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

Unlocking Ampere-Level Nitrate Electroreduction to Ammonia Via the Built-In Electric Field in Monometallic Catalysts

Bimetallic/multimetallic catalysts for nitrate reduction reaction (NO3-RR) have been extensively investigated benefiting from their synergistic effects in optimizing various intermediate adsorptions; however, the interphasic synergistic effects in monometallic catalysts are often overlooked. Here we report an interphasic synergy between electron-rich Co(OH)2 and electron-deficient CoO, in which the asymmetric charge distribution in monometallic cobalt-based heterojunction derived from the built-in electric field (BEF) significantly accelerates electron transfer and lowers the energy barriers for NO3-RR. Theoretical calculations reveal that the chemical affinities of Co atoms toward NO3- and NO2- are significantly enhanced and even NO3- adsorption switches to a spontaneous process. Simultaneously, the BEF in monometallic Co-based heterostructures greatly reduces the energy barrier of the rate-determining step (*NO→*NOH) in the NO3-RR. Therefore, the resultant catalyst exhibits ampere-level NO3-RR performance, achieving a record NH3 yield up to 73.9 mg h-1 cm-2 at a low potential of -0.2 V with a Faradaic efficiency (FE) of 95.6%.

Multi-Dimensional Ni@TiN/CNT Heterostructure with Tandem Catalysis for Efficient Electrochemical Nitrite Reduction to Ammonia

Electrochemical nitrite reduction reaction (NO2RR) is considered a sustainable ammonia (NH3) synthesis strategy. However, there are still significant challenges in designing efficient NO2RR catalysts. Here, carbon nanotube (CNT)-encapsulated Ni nanoparticles (NPs) loaded on MXene-derived TiN (Ni@TiN/CNT) heterostructure is constructed by combining molten salt etching strategy and chemical vapor deposition. Ni@TiN/CNT exhibits an excellent NH3 yield rate (15.6 mg h-1 mgcat. -1), Faradaic efficiency (95.6%), and record cycle stability (NO2RR performance is virtually unattenuated after 60 cycles) at -0.7 V versus reversible hydrogen electrode (versus RHE). In addition, the Zn-nitrite battery with Ni@TiN/CNT as the cathode shows high power density (9.6 mW cm-2) and NH3 synthesis performance. Combining validation experiments and density functional theory calculations reveals that Ni@TiN/CNT follows the tandem catalytic mechanism. The TiN site preferentially adsorbs and activates NO2 -, while the Ni site provides abundant active hydrogen for the subsequent reduction process. Meanwhile, the chainmail structure of CNT prevents the oxidation and leaching of active sites, thereby significantly enhancing the stability of Ni@TiN/CNT. This work provides a new inspiration for the preparation of durable and efficient NO2RR electrocatalysts with tandem catalytic sites.

A Cu0.76Co2.24O4/γ-Cu2(OH)3Cl composite catalyst for efficient neutral nitrate reduction

The electrocatalytic nitrate reduction reaction (eNO3-RR) is an environmentally friendly process that converts nitrate wastewater into high-value ammonia (NH3). However, the multi-step electron and proton transfer in this reaction leads to slow kinetics and competitive reactions, making it challenging to achieve energy-efficient performance. Herein, a Cu0.76Co2.24O4/γ-Cu2(OH)3Cl (CCOC) composite has been prepared as an electrocatalyst for the eNO3-RR. The CCOC catalyst demonstrated an outstanding NH3 yield rate of 10.71 mg h-1 cm-2 and a remarkable faradaic efficiency (FE) of 95.9% in a 0.5 M Na2SO4 neutral solution containing 0.1 M NO3-, surpassing most reported catalysts under neutral conditions. In situ investigations demonstrated that Cu0.76Co2.24O4 with high-valent Cuδ+ and Coδ+ significantly enhances H2O dissociation and proton production while also promoting the adsorption of NO3- and *NH intermediates. These properties contribute to the high NH3 selectivity and activity observed under neutral conditions. This work demonstrates Cu0.76Co2.24O4/γ-Cu2(OH)3Cl as a promising candidate for the sustainable and efficient production of NH3 through the eNO3-RR, offering new insights into efficient nitrate reduction in neutral environments.

Synergistic Cu2O@Ni(OH)2 Core-Shell Electrocatalyst for High-Efficiency Nitrate Reduction to Ammonia

The electrocatalytic reduction reaction of nitrate (NO3RR) is anticipated to convert nitrogen-containing pollutants into valuable ammonia products. Copper-based catalysts have received great attention because of their good performance in the NO3RR due to the strong binding energy with *NO3 intermediates. However, the poor H2O dissociation ability of Cu is unable to provide H• in time for the hydrogenation reaction of NOx, thus hindering the electroreduction of the NO3-. Herein, we designed a shell-core nanocube electrocatalyst Cu2O@Ni(OH)2-x (x represents the molar ratio of Ni/Cu) using the liquid phase reduction combined with the etching and precipitation method for electrocatalytic NO3RR. Due to the synergistic effect between the strong nitrate activation ability of Cu and the excellent H2O dissociation ability of Ni(OH)2, Cu2O@Ni(OH)2-3.3% shows an impressive ammonia yield rate (557.9 μmol h-1 cm-2) and Faradaic efficiency (97.4%) at -0.35 V vs. RHE. Operando Raman and Auger electron spectroscopy observe the reduction of Cu2O to Cu during the NO3RR process. Density functional theory calculations combined with electron paramagnetic resonance analysis reveals that Ni(OH)2 can lower the activation energy barrier of H2O dissociation, thereby promoting the generation of H• and accelerating the hydrogenation of *NO during the NO3RR. This research provides an efficient Cu-based catalyst for reducing NO3- and may motivate the development of effective ammonia electrocatalysts for further experimentation.

