Enhanced H2O2 production by selective electrochemical reduction of O2 on fluorine-doped hierarchically porous carbon

Enhancement of H2O2 yield and TOC removal in electro-peroxone process by electrochemically modified graphite felt: Performance, mechanism and stability

Electro-peroxone (EP) is an emerging advanced oxidation process which combines electro-generation H₂O₂ and ozone for removing organic contaminants. In this paper, a platinum plate as anode, a method of electrochemical oxidation is adopted to modify graphite felt (GF) cathode to promote H₂O₂ yield and TOC removal from oxalic acid solution in EP process, its performance, mechanism and stability were discussed. Compared with original GF cathode, 2.6 times H₂O₂ yield can be achieved by the 5 min electrochemically modified GF (GF-5). The high electrochemical activity of the modified GF can be ascribed to introducing numerous surface oxygen-containing functional groups (OGs), which not only decreased the impedance, but also increased the amount of active site of O₂ reduction. The production of H₂O₂ with GF-5 cathode improved with the increased initial pH, cathodic potential and O₂ flow rate, while this promoting effect was not observed in GF cathode. Compared with GF cathode, TOC removal rate was improved by 21.5% with GF-5 cathode due to higher H₂O₂ yield in EP process. The primary pathway of TOC removal is electrochemically-driven peroxone process, and hydroxyl radical (·OH) is the dominant reactive species. Furthermore, GF-5 cathode had a good stability due to the protection of H₂O₂ and free electrons injected. The results indicate that the electrochemically modified GF severed as the cathode of EP processes has significant efficiency and stability in the removal of ozone-refractory organic contaminants.

Amplifying anti-flooding electrode to fabricate modular electro-fenton system for degradation of antiviral drug lamivudine in wastewater

The advanced oxidation based on in-situ hydrogen peroxide production using carbon air cathode is very potential for wastewater treatment. However, catalyst flooding and complex assembly patterns are the bottleneck limiting the air cathode to the long-term and large-scale application. In this work, a novel anti-flooding air-breathing cathode (ABC) was prepared by a simple rolling-spraying method with relatively low price commercial materials. The novel method changed the morphology of gas diffusion layer as well as adjusted the hydrophobicity of air side of the catalyst layer. As a result, water-air distribution management was achieved and TPI disequilibrium was prevented. Compare with traditional ABC, the H₂O₂ yield and current efficiency (CE) of optimized anti-flooding ABC (ABC_0.9) increased by 13.5% (941 ± 10 mg·L^-1·h^-1 with CE of 84% at 30 mA·cm^-2), the material cost and fabrication time decreased by 10.1% (2.32 ¥·dm^-2, ~0.36 $·dm^-2) and 40%. Amplified ABC coupled with Ti/IrO₂ anodes were integrated into a modular electrode used for H₂O₂generation. When the current density (j) increased from 10 to 30 mA·cm^-2, the energy cost increased from 0.19 to 0.43 ¥·mol^-1 H₂O₂ (from 0.03 to 0.07 $·mol^-1 H₂O₂). The modular electrode was utilized in a 2 L pre-pilot scale reactor for antiviral drug lamivudine degradation by electro-Fenton (EF) process. 100% of lamivudine and 78.1% of total organic carbon (TOC) were removed within 60 min at 20 mA·cm^-2. The susceptible sites on the lamivudine toward hydroxyl radicals were investigated and transformation products (TP) as well as degradation pathway were studied.

Coupling Co-N-C with MXenes Yields Highly Efficient Catalysts for H2O2 Production in Acidic Media

