NH3 Synthesis Research Articles

Green Synthesis of Urea from Carbon Dioxide and Ammonia Catalyzed by Ultraporous Permanently Polarized Hydroxyapatite.

The sustainable synthesis of urea from ammonia (NH3) and carbon dioxide (CO2) using ultraporous permanently polarized hydroxyapatite (upp-HAp) as catalyst has been explored as an advantageous CO2-revalorization strategy. As the simultaneous activation of N2 and CO2 (single-step) demands an increase of the reaction conditions, we have re-visited the industrial two-step Bazarov reaction. upp-HAp has been designed as a stable multifunctional catalyst capable of promoting both CO2 and NH3 adsorption for their subsequent C-N bond formation. Herein we report the synthesis of 1 mmol/gcat of urea with a selectivity of 97% under strictly mild conditions (95-120 ºC and 1 bar of CO2; without applying any electrical currents or UV irradiation) which represents an efficiency of ~2% and ~30% with respect to the NH3 and CO2 content, respectively. The study of the NH3 content, products adsorbed in the catalyst, presence of intermediates and temperature of the reaction allows unveiling the great potential of upp-HAp as a green catalyst for sustainable Bazarov reactions. Results suggest that the double-step approach could be more advantageous for both synthesizing urea and as a CO2-revalorization strategy, which in turn promotes the development of specific technologies for the independent synthesis of green NH3.

Amorphous Ni3B Promotes Electroreduction of Nitrate to Ammonia.

The electrocatalytic nitrate reduction to ammonia (NRA) can address nitrogen cycle imbalance and high carbon emissions; however, the intense competition of hydrogen evolution reaction (HER) restricts the rate of NH3 production. Herein, amorphous Ni3B (a-Ni3B) is designed to balance the NRA and HER. The NH3 yield of a-Ni3B surpasses those of pure Ni and NiO, which is attributed to the preferential adsorption of NO3- on the B and Ni sites of a-Ni3B for the NRA reaction, greatly inhibiting the HER. Furthermore, the a-Ni3B possesses advantages in NRA performance compared to crystalline Ni3B (c-Ni3B) due to more active hydrogen (*H) generated during the catalytic process. The *H in the NRA process on a-Ni3B is verified by the electron spin resonance technique. The NRA mechanism is comprehensively discussed based on the results of in situ characterization and density functional theory calculations. The a-Ni3B can enhance NH3 production by inhibiting HER, which provides ideas for sustainable NH3 synthesis.

Thermodynamic Analysis of Ammonia Synthesis

The paper researches the effect of temperature on ammonia synthesis. The ammonia synthesis reaction is exothermic with a negative entropy change. ∆G < 0 condition is fulfilled at lower temperatures up to 464K (Le Chatelierov principle). It is a constant equilibrium of the ammonia synthesis reaction at lower temperatures, which means that the NH3(g) synthesis reaction is shifted in the direction of NH3(g) formation (higher production of ammonia). The downside of lowering the temperature is that more ammonia is obtained, but the reaction rate slows down. Above 464K, the free enthalpy of the NH3(g) synthesis reaction is greater than zero, so the reaction enters thermodynamically unfavorable conditions. By increasing the reaction temperature, the ammonia yield NH3(g) decreases in the equilibrium mixture. At 400K, it is 0.5127kmol/kmol (51.27%) and at 700K, the synthesis process NH3(g) is practically complete.

Synergistic N/F Dual-Doped MoC/C Catalyst Synthesized via Liquid Phase Plasma for Sustainable Ammonia Production.

NH3 is a versatile solution for the storage and distribution of sustainable energy, offering high energy density and promising applications as a renewable hydrogen carrier. However, electrochemical NH3 synthesis under ambient conditions remains challenging, such as low selectivity and efficiency, owing to the inertness of N≡N and competing reactions. In this study, a catalyst (MoC/NFC) comprising molybdenum carbide evenly dispersed on carbon doped with N and F heteroatoms was successfully synthesized using liquid-phase plasma. The MoC/NFC catalyst exhibited a maximum NH3 yield of 115 μg h-1 mg-1cat. with a faradaic efficiency of 1.15% at -0.7 V vs reversible hydrogen electrode in 0.1 M KOH electrolyte. Pyridinic- and pyrrolic-N atoms adjacent to the carbon pores served as active sites for N2 adsorption and enabled N2 triple bond cleavage. In addition, F doping contributed to N2 activation owing to the high electronegativity of 3.98, resulting in the attraction of more electrons. These findings demonstrate a significant advancement in the development of efficient catalysts for electrochemical ammonia synthesis, potentially paving the way for scalable and sustainable NH3 production methods that can support the growing demand for renewable energy storage solutions.

