Hydroxyl-Functionalized Covalent Organic Frameworks as High-Performance Supercapacitors

Covalent organic frameworks (COFs) have attracted significant interest because of their heteroatom-containing architectures, high porous networks, large surface areas, and capacity to include redox-active units, which can provide good electrochemical efficiency in energy applications. In this research, we synthesized two novel hydroxy-functionalized COFs—TAPT-2,3-NA(OH)2, TAPT-2,6-NA(OH)2 COFs—through Schiff-base [3 + 2] polycondensations of 1,3,5-tris-(4-aminophenyl)triazine (TAPT-3NH2) with 2,3-dihydroxynaphthalene-1,4-dicarbaldehyde (2,3-NADC) and 2,6-dihydroxynaphthalene-1,5-dicarbaldehyde (2,6-NADC), respectively. The resultant hydroxy-functionalized COFs featured high BET-specific surface areas up to 1089 m2 g–1, excellent crystallinity, and superior thermal stability up to 60.44% char yield. When used as supercapacitor electrodes, the hydroxy-functionalized COFs exhibited electrochemical redox activity due to the presence of redox-active 2,3-dihydroxynaphthalene and 2,6-dihydroxynaphthalene in their COF skeletons. The hydroxy-functionalized COFs showed specific capacitance of 271 F g−1 at a current density of 0.5 A g−1 with excellent stability after 2000 cycles of 86.5% capacitance retention. Well-known pore features and high surface areas of such COFs, together with their superior supercapacitor performance, make them suitable electrode materials for use in practical applications.


Introduction
Since the 20th century, the massive usage of fossil fuels including coal, gas, and mineral oil has been essential to economic and industrial advancement [1,2]. Renewable fuel technologies are increasingly frequently integrated with energy storage technologies to make better use of the converted energy, which can lessen the negative effects of poisonous air, global warming, and unsuitable ecosystems [3][4][5]. Energy storage forms for harvesting energy include mechanical, electrochemical, thermal, electrical, and hydrogen-based storage [6]. Among them, electrochemical supercapacitors (Scs) have received a lot of interest because they have a higher energy density, a longer life cycle, quicker storing capabilities, and a faster charge/discharge rate than ordinary dielectric capacitors [7][8][9]. A variety of electrochemical SC technologies for energy storage purposes have been created, including lithium ion batteries, sodium ion batteries, and magnesium ion batteries [10][11][12][13]. There are two types of methods for storing energy in supercapacitors: non-faradaic procedures, in which electrostatic ionic charges gather at the interface between electrolyte and electrode; and faradaic processes, which occur at the solid material's surface via the irreversible redox reaction. Consequentially, the electrode materials must exhibit good thermal stability, accurate pore size distributions, and steady electrochemical activity [14][15][16]. In the strategy to

