Ampere-level current density ammonia electrochemical synthesis using CuCo nanosheets simulating nitrite reductase bifunctional nature

Unraveling the Structural Evolution of Cobalt Sulfides in Electrocatalytic NO3RR: the Inescapable Influence of Cl. Inorg Chem 2025;64:2787-2794. [PMID: 39915902 DOI: 10.1021/acs.inorgchem.4c04780] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Electrochemical nitrate reduction (NO3RR) to ammonia is an attractive approach for mitigating NO3- pollution and producing valuable NH3. Cobalt-sulfur compounds are widely considered to be potential electrocatalysts for NO3RR. However, there is still a lack of research on the probable structural evolution, long-term stability, and reactive sites of cobalt-based sulfides during catalysis. Herein, we have employed three cobalt sulfides (CoSx, where x = 8/9, 2, 1.097) with different sulfur contents as catalysts for electrocatalytic NO3RR under alkaline conditions. At -0.8 V vs RHE, all these CoSx show promising performances that Faradaic efficiencies of >80% and a high yield of >1780 mmol h-1 gcat-1 for NH3 production are achieved. Through a combination of X-ray diffraction (XRD), transmission electron microscopy (TEM), and other characterizations, it is revealed that all these cobalt sulfides are easily converted into cobalt hydroxide during the NO3RR. This phenomenon is seemingly contradictory to the thermodynamic prediction that, according to the Pourbaix diagram, these CoSx compounds should be stable even under the catalytic condition. We suggest that this is due to the presence of Cl- ions in the electrolyte that promote the transformation of CoSx toward Co(OH)2. Chloride ions are commonly found in both industrial settings and natural water bodies and are challenging to remove. The evolved Co(OH)2 species is proposed to be responsible for catalyzing NO3RR, especially during a long-term catalytic process. This study highlights the inevitable structural evolution of CoSx catalysts under current alkaline electrocatalytic NO3RR conditions, offering theoretical guidance for the judicious selection and design of future catalysts.

Enhancing Low-Concentration Electroreduction of NO to NH3 via Potential-Controlled Active Site-Intermediate Interactions

Electronic defect states in catalysts are recognized as highly effective active sites for enhancing the low-concentration electroreduction of NO to NH3 (NORR). Their structures dynamically evolve with applied electrode potentials, allowing the active sites to adjust interactions with intermediates, thereby improving electrocatalytic performance. However, the dynamic changes in these interactions under applied potentials remain poorly understood, hindering the design of more diverse electrocatalytic systems. Herein, we developed a strategy that unitizes electrode potential to control the interactions between active sites and intermediates over oxygen vacancy-modified TiO2 (VO-TiO2-x) to enhance NORR performance. By combining state-of-the-art constant inner potential (CIP) DFT calculations with in situ (spectro)electrochemical measurements, we investigated how the electrode potential influences these interactions in NORR. The results clearly demonstrate that applying an external potential alters the spatial symmetry of degenerate orbitals of Ti3+ to facilitate the generation of key intermediates for NO-to-NH3 conversion. Therefore, the VO-TiO2-x catalyst exhibited superior NORR performance with a NH3 Faradaic efficiency up to 76.4 % and a high NH3 yield rate of 632.9 μg h-1 mgcat. -1 under 1.0 vol % NO atmosphere, which is competitive with those of previously reported works under higher NO concentration (above 10 vol %). Remarkably, the NORR process achieved a record-breaking NH3 yield of 2292.7 μg h-1 mgcat. -1 in a membrane electrode assembly (MEA) electrolyzer under the same conditions. This study opens a new avenue for enhancing electrocatalytic activity by adjusting operating conditions, thereby transcending the limitations of material design.

Precise structural regulation of copper-based electrocatalysts for sustainable nitrate reduction to ammonia

The electrocatalytic reduction of nitrate to ammonia (NRA) technology not only achieves the effective removal of nitrates in the environment but also produces value-added products-NH3. In recent years, copper-based materials have shown tremendous application prospects in this field due to their excellent conductivity, moderate cost, and their proximity of d orbital energy levels to the LUMO π∗ molecular orbitals of nitrate. This review starts with copper-based catalysts to elucidate the reaction mechanisms of NRA and its influencing factors, while summarizing and analyzing the principles and pros and cons of various modification strategies. Then, we will explore the impact of different modification strategies on improving NRA performance and the underlying theoretical mechanisms. Finally, this review proposes the current challenges and prospects of copper-based materials, aiming to provide a reference for the further development and industrial application of copper-based catalysts.

Cascade Design and Facile Fabrication of Cu/Cu2O/CuAl-Layered Double Hydroxides as Efficient Nitrate Reduction Electrocatalysts

Nitrate (NO3¯) reduction reaction (NITRR) presents a promising pathway for the production of renewable NH3 while concurrently decontaminating NO3¯ wastewater. However, the multi-electron transfer sequence and complex reaction network involved in NO3¯ conversion pose significant challenges to achieving high Faradaic efficiency (FE). Herein, this work presents ternary Cu/Cu2O/CuAl-layered double hydroxides (LDHs) catalysts designed through a cascade approach and synthesized via a straightforward one-step electrodeposition method. The resulting catalysts demonstrate peak activity at -0.4 V versus RHE, achieving an impressiveF E N H 3 $F{{E}_{N{{H}_3}}}$ of 92.0%, which significantly surpasses most reported binary and ternary catalysts. Density functional theory calculations and atomic force microscopy reveal that the Cu/Cu2O/CuAl-LDHs exploit cascade design by integrating three distinct functions essential for efficient NO3¯ reduction: CuAl-LDH initiates NO3¯ adsorption, Cu(111) and Cu₂O(111) cooperatively facilitate NO3¯ activation, and Cu(111) promotes NH3 desorption. Durability tests further confirm that both NH3 yield andF E N H 3 $F{{E}_{N{{H}_3}}}$ remain stable after 10 cycles, indicating the excellent stability of the Cu/Cu2O/CuAl-LDHs catalysts. These findings underscore the critical role of cascade design strategies in enhancing the performance of electrocatalysts for NO3¯ reduction to NH3, providing a transformative approach for sustainable ammonia synthesis.

Interfacial Water Regulation for Nitrate Electroreduction to Ammonia at Ultralow Overpotentials

Nitrate electroreduction is promising for achieving effluent waste-water treatment and ammonia production with respect to the global nitrogen balance. However, due to the impeded hydrogenation process, high overpotentials need to be surmounted during nitrate electroreduction, causing intensive energy consumption. Herein, a hydroxide regulation strategy is developed to optimize the interfacial H2O behavior for accelerating the hydrogenation conversion of nitrate to ammonia at ultralow overpotentials. The well-designed Ru─Ni(OH)2 electrocatalyst shows a remarkable energy efficiency of 44.6% at +0.1 V versus RHE and a nearly 100% Faradaic efficiency for NH3 synthesis at 0 V versus RHE. In situ characterizations and theoretical calculations indicate that Ni(OH)2 can regulate the interfacial H2O structure with a promoted H2O dissociation process and contribute to the spontaneous hydrogen spillover process for boosting NO3 - electroreduction to NH3 at Ru sites. Furthermore, the assembled rechargeable Zn-NO3 -/ethanol battery system exhibits an outstanding long-term cycling stability during the charge-discharge tests with the production of high-value-added ammonium acetate, showing great potential for simultaneously achieving nitrate removal, energy conversion, and chemical synthesis. This work can not only provide a guidance for interfacial H2O regulation in extensive hydrogenation reactions but also inspire the design of a novel hybrid flow battery with multiple functions.

