Copper-Based Electrocatalysts for Nitrate Reduction to Ammonia

Abstract

Ammonia (NH₃) is a highly important industrial chemical used as fuel and fertilizer. The industrial synthesis of NH₃ relies heavily on the Haber–Bosch route, which accounts for roughly 1.2% of global annual CO₂ emissions. As an alternative route, the electrosynthesis of NH₃ from nitrate anion (NO₃⁻) reduction (NO₃⁻RR) has drawn increasing attention, since NO₃⁻RR from wastewater to produce NH₃ can not only recycle waste into treasure but also alleviate the adverse effects of excessive NO₃⁻ contamination in the environment. This review presents contemporary views on the state of the art in electrocatalytic NO₃⁻ reduction over Cu-based nanostructured materials, discusses the merits of electrocatalytic performance, and summarizes current advances in the exploration of this technology using different strategies for nanostructured-material modification. The electrocatalytic mechanism of nitrate reduction is also reviewed here, especially with regard to copper-based catalysts.

1. Introduction

As an important industrial chemical, ammonia (NH₃) is widely used in synthetic drugs, fertilizers, dyes, plastics, and so on [1,2]. NH₃ is also regarded as a carbon-free and environmentally friendly clean-energy carrier of hydrogen, since NH₃ is easier to liquefy, store, and transport than hydrogen [3,4]. To date, the industrial synthesis of NH₃ has relied heavily on the Haber–Bosch route. This route requires harsh conditions of high temperature (400–600 °C) and high pressure (200–350 atm) [5]. In fact, the energy consumption of the Haber–Bosch process accounts for 1.4% of the world’s total annual energy consumption, and the resulting carbon dioxide emissions are as high as 450 million tons, accounting for approximately 1.2% of global annual carbon dioxide emissions [6]. Therefore, researchers began to look for low-carbon and environmentally friendly alternatives for producing NH₃ that can be realized under normal temperature and pressure conditions. Among many alternatives, the method of producing NH₃ from nitrogen (N₂) and H₂O driven by renewable electric energy (NRR) has gradually attracted the researchers’ attention [7,8,9]. However, due to the high bond energy (941 kJ mol⁻¹) of the N≡N bond and the low solubility of N₂ in water, the energy utilization of NRR in the aqueous environment is low, and its production yield of NH₃ is 2–3 orders of magnitude lower than the Haber–Bosch process. As the “Renewable Energy to Fuels through Utilization of Energy-dense Liquids” (REFUEL) program of the Department of Energy (DOE) demonstrates, the electrochemical synthesis of NH₃ requires the realization of an NH₃-production Faraday efficiency exceeding 90%, an NH₃ yield of 10⁻⁶ mol s⁻¹ cm⁻², a current density of 300 mA cm⁻², and an energy efficiency of 60% for industrial production [10]. It is very difficult to achieve these indicators through the NRR process, which limits the further development of NRR [11]. For this reason, researchers have tried to apply other nitrogen-containing compounds as nitrogen sources to synthesize NH₃ [12]. Nitrate anion (NO₃⁻) is a good candidate, since the solubility of NO₃⁻ in water is high and the bond energy of N=O bond (204 kJ mol⁻¹) is low [13]. The NO₃⁻ reduction process is a solid–liquid interface reaction, which is more conducive to mass transfer than a gas–liquid–solid interface reaction during NRR and has the potential to realize the target of the REFUEL program. NO₃⁻ is a pollutant that widely exists in industrial and agricultural wastewater and domestic sewage [14]. The residual NO₃⁻ in the environment will not only harm the water body ecosystem and lead to water eutrophication but will also pose a serious threat to human health [15]. Therefore, using NO₃⁻ as a nitrogen source for NH₃ synthesis is not only conducive to improving energy utilization efficiency and reducing greenhouse gas emissions; it can also alleviate the problem of NO₃⁻ pollution in the environment.

In the natural environment, the conversion of NO₃⁻ to NH₃ can be biocatalyzed by a nitrate/nitrite reductase (such as Shewanella oneidensis cytochrome c nitrite reductase) under mild environmental conditions (a process known as bio-NRA) [16]. NH₃ can then be concentrated by physicochemical methods such as ion exchange adsorption and struvite precipitation and used for further industrial applications [6]. However, the biocatalytic process of bio-NRA takes a long time to effect the conversion, and the efficiency of NO₃⁻ removal is low. In addition, the complex structures of the bio-enzymes are difficult to synthesize artificially. Therefore, scientists began to simulate the function of microbial enzymes to design and develop artificially synthesized catalytic materials to satisfy the demands of industrial production [17]. Research on the electrocatalytic reduction of NO₃⁻ (NO₃⁻RR) to NH₃ is also facing great challenges. NO₃⁻RR involves the transfer of 8e⁻ and 9 protons, during which various byproducts such as nitrite (NO₂⁻), nitrogen monoxide (NO), nitrogen (N₂), and nitrous oxide (N₂O) are produced as well [18]. Meanwhile, there is competition between the hydrogen evolution reaction (HER) and NO₃⁻RR. With the increase in the overpotential, HER will be increasingly strengthened, which further reduces the energy utilization efficiency of NH₃ formation. Therefore, it is of great significance to carry out in-depth research on the mechanism of electrocatalytic NO₃⁻RR synthesis of NH₃ to rationally guide the development and design of catalysts with high catalytic activity and high NH₃ formation selectivity.

2. Reaction Mechanism of Electrocatalytic NO₃⁻RR

N has a variety of stable forms of hydrides and oxides from −3 to +5 valence states. The thermodynamic relationship between these nitrides is shown in the Frost–Ebsworth diagram (Figure 1) [13]. A series of reaction intermediate species and final products would be formed during NO₃⁻ reduction, and N₂ and NH₃ are the two most thermodynamically stable products, which can be obtained through the following Equations (1) and (2).

(1)$2{\mathrm{N}\mathrm{O}}_3^{-}+12{\mathrm{H}}^{+}+{10\mathrm{e}}^{-}\to {\mathrm{N}}_2+6{\mathrm{H}}_2\mathrm{O}{E}^0=1.17\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}$

(2)${\mathrm{N}\mathrm{O}}_3^{-}+9{\mathrm{H}}^{+}+{8\mathrm{e}}^{-}\to {\mathrm{N}\mathrm{H}}_3+3{\mathrm{H}}_2\mathrm{O}{E}^0=-0.12\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}$

According to the literature, NO₃⁻ reduction to NO₂⁻ is the rate-determining step of NO₃⁻RR (Figure 2) [20]. NO₃⁻ in the solution is first adsorbed onto the cathode surface (Equation (3)), during which some disturbing anions in the solution may compete with NO₃⁻ adsorption and inhibit NO₃⁻ reduction [21]. The mass transfer rate of NO₃⁻ from the solution to the electrode surface will also affect the adsorption process [22,23]. On the other hand, the number of active sites on the electrode’s surface will affect the reaction rate [23]. Currently, researchers propose two reaction mechanisms for NO₃⁻RR to NH₃: a direct reduction mechanism and an indirect autocatalytic reduction mechanism [19,24].

2.1. Direct Reduction Pathway

In the direct reduction pathway, the adsorbed NO₃⁻ can be reduced by proton-coupling electrons or by adsorbed H atoms (*H) [19]. As shown in Equations (4)–(6), in the proton-coupling electron reduction pathway, the process of NO₃⁻ being reduced to NO₂⁻ involves three steps: (a) electrochemical (Equation (4)), (b) chemical (Equation (5)), and (c) electrochemical process (Equation (6)) [25,26,27]. Koper et al. [20] found that the Tafel slope of NO₃⁻RR over transition metals in acidic solution is approximately 120 mV dec⁻¹, and then they speculated that the first electron-transfer step is the rate-determining step. The slow kinetics of this step was attributed to the high energy of the lowest unoccupied π* orbital (LUMO π*) of NO₃⁻, which made it difficult for electron transfer from the electrode surface into the LUMO π* of *NO₃⁻ [28]. Cu with highly occupied d orbitals has a closed energy level with LUMO π* of NO₃⁻. The electrons are easily transferred from the unclosed d orbital of Cu to the adsorbed NO₃⁻, which is conducive to the electrochemical reduction of NO₃⁻.

