Covalent organic framework based photocatalysts for efficient visible-light driven hydrogen peroxide production

Abstract

Hydrogen peroxide (H₂O₂), as an important environmentally friendly oxidant, has been widely used in bleaching, disinfection, wastewater treatment and chemical synthesis, while the artificial photosynthesis of H₂O₂ from H₂O and O₂ over certain photocatalysts has been considered as a clean and energy-saving approach and has been widely explored in recent years. Covalent organic frameworks (COFs) represent a newly blooming class of crystalline porous materials with periodically ordered structures that possess a set of intriguing features, including diverse structural designability and durability, high porosity and low density, wide light-harvesting ability and semiconducting properties. Benefiting from these particular properties, the blooming developments of COFs in recent years have brought promising impetus for the realization of efficient visible-light driven H₂O₂ production. Thus, a systematic and comprehensive review is timely to provide constructive guidance for the development and rational design of more efficient COF-based photocatalysts. In this review, the up-to-date application of COF-based photocatalysts for H₂O₂ photosynthesis was comprehensively summarized and discussed. Firstly, the general background and fundamental principles of COF-based photocatalysts for photocatalytic H₂O₂ production were briefly introduced, followed by the detailed classification and discussion of the strategies reported thus far for realizing improved photocatalytic performance. In addition, the challenges and prospects of COF-based photocatalysts for visible-light driven H₂O₂ production were addressed.

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Ke-Hui Xie

Ke-Hui Xie received his master's degree in organic chemistry from Shandong Normal University in 2021 under the supervision of Prof. Yan Geng. He is currently a Ph.D. candidate at Shandong Normal University under the supervision of Prof. Yu-Bin Dong. His research focuses on the preparation of photoactive covalent organic frameworks for photocatalysis.

Guang-Bo Wang

Dr Guang-Bo Wang received his master's degree from Dalian University of Technology in 2014 and obtained his PhD degree in chemistry from Ghent University in 2018. He then moved back to China and joined Shandong Normal University in the same year. His current research interest mainly focuses on the task-specific design and preparation of novel covalent organic frameworks for diverse photocatalytic applications.

Yan Geng

Prof. Yan Geng received his B.S. and M.S. degrees in chemistry in 2002 and 2005 from Shandong Normal University under the guidance of Prof. Yu-Bin Dong. He received his Ph.D. degree in chemistry in 2008 from the Technical Institute of Physics and Chemistry, CAS under the supervision of Prof. Li-Zhu Wu and Prof. Chen-Ho Tung. He then worked at Uppsala University, the University of Queensland, the University of Bern and Kyushu University before joining Shandong Normal University in 2017. He is currently a professor focusing on advanced crystalline materials and their applications.

Yu-Bin Dong

Prof. Yu-Bin Dong is the Chang Jiang Scholar of Chemistry at Shandong Normal University. He obtained his PhD degree from Nankai University (under Prof. Li-Cheng Song) in 1996. He then joined Prof. Andreas Mayr's group at the University of Hong Kong and Prof. Hans-Conrad zur Loye's group at the University of South Carolina from 1996 to 2000, and was promoted to full professor at Shandong Normal University in 2000. He has authored over 200 peer-reviewed publications, and his current research interests include COF- and MOF-based materials and their applications.

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Introduction

Hydrogen peroxide (H₂O₂), as a green and powerful detergent, disinfectant or environment friendly oxidant in chemical synthesis, has been widely used in our daily life and various industries since its first discovery in 1818 by Thenard via the reaction of barium peroxide with nitric acid.^1–3 In particular, during the spread of the COVID-19 pandemic, the global demand for H₂O₂ dramatically increased for disinfection purposes. As predicted, the global market for H₂O₂ is estimated to increase at an annual growth rate of 4.6% to approximately 5.7 million tons by 2027.⁴ In addition, H₂O₂ has also been explored as a potential energy carrier in a single chamber fuel cell to generate electricity by replacing H₂ owing to its relatively lower output potential (1.09 V) in comparison with that of hydrogen (1.23 V).⁵ At present, the state-of-the-art technology for H₂O₂ production in industry is the anthraquinone oxidation (AO) process, which involves multiple steps including hydrogenation of anthraquinone, oxidation of the hydrogenated anthraquinone, extraction and purification of H₂O₂.⁶ Although the well-established AO method realizes the practical and large-scale production of H₂O₂, it suffers from several inevitable drawbacks and conflicts with the original idea of green and sustainable chemistry, such as massive energy consumption and usage of organic solvents, generating a large amount of waste.^7,8 In this context, the struggle between the growing market demand and the unsustainability of the conventional AO method promotes extensive research on the urgent exploration and development of eco-friendly alternative H₂O₂ production processes.

Among all the reported strategies, photocatalytic H₂O₂ production has been regarded as an efficient, energy-saving and sustainable process, which only needs water and oxygen as the raw materials and sunlight as the power source, and has attracted increasing interest in recent decades.^5,7,9 Generally, visible-light driven production of H₂O₂ from water can date back to 1921, in which Baur and Neuweiler employed ZnO as the photocatalyst to produce H₂O₂ in the presence of glucose and glycerol.¹⁰ Subsequently, there has been substantial research and breakthroughs over the past decades focusing on developing more efficient photocatalysts for H₂O₂ production, such as metal-oxide semiconductors,¹¹ metal–organic frameworks (MOFs),¹² porous organic polymers (POPs),¹³ graphitic carbon nitride (g-C₃N₄) and covalent organic frameworks (COFs).¹⁴

Covalent organic frameworks (COFs), first reported by the group of Yaghi in 2005,¹⁵ represent a burgeoning class of advanced crystalline porous polymers and have become feasible and promising platforms in diverse artificial photosynthesis in recent years owing to their outstanding photoelectric properties, permanent porosity and remarkable photochemical stability.^16–18 The structural regularity and tunability of COFs can be readily realized by rationally selecting building units with distinct topologies and dimensionalities, endowing them with wide light absorption ability, fast charge carrier mobility and favorable mass transfer, which are rather challenging to be simultaneously realized in previously reported inorganic and organic amorphous photocatalysts. In 2020, Van Der Voort and co-workers pioneered the employment of a set of imine-linked COFs with Wurster-type functional groups for photocatalytic H₂O₂ production, which exhibited redox characteristics and gave a H₂O₂ production rate of up to 97 μmol g⁻¹ h⁻¹ with ethanol as the hole scavenger under visible-light irradiation.¹⁹ Since then, an increasing number of researchers are contributing to this promising field and lots of critical achievements have been made, while there are almost no comprehensive discussions and overviews of this bright and promising research hotspot.^14,20 Herein, the present review is intended to provide useful information related to the up-to-date progress in the field of COF-based photocatalysts and the rational design of effective reaction systems for efficient visible-light driven photocatalytic H₂O₂ production. Specifically, we begin with the introduction of the background for H₂O₂ production together with a brief introduction of photoactive COFs. Then, the fundamental principles of COF-based photocatalysts for photocatalytic H₂O₂ production and the detection of the photogenerated H₂O₂ are discussed in detail. Afterwards, the up-to-date achievements and effective strategies to improve the photocatalytic activities of COF-based photocatalysts are systematically discussed and summarized. Finally, we propose our perspectives on the challenges and opportunities of COF-based photocatalysts for efficient H₂O₂ photosynthesis, aiming to stimulate the increasing design and synthesis of more photoactive and promising COF-based materials for diverse photocatalysis applications.

