Material strategies in the electrochemical nitrate reduction reaction to ammonia production

Wonsang Jung ^ab and Yun Jeong Hwang *^cd
aClean Energy Research Center, Korea Institute of Science and Technology, 5, Hwarang-ro 14-gil, Seongbuk-gu, Seoul, 02792, Republic of Korea
^bDivision of Energy and Environmental Technology, KIST School, Korea University of Science and Technology, Hwarang-ro 14 gil 5, Seongbuk-gu, Seoul, 02792, Republic of Korea
^cDepartment of Chemistry, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul, 08826, Republic of Korea. E-mail: yjhwang1@snu.ac.kr
^dCenter for Nanoparticle Research, Institute for Basic Science (IBS), Seoul, 08826, Republic of Korea

First published on 2nd August 2021

---

---

1. Introduction

Nitrogen is an important element found in living organisms and participates in the nitrogen cycle (N-cycle), which involves nitrogen fixation,¹ nitrification,² and denitrification,³ to balance the production of nitrogen-containing chemicals. The Haber–Bosch process was invented as an artificial nitrogen fixation method to compensate for the insufficient natural nitrogen fixation, and, consequently, humans have access to more useful forms of nitrogen-containing chemicals.⁴ Specifically, 48% of the world's population was estimated to rely on synthetic nitrogenous fertilizers for food production in 2015.⁵ However, artificial nitrogen fixation severely perturbs the natural N-cycle. The main nitrogen sources (N-sources) are fertilizer,⁶ ammonium nitrate (NH₄NO₃), and exhaust from vehicles. Synthetic nitrogenous fertilizers are scattered on the Earth, permeate into the surrounding environment, and cause the accumulation of toxic nitrogen oxyanions in groundwater as well as extensive eutrophication of hydroecological systems.⁷ In addition, vehicles contribute to ∼50% of the total nitrogen oxides (NO_x) emitted into the Earth's atmosphere; once dissolved and decomposed in water, these NO_x species form nitric acid (HNO₃) or nitrous acid (HNO₂).⁸ Thus, nitrate (NO₃⁻) is the prevailing nitrogen oxyanion found in the environment and is in fact the third largest (after dinitrogen and organic nitrogen) freely accessible global N-source.⁹

The unilateral emission of nitrogen by anthropological activity exceeds the natural denitrification capability, thereby causing excess NO₃⁻ to overflow into the N-cycle, which threatens our water sources and causes severe health problems. For instance, NO₃⁻ intake can be fatal to infants due to methemoglobinemia, which involves the oxidation of hemoglobin (Hb) to methemoglobin (metHb), and NO₃⁻ can be reduced to nitrite (NO₂⁻) in the human body, which can act as an oxidant. N-Nitroso compounds, which are carcinogenic, can be formed in the stomach upon the reaction of NO₂⁻ with nitrosatable compounds.¹⁰ Therefore, reverse osmosis,¹¹ ion exchange membranes,¹² electrodialysis reversal,¹³ and electro-capacitive ion capture¹⁴ have been suggested as alternative denitrification processes to remove NO₃⁻. Although they are effective in filtering NO₃⁻ from wastewater or freshwater, the sequestrated NO₃⁻ is eventually released.

In this regard, the electrochemical NO₃⁻ reduction reaction (NO₃RR) used for the production of valuable chemicals is a promising process but is yet to be fully understood. Ammonia (NH₃) is the most desirable product considering that the NO₃RR aims to provide industrially useful chemicals and complete the N-cycle. NH₃ production from the Haber–Bosch process consumes over 1% of global energy supplies and produces >1% of all carbon dioxide (CO₂) emissions.¹⁵ In addition, NH₃ has been proposed as a future liquefied fuel¹⁶ for NH₃ fuel cells and as a carrier for hydrogen storage¹⁷ and transportation owing to its high gravimetric and volumetric hydrogen storage capacity (17.7 wt% and 108 g L⁻¹ at 20 °C and 857 kPa, respectively).¹⁸

In the NO₃RR, NO₃⁻ can be reduced to various chemicals such as nitrite¹⁹ (NO₂⁻), nitric oxide²⁰ (NO), nitrous oxide²¹ (N₂O), dinitrogen^22,23 (N₂), hydrazine²⁴ (N₂H₄), hydroxylamine^25,26 (NH₂OH), and ammonia^19,27–29 (NH₃), depending on the oxidation state of nitrogen produced. The electrochemical reaction can be monitored by measuring the current density and its product can be tuned even at room temperature under ambient pressure conditions using the applied potential, electrolyte, and type of electrocatalyst used. In addition, the recent developments of renewable energy resources, such as wind and solar energy, have provided opportunities to develop sustainable chemical production when integrated with electrocatalytic systems. Thus, the electochemical NO₃RR has recently gained significant attention as an environmentally friendly process to convert NO₃⁻ into NH₃, while avoiding the generation of detrimental chemicals. However, the NO₃RR has mainly focused on the removal of NO₃⁻ for environmental water treatment processes^30–32 and less attention has been paid to product analysis. To date, copper (Cu) is one of the most promising catalyst materials, which exhibits high selectivity toward NH₃ and faradaic efficiencies (FEs) >90%. However, few catalytic materials have been reported to date. The systematic trends and strategy analysis of the NO₃RR used to produce NH₃ are still challenging with a variety of catalyst systems, but recently some successful electrocatalysts have been reported. Thus, it is desirable to review the NO₃RR to produce NH₃, and fundamental mechanism and material studies are essential for the design of high-performance electrocatalysts.

In this review, the recent advances in the electrochemical NO₃RR to NH₃ have been summarized in terms of material design. The fundamentals of the reaction mechanism provide information on the key intermediates, and we can find potential opportunities to improve the activity and selectivity of the heterogeneous electrocatalyst surface by increasing the binding energy of these intermediates. Recent progress in the NO₃RR used to produce NH₃ has demonstrated several successful strategies to tune the activity toward NH₃ production at a decreased overpotential using nanostructuring, alloying, mimicking bio-systems, or controlling the oxygen species present on the catalyst surface. Organizing the origin of the NO₃RR enhancement based on mechanistic understanding can provide the direction of future research. Furthermore, we propose that the NO₃RR and its intermediates can be applied to new types of electrochemical reactions to form carbon–nitrogen (C–N) bond containing products, such as urea or amides. We hope that this review will be helpful for understanding the NO₃RR and its current applications.

2. Reaction mechanism for the electrocatalytic nitrate reduction reaction to ammonia

The NO₃RR to form NH₃ involves the transfer of eight electrons and proceeds via consecutive deoxygenation and hydrogenation steps with the production of OH⁻ in an aqueous electrolyte, as shown in eqn (1).

NO3− + 6H2O + 8e− → NH3 + 9OH−	(1)

The proposed overall reaction pathway is shown in Scheme 1. The electrochemical NO₃RR used to produce NH₃ can be divided into two main steps. One is deoxygenation (*NO₃⁻ → *NO) and the other is hydrogenation (*NO → *NH₃) in the presence of a proton source (H⁺ or H₂O) in the electrolyte. The first deoxygenation involves the reduction of NO₃⁻ to form NO₂⁻ (eqn (2)).

NO3− + H2O + 2e− → NO2− + 2OH−	(2)

According to the Tafel slope reported in a previous study, the first electron transfer step can be considered as the rate-determining step (RDS) of the NO₃RR.³³ Specifically, the reduction of NO₃⁻ on the Cu surface in sulfuric acid has been observed to have a Tafel slope of 130 mV dec⁻¹, which is close to 128 mV dec⁻¹.^34,35 In addition, radiolysis and photoemission studies have shown that the hydrated electrons and NO₃⁻ can form NO₃²⁻.^36–39 This is consistent with the Tafel plot, which shows that the potential of the NO₃⁻-to-NO₃²⁻ reaction via electron transfer has a negative value. Electron transfer into the high-energy state of the lowest unoccupied molecular orbital (LUMO) of NO₃⁻ leads to a high-energy barrier.⁴⁰

NO3− + e− → NO32− (E0 = −0.89 ± 0.02 V (vs. SHE))	(3)

The as-produced NO₃²⁻ can be immediately deoxygenated to form NO₂ because NO₃²⁻ is a highly oxidizing species.^36,41 Subsequently, an electron is transferred to the adsorbed *NO₂ species.

NO32− + H2O → *NO2 + 2OH− (k = 5.5 × 104 s−1)	(4)

*NO2 + e− → NO2− (E = 1.04 (vs. SHE))	(5)

Likewise, electron transfer to the adsorbed *NO₂⁻ species results in the formation of adsorbed NO₂²⁻, which is deoxygenated to give adsorbed *NO.⁴²

*NO2− + e− → *NO22− (E = −0.47 (vs. SHE))	(6)

*NO22− + H2O → *NO + 2OH− (k = 1.0 × 105 s−1)	(7)

Further reduction of the adsorbed *NO species branches into three pathways (Scheme 1): (1) the hydrogenation of the N atom and deoxygenation of the coordinated oxygen atom (*HNOH → *NH₂OH),²⁷ (2) deoxygenation and sequential hydrogenation (*NOH → *N),^43–45 and (3) hydrogenation of the N atom and sequential deoxygenation of the coordinated oxygen atom (*H₂NO → *H₂N).⁴⁶ In particular, the coupling of two NH species and hydrogenation to give NH₃ has been speculated as an alternative to pathway (2) because the stretching vibrations of NN and N–H species were observed on the Rh surface during the electrochemical NO₃RR. Furthermore, coupled intermediates such as N₂H⁺ (m/z = 29) and N₂H₂⁺ (m/z = 30) have been observed using differential electrochemical mass spectroscopy.⁴⁷

One or multiple pathways can coexist depending on the electrocatalytic materials, electrolyte, and applied potential, which can affect the adsorption energy of the key intermediates and the energy barrier for each step. Because the electrochemical conversion of the NO₃RR to form NH₃ is composed of multiple and complex steps, the reaction mechanism is still elusive and other plausible mechanisms and species cannot be excluded. Therefore, further advanced spectroscopic studies performed in situ and under operando conditions are still required to verify the proposed reaction mechanism, in addition to theoretical studies.

3. Electrocatalytic nitrate reduction product analysis methods

In the NO₃RR, NO₃⁻ can be reduced to various chemicals^21,25via multiple deoxygenation and hydrogenation steps, and the determination of NO₃⁻, NH₃, and NO₂⁻ is important as they are the reactant, target product, and main intermediate, respectively, which can be used to evaluate the NH₃ yield rate, conversion rate, FE, and overall NO₃RR performance. In particular, the quantification of NO₂⁻ is an indicator of how effectively the electrocatalyst reduces NO₂⁻ toward further reduced products without their release as free ions into the electrolyte, which is explained in detail later in this review. Because of the acid/base equilibrium, both free ammonia (NH₃) and ionized ammonia (NH₄⁺) are present and the proportion of NH₃/NH₄⁺ is governed by the pH and temperature. NH₄⁺ is the dominant species at pH conditions below the equilibrium point (pK_a = 9.26) and acidic conditions are used to quantify its ionized form in the electrolyte. The concentrations of NH₄⁺, NO₃⁻, and NO₂⁻ can be quantified using a colorimetric method¹⁹ utilizing ultraviolet-visible (UV-vis) spectrophotometry and the Beer–Lambert law (A = εCl, where A, ε, C, and l are the absorbance, absorption coefficient, concentration of chemical species, and distance through which the beam travels, respectively), which is the most popular and convenient method reported to date.

To quantify the amount of NH₃ formed in the reaction, several colorimetric methods depending on the reagent have been reported, which include the indophenol blue (IB) and Nessler's reagent (NR) methods. For the IB method, Berthelot's reagent consisting of phenol (C₆H₆O) and hypochlorite (ClO⁻) is used. Once NH₃ is added, the reagents are converted into indophenol in the presence of a nitroprusside catalyst ([Fe(CN)₅NO]²⁻) (Scheme 2).

NR is composed of K₂HgI₄ and KOH. The tetraiodomercurate anion ([HgI₄]²⁻) reacts with NH₃ in an alkaline solution, leading to the formation of a red-brown complex (eqn (8)).

