Photocatalytic Production of Hydrogen Peroxide from Covalent-Organic-Framework-Based Materials: A Mini-Review

Abstract

Hydrogen peroxide (H₂O₂) is one of the most environmentally friendly and versatile chemical oxidizing agents, with only O₂ and H₂O as reaction products. It is widely used in environmental protection, industrial production, and medical fields. At present, most of the industrial production of H₂O₂ adopts anthraquinone oxidation, but there are shortcomings such as pollution of the environment and large energy consumption. Covalent organic frameworks (COFs) are a class of porous crystalline materials formed by organic molecular building blocks connected by covalent bonds. The ordered conjugated structure of COFs not only facilitates the absorption of light energy but also promotes the transport of excited-state electrons. Therefore, the photochemical synthesis of H₂O₂ from water and oxygen using photocatalysts based on COFs as a green route has attracted much attention. In this review, we provide an overview of recent studies on COFs as photocatalysts and the different mechanisms involved in the photocatalytic production of hydrogen peroxide. Then, we summarize the various strategies to improve the performance. Finally, we outline the challenges and future directions of COFs in practical applications. This review highlights the potential and application prospects of COFs in the photochemical synthesis of H₂O₂, aiming to provide guidance for the design of COF-based catalysts and the optimization for photocatalytic production of H₂O₂, in order to promote scientific development and application in this field.

1. Introduction

As a clean and inexhaustible source of energy, solar energy is a good substitute for traditional fossil energy. Photocatalytic technology is a significant milestone in the exploration of sustainable energy, providing new ideas for stimulating chemical reactions [1,2]. Photocatalytic synthesis of hydrogen peroxide is a significant example that could provide a green and safe route to the field of H₂O₂ synthesis. H₂O₂ was first synthesized by Louis Jacques Thénard in 1818 [3]. Since hydrogen peroxide is regarded as one of the cleanest and universal chemical oxidants [4], with only O₂ and H₂O as reaction products, the scope of its application is constantly expanding, being widely used in environmental protection, industrial production, and medicine [5,6,7].

Currently, the dominant method for industrial production of hydrogen peroxide is the anthraquinone oxidation (AO) process, which includes the hydrogenation of anthraquinone followed by oxidation of the obtained dihydroanthraquinone to produce H₂O₂. However, the AO process is associated with high costs due to the extraction solvent recovery, hydrogenation catalyst, and expensive quinone required [8,9,10]. In addition, this method has limitations such as unsuitability for daily on-site use, environmental pollution, and significant energy consumption, which impede the wide application of hydrogen peroxide as a green oxidant and fuel. In an alternative method, H₂O₂ can be directly synthesized from mixtures of H₂ and O₂ [11,12]. However, one major challenge lies in the precise control of the hydrogen-to-oxygen ratio. The direct oxidation reaction of H₂O and O₂ at high temperatures and pressures increases the risk of safety hazards such as explosions and fires. As a result, a safe and environmentally friendly method for producing H₂O₂ is urgently needed.

In recent years, the process of photocatalytic production of H₂O₂ has received increasing attention [13,14], providing a new perspective on H₂O₂ synthesis. Previous studies have synthesized various photocatalysts for photocatalytic hydrogen peroxide production. Currently, typical catalyst materials include graphitic carbon nitride (g-C₃N₄) [15], metal oxides (TiO₂, WO₃, ZnO) [16,17], metal sulfides (ZnS, CdS, In₂S₃) [18,19,20], and organic frameworks (MOF and COF) [21,22,23]. In 2014, Hirai and co-workers [24] demonstrated the feasibility of producing H₂O₂ with the metal-free polymer photocatalyst g-C₃N₄ for the first time. Despite the good selectivity and innovativeness of g-C₃N₄, its fast electron–hole complexation and insufficient light absorption result in a low H₂O₂ yield (only 30 μmol of H₂O₂ is formed in 12 h of reaction) with ethanol as a sacrificial agent. Since then, researchers have been exploring different methods to exploit the potential of g-C₃N₄ photocatalytic hydrogen peroxide production. Compared to inorganic photocatalysts, organic photocatalysts offer a wider range of precise chemical and electronic modifications at the molecular level due to the diverse constituent units and superior adjustability. COFs were first discovered by Yaghi and colleagues in 2005 [25]. As novel metal-free polymer porous materials, COFs have attracted significant attention owing to their high specific surface area, uniform porosity, and adjustable structural properties [26]. Over the past two decades, researchers have focused on constructing functional COFs with desired properties and expanding their applications.

In 2020, Van Der Voort and co-authors successfully used COFs as photocatalysts to produce H₂O₂ [22], opening up new possibilities for metal-free photocatalysis powered by solar energy. Different from inorganic catalysts, metal-free polymer photocatalysts usually exhibit better H₂O₂ generation performance, probably because of their high selectivity for two-electron O₂ reduction and low activity for H₂O₂ decomposition [27]. Since then, there has been a boom in the study of COFs for photocatalytic hydrogen peroxide production (Table 1). Through in-depth study of structure and properties, it is expected that highly efficient and stable COF-based photocatalysts will be developed to provide a new solution for photocatalytic hydrogen peroxide generation.

