Two-electron Oxygen Reduction Research Articles

Enhanced two-electron oxygen reduction for hydrogen peroxide production via fine-tuning the concentration of oxygen vacancies in MoO3−x

Producing hydrogen peroxide (H2O2) electrochemically is a possible alternative to industrial anthraquinone synthesis. Herein, α-MoO3−x catalysts with controllable oxygen vacancies for two-electron Oxygen Reduction (2e− ORR) electrocatalysis have been reported for the first time. Because of the finely tuned oxygen vacancy concentration of α-MoO3−x-400, it is particularly sensitive to the production of H2O2, providing very high selectivity in H2O2 production reaches as high as 93% with an average yield of 310 mmol gcat−1 h−1 over 12 h. In addition to providing us with the most abundant and attractive transition metal oxide (TMO) catalysts yet, this research provides a fresh approach to the rational development of effective TMO catalysts.

Carbon spheres modified titanium air diffusion cathode for boosting H2O2 and application in disinfection

In situ electrochemical disinfection via H2O2 through two-electron molecular oxygen reduction reaction (2e-ORR) represents a promising strategy for sustainable disinfection. However, its disinfection performance is largely confined by its low selectivity/reactivity towards two-electron oxygen reduction reaction (2e-ORR) for H2O2 production. To tackle this, carbon spheres modified tubular titanium air diffusion cathode (CS@TTC) was developed. It intended to improve interfacial structures for high H2O2 production through a facile hydrothermal carbonation approach. Evolution of morphology and chemical nature of carbon spheres have been systemically investigated under diverse hydrothermal temperature, hydrothermal time and pyrolysis temperature·H2O2 accumulation has improved 6.59-fold at CS@TTC compared to the raw base TTC. Both chemical nature of carbon sphere and a superior oxygen mass transfer (0.0284 s−1) were responsible for this enhancement. Besides, CS@TTC cathode was used in the electrochemical system for disinfection performance, in which 6.42 log of E. coli was deactivated after 120 min. Hence, this study gives an insight into that the H2O2-based electrochemical disinfection is promising to be a next-generation process.

Two–dimensional metal–organic framework nanosheets: An efficient two–electron oxygen reduction reaction electrocatalyst for boosting cathodic luminol electrochemiluminescence

Exploration of electrocatalytic–luminol systems is still in the developmental stages because of the weak electrochemiluminescence (ECL) of cathodic luminol, and it is still a challenge to establish a conformational relationship between electrocatalytic materials and ECL performance. Herein, functionalized two–dimensional NiZn metal–organic framework (MOF) nanosheets were synthesized using controlled heat treatment and were successfully applied to the two-electron (2e−) oxygen reduction reaction (ORR) to produce H2O2. Interestingly, it was found that both the intermediates and products of the ORR significantly affected the ECL emission of luminol. It is worth noting that extending NiM MOF (M = Fe, Co, and Zn) to 2e− ORR reaction and achieving efficient luminol emission is undoubtedly beneficial for constructing cathodic luminol systems. This work demonstrates the relationship between the electrocatalytic materials and luminol ECL performance, thereby, broadening the applications of ECL reactions. Finally, the proposed biosensor was successfully used for the sensitive detection of alpha-fetoprotein (AFP).

Phosphorous and selenium tuning Co-based non-precious catalysts for electrosynthesis of H2O2 in acidic media

Electrosynthesis of hydrogen peroxide (H2O2) is an on-site method that enables independent distribution applications in many fields due to its small-scale and sustainable features. The crucial point remains developing highly active, selective and cost-effective electrocatalysts. The electrosynthesis of H2O2 in acidic media is more practical owing to its stability and no need for further purification. We herein report a phosphorus and selenium tuning Co-based non-precious catalyst (CoPSe) toward two-electron oxygen reduction reaction (2e– ORR) to produce H2O2 in acidic media. The starting point of using both P and Se is finding a balance between strong ORR activity of CoSe and weak activity of CoP. The results demonstrated that the CoPSe catalyst exhibited the optimized 2e– ORR activity compared with CoP and CoSe. It disclosed an onset potential of 0.68 V and the H2O2 selectivity 76%-85% in a wide potential range (0–0.5 V). Notably, the CoPSe catalyst overcomes a significant challenge of a narrow-range selectivity for transition-metal based 2e– ORR catalysts. Finally, combining with electro-Fenton reaction, an on-site system was constructed for efficient degradation of organic pollutants. This work provides a promising non-precious Co-based electrocatalyst for the electrosynthesis of H2O2 in acidic media.

Spectroscopic Identification of Active Sites of Oxygen-Doped Carbon for Selective Oxygen Reduction to Hydrogen Peroxide.