CO2 Laser-Stabilized Ni-Co Dual Single-Atomic Sites for Energy Generation and Ammonia Harvesting

Dual single-atom catalysts (DSACs) hold immense potential in electrochemical nitrate (NO3 -) reduction (EcNR) as a sustainable replacement to the Haber-Bosch process for the production of ammonia (NH3). However, challenges such as synthesis complexity, low purity, scalability, and stability have hindered their practical application. Herein, a rapid and scalable method is introduced to stabilize low-cost 3d transition metals (Ni and Co) as DSACs on Ti3C2Tx MXene in 10 min using continuous-wave CO2-laser irradiation. Ni2+ and Co2+ ions are chelated and stabilized as single atoms onto an L-tryptophan-modified Ti3C2Tx surface via metal─O and metal─N bonds, forming Ni-single atom catalyst (SAC)/MXene, Co-SAC/MXene, and NiCo-DSAC/MXene. This approach enhances MXene properties, enabling the synthesis of efficient atomic-level electrocatalysts. Potential-resolved in situ Raman spectroelectrochemistry and density functional theory reveal that EcNR proceeds through NO3 - reduction to *NO2, *NO, *NH, and *NH2 intermediates, ultimately forming NH3 via final protonation step. This process exhibits a low limiting potential of -0.37 V, with *NO2 protonation identified as the critical step. NiCo-DSAC/MXene exhibited superior EcNR performance for NH3 production in 1.0 M potassium hydroxide with sustained multiple cyclic stability. Furthermore, this catalyst is integrated into a Zn-NO3 - a battery that simultaneously removes NO3 -, generates energy, and synthesizes NH3.

Cu-Co Dual Sites Tandem Synergistic Effect Boosting Neutral Low Concentration Nitrate Electroreduction to Ammonia

Electrochemical nitrate reduction reaction (NO3 -RR) has emerged as an alternative strategy for wastewater treatment and ammonia production in neutral low-concentration nitrate. However, the electrocatalyst faces the challenge of limited NO3 - distribution and deficient active hydrogen (Hads) on the catalyst surface resulting from the low concentration of NO3 - and the difficulty of water splitting under neutral conditions. Here, a Cu-Co dual sites tandem synergistic catalysis mechanism has been proposed by doping Cu into CoP to facilitate the adsorption and conversion of NO3 - on Cu and to accelerate the water splitting on CoP leading to the significantly high NO3 -RR performance. The designed Cu-CoP catalyst exhibits an ammonia yield of 7.65 mg h-1 cm-2 and a Faraday efficiency of 85.1% at -1.0 V under neutral low-concentration nitrate (10 m M), which is the highest ammonia yield in the reported data. In situ characterization and theoretical calculations confirm the tandem synergistic effect, in which the Cu site favors the adsorption and activation of NO3 - to form NO2 -, and concurrently modulates the electronic structure of the Co site with optimized Hads adsorption resulting in the significantly enhanced NO3 -RR at neutral low concentration nitrate.

Industrial-current Ammonia Synthesis by Polarized Cuprous Cyanamide Coupled to Valorization of Glycerol at 4,000 mA cm-2

The electrocatalytic nitrate reduction (NO3RR) holds significance in both NH3 synthesis and nitrate contamination remediation. However, achieving industrial-scale current and high stability in membrane electrode assembly (MEA) electrolyzer remains challenging due to inherent high full-cell voltage for sluggish NO3RR and water oxidation. Here, Cu2NCN with positive surface electrostatic potential VS(r) is applied as highly efficient NO3RR electrocatalysts to achieve industrial-current and low-voltage stable NH3 production in MEA electrolyzer with coupled anodic glycerol oxidation. This paired electro-refinery (PER) system reaches 4000 mA cm-2 at 2.52 V and remains stable at industrial-level 1000 mA cm-2 for 100 h with the NH3 production rate of 97000 µgNH3 h-1 cm-2 and a Faradaic efficiency of 83%. Theoretical calculations elucidate that the asymmetric and electron-withdrawing [N-C≡N] units enhance polarization and VS(r), promoting robust and asymmetric adsorption of NO3 * on Cu2NCN to facilitate O-N bond dissociation. A comprehensive techno-economic analysis demonstrates the profitability and commercial viability of this coupled system. Our work opens a new avenue and marks a significant advancement in MEA systems for industrial NH3 synthesis.