2[HgI4]2− + NH3 + 3OH− → Hg2ONH2I + 7I− + 2H2O	(8)

The concentration of NH₄⁺ is determined by reading the absorbance at 655 nm for IB and 420 nm for NR.⁴⁸ The concentration–absorbance curve is calibrated using standard solutions (e.g., NH₄Cl) prepared at various concentrations of NH₄⁺ because the absorbance is proportional to the NH₃ concentration. The NH₃ concentration can be derived by interpolating the measured absorbance of the analyte into the calibration curve obtained from these standard solutions (Fig. 1a). NR has been found to be the best reagent when considering the linearity of the calibration curve observed over a wide concentration range; the calibration curve deviates from that obtained using NR and ion chromatography (IC) at concentrations >500 μg L⁻¹ when using IB (Fig. 1b). Therefore, when considering the NH₃ concentration, the analyte can be diluted to be in the range of 0–2000 μg L⁻¹ by adding the electrolyte because NR and IB have reliable linearity (0.9991 and 0.9998, respectively) within this concentration range.⁴⁹

The colorimetric method must be carefully chosen considering the pH of the analyte. Under neutral or alkaline conditions, both methods are able to detect the concentration of NH₃, but only NR is suitable for the detection of NH₃ under acidic conditions. This is because NR is relatively less affected by pH, but IB is severely affected by pH. Specifically, when IB and NR were used with a standard solution (1000 μg L⁻¹) at different pH values (pH = 4–12), the NR method showed a smaller deviation (−11%) than IB (−75.6%) at pH = 4 (Fig. 1c). This is because the ClO⁻ in the IB reagent is unstable under acidic conditions. The NO₃RR can be conducted in various media, such as acidic, neutral, or alkaline electrolytes. Thus, adjusting the pH of the analyte prior to the addition of a colour reagent is necessary, especially for the IB method. NR has high applicability over a wide range of pH, but IB is comparable to NR with appropriate pH adjustment and dilution. In fact, due to the toxicity, short lifetime of the colour reagent, and fluctuation as a function of the reaction time of NR,⁴⁹ IB is more convenient to use. In addition, the solubility of NH₃ in the electrolyte can be determined by the pH and temperature due to the acid–base equilibrium of NH₃ in an aqueous solution. Care has to be taken, especially under alkaline and flow cell conditions, in order not to underestimate the amount of NH₃ produced because the preferred species is free NH₃ under high pH conditions.

NH3(g) ⇌ NH3(aq)	(9)

NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH−(aq)	(10)

To corroborate not only the concentration of NH₃, but also the origin of the nitrogen atom, ¹H nuclear magnetic resonance (NMR) spectroscopy has been carried out using either ¹⁴NO₃⁻ or isotopically labeled ¹⁵NO₃⁻. The ¹H NMR spectra obtained for ¹⁴NH₄⁺ and ¹⁵NH₄⁺ exhibit triplet and doublet peaks at ∼7.15 ppm, respectively (Fig. 1d).²⁷ Similarly, the peak integral corresponding to NH₄⁺ in the ¹H NMR spectrum is proportional to the concentration of NH₄⁺ in the solution. The concentration of the analyte can be determined by comparing the integral area of the standard spectrum using an internal standard. Likewise, IC can also be used to detect and analyze the products in their ionic form, such as NH₄⁺ and NO₂⁻.⁴⁶

NO₃⁻ and NO₂⁻ can be quantified in a similar manner because NO₃⁻ absorbs at a wavelength of 220 nm and consequently no colour reagent is required (Fig. 1e).^50–52 However, HCl can be added to alleviate the interference from hydroxide or carbonate, which may absorb at 220 nm. NO₂⁻ can be determined using Griess’ reagent, which consists of sulfanilamide and N-(1-naphthyl)-ethylenediamine dihydrochloride.^27,53 Specifically, hydrochloric acid (HCl) solution is added to an aliquot of the sample at 0–5 °C to form nitrous acid (HNO₂). The colour reagent is then added. An azo dye is formed via a diazonium coupling reaction in the presence of NO₂⁻, as shown below (Scheme 3), and the absorption intensity at 540 nm was observed for the resulting azo compound (Fig. 1f). Likewise, the obtained concentration–absorbance curve is calibrated using a series of standard NO₃⁻ and NO₂⁻ solutions.

4. Electrocatalyst materials for nitrate to ammonia and design strategies

The NO₃RR on various metals and electrolytes has been reported, and the finally evolved product distribution varies depending on the electrode material and applied potential because the interaction between the intermediates and electrode surfaces affects the ability of deoxygenation of NO₃⁻ to NO₂⁻. Various metals, single atom catalysts, metal oxides, and bio-inspired complex structures have been applied as heterogeneous electrocatalysts, and here we focus on the catalytic activity in terms of NH₃ production.

4.1 Metal electrodes

The NO₃RR was studied with noble electrode materials, which provided fundamental understanding and the overall activity trend although the products were not specifically targeted to NH₃. Many of the early studies focused on the initial interaction with NO₃⁻ and their versatile products. Meanwhile, theoretical studies propose the activity trends and product selectivity of NO₃RR on various metallic surfaces to provide guidelines for catalyst design and understanding of the properties. As important descriptors, the adsorption energies of oxygen (ΔE_O) and nitrogen (ΔE_N) are suggested.^54,55 Their relative adsorption energies estimate the surface coverage of key intermediates such as NO₂*, O* and H* and propose the major product among NO, NH₃, N₂ and N₂O (Fig. 2a). For instance, N₂ is proposed as the major evolved product for Fe or Co, which have high ΔE_O and ΔE_N, while NO is the major product for Ag or Co, which have low ΔE_O and ΔE_N. Optimal ΔE_O and ΔE_N are proposed for selective NH₃ production (Fig. 2b).

In addition to the metal type, whose electronic structure determines the adsorption energies, the value of the applied potential and pH of the electrolyte can be critical factors to determine the product selectivity for a competitive NO₃RR over the hydrogen evolution reaction (HER). The applied potential can change the thermodynamics of the reaction pathways and vary the surface coverages of the adsorbed hydrogen and other intermediates of the NO₃RR. At negatively applied potentials (<0 V (vs. RHE)), the surface coverage of *H is dominant on various transition metals (Co, Cu, Rh, Pd, Pt, and Fe) (Fig. 2a). At −0.2 V (vs. RHE), moderate adsorption energies of ΔE_O and ΔE_N are expected to have a preference for NH₃ production from the NO₃RR, and Rh is expected to have high selectivity for NH₃ production (Fig. 2b) compared to other noble metals such as Pt and Pd. However, its activity for NH₃ production decreases and N₂ is rather preferred as the applied potential shifts toward 0–0.4 V (vs. RHE) because moderate hydrogen surface coverage is required to facilitate *NH_x hydrogenation. It shows consistency with the previous experimental results that the NO₃RR activity decreased in the order Rh > Ru > Ir > Pt ≈ Pd in sulfuric acid^34,56,57 (Fig. 2).

High coverage of *H can hinder the adsorption of NO₃⁻ or its reduced intermediates on the active sites leading to a low FE toward the NO₃RR. Therefore, noble metal surfaces are not desirable candidates for an efficient NO₃RR to NH₃ production due to the high adsorption energy of *H and thus efficient HER activity under reduction conditions in aqueous electrolyte. In particular, underpotential deposition of hydrogen (H_UPD) influences significantly noble metals such as Pt, Pd, and Ru. For Pt, H_UPD adsorbed on Pt(110) inhibits the NO₃RR under low potential conditions.⁵⁸ Pd also showed a very low NH₃ FE because the FE of the process from NO₃⁻ to NH₃ was relatively low because of the faster competitive HER and the complicated pathways from NO₃⁻ to NH₃ which produce a number of N species intermediates such as NO_x, N₂H₄, and NH₂OH.⁵⁹ Similarly, bare Ru nanoparticles showed a significant FE decrease of NH₃ and a remarkable increase of the HER from 0 V (vs. RHE).⁴⁶

Pt exhibits low activity (0.01–0.1 mA cm⁻²) for the NO₃RR because of weak adsorption of NO₃⁻ on the Pt surface,^60–63 interfering with adsorbed species such as hydrogen or anions.⁶⁴ Differential electrochemical mass spectrometry (DEMS) and Fourier transform-infrared (FT-IR) spectroscopy were performed to examine the volatile intermediate products and NO₃RR mechanisms on Pt under a limiting potential range of 0.0–0.4 V (vs. RHE), but there is a lack of information on the liquid product quantification. No gas or hydroxylamine was observed on the Pt catalyst during product identification, and it was indirectly speculated that NH₃ was generated from the NO₃RR.^34,65,66 It is expected that strongly adsorbed NO would not desorb from the surface. The examined potential range⁶⁰ (0.0–0.4 V (vs. RHE)) might not be ideal for performance evaluation and NH₃ generation according to simulation studies, and more negative potentials can be investigated, but not too negative to induce a severe HER. Therefore, Pt, an active HER catalyst, has low selectivity for NH₃ production from the NO₃RR. Meanwhile, to tune the activity for the NO₃RR, the surface of Pt was modified with adatoms such as Rh, Cu, Sn, Ge or alloys and a synergistic effect was proposed that the exotic atoms first convert NO₃⁻ to NO₂⁻, and the generated NO₂⁻ is mainly reduced to N₂ or NH₃OH on the Pt electrode.^{61,62,67–70}

On the other hand, recently, an iridium (Ir) electrode⁷¹ was demonstrated to achieve high selectivity for NH₃ production with a one-dimensional nanotube (NT) morphology (Fig. 3a). The Ir NT showed higher NH₃ production activity than Ir nanocrystals, and high surface areas, high conductivity, and optimal atom utilization efficiency are proposed as the advantages of the 1D porous structure for high electrocatalytic activity. However, the origin of the high selectivity for NH₃ requires more study, and the relation between the nanotube morphology and NH₃ selectivity is still obscure regarding whether this is an intrinsic or extrinsic factor. In electrolysis with NO₃⁻, an FE of 84.7% was achieved for NH₃ production but a low current density (<3 mA cm⁻²) at 0.06 V (vs. RHE). The mentioned reactivity trends between the competitive NO₃RR and HER were still observed on the Ir NT electrode in the range of −0.02 to 0.1 V (vs. RHE). When the potential was less than 0 V (vs. RHE), the FE of NH₃ and yield rate abruptly decreased to 25% and 600 μg h⁻¹ mg_cat⁻¹, respectively (Fig. 3b), although the current density increased due to the adsorption of *H, which reduces the coverage of NO₃⁻.

To determine the catalytic potential of noble metals for the NO₃RR to NH₃, more detail quantification of the products should be performed since the previous reports focused on understanding the reaction mechanism. NH₃ quantification with UV-vis, NMR, or IC and efforts to increase the production rate can evaluate precise NO₃RR to NH₃ activities on various noble metal surfaces. Electrocatalytic materials with high NO₃RR activity and moderate H coverage such as bare or designed Cu-, Ti-, Co-, and Ni-based materials and transition metal oxides are reviewed in the following chapter. In particular, design principles applied to Cu-based nanostructured materials are reviewed in detail later since Cu has better intrinsic activity of NO₃⁻-to-NO₂⁻ conversion and sequential NO₂⁻ reduction (small onset potential difference of the NO₃RR and NO₂RR; Fig. 3c) and can manage the parasitic hydrogen evolution reaction better than others such as Ag, Au, Ni, Zn, Al, Sn, Bi and Pb.^{22,51,72–76}

4.2 Facet dependency

The surface of the catalyst is one of the most important parameters because its atomic arrangement determines the physical and chemical properties, such as the surface energy, Fermi energy level,⁷⁷ and bonding geometry (atomic distance and angle) of each intermediate.^78,79 Thus, it is well known that the electrochemical reaction step can vary depending on the facets of the metal catalysts. In particular, when various reactants and electrochemical reaction pathways compete with each other on the catalyst surface, the facets become an important factor in steering the major reaction pathway because of their different preferences for the binding energy of the intermediates, implying that the selectivity and activity can be tuned.^80,81 Here, we focused the discussion on facet dependency with Cu-based catalysts first because it is one of the most active metal electrodes used for the NO₃RR to NH₃ production.

Kang et al.⁸² compared the electrochemical catalytic performance of a series of Cu surfaces by preparing specially designed nanocatalysts in addition to the conventional Cu foil catalyst. Cu nanosheets (CuNSs) covered by {111} basal planes, Cu nanocubes (CuNCs) primarily enclosed by {100}, and irregular Cu nanoparticles (CuNPs) with no preferential facets were prepared. A remarkably high NH₃ production selectivity (99.7% FE; Fig. 4a) at a low bias potential (−0.15 V (vs. RHE)) was achieved using the CuNSs when compared to the other Cu surfaces studied. The CuNSs exhibit four distinguishable reduction peaks in linear sweep voltammetry (LSV) using a 0.1 M KOH + 10 mM KNO₃ electrolyte (Fig. 4b), which can be attributed to the different steps shown below.^83–85

S1: NO3− + H2O + 2e− → NO2− + 2OH−	(11)

S2: NO2− + 4H2O + 4e− → NH2OH + 5OH−	(12)

S3: NO2− + 5H2O + 6e− → NH3 + 7OH−	(13)

S4: Competing adsorption (with Had) of the intermediate N-species

The CuNSs have the highest S1 current density among all the Cu catalysts studied and the high rate of NO₂⁻ generation (S1) and low energy barrier of the NO₂⁻-to-NH₃ reaction (S3) were proposed as the origin of the highest NO₃RR performance observed for the {111} surface of the CuNSs.

