Upgrading of nitrate to hydrazine through cascading electrocatalytic ammonia production with controllable N-N coupling.

Nitrogen oxides (NOx) play important roles in the nitrogen cycle system and serve as renewable nitrogen sources for the synthesis of value-added chemicals driven by clean electricity. However, it is challenging to achieve selective conversion of NOx to multi-nitrogen products (e.g., N2H4) via precise construction of a single N-N bond. Herein, we propose a strategy for NOx-to-N2H4 under ambient conditions, involving electrochemical NOx upgrading to NH3, followed by ketone-mediated NH3 to N2H4. It can achieve an impressive overall NOx-to-N2H4 selectivity of 88.7%. We elucidate mechanistic insights into the ketone-mediated N-N coupling process. Diphenyl ketone (DPK) emerges as an optimal mediator, facilitating controlled N-N coupling, owing to its steric and conjugation effects. The acetonitrile solvent stabilizes and activates key imine intermediates through hydrogen bonding. Experimental results reveal that Ph2CN* intermediates formed on WO3 catalysts acted as pivotal monomers to drive controlled N-N coupling with high selectivity, facilitated by lattice-oxygen-mediated dehydrogenation. Additionally, both WO3 catalysts and DPK mediators exhibit favorable reusability, offering promise for green N2H4 synthesis.

Manipulation of Oxygen Species on an Antimony-Modified Copper Surface to Tune the Product Selectivity in CO2 Electroreduction.

Rational regulation of the electrochemical CO2 reduction reaction (CO2RR) pathway to produce desired products is particularly interesting, yet designing economical and robust catalysts is crucial. Here, we report an antimony-modified copper (CuSb) catalyst capable of selectively producing both CO and multicarbon (C2+) products in the CO2RR. At a current density of 0.3 A/cm2, the faradaic efficiency (FE) of CO was as high as 98.2% with a potential of -0.6 V vs reversible hydrogen electrode (RHE). When the current density increased to 1.1 A/cm2 at -1.1 V vs RHE, the primary products shifted to C2+ compounds with a FE of 75.6%. Experimental and theoretical studies indicate that tuning the potential could manipulate the oxygen species on the CuSb surface, which determined the product selectivity in the CO2RR. At a more positive potential, the existence of oxygen species facilitates the potential-limiting step involving *COOH formation and reduces the adsorption of *CO intermediates, thereby promoting CO production. At a more negative potential, the localized high CO concentration coupled with the enhanced adsorption of *CO intermediates due to Sb incorporation facilitates C-C coupling and deep hydrogenation processes, resulting in an increased C2+ selectivity.

Urea Synthesis via Coelectrolysis of CO2 and Nitrate over Heterostructured Cu-Bi Catalysts.

Electrocatalytic coupling of CO2 and NO3- to urea is a promising way to mitigate greenhouse gas emissions, reduce waste from industrial processes, and store renewable energy. However, the poor selectivity and activity limit its application due to the multistep process involving diverse reactants and reactions. Herein, we report the first work to design heterostructured Cu-Bi bimetallic catalysts for urea electrosynthesis. A high urea Faradaic efficiency (FE) of 23.5% with a production rate of 2180.3 µg h-1 mgcat-1 was achieved in H-cells, which surpassed most reported electrocatalysts in the literature. Moreover, the catalyst had a remarkable recycling stability. Experiments and density functional theory calculations demonstrated that introduction of moderate Bi induced the formation of the Bi-Cu/O-Bi/Cu2O heterostructure with abundant phase boundaries, which are beneficial for NO3-, CO2, and H2O activation and enhance C-N coupling and promote *HONCON intermediate formation. Moreover, favorable *HNCONH2 protonation and urea desorption processes were also validated, further explaining the reason for high activity and selectivity toward urea.

Highly Selective CO2 Electroreduction to Multi-carbon Alcohols via Amine Modified Copper Nanoparticles at Acidic Conditions.