A Copper-Zinc Cyanamide Solid-Solution Catalyst with Tailored Surface Electrostatic Potentials Promotes Asymmetric N-Intermediate Adsorption in Nitrite Electroreduction

The electrocatalytic nitrite reduction (NO2RR) converts nitrogen-containing pollutants to high-value ammonia (NH3) under ambient conditions. However, its multiple intermediates and multielectron coupled proton transfer process lead to low activity and NH3 selectivity for the existing electrocatalysts. Herein, we synthesize a solid-solution copper-zinc cyanamide (Cu0.8Zn0.2NCN) with localized structure distortion and tailored surface electrostatic potential, allowing for the asymmetric binding of NO2-. It exhibits outstanding NO2RR performance with a Faradaic efficiency of ∼100% and an NH3 yield of 22 mg h-1 cm-2, among the best for such a process. Theoretical calculations and in situ spectroscopic measurements demonstrate that Cu-Zn sites coordinated with linear polarized [NCN]2- could transform symmetric [Cu-O-N-O-Cu] in CuNCN-NO2- to a [Cu-N-O-Zn] asymmetric configuration in Cu0.8Zn0.2NCN-NO2-, thus enhancing adsorption and bond cleavage. A paired electro-refinery with the Cu0.8Zn0.2NCN cathode reaches 2000 mA cm-2 at 2.36 V and remains fully operational at industrial-level 400 mA cm-2 for >140 h with a NH3 production rate of ∼30 mgNH3 h-1 cm-2. Our work opens a new avenue of tailoring surface electrostatic potentials using a solid-solution strategy for advanced electrocatalysis.

Industrial-Level Paired Electrosynthesis of Valuable Chemicals over a High-Performance Heterostructural Electrode

Paired electrosynthetic technology is of significance to realize the co-production of high-added value chemicals. However, exploiting efficient bifunctional electrocatalyst of the concurrent electrocatalysis to achieve the industrial-level performance is still challenging. Herein, an amorphous Co2P@MoOx heterostructure is rationally designed by in situ electrodeposition strategy, which is acted as excellent bifunctional catalysts for the electrocatalytic nitrite reduction reaction (NO2RR) and glycerol oxidation reaction (GOR). The membrane-electrode assembly (MEA) electrolyzer realizes a low voltage of 1.30 V, robust stability over 200 h at 100 mA cm-2, high Faraday efficiencies and yield of NH3 (above 95 %, 49.7 mg h-1 cm-2) and formate (above 95 %, 304.4 mg h-1 cm-2) at industrial-level current density of 500 mA cm-2. In situ spectroscopy studies have shown that high-valence CoOOH is the main active material of GOR, and the main catalytic conversion pathway of NO2RR involves key *NH2OH reaction intermediates. In addition, theoretical calculations confirm that the Co2P@MoOx heterostructure has strong interfacial electronic interaction and optimized reaction energy barriers, which endows its intrinsically high electrocatalytic activity for the co-electrosynthesis of NH3 and formate.

Self-Triggering a Locally Alkaline Microenvironment of Co4Fe6 for Highly Efficient Neutral Ammonia Electrosynthesis

Electrochemical nitrate reduction reaction (eNO3-RR) to ammonia (NH3) holds great promise for the green treatment of NO3- and ambient NH3 synthesis. Although Fe-based electrocatalysts have emerged as promising alternatives, their excellent eNO3-RR-to-NH3 activity is usually limited to harsh alkaline electrolytes or alloying noble metals with Fe in sustainable neutral electrolytes. Herein, we demonstrate an unusual self-triggering localized alkalinity of the Co4Fe6 electrocatalyst for efficient eNO3-RR-to-NH3 activity in neutral media, which breaks down the conventional pH-dependent kinetics restrictions and shows a 98.6% NH3 Faradaic efficiency (FE) and 99.9% NH3 selectivity at -0.69 V vs RHE. The synergetic Co-Fe dual sites were demonstrated to enable the optimal free energies of eNO3-RR-to-NH3 species and balance water dissociation and protonation of adsorbed NO2-. Notably, the Co4Fe6 electrocatalysts can attain a high current density of 100 mA cm-2 with a high NH3 FE surpassing 96% and long-term stability for over 500 h eNO3-RR-to-NH3 in a membrane electrode assembly (MEA) electrolyzer. This work provides insight into tailoring the self-reinforced local-alkalinity on the Fe-based alloy electrocatalysts for eNO3-RR-to-NH3 and thus avoids alkaline electrolytes and noble metals for practical sustainable nitrate upcycling technology.

Selenium-Deficient FeSe2/Fe3O4 Electrocatalyst for Nitrate Reduction to Ammonia

Electrocatalytic reduction of NO3 - is a green and sustainable method that not only helps to treat industrial pollutants in wastewater, but also produces valuable chemicals. However, the slow dynamics of the proton-coupled electron transfer process results in a high barrier and low conversion efficiency. In this work, the Se-deficient FeSe2/Fe3O4 heterojunction was synthesized, which showed excellent electrochemical performance in 0.1 M nitrate reduction reaction, superior to most currently reported catalysts. The high activity of Se-deficient FeSe2/Fe3O4 is due to the synergistic effect between FeSe2 and Fe3O4 to achieve relay catalytic NO3 - reduction. Among them, the Se-deficient FeSe2 contributes to NO3 - deoxygenation and subsequent hydrogenation, and Fe3O4 promotes H2O decomposition to provide H proton, jointly promote NO3RR. Finally, the online differential electrochemical mass spectra (DEMS), in situ Raman and DFT calculation confirmed the optimal pathway for NO3RR to NH3 on Se-deficient FeSe2/Fe3O4(100). This strategy of relay catalysis provides a potential way to treat wastewater with high concentration nitrate.

