Comprehensive Insight and Advancements in Material Design for Electrocatalytic Ammonia Production Technologies: An Alternative Clean Energy

5.1.1. Strategical Material Design for NRR

Strategically, precise control and design of materials for noble metal catalysts, such as Ag, Au, Pt, Ru, Rh, Pd, Os, and Ir, involve high atomic utilization processes through single-atom, nanostructure, and alloy formation for the NRR catalytic process. The high cost of noble metals strongly encourages the utilization of non-noble metal catalysts, including Fe, Ni, Bi, Co, Mo, Ti, and V (metal nitrides, sulfides, and oxides) [56, 63, 64]. The precise catalytic activity and selectivity of NRR could be improved through controlled approaches: (1) tuning the local reaction environment and (2) employing specific catalyst design strategies (vacancies, element doping, and construction of heterojunctions) (Table 1) [65–67].

Transition metals possess unoccupied nonbonding d orbitals with a high electron density, which can serve as electron donor sites, promoting the NRR. Transition metal-based materials can donate electrons to the π∗ orbital of the N≡N molecule, facilitating nitrogen adsorption and weakening the N≡N bond. However, it is important to note that the presence of transition metal d orbital electrons also promotes the formation of metal–H bonds, which can enhance the competitive HER while potentially limiting the catalytic efficiency of the NRR [68]. As an alternative, nonmetal elements with abundant valence electrons may offer a more favorable environment for nitrogen activation as well as efficient selectivity due to weak H adsorption (Figures 7(a), 7(b), 7(c), and 7(d)) [19, 69, 70]. Zhang et al. utilized exfoliated few-layer black phosphorus (P) nanosheet as a nonmetallic catalyst for NRR. P (3s²2p³) is a potential candidate for NRR as its valance electron structure is similar to that of the N (2s²2p³) [71]. Exfoliated few-layer P provides a maximum number of intrinsic active sites for NRR (Figures 7(e), 7(f), and 7(g)).

The nonmetallic component in transition metals reduces the hydrogen adsorption capability in the compound, thereby diminishing the hydrogen evolution reaction (HER) activity. Simultaneously, a sufficient number of valence electrons on the nonmetallic compound provide enough active sites for the nitrogen reduction reaction (NRR) process. Transition metal complexes, such as FePc and CoPc, with the assistance of nonmetallic components like phthalocyanine (Pc), are utilized to enhance NRR activity for ammonia (NH₃) production [72–74]. Reducing nitrogen presents significant challenges due to its high bond energy, a substantial energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), and a large ionization potential. Paul et al. demonstrated that the d orbital of the CoPc molecule forms a σ bond with N molecules, utilizing the lone pair of electrons from the N atom (Figure 8) [72]. Additionally, π–back bonding occurs when the π∗ orbital of the N₂ molecule accepts electrons from the filled d orbital (e_g). Consequently, the N₂ molecule can readily attach to the metal center of CoPc, a process facilitated by the combined effects of σ bonding and π–back bonding [72].

In a study by Murmu et al., the nitrogen adsorption energy with d-band center for the NiPc electrocatalyst in NRR and the correlated charge transfer mechanism between Ni and N₂ were investigated [75]. The study reveals that the weak binding of NNH intermediates on carbon, pyrrolicN3,N2, and pyridonicN1 is the reason for the higher free energy and overpotential for NNH∗ than the Ni center in NiPc. The Ni center in NiPc exhibits the lowest overpotential of 1.22 V, compared to pyrrolic N3 (1.65 V), pyrrolic N2 (1.79 V), pyridinic N1 (1.77 V), and carbon (2.29 V). The researchers also discovered that the overpotential of NRR (1.22 V) is lower than that of HER (1.37 V) for the Ni center in NiPc. It appears that for Ni-Pc, the d-band center in the density of states (DOSs) shifts toward negative directions after the adsorption of N₂ (–1.63 eV) and NNH intermediates (–2.23 eV). The electron accumulation on the N–Ni bonds for N₂ and NNH favors nitrogen reduction. The overall scenario demonstrates that the Ni center in NiPc is the optimal active site and suppresses the HER [75].

Current research is primarily centered on optimizing electrocatalysts based on disordered noble and transition metals through manipulation of their shape, size, and composition for the purpose of NRR [76]. Unfortunately, traditional disordered catalysts often exhibit limited activity and stability (Table 2) [77]. Conversely, an ordered electrocatalyst structure can result in stronger electron interactions and have a notable impact on bond lengths, thereby influencing the electrocatalytic performance [78, 79]. Additionally, maintaining high enthalpy mixing in ordered structures contributes to enhanced stability [80]. Tong et al. developed an ordered body-centered cubic (BCC) Pd–Cu electrocatalyst that demonstrates impressive performance, with an NH₃ yield of 35.7 μg h^–1 mg_cat^–1 and a faradaic efficiency of 11.5%, at -0.1 V vs. RHE (reversible hydrogen electrode) [81]. In this context, a robust d–d coupling between Pd and Cu sites connects the coulomb gap related to electron transfer, enabling efficient NRR.

Morphological modulation proves to be an effective strategy for obtaining specific surface facets and active site densities that favor catalytic activities [82]. The presence of oriented spikes on the surface generates a focused electric field at the sharp tips, facilitating the concentration of reactants nearby and promoting catalytic reactions [83, 84]. Zhou et al. designed a dendritic Cu nanostructure with well-oriented nanospikes, leading to efficient catalytic NRR with a NH₃ yield rate of 25.63 μg h^–1 mg_cat^–1 and a FE of 15.1% at –0.4 V vs. RHE [82]. The Du group demonstrated that the thickness of the Cu layer influences catalytic activity; increasing thickness hinders mass transfer [85]. Cu with a monolayer, however, exhibits a lower mass transfer barrier and a mass activity of 12 μg h^–1 mg_cat^–1 at 0.0 V vs. RHE.

The Shi group investigated the Cu_2–xS/MoS₂ catalyst through the epitaxial growth of the MoS₂ layer on Cu_2–xS quantum dots (QDs) for NRR performance, mimicking the natural nitrogenase active sites [86]. The selectivity in NRR improved due to the threefold coordinated Cu sites (Cu_1.81S) [87]. The core-shell structure exposes Mo and Cu active sites more efficiently for electrochemical NRR, thereby increasing the charge transfer rate. The rate-limiting step is ∗NH + e^– + H⁺ → ∗NH₂, and the energy barriers for MoS₂ and Cu_2–xS/MoS₂ are 1.5 and 0.98 eV, respectively [88, 89]. MoS₂ is more prone to HER than Cu_2–xS, so it is crucial to control the thickness of MoS₂ on the core QD Cu_2–xS [90].

In the case of transition, metal binds the N₂ too weakly but binds the NH₃ too strongly, which is imperative for a trade-off. A catalyst should bind molecules or atoms with intermediate strength. It should not be too weak, as this might fail to activate the reactants, nor too strong, to allow for desorption. This concept reflects a volcano-type relationship between activity and bond strength based on the Sabatier principle [91, 92]. Additionally, the d orbital electrons in transition metals contribute to the formation of metal-H bonds, promoting the hydrogen evolution reaction (HER) and, as a consequence, reducing faradic efficiency [76]. Balancing energy in the critical electrocatalytic steps involves modifying the catalyst’s microenvironment through the engineering of O vacancies [93]. This modification can reduce the activation energy barrier and enhance electron transfer at the interface. Yang et al. employ Fe₃C/Fe₃O₄ heterojunctions in the nitrogen reduction reaction (NRR) [94]. Fe₃C plays a crucial role by creating O vacancies on Fe₃O₄, which serve as active sites for NRR, enhancing the adsorption strength of N₂. Fe₃C/Fe₃O₄ heterojunctions paves the distal mechanism of the NRR [94].

Fang et al. developed a freestanding cathode by growing graphdiyne nanosheets on cobalt nitride nanowires in situ. This innovative approach led to impressive results, achieving an ammonia yield rate of 219.7 μg h^–1 mg_cat^–1 and a Faraday efficiency of 58.6% at –0.2 V vs. RHE [95]. Graphdiyne is a unique two-dimensional carbon allotrope known for its porous structure, uneven surface charge distribution, electrochemical stability, and high conductivity. Cobalt nitride (Co₂N) serves as an active electron-pumping center, and the interface with cobalt sites effectively transfers electrons to the graphdiyne layer. The presence of the graphdiyne layer enhances electron transfer toward N₂. An unusual reduction in the d-to-pσ ∗ state transition (p–d coupling) reduces the barrier for initiating the optimal electrochemical nitrogen reduction reaction [95].

