Two-electron Oxygen Reduction Research Articles

O, N Coordination-Mediated Nickel Single-Atom Catalysts for High-Efficiency Generation of H2O2.

Electrochemical selective two-electron oxygen reduction shows great potential for on-site electrochemical production of hydrogen peroxide (H2O2). Herein, we demonstrated the synthesis of Ni single-atom sites coordinated by three oxygen atoms and one nitrogen atom (i.e., Ni-N1O3) supported by oxidized carbon black (OCB) by pyrolyzing nickel-(pyridine-2,5-dicarboxylate) coordination complexes. Aberration-corrected scanning transmission electron microscopy (AC-STEM) combined with X-ray absorption spectroscopy (XAS) proves the presence of atomically dispersed Ni atoms attached on OCB (labeled as Ni-SACs@OCB), in which Ni single atoms are stabilized by a N, O-mediated coordination configuration. This Ni-SACs@OCB catalyst shows high H2O2 selectivity (95%) in a range of 0.2-0.7 V undergoing a two-electron oxygen reduction process, with a kinetic current density of 2.8 mA cm-2 and a mass activity of 24 A gcat.-1 at 0.65 V (vs RHE). In practice, H-cells with Ni-SACs@OCB as catalysts displayed a high H2O2 production rate of 98.5 mmol gcat.-1 h-1 with negligible current loss during testing, suggesting the high H2O2 generation efficiency and robust stability. DFT theoretical calculations revealed that Ni single-atom sites coordinated by O, N coordination exhibit advantages in oxygen adsorption and increased reactivity toward the intermediate species, *OOH, which is beneficial to high selectivity for H2O2 production. This work provides a novel N, O-mediated four-coordinate nickel single-atom catalyst as a promising candidate for practical decentralized production of H2O2.

Preparation and Photocatalytic CO Oxidation Performance Study of Au/Oxygen-Deficient (Anatase/B-Phase) TiO2 Heterojunction Microspheres

Multi-heterojunctions are more promising than single heterojunctions in photocatalysis due to the availability of more interfaces between each component. However, photocatalytic activity is highly dependent on the contact mode of individual components. In this work, we assembled gold nanoparticles/oxygen-deficient (anatase/B-phase) TiO2 multi-heterojunction microspheres using spray pyrolysis and focused on their contact mode-governed photocatalytic activity. The results reveal that using oxygen-deficient (anatase/B-phase) TiO2 microspheres as building blocks could significantly enhance the absorption of visible light and the photocatalytic activity of a gold–TiO2 system toward the photocatalytic carbon monoxide oxidation. Furthermore, loading gold nanoparticles onto B-phase TiO2 could facilitate a more prominent enhancement of activity than that of pure B-phase TiO2, benefiting from the two-electron reduction of oxygen at the interface of TiO2/Au. Meanwhile, the high crystallinity of B-phase TiO2 microspheres allows for a gold loading amount of 1 wt% in the gold/(anatase/B-phase) TiO2 system, which is 1.67 times more active than pure B-phase TiO2 microspheres, in the photocatalytic oxidation of carbon monoxide to generate carbon dioxide.

Photoelectrochemical Production of Hydrogen Peroxide with TiO2 and 1,5-Di(thien-2-yl)-9,10-anthraquinone via a Selective Two-Electron Oxygen Reduction Reaction in the Two-Compartment Cell

The industrial process of hydrogen peroxide (H2O2) synthesis proceeds through the hydrogenation of anthraquinone followed by successive oxidation in an organic solvent. This process is a multistep synthesis that is far away from green synthesis. An alternative photoelectrochemical (PEC) H2O2 production method requires only sunlight and water. Herein, we established the direct production of H2O2 by using a two-compartment PEC cell composed of TiO2 nanorods hydrothermally grown on fluorine-doped tin oxide as a photoanode for water oxidation and 1,5-di(thien-2-yl)-9,10-anthraquinone (AQTh) anchored carbon cloth as a cathode for the selective two-electron (2e–) reduction of oxygen. This photoanode and cathode are positioned in an anodic compartment and a cathodic compartment of a two-compartment cell, respectively. The Nafion membrane (N-117) separates these two compartments. This PEC cell continuously produces H2O2 at −0.1 VRHE applied voltage, which reaches 60 μM after 3 h. The density functional theory study is carried out to better understand the H2O2 production mechanism and the role of thiophene substituents on anthraquinone.

