Electrosynthesis of hydrogen peroxide in a filter-press flow cell using graphite felt as air-diffusion cathode

Defective nitrogen doped carbon material derived from nano-ZIF-8 for enhanced in-situ H2O2 generation and tetracycline hydrochloride degradation in electro-Fenton system

Tetracycline hydrochloride (TC) accumulates in large quantities in the water environment, causing a serious threat to human health and ecological environment safety. This research focused on developing cost-effective catalysts with high 2e- selectivity for electro-Fenton (EF) technology, a green pollution treatment method. Defective nitrogen-doped porous carbon (d-NPC) was prepared using the metal-organic framework as the precursor to achieve in-situ H2O2 production and self-decomposition into high activity ·OH for degradation of TC combined with Co2+/Co3+. The d-NPC produced 172.1 mg L-1 H2O2 within 120 min, and could degrade 96.4% of TC in EF system. The self-doped defects and graphite-nitrogen in d-NPC improved the oxygen reduction performance and increased the H2O2 yield, while pyridine nitrogen could catalyze H2O2 to generate ·OH. The possible pathway of TC degradation was also proposed. In this study, defective carbon materials were prepared by ball milling, which provided a new strategy for efficient in-situ H2O2 production and the degradation of pollutants.

Recent Advances of Electrocatalyst and Cell Design for Hydrogen Peroxide Production

Electrochemical synthesis of H2O2 via a selective two-electron oxygen reduction reaction has emerged as an attractive alternative to the current energy-consuming anthraquinone process. Herein, the progress on electrocatalysts for H2O2 generation, including noble metal, transition metal-based, and carbon-based materials, is summarized. At first, the design strategies employed to obtain electrocatalysts with high electroactivity and high selectivity are highlighted. Then, the critical roles of the geometry of the electrodes and the type of reactor in striking a balance to boost the H2O2 selectivity and reaction rate are systematically discussed. After that, a potential strategy to combine the complementary properties of the catalysts and the reactor for optimal selectivity and overall yield is illustrated. Finally, the remaining challenges and promising opportunities for high-efficient H2O2 electrochemical production are highlighted for future studies.

A novel three-dimensional flow-through graphite felt-matrix cathode for in-situ hydrogen peroxide generation in multi-environment systems-Multiphysics modeling for in-situ hydrogen peroxide generation

In order to achieve in-situ H₂O₂ generation in multi-environment systems, polyvinylpyrrolidone (PVP) and modified carbon nitride (t-g-C₃N₄) are co-doped onto graphite felt-matrix (GF-matrix) by electrodeposition to develop a novel cathode electrode. By means of 3D-X-ray CT, High-Resolution Transmission Electron Microscope (HRTEM), X-ray Photoelectron Spectrometer (XPS), Raman and Electrochemical Workstation, microscopic physical-chemical properties of materials are researched to optimize the electrode structure. Results show that the optimal electrode presents over H₂O₂ production rate of 2000 mgL^-1·h^-1, and as high as current efficiency of 93% to 98% in simulated freshwater (50 mM Na₂SO₄, pH = 1-12) at 20 mAcm^-2. Furthermore, we built an original three-dimensional (3D) flow-through GF-matrix cathode model on H₂O₂ generation in simulated freshwater, explaining solution pH change reasons from solution inlet to outlet.

Electrochemical Reactors for Continuous Decentralized H2 O2 Production

The global utilization of H₂ O₂ is currently around 4 million tons per year and is expected to continue to increase in the future. H₂ O₂ is mainly produced by the anthraquinone process, which involves multiple steps in terms of alkylanthraquinone hydrogenation/oxidation in organic solvents and liquid-liquid extraction of H₂ O₂ . The energy-intensive and environmentally unfriendly anthraquinone process does not meet the requirements of sustainable and low-carbon development. The electrocatalytic two-electron (2 e^- ) oxygen reduction reaction (ORR) driven by renewable energy (e.g. solar and wind power) offers a more economical, low-carbon, and greener route to produce H₂ O₂ . However, continuous and decentralized H₂ O₂ electrosynthesis still poses many challenges. This Minireview first summarizes the development of devices for H₂ O₂ electrosynthesis, and then introduces each component, the assembly process, and some optimization strategies.

Hydroxyl Radical Production via a Reaction of Electrochemically Generated Hydrogen Peroxide and Atomic Hydrogen: An Effective Process for Contaminant Oxidation?

