An experimental comparison of a graphite electrode and a gas diffusion electrode for the cathodic production of hydrogen peroxide

Minerals as catalysts of heterogeneous Electro-Fenton and derived processes for wastewater treatment: a review

Advanced oxidation processes (AOPs) such as Fenton's reagent, which generates highly reactive oxygen species, are efficient in removing biorefractory organic pollutants from wastewater. However, Fenton's reagent has drawbacks such as the generation of iron sludge, high consumption of H₂O₂, and the need for pH control. To address these issues, Electro-Fenton (EF) and heterogeneous Electro-Fenton (HEF) have been developed. HEF, which uses solid catalysts, has gained increasing attention, and this review focuses on the use of mineral catalysts in HEF and derived processes. The reviewed studies highlight the advantages of using mineral catalysts, such as efficiency, stability, affordability, and environmental friendliness. However, obstacles to overcome include the agglomeration of unsupported nanoparticles and the complex preparation techniques and poor stability of some catalyst-containing cathodes. The review also discusses the optimal pH range and dosage of the heterogeneous catalysts and compares the performance of iron sulfides versus iron oxides. Although natural minerals appear to be the best choice for effluents at pH>4, no scale-up reports have been found. The need for further development in this field and the importance of considering the environmental impact of trace toxic metals or catalytic nanoparticles in the treated water on the receiving ecosystem is emphasized. Finally, the article acknowledges the high energy consumption of HEF processes at the lab scale and calls for their performance development to achieve environmentally friendly and cost-effective results using real wastewaters on a pilot scale.

Anodic oxidation of sulfamethoxazole paired to cathodic hydrogen peroxide production

A double chamber electrochemical system is developed consisting of a boron-doped diamond (BDD) anode and a graphite cathode, which not only degrades sulfamethoxazole (SMX) but also simultaneously generates hydrogen peroxide (H₂O₂). The degradation of SMX is carried out by (in)direct oxidation at the BDD anode and H₂O₂ is produced by two electron oxygen (O₂) reduction reaction (ORR) at the cathode. The effect of different parameters on the kinetics of both mechanisms was investigated. The performance of the system at the optimized conditions (pH 3, 0.05 M Na₂SO₄ as electrolyte, and 10 mA as applied current) showed that after 180 min of electrolysis, SMX was almost fully degraded (95% removal and ∼90% COD reduction) as well as about 535 μM H₂O₂ was accumulated. With the help of LC-MS, five intermediates formed during SMX electrolysis were properly identified and a degradation pathway was proposed. This study advocates methods for improving the effectiveness of energy use in advanced wastewater treatment.

Electrosynthesis of H2O2 through a two-electron oxygen reduction reaction by carbon based catalysts: From mechanism, catalyst design to electrode fabrication

Hydrogen peroxide (H₂O₂) is an efficient oxidant with multiple uses ranging from chemical synthesis to wastewater treatment. The in-situ H₂O₂ production via a two-electron oxygen reduction reaction (ORR) will bring H₂O₂ beyond its current applications. The development of carbon materials offers the hope for obtaining inexpensive and high-performance alternatives to substitute noble-metal catalysts in order to provide a full and comprehensive picture of the current state of the art treatments and inspire new research in this area. Herein, the most up-to-date findings in theoretical predictions, synthetic methodologies, and experimental investigations of carbon-based catalysts are systematically summarized. Various electrode fabrication and modification methods were also introduced and compared, along with our original research on the air-breathing cathode and three-phase interface theory inside a porous electrode. In addition, our current understanding of the challenges, future directions, and suggestions on the carbon-based catalyst designs and electrode fabrication are highlighted.

Graphene family for hydrogen peroxide production in electrochemical system

The development of carbon-based materials to catalyze two-electron (2e^-) pathway of oxygen reduction reaction (ORR) offers great potential for hydrogen peroxide (H₂O₂) production. As a class of novel two-dimensional (2D) carbon materials, graphene and its derivatives have raised increasing attention as excellent noble-metal-free catalysts in 2e ORR due to their unique structure, physical and chemical properties. This review focuses on the synthesis of main graphene family members and graphene based electrodes, as well as their applications for H₂O₂ generation in electrochemical systems. We describe the functions of the graphene family in electrochemical systems, such as accelerating electron transfer and increasing oxygen transfer for cathodes in electrochemical systems, aiming to reveal the enhancement mechanisms of graphene and its derivatives on H₂O₂ production. Furthermore, the challenges and prospects for graphene family used as catalyst for H₂O₂ production in the future are also proposed.