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

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

Triple Regulations via Fe Redox Boosting Nitrate Reduction to Ammonia at Industrial Current Densities

Electrochemical nitrate reduction reaction (NO3 -RR) has promising prospects for green synthesis of ammonia and environmental remediation. However, the performance of catalysts at high current density usually suffers from the high energy barrier for the nitrate (NO3 -) to nitrite (NO2 -) and the competitive hydrogen evolution. Herein, we proposed a two-step relay mechanism through spontaneous redox reaction followed electrochemical reaction by introducing low-valence Fe species into Ni2P nanosheets to significantly enhance the NO3 -RR performance at industrial current density. The existence of low-valence Fe species bypasses the NO3 - to NO2 - step through the spontaneous redox with NO3 - to produce NO2 - and Fe2O3, regulates the electronic structure of Ni2P to reduce the barrier of NO2 - to NH3, thirdly prohibits the hydrogen evolution by consuming the excess active hydrogen through reduction of Fe2O3 to recover low-valence Fe species. The triple regulations via Fe redox during the two-step relay reactions guarantee the Fe-Ni2P@NF high ammonia yield of 120.1 mg h-1 cm-2 with Faraday efficiency of more than 90% over a wide potential window and a long-term stability of more than 130 h at ~1000 mA cm-2. This work provides a new strategy to realize the design and synthesis of nitrate reduction electrocatalysts at high current densities.

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.

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

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

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.

Improved Nitrate-to-Ammonia Electrocatalysis through Hydrogen Poisoning Effects

Electrochemical conversion from nitrate to ammonia is a key step in sustainable ammonia production. However, it suffers from low productive efficiency or high energy consumption due to a lack of desired electrocatalysts. Here we report nickel cobalt phosphide (NiCoP) catalysts for nitrate-to-ammonia electrocatalysis that display a record-high catalytic current density of -702±7 mA cm-2, ammonia production rate of 5415±26 mmol gcat -1 h-1 and Faraday efficiency of 99.7±0.2 % at -0.3 V vs. RHE, affording the estimated energy consumption as low as 22.7 kWh kgammonia -1. Theoretical and experimental results reveal that these catalysts benefit from hydrogen poisoning effects, which leave behind catalytically inert adsorbed hydrogen species (HI*) at Co-hollow sites and thereupon enable ideally reactive HII* at secondary Co-P sites. The dimerization between HI* and HII* for H2 evolution is blocked due to the catalytic inertia of HI* thereby the HII* drives nitrate hydrogenation timely. With these catalysts, the continuous ammonia production is further shown in an electrolyser with a real energy consumption of 18.9 kWh kgammonia -1.

MOF-on-MOF Heterostructured Electrocatalysts for Efficient Nitrate Reduction to Ammonia

Electrocatalytic nitrate reduction reaction (NO3 -RR) is an important route for sustainable NH3 synthesis and environmental remediation. Metal-organic frameworks (MOFs) are one family of promising NO3 -RR electrocatalysts, however, there is plenty of room to improve in their performance, calling for new design principles. Herein, a MOF-on-MOF heterostructured electrocatalyst with interfacial dual active sites and build-in electric field is fabricated for efficient NO3 -RR to NH3 production. By growing Co-HHTP (HHTP=2,3,6,7,10,11-hexahydroxytriphenylene) nanorods on Ni-BDC (BDC=1,4-benzenedicarboxylate) nanosheets, experimental and theoretical investigations demonstrate the formation of Ni-O-Co bonds at the interface of MOF-on-MOF heterostructure, leading to dual active sites tailed for NO3 -RR. The Ni sites facilitate the adsorption and activation of NO3 -, while the Co sites boost the H2O decomposition to supply active hydrogen (Hads) for N-containing intermediates hydrogenation on adjacent Ni sites, cooperatively reducing the energy barriers of NO3 -RR process. Together with the accelerated electron transfer enabled by built-in electric field, remarkable NO3 -RR performance is achieved with an NH3 yield rate of 11.46 mg h-1 cm-2 and a Faradaic efficiency of 98.4 %, outperforming most reported MOF-based electrocatalysts. This work provides new insights into the design of high-performance NO3 -RR electrocatalysts.

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

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

Nanocluster-agminated amorphous cobalt nanofilms for highly selective electroreduction of nitrate to ammonia

Developing highly-efficient electrocatalysts for the nitrate reduction reaction (NITRR) is a persistent challenge. Here, we present the successful synthesis of 14 amorphous/low crystallinity metal nanofilms on three-dimensional carbon fibers (M-NFs/CP), including Al, Ti, Mn, Fe, Co, Ni, Cu, Zn, Ag, In, Sn, Pb, Au, or Bi, using rapid thermal evaporation. Among these samples, our study identifies the amorphous Co nanofilm with fine agglomerated Co clusters as the optimal electrocatalyst for NITRR in a neutral medium. The resulting Co-NFs/CP exhibits a remarkable Faradaic efficiency (FENH3) of 91.15 % at - 0.9 V vs RHE, surpassing commercial Co foil (39 %) and Co powder (20 %), despite sharing the same metal composition. Furthermore, during the electrochemical NITRR, the key intermediates on the surface of the Co-NFs/CP catalyst were detected by in situ Fourier-transform infrared (FTIR) spectroscopy, and the possible reaction ways were probed by Density functional theory (DFT) calculations. Theoretical calculations illustrate that the abundant low-coordinate Co atoms of Co-NFs/CP could enhances the adsorption of *NO3 intermediates compared to crystalline Co. Additionally, the amorphous Co structure lowers the energy barrier for the rate-determining step (*NH2→*NH3). This work opens a new avenue for the controllable synthesis of amorphous/low crystallinity metal nano-catalysts for various electrocatalysis reaction applications.

