N-Doped Graphitized Carbon Nanohorns as a Forefront Electrocatalyst in Highly Selective O2 Reduction to H2O2

Integrative Active Sites of Cathode for Electron-Oxygen-Proton Coupling To Favor H2O2 Production in a Photoelectrochemical System

The oxygen reduction process generating H2O2 in the photoelectrochemical (PEC) system is milder and environmentally friendly compared with the traditional anthraquinone process but still lacks the efficient electron-oxygen-proton coupling interfaces to improve H2O2 production efficiency. Here, we propose an integrated active site strategy, that is, designing a hydrophobic C-B-N interface to refine the dearth of electron, oxygen, and proton balance. Computational calculation results show a lower energy barrier for H2O2 production due to synergistic and coupling effects of boron sites for O2 adsorption, nitrogen sites for H+ binding, and the carbon structure for electron transfer, demonstrating theoretically the feasibility of the strategy. Furthermore, we construct a hydrophobic boron- and nitrogen-doped carbon black gas diffusion cathode (BN-CB-PTFE) with graphite carbon dots decorated on a BiVO4 photoanode (BVO/g-CDs) for H2O2 production. Remarkably, this approach achieves a record H2O2 production rate (9.24 μmol min-1 cm-2) at the PEC cathode. The BN-CB-PTFE cathode exhibits an outstanding Faraday efficiency for H2O2 production of ∼100%. The newly formed h-BN integrative active site can not only adsorb more O2 but also significantly improve the electron and proton transfer. Unexpectedly, coupling BVO/g-CDs with the BN-CB-PTFE gas diffusion cathode also achieves a record H2O2 production rate (6.60 μmol min-1 cm-2) at the PEC photoanode. This study opens new insight into integrative active sites for electron-O2-proton coupling in a PEC H2O2 production system that may be meaningful for environment and energy applications.

Selective Oxidation of sp-Bonded Carbon in Graphdiyne/Carbon Nanotubes Heterostructures to Form Dominant Epoxy Groups for Two-Electron Oxygen Reduction

Two-electron oxygen reduction reaction (2e- ORR) is of great significance to H2O2 production and reversible nonalkaline Zn-air batteries (ZABs). Multiple oxygen-containing sp2-bonded nanocarbons have been developed as electrocatalysts for 2e- ORR, but they still suffer from poor activity and stability due to the limited and mixed active sites at the edges as well as hydrophilic character. Herein, graphdiyne (GDY) with rich sp-C bonds is studied for enhanced 2e- ORR. First, computational studies show that GDY has a favorable formation energy for producing five-membered epoxy ring-dominated groups, which is selective toward the 2e- ORR pathway. Then based on the difference in chemical activity of sp-C bonds in GDY and sp2-C bonds in CNTs, we experimentally achieved conductive and hydrophobic carbon nanotubes (CNTs) covering O-modified GDY (CNTs/GDY-O) through a mild oxidation treatment combined with an in situ CNTs growth approach. Consequently, the CNTs/GDY-O exhibits an average Faraday efficiency of 91.8% toward H2O2 production and record stability over 330 h in neutral media. As a cathode electrocatalyst, it greatly extends the lifetime of 2e- nonalkaline ZABs at both room and subzero temperatures.

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.

Understanding Advanced Transition Metal-Based Two Electron Oxygen Reduction Electrocatalysts from the Perspective of Phase Engineering

Non-noble transition metal (TM)-based compounds have recently become a focal point of extensive research interest as electrocatalysts for the two electron oxygen reduction (2e- ORR) process. To efficiently drive this reaction, these TM-based electrocatalysts must bear unique physiochemical properties, which are strongly dependent on their phase structures. Consequently, adopting engineering strategies toward the phase structure has emerged as a cutting-edge scientific pursuit, crucial for achieving high activity, selectivity, and stability in the electrocatalytic process. This comprehensive review addresses the intricate field of phase engineering applied to non-noble TM-based compounds for 2e- ORR. First, the connotation of phase engineering and fundamental concepts related to oxygen reduction kinetics and thermodynamics are succinctly elucidated. Subsequently, the focus shifts to a detailed discussion of various phase engineering approaches, including elemental doping, defect creation, heterostructure construction, coordination tuning, crystalline design, and polymorphic transformation to boost or revive the 2e- ORR performance (selectivity, activity, and stability) of TM-based catalysts, accompanied by an insightful exploration of the phase-performance correlation. Finally, the review proposes fresh perspectives on the current challenges and opportunities in this burgeoning field, together with several critical research directions for the future development of non-noble TM-based electrocatalysts.

