Carbon neutrality refers to the state of having net-zero CO₂ emissions [1]. Since the beginning of the industrial revolution, greenhouse gas emissions have grown rapidly, resulting in the global warming effects considered as serious threats to the environment [2,3,4,5]. One solution to this problem is to upgrade traditional high-carbon emission industries to become carbon-free [6,7,8]. Ammonia, as an essential chemical for the fertilizer industry, is considered as an important strategic resource [9]. Additionally, ammonia serves as a potential hydrogen carrier that can be decomposed into H₂ and N₂, providing hydrogen on demand [10,11,12,13]. Unlike liquid hydrogen, which requires harsh conditions for storage, ammonia can be readily liquefied by increasing the pressure to approximately 10 bar at room temperature or cooling it to -33 °C at atmospheric pressure [12,14,15]. However, industrial-scale ammonia production, known as the Haber-Bosch process, is an energy-intensive chemical process that accounts for approximately 2% of the world’s energy consumption and emits over 450 million tons of carbon dioxide annually [16]. As traditional ammonia production methods no longer align with the goals of achieving carbon neutrality, it is critically urgent to explore carbon-free ammonia synthesis methods.

Electrochemical processes offer a clean pathway for synthesizing green NH₃ at room temperature [16,17,18]. As illustrated in Fig. 1, it has emerged as a promising energy-efficient route for decentralized ammonia production, potentially driven by generated renewable energy [19]. Inspired by the traditional Haber-Bosch process, which converts nitrogen to ammonia, the electrochemical N₂ reduction reaction (NRR) for ammonia synthesis has received extensive attention over the past decade [20,21]. However, NRR poses several challenges, including low solubility of N₂ in aqueous electrolytes, competition from the more facile hydrogen evolution reaction (HER), and large thermodynamic activation barriers [19,22]. Consequently, the Faradaic efficiency (FE) and yield rate (YR) of NH₃ in aqueous systems remain low, making NRR difficult for further reliable application. To overcome these challenges, oxidized nitrogen species can serve as feedstock for efficient electrocatalytic NH₃ production. Nitrogen oxides, including NO₃^-, NO₂^-, NO and et al., with lower dissociation energies of the N=O bond (204 kJ mol^{- 1}) and/or N-O bond (176 kJ mol^{- 1}) than that of the N≡N bond (941 kJ mol^{- 1}), exhibit markedly higher overall kinetic rates [23,24]. Furthermore, nitrogen oxides are common environmental pollutants found in groundwater, industrial wastewater, and industrial exhaust [25]. Upcycling nitrogenous waste in the production of electrochemical green ammonia offers a promising option for treating nitrogenous waste in the environment and provides an alternative to carbon-free and decentralized ammonia production.

Hence, we provide an update on the production of green ammonia from nitrogenous waste. Firstly, we mainly summarize the reports of major environmental nitrogenous wastes in electrocatalytic ammonia production, focusing more on the conversion of common nitrogen pollutants into value-added ammonia. We highlight three kinds of nitrogenous wastes (NO₃^-, NO₂^- and NO) commonly found in nature, and discuss the reaction paths of electrochemical reduction of these three nitrogenous wastes to NH₃, as well as the influence and inhibition methods of competitive reactions. Furthermore, the reported electrocatalysts can be classified based on their active sites, including noble metals, non-noble metals, and metal-free catalysts. Finally, we discuss the current challenges and prospects of nitrogenous waste for ammonia synthesis, emphasizing the significance of this industry for achieving global carbon neutrality.

Compared with precious metals, non-noble metal catalysts have the advantages of low cost, rich resources, high activity, and large-scale application [64]. As shown in Fig. 4a, Cu, Fe, Co, and Ni show high activity for the adsorption of NO₃^-. Most non-precious metal catalysts are designed around these metals with different structures [65]. This section summarizes some outstanding and representative non-noble metal catalysts to demonstrate the great progress in the electrosynthesis of ammonia with nitrates.

