Two-electron Oxygen Reduction Reaction Research Articles

Validating ΔΔG Selectivity Descriptor for Electrosynthesis of H2O2 from Oxygen Reduction Reaction.

Understanding selectivity trends is a crucial hurdle in the developing innovative catalysts for generating hydrogen peroxide through the two-electron oxygen reduction reaction (2e-ORR). The identification of selectivity patterns has been made more accessible through the introduction of a newly developed selectivity descriptor derived from thermodynamics, denoted as ΔΔG introduced in Chem Catal. 2023, 3(3), 100568. To validate the suitability of this parameter as a descriptor for 2e-ORR selectivity, we utilize an extensive library of 155 binary alloys. We validate that ΔΔG reliably depicts the selectivity trends in binary alloys reported for their high activity in the 2e-ORR. This analysis also enables the identification of nine selective 2e-ORR catalysts underscoring the efficacy of ΔΔG as 2e-ORR selectivity descriptor. This work highlights the significance of concurrently considering both selectivity and activity trends. This holistic approach is crucial for obtaining a comprehensive understanding in the identification of high-performance catalyst materials for optimal efficiency in various applications.

Study on Influence Factors of H2O2 Generation Efficiency on Both Cathode and Anode in a Diaphragm-Free Bath

Electrosynthesis of H2O2 via both pathways of anodic two-electron water oxidation reaction (2e-WOR) and cathodic two-electron oxygen reduction reaction (2e-ORR) in a diaphragm-free bath can not only improve the generation rate and Faraday efficiency (FE), but also simplify the structure of the electrolysis bath and reduce the energy consumption. The factors that may affect the efficiency of H2O2 generation in coupled electrolytic systems have been systematically investigated. A piece of fluorine-doped tin oxide (FTO) electrode was used as the anode, and in this study, its catalytic performance for 2e-WOR in Na2CO3/NaHCO3 and NaOH solutions was compared. Based on kinetic views, the generation rate of H2O2 via 2e-WOR, the self-decomposition, and the oxidative decomposition rate of the generated H2O2 during electrolysis in carbonate electrolytes were investigated. Furthermore, by choosing polyethylene oxide-modified carbon nanotubes (PEO-CNTs) as the catalyst for 2e-ORR and using its loaded electrode as the cathode, the coupled electrolytic systems for H2O2 generation were set up in a diaphragm bath and in a diaphragm-free bath. It was found that the generated H2O2 in the electrolyte diffuses and causes oxidative decomposition on the anode, which is the main influent factor on the accumulated concentration in H2O2 in a diaphragm-free bath.

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

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

Electronic Metal-support Interactions Boost *OOH Intermediate Generation in Cu/In2Se3 for Electrochemical H2O2 Production.

Two-electron oxygen reduction reaction (2e- ORR) is a promising method for the synthesis of hydrogen peroxide (H2O2). However, high energy barriers for the generation of key *OOH intermediates hinder the process of 2e- ORR. Herein, we prepared a copper-supported indium selenide catalyst (Cu/In2Se3) to enhance the selectivity and yield of 2e- ORR by employing an electronic metal-support interactions (EMSIs) strategy. EMSIs-induced charge rearrangement between metallic Cu and In2Se3 is conducive to *OOH intermediate generation, promoting H2O2 production. Theoretical investigations reveal that the inclusion of Cu significantly lowers the energy barrier of the 2e- ORR intermediate and impedes the 4e- ORR pathway, thus favoring the formation of H2O2. The concentration of H2O2 produced by Cu/In2Se3 is ~2 times than In2Se3, and Cu/In2Se3 shows promising applications in antibiotic degradation. This research presents a valuable approach for the future utilization of EMSIs in 2e- ORR.

AC-electrochemical synthesis of H2O2 by breathing O2 in three-phase interface

Conventional electrosynthesis of H2O2 through a two-electron oxygen reduction reaction (2e− ORR) requires continuous O2 feeding as the reactant, which is produced by oxygen evolution reaction (OER) in water electrolysis. In principle, H2O2 can be directly synthesized from water once the 2e− ORR is coupled with OER in one electrode. Unfortunately, such effective coupling has not yet been established. Herein, we created a cyclic oxygen exhalation-inhalation for on-demand and on-site H2O2 production by coupling the OER and 2e− ORR with a facile alternating-current (AC) electrocatalytic process. By ingeniously manipulating the reaction interfaces and operating voltage, we achieved efficient and stable in-situ H2O2 production. The rapid oxygen transfer in the localized three-phase interface favors the sufficient electrochemical reactions. The AC-electrocatalytic system exhibits highest performance including H2O2 production rate of 90.16mmolh-1 gcat-1. Moreover, such AC-electrocatalysis inspires a new strategy to combine multiple electrocatalytic reactions into one electrode for fundamental study and practical applications.