However, in recent years, it has been challenging for researchers to screen and summarize the information from the large number of research reports in the field of photocatalytic H₂O₂ production from COFs. In this review, we intend to elaborate on the research results of COFs in H₂O₂ photocatalytic synthesis in the last three years from the perspectives of the reaction mechanism, COF design, and optimization. Following this, the main challenges in the current research as well as the future research directions are analyzed, aiming to advance the further development of COF-based photocatalysts for practical H₂O₂ synthesis applications.

2. Principles of Photocatalytic COFs for H₂O₂ Production

The production of H₂O₂ from solar energy is a complex multistep process involving multiple aspects of photophysics and photochemistry. The process is divided into three stages: photon absorption, carrier dynamics, and surface reactions (Figure 1). Initially, after the photocatalyst absorbs photons with higher energy than the band gap, electrons (e⁻) are excited into the conduction band (CB) and holes (h⁺) remain in the valence band (VB), forming electron-hole pairs. Subsequently, these electron-hole pairs separate into free carriers, which partly undergo a combination of the bulk phase and partly migrate to the surface of the photocatalyst. Finally, if the free carriers migrating to the surface can overcome the overpotential of each electron transfer step, they can drive the oxygen reduction (ORR) or water oxidation (WOR) half-reaction to produce H₂O₂, with some of the surface carriers undergoing combination. The redox capacity of electrons and holes is closely related to the conduction band minimum (CBM) and valence band maximum (VBM) [28,42,43]. Currently, the field of photocatalytic hydrogen peroxide production is mainly limited by poor solar energy response and high photogenerated carrier combination efficiency. COFs, due to their large surface area, large building units in the skeleton, and abundant bonding reactions, provide abundant catalytic sites and a large space for precise control of their electronic structures by adjusting their conformations to satisfy the needs of various redox reactions [44,45]. In this section, we present COF-based photocatalysts for the synthesis of H₂O₂ via different reaction pathways.

Theoretically, we can classify COF-based catalysts into three categories by the pathway of photocatalytic hydrogen peroxide production: catalysts specifically targeting ORR catalysts, specifically targeting WOR catalysts, and catalysts through dual-channel pathway [46] (Figure 1). However, due to the high structural stability of the H₂O molecule, a large activation energy barrier needs to be overcome to achieve the 2e⁻ water oxidation process for H₂O₂ production. This results in a relative scarcity of COF catalysts specifically designed to produce H₂O₂ via the 2e⁻ WOR process in practice. For practical applications in this research area, COF photocatalysts for photocatalytic generation of H₂O₂ can be mainly classified into two main types: catalysts generating H₂O₂ mainly via the ORR and those via dual-channel pathway with ORR and WOR. In this section, we introduce the latest research on these two types of COF catalysts via different reaction paths. Different functional group modifications and different active sites result in different photocatalytic pathways. For example, in the case of ORR, commonly used active sites include benzene, pyrene, diarylamine, pyridine, and so on. For WOR, active sites include triazine, acetylene, diacetylene, and so on.

2.1. Photocatalytic COFs Mainly via ORR Pathway

The oxygen reduction reaction is easier to achieve in photocatalysis than the water oxidation reaction. The current photocatalytic generation of hydrogen peroxide using COF-based photocatalysts mainly depends on the ORR process. The ORR process leads to the generation of H₂O₂ by two routes: the direct ORR route involving two electron steps and the two-step 1e^- ORR route via a proton-coupled electron transfer (PCET) process (Figure 1). In the PCET process, protons are mainly derived from water molecules or organic electron donors. Compared to the consecutive two-step 1e⁻ ORR pathway, the direct one-step 2e⁻ ORR pathway exhibits more beneficial thermodynamic energy levels (O₂/H₂O₂ 0.68 V_NHE vs. O₂/O₂•⁻ −0.33 V_NHE) [29,47].

Moreover, the direct one-step 2e⁻ ORR process avoids the formation of highly reactive O₂•⁻ intermediates, which reduces side reactions. The O₂•⁻ intermediates are susceptible to aqueous matrixes, such as Cl⁻ and dissolved organic matter in natural water, thus decreasing the efficiency of H₂O₂ production in actual applications [48]. Therefore, regulating the ORR process in the one-step 2e⁻ pathway is important for efficient and highly selective in situ H₂O₂ synthesis. Xi and co-authors [30] synthesized a series of COFs modified with different diazines (include pyrazine, pyrimidine, and pyridazine) for H₂O₂ generation without the use of sacrificial agents (Figure 2a). As pyridazine is embedded in TPDz, it stabilizes endoperoxide intermediate species, improving the efficiency of the direct 2e⁻ ORR process (Figure 2b,c). Compared to pyridazine, the stepwise 1e⁻ pathway performs a more essential function in the photosynthesis of H₂O₂ in TpMd- and TpPz-based photocatalytic processes. The results showed that TpDz via the direct 2e⁻ ORR process had the highest yield of 7327 μmol h⁻¹g⁻¹ (Figure 2d), which was one of the best photocatalysts for H₂O₂ production from COF-based catalysts, indicating the role of the reaction pathway in the catalytic performance.

