5. C HAPTER 3. E LECTRONIC C OUPLING E NGINEERING OF C ONJUGATED
5.2. R ESULTS AND D ISCUSSION
5.2.2. Materials Characterization: Solid-State NMR
The TPA-based CMPs were synthesized by previously reported ultrasonic spray synthesis method.15 Spectroscopy investigations were conducted with 13C cross-polarization magic angle spinning solid-state NMR (CP-MAS NMR) to confirm the cross-linked structures of the TPA-based CMPs. Figure 5.4a–5.4c shows the assigned carbon atom of 13C CP-MAS NMR spectra. These assignments were analyzed by 13C NMR spectra of the liquid-state monomers. To clearly observe the cross-linked carbon peak clearly, we performed the peak fitting of 13C CP-MAS NMR spectra for the monomers and CMPs using the Gaussian function (Figure 5.4d–5.4f). Overall, no peak at near 135 ppm was observed in the spectra of the monomers, excluding that for M-TPA of the TPA dimer structure.
After polymerization, the peak intensity of para-carbon of TPA at ~125 ppm (1* in Figure 5.4) decreases, and the cross-linked carbon peak appears at ~135 ppm, as highlighted in Figure 5.4d–5.4f.
Specifically, M-BT has a distinct band at approximately 125 ppm, assigned to carbons 1*, 3, and 6 in Figure 5.4a and 5.4d. After polymerization, the intensity of peak near 125 ppm decreases in the CMP-BT. This is because the para-carbons of TPA units (1* in Figure 5.4a) react with oxidizing agents of FeCl3, resulting in the cross-linked structure of CMP-BT. Although the carbon atoms (3 and 6 in Figure 5.4a) are also reactive sites for oxidative coupling because of their electron sufficient property, the significant steric hindrance of the adjacent phenyl ring prevents reaction. As shown in Figure 5.4d, a new shoulder peak appears at approximately 135 ppm (a* in Figure 5.4a), which corresponds to the cross-linked carbon. This assignment was confirmed using 13C NMR of M-TPA (Figure 5.4b and 5.4e).
M-TPA is the para-position C–C coupled TPA dimer structure, which is the composition of cross-linked structure in CMP-BT. Therefore, the carbon peak (8* in Figure 5.4b) is exhibited at 135 ppm in Figure 5.4e, which is a good agreement with the cross-linked carbon peak of CMP-BT (a* in Figure 5.4d).
Furthermore, the spectra of CMP-TPA and CMP-DTT also exhibit the cross-linked carbon peak at ~135 ppm and the decreased peaks at approximately 125 ppm corresponding to 1*, 3, and 6 in M- TPA and M-DTT (Figure 5.4e and 5.4f).
Figure 5.4. 13C CP-MAS NMR spectra of the TPA-based CMPs. (a–c) Peak assignments of 13C CP- MAS NMR spectra. (d–f) 13C CP-MAS NMR spectra of the TPA-based monomers and CMPs with fitted curved. The sharp peaks are the liquid-state 13C NMR spectra of the monomers. The highlight pattern is the cross-linked carbon peak of the CMPs.
5.2.3. Materials characterizations: FTIR
The network structures of the TPA-based CMPs were confirmed further by FTIR investigations.
The FTIR peaks were assigned by the DFT calculated IR spectra of the monomers. As shown in Figure 5.5, FTIR spectra of the TPA-based monomers exhibit strong vibration bands at approximately 1600, 1500, and 1280 cm−1, which correspond to the carbon-hydrogen bending and stretching of the carbon- carbon double bond of the TPA unit,19,20 as evidence of the DFT calculated IR spectra (Figure 5.6–5.9).
After polymerization reaction, all these TPA peaks are shifted to higher wavenumber because network structures of the CMPs restrict vibrations of the TPA units. This result suggests that the TPA-based CMPs consist of cross-linked TPA units.
Figure 5.5. FTIR characterizations. (a–c) FTIR spectra of the TPA-based monomers and CMPs on quartz glass with DFT-calculation results of the monomers.
