Recent advances in food waste-derived nanoporous carbon for energy storage

Affordable and environmentally friendly electrochemically active raw energy storage materials are in high demand to switch to mass-scale renewable energy. One particularly promising avenue is the feasibility of utilizing food waste-derived nanoporous carbon. This material holds significance due to its widespread availability, affordability, ease of processing, and, notably, its cost-free nature. Over the years, various strategies have been developed to convert different food wastes into nanoporous carbon materials with enhanced electrochemical properties. The electrochemical performance of these materials is influenced by both intrinsic factors, such as the composition of elements derived from the original food sources and recipes, and extrinsic factors, including the conditions during pyrolysis and activation. While current efforts are dedicated to optimizing process parameters to achieve superior performance in electrochemical energy storage devices, it is timely to take stock of the current state of research in this emerging field. This review provides a comprehensive overview of recent developments in the fabrication and surface characterisation of porous carbons from different food wastes. A special focus is given on the applications of these food waste derived porous carbons for energy storage applications including batteries and supercapacitors.

Size Control of Carbon Xerogel Spheres as Key Factor Governing the H2O2 Selectivity in Metal-Free Bifunctional Electro-Fenton Catalysts for Tetracycline Degradation

Carbon xerogel spheres co-doped with nitrogen and eco-graphene were synthesized using a typical solvothermal method. The results indicate that the incorporation of eco-graphene enhances the electrochemical properties, such as the current density (JK) and the selectivity for the four transferred electrons (n). Additionally, nitrogen doping has a significant effect on the degradation efficiency, varying with the size of the carbon xerogel spheres, which could be attributed to the type of nitrogenous group doped in the carbon material. The degradation efficiency improved in the nanometric spheres (48.3% to 61.6%) but decreased in the micrometric-scale spheres (58.6% to 53.4%). This effect was attributed to the N-functional groups present in each sample, with N-CNS-5 exhibiting a higher percentage of graphitic nitrogen (35.7%) compared to N-CMS-5 (15.3%). These findings highlight the critical role of sphere size in determining the type of N-functional groups present in the sample. leading to enhanced degradation of pollutants as a result of the electro-Fenton process.

Polarity reversal for enhanced in-situ electrochemical synthesis of H2O2 over banana-peel derived biochar cathode for water remediation

The fabrication of a cost-efficient cathode is critical for in-situ electrochemical generation of hydrogen peroxide (H2O2) to remove persistent organic pollutants from groundwater. Herein, we tested a stainless-steel (SS) mesh wrapped banana-peel derived biochar (BB) cathode for in-situ H2O2 electrogeneration to degrade bromophenol blue (BPB) and Congo red (CR) dyes. Furthermore, polarity reversal is evaluated for the activation of BB surface via introduction of various oxygen containing functionalities that serve as active sites for the oxygen reduction reaction (ORR) to generate H2O2. Various parameters including the BB mass, current, as well as the solution pH have been optimized to evaluate the cathode performance for efficient H2O2 generation. The results reveal formation of up to 9.4 mg/L H2O2 using 2.0 g BB and 100 mA current in neutral pH with no external oxygen supply with a manganese doped tin oxide deposited nickel foam (Mn-SnO2@NF) anode to facilitate the oxygen evolution reaction (OER). This iron-free electrofenton (EF) like process enabled by the SSBB cathode facilitates efficient degradation of BPB and CR dyes with 87.44 and 83.63% removal efficiency, respectively after 60 min. A prolonged stability test over 10 cycles demonstrates the effectiveness of polarity reversal toward continued removal efficiency as an added advantage. Moreover, Mn-SnO2@NF anode used for the OER was also replaced with stainless steel (SS) mesh anode to investigate the effect of oxygen evolution on H2O2 generation. Although Mn-SnO2@NF anode exhibits better oxygen evolution potential with reduced Tafel slope, SS mesh anode is discussed to be more cost-efficient for further studies.

