateshalu S., Lee J., Lee K., Jin K., Electrochemical Nitrogen Fixation for Green Ammonia: Recent Progress and Challenges. Adv. Sci. 2023, 10, 2300951. 10.1002/advs.202300951