In-Plane Carbon Lattice-Defect Regulating Electrochemical Oxygen Reduction to Hydrogen Peroxide Production over Nitrogen-Doped Graphene

High-Energy Polynitrogen N10 Stabilized on Multi-Walled Carbon Nanotubes

The synthesis of stable polynitrogen compounds with high-energy density has long been a major challenge. The cyclo-pentazolate anion (cyclo-N5 -) is successfully converted into aromatic and structurally symmetric bipentazole (N10) via electrochemical synthesis using highly conductive multi-walled carbon nanotubes (MWCNTs) as the substrate and sodium pentazolate hydrate ([Na(H2O)(N5)]·2H2O) as the raw material. Attenuated total refraction Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and density functional theory calculations confirmed the structure and homogeneous distribution of N10 in the sidewalls of the MWCNTs (named MWCNT-N10-n m). The MWCNT-N10-2.0 m is further used as a catalyst for electrochemical oxygen reduction to synthesize hydrogen peroxide from oxygen with a two-electron selectivity of up to 95%.

Sustainable electro-Fenton simultaneous reduction of Cr (VI) and degradation of organic pollutants via dual-site porous carbon cathode driving uncoordinated molybdenum sites conversion

Simultaneous removal of heavy metals and organic contaminants remains a substantial challenge in the electro-Fenton (EF) system. Herein, we propose a facile and sustainable "iron-free" EF system capable of simultaneously removing hexavalent chromium (Cr (VI)) and para-chlorophenol (4-CP). The system comprises a nitrogen-doped and carbon-deficient porous carbon (dual-site NPC-D) cathode coupled with a MoS2 nanoarray promoter (MoS2 NA). The NPC-D/MoS2 NA system exhibits exceptional synergistic electrocatalytic activity, with removal rates for Cr (VI) and 4-CP that are 20.3 and 4.4 times faster, respectively, compared to the NPC-D system. Mechanistic studies show that the dual-site structure of NPC-D cathode favors the two-electron oxygen reduction pathway with a selectivity of 81 %. Furthermore, an electric field-driven uncoordinated Mo valence state conversion of MoS2 NA enchances the generation of dynamic singlet oxygen and hydroxyl radicals. Notably, this system shows outstanding recyclability, resilience in real wastewater, and sustainability during a 3 L scale-up operation, while effectively mitigating toxicity. Overall, this study presents an effective approach for treating multiple-component wastewater and highlights the importance of structure-activity correlation in synergistic electrocatalysis.

In-Plane Topological-Defect-Enriched Graphene as an Efficient Metal-Free Catalyst for pH-Universal H2O2 Electrosynthesis

Developing efficient metal-free catalysts to directly synthesize hydrogen peroxide (H2O2) through a 2-electron (2e) oxygen reduction reaction (ORR) is crucial for substituting the traditional energy-intensive anthraquinone process. Here, in-plane topological defects enriched graphene with pentagon-S and pyrrolic-N coordination (SNC) is synthesized via the process of hydrothermal and nitridation. In SNC, pentagon-S and pyrrolic-N originating from thiourea precursor are covalently grafted onto the basal plane of the graphene framework, building unsymmetrical dumbbell-like S─C─N motifs, which effectively modulates atomic and electronic structures of graphene. The SNC catalyst delivers ultrahigh H2O2 productivity of 8.1, 7.3, and 3.9 mol gcatalyst -1 h-1 in alkaline, neutral, and acidic electrolytes, respectively, together with long-term operational stability in pH-universal electrolytes, outperforming most reported carbon catalysts. Theoretical calculations further unveil that defective S─C─N motifs efficiently optimize the binding strength to OOH* intermediate and substantially diminish the kinetic barrier for reducing O2 to H2O2, thereby promoting the intrinsic activity of 2e-ORR.

Validating ΔΔG Selectivity Descriptor for Electrosynthesis of H2O2 from Oxygen Reduction Reaction

Understanding selectivity trends is a crucial hurdle in the developing innovative catalysts for generating hydrogen peroxide through the two-electron oxygen reduction reaction (2e-ORR). The identification of selectivity patterns has been made more accessible through the introduction of a newly developed selectivity descriptor derived from thermodynamics, denoted as ΔΔG introduced in Chem Catal. 2023, 3(3), 100568. To validate the suitability of this parameter as a descriptor for 2e-ORR selectivity, we utilize an extensive library of 155 binary alloys. We validate that ΔΔG reliably depicts the selectivity trends in binary alloys reported for their high activity in the 2e-ORR. This analysis also enables the identification of nine selective 2e-ORR catalysts underscoring the efficacy of ΔΔG as 2e-ORR selectivity descriptor. This work highlights the significance of concurrently considering both selectivity and activity trends. This holistic approach is crucial for obtaining a comprehensive understanding in the identification of high-performance catalyst materials for optimal efficiency in various applications.

Catalytically Active Carbon for Oxygen Reduction Reaction in Energy Conversion: Recent Advances and Future Perspectives

The shortage and unevenness of fossil energy sources are affecting the development and progress of human civilization. The technology of efficiently converting material resources into energy for utilization and storage is attracting the attention of researchers. Environmentally friendly biomass materials are a treasure to drive the development of new-generation energy sources. Electrochemical theory is used to efficiently convert the chemical energy of chemical substances into electrical energy. In recent years, significant progress has been made in the development of green and economical electrocatalysts for oxygen reduction reaction (ORR). Although many reviews have been reported around the application of biomass-derived catalytically active carbon (CAC) catalysts in ORR, these reviews have only selected a single/partial topic (including synthesis and preparation of catalysts from different sources, structural optimization, or performance enhancement methods based on CAC catalysts, and application of biomass-derived CACs) for discussion. There is no review that systematically addresses the latest progress in the synthesis, performance enhancement, and applications related to biomass-derived CAC-based oxygen reduction electrocatalysts synchronously. This review fills the gap by providing a timely and comprehensive review and summary from the following sections: the exposition of the basic catalytic principles of ORR, the summary of the chemical composition and structural properties of various types of biomass, the analysis of traditional and the latest popular biomass-derived CAC synthesis methods and optimization strategies, and the summary of the practical applications of biomass-derived CAC-based oxidative reduction electrocatalysts. This review provides a comprehensive summary of the latest advances to provide research directions and design ideas for the development of catalyst synthesis/optimization and contributes to the industrialization of biomass-derived CAC electrocatalysis and electric energy storage.