NO₃⁻ adsorption:

(3)${{\mathrm{N}\mathrm{O}}_3^{-}}_{\mathrm{a}\mathrm{q}}+\ast \to {{\mathrm{N}\mathrm{O}}_3^{-}}_{\mathrm{a}\mathrm{d}}$

NO₃⁻→NO₂⁻:

(4)${{\mathrm{N}\mathrm{O}}_3^{-}}_{\mathrm{a}\mathrm{d}}+{\mathrm{e}}^{-}\to {{\mathrm{N}\mathrm{O}}_3^{2-}}_{\mathrm{a}\mathrm{d}}{E}^0=-0.89\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}$

(5)${{\mathrm{N}\mathrm{O}}_3^{2-}}_{\mathrm{a}\mathrm{d}}+{\mathrm{H}}_2\mathrm{O}\to {{\mathrm{N}\mathrm{O}}_2}_{\mathrm{a}\mathrm{d}}+2{\mathrm{O}\mathrm{H}}^{-}k=5.5\times {10}^4{\mathrm{s}}^{-1}$

(6)${{\mathrm{N}\mathrm{O}}_2}_{\mathrm{a}\mathrm{d}}+{\mathrm{e}}^{-}\to {{\mathrm{N}\mathrm{O}}_2^{-}}_{\mathrm{a}\mathrm{d}}{E}^0=1.04\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}$

NO₂⁻ is a relatively stable intermediate which will become an unstable anion radical NO₂²⁻ through a direct electron transfer reaction pathway (Equation (7)) [29]. Similarly to NO₃²⁻, NO₂²⁻ will be rapidly hydrolyzed into adsorbed NO_ad through Equation (8) (k = 1.0 × 10⁵ s⁻¹) [30].

NO₂⁻→NO:

(7)${{\mathrm{N}\mathrm{O}}_2^{-}}_{\mathrm{a}\mathrm{d}}+{\mathrm{e}}^{-}\to {{\mathrm{N}\mathrm{O}}_2^{2-}}_{\mathrm{a}\mathrm{d}}{E}^0=-0.47\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}$

(8)${{\mathrm{N}\mathrm{O}}_2^{2-}}_{\mathrm{a}\mathrm{d}}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{N}\mathrm{O}}_{\mathrm{a}\mathrm{d}}+2{\mathrm{O}\mathrm{H}}^{-}k=1\times {10}^5{\mathrm{s}}^{-1}$

NO_ad is the key intermediate species that determines the selectivity of the final products, N₂ or NH₃. In the Vooys–Koper pathway, NO_ad reacts with NO_aq in solution to form a short-term dimer byproduct, dinitrogen dioxide (HN₂O₂) (Equation (9)) [31], which is then reduced to dinitrogen oxide (N₂O) by secondary electron transfer (Equation (10)) [23,32]. N₂O can be further reduced to the unstable N₂O⁻ (Equation (11)) [33]. This species will be readily transformed into the final product N₂ (Equation (12)) [34].

NO→N₂—Vooys–Koper pathway:

(9)${\mathrm{N}\mathrm{O}}_{\mathrm{a}\mathrm{d}}+{\mathrm{N}\mathrm{O}}_{\mathrm{a}\mathrm{q}}+{\mathrm{e}}^{-}+{\mathrm{H}}^{+}\to {\mathrm{H}{\mathrm{N}}_2{\mathrm{O}}_2}_{\mathrm{a}\mathrm{d}}{E}^0=0.0\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}$

(10)${\mathrm{H}{\mathrm{N}}_2{\mathrm{O}}_2}_{\mathrm{a}\mathrm{d}}+{\mathrm{e}}^{-}+{\mathrm{H}}^{+}\to {{\mathrm{N}}_2\mathrm{O}}_{\mathrm{a}\mathrm{d}}+2{\mathrm{H}}_2\mathrm{O}{E}^0=1.59\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}$

(11)${{\mathrm{N}}_2\mathrm{O}}_{\mathrm{a}\mathrm{d}}+{\mathrm{e}}^{-}\to {{{\mathrm{N}}_2\mathrm{O}}_{\mathrm{a}\mathrm{d}}}^{-}{E}^0=1.77\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}$

(12)${{{\mathrm{N}}_2\mathrm{O}}_{\mathrm{a}\mathrm{d}}}^{-}+{\mathrm{e}}^{-}+2{\mathrm{H}}^{+}\to {\mathrm{N}}_2+{\mathrm{H}}_2\mathrm{O}$

Based on the NO₃⁻RR study on the Pt (100) crystal plane, researchers proposed the Duca–Feliu–Koper pathway, through which NO₃⁻ will be electrochemically reduced to N₂. In this pathway, stable NH_2ad will be generated by the reduction of NO_ad (Equation (13)) and coexists with NO_ad [35,36]. NO_ad and NH_2ad will recombine according to the Langmuir–Hinshelwood reaction to generate the transient substance nitrosoamide (NONH₂) (Equation (14)), which will be rapidly decomposed to N₂ (Equation (15)).

NO→N₂—Duca–Feliu–Koper pathway:

(13)${\mathrm{N}\mathrm{O}}_{\mathrm{a}\mathrm{d}}+{3\mathrm{H}}_2\mathrm{O}+4{\mathrm{e}}^{-}\to {\mathrm{N}{\mathrm{H}}_2}_{\mathrm{a}\mathrm{d}}+4\mathrm{O}{\mathrm{H}}^{-}$

(14)${\mathrm{N}\mathrm{O}}_{\mathrm{a}\mathrm{d}}+{\mathrm{N}{\mathrm{H}}_2}_{\mathrm{a}\mathrm{d}}\to \mathrm{N}\mathrm{O}\mathrm{N}{{\mathrm{H}}_2}_{\mathrm{a}\mathrm{d}}$

(15)$\mathrm{N}\mathrm{O}\mathrm{N}{{\mathrm{H}}_2}_{\mathrm{a}\mathrm{d}}\to {\mathrm{N}}_2+{\mathrm{H}}_2\mathrm{O}$

Among the mechanism studies for NH₃ formation, the electrochemical–electrochemical (EE) reaction mechanism consisting of several sequential direct electron-transfer reactions is the main pathway [31]. NO successively undergoes reductive hydrogenation to several intermediates, forming nitroxyl (HNO) (Equation (16)) [37], H₂NO (Equation (17)) [31], and hydroxylamine (H₂NOH) (Equation (18)) [38]. Hydroxylamine will be protonated (Equation (19)) and then rapidly electrochemically reduced into NH₃ (Equation (20)) [39].