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 NH3synthesis 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 0.1V. 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 (1.47mmol 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.

Two-Dimensional Bi2Se3 Nanosheets Synthesis for Efficient Electrocatalytic Production of Ammonia from Nitrate

Ammonia (NH3), a critical component for various industries, is produced through the Haber-Bosch process, which, despite its importance, is a significant source of carbon emissions and operates on a centralized model, emphasizing the need for innovative solutions to decarbonize and decentralize its production. Electrochemical nitrate (NO3–) reduction to NH3 presents a promising alternative to the Haber-Bosch process for NH3 synthesis, with the added advantage of transforming waste into a valuable resource while mitigating water pollution concerns. However, achieving a well-defined active center and catalytic selectivity in the electrochemically driven reaction remains a significant challenge. In this study, we successfully fabricated a highly selective and active 2D Bi2Se3 catalyst, which demonstrated better performance in reducing NO3– to NH3 with a Faradaic efficiency (FE) of approximately 80% (−0.3V (RHE)) and a significant yield rate of 45mgh−1mgcat−1 (0.46 mmolh−1cm−2). Furthermore, the fabricated material exhibited exceptional stability and durability, maintaining a high NO3– reduction of 80% FE even after 10 consecutive cycles of extended use and continuous operation, demonstrating its remarkable resilience and reliability. Our findings show that the prepared catalyst selectively promotes the electrochemical reduction of NO3– to NH3 while suppressing the competing hydrogen evolution reaction (HER). The charge density profile indicates a pronounced charge localization around the Se atoms, implying that the Se active sites in the 2D Bi2Se3 catalyst act as the active center for the NO3– reduction mechanism, playing a crucial role in facilitating the reaction kinetics.

Screening of Silver‐Based Single‐Atom Alloy Catalysts for NO Electroreduction to NH3 by DFT Calculations and Machine Learning

AbstractExploring NO reduction reaction (NORR) electrocatalysts with high activity and selectivity toward NH3 is essential for both NO removal and NH3 synthesis. Due to their superior electrocatalytic activities, single‐atom alloy (SAA) catalysts have attracted considerable attention. However, the exploration of SAAs is hindered by a lack of fast yet reliable prediction of catalytic performance. To address this problem, we comprehensively screened a series of transition‐metal atom doped Ag‐based SAAs. This screening process involves regression machine learning (ML) algorithms and a compressed‐sensing data‐analytics approach parameterized with density‐functional inputs. The results demonstrate that Cu/Ag and Zn/Ag can efficiently activate and hydrogenate NO with small Φmax(η), a grand‐canonical adaptation of the Gmax(η) descriptor, and exhibit higher affinity to NO over H adatoms to suppress the competing hydrogen evolution reaction. The NH3 selectivity is mainly determined by the s orbitals of the doped single‐atom near the Fermi level. The catalytic activity of SAAs is highly correlated with the local environment of the active site. We further quantified the relationship between the intrinsic features of these active sites and Φmax(η). Our work clarifies the mechanism of NORR to NH3 and offers a design principle to guide the screen of highly active SAA catalysts.

Dynamic Reconstruction of Cu Catalyst under Electrochemical NO Reduction to NH3.

The electrochemical reduction of nitric oxide (NO) to ammonia (NH3) offers a sustainable way of simultaneously treating the air pollutant and producing a useful chemical. Among catalyst candidates, Cu emerges as a stand-out choice for its superb NH3 selectivity and production rate. However, a comprehensive study concerning its catalytic behaviorin the NO reduction environment is lacking. Here, we unravel the dynamic rearrangement of Cu catalysts during NO reduction: the emergence of a bundled nanowire structure dependent on the applied potential. This unique structure is closely linked to an enhancement in double-layer capacitance, leading to a progressive increase in current density from 236 mA cm-2 by 20% over 1 h, while maintaining a Faradaic efficiency of 95% for NH3. Characterizations of Cu oxidation states suggest that the nanostructure results from the dissolution-redeposition of Cu in the aqueous electrolyte, influenced by the interaction with NO or other reactive intermediates. This understanding contributes to the broader exploration of Cu-based catalysts for sustainable and efficient NH3 synthesis from NO.