Recent advances in electrosynthesis of H2O2via two-electron oxygen reduction reaction

The electrosynthesis of hydrogen peroxide (H2O2) via a selective two-electron oxygen reduction reaction (2e- ORR) presents a green and low-energy-consumption alternative to the traditional, energy-intensive anthraquinone process. This review encapsulates the principles of designing relational electrocatalysts for 2e- ORR and explores remaining setups for large-scale H2O2 production. Initially, the review delineates the fundamental reaction mechanisms of H2O2 production via 2e- ORR and assesses performance. Subsequently, it methodically explores the pivotal influence of microstructures, heteroatom doping, and metal hybridization along with setup configurations in achieving a high-performance catalyst and efficient reactor for H2O2 production. Thereafter, the review introduces a forward-looking methodology that leverages the synergistic integration of catalysts and reactors, aiming to harmonize the complementary characteristics of both components. Finally, it outlines the extant challenges and the promising avenues for the efficient electrochemical production of H2O2, setting the stage for future research endeavors.

Study on Influence Factors of H2O2 Generation Efficiency on Both Cathode and Anode in a Diaphragm-Free Bath

Electrosynthesis of H2O2 via both pathways of anodic two-electron water oxidation reaction (2e-WOR) and cathodic two-electron oxygen reduction reaction (2e-ORR) in a diaphragm-free bath can not only improve the generation rate and Faraday efficiency (FE), but also simplify the structure of the electrolysis bath and reduce the energy consumption. The factors that may affect the efficiency of H2O2 generation in coupled electrolytic systems have been systematically investigated. A piece of fluorine-doped tin oxide (FTO) electrode was used as the anode, and in this study, its catalytic performance for 2e-WOR in Na2CO3/NaHCO3 and NaOH solutions was compared. Based on kinetic views, the generation rate of H2O2 via 2e-WOR, the self-decomposition, and the oxidative decomposition rate of the generated H2O2 during electrolysis in carbonate electrolytes were investigated. Furthermore, by choosing polyethylene oxide-modified carbon nanotubes (PEO-CNTs) as the catalyst for 2e-ORR and using its loaded electrode as the cathode, the coupled electrolytic systems for H2O2 generation were set up in a diaphragm bath and in a diaphragm-free bath. It was found that the generated H2O2 in the electrolyte diffuses and causes oxidative decomposition on the anode, which is the main influent factor on the accumulated concentration in H2O2 in a diaphragm-free bath.

Oxygen-doped carbon nanotubes with dual active cites to enhance •OH formation through three electron oxygen reduction

The electro-Fenton (EF) process generates H2O2 through the 2e- oxygen reduction reaction (ORR), which is subsequently activated to •OH by iron-based catalysts. To alleviate the potential risk of external Fe-based catalysts, along with metal dissolution in acidic or neutral environments, in this study we employed oxygen-doped carbon nanotubes (OCNT) as a bifunctional, metal-free cathode to establish a metal-free EF process for organic pollutant degradation. The results demonstrate that the metal-free electrode has excellent H2O2 accumulation (12 mg L-1 cm-1) and degrades sulfathiazole (STZ) with 97.05 % efficiency in 180 min with an explanation kinetic of 0.0189 min-1. For the first time, this enhancement came from the dual active site centers in OCNT: Ⅰ) -COOH and defects active sites were responsible for H2O2 production, Ⅱ) then -CO triggered H2O2 into •OH, avoiding the introduction of metal-based catalysts. These findings suggest that the EF system with in situ oxygen-doped cathodes have great potential for treating antibiotic wastewater.

Emerging Two-Dimensional Carbonaceous Materials for Electrocatalytic Energy Conversions: Rational Design of Active Structures through High-Temperature Chemistry

Electrochemical energy conversion and storage technologies involving controlled catalysis provide a sustainable way to handle the intermittency of renewable energy sources, as well as to produce green chemicals/fuels in an ecofriendly manner. Core to such technology is the development of efficient electrocatalysts with high activity, selectivity, long-term stability, and low costs. Here, two-dimensional (2D) carbonaceous materials have emerged as promising contenders for advancing the chemistry in electrocatalysis. We review the emerging 2D carbonaceous materials for electrocatalysis, focusing primarily on the fine engineering of active structures through thermal condensation, where the design, fabrication, and mechanism investigations over different types of active moieties are summarized. Interestingly, all the recipes creating two-dimensionality on the carbon products also give specific electrocatalytic functionality, where the special mechanisms favoring 2D growth and their consequences on materials functionality are analyzed. Particularly, the structure-activity relationship between specific heteroatoms/defects and catalytic performance within 2D metal-free electrocatalysts is highlighted. Further, major challenges and opportunities for the practical implementation of 2D carbonaceous materials in electrocatalysis are summarized with the purpose to give future material design guidelines for attaining desirable catalytic structures.

