Synergistic Effect of Co(III) and Co(II) in a 3D Structured Co3O4/Carbon Felt Electrode for Enhanced Electrochemical Nitrate Reduction Reaction

The breaking of charge symmetry at the M-site in octahedral units of KMF3 perovskite induces enhanced electrochemical nitrate reduction in ammonia

Electrochemical production of NH3 from NO3- offers a solution to the environmental issue of excess NO3- but is challenged by the lack of efficient, sensitive electrocatalysts with high NH3 yield rate and Faraday efficiency (FE). Herein, a medium-entropy perovskite fluoride (KMF3, M = Co/Ni/Fe/Ti) was prepared as efficient electrocatalysts to produce NH3 via the NO3- reduction reaction (NO3-RR). By introducing various transition metals at the M-sites, the charge distribution at the M-site octahedral units was adjusted to increase the disorder of KMF3, resulting in an optimized electronic structure with high intrinsic NO3-RR performance. The presence of different transition metals at the M-sites promoted electron transfer, which inhibited the Co 1s-3d transition and strengthened the CoF bond, leading to a decrease in crystal symmetry. Furthermore, owing to differences in valence state, electronegativity, and ionic radius, the coordination number was reduced, inducing the generation of a local twisted coordination environment. This was conducive to catalyst adsorption and the breaking of NO bonds. As a result, the KMF3 exhibited a high NH3 yield rate of 13.9 mg h-1 cm-2 with an NH3 FE close to 100 %. The developed medium-entropy KMF3 electrocatalysts have the potential to recover NH3 from nitrate wastewater.

Constructing Lewis Acid-Base Pairs to Boost Electrocatalytic Hydrogenation of p-Nitrobenzoic Acid to Valuable p-Aminobenzoic Acid Using Water as the Hydrogen Source

Electrocatalytic hydrogenation of toxic nitrobenzene to value-added aniline is of great significance in addressing the issues of energy crisis and environmental pollution. However, it is a considerable challenging and crucial to develop highly efficient and earth-abundant transition metal-based electrocatalysts with superior durability for the electro-hydrogenation of nitrobenzene due to the competitive hydrogen evolution reaction (HER). In this work, a facile approach is designed and introduced to constructing an integrated self-supported heterostructured Co1- xNix(OH)(CO3)/Al(OH)3 nanoarrays (CoNiCH/Al(OH)3) for the electrocatalytic reduction of nitrobenzoic acid (PNBA) to p-aminobenzoic acid (PABA) and its electrocatalytic mechanism for PNBA reduction is investigated. This unique Lewis acid-base pairs with abundant oxygen vacancies (OVs) can effectively regulate the adsorption energy of PNBA and active hydrogen intermediates, facilitate the proton-coupled electrocatalytic reduction process, leading to the high activity and selectivity for PNBA to PABA. The optimal CoNiCH/Al(OH)3-0.5 exhibits outstanding performance for the electrocatalytic hydrogenation of PNBA to PABA with a yield of 92%, selectivity of 95% and Faraday efficiency (FE) of 92% at -0.545 V (vs reversible hydrogen electrode, RHE) under 0.1 m phosphate buffered solution (PBS) neutral electrolyte. Besides, it can maintain a high electrocatalytic activity for at least eight electrocatalytic cycle-test.

Effective Nitrate Electroconversion to Ammonia Using an Entangled Co3O4/Graphene Nanoribbon Catalyst

There has been huge interest among chemical scientists in the electrochemical reduction of nitrate (NO3-) to ammonia (NH4+) due to the useful application of NH4+ in nitrogen fertilizers and fuel. To conduct such a complex reduction reaction, which involves eight electrons and eight protons, one needs to develop high-performance (and stable) electrocatalysts that favor the formation of reaction intermediates that are selective toward ammonia production. In the present study, we developed and applied Co3O4/graphene nanoribbon (GNR) electrocatalysts with excellent properties for the effective reduction of NO3- to NH4+, where NH4+ yield rate of 42.11 mg h-1 mgcat-1, FE of 98.7%, NO3- conversion efficiency of 14.71%, and NH4+ selectivity of 100% were obtained, with the application of only 37.5 μg cm-2 of the catalysts (for the best catalyst ─Co3O4(Cowt %55)GNR, only 20.6 μg cm-2 of Co was applied), confirmed by loadings ranging from 19-150 μg cm-2. The highly satisfactory results obtained from the application of the proposed catalysts were favored by high average values of electrochemically active surface area (ECSA) and low Rct values, along with the presence of several planes in Co3O4 entangled with GNR and the occurrence of a kind of "(Co3(Co(CN)6)2(H2O)12)1.333 complex" structure on the catalyst surface, in addition to the effective migration of NO3- from the cell cathodic branch to the anodic branch, which was confirmed by the experiment conducted using a H-cell separated by a Nafion 117 membrane. The in situ FTIR and Raman spectroscopy results helped identify the adsorbed intermediates, namely, NO3-, NO2-, NO, and NH2OH, and the final product NH4+, which are compatible with the proposed NO3- electroreduction mechanism. The Density Functional Theory (DFT) calculations helped confirm that the Co3O4(Cowt %55)GNR catalyst exhibited a better performance in terms of nitrate electroreduction in comparison with Co3O4(Cowt %75), considering the intermediates identified by the in situ FTIR and Raman spectroscopy results and the rate-determining step (RDS) observed for the transition of *NO to *NHO (0.43 eV).

