Direct insights into the role of epoxy groups on cobalt sites for acidic H2O2 production

Oxygen doping regulation of Co single atom catalysts for electro-Fenton degradation of tetracycline

Electro-Fenton is an effective process for degrading hard-to-degrade organic pollutants, such as tetracycline (TC). However, the degradation efficiency of this process is limited by the activity and stability of the cathode catalyst. Herein, a temperature gradient pyrolysis strategy and oxidation treatment is proposed to modulate the coordination environment to prepare oxygen-doped cobalt monoatomic electrocatalysts (CoNOC). The CoNOC catalysts can achieve the selectivity of 93 % for H2O2 with an electron transfer number close to 2. In the H-cell, the prepared electrocatalysts can achieve more than 100 h of H2O2 production with good stability and the yield of 1.41 mol gcatalyst-1 h-1 with an average Faraday efficiency (FE) of more than 88 %. The calculations indicate that the epoxy groups play a crucial role in modulating the oxygen reduction pathway. The O doping and unique N coordination of Co single-atom active sites (CoN(Pd)3N(Po)1O1) can effectively weaken the O2/OOH* interaction, thereby promoting the production of H2O2. Finally, the electro-Fenton system could achieve a TC degradation rate of 94.9 % for 120 min with a mineralization efficiency of 87.8 % for 180 min, which provides a reliable option for antibiotic treatment. The significant involvement of OH in the electro-Fenton process was confirmed, and the plausible mineralization pathway for TC was proposed.

Ligand Isomerization Driven Electrocatalytic Switching

The prevailing view about molecular catalysts is that the central metal ion is responsible for the reaction mechanism and selectivity, whereas the ligands mainly affect the reaction kinetics. Here, we question this paradigm and show that ligands have a dramatic influence on the selectivity of the product. We show how even a seemingly small change in ligand isomerization sharply alters the selectivity of the well-researched oxygen reduction reaction (ORR) Co phthalocyanine catalyst from an indirect 2e- to a direct 4e- pathway. Detailed analysis reveals that intramolecular hydrogen-bond interactions in the ligand activate the catalytic Co, directing the oxygen binding and thus deciding the final product. The resulting catalyst is the first example of a Co-based molecular catalyst catalyzing a direct 4e- ORR via ligand isomerization, for which it shows an activity close to the benchmark Pt in an actual H2-O2 fuel cell. The effect of the ligand isomerism is demonstrated with different central metal ions, thus highlighting the generalizability of the findings and their potential to open new possibilities in the design of molecular catalysts.

Designing Heterocyclic Covalent Organic Frameworks with Tunable Electronic Structures for Efficient Electrosynthesis of Hydrogen Peroxide

The electrocatalytic production of hydrogen peroxide (H2O2) through the two-electron oxygen reduction reaction (2e- ORR) has garnered significant research attention in recent years due to its numerous appealing advantages, such as being eco-friendly and exhibiting high energy conversion efficiency. Metal-free carbon materials with specific catalytic sites have been recognized as potential electrocatalysts for 2e- ORR; however, the design of highly efficient catalysts with well-defined structures and long-term stability for large-scale H2O2 production remains unsatisfactory. In this study, three covalent organic frameworks (COFs) - imine-linked LZU-1, oxazole-linked LZU-190, and thiazole-linked LZU-190(S), are successfully synthesized to explore their catalytic activity in electrocatalytic H2O2 production. Among these, the carbon sites LZU-190(S) are predominantly activated by the introduced adjacent heteroatoms via electronic effects, resulting in much higher H2O2 selectivity compared to the oxazole and imine linkages. This work provides new insights into developing COFs-based electrocatalysts for efficient H2O2 generation.

Advancing H2O2 electrosynthesis: enhancing electrochemical systems, unveiling emerging applications, and seizing opportunities

Hydrogen peroxide (H2O2) is a highly desired chemical with a wide range of applications. Recent advancements in H2O2 synthesis center on the electrochemical reduction of oxygen, an environmentally friendly approach that facilitates on-site production. To successfully implement practical-scale, highly efficient electrosynthesis of H2O2, it is critical to meticulously explore both the design of catalytic materials and the engineering of other components of the electrochemical system, as they hold equal importance in this process. Development of promising electrocatalysts with outstanding selectivity and activity is a prerequisite for efficient H2O2 electrosynthesis, while well-configured electrolyzers determine the practical implementation of large-scale H2O2 production. In this review, we systematically summarize fundamental mechanisms and recent achievements in H2O2 electrosynthesis, including electrocatalyst design, electrode optimization, electrolyte engineering, reactor exploration, potential applications, and integrated systems, with an emphasis on active site identification and microenvironment regulation. This review also proposes new insights into the existing challenges and opportunities within this rapidly evolving field, together with perspectives on future development of H2O2 electrosynthesis and its industrial-scale applications.

Advanced Nanocarbons Toward two-Electron Oxygen Electrode Reactions for H2O2 Production and Integrated Energy Conversion

Hydrogen peroxide (H2O2) plays a pivotal role in advancing sustainable technologies due to its eco-friendly oxidizing capability. The electrochemical two-electron (2e-) oxygen reduction reaction and water oxidation reaction present an environmentally green method for H2O2 production. Over the past three years, significant progress is made in the field of carbon-based metal-free electrochemical catalysts (C-MFECs) for low-cost and efficient production of H2O2 (H2O2EP). This article offers a focused and comprehensive review of designing C-MFECs for H2O2EP, exploring the construction of dual-doping configurations, heteroatom-defect coupling sites, and strategic dopant positioning to enhance H2O2EP efficiency; innovative structural tuning that improves interfacial reactant concentration and promote the timely release of H2O2; modulation of electrolyte and electrode interfaces to support the 2e- pathways; and the application of C-MFECs in reactors and integrated energy systems. Finally, the current challenges and future directions in this burgeoning field are discussed.