Covalent Organic Frameworks Based Electrocatalysts for Two-Electron Oxygen Reduction Reaction: Design Principles, Recent Advances, and Perspective

Hydrogen peroxide (H2O2) is an environmentally friendly oxidant with a wide range of applications, and the two-electron pathway (2e-) of the oxygen reduction reaction (ORR) for H2O2 production has attracted much attention due to its eco-friendly nature and operational simplicity in contrast to the conventional anthraquinone process. The challenge is to design electrocatalysts with high activity and selectivity and to understand their structure-activity relationship and catalytic mechanism in the ORR process. Covalent organic frameworks (COFs) provide an efficient template for the construction of highly efficient electrocatalysts due to their designable structure, excellent stability, and controllable porosity. This review firstly outlines the design principles of COFs, including the selection of metallic and nonmetallic active sites, the modulation of the electronic structure of the active sites, and the dimensionality modulation of the COFs, to provide guidance for improving the production performance of H2O2. Subsequently, representative results are summarized in terms of both metallic and metal-free sites to follow the latest progress. Moreover, the challenges and perspectives of 2e- ORR electrocatalysts based on COFs are discussed.

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.

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.

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

Hydrogen peroxide (H2O2) has emerged as a kind of multi-functional green oxidants with extensive industrial utility. Oxidized carbon materials exhibit promises as electrocatalysts in the two-electron (2e-) oxygen reduction reaction (ORR) for H2O2 production. However, the precise identification and fabrication of active sites that selectively yield H2O2 present a serious challenge. Herein, a structural engineering strategy is employed to synthesize oxygen-doped carbon quantum dots (o-CQD) for the 2e- ORR. The surface electronic structure of the o-CQDs is systematically modulated by varying isomerization precursors, thereby demonstrating excellent electrocatalyst performance. Notably, o-CQD-3 emerges as the most promising candidate, showcasing a remarkable H2O2 selectivity of 96.2% (n = 2.07) at 0.68 V versus RHE, coupled with a low Tafel diagram of 66.95 mV dec-1. In the flow cell configuration, o-CQD-3 achieves a H2O2 productivity of 338.7 mmol gcatalyst -1 h-1, maintaining consistent production stability over an impressive 120-hour duration. Utilizing in situ technology and density functional theory calculations, it is unveil that edge sites of o-CQD-3 are facilely functionalized by C-O-C groups under alkaline ORR conditions. This isomerization engineering approach advances the forefront of sustainable catalysis and provides a profound insight into the carbon-based catalyst design for environmental-friendly chemical synthesis processes.

Graphene and MOF Assembly: Enhanced Fabrication and Functional Derivative via MOF Amorphization

The integration of graphene and metal-organic frameworks (MOFs) has numerous implications across various domains, but fabricating such assemblies is often complicated and time-consuming. Herein, a one-step preparation of graphene-MOF assembly is presented by directly impregnating vertical graphene (VG) arrays into the zeolitic imidazolate framework (ZIF) precursors under ambient conditions. This approach can effectively assemble multiple ZIFs, including ZIF-7, ZIF-8, and ZIF-67, resulting in their uniform dispersion on the VG with adjustable sizes and shapes. Hydrogen defects on the VG surface are critical in inducing such high-efficiency ZIF assembly, acting as the reactive sites to interact with the ZIF precursors and facilitate their crystallisation. The versatility of VG-ZIF-67 assembly is further demonstrated by exploring the process of MOF amorphization. Surprisingly, this process leads to an amorphous thin-film coating formed on VG (named VG-IL-amZIF-67), which preserves the short-range molecular bonds of crystalline ZIF-67 while sacrificing the long-range order. Such a unique film-on-graphene architecture maintains the essential characteristics and functionalities of ZIF-67 within a disordered arrangement, making it well-suited for electrocatalysis. In electrochemical oxygen reduction, VG-IL-amZIF-67 exhibits exceptional activity, selectivity, and stability to produce H2O2 in acid media.

Toward a Metal Anode-Free Zinc-Air Battery for Next-Generation Energy Storage

Rechargeable aqueous zinc-air batteries (ZABs) promise high energy density and safety. However, the use of conventional zinc anodes affects the energy output from the battery, so that the theoretical energy density is not achievable under operation conditions. A large portion of the zinc is shielded by anode passivation during the discharge process and remains electrochemically unused, making the operation of rechargeable ZABs inefficient up to date. In a metal anode-free ZAB, there is no unnecessary excess zinc if the zinc reservoir can be precisely adjusted by electrodeposition of zinc from the electrolyte. In this respect, an anode-free battery uses the electrolyte offering a dual-mode functionality not only providing ionic conductivity but also being the source of zinc. In addition, it is shown that a defined porous anode architecture is crucial for high rechargeability in this new type of ZAB. 3D-spatially arranged carbon nanotubes as geometrically defined host structures allow a homogeneous zinc deposition from the electrolyte. Together with carbon nanohorns as an active 2e- catalyst on the cathode side, the rechargeability of this new concept reaches up to 92%.

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.

