Gas diffusion electrodes for H2O2 production and their applications for electrochemical degradation of organic pollutants in water: A review

Electrochemical production of HO2- and O2 for sulfide removal from sewage

In this study, a comparative analysis of two electrochemical methods for sulfide control in sewer networks was performed for the first time. In addition, the mechanism of sulfide control by HO2- was elucidated, and an analysis of the device operation and electrolyte selection was performed. The two-electron oxygen reduction reaction (2e--ORR) using untreated gas diffusion electrode (GDE) was superior to the hydrogen evolution reaction (HER) using stainless-steel mesh in terms of cell voltage, product formation, and sulfide suppression. The GDE maintained a stable HO₂⁻ production capacity, achieving a concentration of 4566.6 ± 173.3 mg L⁻¹ with a current efficiency (CE) of 84.13 ± 3.5 %. During the electrolysis period, a stable dissolved oxygen (DO) level in sewage was consistently observed due to continuous in-situ oxygen production in anode. HO2- exhibited a notable increase in sewage pH (10.20 ± 0.01), effectively inhibiting the release of 99.93 % of sulfides. Moreover, the combined treatment of HO2- and DO significantly surpassed that of individual treatments. Seawater treated with cation exchange resin (CER) emerged as the most promising alternative to freshwater as the electrolyte. Overall, this study demonstrates that in-situ generation of HO₂⁻ and oxygen is a more effective strategy for sulfide control in sewer systems.

Application of Biochar-Based Materials for Effective Pollutant Removal in Wastewater Treatment

With the growth of the global population and the acceleration of industrialization, the problem of water pollution has become increasingly serious, posing a major threat to the ecosystem and human health. Traditional water treatment technologies make it difficult to cope with complex pollution, so the scientific community is actively exploring new and efficient treatment methods. Biochar (BC), as a low-cost, green carbon-based material, exhibits good adsorption and catalytic properties in water treatment due to its porous structure and abundant active functional groups. However, BC's pure adsorption or catalytic capacity is limited, and researchers have dramatically enhanced its performance through modification means, such as loading metals or heteroatoms. In this paper, we systematically review the recent applications of BC and its modified materials for water treatment in adsorption, Fenton-like, electrocatalytic, photocatalytic, and sonocatalytic systems, and discuss their adsorption/catalytic mechanisms. However, most of the research in this field is at the laboratory simulation stage and still needs much improvement before it can be applied in large-scale wastewater treatment. This review improves the understanding of the pollutant adsorption/catalytic properties and mechanisms of BC-based materials, analyzes the limitations of the current studies, and investigates future directions.

Influence of Water Hardness and Complexing Agents on Electrochemical Hydrogen Peroxide Generation

Recently, many studies have been published regarding electrochemical oxygen reduction reaction for generating hydrogen peroxide (H2O2) using gas diffusion electrodes (GDEs) for various applications. Sodium salts solved in deionized water were usually used as supporting electrolytes. In technical applications, however, tap water-based electrolytes with hardeners are particularly relevant and have only been considered in a few studies to date. In this work, we investigated the influence of hardeners on H2O2-generation at 150 mA cm-2 and were able to show that scaling occurs predominantly on the GDE-surface and not in its pore structure. With the novel method in electrochemical synthesis by using complexing agents to bind hardeners, we were able to significantly reduce the scaling. Even after 10 h of operation, the reactor still achieves a faradaic efficiency (FE) of above 70 % (>67 mg h-1 cm-2), comparable to the experiments without hardeners and complexing agents in the electrolyte. Furthermore, we demonstrate that the complexing agents are not electrochemically converted at the carbon-based GDE and behave inertly. If the cell is operated with complexing agents and rinsed with acidic liquid (anolyte) between batches, scaling can be completely avoided.