The fundamental principles and quantification approaches of the photogenerated H₂O₂

The fundamental principles of photocatalytic H₂O₂ production over COFs

In principle, the photocatalytic production of H₂O₂ over COF-based materials mainly involves three essential steps: (i) light harvesting for the excitation of the photoinduced charge carriers; (ii) the migration of the photoinduced electrons from the valence band (VB) to the conduction band (CB), leaving holes behind; and (iii) redox reactions of the remaining photogenerated carriers with the surface adsorbed O₂ and H₂O molecules to produce H₂O₂. In general, the photocatalytic H₂O₂ production over certain COF-based photocatalysts can be realized through two common pathways, including the oxygen reduction reaction (ORR) and the water oxidation reaction (WOR).⁵ As for the ORR route, H₂O₂ is commonly photogenerated either via an indirect sequential two-step single-electron reduction (eqn (1) and (5), Fig. 1) or via a direct one-step two-electron reduction (eqn (2), Fig. 1), in which the protons originate from H₂O or organic electron donors (such as ethanol, benzyl alcohol or isopropanol). In some cases, H₂O₂ can also be produced via the four-electron ORR competition reaction (eqn (3), Fig. 1). It should be noted that the CB of the photocatalyst via the 2e⁻ ORR pathway should be more negative than 0.68 V_NHE and indirect 2e⁻ ORR is thermodynamically prohibited when the CB of the photocatalyst is between −0.33 V_NHE and +0.68 V_NHE. Similarly, the WOR route can also be classified as a 2e⁻ WOR process (eqn (6), Fig. 1) together with the competitive single-electron WOR pathway for the generation of hydroxyl radicals (eqn (7), Fig. 1) and the four-electron WOR pathway (eqn (4), Fig. 1). In theory, the VB of the photocatalyst for H₂O₂ evolution through the WOR pathway should be more positive than 1.76 V. Actually, the two-electron ORR pathway seems to be the priority mechanism of COF-based photocatalysts for photocatalytic H₂O₂ evolution in the literature.

Fig. 1 Schematic illustration and energy diagram of the photocatalytic production of H₂O₂ over a photoactive COF-based material via the oxygen reduction reaction (ORR) pathway or water oxidation reaction (WOR) pathway, where potentials are given against the normal hydrogen electrode (NHE) at PH = 0.

Quantification approaches of the photogenerated H₂O₂

The accurate quantification of the photogenerated H₂O₂ is the prerequisite for the scientific evaluation of the photocatalyst's performance. In general, there are various approaches to detect the amount of H₂O₂, such as chemical titration, spectrophotometric, colorimetric test strip, electrochemical, fluorescence, ion chromatography together with an ultraviolet detector and liquid chromatographic approaches.^21,22 Among all these approaches, the most common techniques to accurately quantify the amount of H₂O₂ are the titrimetric and spectrophotometry methods. As for the titration, potassium permanganate is commonly employed to react with H₂O₂, where the dark purple colour of the permanganate is gradually reduced to colourless Mn²⁺ (MnSO₄) by H₂O₂. After H₂O₂ is completely reacted, further addition of the titrant leads to the endpoint of the titration, as evidenced by a persistent faint pinkish colour determined by the naked eye (Fig. 2a).²³ Then the quantification of H₂O₂ can be carried out by the reaction stoichiometry calculation. The mechanism of the spectrophotometry method is utilizing the reaction of coloured reagents (e.g. potassium iodide (KI) and cerium(IV) sulfate) and H₂O₂ to produce coloured substances, which can be exactly determined from the UV-Vis or fluorescence spectrum and features a diminished error from humans compared with the titration method (Fig. 2b). Colorimetric test strips typically operate on a redox/colour change principle similar to that of titration. Typically, the test strip is immersed in the H₂O₂ solution and subsequently exposed to air for a certain duration and the strip undergoes colour changes during the exposure time. The colour of the strip is correlated with the concentration of H₂O₂ and can either be approximated by the naked eye or read using an electronic strip reader (Fig. 2c). Although the use of colorimetric test strips is simple and fast, it can only be considered to be semiquantitative and should only be used as an initial indicator to obtain a quick indication of the produced amount of H₂O₂.²⁴ It should be noted that these methods can effectively detect the amount of H₂O₂ and have been widely used in the literature; however, a standardized approach for the analysis of H₂O₂ would be necessary, which not only can benefit the comparability of different photocatalysts, but can also serve as a guide for researchers to accelerate the fast development of this promising field.

Fig. 2 Comparison of the most common quantification approaches for the photogeneration of H₂O₂: (a) titrimetric method; (b) spectrophotometry approach; and (c) colorimetric test strips. The figure are adapted with permission from ref. 1. Copyright 2023, Nature Publishing Group.

Strategies to realize improved photocatalytic H₂O₂ evolution for COF-based photocatalysts

COFs have been one of the most significant discoveries in the 21^st century pertaining to heterogeneous photocatalysis due to their unique features of (i) almost unlimited structural tunabilities by connecting different molecular building units with different functions; (ii) permanent porosity and well-defined structures, enabling both rapid diffusion of the charges and enhanced accessibility of the substrates or sacrificial agents to the surfaces of the COF samples; and (iii) remarkable light harvesting and charge carrier separation and migration abilities, a prerequisite for all kinds of efficient photocatalysis. As mentioned before, Van Der Voort and coworkers pioneered the utilization of a series of 2D imine-based COFs for photocatalytic H₂O₂ production under visible-light irradiation in 2020.¹⁹ Since then, various strategies have been developed in the literature to realize enhanced photocatalytic H₂O₂ evolution performance over COF-based photocatalysts, such as pristine COFs and the corresponding modifications, construction of donor–acceptor systems, construction of heterojunctions, topology engineering and linkage tailoring (Fig. 3).^25–30 In the following, the most representative achievements in these aspects will be summarized and discussed in detail.

Fig. 3 The strategies that have been employed in the literature to realize improved photocatalytic H₂O₂ production over COF-based photocatalysts.

