Ultrastable Covalent Triazine Organic Framework Based on Anthracene Moiety as Platform for High-Performance Carbon Dioxide Adsorption and Supercapacitors

Conductive and porous nitrogen-rich materials have great potential as supercapacitor electrode materials. The exceptional efficiency of such compounds, however, is dependent on their larger surface area and the level of nitrogen doping. To address these issues, we synthesized a porous covalent triazine framework (An-CTFs) based on 9,10-dicyanoanthracene (An-CN) units through an ionothermal reaction in the presence of different molar ratios of molten zinc chloride (ZnCl2) at 400 and 500 °C, yielding An-CTF-10-400, An-CTF-20-400, An-CTF-10-500, and An-CTF-20-500 microporous materials. According to N2 adsorption–desorption analyses (BET), these An-CTFs produced exceptionally high specific surface areas ranging from 406–751 m2·g−1. Furthermore, An-CTF-10-500 had a capacitance of 589 F·g−1, remarkable cycle stability up to 5000 cycles, up to 95% capacity retention, and strong CO2 adsorption capacity up to 5.65 mmol·g−1 at 273 K. As a result, our An-CTFs are a good alternative for both electrochemical energy storage and CO2 uptake.


Introduction
Over the last few decades, there has been a dramatic surge in the need for future sustainable energy. The cause for this massive demand is the variety of natural resources that supply raw materials to support industrialization and urbanization in our modern era [1][2][3][4][5]. This trend has raised major concerns about the availability of natural resources for future generations, which might lead to disaster in terms of global warming and a shortage of energy supplies [5][6][7][8][9][10][11][12]. To address these issues, researchers across the world have devoted themselves to finding clean and sustainable electrical energy storage devices. Thus, one of the most important energy storage devices discovered is the electric double-layer capacitor (EDLC), also known as a supercapacitor, which piqued the interest of many people due to its substantial power density, high energy density, a wide range of operating temperature, and consistent cycling performance over time [12][13][14][15]. The key point of the working mechanism of these supercapacitors is that they store the charged particles between the interface of electrode and electrolyte with the help of dynamic electrolyte ions. Thus, the performance of these supercapacitors depends on the accessible area of the electrode material with a high surface area and porous morphology to increase the movement of electrolyte ions [12][13][14][15][16][17].

Synthesis and Characterization of An-CN and An-CTFs
As known, the anthracene (An) unit consists of three fused benzene rings with highly planar conjugation structure, and it is widely used in field-effect transistors, solar cells, and light-emitting diodes (LED). The An-CTFs were synthesized in three steps as

Synthesis and Characterization of An-CN and An-CTFs
As known, the anthracene (An) unit consists of three fused benzene rings with highly planar conjugation structure, and it is widely used in field-effect transistors, solar cells, and light-emitting diodes (LED). The An-CTFs were synthesized in three steps as shown in Figure 1. Firstly, anthracene (Figure 1a) was converted into 9,10-dibromoanthracene (An-Br 2 ) using Br 2 in chloroform ( Figure 1b) at 50 • C for 4 h. Then, An-Br 2 was reacted with CuCN in dry DMF to give 9,10-cyanoanthracene (An-CN) (Figure 1c). Then we used the ionothermal method for trimerization of CN groups of An-CN in anhydrous molten ZnCl 2 with different CN-An/ZnCl 2 ratios at two different reaction temperatures of 400 and 500 • C, yielding An-CTF-10-400, An-CTF-20-400, An-CTF-10-500, and An-CTF-20-500 ( Figure 1d). The presence of nitrile groups and their cyclotrimerization reactions in An-CN were confirmed through DSC analysis (Figure 1e). The DSC profile of the An-CN monomer before and after thermal treatment from 25 to 240 • C displayed a maximum exothermic peak of the CN group at 265 • C. We found that the maximum exothermic peak of CN groups was shifted to 295 • C and completely absent when the temperature reaction was 300, 340, 380, and 400 • C, due to the formation of triazine rings and completion of the cyclotrimerization reaction to form An-CTFs precursors.