Trans-electrode pressure of gas-diffusion electrodes significantly influencing the electrochemical hydrogen peroxide production

Hydrogen peroxide (H2O2) synthesis by electrochemical two-electron oxygen reduction has garnered increasing interest as a wide range of potential applications. Gas diffusion electrodes (GDEs) can effectively promote the H2O2 production efficiency by overcoming the oxygen mass transfer limitations but strongly influenced by the electrowetting process along the long-term operation. In this study, the effect of trans-electrode pressure (TEP) of GDE cathode on the electrowetting process was further elucidated. We controlled the TEP values of four types of GDEs: two Ni-based GDEs and two carbon cloth GDEs prepared by hot-pressing or brushing carbon black. SBA-15 was further used to regulate the microstructure of one Ni-based GDE. It was found that an optimal range of TEP occurred for all tested GDEs in terms of the max. concentration, the yield efficiency, the energy consumption, and the stability because TEP may change the triple-phase interface and influence the anti-electrowetting effect. The porosity of hot-pressed Ni GDE can maintain the TEP window and thus enhance the production of H2O2, likely via creating oxygen-containing functional groups and a bimodal pore structure on the electrode, revealed with several characterization techniques including SEM, CA, XPS, Raman spectra, CV and EIS. The porous Ni GDE presented an efficient and stable production of H2O2 for 10 cycles: yielding H2O2 at 4393.2-4602.2 mmol m-2 h-1 with current efficiencies of 94.2-98.7%. The best accumulated H2O2 concentration can reach up to 3.58 ωt% H2O2 at 10 h. The results provide an important reference for the industrial scaleup of electro-production of H2O2 with GDEs.

Electrochemical Generation of Hydroxide and Hydrogen Peroxide for Hydrolysis of Sulfuryl Fluoride Fumigant

The post-harvest fumigant, sulfuryl fluoride (SO2F2), is a >1000-fold more potent greenhouse gas than carbon dioxide and methane. Pilot studies have shown that SO2F2 fumes vented from fumigation chambers can be captured and hydrolyzed by hydroxide (OH-) and hydrogen peroxide (H2O2) at pH ∼ 12 in a scrubber, producing SO42- and F- as waste salts. To reduce the costs and challenges associated with purchasing and mixing these reagents onsite, this study evaluates the electrochemical generation of OH- and H2O2 within spent scrubbing solution, taking advantage of the waste SO42- and F- as free sources of electrolyte. The study used a gas diffusion electrode constructed from carbon paper coated with carbon black as a catalyst selective for the reduction of O2 to H2O2. Under galvanostatic conditions, the study evaluated the effect of electrochemical conditions, including applied cathodic current density and electrolyte strength. Within an electrolyte containing 200 mM SO42- and 400 mM F-, comparable to the waste salts generated by a SO2F2 scrubbing event, the system produced 250 mM H2O2 at pH 12.6 within 4 h with a Faradaic efficiency of 98.8% for O2 reduction to H2O2. In a scrubbing-water sample from lab-scale fumigation, the system generated ∼200 mM H2O2 at pH 13.5 within 4 h with a Faradaic efficiency of 75.6%. A comparison of the costs to purchase NaOH and H2O2 against the electricity costs for electrochemical treatment indicated that the electrochemical approach could be 38-71% lower, depending on the local cost of electricity.

Additive manufacturing-derived free-standing 3D pyrolytic carbon electrodes for sustainable microbial electrochemical production of H2O2

Producing H2O2 via microbial electrosynthesis is a cost-effective and environmentally favorable alternative to the costly and environmentally hazardous anthraquinone method. However, most studies have relied on carbon electrodes with two-dimensional (2D) surfaces (e.g., graphite), which have limited surface area and active sites, resulting in suboptimal H2O2 production. In this study, we demonstrate the enhanced efficiency of microbial H2O2 synthesis using three-dimensional (3D) electrodes produced through additive manufacturing technology due to their larger surface area than conventional carbon electrodes with 2D surfaces. This work innovatively combines 3D printed pyrolytic carbon (3D PyrC) electrodes with highly defined outer geometry and internal mesh structures derived from additive manufacturing with high-temperature resin precursors followed by pyrolysis with microbial electrochemical platform technology to achieve efficient H2O2 synthesis. The 3D PyrC electrode produced a maximum of 129.2 mg L-1 of H2O2 in 12 h, which was 2.3-6.9 times greater than conventional electrodes (e.g., graphite and carbon felt). Furthermore, the scalability, reusability and mechanical properties of the 3D PyrC electrode were exemplary, showcasing its practical viability for large-scale applications. Beyond H2O2 synthesis, the study explored the application of the 3D PyrC electrode in the bio-electro-Fenton process, demonstrating its efficacy as a tertiary treatment technology for the removal of micropollutants. This dual functionality underscores the versatility of the 3D PyrC electrode in addressing both the synthesis of valuable chemicals and environmental remediation. This study shows a novel electrode design for efficient, sustainable synthesis of H2O2 and subsequent environmental remediation.