Sn Bulk Phase Doping and Surface Modification on Ti4 O7 for Oxygen Reduction to Hydrogen Peroxide

Developing stable and highly selective two-electron oxygen reduction reaction (2e- ORR) electrocatalysts for producing hydrogen peroxide (H2 O2 ) is considered a major challenge to replace the anthraquinone process and achieve a sustainable green economy. Here, we doped Sn into Ti4 O7 (D-Sn-Ti4 O7 ) by simple polymerization post-calcination method as a high-efficiency 2e- ORR electrocatalyst. In addition, we also applied plain calcination after the grinding method to load Sn on Ti4 O7 (L-Sn-Ti4 O7 ) as a comparison. However, the performance of L-Sn-Ti4 O7 is far inferior to that of the D-Sn-Ti4 O7 . D-Sn-Ti4 O7 exhibits a starting potential of 0.769 V (versus the reversible hydrogen electrode, RHE) and a high H2 O2 selectivity of 95.7 %. Excitingly, the catalyst can maintain a stable current density of 2.43 mA ⋅ cm-2 for 3600 s in our self-made H-type cell, and the cumulative H2 O2 production reaches 359.2 mg ⋅ L-1 within 50,000 s at 0.3 V. The performance of D-Sn-Ti4 O7 is better than that of the non-noble metal 2e- ORR catalysts reported so far. The doping of Sn not only improves the conductivity but also leads to the lattice distortion of Ti4 O7 , further forming more oxygen vacancies and Ti3+ , which greatly improves its 2e- ORR performance compared with the original Ti4 O7 . In contrast, since the Sn on the surface of L-Sn-Ti4 O7 displays a synergistic effect with Tin+ (3≤n≤4) of Ti4 O7 , the active center Tin+ dissociates the O=O bond, making it more inclined to 4e- ORR.

Monometallic Ultrasmall Nanocatalysts via Different Valence Atomic Interfaces Boost Hydrogen Evolution Catalysis

Synergistic monometallic nanocatalysts have attracted much attention due to their high intrinsic activity properties. However, current synergistic monometallic nanocatalysts tend to suffer from long reaction paths due to restricted nanoscale interfaces. In this paper, we synthesized the interstitial compound N-Pt/CNT with monometallic atomic interfaces. The catalysts are enriched with atomic interfaces between higher valence Ptδ+ and Pt0, allowing the reaction to proceed synergistically within the same component with an ideal reaction pathway. Through ratio optimization, N2.42-Pt/CNT with a suitable ratio of Ptδ+ and Pt0 is synthesized. And the calculated turnover frequency of N2.42-Pt/CNT is about 37.4 s-1 (-0.1 V vs reversible hydrogen electrode (RHE)), six times higher than that of the commercial Pt/C (6.58 s-1), which is the most intrinsically active of the Pt-based catalysts. Moreover, prepared N2.42-Pt/CNT exhibits excellent stability during the chronoamperometry tests of 200 h. With insights from comprehensive experiments and theoretical calculations, Pt with different valence states in monometallic atomic interfaces synergistically accelerates the H2O dissociation step and optimizes the Gibbs free energy of H* adsorption. And the existence of desirable hydrogen transfer paths substantially facilitates hydrogen evolution reaction kinetics.

Carbon Catalysts Empowering Sustainable Chemical Synthesis via Electrochemical CO2 Conversion and Two-Electron Oxygen Reduction Reaction

Carbon materials hold significant promise in electrocatalysis, particularly in electrochemical CO2 reduction reaction (eCO2 RR) and two-electron oxygen reduction reaction (2e- ORR). The pivotal factor in achieving exceptional overall catalytic performance in carbon catalysts is the strategic design of specific active sites and nanostructures. This work presents a comprehensive overview of recent developments in carbon electrocatalysts for eCO2 RR and 2e- ORR. The creation of active sites through single/dual heteroatom doping, functional group decoration, topological defect, and micro-nano structuring, along with their synergistic effects, is thoroughly examined. Elaboration on the catalytic mechanisms and structure-activity relationships of these active sites is provided. In addition to directly serving as electrocatalysts, this review explores the role of carbon matrix as a support in finely adjusting the reactivity of single-atom molecular catalysts. Finally, the work addresses the challenges and prospects associated with designing and fabricating carbon electrocatalysts, providing valuable insights into the future trajectory of this dynamic field.

Heat and carbon co-activated persulfate to regenerate gentamicin-laden activated carbon: Performance, mechanism, and safety assessment

Activated carbon enriched with high concentrations of gentamicin (ACG) was generated in the production process of gentamicin. Inappropriate handling methods for ACG not only squanders carbon resource, but also seriously hinders achieving global carbon neutrality and hazardous to human health. In the present work, thermal and carbon co-activated persulfate method (TC-PS) was developed to regenerate ACG with degrading gentamicin. The results showed that ACG was effectively regenerated by TC-PS, restoring the adsorption performance for gentamicin. When the treatment temperature was 80 °C, the persulfate dosage was 20 mM and the initial pH was 3.0, the degradation efficiency of gentamicin reached 100%. The HO• and SO4•- were the major reactive species for gentamicin degradation. The possible degradation routes of gentamicin were proposed and the safety assessment indicated that the produced intermediates during the regeneration process of ACG by TC-PS have insignificant impact on the biological and ecological environment.

Heteroatom-Coordinated Palladium Molecular Catalysts for Sustainable Electrochemical Production of Hydrogen Peroxide

Currently, hydrogen peroxide (H2O2) manufacturing involves an energy-intensive anthraquinone technique that demands expensive solvent extraction and a multistep process with substantial energy consumption. In this work, we synthesized Pd-N4-CO, Pd-S4-NCO, and Pd-N2O2-C single-atom catalysts via an in situ synthesis approach involving heteroatom-rich ligands and activated carbon under mild reaction conditions. It reveals that palladium atoms interact strongly with heteroatom-rich ligands, which provide well-defined and uniform active sites for oxygen (O2) electrochemically reduced to hydrogen peroxide. Interestingly, the Pd-N4-CO electrocatalyst shows excellent performance for the electrocatalytic reduction of O2 to H2O2 via a two-electron transfer process in a base electrolyte, exhibiting a negligible amount of onset overpotential and >95% selectivity within a wide range of applied potentials. The electrocatalysts based on the activity and selectivity toward 2e- ORR follow the order Pd-N4-CO > Pd-N2O2-C > Pd-S4-NCO in agreement with the pull-push mechanism, which is the Pd center strongly coordinated with high electronegativity donor atoms (N and O atoms) and weakly coordinated with the intermediate *OOH to excellent selectivity and sustainable production of H2O2. According to density functional theory, Pd-N4 is the active site for selectivity toward H2O2 generation. This work provides an emerging technique for designing high-performance H2O2 electrosynthesis catalysts and the rational integration of several active sites for green and sustainable chemical synthesis via electrochemical processes.

