Recent progress in the development of electrocatalysts for the electrochemical N 2 reduction reaction

Ammonia is the second-most produced chemical throughout the world to maintain the global food supply and other chemical stocks. The annual worldwide ammonia production is currently more than 200 million tons through the Haber–Bosch process, which consumes an enormous amount of energy due to the requirement of high pressure ( 4 10 MPa) and relatively high temperature (400–500 1 C). In recent years, electrochemical N 2 reduction reaction (ENRR) under ambient conditions has received paramount attention in the scientific community. However, large-scale production of ammonia from the ENRR is limited by the lack of efficient cost-effective catalysts. The success of ENRR firmly depends on the efficiency of the electrocatalyst in a suitable electrolyte. However, identification and generation of the active sites in the electrocatalysts for ENRR remain elusive, impeding the development of the catalysts. In this review article, recent progress made in the development of efficient electrocatalysts for ENRR under ambient conditions is focused on with special attention on the physicochemical properties and active sites of the catalyst towards the NH 3 production rate by considering experimental as well as theoretical aspects. This review elaborates on key aspects for the development of an efficient and stable electrocatalyst for NH 3 production. In addition, the role of electrolytes and different sources of errors in the ENRR measurement for NH 3 production are outlined briefly.


Introduction
Over the last few decades, the industrialization of populous developing countries has been demanding huge energy that primarily comes from fossil fuels leading to increased greenhouse gas emissions, not only resulting in depletion of nonrenewable fossil fuel resources but also rapidly disturbing the climate. It has thus become an imperative objective of the scientific community to develop cleaner energy resources and technologies to mitigate fossil fuel use and maintain environmental sustainability. Ammonia (NH 3 ) is the second-most manufactured synthetic chemical, which is extensively used as an agricultural fertilizer, chemical feedstock, and hydrogen carrier as well. For more than a century, the Haber-Bosch method has been the primary process of NH 3 production worldwide using nitrogen (N 2 ) and hydrogen (H 2 ) gas as precursors. The N 2 to NH 3 conversion is a multi-step complex process, which requires a large amount of energy due to the high bond dissociation energy of the NRN bond (940.95 kJ mol À1 ). 1 In particular, the Haber-Bosch process requires temperature and pressure in the range of 350-550 1C and 150-350 atm, respectively, to convert N 2 to NH 3 (N 2 + 3H 2 = 2NH 3 ). Most importantly, the H 2 consumed for NH 3 production is exclusively produced from steam reforming of natural gas (CH 4 + 2H 2 O = 4H 2 + CO 2 ), which emits around 450 million metric tons of CO 2 to the earth's atmosphere. Hence, it is one of the primary sources of environmental pollution and cause of rising global temperature. Therefore, an alternative energy-efficient and sustainable route for NH 3 production without hampering its supply chain is urgently required to protect our environment. The electrochemical nitrogen reduction reaction (ENRR) at ambient temperature and pressure is one such highly promising alternative method that requires only N 2 and H 2 O for NH 3 production, making the process completely green if solar/ renewable energy is used. Hence, a great amount of effort has been directed toward ENRR research in the past couple of years indicating the prospects of this method.
In any chemical reaction, the catalyst plays a central role not only in increasing the yield and selectivity of the products but also in seamlessly driving the reaction with minimum inputs.
Catalysts are thus the key and core ingredient for any electrocatalytic reaction. In heterogeneous catalysis, solid materials are majorly used as catalysts and thus their physicochemical properties have paramount importance to catalytic performance. The microstructure, especially steps, kinks, edge atoms, and surface electronic structures of the solid catalyst play significant roles in the efficiency and selectivity of the reaction. The literature reveals that a greater percentage of solid catalysts are noble and rare earth elements that have less abundance and are expensive. For those catalysts, support materials play an equally important role in reducing their amount for use in catalytic reactions while enhancing the overall performance. In particular, the support material controls the catalyst distribution and nucleation, and protects the nanostructure-based catalysts from agglomeration.
The morphology and composition of the catalysts play primary roles in the catalytic reactions including ENRR. The suitably designed surface controls the active sites for the adsorption/desorption of the reactive species while composition determines the surface oxidation state and electronic structure. Therefore, the design of efficient electrocatalysts is highly desirable for ENRR. Electrocatalysts for ENRR can be categorically placed in three groups: (i) metallic catalysts, (ii) metal-based catalysts including oxides, carbides, phosphides, and nitrides, and (iii) metal-free electrocatalysts. Understanding the reaction mechanism at these catalyst surfaces is important for developing and increasing their efficiency. For example, competitive hydrogen evolution reaction (HER) is an obstacle associated with ENRR, which leads to a decrease in its Faradaic efficiency (FE). The suppression of HER and the enhancement of the ENRR process are thus obvious fundamental objectives in the development of ENRR catalysts. So far, noble metal-based electrocatalysts such as Au, 2 Pd, 3 and Ru 4 have shown promising results towards ENRR. Similarly, nonnoble metal-based electrocatalysts have also been explored. As compared to a sole electrocatalyst, a nanostructured electrocatalyst integrated with a suitable support material shows better activity. 5 The high electrocatalytic activity arises from the synergistic effect between the support and the catalyst, which mitigates the competitive HER. Therefore, the rational design of stable, active, and cost-effective catalysts to obtain a high yield rate and selectivity of NH 3 is the prime requirement for ENRR.
Here, RHE and SHE stand for the reversible hydrogen electrode and the standard hydrogen electrode, respectively. On the other hand, the competitive HER reaction requires only two electrons to produce one H 2 molecule. In addition, the potential required for HER is much closer (or lower than) that of the ENRR. Thus HER becomes a complementary reaction at the cathode for ENRR, which leads to poor selectivity towards NH 3 production. The HER at the cathode occurs as per the following equations: 8 2H + (aq.) + 2e À = H 2 (g) E 0 = 0.00 V vs. NHE in an acidic electrolyte (pH = 0) 2H 2 O + 2e À = H 2 (g) + 2OH À E 0 = À0.828 V vs. SHE in an alkaline electrolyte (pH = 14) NHE stands for the normal hydrogen electrode. Both HER and ENRR thus fall into the same category of PCET reaction. Although many in situ techniques have been employed to understand the exact mechanism of ENRR, it remains unclear. Therefore, much more research endeavour is required to establish the ENRR reaction mechanism. Each PCET reaction step depicts its complexity and demands a higher potential for the progression of the reaction. As ENRR proceeds via multiple electron and proton transfer processes, various intermediates are formed as presented in Fig. 1. Even though ENRR appears to be very favourable for NH 3 production as per the standard reduction potential chart, the achieved FE for ENRR remains mostly below 10%, which is far lower than the expected results. This is believed to be due to the kinetically preferred HER, which requires only two electrons whereas ENRR proceeds through a multi-step six electron process. This competing HER thus leads to a comprehensive FE loss for ENRR. The performance of ENRR further depends significantly on the electrolyte medium and its pH. Therefore, elimination of the unwanted HER from the electrolyte reduction becomes the most concerning issue for ENRR as it is difficult to avoid the protic medium. Furthermore, ENRR is highly affected by the mass transport of N 2 . The low solubility of N 2 in water, i.e., only about 2 vol%, is another hurdle for ENRR. Therefore, the choice of electrolyte is another important factor to be considered in which ENRR should be preferred over HER. subdivided into two pathways: (i) associative distal pathway and (ii) associative alternating pathway. The ENRR mechanism usually follows the associative distal pathway or alternating pathway and/or both as shown in Fig. 1. The density functional theory (DFT) calculation reveals that the associative mechanism during the ENRR is governed by the specific active site of the catalyst. 12 The active site controls the adsorption of different active intermediate species during the progression of the reaction. In the distal pathway, the end atoms are attached to the active site, where the terminal atoms get hydrogenated by the PCET process through the formation of *NNH and *NNH 2 intermediates, and then desorb from the catalyst surface as NH 3 . In the alternating pathway, both the terminal and the end atoms are hydrogenated simultaneously by a PCET through the formation of *NNH, *NHNH, and *NH 2 NH 2 intermediates and desorb from the catalyst surface as an NH 3 molecule consequently. Apart from that, in nature, nitrogenase enzymes perform biological N 2 fixation under ambient conditions, where active sites have Fe and S along with Mo atoms, and the detailed mechanism of it remains elusive. 13 Nevertheless, both enzymatic and ENRR mechanisms require six protons and six electrons for the production of NH 3 , which is the common point in all the reaction mechanisms. 13 The ENRR mechanism was theoretically studied in an acid medium over transition metal surfaces by Nørskov and co-workers. 14 The free energy change for the N 2ads molecule and N ads atoms towards catalytic NH 3 conversion was calculated. Fig. 2 shows the volcano diagram of N 2 reduction reaction considering a Heyrovskytype reaction, which includes both dissociative (solid lines) and associative (dashed lines) mechanisms on both flat (black) and stepped (red) surfaces. The H-bonding effect is presented without (solid lines) and with (dotted lines). The DFT calculated negative free energy (ÀDG) values are presented with data points for a given reaction step. The right-hand side of the volcano plot represents the first proton transfer step, i.e., N 2(g) + H + + e À -*N 2 H, indicating the activity of the metals for the associative mechanism whereas the N 2 splitting is the ratedetermining step for the dissociative mechanism. The metals on the left side have the same rate-determining steps for both the mechanisms (associative and dissociative). The x-axis represents the binding energy of N-adatoms. The large grey area inside the plot suggests the surface that is more favourable to be covered with H-adatoms. Mo, Fe, Rh, and Ru can be found at the top of the volcano plot suggesting their higher activity for ENRR. However, the surfaces of these catalysts are also predicted to be active sites for HER thereby leading to low FE for ENRR.
A completely different pathway named the Mars-van Krevelen (MvK) mechanism is proposed for transition metal nitride (TMN) catalysts which require relatively small overpotentials. 15 Different from the associative and dissociative pathways, here lattice N atoms are hydrogenated to produce NH 3 from the surface of the TMNs and thus creating N vacancies. Subsequently, these N vacancies react with the dissolved N 2 and generate a second NH 3 molecule. Recently, Nash et al. experimentally proved the MvK mechanism over the Cr 2 N catalyst. 16 Detailed bulk and surface analyses using XRD and XPS, respectively, reveal that the bulk of the catalyst is a pure Cr 2 N phase whereas the surface possesses CrN, CrN x O y , and CrO x along with Cr 2 N. The bulk CrN phase exhibits negligible ENRR activity compared with Cr 2 N indicating that the latter is the active phase for ENRR. The isotopic batch cell study using 15 N 2 indicates the formation of 14 NH 3 and 15 NH 3 as identified from NMR analysis, which suggests active participation of surface N of Cr 2 N to activate dinitrogen through the MvK mechanism as presented in Fig. 1. The authors also identified two possible reasons for the deactivation of the catalyst. Those are leaching of surface N from Cr 2 N at lower potentials (oÀ0.4 V) and silent conversion of the active Cr 2 N to the inactive CrN at À0.2 V. Using advanced instrumental tools such as surface-enhanced infrared absorption spectroscopy (SEIRAS) and differential electrochemical mass spectrometry (DEMS) Yao et al. studied ENRR on a Rh catalyst. 17 The authors identified the formation of N 2 H x (0 r x r 2) species during the progression of ENRR with an NQN stretching mode at B2020 cm À1 from SEIRAS analysis and the DEMS signal at m/z = 29. The study proposed a new two-step ENRR reaction pathway on the Rh surface, in which the ENRR involved twoelectron transfer to N 2 to form N 2 H 2 first and subsequent decomposition to NH 3 .