Hydrogen peroxide electrogeneration from O2 electroreduction: A review focusing on carbon electrocatalysts and environmental applications

Hydrogen peroxide (H2O2) stands as one of the foremost utilized oxidizing agents in modern times. The established method for its production involves the intricate and costly anthraquinone process. However, a promising alternative pathway is the electrochemical hydrogen peroxide production, accomplished through the oxygen reduction reaction via a 2-electron pathway. This method not only simplifies the production process but also upholds environmental sustainability, especially when compared to the conventional anthraquinone method. In this review paper, recent works from the literature focusing on the 2-electron oxygen reduction reaction promoted by carbon electrocatalysts are summarized. The practical applications of these materials in the treatment of effluents contaminated with different pollutants (drugs, dyes, pesticides, and herbicides) are presented. Water treatment aiming to address these issues can be achieved through advanced oxidation electrochemical processes such as electro-Fenton, solar-electro-Fenton, and photo-electro-Fenton. These processes are discussed in detail in this work and the possible radicals that degrade the pollutants in each case are highlighted. The review broadens its scope to encompass contemporary computational simulations focused on the 2-electron oxygen reduction reaction, employing different models to describe carbon-based electrocatalysts. Finally, perspectives and future challenges in the area of carbon-based electrocatalysts for H2O2 electrogeneration are discussed. This review paper presents a forward-oriented viewpoint of present innovations and pragmatic implementations, delineating forthcoming challenges and prospects of this ever-evolving field.

New Insights on Designing the Next-Generation Materials for Electrochemical Synthesis of Reactive Oxidative Species Towards Efficient and Scalable Water Treatment: A Review and Perspectives

Electrochemical water remediation technologies offer several advantages and flexibility for water treatment and degradation of contaminants. These technologies generate reactive oxidative species (ROS) that degrade pollutants. For the implementation of these technologies at an industrial scale, efficient, scalable, and cost-effective in-situ ROS synthesis is necessary to degrade complex pollutant mixtures, treat large amount of contaminated water, and clean water in a reasonable amount of time and cost. These targets are directly dependent on the materials used to generate the ROS, such as electrodes and catalysts. Here, we review the key design aspects of electrocatalytic materials for efficient in-situ ROS generation. We present a mechanistic understanding of ROS generation, including their reaction pathways, and integrate this with the key design considerations of the materials and the overall electrochemical reactor/cell. This involves tunning the interfacial interactions between the electrolyte and electrode which can enhance the ROS generation rate up to ~ 40% as discussed in this review. We also summarized the current and emerging materials for water remediation cells and created a structured dataset of about 500 electrodes and 130 catalysts used for ROS generation and water treatment. A perspective on accelerating the discovery and designing of the next generation electrocatalytic materials is discussed through the application of integrated experimental and computational workflows. Overall, this article provides a comprehensive review and perspectives on designing and discovering materials for ROS synthesis, which are critical not only for successful implementation of electrochemical water remediation technologies but also for other electrochemical applications.

Heterogeneous electro-Fenton system using Fe-MOF as catalyst and electrocatalyst for degradation of pharmaceuticals

In recent years, heterogeneous electro-Fenton processes have gained considerable attention as an alternative to homogeneous processes. In this context, the aim of this study is the use of a commercial iron metal-organic framework (Fe-MOF), Basolite® F-300, as a base material for the design of a heterogeneous electro-Fenton treatment system for the removal of antipyrine. Initially, the catalyst was applied as powder in aqueous solution and three key parameters of the electro-Fenton process (pH, Fe-MOF concentration and current density) were evaluated and optimized by a Central Composite Design Face Centred (CCD-FC) using antipyrine removal and energy consumption as response functions. Near complete antipyrine removal (94%) was achieved under optimal conditions: pH 3, Fe-MOF 157.78 mg/L and current density 6.67 mA/cm2, obtaining an energy consumption of 0.29 W·h per mg of antipyrine removed. Later, two electrocatalysts (Fe-MOF functionalized cathodes), prepared by different Fe-MOF immobilisation approaches (composite of carbon black/polytetrafluoroethylene or by electrospinning on Ni foam), were synthesized. Their characterisation showed notable Fe-MOF incorporation into the material and favourable properties as electrocatalysts. Both Fe-MOF functionalized cathodes were evaluated in the removal of antipyrine at different pH (acidic and natural) and current density (27.78 and 55.56 mA/cm2), achieving in the best conditions removal levels around 80% in 1 h without any operational problems. In addition, several intermediates generated during the treatment were identified and their toxicity estimated. According to the obtained results, the degradation compounds have less toxicity than the parent compounds, confirming the effectiveness of the treatment.

