Bimodal-Mesoporous hollow carbon nanoreactors with atomically dispersed Co sites for synergistically enhanced electrosynthesis of hydrogen peroxide

Electrosynthesis of hydrogen peroxide (H2O2) from 2e- transfer of the oxygen reduction reaction (2e--ORR) is a potential alternative to the traditional anthraquinone process. Two-dimensional (2D) metal-organic frameworks (MOFs) supported by carbon are frequently reported as promising 2e--ORR catalysts. Herein, a graphene-supported 2D MOF of Ni3(2,3,6,7,10,11-hexahydrotriphenylene)2 is synthesized through a common hydrothermal method, which exhibits high 2e--ORR performance. It is discovered that except for emerging MOFs, exceptional molecularly dispersed Ni sites coexist in the synthesis that have the same coordination sphere of the NiO4C4 moiety as the MOF. The molecular Ni sites are more catalytically active. The graphene support contains a suitable amount of residual oxygen groups, leading to the generation of those molecularly dispersed Ni sites. The oxygen groups exhibit a moderate electron-withdrawing effect at the outer sphere of Ni sites to slightly increase their oxidation state. This interaction decreases overpotentials and kinetically improves the selectivity of the 2e- reaction pathway.

This study aims to evaluate the ability of the graphite granules (GG) recycled from spent Zn-C batteries to act as 3D cathode for hydrogen peroxide (HP) electrosynthesis (HPE) in eco-friendly conditions (unbuffered 0.05 M Na2SO4 solution, air as O2 source). The performances of GG were compared to those of other usual carbonaceous cathode materials for HPE such as graphite bloc, graphite felt and reticulate vitreous carbon (RVC), using a divided filter-press electrochemical reactor. The operational parameters such as the polarization mode, electrolyte and air flow rates, applied potentials or imposed currents, and aeration mode were optimized by 1 h tests of electrosynthesized HP accumulation (EHPA). Considering as optimization criteria the best compromise between the final HP concentration, global current efficiency and electrical energy specific consumption, we find that the most efficient material for EHPA was RVC of 500 ppi, exploited in galvanostatic mode and using an original aeration system. In optimized conditions, for the GG cathode, very promising efficiency indicators were evaluated, suggesting that better results can be obtained by electrode geometry optimization and GG pretreatment.

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.

Most of electrochemical advanced oxidation processes (EAOP´s) such as the electroFenton (EF), photo electroFenton (PEF), solar photo electroFenton (SPEF) and electro-peroxone (E-peroxone) processes use hydrogen peroxide (H2O2) as its core reactant due to its possibility to react with some catalysts (Fe2+, O3) yielding high oxidant hydroxyl radicals to be used in the degradation of persistent organic pollutants [1]. Carbonaceous materials like carbon felt, carbon cloth and reticulated vitreous carbon (RVC) are the most used because of its capability to accumulate H2O2 [2,3]. The use of these materials is often in gas diffusion electrodes which are based in the injection of air from an external source to an air chamber located behind the electrode allowing the dispersion of the gas inside the electrode promoting the oxygen reduction [3]. This work deals with the in situ electrosynthesis of H2O2 in a flow through reactor using RVC as cathode material and Ti|Ir-Ta oxides expanded mesh as anode to improve the oxygen evolution reaction with subsequent reduction of O2 on the RVC to yield H2O2. The electrochemical cell consists of a flow channel where the electrodes are placed along its length, allowing the electrolyte to flow through them forcing the transport of oxygen bubbles inside the porous carbon material for its reduction to H2O2. [1] Sirés I., Brillas., E. Remediation of water pollution caused by pharmaceutical residues sed on electrochemical separation and degradation technologies: A review. Environmental International, 40 (2018) 212-229. [2] Coria G., Pérez T., Sirés I., Nava J.L., Mass transport studies during dissolved oxygen reduction to hydrogen peroxide in a filter-press electrolyzer using graphite felt, reticulated vitreous carbon and boron-doped diamond as cathodes. Journal of Electroanalytical Chemistry, 757 (2015) 225-229. [3] Pérez T., Coria G., Sirés I., Nava J.L., Uribe A.R., Electrosynthesis of hydrogen peroxide in a filter-press flow cell using graphite felt as air-diffusion cathode. Journal of Electroanalytical Chemistry, 812 (2018) 54-58.

