Nitrogen cycle electrocatalysis

Phase-Regulated Active Hydrogen Behavior on Molybdenum Disulfide for Electrochemical Nitrate-to-Ammonia Conversion

Electrochemical reduction of nitrate waste is promising for environmental remediation and ammonia preparation. This process includes multiple hydrogenation steps, and thus the active hydrogen behavior on the surface of the catalyst is crucial. The crystal phase referred to the atomic arrangements in crystals has a great effect on active hydrogen, but the influence of the crystal phase on nitrate reduction is still unclear. Herein, enzyme-mimicking MoS2 in different crystal phases (1T and 2H) are used as models. The Faradaic efficiency of ammonia reaches ≈90 % over 1T-MoS2 , obviously outperforming that of 2H-MoS2 (27.31 %). In situ Raman spectra and theoretical calculations reveal that 1T-MoS2 produces more active hydrogen on edge S sites at a more positive potential and conducts an effortless pathway from nitrate to ammonia instead of multiple energetically demanding hydrogenation steps (such as *HNO to *HNOH) performed on 2H-MoS2 .

Electrocatalysis of nitrogen pollution: transforming nitrogen waste into high-value chemicals

On 16 June 2023, the United Nations Environment Programme highlighted the severity of nitrogen pollution faced by humans and called for joint action for sustainable nitrogen use. Excess nitrogenous waste (NW: NO, NO2, NO2-, NO3-, etc.) mainly arises from the use of synthetic fertilisers, wastewater discharge, and fossil fuel combustion. Although the amount of NW produced can be minimised by reducing the use of nitrogen fertilisers and fossil fuels, the necessity to feed seven billion people on Earth limits the utility of this approach. Compared to current industrial processes, electrocatalytic NW reduction or CO2-NW co-reduction offers a potentially greener alternative for recycling NW and producing high-value chemicals. However, upgrading this technology to connect upstream and downstream industrial chains is challenging. This viewpoint focuses on electrocatalytic NW reduction, a cutting-edge technology, and highlights the challenges in its practical application. It also discusses future directions to meet the requirements of upstream and downstream industries by optimising production processes, including the pretreatment and supply of nitrogenous raw materials (e.g. flue gas and sewage), design and macroscopic preparation of electrocatalysts, and upscaling of reactors and other auxiliary equipment.

Manipulating Reactivity of Ir(CH2)0-2+ Cations toward Dinitrogen at Room Temperature: A Unique Dependence on the Organic Ligand Structures

Nitrogen (N2) activation at room temperature has long been a great challenge. Therefore, the rational design of reactive species to adsorb N2, which is a prerequisite for cleavage of the strong N≡N triple bond in industrial and biological processes, is highly desirable and meaningful. Herein, the N2 adsorption process is controlled by regulating the types and numbers of organic ligands, and the organic ligands are produced through the reactions of Ir+ with methane and ethane. CH4 molecules dissociate on the Ir+ cations to form Ir(CH2)1,2+. The reaction of Ir+ with C2H6 can generate HIrC2H3+, which is different from the structure of Ir(CH2)2+ obtained from Ir+/CH4. The reactivity order of N2 adsorption is Ir(CH2)2+ > HIrC2H3+ ≫ HIrCH+ ≈ Ir+ (almost inert under similar reaction conditions), indicating that different organic ligand structures affect reactivity dramatically. The main reason for this interesting reactivity difference is that the lowest unoccupied molecular orbital (LUMO) level of Ir(CH2)2+ is much closer to the highest occupied molecular orbital (HOMO) level of N2 than those of the other three systems. This study provides new insights into the adsorption of N2 on metal-organic ligand species, in which the organic ligand dominates the reactivity, and it discovers new clues in designing effective transition metal carbine species for N2 activation.

Bimetallic synergistic catalysts based on two-dimensional carbon-rich conjugated frameworks for nitrate electrocatalytic reduction to ammonia: catalyst screening and mechanism insights

The discovery of the 'two birds, one stone' electrochemical nitrate reduction reaction (NO3RR) allows for the removal of harmful NO3-pollutants as well as the production of economically beneficial ammonia (NH3). However, current understanding of the catalytic mechanism of NO3RR is not enough, and this research is still challenging. To determine the mechanism needed to create efficient electrocatalysts, we thoroughly examined the catalytic activity of molybdenum-based diatomic catalysts (DACs) anchored on two-dimensional carbon-rich conjugated frameworks (2D CCFs) for NO3RR. Among the 23 candidate materials, after a four-step screening method and detailed mechanism studies, we discovered that NO3RR can efficiently generate NH3by following the N-end pathway on the MoTi-Pc, MoMn-Pc, and MoNb-Pc, with limiting potential of -0.33 V, -0.13 V, and -0.38 V, respectively. The activity of NO3RR can be attributed to the synergistic effect of the TM1-TM2dimer d orbital coupling to the anti-bonding orbital of NO3-. Additionally, high hybridization between the Mo-4d, TM-3d(4d), and NO3--2p orbitals on the MoTMs-Pc DACs can speed up the flow of electrons from the Mo-TM dual-site to NO3-. The research presented here paves the way for the reasonable design of effective NO3RR catalysts and offers a theoretical basis for experimental research.

Aloe vera-derived graphene-coupled phenosafranin photocatalyst for generation and regeneration of ammonia and NADH by mimicking natural photosynthetic route

Aloe vera-derived graphene (ADG) coupled system photocatalyst, mimicking natural photosynthesis, is one of the most promising ways for converting solar energy into ammonia (NH3 ) and nicotinamide adenine dinucleotide (NADH) that have been widely used to make the numerous chemicals such as fertilizer and fuel. In this study, we report the synthesis of the aloe vera-derived graphene-coupled phenosafranin (ADGCP) acting as a highly efficient photocatalyst for the generation of NH3 and regeneration of NADH from nitrogen (N2 ) and oxidized form of nicotinamide adenine dinucleotide (NAD+ ). The results show a benchmark instance for mimicking natural photosynthesis activity as well as the practical applications for the solar-driven selective formation of NH3 and the regeneration of NADH by using the newly designed photocatalyst.

Enhanced Electrocatalytic Nitrate Reduction to Ammonia Using Functionalized Multi-Walled Carbon Nanotube-Supported Cobalt Catalyst

Ammonia (NH3) is vital in modern agriculture and industry as a potential energy carrier. The electrocatalytic reduction of nitrate (NO3-) to ammonia under ambient conditions offers a sustainable alternative to the energy-intensive Haber-Bosch process. However, achieving high selectivity in this conversion poses significant challenges due to the multi-step electron and proton transfer processes and the low proton adsorption capacity of transition metal electrocatalysts. Herein, we introduce a novel approach by employing functionalized multi-walled carbon nanotubes (MWCNTs) as carriers for active cobalt catalysts. The exceptional conductivity of MWCNTs significantly reduces charge transfer resistance. Their unique hollow structure increases the electrochemical active surface area of the electrocatalyst. Additionally, the one-dimensional hollow tube structure and graphite-like layers within MWCNTs enhance adsorption properties, thus mitigating the diffusion of intermediate and stabilizing active cobalt species during nitrate reduction reaction (NitRR). Using the MWCNT-supported cobalt catalyst, we achieved a notable NH3 yield rate of 4.03 mg h-1 cm-2 and a high Faradaic efficiency of 84.72% in 0.1 M KOH with 0.1 M NO3-. This study demonstrates the potential of MWCNTs as advanced carriers in constructing electrocatalysts for efficient nitrate reduction.