2.2. Photocatalytic COFs via Dual-Channel Pathway

Photocatalytic H₂O₂ synthesis via the ORR and WOR dual-channel pathways is a non-polluting and sustainable method with greater atom utilization compared with the process mainly via ORR [31]. To achieve highly efficient photosynthesis of H₂O₂, WOR is essential to balance the whole photocatalytic system. However, due to the high thermodynamic requirements of WOR, this process is relatively difficult to realize. Previous studies have often relied on the use of sacrificial agents, such as methanol, to reduce the energy barrier of WOR [32,49]. Nevertheless, the use of sacrificial agents may introduce impurities and generate additional by-products, which limits the practical application of photocatalysts and the purity of H₂O₂. Therefore, the development of COFs capable of efficiently generating H₂O₂ via a dual-channel pathway without sacrificial agents is of great scientific significance and application value. Tang and co-workers [33] successfully constructed two new thiophene-containing COFs, TD-COF and TT-COF (Figure 3a), and used them as catalysts for photocatalytic production of H₂O₂. Highly efficient synthesis of H₂O₂ was achieved under visible light irradiation through the two-step 1e^- ORR and direct 2e⁻ WOR pathways. The performance of photocatalytic H₂O₂ generation was optimized by adapting the n-heterocyclic units such as pyridine and triazine in the COFs. The H₂O₂ yields of TD-COF reached 4060 μmol h⁻¹g⁻¹ and 3364 μmol h⁻¹g⁻¹ in deionized water and natural seawater, respectively, without any sacrificial agents (Figure 3b–d). DFT calculations revealed the thiophene unit as the main active center for photoreduction, whereas the benzene ring, which is connected through imine bonding, is the active center for photo-oxidation.

3. Strategies to Improve Photocatalytic H₂O₂ Production

In this section, we present strategies to improve photocatalytic H₂O₂ production. COF-based photocatalysts have a comparatively broad response spectrum, easily adjustable electronic structure, high specific surface area, and diverse pore structure in the field of H₂O₂ production, which are promising for their applications. Most COFs are competent for generating hydrogen peroxide without a sacrificial agent due to the presence of a conjugated structure that favors electron transfer. However, the concentration of H₂O₂ produced at the current level of research is still comparatively low, which makes COFs unsuitable for practical use. The efficiency of H₂O₂ generation can be significantly improved by strategies such as functional group modification, design of redox center-separated COFs, and synthesis of COF nanohybrids.

3.1. Functional Groups Modification

The incorporation of branched functional groups into organic semiconductors is generally regarded as an excellent method of imparting unique properties to these semiconductors [50,51]. Functionalization is an important means of adapting and optimizing the properties of COFs involving pre-synthetic or post-synthetic modifications to the organic backbone or pore interior of the COFs. The introduction of functional groups (hydroxy group [52], amine group [53], and acetylene [54]) into COFs can change the inner electron arrangement and effectively solve the problem of rapid combination of the inner electrons and holes, thus enhancing the photocatalytic hydrogen peroxide production activity [55]. Han and co-authors designed an excellent photocatalyst by modifying sulfone units into COFs (FS-COFs) (Figure 4a) [29]. Sulfone units in FS-COFs accelerated the separation of photogenerated carriers, facilitating the protonation and promoting the adsorption of oxygen (Figure 4b). The FS-COFs achieved highly selective and efficient H₂O₂ generation through the 2e⁻ ORR process (Figure 4c) with a hydrogen peroxide yield of 3904.2 μmol h⁻¹g⁻¹ under visible light irradiation.

The introduction of functional groups not only promotes the separation of light-induced electron-hole pairs but also contributes to the overall catalytic system. For instance, Guo et al. reported a new fluorinated covalent organic framework strategy to enhance the limitation of palladium metal-isolated clusters (Pd-ICs) by COFs for enhanced photosynthesis of H₂O₂ [34]. The introduction of strongly electronegative fluorine into TAPT-TFPA COFs enhances the interaction between COFs and Pd-ICs, which can optimize the d-band center, thus improving the performance of photocatalytic synthesis of H₂O₂ (Figure 4d). The prepared TAPT-TFPA COFs@Pd-ICs have an efficient photocatalytic H₂O₂ yield that can reach 2143 μmol h⁻¹g⁻¹. More significantly, the stability of TAPT-TFPA COFs@Pd-ICs in the photocatalytic H₂O₂ production process is greatly improved, and the photocatalytic activity can remain stable after more than 100 h (Figure 4e), which is the best stability among reported COF-based photocatalytic materials.

