Heteroatom Dopants Promote Two-Electron O2 Reduction for Photocatalytic Production of H2O2 on Polymeric Carbon Nitride

Polymeric carbon nitride modified with selected heteroatom dopants was prepared and used as a model photocatalyst to identify and understand the key mechanisms required for efficient photoproduction of H₂O₂ via selective oxygen reduction reaction (ORR). The photochemical production of H₂O₂ was achieved at a millimolar level per hour under visible-light irradiation along with 100 % apparent quantum yield (in 360–450 nm region) and 96 % selectivity in an electrochemical system (0.1 V vs. RHE). Spectroscopic analysis in spatiotemporal resolution and theoretical calculations revealed that the synergistic association of alkali and sulfur dopants in the polymeric matrix promoted the interlayer charge separation and polarization of trapped electrons for preferable oxygen capture and reduction in ORR kinetics. This work highlights the key features that are responsible for controlling the photocatalytic activity and selectivity toward the two-electron ORR, which should be the basis of further development of solar H₂O₂ production.     

Cyclodextrin-supported Co(OH)2 Clusters as Electrocatalysts for Efficient and Selective H2O2 Synthesis

Co-based material catalysts have shown attractive application prospects in the 2 e⁻ oxygen reduction reaction (ORR). However, for the industrial synthesis of H₂O₂, there is still lack of Co-based catalysts with high production yield rate. Here, novel cyclodextrin-supported Co(OH)₂ cluster catalysts were prepared via a mild and facile method. The catalyst exhibited remarkable H₂O₂ selectivity (94.2 % ~ 98.2 %), good stability (99 % activity retention after 35 h), and ultra-high H₂O₂ production yield rate (5.58 mol g_catalyst⁻¹ h⁻¹ in the H-type electrolytic cell), demonstrating its promising industrial application potential. Density functional theory (DFT) reveals that the cyclodextrin-mediated Co(OH)₂ electronic structure optimizes the adsorption of OOH* intermediates and significantly enhances the activation energy barrier for dissociation, leading to the high reactivity and selectivity for the 2 e⁻ ORR. This work offers a valuable and practical strategy to design Co-based electrocatalysts for H₂O₂ production.     

Atomically Dispersed Iron Regulating Electronic Structure of Iron Atom Clusters for Electrocatalytic H2O2 Production and Biomass Upgrading

The integration of highly active single atoms (SAs) and atom clusters (ACs) into an electrocatalyst is critically important for high-efficiency two-electron oxygen reduction reaction (2e⁻ ORR) to hydrogen peroxide (H₂O₂). Here we report a tandem impregnation-pyrolysis-etching strategy to fabricate the oxygen-coordinated Fe SAs and ACs anchored on bacterial cellulose-derived carbon (BCC) (FeSAs/ACs-BCC). As the electrocatalyst, FeSAs/ACs-BCC exhibits superior electrocatalytic activity and selectivity toward 2e⁻ ORR, affording an onset potential of 0.78 V (vs. RHE) and a high H₂O₂ selectivity of 96.5 % in 0.1 M KOH. In a flow cell reactor, the FeSAs/ACs-BCC also achieves high-efficiency H₂O₂ production with a yield rate of 12.51±0.18 mol g_cat⁻¹ h⁻¹ and a faradaic efficiency of 89.4 %±1.3 % at 150 mA cm⁻². Additionally, the feasibility of coupling the produced H₂O₂ and electro-Fenton process for the valorization of ethylene glycol was explored in detail. The theoretical calculations uncover that the oxygen-coordinated Fe SAs effectively regulate the electronic structure of Fe ACs which are the 2e⁻ ORR active sites, resulting in the optimal binding strength of *OOH intermediate for high-efficiency H₂O₂ production.     

