Enhancing photocatalytic hydrogen peroxide generation by tuning hydrazone linkage density in covalent organic frameworks

Linkages Chemistry of Covalent Organic Frameworks in Photocatalysis and Electrocatalysis

Covalent organic frameworks (COFs) have emerged as promising candidates for electrocatalysis and photocatalysis applications due to their structurally ordered architectures and tunable physicochemical properties. In COFs, organic building blocks are linked via covalent bonds, and the structural and electronic characteristics of COFs are critically governed by their linkage chemistry. These linkages influence essential material attributes including surface area, crystallinity, hydrophobicity, chemical stability, and the optoelectronic behavior (e.g., photoelectron separation efficiency, electron conductivity, and reductive activity), which collectively determine catalytic performance in energy conversion systems. A systematic understanding of linkage engineering in COFs not only advances synthetic methodologies but also provides innovative solutions to global energy and environmental challenges, thereby accelerating the development of sustainable technologies for clean energy production and environmental remediation.

Asymmetric Structural Design in Cobaloxime-Integrated Covalent Organic Frameworks to Facilitate Photocatalytic Overall Water Splitting

Herein, we report a hydrogen-bonded supramolecular hybrid composed of a cobaloxime complex and a covalent organic framework (COF) that achieves photocatalytic overall water splitting. The designed COF features ternary units and dual linkages, which induce asymmetric linkers. Upon photoexcitation, this asymmetry facilitates directed electron transfer and spatially separated redox sites, resulting in a long-lived charge-separated state. Consequently, the COF, hydrogen-bonded with cobaloxime as a hydrogen (H2) production cocatalyst, enables visible-light photocatalytic overall water splitting, simultaneously producing H2 and hydrogen peroxide (H2O2), outperforming the COFs with symmetric linker structures. This strategy of asymmetric structural design offers new insights for the design of photocatalysts with suppressed charge recombination for photocatalysis.

A Dimolybdenum Paddlewheel Embedded Covalent Organic Framework for Photocatalytic Hydrogen Peroxide Generation

In the midst of the global energy crisis, the conversion of solar energy into chemical energy or high-value chemicals has become critical. Hydrogen peroxide (H2O2), a versatile oxidizing agent, plays an important role in chemical synthesis, medical disinfection, and clean energy generation via fuel cells. Recently, photocatalytic H2O2 generation from water and oxygen using covalent organic frameworks (COFs) and metal-organic frameworks (MOFs) has emerged as a sustainable approach. In this context, a novel dimolybdenum paddlewheel-embedded COF (Mo-DHTA COF) is presented and synthesized for photocatalytic H2O2 generation. It is observed that Mo sites help to bind the oxygen molecule, while high-energy valence band electrons localized on the α-hydroquinone moiety of the COF facilitate efficient photoelectron transfer for oxygen reduction. Simultaneously, the reduced electron density above the hydroxy groups in the conduction band serves as a proton source during H2O2 production. Due to these synergistic effects, Mo-DHTA COF exhibited an H2O2 production rate of 626 µmol g-1 h-1 in aqueous ethanol and 4084 µmol g-1 h-1 in aqueous benzyl alcohol, which is found to be higher than the polymeric counterpart of Mo-DHTA (Mo-DHTA-P). This innovative design highlights the potential of metal-embedded crystalline and porous COFs for advanced photocatalytic applications.

Assisting a Type-II Heterojunction with the LSPR Effect for Realizing Photocatalytic Hydrogen Peroxide Evolution with NIR Apparent Quantum Efficiency Exceeding 0.5

Designing heterojunction catalysts for the production of hydrogen peroxide is a crucial strategy for advancing the field of artificial photosynthesis. However, conventional type-II heterojunction catalysts often face challenges of weak redox ability and the utilization of charge carriers. Herein, a distinct strategy is proposed that combines type-II heterojunctions with a localized surface plasmon resonance (LSPR) effect, thereby cooperatively enhancing the utilization of high-energy electrons through the hot electron injection process. The optimized catalyst MoO3-x-ZnIn2S4 (VMZS) exhibits H2O2 production (47.2 μmol g-1 min-1) under simulated sunlight (AM1.5G, 100 mW cm-2) with a filter (λ > 350 nm) and an apparent quantum efficiency of 0.5% at 940 nm, significantly exceeding previously reported state-of-the-art catalysts. Moreover, the prepared film of VMZS enables a H2O2 production rate of 338.1 μM h-1. This work provides a new insight on designing heterojunction catalyst systems through the synergistic contribution of the type-II carrier transfer route and LSPR effect.