3.2. Design of Spatially Separated Redox Centers

According to the above discussion, photocatalytic hydrogen peroxide production can be achieved by oxygen reduction half-reaction (ORR) and water oxidation half-reaction (WOR). If the ORR and WOR occur at the same or close catalytic sites, the photogenerated electrons and holes involved in the half-reactions will be easily recombined, and the overall catalytic performance will be limited. Hence, spatial separation of the oxidation and reduction centers to inhibit charge recombination is an excellent method to improve the efficiency of H₂O₂ generation [56,57]. This design strategy takes advantage of the structural customizability and functional tunability of COFs. By introducing different organic building blocks into the skeleton of COFs, the redox centers are spatially separated so that the electrons and holes are confined to different regions, which facilitates the effective separation and transfer of charge carriers (Figure 5a). Such a design can increase the lifetime of light-generated electrons and holes, thus enhancing the efficiency of the photocatalytic reaction [58]. Chen and co-authors [35] reported s-heptazine-based COFs (Figure 5b), which possessed separated redox centers and were able to efficiently catalyze the production of hydrogen peroxide from O₂ and pure water via a dual-channel pathway without the addition of sacrificial agents. The spatially separated redox centers of HEP-TAPB-COF and HEP-TAPT-COF efficiently promote charge separation, which improves the performance of photocatalytic production of H₂O₂. The triazine and s-heptazine groups in HEP-TAPT-COF constitute a double active center to produce hydrogen peroxide (87.50 μmol  h⁻¹), which has better performance than HEP-TAPB-COF with only the s-heptazine unit involved in the generation of H₂O₂ (49.50 μmol  h⁻¹).

Wang et al. [36] designed benzotrithiophene (Btt)-based COFs with spatially separated redox centers for the photocatalytic synthesis of H₂O₂ from H₂O and O₂ without sacrificial agents (Figure 6a,b). DFT calculations showed that the binding of Btt to various functional groups (Tpa, Tapb, and Tapt) modulates the electron distribution on the C atom near the imine bond, thus enhancing the two-step 1e^- oxygen reduction reaction by facilitating the in-plane charge transfer and the binding intensity of the O₂* and OOH* intermediates. This structural design achieves spatial separation of the redox centers in the COF, thus facilitating efficient charge separation and transfer. The yield of TaptBtt to H₂O₂ was 1407 μmol h⁻¹g⁻¹ with a photochemical conversion efficiency of 0.296%.

3.3. COF Nanohybrids

As most covalent organic frameworks are composed entirely of non-metallic elements, the centers of catalytic activity are limited, resulting in low catalytic efficiency. Numerous studies have demonstrated that combining COFs with different types of nanomaterials such as metals (Au [59]), metal sulfides (In₂S₃ [60], CdS [61]), metal oxides (TiO₂ [62], ZnO [63], WO₃ [64]), metal organic frameworks (MOFs [63]), and carbon-based materials (g-C₃N₄ [65], CQDs [66]) can significantly enhance the efficiency of COFs in photocatalytic reactions. This combination not only greatly enhances the light absorption of COF-based photocatalysts but also forms a heterojunction structure for enhanced photocatalytic performance [67]. Therefore, the development of COF-based heterostructures is not only scientifically prospective but also shows extensive potential and value in practical photocatalytic hydrogen peroxide production reactions. Lu and co-authors synthesized floral inorganic/organic (CdS/TpBpy) composites as an s-type heterojunction using a simple solvothermal–hydrothermal method [37] (Figure 7a–d). The unique floral structure increases the active sites and O₂ absorption rate, and the s-type heterojunction facilitates the photogenerated carrier transfer in the internal embedded electric field. The optimal CdS/TpBpy H₂O₂ yield (3600 μmol  h⁻¹) is 2.4 and 25.6 times higher than that of TpBpy and CdS, respectively, without sacrificial reagents (Figure 7e–g). What is more, for CdS/TpBpy the decomposition of H₂O₂ was suppressed, thus raising the total yield. Wang and co-authors synthesized S-scheme BiOBr/COF photocatalysts by in situ growth of BiOBr nanosheets (NSs) on COFs with large π-conjugated structures [68]. The highest performance of H₂O₂ production by BiOBr/COF photocatalysis was 3749 μmol h⁻¹g⁻¹, which was about 2 and 27 times higher than the H₂O₂ production rates of COF and BiOBr, respectively. The construction of s-type heterojunctions contributes to effective photogenerated carrier separation and transfer, thereby improving redox efficiency. In addition, the lying-down O₂⁻ adsorption configuration on the BiOBr/COF surface facilitates 2e⁻ ORR, which extremely reduces the reduction potential demand for the conversion of oxygen to hydrogen peroxide and favors the reaction. In this system, the synergistic effect between the s-type heterojunction and the apparent interaction between O₂ and COF enhance the activity of photocatalytic generation of H₂O₂.