Ye et al. [66] first prepared cobalt phosphide nanosheet arrays grown on carbon fiber cloth (CoP NAs/CFC) for the NO₃RR and the catalyst achieved a Faradaic efficiency near 100% and an ammonia-evolving rate of 9.56 mol h^-1 m^-2. Figure 5a shows the cumulative amount of ammonia generated by CoP NAs/CFC and the contrast material Co NAs/ CFC over time, indicating that CoP NAs/ CFCS has a higher ammonia-evolving rate. In addition, Fig. 5b illustrates the Operando X-ray absorption near-edge structure (XANES) of the Co K-edge of CoP NAs/CFC. The three peaks A, B, and C at the white line represent the transition of electrons from the 1s orbital to the unoccupied 3d orbital, the ligand, and the 4p orbital, respectively. As the applied potential increases, peak A enhanced while peak B weakened simultaneously, suggesting that electron holes are formed in the 3d orbital and partially occupied in the 4p orbital. Therefore, the catalytic mechanism of Co 4p orbital involvement in NO₃RR was deduced in Fig. 5c. Due to the electric field, electron transition from Co 3d to 4p orbital first occurred. Next, the excited electrons were transferred to the O 2p orbital of NO₃^- through the Co-O-N covalent bond, and finally injected into the p* orbitals of NO₃^- to reduce the adsorbed NO₃^-.

Cu-based materials are generally reported to exhibit the highest catalytic activity for NO₃^- reduction to NH₃ [49,64,67,68,69]. The pioneering work reported by Chen et al. [24] about copper-molecular solid catalysts provided a reference for subsequent work in the NO₃RR field. Figure 5d shows the Faradaic efficiency of reducing NO₃^- into NH₃ (blue) and NO₂^- (lavender) after doping various elements into the organic molecular solid 3,4,9, 10-tetracarboxylic acid (PTCDA). Among all samples, Cu-doped PTCDA catalyst (O-Cu-PTCDA) exhibits the highest total Faradaic efficiency of 83.5% due to the unique electron configuration of Cu element. On the surface of O-Cu-PTCDA, NO₃^- could combine with Cu (3d₁₀) and provide electrons from the highest occupied molecular orbital of NO₃^- to the empty energy level of Cu atoms. Meanwhile, abundant electrons feedback from the fully occupied Cu 3d orbital to the lowest unoccupied molecular orbital of NO₃^-. As depicted in the right illustration of Fig. 5d, the charge density difference of NO₃^- on O-Cu-PTCDA ($\stackrel{-}{1}$03) has a larger electron cloud and a more negative adsorption energy difference (∆E = - 4.258 eV), indicating the strong interaction between NO₃^- and Cu. Furthermore, the adsorption energy of O-Cu-PTCDA for *H is much smaller than that of *NO₃ (in the left illustration of Fig. 5d), indicating a better adsorption effect on NO₃^- and an inhibitory effect on HER. This work deeply unveiled the catalytic mechanism of efficient NO₃^- reduction through electron/proton transfer of O-Cu-PTCDA, which promoted the development of ammonia electrosynthesis from NO₃RR.

Although copper-based catalysts exhibit a high NO₃^- to NO₂^- conversion rate, *NO₂, as the most stable intermediate of NO₃RR, is often desorbed from the electrode surface to reduce the NO₃^- to NH₃ conversion rate. Qiu et al. [70] reported a catalyst that incorporates CuO_x active species within a TiO₂ nanoreactor (TiO₂ NTs/CuO_x). Due to the limited space of the nanotube, this catalyst confines the NO₂^- intermediate to facilitate NO₃RR. Compared with planar electrodes, nanotubular electrodes with encapsulating CuO_x can effectively hinder the diffusion behavior of NO₂^-, significantly promoting the conversion of NO₂^- to NH₃ (Fig. 5e and f). Therefore, TiO₂ NTs/CuO_x increases FE by more than 20% and YR by more than 4 times (Fig. 5g). This work makes rational use of the confinement effect toward active substance given by the catalyst geometry, which can be expected for the multi-scale design of electrode structures in highly selective electrochemical ammonia synthesis.

The introduction of the second element is a common method to design electrocatalysts. The unique characteristics of the second element can endow original catalysts with new properties, such as structural strain, synergistic effect, series reaction, etc. These strategies can significantly adjust the electronic structure of the materials and thus improve the catalytic activity. Wang et al. [71] synthesized a series of alloy catalysts with different Cu/Ni component ratios. In Fig. 6a, the d-band center of CuNi alloys shifts 0.14, 0.28, and 0.32 eV towards the Fermi level with the increase of Ni composition ratio, enhancing the adsorption energy of intermediates. As shown in Fig. 6b, with the increase of Ni concentration in the alloy, the adsorption energy of NO₃^- increases. However, a high proportion of Ni will cause the adsorption value of intermediate *NH₂ to exceed the ideal value and reduce the selectivity of the final product NH₃. In addition, the NO₃RR activity of the Cu₅₀Ni₅₀ is 6 times higher than that of the pure copper electrode, and the half-wave potential increases by 0.12 V (Fig. 6b).

