Dipole-Dipole Tuned Electronic Reconfiguration of Defective Carbon Sites for Efficient Oxygen Reduction into H2O2.

Metal-free carbon-based materials are one of the most promising electrocatalysts toward 2-electron oxygen reduction reaction (2e-ORR) for on-site production of hydrogen peroxide (H2O2), which however suffer from uncontrollable carbonizations and inferior 2e-ORR selectivity. To this end, a polydopamine (PDA)-modified carbon catalyst with a dipole-dipole enhancement is developed via a calcination-free method. The H2O2 yield rate outstandingly reaches 1.8 mol gcat -1 h-1 with high faradaic efficiency of above 95% under a wide potential range of 0.4-0.7 VRHE, overwhelming most of carbon electrocatalysts. Meanwhile, within a lab-made flow cell, the synthesized ORR electrode features an exceptional stability for over 250 h, achieved a pure H2O2 production efficacy of 306 g kWh-1. By virtue of its industrial-level capabilities, the established flow cell manages to perform a rapid pulp bleaching within 30 min. The superior performance and enhanced selectivity of 2e-ORR is experimentally revealed and attributed to the electronic reconfiguration on defective carbon sites induced by non-covalent dipole-dipole influence between PDA and carbon, thereby prohibiting the cleavage of O-O in OOH intermediates. This proposed strategy of dipole-dipole effects is universally applicable over 1D carbon nanotubes and 2D graphene, providing a practical route to design 2e-ORR catalysts.

Highly Efficient Acidic Electrosynthesis of Hydrogen Peroxide at Industrial-Level Current Densities Promoted by Alkali Metal Cations.

Acidic H2O2 synthesis through electrocatalytic 2e- oxygen reduction presents a sustainable alternative to the energy-intensive anthraquinone oxidation technology. Nevertheless, acidic H2O2 electrosynthesis suffers from low H2O2 Faradaic efficiencies primarily due to the competing reactions of 4e- oxygen reduction to H2O and hydrogen evolution in environments with high H+ concentrations. Here, we demonstrate the significant effect of alkali metal cations, acting as competing ions with H+, in promoting acidic H2O2 electrosynthesis at industrial-level currents, resulting in an effective current densities of 50-421 mA cm-2 with 84-100 % Faradaic efficiency and a production rate of 856-7842 µmol cm-2 h-1 that far exceeds the performance observed in pure acidic electrolytes or low-current electrolysis. Finite-element simulations indicate that high interfacial pH near the electrode surface formed at high currents is crucial for activating the promotional effect of K+. In situ attenuated total reflection Fourier transform infrared spectroscopy and ab initio molecular dynamics simulations reveal the central role of alkali metal cations in stabilizing the key *OOH intermediate to suppress 4e- oxygen reduction through interacting with coordinated H2O.

Fe-O4 Motif Activated Graphitic Carbon via Oxo-Bridge for Highly Selective H2O2 Electrosynthesis.

Carbon-based materials have been utilized as effective catalysts for hydrogen peroxide electrosynthesis via two-electron oxygen reduction reaction (2e ORR), however the insufficient selectivity and productivity still hindered the further industrial applications. In this work, we report the Fe-O4 motif activated graphitic carbon material which enabled highly selective H2O2 electrosynthesis operating at high current density with excellent anti-poisoning property. In the bulk production test, the concentration of H2O2 cumulated to 8.6 % in 24 h and the corresponding production rate of 33.5 mol gcat -1 h-1 outperformed all previously reported materials. Theoretical model backed by in situ characterization verified α-C surrounding the Fe-O4 motif as the actual reaction site in terms of thermodynamics and kinetics aspects. The strategy of activating carbon reaction site by metal center via oxo-bridge provides inspiring insights for the rational design of carbon materials for heterogeneous catalysis.

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

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

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

Designing highly efficient and stable electrode-electrolyte interface for hydrogen peroxide (H2 O2 ) electrosynthesis remains challenging. Inhibiting the competitive side reaction, 4 e- oxygen reduction to H2 O, is essential for highly selective H2 O2 electrosynthesis. Instead of hindering excessive hydrogenation of H2 O2 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 H2 O2 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 H2 O2 . 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.

Atomically Dispersed Iron Regulating Electronic Structure of Iron Atom Clusters for Electrocatalytic H2 O2 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 (H2 O2 ). 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 H2 O2 selectivity of 96.5 % in 0.1 M KOH. In a flow cell reactor, the FeSAs/ACs-BCC also achieves high-efficiency H2 O2 production with a yield rate of 12.51±0.18 mol gcat -1 h-1 and a faradaic efficiency of 89.4 %±1.3 % at 150 mA cm-2 . Additionally, the feasibility of coupling the produced H2 O2 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 H2 O2 production.