NiO-Incorporated Cu/Cu2O Nanowires for Highly Efficient Electrochemical Nitrate Reduction to Ammonia

Electrochemical nitrate reduction to ammonia (NRA) is a promising sustainable way to synthesize ammonia (NH3) from nitrate (NO3 -) contaminants. Cu-based electrocatalysts are frequently utilized for NRA due to their strong NO3 - adsorption and de-oxygenation ability. However, this kind of catalyst usually possesses the weak water dissociation ability, resulting in insufficient proton supply in alkaline media to retard the following hydrogenation step of O-containing intermediates (*NOx, typically NO2 -) to target NH3. Herein, NiO-incorporated Cu/Cu2O nanowires grown on nickel foam (p-CuNi@NF, p refers to plasma treatment) were synthesized via hydrothermal growth and subsequent O2 plasma treatment for efficient NRA electrocatalysis. On this p-CuNi@NF catalyst, NiO is able to accelerate the hydrogenation step by promoting the water dissociation to provide protons, ultimately facilitating efficient NRA. p-CuNi@NF exhibits excellent NH3 selectivity and yield in a wide potential range and reaches a high Faradaic efficiency (FENH3) of 97.5 % and a yield (YNH3) of 470 μmol h-1 cm-2 at -0.6 V, both of which largely surpass the Cu/Cu2O catalyst.

Improved Ammonia Synthesis and Energy Output from Zinc-Nitrate Batteries by Spin-State Regulation in Perovskite Oxides

Electrocatalytic nitrate reduction to ammonia (eNRA) is a promising route toward environmental sustainability and clean energy. However, its efficiency is often limited by the slow conversion of intermediates due to spin-forbidden processes. Here, we introduce a novel A-site high-entropy strategy to develop a new perovskite oxide (La0.2Pr0.2Nd0.2Ba0.2Sr0.2)CoO3-δ (LPNBSC) for eNRA. The LPNBSC possesses a higher concentration of high-spin (HS) cobalt-active centers, resulting from an increased concentration of [CoO5] structural motifs compared to conventional LaCoO3. Consequently, this material exhibits a significantly improved electrocatalytic performance toward ammonia (NH3) production, resulting in a 3-fold increase in yield rate (129 μmol h-1 mgcat.-1) and a 2-fold increase in Faradaic efficiency (FE, 76%) compared to LaCoO3 at the optimal potential. Furthermore, the LPNBSC-based Zn-nitrate battery reaches a maximum FE of 82% and an NH3 yield rate of 57 μmol h-1 cm-2. Density functional theory calculations reveal that A-site high-entropy management in perovskites facilitates nitrate activation and potentially optimizes the thermodynamic rate-determining step of the eNRA process, namely, *HNO3 + H+ + e- → *NO2 + H2O. This work presents an efficient concept for modulating the spin state of the B-site metal in perovskites and offers valuable insights for the design of high-performance eNRA catalysts.

Stabilizing Hydrogen Radicals in Two-Dimensional Cobalt-Copper Mesoporous Nanoplates for Complete Nitrate Reduction Electrocatalysis to Ammonia

Ambient electrochemical reduction of waste nitrate (NO3 -) represents an alternative green route for sustainable ammonia (NH3) electrosynthesis in water. Despites some encouraged achievements, sluggish eight electron and nine proton reduction routes that involve multi-step hydrogenation pathways have severely hindered their NH3 Faradaic efficiency (FENH3) and yield rate. Herein, we develop a robust two-dimensional mesoporous cobalt-copper (meso-CoCu) nanoplate electrocatalyst that delivers excellent performance of complete NO3 - reduction reaction (NO3RR), including superior FENH3 of 98.8 %, high NH3 yield rate of 3.39 mol h-1 g-1 and energy efficiency of 49.8 %, and good cycling stability. Mechanism investigations unveil that active hydrogen (*H) radicals produced from water splitting on Co sites spillover to adjacent Cu sites and further stabilize within confined mesopores, which kinetically promote its coupling hydrogenation reactions of nitrogen intermediates and thus facilitate complete NO3RR for favorable NH3 electrosynthesis. Moreover, meso-CoCu nanoplates perform well as a bifunctional electrocatalyst in the two-electrode coupling system that concurrently synthesizes NH3 from NO3 - at cathode and 2,5-furanedicarboxylic acid from 5-hydroxymethylfurfural at anode. This work in stabilizing *H radicals in mesoporous microenvironment provides some insights applied to various hydrogenation reactions for selective electrosynthesis of high value-added chemicals in water.

Reversible Hydrogen Acceptor-Donor Enables Relay Mechanism for Nitrate-to-Ammonia Electrocatalysis

Electrocatalytic nitrate reduction is a crucial process for sustainable ammonia production. However, to maximize ammonia yield efficiency, this technology inevitably operates at the potentials more negative than 0 V vs. RHE, leading to high energy consumption and competitive hydrogen evolution. To eradicate this issue, hydrogen tungsten bronze (HxWO3) as reversible hydrogen donor-acceptor is partnered with copper (Cu) to enable a relay mechanism at potentials positive than 0 V vs. RHE, which involves rapid intercalation of H into HxWO3 lattice, prompt de-intercalation of the lattice H and transfer onto Cu, and spontaneous H-mediated nitrate-to-ammonia conversion on Cu. The resulting catalysts demonstrated a high ammonia yield rate of 3332.9±34.1 mmol gcat -1 h-1 and a Faraday efficiency of ~100 % at 0.10 V vs. RHE, displaying a record-low estimated energy consumption of 17.6 kWh kgammonia -1. Using these catalysts, we achieve continuous ammonia production in an enlarged flow cell at a real energy consumption of 17.0 kWh kgammonia -1.

Recent advances in low-temperature nitrogen oxide reduction: effects of electric field application

This article presents a review of catalytic processes used at low temperatures to reduce emissions of nitrogen oxides (NOx) and nitrous oxide (N2O), which are exceedingly important in terms of their environmental impacts on the Earth. With conventional purification technologies, it has been difficult to remove these compounds under low-temperature conditions. By applying a catalytic process in an electric field for the three reactions of three-way catalysts (TWC), NOx storage reduction catalysts (NSR), and direct decomposition of N2O, we have achieved high catalytic activity even at low temperatures. By promoting ion migration on the catalyst surface, we have filled in the gaps in conventional catalytic technology and have opened the way to more efficient conversion of NOx and N2O.

Ammonia electrosynthesis from nitrate using a stable amorphous/crystalline dual-phase Cu catalyst

Renewable energy-driven electrocatalytic nitrate reduction reaction presents a low-carbon and sustainable route for ammonia synthesis under mild conditions. Yet, the practical application of this process is currently hindered by unsatisfactory electrocatalytic activity and long-term stability. Herein we achieve high-rate ammonia electrosynthesis using a stable amorphous/crystalline dual-phase Cu catalyst. The ammonia partial current density and formation rate reach 3.33 ± 0.005 A cm-2 and 15.5 ± 0.02 mmol h-1 cm-2 at a low cell voltage of 2.6 ± 0.01 V, respectively. Remarkably, the dual-phase Cu catalyst can maintain stable ammonia production with a Faradaic efficiency of around 90% at a high current density of 1.5 A cm-2 for up to 300 h. A scale-up demonstration with an electrode size of 100 cm2 achieves an ammonia formation rate as high as 11.9 ± 0.5 g h-1 at a total current of 160 A. The impressive electrocatalytic performance is ascribed to the presence of stable amorphous Cu domains which promote the adsorption and hydrogenation of nitrogen-containing intermediates, thus improving reaction kinetics for ammonia formation. This work underscores the importance of stabilizing metastable amorphous structures for improving electrocatalytic reactivity and long-term stability.

Electrochemical Nitrate Reduction to Ammonia on AuCu Single-Atom Alloy Aerogels under Wide Potential Window

Electrocatalytic nitrate reduction to ammonia (NO3RR) is very attractive for nitrate removal and ammonia production in industrial processes. However, the nitrate reduction reaction is characterized by intense hydrogen competition at strong reduction potentials, which greatly limits the Faraday efficiency at strong reduction potentials. Herein, we reported an AuxCu single-atom alloy aerogels (AuxCu SAAs) with three-dimensional network structure with significant nitrate reduction performance of Faraday efficiency (FE) higher than 90 % over a wide potential range (0 ~ -1 V RHE). The FE of the catalyst was close to 100 % at a high reduction potential of -0.8 VRHE, accompanying with NH3 yield reaching 6.21 mmol h1 cm2. More importantly, the catalyst maintained a long-term operation over 400 h at 400 mA cm2 for the NO3RR using a continuous flow system in a H-cell. Experimental and theoretical analysis demonstrate that the catalyst can lower the energy barrier for the hydrogenation reaction of *NO2, leading to a rapid consumption of the generated *H, facilitate the hydrogenation process of NO3RR, and inhibit the competitive HER at high overpotentials, which efficiently promotes the nitrate reduction reaction, especially in industrial applications.