Promoted Two-Step Ammonia Synthesis with CoOOH/Co foam at Ampere-Level Current Density and Nearly 100% Faraday Efficiency from Air and Water.

Ammonia (NH3) is regarded as an essential hydrogen storage material in the new energy field, and plasma-electrocatalytic synthesis of NH3 (PESA) is an alternative to the traditional Haber-Bosch process. Here, a bifunctional catalyst CoOOH/CF is proposed to enhance the PESA process. Benefiting from the efficient activation of O2 by CoOOH/CF, NOx - yield rate can reach the highest value of 171.90mmol h-1 to date. Additionally, CoOOH holds a more negative d-band center, thereby exhibiting weaker adsorption toward NO*, lowering the energy barrier for the rate determining step, resulting in a high NH3 yield rate (302.55mg h-1 cm-2 at -0.8V) with ampere-level NH3 current density (2.86 A cm-2 at -0.8V) and nearly 100% Faraday efficiency (FE, 99.8% at -0.6V). Moreover, CoOOH/CF achieves an excellent 4.54g h-1 NH3 yield rate with 97.9% FE in an enlarged electrolyzer, demonstrating the feasibility of PESA on a large scale.

Diatomic Active Sites Embedded Graphyne as Electrocatalysts for Ammonia Synthesis.

Ammonia (NH3) is a vital chemical compound in industry and agriculture, and the electrochemical nitrogen reduction reaction (eNRR) is considered a promising approach for NH3 synthesis. However, the development of eNRR faces the challenge of high overpotential and low Faradaic efficiency. In this work, graphyne (GY) is anchored by 3d, 4d, and 5d dual transition metal atoms to form diatomic catalysts (DACs) and is roundly investigated as an electrocatalyst for eNRR via density functional theory calculations. Due to the protrusion of anchored metal atoms, the active sites of GY are better exposed compared to other substrates, exhibiting higher activity. Through four-step hierarchical high-throughput screening (ΔG*N2 < 0 eV, ΔG*N2→*N2H < 0.4 eV, ΔG*NH2→*NH3 < 0.4 eV, and ΔG*N2 < ΔG*H), the number of selected catalysts in each step is 325, 240, 145, and 20, respectively. Considering a series of factors, including stability, initial potential, and selectivity, 13 kinds of eligible catalysts are identified. Optimal eNRR paths studies show that the best catalyst Mn2@GY features no onset potential. For the three catalysts (Mn2@GY, Ir2@GY, and RhOs@GY), the onset potentials of the most favorable eNRR pathways are -0.07, -0.12, and -0.17 V, respectively. The excellent catalytic activity can be credited to the effective charge transfer and orbital interaction between the active site and adsorbed N2. Our work demonstrates the significance of DACs for ammonia synthesis and provides a paradigm for the study of DACs even for other important reactions.

MULTI-LAYER KESTERITE-BASED PHOTOCATHODES FOR NH3 PHOTOSYNTHESIS FROM N2 REDUCTION REACTION.

The necessity of new methods to substitute the Haber-Bosch process in the NH3 synthesis, generating fewer greenhouse gases, and dispensing less energy, drove the investigation of the photoelectrocatalytic approach in the N2 reduction reaction (N2RR). For that, this work presents the synthesis and characterization of the layered CZTSSe/CdS/TiO2 photocathode decorated with Pt nanoparticles for application in NH3 production using the photoelectrocatalysis technique. The CZTSSe/CdS/TiO2-Pt characterization showed a well-designed and stable photocatalyst synthesized layer by layer with an important contribution of the Pt nanoparticles for the catalyst performance, improving the photocurrent density and the charge transfer. The N2RR in a two-compartment photochemical cell with 0.1 mol L-1 Na2SO3 and 0.05 mol L-1 H2SO4 in the cathodic and anodic chamber, respectively, using CZTSSe/CdS/TiO2-Pt and under 1 sun of light incidence and applied potential of -0.4 VAg/AgCl reached 0.22 mmol L-1 cm-2 NH3, a value 28 folds higher than using the catalyst without Pt modification. The superiority of N2RR under the photoelectrocatalysis technique was demonstrated compared to photocatalytic and electrocatalytic techniques, together with the investigation of the supporting electrolyte influence in the cathodic compartment. Additionally, that is the first time a kesterite-based photocathode has been applied to NH3 photosynthesis, showing excellent photoconversion capability.