The electrochemical synthesis of hydrogen peroxide (H2 O2 ) via a two-electron (2 e- ) oxygen reduction reaction (ORR) process provides a promising alternative to replace the energy-intensive anthraquinone process. Herein, we develop a facile template-protected strategy to synthesize a highly active quinone-rich porous carbon catalyst for H2 O2 electrochemical production. The optimized PCC900 material exhibits remarkable activity and selectivity, of which the onset potential reaches 0.83 V vs. reversible hydrogen electrode in 0.1 M KOH and the H2 O2 selectivity is over 95 % in a wide potential range. Comprehensive synchrotron-based near-edge X-ray absorption fine structure (NEXAFS) spectroscopy combined with electrocatalytic characterizations reveals the positive correlation between quinone content and 2 e- ORR performance. The effectiveness of chair-form quinone groups as the most efficient active sites is highlighted by the molecule-mimic strategy and theoretical analysis.

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

Electrochemical two-electron oxygen reduction reaction (2 e- ORR) to produce hydrogen peroxide (H2 O2 ) is a promising alternative to the energetically intensive anthraquinone process. However, there remain challenges in designing 2 e- ORR catalysts that meet the application criteria. Here, we successfully adopt a microwave-assisted mechanochemical-thermal approach to synthesize hexagonal phase SnO2 (h-SnO2 ) nanoribbons with largely exposed edge structures. In 0.1 M Na2 SO4 electrolyte, the h-SnO2 catalysts achieve the excellent H2 O2 selectivity of 99.99 %. Moreover, when employed as the catalyst in flow cell devices, they exhibit a high yield of 3885.26 mmol g-1  h-1 . The enhanced catalytic performance is attributed to the special crystal structure and morphology, resulting in abundantly exposed edge active sites to convert O2 to H2 O2 , which is confirmed by density functional theory calculations.

Efficient electrocatalytic H2O2 production in simulated seawater on ZnO/reduced graphene oxide nanocomposite

The non-noble metal-based catalysts has been widely studied for two-electron (2e-) oxygen reduction reaction (ORR), and exhibited the excellent 2e- ORR performance in alkaline condition, especially. However, these catalysts are rarely studied at neutral chloride medium. Herein, we synthesized metal oxide nanoparticles supported reduced graphene oxide composites (MxOy/rGO) by adding the precursors of metal salts (Zn, Mn, Co and Fe) and graphene oxide (GO) through a facile lyophilization-pyrolysis strategy. ZnO/rGO exhibits a well 2e- ORR catalytic activity, corresponding to a high 80% H2O2 selectivity and preferable stability in 0.5 M NaCl solution, whereas the MxOy/rGO (M= Mn, Co and Fe) presents even lower selectivity than reduced graphene oxide (rGO). The results suggest that ZnO promotes the 2e- ORR, and the metal oxide (M= Mn, Co and Fe) conducive to 4e- ORR. Interesting finding, the results of rotating ring disk electrode (RRDE) show that chloride ion in the electrolyte promote the H2O2 selectivity of 2e- ORR. These findings provide a promising candidate material for 2e- ORR in neutral chloride environment and valuable insights for future research on seawater-based H2O2 electrosynthesis.

Oxygen Vacancy Boosts the V2O5 Performance for the Electrochemical H2O2 Product

On-site electrochemical hydrogen peroxide (H2O2) generation via the two-electron oxygen reduction reaction (2e– ORR), as a cost-effective alternative to the anthraquinone process, relies on the exploitation of excellent activity and the stability catalyst. In this study, we reveal that by modulating the oxygen vacancy (Ov) on V2O5 with the H2 treatment, high activity and a selective oxygen reduction for H2O2 production can be obtained with the basic electrolyte, showing a 92.32% selectivity within a wide range of potential. Combined spectroscopic results demonstrate that the enhanced performance of the 2e– ORR originates from the introduction of the Ov, which adjusts the electron structure of V2O5. The DFT analysis reveals that the Ov improves the adsorption of the oxygen on V2O5 and optimizes the oxygen reaction path superior to H2O2. Moreover, the V2O5-300 catalyst presented an excellent performance in the electrodegradation of antibiotics, indicating the application potential for environmental protection. This work provides an insight into the design of the metal oxides for effective H2O2 generation.