Static Organic p-n Junctions in Photoelectrodes for Solar Ammonia Production with 86 % Internal Quantum Efficiency

For photoelectrocatalytic cells, a limitation exists in finding appropriate photoelectrode configurations that couple efficient extraction of high-energy electrons from absorbed photons and selective catalysis. Here we report an organic p-n junction approach to fabricate molecular photoelectrodes for conversion of solar energy and nitrate into valuable ammonia product. Solar irradiation of the photoelectrode generates charge-separated states with electrons and holes spatially separated at the n-type and p-type components, as revealed by surface photovoltage mapping, at a quantum yield of 90 %. The high-flux photogenerated electrons are rapidly transferred to the catalyst for solar ammonia production from nitrate reduction at an external quantum efficiency (EQE) and an internal quantum efficiency (IQE) of 57 % and 86 %, respectively. Time-resolved spectroscopic studies reveal that the large IQE originates from the combined high efficiencies for photoelectron generation, catalyst activation and dark catalysis. In a flow-cell setup coupled with a silicon solar cell, the photoelectrode without bias generates photocurrent of 57 mA cm-2 and ammonia at an EQE of 52 %.

Solar energy powered electrochemical reduction of CO2 on In2O3 nanosheets with high energy conversion efficiency at a large current density

Electrochemical CO2 reduction (ECO2R) to value-added chemicals offers a promising approach to both mitigate CO2 emission and facilitate renewable energy conversion. We demonstrate a solar energy powered ECO2R system operating at a relatively large current density (57 mA cm-2) using In2O3 nanosheets (NSs) as the cathode and a commercial perovskite solar cell as the electricity generator, which achieves the high solar to formate energy conversion efficiency of 6.6 %. The significantly enhanced operative current density with a fair solar energy conversion efficiency on In2O3 NSs can be ascribed to their high activity and selectivity for formate production, as well as the fast kinetics for ECO2R. The Faradic efficiencies (FEs) of formate In2O3 NSs are all above 93 %, with the partial current density of formate ranging from 2.3 to 342 mA cm-2 in a gas diffusion flow cell, which is among the widest for formate production on In-based catalysts. In-situ Raman spectroscopy and density functional theory simulations reveal that the exceptional performances of formate production on In2O3 NSs originates from the presence of abundant low coordinated edge sites, which effectively promote the selective adsorption of *OCHO while inhibiting *H adsorption.

Construction of dual sites on FeS2 surface for enhanced electrocatalytic reduction of nitrite to ammonia

Cost-effective iron sulfides (FeS2) hold great potential as high-performance catalysts for NO2- electroreduction to NH3 (NO2ER), which is hindered by the weak NO2 activation. Herein, the design of nonmetal-doped FeS2 electrocatalysts was initially conducted by density functional theory (DFT) computations. We found that doping with different nonmetal atoms effectively not only regulates the electronic structures of the d-electrons of Fe atoms but also creates the unique p-d hybridized dual active sites, thereby boosting the efficient NO2 activation. Owing to the optimal NO2 adsorption strength, N-doped FeS2 demonstrates a low limiting potential for the NO2--to-NH3 conversion, thus significantly improving NO2ER activity. Direct experimental evidence was provided afterward: an NH3 yield rate of 424.5 μmol/hcm-2 with a 92.4 % Faradaic efficiency was achieved. Our findings not only suggest a promising NO2ER catalyst through theoretical computations to guide experiments but also provide a comprehensive understanding of the structure-properties relationship.

Modulation of the Second-Beyond Coordination Structure in Single-Atom Electrocatalysts for Confirmed Promotion of Ammonia Synthesis

Although microenvironments surrounding single-atom catalysts (SACs) have been widely demonstrated to have a remarkable effect on their catalytic performances, it remains unclear whether the local structure beyond the secondary coordination shells works as well or not. Herein, we employed a series of metal-organic frameworks (MOFs) with well-defined and tunable second-beyond coordination spheres as model SAC electrocatalysts to discuss the influence of long-distance structure on the ammonia synthesis from nitrate, which were synthesized and denoted as Cu12-NDI-X (X = NMe2, H, F). It is first experimentally confirmed that the remote substitution of function groups beyond the secondary coordination sphere can remarkably affect the activity of ammonia synthesis. Meanwhile, the -H endowed Cu12-NND-H exhibits a superior ammonia yield (35.1 mg·h-1·mgcat-1) and FE (98.7%) to those modified with -NMe2 and -F, which also shows good stability at 100 mA·cm-2. The remarkable promotion of the modulated second-beyond coordination structure is unraveled to result from the adjustable d-band center of the Cu active site leading to promoted adsorption of the NO3- and protonation of key intermediates. Encouraged by its extraordinary ammonia yield, we employed the Cu12-NND-H electrode as a cathode to assemble one rechargeable Zn-nitrate battery that exhibits an impressive power density of 34.0 mW·cm-2, demonstrating its promising application in energy conversion and storage.

Cu3P/CoP Heterostructure for Efficient Electrosynthesis of Ammonia from Nitrate Reduction Reaction

Electrocatalytic nitrate reduction (ENO3RR) for ammonia production is one of the potential alternatives to Haber-Bosch technology for the realization of artificial ammonia synthesis. However, efficient ammonia production remains challenging due to the complex electron transfer process in ENO3RR. In this study, we fabricated a Cu3P/CoP heterostructure on carbon cloth (CC) by electrodeposition and vapor deposition, which exhibits an exceptional ENO3RR performance in alkaline medium, and showcases a Faradaic efficiency of ammonia (FENH3) and an ammonia yield rate as high as 97.95% and 17,637.3 μg h-1 cm-2 at -0.9 V vs RHE. Moreover, Cu3P/CoP also has excellent catalytic activity for nitrite reduction to ammonia, with an FENH3 up to 98.31% at -0.7 V vs RHE. The experimental and theoretical calculations reveal and confirm that the formation of a heterogeneous interface between Cu3P and CoP effectively promotes the electron transfer, where Cu3P as an electron donor induces the decrease of electron density around Cu and results in an enhancement of NO2- adsorption, thereby accelerating the ENO3RR process while inhibiting the competitive hydrogen evolution reaction (HER). Moreover, the metal phosphide catalyst facilitates the water dissociation, which accelerates the abundant *H generation, thus enhancing the subsequent hydrogenation process toward ENO3RR.

Modulating the Leverage Relationship in Nitrogen Fixation Through Hydrogen-Bond-Regulated Proton Transfer

In the electrochemical nitrogen reduction reaction (NRR), a leverage relationship exists between NH3-producing activity and selectivity because of the competing hydrogen evolution reaction (HER), which means that high activity with strong protons adsorption causes low product selectivity. Herein, we design a novel metal-organic hydrogen bonding framework (MOHBF) material to modulate this leverage relationship by a hydrogen-bond-regulated proton transfer pathway. The MOHBF material was composited with reduced graphene oxide (rGO) to form a Ni-N2O2 molecular catalyst (Ni-N2O2/rGO). The unique structure of O atoms in Ni-O-C and N-O-H could form hydrogen bonds with H2O molecules to interfere with protons being directly adsorbed onto Ni active sites, thus regulating the proton transfer mechanism and slowing the HER kinetics, thereby modulating the leverage relationship. Moreover, this catalyst has abundant Ni-single-atom sites enriched with Ni-N/O coordination, conducive to the adsorption and activation of N2. The Ni-N2O2/rGO exhibits simultaneously enhanced activity and selectivity of NH3 production with a maximum NH3 yield rate of 209.7 μg h-1 mgcat. -1 and a Faradaic efficiency of 45.7 %, outperforming other reported single-atom NRR catalysts.