The FTIR spectra of An-CN and its corresponding synthesized CTFs at different temperatures are displayed in Figure 2a. The FTIR profile of An-CN featured a band at 2220 cm −1 corresponding to the CN unit, which also confirmed its synthesis. Furthermore, as shown in Figure 2a, the FTIR spectra of An-CTFs exhibited absorption bands at 1566 and 1383 cm −1 attributed to the triazine rings in each CTF without any bands of CN, confirming the trimerization or cyclization of the nitrile groups [54,55]. To gain insight into the thermal stability of as-prepared CTFs and An-CN, we performed TGA analysis under N 2 atmosphere from 40-800 • C. As seen in Figure 2b and Table 1, the decomposition temperature (T d10 ) of An-CTF-10-500 was 573 • C, which was the highest among other An-CTFs with a higher char yield of 66 wt.%. The decomposition temperatures of other An-CTFs were smaller compared to An-CTF-10-500 with char yields of 56, 69, and 62 wt.% for An-CTF-10-400, An-CTF-20-400, and An-CTF-20-500, respectively. Therefore, An-CTF-10-500 had a superior decomposition temperature and high char yield, making it more thermally stable. On the other hand, the degrading temperature of the An-CN monomer was 274 • C with no char yield. Overall, our new An-CTFs had excellent thermal stability. Figure 2c represents the XRD patterns of An-CTFs in the range of 5-50 • C. Upon seeing the spectrum, the two broad peaks located at 12 and 26 • C could be attributed to a partially crystalline structure, assigned to the (001) and (100) plane indices [56][57][58][59]. Raman spectroscopy is considered an important method to examine and investigate the graphitization degree and defect properties of carbon materials. The Raman spectra of as-prepared CTFs are shown in Figure 2d, recorded from 1200 to 1700 cm −1 . The spectra of all An-CTFs showed two strong signals characteristic of the D and G bands, revealing the presence of graphitic carbonized structure. These D and G bands represent the carbonaceous materials formed from chemical structures of CTFs, as well as other carbonaceous materials used in electrode preparation. In general, they represent two kinds of hybridization (sp 3 and sp 2 ), corresponding to the second-and first-order Raman scattering. The D and G bands for all four An-CTFs were located at 1350 and 1602 cm −1 , respectively [54,55]. The I D /I G ratios for An-CTF-10-400, An-CTF-20-400, An-CTF-10-500, and An-CTF-20-500 were 1.01, 1.04, 1.11, and 1.03 respectively, suggesting that An-CTF-10-400, An-CTF-20-400, and An-CTF-20-500 had a higher degree of graphitization, indirectly revealing the formation of fewer defects in their morphology with an increase in the condensed aromatic structure when compared to An-CTF-10-500 and other reported CTFs [54][55][56][57][58][59]. An-CTF-10-500 showed a slightly lower degree of graphitization when compared with other samples, possibly due to the low amount of ZnCl 2 and high temperature (500 • C) leading to more defects in the graphene structure.