Similarly, Gewirth et al.⁸⁶ reported higher NO₃RR activity on the Cu(111) surface when compared to Cu(100) in an acid electrolyte because of the more facile formation of surface oxides. Cyclic voltammetry (CV) performed in 0.1 M HClO₄ + 1 mM HNO₃ showed that the Cu(111) surface exhibits different activity in nitric acid solution when compared to Cu(100) (Fig. 4c). The most distinctive point was that Cu(111) shows a valid current density from −0.0 to −0.4 V (vs. Ag/AgCl), while Cu(100) showed almost none. In addition, Cu(111) exhibits the main reduction current at a lower cathodic potential (−0.53 V (vs. Ag/AgCl)) when compared to Cu(100) (−0.58 V (vs. Ag/AgCl)). The negatively shifted onset of the NO₃RR activity indicates that the electron transfer was delayed or required a higher overpotential on the Cu(100) surface relative to Cu(111). Interestingly, in situ electrochemical scanning tunnelling microscopy (EC-STM) analysis revealed that surface oxidation was formed on the Cu(111) surface in the potential window where the NO₃RR occurred. The presence of this oxide adlayer implies that the incorporation of oxygen on Cu(111) but not on Cu(100) can be correlated with the higher deoxygenation activity of NO₃⁻ on Cu(111).

In their subsequent study, they proposed that the (100), (111), and (110) facets of the Cu surfaces have similar intermediates and the same mechanism, and a critical pathway on the bare Cu surface is the reduction of NO₃⁻ to NO₂⁻, a concomitant reaction for the partial oxidation of Cu. Nevertheless, the NO₃RR on Cu(100) occurred at more negative potentials (Fig. 4d). To elucidate the origin of the different reactivity of the facets, the rate of Cu₂O formation was considered because the formation of the Cu₂O active surface was experimentally observed using shell isolated nanoparticle enhanced Raman spectroscopy (SHINERS).⁸⁷ In the SHINERS analysis, no noticeable Cu₂O formation was observed on Cu(100), while strong oxidation on Cu(110) with Cu₂O vibrations was observed at 509 and 619 cm⁻¹. Although the spectroscopic confirmation of oxide formation on the Cu(111) surface was not conclusive, based on the fact that the formation of a uniform Cu₂O layer on Cu(111) was more thermodynamically favourable compared to Cu(100) and Cu(110) and the presence of Cu₂O on Cu(111) observed using CV and EC-STM studies,^86,88,89 the following mechanism is facilitated on Cu(111). An oxygen atom from NO₃⁻ is transferred to the Cu surface, resulting in its reduction to NO₂⁻ at potentials >−0.2 V (vs. Ag/AgCl).

NO3− + Cu(111) → NO2− + Cu2O(111)	(14)

NO3− + Cu(110) → NO2− + Cu2O(110)	(15)

NO3−+ Cu(100) → NR	(16)

It can be concluded that NO₃⁻ is deoxygenated and the oxygen atom is incorporated in either the Cu surface or subsurface, forming Cu₂O. The different susceptibilities of the surfaces to oxidation distinguish their activity toward NO₃⁻-to-NO₂⁻ conversion.

Meanwhile, platonic solid shape controlled nanoparticles (cubes, cuboctahedrons, octahedrons, dodecahedrons, and isosahedrons) can be synthesized through wet chemical synthesis, and the type and ratio of the exposed facets can be determined by controlling the shape. Hatzell et al.⁵⁹ synthesized cube, cuboctahedron, octahedron, and concave shape Pd nanoparticles and studied their electrocatalytic performance for the NO₃RR. In Ar-saturated 0.1 M NaOH and 20 mM NO₃⁻, the Pd cube with six {100} facets shows no electrocatalytic activity for NO₃⁻ reduction to NO₂⁻ (Fig. 4e). Meanwhile, the cuboctahedron enclosed by six {100} and eight {111} facets showed the highest NO₃RR current density (4.14 mA cm⁻²) at −0.2 V (vs. RHE) out of the other nanoparticles and an NH₃ concentration increase (Fig. 4e and g). The Pd octahedron particles enclosed by eight {111} facets reduce NO₃⁻ to NO₂⁻ but barely convert NO₂⁻ to NH₃ (Fig. 4h). It can be inferred that the {100} facet can reduce NO₂⁻ to NH₃ effectively but not reduce NO₃⁻ to NO₂⁻. Therefore, the {100} (NO₂⁻-to-NH₃) and {111} (NO₃⁻-to-NO₂⁻) facets in the cuboctahedron complementarily reduce the chemical species which the other cannot activate.

The NO₃RR has a series of multiple steps toward NH₃ production, and each step can have different active sites. In other words, multiple active sites can be present. Therefore, the intrinsic activity of both the NO₃⁻ and NO₂⁻ reduction reaction should be considered to understand the overall facet dependence of metal catalysts. In the case of Cu, whose surface can be easily oxidized/reduced during electrochemical reactions, the oxygen susceptibility can be an additional factor affecting the NH₃ production activity. These studies provide the insight that a combination of diverse surface states can be promising for the NO₃RR, and doping and alloying can introduce variety in the surface activity.

4.3 Doping and alloys

For catalyst applications, the nanostructuring of solid-state materials is a widely applied strategy used to increase the number of active sites on the catalyst surface because nanomaterials exhibit a high surface-to-volume ratio and the newly exposed surface active sites can exhibit different catalytic activities. In addition, the surface of the nanocatalyst can be more sensitively affected by the interfacial environment or an exotic element atom. The different chemical potentials across the heterostructure interface or defective structures induce electron transfer and shift the Fermi energy level of the electrocatalyst, which is a critical factor in the catalytic reaction. To stabilize the high surface energy of the modified nanocatalyst, the surface atoms can be reconstructed or relaxed, which can vary the chemical and physical properties of the nanocatalyst. Therefore, nanocatalysts have also been a useful platform for developing catalytic strategies.

Although material strategies for the NO₃RR have been much less reported, we can benchmark the concepts that were previously developed in water electrolysis, fuel cells, or the electrochemical CO₂ reduction reaction (CO₂RR). In particular, it is important to develop a catalyst to control the selectivity of the NO₃RR because the NO₃RR used for the synthesis of NH₃ involves eight-electron transfer processes and there are various intermediate species. It also competes with the HER. The NO₃RR step can be divided into two major reduction steps: deoxygenation (*NO₃⁻ → *NO₂⁻) and hydrogenation (*H₂NO → *NH₂ → *NH₃). The presence of readily available proton sources is critical in the NO₃RR for both the deoxygenation and hydrogenation steps. However, the hydrogen intermediate (*H) adsorbed on the catalyst surface can cause the HER, an undesirable competitive reaction, which decreases the selectivity and energy efficiency of the catalyst. Thus, balanced activity has been challenging in various reduction reactions, such as the CO₂RR and dinitrogen reduction reaction (NRR). Efforts have been made to introduce heterogeneity into the nanocatalyst to control the catalytic activity by increasing the binding energy of the target intermediate. In this chapter, we review the recent progress demonstrating that the adsorption energies of the intermediates are effectively modulated upon the introduction of oxygen into the metallic catalyst or bimetallic alloying.

The interfaces between two different materials manifest an interesting phenomenon. Additional electron/chemical transfer through the interface can lead to variation in the electronic structure,⁹⁰ which can be used as a strategy to modulate the thermodynamic reaction barrier of intermediates in the NO₃RR. Zhang et al.²⁷ prepared Cu/Cu₂O nanowire arrays (NWAs) via the in situ electrochemical reconstruction of CuO NWAs. In addition, when comparing the electrocatalytic activity, the Cu/Cu₂O NWAs showed a notably high FE (95.8%), NO₃⁻ conversion rate (97.0%), NH₃ selectivity (81.2%), and yield rate (0.2449 mmol h⁻¹ cm⁻²) compared to the Cu NWAs (43.9%, 51.9%, 26.3%, and 0.0501 mmol h⁻¹ cm⁻², respectively) (Fig. 5a). The intrinsic activity of an electrocatalyst is closely related to its electronic structure. DFT calculations revealed extra electron density of Cu in the Cu/Cu₂O NWAs. The high electron density of Cu was proposed to stabilize the *NOH intermediate involved in the hydrogenation of *NO and facilitates NH₃ production when compared with that of bare Cu (Fig. 5b). In short, electron transfer from Cu₂O to Cu at the interface of the Cu/Cu₂O NWAs leads to the selective and active production of NH₃ by decreasing the energy barrier for the hydrogenation step in the NO₃RR and selectively suppresses the competitive HER. There is still the potential for other methods, such as alternating physical vapour deposition,⁹¹ simple physical gel mixing and sintering,⁹⁰ and surface decoration, to be utilized for the formation of heterostructures and interfaces.

Rational catalyst design with non-metal element introduction based on acid/base theory in chemistry can manage the HER and the incorporated elements play a key role as active sites by strengthening the physiochemical bonding between the catalyst and NO₃⁻. Qiao et al.⁹² designed an active Ni-based electrocatalyst, a surface boron (B)-rich core–shell nickel boride structure (Ni₃B@NiB_2.74), for the NO₃RR. Ni has strong adsorption energy of intermediates such as *NO₃⁻, *NO₂⁻ and *NH₂, while the HER should be managed for high NH₃ selectivity and efficiency. In particular, introduced B can act as a Lewis acidic site with unoccupied 2p orbitals in the valence shell which accept electrons from donors, thereby interacting with weak Lewis basic NO₃⁻.⁹³ Notably, high B content enhanced the FE and NH₃ yield rate. In particular, Ni₃B@NiB_2.74 achieved a 98.7% FE and 107.1 μmol cm⁻² h⁻¹ NH₃ yield rate at −0.3 V (vs. RHE) (Fig. 5c). The current density of Ni₃B@NiB_2.74 significantly decreased upon addition of SCN⁻, implying that B is the active site, because B sites are poisoned by the high Lewis basicity of SCN⁻ and become inactive for NO₃⁻-to-NH₃ (Fig. 5d).

Introducing an exotic element atom into the crystalline structure can also modify the lattice parameters, leading to strain. Strain applied on an electrocatalyst surface can influence the degree of binding strength and the binding sites for the intermediate species, which consequently determine the catalytic activity as well as the selectivity of the reaction. Strain in crystalline solid-state materials can be induced by a lattice mismatch between the substrate and the deposited material, formation of core/shell materials, or alloying of heteroatoms of different sizes. On strained surfaces, the chemical environment of the reactive species can vary. For example, tensile strain increases the oxygen interstitial concentration on the electrocatalyst surface for the high-temperature oxygen reduction reaction (ORR), which facilitates oxygen exchange on the surface.⁹⁴ In addition, compressive strain on the Pt shell of a AuCu@Pt core–shell structured electrocatalyst used for the ORR exhibits superior activity and stability.⁹⁵ Considering these examples of strain engineering, the application of strain on the electrocatalytic surface can affect the reaction pathway and activity to stabilize a key intermediate in the NO₃RR.

Yu et al.⁴⁶ utilized produced hydrogen radical for an effective NO₃RR by introducing strain on the catalyst surface via oxygen doping. They designed ruthenium (Ru)/oxygen-doped Ru core/shell nanoclusters Ru-ST-X (X = 0.6, 5, and 12% strained Ru, respectively) for the NO₃RR to form NH₃. Oxygen doping triggered tensile strain on the surface Ru and the degree of tensile strain was modulated by tailoring the subsurface oxygen content. The strained nanostructures maintained nearly 100% of their NH₃-evolving selectivity at current densities <120 mA cm⁻² (Fig. 5e), and the NO₃RR partial current density (J_NH3) of Ru-ST-12, the most strained catalyst, was improved 77-fold when compared with that of Ru-ST-0.6 at −0.8 V (vs. RHE) (Fig. 5f). The strain on the Ru surface was proposed to increase the energy barrier of the Heyrovsky step (H_ads + H₃O⁺ + e⁻ → H₂ + H₂O) and thus inhibit the dimerization of hydrogen to form H₂. Consequently, parts of the uncoupled H_ads are released away from the Ru surface to form ˙H in the reaction medium. The ˙H in the vicinity of the surface is involved in the hydrogenation intermediates of the NO₃RR, reducing the kinetic energy barrier (Fig. 5g). This hindered H–H dimerization acts as leverage for the formation of a chemical environment that facilitates the hydrogenation steps. Meanwhile, incorporating hetero-metal elements to form an alloyed structure has been used to modulate the d-band centre of the metallic electrocatalyst because individual metals have unique electronic levels. According to d-band theory, the adsorption energy of a key intermediate can be varied by modulating the d-band centre position of transition metal catalysts.⁷³ The more closely the d-band centre is positioned to the Fermi energy level, the less antibonding states are occupied and the more strongly reactive species are prone to adsorption. Increasing the NO₃⁻ bonding strength on the electrocatalyst surface is imperative for facile initialization of the NO₃RR. The RDS of the NO₃RR is the conversion of NO₃⁻ to NO₂⁻,^61,96 which can be divided into two steps. Specifically, the adsorption of NO₃⁻ on the surface, followed by deoxygenation of NO₃⁻ to form NO₂⁻. In the case of the deoxygenation of NO₃⁻ to form *NO₂, the adsorption of NO₃⁻ is critical for initializing the reaction. Therefore, the formation of bimetallic alloys is a feasible strategy to lower the NO₃⁻ adsorption energy barrier.