Performance evaluation of electrocatalysts for NRR
The yield rate and Faradaic efficiency (FE) are the important indicators to define the catalytic conversion efficiency of an electrocatalyst. Therefore, it is important to describe the procedure for yield rate and FE calculation along with the factors that affect them. In an ideal condition, FE is supposed to be one or 100%. However, practically obtained FE for ENRR is much lower than the theoretical value. The Faradaic loss is a prominent issue, which is associated with the catalyst. Two major factors that strongly influence the FE of ENRR are applied potential and competitive HER. The ENRR at a lower potential leads to a higher FE. However, a higher reduction potential facilitates HER at the catalyst surface and thus loss of FE for ENRR. Another important point is the selectivity of ENRR. So far only two nitrogen-containing products are identified from ENRR, i.e., NH 3 and N 2 H 4 . The product selectivity of ENRR is another criterion to obtain high FE for NH 3 conversion.
The yield rate is usually calculated from the salicylate method. The concentration of ammonia and hydrazine produced from ENRR is estimated from the calibration curve obtained from the UV-Vis absorption data. The ammonia yield rate is estimated by using the following equation: 18 3 ] is the concentration (mg mL À1 ) of ammonia, V is the volume (mL) of the electrolyte, t is the electrolysis time (h), and A is the surface area (cm 2 ) of the electrode.
The hydrazine yield is estimated from the following equation: where [N 2 H 4 ] is the concentration (mg mL À1 ) of the produced hydrazine, V is the volume (mL) of the electrolyte, t is the electrolysis time (h), and A is the area (cm 2 ) of the electrode.
Taking into account that three electrons are required for one ammonia molecule synthesis and four electrons for one N 2 H 4 from N 2 , FE is calculated from the following equation: where F is the Faraday constant, Q is the total charge, and other terms have their standard meaning.

Platinum (Pt)
Among the different noble metals, Pt and Pt-based materials are studied in almost every branch of catalyst science due to their high intrinsic conversion efficiency. Especially in fuel cell technology, Pt is considered as the benchmark catalyst due to its inherent HER activity, with HER being a competitive reaction for ENRR. As a result, Pt shows very poor FE towards ENRR as evident from the experimental results. 19,20 A few reports on Pt-based materials as electrocatalysts reveal their poor selectivity towards ENRR and very high activity towards HER. 19

Gold (Au)
Au-based nanostructures are extensively explored in the field of nanoscience and nanotechnology. 26 Due to their specific electronic, optical, plasmonic, and catalytic properties, Au and Au-based materials are extensively studied in the field of biosensors, optoelectronics, and heterogeneous catalysis. 26,27 The intrinsic catalytic as well as the electronic properties of the catalyst can be manipulated by tuning the microstructure shape, size, and surrounding environment of the Au nanostructures. 28,29 The theoretical study revealed that the ENRR on the Au surface follows associative mechanisms that precede adsorption of the N 2 molecule at the electrode surface followed by hydrogenation of the N atoms. The success of the Au-based electrocatalysts towards ENRR is due to the strong N 2 adsorption ability of the Au surface composed of multifaceted facets with various steps and/or nanoclustering that facilitate the adsorption and reduction of N 2 . 30 (Table 1). 32 Zhang et al. correlated the coordination of the surface gold atoms with ENRR. 33 They found that the nanoporous Au film exhibits 5.8 times higher ENRR activity than the {111} facet exposed octahedral Au nanocrystal with an NH 3 production rate  (Fig. 3). The theoretical analysis revealed that the 8 nm size Au NPs have an optimum number of surface edge sites to suppress the HER and hence facilitate the ENRR. The rate-limiting step for NH 3 production was reported to be the formation of *NNH from *NN. 34 The free energy calculation revealed that Au(211) has the potential active site for ENRR. 34 The density of state of different Au models suggested that the 5d band of the Au(211) facet has a higher tendency to overlap with the 2p orbital of N ads in *NNH than that of the Au(111) facet. Such stronger binding stabilizes the intermediate species and hence increases the NH 3 production rate. 34 Nazemi et al. synthesized hollow Au nanocages to enhance ENRR under ambient conditions. 35 The hollow Au nanocages showed a FE of 30.2% at À0.4 V (vs. RHE) with an NH 3 yield of 3.9 mg cm À2 h À1 at À0.5 V ( Table 1). The FE increased from 30.2% to 40.5% with the temperature increasing from 20 1C to 50 1C at À0.4 V indicating the role of temperature. The increase in FE with temperature is due to the high mass transportation rate at higher temperatures even though the solubility of N 2 decreases. The hollow cages offer a large surface area for the reactant molecules than a solid one leading to three times more ENRR activity compared to the latter one. This suggests the urgency to develop an advanced microstructure for ENRR. In addition, a suitable support material for catalysts plays an important role. Support materials not only increase the stability of the catalyst but also facilitate adsorption and diffusion of the reactant molecules. 36 Qin et al. synthesized single Au atom supported hierarchical N-doped porous carbon to enhance ENRR. The N-doped porous carbon (NDPC) supported Au catalyst showed a FE of 12.3% with an NH 3 yield of 2.32 mg h À1 cm À2 at À0.2 V. Both the NDPCs and Au singleatom sites participated in ENRR under ambient conditions. The metal-free NDPCs reached a FE of only 2.1% at a potential of À0.6 V further highlighting the significance of Au-NDPC heterojunction catalyst synthesis. The highly porous architecture of NDPCs provided more active sites and enhanced mass transportation during the ENRR process. The N and C sites stabilized the single atomic Au catalyst and retained the durability of the catalyst. The catalyst also selectively yielded NH 3 without the signature of N 2 H 4 . Li et al. reported amorphous Au NPs supported by the CeO x -RGO hybrid (a-Au/CeO x -RGO) for ENRR under ambient conditions. 2 Amorphous Au has higher chemical reactivity than the crystalline nanostructure towards small molecules. The a-Au/CeO x -RGO showed a FE of 10.10% with an NH 3 yield of 8.3 mg h À1 mg cat À1 at À0.2 V, which is much higher than its crystalline counterpart (c-Au/RGO, FE: 3.67%, NH 3 yield: 3.5 mg h À1 mg cat À1 ). The role of CeO x was to increase the amorphous nature of Au NPs leading to increased catalyst performance. The surface oxidation state (M d+ ) of metal catalysts is another factor that plays a significant role in the electrochemical reaction by the redistribution of the surface electronic structure to enhance the adsorption ability of the catalyst towards reactant and radical species. [37][38][39] Zheng et al. introduced the CoO x layer to manipulate the local electronic structures of Au NPs with a positive valence site (Au + ) to enhance the NH 3 production rate. 37 By using the vapour deposition method, small-sized Au and Co islands were homogeneously deposited onto the Si surface, followed by fast annealing at 800 1C forming a Au NPs/CoO x layer (Au/CoO x ) (Fig. 4a). The CoO x layer created Au + active sites due to the strong charge exchange between them. 40 High-resolution transmission electron microscopy (HRTEM) images ( Fig. 4b) 42 The atomic ratio of the Au-Ni catalyst was tuned by controlling the Au 3+ precursor concentration. The pristine Ni catalyst produced a negligible amount of NH 3 . The NH 3 yield rate significantly increased after the incorporation of Au. The highest NH 3 yield of 7.4 mg h À1 mg cat À1 and FE of 67.8% at À0.14 V were obtained with Au 6 /Ni. In this metallic couple, Ni acts as an electron donor and enriches the Au surface electron density by a combination of support and size effects. XPS analysis provided clear evidence for the gradual shift of the Au 4f spectra towards lower energy binding, whereas Ni 2p shifted towards higher binding energy confirming the acceptor behaviour of the former. Moreover, the optimal Ni-Au donor-acceptor pair enhanced the pre-adsorption and activation of N 2 and the desorption of NH 3 from the catalyst surface.