Highly Efficient Electrocatalytic Nitrate Reduction to Ammonia: Group VIII-Based Catalysts

The accumulation of nitrates in the environment causes serious health and environmental problems. The electrochemical nitrate reduction reaction (e-NO3RR) has received attention for its ability to convert nitrate to value-added ammonia with renewable energy. The key to effective catalytic efficiency is the choice of materials. Group VIII-based catalysts demonstrate great potential for application in e-NO3RR because of their high activity, low cost, and good electron transfer capability. This review summarizes the Group VIII catalysts, including monatomic, bimetallic, oxides, phosphides, and other composites. On this basis, strategies to enhance the intrinsic activity of the catalysts through coordination environment modulation, synergistic effects, defect engineering and hybridization are discussed. Meanwhile, the ammonia recovery process is summarized. Finally, the current research status in this field is prospected and summarized. This review aims to realize the large-scale application of nitrate electrocatalytic reduction in industrial wastewater.

Accelerated ammonia electrosynthesis of cobalt hydroxide through electronic modulation with ultralow noble metal doping

This study introduces an innovative method to ammonia production through electrocatalytic reduction of nitrate using Ru-doped Co(OH)2. The incorporation of Ru into the Co(OH)2 was found to markedly enhance the catalytic activity by optimizing the electronic structure and increasing the number of active sites.

A hydrogen radical pathway for efficacious electrochemical nitrate reduction to ammonia over an Fe-polyoxometalate/Cu electrocatalyst

Electrochemical nitrate (NO3-) reduction to ammonia (NH3), which is a high value-added chemical or high-energy density carrier in many applications, could become a key process overcoming the disadvantages of the Haber-Bosch process; however, current electrocatalysts have severe drawbacks in terms of activity, selectivity, and stability. Here, we report the hydrogen radical (H*) pathway as a solution to overcome this challenge, as demonstrated by efficacious electrochemical NO3- reduction to NH3 over the Fe-polyoxometalate (Fe-POM)/Cu hybrid electrocatalyst. Fe-POM, composed of Preyssler anions ([NaP5W30O110]14-) and Fe cations, facilitates efficient H* generation via H2O + e- → H* + OH-, and H* transfer to the Cu sites of the Fe-POM/Cu catalyst enables selective NO3- reduction to NH3. Operando spectroelectrochemical spectra substantiate the occurrence of the H* pathway through direct observation of Fe redox related to H* generation and Cu redox related to NO3- binding. With the H* pathway, the Fe-POM/Cu electrodes exhibit high activity for NO3- reduction to NH3 with 1.44 mg cm-2 h-1 in a 500 ppm NO3-/1 M KOH solution at -0.2 V vs. RHE, which is about 36-fold higher than that of the pristine Cu electrocatalyst. Additionally, it attains high selectivity with a faradaic efficiency of up to 97.09% at -0.2 V vs. RHE while exhibiting high catalytic stability over cycles.

Regulating the electronic structure by P-doping cobalt-based catalyst for atomic hydrogen mediated electrocatalytic dechlorination

Electrocatalytic dechlorination by atomic hydrogen (H*) is efficient, but limited by the low efficiency of H* production. Herein, a phosphorus-doped cobalt nitrogen carbon catalyst (Co-NP/C) was prepared, which had high catalytic activity in a wide pH range (3-11). The turnover frequency of Co-NP/C (3.54 min-1) was 1.21-59000 times superior to that of current Pd-based and non-noble metal catalysts (0.00006-2.92 min-1). Co-NP/C significantly enhanced H* generation, which was 1.52, 2.44, and 3.77 times stronger than that of Co-N/C, NP/C, and N/C, respectively, since the introduction of phosphorus was found enhanced the electron density of cobalt and regulated the electron transfer. Co-NP/C showed outstanding catalytic performance after ten cycles and could achieve nearly complete chloramphenicol removal. This regulation method was verified to be effective for other non-noble metal (Fe, Mn, Cu, Ni) phosphorus doped catalysts, proposing a general class for efficient electrochemical dechlorination, which would be of great significance for the elimination of chlorinated organic pollutants.