An-CTF-10-500, and An-CTF-20-500 were 1.01, 1.04, 1.11, and 1.03 respectively, suggesting that An-CTF-10-400, An-CTF-20-400, and An-CTF-20-500 had a higher degree of graphitization, indirectly revealing the formation of fewer defects in their morphology with an increase in the condensed aromatic structure when compared to An-CTF-10-500 and other reported CTFs [54][55][56][57][58][59]. An-CTF-10-500 showed a slightly lower degree of graphitization when compared with other samples, possibly due to the low amount of ZnCl2 and high temperature (500 °C) leading to more defects in the graphene structure.    The XPS spectra of all synthesized An-CTFs in this study ( Figure S5) showed three peaks corresponding to the characteristic peaks of the carbon atom (at 284 eV), C-N bond for N 1s orbital (at 400 eV) in triazine unit, and O 1s orbital with absorbed moisture and oxygen (at 530 eV) [54][55][56][57][58][59]. In addition, Figure 3a-h and Table 2 show the fitted XPS curves for N 1s and O 1s orbitals to examine the chemical composition on the surface of these as-prepared An-CTFs. All of the fitted results showed the existence of three different N species: quaternary N species (401.5 eV), pyrrolic species (400 eV), and hexagonal pyridinic N atom due to the triazine unit (398.5 eV). Following a quantitative investigation, pyrrolic N was shown to be the most prevalent, whereas the remaining species of quaternary and pyridinic N were essentially identical in the synthesized CTFs. In addition to this, the three O 1s orbital peaks in Figure 3 reveal that the surface of these CTFs also contained three different O species such as C-O, moisture, and absorbed oxygen at 531.5, 533, and 535 eV, respectively [54,60,61]. ferent N species: quaternary N species (401.5 eV), pyrrolic species (400 eV), and hexagonal pyridinic N atom due to the triazine unit (398.5 eV). Following a quantitative investigation, pyrrolic N was shown to be the most prevalent, whereas the remaining species of quaternary and pyridinic N were essentially identical in the synthesized CTFs. In addition to this, the three O1s orbital peaks in Figure 3 reveal that the surface of these CTFs also contained three different O species such as C-O, moisture, and absorbed oxygen at 531.5, 533, and 535 eV, respectively [54,60,61].

Electrochemical Performance of An-CTFs
The electrochemical performance of the An-CTFs samples was determined using cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) measurements in a 1 M KOH aqueous solution utilizing a three-electrode setup. Figure 7a-d show the corre sponding CV curves of the An-CTFs samples, which were recorded at various sweep speeds ranging from 5 to 200 mV·s −1 in a potential window ranging from 0 to 1.00 V (vs Hg/HgO). The CV curves of all An-CTFs samples had rectangle-like forms, suggesting that this capacitive response was mostly caused by electric double-layer capacitance (EDLC) with little pseudo-capacitance caused by the presence of different nitrogen and oxygen species [54,55]. Furthermore, raising the molar ratio of An-CN/ZnCl2 from 0.05 to 0.1 at 500 °C resulted in An-CTFs materials with higher EDLC capacitance. This behavior can be explained by the reaction with ZnCl2, which produced a unique carbon framework with pyridinic N atoms similar to those seen in N-doped carbon materials [54,55]. BET and XPS studies demonstrated that An-CTFs samples exhibited high specific surface ar eas and N-heteroatom frameworks (primarily pyridinic and pyrrolic N species).

Electrochemical Performance of An-CTFs
The electrochemical performance of the An-CTFs samples was determined using cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) measurements in a 1 M KOH aqueous solution utilizing a three-electrode setup. Figure 7a-d show the corresponding CV curves of the An-CTFs samples, which were recorded at various sweep speeds ranging from 5 to 200 mV·s −1 in a potential window ranging from 0 to 1.00 V (vs. Hg/HgO). The CV curves of all An-CTFs samples had rectangle-like forms, suggesting that this capacitive response was mostly caused by electric double-layer capacitance (EDLC) with little pseudo-capacitance caused by the presence of different nitrogen and oxygen species [54,55]. Furthermore, raising the molar ratio of An-CN/ZnCl 2 from 0.05 to 0.1 at 500 • C resulted in An-CTFs materials with higher EDLC capacitance. This behavior can be explained by the reaction