Electronic Structure Regulation of Single-Atom Catalysts for Electrochemical Oxygen Reduction to H2 O2.

Electrochemical synthesis of hydrogen peroxide (H2 O2 ) via the 2-electron oxygen reduction reaction (ORR) has emerged as a promising alternative to the energy-intensive anthraquinone process and catalysts combining high selectivity with superior activity are crucial for enhancing the efficiency of H2 O2 electrosynthesis. In recent years, single-atom catalysts (SACs) with the merits of maximum atom utilization efficiency, tunable electronic structure, and high mass activity have attracted extensive attention for the selective reduction of O2 to H2 O2 . Although considerable improvements are made in the performance of SACs toward the 2-electron ORR process, the principles for modulating the catalytic properties of SACs by adjusting the electronic structure remain elusive. In this review, the regulation strategies for optimizing the 2-electron ORR activity and selectivity of SACs by different methods of electronic structure tuning, including the altering of the central metal atoms, the modulation of the coordinated atoms, the substrate effect, and alloy engineering are summarized. Finally, the challenges and future prospects of advanced SACs for H2 O2 electrosynthesis via the 2-electron ORR process are proposed.

Boosting Oxygen Reduction for High-Efficiency H2 O2 Electrosynthesis on Oxygen-Coordinated CoNC Catalysts.

Atomically dispersed CoNC is a promising material for H2 O2 selective electrosynthesis via a two-electron oxygen reduction reaction. However, the performance of typical CoNC materials with routine CoN4 active center is insufficient and needs to be improved further. This can be done by fine-tuning its atomic coordination configuration. Here, a single-atom electrocatalyst (Co/NC) is reported that comprises a specifically penta-coordinated CoNC configuration (OCoN2 C2 ) with Co center coordinated by two nitrogen atoms, two carbon atoms, and one oxygen atom. Using a combination of theoretical predictions and experiments, it is confirmed that the unique atomic structure slightly increases the charge state of the cobalt center. This optimizes the adsorption energy towards *OOH intermediate, and therefore favors the two-electron ORR relevant for H2 O2 electrosynthesis. In neutral solution, the as-synthesized Co/NC exhibits a selectivity of over 90% over a potential ranging from 0.36 to 0.8 V, with a turnover frequency value of 11.48 s-1 ; thus outperforming the state-of-the-art carbon-based catalysts.

Efficient H2O2 Synthesis through a Two-Electron Oxygen Reduction Reaction by Electrocatalysts.

The two-electron oxygen reduction reaction (2e-ORR) for the sustainable synthesis of hydrogen peroxide (H2O2) has demonstrated considerable potential for local production of this environmentally friendly chemical oxidant on small, medium, and large scales. This method offers a promising alternative to the energy-intensive anthraquinone approach, placing a primary emphasis on the development of efficient electrocatalysts. Improving the efficiency of electrocatalysts and uncovering their catalytic mechanisms are essential steps in achieving high 2e-ORR activity, selectivity, and stability. This comprehensive review summarizes recent advancements in electrocatalysts for in-situ H2O2 production, providing a detailed overview of the field. In particular, the review delves into the design, fabrication, and investigation of catalytic active sites contributing to H2O2 selectivity. Additionally, it highlights a range of electrocatalysts including pure metals and alloys, transition metal compounds, single-atom catalysts, and carbon-based catalysts for the 2e-ORR pathway. Finally, the review addresses significant challenges and opportunities for efficient H2O2 electrosynthesis, as well as potential future research directions.

Anionic Surfactant-Modulated Electrode-Electrolyte Interface Promotes H2O2 Electrosynthesis.

Conventional strategies for highly selective and active hydrogen peroxide (H2O2) electrosynthesis primarily focus on catalyst design. Electrocatalytic reactions take place at the electrified electrode-electrolyte interface. Well-designed electrolytes, when combined with commercial catalysts, can be directly applied to high-efficiency H2O2 electrosynthesis. However, the role of electrolyte components is equally crucial but is significantly under-researched. In this study, anionic surfactant n-tetradecylphosphonic acid (TDPA) and its analogs are used as electrolyte additives to enhance the selectivity of the two-electron oxygen reduction reaction. Mechanistic studies reveal that TDPA assembled over the electrode-electrolyte interface modulates the electrical double-layer structure, which repels interfacial water and weakens the hydrogen-bond network for proton transfer. Additionally, the hydrophilic phosphonate moiety affects the coordination of water molecules in the solvation shell, thereby directly influencing the proton-coupled kinetics at the interface. The TDPA-containing catalytic system achieves a Faradaic efficiency of H2O2 production close to 100% at a current density of 200 mA cm-2 using commercial carbon black catalysts. This research provides a simple strategy to enhance H2O2 electrosynthesis by adjusting the interfacial microenvironment through electrolyte design.

