Toward the Decentralized Electrochemical Production of H2O2: A Focus on the Catalysis

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.

Enhanced oxygen diffusion and catalytic performance of self-breathing CB/CNT cathodes for high-efficiency H2O2 production (in dual-chamber reactors)

A novel self-breathing gas diffusion electrode was developed by loading carbon nanotubes (CNTs) and carbon black (CB) onto the surface of graphite felt through vacuum filtration. This electrode features a well-structured mesoporous network and a stable three-phase interface, which enable efficient oxygen mass transfer and enhance the self-breathing capability. The incorporation of carbon nanotubes and carbon black significantly boosts the electrode's catalytic performance. In a dual-chamber reactor operating at a current density of 12 mA/cm2 and an initial pH of 3, the system achieved an H2O2 concentration of 4691 mg/L within 1 h, with an energy consumption of 6.58 kWh/kg H2O2 substantially outperforming conventional gas diffusion electrodes. The dynamic pH regulation in the dual-chamber system optimizes the 2e- ORR pathway, leading to corresponding changes in proton transfer pathways and adsorbed species within the Helmholtz plane. Additionally, the presence of reactive hydrogen (H∗) enhances the chemisorption of O2 and facilitates its hydrogenation to form the ∗OOH intermediate. The electrode exhibited excellent stability, maintaining H2O2 yields above 4000 mg/L over 5 cycles and nearly complete degradation of the simulated contaminants within 30 min in an electro-Fenton system application. These results highlight the electrode's potential for efficient H2O2 synthesis and environmental remediation.

High specific surface area graphene-like biochar for green microbial electrosynthesis of hydrogen peroxide and Bisphenol A oxidation at neutral pH

Green electrosynthesis of hydrogen peroxide (H2O2) is a research hotspot in environmental chemistry, particularly for wastewater and sanitation applications, with microbial fuel cells (MFCs) offering a self-sustaining route for in situ production. This investigation showcases the application of chemically activated bagasse biochar (AcBC), a graphene-like carbon material, as a cathode catalyst in a ceramic membrane-fitted MFC for H2O2 generation and bisphenol A (BPA) degradation. The AcBC had an exceptionally high specific surface area of 1604 m2/g and mimicked the physicochemical characteristic of graphene. The MFC having the AcBC-catalysed cathode attained a maximum H2O2 yield of 248. 9 ± 12.5 mg/L (retention time of 12 h) and peak power density of 125.62 ± 5.62 mW/m2. Moreover, this system was tailored into a bioelectro-Fenton system by doping Zn-Fe over AcBC (Zn-Fe/AcBC) that instigated hydroxyl radical formation, thus responsible for removing 95.46 ± 3.50 % of Bisphenol A (BPA, initial concentration = 10 mg/L) in 300 min. Total organic carbon (initial concentration = 47.1 ± 2.3 mg/L) of BPA-containing real wastewater was reduced by 51.4 ± 3.6 % in 300 min while consistently achieving >90 % removal of BPA over eight continuous cycles. Thus, this research demonstrates the potential of biomass-derived graphene-like carbon in catalyzing green H2O2 synthesis for removal of biorefractory organics while achieving sustainable wastewater treatment.

l-Arginine-Functionalized Pd-Ni Catalyst Enhances Direct H2O2 Synthesis in Microreactors

Using an l-arginine (LA)-functionalized Pd-Ni catalyst, hydrogen peroxide (H2O2) was directly synthesized via a self-designed microreactor at ambient temperature and pressure. The yield of H2O2 achieved 574.31g kgcat-1 h-1, with the concentration of H2O2 in the solution reaching 4.05 wt % per hour. After five cycles and online activation, the LA functionalized Pd-Ni catalyst maintained high H2O2 catalytic activity. The catalytic activity evaluation experiments, characterization results, and simulation studies demonstrated that Ni doping and LA-functionalization synergistically modulate the electronic structure of the active Pd component. This modulation increases the concentration of Pd2+ at the active sites, effectively inhibits the dissociation of the O-O bond in O2*, OOH*, and HOOH* species, thereby enhancing the catalytic performance for H2O2 production. This study proposes a systematic strategy for the design of Pd-based catalysts to directly synthesize highly efficient H2O2.

Revealing the Mechanism of Exciton Spontaneous Separation at Room Temperature for Efficient Photocatalytic Hydrogen Peroxide Synthesis

The photocatalytic synthesis of hydrogen peroxide (H2O2) at room temperature has garnered significant attention as an environmentally friendly alternative to traditional anthraquinone oxidation processes. However, the low exciton dissociation efficiency at room temperature often hinders photocatalytic performance. In this study, it is demonstrated that tuning the substitution sites of electron donors in Donor-Acceptor (D-A) conjugated polymers can significantly enhance exciton dissociation by reducing exciton activation energy, which facilitates the spontaneous separation of excitons at room temperature. For comparison, materials with exciton separation energies ≈89 meV exhibit a hydrogen peroxide production rate of 2692 µmol·g-1·h-1. In contrast, the main material developed in this work, O-PTAQ, demonstrates a substantially lower exciton separation energy of 22 meV, resulting in a hydrogen peroxide production rate of 4989 µmol·g-1·h-1 under ambient conditions, outperforming most reported organic semiconductors. This enhancement is attributed to the increased electron delocalization in the electron donors, which lowers exciton activation energy to promote efficient exciton separation. The findings highlight the critical role of molecular-level structural tuning in enhancing exciton dissociation, providing a promising strategy for the development of high-efficiency photocatalysts for sustainable H2O2 production.

