Electrochemical behaviors and influence factors of copper and copper alloys cathode for electrocatalytic nitrate removal

Modulating the Active Hydrogen Adsorption on Fe─N Interface for Boosted Electrocatalytic Nitrate Reduction with Ultra-Long Stability

The electrocatalytic reduction of nitrate (NO3 - ) to nitrogen (N2 ) is an environmentally friendly approach for efficient N-cycle management (toward a nitrogen-neutral cycle). However, poor catalyst durability and the competitive hydrogen evolution reaction significantly impede its practical application. Interface-chemistry engineering, utilizing the close relationship between the catalyst surface/interface microenvironment and electron/proton transfer process, has facilitated the development of catalysts with high intrinsic activity and physicochemical durability. This study reports the synthesis of a nitrogen-doped carbon-coated rice-like iron nitride (RL-Fe2 N@NC) electrocatalyst with excellent electrocatalytic nitrate-reduction reaction activity (high N2 selectivity (≈96%) and NO3 - conversion (≈86%)). According to detailed mechanistic investigations by in situ tests and theoretical calculations, the strong hydrogenation ability of iron nitride and enhanced nitrate enrichment of the system synergistically contribute to the rapid hydrogenation of nitrogen-containing species, increasing the intrinsic activity of the catalyst and reducing the occurrence of the competing hydrogen-evolution side reaction. Moreover, RL-Fe2 N@NC shows excellent stability, retaining good NO3 - -to-N2 electrocatalysis activity for more than 40 cycles (one cycle per day). This paper could guide the interfacial design of Fe-based composite nanostructures for electrocatalytic nitrate reduction, facilitating a shift toward nitrogen neutrality.

Research on the electrochemical treatment of nitrobenzene wastewater: The effects of process parameters and the mechanism of distinct degradation pathways

Nitrobenzene is a typical organic pollutant of petroleum pollutant, which is a synthetic chemical not found naturally in the environment. Nitrobenzene in environment can cause toxic liver disease and respiratory failure in humans. Electrochemical technology provides an effective and efficient method for degrading nitrobenzene. This study, the effects of process parameter (e.g., electrolyte solution type, electrolyte concentration, current density and pH) and distinct reaction pathways for electrochemical treatment of nitrobenzene were investigated. As a result, available chlorine dominates the electrochemical oxidation process compared with hydroxyl radical, thus the electrolyte of NaCl is more suitable for the degradation of nitrobenzene than that of Na2SO4. The concentration and the existence form of available chlorine were mainly controlled by electrolyte concentration, current density and pH, which directly affect the removal of nitrobenzene. Cyclic voltammetry and mass spectrometric analyses suggested that electrochemical degradation of nitrobenzene included two important ways. Firstly, single oxidation: nitrobenzene → other forms of aromatic compounds→ NO-x + organic acids + mineralization products. Secondly, coordination of reduction and oxidation: nitrobenzene → aniline→ N2 + NO-x + organic acid + mineralization products. The results of this study will encourage us to further understand the electrochemical degradation mechanism of nitrobenzene and develop the efficient processes for nitrobenzene treatment.

Advances in iron-based electrocatalysts for nitrate reduction

Excessive nitrate has been a critical issue in the water environment, originating from the burning of fossil fuels, inefficient use of nitrogen fertilizers, and discharge of domestic and industrial wastewater. Among the effective treatments for nitrate reduction, electrocatalysis has become an advanced technique because it uses electrons as green reducing agents and can achieve high selectivity through cathode potential control. The effectiveness of electrocatalytic nitrate reduction (NO₃RR) mainly lies in the electrocatalyst. Iron-based catalysts have the advantages of high activity and low cost, which are well-used in the field of electrocatalytic nitrates. A comprehensive overview of the electrocatalytic mechanism and the iron-based materials for NO₃RR are given in terms of monometallic iron-based materials as well as bimetallic and oxide iron-based materials. A detailed introduction to NO₃RR on zero valent iron, single-atom iron catalysts, and Cu/Fe-based bimetallic electrocatalysts are provided, as they are essential for the improvement of NO₃RR performance. Finally, the advantages of iron-based materials for NO₃RR and the problems in current applications are summarized, and the development prospects of efficient iron-based catalysts are proposed.

