Pulsed electroreduction of low-concentration nitrate to ammonia

Unveiling the Dual Role of Oxophilic Cr4+ in Cr-Cu2O Nanosheet Arrays for Enhanced Nitrate Electroreduction to Ammonia

Cuprous oxide (Cu2O)-based catalysts present a promising activity for the electrochemical nitrate (NO3 -) reduction to ammonia (eNO3RA), but the electrochemical instability of Cu+ species may lead to an unsatisfactory durability, hindering the exploration of the structure-performance relationship. Herein, we propose an efficient strategy to stabilize Cu+ through the incorporation of Cr4+ into the Cu2O matrix to construct a Cr4+-O-Cu+ network structure. In situ and quasi-in situ characterizations reveal that the Cu+ species are well maintained via the strong Cr4+-O-Cu+ interaction that inhibits the leaching of lattice oxygen. Importantly, in situ generated Cr3+-O-Cu+ from Cr4+-O-Cu+ is identified as a dual-active site for eNO3RA, wherein the Cu+ sites are responsible for the activation of N-containing intermediates, while the assisting Cr3+ centers serve as the electron-proton mediators for rapid water dissociation. Theoretical investigations further demonstrated that the metastable state Cr3+-O-Cu+ favors the conversion from the endoergic hydrogenation of the key *ON intermediate to an exoergic reaction in an ONH pathway, and facilitates the subsequent NH3 desorption with a low energy barrier. The superior eNO3RA with a maximum 91.6 % Faradaic efficiency could also be coupled with anodic sulfion oxidation to achieve concurrent NH3 production and sulfur recovery with reduced energy input.

Atomic-Scale Tailoring C-N Coupling Sites for Efficient Acetamide Electrosynthesis over Cu-Anchored Boron Nitride Nanosheets

Electrochemical conversion of carbon and nitrogen sources into valuable chemicals provides a promising strategy for mitigating CO2 emissions and tackling pollutants. However, efficiently scaling up C-N products beyond basic compounds like urea remains a significant challenge. Herein, we upgrade the C-N coupling for acetamide synthesis through coreducing CO and nitrate (NO3-) on atomic-scale Cu dispersed on boron nitride (Cu/BN) nanosheets. The specific form of Cu, such as single atom, nanocluster, and nanoparticles, endows Cu/BN different adsorption capacity for CO and NO3-, thereby dictating the catalytic activity and selectivity for acetamide formation. The Cu nanocluster-anchored BN (Cu NCs/BN) catalyst achieves an industrial-level current density of 178 mA cm-2 for the C-N coupling reaction and an average acetamide yield rate of 137.0 mmol h-1 gcat.-1 at -1.6 V versus the reversible hydrogen electrode. Experimental and theoretical analyses uncover the pivotal role of the strong electronic interaction between Cu nanoclusters and BN, which activates CO and NO3-, facilitates the formation of key *CCO and *NH2 intermediates, and expedites the C-N coupling pathway to acetamide. This work propels the development of atomic structure catalysts for the efficient conversion of small molecules to high-value chemicals through electrochemical processes.

Electrocatalytic systems for NOx upgrading

Chemical manufacturing utilizing renewable resources and energy presents a promising avenue toward sustainability and carbon neutrality. Electrocatalytic upgrading of nitrogen oxides (NOx) into nitrogenous chemicals is a potential strategy for synthesizing chemicals and mitigating NOx pollution. However, this approach is currently hindered by low selectivity and efficiency, limited reaction pathways, and economic challenges, primarily due to the development of suboptimal electrocatalytic systems for NOx upgrading. In this review, we focus on electrocatalytic systems for NOx upgrading and discuss newly developed components, including catalysts, solvents, electrolysers, and upstream/downstream processes. These advancements enable recent developments in NOx upgrading reactions that yield various products, including green ammonia (NH3), dinitrogen (N2), nitrogenous chemicals beyond NH3 and N2 (e.g., hydroxylamine and hydrazine), and organonitrogen compunds. Additionally, we provide an outlook to highlight future directions in the emerging field of novel electrocatalytic systems for NOx upgrading.