Synthesis of Dihydroxynaphthalene Dicarbaldehyde (NADC)
Formamidine acetate (14.98 mmol) and dioxane (30 mL) were added into a 100-mL two-neck round-bottom flask and heated under reflux. Acetic anhydride (3 mL) was added when the target temperature reached 95 • C and then stirred for 30 min. Dihydroxynaphthalene (1.87 mmol) was added once all formamidine acetate was dissolved and was kept for two days. After cooling for minutes, dioxane was evaporated at 50 • C, was added H 2 O (45 mL), and heated at 65 • C for 2 h. Then, we added HCl (1 M, 40 mL) and kept heating at 65 • C for 18 h. The solid was filtered and washed several times with hexane. Dihydroxynaphthalene dicarbaldehydes (2,3-NADC and 2,6-NADC) were obtained after purifying by using column chromatography (Schemes S2 and S3).
We studied the elemental kinds and their ration in our hydroxyl-functionalized COFs using an X-ray Photoelectron Spectroscopy (XPS). As shown in Figure S15, the XPS spectra of hydroxyl-functionalized COFs revealed three distinct peaks for the carbons, nitrogens, and oxygens at 286.21, 400.61, and 533.51 eV, respectively, for the TAPT-2,3-NA(OH) 2 COF and at 286.17, 399.61, and 533.50 eV, respectively, for theTAPT-2,6-NA(OH) 2 COF. The lack of any additional components within XPS detections demonstrates the absence of any perceptible impurities that may have been produced during the synthesis of the hydroxyl-functionalized COFs, as illustrated in Figure S15. We fitted the XPS curves for the C1s, N1s, and O1s orbitals to better understand the kinds of N and O species found in the COFs ( Figure S16). Tables S4 and S5 summarize all XPS fitting data. TAPT-2,3-NA(OH) 2 COF has three primary kinds of C 1s species on their own surfaces: C-OH at 288.30 eV, C=C at 286.30 eV, and C=N at 284.60 eV ( Figure S16a and Table S4). On the other hand, the TAPT-2,6-NA(OH) 2 COF has also three kinds of C1s orbitals at 288.20, 286.17, and 283.45 eV, which we attributed to the C-OH, C=C, and C=N bonds, respectively ( Figure  S16d and Table S4). The fitting of the N1s, and O1s orbitals of both hydroxyl-functionalized COFs revealed a single kind of C1s and O1s orbitals ( Figure S16b,c,e,f and Table S4) [45]. We studied the elemental kinds and their ration in our hydroxyl-functionalized COFs using an X-ray Photoelectron Spectroscopy (XPS). As shown in Figure S15, the XPS spectra of hydroxyl-functionalized COFs revealed three distinct peaks for the carbons, nitrogens, and oxygens at 286.21, 400.61, and 533.51 eV, respectively, for the TAPT-2,3-NA(OH)2 COF and at 286.17, 399.61, and 533.50 eV, respectively, for theTAPT-2,6-NA(OH)2 COF. The lack of any additional components within XPS detections demonstrates the absence of any perceptible impurities that may have been produced during the synthesis of the hydroxyl-functionalized COFs, as illustrated in Figure S15. We fitted the XPS curves for the C1s, N1s, and O1s orbitals to better understand the kinds of N and O species found in the COFs ( Figure S16). Tables S4 and S5 summarize all XPS fitting data. TAPT-2,3-NA(OH)2 COF has three primary kinds of C 1s species on their own surfaces: C-OH at 288.30 eV, C=C at 286.30 eV, and C=N at 284.60 eV ( Figure S16a and Table S4). On the other hand, the TAPT-2,6-NA(OH)2 COF has also three kinds of C1s orbitals at 288.20, 286.17, and 283.45 eV, which we attributed to the C-OH, C=C, and C=N bonds, respectively ( Figure S16d and Table S4). The fitting of the N1s, and O1s orbitals of both hydroxylfunctionalized COFs revealed a single kind of C1s and O1s orbitals ( Figure S16b,c,e,f and Table S4) [45].
We used transmission electron microscopy (TEM) and field-emission scanning electron microscopy (FE-SEM) to visualize the self-assembly morphologies of the TAPT-2,3-NA(OH)2 and TAPT-2,6-NA(OH)2 COFs (Figure 4A-F). Low-magnification TEM images of TAPT-2,3-NA(OH)2 and TAPT-2,6-NA(OH)2 COFs after solvent exfoliation in ethanol revealed that both the hydroxyl-functionalized COFs were assembled into a significant number of long nanofibers with lengths of up to several micrometers, and such nanofibers were linked by their mesoporous sidewalls ( Figure 4A-D and Figures S17 and S18). The statistical analysis of the TEM images of the TAPT-2,3-NA(OH)2 and TAPT-2,6-NA(OH)2 We used transmission electron microscopy (TEM) and field-emission scanning electron microscopy (FE-SEM) to visualize the self-assembly morphologies of the TAPT-2,3-NA(OH) 2 and TAPT-2,6-NA(OH) 2 COFs ( Figure 4A-F). Low-magnification TEM images of TAPT-2,3-NA(OH) 2 and TAPT-2,6-NA(OH) 2 COFs after solvent exfoliation in ethanol revealed that both the hydroxyl-functionalized COFs were assembled into a significant number of long nanofibers with lengths of up to several micrometers, and such nanofibers were linked by their mesoporous sidewalls ( Figure 4A-D and Figures S17 and S18). The statistical analysis of the TEM images of the TAPT-2,3-NA(OH) 2 and TAPT-2,6-NA(OH) 2 COFs revealed average diameters for the nanofibers of 30 ± 50 and 50 ± 70 nm, respectively. As we had documented previously, the degree of planarity of the building monomers can have a significant impact on the morphology of the COFs. We also reported that the COFs constructed from planar building monomers are often built into tubes, rods, or fibers [9,46,47]. Therefore, the nanofiber morphologies of TAPT-2,3-NA(OH) 2 and TAPT-2,6-NA(OH) 2 COFs were apparently formed due to the high planarity of the TAPT-3NH 2 monomer. In Figure 4E,F and Figure S19, the SEM images of the TAPT-2,3-NA(OH) 2 and TAPT-2,6-NA(OH) 2 COFs confirmed their nanofiber morphologies. One of the primary requirements for the possible use of porous polymers in commercial energy storage and electrochemical supercapacitor technologies is their heat stability. Both hydroxyl-functionalized COFs demonstrated exceptional thermal stability when tested using a thermogravimetric analysis (TGA) under a nitrogen environment in a temperature range from 100 to 800 • C. Figure 4G,H and Table S6 show that TAPT-2,3-NA(OH) 2 and TAPT-2,6-NA(OH) 2 COFs were thermally stable materials; the values of their decomposition temperatures (T d10 ) were 435 and 460 • C, respectively, and the char yields were 60.44 and 60.07%, respectively. The desorption of the trapped solvents could be the cause of the initial weight loss of the TAPT-2,3-NA(OH) 2 and TAPT-2,6-NA(OH) 2 COFs.
tionalized COFs demonstrated exceptional thermal stability when tested using a thermogravimetric analysis (TGA) under a nitrogen environment in a temperature range from 100 to 800 °C. Figure 4G,H and Table S6 show that TAPT-2,3-NA(OH)2 and TAPT-2,6-NA(OH)2 COFs were thermally stable materials; the values of their decomposition temperatures (Td10) were 435 and 460 °C, respectively, and the char yields were 60.44 and 60.07%, respectively. The desorption of the trapped solvents could be the cause of the initial weight loss of the TAPT-2,3-NA(OH)2 and TAPT-2,6-NA(OH)2 COFs.