Electroreduction of CO2 into multi-carbon (C2+) products (e.g. C2+ alcohols) offers a promising way for CO2 utilization. Use of strong alkaline electrolytes is favorable to producing C2+ products. However, CO2 can react with hydroxide to form carbonate/bicarbonate, which results in low carbon utilization efficiency and poor stability. Using acidic electrolyte is an efficient way to solve the problems, but it is a challenge to achieve high selectivity of C2+ products. Here we report that the amine modified copper nanoparticles exhibit high selectivity of C2+ products and carbon utilization at acidic condition. The Faradaic efficiency (FE) of C2+ products reach up to 81.8% at acidic media (pH=2) with a total current density of 410 mA cm-2 over n-butylamine modified Cu. Especially the FE of C2+ alcohols is 52.6%, which is higher than those reported for CO2 electroreduction at acidic condition. In addition, the single-pass carbon efficiency towards C2+ production reach up to 60%. Detailed studies demonstrate that the amine molecule on the surface of Cu cannot only enhance the formation, adsorption and coverage of *CO, but also provide a hydrophobic environment, which result in the high selectivity of C2+ alcohols at acidic condition.

Switching Reaction Pathways of CO2 Electroreduction by Modulating Cations in the Electrochemical Double Layer.

Tuning the selectivity of CO2 electroreduction reaction (CO2RR) solely by changing electrolyte is a very attractive topic. In this study, we conducted CO2RR in different aqueous electrolytes over bulk metal electrodes. It was discovered that controlled CO2RR could be achieved by modulating cations in the electrochemical double layer. Specifically, ionic liquid cations in the electrolyte significantly inhibits the hydrogen evolution reaction (HER), while yielding high Faraday efficiencies toward CO (FECO) or formate (FEformate) depending on the alkali metal cations. For example, the product could be switched from CO (FECO=97.3 %) to formate (FEformate=93.5 %) by changing the electrolyte from 0.1 M KBr-0.5 M 1-octyl-3-methylimidazolium bromide (OmimBr) to 0.1 M CsBr-0.5 M OmimBr aqueous solutions over pristine Cu foil electrode. In situ spectroscopy and theoretical calculations reveal that the ordered structure generated by the assembly of Omim+ under an applied negative potential alters the hydrogen bonding structure of the interfacial water, thereby inhibiting the HER. The difference in selectivity in the presence of different cations is attributed to the hydrogen bonding effect caused by Omim+, which alters the solvated structure of the alkali metal cations and thus affects the stabilization of intermediates of different pathways.

CO2 electrolysis to multi-carbon products in strong acid at ampere-current levels on La-Cu spheres with channels.

Achieving satisfactory multi-carbon (C2+) products selectivity and current density under acidic condition is a key issue for practical application of electrochemical CO2 reduction reaction (CO2RR), but is challenging. Herein, we demonstrate that combining microenvironment modulation by porous channel structure and intrinsic catalytic activity enhancement via doping effect could promote efficient CO2RR toward C2+ products in acidic electrolyte (pH ≤ 1). The La-doped Cu hollow sphere with channels exhibits a C2+ products Faradaic efficiency (FE) of 86.2% with a partial current density of -775.8 mA cm-2. CO2 single-pass conversion efficiency for C2+ products can reach 52.8% at -900 mA cm-2. Moreover, the catalyst still maintains a high C2+ FE of 81.3% at -1 A cm-2. The channel structure plays a crucial role in accumulating K+ and OH- species near the catalyst surface and within the channels, which effectively suppresses the undesired hydrogen evolution and promotes C-C coupling. Additionally, the La doping enhances the generation of *CO intermediate, and also facilitates C2+ products formation.

Synthesis of Hydroxylamine via Ketone-Mediated Nitrate Electroreduction.

Hydroxylamine (HA, NH2OH) is a critical feedstock in the production of various chemicals and materials, and its efficient and sustainable synthesis is of great importance. Electroreduction of nitrate on Cu-based catalysts has emerged as a promising approach for green ammonia (NH3) production, but the electrosynthesis of HA remains challenging due to overreduction of HA to NH3. Herein, we report the first work on ketone-mediated HA synthesis using nitrate in water. A metal-organic-framework-derived Cu catalyst was developed to catalyze the reaction. Cyclopentanone (CP) was used to capture HA in situ to form CP oxime (CP-O) with C═N bonds, which is prone to hydrolysis. HA could be released easily after electrolysis, and CP was regenerated. It was demonstrated that CP-O could be formed with an excellent Faradaic efficiency of 47.8%, a corresponding formation rate of 34.9 mg h-1 cm-2, and a remarkable carbon selectivity of >99.9%. The hydrolysis of CP-O to release HA and CP regeneration was also optimized, resulting in 96.1 mmol L-1 of HA stabilized in the solution, which was significantly higher than direct nitrate reduction. Detailed in situ characterizations, control experiments, and theoretical calculations revealed the catalyst surface reconstruction and reaction mechanism, which showed that the coexistence of Cu0 and Cu+ facilitated the protonation and reduction of *NO2 and *NH2OH desorption, leading to the enhancement for HA production.

