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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2024 , Vol. 36 >Issue 1: 48 - 66

DOI: https://doi.org/10.7536/PC230529

Review

Covalent Organic Frameworks for Proton Exchange Membranes

  • Weiyu Zhang ,
  • Jie Li ,
  • Hong Li ,
  • Jiaqi Ji ,
  • Chenliang Gong , * ,
  • Sanyuan Ding
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  • College of Chemistry and Chemical Engineering, State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, China
* e-mail:

Received date: 2023-05-30

  Revised date: 2023-07-14

  Online published: 2023-08-06

Supported by

National Natural Science Foundation of China(21975112)

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Abstract

Covalent organic frameworks (COFs), as a new type of organic porous materials, are highly crystalline and orderly porous, exhibiting functional modifiability, structural tunability and high stability. The regular pore channels of COFs can accommodate a variety of proton carriers and proton donors to build continuous and stable proton transport channels, playing a great role in both aqueous and anhydrous proton conduction. The application of COFs to the field of proton exchange membranes is of great research significance and value. In this paper, the characteristics of different types of proton exchange membranes, such as COFs solid electrolyte membranes, polymer matrix-COFs composite membranes, COFs self-supporting membranes and the modification methods to improve the performance of COFs proton exchange membranes are summarized from the aspects of COFs as proton exchange membranes for low temperature fuel cells and high temperature fuel cells, respectively. The relevant representative research of COFs in the field of fuel cell proton exchange membranes in recent years is reviewed. Finally, the application prospects of COFs proton exchange membranes are discussed and prospected.

Contents

1 Introduction

2 Covalent organic frameworks

2.1 Structure of COFs

2.2 Synthesis of COFs and COFs membrane

2.3 Application of COFs

3 COFs fuel cell proton exchange membrane

3.1 COFs low-temperature fuel cell proton exchange membranes

3.2 COFs high-temperature fuel cell proton exchange membranes

4 Conclusion and outlook

Key words： covalent organic frameworks; proton exchange membranes; proton conduction

Cite this article

Weiyu Zhang , Jie Li , Hong Li , Jiaqi Ji , Chenliang Gong , Sanyuan Ding . Covalent Organic Frameworks for Proton Exchange Membranes[J]. Progress in Chemistry, 2024 , 36(1) : 48 -66 . DOI: 10.7536/PC230529

1 Introduction

Traditional fossil energy reserves are insufficient and polluted seriously, so it is urgent to find alternative energy sources. Proton exchange membrane fuel cell (PEMFC) uses hydrogen as fuel to directly convert chemical energy into electrical energy. As an efficient and clean energy conversion device, PEMFC is popular because of its high energy conversion efficiency and fast start-up[1]. Proton exchange membrane (PEM) is the core component of PEMFC, which plays a role in blocking gas permeation and conducting protons between cathode and anode, and has a profound impact on the performance and operation durability of PEMFC[2]. Traditional proton exchange membrane can be divided into perfluorinated proton exchange membrane and non-fluorinated proton exchange membrane according to the composition of fluorine. Perfluorinated proton exchange membrane, such as Nafion, is the most widely commercialized proton exchange membrane at present. Nafion is composed of polytetrafluoroethylene skeleton and hydrophilic side chain with sulfonic acid group, which has excellent proton conductivity[3]. However, Nafion proton exchange membrane is expensive, easy to swell in water, and has a low glass transition temperature, so it can not be operated in high temperature environment[4]. Non-fluorinated proton exchange membranes are mostly aromatic polymers, such as polybenzimidazole (PBI), polyetheretherketone (PEEK), etc. Compared with perfluorinated proton exchange membranes, they are ideal materials for preparing high-temperature proton exchange membranes because of their low cost, easy availability of materials, good mechanical strength and thermal stability[5][6]. However, the chemical stability of non-fluorinated proton exchange membranes is relatively poor, and it is difficult to meet the requirements of high proton conductivity and good mechanical properties at the same time, so the polymer membrane alone can not be directly used as a proton exchange membrane[7]. Inorganic proton-conducting materials, such as small molecule inorganic acid, heteropolyacid (HPA), inorganic oxide ceramic materials, graphene materials, etc., have also become the research focus in the field of fuel cells. Small molecule inorganic acids (such as phosphoric acid) have high conductivity, but they are usually used on polymers or inorganic solid materials, which are easy to leak[8]. HPA (such as phosphotungstic acid, silicotungstic acid, etc.) is a super inorganic acid with strong temperature tolerance. Its special cage molecular configuration makes it have excellent thermal and chemical stability. Its multi-oxygen molecular structure and strong acidity make it maintain strong water absorption at high and low temperatures. It has good proton conductivity and is often compounded with organic polymers. However, the water solubility of HPA usually makes its binding force with the polymer matrix less than that with water, resulting in serious loss of HPA[9]. Graphene materials have excellent electron mobility and thermal conductivity, which can be used as high-performance electrolytes in fuel cells. Cheng Huiming et al. Directly synthesized a three-dimensional foamy graphene macrostructure, graphene foams (GFs), by directional vapor deposition. GF/poly (dimethylsiloxane) composites show 10 S·cm−1 high conductivity, but there are few practical applications of this material at present[10]. Inorganic oxide ceramic materials have good mechanical properties, but their proton conductivity is relatively low[11]. Therefore, it is of great significance to develop new materials that can be used in proton exchange membranes to achieve high-performance fuel cells, and highly crystalline nanoporous materials have received extensive attention. Nanoporous materials, such as two-dimensional graphyne materials (GDY), metal-organic frameworks (MOFs), covalent organic frameworks (COFs), etc., have regular structures, strong designability, and open frameworks can accommodate a variety of proton carriers, which are easy to assemble into excellent proton conducting materials. In recent years, more and more nanomaterials have been used as proton exchange membranes[12][13][14]. Among them, COFs are new porous crystal materials connected by covalent bonds, which do not contain heavy metal ions, and the porous crystal structure is conducive to the formation of abundant proton transport pathways, so it has attracted more attention[15].
Since Yaghi et al. First reported covalent organic framework materials in 2005, COFs have experienced rapid development, and have shown great potential applications in gas storage and separation, catalysis and other fields[16]. The structural designability and functional controllability of COFs, which are mainly composed of lightweight elements and have highly crystalline and ordered porous structures, also make them strong candidates for proton conduction. In 2014, Banerjee et al. First reported the proton-conducting properties of two phosphate-loaded azo-based COFs, and reports on COFs in the field of proton conduction have increased rapidly[14]. The uniform one-dimensional channel of COFs is similar to the structure of Nafion, which can provide regular pores for functional groups such as acidic molecules to construct continuous proton transport channels, and direct membrane formation can achieve higher proton conductivity[17]. The organic building blocks of COFs have a density similar to that of the polymer, showing good compatibility, and can also be blended with the polymer matrix to prepare proton exchange membranes with good mechanical properties[18]. COFs play an important role in both aqueous and anhydrous proton conduction, and are expected to become an ideal proton conducting material. In this paper, the applications and challenges of COFs in proton exchange membranes are reviewed from the aspects of proton exchange membranes for low temperature fuel cells and high temperature fuel cells.