Research progress on pyrolysis of nitrogen-containing biomass for fuels, materials, and chemicals production

Pyrolysis of nitrogen-containing biomass holds tremendous potential for producing varieties of high value-added products, alleviating energy depletion. Based on the research status about nitrogen-containing biomass pyrolysis, the effect of biomass feedstock composition on pyrolysis products is first introduced from the aspects of elemental analysis, proximate analysis, and biochemical composition. The properties of biomass with high and low nitrogen used in pyrolysis are briefly summarized. Then, with the pyrolysis of nitrogen-containing biomass as the core, biofuel characteristics, nitrogen migration during pyrolysis, the application prospects, unique advantages of nitrogen-doped carbon materials for catalysis, adsorption and energy storage are introduced, as well as their feasibility in producing nitrogen-containing chemicals (acetonitrile and nitrogen heterocyclic) are reviewed. The future outlook for the application of the pyrolysis of nitrogen-containing biomass, specifically, how to realize the denitrification and upgrading of bio-oil, performance improvement of nitrogen-doped carbon materials, as well as separation and purification of nitrogen-containing chemicals, are addressed.

Highly selective electrosynthesis of H2O2 by N, O co-doped graphite nanosheets for efficient electro-Fenton degradation of p-nitrophenol

The activity and selectivity of the cathode towards electrosynthesis of H₂O₂ are critical for electro-Fenton process. Herein, nickel-foam modified with N, O co-doped graphite nanosheets (NO-GNSs/Ni-F) was developed as a cathode for highly efficient and selective electrosynthesis of H₂O₂. Expectedly, the accumulation of H₂O₂ at pH= 3 reached 494.2 mg L^-1 h^-1, with the selectivity toward H₂O₂ generation reaching 93.0%. The synergistic effect of different oxygen-containing functional groups and N species on the performance and selectivity of H₂O₂ electrosynthesis was investigated by density functional theory calculations, and the combination of epoxy and graphitic N (EP + N) was identified as the most favorable configuration with the lowest theoretical overpotential for H₂O₂ generation. Moreover, NO-GNSs/Ni-F was applied in the electro-Fenton process for p-nitrophenol degradation, resulting in 100% removal within 15 min with the kinetic rate constant of 0.446 min^-1 and 97.6% mineralization within 6 h. The efficient removal was mainly attributed to the generation of bulk ·OH. Furthermore, NO-GNSs/Ni-F exhibited excellent stability. This work provides a workable option for the enhancement of H₂O₂ accumulation and the efficient degradation of pollutants in electro-Fenton system.

Co-based Catalysts for Selective H2 O2 Electroproduction via 2-electron Oxygen Reduction Reaction

Electrochemical production of hydrogen peroxide (H₂ O₂ ) via two-electron oxygen reduction reaction (ORR) process is emerging as a promising alternative method to the conventional anthraquinone process. To realize high-efficiency H₂ O₂ electrosynthesis, robust and low cost electrocatalysts have been intensively pursued, among which Co-based catalysts attract particular research interests due to the earth-abundance and high selectivity. Here, we provide a comprehensive review on the advancement of Co-based electrocatalyst for H₂ O₂ electroproduction. The fundamental chemistry of 2-electron ORR is discussed firstly for guiding the rational design of electrocatalysts. Subsequently, the development of Co-based electrocatalysts involving nanoparticles, compounds and single atom catalysts is summarized with the focus on active site identification, structure regulation and mechanism understanding. Moreover, the current challenges and future directions of the Co-based electrocatalysts are briefly summarized in this review.

Evaluating the Growth of Ceria-Modified N-Doped Carbon-Based Materials and Their Performance in the Oxygen Reduction Reaction

Owning to their distinctive electronic structure, rare-earth-based catalysts exhibit good performance in the oxygen reduction reaction (ORR) and can replace commercial Pt/C. In this study, CeO₂-modified N-doped C-based materials were synthesized using salt template and high-temperature calcination methods, and the synthesis conditions were optimized. The successful synthesis of CeO₂-CN-800 was confirmed through a series of characterization methods and electrochemical tests. The test results show that the material has the peak onset potential of 0.90 V and the half-wave potential of 0.84 V, and has good durability and methanol resistance. The material demonstrates good ORR catalytic performance and can be used in Zn-air batteries. Moreover, it is an excellent catalyst for new energy equipment.

