Flatland materials for photochemical and electrochemical nitrogen fixation applications: from lab-door experiments to large-scale applicability

Abstract

Rational design of materials in a catalytic system is the main determinant of the efficiency of sustainable energy sources. Significant efforts have focused on the study and development of flatland two-dimensional (2D) materials (MXenes, MBenes, transition metal dichalcogenides/phosphides (TMDs/TMPs), phosphorene, graphene derivatives) towards energy-driven aspirations in consideration of their superior physiochemical properties as compared to their non-layered counterpart materials, surpassing their transport properties and conductivity. Herein, we aim to provide a detailed account of where the flatland materials currently stand to achieve the goal of sustainability in light of their photochemical and electrochemical nitrogen fixation applications. As of now, numerous challenges have limited the expansion of 2D-material derived nitrogen fixation operations for scalable applications. Therefore, we summarized techno-economic analysis and future perspectives of nitrogen fixation applications in relation to their practical ammonia applicability. We have briefly summarized the functionality of flatland materials and classified them on the basis of their photochemical and electrochemical efficiencies.

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Syed Asim Ali

Syed Asim Ali completed his undergraduate and post-graduate studies at Jamia Millia Islamia, New Delhi. Subsequently, he joined the Nanochemistry Lab under the supervision of Prof. Tokeer Ahmad. His research interests include the design of nanocatalysts, energy conversion and sensing applications, computational chemistry and techno-economic analysis. He is currently working on the synthesis and characterization of molybdenum chalcogenides-based heterojunctions for their multi-functional applications.

Iqra Sadiq

Iqra Sadiq graduated from K. R. Mangalam University and completed her post-graduate studies at G. D. Goenka University, Gurugram. She then joined Jamia Millia Islamia University, New Delhi, as a research scholar under the supervision of Prof. Tokeer Ahmad, and is currently working on the chemical synthesis and structural characterization of some functional nanostructures for environmental remediation and energy-based applications.

Tokeer Ahmad

Prof. Tokeer Ahmad (graduated from IIT Delhi) is the full Professor of Chemistry at JMI Delhi. Prof. Ahmad has supervised 16 PhD's, 79 postgraduates, 10 projects, and published 194 research papers, one patent and three books with research citation of 7610, h-index of 52 and i10-index of 153. Prof. Ahmad is the recipient of the CRSI Medal, MRSI Medal, SMC Bronze Medal, ISCAS Medal, Inspired Teacher's President of India Award, DST-DFG award, Distinguished Scientist Award, Maulana Abul Kalam Azad Excellence Award of Education, Teacher's Excellence Award, and was elected as Member of the National Academy of Sciences, India. Prof. Ahmad is also an admitted Fellow of the Royal Society of Chemistry (FRSC), UK.

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Introduction

The present era of energy utilization has predominantly involved the extensive use of fossil-derived fuels dating back to the dawn of the industrial revolution. The last century paved the way for oil reserves, consolidating the fossils in strengthening economies across the globe.^1,2 In the face of environmental challenges and the urgency to mitigate the effects of global warming, the pursuit of sustainable energy has emerged as a global imperative. Sustainable adaptability can be achieved by subscribing to those energy sources and maneuvers that meet the world's present needs and do not obstruct the scope of future generations for utilizing natural resources. Renewable energy resources such as hydrogen production via overall water splitting and hydrocarbon fuels originating from CO₂ reduction reaction (CO₂RR) are the front-runners aimed at alleviating carbon emissions and promoting a transition to a low-carbon economy.^3–5 Despite significant progress in the research of hydrogen evolution reaction (HER) and CO₂RR, both these processes have some disadvantages⁶ such as extraordinary costs associated with storage and transportation of the hydrogen carrier and separation of the desired product from the mixture of carbon-based products. Utilizing ammonia (NH₃) as the source of renewable energy and as the transport medium for carrying H₂ fuel comes with several benefits such as its compactness and higher efficiency in durable and longer distance energy endeavours.^7,8 The market size of NH₃ is expanding dramatically, which is substantially driven by its conventional use in agricultural and manufacturing sectors alongside its growing interest as a carbon-neutral energy carrier. Among the different routes of NH₃ synthesis, one natural enzymatic reduction phenomenon is the conversion of nitrogen (N₂) via nitrogenase that replenishes the N-cycle.^9–11 Nevertheless, the lethargic kinetics of NH₃ formation restrict the applicability of this pathway. Production of NH₃ from photochemical and electrochemical N₂ fixation processes at ambient conditions is an emerging area of research that has gained attention on account of the industrial significance of NH₃ as the precursor to manufacturing many important chemicals and fertilizers.^12–15 The efficiency of the photochemical and electrochemical nitrogen reduction reaction (NRR) for NH₃ production relies upon the prudent choice of a catalytic system that can accelerate the reaction dynamics.^16–21 A wide array of catalytic systems has been constructed by researchers to augment the yield of the targeted renewable energy carrier during energy-conversion applications. These systems include perovskites,^22,23 mesoporous materials,^24,25 transition metal oxides²⁶/phosphides²⁷/dichalchogenides²⁸/carbonitrides/borides,^29,30 single-atom catalysts (SACs),^31,32 layered double hydroxides (LDHs),^33,34 and carbon-based derivatives.³⁵

Among these materials, the development of flatland materials has been a ground-breaking achievement in materials science and nanotechnology,³⁶ offering unique properties and potential applications across interdisciplinary fields,³⁷ including electronics,³⁸ energy storage,³⁹ catalysis,⁴⁰ and biomedicine.⁴¹ In addition, the recent advances in flatland materials have ascertained their energy conversion applications.⁴² Chemical strategies, such as doping,⁴³ surface modification,⁴⁴ and heterostructure engineering,⁴⁵ have been employed for tuning and engineering the properties of flatland materials for energy conversion applications. The functionalization and integration into a heterojunction formation have led to the development of novel devices with enhanced performance and functionality.⁴⁶ The rise of graphene/graphitic nitride derivatives paved the doors of 2D compounds, as these materials possess remarkably higher surface area and tunable electronic properties that are highly sought after for catalytic transformations.^47,48 As of now, different flatland materials have been designed and exploited in energy conversion applications, viz., hydrogen and oxygen evolution reactions (HER/OER)^28,35 and CO₂ sequestration.⁴⁹ Nevertheless, the research of these layered materials toward NRR is still at its nascent stage, and researchers are showing immense interest in their applicability as state-of-art catalytic systems for the NRR process. Noteworthily, the origin of the catalytic excellence of several 2D materials is linked to their exposed edge facets and defect surfaces that also govern their NRR performance.^49,50 Diverse modification attempts have been made by researchers to augment the catalytic efficiency of flatland materials, such as surface functionalization,⁵¹ intercalation,⁵² and incorporating porous materials or non-layered materials (such as porous organic polymers (POPs) or TiO₂) to develop heterojunctions.^28,53,54 These chemical strategies to tailor the physicochemical properties of flatland materials require optimized reaction conditions at the time of synthesis, such as the solvent selectivity, thermal conditions and exfoliating parameters, to extract monolayers of 2D materials.⁴⁸ To achieve a higher yield during the NRR process, the enhancement of exposed surface sites is the primary factor. The ideal flatland material-based catalytic system is expected to provide an inherent basal plane activation, while possessing higher selectivity for N₂-adsorption as compared to H-adsorption as an HER, making it a thermodynamically more viable process than NRR. The efficient synergetic relationship between the catalytic activity and long-term stability of 2D materials-based catalysts is necessary to have robustness during the NRR process. The stability and durability of flatland materials suffer from several bottlenecks, which restrict their performance. The 2D materials usually comprise defects, such as vacancies, grain boundaries, and edge irregularities, which ideally serve as active sites. However, they can also be prone to degradation, leading to the loss of catalytic activity. The 2D materials can undergo unwanted oxidation or reduction reactions, especially under extreme pH conditions or at high temperatures, which can alter their electronic structure and catalytic properties. When 2D materials are exploited during heterojunctions formation, weak interactions between the 2D material and the support can lead to detachment and loss of the active material. Fabricating 2D materials with consistent quality and physicochemical properties on a large scale is challenging. Variations in synthesis methods can lead to inconsistencies in the catalyst performance. Addressing these challenges involves synthetic advancements, enhancing the understanding of the material properties under operational conditions, and developing strategies to protect and stabilize 2D materials during catalytic transformations.