Ruthenium (Ru)
Ruthenium (Ru), known for its use in the Haber-Bosch process, is another example of an efficient catalyst for ENRR. 43 The theoretical analysis revealed favourable nitrogen adsorption energy leading to a lower ENRR overpotential in both associative and dissociative mechanisms compared to that of Pt and Pd electrocatalysts. 14 Moreover, Ru is placed almost at the top of the volcano plot ( Fig. 2) suggesting it to be one of the most active catalysts for ENRR. Thus, Ru-based nanostructured catalysts have been explored in recent years for ENRR. The fundamental role of the Ru catalyst towards the ENRR mechanism is highlighted here based on experimental and theoretical findings. Back et al. studied the mechanism of ENRR through both associative and dissociative pathways and compared with the energy state of Ru theoretically. 44 The calculated free energy diagram revealed that the kinetically facile intermediate of the dissociative pathway required a thermodynamic limiting potential of À0.71 V, which is comparable to the associative pathway (À0.68 V). The study highlighted that both associative and dissociative reaction pathways are probable during the ENRR. Previously, it was believed that the associative pathway is energetically more feasible than the dissociative pathway because of the high energy requirement to dissociate the NRN bond (9.79 eV). 45 In addition, NH 3 formation is energetically more favourable than N 2 H 4 formation making Ru nanostructures more selective for NH 3 synthesis. However, the competitive HER prohibits the ENRR by blocking the active sites due to the energetically preferential adsorption of *H. 14 Geng et al. developed Ru single atoms on nitrogen-doped carbon (Ru SAs/N-C), which showed very high activity toward ENRR. 46 At À0.2 V, Ru SAs/N-C attained a FE of 29.6% with a partial current density of À0.13 mA cm À2 and a yield rate of 120.9 mg NH 3 mg cat À1 h À1 . The high electrocatalytic activity of Ru SAs/N-C arises from the strong chemical bond between Ru and N 2 as confirmed by the N 2 -temperature program desorption analysis. Zhang 47 The N-doped carbon frameworks not only ensured uniform dispersion of the Ru NPs but also protected the catalyst from dissolution during electrocatalysis. The residual hydrophobic nature of ZIF-8 remained in the carbon matrix which prohibited competitive HER. Furthermore, among the different N-doped carbons, the pyridinic N-type carbon moiety facilitated the adsorption of N 2 in which Ru atoms accelerated dissociation. Yao et al. studied the ENRR kinetics on a Ru thin film using a combination of surface-enhanced infrared absorption spectroscopy and electrochemical measurements. 48 Fig. 5a shows the voltammetry plots of the Ru thin film performed in a N 2 and Ar-saturated HClO 4 electrolyte. The cathodic scan reveals the onset of NRR and HER at 0 V and an increase in reduction current with decreasing potential. It is worth noting that the NRR current (solid red line) is slightly lower than the HER current (solid black line), suggesting that some active species adsorbed on the Ru surface get oxidized in the backward scan and thus showing a small peak between À0.1 and 0 V. To identify the probable intermediate, surface-enhanced infrared absorption spectroscopy was used during the ENRR measurements. The *N 2 H x (0 r x r 2) species was identified and the stability of the species was further studied. The *N 2 H x species was detected at a potential below 0.2 V in an N 2 saturated HClO 4 solution (Fig. 5b). The NQN stretching band at 1940 cm À1 was observed at À0.1 V and remained constant above 0.1 V. The NQN stretching band intensity was accompanied by the oxidation current in between the potential of À0.1 and 0 V during the positive scan (Fig. 5a). The oxidation current is believed to arise from the electrochemical oxidation of the *N 2 H x species. The results ( Fig. 5a and b) reveal that the oxidation current (dashed line) below 0 V potential arises from the oxidation of the *N 2 H x species whereas in the potential range of 0 and 0.4 V, the oxidation of *N 2 H x and H ads surface species occurred. They also studied the ENRR kinetics in alkaline electrolytes and revealed that the N-related signal at the electrode surface is very weak in an alkaline medium. This observation highlights the feasibility of the Ru-based catalyst in an alkaline medium. Recently, Ru NP loaded Ti 3 C 2 MXene (Ru@MXene) was tested for ENRR which showed an NH 3 yield of 2.3 mmol h À1 cm À2 at À0.4 V with a FE of 13.13%. 49

Palladium (Pd)
The Pd-based materials are widely explored as electrocatalysts in the field of the fuel cell, especially for ORR, 50 OER, 51 and HER, 52 as well as anode catalysts for alcohol electrooxidation reaction. Pd has thus been studied as an electrocatalyst for ENRR. Wang et al. reported carbon-supported Pd NPs (Pd/C) for ENRR under ambient conditions. 53 Under a potential, Pd can form Pd hydride, which promotes the surface hydrogenation reaction. The Pd/C catalyst showed an NH 3 yield rate of around 4.5 mg mg Pd À1 h À1 and a high FE of 8.2% at 0.1 V. The Pd/C catalyst showed higher catalytic activity than Pt/C and Au/C with similar wt% mass loading. Interestingly, the HER activity of Pd was effectively suppressed in the neutral PBS electrolyte and N 2 hydrogenation followed the Grotthuss-like hydride transfer mechanism. Free energy calculation suggested that the *N 2 to *N 2 H hydrogenation is the rate-limiting step for ENRR. Lv et al. reported a PdO/Pd heterojunction for ENRR. 54 The desired compositions of PdO/Pd decorated carbon nanotubes (PdO/Pd/CNTs) were synthesized through laser irradiation of PdO/CNTs in distilled water. The optimized PdO (82%)-Pd (18%) interface provided maximum active sites for N 2 activation and proton transportation. In PdO/Pd/CNTs, both Pd and PdO sites were exposed for electrocatalysis where the Pd site strongly bound the N 2 molecule while the PdO site facilitates the subsurface activated H protons to imitate a-PdH and hydrogenate the adsorbed N 2 molecule. The synergy of Pd and PdO contributed to the efficiency of the catalyst. They further studied defect-engineered titanium dioxide and the interfacial effect of supported palladium or platinum for ENRR, which demonstrated 1847.3 mg mg cat. À1 h À1 (Pd/TiO 2 ) and 2520.5 mg mg cat.
respectively. 55 Interestingly, the FE for NRR was higher on Pd/ TiO 2 (2.63%) as compared to Pt/TiO 2 (1.57%). Xu et al. reported nanoporous palladium hydride (np-PdH) as an electrocatalyst for ENRR under ambient conditions, which showed a high NH 3 yield rate of 20.4 mg h À1 mg À1 with a FE of 43.6% at a low overpotential of À150 mV. 56 Isotopic hydrogen labelling studies suggested that the lattice hydrogen atoms present in the np-PdH acted as the active hydrogen source. In situ Raman analysis and DFT calculations further revealed that the *N 2 to *N 2 H reduction energy barrier is the rate-limiting step of ENRR. The unique protonation nature of the np-PdH could provide a new dimension for designing efficient and robust electrocatalysts.
To study the ENRR mechanism, the authors studied the isotopic labelling experiments on np-PdH 0.43 , as presented in Fig. 6. In the presence of 15 N 2 , two peaks with a coupling constant of 73.2 Hz were detected in the baseline-subtracted 1 H NMR spectra (Fig. 6b), which is consistent with a heteronuclear coupling constant between 1 H and 15 N of 15 NH 4 + . The FTIR band at 1439 cm À1 corresponds to the N-H bending, which further provides evidence for NH 3 generation. Three weak FTIR bands that appeared at 1177, 1241, and 1325 cm À1 were ascribed to the bending modes of NH 2 D 2 + and NH 3 D + (Fig. 6c). XRD data suggest that there is no shift in the characteristic (111) peak at different potentials which underlines the structural durability of the catalyst. In situ Raman spectra of np-PdH 0.43 indicate a broad peak at 1644.4 cm À1 that relates to the overlapping of H-N-H/H-O-H bending and the peak intensity increase with potential indicates the adsorption of ammonia on the catalyst surface ( Fig. 6d and e). Recently, Wang et al. achieved a record-high FE of up to 97% for ENRR in aqueous solution using a Pd/activated carbon cloth. 57 The high electrocatalytic activity is demonstrated by adjusting the threephase interface, i.e., gas/solid catalyst/liquid electrolyte, which favours ENRR over HER. The complete suppression of HER is facilitated by the three-interface contact due to high N 2 coverage on the catalyst surface that weakens H adsorption as evidenced by the DFT calculations and experimental results. The authors further confirm the merits of a three-phase interface strategy for selective electrochemical reduction of N 2 to NH 3 using Ir-and RuPd-based catalysts, which are known to have strong H adsorption ability.