MOF-Conductive Polymer Composite Film as Electrocatalyst for Oxygen Reduction in Acidic Media

A metal-organic framework (MOF)-conductive polymer composite film was constructed from PCN-222(Fe) nanoparticles and PEDOT:PSS solution by simple drop-casting approach. The composite film was tested as an electrocatalytic device for oxygen reduction reaction (ORR). The combination of PCN-222(Fe) MOF particles and conductive PEDOT matrix facilitates electron transfer in the composite material and improves the ORR performance of PCN-222(Fe). Levich plot and H2O2 quantification experiment show that PCN-222(Fe)/PEDOT:PSS film mainly catalyzes two-electron oxygen reduction and produces H2O2.

Improved photocatalytic H2O2 production of boron doped g-C3N4 by controllably co-modifying nanosized PDI and Ag

Herein, boron-doped g-C3N4 (BCN) is first modified with perylene diimide (PDI) nanosheets driven by the π−π interactions, and subsequently decorated with nanosized Ag through the photodeposition method. The optimal PDI/BCN-Ag photocatalyst achieves a H2O2 production rate of 143 μmol g−1 h−1 without any sacrificial agents under visible light irradiation, which is about13-fold improvement compared with pristine g-C3N4. The boosted photoactivity is mainly ascribe to the enhanced charge separation due to the cascade S-scheme charge transfer between the coupled PDI and BCN, and the promoted oxygen activation from the introduced nanosized Ag by means of the time-resolved surface photovoltage responses and fluorescence spectra, kelvin probe measurement analyses, O2 temperature-programmed desorption and electron paramagnetic resonance spectroscopy. Notably, there is a dual channel for H2O2 production on PDI/BCN-Ag. The two-electron oxygen reduction dominated by the produced •O2− radicals occurs on the Ag nanoparticles, whereas the two-electron water oxidation occurs on the PDI.

Combination of H2O2-producing microbial desalination cells and UV/H2O2 advanced oxidation process: Water salinity reduction and microbial inactivation

Microbial desalination cells (MDCs) are bioelectrochemical devices that can recover usable energy and chemicals from organic wastes and concurrently desalinate saltwater. MDCs can also produce the green oxidant, H2O2, through a two-electron oxygen reduction reaction occurring on the cathode electrode. If MDCs producing H2O2 are used in the pretreatment of brackish water or seawater for reverse osmosis (RO) processes, the salinity of feed water can be reduced in MDCs and the MDC-produced H2O2 can be utilized onsite to oxidize organic contaminants or disinfect microorganisms in the feed water. This strategy of using MDC pretreatment may substantially alleviate the energy requirements and biofouling tendency of downstream RO units. Herein, we assessed the applicability of a combination of H2O2-producing MDCs and ultraviolet (UV) or UV254nn -based advanced oxidation process (AOP) (i.e., UV/H2O2) as pretreatment for RO processes. We first examined the effect of catholyte conditions, which is one of the most crucial factors directly affecting the H2O2 composition and decomposition, on the performance of MDCs. Then, we evaluated the microorganism disinfection efficiencies of advanced photo-oxidation processes using H2O2 obtained from MDCs. The MDC investigated herein achieved 93.1% desalination efficiency and > 5.0 mM H2O2 concentration. In addition, UV-based AOP with the produced H2O2 demonstrated up to 15-fold higher inactivation kinetics than that by UV254 nm disinfection alone.