Synergistic Effects in N,O-Comodified Carbon Nanotubes Boost Highly Selective Electrochemical Oxygen Reduction to H2 O2

Electrochemical 2-electron oxygen reduction reaction (ORR) is a promising route for renewable and on-site H2 O2 production. Oxygen-rich carbon nanotubes have been demonstrated their high selectivity (≈80%), yet tailoring the composition and structure of carbon nanotubes to further enhance the selectivity and widen working voltage range remains a challenge. Herein, combining formamide condensation coating and mild temperature calcination, a nitrogen and oxygen comodified carbon nanotubes (N,O-CNTs) electrocatalyst is synthesized, which shows excellent selective (>95%) H2 O2 selectivity in a wide voltage range (from 0 to 0.65 V versus reversible hydrogen electrode). It is significantly superior to the corresponding selectivity values of CNTs (≈50% in 0-0.65 V vs RHE) and O-CNTs (≈80% in 0.3-0.65 V vs RHE). Density functional theory calculations revealed that the C neighbouring to N is the active site. Introducing O-related species can strengthen the adsorption of intermediates *OOH, while N-doping can weaken the adsorption of in situ generated *O and optimize the *OOH adsorption energy, thus improving the 2-electron pathway. With optimized N,O-CNTs catalysts, a Janus electrode is designed by adjusting the asymmetric wettability to achieve H2 O2 productivity of 264.8 mol kgcat -1 h-1 .

Radical and non-radical cooperative degradation in metal-free electro-Fenton based on nitrogen self-doped biochar

To achieve sustainable metal-free electron-Fenton, N self-doped biochar air-cathode (BCAC) was prepared by pyrolyzing coffee residues. During the pyrolysis process, the endogenous N transformed from edge-doping to graphite-doping. Particularly, N vacancies started to evolve when the peak temperature exceeded 700 °C. A high Tetracycline removal rate of 70.42% was obtained on the BCAC at the current density of 4 mA cm^-2. Quenching tests incorporated with ESR spectroscopy were adopted to identify the specific oxidants produced on the cathode. The results showed that •OH (37.36%), •O₂^- (29.67%) and ¹O₂ (24.17%) played comparable role in the tetracycline removal, suggesting the coexist of radical and non-radical oxidants in our electro-Fenton system. According to the structure characterization and the DFT calculation, graphitic N was suggested as the critical site for H₂O₂ generation, and both graphitic N and pyridinic N were electroactive sites for H₂O₂ activation to •OH. Graphitic N and N vacancies with stronger capabilities in O₂ adsorption and electron-trapping were proposed as the electroactive sites for ¹O₂ and •O₂^- formation. This work predicts a novel electro-Fenton process with cooperative radical and non-radical degradation on N self-doped carbonaceous catalysts at a mild condition, which is extremely meaningful for boosting sustainable electro-Fenton technology.

Enhancement of E-Peroxone process with waste-tire carbon composite cathode for tinidazole degradation

The cathode is the key component in the electro-peroxone process (E-Peroxone), which is popularly constructed with carbon materials. This study developed an innovative method to fabricate a cathode with waste-tire carbon (WTC) whose performance was evaluated for the degradation of tinidazole (TNZ), an antibiotic frequently detected in water. It was found that the addition of WTC in the cathode can significantly promote the yield of H₂O₂ and the current efficiency: around 2.7 times that of commercial carbon black at the same loading. The critical influencing factors were studied, including the current density, ozone concentration, initial pH value, chlorine ions and initial TNZ concentration. The scavenger tests demonstrated the possible involvement of •OH and O₂^•-. Some transformation products of TNZ were identified with UPLC-MS and the degradation pathway was proposed accordingly. These results demonstrated the potential of WTC for developing E-Peroxone cathodes.

