Interfacial Synthesis of 2D COF Thin Films

*Tao Zhang and Yuxiang Zhao*

## **Abstract**

Two-dimensional covalent organic frameworks (2D COFs) are emerging crystalline 2D organic material comprising planar and covalent networks with long-ranging structural order. Benefiting from their intrinsic porosity, crystallinity, and electrical properties, 2D COFs have displayed great potential for separation, energy conversion, and electronic fields. For the most of these applications, large-area and highly-ordered 2D COFs thin films are required. As such, considerable efforts have been devoted to exploring the fabrication of 2D COF thin films with controllable architectures and properties. In this chapter, we aim to provide the recent advances in the fabrication of 2D COF thin films and highlight the advantages and limitations of different methods focusing on chemical bonding, morphology, and crystal structure.

**Keywords:** interfacial synthesis, 2D material, COF, crystal, thin film

## **1. Introduction**

In 2005, the first covalently bonded crystalline porous polymer was successfully synthesized and named covalent organic frameworks (COFs). In subsequent developments, COFs linked by B∙O, C∙N, C∙C, and other bonds have been reported. The regular network structure of COFs can be fully characterized with the help of existing instruments, which is very beneficial to study the relationship between the performance and structure. COFs can also be divided into two-dimensional (2D) COFs and three-dimensional (3D) COFs according to the specific structure. 2D COFs are emerging crystalline 2D organic material comprising planar and covalent networks with long-ranging structural order [1, 2]. In recent years, 2D COFs have been rapidly developed due to their ease of synthesis and definite structure. Benefiting from their intrinsic porosity, crystallinity, and electrical properties, 2D COFs have displayed great potential for separation [3, 4], energy conversion [5–7], and electronic fields [8]. The preparation of COF materials as thin films is advantageous for most applications. Large-area and highly ordered 2D COFs thin films are widely studied [9–12]. COFs are mostly connected by reversible covalent bonds. If the reaction conditions can be adjusted to make the structure of COFs in a dynamic self-repair process, highly ordered films can be obtained. Clever use of the interface can also give the film a good substrate for growth, and the COFs can be spread out along the interface to produce a smooth film. Common interfaces include gas/solid interface, liquid/liquid interface,

liquid/solid interface, and gas/liquid interface. Hence, in this critical review, we aim to provide the recent advances in the fabrication of 2D COF thin films and highlight the advantages and limitations of different methods focusing on chemical bonding, morphology, and crystal structure.

## **2. Preparation methods of COF film**

At present, the common preparation methods include top-down method and bottom-up method. The bottom-up method is mainly through the interface reaction, so that the formation and rupture of chemical bonds of small organic molecules occur at the interface, and the final product is spread along the interface. The interfaces include gas/solid interface, liquid/liquid interface, liquid/solid interface, and gas/liquid interface. 2D COF films obtained by this method have better orientation and fewer defects, and the relationship between properties and structure can be better studied in the application. But there are some problems such as low yield and slow reaction through interface reaction. Reaction time and efficient utilization of small molecular monomers are also important issues to be considered in the process of synthesizing materials. The top-down preparation method is to peel the powder COFs material into nanosheets by chemical treatment or physical method and then process the nanosheets into large-size films by vacuum extraction and filtration. The prepared materials via this method have many defects, weak orientation, and other problems. How to control the thickness and structure of materials to obtain 2D films with orderly atomic structure is the main challenge at present (**Figure 1**).

#### **Figure 1.**

*Schematic illustration of interface synthesis of COF films. (a) Solid/liquid interface; (b) air/solid interface; (c) air/liquid interface; (d) Langmuir-Blodgett (LB) method; and (e) liquid/liquid interface.*

## **2.1 Bottom-up strategy**

## *2.1.1 Solid/liquid interface*

COFs were first reported by Yaghi et al. in 2005. COF-1 and COF-5 connected by B-O bond were respectively prepared in Pyrex tubes by solvothermal method [1]. The reversible B-O bond gives the material excellent repairability, resulting in a highly regular pore structure and a high specific surface area. Solvothermal synthesis is still the main strategy for preparing COFs powder [13–15].

The preparation of COF film by gas/solid interface is mainly to add solid substrate in the solvent and let COFs grow into film on the surface of the substrate in situ. In 2011, Dichtel et al. prepared COF-5 films from the polymerization of two monomers, 2,3,6,7,10,11-hexahydroxytriphenyl (HHTP) and 1, 4-phenyldiboric acid (PBBA), using a single layer of graphene as a substrate [16] (**Figure 2a**). In this process, the concentration of monomer needs to be regulated. When the concentration of monomer is high, the system will nucleate and generate powder. The characterization of the COF-5 film by grazing incidence X-ray diffraction (GIXRD) and scanning electron microscope (SEM) showed that the films were parallel to the graphene surface and have good orientation (**Figure 2b**–**d**). In this work, COF powder was processed into film for the first time, which is of great significance for the application of COF as organic electronic devices. When COF is processed into a film, it can better characterize its optoelectronic properties. Inspired by this work, different research groups began to choose different substrate materials for the preparation of thin films [17–19]. Base materials include Indium Tin Oxides (ITO) and polyethersulfone (PES) [20, 21], etc. The synthetic conditions of BDT-ETTA COF and the transmission electron microscopy (TEM) images of the powder are shown in **Figure 3a** and **b**, respectively. SEM images and GID patterns analysis of the cross section found that the COF film was spread evenly on the ITO substrate (**Figure 3c**–**f**). The connecting units of COFs include

#### **Figure 2.**

*(a) Schematic illustration of chemical synthesis at solid/liquid interface using a single layer of graphene as a substrate; (b) X-ray scattering data obtained from COF-5 powder; (c) GID data from a COF-5 film on SLG/Cu; (d) top-view SEM image of the COF-5 thin film.*

#### **Figure 3.**

*Schematic illustration of chemical synthesis at solid/liquid interface using the indium tin oxides (ITO) as a substrate; (a) synthesis process of BDT-ETTA COF; (b) TEM image of BDT-ETTA COF powder; (c,d) SEM images (cross-section) of COF film; (e, f) GID patterns of COF film.*

B∙O bond and C∙N bond. These thin films can be well used for water splitting and molecular separation. Solid/liquid interface synthesis is considered to be an effective method for vertically growing high crystallinity films. Since the film grows on the substrate surface, it is easy to collect and characterize. However, there is another problem with this method, that is, the adhesion force between the film and the substrate is strong, and it is challenging to transfer it from the substrate. Since the film cannot be transferred between different substrates, it is only possible to select the appropriate substrate before each experiment [22].