Sargent et al.⁴³ demonstrated a significantly enhanced NO₃RR used to produce NH₃ utilizing a CuNi alloy, especially Cu₅₀Ni₅₀, and proposed that shifting the d-band centre toward the Fermi level was the origin of the enhanced performance. In a 1 M KOH + 100 mM KNO₃ (pH = 14) electrolyte, the overpotential at a current density of 100 mA cm⁻² was a much more negative cathodic potential (−0.2 V (vs. RHE)) for Cu, but only 0.05 V (vs. RHE) for Cu₅₀Ni₅₀ (Fig. 5h), and alloying with Ni effectively increased the NH₃ FE and cathodic NH₃ energy efficiency at low concentrations of NO₃⁻ (Fig. 5i). On pure Cu, the first NO₃⁻ adsorption step is the RDS, in which the maximum reaction free energy was 0.40 eV at −0.14 V (vs. SHE). The energy required for NO₃⁻ adsorption decreased upon increasing the Ni content and the energy barrier for *NO₂ was lower with Cu₅₀Ni₅₀ and Cu₃₀Ni₇₀ when compared to bare Cu and Cu₈₀Ni₂₀ (Fig. 5j). This lower energy barrier for *NO₃⁻ and *NO₂ facilitates the conversion of NO₃⁻ into NO₂⁻ and the sequential deoxygenation step, leading to the prompt conversion of *NO₂ into *NO. This study again underscored the importance of the initial adsorption of the active species without the release of the intermediates into the electrolyte.

4.4 Single-atom catalysts

Beyond traditional solid-state transition metal catalysts, down-sizing of the electrocatalytic materials to several nanometers or single atoms can provide advantages, such as improved catalytic activity due to unsaturated coordination,^97,98 enhanced selectivity ascribed to the uniform active sites,^99,100 and superior stability¹⁰¹ because of the strong interactions formed between the single atom and supporting material. In addition, isolated atoms exhibit unique chemical and physical properties that are not manifested in their bulk materials and various single-atom catalysts (SACs) have been applied to other electrocatalytic reduction reactions, such as the ORR and CO₂RR. Prior to carbon supported SACs, single transition metal center molecular complexes have been studied. Various transition metal (Fe, Co, Ni, Cu, and Rh) center molecular catalysts such as protoporphyrin²⁵ and 1,4,8,11-tetraazacyclotetradecane¹⁰² (cyclam) and metallophthalocyanine¹⁰³ (MPc) have been used as electrocatalysts. Among them, protoporphyrin-based electrocatalys showed low selectivity for NH₄⁺ in 0.1 M LiNO₃ at pH 1 over −1.5 to −0.5 V (vs. RHE), and the performance of cyclam-based electrocatalysts is still elusive because of the limited results of product analysis. MPc (M = Cu, Fe, Ni, Co, Mn, Zn) can reduce NO₃⁻ to NH₃ with high yields, and CuPc particularly lowers the overpotential to a large extent compared to the other MPcs in 0.1 M KOH. Strong NO₃⁻ coordination to Cu active sites was suggested by addition of NO₃⁻ to a Cu tetrasulfonated phthalocyanine dianion complex ([CuTSPc]₄⁻) but not for [FeTSPc]₄⁻. Accordingly, the Cu–N₄ moiety in the form of Cu incorporated in a nitrogen-doped carbon support was investigated for the NO₃RR.

A single Cu atom anchored in solid organic molecular solids is proposed to have superior performance when compared to other transition metals because the intrinsic d-orbital electronic configuration of Cu promotes the adsorption of nitrogen oxyanions.⁴⁵ Specifically, NO₃⁻ can strongly bind to Cu (3d¹⁰) in Cu–PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) by donating a bond from the highest occupied molecular orbital (HOMO) of NO₃⁻ into the empty orbitals of Cu, as well as substantial back-bonding from the fully occupied Cu 3d¹⁰ orbital to the LUMO of NO₃⁻. In addition, the hybridization of the orbitals of adsorbed NO₃⁻ and the Cu single atom is stronger than that between NO₃⁻ and Ni or Ti atoms, which is supported by the larger overlap observed in the projected density of states (PDOS) analysis of the 2p orbitals of two O atoms in NO₃⁻ and the d orbitals of the Cu atom (Fig. 6a). Notably, a high selectivity (FE = 85.9%) and high production rate (436 μg h⁻¹ cm⁻¹) in the NO₃RR to form NH₃ have been achieved using Cu incorporated on PTCDA when compared with other elements, including Ag, Bi, Ir, Pt, Co, Fe, and Ni, because of the unique electronic structure of Cu, which is capable of suppressing the competitive hydrogen evolution reaction (HER) and boosting the H–N combination step in the NO₃RR.

In addition, Cu incorporated on a carbon support doped with nitrogen (Cu–N–C) has been investigated as a Cu SAC and compared with bulk Cu.¹⁹ A series of Cu–N–C–T materials (where T is the annealing temperature) has clearly shown that Cu–N–C-800 exhibited superior NO₃RR activity (NH⁴⁺–N proportion = 80%) when compared to bare Cu plate-800 and Cu–N–C-900 with aggregated bulk Cu nanoparticles (Fig. 6b). The Cu–N–C-800 catalyst was characterized, which showed the successful incorporation of single Cu atoms into the carbon matrix and the predominant Cu(I)–N₂ and Cu(II)–N₄ moieties of the Cu–N bonds. The Cu–N₂ and Cu–N₄ sites in Cu–N–C originate from the high adsorption energies of NO₃⁻ and NO₂⁻ compared to the Cu(111) facet (Fig. 6c) and these high adsorption energies lead to the high conversion selectivity of NO₃⁻ toward NH₃via direct eight-electron transfer on Cu–N–C-800. NO₃⁻ adsorbed on Cu–N–C-800 was reduced to NO₂⁻, which was further reduced without the release of NO₂⁻ into the electrolyte. This illustrates that high adsorption of NO₂⁻ plays a crucial role in accelerating the reduction of NO₂⁻ into NH₃.

Other than Cu, an Fe SAC¹⁰⁴ electrocatalyst was recently investigated in 0.50 M KNO₃/0.10 M K_sSO₄ electrolyte showing the NH₃ FE steadily increasing from 40% at −0.50 V (vs. RHE) to ∼75% at −0.66 V (vs. RHE) (Fig. 6d and e). At −0.85 V (vs. RHE), a large NH₃ partial current density and yield rate are obtained of ∼100 mA cm⁻² and ∼20000 μg h⁻¹ mg_cat⁻¹, respectively (Fig. 6f). DFT calculations pointed out that the potential limiting steps were the NO* reduction to HNO* and HNO* reduction to N*, and a smaller limiting potential of the Fe SAC (0.3 V) was proposed than those of the Co SAC (0.42 V) and Ni SAC (0.39 V). The transition metal–N_x moiety is proposed to bind NO₃⁻ better than bulk transition metal NPs via strong hybridization between the p orbital of oxygen and the d orbital of the center transition metal. Besides, Yu et al.¹⁰⁵ evidenced not only better thermodynamic properties for the NO₃RR but also better kinetics of the NO₃RR on SACs using an electrocatalyst with Fe–N₄ moiety active sites (Fe-PPy SACs). To unveil the origin of the high FE and yield rate, surface interrogation scanning electrochemical microscopy (SI-SECM) was utilized to analyze the time-dependent variation of the site density of the single-site Fe moiety. At 0.2 V (vs. RHE), the Fe-PPy SACs reduce NO₃⁻ to NH₃ in the presence of NO₃⁻, but the Fe NPs showed no ln[Fe] variation. Besides, without NO₃⁻ Fe(II)–N_x showed no activity for the HER. This indicates that because the bonding between Fe(II)–N_x moiety and NO₃⁻ is thermodynamically more favourable, nitrate preoccupies Fe(II)–N_x moiety active sites at higher potential. For the kinetics of the NO₃RR, the rate of the NO₃RR with the Fe(0)–N_x moiety is much higher (1.24 s⁻¹) compared to the Fe(0) NPs (0.1 s⁻¹) (Fig. 6g).

In SACs, the surrounding matrix is an effective parameter used to modulate the electronic states of the incorporated single metal atom because the single metal atom chemically interacts with the coordinated elements. Previous studies have reported that Cu single metal catalysts are superior to the SACs of other metals on a carbon support, but the catalytic performance of SACs can vary depending on the supporting matrix materials used as well as the incorporated metal atoms. Guo et al.⁴⁴ showed that titanium (Ti) and zirconium (Zr) supported on graphitic carbon nitride (g-CN) (Ti/g-CN, and Zr/g-CN, respectively) possibly outperform Cu/g-CN in the NO₃RR. When the potential-determining steps (PDS) (*NO + H⁺ + e⁻ → *NOH and *NH₂ + H⁺ + e⁻ → *NH₃) were drawn as a function of the NO₃⁻ adsorption energy, it forms a volcano shape with a much lower limiting potential (UL = −ΔG_max/e) of −0.39 and −0.41 V for Ti/g-CN and Zr/g-CN, respectively. Ti/g-CN and Zr/g-CN are promising catalysts for the NO₃RR (Fig. 6h).

The results appear to be inconsistent with previous Cu SAC studies because Cu SACs are proposed to have higher intrinsic activity than other transition metal single-atom active sites. However, considering that the NO₃RR performance can be differentiated using single-atom metals as not only the active sites but also the supporting material that determines the coordination environment, the optimal single transition metal atom can vary. As these are emerging material candidates in SACs prepared by modifying the transition metal centre as well as the supporting material, this may be a promising approach for the development of efficient catalysts used in the NO₃RR.

4.5 Metal oxides: oxygen vacancies

Metal oxides such as TiO₂ have also been demonstrated to have electrocatalytic activity in the NO₃RR. Oxygen vacancies (OVs) can be formed in metal oxide catalysts using plasma or thermal treatment and they are considered to act as chemisorption sites and facile charge transfer sites to the active metal atoms.^106,107 Zhang et al.¹⁰⁸ synthesized anatase TiO₂ and oxygen vacancy containing TiO_2−x using a thermal treatment process conducted under an H₂ atmosphere and demonstrated that the high numbers of OVs in TiO_2−x exhibited better NO₃RR performance in terms of conversion rate (95.2%), NH₃ selectivity (87.1%), FE (85.0%), and yield rate (0.045 mmol h⁻¹ mg⁻¹) (Fig. 7a). DFT calculations compared the TiO₂(101) model without OVs, with one OV, and with two OVs (denoted as 0, 1, and 2 OVs). *NO₃ fills the OV site and forms another oxygen bond with a neighbouring unsaturated Ti atom. In this atomic configuration, the NO₃⁻ adsorbed on TiO₂ with two OVs has a much stronger adsorption energy (Fig. 7b). In addition, a high OV concentration steers the reduction pathway to direct NO₂⁻ reduction by lowering the energy barrier for the formation of *NO₂, suppressing the release of NO₂⁻ into the electrolyte (Fig. 7c). By introducing OVs on the surface, the Fermi level moves into the conduction band minimum due to its occupation by the excess 3d electrons of Ti, which increases the metallic properties of TiO_2−x and improves the conductivity, which is beneficial for charge transfer in the electrocatalytic reaction.

This defect engineering strategy has been applied to other transition metal oxides such as CuO. Rose et al.¹⁰⁹ prepared CuO nanoparticles (FSP CuO) through flame spray pyrolysis (FSP) and He plasma treatment of as-prepared CuO for 5 (pCuO-5) and 10 (pCuO-10) min, respectively. They controlled the density of defect sites on CuO with the He plasma treatment duration. Defective pCuO-5 and pCuO-10 attained an NH₄⁺ yield of ∼292 μmol cm⁻² h⁻¹ at −0.6 V (vs. RHE), superior to non-defective FSP CuO (Fig. 7d). The OVs in CuO_x not only strengthen the adsorption energy of NO₃⁻ but also inhibit the HER (Fig. 7e). In addition, most intermediates of the NO₃RR are more energetically stabilized on CuO_x with 2 OVs rather than Cu₂O (Fig. 7f).

