Electrocatalytic conversion of nitrate waste into ammonia: a review

Abstract

The electrocatalytic reduction of nitrate waste into ammonia allows both the removal of nitrate contaminants and an alternative production of ammonia compared to the classical Haber–Bosch industrial process. Ammonia is useful in agriculture for manufacturing fertilizers, and as a reagent in pharmaceuticals, metallurgy, explosives, and the textile industry; ammonia is also an energy carrier in the automobile industry for next-generation fuel cells. Here we review the nitrate-to-ammonia conversion by electrocatalysis of industrial and agricultural waste, with focus on catalysts, reaction intermediates, side reactions, and reaction conditions. Electron transfer is facilitated by electrocatalysts with transition metals having occupied d-orbitals with similar energy levels to that of the nitrate lowest unoccupied molecular orbital. Green electro-conversion using carbon-based materials is also discussed. Results show nitrate conversion from 53 to 99.8% and ammonia selectivity from 70 to 97.4%.

Introduction

Nitrogen is the most abundant element in the atmosphere. Maintenance of the nitrogen cycle is as pivotal as that of the carbon cycle, as one of the biogeochemical processes that integrate the chemistry of earth and life. The ability of nitrogen to exhibit varying oxidation states ranging from + 3 to − 5 accounts for various forms, such as inert nitrogen gas, ammonia, nitrogen oxides such as NO₂ and N₂O, nitrate compounds, and ammonium compounds (Lu et al. 2021; Min et al. 2021). Among the nitrogen compounds, ammonia is one of the most important compounds. Ammonia is extensively used in manufacturing fertilizers and as a reagent in pharmaceuticals, metallurgy, refrigeration, explosives, and the textile industry, among others (Lu et al. 2021; Xiang et al. 2012). Ammonia has great potential to serve as the next-generation carbon-free energy carrier because of energy density of 22.5 MJ kg⁻¹, which is comparable with that of nonrenewable energy resources (Liu et al. 2021b; Valera-Medina et al. 2018). Compared with H₂, ammonia has a lower cost per unit of stored energy, a higher volumetric energy density of 7.1–2.9 MJ L⁻¹, and a more facile production and transportation process (Valera-Medina et al. 2018). Additionally, ammonia, as a hydrogen derivative, can amend the prospective complex aspects of storing and transporting H₂ energy or can serve as an alternative energy carrier to H₂ given high gravimetric density (Fig. 1).

Fig. 1

Reproduced from reference (Hasan et al. 2021) with permission from MDPI

Density of H₂ in H₂-carriers (packing efficiency in solids: 50%). As noticed, ammonia has the high volumetric and gravimetric densities which provides an added advantage for ammonia to be used as alternative storage of sustainable energy since H₂ facing issues on the storage and distribution. By transforming nitrate as targeted waste into value-added product of ammonia as efficient energy carrier that can be liquefied under controlled pressure, renewable energy can be delighted from positions where sustainable energy is excessive or cheap.

The versatile utilization of ammonia in various fields upscaled the global production of ammonia from 131 million metric tons in 2010 to approximately 150 million metric tons by 2021. In this context, the sustainable production of ammonia from the electrochemical reduction of nitrate can be recognized as a waste-to-wealth concept (Wang et al. 2021d). If the electrical energy required for electrochemical reduction is derived from renewable resources, this conversion process can restore the nitrogen cycle in an eco-friendly and economical manner (Min et al. 2021). The rational design and optimization of electrocatalysts with maximum efficiency and selectivity for ammonia over ${\text{N}}_{2}$ are still under dynamic research. As discussed below, fabricating efficient electrocatalysts for electrochemical reduction of nitrate requires a fundamental cognizance of the reaction mechanism.

Ammonia production techniques and applications

Industrial ammonia synthesis is conducted using the Haber–Bosch method (Lu et al. 2021; Shipman and Symes 2017), which involves the reaction between H₂ and N₂ at high pressure and temperature. The extreme reaction conditions necessitate high energy consumption, which is derived from fossil fuel combustion, concomitantly releasing carbon dioxide into the atmosphere (Li et al. 2021a). Statistics show that the Haber–Bosch synthesis of ammonia accounts for 11% of global carbon dioxide emissions (Hasan et al. 2021).

The anthropogenic emission of nitrate from industrial wastewater effluents can seriously pollute water resources, leading to algal blooms and anoxic conditions for aquatic flora and fauna (Morin-Crini et al. 2022; Wang et al. 2020a; Xu et al. 2019). Additionally, the accumulation of nitrates in the air and aquatic and terrestrial environments is a serious threat to human health, the balance of the ecosystem, and a quality-rich atmosphere (Liu et al. 2021a). Furthermore, the contamination of groundwater primarily with a high concentration of nitrates (NO₃⁻), as well as the ingestion of these nitrates through drinking water, can lead to methemoglobinemia, increased heart rate, and nausea in humans and an increased possibility of blue baby syndrome (Barrera and Bala Chandran 2021; Prashantha Kumar et al. 2018; Velusamy et al. 2021). The dissolution of NO₃⁻ in water droplets in the atmosphere and the subsequent emission of nitrogen oxides can cause acid rain. The increasing rates of ammonia production, overfertilization, eutrophication, bioaccumulation of nitrogenous pollutants in the food chain, and nitrate pollutant emission into the biosphere compared with the natural balance maintained by the microorganisms in the nitrogen cycle have induced the deterioration of the nitrogen cycle.

Conventional biological denitrification as a remedy is ineffective, as requires energy inputs up to 45 MJ kgN⁻¹ (Barrera and Bala Chandran 2021) and results in sludge with a high concentration of pathogenic bacteria. Physical wastewater treatment processes, such as reverse osmosis, ion exchange, and electrodialysis, are also not feasible, as they produce secondary brine effluents, which should be further subjected to other treatment methods (Barrera and Bala Chandran 2021; Madhura et al. 2019; Martínez et al. 2017; Mudhoo et al. 2020). There is an increasing demand to develop wastewater treatment technologies combined with renewable energy for the constructive conversion of nitrogen contaminants to value-added products, such as ammonia (Wang et al. 2021i). Another imminent method is when chemical energy is driven by thermal, photoelectrochemical, or electric energy. Electrochemical reduction is a promising remedial approach to decreasing nitrogen-containing pollutants (Wang et al. 2021c). This method involves reducing nitrate-containing wastewater effluents to harmless dinitrogen gas or value-added ammonia (Fig. 2) (Wang et al. 2021d). The electrocatalytic conversion of nitrate-containing wastewater effluents to ammonia can be conducted under ambient temperature and pressure and can serve as an alternative remedial scientific approach to the environmentally unfriendly Haber–Bosch synthesis of ammonia (Lu et al. 2021).

Fig. 2

The concept of waste-to-value added product: Electrocatalytic removal of nitrate as targeted pollutant from the industrial, agricultural waste contaminated groundwater and subsequently, selective production of ammonia as value-added product. The electrochemical cell mentioned with typical two-electrode system in a single compartment with anode and cathode, respectively, since the supporting electrolyte is same medium and pH. In the case of three-electrode system, we termed as counter, reference, and working electrode, respectively. Based on the potential window, the employed working electrode acts as anode or cathode, wherein electrochemical nitrate reduction reaction is mostly taking place on the negative potential window, so the working electrode is considered as cathode. However, we can also perform under H-cell setup where the supporting electrolyte is in different conditions

Experimental aspects of electrocatalytic reduction of nitrates

The electrolysis of nitrates can be conducted through potentiostatic or galvanostatic methods. The potentiostatic method involves providing a constant fixed electric potential from an external source, whereas the galvanostatic method involves maintaining a constant current through the electrolyte. The potentiostatic method is preferred to the galvanostatic method because which can control the selectivity of nitrate reduction at a given potential, causing only the thermodynamically favorable redox reaction to occur. The reaction occurs in a three-electrode electrochemical cell comprising working, reference, and counter electrodes (Garcia-Segura et al. 2018). The working electrode acts as the cathode, providing a platform for the exchange of electrons required for electrochemical nitrate reduction. The counter electrode helps to complete the circuit, and the reference electrode is used as a reference for the measurement and regulation of the working electrode’s potential. All these electrodes are immersed in a conducting electrolyte solution. Working electrodes are coated with electrocatalysts that facilitate the redox reaction through a direct charge transfer.

