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Photocatalytic and electrocatalytic approaches towards atmospheric nitrogen reduction to ammonia under ambient conditions
Nano Convergence volume 6, Article number: 15 (2019)
Ammonia production is essential for sustaining the demand for providing food for the growing population. Being a great source of hydrogen, it has significant potential in turning out to be a viable candidate for the future hydrogen economy. Ammonia has a high hydrogen content of about 17.6 wt %, is easier to liquefy and is produced in large quantities. Even though large-scale production of ammonia is significant globally, it is used predominantly as a fertilizer. It used also as a transport fuel for vehicles because of its low carbon emissions. Ammonia as an energy storage media is realized in many countries with infrastructure for transportation and distribution already put into place. Currently, the Haber–Bosch process is employed globally in industrial ammonia production and is a high energy expending process requiring large capital investment. In realizing a much economic pathway given the large-scale ammonia production growth forecast, it is necessary to seek new and improved methods for large-scale ammonia production. Amongst them, photoelectrochemical and electrochemical approaches stand as most promising towards nitrogen reduction to ammonia owing to their design features, lesser complexity, and economical in terms of the conventional ammonia production system. Several catalyst materials are investigated which include metal oxides, metals sulfides, carbon-based catalysts, and metal nitrides are all currently being pursued better utilization of their catalytic property towards nitrogen fixation and the minimization of the competing hydrogen evolution reaction (HER). In this article, we have summarized the design and reaction mechanisms for photoelectrochemical and electrochemical nitrogen fixation with the inherent challenges and material- related issues in realizing the Nitrogen Reduction Reaction (NRR).
Ammonia (NH3) synthesis by the Haber–Bosch process is regarded as one of the most significant realizations of the 20th century, contributing to a global production of over 150 million tons of NH3 yearly, practically 80% of which is in fertilizer production . Global demands for NH3 production are also likely to increase with its future additional uses such as a carbon-free fuel and particularly as hydrogen storage system owing to its high energy density (15.3 MJ/L), easy handling and storage . The current system of NH3 production through the Haber–Bosch process is energy-intensive with global consumption of 1 to 2% of the total annual energy production . The reason for these high level of energy requirements arises from exothermic N2 reduction reaction as in (1), requires elevated reaction temperatures (~ 500 °C) and high pressures (> 200 atm) and most importantly the need for large volumes of H2 gas. The potential issue that arises from the usage of natural gas as a source for H2 is the substantial levels of CO2 emissions due to its dependence on the natural gas produced through steam reforming [4, 5]. From a thermodynamic standpoint, the NRR is favored much by high pressures and low temperature but in terms of kinetics which requires higher operating temperatures for achieving realistic NH3 production rates .
The fundamental bottleneck in realizing electrochemical reduction of nitrogen can is understood better from the thermodynamic constraints imposed by the intermediate reactions. The NRR equilibrium potentials for different products generated during NRR is as follows:
The electrochemical reduction of N2 to NH3 is comparable to H2 evolution with the following equilibrium potentials. While this is in realizing the fact that H2 is the main side product during NRR in aqueous electrolytes. Regardless of these NRR takes place via multiple proton-electron transfer reactions, wherein several intermediates are involved. Equation (7) indicates the difficulty in the addition of the first H atom as a much negative redox potential is required. A significantly higher pH for Eq. (8) required for the reaction to compete with Eq. (7). Furthermore, the addition of second H atom is much more complex than the addition of the third H atom, thus requiring a larger reduction potential for the reduction of two-electron and four-electron processes when compared to reduction involving six-electron. (Eqs. (5), (6) and (8)). The thermodynamic difficulty for the N2 hydrogenation is further clear from the much negative reduction potentials for the intermediates. Since multiple catalytic reactions associated are with the formation of intermediates, this imposes a much critical limitation to the thermodynamics of NRR [4, 6].
NH3 besides being used as a fertilizer is currently pursued as a potential energy storage medium analogous to its high hydrogen content compared with that of methanol . NH3 is expected to play a crucial role in the future hydrogen economy . It is of great importance to developing an environmentally benign method for NH3 production. Electrocatalytic and photocatalytic approaches are indeed regarded as energy-efficient and offer an environmentally friendly approach towards NH3 production, as these processes are generally carried out under ambient conditions using solar and wind energy which are renewable energy resources  For example, solar cells and wind turbines can power electrolytic cells for N2 reduction, and similarly light (photo) driven N2 reduction can directly access sunlight as a potential light source wherein these two strategies can complement each other for minimizing the carbon emissions by establishing a much realistic NH3 production compared to the traditional approaches [10, 11]. The sole idea of electrocatalytic and photocatalytic NRR is entirely reliant on catalysts which form the core for the entire NRR [12,13,14].
