Efficient reduction of nitrite and nitrate to ammonia using thin-film B-doped diamond electrodes

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.

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.

Electrochemical reduction of nitrate in a catalytic carbon membrane nano-reactor

Nitrate pollution is a critical environmental issue in need of urgent addressing. Electrochemical reduction is an attractive strategy for treating nitrate due to the environmental friendliness. However, it is still a challenge to achieve the simultaneous high activity and selectivity. Here we report the design of a porous tubular carbon membrane as the electrode deposited with catalysts, which provides a large triple-phase boundary area for nitrate removal reactions. The achieved nitrate removal rate is one order of magnitude higher than other literatures with high nitrate conversion and high selectivity of nitrogen. The carbon membrane itself had a limited catalytic property thus Cu-Pd bimetal catalysts were deposited inside the nano-pores to enhance the activity and selectivity. When Na₂SO₄ electrolyte was applied, the achieved single-pass removal of nitrate was increased from 55.15% (for blank membrane) to 97.12% by adding catalysts inside the membrane. In case of NaOH as the electrolyte, the single-pass nitrate removal efficiency, selectivity to nitrogen formation and nitrate removal rate was 90.66%, 96.40% and 1.47 × 10^-3 mmol min^-1 cm^-2, respectively. Density functional theory studies demonstrate that the loading of bimetal catalysts compared with single metal catalysts enhances the adsorption of *NO₃ on membrane surface favorable for N₂ formation than NH₃ on Cu-Pd surface. The application of catalytic carbon membrane nano-reactors can open new windows for nitrate removal due to the high reactor efficiency.

Electrochemical reduction of nitrate on boron-doped diamond electrodes: Effects of surface termination and boron-doping level

This study is among the first to systematically study the electrochemical reduction of nitrate on boron-doped diamond (BDD) films with different surface terminations and boron-doping levels. The highest nitrate reduction efficiency was 48% and the highest selectivity in the production of nitrogen gas was 44.5%, which were achieved using a BDD electrode with a hydrogen-terminated surface and a B/C ratio of 1.0%. C-H bonds served as the anchor points for attracting NO₃^- anions close to the electrode surface, and thus accelerating the formation of NO₃^-_(ads). Compared to oxygen termination, hydrogen-terminated BDD exhibited higher electrochemical reactivity for reducing nitrate, resulting from the formation of shallow acceptor states and small interfacial band bending. The hydrophobicity of the hydrogen-terminated BDD inhibited water electrolysis and the subsequent adsorption of atomic hydrogen, leading to increased selectivity in the production of nitrogen gas. A BDD electrode with a boron-doping level of 1.0% increased the density of acceptor states, thereby enhancing the conductivity and promoting the formation of C-H bonds after the cathodic reduction pretreatment leading to the direct reduction of nitrate.

Combination of Pd-Cu Catalysis and Electrolytic H2 Evolution for Selective Nitrate Reduction Using Protonated Polypyrrole as a Cathode

Pd-Cu catalysis is combined with in situ electrolytic H₂ evolution for NO₃^- reduction with protonated polypyrrole (PPy) as a cathode. The surface of PPy is not only beneficial for H₂ evolution, but exclusive for NO₃^- adsorption, and thus inhibits NO₃^- reduction. Meanwhile, the in situ H₂ generation exhibits a much higher utilization efficiency because of the smaller bubble size and higher dispersion. The Pd-Cu catalysts with the ratios of 6:1 and 4:1 exhibit the highest NO₃^--N removal (100%) and N₂ selectivity (93-95%) after 90 min. In comparison with the results obtained with other cathode materials (Ti, Cu, Co₃O₄, and Fe₂O₃) and obtained by other researchers, the new process shows a faster NO₃^--N reduction rate and much higher N₂ selectivity. However, the O₂ generated on the anode can oxidize Cu to Cu₂O that may work as the catalyst for NO₃^--N reduction to NH₄⁺-N by H₂, resulting in more than 60% NH₄⁺-N generated without a proton exchange membrane. Both the PPy film and Pd-Cu catalyst exhibit good stability and there is no Cu²⁺ or Pd²⁺ in solution after reaction. Real industrial wastewater is further treated in this system, the NO₃^--N is reduced from 670 mg L^-1 to less than 100 mg L^-1 in 90 min, and only little amount of NH₄⁺-N is generated.

Comparison of performance between boron-doped diamond and copper electrodes for selective nitrogen gas formation by the electrochemical reduction of nitrate

The electrochemical nitrate reduction by using boron-doped diamond (BDD) and copper (Cu) electrodes was investigated at various potentials. Product selectivity of nitrate reduction was strongly dependent on the applied potential for both electrodes. The highest selectivity of nitrogen gas production was obtained at -2.0 V (vs. Ag/AgCl) by using a BDD electrode with a faradaic efficiency as high as 45.2%. Compared with Cu electrode, nitrate reduction on BDD electrode occurred at more positive potential, and the production of nitrogen gas was larger. The transformation of surface-adsorbed nitrate into molecular nitrogen would be accelerated on BDD electrode with hindering nitrite production. In addition, low concentration of surface-adsorbed hydrogen on the BDD would also retard the ammonia generation, leading to increase in the selectivity of nitrogen gas formation. Meanwhile, BDD electrode could hinder the hydrogen evolution reaction, which enhanced the efficiency for nitrate reduction and decreased energy consumption. BDD electrode has excellent stability to remain better performance for reducing nitrate during electrolysis without any variation of surface morphology or chemical components.

Investigation of a Cu/Pd Bimetallic System Electrodeposited on Boron-Doped Diamond Films for Application in Electrocatalytic Reduction of Nitrate

The Cu/Pd bimetallic system electrodeposited on boron-doped diamond (BDD) films for application, as electrode material in the electrochemical reduction of nitrate was studied. The electrochemical behavior of Cu, Pd, and Cu/Pd bimetallic system was evaluated by cyclic voltammetry. From these results, the formation of the Cu/Pd composite was verified. In addition, Cu with different phases and a Cu/Pd phase in the composite were obtained. Morphological analysis by scanning electron microscopy (SEM) revealed a homogeneous distribution of Cu/Pd bimetallic particles with intermediary dimensions compared to those observed in Cu or Pd electrodeposits separately. These composites were tested as electrocatalysts for nitrate reduction in Britton-Robinson buffer solution (pH 9). Electrochemical measurements showed that composites with higher Cu content displayed the best electrocatalytic activity for nitrate reduction, and the Cu/Pd phase in the bimetallic system served to improve the Cu adherence on BDD electrode.

Simultaneous determination of nitrate and dissolved oxygen under neutral conditions using a novel silver-deposited gold microelectrode

In this work a novel gold-based microelectrode was successfully fabricated using photolithographic techniques and electrochemical deposition for a simultaneous determination of nitrate and dissolved oxygen (DO) under neutral conditions. Three-dimensional tree-shaped silver nanorods were formed on the gold surface through electrochemical deposition and they had an electrochemical catalytic reductive activity for both nitrate and oxygen under neutral conditions. Thus, the silver nanorods served as the active center of the microelectrode. The microelectrode could be renewed over five times. Linear sweep voltammetry was employed to quantitatively analyze the nitrate and DO in solution. The microelectrode was used to measure the nitrate and DO microprofiles in a nitrifying aerobic granule from a sequencing batch reactor, which shows that denitrification did not occur in the tested granule. The measurement results demonstrate that the microelectrode was able to simultaneously determine the nitrate and DO levels in the granules under neutral conditions accurately and precisely.