Optimizing Nitrate Reduction to Ammonia via Modulating Adsorption-Desorption Dynamics with High-Entropy CuNiCoZnMn Alloy Catalysts

NO3RR synthesis of ammonia is a complex eight-electron reaction involving multiple steps and intermediates, in which NO3- adsorption and NH3 desorption are crucial. The Cu-based high entropy quinary alloy catalyst has good surface adsorption and desorption ability for the reduction of nitric acid to ammonia. Here, the catalytic sites were coordinated by constructing CuNiCoZnMn alloys to adjust the electronic structure of the catalytic sites to facilitate the reaction of the substrate and thus optimize the whole reaction path. Based on the ternary alloy CuNiCo, the introduction of the Zn element continues to reduce the desorption energy barrier, and the introduction of the Mn element continues to enhance the initial adsorption energy so that the target product can be quickly held and released to accelerate the production of ammonia. The NH3 yield and Faraday efficiency obtained for the quinary CuNiCoZnMn alloy catalyst reached 723.7 μmol h-1 cm-2 and 96.6%, respectively, at -0.35 V vs RHE potential. The density functional theory calculations showed that the quinary CuNiCoZnMn alloy (NO3- to *NO3-) initial adsorption-free energy change and (*NH3 to NH3) NH3 desorption-free energy change are -2.50, 0.072 eV, respectively, which are significantly better than those of the ternary CuNiC and quaternary CuNiCoZn of -2.02, 0.544 eV and -1.97, 0.217 eV.

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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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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Electrosynthesis of Ammonia from Nitrate Using a Self-Activated Carbon Fiber Paper

While electrochemically upcycling nitrate wastes to valuable ammonia is considered a very promising pathway for tackling the environmental and energy challenges underlying the nitrogen cycle, the effective catalysts involved are mainly limited to metal-based materials. Here, we report that commercial carbon fiber paper, which is a classical current collector and is typically assumed to be electrochemically inert, can be significantly activated during the reaction. As a result, it shows a high NH3 Faradaic efficiency of 87.39% at an industrial-level current density of 300 mA cm-2 for over 90 h of continuous operation, with a NH3 production rate of as high as 1.22 mmol cm-2 h-1. Through experimental and theoretical analysis, the in situ-formed oxygen functional groups are demonstrated to be responsible for the NO3RR performance. Among them, the C-O-C group is finally identified as the active center, which lowers the thermodynamic energy barrier and simultaneously improves the hydrogenation kinetics. Moreover, high-purity NH4Cl and NH3·H2O were obtained by coupling the NO3RR with an air-stripping approach, providing an effective way for converting nitrate waste into high-value-added NH3 products.

Cu1-Fe Dual Sites for Superior Neutral Ammonia Electrosynthesis from Nitrate

The electrochemical nitrate reduction reaction (NO3RR) is able to convert nitrate (NO3 -) into reusable ammonia (NH3), offering a green treatment and resource utilization strategy of nitrate wastewater and ammonia synthesis. The conversion of NO3 - to NH3 undergoes water dissociation to generate active hydrogen atoms and nitrogen-containing intermediates hydrogenation tandemly. The two relay processes compete for the same active sites, especially under pH-neutral condition, resulting in the suboptimal efficiency and selectivity in the electrosynthesis of NH3 from NO3 -. Herein, we constructed a Cu1-Fe dual-site catalyst by anchoring Cu single atoms on amorphous iron oxide shell of nanoscale zero-valent iron (nZVI) for the electrochemical NO3RR, achieving an impressive NO3 - removal efficiency of 94.8 % and NH3 selectivity of 99.2 % under neutral pH and nitrate concentration of 50 mg L-1 NO3 --N conditions, greatly surpassing the performance of nZVI counterpart. This superior performance can be attributed to the synergistic effect of enhanced NO3 - adsorption on Fe sites and strengthened water activation on single-atom Cu sites, decreasing the energy barrier for the rate-determining step of *NO-to-*NOH. This work develops a novel strategy of fabricating dual-site catalysts to enhance the electrosynthesis of NH3 from NO3 -, and presents an environmentally sustainable approach for neutral nitrate wastewater treatment.

