Improving the Efficiency of Carbon Cloth for the Electrogeneration of H2O2: Role of Polytetrafluoroethylene and Carbon Black Loading

All-in-one continuous electrochemical monitoring of 2-phenylphenol removal from water by electro-Fenton treatment

The biggest allure of heterogeneous electro-Fenton (HEF) processes largely fails on its high efficiency for the degradation of a plethora of hazardous compounds present in water, but still challenging to search for good and cost-effective electrocatalyst. In this work, carbon black (CB) and oxidised carbon black (CBox) materials were investigated as cathodes in the electrochemical production of hydrogen peroxide involved in HEF reaction for the degradation of 2-phenylphenol (2PP) as a target pollutant. The electrodes were fabricated by employing carbon cloth as support, and the highest H2O2 production yields were obtained for the CBox, pointing out the beneficial effect of the hydrophilic character of the electrode and oxygen-type functionalization of the carbonaceous surface. HEF degradation of 2PP was explored at -0.7 V vs. Ag/AgCl exhibiting the best conversion rates and degradation grade (total organic carbon) for the CBox-based cathode. In addition, the incorporation of an electrochemical sensor of 2PP in line with the HEF reactor was accomplished by the use of screen-printed electrodes (SPE) in order to monitor the pollutant degradation. The electrochemical sensor performance was evaluated from the oxidation of 2PP in the presence of Fe2+ ions by using square wave voltammetry (SWV) technique. The best electrochemical sensor performance was based on SPE modified with Meldola Blue showing a high sensitivity, low detection limit (0.12 ppm) and wide linear range (0.5-21 ppm) with good reproducibility (RSD 2.3 %). The all-in-one electrochemical station has been successfully tested for the degradation and quantification of 2PP, obtaining good recoveries analysing spiked waters from different water matrices origins.

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.

Electrosynthesis of Hydrogen Peroxide through Selective Oxygen Reduction: A Carbon Innovation from Active Site Engineering to Device Design

Electrochemical synthesis of hydrogen peroxide (H2 O2 ) through the selective oxygen reduction reaction (ORR) offers a promising alternative to the energy-intensive anthraquinone method, while its success relies largely on the development of efficient electrocatalyst. Currently, carbon-based materials (CMs) are the most widely studied electrocatalysts for electrosynthesis of H2 O2 via ORR due to their low cost, earth abundance, and tunable catalytic properties. To achieve a high 2e- ORR selectivity, great progress is made in promoting the performance of carbon-based electrocatalysts and unveiling their underlying catalytic mechanisms. Here, a comprehensive review in the field is presented by summarizing the recent advances in CMs for H2 O2 production, focusing on the design, fabrication, and mechanism investigations over the catalytic active moieties, where an enhancement effect of defect engineering or heteroatom doping on H2 O2 selectivity is discussed thoroughly. Particularly, the influence of functional groups on CMs for a 2e- -pathway is highlighted. Further, for commercial perspectives, the significance of reactor design for decentralized H2 O2 production is emphasized, bridging the gap between intrinsic catalytic properties and apparent productivity in electrochemical devices. Finally, major challenges and opportunities for the practical electrosynthesis of H2 O2 and future research directions are proposed.

Activity and stability of bifunctional perovskite/carbon-based electrodes for the removal of antipyrine by electro-Fenton process

Bifunctional perovskite/carbon-black(CB)/polytetrafluoroethylene(PTFE) electrodes for electro-generation and catalytic decomposition of hydrogen peroxide to oxidizing hydroxyl radicals have been fabricated. These electrodes were tested for electroFenton (EF) removal of antipyrine (ANT) as a model antipyretic and analgesic drug. The influence of the binder loading (20 and 40 wt % PTFE) and type of solvent (1,3-dipropanediol and water) was studied for the preparation of CB/PTFE electrodes. The electrode prepared with 20 wt % PTFE and water exhibited a low impedance and remarkable H₂O₂ electro-generation (about 1 g/L after 240 min, a production rate of ca. 6.5 mg/h·cm²). The incorporation of perovskite on CB/PTFE electrodes was also studied following two different methods: i) direct deposition on the CB/PTFE electrode surface and ii) addition in the own CB/PTFE/water paste used for the fabrication. Physicochemical and electrochemical characterization techniques were used for the electrode's characterization. The dispersion of perovskite particles in the own electrode matrix (method ii) exhibited a higher EF performance than the immobilisation onto the electrode surface (method i). EF experiments at 40 mA/cm² and pH 7 (non-acidified conditions) showed ANT and TOC removals of 30% and 17%, respectively. The increase of current intensity up to 120 mA/cm² achieved the complete removal of ANT and 92% of TOC mineralisation in 240 min. The bifunctional electrode also proved high stability and durability after 15 h of operation.