3d-and 4d-transition metal catalysts
Even though noble metals such as Au, Pd, and Ru show promising electrocatalytic activity towards ENRR, the scarcity and cost-ineffectiveness associated with these prompt the researchers to explore alternate materials without compromising the performance. In recent years, a variety of noble metalfree transition metal electrocatalysts have been studied for ORR, 58 OER, 59 and HER. 60 These were also extensively employed for ENRR. Transition metals are largely abundant in the mother earth and inexpensive, making them suitable alternatives to noble metal-based catalysts. Mukherjee et al.
reported an atomically dispersed Ni site on N-doped carbon (Ni-N x -C), which exhibited an optimal NH 3 yield of 115 mg cm À2 h À1 at À0.8 V in a neutral medium. 61 The asprepared Ni-N x ÀC catalyst showed a FE of 21 AE 1.9% at À0.2 V under alkaline conditions although the NH 3 yield was lower. The atomically dispersed Ni sites were stabilized with nitrogen in the carbon matrix. The active site and probable reaction pathways through the DFT calculations reveal that the Ni-N 3 sites are responsible for the experimentally observed catalytic activity and selectivity. The N 2 molecule adsorbs on the top of the central Ni atom in an end-on configuration (Fig. 7) in which one N atom is chelated with Ni by a bond length of 1.76 Å, whereas another one is tilted along with a Ni-N bond of the Ni-N 3 site. The proximal N of the adsorbed N 2 undergoes two hydrogenation steps to form *NHNH 2 and *NH 2 NH 2 . The NRN bond length in *NH 2 NH 2 is increased about 28% than that of the N 2 gas molecule. Therefore, subsequent hydrogenation on the distal N is believed to break the NRN bond and release one NH 3 .

Materials Advances Review
Open Zang et al. reported on a single copper atom attached porous N-doped carbon network (NC-Cu SA) for ENRR by the combination of experimental and theoretical calculation. 62 The single-atom copper site exhibits high-density active sites    64 Chen et al. successfully synthesized super small-sized Mo species through reduction of largesized molybdenum oxide in a H 2 atmosphere on a carbon support (carbon cloth). 65 The particle size decreased with increasing synthesis temperature. Remarkably, up to an average particle size of 0.95 nm was demonstrated using the developed method. The synthesized Mo catalyst showed an excellent activity towards ENRR with FE as high as 22.3% and a yield rate of 7.02 mg h À1 mg cat À1 at 0 V vs. RHE under ambient conditions using a 0.1 M KOH electrolyte. The high electrocatalytic activity is attributed to large active sites of super smallsized Mo NPs.

Bismuth (Bi)
The potential of the Bi metal towards ENRR arises from its poor HER activity due to the high Gibbs free energy barrier for hydrogen adsorption on its surface. 66 Thus Bi nanostructures have been widely investigated for ENRR. N 2 molecules selectively adsorb on the Bi surface due to the overlap of the Bi 6p band and the N 2p orbital, which enhances the ENRR. 67,68 Moreover, due to the weak binding of H ad atoms on the Bi surface, it can selectively promote N 2 to N 2 H* reduction, and restrict surface electron accessibility for the HER process thus enhancing the NH 3 production rate. 67  the engineered exposed edge sites/planes (010) are conducive toward the adsorption and activation of N 2 molecules as confirmed by the DFT calculations. Recently, Qiu and co-workers prepared multi-yolk-shell bismuth@porous carbon (MB@PC) composites via a facile simple hydrothermal reaction followed by pyrolyzation. This MB@PC composite showed an excellent performance with an NH 3 yield of 28.63 mg h À1 mg cat. À1 and a FE of 10.58% at À0.5 V in N 2 -saturated 0.1 M HCl solution. The synergetic effect of good conductivity, the highly porous feature derived from the carbon framework, and the intrinsic electrocatalytic ENRR activity of Bi NPs was the primary reason for excellent ENRR performance. 73 Xu et al. demonstrated the ENRR activity using ultrathin Bi NSs prepared through the in situ electrochemical reductionassisted exfoliation of BiOCl nanoplates (BiOCl NPs). This method facilitated an increase in electrochemically active surface area and under-coordinated Bi sites which accelerate electron transport capability. With these unique features, the as-converted Bi NSs exhibited a FE of 14.14% and an NH 3 yield of 11.11 mg h À1 mg cat. À1 at À0.5 V (vs. RHE) in 0.1 M Na 2 SO 4 , and high NH 3 selectivity (nearly 100%). 74 One of the most significant breakthroughs has been achieved by Yan's group, attaining a very high FE of 66% and an NH 3 yield of 200 mmol g À1 h À1 in aqueous solution and under ambient conditions by utilizing a unique strategy of combining Bi nanocrystals (NCs) and K + -supporting cations in water. 67 These studies suggest the potential of Bi-based electrocatalysts towards ENRR for producing a sizable amount of NH 3 .

Bimetallic catalysts
Nanostructured metal catalysts such as Ru, Au, and Pd show significant electrocatalytic responses towards ENRR as discussed earlier. 32 However, the electrocatalytic activity of bimetallic alloy nanostructured catalysts is superior to their monometallic counterparts. The electrocatalytic performances of bimetallic nanostructured electrocatalysts depend on their surface electronic structure and thus on the shape, size, surface area, exposed facets, and compositions. 75 The high electrocatalytic activity of the bimetallic or alloy nanostructures arises from the synergistic effect of the component metals. 76  Strong chemical adsorption between N 2 and IrTe 4 PNRs was indicated by N 2 temperature-programmed desorption (N 2 -TPD) and valence band X-ray spectroscopy measurements, which is the key feature towards the high ENRR of IrTe 4 and suppression of HER. The high adsorption arises from the synergistic effect between the electron-rich Ir and highly electroactive surroundings of the Te atom. Table 1 compares the ENRR performance of single metal and bimetallic/trimetallic alloys under ambient conditions.

Transition metal oxides and defectbased catalysts
Over the years, transition metal oxides (TMOs) are explored almost in every type of energy conversion and storage device with reasonably satisfying results. 94,95 The catalytic activity of TMO-based materials arises from a combination of inherent electronic structure, microstructure, edges, defects, and interfacial structure in heterojunction and/or with support materials. Several TMOs such as TiO 2 , 96 100 and ZnO 65 have been studied as electrocatalysts for ENRR, which have demonstrated quite comparable performances with metal NP based catalysts (Au, Pd, and Ru). In oxides, oxygen vacancy defects play a crucial role in ENRR. An oxygen vacancy is created in an oxide-based semiconductor to form a defect centre. 101 A defect-rich oxygen vacancy exposes co-ordinately unsaturated metal sites, which enhances the chemisorption and activates inert N 2 molecules. Fang et al. reported oxygen vacancy-contained TiO 2 nanosheets as an electrocatalyst towards ENRR. 102 Oxygen vacancies were created by annealing the as-prepared TiO 2 nanostructures under a H 2 / Ar atmosphere. The high activity of the as-prepared oxygen vacant TiO 2 is attributed to the synergistic effect between the structural features and microstructure of the catalyst. The DFT calculations suggest that the oxygen vacancies (OVs) can significantly lower the activation energy barrier of NRN bond dissociation during N 2 fixation (Fig. 9). Without OVs, TiO 2 required a free energy of 4 eV for N 2 activation, which makes TiO 2 (without OV) inert to ENRR. The surface of the OV TiO 2 site has much higher catalytic activity than the perfect TiO 2 surface. The uphill energy of the first hydrogenation step is only 0.25 eV on the OV site, whereas as large as 2.29 eV is required for the perfect surface. After the first hydrogenation step, free energy changes for all steps toward the formation of NH 3 are thermodynamically favourable on the OV site except for the desorption of NH 3 . The two-dimensional TiO 2 nanosheets provide a large number of active sites on their surface. The oxygen vacant TiO 2 shows a 2.83 times higher NH 3 yield rate than that of the as-prepared TiO 2 nanostructures. Li et al. reported Ti 3+ self-doped TiO 2Àx nanowires on Ti mesh (Ti 3+ -TiO 2Àx /TM) as the catalyst for ENRR. 103 The theoretical  113 The Co atom and its adjacent oxygen vacancy created an active   117 Oxygen vacancies (OVs) were created in spinels to enhance the ENRR activity. Various OV-rich nanostructured spinel wrapped hollow nitrogen-doped carbon polyhedra such as OV-rich NiCo 2 O 4 @HNCP, OV-rich ZnCo 2 O 4 @HNCP, and OVrich Co 3 O 4 @HNCP were reported to facilitate the ENRR. Among these, OV-rich NiCo 2 O 4 @HNCP showed a high NH 3 production yield of 4.1 mg h À1 cm À2 /17.8 mg mg À1 h À1 and a FE of 5.3%. 118 However, there is still a vast opportunity to explore these spinelbased materials with rational design for enhancing the efficiency of ENRR.
The ENRR performance of more oxides, perovskites, and spinels under ambient conditions is presented in Table 2 for comparison.

Phosphide-based catalysts
Transition metal phosphides (TMPs) have emerged as highly active and inexpensive electrocatalysts as well as cocatalysts for HER with performance comparable to that of the state-ofthe-art Pt/C. A wide range of TMPs such as CoP, 139 Ni 2 P, 140 MoP, 141 Cu 3 P, 142 (211). The ENRR mechanism follows the distal pathways, which are energetically more favourable than the alternating pathways. The ratedetermining step for ENRR is *N 2 to *NNH. The N 2 adsorption free energy for FeP 2 (101) is higher than that for FeP (211), which suggests the former to be more selective towards ENRR.  147 The IrP 2 @PNPC-NFs showed a FE and NH 3 yield rate of 17.8% and 94.0 mg h À1 mg cat À1 , respectively. The edge site of IrP 2 was found to be very irregular and the low coordinated step atoms at the edge site of the IrP 2 nanocrystals lowered the reaction energy barrier towards the ENRR while inhibiting the HER activity. The plasma-assisted strategy also makes it possible to synthesize other high-melting-point noblemetal phosphides (such as OsP 2 @PNPC-NFs, Re 3 P 4 @PNPC-NFs) at lower temperatures.

