Triphasic Nature of Polymers of Intrinsic Microporosity Induces Storage and Catalysis Effects in Hydrogen and Oxygen Reactivity at Electrode Surfaces

Bio-inspired Cu2O cathode for O2 capturing and oxidation boosting in electro-Fenton for sulfathiazole decay

A hydrophobic Cu2O cathode (CuxO-L) was designed to solve the challenge of low oxidation ability in electro-Fenton (EF) for treating emerging pollutants. This fabrication process involved forming Cu(OH)2 nanorods by oxidizing copper foam (Cu-F) with (NH4)2S2O8, followed by coating them with glucose via hydrothermal treatment. Finally, a self-assembled monolayer of 1-octadecanethiol was introduced to create a low-surface-energy, functionalized CuxO-L cathode. Results exhibited an approximately 7.9-fold increase in hydroxyl radical (·OH) generation compared to the initial Cu-F. This enhancement was attributed to two key factors: (Ⅰ) the superior O2-capturing ability of CuxO-L cathode, which led to high H2O2 production due to a 2 nm thick hydrophobic gas layer facilitated O2-capturing; (Ⅱ) a relative high concentration of Cu+ at the CuxO-L cathode promoted the activation of H2O2 into·OH. In addition, the performance of EF with the CuxO-L cathode using sulfathiazole (STZ) as a model pollutant was evaluated. This study offers valuable insights into the design of O2-capturing cathodes in EF processes, particularly for treating emerging organic pollutants.

Triphasic Oxygen Storage in Wet Nanoparticulate Polymer of Intrinsic Microporosity (PIM-1) on Platinum: An Electrochemical Investigation

The triphasic interaction of gases with electrode surfaces immersed in aqueous electrolyte is crucial in electrochemical technologies (fuel cells, batteries, sensors). Some microporous materials modify this interaction locally via triphasic storage capacity for gases in aqueous environments linked to changes in apparent oxygen concentration and diffusivity (as well as activity and reactivity). Here, a nanoparticulate polymer of intrinsic microporosity (PIM-1) in aqueous electrolyte is shown to store oxygen gas and thereby enhance electrochemical signals for oxygen reduction in aqueous media. Oxygen reduction current transient data at platinum disk electrodes suggest that the reactivity of ambient oxygen in aqueous electrolyte (typically Doxygen = 2.8 × 10-9 m2 s-1; coxygen = 0.3 mM) is substantially modified (to approximately Dapp,oxygen = 1.6 (±0.3) × 10-12 m2 s-1; capp,oxygen = 50 (±5) mM) with important implications for triphasic electrode processes. The considerable apparent concentration of oxygen even for ambient oxygen levels is important. Potential applications in oxygen sensing, oxygen storage, oxygen catalysis, or applications associated with other types of gases are discussed.

Activated carbon filled in a microporous titanium-foam air diffusion electrode for boosting H2O2 accumulation

In the electro-Fenton process, there still suffers concern of low H₂O₂ generation caused by inadequate mass transfer of oxygen and low selectivity of oxygen reduction reaction (ORR). To solve it, in this study, various particle sizes (850 μm, 150 μm, and 75 μm) of granular activated carbon filled in a microporous titanium-foam substate was used to develop a gas diffusion electrode (AC@Ti-F GDE). This facile-prepared cathode has seen a 176.15% improvement in H₂O₂ formation compared to the conventional one. Aside from a much higher oxygen mass transfer by creating gas-liquid-solid three-phase interfaces coupled with much high dissolved oxygen, the filled AC played a significant role in H₂O₂ accumulation. Among these particle sizes of AC, the one in 850 μm has observed the highest H₂O₂ accumulation, reaching 1487 μM in 2 h electrolysis. Because there is a balance between chemical nature for H₂O₂ formation and micropore-dominant porous structure for H₂O₂ decomposition, resulting in an electron transfer of 2.12 and H₂O₂ selectivity of 96.79% during ORR. In a word, the facial AC@Ti-F GDE configuration is promising for H₂O₂ accumulation.

Critical Review on the Mechanisms of Fe2+ Regeneration in the Electro-Fenton Process: Fundamentals and Boosting Strategies

This review presents an exhaustive overview on the mechanisms of Fe³⁺ cathodic reduction within the context of the electro-Fenton (EF) process. Different strategies developed to improve the reduction rate are discussed, dividing them into two categories that regard the mechanistic feature that is promoted: electron transfer control and mass transport control. Boosting the Fe³⁺ conversion to Fe²⁺ via electron transfer control includes: (i) the formation of a series of active sites in both carbon- and metal-based materials and (ii) the use of other emerging strategies such as single-atom catalysis or confinement effects. Concerning the enhancement of Fe²⁺ regeneration by mass transport control, the main routes involve the application of magnetic fields, pulse electrolysis, interfacial Joule heating effects, and photoirradiation. Finally, challenges are singled out, and future prospects are described. This review aims to clarify the Fe³⁺/Fe²⁺ cycling process in the EF process, eventually providing essential ideas for smart design of highly effective systems for wastewater treatment and valorization at an industrial scale.