Efficient C-N coupling for urea electrosynthesis on defective Co3O4 with dual-functional sites.

Urea electrosynthesis under ambient conditions is emerging as a promising alternative to conventional synthetic protocols. However, the weak binding of reactants/intermediates on the catalyst surface induces multiple competing pathways, hindering efficient urea production. Herein, we report the synthesis of defective Co3O4 catalysts that integrate dual-functional sites for urea production from CO2 and nitrite. Regulating the reactant adsorption capacity on defective Co3O4 catalysts can efficiently control the competing reaction pathways. The urea yield rate of 3361 mg h-1 gcat-1 was achieved with a corresponding faradaic efficiency (FE) of 26.3% and 100% carbon selectivity at a potential of -0.7 V vs. the reversible hydrogen electrode. Both experimental and theoretical investigations reveal that the introduction of oxygen vacancies efficiently triggers the formation of well-matched adsorption/activation sites, optimizing the adsorption of reactants/intermediates while decreasing the C-N coupling reaction energy. This work offers new insights into the development of dual-functional catalysts based on non-noble transition metal oxides with oxygen vacancies, enabling the efficient electrosynthesis of essential C-N fine chemicals.

Highly Efficient Electrosynthesis of Glycine over an Atomically Dispersed Iron Catalyst.

Glycine is a nonessential amino acid that plays a vital role in various biological activities. However, the conventional synthesis of glycine requires sophisticated procedures or toxic feedstocks. Herein, we report an electrochemical pathway for glycine synthesis via the reductive coupling of oxalic acid and nitrate or nitrogen oxides over atomically dispersed Fe-N-C catalysts. A glycine selectivity of 70.7% is achieved over Fe-N-C-700 at -1.0 V versus RHE. Synergy between the FeN3C structure and pyrrolic nitrogen in Fe-N-C-700 facilitates the reduction of oxalic acid to glyoxylic acid, which is crucial for producing glyoxylic acid oxime and glycine, and the FeN3C structure could reduce the energy barrier of *HOOCCH2NH2 intermediate formation thus accelerating the glyoxylic acid oxime conversion to glycine. This new synthesis approach for value-added chemicals using simple carbon and nitrogen sources could provide sustainable routes for organonitrogen compound production.

Polymer Modification Strategy to Modulate Reaction Microenvironment for Enhanced CO2 Electroreduction to Ethylene.

Modulation of the microenvironment on the electrode surface is one of the effective means to improve the efficiency of electrocatalytic carbon dioxide reduction (eCO2 RR). To achieve high conversion rates, the phase boundary at the electrode surface should be finely controlled to overcome the limitation of CO2 solubility in the aqueous electrolyte. Herein, we developed a simple and efficient method to structure electrocatalyst with a superhydrophobic surface microenvironment by one-step co-electrodeposition of Cu and polytetrafluoroethylene (PTFE) on carbon paper. The super-hydrophobic Cu-based electrode displayed a high ethylene (C2 H4 ) selectivity with a Faraday efficiency (FE) of 67.3 % at -1.25 V vs. reversible hydrogen electrode (RHE) in an H-type cell, which is 2.5 times higher than a regular Cu electrode without PTFE. By using PTFE as a surface modifier, the activity of eCO2 RR is enhanced and water (proton) adsorption is inhibited. This strategy has the potential to be applied to other gas-conversion electrocatalysts.

Integration of plasma and electrocatalysis to synthesize cyclohexanone oxime under ambient conditions using air as a nitrogen source.