Synthetic advancements in the development of flatland materials

For the systematic exploitation of 2D materials to the maximum extent, the prudent choice of the synthetic protocol is necessary. Improved synthesis techniques lead to higher-quality 2D materials with tailored properties, addressing many of the challenges associated with their stability and durability. There are plenty of top-down and bottom-approaches that have been reported in the literature for the fabrication of functional and advanced materials, such as the facile hydrothermal route, sol–gel/polymeric citrate precursor method, reverse micellar, chemical vapor deposition, mechanical/chemical exfoliation, lithography techniques and many more.^55–60 One of the most important routes of constructing MXene and MBene is their etching from the parental MAX/MAB phases, where A is basically the transition metal (such as Al, Ga or Sn). The MAX/MAB phase precursors are generally prepared via powder metallurgy techniques, followed by etching using stronger acid etchants such as HF/H₂SO₄ to selectively eliminate the A-layers. The etching routes expose the MXene/MBene layers, which are then washed and delaminated to obtain colloidal suspensions or films of MXene/MBene nanosheets.⁶¹ Exfoliation of bulk MXene/MBene materials involve breaking down the layered bulk MXene/MBene into individual nanosheets, which can then be dispersed in solvents to obtain colloidal suspensions or thin films.^62,63 Another interesting method of designing MXenes and MBenes is the intercalation/delamination method, in which the concerned-intercalated precursors are subjected to chemical or electrochemical treatments to selectively eject the interlayer ions or nanoparticles, leading to the delamination of the layered structure into MXene/MBene nanosheets.⁶⁴ Alongside these exfoliation and top-down methodologies, there is the growing interest of researchers toward designing MXenes and MBenes through bottom-up approaches, such as chemical vapor deposition (CVD)⁶⁵ and facile hydrothermal method,⁶⁶ without utilizing environmentally hazardous F-based etchants that are not economical for the scalable synthesis of MXene/MBene nanosheets.⁶⁷ These methodologies offer versatile chemical strategies for developing MXene/MBene materials with tailored properties and structures to ensure their simplified integration into various energy-related applications. Recent attempts have focused on optimizing the synthesis parameters, scaling up the production and exploring novel MXene/MBene catalysts to expand their range of applications.^68,69 For the development of TMDs, there are diverse fabrication routes that ensure morphological and structural tailoring to a larger extent, such as CVD, template-assisted synthesis, mechanical/chemical/liquid-phase exfoliation. CVD provides a simplified protocol to prepare high-quality TMDs nanosheets with higher specific surface areas.⁷⁰ By tuning different reaction parameters like the temperature, pressure, precursor concentration, and substrate type, the rate of nucleation and growth can be modulated to design the desired morphology of TMDs materials required for targeted applications. Exfoliation techniques of synthesizing TMDs generally involve manually peeling off thin layers of bulk TMDs to acquire TMDs monolayers with great control over the thickness of the nanosheets. The facile hydrothermal route ensures the construction of TMDs having controlled size and desirable morphological and structural properties under 180–200 °C temperature range at higher pressure.^26,28 These routes have been comprehensively discussed in reviews elsewhere.⁷⁰ The fabrication routes of TMPs generally encompass different methods based on the precursors of P, such as elemental phosphorus, phosphorenes or organic phosphines. To achieve morphological tunability and higher specific surface area, P-pyrolysis with a C-based compound is an excellent method that hampers the aggregation of TMP nanosheets and improves its electrical conductance. Just like other 2D materials, there are different routes of TMPs fabrication, such as phosphorizing metal sources,^71,72 CVD, solid state method, and hydrothermal synthesis, as detailed in reviews elsewhere.²⁷ The fabrication routes of carbon-based derivatives are more or less similar to those of other 2D materials, such as CVD, exfoliation techniques for g-C₃N₄,⁷³ and the Hummer's method for synthesizing graphene oxide (GO) and reduced-GO. These fabrication routes all fundamentally involve the utilization of different chemical reducing agents, such as hydrazine, hydroiodic acid, or thermal treatment under reducing atmospheres.⁴⁷ Despite the success of these methods in designing flatland materials, the achievement of scalability in their synthesis is fundamental for realizing their industrial applications. State-of-art approaches such as high-throughput screening techniques can be exploited, which involve systematically screening a large number of reaction conditions and parameters to identify optimal conditions for the preparation of 2D materials.⁷⁴ Reaction variables like the reactant concentrations, reaction temperatures, mixing rates, and reactor geometry can be optimized to attain an equilibrium between the maximized yield and minimized production costs.^75–77 Pilot-scale operations and process modelling can be utilized to determine the critical factors affecting scalability and guide the design of large-scale 2D materials synthesis processes. The advancements in the fabrication routes are pivotal for the construction and application of 2D materials in NRR.

Nitrogen fixation

N₂ fixation is regarded as one of the most significant processes to transform atmospheric N₂ into NH₃ and other value-added N-based compounds. This transformation can be achieved through a wide array of pathways, such as the conventional Haber–Bosch method, biological or microorganisms-assisted N₂ fixation. Artificial photochemical N₂ fixation is a sustainable route for developing nitrogenous compounds by utilizing photon energy and replicating the natural N₂ fixation process. Similarly, electrochemical N₂ fixation also should produce N-based chemical feedstock at the expense of electrical energy.^78–81 Among the various nitrogenous compounds, NH₃ holds relatively superior industrial significance and day-to-day applicability, as it serves as the fundamental precursor in manufacturing several fertilizers and purifiers.⁸² Alongside, condensed NH₃ promises applications in the energy conversion on account of its greater gravimetric density and transportability as compared to H₂ fuel. The recent trend of sustainable energy transition suggests an exponential growth in the demand of H₂ fuel by 2050. However, there are challenges in achieving efficient transportation over a larger distance in compact containers. In contrast, NH₃ is relatively easy to transport with the requirement of a less complicated infrastructure.^83,84 Green NH₃ synthesis relies upon the accelerated production of H₂ simultaneously with the extraction of N₂ from the environment via air separating techniques. Photochemical and electrochemical NH₃ production routes are carbon-neutral pathways that aspire to generate NH₃ efficiently and economically in the near future.^85,86 NH₃ is most commonly produced commercially via Haber–Bosch reactions, which require high temperatures (400–500 °C) and pressure (15–25 MPa). A simpler and environmentally benign protocol of NH₃ is sought after to accelerate the pace of NH₃ usage as an energy carrier.

NRR applications of flatland materials

Flatland materials have garnered significant attention for effective NH₃ production due to their unique physicochemical features. These 2D materials offer a diverse range of properties and structures that can be tailored via different compositional and structural modifications, such as surface functionalization, heterostructure engineering, and optimization of electronic and catalytic properties.⁸⁷ Perseverant advancements into the rational design, characterizations and applications of 2D materials affirm their scalability in sustainable nitrogen fixation technologies.

Photochemical applications

The reaction mechanism of photochemical NRR generally progresses via the formation of several reactive intermediate species like N-based radicals and adsorbed N-intermediates that have to effectively undergo the hydrogenation reaction, followed by desorption to generate NH₃.⁸⁸ The excellent optoelectronic and surface properties of 2D materials are of paramount importance in mediating the thermodynamic landscape of NRR by providing the favourable free energies and lower reaction barrier during the surface reactions on the active materials.