In situ evolution of electrocatalysts for enhanced electrochemical nitrate reduction under realistic conditions

Electrochemical nitrate reduction to ammonia (ENRA) is gaining attention for its potential in water remediation and sustainable ammonia production, offering a greener alternative to the energy-intensive Haber-Bosch process. Current research on ENRA is dedicated to enhancing ammonia selectively and productivity with sophisticated catalysts. However, the performance of ENRA and the change of catalytic activity in more complicated solutions (i.e., nitrate-polluted groundwater) are poorly understood. Here we first explored the influence of Ca2+ and bicarbonate on ENRA using commercial cathodes. We found that the catalytic activity of used Ni or Cu foam cathodes significantly outperforms their pristine ones due to the in situ evolution of new catalytic species on used cathodes during ENRA. In contrast, the nitrate conversion performance with nonactive Ti or Sn cathode is less affected by Ca2+ or bicarbonate because of their original poor activity. In addition, the coexistence of Ca2+ and bicarbonate inhibits nitrate conversion by forming scales (CaCO3) on the in situ-formed active sites. Likewise, ENRA is prone to fast performance deterioration in treating actual groundwater over continuous flow operation due to the presence of hardness ions and possible organic substances that quickly block the active sites toward nitrate reduction. Our work suggests that more work is required to ensure the long-term stability of ENRA in treating natural nitrate-polluted water bodies and to leverage the environmental relevance of ENRA in more realistic conditions.

Electrochemical Synthesis of Metasequoia-Like Reduced Graphene Oxide Coated Cobalt-Silver Catalyst for Stable and Efficient Electrocatalytic Nitrate Reduction to Ammonia

Electrocatalytic nitrate (NO3 -) reduction to ammonia (NH3) is a green and efficient NH3 synthesis technology. Metallic silver (Ag) is one of the well-known electrocatalysts for NO3 - reduction. However, under alkaline conditions, its poor water-splitting ability fails to provide sufficient protonic hydrogen required for NH3 synthesis, resulting in low NH3 selectivity. Additionally, metal catalysts are prone to leaching and oxidation during electrocatalysis, resulting in poor stability. Herein, cobalt (Co) into Ag (CoAg) catalyst is doped, which not only increases the NH3 selectivity by 34.4%, but also reduces the reduction potential by 0.1 V. Meanwhile, reduced graphene oxide (rGO) as a protective "armor" is used to encapsulate the CoAg catalyst (rGO2.92@CoAg). The rGO2.92@CoAg catalyst shows excellent stability for over 300 hours (h) of continuous reaction. The Co and Ag contents in the rGO2.92@CoAg catalyst after continuous tests decreases by only 4.3% and 3.1%, respectively, which are much lower than those of the CoAg catalyst without the rGO (90.8%, 52.6%). Moreover, the rGO2.92@CoAg catalyst shows high Faradaic efficiency (99.3%) and NH3 yield rate (1.47 mmol h-1 cm-2). Therefore, a high performance and strong stability rGO2.92@CoAg catalyst is obtained by Co doping and rGO coating, which provides theoretical basis for practical industrial application.

Mechanistic insights on the preparation of 5-methyl-2-hexanone by hydrogenation of 5-methyl-3-hexen-2-one using Pd/Al2O3 catalysts

5-methyl-2-hexanone is used as a versatile polymerization solvent for industrial preparation processes of bulk and fine chemicals. An efficient catalyst, Pd/γ-Al2O3, is reported for the preparation of 5-methyl-2-hexanone by selective hydrogenation of 5-methyl-3-hexen-2-one. The catalyst exhibits remarkable activity and selectivity even at atmospheric pressure and low temperature (1 atm, 80 °C). The influence weight of reaction conditions on the reaction process was obtained through the Artificial Neural Network model, which were reaction pressure, reaction temperature and liquid hourly space velocity in order. The reaction kinetics and mechanism of 5-methyl-2-hexanone preparation by hydrogenation over Pd/γ-Al2O3 catalyst were investigated. The hydrogenation reaction pathway of 5-methyl-3-hexen-2-one was obtained by using Density functional theory calculations, and the mechanism of selective hydrogenation of CC double bonds and CO double bonds was revealed. A kinetic model based on the LHHW model assumption was also proposed and compared with experimental results demonstrating good predictability.

Enrichment of Active Hydrogen at Amorphous CoO/Cu2O Heterojunction Interfaces Enhances Electrocatalytic Nitrate Reduction to Ammonia

The reduction of nitrate into valuable ammonia via electrocatalysis offers a green and sustainable synthetic pathway for ammonia. The electrocatalytic nitrate reduction reaction (NO3RR) encompasses two crucial reaction steps: nitrate deoxygenation and nitrite hydrogenation. Notably, the nitrite hydrogenation reaction is regarded as the rate-determining step of the process. Herein, the amorphous CoO support introduced for the construction of the a-CoO/Cu2O tandem catalyst provides sufficient active hydrogen and synergistically catalyzes the NO3RR. The a-CoO/Cu2O catalyst showed excellent performance with a maximum NH3 Faradaic efficiency of 95.72% and a maximum yield rate of 0.96 mmol h-1 mgcat -1 at -0.4 V. In the flow cell, the maximum NH3 yield rate of 12.14 mmol h-1 mgcat -1 is achieved at -800 mA. The high NO3RR activity of a-CoO/Cu2O is attributed to the synergistic cascade effect of amorphous CoO and Cu2O at the heterojunction interface, where Cu2O serves as the adsorption site for NO3 -, while the accelerated active hydrogen generation of amorphous CoO promotes the nitrite hydrogenation reaction. This work provides a strategy for designing multi-site cascade catalysts centered on amorphous structures to achieve efficient NO3RR.

Leveraging Interfacial Electric Field for Smart Modulation of Electrode Surface in Nitrate to Ammonia Conversion

The efficiency of nitrate reduction reaction (NO3RR) at low nitrate concentration is predominantly hindered by the poor affinity of nitrate ions and competitive hydrogen evolution reaction (HER), particularly in neutral and acidic media. Here, an innovative strategy to leverage the interfacial electric field (IEF) is introduced to boost the NO3RR performance. By in situ constructing tannic acid-metal ion (TA-M2+) crosslinked structure on the electrode surface, the TA-M2+-CuO NW/Cu foam sample exhibits an exceptional Faraday efficiency of 99.4% at -0.2 V versus reversible hydrogen electrode (RHE) and 83.9% at 0.0 V versus RHE under neutral and acidic conditions, respectively. The computational studies unveil that the TA-Cu2+ complex on the CuO (111) plane induces the increasing concentration of nitrate at the interface, accelerating NO3RR kinetics over HER via the IEF effect. This interfacial modulation strategy also contributes the enhanced ammonia production performance when it is employed on commercial electrode materials and flow reactors, exhibiting great potential in practical application. Overall, combined results illustrated multiple merits of the IEF effect, paving the way for future commercialization of NO3RR in the ammonia production industry.

Ampere-level reduction of pure nitrate by electron-deficient Ru with K+ ions repelling effect

Electrochemical nitrate reduction reaction offers a sustainable and efficient pathway for ammonia synthesis. Maintaining satisfactory Faradaic efficiency for long-term nitrate reduction under ampere-level current density remains challenging due to the inevitable hydrogen evolution, particularly in pure nitrate solutions. Herein, we present the application of electron deficiency of Ru metals to boost the repelling effect of counter K+ ions via the electric-field-dependent synergy of interfacial water and cations, and thus largely promote nitrate reduction reaction with a high yield and well-maintained Faradaic efficiency under ampere-level current density. The pronounced electron deficiency of Ru metals boosts the repelling effect on hydrated K+ ions, as indicated by the distance of K+ ions to catalyst surface, which can loosen the water layer to depress hydrogen evolution and accelerate nitrate conversion. Consequently, the optimized electrode loaded with electron-deficient Ru atomic layers can directly produce 0.26 M ammonia solution in pure nitrate solution in 6 h, providing a high yield (74.8 mg mgcat-1 h-1) and well-maintained the Faradaic efficiency for over 120 h under ampere-level reduction.