All the measured potentials are referred against the standard hydrogen potential. Usually, electrolysis occurs when a potential greater than the theoretical potential, known as the overpotential, is applied. Therefore, low overpotential results in low activation energy and great electrocatalytic activity. Studies on the rational design and fabrication of electrocatalysts with low costs and maximum efficiency in the electroreduction of nitrate to ammonia are of great interest. Previous studies have shown that the main factors to be considered to have maximum efficiency are (1) the formation of favorable reaction intermediates for the selective production of ammonia over other side reactions, (2) the dependence of the reactant concentration and the pH of the reaction medium, and (3) the selection of highly stable electrocatalysts that interact limitedly with the reaction intermediates, thus maximizing the electrocatalytic reduction capability (Wang et al. 2021d; Zhang et al. 2021).

Mechanistic aspects of electrocatalytic nitrate reduction

The mechanism of nitrate electroreduction involves complex multielectron reactions involving various stable nitrogen intermediates and products, such as ammonia, nitrite, hydrazine, hydroxylamine, nitric oxide, and nitrous oxide, whose oxidation states range from − 3 to + 5 (Wang et al. 2021d; Zhang et al. 2021). The involvement of various reaction intermediates complicates understanding the reaction mechanism operating at a given electrode surface. However, many factors and reaction conditions can affect selectivity and preferential pathways. High selectivity can be achieved when the elementary reactions and formation of various by-products are correctly understood.

Among the various reaction products, the most thermodynamically stable are dinitrogen and ammonia or ammonium. The conversion of nitrate to dinitrogen is a form of restoration of the disrupted nitrogen cycle, whereas the conversion of nitrate to ammonia at ambient reaction conditions introduces the concept of waste-to-wealth. Although the main focus of nitrate conversion is N₂ and NH₃ as stable products, as illustrated in Reactions (1) and (2), various factors, such as electrodes or crystal planes, can modify the final products (Zhang et al. 2021).

$${\hbox{NO}}_{{3}}^{ - } {\hbox{ + 9 H}}^{ + } {\hbox{ + 8e}}^{ - } { } \rightleftarrows {\hbox{ NH}}_{{3}} {\hbox{ + 3 H}}_{{2}} {\hbox{O}}$$

(1)

E° = −0.12 V versus Standard hydrogen electrode

$${\hbox{2 NO}}_{{3}}^{ - } {\hbox{ + 12 H}}^{ + } {\hbox{ + 10 e}}^{ - } { } \rightleftarrows {\hbox{ N}}_{{2}} {\hbox{ + 6 H}}_{{2}} {\hbox{O}}$$

(2)

E° = 1.17 V versus Standard hydrogen electrode.

The reaction mechanism is classified into direct electrocatalytic and indirect autocatalytic mechanisms (Min et al. 2021; Wang et al. 2021d). In the indirect autocatalytic mechanism, although the overall reaction is the conversion of nitrate to nitrous acid, there is no direct involvement of nitrate ions in the electron transfer reduction process. Rather, ${\hbox{NO}}^{+}$ or ${\hbox{NO}}_{2}$^· are the electroactive species, and they further initiate an autocatalytic mechanism. This mechanism mainly occurs in an acidic medium when the concentration of nitrate ions is greater than 1.0 M. Based on the two predominant electroactive species ${\hbox{NO}}_{2}$, ${{\hbox{NO}}_{2}}^{-}\text{,}$ and ${\text{NO}}^{+}$, the indirect mechanism involves two pathways, namely, the Vetter and Schmid pathways (Wang et al. 2021d). Both pathways involve the production of nitrous acid using ${\text{N}}_{2}{\text{O}}_{4}$ through three elementary steps: (a) electron transfer, (b) intermediate step, and (c) production of nitrous acid.

• 1. Vetter pathway

  • (a) Electron transfer

    $${\text{NO}}_{2} + e ^{ - } \to {\text{NO}}_{2} \cdot$$

    (3)

  • (b) Intermediate step

    $${\text{NO}}_{2}\!\cdot + \,\, {\text{H}}^{ + } \rightleftharpoons {\text{HNO}}_{2}$$

    (4)

  • (c) Production of nitrous acid

    $${\text{HNO}}_{{2}} {\text{ + HNO}}_{{3}} {\,} \to {\text{ N}}_{{2}} {\text{O}}_{{4}} {\text{ + H}}_{{2}} {\text{O}}$$

    (5)
    $${\text{N}}_{2} {\text{O}}_{4} + 2 {\text{NO }} + 2 {\text{H}}_{2} {\text{O}} \, { \leftrightharpoons } \, 4 {\text{HNO}}_{2}$$

    (6)

• 2. Schmid pathway

  • (a) Electron transfer

    $${\text{NO}}^{ + } + \, e^{ - } { } \to {\text{ NO}}$$

    (7)

  • (b) Intermediate step

    $${\text{HNO}}_{{2}} {\text{ + H}}^{ + } { } \to {\text{ NO}}^{ + } {\text{ + H}}_{{2}} {\text{O }}$$

    (8)

  • (c) Production of nitrous acid

    $${\text{HNO}}_{{3}} {\text{ + HNO}}_{{2}} { } \to {\text{ N}}_{{2}} {\text{O}}_{{4}} {\text{ + H}}_{{2}} {\text{O }}$$

    (9)
    $${\text{N}}_{{2}} {\text{O}}_{{4}} {\text{ + 2NO + 2H}}_{{2}} {\text{O }} \rightleftharpoons {\text{ 4HNO}}_{{2}} { }$$

    (10)

The second reaction mechanism for the electrochemical reduction of nitrate is the direct electrocatalytic reduction mechanism, which occurs in an alkaline medium that has a relatively low concentration of nitrate ions. The direct mechanism is usually followed when converting nitrate-containing wastewater effluents to ammonia, as these contain low concentrations of nitrate ions about less than 1.0 M (Min et al. 2021). The direct mechanism can proceed through the hydrogen-adsorbed and electron reduction pathways. The nitrate electroreduction is initiated by the adsorption of nitrate ions onto the cathodic electrode, but the adsorption of binary ions from the solution limits the overall reduction rate. Thus, electrocatalysts with an enhanced selective adsorption of nitrates with a large surface area for adsorption are an important parameter in electroreduction. The adsorbed ${\text{NO}}{{_3^{ - } }}$ is converted into nitrite through a three-step electrochemical–chemical–electrochemical process (Garcia-Segura et al. 2018). Subsequently, the nitrite is converted into nitric oxide (NO_(ads)). NO_(ads) is an important divergent-centered intermediate that branches out into different reactions to form dinitrogen and ammonia.

In the hydrogen-adsorbed pathway, the water molecules of the aqueous medium are reduced to ${\text{H}}_{\text{ads}}$ and ${\text{OH}}^{-}$ through the Volmer process (Reaction (11)).

$${\text{H}}_{2} {\text{O}}~ + ~{\text{e}}^{ - } ~ \to ~{\text{H}}_{{({\text{ads}})}} ~ + ~{\text{OH}}^{ - } ~$$

(11)

Because of high reduction potential ($E^\circ_{{(H ^+ /H^{ \cdot } )}}$ =  − 2.31 V versus standard hydrogen electrode), ${\text{H}}_{{{\rm ads}}}$ acts as a reducing agent and reduces several intermediates, such as adsorbed $~{\text{NO}}{{_2^{ - } ({\text{ads}})}} ,~{\text{NO}}_{{({\text{ads}})}} ,~{\text{N}}_{{({\text{ads}})}} ,~{\text{and}}~{\text{NH}}_{{2({\text{ads}})}} ,$ resulting in the formation of ammonia. Two $\text{N}_{\text{(ads)}}$ can react to form dinitrogen, but the reaction requires high activation energy of approximately 0.75 eV to form an N–N triple bond, which is greater than that of $\text{H}_{\text{ads}}$ (0.10 eV) (Wang et al. 2021d). Additionally, the formation of the N–H bond initiated by $\text{H}_{\text{ads}}$ is kinetically more favorable than that of the N–N bond. Therefore, the $\text{H}_{\text{ads}}$ mediated pathway is selective for the formation of ammonia or ammonium. This pathway usually occurs at a relatively low operational potential and in the presence of noble metal-based electrocatalysts, such as Pt, Pd, Ru, and Ir, because of their high affinity to H₂ (Wen et al. 2022a; Zhang et al. 2021). Since the formation of the N − H bond mediated by $\text{H}_{\text{(ads)}}$ is more kinetically favorable than that of the N − N bond, the dominant end-product is ammonia. The detailed H_ads mediated pathway is shown from Reactions (12–18).

$${\text{NO}}{{_3^{ - } ({\text{aq}})}} ~ \to ~{\text{NO}}{{_3^{ - } ({\text{aqs}})}}$$

(12)

$${\text{NO}}{{_3^{ - } ({\text{ads}})}} ~ + ~2{\text{H}}_{{({\text{ads}})}} ~ \to ~{\text{NO}}{{_2^{ - } ~({\text{ads}})}} ~ + ~{\text{H}}_2{{\text{O}}}$$