Given the slow kinetic rate of NO₃RR, He et al. [11] proposed the concept of an efficient tandem catalyst to achieve high-speed ammonia production at low potential by coupling different transition metal interphases as synergic active sites. Through an electrochemical redox activation process, the Cu/Co disulfide is induced to reconstitute the core-shell Cu/CuO_x and Co/CoO phases (CuCoSP, Fig. 6c) with adjustable potential. In Fig. 6d, CuCoSP reaches the maximum Faradaic efficiency of 95.9%, 14.5% higher than that of CoSP, while almost no ammonia is produced on the CuSP at -0.075 V vs. RHE. In situ Raman spectroscopy reveals the specific tandem catalytic behavior on CuCoSP as follows: the inner Cu/CuO_x phase preferentially catalyzes NO₃^- reduction to NO₂^-, while the NO₂^- intermediates are transferred and selectively converted to NH₃ on the Co/CoO phase. The sequential NO₃^- and NO₂^- reduction enables a tandem system for quickly cascading NO₃^- to NH₃ at low overpotential (Fig. 6e).

As the most abundant transition metal in the world, Fe is active for NO₃RR [72,73]. Inspired by the unique tunable local electronic structure, Li et al. [22] reported a densely populated Fe single-atom catalyst (Fe-PPy SACs) obtained from double pyrolysis of ferric acetylacetone/polypyrrole hydrogels. As shown in Fig. 6f, Fe atoms are isolated from each other and combined with nitrogen species to form Fe-N bonds in Fe-PPy SACs. Additionally, Fe-PPy SACs exhibit the highest ammonia YR of 2.75 mg_NH3 h^{- 1} cm^{- 2} and FE of nearly 100% (Fig. 6g), which are superior to Fe nanoparticles (Fe NP) and PPy. The excellent catalytic performance means that the Fe sites isolated on Fe-PPy SACs make a significant contribution to the electrocatalytic reduction of NO₃^- to ammonia. As shown in Fig. 6h, NO₃^- preoccupies most Fe sites in the Fe-PPy SACs before the transition state Fe(II) is reduced to HER active site Fe(0), resulting in no or barely available adsorption sites for water molecules in the subsequent reduction process and thus the HER will be completely suppressed. Similarly, Wu et al. [74] synthesized Fe single-atom catalysts by using SiO₂ as hard templates, and the catalyst showed the highest Faradaic efficiency of 75% and yield rate of nearly 20,000 µg h^{- 1} mg_cat.^-1.

Although metal catalysts have been widely applied in electrocatalytic NO₃^- reduction due to the high activity, they generally suffer from a series of inevitable challenges such as high cost, poor adaptability, and environmental pollution caused by metal ions leaching [75]. Metal-free catalysts have been widely used in ORR, OER, and CO₂RR because of their low cost, high mechanical strength, and large surface area. Some metal-free materials for NO₃RR have been reported, including carbon nanotubes, graphene, carbon paper, graphite felt, and boron-doped diamond. However, the activity of most metal-free catalysts is still inferior due to complex reaction paths and fierce competitive reactions. To date, heteroatom doping, defect effect, and amorphous regulation are the main strategies for optimizing the activity of metal-free catalysts for ammonia synthesis. In this section, recent representative reports will be summarized, which provide new possibilities for the development of metal-free materials in NO₃RR.

Li et al. [76] reported a series of N-doped carbon aerogel catalysts N-C-T (T = 600,800,1000, and 1200°C) by changing the calcination temperatures. As depicted in Fig. 7a, N-C-1000 shows the highest FE of 95% and a maximum YR of 1.3 mg h^-1 cm^-2, which are both remarkably higher than that of the other materials. In addition, X-ray photoelectron spectroscopy (XPS) analysis revealed that different calcination temperatures would lead to changes in the types of doped nitrogen atoms. According to Fig. 7b, the absolute content of N3 in N-C-1000 is 1.55 at%, much higher than that of N-C-600 (0.47 at%), N-C-800 (1.12 at%) and N-C-1200 (0.51 at%). Notably, with the increase of N3 absolute content, the YR_NH3 increases. Theoretical evidence demonstrates that the N3 structure exhibits higher adsorption energy of NO₃^- (Fig. 7c), lower energy barrier from *NO to *NHO, and exothermic desorption process. This work explores the relationship between the structure type of doping atoms and the NO₃RR activity, which contributes to the fundamental understanding of the structural design of highly efficient metal-free carbon-based catalysts for NO₃RR.

