An Ultrastable Porous Polyhedral Oligomeric Silsesquioxane/Tetraphenylthiophene Hybrid as a High-Performance Electrode for Supercapacitors

In this study, we synthesized three hybrid microporous polymers through Heck couplings of octavinylsilsesquioxane (OVS) with 2,5-bis(4-bromophenyl)-1,3,4-oxadiazole (OXD-Br2), tetrabromothiophene (Th-Br4), and 2,5-bis(4-bromophenyl)-3,4-diphenylthiophene (TPTh-Br2), obtaining the porous organic–inorganic polymers (POIPs) POSS-OXD, POSS-Th, and POSS-TPTh, respectively. Fourier transform infrared spectroscopy and solid state 13C and 29Si NMR spectroscopy confirmed their chemical structures. Thermogravimetric analysis revealed that, among these three systems, the POSS-Th POIP possessed the highest thermal stability (T5: 586 °C; T10: 785 °C; char yield: 90 wt%), presumably because of a strongly crosslinked network formed between its OVS and Th moieties. Furthermore, the specific capacity of the POSS-TPTh POIP (354 F g−1) at 0.5 A g−1 was higher than those of the POSS-Th (213 F g−1) and POSS-OXD (119 F g−1) POIPs. We attribute the superior electrochemical properties of the POSS-TPTh POIP to its high surface area and the presence of electron-rich phenyl groups within its structure.


Synthesis and Character of POSS-OXD, POSS-Th and POSS-TPTh POIPs
Scheme 1 illustrates the syntheses of the three types of hybrid porous POSS frameworks: POSS-OXD, POSS-Th, and POSS-TPTh. We prepared OXD-Br 2 through the reaction of N,N -bis(4-bromobenzoyl)hydrazine with POCl 3 at 80 • C (Scheme S1) and Th-Br 4 through the reaction of Th with Br 2 in CHCl 3 at 0 • C (Scheme S2). We then obtained TPTh (Scheme S3) in high purity and yield through Suzuki coupling of Th-Br 4 with PhB(OH) 2 in toluene in the presence of Pd(PPh 3 ) 4 and K 2 CO 3 . Bromination of TPTh in CH 2 Cl 2 in the presence of Br 2 afforded TPTh-Br 2 as a yellow solid (Scheme S4). Finally, we obtained the POSS-OXD, POSS-Th, and POSS-TPTh POIPs through Heck couplings of OXD-Br 2 , Th-Br 4 , and TPTh-Br 2 , respectively, with OVS in DMF in the presence of Pd(PPh 3 ) 4 , Et 3 N, and K 2 CO 3 at 120 • C for 48 h. These POSS-POIPs (Scheme 1) displayed poor solubility in all organic solvents, consistent with the successful Heck reactions of the OVS units and the formation of POSS materials with high degrees of crosslinking.
FTIR spectroscopy revealed the functional groups present in the synthesized porous POSS-POIP materials ( Figure 1). The spectrum of OVS featured absorption bands at 3066, 1600, and 1106 cm −1 , corresponding to the stretching of C=C-H, C=C, and Si-O-Si bonds, respectively ( Figure 1a). The FTIR spectra of OXD-Br 2 , Th-Br 4 , TPTh-Br 2 , POSS-OXD POIP, POSS-Th POIP, and POSS-TPTh POIP revealed signals for aromatic C-H bonds in the range 3042-3082 cm −1 and for C=C stretching in the range 1600-1640 cm −1 (Figure 1b-g). In addition, the signals for the Si-O-Si units from all three porous POIPs were wider than that of OVS, consistent with the formation of crosslinked networks ( Figure 1). In addition, the FTIR spectra of all the POIPs featured bands representing OH groups, resulting from their absorption of water.  29 Si NMR spectra of the POSS-OXD, POSS-Th, and POSS-TPTh POIPs. * is the side band of solid state nuclear magnetic resonance spectroscopy (NMR).