Advanced Ruthenium-Based Electrocatalysts for NOx Reduction to Ammonia

Ammonia (NH3) is widely recognized as a crucial raw material for nitrogen-based fertilizer production and eco-friendly hydrogen-rich fuels. Currently, the Haber-Bosch process still dominates the worldwide industrial NH3 production, which consumes substantial energy and contributes to enormous CO2 emission. As an alternative NH3 synthesis route, electrocatalytic reduction of NOx species (NO3 -, NO2 -, and NO) to NH3 has gained considerable attention due to its advantages such as flexibility, low power consumption, sustainability, and environmental friendliness. This review timely summarizes an updated and critical survey of mechanism, design, and application of Ru-based electrocatalysts for NOx reduction. First, the reason why the Ru-based catalysts are good choice for NOx reduction to NH3 is presented. Second, the reaction mechanism of NOx over Ru-based materials is succinctly summarized. Third, several typical in situ characterization techniques, theoretical calculations, and kinetics analysis are examined. Subsequently, the construction of each classification of the Ru-based electrocatalysts according to the size of particles and compositions is critically reviewed. Apart from these, examples are given on the applications in the production of valuable chemicals and Zn-NOx batteries. Finally, this review concludes with a summary highlighting the main practical challenges relevant to selectivity and efficiency in the broad range of NOx concentrations and the high currents, as well as the critical perspectives on the fronter of this exciting research area.

Electrodegradation of nitrogenous pollutants in sewage: from reaction fundamentals to energy valorization applications

The excessive accumulation of nitrogen pollutants (mainly nitrate, nitrite, ammonia nitrogen, hydrazine, and urea) in water bodies seriously disrupts the natural nitrogen cycle and poses a significant threat to human life and health. Electrolysis is considered a promising method to degrade these nitrogenous pollutants in sewage, with the advantages of high efficiency, wide generality, easy operability, retrievability, and environmental friendliness. For particular energy devices, including metal-nitrate batteries, direct fuel cells, and hybrid water electrolyzers, the realization of energy valorization from sewage purification processes (e.g., valuable chemical generation, electricity output, and hydrogen production) becomes feasible. Despite the progress in the research on pollutant electrodegradation, the development of electrocatalysts with high activity, stability, and selectivity for pollutant removal, coupled with corresponding energy devices, remains a challenge. This review comprehensively provides advanced insights into the electrodegradation processes of nitrogenous pollutants and relevant energy valorization strategies, focusing on the reaction mechanisms, activity descriptors, electrocatalyst design, and actuated electrodes and operation parameters of tailored energy conversion devices. A feasibility analysis of electrodegradation on real wastewater samples from the perspective of pollutant concentration, pollutant accumulation, and electrolyte effects is provided. Challenges and prospects for the future development of electrodegradation systems are also discussed in detail to bridge the gap between experimental trials and commercial applications.

Promoting Alcohols Electrooxidation Coupled with Hydrogen Production via Asymmetric Pulse Potential Strategy

Electrocatalytic organic oxidation coupled with hydrogen (H2) production emerges as a profitable solution to simultaneously reduce overall energy consumption of H2 production and synthetic high-value chemicals. Noble metal catalysts are highly efficient electrocatalysts in oxidation reactions, but they deactivate easily weakening the benefit in actual production. Herein, we report a universal asymmetric pulse potential strategy to achieve long-term stable operation of noble metals for various alcohol oxidation reactions and noble metal catalysts. For example, by pulsed potentials between 0.8 V and 0 V vs. RHE, palladium (Pd)-catalyzed glycerol (GLY) electrooxidation can continuously proceed for more than 2800 h with glyceric acid (GLA) selectivity of >70 %. Whereas, Pd electrocatalyst becomes nearly deactivated within 6 h of reaction under conventional potentiostatic strategy. Experimental and theoretical calculation results reveal that the generated electrophilic OH* from H2O/OH- oxidation on Pd (denoted as Pd-OH*) acts as main active species for GLY oxidation. However, Pd-OH* is prone to be oxidized to PdOx resulting in performance decay. When a short reduction potential (e.g., 0 V vs. RHE for 5 s) is powered, PdOx can be reversibly reduced to restore the current. Moreover, we tested the feasibility of this strategy in a flow electrolyzer, verifying the practical application potential.