Defect engineering is a common method to improve the performance of electrocatalysts by adjusting the electronic structure of catalysts. For metal catalysts, abundant defects in the base material are conducive to anchoring metal atoms to improve catalyst performance. For non-metallic catalysts, defects can capture and catalyze reactive substances. Huang et al. [77] found that the polymerized g-C₃N₄ with a controllable number of nitrogen vacancies showed good catalytic activity for NO₃RR. The porous C₃N₄ with enriched nitrogen vacancies (PCNV-T) was controlled by calcining Bulk C₃N₄ (BCN) at different temperatures (550, 600, and 650°C). The N1 XPS spectra in Fig. 7d demonstrate that with the increase in calcination temperature, the integral area ratio between N-2 C and N-3 C decreases from 4.30 to 2.79, indicating the information of nitrogen vacancy. Moreover, PCNV-600 exhibits the highest FE of 89.96% and the highest YR of 0.03262 mmol^-1 g^-1 h^-1 (Fig. 7e). This work proves that nitrogen vacancies could promote NO₃^- adsorption, activation, and deionization, which provides new possibilities for the design of non-metallic catalysts.

Amorphization has become another strategy to adjust the structure and optimize the physical and chemical properties of nanomaterials [79]. Cheng et al. [78] first reported a synthetic method for converting small molecules into graphene with an amorphous/heterogeneous structure (sm-LIG). In this work, a series of SM-LiGs was prepared by using different small molecules as precursors, Including 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTCDA), 9,10-phenanthrenequinone (PQ), and 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA). As shown in Fig. 7f, the carbon six-membered rings in NTCDA-LIG material are surrounded by amorphous non-six-membered rings, featuring an intermediate structure between crystalline and amorphous. PTCDA-LIG and PQ-LIG also exhibit similar disordered structural characteristics, suggesting that sm-LIG can be randomly embedded into honeycomb nanocrystals of carbon polygons to form amorphous graphene structures. In addition, these sm-LIG materials all show relatively high NO₃RR activity with FE of 80% and YR of 2200 µg h^-1 cm^-2 at -0.94 V vs. RHE (Fig. 7g). Thereinto, PTCDA-LIG reaches a maximum FE of 83.7% and YR of 2456.8 µg h^-1 cm^-2, attributing to the few structural defects in PTCDA-LIG. This work provides insight into the design of amorphous and metal-free catalysts for the efficient NO₃RR.

NO₂^-, like NO₃^-, is a harmful substance that is widely present in the surface and groundwater [80]. NO₂^- can lead to methemoglobinemia and is classified by the World Health Organization as a Group 2A carcinogen [81,82]. Therefore, it is crucial to reduce NO₂^- in the environment and treat it to value-added species. The electrochemical reduction of NO₂^- to produce ammonia is a newly emerging technology that transforms harmful NO₂^- into high-value ammonia [83]. Notably, the solubility of NO₂^- in water is the highest among the nitrogen-containing species [41]. Therefore, the electrochemical reduction of NO₂^- to ammonia not only offers a green and low-energy alternative solution for the global ammonia industry, but also contributes to the restoration of the disrupted global nitrogen cycle. Meanwhile, the NO₂RR process converts harmful pollutants into high-value ammonia, driving the development of the global nitrogen economy [84].

Currently, catalysts for ammonia production from NO₂^- are mainly focused on transition metals, such as Ag, Pd, Cu, Ni, Co, et al. [82,83,85]. With their distinctive 3d orbital electron configuration, transition metals can hybridize with the p orbitals of NO₂^-, resulting in enhanced adsorption and catalytic performance. For example, the Ag nanoarray using NiO nanosheets array on carbon cloth (Ag@NiO/CC) catalyst synthesized by Sun et al. [86] exhibited the highest FE of 97.7%, and the maximum YR of 5.751 mg h^{- 1} cm^{- 2} for converting NO₂^- to NH₃. Based on DFT calculations, the high activity of Ag@NiO/CC is mainly due to the strong activity of Ag (100) face for NO₂RR, which reduces the reaction energy barriers of *NO₂ to *NO₂H and *NO to *NOH, thus facilitating the NO₂RR process. Meanwhile, Pd has also been extensively studied as a high-performance metal catalyst in the NO₂RR. Wang et al. [87] developed Pd nanoparticles deposited on CuO nanowires (Pd/CuO NOs) as a highly active catalyst for NO₂RR (the inset of Fig. 8a). Pd/CuO NOs forms a heterogeneous interface between Pd and CuO, resulting in the formation of a built-in electric field. As shown in Fig. 8b, a FE of over 90% was achieved at -1.50 V vs. RHE. XPS results indicated that the binding energy of Pd 3d and Cu 2p shifted to higher energy, whereas O 2p shifted towards lower energy, demonstrating a spontaneous electron transfer from Pd to CuO at the Pd-CuO heterogeneous interface. Furthermore, the analysis of the PDOS for the Cu d-band demonstrated that the d-band center of Pd/CuO NOs was shifted upwards by 0.16 eV compared to that of CuO NOs. This result suggests that the formation of Pd/CuO heterointerfaces could improve the electronic structure of Cu, thereby enhancing the adsorption of NO₂^- and reaction intermediates.