Identifying the roles of oxygen-containing functional groups in carbon materials for electrochemical synthesis of H2O2

Electrochemical H2O2 synthesis via a two-electron oxygen reduction reaction (2e− ORR) has emerged as a promising technology. Carbon-based catalysts with oxygen-containing functional groups exhibit superior performance. However, this process has not been examined experimentally in sufficient detail, and the active oxygen-containing components for 2e− ORR remain to be properly identified. In this study, the aromatic molecule-modified CNTs with target single oxygen groups were synthesized and used as catalysts to investigate the explicit role of each oxygen group in the 2e− ORR process. Near-edge X-ray absorption fine structure and cyclic voltammetry demonstrated the existence of strong interactions between the model molecules and CNTs. Electrochemical tests and DFT calculations revealed that the –CO group was the most active component in 2e− ORR followed by the –COOH group. The –CO group with the zigzag and armchair configurations have no apparent influence on the 2e− ORR performance. This work provides new insights into the nature of active sites on the oxygen-doped carbon catalysts for 2e− ORR.

Promotion of the Efficient Electrocatalytic Production of H2O2 by N,O- Co-Doped Porous Carbon

H2O2 generation via an electrochemical two-electron oxygen reduction (2e- ORR) is a potential candidate to replace the industrial anthraquinone process. In this study, porous carbon catalysts co-doped by nitrogen and oxygen are successfully synthesized by the pyrolysis and oxidation of a ZIF-67 precursor. The catalyst exhibits a selectivity of ~83.1% for 2e- ORR, with the electron-transferring number approaching 2.33, and generation rate of 2909.79 mmol g-1 h-1 at 0.36 V (vs. RHE) in KOH solution (0.1 M). The results prove that graphitic N and -COOH functional groups act as the catalytic centers for this reaction, and the two functional groups work together to greatly enhance the performance of 2e- ORR. In addition, the introduction of the -COOH functional group increases the hydrophilicity and the zeta potential of the carbon materials, which also promotes the 2e- ORR. The study provides a new understanding of the production of H2O2 by electrocatalytic oxygen reduction with MOF-derived carbon catalysts.

Efficient photosynthesis of H2O2 via two-electron oxygen reduction reaction by defective g-C3N4 with terminal cyano groups and nitrogen vacancies

The development of photocatalysts for the efficient generation of hydrogen peroxide (H2O2) and degradation of antibiotic pollutants is a mutual-beneficial solution to the chemical resource demands and environmental remediation. Herein, efficient visible-light-driven H2O2 production and antibiotic degradation are achieved by using nitrogen-deficient modified polymeric carbon nitride (g-C3N4) to facilitate the reduction of dioxygen. The N defects modified g-C3N4 reached an excellent H2O2 generation rate of 623.5 μmol g−1h−1 without O2 bubbling. The structural features and entire photocatalytic conversion process were systematically studied via experiments and theoretical calculations. It was found that the N defects in the framework constructed a giant internal electric field (IEF), which effectively inhibited carrier recombination. Revealed the neglected H2O2 production via 1O2 pathway, which effectively promoting the selective 2e− oxygen reduction reaction (ORR). More interestingly, the N defects structure greatly improved the O2 adsorption ability and O2 production ability of the material, while inhibiting the in situ generated H2O2 from being decomposed and maintaining high efficiency of H2O2 production under air conditions. This study thoroughly revealed the synergistic effects of the terminal cyano group and nitrogen vacancies over g-C3N4 for efficient H2O2 photogeneration photosynthesis.

In situ graphitization of small organic molecules on the surface of nano titanium dioxide to construct highly efficient photosynthetic hydrogen peroxide composite catalyst

Hydrogen peroxide (H2O2) is a good strategic choice for developing sustainable energy. In recent years, although many researchers have focused on photocatalytic preparation of H2O2, the progress is still limited for the problems of low efficiency of H2O2 production, complex catalyst preparation, and high cost of catalyst. Here, a novel method to construct a hybrid photocatalyst was developed with one-step graphitization of specific organic small molecules, in which C, N, and O could be introduced simultaneously along with the formation of graphite-like structures within the inner pores of the nano-TiO2 surface. It not only changed the surface defects of TiO2 to successfully reduce its promotion on the photodecomposition of H2O2 but also adjusted the band gap of the catalyst and broadened its visible light absorption range. Besides, it also provided another two-electron oxygen reduction reaction pathway to enhance the catalytic activity of the photosynthesis of H2O2; thus, 11.78 mM/h H2O2, which exceeds all previously reported productivities, was produced under simulated sunlight. The modified catalyst not only has excellent ultraviolet light activity with an apparent quantum efficiency of 34.9% at 365 ± 5 nm but also has very good visible light activity with apparent quantum efficiency of 17.8% at 420 ± 5 nm and more than 1.66% solar-to-chemical conversion (>420 nm), which is very rare for nano-TiO2 and much promising for application. Our present research has proved that the organic molecule tetraethyl 4,4'-(1,4-phenylene)bis(2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxylate) successfully changed the surface defect modes of TiO2, making the oxygen defect that promotes the decomposition of H2O2 change to the C defect that is conducive to the photocatalytic generation of H2O2, and thus, the photocatalytic performance has been greatly improved. Thus, it is an important instruction for developing highly efficient and low-cost photocatalysts for synthesizing H2O2.