Bubble-triggered piezocatalytic generation of hydrogen peroxide by copper nanosheets-modified polyvinylidene fluoride films for organic pollutant degradation and water disinfection

Piezocatalysis has emerged as an attractive technology for environmental remediation by the effective transformation of mechanical energy into electrical energy. Herein, copper nanosheets-modified polyvinylidene fluoride films (CuNS/PVDF) are synthesized via a facile wet-chemistry route and exhibit a much-enhanced piezoelectric property, as compared to pristine PVDF. This is ascribed to CuNS that increases the stress response point and Young's modulus of the PVDF host. Among the series, CuNS4%/PVDF, with a 4 wt% loading of CuNS and a d33 coefficient (39 pC N-1) 2.6 times that of PVDF, exhibits the highest rate of H₂O₂ generation (163.3 μM g⁻¹ h⁻¹) by water oxidation in pure water under air bubbling, which is 3.7 times that of PVDF. This can be exploited for organic pollutant degradation and water disinfection, achieving a degradation rate of 99.8%, 98.37%, 89.02% and 81.60% for chlortetracycline hydrochloride, tetracycline, ofloxacin and ciprofloxacin, respectively, after 80 min's air bubbling, and 99.7% bactericidal efficiency against Escherichia coli after 12 h's co-culture, along with excellent stability and recyclability. Notably, such a performance remains prominent in actual wastewater, seawater, tap water and other water environments. The reaction mechanisms are unraveled by the combined studies of spectroscopic measurements and theoretical calculations. Results from this work highlight the significance of structural engineering in enhancing the piezocatalytic activity of PVDF for water treatment and disinfection.

A novel dual-channel ZnIn2S4/Bi4Ti3O12 S heterojunction significantly enhances photocatalytic hydrogen peroxide synthesis

Photocatalytic synthesis of hydrogen peroxide is an emerging topic for solar-to-chemical energy conversion. ZnIn2S4/Bi4Ti3O12 (ZIS/BTO) - based S-scheme heterojunction semiconductor photocatalysts were prepared via a one-pot water bath method. Under visible light irradiation, the optimum H2O2 yield of ZIS/BTO-2 reaches 1102 µmol L-1, which is 20.0 and 11.5 times higher than those of the pure phases of Bi4Ti3O12 and ZnIn2S4, respectively. Experimental and density functional theory (DFT) calculations demonstrated that the Built-in Electric Field (IEF) present between the ZIS/BTO S-type heterostructures not only facilitated the transfer of photogenerated carriers to surface active sites, but also promoted the adsorption of molecular oxygen and the tuning of the energy band structure. This resulted in an enhancement of the efficiency of photocatalytic oxygen reduction reaction (ORR) and the water oxidation reaction (WOR) for the synthesis of H2O2. Hence, the present study provided a practical route for the efficient synthesis of H2O2 over Bi4Ti3O12 - based photocatalysts and disclosed the reaction mechanism, that is, a heterojunction with a high efficiency for H2O2 synthesis.

π-Stacked organic heterojunction enabled efficient hydrogen peroxide photoproduction

Hydrogen peroxide (H2O2) finds wide application in disinfection, chemical production, and bleaching owing to its sustainability and ease of storage compared to hydrogen. However, the current industrial engineering practices for producing photocatalysts used in H2O2 generation face challenges such as low yields with organic catalysts and high costs associated with inorganic catalysts. In light of these challenges, we investigate a viable method utilizing a simple condensation reflux technique to synthesize pure organic heterojunction photocatalysts featuring a π-π stacked structure derived from graphitized carbon nitride (melem) and meso-tetrakis (4-carboxyphenyl) porphyrin, enabling efficient H2O2 production without the necessity of sacrificial agents. The synthesized catalyst demonstrates an impressive H2O2 production rate of 106 mmol/h.g.L under seawater conditions and 77.67 mmol/h.g.L under pure water conditions in ambient air. Coupling to a solar evaporator, an H2O2 concentration reaches 0.88 wt%. Transient absorption spectroscopy revealed ultrafast charge separation which enabled efficient photocatalysis.

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.

Enhanced two-electron oxygen reduction via Lewis acidic aluminum sites and heterostructures in a nickel-aluminum layered double hydroxide/carbon nitride catalyst

NiAl-LDH/CN heterostructures, featuring Lewis acidic Al sites, were synthesized as highly efficient catalysts for the two-electron oxygen reduction reaction (ORR). Al sites enhanced the two-electron ORR activity, achieving an 87% selectivity for H2O2 and an H2O2 yield rate of 1710 mmol gcat-1 h-1 in an H-type electrolytic cell.

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.

Recent Progress in Oxygen Reduction Reaction Toward Hydrogen Peroxide Electrosynthesis and Cooperative Coupling of Anodic Reactions

Electrosynthesis of hydrogen peroxide (H2O2) via two-electron oxygen reduction reaction (2e- ORR) is a promising alternative to the anthraquinone oxidation process. To improve the overall energy efficiency and economic viability of this catalytic process, one pathway is to develop advanced catalysts to decrease the overpotential at the cathode, and the other is to couple 2e- ORR with certain anodic reactions to decrease the full cell voltage while producing valuable chemicals on both electrodes. The catalytic performance of a 2e- ORR catalyst depends not only on the material itself but also on the environmental factors. Developing promising electrocatalysts with high 2e- ORR selectivity and activity is a prerequisite for efficient H2O2 electrosynthesis, while coupling appropriate anodic reactions with 2e- ORR would further enhance the overall reaction efficiency. Considering this, here a comprehensive review is presented on the latest progress of the state-of-the-art catalysts of 2e- ORR in different media, the microenvironmental modulation mechanisms beyond catalyst design, as well as electrocatalytic system coupling 2e- ORR with various anodic oxidation reactions. This review also presents new insights regarding the existing challenges and opportunities within this rapidly advancing field, along with viewpoints on the future development of H2O2 electrosynthesis and the construction of green energy roadmaps.

Same FeN4 Active Site, Different Activity: How Redox Peaks Control Oxygen Reduction on Fe Macrocycles

Macrocycles show high activity for the electrochemical reduction of oxygen in alkaline media. However, even macrocycles with the same metal centers and MN4 active site can vary significantly in activity and selectivity, and to this date, a quantitative insight into the cause of these staggering differences has not been unambiguously reached. These macrocycles form a fundamental platform, similarly to platinum alloys for metal ORR catalyst, to unravel fundamental properties of FeNx catalysts. In this manuscript, we present a systematic study of several macrocycles, with varying active site motif and ligands, using electrochemical techniques, operando spectroscopy, and density functional theory (DFT) simulations. Our study demonstrates the existence of two families of Fe macrocycles for oxygen reduction in alkaline electrolytes: (i) weak *OH binding macrocycles with one peak in the voltammogram and high peroxide selectivity and (ii) macrocycles with close to optimal *OH binding, which exhibit two voltametric peaks and almost no peroxide production. Here, we also propose three mechanisms that would explain our experimental findings. Understanding what differentiates these two families could shed light on how to optimize the activity of pyrolyzed FeNx catalysts.