2 Covalent organic framework material

2.1 Structure of COFs.

Covalent organic frameworks (COFs) are organic porous materials composed of C, H, O, N and other light elements connected by covalent bonds, which have the characteristics of good crystallinity, low density and high specific surface area[19]. COFs can be pre-designed crystal framework to build porous network structure with desired physicochemical properties, and different covalent bonds connecting COFs endow materials with different characteristics[20]. In 2005, Yaghi et al. First synthesized borate ester COFs linked by B — O bonds based on the principle of dynamic covalent chemistry. Borate ester COFs have good thermal stability, but their hydrolytic stability in humid air or aqueous solution is poor due to the lack of electrons in the boron atom center, which is vulnerable to nucleophilic attack[16]. Subsequently, they reported COF-300 linked by imine bonds. Imine COFs are formed by Schiff base condensation of aromatic amines and aldehydes, which have abundant hydrophilic sites and improved hydrolytic and oxidative stability[21]. Hydrazone bond is a more stable linkage of COFs developed from imine bond. Hydrazone-bonded COFs are obtained by condensation of aldehydes with hydrazides. Compared with simple C = N, N — N significantly reduces the nucleophilicity of imine nitrogen and protects nitrogen atoms from proton attack. Therefore, hydrazone-bonded COFs have higher chemical stability[22]. In 2008, Thomas et al. First obtained a covalent triazine skeleton (CTF) through the trimerization condensation of cyano groups at 400 ℃, which showed good thermal and chemical stability, but low crystallinity[23]. Baek et al. Used phosphorus pentoxide to catalyze the direct condensation of aromatic amides to produce triazine rings, resulting in CTFs with higher crystallinity and high specific surface area[24]. Thomas et al. Combined reversible and irreversible organic synthesis pathways to obtain crystalline chemically stable porous β-ketoenamine-coupled COF, which showed unprecedented chemical stability in both aqueous and acid-base solutions[25]. In 2016, Feng Xinliang et al., Dresden University of Technology, Germany, synthesized two-dimensional conjugated COF (2DPPV) connected by C = C for the first time. The synthesized polymer skeleton has a crystal layered structure and shows excellent carbon yield at high temperature. C = C is more stable than C = N, and the thermal stability of 2DPPV is as high as 400 ℃[26]. Subsequently, Jiang Donglin et al. Of National University of Singapore constructed two-dimensional sp2 carbon conjugated COF by using the condensation reaction between C = C bonds. The COF presents an ordered two-dimensional layered structure, and its electron conjugation energy extends along the topological network structure formed by two-dimensional sp2 carbon. It can be placed in air for a long time and shows good stability in various organic solvents[27].
COFs can be divided into two-dimensional COFs (2D COFs) and three-dimensional COFs (3D COFs) in terms of spatial structure. 2D COFs are composed of stacked planar two-dimensional polymers, and the lamellae of monomers are connected with each other by π-π interactions, with a graphene-like layered structure, forming one-dimensional channels through both ends, which are independent of each other and allow specific chemical functional groups to enter from both ends[28]. In 3D COFs, organic monomers are connected by covalent bonds. Compared with the simple one-dimensional channels of 2D COFs, the pore structure of 3D COFs is more complex, showing high specific surface area and abundant active sites, which is conducive to gas separation and catalysis applications[29]. As porous polymers, COFs structures can be designed by geometric matching of monomers, by applying topological maps to guide the growth of polygonal skeletons. As shown in Figure 1, the unique framework of COFs is pre-designed in terms of the number of vertices, edges, and loops, using organic connections of different symmetry combinations[30].
图1 COFs的拓扑结构[31]

Fig. 1 Topology diagrams for designing COFs[31]. Copyright 2019, Springer Nature

2.2 Synthesis of COFs and COFs Membrane

Common synthesis methods of COFs include solvothermal synthesis, ionothermal synthesis, microwave synthesis, mechanical grinding, etc., as shown in Figure 2. Solvothermal synthesis is relatively mature and widely used. The first covalent organic framework material COF-1 reported by Yaghi et al. Was prepared by solvothermal synthesis. At a certain temperature and pressure, the monomer and solvent were added into a sealed Pyrex tube, and the oxygen in the system was removed through freezing, vacuumizing, thawing and other cycles. The Pyrex tube was sealed with a flame spray gun and placed in a constant temperature oven[16]. However, the reaction time of solvothermal synthesis is longer and the reaction temperature is higher. Ionothermal synthesis is carried out in the presence of ionic liquids or molten salts at high temperature and high pressure in a thick-walled vessel. Ionothermal synthesis is usually only used for the synthesis of covalent triazine frameworks, and the reaction conditions are harsh, so it is relatively rarely used[23]. The microwave synthesis method uses microwave heating, the heated sample itself becomes a heating element, the heat conduction process is not needed, the reaction system is uniformly heated, and the reaction time is short. The reaction rate of COF-5 synthesis by Cooper et al. Using microwave-assisted solvothermal method is 200 times that of traditional solvothermal method, and the specific surface area of COF-5 is up to 2019 m2·g-1[32]. Mechanical milling refers to the mechanical milling of COFs in the presence of a small amount of liquid additives or no solvent. This method is usually carried out at room temperature, which is simple and environmentally friendly. Banerjee et al. Synthesized three kinds of COFs by mechanical milling at room temperature for the first time. During the mechanical milling process, the formation of COFs and exfoliation occur simultaneously. COFs have a layered structure similar to graphene and have significant stability in boiling water, acid and alkali[33]. However, the crystallinity of COFs prepared by mechanical milling is not high.
图2 COFs的合成方法

Fig. 2 Synthesis methods of COFs

COFs with powder microcrystalline morphology are usually difficult to melt and process, which limits their wide application. In order to make full use of the advantages of ordered and regular pores of COFs, shaping COFs crystals in thin film morphology has become the focus and difficulty of research and application in this field. At present, there are two main strategies for COFs film formation: top-down and bottom-up[34]. The top-down strategy directly exfoliates COFs powder into independent COFs films, namely COFs nanosheets. The main methods include: (1) solvent-assisted exfoliation; (2) mechanical stripping; (3) Chemical stripping method, as shown in Fig. 3. Solvent-assisted exfoliation is the most commonly used method to exfoliate COFs, which is usually combined with the sonication method, and the high shear force generated by sonication in an appropriate solvent overcomes the COFs interlayer interaction and promotes the formation of nanosheets[34]. The COFs nanosheets obtained by Zamora et al. Using the solvent-assisted exfoliation method for the first time can be transferred to any substrate without affecting their original properties[35]. Solvent-free mechanical exfoliation method uses the mechanical force generated by grinding and ball milling to destroy the interlayer interaction of COFs and exfoliate COFs nanosheets. This method is simple and safe, but the exfoliation yield is low and the nanosheets are easy to aggregate[36]. The chemical exfoliation method produces high-yield and thickness-tunable COFs nanosheets by adjusting the interlayer interaction through the controllable chemical reaction of interlayer organic molecules. Banerjee et al. Exfoliated N-hexylmaleimide-functionalized DaTp COF nanosheets from anthracene-based COF by Diels-Alder reaction, and used the air/water interface layer-by-layer assembly method to control the thickness of the nanosheets to obtain independent COF films[37].
图3 自上而下策略制备COFs纳米片

Fig. 3 Top-down strategies for preparing COFs nanosheets

The bottom-up strategy is to deposit COFs on a specific substrate or polymerize them at an interface with controlled thickness and surface. The main methods are: (1) solvothermal synthesis; (2) interfacial polymerization; (3) Surface synthesis method, etc., as shown in fig. 4.
图4 自下而上策略制备COFs膜 （a）溶剂热合成法[38] （b）层状自组装聚合法[44] （c）液-液界面聚合法[41]

Fig. 4 Bottom-up strategies for preparing COFs membranes (a) Solvothermal synthesis[38] (b) Laminar assembly polymerization[44] (c) Interfacial polymerization[41]. Ref 38, Copyright 2011, American Association for the Advancement of Science; Ref 44, Copyright 2019, American Association for the Advancement of Science; Ref 41, Copyright 2023, American Chemical Society

Solvothermal synthesis is relatively simple. A suitable substrate is immersed in the reactants for the synthesis of COFs, and a continuous film will be formed on the substrate at a certain temperature. Dichtel et al. Put the substrate covered with single-layer graphene (SLG) into the reaction system of COF-5, and synthesized COF-5 film on the surface of graphene by solvothermal synthesis. The layered film was vertically stacked on the surface of graphene, which improved the crystallinity of COF film[38].
Interfacial polymerization (IP) is a suitable method for the synthesis of polymer films in large quantities. The interfacial polymerization is carried out at the interface of two phases, and the direct film formation can make full use of the advantages of ordered pores of COFs. The reaction conditions are relatively mild, and it is easy to prepare COFs films with adjustable size and thickness, and transfer the films to the required substrate. Traditional interfacial polymerization methods mainly include liquid-liquid interfacial polymerization, liquid-gas interfacial polymerization, liquid-solid interfacial polymerization and so on[39]. Bao Zhenan et al. Of Stanford University prepared COF films with thickness of 1.8 ~ 29 nm at the solution/air interface[40]; Banerjee et al. Synthesized highly crystalline Tp-Bpy thin films at the water/dichloromethane interface at room temperature, and the thickness of the films was adjusted by changing the monomer concentration[41]; Jiang Zhongyi of Tianjin University first used the gas-solid interface reaction in the preparation of ultra-thin COFs membranes. The stable gas-solid interface is conducive to increasing the reaction temperature and accelerating the reaction rate of two-phase monomers[42]. In addition, numerous novel interfacial synthesis strategies have been reported, such as surfactant monolayer assisted interfacial polymerization (SMAIS), layered self-assembly polymerization (LPA), Langmuir-Blodgett (LB) method, inter-buffer layer polymerization, etc[43][44][45][46]. Park et al. Used layered self-assembly polymerization (LPA) to synthesize two-dimensional COF films at the pentane/water interface. Unlike the traditional interfacial polymerization method, LPA directly transports a continuous stream of carrier solution to the pentane/water interface, and the monomer is strictly limited to a single layer. The obtained films have good mechanical properties and uniform thickness, and are transferred to various substrates without fracture or deformation[44]. Lai et al. First used the LB method to construct a two-dimensional COF film (TFP-DHF) at the water/air interface of the LB tank. The film can be transferred to different substrates layer by layer. The TFP-DHF film supported on the porous support of anodic alumina has excellent permeability to organic solvents[45].
The surface synthesis method usually selects a substrate with a good monocrystalline surface, controls the growth direction of COFs nanosheets, and avoids random dispersion of monomers on the substrate. Surface synthesis methods generally include deposition growth under ultra-vacuum (UHV), solution deposition growth, thermal polymerization under ambient pressure, etc[47][48][49]. In 2011, Lackinger et al. First deposited COF on the graphite surface by casting method, placed the sample in a preheating furnace, and obtained a series of high-quality SCOFs with pore sizes ranging from 1. 0 nm to 3. 2 nm by selecting different building monomers[49].
Novel strategies such as electrophoresis, vapor deposition, homopolymerization have also been reported for the fabrication of free-standing COFs membranes[50][51][52]. Wang Haihui et al. Of Tsinghua University and Xue Jian et al. Of South China University of Technology proposed to use a new electrophoresis method to prepare COF films in a parallel plate electric field.Anionic COF nanosheets containing abundant sulfonic acid groups migrate directionally and deposit on porous substrates (anodic aluminum oxide, AAO) under constant electric field force, forming a well-structured and compact COF membrane, which realizes the rapid preparation of COF membranes in aqueous systems[50]. In addition, due to the good compatibility between COFs and polymer matrix, COFs and polymer matrix can be blended to prepare a mixed matrix membrane, which combines the characteristics of polymer and COFs, disperses the filler in the continuous polymer matrix, and improves the selectivity of the membrane while maintaining the mechanical strength and processability[53].

