Simultaneously Providing Iron Source toward Electro-Fenton Process and Enhancing Hydrogen Peroxide Production via a Fe3O4 Nanoparticles Embedded Graphite Felt Electrode

Advancing H2O2 electrosynthesis: enhancing electrochemical systems, unveiling emerging applications, and seizing opportunities

Hydrogen peroxide (H2O2) is a highly desired chemical with a wide range of applications. Recent advancements in H2O2 synthesis center on the electrochemical reduction of oxygen, an environmentally friendly approach that facilitates on-site production. To successfully implement practical-scale, highly efficient electrosynthesis of H2O2, it is critical to meticulously explore both the design of catalytic materials and the engineering of other components of the electrochemical system, as they hold equal importance in this process. Development of promising electrocatalysts with outstanding selectivity and activity is a prerequisite for efficient H2O2 electrosynthesis, while well-configured electrolyzers determine the practical implementation of large-scale H2O2 production. In this review, we systematically summarize fundamental mechanisms and recent achievements in H2O2 electrosynthesis, including electrocatalyst design, electrode optimization, electrolyte engineering, reactor exploration, potential applications, and integrated systems, with an emphasis on active site identification and microenvironment regulation. This review also proposes new insights into the existing challenges and opportunities within this rapidly evolving field, together with perspectives on future development of H2O2 electrosynthesis and its industrial-scale applications.

Recent advances in electrosynthesis of H2O2via two-electron oxygen reduction reaction

The electrosynthesis of hydrogen peroxide (H2O2) via a selective two-electron oxygen reduction reaction (2e- ORR) presents a green and low-energy-consumption alternative to the traditional, energy-intensive anthraquinone process. This review encapsulates the principles of designing relational electrocatalysts for 2e- ORR and explores remaining setups for large-scale H2O2 production. Initially, the review delineates the fundamental reaction mechanisms of H2O2 production via 2e- ORR and assesses performance. Subsequently, it methodically explores the pivotal influence of microstructures, heteroatom doping, and metal hybridization along with setup configurations in achieving a high-performance catalyst and efficient reactor for H2O2 production. Thereafter, the review introduces a forward-looking methodology that leverages the synergistic integration of catalysts and reactors, aiming to harmonize the complementary characteristics of both components. Finally, it outlines the extant challenges and the promising avenues for the efficient electrochemical production of H2O2, setting the stage for future research endeavors.

New Insights on Designing the Next-Generation Materials for Electrochemical Synthesis of Reactive Oxidative Species Towards Efficient and Scalable Water Treatment: A Review and Perspectives

Electrochemical water remediation technologies offer several advantages and flexibility for water treatment and degradation of contaminants. These technologies generate reactive oxidative species (ROS) that degrade pollutants. For the implementation of these technologies at an industrial scale, efficient, scalable, and cost-effective in-situ ROS synthesis is necessary to degrade complex pollutant mixtures, treat large amount of contaminated water, and clean water in a reasonable amount of time and cost. These targets are directly dependent on the materials used to generate the ROS, such as electrodes and catalysts. Here, we review the key design aspects of electrocatalytic materials for efficient in-situ ROS generation. We present a mechanistic understanding of ROS generation, including their reaction pathways, and integrate this with the key design considerations of the materials and the overall electrochemical reactor/cell. This involves tunning the interfacial interactions between the electrolyte and electrode which can enhance the ROS generation rate up to ~ 40% as discussed in this review. We also summarized the current and emerging materials for water remediation cells and created a structured dataset of about 500 electrodes and 130 catalysts used for ROS generation and water treatment. A perspective on accelerating the discovery and designing of the next generation electrocatalytic materials is discussed through the application of integrated experimental and computational workflows. Overall, this article provides a comprehensive review and perspectives on designing and discovering materials for ROS synthesis, which are critical not only for successful implementation of electrochemical water remediation technologies but also for other electrochemical applications.