As previously mentioned, introducing a second metal can significantly enhance the catalytic activity. However, cleverly designing the interaction between the catalyst and substrate can also improve the catalytic activity of the catalyst. Li et al. developed a high metal loading (35 wt%) binary catalyst (Ni₃₅/NC-sd) for NO₂RR via utilizing sulfur diffusion to load nickel onto a N-rich carbon support [82]. The HR-TEM image (inset in Fig. 8c) clearly shows the highly coupled interface between the nickel metal and carbon layer. The XPS spectra of Ni 2p indicate that Ni₃₅/NC-sd has a deficit electron state due to the movement towards higher binding energies. The rectifying contact with electron-rich carbon forms a Schottky barrier that increases the electric field density of the catalyst surface. Kelvin probe force microscopy (KPFM) measurements demonstrate the strength of the surface electric field related to the Ni/NC Schottky barrier (Fig. 8d and e). The enhanced electric field on the surface of the Ni₃₅/NC-sd catalyst leads to a high FE of 99% and a YR of 25.1 mg h^{- 1} cm^{- 2} at -0.5 V vs. RHE (Fig. 8e). The intrinsic electric field effect, as one of the important means to improve catalytic activity in catalyst design. It can be achieved not only by constructing a Schottky barrier, but also by introducing dopants. Zhi et al. reported C-doped Co₃O₄ mesoporous nanotubes (C-Co₃O₄). This catalyst achieved nearly 100% FE during NO₂RR within the potential window of -0.1 V to -0.6 V vs. RHE and a yield rate of 4.10 mg h^{- 1} cm^{- 2} at -0.6 V vs. RHE (Fig. 8f) [41]. DFT calculations were conducted to investigate the effect of C doping on electronic tuning, which revealed an upshift of the d-band center of C/Co₃O₄ compared to Co₃O₄. According to KPFM results, a stronger electric field on the surface of C/Co₃O₄ can promote the adsorption of reactants, thus enhancing the catalytic activity of NO₂RR. These findings suggest that C-doped Co₃O₄ holds great potential as an efficient NO₂RR electrocatalyst.

NO is a significant factor in air pollution, causing a variety of environmental problems such as acid rain, photochemical smog, and ozone depletion [88,89,90]. NO emissions primarily originate from the combustion of fossil fuels in power plants, vehicles, and factories [91]. Currently, the most commonly used method for removing NO is selective catalytic reduction (SCR) technology, which converts NO to harmless nitrogen for release [92]. However, this method is not ideal because it requires valuable ammonia or hydrogen as a reductant [93]. Recently, the electrochemical conversion of NO to NH₃ has garnered extensive attention [94]. This process allows for the conversion of harmful NO into high-value NH₃ through electrochemical synthesis, providing a sustainable and alternative pathway for ammonia synthesis while simultaneously restoring the disrupted global N cycle [95,96,97]. Furthermore, the exploration of the electrochemical reduction of NO to NH₃ is crucial in understanding the fundamentals of electrocatalytic NO₃^-/NO₂^- reduction. Additionally, it is necessary for developing improved electrochemical denitrification and NH₃ production technologies [38]. In summary, the investigation of NO electrochemistry holds fundamental scientific significance, as the developed methods can be easily extended to other environmentally friendly electrocatalytic reduction chemistries. This research serves as a valuable foundation for the development of efficient and sustainable electrochemical processes with minimal environmental impact.