3.4. Linkage Chemistry

COFs are organic monomers linked together by strong covalent bonds, and more than two dozen different linkages have been developed to construct COFs. Among them, triazine, C=C, and imine are common connecting bonds. At present, most COFs are synthesized by solvothermal methods under vacuum or inert atmosphere conditions [69]. Furthermore, microwave synthesis and mechanical synthesis can also synthesize COFs, which are characterized by short reaction times and high yields [70]. Moreover, multi-component reaction (MCR) [64] synthesis methods can promote structural diversification of the constructs. In past studies, it has been shown that linkage chemistry determines COF electronic properties and catalytic behavior. Tang et al. designed a pair of isomeric COFs (TB-COF and TA-COF) with anti-substituted amine bonds [71]. After a series of performance evaluations, it was found that TB-COF exhibits significant advantages over TA-COF in H₂O₂ photosynthesis. This is mainly attributed to the larger dipole moment and superior carrier separation efficiency of TB-COF. Further theoretical calculations reveal the internal structural properties of TB-COF, in which the hydroxyl-rich benzene ring assumes the role of photo-oxidation, while the triazine unit is responsible for the photoreduction process. This finding reveals the possibility of optimizing the electronic structure and energy barriers of COFs by adjusting the bond orientation to effectively enhance the photocatalytic activity, which provides a new strategy and guidance for the design of efficient photocatalytic COF materials. Yu et al. constructed o-COFs and p-COFs with ortho- and para-linkages [72]. O-COFs have higher hydrogen peroxide generation efficiency and stability compared to p-COFs. Furthermore, o-COFs with ortho-linkages are more robust than those with para-linkages, due to the lower energy barrier of o-COFs in the 4e⁻ WOR that determines the rate step. This finding highlights the critical role of the linkage position in COFs in facilitating H₂O₂ generation.

3.5. Dimensions Control

Tuning the dimensions of the COF can make the COF-based material exhibit different properties. Although the topological diversity of COFs has been extensively studied, most COFs consist of two-dimensional (2D) networks stacked on top of each other. For 2D COFs, which are usually stacked in a face-to-face manner, some of the structures in neighboring layers have strong Π–Π interactions. Compared to 2D COFs with laminated structures, 3D COFs usually outperform 2D COFs and show superior competitiveness in terms of performance by virtue of their significantly larger specific surface area, tightly interconnected channel structure, full exposure of the functional part, and highly flexible tunability. In contrast, the dense interlayer stacking of 2D COFs results in a large number of potential active sites being “buried” by interlayer interactions, limiting their efficiency in photocatalytic hydrogen peroxide production [73,74]. However, the structural diversity of 3D COFs is greatly limited by the lack of multifaceted three-dimensional molecular building blocks and crystallization difficulties [75]. Wang et al. demonstrated a study on the synthesis and property characterization of 2D-PdPor-COF and 3D-PdPor-COF to consider the effect of dimensionality on the COF properties [76]. The results show that 3D-PdPor-COF has superior photocatalytic hydrogen peroxide production activity compared with 2D-PdPor-COF. In addition, the pore size of 3D-PdPor-COF is considerably smaller than those of 2D-PdPor-COF, and 3D-PdPor-COF has better size selectivity in photocatalytic reactions. Therefore, three-dimensional COFs are expected to be more promising photocatalysts for the photocatalytic synthesis of H₂O₂.

For example, Zhang et al. designed octa-aldehyde difluorobenzene monomer as 8-connected cubic nodes using a bottom-up [8 + 4] mesh strategy for the construction of three-dimensional COFs with a Tty topology (COF-NUST-16) for the first time (Figure 8a,b) [38]. An 8-connected 3D COF exhibits high photocatalytic activity because of its wide absorption range and high absorption capacity, and it is a superior dual-channel photocatalyst with a hydrogen peroxide production rate of 1081 μmol h⁻¹g⁻¹ compared to 2D COFs (234 μmol h⁻¹g⁻¹). The hydrogen peroxide production activity of 3D COFs is four times higher than 2D COFs with similar chemical structures, which demonstrates the superiority of the 3D COFs.

However, despite the significant theoretical advantages of 3D COFs, they have been reported relatively rarely in practice, mainly due to the challenge of precise structural control during synthesis. Precise control of pore size and the three-dimensional network structure of inter-pore connections requires highly specialized and sophisticated synthesis strategies and techniques, which place higher demands on experimental design and operation.

4. Perspective and Outlook

In summary, significant advancements have been accomplished in the field of photocatalytic hydrogen peroxide production using covalent organic frameworks (COFs) in the last several years. COFs, with high crystallinity and designed structures, are ideal candidates for efficient photocatalytic systems. Despite the positive outlook, the field is still in its initial stages and is still far from commercial application. There is an urgent need to address a number of issues to advance scientific research and practical applications. After reviewing current technological developments, we offer the following suggestions.