Coupling Ni Single Atomic Sites with Metallic Aggregates at Adjacent Geometry on Carbon Support for Efficient Hydrogen Peroxide Electrosynthesis

Single atomic catalysts have shown great potential in efficiently electro-converting O2 to H2O2 with high selectivity. However, the impact of coordination environment and introduction of extra metallic aggregates on catalytic performance still remains unclear. Herein, first a series of carbon-based catalysts with embedded coupling Ni single atomic sites and corresponding metallic nanoparticles at adjacent geometry is synthesized. Careful performance evaluation reveals NiSA/NiNP-NSCNT catalyst with precisely controlled active centers of synergetic adjacent Ni-N4S single sites and crystalline Ni nanoparticles exhibits a high H2O2 selectivity over 92.7% within a wide potential range (maximum selectivity can reach 98.4%). Theoretical studies uncover that spatially coupling single atomic NiN4S sites with metallic Ni aggregates in close proximity can optimize the adsorption behavior of key intermediates *OOH to achieve a nearly ideal binding strength, which thus affording a kinetically favorable pathway for H2O2 production. This strategy of manipulating the interaction between single atoms and metallic aggregates offers a promising direction to design new high-performance catalysts for practical H2O2 electrosynthesis.

Scalable Cathodic H2O2 Electrosynthesis using Cobalt-Coordinated Nanocellulose Electrocatalyst

Converting hierarchical biomass structure into cutting-edge architecture of electrocatalysts can effectively relieve the extreme dependency of nonrenewable fossil-fuel-resources typically suffering from low cost-effectiveness, scarce supplies, and adverse environmental impacts. A cost-effective cobalt-coordinated nanocellulose (CNF) strategy is reported for realizing a high-performance 2e-ORR electrocatalysts through molecular engineering of hybrid ZIFs-CNF architecture. By a coordination and pyrolysis process, it generates substantial oxygen-capturing active sites within the typically oxygen-insulating cellulose, promoting O2 mass and electron transfer efficiency along the nanostructured Co3O4 anchored with CNF-based biochar. The Co-CNF electrocatalyst exhibits an exceptional H2O2 electrosynthesis efficiency of ≈510.58 mg L-1 cm-2 h-1 with an exceptional superiority over the existing biochar-, or fossil-fuel-derived electrocatalysts. The combination of the electrocatalysts with stainless steel mesh serving as a dual cathode can strongly decompose regular organic pollutants (up to 99.43% removal efficiency by 30 min), showing to be a desirable approach for clean environmental remediation with sustainability, ecological safety, and high-performance.

Oxygen Functional Groups Regulate Cobalt-Porphyrin Molecular Electrocatalyst for Acidic H2O2 Electrosynthesis at Industrial-Level Current

Electrosynthesis of hydrogen peroxide (H2O2) based on proton exchange membrane (PEM) reactor represents a promising approach to industrial-level H2O2 production, while it is hampered by the lack of high-efficiency electrocatalysts in acidic medium. Herein, we present a strategy for the specific oxygen functional group (OFG) regulation to promote the H2O2 selectivity up to 92 % in acid on cobalt-porphyrin molecular assembled with reduced graphene oxide. In situ X-ray adsorption spectroscopy, in situ Raman spectroscopy and Kelvin probe force microscopy combined with theoretical calculation unravel that different OFGs exert distinctive regulation effects on the electronic structure of Co center through either remote (carboxyl and epoxy) or vicinal (hydroxyl) interaction manners, thus leading to the opposite influences on the promotion in 2e- ORR selectivity. As a consequence, the PEM electrolyzer integrated with the optimized catalyst can continuously and stably produce the high-concentration of ca. 7 wt % pure H2O2 aqueous solution at 400 mA cm-2 over 200 h with a cell voltage as low as ca. 2.1 V, suggesting the application potential in industrial-scale H2O2 electrosynthesis.

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.

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.

General Design Concept of High-Performance Single-Atom-Site Catalysts for H2O2 Electrosynthesis

Hydrogen peroxide (H2O2) as a green oxidizing agent is widely used in various fields. Electrosynthesis of H2O2 has gradually become a hotspot due to its convenient and environment-friendly features. Single-atom-site catalysts (SASCs) with uniform active sites are the ideal catalysts for the in-depth study of the reaction mechanism and structure-performance relationship. In this review, the outstanding achievements of SASCs in the electrosynthesis of H2O2 through 2e- oxygen reduction reaction (ORR) and 2e- water oxygen reaction (WOR) in recent years, are summarized. First, the elementary steps of the two pathways and the roles of key intermediates (*OOH and *OH) in the reactions are systematically discussed. Next, the influence of the size effect, electronic structure regulation, the support/interfacial effect, the optimization of coordination microenvironments, and the SASCs-derived catalysts applied in 2e- ORR are systematically analyzed. Besides, the developments of SASCs in 2e- WOR are also overviewed. Finally, the research progress of H2O2 electrosynthesis on SASCs is concluded, and an outlook on the rational design of SASCs is presented in conjunction with the design strategies and characterization techniques.

