Highly Efficient Acidic Electrosynthesis of Hydrogen Peroxide at Industrial-Level Current Densities Promoted by Alkali Metal Cations.

Acidic H2O2 synthesis through electrocatalytic 2e- oxygen reduction presents a sustainable alternative to the energy-intensive anthraquinone oxidation technology. Nevertheless, acidic H2O2 electrosynthesis suffers from low H2O2 Faradaic efficiencies primarily due to the competing reactions of 4e- oxygen reduction to H2O and hydrogen evolution in environments with high H+ concentrations. Here, we demonstrate the significant effect of alkali metal cations, acting as competing ions with H+, in promoting acidic H2O2 electrosynthesis at industrial-level currents, resulting in an effective current densities of 50-421 mA cm-2 with 84-100 % Faradaic efficiency and a production rate of 856-7842 µmol cm-2 h-1 that far exceeds the performance observed in pure acidic electrolytes or low-current electrolysis. Finite-element simulations indicate that high interfacial pH near the electrode surface formed at high currents is crucial for activating the promotional effect of K+. In situ attenuated total reflection Fourier transform infrared spectroscopy and ab initio molecular dynamics simulations reveal the central role of alkali metal cations in stabilizing the key *OOH intermediate to suppress 4e- oxygen reduction through interacting with coordinated H2O.

Highly Efficient Hydroxyl Radicals Production Boosted by the Atomically Dispersed Fe and Co Sites for Heterogeneous Electro-Fenton Oxidation.

The heterogeneous electro-Fenton (hetero-e-Fenton)-coupled electrocatalytic oxygen reduction reaction (ORR) is regarded as a promising strategy for ·OH production by simultaneously driving two-electron ORR toward H2O2 and stepped activating the as-generated H2O2 to ·OH. However, the high-efficiency electrogeneration of ·OH remains challengeable, as it is difficult to synchronously obtain efficient catalysis of both reaction steps above on one catalytic site. In this work, we propose a dual-atomic-site catalyst (CoFe DAC) to cooperatively catalyze ·OH electrogeneration, where the atomically dispersed Co sites are assigned to enhance O2 reduction to H2O2 intermediates and Fe sites are responsible for activation of the as-generated H2O2 to ·OH. The CoFe DAC delivers a higher ·OH production rate of 2.4 mmol L-1 min-1 gcat-1 than the single-site catalyst Co-NC (0.8 mmol L-1 min-1 gcat-1) and Fe-NC (1.0 mmol L-1 min-1 gcat-1). Significantly, the CoFe DAC hetero-e-Fenton process is demonstrated to be more energy-efficient for actual coking wastewater treatment with an energy consumption of 19.0 kWh kg-1 COD-1 than other electrochemical technologies that reported values of 29.7∼68.0 kW h kg-1 COD-1. This study shows the attractive advantages of efficiency and sustainability for ·OH electrogeneration, which should have fresh inspiration for the development of new-generation wastewater treatment technology.

High-Efficiency Electrocatalysis of Molecular Oxygen toward Hydroxyl Radicals Enabled by an Atomically Dispersed Iron Catalyst.

Fenton catalysis represents the promising technology to produce super-active ·OH for tackling severe water environment pollution issues, whereas it suffers from low atomic efficiency, poor pH adaptability, and catalyst non-reusability in a homogeneous or heterogeneous system. Here, single-atom iron catalysis is creatively introduced to drive electrochemical ·OH evolution utilizing earth-abundant oxygen and water as raw materials. The atomically dispersed iron settled by defective three-dimensional porous carbon (AD-Fe/3DPC) with unique C, Cl unsaturated coordination can efficiently tune the multi-electron oxygen reduction process, enabling O2-to-·OH conversion. The mass activity in ·OH production by AD-Fe/3DPC is almost two-orders of magnitude higher as compared to that by nanoparticular iron oxide catalyst. Meanwhile, the AD-Fe/3DPC electro-Fenton system exhibits fast elimination of refractory toxic pollutants, surpassing nanoparticular iron oxides in kinetic rate by 59 times or homogeneous Fenton by 10 times under similar experimental conditions. Experimental and theoretical results demonstrate that the remarkable enhanced mass activity of AD-Fe/3DPC in catalyzing O2 to ·OH is contributed by the synergistic effects of the maximized catalysis of atomically dispersed iron and the unique unsaturated coordination environment. The AD-Fe/3DPC catalytic system is demonstrated to be pH-universal, long-term stable, and well recyclable, truly satisfying flexible, sustainable, and green application of wastewater purification. This study gives a new sight into local coordination modulation of single-atom catalysts for selective electrocatalytic oxygen reduction.