A carbon felt cathode modified by acidic oxidised carbon nanotubes for the high H2O2 generation and its application in electro-Fenton

Herein, a carbon felt (CF) cathode modified by the acidic oxidised carbon nanotubes (OCNTs) exhibited a high yield of the H2O2 generation in electro-Fenton. Rotating disk electrode (RDE) measurements showed that the selective generation of H2O2 occurred on the CF cathode coated by OCNTs (OCNTs/CF), which was attributed to the high amount of oxygen-containing functional groups in OCNTs. Moreover, the pollutant degradation efficiency could almost reach 100% within 60 min in electro-Fenton with OCNTs/CF as the cathode. Furthermore, the pollutant removal efficiency was kept constant after five consecutive cycles, indicating the high stability of OCNTs/CF cathode. Besides, the hydrophilicity of OCNTs/CF cathode was significantly enhanced owing to the abundant oxygen-contained functional groups on the surface of the OCNTs/CF cathode, which facilitated the mass transfer between the OCNTs/CF cathode and the reactants in the bulk solution. To reveal the possible mechanism in electro-Fenton equipped with the OCNTs/CF cathode, quenching experiments and electron paramagnetic resonance (EPR) investigations were further conducted. This work provided valuable insights into the fabrication of the non-metallic cathode with a high ability towards H2O2 generation in electro-Fenton for efficient pollutant removal.

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

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

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.

Nanoengineering of Porous 2D Structures with Tunable Fluid Transport Behavior for Exceptional H2O2 Electrosynthesis

Precision nanoengineering of porous two-dimensional structures has emerged as a promising avenue for finely tuning catalytic reactions. However, understanding the pore-structure-dependent catalytic performance remains challenging, given the lack of comprehensive guidelines, appropriate material models, and precise synthesis strategies. Here, we propose the optimization of two-dimensional carbon materials through the utilization of mesopores with 5-10 nm diameter to facilitate fluid acceleration, guided by finite element simulations. As proof of concept, the optimized mesoporous carbon nanosheet sample exhibited exceptional electrocatalytic performance, demonstrating high selectivity (>95%) and a notable diffusion-limiting disk current density of -3.1 mA cm-2 for H2O2 production. Impressively, the electrolysis process in the flow cell achieved a production rate of 14.39 mol gcatalyst-1 h-1 to yield a medical-grade disinfectant-worthy H2O2 solution. Our pore engineering research focuses on modulating oxygen reduction reaction activity and selectivity by affecting local fluid transport behavior, providing insights into the mesoscale catalytic mechanism.

Oxygen reduction reaction (ORR) in alkaline solution catalysed by an atomically precise catalyst based on a Pd(II) complex supported on multi-walled carbon nanotubes (MWCNTs). Electrochemical and structural considerations

A new atomically precise, single-ion catalyst (MWCNT-LPd) for ORR (oxygen reduction reaction), consisting of a Pd(II) complex of a tetraazacycloalkane anchored on multiwalled carbon nanotubes, has been prepared through a supramolecular approach ensuring a uniform distribution of catalytic centres on the support surface. A tetraazacycloalkane was chosen to saturate the four coordination sites of the typical square planar coordination geometry of Pd(II) with the aim of ascertaining whether the metal ion must have free coordination sites to function effectively in the ORR or whether, as predicted by quantum mechanical calculations, the catalytic effect can be originated from an interaction of O2 in the fifth coordinative position. The results clearly demonstrated that tetracoordination of Pd(II) does not influence its catalytic capacity in the ORR. Electrodes based on this catalyst show ORR performance very close to that of commercial Pt electrodes, despite the low Pd(II) content (1.72% by weight) in the catalyst. The onset potential (Eon) value and the half-wave potential (E1/2) of the catalyst are, respectively, only 53 mV and 24 mV less positive than those observed for the Pt electrode and direct conversion of O2 to H2O reaches 85.0%, compared to 89% of the Pt electrode. Furthermore, a preliminary galvanostatic test (simulating a working fuel cell at a fixed potential) showed that the catalyst maintains its efficiency continuing to produce water throughout the process (the average number of electrons exchanged over time per O2 molecule remains close to 4).

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.

N-doped carbon dots for the determination of Al3+ and Fe3+ using aggregation-induced emission

New portable hydrogel sensors for Al3+ and Fe3+ detection were designed based on the aggregation-induced emission (AIE) and color change of N-doped carbon dots (N-CDs). N-CDs with yellow fluorescence were prepared by a one-pot hydrothermal method from 2,5-dihydroxyterephthalic acid and acrylamide. The fluorescence of N-CDs was enhanced by Al3+ about 20 times and quenched by Fe3+. It was interesting that although Fe3+ showed obvious quenching on the fluorescence of N-CDs it did not cause a noticeable change in the fluorescence of N-CDs + Al3+. The colorless solution of N-CDs appeared blue in the presence of Fe3+ without the influence of Al3+. Therefore, the turn-on fluorometry and colorimetry systems based on N-CDs were constructed for the simultaneous detection of Al3+ and Fe3+. Furthermore, the portable sensing of Al3+ and Fe3+ was realized with the assistance of hydrogel, filter paper, cellulose acetate, and cellulose nitrate film. The proposed approach was successfully applied to the detection of Al3+ and Fe3+ in food samples and cell imaging.

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.