Tuning Two-Electron Oxygen-Reduction Pathways for H2 O2 Electrosynthesis via Engineering Atomically Dispersed Single Metal Site Catalysts

The hydrogen peroxide (H₂ O₂ ) generation via the electrochemical oxygen reduction reaction (ORR) under ambient conditions is emerging as an alternative and green strategy to the traditional energy-intensive anthraquinone process and unsafe direct synthesis using H₂ and O₂ . It enables on-site and decentralized H₂ O₂ production using air and renewable electricity for various applications. Currently, atomically dispersed single metal site catalysts have emerged as the most promising platinum group metal (PGM)-free electrocatalysts for the ORR. Further tuning their central metal sites, coordination environments, and local structures can be highly active and selective for H₂ O₂ production via the 2e^- ORR. Herein, recent methodologies and achievements on developing single metal site catalysts for selective O₂ to H₂ O₂ reduction are summarized. Combined with theoretical computation and advanced characterization, a structure-property correlation to guide rational catalyst design with a favorable 2e^- ORR process is aimed to provide. Due to the oxidative nature of H₂ O₂ and the derived free radicals, catalyst stability and effective solutions to improve catalyst tolerance to H₂ O₂ are emphasized. Transferring intrinsic catalyst properties to electrode performance for viable applications always remains a grand challenge. The key performance metrics and knowledge during the electrolyzer development are, therefore, highlighted.

Tailoring surface carboxyl groups of mesoporous carbon boosts electrochemical H2O2 production

Oxygen-doped porous carbon materials have been shown promising performance for electrochemical two-electron oxygen reduction reaction (2e^- ORR), an efficient approach for the safe and continuous on-site generation of H₂O₂. The regulation and mechanism understanding of active oxygen-containing functional groups (OFGs) remain great challenges. Here, OFGs modified porous carbon were prepared by thermal oxidation (MC-12-Air), HNO₃ oxidation (MC-12-HNO₃) and H₂O₂ solution hydrothermal treatment (MC-12-H₂O₂), respectively. Structural characterization showed that the oxygen doping content of three catalysts reached about 20%, with the almost completely maintained specific surface area (exception of MC-12- HNO₃). Spectroscopic characterization further revealed that hydroxyl groups are mainly introduced into MC-12-Air, while carboxyl groups are mainly introduced into MC-12- HNO₃ and MC-12- H₂O₂. Compared with the pristine catalyst, three oxygen-functionalized catalysts showed enhanced activity and H₂O₂ selectivity in 2e^- ORR. Among them, MC-12-H₂O₂ exhibited the highest catalytic activity and selectivity of 94 %, as well as a considerable HO₂^- accumulation of 46.2 mmol L^-1 and excellent stability in an extended test over 36 h in a H-cell. Electrochemical characterization demonstrated the promotion of OFGs on ORR kinetics and the greater contribution of carboxyl groups to the intrinsically catalytic activity. DFT calculations confirmed that the electrons are transferred from carboxyl groups to adjacent carbon and the enhanced adsorption strength toward *OOH intermediate, leading to a lower energy barrier for forming *OOH on carboxyl terminated carbon atoms.

H2O2 production at gas-diffusion cathodes made from agarose-derived carbons with different textural properties for acebutolol degradation in chloride media

The excessive cost, unsustainability or complex production of new highly selective electrocatalysts for H₂O₂ production, especially noble-metal-based ones, is prohibitive in the water treatment sector. To solve this conundrum, biomass-derived carbons with adequate textural properties were synthesized via agarose double-step pyrolysis followed by steam activation. A longer steam treatment enhanced the graphitization and porosity, even surpassing commercial carbon black. Steam treatment for 20 min yielded the greatest surface area (1248 m² g^-1), enhanced the mesopore/micropore volume distribution and increased the activity (E_1/2 = 0.609 V) and yield of H₂O₂ (40%) as determined by RRDE. The upgraded textural properties had very positive impact on the ability of the corresponding gas-diffusion electrodes (GDEs) to accumulate H₂O₂, reaching Faradaic current efficiencies of ~95% at 30 min. Acidic solutions of β-blocker acebutolol were treated by photoelectro-Fenton (PEF) process in synthetic media with and without chloride. In urban wastewater, total drug disappearance was reached at 60 min with almost 50% mineralization after 360 min at only 10 mA cm^-2. Up to 14 degradation products were identified in the Cl^--containing medium.