Promoting CO2 Electroreduction to Ethane by Iodide-Derived Copper with the Hydrophobic Surface.

Electrochemical reduction of CO2 to value-added products provides a feasible pathway for mitigating net carbon emissions and storing renewable energy. However, the low dimerization efficiency of the absorbed CO intermediate (*CO) and the competitive hydrogen evolution reaction hinder the selective electroreduction of CO2 to ethane (C2H6) with a high energy density. Here, we designed hydrophobic iodide-derived copper electrodes (I-Cu/Nafion) for reducing CO2 to C2H6. The Faradaic efficiency of C2H6 reached 23.37% at -0.7 V vs RHE over the I-Cu/Nafion electrode in an H-type cell, which was about 1.7 times higher than that of the I-Cu electrode. The hydrophobic properties of the I-Cu/Nafion electrodes led to an increase in the local CO2 concentration and stabilized the Cu+ species. In situ Raman characterizations and density functional theory calculations indicate that the enhanced performances could be ascribed to the strong *CO adsorption and decreased the formation energy of *COOH and *COCOH intermediates. This study highlights the effect of the hydrophobic surface on Cu-based catalysts in the electroreduction of CO2 and provides a promising way to adjust the selectivity of C2 products.

Highly efficient electro-Fenton process on hollow porous carbon spheres enabled by enhanced H2O2 production and Fe2+ regeneration.

Electro-Fenton (e-Fenton) is a promising method for wastewater treatment that relies on powerful ·OH generated via the decomposition of electro-generated H2O2 catalyzed by Fe2+. In this regard, developing a catalyst capable of simultaneously producing H2O2 and accelerating Fe2+ regeneration is of considerable importance; however, this remains a challenge because of the difficulty in modulating the electronic microenvironment. Herein, a hollow porous carbon sphere catalyst (HPCS) is developed to synchronously enhance H2O2 generation and accelerate Fe3+/Fe2+ cycling by constructing an electron-rich microenvironment via surface curvature regulation. The Fe2+ regeneration efficiency reaches 35.5% on HPCS featuring a larger curvature structure (HPCS-TPOS), which is 1.6 times higher than the smaller curvature HPCS-S catalyst (22.8%). Density functional theory reveals that the electron-rich microenvironment on the outer surface of high curvature structure promotes Fe2+ regeneration. The H2O2 production rate on HPCS-TPOS is 47.2 mmol L-1 h-1, exceeding the state-of-the-art e-Fenton catalysts reported. Benefiting from the concurrent high-efficiency of H2O2 production and Fe2+ regeneration, HPCS-TPOS e-Fenton is demonstrated to be efficient for sulfamethoxazole removal with the kinetic rate of 0.30-0.72 min-1 at pH 3-7. This work offers new insight into the design of efficient catalysts by rationally regulating curvature structures for wastewater treatment.

Enhanced electrochemical-activation of H2O2 to produce •OH by regulating the adsorption of H2O2 on nitrogen-doped porous carbon for organic pollutants removal.

The heterogeneous Fenton oxidation is regarded as a promising technology for refractory organic pollutants removal relying on highly active •OH generated via the decomposition of H2O2 catalyzed by iron-based catalyst that overcomes the issues of pH limitation and iron sludge discharge encountered in conventional Fenton reaction. However, the efficiency of •OH production in heterogeneous Fenton remains low as the limited mass transfer between H2O2 and catalysts caused by the poor H2O2 adsorption. Here, a nitrogen-doped porous carbon (NPC) catalyst with tunable N configuration was prepared for electrochemical-activation of H2O2 to •OH by enhancing the H2O2 adsorption on catalysts. The resultant •OH production yield on NPC reached 0.83 mM in 120 min. Notably, the NPC catalyst could be more energy-efficient for actual coking wastewater treatment with an energy consumption of 10.3 kWh kgCOD-1 than other electro-Fenton catalysts reported (20-29.7 kWh kgCOD-1). Density function theory (DFT) revealed that highly efficient •OH production was ascribed to the graphitic N which enhances the adsorption energy of H2O2 on NPC catalyst. This study provides new insight into the fabrication of efficient carbonaceous catalysts by rationally modulating electronic structures for refractory organic pollutants degradation.