Strategies for promoting the degradation of phenol by electro-Fenton: Simultaneously promoting the generation and utilization of H2O2

The use of the electro-Fenton process to continuously generate H2O2 and efficiently degrade organic pollutants is considered a promising technology. The ratio of generation of H2O2 is usually regarded as the critical step; however, how the H2O2 is utilized is also of particular importance. Herein, activated carbon was activated at different temperatures and used to explore the effect of nitrogen doping on the production and utilization of H2O2 in the electro-Fenton-based degradation of organic pollutants. The experimental results indicate that nitrogen-doped activated carbon simultaneously promotes the generation and utilization of H2O2, which is attributed to the regulation of the competition between phenol and O2 adsorption by the doped nitrogen. Nitrogen doping not only improves 2e-ORR selectivity but also aggregates phenol near the cathode to balance the concentrations of phenol and ·OH. Density functional theory (DFT) calculations further confirmed that pyrrole-N as a dopant promoted the adsorption of phenol, while pyridine-N was more favorable for O2 adsorption. The unique balance of nitrogen types possessed by modified activated carbon NAC-750 permits the efficient synergistic generation and utilization of H2O2 in a balanced manner during the degradation of phenol. This work provides a new direction for the rational nitrogen-doping modification of activated carbon for the electro-Fenton-based degradation of organic pollutants.

Surface oxidation of commercial activated carbon with enriching carboxyl groups for high-yield electrocatalytic H2O2production

Two-electron oxygen reduction reaction (2e-ORR) for H2O2production is regarded as a more ecologically friendly substitute to the anthraquinone method. However, the search of selective and cheap catalysts is still challenging. Herein, we developed a neutral-selective and efficient nonprecious electrocatalyst that was prepared from a commercial activated carbon (AC) by simply microwave-assisted ash impurity elimination and hydrogen peroxide oxidation for surface functional sites optimization. The oxygen configuration can be tuned with enriching carboxyl group up to 6.65 at.% by the dosage of hydrogen peroxide (mass ratio of H2O2/C = ∼0-8.3). Chemical titration experiments identified the carbonyl groups as the most potential active sites, with selectivity boosted by the additional carboxyl groups. The microwave-assisted moderate-oxidized activated carbon (MW-AC5.0) demonstrated optimal 2e-ORR activity and selectivity in neutral electrolyte (0.1 M K2SO4), with H2O2selectivity reaching ∼75%-97%, a maximum H2O2production rate (1.90 mol·gcatal-1·h-1@0.1 V) and satisfying faradaic efficiency (∼85%) in gas-diffusion-electrode. When coupled with Fenton reaction, it can degrade a model organic pollutant (methylene blue [MB], 50 ppm) to colorless in a short time of 20 min, indicating the potential applications in the environmental remediation.

Exploring the Potential of Heteroatom-Doped Graphene Nanoribbons as a Catalyst for Oxygen Reduction

In this study, we created a series of N, S, and P-doped and co-doped carbon catalysts using a single graphene nanoribbon (GNR) matrix and thoroughly evaluated the impact of doping on ORR activity and selectivity in acidic, neutral, and alkaline conditions. The results obtained showed no significant changes in the GNR structure after the doping process, though changes were observed in the surface chemistry in view of the heteroatom insertion and oxygen depletion. Of all the dopants investigated, nitrogen (mainly in the form of pyrrolic-N and graphitic-N) was the most easily inserted and detected in the carbon matrix. The electrochemical analyses conducted showed that doping impacted the performance of the catalyst in ORR through changes in the chemical composition of the catalyst, as well as in the double-layer capacitance and electrochemically accessible surface area. In terms of selectivity, GNR doped with phosphorus and sulfur favored the 2e- ORR pathway, while nitrogen favored the 4e- ORR pathway. These findings can provide useful insights into the design of more efficient and versatile catalytic materials for ORR in different electrolyte solutions, based on functionalized carbon.

Energy-saving and product-oriented hydrogen peroxide electrosynthesis enabled by electrochemistry pairing and product engineering

Hydrogen peroxide (H2O2) electrosynthesis through oxygen reduction reaction (ORR) is drawing worldwide attention, whereas suffering seriously from the sluggish oxygen evolution reaction (OER) and the difficult extraction of thermodynamically unstable H2O2. Herein, we present an electrosynthesis protocol involving coupling ORR-to-H2O2 with waste polyethylene terephthalate (PET) upcycling and the first H2O2 conversion strategy. Ni-Mn bimetal- and onion carbon-based catalysts are designed to catalyze ORR-to-H2O2 and ethylene glycol electrooxidation with the Faradaic efficiency of 97.5% (H2O2) and 93.0% (formate). This electrolysis system runs successfully at only 0.927 V to achieve an industrial-scale current density of 400 mA cm-2, surpassing all reported H2O2 electrosynthesis systems. H2O2 product is upgraded through two downstream routes of converting H2O2 into sodium perborate and dibenzoyl peroxide. Techno-economic evolution highlights the high gross profit of the ORR || PET upcycling protocol over HER || PET upcycling and ORR || OER. This work provides an energy-saving methodology for the electrosynthesis of H2O2 and other chemicals.

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.

