Enhancement of hydrogen peroxide production by electrochemical reduction of oxygen on carbon nanotubes modified with fluorine

Advancing Microbial Electrochemical H2O2 Synthesis by Tailoring the Surface Chemistry of Stereolithography-Derived 3D Pyrolytic Carbon Electrodes

Microbial electrosynthesis of H2O2 offers an economical and eco-friendly alternative to the costly and environmentally detrimental anthraquinone process. Three-dimensional (3D) electrodes fabricated through additive manufacturing demonstrate significant advantages over carbon electrodes with two-dimensional (2D) surfaces in microbial electrosynthesis of H2O2. Nevertheless, the presence of oxygen-containing free acidic groups on the prototype electrode surface imparts hydrophilic properties to the electrode, which affects the efficiency of the two-electron oxygen reduction reaction for H2O2 generation. In this study, we elucidated that the efficiency of microbial H2O2 synthesis is markedly enhanced by utilizing oxygen-free 3D electrodes produced via additive manufacturing techniques followed by surface modifications to eradicate oxygen-containing functional groups. These oxygen-free 3D electrodes exhibit superior hydrophobicity compared to traditional carbon electrodes with 2D surfaces and their 3D printed analogues. The oxygen-free 3D electrode is capable of generating up to 130.2 mg L-1 of H2O2 within a 6-h time frame, which is 2.4 to 13.6 times more effective than conventional electrodes (such as graphite plates) and pristine 3D printed electrodes. Additionally, the reusability of the oxygen-free 3D electrode underscores its practical viability for large-scale applications. Furthermore, this investigation explored the role of the oxygen-free 3D electrode in the bioelectro-Fenton process, affirming its efficacy as a tertiary treatment technology for the elimination of micropollutants. This dual functionality accentuates the versatility of the oxygen-free 3D electrode in facilitating both the synthesis of valuable chemicals and advancing environmental remediation. This research introduces an innovative electrode design that fosters efficient and sustainable H2O2 synthesis while concurrently enabling subsequent environmental restoration.

Advanced Nanocarbons Toward two-Electron Oxygen Electrode Reactions for H2O2 Production and Integrated Energy Conversion

Hydrogen peroxide (H2O2) plays a pivotal role in advancing sustainable technologies due to its eco-friendly oxidizing capability. The electrochemical two-electron (2e-) oxygen reduction reaction and water oxidation reaction present an environmentally green method for H2O2 production. Over the past three years, significant progress is made in the field of carbon-based metal-free electrochemical catalysts (C-MFECs) for low-cost and efficient production of H2O2 (H2O2EP). This article offers a focused and comprehensive review of designing C-MFECs for H2O2EP, exploring the construction of dual-doping configurations, heteroatom-defect coupling sites, and strategic dopant positioning to enhance H2O2EP efficiency; innovative structural tuning that improves interfacial reactant concentration and promote the timely release of H2O2; modulation of electrolyte and electrode interfaces to support the 2e- pathways; and the application of C-MFECs in reactors and integrated energy systems. Finally, the current challenges and future directions in this burgeoning field are discussed.

Understanding the preparative chemistry of atomically dispersed nickel catalysts for achieving high-efficiency H2O2 electrosynthesis

Electrochemical hydrogen peroxide (H2O2) production via two-electron oxygen reduction reaction (2e- ORR) has received increasing attention as it enables clean, sustainable, and on-site H2O2 production. Mimicking the active site structure of H2O2 production enzymes, such as nickel superoxide dismutase, is the most intuitive way to design efficient 2e- ORR electrocatalysts. However, Ni-based catalysts have thus far shown relatively low 2e- ORR activity. In this work, we present the design of high-performing, atomically dispersed Ni-based catalysts (Ni ADCs) for H2O2 production through understanding the formation chemistry of the Ni-based active sites. The use of a precoordinated precursor and pyrolysis within a confined nanospace were found to be essential for generating active Ni-N x sites in high density and increasing carbon yields, respectively. A series of model catalysts prepared from coordinating solvents having different vapor pressures gave rise to Ni ADCs with controlled ratios of Ni-N x sites and Ni nanoparticles, which revealed that the Ni-N x sites have greater 2e- ORR activity. Another set of Ni ADCs identified the important role of the degree of distortion from the square planar structure in H2O2 electrosynthesis activity. The optimized catalyst exhibited a record H2O2 electrosynthesis mass activity with excellent H2O2 selectivity.