## *2.1.2 Liquid/liquid interface*

Water and various organic solvents are incompatible, resulting in a phase interface at the junction of the two phases. If COFs can be spread out during growth, 2D COF

### *Interfacial Synthesis of 2D COF Thin Films DOI: http://dx.doi.org/10.5772/intechopen.106968*

films with excellent orientation can be obtained at the interface with the continuous self-repair of dynamic chemical bonds. The films obtained by this method can be easily transferred to different substrate materials, which is very beneficial in application [23, 24]. In 2016, Xinliang Feng et al. designed and synthesized wafer-sized 2D imine COFs with high mechanical stiffness. Porphyrin amino monomer and 2,5-dihydroxyterephthalaldehyde monomer are dissolved in trichloromethane and water, respectively [25] (**Figure 4a**). The amine group and aldehyde group contact at the interface to form imine bond, leading to 2D polymerization. The authors simulated the structure of COF film and characterized the surface topography of the material by atomic force microscopy (AFM) and TEM, in which a highly ordered arrangement of structures can be seen (**Figure 4b**–**d**). Photographic image of 2DP on 4-inch 300 nm SiO2/Si wafer shows that the entire thin film is macroscopically flat (**Figure 4e**). Liquid/liquid interface synthesis is another method that can efficiently synthesize high-quality films. At present, the main problem is to prepare COF films with higher crystallinity by adjusting the amount of catalyst and the choice of solvent system. On the basis of this work, more efficient catalysts of Schiff base reaction catalysts have also been developed. Dichtel et al. prepared high crystallinity COF films efficiently using Sc(OTf)3 as a catalyst [26, 27]. The catalyst is dissolved in the aqueous phase, and the monomers are dispersed in the organic phase. Amine and aldehyde monomers can be polymerized into films at the phase interface under the catalysis of Sc(OTf)3 (**Figure 4f**–**g**). Bo Wang et al. also used this catalyst to covalently connect 2, 5-dihydroxy-1, 4-phthalate formaldehyde (DOBDA) and 1,3,

#### **Figure 4.**

*Schematic illustration of chemical synthesis at liquid/liquid interface. (a) Synthesis of a 2DP through Schiffbase condensation reaction; (b) molecular structure of the 2DP by DFTB calculation. (c) AFM, and (d) TEM images of 2DP; (e) photographic image of 2DP on 4-inch 300 nm SiO2/Si wafer; (f) schematic explanation and photograph of TAPB-PDA COF; (g) photo of the TAPB-PDA COF film.*

5-tri (4-aminophenyl)-benzene (TAPB) to prepare compact COF films with different thickness of 300 nm ~ 500 nm [28]. Besides, by removing C∙N bonds and using the defects in COF films, a vertical channel with hydrophilic gradient was fabricated.

The reaction time of interfacial synthesis is another problem that needs to be considered. In order to obtain high crystalline films, the rate of reaction is usually controlled to slow down the polymerization kinetics. In 2019, wafer-scale synthesis of monolayer 2D porphyrin polymers was reported by Park Research Group (**Figure 5a**). The authors report that by growing the film at the pentane/water interface, a 2D porphyrin polymer film with sheet-level uniformity was synthesized at the limit of the thickness of a single layer, by growing the film at the pentane/ water interface [29]. Films of different structures have different absorption spectra and colors (**Figure 5b**–**d**). The superposition of films of different colors also reveals

#### **Figure 5.**

*(a) Schematic of monolayer 2DPs and corresponding chemical structures of the molecular precursors; (b) absorption spectra of monolayer 2DPs on fused silica substrates; (c) hyperspectral transmission images and resulting false color images of 1 inch-square 2DP I on a 2-inch fused silica substrate. Transmission images taken at the wavelength of 405 nm, 420 nm, and 440 nm are assigned as red, green, and blue channel, respectively, to generate the false color image. (d) False color images of monolayer 2DPs covering entire 2-inch fused silica wafers.*

### *Interfacial Synthesis of 2D COF Thin Films DOI: http://dx.doi.org/10.5772/intechopen.106968*

different optical properties. The corresponding color and monomer are very close, which provides experience for future film designs of different colors.

In 2017, Banerjee et al. selected p-toluenesulfonic acid (PTSA) to form a selfsupporting COF film at the interface between water and methylene chloride [30] (**Figure 6a**). The hydrogen bond network formed by PTSA can slow down the diffusion rate of monomer and increase the quality of the COF film. The content of catalyst and concentration of monomer have great influence on the thickness of thin films. SEM image and AFM image of Tp–Bpy COF thin film prove that the surface of the film is very smooth and the structural orientation is high (**Figure 6b**–**c**). Zhongyi Jiang et al. prepared ionic covalent organic framework membranes (iCOFMs) with ultrahigh ion exchange capacity via the double-activation interfacial polymerization strategy (**Figure 6d**). Brønsted acid and Brønsted base activate aldehyde monomers and amine monomers with sulfonic groups in aqueous and organic phases, respectively [31]. After the double activation of acid and base, the monomer can react quickly at the interface and form iCOFMs with high crystallinity. At present, the liquid/liquid interface synthesis is developing rapidly. The films synthesized by this method are widely used in the fields of separation and purification and seawater desalination. At the same time, this method has also been continuously improved in the process of development, which can obtain higher quality films in a shorter time.

#### **Figure 6.**

*(a) Schematic illustration of the interfacial synthesis of Tp–Bpy COF thin film; (b) SEM images of Tp–Bpy COF thin film; (c) AFM image of Tp–Bpy COF thin film; (d) schematic illustration of the TpBD-(SO3H)2 iCOFMs fabrication process.*

## *2.1.3 Air/liquid interface*

Zhenan Bao et al. first reported the preparation of polyTB film via DMF/air interface reaction in 2015 [32]. The two types of monomers are 4,8-Bis(octyloxy) thieno[2,3-f][1]benzothiophene-2,6-dicarbaldehyde (BDTA) and Tris(4- Aminophenyl)amine (TAPA). Since the COFs powder is easily formed in the direct polymerization process, the surface of the film obtained by this method is very rough. To solve this problem, the team continued to grow the film using the solution from the first reaction. By controlling the reaction conditions, the polyTB film with different thickness was obtained. The average surface roughness of the material is only 0.2 nm. Lai et al. loaded ultra-thin TFP-DHF 2D COF film (2.9 nm) on porous substrate via the Langmuir-Blodgett (LB) method. This COF was formed through the reaction between 1,3,5-triformylphloroglucinol (TFP) and 9,9-dihexylfluorene-2,7-diamine (DHF).

In 2019, Xinliang Feng et al. reported surfactant-monolayer-assisted interfacial synthesis (SMAIS) as a general method to prepare 2D polymer films with high crystallinity (**Figure 7a**). Sodium oleyl sulfate (SOS) was used as a surfactant to induce aniline to align and polymerize on the surface of aqueous solution and obtain fully conjugated

#### **Figure 7.**

*Schematic illustration of chemical synthesis at air/liquid interface. (a) the synthetic procedure for the 2D polymers; (b) molecular structure of the 2DPI synthesized in the article; (c) optical microscope image of 2DPI film; (d) AFM image of the 2DPI film; (e) molecular structure of the q2D polyaniline synthesized in the article; (f) AC-HRTEM image of q2D polyaniline perpendicular to [001] axis; (g) simulated atomic structure of the q2D polyaniline.*