Sulfone-Modified Covalent Organic Frameworks Enabling Efficient Photocatalytic Hydrogen Peroxide Generation via One-Step Two-Electron O2 Reduction

Photocatalytic oxygen reduction reaction (ORR) offers a promising hydrogen peroxide (H₂O₂) synthetic strategy, especially the one-step two-electron (2e⁻) ORR route holds great potential in achieving highly efficient and selectivity. However, efficient one-step 2e⁻ ORR is rarely harvested and the underlying mechanism for regulating the ORR pathways remains greatly obscure. Here, by loading sulfone units into covalent organic frameworks (FS-COFs), we present an efficient photocatalyst for H₂O₂ generation via one-step 2e⁻ ORR from pure water and air. Under visible light irradiation, FS-COFs exert a superb H₂O₂ yield of 3904.2 μmol h⁻¹ g⁻¹, outperforming most reported metal-free catalysts under similar conditions. Experimental and theoretical investigation reveals that the sulfone units accelerate the separation of photoinduced electron-hole (e⁻-h⁺) pairs, enhance the protonation of COFs, and promote O₂ adsorption in the Yeager-type, which jointly alters the reaction process from two-step 2e⁻ ORR to the one-step one, thereby achieving efficient H₂O₂ generation with high selectivity.     

Photoassisted Oxygen Reduction Reaction in H2–O2 Fuel Cells

The oxygen reduction reaction (ORR) is a key step in H₂–O₂ fuel cells, which, however, suffers from slow kinetics even for state‐of‐the‐art catalysts. In this work, by making use of photocatalysis, the ORR was significantly accelerated with a polymer semiconductor (polyterthiophene). The onset potential underwent a positive shift from 0.66 to 1.34 V, and the current was enhanced by a factor of 44 at 0.6 V. The improvement was further confirmed in a proof‐of‐concept light‐driven H₂–O₂ fuel cell, in which the open circuit voltage (V_oc) increased from 0.64 to 1.18 V, and the short circuit current (J_sc) was doubled. This novel tandem structure combining a polymer solar cell and a fuel cell enables the simultaneous utilization of photo‐ and electrochemical energy, showing promising potential for applications in energy conversion and storage.     

Thiophene-Containing Covalent Organic Frameworks for Overall Photocatalytic H2O2 Synthesis in Water and Seawater

H₂O₂ is a significant chemical widely utilized in the environmental and industrial fields, with growing global demand. Without sacrificial agents, simultaneous photocatalyzed H₂O₂ synthesis through the oxygen reduction reaction (ORR) and water oxidation reaction (WOR) dual channels from seawater is green and sustainable but still challenging. Herein, two novel thiophene-containing covalent organic frameworks (TD-COF and TT-COF) were first constructed and served as catalysts for H₂O₂ synthesis via indirect 2e⁻ ORR and direct 2e⁻ WOR channels. The photocatalytic H₂O₂ production performance can be regulated by adjusting the N-heterocycle modules (pyridine and triazine) in COFs. Notably, with no sacrificial agents, just using air and water as raw materials, TD-COF exhibited high H₂O₂ production yields of 4060 μmol h⁻¹ g⁻¹ and 3364 μmol h⁻¹ g⁻¹ in deionized water and natural seawater, respectively. Further computational mechanism studies revealed that the thiophene was the primary photoreduction unit for ORR, while the benzene ring (linked to the thiophene by the imine bond) was the central photooxidation unit for WOR. The current work exploits thiophene-containing COFs for overall photocatalytic H₂O₂ synthesis via ORR and WOR dual channels and provides fresh insight into creating innovative catalysts for photocatalyzing H₂O₂ synthesis.     

Supramolecular Tuning Enables Selective Oxygen Reduction Catalyzed by Cobalt Porphyrins for Direct Electrosynthesis of Hydrogen Peroxide

We report a supramolecular strategy for promoting the selective reduction of O₂ for direct electrosynthesis of H₂O₂. We utilized cobalt tetraphenylporphyrin (Co‐TPP), an oxygen reduction reaction (ORR) catalyst with highly variable product selectivity, as a building block to assemble the permanently porous supramolecular cage Co‐PB‐1(6) bearing six Co‐TPP subunits connected through twenty‐four imine bonds. Reduction of these imine linkers to amines yields the more flexible cage Co‐rPB‐1(6). Both Co‐PB‐1(6) and Co‐rPB‐1(6) cages produce 90–100 % H₂O₂ from electrochemical ORR catalysis in neutral pH water, whereas the Co‐TPP monomer gives a 50 % mixture of H₂O₂ and H₂O. Bimolecular pathways have been implicated in facilitating H₂O formation, therefore, we attribute this high H₂O₂ selectivity to site isolation of the discrete molecular units in each supramolecule. The ability to control reaction selectivity in supramolecular structures beyond traditional host–guest interactions offers new opportunities for designing such architectures for a broader range of catalytic applications.     