Electrogeneration of H2O2 utilizing anodic O2 on polytetrafluoroethylene-modified cathode in flow-through reactor

Efficient electrogeneration of hydrogen peroxide (H₂O₂) is critical for treatment of refractory pollutants by the electro-Fenton process. An effective strategy is developed by combining a flow-through reactor with a poly- tetrafluoroethylene (PTFE)-modified graphite felt cathode. In this design, anodic oxygen is directly used for efficient H₂O₂ generation at the modified cathode. Experimental results show that the modified cathode with the optimum PTFE content can produce 29.6 mg/L of H₂O₂, which is 16 times higher than the unmodified graphite felt cathode for a flow rate of 3 mL/min. Maximum H₂O₂ production, up to 30.7 mg/L, was obtained under the following conditions: 120 mA, 3 mL/min, initial pH 13, no external aeration.

Carbonaceous cathode materials for electro-Fenton technology: Mechanism, kinetics, recent advances, opportunities and challenges

Electro-Fenton (EF) technique has gained significant attention in recent years owing to its high efficiency and environmental compatibility for the degradation of organic pollutants and contaminants of emerging concern (CECs). The efficiency of an EF reaction relies primarily on the formation of hydrogen peroxide (H₂O₂) via 2e^─ oxygen reduction reaction (ORR) and the generation of hydroxyl radicals (^●OH). This could be achieved through an efficient cathode material which operates over a wide pH range (pH 3-9). Herein, the current progresses on the advancements of carbonaceous cathode materials for EF reactions are comprehensively reviewed. The insights of various materials such as, activated carbon fibres (ACFs), carbon/graphite felt (CF/GF), carbon nanotubes (CNTs), graphene, carbon aerogels (CAs), ordered mesoporous carbon (OMCs), etc. are discussed inclusively. Transition metals and hetero atoms were used as dopants to enhance the efficiency of homogeneous and heterogeneous EF reactions. Iron-functionalized cathodes widened the working pH window (pH 1-9) and limited the energy consumption. The mechanism, reactor configuration, and kinetic models, are explained. Techno economic analysis of the EF reaction revealed that the anode and the raw materials contributed significantly to the overall cost. It is concluded that most reactions follow pseudo-first order kinetics and rotating cathodes provide the best H₂O₂ production efficiency in lab scale. The challenges, future prospects and commercialization of EF reaction for wastewater treatment are also discussed.

O-doped Graphitic Granular Biochar Enables Pollutants Removal via Simultaneous H2O2 Generation and Activation in Neutral Fe-free Electro-Fenton Process

H₂O₂ generation by 2-electron oxygen electroreduction reaction (2eORR) has attracted great attention as an alternative to the industry-dominant anthraquinone process. Electro-Fenton (EF) process, which relies on the H₂O₂ electrogeneration, is regarded as an important environmental application of H₂O₂ generation by 2eORR. However, its application is hindered by the relatively expensive electrode materials. Proposing cathode materials with low cost and facile synthetic procedures are the priority to advance the EF process. In this work, a composite cathode structure that uses graphitic granular bamboo-based biochar (GB) and stainless steel (SS) mesh (GBSS) is proposed, where SS mesh functions as current distributor and GB supports synergistic H₂O₂ electrogeneration and activation. The graphitic carbon makes GB conductive and the oxygen-containing groups serve as active sites for H₂O₂ production. 11.3 mg/L H₂O₂ was produced from 2.0 g GB at 50 mA after 50 min under neutral pH without external O₂/air supply. The O-doped biochar further increased the H₂O₂ yield to 18.3 mg/L under same conditions. The GBSS electrode is also effective for H₂O₂ activation to generate ·OH, especially under neutral pH. Ultimately, a neutral Fe-free EF process enabled by GBSS cathode is effective for removal of various model organic pollutants (reactive blue 19, orange II, 4-nitrophenol) within 120 min, and for their partial mineralization (48.4% to 63.5%). Long-term stability of the GBSS electrode for H₂O₂ electrogeneration, H₂O₂ activation, and pollutants degradation were also examined and analyzed. This work offers a promising application for biomass waste for removals of organic pollutants in neutral Fe-free EF process.