Degradation of atrazine by electroactivation of persulfate using FeCuO@C modified composite cathode: Synergistic activation mechanism

In sulfate radical-based advanced oxidation processes (SR-AOPs), high-efficiency and perdurable materials have drawn considerable interest for use as cathodes, which can effectively degrade refractory organic contaminants through the synergistic electro-activation and transition metal activation of persulfate (PS). Here, the FeCuO@C modified composite cathode (FeCuO@C/AGF) was synthesized via the solvothermal and thermal treatment method based on the CuFe-MOF-74 structure, and the electro-activation PS process (EC/FeCuO@C/AGF/PS) was developed to effectively remove atrazine (ATZ). The surface morphology, electrochemical characteristics, chemical composition, crystal structure, and electrode surface wettability of FeCuO@C/AGF were investigated. It was found that the proposed EC/FeCuO@C/AGF/PS process can successfully remove 100% of ATZ in 20 min at a low current density (2 mA cm^-2) and a low PS concentration (0.4 mM), and PS is successfully activated by combining the electrical and transition metal synergistic activation. The FeCuO@C/AGF cathode exhibits outstanding catalytic functionality over a broad pH range (2-9) and remains stable over five successive cycles. Additionally, the active species involved in the reaction as well as the potential ATZ degradation reaction mechanisms and pathways are discussed. Electrochemical oxidation is a process in which both radicals (SO₄^·-, ·OH, and O₂^·-) and non-radical (¹O₂) participate in the degradation of ATZ. The intermediates of the ATZ degradation process were studied upon the toxicity changing, and the toxicity of the intermediates was found to be reduced during degradation. These results present a novel approach toward the establishment of an effective and reliable electrode in SR-AOPs that can efficiently treat pesticide wastewater.

'Green-to-Green': Iron oxides embedded in lignin-based carbon scaffolds for water remediation via oxidation excluding free-radical pathways

Advanced oxidation processes (AOP) are a common tool to remove organic compounds from the water cycle. The process is mostly relied on free radicals (i.e., SO₄•^- and HO•) with high oxidation power in solution. Surface-mediated mechanism could improve this process to prevent undesired quenching of aqueous radicals that widely exists in free radical pathways and alleviate metal leaching through direct electron transfer. In this work, a facile low-temperature pre-treatment combined with pyrolytic strategy was employed to construct a green catalyst with iron oxides embedded in Kraft-lignin derived bio-char (γ-Fe₂O₃ @KC), upon which radicals stay surface mediated and the activity-stability trade-off is achieved for pollutant degradation. The γ-Fe₂O₃ @KC is capable of activating PMS to generate non-radical species which are more stable (¹O₂ and Fe(V)=O) and of enhancing electron transfer efficiency. A surface-bound reactive complex (Catalyst-PMS*) was identified by electrochemical characterization and was discussed with primary surface-bound radical pairs to explain the contradictions between quenching and EPR detection results. We analyzed the γ-Fe₂O₃ @KC as a PMS-activating catalyst for a wider range of oxidation targets, such as Rhodamine B (∼100%), p-nitrophenol (∼85%), and Ciprofloxacin (∼63%), and found competitive removal efficiencies. The system also shows an encouraging reusability for at least 5 times and high stability at pH 3-9, and the low concentration of iron in γ-Fe₂O₃ @KC/PMS system implies the carbon scaffold of biochar alleviate the leakage process. The combined findings highlight the applicability in 'green (source) to green (application)' processes using cost-effective and bio-friendly iron@carbon catalysts, where alternative oxidation pathways are activated to play a dominant role for water purification.

Degradation of carbamazepine over MOFs derived FeMn@C bimetallic heterogeneous electro-Fenton catalyst

A highly efficient heterogeneous electro-Fenton (Hetero-EF) catalyst with core-shell structure was successfully prepared by calcination of Mn-doped Mil-53 (Fe) precursor at high temperature. FeMn@C-800/2 prepared at pyrolysis temperature of 800 °C and Fe:Mn molar doping ratio of 2:1 showed the best catalytic performance for the degradation of carbamazepine (CBZ). The characterization, properties and stability of FeMn@C-800/2 were systematically investigated, obtaining the apparent first-order reaction rate of Hetero-EF was 8.9 and 17.8 times higher than that on Fe@C-800 and Mn@C-800 at the optimized conditions of current density 10 mA cm^-2, catalyst dosage of 50 mg L^-1 and initial pH 4.0, respectively. The incorporation of Mn promoted the generation of more Fe⁰ and Fe₃C during the pyrolysis process, and enhanced the internal micro-electrolysis between Fe⁰ and carbon shell. At the same time, the presence of Mn⁰ also promoted the regeneration of Fe²⁺, and improved the activity of iron-carbon heterogeneous catalysis in the EF process, so as to degrade organic pollutants more effectively. This work would help to gain insight into the design of MOFs derived Fe-Mn bimetal catalyst and its mechanism for enhanced heterogeneous electro-Fenton.