Synergistic Tuning of Heterovalent States and Oxygen-Vacancy Defect Engineering in Hydrophilic W-Doped Sb2OS2 for Enhanced Nitrogen Photoreduction to Ammonia.

Nitrogen fixation reaction via photocatalysis offers a green and promising strategy for renewable NH3 synthesis, and catalysts with high-efficiency photocatalytic properties are essential to the process. Herein, we demonstrate a W-doped Sb2OS2 bimetal oxysulfide catalyst (labeled as SbWOS) with abundant oxygen vacancies, heterovalent metal states, and hydrophilic surfaces for nitrogen photoreduction to ammonia. The SbWOS-3 with suitable W-doping exhibited excellent nitrogen fixation activity of 408.08 μmol·g-1·h-1 and an apparent quantum efficiency (AQE) of 1.88% at 420 nm and a solar-to-ammonia (STA) conversion efficiency of 0.082% in pure water under AM1.5G light irradiation. The W-doping not only transforms hydrophobic Sb2OS2 into a hydrophilic catalyst, making it easier for H2O molecules adsorbed on the SbWOS surface and catalyzed into protons, but also endows the SbWOS catalyst with rich oxygen vacancies, acting as the active sites for trapping and activating the N2 molecule, and for trapping and activating H2O to produce the protons for the N2 photocatalytic reduction reaction. The hydrazine drives the SbWOS catalyst with the heterovalent metal states, which acts as the photogenerate electrons quickly hopping between W5+ and W6+ to transfer for the N2 reduction reaction. This study provides a feasible scheme for applying oxygen vacancy defects, heterovalent metal states, and surface hydrophobic-to-hydrophilic wetting engineering in bimetal oxysulfide for N2 photoreduction to ammonia.

Cs-Ba-Ru/La2O3-MgO Catalysts from Microdrop-Confined Pyrolysis for Superior NH3 Synthesis under Mild Conditions.

Support modification is an important way to improve the activity of the Ru/MgO-based NH3 synthesis catalyst. This work synthesized 24 composite MgO-based supports of MOx-MgO (M = La, Zn, etc.) via a microdrop-confined pyrolysis (MDCPy) method and then prepared the corresponding Cs-Ba-Ru/MOx-MgO catalysts using the impregnation method. Under the same NH3 synthesis conditions, the activity of the Cs-Ba-Ru/MOx-MgO catalysts composed of 17 elements such as Pr was significantly lower than that of Cs-Ba-Ru/MgO, while the activity of the Cs-Ba-Ru/MOx-MgO catalysts composed of four elements such as Nd was comparable to that of Cs-Ba-Ru/MgO, and the activity of the Cs-Ba-Ru/MOx-MgO catalysts composed of three elements such as La was significantly higher than that of Cs-Ba-Ru/MgO. Among them, it is clear that La2O3-MgO composite supports have the most significant effect on the improvement of NH3 synthesis performance. Through the optimization of the La/Mg ratio and loading of Ru, the optimal activities achieved ∼7 mol(NH3)/g(Ru)/h with corresponding Ru loading of 0.25-0.5 wt % under mild conditions as follows: H2/N2 ratio of 3:1, temperature of 400 °C, pressure of 3 MPa and a weight hourly space velocity (WHSV) of 96.0 L/gcat/h. In addition, the CsBa-Ru/La2O3-MgO catalyst showed good stability.

Two-dimensional Boridene Nanosheets for Efficient Electrochemical Nitrogen Fixation under Ambient Conditions.

The carbon-free electrocatalytic nitrogen reduction reaction (NRR) is an alternative technology to the current Haber-Bosch method, that can be conducted under ambient conditions, and directly converting water and nitrogen (N₂) into ammonia (NH₃). However, the limited activity and selectivity of NH₃ electrosynthesis hinder the practical applications of NRR. In this study, we present a novel type of electrocatalyst called boridene nanosheets enriched with metal vacancies that are specifically designed for efficient electrocatalytic NRR under ambient conditions. Electrochemical testing in a 0.1 M phosphate-buffered saline (PBS) electrolyte demonstrates that boridene exhibits a high Faradaic efficiency of 66.7% for NH₃ production at -0.2 V vs. RHE, with a maximum NH₃ yield rate of 23.6 µg h-1 mg cat-1 at -0.4 V vs. RHE. Durability tests show that boridene maintains significant stability throughout multiple cycles of NRR. Mechanistic insights are obtained through in situ FTIR spectroscopy, revealing that boridene exhibits a preference for the distal pathway during the process of NRR. These findings highlight the potential of boridene as an efficient and stable catalyst for sustainable NH₃ synthesis.