Oxygen reduction reaction electrocatalysis inducing Fenton-like processes with enhanced electrocatalytic performance based on mesoporous ZnO/CuO cathodes: Treatment of organic wastewater and catalytic principle

To treat typical organic wastewater efficiently, a novel Fenton-like processes based on ZnO/CuO composite cathode induced by oxygen reduction reaction (ORR) electrocatalysis with enhanced electrocatalytic performance was established successfully. Electrochemical testing investigation indicated that the ZnO/CuO cathode possessed conspicuous redox peak and better conductivity than uncompounded electrodes. Additionally, the removal efficiency of methylene blue and its chemical oxygen demand (COD) reached 96.4% and 70.8% after 120 min, respectively. Next, the feasibility of the material in practical application was also discussed. Subsequently, electrocatalytic principle based on valence state changes of metal elements on the electrode surface were also studied by x-ray photoelectron spectroscopy (XPS). Redox reactions between the active species H₂O₂ and the species Cu⁺ promoting Fenton-like processes were deduced. Namely, the conversion of Cu(I) and Cu(II) on the electrode surface was accompanied by OH generation. The combination of ZnO and CuO improved the surface morphology, increasing the active site of ORR and the yield of H₂O₂, thus greatly enhanced the Fenton-like activity. Finally, the main intermediates were identified by Gas chromatography-mass spectrometer (GC-MS), and possible pathways for dye degradation were proposed. In short, the research of ZnO/CuO cathode provided great significance for heterogeneous Fenton-like degradation and also showed its application potential in water treatment and remediation.

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 flow-through electro-Fenton process using modified activated carbon fiber cathode for orange II removal

This study investigated the removal of Orange II by an electro-Fenton process using a novel recirculation flow-through reactor. The hydrogen peroxide was generated in-situ on the activated carbon fiber (ACF) modified with carbon black and polytetrafluoroethylene (PTFE). The modified ACF cathode was characterized by scanning electron microscopy (SEM) and nitrogen adsorption-desorption study. In light of the production of H₂O₂ and removal of Orange II, the optimum weight percentage of PTFE in the mixture of carbon black and PTFE was 75%. The effects of some important operating parameters such as current and flow rate were investigated. The best Orange II removal reached 96.7% with mineralization efficiency of 55.4% at 120 min under the current of 100 mA, initial pH 3, Fe²⁺ 0.3 mM and the flow rate of 7 mL min^-1. The cathode exhibited good regeneration ability and stability. OH was proved to be the main oxidizing species in this flow-through electro-Fenton system. This work demonstrated that such electro-Fenton process using modified ACF cathode was promising for the degradation of organic pollutants.

Direct electrosynthesis of pure aqueous H2O2 solutions up to 20% by weight using a solid electrolyte

Hydrogen peroxide (H2O2) synthesis generally requires substantial postreaction purification. Here, we report a direct electrosynthesis strategy that delivers separate hydrogen (H2) and oxygen (O2) streams to an anode and cathode separated by a porous solid electrolyte, wherein the electrochemically generated H+ and HO2– recombine to form pure aqueous H2O2 solutions. By optimizing a functionalized carbon black catalyst for two-electron oxygen reduction, we achieved >90% selectivity for pure H2O2 at current densities up to 200 milliamperes per square centimeter, which represents an H2O2 productivity of 3.4 millimoles per square centimeter per hour (3660 moles per kilogram of catalyst per hour). A wide range of concentrations of pure H2O2 solutions up to 20 weight % could be obtained by tuning the water flow rate through the solid electrolyte, and the catalyst retained activity and selectivity for 100 hours.

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.

Activated carbon as effective cathode material in iron-free Electro-Fenton process: Integrated H2O2 electrogeneration, activation, and pollutants adsorption

Major challenges for effective implementation of the Electro-Fenton (EF) water treatment process are that conventional efficient cathodes are relatively expensive, and H2O2 activation by Fe2+ may cause secondary pollution. Herein, we propose a low-cost activated carbon/stainless steel mesh (ACSS) composite cathode, where the SS mesh distributes the current and the AC simultaneously supports H2O2 electrogeneration, H2O2 activation, and organic compounds (OCs) adsorption. The oxygen-containing groups on the AC function as oxygen reduction reaction (ORR) sites for H2O2 electrogeneration; while the porous configuration supply sufficient reactive surface area for ORR. 8.9 mg/L H2O2 was obtained with 1.5 g AC at 100 mA under neutral pH without external O2 supply. The ACSS electrode is also effective for H2O2 activation to generate ‧OH, especially under neutral pH. Adsorption shows limited influence on both H2O2 electrogeneration and activation. The iron-free EF process enabled by the ACSS cathode is effective for reactive blue 19 (RB19) degradation. 61.5% RB19 was removed after 90 min and 74.3% TOC was removed after 720 min. Moreover, long-term stability test proved its relatively stable performance. Thus, the ACSS electrode configuration is promising for practical and cost-effective EF process for transformation of OCs in water.