Hydrogen Peroxide Production with High Solar to Chemical Conversion (0.16%) and Efficient Water Purification Using Carbon Quantum Dots-Modified Indium Sulfide

Photocatalytic production of hydrogen peroxide (H2O2) is the need of the hour for green synthesis and sustainability. Herein, we designed hierarchical carbon quantum dots (CQDs)-supported β-indium sulfide (β-In2S3) nanoflakes as an efficient catalyst for H2O2 generation via a two-electron oxygen reduction reaction. The composite material exhibited a high H2O2 production rate of 634 μM h–1, a solar-to-chemical conversion (SCC) efficiency of 0.16%, a high forward rate constant of ∼19 μM min–1, and a low H2O2 dissociation constant of ∼0.004 min–1, all of which are significantly higher than those of pure β-In2S3. Moreover, this material also manifested good water remediation photocatalytic performance by way of removal of 93% chromium(VI), 58% nickel(II), and 94% rhodamine B in 2 h. The apparent rate constant for chromium(VI) reduction of the composite was 0.023 min–1, which is 10-fold higher than that of the pure material. β-In2S3-CQDs exhibited superior results, which can be attributed to their large electrochemical surface area, good light harnessing ability, upconverted photoluminescence, and the ability to act as an electron reservoir preventing recombination.

A Pentagonal Defect-Rich Metal-Free Carbon Electrocatalyst for Boosting Acidic O2 Reduction to H2O2 Production

Metal-free carbon-based materials are considered one of the most promising two-electron oxygen reduction reaction (2e- ORR) electrocatalysts for the green synthesis of hydrogen peroxide (H2O2). However, most reported carbon electrocatalysts perform much more effectively in alkalis than in acids. Herein, by creatively using fullerene (C60) as the precursor subject to ammonia treatment, we designed and synthesized a pentagonal defect-rich nitrogen-doped carbon nanomaterial (PD/N-C). It achieves outstanding ORR activity, 2e- selectivity, and stability in acidic electrolytes, surpassing the benchmark PtHg4 alloy catalyst. Impressively, the flow cell based on the PD/N-C catalyst achieves nearly 100% Faraday efficiency with a remarkable H2O2 yield, representing the best improvement among all the metal-free catalysts. Experimental and theoretical results reveal that such superb 2e- ORR performance of PD/N-C originates from the synergism between pentagonal defects and nitrogen dopants. This work presents an effective strategy for the design and construction of highly efficient acid-resistant carbon electrocatalysts for H2O2 production and beyond.

Machine learning screening of high-performance single-atom electrocatalysts for two-electron oxygen reduction reaction

Electrocatalysis has emerged as one of the most promising alternatives to conventional anthraquinone for preparing hydrogen peroxide (H2O2) with high energy consumption and pollution because of its simplicity, convenience, and environmental friendliness. However, the oxygen reduction reaction (ORR) generating H2O2viathe2e- path is acompetitive path for 4e–ORR to generate H2O. Therefore, it is crucial to identify an electrocatalyst with high selectivity and activity of 2e-ORR. Here, we established five machine learning (ML) models based on the adsorption free energy of O* (△G (O*)) of 149 single-atom catalysts (SACs) collected and the limiting potential (UL) of 31 SACs calculated using density functional theory (DFT) from the literature. We then obtained descriptors that could accurately describe SACs. Furthermore, 690 unknown SACs’ 2e-ORR catalytic performance was well predicted. Four 2e-ORR materials with high selectivity and activity were screened: Zn@Pc-N3C1, Au@Pd-N4, Au@Pd-N1C3, and Au@Py-N3C1. We verified the UL of these SACs through DFT calculation, which was higher than the standard value, proving the ML model’s validity. The ML-based method to predict the material properties with highly selective and active electrocatalysts provides an efficient, rapid, and low-cost method for discovering and designing more valuable SACs catalysts.

A thioether-decorated triazine-based covalent organic framework towards overall H2O2 photosynthesis without sacrificial agents

Covalent organic frameworks (COFs) have been rapidly applied in photocatalytic H2O2 production owing to their strong light absorption, suitable band structure, etc. However, sluggish water oxidation reaction (WOR) with low selectivity restricts the H2O2 photosynthesis efficiency of COFs. Herein, a thioether-decorated triazine-based COF (TDB-COF) was designed and developed for efficient overall H2O2 photosynthesis. TDB-COF achieves a photocatalytic H2O2 production rate of 723.5 μmol g−1 h−1 without any sacrificial agents and exhibits remarkable cyclic stability. The modification of thioether groups broadens the visible light absorption of TDB-COF and narrows its optical band gap. Such a strategy can not only effectively regulate the band structure suitable for two-electron two-steps WOR but also promote the photogenerated carriers transfer and accelerate a two-electron two-steps oxygen reduction process to generate H2O2, thus ultimately boosting the overall H2O2 photosynthesis. This work provides valuable suggestions for the development of the functional COFs towards overall H2O2 photosynthesis.