(13)

$${\text{NO}}{{_2^{ - } ({\text{ads}})}} ~ + ~{\text{H}}_{{({\text{ads}})}} ~ \to ~{\text{NO}}_{{({\text{ads}})}} ~ + ~{\text{OH}}^{ - }$$

(14)

$${\text{NO}}_{{\rm (ads)}} {\text{ + 2H}}_{{\rm (ads)}} { } \to {\text{ N}}_{{\rm (ads)}} {\text{ + H}}_{{2}} {\text{O}}$$

(15)

$${\text{N}}{}_{{\rm (ads)}}{\text{ + H}}_{{\rm (ads)}} \to {\text{ NH}}_{{\rm (ads)}}$$

(16)

$${\text{NH}}_{{\rm (ads)}} {\text{ + H}}_{{\rm (ads)}} \to {\text{ NH}}_{{\rm 2(ads)}}$$

(17)

$${\text{NH}}_{{\rm 2(ads)}} {\text{ + H}}_{{\rm (ads)}} \to {\text{ NH}}_{{\rm 3(ads)}}$$

(18)

In the electrocatalytic pathway, the formation of nitrite is the first step (Reaction (19)). The conversion of nitrate to nitrite is the rate-determining step (de Groot and Koper 2004); hence, the fundamental step in initiating charge transfer kinetics. Transition metal-based electrocatalysts with an unpaired d-electron, such as Cu, Ag, and Pt, can enhance reaction kinetics, as they have similar molecular orbital energies to those of nitrate. This similarity in their lowest unoccupied molecular orbital energies can help in the electron ejection required for the reduction to nitrite (Min et al. 2021). Furthermore, nitrite-to-nitric oxide reduction is a critical step in monitoring the formation of various products, such as ammonia, dinitrogen, and nitrous oxide.

$${\text{NO}}{{{_3}^{ - } {\text{(ads)}}}} {\text{ + 2H}}^{ + } {\text{ + 2e}}^{-} \to {\text{ NO}}{{{_2}^{-} {\text{(ads)}}}} {\text{ + H}}_{{2}} {\text{O}}$$

(19)

$${\text{NO}}_{{\text{2(ads)}}} {\text{ + 2H}}^{ + } {\text{ + 2e}}^{-} \to {\text{ NO}}_{{\text{(ads)}}} {\text{ + H}}_{{2}} {\text{O}}$$

(20)

The existence of NO_(ads) and in the solution $\left( {{\text{NO}}_{{\text{(aq)}}} } \right)$ can result in ${\text{N}}_{{2}} {\text{O}}_{{\text{(ads)}}}$ formation (Reactions (21) and (22)), which can further be reduced to ${\text{N}}_{{2}}$.

$${\text{NO}}_{{\text{(ads)}}} {\text{ + NO}}_{{\text{(aq)}}} {\text{ + 2H}}^{+} {\text{ + 2e}}^{-} \to {\text{ N}}_{{2}} {\text{O}}_{{\text{(ads)}}} {\text{ + H}}_{{2}} {\text{O}}$$

(21)

$${\text{N}}_{{2}} {\text{O}}_{{\text{(ads)}}} {\text{ + 2H}}^{+} {\text{ + 2e}}^{-} \to {\text{ N}}_{{2}} {\text{ + H}}_{{2}} {\text{O}}$$

(22)

The dissociation of ${\text{NO}}_{{\text{(ads)}}}$ into adsorbed nitrogen (${\text{N}}_{{\text{(ads)}}}$) and the combination of the two ${\text{N}}_{{\text{(ads)}}}$ can also generate ${\text{N}}_{{2}}$ (Reactions (23) and (24)). However, this ${\text{N}}_{2}$ formation pathway has not been experimentally proven.

$${\text{NO}}_{{\text{(ads)}}} {\text{ + 2H}}^{ + } {\text{ + 2e}}^{ - } { } \to {\text{ N}}_{{\text{(ads)}}} {\text{ + H}}_{{2}} {\text{O}}$$

(23)

$${\text{N}}_{{\text{(ads)}}} {\text{ + N}}_{{\text{(ads)}}} { } \to {\text{ N}}_{{2}} { }$$

(24)

The interaction of ${\text{NO}}_{{\text{(ads)}}}$ with protons and electrons can also result in the formation of the intermediate azanone ${\text{HNO}}_{{\text{(ads)}}}$ (Reaction (25)). The ammonia formation pathway can be explained by the electrochemical–electrochemical mechanism (Garcia-Segura et al. 2018), which comprises two direct charge transfer reactions. Reaction (26), which involves the conversion of ${\text{NO}}_{{({\text{ads}})}}$ to ${\text{HNO}}_{{({\text{ads}})}}$, is the rate-determining step. Furthermore, the subsequent reactions will lead to the generation of hydroxylamine, and the electrochemical reduction of hydroxylamine will yield ammonia (Reactions (27) and (28)).

$${\text{NO}}_{{({\text{ads}})}} + ~{\text{H}}^{ + } + ~{\text{e}}^{ - } \to {\text{HNO}}_{{({\text{ads}})}}$$

(25)

$${\text{HNO}}_{{({\text{ads}})}} + ~{\text{H}}^{ + } + ~{\text{e}}^{ - } \to {\text{H}}_{2} {\text{NO}}_{{({\text{ads}})}}$$

(26)

$${\text{H}}_{2} {\text{NO}}_{{({\text{ads}})}} + ~{\text{H}}^{ + } + ~e^{ - } \to {\text{NH}}_{2} {\text{OH}}~_{{({\text{ads}})}}$$

(27)

$${\text{NH}}_{2} {\text{OH}}_{{({\text{ads}})}} + ~2{\text{H}}^{ + } + ~2e^{ - } \to {\text{NH}}_{3} + ~{\text{H}}_{2} {\text{O}}$$

(28)

From the above discussion, the several multielectron transfer reactions involve numerous intermediates and by-products. The identification and quantification of these stable, unstable, and instantaneous intermediates are necessary for understanding the fundamental reaction pathways for the formation of desired products. Hence, identifying and fabricating suitable electrocatalytic materials for the reduction reaction with desirable products is achievable by understanding the fundamentals of the reaction mechanism.

Catalyst design and applications of nitrate-to-ammonia technology

Metal oxide-based electrocatalysts

Oxides mostly based on transition metals, such as Cu (combined or not with Co or Al or Mn or Pd), Co, Ag, Ti (doped or not with Pd), Nb, Bi, and Ru, with different interfaces, phases, structures, defects, and oxygen vacancies. have performed excellently as electrocatalysts in reducing nitrate to ammonia, showing great nitrate conversion efficiency, substantial ammonia yield or conversion, high Faradaic efficiency, and large ammonia selectivity. In this section, we present a summary of different oxides used in the electroreduction of nitrate to ammonia. Wang et al. (2020b) produced CuO nanowire arrays from the thermal treatment of Cu(OH)₂ nanowire arrays and observed the electrochemical reconstruction of CuO nanowire arrays to Cu-Cu₂O nanowire arrays, which served as the active phase in nitrate-to-ammonia reduction. The authors found that at an optimal potential of − 0.85 V versus reversible hydrogen electrode, the nitrate conversion rate was approximately 97.0%, the ammonia yield rate was 0.2449 mmol h⁻¹ cm⁻², Faradaic efficiency was 95.8%, and the ammonia selectivity reached 81.2% in a 0.5 M Na₂SO₄ solution. Using density functional theory calculations, the authors suggested that the formation of the *NOH reaction intermediate is aided by the electron transfer occurring at the Cu-Cu₂O interface, which decreases the H₂ production competition.

Xu et al. (2022b), applied the hierarchical-defect approach to producing a low-content Pd (2.93 at%) incorporated into Cu₂O. This Pd incorporation provoked the structural formation of corner-etched octahedra (Fig. 3), cavity defects, and the appearance of surface oxygen vacancy defects. Additionally, this synergic effect of defects helped to promote nitrate adsorption, resulting in the N–O bond being weakened and the by-product formation being restricted. The authors claimed that the electrochemical reduction from part of Cu₂O to Cu⁰ signified that the in situ occurrence of Cu⁺ and Cu⁰ acted as an active site for nitrate electroreduction and that the catalytic hydrogenation to produce ammonia would be provided by the Pd active sites, forming a Pd-H intermediate from the H ion adsorption. The authors found that the ammonia obtained from the electroreduction of nitrate at an optimum potential of − 1.3 V versus saturated calomel electrode had a yield of 925.11 µg h⁻¹ mg_cat⁻¹, a Faradaic efficiency of 96.56%, and a maximum selectivity of 95.31% in a 0.5 M K₂SO₄ electrolyte solution. Wang et al. (2022a) produced different oxides, including Cu₂O, on an island-like Cu surface obtained from electrodeposition, which showed that at − 0.8 V versus reversible hydrogen electrode and in a 0.5 M Na₂SO₄ electrolyte solution, nitrate conversion reached 99.14%, Faradaic efficiency was 98.28%, and selectivity toward ammonia production was 96.6%. Based on the density functional theory calculations, the authors suggested that the conversion of *HNOH to *HNHOH was promoted by the stable presence of Cu(I) at the Cu₂O surface and that the presence of oxygen atoms on this surface impedes H₂ coupling and the generation of by-products.