Synthesis and characterization of iron-nitrogen-doped biochar catalysts for organic pollutant removal and hexavalent chromium reduction

Fe, N atoms deposited on porous biochar (Fe-N@BC) composites were synthesized and employed as an efficient catalyst for organic pollutant removal and Cr^VI reduction. Naturally abundant, renewable and N-rich pomelo peel as a carbon and nitrogen source and unsubstituted phthalocyanine/iron phthalocyanine complexes as a Fe and nitrogen resource were used to develop the Fe-N@BC via a carbonization process. It is found that Fe-N@BC hybrids have homogeneous dispersion of Fe and N atoms on 3D hierarchically porous biochar, which significantly improves the performance toward the detoxification of organic pollutants using peroxymonosulfate as an oxidant, as well as the reduction of hexavalent chromium by formic acid as a reductant. Furthermore, the effects of Fe loading and pyrolytic temperature on catalysis were comprehensively analyzed and optimized. The excellent activity of Fe-N@BC in acid media can be attributed to the high dispersion of Fe species, high content of doped nitrogen as well as hierarchical micro-mesopores, which induce to expose more active sites for catalysis. Owing to the structure-enabled acidic stability, Fe-N@BC efficiently retains its activity as well as its structural stability after several cycles of reactions. This work provides a new approach to construct Fe, N-doped biochar as an effective catalyst for the detoxification of organic and inorganic pollutants.

Interfacial Electronic Effects in Co@N-Doped Carbon Shells Heterojunction Catalyst for Semi-Hydrogenation of Phenylacetylene

Metal-supported catalyst with high activity and relatively simple preparation method is given priority to industrial production. In this work, this study reported an easily accessible synthesis strategy to prepare Mott-Schottky-type N-doped carbon encapsulated metallic Co (Co@N_p+gC) catalyst by high-temperature pyrolysis method in which carbon nitride (g-C₃N₄) and dopamine were used as support and nitrogen source. The prepared Co@N_p+gC presented a Mott-Schottky effect; that is, a strong electronic interaction of metallic Co and N-doped carbon shell was constructed to lead to the generation of Mott-Schottky contact. The metallic Co, due to high work function as compared to that of N-doped carbon, transferred electrons to the N-doped outer shell, forming a new contact interface. In this interface area, the positive and negative charges were redistributed, and the catalytic hydrogenation mainly occurred in the area of active charges. The Co@N_p+gC catalyst showed excellent catalytic activity in the hydrogenation of phenylacetylene to styrene, and the selectivity of styrene reached 82.4%, much higher than those of reference catalysts. The reason for the promoted semi-hydrogenation of phenylacetylene was attributed to the electron transfer of metallic Co, as it was caused by N doping on carbon.

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.

Accelerated Fe2+ Regeneration in an Effective Electro-Fenton Process by Boosting Internal Electron Transfer to a Nitrogen-Conjugated Fe(III) Complex

The regeneration rate of Fe²⁺ from Fe³⁺ dictates the performance of the electro-Fenton (EF) process, represented by the amount of produced hydroxyl radicals (·OH). Current strategies for the acceleration of Fe²⁺ regeneration normally require additional chemical reagents, to vary the redox potential of Fe²⁺/Fe³⁺. Here, we report an attempt at using the intrinsic property of the electrode to our advantage, i.e., nitrogen-doped carbon aerogel (NDCA), as a reducing agent for the regeneration of Fe²⁺ without using foreign reagents. Moreover, the pyrrolic N in NDCA provides unpaired electrons through the carbon framework to reduce Fe³⁺, while the graphitic and pyridinic N coordinate with Fe³⁺ to form a C-O-Fe-N₂ bond, facilitating electron transfer from both the external circuit and pyrrolic N to Fe³⁺. Our Fe²⁺/NDCA-EF system exhibits a 5.8 ± 0.3 times higher performance, in terms of the amount of generated ·OH, than a traditional Fenton system using the same Fe²⁺ concentration. In the subsequent reaction, the Fe²⁺/NDCA-EF system demonstrates 100.0% removal of dimethyl phthalate, 3-chlorophenol, bisphenol A, and sulfamethoxazole with a low specific energy consumption of 0.17-0.36 kW·h·g^-1. Furthermore, 90.1 ± 0.6% removal of dissolved organic carbon and 83.3 ± 0.9% removal of NH₃-N are achieved in the treatment of domestic sewage. The purpose of this work is to present a novel strategy for the regeneration of Fe²⁺ in the EF process and also to elucidate the role of different N species of the carbonaceous electrode in contributing to the redox cycle of Fe²⁺/Fe³⁺.