Titanium-based materials are promising electrocatalysts due to their stronger affinity for binding to nitrogen (adsorption energy: –2.7 eV). In contrast, their affinity for binding to hydrogen is lower (adsorption energy: –0.9 eV) [96]. In addition, doping TiO₂ creates oxygen vacancies that promote N₂ activation rather than the hydrogen evolution reaction (HER) [97]. Li et al. demonstrated that lattice strain on Ti³⁺ sites is more conducive to N₂ activation than pristine TiO₂, resulting in an NH₃ yield rate of 5.5 μg h^–1 cm^–2 and a faraday efficiency of 26% at –0.6 V vs. RHE [98].

Liu et al. investigated on P-doped Fe-Ni-layered double hydroxides (LDHs) supported on carbon nanofibers for NH₃ production. Their study revealed an ammonia yield rate of 1.7 × 10^–10 mol s^–1 cm^–2 and a faraday efficiency of 23% at –0.5 V vs. RHE [48]. P doping reduces the electron density around metallic sites, promoting empty d–d orbitals.

Palladium- (Pd-) based catalysts facilely generate α-palladium hydride, a key intermediate initiating the NRR process via the hydride transfer pathway [99]. In pursuit of enhanced catalytic activity, metalloid boron (B) is introduced via doping, facilitating weak hydrogen adsorption and inducing rich valence electrons. This strategic introduction results in an intrinsically more catalytically active surface, fostering N reduction while mitigating the HER. Moreover, B doping on Pd stimulates electron transfer between the p orbitals of B and the d orbital of Pd, thereby optimizing nitrogen adsorption on the catalysts [100, 101]. In an effort, Wang et al. successfully synthesized a B-doped nanoporous Pd film on a nickel foam (Ni foam) substrate [102]. This engineered catalyst exhibited a considerable ammonia yield of 29 μg h^–1 mg_cat^–1 and 17.9% FE under neutral conditions at –0.1 V vs. RHE. The self-supported nanoporous architecture, crafted with the assistance of micelles, provides an intricate network of numerous active site transfer channels [102].

Xu et al. have developed nanoporous Pd–H catalysts for NRR under ambient conditions [103]. In this process, the formation of ∗N₂H (dinitrogen hydride) involves rate-limiting steps. The presence of hydrogen in Pd–H catalysts significantly enhances both catalytic activity and selectivity compared to pristine Pd. The hydrogen atoms in Pd–H actively participate in the formation of ammonia (Figure 9). The reaction involves the interaction between adsorbed nitrogen on Pd and hydrogen extracted from the outermost Pd–H lattice. This interaction results in the creation of hydrogen vacancies, which play a crucial role in activating water molecules. Notably, Pd–H (with an energy of 0.65 eV) leads to a lower Gibbs free energy (ΔG) for the formation of ∗N₂H compared to pristine Pd (with an energy of 0.91 eV) [103].

The Zhang group also designed and synthesized palladium hydride nanorods (PdH0.43 NRs) as an electrocatalyst for NRR, demonstrating a 17.5 μg h^–1 mg_cat^–1 ammonia yield rate and an 18.5% FE in a neutral electrolyte at –0.2 V vs. RHE [104]. The use of 1D palladium hydride proves advantageous for efficient electroreduction in ammonia production. One-dimensional materials, characterized by a high aspect ratio, inherent anisotropy, and faster charge/mass transport, contribute to the enhanced performance in electrocatalysis [105, 106].

Strategically, single-atom catalysts (SACs) are prepared by anchoring isolated noble metals (Ru, Pd, and Au) or transition metals (Mo, Cu, Fe, Fe/Zn, and Co) onto a conductive carbon-based solid support to achieve efficient catalytic activity in the NRR. These single atoms stabilize through M-N_x, M-O_x, or M-C_x coordination bonds with the carbon support, allowing precise control over NRR reaction pathways [107, 108]. Additionally, the introduction of alternative dopants such as Fe, Zr, V, and Cu into metal oxides creates oxygen vacancies that serve as active sites, promoting nitrogen adsorption and reducing the energy barrier for NRR [109, 110]. Chen et al. have mimicked the natural nitrogenase enzyme by synthesizing S-coordinated Fe single-atom catalysts on TiO₂, adopting a lattice-confined strategy to enhance efficient electrocatalytic NRR [58]. The Fe₁S_x@TiO₂ catalyst exhibits an NH₃ yield rate of 18.3 μg h^–1 mg_cat^–1 with a faradaic efficiency (FE) of 17.3% at –0.2 V vs. RHE [58].

NRR faces challenges from the competitive HER, whether it involves hydrogen evolution (Volmer and Heyrovsky steps: 2H⁺ + 2e^– → 2H∗ → H₂) or the initial hydrogenation step of NRR (∗NN → ∗NNH) [111, 112]. The selectivity of NRR can be guided by suppressing the adsorption of H⁺ on the catalysts. Chung et al. devised a catalyst composed of carbon nanofibers embedded with Co and Mo, denoted as Co–Mo–CNF [57]. By harnessing the varying adsorption strengths of the catalysts, Co serves as a robust catalyst for water dissociation, providing a local proton source in the vicinity of Mo, where the NRR takes place. In the Co–Mo–CNF catalyst, water dissociation at the Co catalyst sites supplies concentrated protons to the nearby N atoms, which are adsorbed at the Mo catalyst sites, promoting the selective hydrogenation of ∗N–Mo. Consequently, Co–Mo–CNF exhibits an NH₃ formation rate of 72.7 μg h^–1 mg_cat^–1 and achieves a Faradaic efficiency of 34.5% at –0.5 V vs. RHE, compared to only 48 and 10 μg h^–1 mg_cat^–1 for Mo–CNF and Co–CNF, respectively [57]. The Wu group prepared a unique sub-monolayer of MoS_2–x, generating in-plane S vacancies [113]. These S vacancies create compressive strain effects, allowing for the efficient activation of N₂ and adjustment of N intermediates’ affinity, thereby tuning the catalytic properties. Additionally, the strong H adsorption at the 2D surface suppresses the competitive HER process [113]. Zi et al. designed and utilized a single-layer MoS₂ with S vacancies coated on MoO₃ (SV–1 T–MoS₂@MoO₃) [114]. The charge density difference reveals a depletion of electrons in the MoO₃ matrix, promoting the transfer of charge from MoO₃ to MoS₂ in the SV–1 T–MoS₂@MoO₃ catalyst. This facilitates fast reaction kinetics on the surface. Moreover, the charge transfer from N₂ to the bare Mo atom favors the formation of Mo–N bonds, lengthening the N–N bond from 1.10 Å to 1.13 Å due to N₂ activation by the S vacancies. Simultaneously, this hybrid configuration prevents the agglomeration of MoS₂ and enhances the stability of the catalyst. The hybrid catalysts exhibit a superb NH₃ yield rate of 116.1 μg h^–1 mg_cat^–1 and 18.9% FE at –0.2 V vs. RHE [114].

The Meng group encapsulated oxidized CoP particles in carbon nanotubes (O–CoP/CNT) and utilized them as NRR catalysts, demonstrating a NH₃ yield of 39.5 μg h^–1 mg_cat^–1 and a FE of 19.4% at –0.5 V vs. RHE [115]. Oxygen-rich CoP stabilizes N intermediates (NH–NH, NH₂–NH, and NH₂–NH₂) through H bonds, enhancing NRR activity. Additionally, the hydrophobic nature of CNT inhibits proton contact with the catalyst surface, promoting the hydrogenation of N intermediates [115].