High-Throughput Screening of Effective Dual Atom Catalysts for the Nitric Oxide Reduction Reaction

Electrochemical NO-to-NH3 conversion has been attracting more attention in the field of green energy, which, however, imposes restrictions on the catalysts. We therefore design a family of dual atom catalysts (DACs) TM1TM2@g-CN and perform high-throughput screening to position the effective catalysts for electrocatalytic NO-to-NH3 conversion from first-principles computations. We identify that TiCr@g-CN (-0.37 V), TiMo@g-CN (-0.36 V), and MnMo@g-CN (-0.43 V) are promising candidates with low overpotentials. In particular, we find that MoMo@g-CN can spontaneously reduce NO to NH3, which makes it an excellent electrocatalyst for the NO reduction reaction (NORR) to be translated to experiments. In terms of the local geometry feature and local electronic structures, we unravel the origin of the high NORR activity and high selectivity of the DAC MoMo@g-CN.

Understanding the Activity Trends in Electrocatalytic Nitrate Reduction to Ammonia on Cu Catalysts

Cu-based catalysts possess great potential in the electrocatalytic nitrate (NO3-) reduction reaction for ammonia (NH3) synthesis. However, the low atomic economy limits their further application. Here we report a Cu single-atom (SA) incorporated in nitrogen-doped carbon (Cu SA/NC) with high atomic economy, which exhibits superior NH3 Faradaic efficiency (FE) of 100% along with an impressive NH3 yield rate of 7480 μg h-1 mgcat.-1. As counterparts, Cus+n/NC, with mixed SA and nanoparticles (NPs), shows decreasing NH3 FE with decreasing SA content, but the production of N2 and N2O increases gradually, which reaches the maximum on pure Cu NPs. In situ characterizations and theoretical calculations reveal that a higher NH3 FE of Cu SA/NC is ascribed to a lower free energy of the rate-limiting step (HNO* → N*) and effective inhibition for the N-N coupled process. This work provides the intuitive activity trends of Cu-based catalysts, opening an avenue for subsequent catalysts design.

Unraveling the Activity Trends and Design Principles of Single-Atom Catalysts for Nitrate Electrocatalytic Reduction

Electrocatalytic nitrate (NO3-) reduction represents one of the most promising approaches to mitigate NO3- pollution and yield NH3, but it is still challenged by the atomic economy and selectivity issues of substantial active sites. Here, we describe a comprehensive investigation on a series of single-atom catalysts (SACs) using nitrogen-doped carbon as substrate (metal/NC). The essence of activity is related to the extent of the electron transfer capacity (SAs → NO3-). Among these examined SACs, the Cu/NC presents good performance toward NH3 synthesis, i.e., a maximum NH3 Faradaic efficiency of 100% with a high NH3 yield rate of up to 32,300 μg h-1 mgcat.-1. X-ray absorption fine structure spectra and density functional theory calculations provide evidence that the electronic structure of Cu-N4 coordination prohibits the formation of N2, N2O, and H2 and facilitates the orbital hybridization between the 2p orbitals of NO3- and 3d orbitals of Cu single-atom sites. Our study is believed to provide fundamental guidance for the future design of highly efficient electrocatalysts in NO3- reduction to NH3.

Theoretical Insight into the Special Synergy of Bimetallic Site in Co/MoC Catalyst to Promote N2 -to-NH3 Conversion

The catalytic mechanisms of nitrogen reduction reaction (NRR) on the pristine and Co/α-MoC(001) surfaces were explored by density functional theory calculations. The results show that the preferred pathway is that a direct N≡N cleavage occurs first, followed by continuous hydrogenations. The production of second NH3 molecule is identified as the rate-limiting step on both systems with kinetic barriers of 1.5 and 2.0 eV, respectively, indicating that N2 -to-NH3 transformation on bimetallic surface is more likely to occur. The two components of the bimetallic center play different roles during NRR process, in which Co atom does not directly participate in the binding of intermediates, but primarily serves as a reservoir of H atoms. This special synergy makes Co/α-MoC(001) have superior activity for ammonia synthesis. The introduction of Co not only facilitates N2 dissociation, but also accelerates the migration of H atom due to the antibonding characteristic of Co-H bond. This study offers a facile strategy for the rational design and development of efficient catalysts for ammonia synthesis and other reactions involving the hydrogenation processes.

Synergy between Cu and Co in a Layered Double Hydroxide Enables Close to 100% Nitrate-to-Ammonia Selectivity

Nitrate electroreduction (NO3RR) holds promise as an energy-efficient strategy for the removal of toxic nitrate to restore the natural nitrogen cycle and mitigate the adverse impacts caused by overfertilization from suboptimal agricultural practices. However, existing catalysts suffer from limited electrocatalytic activity, poor selectivity, inadequate durability, and low scalability. To address this quadrilemma, in this study, we developed a cost-effective layered double hydroxide (LDH) electrocatalyst with a lamellar structure that presents trimetallic CuCoAl active sites on the nanomaterial surface. This codoping design enabled electrochemical upcycling of nitrate into ammonia exclusively and efficiently with an onset potential at 0 V vs RHE, where the electrocatalytic process is less energy intensive and has a lower carbon footprint than conventional practices. The synergistic interaction among Cu, Co, and Al further afforded a 99.5% Faradic efficiency (FE) and a yield rate of 0.22 mol h-1 g-1 for nitrate-to-ammonia electroreduction, surpassing the performance of state-of-the-art nonprecious metal NO3RR electrocatalysts over an extended operation period. To gain insights into the origin of the catalytic performance observed on LDH, control materials were employed to elucidate the roles of Cu and Co. Cu was found to improve the NO3RR onset potential despite displaying limited FE for ammonia synthesis, while Co was discovered to suppress the formation of nitrite byproduct though requiring large overpotential. Simulated wastewater containing phosphate and sulfate, which are typically present in industrial effluents, was used to further investigate the effect of electrolytes on NO3RR. Intriguingly, the use of phosphate buffer resulted in a superior yield rate and FE for ammonia production while simultaneously inhibiting nitrite byproduct formation compared with the sulfate case. These experimental findings were supported by density functional theory (DFT) calculations, which explored the adsorption strength of nitrate adducts adjacent to coadsorbed electrolytes on the LDH surface. Additionally, the relative free energies of NO3RR species were also computed to examine the proton-coupled electron transfer (PCET) mechanism on CuCoAl LDH, shedding light on the potential-dependent step (PDS) and the exclusive selectivity for nitrate-to-ammonia conversion. The CuCoAl LDH developed here offers scalability by eliminating the need for precious metals, rendering this earth-abundant catalyst particularly appealing for sustainable nitrate electrovalorization technology.

Unconventional Synthesis of Hierarchically Twinned Copper as Efficient Electrocatalyst for Nitrate-Ammonia Conversion

Twin boundary (TB) engineering provides exciting opportunities to tune the performance levels of metal-based electrocatalysts. However, the controllable construction of TB greatly relies on surfactants, blocking active sites, and electron transfer by surfactants. Here, a surfactant-free and facile strategy is proposed for synthesizing copper (Cu) nanocatalysts with dense hierarchical TB networks (HTBs) by the rapid thermal reductions in metastable CuO nanosheets in H2 . As revealed by in situ transmission electron microscopy, the formation of HTBs is associated with the fragmentation of nanosheets in different directions to generate abundant crystal nuclei and subsequently unconventional crystal growth through the collision and coalescence of nuclei. Impressively, the HTBs endow Cu with excellent electrocatalytic performance for direct nitrate-ammonia conversion, superior to that of Cu with a single-oriented TB and without TB. It is discovered that the HTBs induce the formation of compressive strains, thereby creating a synergistic effect of TBs and strains to efficiently tune the binding energies of Cu with nitrogen intermediates (i.e., NO2 *) and thus promote the tandem reaction process of NO3 - -to-NO2 - and subsequent NO2 - -to-NH3 electrocatalysis. This work demonstrates the crucial role of HTBs for boosting electrocatalysis via the synergistic effect of TBs and strains.