Coherent Sub-Nanometer Interface between Crystalline and Amorphous Materials Boosts Electrochemical Synthesis of Hydrogen Peroxide

There are enormous yet largely underexplored exotic phenomena and properties emerging from interfaces constructed by diverse types of components that may differ in composition, shape, or crystal structure. It remains poorly understood the unique properties a coherent interface between crystalline and amorphous materials may evoke, and there lacks a general strategy to fabricate such interfaces. It is demonstrated that by topotactic partial oxidation heterostructures composed of coherently registered crystalline and amorphous materials can be constructed. As a proof-of-concept study, heterostructures consisting of crystalline P3 N5 and amorphous P3 N5 Ox can be synthesized by creating amorphous P3 N5 Ox from crystalline P3 N5 without interrupting the covalent bonding across the coherent interface. The heterostructure is dictated by nanometer-sized short-range-ordered P3 N5 domains enclosed by amorphous P3 N5 Ox matrix, which entails simultaneously fast charge transfer across the interface and bicomponent synergistic effect in catalysis. Such a P3 N5 /P3 N5 Ox heterostructure attains an optimal adsorption energy for *OOH intermediates and exhibits superior electrocatalytic performance toward H2 O2 production by adopting a selectivity of 96.68% at 0.4 VRHE and a production rate of 321.5 mmol h-1 gcatalyst -1 at -0.3 VRHE . The current study provides new insights into the synthetic strategy, chemical structure, and catalytic property of a sub-nanometer coherent interface formed between crystalline and amorphous materials.

Dual-functional single-atomic Mo/Fe clusters-decorated C3N5 via three electron-pathway in oxygen reduction reaction for tandemly removing contaminants from water

Inspired by the development of single-atom catalysts (SACs), the fabrication of multimetallic SACs can be a promising technical approach for the in situ electro-Fenton (EF) process. Herein, dual-functional atomically dispersed Mo-Fe sites embedded in carbon nitride (C3N5) (i.e., MoFe/C3N5) were synthesized via a facile SiO2 template method. The atomically isolated bimetallic configuration in MoFe/C3N5 was identified by combining the microscopic and spectroscopic techniques. The MoFe/C3N5 catalyst on the cathode exhibited a remarkable catalytic activity toward the three electron-dominated oxygen reduction reaction in sodium sulfate, leading to a highly effective EF reaction with a low overpotential for the removal of organic contaminants from wastewater. The new catalyst showed a superior performance over its conventional counterparts, owing to the dual functions of the dual-metal active sites. Density functional theory (DFT) analysis revealed that the dual-functional 50-MoFe/C3N5 catalyst enabled a synergistic action of the Mo-Fe dual single atomic centers, which can alter the adsorption/dissociation behavior and decrease the overall reaction barriers for effective organic oxidation during the EF process. This study not only sheds light on the controlled synthesis of atomically isolated catalyst materials but also provides deeper understanding of the structure-performance relationship of the nanocatalysts with dual active sites for the catalytic EF process. Additionally, the findings will promote the advanced catalysis for the treatment of emerging organic contaminants in water and wastewater.

Single-atom Iron Catalyst with Biomimetic Active Center to Accelerate Proton Spillover for Medical-level Electrosynthesis of H2 O2 Disinfectant

Electrosynthesis of H2 O2 has great potential for directly converting O2 into disinfectant, yet it is still a big challenge to develop effective electrocatalysts for medical-level H2 O2 production. Herein, we report the design and fabrication of electrocatalysts with biomimetic active centers, consisting of single atomic iron asymmetrically coordinated with both nitrogen and sulfur, dispersed on hierarchically porous carbon (FeSA -NS/C). The newly-developed FeSA -NS/C catalyst exhibited a high catalytic activity and selectivity for oxygen reduction to produce H2 O2 at a high current of 100 mA cm-2 with a record high H2 O2 selectivity of 90 %. An accumulated H2 O2 concentration of 5.8 wt.% is obtained for the electrocatalysis process, which is sufficient for medical disinfection. Combined theoretical calculations and experimental characterizations verified the rationally-designed catalytic active center with the atomic Fe site stabilized by three-coordinated nitrogen atoms and one-sulfur atom (Fe-N3 S-C). It was further found that the replacement of one N atom with S atom in the classical Fe-N4 -C active center could induce an asymmetric charge distribution over N atoms surrounding the Fe reactive center to accelerate proton spillover for a rapid formation of the OOH* intermediate, thus speeding up the whole reaction kinetics of oxygen reduction for H2 O2 electrosynthesis.

Carbon Gels-Green Graphene Composites as Metal-Free Bifunctional Electro-Fenton Catalysts

The Electro-Fenton (EF) process has emerged as a promising technology for pollutant removal. However, the EF process requires the use of two catalysts: one acting as an electrocatalyst for the reduction of oxygen to H2O2 and another Fenton-type catalyst for the generation of ·OH radicals from H2O2. Thus, the search for materials with bifunctionality for both processes is required for a practical and real application of the EF process. Thus, in this work, bifunctional electrocatalysts were obtained via doping carbon microspheres with Eco-graphene, a form of graphene produced using eco-friendly methods. The incorporation of Eco-graphene offers numerous advantages to the catalysts, including enhanced conductivity, leading to more efficient electron transfer during the Electro-Fenton process. Additionally, the synthesis induced structural defects that serve as active sites, promoting the direct production of hydroxyl radicals via a 3-electron pathway. Furthermore, the spherical morphology of carbon xerogels enhances the accessibility of the reagents to the active sites. This combination of factors results in the effective degradation of Tetracycline (TTC) using metal-free catalysts in the Electro-Fenton process, achieving up to an impressive 83% degradation without requiring any other external or additional catalyst.