The consideration of metal-free catalysts for NRR arises due to the scarcity of transition metals and the extensive hydrogen evolution reaction associated with them. However, it is worth noting that the performance of metal-free catalysts in NRR is often unsatisfactory due to the lack of dedicated active sites [116, 117]. Lin et al. explored the use of defective boron carbon nitride (BCN) for NRR electrocatalysis [118]. The empty sp² orbital of boron (B) can interact with the lone pair of N₂, activating and adsorbing N₂, thereby hindering the binding of H⁺ ions and suppressing the competing hydrogen evolution reaction (HER). BCN, with its unsaturated B and N atoms, serves as dual catalyst sites, forming frustrated Lewis pairs (FLPs) that can adsorb, activate, and selectively catalyze NRR (Figures 10(a) and 10(b)). In this context, the adjacent electron-deficient boron and electron-rich nitrogen transform the single Lewis acid catalyst into FLPs, resulting in an ammonia yield rate of 20.9% and an 18.9% faradaic efficiency (FE) at –0.1 V vs. RHE. FLPs facilitate the adsorption of nitrogen, forming a six-membered intermediate ring, which can break down N₂ through pull-pull effects. As a consequence, this significantly reduces the energy barrier for the rate-limiting steps in the NRR process [118].

Wu et al. consider the earth-abundant and low-cost non-noble-metal tri-manganese tetraoxide (Mn₃O₄) nanocubes as electrocatalysts for NRR at ambient conditions. The catalyst shows the NH₃ yield of 11.6 μg h^–1 mg_cat^–1 and FE of 3% at –0.8 V vs. RHE [119]. Huang et al. utilized 2D Co-doped FePS₃ (Co–FePS₃) nanosheets as an electrocatalyst for NRR [120]. In biological nitrogen fixation nitrogenases, Fe-S (iron protein) plays a critical for electron transfer. Also, the P cluster is an inevitable constituent as its bridging elements for transfers of electrons from Fe-S to the substrate [121–123]. Based on the understanding of the nitrogenase process, 2D FePS₃ is employed for the NRR process (Figures 10(c), 10(d), and 10(e)). Cobalt (Co) incorporation enhances the conductivity as well as catalytic activity at Fe-edge sites. Co-FePS₃ shows NH₃ yield rate of 90.6 μg h^–1 mg_cat^–1 (29 μg h^–1 cm^–2) and FE of 3.38% at –0.4 V vs. RHE [120]. Hao et al. consider isolated Pt on WO₃ nanoplates (Pt SAs/WO₃) for electrochemical fixation of N₂ [124]. Single-atom active sites are believed to suppress the strong HER. Pt-based electrocatalyst shows strong HER activity due to the surface of Pt nanoparticles readily adsorbed H atoms than N₂ at negative potential [125]. Excessive adsorption of H atoms promotes the HER and suppresses the NRR process in Pt nanoparticles for occupying the active sites by H [126, 127]. In Pt nanoparticle, ∗H adsorption (ΔG(∗H) = −0.70 eV) is much spontaneous than N₂ adsorption (ΔG(∗N₂) = −0.17 eV). Here, ∗H adsorption energies ΔG(∗H) on the isolated Pt sites for Pt–3O, and Pt–2O are –0.94 and –0.82 eV, respectively, for Pt SAs/WO₃. In the case of ∗N_2, adsorption energies ΔG(∗N₂) for Pt–3O and Pt–2O are –1.34 and –0.30 eV, respectively, which shows that the N₂ adsorption could be less hindered by the ∗H adsorption if the Pt incorporate as single atom on the catalysts. Also, ∗H adsorption on the WO₃ (ΔG(∗H) = −2.39 eV) is so strong that could not initiate the HER process on the catalyst [124].

Tan et al. designed the electrocatalyst of Au₂₅–Cys–M (M: Mo⁶⁺, Fe³⁺, Co²⁺, Ni²⁺) for the NRR process. Here, Mo⁶⁺, Fe³⁺, Co²⁺, and Ni²⁺ are atomically decorated on Au₂₅ nanoclusters via thiol bridging [128]. Among Mo⁶⁺, Fe³⁺, Co²⁺, and Ni²⁺, Au₂₅–Cys–Mo catalyst shows the highest catalytic efficiency of FE (26.5%) and NH₃ yield (34.5 μg h⁻¹ mg_cat⁻¹) in 0.1 m HCl solution. Formation of Au–S and Mo–S interactions that modulated the electronic structure of Mo sites helps to activate N₂. Also, Mo sites promising for the conversion of adsorbed N₂ molecules as well as the desorption of NH₃ [128].

Cu generally shows weak NRR activities due to weak binding to N-containing intermediates [82]. Forming heterojunction by optimizing charge distribution or oxygen vacancies or improving surface hydrophobicity inhibits the hydrogen evolution or results in electron-rich active sites for increasing adsorption and activation of the NRR process [129, 130]. In the case of CeO₂, flexible oxidation state transition properties of CeO₂ between Ce³⁺ and Ce⁴⁺ show drastic electronic interactions with the catalysts [131, 132]. CeO₂ coupling with electrocatalyst boosts the electrocatalytic performance and induces O vacancies on the CeO₂ promoting it to NRR active substance [133, 134]. Jing et al. developed Cu₂O–CeO₂–C nanorods on Cu mesh by MOF templated for the NRR process [135]. Cu₂O–CeO₂ construction provides the formation of interfacial Cu–[O_x]–Ce structure, where the removal of the O atom in Cu–[O_x]–Ce to form Cu–[V_o]–Ce of oxygen vacant catalyst. Here, Cu–[Vo]–Ce gives the highest free energy of ∗H adsorption (0.52 eV) and results in the lowest HER activity than Cu₂O with oxygen vacancy (0.32 eV) and Cu-doped CeO₂ with 1 oxygen vacancy (0.46 eV). Cu₂O–CeO₂–C provides an NH₃ yield of 6.37 × 10⁻³ μg s⁻¹ cm⁻² and a FE of 18.21% at −0.3 V vs. RHE [135].

Liu et al. developed an n-n heterojunction of Cu–N and B–N dual active sites by in situ growth of a conductive MOF on hexagonal boron nitride [136]. Here, heterojunction can modulate the band gap energy and density of state near the Fermi level which effectively improves the electron capability to reduce intermediates of electrocatalytic NRR. The catalysts show the146 μg h⁻¹ mg_cat⁻¹ NH₃ production and 42.5% FE efficiency due to amounts of oxygen vacancies, porosity, and dual active sites [136].

Perovskite oxides are utilized in electrocatalytic activities due to their high conductivity and the capacity to modify A-sites and B-sites [137]. These modifications induce oxygen vacancies, influencing N₂ adsorption and activation. Both Ru single atoms and Ru cluster–embedded perovskites exhibit efficient electrocatalytic activity, with an NH₃ yield rate of 137.5 μg h⁻¹ mg_cat⁻¹ and a FE of 56.9% [138]. The Ru cluster modifies the electronic structure of the Ru single atom, reducing the energy barrier for the initial hydrogenation (∗NN → ∗NNH) in ammonia generation [138]. Ru single atoms anchored on graphdiyne (GDY) generate H radicals (•H) and consecutively activate nitrogen to produce NNH radicals (•NNH) (Figure 11(a)) [139]. Here, combined dual-active sites are employed, with GDY acting as H adsorption sites and Ru acting as •NNH adsorption sites. The dedicated catalyst (Ru SAs/GDY/G) exhibits a production rate of 56.8 μg h⁻¹ mg_cat⁻¹ (4.7 mg h⁻¹ mg_Ru⁻¹) and a FE of 37.6% at –0.1 V vs. RHE [139].

Hollow hierarchical nanotubes of cobalt vanadium phosphide (CoVP) @NiFeV-LDHs were designed, with NiFeV ternary LDHs growing in situ on the hollow tube of CoVP [140]. The inclusion of phosphorus on the transition metal (TM) enhances N₂ reduction by weakening the N≡N bond and strengthening the TM-N bond. The ion transport path is shortened by the hollow nanotube, providing maximum active exposed sites. This is due to the accessible surface area for water and nitrogen reactants, facilitating the reaction [140]. The Ren group investigated N, B codoping on porous carbon for nitrogen reduction reaction (NRR) (Figure 11(b)) [141]. They observed that the porous structure of carbon facilitates the trapping of N₂ and stabilizes intermediate reactants (N₂H_y). Additionally, B doping promotes the formation of pyridinic N and B–N bonds, both of which effectively enhance NRR activity [141]. Wang et al. similarly designed hollow bimetallic CoFe₂O₄ nanocubes wrapped in a thin carbon (C) layer and modified with O vacancies [142]. The binding energy of the nanocubes in C@CoFe₂O_4–x is higher than that of pure CoFe₂O₄. The strong adsorption capability of N₂ is beneficial for initiating the NRR, attributed to the synergetic effects of O vacancies and the presence of wrapped carbon [142].