2.3 Application of COFs

The properties of covalent bonds are diverse, and a small change in atoms or bonds may completely change the properties of the framework of COFs. Diversified monomers also endow COFs with different functions, making COFs have broad application prospects in catalysis, gas separation, electrochemistry and other fields, as shown in Figure 5[20]. For example, the two-dimensional azo-linked ACOF-1 membrane prepared on the surface of porous alumina by Caro et al. Has high selective adsorption for CO2/CH4 mixtures and can efficiently separate CO2[54]. COFs materials have abundant active sites, making them potential efficient catalysts. Wang Wei et al. Of Lanzhou University combined COF-LZU-1 with two-dimensional overlapping layered structure and palladium acetate （Pd（OAc）2） to construct Pd (II) -containing Pd/COF-LZU-1 catalyst, which has high stability, easy recovery and good catalytic activity for Suzuki-Miyaura coupling reaction[55]. Thomas et al. Prepared porous β-ketoenamine-linked two-dimensional COFs, and introduced alkynyl groups to enhance the photocatalytic hydrogen production capacity. The photocatalytic hydrogen production rate of COFs containing double alkynyl groups was more than 10 times higher than that of COFs containing single alkynyl group[56]. COFs have achieved certain application value in catalysis, optoelectronic devices and so on.
图5 COFs的应用[54,55,60]

Fig. 5 Applications of COFs[54,55,60]. ref 54, Copyright 2018, Royal Society of Chemistry; ref 55, Copyright 2011, American Chemical Society; ref 60, Copyright 2023, American Chemical Society

In recent years, researchers have found that COFs have shown great potential in proton conduction due to their good stability, structural designability and unique one-dimensional open channels, and the research on COFs has increasingly focused on the field of fuel cells. In 2014, Banerjee et al. First reported the conductivity of phosphoric acid supported Azo-based COFs (PA @ TP-Azo). The interaction between the phosphate group and the azo group makes the doped phosphoric acid form a hydrogen bond network with the protonated azo group under the cooperation of water, which promotes the proton transfer through the skeleton[14]. In 2016, Banerjee et al. First assembled PA @ TpBpy-MC with good proton conduction performance as a solid electrolyte into a fuel cell membrane electrode assembly. The membrane electrode composed of PA @ TpBpy-MC effectively inhibited fuel crossing and established a stable open circuit voltage of 0.93 V at 50 ℃[57]. Beijing Institute of Technology Wang Bo et al. Constructed porous covalent organic framework ionomer suitable for fuel cell catalyst layer, which significantly improved the mass transfer efficiency of catalyst layer and the power density of fuel cell, and effectively solved the problems of low utilization rate of platinum catalyst and high cost of platinum[58]. For the first time, Jiang Donglin et al. Of the National University of Singapore used COF materials doped with imidazole groups for high temperature anhydrous proton conduction, and initiated the research on high temperature proton conduction materials of COFs[59]. COFs materials with high porosity and enriched functional groups exhibit excellent proton conductivity.

3 COFs fuel cell proton exchange membrane

3.1 COFs proton exchange membrane for low temperature fuel cell

There are two main conduction mechanisms of protons in water environment: Vehicle mechanism and Grotthuss mechanism. The transport mechanism means that protons combine with water molecules in the surrounding microenvironment to form hydrated protons, which diffuse from one side of the electrolyte to the other side in the hydrophilic channel driven by chemical potential and electric field, releasing protons and realizing proton transport conduction[61]. According to the hopping mechanism, proton donors and acceptors in the proton exchange membrane are connected with each other to form a hydrogen bond network, and protons migrate from one carrier molecule to another along the hydrogen bond to complete proton conduction[62]. Generally speaking, there are two mechanisms of proton transport in the membrane at the same time, and there is a clear range of activation energy （Ea） for the two mechanisms, which is often used to infer the proton transport mechanism. If the activation energy is lower than the 40 kJ·mol-1, the hopping mechanism is considered to dominate the transfer process, and if it is higher than the 40 kJ·mol-1, the transport mechanism is considered to dominate. However, in the actual pores of COFs, the microscopic environment of protons may be very complex, and there may be transfer mechanisms other than the above two basic transfer mechanisms in COFs. Temperature, pore size, etc. May affect the transfer of protons in COFs[63].

3.1.1 COFs solid electrolyte membrane

The unique pore structure and rigid framework of COFs can provide stable proton transport nanochannels. Through post-treatment, guest molecules such as sulfuric acid, phytic acid and phosphoric acid can be incorporated into the pores to construct continuous proton conduction channels and improve proton conductivity[15]. In recent years, researchers have designed and synthesized a variety of solid electrolytes based on COFs with excellent performance, aiming at chemically arranging guest molecules in the form of ordered channels in solid materials.
Mirica et al., inspired by the rigid structure of o-phenanthroline, synthesized pyrazine-linked powdered covalent organic frameworks aza-COF-1 and aza-COF-2 using hexanone cyclohexane octahydrate with 2,3,6,7,10,11-hexaaminotriphenyl hexahydrochloride and 1,2,4,5-benzenetetramine tetrahydrochloride, respectively, and both COFs showed good thermal stability, retaining 95% of their weight at 250 ° C[64]. The rich nitrogen environment and uniform one-dimensional channel are easier to absorb water molecules and load acidic groups, which is beneficial to proton transport. The activation energies of the two kinds of COFs after phosphoric acid acidification are 29 kJ·mol-1 and 45 kJ·mol-1, respectively, which are smaller than that of the original COFs. AC electrochemical impedance spectroscopy was used to test the proton conductivity of the materials, and the particle pellets showed good mechanical stability after humidification. Compared with the original particles, the proton conductivity of the acidified COFs was improved by several orders of magnitude, and the proton conductance of PA @ aza-COF-2 with 4. 6 wt% phosphoric acid loading was 4.8×10-3S·cm-1 at 50 ℃ and 97% RH. Trialdehyde phloroglucinol (TFP) was milled with 2,5-diaminobenzenesulfonic acid (DABA) and 2,5-diaminobenzene-1,4-disulfonic acid (DABDA) at room temperature by mechanical synthesis, and two sulfonated covalent organic frameworks NUS-9 (R) and NUS-10 (R) were obtained by solvothermal synthesis[65]. The abundant sulfonic acid groups in the nanospace of the two materials enhance the absorption of water molecules, and the water absorption of NUS-10 (R) is about 52% higher at 298 K and 100% RH, which provides a favorable way for proton conduction. The proton conductivity of NUS-9 (R) and NUS-10 (R) sheets with a thickness of 0. 10 ~ 0.20 cm was tested, and the intrinsic proton conductivity of NSU-10 (R) was as high as 3.96×10-2S·cm-1 at room temperature and 97% RH, and maintained good long-term stability.
Zhang Zhenjie et al. Of Nankai University used a step-by-step synthesis method, first using p-phenylenediamine and phloroglucinol to synthesize a C3 symmetrical precursor with three azo groups and three phenolic hydroxyl groups, and then using trialdehyde phloroglucinol, 2,A series of NKCOFs with high crystallinity and high stability were prepared by the reaction of different aldehyde monomers such as 5-dihydroxyterephthalaldehyde and 1,3,5-tris (4-formylphenyl) benzene, as shown in Figure 6[66]. The phenolic hydroxyl group in the skeleton is used as an acid to directly provide protons, and the high-density azo group is used as a proton acceptor and an acid load point, and both groups have good hydrophilicity, which is beneficial to absorbing water molecules to form hydrogen bonds and provide additional proton conduction capacity for the material. Phosphoric acid can be fixed by the azo bond, and the proton conductivity of H3PO4@NKCOFs loaded with phosphoric acid is significantly improved, and the proton conductivity of H3PO4@NKCOF-1 is as high as 1.13×10-1S·cm-1 at 80 ° C and 98% RH. The activation energies of NKCOF-1 and H3PO4@NKCOF-1 are 24 kJ·mol-1 and 14 kJ·mol-1, respectively, indicating that proton conduction follows a hopping mechanism. The H3PO4@NKCOF-1 particles were fabricated into thin sheets to investigate their performance as PEMFC solid electrolyte membrane under H2/O2 operating conditions, which prevents fuel crossover with a maximum power density of 81 mW·cm-2 and a maximum current density of 456 mA·cm-2, showing good proton conduction ability and fuel cell performance.
图6 （a）NKCOFs的合成路线；（b）NKCOFs的六方结构；（c~f）NKCOFs的AA堆叠俯视图[66]