An electrochemical advanced oxidation process (EAOP) is demonstrated with a catalytic cathode capable of simultaneously catalyzing the hydrogen evolution reaction (HER) and the oxygen reduction reaction (ORR) with resultant in situ generation of atomic hydrogen (H*) and hydrogen peroxide (H₂O₂). A palladium-coated carbon-PTFE gas diffusion electrode (Pd/C GDE) was used as a catalytic cathode with hydroxyl radical (^•OH) formed as a result of the reaction of electrogenerated H* with H₂O₂. As both the HER and ORR can be induced to occur at the same cathode, the H*/GDE process results in more effective degradation of organic contaminants than can be achieved by a conventional H*/H₂O₂ process involving direct addition of H₂O₂. At circumneutral pH, 82.7% of added formate was degraded after 2 h treatment at an applied potential of -1.0 V vs Ag/AgCl with relatively low concentrations of generated H₂O₂ remaining in the solution. We also show that H* and H₂O₂ (and thus ^•OH) can be electrogenerated effectively over a wide range of pH (3.2-7.0). These results suggest that by in situ generation of H* and H₂O₂, the H*/GDE process is able to produce significant amounts of ^•OH without external chemical addition and thus offers an alternative method for abatement of aqueous organic contaminants.

A novel UV-LED hydrogen peroxide electrochemical photoreactor for point-of-use organic contaminant degradation

The degradation of organic contaminants is typically achieved by exposure of hydrogen peroxide (H₂O₂) containing influent to ultraviolet (UV) lamps as the source of radiation that can convert H₂O₂ to hydroxyl radicals (·OH), which oxidize organic pollutants. However, two factors prevent this process from being scaled down: the need to introduce H₂O₂, which requires special handling, and the use of bulky UV lamps, which have a high electric power consumption. In this work, an electrochemical cell was developed for the efficient in situ generation of H₂O₂ from water and atmospheric oxygen in a process called a two-electron oxygen reduction reaction (2e-ORR), so that the external addition of H₂O₂ is no longer needed. Moreover, the electrochemical cell was equipped with ultraviolet light-emitting diodes (UV-LEDs) to convert H₂O₂ to ·OH. The reactor exhibited a current efficiency of ∼90% for the H₂O₂ production at a flow rate of 50 mL min^-1. The degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) was studied at 277 nm based on different operational parameters, such as UV fluence rate, initial concentration, and initial pH. A high degradation of >70% was obtained at a UV output of 900 mW. Our approach, the first of its kind, has novel features applied, including: optimal radiation distribution in the reactor by applying a new UV source, UV-LEDs that offer much more control for the radiation profile in the reaction system compared to traditional UV lamps, controlled hydrodynamics by implementing special flow channels to provide a more uniform residence time and offer enhanced mixing, and integrating UV reactor and electrochemical cell in a single unit which could lead to superior performance and space efficiency of the device. These features make the device very suitable for point-of-use (POU) water treatment applications to eliminate both microbial and chemical contaminants.

Degradation of Acid Violet 19 textile dye by electro-peroxone in a laboratory flow plant

This paper deals with the degradation of Acid Violet 19 (AV19) textile dye by the electro-peroxone (E-peroxone) process in a laboratory flow plant using a filter press cell fitted with a 3D gas diffusion electrode (3D GDE) containing a graphite felt positioned on carbon-cloth PTFE as cathode, and a Ti|IrSnSb-oxides plate as anode. H₂O₂ was formed by the oxygen reduction reaction (ORR) in the cathode; the air was supplied by an external compressor. The O₃ produced externally by an ozonator was added in the pipeline at the outlet of the electrolyzer to promote the reaction between the H₂O₂ and O₃ to produce OH, which is the responsible for the mineralization of the dye. The effect of electrolyte flow rate (Q), current density (j), and initial concentration of AV19 dye on its degradation was addressed. The best electrolysis in a solution containing 40 mg TOC L^-1, 0.05 M Na₂SO₄, at pH 3, was obtained at j = 20 mA cm^-2, Q = 2.0 L min^-1, using a pressure of the air fed to the 3D GDE of P_GDE = 3 psi, and an ozone inlet mass flow rate of [Formula: see text]  = 14.5 mg L^-1, achieving 100% discoloration, 60% mineralization, with mineralization current efficiency and energy consumption of 36% and 0.085 kWh(gTOC)^-1. The degradation of AV19 dye was also performed by anodic oxidation plus H₂O₂ electrogenerated (AO-H₂O₂) and ozonation. The oxidation power was AO-H₂O₂ < ozonation < E-peroxone. Three carboxylic acids were quantified by chromatography as oxidation end products.