Au serves as an excellent electrocatalyst, as N₂ adsorbs on the Au surface, promoting the hydrogenation of N intermediates to generate N₂H_x (x = 1 to 4) species [143, 144]. The Sun group synthesized well-dispersed ultrafine nanoparticles of Au (1.9 nm) on the MOF (Zn (II) constructing a 3D disulfide trimer), where the MOF acts as host material with high porosity and stability (Figure 11(c)) [145]. Au@MOF surfaces were modified with organosilicon to induce hydrophobicity. Here, sulfides serve as carriers for Au nanoparticles, and the hydrophobic organosilicon limits competitive HER [145].

The pristine form of carbon nitride (CN) demonstrates limited catalytic activity for the NRR process, primarily due to a lack of catalytically active sites. However, its intrinsic properties offer an opportunity for straightforward regulation, potentially leading to favorable catalytic characteristics. This is attributed to the material’s inherent stability and a high content of nitrogen-rich sites (N-rich) [146]. The Zhao group investigated the potential of nonmetal phosphorus (P) doping on CN as an NRR catalyst (Figure 11(d)) [147]. The study involved the synthesis and preparation of P-doped CN with a rich nitrogen vacancy (P–NV–CN) catalyst, demonstrating a substantial ammonia yield rate of 28.6 mg h⁻¹ μg_cat⁻¹ and a FE of 22.1% at –0.3 V vs. RHE. The nitrogen vacancy (N vacancy) inhibits the HER of Heyrovsky steps (∗H → H₂) and promotes N₂ adsorption and activation. Nitrogen vacancy effect is attributed to the enriched electron density around the N vacancy, facilitating transfer at the unoccupied 2π ∗ orbits in N₂. The P doping contributes to catalytic water splitting, providing a source of hydrogen (H) for the hydrogenation of nitrogen. The synergistic effects of the P–NV–CN catalyst reduce the energy barrier of the potential determining step of NRR and contribute to the stabilization of reaction intermediates [147].

Arif et al. developed an electrocatalyst comprising CoFeB nanospheres wrapped around the edges of reduced graphene oxide (rGO) nanosheets, forming a CoFeB@rGO heterostructure [148]. The strategic positioning of CoFeB nanospheres along the edges significantly improved catalytic activity, providing a higher surface area, more accessible active sites, and facilitating fast electronic transfer between the rGO nanosheets and CoFeB nanospheres. The ultrathin rGO encapsulating the CoFeB nanospheres enhances the structural stability of the nanospheres, while the interconnected nanosheets contribute to improved conductivity. CoFeB plays a crucial role in lowering the energy threshold for N₂ adsorption and protonation. The synergistic effects of the electrocatalyst result in a FE of 31.6% and an NH₃ yield rate of 35 μg h^–1 mg^–1 at –0.2 V in 0.05 M H₂SO₄ [148].

Mushtaq et al. have developed a hierarchical heterostructure hollow microsphere composed of Mo-doped WO₃@CdS, achieved through the in situ growth of dispersed CdS nanoparticles onto Mo-WO₃ for the purpose of photoelectrochemical NRR to produce NH₃ [149]. The introduction of Mo doping on WO₃ effectively reduces the energy barrier for nitrogen activation and protonation. The resulting photoelectrocatalyst demonstrates a FE of 36.7% and an ammonia yield rate of 38.9 μg h^–1 mg^–1 at –0.3 V vs. RHE in neutral solution under ambient conditions. The hollow structure of the microsphere offers advantages, allowing for multiple scattering of sequentially absorbed light to enhance light harvesting capabilities. The voids within the structure provide abundant surface areas for catalytic reactions. The Mo-W dimer serves as an active center for N₂ adsorption and facilitates the rapid generation of ∗NNH. The incorporation of Mo doping and the presence of O vacancies contribute to a decreased overpotential (1.48 eV) compared to pure WO₃ (1.82 eV). CdS aids in the swift transfer of photogenerated electrons from CdS to Mo-WO₃, thereby accelerating the photoelectrochemical performance. The observed slow photon light scattering effects effectively increase light absorption ability, decrease charge transfer distance, and mitigate the recombination of photogenerated charges. Additionally, plasmonic nanoparticles of MoO₂ and MoO_3–x within the hollow structure attribute localized surface plasmon resonance effects. The synergetic effects improve the NH₃ yield [149].

In another study, Mushtaq et al. also synthesized a hybrid micro/nanostructure of MoSe₂@g-CN. The hierarchical micro/nanoflowers of MoSe₂, integrated with exfoliated g-CN nanosheets, demonstrated a photoelectrocatalyst with a 28.9% FE and a 7.7 μmol h^–1 cm^–2 ammonia yield rate at –0.3 V vs. RHE in a 0.1 M KOH electrolyte [150]. The Mo atoms in MoSe₂ served as the primary catalytic active sites, while g-CN facilitated the photoexcited electrons for the N₂ reduction process. The flower-like micro/nanostructure of MoSe₂ enhanced light harvesting capability, mass transport, and charge separation efficiency. The homogeneous dispersion of g-CN played a crucial role in forming a heterojunction, promoting interfacial charge transfer, and enhancing the efficiency of electron-hole pair separation. This synergy effects contributed to a reduced recombination rate, extending the lifetime of photoinduced electron-hole pairs. Simultaneously, g-CN provided active sites for N-containing groups, such as pyrrolic and pyridine N, supporting the NRR [150].

Single-metal moiety (M–N₄) is utilized for the various catalytic activities. The high electronegativity of the systematic bordering of nitrogen atoms around metal sites could result in the improper adoption of energy for the intermediate species. Zhao groups designed noble catalyst, introduced the sulfur, removed the one nitrogen atom from the M–N₄, and created an asymmetric atomic boundary to obtain an enhanced kinetics effect. Dual metals anchoring onto heteroatom-doped C (Fe/Co–atom microenvironment with N/S atom) electronically engineered microenvironments of active sites efficiently standardize the distribution of local charge density of the catalyst and implicate the synergetic effects and enriched active center on the catalyst [151].

Defects in porous catalysts definitively induce modifications in the catalyst’s active sites. Combining these defects with heteroatom doping can enhance the localization state and bring it closer to the Fermi level. This atomic modulation approach, which involves edge/surface modifications, graphitization, and heteroatom doping, influences the spin/charge status and significantly improves the performance of oxygen reactions [152]. Furthermore, strategies for the precise control of the microenvironments of single-atom catalysts (SACs) have the potential to facilitate various catalytic modulations by controlling the coordination environment and electronic states [153]. Adjusting the coordination environment can standardize the d-band center of the central atoms, which helps to enable different density transfers and optimize the adsorption and desorption properties of various intermediates. Tuning the coordination number is utilized to activate synergistic catalytic properties. Engineering surface defects prevent the aggregation of SACs by increasing charge transfer between the single atom and the defects. Dual metal coordination induces a synergistic interaction and redistributes electron density, and altering the metal charge state can result in superior catalytic activity [154]. Multistep catalytic reaction is limited by the linear scaling relationship of the adsorbate, even though the SACs by heteroatom doping, the addition of p-sites, and lowering coordination numbers were not able to break the linear scaling relationship of multistep catalytic activity. However, a heteronuclear dual-atomic catalyst could be a promising model for different metal sites at the distance limit of the electronic interaction and could offer different interactions to break the linear scaling relationship of multistep catalytic activity [155]. Crystal facet engineering by rational order, effective atomic structure, and synchronization of crystal facet could be an alternative model to improve the electrocatalytic NRR activity. The optimal and exposed facet could improve the adsorption and catalytic activity [156].

Nitric oxide and hydrazine are byproducts of the nitrogen reduction reaction, and high selectivity for ammonia production is essential. Hydrazine production can be anticipated through associative alternating pathways [157]. Dissociative and distal pathways may reduce the formation of hydrazine byproducts. Additionally, it needs to be considered that multiple catalytic steps involve numerous intermediates, each with adsorption energetic relationships, termed scaling relations [92, 158]. Unfavorable scaling relationships among intermediates can make achieving optimal products or selectivity challenging [91]. Countering the effects of unwanted byproducts or achieving higher selectivity requires designing active sites with optimal adsorption energy of the intermediates by changing the binding configurations of energy. The integrity of the homogeneous distribution of catalytic active sites while maintaining the perfect and precise synthesis protocol of catalyst material is paramount to reducing byproducts and facilitating the efficient formation of selective products [157].