with ZnCl 2 , which produced a unique carbon framework with pyridinic N atoms similar to those seen in N-doped carbon materials [54,55]. BET and XPS studies demonstrated that An-CTFs samples exhibited high specific surface areas and N-heteroatom frameworks (primarily pyridinic and pyrrolic N species). Figure 8a-d show the GCD curves of An-CTFs samples recorded at different current densities ranging from 0.5 to 20 A·g −1 . The An-CTFs sample GCD curves displayed triangular forms with a small bend, indicating both pseudo-capacity and EDLC features [54,55]. As shown in Figure 8c,d, the discharging time of An-CTF-10-500 was longer than that of the An-CTF-20-500, showing that the former's capacitance was greater than that of the latter.  The An-CTFs sample GCD curves displayed triangular forms with a small bend, indicating both pseudo-capacity and EDLC features [54,55]. As shown in Figure 8c,d, the discharging time of An-CTF-10-500 was longer than that of the An-CTF-20-500, showing that the former's capacitance was greater than that of the latter.    The An-CTFs sample GCD curves displa angular forms with a small bend, indicating both pseudo-capacity and EDLC [54,55]. As shown in Figure 8c,d, the discharging time of An-CTF-10-500 was lon that of the An-CTF-20-500, showing that the former's capacitance was greater tha the latter.    Table S1 show the specific capacitances of An-CTF samples calculated from GCD curves using Equation (S1). At a current density of 0.5 A·g -1 , An-CTF-10-500 demonstrated good capacitance with a value of 589 F·g -1 . The existence of graphitic microporous carbon structures with diverse functionalized units (pyridinic and pyrrolic N atoms, C=O, and phenolic OH groups) might explain An-CTF-10-500's remarkable performance. In addition to this, the superior behavior of An-CTF-10-500 could be due to the increased molar ratio of An-CN/ZnCl 2 from 0.05 to 0.1 at 500 • C, which helped to form a carbon-rich network with high N content. Consequently, An-CTF-20-500 also exhibited excellent specific capacitance quite similar to that of An-CTF-10-500; however, due to the low molar ratio of An-CN/ZnCl 2 , it could not outperform An-CTF-10-500. As shown in Figure 9a, the specific capacitance of all four samples, i.e., An-CTF-10-400, An-CTF-20-400, An-CTF-10-500, and An-CTF-20-500, decreased upon varying the current density from 0.5 to 20 A·g −1 . This unique behavior of these An-CTF materials can be attributed to the rapid adsorption of electrolyte ions on the electrodes. Furthermore, An-CTF-10-500 was superior owing to its highest surface area during the cycling performance. In addition, to examine the stability of these as-prepared electrode materials over long cycling tests, we investigated the cycling stability profiles of all An-CTF materials with GCD evaluation at a current density of 10 A·g −1 over 5000 cycles (Figure 9b). The results revealed their extraordinary stability in 1 M KOH electrolyte as An-CTF-10-400, An-CTF-20-400, An-CTF-10-500, and An-CTF-20-500 retained 56%, 72%, 95%, and 90% of their initial capacity. We previously reported two ultrastable conductive CTFs based on pyrene, and these materials exhibited high capacitance of 380 and 500 F·g −1 . These high capacitances were attributed to their high specific surface areas of 819 and 1019 m 2 ·g −1 and high N content [55]. Additionally, Hao et al. reported 2D microporous CTFs with suitable low capacitance (151.3 F·g −1 at 0.1 A·g −1 ) [57]. Thus, these An-CTFs materials show great potential to be employed as electrochemical energy storage systems.  Table S1 show the specific capacitances of An-CTF samples calculated from GCD curves using Equation (S1). At a current density of 0.5 A·g -1 , An-CTF-10-500 demonstrated good capacitance with a value of 589 F·g -1 . The existence of graphitic microporous carbon structures with diverse functionalized units (pyridinic and pyrrolic N atoms, C=O, and phenolic OH groups) might explain An-CTF-10-500's remarkable performance. In addition to this, the superior behavior of An-CTF-10-500 could be due to the increased molar ratio of An-CN/ZnCl2 from 0.05 to 0.1 at 500 °C, which helped to form a carbon-rich network with high N content. Consequently, An-CTF-20-500 also exhibited excellent specific capacitance quite similar to that of An-CTF-10-500; however, due to the low molar ratio of An-CN/ZnCl2, it could not outperform An-CTF-10-500. As