### *Interfacial Synthesis of 2D COF Thin Films DOI: http://dx.doi.org/10.5772/intechopen.106968*

2D polyaniline films with lateral size ~50 cm [2] and tunable thickness (2.6–30 nm) (**Figure 7e**–**g**) [33]. In another work, the researchers prepared polyimide COF film by the reaction of 4,4′,4′′,4′′′-(porphyrin-5,10,15,20-tetrayl)tetraaniline and disochromeno (**Figure 6b**) [34]. The polyimide COF film was characterized by X-ray scattering and AFM (**Figure 7c** and **d**). It is found that the thickness is about 2 nm and the average domain size is about 3.5 μm [2]. Later, on the basis of the above two works, Xinliang Feng et al. also synthesized a series of COF films connected by C∙N and B∙O bonds [35, 36]. Bien Tan et al. reported an aliphatic amine-assisted interfacial polymerization method to obtain independent covalent triazine frameworks (CTFs) films [37]. The structure was investigated by grazing incidence wide-angle X-ray scattering (GIWAXS) and small-angle X-ray scattering (SAXS). The lateral size of the film was up to 250 cm [2], and average thickness can be tuned from 30 to 500 nm. The unique conjugated structure of the material endows it with excellent photocatalytic performance for hydrogen evolution reaction. Zhikun Zheng et al. synthesized a series of imine COF films assisted by charged polymers [38]. The morphology and diffusion of preorganized monomers in charged polymers on water surface are very important for the formation of organic two-dimensional crystals. The monomers were 5,10,15,20-Tetrakis (4-aminophenyl)-21H,23H-porphyrin (TAPP) and four aldehyde monomers with different structures. Air/liquid interfacial synthesis is the most common method for preparing 2D COF films. The film is easy to operate and has good crystallinity and orientation. The air/liquid interface is very helpful for the preparation of single-layer or few-layer COF films. First, the monomers should be induced to arrange at the interface, and then the crystallinity of the film can be better guaranteed in the process of polymerization. So how to arrange the monomers on the surface is the main consideration in this method. The transfer and characterization of monolayers are also not easy. Generally, advanced electron microscope is needed to observe its morphology.

### *2.1.4 Air/solid interface*

In 2008, Porte et al. prepared monolayer surface covalent organic frameworks (SCOFs) on the surface of Ag (111) via chemical vapor deposition (CVD) [39]. The polymerization process is achieved by the reaction of 1, 4-Benzenediboronic acid (BDBA) and boronic acid. The experiment was performed under ultrahigh vacuum (UHV) conditions. Two monomers were sublimated from two heated molybdenum crucible evaporators to the clean Ag (111) surface to obtain a molecular array.

Dong Wang et al. reported the formation of highly ordered 2D COF film via dehydration reaction of boronic acid (**Figure 8a**). The molecular layers were imaged at room temperature using scanning tunneling microscopy (STM) (**Figure 8b**–**c**). In 2013, Dong Wang et al. reported another method for the synthesis of imine COF films at the air/solid interface [40]. The solution of the two monomers was first coated on the substrate and then sealed in a reactor with copper sulfate pentahydrate as a thermodynamic regulator. By heating the reactor to a specified temperature to control the evaporation of aldehyde monomers, the aldehyde monomers condense on the surface covered by amine monomers and polymerize with them, high-quality monolayer imine COF films can be prepared at the air/solid interface [41]. Recently, Yunqi Liu et al. reported the preparation of large area imine PyTTA-TPA COF film with controllable thickness by gas-phase induced conversion in a CVD system (**Figure 8d**) [42]. The assembly process is achieved by reversible reaction between 4,4′,4″,4″′-(1,3,6,8-Tetrakis(4-aminophenyl)pyrene (PyTTA) film and terephthalaldehyde (TPA) vapor. Driven by π-π superposition and catalyzed by acid vapors,

#### **Figure 8.**

*Schematic illustration of chemical synthesis at air/solid interface. (a) the synthesis route to SCOF-1; (b) STM image of SCOF-1 on HOPG formed after dehydration of BPDA precursors at 150°C; (c) a high-resolution STM image showing the hexagonal structure of SCOF-1; (d) schematic representation for the growth of imine-linked 2D COF films on SiO2/Si substrates.*

a uniform organic frame film was formed on a growing substrate. The COF films obtained by air/solid interface synthesis have adjustable thickness and highly ordered structure, which is an effective method to grow high-quality thin films [43–45]. However, this method requires high temperature and vacuum environment and has high requirements for equipment.

#### *2.1.5 Vapor-assisted conversion*

Some organic solvents with low boiling point can evaporate at room temperature. Combined with this feature, Bein et al. reported the strategy of steam-assisted conversion at room temperature in 2014 [46]. They succeeded in making three thin films named BDT-COF, COF-5, and pyrene-COF (**Figure 9a**). The monomers are first mixed into a solution of acetone/ethanol, which is then dripped onto the matrix. Finally, the material is transformed into crystalline porous COF films in a vapor atmosphere of homotrimethylbenzene/dioxane. The presence of vapor plays an important role in the growth of thin films.

This method can accurately prepare COF films from 100 nm to a few microns thick. SEM characterization showed that the structure of the film was composed of small particles stacked irregularly, and there were submicron intervals between the particles (**Figure 9b** and **c**).

*Interfacial Synthesis of 2D COF Thin Films DOI: http://dx.doi.org/10.5772/intechopen.106968*

## *2.1.6 Continuous flow condition synthesis*

Because COFs are easy to form powder particles under thermodynamic conditions, the films synthesized by liquid/solid interface method may have the problem of high surface roughness. To remedy this problem, Dichtel et al. converted the solution from a static state to a flowing state [47]. The flow rate affects the surface morphology, thickness, and crystallization degree of films. High-quality films with different thickness can be prepared by adjusting this condition (**Figure 10**). A quartz crystal microbalance can be used as the base of the flow tank to monitor the quality of film deposition at any time.

## **2.2 Other promising preparation methods of COF films**

COFs powders are formed by layer upon layer of planes through π stacking and the interlayer forces are weak relative to covalent bonds. Similar to graphene, a large

#### **Figure 9.**

*(a) Schematic representation of BDT-COF and COF-5; (b) top view SEM micrograph of BDT-COF film synthesized by room temperature vapor-assisted conversion, representing the surface morphology; (c) crosssectional SEM micrograph shows a uniform film thickness.*

#### **Figure 10.**

*(a) Turbidity as a function of reaction time during the formation of COF from homogeneous conditions provides an induction period amenable to a flow cell configuration. (b) Schematic of flow setup designed with variable induction period.*

**Figure 11.**

*(a) Schematic illustration of the preparation of a COF-1 membrane via the assembly of exfoliated COF-1 nanosheets; (b) overview of acid exfoliation and film casting procedures; (c) pore structure of the BND-TFB COF.*

area of smooth COF film can be obtained if the powdered COFs can be stripped into a single layer of nanosheets, which can then be self-assembled into films [48]. The common preparation methods of nanosheets include physical exfoliation and chemical exfoliation [49, 50].

The physical method is to disperse COFs powder in the solvent and then form nanosheets assisted by ultrasound. Chemical methods require the addition of a chemical agent to the solvent to promote lamellar abscission. Compared with COFs powder, COFs nanosheets show more advantages in photoelectric applications. In addition, the processing of nanosheets into films is also a key point in practical applications. In 2017, Tsuru et al. applied the obtained COF-1 nanosheet solution drops on an α-Al2O3 macroporous support with a SiO2-ZrO2 intermediate layer and obtained uniform and smooth COF films after several drops (**Figure 11a**) [51]. Dichtel et al. protonated imine bond using trifluoroacetic acid to promote the stripping of COFs powder into nanosheets dispersed in a solvent (**Figure 11b** and **c**) [52]. COF films with thickness ranging from 50 nm to 20 μm can be prepared by deposition on any substrate.