Evaluation of the hydrogenation reaction on the electrocatalytic nitrobenzene degradation over (FeCoNiCuZn)xOy high entropy oxides (HEOs)

Environmental pollution is one of the main challenges currently faced by mankind; especially industrial waste-waters treatment and remediation using energy-efficient methodologies. This research focused on the electrochemical degradation of the hazardous nitrobenzene (NB) in aqueous solutions, using novel high-entropy oxide (HEO) electrodes to elucidate the influence of the electrochemical reduction step on the degradation process. The effect of solution pH, dissolved oxygen concentration, anodic applied potential, and cell configurations on NB hydrogenation reaction were evaluated. A synergistic effect between the hydrogenation of nitrobenzene and the degradation was found to drastically enhance the conversion rates. The presence of dissolved oxygen promoted the oxidation reactions, reaching more than 90% nitrobenzene removal at 1.7 V and pH 14. The degradation of the organic by-products was attributed to the formation of the superoxide radical (O2*-) as the main oxidant species. A degradation mechanism was proposed based on the detected intermediates and the decoupling of electrochemical surface reactions observed by EIS at different solution pH.

Recent Progress on Electrode Design for Efficient Electrochemical Valorisation of CO2 , O2 , and N2

CO2 reduction, two-electron O2 reduction, and N2 reduction are sustainable technologies to valorise common molecules. Their further development requires working electrode design to promote the multistep electrochemical processes from gas reactants to value-added products at the device level. This review proposes critical features of a desirable electrode based on the fundamental electrochemical processes and the development of scalable devices. A detailed discussion is made to approach such a desirable electrode, addressing the recent progress on critical electrode components, assembly strategies, and reaction interface engineering. Further, we highlight the electrode design tailored to reaction properties (e.g., its thermodynamics and kinetics) for performance optimisation. Finally, the opportunities and remaining challenges are presented, providing a framework for rational electrode design to push these gas reduction reactions towards an improved technology readiness level (TRL).

Iron (II) phthalocyanine loaded tourmaline efficiently activates PMS to degrade pharmaceutical contaminants under solar light

Iron (II) phthalocyanine (FePc) is loaded on the surface of the tourmaline (TM) by the reflow method to obtain FePc/TM. This research effectively prevents the π-π stacking of FePc, increased the effective utilization rate of PMS activation under solar light, and further improved the catalytic performance of the catalytic system. The catalytic oxidation efficiency of FePc/TM on carbamazepine (CBZ) and sulfadiazine (SD) can reach 99% under solar light for 15 and 5 min, the total organic carbon (TOC) removal rate can reach 58% and 69% under solar light for 120 min. After 6 cycles, the CBZ removal rate remained above 95%. In addition, the FePc/TM catalytic system has an excellent removal rate for other pharmaceuticals. The results of spin-trapped electron paramagnetic resonance and classical quenching experiments show that FePc/TM can effectively activate PMS to generate active species under solar light, including superoxide radical (•O2-), singlet oxygen (1O2), hydroxyl radicals(•OH), and sulphate radicals (SO4•-). The intermediates of CBZ were identified by Ultra-high performance liquid chromatography and high resolution mass spectrometry, and the degradation pathway was proposed. As the reaction progresses, all CBZ and intermediates are reduced and converted into small acids, or mineralized to H2O, CO2. This work provides an alternative method for the design of efficient activation of PMS activation catalysts under solar light to eliminate residual pharmaceuticals in actual water bodies.