Single-atom Iron Catalyst with Biomimetic Active Center to Accelerate Proton Spillover for Medical-level Electrosynthesis of H2O2 Disinfectant

Electrosynthesis of H₂O₂ has great potential for directly converting O₂ into disinfectant, yet it is still a big challenge to develop effective electrocatalysts for medical-level H₂O₂ production. Herein, we report the design and fabrication of electrocatalysts with biomimetic active centers, consisting of single atomic iron asymmetrically coordinated with both nitrogen and sulfur, dispersed on hierarchically porous carbon (Fe_SA-NS/C). The newly-developed Fe_SA-NS/C catalyst exhibited a high catalytic activity and selectivity for oxygen reduction to produce H₂O₂ at a high current of 100 mA cm⁻² with a record high H₂O₂ selectivity of 90 %. An accumulated H₂O₂ concentration of 5.8 wt.% is obtained for the electrocatalysis process, which is sufficient for medical disinfection. Combined theoretical calculations and experimental characterizations verified the rationally-designed catalytic active center with the atomic Fe site stabilized by three-coordinated nitrogen atoms and one-sulfur atom (Fe-N₃S-C). It was further found that the replacement of one N atom with S atom in the classical Fe-N₄-C active center could induce an asymmetric charge distribution over N atoms surrounding the Fe reactive center to accelerate proton spillover for a rapid formation of the OOH* intermediate, thus speeding up the whole reaction kinetics of oxygen reduction for H₂O₂ electrosynthesis.     

High‐Efficiency Oxygen Reduction to Hydrogen Peroxide Catalyzed by Nickel Single‐Atom Catalysts with Tetradentate N2O2 Coordination in a Three‐Phase Flow Cell

Carbon‐supported Ni^II single‐atom catalysts with a tetradentate Ni‐N₂O₂ coordination formed by a Schiff base ligand‐mediated pyrolysis strategy are presented. A Ni^II complex of the Schiff base ligand (R,R)‐(?)‐N,N′‐bis(3,5‐di‐tert‐butylsalicylidene)‐1,2‐cyclohexanediamine was adsorbed onto a carbon black support, followed by pyrolysis of the modified carbon material at 300 °C in Ar. The Ni‐N₂O₂/C catalyst showed excellent performance for the electrocatalytic reduction of O₂ to H₂O₂ through a two‐electron transfer process in alkaline conditions, with a H₂O₂ selectivity of 96 %. At a current density of 70 mA cm^?2, a H₂O₂ production rate of 5.9 mol g_cat.^?1 h^?1 was achieved using a three‐phase flow cell, with good catalyst stability maintained over 8 h of testing. The Ni‐N₂O₂/C catalyst could electrocatalytically reduce O₂ in air to H₂O₂ at a high current density, still affording a high H₂O₂ selectivity (>90 %). A precise Ni‐N₂O₂ coordination was key to the performance.     

Boosting Hydrogen Peroxide Electrosynthesis via Modulating the Interfacial Hydrogen-Bond Environment