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

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

Metal Atom-Support Interaction in Single Atom Catalysts toward Hydrogen Peroxide Electrosynthesis.

Single metal atom catalysts (SACs) have garnered considerable attention as promising agents for catalyzing important industrial reactions, particularly the electrochemical synthesis of hydrogen peroxide (H2O2) through the two-electron oxygen reduction reaction (ORR). Within this field, the metal atom-support interaction (MASI) assumes a decisive role, profoundly influencing the catalytic activity and selectivity exhibited by SACs, and triggers a decade-long surge dedicated to unraveling the modulation of MASI as a means to enhance the catalytic performance of SACs. In this comprehensive review, we present a systematic summary and categorization of recent advancements pertaining to MASI modulation for achieving efficient electrochemical H2O2 synthesis. We start by introducing the fundamental concept of the MASI, followed by a detailed and comprehensive analysis of the correlation between the MASI and catalytic performance. We describe how this knowledge can be harnessed to design SACs with optimized MASI to increase the efficiency of H2O2 electrosynthesis. Finally, we distill the challenges that lay ahead in this field and provide a forward-looking perspective on the future research directions that can be pursued.

Low-Coordinated Pd Site within Amorphous Palladium Selenide for Active, Selective, and Stable H2 O2 Electrosynthesis.

The development of high-performance catalysts with high activity, selectivity, and stability are essential for the practical applications of H2 O2 electrosynthesis technology, but it is still formidably challenging. It is reported that the low-coordinated structure of Pd sites in amorphous PdSe2 nanoparticles (a-PdSe2 NPs) can significantly boost the electrocatalytic synthesis of H2 O2 . Detailed investigations and theoretical calculations reveal that the disordered arrangement of Pd atoms in a-PdSe2 NPs can promote the activity, while the Pd sites with low-coordinated environment can optimize the adsorption toward oxygenated intermediate and suppress the cleavage of O-O bond, leading to a significant enhancement in both the H2 O2 selectivity and productivity. Impressively, a-PdSe2 NPs/C exhibits high H2 O2 selectivity over 90% in different pH electrolytes. H2 O2 productivities with ≈3245.7, 1725.5, and 2242.1 mmol gPd -1  h-1 in 0.1 m KOH, 0.1 m HClO4 , and 0.1 m Na2 SO4 can be achieved, respectively, in an H-cell electrolyzer, being a pH-universal catalyst for H2 O2 electrochemical synthesis. Furthermore, the produced H2 O2 can reach 1081.8 ppm in a three-phase flow cell reactor after 2 h enrichment in 0.1 m Na2 SO4 , showing the great potential of a-PdSe2 NPs/C for practical H2 O2 electrosynthesis.

Electrochemically enhanced iron oxide-modified carbon cathode toward improved heterogeneous electro-Fenton reaction for the degradation of norfloxacin.

In this work, different iron-based cathode materials were prepared using two different approaches: a novel one-step approach, which involved the incorporation of iron oxide with Printex® L6 carbon/PTFE (PL6C/PTFE) on bare carbon felt (CF) and a two-step approach, where iron oxide is deposited onto CF previously modified with PL6C/PTFE. The results obtained from the physical characterization indicated that the presence of iron oxide homogeneously dispersed on the felt fibers with the CF 3-D network kept intact in the one-step approach; whereas the formation of iron oxide aggregates between the felt fibers for material obtained using the two-step approach. Among the iron oxide-based cathodes investigated, the iron-incorporated electrode exhibited the greatest efficiency in terms of the removal and mineralization of norfloxacin (NOR) under neutral pH (complete NOR removal in less than 30 min with around 50% mineralization after 90 min). The findings of this study show that the low cost and simple-to-prepare iron-modified carbon-based materials in HEF process led to the enhanced degradation of organic contaminants in aqueous solutions.

Unveiling the Cationic Promotion Effect of H2O2 Electrosynthesis Activity of O-Doped Carbons.