The electrochemical oxygen reduction reaction (ORR) offers a promising method to replace the anthraquinone process for hydrogen peroxide (H₂O₂) production. However, the efficiency of this process suffers from sluggish kinetics, particularly in an acidic environment. Therefore, employing catalysts with high electroactivity is highly desirable for H₂O₂ synthesis. Here, an effective strategy for preparing Co-N-C/Ti₃C₂T_x with high H₂O₂ selectivity and ORR reactivity is proposed. The acquired Co-N-C/Ti₃C₂T_x shows excellent H₂O₂ electrosynthesis performance in acidic media with H₂O₂ productivity of up to 3200 ppm h^-1, superior to state-of-the-art catalysts. Interestingly, a H₂O₂ concentration of 6.0 wt % was obtained after the stability test, and the Co-N-C/Ti₃C₂T_x catalyst was found to effectively catalyze organic dye degradation. Further analysis reveals that the enhanced H₂O₂ electrosynthesis performance originates from the layered structure and the oxygen functional groups of Ti₃C₂T_x. The layered structure can effectively promote increased exposure of active sites, while the oxygen functional groups will fine-tune the electronic structure of Co atoms, allowing a selective ORR pathway to produce H₂O₂. This work provides a strategy to design and fabricate highly efficient catalysts for H₂O₂ production and degradation of organic pollutants.

Electro-Fenton improving fouling mitigation and microalgae harvesting performance in a novel membrane photobioreactor

An innovative electro-Fenton enhanced membrane photobioreactor with satisfactory membrane fouling mitigation was constructed for microalgae harvesting. The porous carbon and carbon nanotubes hollow fiber membranes (PC-CHFMs) were used as the separation unit and cathode, simultaneously. H₂O₂ was generated by cathode reducing O₂ in-situ, which would further produce •OH as the main oxidant by coupling H₂O₂ with Fe²⁺. The •OH could deeply remove the extracellular organic matter (EOM) deposited on the membrane surface or inside the pores. Experimental results showed that the permeate flux recovery rates of PC-CHFMs by electro-Fenton at the 18th, 29th and 41st day were 100%, 100% and 98.3%, respectively. The corresponding recovery rates by chemical cleaning at the same time were 99.8%, 81.7% and 54.4%. The stable and high permeate flux of PC-CHFMs made a great contribution to the microalgae harvesting efficiency, where the concentration factor could be 4.8 times higher than that of the control group. Filtrating superiority of PC-CHFMs was becoming more prominent with the extension of operating time. In addition, the removal efficiency of NH₄⁺-N and TP in wastewater was approximately 100% at stable culture period.

Treatment of organic wastewater by a synergic electrocatalysis process with Ti3+ self-doped TiO2 nanotube arrays electrode as both cathode and anode

Electrochemical anodic oxidation (AO) is a promising technology for wastewater treatment due to its strong oxidation property and environmental compatibility. However, it suffers from high energy consumption for pollutants removal due to the side-reactions of hydrogen evolution reaction on cathode and oxygen evolution reaction on anode. Combining electro-Fenton (EF) with AO not only generated •OH for pollutants degradation but also increased current efficiency. This work investigated a synergic electrocatalysis process between EF and AO with Ti³⁺ self-doped TiO₂ nanotube arrays (Ti³⁺/TNTAs) electrode as both cathode and anode for wastewater treatment. The pseudo-first-order kinetic rate constant of phenol degradation by EF+AO (0.107 min^-1) was 9.7 or 6.3 times as much as that of only EF (0.011 min^-1) or AO (0.017 min^-1) process, respectively. Enhanced pollutants removal of EF+AO could be attributed to the coexistence of •OH oxidation and direct oxidation on Ti³⁺/TNTAs surface. The COD of secondary effluent of coking wastewater decreased from 159.3 mg L^-1 to 47.0 mg L^-1 by EF+AO within 120 min with low specific energy consumption (9.5 kWh kg^-1 COD^-1). This work provided a new insight into design of the energy-efficient synergic electrocatalysis process for refractory pollutants degradation.