NO→NH₃:

(16)${\mathrm{N}\mathrm{O}}_{\mathrm{a}\mathrm{d}}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\to {\mathrm{H}\mathrm{N}\mathrm{O}}_{\mathrm{a}\mathrm{d}}{E}^0=-0.78\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}$

(17)${\mathrm{H}\mathrm{N}\mathrm{O}}_{\mathrm{a}\mathrm{d}}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\to {{\mathrm{H}}_2\mathrm{N}\mathrm{O}}_{\mathrm{a}\mathrm{d}}{E}^0=0.52\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}$

(18)${{\mathrm{H}}_2\mathrm{N}\mathrm{O}}_{\mathrm{a}\mathrm{d}}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\to {{\mathrm{H}}_2\mathrm{N}\mathrm{O}\mathrm{H}}_{\mathrm{a}\mathrm{d}}{E}^0=0.90\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}$

(19)${\mathrm{H}}_2\mathrm{N}\mathrm{O}\mathrm{H}+{\mathrm{H}}^{+}\rightleftharpoons {{\mathrm{H}}_3\mathrm{N}\mathrm{O}\mathrm{H}}^{+}p{K}_a=5.92$

(20)${{\mathrm{H}}_2\mathrm{N}\mathrm{O}\mathrm{H}}^{+}+{2\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to \mathrm{N}{\mathrm{H}}_3+{\mathrm{H}}_2\mathrm{O}{E}^0=0.42\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}$

In addition to the above direct reduction pathway, another hydrogenation pathway using adsorbed active hydrogen species (H_ad) as a reductant is considered to be an important pathway to promote the production of NH₃ [40,41]. In the potential region where hydrogen adsorption occurs on the Pt catalyst, the formation of NH₃ is highly promoted [42], since H_ad is stable on the electrode surface in that potential window. H_ad will efficiently deoxygenate NO₃⁻ and then hydrogenate the intermediates into NH₃. A hydrogenation mechanism is postulated according to the theoretical calculation results (Equations (21)–(26)): [43]

(21)${{\mathrm{N}\mathrm{O}}_3^{-}}_{\mathrm{a}\mathrm{d}}+2{\mathrm{H}}_{\mathrm{a}\mathrm{d}}\to {{\mathrm{N}\mathrm{O}}_2^{-}}_{\mathrm{a}\mathrm{d}}+{\mathrm{H}}_2\mathrm{O}$

(22)${{\mathrm{N}\mathrm{O}}_2^{-}}_{\mathrm{a}\mathrm{d}}+{\mathrm{H}}_{\mathrm{a}\mathrm{d}}\to {\mathrm{N}\mathrm{O}}_{\mathrm{a}\mathrm{d}}+{\mathrm{O}\mathrm{H}}^{-}$

(23)${\mathrm{N}\mathrm{O}}_{\mathrm{a}\mathrm{d}}+{2\mathrm{H}}_{\mathrm{a}\mathrm{d}}\to {\mathrm{N}}_{\mathrm{a}\mathrm{d}}+{\mathrm{H}}_2\mathrm{O}$

(24)${\mathrm{N}}_{\mathrm{a}\mathrm{d}}+{\mathrm{H}}_{\mathrm{a}\mathrm{d}}\to {\mathrm{N}\mathrm{H}}_{\mathrm{a}\mathrm{d}}$

(25)${\mathrm{N}\mathrm{H}}_{\mathrm{a}\mathrm{d}}+{\mathrm{H}}_{\mathrm{a}\mathrm{d}}\to {\mathrm{N}{\mathrm{H}}_2}_{\mathrm{a}\mathrm{d}}$

(26)${\mathrm{N}{\mathrm{H}}_2}_{\mathrm{a}\mathrm{d}}+{\mathrm{H}}_{\mathrm{a}\mathrm{d}}\to {\mathrm{N}{\mathrm{H}}_3}_{\mathrm{a}\mathrm{d}}$

However, how H_ad participates in the NO₃⁻RR process is still not clear, and the key intermediates of the reaction have not been experimentally proved. The intrinsic mechanism of the hydrogenation pathway remains to be further explored.

2.2. Indirectly Autocatalytic Reduction Pathway

The indirectly autocatalytic reduction pathway occurs only in the strongly acidic solution of high NO₃⁻ concentrations (1–4 mol L⁻¹) with the presence of HNO₂ [23] and mainly affects the step involving the reduction of HNO₃ to HNO₂. In this pathway, NO₃⁻ itself is not the electroactive species and does not participate in the electron transfer process. NO₃⁻ and HNO₂⁻ will be transformed into NO₂ or NO⁺ that is postulated as the electroactive species. With the accumulation of electroactive intermediates, the reaction will be highly enhanced. These two active substances can be generated via the Vetter pathway and the Schmid pathway, respectively (Equations (27)–(34)) [19]:

Vetter pathway:

(27)${\mathrm{N}{\mathrm{O}}_2}^{-}+{\mathrm{H}}^{+}\rightleftharpoons \mathrm{H}\mathrm{N}{\mathrm{O}}_2$

(28)$\mathrm{H}\mathrm{N}{\mathrm{O}}_2+\mathrm{H}\mathrm{N}{\mathrm{O}}_3\rightleftharpoons {\mathrm{N}}_2{\mathrm{O}}_4+{\mathrm{H}}_2\mathrm{O}$

(29)${\mathrm{N}}_2{\mathrm{O}}_4\rightleftharpoons 2\mathrm{N}{\mathrm{O}}_2$

(30)$\mathrm{N}{\mathrm{O}}_2+{\mathrm{e}}^{-}\to {\mathrm{N}{\mathrm{O}}_2}^{-}$

Schmid pathway:

(31)$\mathrm{H}\mathrm{N}{\mathrm{O}}_2+{\mathrm{H}}^{+}\rightleftharpoons {\mathrm{N}\mathrm{O}}^{+}+{\mathrm{H}}_2\mathrm{O}$

(32)${\mathrm{N}\mathrm{O}}^{+}+{\mathrm{e}}^{-}\to \mathrm{N}\mathrm{O}$

(33)${\mathrm{N}}_2{\mathrm{O}}_4+2\mathrm{N}\mathrm{O}+2{\mathrm{H}}_2\mathrm{O}\rightleftharpoons 4\mathrm{H}\mathrm{N}{\mathrm{O}}_2$

(34)$\mathrm{H}\mathrm{N}{\mathrm{O}}_3+2\mathrm{N}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons 3\mathrm{H}\mathrm{N}{\mathrm{O}}_2$

The following pathway of HNO₂ to NH₃ is similar to the direct reduction pathway mentioned previously. The indirect autocatalytic reduction mechanism could effectively accelerate the reaction rate of NO₃⁻RR, which plays an important role in accelerating the conversion rate and reducing the reaction cost.

3. Research Status of Electrocatalytic NO₃⁻RR to NH₃ Production

3.1. Evaluation Criteria for NO₃⁻RR Performance

Since nitrogen oxides and NH₃ widely exist in air and water, N species pollution during the tests must be strenuously avoided to guarantee the reproducibility of the research results. MacFarlane and Ib Chorkendorff et al. [44,45] proposed a set of testing procedures for NH₃ detection during NRR to ensure the accuracy of experimental results. The interference of N species pollution must be eliminated, and it must be determined that N in the formed NH₃ is derived from N₂ rather than from pollution from other N sources (catalysts, electrolytes, air, etc.) [11]. Similarly, the source of N in the formed NH₃ in the NO₃⁻RR process should also be strictly verified to obtain the real experimental results. Therefore, we need to have a unified understanding of the performance evaluation criteria for NH₃ synthesis during the NO₃⁻RR process.

There are two main factors affecting the NO₃⁻RR test results: (1) pollution from other N sources and (2) the accuracy of the NH₃ detection method. The pollution from other N sources mainly comes from air, reaction devices, electrolytes, and catalysts. Strict requirements on the cleanliness of the experimental equipment and accessories are essential to absolutely exclude these contaminations. Gases and chemicals used in experiments need to be at least analytically pure. The N element in the catalyst is another common contaminant affecting the experimental results. Many researchers who study single-atom catalyst systems should pay special attention to this if they use the MNC as the single-atom catalyst (M is the metal). It is necessary to verify whether the N content in the catalyst is consistent before and after the reaction. In addition, the ¹⁵N isotope labeling experiment is also one of the methods of confirming that the ¹⁵N in the product ¹⁵NH₃ comes from ¹⁵NO₃⁻ instead of pollution from other N sources. The Macfarlane and Zhang groups [46,47] proposed strict experimental methods for the ¹⁵N isotope labeling experiment for NRR. Quantitative detection methods for NH₃ include UV–Vis spectroscopic/colorimetric methods (Nessler reagent and indophenol blue method) [47,48], the o-phthalaldehyde fluorescence method [49], ion chromatography (IC) [50], ion selective electrode (ISE) [51], hydrogen nuclear magnetic resonance spectroscopy (¹H NMR) [46,52], etc. The accuracy of these detection methods can be affected by environmental factors. For example, the pH will affect the complexation of the chromogenic agent with NH₃, thereby affecting the coloration. The presence of other cations may affect the peak signal of NH₄⁺ when using ion chromatography detection. Two different detection methods are usually required for a simultaneous detection to ensure the accuracy of the experimental results. This involves using isotope-labeled ¹⁵NO₃⁻ as the N source for the reaction and then using ¹H NMR to detect the ¹⁵NH₃ product so that we can track the source of N, which is currently considered to be the safest measurement.