Lattice Distortion and H-passivation in Pure Carbon Electrocatalysts for Efficient and Stable Two-electron Oxygen Reduction to H2 O2

The exploration of inexpensive and efficient catalysts for oxygen reduction reaction (ORR) is crucial for chemical and energy industries. Carbon materials have been proved promising with different catalysts enabling 2 and 4e- ORR. Nevertheless, their ORR activity and selectivity is still complex and under debate in many cases. Many structures of these active carbon materials are also chemically unstable for practical implementations. Unlike the well-discussed structures, this work presents a strategy to promote efficient and stable 2e- ORR of carbon materials through the synergistic effect of lattice distortion and H-passivation (on the distorted structure). We show how these structures can be formed on carbon cloth, and how the reproducible chemical adsorption can be realized on these structures for efficient and stable H2 O2 production. The work here gives not only new understandings on the 2e- ORR catalysis, but also the robust catalyst which can be directly used in industry.

Insights on the Roles of Nitrogen Configuration in Enhancing the Performance of Electrocatalytic Methanol Oxidation over Pt Nanoparticles

Stabilization of the Pt in N-doped carbon materials is an effective method to improve the performance of electrocatalytic methanol oxidation reaction (MOR). Nevertheless, the roles of different N configurations (pyridinic N, pyrrolic N, and graphitic N) toward the electrochemical performance of Pt-based catalysts remain unclear. Herein, Density Functional Theory calculations are adopted to elucidate the synergistic promotion of MOR by different N-configurations with Pt nanoparticles (NPs). Guided by the theoretical study, a series of MOR electrocatalysts with different ratios of pyridinic N and pyrrolic N (denoted as Pt/N-CNT-X (500, 600, 700, 800, and 900)) are designed and synthesized. Surprisingly, the electrocatalytic activity of Pt/N-CNT-600 with a suitable ratio of pyrrolic-N and pyridinic-N for MOR reaches 2394.7 mA mg-1 Pt and 5515.8 mA mg-1 Pt in acidic and alkaline media, respectively, which are superior to the Pt/CNTs, commercial Pt/C, and the ever-reported Pt-based electrocatalysts. The strong metal-support interaction induced by the N-doping is the crucial reason for the superior electrocatalytic performance. More importantly, the ability of pyrrolic-N and pyridinic-N in promoting the adsorption and oxidation of CH3 OH and the oxidation of CO* is substantiated for the first time in methanol oxidation. This work provides new insights on the design of efficient electrocatalysts for MOR.

Tailoring the Atomic-Local Environment of Carbon Nanotube Tips for Selective H2 O2 Electrosynthesis at High Current Densities

The atomic-local environment of catalytically active sites plays an important role in tuning the activity of carbon-based metal-free electrocatalysts (C-MFECs). However, the rational regulation of the environment is always impeded by synthetic limitations and insufficient understanding of the formation mechanism of the catalytic sites. Herein, the possible cleavage mechanism of carbon nanotubes (CNTs) through the crossing points during ball-milling is proposed, resulting in abundant CNT tips that are more susceptible to be modified by heteroatoms, achieving precise modulation of the atomic environment at the tips. The obtained CNTs with N,S-rich tips (N,S-TCNTs) exhibit a wide potential window of 0.59 V along with H2 O2 selectivity for over 90.0%. Even using air as the O2 source, the flow cell system with N,S-TCNTs catalyst attains high H2 O2 productivity up to 30.37 mol gcat. -1  h-1 @350 mA cm-2 , superior to most reported C-MFECs. From a practical point of view, a solid electrolyzer based on N,S-TCNTs is further employed to realize the in-situ continuous generation of pure H2 O2 solution with high productivity (up to 4.35 mmol cm-2  h-1 @300 mA cm-2 ; over 300 h). The CNTs with functionalized tips hold great promise for practical applications, even beyond H2 O2 generation.

Regulating N Species in N-Doped Carbon Electro-Catalysts for High-Efficiency Synthesis of Hydrogen Peroxide in Simulated Seawater

Electrochemical oxygen reduction reaction (ORR) is an attractive and alternative route for the on-site production of hydrogen peroxide (H2 O2 ). The electrochemical synthesis of H2 O2 in neutral electrolyte is in early studying stage and promising in ocean-energy application. Herein, N-doped carbon materials (N-Cx ) with different N types are prepared through the pyrolysis of zeolitic imidazolate frameworks. The N-Cx catalysts, especially N-C800 , exhibit an attracting 2e- ORR catalytic activity, corresponding to a high H2 O2 selectivity (≈95%) and preferable stability in 0.5 m NaCl solution. Additionally, the N-C800 possesses an attractive H2 O2 production amount up to 631.2 mmol g-1  h-1 and high Faraday efficiency (79.8%) in H-type cell. The remarkable 2e- ORR electrocatalytic performance of N-Cx catalysts is associated with the N species and N content in the materials. Density functional theory calculations suggest carbon atoms adjacent to graphitic N are the main catalytic sites and exhibit a smaller activation energy, which are more responsible than those in pyridinic N and pyrrolic N doped carbon materials. Furthermore, the N-C800 catalyst demonstrates an effective antibacterial performance for marine bacteria in simulated seawater. This work provides a new insight for electro-generation of H2 O2 in neutral electrolyte and triggers a great promise in ocean-energy application.

Unveiling the dynamic active site of defective carbon-based electrocatalysts for hydrogen peroxide production

Active sites identification in metal-free carbon materials is crucial for developing practical electrocatalysts, but resolving precise configuration of active site remains a challenge because of the elusive dynamic structural evolution process during reactions. Here, we reveal the dynamic active site identification process of oxygen modified defective graphene. First, the defect density and types of oxygen groups were precisely manipulated on graphene, combined with electrocatalytic performance evaluation, revealing a previously overlooked positive correlation relationship between the defect density and the 2 e- oxygen reduction performance. An electrocatalytic-driven oxygen groups redistribution phenomenon was observed, which narrows the scope of potential configurations of the active site. The dynamic evolution processes are monitored via multiple in-situ technologies and theoretical spectra simulations, resolving the configuration of major active sites (carbonyl on pentagon defect) and key intermediates (*OOH), in-depth understanding the catalytic mechanism and providing a research paradigm for metal-free carbon materials.