4.6 Bio-inspired structures

Organisms give us a hint of the elemental combinations and molecular structures that can be the most efficient for catalytic reactions. In electrocatalysts, the catalyst structures have been artificially designed based on inspiration provided by enzymes. For example, Mn₄Ca catalysts¹¹⁰ for oxygen evolution from photosynthesis, [NiFe]-hydrogenase analogues as HER catalysts,¹¹¹ and superoxide dismutase mimetics¹¹² for oxygen reduction in lithium–oxygen batteries. In nature, NO₃⁻ reductase (NRase) exclusively has Mo–S as the catalytic centre and is the most efficient under neutral conditions.^113–115 In contrast, a noble metal catalyst is active under acidic conditions during the NO₃RR. Owing to the planar symmetrical (D_3h) resonant structure of NO₃⁻, it exhibits a low binding affinity for transition metals and the NO₃RR requires harsh conditions, such as a strongly acidic pH, in which the protonated species (HNO₃, pK_a = −1.3) are more reactive. For instance, Pt effectively catalyzes the NO₃RR only under strongly acidic conditions (pH < 1) and shows negligible activity at neutral pH. It has been hypothesized that this distinctive activity of NRase under neutral conditions is induced by the configuration of the mononuclear Mo atom coordinated by oxo and dithiolene sulfur ligands (Fig. 8a). The bio-mimic strategy provides reasons for the use of specific elements and unique structures in electrocatalyst design.

A bio-inspired molybdenum sulfide (oxo-MoS_x) catalyst has been prepared using a hydrothermal method using MoO₄²⁻ and L-cysteine (C₃H₇NO₂S) as precursors, and its superior NO₃RR performance was achieved over a wide range of pH (pH = 3–11).³⁵ An FE of >90% for NH₃ production was obtained at 0 V (vs. RHE) and pH 7. However, the FeS moiety in the as-synthesized greigite (Fe₃S₄) did not show any activity under neutral pH conditions for both the NO₃RR and NO₂⁻ reduction reaction, but a low activity was achieved when 5% Mo was doped into Fe₃S₄ for the NO₂⁻ reduction reaction step.¹¹⁶ This implies that Mo–S plays a key role in the active site. The intermediate valence state change of Mo (Mo^VI → Mo^V) promoted by proton-coupled electron transfer (PCET) was proposed to increase the binding energy of NO₃⁻, a prevailing deprotonated form of NO₃⁻ under neutral pH conditions, and thus led to its high activity in the NO₃RR.

In contrast to the high activity of the oxo-MoS_x catalyst, a crystalline MoS₂ (c-MoS₂) catalyst showed negligible NO₃RR activity, implying that the bio-inspired Mo-S bond structure is crucial in the reaction. For the oxo-MoS_x catalyst, the NH₄⁺ FE exceeded 96% at 0 mV, while a low FE of <5% was observed for c-MoS₂ (Fig. 8b). In addition, oxo-MoS_x exhibited an 18.8-fold higher NO₃⁻ consumption rate than that of c-MoS₂ (Fig. 8c). The RDS of oxo-MoS_x was measured to be 1e⁻/1H⁺via PCET during the conversion of NO₃⁻ into NO₂⁻. Electron paramagnetic resonance (EPR) and Raman spectroscopy showed that Mo_VOS₄ was the specific active species in the NO₃RR, which was only observed in the oxo-MoS_x catalyst (Fig. 8d). For oxo-MoS_x, Mo^VI(O)S₄ was used as a precursor to generate the active Mo^VO species. When NO₃⁻ was added to the electrochemical reduction, the signals corresponding to Mo^V(O)S₄ were diminished, while the signal corresponding to Mo^VI(O)S₄ was regenerated, indicating the interaction between NO₃⁻ and the Mo^VOS₄ species. Specifically, one oxygen atom of NO₃⁻ was proposed to coordinate with oxo-Mo by overlapping the 4d_xy orbital of Mo and p* orbital of NO₃⁻. This molecular arrangement was speculated to be significantly lower in energy when compared to the π* orbital of NO₃⁻. This particular electronic configuration enables facile electron transfer from Mo^V to NO₃⁻. Meanwhile, the Mo^V–S and S radical species in oxo-MoS_x were almost unchanged, implying their inactivity in the NO₃RR. However, Mo^V–S and S species have been suggested to stabilize the Mo^V(O)S₄ active sites in oxo-MoS_xvia electronic interactions.¹¹⁷ Operando spectroscopy revealed the absence of Mo^VI(O)S₄ or Mo^V(O)S₄ in the c-MoS₂ sample, which was consistent with its low activity in the NO₃RR (Fig. 8e). Therefore, we can use a bio-inspired NO₃RR, which also provide insights for the design of new, highly efficient catalysts.

5. Summary and perspectives

Various electrocatalysts for the efficient NO₃RR used to produce NH₃ have been demonstrated by developing material parameters such as facets, strain, heterostructures, bimetallic alloys, vacancies, down-sizing to single atoms and bio-mimic strategies, and the origins of their enhanced activity have been proposed based on the understanding of fundamental aspects, such as the reaction pathway and binding energy of the reaction intermediates (Table 1). Based on previous studies, design principles are proposed for high-performance electrocatalysts of NO₃⁻ reduction to NH₃. First, the susceptibility to oxygen from NO₃⁻ is important for improving the NO₃⁻ reduction rate because the deoxygenation step in the NO₃⁻-to-NO₂⁻ reaction is an RDS and thereby oxide formation distinguishes the electrocatalytic activity of catalysts. In addition, the binding energy of nitrogen and oxygen is proposed to affect the final product. NO₃RR activities are influenced by the atomic arrangement on the surface which determines the binding states of the intermediates during multiple steps. Second, strong NO₃⁻ adsorption is a critical factor because it initializes the NO₃⁻-to-NO₂⁻ reaction and possibly modulates the limiting potential of the sequential reduction reaction. The effectiveness of this principle is realized in electronic state modification strategies, such as the hybridization of orbitals due to the intrinsic properties, Lewis base/acid interaction, d-band centre shift, down-sizing to single atoms, and OVs. Third, stabilizing intermediates such as *NO₂ or *NOH is important toward increasing the conversion efficiency and selectivity. In particular, the weak binding of the *NO₂ intermediate can cause its release into the electrolyte in its solvated anion (NO₂⁻) or gaseous (NO₂) form, which further retards its reduction to NH₃ and decreases the selectivity of the reaction. The strong adsorption of *NO₂ and *NOH steers the reaction pathway toward the formation of intermediates for the production of NH₃ and promotes the sequential hydrogenation steps, respectively. Fourth, suppression of the competitive HER is effective and necessary to increase the FE, but trade-offs should be considered. Single copper atoms and hetero-nanostructured copper electrocatalysts have high selectivity toward the NO₃RR used to produce NH₃ because of the high-energy barrier for hydrogen formation. However, adsorbed hydrogen production can be a favourable chemical environment because a proton source is necessary for the deoxygenation and hydrogenation reactions. By adjusting the degree of oxygen doping in the subsurface, the HER is hindered, thereby providing uncoupled hydrogen radicals for the hydrogenation of the reaction intermediates (*HNO and *NH₂).

---

At this early stage of the development of the electrochemical NO₃RR used to selectively produce NH₃, more studies are required to determine the detailed reaction mechanism occurring on the catalyst surface and thus provide a descriptor for the NO₃RR activity. Based on previous studies, we can design active catalyst materials varying from noble metals, low cost transition metals, single atom catalysts and metal oxides to bio-mimic moieties. Although early studies have been performed to analyse the various products and monitor the intermediate states, insufficient efforts are given to target selective NH₃ production. Also, only limited material combinations or reaction conditions have been studied. Noble metals have to be modified with nanostructuring when their flat surface exhibited overwhelming HER activity or preferred other NO₃RR products under high current density conditions. Recent studies show the potential of using single atom catalysts for the NO₃RR to NH₃ production in addition to cost efficient transition metal catalysts such as Cu or Ni, which can be investigated further with a variety of metal elements or their alloys. The NO₃RR to NH₃ production involves multi-electron and proton transfer, which places a high burden on simulation studies. With the assistance of simulation studies, more systematic studies have to be designed to investigate the effect of the morphological or elemental combination, which can affect the intrinsic activity of catalysts.

Moreover, to suppress the HER and promote a targeted NO₃RR, extrinsic properties such as the porosity of a catalyst, mass transport in the electrolyte or device fabrication can be modified. The mass transfer can be affected by a hierarchical nanostructure, the porosities of the electrode, or the cations/anions in the electrolyte.^71,118–120 Depending on the material morphology, micro mass transfer can change the available chemical distribution on the interface and local pH, and thus the surface coverage of species is varied as well.¹²¹ Specifically, the porosity of an electrocatalyst has an influence on the retention mean time (confinement of chemical species inside the pores) of chemical species before releasing to the bulk electrolyte depending on the porous geometry. This can be reflected in the electrocatalytic performance such as the FE, conversion rate, selectivity and yield rate.^122,123 Therefore, there are some progressing directions in terms of electrode morphology and cell design. Firstly, it is needed to build up mass transport models with reactant species (NO₃⁻, NO₂⁻, NO or OH⁻) on various micro morphologies through computational simulations and controlled electrode fabrication for future efficient NH₃ production. Secondly, the produced NH₃/NH₄⁺ could hinder adsorption of reactant species, and this affects performance of the NO₃RR. To evaluate how competitive NH₃/NH₄⁺ adsorption can interfere with NO₃⁻ or NO₂⁻ adsorption and to exclude the NH₃/NH₄⁺ adsorption in electrocatalyst performance examination, a rotating disk electrode (RDE) under controlled convection can be employed to decrease the produced NH₃/NH₄⁺ concentration on the electrode surface, by releasing them toward bulk electrolyte. Thirdly, cell modelling for verifying geometric and macroscopic parameters at the device level (i.e., electrode dimensions, flow field width or flow rate) was tried and conducted with NO₂⁻ for the electrochemical NO₂RR.¹²⁴ This trial and approach should be extended to NO₃RR study. The electrochemical synthesis of nitrogen-containing chemicals can provide new opportunities for sustainable chemical cycles and further research directions can be diversified, but we have to consider how to access a concentrated source of NO₃⁻ for the scale-up system. In fact, direct electrochemical dinitrogen reduction reactions used for the synthesis of NH₃ have been studied in the field of electrochemistry, but their efficiencies are low. Due to the strong triple bond in N₂ and the low solubility of N₂ in aqueous solutions, the current density and FE are very low. To boost the current density and FE during the synthesis of NH₃, the direct dinitrogen oxidation reaction (NOR) for the production of NO₃⁻ as a counter oxidation reaction rather than the ORR is valuable (Fig. 9a).^125,126 By replacing the irrelevant ORR at the anode, this combined system can constantly provide an N-source for reduction using both gas- and solution-fed systems.

In addition to the production of NH₃, we expect that the NO₃RR or the reduction reaction of its intermediates can be potentially utilized to develop value-added chemical production containing both carbon and nitrogen atoms by combining them with the electrochemical CO₂RR or CO reduction reaction (CORR). Fortunately, Cu has been widely studied as an effective catalyst for both the CO₂RR and NO₃RR, and thus the potential of C–N coupling has been proposed. Under electrochemical reduction conditions, electrochemical carbon–nitrogen (C–N) bonding forming reactions to form products such as acetamide,¹²⁷ CH₃CONH₂, and urea¹²⁸ have been studied starting from small molecules, such as CO₂, CO, NO₃⁻, N₂, and NH₃. During multiple reduction reaction pathways, the intermediate species can be cross-linked to produce C–N coupling chemicals. One of the pioneering studies has been demonstrated using the CORR with an NH₃ co-gas feed to produce acetamide. The *CCO intermediate of the CORR can undergo a C–N coupling reaction with NH₃via nucleophilic attack on the same Cu electrode surface. In addition, other types of amines have been used for the synthesis of amides. In these early stage studies, further consideration of how to target a wider range of products is required and once the application direction is determined the selectivity and efficiency of the C–N bond coupling can be improved.

Relatively active and reduced forms of the reactants (CO and NH₃) are used in this C–N coupling reaction, but the direct coupling of CO₂ and N₂ or CO₂ and NO₃⁻ is also electrochemically possible because CO and NH₃ can be produced via these reduction reactions. For the synthesis of urea, it has been reported that urea can be produced via the co-reduction of CO₂ and NO₂⁻ using electrochemical co-reduction by tellurium (Te)-doped palladium (Pd) nanocrystals (Te–Pd NCs) (Fig. 9b).⁵³ Although this study utilizes NO₂⁻ as a nitrogen precursor, it can possibly be utilized as a nitrogen precursor because NO₂⁻ is the first deoxygenated intermediate formed in the NO₃RR. By combining the electrochemical CO₂RR and NO₃RR, we can produce value-added gas products containing C–N bonds, such as urea and acetamide. This is highly promising because urea is used as a fertilizer, in the production of resin, and in moisturizers, and acetamide is used in pharmaceuticals and pesticides, and as an antioxidant in plastics. In addition, this showed the possibility of removing harmful chemicals, one in the atmosphere and the other in solution. Although the utilization of the NO₃RR is at an early stage, NO₃⁻ can be a worthwhile starting chemical to balance the N-cycle and to produce value-added chemicals. Recent progress has shown that a high conversion rate and selectivity for the production of NH₃ are possible through the development of electrocatalyst materials, which makes the application of the NO₃RR promising.