Pristine COFs and the corresponding modifications

In 2022, Cooper and coworkers developed a high-throughput sonochemical synthesis strategy of imine COFs, which was applied for the discovery of functional photoactive COFs as efficient photocatalysts for the photosynthesis of H₂O₂. More importantly, some of their findings are of significant importance in this study for the following rational design and synthesis of COF-based photocatalysts: (1) High crystallinity and porosity are thought to be beneficial for H₂O₂ photosynthesis. (2) Triazine moieties within the structures of COFs are favorable for better photocatalytic performance. (3) Donor–acceptor systems are beneficial for high photoactivities. (4) Long-term stability of the imine-linked COFs is still far away from practical applications.³¹ There is no doubt that these findings will provide useful and constructive guidance for the fast development of COF-based photocatalysts. The functionalization strategy arising from organic chemistry has been widely employed for the construction of functionalized COFs to realize enhanced photocatalytic performance, where the functional groups can be introduced into the backbones of the COFs by rationally selecting the functional building units.^32,33 As an example, Han et al. rationally engineered the partial-substitution of fluorine atoms on a triazine COF to modulate the electronic structures (Fig. 4a) with enhanced visible-light absorption ability, promoted charge separation and migration and facilitated O₂ adsorption for two-electron ORR to H₂O₂. Density functional theory (DFT) calculations demonstrated that the endoperoxide species of the fluorinated COF (TF-COF) possessed much lower adsorption energies on different adsorption sites compared with those of non-fluorinated H-COF (Fig. 4b). Meanwhile, the positive value of carbon atoms of TF-COF exhibited strong electron-withdrawing capability, which was beneficial for the in-plane charge migration (Fig. 4c). As a result, the TF₅₀-COF, consisting of equimolar amounts of fluorinated and non-fluorinated building units, exhibited the highest crystallinity and the strongest interaction effect, thus leading to the best H₂O₂ production rate of 1739 μmol g⁻¹ h⁻¹ and an impressive apparent quantum yield (AQY) of 5.1% at 400 nm under visible-light irradiation, higher than that of TF-COF (1239 μmol g⁻¹ h⁻¹) and non-fluorinated H-COF (516 μmol g⁻¹ h⁻¹) (Table 1).³⁴ Very recently, the same group synthesized a cyano-substituted COF (CN-COF) by the incorporation of cyano groups into the COF structures to improve its photocatalytic activity. As expected, the introduction of cyano groups endowed the CN-COF with a broader visible-light absorption ability and significantly enhanced charge separation efficiency, thus resulting in a dramatically improved H₂O₂ evolution rate of 2623 μmol g⁻¹ h⁻¹ and a superior AQY of 9.8% at 420 nm compared with that of the pristine non-substituted COF (1640 μmol g⁻¹ h⁻¹).³⁵ Similarly, Yan and co-workers prepared the same fluorinated COF (TFA-TaPt-COF) and they also developed an acetal strategy to improve the crystallinity of the TFA-TaPt-COF. Interestingly, the TFA-TaPt-COF-E synthesized in the presence of ethanol exhibited higher crystallinity together with a red-shift in the absorption edge and lower photoluminescence (PL) intensity, thus leading to significantly improved photocatalytic activity.³⁶

Fig. 4 (a) Schematic representative structures of the synthesized COFs. (b) The calculated O₂ adsorption energy on different sites of TF-COF and H-COF. (c) The calculated O₂ adsorption on site 3 for TF-COF and H-COF. The figures are adapted with permission from ref. 34. Copyright 2022, Wiley-VCH.

Table 1 Summary of the representative COF-based photocatalysts for H₂O₂ photosynthesis

Entry	COFs	Reaction conditions^a	Irradiation (nm)	H2O2 yield^b (μmol h^−1 g^−1)	AQY	Ref.
a EtOH: ethanol; IPA: isopropanol; BA: benzyl alcohol. b O2-presaturated water. c Continuous O2 bubbling. d Biphasic reaction system.
1	COF-TpHt	H2O : BA (9 : 1)	λ > 420	11 986	38% @ 420 nm	38
2	TpDz	H2O	λ > 420	7327	11.9% @ 420 nm	41
3	TpMd	H2O	λ > 420	6034	11.9% @ 420 nm	41
4	PMCR-1	H2O : BA (10 : 1)	420 ≤ λ ≤ 700	5499	14% @ 420 nm	62
5	COF-TTA-TTTA	H2O : EtOH (9 : 1)	λ > 420	4347	—	53
6	Bpy-TAPT	H2O	λ > 420	4038	8.6% @ 420 nm	37
7	FS-COF	H2O	λ > 420	3904	6.21% @ 420 nm	46
8	Bpt-CTFB	H2O	350 ≤ λ ≤ 780	3268	8.6% @ 400 nm	71
9	TTF-BT-COF	H2O	λ > 420	2760	11.19% @ 420 nm	51
10	DMCR-1NH	H2O : IPA (10 : 1)	λ > 420	2588	10.2% @ 420 nm	61
11	ZT-5	H2O : EtOH (9 : 1)	λ > 420	2443	13.2% @ 365 nm	75
12	DMCR-1	H2O : IPA (10 : 1)	λ > 420	2264	8.8% @ 420 nm	61
13	4PE-N-S	H2O : EtOH (9 : 1)	λ > 420	2237	—	63
14	TAPT-TFPA COFs@Pd Ics	H2O : EtOH (9 : 1)	λ > 420	2143	6.5% @ 400 nm	77
15	TAPB-PDA-OH	H2O : EtOH (9 : 1)	λ > 420	2117	2.99% @ 420 nm	44
16	CoPc-BTM-COF	H2O : EtOH (9 : 1)	λ > 420	2096	7.2% @ 630 nm	64
17	COF-TfpBpy	H2O	λ > 420	2084	8.1% @ 420 nm	40
18	Bpy-TAPT-CN	H2O	λ > 420	2064	8.0% @ 420 nm	37
19	Bpy-TAPB	H2O	λ > 420	1910	7.8% @ 420 nm	37
20	CoPc-DAB-COF	H2O : EtOH (9 : 1)	λ > 420	1857	5.2% @ 630 nm	64
21	EBA-COF	H2O : EtOH (9 : 1)	λ > 420	1830	4.4% @ 420 nm	67
22	HEP-TAPT-COF	H2O	λ > 420	1750^c	15.35% @ 420 nm	55
23	TF50-COF	H2O : EtOH (9 : 1)	λ > 420	1739	5.1% @ 400 nm	34
24	CTF-NS-5BT	H2O : BA (9 : 1)	λ > 420	1630	6.6% @ 420 nm	70
25	Imine-1	H2O : IPA (10 : 1)	λ > 420	1617	7.2% @ 420 nm	61
26	TAPB-PDA-H2/TAPB-PDA-OCH3	H2O : EtOH (9 : 1)	λ > 420	1593	2.99% @ 420 nm	44
27	N0-COF	H2O	λ > 495	1570	—	56
28	TPB-DMTP-COF	H2O	λ > 420	1565^c	—	30
29	FS-COF	H2O	λ > 400	1510.6	—	46
30	1H-COF	H2O : IPA (9 : 1)	λ > 420	1483	5.4% @ 420 nm	32
31	TpPz	H2O	λ > 420	1418	11.9% @ 420 nm	41
32	TaptBtt	H2O	λ > 420	1407	4.6% @ 450 nm	54
33	Bpu-CTF	H2O	350 ≤ λ ≤ 780	1353	6.6% @ 420 nm	71
34	SonoCOF-F2	H2O	λ > 420	1244	4.8% @ 420 nm	31
35	Py-Da-COFPY	H2O : BA (9 : 1)	λ > 420	1242^d	4.5% @ 420 nm	25
36	COF-TAPB-BPDA	H2O : BA (4 : 1)	λ > 420	1240	—	42
37	TF-COF	H2O : EtOH (9 : 1)	λ > 400	1239	—	34
38	TAPB-PDA-CH3	H2O : EtOH (9 : 1)	λ > 420	1148	2.99% @ 420 nm	44
39	TAPT-PBA COFs@Pd ICs	H2O : EtOH (9 : 1)	λ > 420	1140	4.1% @ 400 nm	77
40	2H-COF	H2O : IPA (9 : 1)	λ > 420	1130	5.4% @ 420 nm	32
41	COF-NUST-16	H2O : EtOH (9 : 1)	λ > 420	1081	—	57
42	3H-COF	H2O : IPA (9 : 1)	λ > 420	1010	5.4% @ 420 nm	32
43	CTF-BDDBN	H2O	λ > 420	997	—	69
44	TTF-pT-COF	H2O	λ > 420	996	—	51
45	HEP-TAPB-COF	H2O	λ > 420	990^c	—	55
46	BTEA-COF	H2O : EtOH (9 : 1)	λ > 420	780	—	67
47	0H-COF	H2O : IPA (9 : 1)	λ > 420	633	5.4% @ 420 nm	32
48	4PE-TT	H2O : EtOH (9 : 1)	λ > 420	624	—	63
49	TPB-DMTP-COF	H2O	λ > 420	606	—	30
50	COF-N32	H2O	λ > 420	605	6.9% @ 459 nm	26
51	TPE-pT-COF	H2O	λ > 420	592	—	51
52	C-COF		λ > 400	587.6	—	46
53	TapbBtt	H2O	λ > 420	557	—	54
54	4PE-N	H2O : EtOH (9 : 1)	λ > 420	546	—	63
55	H-COF	H2O : EtOH (9 : 1)	λ > 400	516	—	34
56	TiCOF-spn	H2O : EtOH (9 : 1)	420 ≤ λ ≤ 780	490	—	57
57	COF-N31	H2O	λ > 420	442	—	26
58	TpPa-Cl	H2O : EtOH (9 : 1)	λ > 420	281	—	75
59	TpaBtt	H2O	λ > 420	252	—	54
60	COF-NUST-17	H2O : EtOH (9 : 1)	λ > 420	234	—	57
61	COF-N33	H2O	λ > 420	155	—	26
62	TAPD-(Me)2-COF	H2O : EtOH (9 : 1)	λ > 420	97	—	19
63	TAPD-(OMe)2-COF	H2O : EtOH (9 : 1)	λ > 420	91	—	19
64	CTF-EDDBN	H2O	λ > 420	57	—	69
65	CTF-BPDCN	H2O	λ > 420	28	—	69