Fig. 3

Detailed schematic design for the fabrication of Pd incorporated Cu₂O. One-pot solution synthetic route involved for the preparation of Pd incorporated Cu₂O, wherein PdCl₂ was reduced to Pd(0) seeds by glucose and then catalyzed from Cu²⁺-tartrate to Cu₂O octahedra by reductive transformation. Consequently, the Cu₂O octahedra grew into corner-etched octahedra Cu₂O via a Pd(0)-catalysis, the process is the so-called as oxygen-engaged oxidation method. During the reaction, Pd with low content was deposited on Cu₂O structure. Reproduced from reference (Xu et al. 2022b) with permission from Elsevier

He et al. (2022) synthesized the core–shell Cu-CuO_x and Co-CoO phases on a Cu foil from binary Cu–Co sulfides through electrochemical redox activation. This electrocatalyst reduced nitrate to ammonia with a 90.6% Faradaic efficiency and 1.17 mmol h⁻¹ cm⁻² yield rate at − 0.175 V versus reversible hydrogen electrode in a 0.1 M KOH electrolyte solution, besides 36% of energy efficiency to a half-cell. The authors proposed that NO₃⁻ reduction to NO₂⁻ occurs preferentially in the inner Cu-CuO_x phase, whereas NO₂⁻ reduction to NH₃ occurs selectively in the outer-layer Co-CoO phase, both working together in tandem to produce ammonia. Fu et al. (2022) produced a Cu₂O-Cu interface on a Cu-based electrode through pulse electrodeposition and electroreduction. Furthermore, they applied this electrocatalyst in nitrate-to-ammonia reduction at − 0.25 V versus reversible hydrogen electrode in a 1 M KOH electrolyte solution, obtaining a yield rate of 2.17 mg h⁻¹ cm⁻², a Faradaic efficiency of 84.36%, and a selectivity of 94.4%. The authors attributed the improved activity of the Cu₂O-Cu interface in the nitrate-to-ammonia reduction to the tuned mobility of NO₂⁻ in the interface and the electron transfer from the Cu-based electrocatalyst to NO₃⁻. Jiang et al. (2022a) produced a strong metal-support interaction Cu − Al₂O₃ interface with Cu − O − Al bonds using CuAl mixed-oxide electroreduction. They found a 97.4% of ammonia selectivity to the Cu₅₂Al₄₈ catalyst, with 661.6 mg-N h⁻¹ m⁻² of specific activity, and 70.4% of Faradaic efficiency to ammonia production at − 1.10 V versus Ag–AgCl, 0.05 mM Na₂SO₄ of electrolyte solution, and when reducing 22.5 mg L⁻¹ NO₃⁻–N. The authors assumed that the Cu^δ+ resulting from the Cu–O − Al bonds shows a great affinity to the N-intermediates, thus promoting the activation of the molecule and preventing intercoupling of the dinitrogen species.

Ren et al. (2021) produced Cu@Cu₂₊₁O core-sheath nanowires contained at the surface concave–convex oxide layer) by drying Cu nanowires under air, which was achieved at − 1.2 V versus saturated calomel electrode in a 0.5 M K₂SO₄ electrolyte solution, an NO₃^– − N conversion rate of 78.57%, ammonia selectivity of 76%, Faradaic efficiency of 87.07%, and ammonia yield rate of 576.53 µg h⁻¹ mg_cat⁻¹. The authors assumed that the electron transfer was facilitated by the metallic Cu components in the core, although the high density of the catalytic active sites was due to concave-convex Cu₂₊₁O layers on the exterior. Additionally, the intermediates’ adsorption energies during the nitrate reduction were modulated by the electronic interaction between Cu-Cu₂₊₁O and the interface effect, which tunes the d-band center of Cu. Wang et al. (2021a) synthesized ultrathin heterostructured nanosheet arrays of Cu-oxygen vacancy-rich Cu-Mn₃O₄ on a Cu foam using the hydrothermal method, which was achieved at − 1.3 V versus saturated calomel electrode in a 0.5 M K₂SO₄ electrolyte solution, a 95.8% nitrate conversion, 87.6% selectivity, 92.4% Faradaic efficiency, and 0.21 mmolh⁻¹ cm⁻² ammonia production rate. The authors assumed that the numerous oxygen vacancies and Cu-Cu-Mn₃O₄ interfaces modulated the electronic structure of the surface, promoting nitrate adsorption and nitrate-to-ammonia electroreduction.

Wang et al. (2021b) synthesized nanosheets of CoO_x with numerous adsorbed oxygen species at a potential of − 0.3 V versus reversible hydrogen electrode and a maximum ammonia production of 82.4 ± 4.8 mg h⁻¹ mg_cat⁻¹ or 36.62 mg h⁻¹ cm⁻² from the reduction of nitrate at 0.1 M KOH and 93.4 ± 3.8% Faradaic efficiency. Using density functional theory calculations, the authors assumed that the adsorbed oxygen improved the H₂ adsorption, which restrained the H₂ evolution reaction, thus favoring a more exothermic reaction pathway. Yu et al. (2020) synthesized nanosheet arrays of Co–CoO through electrodeposition and annealing methods, which electrocatalyzed the nitrate-to-ammonia reduction with a 93.8% Faradaic efficiency, 91.2% selectivity, and 194.46 μmol h⁻¹ cm⁻² yield rate at − 1.3 V versus saturated calomel electrode in a 0.1 M Na₂SO₄ electrolyte solution. The authors concluded that the Schottky-rectifying effect prompted the Co to CoO electron transfer, which resulted in a Co electron deficiency that obliterated the H₂ production and prevented the by-product formation due to their high energy barriers. Liu et al. (2021a) produced oxide-derived Ag through a square-wave pulse potential applied on an Ag foil, which worked as a selective electrocatalyst in NO₃⁻ − to − NO₂⁻ or NO₃⁻ − to − NH₄⁺, depending on the cathodic potential applied where potential exceeded the theoretical charge of 116 C, NO₃⁻ − to − NH₄⁺. They achieved an 89% Faradaic efficiency to NH₄⁺ at − 1.35 V versus Ag–AgCl in a 0.1 M KCl electrolyte solution.

Jia et al. (2020) produced nanotubes of TiO₂ with oxygen vacancies, TiO_2-x, by heating in a H₂ atmosphere and found that the conversion of nitrate to ammonia at − 1.6 V versus saturated calomel electrode in a 0.5 M Na₂SO₄ electrolyte solution had a 95.2% conversion rate, 87.1% ammonia selectivity, 0.045 mmol h⁻¹ mg⁻¹ yield rate, and 85% Faradaic efficiency. The experimental H-cell setup for the nitrate-to-ammonia technology over the TiO₂ catalyst is depicted in Fig. 4. The authors assumed that the oxygen atom from the nitrate occupied the oxygen vacancies existing at TiO_2-x, weakening the N − O bond and limiting the formation of by-products. Guo et al. (2021) produced nanorod arrays of Pd-doped TiO₂ on carbon cloth using the ion exchange method, which reached a 99.6% nitrate conversion efficiency, 92.1% Faradaic efficiency, and 1.12 mg h⁻¹ cm⁻² ammonia yield rate at − 0.7 V versus reversible hydrogen electrode in a 1 M LiCl electrolyte solution. The authors concluded that the introduction of Pd heteroatoms induced TiO₂ lattice enlargement, which decreased the function of the catalyst surface, diminishing the intermediate adsorption energy.

Fig. 4

Reproduced from reference (Jia et al. 2020) with permission from American Chemical Society

Typical H-cell experimental setup for the electrochemical conversion of nitrate to ammonia over the TiO₂ catalyst, and the resulting removal efficiency of nitrate and ammonium yield production with selectivity. The fabricated TiO₂ and TiO_2-x electrocatalysts on carbon cloth substrate employed as working electrode, along with saturated calomel electrode and platinum foil as reference and counter electrodes, respectively. 50 ppm of nitrate solution in 0.5 M Na₂SO₄ served as supporting electrolyte. The optimal TiO_2-x electrocatalyst exhibited an enhanced nitrate conversion rate of 95.2%, ammonium production yield of 0.045 mmolh⁻¹ mg.⁻¹, and Faradaic efficiency of 85% along with selectivity of 87.1%.