Author contributions

Both Wonsang Jung and Yun Jeong Hwang wrote the manuscript and perspectives.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2019R1A2C2005521) and by the Research Center Program of the IBS (IBS-R006-D1) in Korea.

Notes and references

1. M. H. Ryu, J. Zhang, T. Toth, D. Khokhani, B. A. Geddes, F. Mus, A. Garcia-Costas, J. W. Peters, P. S. Poole, J. M. Ane and C. A. Voigt, Control of nitrogen fixation in bacteria that associate with cereals, Nat. Microbiol., 2020, 5, 314–330 CrossRef CAS PubMed .
2. K. D. Kits, C. J. Sedlacek, E. V. Lebedeva, P. Han, A. Bulaev, P. Pjevac, A. Daebeler, S. Romano, M. Albertsen, L. Y. Stein, H. Daims and M. Wagner, Kinetic analysis of a complete nitrifier reveals an oligotrophic lifestyle, Nature, 2017, 549, 269–272 CrossRef CAS PubMed .
3. W. G. Zumft, Cell biology and molecular basis of denitrification, Microbiol. Mol. Biol. Rev., 1997, 61, 533–616 CAS .
4. J. W. Erisman, M. A. Sutton, J. Galloway, Z. Klimont and W. Winiwarter, How a century of ammonia synthesis changed the world, Nat. Geosci., 2008, 1, 636–639 CrossRef CAS .
5. Our world in data, https://ourworldindata.org/how-many-people-does-synthetic-fertilizer-feed, accessed Feburary 2021.
6. J. M. McEnaney, S. J. Blair, A. C. Nielander, J. A. Schwalbe, D. M. Koshy, M. Cargnello and T. F. Jaramillo, Electrolyte Engineering for Efficient Electrochemical Nitrate Reduction to Ammonia on a Titanium Electrode, ACS Sustainable Chem. Eng., 2020, 8, 2672–2681 CrossRef CAS .
7. D. He, Y. Li, H. Ooka, Y. K. Go, F. Jin, S. H. Kim and R. Nakamura, Selective Electrocatalytic Reduction of Nitrite to Dinitrogen Based on Decoupled Proton-Electron Transfer, J. Am. Chem. Soc., 2018, 140, 2012–2015 CrossRef CAS PubMed .
8. Technology transfer network U.S.-Mexico border information center on air pollution (CICA), https://www3.epa.gov/ttncatc1/cica/other7_e.html, accessed Feburary 2021.
9. M. M. M. Kuypers, H. K. Marchant and B. Kartal, The microbial nitrogen-cycling network, Nat. Rev. Microbiol., 2018, 16, 263–276 CrossRef CAS PubMed .
10. G. J. A. Speijers, Nitrate and nitrite in drinking-water, background document for development of WHO guidelines for drinking-water quality, World Health Organization Press, Geneva, 2011 Search PubMed .
11. R. Epsztein, O. Nir, O. Lahav and M. Green, Selective nitrate removal from groundwater using a hybrid nanofiltration–reverse osmosis filtration scheme, Chem. Eng. J., 2015, 279, 372–378 CrossRef CAS .
12. A. D. Fonseca, J. G. Crespo, J. S. Almeida and M. A. Reis, Drinking water denitrification using a novel ion-exchange membrane bioreactor, Environ. Sci. Technol., 2000, 34, 1557–1562 CrossRef CAS .
13. A. Elmidaoui, M. A. Menkouchi Sahli, M. Tahaikt, L. Chay, M. Taky, M. Elmghari and M. Hafsi, Selective nitrate removal by coupling electrodialysis and a bioreactor, Desalination, 2002, 153, 389–397 CrossRef .
14. Y. Qu, P. G. Campbell, A. Hemmatifar, J. M. Knipe, C. K. Loeb, J. J. Reidy, M. A. Hubert, M. Stadermann and J. G. Santiago, Charging and Transport Dynamics of a Flow-Through Electrode Capacitive Deionization System, J. Phys. Chem. B, 2018, 122, 240–249 CrossRef CAS PubMed .
15. S. Wood and A. Cowie, A review of greenhouse gas emission factors for fertiliser production, Climate technology center & network, Copenhagen, 2004 Search PubMed .
16. J. Choi, B. H. R. Suryanto, D. Wang, H. L. Du, R. Y. Hodgetts, F. M. Ferrero Vallana, D. R. MacFarlane and A. N. Simonov, Identification and elimination of false positives in electrochemical nitrogen reduction studies, Nat. Commun., 2020, 11, 5546 CrossRef CAS PubMed .
17. K. E. Lamb, M. D. Dolan and D. F. Kennedy, Ammonia for hydrogen storage; A review of catalytic ammonia decomposition and hydrogen separation and purification, Int. J. Hydrogen Energy, 2019, 44, 3580–3593 CrossRef CAS .
18. J. Cha, T. Lee, Y.-J. Lee, H. Jeong, Y. S. Jo, Y. Kim, S. W. Nam, J. Han, K. B. Lee, C. W. Yoon and H. Sohn, Highly monodisperse sub-nanometer and nanometer Ru particles confined in alkali-exchanged zeolite Y for ammonia decomposition, Appl. Catal., B, 2021, 283, 119627 CrossRef CAS .
19. T. Zhu, Q. Chen, P. Liao, W. Duan, S. Liang, Z. Yan and C. Feng, Single-Atom Cu Catalysts for Enhanced Electrocatalytic Nitrate Reduction with Significant Alleviation of Nitrite Production, Small, 2020, 16, e2004526 CrossRef PubMed .
20. M. Tiso and A. N. Schechter, Nitrate reduction to nitrite, nitric oxide and ammonia by gut bacteria under physiological conditions, PLoS One, 2015, 10, e0119712 CrossRef PubMed .
21. T. Yoshioka, K. Iwase, S. Nakanishi, K. Hashimoto and K. Kamiya, Electrocatalytic Reduction of Nitrate to Nitrous Oxide by a Copper-Modified Covalent Triazine Framework, J. Phys. Chem. C, 2016, 120, 15729–15734 CrossRef CAS .
22. I. Katsounaros, D. Ipsakis, C. Polatides and G. Kyriacou, Efficient electrochemical reduction of nitrate to nitrogen on tin cathode at very high cathodic potentials, Electrochim. Acta, 2006, 52, 1329–1338 CrossRef CAS .
23. W. Teng, N. Bai, Y. Liu, Y. Liu, J. Fan and W. X. Zhang, Selective Nitrate Reduction to Dinitrogen by Electrocatalysis on Nanoscale Iron Encapsulated in Mesoporous Carbon, Environ. Sci. Technol., 2018, 52, 230–236 CrossRef CAS PubMed .
24. J. Soto-Hernández, C. R. Santiago-Ramirez, E. Ramirez-Meneses, M. Luna-Trujillo, J.-A. Wang, L. Lartundo-Rojas and A. Manzo-Robledo, Electrochemical reduction of NO_x species at the interface of nanostructured Pd and PdCu catalysts in alkaline conditions, Appl. Catal., B, 2019, 259, 118048 CrossRef .
25. J. Shen, Y. Y. Birdja and M. T. Koper, Electrocatalytic Nitrate Reduction by a Cobalt Protoporphyrin Immobilized on a Pyrolytic Graphite Electrode, Langmuir, 2015, 31, 8495–8501 CrossRef CAS PubMed .
26. D. H. Kim, S. Ringe, H. Kim, S. Kim, B. Kim, G. Bae, H. S. Oh, F. Jaouen, W. Kim, H. Kim and C. H. Choi, Selective electrochemical reduction of nitric oxide to hydroxylamine by atomically dispersed iron catalyst, Nat. Commun., 2021, 12, 1856 CrossRef CAS PubMed .
27. Y. Wang, W. Zhou, R. Jia, Y. Yu and B. Zhang, Unveiling the Activity Origin of a Copper-based Electrocatalyst for Selective Nitrate Reduction to Ammonia, Angew. Chem., Int. Ed., 2020, 59, 5350–5354 CrossRef CAS PubMed .
28. X. Deng, Y. Yang, L. Wang, X. Z. Fu and J. L. Luo, Metallic Co Nanoarray Catalyzes Selective NH₃ Production from Electrochemical Nitrate Reduction at Current Densities Exceeding 2 A cm⁻², Adv. Sci., 2021, 8, 2004523 CrossRef CAS PubMed .
29. J. Sun, D. Alam, R. Daiyan, H. Masood, T. Zhang, R. Zhou, P. J. Cullen, E. C. Lovell, A. Jalili and R. Amal, A hybrid plasma electrocatalytic process for sustainable ammonia production, Energy Environ. Sci., 2021, 14, 865–872 RSC .
30. S. Guo, K. Heck, S. Kasiraju, H. Qian, Z. Zhao, L. C. Grabow, J. T. Miller and M. S. Wong, Insights into Nitrate Reduction over Indium-Decorated Palladium Nanoparticle Catalysts, ACS Catal., 2017, 8, 503–515 CrossRef .
31. J. Li, M. Li, X. Yang, S. Wang, Y. Zhang, F. Liu and X. Liu, Sub-nanocatalysis for Efficient Aqueous Nitrate Reduction: Effect of Strong Metal-Support Interaction, ACS Appl. Mater. Interfaces, 2019, 11, 33859–33867 CrossRef CAS PubMed .
32. Y. Lan, J. Chen, H. Zhang, W.-X. Zhang and J. Yang, Fe/Fe₃C nanoparticle-decorated N-doped carbon nanofibers for improving the nitrogen selectivity of electrocatalytic nitrate reduction, J. Mater. Chem. A, 2020, 8, 15853–15863 RSC .
33. Y. Zeng, C. Priest, G. Wang and G. Wu, Restoring the Nitrogen Cycle by Electrochemical Reduction of Nitrate: Progress and Prospects, Small Methods, 2020, 4, 2000672 CrossRef CAS .
34. G. E. Dima, A. C. A. de Vooys and M. T. M. Koper, Electrocatalytic reduction of nitrate at low concentration on coinage and transition-metal electrodes in acid solutions, J. Electroanal. Chem., 2003, 554–555, 15–23 CrossRef CAS .
35. Y. Li, Y. K. Go, H. Ooka, D. He, F. Jin, S. H. Kim and R. Nakamura, Enzyme Mimetic Active Intermediates for Nitrate Reduction in Neutral Aqueous Media, Angew. Chem., Int. Ed., 2020, 59, 9744–9750 CrossRef CAS PubMed .
36. A. R. Cook, N. Dimitrijevic, B. W. Dreyfus, D. Meisel, L. A. Curtiss and D. M. Camaioni, Reducing radicals in nitrate solution. The NO₃²⁻ system revisited, J. Phys. Chem. A, 2001, 105, 3658–3666 CrossRef CAS .
37. Y. V. Pleskov, Z. A. Rotenberg, V. V. Eletsky and V. I. Lakomov, Investigation of intermediates by electron photoemission from metal into electrolyte solution, Faraday Discuss. Chem. Soc., 1973, 56, 52–61 RSC .
38. G. C. Barker, P. Fowles and B. Stringer, Pulse radiolytic induced transient electrical conductance in liquid solutions Part 2.-Radiolysis of aqueous solutions of NO₃⁻, NO₂⁻ and Fe(CN)₆³⁻, Trans. Faraday Soc., 1970, 66, 1509–1519 RSC .
39. M. C. Gonzalez and A. M. Braun, VUV photolysis of aqueous solution of nitrate and nitrite, Res. Chem. Intermed., 1995, 21, 837–859 CrossRef CAS .
40. M. C. P. M. da Cunha, M. Weber and F. C. Nart, On the adsorption and reduction of NO₃⁻ ions at Au and Pt electrodes studied by in situ FTIR spectroscopy, J. Electroanal. Chem., 1996, 414, 163–170 Search PubMed .
41. H. O. Tugaoen, S. Garcia-Segura, K. Hristovski and P. Westerhoff, Challenges in photocatalytic reduction of nitrate as a water treatment technology, Sci. Total Environ., 2017, 599–600, 1524–1551 CrossRef PubMed .
42. Z. Zhang, W. Shi, W. Wang, Y. Xu, X. Bao, R. Zhang, B. Zhang, Y. Guo and F. Cui, Interfacial electronic effects of palladium nanocatalysts on the by-product ammonia selectivity during nitrite catalytic reduction, Environ. Sci.: Nano, 2018, 5, 338–349 RSC .