Screening of catalysts based on the composition and structure through experimental and theoretical approaches have over time has been done, and many of these electro/photocatalysts have already been designed and fabricated for NH3 production under ambient conditions . For realizing a suitable electrocatalyst for electrocatalytic NRR, over the past few years, some of the highly researched materials include noble metals, non-noble metals based, and conducting polymer-based electrocatalysts is reported over the past for electrocatalytic NRR operated under ambient conditions. Recently, the focus has shifted towards more potential electrocatalysts including transition metals with flat and stepped surfaces and metal nitrides, which have been explored much in much detail through theoretically calculating the free energy changes after finding the possible intermediates formed on these surfaces .
The discovery of TiO2 as a photocatalytic material for splitting water through irradiating ultraviolet light by Fujishima and Honda completely changed the perspective at which water-splitting viewed in the past . Not until 1977 when Guth and Schrauzer reported that electron–hole pairs were generated by the absorption of light by rutile TiO2 to reduce N2 to NH3. Here they also required the use of ultraviolet light to excite the photocatalyst . The solar irradiance observed on the earth’s surface comprises about 5% ultraviolet radiation (< 400 nm), visible light forms 50% (400–800 nm) and lastly infrared light constitutes for almost 45% (> 800 nm). However, the generated NH3 is oxidized immediately to nitrate making it difficult to generate NH3 photocatalytically. In the case of photocatalysts, much importance is given specifically to three classes which include inorganic hybrids , biomimetic chalcogels , and inorganic semiconductor  based electrodes. Among these hydrogen-terminated diamond , and black silicon  have been employed in the conversion of N2 to NH3 under ambient conditions. Electrocatalysts and photocatalysts play vital roles in N2 fixation to NH3, for which we would like to discuss the involved mechanisms. Understanding the underlying mechanism of the NRR is crucial for designing or fabricating efficient catalysts, and with the right knowledge, it would be appropriate to fine-tune or modify the properties of the existing electro/photocatalysts.
In this review, we would like to address the key reaction mechanisms involved in realizing electrocatalytic and photocatalytic NRR under ambient conditions from a futuristic perspective. We begin by introducing the underlying reactions mechanisms employed for electrocatalytic and photocatalytic N2 reduction to NH3 on a heterogeneous catalyst surface. When discussing the reaction mechanisms involved in realizing N2 reduction to NH3, it is necessary to identify the bottlenecks that hinder the natural process. Finally, we list the most promising state-of-the-art electro/photocatalysts reported so far in this area.
Reaction mechanisms for electrochemical NRR
The proposed reaction mechanism on a heterogeneous catalyst for the electrochemical N2 reduction to NH3 is classified into (1) associative pathway and (2) dissociative pathway . In associative pathway, the N2 molecule binds to the catalyst surface and further undergoes hydrogenation with the two nitrogen centers bound to each other, and the NH3 molecule is released after the final N≡N is broken. The associative pathway can be subdivided further into two categories, the distal pathway and alternating pathway based on different hydrogenation sequences. In distal pathway the NH3 molecule is released after the remote N atom is hydrogenated first, and the hydrogenation process continues further to produce the other NH3 molecule, while in the alternating pathway, the two N atoms are hydrogenated synchronously as shown in Fig. 1 . In the dissociative pathway, the N≡N bond is broken much before hydrogenation; thus, leaving two adsorbed N-atoms on the catalyst surface. Finally, the adsorbed N atom undergoes independent hydrogenation before conversion to NH3. Recently, Abghoui and Skúlasson proposed a possible reaction mechanism for N2 reduction to NH3 via Mars-van Krevelen (Mvk) as shown in the Scheme 1, which provides a much favorable reaction mechanism than compared to typical dissociative and associative mechanisms on the surface of transition metal nitride’s (TMN) . In the Mvk mechanism for N2 reduction to NH3 on TMN, the N atom is reduced to NH3, and the catalyst regenerated through the gaseous N2 while differing from the conventional associative and dissociative mechanism. Further, density functional theory (DFT) calculations revealed that the dissociative mechanism hinders the dissociation of N2 on the clean surfaces of TMNs is endothermic while the activation barriers being large. The overpotential for N2 reduction to NH3 is predicted to be much smaller via the MvK mechanism than in the case of the associative mechanism [26, 27].