shown in Figure 9a, the specific capacitance of all four samples, i.e., An-CTF-10-400, An-CTF-20-400, An-CTF-10-500, and An-CTF-20-500, decreased upon varying the current density from 0.5 to 20 A·g −1 . This unique behavior of these An-CTF materials can be attributed to the rapid adsorption of electrolyte ions on the electrodes. Furthermore, An-CTF-10-500 was superior owing to its highest surface area during the cycling performance. In addition, to examine the stability of these as-prepared electrode materials over long cycling tests, we investigated the cycling stability profiles of all An-CTF materials with GCD evaluation at a current density of 10 A·g −1 over 5000 cycles (Figure 9b). The results revealed their extraordinary stability in 1 M KOH electrolyte as An-CTF-10-400, An-CTF-20-400, An-CTF-10-500, and An-CTF-20-500 retained 56%, 72%, 95%, and 90% of their initial capacity. We previously reported two ultrastable conductive CTFs based on pyrene, and these materials exhibited high capacitance of 380 and 500 F·g −1 . These high capacitances were attributed to their high specific surface areas of 819 and 1019 m 2 ·g −1 and high N content [55]. Additionally, Hao et al. reported 2D microporous CTFs with suitable low capacitance (151.3 F·g −1 at 0.1 A·g −1 ) [57]. Thus, these An-CTFs materials show great potential to be employed as electrochemical energy storage systems.   Figure S7 shows that the An-CTFs displayed superior capacitance performance compared to other porous materials such as Car-CTFs [54], Py-CTFs [55], TCNQ-CTFS [52], TDFP-1 [53], and TPE-CPOP-800 [63]. Electrochemical impedance spectroscopy is an outstanding characterization method to study the interface of electrodes and electrolytes  Figure S7 shows that the An-CTFs displayed superior capacitance performance compared to other porous materials such as Car-CTFs [54], Py-CTFs [55], TCNQ-CTFS [52], TDFP-1 [53], and TPE-CPOP-800 [63]. Electrochemical impedance spectroscopy is an outstanding characterization method to study the interface of electrodes and electrolytes in a given frequency domain. EIS provides information about the internal resistances offered by the electrode material and used electrolyte system. Figure 10 shows the Nyquist plots of as-prepared electrode materials in 1.0 M KOH electrolyte, in which the compounds An-CTF-10-500 and An-CTF-20-500 showed depressed semicircles and a sloping line at high frequencies and low frequencies, respectively, attributed to the charge transfer during the faradic reactions at the electrode/electrolyte interface [64,65]. The EIS results demonstrated that the best electric charge transfer performance was observed with compound An-CTF-10-500, followed by An-CTF-20-500, An-CTF-10-400, and An-CTF-20-400, due to the better conductivity of An-CTF-10-500. This trend matches well with the capacitance plots of all the compounds shown in Figure 9a. In order to study the impedance properties of the electrode materials, electrochemical impedance spectroscopy (EIS) was carried out, as displayed in Figure 10a,b. The equivalent circuit was fitted with the equivalent model in order to compare the charge transfer from each electrode. Rs, Rct, CPE-EDL, CPE-P, and Zw represent the equivalent series resistance, charge transfer resistance, constant phase element representing EDLC, pseudocapacitive behavior, and Warburg element, respectively. As shown in Table S2, the initial ohmic resistance values of all electrodes were 9.951, 10.75, 4.068, and 7.08 for An-CTF-10-400, An-CTF-20-400, An-CTF-10-500, and An-CTF-20-500, respectively. Among them, An-CTF-10-500 delivered the smallest ohmic resistance; thus, it was the most conductive in nature due to the charge transfer during the faradic reactions at electrode/electrolyte interface [64][65][66][67][68]. Furthermore, Figure 10c represents the frequency-dependent Bode plots of triazine-based electrodes. The figures reveal slanted lines with a negative slope at low frequency with tiny resistance at high frequency, demonstrating the ideal capacitive behavior of the as-prepared electrodes. Figure 10d shows the frequency-dependent Bode phase plot. The knee frequency can be defined as the characteristic frequency where the phase angle reaches 45 • . At this stage, both the capacitive and resistance impedance become equal in magnitude. Beyond this point at higher frequency, the supercapacitors become resistive. This