43. Y. Wang, A. Xu, Z. Wang, L. Huang, J. Li, F. Li, J. Wicks, M. Luo, D. H. Nam, C. S. Tan, Y. Ding, J. Wu, Y. Lum, C. T. Dinh, D. Sinton, G. Zheng and E. H. Sargent, Enhanced Nitrate-to-Ammonia Activity on Copper-Nickel Alloys via Tuning of Intermediate Adsorption, J. Am. Chem. Soc., 2020, 142, 5702–5708 CrossRef CAS PubMed .
44. H. Niu, Z. Zhang, X. Wang, X. Wan, C. Shao and Y. Guo, Theoretical Insights into the Mechanism of Selective Nitrate-to-Ammonia Electroreduction on Single-Atom Catalysts, Adv. Funct. Mater., 2021, 31, 2008533 CrossRef CAS .
45. G.-F. Chen, Y. Yuan, H. Jiang, S.-Y. Ren, L.-X. Ding, L. Ma, T. Wu, J. Lu and H. Wang, Electrochemical reduction of nitrate to ammonia via direct eight-electron transfer using a copper–molecular solid catalyst, Nat. Energy, 2020, 5, 605–613 CrossRef CAS .
46. J. Li, G. Zhan, J. Yang, F. Quan, C. Mao, Y. Liu, B. Wang, F. Lei, L. Li, A. W. M. Chan, L. Xu, Y. Shi, Y. Du, W. Hao, P. K. Wong, J. Wang, S. X. Dou, L. Zhang and J. C. Yu, Efficient Ammonia Electrosynthesis from Nitrate on Strained Ruthenium Nanoclusters, J. Am. Chem. Soc., 2020, 142, 7036–7046 CrossRef CAS PubMed .
47. Y. Yao, S. Zhu, H. Wang, H. Li and M. Shao, A Spectroscopic Study of Electrochemical Nitrogen and Nitrate Reduction on Rhodium Surfaces, Angew. Chem., Int. Ed., 2020, 59, 10479–10483 CrossRef CAS PubMed .
48. X. Zhao, X. Lan, D. Yu, H. Fu, Z. Liu and T. Mu, Deep eutectic-solvothermal synthesis of nanostructured Fe₃S₄ for electrochemical N₂ fixation under ambient conditions, Chem. Commun., 2018, 54, 13010–13013 RSC .
49. Y. Zhao, R. Shi, X. Bian, C. Zhou, Y. Zhao, S. Zhang, F. Wu, G. I. N. Waterhouse, L. Z. Wu, C. H. Tung and T. Zhang, Ammonia Detection Methods in Photocatalytic and Electrocatalytic Experiments: How to Improve the Reliability of NH₃ Production Rates?, Adv. Sci., 2019, 6, 1802109 CrossRef PubMed .
50. L. Li, C. Tang, D. Yao, Y. Zheng and S.-Z. Qiao, Electrochemical Nitrogen Reduction: Identification and Elimination of Contamination in Electrolyte, ACS Energy Lett., 2019, 4, 2111–2116 CrossRef CAS .
51. C. Polatides and G. Kyriacou, Electrochemical reduction of nitrate ion on various cathodes? reaction kinetics on bronze cathode, J. Appl. Electrochem., 2005, 35, 421–427 CrossRef CAS .
52. A. P. Carvalho, L. A. Meireles and F. X. Malcata, Rapid spectrophotometric determination of nitrates and nitrites in marine aqueous culture media, Analusis, 1998, 26, 347–351 CrossRef CAS .
53. Y. Feng, H. Yang, Y. Zhang, X. Huang, L. Li, T. Cheng and Q. Shao, Te-Doped Pd Nanocrystal for Electrochemical Urea Production by Efficiently Coupling Carbon Dioxide Reduction with Nitrite Reduction, Nano Lett., 2020, 20, 8282–8289 CrossRef CAS PubMed .
54. J.-X. Liu, D. Richards, N. Singh and B. R. Goldsmith, Activity and Selectivity Trends in Electrocatalytic Nitrate Reduction on Transition Metals, ACS Catal., 2019, 9, 7052–7064 CrossRef CAS .
55. Z. Wang, S. D. Young, B. R. Goldsmith and N. Singh, Increasing electrocatalytic nitrate reduction activity by controlling adsorption through PtRu alloying, J. Catal., 2021, 395, 143–154 CrossRef CAS .
56. J. Yang, P. Sebastian, M. Duca, T. Hoogenboom and M. T. Koper, pH dependence of the electroreduction of nitrate on Rh and Pt polycrystalline electrodes, Chem. Commun., 2014, 50, 2148–2151 RSC .
57. P. M. Tucker, M. J. Waite and B. E. Hayden, Electrocatalytic reduction of nitrate on activated rhodium electrode surfaces, J. Appl. Electrochem., 2004, 34, 781–796 CrossRef CAS .
58. S. Taguchi and J. M. Feliu, Kinetic study of nitrate reduction on Pt(110) electrode in perchloric acid solution, Electrochim. Acta, 2008, 53, 3626–3634 CrossRef CAS .
59. J. Lim, C.-Y. Liu, J. Park, Y.-H. Liu, T. P. Senftle, S. W. Lee and M. C. Hatzell, Structure Sensitivity of Pd Facets for Enhanced Electrochemical Nitrate Reduction to Ammonia, ACS Catal., 2021, 11, 7568–7577 CrossRef CAS .
60. G. E. Dima, G. L. Beltramo and M. T. M. Koper, Nitrate reduction on single-crystal platinum electrodes, Electrochim. Acta, 2005, 50, 4318–4326 CrossRef CAS .
61. T. Chen, H. Li, H. Ma and M. T. Koper, Surface modification of Pt(100) for electrocatalytic nitrate reduction to dinitrogen in alkaline solution, Langmuir, 2015, 31, 3277–3281 CrossRef CAS PubMed .
62. M. A. Hasnat, N. Ahamad, S. M. Nizam Uddin and N. Mohamed, Silver modified platinum surface/H⁺ conducting Nafion membrane for cathodic reduction of nitrate ions, Appl. Surf. Sci., 2012, 258, 3309–3314 CrossRef CAS .
63. C. Roy, J. Deschamps, M. H. Martin, E. Bertin, D. Reyter, S. Garbarino, L. Roué and D. Guay, Identification of Cu surface active sites for a complete nitrate-to-nitrite conversion with nanostructured catalysts, Appl. Catal., B, 2016, 187, 399–407 CrossRef CAS .
64. M. Duca and M. T. M. Koper, Powering denitrification: the perspectives of electrocatalytic nitrate reduction, Energy Environ. Sci., 2012, 5, 9726–9742 RSC .
65. J. F. E. Gootzen, P. G. J. M. Peeters, J. M. B. Dukers, L. Lefferts, W. Visscher and J. A. R. van Veen, The electrocatalytic reduction of NO₃⁻ on Pt, Pd and Pt + Pd electrodes activated with Ge, J. Electroanal. Chem., 1997, 434, 171–183 CrossRef CAS .
66. G. E. Dima, V. Rosca and M. T. M. Koper, Role of germanium in promoting the electrocatalytic reduction of nitrate on platinum: an FTIR and DEMS study, J. Electroanal. Chem., 2007, 599, 167–176 CrossRef CAS .
67. J. Yang, M. Duca, K. J. P. Schouten and M. T. M. Koper, Formation of volatile products during nitrate reduction on a Sn-modified Pt electrode in acid solution, J. Electroanal. Chem., 2011, 662, 87–92 CrossRef CAS .
68. K.-W. Kim, S.-M. Kim, Y.-H. Kim, E.-H. Lee and K.-Y. Jee, Sn Stability of Sn-Modified Pt Electrode for Reduction of Nitrate, J. Electrochem. Soc., 2007, 154, E145 CrossRef CAS .
69. T. Chen, Y. Li, L. Li, Y. Zhao, S. Shi, R. Jiang and H. Ma, Cu Modified Pt Nanoflowers with Preferential (100) Surfaces for Selective Electroreduction of Nitrate, Catalysts, 2019, 9(6), 536 CrossRef CAS .
70. J. Yang, Y. Kwon, M. Duca and M. T. Koper, Combining voltammetry and ion chromatography: application to the selective reduction of nitrate on Pt and PtSn electrodes, Anal. Chem., 2013, 85, 7645–7649 CrossRef CAS PubMed .
71. J. Y. Zhu, Q. Xue, Y. Y. Xue, Y. Ding, F. M. Li, P. Jin, P. Chen and Y. Chen, Iridium Nanotubes as Bifunctional Electrocatalysts for Oxygen Evolution and Nitrate Reduction Reactions, ACS Appl. Mater. Interfaces, 2020, 12, 14064–14070 CrossRef CAS PubMed .
72. H. Liu, J. Park, Y. Chen, Y. Qiu, Y. Cheng, K. Srivastava, S. Gu, B. H. Shanks, L. T. Roling and W. Li, Electrocatalytic Nitrate Reduction on Oxide-Derived Silver with Tunable Selectivity to Nitrite and Ammonia, ACS Catal., 2021, 8431–8442,  DOI:10.1021/acscatal.1c01525 .
73. I. Katsounaros and G. Kyriacou, Influence of nitrate concentration on its electrochemical reduction on tin cathode: identification of reaction intermediates, Electrochim. Acta, 2008, 53, 5477–5484 CrossRef CAS .
74. M. Dortsiou and G. Kyriacou, Electrochemical reduction of nitrate on bismuth cathodes, J. Electroanal. Chem., 2009, 630, 69–74 CrossRef CAS .
75. S. Cattarin, Electrochemical reduction of nitrogen oxyanions in 1 M sodium hydroxide solutions at silver, copper and CuInSe₂ electrodes, J. Appl. Electrochem., 1992, 22, 1077–1081 CrossRef CAS .
76. K. Bouzek, M. Paidar, A. Sadilkova and H. Bergmann, Electrochemical reduction of nitrate in weakly alkaline solutions, J. Appl. Electrochem., 2001, 31, 1185–1193 CrossRef CAS .
77. Y. Gao, Q. Wu, X. Liang, Z. Wang, Z. Zheng, P. Wang, Y. Liu, Y. Dai, M. H. Whangbo and B. Huang, Cu₂O Nanoparticles with Both {100} and {111} Facets for Enhancing the Selectivity and Activity of CO₂ Electroreduction to Ethylene, Adv. Sci., 2020, 7, 1902820 CrossRef CAS PubMed .
78. H. Dong, L. Zhang, P. Yang, X. Chang, W. Zhu, X. Ren, Z.-J. Zhao and J. Gong, Facet design promotes electroreduction of carbon dioxide to carbon monoxide on palladium nanocrystals, Chem. Eng. Sci., 2019, 194, 29–35 CrossRef CAS .
79. H. Won da, H. Shin, J. Koh, J. Chung, H. S. Lee, H. Kim and S. I. Woo, Highly Efficient, Selective, and Stable CO₂ Electroreduction on a Hexagonal Zn Catalyst, Angew. Chem., Int. Ed., 2016, 55, 9297–9300 CrossRef PubMed .
80. I. Takahashi, O. Koga, N. Hoshi and Y. Hori, Electrochemical reduction of CO₂ at copper single crystal Cu(S)–[n(111) × (111)] and Cu(S)–[n(110) × (100)] electrodes, J. Electroanal. Chem., 2002, 533, 135–143 CrossRef CAS .
81. W. Luc, X. Fu, J. Shi, J.-J. Lv, M. Jouny, B. H. Ko, Y. Xu, Q. Tu, X. Hu, J. Wu, Q. Yue, Y. Liu, F. Jiao and Y. Kang, Two-dimensional copper nanosheets for electrochemical reduction of carbon monoxide to acetate, Nat. Catal., 2019, 2, 423–430 CrossRef CAS .
82. X. Fu, X. Zhao, X. Hu, K. He, Y. Yu, T. Li, Q. Tu, X. Qian, Q. Yue, M. R. Wasielewski and Y. Kang, Alternative route for electrochemical ammonia synthesis by reduction of nitrate on copper nanosheets, Appl. Mater. Today, 2020, 19, 100620 CrossRef .
83. V. Rosca, M. Duca, M. T. de Groot and T. M. Koper, Nitrogen cycle electrocatalysis, Chem. Rev., 2009, 109, 2209–2244 CrossRef CAS PubMed .
84. S. Garcia-Segura, M. Lanzarini-Lopes, K. Hristovski and P. Westerhoff, Electrocatalytic reduction of nitrate: Fundamentals to full-scale water treatment applications, Appl. Catal., B, 2018, 236, 546–568 CrossRef CAS .
85. J. Martínez, A. Ortiz and I. Ortiz, State-of-the-art and perspectives of the catalytic and electrocatalytic reduction of aqueous nitrates, Appl. Catal., B, 2017, 207, 42–59 CrossRef .
86. S. Bae and A. A. Gewirth, Differential reactivity of Cu(111) and Cu(100) during nitrate reduction in acid electrolyte, Faraday Discuss., 2008, 140, 113–123 RSC .
87. D. P. Butcher and A. A. Gewirth, Nitrate reduction pathways on Cu single crystal surfaces: effect of oxide and Cl⁻, Nano Energy, 2016, 29, 457–465 CrossRef CAS .