Nitride-based catalysts
Transition metal nitrides (TMNs) are another class of materials that have high electronic conductivity with rich N ad atoms. Theoretical as well as experimental investigations suggest that TMNs have huge potential for ENRR. 148,149 However, the chemical stability of the nitride-based materials is an obstacle in addition to competent HER. 150 Moreover the generation of NH 3 from the nitrogen atom of the catalyst can give falsepositive results (discussed later). Abghoui and Skúlason theoretically investigated TMNs for ENRR under ambient conditions and provided possible mechanistic insights considering several nitrides such as ZrN, MoN, CrN, MnN, NbN, and VN with a rocksalt structure. 151 The free energy diagrams were constructed on the rocksalt (100) structure using the associated mechanism to evaluate their performance in terms of onset potential for ENRR. The free energy of each intermediate was calculated at room temperature, pH = 0, and an applied potential of 0.0 V, which indicates that the potential determining step (PDS) is the first protonation of *N 2 to *NNH for all the candidates except for VN and NdN where the formation of *NNH 3 is the PDS. Among these nitrides, MoN showed promising results with DG PDS = 0.83 eV. 151 153 The ex situ synchrotron-based characterization of the catalyst before and after the catalytic cycle revealed the stability of 2D W 2 N 3 , which was attributed to the vacancies on 2D W 2 N 3 with the combinational effect of the high valence state of tungsten atoms and 2D morphology induced surface distortion. The DFT calculations suggested that the electron-deficient environment in the 2D layered W 2 N 3 effectively facilitates the electron acceptance from N 2 and enhances the subsequent reduction rate.

Carbide-based catalysts
Transition metals carbides (TMCs) have also been explored in the search for a noble metal-free catalyst for ENRR. TMCs exhibit specific properties which are desirable for electrocatalysis, such as corrosion resistance, and high stability, melting point, and mechanical strength. [157][158][159] These specific properties have attracted increasing research interest for their potential application as electrocatalysts, 160 catalyst supports, 161 lithiumion battery materials, 162 and solar cell materials, 163 particularly for energy generation and storage purpose. 164 The carbides are referred as an interstitial alloy. The electronic structure of the host metal of the carbide is altered due to the inclusion of the carbon atoms through the charge transfer process. The distinct electrocatalytic activity of the TMC arises from the electronic structure, due to the presence of carbon atoms in the metal lattice. For example, the excellent HER performance of the tungsten carbide arises from the filling of the d-states at the Fermi level of tungsten by alloying it with carbon. 165 Thereby, TMCs are investigated as electrocatalysts without certain disadvantages such as chemical susceptibility and durability. So far, TMCs are employed as HER, OER, and ORR cathode catalysts with excellent activity. 165,166 However, there are relatively few reports on the use of TMC materials for ENRR through nanostructuring, 167 combination with another material, 168 and exploiting hybrid structures. The DFT calculation suggests that the metal site of the carbide can activate the N 2 molecule and stabilize the intermediate N x H y species while destabilizing the -NH 2 species during the desorption as NH 3 . 82 Although the carbide-based materials show promising ENRR activity theoretically, the experimental performance is not up to the mark due to the competitive HER under similarly applied potentials. 169 Therefore different fabrication methods have been adopted to enhance the ENRR performance of the carbide-based catalysts by creating heterojunction interfaces with the generation of active sites for various elementary steps. DFT analysis indicates that all the crystallographic surfaces of the cubic MoC catalyst have the ability for adsorption and dissociation. 170 Ramaiyan et al. reported an origami-like Mo 2 C cathode catalyst for ENRR and achieved a maximum NH 3 synthesis rate of 2.16 Â 10 À11 mol cm À2 s À1 with a FE of 1.8% at 30 1C using Nafion-212 as an electrolyte. 171 The authors revealed that numerous kinks at the surface of the origami-like structures are responsible for the catalytic ENRR. However, the synthesized origami-like Mo 2 C catalyst was found to be unstable during the ENRR process. XPS analysis after the electrolysis process indicates the formation of molybdenum oxycarbide which raises concern about catalyst susceptibility for long term uses. Qu  ENRR decreases. DFT analysis suggested that the free energy change of the rate-limiting step over the Fe 3 C@C core-shell towards formation and desorption of NH 3 is more favourable than that of Fe 3 C. The observation indicates that the interaction between the carbon shell and the Fe 3 C core facilitates the charge transport in the catalyst. MXene represents another group of novel 2D materials with the general formula M n+1 C n , where M represents transition metals such as Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W with n = 1, 2, 3 along with the terminal functional groups (F, OH, and/or O). 175,176 Azofra et al. computationally investigated the possibility of d 2 -d 4 transition metal-based MXene (M 3 C 2 ) as a model material for N 2 capture and reduction. The DFT calculation revealed that the transition metal atoms present at the terminal surface can activate the adsorbed N 2 and facilitate the ENRR under mild conditions. 175 Zheng et al. theoretically investigated the possible prospect of boron-doped MXene towards ENRR. 176 The B-doped Mo 2 CO 2 and W 2 CO 2 MXenes exhibited decent catalytic activity and selectivity with limiting potentials of À0.20 and À0.24 V, respectively. The boron centre mimics the empty and filled d-orbital electronic structure of transition metals and activates the N 2 molecule effectively. In addition, a high B-toadsorbate electron donation tendency facilitates the hydrogenation of *N 2 to *N 2 H during the ENRR. A recent study further suggested that the terminal oxygen groups act as active sites for the HER and CO 2 reduction. 160 Even though there is a huge prospect for MXene materials toward ENRR, competitive HER mitigation is the main issue that needs to be addressed to unveil the higher efficiency of MXene towards ENRR. Luo et al.
reported Ti 3 C 2 O 2 2D MXene for ENRR. 177 The surface Ti atoms act as active sites for N 2 adsorption in the single-layer T-Ti-C-Ti-C-T-T (T = terminal atoms) sandwich structure with maximum binding energy. The N 2 adsorption energy indicates that the middle Ti atoms are energetically preferable to the C and O sites. The terminal O atoms cannot overcome the high energy barrier for ENRR whereas the Ti site at the edge plane is preferred. Ti 3 C 2 T x MXene with a smaller sheet size shows higher performance than that with a large sheet size under À0.1 V. The study also revealed that vertically aligned Ti 3 C 2 T x MXene has more active sites than the randomly oriented one. Vertically aligned Ti 3 C 2 T x MXene with a FeOOH nanosheet host was synthesized to overcome the thermodynamic obstacles. The FE of MXene/FeOOH reached 5.78% under À0.2 V, which is 1.25 times higher than the maximum value of the MXene. The improved performance of the MXene was ascribed to a higher number of active sites of the vertically aligned MXene and the sluggish nature of FeOOH towards HER.

Sulfide-based catalysts
Over the past few years, metal sulfides have been widely studied as a potential candidate for various electrocatalytic activities. [178][179][180] Inspired by the natural nitrogenase enzymes (Mo-Fe protein), MoS 2 has been investigated as a catalyst for ENRR. 181 Sun and co-workers were the first to introduce and experiment on MoS 2 as a catalyst under room temperature and atmospheric pressure. The catalyst showed a FE of 1.17% and an NH 3 yield of 8.08 Â 10 À11 mol s À1 cm À1 at À0.5 V in 0.1 M Na 2 SO 4 . 182 Later on, they further improved the FE to 8.34% with an NH 3 yield of 29.28 mg mg cat. À1 h À1 at À0.40 V, by designing defect-rich MoS 2 nanoflowers. 183 Zeng et al. developed nanoflower-like N-doped MoS 2 electrocatalysts and introduced rich sulfur vacancies to further enhance the FE to 9.14% with an NH 3 yield of 69.82 mg mg cat. À1 h À1 at À0.3 V in 0.1 M Na 2 SO 4 solution. 184 Suryanto et al. developed MoS 2 nanosheets decorated with Ru clusters and obtained a FE of 17.6% and an NH 3 yield rate of 1.14 Â 10 À10 mol s À1 cm À2 attributed to the polymorphic engineering in Ru/2H-MoS 2 . 185 Thereafter several other studies based on MoS 2 have been reported to date achieving FE from as low as 4.58% to 20.6% using different strategies. [186][187][188][189][190] Apart from MoS 2 , other transition metal dichalcogenides such as WS 2 have been tested as an ENRR catalyst with the lowest potential of À0.24 V through an enzymatic mechanism using DFT. 191 Flower-like SnS 2 and forest-like ZnS nanoarrays directly grown on Ni foam showed certain abilities towards ENRR. The NH 3 yield and FE of 15 N 2 with flower-like SnS 2 were 9.08 Â 10 À10 mol s À1 cm À2 and 10.3%, whereas for ZnS, they were 5.27 Â 10 À10 mol s À1 cm À2 and 5.62%, at À0.5 V, respectively. 192