Electrochemical nitrate reduction in acid enables high-efficiency ammonia synthesis and high-voltage pollutes-based fuel cells

Most current research is devoted to electrochemical nitrate reduction reaction for ammonia synthesis under alkaline/neutral media while the investigation of nitrate reduction under acidic conditions is rarely reported. In this work, we demonstrate the potential of TiO2 nanosheet with intrinsically poor hydrogen-evolution activity for selective and rapid nitrate reduction to ammonia under acidic conditions. Hybridized with iron phthalocyanine, the resulting catalyst displays remarkably improved efficiency toward ammonia formation owing to the enhanced nitrate adsorption, suppressed hydrogen evolution and lowered energy barrier for the rate-determining step. Then, an alkaline-acid hybrid Zn-nitrate battery was developed with high open-circuit voltage of 1.99 V and power density of 91.4 mW cm-2. Further, the environmental sulfur recovery can be powered by above hybrid battery and the hydrazine-nitrate fuel cell can be developed for simultaneously hydrazine/nitrate conversion and electricity generation. This work demonstrates the attractive potential of acidic nitrate reduction for ammonia electrosynthesis and broadens the field of energy conversion.

Tuning the Interfaces of ZnO/ZnCr2 O4 Derived from Layered-Double-Hydroxide Precursors to Advance Nitrogen Photofixation

Drawing inspiration from the enzyme nitrogenase in nature, researchers are increasingly delving into semiconductor photocatalytic nitrogen fixation due to its similar surface catalytic processes. Herein, we reported a facile and efficient approach to achieving the regulation of ZnO/ZnCr2 O4 photocatalysts with ZnCr-layered double hydroxide (ZnCr-LDH) as precursors. By optimizing the composition ratio of Zn/Cr in ZnCr-LDH to tune interfaces, we can achieve an enhanced nitrogen photofixation performance (an ammonia evolution rate of 31.7 μmol g-1  h-1 using pure water as a proton source) under ambient conditions. Further, photo-electrochemical measurements and transient surface photovoltage spectroscopy revealed that the enhanced photocatalytic activity can be ascribed to the effective carrier separation efficiency, originating from the abundant composite interfaces. This work further demonstrated a promising and viable strategy for the synthesis of nanocomposite photocatalysts for nitrogen photofixation and other challenging photocatalytic reactions.

Pulsed electroreduction of low-concentration nitrate to ammonia

Electrocatalytic nitrate (NO3-) reduction to ammonia (NRA) has emerged as an alternative strategy for effluent treatment and ammonia production. Despite significant advancements that have been achieved in this field, the efficient conversion of low-concentration nitrate to ammonia at low overpotential remains a formidable challenge. This challenge stems from the sluggish reaction kinetics caused by the limited distribution of negatively charged NO3- in the vicinity of the working electrode and the competing side reactions. Here, a pulsed potential approach is introduced to overcome these issues. A good NRA performance (Faradaic efficiency: 97.6%, yield rate: 2.7 mmol-1 h-1 mgRu-1, conversion rate: 96.4%) is achieved for low-concentration (≤10 mM) nitrate reduction, obviously exceeding the potentiostatic test (Faradaic efficiency: 65.8%, yield rate: 1.1 mmol-1 h-1 mgRu-1, conversion rate: 54.1%). The combined results of in situ characterizations and finite element analysis unveil the performance enhancement mechanism that the periodic appearance of anodic potential can significantly optimize the adsorption configuration of the key *NO intermediate and increase the local NO3- concentration. Furthermore, our research implies an effective approach for the rational design and precise manipulation of reaction processes, potentially extending its applicability to a broader range of catalytic applications.

Symmetry-Breaking p-Block Antimony Single Atoms Trigger N-Bridged Titanium Sites for Electrocatalytic Nitrogen Reduction with High Efficiency

The electrochemical nitrogen reduction reaction (eNRR) under mild conditions emerges as a promising approach to produce ammonia (NH3) compared to the typical Haber-Bosch process. Herein, we design an asymmetrically coordinated p-block antimony single-atom catalyst immobilized on nitrogen-doped Ti3C2Tx (Sb SA/N-Ti3C2Tx) for eNRR, which exhibits ultrahigh NH3 yield (108.3 μg h-1 mgcat-1) and excellent Faradaic efficiency (41.2%) at -0.3 V vs RHE. Complementary in situ spectroscopies with theoretical calculations reveal that the nitrogen-bridged two titanium atoms triggered by an adjacent asymmetrical Sb-N1C2 moiety act as the active sites for facilitating the protonation of the rate-determining step from *N2 to *N2H and the kinetic conversion of key intermediates during eNRR. Moreover, the introduction of Sb-N1C2 promotes the formation of oxygen vacancies to expose more titanium sites. This work presents a strategy for single-atom-decorated ultrathin two-dimensional materials with the aim of simultaneously enhancing NH3 yield and Faradaic efficiency for electrocatalytic nitrogen reduction.

Oxygen-Coordinated Single Mn Sites for Efficient Electrocatalytic Nitrate Reduction to Ammonia

Electrocatalytic nitrate reduction reaction has attracted increasing attention due to its goal of low carbon emission and environmental protection. Here, we report an efficient NitRR catalyst composed of single Mn sites with atomically dispersed oxygen (O) coordination on bacterial cellulose-converted graphitic carbon (Mn-O-C). Evidence of the atomically dispersed Mn-(O-C2)4 moieties embedding in the exposed basal plane of carbon surface is confirmed by X-ray absorption spectroscopy. As a result, the as-synthesized Mn-O-C catalyst exhibits superior NitRR activity with an NH3 yield rate (RNH3) of 1476.9 ± 62.6 μg h-1 cm-2 at - 0.7 V (vs. reversible hydrogen electrode, RHE) and a faradaic efficiency (FE) of 89.0 ± 3.8% at - 0.5 V (vs. RHE) under ambient conditions. Further, when evaluated with a practical flow cell, Mn-O-C shows a high RNH3 of 3706.7 ± 552.0 μg h-1 cm-2 at a current density of 100 mA cm-2, 2.5 times of that in the H cell. The in situ FT-IR and Raman spectroscopic studies combined with theoretical calculations indicate that the Mn-(O-C2)4 sites not only effectively inhibit the competitive hydrogen evolution reaction, but also greatly promote the adsorption and activation of nitrate (NO3-), thus boosting both the FE and selectivity of NH3 over Mn-(O-C2)4 sites.

Effects of anode materials on nitrate reduction and microbial community in a three-dimensional electrode biofilm reactor with sulfate

Graphite rod corrosion and peeling are serious problems in three-dimensional electrode biofilm reactors (3D-BERs). In this study, titanium rods, titanium suboxide-coated titanium rods and graphite rods were used as anodes to investigate the effect of anodic materials on the electrochemical and bioelectrochemical reduction of nitrate and sulfate. The results showed that the reactor with the titanium suboxide-coated titanium rod anode (3D-ER-T) exhibited a stable NO3--N removal efficiency (46%-95%) with a current range of 160-320 mA in the electrochemical reduction process. In the bioelectrochemical reduction, the removal efficiencies of NO3--N and SO42- and nitrogen selectivity in the 3D-BER with titanium suboxide-coated titanium rod anode (3D-BER-T) were higher than those in the 3D-BER with titanium suboxide-coated graphite rod anode (3D-BER-G). The removal efficiencies of NO3--N and SO42- and nitrogen selectivity were 92%, 43% and 86%, respectively, in 3D-BER-T under 320 mA and HRT 12 h. Anode materials affected the microbial community. Hydrogenophaga and Dethiobacter were the dominant bacteria in 3D-BER-T, while OPB41 and Sulfurospirillum were dominant in 3D-BER-G. Nitrate and sulfate were effectively removed in 3D-BER-T by the synergistic work of electrochemical reduction, bioelectrochemical reduction and indirect electrochemical reduction. The resupply/reserve mode of the electron donor promoted the load of shock resistance of 3D-BER-T via the sulfur cycle. Titanium suboxide coating could significantly enhance the anti-corrosion ability of matrix anodes.