State-of-the-art review and bibliometric analysis on electro-Fenton process

The electro-Fenton (EF) process was first proposed in 1996 and, since then, considerable development has been achieved for its application in wastewater treatment, especially at lab and pilot scale. After more than 25 years, the high efficiency, versatility and environmental compatibility of EF process has been demonstrated. In this review, bibliometrics has been adopted as a tool that allows quantifying the development of EF as well as introducing some useful correlations. As a result, information is summarized in a more visual manner that can be easily analyzed and interpreted as compared to conventional reviewing. During the recent decades under review, 83 countries have contributed to the dramatic growth of EF publications, with China, Spain and France leading the publication output. The top 12 most cited articles, along with the top 32 most productive authors in the EF field, have been screened. Four stages have been identified as main descriptors of the development of EF throughout these years, being each stage characterized by relevant breakthroughs. To conclude, a general cognitive model for the EF process is proposed, including atomic, microscopic and macroscopic views, and future perspectives are discussed.

Tröger's Base Network Polymers of Intrinsic Microporosity (TB-PIMs) with Tunable Pore Size for Heterogeneous Catalysis

Heterogeneous catalysis plays a pivotal role in the preparation of value-added chemicals, and it works more efficiently when combined with porous materials and supports. Because of that, a detailed assessment of porosity and pore size is essential when evaluating the performance of new heterogeneous catalysts. Herein, we report the synthesis and characterization of a series of novel microporous Tröger’s base polymers and copolymers (TB-PIMs) with tunable pore size. The basicity of TB sites is exploited to catalyze the Knoevenagel condensation of benzaldehydes and malononitrile, and the dimension of the pores can be systematically adjusted with an appropriate selection of monomers and comonomers. The tunability of the pore size provides the enhanced accessibility of the catalytic sites for substrates, which leads to a great improvement in conversions, with the best results achieving completion in only 20 min. In addition, it enables the use of large benzaldehydes, which is prevented when using polymers with very small pores, typical of conventional PIMs. The catalytic reaction is more efficient than the corresponding homogeneous counterpart and is ultimately optimized with the addition of a small amount of a solvent, which facilitates the swelling of the pores and leads to a further improvement in the performance and to a better carbon economy. Molecular dynamic modeling of the copolymers’ structures is employed to describe the swellability of flexible chains, helping the understanding of the improved performance and demonstrating the great potential of these novel materials.

Effects of g-C3N4 Heterogenization into Intrinsically Microporous Polymers on the Photocatalytic Generation of Hydrogen Peroxide

Graphitic carbon nitride (g-C₃N₄) is known to photogenerate hydrogen peroxide in the presence of hole quenchers in aqueous environments. Here, the g-C₃N₄ photocatalyst is embedded into a host polymer of intrinsic microporosity (PIM-1) to provide recoverable heterogenized photocatalysts without loss of activity. Different types of g-C₃N₄ (including Pt@g-C₃N₄, Pd@g-C₃N₄, and Au@g-C₃N₄) and different quenchers are investigated. Exploratory experiments yield data that suggest binding of the quencher either (i) directly by adsorption onto the g-C₃N₄ (as shown for α-glucose) or (ii) indirectly by absorption into the microporous polymer host environment (as shown for Triton X-100) enhances the overall photochemical H₂O₂ production process. The amphiphilic molecule Triton X-100 is shown to interact only weakly with g-C₃N₄ but strongly with PIM-1, resulting in accumulation and enhanced H₂O₂ production due to the microporous polymer host.

H2O2 production at gas-diffusion cathodes made from agarose-derived carbons with different textural properties for acebutolol degradation in chloride media

The excessive cost, unsustainability or complex production of new highly selective electrocatalysts for H₂O₂ production, especially noble-metal-based ones, is prohibitive in the water treatment sector. To solve this conundrum, biomass-derived carbons with adequate textural properties were synthesized via agarose double-step pyrolysis followed by steam activation. A longer steam treatment enhanced the graphitization and porosity, even surpassing commercial carbon black. Steam treatment for 20 min yielded the greatest surface area (1248 m² g^-1), enhanced the mesopore/micropore volume distribution and increased the activity (E_1/2 = 0.609 V) and yield of H₂O₂ (40%) as determined by RRDE. The upgraded textural properties had very positive impact on the ability of the corresponding gas-diffusion electrodes (GDEs) to accumulate H₂O₂, reaching Faradaic current efficiencies of ~95% at 30 min. Acidic solutions of β-blocker acebutolol were treated by photoelectro-Fenton (PEF) process in synthetic media with and without chloride. In urban wastewater, total drug disappearance was reached at 60 min with almost 50% mineralization after 360 min at only 10 mA cm^-2. Up to 14 degradation products were identified in the Cl^--containing medium.