Electrochemical Pilot H2O2 Production by Solid-State Electrolyte Reactor: Insights From a Hybrid Catalyst for 2-Electron Oxygen Reduction Reaction

The electrochemical oxygen reduction reaction (ORR) offers an alluring and sustainable alternative to the traditional anthraquinone process for hydrogen peroxide (H₂O₂) synthesis. However, challenges remain in developing scalable electrocatalysts and cost-effective reactors for high-purity H₂O₂ production. This study introduces a simple yet effective mechanical mixing method to fabricate a hybrid electrocatalyst from oxidized carbon nanotubes and layered double hydroxides (LDHs). This easily accessible and low-cost catalyst achieves near-perfect Faradaic efficiency (∼100%) with low overpotentials of 73 mV at 10 mA cm⁻2 and 588 mV at 400 mA cm⁻2 in a solid electrolyte cell. Through theoretical calculations and in-situ analyses, we uncover the pivotal role played by the LDH co-catalyst in fine-tuning the local pH at the catalyst/solid-electrolyte interface that drives both the activity and selectivity. We also design a low-cost solid-state reactor using cation-exchange resin (CER) as both a proton conductor and a microchannel for efficient mass transfer, achieving a production rate of 5.29 mmol cm⁻2 h⁻¹ and continuous output concentrations of 11.8 wt.% H₂O₂. Scaled to an industrial area of 2 × 100 cm2, the pilot reactor achieves an impressive H₂O₂ production rate of approximately 127.0 mmol h⁻¹ at 15 A, marking a significant advancement in sustainable H₂O₂ production.

Synergistic Zn-Cd Bimetallic Engineering in ZIFs for High-Chloride 2e- ORR to H2O2 in Simulated Neutral Seawater

Marine biofouling causes significant economic losses, and conventional antifouling methods are often associated with environmental pollution. Hydrogen peroxide (H2O2), as a clean energy source, has gained increasing attention in recent years. Meanwhile, electrocatalytic 2e- oxygen reduction reaction (ORR) for H2O2 production has received growing interest. However, the majority of current studies are conducted on acidic or alkaline electrolytes, and research on 2e- ORR in neutral NaCl solutions remains rare. Here, a bimetallic Zn-Cd zeolitic imidazolate framework (ZnCd-ZIF) is rationally designed to achieve chloride-resistant 2e- ORR catalysis under simulated seawater conditions (pH 7.5, 3.5% Cl-). Experimental results demonstrate that the ZnCd-ZIF catalyst exhibits an exceptional H2O2 selectivity of 70% at 0.3 VRHE, surpassing monometallic Zn-ZIF (60%) and Cd-ZIF (50%). Notably, H2O2 production reaches 120 mmol g-1 in a Cl--containing neutral electrolyte, exhibiting strong resistance to structural corrosion and Cl- poisoning. This work not only pioneers an effective strategy for designing ORR catalysts adapted to marine environments but also advances the practical implementation of seawater-based electrochemical H2O2 synthesis.

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.

Optimization and scaling-up of porous solid electrolyte electrochemical reactors for hydrogen peroxide electrosynthesis

The recently developed porous solid electrolyte (PSE) reactor for electrosynthesis of hydrogen peroxide (H2O2) has attracted significant global interest. However, scaling up the PSE reactor for practical applications poses challenges, particularly due to performance decline in enlarged reactors. Here we systematically investigate how factors such as material selection, assembly parameters, flow field patterns, and operating conditions influence H2O2 electrosynthesis in the PSE reactor. Our findings reveal that the performance decline during reactor scale-up is primarily caused by the uneven flow field in the PSE layer. Based on these insights, we optimize the reactor design and develop a 12-unit modular electrode stack PSE reactor with a total electrode area of 1200 cm2. The scaled-up reactor maintains efficient H2O2 electrosynthesis without significant performance decline. It operates stably for over 400 h and can produce up to 2.5 kg pure H2O2 (~83 kg 3% H2O2 solutions) per day with considerably lower energy costs (0.2‒0.8 USD/kg H2O2) than the market prices of H2O2 stocks. This work represents a crucial advancement in the development of PSE reactor technology for practical H2O2 electrosynthesis.

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.

Advanced development of finite element analysis for electrochemical catalytic reactions

The development of robust simulation techniques is crucial for elucidating electrochemical catalytic mechanisms and can even provide guidance for the tailored design and regulation of highly efficient catalysts. Finite element analysis (FEA), as a powerful numerical simulation tool, can effectively simulate and analyze the sophisticated processes involved in electrochemical catalytic reactions and unveil the underlying microscopic mechanisms. By employing FEA, researchers can gain better insights into reaction kinetics and transport processes, optimize electrode design, and predict electrochemical performance under various reaction conditions. Consequently, the application of FEA in electrochemical catalytic reactions has emerged as a critical area of current research and a summary of the advanced development of FEA for electrochemical catalytic reactions is urgently required. This review focuses on exploring the applications of FEA in investigating the crystal structure effect, tip effect, multi-shell effect, porous structure effect, and mass transfer phenomena in electrochemical reactions. Particularly emphasized are its applications in the fields of CO2 reduction, oxygen evolution reaction, and nitrogen reduction reaction. Finally, the challenges encountered by this research field are discussed, along with future directions for further advancement. We aim to provide comprehensive theoretical and practical guidance on FEA methods for researchers in the field of electrochemical catalysis, thereby fostering the advancement and wider implementation of FEA within this domain.

Rational synthesis of carbon-rich hollow carbon nitride spheres for photocatalytic H2O2 production and Cr(VI) reduction

Hollow carbon nitride spheres with a well-designed architecture and excellent optical properties serve as promising polymers for solar fuel production. In this study, carbon-rich hollow carbon nitride nanospheres were rationally designed for photocatalytic hydrogen peroxide production. Experimental results revealed that the doping of carbon species in the heptazine unit enhanced light absorption and promoted charge separation and transport. Accordingly, the optimized carbon-rich hollow carbon nitride nanospheres exhibited significantly enhanced photocatalytic performance for solar-driven hydrogen peroxide production and Cr(VI) reduction in comparison with pristine polymeric carbon nitride and hollow carbon nitride nanospheres.