In the NRR electrocatalytic process, challenges arise for competitive HER due to the high surface coverage of ∗H on the active atomic sites. Poor selective NRR is a result of the high coverage of adsorbed H on pure metals, hindering the active sites for the NRR. The adsorption Gibbs free energy difference among ∗H, ∗N₂, and ∗NNH serves as the theoretical descriptor for the first steps of the NRR, whether it is N₂ adsorption or NNH formation (Figure 12) [159]. In Figure 12(b), the atomic site catalyst (ASC) mint region shows that ΔGN₂–ΔGH is negative, implying that nitrogen adsorption is favorable, and higher NRR selectivity is expected. The reaction with lower free energy is more selective. In this context, the HER is more selective for Re₁C₃, Mo₁C₂N₁, and Ru₁C₂N₁ than NRR [160]. Long et al. have demonstrated that N2 + H⁺ + e⁻ → ∗NNH is the first step of the NRR process, and the selectivity descriptors include ΔG[N₂ (g) + H⁺ + e⁻⟶∗NNH] − ΔG(H⁺ + e⁻⟶∗H) [161]. It is observed that in the lower right area, where ΔG[N₂ (g) + H⁺ + e−⟶∗NNH]˃ΔG(H⁺ + e⁻⟶∗H), ∗H is preferred, and HER is favorable, while NRR is hindered (Figure 12(a)). On the other hand, in the upper left area of the figure, ∗NNH adsorption dominates, resulting in higher NH₃ selectivity [161]. The NRR volcano is governed by two elementary reaction steps. Volcano plots were constructed using the scaling relation between the binding energies of various intermediates in the NRR process on transition metal oxide (Figure 11(c)) [162, 163]. Electrochemical ammonia formation potential determining steps (PDS) on transition metal oxide are plotted against ΔENNH (binding energy of NNH). The mechanistic descriptors on the left leg provide insight into ammonia formation, while the right leg elucidates the formation of the ∗NNH adsorbate with potential determining steps. These descriptors offer a general understanding of the phenomena underlying the NRR process on solid-state electrodes. Hydrazine is a competitive side reaction in the volcano plots. Weak bindings of ∗NNH and catalyst, along with hydrazine formation, compete with ammonia formation [162].

5.2.1. Strategical Material Design for NORR

Long et al. conducted a study employing density functional theory (DFT) to explore the selection of an efficient electrocatalyst for the nitrogen oxide reduction reaction (NORR) [170]. Their research revealed that copper (Cu) exhibits higher NORR activity than nitrogen reduction reaction (NRR) while maintaining superior selectivity for ammonia over hydrogen production. Experimental results demonstrated that a Cu foam electrode achieved an electrochemical ammonia synthesis rate of 517.1 μmol h^–1 cm^–2 and a faraday efficiency of 93.5% at –0.9 V vs. RHE [170].

Single-atom catalyst was implemented for the NORR mechanism in perspective of the high metal utilization and catalytic efficiency [171]. Niu et al. employed density functional theory (DFT) to systematically screen efficient single-atom catalysts (transition metals, TM) within TM–C₂N for the conversion of NO to NH₃ (Figures 13(a), 13(b), 13(c), 13(d), and 13(e)) [172]. Among the 23 transition metals studied, spanning from Ti to Au, TM–C₂N (specifically Ti, V, and Zr) exhibited promising nitrogen oxide reduction reaction (NORR) activity, with limiting potentials of –0.35 V, –0.29 V, and –0.33 V, respectively. Notably, among Ti, V, and Zr in TM–C₂N, Zr–C₂N emerged as a promising catalyst for NH₃ production, effectively suppressing the formation of byproducts such as N₂O, N₂, and H₂ [172].

Li et al. developed an electrocatalyst for the reduction of NO to NH₃, consisting of a TiO₂ nanoarray supported on a Ti plate (TiO_2-x/TP) [165]. While TiO₂ is known for being a cost-effective, eco-friendly, and highly durable material, its inherent limitations include low active sites and poor electrical conductivity, resulting in reduced catalytic activity [173]. To address these issues, the introduction of oxygen vacancies on TiO_2-x and defect engineering enhances the availability of active sites. Additionally, the formation of nanoarrays serves to maximize catalytic sites, improving NO adsorption and activation (NH₃ yield 1233.2 μg h⁻¹ cm⁻² at −0.7 V and FE 92.5% at −0.4 V in neutral media) [165]. Wang et al. designed and reported that hexagonal-closed-packed Co nanosheets (hcp–Co) show a unique electronic structure and proton shuttle effect that shows efficient NORR catalytic activity of 439.5 μmol h^–1 cm⁻² NH₃ yield and 72.5% FE [168]. Zn-O batteries with hcp–Co were assembled as the cathode, and they performed better than what has been reported before, with a power density of 4.66 mW cm⁻² [168].

MoS₂ is considered a comparatively efficient electrocatalyst for the NORR, selectively converting NO to NH₃ with an FE of 76.6% [174]. Notably, MoS₂ edges exhibit catalytic activity, while the basal plane remains catalytically inactive [175]. Efficient NO adsorption and activation occur at these edge sites. Considering previous discussions, Co is recognized as an efficient element for catalytic NO reduction. Li groups introduced single Co atoms on MoS₂ to induce catalytic activity on the basal plane (Figures 13(f) and 13(g)). The research yielded a robust and highly active catalyst, Co₁/MoS₂, providing an NH3 yield of 217.6 μmol h^–1 cm^–2 and an FE of 87.7% at –0.5 V vs. RHE. The energy barrier for the rate-determining step of NO adsorption on bare MoS₂ is 0.80 eV [176], which significantly reduced to 0.48 eV on the modified Co₁/MoS₂. Additionally, the binding energy of hydrogenation intermediates on Co₁/MoS₂ is considerably lower than on MoS₂. The presence of Co₁–S₃ moieties selectively activates and ultimately improves the catalytic activity of NORR [176].

Bai et al. designed and synthesized a hollow nanoreactor, Cu₂O@CoMn₂O₄ (nanocube, with Cu₂O entrapped in CoMn₂O₄), for NO reduction. In the synthesis process, different etching times were employed to control the formation of various structures [177]. The void confinement effects enhance the reactivity of reactants within the internal spaces of the nanoreactor, improving electron transfer efficiency. As the internal space increases, there is a greater concentration of intermediates within. The Cu site in Cu₂O@CoMn₂O₄ exhibits the lowest energy barrier (0.6 eV) for the formation of ∗NOH, a crucial intermediate in the rate-determining steps of NO reduction. The 3d states of Cu in Cu₂O@CoMn₂O₄ lie below the Fermi level and shift toward the Fermi level, making electrons more available and increasing their activity. The synthesized hollow nanoreactor demonstrates considerable electrochemical NO reduction activity, yielding an NH₃ production rate of 94.1 mmol g⁻¹ h⁻¹ and FE of 75% at −0.8 V vs. RHE. The Tafel slope indicates that the Cu₂O@CoMn₂O₄ nanoreactor with an etching time of 8 minutes (278 mV dec^–1) before calcination at 350°C exhibits more favorable kinetics compared to etching times of 2, 3, and 8 minutes, which result in Tafel slopes of 449, 408, and 399 mV dec^–1, respectively [177].

5.3.1. Strategical Material Design for NITRR

A MXene-like layered structure is employed for electrocatalytic reactions, with its surface metal atoms passivated by various functional groups. An alternative approach involves the technical modification or removal of these surface functional groups to create a clean surface of metal atoms in MBenes (metal/boron materials), taking advantage of their physiochemical properties. This clean surface in MBenes promotes the full exposure of active metal/boron sites, enabling the efficient progression of tandem electrocatalytic reactions [183, 184]. Zhang et al. employ MBenes (FeB₂) for electrocatalytic nitrate reduction to ammonia [185]. In this process, nitrogenous molecules are activated at the B-sites, while the metal Fe-sites facilitate water splitting (Figure 14). The Fe-active sites enhance water dissociation, leading to an increase in the production of ∗H, which subsequently spillovers from Fe to B, thereby enhancing the hydrogenation reaction in nitrate reduction to ammonia [185].