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.

Additive manufacturing-derived free-standing 3D pyrolytic carbon electrodes for sustainable microbial electrochemical production of H2O2

Producing H2O2 via microbial electrosynthesis is a cost-effective and environmentally favorable alternative to the costly and environmentally hazardous anthraquinone method. However, most studies have relied on carbon electrodes with two-dimensional (2D) surfaces (e.g., graphite), which have limited surface area and active sites, resulting in suboptimal H2O2 production. In this study, we demonstrate the enhanced efficiency of microbial H2O2 synthesis using three-dimensional (3D) electrodes produced through additive manufacturing technology due to their larger surface area than conventional carbon electrodes with 2D surfaces. This work innovatively combines 3D printed pyrolytic carbon (3D PyrC) electrodes with highly defined outer geometry and internal mesh structures derived from additive manufacturing with high-temperature resin precursors followed by pyrolysis with microbial electrochemical platform technology to achieve efficient H2O2 synthesis. The 3D PyrC electrode produced a maximum of 129.2 mg L-1 of H2O2 in 12 h, which was 2.3-6.9 times greater than conventional electrodes (e.g., graphite and carbon felt). Furthermore, the scalability, reusability and mechanical properties of the 3D PyrC electrode were exemplary, showcasing its practical viability for large-scale applications. Beyond H2O2 synthesis, the study explored the application of the 3D PyrC electrode in the bio-electro-Fenton process, demonstrating its efficacy as a tertiary treatment technology for the removal of micropollutants. This dual functionality underscores the versatility of the 3D PyrC electrode in addressing both the synthesis of valuable chemicals and environmental remediation. This study shows a novel electrode design for efficient, sustainable synthesis of H2O2 and subsequent environmental remediation.

Carbon Catalysts Empowering Sustainable Chemical Synthesis via Electrochemical CO2 Conversion and Two-Electron Oxygen Reduction Reaction

Carbon materials hold significant promise in electrocatalysis, particularly in electrochemical CO2 reduction reaction (eCO2 RR) and two-electron oxygen reduction reaction (2e- ORR). The pivotal factor in achieving exceptional overall catalytic performance in carbon catalysts is the strategic design of specific active sites and nanostructures. This work presents a comprehensive overview of recent developments in carbon electrocatalysts for eCO2 RR and 2e- ORR. The creation of active sites through single/dual heteroatom doping, functional group decoration, topological defect, and micro-nano structuring, along with their synergistic effects, is thoroughly examined. Elaboration on the catalytic mechanisms and structure-activity relationships of these active sites is provided. In addition to directly serving as electrocatalysts, this review explores the role of carbon matrix as a support in finely adjusting the reactivity of single-atom molecular catalysts. Finally, the work addresses the challenges and prospects associated with designing and fabricating carbon electrocatalysts, providing valuable insights into the future trajectory of this dynamic field.

Fabrication of a Co-Mo-Based Metal-Organic Framework for Growth of Double-Walled Carbon Nanotubes