Photocatalytic Synthesis of Hydrogen Peroxide from Molecular Oxygen and Water

Hydrogen peroxide is a powerful and green oxidant that allows for the oxidation of a wide span of organic and inorganic substrates in liquid media under mild reaction conditions, and forms only molecular water and oxygen as end products. Hydrogen peroxide is therefore used in a wide range of applications, for which the well-documented and established anthraquinone autoxidation process is by far the dominating production method at the industrial scale. As this method is highly energy consuming and environmentally costly, the search for more sustainable synthesis methods is of high interest. To this end, the article reviews the basis and the recent development of the photocatalytic synthesis of hydrogen peroxide. Different oxygen reduction and water oxidation mechanisms are discussed, as well as several kinetic models, and the influence of the main key reaction parameters is itemized. A large range of photocatalytic materials is reviewed, with emphasis on titania-based photocatalysts and on high-prospect graphitic carbon nitride-based systems that take advantage of advanced bulk and surface synthetic approaches. Strategies for enhancing the performances of solar-driven photocatalysts are reported, and the search for new, alternative, photocatalytic materials is detailed. Finally, the promise of in situ photocatalytic synthesis of hydrogen peroxide for water treatment and organic synthesis is described, as well as its coupling with enzymes and the direct in situ synthesis of other technical peroxides.

Electrochemical generation of hydrogen peroxide from a zinc gallium oxide anode with dual active sites

Electrochemical water oxidation enables the conversion of H₂O to H₂O₂. It holds distinct advantages to the O₂ reduction reaction, which is restricted by the inefficient mass transfer and limited solubility of O₂ in aqueous media. Nonetheless, most reported anodes suffer from high overpotentials (usually >1000 mV) and low selectivity. Electrolysis at high overpotentials often causes serious decomposition of peroxides and leads to declined selectivity. Herein, we report a ZnGa₂O₄ anode with dual active sites to improve the selectivity and resist the decomposition of peroxides. Its faradaic efficiency reaches 82% at 2.3 V versus RHE for H₂O₂ generation through both direct (via OH^-) and indirect (via HCO₃^-) pathways. The percarbonate is the critical species generated through the conversion of bicarbonate at Ga-Ga dual sites. The peroxy bond is stable on the surface of the ZnGa₂O₄ anode, significantly improving faradaic efficiency.

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.

Electro-Fenton-Based Technologies for Selectively Degrading Antibiotics in Aqueous Media

The viability of the Electro-Fenton (EF) process in the selective degradation of penicillin G (PenG) in complex solutions has been studied. The role of the anode material (boron-doped diamond (BDD) or mixed metal oxide (MMO)) and the cathode 3D support (foam or mesh), as well as the synergistic effect of UVC light irradiation (photoelectron-Fenton, PEF), have been evaluated. The results show that Pen G can be efficiently and selectively removed by EF, obtaining higher PenG removal rates when using the BDD anode (100%) than when using the MMO anode (75.5%). Additionally, mineralization is not favored under the experimental conditions tested (pH 3, 5 mA cm−2), since both aromatic and carboxylic acids accumulate in the reaction system as final products. In this regard, the EF-treated solution presents a high biological oxygen demand and a low percentage of Vibrio fischeri inhibition, which leads to high biodegradability and low toxicity of this final effluent. Furthermore, the combination with UVC radiation in the PEF process shows a clear synergistic effect on the degradation of penicillin G: 166.67% and 83.18% using MMO and BBD anodes, respectively. The specific energy required to attain the complete removal of PenG and high inhibition of the antibiotic effect is less than 0.05 Ah dm−3. This confirms that PEF can be efficiently used as a pretreatment of conventional wastewater treatment plants to decrease the chemical risk of complex solutions polluted with antibiotics.