Inactivation of Microcystis aeruginosa by H2O2 generated from a carbon black polytetrafluoroethylene gas diffusion electrode in electrolysis by low-amperage electric current

Frequent outbreaks of cyanobacterial blooms have seriously threatened aquatic ecological environments and human health. Electrolysis by low-amperage electric current is effective for algae inactivation; however, it has no selectivity. Hydrogen peroxide (H₂O₂) is considered to be an efficient and selective suppressor of algae. Therefore, it is necessary to develop an electrode that can generate H₂O₂ to improve electrolysis technology. In this study, a carbon black polytetrafluoroethylene gas diffusion electrode (C-PTFE GDE) with good stability was prepared by a simple adhesive coating method. Then, the inactivation of Microcystis aeruginosa was conducted with electrolysis by low-amperage electric current using Ti/RuO₂ as the anode and C-PTFE GDE as the cathode. When the electrode spacing was 4 cm, the current density was 20 mA cm^-2, and the gas flow was 0.4 L min^-1, 85% of the algae could be inactivated in 20 min. Comparing the inactivation effect of the electric field and electrogenerated oxidants, it was found that electrolysis more rapidly and strongly inactivated algae when an electric field existed. However, electrogenerated oxidants dominated algae inactivation. The concentration of H₂O₂ was as high as 58 mg L^-1, while the concentration of chlorines was only 0.57 mg L^-1, and the generation rate of H₂O₂ was 65 times that of chlorines. Consequently, electrogenerated oxidants dominated by H₂O₂ attacked photosystem II of the algae and caused oxidative damage to membrane lipids, affecting the photosynthetic capacity. Eventually, most of the algae were inactivated. The study suggested that C-PTFE GDE was promising for the inactivation of Microcystis aeruginosa in this electrochemical system.

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.

From Fenton and ORR 2e−-Type Catalysts to Bifunctional Electrodes for Environmental Remediation Using the Electro-Fenton Process

Currently, the presence of emerging contaminants in water sources has raised concerns worldwide due to low rates of mineralization, and in some cases, zero levels of degradation through conventional treatment methods. For these reasons, researchers in the field are focused on the use of advanced oxidation processes (AOPs) as a powerful tool for the degradation of persistent pollutants. These AOPs are based mainly on the in-situ production of hydroxyl radicals (OH•) generated from an oxidizing agent (H2O2 or O2) in the presence of a catalyst. Among the most studied AOPs, the Fenton reaction stands out due to its operational simplicity and good levels of degradation for a wide range of emerging contaminants. However, it has some limitations such as the storage and handling of H2O2. Therefore, the use of the electro-Fenton (EF) process has been proposed in which H2O2 is generated in situ by the action of the oxygen reduction reaction (ORR). However, it is important to mention that the ORR is given by two routes, by two or four electrons, which results in the products of H2O2 and H2O, respectively. For this reason, current efforts seek to increase the selectivity of ORR catalysts toward the 2e− route and thus improve the performance of the EF process. This work reviews catalysts for the Fenton reaction, ORR 2e− catalysts, and presents a short review of some proposed catalysts with bifunctional activity for ORR 2e− and Fenton processes. Finally, the most important factors for electro-Fenton dual catalysts to obtain high catalytic activity in both Fenton and ORR 2e− processes are summarized.

Mesoscale Mass Transport Enhancement on Well-Defined Porous Carbon Platform for Electrochemical H2O2 Synthesis

Two-electron oxygen reduction toward hydrogen peroxide (H₂O₂) offers a promising alternative for H₂O₂ production, but its commercial utilization is still hindered by the difficulty of transferring lab-observed catalyst performance to the practical reactor. Here we report the investigation of the porosity engineering effect on catalytic performance inconsistency through a material platform consisting of a series of hollow mesoporous carbon sphere (HMCS) samples. The performance comparison of HMCS samples in rotating ring-disk electrode and Zn-air battery together with the simulation of diffusion behavior reveals that, in low current density conditions, large surface area is preferred, but the mass transport governs the performance in high current density regions. On account of the favorable porous structure, HMCS-8 nm delivers the most excellent practical performance (166 mW cm^-2) and performs well in the bifunctional Zn-air battery for the wastewater purification (70% RhB degraded after 2 min and 99% after 32 min).

Micromodel of a Gas Diffusion Electrode Tracks In-Operando Pore-Scale Wetting Phenomena

Utilizing carbon dioxide (CO₂ ) as a resource for carbon monoxide (CO) production using renewable energy requires electrochemical reactors with gas diffusion electrodes that maintain a stable and highly reactive gas/liquid/solid interface. Very little is known about the reasons why gas diffusion electrodes suffer from unstable long-term operation. Often, this is associated with flooding of the gas diffusion electrode (GDE) within a few hours of operation. A better understanding of parameters influencing the phase behavior at the electrolyte/electrode/gas interface is necessary to increase the durability of GDEs. In this work, a microfluidic structure with multi-scale porosity featuring heterogeneous surface wettability to realistically represent the behavior of conventional GDEs is presented. A gas/liquid/solid phase boundary was established within a conductive, highly porous structure comprising a silver catalyst and Nafion binder. Inoperando visualization of wetting phenomena was performed using confocal laser scanning microscopy (CLSM). Non-reversible wetting, wetting of hierarchically porous structures and electrowetting were observed and analyzed. Fluorescence lifetime imaging microscopy (FLIM) enabled the observation of reactions on the model electrode surface. The presented methodology enables the systematic evaluation of spatio-temporally evolving wetting phenomena as well as species characterization for novel catalyst materials under realistic GDE configurations and process parameters.