Double-walled carbon nanotubes (DWCNTs) make up a unique class of carbon nanotubes (CNTs) that are particularly intriguing for scientific research and are promising candidates for technological applications. A more precise level of control and greater yields can be achieved via catalytic chemical vapor deposition (CCVD), which involves the breakdown of a carbonaceous gas over nanoparticles. The addition of molybdenum to the system can increase the selectivity with regard to the number of walls that exist in the obtained CNTs. As reported herein, we have designed and synthesized a novel Co-Mo-MOF, [Co(3-bpta)1.5(MoO4)]·H2O (where 3-bpta = N,N'-bis(3-pyridyl)terephthalamide), and employed the Co-Mo-MOF as a bimetallic catalyst precursor for the CCVD approach to prepare high-quality DWCNTs. The Co-Mo-MOF was employed after being calcined in N2 and H2 at 1100 °C and decomposing into CoO, CoMoO4, and MoO3. Existing CoMoO4 is unaltered after reduction in H2 at 1100 °C, while CoO and MoO3 are converted into Co0 and MoO2, and more CoMoO4 is created at the expense of Co0 and MoO2 without clearly defining agglomeration. Finally, the interaction between metallic Co particles and C2H4 is what initiates the formation of DWCNTs. In-depth discussion is provided in this paper regarding the mechanism underlying the high selectivity and activity of Co-Mo catalysts in regulating the development and structure of DWCNTs. The DWCNTs also offer excellence performance when they are used as water purification agents and as selective sorbents. This work opens a feasible way to use MOFs as a way to produce MWCNTs, thus blazing a new trail in the field of MOF-derived carbon-based materials.

Synthesis of F-doped materials and applications in catalysis and rechargeable batteries

Elemental doping is one of the most essential techniques for material modification. It is well known that fluorine is considered to be a highly efficient and inexpensive dopant in the field of materials. Fluorine is one of the most reactive elements with the highest electronegativity (χ = 3.98). Compared to cationic doping, anionic doping is another valuable method for improving the properties of materials. Many materials have physicochemical limitations that affect their practical application in the field of catalysis and rechargeable ion batteries. Many researchers have demonstrated that F-doping can significantly improve the performance of materials for practical applications. This paper reviews the applications of various F-doped materials in photocatalysis, electrocatalysis, lithium-ion batteries, and sodium-ion batteries, as well as briefly introducing their preparation methods and mechanisms to provide researchers with more ideas and options for material modification.

Promotion of the Efficient Electrocatalytic Production of H2O2 by N,O- Co-Doped Porous Carbon

H₂O₂ generation via an electrochemical two-electron oxygen reduction (2e^- ORR) is a potential candidate to replace the industrial anthraquinone process. In this study, porous carbon catalysts co-doped by nitrogen and oxygen are successfully synthesized by the pyrolysis and oxidation of a ZIF-67 precursor. The catalyst exhibits a selectivity of ~83.1% for 2e^- ORR, with the electron-transferring number approaching 2.33, and generation rate of 2909.79 mmol g^-1 h^-1 at 0.36 V (vs. RHE) in KOH solution (0.1 M). The results prove that graphitic N and -COOH functional groups act as the catalytic centers for this reaction, and the two functional groups work together to greatly enhance the performance of 2e^- ORR. In addition, the introduction of the -COOH functional group increases the hydrophilicity and the zeta potential of the carbon materials, which also promotes the 2e^- ORR. The study provides a new understanding of the production of H₂O₂ by electrocatalytic oxygen reduction with MOF-derived carbon catalysts.

Singlet oxygen generation for selective oxidation of emerging pollutants in a flow-by electrochemical system based on natural air diffusion cathode

The decay of free radicals involved in side reactions is one of the challenges faced by electrochemical degradation of organic pollutants. To this end, a non-radical oxidation system was constructed by a natural air diffusion cathode (ADC) and a Ti-based dimensional stable anode coated by RuO₂ (RuO₂-Ti anode) for cathodic hydrogen peroxide activation by anodic chlorine evolution. The ADC fabricated by the carbon black of BP2000 produced a stable concentration of hydrogen peroxide of 339.94 mg L^-1 (current efficiency of 73.4%) without aeration, which was superior to the cathode made by the XC72 carbon black. The flow-by ADC-RuO₂ system consisted of an ADC and a RuO₂-Ti anode showed high selectivity to aniline (AN) compared to benzoate (BA) in a NaCl electrolyte, whose degradation efficiencies were 97.72% and 1.3%, respectively. Rapid degradations of a mixture of emerging pollutants and AN were also observed in the ADC-RuO₂ system, with pseudo-first-order kinetic constants of 0.51, 1.29, 0.89, and 0.99 min^-1 for Bisphenol A (BPA), tetracycline (TC), sulfamethoxazole (SMX) and AN, respectively. Quenching experiments revealed the main reactive oxygen species for the pollutant degradation was singlet oxygen (¹O₂), which was also identified by the electron spin resonance (ESR) analysis. Finally, the steady-stable content of ¹O₂ was quantitatively determined to be 6.25 × 10^-11 M by the method of furfuryl alcohol (FFA) probe. Our findings provide a fast, low energy consumption and well controlled electrochemical oxidation method for selective degradation of organic pollutants. H₂O₂ generated on an air diffusion cathode by naturally diffused O₂, reacts with ClO^- produced from chloride oxidation on the RuO₂-Ti anode to form singlet oxygen (¹O₂). The electrochemical system shows an efficient oxidation to electron-rich emerging pollutants including bisphenol A, tetracycline, sulfamethoxazole and aniline, but a poor performance on the electron-deficient compounds (e.g., benzoate).