A highly selective colorimetric sensor of mercury(ii) ions and hydrogen peroxide by biosynthesized silver nanoparticles in water and investigations of the interaction between silver and mercury

Silver nanoparticles (AgNPs) are promising for their exceptional properties for various applications. This study applied a facile and green method to synthesize AgNPs in an aqueous medium using Averrhoa bilimbi fruit extract as a reducing and stabilizing agent. The formation of AgNPs was confirmed by using UV-visible spectroscopy, X-ray diffraction pattern (XRD), and High-resolution transmission electron microscopy (HRTEM). The synthesized AgNPs consist of face-centered cubic crystals and exhibit homogeneous spherical morphology with an average size of 11 nm. Heavy metals like mercury contamination in water and food pose global health risks, leading to disability issues, even at trace levels. Beside, H2O2 is a reactive oxygen species. Thus, elevated H2O2 levels can harm cell membranes, proteins, and DNA in aquatic creatures, resulting in oxidative stress that may affect physiological processes. Therefore, there is an urgent need for effective monitoring and prevention. The synthesized AgNPs were utilized as a colorimetric probe for the detection of mercury (Hg2+) ions in water at room temperature and found to be highly sensitive and selective with a limit of detection (LOD) of 1.58 μM and a limit of quantification (LOQ) of 5.27 μM. Furthermore, the detection of Hg2+ was unaffected in the presence of other pertinent metal ions. The prepared AgNPs probe can also enable detection of Hg2+ with the naked eye. In addition, the AgNPs probe was investigated for detecting Hg2+ ions in real water samples, which offered satisfying recovery rates ranging from 90.60 ± 2.60 to 96.73 ± 2.83%, confirming the probe's practicability. The capping agent stabilized on the surface of AgNPs can effectively pre-concentrate Hg2+ ions through the chemical interaction between AgNPs and Hg2+ ions to form Ag-Hg amalgam. This leads to a decrease in the SPR peak from AgNPs. The interaction between Ag and Hg was investigated using synchrotron radiation-induced X-ray photoelectron spectroscopy (SR-XPS). In addition, the AgNPs probe effectively detected hydrogen peroxide (H2O2) in an aqueous medium with a LOD of 3.21 μM and LOQ of 10.70 μM. This study aimed to develop a rapid, easy-to-use, eco-friendly, and reliable colorimetric sensor that may quickly identify dangerous pollutants in aqueous samples.

Tungsten Oxide/g-C3N4 Heterostructures: Composition, Structure, and Photocatalytic Applications

The construction of heterostructures promotes extending the light adsorption range of graphitic carbon nitride (g-C3N4) materials, improving the photogenerated charge carrier separation/transfer efficiency for attaining much enhanced performances. Because defective tungsten oxide (WOx) materials possess rich composition/morphology and an extended light response in the near-infrared region, WOx is a quite popular nanocomponent for modifying g-C3N4, forming heterostructures that can be used for various photocatalytic applications involving water splitting, CO2 reduction, NOx removal, H2O2 generation, and related chemical to fuel conversion reactions. In this review, important aspects of WOx/g-C3N4 heterostructure photocatalysts are reviewed to provide paradigms for composition adjustment, structural design, and photocatalytic applications of these materials. The WOx growth control in amorphous and crystalline g-C3N4, adjustment on heterostructure types (e.g., type II and Z-scheme), and the catalytic performances of the composite system are also discussed in detail. Moreover, the effects of synthetic methodologies and preparation parameters on the formation of two-dimensional layered heterostructures are discussed to provide inspiration for the construction of state-of-the-art WOx/g-C3N4 heterostructures that can be utilized for photoredox reactions. The challenges and prospects of the heterostructure formation and the photocatalytic applications of the heterostructures in future research are also summarized.

Vacancy-Activated B-Doping for Efficient 2e- Oxygen Reduction through Suppressing H2O2 Decomposition at High Overpotential

The production of hydrogen peroxide (H2O2) through two-electron oxygen reduction reaction (2e- ORR) has emerged as a more environmentally friendly alternative to the traditional anthraquinone method. Although oxidized carbon catalysts have intensive developed due to their high selectivity and activity, the yield and conversion rate of H2O2 under high overpotential still limited. The produced H2O2 was rapidly consumed by the increased intensity of H2O2 reduction, which could ascribe to decomposition of peroxide radicals under high voltage in the carbon catalyst. To overcome this issue, a B doped carbon have been developed to catalyze 2e- ORR with high efficient through suppressing H2O2 decomposition at high potential. Thus, thermal reducing of oxygen containing groups (OCGs) on graphite could construct defects and vacancies, which in situ convert to B-Cx subunits on the edge of graphene sheets. The introduction of B-Cx effectively prevented the decomposition of the *O-O bond and provided suitable adsorption capacity for *OOH, achieving excellent selectivity for the 2e- ORR across a wide voltage range. Finally, a remarkable H2O2 yield of 7.91 mmol cm-2 h-1 was delivered at an industrial current density of 600 mA cm-2, which could provide "green" pathway for scale-upable synthesis H2O2.

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.

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.

Solid-State-Electrolyte Reactor: New Opportunity for Electrifying Manufacture

Electrocatalysis, which leverages renewable electricity, has emerged as a cornerstone technology in the transition toward sustainable energy and chemical production. However, traditional electrocatalytic systems often produce mixed, impure products, necessitating costly purification. Solid-state electrolyte (SSE) reactors represent a transformative advancement by enabling the direct production of high-purity chemicals, significantly reducing purification costs and energy consumption. The versatility of SSE reactors extends to applications such as CO2 capture and tandem reactions, aligning with the green and decentralized production paradigm. This Perspective provides a comprehensive overview of SSE reactors, discussing their principles, design innovations, and applications in producing pure chemicals-such as liquid carbon fuels, hydrogen peroxide, and ammonia-directly from CO2 and other sources. We further explore the potential of SSE reactors in applications such as CO2 capture and tandem reactions, highlighting their compatibility with versatile production systems. Finally, we outline future research directions for SSE reactors, underscoring their role in advancing sustainable chemical manufacturing.