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

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

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

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

Thermally treated candle soot as a novel catalyst for hydrogen peroxide in-situ production enhancement in the bio-electro-Fenton system

Thermally treated candle soot (TCS) was used as a two-electron (2e¯) oxygen reduction reaction (ORR) catalyst to in situ produce H₂O₂ in a bio-electro-Fenton (BEF) system. Compared with the pristine candle soot (CS), TCS showed larger Brunauer-Emmett-Teller (BET) surface area (102.54 m² g^-1 vs. 61.79 m² g^-1), higher mesoporous ratio (50.39% vs. 34.98%), and improved hydrophilicity. X-ray photoelectron spectra (XPS) results revealed that the C-O-C was the dominant oxygen-containing group of the CS, and its percentage reached at 80.55%. However, the C-O-C ratio of the TCS decreased to 48.93%, whilst it's CO and OC-O ratios significantly increased to 27.92% and 23.15%. The TCS showed a high H₂O₂ selectivity (87.5%∼97.0%) at the neutral pH condition, which was much higher than that of the commonly used carbon black (CB) catalyst. Finally, the H₂O₂ concentration maxima (C_max-_H2O2) of the bio-electro-Fenton system running with the TCS air-cathode (BEF-TCS) achieved at 32.02 mg/L, which was 6.29 times higher than that of the BEF-CB (5.09 mg/L). The removal and mineralization ratios of the SMX in the BEF-TCS reached at 83.0% and 79.0%, respectively. This paper reported a novel 2e¯ ORR electro-catalyst which was low-cost, easily available and highly efficiency.

Tailoring the Electrochemical Production of H2 O2 : Strategies for the Rational Design of High-Performance Electrocatalysts

The production of H₂ O₂ via the electrochemical oxygen reduction reaction (ORR) presents an attractive decentralized alternative to the current industry-dominant anthraquinone process. However, in order to achieve viable commercialization of this process, a state-of-the-art electrocatalyst exhibiting high activity, selectivity, and long-term stability is imperative for industrial applications. Herein, an in-depth discussion on the current frontiers in electrocatalyst design is provided, emphasizing the influences of electronic and geometric effects, surface structure, and the effects of heteroatom functionalization on the catalytic performance of commonly studied materials (metals, alloys, carbons). The limitations on the performance of the current catalyst materials are also discussed, together with alternative strategies to overcome the impediments. Finally, directions of future research efforts for the discovery of next-generation ORR electrocatalysts are highlighted.

Recent Progress on Carbonaceous Material Engineering for Electrochemical Hydrogen Peroxide Generation

Electrochemical synthesis of hydrogen peroxide (H2O2) provides a clean and safe technology for large-scale H2O2 production. The core of this project is the development of highly active and highly selective catalysts. Recent studies demonstrate that carbonaceous materials are favorable catalysts because of their low-cost and tunable surface structures. This brief review first summarizes the strategies of carbonaceous material engineering for selective two-electron O2 reduction reaction and discusses potential mechanisms. In addition, several device designs using carbonaceous materials as catalysts for H2O2 production are introduced. Finally, research directions are proposed for practical application and performance improvement.

Selective electrochemical H2O2 generation and activation on a bifunctional catalyst for heterogeneous electro-Fenton catalysis

Heterogeneous electro-Fenton is attractive for pollutants removal, where H₂O₂ is in-situ generated and simultaneously activated to ·OH at the cathodic catalyst. However, the heterogeneous electro-Fenton efficiency is limited by low H₂O₂ production and slow Fe(II) regeneration, which can be improved by tuning oxygen reduction selectivity and facilitating electron transfer to Fe(III) centers. Herein, we designed a bifunctional catalyst with FeOx nanoparticles embedded into N-doped hierarchically porous carbon (FeOx/NHPC). The activity and selectivity for H₂O₂ production were improved by regulating N doping configurations and contents. The obtained FeOx/NHPC750 presented high catalytic activity for H₂O₂ production with a low overpotential of 190 mV and high H₂O₂ selectivity of 95%˜98% at -0.3 V to -0.8 V. The Fe(II) regeneration was enhanced by the strong interfacial interaction between FeOx and N-doped porous carbon support, which leaded to a rapid decomposition of H₂O₂ into ·OH. FeOx/NHPC750 exhibited excellent electro-Fenton performance for the degradation and mineralization of phenol, sulfamethoxazole, atrazine, rhodamine B and 2,4-dichlorophenol in neutral reaction solution. This study offered a new strategy to construct an efficient and durable bifunctional catalyst for heterogeneous electro-Fenton system for advanced treatment of refractory wastewater.