Wan et al. (2022) constructed a NbO_x catalyst containing oxygen vacancies using hydrothermal and annealing methods that showed a 94.5% Faradaic efficiency and 55.0 μg h⁻¹ mg_cat⁻¹ formation rate to ammonia from the nitrate reduction at − 1.15 V versus reversible hydrogen electrode in a 0.5 M K₂SO₄ electrolyte solution. The authors assumed that the NbO_x activity was due to the nature and the oxygen vacancies, which improve the key intermediates’ binding energies. Chen et al. (2021) produced Bi₂O₃ nanosheets grown in situ on carbon cloth using the hydrothermal method, which reached an 84.9% nitrate reduction efficiency and an 80.3% ammonia selectivity after treatment for 180 min and a 10 mA cm⁻² current density in a 0.5 M Na₂SO₄ electrolyte solution. The authors assumed that atomic H* and the direct electron transfer were responsible for the ammonia produced from the nitrate electroreduction. Wang et al. (2022c) grew nanosheets of amorphous RuO₂ on a carbon paper surface through annealing of the salt method, which reached 97.46% Faradaic efficiency and 96.42% ammonia selectivity from nitrate electroreduction in a 0.5 M Na₂SO₄ electrolyte solution. The authors concluded that the enriched oxygen vacancies at amorphous RuO₂ due to the disordered atomic distribution at a-RuO₂ adjusted the H₂ affinity and d-band center, decreasing the required energy to the speed-determining step of NH₂* → NH₃* for ammonia production. The summary of metal oxide-based electrocatalysts in the performance of nitrate reduction for ammonia production is presented in Table 1.

Table 1 Overview of metal oxide and alloy-based catalysts, potentials, nitrate conversion efficiency, ammonia yields or production rates, Faradaic efficiencies, ammonia selectivity, and electrolyte solutions involved in the electrochemical reduction of nitrate

Metal alloys

A brief summary of the different kinds of alloys used in nitrate-to-ammonia electroreduction based on CuNi, CuPd, CuAu, CoP, and PtRu is discussed in this section. Wang et al. (2020a) produced a Cu₅₀Ni₅₀ alloy through electrodeposition and converted nitrate to ammonia with an Faradaic efficiency of 99 ± 1% at around − 0.15 V versus reversible hydrogen electrode in a 1.0 M KOH electrolyte solution. The authors suggested that the Cu₅₀Ni₅₀ alloy catalyst d-band center upshifted toward the Fermi level, enhancing the intermediates’ adsorption energies. They also suggested that the insertion of Ni atoms displaced the potential-dependent step from the adsorption of NO₃⁻ to *NH₂ hydrogenation due to the improved adsorption energy of NO₃⁻ to the surface of CuNi alloy, resulting in an overpotential diminution. Zhang et al. (2022b) prepared a Cu(111) surface containing atomically dispersed Au in small amounts with Cu vacancies through galvanic replacement and dealloying. This catalyst exhibited a Faradaic efficiency of 98.7% and a production rate of 555 μg h⁻¹ cm⁻² at − 0.2 V versus reversible hydrogen electrode in a 0.1 M KOH electrolyte solution to reduce nitrate to ammonia. The authors assumed that the isolated Au atoms and neighboring Cu vacancies were responsible for regulating the geometric structure and local electronic at the surface of the catalyst, facilitating *NH₃ desorption and the H₂O to *H activation to advance the kinetic energy hydrogenation of NO₃⁻.

Li et al. (2021d) produced a cobalt-phosphorous alloy film supported at the Ti plate using electrodeposition and found a yield rate of 416.0 ± 7.2 mg h⁻¹ cm⁻² and an Faradaic efficiency of 93.6 ± 3.3% at − 0.6 V versus reversible hydrogen electrode in a 0.2 M Na₂SO₄ electrolyte solution, with a conversion rate of 86.9% after 10 h of bulk electrolysis to reduce nitrate to ammonia. The authors assumed that as the transition metal phosphide showed a high ability for H-adsorption, they can active catalytically hydrogenation reactions. Furthermore, the amorphous structure of Co-P, which is rich in defects, showed a high density of catalytic sites. Xu et al. (2021) synthesized aerogels of Cu3Pd1 by alloying Pd with Cu using the wet chemical method and co-reducing the metal precursors with NaBH₄. At − 0.46 V versus reversible hydrogen electrode in a 0.5 M K₂SO₄ electrolyte solution, the Cu3Pd1 aerogels showed a 95.27% NO₃⁻ conversion rate, 77.49% selectivity, 90.02% Faradaic efficiency to ammonia, and 784.37 mg h⁻¹ mg_cat⁻¹ ammonia production rate. The authors assumed that Pd alloying adjusted the Cu d-band center, favoring the nitrate and related intermediates’ adsorption energies at the Cu sites during the electroreduction of nitrate. Additionally, the Pd sites may have acted as active centers for H-atom adsorption, which accumulated at the catalyst surface of the hydrogenated species and promoted NH₃ formation. Yin et al. (2021) synthesized PdCu-Cu₂O mesoporous hollow sphere hybrids containing highly dispersed PdCu alloys and an ultrathin Cu₂O shell by heating Cu₂O cubes and Pd salt in an aqueous solution, which reached a 99.82% nitrate conversion rate, 96.70% selectivity, 94.32% Faradaic efficiency, and 0.190 mmol h⁻¹ cm⁻² ammonia yield rate at − 0.80 V versus reversible hydrogen electrode in a 0.5 M Na₂SO₄ electrolyte solution (Fig. 5). The authors concluded that the highly dispersed PdCu alloys adjusted the synthesis path of Cu₂O ammonia production due to the change of nitrate in the adsorption sites, which blocked the generation of the *NOH intermediate and facilitated the formation of the *N intermediate.

Fig. 5

Reproduced from reference (Yin et al. 2021) with permission from Elsevier

Electrochemical nitrate reduction reaction over the PdCu-Cu₂O catalyst. A Linear sweep voltammetry curves in a 0.5 M Na₂SO₄ electrolyte, B Faradaic efficiencies of the ammonia generation and nitrate removal rate at different applied potentials, C change in the concentrations of nitrate, nitrite, and ammonia changes with respect to time, D ammonium production yield alongside the Faradaic efficiency for eight consecutive cycles, E ammonia selectivity and nitrate conversion with different initial concentrations of the nitrate solution, and F conversion rate of nitrate and the selectivity and Faradaic efficiency of ammonia generation over commercial Cu₂O, synthesized Cu₂O, and PdCu-Cu₂O catalysts. Thus, the obtained plot revealed that the optimal PdCu-Cu₂O catalysts exhibited highest electrochemical nitrate reduction performance when compare to commercial Cu₂O and synthesized pure Cu₂O materials. The optimal potential for the electrochemical reduction reaction over PdCu-Cu₂O catalysts was −0.8 V versus reversible hydrogen electrode which showed an enhanced ammonia yield and Faradaic efficiency with highest selectivity. RHE—reversible hydrogen electrode; ppm—parts per million; vs. —versus.

Wang et al. (2021h) compared the conversion of nitrate to ammonia using PtRu alloys and thermocatalytic and electrocatalytic methods. The Pt₇₅Ru₂₅-C was synthesized using NaBH₄ chemical reduction of metal salt precursors, showing the optimal nitrate conversion rate when H₂ pressure is increased from 0.1 to 1 atm H₂ and the potential is changed from 0.15 to 0.05 V versus reversible hydrogen electrode. At 0.1 V versus reversible hydrogen electrode, the Faradaic efficiency in ammonia production reached 93% at a pH of 1 and 54% at a pH of 7. The authors concluded that increasing the content of Ru in the alloy increased the adsorption strengths of nitrate, H₂, and the intermediates. Wang et al. (2021i) synthesized Pt_xRu_y alloy nanoparticles on vulcan carbon using the NaBH₄ reduction method and found Faradaic efficiency to be higher than 93% in nitrate-to-ammonia reduction in a 1.0 M H₂SO₄ electrolyte solution. The authors attributed the improved reduction of nitrate to ammonia using an alloying effect on the nitrate and H₂ binding energies such as the binding of the intermediates to the Pt₇₈Ru₂₂-vulcan carbon catalyst are either not too strong or not too weak. The alloys based at CuNi, CuPd, CuAu, CoP, and PtRu were shown to be effective electrocatalysts for nitrate reduction to ammonia, achieving high nitrate conversion efficiency, considerable ammonia yield or conversion, large Faradaic efficiency, and great ammonia selectivity. The summary of metal alloy-based electrocatalysts in the performance of nitrate reduction for ammonia production is presented in Table 1.