Thermoplasmonic In Situ Fabrication of Nanohybrid Electrocatalysts over Gas Diffusion Electrodes for Enhanced H2O2 Electrosynthesis

Large-scale development of electrochemical cells is currently hindered by the lack of Earth-abundant electrocatalysts with high catalytic activity, product selectivity, and interfacial mass transfer. Herein, we developed an electrocatalyst fabrication approach which responds to these requirements by irradiating plasmonic titanium nitride (TiN) nanocubes self-assembled on a carbon gas diffusion layer in the presence of polymeric binders. The localized heating produced upon illumination creates unique conditions for the formation of TiN/F-doped carbon hybrids that show up to nearly 20 times the activity of the pristine electrodes. In alkaline conditions, they exhibit enhanced stability, a maximum H2O2 selectivity of 90%, and achieve a H2O2 productivity of 207 mmol gTiN-1 h-1 at 0.2 V vs RHE. A detailed electrochemical investigation with different electrode arrangements demonstrated the key role of nanocomposite formation to achieve high currents. In particular, an increased TiOxNy surface content promoted a higher H2O2 selectivity, and fluorinated nanocarbons imparted good stability to the electrodes due to their superhydrophobic properties.

Carbon Dots Strongly Immobilized onto Carbon Nanohorns as Non-Metal Heterostructure with High Electrocatalytic Activity towards Protons Reduction in Hydrogen Evolution Reaction

Highly performing, non-metal inexpensive electrocatalysts for the production of hydrogen via electrochemical water splitting are called for the replacement of current platinum-based ones. In order to speed up the electrocatalytic hydrogen evolution, abundant active sites but also efficient charge transfer is needed. In this context, 0D carbon dots (CDs) with large specific surface area, low cost, high conductivity, and rich functional groups emerge as promising non-metal electrocatalysts. Additionally, the use of conductive substrates provides an effective strategy to boost their electrocatalytic performance. Herein, the unique 3D superstructure of carbon nanohorns (CNHs), as well as without any metal content in their structure, is used to provide a conductive support of high porosity, large specific surface area, and good electrical conductivity, for the in situ growth and immobilization of CDs, via a simple hydrothermal method. The direct contact of CDs with the 3D conductive network of CNHs promotes charge transfer, accelerating hydrogen evolution. The all-carbon non-metal CDs/CNHs nanoensembleshows an onset potential close to the one of Pt/C, low charge transfer resistance, and excellent stability.

N-Doped porous carbons obtained from chitosan and spent coffee as electrocatalysts with tuneable oxygen reduction reaction selectivity for H2O2 generation

Nitrogen-containing porous carbons prepared by the pyrolysis of adequate biopolymer-based precursors have shown potential in several electrochemical energy-related applications. However, it is still of crucial interest to find the optimal precursors and process conditions which would allow the preparation of carbons with adequate porous structure as well as suitable nitrogen content and distribution of functional groups. In the present work we suggested a straightforward approach to prepare N-doped porous carbons by direct pyrolysis under nitrogen of chitosan : coffee blends of different compositions and using KOH for simultaneous surface activation. The synthetized carbon materials were tested for the electrochemical oxygen reduction to hydrogen peroxide (H₂O₂). A higher fraction of chitosan in the precursor led to a decrease in meso- and nano-porosity of the formed porous carbons, while their activity towards H₂O₂ generation increased. The nitrogen species derived from chitosan seem to play a very important role. Out of the synthesized catalysts the one with the largest content of pyridinic nitrogen sites exhibited the highest faradaic efficiency. The faradaic efficiencies and current densities of the synthesized materials were comparable with the ones of other commercially available carbons obtained from less renewable precursors.

Facilitating two-electron oxygen reduction with pyrrolic nitrogen sites for electrochemical hydrogen peroxide production

Electrocatalytic hydrogen peroxide (H₂O₂) production via the two-electron oxygen reduction reaction is a promising alternative to the energy-intensive and high-pollution anthraquinone oxidation process. However, developing advanced electrocatalysts with high H₂O₂ yield, selectivity, and durability is still challenging, because of the limited quantity and easy passivation of active sites on typical metal-containing catalysts, especially for the state-of-the-art single-atom ones. To address this, we report a graphene/mesoporous carbon composite for high-rate and high-efficiency 2e^- oxygen reduction catalysis. The coordination of pyrrolic-N sites -modulates the adsorption configuration of the *OOH species to provide a kinetically favorable pathway for H₂O₂ production. Consequently, the H₂O₂ yield approaches 30 mol g^-1 h^-1 with a Faradaic efficiency of 80% and excellent durability, yielding a high H₂O₂ concentration of 7.2 g L^-1. This strategy of manipulating the adsorption configuration of reactants with multiple non-metal active sites provides a strategy to design efficient and durable metal-free electrocatalyst for 2e^- oxygen reduction.

