Electrochemical nitrate reduction to ammonia on copper-based materials for nitrate-containing wastewater treatment

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.

Accelerating Tandem Electroreduction of Nitrate to Ammonia via Multi-Site Synergy in Mesoporous Carbon-Supported High-Entropy Intermetallics

The electrochemical nitrate reduction reaction (NO3 -RR) for ammonia (NH3) synthesis represents a significant technological advancement, yet it involves a cascade of elementary reactions alongside various intermediates. Thus, the development of multi-site catalysts for enhancing NO3 -RR and understanding the associated reaction mechanisms for NH3 synthesis is vital. Herein, a versatile approach is presented to construct platinum based high-entropy intermetallic (HEI) library for NH3 synthesis. The HEI nanoparticles (NPs) are uniformly supported on a 2D nitrogen doped mesoporous carbon (N-mC) framework, featured with adjustable compositions (up to eight elements) and a high degree of atomic order (over 90%). Guided by the density functional theory (DFT) calculations and atomic structural analysis, a quinary Pt0.8Fe0.2Co0.2Ni0.2Cu0.2 HEI NPs based N-mC catalyst is designed, which demonstrates a large ammonia Faradaic efffciency (>97%) and a remarkable recyclability (>20 cycles) under both acidic and basic conditions. The combined in situ experimental analysis and further DFT calculation suggests that the well-defined multi-sites nature of the HEI NPs cooperate for a tandem reduction mechanism, in which the Pt-X (X represents the other four transition elements) bridging sites offer optimal adsorption for key nitrogen-oxygen species while the Pt sites facilitate the generation and adsorption of *H species.

Boosted direct electrochemical reduction of As(III) from arsenic wastewater via Cu(II)-assisted codeposition on a CuIn alloy electrode

Arsenic contamination is a severe environmental problem. A promising strategy for addressing this issue is the direct conversion of highly toxic As(III) to less toxic elemental arsenic (As(0)) using electrochemical reduction technology. In this study, a novel CuIn alloy nanoparticles-modified copper foam (CuIn NPs/CF) was prepared as an efficient cathode for the electrocatalytic reduction of highly mobile As(III) to solid As(0). Density functional theory (DFT) results revealed that the Cu-In bimetallic system exhibited weaker H atom bonding, and the Cu-ln surface was more favorable for the adsorption of *AsO₃ species than the Cu surface. Compared to the pristine CF electrode, CuIn NPs/CF was demonstrated to effectively suppressed the hydrogen evolution reaction with an enlarged hydrogen evolution potential of 1.45 V, and displayed a superior As(0) recovery yield. The conversion of As(III) to As(0) was further enhanced by adding Cu²⁺ to the electrolyte, facilitating a Cu-As co-deposition process. Notably, the CuIn NPs/CF electrode achieved an As(0) recovery yield of 5.38 mg cm⁻² after eight successive recycling tests. This work not only presents a green and sustainable strategy for As(III) removal, but also provides valuable insights into the rational design of Cu-based alloy cathodes for electrocatalytic reduction.

Functional partitioning synergistically enhances multi-scenario nitrate reduction

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

Atomic-Dispersed Cu Catalysts for Electrochemical Nitrate Reduction: Coordination Engineering and Fundamental Insights

The development of Cu-based atomic dispersed catalysts with tailored coordination environments represents a significant step forward in enhancing the electrocatalytic reduction of nitrate to ammonia. By precisely modulating the electronic structures of Cu active centers, the binding strength of the *NO3 intermediates is successfully tuned, thereby substantially improving the catalytic activity toward electrochemical nitrate reduction reaction (eNO3RR). This study reveals that the N4-coordinated Cu single-atom catalyst (Cu-SAC) exhibits superior performance due to its robust interaction with coordinating atoms. Notably, this optimized catalyst achieves a low limiting potential of -0.38 V, while the dual-atom system further reduces this value to -0.32 V, demonstrating exceptional activity. Detailed electronic structure analysis, including the examination of d-band centers, Bader charges, and projected density of states (PDOS), provides a comprehensive understanding of the origin of this high activity. Specifically, the high and concentrated density of states near the Fermi level plays a crucial role in facilitating the electrocatalytic nitrate reduction process. This work not only offers crucial insights into the underlying mechanisms of eNO3RR but also provides valuable guidelines for the rational design of highly efficient electrocatalysts for this important reaction.