Nanoporous copper titanium tin (np-Cu2TiSn) Heusler alloy prepared by dealloying-induced phase transformation for electrocatalytic nitrate reduction to ammonia

Heusler alloys are a series of well-established intermetallic compounds with abundant structure and elemental substitutions, which are considered as potentially valuable catalysts for integrating multiple reactions owing to the features of ordered atomic arrangement and optimized electronic structure. Herein, a nanoporous copper titanium tin (np-Cu2TiSn) Heusler alloy is successfully prepared by the (electro)chemical dealloying transformation method, which exhibits high nitrate (NO3-) reduction performance with an NH3 Faradaic efficiency of 77.14 %, an NH3 yield rate of 11.90 mg h-1 mg-1cat, and a stability for 100 h under neutral condition. Significantly, we also convert NO3- to high-purity ammonium phosphomolybdate with NH4+ collection efficiency of 83.8 %, which suggests a practical approach to convert wastewater nitrate into value-added ammonia products. Experiments and theoretical calculations reveal that the electronic structure of Cu sites is modulated by the ligand effect of surrounding Ti and Sn atoms, which can simultaneously enhance the activation of NO3-, facilitate the desorption of NH3, and reduce the energy barriers, thereby boosting the electrochemical nitrate reduction reaction.

Copper-nickel-MOF/nickel foam catalysts grown in situ for efficient electrochemical nitrate reduction to ammonia

Reducing nitrate (NO3-) in an aqueous solution to ammonia under ambient conditions can provide a green and sustainable NH3-synthesis technology and mitigate global energy and pollution issues. In this work, a CuNi0.75-1,3,5-benzenetricarboxylic acid/nickel foam (CuNi0.75-MOF/NF) catalyst grown in situ was prepared via a one-pot method as an efficient cathode material for electrocatalytic nitrate reduction reaction (NO3RR). The CuNi0.75-MOF/NF catalyst exhibited excellent electrocatalytic NO3RR performance at -1.0 V versus a reversible hydrogen electrode, achieving an outstanding faradaic efficiency of 95.88 % and an NH3 yield of 51.78 mg h-1 cm-2. The 15N isotope labeling experiments confirmed that the sole source of N in the electrocatalytic NO3RR was the NO3- in the electrolyte. The reaction pathway for the electrocatalytic NO3RR was derived by in situ Fourier transform infrared spectroscopy and in situ differential electrochemical mass spectrometry. Density functional theory calculations revealed that the Ni element in the CuNi0.75-MOF/NF catalyst had excellent O-H activation ability and strong *H adsorption capacity. These *H species were transferred from the Ni sites to the *NO adsorption intermediates located on the Cu sites, providing a continuous supply of *H to Cu, thereby promoting the formation of *NOH intermediates and enhancing the hydrogenation process of the electrocatalytic NO3RR.

Transition Metal-Gallium Intermetallic Compounds with Tailored Active Site Configurations for Electrochemical Ammonia Synthesis

Gallium (Ga) with a low melting point can serve as a unique metallic solvent in the synthesis of intermetallic compounds (IMCs). The negative formation enthalpy of transition metal-Ga IMCs endows them with high catalytic stability. Meanwhile, their tunable crystal structures offer the possibility to tailor the configurations of active sites to meet the requirements for specific catalytic applications. Herein, we present a general method for preparing a range of transition metal-Ga IMCs, including Co-Ga, Ni-Ga, Pt-Ga, Pd-Ga, and Rh-Ga IMCs. The structurally ordered CoGa IMCs with body-centered cubic (bcc) structure are uniformly dispersed on the nitrogen-doped reduced graphene oxide substrate (O-CoGa/NG) and deliver outstanding nitrate reduction reaction (NO3RR) performance, making them excellent catalysts to construct highly efficient rechargeable Zn-NO3 - battery. Operando studies and theoretical simulations demonstrate that the electron-rich environments around the Co atoms enhance the adsorption strength of *NO3 intermediate and simultaneously suppress the formation of hydrogen, thus improving the NO3RR activity and selectivity.

Self-Reconstruction Induced Electronic Metal-Support Interaction for Modulated Cu+ Sites on TiO2 Nanofibers in Electrocatalytic Nitrate Conversion

The Cu+ active sites have gained great attention in electrochemical nitrate reduction, offering a highly promising method for nitrate removal from water bodies. However, challenges arise from the instability of the Cu+ state and microscopic structure over prolonged operation, limiting the selectivity and durability of Cu+-based electrodes. Herein, a self-reconstructed Cu2O/TiO2 nanofibers (Cu2O/TiO2 NFs) catalyst, demonstrating exceptional stability over 50 cycles (12 h per cycle), a high NO3 --N removal rate of 90.2%, and N2 selectivity of 98.7% is reported. The in situ electrochemical reduction contributes to the self-reconstruction of Cu2O/TiO2 nanofibers with stabilized Cu+ sites via the electronic metal-support interaction between TiO2 substrates, as evidenced by in situ characterizations and theoretical simulations. Additionally, density functional theory (DFT) calculations also indicate that the well-retained Cu+ sites enhance catalytic capability by inhibiting the hydrogen evolution reaction and optimizing the binding energy of *NO on the Cu2O/TiO2 NFs heterostructure surface. This work proposes an effective strategy for preserving low-valence-state Cu-based catalysts with high intrinsic activity for nitrate reduction reaction (NO3RR), thereby advancing the prospects for sustainable nitrate remediation technologies.

Enhanced Charge-Carrier Dynamics and Efficient Photoelectrochemical Nitrate-to-Ammonia Conversion on Antimony Sulfide-Based Photocathodes

The photoelectrochemical reduction of nitrate to ammonia (PEC NO3RR) has emerged as a promising pathway for facilitating the natural nitrogen cycle. The PEC NO3RR can lower the reduction potential needed for ammonia synthesis through photogenerated voltage, showcasing the significant potential for merging abundant solar energy with sustainable nitrogen fixation. However, it is influenced by the selective photocathodes with poor carrier kinetics, low catalytic selectivity, and ammonia yields. There are few reports on suitable photoelectrodes owning efficient charge transport on PEC NO3RR at low overpotentials. Herein, we rationally constructed the CuSn alloy co-catalysts on the antimony sulfides with a highly selective PEC ammonia and an ultra-low onset potential (0.62 VRHE). CuSn/TiO2/Sb2S3 photoelectrodes achieved an ammonia faradic efficiency of 97.82 % at a low applied potential of 0.4 VRHE, and an ammonia yield of 16.96 μmol h-1 cm-2 at 0 VRHE under one sun illumination. Dynamics experiments and theoretical calculations have demonstrated that CuSn/TiO2/Sb2S3 has an enhanced charge separation and transfer efficiency, facilitating photogenerated electrons to participate in PEC NO3RR quickly. Meanwhile, moderate NO2* adsorption on this photocathode optimizes the catalytic activity and increases the NH4 + yield. This work opens an avenue for designing sulfide-based photocathodes for the efficient route of solar-to-ammonia conversion.

Selective electrosynthesis of hydroxylamine from aqueous nitrate/nitrite by suppressing further reduction

The electrocatalytic reduction of nitrogenous waste offers a sustainable approach to producing nitrogen-containing chemicals. The selective synthesis of high-value hydroxylamine (NH2OH) is challenging due to the instability of NH2OH as an intermediate. Here, we present a rational electrocatalyst design strategy for promoting NH2OH electrosynthesis by suppressing the competing pathways of further reduction. We screen zinc phthalocyanines (ZnPc) with a high energy barrier for NH2OH reduction by regulating their intrinsic activity. Additionally, we discover that carbon nanotube substrates exhibit significant NH3-producing activity, which can be effectively inhibited by the high coverage of ZnPc molecules. In-situ characterizations reveal that NH2OH and HNO are generated as intermediates in nitrate reduction to NH3, and NH2OH can be enriched in the ZnPc electrode. In the H-cell, the optimized ZnPc catalyst demonstrates a Faradaic efficiency (FE) of 53 ± 1.7% for NH2OH with a partial current density exceeding 270 mA cm-2 and a turnover frequency of 7.5 ± 0.2 s-1. It also enables the rapid electrosynthesis of cyclohexanone oxime from nitrite with a FE of 64 ± 1.0%.

