Two-electron Oxygen Reduction Research Articles

Effects of porous structure and oxygen functionalities on electrochemical synthesis of hydrogen peroxide on ordered mesoporous carbon

Two-electron oxygen reduction reaction (2e− ORR) is a promising alternative to energy-intensive anthraquinone process for hydrogen peroxide (H2O2) production. Metal-free nanocarbon materials have garnered intensive attention as highly prospective electrocatalysts for H2O2 production, and an in-depth understanding of their porous structure and active sites have become a critical scientific challenge. The present research investigates a range of porous carbon catalysts, including non-porous, microporous, and mesoporous structures, to elucidate the impacts of porous structures on 2e− ORR activity. The results highlighted the superiority of mesoporous carbon over other porous materials, demonstrating remarkable H2O2 selectivity. Furthermore, integration of X-ray photoelectron spectroscopy (XPS) data analysis with electrochemical assessment results unravels the moderate surface oxygen content is the key to increase 2e− ORR activity. These results not only highlight the intricate interplay between pore structure and oxygen content in determining catalytic selectivity, but also enable the design of carbon catalysts for specific electrochemical reactions.

Electrochemical selection of ferrous phosphide (FexP) as efficient catalyst for wastewater treatment: Performance and mechanism

Iron phosphide (FexP) has a great potential for electrochemical catalysis attributing to its outstanding electrical conductivity and activity. In the present study, a series of FexP (FeP, Fe2P and Fe3P) were carried out as catalysts for the oxidation of phenol from wastewater by the heterogeneous EF processes. Compared with FeP and Fe2P catalysts, Fe3P has shown significant two-electron oxygen reduction activity and H2O2 selectivity (90–100 %). The degradation efficiencies of phenol in Fe3P/EF (100 %) were higher than those in Fe2P/EF (91 %) and FeP/EF (67 %) upon 60-min electrochemical oxidation under pH 7. The Fe3P-CF/EF system could broaden the range of solution pH, as well the decay kinetics of phenol was enhanced by 1.5–2.4 times and the disposal cost was reduced by 33–41 % compared with FeP/EF and Fe2P/EF. Feɑ+ in Fe3P could promote the generation of •OH via accelerating Fe(II)/Fe(III) cycle and accelerating the transfer rate of electrons between Fe3P and H2O2. It was proved that •OH played a dominant role in removing phenol with the electron spin resonance (ESR) tests and trapping experiments. This work selected the most effective FexP catalyst in heterogeneous EF process and would bring new insights for organics removal from wastewater via H2O2 activation.

Heavily Doped Carbon Nitride Nanocrystal Promotes Visible-Near-Infrared Photosynthesis of Hydrogen Peroxide with Near-Unit Photon Utilization.

Direct photosynthesis of hydrogen peroxide (H2O2) from water and oxygen represents an intriguing alternative to the current indirect process involving the reduction and oxidation of quinones. However, limited light utilization and sluggish charge transfer largely impede overall photocatalytic efficiency. Herein, we present a heavily doped carbon nitride (CNKLi) nanocrystal for efficient and selective photoproduction of H2O2 via a two-electron oxygen reduction reaction (ORR) pathway. CNKLi induces metal-to-ligand charge transfer (MLCT) and electron trapping, which broadens the light absorption to the visible-near-infrared (vis-NIR) spectrum and prolongs the photoelectron lifetime to the microsecond time scale with an exceptional charge diffusion length of ∼1200 nm. Near-unit photoutilization with an apparent quantum yield (AQY) of 100% for H2O2 generation is achieved below 420 nm. Impressively, CNKLi exhibits an appreciable AQY of 16% at 700 nm, which reaches the absorption capacity (∼16%), thus suggesting a near-unit photon utilization <700 nm. In situ characterization and theoretical calculations reveal the facilitated charge transfer from K+ to the heptazine ring skeleton. These findings provide an approach to improve the photosynthetic efficiency of direct H2O2 preparation in the vis-NIR region and expand applications for driving kinetically slow and technologically desirable oxidations or high-value chemical generation.

Isomerization Engineering of Oxygen-Enriched Carbon Quantum Dots for Efficient Electrochemical Hydrogen Peroxide Production.