Full and Sustainable Electrochemical Production of Chlorine Dioxide

With the final purpose of manufacturing electrochemically-based devices that produce chloride dioxide efficiently, this paper focuses on the production of chlorates and hydrogen peroxide in two different electrochemical cells, in which operation conditions are selected to obtain high efficiencies, and in the subsequent combination of both electrochemically manufactured solutions to produce chlorine dioxide. Results demonstrate that suitable reagents can be produced by electrolyzing 20 g L−1 sodium chloride solutions at 50 mA cm−2 and 50 °C, and 3000 mg L−1 NaClO4 solutions at 5.0 mA cm−2 and 15 °C with current efficiencies of 30.9% and 48.0%, respectively. Different tests performed with these electrolyzed solutions, and also with commercial hydrogen peroxide and chlorate solutions, demonstrate that the ratio between both reagents plays a very important role in the efficiency in the production of chlorine dioxide. Results clearly showed that, surplus chlorate should be contained in the reagent media to prevent further reduction of chlorine dioxide by hydrogen peroxide and consequently, loses of efficiency in the process. During the reaction, a gas with a high oxidation capacity and consisting mainly in chloride dioxide is produced. The results contributed to the maximum conversion reached being 89.65% using electrolyzed solutions as precursors of ClO2, confirming that this technology can be promising to manufacture portable ClO2 devices.

Electrochemical Production of Hydrogen Peroxide in Perchloric Acid Supporting Electrolytes for the Synthesis of Chlorine Dioxide

This work focuses on the electrochemical production of hydrogen peroxide in supporting electrolytes containing perchlorate ions for being used as a reagent in the reduction of chlorates to produce chlorine dioxide, as a first step in the manufacture of portable ClO₂ production devices. This study evaluates the effect of the current density, pressure, and temperature on the production of hydrogen peroxide, and concentrations over 400 mg L^-1 are reached. The average rate for the formation of hydrogen peroxide is 9.85 mg h^-1, and the effect of increasing electrolyte concentration (3.0 and 30.0 g L^-1 perchloric acid), intensity, and pressure results in values of, respectively, -2.99, -4.49, and +7.73 mg h^-1. During the manufacturing process, hydrogen peroxide is decomposed through two mechanisms. The average destruction rate is 1.93 mg h^-1, and the effects of the three factors results in values of, respectively, +0.07, +0.11, and -0.12 mg h^-1. Solutions of this hydrogen peroxide produced electrochemically in a perchloric acid aqueous electrolyte were used to reduce chlorates in strongly acidic media and produce chlorine dioxide. Conversions of around 100% were obtained, demonstrating that this electrochemical product can be used efficiently to reduce chlorates to chlorine dioxide.

Electrosynthesis of >20 g/L H2O2 from Air

Hydrogen peroxide (HP) production via electrochemical oxygen reduction reaction (ORR-HP) is a critical reaction for energy storage and environmental remediation. The onsite production of high-concentration H₂O₂ using gas diffusion electrodes (GDEs) fed by air is especially attractive. However, many studies indicate that the air-GDE combination could not produce concentrated H₂O₂, as the [H₂O₂] leveled off or even decreased with the increasing reaction time. This study proves that the limiting factors are not the oxygen concentration in the air but the anodic and cathodic depletion of the as-formed H₂O₂. We proved that the anodic depletion could be excluded by adopting a divided electrolytic cell. Furthermore, we demonstrated that applying poly(tetrafluoroethylene) (PTFE) as an overcoating rather than a catalyst binder could effectively mitigate the cathodic decomposition pathways. Beyond that, we further developed a composite electrospun PTFE (E-PTFE)/carbon black (CB)/GDE electrode featuring the electrospun PTFE (E-PTFE) nanofibrous overcoating. The E-PTFE coating provides abundant triphase active sites and excludes the cathodic depletion reaction, enabling the production of >20 g/L H₂O₂ at a current efficiency of 86.6%. Finally, we demonstrated the efficacy of the ORR-HP device in lake water remediation. Cyanobacteria and microcystin-LR were readily removed along with the onsite production of H₂O₂.