Fig. 6 （a）Synthetic route of NKCOFs; （b）The hexagonal structural of NKCOFs; （c~f）The side views of NKCOFs, resulting from the eclipsed AA stacking[66]. Copyright 2019, Wiley-VCH

Zang Hongying et al., Northeast Normal University, prepared charged COFs by ion exchange for the first time, and synthesized cationic covalent organic frameworks with high thermal stability[67]. As shown in Fig. 7, EB-COF: Br with a highly stable β-ketoenamine structure was synthesized by trialdehyde phloroglucinol (TP) and ethidium bromide (EB) using a solvothermal synthesis method, and EB-COF: Br was exchanged with polyoxometallate PW12O403− to obtain EB-COF：PW12. The water vapor adsorption isotherm at room temperature shows that the water absorption capacity of EB-COF：PW12 is significantly higher than that of EB-COF: Br in the whole tested relative humidity range. Water clusters are formed around the hydrophilic PW12O403− anions, which interact with water molecules to form a hydrogen bond network to build effective proton transport channels and increase the proton conductivity, and the proton conductivity of EB-COF:PW12 can reach up to 3.32×10-3S·cm-1 at 25 ° C and 97% RH. The activation energy (Ea) of EB-COF：PW12 is 24 kJ·mol-1, which is comparable to that of Nafion film （21 kJ·mol-1）. Cationic COFs become an effective platform for ion exchange and proton conduction, which lays a foundation for the study of ionic COFs.
图7 （a）EB-COF：Br的合成；（b）EB-COF：Br的偏移AA堆叠；（c）PW12O403−掺杂原理[67]

Fig. 7 (a) The synthesis of EB-COF：Br; (b) Top views and side views of the offset AA stacking structure of the EB-COF：Br;(c) Schematic of PW12O403− doping in COF[67]. Copyright 2016, American Chemical Society

Li Gang et al. Of Zhengzhou University used 2,4,6-tris (4-aminophenyl) -1,3,5-triazine and trialdehyde phloroglucinol to prepare a covalent organic framework TpTta with a β-ketoenamine structure, introduced CuCl2 into the pores of TpTta, and the N and O complexation sites in TpTta formed coordination with metal ions, which stably fixed them in the pores[68]. The addition of the metal salt enhances the interaction with water, improves the water absorption of the material, and enhances the proton conduction ability of the particles. When P/P0=0.9, the maximum water vapor uptake of CuCl2@TpTta-3 with 3 wt% CuCl2 content is 310 mg·g-1, which is higher than the 258 mg·g-1 of TpTta under the same conditions. At 100 ° C and 98% RH, the highest proton conductivity of CuCl2@TpTta-10 is 8.81×10-3S·cm-1, which is two orders of magnitude higher than that of the original particle, providing an idea for the development of new proton conductive materials.

3.1.2 COFs composite membrane

COFs particles can be directly used as solid electrolyte to assemble membrane electrode, but the biggest problem of proton exchange membrane is that it is difficult to prepare membrane materials with excellent mechanical properties by using COFs particles alone, and the solid membrane is easily broken down, resulting in fuel leakage and limiting its practical application[69]. The high specific surface area of COFs can expose more active sites, which is easy to modify the interface, strengthen the synergistic effect between COFs particles and the base membrane material, and prepare a composite membrane with strong compatibility with the polymer matrix. The composite membrane can take into account the easy processability of polymers and the versatility of COFs, show high mechanical strength, and broaden its application in fuel cells[70].
In 2016, Wu Hong et al. Of Tianjin University synthesized a covalent organic framework SNW-1 containing triazine structure by using melamine and terephthalaldehyde, impregnated phosphoric acid molecules into SNW-1 by vacuum impregnation method, and introduced it into Nafion matrix for the first time to prepare a new composite membrane[71]. The acid-base pair formed between the H3PO4@SNW-1 network and Nafion optimized the interfacial interaction and hydrophilic domain, and an appropriate amount of H3PO4@SNW-1 increased the water uptake of the membrane and effectively inhibited the swelling of the membrane. The Nafion/H3PO4@S1-20 composite membrane showed the highest water uptake of 28.8% at 25 ℃, which was 33.9% higher than that of the Nafion casting membrane, while the swelling rate was only 16.4%, which was still lower than that of the Nafion casting membrane. There is a good compatibility between the H3PO4@SNW-1 framework and Nafion, and the thermal stability of the composite membrane is enhanced, and the initial decomposition temperature is 389 ℃, which meets the working requirements of fuel cells. The acidic group —PO3H2 provides abundant proton transfer sites, as shown in Fig. 8, a continuous proton transport channel is formed in the composite membrane, and the proton conductivity of the Nafion/H3PO4@S1-15 composite proton exchange membrane with SNW-1 content of 15 wt% is 0.0604 S·cm-1 at 80 ° C and 51% RH,At 60 ° C with humidification, the peak power density of the hydrogen-oxygen fuel cell is 277.8 mW·cm-2, which is increased by 60.3% compared with the Nafion casting membrane, and the maximum current density is also increased by 30.8%, indicating that the introduction of COF effectively improves the fuel cell performance.
图8 （a）复合膜的质子传导机理；（b）Nafion膜与Nafion/H3PO4@S1-15复合膜的截面SEM图[71]

Fig. 8 (a)Schematic of proton transfer in composite membranes; (b)SEM images of the cross sections of Nafion membrane and Nafion/H3PO4@S1-15 composite membrane[71]. Copyright 2016, Elsevier

Two years later, on the basis of SNW-1, they used sultone to synthesize zwitterionic functionalized covalent organic framework Z-COF with sulfonic acid group side chain, as shown in Fig. 9, and blended with Nafion to prepare Nafion/Z-COF composite proton exchange membrane[72]. The addition of Z-COF improves the water retention performance of the membrane, and the sulfonic acid group provides additional proton transport sites in the membrane, which improves the proton conductivity at low humidity. There is a good compatibility between Nafion matrix and Z-COF. On the one hand, the strong interaction improves the mechanical properties of the composite film, and the stress-strain curve shows that the tensile strength of the composite film is higher than that of the Nafion casting film; On the other hand, the N — H group in Z-COF forms a proton transport path with the Nafion matrix, which improves the proton conduction ability. At 80 ℃ and 100% RH, the proton conductivity of Nafion/Z-COF-10 composite membrane with 10 wt% Z-COF reached the maximum of 0.22 S·cm-1, and the peak power density of fuel cell composed of the composite membrane was 61. 4% higher than that of Nafion cast membrane.
图9 SNW-1及Z-COF的合成路线[72]

Fig. 9 Synthetic route of SNW-1 and Z-COF[72]