Electrochemical disinfection using a modified reticulated vitreous carbon cathode for drinking water treatment

A reticulated vitreous carbon (RVC) cathode modified by anodic polarization in 20 wt% H₂SO₄ solution was used for drinking water disinfection under a neutral low electrolyte concentration (0.25 g/L Na₂SO₄) condition. The contribution of the modified RVC anode and the Ti/RuO₂ cathode to disinfection was investigated. The influences of current, initial Escherichia coli load, temperature and water volume were studied. The results show that H₂O₂ generation increased to approximately three times using the modification of the RVC. E. coli was mainly deactivated by the H₂O₂ generated at the cathode. For water with about 10⁶ CFU/mL E. coli, the detection limit (<4 CFU/mL) was reached under different conditions. Increasing current could simultaneously shorten the treatment time and increase the energy consumption (EC) simultaneously. Although decreasing the initial load reduced the treatment time, the EC for per log E. coli removal increased. The time required for disinfection shortened from 3.5 to 2.5 h and the EC for per log removal decreased from 218.5 to 123.2 Wh/m³ when the temperature increased from 20 to 40 °C. Although more time was required for disinfection, the EC decreased from 218.5 to 141.4 Wh/m³ when the volume was doubled.

"Floating" cathode for efficient H2O2 electrogeneration applied to degradation of ibuprofen as a model pollutant

The performance of the Electro-Fenton (EF) process for contaminant degradation depends on the rate of H2O2 production at the cathode via 2-electron dissolved O2 reduction. However, the low solubility of O2 (≈1×10-3 mol dm-3) limits H2O2 production. Herein, a novel and practical strategy that enables the synergistic utilization of O2 from the bulk electrolyte and ambient air for efficient H2O2 production is proposed. Compared with a conventional "submerged" cathode, the H2O2 concentration obtained using the "floating" cathode is 4.3 and 1.5 times higher using porous graphite felt (GF) and reticulated vitreous carbon (RVC) foam electrodes, respectively. This surprising enhancement results from the formation of a three-phase interface inside the porous cathode, where the O2 from ambient air is also utilized for H2O2 production. The contribution of O2 from ambient air varies depending on the cathode material and is calculated to be 76.7% for the GF cathode and 35.6% for the RVC foam cathode. The effects of pH, current, and mixing on H2O2 production are evaluated. Finally, the EF process enhanced by the "floating" cathode degraded 78.3% of the anti-inflammatory drug ibuprofen after 120 min compared to only 25.4% using a conventional "submerged" electrode, without any increase in the cost.

Rates of H2O2 Electrogeneration by Reduction of Anodic O2 at RVC Foam Cathodes in Batch and Flow-through Cells

The Electro-Fenton process for in-situ H₂O₂ electrogeneration is impacted by low O₂ utilization efficiency (<0.1%) and the need of acid for pH adjustment. An electrochemical flow-through cell can develop localized acidic conditions, coupled with simultaneous formation and utilization of O₂ to enhance H₂O₂ formation. Multiple electrode configurations using reticulated vitreous carbon (RVC) foam and Ti/mixed metal oxides (MMO) are proposed to identify the optimum conditions for H₂O₂ formation in batch and flow-through cells. A pH of 2.75±0.25 is developed locally in the flow-through cell that supports effective O₂ reduction. Up to 9.66 mg/L H₂O₂ is generated in a 180 mL batch cell under 100 mA, at pH 2, and mixing at 350 rpm. In flow-through conditions, both flow rate and current significantly influence H₂O₂ production. A current of 120 mA produced 2.27 mg/L H₂O₂ under a flow rate of 3 mL/min in a 3-electrode cell with one RVC foam cathode at 60 min. The low current of 60 mA does not enable effective H₂O₂ production, while the high current of 250 mA produced less H₂O₂ due to parasitic reactions competing with O₂ reduction. Higher flow rates decrease the retention time, but also increase the O₂ mass transfer. Furthermore, 3-electrode flow-through cell with two RVC foam cathodes was not effective for H₂O₂ production due to the limited O₂ supply for the secondary cathode. Finally, a coupled process that uses both O₂ and H₂ from water electrolysis is proposed to improve the H₂O₂ yield further.

Enhancement of H2O2 production and AYR degradation using a synergetic effect of photo-electrocatalysis for carbon nanotube/g-C3N4 electrodes

In this work, a new gas diffusion electrode (GDE) of carbon nanotube/graphitic carbon nitride (CNT/g-C₃N₄) was prepared, which enables the substantially improved production of H₂O₂ (up to 1083.54 mg L⁻¹) compared to generation without g-C₃N₄ (400 mg L⁻¹).

Highly efficient degradation of phenol from wastewater via an electro-catalytic oxidation approach with a CeO2–CuO cathode

The development of highly efficient cathode materials for the electro-catalytic oxidation of phenol from wastewater is of vital importance for environment protection.

Sulfonic acid-grafted polyvinylidene fluoride electrospun mats as electro-Fenton reactor membrane components

Illustration of reaction mechanism of the EF process.