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

Electrocatalysts for Inorganic and Organic Waste Nitrogen Conversion

Anthropogenic activities have disrupted the natural nitrogen cycle, increasing the level of nitrogen contaminants in water. Nitrogen contaminants are harmful to humans and the environment. This motivates research on advanced and decarbonized treatment technologies that are capable of removing or valorizing nitrogen waste found in water. In this context, the electrocatalytic conversion of inorganic- and organic-based nitrogen compounds has emerged as an important approach that is capable of upconverting waste nitrogen into valuable compounds. This approach differs from state-of-the-art wastewater treatment, which primarily converts inorganic nitrogen to dinitrogen, and organic nitrogen is sent to landfills. Here, we review recent efforts related to electrocatalytic conversion of inorganic- and organic-based nitrogen waste. Specifically, we detail the role that electrocatalyst design (alloys, defects, morphology, and faceting) plays in the promotion of high-activity and high-selectivity electrocatalysts. We also discuss the impact of wastewater constituents. Finally, we discuss the critical product analyses required to ensure that the reported performance is accurate.

Nitrate Sensor with a Wide Detection Range and High Stability Based on a Cu-Modified Boron-Doped Diamond Electrode

This paper describes an electrochemical sensor based on a Cu-modified boron-doped diamond (BDD) electrode for the detection of nitrate-contaminated water. The sensor utilizes the catalytic effect of copper on nitrate and the stability of the BDD electrode. By optimizing the electrolyte system, the linear detection range was expanded, allowing the sensor to detect highly concentrated nitrate samples up to 100 mg/L with a low detection limit of 0.065 mg/L. Additionally, the stability of the sensor was improved. The relative standard deviation of the current responses during 25 consecutive tests was only 1.03%. The wide detection range and high stability of the sensor makes it suitable for field applications and the on-site monitoring of nitrate-contaminated waters.

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.

Nitrate removal from groundwater using a batch and continuous flow hybrid Fe-electrocoagulation and electrooxidation system

During the last two decades nitrate contaminated groundwater has become an extensive worldwide problem with wide-reaching negative effects on human health and the environment. In this study, a combination of electrocoagulation (EC) and electrooxidation (EO) was studied as a denitrification process to efficiently remove nitrates and ammonium (a by-product produced during EC) from real polluted groundwater. Initially, EC experiments under batch operating mode were performed using iron electrodes at different applied current density values (20-40 mA cm^-2). Nitrate percentage removal of 100 % was recorded, however high ammonium concentrations were performed (4.5-6.5 mg NH₄⁺-Ν L^-1). Therefore, a continuous flow system was examined for the complete removal of both nitrates and EC-generated ammonium cations. The system comprised an EC reactor, a settling tank and an EO reactor. The applied current densities to the EC process were the same as those in the batch experiments, while the volumetric flow rates were 4, 6 and 8 mL min^-1. Regarding the current density of the EO process was kept constant at the value of 75 mA cm^-2. The percentage nitrate removal recorded during the EC process ranged between 52.0 and 100 %, while the NH₄⁺-N concentration at the outlet of the EO reduced significantly (53-100 %) depending on the applied current density and the volumetric flow rate. Also, the dissolved iron concentration in the treated water was always below the legislated limit of 0.2 mg L^-1 (up to 0.027 mg L^-1). These results indicate that the proposed hybrid system is capable of denitrifying real nitrate contaminated groundwater without generating toxic by-products, therefore making the water suitable for human consumption.