Non-oxide electrocatalysts

Because of high efficiency and eco-friendliness, the elimination of NO₃⁻ through electrocatalytic reduction has become a promising technique. However, during the electrocatalytic reduction, the metal oxides and alloy-based catalysts were applied as the electrodes in the consecutive reduction pathway of NO₃⁻, and N₂ and NH₃ evolved as products, which is a point of concern (Balaji et al. 2021; Das et al. 2022). Other important aspects to be considered are the high dependence of the selectivity and reactivity on the electrode category applied and the medium pH. For example, in the reduction of NO₃⁻, Pd and Pt have exhibited favorable selectivity in producing N₂O, N₂ gaseous components, and NH₃ (Gootzen et al. 1997). However, in the selective reduction of NO₃⁻ to NH₃·H₂O, Pd has shown superior activity (Pintar et al. 2004). However, the prohibitive cost and scarcity of Pd have restricted large-scale usage in wastewater purification techniques. However, in this section, we elaborate on non-oxide catalysts and their future scopes in the electrocatalytic reduction process.

Metal phosphides

Because of their low cost, hybrid orbitals, and superior electronic conductivity, metal phosphides or transition metal phosphides are decent candidates and have shown impressive electrocatalytic activity. Nickel phosphide is a frequently utilized electrocatalyst for examining activity in H₂ evolution reaction and oxygen evolution reaction, among others. (Liu et al. 2018; Popczun et al. 2013; Roberts et al. 2018; Stern et al. 2015). In a recent study, both NO₃⁻ reduction and HER were examined using a cathode made of nickel phosphide fabricated on carbon cloth where the generated active *H recombined with each other and helped in reducing NO₃⁻, which finally led to the relatively low efficiency of the nitrate electrocatalytic reduction (Huo et al. 2020b). Yao et al. (2021) fabricated nickel phosphide with (111) facets on nickel foam and applied them as a self-supporting electrode for nitrate reduction to synthesize NH₃ with excellent Faradaic efficiency of 99.23% and a high production yield rate of 0.056 mmol h⁻¹ mg⁻¹. Additionally, the as-prepared cathode exhibited high selectivity for NH₃ of 89.1%. Cobalt phosphide is another electrocatalyst utilized in NO_x⁻ reduction to ammonia (Zhang et al. 2022a).

Recently, Zhang et al. (2022a) tested the electrocatalytic conversion of nitrate to NH₃ in an alkaline medium using a cathode formed with cobalt phosphide on a carbon cloth substrate. The as-prepared nanowire catalyst exhibited a high Faradaic efficiency and conversion rate and showed durable stability and a low onset potential for NO_x⁻ reduction. Another group utilized Mg²⁺ ion-doped calcium carbonate nanoshuttles to develop amorphous and porous cobalt phosphide nanoshuttles through precipitation, followed by a high-temperature phosphidation process (Fig. 6 i) (Jia et al. 2021). The electrocatalysts exhibited a high ammonia yield rate of 19.28 ± 0.53 mg h⁻¹ mg⁻¹ and Faradaic efficiency of 94.24% ± 2.8%. The authors specifically illustrated that the shuttle-like morphology of the synthesized material has proven beneficial for the electrocatalytic application (Fig. 6 ii). The amorphous crystal structure, porous architecture, and improved surface area are responsible for the enhanced ammonia production rate (Jia et al. 2021). Another amorph of CoP with high-quality nanorings has also exhibited a high ammonia yield of 30.1 mg h⁻¹ mg⁻¹ with an Faradaic efficiency of 97.1% toward nitrate-to-ammonia conversion (Hong et al. 2021b). A few beneficial features of the catalyst highlighted in the article were its high surface area, substantial coordinately unsaturated active sites, and available mass transfer pathways, all highlighted by the authors as the characteristics responsible for enhanced ammonia production. Other aspects mentioned were high stability, high selectivity, and an earlier onset reduction potential. In another study, Wu and Yu (2021) illustrated the pathway of Ru-doped CO₂P monolayers, which were thermodynamically stable, and the Gibbs free energy for NO₃⁻ adsorption was made to be more negative than that of the proton, with an electrode potential of − 0.5 V.

Fig. 6

Reproduced from reference (Jia et al. 2021) with permission from American Chemical Society. NSs—nanoshuttles; PANSs—porous and amorphous nanoshuttles

(i) Fabrication procedure of Mg-doped porous and amorphous CoP nanoshuttles (Mg-CoP PANSs) through precipitation, followed by a high-temperature phosphidation process. (ii) TEM and SAED patterns of Mg-CoP PANSs. At first, the Mg-CaCo₃ nanoshuttles as precursor was synthesized by the solution mixture of MgCl₂ and CaCl₂ in the existence of polyethylene glycol as a surfactant. Then, the two-step synthesis route followed to synthesis CoP nanoshuttles via the simple precipitation of Mg-CaCo₃ to CoCo₃ nanoshuttles, followed by high-temperature phosphidation treatment to obtain CoP nanoshuttles product.

In recent studies (Liang et al. 2022; Wen et al. 2022b; Ye et al. 2022), nanosheet arrays of both cobalt and nickel phosphide were developed by different groups, which almost exhibited a 90%–100% Faradaic efficiency and a great ammonia-evolving rate. Wu et al. (2021a) recently investigated the potential of transition metal-doped hexagonal boron phosphide in nitrate reduction using a theoretical approach. This monolayer compound showed superior electronic and mechanical characteristics and acted favorably as a substrate for a single atom catalyst. The catalyst seemed facile for synthesis and had high thermal stability, which is suitable for experimental studies. However, electrocatalysts such as FeCoP, WP, FeP, and NiP have already been examined as H₂ evolution reaction catalysts and should be explored for nitrate reduction (Kibsgaard et al. 2015; Laursen et al. 2015; McEnaney et al. 2014; Xu et al. 2013).

Metal sulfides

In recent studies, various reduced sulfur compounds, such as H₂S and FeS, were examined as electron donors for the reduction of nitrate to ammonia. Garcia-Gil (1996) presented in their investigation that the addition of NO₃⁻ to sulfide-containing freshwater sediments, in both laboratory and natural conditions, results in the oxidation of sulfide and the concomitant accumulation of ammonia. The report also explained that the NO₃⁻ reduction pathway represents up to 30% of the total nitrate reduction in sulfide-dependent ammonia production. Molybdenum sulfide is another electrocatalyst considered suitable for nitrate-to-ammonia electrocatalytic reduction, over an extensive pH range ~ 3–11 (Li et al. 2016b). The onset potential for the pH dependence denitrification shows that the high activity was due to the ability of molybdenum sulfide to induce a concerted proton-electron transfer, signifying a turnover frequency comparable to that of Mo-dependent nitrate reductases. The protonated Mo–S reductive species can facilitate the nitrate and nitrite activation by establishing H₂ bonds among the substrates and active sites. Sato et al. (1999) investigated the impact of titanate on CdS nanocomposites toward nitrate reduction. Moreover, the catalytic activity greatly improved when the Pt nanoparticles were doped in the interlayer of the composite and furthermore, when methanol was added and acted as a sacrificial hole acceptor suitable for endorsing the NO₃⁻ reduction.

Metal carbides

Reports on utilizing transition metal carbides for electrochemical nitrate reduction are available (Xu et al. 2022a). The carbides with an unoccupied d-orbital exhibited a strong adsorption ability toward NO₃⁻. Meanwhile, the sp-hybridized state of the transition metal in the carbides moves to the center of the surface with the d-state of the transition metal and the s-state of carbon. The excessively occupied orbital offers precise prospects for reverse donation to the p-orbital of N₂, which benefits the activation of the N- to O-triple bond with improved nitrate reduction activity. One of the extensively utilized electrochemical nitrate reduction catalysts for easy NH₃ synthesis and NO₃⁻ removal is iron carbide of Fe₃C phase. The excellent electronic structure and morphology enable the carbide to activate the catalytic sites and prevent sluggish kinetics. Wang et al. (2021a, b, c, d, e, f, g, h, i) reported that Fe₃C nanoflakes embedded in N-doped carbon nanosheets possess a high surface area of 860.024 m² g⁻¹ for superior selectivity and nitrate reduction. At − 0.5 V, the NH₃ yield, Faradaic efficiency, selectivity, and current density reached 1.19 mmol h⁻¹ mg⁻¹, 96.7%, 79.0%, and 85.6 mA cm⁻², respectively, exceeding most reported results. Such exceptional performances mainly originated from the optimized electronic structures that optimized nitrate adsorption and the reaction kinetics with Tafel slope of 56.7 mV dec⁻¹. The NO₃⁻ reduction to NH₃ occurs through NO₂⁻, where the Fe³⁺-Fe²⁺ ratios and d-band centers play a vital role as the rate-determining step (Wang et al. 2021e).

