Electrochemical synthesis of hydrogen peroxide from water and oxygen

Waste corn stalk-derived biomass carbon materials as two-electron ORR electrocatalysts for dye contaminant degradation and water disinfection

Porous carbon materials as efficient two-electron oxygen reduction reaction (ORR) electrocatalysts for on-situ production of hydrogen peroxide (H2O2) is one of the promising alternatives to the traditional anthraquinone process. Herein, waste corn stalks-derived porous carbon composites (CSDC-O, Fe/CSDC-O-12) were developed as two-electron ORR electrocatalysts for H2O2 generation and the further organic dye pollutants degradation and water disinfection. The high-temperature pyrolysis and oxidation treatment enriched the hierarchical porous structure of the biomass carbon materials, improved graphitization degree and the content of oxygen-containing functional groups, which facilitated the increase of active sites density, the mass and charge transfer rates acceleration, and the active and selective H2O2 generation. Based on the remarkable two-electron ORR selectivity and long-term stability in both alkaline and acidic media exhibited by Fe/CSDC-O-12, it was used to completely degrade 25 mg L-1 of rhodamine B and methyl orange within 70 and 80 min, respectively. Moreover, the CSDC-O electrocatalyst demonstrated disinfection efficiency exceeding 99.9999 % against Escherichia coli and Staphylococcus aureus within 20 and 60 min, respectively. Thus, our work provides a feasibility verification for the transformation of abundant biomass corn stalk waste into low-cost, sustainable, and high-value-added two-electron ORR electrocatalysts, and expand their application in dye contaminant degradation and water disinfection.

Rational Dual-Atom Design to Boost Oxygen Reduction Reaction on Iron-Based Electrocatalysts

The oxygen reduction reaction (ORR) is critical for energy conversion technologies like fuel cells and metal-air batteries. However, advancing efficient and stable ORR catalysts remains a significant challenge. Iron-based single-atom catalysts (Fe SACs) have emerged as promising alternatives to precious metals. However, their catalytic performance and stability remain constrained. Introducing a second metal (M) to construct Fe─M dual-atom catalysts (Fe─M DACs) is an effective strategy to enhance the performance of Fe SACs. This review provides a comprehensive overview of the recent advancements in Fe-based DACs for ORR. It begins by examining the structural advantages of Fe─M DACs from the perspectives of electronic structure and reaction pathways. Next, the precise synthetic strategies for DACs are discussed, and the structure-performance relationships are explored, highlighting the role of the second metal in improving catalytic activity and stability. The review also covers in situ characterization techniques for real-time observation of catalytic dynamics and reaction intermediates. Finally, future directions for Fe─M DACs are proposed, emphasizing the integration of advanced experimental strategies with theoretical simulations as well as artificial intelligence/machine learning to design highly active and stable ORR catalysts, aiming to expand the application of Fe─M DACs in energy conversion and storage technologies.

Boosting H2O2 production via two-electron oxygen reduction with O-doped g-C3N4 decorated with Ti3C2Tx quantum dots

Photocatalytic synthesis of H2O2 with g-C3N4 holds great promise for converting solar energy into chemical energy, but it remains constrained by the narrow optical absorption range and rapid charge recombination. To overcome these challenges, Ti3C2Tx MXene quantum dots (TQDs), known for their ease of carrier regulation and strong visible light absorption, were incorporated into O-doped g-C3N4 (O-CN) to form TQDs-modified O-CN (O-CN@TQDs) with a Schottky heterojunction. Attributed to such structural design, the H2O2 production was promoted through the two-step two-electron oxygen reduction pathway, with O2- serving as the primary intermediate. The photocatalytic H2O2 production rate over optimized O-CN@TQDs reached 868.9 μmol g-1 h-1 under visible light irradiation, which was 10.8 times higher than that of the pristine g-C3N4. This study underscores the potential of judiciously selecting suitable semiconductor and metal-like materials to construct Schottky heterojunctions for the efficient photocatalytic production of H2O2.

Innovative engineering strategies and mechanistic insights for enhanced carbon-based electrocatalysts in sustainable H2O2 production

Hydrogen peroxide (H2O2) plays a crucial role in various industrial sectors and everyday applications. Given the energy-intensive nature of the current anthraquinone process for its production, the quest for cost-effective, efficient, and stable catalysts for H2O2 synthesis is paramount. A promising sustainable approach lies in small-scale, decentralized electrochemical methods. Carbon nanomaterials have emerged as standout candidates, offering low costs, high surface areas, excellent conductivity, and adjustable electronic properties. This review presents a thorough examination of recent strides in engineering strategies of carbon-based nanomaterials for enhanced electrochemical H2O2 generation. It delves into tailored microstructures (e.g., 1D, 2D, porous architectures), defect/surface engineering (e.g., edge sites, heteroatom doping, surface modification), and heterostructure assembly (e.g., semiconductor-carbon composites, single-atom, dual-single-atom catalysts). Moreover, the review explores structure-performance interplays in these carbon electrocatalysts, drawing from advanced experimental analyses and theoretical models to unveil the mechanisms governing selective electrocatalytic H2O2 synthesis. Lastly, this review identifies challenges and charts future research avenues to propel carbon electrocatalysts towards greener and more effective H2O2 production methods.

Two-step preparation of cyano-functionalized linkage-hybrid covalent organic frameworks for efficient H2O2 photosynthesis in air

We report two cyano-functionalized hybrid-linkage covalent organic frameworks (COFs) for efficient H2O2 photosynthesis from air and water, overcoming the critical O2 dependence limitation of conventional systems, achieving 97% oxygen utilization efficiency.

Unraveling Dimensional Tuning: From 2D to 3D in Covalent Organic Frameworks for Enhanced 2e- Oxygen Reduction Reaction

Covalent organic frameworks (COFs) with a two-dimensional (2D) topology have recently emerged as promising catalyst systems for the electrosynthesis of hydrogen peroxide (H2O2) from oxygen (O2). However, designing 2D catalysts to achieve higher H2O2 selectivity presents a significant challenge because of the extensive layer stacking and the aggregated active sites located in the basal planes. It results in lower atom utilization, which requires attention. In this study, we present two functionally similar COFs: one with a 2D rhombus topology (2D@BT_TPA-COF) and another with a three-dimensional (3D) noninterpenetrated pts topology (3D@BT_TPA-COF). Both COFs were utilized for the 2e- oxygen reduction reaction (2e- ORR). Tunning the dimensionality from 2D to 3D resulted in an increase in H2O2 selectivity from approximately ∼56% to approximately ∼96% (at 0.4 V) and a rise in the turnover frequency (TOF) from 0.05 to 0.08 s-1 at 0.3 V. Nonaggregated active site distribution over 3D topology, featuring higher active site exposure, provides better access to the O2/electrolyte and facilitates electron transfer leading to higher 2e- ORR activity and selectivity compared to the 2D counterpart.

Tuning Local Proton Concentration and *OOH Intermediate Generation for Efficient Acidic H2O2 Electrosynthesis at Ampere-Level Current Density

Electrocatalytic oxygen reduction is a sustainable method for on-site H2O2 synthesis. The H2O2 in acidic media has wide downstream applications, but acidic H2O2 electrosynthesis suffers from poor efficiency due to high proton concentration and unfavourable *OOH (key intermediate) generation. Herein, acidic H2O2 electrosynthesis was enhanced by regulating local proton availability and *OOH generation via fluorine-doped on inner and outer walls of carbon nanotubes (F-CNTs). It was efficient and stable for H2O2 electrosynthesis with Faradaic efficiency of 95.6% and H2O2 yield of 606.6 mg cm-2 h-1 at 1.0 A cm-2 and 0.05 M H2SO4, outperforming the state-of-the-art electrocatalysts. The F-doping regulated the electronic structure of CNTs with elevated p-band center, and F-doping on its inner and outer walls also enhanced nanoconfinement effect and superhydrophobicity, respectively. As a result, a local alkaline microenvironment was created on F-CNTs surface during acidic H2O2 electrosynthesis. The energy barrier for *OOH generation was significantly reduced and oxygen mass transfer was boosted. Their synergistic effects promoted acidic H2O2 electrosynthesis. This work provides new insights into the mechanism for regulating H2O2 electrosynthesis.