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To date, various nitrogen-heterocycles, such as pyridine, triazine, bipyridine and heptazine, have been integrated into the structures of COFs for accelerated photosynthesis of H₂O₂,^37–40 while the relative locations of the nitrogen atoms within the N-heterocycles also significantly affect their photocatalytic activities. In the recent work of Xi and co-workers, three isomeric COFs functionalized with two nitrogen-containing N-heterocycles (diazines) including pyridazine (TpDz), pyrimidine (TpMd), and pyrazine (TpPz) were synthesized for photocatalytic production of H₂O₂ (Fig. 5a). The optical and electronic properties revealed that the electron–hole separation efficiency of TpDz was more effective and its charge transfer resistance was the lowest among these studied COFs, thus exhibiting the best H₂O₂ evolution rate of 7327 μmol g⁻¹ h⁻¹ in pure water and a remarkable AQY of 11.9% at 420 nm, ranking as one of the best COF-based photocatalysts thus far (Fig. 5b and c). DFT calculations confirmed that compared with pyrimidine and pyrazine, pyridazine embedded in the TpDz tended to stabilize the endoperoxide intermediate species, leading to the more efficient direct two-electron ORR pathway.⁴¹ Along this line, Kong and co-workers employed the same strategy and synthesized three imine-linked COFs with different numbers of nitrogen atoms in the frameworks by the condensation of polycyclic aromatic benzene and a spatially separated acetylene unit with different benzene or triazine type building units. Interestingly, COF-TAPB-BPDA without nitrogen atoms in the structures exhibited the best H₂O₂ evolution rate of 1240 μmol g⁻¹ h⁻¹ in the H₂O-benzyl alcohol two-phase system and the two-step single-electron ORR pathway was dominant for H₂O₂ photosynthesis over the studied COFs.⁴² More recently, the same group reported other three newly designed COFs by altering the linker length of the building units, which were further evaluated as photocatalysts for photocatalytic H₂O₂ production. It was found that the obtained COF with a longer linker (COF-BPDA-DTP) exhibited better photocatalytic performance due to its more negative conductor band, higher specific surface area and enhanced photogenerated carrier separation efficiency.⁴³ To systematically examine the effects of different functional groups on the photocatalytic performance of a specific COF, Wang and co-workers synthesized a series of imine-linked COFs (TAPB-PDA-X) by the condensation of 1,3,5-tris(4-aminophenyl) benzene (TAPB) and the teraphthaldehyde with different functional groups (PDA-X, X = H₂, CH₃, OH, and OCH₃). Among all the obtained COFs, the TAPB-PDA-OH modified with hydroxyl groups exhibited the best H₂O₂ production rate of 2117.6 μmol g⁻¹ h⁻¹ and an AQY of 2.99% at 420 nm in H₂O : EtOH (9 : 1) aqueous solution. The hydroxyl groups within the TAPB-PDA-OH could effectively trap the holes to reduce the recombination of photo-generated carriers and improve the stability of TAPB-PDA-OH, thus resulting in its superior photocatalytic performance.⁴⁴ The incorporation of the sulfone group, a hydrophilic and strong electron-deficient functional group, to the backbones of COFs has been demonstrated to be impressive for improved photocatalytic hydrogen evolution.⁴⁵ Following this strategy, Han et al. synthesized a set of sulfone-modified COFs by the Schiff-base condensation between triformylphloroglucinol and 2,7-diaminofluorene (DAF) or the aromatic diamine ligands bearing either a dibenzothiophene sulfone (SA) or a benzo-bis-benzothiophene sulfone group (FSA) for photocatalytic H₂O₂ production (Fig. 6a). Experimental and theoretical calculations confirmed that the presence of sulfone groups within the COF structures can not only improve the separation of the photogenerated carriers and increase the hydrophilicity of the COFs, but also can alter the initial O₂ adsorption with the Yeager-type configuration, which is more likely to acquire electrons and favorable for the formation of 1,4-endoperoxide intermediates instead of *OOH, thus avoiding the generation of superoxide radicals and promoting the direct two-electron ORR pathway other than the two-step single-electron ORR pathway for the production of H₂O₂ over the FS-COF (Fig. 6b). As such, FS-COFs exhibited an H₂O₂ production rate of 1501.6 μmol h⁻¹ g⁻¹ in pure water under LED visible light (λ ≥ 400 nm) and a remarkable AQY of 6.21% at 420 nm, which was about 3-fold higher than those of C-COFs (487.6 μmol h⁻¹ g⁻¹) (Fig. 6c).⁴⁶ Apart from the aforementioned functionalization strategies, Wang and coworkers recently developed an intriguing strategy to boost the exciton dissociation in COFs by integrating highly polar ionic moieties into the framework of COFs. Photoelectrochemical and time-resolved fluorescence analyses verified that the ionic moieties and crystalline structure in DBTP synergistically facilitated the efficiencies of exciton dissociation and charge carrier mobility, thus leading to a record-high H₂O₂ production rate of 10.01 mmol g⁻¹ h⁻¹ for iCOF-DBTP without any sacrificial reagent under visible-light irradiation (420 ≤ λ ≤ 780 nm), much higher than that of the neutral COF-DPTP (1.47 mmol g⁻¹ h⁻¹).⁴⁷ This strategy can be employed as valuable guidance for regulating the exciton properties of COFs to realize more efficient photocatalytic performance.