Li Shenghai et al. of Changchun Institute of Applied Chemistry synthesized sulfonic acid functionalized SCOFs through trialdehyde phloroglucinol and 2,5-diaminobenzenesulfonic acid by solvothermal synthesis method. As shown in Fig. 10, SCOFs were stripped into nanosheet SCONs, and the well-dispersed SCONs were combined with Nafion by casting method to prepare Nafion-SCONs composite membrane[73]. The electrostatic interaction between SCONs and Nafion polymer molecules effectively inhibits the swelling of the composite membrane, provides a large number of continuous proton transport channels, and reduces the permeability of methanol. The Nafion-SCONs-0. 6 composite membrane with 0. 6 wt% SCONs showed a good water absorption of 33. 9% ± 1.30%, a low swelling rate of 15. 4% ± 0.4%, a tensile strength of 27. 3 ± 0.4 MPa, and an oxidation stability of 98. 5% ± 0.3% at 30 ℃, which fully met the working requirements of fuel cells. The Nafion-SCONs-0.6 composite membrane exhibits a proton conductivity of 0.265 S·cm-1 at 80 ° C, and the fuel cell composed of it has a higher power density of 118.2 mW·cm-2, which is 44% higher than that of the Nafion-cast membrane.
图10 SCONs与Nafion分子结构图[73]

Fig. 10 Structural illustration of SCONs and Nafion molecule[73]. Copyright 2019, Elsevier

Based on the synthesis of high proton conductivity NUS-9 (R) and NUS-10 (R), Zhao Dan et al. Of National University of Singapore prepared COF @ PVDF-X composite membrane by blending with polyvinylidene fluoride (PVDF) by casting method[65]. Due to the organic characteristics of COFs fillers and the good compatibility between COFs and PVDF matrix, the composite membrane showed excellent mechanical properties and could be directly immersed in water for proton conductivity testing. At 353 K, the NUS-10 (R) @ PVDF-50 composite membrane with 50 wt% COF filler exhibited the highest proton conductivity of 1.58×10−2S·cm−1 among many composite membranes. And the activation energy of NUS-9 (R) @ PVDF composite membrane filled with different contents of COF was 20 kJ·mol-1, and that of NUS-10 (R) @ PVDF was 21 kJ·mol-1, indicating that the proton conduction followed the hopping mechanism, and the proton was transported by hopping in the network formed by the interaction between sulfonic acid groups and water molecules through hydrogen bonds. Wu Hong et al. Of Tianjin University prepared phosphotungstic acid (HPW) -loaded triazole covalent organic framework HPW @ COF by solvothermal synthesis through trialdehyde phloroglucinol and 3,5-diamino-1,2,4-triazole, and blended it with sulfonated polyether ether ketone (SPEEK) matrix to prepare composite membrane[74]. The N atom in the triazole ring acts as a host at low relative humidity, and the HPW is fixed through the electrostatic interaction between the N atom and the HPW and the constraint effect of the microporous COF channel, so that the composite membrane has higher proton conduction capacity at low relative humidity. The hygroscopic HPW, triazole group, PO40W123− in the COF one-dimensional channel act as proton donors and acceptors to form an interconnected hydrogen bond network, and the basic triazole group forms an acid-base pair with the sulfonic acid group in SPEEK, as shown in Fig. 11, which promotes the transition of protons through the hopping mechanism. The SPEEK/HPW @ COF-15 composite membrane with 15 wt% HPW @ COF filler has a tensile strength of 75.0 MPa, an activation energy of 23.76 kJ·mol-1, and a proton conductivity of 6.2×10-3S·cm-1 at 65 ° C and 40% RH, which is 35.5 times that of the SPEEK membrane, and the proton conductivity remains basically unchanged after 15 days of immersion in water at 30 ° C, showing good stability. COFs nanomaterials and polymer molecules rely on strong compatibility to construct a variety of composite membranes with enhanced mechanical properties and excellent proton conductivity.
图11 SPEEK/HPW@COF复合膜质子传导示意图[74]

Fig. 11 Schematic of proton transfer in SPEEK/HPW@COF composite membranes[74]. Copyright 2020, Elsevier

3.1.3 Free-standing COFs membrane

The mixed matrix membrane can balance the characteristics of polymer matrix and COFs particles, but with the increase of particle content, the particles are easy to agglomerate, which affects the proton transport and reduces the performance of the membrane. The free-standing membrane has a nano-scale thickness, is dense and uniform, and can make full use of the advantages of ordered pores of COFs to achieve high proton conduction[75].
For the first time, Jiang Zhongyi et al. Of Tianjin University used the strategy of interfacial polymerization to synthesize COF nanosheet NUS-9 with proton conduction characteristics in aqueous solution by using trialdehyde phloroglucinol and 2,5-diaminobenzene sulfonic acid through synergistic regulation of monomer diffusion and interaction between COF and solvent[76]. The NUS-9 nanosheets were assembled into dense and robust free-standing IPC-COF membranes by using vacuum-assisted suction filtration method, and the IPC-COF membranes have excellent mechanical properties, showing a higher tensile strength of 91.2 ± 6 MPa, which is higher than most polymer-based proton exchange membranes. The strong capillary effect in the crystalline, rigid ionic nanochannels of the membrane makes it show low humidity dependence, and the swelling ratio is still close to zero even at 98% RH. As shown in Fig. 12, the presence of —SO3H provided additional proton delivery sites, and the proton conductivity of IPC-COF membrane was up to 0.38 S·cm-1 at 80 ° C, 98% RH. In the cell performance test, the membrane electrode showed a high peak power density of 1.1 W·cm-2 at 80 ℃ and 95% RH, and even when the humidity was reduced to 35% RH, it could still maintain a 0.93 W·cm-2, which was three times higher than that of the commercial Nafion 212 membrane.
图12 （a）IPC-COF膜组装和孔隙结构示意图；（b）IPC-COF的SEM图像；（c）IPC-COF的XRD图像；（d）IPC-COF膜和文献中报道的PEMs的溶胀率与IEC之间的关系[76]

Fig. 12 (a)Schematic illustration of IPC-COF membrane assembly and pore structures; (b)SEM image of IPC-COF; (c)XRD pattern of IPC-COF; (d)Swelling ratio versus IEC value from IPC-COF membrane and existing PEMs as reported in the literature[76]. Copyright 2020, Wiley-VCH

Jiang Zhongyi et al. Then used the same monomer and reported a method to synthesize covalent organic framework nanosheets (COF-NSs) with nanoscale thickness and abundant charge groups in single-phase solution, as shown in Fig. 13[77]. The single-phase solution provides an open space, which can accommodate sufficient COF-NS to achieve high volume utilization and atom utilization. The key to the synthesis of nanosheets is the electrostatic repulsion between the charged COFs, and the charge-induced electrostatic repulsive force suppresses the aggregation of the polymer, and the disorder-to-order transition occurs. The charged COF-NS colloidal suspension was used to prepare a dense self-supporting SPC-COF-NS film with a thickness of 25 μm and a layered morphology, which had good mechanical properties and solvent stability, with a tensile strength of 24. 3 MPa, and remained intact in ethanol, dimethyl sulfoxide and other solvents. The abundant nanopores in SPC-COF-NS can accommodate a large amount of water with water absorption up to 50.2 wt%, while the expansion rate is negligible. The high ion density and ordered proton transport channels promote the SPC-COF-NS to show a proton conductivity of 364.1 mS·cm-1 at 80 ℃, and the peak power density of the H2/O2 fuel cell composed of SPC-COF-NS can reach 891.7 mW·cm-2 at 80 ℃ and 70% RH.
图13 单相溶液法合成DABA-TFP-COF-NS纳米片[77]

Fig. 13 Illustration of DABA-TFP-COF-NS synthesis process in single solution-phase[77]. Copyright 2022, Wiley-VCH

Banerjee et al. Prepared highly flexible free-standing COFMs membranes by baking trialdehyde phloroglucinol with three diamino-compounds at 50 – 90 ° C for 3 – 4 d in the presence of p-toluenesulfonic acid （PTSA·H2O） and water using the in situ growth method, as shown in Fig. 14[78]. The film has smooth and compact surface, good stability and mechanical properties. The existence of PTSA·H2O can not only be used as an auxiliary reagent to increase the crystallinity and porosity of COFMs, but also the sulfonic acid group itself makes it an effective proton carrier to enhance the proton conductivity of the membrane, and the proton conductivity of the membrane can be stabilized at 7.8×10-2S·cm-1 at 80 ℃ and 95% RH. The activation energy of PTSA @ COFMs lies between 11~23 kJ·mol-1, indicating that the proton conduction mechanism follows the hopping mechanism. The fabricated flexible membrane was used to assemble and test the performance of the proton exchange membrane fuel cell, and the single cell showed a stable open circuit voltage of 0. 81 V, and the maximum output power of the membrane electrode was 24 mW·cm-2. The different thickness of the membrane can be achieved by changing the thickness of the mold, and the proton conductivity gradually increases with the decrease of the membrane thickness. It is predicted that when the thickness of COFMs reaches 50 μm, the performance of a single cell is expected to surpass that of a Nafion proton exchange membrane fuel cell.
图14 （a~d）COFMs的合成示意图；（e）原始和水洗后的PTSA@COFMs的PXRD谱图[78]