Recent advances in carbonaceous catalyst design for the in situ production of H2O2 via two-electron oxygen reduction

The electrochemical oxygen reduction reaction has received increasing attention as a relatively green, safe and sustainable method for in situ hydrogen peroxide (H₂O₂) production. Recently, significant achievements have been made to explore carbon-based (noble metal-free) low-cost and efficient electrocatalysts for H₂O₂ electroproduction, which could potentially replace the traditional anthraquinone process. However, to realize industrial-scale implementation, a highly active and selective catalytic material is needed. In this review paper, we first expound on the oxygen reduction reaction (ORR) mechanism, which is the origin of in situ H₂O₂ production. Then, the recent progress in the development of modified carbon-based catalysts is reviewed and classified, corresponding to their physical or chemical modulation. Furthermore, an overview is provided of the available examples from pilot/large-scale applications. Finally, an outlook on the current challenges and future research prospects to transfer the lab-developed catalysts into pilot or industrial-scale reactors is briefly discussed.

Effect of Persulfate Activation by Electrogenerated H2O2 and Anodic Oxidation on the Color Removal of Dye Solutions at Pt and BDD Anodes

In this study, tartrazine solutions were oxidized using innovative electrochemical advanced oxidation processes (EAOPs) that combined persulfate (PS) activation with electrogenerated H₂O₂, cathodic reduction and anodic oxidation at Pt and BDD anodes, and graphite cathode in an undivided stirred reactor. For the Pt anode, SO₄·^- was generated from a reduction reaction at the cathode and a reaction between the PS and electrogenerated H₂O₂. For the BDD anode, SO₄·^- was generated from a reduction reaction at the cathode, an oxidation reaction at the anode, and a reaction between PS and electrogenerated H₂O₂. Among these activation methods, the activation efficiency of PS by electrogenerated H₂O₂ is much better than other methods. The effects of PS concentration up to 36 mM, applied current density between 6 to 15 mA cm^-2, and temperatures between 25 to 45 °C were investigated. For the electro-Fenton process with Pt anode (Pt-H₂O₂/PS process), the best result for oxidizing 250 mg L^-1 tartrazine solution was obtained with 37.5 mM Na₂SO₄ + 9.0 mM Na₂S₂O₈, applied current density at 12 mA cm^-2 and 45 °C, acquiring total color removal after 30 min reaction. For the electro-Fenton process with BDD anode (BDD-H₂O₂/PS process), the best result for oxidizing 250 mg L^-1 tartrazine solution was obtained with 25 mM Na₂SO₄ + 18 mM Na₂S₂O₈, applied current density at 12 mA cm^-2 and 45 °C, yielding 100% color removal after 30 min reaction. The main oxidizing agents are SO₄·^- and OH· in the anodic oxidation process with PS and the electro-Fenton process with PS. It is concluded that the additions of PS tremendously improve the oxidation power of electro-Fenton processes with PS, especially the Pt-H₂O₂/PS process.

Highly Efficient Electrochemical Production of Hydrogen Peroxide Using the GDE Technology

This work examines the role of oxygen supply in the improvement of the hydrogen peroxide (H₂O₂) electrochemical production efficiency and the generation of high H₂O₂ concentrations in electrochemical processes operated in a discontinuous mode. To conduct this study, a highly efficient Printex L6 carbon-based gas diffusion electrode (GDE) as a cathode was employed for the electrogeneration of H₂O₂ in a flow-by reactor and evaluated the effects of lowering the operation temperature (to increase solubility) and increasing the air supply in the system on H₂O₂ electrogeneration. The results obtained in this study show that unlike what is expected in flow-through reactors, the efficiency in the H₂O₂ production is not affected by the solubility of oxygen when GDE is employed in the electrochemical process (using the flow-by reactor); i.e., the efficiency of H₂O₂ production is not significantly dependent on O₂ solubility, temperature, and pressure. The application of the proposed PL6C-based GDE led to the generation of accumulated H₂O₂ of over 3 g L^-1 at a high current density. It should be noted, however, that the application of the electrocatalyst at lower current densities resulted in higher energy efficiency in terms of H₂O₂ production. Precisely, a specific production of H₂O₂ as high as 131 g kWh^-1 was obtained at 25 mA cm^-2; the energy efficiency (in terms of H₂O₂ production) values obtained in this study based on the application of the proposed GDE in a flow-by reactor at low current densities were found to be within the range of values recorded for H₂O₂ production techniques that employ flow-through reactors.