Direct fixation of N2 to N-containing value-added chemicals is a promising pathway for sustainable chemical manufacturing. There is extensive demand for cyclohexanone oxime because it is the essential feedstock of Nylon 6. Currently, cyclohexanone oxime is synthesized under harsh conditions that consume a considerable amount of energy. Herein, we report a novel approach to synthesize cyclohexanone oxime by in situ NO3- generation from air under ambient conditions. This process was carried out through an integrated strategy including plasma-assisted air-to-NOx and co-electrolysis of NOx and cyclohexanone. A high rate of cyclohexanone oxime formation at 20.1 mg h-1 cm-2 and a corresponding faradaic efficiency (FE) of 51.4% was achieved over a Cu/TiO2 catalyst, and the selectivity of cyclohexanone oxime was >99.9% on the basis of cyclohexanone. The C-N bond formation mechanism was examined by in situ experiments and theoretical calculations, which showed that cyclohexanone oxime forms through the reaction between an NH2OH intermediate and cyclohexanone.

Ultra-fast synthesis of three-dimensional porous Cu/Zn heterostructures for enhanced carbon dioxide electroreduction.

The construction of metal hetero-interfaces has great potential in the application of electro-catalytic carbon dioxide reduction (ECR). Herein, we report a fast, efficient, and simple electrodeposition strategy for synthesizing three-dimensional (3D) porous Cu/Zn heterostructures using the hydrogen bubble template method. When the deposition was carried out at -1.0 A for 30 s, the obtained 3D porous Cu/Zn heterostructures on carbon paper (CP) demonstrated a nearly 100% CO faradaic efficiency (FE) with a high partial current density of 91.8 mA cm-2 at -2.1 V vs. Ag/Ag+ in the mixed electrolyte of ionic liquids/acetonitrile in an H-type cell. In particular, the partial current density of CO could reach 165.5 mA cm-2 and the FE of CO could remain as high as 94.3% at -2.5 V vs. Ag/Ag+. The current density is much higher than most reported to date in an H-type cell (Table S1). Experimental and density functional theory (DFT) calculations reveal that the outstanding electrocatalytic performance of the electrode can be ascribed to the formation of 3D porous Cu/Zn heterostructures, in which the porous and self-supported architecture facilitates diffusion and the Cu/Zn heterostructures can reduce the energy barrier for ECR to CO.

Switching between C2+ Products and CH4 in CO2 Electrolysis by Tuning the Composition and Structure of Rare-Earth/Copper Catalysts.

Rational regulation of the reaction pathway to produce the desired products is one of the most significant challenges in the electrochemical CO2 reduction reaction (CO2RR). Herein, we designed a series of rare-earth Cu catalysts with mixed phases. It was found that the products could be switched from C2+ to CH4 by tuning the composition and structure of the catalysts. Particularly at the Cu/Sm atomic ratio of 9/1 (Cu9Sm1-Ox), the Faradaic efficiency (FE) for C2+ products (FEC2+) could reach 81% at 700 mA cm-2 with negligible CH4. However, the FE of CH4 (FECH4) was 65% at 500 mA cm-2 over Cu1Sm9-Ox (Cu/Sm = 1/9), and the FEC2+ was extremely low. Experiments and theoretical studies indicated that the stable CuSm2O4 phase existed in all the catalysts within the Cu/Sm range of 9/1 to 1/9. At a high Cu content, the catalyst was composed of CuSm2O4 and Cu phases. The small amount of Sm could enhance the binding strength of *CO and facilitate C-C coupling. Conversely, at a high Sm content, the catalyst was composed of CuSm2O4 and Sm2O3 phases. Sm could effectively stabilize bivalent Cu and enrich proton donors, lowering the reaction energy of *CO for deep hydrogenation to generate CH4. In both pathways, the stable CuSm2O4 phase could cooperate with the Cu or Sm2O3 phases, which induced the formation of different microenvironments to generate different products. This strategy also had commonality with other Cu-rare-earth (La, Pr, and Eu) catalysts to boost the CO2RR for C2+ or CH4 production.

Highly efficient zinc electrode prepared by electro-deposition in a salt-induced pre-phase separation region solution.