Electrochemical valorization of dilute reactive nitrogen compounds into ammonia: advances in catalysis and reactive separations

The global demand for sustainable nitrogen management has brought attention to the challenge of efficiently converting dilute nitrogen compounds, such as nitrates and nitrogen oxides, into valuable ammonia. This review emphasizes on innovative catalyst designs, including homogeneous and heterogenous catalysts tailored to low-concentration reactive nitrogen species. Moreover it explores the integration of advanced separation and concentration techniques, such as electrosorption and dialysis, to overcome mass transport limitations and enable effective electrochemical valorization. This review also examines reactive separation strategies for post-purification, focusing on the integration of recovery processes with catalysis in a direct manner. By detailing these approaches, this work outlines pathways to scalable and energy-efficient solutions for converting waste nitrogen streams into ammonia, addressing critical challenges in nitrogen valorization and offering prospects for industrial applications.

Functional partitioning synergistically enhances multi-scenario nitrate reduction

The promising electrocatalytic nitrate reduction reaction (eNitRR) for distributed ammonia synthesis requires the fine design of functionally compartmentalised and synergistically complementary integrated catalysts to meet the needs of low-cost and efficient ammonia synthesis. Herein, the partitionable CoP3 and Cu3P modules were built on the copper foam substrate, and the functional differentiation promoted the catalytic performance of the surface accordion-like CoP3/Cu3P@CF for eNitRR in complex water environment. Where the ammonia yield rate is as high as 23988.2 μg h-1 cm-2, and the Faradaic efficiency is close to 100 %. With CoP3/Cu3P@CF as the core, the assembled high-performance Zn-nitrate flow battery can realize the dual function of ammonia production and power supply, and can also realize the continuous production of ammonia with high selectivity driven by solar energy. The ammonia recovery reaches 753.9 mg L-1, which shows the superiority of CoP3/Cu3P@CF in multiple application scenarios and provides important experience for the vigorous development of eNitRR. Density functional theory calculation reveal that CoP3 and Cu3P sites play a relay synergistic role in eNitRR catalyzed by CoP3/Cu3P@CF. CoP3 first promotes the activation of NO3- to *NO3H, and then continuously provides proton hydrogen for the eNitRR on the surface of Cu3P, which relays the synergistic catalytic effect to promote the efficient conversion of NO3- to NH3. This study not only develops a catalyst that can promote the efficient reduction of NO3- to ammonia through an easy-to-obtain innovative strategy, but also provides an alternative strategy for the development of eNitRR that is suitable for multiple scenarios and meets the production conditions.

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

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

Microenvironment Engineering of Heterogeneous Catalysts for Liquid-Phase Environmental Catalysis

Environmental catalysis has emerged as a scientific frontier in mitigating water pollution and advancing circular chemistry and reaction microenvironment significantly influences the catalytic performance and efficiency. This review delves into microenvironment engineering within liquid-phase environmental catalysis, categorizing microenvironments into four scales: atom/molecule-level modulation, nano/microscale-confined structures, interface and surface regulation, and external field effects. Each category is analyzed for its unique characteristics and merits, emphasizing its potential to significantly enhance catalytic efficiency and selectivity. Following this overview, we introduced recent advancements in advanced material and system design to promote liquid-phase environmental catalysis (e.g., water purification, transformation to value-added products, and green synthesis), leveraging state-of-the-art microenvironment engineering technologies. These discussions showcase microenvironment engineering was applied in different reactions to fine-tune catalytic regimes and improve the efficiency from both thermodynamics and kinetics perspectives. Lastly, we discussed the challenges and future directions in microenvironment engineering. This review underscores the potential of microenvironment engineering in intelligent materials and system design to drive the development of more effective and sustainable catalytic solutions to environmental decontamination.

Bimetallic Cu11Ag3 Nanotips for Ultrahigh Yield Rate of Nitrate-to-Ammonium

Electrochemical reduction of nitrate to ammonia (NRA) offers a sustainable approach for NH3 production and NO3 - removal but suffers from low NH3 yield rate (<1.20 mmol h-1 cm-2). We present bimetallic Cu11Ag3 nanotips with tailored local environment, which achieve an ultrahigh NH3 yield rate of 2.36 mmol h-1 cm-2 at a low applied potential of -0.33 V vs. RHE, a high Faradaic efficiency (FE) of 98.8 %, and long-term operation stability at 1800 mg-N L-1 NO3 -, outperforming most of the recently reported catalysts. At a NO3 - concentration as low as 15 mg-N L-1, it still delivers a high FE of 86.9 % and an NH3 selectivity of 93.8 %. Finite-element method and density functional theory calculations reveal that the Cu11Ag3 exhibits reduced adsorption energy barrier of *N intermediates, favorable water dissociation for *H generation and high energy barrier for H2 formation, while its tip-enhanced enrichment promoting NO3 - accumulation.

