Constructing Ru@C3 N4 /Cu Tandem Electrocatalyst with Dual-Active Sites for Enhanced Nitrate Electroreduction to Ammonia

Unveiling Cutting-Edge Developments in Electrocatalytic Nitrate-to-Ammonia Conversion

The excessive enrichment of nitrate in the environment can be converted into ammonia (NH3) through electrochemical processes, offering significant implications for modern agriculture and the potential to reduce the burden of the Haber-Bosch (HB) process while achieving environmentally friendly NH3 production. Emerging research on electrocatalytic nitrate reduction (eNitRR) to NH3 has gained considerable momentum in recent years for efficient NH3 synthesis. However, existing reviews on nitrate reduction have primarily focused on limited aspects, often lacking a comprehensive summary of catalysts, reaction systems, reaction mechanisms, and detection methods employed in nitrate reduction. This review aims to provide a timely and comprehensive analysis of the eNitRR field by integrating existing research progress and identifying current challenges. This review offers a comprehensive overview of the research progress achieved using various materials in electrochemical nitrate reduction, elucidates the underlying theoretical mechanism behind eNitRR, and discusses effective strategies based on numerous case studies to enhance the electrochemical reduction from NO3 - to NH3. Finally, this review discusses challenges and development prospects in the eNitRR field with an aim to guide design and development of large-scale sustainable nitrate reduction electrocatalysts.

Ru-Doped Ultrasmall Cu Nanoparticles Decorated with Carbon for Electroreduction of Nitrate to Ammonia

Electrocatalytic nitrate reduction reaction offers a sustainable approach to treating wastewater and synthesizing high-value ammonia under ambient conditions. However, electrocatalysts with low faradaic efficiency and selectivity severely hinder the development of nitrate-to-ammonia conversion. Herein, Ru-doped ultrasmall copper nanoparticles loaded on a carbon substrate (Cu-Ru@C) were fabricated by the pyrolysis of Cu-BTC metal-organic frameworks (MOFs). The Cu-Ru@C-0.5 catalyst exhibits a high faradaic efficiency (FE) of 90.4% at -0.6 V (vs RHE) and an ammonia yield rate of 1700.36 μg h-1mgcat.-1 at -0.9 V (vs RHE). Moreover, the nitrate conversion rate is almost 100% over varied pHs (including acid, neutral, and alkaline electrolytes) and different nitrate concentrations. The remarkable performance is attributed to the synergistic effect between Cu and Ru and the excellent conductivity of the carbon substrate. This work will open an exciting avenue to exploring MOF derivatives for ambient ammonia synthesis via selective electrocatalytic nitrate reduction.

Bioinspired Tandem Electrode for Selective Electrocatalytic Synthesis of Ammonia from Aqueous Nitrate

The electrocatalytic nitrate reduction reaction (NO3RR) has recently emerged as a promising technique for readily converting aqueous nitrate (NO3-) pollutants into valuable ammonia (NH3). It is vital to thoroughly understand the mechanism of the reaction to rationally design and construct advanced electrocatalytic systems that can effectively and selectively drive the NO3RR. There are several natural enzymes that incorporate molybdenum (Mo) and that can activate NO3-. Based on this, a cadmium (Cd) single-atom anchored Mo2TiC2Tx electrocatalyst (referred to as CdSA-Mo2TiC2Tx) through the NO3RR to generate NH3 was rationally designed and demonstrated. In an H-type electrolysis cell and at a current density of 42.5 mA cm-2, the electrocatalyst had a Faradaic efficiency of >95% and an impressive NH3 yield rate of 48.5 mg h-1 cm-2. Moreover, the conversion of NO3- to NH3 on the CdSA-Mo2TiC2Tx surface was further revealed by operando attenuated total reflection Fourier-transform infrared spectroscopy and an electrochemical differential mass spectrometer. The electrocatalyst significantly outperformed Mo2TiC2Tx as well as reported state-of-the-art catalysts. Density functional theory calculations revealed that CdSA-Mo2TiC2Tx decreased the ability of the d-p orbital to hybridize with NH3* intermediates, thereby decreasing the activation energy of the potential-determining step. This work not only highlights the application prospects of heavy metal single-atom catalysts in the NO3RR but also provides examples of bio-inspired electrocatalysts for the synthesis of NH3.