Solvent Engineering of Oxygen-Enriched Carbon Dots for Efficient Electrochemical Hydrogen Peroxide Production

The development of cost-effective and reliable metal-free carbon-based electrocatalysts has gained significant attention for electrochemical hydrogen peroxide (H2 O2 ) generation through a two-electron oxygen reduction reaction. In this study, a scalable solvent engineering strategy is employed to fabricate oxygen-doped carbon dots (O-CDs) that exhibit excellent performance as electrocatalysts. By adjusting the ratio of ethanol and acetone solvents during the synthesis, the surface electronic structure of the resulting O-CDs can be systematically tuned. The amount of edge active CO group was strongly correlated with the selectivity and activity of the O-CDs. The optimum O-CDs-3 exhibited extraordinary H2 O2 selectivity of up to 96.55% (n = 2.06) at 0.65 V (vs RHE) and achieved a remarkably low Tafel plot of 64.8 mV dec-1 . Furthermore, the realistic H2 O2 productivity yield of flow cell is measured to be as high as 111.18 mg h-1  cm-2 for a duration of 10 h. The findings highlight the potential of universal solvent engineering approach for enabling the development of carbon-based electrocatalytic materials with improved performance. Further studies will be undertaken to explore the practical implications of the findings for advancing the field of carbon-based electrocatalysis.

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.

Concurrent oxygen reduction and water oxidation at high ionic strength for scalable electrosynthesis of hydrogen peroxide

Electrosynthesis of hydrogen peroxide via selective two-electron transfer oxygen reduction or water oxidation reactions offers a cleaner, cost-effective alternative to anthraquinone processes. However, it remains a challenge to achieve high Faradaic efficiencies at elevated current densities. Herein, we report that oxygen-deficient Pr1.0Sr1.0Fe0.75Zn0.25O4-δ perovskite oxides rich of oxygen vacancies can favorably bind the reaction intermediates to facilitate selective and efficient two-electron transfer pathways. These oxides exhibited superior Faradic efficiencies (~99%) for oxygen reduction over a wide potential range (0.05 to 0.45 V versus reversible hydrogen electrode) and current densities surpassing 50 mA cm-2 under high ionic strengths. We further found that the oxides perform a high selectivity (~80%) for two-electron transfer water oxidation reaction at a low overpotential (0.39 V). Lastly, we devised a membrane-free electrolyser employing bifunctional electrocatalysts, achieving a record-high Faradaic efficiency of 163.0% at 2.10 V and 50 mA cm-2. This marks the first report of the concurrent oxygen reduction and water oxidation catalysed by efficient bifunctional oxides in a novel membrane-free electrolyser for scalable hydrogen peroxide electrosynthesis.

Continuous On-Site H2O2 Electrosynthesis via Two-Electron Oxygen Reduction Enabled by an Oxygen-Doped Single-Cobalt Atom Catalyst with Nitrogen Coordination

Single-Co atom catalysts are suggested as an efficient platinum metal group-free catalyst for promoting the oxygen reduction into water or hydrogen peroxide, while the relevance of the catalyst structure and selectivity is still ambiguous. Here, we propose a thermal evaporation method for modulating the chemical environment of single-Co atom catalysts and unveil the effect on the selectivity and activity. It discloses that nitrogen functional groups prefer to proceed the oxygen reduction via a 4e- pathway and notably improve the intrinsic activity, especially when being coordinated with the Co center, while oxygen doping tempts the electron delocalization around cobalt sites and decreases the binding force toward HOO* intermediates, thereby increasing the 2e- selectivity. Consequently, the well-designed oxygen-doped single-Co atom catalysts with nitrogen coordination deliver an impressive 2e- oxygen reduction performance, approaching the onset potential of 0.78 V vs RHE and selectivity of >90%. As an impressive cathode catalyst of an electrochemical flow cell, it generates H2O2 at a rate of 880 mmol gcat-1 h-1 and faradaic efficiency of 95.2%, in combination with an efficient nickel-iron oxygen evolution anode.

Fabrication of superhydrophilic porous carbon materials through a porogen-free method: Surface and structure modification promoting the two-electron oxygen reduction activity

HYPOTHESIS

Electrochemical manufacture of H₂O₂ through the two-electron oxygen reduction reaction (2e^- ORR), providing prospects of the distributed production of H₂O₂ in remote regions, is considered a promising alternative to the energy-intensive anthraquinone oxidation process.

EXPERIMENTS

In this study, one glucose-derived oxygen-enriched porous carbon material (labeled as HGC₅₀₀) is developed through a porogen-free strategy integrating structural and active site modification.

FINDINGS

The superhydrophilic surface and porous structure together promote the mass transfer of reactants and accessibility of active sites in the aqueous reaction, while the abundant CO species (e.g., aldehyde groups) are taken for the main active site to facilitate the 2e^- ORR catalytic process. Benefiting from the above merits, the obtained HGC₅₀₀ possesses superior performance with a selectivity of 92 % and mass activity of 43.6 A g_cat^-1 at 0.65 V (vs. RHE). Besides, the HGC₅₀₀ can operate steadily for 12 h with the accumulation of H₂O₂ reaching up to 4090±71 ppm and a Faradic efficiency of 95 %. The H₂O₂ generated from the electrocatalytic process in 3 h can degrade a variety of organic pollutants (10 ppm) in 4-20 min, displaying the potential in practical applications.