Fig. 5 (a) Synthetic routes and structures of TpDz, TpMd and TpPz. (b) The photocatalytic performance comparison of the studied COFs with other reported COF-based photocatalysts. (c) The AQYs of TpDz measured at different wavelengths (left to right, 420, 450, 500 and 600 nm). The figures are reproduced with permission from ref. 41. Copyright 2023, Wiley-VCH.

Fig. 6 (a) Synthetic scheme and structures of C-COFs, S-COFs and FS-COFs. (b) The theoretical calculation results (e.g. the O₂ adsorption sites and energies together with the key steps of H₂O₂ production over FS-COFs). (c) The H₂O₂ production rates of the C-COFs, S-COFs and FS-COFs, and the AQYs of the FS-COFs measured at different wavelengths. The figures are reproduced with permission from ref. 46. Copyright 2023, Wiley-VCH.

Construction of donor–acceptor systems

It has been well documented that the construction of donor–acceptor (D–A) type structures is a promising strategy to realize improved photocatalytic performance of the photocatalyst owing to the following merits: (1) Tuning the bandgap of the materials and energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) more readily. (2) Promoting the intramolecular or intermolecular charge separation and migration. (3) Enhancing the π–π interactions and stacking.^48–50 As an example, Lan and coworkers synthesized a D–A type COF, namely TTF-BT-COF, with tetrathiafulvalene (TTF) as the donor (photo-oxidation sites) and benzothiazole (BT) as the acceptor (photo-reduction sites) for the full reaction photosynthesis of H₂O₂ (Fig. 7). The covalent connection generates the oxidation–reduction junction that enables the photogenerated electrons and holes to be efficiently separated from the acceptor to the donor, resulting in the simultaneously accomplished ORR and WOR. Notably, the obtained TTF-BT-COF exhibited an impressive H₂O₂ evolution rate of ∼2760 μmol h⁻¹ g⁻¹ in pure water without any sacrificial agent. Experimental and theoretical calculations confirmed that the photocatalytic mechanism for TTF-BT-COF was the synergistic effects of the ORR and WOR coupling with different intermediates produced simultaneously.⁵¹ Similarly, the same group synthesized a MCOF (Cu₃-BT-COF) with redox molecular junction by the connection of Cu₃(PyCA)₃ (photo-oxidation sites) and 4,4′-(benzo-2,1,3-thiadiazole-4,7-diyl)dianiline (photo-reduction sites), which was employed as an efficient photocatalyst for H₂O₂ photosynthesis coupled with furfuryl alcohol photo-oxidation to furoic acid. For comparison, two other COFs with only oxidation centers (Cu₃-pT-COF) and only reduction centers (TFP-BT-COF) were also synthesized. As expected, the D–A type Cu₃-BT-COF exhibited the best photocatalytic performance with an H₂O₂ evolution rate of 1870 μmol g⁻¹ h⁻¹, much higher than that of Cu₃-pT-COF (646 μmol g⁻¹ h⁻¹) and TFP-BT-COF (940 μmol g⁻¹ h⁻¹) due to its strong visible light absorption ability, narrowed band gaps and efficient electron transfer ability.⁵² Zheng and coworkers developed a novel liquid crystal (LC)-directed synthesis approach for up to gram-scale production of seventeen types of D–A COFs in water under air conditions, which were further explored for photocatalytic H₂O₂ production and among all the obtained COFs, COF-TTA-TTTA exhibited the best H₂O₂ evolution rate of 2406 μmol g⁻¹ h⁻¹ in pure water and 4347 μmol g⁻¹ h⁻¹ with ethanol as the sacrificial agent, approximately 4.67-fold better than that of the benchmark g-C₃N₄. Meanwhile, the mechanism study revealed the direct two-electron ORR pathway for H₂O₂ production over the studied COFs.⁵³ In a recent study of Wang and co-workers, a set of dual D–A type benzotrithiophene-based COFs with spatially separated redox centers were synthesized by the combination of benzo[1,2-b:3,4-b′:5,6-b′′]trithiophene-2,5,8-tricarbaldehyde (Btt) and various monomers with different electron-acceptor capabilities, including 4,4,4-triaminotriphenylamine (Tpa) (TpaBtt), 1,3,5-tris(4-aminophenyl)benzene (Tapb) (TapbBtt), and 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (Tapt) (TaptBtt), for photocatalytic H₂O₂ production from H₂O and O₂ without any sacrificial agent. As a result, the optimized H₂O₂ evolution rate of TaptBtt can reach up to 1407 μmol g⁻¹ h⁻¹ with an SCC of 0.297%, much higher than that of TpaBtt (252 μmol g⁻¹ h⁻¹) and TapbBtt (557 μmol g⁻¹ h⁻¹). Mechanism study and theoretical calculations suggested that the electron-deficient fragments near the imine bonds were the active sites for the reaction and the unique electronic structure of the TaptBtt was beneficial for the synchronous two-electron ORR and WOR.⁵⁴ Along this line, Chen et al. rationally designed two 2D s-heptazine-based COFs (HEP-TAPT-COF and HEP-TAPB-COF) by integrating the triazine or benzene moieties into the frameworks (Fig. 8a). The spatially and orderly separated active centers within these HEP-COFs are favorable for suppressing charge recombination and promoting their photocatalytic activities. Significantly, the HEP-TAPT-COF exhibited an excellent H₂O₂ production rate of 1750 μmol g⁻¹ h⁻¹ and an impressive SCC of 0.65% under AM 1.5G simulated sunlight, much higher than that of HEP-TAPB-COF (990 μmol g⁻¹ h⁻¹ and 0.38%) (Fig. 8b and c) and they can be recycled and reused for 96 h without significant loss of their activities (Fig. 8d). The mechanism study revealed that the s-heptazine and benzene groups of the HEP-TAPB-COF served as the ORR and WOR sites, respectively, while both s-heptazine and triazine moieties of the HEP-TAPT-COF acted as the ORR sites and the benzene groups were the WOR centers (Fig. 8e).⁵⁵ To further confirm the advantages of D–A systems, the group of Chen recently reported two benzothiadiazole-based COFs by the condensation of benzo[c][1,2,5]thiadiazole-4,7-dicarbaldehyde (BT-CHO) with tris(4-aminophenyl)benzene (TAPB) and tris-(4-aminophenyl)triazine (TAPTA) to afford the D–A type N₀-COF and N₃-COF. Notably, N₀-COF exhibited an average H₂O₂ evolution rate of 1500 μmol g⁻¹ h⁻¹ in pure water, which is approximately 9-fold higher than that of N₃-COF and this is mainly due to the fact that the push–pull effects between the donor (TAPB) and acceptor (BT) moieties probably decrease the electrostatic Coulomb attraction of charge carriers, enabling enhanced exciton separation, thus leading to improved photocatalytic activity of N₀-COF.⁵⁶