Heterogeneous Electro-Fenton-like Designs for the Disposal of 2-Phenylphenol from Water

The hunt for efficient and environmentally friendly degradation processes has positioned the heterogeneous advanced oxidation processes as an alternative more interesting and economical rather than homogenous processes. Hence, the current study lies in investigating the efficiency of different heterogeneous catalysts using transition metals in order to prevent the generation of iron sludge and to extend the catalogue of possible catalysts to be used in advanced oxidation processes. In this study, nickel and zinc were tested and the ability for radical-generation degradation capacity of both ions as homogeneous was evaluated in the electro-Fenton-like degradation of 2-phenylphenol. In both cases, the degradation profiles followed a first-order kinetic model with the highest degradation rate for nickel (1 mM) with 2-phenylphenol removal level of 90.12% and a total organic reduction near 70% in 2 h. To synthesise the heterogeneous nickel catalyst, this transition metal was fixed on perlite by hydrothermal treatment and in a biochar or carbon nanofibers by adsorption. From the removal results using the three synthesized catalysts, it is concluded that the best catalysts were obtained by inclusion of nickel on biochar or nanofibers achieving in both with removal around 80% before 1 h. Thus, to synthetize a nickel electrocatalyst, nickel doped nanofibers were included on carbon felt. To do this, the amount of carbon black, nickel nanofibers and polytetrafluoroethylene to add on the carbon felt was optimized by Taguchi design. The obtained results revealed that under the optimised conditions, a near-complete removal was achieved after 2 h with high stability of the nickel electrocatalyst that open the applicability of this heterogeneous system to operate in flow systems.

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.

Janus graphite felt cathode dramatically enhance the H2O2 yield from O2 electroreduction by the hydrophilicity-hydrophobicity regulation

Hydrogen peroxide (H₂O₂) electrosynthesis from 2-electron O₂ reduction reaction (2eORR) is widely regarded as a promising alternative to the current industry-dominant anthraquinone process. Design and fabrication of effective, low-cost carbon-based electrodes is one of the priorities. Many previous work well confirmed that hydrophilic carbon-based electrodes are preferable for 2eORR. Here, we proposed a strategy of hydrophilicity-hydrophobicity regulation. By using commercially available graphite felt (GF) as electrodes, we showed that both hydrophilic GF, hydrophobic GF, and Janus GF yielded significantly higher H₂O₂ production, which is 7.3 times, 7.6 times, and 7.7 times higher than the original GF, respectively. Results showed that currents and stirring rates affect the H₂O₂ yields. The enhancement of hydrophilic GF is due to the incorporation of oxygen-containing functional groups, while the hydrophobic and Janus GF comes from the locally confined O₂ bubbles, which built a gas-liquid-solid interface inside GF and thus enhance the H₂O₂ formation kinetics. Finally, the effectiveness of the hydrophilicity-hydrophobicity regulation concept was tested in Electro-Fenton process by removing typical dyes and antibiotics. This work supply an effective but facile strategy to enhance the performance of carbon-based electrodes towards 2eORR by regulating the micro-environment of electrodes.

Photo-assisted electrochemical advanced oxidation processes for the disinfection of aqueous solutions: A review

Disinfection is usually the final step in water treatment and its effectiveness is of paramount importance in ensuring public health. Chlorination, ultraviolet (UV) irradiation and ozone (O3) are currently the most common methods for water disinfection; however, the generation of toxic by-products and the non-remnant effect of UV and O3 still constitute major drawbacks. Photo-assisted electrochemical advanced oxidation processes (EAOPs) on the other hand, appear as a potentially effective option for water disinfection. In these processes, the synergism between electrochemically produced active species and photo-generated radicals, improve their performance when compared with the corresponding separate processes and with other physical or chemical approaches. In photo-assisted EAOPs the inactivation of pathogens takes place by means of mechanisms that occur at different distances from the anode, that is: (i) directly at the electrode's surface (direct oxidation), (ii) at the anode's vicinity by means of electrochemically generated hydroxyl radical species (quasi-direct), (iii) or at the bulk solution (away from the electrode surface) by photo-electrogenerated active species (indirect oxidation). This review addresses state of the art reports concerning the inactivation of pathogens in water by means of photo-assisted EAOPs such as photo-electrocatalytic process, photo-assisted electrochemical oxidation, photo-electrocoagulation and cathodic processes. By focusing on the oxidation mechanism, it was found that while quasi-direct oxidation is the preponderant inactivation mechanism, the photo-electrocatalytic process using semiconductor materials is the most studied method as revealed by numerous reports in the literature. Advantages, disadvantages, trends and perspectives for water disinfection in photo-assisted EAOPs are also analyzed in this work.