Construction of an electron-transfer channel via Cu-O-Ni to inhibit the overoxidation of Ni for durable methanol oxidation at industrial current density

The electrocatalytic methanol oxidation reaction (MOR) is a viable approach for realizing high value-added formate transformation from biomass byproducts. However, usually it is restricted by the excess adsorption of intermediates (COad) and overoxidation of catalysts, which results in low product selectivity and inactivation of the active sites. Herein, a novel Cu-O-Ni electron-transfer channel was constructed by loading NiCuO x on nickel foam (NF) to inhibit the overoxidation of Ni and enhance the formate selectivity of the MOR. The optimized NiCuO x -2/NF demonstrated excellent MOR catalytic performance at industrial current density (E 500 = 1.42 V) and high faradaic efficiency of ∼100%, as well as durable formate generation up to 600 h at ∼500 mA cm-2. The directional electron transfer from Cu to Ni and enhanced lattice stability could alleviate the overoxidation of Ni(iii) active sites to guarantee reversible Ni(ii)/Ni(iii) cycles and endow NiCuO x -2/NF with high stability under increased current density, respectively. An established electrolytic cell created by coupling the MOR with the hydrogen evolution reaction could produce H2 with low electric consumption (230 mV lower voltage at 400 mA cm-2) and concurrently generated the high value-added product of formate at the anode.

Synergistic Effect of Ni/Ni(OH)2 Core-Shell Catalyst Boosts Tandem Nitrate Reduction for Ampere-Level Ammonia Production

Electrocatalytic reduction of nitrate to ammonia provides a green alternate to the Haber-Bosch method, yet it suffers from sluggish kinetics and a low yield rate. The nitrate reduction follows a tandem reaction of nitrate reduction to nitrite and subsequent nitrite hydrogenation to generate ammonia, and the ammonia Faraday efficiency (FE) is limited by the competitive hydrogen evolution reaction. Herein, we design a heterostructure catalyst to remedy the above issues, which consists of Ni nanosphere core and Ni(OH)2 nanosheet shell (Ni/Ni(OH)2). In situ Raman spectroscopy reveals Ni and Ni(OH)2 are interconvertible according to the applied potential, facilitating the cascade nitrate reduction synergistically. Consequently, it attains superior electrocatalytic nitrate reduction performance with an ammonia FE of 98.50 % and a current density of 0.934 A cm-2 at -0.476 V versus reversible hydrogen electrode, and exhibits an average ammonia yield rate of 84.74 mg h-1 cm-2 during the 102-hour stability test, which is highly superior to the reported catalysts tested under similar conditions. Density functional theory calculations corroborate the synergistic effect of Ni and Ni(OH)2 in the tandem reaction of nitrate reduction. Moreover, the Ni/Ni(OH)2 catalyst also possesses good capability for methanol oxidation and thus is used to establish a system coupling with nitrate reduction.

Ultrathin High-Entropy Fe-Based Spinel Oxide Nanosheets with Metalloid Band Structures for Efficient Nitrate Reduction toward Ammonia

Spinel oxides with tunable chemical compositions have emerged as versatile electrocatalysts, however their performance is greatly limited by small surface area and low electron conductivity. Here, ultrathin high-entropy Fe-based spinel oxides nanosheets are rationally designed (i.e., (Co0.2Ni0.2Zn0.2Mg0.2Cu0.2)Fe2O4; denotes A5Fe2O4) in thickness of ≈4.3 nm with large surface area and highly exposed active sites via a modified sol-gel method. Theoretic and experimental results confirm that the bandgap of A5Fe2O4 nanosheets is significantly smaller than that of ordinary Fe-based spinel oxides, realizing the transformation of binary spinel oxide from semiconductors to metalloids. As a result, such A5Fe2O4 nanosheets manifest excellent performance for the nitrate reduction reaction (NO3 -RR) to ammonia (NH3), with a NH3 yield rate of ≈2.1 mmol h-1 cm-2 at -0.5 V versus Reversible hydrogen electrode, outperforming other spinel-based electrocatalysts. Systematic mechanism investigations reveal that the NO3 -RR is mainly occurred on Fe sites, and introducing high-entropy compositions in tetrahedral sites regulates the adsorption strength of N and O-related intermediates on Fe for boosting the NO3 -RR. The above findings offer a high-entropy platform to regulate the bandgap and enhance the electrocatalytic performance of spinel oxides.

Controlled Synthesis of Unconventional Phase Alloy Nanobranches for Highly Selective Electrocatalytic Nitrite Reduction to Ammonia

The controlled synthesis of metal nanomaterials with unconventional phases is of significant importance to develop high-performance catalysts for various applications. However, it remains challenging to modulate the atomic arrangements of metal nanomaterials, especially the alloy nanostructures that involve different metals with distinct redox potentials. Here we report the general one-pot synthesis of IrNi, IrRhNi and IrFeNi alloy nanobranches with unconventional hexagonal close-packed (hcp) phase. Notably, the as-synthesized hcp IrNi nanobranches demonstrate excellent catalytic performance towards electrochemical nitrite reduction reaction (NO2RR), with superior NH3 Faradaic efficiency and yield rate of 98.2 % and 34.6 mg h-1 mgcat -1 (75.5 mg h-1 mgIr -1) at 0 and -0.1 V (vs reversible hydrogen electrode), respectively. Ex/in situ characterizations and theoretical calculations reveal that the Ir-Ni interactions within hcp IrNi alloy improve electron transfer to benefit both nitrite activation and active hydrogen generation, leading to a stronger reaction trend of NO2RR by greatly reducing energy barriers of rate-determining step.