MXenes- and MBenes-based photocatalysts

MXenes and MBenes have emerged as two of the latest classes of flatland materials that hold a plethora of potential for heterogeneous catalysis, especially in energy-conversion applications, owing to their enormously higher exposed active sites and unusual layered structures.⁸⁹ Current research has hinted that MXenes- and MBenes-based catalytic systems have advanced the enhancement in the rate of N₂-adsorption/activation and its conversion into NH₃ through photochemical and electrochemical NRR processes.^90–92 As of now, the quantum yield of MXenes-led NRR remains lower as compared to the non-sustainable routes of NH₃ synthesis. This is the main reason that a deeper understanding of the mechanism is requisite to optimize the activity of MXenes. Gao et al.⁹³ fabricated a unique MXene-based heterostructured photocatalyst composed of Co-decorated Ti₃C₂/TiO₂ for effective nitrogen fixation. The group demonstrated that the tailoring of the surface activity of N₂ was due to the presence of Co-atoms, as the extent of chemisorption was altered. The optimized photocatalyst achieved as high as 110 μmol g⁻¹ h⁻¹ NH₄⁺ yield in the absence of any sacrificial hole acceptor species. Hao et al.⁹⁴ hydrothermally constructed a RuO₂-incorporated TiO₂–Ti₃C₂T_x photocatalytic system to examine its nitrogen fixation performance at ambient conditions. The group revealed that the augmented activity of the as-developed catalytic system was due to the symbiotic effects between the MXene/metal oxides, and to the efficient N₂-activation over light exposure. Liao et al.⁹⁵ showcased the role of Ti₃C₂ in augmenting the NRR activity of the P25 (TiO₂) photocatalyst, attributed to its higher exposed active sites and marvelous electrical conductance. In addition to that, the improved N₂-adsorption over the Ti₃C₂-P25 photocatalyst resulted in its 5-fold higher NRR performance as compared to pristine P25. The close-knit relationship between Ti₃C₂-P25 was realized by X-ray photoelectron spectroscopy (XPS), as illustrated in Fig. 1(a)–(d). The photochemical results inferred enhanced NH₃ selectivity over H₂ evolution on account of the relatively higher chemisorption of N₂ over the H-atoms. Furthermore, the lower conduction band position of Ti₃C₂ also favoured the NRR, as can be seen in Fig. 1(e). Shen et al.⁹⁶ engineered a ligand-bridged Ti₃C₂O_x/MIL-125(Ti)-based MXene/MOF photocatalyst to favourably couple the O-terminal groups (MXene) with the COO-groups (MOF). This type of coordination linkage facilitated the charge migration via heterojunction formation during the NRR process. The as-prepared photocatalyst displayed a remarkable NH₃ production rate as high as ∼103 μmol g_cat⁻¹ h⁻¹, which was ascribed to the monodirectional channelization of electron carriers from MOF to MXene. Intermediary species formed during the course of the NRR experiment over the optimized Ti₃C₂O_x/MIL-125(Ti) photocatalyst were assessed via DRIFTS technique, which proved the N₂-adsorption and activation due to the advent of peaks corresponding to N-based intermediates, as illustrated in Fig. 2(a). These observations indicated the probable symmetric side-on mechanistic route for the NH₃ production. The authors utilized density functional theory (DFT) calculations to investigate the impact of ligand-bridging on the dynamics of N₂-activation. Bader charge analysis and the corresponding 2D curve of the Ti₃C₂O_x/MIL-125(Ti) photocatalyst revealed the extensive charge migration occurring through the ligand molecular bridge (LMB). Based on the hydrogenation reaction step of adsorbed-N₂ over Ti₃C₂O_x/MIL-125(Ti), NRR could have proceeded via two different distal and alternating pathways, as shown in Fig. 2(b). Both reaction mechanisms exhibited varying energy barriers, where the alternating route inferred a relatively lesser uphill energy barrier. In addition, the authors analyzed the modifications in the N–N bond order in the span of the reaction mechanism to further supplement their identification of the probable NRR route. Based on the bond length values of the N–N bonds in both pathways, the pronounced bond elongation in the alternating route provided the evidence that it was the predominant course of NRR. Just like their MXenes counterparts, MBenes have also recently emerged as active materials for the photocatalytic NRR process. Zhang et al.⁹⁷ developed the unique Ni₃B-incorporated N-vacant g-C₃N₄ (VN-CN) to augment the N₂ fixation efficiency for NH₃ generation. The group ascertained the Schottky junction-formation between MBene and VN-CN, which led to the improved light harvesting and higher exposed sites availability for adsorbing and activating N₂ molecules. The optimized Ni₃B/N-vacant g-C₃N₄ photocatalyst revealed a rate of NH₃ production as high as 7.68 mM g⁻¹ h⁻¹, which was nearly 7-fold higher than bare g-C₃N₄. These recent achievements in MXenes and MBenes infer the potential of these 2D-materials toward NRR applications.

Fig. 1 Core-level spectra of (A) C 1s, (B) Ti 2p, (C) O 1s of bare P25(TiO₂), Ti₃C₂ and the optimized Ti₃C₂-P25 heterostructure. (D) Chemical interactions between TiO₂ and Ti₃C₂. (E) Reaction mechanism of the photochemical NRR for NH₃ formation over the Ti₃C₂-P25 heterostructure. Reproduced with permission.⁹⁵ Copyright 2020, Elsevier.

Fig. 2 (a) In situ infrared spectra of the optimized Ti₃C₂O_x/MIL-125(Ti) heterojunctions analyzed at the time of photochemical NH₃ generation. (b) Thermodynamic landscape for alternating and distal reaction mechanisms over Ti₃C₂O_x/MIL-125(Ti). Reproduced with permission.⁹⁶ Copyright 2024, Elsevier.

TMDs-based photocatalysts

TMDs have attained significant benchmarks in the field of photochemical NRR on account of their uncommon electronic properties, higher specific surface area, lower band energy and facilitated visible light absorption. TMDs possess active sites with favourable free energies that accelerate the adsorption/activation of nitrogen molecules, leading to efficient NH₃ conversion.^98,99 Alongside their unique physicochemical properties, two important chemical strategies to enhance the NRR activity of TMDs are structural and compositional modifications. The ability of TMDs to accommodate various dopants and functional groups, in addition to their ability to form heterojunctions with other conducting materials, remarkably augment their NRR response. Nevertheless, TMDs suffer from several bottlenecks, such as their limited chemical and thermal stability during photochemical environment, which consequently lead to the oxide formation and photo-corrosiveness of TMD-photocatalysts. Moreover, a profound understanding of the underlying NRR mechanisms is requisite to optimize the catalytic efficiency and stability of TMDs for practical applications. By developing novel TMDs-based heterojunction photocatalysts for energy conversion applications,^26,28 researchers have made satisfactory progress in the past few years, especially for realizing their potential in NRR applications. Recently, Ma et al.¹⁰⁰ illustrated the unique interfacial chemical bonding between MoS₂/In-Bi₂MoO₆, which resulted in the enhancement of the NRR performance of the as-developed heterostructured photocatalyst fabricated via electrostatic self-assembly route. The group ascertained the advent of the Mo–S intermolecular chemical bonding between the heterostructured photocatalyst to be the primary influencing factor that facilitated the charge-transfer dynamics at the interfacial contact. The theoretical investigations inferred the transfer of electron carriers from MoS₂ to the antibonding orbitals of N₂-molecules for its activation. The optimized MoS₂–In-Bi₂MoO₆ heterostructured photocatalyst exhibited ∼90 μmol g_cat⁻¹ h⁻¹ rate of NH₃ production. Based on the experimental observations, the authors revealed the significant role of MoS₂ in impeding the generation of NO₃⁻ species, which is an offshoot of the oxidation reaction of the NH₃/NH₄⁺ present in the aqueous solution. Therefore, this study clearly depicted the role of the TMDs-based photocatalyst for the selective N₂ photo-reduction reaction. To further highlight the role of MoS₂ in achieving higher photocatalytic NRR performance, Hu et al.¹⁰¹ proposed an excellent study where they regulated the edge sites of MoS₂ to augment the NRR performance. The S-rich edge sites of MoS₂ are relatively exposed active sites that are responsible for the reduction reaction as compared to the S-atoms present on the basal planes. It was revealed that Mn-doping effectively enhanced the NRR activity of MoS₂ by improving its edges. This led to the modulation of S-edges that facilitated the adsorption of N₂ molecules and lowered the NRR energy barrier. Hydrothermally-derived optimized Mn-doped MoS₂ depicted an NH₃ yield as high as ∼213 μmol g_cat⁻¹ h⁻¹ without using any sacrificial electron donor species, which was found to be nearly 5-fold higher than that for the bare MoS₂ photocatalyst. Recently, Yao et al.¹⁰² constructed a TMDs/metal organic framework (MOF) heterostructured photocatalyst composed of WS₂/ZIF-8 to examine its photochemical NRR activity using tap water in real/simulated solar energy. The optimized photocatalyst revealed an NH₃ production rate as high as ∼191.6 μmol g_cat⁻¹ h⁻¹, as can be seen in Fig. 3(a). The accelerated NRR performance was attributed to the enhanced light harvesting and higher separation of photo-excited charge carriers. In addition, the close-knit interfacial contact between WS₂/ZIF-8 led to the pronounced weakening of the N₂ bond and facilitated its activation dynamics during photochemical NRR. Fig. 3(b and c) demonstrates the Bader charge analysis and photocatalytic NRR mechanism for the generation of NH₃. The transfer of electron-carriers from ZIF-8 to WS₂ led to the facilitated reduction of N₂, which was ascribed to the higher availability of electrons on the conduction band of WS₂. Qin et al.¹⁰⁴ explored the reaction mechanism of N₂ adsorption/activation over the hexagonal-triclinic biphasic Mo_1−xW_xS₂ photocatalyst for advancing the rate of NH₃ production. The optimized Mo_1−xW_xS₂ photocatalyst exhibited an NH₃ evolution rate of ∼111 μmol g_cat⁻¹ h⁻¹, which was found to be 3.7- and 3-fold higher than that of bare MoS₂ and WS₂. Experimental investigations manifested the favourable doping of W-atoms, which caused the exploitation of electron density states from the W 5d-orbital for the polarization of the adsorbed-N₂ species. This report highlighted the significance of fabricating intramolecular TMDs to achieve an efficient NRR process. These recent advancements in TMDs-led NRR experiments reveal the scope of MoS₂, MoSe₂ and WS₂ in accelerating the NH₃ production efficiency by acting either as an active material or as the cocatalyst in heterostructured materials during photochemical transformations.

