Cu Modified Pt Nanoflowers with Preferential (100) Surfaces for Selective Electroreduction of Nitrate

High Ammonia Yield Rate from Dilute Nitrate Solutions Using a Cu(100)-Rich Foil: A Step Closer to Large-Scale Production

The electrochemical reduction of nitrate (NO3-) ions to ammonia (NH3) provides an alternative method to eliminate harmful NO3- pollutants in water as well as to produce highly valuable NH3 chemicals. The NH3 yield rate however is still limited to the μmol h-1 cm-2 range when dealing with dilute NO3- concentrations found in waste streams. Copper (Cu) has attracted much attention because of its unique ability to effectively convert NO3- to NH3. We have reported a simple and scalable electrochemical method to produce a Cu foil having its surface covered with a porous Cu nanostructure enriched with (100) facets, which efficiently catalyzes NO3- to NH3. The Cu(100)-rich electrocatalyst showed a very high NH3 production rate of 1.1 mmol h-1 cm-2 in NO3- concentration as low as 14 mM NO3-, which is 4-5 times higher than the best-reported values. Increasing the NO3- concentration (140 mM) resulted in an NH3 production yield rate of 3.34 mmol h-1 cm-2. The durability test conducted for this catalyst foil in a flow cell system showed greater than 100 h stability with a Faradaic efficiency greater than 98%, demonstrating its potential to be used on an industrially relevant scale. Further, density functional theory (DFT) calculations have been performed to understand the better catalytic activity of Cu(100) compared to Cu(111) facets toward NO3-RR.

One Bicopper Complex with Good Affinity to Nitrate for Highly Selective Electrocatalytic Nitrate Reduction to Ammonia

Ammonia (NH3) plays an irreplaceable role in human life as a promising energy carrier and indispensable chemical raw material. Nitrate electroreduction to ammonium (NRA) not only removes nitrate pollutants, but also can be used for efficient NH3 production under ambient conditions. However, achieving high efficiency and selectivity of electrocatalysts is still a great challenge. Herein, a complex Cu2(NO3)4(BMMB)·H2O with a bicopper core is assembled by Cu(NO3)2·3H2O and 1,4-bis{[2-(2’-pyridyl)benzimidazolyl]methyl}benzene (BMMB) for NRA under alkaline conditions. The optimal sample showed excellent nitrate reduction performance with the NO3− conversion rate of 70%, Faradaic efficiency of up to 90%, and NH3 selectivity of more than 95%. The high-catalytic activity is mainly due to the ingeniously designed copper cores with strong affinity for NO3−, which accelerates the transferring rate of adsorbed nitrate on the Cu surface and increases the efficiency of rate-determining step (NO3− → NO2−) in the whole catalytic process. Therefore, the transformation of surface-exposed nitrate can be rapidly catalyzed by the Cu active sites, facilitating the conversion efficiency of nitrate.

Revealing the pH-dependent mechanism of nitrate electrochemical reduction to ammonia on single-atom catalysts

Nitrate electrochemical reduction to ammonia (NO₃RR) catalyzed by single-atom catalysts (SACs) is an attractive and efficient way for solving the problem of nitrate pollution in water and obtaining valuable product ammonia through low temperature synthesis. It is well known that the pH conditions can be regulated to tune the performance of NO₃RR, however, there have been few studies aimed at gaining theoretical insight into the origin of pH-dependent catalytic performance among SACs. Herein, taking 3d-transition metal (Fe, Co, Ni and Mn) single-atoms supported on diverse anchor sites of MoS₂ as an example (SA-MoS₂), we explore the activity and selectivity for NO₃RR towards ammonia (NH₃ and NH₄⁺) under different pH conditions by density functional theory calculations. It is found that priority reaction pathways, the potential determining step and limiting potentials of SA-MoS₂ exhibit pH-dependent characteristics, which can be described by a contour map of catalytic reactivity, spanned by adsorption free energies (G_NO* and G_NH2*), and further determined by local coordination environment and electronic states of active sites. Our three-step screening method reveals that the Co single-atom adsorbed MoS₂ edge catalyst is the most promising catalyst among the studied SA-MoS₂ because of its low limiting potential (-0.3-0.4 V, RHE), excellent selectivity in the competition with the hydrogen evolution reaction (HER), as well as stability against aggregation and electrochemical dissolution across the full pH range. This work demonstrates a theoretical insight into the pH-dependent mechanism of supported SA catalyzed NO₃RR, which proposes a screening strategy for finding new SACs, and provides motivation for further experimental exploration.

Electrocatalytic reduction of nitrate - a step towards a sustainable nitrogen cycle

Nitrate enrichment, which is mainly caused by the over-utilization of fertilisers and industrial sewage discharge, is a major global engineering challenge because of its negative influence on the environment and human health. To solve this serious problem, many technologies, such as the activated sludge method, reverse osmosis, ion exchange, adsorption, and electrodialysis, have been developed to reduce the nitrate levels in water bodies. However, the applications of these traditional techniques are limited by several drawbacks, such as a long sludge retention time, slow kinetics, and undesirable by-products. From an environmental perspective, the most promising nitrate reduction technology is enabled to convert nitrate into benign N₂, and features low cost, high efficiency, and environmental friendliness. Recently, electrocatalytic nitrate reduction has been proven by satisfactory research achievements to be one of the most promising methods among these technologies. This review provides a comprehensive account of nitrate reduction using electrocatalysis methods. The fundamentals of electrocatalytic nitrate reduction, including the reaction mechanisms, reactor design principles, product detection methods, and performance evaluation methods, have been systematically summarised. A detailed introduction to electrocatalytic nitrate reduction on transition metals, especially noble metals and alloys, Cu-based electrocatalysts, and Fe-based electrocatalysts is provided, as they are essential for the accurate reporting of experimental results. The current challenges and potential opportunities in this field, including the innovation of material design systems, value-added product yields, and challenges for products beyond N₂ and large-scale sewage treatment, are highlighted.