Fig. 14 (a~d)Schematic representation of the synthesis of COFMs; (e)PXRD patterns of as-obtained PTSA@COFMs and those obtained after washing with water[78]. Copyright 2018, Wiley-VCH

Northeast Normal University Zhang Ning and Zhu Guangshan et al. Used the surface-initiated polymerization strategy to prepare continuous free-standing SCOF films with controllable thickness on the surface of silicon wafers[75]. A self-assembled monolayer (SAM) of 3-aminopropyltrimethoxysilane (APTMS) was prepared on silicon wafer with a thin native oxide layer, and the reaction between trialdehyde phloroglucinol and 2,5-diaminobenzenesulfonic acid was initiated by the —NH2 at the end of SAM to form a sulfonated covalent organic framework (SCOF) coating. By adjusting the polymerization time to control the thickness of the film within 10 ~ 100 nm, the free-standing film can be transferred to any substrate, has better mechanical properties and thermal stability, and still maintains good chemical stability in harsh environments. The Young's modulus determined by AFM is 2.3 GPa, which is much higher than 0.24 GPa of the commercial Nafion membrane. The activation energy of the SCOF membrane was 19 KJ·mol-1, indicating that protons were transported by hopping in the membrane, and sulfonic acid groups were introduced into the skeleton in advance. Without the incorporation of polymers and acidic small molecules, the proton conductivity of the SCOF membrane was outstanding. At 80 ℃, the proton conductivity of the SCOF membrane could be stabilized at 0.54 S·cm-1. The research of free-standing COFs proton exchange membranes is still in its infancy, and the preparation of COFs membranes with high crystallinity and higher chemical stability will be an important research content of free-standing COFs membranes as PEM.

3.1.4 Brief summary

COFs are mostly powdery and can be directly used as electrolytes, but it is difficult for pure solid COFs membranes to obtain good mechanical properties. The mixed matrix membrane solves this problem by using the strong interaction between COFs particles and polymer matrix to improve the tensile strength and proton conductivity of the membrane. In addition, the direct film formation of COFs can make full use of the ordered and nanoscale pores of the materials to obtain smooth and dense COFs films. In order to better summarize the conduction characteristics of different types of COFs low-temperature proton exchange membranes, Table 1 lists various types of low-temperature proton exchange membranes and their performance.
表1 不同COFs质子交换膜的性能

Table 1 Properties of different COFs proton exchange membranes

COFs Sample type Thermal decomposition temperature (℃) Tensile strength（MPa） Activation energy
(kJ·mol-1)		 σ (S·cm-1) Maximum power density
(mW·cm-2)		 ref
PA@aza-COF-2 Pellet 250 — 45 4.8×10-3 (50°C, 97% RH) — 64
NSU-10（R） Pellet — — — 3.96×10-2 (room temperature, 97% RH) — 65
H3PO4@NKCOF-1 Pellet 260 — 14 1.13×10-1 (80 ℃, 98% RH) 81 (60°C) 66
EB-COF：PW12 Pellet 300~400 — 24 3.32×10-3 (25 ℃, 97% RH) — 67
Nafion/H3PO4@S1-15 Composite membrane 389 — — 6.04×10-2 (80 ℃, 51% RH) 277.8 (60°C) 71
Nafion/Z-COF-10 Composite membrane 359.5 25~28 7.7 2.2×10-1 (80 ℃, 100% RH) — 72
Nafion-SCONs-0.6 Composite membrane 280~350 27.3±0.4 — 2.65×10-1 (80 ℃) 118.2 (80°C) 73
SPEEK/HPW@COF-15 Composite membrane 250~360 75.0 23.76 6.2×10-3 (65 ℃, 40% RH) — 74
IPC-COF Membrane — 91.2±6 12 3.8×10-1 (80 ℃, 98% RH) 1100 (80°C) 76
SPC-COF-NS Membrane — 24.3 — 3.64×10-1 (80 ℃) 891.7 (80°C) 77
SCOF Membrane — — 19 5.4×10-1 (80 ℃) — 75

3.2 COFs high temperature fuel cell proton exchange membrane

The performance and durability of low temperature proton exchange membrane fuel cell (PEMFC) are restricted when it works below 100 ℃. The impurities in the fuel are easy to cause catalyst poisoning and reduce the catalytic activity; The water and heat management system is complex and costly; The coexistence of gas and liquid phases in the cell leads to a decrease in mass transfer efficiency[79]. To solve the above problems, increasing the working temperature of fuel cell is an effective solution. Therefore, the development of high temperature proton exchange membrane fuel cell (HT-PEMFC) has been widely concerned. The working temperature of HT-PEMFC is 100 ~ 200 ℃, which has more advantages: (1) the catalytic reaction efficiency of anode and cathode can be improved at the same time in high temperature environment; (2) the tolerance of the catalyst to carbon monoxide (CO) is improved, and the poisoning of the catalyst is effectively prevented; (3) The water and heat management system is simple[80]. In recent years, more and more scholars have devoted themselves to the development of high-temperature proton exchange membranes with high temperature and low humidity.

3.2.1 Build an efficient and stable transmission network

High temperature proton conduction is generally anhydrous proton conduction, proton transport can only be carried out by hopping mechanism, and the conduction system is usually based on heterocycles and phosphoric acid, so the design of a material with high stability and good proton conduction ability is the key[81]. The covalent bonding in the COFs layer and the strong interlayer force make the structure of COFs stable enough, and the hydrogen bond anchoring between the proton network and the channel wall of COFs can form a stable proton conduction network. More and more studies on COFs are focused on high temperature proton exchange membranes[82].
Based on the topological design concept, National University of Singapore Jiang Donglin et al. Used 1,3,5-tris (4-aminophenyl) benzene (TPB) with C3 symmetry as the node and dimethyl terephthalaldehyde (DMeTP) with C2 symmetry as the connecting segment to synthesize TPB-DMeTP-COF materials with high crystallinity and porosity by adjusting the monomer concentration, reaction temperature, solvent, etc. Under solvothermal conditions, as shown in Fig. 15[83]. The hyperconjugation effect and induction effect caused by the two methyl groups on the benzene ring of DMeTP weaken the polarity of the C = N bond and the interlayer repulsion; After the protonophore phosphoric acid is loaded, the hydrogen bond network of the phosphoric acid and the nitrogen atom in the C = N are anchored by hydrogen bonds, and the double stabilizing effect ensures that the material has higher stability. The material still maintained high crystallinity and porosity after soaking in tetrahydrofuran, acetonitrile, hydrochloric acid, sodium hydroxide and other solutions for 7 days. The activation energy of H3PO4@TPB-DMeTP-COF is 34 kJ·mol-1, indicating the presence of low energy hopping on the proton network in the nanochannel. The proton conductivity of H3PO4@TPB-DMeTP-COF was 1.91×10-1S·cm-1 at 160 ℃, and the performance did not decay after 20 H of continuous operation. This experiment provides a new idea for the design and preparation of anhydrous proton conduction of COFs with highly stable structure.
图15 （a）TPB-DMeTP-COF的合成路线；（b）TPB-DMeTP- COF的单个六边形大环结构；（c）TPB-DMeTP-COF单个六边形通道结构（灰色：C原子；绿色：N原子；省略了CH3和H单元）；（d）77 K下TPB-DMeTP-COF的氮气吸附等温线（圆圈：吸附；三角形：解吸）[83]

Fig. 15 (a)The synthesis of TPB-DMeTP-COF; (b)The structure of one hexagonal macrocycle; (c)The structure of a 1D channel (grey, C；green, N；CH3 units and H are omitted for clarity); (d)Nitrogen sorption isotherms of TPB-DMeTP-COF measured at 77 K (circle, adsorption; triangle, desorption)[83]. Copyright 2020, Spring Nature