The production yield per unit area (mmol h⁻¹ cm⁻²) and the production yield per unit mass (mmol h⁻¹ g⁻²) are the key indicators in evaluating the NH₃ production rate. Faradaic efficiency (%) and partial current density (mA cm⁻²) are important indicators for the measurement of the energy utilization efficiency of the electrocatalytic process. In addition, the overpotential (V vs. RHE), stability, and NO₃⁻ removal rate (%) of the reaction are also important factors in evaluating the overall catalytic performances.

3.2. Electrocatalysts Designed for NO₃⁻RR to NH₃

Noble metal catalysts have shown good electrocatalytic performances in many electrocatalytic reactions, such as hydrogen evolution, alcohol oxidation, nitrogen reduction, and carbon dioxide reduction reactions. In their study of electrocatalytic NO₃⁻RR, DIMA et al. [20] analyzed the catalytic performance of the Pt group catalysts and found that the corresponding NO₃⁻RR activity decreased following the sequence Rh > Ru > Ir > Pd > Pt (Figure 3a–e). The adsorption of NO₃⁻ over Pt and Pd is weak, while the adsorption of *H is too strong, making it difficult for NO₃⁻RR to compete with HER. Rh, Ru, and Ir have a higher ability to adsorb NO₃⁻ and so those catalysts show better catalytic activity. Li et al. [53] reported that nitrate electroreduction overstrained Ru nanoclusters gave a high rate of NH₃ formation (5.56 mol g_cat⁻¹ h⁻¹). The primary contributor to such performance is the *H, which are generated by suppressing H-H dimerization during the water splitting enabled by the tensile lattice strains. The *H expedited NO₃⁻-to-NH₃ conversion by hydrogenating intermediates of the rate-limiting steps at lower kinetic barriers. The strained nanostructures can maintain nearly 100% NH₃ selectivity at >120 mA cm⁻² current densities for 100 h. Feliu et al. [42] found that electrocatalytic nitrate reduction was hardly perceptible on Pt (111), while electrocatalytic activity was improved with an increase in the step density of the Pt electrode surface. Inactivation was observed on the step sites in the presence of adsorbed NO₃⁻-derived reduced adsorbates, i.e., adsorbed NO. It was, therefore, concluded that the electrocatalytically active NO₃⁻ species did not adsorb on the (111) terraces but on the (111) monoatomic steps (as shown in Figure 3f,g). In addition, Han et al. [54] prepared Pd nanocrystalline with well-designed facets that could act as an efficient NO₃⁻RR electrocatalyst for ambient NH₃ synthesis and found that Pd (111) exhibited excellent activity and selectivity in reducing NO₃⁻ to NH₃ with a Faradaic efficiency of 79.91% and an NH₃ production of 0.55 mmol h⁻¹ cm⁻² (2.74 mmol h⁻¹ mg⁻¹) in 0.1 M Na₂SO₄ (containing 0.1 M NO₃⁻), which was 1.4 times higher than Pd (100) and 1.9 times higher than Pd (110), respectively. However, although many studies have been conducted on noble metals and some interesting results have been obtained, more effort should be made to lower the cost of the fabrication of catalysts and apply them to the industrial production of NH₃ formation. Therefore, many researchers have turned their attention to the development and design of non-precious metal catalysts.

Many transition metal catalysts exhibit a good electrocatalytic activity for NO₃⁻RR. Rossmeissl et. al. [55] studied the thermodynamics of various transition metal catalysts based on DFT calculations and found that Cu has a good adsorption capability for the intermediate species such as *NO₃⁻, *NO₂⁻, and *NO (Figure 4a–d), which is conducive to the continuous reduction and transformation of NO₃⁻. In addition, the adsorption of *H over Cu is weak, which hinders HER competition. Compared with other catalysts (as shown in Figure 4e), *N on Cu is more easily hydrogenated to NOH, which indicates that it has a better NH₃ selectivity [12,56]. Jiao et al. [57] experimentally investigated various transition metals (Fe, Co, Ni, Cu) for the electrochemical reduction of NO and N₂O to reveal the role of electrocatalysts in determining the product selectivity. Specifically, Cu was highly selective toward NH₃ formation with >80% Faradaic efficiency in NO electroreduction. Furthermore, a high NO coverage facilitated the N–N coupling reaction. In acidic electrolytes, the formation of NH₃ is greatly favored, whereas N₂ production is suppressed.

4. Cu-Based Catalyst for NO₃⁻RR

4.1. Modifications for Cu-Based Catalysts

Currently, the NH₃ production rate over Cu-based catalysts is far lower than the traditional Haber–Bosch method (200 mmol g_cat⁻¹ h⁻¹), which hinders its further development in industrial applications. Due to the complex reaction process of NO₃⁻RR to NH₃ and the low kinetics of NO₃⁻RR, the current density of NO₃⁻ reduction over Cu-based electrocatalysts is mostly below 100 mA cm⁻², especially in solutions with a low NO₃⁻ concentration. Therefore, it is urgent to improve the intrinsic activity of Cu-based catalysts for NH₃ formation. Research published in the literature describes the following methods of modification for Cu-based catalysts, which have been studied extensively; they are summarized below.

4.1.1. Size Modulation

The catalytic activity of the catalyst is highly affected by its particle size [58]. By optimizing the particle size of the catalyst, the catalytic activity of the catalyst can be significantly improved. As shown in Figure 5, from nanoparticles to nanoclusters and even single-atom catalysts, the density of active sites per unit mass increases, normally giving an increased catalytic activity [58]. Fu et al. [59] reported that the current density of NO₃⁻ reduction over Cu nanoparticles is 13 times higher than that over Cu foil, and the Faraday efficiency of NH₃ production has also increased from 30% over Cu foil to approximately 61% over Cu nanoparticles. In addition, the particle size of the catalyst will affect the interaction of reactants with the catalyst’s surface [60]. Yang et al. [61] prepared a Cu single-atom catalyst (Cu SAC) for NO₃⁻RR during which the maximum NH₃ yield reached 0.26 mmol cm⁻² h⁻¹ (12.5 mol g_Cu⁻¹ h⁻¹) and the Faraday efficiency of NH₃ production reached 84.7%. According to in situ X-ray absorption spectroscopy coupled with advanced electron microscopy technology, it was found that Cu SAC would be reconstructed into Cu nanoparticles (Cu₁₃) with a particle size of approximately 5 nm during NO₃⁻RR. DFT calculation results also showed that there was a high thermodynamic energy barrier of *NO₂ reduction over Cu SAC, while the thermodynamic energy barrier of *NO₃ to NH₃ on a Cu₁₃ cluster presented a continuous downward trend. These results showed that it was the Cu nanoparticles that were the active sites and not the Cu single atom sites.