Charge Transfer Modulation in g-C3N4/CeO2 Composites: Electrocatalytic Oxygen Reduction for H2O2 Production

The electrocatalytic two-electron oxygen reduction reaction (2e-ORR) represents one of the most prospective avenues for the in situ synthesis of hydrogen peroxide (H2O2). However, the four-electron competition reaction constrains the efficiency of H2O2 synthesis. Therefore, there is an urgent need to develop superior catalysts to facilitate the H2O2 synthesis. In this study, graphite-phase carbon nitride and cerium dioxide composites (g-C3N4/CeO2) with varying CeO2 loadings were prepared with favorable 2e-ORR electrocatalysts. The optimized composite, containing 20 wt % CeO2 (g-C3N4/CeO2-20%) exhibited the highest Faradaic efficiency (FE) of 97% and notable H2O2 yielding of 9.84 mol gcat.-1 h-1 at the potential of 0.3 V (vs RHE) in a 0.1 M KOH electrolyte. Density functional theory calculations revealed that the improvement of the selectivity and yield of H2O2 for the composites were attributed to the charge transfer between g-C3N4 and CeO2, which causes the active site C atom donating electrons to form C+, thereby enhancing the adsorption and desorption of *OOH intermediates. Additionally, the g-C3N4/CeO2-20% composite exhibits excellent 2e-ORR performance in neutral and acidic electrolytes and demonstrates superior capability in electro-Fenton degradation of organic pollutants. This study not only provides new insights into the electrocatalytic mechanism of g-C3N4/CeO2 composites but also demonstrates an effective method for designing 2e-ORR catalysts.

Hydrogen Radical Enabling Industrial-Level Oxygen Electroreduction to Hydrogen Peroxide

The electrochemical synthesis of hydrogen peroxide from oxygen and water, powered by renewable electricity, provides a highly attractive alternative to the energy-intensive autoxidation process presently used in industry, but much remains unknown about this two-electron oxygen reduction reaction (2e-ORR), especially the local proton effect. Here, we have investigated the function of hydrogen-associated intermediates in the 2e-ORR using a rationally designed cooperative electrode material with cobalt (II) clusters embedded onto the oxidized carbon nanotube composites (Co-OCNT). We found that the local proton availability can determine both the reaction kinetics and selectivity. A 2e-ORR process involving hydrogen radical transfer is confirmed. Specifically, the carbon sites from the OCNTs promote proton production, and the cobalt sites from the Co cluster facilitate ORR intermediate formation. The high local proton availability and the cooperative dual-active sites both contribute to the superior reaction kinetics and selectivity of the Co-OCNT, reaching an H2O2 production rate of ~40.6 mol gcat -1 h-1 and a faradaic efficiency of 90 % at a current density of 300 mA cm-2. Further cascading the 2e-ORR with the electro-Fenton process shows a high selectivity of oxalic acid up to 97 % for the valorization of ethylene glycol.

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.

Synthesis of UiO-66-NH2@PSF Hollow Fiber Membrane with Enhanced Simultaneous Adsorption of Pb2+ and Phosphate for Hydrogen Peroxide Purification

Electronic grade hydrogen peroxide plays a crucial role in the fabrication of large-scale integrated circuits. However, hydrogen peroxide prepared by the anthraquinone method contains impurities such as lead ions (Pb2+) and phosphate, which can seriously affect the yield of the circuit. Traditional adsorbent materials have difficulty in solving the problem of simultaneous adsorption of trace anions and cations in hydrogen peroxide due to the single adsorption site and poor adsorption kinetics. In this study, UiO-66-NH2 was prepared by introducing a -NH2 group on the terephthalic acid ligand, and a series of hybrid matrix hollow fiber membranes with different UiO-66-NH2 contents were prepared by loading it on polysulfone (PSF). This initiative not only improved the pore size and water flux of hollow fiber membranes but also enhanced the removal efficiency of ions from hydrogen peroxide solution, thereby facilitating practical application. Among them, UiO-66-NH2@PSF-1.5 showed the best adsorption of phosphate and lead ions with adsorption capacities of 3.099 and 2.160 mg g-1 and reached the removal efficiency of 67.1 and 60.1%, which fully confirms the practicability in the purification of electronic chemicals. This work innovatively proposes that UiO-66-NH2@PSF hybrid matrix hollow fiber membranes have great potential as simultaneous adsorbents for cations and anions in the efficient purification of electronic grade solvents.

Ambient Synthesis of Cyclohexanone Oxime via In Situ Produced Hydrogen Peroxide over Cobalt-Based Electrocatalyst

Cyclohexanone oxime, a critical precursor for nylon-6 production, is traditionally synthesized via the hydroxylamine method under industrial harsh conditions. Here is present a one-step electrochemical integrated approach for the efficient production of cyclohexanone oxime under ambient conditions. This approach employed the coupling of in situ electro-synthesized H2O2 over a cobalt (Co)-based electrocatalyst with the titanium silicate-1 (TS-1) heterogeneous catalyst to achieve the cyclohexanone ammoximation process. The cathode electrocatalyst is consisted of atomically dispersed Co sites and small Co nanoparticles co-anchored on carboxylic multi-walled carbon nanotubes (CoSAs/SNPs-OCNTs), which delivered superior electrocatalytic activity toward the two-electron oxygen reduction reaction (2e- ORR) with high-efficient H2O2 production in 0.1 m sodium phosphate (NaPi). Theoretical calculations revealed that the introduction of Co nanoparticles effectively optimized the binding strength of *OOH species on Co atomic sites, thus facilitating the 2e- ORR. The subsequent tandem catalytic system achieved a high cyclohexanone conversion of 71.7% ± 1.1% with a cyclohexanone oxime selectivity of 70.3% ± 0.6%. In this system, the TS-1 catalyst effectively captured the *OOH intermediate and activated the in situ generated H2O2 to form Ti-OOH species, which promoted the formation of hydroxylamine and thereby enhanced the oxime production performance.

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.

Single-Atom-Induced Hybridization States Promote the Direct Trapping of Hot Carriers by Reactants for Photocatalysis

Single-atom manipulation has emerged as an effective strategy for enhancing the photocatalytic efficiency. However, the mechanism of photogenerated carrier dynamics under single-atom modulation remains unclear. Combining first-principles calculations and non-adiabatic molecular dynamics simulations, we systematically studied carrier transfer and recombination in the oxygen reduction reaction of single-atom-doped C3N4 systems. Unlike the conventional two-step process, where single atoms trap photogenerated carriers before transferring them to reactants, our findings reveal a direct one-step electron transfer process, where single-atom-induced hybridization states facilitate the direct trapping of hot carriers by reactants from photocatalysts. Specifically, photogenerated electron transfer time through the one-step process is 237 and 325 fs for Sb and Cu single-atom-doped systems, respectively, considerably faster than the two-step process (hundreds of picoseconds). Moreover, these systems exhibit a nanosecond-level photogenerated carrier lifetime, driving a high photocatalytic efficiency. This study elucidates the carrier dynamics in single-atom photocatalysts, facilitating the screening of high-performance photocatalysts.