Surface-Reconstructed Copper Foil Free-Standing Electrode with Nanoflower Cu/Ce2O3 by In Situ Electrodeposition Reduction for Electrocatalytic Nitrate Reduction to Ammonia

The electrocatalytic reduction of nitrate to ammonia (NRA) is considered to be a promising and environmentally friendly alternative to the Haber-Bosch process. Herein, a simple in situ electrodeposition-reduction method is proposed, which allows for the surface reconstruction and modification of copper foil by introducing cerium. The surface modification results in the formation of Cu/Ce2O3 nanoflowers on the copper electrode surface. Additionally, density functional theory calculations demonstrate that Ce2O3 enhances the adsorption of NO3-. Compared with a pure copper electrode, the NRA performance of surface-reconstructed copper is greatly improved, with the maximum ammonia yield rate and the highest Faradaic efficiency from 24.8% to 73.0% and from 0.081 to 0.386 mmol h-1 cm-2, respectively. The 15N isotope labeling experiment confirms that the nitrogen source in the ammonia originated from nitrate.

Transition Metal Single-Atom Catalysts for the Electrocatalytic Nitrate Reduction: Mechanism, Synthesis, Characterization, Application, and Prospects

Excessive accumulation of nitrate in the environment will affect human health. To combat nitrate pollution, chemical, biological, and physical technologies have been developed recently. The researcher favors electrocatalytic reduction nitrate reaction (NO3 RR) because of the low post-treatment cost and simple treatment conditions. Single-atom catalysts (SACs) offer great activity, exceptional selectivity, and enhanced stability in the field of NO3 RR because of their high atomic usage and distinctive structural characteristics. Recently, efficient transition metal-based SACs (TM-SACs) have emerged as promising candidates for NO3 RR. However, the real active sites of TM-SACs applied to NO3 RR and the key factors controlling catalytic performance in the reaction process remain ambiguous. Further understanding of the catalytic mechanism of TM-SACs applied to NO3 RR is of practical significance for exploring the design of stable and efficient SACs. In this review, from experimental and theoretical studies, the reaction mechanism, rate-determining steps, and essential variables affecting activity and selectivity are examined. The performance of SACs in terms of NO3 RR, characterization, and synthesis is then discussed. In order to promote and comprehend NO3 RR on TM-SACs, the design of TM-SACs is finally highlighted, together with the current problems, their remedies, and the way forward.

Coordinative Stabilization of Single Bismuth Sites in a Carbon-Nitrogen Matrix to Generate Atom-Efficient Catalysts for Electrochemical Nitrate Reduction to Ammonia

Electrochemical nitrate reduction to ammonia powered by renewable electricity is not only a promising alternative to the established energy-intense and non-ecofriendly Haber-Bosch reaction for ammonia generation but also a future contributor to the ever-more important denitrification schemes. Nevertheless, this reaction is still impeded by the lack of understanding for the underlying reaction mechanism on the molecular scale which is necessary for the rational design of active, selective, and stable electrocatalysts. Herein, a novel single-site bismuth catalyst (Bi-N-C) for nitrate electroreduction is reported to produce ammonia with maximum Faradaic efficiency of 88.7% and at a high rate of 1.38 mg h-1 mgcat -1 at -0.35 V versus reversible hydrogen electrode (RHE). The active center (described as BiN2 C2 ) is uncovered by detailed structural analysis. Coupled density functional theory calculations are applied to analyze the reaction mechanism and potential rate-limiting steps for nitrate reduction based on the BiN2 C2 model. The findings highlight the importance of model catalysts to utilize the potential of nitrate reduction as a new-generation nitrogen-management technology based on the construction of efficient active sites.

Electrochemical Wastewater Refining: A Vision for Circular Chemical Manufacturing

Wastewater is an underleveraged resource; it contains pollutants that can be transformed into valuable high-purity products. Innovations in chemistry and chemical engineering will play critical roles in valorizing wastewater to remediate environmental pollution, provide equitable access to chemical resources and services, and secure critical materials from diminishing feedstock availability. This perspective envisions electrochemical wastewater refining─the use of electrochemical processes to tune and recover specific products from wastewaters─as the necessary framework to accelerate wastewater-based electrochemistry to widespread practice. We define and prescribe a use-informed approach that simultaneously serves specific wastewater-pollutant-product triads and uncovers a mechanistic understanding generalizable to broad use cases. We use this approach to evaluate research needs in specific case studies of electrocatalysis, stoichiometric electrochemical conversions, and electrochemical separations. Finally, we provide rationale and guidance for intentionally expanding the electrochemical wastewater refining product portfolio. Wastewater refining will require a coordinated effort from multiple expertise areas to meet the urgent need of extracting maximal value from complex, variable, diverse, and abundant wastewater resources.

Electrochemical nitrogen fixation on single metal atom catalysts

The electrochemical reduction of nitrogen (eNRR) offers a promising alternative to the Haber-Bosch (H-B) process for producing ammonia under moderate conditions. However, the inertness of dinitrogen and the competing hydrogen evolution reaction pose significant challenges for eNRR. Thus, developing more efficient electrocatalysts requires a deeper understanding of the underlying mechanistic reactions and electrocatalytic activity. Single atom catalysts, which offer tunable catalytic properties and increased selectivity, have emerged as a promising avenue for eNRR. Carbon and metal-based substrates have proven effective for dispersing highly active single atoms that can enhance eNRR activity. In this review, we explore the use of atomically dispersed single atoms on different substrates for eNRR from both conceptual and experimental perspectives. The review is divided into four sections: the first section describes eNRR mechanistic pathways, the second section focuses on single metal atom catalysts (SMACs) with metal atoms dispersed on carbon substrates for eNRR, the third section covers SMACs with metal atoms dispersed on non-carbon substrates for eNRR, and the final section summarizes the remaining challenges and future scope of eNRR for green ammonia production.

Supramolecular Enhancement of Electrochemical Nitrate Reduction Catalyzed by Cobalt Porphyrin Organic Cages for Ammonia Electrosynthesis in Water

The electrochemical nitrate (NO3 - ) reduction reaction (NO3 RR) to ammonia (NH3 ) represents a sustainable approach for denitrification to balance global nitrogen cycles and an alternative to traditional thermal Haber-Bosch processes. Here, we present a supramolecular strategy for promoting NH3 production in water from NO3 RR by integrating two-dimensional (2D) molecular cobalt porphyrin (CoTPP) units into a three-dimensional (3D) porous organic cage architecture. The porphyrin box CoPB-C8 enhances electrochemical active site exposure, facilitates substrate-catalyst interactions, and improves catalyst stability, leading to turnover numbers and frequencies for NH3 production exceeding 200,000 and 56 s-1 , respectively. These values represent a 15-fold increase in NO3 RR activity and 200-mV improvement in overpotential for the 3D CoPB-C8 box structure compared to its 2D CoTPP counterpart. Synthetic tuning of peripheral alkyl substituents highlights the importance of supramolecular porosity and cavity size on electrochemical NO3 RR activity. These findings establish the incorporation of 2D molecular units into 3D confined space microenvironments as an effective supramolecular design strategy for enhancing electrocatalysis.

Renewable energy driven electroreduction nitrate to ammonia and in-situ ammonia recovery via a flow-through coupled device

Green ammonia production from wastewater via electrochemical nitrate reduction contributes substantially to the realization of carbon neutrality. Nonetheless, the current electrochemical technology is largely limited by the lack of suitable device for efficient and continuous electroreduction nitrate into ammonia and in-situ ammonia recovery. Here, we report a flow-through coupled device composed of a compact electrocatalytic cell for efficient nitrate reduction and a unit to separate the produced ammonia without any pH adjustment and additional energy-input from the circulating nitrate-containing wastewater. Using an efficient and selective Cl-modified Cu foam electrode, nearly 100% NO3- electroreduction efficiency and over 82.5% NH3 Faradaic efficiency was realized for a wide range of nitrate-containing wastewater from 50 to 200 mg NO3--N L-1. Moreover, this flow-through coupled device can continuingly operate at a large current of 800 mA over 100 h with a sustained NH3 yield rate of 420 μg h-1 cm-2 for nitrate-containing wastewater treatment (50 mg NO3--N L-1). When driven by solar energy, the flow-through coupled device can also exhibit exceptional real wastewater treatment performance, delivering great potential for practical application. This work paves a new avenue for clean energy production and environmental sustainability as well as carbon neutrality.