Hydrogen peroxide (H2O2) has emerged as a kind of multi-functional green oxidants with extensive industrial utility. Oxidized carbon materials exhibit promises as electrocatalysts in the two-electron (2e-) oxygen reduction reaction (ORR) for H2O2 production. However, the precise identification and fabrication of active sites that selectively yield H2O2 present a serious challenge. Herein, a structural engineering strategy is employed to synthesize oxygen-doped carbon quantum dots (o-CQD) for the 2e- ORR. The surface electronic structure of the o-CQDs is systematically modulated by varying isomerization precursors, thereby demonstrating excellent electrocatalyst performance. Notably, o-CQD-3 emerges as the most promising candidate, showcasing a remarkable H2O2 selectivity of 96.2% (n = 2.07) at 0.68V versus RHE, coupled with a low Tafel diagram of 66.95mV dec-1. In the flow cell configuration, o-CQD-3 achieves a H2O2 productivity of 338.7mmol gcatalyst -1 h-1, maintaining consistent production stability over an impressive 120-hour duration. Utilizing in situ technology and density functional theory calculations, it is unveil that edge sites of o-CQD-3 are facilely functionalized by C-O-C groups under alkaline ORR conditions. This isomerization engineering approach advances the forefront of sustainable catalysis and provides a profound insight into the carbon-based catalyst design for environmental-friendly chemical synthesis processes.

Towards highly-selective H2O2 photosynthesis: In-plane highly ordered carbon nitride nanorods with Ba atoms implantation

Graphitic carbon nitride (g-C3N4) shows great potential in photocatalytic H2O2 production. However, challenges arise from its low in-plane crystallinity and selectivity in two-electron oxygen reduction reaction (2e−-ORR), greatly limiting its H2O2 photosynthesis efficiency. Herein, we develop an ingenious strategy to simultaneously increase the in-plane crystallinity and induce the highly-selective 2e−-ORR by rationally designing barium (Ba) atom-implanted in-plane highly ordered g-C3N4 nanorods. The approach involves controllable synthesis of in-plane high crystallinity g-C3N4 nanorods with Ba implantation (BI-CN) using a BaCl2-mediated in-plane polymerization strategy. The unique Ba-N interaction induces the oriented polymerization of 3-s-triazine units to form well-arranged in-plane structures. Experimental and theoretical calculations clarify that the implanted Ba atoms function as positive charge centers, resulting in a Pauling-type O2 adsorption configuration. This minimizes O–O bond breaking energy, thus suppressing the four-electron oxygen reduction reaction (4e−-ORR) and facilitating a highly-selective 2e−-ORR pathway for efficient photocatalytic H2O2 production. Consequently, the optimized BI-CN3 photocatalyst exhibits an outstanding H2O2 production rate of as high as 353 μmol L−1 h−1, surpassing the pristine g-C3N4 by 6.1 times. This study concurrently optimizes the in-plane crystallinity and O2 adsorption sites of g-C3N4 photocatalysts for highly-selective H2O2 production, providing innovative insights for designing efficient photocatalysts with diverse applications.

Steering sp-Carbon Content in Graphdiynes for Enhanced Two-Electron Oxygen Reduction to Hydrogen Peroxide.

Compared to sp2-hybridized graphene, graphdiynes (GDYs) composed of sp and sp2 carbon are highly promising as efficient catalysts for electrocatalytic oxygen reduction into oxygen peroxide because of the high catalytic reactivity of the electron-rich sp-carbon atoms. The desired catalytic capacity of GDY, such as catalytic selectivity and efficiency, can theoretically be achieved by strategically steering the sp-carbon contents or the topological arrangement of the acetylenic linkages and aromatic bonds. Herein, we successfully tuned the electrocatalytic activity of GDYs by regulating the sp-to-sp2 carbon ratios with different organic monomer precursors. As the active sp-carbon atoms possess electron-sufficient π orbitals, they can donate electrons to the lowest unoccupied molecular orbital (LUMO) orbitals of O2 molecules and initiate subsequent O2 reduction, GDY with the high sp-carbon content of 50 at % exhibits excellent capability of catalyzing O2 reduction into H2O2. It demonstrates exceptional H2O2 selectivity of over 95.0 % and impressive performance in practical H2O2 production, Faraday efficiency (FE) exceeding 99.0 %, and a yield of 83.3 nmol s-1 cm-2. Our work holds significant importance in effectively steering the inherent properties of GDYs by purposefully adjusting the sp-to-sp2 carbon ratio and highlights their immense potential for research and applications in catalysis and other fields.

Bimetallic Center Metal–Organic Framework Catalysts for the Two-Electron Oxygen Reduction Reaction

Bimetallic Center Metal–Organic Framework Catalysts for the Two-Electron Oxygen Reduction Reaction

Mo-based MXenes as highly selective two-electron oxygen reduction catalysts for H2O2 production

Hydrogen peroxide production through the electrochemical two-electron oxygen reduction presents a compelling alternative to the anthraquinone process due to the cheap starting materials and low operating temperature and pressure. Transition metal species featuring with multi-active sites, offer high electrochemical activity and selectivity. MXenes, distinguished by excellent electrical conductivity and the two-dimensional structure facilitating smooth ion transportation, further enhance the prospects of this production approach. In this study, three Mo-based MXenes were synthesized: Mo2TiC2Fx, Mo2Ti2C3Fx and Mo2CFx. Among these, Mo2TiC2Fx emerged as the most promising catalyst for 2e−ORR, achieving a remarkable H2O2 yield of up to 0.54 mol·gcat−1·h−1. Moreover, it also exhibits a high H2O2 selectivity ranging from 81.3 % at 0.2 V to 94.1 % at 0.6 V vs. RHE, coupled with an impressive faradaic efficiency of 76.4 % at 0.4 V using H-cell. Further analysis indicates that the superior 2e−ORR performance can be attributed to the presence of metallic Mo on the surface of Mo2TiC2Fx.