A GO regulated bimetallic CoFe catalyst for efficient electrochemical nitrate reduction to ammonia under acidic conditions

Electrocatalytic nitrate reduction to ammonia (ENRA) in acidic media is currently a challenge. Herein, we presented a CoFe2O4/GO catalyst that exhibited a high faradaic efficiency (95.51%) and ammonia yield rate (1268.04 μmol h-1 cm-2) for ENRA under acidic conditions. The ENRA reaction mechanism was investigated through in situ ATR-FTIR spectroscopy and isotope labeling experiments.

Evidence for Distinct Active Sites on Oxide-Derived Cu for Electrochemical Nitrate Reduction

Cu is a promising catalyst for electrochemical nitrate (NO3-) reduction. However, desorption of the nitrite (NO2-) intermediate can occur, leading to lowered ammonia productivity and Faradaic efficiency. Here, we discovered that this does not occur with oxide-derived Cu due to the presence of at least two distinct types of cooperative active sites: one for NO3- → NO2- and another for NO2- → NH3. As a result, oxide-derived Cu exhibits enhanced ammonia productivity with a mixed NO3-/NO2- feed relative to pure NO3- or NO2-. In contrast, this was not observed with a standard Cu sample, implying the presence of only a single type of active site. Our dual-site hypothesis was supported by attenuated total reflection surface enhanced infrared absorption spectroscopy and isotopic labeling experiments involving co-reduction of 15NO3-/14NO2-. We also successfully simulated our experimental results using a mathematical model involving two different adsorption sites. These findings motivate the need for further study and rational design of such active sites.

Cu/CuxO/Graphdiyne Tandem Catalyst for Efficient Electrocatalytic Nitrate Reduction to Ammonia

The electrocatalytic reduction reaction of nitrate (NO3 -) to ammonia (NH3) is a feasible way to achieve artificial nitrogen cycle. However, the low yield rate and poor selectivity toward NH3 product is a technical challenge. Here a graphdiyne (GDY)-based tandem catalyst featuring Cu/CuxO nanoparticles anchored to GDY support (termed Cu/CuxO/GDY) for efficient electrocatalytic NO3 - reduction is presented. A high NH3 yield rate of 25.4 mg h-1 mgcat. -1 (25.4 mg h-1 cm-2) with a Faradaic efficiency of 99.8% at an applied potential of -0.8 V versus RHE using the designed catalyst is achieved. These performance metrics outperform most reported NO3 - to NH3 catalysts in the alkaline media. Electrochemical measurements and density functional theory reveal that the NO3 - preferentially attacks Cu/CuxO, and the GDY can effectively catalyze the reduction of NO2 - to NH3. This work highlights the efficacy of GDY as a new class of tandem catalysts for the artificial nitrogen cycle and provides powerful guidelines for the design of tandem electrocatalysts.

Regulating the Electrochemical Nitrate Reduction Performance with Controllable Distribution of Unconventional Phase Copper on Alloy Nanostructures

Electrochemical nitrate reduction reaction (NO3RR) is emerging as a promising strategy for nitrate removal and ammonia (NH3) production using renewable electricity. Although great progresses have been achieved, the crystal phase effect of electrocatalysts on NO3RR remains rarely explored. Here, the epitaxial growth of unconventional 2H Cu on hexagonal close-packed (hcp) IrNi template, resulting in the formation of three IrNiCu@Cu nanostructures, is reported. IrNiCu@Cu-20 shows superior catalytic performance, with NH3 Faradaic efficiency (FE) of 86% at -0.1 (vs reversible hydrogen electrode [RHE]) and NH3 yield rate of 687.3 mmol gCu -1 h-1, far better than common face-centered cubic Cu. In sharp contrast, IrNiCu@Cu-30 and IrNiCu@Cu-50 covered by hcp Cu shell display high selectivity toward nitrite (NO2 -), with NO2 - FE above 60% at 0.1 (vs RHE). Theoretical calculations have demonstrated that the IrNiCu@Cu-20 has the optimal electronic structures for NO3RR due to the highest d-band center and strongest reaction trend with the lowest energy barriers. The high electroactivity of IrNiCu@Cu-20 originates from the abundant low coordination of Cu sites on the surface, which guarantees the fast electron transfer to accelerate the intermediate conversions. This work provides a feasible tactic to regulate the product distribution of NO3RR by crystal phase engineering of electrocatalysts.

Enhancing Compatibility of Two-Step Tandem Catalytic Nitrate Reduction to Ammonia Over P-Cu/Co(OH)2

Electrochemical nitrate reduction reaction (NO3RR) is a promising approach to realize ammonia generation and wastewater treatment. However, the transformation from NO3 - to NH3 involves multiple proton-coupled electron transfer processes and by-products (NO2 -, H2, etc.), making high ammonia selectivity a challenge. Herein, a two-phase nanoflower P-Cu/Co(OH)2 electrocatalyst consisting of P-Cu clusters and P-Co(OH)2 nanosheets is designed to match the two-step tandem process (NO3 - to NO2 - and NO2 - to NH3) more compatible, avoiding excessive NO2 - accumulation and optimizing the whole tandem reaction. Focusing on the initial 2e- process, the inhibited *NO2 desorption on Cu sites in P-Cu gives rise to the more appropriate NO2 - released in electrolyte. Subsequently, P-Co(OH)2 exhibits a superior capacity for trapping and transforming the desorbed NO2 - during the latter 6e- process due to the thermodynamic advantage and contributions of active hydrogen. In 1 m KOH + 0.1 m NO3 -, P-Cu/Co(OH)2 leads to superior NH3 yield rate of 42.63 mg h- 1 cm- 2 and NH3 Faradaic efficiency of 97.04% at -0.4 V versus the reversible hydrogen electrode. Such a well-matched two-step process achieves remarkable NH3 synthesis performance from the perspective of optimizing the tandem catalytic reaction, offering a novel guideline for the design of NO3RR electrocatalysts.

Shear-Strained Pd Single-Atom Electrocatalysts for Nitrate Reduction to Ammonia

Electrochemical nitrate reduction method (NitRR) is a low-carbon, environmentally friendly, and efficient method for synthesizing ammonia, which has received widespread attention in recent years. Copper-based catalysts have a leading edge in nitrate reduction due to their good adsorption of *NO3. However, the formation of active hydrogen (*H) on Cu surfaces is difficult and insufficient, resulting in a large amount of the by-product NO2 -. In this work, Pd single atoms suspended on the interlayer unsaturated bonds of CuO atoms formed due to dislocations (Pd-CuO) were prepared by low temperature treatment, and the Pd single atoms located on the dislocations were subjected to shear stress and the dynamic effect of support formation to promote the conversion of nitrate into ammonia. The catalysis had an ammonia yield of 4.2 mol. gcat -1. h-1, and a Faraday efficiency of 90 % for ammonia production at -0.5 V vs. RHE. Electrochemical in situ characterization and theoretical calculations indicate that the dynamic effects of Pd single atoms and carriers under shear stress obviously promote the production of active hydrogen, reduce the reaction energy barrier of the decision-making step for nitrate conversion to ammonia, further promote ammonia generation.

Integrating few-atom layer metal on high-entropy alloys to catalyze nitrate reduction in tandem

While high-entropy alloy (HEA) catalysts seem to have the potential to break linear scaling relationships (LSRs) due to their structural complexity, the weighted averaging of properties among multiple principal components actually makes it challenging to diverge from the symmetry dependencies imposed by the LSRs. Herein, we develop a 'surface entropy reduction' method to induce the exsolution of a component with weak affinity for others, resulting in the formation of few-atom-layer metal (FL-M) on the surface of HEAs. These exsolved FL-M surpass the confines of the original configurational space of conventional HEAs, and collaborate with the HEA substrate, serving as geometrically separated active sites for multiple intermediates in a complex reaction. This FL-M-covered HEA shows an outstanding performance for electrocatalytic reduction of nitrate to ammonia (NH3) with a Faradaic efficiency of 92.7%, an NH3 yield rate of 2.45 mmol h-1 mgcat.-1, and high long-term stability (>200 h). Our work achieves the precise manipulation of atomic arrangement, thereby expanding both the chemical space occupied by known HEA catalysts and their potential application scenarios.