A comparative perspective of electrochemical and photochemical approaches for catalytic H2O2 production

Recent advances in the design, preparation, and applications of different catalysts for electrochemical and photochemical H₂O₂ production are summarized, and some invigorating perspectives for future developments are also provided.

Luffa sponge-derived hierarchical meso/macroporous boron nitride fibers as superior sorbents for heavy metal sequestration

Boron nitride (BN) has received tremendous attention as a promising adsorbent material. However, unsatisfactory uptake capacities over heavy metal ions limit their practical applications. Herein, we have synthesized a novel hierarchical meso/macroporous BN fibers (MBNFs) via a simple carbothermal reduction method using luffa sponge as a template. The as-obtained MBNFs comprise densely arranged parallel macrochannels on a micrometer scale, with mesopores on the surface of the channel. The resulting MBNFs exhibited remarkable adsorption performance for different heavy metal ions including Cd²⁺, Zn²⁺, Cr³⁺, and Pb²⁺ with maximum uptake capacities as high as 2989, 1885, 723, and 453 mg/g, respectively. In particular, the adsorption capacity for Cd²⁺ and Zn²⁺ exceed the highest values reported for BN materials. In addition, the MBNFs showed excellent stability to re-use for a few times. The present MBNFs materials prepared using cheap and earth abundant luffa sponge may find broad applications such as adsorbent for environmental remediation applications.

Activity-Selectivity Trends in the Electrochemical Production of Hydrogen Peroxide over Single-Site Metal-Nitrogen-Carbon Catalysts

Nitrogen-doped carbon materials featuring atomically dispersed metal cations (M-N-C) are an emerging family of materials with potential applications for electrocatalysis. The electrocatalytic activity of M-N-C materials toward four-electron oxygen reduction reaction (ORR) to H₂O is a mainstream line of research for replacing platinum-group-metal-based catalysts at the cathode of fuel cells. However, fundamental and practical aspects of their electrocatalytic activity toward two-electron ORR to H₂O₂, a future green "dream" process for chemical industry, remain poorly understood. Here we combined computational and experimental efforts to uncover the trends in electrochemical H₂O₂ production over a series of M-N-C materials (M = Mn, Fe, Co, Ni, and Cu) exclusively comprising atomically dispersed M-N_x sites from molecular first-principles to bench-scale electrolyzers operating at industrial current density. We investigated the effect of the nature of a 3d metal within a series of M-N-C catalysts on the electrocatalytic activity/selectivity for ORR (H₂O₂ and H₂O products) and H₂O₂ reduction reaction (H₂O₂RR). Co-N-C catalyst was uncovered with outstanding H₂O₂ productivity considering its high ORR activity, highest H₂O₂ selectivity, and lowest H₂O₂RR activity. The activity-selectivity trend over M-N-C materials was further analyzed by density functional theory, providing molecular-scale understandings of experimental volcano trends for four- and two-electron ORR. The predicted binding energy of HO* intermediate over Co-N-C catalyst is located near the top of the volcano accounting for favorable two-electron ORR. The industrial H₂O₂ productivity over Co-N-C catalyst was demonstrated in a microflow cell, exhibiting an unprecedented production rate of more than 4 mol peroxide g_catalyst^-1 h^-1 at a current density of 50 mA cm^-2.