Single-atom catalysts (SACs) are used in nitrate reduction reactions because they offer improved atomic utilization efficiency and trigger specific electronic structure changes (Figure 15(a)). In the context of nitrate reduction, SACs overcome the challenge of N–N coupling (∗N intermediates), which typically occurs between two neighboring catalytic sites [186]. This is achieved through the presence of isolated metal active sites [187, 188]. A proposed strategy to enhance the catalytic efficiency involves a symmetric planar four-ligand structure (M–N₄, where M can be Fe, Co, Cu, etc.) [189, 190]. However, it is important to note that if the four coordinated N atoms share electronic density symmetrically with the metal sites, it can have a detrimental effect. This symmetric distribution of electronic density can impede the adsorption and activation of intermediate species on the active sites, limiting the overall reaction kinetics and performance [191]. To address this issue, a strategic approach involves designing heterojunction materials and modulating the electronic structure of active sites by introducing heteroatoms such as P, O, and S. These heteroatoms play a critical role in efficiently accumulating and diffusing $$ during the catalytic process. Xu et al. reported that Fe atoms, which atomically coordinated with N and P on a hollow carbon substrate (Fe–N/P–C), were used for nitrate reduction to produce NH₃. This modification achieved a Faradaic efficiency (FE) of 93% and a yield rate of 1800 μg h^–1 mg_cat^–1 at �0.8 V vs. RHE [18]. Modifying the presence of phosphorus (P) on the electrocatalyst increases the electron density around the iron (Fe) center. This adjustment has the potential to improve the adsorption and proton transfer processes of nitrate ($$) ions and their intermediates on the catalyst’s surface. Ultimately, these changes contribute to enhanced catalytic performance [18]. Also, dual-atom catalysts (DACs) synergistically activate reactants and stabilize key intermediates in the complex multistep process (Figures 15(b), 15(c), and 15(d)). Zhao et al.’s group investigated homonuclear dual-metal catalysts on N-doped graphene (Cu₂@N₃–6) [192]. These catalysts exhibit an ammonia yield rate of 18.2 mg h^–1 cm^–2 and an FE of 97.4% at –0.8 V vs. RHE. The NITRR process and energy diagram show the following steps: $$ → ∗NO₃ → ∗NO₂ + ∗OH → ∗NO₂ → ∗NO₂H → ∗NO → ∗HNO → ∗H₂NO → ∗O + NH₃ → ∗OH → H₂O.

Cu-based catalysts are useful for catalyzing the reduction of nitrate to ammonia, but their performance is hindered by their vulnerability to poisoning by ∗NO₂ intermediates, primarily due to strong adsorption, resulting in lower reaction activity and stability [193, 194]. Wu et al. used Zn-doped Cu catalysts to control the adsorption strength of N-containing intermediates (∗NO₃, ∗NO₂) at a moderate level, following the Sabatier principle [195]. Bare Cu exhibits top NO adsorption, while Zn doping on Cu increases the electron density on Cu and decreases it on Zn, leading to bridge adsorption of NO due to the higher oxygen (O) affinity to Zn. As a result, this promotes the breaking of N–O bonds and favors the formation of NH₃ [195]. Wang et al. employed CuO nanowire arrays (NWAs) as a cathode material for the electrocatalytic reduction of nitrate (NITRR) [196]. They found that CuO NWAs underwent electrochemical conversion to Cu/Cu₂O NWAs. Cu/Cu₂O served as the active species for the NITRR process. Initially, nitrate adsorbed onto the catalyst to form ∗NO₃. Subsequently, the N–O bond of ∗NO₃ broke, leading to the production of ∗NO₂ and ∗NO. During the hydrogenation steps, ∗NO transformed into ∗NOH, and further hydrogenation resulted in ∗NH₂OH, which eventually transformed into ∗NH₃. In the final step, ∗NH₃ is desorbed from the catalyst. The formation of ∗NOH from ∗NO was identified as the rate-limiting step, characterized by a higher energy barrier in CuO NWAs. However, in Cu/Cu₂O NWAs, this energy barrier was significantly reduced. Additionally, the electron density of Cu in Cu/Cu₂O NWAs was lower compared to pure Cu. Consequently, Cu/Cu₂O NWAs suppressed the hydrogen evolution reaction (HER), as the energy barrier for H₂ formation in Cu/Cu₂O NWAs (0.33 eV) was much higher than in CuO NWAs (0.12 eV) [197]. As a result, a high nitrate conversion rate (97%), ammonia yield rate (0.24 mmol h^–1 cm^–2), faradic efficiency (95.8%), and selectivity (81.2%) were achieved at 0.85 V vs. RHE [196]. Fang and Wang proposed the in situ formation of a new phase with atom defects, which could modify surface properties during electroreduction and potentially enhance the efficiency of the catalytic nitrate-to-ammonia process [186]. Their investigation unveiled that CuO can undergo in situ transformation into oxide-derived Cu, characterized by infused stacking faults and tensile strain. This transformation promotes increased Cu activity and enhances nitrate adsorption. The in situ generation of stacking faults proves effective for catalytic activation, providing insights applicable to various catalysts [186].

It is worth noting that while Cu-based catalysts demonstrate a high FE of over 90%, they operate at very negative potentials ranging from –0.4 to –0.7 V vs. RHE, which is energetically inefficient [196]. This negative potential is necessary to optimize the weak adsorption of hydrogen (H) on the Cu surface, ensuring sufficient coverage of ∗H to achieve a significant hydrogenation rate. However, it is expected that Cu-based catalysts could exhibit higher hydrogenation ability at more positive applied potentials [198, 199]. To address this challenge, Liu et al. explored the use of rhodium (Rh) clusters or single atoms on Cu nanowires [20]. Rh has excellent adsorption-desorption properties for hydrogen (H) and can compensate for Cu’s limited hydrogenation ability, thereby improving NITRR. As a result, the combination of Rh with Cu nanowires achieved an FE of 93% at –0.2 V vs. RHE and an ammonia yield rate of 1.27 mmol h^–1 cm^–2 at –0.4 V vs. RHE, outperforming pure Cu (100) catalysts which achieved only 0.65 mmol h^–1 cm^–2 at –0.15 V vs. RHE [20, 200]. The Zhang group dispersed small Au on Cu (111) nanosheets and introduced copper vacancies (V_Cu) [201]. Here, V_cu and single Au atoms work synergistically, where electron transfer from Cu to Au enhances the activation of H₂O to ∗H and promotes nitrate hydrogenation kinetics.

The intrinsic catalytic activity and selectivity are enhanced through the modification or tuning of textural properties, considering size, shape, and specific morphological structure [202, 203]. Introducing a one-dimensional (1D) nano-wire-based structure proves beneficial for improving surface chemistry and electron transfer while eliminating issues such as agglomeration, dissolution, and Ostwald ripening of the catalyst [204, 205]. Furthermore, precise engineering of the metal–oxide interface through partial oxidation contributes to heightened catalytic activity by facilitating electronic interactions at the interface of metal and metal oxide [206–208]. Building on this assumption, the Ren group devised a Cu₂₊₁O concave–convex layer on Cu nanowires [209]. The interior Cu components efficiently transmit electrons along the nanowire structure, and the concave–convex Cu₂₊₁O layers offer abundant catalytic active surfaces. Electronic interactions between the metal and metal oxide in the interface tune the Cu d-band and modulate the adsorption energies of intermediates. Utilizing 1D nanowires helps prevent agglomeration, dissolution, and Ostwald ripening of the catalyst. Ultimately, the Cu/Cu₂₊₁O catalyst achieved notable results, including a 78.5% nitrate conversion rate, 76% NH₃-N selectivity, 87% FE, and 576.5 μg h^–1 mg_cat^–1 of NH₃ yield rate, respectively, at –1.2 V vs. SCE [209].