4.1. Clarify the Reaction Mechanism and the Effect of Structure on Catalytic Performance

This includes determining the pathways of hydrogen peroxide production by photocatalysis of different structures, photogenerated carrier dynamics, hydrophobicity/hydrophilicity of COFs, and adsorption of oxygen by COFs. Various studies have demonstrated that different functional groups in COFs are used as active sites for different reaction pathways [77]. For instance, bipyridine is mainly used as an ORR site [78], and acetylene is mainly used as a WOR site [31]. More research is needed in the future to determine the role of multiple functional groups in the photocatalytic production of hydrogen peroxide. Analyzing reaction mechanisms relies mainly on a number of characterization techniques. While indirect characterization techniques such as X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR) techniques, and photoluminescence (PL) spectroscopy provide useful proof for the kinetics of the photogenerated carriers and the presence of reactive intermediates, they do not provide direct evidence of the real-time involvement of electrons/holes (e⁻/h⁺) and the reactive intermediates in the redox reactions at the surface. Therefore, further development of in situ characterization techniques is required to accurately provide proof of reaction mechanisms. For example, an in situ environmental transmission electron microscope (ETEM) [79] provides a way to observe the structural changes in photocatalysts at the microscopic level. In situ Fourier-transform infrared spectroscopy (FT-IR) allows for real-time monitoring of the formation and adsorption/desorption processes of intermediates on the catalyst surface. The integration of these in situ techniques will help to uncover the underlying reaction mechanisms and provide guidance for the appropriate design of COFs for efficient H₂O₂ production.

4.2. Improve Catalytic Performance and Applicability

This includes photocatalytic activity, stability, selectivity, and applicability in different water bodies. Enhancing the photocatalytic activity of the catalysts was mainly achieved by enhancing light absorption, promoting carrier separation and inhibiting carrier recombination. Various strategies have been developed, such as the design of redox center-separated COFs, to inhibit carrier recombination and the growth of other semiconductor photocatalyst materials on COFs to enhance light absorption and promote carrier separation. However, the activity is still far from practical application, and better strategies are needed to improve the overall activity in the future. A good photocatalyst must have high stability. Compared with inorganic photocatalysts, COFs linked by covalent bonds obviously have higher structural stability. However, the structure of COFs can be easily damaged by H₂O₂ with oxidizing and reducing properties as the concentration of H₂O₂ in the reaction system increases during the photocatalytic process, which is the major reason why a number of COF materials exhibit unstable performance in the photocatalytic synthesis of H₂O₂. As a result, there is a need to further explore additional improvement methods, such as functionalized modifications, formation of heterojunctions, and establishment of multiphase reaction systems to enhance the stability of COFs during the reaction process. In addition, the synthesis method also has a higher influence on the catalytic performance. Different synthesis strategies can significantly change the crystallinity, pore structure, and specific surface area of COFs which are directly related to the distribution of active sites and substrate contact efficiency during catalysis. For example, solvothermal synthesis provides more active sites and improves catalytic activity and stability, while microwave-assisted synthesis helps to obtain materials with large specific surface area and high porosity. Moreover, the development of simple synthesis methods is very important. Such methods can reduce the difficulty of synthesizing COFs and accelerate the preparation process, such as the simple synthesis method of COFs based on the imine exchange reaction [80] and the simple solution-refluxing synthesis method of 3D COF-300 [81], which contributes to the industrial application of COFs. Furthermore, the application range of COF photocatalytic hydrogen peroxide production should be improved. For instance, the pure water system is replaced by a simulated or real seawater system [82]. Seawater is a widely available resource, and the use of seawater as a reaction medium reduces dependence on freshwater resources, which contributes to sustainable development. Furthermore, it has recently been shown that different synthesis methods lead to different catalytic properties [83].