Pulsed-Current Operation Enhances H2O2 Production on a Boron-Doped Diamond Mesh Anode in a Zero-Gap PEM Electrolyzer

A niobium (Nb) mesh electrode was coated with boron-doped diamond (BDD) using chemical vapor deposition in a custom-built hot-filament reactor. The BDD-functionalized mesh was tested in a zero-gap electrolysis configuration and evaluated for the anodic formation of H2O2 by selective oxidation of water, including the analysis of the effects on Faradaic efficiency towards H2O2FE H 2 O 2 ${\left( {{\rm{FE}}_{{\rm{H}}_2 {\rm{O}}_2 } } \right)}$ induced by pulsed electrolysis. A low electrolyte flow rate (V ˙ anolyte ${{\dot {\rm V}}_{{\rm{anolyte}}} }$ ) was found to result in a relatively high concentration of H2O2 in single-pass electrolysis experiments. Regarding pulsed electrolysis, we show an optimal ratio of on-time to off-time to obtain the highest concentration of H2O2. Off-times that are "too short" result in decreasedFE H 2 O 2 ${{\rm{FE}}_{{\rm{H}}_2 {\rm{O}}_2 } }$ , whereas "too long" off-times dilute the product in the electrolyte stream. Using our electrolyzer setup with an anodic pulse of 2 s with 4 s intervals, and aV ˙ anolyte ${{\dot {\rm V}}_{{\rm{anolyte}}} }$ of 0.75 cm3 min-1, resulted in the best performance. This adjustment increased theFE H 2 O 2 ${{\rm{FE}}_{{\rm{H}}_2 {\rm{O}}_2 } }$ by 70 % compared to constant current electrolysis, at industrially relevant current densities (150 mA cm-2). Fine tuning of BDD morphology, flow patterns, and anolyte composition might further increase the performance of zero-gap electrolyzers in pulsed operation modes.

Engineering Asymmetric Electronic Structure of Co─N─C Single-Atomic Sites Toward Excellent Electrochemical H2O2 Production and Biomass Upgrading

To advance electrochemical H2O2 production and unravel catalytic mechanisms, the precise structural coordination of single-atomic M-N-C electrocatalysts is urgently required. Herein, the Co─N5 site with an asymmetric electronic configuration is constructed to boost the two-electron oxygen reduction reaction (2e- ORR) compared to symmetric Co─N4, effectively overcoming the trade-off between activity and selectivity in H2O2 production. Both experimental and theoretical analyses demonstrate that breaking the symmetry of Co─N sites promotes the activation of O2 molecules and moderates the adsorption of the key *OOH intermediate by disrupting the linear scaling relationship for intermediates adsorption. This modulation enables efficient H₂O₂ production and its effective retention for subsequent applications. As a proof of concept, Co─N5 achieves a H2O2 production rate as high as 16.1 mol gcat -1 h-1 in a flow cell, outperforming most recently reported counterparts. Furthermore, the coupling of 2e- ORR with the oxidation of cellulose-derived carbohydrates accomplishes high formic acid yields (84.1% from glucose and 62.0%-92.1% from other substrates), underpinning the sustainable electro-refinery for biomass valorization at ambient conditions. By elucidating the intrinsic relationship between 2e⁻ ORR and the asymmetry of single-atomic sites, this work paves the way for high-performance electrosynthesis.

Internal hydrogen-bond enhanced two-electron oxygen reduction reaction for π-d conjugated metal-organic framework to H2O2 synthesis

Tailoring the electronic structure of electrocatalysts for oxygen reduction reaction (ORR) has been widely adopted to optimize their performance. However, the steric effect originating from the layered or crystal structure of a catalyst is often neglected. Herein, we demonstrate the importance of such steric effect in a one-dimensional π-d conjugated metal-organic framework with Ni-(NH)4 nodes (Ni-BTA) for optimizing its electrocatalytic performance, where the activity and selectivity towards two-electron ORR for H2O2 production are largely enhanced. Theoretical simulation and in-situ characterization demonstrate the formation of inter-layer H-bonds between *OOH intermediates and -N-H groups in the adjacent top layers of the Ni-sites, enhancing the *OOH binding energy to an optimum value. Thus, the as-prepared Ni-BTA catalyst exhibits an outstanding electrocatalytic 2e--ORR performances under neutral and alkaline conditions (e.g., >85% H2O2 selectivity from -0.1-0.4 V vs. RHE and >13.5 mol g-1 h-1 H2O2 yield in neutral electrolytes), also showing great potential on water treatment and disinfection. Here, we highlight the alternative avenues for utilizing the non-coordinated structure to regulate the catalytic performance, thus providing opportunities for the design of catalysts and beyond.

Redox-Mediated TEMPO-Based Donor-Acceptor Covalent Organic Framework for Efficient Photo-Induced Hydrogen Peroxide Generation

Molecular engineering of covalent organic frameworks (COFs) offers an alternative approach to conventional anthraquinone oxidation via photo-induced H2O2 production from O2 reduction. Despite their potential, reported photocatalysts suffer limited proton mobility, low selectivity, and insufficient charge separation and utilization. Herein, we report a nitroxyl radical (TEMPO) decorated two-dimensional (2D) donor-acceptor (D-A)-COF photocatalyst via a one-pot strategy. Under visible light irradiation, highly crystalline TAPP-TPDA-TEMPO-COF (TT-T-COF) exhibits a remarkable photocatalytic H2O2 yield of 10066 μmol g-1 h-1 in two-phase water-benzyl alcohol (10 % BA) system through direct two-electron (2e-) pathway. The mechanistic study by DFT calculations and in situ DRIFT spectra suggests Yeager-type adsorption of *O2⋅- intermediate on the nitroxyl radical site (N-O⋅). The efficient photocatalytic performance and stability of TT-T-COF are attributed to the involvement of the nitroxyl radical, which enhances selective O2 adsorption, establishes a distinct electron density distribution, and facilitates photogenerated charge separation compared to TT-HT-COF and TT-COF counterparts. This study uncovers a new perspective for constructing metal-free, redox-mediated radical-based COFs for sustainable energy conversion, storage, and biomedical applications.

Selective two-electron and four-electron oxygen reduction reactions using Co-based electrocatalysts

The oxygen reduction reaction (ORR) can take place via both four-electron (4e-) and two-electron (2e-) pathways. The 4e- ORR, which produces water (H2O) as the only product, is the key reaction at the cathode of fuel cells and metal-air batteries. On the other hand, the 2e- ORR can be used to electrocatalytically synthesize hydrogen peroxide (H2O2). For the practical applications of the ORR, it is very important to precisely control the selectivity. Understanding structural effects on the ORR provides the basis to control the selectivity. Co-based electrocatalysts have been extensively studied for the ORR due to their high activity, low cost, and relative ease of synthesis. More importantly, by appropriately designing their structures, Co-based electrocatalysts can become highly selective for either the 2e- or the 4e- ORR. Therefore, Co-based electrocatalysts are ideal models for studying fundamental structure-selectivity relationships of the ORR. This review starts by introducing the reaction mechanism and selectivity evaluation of the ORR. Next, Co-based electrocatalysts, especially Co porphyrins, used for the ORR with both 2e- and 4e- selectivity are summarized and discussed, which leads to the conclusion of several key structural factors for ORR selectivity regulation. On the basis of this understanding, future works on the use of Co-based electrocatalysts for the ORR are suggested. This review is valuable for the rational design of molecular catalysts and material catalysts with high selectivity for 4e- and 2e- ORRs. The structural regulation of Co-based electrocatalysts also provides insights into the design and development of ORR electrocatalysts based on other metal elements.

Enhancing Solar-to-Hydrogen Peroxide Conversion Efficiency by Promoting the Two-Electron Water Oxidation Pathway via Modulating the Main Electron Transition Orbital

Photosynthetic hydrogen peroxide (H2O2) production involves coupling oxygen reduction and water oxidation half-reactions. However, the low efficiency of water oxidation constrains the overall solar-to-hydrogen peroxide conversion efficiency under natural conditions. The two-electron water oxidation pathway holds potential for enhanced photocatalytic H2O2 synthesis, yet its regulatory mechanisms and detailed understanding remain inadequately explored. Herein, we construct donor-acceptor (D-A) conjugated polymers with pyrene as the electron donor and triazine as the electron acceptor. By optimizing the connecting positions of the electron acceptors at the 2,7 positions of the electron donors, the main excited state is regulated from S1 to S2, leading to the electron transition from the lower HOMO-1 orbital. This modulation effectively enhances the oxidation capacity of the photocatalyst, enabling it to undergo two-electron water oxidation reaction (2e- WOR) for H2O2 production. Consequently, the WOR activity reaches a remarkable efficiency of 2560 µmol g-1 h-1, corresponding to a solar-to-chemical conversion (SCC) efficiency of up to 0.94%. This strategy of modulating electronic transition orbitals to enhance the water oxidation capacity of the material significantly improves photocatalytic performance and facilitates its application in natural environments.