Mineralization of Methyl Orange azo dye by processes based on H2O2 electrogeneration at a 3D-like air-diffusion cathode

This work addresses the mineralization of the widely used Methyl Orange (MO) azo dye by technologies based on H₂O₂ electrogeneration at a 3D-like air-diffusion cathode. These include two Fe²⁺-catalyzed processes such as electro-Fenton (EF) and photoelectro-Fenton (PEF). Bulk electrolyses were performed in a recirculation flow plant, in which the Eco-Cell filter-press electrochemical reactor was connected in series with a UVA photoreactor. The former reactor was equipped with a Ti|Ir-Sn-Sb oxide plate anode alongside a 3D-like air-diffusion cathode made from graphite felt and hydrophobized carbon cloth, aimed at electrogenerating H₂O₂ on site. The influence of current density (j), volumetric flow rate (Q) and initial MO concentration was examined. The greatest oxidation power corresponded to PEF process. The best operation conditions to treat 30 mg L^-1 of total organic carbon of MO in a 50 mM Na₂SO₄ solution by PEF were found at 0.50 mM Fe²⁺, pH 3.0, j = 20 mA cm^-2 and Q = 2.0 L min^-1, obtaining 100% and 94% of color and TOC removals at 30 and 240-300 min, respectively. This accounted for 35% of mineralization current efficiency and 0.12 kWh (g TOC)^-1 of energy consumption at the end of the electrolysis. The oxidation power of EF and PEF was compared with that of anodic oxidation (AO), and the sequence obtained was: PEF > EF > AO. The dye was gradually degraded, yielding non-toxic short carboxylic acids, like maleic, fumaric, formic, oxalic and oxamic, whose Fe(III) complexes were rapidly photolyzed.

Effective treatment of levofloxacin wastewater by an electro-Fenton process with hydrothermal-activated graphite felt as cathode

The performance of the cathode significantly affects the ability of the electro-Fenton (EF) process to degrade chemicals. In this study, a simple method to modify the graphite felt (GF) cathode was proposed, i.e. oxidizing GF by hydrothermal treatment in nitric acid. The surface physical and electrochemical properties of modified graphite felt were characterized by several techniques: scanning electron microscope (SEM), water contact angle, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and linear scanning voltammetry (LSV). Compared with an unmodified GF (GF-0), the oxygen reduction reaction (ORR) activity of a modified GF was significantly improved due to the introduction of more oxygen-containing functional groups (OGs). Furthermore, the results showed that GF was optimally modified after 9 h (GF-9) of treatment. As an example, the H₂O₂ generation by GF-9 was 2.26 times higher than that of GF-0. After optimizing the process parameters, which include the initial Fe²⁺ concentration and current density, the apparent degradation rate constant of levofloxacin (LEV) could reach as high as 0.40 min^-1. Moreover, the total organic carbon (TOC) removal rate and mineralization current efficiency (MCE) of the modified cathode were much higher than that of the GF-0. Conclusively, GF-9 is a promising cathode for the future development in organic pollutant removal via EF.

Mechanism and stability of an Fe-based 2D MOF during the photoelectro-Fenton treatment of organic micropollutants under UVA and visible light irradiation

This work reports the novel application of an Fe-based 2D metal-organic framework (MOF), prepared with 2,2'-bipyridine-5,5'-dicarboxylate (bpydc) as organic linker, as highly active catalyst for heterogeneous photoelectro-Fenton (PEF) treatment of the lipid regulator bezafibrate in a model matrix and urban wastewater. Well-dispersed 2D structures were successfully synthesized and their morphological, physicochemical and photocatalytic properties were assessed. UV/Vis PEF using an IrO₂/air-diffusion cell with an extremely low catalyst concentration (0.05 g L^-1, tenfold lower than reported 3D MOFs) outperformed electro-oxidation with electrogenerated H₂O₂, electro-Fenton and visible-light PEF. Its excellent performance was explained by: (i) the enhanced mass transport of H₂O₂ (and organic molecules) at the 2D structure, providing active sites for heterogeneous Fenton's reaction and in-situ Fe(II) regeneration; (ii) the ability of photoinduced electrons to reduce H₂O₂ to ^•OH, and Fe(III) to Fe(II); and (iii) the enhanced charge transfer and excitation of Fe-O clusters, which increased the number of electron-hole pairs. LC-QToF-MS and GC-MS allowed the identification of 16 aromatic products of bezafibrate. The complete removal of four micropollutants mixed in urban wastewater at pH 7.4 revealed the great potential of (Fe-bpydc)-catalyzed PEF process.