Approaching Theoretical Performances of Electrocatalytic Hydrogen Peroxide Generation by Cobalt-Nitrogen Moieties

Electrocatalytic oxygen reduction reaction (ORR) has been intensively studied for environmentally benign applications. However, insufficient understanding of ORR 2 e- -pathway mechanism at the atomic level inhibits rational design of catalysts with both high activity and selectivity, causing concerns including catalyst degradation due to Fenton reaction or poor efficiency of H2 O2 electrosynthesis. Herein we show that the generally accepted ORR electrocatalyst design based on a Sabatier volcano plot argument optimises activity but is unable to account for the 2 e- -pathway selectivity. Through electrochemical and operando spectroscopic studies on a series of CoNx /carbon nanotube hybrids, a construction-driven approach based on an extended "dynamic active site saturation" model that aims to create the maximum number of 2 e- ORR sites by directing the secondary ORR electron transfer towards the 2 e- intermediate is proven to be attainable by manipulating O2 hydrogenation kinetics.

Geometric Tuning of Single-Atom FeN4 Sites via Edge-Generation Enhances Multi-Enzymatic Properties

Single-atom nanozymes (SAzymes) are considered promising alternatives to natural enzymes. The catalytic performance of SAzymes featuring homogeneous, well-defined active structures can be enhanced through elucidating structure-activity relationship and tailoring physicochemical properties. However, manipulating enzymatic properties through structural variation is an underdeveloped approach. Herein, the synthesis of edge-rich Fe single-atom nanozymes (FeNC-edge) via an H₂ O₂ -mediated edge generation is reported. By controlling the number of edge sites, the peroxidase (POD)- and oxidase (OXD)-like performance is significantly enhanced. The activity enhancement results from the presence of abundant edges, which provide new anchoring sites to mononuclear Fe. Experimental results combined with density functional theory (DFT) calculations reveal that FeN₄ moieties in the edge sites display high electron density of Fe atoms and open N atoms. Finally, it is demonstrated that FeNC-edge nanozyme effectively inhibits tumor growth both in vitro and in vivo, suggesting that edge-tailoring is an efficient strategy for developing artificial enzymes as novel catalytic therapeutics.

Defects in Carbon-Based Materials for Electrocatalysis: Synthesis, Recognition, and Advances

ConspectusOwing to climate change and over-reliance on fossil fuels, the study and development of sustainable energy is of essential importance in the next few decades. In recent years, rapid advances have been witnessed in various power to gas electrocatalysis technologies including oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) for realizing the target of blue planet with carbon neutrality. Nevertheless, practical applications with superior performance and affordable cost are largely limited by the electrode materials because the reactions are regularly driven by precious metals such as platinum (Pt) or iridium (Ir) based catalysts. Therefore, it is of significance to develop novel electrocatalysts with high electroactivity and limited cost for boosting the commercialization of green hydrogen technology.Since nitrogen-doped carbon nanotubes were first reported for enhanced ORR performance in 2009, the exploitation of carbon-based metal-free catalysts (CMFCs) as potential replacements for the precious metal electrocatalysts has become an attractive research field. To date, great progress has been made in developing new dopant strategies for CMFCs; however, the details of the catalytic mechanism and identification of active sites remain unclear, owing to the complexity in controlling the dopants and their homogeneity in carbon-based materials. To tackle this issue, our group has presented a series of works on defects catalyzing electrochemical reactions and proposed a defect catalysis mechanism since 2015. This theory is now widely accepted by the research community and has become a very important area in electrocatalysis worldwide.In this Account, we first present the defect theory for the reasonable design of defective carbon-based materials (DCMs) and subsequently summarize our previous works on the state-of-the-art defect engineering strategies to design DCMs possessing high activity, with the particular emphasis on the conjunction between defect structures and electrochemical performances. We also categorize recent defect modulation approaches on active sites in DCMs as well as showcase the advanced characterization techniques to confirm the types and densities of defects in DCMs. Finally, several perspectives on the challenges and future research opportunities of this exciting field are proposed. Remarkably, rapid advances of DCMs possessing both high electrochemical activities and low cost as a new generation of electrode materials may greatly facilitate the deployment of sustainable energy infrastructures.

Photocatalytic and Electrocatalytic Generation of Hydrogen Peroxide: Principles, Catalyst Design and Performance

Hydrogen peroxide (H2O2) is a high-demand organic chemical reagent and has been widely used in various modern industrial applications. Currently, the prominent method for the preparation of H2O2 is the anthraquinone oxidation. Unfortunately, it is not conducive to economic and sustainable development since it is a complex process and involves unfriendly environment and potential hazards. In this context, numerous approaches have been developed to synthesize H2O2. Among them, photo/electro-catalytic ones are considered as two of the most promising manners for on-site synthesis of H2O2. These alternatives are sustainable in that only water or O2 is required. Namely, water oxidation (WOR) or oxygen reduction (ORR) reactions can be further coupled with clean and sustainable energy. For photo/electro-catalytic reactions for H2O2 generation, the design of the catalysts is extremely important and has been extensively conducted with an aim to obtain ultimate catalytic performance. This article overviews the basic principles of WOR and ORR, followed by the summary of recent progresses and achievements on the design and performance of various photo/electro-catalysts for H2O2 generation. The related mechanisms for these approaches are highlighted from theoretical and experimental aspects. Scientific challenges and opportunities of engineering photo/electro-catalysts for H2O2 generation are also outlined and discussed.

Promotion of the Efficient Electrocatalytic Production of H2O2 by N,O- Co-Doped Porous Carbon

H₂O₂ generation via an electrochemical two-electron oxygen reduction (2e^- ORR) is a potential candidate to replace the industrial anthraquinone process. In this study, porous carbon catalysts co-doped by nitrogen and oxygen are successfully synthesized by the pyrolysis and oxidation of a ZIF-67 precursor. The catalyst exhibits a selectivity of ~83.1% for 2e^- ORR, with the electron-transferring number approaching 2.33, and generation rate of 2909.79 mmol g^-1 h^-1 at 0.36 V (vs. RHE) in KOH solution (0.1 M). The results prove that graphitic N and -COOH functional groups act as the catalytic centers for this reaction, and the two functional groups work together to greatly enhance the performance of 2e^- ORR. In addition, the introduction of the -COOH functional group increases the hydrophilicity and the zeta potential of the carbon materials, which also promotes the 2e^- ORR. The study provides a new understanding of the production of H₂O₂ by electrocatalytic oxygen reduction with MOF-derived carbon catalysts.