Roles of Oxygen-Containing Functional Groups in Carbon for Electrocatalytic Two-Electron Oxygen Reduction Reaction

Recent years have witnessed great research interests in developing high-performance electrocatalysts for the two-electron (2e-) oxygen reduction reaction (ORR) that enables the sustainable and flexible synthesis of H2O2. Carbon-based electrocatalysts exhibit attractive catalytic performance for the 2e- ORR, where oxygen-containing functional groups (OFGs) play a decisive role. However, current understanding is far from adequate, and the contribution of OFGs to the catalytic performance remains controversial. Therefore, a critical overview on OFGs in carbon-based electrocatalysts toward the 2e- ORR is highly desirable. Herein, we go over the methods for constructing OFGs in carbon including chemical oxidation, electrochemical oxidation, and precursor inheritance. Then we review the roles of OFGs in activating carbon toward the 2e- ORR, focusing on the intrinsic activity of different OFGs and the interplay between OFGs and metal species or defects. At last, we discuss the reasons for inconsistencies among different studies, and personal perspectives on the future development in this field are provided. The results provide insights into the origin of high catalytic activity and selectivity of carbon-based electrocatalysts toward the 2e- ORR and would provide theoretical foundations for the future development in this field.

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.

Recent Advances in Carbon-Based Single-Atom Catalysts for Electrochemical Oxygen Reduction to Hydrogen Peroxide in Acidic Media

Electrochemical oxygen reduction reaction (ORR) via the 2e- pathway in an acidic media shows great techno-economic potential for the production of hydrogen peroxide. Currently, carbon-based single-atom catalysts (C-SACs) have attracted extensive attention due to their tunable electronic structures, low cost, and sufficient stability in acidic media. This review summarizes recent advances in metal centers and their coordination environment in C-SACs for 2e--ORR. Firstly, the reaction mechanism of 2e--ORR on the active sites of C-SACs is systematically presented. Secondly, the structural regulation strategies for the active sites of 2e--ORR are further summarized, including the metal active center, its species and configurations of nitrogen coordination or heteroatom coordination, and their near functional groups or substitute groups, which would provide available and proper ideas for developing superior acidic 2e--ORR electrocatalysts of C-SACs. Finally, we propose the current challenges and future opportunities regarding the acidic 2e--ORR pathway on C-SACs, which will eventually accelerate the development of the distributed H2O2 electrosynthesis process.

Enhancing H2O2 Electrosynthesis at Industrial-Relevant Current in Acidic Media on Diatomic Cobalt Sites

Electrocatalytic synthesis of hydrogen peroxide (H2O2) in acidic media is an efficient and eco-friendly approach to produce inherently stable H2O2, but limited by the lack of selective and stable catalysts under industrial-relevant current densities. Herein, we report a diatomic cobalt catalyst for two-electron oxygen reduction to efficiently produce H2O2 at 50-400 mA cm-2 in acid. Electrode kinetics study shows a >95% selectivity for two-electron oxygen reduction on the diatomic cobalt sites. In a flow cell device, a record-high production rate of 11.72 mol gcat-1 h-1 and exceptional long-term stability (100 h) are realized under high current densities. In situ spectroscopic studies and theoretical calculations reveal that introducing a second metal into the coordination sphere of the cobalt site can optimize the binding strength of key H2O2 intermediates due to the downshifted d-band center of cobalt. We also demonstrate the feasibility of processing municipal plastic wastes through decentralized H2O2 production.

Elucidating the impact of oxygen functional groups on the catalytic activity of M-N4-C catalysts for the oxygen reduction reaction: a density functional theory and machine learning approach

Efforts to enhance the efficiency of electrocatalysts for the oxygen reduction reaction (ORR) in energy conversion and storage devices present formidable challenges. In this endeavor, M-N4-C single-atom catalysts (MN4) have emerged as promising candidates due to their precise atomic structure and adaptable electronic properties. However, MN4 catalysts inherently introduce oxygen functional groups (OGs), intricately influencing the catalytic process and complicating the identification of active sites. This study employs advanced density functional theory (DFT) calculations to investigate the profound influence of OGs on ORR catalysis within MN4 catalysts (referred to as OGs@MN4, where M represents Fe or Co). We established the following activity order for the 2eORR: for OGs@CoN4: OH@CoN4 > CoN4 > CHO@CoN4 > C-O-C@CoN4 > COC@CoN4 > COOH@CoN4 > CO@CoN4; for OGs@FeN4: COC@FeN4 > CO@FeN4 > OH@FeN4 > FeN4 > COOH@FeN4 > CHO@FeN4 > C-O-C@FeN4. Multiple oxygen combinations were constructed and found to be the true origin of MN4 activity (for instance, the overpotential of 2OH@CoN4 as low as 0.07 V). Furthermore, we explored the performance of the OGs@MN4 system through charge and d-band center analysis, revealing the limitations of previous electron-withdrawing/donating strategies. Machine learning analysis, including GBR, GPR, and LINER models, effectively guides the prediction of catalyst performance (with an R2 value of 0.93 for predicting ΔG*OOH_vac in the GBR model). The Eg descriptor was identified as the primary factor characterizing ΔG*OOH_vac (accounting for 62.8%; OGs@CoN4: R2 = 0.9077, OGs@FeN4: R2 = 0.7781). This study unveils the significant impact of OGs on MN4 catalysts and pioneers design and synthesis criteria rooted in Eg. These innovative findings provide valuable insights into understanding the origins of catalytic activity and guiding the design of carbon-based single-atom catalysts, appealing to a broad audience interested in energy conversion technologies and materials science.