Samples, such as Mo₂C-based nanocomposites, are highly recommended for superior nitrogen reduction reaction, electrocatalytic ammonia synthesis, and H₂ evolution inhibition, which could be utilized for the nitrate reduction reaction (Cheng et al. 2018). The density functional theory and electronic state density simulation studies confirmed that carbides provide active center sites for nitrate reduction. Additionally, two-dimensional atomic layer MXenes have exhibited beneficial electrochemical nitrogen reduction reaction performance with numerous active transition metal sites, excellent ion transmission, satisfactory conductivity, and stability in aqueous solutions (Sun et al. 2019). Generally, MXenes are referred to as two-dimensional transition metal nitrides and carbides. Majorly, M₃C₂, M₂C, and M₄C₃ are the three carbides of MXenes frequently applied for nitrate-to-ammonia reduction. Nevertheless, the MXene synthesis environments and conditions are harsh, as preparation at ambient temperature and pressure is difficult. The easy oxidization of MXenes in air made many characteristics acquirable using calculation and simulation. Major efforts have been made to tune the geometric and electronic structures of MXenes for fast nitrate reduction performance. As one of the typical M₃C₂T_x structures, Ti₃C₂T_x is used in the electrocatalytic reduction of nitrate due to its mechanical flexibility and stability (Li et al. 2021c). Surface and interface engineering, doping, or alloying is preferred in carbides to adsorb more nitrates and improve catalytic conversion to ammonia.

Carbon-based electrocatalysts in nitrate reduction

Casella and Contursi (2014) combined rhodium particles and multiwalled carbon nanotubes in an sulfuric acid medium to examine the nitrate reduction performance in ammonia production. Thus, the Rh-catalytic sites sufficiently adsorbed NO₂⁻ ions. During the electrocatalytic activity, the alternating development of Rh(OH)_n enabled the desirable desorption of fouling Rh(NOx)_ads adsorbed species and conserved the initial electrocatalytic performance of the Rh nanoparticles. By contrast, the initial steps of the electrocatalytic nitrite reduction proceeded through the direct and competitive reaction mechanisms of adsorption and desorption of nitrite and H₂ species, respectively. The authors also tested the redox activity of gold particles dispersed into multiwalled carbon nanotubes (Casella et al. 2014). Similarly, the vast potential usage of Pd-Fe-multiwalled carbon nanotubes can also be explored, as they have already been used to examine the denitrification process (Wang et al. 2021g).

The electrochemical results and index correlation analysis confirmed that the electron exchange capacity ~ 1.401 mmol eg⁻¹ of Pd-Fe-multiwalled carbon nanotubes is positively associated with a reduction in nitrite performance (Wang et al. 2019, 2020c). Similarly, Fe–Pd–Fe₃O₄-multiwalled carbon nanotubes have also been studied in nitrate reduction, where the NO₃⁻–N was completely eliminated within 2 h, 39% and 25% of which were converted into NO₂⁻–N and NH₄⁺–N, respectively. Figure 7 shows the resulting nitrate reduction mechanism over the Fe–Pd–Fe₃O₄-multiwalled carbon nanotube catalyst (Wang et al. 2021f). The reusability and recoverability test of Fe–Pd–Fe₃O₄-multiwalled carbon nanotubes through five consecutive cycles showed that a 43% NO₃⁻–N removal rate was reached, revealing that multiwalled carbon nanotubes prolonged the reduction efficiency of NO₃⁻–N by controlling the aggregation of catalytic nanoparticles. Yin et al. (2022) fabricated a corrosion-resistant Cu-based catalyst through thermolysis of the Cu-salophen coordination organic polymer on multiwalled carbon nanotube wires. This experiment showed an N₂–N selectivity of 93% and efficient catalytic performance for NO₃–N, with a NO₃⁻–N conversion yield of about 66% in Cl⁻-free neutral solution within 12 h. Furthermore, the study revealed that negligible Cu degradation occurred in continuous electrolysis for 60 h, and catalytic performance was not heavily deteriorated. The low Cu content ~ 1.6% and special core–shell structures enable this Cu@N-carbon catalyst to possess better corrosion-resistant properties in long-term electrolysis for nitrate reduction.

Fig. 7

Surface mechanism for the adsorption and electrochemical reduction of nitrate over the Fe–Pd–Fe₃O₄-multiwalled carbon nanotube catalyst. During nitrate reduction process, initially, adsorption takes place initially on the surface of metal active centers. Then, the adsorbed species get reduced at the surface of catalysts and followed by the re-adsorption which is known as removal process of ammonium product. Reproduced from reference (Wang et al. 2021f) with permission from Elsevier. nZVI—zero valent iron nanoparticles

In another study, Liu et al. 2022a examined the ability of a nitrogen-doped carbon matrix in nitrate-to-ammonia reduction, where the Faradaic efficiency was found to be 79.6% and selectivity was almost 94.4%. Additionally, the density functional theory calculations revealed that both the electronic and geometric structures of the catalysts were improved because of the introduction of nickel into the modified copper lattice. The nickel and copper sites in the CuNi alloy activated synergistically to accelerate the NO₂* to HNO₂* hydrogenation and suppress the H₂ evolution reaction, enhancing the selective production of ammonia. Nitrate conversion using this catalyst was found to be almost 81.2%. Duan et al. (2019) developed N-doped graphitic carbon-encapsulated iron nanoparticles that have shown long-term durability, high electrocatalytic activity, and 83% nitrate removal. The selectivity of nitrogen was ~ 25.0% in the absence of Cl⁻ and was enhanced to 100% when accompanied by NaCl ~ 1.0 gL⁻¹. Notably, there was no substantial difference at ~ 95% confidence interval in the percentage of removal measured over 20 cycles for the fabricated cathode where 20% iron was applied. The author further added that potential efforts can be made to improve selectivity and performance for nitrogen in the absence of Cl⁻ and that the upscaling strategy preparation of the catalyst should be advanced for nitrate control in wastewater treatment. Waste-derived carbon has also attracted many researchers in the nitrate reduction process. Li et al. (2021b) carbonized natural wood to prepare charcoal cathodes, which exhibited an ammonia production rate of 0.570 mmol L⁻¹ h⁻¹ cm⁻², a nitrate removal rate of 91.2%, and an outstanding selectivity of around 98.5% at an optimal − 3.6 V potential.

Metal–organic frameworks

The metal–organic frameworks are well known for their porous structure and high surface area. Two-dimensional and three-dimensional networks are easily constructed by altering the synthesis precursor of polydented organic ligands, linkers, and metal ions (Wang and Astruc 2020), which can vary the secondary building units or metal clusters. Different designs or metal frameworks can be implemented in various activities. Different metal–organic framework morphology-governed experiments in the selective catalytic reduction of nitrate to ammonia were conducted by Gao et al. (2021a). The performances varied due to the presence of active pores, catalytic sites, and stability in the acidic medium. The microporous and mesoporous metal–organic frameworks have surface areas up to 10,000 m²g⁻¹, which provide a large platform for electrocatalytic reactions (Li et al. 2016a). Furthermore, researchers’ investigations of the synergistic effect of metal–organic frameworks with conductive carbon bases, such as graphene or multi-walled carbon nanotubes, can be more conducive for electrocatalytic nitrate reduction to ammonia (Petit and Bandosz 2009). Hybrid nanocatalysts, such as heterometallic or nanocomposite metal–organic frameworks, can enhance catalytic reactions and gas absorption (Liu et al. 2022a).

The bridges and cavities in metal–organic frameworks can act as cushions for the transport and diffusion of products with a low mass transfer limit (Humphreys et al. 2018). A particular shape and size of the catalytic active sites in metal–organic frameworks are proven to be highly selective and sensitive to electrocatalytic nitrate reduction (Qin et al. 2022). The bridging ligands also contribute to the thermal and chemical stability during the adsorption of hazardous nitrates in wastewater treatment. Furtado et al. (2011) investigated Cu-benzene 1,3,5-tricarboxylate with an ordered mesoporous silica template for the stability of metal–organic frameworks during ammonia synthesis.