4.1.2. Crystalline Facet Engineering

Feliu and Koper [42,62] found that NO₃⁻RR on the Pt-based catalyst was highly affected by the surface crystalline facet. Recent NO₃⁻RR production also proved that NO₃⁻RR on the Cu-based catalyst was sensitive to the crystalline facet. Hu et al. [44] studied the NO₃⁻RR and HER processes on different basal facets of Cu by using theoretical calculations and found that the competition between NO₃⁻RR and HER was affected by the solution pH. Cu (100) showed a catalytic performance of NO₃⁻ reduction to NH₃ in a strong acidic environment that was higher than that of the other basal facet, while Cu (111) had a higher catalytic activity in a neutral or alkaline environment (Figure 6). The difference in NO₃⁻RR activities between the different crystalline facets of Cu are attributed to the different atomic spacing and electronic state of the surface atom. The Cu–Cu bond length on Cu (111) was closer to the distance of two oxygen atoms in NO₃⁻, which is more conducive to the adsorption of NO₃⁻ on Cu (111). In addition, in the theoretical calculation, the d-band center on the Cu (111) was higher than that of Cu (100), due to which there was a stronger adsorption ability to NO₃⁻ on Cu (111). The electronic state of the catalyst surface affected its adsorption energy to NO₃⁻ and other intermediate species that affected the NO₃⁻RR activity. At a potential of 0 V vs. RHE, *NO→*NOH was theoretically proved as the rate-limiting step. The low adsorption energy of *NOH can reduce the reaction energy barrier. Fu et al. [59] prepared the Cu catalysts with different basal facets and found that Cu (111) exhibited its highest catalytic activity in alkaline electrolytes. Koper et al. [63] studied the performance of the electrocatalytic NO₃⁻ over Cu single crystal electrode in different pH media. In acidic and alkaline media, the onset potential of Cu (111) was slightly more positive than Cu (100). However, the HER activity on Cu (111) was relatively higher than Cu (100), leading to better NO₃⁻RR performances over the latter crystalline facet within a wide electrode potential window. Based on the advantages of electrochemically controllable technology in nanocrystals synthesis, our research group prepared Cu nanocrystals with different crystal plane, including high-index faceted Cu tetrahexahedron (THH Cu NCs, mainly composed of (211) and (311) crystal planes), low-index faceted Cu cube (100), and Cu octahedron (111) [64]. We found that, compared with Cu cube (100) and Cu octahedron (111), the THH Cu NCs with higher surface stepped sites had a higher reduction current density and a higher Faraday efficiency of NH₃ formation of 98.3% (Figure 7). This is consistent with the results on the Pt catalyst reported by Feliu et al. [42]. The surface stepped sites of the catalyst are the main active center. The higher the density of the steps, the better the NO₃⁻RR activity.

4.1.3. Surface Defects Engineering

By constructing the defective structure and decreasing the coordination number of the active site on the catalysts surface, NO₃⁻ adsorption and activation on the surface can be significantly enhanced and the intrinsic activity of the catalysts can then be improved. Hu et al. [65] studied the effect of defect exposure on the surface of Cu (100) on NO₃⁻ reduction activity. They found that under alkaline conditions, ultra-thin two-dimensional CuO (111) nanoribbons reconstituted into Cu (100) nanoribbons (Cu-NBS -100) in the process of NO₃⁻ electrochemical reduction, and, due to the high surface energy of ultra-thin nanoribbons, certain surface defects were formed in the process of structure reconstruction. Cu (100) -D1~7 models with different defects were constructed for DFT calculation. After comprehensive study, it was found that the d-band center of Cu (100)-D upshifted towards the Fermi level after the construction of surface defects on which the adsorption of intermediate species was enhanced. The Gibbs free energy of Cu (100)-D7 for NO₃⁻ was significantly lower than that of Cu (100). The defective sites on Cu (100)-D7 had a strong adsorption effect on *H intermediates, which makes *H difficult to desorb from the Cu surface, thus inhibiting HER. Xu et al. [66] also proved that Cu nanosheets with rich surface defects had a higher catalytic activity than those with smooth surfaces. At the potential of −1.3 V vs. SCE, the production rate of NH₃ reached 781.25 μg h⁻¹ mg⁻¹, and the Faraday efficiency arrived at 81.99%. Surface defects were able to change charge distribution on the catalyst surface and effectively improved electron transfer between the catalyst and the adsorbed species. They were able to enhance the adsorption and conversion of reaction intermediates on the catalyst surface [67]. In addition, surface atomic defects also brought more irregular edge steps to the catalyst surface and increased the density of surface-active sites.

4.1.4. Interfacial Engineering

In photocatalysis, constructing heterojunction structures to apply interfacial interactions to improve photocatalytic activity is a common catalyst-design strategy [68,69,70]. In electrocatalysis, the interfacial interaction between different phases also has a significant impact on catalytic performances [71,72]. Deng et al. [73] proved that the electrocatalytic NO₃⁻RR activity of CoP can be effectively enhanced by constructing a p-n heterojunction of a CoP/TiO₂ nanoarray on titanium plate (CoP/TiO₂@TP). According to DFT, the p-n heterojunction in CoP/TiO₂ could induce the charge redistribution of CoP and TiO₂, enhance the electron transport between the interfaces, effectively reduce the free energy of each reaction step, and improve the performance of NO₃⁻RR. Benefiting from the interaction of heterostructures, such CoP/TiO₂@TP exhibited an excellent Faradaic efficiency of 95.0% and a large NH₃ yield as high as 499.8 μmol h⁻¹ cm⁻², which was superior to CoP@TP and TiO₂@TP. For a Cu-based catalyst, Liu et al. [74] grew Pd particles on CuO nano-olives (Pd/CuO NOs) to form a Pd/CuO NOs catalyst with a unique Pd/CuO heterogeneous interface for nitrite reduction (NO₂⁻RR). The heterogeneous interface formation of Pd/CuO NOs was analyzed by means of XPS characterization and DFT calculation. CuO and Pd show the characteristics of negative charge and positive charge, respectively, because of the transfer of electrons. Additionally, the built-in electric field constructed at the Pd/CuO heterointerface promotes local charge polarization. The formation of a Pd/CuO heterointerface optimizes the electronic structure of Cu, which is more favorable for the adsorption of NO₂⁻ and reaction intermediates. Pd/CuO NOs showed an NH₃ yield of 906.4 μg h⁻¹ mg⁻¹ _cat and a Faraday efficiency of 91.8%. Wang et al. [75] designed the Cu/Cu₂O nanowire array and revealed that CuO would be electrochemically converted into Cu/Cu₂O, which was observed by performing in situ Raman and ex situ experiments. The Cu/Cu₂O interface was proved as the active site. The combined results of online differential electrochemical mass spectrometry (DEMS) and DFT calculations demonstrated that the electron transfer from Cu₂O to Cu at the interface could facilitate the formation of *NOH intermediate and suppress the HER, leading to 95.8% of NH₃ Faraday efficiency and 0.2449 mmol h⁻¹ cm⁻² NH₃.

In summary, the optimization of Cu catalysts by various modification methods can significantly improve the Faraday efficiency of NH₃ production, but it was also found that the current density of NO₃⁻RR and the NH₃ production rate were still too low to be applied in industrial applications (Table 1). Moreover, the required overpotential was also too high and was not conducive to being powered by regeneration energy. To this end, researchers have gradually designed Cu-based bimetallic catalysts by introducing second metals, and tried to further improve the catalytic activity of Cu-based bimetallic catalysts by regulating their electronic structure and synergistic effect, and they have promoted the further development of NO₃⁻RR.