Cu/Fe oxide integrated on graphite felt for degradation of sulfamethoxazole in the heterogeneous electro-Fenton process under near-neutral conditions

In the heterogeneous electro-Fenton (EF) system, high-efficiency and durable materials have attracted widespread attention as cathodes for degradation of refractory organic pollutants. In this study, a stable Cu/Fe oxide modified graphite felt electrode (Cu_0.33Fe_0.67NBDC-300/GF) was fabricated via a one-step hydrothermal method and subsequent thermal treatment, which used a bimetallic metal-organic framework (MOF) with 2-aminoterephthalic acid (NH₂BDC) ligand as the precursor. The Cu_0.33Fe_0.67NBDC-300/GF electrode was used as the cathode for sulfamethoxazole (SMX) degradation in the heterogeneous EF process. The coexistence of the Fe^II/Fe^III and Cu^I/Cu^II redox couples significantly accelerates the regeneration of Fe^II and promotes the generation of active free radicals (•OH and •O₂^-). Fe^IV was detected during the process, which indicates that the high-valent iron-oxo species was produced in near-neutral pH conditions. The removal efficiency of SMX (10 mg L^-1) can reach 100.0% within 75 min over a wide pH range (4.0-9.0). After five cycles, the electrode retained a high stability and an outstanding catalytic capacity. Furthermore, the mechanisms and pathways for SMX degradation were proposed, the products and intermediates of SMX were analyzed, and the toxicity was evaluated. It was found that the toxicity decreased after degradation. This study displays a novel strategy for building an efficient and stable self-supporting electrode for treating antibiotic wastewater.

The Feasibility of Targeted Magnetic Iron Oxide Nanoagent for Noninvasive IgA Nephropathy Diagnosis

IgA nephropathy is the most common glomerular disease in the world and has become a serious threat to human health. Accurate and non-invasive molecular imaging to detect and recognize the IgA nephropathy is critical for the subsequent timely treatment; otherwise, it may progress to end-stage renal disease and lead to glomerular dysfunction. In this study, we have developed a sensitive, specific, and biocompatible integrin αvβ3-targeted superparamagnetic Fe₃O₄ nanoparticles (NPs) for the noninvasive magnetic resonance imaging (MRI) of integrin αvβ3, which is overexpressed in glomerular mesangial region of IgA nephropathy. The rat model of IgA nephropathy was successfully established and verified by biochemical tests and histological staining. Meanwhile, the clinical ¹⁸F-AlF-NOTA-PRGD2 probe molecule was utilized to visualize and further confirmed the IgA nephropathy in vivo via positron emission computed tomography. Subsequently, the Fe₃O₄ NPs were conjugated with arginine–glycine–aspartic acid (RGD) molecules (Fe₃O₄-RGD), and their integrin αvβ3-targeted T2-weighted imaging (T2WI) potential has been carefully evaluated. The Fe₃O₄-RGD demonstrated great relaxation in vivo. The T2WI signal of renal layers in the targeted group at 3 h after intravenous injection of Fe₃O₄-RGD was distinctly lower than baseline, indicating MRI signal decreased in the established IgA nephropathy rat model. Moreover, the TEM characterization and Prussian blue staining confirmed that the Fe₃O₄-RGD was located at the region of glomerulus and tubular interstitium. Moreover, no obvious signal decreased was detected in the untargeted Fe₃O₄ treated and normal groups. Collectively, our results establish the possibility of Fe₃O₄-RGD serving as a feasible MRI agent for the noninvasive diagnosis of IgA nephropathy.