Thus, because of their excellent catalytic properties for adsorption, stability, sensitivity, and selective reduction of nitrates, metal–organic frameworks have been exponentially investigated by various researchers. For example, Li et al. (2016a) reported Cu- benzene 1,3,5-tricarboxylate as a selective low-temperature catalyst. The paddlewheel units with Cu²⁺ dimers produce a high number of metal sites for catalytic activity and also have good stability. Overall, the low amount of Pd in high surface area catalysts was examined to make the nanocatalyst cost-effective with better catalyst performance (Xu et al. 2022b). The flawed, edge-etched Pd-CuO₂ octahedral structure was proven to have high electrochemical selectivity toward nitrate reduction. The dual active sites, cavity structure, and rich surface oxygen vacancies were found to be a scalable medium for ample ammonia synthesis. The high Faradaic efficiency of Cu-based metal–organic frameworks and nitrates may be due to the similarity in their molecular energy levels during the electroreduction reaction. Nitrate electroreduction suffers from the generation of by-products, such as nitrogen oxyions or H₂; thus, a selective catalyst is essential. To overcome this issue, Qin et al. (2022) reported Ni- metal–organic frameworks-supported Ru_xO_y clusters as having high selectivity for NH₄⁺ and a high yield of ammonia. The study of selectivity was also supported by theoretical analysis, which stated that by-products such as NO₂ are easily converted to NH₄⁺ and then to ammonia. A density functional theory calculation by Liu et al. 2022a proved that the insertion of Cu into Ni sites and a supported nitrogen-doped carbon base could improve the ammonia yield by suppressing H₂ evolution (Fig. 8). Interestingly, Gao et al. (2021b) investigated a Cu^II-bipyridine-based thorium metal–organic framework for NH₃ storage through electroreduction reaction with a 92.5% Faradaic efficiency and a 225.3 μmol h⁻¹ cm⁻² yield. The low capacity and recyclability of nanocatalysts hinder the usability of NO₃⁻ to NH₃ in electroreduction. In that context, metal–organic frameworks supported with graphene oxide composites in a sandwich-like structure can maintain structural and chemical stability (Petit and Bandosz 2009).

Fig. 8

Electrocatalytic nitrate reduction activity of CuNi alloy embedded in an N-doped carbon matrix (CuNi-NC) obtained by the pyrolysis of bimetallic metal–organic frameworks. a Linear sweep voltammetry curves of CuNi-NC in a 1.0 M phosphate-buffered saline solution with and without nitrate, the resulting b nitrate removal efficiency, and c and d ammonia and nitrate selectivity with differing ratios of Cu and Ni in the CuNi-nitrogen-doped carbon catalyst. Overall, the linear sweep voltammetry indicates that the current density was enhanced by adding nitrate into the supporting electrolyte confirms the active response of CuNi-nitrogen-doped carbon electrocatalyst for the electrocatalytic reduction of nitrate. The optimal Cu5Ni1 exhibited an enhanced electrochemical reduction reaction with highest ammonia product selectivity with Faradic efficiency. NC—nitrogen-doped carbon; FE—Faradaic efficiency. Reproduced from reference (Liu et al. 2022a) with permission from Elsevier

The interconnected bonds and porousness of metal–organic frameworks and graphene oxide enhance ammonia intake. Besides Cu-metal–organic frameworks, Materials Institute Lavoisier-100(Fe) is known for the catalytic conversion of nitrate to ammonia due to the redox nature of iron species (Wang et al. 2014). The different metal-loaded metal–organic frameworks have been proven to be more effective catalysts than regular catalysts due to the synergistic effect of metal ions, metal–organic frameworks (Mounfield et al. 2016; Zhang et al. 2017), and metal–organic framework-derived oxide nanocatalysts such as Mn-doped Fe₂O₃ derived from a bimetallic metal–organic framework and also act as good avenues for the catalytic removal of nitrates (Lee and Kwak 2018). Such studies provide a scope for further investigation of other metal–organic framework-derived nanocatalysts for cost-effective and eco-friendly nitrate reduction.

Recently, zirconia-based metal–organic framework has been studied for the high gas uptake capacity (Binaeian et al. 2021). A stable and electroactive Zr-metal–organic framework with Zr₆-nanoclusters and redox-reversible tetrathiafulvalene derivatives as inorganic nodes and organic linkers, respectively, was explored using metal nanodots where M = Pd, Ag, or Au. Among the metal-nanodots-Zr-metal–organic framework catalysts, Pd-nanodots-Zr-metal–organic framework was found to have a high ammonia yield due to the high porosity, conductivity, and efficiency, which enhance the mass transport process (Jiang et al. 2022c). Hence, the various metal-based metal–organic frameworks exhibit extraordinary feasibility for electrochemical nitrate-to-ammonia technology. Metal–organic frameworks are also recognized for the photocatalytic reduction of nitrate and aid in the green synthesis of ammonia (Choi et al. 2019; Wang et al. 2022b). Table 2 presents the summary of non-oxide electrocatalysts toward the performance of nitrate reduction in ammonia production.

Table 2 Summary of non-oxide-based catalysts, potentials, nitrate conversion efficiencies, ammonia yields or production rates, Faradaic efficiency, ammonia selectivity, and electrolyte solutions involved in the electrochemical reduction of nitrate

Perspective

Various types of wastewater were investigated to find the most convenient reaction demands. The resultant studies proved that the wastewater effluents must contain a minimum concentration of nitrate. A high level of nitrate concentration was found in nuclear wastewater effluents, whereas a low concentration was found in industrial and polluted groundwater, among others. Although polluted groundwater contains nitrate levels unfit for human consumption, the removal through electroreduction is not conducive due to the low nitrate concentration, neutral pH conditions, and low ionic conduction. Consequently, additional steps have to be installed to increase the concentration of nitrate ions before the reduction process. Various factors, such as the presence of contaminants like hexavalent chromium and a high chemical oxygen demand, can be challenging to the reduction process. The side reaction of water electrolysis, producing H₂ in large quantities, can lead to the risk of explosion. Hence, the removal of H₂ or transient utilization must be thoroughly studied. The surface modulation of the electrocatalysts can be applied as a remedial strategy to enhance product selectivity over H₂ evolution reaction, as the electrocatalyst with negatively adsorbed charged species is expected to reduce the H₂ evolution reaction. For example, oxygen- or chlorine-based anions adsorbed on the copper electrocatalyst can capture protons and suppress H₂ evolution reaction while significantly promoting cathodic nitrate reductions. Electrode materials have a direct role in the reaction kinetics and the product selectivity of electrochemical reduction. The catalyst materials for nitrate removal should possess selective adsorption properties toward electron-rich N and O species for enhanced conversion to ammonia. Studies have shown that nitrate is a more suitable source for ammonia than dinitrogen, as a low dissociation energy ~ 204 kJ mol⁻¹ and greater solubility in aqueous solvents.

For future energy, the potential of selective nitrate-to-ammonia electroreduction can be summarized by the recent developments of efficient electrocatalysts with long-term stability. Transition metal-based electrocatalysts that exhibit variable oxidation states can enhance their electrocatalytic performance. The class of metal alloy-based electrocatalysts has the unique property of altering the charge distribution, making them suitable candidates for nitrate reduction. Transition metals with occupied d-orbitals having similar energy levels to that of the nitrate lowest unoccupied molecular orbital can significantly facilitate the electron transfer process. Beyond electrochemical performance, attention also needs to be paid to energy efficiency for proper electrode selection. Copper-based electrodes with unique electronic structures have proved to be the electrocatalysts of choice with optimal activity in an alkaline medium, but they lack long-term stability. Based on the concept of the green electro-conversion process, the development of economical and carbon-based materials must be explored. Selectivity and Faradaic efficiency are still challenging because of the complex reaction mechanism involving an eight-electron reduction course and competitive H₂ evolution reaction. Highly efficient and selective electrocatalysts for the large-scale production of ammonia from nitrate-containing wastewater remain lacking, providing plenty of research opportunities.

Conclusion

The versatile utilization of ammonia in manufacturing fertilizers and in chemical industries, as well as the recent exploration as a substitute energy carrier source, has increased the significance and production in the past few decades. The application of ammonia as a renewable energy carrier plays an important role as a carbon-free energy carrier and as a promising means for the efficient and economical indirect storage and transportation of H₂ energy. The direct conversion of ammonia in fuel cells can result in power generation. Nevertheless, the prevailing conventional production methods of ammonia, such as from the Haber–Bosch process and natural gas feedstock, result in the emission of greenhouse gases, which can be rectified by the waste-to-wealth concept reviewed here. Nitrate-containing wastewater effluents as a source for the electrochemical conversion of nitrate into ammonia results in value-added products at ambient reaction conditions. This method can be associated with the zero emission of greenhouse gases when the power supply is derived from renewable energy resources, which facilitates the removal of the nitrate pollutant from the wastewater and aids the restoration of the disrupted nitrogen cycle. The mechanistic aspects in electrocatalytic nitrate reduction is reviewed, along with the recent progress on the various electrocatalysts in the nitrate-to-ammonia conversion applications. Transition metals with occupied d-orbitals having similar energy levels to that of the nitrate lowest unoccupied molecular orbital can significantly facilitate the electron transfer process in the electrochemical nitrate reduction reaction.