Engaging the Concepts of Bimetallicity and Mechanical Strain for N2 Activation: A Computational Exploration.

N2 activation is a vital step in the process toward NH3 production. NH3 synthesis has been considered a crucial process for the production of value-added chemicals and/or hydrogen carriers over recent years. In this work, density functional theory (ab initio) calculations are implemented for a thorough screening of bimetallic alloy surfaces using Fe, Ru, and Mo as the matrix (host) metals and Ag, Au, Co, Cu, Fe, Mo, Ni, Pd, Pt, Rh, and Rh as heterometals toward exploring the N2 catalytic activation (electronic and chemical characteristics); the monometallic surfaces are used for critical comparison in terms of their N2 activation behavior. In particular, adsorption geometries/energetics, density of states (DOS), and charge transfer are discussed. From the N2 activation on the surfaces, we could precisely capture the transition state of the N2 dissociation reaction/step. The effect of the metal alloying (geometrical and electronic factors) as well as the effect of applied mechanical strain, as a tuning factor of alloying, are both studied and thoroughly discussed. DOS studies revealed that the d-band center moved toward the negative direction for all late-TM-based alloys, thereby allowing the nitrogen molecule to adsorb weakly as compared to the early-TM surface alloys. In terms of the mechanical strain, for most of the alloy surfaces studied, apart from the Mo/Fe(110) one, the N2 binding energy varies as a linear function of the applied strain. The mechanical effect trend is in agreement with the charge transfer descending order followed: Fe/Mo(110) > Rh/Mo(110) > Au/Mo(110) > Pt/Mo(110) > Ni/Mo(110) > Ru/Mo(110) > Cu/Mo(110) > Ag/Mo(110) > Pd/Mo(110) > Au/Mo(110), pointing out that Fe-functionalized Mo(110) surface presents the highest charge transfer of -2.14 |e| to the N2 molecule. This study aspires to provide navigation criteria through the abundant design criteria of N2 activation catalysts.

Two-Dimensional MOF Constructed by a Binuclear-Copper Motif for High-Performance Electrocatalytic NO Reduction to NH3.

Ambient electrochemical NO reduction presents a dual solution for sustainable NO reduction and NH3 synthesis. However, their complex kinetics and energy demands necessitate high-performance electrocatalysts to ensure effective and selective process outcomes. Herein, we report that a two-dimensional Cu-based metal-organic framework (MOF), {[Cu(HL)]·H2O} n , (Cu-OUC, H3L = 5-(2'-carboxylphenoxy)isophthalic acid) acts as a stable electrocatalyst with high efficiency for NO-to-NH3 conversion. Electrochemical experimental studies showed that in 0.1 M K2SO4 solution, the as-prepared Cu-OUC achieved a peak Faradaic efficiency of 96.91% and a notable NH3 yield as high as 3415.82 μg h-1 mg-1. The Zn-NO battery in aqueous solution can produce electricity possessing a power density of 2.04 mW cm-2 while simultaneously achieving an NH3 yield of 616.92 μg h-1 mg-1. Theoretical calculations revealed that the surface of Cu-OUC effectively facilitates NO activation through a two-way charge transfer mechanism of "electron acceptance and donation", with the *NO formation step being the potential-determining stage. The study pioneers the use of a MOF as an electrocatalyst for ambient NO-to-NH3 conversion.

Computational Insight into Transition Metal Atoms Anchored on B2C3P as Single‐Atom Electrocatalysts for Nitrogen Reduction Reaction

AbstractNH3 is not only an important chemical raw material but also a high‐energy storage chemical with zero carbon. Electrocatalytic nitrogen reduction reaction (NRR), which can be driven by clean electric energy under ambient conditions, has become a promising technology for NH3 synthesis due to its environmentally friendly properties. Because of the limitations of low yield and high overpotential, efficient catalysts are urgently needed to solve this problem. In this study, based on density functional theory method and high throughput screening strategy, the NRR was investigated on transition metal single atom anchored to 2D B2C3P surface (TM@B2C3P) as single‐atom catalysts (SACs). The results showed that V@B2C3P and Ti@B2C3P have good catalytic properties, and the limiting potentials were −0.10 and −0.24 V, respectively. Furthermore, the charge density difference and crystal orbital Hamilton population calculations demonstrated that the high catalytic activity can be attributed to the obvious charge transfer between TM@B2C3P and the adsorption intermediates. It is hoped that this work can play a certain role in exploring the application of SACs in NRR.