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.

Engineering the Co(II)/Co(III) Redox Cycle and Coδ+ Species Shuttle for Nitrate-to-Ammonia Conversion

Electroreduction of waste nitrate to valuable ammonia offers a green solution for environmental restoration and energy storage. However, the electrochemical self-reconstruction of catalysts remains a huge challenge in terms of maintaining their stability, achieving the desired active sites, and managing metal leaching. Herein, we present an electrical pulse-driven Co surface reconstruction-coupled Coδ+ shuttle strategy for the precise in situ regulation of the Co(II)/Co(III) redox cycle on the Co-based working electrode and guiding the dissolution and redeposition of Co-based particles on the counter electrode. As result, the ammonia synthesis performance and stability are significantly promoted while cathodic hydrogen evolution and anodic ammonia oxidation in a membrane-free configuration are effectively blocked. A high rate of ammonia production of 1.4 ± 0.03 mmol cm-2 h-1 is achieved at -0.8 V in a pulsed system, and the corresponding nitrate-to-ammonia Faraday efficiency is 91.7 ± 1.0%. This work holds promise for the regulation of catalyst reactivity and selectivity by engineering in situ controllable structural and chemical transformations.

Cobalt etched graphite felt electrode for enhanced removal of organic pollutant in aqueous solution with a solid polymer electrolyte

In this study, cobalt etched graphite felt electrodes were produced using a simple etching technique. It was used in combination with a solid polymer electrolyte (SPE) for the degradation of the target contaminant Orange II by Electro-Fenton (EF) technique in low conductivity water. In this method, 94% of Orange II in low conductivity water was removed in 90 min. The characterization analysis substantiates the hypothesis that the electrodes produced exhibit a three-dimensional porous structure, augmented defect concentration, and enhanced electron transfer capability. In addition, the potential reaction mechanism was inferred from the radical quenching experiments, and hydroxyl radicals (·OH) were deemed the main reactive substances. The combination of cobalt etched graphite felt electrodes with SPE demonstrates remarkable efficacy in the treatment of organic wastewater characterized by low electrical conductivity.

Electrochemical Nitrate Reduction Catalyzed by Two-Dimensional Transition Metal Borides

We investigated 2D transition metal borides (MBenes) for the efficient conversion of nitrate to ammonia. MBenes have been previously shown to bind oxygen with extraordinary strength, which should translate toward selective adsorption of nitrate in aqueous media. Using Density Functional Theory, we screened MBenes by computing their nitrate and water adsorption energies, seeking materials with strong nitrate binding and weak water binding. We identified MnB, CrB, and VB as the best materials for selective nitrate adsorption and proceeded by computing their free energies for generating ammonia. Of the three candidates, CrB requires the lowest overpotential, making it the best candidate. To further decrease the overpotential, we doped the CrB MBene with secondary transition metals and found the addition of Mn to the active site further reduced the overpotential. We then computed the reaction mechanism grand canonically to observe the effect of applied potential on the free energy landscape.

Spin-related Cu-Co pair to increase electrochemical ammonia generation on high-entropy oxides

The electrochemical conversion of nitrate to ammonia is a way to eliminate nitrate pollutant in water. Cu-Co synergistic effect was found to produce excellent performance in ammonia generation. However, few studies have focused on this effect in high-entropy oxides. Here, we report the spin-related Cu-Co synergistic effect on electrochemical nitrate-to-ammonia conversion using high-entropy oxide Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O. In contrast, the Li-incorporated MgCoNiCuZnO exhibits inferior performance. By correlating the electronic structure, we found that the Co spin states are crucial for the Cu-Co synergistic effect for ammonia generation. The Cu-Co pair with a high spin Co in Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O can facilitate ammonia generation, while a low spin Co in Li-incorporated MgCoNiCuZnO decreases the Cu-Co synergistic effect on ammonia generation. These findings offer important insights in employing the synergistic effect and spin states inside for selective catalysis. It also indicates the generality of the magnetic effect in ammonia synthesis between electrocatalysis and thermal catalysis.