Metal-free electrocatalysts: heteroatom doped carbon-based catalysts
Metal and metal-based oxide, phosphide, carbide, nitride, and sulfide catalysts have been explored as potential ENRR electrocatalysts as discussed earlier. In addition to these, metal-free electrocatalysts have also been studied as ENRR electrocatalysts due to their N 2 adsorption ability and weak interaction with H + ions that specifically suppressed the HER. [194][195][196] However, pure carbon-based materials are inactive towards ENRR. 197 This led to the synthesis of a variety of heteroatom doped carbons for ENRR that exhibit considerable ENRR performance. In particular, N-doped porous carbon (NC) exhibited ENRR activity, where pyridinic and pyrrolic N-sites are identified as active sites for N 2 adsorption and subsequent dissociation. 198 Polymeric carbon nitride (PCN), 199 N-doped carbon synthesized from biomass, 200 and zeolitic imidazolate frameworks (ZIF-8) have shown considerable ENRR activity. 201 The ENRR performance of these catalysts is attributed to the defects arising from the heteroatom doping. However, a recent study has shown that heteroatom-doped carbon materials are also efficient HER electrocatalysts. 202 Thus, suppression of HER is the fundamental task to obtain efficient carbon-based catalysts for ENRR. Yu et al. studied boron-doped graphene for ENRR. 203 Boron doping redistributed the electron density in the graphene framework, where the electron-deficient boron sites enhanced the binding capability of N 2 molecules. DFT calculations based on different boron carbon models revealed that the BC 3 site facilitates the lowest energy barrier for ENRR. At 6.2% of boron doping on graphene, an NH 3 yield of 9.8 mg h À1 cm À2 and a FE of 10.8% were achieved at À0.5 V in aqueous solutions under ambient conditions. Qiao et al. revealed that the addition of boron (B) to nitrogen-doped graphene yields a higher Gibbs free energy of hydrogen adsorption (DGH*) indicating that *H species were not easily adsorbed onto the active sites and hence suppressed HER. 204 BC 3 sites in the boron-doped graphene exhibit as active centres for better ENRR activity compared to that of raw graphene. 203 Song et al. reported ENRR using N and P co-doped porous carbon with a trace of iron (0.028 wt%) (Fe-NPC). 205 The Fe-NPC catalyst was synthesized from solid-state pyrolysis of polyaniline aerogels with phytic acids and FeCl 3 , which showed significantly enhanced electrocatalytic activity in an alkaline medium as compared to NPC. The Fe sites acted as active sites towards ENRR. In the presence of SCN À , the ENRR was significantly decreased due to the strong affinity of the SCN À ions towards the Fe site. Fe-NPC showed a FE and NH 3 yield rate of 5.3% and 4.36 mg h À1 mg À1 at À0.1 V. The in situ FTIR spectra at À0.1 V for the Fe-NPC catalyst showed four peaks at 1126, 1273, 1412, and 3256 cm À1 . The peak at 1126 cm À1 indicates the cleavage of NRN whereas the peaks at 1273, 1412, and 3256 cm À1 correspond to -NH 2 wagging and H-N-H bending, suggesting the intermediate species formed during the ENRR process. It is to be noted that the ENRR process over the Fe-NPC catalyst follows associative pathways.
Wu et al. reported P-doped graphene (PG) for ENRR with a FE and NH 3 yield of 20.82% and 32.33 mg h À1 mg À1 at À0.65 V, respectively. 197 Two different types of bonded P atoms were present in PG, namely P-O and P-C, with an atomic percent of 0.47 wt%. The DFT calculation showed that the O functional group prefers to adsorb on P than the OH functional group energetically. Therefore, the O functional group is considered to be active for ENRR, and N 2 adsorbs preferentially at the C site adjacent to the P-O bonding. This study also revealed that the PDS is *N 2 to *NNH hydrogenation. Song et al. modified reduced graphene oxide (rGO) with tannic acid (TA) and studied the ENRR. 206 TA-rGO showed a yield rate and FE of 17.02 mg h À1 mg À1 and 4.83% at À0.75 V, respectively. Qiu et al. reported boron carbide (B 4 C) nanosheets as an electrocatalyst for ENRR with an NH 3 yield rate of 26.57 mg h À1 mg cat.
À1 and a FE of 15.95% at À0.75 V. 196 The N 2 atom adsorption on the B 4 C nanosheets (110) follows either end-on terminal N atom adsorption or adsorption to the adjacent B 4 C site. Both configurations show a similar energy profile for N 2 adsorption on the B 4 C nanosheet. Free energy calculations showed that hydrogenation of *NN to *NH 2 NH 2 proceeds almost without a barrier. The *NH 2 -*NH 2 -*NH 2 + *NH 3 step is the ratelimiting step of the ENRR on the B 4 C(110) surface, which requires a free energy of 0.34 eV at a potential of 0 V. Liu et al. developed a metal-free F-doped carbon catalyst by introducing F atoms into a 3D porous carbon framework for ENRR. 207 The F-doped carbon catalyst exhibited the highest FE of 54.8% for NH 3 at À0.2 V, which is 3.0 times higher than that of pristine carbon frameworks (18.3%), and a yield rate of 197.7 mg NH 3 mg À1 cat. h À1 at À0.3 V. The improved performance of the F-doped carbon catalyst is due to the binding strength of N 2 which facilitates dissociation of N 2 into *N 2 H as revealed by mechanistic studies. Due to the different electronegativity between the F and C atoms, a Lewis acid site is created when the F atom is bonded to the C atom. As a result, the HER activity is suppressed significantly. On the other hand, the selectivity of N 2 electroreduction into NH 3 is enhanced because of the repulsive interaction between the Lewis acid site and proton H. 207 Wang and co-workers identified oxygen-doped graphene (O-G) derived from sodium gluconate as a new promising catalyst for the effective production of NH 3 by catalyzing ambient electro-hydrogenation of N 2 . 208  RHE) and a high FE of 11.5% at À0.5 V (vs. RHE), which are significantly higher compared to the undoped graphene catalyst (6.25 mg h À1 mg cat. À1 ; 0.52%), were achieved. The ENRR mechanism was further explored by DFT calculations, which revealed that carbon atoms nearest to substituted sulfur atoms act as underlying active sites for the ENRR on S-G. Further, they prepared S doped carbon nanospheres (S-CNSs) as an electrocatalyst with excellent selectivity towards ENRR, demonstrating an NH 3 yield of 19.07 mg h À1 mg cat. À1 and a FE of 7.47% at À0.7 V in a 0.1 M Na 2 SO 4 electrolyte. 210 The S-doped CNS showed much higher catalytic activity than that of the undoped CNS (3.70 mg h À1 mg cat. À1 , 1.45%) which is similar to the previous discussion. 209 Importantly, this S-CNS catalyst also showed high stability for ambient N 2 to NH 3 conversion. Cheng et al. adopted a novel one-step synthesis protocol to synthesize defect-rich S and N co-doped carbon cloth (CC) by using ammonium persulfate (APS) as the source of nitrogen and sulfur. The S/N co-doped CC catalyst, prepared at 800 1C (CC-APS 800), exhibited a higher number of defects and heteroatom sites and acted as an active and stable electrocatalyst for ENRR with an NH 3 yield of 9.87 Â 10 À10 mol s À1 cm À2 and a FE of 8.11% at À0.3 V in 0.05 M H 2 SO 4 solution. 211 Kong et al. synthesized B and N co-doped porous carbon nanofibers (B/N-CNFs) that exhibited much higher electrocatalytic performance than the only N-or B-doped carbon materials towards ENRR with a FE of 13.2% at À0.5 V and an NH 3

À1
with a FE of 16% at À0.4 V (vs. RHE) was achieved. This is due to the capability of water dissociation and adsorption of generated protons by the doped nitrogen atoms present in BNFC electrocatalysts. 214 Table 3 presents the ENRR performance of recently reported metal phosphide, nitride, carbide, sulfide, and metal-free electrocatalysts, indicating their potential.