4.1.5. Electronic Structure Modulation

Wang et al. [82] designed Cu_100−xNi_x alloy with different proportions of the Cu and Ni elements and investigated the adsorption energy of intermediate products over the Cu_100−xNi_x alloy. Since the NO₃⁻RR activity on Ni is weak, the increase in the Ni element proportion should decrease the number of active sites on the surface, possibly resulting in a decreased performance. However, compared with pure Cu, Cu₅₀Ni₅₀ showed an increase of 0.12 V in the half-wave potential and a sixfold increase in activity at 0 V vs. RHE. The electronic structure of Cu_100−xNi_x was analyzed by performing X-ray photoelectron spectroscopy (XPS), X-ray adsorption spectroscopy (XAS), and ultraviolet photoelectron spectroscopy (UPS). It was found that the binding energy of Cu 2p in Cu_100−xNi_x alloy shifted to a lower binding energy with the increase in the Ni proportion. The binding energy of the Ni 2p shifted towards a higher binding energy, indicating that the charge in the Cu_100−xNi_x alloy was redistributed and electrons were transferred from the Ni to the Cu. At the same time, the Cu d-band center upshifted, which enhanced the adsorption of the intermediate products on the Cu. Further study by means of DFT calculation found that the adsorption energy of the intermediate products showed a volcanic distribution with the overall reactivity. When the Ni accounted for 50%, the catalyst had moderate adsorption on all intermediate species, and then the highest reaction performances arrived. The establishment of the relationship between the electronic structure of the catalysts and NO₃⁻RR activity provided an idea for guiding the design of novel NO₃⁻RR catalysts. Ge et al. [83] reported polyallylamine (PA) functionalized RhCu bimetallic nanocubes (PA-RhCu cNCs) as a robust electrocatalyst for the reduction of NO₃⁻ to NH₃. PA-RhCu cNCs showed a remarkable NH₃ production yield of 2.40 mg h⁻¹ mg_cat⁻¹ and a high Faradaic efficiency of 93.7% at 0.05 V. DFT calculations and experimental results indicated that Cu and PA (adsorbed amino) co-regulated the Rh d-band center, which slightly weakens the adsorption energy of reaction-related species on Rh. These findings may open an avenue to construct other advanced catalysts based on organic molecule-mediated interfacial engineering. Goldsmith’s group [84] predicted NO₃⁻RR activity by calculating the adsorption energy of N and O atoms on transition metals. They proposed that alloyed Fe₃Ru, Fe₃Ni, Fe₃Cu, and Pt₃Ru have a strong adsorption capability to N and O atoms and suggested that those bimetals should be the potential NO₃⁻ reduction catalysts.

Based on the above analysis, the appropriate adsorption energy of catalysts for intermediate species such as *NO₃, *H, and *NH₃ is the key factor affecting their performance, which can be achieved by modulating the electronic structure of bimetal alloys to affect the d-band center or by forming a poor/rich-electron center.

4.1.6. Synergistic Effect

Protons (8e⁻) are involved in the process of NO₃⁻ reduction to NH₃, giving a series of elementary reactions before the final products are formed. The active sites on single metallic catalysts are not well adapted to simultaneous multiple elementary steps. To this end, the construction of a synergistic effect between multiple active sites become one of the research hotspots in the field of NO₃⁻RR. Precious metals alloying with non-precious metals is one of the most useful strategies for fabricating bimetallic catalysts (Table 2). The introduction of precious metals into Cu metal was proved to facilitate a much higher catalytic activity than that of pure Cu [85,86,87,88]. Kerkeni et al. [89] modified the platinum catalyst’s surface through a deposition on the Cu (sub)monolayer of a certain amount of Cu (Cu_ads/PT) using electrochemical methods. They found that the intrinsic catalytic activity (at t = 0) and the NH₃ selectivity of Cu_ads/Pt depended on the fraction of the platinum surface occupied by copper adatoms (θ_Cu). The highest activities and selectivity of NH₃ were obtained with θ_Cu ≈ 0.4–0.6. Moreover, higher activities were obtained in the reduction of NO₃⁻ than in the reduction of NO₂⁻. Barrabés [85] and Wang et al. [90] believed that the existence of Cu was conducive to NO₃⁻ adsorption and reduction into NO₂⁻ and that NO₂⁻ would then be transferred to Pt/Pd to continue being hydrogenated. The presence of Pt/Pd would produce *H, which was conducive to maintaining the existence of Cu⁰ activity and promoting the intermediate species to further hydrogenation to N₂ or NH₃. Luo et al. [91] designed the Cu nanowire-supported Rh cluster and single-atom electrocatalyst (Rh@Cu) for a NO₃⁻ reduction reaction. Benefiting from the catalytic cooperation between Rh and Cu, whereby Rh activates the hydrogenation of the *NO intermediate on Cu, Rh@Cu systems achieved a Faradaic efficiency up to 93% at −0.2 V vs. an RHE and NH₃ yield rate of 1.27 mmol h⁻¹ cm⁻². Detailed investigations by means of electron paramagnetic resonance, in situ infrared spectroscopy, differential electrochemical mass spectrometry, and DFT calculations suggested that the high activity originates from the synergistic cooperation between the Rh and Cu sites, whereby adsorbed hydrogen on the Rh site transferred to vicinal *NO intermediate species adsorbed on the Cu. Schuhmann et al. [92] presented a design concept of tandem catalysts, which involves coupling intermediate phases of different transition metals, existing at low applied overpotentials, as cooperative active sites that enable cascade NO₃⁻-to-NH₃ conversion, in turn, avoiding the generally encountered scaling relations. They implemented the concept by means of the electrochemical transformation of Cu−Co binary sulfides into potential-dependent core−shell Cu/CuO_x and Co/CoO phases. Electrochemical evaluation, kinetic studies, and in situ Raman spectra reveal that the inner Cu/CuO_x phases preferentially catalyzed NO₃⁻ reduction to NO₂⁻, which was rapidly reduced to NH₃ at the nearby Co/CoO shell. This unique tandem catalyst system led to a NO₃⁻-to-NH₃ Faradaic efficiency of 93.3 ± 2.1% in a wide range of NO₃⁻ concentrations at pH 13, a high NH₃ yield rate of 1.17 mmol cm⁻² h⁻¹ in 0.1 M NO₃⁻ at −0.175 V vs. RHE, and a half-cell energy efficiency of ~36%, surpassing most previous reports.

4.2. Study on Mechanism of Electrocatalysis of NO₃⁻RR Synthesis of NH₃ by Cu-Based Catalyst

Hu et al. [44] studied the pathway of the electroreduction of NO₃⁻ to NH₃ by employing DFT calculation. According to their comprehensive thermodynamic and dynamic analysis, the pathway of NO₃⁻RR on Cu should be as follows: NO₃⁻ → *NO₃ → *NO₂ → *NOH → *NHOH → *NH → *NH₂ → *NH₃ → NH₃ (g) (Figure 8). Butcher et al. [99] detected a series of intermediates in the NO₃⁻RR process in the acid media by means of SHINERS analysis, including NO₂⁻ and HNO, NH₂, NH₄⁺, etc. This was consistent with DFT calculation results. In an alkaline medium, Koper et al. [63] observed the appearance of hydroxylamine on the Cu catalyst using Fourier infrared spectra analysis, indicating that hydroxylamine was an intermediate species in the alkaline environment.

Gewirth et al. [100] analyzed the structural evolution of the adsorption intermediates on the surface of the Cu (100) by using an in situ electrochemical scanning tunnel microscope (EC-STM). They intuitively observed the process of adsorption of NO₃⁻ being transformed into NO₂⁻. The adsorption of oxyanions was always accompanied by the synergistic adsorption of H₂O or H₃O⁺ [101]. H₂O or H₃O⁺ can stabilize the structure of oxyanions by forming hydrogen bonds with the lone electron pairs of oxygen atoms [102]. Researchers proposed that the NO₃⁻ adsorption layer on Cu (100) was also involved with H₃O⁺. In addition, DFT calculation results showed that NO₃⁻ and NO₂⁻ were adsorbed on the Cu (100) surface by the bridging of their two oxygen atoms. Koper et al. [20] conducted an electrochemically dynamic analysis of the NO₃⁻RR process using cyclic voltammetry and found that the Tafel slope on the Cu-based catalyst was slightly higher than the 120 mV dec⁻¹. Therefore, they proposed that the first electron transfer step was the rate-limiting step. There are two possible mechanisms: *H reduction (Equations (35) and (36)) and the proton-coupling electron reduction mechanism (Equations (37) and (38)).