Low-nitrite generation Cu-Co/Ti cathode materials for electrochemical nitrate reduction

In order to reduce by-product nitrite, a more toxic compound than nitrate, and increase high value-added products ammonia in the electrochemical reduction nitrate process, the novel Cu-Co/Ti cathode material was applied in this process. In this paper, the electrochemical process was carried out in a single compartment electrolytic cell, and with Cu-Co/Ti electrode as cathode, identifying the effects of current density, pH, electrolytes in the nitrate reduction, and the distribution of products. The Cu-Co/Ti cathode exhibited 94.65% NO₃^--N (nitrate-N) removal, 0.18% NO₂^--N (nitrite-N) generation, and 40.86% NH₄^--N (ammonia-N) generation with the assistance of Na₂SO₄ electrolyte in 6 h at 10 mA cm^-2 and pH 6. Compared with the Cu/Ti cathode, the higher nitrate removal ratio and lower nitrite generation ratio were obtained on the Cu-Co/Ti cathode. The excellent performance of Cu-Co/Ti cathode is ascribed to the synergy of Cu and Co, which couples the facilitation of nitrate conversion to nitrite and the acceleration of nitrite reduction on the Cu-Co/Ti cathode. The LSV curves showed that nitrate and nitrite might undergo indirect and direct reduction reactions on Cu-Co/Ti cathode. The possible pathways of nitrate reduction on the Cu-Co/Ti cathodes were proposed. These results highlight the viability of using the Cu-Co/Ti cathode developed at this work for the nitrate removal from contaminated waters. This study achieved low-nitrite generation by Cu-Co/Ti cathode during electrochemical nitrate reduction.

Comparing the effects of Cu-Ti/RuO2-IrO2 electrode configuration on the electro-reduction of nitrate

Nitrate pollution is a global problem as it affects both the environment and human health. The objective of this research was to study the effect of electrode configuration on the electro-reduction of nitrate. Coaxial cylindrical (inner rod and outer tube copper cathodes) and vertical plate parallel copper cathodes paired with Ti/RuO₂-IrO₂ (rod, tube, and plate) configurations were studied under various current densities and initial nitrate concentrations. The efficiency of each configuration was determined based on the removal efficiency of nitrate, specific energy consumption, mass transfer coefficients, and first order rate constants. Additionally, the overall systems' resistance and geometric factors are discussed. It was found that the performances of the inner rod and outer tube copper cathodes were similar. The vertical plate parallel configuration was superior to the coaxial cylindrical electrode setup, as evident from a higher maximum nitrate removal of 74 and 56% at a current density of 7 mA/cm² and lower energy consumption of 46.7 × 10^-3 and 54.3 × 10^-3 kWh/mmol NO₃^- at 36.4 mA/cm², respectively. In addition, the mass transfer coefficients and first order rate constants were higher in all conditions tested for the vertical plate parallel configuration.

Iron-Based Nanocatalysts for Electrochemical Nitrate Reduction

Nitrate has a high level of stability and persistence in water, endangering human health and aquatic ecosystems. Due to its high reliability and efficiency, the electrochemical nitrate reduction reaction (NO₃ RR) is regarded as the best available option for mitigating excess nitrate in water and wastewater, especially for the removal of trace levels of nitrate. One of the most critical factors in the electrochemical reduction are the catalysts, which directly affect the reaction efficiency of nitrate removal. Iron-based nanocatalysts, which have the advantages of nontoxicity, wide availability, and low cost, have emerged as a promising electrochemical NO₃ RR material in recent years. This review covers major aspects of iron-based nanocatalysts for electrochemical NO₃ RR, including synthetic methods, structural design, performance enhancement, electrocatalytic nitrate reduction test, and reduction mechanism. The recent progress of iron-based nanocatalysts for electrochemical NO₃ RR and the mechanism of functional advantages for modified structures are reviewed from the perspectives of loading, doping, and assembly strategies, in order to realize the conversion from pollutant nitrate to harmless nitrogen or ammonia and other sustainable products. Finally, challenges and future directions for the development of low-cost and highly-efficient iron-based nanocatalysts are explored.

Boosting electrochemical nitrite-ammonia conversion properties by a Cu foam@Cu2O catalyst

Electrocatalytic reduction of nitrite (NO₂^-) to ammonia (NH₃) can simultaneously achieve wastewater treatment and ammonia production, but it needs efficient catalysts. Herein, Cu₂O particles self-supported on Cu foam with enriched oxygen vacancies are developed to enable selective NO₂^- reduction to NH₃, exhibiting a maximum NH₃ yield rate of 7510.73 μg h^-1 cm^-2 and high faradaic efficiency of 94.21% at -0.6 V in 0.1 M PBS containing 0.1 M NaNO₂. Density functional theory calculations reveal the vital role of oxygen vacancies during the nitrite reduction process, as well as the reaction mechanisms and the potential limiting step involved. This work provides a new avenue to the rational design of Cu-based catalysts for NH₃ electrosynthesis.