Fig. 3 (a) Photochemical efficiency of the WS₂/ZIF-8 catalytic system towards NRR in varying sources of photon energy. (b) Bader charge analysis and optimized crystal structure of WS₂/ZIF-8. (c) Photochemical NRR mechanism over WS₂/ZIF-8. Reproduced with permission.¹⁰² Copyright 2024, Elsevier. (d) Recyclability test of the optimized Ni₂P/Cd_0.5Zn_0.5S photocatalyst under visible light irradiation. (e) Photochemical NRR mechanism over Ni₂P/Cd_0.5Zn_0.5S for NH₃ production. Reproduced with permission.¹⁰³ Copyright 2017, Elsevier.

TMPs and phosphorene-based photocatalysts

TMPs are an emerging class of 2D materials that offer potential eminence when employed as a photocatalyst during the NRR process on account of their unique physicochemical properties that accelerate the transformation of N₂ into NH₃ under light irradiation. These flatland materials possess an edge over other photocatalysts in terms of their improved activity, selectivity, and stability during photocatalytic transformations.^105,106 The excellent optoelectronic properties of TMPs assist in achieving higher efficiency in photochemical NRR processes, ascribed to the improved light absorption and their layered structure that promote the separation and migration of photo-induced charge carriers during photocatalysis. Alongside, TMPs exhibit exceptional surface properties with mesoporous exposed active sites that favour the adsorption/activation of N₂ molecules during NRR. Recently, researchers have attempted to design the heterostructured photocatalytic systems by exploiting the remarkable physicochemical properties of TMPs. TMPs exhibit superior stability, robustness and prolonged lifetime during photochemical runs, which particularly make them viable photocatalysts for durable NRR experiments. To demonstrate the higher stability of the TMP-based photocatalysts, Gao et al.¹⁰⁷ recently reported on the construction of a g-C₃N₄/Ni₂P/Ni-3D reticulated foam photocatalytic system (CNNPF) for visible light-driven NRR application. The as-designed CNNPF photocatalyst displayed excellent NRR activity as high as ∼373 μmol g_cat⁻¹ h⁻¹ rate owing to the synergetic relationship of g-C₃N₄/Ni₂P, which resulted in the ameliorated charge-transfer dynamics. The group revealed that the 3D-scaffold was responsible for providing the exposed surface-sites availability for N₂ capture. The CNNPF photocatalytic system has shown advanced stability and recyclability after 5 consecutive cycles without significant loss in the activity toward the NRR process. This study demonstrates the significance of designing a stable photocatalytic system for durable NRR application. To highlight the simultaneous HER and NRR applications of TMP-based photocatalysts, Wang et al.¹⁰⁸ recently came up with an interesting study where they reported the simple route of designing a unique Ohm–Schottky heterostructured photocatalytic system by utilizing Co–Ni₂P nanosheets. The as-designed modified-CdS/Co–Ni₂P/O-rich Ti₃C₂ photocatalyst revealed a H₂ yield as high as 1.72 mmol h⁻¹ and NH₄⁺ yield of ∼656.8 μmol L⁻¹ h⁻¹ with 59.81% AQE. The enhanced HER and NRR performances were ascribed to the improved interfacial charge transfer via Ohm–Schottky heterojunctions. Theoretical investigations ascertained the positive impact of decorating the Co–Ni₂P nanosheets over CdS/MXene by providing favourable N₂ adsorption energy and by increasing the conductivity of photocatalyst. Ye et al.¹⁰³ fabricated a unique Ni₂P/Cd_0.5Zn_0.5S photocatalyst. The TMP incorporation into Cd_0.5Zn_0.5S incredibly ameliorated the charge-transfer dynamics during the photochemical NRR process. In the visible-light driven photocatalysis, the NRR activity to achieve NH₃ was found to be ∼101.5 μmol L⁻¹ h⁻¹ with 4.32% AQE. The heterostructured photocatalyst depicted no loss of activity after four consecutive cycles, as can be seen in Fig. 3(d). Moreover, the advanced performance of the TMP-based Cd_0.5Zn_0.5S photocatalyst was realized by the modulations of the conduction band (CB) positions after Ni₂P loading, which tuned the photo-reduction efficiency during NRR, as shown in Fig. 3(e). As compared to Cd_0.5Zn_0.5S, the relatively higher CB position of the heterostructured photocatalyst facilitated the N₂ reduction and lowered the dynamics of oxidation of NH₃ into NO₃⁻, which was confirmed by the low yield of the latter (∼0.35 μmol L⁻¹ h⁻¹). Phosphorene is the P-analog of graphene that is composed of a honeycomb lattice composed of P-atoms. Phosphorene has gained significant research interest owing to its unusual physicochemical properties, such as its higher specific surface area, and its peculiar electronic properties that favour its energy conversion applications.¹⁰⁹ The effectiveness of phosphorene materials in the photochemical NRR application is supported by its recent reports. Wang et al.¹¹⁰ designed the unique non-metallic B-decorated phosphorene (BP) photocatalytic system for investigating its photochemical NRR performance. The group revealed that in the case of P-phosphorene, the adsorption limit of the photocatalyst towards N₂ remains limited. However, the BP-photocatalyst has shown its structural superiority to enhance the rate of N₂ capture by the utilization of vacant and filled sp²- and p-orbitals, respectively. This ensured the better electron-acceptance and -donation between the σ–π* orbitals for efficiently adsorbing N₂, as illustrated in Fig. 4(a). On account of the boron-decoration, the band structure of phosphorene was considerably tailored and the electrical conductance of the photocatalyst was uplifted to favour the NRR performance, as revealed in Fig. 4(b). The B-sites played a crucial role in transmitting electron-carriers from BP to N–H composites during N₂ activation.

Fig. 4 (a and b) Mechanistic sketch of the photo-reduction of N₂via the B-decorated phosphorene catalyst. Reproduced with permission.¹¹⁰ Copyright 2021, Elsevier. (c) Photochemical NH₃ production performance of g-C₃N₄ and boron-decorated g-C₃N₄. Reproduced with permission.¹¹¹ Copyright 2020, John Wiley and Sons. (d) Photochemical NRR activities of bulk g-C₃N₄ and carbon-vacant g-C₃N₄. Reproduced with permission.¹¹² Copyright 2019, Elsevier.