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

Nowadays, it is a great challenge to minimize the negative impact of hazardous organic compounds in the environment. Highly efficient hydrogen peroxide (H₂O₂) production through electrochemical methods with gas diffusion electrodes (GDEs) is greatly demand for degradation of organic pollutants that present in drinking water and industrial wastewater. The GDEs as cathodic electrocatalyst manifest more cost-effective, lower energy consumption and higher oxygen utilization efficiency for H₂O₂ production as compared to other carbonaceous cathodes due to its worthy chemical and physical characteristics. In recent years, the crucial research and practical application of GDE for degradation of organic pollutants have been well developed. In this review, we focus on the novel design, fundamental aspects, influence factors, and electrochemical properties of GDEs. Furthermore, we investigate the generation of H₂O₂ through electrocatalytic processes and degradation mechanisms of refractory organic pollutants on GDEs. We describe the advanced methodologies towards electrochemical kinetics, which include the enhancement of GDEs electrochemical catalytic activity and mass transfer process. More importantly, we also highlight the other technologies assisted electrochemical process with GDEs to enlarge the practical application for water treatment. In addition, the developmental prospective and the existing research challenges of GDE-based electrocatalytic materials for real applications in H₂O₂ production and wastewater treatment are forecasted. According to our best knowledge, only few review articles have discussed GDEs in detail for H₂O₂ production and their applications for degradation of organic pollutants in water.

Does intensification with UV light and US improve the sustainability of electrolytic waste treatment processes?

This work aims to assess the influence of ultrasounds (US) application or ultraviolet (UV) light irradiation on the efficiency and sustainability of the treatment of wastes by conductive diamond electrochemical oxidation (CDEO). To do this, a life cycle assessment (LCA) is carried out in order to quantify the environmental impacts of the intensified CDEO processes. Inventories of three bench scale remediation plants (CDEO, Sono-CDEO and Photo-CDEO) in which the different technologies are implemented are performed by means of Ecoinvent 3.3 data base. AWARE, USEtox, IPPC and ReCiPe methodologies are used to quantify the environmental burden into 5 midpoint (water footprint, global warming 100a, ozone layer depletion, human toxicity, freshwater ecotoxicity) and 17 endpoint impact categories. Photo-CDEO attains the faster and more efficient removal in terms of energy consumed. All impact categories are lower in the case in which UV light irradiation is coupled to the CDEO treatment, particularly if the electrolyte does not contain chloride anions. From the point of view of toxicity and ecotoxicity, it is essential to achieve a complete mineralization, because of the intermediates generated into wastes containing chloride anions can become more hazardous than the initial pesticide. The operation of these technologies at large current densities shows positive results from the sustainability point of view, despite the huge environmental impact related to the energy production. Data notice that almost a 99.0% of the total global warming potential is mainly due to the electricity required during the electrochemical treatment, being higher by the sono and photo CDEO treatments because of the use of additional devices. Nevertheless, this issue can be overcome by means of using renewable energies as power sources of these remediation treatments. According to results, it can be claimed that the electrochemical technologies may successfully compete with other AOPs in terms of sustainability.

Supported Atomically-Precise Gold Nanoclusters for Enhanced Flow-through Electro-Fenton

Gold (Au) has been considered catalytically inert for decades, but recent reports have described the ability of Au nanoparticles to catalyze H₂O₂ decomposition in the Haber-Weiss cycle. Herein, the design and demonstration of a flow-through electro-Fenton system based on an electrochemical carbon nanotube (CNT) filter functionalized with atomically precise Au nanoclusters (AuNCs) is described. The functionality of the device was then tested for its ability to catalyze antibiotic tetracycline degradation. In the functional filters, the Au core of AuNCs served as a high-performance Fenton catalyst; while the AuNCs ligand shells enabled CNT dispersion in aqueous solution for easy processing. The hybrid filter enabled in situ H₂O₂ production and catalyzed the subsequent H₂O₂ decomposition to HO·. The catalytic function of AuNCs lies in their ability to undergo redox cycling of Au⁺/Au⁰ under an electric field. The atomically precise AuNCs catalysts demonstrated superior catalytic activity to larger nanoparticles; while the flow-through design provided convection-enhanced mass transport, which yielded a superior performance compared to a conventional batch reactor. The adsorption behavior and decomposition pathway of H₂O₂ on the filter surfaces were simulated by density functional theory calculations. The research outcomes provided atomic-level mechanistic insights into the Au-mediated Fenton reaction.