Electrochemical nitrate reduction of brines: Improving selectivity to N2 by the use of Pd/activated carbon fiber catalyst

Contamination of water by nitrate has become a worldwide problem, being high levels of this ion detected in the surface, and groundwater, mainly due to the intensive use of fertilizers, and to the discharge of not properly treated effluents. This study aims to evaluate the electrocatalytic process, carried out in a cell divided into two compartments by a cation exchange membrane, and with a copper plate electrode as cathode, identifying the effects of current density, pH, the use of a catalyst in the nitrate reduction, and the production of gaseous compounds. The highest nitrate reduction was obtained with a current density of 2.0 mA cm^-2, without pH adjustment and, in this condition, nitrite ion was mainly formed. The application of activated carbon fibers with palladium (1% wt. and 3% wt.) in an alkaline medium presented an increase in gaseous compounds formation. With 2.0 mA cm^-2, pH adjustment, and applying 3% wt. Pd catalyst, the highest selectivity to gaseous compounds was obtained (95%) with no nitrite detection. These results highlight the viability of using the process developed at this work for the treatment of nitrate contaminated waters.

Nitrate reduction by electrochemical processes using copper electrode: evaluating operational parameters aiming low nitrite formation

This work aims to present different electroreduction and electrocatalytic processes configurations to treat nitrate contaminated water. The parameters tested were: current density, cell potential, electrode potential, pH values, cell type and catalyst use. It was found that the nitrite ion is present in all process variations used, being the resulting nitrite concentration higher in an alkaline pH. The increase in current density on galvanostatic operation mode provides a greater reduction of nitrate (64%, 1.4 mA cm^-2) if compared to the potentiostatic (20%) and constant cell potential (37%) configurations. In a dual-chamber cell the nitrate reduction with current density of 1.4 mA cm^-2 was tested and obtained as a NO₃^- reduction of 85%. The use of single chamber cell presented 32 ± 3% of nitrate reduction, indicating that in this cell type the nitrate reduction is smaller than in dual-chamber cell (64%). The presence of a Pd catalyst with 3.1% wt. decreased the nitrite (1.0 N-mg L^-1) and increased the gaseous compounds (9.4 N-mg L^-1) formation. The best configuration showed that, by fixing the current density, the highest nitrate reduction is obtained and the pH presents a significant influence during the tests. The use of the catalyst decreased the nitrite and enhanced the gaseous compounds formation.

Recent discoveries in the reaction mechanism of heterogeneous electrocatalytic nitrate reduction

We review advances in the electrocatalytic nitrate reduction mechanism and evaluate future efforts. Existing work could be supplemented by controlling reaction conditions and quantifying active sites to determine activity on a per-site basis.

Efficient Nitrate Reduction over Novel Covalent Ag-Salophen Polymer-Derived "Vein-Leaf-Apple"-like Ag@Carbon Structures

Efficient electrocatalysts for nitrate reduction reaction (NO₃^--RR) that could selectively transfer nitrate into harmless nitrogen are required for water-denitrification treatment. The most widely used electrodes for NO₃^--RR including noble metals, transition metals, and their alloys still face many challenges such as lower selectivity and efficiency, high cost, and easy corrosion properties. Metallic Ag with acceptable cost possesses strong corrosion resistance in electrolysis, but its activity is often incompetent for NO₃^--RR. In this work, Ag nanoparticles with a lower loading content (1.99 wt %) on a nitrogen-doped carbon support was successfully used as the robust electrocatalyst for NO₃^--RR in a Cl^--free neutral solution. This Ag@carbon catalyst exhibited an impressive electrochemical performance for NO₃^--RR, with a NO₃^--N conversion yield of 53% and a N₂-N selectivity of 97% at a low electrolysis overpotential (-0.29 V vs RHE). In particular, the prepared Ag@carbon showed better stability and no secondary Ag ion pollution in electrolysis. Its impressive electrocatalytic performance was attributed to the unique "vein-leaf-apple"-like Ag@carbon structures, prepared by thermal conversion of Ag-salophen polymers@CNTs. CNTs served as veins to enhance the electron transportation in electrocatalysts. Salophen polymer-derived mesoporous N-doped carbon plates acted as leaves to concentrate NO₃^- from the electrolyte. Like apples on trees, Ag nanoparticles of about 10-20 nm highly dispersed on carbons selectively converted NO₃^--N into N₂-N. It opens up a cost-acceptable and corrosion-resistant Ag-less electrocatalytic pathway for NO₃^--RR, and the special "vein-leaf-apple"-like Ag@carbon structure could enhance the electrolytic efficiency and N₂-N selectivity for NO₃^--RR.