Revealing the Mechanism of Exciton Spontaneous Separation at Room Temperature for Efficient Photocatalytic Hydrogen Peroxide Synthesis

The photocatalytic synthesis of hydrogen peroxide (H2O2) at room temperature has garnered significant attention as an environmentally friendly alternative to traditional anthraquinone oxidation processes. However, the low exciton dissociation efficiency at room temperature often hinders photocatalytic performance. In this study, it is demonstrated that tuning the substitution sites of electron donors in Donor-Acceptor (D-A) conjugated polymers can significantly enhance exciton dissociation by reducing exciton activation energy, which facilitates the spontaneous separation of excitons at room temperature. For comparison, materials with exciton separation energies ≈89 meV exhibit a hydrogen peroxide production rate of 2692 µmol·g-1·h-1. In contrast, the main material developed in this work, O-PTAQ, demonstrates a substantially lower exciton separation energy of 22 meV, resulting in a hydrogen peroxide production rate of 4989 µmol·g-1·h-1 under ambient conditions, outperforming most reported organic semiconductors. This enhancement is attributed to the increased electron delocalization in the electron donors, which lowers exciton activation energy to promote efficient exciton separation. The findings highlight the critical role of molecular-level structural tuning in enhancing exciton dissociation, providing a promising strategy for the development of high-efficiency photocatalysts for sustainable H2O2 production.

Regulating Electron Distribution in Regioisomeric Covalent Organic Frameworks for Efficient Solar-Driven Hydrogen Peroxide Production

Covalent organic frameworks (COFs) are emerging as a transformative platform for photocatalytic hydrogen peroxide (H2O2) production due to their highly ordered structures, intrinsic porosity, and molecular tunability. Despite their potential, the inefficient utilization of photogenerated charge carriers in COFs significantly restrains their photocatalytic efficiency. This study presents two regioisomeric COFs, α-TT-TDAN COF and β-TT-TDAN COF, both incorporating thieno[3,2-b]thiophene moieties, to investigate the influence of regioisomerism on the excited electron distribution and its impact on photocatalytic performance. The β-TT-TDAN COF demonstrates a remarkable solar-to-chemical conversion efficiency of 1.35%, outperforming its α-isomeric counterpart, which is merely 0.44%. Comprehensive spectroscopic and computational investigations reveal the critical role of subtle substitution change in COFs on their electronic properties. This structural adjustment intricately connects molecular structure to charge dynamics, enabling precise regulation of electron distribution, efficient charge separation and transport, and localization of excited electrons at active sites. Moreover, this finely tuned interplay significantly enhances the efficiency of the oxygen reduction reaction. These findings establish a new paradigm in COF design, offering a molecular-level strategy to advance COFs and reticular materials toward highly efficient solar-to-chemical energy conversion.

Rational Design of Covalent Organic Frameworks for Photocatalytic Hydrogen Peroxide Production

Photocatalytic production of hydrogen peroxide (H2O2) represents a significant approach to achieving sustainable energy generation through solar energy, addressing both energy shortages and environmental pollution. Among various photocatalytic materials, covalent organic frameworks (COFs) have gained widespread attention and in-depth research due to their unique advantages, including high porosity, predesignability, and atomic-level tunability. In recent years, significant progress has been made in the development, performance enhancement, and mechanistic understanding of COF-based photocatalysts. This review focuses on the latest advancements in photocatalytic H2O2 production using COFs, particularly emphasizing the rational design of COF structures to regulate catalytic performance and exploring the fundamental processes involved in photocatalysis. Based on current research achievements in this field, this paper also discusses existing challenges and future opportunities, aiming to provide a reference for the application of COFs in photocatalytic H2O2 production.