Boundary-Rich Carbon-Based Electrocatalysts with Manganese(II)-Coordinated Active Environment for Selective Synthesis of Hydrogen Peroxide

Coordinated manganese (Mn) electrocatalysts owing to their electronic structure flexibility, non-toxic and earth abundant features are promising for electrocatalytic reactions. However, achieving selective hydrogen peroxide (H2 O2 ) production through two electron oxygen reduction (2e-ORR) is a challenge on Mn-centered catalysts. Targeting this goal, we report on the creation of a secondary Mn(II)-coordinated active environment with reactant enrichment effect on boundary-rich porous carbon-based electrocatalysts, which facilitates the selective and rapid synthesis of H2 O2 through 2e-ORR. The catalysts exhibit nearly 100 % Faradaic efficiency and H2 O2 productivity up to 15.1 mol gcat -1  h-1 at 0.1 V versus reversible hydrogen electrode, representing the record high activity for Mn-based electrocatalyst in H2 O2 electrosynthesis. Mechanistic studies reveal that the epoxide and hydroxyl groups surrounding Mn(II) centers improve spin state by modifying electronic properties and charge transfer, thus tailoring the adsorption strength of *OOH intermediate. Multiscale simulations reveal that the high-curvature boundaries facilitate oxygen (O2 ) adsorption and result in local O2 enrichment due to the enhanced interaction between carbon surface and O2 . These merits together ensure the efficient formation of H2 O2 with high local concentration, which can directly boost the tandem reaction of hydrolysis of benzonitrile to benzamide with nearly 100 % conversion rate and exclusive benzamide selectivity.

Improved H2O2 Electrosynthesis on S-doped Co-N-C through Cooperation of Co-S and Thiophene S

Co-N-C based catalysts have emerged as a prospective alternative for H2O2 electrosynthesis via a selective 2e- oxygen reduction reaction (ORR). However, conventional Co-N-C with Co-N4 configurations usually exhibits low selectivity toward 2e- ORR for H2O2 production. In this study, the S-doped Co-N-C (Co-N-C@S) catalysts were designed and synthesized for enhancing the electrosynthesis of H2O2, and their S doping levels and species were tuned to investigate their relationship with the H2O2 yield. The results showed that the S doping greatly enhanced the activity and selectivity of Co-N-C@S for H2O2 production. The optimal Co-N-C@S(12) displayed a high H2O2 production rate of 395 mmol gcat-1 h-1, H2O2 selectivity of 76.06%, and Faraday efficiency of 91.66% at 0.2 V, which were obviously better than those of Co-N-C (H2O2 production rate of 44 mmol gcat-1 h-1, H2O2 selectivity of 26.63%, and Faraday efficiency of 17.37%). Moreover, the Co-N-C@S(12) based electron-Fenton system displayed effective rhodamine B (RhB) removal, significantly outperforming the Co-N-C-based system. Experimental results combined with density functional theory unveiled that the enhanced performance of Co-N-C@S(12) stemmed from the combined effect of Co-S and thiophene S, which jointly enhanced electron density of the Co center, reduced the desorption energy of the *OOH intermediate, and then promoted the production of H2O2.

Carboxylated Hexagonal Boron Nitride/Graphene Configuration for Electrosynthesis of High-Concentration Neutral Hydrogen Peroxide

The electrosynthesis of hydrogen peroxide (H2 O2 ) via two-electron (2e- ) oxygen (O2 ) reduction reaction (ORR) has great potential to replace the traditional energy-intensive anthraquinone process, but the design of low-cost and highly active and selective catalysts is greatly challenging for the long-term H2 O2 production under industrial relevant current density, especially under neutral electrolytes. To address this issue, this work constructed a carboxylated hexagonal boron nitride/graphene (h-BN/G) heterojunction on the commercial activated carbon through the coupling of B, N co-doping with surface oxygen groups functionalization. The champion catalyst exhibited a high 2e- ORR selectivity (>95 %), production rate (up to 13.4 mol g-1  h-1 ), and Faradaic efficiency (FE, >95 %). The long-term H2 O2 production under the high current density of 100 mA cm-2 caused the cumulative concentration as high as 2.1 wt %. The combination of in situ Raman spectra and theoretical calculation indicated that the carboxylated h-BN/G configuration promotes the adsorption of O2 and the stabilization of the key intermediates, allowing a low energy barrier for the rate-determining step of HOOH* release from the active site and thus improving the 2e- ORR performance. The fast dye degradation by using this electrochemical synthesized H2 O2 further illustrated the promising practical application.