Boron-Modulated Electronic-Configuration Tuning of Cobalt for Enhanced Nitric Oxide Fixation to Ammonia

Electrocatalytic nitric oxide reduction (eNORR) to ammonia (NH3) provides an environmental route to alleviate NO pollution and yield great-value chemicals. The evolution of eNORR has been primarily hindered, however, by the poor reaction kinetics and low solubility of the NO in aqueous electrolytes. Herein, we have rationally designed a cobalt-based composite with a heterostructure as a highly efficient eNORR catalyst. In addition, by integrating boron to modulate the electronic structure, the catalyst CoB/Co@C delivered a significant NH3 yield of 315.4 μmol h-1 cm-2 for eNORR and an outstanding power density of 3.68 mW cm-2 in a Zn-NO battery. The excellent electrochemical performance of CoB/Co@C is attributed to the enrichment of NO by cobalt and boron dual-site adsorption and fast charge-transfer kinetics. It is demonstrated that the boron is pivotal in the enhancement of NO, the suppression of hydrogen evolution, and Co oxidation to boost eNORR performance.

Dual-Site W-O-CoP Catalysts for Active and Selective Nitrate Conversion to Ammonia in a Broad Concentration Window

Environmentally friendly electrochemical reduction of contaminated nitrate to ammonia (NO3 - RR) is a promising solution for large quantity ammonia (NH3 ) production, which, however, is a complex multi-reaction process involving coordination between different reaction intermediates of nitrate reduction and water decomposition-provided active hydrogen (Hads ) species. Here, a dual-site catalyst of [W-O] group-doped CoP nanosheets (0.6W-O-CoP@NF) has been designed to synergistically catalyze the NO3 - RR and water decomposition, especially the reactions between the intermediates of NO3 - RR and water decomposition-provided Hads species. This catalytic NO3 - RR exhibits an extremely high NH3 yield of 80.92 mg h-1 cm-2 and a Faradaic efficiency (FE) of 95.2% in 1 m KOH containing 0.1 m NO3 - . Significantly, 0.6W-O-CoP@NF presents greatly enhanced NH3 yield and FE in a wide NO3 - concentration ranges of 0.001-0.1 m compared to the reported. The excellent NO3 - RR performance is attributed to a synergistic catalytic effect between [W-O] and CoP active sites, in which the doped [W-O] group promotes the water decomposition to supply abundant Hads , and meanwhile modulates the electronic structure of Co for strengthened adsorption of Hads and the hydrogen (H2 ) release prevention, resultantly facilitating the NO3 - RR. Finally, a Zn-NO3 - battery has been assembled to simultaneously achieve three functions: electricity output, ammonia production, and nitrate treatment in wastewater.

Enhanced Nitrate-to-Ammonia Efficiency over Linear Assemblies of Copper-Cobalt Nanophases Stabilized by Redox Polymers

Renewable electricity-powered nitrate (NO3 - ) reduction reaction (NO3 RR) offers a net-zero carbon route to the realization of high ammonia (NH3 ) productivity. However, this route suffers from low energy efficiency (EE, with a half-cell EE commonly <36%), since high overpotentials are required to overcome the weak NO3 - binding affinity and sluggish NO3 RR kinetics. To alleviate this, a rational catalyst design strategy that involves the linear assembly of sub-5 nm Cu/Co nanophases into sub-20 nm thick nanoribbons is suggested. The theoretical and experimental studies show that the Cu-Co nanoribbons, similar to enzymes, enable strong NO3 - adsorption and rapid tandem catalysis of NO3 - to NH3 , owing to their richly exposed binary phase boundaries and adjacent Cu-Co sites at sub-5 nm distance. In situ Raman spectroscopy further reveals that at low applied overpotentials, the Cu/Co nanophases are rapidly activated and subsequently stabilized by a specifically designed redox polymer that in situ scavenges intermediately formed highly oxidative nitrogen dioxide (NO2 ). As a result, a stable NO3 RR with a current density of ≈450 mA cm-2 is achieved, a Faradaic efficiency of >97% for the formation of NH3 , and an unprecedented half-cell EE of ≈42%.

Decoration of boron nanoparticles on a graphene sheet for ammonia production from nitrate

Clean water and sanitation are two of the most important challenges worldwide and the main source for freshwater is groundwater. Nowadays, water is polluted by human activities. Concern about the presence of nitrates (NO₃^-) in groundwater is increasing day-by-day due to the intensive use of fertilizers and other anthropogenic sources, such as sewage or industrial wastewater discharge. Thus, the main solution available is to remove NO₃^- from groundwater and transfer it back to a usable nitrogen source. Electrochemical reduction of NO₃^- to ammonia (NH₃) under ambient conditions is a highly desirable method and it needs an efficient electrocatalyst. In this work, we synthesized a composite of amorphous boron with graphene oxide (B@GO) as an efficient catalyst for the nitrate reduction reaction. XRD and TEM analysis revealed an amorphous boron decoration on the graphene oxide sheet, and XPS confirmed that no bonding between boron and carbon occurs. In B@GO, a stronger defect carbon peak was observed than in GO and there was a random distribution of boron particles on the surface of the graphene nanosheets. Amorphous boron exhibits a higher bond energy, more reactivity, and chemical activity toward nitrate ions, which could be due to the lone pair present in the B atoms and could also be due to the edge oxidized B atoms. B@GO has a high number of active sites exposed leading to excellent nitrate reduction performance with a faradaic efficiency of 61.88% and good ammonia formation rate of 40006 μg h^-1 m_cat^-1 at -0.8 V versus reversible hydrogen electrode.

Intermetallic Single-Atom Alloy In-Pd Bimetallene for Neutral Electrosynthesis of Ammonia from Nitrate

Harvesting recyclable ammonia (NH₃) from the electrocatalytic reduction of nitrate (NO₃RR) offers a sustainable strategy to close the ecological nitrogen cycle from nitration contamination in an energy-efficient and environmentally friendly manner. The emerging intermetallic single-atom alloys (ISAAs) are recognized to achieve the highest site density of single atoms by isolating contiguous metal atoms into single sites stabilized by another metal within the intermetallic structure, which holds promise to couple the catalytic benefits from intermetallic nanocrystals and single-atom catalysts for promoting NO₃RR. Herein, ISAA In-Pd bimetallene, in which the Pd single atoms are isolated by surrounding In atoms, is reported to boost neutral NO₃RR with a NH₃ Faradaic efficiency (FE) of 87.2%, a yield rate of 28.06 mg h^-1 mg_Pd^-1, and an exceptional electrocatalytic stability with increased activity/selectivity over 100 h and 20 cycles. The ISAA structure induces substantially diminished overlap of Pd d-orbitals and narrowed p-d hybridization of In-p and Pd-d states around the Fermi level, resulting in a stronger NO₃^- adsorption and a depressed energy barrier of the potential-determining step for NO₃RR. Further integrating the NO₃RR catalyst into a Zn-NO₃^- flow battery as the cathode delivers a power density of 12.64 mW cm^-2 and a FE of 93.4% for NH₃ production.