Flower-like molybdenum disulfide decorated ZIF-8-derived nitrogen-doped dodecahedral carbon for electro-catalytic degradation of phenol

In this work, flower-like molybdenum disulfide was constructed on the surface of ZIF-8-derived nitrogen-doped dodecahedral carbon (ZNC) for the electrocatalytic degradation of phenol. The flower-like nanostructure of MoS₂@ZNC contributed to the exposure of more edge-active sites of MoS₂. At the same time, Mo⁴⁺ and Mo⁶⁺ co-existed in MoS₂@ZNC, which promoted the generation of H₂O₂ and •OH, and improved the catalytic activity of composite materials. In addition, electrochemical performance analysis showed that MoS₂ loaded on the surface of ZNC significantly improved the redox capacity of the material, and the composite ratio of MoS₂ and ZNC affected the structure and properties of MoS₂@ZNC composites. Moreover, the electrochemical performance of prepared MoS₂@ZNC was evaluated by the generation of hydroxyl (•OH) and the degradation efficiency of phenol. The results showed that MoS₂@ZNC-2 had an excellent phenol degradation efficiency (98.8%) and COD removal efficiency (86.8%) within 120 min. Furthermore, MoS₂@ZNC cathode still maintained good performance after being experimented with 20 times, indicated the excellent stability of MoS₂@ZNC.

A state of the art review on electrochemical technique for the remediation of pharmaceuticals containing wastewater

Pharmaceutical wastewater is a frequent kind of wastewater with high quantities of organic pollutants, although little research has been done in the area. Pharmaceutical wastewaters containing antibiotics and high salinity may impair traditional biological treatment, resulting in the propagation of antibiotic resistance genes. The potential for advanced oxidation processes (AOPs) to break down hazardous substances instead of present techniques that essentially transfer contaminants from wastewater to sludge, a membrane filter, or an adsorbent has attracted interest. Among a variety of AOPs, electrochemical systems are a feasible choice for treating pharmaceutical wastewater. Many electrochemical approaches exist now to remediate rivers polluted by refractory organic contaminants, like pharmaceutical micro-pollutants, which have become a severe environmental problem. The first part of this investigation provides the bibliometric analysis of the title search from 1970 to 2021 for keywords such as wastewater and electrochemical. We have provided information on relations between keywords, countries, and journals based on three fields plot, inter-country co-authorship network analysis, and co-occurrence network visualization. The second part introduces electrochemical water treatment approaches customized to these very distinct discarded flows, containing how processes, electrode materials, and operating conditions influence the results (with selective highlighting cathode reduction and anodic oxidation). This section looks at how electrochemistry may be utilized with typical treatment approaches to improve the integrated system's overall efficiency. We discuss how electrochemical cells might be beneficial and what compromises to consider when putting them into practice. We wrap up our analysis with a discussion of known technical obstacles and suggestions for further research.

A novel electrokinetic remediation with in-situ generation of H2O2 for soil PAHs removal

Electrokinetic-Fenton (EK-Fenton) technology requires a high dose of H₂O₂ to produce •OH radicals, which adds a high cost to the remediation process and raises safety concerns during transportation and storage of H₂O₂. Moreover, the remediation efficiency of the conventional EK-Fenton process is low due to the meaningless consumption of H₂O₂ on the electrodes and the alkaline environment near the cathode. In this work, a modified CMK3-gas diffusion electrode (CMK3-GDE) is fabricated. This cathode can continuously generate H₂O₂, and the cumulative H₂O₂ concentration can reach 0.23 M during 10 days of the test. The utilization of cation exchange membranes (CEMs) efficiently restricts the decomposition of H₂O₂ on the electrodes and prevents the alkalization of the soil near the cathode, resulting in a 13.7-43.2% increase of the removal efficiency of polycyclic aromatic hydrocarbons (PAHs). In this new treatment process, PAHs are mainly oxidized into quinones, ketones, alcohols, and small molecule acids, and all these products have lower toxicities than PAHs. The EK-Fenton/CMK3-GDE-CEM system exhibits excellent remediation efficiency for treating PAHs polluted soil, which could be a sustainable, eco-friendly, and low-cost strategy for soil remediation.