The Ratio of sp2 and sp3 Hybridized Carbon Determines the Performance of Carbon-based Catalysts in H2O2 Electrosynthesis from O2

Introducing oxygen- or carbon-containing functional groups is a widely adopted strategy to optimize the performance of carbon-based catalysts for the two-electron oxygen reduction reaction (2 e- ORR). Nevertheless, the specific contributions of these functional groups to enhance activity and selectivity are not well-defined and continue under debate. In this study, we systematically modified carbon materials by controlling the contents of oxygen functional groups (OFGs) and carbon functional groups (CFGs). Surface compositions were accurately quantified using X-ray photoelectron spectroscopy, and 2 e- ORR performance was evaluated using a rotating ring-disk electrode (RRDE) in 0.1 M KOH. Through reliable statistical analyses, including partial least squares regression and linear regression, we explored the correlations between the surface compositional features - specifically the ratios of OFGs, CFGs, and sp2/sp3 hybridized carbon - and the catalytic performance metrics such as onset potential and H2O2 selectivity. Our findings challenge existing paradigms by demonstrating that the sp2/sp3 ratio is a critical factor in determining catalytic selectivity and a certain correlate with the 2 e- ORR onset potential. By tuning this ratio, we achieved nearly 100 % H2O2 selectivity within 0.4-0.6 V vs. RHE, and onset potential approached the thermodynamic potential (0.766 V vs. RHE for the O2/HO2 - process), pointing a new direction in designing and developing advanced electrocatalysts for sustainable H2O2 synthesis.

Spatially Separated Redox Centers in Anthraquinone-grafted Metal-Organic Frameworks for Efficient Piezo-photocatalytic H2O2 Production

Piezo-photocatalytic production of hydrogen peroxide (H2O2) from water and air is promising but its large-scale application is still challenging as insufficient reaction active sites and low reaction efficiency. We have applied molecular engineering methods to design an anthraquinone molecularly (AQ) grafted metal-organic framework piezo-photocatalyst (UiO-66-AQ) for H2O2 generation from water and air. The catalyst achieves a peak H2O2 yield of 7872.4 μM g-1 h-1 by facilitating two critical reactions: single-electron water oxidation (WOR) and two-electron oxygen reduction (ORR) on spatially separated redox sites. Experiments and computational simulations reveal efficient charge separation through a ligand-to-chain transfer mechanism. Electrons and holes are selectively transferred to AQ and UiO-66 promoting ORR and WOR under ultrasound and visible light. The high reaction rate of ORR (rapid generation of endoperoxide) compensates for the slow kinetics of WOR (generation of OH*) and greatly increases the rate of full-reaction of H2O2 production. Additionally, a continuous flow tubular reactor equipped with UiO-66-AQ catalytic membranes affords 96 % removal of organic dyes by a in situFenton process under visible light and water flow, confirming the significant potential of the catalyst for practical applications. This work deepens the understanding of directional carrier migration at piezo-photocatalytic spatial separation sites, opening new pathways for environmentally friendly and efficient H2O2 synthesis.

Intramolecular Double Activation by Biligands Sharing a Single Metal Atom for Preferred Two-Electron Oxygen Reduction

It is challenging to selectively promote the two-electron oxygen reduction reaction (2e-ORR) since highly ORR-active electrocatalysts are not satisfied with 2e-ORR and are most likely to go all the way to 4e-ORR, completely reducing dioxygen to water. Recently, however, the possibility of a 2e-ORR preference over 4e-ORR was raised by extensively considering multiple ORR mechanisms and employing a potential-dependent activity measure for constructing volcano plots. Here, we realized the preferred 2e-ORR via an intramolecular double activation of the peroxide intermediate (*OOH) by allowing the intermediate to be easily desorbed before proceeding to 4e-ORR. Dioxygen was transformed to *OOH on a carbon atom of the imidazole ligand of zeolitic imidazolate framework-8 (ZIF-8). When an amine group was introduced via ligand exchange, the selectivity of 2e-ORR was enhanced by 11%. The added amine attracted the oxygen atom of *OOH via a hydrogen bond to weaken the binding strength of *OOH to the carbon active site (double activation). The amine-decorated ZIF-8 exhibited H2O2 faradaic efficiency at 98.5% at ultrahigh-rate production at 625 mg cm-2 h-1 by 1 A cm-2 in a flow cell.

A ZnO-based Catalytic System for the Synthesis of Hydrogen Peroxide from Air

Hydrogen peroxide (H2O2) has a wide range of applications as an eco-friendly and sustainable oxidant. However, the clean, efficient and convenient synthesis of this compound remains challenging. This work demonstrates a rationally designed electron-self-supplied catalytic system capable of generating H2O2 from water and atmospheric oxygen without extra energy input. This catalytic system is made of a ZnO coating containing oxygen vacancies and a Zn substrate. The ZnO catalyst layer obtains electrons from the Zn substrate to synthesize H2O2. The H2O2 concentration produced by this catalytic system is up to 17.9 mM without any secondary processing. This remarkably high concentration is attributed to the formation of a liquid film on the hydrophilic ZnO surface that promotes the oxygen reduction reaction by accelerating the transfer of oxygen from the ambient air to the catalyst surface. By integrating with atmospheric fog collection, this system can continuously collect H2O2 directly from the air.

Molecular coordination inheritance of single Co atom catalysts for two-electron oxygen reduction reaction

Electrosynthesis of hydrogen peroxide (H2O2) through the two-electron oxygen reduction reaction (2e-ORR) is environmentally friendly and sustainable. Transition-metal single-atom catalysts (SACs) have gained attention for this application due to their low cost, high atom utilization, adjustable coordination, and geometric isolation of active metal sites. Although various synthetic methods of SACs have been reported, the specific mechanism of the formation of active sites is still less studied. Herein, we presented the molecular coordination inheritance strategy for synthesizing 2e-ORR SACs with well-defined coordination environments and investigated the formation mechanism of the active sites. We select precursors including [Co(II)Salen], CoPc, Co(acac)2 to achieve specific configurations (Co-N2O2, Co-N4, Co-O4). Our results indicate that the precursors undergo decomposition and are partially embedded in the carbon substrate at lower temperatures, facilitating the inheritance of the desired configurations. As the temperature increases, the inherited configurations will rearrange, forming dual-atom structures and metal particles gradually. Among the Co-N2O2, Co-N4, and Co-O4 catalysts, the Co-N2O2 catalyst demonstrates the highest 2e-ORR selectivity. This work reveals the mechanism of regulating SAC's active site structure by the molecular coordination inheritance strategy, which may provide new insights for further research on the precise regulation and formation mechanism of SAC's active site.

Monomolecule Coupled to Oxygen-Doped Carbon for Efficient Electrocatalytic Hydrogen Peroxide Production

The electrocatalytic production of hydrogen peroxide (H2O2) is an ideal alternative for the industrial anthraquinone process because of environmental friendliness and energy efficiency, depending on the activity and selectivity of catalysts. Carbon-based materials possess prospects as candidate catalysts for the production of H2O2. Herein, cedar-derived monolithic carbon catalysts modified with coupling oxygen doping and phthalocyanine molecules are synthesized. Cobalt phthalocyanine (CoPc) molecules are introduced onto the carbon surface to construct monomolecular active sites via π-π stacking. The electronic structure of CoPc is modulated by oxygen doping on carbon substrates, mediated by monomolecular π-π stacking. A synergistic effect optimally modulated the interaction between CoPc and key intermediate to H2O2. The energy barrier for oxygen reduction is reduced to optimize the selectivity to H2O2. CoPc@OCW provided up to 99% selectivity to H2O2 at 0.7 V versus RHE. In a three-phase flow cell, CoPc@OCW achieved an H2O2 yield up to 10.4 mol·g-1·h-1 at 0.2 V versus RHE with stable running for 24 h. The advantages of carbon-based catalysts including the adjustable chemical structure depending on π-π stacking and electronic structure of carbon atoms through oxygen doping improved the catalytic performances in the production of H2O2. This proof-to-concept research demonstrates the potential application of carbon-based molecular catalysts for electrochemical synthesis.