Boosting Nitrate to Ammonia Electroconversion through Hydrogen Gas Evolution over Cu-foam@mesh Catalysts

The hydrogen evolution reaction (HER) is often considered parasitic to numerous cathodic electro-transformations of high technological interest, including but not limited to metal plating (e.g., for semiconductor processing), the CO₂ reduction reaction (CO₂RR), the dinitrogen → ammonia conversion (N₂RR), and the nitrate reduction reaction (NO₃^-RR). Herein, we introduce a porous Cu foam material electrodeposited onto a mesh support through the dynamic hydrogen bubble template method as an efficient catalyst for electrochemical nitrate → ammonia conversion. To take advantage of the intrinsically high surface area of this spongy foam material, effective mass transport of the nitrate reactants from the bulk electrolyte solution into its three-dimensional porous structure is critical. At high reaction rates, NO₃^-RR becomes, however, readily mass transport limited because of the slow nitrate diffusion into the three-dimensional porous catalyst. Herein, we demonstrate that the gas-evolving HER can mitigate the depletion of reactants inside the 3D foam catalyst through opening an additional convective nitrate mass transport pathway provided the NO₃^-RR becomes already mass transport limited prior to the HER onset. This pathway is achieved through the formation and release of hydrogen bubbles facilitating electrolyte replenishment inside the foam during water/nitrate co-electrolysis. This HER-mediated transport effect "boosts" the effective limiting current of nitrate reduction, as evidenced by potentiostatic electrolyses combined with an operando video inspection of the Cu-foam@mesh catalysts under operating NO₃^-RR conditions. Depending on the solution pH and the nitrate concentration, NO₃^-RR partial current densities beyond 1 A cm^-2 were achieved.

Simple and Low-Cost Electroactive Membranes for Ammonia Recovery

Ammonia is considered a contaminant to be removed from wastewater. However, ammonia is a valuable commodity chemical used as the primary feedstock for fertilizer manufacturing. Here we describe a simple and low-cost ammonia gas stripping membrane capable of recovering ammonia from wastewater. The material is composed of an electrically conducting porous carbon cloth coupled to a porous hydrophobic polypropylene support, that together form an electrically conductive membrane (ECM). When a cathodic potential is applied to the ECM surface, hydroxide ions are produced at the water-ECM interface, which transforms ammonium ions into higher-volatility ammonia that is stripped across the hydrophobic membrane material using an acid-stripping solution. The simple structure, low cost, and easy fabrication process make the ECM an attractive material for ammonia recovery from dilute aqueous streams, such as wastewater. When paired with an anode and immersed into a reactor containing synthetic wastewater (with an acid-stripping solution providing the driving force for ammonia transport), the ECM achieved an ammonia flux of 141.3 ± 14.0 g.cm^-2.day^-1 at a current density of 6.25 mA.cm^-2 (69.2 ± 5.3 kg(NH₃-N)/kWh). It was found that the ammonia flux was sensitive to the current density and acid circulation rate.

Thiol and H2S-Mediated NO Generation from Nitrate at Copper(II)

Reduction of nitrate is an essential, yet challenging chemical task required to manage this relatively inert oxoanion in the environment and biology. We show that thiols, ubiquitous reductants in biology, convert nitrate to nitric oxide at a Cu(II) center under mild conditions. The β-diketiminato complex [Cl2NNF6]Cu(κ2-O2NO) engages in O-atom transfer with various thiols (RSH) to form the corresponding copper(II) nitrite [CuII](κ2-O2N) and sulfenic acid (RSOH). The copper(II) nitrite further reacts with RSH to give S-nitrosothiols RSNO and [CuII]2(μ-OH)2 en route to NO formation via [CuII]-SR intermediates. The gasotransmitter H2S also reduces nitrate at copper(II) to generate NO, providing a lens into NO3-/H2S crosstalk. The interaction of thiols with nitrate at copper(II) releases a cascade of N- and S-based signaling molecules in biology.

Electrosynthesis of a nylon-6 precursor from cyclohexanone and nitrite under ambient conditions

Cyclohexanone oxime, an important nylon-6 precursor, is conventionally synthesized through cyclohexanone-hydroxylamine (NH₂OH) and cyclohexanone ammoxidation methodologies. These strategies require complicated procedures, high temperatures, noble metal catalysts, and toxic SO₂ or H₂O₂ usage. Here, we report a one-step electrochemical strategy to synthesize cyclohexanone oxime from nitrite (NO₂^-) and cyclohexanone under ambient conditions using a low-cost Cu-S catalyst, avoiding complex procedures, noble metal catalysts and H₂SO₄/H₂O₂ usage. This strategy produces 92% yield and 99% selectivity of cyclohexanone oxime, comparable to the industrial route. The reaction undergoes a NO₂^- → NH₂OH→oxime reaction pathway. This electrocatalytic strategy is suitable for the production of other oximes, highlighting the methodology universality. The amplified electrolysis experiment and techno-economic analysis confirm its practical potential. This study opens a mild, economical, and sustainable way for the alternative production of cyclohexanone oxime.

Single atomic cerium sites anchored on nitrogen-doped hollow carbon spheres for highly selective electroreduction of nitric oxide to ammonia

Electrocatalytic nitric oxide reduction reaction (NORR) at ambient environments not only offers a promising strategy to yield ammonia (NH₃) but also degrades the NO contaminant; however, its application depends on searching for high-performance catalysts. Herein, we present single atomic Ce sites anchored on nitrogen-doped hollow carbon spheres that are capable of electro-catalyzing NO reduction to NH₃ in an acidic solution, achieving a maximal Faradaic efficiency of 91 % and a yield rate of 1023 μg h^-1 mg_cat.^-1 at -0.7 V vs RHE for NH₃ formation, both of which outperform these on Ce nanoclusters and approach the best-reported results. Meanwhile, the single atomic Ce catalyst shows good structural and electrochemical stability during the 30-h NO electrolysis. Furthermore, when the single atomic Ce catalyst was used as cathodic material in a proof-of-concept of Zn-NO battery, it delivers a maximal power density of 3.4 mW cm^-2 and a high NH₃ yield rate of 309 μg h^-1 mg_cat.^-1. Theoretical simulations suggest that the Ce-N4 active moiety can not only activate NO molecules via a strong electronic interaction but also reduce the free energy barrier of *NO transition to *NOH intermediate as the limiting step, and therefore boosting the NORR kinetics and suppressing the competitive hydrogen evolution.

Theoretical Investigation of Electrocatalytic Reduction of Nitrates to Ammonia on Highly Efficient and Selective g-C2N Monolayer-Supported Single Transition-Metal Atoms

Electrocatalytic reduction of nitrate (NO₃RR) to synthesize ammonia (NH₃) can effectively degrade nitrate while producing a valuable product. By utilizing density functional theory calculations, we investigate the potential catalytic performance of a range of single transition-metal (TM) atoms supported on nitrogenated holey doped graphene (g-C₂N) (TM/g-C₂N) for the reduction of nitrates to NH₃. Based on the screening procedure, Zr/g-C₂N and Hf/g-C₂N are predicted as potential electrocatalysts for the NO₃RR with limiting potential (U_L) values of -0.28 and -0.27 V, respectively. The generation of byproducts such as dioxide (NO₂), nitric oxide (NO), and nitrogen (N₂) is hindered on Zr/g-C₂N and Hf/g-C₂N due to the high energy cost. The NO₃RR activity of TM/g-C₂N is closely related to the adsorption free energy of NO₃^-. The study not only proposes a competent electrocatalyst for enhancing NO₃RR in ammonia synthesis but also provides a comprehensive understanding of the NO₃RR mechanism.

Extracting Features of Active Transition Metal Electrodes for NO Electroreduction with Catalytic Matrices

Electrocatalytic reduction of oxidized nitrogen compounds (NO_x) promises to help rebalance the nitrogen cycle. It is widely accepted that nitrate reduction to NH₄⁺/NH₃ involves NO as an intermediate, and NO hydrogenation is the potential-limiting step of NO reduction. Whether *NO hydrogenates to *NHO or *NOH is still a matter of debate, which makes it difficult to optimize catalysts for NO_x electroreduction. Here, "catalytic matrices" are used to swiftly extract features of active transition metal catalysts for NO electroreduction. The matrices show that active catalysts statistically stabilize *NHO over *NOH and have undercoordinated sites. Besides, square-symmetry active sites with Cu and other elements may prove active for NO electroreduction. Finally, multivariate regressions are able to reproduce the main features found by the matrices, which opens the door for more sophisticated machine-learning studies. In sum, catalytic matrices may ease the analysis of complex electrocatalytic reactions on multifaceted materials.