Designing highly efficient and stable electrode-electrolyte interface for hydrogen peroxide (H₂O₂) electrosynthesis remains challenging. Inhibiting the competitive side reaction, 4 e⁻ oxygen reduction to H₂O, is essential for highly selective H₂O₂ electrosynthesis. Instead of hindering excessive hydrogenation of H₂O₂ via catalyst modification, we discover that adding a hydrogen-bond acceptor, dimethyl sulfoxide (DMSO), to the KOH electrolyte enables simultaneous improvement of the selectivity and activity of H₂O₂ electrosynthesis. Spectral characterization and molecular simulation confirm that the formation of hydrogen bonds between DMSO and water molecules at the electrode-electrolyte interface can reduce the activity of water dissociation into active H* species. The suitable H* supply environment hinders excessive hydrogenation of the oxygen reduction reaction (ORR), thus improving the selectivity of 2 e⁻ ORR and achieving over 90 % selectivity of H₂O₂. This work highlights the importance of regulating the interfacial hydrogen-bond environment by organic molecules as a means of boosting electrochemical performance in aqueous electrosynthesis and beyond.     

Partially Pyrolyzed Binary Metal–Organic Framework Nanosheets for Efficient Electrochemical Hydrogen Peroxide Synthesis

Herein, we developed a partially controlled pyrolysis strategy to create evenly distributed NiO nanoparticles within NiFe‐MOF nanosheets (MOF NSs) for electrochemical synthesis of H₂O₂ by a two‐electron oxygen reduction reaction (ORR). The elemental Ni can be partially transformed to NiO and uniformly distributed on the surface of the MOF NSs, which is crucial for the formation of the particular structure. The optimized MOF NSs‐300 exhibits the highest activity for ORR with near‐zero overpotential and excellent H₂O₂ selectivity (ca. 99 %) in 0.1 m KOH solution. A high‐yield H₂O₂ production rate of 6.5 mol g_cat^?1 h^?1 has also been achieved by MOF NSs‐300 in 0.1 m KOH and at 0.6 V (vs. RHE). In contrast to completely pyrolyzed products, the enhanced catalytic activities of partially pyrolyzed MOF NSs‐300 originates mainly from the retained MOF structure and the newly generated NiO nanoparticles, forming the coordinatively unsaturated Ni atoms and tuning the performance towards electrochemical H₂O₂ synthesis.     

Lattice Distortion and H-passivation in Pure Carbon Electrocatalysts for Efficient and Stable Two-electron Oxygen Reduction to H2O2

The exploration of inexpensive and efficient catalysts for oxygen reduction reaction (ORR) is crucial for chemical and energy industries. Carbon materials have been proved promising with different catalysts enabling 2 and 4e⁻ ORR. Nevertheless, their ORR activity and selectivity is still complex and under debate in many cases. Many structures of these active carbon materials are also chemically unstable for practical implementations. Unlike the well-discussed structures, this work presents a strategy to promote efficient and stable 2e⁻ ORR of carbon materials through the synergistic effect of lattice distortion and H-passivation (on the distorted structure). We show how these structures can be formed on carbon cloth, and how the reproducible chemical adsorption can be realized on these structures for efficient and stable H₂O₂ production. The work here gives not only new understandings on the 2e⁻ ORR catalysis, but also the robust catalyst which can be directly used in industry.     

Regulating Relative Nitrogen Locations of Diazine Functionalized Covalent Organic Frameworks for Overall H2O2 Photosynthesis

Nitrogen-heterocycle-based covalent organic frameworks (COFs) are considered promising candidates for the overall photosynthesis of hydrogen peroxide (H₂O₂). However, the effects of the relative nitrogen locations remain obscured and photocatalytic performances of COFs need to be further improved. Herein, a collection of COFs functionalized by various diazines including pyridazine, pyrimidine, and pyrazine have been judiciously designed and synthesized for photogeneration of H₂O₂ without sacrificial agents. Compared with pyrimidine and pyrazine, pyridazine embedded in TpDz tends to stabilize endoperoxide intermediate species, leading toward the more efficient direct 2e^- oxygen reduction reaction (ORR) pathway. Benefiting from the effective electron-hole separation, low charge transfer resistance, and high-efficiency ORR pathway, an excellent production rate of 7327 μmol g⁻¹ h⁻¹ and a solar-to-chemical conversion (SCC) value of 0.62 % has been achieved by TpDz, which ranks one of the best COF-based photocatalysts. This work might shed fresh light on the rational design of functional COFs targeting photocatalysts in H₂O₂ production.     