Confinement-Modulated Singlet-Oxygen Nanoreactors for Water Decontamination

Water decontamination with singlet oxygen (1O2) has shown advantages over the traditional radical-based treatment processes, which are frequently inhibited by the background inorganic/organic substances and produce toxic byproducts. However, earlier reported treatment systems mostly suffer from side reactions against efficient and cost-effective production of 1O2, together with insufficient utilization of 1O2 limited by the extremely short diffusion length. To overcome the drawbacks, we here designed high-performance nanoreactors by modulating the MnO2 phase to nanotube structures (with ∼5 nm diameter, termed "NT5"). With nanoconfinement, our developed NT5 directed efficient and almost 100% utilization of peroxymonosulfate (PMS) to produce 1O2 and achieved maximal kinetics on organic pollutant elimination. The mechanism study revealed that the geometric strain of NT5 together with spatial confinement modulated the adsorption properties of PMS molecules and led to their transformation to 1O2. To demonstrate the applicability of NT5, we developed a reactive filter with a particulate catalyst (NT5 grown on an alumina substrate) that can effectively and stably work in a broad range of contaminated scenarios (surface water, groundwater, municipal secondary effluent, and industrial wastewaters), due to the confined treatment together with the fouling-resistance nature. Our study may boost the deployment of nanomaterials with confined catalysis and their applications in practical water treatment scenarios.

Efficient H2O2 Electrosynthesis in Acidic media via Multiscale Catalyst Optimization

Electrochemically generating hydrogen peroxide (H2O2) from oxygen offers a more sustainable and cost-effective alternative to conventional anthraquinone process. In alkaline conditions, H2O2 is unstable as HO2 -, and in neutral electrolytes, alkali cation crossover causes system instability. Producing H2O2 in acidic electrolytes ensures enhanced stability and efficiency. However, in acidic conditions, the oxygen reduction reaction mechanism is dominated by the inner-sphere electron transfer pathway, requiring careful consideration of both reaction and mass transfer kinetics. These stringent requirements limit H2O2 production efficiency, typically below 10-20% at industrial-relevant current densities (>300 mA cm-2). Using a multiscale approach that combines active site tuning with macrostructure tuning, this work presents an octahedron-like cobalt structure on interconnected hierarchical porous nanofibers, achieving a faradaic efficiency exceeding 80% at 400 mA cm-2 and stable operation for over 120 h at 100 mA cm-2. At 300 mA cm-2, the optimized catalyst demonstrates a cell potential of 2.14 V, resulting in an energy efficiency of 26%.

Highly Ordered Conductive Metal-Organic Frameworks with Chemically Confined Polyoxometalate Clusters: A Dual-Functional Electrocatalyst for Efficient H2O2 Synthesis and Biomass Valorization

The design of bifunctional and high-performance electrocatalysts that can be used as both cathodes and anodes for the two-electron oxygen reduction reaction (2e- ORR) and biomass valorization is attracting increasing attention. Herein, a conserved ligand replacement strategy is developed for the synthesis of highly ordered conductive metal-organic frameworks (Ni-HITP, HITP = 2, 3, 6, 7, 10, 11-hexaiminotriphenylene) with chemically confined phosphotungstic acid (PW12) nanoclusters in the nanopores. The newly formed Ni-O-W bonds in the resultant Ni-HITP/PW12 electrocatalysts modulate the electronic structures of both Ni and W sites, which are favorable for cathodic 2e- ORR to H2O2 production and anodic 5-hydroxymethylfurfural oxidation reaction (HMFOR) to 2, 5-furandicarboxylic acid (FDCA), respectively. In combination with the deliberately retained conductive frameworks and ordered pores, the dual-functional Ni-HITP/PW12 composites enable a H2O2 production rate of 9.51 mol gcat -1 h-1 and an FDCA yield of 96.8% at a current density of 100 mA cm-2/cell voltage of 1.38 V in an integrated 2e- ORR/HMFOR system, significantly improved than the traditional 2e- ORR/oxygen evolution reaction system. This work has provided new insights into the rational design of advanced electrocatalysts and electrocatalytic systems for the green synthesis of valuable chemicals.

p-π Conjugated Covalent Organic Frameworks Expedite Molecular Triplet Excitons for H2O2 Production Coupled with Biomass Upgrading

High-efficiency production of triplet states in covalent organic framework photocatalysts is crucial for high-selectivity oxygen (O2) reduction to hydrogen peroxide (H2O2). Herein, fluorine and partial fluorine atoms are incorporated into an olefin-linked triazine covalent organic framework (F-ol-COF and HF-ol-COF), in which the adjacent fluorine (F) atoms-olefinic bond forms p-π conjugation that induces spin-polarization under irradiation, thus expediting triplet excitons for activating O2 to singlet oxygen (1O2) and contributing to a high H2O2 selectivity (91%). Additionally, the feasibility of coupling H2O2 production with the valorization of 5-hydroxymethylfurfural (HMF) is exhibited. The F-ol-COF demonstrates a highly stable H2O2 yield rate of 12558 µmol g-1 h-1 with the HMF-to-functionalized furan conversion yield of 95%, much higher than the partially fluorinated COF (HF-ol-COF) and the non-fluorinated COF (H-ol-COF). Mechanistic studies reveal that F-incorporation promotes charge separation, intensifies the Lewis acidity of the carbon atoms on the olefinic bond as active sites for O2 adsorption, and provides highly concentrated holes at the triazine unit for HMF oxidation upgrading. This study suggests the attractive potential of rational design of porous-crystalline photocatalysts for high-efficiency photocatalytic O2 reduction to H2O2 and biomass upgrading.

Efficient Photocatalytic Synthesis of Hydrogen Peroxide Facilitated by Triptycene-Based 3D Covalent Organic Frameworks

Covalent organic frameworks (COFs) are widely studied for hydrogen peroxide (H₂O₂) photosynthesis, with 3D COFs standing out for their porous structures and chemical stability. However, the difficult preparation of 3D COFs and the low efficiency in separating photo-generated electrons and holes (e- and h+) limits the efficient production of H2O2. In this study, two kinds of [6+3] 3D COFs (XJU-1, XJU-2) with significant charge separation, achieving record-breaking H₂O₂ photocatalysis rates of 34 777 and 11 922 µmol g⁻¹ h⁻¹, respectively. XJU-1's superior efficiency stems from its larger pores, enhancing material transport and oxygen (O2) activation. Experimental and theoretical studies have demonstrated that triptycene monomers achieve significant charge separation toward triazine via imine bonds. Moreover, the dimer's smaller singlet-triplet energy gap (∆ES-T) and triptycene's orthogonal configuration enhance singlet oxygen (1O2) production, enabling multiple H2O2 generation pathways. Ultimately, through the oxygen reduction reaction (ORR) pathway, rapid generation of H2O2 can be achieved at multiple catalytic sites. XJU-1 mainly follows a mixed pathway involving 1e--ORR and 2e--ORR, and XJU-2 primarily follows the 2e--ORR pathway, respectively. These open the door of triptycene-based 3D COFs applied in continuous, efficient, and stable photosynthesis of H2O2.