Highly efficient electrosynthesis of hydrogen peroxide on a superhydrophobic three-phase interface by natural air diffusion

Hydrogen peroxide (H₂O₂) synthesis by electrochemical oxygen reduction reaction has attracted great attention as a green substitute for anthraquinone process. However, low oxygen utilization efficiency (<1%) and high energy consumption remain obstacles. Herein we propose a superhydrophobic natural air diffusion electrode (NADE) to greatly improve the oxygen diffusion coefficient at the cathode about 5.7 times as compared to the normal gas diffusion electrode (GDE) system. NADE allows the oxygen to be naturally diffused to the reaction interface, eliminating the need to pump oxygen/air to overcome the resistance of the gas diffusion layer, resulting in fast H₂O₂ production (101.67 mg h^-1 cm^-2) with a high oxygen utilization efficiency (44.5%–64.9%). Long-term operation stability of NADE and its high current efficiency under high current density indicate great potential to replace normal GDE for H₂O₂ electrosynthesis and environmental remediation on an industrial scale.

H₂O₂ electrosynthesis has garnered great attention as a green alternative to the anthraquinone process. Here the authors propose a cost-effective cathode to greatly improve the O₂ diffusion coefficient, resulting in a high H₂O₂ production without the need for aeration.

Flexible 3D carbon cloth as a high-performing electrode for energy storage and conversion

High-performance energy storage and conversion devices with high energy density, power density and long-term cycling life are of great importance in current consumer electronics, portable electronics and electric vehicles. Carbon materials have been widely investigated and utilized in various energy storage and conversion devices due to their excellent conductivity, mechanical and chemical stability, and low cost. Abundant excellent reviews have summarized the most recent progress and future outlooks for most of the current prime carbon materials used in energy storage and conversion devices, such as carbon nanotubes, fullerene, graphene, porous carbon and carbon fibers. However, the significance of three-dimensional (3D) commercial carbon cloth (CC), one of the key carbon materials with outstanding mechanical stability, high conductivity and flexibility, in the energy storage and conversion field, especially in wearable electronics and flexible devices, has not been systematically summarized yet. In this review article, we present a careful investigation of flexible CC in the energy storage and conversion field. We first give a general introduction to the common properties of CC and the roles it has played in energy storage and conversion systems. Then, we meticulously investigate the crucial role of CC in typical electrochemical energy storage systems, including lithium-ion batteries, sodium-ion batteries, lithium-sulfur batteries and supercapacitors. Following a description of the wide application potential of CC in electrocatalytic hydrogen evolution, oxygen evolution/reduction, full-water splitting, etc., we will give a brief introduction to the application of CC in the areas of photocatalytically and photoelectrochemically induced solar energy conversion and storage. The review will end with a brief summary of the typical superiorities that CC has in current energy conversion and storage systems, as well as providing some perspectives and outlooks on its future applications in the field. Our main interest will be focused on CC-based flexible devices due to the inherent superiority of CC and the increasing demand for flexible and wearable electronics.

A comparison between flow-through cathode and mixed tank cells for the electro-Fenton process with conductive diamond anode

This work focusses on the production of hydrogen peroxide and in the removal of bromacil by the electro-Fenton process using two different electrochemical cells: mixed tank cell (MTC) and flow-through cell (FTC). Both cells use boron doped diamond (BDD) as anode and carbon felt as cathode to promote the formation of hydrogen peroxide. In the case of the MTC, two surface area ratios, A_cathode/A_anode, have been used. Results show that the H₂O₂ produced by MTC and FTCPSC increases with the time until a stabilization state. For the FTCPSC, the average hydrogen peroxide concentration produced increases progressively with the current, while for MTC the maximum values are found in applying very low current densities. In addition, the FTCPSC provides higher concentrations of hydrogen peroxide for the same current density applied. Regarding the MTC, it can be stated that the higher the area of the cathode, the higher is the amount of H₂O₂ produced and the lower is the cell voltage (because of a more efficient current lines distribution). The initial oxidation of bromacil is very efficiently attained being rapidly depleted from wastewater. However, the higher production of hydrogen peroxide obtained by the FTCPSC cell does not reflect on a better performance of the electro-Fenton process. Thus, bromacil is better mineralized using the MTC cell with the lowest cathode area. This observation has been explained because larger concentrations of produced hydrogen peroxide seems to benefit the oxidation of intermediates and not the mineralization.