Polymers of Intrinsic Microporosity in the Design of Electrochemical Multicomponent and Multiphase Interfaces

Polymers of intrinsic microporosity (or PIMs) provide porous materials due to their highly contorted and rigid macromolecular structures, which prevent space-efficient packing. PIMs are readily dissolved in solvents and can be cast into robust microporous coatings and membranes. With a typical micropore size range of around 1 nm and a typical surface area of 700-1000 m² g^-1, PIMs offer channels for ion/molecular transport and pores for gaseous species, solids, and liquids to coexist. Electrode surfaces are readily modified with coatings or composite films to provide interfaces for solid|solid|liquid or solid|liquid|liquid or solid|liquid|gas multiphase electrode processes.

Polymers with intrinsic microporosity (PIMs) for targeted CO2 reduction to ethylene

CO₂ reduction offers an attractive alternative green synthetic route for ethylene, especially where CO₂ could be sourced from industrial exhausts and in combination with green power sources. However, practical applications are currently limited due to the unfortunately low selectivity of cathode materials towards ethylene. This work uses polymers with intrinsic microporosity (PIMs) to improve the performance of copper gas diffusion electrodes for CO₂ reduction to ethylene. We report an improved selectivity and activity towards ethylene with the addition of a thin PIMs layer, which is seen as improved Faradaic efficiency, increased stability and a shift in the reduction to lower overpotential. This improvement is highly dependent on the thickness of the added polymer layer, with too thick a layer having a detrimental impact on the hydrophobicity of the gas diffusion layer. With a compromise in loading, PIMs can be used to enhance the activity and selectivity of catalysts for targeted CO₂ reduction to ethylene.

Indirect photo-electrochemical detection of carbohydrates with Pt@g-C3N4 immobilised into a polymer of intrinsic microporosity (PIM-1) and attached to a palladium hydrogen capture membrane

An "indirect" photo-electrochemical sensor is presented for the measurement of a mixture of analytes including reducing sugars (e.g. glucose, fructose) and non-reducing sugars (e.g. sucrose, trehalose). Its innovation relies on the use of a palladium film creating a two-compartment cell to separate the electrochemical and the photocatalytic processes. In this original way, the electrochemical detection is separated from the potential complex matrix of the analyte (i.e. colloids, salts, additives, etc.). Hydrogen is generated in the photocatalytic compartment by a Pt@g-C₃N₄ photocatalyst embedded into a hydrogen capture material composed of a polymer of intrinsic microporosity (PIM-1). The immobilised photocatalyst is deposited onto a thin palladium membrane, which allows rapid pure hydrogen diffusion, which is then monitored by chronopotentiometry (zero current) response in the electrochemical compartment. The concept is demonstrated herein for the analysis of sugar content in commercial soft drinks. There is no requirement for the analyte to be conducting with electrolyte or buffered. In this way, samples (biological or not) can be simply monitored by their exposition to blue LED light, opening the door to additional energy conversion and waste-to-energy applications.

Platinum Nanoparticle Inclusion into a Carbonized Polymer of Intrinsic Microporosity: Electrochemical Characteristics of a Catalyst for Electroless Hydrogen Peroxide Production

The one-step vacuum carbonization synthesis of a platinum nano-catalyst embedded in a microporous heterocarbon (Pt@cPIM) is demonstrated. A nitrogen-rich polymer of an intrinsic microporosity (PIM) precursor is impregnated with PtCl₆2- to give (after vacuum carbonization at 700 °C) a nitrogen-containing heterocarbon with embedded Pt nanoparticles of typically 1⁻4 nm diameter (with some particles up to 20 nm diameter). The Brunauer-Emmett-Teller (BET) surface area of this hybrid material is 518 m² g-1 (with a cumulative pore volume of 1.1 cm³ g-1) consistent with the surface area of the corresponding platinum-free heterocarbon. In electrochemical experiments, the heterocarbon-embedded nano-platinum is observed as reactive towards hydrogen oxidation, but essentially non-reactive towards bigger molecules during methanol oxidation or during oxygen reduction. Therefore, oxygen reduction under electrochemical conditions is suggested to occur mainly via a 2-electron pathway on the outer carbon shell to give H₂O₂. Kinetic selectivity is confirmed in exploratory catalysis experiments in the presence of H₂ gas (which is oxidized on Pt) and O₂ gas (which is reduced on the heterocarbon surface) to result in the direct formation of H₂O₂.