DFT has played a crucial role in evaluating the activity of different metals towards NORR. As shown in Fig. 9a, Wan et al. [92] employed DFT simulations to calculate the activity and selectivity of different metals towards NORR. The red line represents the strong binding side of the volcano where the hydrogenation of *NH to *NHH is the limiting step. The blue line shows metals with weak binding of *N. Specifically, when ΔG_*N is less than - 1.5 eV, ΔG_*H becomes negative and H₂ becomes the main product. Metals with ΔG_*N greater than - 0.83 eV are not favorable for NO adsorption, resulting in lower NORR activity. When ΔG_*N is between - 0.83 eV and 1.5 eV, there is a large N adsorption energy and the limiting potential is also close to or greater than 0 V vs. RHE. Theoretically, the thermodynamics of HER is unfavorable under these conditions, leading to high activity for NO reduction to NH₃. Prior research has demonstrated that altering the shape, coordination number, and electronic structure of metal catalysts can modify their capacity to adsorb NO and compete with HER.

For instance, Kim et al. [98] reported a nanostructured Ag with close to 100% FE for NORR at -0.165 V vs. RHE (Fig. 9b). The nanostructured catalysts provide more active sites for NORR due to the increased surface area. Additionally, due to the size effect, nanostructured metal catalysts often exhibit improved intrinsic catalytic activity. Li et al. [99] demonstrated a low-coordinated Ru nanoarray (Ru-LCN). Differential charge density analysis (Fig. 9c) reveals that reducing the coordination number is more conducive to the adsorption and activation of the N=O bond compared to high-coordinated Ru sites. Therefore, in the NORR process, Ru-LCN exhibits significantly higher catalytic performance than high-coordinate Ru nanoarray (Ru-HCN) at low NO concentrations (Fig. 9d). Under 1% NO concentration, Ru-LCN achieved a FE of 65.96% and a YR of 45.02 µmol h^{- 1} mg_cat^-1 for NH₃ at -0.2 V vs. RHE. Similarly, Zhang et al. [100] synthesized body-centered cubic intermetallic compounds of Ru and Ga (bcc RuGa) to manipulate the electron density of Ru. Surface analysis by differential charge density, as depicted in Fig. 9e, revealed that HNO adsorption is particularly favorable at the RuGa, thereby facilitating the protonation of the rate-limiting step *HNO to *HNOH. In the second protonation step, the *HNOH intermediate adsorbs on Ru in a top adsorption form, resulting in the breaking of the Ru—O bond. At this point, the Ru(O) atom no longer donates electrons, and all necessary electrons for the *HNOH intermediate are provided by the Ru(N) atom. In contrast, when the *HNOH intermediate adsorbs on RuGa, electron transfer still occurs from Ru(O) to the O atom and Ru(N) to the N atom simultaneously. The above computational results demonstrate that RuGa has a lower energy barrier in the critical hydrogenation step, facilitating the five-electron transfer involved in NORR to form NH₃. As shown in Fig. 9f, the electrocatalyst demonstrates a remarkable NH₃ production rate of 320.6 µmol h^{- 1} mg_Ru^-1 with a corresponding FE of 72.3% at a very low potential of -0.2 V vs. RHE in neutral media.

As the most ideal catalyst for NORR, copper-based catalysts have shown outstanding performance in both DFT theoretical calculations and experimental tests (Figs. 9a and 10a). Long et al. [38] tested the performance of Cu foam for NORR through a combination of theoretical calculations and experiments. The results showed (Fig. 10b) that the copper-based catalyst exhibited excellent activity and selectivity for NORR, achieving over 90% FE at -0.9 V vs. RHE. Single-atom catalysts are highly efficient catalysts that have been developed in recent years, with a theoretical atomic utilization efficiency of 100%. As the active sites are single metal atoms, SACs possess the advantages of both homogeneous and heterogeneous catalysts. Therefore, these catalysts have inherent advantages in catalytic processes, and many previous studies have reported highly efficient SACs in electrocatalytic NORR. Chen et al. [101] synthesized a Sb₁/a-MoO₃ single-atom catalyst for regulating the adsorption energy between MoO₃ and NO via Sb single atoms. Figure 10c reveals that the Sb site has fewer electronic states crossing the Fermi level compared to the Mo site upon the NO adsorption. This reduced interaction between Sb₁-*NO electrons indicates the weak binding of *NO on the Sb₁ site. Additionally, the PDOS analysis of *NO/*NHO on both catalysts demonstrated that the highest peak below the Fermi level (Ep) position of *NO on the Sb₁ site was significantly lower than that on the Mo site, while the Ep positions of *NHO on both Sb₁ and Mo sites were similar. The findings suggest that the Sb₁ site stabilized the crucial *NHO intermediate, despite exhibiting reduced *NO adsorption. Besides, the Sb₁ site maintained an optimal balance of the binding free energies of *NO and *NHO, thereby minimizing the reaction energy barrier. Consequently, this led to an acceleration of the reaction energetics for the conversion of NO to NH₃ on the Sb₁/a-MoO₃ catalyst. Remarkably, Sb₁/a-MoO₃ exhibits a FE of 91.7% and the highest NH₃ YR of 273.5 µmol h^{- 1} cm^{- 2} at - 0.6 V vs. RHE (Fig. 10d). By introducing heteroatoms such as B, N, P, etc., the electronic structure of metal sites in SACs can be directly tuned, thereby improving the catalytic activity. Nb-SA/BNC catalyst reported by Peng et al. [102] for NORR exhibited an ammonia YR of 8.2 × 10^{- 8} mol cm^{- 2} s^{- 1} at -0.6 V vs. RHE in Fig. 10e. Theoretical calculations demonstrated that the single-atomic Nb sites played a dual role in enhancing the NORR activity by promoting NO adsorption and reducing the energy barrier of the potential determining step (Fig. 10f).