Principles for designing NRR electrocatalysts
In electrochemical NRR usually, the formation of protons (H+) occurs at the anode/electrolyte interface. The pivotal step in NRR is the N binding to the catalyst surface and considering this for an ideal electrode material should have an optimized N binding. Various studies conducted on electrochemical NRR on transition metal under ambient conditions report the importance of N binding  A noteworthy point is that with weak binding the catalyst will be unable to activate the reactant, whereas when the binding is firm, there is a possibility that the catalyst being poisoned with the strongly adsorbed intermediates and in turn, creates a volcano-like trend between the bond strength and catalytic activity .
These “volcano” diagrams, as displayed in Fig. 2 predict that metals such as Mo, Fe, Rh, and Ru are the most suitable catalyst for NRR activity . However, at the same time, the HER volcano plot also summarizes that these metals are more active for HER than NRR at the same potentials, which reduces the Faradaic efficiency for the NRR while HER is the competing reaction. From the volcano plot, it was found that Mo and Fe to be the most promising candidates for electrochemical N2 reduction to NH3 via the associative mechanism. However, another issue is that these active sites on the catalyst surface could be occupied by oxygen rather than N2 in the presence of water thereby minimizing the efficiency of these catalysts. A recent computational study conducted on Mo nanoclusters shows that the active sites specifically bind oxygen over N2 and H2 . It is therefore imperative that much negative potential is required to reduce the oxygen adsorption on the surface and promote N2 adsorption. The studies conducted for NRR on the surfaces of Ag and Cu with weak N binding showed that it was limited by the adsorption of N2 as *N2H as the first step and protonation of *NH to *NH2. However, in case of metals like Re that bind strongly to nitrogen, the removal of *NH2 to release NH3 is the limiting step . In the case of noble metals like Pt  it has been found that the N-coverage is smaller when compared to H-adatoms at the same onset potential for NH3 formation, which is indicative of the fact that the HER is the most competing reaction with NRR.
The possibility of N2 reduction for NH3 activation under ambient conditions have been recently explored intensively on the (110) facet of several metal oxides , of them the most promising candidates turned out to be NbO2, ReO2, and TaO2. Metal nitrides are another class of highly researched materials that show promising results than its metal counterpart towards N2 reduction to NH3 than the competing HER. A DFT study performed by Abghoui and Skúlasson on TMN surfaces concluded by stating that a suitable catalyst should a have high reactivity and stability towards NRR and studies conducted on the surfaces of the mononitrides with Zincblende (ZB) and rocksalt (RS) structures shows that VN, CrN, NbN, ZrN having a rocksalt structure showed lower onset potential for NRR .
Among these polycrystalline VN exhibited lower overpotential (− 0.5 V) and prevented both HER and catalyst decomposition  2D materials have also been researched, but these have not been employed much towards NRR. Recently, Azofra et al.  implied that 2D transition metal carbides such as MXenes are promising towards N2 reduction to NH3 based on DFT calculations. The N2 chemisorbed on the MXene nanosheets can elongate and weaken the N–N triple bond thereby promoting NRR. Despite these, the critical point to consider while designing an electrocatalyst for efficient NRR is the right material composition, crystal structure, textural structure (porosity), good electrical conductivity to promote electron transfer, and surface properties which would promote strong affinity towards N-adatoms binding rather than H-adatoms.