Integrated Ozonation Ni-NiO/Carbon/g-C3N4 Nanocomposite-Mediated Catalytic Decomposition of Organic Contaminants in Wastewater under Visible Light

Developing a hybrid process for wastewater purification is of utmost importance to make conventional methods more efficient and faster. Herein, an effective visible light-active nickel-nickel oxide/carbon/graphitic carbon nitride (Ni-NiO/C/g-C3N4)-based nanocatalyst was developed. A hybrid process based on ozonation and Ni-NiO/C/g-C3N4 visible light photocatalysis was applied to decolourize the Congo red (CR), Alizarin Red S (ARS), and real dairy industry wastewater. The synthesized catalyst was characterized using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), Χ-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV-Vis diffuse reflectance spectrophotometry (UV-Vis DRS). The factors affecting the catalytic process were evaluated, including contact time, solution pH, initial dye concentration, etc. The degradation rate of CR and ARS was compared between the photocatalysis, ozonation, and integrated photocatalytic ozonation (PC/O3) methods. The results showed 100% degradation of CR and ARS within 5 min and 40 min, respectively, by integrated PC/O3. The reusability of the modified catalyst was evaluated, and four successive regenerations were achieved. The modified Ni-NiO/C/g-C3N4 composite could be considered an effective, fast, and reusable catalyst in an integrated PC/O3 process for the complete decolourization of wastewater.

Cu/NiO nanorods for efficiently promoting the electrochemical nitrate reduction to ammonia

The electrochemical nitrate reduction reaction (ENO3RR) is a green ammonia synthesis method under ambient conditions relative to the traditional Haber-Bosch technology, which does not require high-temperature or high-pressure conditions and can convert nitrate pollutants in the environment into value-added NH3, thus achieving a dual purpose. However, more electrocatalysts with a remarkable performance towards high-efficiency ENO3RR need to be developed. In this work, a Cu/NiO-NF composite electrocatalyst with a nanorod structure on nickel foam was successfully fabricated, which contains heterogeneous interfaces between Cu and NiO toward selective electrocatalytic nitrate reduction for ammonia synthesis. The steric nanorod morphology of the catalyst can significantly increase the surface area, expose more active sites, and improve the reaction activity. Moreover, the construction of the composite and the interface effectively boosts the synergistic effect of the active species Cu and NiO, which can regulate the electronic structure of the catalyst, expose more active sites, enhance the conductivity of the material, and accelerate the interfacial electron transfer, thereby further promoting the ENO3RR performance. This Cu/NiO-NF composite exhibits a high NH3 yield of 0.6 mmol h-1 cm-2 and up to 97.81% faradaic efficiency at the optimal applied potential of -1.0 V (vs. RHE) in a concentration of 0.1 M NO3--containing 0.1 M PBS. Furthermore, it demonstrates excellent electrochemical cycle stability. This work provides insights into the rational design and fabrication of ENO3RR electrocatalysts for potential electrocatalytic applications.

Boosting Electrocatalytic Nitrate-to-Ammonia via Tuning of N-Intermediate Adsorption on a Zn-Cu Catalyst

The renewable-energy-powered electroreduction of nitrate (NO3 - ) to ammonia (NH3 ) has garnered significant interest as an eco-friendly and promising substitute for the Haber-Bosch process. However, the sluggish kinetics hinders its application at a large scale. Herein, we first calculated the N-containing species (*NO3 and *NO2 ) binding energy and the free energy of the hydrogen evolution reaction over Cu with different metal dopants, and it was shown that Zn was a promising candidate. Based on the theoretical study, we designed and synthesized Zn-doped Cu nanosheets, and the as-prepared catalysts demonstrated excellent performance in NO3 - -to-NH3 . The maximum Faradaic efficiency (FE) of NH3 could reach 98.4 % with an outstanding yield rate of 5.8 mol g-1  h-1 , which is among the best results up to date. The catalyst also had excellent cycling stability. Meanwhile, it also presented a FE exceeding 90 % across a wide potential range and NO3 - concentration range. Detailed experimental and theoretical studies revealed that the Zn doping could modulate intermediates adsorption strength, enhance NO2 - conversion, change the *NO adsorption configuration to a bridge adsorption, and decrease the energy barrier, leading to the excellent catalytic performance for NO3 - -to-NH3 .