Rational design of heterogenized molecular phthalocyanine hybrid single-atom electrocatalyst towards two-electron oxygen reduction

Single-atom catalysts supported on solid substrates have inspired extensive interest, but the rational design of high-efficiency single-atom catalysts is still plagued by ambiguous structure determination of active sites and its local support effect. Here, we report hybrid single-atom catalysts by an axial coordination linkage of molecular cobalt phthalocyanine with carbon nanotubes for selective oxygen reduction reaction by screening from a series of metal phthalocyanines via preferential density-functional theory calculations. Different from conventional heterogeneous single-atom catalysts, the hybrid single-atom catalysts are proven to facilitate rational screening of target catalysts as well as understanding of its underlying oxygen reduction reaction mechanism due to its well-defined active site structure and clear coordination linkage in the hybrid single-atom catalysts. Consequently, the optimized Co hybrid single-atom catalysts exhibit improved 2e− oxygen reduction reaction performance compared to the corresponding homogeneous molecular catalyst in terms of activity and selectivity. When prepared as an air cathode in an air-breathing flow cell device, the optimized hybrid catalysts enable the oxygen reduction reaction at 300 mA cm−2 exhibiting a stable Faradaic efficiency exceeding 90% for 25 h.

Enhanced photo-electro-Fenton degradation performance using graphene structure-oriented MOFs (2Fe/Co)/CNF cathode membrane

Difficulties in mineralizing organic dyes and antibiotics have been bottlenecks limiting wastewater treatment technology development. Electrocatalytic advanced oxidation through selective two-electron oxygen reduction on cathodes is a promising method of mineralizing organic wastewater. However, most developed cathodes cannot efficiently synthesize H2O2 in situ via 2e− pathways and their durability cannot be guaranteed due to Fe2+ leaching. Therefore, the electrospun cathode (MOFs (2Fe/Co)/CNF) with both photocatalytic and electrocatalytic functions was ingeniously fabricated for efficient degradation of dyes and antibiotics through graphene template effect, carbonization and in-situ growing strategies. On the cathode surface, H2O2 was continuously generated in situ via 2e− reduction reactions, activating Fenton-like reactions. In addition, the developed cathode (MOFs (2Fe/Co)/CNF) had a low band gap (1.32 eV), which enhanced visible light response and improved catalytic active sites of the cathode. Degradation experiments of methylene blue and erythromycin solution indicate that the developed cathode showed excellent synergistic photoelectrocatalysis performance. In a photo-electro-Fenton system (PEF), the mineralization rate of methylene blue and erythromycin reached 86.5% and 90.2% within 10 and 30 min, respectively. The cathode in this research is expected to have great potential to efficiently mineralize wastewater via PEF.

Highly efficient heterogeneous electro-Fenton degradation of organic pollutants using a FeNi-OH/NF cathode

The electro-Fenton (EF) technology provided a promising way to degrade organic pollutants by in-situ producing H2O2 and decomposing it into hydroxyl radicals (OH). However, the degradation rate is restricted by the two-electron oxygen reduction reaction (ORR) rate of H2O2 generation and the production efficiency of OH, and the efficient EF cathode is important. Herein, we designed an efficient heterogeneous EF system with a Fe-doped β-Ni(OH)2 nanosheets/nickel foam (FeNi-OH/NF) cathode for phenol removal in water. FeNi-OH enhanced the electrochemically active area for H2O2 production and produced OH for pollutant degradation. The FeNi-OH/NF cathode demonstrates excellent heterogeneous EF catalytic activity with a degradation efficiency of phenol (99.4%) and total organic carbon (67.4%) in 2 h. The experiments and density functional theory (DFT) calculations further confirmed that the Fe-doped β-Ni(OH)2 on the nickel foam enhanced H2O2 adsorption and accelerated Fe3+/Fe2+ circulation, promoting the reaction of H2O2 with Fe2+ to produce OH for phenol degradation. This study provides a novel and efficient heterogeneous EF cathode for accelerating pollutant degradation securely and quickly.

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

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

Crystal Engineering Enables Cobalt-Based Metal-Organic Frameworks as High-Performance Electrocatalysts for H2O2 Production.