Phosphorization of α-Fe2O3 Boosts Active Hydrogen Mediated Electrochemical Nitrate Reduction to Ammonia.

Inexpensive iron-based materials are considered promising electrocatalysts for nitrate (NO3 -) reduction, but their catalytic activity and spontaneous corrosion remain challenges. Here, the α-Fe2O3 active surface is reconstructed by gradient phosphorization to obtain FePx with higher electrochemical activity. FeP2.0 optimizes the adsorption energy of NO3 - and its reduction intermediates, meanwhile promote the generation of active hydrogen (*H) but inhibit its generation of H2. More importantly, Fe and P can serve as binding sites for NO3 - and *H, respectively, which improves the electron utilization of NO3 - deoxygenation and the efficiency of the subsequent hydrogenation for the selective synthesis of NH3. 91.7% NO3 - conversion rate is achieved for the reduction of 100mL 200mg L-1 NO3 --N, 99.3% ammonia (NH3 selectivity (yield of 1.79 mg h-1 cm-2), and 91.4% Faraday efficiency in 3 h. The high-purity solid NH4Cl is finally extracted by gas extraction and vacuum distillation (81.4% recovery). This study provides new insights and strategies for the conversion of NO3 - to NH3 products over iron-based electrocatalysts.

Toward High-Performance Electrochemical Ammonia Synthesis by Circumventing the Surface H-Mediated N2 Reduction.

The rapid performance decay with potentials is a significant obstacle to achieving an efficient electrocatalytic N2 reduction reaction (eNRR), which is typically attributed to competition from hydrogen evolution. However, the potential-dependent competitive behavior and reaction mechanism are still under debate. Herein, we theoretically defined N2 adsorption, H mediation, and H2 evolution as three crucial regions along the potentials by revisiting the potential-dependent competitive adsorption between N2 and H on FeN4 and RuN4 catalysts. We revealed that the surface H-mediated mechanism makes eNRR feasible at low potentials but introduces sluggish reaction kinetics, showing a double-edged sword nature. In view of this, we proposed a new possibility to achieve high-performance NH3 synthesis by circumventing the H-mediated mechanism, where the ideal catalyst should have a wide potential interval with N2-dominated adsorption to trigger direct eNRR. Using this mechanistic insight as a new criterion, we proposed a theoretical protocol for eNRR catalyst screening, but almost none of the theoretically reported electrocatalysts passed the assessment. This work not only illustrates the intrinsic mechanism behind the low-performance dilemma of eNRR but also points out a possible direction toward designing promising catalysts with high selectivity and high current density.

Tandem Electroreduction of Nitrate to Ammonia Using a Cobalt-Copper Mixed Single-Atom/Cluster Catalyst with Synergistic Effects.

Electrochemical conversion of waste nitrate (NO3 -) to ammonia (NH3) for environmental applications, such as carbon-neutral energy sources and hydrogen carriers, is a promising alternative to the energy-intensive Haber-Bosch process. However, increasing the energy efficiency is limited by the high overpotential and selectivity. Herein, a Co─Cu mixed single-atom/cluster catalyst (Co─Cu SCC) is demonstrated-with well-dispersed Co and Cu active sites anchored on a carbon support-that delivers high NH3 Faradaic efficiency of 91.2% at low potential (-0.3V vs. RHE) due to synergism between the heterogenous active sites. Electrochemical analyses reveal that Cu in Co─Cu SCC preferentially catalyzes the NO3 --to-NO2 - pathway, whereupon Co catalyzes the NO2 --to-NH3 pathway. This tandem electroreduction bypasses the rate-determining steps (RDSs) for Co and Cu to lower the reaction energy barrier and surpass scaling relationship limitations. The electrocatalytic performance is amplified by the subnanoscale catalyst to increase the partial current density of NH3 by 2.3 and 5.4 times compared to those of individual Co, Cu single-atom/cluster catalysts (Co SCC, Cu SCC), respectively. This Co─Cu SCC is operated stably for 32 h in a long-term bipolar membrane (BPM)-based membrane electrode assembly (MEA) system for high-concentration NH3 synthesis to produce over 1m NH3 for conversion into high-purity NH4Cl at 2.1g day-1.