## **3. Conclusion**

The above is a review of common methods of film synthesis, including interfacial synthesis and some other new and promising preparation methods. By replacing different organic monomers, COF films with different functions can be designed in a targeted manner. Combined with the final different applications, the appropriate preparation method can be selected. At present, the solid/liquid interface synthesis

#### *Interfacial Synthesis of 2D COF Thin Films DOI: http://dx.doi.org/10.5772/intechopen.106968*

method is mature and suitable for most COFs. However, if organic solvents are used for solvothermal reaction at high temperature, only inorganic materials can be selected as the substrate. COF films grown on solid surfaces cannot be transferred to other substrates. The air/solid interface synthesis also has the problem that the grown film is difficult to transfer. For liquid/liquid interface synthesis, organic solvents and aqueous solutions will form an obvious contact surface, which is an ideal substrate for the efficient growth of films at the interface. Liquid/liquid interface synthesis is also the most promising method to grow single crystal thin films. This method usually requires dissolving aldehydes and amines in two solvents, respectively, and reacting at the interface to form a film. The growth of COF thin films depends on the diffusion of monomers in solvents. The process of film formation at the interface will affect the diffusion of molecules, and the subsequent polymerization reaction will also be limited. Therefore, the surface of the grown film will be rough. If the two monomers are dissolved in the same organic solvent, and then the catalyst is dissolved in water, the monomers are catalyzed at the interface, and the polymerization reaction takes place to form a film, which will significantly improve this problem. Air/solid interface synthesis is also a strategy to prepare large-scale single crystal films. This strategy can prepare ultrathin COF films with a thickness of several nanometers. Generally, surfactants are used to induce the regular arrangement of monomers on the surface of solvents. It usually takes a week to produce a better crystalline film.

Up to now, there are still few reports of the synthesis of wafer-scale films, and the methods are not necessarily universal. To obtain large-scale crystalline films, the reaction rate of monomers needs to be very slow. It usually takes a week. For imine COF films, it is very important to find a suitable reagent to inhibit the diffusion of monomers because of their high reactivity. In addition, surfactant is a good choice to induce crystallization of thin films. In future research, the efficiency of interfacial film formation and the crystallinity of the film are still important factors to be considered. Improving the existing methods, judging the catalyst activity in the reaction, and adjusting the diffusion rate are all promising research contents.

The currently reported COF linkage bonds mainly include boron-oxygen bonds, imide bonds, and carbon-carbon double bonds. COF films with higher reversibility of boron-oxygen bonds and imine bonds are reported the most. However, sp2 carboncarbon COF films have not been reported. This type of COF powder with a high degree of conjugate has been well applied in the field of photocatalysis. In future research, it is a good research direction to prepare this type of COF thin films by advanced interfacial methodologies. With its excellent light absorption ability and photoelectron migration ability, sp2 carbon-carbon COF films can show excellent performance in seawater evaporation and photocatalysis.

## **Acknowledgements**

T.Z. acknowledges the Excellent Youth Foundation of Zhejiang Province of China (Grant No. LR21E030001) and the National Natural Science Foundation of China (Grant No. 52005491 and No. 52003279).

## **Conflict of interest**

The authors declare no conflict of interest.

*Covalent Organic Frameworks*

## **Author details**

Tao Zhang\* and Yuxiang Zhao

Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, China

\*Address all correspondence to: tzhang@nimte.ac.cn

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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## **Chapter 2**

## Covalent Organic Frameworks for Ion Conduction

*Fei Lu and Yanan Gao*

## **Abstract**

Covalent organic frameworks (COFs) are an emerging class of crystalline porous materials constructed by the precise reticulation of organic building blocks through dynamic covalent bonds. Due to their facile preparation, easy modulation and functionalization, COFs have been considered as a powerful platform for engineering molecular devices in various fields, such as catalysis, energy storage and conversion, sensing, and bioengineering. Particularly, the highly ordered pores in the backbones with controlled pore size, topology, and interface property provide ideal pathways for the long-term ion conduction. Herein, we summarized the latest progress of COFs as solid ion conductors in energy devices, especially lithium-based batteries and fuel cells. The design strategies and performance in terms of transporting lithium ions, protons, and hydroxide anions are systematically illustrated. Finally, the current challenges and future research directions on COFs in energy devices are proposed, laying the groundwork for greater achievements for this emerging material.

**Keywords:** COFs, ion conduction, lithium ion, proton, hydroxide

## **1. Introduction**

The development of society depends on the effective use of new energy, which relies on the innovation of novel energy storage technology. Environmentally friendly energy storage devices such as lithium (Li)-ion batteries have achieved great success in the fields of consumer electronics and electric vehicles due to their excellent energy density. However, the large application of Li-ion batteries is limited by the use of liquid electrolytes, which suffer from the potential risk of leakage, flammability, and narrow voltage windows [1]. In contrast, solid polymer electrolytes with greater thermal and chemical stability have been considered as the promising candidates for applications in commercial energy devices including rechargeable batteries and fuel cells [2–4]. Recent representative solid-polymer electrolytes mostly involve with high-molecularweight fluoro-containing polymers such as Nafion and polyolefin-type membranes [5]. However, the sever capacity fade is inevitable for these membranes due to the low ionic conductivity, especially under some extreme conditions including but not limited to high temperature and low relative humidity [6, 7]. Thus, solid-state electrolytes with outstanding ionic conductivity and superior stability are in high requirement.

The porous materials with high porosity and facial functionality, such as metalorganic frameworks (MOFs), polymers of intrinsic microporosity (PIMs), porous

aromatic frameworks (PAFs), and covalent organic frameworks (COFs), offer potential high performance as solid electrolytes for energy devices. However, MOFs tend to decompose during battery cycling due to the low thermal and electrochemical stabilities. However, PIMs would reshaped their ultra-micropores to meso- and macropores under alkaline conditions, which reduces their electrochemical performance. PAFs with strong carbon-carbon bonds have a stable framework in harsh acidic and alkaline environments. But the synthesis methodologies of PAFs are very limited. Compared with all these porous materials, COFs can overcome the abovementioned disadvantages and therefore act as suitable candidates for energy device applications. COFs are synthesized by the polycondensation reaction to form covalent bonds between lightweight-atom-containing monomers, such as carbon (C), oxygen (O), nitrogen (N), hydrogen (H), boron (B), etc. [8, 9]. Most COFs are synthesized under thermodynamic control to modulate the reversibility of bond formation and breakage. As a result, highly periodic networks with defined pore size, shape, topology, and crystalline lattice will be formed according to the building blocks. The porosity generated by the geometries of monomers as well as the stable covalent bonds makes COFs preferable platforms for solid-state ion conduction. To be specific, 2D or 3D nanochannels can be constructed by the pores of COFs, through which the ions can transport. For the ions such as lithium ions (Li+ ), protons (H+ ), and hydroxides (OH− ), the conduction behavior mainly dominated by two mechanisms, which are hopping mechanism and vehicular mechanism [10–12]. For hopping mechanism, ions are preferable to hop between counter-charged adjacent sites with lower energy requirement to transport. While for vehicular mechanism, ions prefer bonding to some vehicular carriers such as H2O (for H+ ) or anions (for Li+ ) to form large clusters, which needs higher energy to move. From this sense, the well-defined nanochannels of COFs, which could be further precisely installed ionic groups on the pore walls, provide an ideal pattern for ion conduction [13, 14]. In addition, the possibility to introduce extra charged species into the pores of COFs offers another opportunity for the high performance as ion conductors [15].