Effect of electrolytes
To achieve ENRR activity with high FEs, several other factors are needed to be considered to tackle the challenges in addition to suppressing the competing HER. In particular, besides having an efficient electrocatalyst, the choice of a suitable electrolyte (in terms of ions and pH) is one of the fundamentally important factors to enhance ENRR activity. For instance, acidic, neutral, and alkaline electrolytes have been tested taking into consideration pH adjustment. The pH value for ENRR systems is needed to be well adjusted using dilute H 2 SO 4 , HCl, Na 2 SO 4 , KHCO 3 , and KOH aqueous solutions. Chen et al. tested ENRR using a 30% Fe 2 O 3 -CNT electrocatalyst in different electrolytes with different pH values at a constant potential. 225 The highest NH 3 yield rate of 1.06 Â 10 À11 mol cm À2 s À1 was obtained in the 0.5 M KOH electrolyte, which was higher than that obtained in 0.25 M KHSO 4 (7.87 Â 10 À12 mol cm À2 s À1 ). In acidic electrolytes, due to the higher proton concentration, HER is significantly increased and thereby suppressing ENRR. In contrast, HER activity is significantly reduced in alkaline electrolytes. Recently, the effective suppression of HER activities was found for the ENRR using the neutral phosphate buffer solution (PBS) electrolyte. 53,226 PBS has a higher barrier for mass and charge transfer, which prohibits HER kinetics. Upon using Pd/C as an electrocatalyst, the authors were able to obtain a FE of 2.4% in 0.1 M PBS (pH = 7.2) as compared to o0.1% with 0.05 M H 2 SO 4 (pH = 1.2) and 0.1 M NaOH (pH = 12.9) electrolytes. 53 All these electrolytes were Ar-saturated. Zhang et al. showed a higher ENRR selectivity and a FE of 1.17% with MoS 2 /CC in 0.1 M Na 2 SO 4 as compared to 0.09% in 0.1 M HCl due to the strong HER activity of the latter. 182 Moreover, to suppress HER and increase N 2 solubility in the electrolyte, a mixture of water and low-proton (alcohols) solvents was also taken. 227 Kim et al. studied the water and 2-propanol (1 : 9, v/v) mixture as an electrolyte along with H 2 SO 4 as the supporting electrolyte. 228 The mixed solvent gave rise to a FF of 0.89% as compared to 0.07% for pure water. The observed result is far from being optimal as the FE could not exceed more than 1%. Alkali metal ions are also known to suppress the HER and have a strong ability to interact with molecular N 2 in an electrolyte. Selection of counterions in the aqueous electrolyte was found to have a significant role with an order of Li + 4 Na + 4 K + in terms of NH 3 yield rate, which suggests a most favourable role of the smallest counterions. The steric effect and the relatively strong interaction between counterions and N 2 play important roles in ENRR. 35,229 To enhance the FE, 0.1 M LiCl/ethylenediamine (EDA) and 0.05 M H 2 SO 4 solution were used as an electrolyte in the cathode and anode compartment, respectively, to study the effect of EDA towards ENRR. A much higher FE of 17.2% was obtained under ambient conditions under this condition, possibly due to the wide electrochemical window of EDA in the negative potential region. 230 Similarly, a mixture of water/alcohols and aprotogenic (tetrahydrofuran, dimethyl sulfoxide) solvents were used as an electrolyte for ENRR. 231,232  weakly, whereas they strongly interact with [eFAP] À . Two modes of favourable interactions were identified, namely in complex 1, the N 2 atoms interact with the F atoms of the alkaline chains, and in complex 2, the N 2 atoms interact with F atoms bonded with phosphorus as presented in Fig. 12. The N 2 binding energy depends strongly on the delocalization of the negative charge.
The interaction further increases with the addition of the cation. 236 Licht et al. used a molten hydroxide suspension of nano-Fe 2 O 3 named the NaOH-KOH eutectic electrolyte, which showed high rates of ammonia generation upon addition of nano-Fe 2 O 3 at modest temperatures of 200-250 1C. 5 As per the recent reports on ENRR, electrolytes play a vital role in HER suppression and enhancement of ENRR. Nevertheless, more experimental and theoretical studies could further improve ENRR under ambient conditions.

Lithium-mediated N 2 reduction reaction
As mentioned in the previous section, not only the catalyst but also the electrolyte plays an important role in ENRR. Among the different electrolytes tested for ENRR, a lithium-mediated ENRR is found to be the most promising one. Typically the lithium-mediated ENRR is performed in tetrahydrofuran solvent that can dissolve a high amount of N 2 (as high as B6 mM) at 25 1C. The mechanism of Li-mediated N 2 reduction is hypothesized as the reduction of Li + ions in the presence of N 2 forming lithium nitride (Li 3 N) at the cathode. The Li 3 N species react with a proton source and produce NH 3 . Ethanol has been used as a proton source for nitride to NH 3 conversion. In 1994, Tsuneto et al. reported a Li-mediated ammonia synthesis with a yield rate of B2 to 4 nmol s À1 cm À2 at 50 bar of N 2 pressure. 231 McEnaney et al. experimentally proposed Lirecycling as a tool to produce NH 3 using renewable energy. 237 First, a highly active Li metal surface was synthesized from LiOH electrolysis in a proton-free environment and then exposed to N 2 to form Li 3 N. Suryanto et al. recently demonstrated a phosphonium based cation such as trihexyltetradecylphosphonium ([P 6,6,6,14 ] + ) as a proton source for lithiummediated NH 3 synthesis. 238 [P 6,6,6,14 ] + is known to have excellent thermal, chemical, and electrochemical durability. The schematic reaction of electrochemical synthesis of NH 3 is presented in Fig. 13. Herein, electrochemically generated Li 3 N is reacted with the phosphonium cation to produce NH 3 and form a ylide as a derivative. The original phosphonium cation is regenerated from the protonation of phosphonium ylide which is a strong base. The author and his group have achieved the best performance at À0.75 V versus Li 0/+ with N 2 to NH 3 conversion FE as high as 69 AE 7% and an NH 3 yield rate of 58 AE 14 nmol s À1 cm À2 , at an average current density of about À22.5 mA cm À2 . The high reduction potential for the generation of the Li plate simultaneously leads to degradation of the electrode interface over time. The continuous regeneration from the Li salt thus limits the scalability of the process. Moreover, Schwalbe et al. observed the development of a solid electrolyte interface (SEI) layer as well as the accumulation of lithium and lithium-containing species at the electrode surface. 239 In this regard, Andersen et al. succeeded in developing a stable cycle of Li-mediated NRR through the implementation of the pulse deposition strategy as presented in Fig. 14. 240 Using this method, the authors achieved a FE of B30%. They revealed that the SEI layer not only prevents the catalyst from poisoning but also maintains its long-term Li cycling ability. The SEI layer is further found to control the diffusion rate during the electrolysis process.

Sources of false positives in ENRR and their elimination
Electrochemical N 2 reduction has gained momentum in recent years due to its potential feasibility under ambient conditions, unlike the Haber-Bosch process. However, the large difference in yield rate and FE raised serious concerns about its actual applicability. Therefore, it is important to evaluate the pitfalls of ENRR, in particular, the inconvenient sources of error and their elimination from ENRR during the estimation of FE and yield rate. The recent developments of the catalyst for ENRR have been discussed in detail in the previous sections. However, a question may arise whether all the catalysts catalyze ENRR under ambient conditions? As we have seen, the conversion rates are in the range of nano or micromolar range with most of the catalysts. Such a low value could be contributed from other sources and/or impurities. For example, Ru is considered as one of the prominent candidates for ENRR in aqueous electrolytes. Theoretical as well as experimental evidence for the same is discussed earlier. However, Andersen et al. reported false-positive results with Ru-based catalysts. 241 Interestingly, the same catalyst does not reduce N 2 at all under ambient conditions in aqueous electrolytes. The author and his group carried out a thorough investigation and revealed that the false-positives arise from NO x impurities present in the N 2 feed stream. Thus it is very important to eliminate such impurities to find the true nature of the catalyst in ENRR. In this regard, the authors have further proposed modified methods to carry out ENRR as presented in Fig. 15. The readers are urged to go through the modified methods for accurate measurement of NH 3 production through ENRR using a suitable catalyst. In this regard, Suryanto et al. and Choi et al. reported excellent perspectives on the steps and missteps made in the ENRR study and suggested the directions to minimize Fig. 13 Schematic illustration of sustainable electrosynthesis of ammonia from H 2 and N 2 . The cathode process involves the reduction of Li + to the lithium metal that rapidly reacts with N 2 to produce Li 3 N, which is protonated to release the target product, NH 3 . The proton carrier is the phosphonium cation, which forms an ylide structure in its deprotonated state; this cycles to the anode where it is regenerated back to the phosphonium form through the reaction with H + derived from the oxidation of H 2 . Reproduced with permission from ref. 238. Copyright 2021, AAAS. missteps during the ENRR under ambient conditions. 242,243 A few recent studies thus clearly suggest sources of error for the estimation FE and yield rate of ENRR under ambient conditions in an aqueous electrolyte. It is important to identify such errors and eliminate those for correct measurements of NH 3 yield and FE. In the following sections, the sources of errors that could potentially provide false positives in ENRR are elaborated.

Nitrogen-containing catalyst
Nitrogen-containing electrocatalysts, especially metal nitrides have been explored as efficient catalysts for ENRR. The mechanism of ENRR is generally explained by the Mars van Krevelen (MvK) mechanism which is discussed earlier. Recently Du et al. investigated the ENRR on vanadium nitride (VN) and niobium nitride (Nb 4 N 5 ) as model electrocatalysts to validate the ENRR at different pH and raised serious questions about the feasibility of these catalysts toward ENRR. 150 The authors revealed that VN and Nd 4 N 5 do not catalyze N 2 under ambient conditions. The false-positive for NH 3 arises from the nitrogen atoms present in the catalyst instead of true ENRR. The Nspecies present in the catalyst surface may lead to an erroneous conclusion. Therefore, it is important to take necessary precautions to avoid such missteps during ENRR. One simple strategy that could be implemented to avoid and remove the contaminants from the catalyst is rigorous prewashing and following the pre-treatment method several times before studying the ENRR performance. The obtained pretreatment results must be subtracted from the final FE and yield rate.