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

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

Controlled Defective Engineering on CuIr Catalyst Promotes Nitrate Selective Reduction to Ammonia

Electrochemical nitrate reduction reaction (NO3-RR) is a promising low-carbon and environmentally friendly approach for the production of ammonia (NH3). Herein, we develop a high-temperature quenched copper (Cu) catalyst with the aim of inducing nonequilibrium phase transformation, revealing the multiple defects (distortion, dislocations, vacancies, etc.) presented in Cu, which lead to low overpotential for NO3-RR and high efficiency for NH3 production. Further loading a low content of iridium (Ir) species on the Cu surface improves the reactivity and ammonia selectivity. The resultant CuIr electrode exhibits a Faradaic efficiency of 93% and a record yield of 6.01 mmol h-1 cm-2 at -0.22 VRHE exceeding those of state-of-the-art NO3-RR catalysts. Detailed investigations have demonstrated that the synergistic effect between multiple defects and Ir decoration effectively regulate the d-band center of copper, change the adsorption state of the catalyst surface, and promote the adsorption and reduction of intermediates and reactants. The strong H* adsorption ability of the Ir element provides more active hydrogen for the generation of ammonia, promoting the reduction of nitrate to NH3.

Electrosynthesis of adipic acid with high faradaic efficiency within a wide potential window

Electrosynthesis of adipic acid (a precursor for nylon-66) from KA oil (a mixture of cyclohexanone and cyclohexanol) represents a sustainable strategy to replace conventional method that requires harsh conditions. However, its industrial possibility is greatly restricted by the low current density and competitive oxygen evolution reaction. Herein, we modify nickel layered double hydroxide with vanadium to promote current density and maintain high faradaic efficiency (>80%) within a wide potential window (1.5 ~ 1.9 V vs. reversible hydrogen electrode). Experimental and theoretical studies reveal two key roles of V modification, including accelerating catalyst reconstruction and strengthening cyclohexanone adsorption. As a proof-of-the-concept, we construct a membrane electrode assembly, producing adipic acid with high faradaic efficiency (82%) and productivity (1536 μmol cm-2 h-1) at industrially relevant current density (300 mA cm-2), while achieving >50 hours stability. This work demonstrates an efficient catalyst for adipic acid electrosynthesis with high productivity that shows industrial potential.

Efficient Electroreduction of Low Nitrate Concentration via Nitrate Self-Enrichment and Active Hydrogen Inducement on the Ce(IV)-Co3O4 Cathode

Low concentrations of nitrate (NO3-) widely exist in wastewater, post-treated wastewater, and natural environments; its further disposal is a challenge but meaningful for its discharge goals. Electroreduction of NO3- is a promising method that allows to eliminate NO3- and even generate higher-value NH3. However, the massive side reaction of hydrogen evolution has raised great obstacles in the electroreduction of low concentrations of NO3-. Herein, we present an efficient electroreduction method for low or even ultralow concentrations of NO3- via NO3- self-enrichment and active hydrogen (H*) inducement on the Ce(IV)-Co3O4 cathode. The key mechanism is that the strong oxytropism of Ce(IV) in Co3O4 resulted in two changes in structures, including loose nanoporous structures with copious dual adsorption sites of Ce-Co showing strong self-enrichment of NO3- and abundant oxygen vacancies (Ovs) inducing substantial H*. Ultimately, the bifunctional role synergistically promoted the selective conversion of NH3 rather than H2. As a result, Ce(IV)-Co3O4 demonstrated a NO3- self-enrichment with a 4.3-fold up-adsorption, a 7.5-fold enhancement of NH3 Faradic efficiency, and a 93.1% diminution of energy consumption when compared to Co3O4, substantially exceeding other reported electroreduction cathodes for NO3- concentrations lower than 100 mg·L-1. This work provides an effective treatment method for low or even ultralow concentrations of NO3-.