As a renewable energy source, ammonia is considered to have great potential and will be the research focus in the future energy field. The electrocatalytic process, which can be conducted under ambient conditions without carbon emissions, offers a promising strategy for environmentally friendly ammonia production. Therefore, it is promising to realize the green cycle of nitrogen species via the electrochemical process. The upcycling of nitrogenous wastes into green ammonia by a carbon-free process using electrochemical technology at room temperature is very attractive. In this review, we summarized the electroreduction pathways of various nitrogenous wastes (NO₃^-, NO₂^-, NO) to green ammonia and the current research progress. Although some promising catalysts have been reported for these three catalytic reactions (NO₃RR, NO₂RR, NORR) as shown in Table 1, there are still several significant challenges in the future development of this field.

(1) The ideal electrode material should have high stability for long-period cycle electrolysis. However, the stabilities of the reported catalysts are unable to meet the durability standard of industrial applications. After long-time catalytic reactions, generally, electrode materials will face problems such as electrode corrosion, structure collapse, active site inactivation, and others. Therefore, one could focus on the design of catalysts with higher stability, which would facilitate the early industrialization of the field. Effective solutions could be developed from constructing corrosion-resistant electrodes, designing tandem catalysts to avoid the aggregation of intermediates on the catalyst surface, and improving the chelation strength between the active center and ligand.

(2) The catalysts reported so far are specific to a single nitrogenous waste. These nitrogenous wastes are mostly found in industrial and agricultural effluent/gas emissions, which often contain multiple nitrogenous wastes (NO₃^-, NO₂^-, NO_x). Considering that the conversion of all nitrogenous wastes to ammonia in wastewater requires complicated processes for different catalysts, designing universal catalysts for all nitrogenous wastes is promising to reduce process steps, energy consumption, and cost. At present, some existing catalysts have shown high catalytic activity for both NO₃^- and NO₂^-, but there is still no relevant research on the simultaneous catalysis of liquid and gas phase substances. The main reason is that the catalysis of gaseous nitrogenous waste requires the formation of a gas-liquid-solid three-phase steady state on the catalyst surface, which puts forward high requirements on the selectivity of the catalyst active site. It is possible to apply the design strategy of tandem catalysts, photocatalysts, and synergistic catalysts to the reduction of nitrogenous waste to ammonia.

(3) The reported experimental results of highly efficient catalysts are obtained under specific laboratory conditions, neglecting the variations in catalytic environments encountered in practical applications. These variations include factors such as electrolyte composition and concentration, supported electrolyte types, pH value, electrode potential, mass transport, gas atmosphere, and interference from other ions, among others. These factors would significantly affect the Faradaic efficiency and yield rate when dealing with nitrogenous wastes in the real environment. Therefore, researchers are suggested to simulate a more realistic catalytic environment, such as textile and nuclear wastewaters [19]. which would provide useful experimental data for large-scale treatment of nitrogen pollution.

(4) In the real environment, the concentration of nitrogenous compounds is generally lower than that reported in most articles. For example, in the case of nitrate, its content in industrial wastewater, textile wastewater, and contaminated groundwater is only 41.6 mM, 7.4 mM, and 0.9 ~ 1.2 mM, respectively. Consequently, achieving the efficient reduction of diluted nitrogenous compounds for ammonia production poses a significant challenge. Nevertheless, potential solutions include concentrating the diluted nitrogenous reagents into high nitrogenous reagents and designing highly selective catalysts tailored for treating low-concentration wastewater. These strategies hold promise for addressing the aforementioned challenge effectively [19,107].