Electrocatalysts for N2 fixation to ammonia
Electrocatalysts designed for N2 fixation to NH3 have been on the rise, and these catalysts could be categorized based on their ability to reduce N2 to NH3 under ambient conditions. Here we would like to discuss a few emerging electrocatalysts for NRR. In a recent study, Cui et al. demonstrated that both the NH3 yield and Faradaic efficiency could be improved by surface modification of hematite nanostructure, with a proof-of-concept wherein hematite electrocatalysts were fabricated with varying concentration of oxygen vacancies (OVs) while annealing in Argon (Ar) and air to modify OV concentration. They were able to achieve better catalytic performance with a higher concentration of OVs. Their o-Fe2O3-Ar/CNT catalyst produced a maximum NH3 production rate of 0.46 μg h−1 cm−2 at − 0.9 V vs. Ag/AgCl in 0.1 M KOH electrolyte with a Faradaic efficiency of 6.0%. The durability of the system was further evaluated and was surprisingly able to achieve a high NH3 production rate of 1.45 μg h−1 cm−2 with a Faradaic efficiency of 8.28% respectively. This work further elucidates the importance of active sites (oxygen vacancy defects) for N2 adsorption and activation. Carbon-based materials have received widespread attention in recent years owing to the natural abundance and feasibility in preparation methods. Recently Liu et al.  reported the N-doped porous carbon as an efficient electrocatalyst for N2 fixation to NH3. N-doped porous carbon showed an enhanced electrocatalytic activity with a high NH3 production rate of 1.40 mmol g−1 h−1 (− 0.9 V) with a maximum current efficiency of 1.42%. The authors believe that the high content of the pyridinic and pyrrolic N might be responsible for promoting the NH3 formation. DFT studies performed indicate that the preferred pathway for NH3 formation was through *N≡N→*NH=NH→*NH2–NH2→2NH3. The current study opens up the possibilities of exploring carbon-based materials for NRR under ambient conditions.
Recently Zhang et al. reported MoS2 as a proof-of-concept for NRR which was theoretically and experimentally confirmed to be active for NRR under ambient conditions. Density functional theory (DFT) calculations performed to study if the MoS2 edge sites are electrocatalytically active for NRR as it is well known that these edge sites are active for HER. The DFT calculations further performed suggests that the positively charged Mo-edge plays a vital role in polarizing and activating the N2 molecules. The potential-determining step (PDS) happens to be the reductive protonation of adsorbed N2, with a barrier potential of 0.68 eV without any externally applied potential. This relatively low barrier in terms of PDS on Mo-edge further strengthens the claim that MoS2 as a potential candidate for NRR. Appreciable NH3 yield of 8.08 × 10−11 mol s−1 cm−2 with a faradaic efficiency of 1.17% respectively. The MoS2 catalyst exhibited inferior NRR selectivity under acidic conditions than in alkaline conditions, as the electrolyte is infested mostly with protons which would ultimately result in hydrogen evolution. This study offers an exciting new avenue to explore transition metal sulfides as an attractive NRR electrocatalyst for N2 reduction to NH3 .
Transition metal nitrides-based electrocatalysts have gathered tremendous as they offer an added advantage for the presence of N-adatoms in the crystal structure of the metal atom. Zhang et al.  reported the synthesis of MoN nanosheet array as a promising NRR electrocatalyst. They were able to achieve an NH3 production rate of 3.01 × 10−10 mol s−1 cm−2 with a Faradaic efficiency of 1.15%. Further DFT calculations proposed that the MoN nanoarray catalyzes NRR via MvK mechanism. This study opened new doors for the rational design of MoN nanocatalysts for efficient NRR.
Another essential breakthrough for the use of TMN catalysts in electrocatalytic NRR recently was proposed by Yang et al. based on Vanadium Nitride nanoparticles as an electrocatalyst for NRR under ambient conditions. They were able to achieve a high NH3 yield rate of 3.3 × 10−10 mol s−1 cm−2 with high faradaic efficiency of 6.0% (− 0.1 V) in the very first hour of NRR studies. A steady rate of NH3 production was reported with a yield rate of 1.1 × 10−10 mol s−1 cm−2 and a Faradaic efficiency of 1.6% even after 116 h. Further studies conducted provided elucidative results that the electrochemical NRR proceeded via MvK mechanism. This was supported through 15N2 isotope tests. Combined with the ex situ and operando studies that the surface of VN0.7O0.45 is the active phase for electrochemical NRR and the conversion to VN would result in deactivation of the catalyst. This hypothesis is supported also by the DFT studies performed .
