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

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.

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.

The interface-mediated electron structure tuning of RuOx-Co3O4 nano-particles for efficient electrocatalytic nitrate reduction

The energy-intensive processes for the industrial production of ammonia necessitates the development of new methods to be proposed that will aid in reducing the global energy consumption. Specifically, the electrocatalytic nitrate reduction reaction (NO3RR) to produce ammonia is more thermodynamically feasible than the electrocatalytic nitrogen reduction reaction (NRR). However, it is hindered by a low catalytic activity due to its complex reaction pathways. Herein, we synthesized a novel electrocatalyst, RuOx-Co3O4 nanoparticles, with abundant interfaces, which exhibited an enhanced catalytic activity for efficient ammonia synthesis. This catalyst delivered a partial current density of 65.8 mA cm-2 for NH3 production, a faradaic efficiency (FE) of 89.7%, and a superior ammonia yield rate of up to 210.5 μmol h-1 cm-2 at -0.6 V vs. RHE. X-ray photoelectron and Raman spectroscopy revealed that the formed interfacial Ru-O-Co bond can decorate the electronic structures of the active sites and accelerate the absorption of NO3-, thus promoting the production of ammonia.

Electrocatalytic nitrite-to-ammonia reduction on isolated Cu sites

We report that isolated Cu atoms anchored on MnO2 nanowires (Cu1/MnO2) can be an effective catalyst towards the electrocatalytic NO2--to-NH3 reduction (NO2RR). A combination of experiments and theoretical calculations reveals that isolated Cu sites can effectively activate NO2-, lower the energy barrier of *NO→*NOH rate-determining step and suppress the competitive hydrogen evolution, thus facilitating both activity and selectivity towards the NO2RR. As a result, Cu1/MnO2 shows the maximum NH3-Faradaic efficiency of 93.3% with a corresponding NH3 yield rate of 439.8 μmol h-1 cm-2 at -0.7 V vs. RHE, together with an excellent electrocatalytic durability.

Nb-doped NiO nanoflowers for nitrite electroreduction to ammonia

Electrocatalytic reduction of nitrite to ammonia (NO2RR) is considered as an appealing route to simultaneously achieve sustainable ammonia production and abate hazardous nitrite pollution. Herein, atomically Nb-doped NiO nanoflowers are designed as a high-performance NO2RR catalyst, which exhibits the highest NH3-Faradaic efficiency of 92.4% with an NH3 yield rate of 200.5 μmol h-1 cm-2 at -0.6 V RHE. Theoretical calculations unravel that Nb dopants can act as Lewis acid sites to render effective NO2- activation, decreased protonation energy barriers, and restricted hydrogen evolution, ultimately leading to a high NO2RR selectivity and activity.

Ca substitution improves the catalytic activity of perovskite LaCoO3 toward toluene: comprehension of electronic structure alteration

For perovskite La1-xCaxCoO3 (Ca-x, x = 0-0.3), Ca-0.2 with the closest O p band center to the Fermi level, displays the best catalytic activity for toluene oxidation. The O p band center determines the reducibility and active oxygen content. This finding is beneficial for the design of highly active perovskite catalysts.

Electrochemical Postmodification-Induced Surface Atom Rearrangement over Cu Nanodendrites for Enhanced Electrosynthesis of Ammonia from Nitrate

Utilizing nitrate from wastewater as a N-source for ammonia synthesis via electrocatalysis is of significance for both environmental protection and ecological nitrogen cycle balance, which requires high-performance electrocatalysts to drive selective nitrate-to-ammonia transformation. In this work, an electrochemical postmodification strategy was developed to regulate the surface structure of presynthesized Cu nanodendrites at the atomic level. A combination of physicochemical characterization and electrochemical study demonstrates that such a treatment could induce surface Cu atom rearrangement and result in increased electrochemically active surface area and high density of surface-active sites, disclosing a high electrocatalytic nitrate-to-ammonia capability, with an optimal NH3 yield rate of 0.2238 mmol h-1 cm-2 and a corresponding Faradaic efficiency of 94.43%. This study may provide a guiding design avenue for atomic arrangement engineering of metallic nanocrystals via electrochemical postmodification for nitrate reduction reaction and other energy conversion electrocatalysis.

Cr3C2 nanoparticles decorated carbon nanofibers for efficient nitrate reduction to ammonia at ambient conditions

Electrochemical nitrate (NO₃^-) reduction is a promising approach to relieve nitrate pollution and produce value-added ammonia (NH₃), but efficient and durable catalysts are required due to the large bond dissociation energy of nitrate and low selectivity. Herein, we propose chromium carbide (Cr₃C₂) nanoparticles loaded carbon nanofibers (Cr₃C₂@CNFs) as electrocatalysts to convert nitrate to ammonia. In phosphate buffer saline containing 0.1 mol L^-1 NaNO₃, such catalyst achieves a large NH₃ yield of 25.64 mg h^-1 mg^-1_cat. and a high faradaic efficiency of 90.08% at -1.1 V vs the reversible hydrogen electrode, which also shows excellent electrochemical durability and structural stability. Theoretical calculations reveal the adsorption energy for nitrate at Cr₃C₂ surfaces reaches -1.92 eV and the potential determining step (*NO→*N) for Cr₃C₂ hits a low energy increase of 0.38 eV.