4.3. Standardize Performance Judgement Criteria

The standardization of COF photocatalytic hydrogen peroxide production performance evaluation criteria is a key step in promoting the application of COF-based materials in photocatalysis. When evaluating the photocatalytic performance of COF-based materials, there does not yet exist a universally accepted standard protocol or benchmark to ensure consistency between different studies and comparability of results. However, there are some key evaluation criteria, including H₂O₂ generation rate, H₂O₂ concentration, H₂O₂ decomposition, apparent quantum yield (AQY), and solar-to-chemical energy conversion efficiency (SCC efficiency). These criteria provide a quantitative approach to photocatalytic H₂O₂ generation performance. The judgment criteria can be regulated by standardizing the test conditions and determining the key indicators for evaluating the photocatalytic performance of COFs. What is more, there are no standardized judgement criteria for detecting the yield of H₂O₂ due to the difficulty of H₂O₂ separation and instability. Currently, the main methods for the detection of H₂O₂ are UV spectrophotometry and titration. However, for the same reaction system, the results obtained by different testing approaches may be very different. Authoritative detection methods and standards are needed for photocatalytic production of H₂O₂ driven by COF-based materials. Considering the actual production process and economic benefits, it is essential to choose the appropriate reaction conditions. Firstly, the light source is recommended to be simulated sunlight or LED lamps, which not only ensures the green sustainability of the reaction but also reduces energy consumption by effectively using natural or artificial light sources. Meanwhile, air and seawater are selected as the reaction medium, which can significantly reduce the dependence on pure oxygen and freshwater resources. In terms of temperature control, conducting photocatalytic reactions at room temperature can avoid the need for additional heating processes, thereby reducing energy consumption and production costs. In addition, the design of a simple catalyst recovery and recycling process not only reduces the cost of catalyst usage but also reduces the environmental impact of the production process. Through these combined measures, the COF photocatalytic H₂O₂ production process can be optimized to achieve the goal of cost-effective and environmentally friendly production.

4.4. Combine with Other Fields

In the research in the field of photocatalytic H₂O₂ production by COFs, the photocatalytic performance of COFs can be significantly enhanced by combining multidisciplinary knowledge from materials science, computational chemistry, and machine learning. Firstly, using innovations in materials science, novel COFs with specific functionalities can be designed and synthesized to regulate their electronic structure and pore properties through precise control at the molecular level in order to optimize light absorption and carrier transport efficiency. Secondly, applications of computational chemistry can simulate and predict the photocatalytic mechanisms of COFs, providing a theoretical basis for experimental design, such as predicting the electronic properties and reaction pathways of materials using first-principles calculations. Considering the increasing application of artificial intelligence and machine learning in materials science, we can foresee that these technologies will transform the design and synthesis of COFs. Through machine learning algorithms, researchers are able to analyze large amounts of existing data, identify key parameters that affect the performance of COFs, and predict the potential performance of new structures. For example, the crystallinity of COFs is optimized through artificial intelligence. Although this review has explored a variety of strategies to improve the photocatalytic performance of COFs, the potential for the application of interdisciplinary approaches remains enormous.

4.5. Assess the Environmental and Safety Performance of COFs

Due to their high specific surface area, high porosity, and designable chemical composition, COFs are able to efficiently utilize light energy for catalytic reactions, reducing energy consumption and the use of chemical additives and thus potentially lowering the burden on the environment. The chemical and thermal stability of COFs contributes to the safety of the reactions and reduces the generation of harmful by-products. Although there is a lack of extensive research directly addressing the life cycle assessment (LCA) of COFs, it is expected that their lower environmental footprint and potential recyclability or degradability will further enhance their environmental friendliness. However, further life cycle assessment and risk analysis studies are needed to fully assess the environmental and safety performance of COFs.

Author Contributions

J.M. and Y.H. conducted the information search and wrote the first draft of the manuscript, and X.W., Y.L., H.Z. and W.D. discussed and revised parts of the manuscript. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures and Table

Figure 1. Schematic diagram of photocatalytic H2O2 synthesis mechanism and pathways of water oxidation and oxygen reduction reaction.

Figure 2. (a) Schematic chemical structures of DAzCOFs with different relative nitrogen positions; (b) EPR spectra of TpDz photocatalytic system after 5 min of dark and visible irradiation; (c) Koutecky–Levich plot acquired from RDE measurements; (d) Comparison of hydrogen peroxide production performance of DAzCOFs with other COF-based photocatalysts [30]. Copyright 2023 Wiley-VCH.

Figure 3. (a) Schematic diagram of the synthesis of TD-COF and TT-COF. Performance of TD-COF and TT-COF for photocatalytic production of hydrogen peroxide in water and seawater under (b) O2, (c) air, and (d) Ar atmospheres [33]. Copyright 2023 Wiley-VCH.

Figure 4. (a) Schematic synthetic pathways of C-COFs, S-COFs, and FS-COFs; (b) mechanistic schematic of the key steps in the production of hydrogen peroxide by FS-COFs (I: O2 adsorption, II: photogenerated carrier separation, III: proton coupling, IV: H2O2 desorption); (c) comparative plots of metal-free photocatalysts for photocatalytic hydrogen peroxide production without sacrificial agents [29]. Copyright 2023 Wiley-VCH. (d) Schematic representation of palladium metal-isolated clusters interacting with TAPT-PBA COFs and TAPT-TFPA COFs; (e) photocatalytic stability for the production of H2O2 using TAPT-TFPA COFs@Pd-ICs [34]. Copyright 2023 American Chemical Society.

Figure 5. (a) Schematic diagram of the molecular engineering strategy for the photocatalytic hydrogen peroxide evolution of COFs with isolated redox centers. (b) Schematic route of synthesis of HEP-TAPT-COF and HEP-TAPB-COF [35]. Copyright 2022 Wiley-VCH.