Modulating the Electrolyte Microenvironment in Electrical Double Layer for Boosting Electrocatalytic Nitrate Reduction to Ammonia

Electrochemical nitrate reduction reaction (NO3RR) is a promising approach to achieve remediation of nitrate-polluted wastewater and sustainable production of ammonia. However, it is still restricted by the low activity, selectivity and Faraday efficiency for ammonia synthesis. Herein, we propose an effective strategy to modulate the electrolyte microenvironment in electrical double layer (EDL) by mediating alkali metal cations in the electrolyte to enhance the NO3RR performance. Taking bulk Cu as a model catalyst, the experimental study reveals that the NO3 --to-NH3 performance in different electrolytes follows the trend Li+ Collapse

Key Words

• alkali metal cation
• electrical double layer
• nitrate reduction
• proton transport
• reaction kinetics

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MESH Headings Collapse

Grants

• GXXT-2022-026 University Synergy Innovation Program of Anhui Province
• 2022AH050112 and 2022AH010004 Project of Anhui Provincial Department of Education
• KJ2021A0070 and KJ2021A1168 Natural Science Research Project of Universities in Anhui Province

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Affiliation(s)

Weidong Wen School of Materials Science and Engineering, Anhui University, Hefei, 230601, P. R. China
Shidong Fang Institute of Energy, Hefei Comprehensive National Science Centre (Anhui Energy Laboratory), Hefei, 230051, P. R. China Hefei Institutes of Physical Science, Chinese Academy of Sciences (CAS), Hefei, 230031, P. R. China
Yitong Zhou Institutes of Physical Science and Information Technology, Anhui University, Hefei, 230601, P. R. China
Ying Zhao School of Pharmacy, Anhui Xinhua University, Hefei, 230088, P. R. China
Peng Li School of Materials Science and Engineering, Anhui University, Hefei, 230601, P. R. China
Xin-Yao Yu School of Materials Science and Engineering, Anhui University, Hefei, 230601, P. R. China

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Gradient-concentration RuCo electrocatalyst for efficient and stable electroreduction of nitrate into ammonia

Electrocatalytic nitrate reduction to ammonia holds great promise for developing green technologies for electrochemical ammonia energy conversion and storage. Considering that real nitrate resources often exhibit low concentrations, it is challenging to achieve high activity in low-concentration nitrate solutions due to the competing reaction of the hydrogen evolution reaction, let alone considering the catalyst lifetime. Herein, we present a high nitrate reduction performance electrocatalyst based on a Co nanosheet structure with a gradient dispersion of Ru, which yields a high NH3 Faraday efficiency of over 93% at an industrially relevant NH3 current density of 1.0 A/cm2 in 2000 ppm NO3- electrolyte, while maintaining good stability for 720 h under -300 mA/cm2. The electrocatalyst maintains high activity even in 62 ppm NO3- electrolyte. Electrochemical studies, density functional theory, electrochemical in situ Raman, and Fourier-transformed infrared spectroscopy confirm that the gradient concentration design of the catalyst reduces the reaction energy barrier to improve its activity and suppresses the catalyst evolution caused by the expansion of the Co lattice to enhance its stability. The gradient-driven design in this work provides a direction for improving the performance of electrocatalytic nitrate reduction to ammonia.

Effects of Ionic Interferents on Electrocatalytic Nitrate Reduction: Mechanistic Insight

Nitrate, a prevalent water pollutant, poses substantial public health concerns and environmental risks. Electrochemical reduction of nitrate (eNO3RR) has emerged as an effective alternative to conventional biological treatments. While extensive lab work has focused on designing efficient electrocatalysts, implementation of eNO3RR in practical wastewater settings requires careful consideration of the effects of various constituents in real wastewater. In this critical review, we examine the interference of ionic species commonly encountered in electrocatalytic systems and universally present in wastewater, such as halogen ions, alkali metal cations, and other divalent/trivalent ions (Ca2+, Mg2+, HCO3-/CO32-, SO42-, and PO43-). Notably, we categorize and discuss the interfering mechanisms into four groups: (1) loss of active catalytic sites caused by competitive adsorption and precipitation, (2) electrostatic interactions in the electric double layer (EDL), including ion pairs and the shielding effect, (3) effects on the selectivity of N intermediates and final products (N2 or NH3), and (4) complications by the hydrogen evolution reaction (HER) and localized pH on the cathode surface. Finally, we summarize the competition among different mechanisms and propose future directions for a deeper mechanistic understanding of ionic impacts on eNO3RR.