High-precision and on-line measurement of dissolved organic matter in Electro-Fenton process based on dual wavelength analysis with combination of fluorescence emission and ultraviolet absorption spectroscopy

Electro-Fenton (EF) process is a significant water treatment method for the degradation of dissolved organic matter (DOM) in water and has been widely studied and applied in the past decade to degrade various dissolved organics. During water treatment, in order to monitor the degradation efficiency, it is in dire need to develop rapid and accurate methods that are favorable to assay on-line and on-site to provide feedback in a timely manner. UV-Visible absorption spectroscopy and Excitation-Emission Matrix (EEM) fluorescence spectroscopy techniques are most potential to realize on-line DOM measurement, but the measurement accuracy is unsatisfactory because of the obligatory involvement of iron-containing interferents. This study aims to simplify the measurement system complexity while overcoming the effect of iron-containing interferents during the measurement. An intrinsic relationship between the measured DOM concentration and the ultraviolet absorption at λ₁ and the light intensities of the fluorescence emission at λ₂ is derived theoretically and proved, based on which the influence of iron ions and their complexes on the spectrum can be eliminated, thus the content of DOM in the Electro-Fenton process is accurately determined. The proposed dual wavelength analysis with combination of fluorescence emission and ultraviolet absorption spectroscopy can achieve high precision (R²=0.9882,RMSE=0.0131mg/L). Furthermore, the on-line measurement design, called ultraviolet absorption-fluorescence emission dual wavelength analyzer, only includes one ultraviolet LED and two photodetectors. Its structure is simple and suitable for on-line monitoring DOM in EF process.

Recent Progress of Electrochemical Production of Hydrogen Peroxide by Two-Electron Oxygen Reduction Reaction

Shifting electrochemical oxygen reduction reaction (ORR) via two-electron pathway becomes increasingly crucial as an alternative/green method for hydrogen peroxide (H2 O2 ) generation. Here, the development of 2e- ORR catalysts in recent years is reviewed, in aspects of reaction mechanism exploration, types of high-performance catalysts, factors to influence catalytic performance, and potential applications of 2e- ORR. Based on the previous theoretical and experimental studies, the underlying 2e- ORR catalytic mechanism is firstly unveiled, in aspect of reaction pathway, thermodynamic free energy diagram, limiting potential, and volcano plots. Then, various types of efficient catalysts for producing H2 O2 via 2e- ORR pathway are summarized. Additionally, the catalytic active sites and factors to influence catalysts' performance, such as electronic structure, carbon defect, functional groups (O, N, B, S, F etc.), synergistic effect, and others (pH, pore structure, steric hindrance effect, etc.) are discussed. The H2 O2 electrogeneration via 2e- ORR also has various potential applications in wastewater treatment, disinfection, organics degradation, and energy storage. Finally, potential future directions and prospects in 2e- ORR catalysts for electrochemically producing H2 O2 are examined. These insights may help develop highly active/selective 2e- ORR catalysts and shape the potential application of this electrochemical H2 O2 producing method.

Enhanced electro-Fenton catalytic performance with in-situ grown Ce/Fe@NPC-GF as self-standing cathode: Fabrication, influence factors and mechanism

Heterogeneous electro-Fenton (E-F) is considered as an attractive technique for efficient removal of refractory organic pollutants in wastewater. The regeneration of Fe^II and catalyst reusability are key issues for effective and sustainable degradation. Developing binder-free iron phase/carbon composite cathode is a feasible strategy. In this work, the stable Ce/Fe-nanoporous carbon modified graphite felt electrode (Ce/Fe@NPC-GF) was fabricated using in situ solvothermal method and subsequent carbonization treatment, which worked as the cathode in a heterogeneous electro-Fenton system to degrade sulfamethoxazole. The electrocatalytic activity was significantly improved with doping of Ce. It was found that mesoporous Ce/Fe@NPC-GF cathode demonstrated high oxygen reduction activity and low resistance. The co-existence of Fe^Ⅱ/Fe^Ⅲ and Ce^Ⅲ/Ce^Ⅳ redox couples enhanced remarkably interfacial electron transfer, promoting in-situ H₂O₂ generation and decomposition, sequentially boosting the production of reactive radicals (·OH and ·O₂^-). Under 20 mA and pH 3, Sulfamethoxazole (SMX) was basically degraded in 120 min, and the removal rate was satisfactory in wide pH (2-6). After 8 cycles, the electrode could still maintain high stability and outstanding catalytic capacity. This work displayed a novel in-situ preparation method of composite cathode with excellent catalytic performance in E-F system, which offered inspiration for developing efficient heterogeneous electro-Fenton cathode material.