Efficient Electrochemical Co-Reduction of Carbon Dioxide and Nitrate to Urea with High Faradaic Efficiency on Cobalt-Based Dual-Sites

Renewable electricity-powered nitrate/carbon dioxide co-reduction reaction toward urea production paves an attractive alternative to industrial urea processes and offers a clean on-site approach to closing the global nitrogen cycle. However, its large-scale implantation is severely impeded by challenging C-N coupling and requires electrocatalysts with high activity/selectivity. Here, cobalt-nanoparticles anchored on carbon nanosheet (Co NPs@C) are proposed as a catalyst electrode to boost yield and Faradaic efficiency (FE) toward urea electrosynthesis with enhanced C-N coupling. Such Co NPs@C renders superb urea-producing activity with a high FE reaching 54.3% and a urea yield of 2217.5 µg h-1 mgcat. -1, much superior to the Co NPs and C nanosheet counterparts, and meanwhile shows strong stability. The Co NPs@C affords rich catalytically active sites, fast reactant diffusion, and sufficient catalytic surfaces-electrolyte contacts with favored charge and ion transfer efficiencies. The theoretical calculations reveal that the high-rate formation of *CO and *NH2 intermediates is crucial for facilitating urea synthesis.

Enabling Logistics Automation in Nanofactory: Cobalt Phosphide Embedded Metal-Organic Frameworks for Efficient Electrocatalytic Nitrate Reduction to Ammonia

Electrocatalytic nitrate reduction reaction (NitRR) in neutral condition offers a promising strategy for green ammonia synthesis and wastewater treatment, the rational design of electrocatalysts is the cornerstone. Inspired by modern factory design where both machines and logistics matter for manufacturing, it is reported that cobalt phosphide (CoP) nanoparticles embedded in zinc-based zeolite imidazole frameworks (Zn-ZIF) function as a nanofactory with high performance. By selective phosphorization of ZnCo bimetallic zeolite imidazole framework (ZnCo-ZIF), the generated CoP nanoparticles act as "machines" (active sites) for molecular manufacturing (NO3 - to NH4 + conversion). The purposely retained framework (Zn-ZIFs) with positive charge promotes logistics automation, i.e., the automatic delivery of NO3 - reactants and timely discharge of NH4 + products in-and-out the nanofactory due to electrostatic interaction. Moreover, the interaction between Zn-ZIF and CoP modulates the Co sites into electron insufficient state with upshifted d-band center, facilitating the reduction/hydrogenation of NO3 - to ammonia and restricting the competitive hydrogen evolution. Consequently, the assembled CoP/Zn-ZIF nanofactory exhibits superior NitRR performances with a high Faraday efficiency of ≈97% and a high ammonia yield of 0.89 mmol cm-1 h-1 in neutral condition, among the best of reported electrocatalysts. The work provides new insights into the design principles of efficient NitRR electrocatalysts.

Ru-Doped NiFe-MIL-53 with Facilitated Reconstruction and Active Hydrogen Supplement for Enhanced Nitrate Reduction

The Electrochemical reduction of nitrate to ammonia (NH3) is a process of great significance to energy utilization and environmental protection. However, it suffers from sluggish multielectron/proton-involved steps involving coupling reactions between different reaction intermediates and active hydrogen species (Hads) produced by water decomposition. In this study, a Ru-doped NiFe-MIL-53 (NiFeRu-MIL-53) supported on Ni foam (NF) has been designed for the nitrate reduction reaction (NO3RR). The NiFeRu-MIL-53 exhibits excellent NO3RR activity with a maximum Faradaic efficiency (FE) of 100% at -0.4 V vs. RHE for NH3 and a maximum NH3 yield of 62.39 mg h-1 cm-2 at -0.7 V vs. RHE in alkaline media. This excellent performance for the NO3RR is attributed to a strong synergistic effect between Ru and reconstructed NiFe(OH)2. Additionally, the doped Ru facilitates water dissociation, leading to an appropriate supply of Hads required for N species hydrogenation during NO3RR, thereby further enhancing its performance. Furthermore, in situ Raman analysis reveals that incorporating Ru facilitates the reconstruction of MOFs and promotes the formation of hydroxide active species during the NO3RR process. This work provides a valuable strategy for designing electrocatalysts to improve the efficiency of the reduction of electrochemical nitrate to ammonia.

Unveiling the Reaction Mechanism of Nitrate Reduction to Ammonia Over Cobalt-Based Electrocatalysts

Electrocatalytic reduction of nitrate to ammonia (NRA) has emerged as an alternative strategy for sewage treatment and ammonia generation. Despite excellent performances having been achieved over cobalt-based electrocatalysts, the reaction mechanism as well as veritable active species across a wide potential range are still full of controversy. Here, we adopt CoP, Co, and Co3O4 as model materials to solve these issues. CoP evolves into a core@shell structured CoP@Co before NRA. For CoP@Co and Co catalysts, a three-step relay mechanism is carried out over superficial dynamical Coδ+ active species under low overpotential, while a continuous hydrogenation mechanism from nitrate to ammonia is unveiled over superficial Co species under high overpotential. In comparison, Co3O4 species are stable and steadily catalyze nitrate hydrogenation to ammonia across a wide potential range. As a result, CoP@Co and Co exhibit much higher NRA activity than Co3O4 especially under a low overpotential. Moreover, the NRA performance of CoP@Co is higher than Co although they experience the same reaction mechanism. A series of characterizations clarify the reason for performance enhancement highlighting that CoP core donates abundant electrons to superficial active species, leading to the generation of more active hydrogen for the reduction of nitrogen-containing intermediates.