Nature-Inspired N, O Co-Coordinated Manganese Single-Atom Catalyst for Efficient Hydrogen Peroxide Electrosynthesis

The two-electron oxygen reduction reaction (2e- ORR) is a pivotal pathway for the distributed production of hydrogen peroxide (H2O2). In nature, enzymes containing manganese (Mn) centers can convert reactive oxygen species into H2O2. However, Mn-based heterogeneous catalysts for 2e- ORR are scarcely reported. Herein, we developed a nature-inspired single-atom electrocatalyst comprising N, O co-coordinated Mn sites, utilizing carbon dots as the modulation platform (Mn CD/C). As-synthesized Mn CD/C exhibited exceptional 2e- ORR activity with an onset potential of 0.786 V and a maximum H2O2 selectivity of 95.8 %. Impressively, Mn CD/C continuously produced 0.1 M H2O2 solution at 200 mA/cm2 for 50 h in the flow cell, with negligible loss in activity and H2O2 faradaic efficiency, demonstrating practical application potential. The enhanced activity was attributed to the incorporation of Mn atomic sites into the carbon dots. Theoretical calculations revealed that the N, O co-coordinated structure, combined with abundant oxygen-containing functional groups on the carbon dots, optimized the binding strength of intermediate *OOH at the Mn sites to the apex of the catalytic activity volcano. This work illustrates that carbon dots can serve as a versatile platform for modulating the microenvironment of single-atom catalysts and for the rational design of nature-inspired catalysts.

Bismuth titanate microplates with tunable oxygen vacancies for piezocatalytic hydrogen peroxide production

Piezocatalysis offers an encouraging alternative for the sustainable, on-demand, and decentralized production of hydrogen peroxide (H2O2), underscoring the importance of enhancing piezocatalytic efficiency. Enhancing piezocatalysts through defect engineering has shown considerable potential in boosting H2O2 production efficiency. However, the impact of oxygen vacancies on piezocatalytic activity remains unclear. Herein, we used a chemical probe method to quantify negative charges (q-) and superoxide radicals (O2-) to explore the relation between the oxygen vacancy concentration and piezocatalytic performance of bismuth titanate (Bi4Ti3O12) based catalysts. Results indicate that piezocatalytic H2O2 production in pure water demonstrates a volcanic trend with increasing oxygen vacancy concentration. This trend is attributed to the dual role of oxygen vacancies, which reduce the piezoelectric property of the piezocatalyst while simultaneously increasing the concentration of O2-, which is crucial for H2O2 formation through the O2 reduction pathway. This study provides insights into the interplay between oxygen vacancies, piezoelectric properties, and piezocatalytic activity, offering valuable guidance for the design of piezocatalysts for sustainable H2O2 production.

Fragmented Polymetric Carbon Nitride with Rich Defects for Boosting Electrochemical Synthesis of Hydrogen Peroxide in Alkaline and Neutral Media

Electrocatalytic oxygen reduction reaction via 2e- pathway is a safe and friendly route for hydrogen peroxide (H2O2) synthesis. In order to achieve efficient synthesis of H2O2, it is essential to accurately control the active sites. Here, fragmented polymetric carbon nitride with rich defects (DCN) is designed for H2O2 electrosynthesis. The multi-type defects, including the sodium atom doping in six-fold cavities, the boron atom doping at N-B-N sites and the cyano groups, are successfully created. Owing to the synergistic effect of these defects, the fragmented DCN achieves a high H2O2 production rate of 2.28 mol gcat. -1 h-1 and a high Faradic efficiency of nearly 90 % in alkaline media at 0.4 V vs. RHE in H-type cell. In neutral media, the H2O2 concentration produced by DCN can reach 1815 μM within 6 h at a potential of 0.2 V vs. RHE, and the H2O2 production rate of DCN is 0.23 mol gcat. -1 h-1. In addition, DCN shows excellent long-term durability in alkaline and neutral media. This study provides a new approach for the development of the boron, nitrogen doped carbon-based electrocatalysts for H2O2 electrochemical synthesis.

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.

Designed Synthesis of Mesoporous sp2 Carbon-Conjugated Benzothiadiazole Covalent Organic Frameworks for Artificial Photosynthesis of Hydrogen Peroxide

Artificial photosynthesis of hydrogen peroxide (H2O2) from ambient air, water, and sunlight has attracted considerable attention recently. Despite being extremely challenging to synthesis, sp2 carbon-conjugated covalent organic frameworks (COFs) can be powerful and efficient materials for the photosynthesis of H2O2 due to desirable properties. Herein, we report the designed synthesis of an sp2 carbon-conjugated COF, BTD-sp2c-COF, from benzothiadiazole and triazine units with high crystallinity and ultralarge mesopores (∼4 nm). The sp2 carbon-conjugated skeletons guarantee BTD-sp2c-COF superior optoelectronic properties and chemical stability. BTD-sp2c-COF exhibits an exceptional efficiency of 3066 μmol g-1 h-1 from pure water and air, much better than that of BTD-imine-COF. In contrast, the resilience of BTD-imine-COF is compromised due to the participation of imine linkages in the oxygen reduction reaction. Importantly, in situ characterization and theoretical calculation results reveal that both benzothiadiazole and triazine units serve as oxygen reduction reaction centers for H2O2 photosynthesis through a sequential electron transfer pathway, while the vinylene bridged phenyls serve as water oxidation reaction centers. The sp2 carbon-conjugated COFs pave the way for potent artificial photosynthesis.