Metal-organic frameworks (MOFs) with highly adjustable structures are an emerging family of electrocatalysts in two-electron oxygen reduction reaction (2e-ORR) for H2O2 production. However, the development of MOF-based 2e-ORR catalysts with high H2O2 selectivity and production rate remains challenging. Herein, an elaborate design with fine control over MOFs at both atomic and nano-scale is demonstrated, enabling the well-known Zn/Co bimetallic zeolite imidazole frameworks (ZnCo-ZIFs) as excellent 2e-ORR electrocatalysts. Experimental results combined with density functional theory simulation have shown that the atomic level control can regulate the role of water molecules participating in the ORR process, and the morphology control over desired facet exposure adjusts the coordination unsaturation degree of active sites. The structural regulation at two length scales leads to synchronous control over both the kinetics and thermodynamics for ORR on bimetallic ZIF catalysts. The optimized ZnCo-ZIF with a Zn/Co molar ratio of 9/1 and predominant {001} facet exposure exhibits a high 2e- selectivity of ∼100% and a H2O2 yield of 4.35 mol gcat-1 h-1. The findings pave a new avenue toward the development of multivariate MOFs as advanced 2e-ORR electrocatalysts.

Investigation on the reaction kinetic mechanism of polydopamine-loaded copper as dual-functional catalyst in heterogeneous electro-Fenton process

Heterogeneous electro-Fenton (HEF) process has been regarded as a promising method in environmental remediation. However, the reaction kinetic mechanism of the HEF catalyst for simultaneous production and activation of H2O2 remained confounded. Herein, the copper supported on polydopamine (Cu/C) was synthesized by a facile method and employed as a bifunctional HEFcatalyst, and the catalytic kinetic pathways were deeply investigated by using rotating ring-disk electrode (RRDE) voltammetry based on the Damjanovic model. Experimental results substantiated that a two-electron oxygen reduction reaction (2e− ORR) and a sequential Fenton oxidation reaction were proceeded on 1.0-Cu/C, where metallic copper played a crucial role in the fabrication of 2e− active sites as well as utmost H2O2 activation to produce highly reactive oxygen species (ROS), resulting in the high H2O2 productivity (52.2%) and the almost complete removal of contaminant ciprofloxacin (CIP) after 90 min. The work not only expanded the idea of reaction mechanism on Cu-based catalyst in HEF process but also provided a promising catalyst for pollutants degradation in wastewater treatment.

CO2-derived edge-boron-doped hierarchical porous carbon catalysts for highly effective electrochemical H2O2 production

Electrochemical hydrogen peroxide (H2O2) synthesis via the two-electron oxygen reduction reaction (2e- ORR) is considered a promising alternative to the anthraquinone process due to its eco-friendliness and on-site production. Recently, although B-doped carbon (BC) has been suggested as a promising 2e- ORR catalyst, the question of whether BC can further improve catalytic activity by tuning the doping configuration and site still remains unanswered. This work demonstrates CO2-derived edge-B-doped porous carbon (E-BPC) for highly effective electrochemical H2O2 production. Herein, it is revealed that the oxygenated B-doping configurations (BCO2 and BC2O) at edge sites are responsible for enhanced 2e- ORR activity and stability. Outstanding mass activity (54.7 A g−1 at 0.65 V vs. RHE) is demonstrated with the highest high production rate in a flow reactor among the reported studies, of 24.3 mol gcat−1 h−1. The faradaic efficiency of the E-BPC was maintained (∼82%) for over 100 h without performance degradation.

Ultrahigh Photocatalytic Oxygen Reduction to Hydrogen Peroxide by a True Oxygen-Bridged Heptazine Polymer

Solar-H2O2 production presents an eco-friendly and sustainable strategy over the existing anthraquinone process. The present photocatalytic systems suffer from low two-electron oxygen reduction reaction (ORR) selectivity. The current study reports an easy approach to produce a true oxygen-bridged heptazine polymer with periodic arrangement of oxygen atoms (CNO) by introducing C–O–C bridging species through the right precursor selection. The C–O–C species in CNO dramatically promotes ORR compared to the benchmark carbon nitride synthesized from melamine (CNM), which has been proved by electrochemical analysis. To our surprise, the CNO has produced 1.5 mmol/g of H2O2 after 4 h in natural sunlight, which is the highest among the existing carbon-nitride-based photocatalysts. In addition, the challenging biomass-derived chemicals, glycerol, glucose, and 1,2-propanediol were also utilized as sacrificial electron donors in H2O2 production to synthesize fine chemicals. Overall, this work provides an easy approach to tune the two-electron ORR to boost H2O2 production and paves the way for practical solar-H2O2 production from biomass using polymeric O-bridged heptazines (CNO).