Using Palladium and Gold Palladium Nanoparticles Decorated with Molybdenum Oxide for Versatile Hydrogen Peroxide Electroproduction on Graphene Nanoribbons

Electrocatalytic production of H₂O₂ via a two-electron oxygen reduction reaction (ORR-2e^-) is regarded as a highly promising decentralized and environmentally friendly mechanism for the production of this important chemical commodity. However, the underlying challenges related to the development of catalytic materials that contain zero or low content of noble metals and that are relatively more active, selective, and resistant for long-term use have become a huge obstacle for the electroproduction of H₂O₂ on commercial and industrial scales. The present study reports the synthesis and characterization of low metal-loaded (≤6.4 wt %) catalysts and their efficiency in H₂O₂ electroproduction. The catalysts were constructed using gold palladium molybdenum oxide (AuPdMoO_x) and palladium molybdenum oxide (PdMoO_x) nanoparticles supported on graphene nanoribbons. Based on the application of a rotating ring-disk electrode, we conducted a thorough comparative analysis of the electrocatalytic performance of the catalysts in the ORR under acidic and alkaline media. The proposed catalysts exhibited high catalytic activity (ca. 0.08 mA g_noble metal^-1 in an acidic medium and ca. 6.6 mA g_noble metal^-1 in an alkaline medium), good selectivity (over 80%), and improved long-term stability toward ORR-2e^-. The results obtained showed that the enhanced ORR activity presented by the catalysts, which occurred preferentially via the two-electron pathway, was promoted by a combination of factors including geometry, Pd content, interparticle distance, and site-blocking effects, while the electrochemical stability of the catalysts may have been enhanced by the presence of MoO_x.

Fast Generation of Hydroxyl Radicals by Rerouting the Electron Transfer Pathway via Constructed Chemical Channels during the Photo-Electro-Reduction of Oxygen

A strategy for the fast generation of hydroxyl radicals (HO·) via photo-electro-reduction of oxygen by rerouting the electron transfer pathway was proposed. The rate-determining step of HO· production is the formation of H₂O₂ and the simultaneous reduction of H₂O₂. Engineering of F-TiO₂ with single atom Pd bonded with four F and two O atoms favored the electrocatalytic 2-electron oxygen reduction to H₂O₂ with as high as 99% selectivity, while the additional channel bond HO-O···Pd-F-TiO₂ facilitates the photogenerated electron transfer from the conduction band to single atom Pd to reduce Pd···O-OH to HO·. The optimized HO· production rate is 9.18 μ mol L^-1 min^-1, which is 2.6-52.5 times higher than that in traditional advanced oxidation processes. In the application of wastewater treatment, this proposed photoelectrocatalytic oxygen reduction method, respectively, shows fast kinetics of 0.324 and 0.175 min^-1 for removing bisphenol A and acetaminophen. Around 93.2% total organic carbon and 99.3% acute toxicity removal were achieved. Additionally, the degradation efficiency was less affected by the water source and pH value because of the evitable usage of metallic active sites. This work represents a fundamental investigation on the generation rate of HO·, which would pave the way for the future development of photoelectrocatalytic technologies for water purification.

Regulable pyrrolic-N-doped carbon materials as an efficient electrocatalyst for selective O2 reduction to H2O2

Metal-free nitrogen-doped carbon catalysts with pyrrolic-N as the active site were efficiently synthesized via a direct carbonization method, which exibited a high two-electron oxygen reduction reaction performance.

Carbon-Based Electrocatalysts for Efficient Hydrogen Peroxide Production

Hydrogen peroxide (H₂ O₂ ) is an environment-friendly and efficient oxidant with a wide range of applications in different industries. Recently, the production of hydrogen peroxide through direct electrosynthesis has attracted widespread research attention, and has emerged as the most promising method to replace the traditional energy-intensive multi-step anthraquinone process. In ongoing efforts to achieve highly efficient large-scale electrosynthesis of H₂ O₂ , carbon-based materials have been developed as 2e^- oxygen reduction reaction catalysts, with the benefits of low cost, abundant availability, and optimal performance. This review comprehensively introduces the strategies for optimizing carbon-based materials toward H₂ O₂ production, and the latest advances in carbon-based hybrid catalysts. The active sites of the carbon-based materials and the influence of coordination heteroatom doping on the selectivity of H₂ O₂ are extensively analyzed. In particular, the appropriate design of functional groups and understanding the effect of the electrolyte pH are expected to further improve the selective efficiency of producing H₂ O₂ via the oxygen reduction reaction. Methods for improving catalytic activity by interface engineering and reaction kinetics are summarized. Finally, the challenges carbon-based catalysts face before they can be employed for commercial-scale H₂ O₂ production are identified, and prospects for designing novel electrochemical reactors are proposed.