Artificial photosynthesis has been regarded as a promising solution toward solar energy conversion, generating storable and transportable chemical fuels such as hydrogen (H2) and hydrogen peroxide (H2O2). However, the design of robust catalytic sites not only affects the activity, but also identify the atomic‐level correlation between active sites and natural photosynthesis performance. Herein, a synthesis method of single‐atomic Iron (Fe) active sites anchored on novel covalent organic framework (COF) for the production of H2O2 under visible light irradiation. When benzyl alcohol is the most sacrificial agent, the state‐of‐the‐art Fe‐based COF exhibits an excellent H2O2 generation rate of 4130 µmol g−1 h−1, over 5.3 times higher than that of pristine COF, achieving an apparent quantum yield of 6.4% at 420 nm. The enhanced photocatalytic performance is ascribed to the synergistic effect of atomically dispersed Fe sites and COF hosts, reducing the reaction energy barrier for the formation of *OOH intermediates and optimizing the adsorption of O2 and thus promoting two‐electron oxygen reduction reaction (ORR). This work establishes an atomic‐level engineering approach to build atomically dispersed Fe active sites on COF photocatalysts and provides in‐depth insight upon the ORR mechanism for promising artificial photosynthesis of H2O2.

Electrosynthesis of hydrogen peroxide via two-electron oxygen reduction (2e- ORR) provides a green, sustainable, and cost-effective alternative to anthraquinone processes. However, scaling up from laboratory evaluations to practical applications remains challenging. Herein, an interfacial microenvironment regulation strategy using cetyltrimethylammonium bromide cationic surfactant is reported to boost the hydrogen peroxide (H2O2) production rate of commercial carbon black catalysts in alkaline flow-cell reactors. The modified interfacial microenvironment creates an ideal environment for H2O2 production, resulting in a 1.40-fold improvement in 2e- ORR current density (from 227.0 to 320.0 mA cm-2) and a 1.58-fold improvement in H2O2 production rate (from 137.0 to 217.8 mM L-1 h-1). Additionally, a boron-doped mesoporous carbon catalyst is developed, demonstrating superior catalytic performance, achieving a 1.80-fold improvement in H2O2 production rate (246.7 mM L-1 h-1) comparing with commercial carbon black. These results highlight the potential of microenvironment regulation and catalyst design for developing highly efficient and scalable H2O2 electrosynthesis system.

Fabricating highly efficient and long-life redox bifunctional electrocatalysts is vital for oxygen-related renewable energy devices. To boost the bifunctional catalytic activity of Fe-N-C single-atom catalysts, it is imperative to fine-tune the coordination microenvironment of the Fe sites to optimize the adsorption/desorption energies of intermediates during oxygen reduction/evolution reactions (ORR/OER) and simultaneously avoid the aggregation of atomically dispersed metal sites. Herein, a strategy is developed for fabricating a free-standing electrocatalyst with atomically dispersed Fe sites (≈0.89wt.%) supported on N, F, and S ternary-doped hollow carbon nanofibers (FeN4 -NFS-CNF). Both experimental and theoretical findings suggest that the incorporation of ternary heteroatoms modifies the charge distribution of Fe active centers and enhances defect density, thereby optimizing the bifunctional catalytic activities. The efficient regulation isolated Fe centers come from the dual confinement of zeolitic imidazole framework-8 (ZIF-8) and polymerized ionic liquid (PIL), while the precise formation of distinct hierarchical three-dimensional porous structure maximizes the exposure of low-doping Fe active sites and enriched heteroatoms. FeN4 -NFS-CNF achieves remarkable electrocatalytic activity with a high ORR half-wave potential (0.90V) and a low OER overpotential (270mV) in alkaline electrolyte, revealing the benefit of optimizing the microenvironment of low-doping iron single atoms in directing bifunctional catalytic activity.