Isolated Metal Centers Activate Small Molecule Electrooxidation: Mechanisms and Applications

Electrochemical oxidation of small molecules shows great promise to substitute oxygen evolution reaction (OER) or hydrogen oxidation reaction (HOR) to enhance reaction kinetics and reduce energy consumption, as well as produce high-valued chemicals or serve as fuels. For these oxidation reactions, high-valence metal sites generated at oxidative potentials are typically considered as active sites to trigger the oxidation process of small molecules. Isolated atom site catalysts (IASCs) have been developed as an ideal system to precisely regulate the oxidation state and coordination environment of single-metal centers, and thus optimize their catalytic property. The isolated metal sites in IASCs inherently possess a positive oxidation state, and can be more readily produce homogeneous high-valence active sites under oxidative potentials than their nanoparticle counterparts. Meanwhile, IASCs merely possess the isolated metal centers but lack ensemble metal sites, which can alter the adsorption configurations of small molecules as compared with nanoparticle counterparts, and thus induce various reaction pathways and mechanisms to change product selectivity. More importantly, the construction of isolated metal centers is discovered to limit metal d-electron back donation to CO 2p* orbital and reduce the overly strong adsorption of CO on ensemble metal sites, which resolve the CO poisoning problems in most small molecules electro-oxidation reactions and thus improve catalytic stability. Based on these advantages of IASCs in the fields of electrochemical oxidation of small molecules, this review summarizes recent developments and advancements in IASCs in small molecules electro-oxidation reactions, focusing on anodic HOR in fuel cells and OER in electrolytic cells as well as their alternative reactions, such as formic acid/methanol/ethanol/glycerol/urea/5-hydroxymethylfurfural (HMF) oxidation reactions as key reactions. The catalytic merits of different oxidation reactions and the decoding of structure-activity relationships are specifically discussed to guide the precise design and structural regulation of IASCs from the perspective of a comprehensive reaction mechanism. Finally, future prospects and challenges are put forward, aiming to motivate more application possibilities for diverse functional IASCs.

Sustainable Strategies for Converting Organic, Electronic, and Plastic Waste From Municipal Solid Waste Into Functional Materials

The valorization of municipal solid waste permits to obtain sustainable functional materials. As the urban population burgeons, so does the volume of discarded waste, presenting both a challenge and an opportunity. Harnessing the materials and the latent energy within this solid waste not only addresses the issue of disposal but also contributes to the innovation of functional materials with applications in the energy, electronics, and environment sectors. In this perspective, technologies for converting, after sorting, municipal solid waste into valuable metals, chemicals, and fuels are critically analyzed. Innovative approaches to convert organic waste into functional carbon materials and to create, from plastic and electronic wastes, metal-organic frameworks for energy conversion, storage, and CO2 adsorption and conversion are proposed. Green hydrometallurgy routes that permit the recovery of precious metals avoiding noble metals' oxidative leaching, thus avoiding their downcycling, are also highlighted. The reclaimed precious metals hold promise for use in optoelectronic devices.

Proton Reservoir in Covalent Organic Framework Compensating Oxygen Reduction Reaction Enhances Hydrogen Peroxide Photosynthesis

The water oxidation reaction (WOR) with sluggish kinetics usually fails to adequately furnish protons to oxygen reduction reaction (ORR), which ultimately constrains the overall efficiency of photocatalytic synthesis of H2O2, particularly in the absence of sacrificial agents. Herein, a class of hydroxyl groups (─OH) functionalized covalent organic framework (COF) is reported as a proton reservoir to compensate ORR for greatly improving the efficiency of H2O2 photosynthesis. It has been demonstrated that the incorporation of ─OH into the TFBP-DHBD (TFBP: 1,2,4,5-tetrakis-(4-formylphenyl)benzene, DHBD: 3,3'-dihydroxybenzidine) COF can achieve a 3.3 times higher H2O2 photosynthetic activity than that of TFBP-BD (BD: benzidine) COF without ─OH groups. Isotope labeling experiments and in situ infrared spectroscopy analysis demonstrate that the proton reservoir can donate protons for ORR and subsequently regain the released protons from WOR. Theoretical calculations further confirm that the ─OH functionalized COF provides the protons for the formation of *OOH intermediate and reduces its energy barrier, thereby facilitating the photosynthesis of H2O2. A novel COF loaded with Al2O3 spheres-based streamlined microreactor is built that can simultaneously achieve production of H2O2 and elimination of bacteria over 1.3 × 104 CFU mL-1, superior to the reported continuous flow photocatalytic reactors.

Advancing Seawater Electrochemical Reaction for Fuel and Chemical Production

As global demand for sustainable chemical processes intensifies, seawater, with its vast availability and rich composition, represents a promising resource for advancing green chemical technologies. Seawater can serve as a feedstock or intermediate for producing fuels and chemicals, including hydrogen, chlorine gas and chloride, sodium, magnesium, and carbon-based compounds through specific electrochemical reactions. While extensive studies have been focused on seawater hydrogen production, systematic exploration of its broader electrochemical reactions remains limited. This review provides a comprehensive overview of current progress in seawater electrochemical reactions, covering its composition, fundamental reaction principles, and existing challenges. Specific examples on the use of seawater to produce fuels and chemicals beyond hydrogen are reviewed, with an emphasis on innovative electrochemical reaction mechanisms, advanced catalyst development, and integrated system designs. Apart from catalyst optimization for existing reactions, we highlight the importance of exploring alternative reactions and scalable systems. Future perspectives focus on expanding research scope, developing efficient catalysts and electrolyzers, testing in real seawater, advancing product separation, and evaluating practical systems to enable sustainable processes for clean fuel and high-value chemical production, supporting global carbon neutrality.

On-Demand Catalytic Platform for Glycerol Upgrade and Utilization

Surplus byproducts generated during biomass exploitation, such as glycerol from biodiesel manufacturing, seriously undermine the credibility of renewable energy policies. Here, we establish an on-demand catalytic platform for the upgrade and utilization of glycerol via photoelectro-bioelectro-heterogeneous coupling catalysis. Combining theoretical descriptors, specifically the highest occupied molecular orbital energy levels and dual local softness values, along with systematic experimental validation, we demonstrated the reaction activity of glycerol and its upgraded products on BiVO4 photoelectrodes. Glyceric acid was identified as the optimal biofuel candidate through monohydroxyl oxidation of glycerol. Coupling the preferential upgrading of glycerol to glyceric acid by night and its reuse as biofuel by day, a hybrid biophotoelectrochemical system delivered an open-circuit voltage of 0.89 ± 0.02 V and a maximum power density of 0.41 ± 0.03 mW cm-2 with stable diurnal operation for over 10 days. This successful model construction provides valuable insights into the strategic integration of multiple energy sources and the exploration of coupling-catalytic platforms, charting new territory for the next-generation sustainable energy systems.

Electrocatalysts for the Formation of Hydrogen Peroxide by Oxygen Reduction Reaction

Hydrogen peroxide (H2O2) is a widely used strong oxidant, and its traditional preparation methods, anthraquinone method, and direct synthesis method, have many drawbacks. The method of producing H2O2 by two-electron oxygen reduction reaction (2e- ORR) is considered an alternative strategy for the traditional anthraquinone method due to its high efficiency, energy saving, and environmental friendliness, but it remains a big challenge. In this review, we have described the mechanism of ORR and the principle of electrocatalytic performance testing, and have summarized the standard performance evaluation techniques for electrocatalysts to produce H2O2. Secondly, according to the theoretical calculation and experimental results, several kinds of efficient electrocatalysts are introduced. It is concluded that noble metal-based materials, carbon-based materials, non-noble metal composites, and single-atom catalysts are the preferred catalyst materials for the preparation of H2O2 by 2e- ORR. Finally, the advantages and novelty of 2e- ORR compared with traditional methods for H2O2 production, as well as the advantages and disadvantages of the above-mentioned high-efficiency catalysts, are summarized. The application prospect and development direction of high-efficiency catalysts for H2O2 production by 2e- ORR has been prospected, which is of great significance for promoting the electrochemical yield of H2O2 and developing green chemical production.

Hydrogen-bonded organic frameworks for photocatalytic synthesis of hydrogen peroxide

Photocatalysis provides a sustainable and environment-friendly strategy to produce H2O2, yet the catalytic efficiency of H2O2 overall photosynthesis (O2 + 2H2O → 2H2O2) needs to be further improved, especially in the absence of additional cocatalysts, photosensitizers and sacrificial agents. Here we find that hydrogen-bonded organic frameworks can serve as photocatalysts for H2O2 overall photosynthesis under the above-mentioned conditions. Specifically, we constructed a donor-acceptor hydrogen-bonded organic framework that exhibits a high photocatalytic activity for H2O2 overall photosynthesis, with a production rate of 681.2 μmol g-1 h-1. The control experiments and theoretical calculation revealed that the hydrogen-bonded organic frameworks with donor-acceptor structures can not only accelerate the charge separation and transfer but also optimize the reaction pathways, which significantly boosts the photocatalytic efficiency in H2O2 overall photosynthesis. This work provides insights into the design and development of efficient photocatalysts for overall H2O2 photosynthesis.