The superiority of COFs as high-performance ion conductors can also be described by the point of diffusion energy barrier of ionic migration [16]. For the liquid electrolytes, the charge carriers are surrounded by uniform and homogeneous solvents and thus can be quickly conducted by the exchange with solvating molecules, which

#### **Figure 1.**

*Schematics of the ion-conducting behavior and diffusion energy barrier in (a) liquid conductor, (b) typical solid conductor, and (c) COF conductor, respectively. Reproduced with permission from Ref. [16]. Copyright 2022 Wiley-VCH GmbH.*

*Covalent Organic Frameworks for Ion Conduction DOI: http://dx.doi.org/10.5772/intechopen.108291*

generally produce high ionic conductivity. Thus, the diffusion energy barrier for ion conducting in liquid electrolytes can be considered flat (**Figure 1a**). However, the ion transporting in the solid-state conductors needs to overcome high energy barrier, which is related to the migration of charge carriers through the segmental motion of polymers or periodic crystalline space of inorganic solid (**Figure 1b**) [17, 18]. While for COFs, the charge species tend to migrate along the nanochannels due to the large free volume combined with the inside ionic sites, thus resulting in a lower diffusion energy barrier than that in typical solid-state conductor (**Figure 1c**). Thus, COFs are considered to be an excellent solid ion conductor.

In this context, we focus on the application of COFs as solid-state polyelectrolytes in energy devices, especially lithium-based batteries and fuel cells. The design strategies, nanostructures, and performance in terms of transporting Li<sup>+</sup> , H+ , and OH− are systematically illustrated. Finally, the current challenges and future research directions for the utilization of COFs in energy devices are proposed.

## **2. COFs for lithium-ion conduction**

Li-ion batteries have been considered as the mainstream devices in commercial portable electronics. However, traditional Li-ion batteries suffer from serious safety risk due to the utilization of liquid electrolytes consisting of Li salts and flammable organic solvents. The long-term performance of Li-ion batteries is also limited due to the narrow voltage windows of liquid electrolytes. It means that the charging process would induce the decomposition of organic solvents and result in obvious capacity fade [19]. Additionally, the separators that are essential in liquid-electrolyte batteries commonly exhibit low conductivity, also inducing the decrease in the performance of Li-ion batteries [20]. Thus, the development of polymer electrolytes for all-solidstate batteries can address the above safety problems. The third key parameter is the Li+ transference number (*t*Li+), which represents the number of Li+ transferred per Faraday of charge during charge and discharge process. The values of *t*Li+ for most polymer electrolytes are always lower than 0.5. These low *t*Li+ values would generate a ion concentration gradient between cathode and anode, leading to the polarization and limiting the charging/discharging rate and lifetime of batteries [3, 21]. Based on this background, the state-of-the-art polymer electrolytes should possess high ionic conductivity, high *t*Li+ values, and better electrochemical stability.

Due to the particular nanostructures and properties, COFs can act as an excellent Li+ conductor due to some intrinsic merits. Firstly, the well-defined open channels of COFs provide fast pathways for the conduction of ions by reduced diffusion energy barrier. Second, the organic nature endows COFs' flexibility to introduce functional group to facilitate the dissociation of lithium salt and enhance the trapping of anions, which is beneficial to improve *t*Li+ value. Thirdly, the superior electrochemical and thermal stabilities make COFs available in the practical application.

In 2016, Zhang and coworkers synthesized a novel type of ionic COFs (ICOFs) containing *sp*<sup>3</sup> hybridized boron anions by the formation of spiroborate linkages [22]. After immobilization of Li+ into the nanochannels, the obtained ICOF-2 exhibited an ionic conductivity of 3.05 × 10−5 S/cm and an average *t*Li+ value of 0.80 at room temperature, which was ascribed to the predesigned pathway for ion conduction. However, insertion of Li+ into the bare nanochannels of COF can only produce a limited ionic conductivity. Inspired by the poly(ethylene oxide) (PEO)-based electrolytes, which can solvate Li+ for fast ion conduction by the segmental motion, Jiang and coworkers firstly integrated

the flexible oligo(ethylene oxide) chains on the pore walls of COFs [23]. Compared with the bare COF (Li+ @TPB-DMTP-COF), the EO-modified COF (Li+ @TPB-BMTP-COF) can exhibit an enhanced ionic conductivity and a lower energy barrier (**Figure 2a**). Furthermore, Horike *et al.* developed a bottom-up method to accumulate different concentration of glassy PEO moieties into the nanochannels of COF (**Figure 2b**) [24]. As a result, the material containing highest density accumulation of PEO (COF-PEO-9-Li) exhibited the superior ionic conductivity over 10−3 S/cm at 200°C. The similar trend was also demonstrated by branched PEO-functionalized COFs as solid conductors [26]. Recently, Horike used the similar side-chain engineering strategy to fabricate a gel-state COF (COF-Gel) with facile processability by implanting soft branched alkyl chains as internal plasticizers into the nanopores of COFs (**Figure 2c**) [25]. Benefiting from the gel-state morphology, the COF-Gel can be easily manufactured into gel electrolyte with controlled shape and thickness with enhanced ion conducting kinetics and reduced interfacial resistance, which showed a better performance in LiFePO4-Li cell than the typical liquid electrolyte (**Figure 2d**).

The simple permeation of lithium salts into the nanopores of COFs usually results in a relatively low ionic conductivity and poor ion diffusion kinetics due to the closely associated ion pairs. While enhancing the binding interaction between the anions of lithium salt and COF backbones, the transfer of Li<sup>+</sup> would be promoted. In 2018, Chen et al. synthesized a cationic COF incorporated with LiTFSI to improve the ionic conductivity [27]. Compared with the neutral framework, the cationic skeleton can generate stronger dielectric screening to split Li+ and the related anions, increasing the amount of free Li+ and resulting in an improved solid-state ionic conductivity up to 2.09 × 10−4 S/cm at 70°C (**Figure 3a**). Similarly, Feng and coworkers employed imidazolium monomer as the building block to construct cationic COF to enhance the trapping of counter ions of lithium salts and improve the lithium ionic conductivity (**Figure 3b**) [28]. The obtained Im-COF-TFSI possessed the ionic conductivity as

#### **Figure 2.**

*(a) Structural representation of Li+ @TPB-DMTP-COF with bare pore walls and Li+ @TPB-DMTP-COF with EO-modified pore walls. Reproduced with permission from Ref. [23]. Copyright 2018 American Chemical Society. (b) Structural representation of COFs containing different concentration of PEO moieties. Reproduced with permission from Ref. [24]. Copyright 2018 American Chemical Society. (c) Structural representation and the corresponding state of COF with alkyl chains. (d) Rate performance of COF-Gel in a LiFePO4-Li cell. Reproduced with permission from Ref. [25]. Copyright 2021 Wiley-VCH GmbH.*

*Covalent Organic Frameworks for Ion Conduction DOI: http://dx.doi.org/10.5772/intechopen.108291*

#### **Figure 3.**

*(a) Structural representation of the cationic COF for binding with anions as Li+ conductors. Reproduced with permission from Ref. [27]. Copyright 2018 American Chemical Society. (b) Synthesis of cationic Im-COF-TFSI. Reproduced with permission from Ref. [28]. Copyright 2020 Royal Society of Chemistry.*

high as 4.64 × 10−4 S/cm at 80°C and 4.04 × 10−3 S/cm at 150°C, respectively. Recently, Han's group utilized the defects of COFs to introduce imidazolium groups onto the pore walls via the Schiff-base reaction [29]. After ion exchange with TFSI− , the resultant dCOF-ImTFSI-Xs not only had the 2D pathways for Li<sup>+</sup> transport, but also contained the cationic moieties to promote the dissociation of lithium salts.