Oxygen vacancies-rich Cu-W18O49 nanorods supported on reduced graphene oxide for electrochemical reduction ofN2to NH3

High-performance nitrogen fixation is severely limited by the efficiency and selectivity of a catalyst of electrochemical nitrogen reduction reaction (NRR) under ambient conditions. Here, the RGO/WOCu (reduced graphene oxide and Cu-doping W₁₈O₄₉) composite catalysts with abundant oxygen vacancies are prepared by the hydrothermal method. The obtained RGO/WOCu achieves an enhanced NRR performance (NH₃ yield rate:11.4 μg h^-1 mg_cat^-1, Faradaic efficiency: 4.4%) at -0.6 V (vs. RHE) in 0.1 mol L^-1 Na₂SO₄ solution. Furthermore, the NRR performance of the RGO/WOCu still keeps at 95% after four cycles, demonstrating its excellent stability. The Cu⁺-doping increases the concentration of oxygen vacancies, which is conducive to the adsorption and activation of N₂. Meanwhile, the introduction of RGO further improves the electrical conductivity and reaction kinetics of the RGO/WOCu due to the high specific surface area and conductivity. This work provides a simple and effective method for efficient electrochemical reduction ofN₂.

In situ interface engineered Co/NC derived from ZIF-67 as an efficient electrocatalyst for nitrate reduction to ammonia

Electrocatalytic nitrate (NO₃^-) reduction to ammonia (NH₃) is a promising alternative approach for simultaneous NH₃ green synthesis and NO₃^- contaminants removal. However, the complex eight-electron reaction requires catalysts with superb performance due to the low NH₃ selectivity and yield. In this work, the Co nanoparticles decorated N-doped carbon (NC) by in situ interface engineering were prepared by deriving ZIF-67 at 800 ℃ (Co/NC-800) for the selective NH₃ synthesis. This catalyst exhibits a remarkable performance and excellent cycle stability, achieving a great NH₃ yield of 1352.5 μg h^-1 mg_cat^-1 at -1.7 V vs Ag/AgCl, with a high NH₃ selectivity of up to 98.2 %, and a maximum Faradic efficiency of 81.2 % at -1.2 V vs Ag/AgCl. Moreover, DFT calculation results indicate that the interfacial effect between Co nanoparticle and NC could enhance electron transfer, and the composite Co/NC-800 shows a lower adsorption and conversion free energy, which promotes the production of ammonia.

In Situ/Operando Methods for Understanding Electrocatalytic Nitrate Reduction Reaction

With the development of industrial and agricultural, a large amount of nitrate is produced, which not only disrupts the natural nitrogen cycle, but also endangers public health. Among the commonly used nitrate treatment techniques, the electrochemical nitrate reduction reaction (eNRR) has attracted extensive attention due to its mild conditions, pollution-free nature, and other advantages. An in-depth understanding of the eNRR mechanism is the prerequisite for designing highly efficient electrocatalysts. However, some traditional characterization tools cannot comprehensively and deeply study the reaction process. It is necessary to develop in situ and operando techniques to reveal the reaction mechanism at the time-resolved and atomic level. This review discusses the eNRR mechanism and summarizes the possible in situ techniques used in eNRR. A detailed introduction of various in situ techniques and their help in understanding the reaction mechanism is provided. Finally, the current challenges and future opportunities in this research area are discussed and highlighted.

Electrocatalyst Microenvironment Engineering for Enhanced Product Selectivity in Carbon Dioxide and Nitrogen Reduction Reactions

Carbon and nitrogen fixation strategies are regarded as alternative routes to produce valuable chemicals used as energy carriers and fertilizers that are traditionally obtained from unsustainable and energy-intensive coal gasification (CO and CH₄), Fischer-Tropsch (C₂H₄), and Haber-Bosch (NH₃) processes. Recently, the electrocatalytic CO₂ reduction reaction (CO₂RR) and N₂ reduction reaction (NRR) have received tremendous attention, with the merits of being both efficient strategies to store renewable electricity while providing alternative preparation routes to fossil-fuel-driven reactions. To date, the development of the CO₂RR and NRR processes is primarily hindered by the competitive hydrogen evolution reaction (HER); however, the corresponding strategies for inhibiting this undesired side reaction are still quite limited. Considering such complex reactions involve three gas-liquid-solid phases and successive proton-coupled electron transfers, it appears meaningful to review the current strategies for improving product selectivity in light of their respective reaction mechanisms, kinetics, and thermodynamics. By examining the developments and understanding in catalyst design, electrolyte engineering, and three-phase interface modulation, we discuss three key strategies for improving product selectivity for the CO₂RR and NRR: (i) targeting molecularly defined active sites, (ii) increasing the local reactant concentration at the active sites, and (iii) stabilizing and confining product intermediates.

Microenvironment Engineering to Promote Selective Ammonia Electrosynthesis from Nitrate over a PdCu Hollow Catalyst

The electrosynthesis of recyclable ammonia (NH₃ ) from nitrate under ambient conditions is of great importance but still full of challenges for practical application. Herein, an efficient catalyst design strategy is developed that can engineer the surface microenvironment of a PdCu hollow (PdCu-H) catalyst to confine the intermediates and thus promote selective NH₃ electrosynthesis from nitrate. The hollow nanoparticles are synthesized by in situ reduction and nucleation of PdCu nanocrystals along a self-assembled micelle of a well-designed surfactant. The PdCu-H catalyst shows a structure-dependent selectivity toward the NH₃ product during the nitrate reduction reaction (NO₃ ^- RR) electrocatalysis, enabling a high NH₃ Faradaic efficiency of 87.3% and a remarkable NH₃ yield rate of 0.551 mmol h^-1 mg^-1 at -0.30 V (vs reversible hydrogen electrode). Moreover, this PdCu-H catalyst delivers high electrochemical performance in the rechargeable zinc-NO₃ ^- battery. These results provide a promising design strategy to tune catalytic selectivity for efficient electrosynthesis of renewable NH₃ and feedstocks.

Accelerating ammonia synthesis in a membraneless flow electrolyzer through coupling ambient dinitrogen oxidation and water splitting

An electrochemical approach for ammonia production is successfully developed by coupling the anodic dinitrogen oxidation reaction (NOR) and cathodic hydrogen evolution reaction (HER) within a well-designed membraneless flow electrolyzer. The obtained reactor shows the preferential yield of ammonia over nitrogen oxides on the vanadium nitride catalyst surface. At an applied oxidation potential of 2.25 V versus the reversible hydrogen electrode (vs RHE), a promoted ammonia production rate and Faradaic efficiency (FE) were obtained with 9.9 mmol g^-1 h^-1 (0.029 mmol cm^-2 h^-1) and 4.8%, respectively. Besides, the negative affection of ammonia contamination is efficiently alleviated. Density functional theory calculations revealed that the thermodynamic energy needed to produce ammonia (-0.63 eV) is far lower than that of producing nitrogen oxide (0.96 eV) from hydrogenated nitrogen oxides [∗N₂OH] splitting, confirming the coupling of NOR and HER.