Groundwater and surface water are currently experiencing significant issues due to the increasing presence of low concentrations of nitrate, which can lead to acid rain and photochemical smog, as well as health risks such as methemoglobinemia and cancer [210]. The Environmental Protection Agency of the United States has set a limit for nitrate levels in public water systems, requiring them to be kept below 10 mg L^–1 (WHO recommendation 50 mg L^–1) [211]. The key concern lies in the necessity of removing these low-concentration nitrate pollutants from the water through electroreduction, thereby transforming them into value-added products. Sun et al. modulated and stacked the CuCl (111) and rutile TiO₂ (110) and induced a built-in electric field by transferring an electron from the TiO₂ to CuCl [211]. The presence of a built-in electric field in the system effectively leads to the accumulation of a higher concentration of nitrate ions in proximity to the electrocatalyst’s surface. This, in turn, streamlines and enhances the process of removing low-concentration nitrates from the system, ultimately enabling the production of ammonia as a valuable outcome. In this context, the crucial reaction step, ∗NO → ∗NOH, exhibits a lower free energy barrier (ΔG) of 0.18 eV when the system includes a built-in electric field with CuCl, as opposed to pure CuCl (0.42 eV) and TiO₂ (0.78 eV) [211].

The large-scale production of catalysts based on single-atom catalysts or modified metal molecular solid catalysts presents a significant challenge [212, 213]. Crawford et al. introduced an alternative electrocatalyst based on Galinstan, a eutectic alloy comprising 68.5% Ga, 21.5% In, and 10% Sn by weight [214]. This liquid metal-based catalyst is designed for the reduction of nitrate to ammonia and has the potential to enhance the selectivity of converting $$ to NH₃, primarily due to its poor hydrogen evolution reaction (HER) properties. Galinstan immobilized on the copper substrate, with In₃Sn serving as the active site for the reaction. This active site demonstrates selectivity for NH₃ by effectively suppressing the competing hydrogen evolution reaction. The free energy for each reaction intermediate, such as ∗NO₃, ∗NO₂, ∗NO, ∗N, ∗NH, ∗NH₂, and ∗NH₃, exhibits a downhill trend at each step of the reaction, from $$ (liquid) reduction to NH₃ (gas) on In₃Sn, with free energy decreasing from 0 to –5.56 eV [214, 215].

Wang et al. demonstrated and confirmed the crystallinity-dependent activity of the catalyst in the NITRR process. They synthesized RuO₂ catalysts with varying degrees of crystallinity, including amorphous, low crystalline, and high crystalline structures, and compared their performance [216]. Their results indicate that the FE of the NITRR process for amorphous, low-crystalline, and high-crystalline RuO₂ catalysts are 97.4%, 55.27%, and 7%, respectively. The disordered atomic arrangement in RuO₂ with oxygen vacancies facilitates the formation of ∗NH₃ intermediates and enhances hydrogen affinity, leading to high selectivity and FE. The presence of abundant oxygen vacancies modulates the d-band center and hydrogen affinity of amorphous RuO₂, consequently reducing the energy barrier for the formation of ∗NH₂ → ∗NH₃ to a mere 0.08 eV [216]. Du et al. also fabricated pseudobrookite (Fe₂TiO₅) with abundant oxygen vacancies for NITRR, achieving an ammonia yield of 0.73 mmol h^–1 mg_cat^–1 at –1.0 V vs. RHE. In this configuration, Fe atoms serve as active sites for NO₃ adsorption, and the oxygen vacancies shift the d-band center to a higher level, enhancing catalytic activity [217].

Wang et al. developed a Cu–Ni dual-single-atom catalyst in N-doped carbon (Cu–Ni–NC) to the reduction of nitrate to ammonia [218]. Coupling of ∗N–∗N occurs at neighboring Cu sites of Cu nanoparticles generated byproducts of N₂ and N₂O [219]. Cu isolated as a Cu-based single-atom catalyst could inhibit the ∗N–∗N coupling [220]. Ni serves as a strong binding site for H∗ and exhibits strong adsorption for intermediates (∗NO₂, ∗NH₂), which aid in the hydrogenation of neighboring Cu intermediates [215, 221]. Combined dual-active sites (Cu–Ni) synergistically maximize the catalytic activity and show 97.2% FE of NH₃ formation [218]. Also, the Pd–Cu hollow (PdCu–H) catalyst developed to confine the intermediates in a hollow cavity thus promotes NH₃ electrosynthesis by the longer retention time of intermediates which could suppress the nitrate ($$) to N₂ [222]. The PdCu–H catalyst shows the 0.55 mmol h^–1 mg_cat^–1 of NH₃ yield and 87.3% FE at –0.3 V vs. RHE. Impregnation of Pd nanorod arrays onto the surface of nickel foam (Pd–NF) results in pine needle-like Pd nanorods, which are employed for nitrate reduction to ammonia [223]. The Pd reactive sites facilitate the adsorption of ∗H and the hydrogenation of nitrate and reactive intermediates, resulting in an NH₃ yield rate of 1.52 mmol h^–1 cm^–2 and a FE of 78% at –1.4 V vs. RHE [223]. The Pd-doped Co₃O₄ nanoarray on a titanium mesh (Pd–Co₃O₄/TM) catalyst is designed for nitrate reduction [224]. Pd–Co₃O₄/TM exhibits highly selective nitrate reduction to ammonia at a rate of 745.6 μmol h^–1 cm^–2 and a FE of 98.7% at –0.3 V vs. RHE. The Pd doping on Co₃O₄ enhances the adsorption properties (–1.6 eV), surpassing Co₃O₄ (–1.03 eV), and adjusts the free energies of intermediates, thereby facilitating reaction kinetics [224]. Pd doping also influences the d-band center location to the Fermi level, regulating the electronic structure and adsorption properties.

Some investigations have reported that the metal/metal oxide interface can effectively suppress the HER during the electrocatalytic reduction of CO₂ [225, 226]. This suggests a possible assumption that the metal/metal oxide interface provides efficient FE for nitrate reduction to ammonia production by inhibiting the HER. Based on this assumption, Cu/MnO_x hybrids were utilized for NIRR at room temperature, demonstrating an ammonia production rate of 5.5 mg h^–1 mg_cat^–1 and an FE of 98.2% at –0.6 V vs. RHE [227].

Ordered intermetallic single-atom alloys (ISAAs) feature a dedicated structure with the highest density of isolated atoms in proximity to the host metals [228]. In a Cu/CuAu core/shell nanocrystal, there is a high density of Au single atoms within the Cu matrix. This configuration exhibits a unique ligand effect, demonstrating efficient nitrate reduction (NITRR) to ammonia through optimized chemisorption of intermediates (∗NO₃ and ∗N) [229, 230]. In ISAAs, reducing the dimension of a metal single atom and hosting it in an atomically thin layer provides a high surface-to-volume ratio, resulting in an ISAA metallene with the highest density of single-atom sites (Figure 16(a)). Thus, the atomic utilization efficiency increases by controlling the nanostructure of the metal host in ISSAs, maximizing the exposure of single atoms [231–233]. Xie et al. reported and developed an ISAA In−Pd bimetallene, where Pd single atoms are isolated by surrounding In atoms [234]. The In−Pd bimetallene exhibits an FE of 87.2%, a yield rate of 28.06 mg h⁻¹ mg_Pd⁻¹, and electrocatalytic stability over 100 hours [234].

A hydrogel electrode based on a poly(N, N-dimethylacrylamide) network chemically crosslinked with carbazole and thiophene and with a purpurin–coordinate Fe catalyst anchored on the hydrogel [235]. The Fe-modified hydrogel, utilized for reducing nitrate to ammonia, demonstrates efficient selectivity with a high production rate at pH 4.5. The hydrophobicity of polycarbazole on the hydrogel increases under negative bias due to the electric field-induced detachment of the hydrophilic anion. As a consequence, this spurs the hydrogen evolution reaction (HER) due to the internal hydrophobic microenvironment. Additionally, the external hydrophilic hydrogel provides a high density of ions, aiding the mass transfer process. The Fe hydrogel catalyst demonstrates consistent and stable selective NH₃ production for over 100 hours [235].