Carbon-based metal-free electrocatalysts: from oxygen reduction to multifunctional electrocatalysis

Since the discovery of N-doped carbon nanotubes as the first carbon-based metal-free electrocatalyst (C-MFEC) for oxygen reduction reaction (ORR) in 2009, C-MFECs have shown multifunctional electrocatalytic activities for many reactions beyond ORR, such as oxygen evolution reaction (OER), hydrogen evolution reaction (HER), carbon dioxide reduction reaction (CO₂RR), nitrogen reduction reaction (NRR), and hydrogen peroxide production reaction (H₂O₂PR). Consequently, C-MFECs have attracted a great deal of interest for various applications, including metal-air batteries, water splitting devices, regenerative fuel cells, solar cells, fuel and chemical production, water purification, to mention a few. By altering the electronic configuration and/or modulating their spin angular momentum, both heteroatom(s) doping and structural defects (e.g., atomic vacancy, edge) have been demonstrated to create catalytic active sites in the skeleton of graphitic carbon materials. Although certain C-MFECs have been made to be comparable to or even better than their counterparts based on noble metals, transition metals and/or their hybrids, further research and development are necessary in order to translate C-MFECs for practical applications. In this article, we present a timely and comprehensive, but critical, review on recent advancements in the field of C-MFECs within the past five years or so by discussing various types of electrocatalytic reactions catalyzed by C-MFECs. An emphasis is given to potential applications of C-MFECs for energy conversion and storage. The structure-property relationship for and mechanistic understanding of C-MFECs will also be discussed, along with the current challenges and future perspectives.

Structural Modulation on NiCo2 S4 Nanoarray by N Doping to Enhance 2e-ORR Selectivity for Photothermal AOPs and Zn-O2 Batteries*

As a H₂ O₂ generator, a 2e^- oxygen reduction reaction active electrocatalyst plays an important role in the advanced oxidation process to degrade organic pollutants in sewage. To enhance the tendency of NiCo₂ S₄ towards the 2e^- reduction reaction, N atoms are doped in its structure and replace S^2- . The result implies that this weakens the interaction between NiCo₂ S₄ and OOH*, suppresses O-O bond breaking and enhances H₂ O₂ selectivity. This electrocatalyst also shows photothermal effect. Under photothermal heating, H₂ O₂ produced by the oxidation reduction reaction can decompose and releaseOH, which degrades organic pollutants through the advanced oxidation process. Photothermal effect induced by the advance oxidation process shows obvious advantages over the traditional Fenton reaction, such as wide pH adaptation scope and low secondary pollutant due to its Fe²⁺ free character. With Zn as anode and the electrocatalyst as cathode material, a Zn-O₂ battery is assembled. It achieves electricity generation and photothermal effect induced by the advance oxidation process simultaneously.

Recent Progress of Electrochemical Production of Hydrogen Peroxide by Two-Electron Oxygen Reduction Reaction

Shifting electrochemical oxygen reduction reaction (ORR) via two-electron pathway becomes increasingly crucial as an alternative/green method for hydrogen peroxide (H2 O2 ) generation. Here, the development of 2e- ORR catalysts in recent years is reviewed, in aspects of reaction mechanism exploration, types of high-performance catalysts, factors to influence catalytic performance, and potential applications of 2e- ORR. Based on the previous theoretical and experimental studies, the underlying 2e- ORR catalytic mechanism is firstly unveiled, in aspect of reaction pathway, thermodynamic free energy diagram, limiting potential, and volcano plots. Then, various types of efficient catalysts for producing H2 O2 via 2e- ORR pathway are summarized. Additionally, the catalytic active sites and factors to influence catalysts' performance, such as electronic structure, carbon defect, functional groups (O, N, B, S, F etc.), synergistic effect, and others (pH, pore structure, steric hindrance effect, etc.) are discussed. The H2 O2 electrogeneration via 2e- ORR also has various potential applications in wastewater treatment, disinfection, organics degradation, and energy storage. Finally, potential future directions and prospects in 2e- ORR catalysts for electrochemically producing H2 O2 are examined. These insights may help develop highly active/selective 2e- ORR catalysts and shape the potential application of this electrochemical H2 O2 producing method.