Highly efficient nitrogen fixation over S-scheme heterojunction photocatalysts with enhanced active hydrogen supply

Photocatalytic N2 fixation is a promising strategy for ammonia (NH3) synthesis; however, it suffers from relatively low ammonia yield due to the difficulty in the design of photocatalysts with both high charge transfer efficiency and desirable N2 adsorption/activation capability. Herein, an S-scheme CoSx/ZnS heterojunction with dual active sites is designed as an efficient N2 fixation photocatalyst. The CoSx/ZnS heterojunction exhibits a unique pocket-like nanostructure with small ZnS nanocrystals adhered on a single-hole CoSx hollow dodecahedron. Within the heterojunction, the electronic interaction between ZnS and CoSx creates electron-deficient Zn sites with enhanced N2 chemisorption and electron-sufficient Co sites with active hydrogen supply for N2 hydrogenation, cooperatively reducing the energy barrier for N2 activation. In combination with the promoted photogenerated electron-hole separation of the S-scheme heterojunction and facilitated mass transfer by the pocket-like nanostructure, an excellent N2 fixation performance with a high NH3 yield of 1175.37 μmol g-1 h-1 is achieved. This study provides new insights into the design of heterojunction photocatalysts for N2 fixation.

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.

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.

The interface-mediated electron structure tuning of RuOx-Co3O4 nano-particles for efficient electrocatalytic nitrate reduction

The energy-intensive processes for the industrial production of ammonia necessitates the development of new methods to be proposed that will aid in reducing the global energy consumption. Specifically, the electrocatalytic nitrate reduction reaction (NO3RR) to produce ammonia is more thermodynamically feasible than the electrocatalytic nitrogen reduction reaction (NRR). However, it is hindered by a low catalytic activity due to its complex reaction pathways. Herein, we synthesized a novel electrocatalyst, RuOx-Co3O4 nanoparticles, with abundant interfaces, which exhibited an enhanced catalytic activity for efficient ammonia synthesis. This catalyst delivered a partial current density of 65.8 mA cm-2 for NH3 production, a faradaic efficiency (FE) of 89.7%, and a superior ammonia yield rate of up to 210.5 μmol h-1 cm-2 at -0.6 V vs. RHE. X-ray photoelectron and Raman spectroscopy revealed that the formed interfacial Ru-O-Co bond can decorate the electronic structures of the active sites and accelerate the absorption of NO3-, thus promoting the production of ammonia.

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.

Electrocatalytic nitrate-to-ammonia conversion on CoO/CuO nanoarrays using Zn-nitrate batteries

Zn-NO3- batteries can generate electricity while producing NH3 in an environmentally friendly manner, making them a very promising device. However, the conversion of NO3- to NH3 involves a proton-assisted 8-electron (8e-) transfer process with a high kinetic barrier, requiring high-performance catalysts to realize the potential applications of this technology. Herein, we propose a heterostructured CoO/CuO nanoarray electrocatalyst prepared on a copper foam (CoO/CuO-NA/CF) that can electrocatalytically and efficiently convert NO3- to NH3 at low potential and achieves a maximum NH3 yield of 296.9 μmol h-1 cm-2 and the Faraday efficiency (FE) of 92.9% at the -0.2 V vs. reversible hydrogen electrode (RHE). Impressively, Zn-NO3- battery based on the monolithic CoO/CuO-NA/CF electrode delivers a high NH3 yield of 60.3 μmol h-1 cm-2, FENH3 of 82.0%, and a power density of 4.3 mW cm-2. This study provides a paradigm for heterostructured catalyst preparation for the energy-efficient production of NH3 and simultaneously generating electrical energy.

Strong electron coupling of FeP4/Ni2P to boost highly-efficient electrochemical nitrate reduction to ammonia

Electrochemical reduction of contaminated nitrate to ammonia (NRA) opens a new window for mass production of ammonia and the alleviation of energy crises and environmental pollution. However, fabricating effective catalysts for the NRA still faces significant challenges. Herein, a highly-efficient NRA catalyst, FeP4/Ni2P, was successfully constructed. The strong electron coupling at heterointerfaces of FeP4/Ni2P promoted the generation of abundant active hydrogen *H, inhibited the competition of the HER, accelerated the hydrogenation of the NRA. Benefiting from these, the catalyst displays good NRA catalytic activity in the neutral electrolyte, with the NH3 FE of 97.83 ± 0.091 %, NH3 selectivity of 98.67 ± 0.50 %, NH3 yield rate of 0.262 ± 0.01 mmol·h-1·cm-2, and NO3- conversion rate of 93.02 ± 0.14 %. The DFT theoretical calculations demonstrated that the FeP4/Ni2P heterointerfaces played a critical role in shearing the H-OH bonds of water, resulting in generating more active hydrogen as a key NRA hydrogenation source, and hindering the *H dimerization to form H2, enhancing the NH3 selectivity. This work has a certain reference value for designing excellent catalysts for the NRA.