Construction of a Microchannel Aeration Cathode for Producing H2O2 via Oxygen Reduction Reaction

Electrochemical oxygen reduction is a promising method for in situ H₂O₂ production. Its important precondition is that dissolved oxygen molecules have to diffuse to and arrive at the cathode surface for reacting with electrons. Obviously, shortening the diffusion distance is beneficial to improve the reaction efficiency. In this study, a novel microchannel aeration mode was proposed to confine the diffusion distance of O₂ to the micrometer level. For this mode, an aeration cathode was fabricated from a carbon block with microchannel arrays. The diameter of each channel was only 10-40 μm. Oxygen will be pumped into every microchannel from the top entry, while an aqueous solution will permeate into microchannels through the bottom entry and pores in the channel wall. This microchannel aeration cathode exhibited a H₂O₂ yield of up to 4.34 mg h^-1 cm^-2, about eight times higher than that of the common bubbling aeration mode. The corresponding energy consumption was only 7.35 kWh kg^-1, which was superior to most reported results. In addition to H₂O₂, this aeration cathode may also produce ^•OH via a one-electron reduction of H₂O₂. In combination with H₂O₂ and ^•OH, phenol, sulfamethoxazole, and atrazine were degraded effectively. We expect that this microchannel aeration cathode may inspire researchers focused on H₂O₂ production, water pollutant control, and other multiphase interfacial reactions.

Cost-efficient microbial electrosynthesis of hydrogen peroxide on a facile-prepared floating electrode by entrapping oxygen

Microbial electrosynthesis of hydrogen peroxide is receiving growing interest for a green substitute for anthraquinone process.However, poor oxygen transmission of electrode remains an obstacle to enhance H₂O₂ production rate without aeration. Here, a superhydrophobic natural air diffusion floating electrode (NADFE), which naturally and efficiently entraps O₂ in the air, was proposed for the first time to improve microbial electrosynthesis of H₂O₂. Furthermore, a one-step calcined electrode preparation method was developed to reduce energy consumption further. In the microbial electrolysis cell with the NADFE, a high H₂O₂ production rate of 39 mg/L/h and current efficiency of 86% were achieved without aeration. The production rate of H₂O₂ was 2.2 times that of a gas diffusion electrode. Importantly, the energy consumption was 34.3 times lower than an electrochemical system. Therefore, the high H₂O₂ production rate and current efficiency, and low energy consumption of the process provide a superior alternative for environmental remediation.

Scaling-up of microbial electrosynthesis with multiple electrodes for in situ production of hydrogen peroxide

Microbial electrosynthesis system (MES) has recently been shown to be a promising alternative way for realizing in situ and energy-saving synthesis of hydrogen peroxide (H₂O₂). Although promising, the scaling-up feasibility of such a process is rarely reported. In this study, a 20-L up-scaled two-chamber MES reactor was developed and investigated for in situ and efficient H₂O₂ electrosynthesis. Maximum H₂O₂ production rate of 10.82 mg L⁻¹ h⁻¹ and cumulative H₂O₂ concentration of 454.44 mg L⁻¹ within 42 h were obtained with an input voltage of 0.6 V, cathodic aeration velocity of 0.045 mL min⁻¹ mL⁻¹, 50 mM Na₂SO₄, and initial pH 3. The electrical energy consumption regarding direct input voltage was only 0.239 kWh kg⁻¹ H₂O₂, which was further much lower compared with laboratory-scale systems. The obtained results suggested that the future industrialization of MES technology for in situ synthesis of H₂O₂ and further application in environmental remediation have broad prospects.

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Up-scaled microbial electrosynthesis with multiple electrodes to synthesize H₂O₂

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The H₂O₂ yield was higher than that of laboratory-scale systems using graphite cathode

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Energy consumption was lower than that of laboratory-scale (bio)electrochemical systems

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Systematic evaluation of the influence of operating parameters on H₂O₂ production