Heterostructured Co-Doped-Cu2 O/Cu Synergistically Promotes Water Dissociation for Improved Electrochemical Nitrate Reduction to Ammonia

Electrochemical nitrate reduction reaction (NO3 RR) has recently emerged as a promising approach for sustainable ammonia synthesis and wastewater treatment, while the activity and selectivity for ammonia production have remained low. Herein, rational design and controllable synthesis of heterostructured Co-doped Cu2 O/Cu nanoparticles embedded in carbon framework (Co-Cu2 O/Cu@C) is reported for NO3 RR. The Co-Cu2 O/Cu@C exhibits a high ammonia yield rate of 37.86 mg h-1  mg-1 cat. with 98.1% Faraday efficiency, which is higher than those obtained for most of the Cu-based catalysts under similar conditions. Density functional theory calculations indicated that the strong electronic interactions at Cu/Co-Cu2 O interface facilitate the N species deoxygenation process and doping of Co promotes water dissociation to generate * H for the N species hydrogenation process, leading to enhanced NO3 RR performance. This work provides a new design strategy toward high-performance catalysts toward NO3 RR for ammonia generation.

Ag-Co3 O4 -CoOOH-Nanowires Tandem Catalyst for Efficient Electrocatalytic Conversion of Nitrate to Ammonia at Low Overpotential via Triple Reactions

The electrocatalytic conversion of nitrate (NO3 ‾) to NH3  (NO3 RR) offers a promising alternative to the Haber-Bosch process. However, the overall kinetic rate of NO3 RR is plagued by the complex proton-assisted multiple-electron transfer process. Herein, Ag/Co3 O4 /CoOOH nanowires (i-Ag/Co3 O4  NWs) tandem catalyst is designed to optimize the kinetic rate of intermediate reaction for NO3 RR simultaneously. The authors proved that NO3 ‾ ions are reduced to NO2 ‾ preferentially on Ag phases and then NO2 ‾ to NO on Co3 O4  phases. The CoOOH phases catalyze NO reduction to NH3  via NH2 OH intermediate. This unique catalyst efficiently converts NO3 ‾ to NH3  through a triple reaction with a high Faradaic efficiency (FE) of 94.3% and a high NH3  yield rate of 253.7 μmol h-1  cm-2  in 1 M KOH and 0.1 M KNO3  solution at -0.25 V versus RHE. The kinetic studies demonstrate that converting NH2 OH into NH3  is the rate-determining step (RDS) with an energy barrier of 0.151 eV over i-Ag/Co3 O4  NWs. Further applying i-Ag/Co3 O4  NWs as the cathode material, a novel Zn-nitrate battery exhibits a power density of 2.56 mW cm-2  and an FE of 91.4% for NH3  production.

Fe-Doped CoS2 Nanoarrays: Efficient Electrocatalytic Nitrate Reduction to Ammonia under Ambient Conditions

The electrocatalytic nitrate reduction reaction (NO3 - RR) enables the reduction of nitrate to ammonium ions under ambient conditions. It was considered as an alternative reaction for the production of ammonia (NH3 ) in recent years. In this paper, we report that the Fe doping CoS2 nanoarrays can effectively catalyze the formation of NH3 from nitrate (NO3 - ) under ambient conditions. This is mainly due to the increase of the NO3 - reaction active site by Fe doping and the porous nanostructure of the catalyst, which greatly improves the catalytic activity. Specifically, at -0.9 V vs. RHE, the NH3 yield rate (RNH3 ) of Fe-CoS2 /CC is 17.8×10-2  mmol h-1  cm-2 with Faraday Efficiency (FE) of 88.93 %. Besides, such catalyst shows good durability and catalytic stability, which provides the possibility for the future application of electrocatalytic NH3 production.

Fe and Cu Double-Doped Co3O4 Nanorod with Abundant Oxygen Vacancies: A High-Rate Electrocatalyst for Tandem Electroreduction of Nitrate to Ammonia

The electrochemical nitrate reduction reaction (NO3RR) is an attractive green alternative to the conventional Haber-Bosch method for the synthesis of NH3. However, this reaction is a tandem process that involves multiple steps of electrons and protons, posing a significant challenge to the efficient synthesis of NH3. Herein, we report a high-rate NO3RR electrocatalyst of Fe and Cu double-doped Co3O4 nanorod (Fe1/Cu2-Co3O4) with abundant oxygen vacancies, where the Cu preferentially catalyzes the rapid conversion of NO3- to NO2- and the oxygen vacancy in the Co3O4 substrate can accelerate NO2- reduction to NH3. In addition, the introduction of Fe can efficiently capture atomic H* that promotes the dynamics of NO2- to NH3, improving Faradaic efficiency of the produced NH3. Controlled experimental results show that the optimal electrocatalyst of Fe1/Cu2-Co3O4 exhibits good performance with high conversion (93.39%), Faradaic efficiency (98.15%), and ammonia selectivity (98.19%), which is significantly better than other Co-based materials. This work provides guidance for the rational design of high-performance NO3RR catalysts.