Porous Electropolymerized Films of Ruthenium Complex: Photoelectrochemical Properties and Photoelectrocatalytic Synthesis of Hydrogen Peroxide

The photoelectrochemical cells (PECs) performing high-efficiency conversions of solar energy into both electricity and high value-added chemicals are highly desirable but rather challenging. Herein, we demonstrate that a PEC using the oxidatively electropolymerized film of a heteroleptic Ru(II) complex of [Ru(bpy)(L)2](PF6)2Ru1 {bpy and L stand for 2,2'-bipyridine and 1-phenyl-2-(4-vinylphenyl)-1H-imidazo[4,5-f][1,10]phenanthroline respectively}, polyRu1, as a working electrode performed both efficient in situ synthesis of hydrogen peroxide and photocurrent generation/switching. Specifically, when biased at -0.4 V vs. saturated calomel electrode and illuminated with 100 mW·cm-2 white light, the PEC showed a significant cathodic photocurrent density of 9.64 μA·cm-2. Furthermore, an increase in the concentrations of quinhydrone in the electrolyte solution enabled the photocurrent polarity to switch from cathodic to anodic, and the anodic photocurrent density reached as high as 11.4 μA·cm-2. Interestingly, in this single-compartment PEC, the hydrogen peroxide yield reached 2.63 μmol·cm-2 in the neutral electrolyte solution. This study will serve as a guide for the design of high-efficiency metal-complex-based molecular systems performing photoelectric conversion/switching and photoelectrochemical oxygen reduction to hydrogen peroxide.

Boosted Electrocatalytic Degradation of Levofloxacin by Chloride Ions: Performances Evaluation and Mechanism Insight with Different Anodes

As chloride (Cl-) is a commonly found anion in natural water, it has a significant impact on electrocatalytic oxidation processes; yet, the mechanism of radical transformation on different types of anodes remains unexplored. Therefore, this study aims to investigate the influence of chlorine-containing environments on the electrocatalytic degradation performance of levofloxacin using BDD, Ti4O7, and Ru-Ti electrodes. The comparative analysis of the electrode performance demonstrated that the presence of Cl- improved the removal and mineralization efficiency of levofloxacin on all the electrodes. The enhancement was the most pronounced on the Ti4O7 electrode and the least significant on the Ru-Ti electrode. The evaluation experiments and EPR characterization revealed that the increased generation of hydroxyl radicals and active chlorine played a major role in the degradation process, particularly on the Ti4O7 anode. The electrochemical performance tests indicated that the concentration of Cl- affected the oxygen evolution potentials of the electrode and consequently influenced the formation of hydroxyl radicals. This study elucidates the mechanism of Cl- participation in the electrocatalytic degradation of chlorine-containing organic wastewater. Therefore, the highly chlorine-resistant electrocatalytic anode materials hold great potential for the promotion of the practical application of the electrocatalytic treatment of antibiotic wastewater.

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.

Langmuir-Blodgett Monolayer of Cobalt Phthalocyanine as Ultralow Loading Single-Atom Catalyst for Highly Efficient H2O2 Production

The electrochemical production of H2O2 via the two-electron oxygen-reduction reaction (2e- ORR) has been actively studied using systems with atomically dispersed metal-nitrogen-carbon (M-N-C) structures. However, the development of well-defined M-N-C structures that restrict the migration and agglomeration of single-metal sites remains elusive. Herein, we demonstrate a Langmuir-Blodgett (LB) monolayer of cobalt phthalocyanine (CoPc) on monolayer graphene (LB CoPc/G) as a single-metal catalyst for the 2e- ORR. The as-prepared CoPc LB monolayer has a β-form crystalline structure with a lattice space for the facile adsorption of oxygen molecules on the cobalt active sites. The CoPc LB monolayer system provides highly exposed Co atoms in a well-defined structure without agglomeration, resulting in significantly improved catalytic activity, which is manifested by a very high H2O2 production rate per catalyst (31.04 mol gcat-1 h-1) and TOF (36.5 s-1) with constant production stability for 24 hours. To the best of our knowledge, the CoPc LB monolayer system exhibits the highest H2O2 production rate per active site. This fundamental study suggests that an LB monolayer of molecules with single-metal atoms as a well-defined structure works for single-atom catalysts.

Single-Atom Zinc Sites with Synergetic Multiple Coordination Shells for Electrochemical H2 O2 Production

Precise manipulation of the coordination environment of single-atom catalysts (SACs), particularly the simultaneous engineering of multiple coordination shells, is crucial to maximize their catalytic performance but remains challenging. Herein, we present a general two-step strategy to fabricate a series of hollow carbon-based SACs featuring asymmetric Zn-N2 O2 moieties simultaneously modulated with S atoms in higher coordination shells of Zn centers (n≥2; designated as Zn-N2 O2 -S). Systematic analyses demonstrate that the synergetic effects between the N2 O2 species in the first coordination shell and the S atoms in higher coordination shells lead to robust discrete Zn sites with the optimal electronic structure for selective O2 reduction to H2 O2 . Remarkably, the Zn-N2 O2 moiety with S atoms in the second coordination shell possesses a nearly ideal Gibbs free energy for the key OOH* intermediate, which favors the formation and desorption of OOH* on Zn sites for H2 O2 generation. Consequently, the Zn-N2 O2 -S SAC exhibits impressive electrochemical H2 O2 production performance with high selectivity of 96 %. Even at a high current density of 80 mA cm-2 in the flow cell, it shows a high H2 O2 production rate of 6.924 mol gcat -1  h-1 with an average Faradaic efficiency of 93.1 %, and excellent durability over 65 h.

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.

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.