Hydrogen Peroxide Electrosynthesis via Selective Oxygen Reduction Reactions Through Interfacial Reaction Microenvironment Engineering

The electrochemical two-electron oxygen reduction reaction (2e- ORR) offers a compelling alternative for decentralized and on-site H2O2 production compared to the conventional anthraquinone process. To advance this electrosynthesis system, there is growing interest in optimizing the interfacial reaction microenvironment to boost electrocatalytic performance. This review consolidates recent advancements in reaction microenvironment engineering for the selective electrocatalytic conversion of O2 to H2O2. Starting with fundamental insights into interfacial electrocatalytic mechanisms, an overview of various strategies for constructing the favorable local reaction environment, including adjusting electrode wettability, enhancing mesoscale mass transfer, elevating local pH, incorporating electrolyte additives, and employing pulsed electrocatalysis techniques is provided. Alongside these regulation strategies, the corresponding analyses and technical remarks are also presented. Finally, a summary and outlook on critical challenges, suggesting future research directions to inspire microenvironment engineering and accelerate the practical application of H2O2 electrosynthesis is delivered.

Pairing Oxygen Reduction and Water Oxidation for Dual-Pathway H2O2 Production

Hydrogen peroxide (H2O2) is a crucial chemical applied in various industry sectors. However, the current industrial anthraquinone process for H2O2 synthesis is carbon-intensive. With sunlight and renewable electricity as energy inputs, photocatalysis and electrocatalysis have great potential for green H2O2 production from oxygen (O2) and water (H2O). Herein, we review the advances in pairing two-electron O2 reduction and two-electron H2O oxidation reactions for dual-pathway H2O2 synthesis. The basic principles, paired redox reactions, and catalytic device configurations are introduced initially. Aligning with the energy input, the latest photocatalysts, electrocatalysts, and photo-electrocatalysts for dual-pathway H2O2 production are discussed afterward. Finally, we outlook the research opportunities in the future. This minireview aims to provide insights and guidelines for the broad community who are interested in catalyst design and innovative technology for on-site H2O2 synthesis.

Synthesis of Hydrogen Peroxide in Neutral Media by an Iron-Doped Nickel Phosphide Catalyst

The two-electron oxygen reduction reaction (2e- ORR) for electrochemical hydrogen peroxide (H2O2) synthesis has drawn much attention due to its eco-friendly, cost-effective, and highly efficient properties. Developing catalysts with excellent H2O2 production rates and selectivity is still a big challenge. In this work, an iron-doped nickel phosphide (Fe-Ni-P) catalyst was synthesized by a solvent thermal method. The synthesis temperature of 180 °C could obtain the best 2e- ORR catalyst, i.e., Fe-Ni-P-180, since the crystallization of metal phosphide under this temperature was promoted. In addition, Fe-Ni-P-180 had a high catalytic activity, high electron transfer rate, and low electrochemical resistance. The H2O2 production rate constant of Fe-Ni-P-180 was 9.99 ± 0.63 μM/(min cm2) and the Faradaic efficiency was 94.38 ± 4.68% at 0.25 V vs RHE, which increased by 57.9 and 15.4% compared with Ni-P, respectively. Fe-Ni-P-180 could work in a wide pH range of 5-9 with the optimized pH of 7, and it exhibited low specific energy consumption and great reusability. The elemental state analysis demonstrated that Niδ+, Feδ+, and Pδ- are all active species, and the doping of Fe increases the crystallization of metal phosphide.

Refining Metal-Free Carbon Nanoreactors through Electronic and Geometric Comodification for Boosted H2O2 Electrosynthesis toward Efficient Water Decontamination

Hydrogen peroxide (H2O2) electrosynthesis using metal-free carbon materials via the 2e- oxygen reduction pathway has sparked considerable research interest. However, the scalable preparation of carbon electrocatalysts to achieve satisfactory H2O2 yield in acidic media remains a grand challenge. Here, we present the design of a carbon nanoreactor series that integrates precise O/N codoping alongside well-regulated geometric structures targeting high-efficiency electrosynthesis of H2O2. Theoretical computations reveal that strategic N/O codoping facilitates partial electron transfer from C sites to O sites, realizing electronic rearrangement that optimizes C-site adsorption of *OOH. Concurrently, the O-O bond in *OOH is strengthened by charge transfer from antibonding to π-orbitals, stabilizing the O-O bond and preventing its dissociation. The carbon nanoreactor with a hollow bowl geometry also facilitates the mass transport of O2 and H2O2, achieving an H2O2 selectivity of 96% in acidic media. Furthermore, a flow cell integrated with the refined nanoreactor catalyst achieves an impressive H2O2 production rate of 2942.4 mg L-1 h-1, coupled with stable operation of nearly 80 h, surpassing the state-of-the-art metal-free analogs. The feasibility of the electro-synthesized H2O2 is further demonstrated to be highly efficient in wastewater remediation.

Error Awareness in the Volcano Plots of Oxygen Electroreduction to Hydrogen Peroxide

Electrocatalysis holds the key to the decentralized production of hydrogen peroxide via the two-electron oxygen reduction reaction (ORR, O2(g)+2H++2e-->H2O2(aq)). However, cost-effective, active, and selective catalysts are still sought after. While density functional theory (DFT) has already led to the discovery of various enhanced catalysts, it has a severe yet often unnoticed drawback: the ill description of O2 and H2O2. Here, we analyze the impact of the errors in those two species on the most widespread activity plots in the literature, namely free-energy diagrams and Sabatier-type volcano plots. Uncorrected or partially corrected gas-phase energies lead to appreciably different activity plots that may provide inaccurate predictions. Indeed, we show for a variety of electrocatalysts that only when the errors in O2 and H2O2 are corrected can DFT mimic the experiments. In sum, this work provides concrete guidelines to avoid a common pitfall of computational models for electrocatalytic hydrogen peroxide production.

Site engineering of linear conjugated polymers to regulate oxygen adsorption affinity for boosting photocatalytic production of hydrogen peroxide without sacrificial agent

Artificial photosynthesis of hydrogen peroxide (H2O2) is a hopeful alternative to the industrial anthraquinone process. However, rational fabrication of the photocatalysts for the production of H2O2 without any sacrificial agents is still a formidable challenge. Herein, two kinds of linear conjugated polymers (LCPs) including pyridinic N functionalized polymer (DEB-N2) and pyridinic N non-contained polymer (DEB-N0) were successfully synthesized. DEB-N2 displays enhanced light capturing ability and good dispersion in water, leading to a substantial initial H2O2 generation rate of 3492μmol g-1h-1 as well as remarkable photocatalytic stability in pure water. Furthermore, the temperature programmed desorption (TPD) and density functional theory (DFT) analysis reveal that highly electronegative pyridine-N atoms in DEB-N2 boost the adsorption affinity of oxygen molecules, which facilitates the occurrence of the oxygen reduction reaction, therefore enhancing the performance of photocatalytic H2O2 production. This study unveils that the presence of pyridinic N in DEB-N2 has a significant impact on photocatalytic H2O2 production, suggesting the precise manipulation of the chemical structure of polymer photocatalysts is essential to achieve efficient solar-to-chemical energy conversion.