Fully Biodegradable and Long-Term Operational Primary Zinc Batteries as Power Sources for Electronic Medicine

Given the advantages of high energy density and easy deployment, biodegradable primary battery systems remain as a promising power source to achieve bioresorbable electronic medicine, eliminating secondary surgeries for device retrieval. However, currently available biobatteries are constrained by operational lifetime, biocompatibility, and biodegradability, limiting potential therapeutic outcomes as temporary implants. Herein, we propose a fully biodegradable primary zinc-molybdenum (Zn-Mo) battery with a prolonged functional lifetime of up to 19 days and desirable energy capacity and output voltage compared with reported primary Zn biobatteries. The Zn-Mo battery system is shown to have excellent biocompatibility and biodegradability and can significantly promote Schwann cell proliferation and the axonal growth of dorsal root ganglia. The biodegradable battery module with 4 Zn-Mo cells in series using gelatin electrolyte accomplishes electrochemical generation of signaling molecules (nitric oxide, NO) that can modulate the behavior of the cellular network, with efficacy comparable with that of conventional power sources. This work sheds light on materials strategies and fabrication schemes to develop high-performance biodegradable primary batteries to achieve a fully bioresorbable electronic platform for innovative medical treatments that could be beneficial for health care.

Highly Selective N2 Electroreduction to NH3 Using a Boron-Vacancy-Rich Diatomic NbB Catalyst

The ambient electrochemical N₂ reduction reaction (NRR) is a future approach for the artificial NH₃ synthesis to overcome the problems of high-energy consumption and environmental pollution by Haber-Bosch technology. However, the challenge of N₂ activation on a catalyst surface and the competitive hydrogen evolution reaction make the current NRR unsatisfied. Herein, this work demonstrates that NbB₂ nanoflakes (NFs) exhibit excellent selectivity and durability in NRR, which produces NH₃ with a production rate of 30.5 µg h^-1 mg_cat ^-1 and a super-high Faraday efficiency (FE) of 40.2%. The high-selective NH₃ production is attributed to the large amount of active B vacancies on the surface of NbB₂ NFs. Density functional theory calculations suggest that the multiple atomic adsorption of N₂ on both unsaturated Nb and B atoms results in a significantly stretched N₂ molecule. The weakened NN triple bonds are easier to be broken for a biased NH₃ production. The diatomic catalysis is a future approach for NRR as it shows a special N₂ adsorption mode that can be well engineered.

Advances in iron-based electrocatalysts for nitrate reduction

Excessive nitrate has been a critical issue in the water environment, originating from the burning of fossil fuels, inefficient use of nitrogen fertilizers, and discharge of domestic and industrial wastewater. Among the effective treatments for nitrate reduction, electrocatalysis has become an advanced technique because it uses electrons as green reducing agents and can achieve high selectivity through cathode potential control. The effectiveness of electrocatalytic nitrate reduction (NO₃RR) mainly lies in the electrocatalyst. Iron-based catalysts have the advantages of high activity and low cost, which are well-used in the field of electrocatalytic nitrates. A comprehensive overview of the electrocatalytic mechanism and the iron-based materials for NO₃RR are given in terms of monometallic iron-based materials as well as bimetallic and oxide iron-based materials. A detailed introduction to NO₃RR on zero valent iron, single-atom iron catalysts, and Cu/Fe-based bimetallic electrocatalysts are provided, as they are essential for the improvement of NO₃RR performance. Finally, the advantages of iron-based materials for NO₃RR and the problems in current applications are summarized, and the development prospects of efficient iron-based catalysts are proposed.

Highly Selective Reduction of Nitrate by Zero-Valent Aluminum (ZVAI) Ball-Milled Materials at Circumneutral pH: Important Role of Microgalvanic Cells for Depassivation of ZVAl and N2-Selectivity

The passivation of zero-valent aluminum (ZVAl) limits its application in environmental remediation. Herein, a ternary composite material Al-Fe-AC is synthesized via a ball-milling treatment on a mixture of Al⁰, Fe⁰, and activated carbon (AC) powders. The results show that the as-prepared micronsized Al-Fe-AC powder could achieve highly efficient nitrate removal and a nitrogen (N₂)-selectivity of >75%. The mechanism study reveals that, in the initial stage, numerous Al//AC and Fe//AC microgalvanic cells in the Al-Fe-AC material could lead to a local alkaline environment in the vicinity of the AC cathodes. The local alkalinity depassivated the Al⁰ component and enabled its continuous dissolution in the subsequent second stage of reaction. The functioning of the AC cathode of the Al//AC microgalvanic cell is revealed as the primary reason accounting for the highly selective reduction of nitrate. The investigation on the mass ratio of raw materials manifested that an Al/Fe/AC mass ratio of 1:1:5 or 1:3:5 was preferable. The test in simulated groundwater suggested that the as-prepared Al-Fe-AC powder could be injected into aquifers to achieve a highly selective reduction of nitrate to nitrogen. This study provides a feasible method to develop high-performance ZVAl-based remedial materials that could work in a wider pH range.

Enhancing electro-reduction of nitrite to ammonia by loading Co3O4 on CuO to construct elecrocatalytic dual-sites

Improving the performance of CuO in electrocatalytic nitrite reduction to ammonia (NIRA) is the priority for designing efficient NIRA electrocatalysts. The electrocatalytic activity of CuO was enhanced by growing Co₃O₄ nanospheres on it. By comparing Co₃O₄@CuO with the mechanically mixed CuO and Co₃O₄ on a rotating ring-disk electrode, we discovered that the enhancement was attributed to a dual-site catalytic pathway.

Highly distributed amorphous copper catalyst for efficient ammonia electrosynthesis from nitrate

Electroreduction of nitrate to ammonia, instead of N₂, is beneficial toward pollution control and value-added chemical production. Metallic catalysts have been developed for enhancing ammonia evolution efficiency from nitrate based on the crystalline state of the catalyst. However, the development of amorphous metallic catalysts with more active sites is still unexplored. Herein, a highly distributed amorphous Cu catalyst exhibiting an outstanding ammonia yield rate of 1.42 mol h^-1 g^-1 and Faradaic efficiency of 95.7%, much superior to crystallized Cu, is demonstrated for nitrate-reduction to ammonia. Experimental and computational results reveal that amorphizing Cu increases the number of catalytic sites, enhances the NO₃^- adsorption strength with flat adsorption configurations, and facilitates the potential determining step of *NO protonation to *NHO. The amorphous Cu catalyst shows good electrochemical stability at - 0.3 V, while crystallization weakens the activity at a more negative potential. This study confirms the crystallinity-activity relationship of amorphous catalysts and unveils their potential-limited electrochemical stability.

Electrocatalytic reduction of nitrate ions in neutral medium at coinage metal-modified platinum electrodes

Nitrate is a water-soluble toxic pollutant that needs to be excluded from the environment. For this purpose, several electrochemical studies have been conducted but most of them focused on the nitrate reduction reaction (NRR) in alkaline and acidic media while insignificant research is available in neutral media with Pt electrode. In this work, we explored the effect of three coinage metals (Cu, Ag, and Au) on Pt electrode for the electrochemical reduction of nitrate in neutral solution. Among the three electrodes, Pt-Cu exhibited the best catalytic activity toward NRR, whereas Pt-Au electrode did not show any reactivity. An activity order of Pt-Cu > Pt-Ag > Pt-Au was observed pertaining to NRR. The Pt-Ag electrode produces nitrite ions by reducing nitrate ions ([Formula: see text]. Meanwhile, at Pt-Cu electrode, nitrate reduction yields ammonia via both direct ([Formula: see text] and indirect ([Formula: see text] reaction pathways depending on the potential. The cathodic transfer coefficients were estimated to be ca. 0.40 and ca. 0.52, while the standard rate constants for nitrate reduction were calculated as ca. 2.544 × 10^-2 cm.s^-1 and ca. 1.453 × 10^-2 cm.s^-1 for Pt-Cu and Pt-Ag electrodes, respectively. Importantly, Pt-Cu and Pt-Ag electrodes execute NRR in the neutral medium between their respective Hydrogen-Evolution Reaction (HER) and Open-Circuit Potential (OCP), implying that on these electrodes, HER and NRR do not compete and the latter is a corrosion-free process.