Although COFs have been demonstrated as a new concept of solid-state Li<sup>+</sup> conductors, most of the reported works are involved with the incorporation of lithium salts into the nanopores of COFs to facilitate ion transfer, thus failing to realize a single Li+ conduction, which performs the *t*Li+ value close to unity (**Figure 4a**). To address this issue, Lee and coworkers presented a sulfonated COF (TpPa-SO3Li) with anionic framework and well-defined directional channels (**Figure 4b**) [30]. As the sulfonates were covalently anchored to the pore walls, a real single Li+ conductor can be achieved. The obtained TpPa-SO3Li showed the *t*Li+ value as high as 0.9 at room temperature, thereby possessing a more stable Li plating/stripping on lithium anode than the previously reported COF-based ion conductors (**Figure 4c**). Recently, Loh et al. used a post-synthetic method to graft sulfonate groups on the pore wall of COFs and exfoliated COF powders to nanosheets to create better ion conduction channels because the repulsive forces between Li+ were stronger than the van der Waals interactions between frameworks (**Figure 4d**) [31]. As a result, the sulfonated COF nanosheets achieved the ionic conductivity of 0.9 × 10−5 S/cm at −40°C and 1.17 × 10−4 S/cm at 100°C with *t*Li+ value of 0.92 when equipped in an all-solid-state battery, indicating the possibility to be practically applied under harsh conditions.

## **3. COFs for proton conduction**

Due to the high energy conversion efficiency, low emission, fuel flexibility, and mild operation accessibility, fuel cells are considered as electrochemical power plants,

#### **Figure 4.**

*(a) Conceptual illustrations of ion transport phenomena in the porous crystalline ion conductors: previous approaches (top) and this study (bottom). (b) Chemical structure of lithium sulfonated COF (TpPa-SO3Li). (c) Galvanostatic Li plating/stripping profile of the Li/Li symmetric cell containing TpPa-SO3Li. Reproduced with permission from Ref. [30]. Copyright 2019 American Chemical Society. (d) Schematic illustration describing the fabrication of all-solid-state organic Li-ion battery using lithiated COF nanosheets. Reproduced with permission from Ref. [31]. Copyright 2020 American Chemical Society.*

which convert chemical energy to electrical energy by the cost of specific fuels. Among the typical fuel cells, which differentiated by the type of electrolytes (phosphoric acid, proton exchange membrane, oxide, and alkaline), the proton exchange membrane fuel cell (PEMFC) has attracted the most attentions due to the relatively lower operation temperature [32]. Except for PEMFC, proton-conducting materials are also the key component for other electrochemical devices such as supercapacitors, proton sieving, proton transistors and hydrogen sensors [33]. Up to now, the

*Covalent Organic Frameworks for Ion Conduction DOI: http://dx.doi.org/10.5772/intechopen.108291*

universally utilized proton-conducting membranes are sulfonated polymers such as Nafion. Nafion can display proton conductivity as high as 10−1 S/cm in fully hydrated state at a moderate temperature [5]. However, when the temperature exceeds 100°C, the proton conductivity would dramatically decrease due to the loss of water. Other polymer-based proton conducting materials such as polybenzimidazole (PBI)-based membranes can perform excellent properties in 150–200°C. While the heterogeneous random structures and the amorphous nature hinder the efficient analysis of conducting mechanism and determination of structure-property relationship at the molecular level [34]. In this regard, COFs with precise structural designability, synthetic controllability, and available functionality hold the promise to work as superior proton conductors.

Inspired by the structure of Nafion, sulfonated COFs would be the ideal candidates for proton conduction and fuel cell practical application. In 2016, Banerjee's group firstly synthesized sulfonic-acid-based COFs (TpPa-SO3H) by a Schiff-base reaction of 1,3,5-triformylphloroglucinol and 2,5-diaminobenzenesulfonic acid to form periodic intervals with free -SO3H groups within COF backbones to promote the proton hopping along the hexagonal 1D channels [35]. The intrinsic proton conductivity of TpPa-SO3H can reach 1.7 × 10−5 S/cm at 120°C under anhydrous condition. After shortly, Zhao et al. reported two sulfonated COFs, NUS-9(G) and NUS-10 (G), by liquid-assisted grinding strategy at room temperature (**Figure 5a**) [36]. The obtained NUS-9(G) and NUS-10 (G) exhibited hexagonal architecture and displayed eclipsed AA stacking layers (**Figure 5b**). Due to the pre-implanted free -SO3H groups, NUS-9(G) showed a proton conductivity of 1.5 × 10−4 S/cm at room temperature and 33% relative humidity (RH), which was increased to 3.96 × 10−2 S/cm at 97% RH. While for NUS-10 (G), which possessed twice as many free -SO3H groups as NUS-9(G), the intrinsic proton conductivity increased to 2.8 × 10−4 S/cm at 33% RH and 3.96 × 10−2 S/cm at 97% RH with long-term stability. Very recently, Zhu's group used surface-initiated condensation polymerization to synthesize the same sulfonic COF TpPa-SO3H (**Figure 5c**) [37]. Through the precise control of polymerization time, the thickness of SCOF layer can be tuned from 10 to 100 nm, overcoming the processable challenge of COFs. The obtained free-standing COF membrane exhibited a proton conductivity of 0.54 S/cm at 80°C under fully hydrated state.

For these sulfonated COFs, the high water-assisted proton conductivity could be attributed to the presence of aligned -SO3H groups on the pore walls of COFs, which not only enhance the adsorption of water but also facilitate the formation of hydrophilic domains to generate proton conducting pathways. At a high RH percentage, the proton transportation is mainly dominated by the hopping mechanism benefiting from the continuous hydrogen bonds between H2O and -SO3H groups. However, under low humidity condition, the significant decrease of proton conductivity would be still inevitable, which is similar to Nafion. Otherwise, the preinstallation of proton conducting groups onto the pore wall of COFs is sometimes difficult. Thus, incorporation of guest protonic species into the nanochannels of COFs would provide a more effective approach to improve the proton conductivity.

In one aspect, some proton donors can incorporate with COFs to trigger proton conductivity by donating more protons or facilitating the formation of hydrogen bonds. In 2016, Jiang and coworkers developed a highly robust COF, TPB-DMTP-COF, with hexagonally aligned, dense, mesoporous channels (**Figure 6a**) [38]. By loading the N-heterocyclic proton carriers, 1,2,4-triazole (trz) and imidazole (im), the anhydrous proton conductivity of trz@TPB-DMTP-COF and im@TPB-DMTP-COF can reach the maximum of 1.1 × 10−3 S/cm and 4.37 × 10−3 S/cm at 130°C,

#### **Figure 5.**

*(a) Synthetic scheme of NUS-9(G) and NUS-10(G) via liquid-assisted grinding at room temperature. (b) Crystal structure and representation of the eclipsed AA stacking for NUS-9(G) and NUS-10(G), respectively. Reproduced with permission from Ref. [36]. Copyright 2016 American Chemical Society. (c) Preparation of the SCOF membrane grafted on silicon wafers and the molecular structures TpPa-SO3H. Reproduced with permission from Ref. [37]. Copyright 2021 Wiley-VCH GmbH.*

respectively. The activation energy calculation also demonstrated that protons were transported by hopping along the interconnected hydrogen bonding networks of proton carriers.