Mechanism insights into H2O2in situ generation and activation for ciprofloxacin degradation by electro-peroxidation using the ACF@RuO2/Ti cathode

Electrocatalytic H2O2 production via the two-electron oxygen reduction reaction on the cathode surface plays a crucial role in the electro-peroxidation (EO-H2O2). In this work, the H2O2 generation potential of the EO-H2O2 process using an activated carbon fiber attached RuO2/Ti cathode (ACF@RuO2/Ti) was evaluated by simulation experiments with a high-precision syringe pump. Results shown that the actual average H2O2 accumulation rate reached up to 4.92 mg h−1 cm−2, while more than 95 % of the electrogenerated H2O2 was decomposed quickly. Electron paramagnetic resonance and quenching tests confirmed that the primary oxidant •OH was produced from H2O2 activation by ACF, RuO2 catalyst and their synergistic effect, while RuO2 played dominant role. The production of •OH in the EO-H2O2 process was also confirmed by the degradation and mineralization of antibiotic ciprofloxacin. The characterization results indicated that heterogeneous catalysis of cathode was due to both the pyridinic nitrogen and oxygen-containing groups of ACF, along with RuO2 catalyst on the RuO2/Ti surface. This study provides new insights into the mechanisms of the in situ H2O2 generation and activation in the EO-H2O2 process.

Magnetic field-enhanced oxygen reduction reaction for electrochemical hydrogen peroxide production with different cerium oxide nanostructures

We investigated cerium oxide nanoparticles of various morphologies (nanosheets, nanocubes, and nanoparticles) supported on carbon Vulcan XC-72 for the two-electron oxygen reduction reaction (ORR). A continuous magnetic field (2000 Oe) was used for the first time in the literature. The best results were for 5 % (w/w) CeO2 for all three different morphologies, more than doubling the ring current, enhancing the hydrogen peroxide selectivity from 51 % (Vulcan XC-72) to 84–89 %, and modifying the onset potential to lesser negative values. The presence of the magnetic field led to even higher ring currents with 5 % (w/w) CeO2, H2O2 selectivity from 54 % (Vulcan XC-72) to 88–96 % and changing even more the onset potential. Those results were correlated with the Zeeman effect, the Lorentz force, generating magnetohydrodynamic effects, the Kelvin force, and the formation of Bound Magnetic Polarons. This pioneering research introduces an innovative approach, highlighting the potential of an external continuous magnetic field.

N-coordinated Ag sites on ordered mesoporous carbons for highly efficient electrolytic production of H2O2 in acidic electrolyte

N-coordinated Ag (Ag-Nx) sites embedded in ordered mesoporous carbon (Ag-Nx/C) were prepared by pyrolyzation of Ag-2, 2-bipyridine complexes in the confined channels of mesoporous SBA-15 silica. This nanocasting method allowed high dispersion state of Ag-Nx sites on carbons with ordered mesoporous channels and higher specific surface area. Ag-Nx/C was found to be very active electrocatalyst for two-electron oxygen reduction reaction (2e- ORR) in 0.1 M HClO4 acidic electrolyte. The bulk electrolytic production for hydrogen peroxides (H2O2) could be achieved with a high production rate of 1.67 mol h−1 gcat−1 together with high electrolysis Faraday efficiency of about 93%. Ag-Nx/C also revealed better acid-resistant durability during H2O2 electrolysis production. This work provided an efficient and durable Ag-Nx/C electrocatalyst for electrolytic H2O2 production in acids.

Selective oxygen-doping in self-supportive carbon electrode by electrochemical oxidation for electrocatalytic synthesis of hydrogen peroxide

We report the selective introduction of oxygen-containing functional groups for two-electron oxygen reduction reaction (2e-ORR) in self-supportive carbon electrode using electrochemical oxidation method. The optimal electrode shows hydrogen peroxide (H2O2) selectivity up to 95 %. CO is found active to control 2e-ORR selectivity and mechanism of oxygen species variation is elucidated by in situ infrared reflection absorption spectroscopy. We further improve mass transfer by introducing highly interconnected macropores in electrode, which shows H2O2 yield of 144 mmol cm−2 h−1 and Faradaic efficiency of nearly 100 %. These may inspire advanced active electrode creation for applications through selective electrochemical doping and structural design.