Electrochemical Methods for Water Purification, Ion Separations, and Energy Conversion

Agricultural development, extensive industrialization, and rapid growth of the global population have inadvertently been accompanied by environmental pollution. Water pollution is exacerbated by the decreasing ability of traditional treatment methods to comply with tightening environmental standards. This review provides a comprehensive description of the principles and applications of electrochemical methods for water purification, ion separations, and energy conversion. Electrochemical methods have attractive features such as compact size, chemical selectivity, broad applicability, and reduced generation of secondary waste. Perhaps the greatest advantage of electrochemical methods, however, is that they remove contaminants directly from the water, while other technologies extract the water from the contaminants, which enables efficient removal of trace pollutants. The review begins with an overview of conventional electrochemical methods, which drive chemical or physical transformations via Faradaic reactions at electrodes, and proceeds to a detailed examination of the two primary mechanisms by which contaminants are separated in nondestructive electrochemical processes, namely electrokinetics and electrosorption. In these sections, special attention is given to emerging methods, such as shock electrodialysis and Faradaic electrosorption. Given the importance of generating clean, renewable energy, which may sometimes be combined with water purification, the review also discusses inverse methods of electrochemical energy conversion based on reverse electrosorption, electrowetting, and electrokinetic phenomena. The review concludes with a discussion of technology comparisons, remaining challenges, and potential innovations for the field such as process intensification and technoeconomic optimization.

Microbial Electrolysis Cell as a Diverse Technology: Overview of Prospective Applications, Advancements, and Challenges

Microbial electrolysis cells (MECs) have been explored for various applications, including the removal of industrial pollutants, wastewater treatment chemical synthesis, and biosensing. On the other hand, MEC technology is still in its early stages and faces significant obstacles regarding practical large-scale implementations. MECs are used for energy generation and hydrogen peroxide, methane, hydrogen/biohydrogen production, and pollutant removal. This review aimed to investigate the aforementioned uses in order to better understand the different applications of MECs in the following scenarios: MECs for energy generation and recycling, such as hydrogen, methane, and hydrogen peroxide; contaminant removal, particularly complex organic and inorganic contaminants; and resource recovery. MEC technology was examined in terms of new concepts, configuration optimization, electron transfer pathways in biocathodes, and coupling with other technologies for value-added applications, such as MEC anaerobic digestion, combined MEC–MFC, and others. The goal of the review was to help researchers and engineers understand the most recent developments in MEC technologies and applications.

Enhancing catalytic ozonation activity of MCM-41 via one-step incorporating fluorine and iron: The interfacial reaction induced by hydrophobic sites and Lewis acid sites

Fe-MCM-41 had been widely used as ozonation catalyst, however, the existence of large amount of hydrophilic silanol hindered its interfacial reaction with O₃ and pollutants. To solve this problem, F-Fe-MCM-41 was synthesized by co-doping F and Fe into the framework of MCM-41 to replace silanol with Si-F groups through a one-step hydrothermal method. F introduced hydrophobic sites which contributed to more ibuprofen (IBP) chemisorption on the surface of F-Fe-MCM-41. Moreover, doping F also enhanced the acidity, which accelerated O₃ decomposition into •OH. F-Fe-MCM-41/O₃ exhibited notably activity with 96.6% IBP removal efficiency within 120 min, while only 78.5% and 80.9% in O₃ alone and Fe-MCM-41/O₃, respectively. Surface Lewis acid sites and metal hydroxyl groups were considered as important factors for O₃ activation and •OH generation. F-Fe-MCM-41 exhibited excellent catalytic performance under acidic and alkaline conditions. Comparative experiments revealed that F doping improved the interfacial reaction, especially the interfacial electron transfer, which resulted in the high catalytic activity of F-Fe-MCM-41. F-Fe-MCM-41 possessed good stability and reusability, with only 5.7% decline for IBP removal in five successive cycles. Furthermore, the possible degradation path of IBP was proposed according to DFT calculation and GC-MS analysis.