Unsymmetric Protonation Driven Highly Efficient H2O2 Photosynthesis in Supramolecular Photocatalysts via One-Step Two-Electron Oxygen Reduction

Photocatalytic hydrogen peroxide (H2O2) production has emerged as an attractive alternative to the traditional anthraquinone process. However, its performance is often hindered by low selectivity and sluggish kinetics of oxygen reduction reaction (ORR). Herein, we report an anthrazoline-based supramolecular photocatalyst, SA-SADF-H+, featuring an unsymmetric protonation structure for H2O2 photosynthesis from water and air. The introduction of unsymmetric protonation disrupts the initial mirror symmetry of SADF, significantly enhancing the molecular dipole and facilitating efficient charge separation and electron transfer. Additionally, this modification increases the hydrophilicity of SA-SADF-H+, enabling the interaction of water and dissolved oxygen with the catalytic sites. The altered electron density distribution creates numerous dual active sites for Yeager-type O2 adsorption, facilitating an efficient ORR towards H2O2 via a direct one-step two-electron pathway. Notably, SA-SADF-H+ achieves an outstanding photocatalytic H2O2 production at a rate of 4666.7 μmol L-1 h-1, with a remarkable solar-to-chemical conversion (SCC) of 1.35 %, surpassing most organic photocatalytic systems. Furthermore, SA-SADF-H+ demonstrates remarkable photocatalytic antibacterial activity, achieving 100 % antibacterial efficiency against Staphylococcus aureus within 60 min.

Ultrafast Charge Transfer on Ru-Cu Atomic Units for Enhanced Photocatalytic H2O2 Production

Photosensitizer-assisted photocatalytic systems offer a solution to overcome the limitations of inherent light harvesting capabilities in catalysts. However, achieving efficient charge transfer between the dissociative photosensitizer and catalyst poses a significant challenge. Incorporating photosensitive components into reactive centers to establish well-defined charge transfer channels is expected to effectively address this issue. Herein, the electrostatic-driven self-assembly method is utilized to integrate photosensitizers into metal-organic frameworks, constructing atomically Ru-Cu bi-functional units to promote efficient local electron migration. Within this newly constructed system, the [Ru(bpy)2]2+ component and Cu site serve as photosensitive and catalytic active centers for photocarrier generation and H2O2 production, respectively, and their integration significantly reduces the barriers to charge transfer. Ultrafast spectroscopy and in situ characterization unveil accelerated directional charge transfer over Ru-Cu units, presenting orders of magnitude improvement over dissociative photosensitizer systems. As a result, a 37.2-fold enhancement of the H2O2 generation rate (570.9 µmol g-1 h-1) over that of dissociative photosensitizer system (15.3 µmol g-1 h-1) is achieved. This work presents a promising strategy for integrating atomic-scale photosensitive and catalytic active centers to achieve ultrafast photocarrier transfer and enhanced photocatalytic performance.

Extracellular Electron Uptake Mediated by H2O2

Harvesting electricity from microbial electron transfer is believed as a promising way of renewable energy generation. However, a major challenge lies in the still-unknown mechanisms of extracellular electron transfer, especially how microbes consume electrons from the cathode to catalyze oxygen reduction. Here we report a previously undescribed yet significant extracellular electron uptake pathway mediated by inevitably produced H2O2, contributing up to 45% of the total biocurrent. This new H2O2-based bioelectrochemical respiration depends on the continuous supply of electrons from the electrode and the presence of the catalase katG. Selective enhancement of two-electron oxygen reduction on the cathode results in a 2.4-fold increase in biocurrent, and both autotrophic biosynthesis and energy production pathways are upregulated to sustain the H2O2-based respiration. Our results highlight the importance of two-electron oxygen reduction in bioelectron uptake at the cathode and provide a basis for the design of bioelectricity production systems.

Constructing Donor-Acceptor Covalent Organic Frameworks for Highly Efficient H2O2 Photosynthesis Coupled with Oxidative Organic Transformations

The development of photocatalytic systems that enable the simultaneous production of H2O2 and value-added organic chemicals presents a dual advantage: generating valuable products while maximizing the utilization of solar energy. Despite the potential, there are relatively few reports on photocatalysts capable of such dual functions. In this study, we synthesized a series of donor-acceptor covalent organic frameworks (COFs), designated as JUC-675 to JUC-677, to explore their photocatalytic efficiency in the co-production of H2O2 and N-benzylbenzaldimine (BBAD). Among them, JUC-675 exhibited exceptional performance, achieving a H2O2 production rate of 22.8 mmol g-1 h-1 with an apparent quantum yield of 15.7 %, and its solar-to-chemical conversion efficiency was calculated to be 1.09 %, marking it as the most effective COF-based photocatalyst reported to date. Additionally, JUC-675 demonstrated a high selectivity (99.9 %) and yield (96 %) for BBAD in the oxidative coupling of benzylamine. The underlying reaction mechanism was thoroughly investigated through validation experiments and density functional theory (DFT) calculations. This work represents a significant advancement in the design of COF-based photocatalysts and the development of efficient dual-function photocatalytic platforms, offering new insights and methodologies for enhanced solar energy utilization and the synthesis of value-added products.

Synergistic Single-Atom and Clustered Cobalt Sites on N/S Co-Doped Defect Nano-Carbon for Efficient H2O2 Electrosynthesis

Non-noble-based single atomic catalysts have exhibited significant potential in electrochemical production of H2O2 via two-electron oxygen reduction reactions (2e- ORR). However, constructing highly efficient and acid-resistant catalysts remains a challenge but significant. In this work, fullerene (C60) with abundant pentagonal inherent defects was employed as a carbon substrate to synthesize defect-rich nanocarbon electrocatalysts doped with NSCo single atoms and accompanied by metallic Co nanoparticles (CoSA/CoNP-NSDNC) for the first time. The electrochemical experiments demonstrate that the active sites of CoSA/CoNP-NSDNC are formed through the synergistic interaction between NSCo single atoms and Co nanoparticle clusters embedded within the carbon framework. The obtained CoSA/CoNP-NSDNC catalyst exhibits an onset potential as 0.72 V versus RHE and achieves up to 90% H2O2 selectivity over a wide potential range of 500 mV. Moreover, the as-obtained CoSA/CoNP-NSDNC configured as the cathode in a self-assembled flow cell under acidic conditions achieves a high H2O2 production rate of 4206.96 mmol gcat⁻1 h⁻1 with a Faraday efficiency of ∼ 95% and exhibit ultra fast degradation of organic pollutants. This work focuses on the synergistic effect of non-noble metal nanoparticles, metal single-atom sites, and topological defects on the 2e- ORR process, which provides a new direction for designing carbon-based catalysts for efficient H2O2 electrosynthesis.

Valorization of Hydrogen Peroxide for Sodium Percarbonate and Hydrogen Coproduction via Alkaline Water Electrolysis: Conceptual Process Design and Techno-Economic Evaluation

The recent interest in the production of green hydrogen through water electrolysis is hampered by its high cost when compared to steam methane reforming. To overcome this disadvantage, some studies explore replacing oxygen production with hydrogen peroxide at the anode, which has a higher value. Existing electrocatalysis research primarily focuses on hydrogen peroxide synthesis, neglecting process design and separation. Additionally, hydrogen peroxide's thermodynamic instability in alkaline conditions and the existence of other ions make the separation difficult. This paper proposes a novel concept for the paired water electrolysis process that can be used to improve green hydrogen production economics through valuable chemical coproductions. Valorizing hydrogen peroxide to sodium percarbonate as the final product was chosen to address hydrogen peroxide separation challenges. An electrolyzer stack of 2 MW was chosen, incorporating a recirculating structure, and a boron-doped diamond anode to enhance the hydrogen peroxide production as the base case. According to the techno-economic analysis, for a 2 MW electrolyzer stack, capital expenditure was calculated as 64.5 M€, operational expenses as 21.6 M€, and revenue was calculated as 2.5 M€, resulting in a negative cash flow of -19.1 M€. Results revealed that the process can be profitable (breakeven point) at a capacity of approximately 308 electrolyzer stacks, which is 616 MW in capacity. A sensitivity analysis was conducted to determine how cost drivers including electricity price, anode price, Faradaic efficiency, price of the products and tax subsidy affect the breakeven point. A breakeven point of 60 electrolyzer stacks (120 MW) was found with a 100% increase in the sodium percarbonate sale price. In comparison, a theoretical 100% Faradaic efficiency in the anode material would result in a breakeven point of 38 electrolyzer stacks (76 MW). Even a more realistic 75% Faradaic efficiency leads to a breakeven plant size of 75 stacks (150 MW). Further, multiple two-parameter sensitivity analyses were conducted to assess the relations between Faradaic efficiency, sodium percarbonate sale price and anode material price. For instance, if sodium percarbonate price increases by 100% and Faradaic efficiency increases to 75%, the breakeven capacity drops down to 13 stacks (26 MW). Despite facing economic challenges for the proposed process design based on available technologies, the techno-economic analysis highlights key targets for future works. It also provides valuable insights into the economic feasibility of simultaneously producing hydrogen and sodium percarbonate through water electrolysis, indicating promising potential for the future.