(5) The upcycling of nitrogenous wastes into green ammonia is a very attractive catalytic field. In addition to evaluating the selectivity and yield rate of NH₃, how to extract and collect the converted ammonia is the key factor to realizing the green production of ammonia energy. Meanwhile, researchers should try to adopt methods with low energy consumption and low carbon when designing feasible solutions for ammonia extraction, collection, and transportation, which does not contradict the original intention of environmental protection. Furthermore, ammonia is also a pollutant present in environment. Therefore, efficient separation of ammonia from water is equally important and must be considered in the electrochemical ammonia production process. Under alkaline conditions, the potential approaches of steam stripping and solar-driven ammonia distillation offer promising pathways for the separation of ammonia. Additionally, emerging ammonia-ion batteries present a prospective solution for ammonia separation.

(6) Metal catalysts are currently the mainstream materials for electrocatalytic NO₃RR, NO₂RR, and NORR, and only a few publications on metal-free materials have been reported in NO₃RR. However, metal-based catalysts are confronted with some challenges such as high cost, easy poisoning of metal sites, and secondary pollution caused by metal ion leaching. The development of more low-cost and environmentally friendly non-metallic catalysts holds greater promise as well as lays the foundation for future industrialization. To solve the above problems, both single-atom catalysts and nonmetallic catalysts show great potential. However, the synthesis methods of single-atom catalysts are still very complicated at present. The development of simplified synthesis strategies for the preparation of atomically dispersed catalysts is more conducive to the wide application of metal catalysts with low loading capacity. In addition, the substrate with strong anchoring ability could be selected to prevent the accumulation or overflow of metal atoms to cause pollution.

In conclusion, electrocatalytic upcycling of nitrogenous wastes into more valuable ammonia is a fascinating and promising field with advantages in both nitrogen pollution reduction and green ammonia production. The promising field has attracted a steady stream of researchers to invest considerable efforts. In this review, the progress of NO₃RR, NO₂RR, and NORR catalysts is reviewed from the perspective of noble metal-based catalysts, non-noble metal-based catalysts, and metal-free catalysts (only NO₃RR). In addition, researchers are encouraged to refer to the above six suggestions and it is hoped that this paper could provide more enlightenment for the future development of electrocatalytic nitrogenous wastes into green ammonia.

NO₃RR$\ \ \ \ \ \ \ \ $ Nitrate Reduction Reaction

NO₂RR$\ \ \ \ \ \ \ \ $ Nitrite Reduction Reaction

NORR$\ \ \ \ \ \ \ \ $ Nitric Oxide Reduction Reaction

NRR$\ \ \ \ \ \ \ \ $ N₂ Reduction Reaction

HER$\ \ \ \ \ \ \ \ $ Hydrogen Evolution Reaction

FE $\ \ \ \ \ \ \ \ $ Faradaic Efficiency

YR$\ \ \ \ \ \ \ \ $ Yield Rate

ECE$\ \ \ \ \ \ \ \ $ Electrochemical-Chemical-Electrochemical

g-CN$\ \ \ \ \ \ \ \ $ Graphite Carbon Nitride

TM$\ \ \ \ \ \ \ \ $ Transition Metal

ΔG$\ \ \ \ \ \ \ \ $ Adsorption Energy

DFT$\ \ \ \ \ \ \ \ $ Density Functional Theory

PDOS$\ \ \ \ \ \ \ \ $ Partial Density of States

XANES$\ \ \ \ \ \ \ \ $ X-Ray Absorption Near-Edge Structure

XPS$\ \ \ \ \ \ \ \ $ X-Ray Photoelectron Spectroscopy

KPFM$\ \ \ \ \ \ \ \ $ Kelvin Probe Force Microscopy

The authors acknowledge the financial support by the National Nature Science Foundation of China (52202372), the Sichuan Science and Technology Program (2023NSFSC0436, 2023NSFSC0089), the National Key Research and Development Project (2022YFA1505300), and the Fundamental Research Funds for the Central Universities (YJ2021151).

P.L. and Z.J. conceived this review. Y.L. and K.L. were major contributors to writing the manuscript. P.W. participated in the discussion of this work. Z.J. and P.L. reviewed and edited the manuscript. All authors read and approved the final manuscript.

Open access funding provided by Shanghai Jiao Tong University. This work was supported by the National Nature Science Foundation of China (52202372), the Sichuan Science and Technology Program (2023NSFSC0436, 2023NSFSC0089), the National Key Research and Development Project (2022YFA1505300), and the Fundamental Research Funds for the Central Universities (YJ2021151).

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All authors declare that there are no competing interests.

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