Construction of integrated oxygen-rich carbon-based metal-free cathode to simultaneous boost wastewater treatment performance and energy recovery in bio-electro-Fenton system

Fenton coupled microbial fuel cells (MFCs) used for antibiotic pollution purification are limited by energy recovery and metal leakage of electrode. Here, we constructed a two-compartment bio-electro-Fenton (MFC-Fenton) with biological anode and Fenton cathode, which using carbon nanotube based on carbon brushes (OCNT-80/CB) with active centers rich in oxygen-containing functional groups as metal-free cathode. The results show that C = O locked environmental oxygen in the cathode can in-situ use the electrons produced by the anode through two-electron oxygen reduction reaction (2e− ORR) synthesize H2O2. •OH is further generated under activation of H2O2, achieving effective degradation of tetracycline (TC) in the cathode chamber. This study breaks through the limitations of poor pH adaptability and environmental secondary pollution caused by metal dissolution. Moreover, with the advantageous features of oxygen-containing functional groups active centers, good conductivity, and cyclic stability, the OCNT-80/CB achieves a maximum power density increased by 79.4 % as compared to that of the CB control. More notably, Microbial analysis further indicated that OCNT-80/CB electrode could simultaneously enhance Proteobacteria and Firmicutes abundance in bioanode. Dominant bacteria can self-metabolize environmental wastewater generate and recycle green energy. This study proved that careful structural design of cathode could promote the electrode performance. MFC-Fenton can be a competitive technology to enhance wastewater treatment and energy recovery simultaneously.

Preparation of nitrogen-doped carbon pyrolyzed from ZIF-8 and its performance in electrocatalytic oxygen reduction to hydrogen peroxide

As an environment-friendly strategy, electrocatalytic hydrogen peroxide (H2O2) generation via two-electron oxygen reduction reaction (2e– ORR) has attracted significant scientific and technological attention. Nevertheless, limited to the low catalytic activity and high cost of noble metal-containing catalysts, forging desired electrocatalysts with high catalytic activity, low cost, and sustainability is still challenging. In this work, nitrogen doped porous carbon materials named p-ZIF-800 and p-ZIF-950 were obtained by pyrolysis of zeolitic imidazolate framework-8 (ZIF-8) nanocrystals at 800 °C and 950 °C, respectively. Then, they were employed as a catalytic layer in the gas diffusion electrodes p-ZIF-n/Ni/CB (n = 800 °C or 950 °C, CB = acetylene black). The production rates of H2O2 are up to 1.18 mg h−1 cm−2 and 1.25 mg h−1 cm−2 for p-ZIF-800/Ni/CB and p-ZIF-950/Ni/CB, respectively. Noticeably, although p-ZIF-800 had a lower degree of graphitization the difference in catalytic activity between p-ZIF-800 and p-ZIF-950 is negligible. This can be construed as a higher total nitrogen and richer mesoporous structure in p-ZIF-800 than in p-ZIF-950. Under optimized conditions of an electrode spacing of 3 cm, a pH value of 6.5, and a current density of 7.5 mA cm−2, the H2O2 production rate of p-ZIF-800/Ni/CB in the “floating-cathode” cell is up to as high as 2.09 mg h−1 cm−2, which is about 3.3 times higher than that of the cell with the “submerged-cathode” under aeration and stirring. Also, the p-ZIF-800/Ni/CB cathode demonstrates remarkable stability, it maintains its structure without losing catalytic activity after 5 catalytic cycle experiments. These results indicate that the state-of-the-art pyrolysis-based nitrogen-doped porous carbon materials exhibit a high potential for the advanced electrocatalyst of H2O2 electrogeneration.

B–F dual-doped carbon nanotubes for multi-site and high-rate two-electron oxygen reduction reaction electrocatalysis

Electrocatalytic two-electron oxygen reduction reaction (2e−-ORR) to produce hydrogen peroxide (H2O2) is a promising green way to replace the conventional anthraquinone reaction. Non-metallic catalysts are considered as low-cost catalysts due to their excellent stability and versatile surface chemistry. The performance of non-metallic electrocatalysts is usually limited by the limited catalytic active sites on the surface. Here, we report fluorine and boron dual-doped carbon nanotubes (B–F–C) as a metal-free electrocatalyst for H2O2 production, fabricated via a fast microwave treatment. With the synergistic effect of F and B, this strategy can effectively convert carbon and boron atoms on carbon nanotubes into highly active sites. As a result, an H2O2 yield up to 24.3 mol g−1 L−1 is achieved over B–F–C with a stable selectivity of above 90% within 0.2–0.7 V vs. RHE in 0.1 M KOH. Density functional theory (DFT)calculations further confirm that the multiple active sites created by the dual doping of fluorine and boron are the key factor in improving the performance of 2e−-ORR electrocatalysis. Therefore, this work offers a promising prospect for the design and preparation of catalysts with high catalytic activity for H2O2 production and other catalytic areas of industrial significance.