Pd-Doped Co3 O4 Nanoarray for Efficient Eight-Electron Nitrate Electrocatalytic Reduction to Ammonia Synthesis

Ammonia (NH3 ) is an indispensable feedstock for fertilizer production and one of the most ideal green hydrogen rich fuel. Electrochemical nitrate (NO3 - ) reduction reaction (NO3 - RR) is being explored as a promising strategy for green to synthesize industrial-scale NH3 , which has nonetheless involved complex multi-reaction process. This work presents a Pd-doped Co3 O4 nanoarray on titanium mesh (Pd-Co3 O4 /TM) electrode for highly efficient and selective electrocatalytic NO3 - RR to NH3 at low onset potential. The well-designed Pd-Co3 O4 /TM delivers a large NH3 yield of 745.6 µmol h-1 cm-2 and an extremely high Faradaic efficiency (FE) of 98.7% at -0.3 V with strong stability. These calculations further indicate that the doping Co3 O4 with Pd improves the adsorption characteristic of Pd-Co3 O4 and optimizes the free energies for intermediates, thereby facilitating the kinetics of the reaction. Furthermore, assembling this catalyst in a Zn-NO3 - battery realizes a power density of 3.9 mW cm-2 and an excellent FE of 98.8% for NH3 .

Heterostructured Co/Co3O4 anchored on N-doped carbon nanotubes as a highly efficient electrocatalyst for nitrate reduction to ammonia

The electrochemical reduction of nitrate (NO₃^-) to ammonia (NH₃) has emerged as an attractive approach for selectively reducing NO₃^- to highly value-added NH₃ and removing NO₃^- pollutants simultaneously. In this work, a heterostructured Co/Co₃O₄ electrocatalyst anchored on N-doped carbon nanotubes was prepared and applied for the NO₃^- reduction towards NH₃ under alkaline conditions. The catalyst achieves outstanding performance with up to 67% NH₃ faradaic efficiency at -1.2 V vs. Hg/HgO and 8.319 mg h^-1 mg_cat^-1 yield at -1.7 V vs. Hg/HgO. In addition, it also exhibits good long-term stability. ¹⁵N isotopic labelling experiments prove that the yielded NH₃ is derived from NO₃^- species. In situ electrochemical Raman spectra revealed that the structure of the as-prepared catalyst showed outstanding stability and identified possible intermediates during the electrocatalytic NO₃^- reduction reaction (NO₃RR).

Efficient electrocatalytic reduction of nitrate to ammonia over fibrous SmCoO3 under ambient conditions

We report SmCoO₃ nanofibers as an efficient catalyst for nitrate reduction to ammonia. This catalyst achieves a large NH₃ yield of 14.4 mg h^-1 mg_cat.^-1 and a high faradaic efficiency of 81.3% at -1.0 V vs. RHE in 0.1 M PBS with 0.1 M NaNO₃, and it also displays excellent electrochemical durability and structural stability. Theoretical calculations indicate that Sm-O and Co-O bonds have an incredibly low adsorption energy of -0.1 eV, which can significantly reduce the applied potential and hence enhance the catalytic activity.

Three-dimensional structured electrode for electrocatalytic organic wastewater purification: Design, mechanism and role

Considering the growing need in decentralized water treatment, the application of electrocatalytic processes (EP) to achieve organic wastewater purification will be dominant in the near future due to high efficiency, small reactor assembly as well as the flexibility of operation and management. The catalytic performance of electrode materials determines the development of this technology. Among them, the unique three-dimensional (3D) structure electrode shows better performance than two-dimensional (2D) electrode in increasing mass transfer, enhancing adsorption and exposing more active sites. Hence, this review starts with the introduction of definition, classification, advantages and disadvantages of 3D electrode materials. Then a critical discussion on the design and construction of 3D electrode materials for organic wastewater purification application is provided. Next, the removal mechanism of organic pollutants on the surface of 3D electrode, the role of 3D structure, the design of reactor with 3D electrode, the conversion and toxicity of degradation products, electrode energy efficiency, stability and cost, are comprehensively reviewed. At last, current challenges and future perspectives for the development of 3D electrode materials are addressed. We deem that this review will provide a valuable insight into the design and application of 3D electrodes in environmental water purification.