Efficient electrode design is crucial for the electrochemical reduction of CO2 to produce valuable chemicals. The solution used for the preparation of electrodes can affect their overall properties, which in turn determine the reaction efficiency. In this work, we report that transition metal salts could induce the change of two-phase ionic liquid/ethanol mixture into miscible one phase. Pre-phase separation region near the phase boundary of the ternary system was observed. Zinc nanoparticles were electro-deposited along the fibres of carbon paper (CP) substrate uniformly in the salt-induced pre-phase separation region solution. The as-prepared Zn(1)/CP electrode exhibits super-wettability to the electrolyte, rendering very high catalytic performance for CO2 electro-reduction, and the Faradaic efficiency towards CO is 97.6% with a current density of 340 mA cm-2, which is the best result to date in an H-type cell.

Boosting Electrocatalytic Nitrate-to-Ammonia via Tuning of N-Intermediate Adsorption on a Zn-Cu Catalyst.

The renewable-energy-powered electroreduction of nitrate (NO3 - ) to ammonia (NH3 ) has garnered significant interest as an eco-friendly and promising substitute for the Haber-Bosch process. However, the sluggish kinetics hinders its application at a large scale. Herein, we first calculated the N-containing species (*NO3 and *NO2 ) binding energy and the free energy of the hydrogen evolution reaction over Cu with different metal dopants, and it was shown that Zn was a promising candidate. Based on the theoretical study, we designed and synthesized Zn-doped Cu nanosheets, and the as-prepared catalysts demonstrated excellent performance in NO3 - -to-NH3 . The maximum Faradaic efficiency (FE) of NH3 could reach 98.4 % with an outstanding yield rate of 5.8 mol g-1 h-1 , which is among the best results up to date. The catalyst also had excellent cycling stability. Meanwhile, it also presented a FE exceeding 90 % across a wide potential range and NO3 - concentration range. Detailed experimental and theoretical studies revealed that the Zn doping could modulate intermediates adsorption strength, enhance NO2 - conversion, change the *NO adsorption configuration to a bridge adsorption, and decrease the energy barrier, leading to the excellent catalytic performance for NO3 - -to-NH3 .

Oxophilicity-Controlled CO2 Electroreduction to C2+ Alcohols over Lewis Acid Metal-Doped Cuδ+ Catalysts.

Cu-based electrocatalysts have great potential for facilitating CO2 reduction to produce energy-intensive fuels and chemicals. However, it remains challenging to obtain high product selectivity due to the inevitable strong competition among various pathways. Here, we propose a strategy to regulate the adsorption of oxygen-associated active species on Cu by introducing an oxophilic metal, which can effectively improve the selectivity of C2+ alcohols. Theoretical calculations manifested that doping of Lewis acid metal Al into Cu can affect the C-O bond and Cu-C bond breaking toward the selectively determining intermediate (shared by ethanol and ethylene), thus prioritizing the ethanol pathway. Experimentally, the Al-doped Cu catalyst exhibited an outstanding C2+ Faradaic efficiency (FE) of 84.5% with remarkable stability. In particular, the C2+ alcohol FE could reach 55.2% with a partial current density of 354.2 mA cm-2 and a formation rate of 1066.8 µmol cm-2 h-1. A detailed experimental study revealed that Al doping improved the adsorption strength of active oxygen species on the Cu surface and stabilized the key intermediate *OC2H5, leading to high selectivity toward ethanol. Further investigation showed that this strategy could also be extended to other Lewis acid metals.

Zn-induced electron-rich Sn catalysts enable highly efficient CO2 electroreduction to formate.

Renewable-energy-driven CO2 electroreduction provides a promising way to address the growing greenhouse effect issue and produce value-added chemicals. As one of the bulk chemicals, formic acid/formate has the highest revenue per mole of electrons among various products. However, the scaling up of CO2-to-formate for practical applications with high faradaic efficiency (FE) and current density is constrained by the difficulty of precisely reconciling the competing intermediates (*COOH and HCOO*). Herein, a Zn-induced electron-rich Sn electrocatalyst was reported for CO2-to-formate with high efficiency. The faradaic efficiency of formate (FEformate) could reach 96.6%, and FEformate > 90% was maintained at formate partial current density up to 625.4 mA cm-1. Detailed study indicated that catalyst reconstruction occurred during electrolysis. With appropriate electron accumulation, the electron-rich Sn catalyst could facilitate the adsorption and activation of CO2 molecules to form a intermediate and then promoted the carbon protonation of to yield a HCOO* intermediate. Afterwards, the HCOO* → HCOOH* proceeded via another proton-coupled electron transfer process, leading to high activity and selectivity for formate production.