Besides, phosphoric acid (H3PO4) is also a good proton donors for the extrinsic incorporation with COFs [40]. In 2020, Horike's group reported perfluoroalkylfunctionalized COFs (COF-Fx-H) with super hydrophobic well-defined 1D channels to accommodate a large amount of H3PO4 (**Figure 6b**) [39]. Due to the interactions between H3PO4 and the NH groups of framework as well as the fluorinated side chains, the guest H3PO4 could be anchored onto the pore walls through P∙O…H∙N, OH… N∙C, and O∙H…F∙C hydrogen bonding networks, which further generated an efficient proton conducting pathway (**Figure 6c**). After 62 wt% loading of H3PO4, the maximum anhydrous proton conductivity reached 4.2 × 10−2 S/cm. Recently, Jiang's group also designed polybenzimidazole COFs in conjunction with H3PO4 to achieve stable and ultrafast proton conduction over a wide range of temperature [41]. Due to the presence of imine linkage and the benzimidazole chains, H3PO4 could be tightly locked by the electrostatic and hydrogen binding interactions. More importantly, the N atom of benzimidazole moieties could be protonated by H3PO4 and release open H2PO4− anion. Thus the proton conduction would be facilitated by the activated proton networks. As a result, the H3PO4@TPB-DABI-COF realized a hydrous proton conductivity of 8.35 × 10−3 S/cm at 160°C.

Although great progress has been achieved for improving the proton conductivity by extrinsic incorporation with proton carriers such as acids and N-heterocycles, *Covalent Organic Frameworks for Ion Conduction DOI: http://dx.doi.org/10.5772/intechopen.108291*

#### **Figure 6.**

*(a) Chemical structure and hexagonal structure of TPB-DMTP-COF and graphic representation of 1,2,4-triazole and imidazole in the channels. Reproduced with permission from Ref. [38]. Copyright 2016 Nature Publishing Group. (b) Synthesis route of COF-Fx-H. (c) Illustration of proposed proton conducting mechanism. Reproduced with permission from Ref. [39]. Copyright 2020 American Chemical Society.*

#### **Figure 7.**

*(a) Schematic of the synthesis and structure of IL-COF-SO3H. Reproduced with permission from Ref. [42]. Copyright 2021 Elsevier B. V. (b) Structure and illustration of the proton transfer of PIL-TB-COF. Reproduced with permission from Ref. [43]. Copyright 2022 The Royal Society of Chemistry.*

relatively less attention has been paid on the proton conducting property of ionic liquids (ILs) impregnated COFs. In 2021, Tang and coworkers firstly reported an IL impregnated sulfonic-acid-based COF (IL-COF-SO3H), which further combined with silk nanofibrils (SNFs) to fabricate a composite membrane (**Figure 7a**) [42]. The electrostatic interactions between imidazolium anions and sulfonic acids promoted the

deprotonation to release more protons and immobilized ILs. The uniform distribution of ILs in the channels of COF-SO3H could provide a large amount of hopping sites for protons. Moreover, the hydrogen bonding networks between SNFs and IL-COF-SO3H could provide additional proton conduction pathways. Particularly, the IL-COF-SO3H@SNF-35, which loaded 35 wt% SNFs, acquired an ionic conductivity of 224 mS/cm at 90°C and 100% RH. Very recently, Yan's group developed a protic ionic liquid (PIL), 1-methyl-3-(3-sulfopropyl) imidazolium hydrogensulphate ([PSMIm] [HSO4]) to incorporate with a high-density ∙SO3H functionalized COF (TB-COF) for efficient anhydrous proton conduction (**Figure 7b**) [43]. As expected, the addition of PIL into the nanochannels of COFs can significant increase the ionic conductivity from 1.52 × 10−4 S/cm to 2.21 × 10−3 S/cm at 120°C due to the increase of hopping sites for protons.

## **4. COFs for hydroxide anion conduction**

Compared with PEMFC, alkaline fuel cells, which are operated on hydroxide anion transport, have attracted increasing attention due to the high energy density, rapid reaction kinetics, and low-cost catalyst [44]. As one of the critical components, the hydroxide conducting membrane affords the transfer of anions and determines the terminal electrochemical output. However, the high-performance hydroxide anion conduction is challenging because of its lower diffusion coefficient compared with protons [45]. Similar to proton conducting materials, the typical anion conducting membranes depend on polymer system, which can form percolated water channels via the microphase separation of hydrophobic/hydrophilic domains [46–48]. Multiple factors including the polarity of segments, distribution of charged moieties, and anion concentration have influence on the physical phase-separation process. Thus, precise control of the phase-segregated morphologies usually has of a large difficulty. Based on this background, COFs with structural tunability and functional pore surface build a powerful platform to achieve fast anion transport.

#### **Figure 8.**

*(a) Synthesis route of [OH− ]100-TPB-BPTA-COF. (b) Structure of [OH− ]50-TPB-BPTA-COF. Reconstructed structures of (c) [OH− ]100-TPB-BPTA-COF and (d) [OH− ]50-TPB-BPTA-COF (red O, blue N, gray C, white H). Reproduced with permission from Ref. [49]. Copyright 2021 American Chemical Society.*

*Covalent Organic Frameworks for Ion Conduction DOI: http://dx.doi.org/10.5772/intechopen.108291*

For example, Jiang and coworkers constructed an anion-surfaced channels for hydroxide anion conduction via supramolecular self-assembly while maintaining both the ordering topology and skeleton stability of COFs [49]. The precursor COF with ethynyl side groups was synthesized by the condensation of C3-symmetric 1,3,5-tris(4 aminophenyl)benzene (TAPB) as knot and 2,5-bis(2-propynyloxy)terephthalaldehyde (BPTA) and 2,5-dimethoxyterephthalaldehyde at different molar ratios (DMTA) as linker (**Figure 8a**). Then an azide-imidazolium salt was introduced into the pores by the click reaction with the ethynyl sides. After anion exchange, the hydroxide anion conducting [OH− ]100-TPB-BPTA-COF was constructed (**Figure 8a**). The crystal structural analysis demonstrated that the imidazolium cations were extruded from pore walls and concentrated in the channel center aligning with OH− at the end of cationic chains, thus creating a continuous anionic phase (**Figure 8c**). While for [OH− ]50- TPB-BPTA-COF in which half of the edge units were appended with ethynyl groups (**Figure 8b**), the hydroxide anion interface was not continuous due to the reduced anion density (**Figure 8d**). Consequently, the conductivity of [OH− ]100-TPB-BPTA-COF was 2–8 times higher than that of [OH− ]50-TPB-BPTA-COF.

The poor processability of COFs generally produces insoluble powders, which dramatically limits their practical application as free-standing membranes. Thus, it is highly desirable to develop efficient methods to fabricate COF-based anion conducting membranes. Recently, Jiang's group has made remarkable achievements on engineering COF membranes via interfacial polymerization strategy [50–52].