Engineering Co-N-Cr Cross-Interfacial Electron Bridges to Break Activity-Stability Trade-Off for Superdurable Bifunctional Single Atom Oxygen Electrocatalysts.

Atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts have exhibited encouraging oxygen reduction reaction (ORR) activity. Nevertheless, the insufficient long-term stability remains a widespread concern owing to the inevitable 2-electron byproducts, H2O2. Here, we construct Co-N-Cr cross-interfacial electron bridges (CIEBs) via the interfacial electronic coupling between Cr2O3 and Co-N-C, breaking the activity-stability trade-off. The partially occupied Cr 3d-orbitals of Co-N-Cr CIEBs induce the electron rearrangement of CoN4 sites, lowering the Co-OOH* antibonding orbital occupancy and accelerating the adsorption of intermediates. Consequently, the Co-N-Cr CIEBs suppress the two-electron ORR process and approach the apex of Sabatier volcano plot for four-electron pathway simultaneously. As a proof-of-concept, the Co-N-Cr CIEBs is synthesized by the molten salt template method, exhibiting dominant 4-electron selectively and extremely low H2O2 yield confirmed by Damjanovic kinetic analysis. The Co-N-Cr CIEBs demonstrates impressive bifunctional oxygen catalytic activity (▵E=0.70 V) and breakthrough durability including 100 % current retention after 10 h continuous operation and cycling performance over 1500 h for Zn-air battery. The hybrid interfacial configuration and the understanding of the electronic coupling mechanism reported here could shed new light on the design of superdurable M-N-C catalysts.

In Situ Generation of H2O2 over MoOx Decorated on Cu2O@CuO Core-Shell Particle Nanoarchitectonics for Boosting Photocatalytic Oxidative Desulfurization.

Photocatalytic oxidation desulfurization (PODS) has emerged as a promising, ecofriendly alternative to traditional, energy-intensive fuel desulfurization methods. Nevertheless, its progress is still hindered due to the slow sulfide oxidation kinetics in the current catalytic systems. Herein, we present a MoOx decorated on a Cu2O@CuO core-shell catalyst, which enables a new, efficient PODS pathway by in situ generation of hydrogen peroxide (H2O2) with saturated moist air as the oxidant source. The photocatalyst delivers remarkable specific activity in oxidizing dibenzothiophene (DBT), achieving a superior rate of 7.8 mmol g-1 h-1, while maintaining a consistent performance across consecutive reuses. Experimental investigations reveal that H2O2 is produced through the two-electron oxygen reduction reaction (ORR), and both H2O2 and the hydroxyl radicals (•OH) generated from it act as the primary reactive species responsible for sulfide oxidation. Importantly, our catalyst accomplishes complete PODS of real diesel fuel, underscoring an appealing industrial prospect for our photocatalyst.

Boosting excitons dissociation in defective-rich graphitic carbon nitride for efficient hydrogen peroxide photosynthesis and on-site environmental governance

Solar-driven hydrogen peroxide (H2O2) production offers a sustainable strategy to address the ever-growing demand of eco-friendly oxidant and clean fuel, while its efficiency highly relies on the generation of desired reactive oxygen species (ROSs). Herein, we demonstrate that introducing boron-nitrogen vacancy (B-NV) associates in g-C3N4 enables to weaken the robust excitonic effect and promote bound excitons spontaneously dissociating into free charge carriers under ambient conditions. Specifically, the doped B atoms introduced a donor state below the conduction band of g-C3N4, which disturbs charge distribution around heptazine rings and further destabilizes excitons. The concomitant NV, serving as an accepter state locating above the valence band, cooperates with the doped B atoms to form B-NV associates that simultaneously downshifts the conduction band and valence band of g-C3N4. Distinct from the counterpart preferring to activate O2 into 1O2 via an energy-transfer-involved pathway, g-C3N4 featuring with B-NV associates displayed a superior photoactivity of visible-light-driven two-electron oxygen reduction reaction for H2O2 production with a yield of 182 μmol h−1 and selectivity of 100%. The produced H2O2 could effectively degrade organic contaminants and kill typical bacteria. This study highlights the importance of ROSs generation in two-dimensional photocatalysts for sustainable solar-to-chemical conversion and on site environmential governance.