Song et al. reported N-doped Carbon Nanospikes (CNS) as an efficient electrocatalyst for efficient N2 fixation to NH3. The proposed mechanism for the reaction depends on the physical interactions on the sharp surface of the catalyst in the absence of a transition metal on CNS. This mechanism was supported by a control experiment performed with an O-etched CNS where the blunt tips produced small fractions of NH3 under the same conditions. They were able to achieve a maximum yield of 97.18 μg h−1 cm−2 with high Faradaic efficiency of 11.56%. The study also stressed on the importance of counterions in the aqueous electrolyte in enhancing the NH3 production rates in the order of Li+ > Na+>K+ wherein these small counterions increased the N2 concentration within the Stern layer. H2 evolution was suppressed with the formation of the dehydrated cation layer surrounding the tip while minimizing the access to water molecules and allowing access to N2 molecules in a high electric field. Although this theory requires further elucidation to fully understand this reaction mechanism, which includes the energy needed for the injection of N2 and the solvation of N2 and both the counterions. This study provides a viable physical reaction mechanism for electrolysis of N2 to NH3 .
Nanoscale confinement is a crucial aspect of improving N2 selectivity on the electrocatalysts surface. Recently Nazemi et al. reported the electrosynthesis of NH3 from N2 and water under ambient conditions with hollow gold nanocages (AUHNCs) as the catalyst. The significant findings from this study are the interdependency of the Ag content in the interior of the Au hollow nanoparticles, pore size/density and finally, the total surface area of the nanoparticles contributed for the enhanced electrocatalytic activity for NRR. Another important finding was the presence of Ag in the cavity of AuHNCs-635 was found to decrease the electrocatalytic activity towards NRR as Ag enhances H2 evolution. Among the various shapes tested for NRR AuHNCs with Localized surface plasmon resonance (LSPR) value of 715 showcased highest NH3 yield rates of 3.74 μg cm−2 h−1 and high Faradaic efficiency of 35.9% was achieved at − 0.4 V vs. RHE. The higher FE and NH3 yield rates of AuHNCs-715 resulted from the better selectivity for N2 reduction as the pore size, Ag content in the cavity and the active surface area of the nanoparticle played a crucial role in enhancing N2 selectivity. As primarily NRR happens in the cavities of AuHNCs, a smaller pore didn’t enhance the N2 selectivity as a higher Ag content resulted in lower selectivity with increased H2 evolution. Further increase in pore size results in decreased NRR selectivity due to the decreased surface area and lack of confinement of the reactants within the cavities (AuHNCs-795). Compared to AuHNCs with larger pore size but do not have Ag in the cavities. Further increase in pore size results in decreased NRR selectivity due to the decreased surface area and lack of confinement of the reactants within the cavities (AuHNCs-795) .
Recently Yao et al. reported the use of Chromium Oxynitride (CrO0.66N0.56) as an efficient electrocatalyst for N2 fixation to NH3 using a homemade proton exchange membrane electrolyzer (PEMEL) under ambient conditions. A high NH3 yield rate of 8.9 × 10−11 mol s−1 cm−2 with high Faradaic efficiency of 6.7% was achieved at 2.0 and 1.8 V respectively. The NRR activity of CrO0.66N0.56 is better than pure Cr2O3 and CrN catalysts. The study also provided more insights into the improved electronic properties of metal nitrides through partial oxidation which opens the door towards designing rational electrocatalysts for NRR .
Reaction mechanism for photocatalytic NRR
Photocatalytic N2 reduction to NH3 typically follows several steps before the release of the NH3 molecule. The first step involves the occupation of the conduction band by the photogenerated electrons which leaves behind a vacancy in the valance band that will be occupied by holes. Subsequently, some these photogenerated electrons and holes recombine while some drift to the surface of the catalyst and take part in the redox reactions on the surface. At this point, H2O can be oxidized to O2 by the holes whereas N2 reduced to NH3 after a series of multiple generations of electrons and water-derived protons [39, 40].
A well-known fact regarding photocatalytic redox reaction is that it is entirely reliant on the reduction potential of the adsorbate and band position of the semiconductor electrode [41,42,43] For instance, it is necessary that the position of the conduction band of a semiconductor electrode should be higher (more negative) in relative to the reduction potential required for N2 hydrogenation, while the valance band position is located much lower (more positive) for oxygen evolution potential [44,45,46]. This leads to limitations which mainly revolves around two aspects one being the ability to active N2 molecule for NH3 formation. This also requires developing semiconductor electrodes with small bandgap preferably along the visible light region, which would still fulfill the necessary thermodynamic conditions for the reduction of N2 to NH3. The second limitation is the necessity to suppress the recombination of the charge carriers to obtain a higher solar conversion efficiency and apparent quantum yield for the reaction .