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

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

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

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

Enhancing the Selectivity of H2O2 Electrogeneration by Steric Hindrance Effect

Selective hydrogen peroxide (H₂O₂) electrogeneration by oxygen reduction reaction (ORR) is an efficient and promising synthetic method for H₂O₂ production. Herein, we build a particular inorganic-organic interface to enhance the electrocatalytic selectivity of reduced graphene oxide (rGO) aerogels for H₂O₂ electrogeneration by modifying rGO aerogels with polyethyleneimine (PEI). The three-dimensional porous structure of aerogels and the steric hindrance effect of PEI on rGO endow PEI-functionalized rGO (rGO/PEI) aerogels with enhanced selectivity (90.7%), production rate (106.4 mmol g_cat^-1 h^-1), and durability for H₂O₂ electrogeneration by the two-electron pathway of ORR.

Structure, Activity, and Faradaic Efficiency of Nitrogen-Doped Porous Carbon Catalysts for Direct Electrochemical Hydrogen Peroxide Production

Carbon materials doped with nitrogen are active catalysts for the electrochemical two-electron oxygen reduction reaction (ORR) to hydrogen peroxide. Insights into the individual role of the various chemical nitrogen functionalities in the H₂ O₂ production, however, have remained scarce. Here, we explore a catalytically very active family of nitrogen-doped porous carbon materials, prepared by direct pyrolysis of ordered mesoporous carbon (CMK-3) with polyethylenimine (PEI). Voltammetric rotating ring-disk analysis in combination with chronoamperometric bulk electrolysis measurements in electrolysis cells demonstrate a pronounced effect of the applied potentials, current densities, and electrolyte pH on the H₂ O₂ selectivity and absolute production rates. H₂ O₂ selectivity up to 95.3 % was achieved in acidic environment, whereas the largest H₂ O₂ production rate of 570.1 mmol g^-1 _catalyst  h^-1 was observed in neutral solution. X-ray photoemission spectroscopy (XPS) analysis suggests a key mechanistic role of pyridinic-N in the catalytic process in acid, whereas graphitic-N groups appear to be catalytically active moieties in neutral and alkaline conditions. Our results contribute to the understanding and aid the rational design of efficient carbon-based H₂ O₂ production catalysts.

Highly effective electrosynthesis of hydrogen peroxide from oxygen on a redox-active cationic covalent triazine network

Direct electrosynthesis of hydrogen peroxide (H2O2) by oxygen reduction is a green and safe strategy to replace the traditional anthraquinone process. Herein, we have designed a two-dimensional redox-active cationic covalent triazine network to be used directly as a cost-effective metal-free electrocatalyst for the oxygen reduction reaction (ORR) to form H2O2. Such a dicationic 2D polymer possesses a porous structure with pore diameters of 2-10 nm and a total N content of 13.3 wt%. The electron paramagnetic resonance experiment confirms the reduction of a viologen-based polymer to radical cations and the subsequent generation of superoxygen radicals. The radical characteristics and high N content within this polymer are the essential for the efficient ORR via a two-electron pathway. As a result, the present electrocatalyst exhibits a high ORR activity and excellent H2O2 selectivity (∼85%), thus providing a feasible possibility of designing highly selective metal-free electrocatalysts for electrocatalytic production of H2O2 from O2.

Three dimensional metal/N-doped nanoplate carbon catalysts for oxygen reduction, the reason for using a layered nanoreactor

A layered nanoreactor (zinc hydroxide gallate/nitrate nanohybrid) has been designed as a nano-vessel to confine the gallate/nitrate reaction inside zinc hydroxide layers for production of metal/nitrogen-doped carbon catalysts. Metals (Fe²⁺, Co²⁺ and Ni²⁺) doped and bare zinc hydroxide nitrates (ZHN) were synthesized as the α-phase hydroxide hosts. By an incomplete ion-exchange process, nitrate anions between the layers of the hosts were then partially replaced by the gallate anions to produce the layered nanoreactors. Under heat-treatment, the reaction between the remaining un-exchanged nitrate anions and the organic moiety inside the basal spacing of each nanohybrid plate resulted in obtaining highly porous 3D metal/nitrogen-doped carbon nanosheets. These catalysts were then used as extremely efficient electrocatalysts for catalyzing oxygen reduction reaction (ORR). This study is intended to show the way to get maximum electrocatalytic activity of the metal/N-doped carbon catalysts toward the ORR. This exceptionally high ORR performance originates from the increased available surface, the best pore size range and the uniform distribution of the active sites in the produced catalysts, all provided by the use of new idea of the layered nanoreactor.