Highly active and selective oxygen reduction to H2O2 on boron-doped carbon for high production rates

Oxygen reduction reaction towards hydrogen peroxide (H₂O₂) provides a green alternative route for H₂O₂ production, but it lacks efficient catalysts to achieve high selectivity and activity simultaneously under industrial-relevant production rates. Here we report a boron-doped carbon (B-C) catalyst which can overcome this activity-selectivity dilemma. Compared to the state-of-the-art oxidized carbon catalyst, B-C catalyst presents enhanced activity (saving more than 210 mV overpotential) under industrial-relevant currents (up to 300 mA cm^-2) while maintaining high H₂O₂ selectivity (85-90%). Density-functional theory calculations reveal that the boron dopant site is responsible for high H₂O₂ activity and selectivity due to low thermodynamic and kinetic barriers. Employed in our porous solid electrolyte reactor, the B-C catalyst demonstrates a direct and continuous generation of pure H₂O₂ solutions with high selectivity (up to 95%) and high H₂O₂ partial currents (up to ~400 mA cm^-2), illustrating the catalyst's great potential for practical applications in the future.

Chemical Identification of Catalytically Active Sites on Oxygen-doped Carbon Nanosheet to Decipher the High Activity for Electro-synthesis Hydrogen Peroxide

Electrochemical production of hydrogen peroxide (H₂ O₂ ) through two-electron (2 e^- ) oxygen reduction reaction (ORR) is an on-site and clean route. Oxygen-doped carbon materials with high ORR activity and H₂ O₂ selectivity have been considered as the promising catalysts, however, there is still a lack of direct experimental evidence to identify true active sites at the complex carbon surface. Herein, we propose a chemical titration strategy to decipher the oxygen-doped carbon nanosheet (OCNS₉₀₀ ) catalyst for 2 e^- ORR. The OCNS₉₀₀ exhibits outstanding 2 e^- ORR performances with onset potential of 0.825 V (vs. RHE), mass activity of 14.5 A g^-1 at 0.75 V (vs. RHE) and H₂ O₂ production rate of 770 mmol g^-1  h^-1 in flow cell, surpassing most reported carbon catalysts. Through selective chemical titration of C=O, C-OH, and COOH groups, we found that C=O species contributed to the most electrocatalytic activity and were the most active sites for 2 e^- ORR, which were corroborated by theoretical calculations.

Design Strategies of Non-Noble Metal-Based Electrocatalysts for Two-Electron Oxygen Reduction to Hydrogen Peroxide

Hydrogen peroxide (H₂ O₂ ) is a highly value-added and environmentally friendly chemical with various applications. The production of H₂ O₂ by electrocatalytic 2e^- oxygen reduction reaction (ORR) has drawn considerable research attention, with a view to replacing the currently established anthraquinone process. Electrocatalysts with low cost, high activity, high selectivity, and superior stability are in high demand to realize precise control over electrochemical H₂ O₂ synthesis by 2e^- ORR and the feasible commercialization of this system. This Review introduces a comprehensive overview of non-noble metal-based catalysts for electrochemical oxygen reduction to afford H₂ O₂ , providing an insight into catalyst design and corresponding reaction mechanisms. It starts with an in-depth discussion on the origins of 2e^- /4e^- selectivity towards ORR for catalysts. Recent advances in design strategies for non-noble metal-based catalysts, including carbon nanomaterials and transition metal-based materials, for electrochemical oxygen reduction to H₂ O₂ are then discussed, with an emphasis on the effects of electronic structure, nanostructure, and surface properties on catalytic performance. Finally, future challenges and opportunities are proposed for the further development of H₂ O₂ electrogeneration through 2e^- ORR, from the standpoints of mechanistic studies and practical application.

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.