Electro Fenton removal of clopyralid in soil washing effluents

The removal of a commercial herbicide, based on clopyralid, by means of Electro-Fenton (EF) was studied using a soil washing effluent obtained using synthetic ground water as washing fluid. From the results, it was observed that the degradation and mineralization yields of clopyralid were high, even without the addition of supporting electrolyte. The groundwater could be then used as a sustainable supporting electrolyte. The influence of the minerals constituents, the current and the ferrous ions regeneration was evaluated. The highest hydrogen peroxide production was achieved working at 200 mA but regeneration of ferrous ions was not efficient at this current. Iodide ions were one of the main responsible in the EF efficiency decrease due to their reaction with the produced hydrogen peroxide. Electrochemical study proved that clopyralid was not electroactive and that its degradation was mainly due to radical oxidation. Long duration electrolysis carried out at 200 mA in groundwater provided an improvement of the solution biodegradability after 480 min that can be linked to a significant increase in the carboxylic acids production. These results support the feasibility of applying an EF process in order to carry out a subsequent biological mineralization.

Superhydrophobic Air-Breathing Cathode for Efficient Hydrogen Peroxide Generation through Two-Electron Pathway Oxygen Reduction Reaction

Electrochemical catalysis of carbon-based material via two-electron pathway oxygen reduction reaction (ORR) offers great potential for in situ hydrogen peroxide (H₂O₂) production. In this work, we tuned catalyst mesostructure and hydrophilicity/hydrophobicity by adjusting polytetrafluoroethylene (PTFE) content in graphite/carbon black/PTFE hybrid catalyst layer (CL), aimed to improving the two-electron ORR activity for efficient H₂O₂ generation. As the only superhydrophobic CL with initiating contact angles of 141.11°, PTFE_0.57 obtained the highest H₂O₂ yield of 3005 ± 58 mg L^-1 h^-1 (at 25 mA cm^-2) and highest current efficiency (CE) of 84% (at 20 mA cm^-2). Rotating ring disk electrode (RRDE) results demonstrated that less PTFE content in CLs results in less electrons transferred and better selectivity toward two-electron ORR. Though the highest H₂ concentration (2 μmol L^-1 at 25 mA cm^-2) was monitored from PTFE_0.57 which contained the lowest PTFE, the CE decreased inversely with increasing content of PTFE, which proved that the H₂O₂ decomposition reaction was the major side reaction. Higher PTFE content increased the hydrophilicity of CL for excessive H⁺ and insufficient O₂ diffusion, which induced H₂O₂ decomposition into H₂O. Simultaneously, the electroactive surface area of CLs decreased with higher PTFE content, from 0.0041 m² g^-1 of PTFE_0.57 to 0.0019 m² g^-1 of PTFE_4.56. Besides, higher PTFE content in CL leads to the increase of total impedance (from 14.5 Ω of PTFE_0.57 to 18.3 Ω of PTFE_4.56), which further hinders the electron transfer and ORR activity.

Antituberculosis drug isoniazid degraded by electro-Fenton and photoelectro-Fenton processes using a boron-doped diamond anode and a carbon-PTFE air-diffusion cathode

Solutions with 0.65 mM of the antituberculosis drug isoniazid (INH) in 0.050 M Na₂SO₄ at pH 3.0 were treated by electro-Fenton (EF) and UVA photoelectro-Fenton (PEF) processes using a cell with a BDD anode and a carbon-PTFE air-diffusion cathode. The influence of current density on degradation, mineralization rate, and current efficiency has been thoroughly evaluated in EF. The effect of the metallic catalyst (Fe²⁺ or Fe³⁺) and the formation of products like short-chain linear aliphatic carboxylic acids were assessed in PEF. Two consecutive pseudo-first-order kinetic regions were found using Fe²⁺ as catalyst. In the first region, at short time, the drug was rapidly oxidized by ^●OH, whereas in the second region, at longer time, a resulting Fe(III)-INH complex was much more slowly removed by oxidants. INH disappeared completely at 300 min by EF, attaining 88 and 94% mineralization at 66.6 and 100 mA cm^-2, respectively. Isonicotinamide and its hydroxylated derivative were identified as aromatic products of INH by GC-MS and oxalic, oxamic, and formic acids were quantified by ion-exclusion HPLC. The PEF treatment of a real wastewater polluted with the drug led to slower INH and TOC abatements because of the parallel destruction of its natural organic matter content.