Fig. 7 Schematic illustration of the oxidation–reduction molecular junction COF (TTF-BT-COF) for full reaction H₂O₂ photosynthesis. The figure is reproduced with permission from ref. 51. Copyright 2023, Wiley-VCH.

Fig. 8 (a) Schematic illustration and the synthetic route of HEP-COFs with separated redox centers. (b) The comparison of the H₂O₂ production rates of the studied COFs and g-C₃N₄. (c) The solar-to-chemical conversion (SCC) efficiencies of the HEP-COFs and other reported COFs. (d) Long-term recycling tests of HEP-COFs. (e) The proposed mechanism for H₂O₂ production over HEP-COFs. The figures are reproduced with permission from ref. 55. Copyright 2023, Wiley-VCH.

Topology engineering of COFs

The rational design of three-dimensional (3D) COFs has been proven to be a promising approach to realize superior photocatalytic performance due to the significant merits of their large surface areas and pore volumes, which can not only exhibit better photosensitizing activity and promote more efficient photogenerated electron and hole separation and migration, but is also beneficial for the diffusion of the substrates and products, while there are still quite a few reports focusing on the H₂O₂ photosynthesis by 3D COF-based materials. In 2022, Gu et al. reported a 3D titanium-based COF with spn topology (TiCOF-spn) via [6 + 3] imine condensation of Ti(IV) catecholate complex Na₂Ti(2,3-DHTA)₃ (2,3-DHTA = 2,3-dihydroxyterephthalaldehyde) and 1,3,5-tris(4-aminophenyl)triazine (TAPT), which was applied for photocatalytic H₂O₂ production for the first time (Fig. 9a), exhibiting an efficient H₂O₂ evolution rate of 489.94 μmol g⁻¹ h⁻¹ in water/ethanol (1 : 9) under visible-light irradiation (Fig. 9b) and the mechanism study revealed that H₂O₂ was produced via the typical sequential two-step single-electron ORR pathway (Fig. 9c and d).⁵⁷ Very recently, Zhang and coworkers developed a bottom-up [8 + 4] reticular strategy to synthesize a novel 3D COF (COF-NUST-16) with tty topology (Fig. 10a), which exhibited broad and strong light absorption in the visible region and narrow bandgap, serving as an efficient photocatalyst for H₂O₂ photosynthesis with a H₂O₂ evolution rate of 1081 μmol g⁻¹ h⁻¹ with ethanol as the sacrificial agent. More importantly, the photocatalytic activity of 3D COF-NUST-16 is 4-fold higher than that of 2D COF (COF-NUST-17) with a similar structure under identical conditions and the mechanism study confirmed the production of H₂O₂ over the studied COFs via both ORR and WOR pathways (Fig. 10b),⁵⁸ which is the first example for the experimental comparison of 2D and 3D COFs towards H₂O₂ photosynthesis.

Fig. 9 (a) Representation of the (3,6)-connected eea and spn topologies together with the building units employed for the construction of TiCOF-spn. (b) Time dependent H₂O₂ production under different conditions for TiCOF-spn. (c) EPR results of TiCOF-spn. (d) Proposed mechanism for the production of H₂O₂. The figures are adapted with permission from ref. 57. Copyright 2022, Elsevier.

Fig. 10 (a) Schematic of synthetic routes to COF-NUST-16 and COF-NUST-17. (b) The proposed mechanism for H₂O₂ production over the studied COFs. The figures are reproduced with permission from ref. 58. Copyright 2023, Elsevier.