Carbon Catalysts Empowering Sustainable Chemical Synthesis via Electrochemical CO2 Conversion and Two-Electron Oxygen Reduction Reaction.

Carbon materials hold significant promise in electrocatalysis, particularly in electrochemical CO2 reduction reaction (eCO2 RR) and two-electron oxygen reduction reaction (2e- ORR). The pivotal factor in achieving exceptional overall catalytic performance in carbon catalysts is the strategic design of specific active sites and nanostructures. This work presents a comprehensive overview of recent developments in carbon electrocatalysts for eCO2 RR and 2e- ORR. The creation of active sites through single/dual heteroatom doping, functional group decoration, topological defect, and micro-nano structuring, along with their synergistic effects, is thoroughly examined. Elaboration on the catalytic mechanisms and structure-activity relationships of these active sites is provided. In addition to directly serving as electrocatalysts, this review explores the role of carbon matrix as a support in finely adjusting the reactivity of single-atom molecular catalysts. Finally, the work addresses the challenges and prospects associated with designing and fabricating carbon electrocatalysts, providing valuable insights into the future trajectory of this dynamic field.

Mesoporous carbon spheres with programmable interiors as efficient nanoreactors for H2O2 electrosynthesis

The nanoreactor holds great promise as it emulates the natural processes of living organisms to facilitate chemical reactions, offering immense potential in catalytic energy conversion owing to its unique structural functionality. Here, we propose the utilization of precisely engineered carbon spheres as building blocks, integrating micromechanics and controllable synthesis to explore their catalytic functionalities in two-electron oxygen reduction reactions. After conducting rigorous experiments and simulations, we present compelling evidence for the enhanced mass transfer and microenvironment modulation effects offered by these mesoporous hollow carbon spheres, particularly when possessing a suitably sized hollow architecture. Impressively, the pivotal achievement lies in the successful screening of a potent, selective, and durable two-electron oxygen reduction reaction catalyst for the direct synthesis of medical-grade hydrogen peroxide disinfectant. Serving as an exemplary demonstration of nanoreactor engineering in catalyst screening, this work highlights the immense potential of various well-designed carbon-based nanoreactors in extensive applications.

Recent advances in the electrochemical production of hydrogen peroxide

Hydrogen peroxide (H2O2) is an innovative and environmentally friendly oxidant that finds wide-ranging applications across multiple industries. In the past, H2O2 production predominantly relied on the anthraquinone method, which had drawbacks such as the generation of organic waste and the requirement for energy-intensive reactions. A cheap, efficient, and sustainable way of producing H2O2 may be achieved through the redox reaction between oxygen and water. On both small and large scales, the electrosynthesis of H2O2 is practical and affordable. In recent years, it has been thought that the energy-intensive anthraquinone process may be replaced by the electrochemical synthesis of H2O2 via the two-electron oxygen reduction reaction (ORR) route. To eliminate the organic pollutants found in drinking water and industrial effluent, highly effective hydrogen peroxide (H2O2) must be produced electrochemically using gas diffusion electrodes (GDEs). Compared to other carbonaceous cathodes, the GDEs as cathodic electrocatalysts demonstrate greater cost-effectiveness, lower energy consumption, and higher oxygen utilization efficiency for the formation of H2O2. A promising alternative for enabling the growth of sustainable economics in the W&W sector is microbial electrochemical systems (MESs) that create H2O2. To enhance the efficiency and predictability of H2O2 production in MESs, a machine-learning approach was adopted, incorporating a meta-learning methodology to forecast the generation rate of H2O2 in MES based on the seven input variables, comprising several design and operational parameters.

Recent progress in two-dimensional materials for generation of hydrogen peroxide by two-electron oxygen reduction reaction

Hydrogen peroxide (H2O2) is an important green oxidant with a wide range of applications in wastewater treatment, pulp bleaching and chemical synthesis. In recent years, two-electron oxygen reduction reaction (2e-ORR) is expected to be a greener and safer alternative to the more energy-intensive and also carcinogenic anthraquinone process. Among the various electrocatalysts for 2e-ORR, two-dimensional nanomaterials have special advantages in the electrosynthesis of H2O2 due to their large specific surface area, abundant basal and edge sites, and tunable atomic structure. This review article summarizes the synthesis methods, and the strategies and mechanisms for enhancing 2e-ORR properties of 2D materials. Practical problems and serious challenges faced by 2D nanomaterials in the synthesis and 2e-ORR applications are elaborated. The aim of this review is to provide researchers with an understanding of 2D nanomaterials in electrocatalytic applications and to promote the further development of novel 2D nanoelectrocatalytic materials.

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

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