H2O2 electrosynthesis is an emerging clean chemical technology, whose efficiency critically depends on the activity and selectivity of electrocatalysts for two-electron oxygen reduction reaction (2e- ORR). Here, we demonstrate that 2e- ORR activity of oxygen-doped carbons, which have been one of the most promising catalysts for this reaction, can be substantially influenced by the types and concentrations of cations in electrolytes. Heat-treated carbon comprising active oxygen functional groups exhibits cation-dependent 2e- ORR activity trends in alkaline media, following the order Cs+ > K+ > Li+. Importantly, an electrolyte with a high cation concentration (0.1 M KOH + 0.5 M KCl) afforded the highest 2e- ORR mass activity (250 ± 30 A gcat-1 at 0.70 V vs reversible hydrogen electrode) ever reported. We have established that the cation promotion effect correlates with cation-dependent electron-transfer kinetics, which regulates the rate-determining first electron transfer to O2.

Degradation of Acid Violet 19 textile dye by electro-peroxone in a laboratory flow plant.

This paper deals with the degradation of Acid Violet 19 (AV19) textile dye by the electro-peroxone (E-peroxone) process in a laboratory flow plant using a filter press cell fitted with a 3D gas diffusion electrode (3D GDE) containing a graphite felt positioned on carbon-cloth PTFE as cathode, and a Ti|IrSnSb-oxides plate as anode. H2O2 was formed by the oxygen reduction reaction (ORR) in the cathode; the air was supplied by an external compressor. The O3 produced externally by an ozonator was added in the pipeline at the outlet of the electrolyzer to promote the reaction between the H2O2 and O3 to produce OH, which is the responsible for the mineralization of the dye. The effect of electrolyte flow rate (Q), current density (j), and initial concentration of AV19 dye on its degradation was addressed. The best electrolysis in a solution containing 40 mg TOC L-1, 0.05 M Na2SO4, at pH 3, was obtained at j = 20 mA cm-2, Q = 2.0 L min-1, using a pressure of the air fed to the 3D GDE of PGDE = 3 psi, and an ozone inlet mass flow rate of [Formula: see text]  = 14.5 mg L-1, achieving 100% discoloration, 60% mineralization, with mineralization current efficiency and energy consumption of 36% and 0.085 kWh(gTOC)-1. The degradation of AV19 dye was also performed by anodic oxidation plus H2O2 electrogenerated (AO-H2O2) and ozonation. The oxidation power was AO-H2O2 < ozonation < E-peroxone. Three carboxylic acids were quantified by chromatography as oxidation end products.

Simultaneous persulfate activation by electrogenerated H2O2 and anodic oxidation at a boron-doped diamond anode for the treatment of dye solutions.

The development of new or upgraded electrochemical water treatment technologies is considered a topic of great interest. Here, Tartrazine azo dye solutions were treated by means of a quite innovative dual electrochemical persulfate (S2O82-, PS) activation that combines H2O2 generation at an air-diffusion cathode and anodic oxidation (AO) at a boron-doped diamond (BDD) anode using a stirred tank reactor. This so-called AO-H2O2/PS process was compared to AO with stainless steel cathode, both in 50 mM Na2SO4 medium, finding the oxidation power increasing as: AO < AO-H2O2 < AO/PS < AO-H2O2/PS. In the latter, the dye and its products were mainly destroyed by: (i) hydroxyl radicals, formed either from water oxidation at BDD surface or via reaction between H2O2 and S2O82-, and (ii) sulfate radical anion, formed from the latter reaction, thermal PS activation and cathodic S2O82- reduction. Hydroxyl radicals prevailed as oxidizing agents, as deduced from trials with tert-butanol and methanol. The reaction between S2O82- and accumulated H2O2 was favored as temperature increased from 25 to 45 °C. The effect of PS content up to 36 mM, dye concentration within the range 0.22-0.88 mM, current density (j) between 8.3 and 33.3 mA cm-2 and pH between 3.0 and 9.0 on the process performance was examined. All decolorization profiles agreed with a pseudo-first-order kinetics. The best results for treating 0.44 mM dye were attained with 36 mM PS at pH 3.0, j = 16.7 mA cm-2 and 45 °C, yielding total loss of color, 62% TOC removal and 50% mineralization current efficiency after 360 min. The slow mineralization was attributed to the persistence of recalcitrant byproducts like maleic, acetic, oxalic, formic and oxamic acids. It is concluded that the novel AO-H2O2/PS process is more effective than AO/PS to treat Tartrazine solutions, being advisable to extend the study to other organic pollutants.