Zhang Gen et al. of Nanjing University of Science and Technology proposed a bottom-up self-assembly strategy to construct perfluoroalkyl-functionalized two-dimensional COFs. The corresponding COF-Fx (X represents the number of carbon atoms in the fluoroalkyl chain) was obtained by solvothermal reaction of trimellitic dialdehyde with three fluorocarbon hydrazide monomers of different lengths, and the effect of fluoroalkyl chains of different lengths on proton conduction was studied[17]. Fluorinated COFs have high stability to strong acids due to their enhanced hydrophobicity and water contact angle of more than 140 °, and superhydrophobic 1D nanochannels can accommodate a large amount of phosphoric acid.Phosphoric acid molecules form a hydrogen bond network with the rigid backbone and fluoroalkyl chains of COFs, and most phosphoric acids are dynamic and can move freely in the channel, undergoing rapid proton conduction under anhydrous conditions and a wide temperature range. Because the longer fluoroalkyl chain will hinder the proton transport path, the shorter fluoroalkyl chain will obtain higher proton conductivity at the same phosphoric acid loading, and the highest anhydrous proton conductivity of COF-F6-H loaded with phosphoric acid is 4.2×10-2S·cm-1 at 140 ℃.
Du Li et al. Of South China University of Technology proposed a strategy to enhance the interlayer interaction of two-dimensional COFs, as shown in Fig. 16, the planar and rigid triazine units were introduced into the center of C3 symmetrical monomers to adjust the interlayer interaction, which effectively solved the problem of instability of imine COFs, and the closely stacked structure improved the crystallinity and porosity of the material at the same time[84]. TPT-COF was synthesized from two triazine monomers, 2,4,6-tris (4-formylphenyl) -1,3,5-triazine and 2,4,6-tris (4-aminophenyl) -1,3,5-triazine, by solvothermal method. TPT-COF has high thermal and chemical stability, the initial decomposition temperature is 525 ℃, and it still maintains a long-range ordered structure after immersion in 14.6 mol·L-1 phosphoric acid for 2 months. The presence of the triazine group in the center of TPT-COF enhances the interlayer interaction, and after doping with phosphoric acid, the H3PO4 network occupies an efficient and stable long-range ordered framework, and the multi-point polytype N sites form a stable and dense proton network with H3PO4. TPT-COF shows a high proton conductivity of 1.27×10-2S·cm-1 at 160 ℃ and remarkable long-term durability. Jiang Donglin et al. Prepared polybenzimidazole covalent organic framework, designed benzimidazole into a monomer structure containing symmetrical diamines, and integrated it into the framework of COFs to construct stable COFs with one-dimensional unidirectional channels[85]. 1,3,5-diamino-3,4-dimethylbenzimidazole (DABI) was condensed with 1,3,5-tris (4-formylphenyl) benzene (TPB) under solvothermal conditions to form TPB-DABI-COF. TPB-DABI-COF maintains good chemical stability in tetrahydrofuran, boiling water, 9 M HCl and other solutions. After phosphate loading, the TPB-DABI-COF built-in channel is fully occupied by neat phosphate, and the benzimidazole wall triggers multipoint and multitype interactions. The N atom in the benzimidazole unit is protonated by H3PO4 to form an imidazolium cation, H3PO4 is deprotonated to form H2PO4-, and the two ions form an electrostatic interaction to promote proton conduction. The activation energy of H3PO4@TPB-DABI-COF is low 17 kJ·mol-1, and the proton conductivity of H3PO4@TPB-DABI-COF can reach up to 1.52×10-1S·cm-1 at 160 ℃ and 66.1 wt% H3PO4 loading, which realizes the ultrafast and stable proton conduction at low PA doping.
图16 （a）TPT-COF的合成；（b）反平行堆积模型下TPT-COF相应细化晶体结构的俯视图和侧视图（灰色、蓝色和白色球体分别代表C、N和H原子）；（c）TPT-COF的PXRD谱图[84]

Fig. 16 (a)The synthesis of TPT-COF; (b)Top and side views of the corresponding refined crystal structures of TPT-COF with the antiparallel stacking model (gray, blue, and white spheres represent C, N, and H atoms, respectively); (c)The PXRD patterns of TPT-COF[84]. Copyright 2022, Wiley-VCH

3.2.2 Doped guest molecule

The pore channels of COFs are long-range ordered and easy to be functionalized, and the proton conduction with high proton concentration is realized by immobilizing functionalized guest molecules into COFs under high temperature and anhydrous conditions[86].
A two-dimensional mesoporous covalent organic framework TPB-DMTP-COF was prepared from 1,3,5-tris (4-aminophenyl) benzene (TPB) and 2,5-dimethoxyterephthalaldehyde (DMTP) by solvothermal synthesis. The stable framework structure provides a good environment for doping guest molecules[59]. The two solid materials (trz @ TPB-DMTP-COF and im @ TPB-DMTP-COF) showed excellent stability, maintained the original crystallinity and porosity in N, N-dimethylformamide, tetrahydrofuran and other organic solvents, and the thermal decomposition temperature was above 200 ℃. When the proton conductivity of the material is measured by the AC impedance method, the pressed particle pellet has good mechanical strength without cracks or fragments during repeated use. In the two materials, there are two lone pair nitrogen atoms in the trz @ TPB-DMTP-COF structure, and the proton concentration is relatively low, while the imidazole group in the pore of im @ TPB-DMTP-COF forms a continuous hydrogen bond network to enhance the proton conduction ability, so im @ TPB-DMTP-COF has a higher proton conductivity, which is 4.37×10-3S·cm-1 at 130 ℃, four times that of trz @ TPB-DMTP-COF at the same temperature. Mesoporous COFs loaded with N-heterocyclic proton carriers provide a new strategy for anhydrous proton conduction.
Yan et al. of Soochow University designed and synthesized a high-density sulfonic acid functionalized proton-conducting covalent organic framework TB-COF by using trialdehyde phloroglucinol and 4,4 '-diaminobiphenyl-3,3' -disulfonic acid, as shown in Fig. 17.The presence of high-density sulfonic acid groups in the framework and one-dimensional ordered nanochannels make TB-COF an intrinsic proton conductor, and the proton ionic liquid (PIL) 1-methyl-3- (3-propanesulfonyl) imidazole hydrogen sulfate was introduced to increase the proton transfer sites and external proton source[87]. The electrostatic interaction between the —SO3H on the framework and the imidazole group on the PIL ensures that the PIL does not leak from the COF channel, and the stable framework structure ensures that the PIL-TB-COF can be used in a high-temperature fuel cell working environment under high-temperature and anhydrous conditions. The activation energies of TB-COF and PIL-TB-COF are 37 kJ·mol-1 and 30 kJ·mol-1, respectively, indicating that protons are transported by hopping mechanism. The internal proton source and the external proton source simultaneously provide a large amount of movable H+, and the temperature rises to accelerate the directional movement of the H+ in the pore. The proton conductivity of PIL-TB-COF was measured by pressing the COF powder into thin sheets in a glove box under Ar atmosphere. The proton conductivity of PIL-TB-COF was 2.21×10-3S·cm-1 at 120 ℃, and the proton conductivity remained almost unchanged after 72 H at this temperature, showing good thermal stability. This work shows the great potential of COFs as anhydrous proton-conducting materials.
图17 （a）PIL-TB-COF的合成路线 ；（b）质子传输机理[87]

Fig. 17 (a)Structure of and synthetic route to TB-COF and PIL-TB-COF (b)Schematic illustration of the proton transfer in PIL-TB-COF[87]. Copyright 2022, Royal Society of Chemistry

Nanjing University of Science and Technology Zhang et al. Assembled perfluoroalkyl functional groups into the channels of COFs in a bottom-up manner. The perfluoroalkyl chain with appropriate chain length makes the material have good thermal and chemical stability. Then the proton ionic liquid was introduced into the one-dimensional nanochannels of COFs to assemble the hydrogen bond network and achieve rapid anhydrous proton conduction[88]. The ionic liquid diethylmethylamine bisulfate （[dema]HSO4） was doped into COF-F6 with good hydrophobicity by grinding method. With the increase of ionic liquid content, the internal hydrogen bond network of COFs became more dense and the activation energy decreased. The 1.5 wt%[dema]HSO4 loaded COF-F6（F6-[dema]HSO4-1.5） particles have the highest proton conductivity 1.33×10-2S·cm-1 at 140 ° C in the absence of water and maintain good long-term durability. Banerjee et al. Prepared hybrid COF（TpPa-（SO3H-Py）） containing both pyridine and sulfonic acid groups by solvothermal synthesis. Phytic acid was introduced into the porous structure of COFs as a proton source and carrier. The low volatility of phytic acid increases the possibility of working at high temperatures. The molecular size of phytic acid is equivalent to the pore size of COFs, which can prevent leaching problems[82]. After loading phytic acid, the intrinsic conductivity of phytic@TpPa-（SO3H-Py） is produced by the combination of sulfonic acid group and adjacent pyridine, and the external conductivity is caused by the fixation of phytic acid by pyridine nitrogen. The interaction between internal and external promotes proton transport. At 120 ℃, the proton conductivity of phytic@TpPa-（SO3H-Py） can reach 5×10-4S·cm-1.
Zhang Zhenjie et al. Of Nankai University used pore engineering to synthesize a series of covalent organic frameworks with high crystallinity and porosity, and used the pre-design ability of COFs to design hexagonal channels into concave dodecagonal nanopores, and at the same time, functional groups such as — N = N, —CF3, and — OH were installed on the pore surface[81]. — The N = N bond can act as an effective site for phosphate to promote hydrogen bond formation; The —CF3 group can enhance the hydrophobicity of the nanochannel; As a proton source, phenolic hydroxyl groups can assist phosphoric acid to establish a proton transmission channel at low humidity, and the obtained three COFs can load a large amount of phosphoric acid, thereby improving the utilization rate of the proton carrier and the interaction between the host and the guest, and obtaining higher anhydrous proton conduction capacity. The proton conductivity of the most —CF3 substituted H3PO4@NKCOF-54 particles was up to 2.33×10-2S·cm-1 at 160 ° C in the absence of water.