Linkage tailoring

It should be noted that most of the COF-based photocatalysts reported thus far are based on imine linkages and recent studies have demonstrated that the construction of suitable linkages or linkage tailoring could also significantly affect the performance of the corresponding COFs.^59,60 As an example, the group of Thomas reported four chemically stable quinoline-4-carboxylic acid DMCR-COFs via the one-pot Doebner reaction for photocatalytic H₂O₂ production, while the corresponding imine-based COF (imine-1) was also synthesized to compare its performance with those of DMCR-COFs (Fig. 11a). As expected, the photocatalytic activities of DMCR-COFs are better than that of the corresponding imine-based COF. For example, the H₂O₂ production rate can reach up to 2588 and 2264 μmol g⁻¹ h⁻¹ for the DMCR-1NH and DMCR-1 in water with isopropanol as the sacrificial reagent in the presence of O₂, while it is only 1914 μmol g⁻¹ h⁻¹ for the imine-1 COF under identical conditions (Fig. 11b), which was attributed to the extended conjugated structures and the presence of additional reaction sites (e.g. carboxyl-groups and amine groups) within the structures.⁶¹ Later, the same group reported another chemically stable quinoline-linked COF (PMCR-1) via the multicomponent Povarov reaction, which produced high amounts of H₂O₂ in pure water (1294 μmol g⁻¹ h⁻¹) over a long period of time without loss of its photocatalytic activity and the yield can increase to 5500 μmol g⁻¹ h⁻¹ with benzyl alcohol (BA) as the sacrificial agent due to the benzene–benzene interactions between the phenyl groups within the pores and BA, resulting in efficient transfer of holes.⁶² Similarly, Van Der Voort et al. recently synthesized a thiazole-linked COF (4PE-N-S-COF) via post-sulfurization of the corresponding imine-linked COF (4PE-N-COF) for the photosynthesis of H₂O₂. In addition, another imine-linked COF (4PE-TT-COF) was also synthesized for comparison. Remarkably, the 4PE-N-S-COF exhibited a H₂O₂ evolution rate of 1574 μmol g⁻¹ h⁻¹ in the presence of ethanol, which was about 5.8-fold and 3.7-fold higher than those of 4PE-N-COF and 4PE-TT-COF, respectively. Their experimental results confirmed that the extended conjugation of the structure not only broadened its visible-light absorption ability but also promoted electron separation and migration.⁶³ The group of Jiang recently reported two new piperazine-linked CoPc-based COFs, namely CoPc-BTM-COF and CoPcDAB-COF, by the condensation of hexadecafluorophthalocyaninato cobalt(II) (CoPcF₁₆) with 1,2,4,5-benzenetetramine (BTM) or 3,3′-diaminobenzidine (DAB). Significantly, the CoPc-BTM-COF and CoPcDAB-COF exhibited excellent H₂O₂ production rates of 2096 and 1851 μmol g⁻¹ h⁻¹, respectively with ethanol as the sacrificial agent and impressive AQYs of 7.2% and 5.2% at 630 nm, which were attributed to their wide-range light absorption capacity together with their remarkable efficiently fused conjugated structures.⁶⁴ Recently, vinylene-linked COFs have attracted increasing attention due to their extended π-conjugation, excellent visible-light absorption and charge carrier mobility, and superior chemical stability even under harsh conditions,^65,66 while there are still quite a few reports focusing on H₂O₂ photosynthesis over vinylene-linked COFs. Very recently, Mi and coworkers synthesized two vinylene-linked COFs (BTEA-COF and EBA-COF) with triazine and different acetylene-containing building units for photocatalytic H₂O₂ production (Fig. 12a). The spatial separation of triazine and acetylene cores leads to efficient charge separation and suppressed charge recombination, and C=C linkages facilitate the electrons migration over the structures. Remarkably, the obtained EBA-COF and BTEA-COF exhibited an attractive H₂O₂ production rate of 1830 μmol g⁻¹ h⁻¹ and 780 μmol g⁻¹ h⁻¹ with ethanol as the sacrificial agent under monochromatic light (420 nm LED) and it can be reused for five cycles without significant loss of its photocatalytic activity. Both experimental and theoretical calculations confirmed that the triazine and acetylene units synergistically promoted H₂O₂ photosynthesis via a two-electron ORR pathway (Fig. 12b).⁶⁷ Apart from the aforementioned linkages, covalent triazine frameworks (CTFs),⁶⁸ a subclass of COFs, have also been examined for efficient photocatalytic H₂O₂ production due to their structural tunability together with their excellent stability and nitrogen-rich features.^69–71

Fig. 11 (a) Synthetic routes to DMCR-COFs. (b) The photocatalytic H₂O₂ production under different gas atmospheres and different sacrificial agents after 3 h at 25 °C and λ > 420 nm. The figures are reproduced with permission from ref. 61. Copyright 2023, American Chemical Society.

Fig. 12 (a) Scheme of the synthetic routes to BTEA-COF and EBA-COF. (b) Calculated free energy diagrams of H₂O₂ production photocatalyzed by EBA-COF and BTEA-COF. The figures are adapted with permission from ref. 67. Copyright 2022, American Chemical Society.

Construction of heterojunctions

The inevitably rapid recombination of the photogenerated charges together with the corresponding limited light absorption ability of a photocatalyst commonly inhibits its photocatalytic performance. To avoid or weaken this issue, the construction of heterojunctions by connecting two or more compounds has been a promising approach to realize enhanced photocatalytic activities.^72,73 For example, Wang and co-workers proposed a Z-scheme heterojunction of WO₃/COF-Tp-TAPB, which can not only accelerate the separation and migration of the charges at the interface but also retain the oxidative and reductive ability of WO₃ and COF-Tp-TAPB. Notably, the obtained WO₃/COF-Tp-TAPB exhibited a significant H₂O₂ evolution rate of 1488.4 μmol g⁻¹ h⁻¹ in pure water without any sacrificial agent, which is even 72.3-fold and 2.8-fold higher than that of the pristine WO₃ and COF-Tp-TAPB.⁷⁴ Along this line, an S-scheme heterojunction of ZnO/COF (TpPa-Cl) was synthesized by a simple self-assembly approach and the optimized composite exhibited the maximum H₂O₂ evolution rate of 2443 μmol g⁻¹ h⁻¹ in pure water, approximately 3.3-fold and 8.7-fold higher than those of pristine ZnO and COF-TaPa-Cl, respectively, and the apparent quantum efficiency (AQE) of ZnO/COF(TpPa-Cl) can reach up to 13.12% at 365 nm.⁷⁵ In addition, some other S-scheme heterojunction materials, such as BiOBr/COF and TiO₂/COF have also been developed and exhibited remarkable photocatalytic performance compared with the corresponding pristine materials.^73,76 In photocatalysis, doping modification (such as nanoparticles, nanoclusters, carbon dots (CDs), and quantum dots (QDs)) is an effective approach to regulate the active sites, thus promoting the separation and migration of charges for improved photocatalytic performance. For instance, Guo and coworkers reported a novel strategy of fluorinated COFs to confine Pd isolated clusters (Pd ICs) for significantly boosting their photocatalytic H₂O₂ production ability (Fig. 13a). They found that the introduction of fluorine groups can not only increase the metal–support interactions between Pd ICs and the COF backbones, but also optimize the d-band center of the Pd ICs, resulting in a remarkable H₂O₂ evolution rate of 2143 μmol g⁻¹ h⁻¹ and excellent stability over 100 h for the obtained TAPT-TFPA@Pd ICs (Fig. 13b and c). Meanwhile, the calculated charge density differences of the TAPT-TFPA@Pd ICs confirmed that the existence of fluorine groups promoted the electron transfer between TAPT-TFPA COFs and Pd ICs and the mechanism study revealed a two-step single-electron ORR pathway for the production of H₂O₂ (Fig. 13d).⁷⁷ As an example of noble metal doping, Liao et al. synthesized a carboxyquinoline-linked COF (COF-COOH) via the Doebner–Miller reaction, where the carboxyl groups not only served as the growth sites for the Au nanoclusters anchoring in the pores of the COF backbone, but also accelerated the separation and migration of the carriers, resulting in a superior H₂O₂ evolution rate of 18.93 mmol g⁻¹ h⁻¹ for the obtained Au@COF with benzyl alcohol as the sacrificial agent at pH = 3, which is almost two-fold that of the pristine COF-COOH (9.48 mmol g⁻¹ h⁻¹) under identical conditions. The mechanism study suggested that H₂O₂ was produced over the Au@COF via the indirect two-step single electron ORR pathway.⁷⁸ Hu et al. reported a carbon dot-modulated covalent triazine framework (CDs@CTF) for rapid H₂O₂ photosynthesis, the resulting CDs@CTF exhibited both ORR and WOR pathways to generate H₂O₂ simultaneously and the H₂O₂ evolution rate is approximately 22.6-fold that of the pristine CTF materials with an SCC efficiency of 0.16%.⁷⁹ Similarly, Lv and co-workers reported two kinds of novel polyimide COFs decorated with carbon quantum dots (CQDs), termed MPa-COFs/CQDs and MNd-COFs/CQDs. Interestingly, the embedding of the CQDs not only boosted the visible-light absorption ability of the polyimide COFs but also reduced the impedance and charge separation and migration efficiency. As a result, the obtained MPa-COFs/CQD-2 exhibited an H₂O₂ evolution rate of up to 540 μmol g⁻¹ h⁻¹ in pure water at pH = 5, much higher than that of the pristine polyimide COFs. Mechanism analysis revealed that both the step-by-step single-electron ORR and the direct two-electron WOR were involved in the H₂O₂ photosynthesis.⁸⁰ Very recently, another quantum dot decorated CTF-based nanocomposite (CsPbBr₃/CTF), further confirmed the advantages of the construction of hybrid materials for improved photocatalytic performance.⁸¹ To make it clear for the readers, the detailed photocatalytic performance of the reported representative COF-based photocatalysts is summarized in Table 1.