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.

Charge-Polarized Selenium Vacancy in Nickel Diselenide Enabling Efficient and Stable Electrocatalytic Conversion of Oxygen to Hydrogen Peroxide

Vacancy engineering is deemed as one of the powerful protocols to tune the catalytic activity of electrocatalysts. Herein, Se-vacancy with charge polarization is created in the NiSe₂ structure (NiSe₂ -V_Se ) via a sequential phase conversion strategy. By a combined analysis of the Rietveld method, transient photovoltage spectra (TPV), in situ Raman and density functional theory (DFT) calculation, it is unequivocally discovered that the presence of charge-polarized Se-vacancy is beneficial for stabilizing the structure, decreasing the electron transfer kinetics, as well as optimizing the free adsorption energy of reaction intermediate during two-electron oxygen reduction reaction (2e^- ORR). Benefiting from these merits, the as-prepared NiSe₂ -V_Se delivered the highest selectivity of 96% toward H₂ O₂ in alkaline media, together with a selectivity higher than 90% over the wide potential range from 0.25 to 0.55 V, ranking it in the top level among the previously reported transition metal-based electrocatalysts. Most notably, it also displayed admirable stability with only a slight selectivity decay after 5000 cycles of accelerated degradation test (ADT).

Selective H2O2 Electrosynthesis over Defective Carbon from Electrochemical Etching of Molybdenum Carbide

The controllable synthesis of specific defective carbon catalysts is crucial for two-electron oxygen reduction reaction (2e^- ORR) to generate H₂O₂ due to the great potential applications. Herein, the defective carbon catalysts (Mo-CDC-ns) were prepared by an electrochemical activation (ECA) method with Mo₂C/C as a parent. Electrochemical cyclic voltammetry curves, X-ray photoelectron spectroscopy, inductively coupled plasma-mass spectrometry, scanning electron microscopy, and high-resolution transmission electron microscopy confirm the evolution process of a defective carbon structure from the Mo₂C phase in which Mo species are first oxidized to Mo⁶⁺ species and then the latter are dissolved into the solution and defective carbon is simultaneously formed. Raman and electron paramagnetic resonance spectra reveal that the defect types in Mo-CDC-ns are the edge defect and vacancy defect sites. Compared with the parent Mo₂C/C, Mo-CDC-ns exhibit gradually increased kinetic current density and selectivity for H₂O₂ generation with an extension of activation cycles from 10 (Mo-CDC-10) to 30 (Mo-CDC-30). Over Mo-CDC-30, a kinetic current density of 19.4 mA cm^-2 and a selectivity close to 90% in 0.1 M KOH solution were achieved, as well as good stability for H₂O₂ production in an extended test up to 12 h in an H-cell. Graphene planes and Stone Wales 5757-carbon were constructed as basic models for density functional theory calculations. It revealed that the obtained defective structure after the removal of Mo atoms contains the double vacancy at the edge of graphene (Edge-DVC) and the topological defect on the plane of 5757-carbon (5757C-D), which show more moderate reaction free energy for forming *OOH and smaller energy barrier of 2e^- ORR.

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

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

Active Oxygen Functional Group Modification and the Combined Interface Engineering Strategy for Efficient Hydrogen Peroxide Electrosynthesis

Cathodic catalytic activity and interfacial mass transfer are key factors for efficiently generating hydrogen peroxide (H₂O₂) via a two-electron oxygen reduction reaction (ORR). In this work, a carbonized carboxymethyl cellulose (CMC)-reduced graphene oxide (rGO) synthetic fabric cathode was designed and constructed to improve two-electron ORR activity and interfacial mass transfer. Carbonized CMC exhibits abundant active carboxyl groups and excellent two-electron ORR activity with an H₂O₂ selectivity of approximately 87%, higher than that of rGO and other commonly used carbonaceous catalysts. Carbonizing CMC and the agglomerates formed from it restrain the restacking of rGO sheets and thus create abundant meso/macroporous channels for the interfacial mass transfer of oxygen and H₂O₂. Thus, the as-constructed carbonized CMC-rGO synthetic fabric cathode exhibits exceptional H₂O₂ electrosynthesis performance with 11.94 mg·h^-1·cm^-2 yield and 82.32% current efficiency. The sufficient active sites and mass-transfer channels of the cathode also ensure its practical application performance at high current densities, which is further illustrated by the rapid organic pollutant degradation via the H₂O₂-based electro-Fenton process.

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.

S-Doping Promotes Pyridine Nitrogen Conversion and Lattice Defects of Carbon Nitride to Enhance the Performance of Zn-Air Batteries

The efficient operation of Zn-air batteries (ZABs) requires highly active and stable reversible air catalysts. Studies have shown that heteroatom-doped carbonaceous nanomaterials are effective metal-free electrocatalysts for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR). Herein, we design a facile and scalable catalyst doping scheme to manufacture S-doped carbon nitride (S-C₃N₄). Surprisingly, this metal-free catalyst exhibits excellent OER and ORR electrocatalytic activities in alkaline electrolytes, being comparable to those of commercial Pt/C. For the first time, it is proved by experiments that S doping can not only effectively increase the lattice defects of C₃N₄ but also promote the conversion of pyrrolic nitrogen to pyridine nitrogen, thereby enhancing the bifunctional catalytic activity (OER and ORR). When the catalyst is used as an air electrode for rechargeable ZABs, its performance is obviously better than that provided by commercial Pt/C. Our findings and material design strategies are expected to provide new ideas for the synthesis of various high-performance carbon-based electrocatalysts.