Ultrafine Co nanoparticles confined in nitrogen-doped carbon toward two-electron oxygen reduction reaction for H2O2 electrosynthesis in acidic media

Electrocatalytic production of hydrogen peroxide (H₂O₂) by two-electron oxygen reduction reaction (2e^– ORR) under acidic condition has been considered to have great application value. Co nanoparticles (CoNPs) coupled with N-doped carbon are a class of potential electrocatalysts. The effective strategies to further enhance their performances are to improve the active sites and stability. Herein, the material containing ultrafine CoNPs confined in a nitrogen-doped carbon matrix (NC@CoNPs) was synthesized by pyrolyzing corresponding precursors, which was obtained through regulating the topological structure of ZIF-67/ZIF-8 with dopamine (DA). The DA self-polymerization process induced the formation of CoNPs with smaller sizes and formed polydopamine film decreased the detachment of CoNPs from the catalyst. High density of Co-N_x active sites and defective sites could be identified on NC@CoNPs, leading to high activity and H₂O₂ selectivity, with an onset potential of 0.57 V (vs. RHE) and ∼90% selectivity in a wide potential range. An on-site electrochemical removal of organic pollutant was achieved rapidly through an electro-Fenton process, demonstrating its great promise for on-site water treatment application.     

Supramolecular Tuning Enables Selective Oxygen Reduction Catalyzed by Cobalt Porphyrins for Direct Electrosynthesis of Hydrogen Peroxide

We report a supramolecular strategy for promoting the selective reduction of O₂ for direct electrosynthesis of H₂O₂. We utilized cobalt tetraphenylporphyrin (Co-TPP), an oxygen reduction reaction (ORR) catalyst with highly variable product selectivity, as a building block to assemble the permanently porous supramolecular cage Co-PB-1(6) bearing six Co-TPP subunits connected through twenty-four imine bonds. Reduction of these imine linkers to amines yields the more flexible cage Co-rPB-1(6). Both Co-PB-1(6) and Co-rPB-1(6) cages produce 90–100 % H₂O₂ from electrochemical ORR catalysis in neutral pH water, whereas the Co-TPP monomer gives a 50 % mixture of H₂O₂ and H₂O. Bimolecular pathways have been implicated in facilitating H₂O formation, therefore, we attribute this high H₂O₂ selectivity to site isolation of the discrete molecular units in each supramolecule. The ability to control reaction selectivity in supramolecular structures beyond traditional host–guest interactions offers new opportunities for designing such architectures for a broader range of catalytic applications.     

Coordination Tunes Selectivity: Two‐Electron Oxygen Reduction on High‐Loading Molybdenum Single‐Atom Catalysts

Single‐atom catalysts (SACs) have great potential in electrocatalysis. Their performance can be rationally optimized by tailoring the metal atoms, adjacent coordinative dopants, and metal loading. However, doing so is still a great challenge because of the limited synthesis approach and insufficient understanding of the structure–property relationships. Herein, we report a new kind of Mo SAC with a unique O,S coordination and a high metal loading over 10 wt %. The isolation and local environment was identified by high‐angle annular dark‐field scanning transmission electron microscopy and extended X‐ray absorption fine structure. The SACs catalyze the oxygen reduction reaction (ORR) via a 2 e^? pathway with a high H₂O₂ selectivity of over 95 % in 0.10 m KOH. The critical role of the Mo single atoms and the coordination structure was revealed by both electrochemical tests and theoretical calculations.     

Enhanced Electrocatalytic Performance of Anthraquinonemonosulfonate‐Doped Polypyrrole Composite: Electroanalysis for the Specific Roles of Anthraquinone Derivative and Polypyrrole Layer on Oxygen Reduction Reaction

To analyze the specific roles of anthraquinone‐2‐sulfonate (AQS) and polypyrrole (PPy) layer on oxygen reduction reaction (ORR), the electrocatalytic reduction of oxygen was investigated on the AQS/PPy composite modified graphite electrode. Results show that the enhanced electrocatalytic performance is attributed to the excellent electrocatalytic activity of the immobilized AQS functional groups to mediate two‐electron reduction of O₂ to H₂O₂. The PPy layer may not participate in ORR, but it can further catalyze the two‐electron reduction of H₂O₂ to produce H₂O in the potential range more negative than that the two‐electron reduction of oxygen proceeds efficiently on the AQS sites.     