Dual-site electrocatalytic nitrate reduction to ammonia on oxygen vacancy-enriched and Pd-decorated MnO2 nanosheets

Electrocatalytic nitrate reduction (NRR) represents one promising alternative to the Haber-Bosch process for NH₃ production due to the lower reaction energy barrier compared to N₂ reduction and the potential recycling of nitrogen source from nitrate wastewater. The metal oxides with oxygen vacancy (O_v) display high NH₃ selectivities in NRR (NO₂^-/N₂ as side products), but the complexity in O_v enrichment and the inferior hydrogen adsorption on oxides make NRR an inefficient process. Herein, one superior dual-site NRR electrocatalyst that is composed of O_v-enriched MnO₂ nanosheets (MnO₂-O_v) and Pd nanoparticles (deposited on MnO₂) is constructed over the three-dimensional porous nickel foam (Pd-MnO₂-O_v/Ni foam). In a continuous-flow reaction cell, this electrode delivers a NO₃^--N conversion rate of 642 mg N m^-2_electrode h^-1 and a NH₃ selectivity of 87.64% at -0.85 V vs. Ag/AgCl when feeding 22.5 mg L^-1 of NO₃^--N (0.875 mL min^-1), outperforming the Pd/Ni foam (369 mg N m^-2_electrode h^-1, 85.02%) and MnO₂-O_v/Ni foam (118 mg N m^-2_electrode h^-1, 32.25%). Increasing the feeding NO₃^--N concentration and flow rate to 180.0 mg L^-1 and 2.81 mL min^-1 can further lift the conversion rate to 1933 and 1171 mg N m^-2_electrode h^-1, respectively. The combination of experimental characterizations and theoretical calculations reveal that the MnO₂-O_v adsorbs, immobilizes, and activates the NO₃^- and N-intermediates, while the Pd supplies the O_v sites with sufficient adsorbed hydrogen (H*) for both the NRR and O_v refreshment. Our work presents a good example of utilizing dual-site catalysis in the highly selective conversion of NO₃^- to NH₃ that is important for nitrate pollution abatement, nitrogen resource recycling, as well as sustainable NH₃ production.

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.

PEM Electrolysis-Assisted Catalysis Combined with Photocatalytic Oxidation towards Complete Abatement of Nitrogen-Containing Contaminants in Water

Electrolysis-assisted nitrate (NO₃ ^- ) reduction is a promising approach for its conversion to harmless N₂ from waste, ground, and drinking water due to the possible process simplicity by in-situ generation of H₂ /H/H⁺ by water electrolysis and to the flexibility given by tunable redox potential of electrodes. This work explores the use of a polymer electrolyte membrane (PEM) electrochemical cell for electrolysis-assisted nitrate reduction using SnO₂ -supported metals as the active cathode catalysts. Effects of operation modes and catalyst materials on nitrate conversion and product selectivity were studied. The major challenge of product selectivity, namely complete suppression of nitrite (NO₂ ^- ) and ammonium (NH₄ ⁺ ) ion formation, was tackled by combining with simultaneous photocatalytic oxidation to drive the overall reaction towards N₂ formation.

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.

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.

Electrochemical nitrate removal with simultaneous magnesium recovery from a mimicked RO brine assisted by in situ chloride ions

Electrochemical reduction is effective to remove nitrate but byproducts such as ammonia and nitrite would need chloride addition for indirect oxidation to nitrogen gas. Herein, electrochemical nitrate reduction was investigated to remove nitrate from a mimicked reverse osmosis (RO) brine containing chloride that eliminates the need for external chloride addition. Both Cu/Zn and Ti nano cathodes exhibited the best performance of nitrate removal with >97 % removal in either Na₂SO₄ or NaCl electrolyte, although with different products. Complete nitrate reduction to nitrogen gas was realized in the RO brine whose complex composition decreased the electrode efficiency, for example from 71.4 ± 0.2%-49.4 ± 0.3 % with the Cu/Zn cathode after 5 cycles of operation. Magnesium was recovered at the same time of nitrate removal and the purity of Mg(II) could reach 96.8 ± 2.0 % after proper pH pre-treatment. In a preliminary adsorption study, a key byproduct - chlorate was reduced by 49.8 ± 2.7 % after 3-h adsorption by 100 g L^-1 activated carbon. These results have demonstrated the simultaneous electrochemical nitrate removal and resource recovery from a complex water like a RO brine and provided new information such as byproduct management and electrode deterioration.