High-Rate CO2 Electrolysis to Formic Acid over a Wide Potential Window: An Electrocatalyst Comprised of Indium Nanoparticles on Chitosan-Derived Graphene.

Realizing industrial-scale production of HCOOH from the CO2 reduction reaction (CO2 RR) is very important, but the current density as well as the electrochemical potential window are still limited to date. Herein, we achieved this by integration of chemical adsorption and electrocatalytic capabilities for the CO2 RR via anchoring In nanoparticles (NPs) on biomass-derived substrates to create In/X-C (X=N, P, B) bifunctional active centers. The In NPs/chitosan-derived N-doped defective graphene (In/N-dG) catalyst had outstanding performance for the CO2 RR with a nearly 100 % Faradaic efficiency (FE) of HCOOH across a wide potential window. Particularly, at 1.2 A ⋅ cm-2 high current density, the FE of HCOOH was as high as 96.0 %, and the reduction potential was as low as -1.17 V vs RHE. When using a membrane electrode assembly (MEA), a pure HCOOH solution could be obtained at the cathode without further separation and purification. The FE of HCOOH was still up to 93.3 % at 0.52 A ⋅ cm-2 , and the HCOOH production rate could reach 9.051 mmol ⋅ h-1 ⋅ cm-2 . Our results suggested that the defects and multilayer structure in In/N-dG could not only enhance CO2 chemical adsorption capability, but also trigger the formation of an electron-rich catalytic environment around In sites to promote the generation of HCOOH.

Construction of 3D copper-chitosan-gas diffusion layer electrode for highly efficient CO2 electrolysis to C2+ alcohols.

High-rate electrolysis of CO2 to C2+ alcohols is of particular interest, but the performance remains far from the desired values to be economically feasible. Coupling gas diffusion electrode (GDE) and 3D nanostructured catalysts may improve the efficiency in a flow cell of CO2 electrolysis. Herein, we propose a route to prepare 3D Cu-chitosan (CS)-GDL electrode. The CS acts as a "transition layer" between Cu catalyst and the GDL. The highly interconnected network induces growth of 3D Cu film, and the as-prepared integrated structure facilitates rapid electrons transport and mitigates mass diffusion limitations in the electrolysis. At optimum conditions, the C2+ Faradaic efficiency (FE) can reach 88.2% with a current density (geometrically normalized) as high as 900 mA cm-2 at the potential of -0.87 V vs. reversible hydrogen electrode (RHE), of which the C2+ alcohols selectivity is 51.4% with a partial current density of 462.6 mA cm-2, which is very efficient for C2+ alcohols production. Experimental and theoretical study indicates that CS induces growth of 3D hexagonal prismatic Cu microrods with abundant Cu (111)/Cu (200) crystal faces, which are favorable for the alcohol pathway. Our work represents a novel example to design efficient GDEs for electrocatalytic CO2 reduction (CO2RR).

Improving CO2-to-C2+ Product Electroreduction Efficiency via Atomic Lanthanide Dopant-Induced Tensile-Strained CuOx Catalysts.

Cu is a promising electrocatalyst in CO2 reduction reaction (CO2RR) to high-value C2+ products. However, as important C-C coupling active sites, the Cu+ species is usually unstable under reduction conditions. How atomic dopants affect the performance of Cu-based catalysts is interesting to be studied. Herein, we first calculated the difference between the thermodynamic limiting potentials of CO2RR and the hydrogen evolution reaction, as well as the *CO binding energy over Cu2O doped with different metals, and the results indicated that doping atomic Gd into Cu2O could improve the performance of the catalyst effectively. On the basis of the theoretical study, we designed Gd1/CuOx catalysts. The distinctive electronic structure and large ion radii of Gd not only keep the Cu+ species stable during the reaction but also induce tensile strain in Gd1/CuOx, resulting in excellent performance of the catalysts for electroreduction of CO2 to C2+ products. The Faradic efficiency of C2+ products could reach 81.4% with a C2+ product partial current density of 444.3 mA cm-2 at -0.8 V vs a reversible hydrogen electrode. Detailed experimental and theoretical studies revealed that Gd doping enhanced CO2 activation on the catalyst, stabilized the key intermediate O*CCO, and reduced the energy barrier of the C-C coupling reaction.