Electrocatalytic transformation of oxygen to hydroxyl radicals via three-electron pathway using nitrogen-doped carbon nanotube-encapsulated nickel nanocatalysts for effective organic decontamination

The selective electrochemical reduction of oxygen (O₂) via 3e^- pathway for the production of hydroxyl radicals (HO) is a promising alternative to conventional electro-Fenton process. Here, we developed a nitrogen-doped CNT-encapsulated Ni nanoparticle electrocatalyst (Ni@N-CNT) with high O₂ reduction selectivity for the generation of HO^•via 3e^- pathway. Exposed graphitized N on the CNT shell, and Ni nanoparticles encapsulated within the tip of the N-CNT, played a key role in the generation of H₂O₂ intermediate (*HOOH) via a 2e^- oxygen reduction reaction. Meanwhile, those encapsulated Ni nanoparticles at the tip of the N-CNT facilitated the sequential HO^• generation by directly decomposing the electrogenerated *H₂O₂ in a 1e^- reduction reaction on the N-CNT shell without inducing Fenton reaction. Improved bisphenol A (BPA) degradation efficiency were observed when compared with conventional batch system (97.5% vs 66.4%). Trials using Ni@N-CNT in a flow-through configuration demonstrated a complete removal of BPA within 30 min (k = 0.12 min^-1) with a limited energy consumption of 0.068 kW·h·g^-1 TOC.

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.

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.

Investigation on the reaction kinetic mechanism of polydopamine-loaded copper as dual-functional catalyst in heterogeneous electro-Fenton process

Heterogeneous electro-Fenton (HEF) process has been regarded as a promising method in environmental remediation. However, the reaction kinetic mechanism of the HEF catalyst for simultaneous production and activation of H₂O₂ remained confounded. Herein, the copper supported on polydopamine (Cu/C) was synthesized by a facile method and employed as a bifunctional HEFcatalyst, and the catalytic kinetic pathways were deeply investigated by using rotating ring-disk electrode (RRDE) voltammetry based on the Damjanovic model. Experimental results substantiated that a two-electron oxygen reduction reaction (2e^- ORR) and a sequential Fenton oxidation reaction were proceeded on 1.0-Cu/C, where metallic copper played a crucial role in the fabrication of 2e^- active sites as well as utmost H₂O₂ activation to produce highly reactive oxygen species (ROS), resulting in the high H₂O₂ productivity (52.2%) and the almost complete removal of contaminant ciprofloxacin (CIP) after 90 min. The work not only expanded the idea of reaction mechanism on Cu-based catalyst in HEF process but also provided a promising catalyst for pollutants degradation in wastewater treatment.

Molecular Design of Porous Organic Polymer-Derived Carbonaceous Electrocatalysts for Pinpointing Active Sites in Oxygen Reduction Reaction

The widespread application of fuel cells is hampered by the sluggish kinetics of the oxygen reduction reaction (ORR), which traditionally necessitates the use of high-cost platinum group metal catalysts. The indispensability of these metal catalysts stems from their ability to overcome kinetic barriers, but their high cost and scarcity necessitate alternative strategies. In this context, porous organic polymers (POPs), which are built up from the molecular level, are emerging as promising precursors to produce carbonaceous catalysts owning to their cost-effectiveness, high electrical conductivity, abundant active sites and extensive surface area accessibility. To enhance the intrinsic ORR activity and optimize the performance of these electrocatalysts, recognizing, designing, and increasing the density of active sites are identified as three crucial steps. These steps, which form the core of our review, serve to elucidate the link between the material structure design and ORR performance evaluation, thereby providing valuable insights for ongoing research in the field. Leveraging the precision of polymer skeletons based on molecular units, POP-derived carbonaceous catalysts provide an excellent platform for in-depth exploration of the role and working mechanism for the specific active site during the ORR process. In this review, the recent advances pertaining to the synthesis techniques and electrochemical functions of various types of active sites, pinpointed from POPs, are systematically summarized, including heteroatoms, surficial substituents and edge/defects. Notably, the structure-property relationship, between these active sites and ORR performance, are discussed and emphasized, which creates guidelines to shed light on the design of high-performance ORR electrocatalysts.

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.