Recent Progress on Electrode Design for Efficient Electrochemical Valorisation of CO2 , O2 , and N2

CO2 reduction, two-electron O2 reduction, and N2 reduction are sustainable technologies to valorise common molecules. Their further development requires working electrode design to promote the multistep electrochemical processes from gas reactants to value-added products at the device level. This review proposes critical features of a desirable electrode based on the fundamental electrochemical processes and the development of scalable devices. A detailed discussion is made to approach such a desirable electrode, addressing the recent progress on critical electrode components, assembly strategies, and reaction interface engineering. Further, we highlight the electrode design tailored to reaction properties (e.g., its thermodynamics and kinetics) for performance optimisation. Finally, the opportunities and remaining challenges are presented, providing a framework for rational electrode design to push these gas reduction reactions towards an improved technology readiness level (TRL).

Epoxy-rich Fe Single Atom Sites Boost Oxygen Reduction Electrocatalysis

Electrocatalysts for highly efficient oxygen reduction reaction (ORR) are crucial for energy conversion and storage devices. Single-atom catalysts with maximized metal utilization and altered electronic structure are the most promising alternatives to replace current benchmark precious metals. However, the atomic level understanding of the functional role for each species at the anchoring sites is still unclear and poorly elucidated. Herein, we report Fe single atom catalysts with the sulfur and oxygen functional groups near the atomically dispersed metal centers (Fe1/NSOC) for highly efficient ORR. The Fe1/NSOC delivers a half-wave potential of 0.92 V vs. RHE, which is much better than those of commercial Pt/C (0.88 V), Fe single atoms on N-doped carbon (Fe1/NC, 0.89 V) and most reported nonprecious metal catalysts. The spectroscopic measurements reveal that the presence of sulfur group induces the formation of epoxy groups near the FeN4S2 centers, which not only modulate the electronic structure of Fe single atoms but also participate the catalytic process to improve the kinetics. The density functional theory calculations demonstrate the existence of sulfur and epoxy group engineer the charges of Fe reactive center and facilitate the reductive release of OH* (rate-limiting step), thus boosting the overall oxygen reduction efficiency.

Regulation of Redox Molecular Junctions in Covalent Organic Frameworks for H2 O2 Photosynthesis Coupled with Biomass Valorization

H2 O2 photosynthesis coupled with biomass valorization can not only maximize the energy utilization but also realize the production of value-added products. Here, a series of COFs (i.e. Cu3 -BT-COF, Cu3 -pT-COF and TFP-BT-COF) with regulated redox molecular junctions have been prepared to study H2 O2 photosynthesis coupled with furfuryl alcohol (FFA) photo-oxidation to furoic acid (FA). The FA generation efficiency of Cu3 -BT-COF was found to be 575 mM g-1 (conversion ≈100 % and selectivity >99 %) and the H2 O2 production rate can reach up to 187 000 μM g-1 , which is much higher than Cu3 -pT-COF, TFP-BT-COF and its monomers. As shown by theoretical calculations, the covalent coupling of the Cu cluster and the thiazole group can promote charge transfer, substrate activation and FFA dehydrogenation, thus boosting both the kinetics of H2 O2 production and FFA photo-oxidation to increase the efficiency. This is the first report about COFs for H2 O2 photosynthesis coupled with biomass valorization, which might facilitate the exploration of porous-crystalline catalysts in this field.

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.

Z-scheme cobalt-iron oxide/perylene diimide supermolecule heterojunction for high-efficiency ciprofloxacin removal in a photocatalysis-self-Fenton system

Fenton technology has been famous on antibiotics removal, but seriously restricted by the extra addition of H₂O₂ and low mineralization efficiency. Herein, we develop a novel cobalt-iron oxide/perylene diimide organic supermolecule (CoFeO/PDIsm) Z-scheme heterojunction under photocatalysis-self-Fenton system, in which the holes (h⁺) of photocatalyst can mineralize organic pollutants and the photo-generated electrons (e^-) are used to in-situ H₂O₂ production with high efficiency. The CoFeO/PDIsm exhibits superior in-situ H₂O₂ production at a rate of 281.7 µmol g^-1 h^-1 in contaminating solution, correspondingly of total organic carbon (TOC) removal rate of ciprofloxacin (CIP) is 63.7 %, far exceeding current photocatalysts. The high H₂O₂ production rate and remarkable mineralization ability are ascribed to great charge separation in Z-scheme heterojunction. This work provides a novel Z-scheme heterojunction with photocatalysis-self-Fenton system for environmental-friendly removing the organic containment.

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.

Spectroscopic Identification of Active Sites of Oxygen-Doped Carbon for Selective Oxygen Reduction to Hydrogen Peroxide

The electrochemical synthesis of hydrogen peroxide (H2 O2 ) via a two-electron (2 e- ) oxygen reduction reaction (ORR) process provides a promising alternative to replace the energy-intensive anthraquinone process. Herein, we develop a facile template-protected strategy to synthesize a highly active quinone-rich porous carbon catalyst for H2 O2 electrochemical production. The optimized PCC900 material exhibits remarkable activity and selectivity, of which the onset potential reaches 0.83 V vs. reversible hydrogen electrode in 0.1 M KOH and the H2 O2 selectivity is over 95 % in a wide potential range. Comprehensive synchrotron-based near-edge X-ray absorption fine structure (NEXAFS) spectroscopy combined with electrocatalytic characterizations reveals the positive correlation between quinone content and 2 e- ORR performance. The effectiveness of chair-form quinone groups as the most efficient active sites is highlighted by the molecule-mimic strategy and theoretical analysis.

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.