Design principles for photocatalytic NRR
Several recent investigations carried out from the experimental and theoretical viewpoint on the N2 reduction mechanisms over various photocatalysts [31, 47, 48]. For a better understanding of the N2 reduction to NH3 especially on hydrogenation on photocatalysts it is necessary to understand the process in greater detail. For instance, in the first photogenerated electron transfer, the N2 adsorbed on the catalyst surface attains a proton (H +) from the environment and a photogenerated electron from the catalyst to generate the necessary chemical species. Photocatalytic N2 fixation differs from conventional N2 reduction on transition metals because the former is a chemical process occurring on a semiconductor surface. Which makes the entire catalytic activity becomes reliant on the surface chemistry of semiconductors. Electrocatalytic NRR can be realized through electricity obtained from solar cells and wind turbines but in the case of photocatalytic NRR which can utilize the renewable energy source (solar) for N2 fixation to NH3 (Fig. 3).
Photocatalysts for N2 fixation to NH3
Recently N2 fixation to NH3 on TiO2 surface was studied with the catalytic conversion taking place on surface oxygen vacancies . Various catalysts were tested among them JRC-TIO-6 (rutile phase) displayed highest N2 reduction activity, and it showcased an enhancement of 2.7 times in the catalytic activity when 2-PrOH used as a sacrificial e− donor for the entire duration of the reaction (12 h). From Fig. 4 it can be seen that the superficial Ti3+ surface acts as an electron donor by providing numerous number of active sites for N2 reduction to NH3 which eases the process of N ≡ N bond dissociation. They were able to achieve an NH3 yield rate of 1.75 mmol h−1 with a solar-to-chemical conversion efficiency of 0.02% (Figs. 5, 6, 7, 8, 9 and 10).
Two-dimensional (2D) nanosheets represent important class photocatalysts among which layered double hydroxides (LDH) show promising results for NRR. Zhao et al. reported the synthesis of ultrathin LDH nanosheets of the type MII MIII (where MII = Mg, Zn, Ni, Cu; and MIII = Al, Cr) as a photocatalyst material for NRR. The NRR activity followed the trend of CuCr-NS (184.8 μmol L−1) > NiCr-NS (56.3 μmol L−1) > ZnCr-NS (31.2 μmol L−1) > ZnAl-NS (38.2 μmol L−1) > NiAl-NS (22.3 μmol L−1). Furthermore, MgAl-NS was found to be inactive which presumably is due to its larger bandgap (~ 5.0 eV). CuCr-NS showed better photocatalytic performance with a Faradaic efficiency of 0.44% and displayed excellent photocatalytic stability without any obvious decrease even after five consecutive cycles. The reason for the enhanced catalytic activity was the distortions in the MO6 octahedra caused by the incorporation of oxygen vacancy defects within the ultrathin LDH nanosheets. Density functional theory (DFT) studies indicate that the introduction of oxygen vacancy defects further introduces gap states that promote N2 adsorption and facilitate photoinduced charge transfer from LDH to N2, while at the same time serving as active sites for the chemical reaction of N2 and H2O to form NH3 and O2 respectively. The authors also compared the above results for different operating atmospheres of N2 and Ar. These LDH nanosheets with a large abundance of oxygen vacancies that enhanced the adsorption and activation of N2 with excellent photocatalytic activity in transforming N2 to NH3 under UV–vis excitation and in some selected cases visible excitation (CuCr-NS under wavelength > 500 nm). Further, the introduction of Cu (II) ions in the LDH nanosheets created additional structural distortions and compressive strains, thus leading to increased interaction between the LDH and N2, therefore promoting NH3 evolution. The study conducted here demonstrates a promising new strategy for reduction of N2 to NH3 using LDH nanosheets as photocatalysts with good NH3 yields under ambient conditions .
Dong et al. reported the synthesis of nitrogen vacancy (NV) incorporated g-C3N4 as a photocatalyst for NRR which was synthesized by annealing in nitrogen atmosphere. A high NH3 yield of 1240 μmol h−1 was obtained with g-C3N4 photocatalyst. It was believed that introducing nitrogen vacancies in g-C3N4 would generate N2 activation sites to enhance the photocatalytic activity towards NRR. These induced NV sites can selectively adsorb and activate N2 molecule. In addition to this NVs improved the separation efficiency of the photogenerated carriers and the electron transfer from g-C3N4 to the adsorbed N2. The study conducted here was the first of its kind to report how NVs affected the reactivity of semiconductors for photocatalytic N2 fixation .