H2O2 production on a carbon cathode loaded with a nickel carbonate catalyst and on an oxide photoanode without an external bias

Efficient H₂O₂ production both on a carbon cathode modified with various metal salts and on an oxide photoanode was investigated. The cathodic current density and faradaic efficiency for H₂O₂ production (FE(H₂O₂)) on a carbon cathode in KHCO₃ aqueous solution were significantly improved by the loading of an insoluble nickel carbonate basic hydrate catalyst. This electrode was prepared by a precipitation method of nickel nitrate and KHCO₃ aqueous solution at ambient temperature. The nickel carbonate basic hydrate electrode was very stable, and the accumulated concentration of H₂O₂ was reached at 1.0 wt% at a passed charge of 2500C (the average FE(H₂O₂) was 80%). A simple photoelectrochemical system for H₂O₂ production from both the cathode and a BiVO₄/WO₃ photoanode was demonstrated without an external bias or an ion-exchange membrane in a one-compartment reactor under simulated solar light. The apparent FE(H₂O₂) from both electrodes was calculated to be 168% in total, and the production rate of H₂O₂ was approximately 0.92 μmol min^-1 cm^-2. The solar-to-chemical energy conversion efficiency for H₂O₂ production (STC_H2O2 ) without an external bias was approximately 1.75%.

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.

Docking MOF crystals on graphene support for highly selective electrocatalytic peroxide production

Tailoring the reaction kinetics is the central theme of designer electrocatalysts, which enables the selective conversion of abundant and inert atmospheric species into useful products. Here we show a supporting effect in tuning the electrocatalytic kinetics of oxygen reduction reaction (ORR) from four-electron to two-electron mechanism by docking metalloporphyrin-based metal-organic frameworks (MOFs) crystals on graphene support, leading to highly selective peroxide production with faradaic efficiency as high as 93.4%. A magic angle of 38.1° tilting for the co-facial alignment was uncovered by electron diffraction tomography, which is attributed to the maximization of π-π interaction for mitigating the lattice and symmetry mismatch between MOF and graphene. The facilitated electron migration and oxygen chemisorption could be ascribed to the supportive effect of graphene that disperses of the electron state of the active center, and ultimately regulates rate-determining step.

ELECTRONIC SUPPLEMENTARY MATERIAL

Supplementary material (synthesis protocols for control samples, morphological and structural characterizations, porosity, electrochemical properties and activities including SEM, TEM, XPS, Raman, AFM investigations) is available in the online version of this article at 10.1007/s12274-021-3382-3.

Degreasing cotton used as pore-creating agent to prepare hydrophobic and porous carbon cathode for the electro-Fenton system: enhanced H2O2 generation and RhB degradation

A porous carbon cathode was prepared using graphite, polytetrafluoroethylene (PTFE), and degreasing cotton (DC) through sintering treatment. The carbonization of DC by heat treatment played an ideal role in pore-creating, which weakened the mass transfer resistance of O2, and as a result, the adoption of degreasing cotton significantly improved the performance of H2O2 electro-generation. The optimized cathode was able to generate 567 mg L-1 H2O2 with a current efficiency (CE) of 86.7% by the electrochemical reaction of 60 min in a divided reactor. Furthermore, the degradation of rhodamine B (RhB) was carried out by an electro-Fenton system using the optimal cathode selected. The developed electro-Fenton system exhibited an excellent RhB degradation performance. The RhB solution of 50 mg L-1 was decolorized completely by the treatment of 10 min. Moreover, the degradation of 50~90 mg L-1 RhB solution presented over 90% TOC removal by the treatment of 120 min, indicating the ideal mineralization of organic pollutants. In addition, it was found that •OH was the major oxidizing specie responsible for the organics degradation. Finally, the possible pathway of RhB degradation in the electro-Fenton system was proposed by GC-MS analysis. The adoption of natural fibers for pore-creating provides an innovative and low-cost method to prepare porous cathode, which may promote the application of electro-Fenton oxidation in wastewater treatment.