On-Site H2O2 Synthesis Using a Series of Ni(II) Furan-2-thiocarboxylate Complexes Integrated on Multi-Walled Carbon Nanotubes: Soft Donor Sites Stimulated Selective Two-Electron Oxygen Reduction

A series of Ni(II) complexes [Ni(tmeda)(fCOS)2] (1a), [Ni(2,2′-bipy)(fCOS)2] (1b), [Ni(1,10-Phen)(fCOS)2] (1c), [Ni(opda)2(fCOS)2] (2), [Ni(dppe)(fCOS)2] (3a), and [Ni(dppee)(fCOS)2] (3b) (fCOS = furan-2-thiocarboxylate) have been synthesized. All the complexes, except 1b, were characterized by single-crystal X-ray diffraction analysis and spectroscopic techniques. In the solid state, the geometry around the Ni(II) center is square planar in the cases of 3a and 3b, while the same is octahedral (with varying degrees of distortion) in all other complexes. The complexes were integrated separately on multi-walled carbon nanotubes (MWCNTs) to prepare their respective composites. Electrochemical characteristics and oxygen reduction response of all the composites were studied. All composites exhibited a two-electron oxygen reduction process. The composite of complex 2 (2@MWCNTs) having S and N donor sites shows the highest onset potential and the highest rate of oxygen reduction. Composites of complexes having P and S coordination sites (3a@MWCNTs and 3b@MWCNTs) show relatively high selectivity toward the two-electron oxygen reduction. The composites of complexes having S, N, and O coordination sites (1a@MWCNTs, 1b@MWCNTs, and 1c@MWCNTs) show moderate selectivity and activity for the two-electron oxygen reduction. 3a@MWCNTs show the optimum combination of activity and selectivity for H2O2 production. Remarkably, all the composites show high durability and may be suitable for the long-term oxygen-reduction process to synthesize H2O2.

Electro-synthesis of pure aqueous H2O2 on nitrogen-doped carbon in a solid electrolyte flow cell without using anion exchange membrane

The two-electron oxygen reduction reaction (2e-ORR) generates on-site hydrogen peroxide (H2O2) more sustainably than the industrial anthraquinone process. Pure aqueous H2O2 electrosynthesis is the most desirable approach, as it is ready-to-use and pH-adjustable. Recently an innovative dual membrane-based solid-electrolyte flow cell (SE-FCAEM/CEM) was reported [Science, 2019, 366, 226-231], in which the anode and cathode “sandwiched” the cation-exchange-membrane (CEM) and anion-exchange-membrane (AEM), separated by a solid-electrolyte, thus allowing H+ and HO2– ions to recombine to form pure H2O2 in deionized (DI)-water stream. One key research needs to effectively deploy this flow cell is to address the stability and engineering difficulties of using an AEM, creating significant drawbacks in cell performance and lifespan. Here, we report a modified SE-FC without involving AEM (SE-FCAEM-FREE) to achieve better performance of H2O2 electrosynthesis. To validate SE-FCAEM-FREE for industrial-relevant production rates, we first developed a nitrogen-doped-carbon catalyst (N-C) with varied nitrogen-to-carbon ratios. Among all samples, the catalyst N-C(2:3) contains high carbon and a proper nitrogen precursor that boosted its activity, resulting in excellent half-cell performance with faradaic efficiency (FE) above 90% at different pH-electrolytes. Secondly, we optimized the catalyst microenvironment by applying a PTFE layer. The Layered-PTFE (5 wt.%) arrangement suppresses hydrogen evolution reaction (HER) and exhibits a high 2e-ORR activity with high current density of 389 mA cm−2 (about 6.53 mmol cm-2h−1) at 90% FEH2O2 without degradation for a 50-hour durability test.

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.