Covalent organic frameworks (COFs) are highly desirable for achieving high-efficiency overall photosynthesis of hydrogen peroxide (H2O2) via molecular design. However, precise construction of COFs toward overall photosynthetic H2O2 remains a great challenge. Herein, we report the crystalline s-heptazine-based COFs (HEP-TAPT-COF and HEP-TAPB-COF) with separated redox centers for efficient H2O2 production from O2 and pure water. The spatially and orderly separated active sites in HEP-COFs can efficiently promote charge separation and enhance photocatalytic H2O2 production. Compared with HEP-TAPB-COF, HEP-TAPT-COF exhibits higher H2O2 production efficiency for integrating dual O2 reduction active centers of s-heptazine and triazine moieties. Accordingly, HEP-TAPT-COF bearing dual O2 reduction centers exhibits a remarkable solar-to-chemical energy efficiency of 0.65% with a high apparent quantum efficiency of 15.35% at 420 nm, surpassing previously reported COF-based photocatalysts.

Covalent Organic Frameworks Containing Dual O2 Reduction Centers for Overall Photosynthetic Hydrogen Peroxide Production.

Photocatalytic oxygen reduction reaction (ORR) offers a promising hydrogen peroxide (H2O2) synthetic strategy, especially the one-step two-electron (2e-) ORR route holds great potential in achieving highly efficient and selectivity. However, efficient one-step 2e- ORR is rarely harvested and the underlying mechanism for regulating the ORR pathways remains greatly obscure. Here, by loading sulfone units into covalent organic frameworks (FS-COFs), we present an efficient photocatalyst for H2O2 generation via one-step 2e- ORR from pure water and air. Under visible light irradiation, FS-COFs exert a superb H2O2 yield of 3904.2 μmol h-1 g-1, outperforming most reported metal-free catalysts under similar conditions. Experimental and theoretical investigation reveals that the sulfone units accelerate the separation of photoinduced electron-hole (e--h+) pairs, enhance the protonation of COFs, and promote O2 adsorption in the Yeager-type, which jointly alters the reaction process from two-step 2e- ORR to the one-step one, thereby achieving efficient H2O2 generation with high selectivity.

Sulfone-Modified Covalent Organic Frameworks Enabling Efficient Photocatalytic Hydrogen Peroxide Generation via One-Step Two-Electron O2 Reduction.

In the last decade, interfacial solar steam generation (ISSG), powered by natural sunlight garnered significant attention due to its great potential for low-cost and environmentally friendly clean water production in alignment with the global decarbonization efforts. In this review, we aim to share the knowledge and engage with a broader readership about the current progress of ISSG technology and the facing challenges to promote further advancements towards practical applications. The first part of this review assesses the current strategies for enhancing energy efficiency of ISSG systems, including optimizing light absorption, reducing energy losses, harvesting additional energy, and lowering evaporation enthalpy. Subsequently, we elucidate the current challenges faced by ISSG technologies, notably salt accumulation and bio-fouling issues in practical applications and discuss contemporary methods to overcome these challenges. In the end, we introduce potential applications of ISSG, ranging from initial seawater desalination and industrial wastewater purification to power generation, sterilization, soil remediation, and innovative concept of solar sea farm, highlighting the promising potential of ISSG technology in contributing to sustainable and environmentally conscious practices. Based on the review and in-depth understanding of these aspects, we propose the future research focuses to address potential issues in both fundamental research and practical applications. This article is protected by copyright. All rights reserved.

Interfacial Solar Evaporation: from Fundamental Research to Applications.

Artificial photosynthesis is a promising strategy for efficient hydrogen peroxide production, but the poor directional charge transfer from bulk to active sites restricts the overall photocatalytic efficiency. To address this, a new process of dipole field-driven spontaneous polarization in nitrogen-rich triazole-based carbon nitride (C3N5) to harness photogenerated charge kinetics for hydrogen peroxide production is constructed. Here, C3N5 achieves a hydrogen peroxide photosynthesis rate of 3809.5 µmol g-1 h-1 and a 2e- transfer selectivity of 92% under simulated sunlight and ultrasonic forces. This high performance is attributed to the introduction of rich nitrogen active sites of the triazole ring in C3N5, which brings a dipole field. This dipole field induces a spontaneous polarization field to accelerate a rapid directional electron transfer process to nitrogen active sites and therefore induces Pauling-type adsorption of oxygen through an indirect 2e- transfer pathway to form hydrogen peroxide. This innovative concept using a dipole field to harness the migration and transport of photogenerated carriers provides a new route to improve photosynthesis efficiency via structural engineering. 