3.2.3 Composite with polymer matrix

Powdered COFs doped with guest molecules can construct continuous and stable proton transport channels and obtain high proton conductivity, but they are easily broken down at high temperature and their stability is weakened, which limits their application in high temperature proton exchange membrane fuel cells. The polymer has remarkable processability, the mixing of COFs and the polymer matrix can avoid the problem of poor processability of COFs, and the rigid frame can inhibit the expansion of the membrane to obtain the COFs membrane with high mechanical strength and low gas permeability[89]. The research on high temperature proton exchange membrane has realized the transformation from unstable powder material to stable and processable membrane material.
Polybenzimidazole (PBI) is a rigid, high temperature resistant aromatic heterocyclic polymer with high thermal decomposition temperature, which has become the most concerned high temperature proton exchange membrane material. However, PBI itself can not conduct protons, and inorganic acids such as phosphoric acid (PA) are usually used as proton conducting media[90]. Although the PA/PBI composite membrane can obtain relatively high proton conductivity, the leakage of PA is serious and the mechanical properties are seriously reduced. It is a feasible solution to construct a continuous and stable proton transport channel[91]. COFs with stable, ordered porous structures hold great promise for improving the performance of PA-doped PBI membranes.
Wang Lei et al. Of Shenzhen University used polybenzimidazole to prepare covalent triazine frameworks (CTFs) in situ under the catalysis of trifluoromethanesulfonic acid.The aromatic skeleton of CTFs without thermosensitive groups can meet the stability requirements of high temperature work, the rigid framework rich in nitrogen sites can improve the affinity for acid, and the inherent porous structure can provide enough gaps to retain PA and establish a continuous and efficient proton transport channel[92]. The classical method of mixing nanomaterials with polymer matrix usually has the problems of poor dispersion and low loading of nanomaterials. The CTFs in the composite membrane obtained by in-situ method are uniformly dispersed, which is conducive to enhancing the interaction between the matrix.The tensile strength of the 30% -CTFs-OPBI in-situ composite membrane with 30 wt% CTFs can reach 7. 7 MPa, which is better than that of the direct blend membrane and meets the working requirements of high temperature fuel cells. At 160 ℃, the proton conductivity of 30% -CTFs-OPBI composite membrane can reach 71.7 mS·cm-1. The performance of PEMFC was tested by non-wet H2 and O2.The peak power density of the cell was 534.4 mW·cm-2, and the long-term stability test was carried out at a constant current density of 200 mA·cm-2 at 160 ° C for 250 H. The composite membrane showed good stability due to its stable voltage and low H2 permeability. The novel strategy of introducing covalent triazine organic frameworks to construct proton high-speed conduction channels in situ provides a way to fabricate high-performance proton exchange membranes. Using the same in situ growth method, as shown in Fig. 18, they designed and prepared a series of polyarylether-based COF-316 containing cyano groups synthesized in situ in OPBI composite membranes[93]. The in situ grown COF-316 improves the inherent mechanical properties of the membrane, the rigid framework enhances the dimensional stability of the membrane, and the porous structure effectively inhibits the loss of PA. After doping phosphoric acid, the tensile strength of 40% -COF-OPBI composite membrane with COF content of 40 wt% can reach up to 12. 2 MPa, and the PA retention rate at 80 ℃ and 40% RH can reach up to 86. 8%. Nitrogen-containing cyano groups can provide proton transfer sites and form a continuous proton transport channel. At 160 ℃, the proton conductivity of 40% -COF-OPBI composite membrane can reach 177.7 mS·cm-1. The peak power density of proton exchange membrane fuel cell is 774.7 mW·cm-2. The fuel cell has low internal resistance, no gas permeation, and maintains excellent durability. The composite membrane loaded with in-situ grown COFs shows remarkable fuel cell performance in a wide temperature range, and has broad prospects for practical application of high-temperature proton exchange membrane fuel cells.
图18 （a）OPBI溶液中COFs的原位生长反应；（b）原始OPBI膜的SEM图；（c）40%-COF-OPBI复合膜的SEM图[93]

Fig. 18 (a)Reaction for the in situ growth of COFs in OPBI solution; (b)SEM image of pristine OPBI membrane; (c)SEM image of 40%-COF-OPBI composite membrane[93]. Copyright 2022, Elsevier

3.2.4 Brief summary

Under high temperature and anhydrous conditions, protons are mainly transported by hopping mechanism, that is, hopping from one carrier site to another through hydrogen bonds. At this time, it is very important to construct a continuous and stable channel structure and install beneficial functional groups on the pore surface. Different types of modified COFs high temperature proton exchange membranes show different conduction characteristics. Table 2 lists the main properties of typical COFs high temperature proton exchange membranes.
表2 不同高温COFs质子交换膜的主要性能

Table 2 Properties of different COFs HTPEMs

COFs Sample type Thermal decomposition temperature (°C) Tensile strength(MPa) Activation energy
(kJ·mol-1)		 σ (S·cm-1) Maximum power density
(mW·cm-2)		 ref
H3PO4@TPB-DMeTP-COF Pellet — — 34 1.91×10-1 (160 ℃, 0 RH) — 83
TPT-COF Pellet 525 — 17 1.27×10-2 (160 ℃, 0 RH) — 84
H3PO4@TPB-DABI-COF Pellet — — 17 1.52×10-1 (160 ℃, 0 RH) — 85
im@TPB-DMTP-COF Pellet 220 — 25 4.37×10-3 (130 ℃, 0 RH) — 59
PIL-TB-COF Pellet 221 — 30 2.21×10-3 (120 ℃, 0 RH) — 87
F6-[dema]HSO4-1.5 Pellet 300 — 34 1.33×10-2 (140 ℃, 0 RH) — 88
30%-CTFs-OPBI Membrane 300 7.7 — 7.71×10-2 (160 ℃, 0 RH) 534.3 92
40%-COF-OPBI Membrane 300~400 12.2 — 1.777×10-1 (160 ℃, 0 RH) 774.7 93

4 Conclusion and prospect

As ordered porous materials, the regular pore channels of COFs can accommodate proton carriers and proton donors such as imidazole, phosphoric acid and sulfuric acid, and the rigid organic framework can effectively inhibit swelling and improve the stability of the materials. Therefore, it is of great significance and value to apply COFs to proton exchange membranes for low temperature fuel cells and high temperature fuel cells.
In recent years, COFs materials have made significant progress in the field of proton exchange membranes. For example, Jiang Zhongyi of Tianjin University used interfacial polymerization to prepare free-standing sulfonated COFs membranes with adjustable thickness, which showed 0.38 S·cm-1 proton conductivity. Zhu Guangshan of Northeast Normal University used surface-initiated polymerization strategy to prepare continuous SCOF membranes with proton conductivity up to 0.54 S·cm-1.However, the practical application of COFs proton exchange membranes still faces many challenges: (1) The nano-pores of COFs materials have directionality, but in most of the existing membrane preparation processes, the direction of pore arrangement changes randomly, which changes the orderly arrangement of pores[76][75]; (2) The mechanical properties of COFs proton exchange membranes are relatively poor, and the membranes face problems such as fuel crossover and gas crossover during long-term use; (3) The development of COFs is mostly in laboratory-scale synthesis and battery performance testing, and has not yet been applied in real life on a large scale.
In view of the above challenges, first of all, we should focus on improving the mechanical properties of the membrane. How to obtain higher mechanical integrity and high-quality crystalline COFs films and achieve the balance between mechanical properties and proton conduction is a problem that needs to be solved. Secondly, for the composite membrane of COFs and polymer matrix, the compatibility between the matrix should be strengthened to minimize the fragility of the membrane; Thirdly, efforts should be made to improve the mechanical properties of COFs self-supporting membranes, strengthen the optimization of membrane electrode assemblies, and reduce the fuel permeability of membranes. Finally, it is necessary to continue to explore new methods for direct film formation of COFs, give full play to the advantages of ordered and regular nano-pores of materials, and overcome the defects of long-range disorder of polymer membranes. At the same time, it will promote the application development and industrial scale progress of COFs proton exchange membrane, and realize large-scale application as soon as possible.

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