Pd-Sn Alloy Catalysts for Direct Synthesis of Hydrogen Peroxide from H2 and O2 in a Microchannel Reactor

Direct synthesis of hydrogen peroxide (DSHP) from H₂ and O₂ offers a promising alternative to the present commercial anthraquinone method, but it still faces the challenges of low H₂O₂ productivity, low stability of catalysts, and high risk of explosion. Herein, by loading in a microchannel reactor, the as-synthesized Pd-Sn alloy materials exhibit high catalytic activity for H₂O₂ production, presenting a H₂O₂ productivity of 3124 g kg_Pd^-1 h^-1. The doped Sn atoms on the surface of Pd not only facilitate the release of H₂O₂ but also effectively slow down the deactivation of catalysts. Theoretical calculations demonstrate that the Pd-Sn alloy surface has the property of antihydrogen poisoning, showing higher activity and stability than pure Pd catalysts. The deactivation mechanism of the catalyst was elucidated, and the online reactivation method was developed. In addition, we show that the long-life Pd-Sn alloy catalyst can be achieved by supplying an intermittent flow of hydrogen gas. This work provides guidance on how to prepare high performance and stable Pd-Sn alloy catalysts for the continuous and direct synthesis of H₂O₂.

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.

Dual molecules engineered carbon nitride for achieving outstanding photocatalytic H2O2 production

Molecular engineering of carbon nitride (CN) was considered as a suitable and compelling strategy to overcome the intrinsic imperfections and enhance photocatalytic H₂O₂ production. However, the photocatalytic H₂O₂ production of conventional single molecular engineering is still unsatisfactory, and the comprehension of photogenerated carrier migration and separation is still indistinct. Herein, dual molecules were engineered on CN molecular skeleton for achieving an outstanding photocatalytic rate of H₂O₂ production. The photocatalytic H₂O₂ production rate of the dual molecules engineered CN was up to 3320 μmol g^-1 h^-1, which was approximately 25 times than that of the pristine CN. After the dual-molecular engineering, pyrimidine and cyano group were co-grafted. Synchronously, K ion and Na ion were co-embedded near the interlamination of CN layers. The synergistic effect of the dual molecules in CN not only restrained photogenerated carrier recombination and broadened visible light response by modulating the intrinsic energy band structure, but also enhanced the capture of the photogenerated electrons and accelerated the migration of proton. Hence, the photocatalytic 2e^- oxygen reduction reaction, the rate-determining step, was significantly strengthened. Additionally, caused by the positive valence band potential, the H₂O oxidation reaction became an indispensable role in photocatalytic H₂O₂ production. This work provided a viable route to modulate the molecular skeleton of organic semiconductors and presented a promising strategy to obtain high-efficient photocatalytic H₂O₂ production.

Correlation of the spin state and catalytic property of M-N4 single-atom catalysts in oxygen reduction reactions

The rational design of high-performance catalysts for oxygen reduction reactions (ORRs) is of great importance for large-scale applications in the field of proton-exchange membrane fuel cells and the green synthesis of H₂O₂. The effect of spin states of paramagnetic metal ions on the selectivity of ORRs is significant for single-atom catalysts (SACs). In this work, via spin-polarization density functional theory (DFT) calculations, we systematically investigated the popular paramagnetic metal-nitrogen graphene (M-N₄-C, M = Mn, Fe, and Co) SACs to mainly focus on the correlation of spin states and catalytic performance (e.g. activity and selectivity). Both thermodynamically and kinetically, it was found that Co-N₄-C (S = 1/2) has excellent 2e^- oxygen reduction performance (hydrogen peroxide production) with an ultralow overpotential of 0.03 V, and the hydrogenation of OOH* is the rate-determining step (RDS) with an energy barrier of 1.20 eV. The 4e^- ORR tends to occur along the OOH dissociation pathway (O* + OH*) on Co-N₄-C (S = 3/2), in which OOH* decomposition is the RDS with an energy barrier of 1.01 eV. It is proved that the spin magnetic moment is the key factor to regulate the ORR property via multi-angle electronic analysis. The spin states of catalysts play a crucial role in the activity and selectivity of ORRs mainly by manipulating the bond strength between OOH and catalysts. This will provide new insights for the rational design of ORR catalysts with magnetic metals.

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.

CuC(O) Interfaces Deliver Remarkable Selectivity and Stability for CO2 Reduction to C2+ Products at Industrial Current Density of 500 mA cm-2

The electrocatalytic CO₂ reduction reaction (CO₂ RR) is an attractive technology for CO₂ valorization and high-density electrical energy storage. Achieving a high selectivity to C₂₊ products, especially ethylene, during CO₂ RR at high current densities (>500 mA cm^-2 ) is a prized goal of current research, though remains technically very challenging. Herein, it is demonstrated that the surface and interfacial structures of Cu catalysts, and the solid-gas-liquid interfaces on gas-diffusion electrode (GDE) in CO₂ reduction flow cells can be modulated to allow efficient CO₂ RR to C₂₊ products. This approach uses the in situ electrochemical reduction of a CuO nanosheet/graphene oxide dots (CuOC(O)) hybrid. Owing to abundant CuOC interfaces in the CuOC(O) hybrid, the CuO nanosheets are topologically and selectively transformed into metallic Cu nanosheets exposing Cu(100) facets, Cu(110) facets, Cu[n(100) × (110)] step sites, and Cu⁺ /Cu⁰ interfaces during the electroreduction step, the faradaic efficiencie (FE) to C₂₊ hydrocarbons was reached as high as 77.4% (FE_ethylene  ≈ 60%) at 500 mA cm^-2 . In situ infrared spectroscopy and DFT simulations demonstrate that abundant Cu⁺ species and Cu⁰ /Cu⁺ interfaces in the reduced CuOC(O) catalyst improve the adsorption and surface coverage of *CO on the Cu catalyst, thus facilitating CC coupling reactions.