This study provided insight into the reaction mechanism for improving the NRR activity of phosphorene due to the anchoring effect of the Boron atoms. Cheng et al.¹¹³ theoretically anticipated the NRR efficiency of the B-incorporation and oxidation of black/blue phosphorene. Their study also included the lowering of the band energy due to boron atoms, which enhanced the light absorption range in the visible-IR region. The thermodynamic screening also indicated the favourable formation energy of the B-introduction into phosphorene at ambient temperature. Furthermore, it was ascertained that the oxidation of the black/blue phosphorene augmented the NRR response via end-on/side-on modes during the reaction mechanism.

Carbon derivatives-based photocatalysts

Carbon-based derivatives have shown enormous growth in their catalytic applicability, which is ascribed to their peculiar layered structure and advanced stability. Graphitic-carbon nitride (g-C₃N₄) has been most frequently exploited for photochemical NRR application among the different carbon derivatives, as it is cost-efficient, easy to fabricate, and possesses visible light band energy. Experimental characterization techniques indicate its adequate band positions of conduction and valence bands to fulfil the thermodynamic criteria for the reduction of N₂.^114,115 To highlight the catalytic efficiency of g-C₃N₄, Liu et al.¹¹⁶ designed an Fe₂O₃/g-C₃N₄ photocatalytic system for NH₃ synthesis through NRR. The Z-scheme type heterojunction formation facilitated the interfacial charge transfer. Furthermore, it was ascertained that g-C₃N₄ was the reduction photocatalyst responsible for the NRR, whereas Fe₂O₃ was the oxidation photocatalyst. The higher specific surface area with mesoporous active sites favoured the extensive N₂ adsorption/activation as an optimized Fe₂O₃/g-C₃N₄ photocatalyst with an NH₃ generation rate of 47.9 mg L⁻¹ h⁻¹, which was 6-fold higher than that of pristine g-C₃N₄. In order to activate the N-atom active sites in g-C₃N₄ such as edges/amino-N sites during NRR, Wang et al.¹¹¹ came up with a unique study in which, by anchoring B-atoms over g-C₃N₄, the N-atom active sites were stabilized via the development of B–N–C bonding that ultimately boosted the photochemical activity of g-C₃N₄ towards NH₃ production. The limited loading of boron into g-C₃N₄ enhanced the range of light absorption, and also impeded the recombination dynamics of photo-excited e⁻–h⁺ pairs. This was confirmed by the higher activity of the optimized photocatalyst for NH₃ evolution (∼314 μmol g⁻¹ h⁻¹), which was found to be 10-fold higher than bare g-C₃N₄, as can be seen in Fig. 4(c). Dong et al.¹¹⁷ investigated the role of the nitrogen vacancy (N_v) in ameliorating the photochemical NRR performance of g-C₃N₄. As the shape and size of N_v was found to be identical to N-atoms of N₂ molecules, it thus promoted the adsorption and activation of the reactant N₂ during the reaction mechanism. Similar to this study, for the utilization of the carbon-vacancies (V_C), Zhang et al.¹¹² developed an ultrathin g-C₃N₄ having a higher V_Cvia thermal peeling technique that augmented the NRR efficiency. The group ascertained that the NRR activity of the carbon-based materials can be substantially increased by structural control at the atomic level and by the introduction of V_C in g-C₃N₄. A photochemical NRR activity test was performed in N₂ and Ar gases to determine the origin of the N-precursor for N₂ fixation, as the catalyst in itself was composed of N. The control test inferred the fixed amount of NH₄⁺ release in both catalytic systems, as can be seen in Fig. 4(d). The V_C-rich g-C₃N₄ showed nearly 2.25-fold higher NRR efficiency as compared to bulk-g-C₃N₄, thus manifesting the significance of V_C in augmenting the photocatalytic activity of g-C₃N₄. Other carbon-based derivatives such as graphene also exhibited a propensity to be utilized as a photocatalyst for NRR application owing to their ability to induce a higher hot electron density in the presence of visible light irradiation.¹¹⁸ To support this claim, Lu et al.¹¹⁹ demonstrated the induction of hot electrons from an Fe-decorated 3D-graphene photocatalyst to accelerate its activity and selectivity for NH₃ production at ambient conditions in the presence of a simulated light source. The as-prepared Fe@graphene photocatalyst exhibited remarkable stability without losing its activity up to 50 h. Based on the aforementioned recent advances in the photochemical NRR process via flatland materials, we have highlighted the strategies of ameliorating the scalability of this carbon-neutral vector in Fig. 5.

Fig. 5 Chemical strategies for advancing the photochemical NRR efficiency of catalytic systems.

Electrochemical applications

Aiming to supplant the energy-intensive Haber–Bosch method, the electrochemical NRR to form NH₃ takes place under ambient conditions and can be propelled by renewable energy comprising solar energy, wind energy, etc. Thermodynamically, it has been considered that electrocatalytic NRR holds the advantage of 20% greater energy efficiency in comparison to the Haber–Bosh process.¹²⁰ However, an effective electrochemical NRR has been evinced to be exceptionally challenging due to the need of a considerable amount of energy to break the N₂ triple bond.¹²¹ Additionally, HER is the competitive process in electrocatalytic NRR as its theoretical reductive potential is less than NRR.¹²² It is widely known that one of the most crucial parameters in electrochemical NRR is the conductivity of materials; hence, the search for novel electrocatalysts to surpass the challenging issues is necessary for the electrocatalytic NRR industrial applications.¹²³ In general, noble metal catalysts display significant activity, but their scarcity and high cost limit their far-reaching applications. As a result, research studies have changed focus to non-noble metals-derived catalytic systems, such as carbon derivatives, MXenes, TMDs, TMPs. Electrochemical NRR is a typical heterogeneous reaction that comprises the adsorption of N₂ molecule at the catalytically active surface of the catalyst, hydrogenation and desorption of NH₃ molecule or other intermediates. According to reduction and protonation sequences and the breaking of N≡N bonds, electrochemical NRR can be classified into dissociative and associative mechanisms. In the dissociative mechanism, the N≡N bond is broken at the time of adsorption, resulting in the adsorption of discrete N atoms separated by a defined distance. In the successive step, hydrogenation takes place on each N atom, leading to the construction of NH₃ that is released in the last step. In the associative mechanism, the N≡N bond stays intact after the adsorption of N₂ and breaks at the time of hydrogenation. Consequently, the associative pathway is more complex than the dissociative route, and can be further simplified according to the hydrogenation sequences into distal, alternating, and enzymatic pathways. In the distal route, the hydrogenation preferentially takes place on the N atom that is away from the catalytic surface, inclining to the adsorption of one nitrogen atom after the release of the first NH₃ on the catalytic surface. In the following steps, the successive hydrogenation of the remaining N atoms leads to the construction of a second NH₃ molecule. In the alternating route, the hydrogenation takes place on two N atoms alternatively and N≡N bond breaks with the generation of the first NH₃ at the last step, which leaves one NH₃ molecule behind.¹²⁴ In contrast to the alternating route, N₂ adsorbs at the catalytic surface in the very first step in the enzymatic route. The subsequent steps are similar to those in the alternating route.¹²⁵ Recently, the Mars-van Krevelen mechanism was developed by Abghoui and Skulason. In this mechanism, the lattice N atoms are reduced to NH₃ on the surface of transition-metal nitrides (TMNs) and N vacancies are thus created.¹²⁶ These vacancies chemically adsorb N₂ molecules for electrochemical NRR to occur continuously. Therefore, a suitable electrocatalyst for effective NRR is rather critical that can ensure the convenient reaction mechanism in order to attain higher NH₃ efficiency.