Metal single-site catalyst design for electrocatalytic production of hydrogen peroxide at industrial-relevant currents.

Direct hydrogen peroxide (H2O2) electrosynthesis via the two-electron oxygen reduction reaction is a sustainable alternative to the traditional energy-intensive anthraquinone technology. However, high-performance and scalable electrocatalysts with industrial-relevant production rates remain to be challenging, partially due to insufficient atomic level understanding in catalyst design. Here we utilize theoretical approaches to identify transition-metal single-site catalysts for two-electron oxygen reduction using the *OOH binding energy as a descriptor. The theoretical predictions are then used as guidance to synthesize the desired cobalt single-site catalyst with a O-modified Co-(pyrrolic N)4 configuration that can achieve industrial-relevant current densities up to 300 mA cm-2 with 96-100% Faradaic efficiencies for H2O2 production at a record rate of 11,527 mmol h-1 gcat-1. Here, we show the feasibility and versatility of metal single-site catalyst design using various commercial carbon and cobalt phthalocyanine as starting materials and the high applicability for H2O2 electrosynthesis in acidic, neutral and alkaline electrolytes.

Highly efficient metal-free electro-Fenton degradation of organic contaminants on a bifunctional catalyst.

Heterogeneous electro-Fenton (e-Fenton) is a promising technology for the treatment of persistent organic pollutants, in which H2O2 is produced via 2e- oxygen reduction and is simultaneously activated into •OH by the iron-based catalyst. This system often suffers from the inevitable metal dissolution in the acidic or even neutral environment, leading to poor pH adaptation and low stability. In this work, we designed a metal-free e-Fenton system, using O-doped carbon nanotubes (OCNTs) as the bifunctional metal-free cathode for the degradation of organic pollutants. The system showed the excellent e-Fenton performance under neutral conditions with the phenol degradation kinetic constant of 0.071 min-1, and the corresponding TOC removal was 76.6% within 300 min. It also exhibited excellent performance for actual coking wastewater treatment with the specific energy consumption of 7.4 kW h kg-1 COD-1, which was lower than that reported heterogeneous electro-Fenton system (9.2-14.4 kW h kg-1 COD-1). The in-situ metal-free e-Fenton system could be regarded as a promising strategy for actual wastewater treatment.

Selective electrochemical H2O2 generation and activation on a bifunctional catalyst for heterogeneous electro-Fenton catalysis.

Heterogeneous electro-Fenton is attractive for pollutants removal, where H2O2 is in-situ generated and simultaneously activated to ·OH at the cathodic catalyst. However, the heterogeneous electro-Fenton efficiency is limited by low H2O2 production and slow Fe(II) regeneration, which can be improved by tuning oxygen reduction selectivity and facilitating electron transfer to Fe(III) centers. Herein, we designed a bifunctional catalyst with FeOx nanoparticles embedded into N-doped hierarchically porous carbon (FeOx/NHPC). The activity and selectivity for H2O2 production were improved by regulating N doping configurations and contents. The obtained FeOx/NHPC750 presented high catalytic activity for H2O2 production with a low overpotential of 190 mV and high H2O2 selectivity of 95%˜98% at -0.3 V to -0.8 V. The Fe(II) regeneration was enhanced by the strong interfacial interaction between FeOx and N-doped porous carbon support, which leaded to a rapid decomposition of H2O2 into ·OH. FeOx/NHPC750 exhibited excellent electro-Fenton performance for the degradation and mineralization of phenol, sulfamethoxazole, atrazine, rhodamine B and 2,4-dichlorophenol in neutral reaction solution. This study offered a new strategy to construct an efficient and durable bifunctional catalyst for heterogeneous electro-Fenton system for advanced treatment of refractory wastewater.