Water Spillover to Expedite Two-Electron Oxygen Reduction

Limited by the activity-selectivity trade-off relationship, the electrochemical activation of small molecules (like O2, N2, and CO2) rapidly diminishes Faradaic efficiencies with elevated current densities (particularly at ampere levels). Nevertheless, some catalysts can circumvent this restriction in a two-electron oxygen reduction reaction (2e- ORR), a sustainable pathway for activating O2 to hydrogen peroxide (H2O2). Here we report 2e- ORR expedited in a fluorine-bridged copper metal-organic framework catalyst, arising from the water spillover effect. Through operando spectroscopies, kinetic and theoretical characterizations, it demonstrates that under neutral conditions, water spillover plays a dual role in accelerating water dissociation and stabilizing the key *OOH intermediate. Benefiting from water spillover, the catalyst can expedite 2e- ORR in the current density range of 0.1-2.0 A cm-2 with both high Faradaic efficiencies (99-84.9%) and H2O2 yield rates (63.17-1082.26 mg h-1 cm-2). Further, the feasibility of the present system has been demonstrated by scaling up to a unit module cell of 25 cm2, in combination with techno-economics simulations showing H2O2 production cost strongly dependent on current densities, giving the lowest H2O2 price of $0.50 kg-1 at 2.0 A cm-2. This work is expected to provide an additional dimension to leverage systems independent oftraditional rules.

Customized Design of Covalent Organic Framework with Asymmetric Dual-Function Hybrid Linkages for Promoting H2O2 Photosynthesis in Air and Water

Efficient sacrificial-agent-free photosynthesis of H2O2 from air and water represents the greenest, lowest-cost, most real-time avenue for H2O2 production but remains a challenging issue. Here, we show a general and effective approach through a structural design on covalent organic frameworks (COFs) with asymmetric dual-function hybrid linkages for boosting the H2O2 photosynthesis of the COFs. Through such design we can equip a COF with not only a catalytic active center but also a special function for isolating the D-A motif, which consequently endows the COF (CI-COF) built on asymmetric dual-function hybrid linkages with a significantly enhanced efficiency in the generation, transmission, and separation of photogenerated carriers, relative to the COF (II-COF and CC-COF) built on symmetric single-function single linkages. Correspondingly, the performance of H2O2 photosynthesis is enhanced by three or five times. Accompanied is the largely promoted O2 utilization and conversion efficiency from 36.6% to 99.9%. A rare dual-channel H2O2 photosynthesis is suggested for the CI-COF.

Constructing Interfacial B-P Bonding Bridge to Promote S-Scheme Charge Migration within Heteroatom-Doped Carbon Nitride Homojunction for Efficient H2O2 Photosynthesis

The emerging step (S)-scheme heterojunction systems became a powerful strategy in promoting photogenerated charge separation while maintaining their high redox potentials. However, the weak interfacial interaction limits the charge migration rate in S-scheme heterojunctions. Herein, we construct a unique S-scheme carbon nitride (CN) homojunction with boron (B)-doped CN and phosphorus (P)-doped CN (B-CN/P-CN) for hydrogen peroxide (H2O2) photosynthesis. The B-CN/P-CN nanosheet composites revealed extensively tight interfacial contact, improved visible-light harvesting, and reduced carrier lifetime. The structural investigation results also indicate that the interfacial chemical B-P bonding is formed between B-CN and P-CN nanosheets, inducing an accelerated interfacial S-scheme charge migration. Density functional theory calculations further clarify the S-scheme charge transfer mechanism. Consequently, the 2e- oxygen reduction reaction was the predominant pathway of H2O2 production, facilitated by the B-CN/P-CN homojunction. The optimal H2O2 yield rate reached 2199.5 μmol L-1 h-1 over the B-CN/P-CN homojunction (S3) photocatalyst under monochromatic LED irradiation, increasing 2-8 times as against most of the C3N4 photocatalysts. This study highlights the crucial role of interfacial charge transfer between heterojunction/homojunction materials, accompanied by an unveiling reaction mechanism for solar-energy conversions.

p-Type BiVO4 for Solar O2 Reduction to H2O2

Photoelectrochemical cells (PECs) can directly utilize solar energy to drive chemical reactions to produce fuels and chemicals. Oxide-based photoelectrodes in general exhibit enhanced stability against photocorrosion, which is a critical advantage for building a sustainable PEC. However, most oxide-based semiconductors are n-type, and p-type oxides that can be used as photocathodes are limited. In this study, we report the synthesis, characterization, and application of p-type BiVO4 with a monoclinic scheelite (ms) structure. ms-BiVO4 is inherently n-type, and it has been investigated only as a photoanode to date. In this study, we prepared p-type ms-BiVO4 (bandgap of 2.4 eV) via atomic doping of Ca2+ at the Bi3+ site under an O2-rich environment and examined its performance as a photocathode. We then demonstrated that the Ca-doped ms-BiVO4 photocathode can be used for solar O2 reduction to H2O2 when coupled with appropriate catalysts. Our computational investigation using hybrid density functional theory revealed that holes are stable as polarons in ms-BiVO4 and have a low self-trapping energy, that may lead to free carriers in the valence band at finite temperature. Our calculations also show that Ca is an effective shallow acceptor dopant with low formation energy and thermal ionization energy leading to p-type conductivity. Our joint experimental and computational results provide critical insights into the design of p-type ms-BiVO4, enabling its use as a polaronic oxide photocathode.

Edge-doped substituents as an emerging atomic-level strategy for enhancing M-N4-C single-atom catalysts in electrocatalysis of the ORR, OER, and HER

M-N4-C single-atom catalysts (MN4) have gained attention for their efficient use at the atomic level and adjustable properties in electrocatalytic reactions like the ORR, OER, and HER. Yet, understanding MN4's activity origin and enhancing its performance remains challenging. Edge-doped substituents profoundly affect MN4's activity, explored in this study by investigating their interaction with MN4 metal centers in ORR/OER/HER catalysis (Sub@MN4, Sub = B, N, O, S, CH3, NO2, NH2, OCH3, SO4; M = Fe, Co, Ni, Cu). The results show overpotential variations (0 V to 1.82 V) based on Sub and metal centers. S and SO4 groups optimize FeN4 for peak ORR activity (overpotential at 0.48 V) and reduce OER overpotentials for NiN4 (0.48 V and 0.44 V). N significantly reduces FeN4's HER overpotential (0.09 V). Correlation analysis highlights the metal center's key role, with ΔG*H and ΔG*OOH showing mutual predictability (R2 = 0.92). Eg proves a reliable predictor for Sub@CoN4 (ΔG*OOH/ΔG*H, R2 = 0.96 and 0.72). Machine learning with the KNN model aids catalyst performance prediction (R2 = 0.955 and 0.943 for ΔG*OOH/ΔG*H), emphasizing M-O/M-H and the d band center as crucial factors. This study elucidates edge-doped substituents' pivotal role in MN4 activity modulation, offering insights for electrocatalyst design and optimization.