Engineering of Single-Atomic Sites for Electro- and Photo-Catalytic H2O2 Production

Direct electro- and photo-synthesis of H2O2 through the 2e- O2 reduction reaction (ORR) and H2O oxidation reaction (WOR) offer promising alternatives for on-demand and on-site production of this chemical. Exploring robust and selective active sites is crucial for enhancing H2O2 production through these pathways. Single-atom catalysts (SACs), featuring isolated active sites on supports, possess attractive properties for promoting catalysis and unraveling catalytic mechanisms. This review first systematically summarizes significant advancements in atomic engineering of both metal and nonmetal single-atom sites for electro- and photo-catalytic 2e- ORR to H2O2, as well as the dynamic behaviors of active sites during catalytic processes. Next, the progress of single-atom sites in H2O2 production through 2e- WOR is overviewed. The effects of the local physicochemical environments on the electronic structures and catalytic behaviors of isolated sites, along with the atomic catalytic mechanism involved in these H2O2 production pathways, are discussed in detail. This work also discusses the recent applications of H2O2 in advanced chemical transformations. Finally, a perspective on the development of single-atom catalysis is highlighted, aiming to provide insights into future research on SACs for electro- and photo-synthesis of H2O2 and other advanced catalytic applications.

Tandem Oxidation Activation of Carbon for Enhanced Electrochemical Synthesis of H2O2: Unveiling the Role of Quinone Groups and Their Operando Derivatives

Oxygen-doped carbon materials show great promise to catalyze two-electron oxygen reduction reaction (2e-ORR) for electrochemical synthesis of hydrogen peroxide (H2O2), but the identification of the active sites is the subject of ongoing debate. In this study, a tandem oxidation strategy is developed to activate carbon black for achieving highly efficient electrochemical synthesis of H2O2. Acetylene black (AB) is processed with O2 plasma and subsequent electrochemical oxidation, resulting in a remarkable selectivity of >96% over a wide potential range, and a record-setting high yield of >10 mol gcat -1 h-1 with good durability in gas diffusion electrode. Comprehensive characterizations and calculations revealed that the presence of abundant C═O groups at the edge sites positively correlated to and accounted for the excellent 2e-ORR performance. Notably, the edge hydroquinone group formed from quinone under operando conditions, which is overlooked in previous research, is identified as the most active catalytic site.

In Situ Carbon Thermal Reduction to Enrich Sulfur-Vacancy in Nickel Disulfide Cathode for Efficient Synthesizing Hydrogen Peroxide

Transition metal catalysts are widely used in the 2e- ORR due to their cost-effectiveness. However, they often encounter issues related to low activity. Defect engineering are used on developing highly active catalysts, which can effectively modify active sites and promote electron transfer. Here, carbon-coated Ni3S2 (Ni3S2@C), where the additional sulfur vacancies (VS) is prepared induced by the carbon layer is coupled with active nickel sites. Through in situ and ex situ experiments combined with DFT calculations, it is demonstrated that the carbon layer can regulate the quantity of VS in Ni3S2. Materials with a higher concentration of VS exhibit enhanced 2e- ORR activity and higher H2O2 selectivity. In situ Raman spectroscopy confirms that Ni serves as the key active site in this catalyst. DFT calculations indicate that the OOH binding energy (ΔG) decreases with an increase in the number of VS, favoring the protonation of *OOH to generate H2O2. Upon performance testing, the average H2O2 selectivity is 92.3%, with the highest yield reaching up to 3860 mmol gcat-1 h-1. It is noteworthy that Ni3S2@C exhibits high stability, with only a slight decrease in 2e- pathway selectivity after 5000 cycles of ADT.

The Still Elusive Role of Lightweight Doping in Carbon-Based Electrocatalysts for the Selective Oxygen Reduction Reaction to Hydrogen Peroxide

The two-electron electrocatalytic oxygen reduction reaction (ORR) to hydrogen peroxide (H2O2) is a valuable alternative to the more conventional and energy-intensive anthraquinone process. From a circularity viewpoint, metal-free catalysts constitute a sustainable alternative for the process. In particular, lightweight hetero-doped C-materials are cost-effective and easily scalable samples that replace - more and more frequently - the use of critical raw elements in the preparation of highly performing (electro)catalysts. Anyhow, their large-scale exploitation in industrial processes still suffers from technical limits of samples upscale and reproducibility other than a still moderate comprehension of their action mechanism in the process. This concept article offers a comprehensive and exhaustive "journey" through the most representative lightweight hetero-doped C-based electrocatalysts and their performance in the 2e- ORR process. It provides an interpretation of phenomena at the triple-phase interface of solid catalyst, liquid electrolyte and gaseous oxygen based on the doping-driven generation of ideal electronic microenvironments at the catalyst surface.

Challenges and opportunities for large-scale applications of the electro-Fenton process

As an electrochemical advanced oxidation process, the electro-Fenton (EF) process has gained significant importance in the treatment of wastewater and persistent organic pollutants in recent years. As recently reported in a bibliometric analysis, the number of scientific publications on EF have increased exponentially since 2002, reaching nearly 500 articles published in 2022 (Deng et al., 2022). The influence of the main operating parameters has been thoroughly investigated for optimization purposes, such as type of electrode materials, reactor design, current density, and type and concentration of catalyst. Even though most of the studies have been conducted at a laboratory scale, focusing on fundamental aspects and their applications to degrade specific pollutants and treat real wastewater, important large-scale attempts have also been made. This review presents and discusses the most recent advances of the EF process with special emphasis on the aspects more closely related to future implementations at the large scale, such as applications to treat real effluents (industrial and municipal wastewaters) and soil remediation, development of large-scale reactors, costs and effectiveness evaluation, and life cycle assessment. Opportunities and perspectives related to the heterogeneous EF process for real applications are also discussed. This review article aims to be a critical and exhaustive overview of the most recent developments for large-scale applications, which seeks to arouse the interest of a large scientific community and boost the development of EF systems in real environments.