Highly Efficient Electrocatalytic Nitrate Reduction to Ammonia: Group VIII-Based Catalysts

The accumulation of nitrates in the environment causes serious health and environmental problems. The electrochemical nitrate reduction reaction (e-NO3RR) has received attention for its ability to convert nitrate to value-added ammonia with renewable energy. The key to effective catalytic efficiency is the choice of materials. Group VIII-based catalysts demonstrate great potential for application in e-NO3RR because of their high activity, low cost, and good electron transfer capability. This review summarizes the Group VIII catalysts, including monatomic, bimetallic, oxides, phosphides, and other composites. On this basis, strategies to enhance the intrinsic activity of the catalysts through coordination environment modulation, synergistic effects, defect engineering and hybridization are discussed. Meanwhile, the ammonia recovery process is summarized. Finally, the current research status in this field is prospected and summarized. This review aims to realize the large-scale application of nitrate electrocatalytic reduction in industrial wastewater.

Accelerated ammonia electrosynthesis of cobalt hydroxide through electronic modulation with ultralow noble metal doping

This study introduces an innovative method to ammonia production through electrocatalytic reduction of nitrate using Ru-doped Co(OH)2. The incorporation of Ru into the Co(OH)2 was found to markedly enhance the catalytic activity by optimizing the electronic structure and increasing the number of active sites.

Ultrathin cobalt-based nanosheets containing surface oxygen promoted near-complete nitrate removal

Electrocatalytic nitrate removal offers a sustainable approach to alleviate nitrate pollution and to boost the anthropogenic nitrogen cycle, but it still suffers from limited removal efficiency at high rates, especially at low levels of nitrate. Herein, we report the near-complete removal of low-level nitrate (10-200 ppm) within 2 h using ultrathin cobalt-based nanosheets (CoNS) containing surface oxygen, which was fabricated from in-situ electrochemical reconstruction of conventional nanosheets. The average nitrate removal of 99.7 % with ammonia selectivity of 98.2 % in 9 cyclic runs ranked in the best of reported catalysts. Powered by a solar cell under the winter sun, the full-cell nitrate electrolysis system, equipped with ultrathin CoNS, achieved 100 % nitrogen gas selectivity and 99.6 % total nitrogen removal. The in-situ Fourier Transform Infrared included experiments and theoretical computations revealed that in-situ electrochemical reconstruction not only increased electrochemical active surface area but also constructed surface oxygen in active sites, leading to enhanced stabilization of nitrate adsorption in a symmetry breaking configuration and charge transfer, contributing to near-complete nitrate removal on ultrathin CoNS. This work provides a strategy to design ultrathin nanocatalysts for nitrate removal.

Advances in designing efficient electrocatalysts for nitrate reduction from a theoretical perspective

Ammonia (NH3), an important raw material for producing fertilizers and useful chemicals, plays a crucial role in modern human society. As the Haber-Bosch process is energy- and emission-intensive, it is critical to develop a green and energy-efficient route for massive NH3 production under ambient conditions. The electrochemical nitrate reduction reaction to ammonia (eNO3-RR) is a potential way for producing NH3 while harmonizing the nitrogen cycle. In this feature article, we summarize the advances in designing eNO3-RR electrocatalysts from a theoretical perspective. First, the mechanisms and pathways of the eNO3-RR are summarized. Then, the recently developed electrocatalysts, including Cu-based catalysts, single-atom catalysts (SACs), dual-atom catalysts (DACs), and MXene catalysts, are categorically discussed. Finally, the challenges and prospects of designing highly efficient eNO3-RR catalysts through theoretical simulations are discussed. This feature article will provide valuable guidance for the future development of advanced eNO3-RR electrocatalysts for NH3 production.

Homogeneously Mixed Cu-Co Bimetallic Catalyst Derived from Hydroxy Double Salt for Industrial-Level High-Rate Nitrate-to-Ammonia Electrosynthesis

Electrocatalytic nitrate reduction reaction (NO3RR) presents an innovative approach for sustainable NH3 production. However, selective NH3 production is hindered by the multiple intermediates involved in the NO3RR process and the competitive hydrogen evolution reaction. Hence, the development of highly efficient NO3RR catalysts is paramount. Herein, we report highly efficient bimetallic catalysts derived from hydroxy double salt (HDS). Under NO3RR conditions, Cu1Co1-HDS undergoes in situ reconstruction, forming nanocomposites of homogeneously distributed metallic Cu0 and Co(OH)2. Reconstruction-induced Cu0 rapidly converts NO3- to NO2-, which is further hydrogenated to NH3 by Co(OH)2. Homogeneously mixed Cu and Co species maximize this synergistic effect, achieving outstanding NO3RR performance including the highest NH3 yield rate (4.625 mmol h-1 cm-2) reported for powder-type NO3RR catalysts. Integration of Cu1Co1-HDS with a commercial Si solar cell attained 4.53% solar-to-ammonia efficiency from industrial wastewater-level concentrations of NO3- (2000 ppm), demonstrating practical application potential for solar-driven NH3 production. This study provides a strategy for enhancing the NH3 yield rate by optimizing the compositions and distributions of Cu and Co.

Ambient electrosynthesis of urea with nitrate and carbon dioxide over a CuRu alloy catalyst

Urea synthesis under mild conditions starting from the electrocatalytic coupling of carbon dioxide (CO2) and nitrate represents a promising alternative experimentally to conquer the huge energy consumption in the industrial Haber-Bosch process. Herein, an electrocatalyst consisting of CuRu alloy nanoparticles on carbonized cellulose (CuRu-CBC) is designed and realizes the urea yield rate of 394.85 ± 16.19 μg h-1 mgcat-1 and an ultrahigh faradaic efficiency (FE) of 68.94 ± 3.05% at -0.55 V (vs. RHE) under ambient conditions. Further XAS analyses indicated that the favored internal electron transfer between Cu and Ru dual active sites significantly improved the C-N coupling activity. Various characterizations, including in situ ATR-SEIRAS and DEMS analysis highlighted the favorable generation of key intermediates *CO and *NH, making CuRu-CBC a promising catalyst for urea synthesis.

Conversion of Nitrate to Ammonia by Amidinothiourea-Coordinated Metal Molecular Electrocatalysts with d-π Conjugation

The electrochemical reduction of nitrate to ammonia (NO3RR) provides a desired alternative of the traditional Haber-Bosch route for ammonia production, igniting a research boom in the development of electrocatalysts with high activity. Among them, molecular electrocatalysts hold considerable promise for the NO3RR, suppressing the competing hydrogen evolution reaction. However, the complicated synthesis procedure, usage of environmentally unfriendly organic solvents, and poor stability of Cu-based molecular electrocatalysts greatly limit their employment in NO3RR, and the development of desired Cu-based molecular catalysts remains challenging. Herein, a simple nonorganic solvent involving a one-step strategy was proposed to synthesize d-π-conjugated molecular electrocatalysts metal-amidinothiourea (M-ATU). Cu-ATU is composed of Cu coordinated with two S and two N atoms, whereas Ni-ATU is formed by Ni with four N atoms from two ATU ligands. Remarkably, Cu-ATU with a Cu-N2S2 coordination configuration exhibits superior NO3RR activity with a NH3 yield rate of 159.8 mg h-1 mgcat-1 (-1.54 V) and Faradaic efficiency of 91.7% (-1.34 V), outperforming previously reported molecular catalysts. Compared to Ni-ATU, Cu-ATU transfers more electrons to the *NO intermediate, effectively breaking the strong sp2 hybridization system and weakening the energy of N═O bonds. The increase in free energy of *NO reduced the energy barriers of the rate-determining step, facilitating the further hydrogenation process over Cu-ATU. Our work opened up a new horizon for exploring molecular electrocatalysts for nitrate activation and paved a way for the in-depth understanding of catalytic behaviors, aligning more closely with industrial demands.