Electric Field Generated at the Millisecond Pulse-Polarized Interface Facilitates the Electrolytic Conversion of SO2 into H2S

Interfacial electric field holds significant importance in determining both the polar molecular configuration and surface coverage during electrocatalysis. This study introduces a methodology leveraging the varying electric dipole moment of SO2 under distinct interfacial electric field strengths to enhance the selectivity of the SO2 electroreduction process. This approach presented the first attempt to utilize pulsed voltage application to the Au/PTFE membrane electrode for the control of the molecular configuration and coverage of SO2 on the electrode surface. Remarkably, the modulation of pulse duration resulted in a substantial inhibition of the hydrogen evolution reaction (HER) (FEH2 < 3%) under millisecond pulse conditions (ta = 10 ms, tc = 300 ms, Ea = -0.8 V (vs Hg/Hg2SO4), Ec = -1.8 V (vs Hg/Hg2SO4)), concomitant with a noteworthy enhancement in H2S selectivity (FEH2S > 97%). A comprehensive analysis, incorporating in situ Raman spectroscopy, electrochemical quartz crystal microbalance, COMSOL simulations, and DFT calculations, corroborated the increased selectivity of H2S products was primarily associated with the inherently large dipole moment of the SO2 molecule. The enhancement of the interfacial electric field induced by millisecond pulses was instrumental in amplifying SO2 coverage, activating SO2, facilitating the formation of the pivotal intermediate product *SOH, and effectively reducing the reaction energy barrier in the SO2 reduction process. These findings provide novel insights into the influences of ion and molecular transport dynamics, as well as the temporal intricacies of competitive pathways during the SO2 electroreduction process. Moreover, it underscores the intrinsic correlation between the electric dipole moment and surface-molecule interaction of the catalyst.

Pulsed electro-catalysis enables effective conversion of low-concentration nitrate to ammonia over Cu2O@Pd tandem catalyst

Electro-catalytic conversion of nitrate (NO3-) to ammonia (NH3) via the Nitrate Reduction to Ammonia (NORA) process represents a promising strategy for both ammonia synthesis and environmental remediation. Despite its potential, the efficiency of low-concentration NORA is often hindered by mass transfer limitations, competing byproducts (N2 and NO2-), and side reactions such as hydrogen evolution. This study introduces a novel pulsed electro-synthesis technique that alternates the potential to periodically accumulate and transform NO2- intermediates near a Cu2O@Pd electrode, enhancing the NORA process. Compared with that under potentiostatic conditions, the Cu2O@Pd electrodes exhibited a higher NORA activity under the optimized pulsed condition, where a NH3-N Faradaic efficiency (FE) of 81.2%, a yield rate of 1.08 mg h-1 cm-2 and a selectivity efficiency (SE) of 81.5%, were achieved. In-situ characterization revealed an enhancement mechanism characterized by optimized adsorption of the key *NO intermediate, followed by the hydrogenation path "*N → *NH → *NH2→ *NH3". Further investigations indicated the electro-catalytic synergies between Pd sites and Cu species, where the Pd atoms were the reaction sites for the H adsorption while the Cu species were responsible for the NO3- activation. This research offers a novel insight into a method of enhancing low-concentration NORA.

Overcoming Electrostatic Interaction via Pulsed Electroreduction for Boosting the Electrocatalytic Urea Synthesis

Electrocatalytic urea synthesis under ambient conditions offers a promising alternative strategy to the traditional energy-intensive urea industry protocol. Limited by the electrostatic interaction, the reduction reaction of anions at the cathode in the electrocatalytic system is not easily achievable. Here, we propose a novel strategy to overcome electrostatic interaction via pulsed electroreduction. We found that the reconstruction-resistant CuSiOx nanotube, with abundant atomic Cu-O-Si interfacial sites, exhibits ultrastability in the electrosynthesis of urea from nitrate and CO2. Under a pulsed potential approach with optimal operating conditions, the Cu-O-Si interfaces achieve a superior urea production rate (1606.1 μg h-1 mgcat. -1) with high selectivity (79.01 %) and stability (the Faradaic efficiency is retained at 80 % even after 80 h of testing), outperforming most reported electrocatalytic synthesis urea catalysts. We believe our strategy will incite further investigation into pulsed electroreduction increasing substrate transport, which may guide the design of ambient urea electrosynthesis and other energy conversion systems.