MXenes and MBenes-based electrocatalysts

MXenes are a novel type of 2D materials possessing a high specific surface area, good electrical conductivity, stability and hydrophilic behaviours, making these flatland materials potential candidates for electrocatalytic NRR applications. Shi et al.¹²⁷ fabricated the N-doped-Ti_V-Ti_3−xC₂T_y-1.2 MXene for electrocatalytic NRR. The introduction of Ti vacancies improved the adsorption and activation of N₂. Furthermore, the N-dopant species enhanced the desorption of NH₃. The optimized level of the N doping content and Ti vacancy density in the as-synthesized MXene boosted the electrocatalytic NRR with the rate of 24.33 μg h⁻¹ mg_cat⁻¹ of NH₃ and around 10% faradaic efficiency (FE) through the promotion of kinetic desorption and activation barriers of NH₃. Ba et al.¹²⁸ designed Ti₃AlC₂ MAX/Ti₃C₂ MXene electrocatalysts for N₂ fixation. The difference of the surface potential between MXene and MAX was around 40 mV, indicating that the electron can be easily migrated from MAX to MXene over the interfaces. This was beneficial for N₂ fixation, resulting in a remarkable FE of ∼40% and rate of 2.73 μg h⁻¹ cm⁻² of NH₃. The outstanding NRR activity was ascribed to the unique MXene/MAX heterostructure framework that offered suitable and rich active centers to assist in the N₂ adsorption billiard sequence, hydrogenation and N≡N bond breaking. Guo et al.¹²⁹ altered the surface of Ti₃C₂T_x (MXene) by introducing Fe to attain high surface reactivity for the NRR. The optimized MXene/TiFeO_x-700 electrocatalyst exhibited remarkable performance with an NH₃ production rate of 2.19 μg cm⁻² h⁻¹ and FE of 25.44% for the NRR. The improved catalytic activity was credited to the high surface reactivity of as-prepared optimized electrocatalyst generated through the reduction of inactive F*/OH* terminals and the rational decrement in the surface work function, which revealed the higher number of active sites to bind N₂ and accelerated the electron migration from the catalytic surface to the intermediates of the reaction, enhancing the NRR. Du et al.¹³⁰ incorporated Ni nanoparticles on V₄C₃T_x MXene (Ni/MX) for effective electrocatalytic N₂ fixation that performed exceptionally well by achieving ∼21.3 μg h⁻¹ mg_cat⁻¹ yield of NH₃. The enhanced performance was credited to the synergistic pathway by the O vacancy of V₄C₃T_x and the Ni sites in the nanoparticles, achieving excellent enhancement of the NRR performance on the MXene based nanocomposites. Niu et al.¹³¹ examined the transition metal atom-assisted Ti₃C₂T₂ MXene along with O/OH terminals for electrocatalytic NRR. The catalytic activity of the transition metal single atoms was investigated by their capability of adsorption on MXene, and also their potential to bind and desorb N₂ and NH₃, respectively. The OH termination in the as-synthesized electrocatalyst improved the N₂ adsorption and suppressed the adsorption of NH₃. The results displayed that Ni/Ti₃C₂O_0.19(OH)_1.81 showed rational kinetics and thermodynamics for electrocatalytic NRR. Another class of flatland materials having unique characteristics and structure, transition metal borides termed as MBenes have recently shown immense potential in energy-driven applications. The exotic nature of B in MBenes in terms of bonding and conductivity has led to growing research interest in these 2D materials. Zhang et al.¹³² demonstrated FeB₂ as a novel catalytic system for electrochemical nitrate reduction into NH₃. The synergetic relationship between Fe and the B atomic-sites augmented the rate of activation of nitrate and the rate of hydrogenation of the intermediary species to achieve higher NH₃ selectivity. As high as 96.8% FE was obtained with 25.5 mg h⁻¹ cm⁻² rate of NH₃ generation at −0.6 V overpotential. Recently, Feng et al.¹³³ carried out a high-throughput screening of hexagonal-MBene electrocatalysts to determine their NRR efficiency. The authors ascertained that the catalytic response of MBenes can be tuned by bimetallic anchoring effects. It was concluded that this compositional modification leads to the regulation of the orbital energy redistribution in MBenes, and ultimately ameliorates the binding limit of the N-based intermediate species over the MBene-active sites. Gao et al.¹³⁴ examined the electrochemical NRR activity of hexagonal-Zr(Hf)₂B₂O₂ nanosheets-decorated SAC. The theoretical simulations inferred the low overpotential value of −0.10 V with higher NRR selectivity against the competitive HER process. These recent studies endorse the MXenes and MBenes tandem electrocatalysts as the state-of-art catalytic systems for efficient electrochemical NRR application.

TMDs-based electrocatalysts

TMDs have been extensively examined for a number of electrochemical reactions such as HER, oxygen reduction reaction (ORR), CO₂RR, and OER because of their tunable electronic structures to attain high catalytic performance.¹³⁵ It is quite obvious that the revealed edge defects of TMDs are the favourable active sites for HER catalysis. Therefore, decreasing edge defects can activate their capability for NRR catalysis applications. Chen et al.¹³⁶ fabricated a series of transition metal (TM)/WS₂ and examined their electrocatalytic performance for NRR. The results illustrated that Os/WS₂, Tc/WS₂ and Cr/WS₂ showed the best catalytic performance. The adsorbed N₂ can be efficiently transformed into NH₃ following the distal mechanism pathway together with −0.33, −0.27 and −0.37 V limiting potentials, respectively, as a consequence of the suppressed dynamics of the HER competing process. Yang et al.¹³⁷ synthesized hollow MoSe₂ nanospheres as an effective electrocatalyst for NRR. The results exhibited the rate of 11.2 μg h⁻¹ mg_cat⁻¹ of NH₃ production and 14.2% FE that was credited to the greater number of active sites, higher surface area and the longer retention time of N₂ in the shells due to the hollow structure of the as-prepared electrocatalyst. Chu et al.¹³⁸ investigated the MoS₂/C₃N₄ heterostructure for electrochemical NRR via in situ growth of MoS₂ on C₃N₄ nanosheets utilizing the solvothermal method, as represented in Fig. 6(a). The proton nuclear magnetic resonance (¹H NMR) spectrum showed a doublet chemical shift upon providing ¹⁵N₂ gas after electrochemical NRR. A triplet chemical shift was also displayed upon providing ¹⁴N₂ gas. Meanwhile, the peaks of ¹⁵NH₄⁺ and ¹⁴NH₄⁺ were undetectable in Ar gas, as shown in Fig. 6(b). The current density for 2 hours at various potentials is displayed in Fig. 6(c). The as-synthesized electrocatalyst exhibited enhanced NRR activity with the yield of 18.5 μg h⁻¹ mg⁻¹ of NH₃ and FE of 17.8%, which was much better than that of pristine MoS₂ and C₃N₄, as illustrated in Fig. 6(d) and (e). DFT calculations showed that the interfacial charge migration from C₃N₄ to MoS₂ can improve the NRR activity of MoS₂/C₃N₄ by boosting the stability of the intermediate and simultaneously reducing the reaction energy barrier. Ling et al.¹³⁹ reported the WS₂/WO₂ electrocatalyst for efficient NRR that exhibited the rate of 8.53 μg_NH3 h⁻¹ mg_cat⁻¹ for NH₃ production with 13.5% FE, which was better than that of pristine WS₂. The enhanced activity was ascribed to the incorporation of WO₂ that efficiently suppressed the HER due to the blocking of the edge defects in WS₂ and concurrent formation of planar defects at the heterostructure as active sites for effective NRR. Liu et al.¹⁴⁰ reported an interesting study in which synergetic effects between bridged-S atoms and metallic sites over the 1T-MoS₂ phase were exploited in achieving higher electrochemical NRR performance. S-rich metallic MoS₂ yielded as high as ∼99 μg h⁻¹ mg_cat⁻¹ NH₃ with nearly 23% FE, which was found to be one of the highest among the MoS₂-based reports of NRR as of now. Theoretical studies also corroborated the favourable adsorption energy and lowered energy barrier during the reaction mechanism. The as-designed electrocatalyst also depicted the higher NH₃ selectivity and remarkable competitiveness during NRR.

Fig. 6 (a) Schematic representation of the MoS₂/C₃N₄ heterostructure fabrication. (b) NMR depiction of MoS₂/C₃N₄. (c) Current densities of MoS₂/C₃N₄ at different potentials. (d) Yield and FEs of NH₃ at various potentials. (e) NRR activities of MoS₂, C₃N₄ and MoS₂/C₃N₄. Reproduced with permission.¹³⁸ Copyright 2020, American Chemical Society.