Electrolyte
The false-positive detection of NH 3 can arise from the electrolyte. Li et al. reported the identification process and elimination of contamination in an electrolyte which provides the falsepositive. 244 The investigation found that a trace amount of nitrate and nitrite salts exist in the lithium-based electrolyte. Thus a significant amount of NH 3 is produced with excellent reproducibility in the presence of bare electrodes (Ti foil, carbon paper, or copper foam) without loading any catalyst in both N 2 and Ar-saturated Li 2 SO 4 solution. The detected NH 3 arises from the trace amount of nitrate and nitrite present in the electrolyte which is electrochemically reduced to NH 3 rather than N 2 . In this regard, the author studied the ENRR using bare Ti foil as the working electrode and Li 2 SO 4 as the electrolyte. The false ENRR to NH 3 as measured using the indophenol method is presented in Fig. 16. In particular, an apparent increase in absorbance is found in the experiments performed in both N 2 and Ar atmospheres, which confirms that the NH 3 yield is not from ENRR but due to contamination in the electrolyte. To eliminate the NO x contamination from the Fig. 14 (a) Cycling method between À2.0 and 0.0 mA cm À2 (red), for a total of 100C of charge passed (black). The WE potential (blue) and the CE potential (green) are stable across the entire experiment by varying the resting time. (b) A close-up of the cycling. Immediately after switching to a deposition current, the absolute value of the WE potential increases for the entire 1 min duration. When switching back to resting, the WE potential is initially stable just above 0 V vs. Li + /Li, until it eventually starts becoming anodic after some minutes, suggesting dissolution of all surface Li species that were deposited in the preceding deposition pulse. At this point, another Li depositing pulse is applied. As the experiment did not have constant supervision, there are occasional points where the resting WE potential becomes very positive (e.g. around 22 and 40 hours) before the deposition pulse is applied. This is simply changed by shortening the resting time. Reproduced with permission from ref. 240   (a) UV-Vis spectra for the indophenol blue indicator stained electrolytes after electrolysis at À0.1, À0.5, À1.0, and À2.0 mA cm À2 in N 2 for 1 h and (b) the corresponding ammonia yield rates at each current density. (c) UV-Vis spectra for the indophenol blue indicator stained electrolytes after electrolysis at À1.0 mA cm À2 in N 2 for 1, 2, and 3 h. (d) UV-Vis spectra for the indophenol blue indicator stained electrolytes after electrolysis at À0.5 and À1.0 mA cm À2 in Ar for electrolyte, the author annealed the Li 2 SO 4 salt at 800 1C in an argon atmosphere. After annealing the electrolyte, the NH 3 yield was found to be significantly reduced. This suggests that the presence of contamination in the electrolyte contributed to NH 3 formation, thus giving the false-positive test. Therefore it is important to identify the source of contamination in aqueous electrolytes and eliminate it to estimate the correct NH 3 yield rate.

Membrane separator
Currently, most of the reported ENRR studies are carried out in an H-cell with Nafion membrane as a separator. However, the Nafion membrane can absorb and release NH 3 during the ENRR as reported by Liu et al. 245 This suggests that the Nafion membrane is a possible source of error in the ENRR process.
On the other hand, Celgard 3401 and 3501 has poor ability to absorb NH 3 as compared to Nafion and other membranes and thus must be considered for use in ENRR studies.

Gas purity
The feed gas must be ultrapure and devoid of any atmospheric contamination. The main source of contamination is NO x species present in the N 2 feed gas ( 14 N 2 as well as 15 N 2 ). Dabundo et al. reported that the trace amount of NO x in the N 2 feedstock is enough to give a false positive test. 246 It must be noted that the electrochemical reduction of NO x species to N 2 is energetically favourable. 243 Thus enough care must be taken to eliminate any signal due to the contamination in the gas feed. Choi et al. suggested NO x scrubbing to eliminate NO x from the N 2 feed gas. 243 To evaluate the FE and yield rate of NH 3 , it is thus necessary to subtract the background signal from the ENRR signal obtained in the presence of N 2 .

Background subtraction
To eliminate false positives from the experimental results, it is necessary to perform control experiments at open circuit potential and in the absence/presence of a catalyst in the absence and presence of Ar/N 2 . The control experiments will provide clarity on the actual performance of the catalyst. However, many studies either ignore or do not perform sufficient and satisfactory control experiments. 242,243 This leads to potential errors in the assessment of the FE and yield rate of ENRR. Likewise, some reports have presented control experiments where the background NH 3 is significant, and thus it cannot be ignored. 242,243 Therefore the researchers are suggested to perform control experiments as prescribed above to eliminate the potential error from the results.

Conclusions
For a sustainable global energy economy, persistent effort is needed through research and development to generate products from renewable resources through different strategies. The present context reveals that ENRR could serve as a potential alternative to synthesize NH 3 against the conventional fossil fuel-based energy economy even though ENRR is an energy-intensive reaction. In the present review, we have detailed the roles of the electrocatalysts in facilitating the ENRR at room temperature, which is the ultimate goal of the scientific community. To date, noble metal electrocatalysts such as Pd, Au, and Ru along with a few non-noble metal electrocatalysts have exhibited promising ENRR efficiency while keeping a lot of space for further improvement. This review highlights several key aspects that are important to develop efficient catalysts while taking into account the key features that control the catalyst efficiency and selectivity including nicks, steps, facets, porosity, and coordination number of the surface atoms. Designing nanostructured electrocatalysts with an optimized surface structure facilitates N 2 adsorption and thereby ENRR.
In particular, nanostructuring increases the active site density with a specific structure. To obtain enormous active sites on the catalyst surface, various synthesis routes have been developed in recent years. Furthermore, to improve the mass transport kinetics across the electrode/electrolyte interface, the catalyst can be anchored on a suitable support material, which allows minimal use of the catalyst while improving the product yield. Herein, we summarize the key achievements in catalyst development for ENNR.
(1) The catalytic activity significantly increases especially for the single atomic electrocatalyst supported and/or stabilized by the N-site over the pure bulk catalyst. The single atomic catalyst site at the surface is coordinated by different numbers of N-sites, which act as active sites to control the N 2 reduction kinetics steps. The metal to N site coordination number strongly influences the rate-limiting step for ENRR.
(2) The oxygen vacancy in the oxide-based catalysts intrinsically enhances the catalytic conversion rate of N 2 to NH 3 . The oxygen vacant site of the catalysts plays a predominant role in the activation of N 2 during the progression of the reaction and hence decreases the activation energy barrier for ENRR.
(3) Doping is another approach to obtain efficient catalysts that generate active sites for ENRR. The doping sites facilitate the adsorption as well as activation of N 2 .
(4) Support materials integrated with catalysts play a crucial role in the suppression of HER. Especially, carbon shelling around the catalyst is found to play a prominent role in suppressing HER activity at the catalyst surface under a potential and facilitate the ENRR. Support materials also increase the catalyst distribution in the synthesis process and enhance mass transfer kinetics during the electrolysis process.

Challenges
Although impressive progress has been made in ENRR, there are still several major challenges that need to be addressed for the commercialization of the process. One of the major challenges associated with ENRR is the yield rate and FE. So far, the yield rate and FE are not up to the mark. In particular, the yield rate is mostly in micrograms per hour per milligram of catalyst or per centimetre square and FE is mostly less than 10% with a few exceptions. Therefore, more efforts are needed for the enhancement of the yield rate and FE of ENRR. The requirement of higher negative potential for ENRR is another major obstacle that is associated with HER with almost every catalyst. Thus, efficient electrocatalysts must be explored which could show ENRR at lower negative potentials to make it viable for commercial application. Hence, efforts should be directed to suppress the HER activity while increasing the FE of the catalyst for ENRR. The theoretical calculation provides a certain model to mitigate HER; however, the feasibility of this active model is still unclear. Identification of suitable electrolytes is another feature that can increase the solubility of N 2 while reducing the mass transport for HER and thereby increase the ENRR efficiency. Another important challenge associated with ENRR is measuring the evolved NH 3 through the ENRR correctly and precisely. Considering a very small yield of NH 3 (in microgram quantity per hour) through ENRR, it could be from other sources such as the atmosphere, supplied N 2 , and nitrogencontaining chemicals or catalysts used in ENRR. Among these, the supplied N 2 source is the main source of error as it often contains NO x species that are easily reducible to NH 3 . Similarly, nitrogen-containing catalysts and electrolytes are other potential sources of NH 3 release through ENNR. Thus precautions must be taken to avoid these errors which show erroneous results. Furthermore, the most commonly used indophenol blue method is not suitable for low NH 3 yield measurement. For detecting small quantities of NH 3 yield, the need for sophisticated spectroscopy techniques including isotope labelling is another challenge. To meet these challenges more attention is required from the scientific community.

Future directions
As highlighted in the previous sections, the development of catalysts, effective methods of measurement, and improvement of NH 3 yield are the immediate needs to make progress in realizing the ENRR in future. Although several electrocatalysts have been studied and they were suitably modified, the research on the same remains not substantial. In particular, the small-sized or single atomic catalytic centre could be a possible candidate to increase the active sites drastically and thus increase the NH 3 yield and FE. The synthesis of desired reactive facets and forming uniform defects such as oxygen vacancies appear to be a couple of promising strategies for the development of catalysts for ENRR. Not only an electrocatalyst but also an appropriate electrolyte is another important aspect to make the ENRR process practically viable. The theoretical investigation along with experimental study can rationalize the ENRR and provide further insights to improve the performance. As ENRR is a more complex process than the HER and not well understood at the molecular level, computational studies could be highly beneficial. Considering that the active intermediates form during the ENRR, the role of the active site in the stabilization of active intermediates remains ambiguous. Some investigations suggested that the N 2 adsorption ability of the catalyst is the key step whereas others suggested that coordinately unsaturated atoms are primarily responsible. Therefore, further experimental as well as theoretical investigation is necessary to understand the active species of the catalysts under the given conditions. The adsorption of N 2 and electron-coupled proton transfer to adsorbed N 2 are energetically uphill processes. Therefore, in situ measurements could provide clues to precisely identify the active site for ENRR. Finally, it is very important to follow the correct steps to measure the effective reduction of N 2 to NH 3 and measure the evolved NH 3 accurately. The possible sources of NH 3 contamination must be avoided and appropriate spectroscopic techniques must be followed to measure the low yield NH 3 .
In summary, the ENRR to form NH 3 has strong potential for the production of fertilizer and for use as an energy carrier after addressing the shortcomings which might take a few years from now. If progress is made in the right direction, reduction of N 2 to NH 3 will significantly reduce the global energy demand and carbon emission.

Conflicts of interest
There are no conflicts to declare.