Sulfur poisoned PtNi/C catalysts toward two-electron oxygen reduction reaction for acidic electrosynthesis of hydrogen peroxide

Selective electrocatalysis of two-electron oxygen reduction reaction (2e– ORR) has been recognized as a sustainable and on-site process for hydrogen peroxide (H2O2) production. Great progress has been achieved for 2e– ORR in alkaline media. However, it is challenged by insufficient activity and selectivity of the catalysts in acidic electrolytes. Herein, we report sulfur-poisoned PtNi/C catalysts (PtNiSx/C) that could regulate ORR from the 4e– to 2e– pathway. The identified PtNiS0.6/C offers high activity in terms of onset potential of ∼0.69 V (vs. RHE) and ∼80 % selectivity. The mass activity is also comparable and outperforms representative Pt-based precious and transition-metal-based catalysts. In addition, it is interestingly found that the Faradaic efficiency further increased to 95 % during the long-term electrolysis test due to Ni atom surface migration. The electrochemical production of the H2O2 system was applied to the electro-Fenton process, which has realized the effective degradation of organic pollutants. This work offers a strategy by sulfur poisoning PtNi/C catalyst to realize Pt-based 2e– ORR active catalysts to electrolysis of H2O2 in acidic media.

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.

Efficient Electrosynthesis of Hydrogen Peroxide Using Oxygen-Doped Porous Carbon Catalysts at Industrial Current Densities.

Metal-free carbon catalysts (MFCCs) are one of the commonly used catalysts for electrocatalytic two-electron oxygen reduction (2e- ORR) synthesis of hydrogen peroxide (H2O2). Oxygen doping is an effective means to improve the performance of MFCCs, but the performance of oxygen-doped carbon catalysts is still not high enough, and the contribution of different oxygen functional groups (OFGs) to the catalytic performance is still inconclusive. In this paper, carbon-based catalysts with different oxygen contents and ratios of OFGs were prepared, and the high 2e- ORR activity of COOH + C-OH was demonstrated by combining the results of experiments and theoretical calculations. The prepared oxygen-doped carbon-based catalyst C-0.1M80 achieved an onset potential of 0.795 V (vs RHE), a selectivity of up to 98.2% (0.6 V vs RHE), and a H2O2 oxidation current of 1.33 mA cm-2 (0.5 V vs RHE) in a rotating ring-disk electrode test (0.1 M KOH solution), which was an outstanding performance in MFCCs. In a solid electrolyte flow cell, C-0.1M80 achieved a Faraday efficiency of 97.5% at 200 mA cm-2 with a corresponding H2O2 production rate of 123.7 mg cm-2 h-1. In addition, a flow cell stability test was performed at an industrial current density (100 mA cm-2) with an astounding 200 h of uninterrupted operation, also achieving an outstanding average Faradaic efficiency (95.8%).

Heterostructure and interface engineering of Ni0.85Se-MoSe2@PEI for enhancing electrocatalytic acidic oxygen reduction to hydrogen peroxide

Two-electron oxygen reduction reaction (2e− ORR) for hydrogen peroxide (H2O2) production in acidic solution is becoming a promising synthetic method due to its mild and green process. However, it is still hindered by sluggish kinetics and low selectivity. Given that heterostructure can enhance electron transfer and provide abundant active sites, interface modification can optimize O2 adsorption configurations and the adsorption energy of the reaction intermediates to improve ORR activity and selectivity. Here, we coupled heterostructures of Ni0.85Se-MoSe2 and polyethyleneimine (PEI) functionalization to enhance the activity and selectivity of the 2e− ORR for the efficient production of H2O2 in acidic solutions. The as-fabricated Ni0.85Se-MoSe2@PEI performed 91.7 % selectivity of H2O2 in the rotating ring-disk electrode test, while the H2O2 yield rate was up to 3.57 mol gcat.−1h−1 and the Faraday efficiency was 95.5 % in the H-type cell. In addition, Ni0.85Se-MoSe2@PEI maintains excellent stability in long-term operation, verifying the practicability of the catalyst. The combination of heterostructure and interface engineering provides research ideas for the electrochemical synthesis of H2O2.

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.

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 90.16 mmol h−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.

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.

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.