Hollow carbon sphere featuring highly dispersed Co-Nx sites for efficient and controllable syngas electrosynthesis from CO2

A zinc-quinone battery for paired hydrogen peroxide electrosynthesis

Hollow N-doped carbon spheres with anchored single-atom Fe sites for efficient electrocatalytic oxygen reduction

The exploration of efficient and low-consumption catalysts for carbon dioxide (CO2) conversion is desirable yet remains a great challenge. Herein, a novel catalyst composed of a hollow nitrogen-doped carbon framework (HNF) enriched with high-loading (9.8 wt %) atomically dispersed iron sites (defined as FeSAs/HNF) has been fabricated by a polymer-assisted strategy. As a result, FeSAs/HNF has an excellent performance on the cycloaddition reactions of CO2 with epoxides (the conversion >96%) under milder conditions because of its ultrahigh loading of atomically dispersed iron sites. This study not only provides an advanced catalyst for driving CO2 cycloaddition but also furnishes a novel perspective on the rational design of superior catalysts with high-loading active sites for diverse heterogeneous catalytic reactions.

Discharge of recalcitrant pharmaceuticals into aquatic systems has caused severe impacts on public health and ecosystem. Advanced oxidation processes (AOPs) are effective for eliminating these refractory pollutants, for which single-atom catalysts (SACs) become the state-of-the-art materials owing to the maximized exposure of active metal sites. In this work, hollow spherical graphitic carbon nitride (hsCN) was fabricated to incorporate copper species to develop Fenton-like catalysts for acetaminophen (ACT) removal. Through pyrolysis of supramolecular assemblies derived from melamine-Cu complex and cyanuric acid, single atom Cu-N3 sites were anchored on hsCN by N-coordination to obtain SACu-hsCN. In virtue of the atomically dispersed Cu-N3 sites as well as the hollow structure of hsCN providing smooth channels for the interactions between single Cu atoms and reactants, the optimal 5.5SACu-hsCN removed 94.8% of ACT after 180 min of Fenton-like reactions, which was superior to that of 5.5AGCu-hsCN with aggregated Cu particles on hsCN (56.7% in 180 min). Moreover, 5.5SACu-hsCN was still active after four cycles of regeneration. The mechanism investigation demonstrated that both hydroxyl radicals (OH) and singlet oxygen (1O2) contributed to ACT degradation in 5.5SACu-hsCN/H2O2 system, in which non-radical 1O2 played the dominant role.

Hollow Nitrogen-Doped porous carbon spheres decorated with atomically dispersed Ni-N3 sites for efficient electrocatalytic CO2 reduction

Carbon materials with ordered frameworks and atomically dispersed metal sites, referred to as ordered carbonaceous frameworks (OCFs), have attracted considerable attention for their promising potential in fundamental research and diverse practical applications, particularly in electrocatalysis. In this work, we synthesize Fe-incorporated OCF (Fe-OCF) with a heme-like structure through structure-preserving pyrolysis of Fe-porphyrin with four ethynyl groups. Fe-OCF is characterized by its ordered microporous framework, incorporating atomically dispersed Fe(III) sites with a high content of 6.9 wt%, analogous to metal-organic frameworks. At the same time, Fe-OCF possesses the advantages of carbon materials, including chemical stability, thermal stability, and electrical conductivity. Remarkably, Fe-OCF mimics the functionality of a sensor enzyme by facilitating the redox reaction of hydrogen peroxide, which is regulated by an applied potential, thereby enabling bidirectional catalytic behavior. Fe-OCF exhibits a linear reduction current response to hydrogen peroxide, underscoring its efficient electron transfer and catalytic properties. Moreover, Fe-OCF demonstrates superior stability compared to molecular Fe-porphyrin, further emphasizing its potential application as a novel hydrogen peroxide sensor. These results emphasize the significant potential of Fe-based OCFs as advanced materials for artificial enzyme applications and next-generation hydrogen peroxide sensing technology.

Single atomic Fe-dispersed hollow carbon spheres coated with Co3O4 synergistically catalyze oxygen reduction and oxygen evolution reactions