We used TGA to examine the thermal stabilities of the POSS-OXD, POSS-Th, and POSS-TPTh POIPs ( Figure 3a). The thermal decomposition temperatures Td5 and Td10 and the char yields of the POSS-OXD POIP were 260 °C, 386 °C, and 69 wt%, respectively; for POSS-Th POIP, they were 586 °C, 786 °C, and 90 wt%, respectively; and for the POSS-TPTh POIP, they were 513 °C, 586 °C, and 86 wt%, respectively. For all three porous POIPs, these high values are consistent with the presence of highly crosslinked networks formed between the OVS and OXD, Th, and TPTh moieties. Notably, the thermal stability and char yield of the POSS-Th POIP were higher than those of the POSS-OXD and POSS-TPTh POIPs, as well as those of all other previously reported porous POSS systems [57,58], implying its higher crosslinking density. Table 1 summarizes the thermal properties of the POSS-OXD, POSS-Th, and POSS-TPTh POIPs. X-ray diffraction revealed that the POSS-OXD, POSS-Th, and POSS-TPTh POIPs were all amorphous materials with no long-range order in their characteristics (Figure 3b), consistent with data reported previously for porous hybrid POSS materials [57,58].  29 Si NMR spectra of the POSS-OXD, POSS-Th, and POSS-TPTh POIPs. * is the side band of solid state nuclear magnetic resonance spectroscopy (NMR).
We used TGA to examine the thermal stabilities of the POSS-OXD, POSS-Th, and POSS-TPTh POIPs ( Figure 3a). The thermal decomposition temperatures T d5 and T d10 and the char yields of the POSS-OXD POIP were 260 • C, 386 • C, and 69 wt%, respectively; for POSS-Th POIP, they were 586 • C, 786 • C, and 90 wt%, respectively; and for the POSS-TPTh POIP, they were 513 • C, 586 • C, and 86 wt%, respectively. For all three porous POIPs, these high values are consistent with the presence of highly crosslinked networks formed between the OVS and OXD, Th, and TPTh moieties. Notably, the thermal stability and char yield of the POSS-Th POIP were higher than those of the POSS-OXD and POSS-TPTh POIPs, as well as those of all other previously reported porous POSS systems [57,58], implying its higher crosslinking density. Table 1 summarizes the thermal properties of the POSS-OXD, POSS-Th, and POSS-TPTh POIPs. X-ray diffraction revealed that the POSS-OXD, POSS-Th, and POSS-TPTh POIPs were all amorphous materials with no long-range order in their characteristics (Figure 3b), consistent with data reported previously for porous hybrid POSS materials [57,58].   We performed N2 adsorption/desorption measurements to obtain the porosity parameters of the POIPs (Figure 4a-c, Table 1). With reference to IUPAC classifications, the adsorption isotherm of the POSS-OXD POIP was of type III; that of POSS-Th POIP was of types II and IV; and that of POSS-TPTh POIP had type II characteristics. Furthermore, the N2 absorption uptake potentials for the POSS-Th and POSS-TPTh POIPs increased in the low-and high-pressure regions, confirming the existence of meso-and microspores in their structures; in contrast, POSS-OXD POIP had high N2 absorption uptake at high pressure, suggesting the presence of only macropores. The specific surface area of the POSS-TPTh POIP (682 m 2 g −1 ) was higher than those of the POSS-Th (605 m 2 g −1 ) and POSS-OXD (94 m 2 g −1 ) POIPs. The pore diameters of the POSS-OXD, POSS-Th, and POSS-TPTh POIPs were 2.34, 1.96, and 1.88 nm, respectively (Figure 4d-f, Table 1); their total pore volumes were 0.02, 0.23, and 0.23 cm 3 g −1 , respectively.  We performed N 2 adsorption/desorption measurements to obtain the porosity parameters of the POIPs (Figure 4a-c, Table 1). With reference to IUPAC classifications, the adsorption isotherm of the POSS-OXD POIP was of type III; that of POSS-Th POIP was of types II and IV; and that of POSS-TPTh POIP had type II characteristics. Furthermore, the N 2 absorption uptake potentials for the POSS-Th and POSS-TPTh POIPs increased in the low-and high-pressure regions, confirming the existence of meso-and microspores in their structures; in contrast, POSS-OXD POIP had high N 2 absorption uptake at high pressure, suggesting the presence of only macropores. The specific surface area of the POSS-TPTh POIP (682 m 2 g −1 ) was higher than those of the POSS-Th (605 m 2 g −1 ) and POSS-OXD     We tested these three porous POSS materials for their ability to capture CO 2 at 298 • C ( Figure 6). The POSS-OXD, POSS-Th, and POSS-TPTh POIPs had CO 2 capture capacities of 18.27, 35.68, and 50.42 mg g −1 , respectively. The relatively high CO 2 capture capacity of the POSS-TPTh POIP was presumably due to its Brunauer-Emmett-Teller (BET) surface area being higher than those of the POSS-Th and POSS-OXD POIPs. We tested these three porous POSS materials for their ability to capture CO2 at 298 °C ( Figure 6). The POSS-OXD, POSS-Th, and POSS-TPTh POIPs had CO2 capture capacities of 18.27, 35.68, and 50.42 mg g -1 , respectively. The relatively high CO2 capture capacity of the POSS-TPTh POIP was presumably due to its Brunauer-Emmett-Teller (BET) surface area being higher than those of the POSS-Th and POSS-OXD POIPs.