*H reduction:

(35)${\mathrm{H}}^{-}+{\mathrm{e}}^{-}\to {\mathrm{H}}_{\mathrm{a}\mathrm{d}}$

(36)${{\mathrm{N}\mathrm{O}}_3^{-}}_{\mathrm{a}\mathrm{d}}+{\mathrm{H}}_{\mathrm{a}\mathrm{d}}\to \mathrm{a}\mathrm{d}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{a}$

Proton-coupling electron reduction:

(37)${{\mathrm{N}\mathrm{O}}_3^{-}}_{\mathrm{a}\mathrm{d}}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\to {{\mathrm{H}\mathrm{N}\mathrm{O}}_3^{-}}_{\mathrm{a}\mathrm{d}}$

(38)${{\mathrm{H}\mathrm{N}\mathrm{O}}_3^{-}}_{\mathrm{a}\mathrm{d}}\to \mathrm{a}\mathrm{d}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{a}$

The HER performance over the Cu catalyst is relatively poor, so it is difficult to produce *H species. Therefore, it is generally believed that the proton-coupling electron reduction mechanism is most effective on a pure Cu catalyst. For catalysts that are more susceptible to activating protons, such as the precious metals contained in bimetals, *H was more conducive to NO₃⁻ reduction.

The existence of other anions/cations in electrolytes usually affects the target catalytic reactions. For example, the existence of Cl⁻ ions would lead to different final products. Butcher et al. [99], analyzing to Raman spectra, found that there were more bands of N-H_x species on the Cu basal facets after adding 10 mmol L⁻¹ Cl⁻, revealing that the presence of Cl⁻ was beneficial for NH₃ formation. Gao et al. [103] found that the amount of NH₃ formation was dramatically decreased, although a similar removal efficiency of NO₃⁻ was obtained when 100 mg L⁻¹ Cl⁻ was added. As the Cl⁻ ion concentration increased, the NH₃ selectivity rapidly decreased. This was due to the Cl⁻ indirectly oxidizing NH₃ into N₂. The Cl⁻ was oxidized by the anode to produce Cl₂ (Equation (39)), which was then transformed into HOCl with strong oxidation (Equation (40)) [104]. The HOCl could efficiently oxidize the NH₃ into N₂ (Equation (41)).

(39)$2{\mathrm{C}\mathrm{l}}^{-}\to {\mathrm{C}\mathrm{l}}_2+2{\mathrm{e}}^{-}$

(40)${\mathrm{C}\mathrm{l}}_2+{\mathrm{H}}_2\mathrm{O}\to \mathrm{H}\mathrm{O}\mathrm{C}\mathrm{l}+{\mathrm{H}}^{+}+{\mathrm{C}\mathrm{l}}^{-}$

(41)$3\mathrm{H}\mathrm{O}\mathrm{C}\mathrm{l}+2\mathrm{N}{\mathrm{H}}_3\to {\mathrm{N}}_2+{3\mathrm{H}}_2\mathrm{O}+3{\mathrm{C}\mathrm{l}}^{-}+3{\mathrm{H}}^{+}$

Currently, mechanistic studies of NO₃⁻RR over Cu-based catalysts are relatively scarce, and most of them have followed the results from Pt-based catalysts. Therefore, the mechanism of electrocatalytic NO₃⁻ reduction to NH₃ over Cu-based catalysts requires further study.

5. Conclusions and Perspectives

Electrocatalytic NO₃⁻RR is a clean and pollution-free process for NH₃ production, which has the potential to realize large-scale industrial NH₃ production and could solve the problem of NO₃⁻ pollution at the same time. This review mainly presents contemporary views on the state of the art in electrocatalytic NO₃⁻ reduction over Cu-based nanostructured materials. The effective methods, which were reported to modify the performance of Cu-based catalysts for electrocatalytic NO₃⁻RR to NH₃, include size modulation, crystalline facet engineering, surface defect engineering, interfacial engineering, electronic structure modulation, and synergistic effect. A reduction in metal catalyst particle size can effectively improve the utilization efficiency of atoms and enhance reactivity. Theoretically, the single-atom catalyst has nearly 100% atomic utilization efficiency. However, for Cu single-atom catalysts, the active sites were Cu nanoparticles instead of the Cu single atom sites. For the optimization of crystal surfaces, Cu nanocrystalline catalysts with a high crystal index have higher activity than those with a low crystal index. Cu catalysts with a high crystal index can be constructed to further improve the performance of NH₃ synthesis. Moreover, by constructing the defective structure and decreasing the coordination number of the active site on the catalyst’s surface, NO₃⁻ adsorption and activation on the surface can be significantly enhanced, and the intrinsic activity of the catalysts can then be improved. Cu active sites can be optimized effectively by the three methods mentioned above. However, since the reduction of NO₃⁻ to NH₃ involves the transfer of nine protons and eight electrons, it is difficult for the single active site to work effectively with simultaneous multiple elementary steps. To this end, catalysts with multiple active sites were designed. The heterogeneous interface between the Cu-based catalysts and other materials could promote local charge polarization by means of the formation of a built-in electric field and change the adsorption of intermediate species of NO₃⁻RR. The alloying of Cu with a secondary metal can regulate the electronic structure of Cu and the synergistic effect. Additionally, different elements/active sites with different activities may have synergistic effects by providing a reduction in intermediate species or tandem reaction mechanisms to effectively reduce the reaction energy barrier. In conclusion, there are broad development spaces in the design and development of catalysts for the electrocatalytic NO₃⁻RR of synthetic NH₃, and this paper can provide a reference for subsequent research.

The research of nitrate reduction reaction has developed vigorously and made some progress in recent years, but there are still many problems. Below, we list some of the shortcomings and further research directions.

Research on the mechanism of NO₃⁻RR is still insufficient. The reaction pathway of NO₃⁻RR is affected by many environmental parameters, such as other ions, pH, NO₃⁻ concentration, and applied potential. Different systems accrue different basic pathways, which require a more systematic and detailed analysis, including the identification of key reaction intermediates by means of various in situ spectra and the postulation of basic reaction steps using theoretical calculations.

Electrocatalysts that are more economical, efficient, and stable need to be developed. At present, although modified Cu-based catalysts show improved electrocatalytic reduction activities, there are oxidation, dissolution, and leaching problems in catalysts, and these lead to the decay of electrochemical activity and could cause adverse effects on the environment. In addition, structural evolution during electrocatalysis makes it more difficult to identify the real active sites and design strategies for protecting these active sites. Various electrochemical in situ spectral analyses and electron microscopic observations can help track the evolution of active sites during electrocatalysis and provide guidance and assistance for catalyst design. Furthermore, the predictive power of machine learning based on the analysis of large experimental data will help to develop the rapid screening of suitable and efficient NO₃⁻RR catalysts.

There are differences between the actual sewage and the experimental test electrolyte. The potential and limitations of electrocatalytic NO₃⁻RR in practical water-remediation systems should be investigated by fully considering the effects of possible ion pollution, organic compound pollution, and solid sediment on NO₃⁻RR in actual sewage and then designing the corresponding solutions. For example, pretreatment devices could be designed to purify the pollution.

It is necessary to design and build a reasonable and efficient reaction system and device. First, in order to form a complete chemical reaction process with NO₃⁻RR, another set of electrocatalytic reactions (such as oxygen evolution) would be required, which would not only increase the complexity and cost of the reaction but also mean that the oxidation process and reaction byproducts might interfere with NO₃⁻ reduction. Therefore, we need to optimize the electrolytic cell. In addition, it is necessary to overcome the influence of mass transfer on scale-up experiments. The design of more efficient flow electrolytic cells is also one of the keys to improving the efficiency of NO₃⁻RR.