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

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

Highly sensitive determination of paracetamol, uric acid, dopamine, and catechol based on flexible plastic electrochemical sensors

Flexible sensing is an alternative to traditional sensing and possesses good flexibility and wearability. Intrinsically conductive polymers, particularly poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS), have received significant attention due to their high mechanical flexibility and good biocompatibility. Here, we report the design of highly conductive and electrochemically active PEDOT:PSS-coated plastic substrate electrodes by combining N-doped graphene (NG) or S-doped graphene (SG) with methanesulfonic acid-treated PEDOT:PSS (denoted as NG-f-MSA-PEDOT:PSS/PET and SG-f-MSA-PEDOT:PSS/PET) by a simple drop-coating method. At room temperature, the NG-f-MSA-PEDOT:PSS/PET electrode demonstrated the lowest detection limits of 17.09, 33.84, 28.30, and 44.96 nM for paracetamol, uric acid, dopamine, and catechol (S/N = 3), respectively. The NG-f-MSA-PEDOT:PSS/PET electrode had good anti-interference ability and reproducibility without employing expensive noble metals and requiring much effort to polish the surface of traditional glass carbon electrodes. Most importantly, this film electrode could maintain a stable electrochemical response under different bending and crease states and had excellent mechanical stability and flexibility.

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.

Electrocatalytic two-electron oxygen reduction over nitrogen doped hollow carbon nanospheres

The two-electron oxygen reduction reaction (2e^- ORR) has become a hopeful alternative for production of hydrogen peroxide (H₂O₂), but its practical feasibility is hindered by the lack of efficient electrocatalysts to achieve high activity and selectivity. Herein, we successfully synthesized outstanding nitrogen doped hollow carbon nanospheres (NHCSs) for electrochemical production of H₂O₂. In 0.1 M KOH, NHCSs exhibit superior and sustained catalytic activity for the 2e^- ORR with an unordinary selectivity of 96.6%. Impressively, such NHCSs manifest an ultrahigh H₂O₂ yield rate of 7.32 mol g_cat.^-1  h^-1 and a high faradaic efficiency of 96.7% at 0.5 V in an H-cell system. Density functional theory calculations were performed to further reveal the catalytic mechanism involved.

Emerging 2D Materials for Electrocatalytic Applications: Synthesis, Multifaceted Nanostructures, and Catalytic Center Design

Currently, the development of advanced 2D nanomaterials has become an interdisciplinary subject with extensive studies due to their extraordinary physicochemical performances. Beyond graphene, the emerging 2D-material-derived electrocatalysts (2D-ECs) have aroused great attention as one of the best candidates for heterogeneous electrocatalysis. The tunable physicochemical compositions and characteristics of 2D-ECs enable rational structural engineering at the molecular/atomic levels to meet the requirements of different catalytic applications. Due to the lack of instructive and comprehensive reviews, here, the most recent advances in the nanostructure and catalytic center design and the corresponding structure-function relationships of emerging 2D-ECs are systematically summarized. First, the synthetic pathways and state-of-the-art strategies in the multifaceted structural engineering and catalytic center design of 2D-ECs to promote their electrocatalytic activities, such as size and thickness, phase and strain engineering, heterojunctions, heteroatom doping, and defect engineering, are emphasized. Then, the representative applications of 2D-ECs in electrocatalytic fields are depicted and summarized in detail. Finally, the current breakthroughs and primary challenges are highlighted and future directions to guide the perspectives for developing 2D-ECs as highly efficient electrocatalytic nanoplatforms are clarified. This review provides a comprehensive understanding to engineer 2D-ECs and may inspire many novel attempts and new catalytic applications across broad fields.

A selective copper based oxygen reduction catalyst for the electrochemical synthesis of H2O2 at neutral pH

H₂O₂ is a bulk chemical used as “green” alternative in a variety of applications, but has an energy and waste intensive production method. The electrochemical O₂ reduction to H₂O₂ is viable alternative with examples of the direct production of up to 20% H₂O₂ solutions. In that respect, we found that the dinuclear complex Cu₂(btmpa) (6,6’‐bis[[bis(2‐pyridylmethyl)amino]methyl]‐2,2’‐bipyridine) reduces O₂ to H₂O₂ with a selectivity up to 90 % according to single linear sweep rotating ring disk electrode measurements. Microbalance experiments showed that complex reduction leads to surface adsorption thereby increasing the catalytic current. More importantly, we kept a high Faradaic efficiency for H₂O₂ between 60 and 70 % over the course of 2 h of amperometry by introducing high potential intervals to strip deposited copper (^depCu). This is the first example of extensive studies into the long term electrochemical O₂ to H₂O₂ reduction by a molecular complex which allowed to retain the high intrinsic selectivity of Cu₂(btmpa) towards H₂O₂ production leading to relevant levels of H₂O₂.

Highly Selective Metal-Free Electrochemical Production of Hydrogen Peroxide on Functionalized Vertical Graphene Edges

Electrochemical generation of hydrogen peroxide (H₂ O₂ ) is an attractive alternative to the energy-intensive anthraquinone oxidation process. Metal-free carbon-based materials such as graphene show great promise as efficient electrocatalysts in alkaline media. In particular, the graphene edges possess superior electrochemical properties than the basal plane. However, identification and enhancement of the catalytically active sites at the edges remain challenging. Furthermore, control of surface wettability to enhance gas diffusion and promote the performance in bulk electrolysis is largely unexplored. Here, a metal-free edge-rich vertical graphene catalyst is synthesized and exhibits a superior performance for H₂ O₂ production, with a high onset potential (0.8 V versus reversible hydrogen electrode (RHE) at 0.1 mA cm^-2 ) and 100% Faradaic efficiency at various potentials. By tailoring the oxygen-containing functional groups using various techniques of electrochemical oxidation, thermal annealing and oxygen plasma post-treatment, the edge-bound in-plane ether-type (COC) groups are revealed to account for the superior catalytic performance. To manipulate the surface wettability, a simple vacuum-based method is developed to effectively induce material hydrophobicity by accelerating hydrocarbon adsorption. The increased hydrophobicity greatly enhances gas transfer without compromising the Faradaic efficiency, enabling a H₂ O₂ productivity of 1767 mmol g_catalyst ^-1 h^-1 at 0.4 V versus RHE.

Electrochemically functionalized graphene for highly sensitive detection of nitrofurazone

The electrochemically functionalized graphene nanosheets (EGS) possesses more oxygen-containing groups and higher defect level, showing superior electrochemical sensing performance.

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.