Regulating the Electrochemical Nitrate Reduction Performance with Controllable Distribution of Unconventional Phase Copper on Alloy Nanostructures

Electrochemical nitrate reduction reaction (NO3RR) is emerging as a promising strategy for nitrate removal and ammonia (NH3) production using renewable electricity. Although great progresses have been achieved, the crystal phase effect of electrocatalysts on NO3RR remains rarely explored. Here, the epitaxial growth of unconventional 2H Cu on hexagonal close-packed (hcp) IrNi template, resulting in the formation of three IrNiCu@Cu nanostructures, is reported. IrNiCu@Cu-20 shows superior catalytic performance, with NH3 Faradaic efficiency (FE) of 86% at -0.1 (vs reversible hydrogen electrode [RHE]) and NH3 yield rate of 687.3 mmol gCu -1 h-1, far better than common face-centered cubic Cu. In sharp contrast, IrNiCu@Cu-30 and IrNiCu@Cu-50 covered by hcp Cu shell display high selectivity toward nitrite (NO2 -), with NO2 - FE above 60% at 0.1 (vs RHE). Theoretical calculations have demonstrated that the IrNiCu@Cu-20 has the optimal electronic structures for NO3RR due to the highest d-band center and strongest reaction trend with the lowest energy barriers. The high electroactivity of IrNiCu@Cu-20 originates from the abundant low coordination of Cu sites on the surface, which guarantees the fast electron transfer to accelerate the intermediate conversions. This work provides a feasible tactic to regulate the product distribution of NO3RR by crystal phase engineering of electrocatalysts.

Electrochemical Growth of Copper Crystals on SPCE for Electrocatalysis Nitrate Reduction

Copper is efficient, has a high conductivity (5.8 × 107 S/m), and is cost-effective. The use of copper-based catalysts is promising for the electrocatalytic reduction of nitrates. This work aims to grow and characterize copper micro-crystals on Screen-Printed Electrodes (SPEs) for NO3- reduction in water. Copper micro-crystals were grown by cyclic voltammetry. Different cycles (2, 5, 7, 10, 12, 15) of copper electrodeposition were investigated (potential ranges from -1.0 V to 0.0 V, scan rate of 0.1 V s-1). Electrodeposition generated different morphologies of copper crystals on the electrodes, as a function of the number of cycles, with various performances. The presence of numerous edges and defects in the copper micro-crystal structures creates highly reactive active sites, thus favoring nitrate reduction. The manufactured material can be successfully employed for environmental applications.

Copper-based electro-catalytic nitrate reduction to ammonia from water: Mechanism, preparation, and research directions

Global water bodies are increasingly imperiled by nitrate pollution, primarily originating from industrial waste, agricultural runoffs, and urban sewage. This escalating environmental crisis challenges traditional water treatment paradigms and necessitates innovative solutions. Electro-catalysis, especially utilizing copper-based catalysts, known for their efficiency, cost-effectiveness, and eco-friendliness, offer a promising avenue for the electro-catalytic reduction of nitrate to ammonia. In this review, we systematically consolidate current research on diverse copper-based catalysts, including pure Cu, Cu alloys, oxides, single-atom entities, and composites. Furthermore, we assess their catalytic performance, operational mechanisms, and future research directions to find effective, long-term solutions to water purification and ammonia synthesis. Electro-catalysis technology shows the potential in mitigating nitrate pollution and has strategic importance in sustainable environmental management. As to the application, challenges regarding complexity of the real water, the scale-up of the commerical catalysts, and the efficient collection of produced NH3 are still exist. Following reseraches of catalyst specially on long term stability and in situ mechanisms are proposed.