The design and evaluation of the whole chain of the NH₃ synthesis process by electrochemical NO₃⁻RR require further work. At present, the low efficiency of NH₃ extraction is an important factor hindering the development of NH₃ production by NO₃⁻RR. Therefore, the overall efficiency and the feasibility of NO₃⁻RR in practical applications still need to be further evaluated. For example, the efficiency and cost of electrocatalytic NO₃⁻RR technology and the Haber–Bosch process, as well as other important parameters related to scalability and commercial feasibility, should be investigated to make a comprehensive technological and economic evaluation.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

Full name	Abbreviation
ammonia	NH3
nitrate	NO3−
nitrate reduction reaction	NO3−RR
nitrogen	N2
nitrogen reduction reaction	NRR
nitrite	NO2−
nitrogen monoxide	NO
nitrous oxide	N2O
dinitrogen dioxide	HN2O2
nitroxyl	HNO
hydroxylamine	H2NOH
nitrosoamide	NONH2
Renewable Energy to Fuels through Utilization of Energy-dense Liquids	REFUEL
Department of Energy	DOE
biocatalyzed by nitrate/nitrite reductase	bio-NRA
hydrogen evolution reaction	HER
adsorbed active hydrogen species	Had
ion chromatography	IC
ion selective electrode	ISE
hydrogen nuclear magnetic resonance spectroscopy	1H NMR
single-atom catalyst	SAC
online differential electrochemical mass spectrometry	DEMS
X-ray photoelectron spectroscopy	XPS
X-ray adsorption spectroscopy	XAS
ultraviolet photoelectron spectroscopy	UPS
polyallylamine	PA

Footnotes

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Figures and Tables

Figure 1. Frost–Ebsworth diagram [19].

Figure 2. Schematic diagram of the mechanism of NO3−RR in aquatic media [13].

Figure 3. Cyclic voltammograms on (a) Pt, (b) Pd, (c) Ir, (d) Ru, and (e) Rh in 0.5 M H2SO4 with (solid line) or without (dashed line) the presence of 0.1 M NaNO3. Inset in (a) is the steady-state NO3− reduction current at different potentials [20]. (f) Cyclic voltammograms at Pt(1 1 1) and Pt(S)[n(111) × 111]] electrodes (n = 14, 10, 7, 6, 5, 4, 3, 2) in 0.1 M 10 mM KNO3 + HClO4. (g) The cyclic voltammograms of the expansion of current density scale of (f) [42].

Figure 4. The adsorption energies of the intermediates (a) [Forumla omitted. See PDF.]; (b) [Forumla omitted. See PDF.] and (c) [Forumla omitted. See PDF.] corresponding to ΔE*H [55]. (d) A two-dimensional activity map for producing NH3, where all the reaction free energies are shown at 0 V vs. RHE [12]. (e) The logarithm of partial current density of NH3 for NORR on different metal electrodes at 0 V vs. RHE [56].

Figure 5. (a) Schematic diagrams of the geometric structures and electronic structures of single atoms, clusters, and nanoparticles [58]. Free energy diagrams of the electrochemical NO3−RR to NO2−/NH3 on (b) Cu-N3 and (c) Cu13 sites [61].

Figure 6. Competing relationship between NRA and HER on (a) Cu (111) and (b) comparison between Cu (111), Cu (110) and Cu (100). (c) Surface structure, (d) d-band centers of Cu (111), Cu (120), and Cu (100), and (e) intermediate adsorption energies on Cu (111), Cu (110), and Cu (100) [44].

Figure 7. SEM images of (a) THH Cu NCs, (b) Cu nanocubes, and (c) Cu octahedrons; (d) FE of NH3 on THH Cu NCs, Cu nanocubes and Cu octahedrons [64].

Figure 8. (a) The three possible NO3−RR pathways on Cu (111) and (b) the activation energy diagram of each step of NO3−RR synthesis of NH3 on Cu (111) [44].

Table 1

Recently reported Cu-based metallic catalysts for NO₃⁻ reduction to NH₃.

CathodeMaterial	FE to NH3	Current Density(mA cm−2)	NH3 ProductionReported	Conditions	Potential(V vs. RHE)	Ref.
Cu foil	30%	~−0.36	3.9 ugNH3 mgcat−1 h−1 (3.9 ugNH3 h−1 cm−2)	10 mM KNO3, 0.1 M KOH	−0.15	[59]
Cu nanoparticle	61%	~−4.65	225.8 ugNH3 mgcat−1 h−1 (225.8 ugNH3 h−1 cm−2)	10 mM KNO3, 0.1 M KOH	−0.15	[59]
Cu nanosheets	99.7%	~−4.92	390.1 ugNH3 mgcat−1 h−1 (390.1 ugNH3 h−1 cm−2)	10 mM KNO3, 0.1 M KOH	−0.15	[59]
Cu SAC	84.7%	~−75	0.26 mmol cm−2 h−1 (12.5 mol gCu−1 h−1)	0.1 M KNO3, 0.1 M KOH	−1.00	[61]
THH Cu NCs	98.3%	~−70	-	0.1 M K2SO4, 50.1 M KNO3	−0.9	[64]
Cu-NBs-100	95.3%	~−300	650 mmol gcat−1 h−1	0.1 M KNO3, 1 M KOH	−0.15	[65]
dr-Cu-NPs	81.99%	~−30	781.25 μg h−1 mg−1	0.5 M K2SO4, 50 ppm KNO3	−0.646	[66]
Cu-incorporated PTCDA	77%	-	0.0256 mmol h−1 cm−2	0.1 mM PBS, 36 mM NO3−	−0.4	[76]
Cu/Cu2O NWAs	95.8%	-	0.2449 mmol h−1 cm−2	0.5 M Na2SO4, 14.3 mM NO3−	−0.196	[75]
Cu crystallite	97%	~−6.4	-	0.1 M NaNO3, 1 M NaOH	−0.376	[77]
FOSP-Cu	93.9%	-	101.4 μmol h−1 cm−2	0.1 M KNO3, 0.5 M Na2SO4	−0.266	[78]
OD-Cu	92%	-	1.1 mmol cm−2 h−1	0.1 M KNO3, 1 M KOH	−0.15	[79]
BCN-Cu	98.2%	~−40	3358.74 μg h−1 cm−2	0.1 M KNO3, 1 M KOH	−0.6	[80]
Cu2O(100)	82.5%	-	743 μg h−1 mgcat−1	0.1 M NaSO4, 50 ppm NaNO3	−0.6	[81]

Table 2

Recently reported Cu-based bimetallic catalysts for NO₃⁻ reduction to NH₃.

CathodeMaterial	FE to NH3	Current Density(mA cm−2)	NH3 ProductionReported	Conditions	Potential	Ref.
Cu50Ni50	99%	~−90	-	100 mM NO3−, 1 M KOH	−0.1V vs. RHE	[82]
Cu70Ni30 porous	95.9%	~−400	-	0.1 M NaNO3, 1 M NaOH	−1.20 V vs. Hg/HgO	[93]
Cu70Zn30 porous	97%	~−400	-	0.1 M NaNO3, 1 M NaOH	−1.40 V vs. Hg/HgO	[94]
Au1Cu (111)	98.7%		555 μg h−1 cm−2	0.1 M KOH, 7.14 mM NO3−	−0.2 V vs. RHE	[95]
Cu3Pd1	90.02%		784.37 mg h−1 mgcat−1	0.5 M K2SO4, 50 ppm KNO3-N	−0.46 V vs. RHE	[96]
PdCu-Cu2O	94.32%		0.190 mmol h−1 cm−2	0.5 M Na2SO4, 100 ppm NO3−-N	−0.80 V vs. RHE	[97]
PA-RhCu cNCs	93.7%	-	2.40 mg h−1 mgcat−1	0.1 M HClO4, 0.05 M KNO3	+0.05 V vs. RHE	[83]
Rh@Cu	93.0%	−162	1.27 mmol h−1 cm−2	100 mM NO3−, 0.1 M Na2SO4	−0.2 V vs. RHE	[91]
CuCoSP	93.3%	−300	1.17 mol cm−2 h−1	100 mM NO3−, 0.1 M KOH	−0.175 V vs. RHE	[92]
CuCo nanosheet	100%	−1035	960 mmol h−1 mgcat−1	100 mM NO3−, 0.1 M KOH	0.4 V vs. RHE	[98]

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