DNA origami nanocalipers for pH sensing at the nanoscale

A DNA origami nanocaliper is employed as a shape-resolved nanomechanical device, with pH-responsive triplex DNA integrated into the two arms. The shape transition of the nanocaliper results in a subtle difference depending on the local pH that is visible via TEM imaging, demonstrating the potential of these nanocalipers to act as a universal platform for pH sensing at the nanoscale.

A review of recent advances in electrode materials for emerging bioelectrochemical systems: From biofilm-bearing anodes to specialized cathodes

Bioelectrochemical systems (BES), mainly microbial fuel cells (MEC) and microbial electrolysis cells (MFC), are unique biosystems that use electroactive bacteria (EAB) to produce electrons in the form of electric energy for different applications. BES have attracted increasing attention as a sustainable, low-cost, and neutral-carbon option for energy production, wastewater treatment, and biosynthesis. Complex interactions between EAB and the electrode materials play a crucial role in system performance and scalability. The electron transfer processes from the EAB to the anode surface or from the cathode surface to the EAB have been the object of numerous investigations in BES, and the development of new materials to maximize energy production and overall performance has been a hot topic in the last years. The present review paper discusses the advances on innovative electrode materials for emerging BES, which include MEC coupled to anaerobic digestion (MEC-AD), Microbial Desalination Cells (MDC), plant-MFC (P-MFC), constructed wetlands-MFC (CW-MFC), and microbial electro-Fenton (BEF). Detailed insights on innovative electrode modification strategies to improve the electrode transfer kinetics on each emerging BES are provided. The effect of materials on microbial population is also discussed in this review. Furthermore, the challenges and opportunities for materials scientists and engineers working in BES are presented at the end of this work aiming at scaling up and industrialization of such versatile systems.

Scaling-up of microbial electrosynthesis with multiple electrodes for in situ production of hydrogen peroxide

Microbial electrosynthesis system (MES) has recently been shown to be a promising alternative way for realizing in situ and energy-saving synthesis of hydrogen peroxide (H₂O₂). Although promising, the scaling-up feasibility of such a process is rarely reported. In this study, a 20-L up-scaled two-chamber MES reactor was developed and investigated for in situ and efficient H₂O₂ electrosynthesis. Maximum H₂O₂ production rate of 10.82 mg L⁻¹ h⁻¹ and cumulative H₂O₂ concentration of 454.44 mg L⁻¹ within 42 h were obtained with an input voltage of 0.6 V, cathodic aeration velocity of 0.045 mL min⁻¹ mL⁻¹, 50 mM Na₂SO₄, and initial pH 3. The electrical energy consumption regarding direct input voltage was only 0.239 kWh kg⁻¹ H₂O₂, which was further much lower compared with laboratory-scale systems. The obtained results suggested that the future industrialization of MES technology for in situ synthesis of H₂O₂ and further application in environmental remediation have broad prospects.

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Up-scaled microbial electrosynthesis with multiple electrodes to synthesize H₂O₂

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The H₂O₂ yield was higher than that of laboratory-scale systems using graphite cathode

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Energy consumption was lower than that of laboratory-scale (bio)electrochemical systems

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Systematic evaluation of the influence of operating parameters on H₂O₂ production

A computational study on the reduction of O2 to H2O2 using small polycyclic aromatic molecules

This work presents the complete autoxidation pathway for the anthraquinone process and one alternative catalyst that overcomes its kinetic challenges.