Perovskite oxides (ABO₃, where A represents alkaline-earth or rare earth metals and B represents transition metals) are typically prepared using sol–gel or ball milling methods, offering high flexibility for incorporating bimetallic perovskites at the B-sites [236]. The tailoring of octahedral sites allows control over B–O hybridization, inducing oxygen vacancies to modulate catalytic activity [237]. Substituting B-sites influences the surface potential and work function, facilitating electron transmission from the catalyst to the reactant. Perovskite-based materials demonstrate efficient catalytic activity by exposing active sites [238]. The use of submicron or 1D perovskite structures prevents agglomeration, promoting efficient reactant diffusion and electron transport during catalytic activity [239]. Consequently, this design maximizes exposure to intermediates for the adsorption and activation processes. Chu et al. reported that LaFeCuO sub-microfibers, with strong Fe–O hybridization, exhibit a 349 μg h⁻¹ mg_cat⁻¹ ammonia yield rate and 48% FE compared to LaFO [15]. The findings indicate that more oxygen vacancies result in a more positive surface potential and a higher ammonia yield rate (Figures 16(b)–16(d)). The positive surface of LFCuO induces nitrate and proton-electron coupling (rate-determining step: ∗NO₃ + H⁺ + e⁻ → ∗HNO₃), decreases the energy barrier (LaFCuO: 1.84 eV, LFO: 2.07 eV), and suppresses the HER [15].

Metal–organic frameworks (MOFs) are considered for eNITRR (electrochemical nitrate reduction reaction) due to their abundant active metal sites, porous structure, and the opportunity to tune electronic properties [240, 241]. However, their low electrical conductivity puts them at a disadvantage in terms of catalytic activity. It is auspicious that under electrochemical catalytic conditions, MOFs undergo structural reconstruction or the formation of multiphase species, which can boost catalytic activity [242, 243]. Yang’s group has shown that a Co-based MOF (Co–TPA) transforms into high-valent Co-active sites (CoOOH) to become an electrochemical active catalyst (Co–TPA–E) under electrochemical conditions [244]. The introduction of Cu to the Co–TPA tailors the electronic cloud distribution, enhancing nitrate adsorption and ammonia desorption on the Co sites. Cobalt is also considered an effective doping agent due to its abundant electronic state. Zhang’s group engineered Co doping on defect-engineered (oxygen vacancy) TiO₂ nanotubes, which efficiently facilitate electron transfer to nitrate and exhibit high selectivity for ammonia production [245]. In this context, codoping suppresses the desorption of reactant intermediates (∗NO/∗NO₂), while defect engineering increases electrical conductivity and charge density, ultimately boosting ammonia production [245]. The Xiaohui group drew inspiration from the biological conversion of nitrate to ammonia through a tandem process, employing a sequential active-site-switching (SASS) mechanism [246]. In this mechanism, two different intermediate species alternate between active sites. Cu–Fe heterophase interface nanoparticles, anchored on TiO₂, facilitate the nitrate adsorption on in-plane Fe sites, which then switch toward the Fe–Cu interface for catalysis into ∗NH₃. Subsequently, the reaction switches to the in-plane Cu phase, leading to desorption [246]. In situ ATR–SERIES (attenuated total refection Fourier transform infrared spectroscopy) technology is employed to identify key reaction intermediates on NITRR. The electrochemical reduction of nitrate to ammonia is confirmed by the stretching peaks at 1494 cm^–1 (N–H stretching/bending mode) and the broadening peak at 3460 cm^–1 (Figure 17(a)). Intermediate peaks of ∗NO₃, ∗NO₂, and electrolyte ∗SO₄ species on the electrode are identified at 948, 1241, and 1110 cm^–1, respectively. The strongest peak at 1494 cm^–1 (∗NH₃) on Fe, compared to Cu, indicates that the Cu phase is more favorable for the desorption of ∗NH₃ (Figure 17(b)). It is demonstrated that $$ in the electrolyte adsorbs on the in-plane Fe phase, and ∗NO₃ species intelligently switch toward the Cu/Fe–TiO₂ heterophase interface, where they undergo reduction to ∗NH₃ through consecutive protonation and hydrogenation processes (Figure 17(c)). In the final steps, ∗NH₃ switches toward the Cu phase, and facile desorption occurs.

Pd-incorporated Cu-based MOF (CuPd–MOF) precatalysts were employed, leading to an in situ reconstitution into a multiphase heterostructure of Cu/Pd/CuO_x at the electrocatalytic operating conditions [242]. The interface among Cu, Pd, and CuO_x generates uneven spatial charge distribution. This uneven spatial charge distribution induces interface polarization, causing the transfer of electrons from Pd to Cu, thereby increasing the adsorption of nitrate on electron-deficient Pd sites. Furthermore, it promotes the reduction on the electron-rich Cu sites [242].

Co-doped Fe@Fe₂O₃ was employed for NITRR, where the Co dopant shifted the Fe d-band center, altering the adsorption energy of intermediates and the free energies of the PDS [247]. The adsorption energy of ∗NO₃^– in Co-Fe@Fe₂O₃ is –2.60 eV, stronger than Co (–2.43 eV) and weaker than Fe@Fe₂O₃ (–2.61 eV). The adsorption strength follows the order Fe@Fe₂O₃ > Co-Fe@Fe₂O₃ > Co, indicating that Co-Fe@Fe₂O₃ possesses moderate strength, favorable for the adsorption and desorption of intermediates, facilitating efficient NITRR. From the Tafel slope, the Co-Fe@Fe₂O₃ catalyst (70.7 mV dec^–1) demonstrates faster NITRR kinetics than Fe@Fe₂O₃ (87.7 mV dec^–1). The electrocatalyst exhibits a nitrate removal capacity of 100.8 mg N g_cat^–1 h^–1, an ammonium selectivity of 99%, an NH₃ yield rate of 1505.9 μg h^–1 cm^–2, and an FE of 85.2% [247]. In another study, Deng et al. synthesized metallic Co nanoarrays (Co–NAs) by reducing Co(OH)₂ for NITRR [248]. The intrinsic activity of Co⁰ is evident due to the intimate contact between active species and the conductive substrate. Co–NAs exhibit a noteworthy ammonia production rate of 104 mmol h^–1 cm^–2 and a current density of –2.2 A cm^–2 at 0.24 V RHE under alkaline conditions. The Tafel slope indicates that Co–NAs facilitate faster kinetics with a slope of 24 mV dec^–1 compared to Co (OH)₂–NAs, which exhibit a Tafel slope of 197 mV dec^–1. Notably, the metallic Co in Co–NAs provides optimized intermediate adsorption compared to pristine Co (OH)₂ [248].

Phthalocyanine- (Pc-) based electrocatalysts, characterized by a planar conjugated array in a 2D structure, hold promise for the electrochemical reduction of CO₂ and N₂, with examples such as CoPc, NiPc, and CuPc. The Rajmohan group employed copper phthalocyanine (CuPc) supported on carbon nanotubes (CNT) for nitrate reduction, achieving an impressive 76% nitrate removal efficiency [249]. Simultaneously, the Li group homogeneously doped nickel phthalocyanine into a carbon nanotube sponge fiber, resulting in outstanding performance with 97.6% nitrate removal, 88.4% ammonia selectivity, and 86.8% FE at a nitrate concentration of 50 mg-N L^–1 at –1.2 vs. Ag/AgCl [250]. The Ni–N₄ active sites play a crucial role in suppressing the HER, while the reduction of dinitrogen enhances ammonia selectivity and FE. The impressive capacitance (11.7 mF cm^–2) and electrochemical durability of NiPc–CNT demonstrate improved nitrate reduction performance.

In another study, the Ghorai group utilized 2D graphene sheets with 1D Mn (II) Pc, producing a pyrrolic-N-coordinated electron-deficient Mn center [251]. This Mn center interacts to generate vital intermediates of the NITRR process. The MnPc/RGO electrocatalyst exhibits an ammonia yield rate of 20316 μg h⁻¹ mg_cat⁻¹, an FE of 98.3%, and over 2 days of electrocatalytic activity [251]. Similarly, in another study, the Ghorai group designed hollow materials of FePc rectangular nanotubes for nitrate reduction, achieving 100% FE and a 35067 μg h⁻¹ mg_cat⁻¹ ammonia yield rate at –1.5 V vs. RHE [252]. The main active sites for capturing nitrate molecules for NITRR are the Fe centers surrounding the pyridinic N, compared to the HER. Moreover, rectangular nanotubes contain more exposed active sites at the inner and outer surfaces, and the binding strength facilitates selective electrochemical NH₃ synthesis. The smaller 0.2 nm hydrodynamic radius nitrate ion can easily enter into the 120 nm width tube, and nitrate residence in the tube is quite large due to the 30–50 μM longer length of the tube. Additionally, FePc is more selective for NITRR due to the stronger exothermic adsorption behavior of NO₃ than H [252].