Electrochemical Performance of the POSS-OXD, POSS-Th, and POSS-TPTh POIPs
We employed cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) measurements to examine the electrochemical behavior of our three POIPs in aqueous 1 M KOH, using a three-electrode configuration. We recorded the CV profiles of the POSS-OXD, POSS-Th, and POSS-TPTh POIPs at various scan rates between 1.00 to 0 V ( Figure  7a-c). The rectangular CV profiles with slight humps for all three POSS systems indicate that their capacitive effects were caused by EDLC [76,77]. The humps suggest the involvement of pseudocapacitance responses emerging from the presence of the N and S heteroatoms and electron-rich phenyl groups in the OXD, Th, and TPTh structures, respectively, facilitating the passage of the electrolyte to the electrode area and for the electrons to keep moving from one electrode to another [78,79]. Upon increasing the scan rate, the current densities improved without affecting the shapes of the CV curves, confirming the highrate capabilities, stabilities, and kinetic features of all three porous POSS systems. The electrochemical properties of the POSS-TPTh POIP were superior to those of the POSS-Th and POSS-OXD POIPs, presumably because of its electron-rich phenyl groups and S heteroatoms, high surface area, and blend of micro-and mesoporosity. All these factors would ensure higher mobility of the electrolyte to the surface of the electrode, resulting in accelerated mass transport and enhanced electrochemical performance. Figure 7d-f display the GCD behavior of the POSS-OXD, POSS-Th, and POSS-TPTh POIPs, respectively, measured at various current densities. In the discharge profiles, each porous POSS system provided a nearly triangular GCD curve with a small bend, indicating the combined impacts of pseudocapacitance and EDLC.

Electrochemical Performance of the POSS-OXD, POSS-Th, and POSS-TPTh POIPs
We employed cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) measurements to examine the electrochemical behavior of our three POIPs in aqueous 1 M KOH, using a three-electrode configuration. We recorded the CV profiles of the POSS-OXD, POSS-Th, and POSS-TPTh POIPs at various scan rates between 1.00 to 0 V (Figure 7a-c). The rectangular CV profiles with slight humps for all three POSS systems indicate that their capacitive effects were caused by EDLC [76,77]. The humps suggest the involvement of pseudocapacitance responses emerging from the presence of the N and S heteroatoms and electron-rich phenyl groups in the OXD, Th, and TPTh structures, respectively, facilitating the passage of the electrolyte to the electrode area and for the electrons to keep moving from one electrode to another [78,79]. Upon increasing the scan rate, the current densities improved without affecting the shapes of the CV curves, confirming the high-rate capabilities, stabilities, and kinetic features of all three porous POSS systems. The electrochemical properties of the POSS-TPTh POIP were superior to those of the POSS-Th and POSS-OXD POIPs, presumably because of its electron-rich phenyl groups and S heteroatoms, high surface area, and blend of micro-and mesoporosity. All these factors would ensure higher mobility of the electrolyte to the surface of the electrode, resulting in accelerated mass transport and enhanced electrochemical performance. Figure 7d-f display the GCD behavior of the POSS-OXD, POSS-Th, and POSS-TPTh POIPs, respectively, measured at various current densities. In the discharge profiles, each porous POSS system provided a nearly triangular GCD curve with a small bend, indicating the combined impacts of pseudocapacitance and EDLC. Furthermore, the charging profiles were smaller than the discharging curves throughout the GCD analyses, indicating that all the porous POIPs displayed excellent capacitance. The discharging profile of the POSS-TPTh POIP was more significant than those of the POSS-Th and POSS-OXD POIPs, indicating its relatively high specific capacitance. It is worth mentioning that the IR drop in the galvanostatic charge-discharge profile is related to the equivalent series resistance, charge-transfer resistance, and doublelayer capacitance of