Coupling nitrate capture with ammonia production through bifunctional redox-electrodes

Nitrate is a ubiquitous aqueous pollutant from agricultural and industrial activities. At the same time, conversion of nitrate to ammonia provides an attractive solution for the coupled environmental and energy challenge underlying the nitrogen cycle, by valorizing a pollutant to a carbon-free energy carrier and essential chemical feedstock. Mass transport limitations are a key obstacle to the efficient conversion of nitrate to ammonia from water streams, due to the dilute concentration of nitrate. Here, we develop bifunctional electrodes that couple a nitrate-selective redox-electrosorbent (polyaniline) with an electrocatalyst (cobalt oxide) for nitrate to ammonium conversion. We demonstrate the synergistic reactive separation of nitrate through solely electrochemical control. Electrochemically-reversible nitrate uptake greater than 70 mg/g can be achieved, with electronic structure calculations and spectroscopic measurements providing insight into the underlying role of hydrogen bonding for nitrate selectivity. Using agricultural tile drainage water containing dilute nitrate (0.27 mM), we demonstrate that the bifunctional electrode can achieve a 8-fold up-concentration of nitrate, a 24-fold enhancement of ammonium production rate (108.1 ug h^-1 cm^-2), and a >10-fold enhancement in energy efficiency when compared to direct electrocatalysis in the dilute stream. Our study provides a generalized strategy for a fully electrified reaction-separation pathway for modular nitrate remediation and ammonia production.

Facile and Scalable Synthesis of Self-Supported Zn-Doped CuO Nanosheet Arrays for Efficient Nitrate Reduction to Ammonium

CuO has been regarded as a promising catalyst for the electrochemical reduction of nitrate (NO₃^-RR) to ammonium (NH₃); however, the intrinsic activity is greatly restricted by its poor electrical property. In this work, self-supported Zn-doped CuO nanosheet arrays (Zn-CuO NAs) are synthesized for NO₃^-RR, where the Zn dopant regulates the electronic structure of CuO to significantly accelerate the interfacial charge transfer and inner electron transport kinetics. The Zn-CuO NAs are constructed by a one-step etching of commercial brass (Cu₆₄Zn₃₆ alloy) in 0.1 M NaOH solution, which experiences a corrosion-oxidation-reconstruction process. Initially, the brass undergoes a dealloying procedure to produce nanosized Cu, which is immediately oxidized to the Cu₂O unit with a low valence state. Subsequently, Cu₂O is further oxidized to the CuO unit and reconstructed into nanosheets with the coprecipitation of Zn²⁺. For NO₃^-RR, Zn-CuO NAs show a high NH₃ production rate of 945.1 μg h^-1 cm^-2 and a Faradaic efficiency of up to 95.6% at -0.7 V in 0.1 M Na₂SO₄ electrolyte with 0.01 M NaNO₃, which outperforms the majority of the state-of-the-art catalysts. The present work offers a facile yet very efficient strategy for the scale-up synthesis of Zn-CuO NAs for high-performance NH₃ production from NO₃^-RR.

Highly selective electrocatalytic reduction of nitrate to nitrogen in a chloride ion-free system by promoting kinetic mass transfer of intermediate products in a novel Pd-Cu adsorption confined cathode

The mass transfer on the catalyst surface has a great influence on the selectivity of electrocatalytic nitrate reduction to nitrogen. In this study, a Pd-Cu adsorption confined nickel foam cathode is designed in the absence of both proton exchange membranes and chloride ions. The repulsion of the cathode enables intermediate products such as nitrite to accumulate in the confined region, resulting in an increase in the possibility of a second-order reaction to form nitrogen. The system can obtain more than 92% continuous N₂ selectivity when it is used to treat 200 mg L^-1 NO₃^--N under a current density of 8 mA cm^-2, which is not only higher than those of semiconfined and nonconfined systems but also significantly better than the results obtained by Pd-Cu directly modified cathodes prepared by electrodeposition or impregnation. It is found that a high initial nitrate concentration and low current density are more beneficial for the accumulation of intermediates on Pd-Cu catalysts, thus improving the formation of nitrogen. A mechanism study reveals that the intermediates can completely occupy the active sites on the surface of Pd, avoiding the generation of active hydrogen, and therefore inhibiting the first-order reaction to produce ammonia. Moreover, the reducibility of Pd-Cu can also be gradually improved under the function of the cathode so that the system exhibits good stability. This study demonstrates an environmentally friendly and promising method for total nitrogen removal from industrial wastewater with high conductivity.