Zhang et al. reported Mo-doped defect rich W18O49 ultrathin nanowires as efficient photocatalyst towards solar-driven nitrogen fixation. An NH3 yield of 195.5 μmol h−1 g−1 with a faradaic efficiency of 0.33% at 400 nm and a solar-to-NH3 efficiency of 0.028% was achieved under illumination from an AM 1.5G light irradiation in pure water. Further investigations revealed that Mo doping induces multi-synergetic effect on N2 activation and dissociation through the defect states in W18O49. Active sites were found to be the Mo-W centers for the chemisorption of N2 molecules whose heterogeneity polarizes the N2 molecule for better activation of the catalyst. The metal–oxygen covalency in the photocatalyst lattice promotes better electron transfer to N2 molecules. Mo-doping introduces an elevation of the defect band center towards the Fermi level which generates more energy into the photoexcited electrons for N2 fixation to NH3 .
Yang et al. reported nitrogen photofixation using plasmonic gold nanocrystal on ultrathin nanosheets. Based on the “working-in-tandem” concept of natural nitrogenase herein they have constructed a N2 photofixation system comprising of inorganic Au/TiO2-OV catalysts. The oxygen vacancies (OVs) acted as the activation sites for N2 molecules, whereas the plasmonic electrons acting as the reducing agents for the final fixation of the N2 to NH3. Au/TiO2-OV catalyst exhibited a high NH3 yield of 130.5 mmol h−1g−1 with an apparent quantum efficiency of 0.82% at 550 nm. The authors reported that the “working-in-tandem” strategy effectively tackles two important criteria for activation and reduction of N2 to form NH3. Thus, optimizing the absorption across the overall visible range with the mixture of Au nanospheres and nanorods which further elevates the N2 photofixation rate by 66.2% in comparison with the Au nanospheres solely  Tables 1 and 2 describes a brief summary on the representative NRR photocatalysts and electrocatalysts.
Strategies for improving selectivity towards N2 fixation to NH3
Current systems for NRR lack efficient electro/photocatalyst for N2 reduction to NH3. Most of the catalysts reported so far are limited in terms of the large overpotential required for N2 reduction to NH3 and the low faradaic efficiencies of these systems are major concerns. The thermodynamics of NRR suggest that the reaction should proceed towards negative potentials wherein HER is more influenced this raises the question of selectivity. The other factors that affect N2 reduction to NH3 lies in the electrode material, conductivity of the catalyst material, electrolyte used, operating conditions (temperature), and applied potential.
The intrinsic reactivity can be enhanced by incorporating various strategies which include effects of crystal facets, [54, 55] size effects,  and amorphous nature of the catalyst,  have been found to be very decisive for electro/photocatalytic NRR. As discussed earlier limiting the availability of protons or electrons at the surface of the catalysts is another important aspect to suppress HER and provide selectivity towards NRR. Many strategies have been explored and recently to limit the concentration in the solution researchers have made use of mixture of electrolytes for aqueous based systems . It is also very important to validate that the origin of NH3 produced from N2 is appropriately quantified using experiments such as 15N2 reduction and measured using 1H-NMR. Finally, the electron transfer rate at the interface between the current collector and active material and the interface between the electrode and electrolyte can also play an important role in minimizing HER and showing selectivity towards NRR. One must also take into consideration the underlying drawbacks that needs to be dealt while trying to limit the electron transfer rate towards HER as would result in lesser faradaic efficiencies of the system. It would be therefore astute to have an optimum balance between selectivity to attain a higher performance.
In summary, we have discussed the advancement in electro/photocatalytic NRR and the underlying theoretical and experimental progress so far. Despite all the challenges and obstacles ahead we believe electro/photocatalytic NRR is much more realizable with its current growth. Moreover, in the near future, theoretical and experimental advancement coupled with operando/in situ studies will further shift the tide in developing much efficient electro/photocatalysts which much higher selectivity towards N2 reduction for NH3 formation.
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JJ, DKL, and US contributed to this work in manuscript preparation. The authors have read and approved the final manuscript.
The authors declare that they have no competing interests.
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The review is based on the published data and sources of data upon which conclusions have been drawn can be found in the reference list.
This work has supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (No. 2018R1C1B60012167 and 2018R1A5A1025224).
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