Fig. 13 (a) Schematic illustration of TAPT-TFPA@Pd ICs. (b) Comparison of the H₂O₂ production performance of the studies COFs with the reported photocatalyst. (c) Long-term recycling experiments of TAPT-TFPA@Pd ICs. (d) Proposed mechanism for H₂O₂ photosynthesis. The figures are adjusted with permission from ref. 77. Copyright 2023, American Chemical Society.

Conclusions and perspectives

H₂O₂ has received gradually increasing attention in recent years, and it can serve not only as an efficient oxidant in various fields but also as a prospective energy carrier. Photosynthesis of H₂O₂ over COF-based materials has attracted more and more interest in recent years and researchers are devoted to regulating the structures of the COF-based photocatalysts to realize remarkable H₂O₂ production performance. In this review, we have elaborated the general background and the fundamental principles of COF-based photocatalysts for H₂O₂ photosynthesis. More importantly, the modification strategies that have been reported thus far for realizing improved photocatalytic H₂O₂ production performance over COFs were comprehensively summarized and discussed. Although significant progress has been achieved for efficient H₂O₂ production over COF-based photocatalysts, it is still in its infant stage and there are still some challenging issues that should be addressed for further possible commercial viability and practical applications.

Large-scale productivity and viability

To advance the photosynthesis of H₂O₂ from the current laboratory research phase to practical industrialization, one of the most challenging issues for COF-based materials is the large-scale productivity and viability. On the one hand, the most conventional preparation strategy of COFs thus far is the solvothermal synthesis, which requires complex organic solvents and expensive building units in sealed Pyrex tubes with relatively poor reproducibility and low yields of COF powders, significantly limiting the industrialization of COFs. On the other hand, the large-scale and long-term photosynthesis of H₂O₂ still lacks exploitation and remains to be further investigated, which is of significant importance and a prerequisite for future practical viability. Very recently, Zhang and coworkers have developed a melt polymerization approach for the kilogram fabrication of olefin-linked COFs in one pot.⁸² Ma and coworkers have developed a general high pressure homogenization approach and an industrial homogenizer can produce as high as 0.96–580.48 tons of COFs per day.⁸³ Thus, the further exploration of these approaches for the large-scale preparation of more efficient COF-based photocatalysts is urgently needed and their possible large-scale and long-term H₂O₂ photosynthesis study should be further investigated.

In-depth study and understanding of the mechanism

It is of note that the study and understanding of the photocatalysis mechanism is still deficient at present due to the lack of direct experimental evidence and advanced characterization techniques. In situ and operando investigations of the photocatalysts have opened up the possibility of verifying the actual reactive intermediates and accessing unprecedented photocatalysis mechanisms. Furthermore, the combination of experimental and theoretical calculations (e.g. band structures, adsorption energies, possible reaction sites and pathways) is of significant importance to reveal the photocatalytic mechanisms and guide the researchers to rationally design and synthesize more efficient COF-based photocatalysts for various photocatalysis applications. In addition, more efforts are suggested to be made to study and reveal the relationships between the structures and properties, which will be beneficial for the fast development of this field.

Direct air capture for H₂O₂ photosynthesis

At present, O₂ is widely employed as the reactant and continuously bubbled into the reaction system for H₂O₂ production, which inevitably increases the cost and limits its widespread practical applications. To some extent, direct air capture and utilization of the ubiquitous O₂ (approximately 21 vol%) for carrying out the photocatalytic H₂O₂ production probably would be promising. However, it is still challenging to efficiently and directly utilize O₂ in air for this reaction due to the low solubility and slow mass transfer of O₂ in the reaction solutions. Thus, exploring the O₂ absorption ability of the COF-based materials by rational material design, such as creation of hydrophobicity and integration with other nanomaterials with remarkable O₂ adsorption capacity, may promote the enrichment of the O₂ on the surfaces and interfaces of the photocatalysts, thus leading to improved photocatalytic performance.

Uniform analysis and evaluation of the photocatalyst

If we carefully check the reported studies, the direct comparison between different studies is particularly challenging as their detailed experimental conditions are different. Generally, the H₂O₂ production rate (μmol g⁻¹ h⁻¹) can provide an easy-to-compare metric and it should be widely adopted in the literature and the AQY or SCC efficiency must be reported to scientifically evaluate the photocatalytic performance of the photocatalysts. Furthermore, the addition of the sacrificial agent needs to be carefully evaluated as they may lead to false-positive results and will also increase the costs of the photocatalytic systems. Finally, a standardized way for the detection and analysis of the photogenerated H₂O₂ will not only facilitate the direct comparison of the photocatalysts, but also serves as a useful guide for the researchers.

Overall, we believe that this timely review has summarized the up-to-date progress on COF-based photocatalysts for impressive photocatalytic H₂O₂ evolution and we envision that it can provide new insights to assist and inspire future rational design and development of more efficient photocatalysts for sustainable and possible practical H₂O₂ production.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are grateful for the financial support from the National Natural Science Foundation of China (no. 22371172, 22171169, 22001153 and 21971153), the Shandong Provincial Natural Science Foundation (no. ZR2020QB036), the Taishan Scholars Climbing Program of Shandong Province, the Taishan Youth Scholars Project of Shandong Province and the Major Basic Research Projects of Shandong Natural Science Foundation (no. ZR2020ZD32).

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Footnote

† These authors contributed equally to this work.

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