A novel Electro-Fenton process characterized by aeration from inside a graphite felt electrode with enhanced electrogeneration of H2O2 and cycle of Fe3+/Fe2

A novel Electro-Fenton process characterized by aeration from inside a graphite felt electrode with enhanced generation of H₂O₂ and cycle of Fe³⁺/Fe²⁺ was proposed. The new type of Electro-Fenton process was used to degrade organic pollutants via graphite felt electrode aeration (GF-EA). The H₂O₂ concentration by GF-EA could reach 152-169 mg/L in a wide pH range (3-10), which was much higher than that achieved by graphite felt using solution aeration (GF-SA, 37-113 mg/L). For the degradation of nitrobenzene (NB), benzoic acid (BA), bisphenol A (BPA), and sulfamethoxazole (SMX) at pH 5.5, the percentage degradation by GF-EA could reach 55%, 56%, 80%, and 60% higher than those obtained by GF-SA, respectively. The solution TOC removal by GF-EA were enhanced by 29-51% relative to GF-SA. Mechanism analysis showed both OH and ferryl species were involved in the reaction system, and the amounts of OH and dissolved iron species in GF-EA group were 7.7 times and 4-8 times higher than those in GF-SA group, respectively. Besides, the mass transfer rate of GF-EA system was 5.4 times higher than that of GF-SA system. High amounts of H₂O₂, dissolved iron species and OH were attributed to the enhanced mass transfer of O₂ and the solution.

A Highly Stable Metal-Organic Framework-Engineered FeS2/C Nanocatalyst for Heterogeneous Electro-Fenton Treatment: Validation in Wastewater at Mild pH

Herein, the novel application of FeS₂/C nanocomposite as a highly active, stable, and recyclable catalyst for heterogeneous electro-Fenton (EF) treatment of organic water pollutants is discussed. The simultaneous carbonization and sulfidation of an iron-based metal-organic framework (MOF) yielded well-dispersed pyrite FeS₂ nanoparticles of ∼100 nm diameter linked to porous carbon. XPS analysis revealed the presence of doping N atoms. EF treatment with an IrO₂/air-diffusion cell ensured the complete removal of the antidepressant fluoxetine spiked into urban wastewater at near-neutral pH after 60 min at 50 mA with 0.4 g L^-1 catalyst as optimum dose. The clear enhancement of catalytic activity and stability of the material as compared to natural pyrite was evidenced, as deduced from its characterization before and after use. The final solutions contained <1.5 mg L^-1 dissolved iron and became progressively acidified. Fluorescence excitation-emission spectroscopy with parallel factor analysis demonstrated the large mineralization of all wastewater components at 6 h, which was accompanied by a substantial decrease of toxicity. A mechanism with ^•OH as the dominant oxidant was proposed: FeS₂ core-shell nanoparticles served as Fe²⁺ shuttles for homogeneous Fenton's reaction and provided active sites for the heterogeneous Fenton process, whereas nanoporous carbon allowed minimizing the mass transport limitations.

Expanding the application of photoelectro-Fenton treatment to urban wastewater using the Fe(III)-EDDS complex