Dipole field in nitrogen-enriched carbon nitride with external forces to boost the artificial photosynthesis of hydrogen peroxide

Designing delocalized excitons with low binding energy (Eb) in organic semiconductors is urgently required for efficient photochemistry because the excitons in most organic materials are localized with a high Eb of >300 meV. In this work, we report the achievement of a low Eb of ∼50 meV by constructing phenothiazine-based covalent organic frameworks (COFs) with inherent crystallinity, porosity, chemical robustness, and feasibility of bandgap engineering. The low Eb facilitates effective exciton dissociation and thus promotes photocatalysis by using these COFs. As a demonstration, we subject these COFs to photocatalytic polymerization to synthesize polymers with remarkably high molecular weight without any requirement of the metal catalyst. Our results can facilitate the rational design of porous materials with low Eb for efficient photocatalysis.

/pdf/phenothiazine-based-covalent-organic-frameworks-with-low-2kwlx9vy.pdf

Phenothiazine-based covalent organic frameworks with low exciton binding energies for photocatalysis

Progress in low dielectric polyimide film – A review

Boosting exciton dissociation by regulating dielectric constant in covalent organic framework for photocatalysis

Conversion of solar energy into heat for water evaporation is of great significance to provide clean and sustainable technology for water purification by using inexhaustible sunlight. In this field, one of the challenges comes from the design of high-performance photothermal materials powerful in light harvesting, light-to-heat conversion, and water activation. Herein, it is demonstrated that rationalization of the ionic covalent organic framework (iCOF) can simultaneously satisfy these multiple requirements and a new iCOF STTP is constructed through the Schiff base chemistry in a rapid microwave-assisted solvothermal route by using a hydrophilic dye molecule safranineT as the ionic building block. The integrated dye-related ionic moieties greatly strengthen the light absorbance (>97%) throughout the entire solar spectrum from UV–vis to the infrared region. The framework ionic moieties provide strong polarization to reduce the exciton dissociation energy for enhanced photothermal effect, and in addition, promote sufficient water activation to decrease the water evaporation enthalpy. As an outcome, the STTP driven solar water evaporator affords a fast water evaporation rate of 3.55 kg h−1 m−2 and high solar-to-vapor efficiency of 95.8%. This study highlights the potential of designing iCOF materials for photothermal applications. 

Ionic Dye Based Covalent Organic Frameworks for Photothermal Water Evaporation

Photocatalytic hydrogen (H2) production from seawater over a photocatalyst is of great significance for the utilization of the earth’s abundant seawater resources and the storage of inexhaustible solar energy with low-energy density. However, the designation of efficient photocatalytic systems by using seawater usually suffers from activation decline when replacing pure water with seawater. Herein, we synthesized three β-ketoenamine-linked covalent organic frameworks (COFs), demonstrating their promotion in the photocatalytic decomposition of seawater relative to pure water under visible-light irradiation. The target COFs were synthesized via a microwave assisted solvothermal method by using 1,3,5-triformylphloroglucinol (Tp) as the aldehyde monomer to react with diamino units with different numbers of benzene rings. By surveying the building blocks, the constructed COF with a high density of β-ketoenamine units exhibited the so far highest photocatalytic H2 evolution rate (41.3 mmol·g−1·h−1) in seawater, about 1.66 times higher than that in pure water. The β-ketoenamine units allowed the in-situ polarization of the COF framework through the adsorption of metal salts when proceeding with the photocatalytic H2 production in seawater. This polarization effect significantly increases the dielectric constant of the organic semiconductor to lower exciton dissociation energy and thus enhances the charge separation and transfer to promote the H2 photoproduction in seawater. 

https://link.springer.com/content/pdf/10.1007/s12274-022-5332-0.pdf

In-situ polarization of covalent organic frameworks in seawater enables enhanced photocatalytic hydrogen evolution under visible-light irradiation

High tunability of both ionic liquids (ILs) and metal organic frameworks (MOFs) enables great opportunity in the rational designation of IL/MOF composites for physical adsorption and separation. Traditionally, cations and anions of ILs as an entirety are combined with MOFs either inside or outside the microchannels. Herein, organic cations of ILs were confined into Cu-BTC and the champion adsorbent is obtained by using 1-propionic acid-3-vinylimidazole bromide as the precursor with a moderate loading amount, exhibiting higher CO2 uptakes of 8/5 mmol g−1 than Cu-BTC (6.0/3.5 mmol g−1) at 273/298 K and 100 kPa, associating with significantly improved CO2/N2(CH4) selectivities. The organic cations are interacted with two adjacent CuII2(CRO2)2 paddle wheel units of Cu-BTC, expanding the CuO bond to strengthen the CO2 affinity of open Cu sites and also serving as an additive CO2 adsorptive site. The promotion of CO2 capture ability is further reflected in the dynamic column breakthrough experiment. 

Confining organic cations in metal organic framework allows molecular level regulation of
 
 CO
 2
 
 capture

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Progress in low dielectric polyimide film – A review

Boosting exciton dissociation by regulating dielectric constant in covalent organic framework for photocatalysis

Ionic Dye Based Covalent Organic Frameworks for Photothermal Water Evaporation

In-situ polarization of covalent organic frameworks in seawater enables enhanced photocatalytic hydrogen evolution under visible-light irradiation

Confining organic cations in metal organic framework allows molecular level regulation of CO 2 capture