TMPs and phosphorene-based electrocatalysts

Much attraction has been garnered by TMPs due to their low cost, distinct physical and chemical features, controllable components, in addition to TMPs specifically exhibiting remarkable results as electrocatalysts. Integrating phosphorus (P) into the transition metal lattices can alter the internal electronic structure of metals, thus enhancing the intrinsic catalytic performance. Although the utilization of TMPs in NRR has not been explored more, Rytelewska et al.¹⁴¹ reported Fe₂P for electrochemical NRR. The as-prepared electrocatalyst showed inhibition for HER, providing around 60% FE and an NH₃ production rate of 7.5 μmol cm⁻² h⁻¹. The improved results were attributed to the interfacially reduced iron in the presence of P network sites that activated and reduced N₂. Su et al.¹⁴² designed RhP_x and a N, P-doped carbon framework as a highly efficient electrocatalyst for NRR. The catalytic behaviour and electronic structure of RhP_x were altered by tuning the P contents in the composites. DFT calculations illustrated that the incorporation of P into Rh eased the intermediate process of the catalytic reaction and reduced the catalytic reaction barrier. The results exhibited 7.64% FE and 37.6 μg h⁻¹ mg_cat⁻¹ NH₃ production in accordance with the higher stability and selectivity possessed by the optimized electrocatalyst. Luo et al.¹⁴³ engineered Mo/FeP through the thermo-assisted hydrolysis method, as shown in Fig. 7(a), for the electrochemical transformation of N₂ to NH₃. The chronoamperometry results displayed steady current densities at various potentials under 2 hours electrolysis, indicating the outstanding stability (Fig. 7(b)). Also, the ultraviolet visible (UV-vis) spectra at the potential range from −0.2 V to −0.6 V is shown in Fig. 7(c). The as-synthesized nanosphere possessed remarkable catalytic activity with the rate of 13.1 μg h⁻¹ mg⁻¹ for NH₄⁺ yield and ∼7.5% FE, as illustrated in Fig. 7(d) and (e). The enhanced results stemmed from the Mo doping that was beneficial for the adsorbed N₂ molecule polarization, facilitating the easier dissociation of the N≡N bond.

Fig. 7 (a) The synthesis method of the Mo–FeP nanosphere. (b) Current density plots of Mo–FeP at distinct applied potentials. (c) UV-vis absorption spectra tinted with the indophenol indicator for the electrolytes after electrochemical NRR from −0.2 V to −0.6 V. (d) Yield of NH₄⁺ and (e) FEs of Mo–FeP and FeP at various potentials. Reproduced with permission.¹⁴³ Copyright 2020, American Chemical Society.

Wu et al.¹⁴⁴ examined the NRR taking place on the monolayers of iron-based phosphides. The Mo-doped Fe₃P and Fe₂P monolayers effectively boosted the electrocatalytic NRR with −0.17 and −0.30 V onset potential. Additionally, the Fe and Mo atoms served as bimetallic active sites that corroborated the linear relationship at the time of the NRR pathway. Wang et al.¹⁴⁵ highlighted the anchoring effects of bimetallic Cu–Ru atoms over N-loaded phosphorene (NP) in enhancing the rate of the electrochemical NRR. The results manifested the lower overpotential value of 0.3 V of the as-designed electrocatalyst for N₂ reduction into NH₃. The electron donor–acceptor relationship of π–d electrons facilitated the activation of N₂ that is fundamental for its hydrogenation into NH₃. The symbiotic relationship between Cu–Ru and NP was further confirmed by the lowering of the NH₃-desorption energy ascribed to the higher availability of the exposed active sites.

Carbon derivatives-based electrocatalysts

At present, graphene and its derivatives have become propitious metal-free electrocatalysts possessing several distinctive features, such as tunable porosity, high surface area, outstanding stability, and conductivity and abundant defects. These merits of mass transfer and reinforcing electrons and higher active sites offer efficient NRR. Wang et al.¹⁴⁶ synthesized broken holey graphene oxide (BHGO) for electrocatalytic NRR, and compared the activity of the optimized electrocatalyst with holey graphene oxide (HGO) and graphene oxide (GO). The BHGO displayed higher active sites and greater electron migration capability. Therefore, it showed outstanding NRR activity and stability with an NH₃ yield of 22.27 μg h⁻¹ mg⁻¹ and ∼11% FE at ambient conditions, as shown in Fig. 8(a) and (b). Also, the yield and FE of NH₃ almost remained constant for 10 cycles, as displayed in Fig. 8(c). The source of the N element had also been examined by carrying out the experiment in blank and control environments. The results demonstrated that there was no NH₃ detected except for when N₂ was used as the feed gas, as shown in Fig. 8(d). Moreover, DFT simulations showed that the insertion of multi-dimensional coordinated defects improved the adsorption of N₂, decreased the energy barrier at the rate-determining step, and profoundly redistributed the charge. Zhang et al.¹⁴⁷ designed reduced graphene oxide (rGO) consisting of tuned defects for NRR. The as-synthesized rGO having defect sites (including unsaturated carbon) displayed enhanced NH₃ selectivity because of the robust binding of N₂ in place of the H⁺ ions. The as-prepared dopant-free electrocatalyst showed the FE as high as 22% and an NH₃ production yield of ∼7.5 μg h⁻¹ mg⁻¹. Liu et al.¹⁴⁸ fabricated a MoS₂/rGO electrocatalyst for N₂ fixation. The homogeneous distribution of MoS₂ on rGO and the C–S–C linking bonds among them benefitted the electrocatalyst and offered abundant active sites, boosting the electron migration in the reaction that galvanized an outstanding FE of ∼28% and nearly 16.5 μg h⁻¹ mg_cat⁻¹ yield of NH₃. Moreover, the as-prepared electrocatalyst displayed high electrochemical stability and selectivity.

Fig. 8 (a) FEs and yield of NH₃ at distinct potentials for BHGO. (b) Current density curve of BHGO for 80 000 seconds. (c) Ten electrocatalytic cycles at −0.6 V for BHGO. (d) Yield of NH₃ of blank and after 2 hours electrolysis at different conditions. Reproduced with permission.¹⁴⁶ Copyright 2020, Elsevier.

Wang et al.¹⁴⁹ synthesized an Fe₃O₄/rGO electrocatalyst for NRR having remarkable stability, selectivity and activity, with an achieved NH₃ rate of ∼28 μg h⁻¹ mg⁻¹ and FE of ∼19% at ambient conditions. The enhanced NRR activity stemmed from the chemical coupling impact between the Fe₃O₄ particles and rGO that resulted in the improvement of the binding affinity of N₂ molecules, decreasing the free energy of the reaction and notably ameliorating the conductivity. Wang et al.¹⁵⁰ engineered In₂O₃/rGO, and determined the experimental and theoretical evidences to support the remarkable NRR activity of the heterostructured electrocatalyst. DFT studies manifested that the exposed facets of In₂O₃ favoured the efficient N₂ adsorption, which ultimately led to the improved cleaving of N–N bonds and following hydrogenation steps during NRR. This ultimately restricted the HER kinetics and resulted in a lower NRR overpotential. Therefore, the as-synthesized electrocatalyst showed excellent NRR activity with ∼8% FE and the rate of NH₃ yield was found to be 18.4 μg h⁻¹ mg⁻¹.

Techno-economic assessment of NRR

It is known that nearly 50% of the H₂ evolved globally is consumed as one of the primary reactant feeds of the NH₃ preparation process via Haber–Bosch technique. Therefore, sustainable NH₃ production holds great significance, which is very much achievable due to the lowering prices of renewable electricity across the globe and because of the abundant solar energy.¹⁵¹ For the determination of the economic viability of the NH₃ synthesis via NRR process, techno-economic analysis (TEA) is an important indicator that encompasses the various segments of the proposed technology and its market size globally. It is a methodology that can be employed to estimate the economic feasibility of NRR, as it combines the engineering principles with economic analysis to assess the cost of green NH₃ fuel, benefits, and risks associated with it.^151–154 To obtain the balanced equilibrium between the cost and production of NH₃, the photochemical and electrochemical NRR should be more efficient than the conventional Haber–Bosch process for its industrial competitiveness. As of now, the production cost of the latter technology is estimated to be nearly 600 dollars per ton by comprising different costs of transporting and storing NH₃.^155,156 Pozo et al.¹⁵⁷ evaluated the viability of green NH₃ production as a carbon-neutral energy vector. The group carried out cash flow investigations, which inferred that based on the gas-switching reforming, a levelized cost of ∼332 € per ton is achievable as compared to traditional NH₃ production plants (∼385.5 € per ton). Pawar et al.¹⁵⁴ examined the TEA of NH₃ generation by employing renewable resources of wind, solar and electrical energies. The group revealed that India can achieve the higher NH₃ efficiency with cost as low as 723 and 765 EUR per t_NH3 towards national and international usage. Progress in TEA of green NH₃ will play a vital role in channelizing the NRR technology to a scalable platform. However, TEA alongside life-cycle assessments (LCA) have certain disadvantages which limit their effectiveness in providing a complete scenario of green NH₃ generation refinery, such as resource intensiveness, limited scope of analysis, and assumption sensitivity. Nevertheless, TEA remains a valuable technique for providing detailed insights into the economic viability and risks of NH₃ production via NRR. It is essential to recognize and address these limitations to ensure that the TEA results are robust, reliable, and actionable.