Metal Doped Unconventional Phase IrNi Nanobranches: Tunable Electrochemical Nitrate Reduction Performance and Pollutants Upcycling

Electrochemical nitrate reduction (NO3RR) provides a new option to abate nitrate contamination with a low carbon footprint. Restricted by competitive hydrogen evolution, achieving satisfied nitrate reduction performance in neutral media is still a challenge, especially for the regulation of this multielectron multiproton reaction. Herein, facile element doping is adopted to tune the catalytic behavior of IrNi alloy nanobranches with an unconventional hexagonal close-packed (hcp) phase toward NO3RR. In particular, the obtained hcp IrNiCu nanobranches favor the ammonia production and suppress byproduct formation in a neutral electrolyte indicated by in situ differential electrochemical mass spectrometry, with a high Faradaic efficiency (FE) of 85.6% and a large yield rate of 1253 μg cm-2 h-1 at -0.4 and -0.6 V (vs reversible hydrogen electrode (RHE)), respectively. In contrast, the resultant hcp IrNiCo nanobranches promote the formation of nitrite, with a peak FE of 33.1% at -0.1 V (vs RHE). Furthermore, a hybrid electrolysis cell consisting of NO3RR and formaldehyde oxidation is constructed, which are both catalyzed by hcp IrNiCu nanobranches. This electrolyzer exhibits lower overpotential and holds the potential to treat polluted air and wastewater simultaneously, shedding light on green chemical production based on contaminate degradation.

Element-dependent effects of alkali cations on nitrate reduction to ammonia

Catalytic conversion of nitrate (NO3-) pollutants into ammonia (NH3) offers a sustainable and promising route for both wastewater treatment and NH3 synthesis. Alkali cations are prevalent in nitrate solutions, but their roles beyond charge balance in catalytic NO3- conversion have been generally ignored. Herein, we report the promotion effect of K+ cations in KNO3 solution for NO3- reduction over a TiO2-supported Ni single-atom catalyst (Ni1/TiO2). For photocatalytic NO3- reduction reaction, Ni1/TiO2 exhibited a 1.9-fold NH3 yield rate with nearly 100% selectivity in KNO3 solution relative to that in NaNO3 solution. Mechanistic studies reveal that the K+ cations from KNO3 gradually bonded with the surface of Ni1/TiO2, in situ forming a K-O-Ni moiety during reaction, whereas the Na+ ions were unable to interact with the catalyst in NaNO3 solution. The charge accumulation on the Ni sites induced by the incorporation of K atom promoted the adsorption and activation of NO3-. Furthermore, the K-O-Ni moiety facilitated the multiple proton-electron coupling of NO3- into NH3 by stabilizing the intermediates.

Electrochemical Nitrate Reduction: Ammonia Synthesis and the Beyond

Natural nitrogen cycle has been severely disrupted by anthropogenic activities. The overuse of N-containing fertilizers induces the increase of nitrate level in surface and ground waters, and substantial emission of nitrogen oxides causes heavy air pollution. Nitrogen gas, as the main component of air, has been used for mass ammonia production for over a century, providing enough nutrition for agriculture to support world population increase. In the last decade, researchers have made great efforts to develop ammonia processes under ambient conditions to combat the intensive energy consumption and high carbon emission associated with the Haber-Bosch process. Among different techniques, electrochemical nitrate reduction reaction (NO3RR) can achieve nitrate removal and ammonia generation simultaneously using renewable electricity as the power, and there is an exponential growth of studies in this research direction. Here, a timely and comprehensive review on the important progresses of electrochemical NO3RR, covering the rational design of electrocatalysts, emerging CN coupling reactions, and advanced energy conversion and storage systems is provided. Moreover, future perspectives are proposed to accelerate the industrialized NH3 production and green synthesis of chemicals, leading to a sustainable nitrogen cycle via prosperous N-based electrochemistry.

Highly distributed amorphous copper catalyst for efficient ammonia electrosynthesis from nitrate

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