Sulphur-Boosted Active Hydrogen on Copper for Enhanced Electrocatalytic Nitrate-to-Ammonia Selectivity

Electrocatalytic nitrate reduction to ammonia is a promising approach in term of pollutant appreciation. Cu-based catalysts performs a leading-edge advantage for nitrate reduction due to its favorable adsorption with *NO3. However, the formation of active hydrogen (*H) on Cu surface is difficult and insufficient, leading to the significant generation of by-product NO2 -. Herein, sulphur doped Cu (Cu-S) is prepared via an electrochemical conversion strategy and used for nitrate electroreduction. The high Faradaic efficiency (FE) of ammonia (~98.3 %) and an extremely low FE of nitrite (~1.4 %) are achieved on Cu-S, obviously superior to its counterpart of Cu (FENH3: 70.4 %, FENO2 -: 18.8 %). Electrochemical in situ characterizations and theoretical calculations indicate that a small amount of S doping on Cu surface can promote the kinetics of H2O dissociation to active hydrogen. The optimized hydrogen affinity validly decreases the hydrogenation kinetic energy barrier of *NO2, leading to an enhanced NH3 selectivity.

Matched Kinetics Process Over Fe2O3-Co/NiO Heterostructure Enables Highly Efficient Nitrate Electroreduction to Ammonia

Tandem nitrate electroreduction reaction (NO3 -RR) is a promising method for green ammonia (NH3) synthesis. However, the mismatched kinetics processes between NO3 --to-NO2 - and NO2 --to-NH3 results in poor selectivity for NH3 and excess NO2 - evolution in electrolyte solution. Herein, a Ni2+ substitution strategy for developing oxide heterostructure in Co/Fe layered double oxides (LDOs) was designed and employed as tandem electrocataltysts for NO3 -RR. (Co0.83Ni0.16)2Fe exhibited a high NH3 yield rate of 50.4 mg ⋅ cm-2 ⋅ h-1 with a Faradaic efficiency of 97.8 % at -0.42 V vs. reversible hydrogen electrode (RHE) in a pulsed electrolysis test. By combining with in situ/operando characterization technologies and theoretical calculations, we observed the strong selectivity of NH3 evolution over (Co0.83Ni0.16)2Fe, with Ni playing a dual role in NO3 -RR by i) modifying the electronic behavior of Co, and ii) serving as complementary site for active hydrogen (*H) supply. Therefore, the adsorption capacity of *NO2 and its subsequent hydrogenation on the Co sites became more thermodynamically feasible. This study shows that Ni substitution promotes the kinetics of the NO3 -RR and provides insights into the design of tandem electrocatalysts for NH3 evolution.

Nitrogenous Intermediates in NOx-involved Electrocatalytic Reactions

Chemical manufacturing utilizing renewable sources and energy emerges as a promising path towards sustainability and carbon neutrality. The electrocatalytic reactions involving nitrogen oxides (NOx) offered a potential strategy for synthesizing various nitrogenous chemicals. However, it is currently hindered by low selectivity/efficiency and limited reaction pathways, mainly due to the difficulties in controllable generation and utilization of nitrogenous intermediates. In this minireview, focusing on nitrogenous intermediates in NOx-involved electrocatalytic reactions, we discuss newly developed methodologies for studying and controlling the generation, conversion, and utilizing of nitrogenous intermediates, which enable recent developments in NOx-involved electrocatalytic reactions that yield various products, including ammonia (NH3), organonitrogen molecules, and nitrogenous compounds exhibiting unconventional oxidation states. Furthermore, we also make an outlook to highlight future directions in the emerging field of NOx-involved electrocatalytic reactions.

Unveiling Cutting-Edge Developments in Electrocatalytic Nitrate-to-Ammonia Conversion

The excessive enrichment of nitrate in the environment can be converted into ammonia (NH3) through electrochemical processes, offering significant implications for modern agriculture and the potential to reduce the burden of the Haber-Bosch (HB) process while achieving environmentally friendly NH3 production. Emerging research on electrocatalytic nitrate reduction (eNitRR) to NH3 has gained considerable momentum in recent years for efficient NH3 synthesis. However, existing reviews on nitrate reduction have primarily focused on limited aspects, often lacking a comprehensive summary of catalysts, reaction systems, reaction mechanisms, and detection methods employed in nitrate reduction. This review aims to provide a timely and comprehensive analysis of the eNitRR field by integrating existing research progress and identifying current challenges. This review offers a comprehensive overview of the research progress achieved using various materials in electrochemical nitrate reduction, elucidates the underlying theoretical mechanism behind eNitRR, and discusses effective strategies based on numerous case studies to enhance the electrochemical reduction from NO3 - to NH3. Finally, this review discusses challenges and development prospects in the eNitRR field with an aim to guide design and development of large-scale sustainable nitrate reduction electrocatalysts.