Conclusions

Photochemical and electrochemical NRR have emerged as potential alternative pathways of the energy-exhaustive conventional Haber–Bosch (HB) route to produce NH₃ efficiently at lower cost. The generation of green NH₃ alongside HER and CO₂RR is the environmentally benign energy carrier that promises to achieve a carbon-neutral society. Herein, we have discussed photochemical and electrochemical NRR applications in light of the ever-evolving flatland materials by taking into consideration their remarkable physicochemical properties. Recent advances in 2D-materials-utilized photochemical NRR operations have been described with the main preference given to those materials which have achieved decent progress in green NH₃ synthesis in the past few years, such as MXenes and MBenes, TMDs, TMPs, carbon derivatives and phosphorene. In addition, the electrochemical NRR applications of 2D materials have been summarized to highlight their experimental achievements in achieving higher faradaic efficiency and NH₃ selectivity. Different types of reports have been discussed to bolster the catalytic design of flatland materials for efficient photochemical and electrochemical NRR performance. The catalytic efficiency of flatland materials can be augmented by engineering unique heterostructured catalysts and introducing surface defects to improve the optoelectronic and surface properties in order to ameliorate the charge-transfer dynamics and electrical conductance of 2D materials. Techno-economic assessment of NH₃ has also been surveyed to indicate the current status of NRR, and where this technology stands as the contender of a sustainable energy resource.

Future outlook

Photochemical and electrochemical pathways of NRR offer promising perspectives for sustainable NH₃ synthesis, providing opportunities to address environmental challenges and transition towards a greener and more energy-efficient nitrogen fixation process. Here, we have aimed to discuss the most important factors that should be taken into consideration for an efficient NRR process via flatland materials-based catalytic systems, as shown in Fig. 9.

Fig. 9 Significant variables to enhance the effectiveness of flatland materials-based catalytic systems for NRR.

(1) As compared to the exploitation of 2D materials in other energy conversion applications of HER and CO₂RR, the literature of NRR is still in its nascent stage. This is attributed to several contributing factors, such as the complexity of the reaction mechanism, low yield, sluggish dynamics, and its lower thermodynamic feasibility as compared to the competitive HER process.^158–160

Nevertheless, various chemical strategies are in the developing phase for impeding the rate of the competitive HER during nitrogen fixation, such as: (1) designing catalysts with tailored surface properties and active sites that selectively promote the NRR over the HER by providing favourable free energies for N₂ capture in contrast to the H⁺-adsorption. In this quest, Liao et al.⁹⁵ provided the superior chemisorption of N₂ over H-atoms during photochemical NRR process, as also discussed in aforementioned reports. Furthermore, engineering flatland materials with controlled surface morphology/hierarchical structure are the other prospects of ameliorating the NRR efficiency. (2) Decoration of exposed facets of 2D-materials can also lead to the modification of their electronic properties, thus assisting in improving the selectivity towards NRR. For instance, the incorporation of metallic/non-metallic dopants (as highlighted by Wang et al.¹⁰⁸) by utilizing Co-dopant in Ni₂P nanosheets. (3) Surface modification of 2D materials in such fashion that functionalizes their catalytic surface with ligands/functional groups for preferential binding of N₂ molecules and facilitates their activation and reduction, such as introduction of heteroatoms (N, P, B) into the catalyst. For instance, Wang et al.¹¹⁰ ascertained the anchoring effects of B-atoms in augmenting the N₂ adsorption ability of P-phosphorene on account of exploiting vacant and filled sp²- and p-orbitals during the NRR process. In a similar manner, Dong et al.¹¹⁷ examined the role of the nitrogen vacancy (N_v) in improving the N₂ capture ability of g-C₃N₄ by exploiting the identical geometry of N_v and N-atoms. This report uncovered an excellent surface engineering strategy to augment the NRR activity by enhancing the N₂ binding. (4) Optimization of the operational reaction conditions (such as temperature, pressure, pH, electrolyte composition) and utilization of sacrificial electron donor species to improve the NRR performance over its HER counterpart. Moreover, in electrochemical processes, the applied potential can be equilibrated to shift the reaction dynamics towards NRR, while minimizing the rate of HER. For instance, utilization of electrolytes with high ionic conductivity and low HER overpotential can suppress the HER and enhance the NRR. (5) Hampering the HER-active sites by introducing inhibitors that selectively block or deactivate sites on the catalyst surface is another strategy of promoting the rate of NRR. This can be achieved by employing strategies that can passivate or modify specific exposed sites on the surface of flatland materials that otherwise are favourable for HER activity. For instance, the edge sites of TMDs are naturally inclined towards the adsorption of H⁺ ions, ascribed to their favourable free energies. Therefore, modifying these edge sites in such fashion that NRR active sites are exposed can lead to an efficient NRR process. These are some of the most significant chemical strategies that can mitigate the rate of the competitive HER process during NRR, and enhance the selectivity and efficiency of the NH₃ production.

(2) Another important factor that governs the NRR efficiency is the stability of the catalyst and separation of NH₃ from the side reactions that take place simultaneously during the photo/electro fixing of N₂ through reductive pathways. As of now, the catalytic activity of the flatland materials is limited towards NRR with stability up to hardly hundreds of hours only. Based on this, the scalable applicability of these 2D materials is still far away and requires the addressing of factors that restrict their activity, such as robustness, recyclability during NRR, separation of NH₃ and its oxidation into nitrite/nitrate species via activated O-radicals.¹⁶¹

(3) To fully optimize the catalytic performance of flatland materials, a comprehension of the mechanistic sketch is requisite during photochemical and electrochemical NRR applications. Owing to the complex reaction mechanism of NRR, machine learning tools and theoretical modelling should be employed to investigate the surface-active sites that involve favourable free energies for N₂ adsorption/activation. The role of non-covalent interactions, chemical modifications and chemical environment effects during surface reactions should be extensively scrutinized using different experimental techniques to determine the kinetics of intermediates that are formed during the reaction mechanism of N₂ fixation.³ Existing computational models are able to ascertain the formation energies of different intermediates. However, the anticipations of theoretical data should be confirmed via experimental tools. Therefore, researchers should focus on developing theoretical-experimental problems to accelerate the pace of NH₃ formation via NRR process.¹⁰ The state-of-art atomic-level characterization and in situ techniques can play a crucial role in obtaining scalability in NH₃ synthesis.

(4) One of the most significant approaches of enhancing the quantum efficiency during NRR operations is the fabrication of catalytic systems that exhibit an ideal band structure with visible light-driven band energy, as the driving potential involved in N₂ reduction is 1.17 V. However, the role of other half-oxidation reactions should not be undermined as the produced NH₃ is prone to the oxidation reaction. For identifying the reactions that take place at the valence band of the photocatalyst, high-throughput screening of HER can be utilized, which has been discussed in detail in reviews elsewhere.¹⁶²

Data availability

No primary research results, software or code have been included, and no new data were generated or analysed as part of this review.

Conflicts of interest

The authors declare no financial competing interest.

Acknowledgements

T. A. thanks the SERB sponsored research scheme (CRG/2023/000480) and UGC, Govt. of India for Research Grants for In-service Faculty Members for financial support. S. A. A. and I. S. acknowledge UGC for research fellowships.

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