Figure 6. (a) Schematic route of synthesis of TpaBtt, TapbBtt, and TaptBtt. (b) Schematic illustrations of the direction of electron transfer between functional motifs and imine bonds in TpaBtt, TapbBtt, and TaptBtt. The yellow dashed line represents the motifs and the red dashed line represents the imine bond. Shades of color represent differences in energy [36]. Copyright 2023 Springer Nature.

Figure 7. (a) Schematic route of CdS/TpBpy synthesis; (b–d) charge-transfer mechanism of CdS/TpBpy; (e) photocatalytic H2O2 synthesis of CdS, TpBpy, and CdS/TpBpy; (f,g) Kf and Kd values for photocatalytic H2O2 decomposition of CdS, TpBpy, and CdS/TpBpy [37]. Copyright 2023 Elsevier.

Figure 8. (a) Schematic synthetic routes of COF-NUST-16 with extended [8 + 4] Tty network and (b) COF-NUST-17 with extended [4 + 2] bex network. Different colors represent different building blocks, purple, orange, and blue represent FBTA-8CHO, TAPPy, and 2D-2F-CHO [38]. Copyright 2022 Elsevier.

Table 1

The photocatalyst for hydrogen peroxide production.

Photocatalysts	Irradiation Condition	Catalysts Amount	Reaction Solution	Reaction Pathway	H2O2 Generation Rate	SCC Efficiency (%)	AQY (%)	Ref.
TDB-COF	Simulated AM 1.5 G illumination	10 mg	10 mL water	Two-step 1e− ORR and two-step 1e− WOR	723.5 μmol h−1 g−1	0.39%	1.4%	[28]
FS-COFs	300 W Xe lamp (λ > 420 nm)	5 mg	20 mL water	2e− ORR	3904.2 μmol h−1 g−1		6.21%	[29]
TpDz-COF	300 W Xe lamp (λ > 420 nm)	3 mg	18 mL water	2e− ORR	7327 μmol h−1 g−1	0.62%	11.9%	[30]
CTF-EDDBN	300 W Xe lamp (λ > 420 nm)	30 mg	50 mL water	2e− ORR and 2e− WOR	54 μmol h−1 g−1	0.07%		[31]
CTF-BDDBN	300 W Xe lamp (λ > 420 nm)	30 mg	50 mL water	2e− ORR and 2e− WOR	97 μmol h−1 g−1	0.14%		[31]
TTF-BT-COF	300 W Xe lamp (λ > 420 nm)	5 mg	10 mL water	Two-step 1e− ORR and two-step 1e− WOR	2760 μmol h−1 g−1	0.49%	11.19%	[32]
TD-COF	LED (400–700 nm, 100 mW cm−2)	1 mg	4 mL seawater	Two-step 1e− ORR and 2e− WOR	3364 μmol h−1 g−1	0.15%		[33]
TT-COF	LED (400–700 nm, 100 mW cm−2)	1 mg	4 mL seawater	Two-step 1e− ORR and 2e− WOR	2890 μmol h−1 g−1	0.14%		[33]
TAPT-TFPA COFs@Pd-ICs	300 W Xe lamp (λ > 400 nm)	10 mg	20 mL 10%ethanol water	Two-step 1e− ORR	2143 μmol h−1 g−1	0.82%	6.5%	[34]
HEP-TAPT-COF	300 W Xe lamp (λ > 420 nm)	50 mg	100 mL water	2e− ORR	87.50 μmol h−1	0.65%	15.35%	[35]
HEP-TAPB-COF	300 W Xe lamp (λ > 420 nm)	50 mg	100 mL water	2e− ORR	49.50 μmol h−1	0.38%	9.98%	[35]
TaptBtt-COF	300 W Xe lamp (λ > 420 nm)	15 mg	50 mL water	Two-step 1e− ORR and 2e− WOR	1407 μmol h−1 g−1	0.296%	4.6%	[36]
CdS/TpBpyCOF	300 W Xe lamp (λ > 420 nm)	10 mg	50 mL water	Two-step 1e− ORR	3600 μmol h−1 g−1		13.4%	[37]
COF- NUST -16	300 W Xe lamp (λ > 420 nm)	5 mg	50 mL 10%ethanol water	Two-step 1e− ORR and Two-step 1e− WOR	1081 μmol h−1 g−1			[38]
MIL-125-NH2	500 W Xe lamp (λ > 420 nm)	5 mg	Acetonitrile/water (4:1) 5 mL	Two-step 1e− ORR	8 mM h−1			[39]
MIL-125-R7	visible light (λ > 420 nm)	5 mg	Water/benzyl alcohol (2:5) 7 mL	Two-step 1e− ORR	about 400 μM h−1			[40]
TiO2/In2S3	300 W Xe lamp	20 mg	40 mL 10%ethanol water	Two-step 1e− ORR	376 μmol h−1 L−1		3.42%	[41]

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