the energy storage devices, such as supercapacitors. A supercapacitor has its own stored energy in the form of internal resistance which is lost during this process [80]. Figure 8a reveals that the specific capacitances of the POSS-OXD, POSS-Th, and POSS-TPTh POIPs were 119, 213, and 354 F g -1 , respectively, at a current density of 0.5 A g -1 . Furthermore, the specific capacitance gap remained large when measured at a higher current density of 20 A g -1 , where the POSS-OXD, POSS-Th, and POSS-TPTh POIPs had specific capacities of 16, 15, and 30.4 F g -1 , respectively. We suggest that the structure of the POSS-TPTh POIP was responsible for its relatively outstanding performance, with its electron-rich phenyl rings and S heteroatoms allowing the electrolytes to approach the electrode surface more rapidly than in the POSS-OXD and POSS-Th POIPs. The specific capacitances of all three porous POIPs decreased upon increasing the current density from 0.5 to 20 A g -1 , presumably because of insufficient time for ion transportation at higher current densities. Moreover, the energy density of the POSS-TPTh POIP (176.75 W h kg -1 ) was also higher than those of the POSS-Th (106.59 W h kg -1 ) and POSS-OXD (59.57 W h kg -1 ) POIPs (Figure 8b). Furthermore, the charging profiles were smaller than the discharging curves throughout the GCD analyses, indicating that all the porous POIPs displayed excellent capacitance. The discharging profile of the POSS-TPTh POIP was more significant than those of the POSS-Th and POSS-OXD POIPs, indicating its relatively high specific capacitance. It is worth mentioning that the IR drop in the galvanostatic charge-discharge profile is related to the equivalent series resistance, charge-transfer resistance, and double-layer capacitance of the energy storage devices, such as supercapacitors. A supercapacitor has its own stored energy in the form of internal resistance which is lost during this process [80]. Figure 8a reveals that the specific capacitances of the POSS-OXD, POSS-Th, and POSS-TPTh POIPs were 119, 213, and 354 F g −1 , respectively, at a current density of 0.5 A g −1 . Furthermore, the specific capacitance gap remained large when measured at a higher current density of 20 A g −1 , where the POSS-OXD, POSS-Th, and POSS-TPTh POIPs had specific capacities of 16, 15, and 30.4 F g −1 , respectively. We suggest that the structure of the POSS-TPTh POIP was responsible for its relatively outstanding performance, with its electron-rich phenyl rings and S heteroatoms allowing the electrolytes to approach the electrode surface more rapidly than in the POSS-OXD and POSS-Th POIPs. The specific capacitances of all three porous POIPs decreased upon increasing the current density from 0.5 to 20 A g −1 , presumably because of insufficient time for ion transportation at higher current densities. Moreover, the energy density of the POSS-TPTh POIP (176.75 W h kg −1 ) was also higher than those of the POSS-Th (106.59 W h kg −1 ) and POSS-OXD (59.57 W h kg −1 ) POIPs (Figure 8b Finally, we examined the stabilities of all three POIPs through GCD analyses over 2000 cycles at 10 A g -1 (Figures 9a and S12-14). The POSS-OXD, POSS-Th, and POSS-TPTh POIPs displayed outstanding cycling stabilities, with retention of their capacitance of 93.41, 96.27, and 97.56%, respectively. In addition, the specific capacitances of both the POSS-Th and POSS-TPTh POIPs were excellent when compared with those of previously published porous frameworks and porous polymer composite materials (Figure 9b) [27,28,58].  Finally, we examined the stabilities of all three POIPs through GCD analyses over 2000 cycles at 10 A g −1 (Figures 9a and S12-14). The POSS-OXD, POSS-Th, and POSS-TPTh POIPs displayed outstanding cycling stabilities, with retention of their capacitance of 93.41, 96.27, and 97.56%, respectively. In addition, the specific capacitances of both the POSS-Th and POSS-TPTh POIPs were excellent when compared with those of previously published porous frameworks and porous polymer composite materials (Figure 9b) [27,28,58]. Finally, we examined the stabilities of all three POIPs through GCD analyses over 2000 cycles at 10 A g -1 (Figures 9a and S12-14). The POSS-OXD, POSS-Th, and POSS-TPTh POIPs displayed outstanding cycling stabilities, with retention of their capacitance of 93.41, 96.27, and 97.56%, respectively. In addition, the specific capacitances of both the POSS-Th and POSS-TPTh POIPs were excellent when compared with those of previously published porous frameworks and porous polymer composite materials (Figure 9b) [27,28,58].  