The Loss of Interfacial Water-Adsorbate Hydrogen Bond Connectivity Position Surface-Active Hydrogen as a Crucial Intermediate to Enhance Nitrate Reduction Reaction

The electrochemical nitrate reduction reaction (NO3RR) offers a promising solution for remediating nitrate-polluted wastewater while enabling the sustainable production of ammonia. The control strategy of surface-active hydrogen (*H) is extensively employed to enhance the kinetics of the NO3RR, but atomic understanding lags far behind the experimental observations. Here, we decipher the cation-water-adsorbate interactions in regulating the NO3RR kinetics at the Cu (111) electrode/electrolyte interface using AIMD simulations with a slow-growth approach. We demonstrate that the key oxygen-containing intermediates of the NO3RR (e.g., *NO, *NO2, and *NO3) will stably coordinate with the cations, impeding their integration with the hydrogen bond network and further their hydrogenation by interfacial water molecules due to steric hindrance. The *H can migrate across the interface with a low energy barrier, and its hydrogenation barrier with oxygen-containing species remains unaffected by cations, offering a potent supplement to the hydrogenation process, playing the predominant factor by which the *H facilitates NO3RR reaction kinetic. This study provides valuable insights for understanding the reaction mechanism of NO3RR by fully considering the cation-water-adsorbate interactions, which can aid in the further development of the electrolyte and electrocatalysts for efficient NO3RR.

Regulating RuxMoy Nanoalloys Anchored on Porous Nitrogen-Doped Carbon via Domain-Confined Etching Strategy for Neutral Efficient Ammonia Electrosynthesis

Neutral electrochemical nitrate (NO3-) reduction to ammonia involves sluggish and complex kinetics, so developing efficient electrocatalysts at low potential remains challenging. Here, we report a domain-confined etching strategy to construct RuxMoy nanoalloys on porous nitrogen-doped carbon by optimizing the Ru-to-Mo ratio, achieving efficient neutral NH3 electrosynthesis. Combining in situ spectroscopy and theoretical simulations demonstrated a rational synergic effect between Ru and Mo in nanoalloys that reinforces *H adsorption and lowers the energy barrier of NO3- hydrodeoxygenation for NH3 production. The resultant Ru5Mo5-NC surpasses 92.8% for NH3 selectivity at the potential range from -0.25 to -0.45 V vs RHE under neutral electrolyte, particularly achieving a high NH3 selectivity of 98.3% and a corresponding yield rate of 1.3 mg h-1 mgcat-1 at -0.4 V vs RHE. This work provides a synergic strategy that sheds light on a new avenue for developing efficient multicomponent heterogeneous catalysts.

p-d Orbital Hybridization in Ag-based Electrocatalysts for Enhanced Nitrate-to-Ammonia Conversion

Considering the substantial role of ammonia, developing highly efficient electrocatalysts for nitrate-to-ammonia conversion has attracted increasing interest. Herein, we proposed a feasible strategy of p-d orbital hybridization via doping p-block metals in an Ag host, which drastically promotes the performance of nitrate adsorption and disassociation. Typically, a Sn-doped Ag catalyst (SnAg) delivers a maximum Faradaic efficiency (FE) of 95.5±1.85 % for NH3 at -0.4 V vs. RHE and reaches the highest NH3 yield rate to 482.3±14.1 mg h-1 mgcat. -1. In a flow cell, the SnAg catalyst achieves a FE of 90.2 % at an ampere-level current density of 1.1 A cm-2 with an NH3 yield of 78.6 mg h-1 cm-2, during which NH3 can be further extracted to prepare struvite as high-quality fertilizer. A mechanistic study reveals that a strong p-d orbital hybridization effect in SnAg is beneficial for nitrite deoxygenation, a rate-determining step for NH3 synthesis, which as a general principle, can be further extended to Bi- and In-doped Ag catalysts. Moreover, when integrated into a Zn-nitrate battery, such a SnAg cathode contributes to a superior energy density of 639 Wh L-1, high power density of 18.1 mW cm-2, and continuous NH3 production.

Spin Manipulation of Co sites in Co9S8/Nb2CTx Mott-Schottky Heterojunction for Boosting the Electrocatalytic Nitrogen Reduction Reaction

Regulating the adsorption of an intermediate on an electrocatalyst by manipulating the electron spin state of the transition metal is of great significance for promoting the activation of inert nitrogen molecules (N2) during the electrocatalytic nitrogen reduction reaction (eNRR). However, achieving this remains challenging. Herein, a novel 2D/2D Mott-Schottky heterojunction, Co9S8/Nb2CTx-P, is developed as an eNRR catalyst. This is achieved through the in situ growth of cobalt sulfide (Co9S8) nanosheets over a Nb2CTx MXene using a solution plasma modification method. Transformation of the Co spin state from low (t2g 6eg 1) to high (t2g 5eg 2) is achieved by adjusting the interface electronic structure and sulfur vacancy of Co9S8/Nb2CTx-P. The adsorption ability of N2 is optimized through high spin Co(II) with more unpaired electrons, significantly accelerating the *N2→*NNH kinetic process. The Co9S8/Nb2CTx-P exhibits a high NH3 yield of 62.62 µg h-1 mgcat. -1 and a Faradaic efficiency (FE) of 30.33% at -0.40 V versus the reversible hydrogen electrode (RHE) in 0.1 m HCl. Additionally, it achieves an NH3 yield of 41.47 µg h-1 mgcat. -1 and FE of 23.19% at -0.60 V versus RHE in 0.1 m Na2SO4. This work demonstrates a promising strategy for constructing heterojunction electrocatalysts for efficient eNRR.

Boosting Electrochemical Nitrate Reduction at Low Concentrations Through Simultaneous Electronic States Regulation and Proton Provision

Electrochemically converting nitrate (NO3 -) into ammonia (NH3) has emerged as an alternative strategy for NH3 production and effluent treatment. Nevertheless, the electroreduction of dilute NO3 - is still challenging due to the competitive adsorption between various aqueous species and NO3 -, and unfavorable water dissociation providing *H. Herein, a new tandem strategy is proposed to boost the electrochemical nitrate reduction reaction (NO3RR) performance of Cu nanoparticles supported on single Fe atoms dispersed N-doped carbon (Cu@Fe1-NC) at dilute NO3 - concentrations (≤100 ppm NO3 --N). The optimized Cu@Fe1-NC presents a FENH3 of 97.7% at -0.4 V versus RHE, and a significant NH3 yield of 1953.9 mmol h-1 gCu -1 at 100 ppm NO3 --N, a record-high activity for dilute NO3RR. The metal/carbon heterojunctions in Cu@Fe1-NC enable a spontaneous electron transfer from Cu to carbon substrate, resulting in electron-deficient Cu. As a result, the electron-deficient Cu facilitates the adsorption of NO3 - compared with the pristine Cu. The adjacent atomic Fe sites efficiently promote water dissociation, providing abundant *H for the hydrogenation of *NOx e at Cu sites. The synergistic effects between Cu and single Fe atom sites simultaneously decrease the energy barrier for NO3 - adsorption and hydrogenation, thereby enhancing the overall activity of NO3 - reduction.

Ruthenium Anchored Laser-Induced Graphene as Binder-Free and Free-Standing Electrode for Selective Electrosynthesis of Ammonia from Nitrate

Developing effective electrocatalysts for the nitrate reduction reaction (NO3RR) is a promising alternative to conventional industrial ammonia (NH3) synthesis. Herein, starting from a flexible laser-induced graphene (LIG) film with hierarchical and interconnected macroporous architecture, a binder-free and free-standing Ru-modified LIG electrode (Ru-LIG) is fabricated for electrocatalytic NO3RR via a facile electrodeposition method. The relationship between the laser-scribing parameters and the NO3RR performance of Ru-LIG electrodes is studied in-depth. At -0.59 VRHE, the Ru-LIG electrode exhibited the optimal and stable NO3RR performance (NH3 yield rate of 655.9 µg cm-2 h-1 with NH3 Faradaic efficiency of up to 93.7%) under a laser defocus setting of +2 mm and an applied laser power of 4.8 W, outperforming most of the reported NO3RR electrodes operated under similar conditions. The optimized laser-scribing parameters promoted the surface properties of LIG with increased graphitization degree and decreased charge-transfer resistance, leading to synergistically improved Ru electrodeposition with more exposed NO3RR active sites. This work not only provides a new insight to enhance the electrocatalytic NO3RR performance of LIG-based electrodes via the coordination with metal electrocatalysts as well as identification of the critical laser-scribing parameters but also will inspire the rational design of future advanced laser-induced electrocatalysts for NO3RR.