88. C. Milhano, D. Pletcher and R. E. White, Modern aspects of electrochemistry, Springer, New York, 2009 Search PubMed .
89. G. Zhou and J. C. Yang, Initial Oxidation Kinetics of Cu(100), (110), and (111) Thin Films Investigated by in Situ Ultra-high-vacuum Transmission Electron Microscopy, J. Mater. Res., 2005, 20, 1684–1694 CrossRef CAS .
90. Y. Zheng, C. Zhao, T. Wu, Y. Li, W. Zhang, J. Zhu, G. Geng, J. Chen, J. Wang, B. Yu and J. Zhang, Enhanced oxygen reduction kinetics by a porous heterostructured cathode for intermediate temperature solid oxide fuel cells, Energy and AI, 2020, 2, 100027 CrossRef .
91. Y. Zheng, Y. Li, T. Wu, C. Zhao, W. Zhang, J. Zhu, Z. Li, J. Chen, J. Wang, B. Yu and J. Zhang, Controlling crystal orientation in multilayered heterostructures toward high electro-catalytic activity for oxygen reduction reaction, Nano Energy, 2019, 62, 521–529 CrossRef CAS .
92. L. Li, C. Tang, X. Cui, Y. Zheng, X. Wang, H. Xu, S. Zhang, T. Shao, K. Davey and S. Z. Qiao, Efficient Nitrogen Fixation to Ammonia through Integration of Plasma Oxidation with Electrocatalytic Reduction, Angew. Chem., Int. Ed., 2021, 60, 14131–14137 CrossRef CAS PubMed .
93. S. Liu, M. Wang, T. Qian, H. Ji, J. Liu and C. Yan, Facilitating nitrogen accessibility to boron-rich covalent organic frameworks via electrochemical excitation for efficient nitrogen fixation, Nat. Commun., 2019, 10, 3898 CrossRef PubMed .
94. N. Tsvetkov, Q. Lu, Y. Chen and B. Yildiz, Accelerated oxygen exchange kinetics on Nd₂NiO₄ thin films with tensile strain along c-axis, ACS Nano, 2015, 9, 1613–1621 CrossRef CAS .
95. J. Yang, X. Chen, X. Yang and J. Y. Ying, Stabilization and compressive strain effect of AuCu core on Pt shell for oxygen reduction reaction, Energy Environ. Sci., 2012, 5, 8976–8981 RSC .
96. Z. Wang, D. Richards and N. Singh, Recent discoveries in the reaction mechanism of heterogeneous electrocatalytic nitrate reduction, Catal. Sci. Technol., 2021, 11, 705–725 RSC .
97. T. Zhang, D. Zhang, X. Han, T. Dong, X. Guo, C. Song, R. Si, W. Liu, Y. Liu and Z. Zhao, Preassembly Strategy To Fabricate Porous Hollow Carbonitride Spheres Inlaid with Single Cu-N₃ Sites for Selective Oxidation of Benzene to Phenol, J. Am. Chem. Soc., 2018, 140, 16936–16940 CrossRef CAS PubMed .
98. Z. Li, S. Ji, Y. Liu, X. Cao, S. Tian, Y. Chen, Z. Niu and Y. Li, Well-Defined Materials for Heterogeneous Catalysis: From Nanoparticles to Isolated Single-Atom Sites, Chem. Rev., 2020, 120, 623–682 CrossRef CAS PubMed .
99. M. Li, H. Wang, W. Luo, P. C. Sherrell, J. Chen and J. Yang, Heterogeneous Single-Atom Catalysts for Electrochemical CO₂ Reduction Reaction, Adv. Mater., 2020, 32, e2001848 CrossRef PubMed .
100. Y. Wang, Y. Liu, W. Liu, J. Wu, Q. Li, Q. Feng, Z. Chen, X. Xiong, D. Wang and Y. Lei, Regulating the coordination structure of metal single atoms for efficient electrocatalytic CO₂ reduction, Energy Environ. Sci., 2020, 13, 4609–4624 RSC .
101. Y. Guan, Y. Feng, J. Wan, X. Yang, L. Fang, X. Gu, R. Liu, Z. Huang, J. Li, J. Luo, C. Li and Y. Wang, Ganoderma-Like MoS₂/NiS₂ with Single Platinum Atoms Doping as an Efficient and Stable Hydrogen Evolution Reaction Catalyst, Small, 2018, 14, e1800697 CrossRef PubMed .
102. L. Ma and H. Li, Electrocatalysis of adsorbed Co-cyclam at Au electrodes for nitrate reduction in concentrated alkaline solution, Electroanulysis, 1995, 8, 7 Search PubMed .
103. N. Chebotareva and T. Nyokong, Metallophthalocyanine catalysed electroreduction of nitrate and nitrite ions in alkaline media, J. Appl. Electrochem., 1997, 27, 975–981 CrossRef CAS .
104. Z. Y. Wu, M. Karamad, X. Yong, Q. Huang, D. A. Cullen, P. Zhu, C. Xia, Q. Xiao, M. Shakouri, F. Y. Chen, J. Y. T. Kim, Y. Xia, K. Heck, Y. Hu, M. S. Wong, Q. Li, I. Gates, S. Siahrostami and H. Wang, Electrochemical ammonia synthesis via nitrate reduction on Fe single atom catalyst, Nat. Commun., 2021, 12, 2870 CrossRef CAS PubMed .
105. P. Li, Z. Jin, Z. Fang and G. Yu, A single-site iron catalyst with preoccupied active centers that achieves selective ammonia electrosynthesis from nitrate, Energy Environ. Sci., 2021, 14, 3522–3531 RSC .
106. J. Zhang, R. Yin, Q. Shao, T. Zhu and X. Huang, Oxygen Vacancies in Amorphous InO_x Nanoribbons Enhance CO₂ Adsorption and Activation for CO₂ Electroreduction, Angew. Chem., Int. Ed., 2019, 58, 5609–5613 CrossRef CAS PubMed .
107. H. Li, N. Xiao, Y. Wang, C. Liu, S. Zhang, H. Zhang, J. Bai, J. Xiao, C. Li, Z. Guo, S. Zhao and J. Qiu, Promoting the electroreduction of CO₂ with oxygen vacancies on a plasma-activated SnO_x/carbon foam monolithic electrode, J. Mater. Chem. A, 2020, 8, 1779–1786 RSC .
108. R. Jia, Y. Wang, C. Wang, Y. Ling, Y. Yu and B. Zhang, Boosting Selective Nitrate Electroreduction to Ammonium by Constructing Oxygen Vacancies in TiO₂, ACS Catal., 2020, 10, 3533–3540 CrossRef CAS .
109. R. Daiyan, T. Tran-Phu, P. Kumar, K. Iputera, Z. Tong, J. Leverett, M. H. A. Khan, A. Asghar Esmailpour, A. Jalili, M. Lim, A. Tricoli, R.-S. Liu, X. Lu, E. Lovell and R. Amal, Nitrate reduction to ammonium: from CuO defect engineering to waste NO_x-to-NH₃ economic feasibility, Energy Environ. Sci., 2021, 14, 3588–3598 RSC .
110. C. Chen, Y. Chen, R. Yao, Y. Li and C. Zhang, Artificial Mn₄Ca Clusters with Exchangeable Solvent Molecules Mimicking the Oxygen-Evolving Center in Photosynthesis, Angew. Chem., Int. Ed., 2019, 58, 3939–3942 CrossRef CAS PubMed .
111. L. Gan, T. L. Groy, P. Tarakeshwar, S. K. Mazinani, J. Shearer, V. Mujica and A. K. Jones, A nickel phosphine complex as a fast and efficient hydrogen production catalyst, J. Am. Chem. Soc., 2015, 137, 1109–1115 CrossRef CAS PubMed .
112. C. Hwang, J. Yoo, G. Y. Jung, S. H. Joo, J. Kim, A. Cha, J. G. Han, N. S. Choi, S. J. Kang, S. Y. Lee, S. K. Kwak and H. K. Song, Biomimetic Superoxide Disproportionation Catalyst for Anti-Aging Lithium–Oxygen Batteries, ACS Nano, 2019, 13, 9190–9197 CrossRef CAS PubMed .
113. C. L. Ford, Y. J. Park, E. M. Maston, Z. Gordon and A. R. Fout, A bioinspired iron catalyst for nitrate and perchlorate reduction, Science, 2016, 354, 741–743 CrossRef CAS PubMed .
114. R. Silaghi-Dumitrescu, M. Mich, C. Matyas and C. E. Cooper, Nitrite and nitrate reduction by molybdenum centers of the nitrate reductase type: computational predictions on the catalytic mechanism, Nitric oxide, 2012, 26, 27–31 CrossRef CAS PubMed .
115. C. Sparacino-Watkins, J. F. Stolz and P. Basu, Nitrate and periplasmic nitrate reductases, Chem. Soc. Rev., 2014, 43, 676–706 RSC .
116. Y. Li, A. Yamaguchi, M. Yamamoto, K. Takai and R. Nakamura, Molybdenum Sulfide: A Bioinspired Electrocatalyst for Dissimilatory Ammonia Synthesis with Geoelectrical Current, J. Phys. Chem. C, 2016, 121, 2154–2164 CrossRef .
117. T. A. Okamura, M. Tatsumi, Y. Omi, H. Yamamoto and K. Onitsuka, Selective and effective stabilization of Mo(VI) horizontal lineO bonds by NH⋯S hydrogen bonds via trans influence, Inorg. Chem., 2012, 51, 11688–11697 CrossRef CAS PubMed .
118. K. M. Cho, W.-B. Jung, D. Kim, J. Y. Kim, Y. Kim, G.-T. Yun, S. Ryu, A. Al-Saggaf, I. Gereige and H.-T. Jung, Confined cavity on a mass-producible wrinkle film promotes selective CO₂ reduction, J. Mater. Chem. A, 2020, 8, 14592–14599 RSC .
119. G. Hyun, J. T. Song, C. Ahn, Y. Ham, D. Cho, J. Oh and S. Jeon, Hierarchically porous Au nanostructures with interconnected channels for efficient mass transport in electrocatalytic CO₂ reduction, Proc. Natl. Acad. Sci. U. S. A., 2020, 117, 5680–5685 CrossRef CAS PubMed .
120. J. Kim, J. T. Song, H. Ryoo, J.-G. Kim, S.-Y. Chung and J. Oh, Morphology-controlled Au nanostructures for efficient and selective electrochemical CO₂ reduction, J. Mater. Chem. A, 2018, 6, 5119–5128 RSC .
121. K. D. Yang, W. R. Ko, J. H. Lee, S. J. Kim, H. Lee, M. H. Lee and K. T. Nam, Morphology-Directed Selective Production of Ethylene or Ethane from CO₂ on a Cu Mesopore Electrode, Angew. Chem., Int. Ed., 2017, 56, 796–800 CrossRef CAS PubMed .
122. A. S. Malkani, J. Li, J. Anibal, Q. Lu and B. Xu, Impact of Forced Convection on Spectroscopic Observations of the Electrochemical CO Reduction Reaction, ACS Catal., 2019, 10, 941–946 CrossRef .
123. D. Raciti, M. Mao and C. Wang, Mass transport modelling for the electroreduction of CO₂ on Cu nanowires, Nanotechnology, 2018, 29, 044001 CrossRef PubMed .
124. C. Yan, S. Kakuturu, A. H. Butzlaff, D. M. Cwiertny, S. Mubeen and C. J. Werth, Scalable Reactor Design for Electrocatalytic Nitrite Reduction with Minimal Mass Transfer Limitations, ACS ES&T Engg, 2020, 1, 204–215 Search PubMed .
125. C. Dai, Y. Sun, G. Chen, A. C. Fisher and Z. J. Xu, Electrochemical Oxidation of Nitrogen towards Direct Nitrate Production on Spinel Oxides, Angew. Chem., Int. Ed., 2020, 59, 9418–9422 CrossRef CAS PubMed .
126. S. Han, C. Wang, Y. Wang, Y. Yu and B. Zhang, Electrosynthesis of Nitrate via the Oxidation of Nitrogen on Tensile-Strained Palladium Porous Nanosheets, Angew. Chem., Int. Ed., 2021, 133, 4524–4528 CrossRef .
127. M. Jouny, J. J. Lv, T. Cheng, B. H. Ko, J. J. Zhu, W. A. Goddard, 3rd and F. Jiao, Formation of carbon-nitrogen bonds in carbon monoxide electrolysis, Nat. Chem., 2019, 11, 846–851 CrossRef CAS PubMed .
128. C. Chen, X. Zhu, X. Wen, Y. Zhou, L. Zhou, H. Li, L. Tao, Q. Li, S. Du, T. Liu, D. Yan, C. Xie, Y. Zou, Y. Wang, R. Chen, J. Huo, Y. Li, J. Cheng, H. Su, X. Zhao, W. Cheng, Q. Liu, H. Lin, J. Luo, J. Chen, M. Dong, K. Cheng, C. Li and S. Wang, Coupling N₂ and CO₂ in H₂O to synthesize urea under ambient conditions, Nat. Chem., 2020, 12, 717–724 CrossRef CAS PubMed .

This journal is © the Partner Organisations 2021