Supercapacitor Application of Hydroxyl-Functionalized COFs
The above PXRD, BET, XPS, and TEM measurements reveal that our TAPT-2,3-NA(OH) 2 and TAPT-2,6-NA(OH) 2 COFs had excellent crystallinities, high surface areas, redox-active structures, and microporous frameworks, suggesting that these hydroxylfunctionalized COFs might well be employed as promising supercapacitor materials. Therefore, we evaluated the electrochemical performance of TAPT-2,3-NA(OH) 2 and TAPT-2,6-NA(OH) 2 COFs using cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) in a three-electrode setup with an aqueous solution of potassium hydroxide (1 M) as electrolyte. Figure 5A,B and Figure S20 reveal the CV curves of the hydroxyl-functionalized COFs in the potential window from −0.8 to +0.2 V which were recorded at various rates ranging from 5 to 200 mV s −1 . The CV curves of TAPT-2,3-NA(OH) 2 and TAPT-2,6-NA(OH) 2 COFs showed quasi-rectangular shapes with small humps around −0.44 V for the TAPT-2,3-NA(OH) 2 and around −0.39 V for the TAPT-2,6-NA(OH) 2 COFs ( Figure S21a,b), suggesting that the capacitive responses were originated mainly from the EDL capacitance and minor from pseudo-capacitance [48][49][50][51][52]. The redox reactions of 2,3-dihydroxynaphthalene or 2,6-dihydroxynaphthalene on the COF surfaces and/or the N-heteroatom functionalities of the substance can be responsible for the formation of small humps in CV curves ( Figure S21c,d) [48][49][50][51][52]. The apparent peak separations between the waves of oxidation and reduction of both COFs were quite small, indicating a rapid electron transfer between the GC electrode and the dihydroxynaphthalene units in the COFs [53,54]. Additionally, the shape of the CV curves of TAPT-2,3-NA(OH) 2 and TAPT-2,6-NA(OH) 2 COFs were well maintained, but as the sweep rate was raised, the current density rose ( Figure 5A,B), indicating a strong rate capacity and fast kinetics [55]. The GCD measurements of TAPT-2,3-NA(OH) 2 and TAPT-2,6-NA(OH) 2 COFs were performed in the same voltage window, but at various current densities ranging from 0.5 A g −1 to 20 A g −1 . As shown in Figure 5C,D, both TAPT-2,3-NA(OH) 2 and TAPT-2,6-NA(OH) 2 COFs exhibited triangular-shaped GCD curves with modest bends, confirming the co-contribution of both EDLC and pseudo-capacitance, which is consistent with the CV curves. Figure 5C,D and Figure S22 show that the discharge time of the TAPT-2,3-NA(OH) 2 COF was longer than that of the TAPT-2,6-NA(OH) 2 COF, indicating that the capacitance of the former was greater than the latter.

Conclusions
In summary, two hydroxy-functionalized COFs, namely, TAPT-2,3-NA(OH) 2, TAPT-2,6-NA(OH) 2 COFs, were synthesized through the Schiff-base [3 + 2] polycondensations of TAPT-3NH 2 with 2,3-NADC and 2,6-NADC, respectively, using n-butanol/odichlorobenzene (1:1) as a co-solvent over 72 h at 115 • C in Schlenk tubes, in the presence of acetic acid (6 M, 10 vol%) as an acidic catalyst. The molecular structures of the hydroxylfunctionalized COFs were verified using FTIR and a solid state 13 C NMR spectroscopy. According to the PXRD, BET, TGA, and XPS measurements, hydroxyl-functionalized COFs featured high BET-specific surface areas up to 1089 m 2 g −1 , excellent crystallinity, and a superior thermal stability up to 60.44% char yield. In addition, we evaluated the applicability of our hydroxyl-functionalized COFs for a supercapacitor application. The resultant hydroxyl-functionalized COFs demonstrated an outstanding electrochemical efficiency (271 F g −1 at a current density of 0.5 A g −1 ) because of the presence of redox-active 2,3-dihydroxynaphthalene and 2,6-dihydroxynaphthalene in their COF skeletons. Furthermore, we believe that these novel hydroxyl-functionalized COFs might be useful in a variety of applications, including energy storage technologies.