Harnessing Nickel Phthalocyanine-Based Electrochemical CNT Sponges for Ammonia Synthesis from Nitrate in Contaminated Water

Electrochemical reduction of nitrate to ammonia is of great interest in water treatment with regard to the conversion of contaminants to value-added products, which requires the development of advanced electrodes to achieve high selectivity, stability, and Faradaic efficiency (FE). Herein, nickel phthalocyanine was homogeneously doped into the fiber of a carbon nanotube (CNT) sponge, enabling the production of an electrode with high electrochemical double-layer capacitance (C_DL) and a large electrochemically active surface area (ECSA). The as-prepared NiPc-CNT sponge could achieve 97.6% nitrate removal, 88.4% ammonia selectivity, and 86.8% FE at a nitrate concentration of 50 mg-N L^-1 under an optimized potential of -1.2 V (vs Ag/AgCl). Meanwhile, the ammonia selectivity could be further improved at the high nitrate concentration. Density functional theory calculations showed that the exposure of Ni-N₄ active sites could effectively suppress the hydrogen evolution reaction and dinitrogen generation, enhancing the ammonia selectivity and Faradaic efficiency. Overall, this work sheds light on the conversion of nitrate to ammonia on the metal phthalocyanine-based electrode, offering a novel strategy for managing nitrate in wastewater.

Efficient electrocatalytic nitrate reduction in neutral medium by Cu/CoP/NF composite cathode coupled with Ir-Ru/Ti anode

In this work, a three-dimensional self-supporting copper/cobalt phosphide/nickel foam (Co/CoP/NF) composite was fabricated and employed as the cathode for electrochemical nitrate removal from surface water with the assistance of a commercial Ir-Ru/Ti anode. The experimental results demonstrate that the introduction of Cu nanoparticles on CoP nanosheets is favorable for the electrocatalytic nitrate reduction. The influences of operating parameters (pH value, current density and initial nitrate concentration) on the nitrate reduction were assessed with the presence of Cl^-. At the optimized conditions, the removal of nitrate exhibits an efficiency ca. 100% via the coupling electrochemical reduction and oxidation processes. Moreover, the nitrogen selectivity is found to be as high as 98.8% within 210 min, accompanied with a promising test endurance (>94.0% for total nitrogen (TN) and NO₃^- removal efficiencies after an electrochemical run of 24.5 h). Importantly, as for the treated actual surface water, the concentration of TN is smaller than 1.5 mg L^-1, in accordance with the limit of Ⅳ-level standard of the surface water environmental quality in China (GB 3838-2002). The efficient removal of nitrate can be attributed to the synergistic effect of Cu and CoP microparticles to enhance the reduction activity, as well as the subsequent chloride oxidation for the major intermediate of ammonium.

High-Efficiency Electrochemical Nitrate Reduction to Ammonia on a Co3O4 Nanoarray Catalyst with Cobalt Vacancies

Electrocatalytic nitrate reduction reaction (NO₃RR) affords a bifunctional character in the carbon-free ammonia synthesis and remission of nitrate pollution in water. Here, we fabricated the Co₃O₄ nanosheet array with cobalt vacancies on carbon cloth (v_Co-Co₃O₄/CC) by in situ etching aluminum-doped Co₃O₄/CC, which exhibits an excellent Faradaic efficiency of 97.2% and a large NH₃ yield as high as 517.5 μmol h^-1 cm^-2, better than the pristine Co₃O₄/CC. Theoretical calculative results imply that the cobalt vacancies can tune the local electronic environment around Co sites of Co₃O₄, increasing the charge and reducing the electron cloud density of Co sites, which is thus conducive to adsorption of NO₃^- on Co sites for greatly enhanced nitrate reduction. Furthermore, the v_Co-Co₃O₄ (311) facet presents excellent NO₃RR activity with a low energy barrier of about 0.63 eV on a potential-determining step, which is much smaller than pristine Co₃O₄ (1.3 eV).

Electrocatalytic performance of oxygen-activated carbon fibre felt anodes mediating degradation mechanism of acetaminophen in aqueous environments