Promoting Piezocatalytic H2O2 Production in Pure Water by Loading Metal-Organic Cage-Modified Gold Nanoparticles on Graphitic Carbon Nitride

Piezocatalytic hydrogen peroxide (H₂O₂) production is a green synthesis method, but the rapid complexation of charge carriers in piezocatalysts and the difficulty of adsorbing substrates limit its performance. Here, metal-organic cage-coated gold nanoparticles are anchored on graphitic carbon nitride (MOC-AuNP/g-C₃N₄) via hydrogen bond to serve as the multifunctional sites for efficient H₂O₂ production. Experiments and theoretical calculations prove that MOC-AuNP/g-C₃N₄ simultaneously optimize three key parts of piezocatalytic H₂O₂ production: i) the MOC component enhances substrate (O₂) and product (H₂O₂) adsorption via host–guest interaction and hinders the rapid decomposition of H₂O₂ on MOC-AuNP/g-C₃N₄, ii) the AuNP component affords a strong interfacial electric field that significantly promotes the migration of electrons from g-C₃N₄ for O₂ reduction reaction (ORR), iii) holes are used for H₂O oxidation reaction (WOR) to produce O₂ and H⁺ to further promote ORR. Thus, MOC-AuNP/g-C₃N₄ can be used as an efficient piezocatalyst to generate H₂O₂ at rates up to 120.21 μmol g⁻¹ h⁻¹ in air and pure water without using sacrificial agents. This work proposes a new strategy for efficient piezocatalytic H₂O₂ synthesis by constructing multiple active sites in semiconductor catalysts via hydrogen bonding, by enhancing substrate adsorption, rapid separation of electron-hole pairs and preventing rapid decomposition of H₂O₂.     

Molecular engineering of carbon nitrides for overall photosynthesis of H2O2

Artificial photocatalysis offers a promising strategy to sustainably produce hydrogen peroxide (H₂O₂) that is one of the most valuable multifunctional chemicals. Among various photocatalysts, polymeric carbon nitride (pCN) has drawn continuous attention in non-sacrificial H₂O₂ production. However, the poor activity of half reactions, i.e., the oxygen reduction reaction (ORR) and water oxidation reaction (WOR), greatly restricts the efficiency of photocatalytic H₂O₂ production. In this highlight, we discuss the significant advances in molecular engineering of carbon nitrides for H₂O₂ photosynthesis and the importance of the deep understanding of the photocatalysis process for rational design and reaction pathways of organic conjugated polymers to address the growing H₂O₂ demand. Furthermore, we summarize the emerging applications of photocatalytic H₂O₂ productions beyond energy and environment.     

Metastable Hexagonal Phase SnO2 Nanoribbons with Active Edge Sites for Efficient Hydrogen Peroxide Electrosynthesis in Neutral Media

Electrochemical two-electron oxygen reduction reaction (2 e⁻ ORR) to produce hydrogen peroxide (H₂O₂) is a promising alternative to the energetically intensive anthraquinone process. However, there remain challenges in designing 2 e⁻ ORR catalysts that meet the application criteria. Here, we successfully adopt a microwave-assisted mechanochemical-thermal approach to synthesize hexagonal phase SnO₂ (h-SnO₂) nanoribbons with largely exposed edge structures. In 0.1 M Na₂SO₄ electrolyte, the h-SnO₂ catalysts achieve the excellent H₂O₂ selectivity of 99.99 %. Moreover, when employed as the catalyst in flow cell devices, they exhibit a high yield of 3885.26 mmol g⁻¹ h⁻¹. The enhanced catalytic performance is attributed to the special crystal structure and morphology, resulting in abundantly exposed edge active sites to convert O₂ to H₂O₂, which is confirmed by density functional theory calculations.