Additionally, we investigated the electrochemical characteristics of all three POSS-POIP samples in two electrode systems for a symmetric supercapacitor through coin cells. The electrochemical characteristics were conducted between a potential range of 0 to −0.2 V at different scan rates, as shown in Figure S15. The CV profiles of all the POSS-POIPs electrodes (Figure S15a-c) were approximately rectangular with the presence of humps in the low potential window-the expected response of supercapacitors due to the existence of EDLC and pseudocapacitive capacitive characteristics. The pseudocapacitance response was caused by the introduction of heteroatoms (N, O, and S) in the backbone of the POSS-POIPs. The functionality of the electrodes seemed to be stable at high scan rates, indicating that the current density also increased, revealing improved stability and a symmetric capacity of all POSS-POIPs materials. Furthermore, the GCD profiles (Figure S15d-f) were nearly triangular with a bent shape due to the presence of heteroatoms in all the POSS-POIPs, demonstrating EDLC and pseudocapacitance effects. The specific capacitance was observed as 41.05, 27.63 and 7.11 F g −1 for POSS-TPTh POIP, POSS-Th POIP, and POSS-OXD POIP electrode samples, respectively, at 0.5 A g −1 . Thus, these results revealed that POSS-TPTh POIP experienced superior electrochemical properties to POSS-Th POIP, and POSS-OXD POIP. The superior electrochemical properties of the POSS-TPTh POIP sample were attributed to its high surface area and electron-rich phenyl group in its structure. In order to understand the resistance behavior of these as-prepared electrodes as symmetric supercapacitors devices, we carried out the impedance measurement, as shown in Figure S16. The EIS plots reveal the ohmic resistance, which is also known as the series resistance, of these electrode materials to be 2.083, 3.799 and 4.491 Ω for POSS-TPTh POIP, POSS-OXD POIP and POSS-Th POIP, respectively. This confirms the superior performance of the POSS-TPTh POIP electrodes compared to the others by virtue of their smallest series resistance offering better conductivity. Apart from this, all the electrodes displayed a clear semicircle in the higher frequency region which corresponds to their charge-transfer resistance. As indicated in the plots, the charge-transfer values obtained from these electrodes were 4.479, 13.72 and 20.16 Ω, for POSS-TPTh POIP, POSS-OXD POIP and POSS-Th POIP, respectively. The electrode composed of POSS-TPTh POIP offers almost negligible charge-transfer resistance, with improved conductivity compared to the others. Thus, POSS-TPTh POIP is an outstanding electrode material for energy storage which has the potential to be used in real-life applications.

Conclusions
In conclusion, we used Heck reactions to synthesize the hybrid OVS-based POIPs POSS-OXD, POSS-Th, and POSS-TPTh. TGA revealed that the thermal stability of the POSS-Th POIP (T 10 : 785 • C; char yield: 90 wt%) was higher than those of the POSS-OXD and POSS TPTh POIPs. N 2 isothermal profiles and electrochemical analyses indicated that POSS-TPTh POIP had the highest BET surface area (682 m 2 g −1 ) and an excellent specific capacity (354 F g −1 ) when compared with those of the other two POIPs. The superior energy storage performance of the POSS-TPTh POIP was due to its high surface area and electron-rich phenyl groups, which were not present in the POSS-OXD and POSS-Th POIPs. In addition to this, we evaluated the electrochemical performance of a symmetric supercapacitor two-electrode device which exhibited great potential for energy-storage. These findings suggest that the POSS-TPTh and POSS-Th POIP materials, with their high surface areas, might find application in wastewater treatment and in Li and Li-S batteries.