An efficient electrocatalytic in-situ hydrogen peroxide generation for ballast water treatment with oxygen groups

The in-situ electrochemical production of hydrogen peroxide (H2O2) offers a promising approach for ballast water treatment. However, further advancements are required to develop electrocatalysts capable of achieving efficient H2O2 generation in seawater environments. Herein, we synthesized two-dimensional lamellated porous carbon nanosheets enriched with oxygen functional groups, which exhibited exceptional performance in H2O2 electrosynthesis. The carbon nanosheet electrocatalysts demonstrated high selectivity for H2O2 production, reaching 90 % at 0.33 V vs. RHE under neutral conditions. Maximum yields were achieved at 2238 mmol gcat-1 h-1 at -0.5 V in an H-type electrolysis cell and 3681 mmol gcat-1 h-1 at a current density of 150 mA cm-2 in a flow cell, with Faraday efficiencies exceeding 70 %. Notably, a continuous 9-hour electrosynthesis test produced a high cumulative H2O2 concentration of 1.2 wt% at a current density of 100 mA cm-2, highlighting the stability and scalability of carbon nanosheets. The outstanding performance of carbon nanosheets is attributed to the abundant basal plane C-O-C group, which provide optimal *OOH binding energy and minimal overpotential. Additionally, the in-situ generated H2O2 from the electrocatalytic system achieved complete sterilization within 60 min against Escherichia coli and several marine bacterial strains isolated from seawater. Furthermore, treatment of real seawater with H2O2 significantly altered the bacterial population abundance at both the phylum and genus levels, highlighting its effectiveness in microbial control. This study presents a high-performance electrocatalytic system for ballast water treatment, offering both scalability and environmental sustainability.

The steric hindrance effect of Co porphyrins promoting two-electron oxygen reduction reaction selectivity

A new Co 5,10,15,20-tetrakis(2',6'-dipivaloyloxyphenyl)porphyrin (1) with eight ester groups in all ortho and ortho' positions of phenyl groups was designed, which displayed significantly improved 2e oxygen reduction reaction (ORR) selectivity compared with a 5,10,15,20-tetrakis(para-dipivaloyloxyphenyl) porphyrin (2) without large steric groups. This work is significant to reveal the steric hindrance effect of metal porphyrins on electrocatalytic ORR selectivity.

Interfacial design of pyrene-based covalent organic framework for overall photocatalytic H2O2 synthesis in water

Covalent organic frameworks (COFs) have shown great potential in the photocatalytic production of hydrogen peroxide (H2O2) due to their precisely designed and customized ability. Nevertheless, the quest for efficient overall photosynthesis of H2O2 in pure water without sacrificial agents using COF photocatalysts remains a formidable challenge. Herein, three pyrene-based covalent organic frameworks are synthesized using an advanced interfacial design strategy. By incorporating functional groups of F, H, and OH into a COF skeleton, their wettability and charge-separation properties are fine-tuned. These COFs show great performances as photocatalysts for H2O2 production from water and air by utilizing both the oxygen reduction reaction and water oxidation reaction pathways. Compared to PyCOF-F and PyCOF-H, PyCOF-OH demonstrates superior H2O2 production efficiency due to its improved hydrophilicity and enhanced carrier separation, achieving a remarkable rate of 2961 µmol g-1 h-1 from 25 mL pure water and air. Further, the mechanism of H2O2 production over PyCOF-OH is clarified by combining a series of control experiments, in situ characterizations, and theoretical calculations. This study offers valuable insights into the interfacial design of high-performance photocatalysts for H2O2 synthesis.

Enhancing Oxygen Reduction Reaction of Single-Atom Catalysts by Structure Tuning

Deciphering the fine structure has always been a crucial approach to unlocking the distinct advantages of high activity, selectivity, and stability in single-atom catalysts (SACs). However, the complex system and unclear catalytic mechanism have obscured the significance of exploring the fine structure. Therefore, we endeavored to develop a three-component strategy to enhance oxygen reduction reaction (ORR), delving deep into the profound implications of the fine structure, focusing on central atoms, coordinating atoms, and environmental atoms. Firstly, the mechanism by which the chemical state and element type of central atoms influence catalytic performance is discussed. Secondly, the significance of coordinating atoms in SACs is analyzed, considering both the number and type. Lastly, the impact of environmental atoms in SACs is reviewed, encompassing existence state and atomic structure. Thorough analysis and summarization of how the fine structure of SACs influences the ORR have the potential to offer valuable insights for the accurate design and construction of SACs.

Advances in Two-Electron Water Oxidation Reaction for Hydrogen Peroxide Production: Catalyst Design and Interface Engineering

Hydrogen peroxide (H2O2) is a versatile and zero-emission material that is widely used in the industrial, domestic, and healthcare sectors. It is clear that it plays a critical role in advancing environmental sustainability, acting as a green energy source, and protecting human health. Conventional production techniques focused on anthraquinone oxidation, however, electrocatalytic synthesis has arisen as a means of utilizing renewable energy sources in conjunction with available resources like oxygen and water. These strides represent a substantial change toward more environmentally and energy-friendly H2O2 manufacturing techniques that are in line with current environmental and energy goals. This work reviews recent advances in two-electron water oxidation reaction (2e-WOR) electrocatalysts, including design principles and reaction mechanisms, examines catalyst design alternatives and experimental characterization techniques, proposes standardized assessment criteria, investigates the impact of the interfacial milieu on the reaction, and discusses the value of in situ characterization and molecular dynamics simulations as a supplement to traditional experimental techniques and theoretical simulations. The review also emphasizes the importance of device design, interface, and surface engineering in improving the production of H2O2. Through adjustments to the chemical microenvironment, catalysts can demonstrate improved performance, opening the door for commercial applications that are scalable through tandem cell development.

Rational Construction of Cyanide-Functionalized D-A-π-D Covalent Organic Framework for Highly Efficient Overall H2O2 Photosynthesis from Air and Water

Sacrificial-agent-free overall photosynthesis of H2O2 from water and air represents currently a promising route to reform the industrial anthraquinone production manner, but, still blocks by the requirement of pure O2 feedstock, due to the insufficient oxygen supply from water under air. Herein, we report a rational molecule design on COFs (covalent organic frameworks) equiped with cyanide-functionalized D-A-π-D system for highly efficient overall H2O2 production from air and water through photocatalytic oxygen reduction reactions (ORR) and water oxidation reaction (WOR). Without using any sacrificial agent, the as-synthesized D-A-π-D COF is found to enable a H2O2 production rate as high as 4742 μmol h-1 g-1 from water and air and an O2 utilization and conversion rate up to 88 %, exceeding the other D-A-π-A COF by respectively 1.9- and 1.3-fold. Such high performance is attributed to the tuned electronic structure and prolonged charge lifetime facilitated by the unique D-A-π-D structure and cyanide groups. This work highlights a fundamental molecule design on advanced photocatalytic COFs with complicated D-A system for low-cost and massive H2O2 production.

Metal-organic frameworks as thermocatalysts for hydrogen peroxide generation and environmental antibacterial applications

Reactive oxygen species (ROS) are highly reactive, making them useful for environmental and health applications. Traditionally, photocatalysts and piezocatalysts have been used to generate ROS, but their utilization is limited by various environmental and physical constraints. This study introduces metal-organic frameworks (MOFs) as modern thermocatalysts efficiently producing hydrogen peroxide (H2O2) from small temperature differences. Temperature fluctuations, abundant in daily life, offer tremendous potential for practical thermocatalytic applications. As proof of concept, MOF materials coated onto carbon fiber fabric (MOF@CFF) created a thermocatalytic antibacterial filter. The study compared three different MOFs (CuBDC, MOF-303, and ZIF-8) with bismuth telluride (Bi2Te3), a known thermocatalytic material. ZIF-8 demonstrated superior H2O2 generation under low-temperature differences, achieving 96% antibacterial activity through temperature variation cycles. This work advances potential in thermoelectric applications of MOFs, enabling real-time purification and disinfection through H2O2 generation. The findings open interdisciplinary avenues for leveraging thermoelectric effects in catalysis and various technologies.

Activity-Stability Relationships in Oxygen Evolution Reaction

The oxygen evolution reaction (OER) is a critical process in various sustainable energy technologies. Despite substantial progress in catalyst development, the practical application of OER catalysts remains hindered by the ongoing challenge of balancing high catalytic activity with long-term stability. We explore the inverse trends often observed between activity and stability, drawing on key insights from both experimental and theoretical studies. Special focus is placed on the performance of different electrodes and their interaction with acidic and alkaline media across a range of electrochemical conditions. This Perspective integrates recent advancements to present a thorough framework for understanding the mechanisms underlying the activity-stability relationship, offering strategies for the rational design of next-generation OER catalysts that successfully meet the dual demands of activity and durability.