Copper single-atom catalyst as a high-performance electrocatalyst for nitrate-ammonium conversion

Nickel-based dual single atom electrocatalysts for the nitrate reduction reaction

Electrochemical nitrate (NO3-) reduction reaction (NO3-RR) to ammonium (NH4+) or nitrogen (N2) provides a green route for nitrate remediation. However, nitrite generation and hydrogen evolution reactions hinder the feasibility of the process. Herein, dual single atom catalysts were rationally designed by introducing Ag/Bi/Mo atoms to atomically dispersed NiNC moieties supported by nitrogen-doped carbon nanosheet (NCNS) for the NO3-RR. Ni single atoms loaded on NCNS (Ni/NCNS) tend to reduce NO3- to valuable NH4+ with a high selectivity of 77.8 %. In contrast, the main product of NO3-RR catalyzing by NiAg/NCNS, NiBi/NCNS, and NiMo/NCNS was changed to N2, giving rise to N2 selectivity of 48.4, 47.1 and 47.5 %, respectively. Encouragingly, Ni/NCNS, NiBi/NCNS, and NiAg/NCNS showed excellent durability in acidic electrolytes, leading to nitrate conversion rates of 70.3, 91.1, and 93.2 % after a 10-h reaction. Simulated wastewater experiments showed that NiAg/NCNS could remove NO3- up to 97.8 % at -0.62 V after 9-h electrolysis. This work afforded a new strategy to regulate the reaction pathway and improve the conversion efficiency of the NO3-RR via engineering the dual atomic sites of the catalysts.

Advances in designing efficient electrocatalysts for nitrate reduction from a theoretical perspective

Ammonia (NH3), an important raw material for producing fertilizers and useful chemicals, plays a crucial role in modern human society. As the Haber-Bosch process is energy- and emission-intensive, it is critical to develop a green and energy-efficient route for massive NH3 production under ambient conditions. The electrochemical nitrate reduction reaction to ammonia (eNO3-RR) is a potential way for producing NH3 while harmonizing the nitrogen cycle. In this feature article, we summarize the advances in designing eNO3-RR electrocatalysts from a theoretical perspective. First, the mechanisms and pathways of the eNO3-RR are summarized. Then, the recently developed electrocatalysts, including Cu-based catalysts, single-atom catalysts (SACs), dual-atom catalysts (DACs), and MXene catalysts, are categorically discussed. Finally, the challenges and prospects of designing highly efficient eNO3-RR catalysts through theoretical simulations are discussed. This feature article will provide valuable guidance for the future development of advanced eNO3-RR electrocatalysts for NH3 production.

Single-atom catalysts for electrocatalytic nitrate reduction into ammonia

Ammonia (NH3) is a versatile and important compound with a wide range of uses, which is currently produced through the demanding Haber-Bosch process. Electrocatalytic nitrate reduction into ammonia (NRA) has recently emerged as a sustainable approach for NH3synthesis under ambient conditions. However, the NRA catalysis is a complex multistep electrochemical process with competitive hydrogen evolution reaction that usually results in poor selectivity and low yield rate for NH3synthesis. With maximum atom utilization and well-defined catalytic sites, single atom catalysts (SACs) display high activity, selectivity and stability toward various catalytic reactions. Very recently, a number of SACs have been developed as promising NRA electrocatalysts, but systematical discussion about the key factors that affect their NRA performance is not yet to be summarized to date. This review focuses on the latest breakthroughs of SACs toward NRA catalysis, including catalyst preparation, catalyst characterization and theoretical insights. Moreover, the challenges and opportunities for improving the NRA performance of SACs are discussed, with an aim to achieve further advancement in developing high-performance SACs for efficient NH3synthesis.

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.

Atomic-Level Regulation of Cu-Based Electrocatalyst for Enhancing Oxygen Reduction Reaction: From Single Atoms to Polymetallic Active Sites

The slow kinetics of cathodic oxygen reduction reactions (ORR) in fuel cells and the high cost of commercial Pt-based catalysts limit their large-scale application. Cu-based single-atom catalysts (SACs) have received increasing attention as a promising ORR catalyst due to their high atom utilization, high thermodynamic activity, adjustable electronic structure, and low cost. Herein, the recent research progress of Cu-based catalysts is reviewed from single atom to polymetallic active sites for ORR. First, the design and synthesis method of Cu-based SACs are summarized. Then the atomic-level structure regulation strategy of Cu-based catalyst is proposed to improve the ORR performance. The different ORR catalytic mechanism based on the different Cu active sites is further revealed. Finally, the design principle of high-performance Cu-based SACs is proposed for ORR and the opportunities and challenges are further prospected.

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.

Conjugated Coordination Polymer as a New Platform for Efficient and Selective Electroreduction of Nitrate into Ammonia

Electroreduction of nitrate into ammonia (NRA) provides a sustainable route to convert the widespread nitrate pollutants into high-value-added products under ambient conditions, which unfortunately suffers from unsatisfactory selectivity due to the competitive hydrogen evolution reaction (HER). Previous strategies of modifying the metal sites of catalysts often met a dilemma for simultaneously promoting activity and selectivity toward NRA. Here, a general strategy is reported to enable an efficient and selective NRA process through coordination modulation of single-atom catalysts to tailor the local proton concentration at the catalyst surface. By contrast, two analogous Ni-single-atom enriched conjugated coordination polymers (NiO₄ -CCP and NiN₄ -CCP) with different coordination motifs are investigated for the proof-of-concept study. The NiO₄ -CCP catalyst exhibits an ammonia yield rate as high as 1.83 mmol h^-1 mg^-1 with a Faradaic efficiency of 94.7% under a current density of 125 mA cm^-2 , outperforming the NiN₄ -CCP catalyst. These experimental and theoretical studies both suggest that the strategy of coordination modulation can not only accelerate the NRA by adjusting the adsorption energies of NRA intermediates on the metal sites but also inhibit the HER through regulating the proton migration with contributions from the metal-hydrated cations adsorbed at the catalyst surface, thus achieving simultaneous enhancement of NRA selectivity and activity.

Constructing Cu-CuO heterostructured skin on Cu cubes to promote electrocatalytic ammonia production from nitrate wastewater

Electroreducting nitrate (NO₃^-) into ammonia (NH₃), i.e. NO₃^-RA, can not only relieve NO₃^- pollution but also produce high value-added NH₃. Cu-based species have been taken as a promising catalyst for NO₃^-RA because of their relatively high Faradaic efficiency (FE), benefiting from the weak side hydrogen evolution reaction (HER). However, their NH₃ yield rates are still unsatisfactory due to the multiple electron paths of NO₃^-RA. Herein, we report a Cu cube with Cu-CuO heterostructured skin, prepared by electrochemically induced reconstruction from a Cu₂O cube. This novel Cu-based catalyst presents a mol-level NH₃ yield rate of 3.17 mol h^-1 g^-1 ranking at the top level among non-noble metal catalysts and an ultrahigh FE of 98.7 %. These excellent performances attribute to the Cu-CuO heterostructured skin of Cu cubes, which has favorable energy for the hydrogenation of *NO to *NOH during the NO₃^-RA process and an unfavorable one for HER. For the NO₃^- removal of real river water, this novel Cu-based catalyst presents a high NO₃^- removal rate of 95.5 % after the NO₃^-RA test for 12 h, resulting in a lower NO₃^- concentration than the maximum residual amount of NO₃^- in drinking water limited by WHO and China. This study provides a feasible strategy by the electrochemical reconstruction method to prepare superior Cu-based electrocatalysts with mol-level NO₃^-RA performances for the purification of nitrate wastewater.