Carbon felts are flexible and scalable, have high specific areas, and are highly conductive materials that fit the requirements for both anodes and cathodes in advanced electrocatalytic processes. Advanced oxidative modification processes (thermal, chemical, and plasma-chemical) were applied to carbon felt anodes to enhance their efficiency towards electro-oxidation. The modification of the porous anodes results in increased kinetics of acetaminophen degradation in aqueous environments. The utilised oxidation techniques deliver single-step, straightforward, eco-friendly, and stable physiochemical reformation of carbon felt surfaces. The modifications caused minor changes in both the specific surface area and total pore volume corresponding with the surface morphology. A pristine carbon felt electrode was capable of decomposing up to 70% of the acetaminophen in a 240 min electrolysis process, while the oxygen-plasma treated electrode achieved a removal yield of 99.9% estimated utilising HPLC-UV-Vis. Here, the electro-induced incineration kinetics of acetaminophen resulted in a rate constant of 1.54 h^-1, with the second-best result of 0.59 h^-1 after oxidation in 30% H₂O₂. The kinetics of acetaminophen removal was synergistically studied by spectroscopic and electrochemical techniques, revealing various reaction pathways attributed to the formation of intermediate compounds such as p-aminophenol and others. The enhancement of the electrochemical oxidation rates towards acetaminophen was attributed to the appearance of surface carbonyl species. Our results indicate that the best-performing plasma-chemical treated CFE follows a heterogeneous mechanism with only approx. 40% removal due to direct electro-oxidation. The degradation mechanism of acetaminophen at the treated carbon felt anodes was proposed based on the detected intermediate products. Estimation of the cost-effectiveness of removal processes, in terms of energy consumption, was also elaborated. Although the study was focussed on acetaminophen, the achieved results could be adapted to also process emerging, hazardous pollutant groups such as anti-inflammatory pharmaceuticals.

Electrocatalytic nitrate reduction to ammonia via amorphous cobalt boride

Electrocatalytic nitrate reduction reaction (NitRR) is an energy-saving and environmentally benign approach to synthesizing ammonia under ambient conditions. However, the development of noble metal-free catalysts with high activity and selectivity is still a significant challenge. In this study, uniformly dispersed amorphous CoB_x nanoparticles supported on carbon paper were synthesized VIA a simple wet chemical reduction method. As an efficient nitrate reduction electrocatalyst, CoB_x exhibited a maximum faradaic efficiency of 94.00 ± 1.67% and a yield rate of up to 0.787 ± 0.028 mmol h^-1 cm^-2 for ammonia production. The enhanced NitRR performance could be attributed to a partial electron transfer from B to Co, which is necessary for optimizing the adsorption energies of the reaction intermediates and facilitating electron transport. Thus, selective and cost-effective electroreduction of nitrates to ammonia can be achieved using CoB_x nanoparticles.

A FeCo2O4 nanowire array enabled electrochemical nitrate conversion to ammonia

Electrocatalytic nitrate (NO₃^-) reduction not only generates high-value ammonia (NH₃) but holds significant potential in the control of NO₃^- contaminants in natural environments. Here, a bimetallic FeCo₂O₄ spinel nanowire array grown on carbon cloth is proposed as an efficient electrocatalyst for the conversion of NO₃^- to NH₃ with a high faradaic efficiency of up to 95.9% and a large NH₃ yield of 4988 μg h^-1 cm^-2. Furthermore, it also exhibits excellent stability during 16 h electrolysis.

Controlled Growth of Donor-Bridge-Acceptor Interface for High-Performance Ammonia Production

The intrinsic catalytic activity and active sites of the catalyst originate from the interface efficient charge transfer. A 2D graphdiyne (GDY) layer grown on the surface of zeolitic imidazolate framework nanocubes (ZIFNC@GDY) forms a novel structure of a perfect "donor-bridge-acceptor" interface, in which the ZIFNC and GDY act as electron donor and acceptor, respectively, linked by the sp-C-Co and sp-C-N bonds as bridges. Importantly, the as-prepared catalyst exhibits intrinsically high reactivity for ammonia production through the nitrate reduction reaction (NtRR) in neutral aqueous solutions at ambient pressures and temperatures. The NtRR performance of the as-prepared electrocatalyst is confirmed by the high NH₃ yield rate (Y_NH3 ) of 0.40 ± 0.02 mmol h^-1  cm^-2 at potential of -0.745V versus RHE and Faradaic efficiency (FE) of 98.51 ± 0.75%, as well as the excellent stability. We show that such unique interfacial structures can accelerate the efficient electron transfers between the zeolitic imidazolate framework nanocubes (ZIFNC) core and GDY shell, enrich the electron density on the GDY surface, and thereby promote fast redox switching, creating more active sites, and improving the catalytic performances.

Direct eight-electron NO3−-to-NH3 conversion: using a Co-doped TiO2 nanoribbon array as a high-efficiency electrocatalyst

A Co-TiO2 nanoribbon array supported on a Ti plate is a high-efficiency catalyst for electrochemical NO3–-to-NH3 conversion, capable of attaining a large NH3 yield of 1127 μmol h−1 cm−2 and high Faradaic efficiency of 98.2%.

CoO nanoparticle decorated N-doped carbon nanotube: a high-efficiency catalyst for nitrate reduction to ammonia

Ambient electrochemical nitrate (NO3–) reduction emerges as an appealing approach toward eliminating NO3– contaminant and generating ammonia (NH3) simultaneously, while its efficiency is challenged by lack of active and selective...