Strategies for Sustainable Production of Hydrogen Peroxide via Oxygen Reduction Reaction: From Catalyst Design to Device Setup

An environmentally benign, sustainable, and cost-effective supply of H2O2 as a rapidly expanding consumption raw material is highly desired for chemical industries, medical treatment, and household disinfection. The electrocatalytic production route via electrochemical oxygen reduction reaction (ORR) offers a sustainable avenue for the on-site production of H2O2 from O2 and H2O. The most crucial and innovative part of such technology lies in the availability of suitable electrocatalysts that promote two-electron (2e-) ORR. In recent years, tremendous progress has been achieved in designing efficient, robust, and cost-effective catalyst materials, including noble metals and their alloys, metal-free carbon-based materials, single-atom catalysts, and molecular catalysts. Meanwhile, innovative cell designs have significantly advanced electrochemical applications at the industrial level. This review summarizes fundamental basics and recent advances in H2O2 production via 2e--ORR, including catalyst design, mechanistic explorations, theoretical computations, experimental evaluations, and electrochemical cell designs. Perspectives on addressing remaining challenges are also presented with an emphasis on the large-scale synthesis of H2O2 via the electrochemical route.

Insights into the Electrochemical Production of Hydrogen Peroxide over Single-Atom Co-N-C Catalysts with the Introduction of Carbon Vacancy Defect near the Co-N4 Site

With the introduction of carbon divacancy, trivacancy, and tetravacancy defects near the Co-N₄ site, we have explored the 2e^- ORR activity at the Co-N₄ site from the perspective of spatial structure and the atomic orbital by DFT calculations. We demonstrate the hybridization strength between Co 3dyz (3dxz) and O 2py (2px) orbitals is the origin of 2e^- ORR activity at the Co-N₄ site and the hybridization strength relates to the height of the Co 3d projected orbital in the Z direction. The bond length (L_Co-O, L_O-O), the charge transfer from the Co site to the *OOH adsorbate (ΔQ_Co-O), the d-band center of the Co site (ε_d), and the ICOHP value between Co 3d and O 2p orbitals as descriptors can well predict the 2e^- ORR activity at the Co-N₄ site. This work provides original insights into the 2e^- ORR activity over the single-atom Co-N-C catalysts.

Recent Advances of Electrocatalyst and Cell Design for Hydrogen Peroxide Production

Electrochemical synthesis of H2O2 via a selective two-electron oxygen reduction reaction has emerged as an attractive alternative to the current energy-consuming anthraquinone process. Herein, the progress on electrocatalysts for H2O2 generation, including noble metal, transition metal-based, and carbon-based materials, is summarized. At first, the design strategies employed to obtain electrocatalysts with high electroactivity and high selectivity are highlighted. Then, the critical roles of the geometry of the electrodes and the type of reactor in striking a balance to boost the H2O2 selectivity and reaction rate are systematically discussed. After that, a potential strategy to combine the complementary properties of the catalysts and the reactor for optimal selectivity and overall yield is illustrated. Finally, the remaining challenges and promising opportunities for high-efficient H2O2 electrochemical production are highlighted for future studies.

NBOH Site-Activated Graphene Quantum Dots for Boosting Electrochemical Hydrogen Peroxide Production

Carbon materials are considered promising 2/4 e^- oxygen reduction reaction (ORR) electrocatalysts for synthesizing H₂ O₂ /H₂ O via regulating heteroatom dopants and functionalization. Here, various doped and functionalized graphene quantum dots (GQDs) are designed to reveal the crucial active sites of carbon materials for ORR to produce H₂ O₂ . Density functional theory (DFT) calculations predict that the edge structure involving edge N, B dopant pairs and further OH functionalization to the B (NBOH) is an active center for 2e^- ORR. To verify the above predication, GQDs with an enriched density of NBOH (NBO-GQDs) are designed and synthesized by the hydrothermal reaction of NH₂ edge-functionalized GQDs with H₃ BO₃ forming six-member heterocycle containing the NBOH structure. When dispersed on conductive carbon substrates, the NBO-GQDs show H₂ O₂ selectivity of over 90% at 0.7 -0.8 V versus reversible hydrogen electrode in the alkaline solution in a rotating ring-disk electrode setup. The selectivity retains 90% of the initial value after 12 h stability test. In a flow cell setup, the H₂ O₂ production rate is up to 709 mmol g_catalyst ^-1  h^-1 , superior to most reported carbon- and metal-based electrocatalysts. This work provides molecular insight into the design and formulation of highly efficient carbon-based catalysts for sustainable H₂ O₂ production.

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

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

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.

Revealing the atomic-scale configuration modulation effect of boron dopant on carbon layers for H2O2 production

This work reports an atomic-scale carbon layer configuration tuning strategy induced by a boron dopant. Through regulating the doping level of boron, it was found that the boron dopant not only favors carbon layer growth by strengthening the metallic state of the Ni core, but also enhances the abundance of pyrrolic N species and graphitization degree of carbon by tailoring the carbon/nitrogen atom configuration, thereby contributing to more active pyrrolic N/carbon sites and accelerated interface reaction dynamics. Consequently, the developed Ni@B,N-C catalyst achieves remarkable electrochemical H₂O₂ production performances with a high selectivity of 95.5% and a yield of 795 mmol g^-1 h^-1. In comparison with previous reports in which the boron dopant mainly acts as an electronic structure regulator, this study reveals the tuning effect of boron dopants on the atomic-scale carbon layer configuration, opening up a new avenue for the development of advanced catalysts.

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).