Multi-Functional Formaldehyde-Nitrate Batteries for Wastewater Refining, Electricity Generation, and Production of Ammonia and Formate

As bulky pollutants in industrial and agricultural wastewater, nitrate and formaldehyde pose serious threats to the human health and ecosystem. Current purification technologies including chemical and bio-/photo-/electro-chemical methods, are generally high-cost, time-consuming, or energy-intensive. Here, we report a novel formaldehyde-nitrate battery by pairing anodic formaldehyde oxidation with cathodic nitrate reduction, which simultaneously enables wastewater purification, electricity generation, and the production of high-value-added ammonia and formate. As a result, the formaldehyde-nitrate battery remarkably exhibits an open-circuit voltage of 0.75 V, a peak power density of 3.38 mW cm-2 and the yield rates of 32.7 mg h-1  cm-2 for ammonia and 889.4 mg h-1  cm-2 for formate. In a large-scale formaldehyde-nitrate battery (25 cm2 ), 99.9 % of nitrate and 99.8 % of formaldehyde are removed from simulated industrial wastewater and the electricity of 2.03 W⋅h per day is generated. Moreover, the design of such a multi-functional battery is universally applicable to the coupling of NO3 - or NO2 - reduction with various aldehyde oxidization, paving a new avenue for wastewater purification and chemical manufacturing.

Recent research progress on building C-N bonds via electrochemical NOx reduction

The release of NOx species (such as nitrate, nitrite and nitric oxide) into water and the atmosphere due to human being's agricultural and industrial activities has caused a series of environmental problems, including accumulation of toxic pollutants that are dangerous to humans and animals, acid rain, the greenhouse effect and disturbance of the global nitrogen cycle balance. Electrosynthesis of organonitrogen compounds with NOx species as the nitrogen source offers a sustainable strategy to upgrade the waste NOx into value-added organic products under ambient conditions. The electrochemical reduction of NOx species can generate surface-adsorbed intermediates such as hydroxylamine, which are usually strong nucleophiles and can undergo nucleophilic attack to carbonyl groups to build C-N bonds and generate organonitrogen compounds such as amine, oxime, amide and amino acid. This mini-review summarizes the most recent progress in building C-N bonds via the in situ generation of nucleophilic intermediates from electrochemical NOx reduction, and highlights some important strategies in facilitating the reaction rates and selectivities towards the C-N coupling products. In particular, the preparation of high-performance electrocatalysts (e.g., nano-/atomic-scale catalysts, single-atom catalysts, alloy catalysts), selection of nucleophilic intermediates, novel design of reactors and understanding the surface adsorption process are highlighted. A few key challenges and knowledge gaps are discussed, and some promising research directions are also proposed for future advances in electrochemical C-N coupling.

Aqueous Electroreduction of Nitric Oxide to Ammonia at Low Concentration via Vacancy Engineered FeOCl

Electroreduction of nitric oxide (NO) to NH3 (NORR) has gained extensive attention for the sake of low carbon emission and air pollutant treatment. Unfortunately, NORR is greatly hindered by its sluggish kinetics, especially under low concentrations of NO. Herein, we developed a chlorine (Cl) vacancy strategy to overcome this limitation over FeOCl nanosheets (FeOCl-VCl ). Density functional theory (DFT) calculations revealed that the Cl vacancy resulted in defective Fe with sharp d-states characteristics in FeOCl-VCl to enhance the absorption and activation of NO. In situ X-ray absorption near-edge structure (XANES) and attenuated total reflection-infrared spectroscopy (ATR-IR) verified the lower average oxidation state of defective Fe to enhance the electron transfer for NO adsorption/activation and facilitate the generation of key NHO and NHx intermediates. As a result, the FeOCl-VCl exhibited superior NORR activities with the NH3 Faradaic efficiency up to 91.1 % while maintaining a high NH3 yield rate of 455.4 μg cm-2  h-1 under 1.0 vol % NO concentration, competitive with those of previously reported literatures under higher NO concentration. Further, the assembled Zn-NO battery utilizing FeOCl-VCl as cathode delivered a record peak power density of 6.2 mW cm-2 , offering a new route for simultaneous NO removal, NH3 production, and energy supply.