#### **Figure 9.**

*(a) Scheme for the synthesis of COF-QAs. (b) Schematic of anion transport through the 1D channel of COF-QAs. (c) Schematic of COF-QAs membrane fabrication process. Reproduced with permission from Ref. [53]. Copyright 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.*

For instance, they used hydrazide units functionalized with quaternary ammonium (QA) groups bearing different length of alkyl chains and aldehyde units to construct four quaternized COFs (COF-QAs) (**Figure 9a**) [53]. The well-defined ordered nanochannels with aligned QA cations provided an ultrafast pathway for anion transport (**Figure 9b**). In order to realize the self-standing membrane with robust crystalline framework, a phase-transfer polymerization process, which involved phase transfer of 1,3,5-triformylbenzene to polymerize with QA-functionalized hydrazide in a mesitylene-water system (**Figure 9c**). Due to the slight solubility in water, the aldehyde units would gradually diffuse from organic phase to aqueous solution to react with hydrazides, resulting in a stable colloidal suspension. Upon solvent removal, the COF nanoplates could further be assembled into free-standing membrane with identical crystalline structures, which exhibited the hydroxide conductivity as high as 212 mS/ cm at 80°C. Very recently, they developed six QA-functionalized COFs via the assembly of hydrazides and aldehyde precursors by interfacial polymerization to systematically elucidate the impact of aldehyde size, electrophilicity, and hydrophilicity on the synthesis process as well as the anion conducting property of COFs [54]. Particularly, more hydrophilic aldehydes were preferable to react with hydrazides in the aqueous solution rather than the interface region, which led to the tight membrane. Compared with the loose membranes, the anion conductivity could improve around 4–8 times.

## **5. COFs for other ion conduction**

Among various electrical devices to date, sodium-ion batteries have gained considerable attention due to its low cost and sustainability. Similar with lithium-ion batteries, sodium-ion batteries also suffer from the easy formation of dendrite in traditional liquid electrolyte and have high desire to develop solid ion conductors. To tackle these bottlenecks, Sun and coworkers studied the first example of carboxylic acid sodium functionalized COF (NaOOC-COF) as quasi-solid-state electrolyte to accelerate the transporting of Na+ and simultaneously restrain the dendrite growth (**Figure 10a**). The covalently tethered carboxylic acid sodium groups in the pore wall of COFs provided sufficient content of Na+ and favorable nanostructures for Na+ migration. Benefiting from the well-defined ion channels, NaOOC-COF displayed an excellent conductivity of 2.68 × 10−4 S/cm at room temperature and high transference number of 0.9. Finally, NaOOC-COF devoted to durable cycling performance of Na plating/stripping and outstanding performance in solid-state battery [55].

Aqueous Zn-ion batteries are also a great promising energy storage system owing to the high energy density driven by multielectron redox (Zn0/2+) and prominent safety supported by water-based electrolytes. However, the practical application has still been limited due to the lack of suitable electrolytes to ensure stable interface with electrodes. Recently, Lee and coworkers demonstrated for the first time to use COF-based single Zn2+ conductors, which can both secure interfacial stability with electrodes and exhibit competitive ionic conductivity [56]. A zinc sulfonated COF (TpPa-SO3Zn0.5, **Figure 10b**) with well-defined directional channels in which covalently anchored and delocalized sulfonates was designed to realize single Zn2+ conduction. From the molecular dynamics (MD) simulations, a significantly uniform Zn2+ flux was observed due to the anionic groups along the directional pores (**Figure 10c**). While in the control model of liquid electrolyte (LE), which is 2 M ZnSO4 in H2O, only randomly spread Zn2+ clusters can be observed due to the freely

*Covalent Organic Frameworks for Ion Conduction DOI: http://dx.doi.org/10.5772/intechopen.108291*

#### **Figure 10.**

*(a) The synthetic route of NaOOC-COF. Reproduced with permission from ref. 55. Copyright 2021 Elsevier Ltd. (b) Chemical structure of TpPa-SO3Zn0.5. Representative snapshots obtained from the MD simulations showing time-dependent ion distributions in (c) TpPa-SO3Zn0.5 and (d) LE. Zn2+: colored diversely for a clear representation of the movement, TpPa-SO3 − : gray, SO4 2−: green, H2O: omitted for clarity. Reproduced with permission from Ref. [56]. Copyright 2020 The Royal Society of Chemistry.*

mobile SO4 2− (**Figure 10d**). As a result, TpPa-SO3Zn0.5 enabled the Zn-MnO2 cells to exhibit a long-term cycling performance.

## **6. Conclusion and outlook**

In this chapter, we summarized the recent progress of COFs as solid-state ion conductors in energy devices, especially lithium-based batteries and fuel cells. As the emerging crystalline porous materials with controllable chemistry, tunable topology, and well-defined order channels, COFs exhibit a promising performance to conduct lithium ion, proton, and hydroxide anion. However, the development of COF-based solid ion conductors is still in its infancy, and many challenges remain to be issued.

Firstly, most ion-conducting COFs relate to ionic frameworks. Compared with neutral COFs, the examples of ionic COFs are still limited due to the more restricted synthesis conditions for crystallization and ionization. Although a series of covalent bonds have been successfully applied to construct COFs, only a few of linkages afford the formation of ionic COFs. To date, most ionic COFs are formed by the imine bonds. Thus, deep chemistry insight and novel synthetic approach to ionize COFs are in high demand. In addition, universal strategies to construct 3D ionic COFs, which are scientifically intriguing with unique properties, are also requiring since most COFs have 2D frameworks.

Secondly, most COFs are synthesized via a solvothermal method under harsh reaction conditions with powder products. Thus, the large-scale synthesis of COFs at industrial level is still challenging. To achieve more practical application, the large-scale synthesis with retaining the crystallinity and porosity of COFs is of critical significance. Moreover, the powder nature also hinders their application in electronic devices. Although some strategies such as interfacial polymerization can develop free-standing COF membranes to some extent, the limited mechanical property is always hard to meet the practical requirement. Therefore, efficient approaches to prepare COF membrane with good mechanical stability should be explored to enhance their practicality, especially in flexible electronic devices.

Thirdly, the development of COFs as ion conductors is in the initial state with most research interests focusing on the improvement of apparent performance via experimental investigations. To better clarify the structure-property relationship and guide the structural design, the theoretical simulations, which can provide more thorough insight on the ion transport mechanism in COFs, should be probed.

To sum up, COFs offer new opportunities for the solid ion conductors and exhibit tremendous advantages over other materials such as highly ordered pores, tailorable pore surface, tunable chemical composition, etc. Benefiting from the rapid development of experimental and theoretical tools, the electrochemical performance of COFs is expected to gain greater achievements.

## **Acknowledgements**

The authors acknowledge financial support from the National Natural Science Foundation of China (21965011 and 21902092) and the Major Science and Technology Plan of Hainan Province (ZDKJ202016).

## **Conflict of interest**

The authors declare no conflict of interest.

## **Author details**

Fei Lu and Yanan Gao\* Key Laboratory of Ministry of Education for Advanced Materials in Tropical Island Resources, Hainan University, Haikou, China

\*Address all correspondence to: ygao@hainanu.edu.cn

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Covalent Organic Frameworks for Ion Conduction DOI: http://dx.doi.org/10.5772/intechopen.108291*

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## **Chapter 3**