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.

Photoredox-Promoted Co-Production of Dihydroisoquinoline and H2 O2 over Defective Zn3 In2 S6

One of the most sustainable and promising approaches for hydrogen peroxide (H₂ O₂ ) production in a low-cost and environment-friendly way is photosynthesis, which, however, suffers from poor carrier utilization and low H₂ O₂ productivity. The addition of proton donors such as isopropanol or ethanol can increase H₂ O₂ production, which, unfortunately, will inevitably elevate the entire cost while wasting the oxidizing power of holes (h⁺ ). Herein, the tetrahydroisoquinolines (THIQs) is employed as a distinctive proton donor for the thermodynamically feasible and selective semi-dehydrogenation reaction to highly valuable dihydroisoquinolines (DHIQs), and meanwhile, to couple with and promote H₂ O₂ generation in one photoredox reaction under the photocatalysis by dual-functional Zn₃ In₂ S₆ photocatalyst. Surprisingly, the suitably defective Zn₃ In₂ S₆ offers an excellent and near-stoichiometric co-production performance of H₂ O₂ and DHIQs at unprecedentedly high rates of 66.4 and 62.1 mmol h^-1 g^-1 under visible light (λ ≥ 400 nm), respectively, which outperforms all the previously available reports even though sacrificial agents were employed in those reports. Additionally, photocatalytic redox reaction mechanism demonstrates that H₂ O₂ can be generated through multiple pathways, highlighting the synergistic effect among ROS (·O₂ ^- and ¹ O₂ ), h⁺ and proton donor, which has been ignored in previous studies.

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

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

Toward More Efficient Carbon-Based Electrocatalysts for Hydrogen Peroxide Synthesis: Roles of Cobalt and Carbon Defects in Two-Electron ORR Catalysis

Electrochemical production of H₂O₂ is a cost-effective and environmentally friendly alternative to the anthraquinone-based processes. Metal-doped carbon-based catalysts are commonly used for 2-electron oxygen reduction reaction (2e^-ORR) due to their high selectivity. However, the exact roles of metals and carbon defects on ORR catalysts for H₂O₂ production remain unclear. Herein, by varying the Co loading in the pyrolysis precursor, a Co-N/O-C catalyst with Faradaic efficiency greater than 90% in alkaline electrolyte was obtained. Detailed studies revealed that the active sites in the Co-N/O-C catalysts for 2e^-ORR were carbon atoms in C-O-C groups at defect sites. The direct contribution of cobalt single atom sites and metallic Co for the 2e^-ORR performance was negligible. However, Co plays an important role in the pyrolytic synthesis of a catalyst by catalyzing carbon graphitization, tuning the formation of defects and oxygen functional groups, and controlling O and N concentrations, thereby indirectly enhancing 2e^-ORR performance.

Metal single-site catalyst design for electrocatalytic production of hydrogen peroxide at industrial-relevant currents

Direct hydrogen peroxide (H₂O₂) electrosynthesis via the two-electron oxygen reduction reaction is a sustainable alternative to the traditional energy-intensive anthraquinone technology. However, high-performance and scalable electrocatalysts with industrial-relevant production rates remain to be challenging, partially due to insufficient atomic level understanding in catalyst design. Here we utilize theoretical approaches to identify transition-metal single-site catalysts for two-electron oxygen reduction using the *OOH binding energy as a descriptor. The theoretical predictions are then used as guidance to synthesize the desired cobalt single-site catalyst with a O-modified Co-(pyrrolic N)₄ configuration that can achieve industrial-relevant current densities up to 300 mA cm^-2 with 96-100% Faradaic efficiencies for H₂O₂ production at a record rate of 11,527 mmol h^-1 g_cat^-1. Here, we show the feasibility and versatility of metal single-site catalyst design using various commercial carbon and cobalt phthalocyanine as starting materials and the high applicability for H₂O₂ electrosynthesis in acidic, neutral and alkaline electrolytes.

Edge-hosted Atomic Co-N4 Sites on Hierarchical Porous Carbon for Highly Selective Two-electron Oxygen Reduction Reaction

Not only high efficiency but also high selectivity of the electrocatalysts is crucial for high-performance, low-cost, and sustainable energy storage applications. Herein, we systematically investigate the edge effect of carbon-supported single-atom catalysts (SACs) on oxygen reduction reaction (ORR) pathways (two-electron (2 e^- ) or four-electron (4 e^- )) and conclude that the 2 e^- -ORR proceeding over the edge-hosted atomic Co-N₄ sites is more favorable than the basal-plane-hosted ones. As such, we have successfully synthesized and tuned Co-SACs with different edge-to-bulk ratios. The as-prepared edge-rich Co-N/HPC catalyst exhibits excellent 2 e^- -ORR performance with a remarkable selectivity of ≈95 % in a wide potential range. Furthermore, we also find that oxygen functional groups could saturate the graphitic carbon edges under the ORR operation and further promote electrocatalytic performance. These findings on the structure-property relationship in SACs offer a promising direction for large-scale and low-cost electrochemical H₂ O₂ production via the 2 e^- -ORR.