This work reports the first investigation on the use of EDDS as chelating agent in photoelectro-Fenton (PEF) treatment of water at near-neutral pH. As a case study, the removal of the antidepressant fluoxetine was optimized, using an electrochemical cell composed of an IrO₂-based anode an air-diffusion cathode for in-situ H₂O₂ production. Electrolytic trials at constant current were made in ultrapure water with different electrolytes, as well as in urban wastewater (secondary effluent) at pH 7.2. PEF with Fe(III)-EDDS (1:1) complex as catalyst outperformed electro-Fenton and PEF processes with uncomplexed Fe(II) or Fe(III). This can be explained by: (i) the larger solubilization of iron ions during the trials, favoring the production of ^•OH from Fenton-like reactions between H₂O₂ and Fe(II)-EDDS or Fe(III)-EDDS, and (ii) the occurrence of Fe(II) regeneration from Fe(III)-EDDS photoreduction, which was more efficient than conventional photo-Fenton reaction with uncomplexed Fe(III). The greatest drug concentration decays were achieved at low pH, using only 0.10 mM Fe(III)-EDDS, although complete removal in wastewater was feasible only with 0.20 mM Fe(III)-EDDS due to the greater formation of ^•OH. The effect of the applied current and anode nature was rather insignificant. A progressive destruction of the catalytic complex was unveiled, whereupon the mineralization mainly progressed thanks to the action of ^•OH adsorbed on the anode surface. Despite the incomplete mineralization using BDD as the anode, a remarkable toxicity decrease was determined. Fluoxetine degradation yielded F^- and NO₃^- ions, along with several aromatic intermediates. These included two chloro-organics, as a result of the anodic oxidation of Cl^- to active chlorine. A detailed mechanism for the Fe(III)-EDDS-catalyzed PEF treatment of fluoxetine in urban wastewater is finally proposed.

Hydrogen peroxide generation from O2 electroreduction for environmental remediation: A state-of-the-art review

The electrochemical production of hydrogen peroxide (H2O2) by 2-electron oxygen reduction reaction (ORR) is an attractive alternative to the present complex anthraquinone process. The objective of this paper is to provide a state-of-the-arts review of the most important aspects of this process. First, recent advances in H2O2 production are reviewed and the advantages of H2O2 electrogeneration via 2-electron ORR are highlighted. Second, the selectivity of the ORR pathway towards H2O2 formation as well as the development process of H2O2 production are presented. The cathode characteristics are the decisive factors of H2O2 production. Thus the focus is shifted to the introduction of commonly used carbon cathodes and their modification methods, including the introduction of other active carbon materials, hetero-atoms doping (i.e., O, N, F, B, and P) and decoration with metal oxides. Cathode stability is evaluated due to its significance for long-term application. Effects of various operational parameters, such as electrode potential/current density, supporting electrolyte, electrolyte pH, temperature, dissolved oxygen, and current mode on H2O2 production are then discussed. Additionally, the environmental application of electrogenerated H2O2 on aqueous and gaseous contaminants removal, including dyes, pesticides, herbicides, phenolic compounds, drugs, VOCs, SO2, NO, and Hg0, are described. Finally, a brief conclusion about the recent progress achieved in H2O2 electrogeneration via 2-electron ORR and an outlook on future research challenges are proposed.

Ferrocene functionalized graphene based electrode for the electro-Fenton oxidation of ciprofloxacin

Ferrocene functionalized graphene based graphite felt electrode was firstly investigated for heterogeneous electro-Fenton oxidation of ciprofloxacin in neutral pH condition. Electrochemical reduction of Ferrocene functionalized graphene oxide (Fc-ErGO) was performed by cyclic voltammetry technique. At neutral pH condition, Fc-ErGO electrode (0.035 min^─1) exhibited ∼3 times and ∼9 times higher removal rates in comparison with plane ErGO (0.010 min^─1) and plane graphite felt (0.004 min^─1) electrodes respectively. The effect of pH and applied potential were studied for the degradation of ciprofloxacin in Fc-ErGO based electrode. Higher removal rate was observed at acidic pH (0.222 min^─1), whereas alkaline pH showed lower removal efficiency (0.014 min^─1). > 99% removal of ciprofloxacin was achieved with in 15 min and 120 min of reactions period at pH 3.0 and pH 7.0, respectively. H₂O₂ generation was found to be high in plane ErGO electrode system in all of the pH conditions. Owing to the high redox ability of ferrocene, Fc-ErGO electrode generated high concentration of OH radicals (426 μM pH 3.0; 247 μM pH 7.0; 210 μM pH 9.0) than ErGO and plane graphite felt electrodes; The electrode reusability study was performed to understand the electrode stability. There was no significant change in removal efficiency even after the 5th cycle of reusability study at both acidic and neutral conditions. The possible mechanism of oxidation in Fc-ErGO based electro-Fenton process was also proposed based on the continuous monitoring of H₂O₂ and OH radicals generated in the system.