Differential control of anode/cathode potentials of paired electrolysis for simultaneous removal of chemical oxygen demand and total nitrogen

Paired electrolysis can take advantage of both anodic oxidation and cathodic reduction, and thus improve current efficiency for electrochemical wastewater treatment. In this work, differential control of anode/cathode potentials of paired electrolysis for simultaneous removal of chemical oxygen demand (COD) and total nitrogen (TN, including ammonia, nitrate, and nitrite) was studied. We first determined the optimal potentials for anodic oxidation of COD/NH₄⁺ or cathodic reduction of NO₃^-/NO₂^- (minimization of over-oxidation or over-reduction) by preliminary cyclic voltammetry and constant-potential electrolysis experiments, i.e., 1.6 V for anodic oxidation and -1.26 V for cathodic reduction in this case. The optimal working potential of the cathode was achieved at appropriate current density in the paired electrolysis system, the working potential of the anode was independently controlled by adjusting the ratio of its surface area to that of the cathode. In this way, both the cathode and anode could work under optimal potentials. At an optimized cathodic current density of 5.0 mA cm^-2 and cathode/anode surface area ratio of 2:1, the removal efficiencies of COD and TN from simulated wastewater reached 91.9% and 86.2%, respectively. Additionally, the developed paired electrolysis system was validated by treating an actual pharmaceutical wastewater, results for which showed that a total current efficiency of 84.8% was achieved, which was at least twice as high as that of traditional electrochemical processes.

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

Improving surface selectivity and maximizing electrode surface area are critical needs for the electroreduction of nitrate. Herein, preferential (100) oriented Pt nanoflowers with an extended surface area were prepared by potentiostatic deposition on carbon cloth (Pt NFs/CC), and then Cu atoms were adsorbed on the Pt NFs (Cu/Pt NFs/CC) for application of nitrate electroreduction. The results reveal that Cu/Pt NFs/CC with 8.7% Cu coverage exhibits a high selectivity for nitrate electroreduction to N2 following two steps: Nitrate firstly converts into nitrite on Cu sites adsorbed on Pt NFs, then nitrite subsequently selective reduction and ammonia oxidation to N2 occur on the large exposed (100) terraces in Pt NFs. In addition, electrocatalytic activity and selectivity of nitrate reduction strongly rely on the Cu surface coverage on Pt NFs, the lower activity of nitrate reduction is displayed with increase of Cu coverage. Accordingly, the selective reduction of nitrate to N2 is feasible at such nanostructured Pt nanoflowers modified with Cu.

Sonocatalytic reduction of nitrate using magnetic layered double hydroxide: Implications for removal mechanism

In this study, magnetic layered double hydroxides (mag-LDHs) were synthesized through compositing magnetite with three different metals (Mg, Cu and Al) under ultrasound (US, 100 kHz frequency and 50 W power). For the first time, mag-LDHs were applied to sonocatalytic reduction of nitrate (NO₃^-) and the reduction mechanism were determined by conducting kinetic tests and various spectroscopic analyses. Based on the kinetic data, NO₃^- reduction and the selectivity for N₂ highly depends on the ratio between Mg/Al, solution pH and sonication frequency. The best condition for sonocatalytic denitrification was found to be pH 7 operated under 100 kHz (50% power) using the catalyst with lowest amount of Al (mag-LDH-Al_0.3Mg_1.5). As a proposed mechanism, NO₃^- is initially reduced to NO₂^- by Cu⁰, and then further reduced to N₂/NH₄⁺ by Mg⁰. Hypothetically Al⁰ could provide sorption sites for hydrogen radicals (·H) dissociated from ultrasound, hence served as reducing sites in denitrification process. The XPS analysis showed an increased peak of Cu⁰ after the sonocatalytic reduction when catalyst has lower amount of Al. The excessive hydrogen adsorbed on Al⁰ might spill-over to the adjacent Cu, thus reducing the CuO into Cu⁰ at high temperature created by the implosion of the microbubbles. Without the use of consumable reducing agents (i.e. H₂ gas), sonocatalytic reduction could be a potential candidate of remediation method to treat NO₃^- polluted water with high N₂ selectivity and easy magnetic recovery.