also indicates the relaxation time for the device to discharge its energy with efficiency greater than 50%. The knee frequency is directly proportional to the rate capability of electrode materials. in a given frequency domain. EIS provides information about the internal resist fered by the electrode material and used electrolyte system. Figure 10 shows the plots of as-prepared electrode materials in 1.0 M KOH electrolyte, in which t pounds An-CTF-10-500 and An-CTF-20-500 showed depressed semicircles and a line at high frequencies and low frequencies, respectively, attributed to the transfer during the faradic reactions at the electrode/electrolyte interface [64,65] results demonstrated that the best electric charge transfer performance was o with compound An-CTF-10-500, followed by An-CTF-20-500, An-CTF-10-4 An-CTF-20-400, due to the better conductivity of An-CTF-10-500. This trend matc with the capacitance plots of all the compounds shown in Figure 9a. In order to s impedance properties of the electrode materials, electrochemical impedance sp py (EIS) was carried out, as displayed in Figure 10a,b. The equivalent circuit w with the equivalent model in order to compare the charge transfer from each e Rs, Rct, CPE-EDL, CPE-P, and Zw represent the equivalent series resistance transfer resistance, constant phase element representing EDLC, pseudocapac havior, and Warburg element, respectively. As shown in Table S2, the initial o sistance values of all electrodes were 9.951, 10.75, 4.068, and 7.08 for An-CTF An-CTF-20-400, An-CTF-10-500, and An-CTF-20-500, respectively. Amon An-CTF-10-500 delivered the smallest ohmic resistance; thus, it was the most co in nature due to the charge transfer during the faradic reactions at electrode/el interface [64][65][66][67][68]. Furthermore, Figure 10c represents the frequency-depende plots of triazine-based electrodes. The figures reveal slanted lines with a negative low frequency with tiny resistance at high frequency, demonstrating the ideal ca behavior of the as-prepared electrodes. Figure 10d shows the frequency-depend phase plot. The knee frequency can be defined as the characteristic frequency w phase angle reaches 45°. At this stage, both the capacitive and resistance imped come equal in magnitude. Beyond this point at higher frequency, the superca become resistive. This also indicates the relaxation time for the device to disc energy with efficiency greater than 50%. The knee frequency is directly propor the rate capability of electrode materials.

Preparation of 9,10-Cyanoanthracene (An-CN)
A mixture of An-Br 2 (2 g) and CuCN (2.13 g) was added to a flask containing 60 mL of DMF under reflux at 140 • C for 2 days. After the reaction, the beaker was cooled to room temperature and filtered to remove excess CuCN. The DMF solution was then poured into ethylenediamine and water to afford a yellow solid. The obtained solid was crystallized in a hot solution of DMF to afford An-CN as yellow needles (85%). 1 H-NMR (500 MHz, DMSO-d 6 , Figure S3

Preparation of An-CTFs
The An-CTFs were prepared using An-CN as the monomer precursor, mixed with anhydrous molten ZnCl 2 at molar ratios of 0.1 and 0.05. They were subjected to two different temperatures of 400 and 500 • C and maintained for 2 days under nitrogen atmosphere to afford An-CTFs (Figure 1). The obtained solid was washed with 1 M HCl, water, THF, DCM, and methanol, before drying in an oven to afford An-CTFs.

Conclusions
We discovered a new class of nitrogen-enriched covalent organic frameworks based on triazine and An in the presence of ZnCl 2 at two distinct temperatures and molar ratios. The structural characterization demonstrated that the synthesized An-CTFs had fewer morphological differences than previously reported CTFs. These An-CTFs had thermal stability, high BET surface area and porosity, and excellent cycle stability, which enabled us to examine the CO 2 uptake and energy storage performance of as-prepared An-CTFs. An-CTF-10-500 outperformed the other three CTFs with a higher specific capacitance of 589 F·g −1 and higher capacity retention of 95% over 5000 cycles. It exhibited excellent charge transport from the electrode to electrolyte and a unique higher level of CO 2 uptake of 5.25 mmol·g −1 at 273 K. From the results obtained, it is well understood that the CTFs based on the An moiety can set a new bar for applications in gas and energy storage.