Enhanced electro-Fenton degradation of sulfonamides using the N, S co-doped cathode: Mechanism for H2O2 formation and pollutants decay

Facing low reactivity/selectivity of oxygen reduction reaction (ORR) in electro-Fenton (EF), N, S atoms were introduced into carbon-based cathode. "End-on" O₂ adsorption was achieved by adjusting electronic nature via N doping, while *OOH binding capability was tuned by spin density variation via S doping. Results showed the optimized N, S co-doped cathode presented a 42.47% improvement of H₂O₂ accumulation (7.95 ± 0.02 mg L^-1 cm^-2). According to density functional theory (DFT), N, S co-doped structure favored the "end-on" O₂ adsorption as adsorption energy dropped to - 2.24 eV. Moreover, O-O/C-O bond lengths variation proved a possibility for *OOH desorption. The elaborated cathode was used in EF for sulfonamides (SAs) decay. A 100% removal rate of sulfadiazine (SDZ), sulfathiazole (STZ) and sulfadimethoxine (SDM) was achieved within 60 min, among which SDZ tended to be degraded easily. Because the absolute hardness (η) of those pollutants is ranked as follows: η_SDM> η_STZ> η_SDZ. Degradation pathways were proposed based on the detected byproducts, along with toxicity was evaluated by ecological structure-activity relationship (ECOSAR) program. Results showed that toxic intermediates generated were reduced or even disappeared. EF with N, S co-doped cathode provides a promising process for antibiotics wastewater treatment.

Biochar cathode: Reinforcing electro-Fenton pathway against four-electron reduction by controlled carbonization and surface chemistry

Porous biochars have attracted tremendous interests in electrochemical applications. In this study, a family of biochars were prepared from cellulose subject to different carbonization temperatures ranging from 400 to 700 °C, and the biochars were in-situ activated by a molten salt (ZnCl₂) to construct a hierarchically porous architecture. The activated porous biochars (ZnBC) were used as a carbocatalyst for electro-Fenton (EF) oxidation of organic contaminants. Results showed that high-temperature carbonization improved the activity of biochar for four-electron oxygen reduction reaction (ORR) due to the rich carbon defects, while the mild-temperature treatment regulated the species and distribution of oxygen functional groups to increase the production of hydrogen peroxide (H₂O₂) via a selective two-electron ORR pathway. ZnBC-550 was the best cathode material with a high ORR activity without compromise in H₂O₂ selectivity; a high production rate of H₂O₂ (796.1 mg/g/h) was attained at -0.25 V vs RHE at pH of 1. Furthermore, Fe(II) addition induced an electro-Fenton system to attain fast decomposition of various organic pollutants at -0.25 V vs RHE (reversible hydrogen electrode) and pH of 3 with a satisfactory mineralization efficiency toward phenolic pollutants. The EF system maintains its excellent stability for 10 cycles. Hydroxyl radicals were identified as the dominant reactive oxygen species based on in situ electron paramagnetic resonance (EPR) analysis and radical quenching tests. This study gains new insights into electrocatalytic H₂O₂ production over porous biochars and provides a low-cost, robust and high-performance electro-Fenton cathode for wastewater purification.

Heteroatom-doped carbon-based oxygen reduction electrocatalysts with tailored four-electron and two-electron selectivity

Oxygen reduction reaction (ORR) plays a pivotal role in electrochemical energy conversion and commodity chemical production. Oxygen reduction involving a complete four-electron (4e-) transfer is important for the efficient operation of polymer electrolyte fuel cells, whereas the ORR with a partial 2e- transfer can serve as a versatile method for producing industrially important hydrogen peroxide (H2O2). For both the 4e- and 2e- pathway ORR, platinum-group metals (PGMs) have been materials of prevalent choice owing to their high intrinsic activity, but they are costly and scarce. Hence, the development of highly active and selective non-precious metal catalysts is of crucial importance for advancing electrocatalysis of the ORR. Heteroatom-doped carbon-based electrocatalysts have emerged as promising alternatives to PGM catalysts owing to their appreciable activity, tunable selectivity, and facile preparation. This review provides an overview of the design of heteroatom-doped carbon ORR catalysts with tailored 4e- or 2e- selectivities. We highlight catalyst design strategies that promote 4e- or 2e- ORR activity. We also summarise the major active sites and activity descriptors of the respective ORR pathways and describe the catalyst properties controlling the ORR mechanisms. We conclude the review with a summary and suggestions for future research.

Nitrogen and oxygen tailoring of a solid carbon active site for two-electron selectivity electrocatalysis

The second nearest C atoms of pyridinic N were predicted to be an active site for 2e⁻ ORR using DFT calculations, and were experimentally demonstrated to possess a tailoring function of a pyridinic N structure and O dopants on carbon materials.

Design and application of metal-organic frameworks and derivatives as heterogeneous Fenton-like catalysts for organic wastewater treatment: A review

Advanced oxidation process (AOP), with a high oxidation efficiency, fast reaction speed (relatively no secondary pollution), has become one of the core technologies of industrial wastewater and advanced drinking water treatment. Heterogeneous Fenton-like oxidation process (HFOP) is a kind of AOP, which developed rapidly in recent years in such a way to overcome the disadvantages of traditional Fenton reaction. Metal-organic frameworks (MOFs) and their derivatives become essential heterogeneous catalysts for organics mineralization due to the large specific surface area, abundant active sites, and ease of structural regulation. However, the knowledge gap on the mechanism and the fate of heterogeneous catalyst species during organics degradation activities by MOFs presents considerable impediments, particularly for a wide application and scaling up the process. This work has the potential to provide guidance and ideas for researchers and engineers in the fields of environmental remediation, environmental catalysis and functional materials. This review focuses on clarifying the critical mechanism of •OH production from MOFs and derivatives as well as its action on the organic's degradation process. The recent developments in MOF based HFOP are compared, and more attention is paid for the following aspects in this review: (1) classifies systematically progressive modification methods of MOFs by chemical and physical treatments; (2) analyzes the fate of catalytic species during treating organic wastewater; (3) proposes design ideas and principles for improving the performance of MOFs catalysts; (4) discusses the main factors influencing the catalytic properties and practical application; (5) summarizes the possible research challenges and directions for MOFs and their derivatives as catalysts applied to wastewater treatment in the future.

Cobalt single atom catalysts for the efficient electrosynthesis of hydrogen peroxide

Cobalt single atoms anchored on N-doped graphitic carbon were successfully synthesised and exhibited superior two-electron oxygen reduction reaction activity with a H₂O₂ selectivity of ∼76.0% at 0.5 V (vs. RHE).

High-Efficient Generation of H2O2 by Aluminum-Graphite Composite through Selective Oxygen Reduction for Degradation of Organic Contaminants

Hydrogen peroxide (H₂O₂) is an effective green oxidant, which has been widely applied for environmental remediation. Here, we prepared a novel aluminum-graphite (Al-Gr) composite, which was capable of high-efficient production of H₂O₂ through selective O₂ reduction via a two-electron pathway. We discovered the production of H₂O₂ at a wide pH range, which could be enhanced by optimizing Al-Gr synthesis conditions. Poly(ethylene glycol) (PEG) addition could promote the formation of a welding interface and porous structure between Al and Gr in the Al-Gr composite, which enhanced the galvanic corrosion of Al⁰, the selectivity of oxygen reduction via the two-electron pathway, and the mass transfer of O₂ in the Al-Gr/O₂ system. The formation of Al₄C₃ could be regulated by sintering temperature and sintering time, which could promote the intergranular corrosion of Al⁰ and enhance the mass transfer of O₂ by reaction with water to generate the porous structure in the Al-Gr composite. The concentration of H₂O₂ reached 777.5 mg/L at an initial pH of 9.0, an Al-Gr dosage of 8 g/L, and an O₂ gas flow rate of 400 mL/min. The possible mechanisms of Al-Gr synthesis and H₂O₂ production in the Al-Gr/O₂ system were proposed. The Al-Gr composite was effective for the in situ production of H₂O₂, which could be further decomposed into a hydroxyl radical (^•OH) by Al⁰ in the Al-Gr composite. This composite could be used not only to decolorize the Rhodamine B dye but also to degrade various organic contaminants in different water matrices, indicating its environmental significance.

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

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