Figure 5.6. DFT calculated IR spectra of monomers. These DFT calculation results were corrected by −30 cm−1.
Figure 5.8. DFT-calculated vibrational modes of M-DTT.
Figure 5.9. DFT-calculated vibrational modes of M-BT.
5.2.4. Materials characterizations: UV–Vis Spectroscopy
The extension of π-conjugated system in the TPA-based CMPs was confirmed by ground-state electronic absorption spectra. As shown in Figure 5.10, two characteristic absorption bands of the TPA- based monomers exhibit at 300–550 nm. DFT calculations were performed for peak assignments (Figure 5.11). The absorption peaks at 300–350 nm are assigned to local aromatic π–π* transitions, and the absorption peaks at 350–400 nm correspond to HOMO–LUMO transition. It is noteworthy that an ICT peak was observed at ~470 nm in M-BT owing to the strong electronic coupling of HTPA|BT (3100 cm−1). The TPA-based CMPs show broader and bathochromic-shifted electronic absorption spectra compared with corresponding to the monomers, which is attributed to the extension of π-conjugated system.21,22
Figure 5.10. Electronic absorption spectra of the TPA-based monomers and CMPs. Inset:
photographs of CMPs on quartz glass. (a) CMP-BT and M-BT, (b) CMP-TPA and M-TPA, and (c) CMP-DTT and M-DTT.
Figure 5.11. DFT calculation results of the TPA-based monomers. (a) Ground-state optimized geometries of the TPA-based monomers and corresponding to their HOMO and LUMO. (b) PDOS of the entire monomers and the molecular fragments (part 1 and 2). Percentage contributions of part 1 and 2 to MO density in HOMO and LUMO are indicated in each panel. (c) TD-DFT calculated electronic absorption spectra of the TPA-based monomers. MOs composition in the electronic transition (S1) is given in each panel.
5.2.5. Materials characterizations: Porosity
Scanning electron microscopy (SEM) measurements were performed to observe the porous structures of the TPA-based CMPs. The surface and cross-sectional SEM images are shown in Figure 5.12. All the TPA-based CMPs exhibit ca. 300 nm-thick films with porous surface morphologies (Figure 5.12). Specific surface areas (SSAMB) were evaluated by methylene blue (MB) analysis method.14,23 Figure 5.13 elucidates that CMP-BT has the largest SSAMB (542 ± 13 m2/g), followed by CMP-TPA (372 ± 17 m2/g) and CMP-DTT (336 ± 17 m2/g). This is because the strong electronic coupling of HTPA|BT (3100 cm−1) in CMP-BT would lead to better cross-linking during the polymerization. Furthermore, the electron-withdrawing substituent in monomers significantly accelerates the reaction activity during the oxidative coupling reaction.24,25
Figure 5.12. SEM images of the TPA-based CMPs. (a–c) Cross-section and (d–f) surface images.
CMPs electrodes were fabricated on ITO glass substrates.
Figure 5.13. Evaluation of surface area using MB adsorption method. UV–vis absorption spectra of initial and equilibrium MB solutions.
The porous characteristics of the TPA-based CMPs were further confirmed by N2 sorption measurements. We focused on CMP-BT because of its high SSAMB and superior supercapacitor properties compared with CMP-DTT and CMP-TPA. As shown in Figure 5.14a, CMP-BT yields combined type-I and -II N2 sorption isotherms involving rapid N2 sorption at low relative pressures (P/P0 < 0.05) and some N2 sorption at high relative pressures (P/P0 > 0.9). Both trends can be interpreted as resulting from microporosity and inter-particulate porosity associated with the meso- and macroporosity, respectively.26,27 The Brunauer–Emmett–Teller (BET) specific surface areas (SSABET) and t-plot micropore areas were calculated from N2 sorption isotherms. CMP-BT exhibits SSABET of 280 m2/g.
In particular, the microporosity contributes significantly to the SSABET considering the high micropore area; the microporosity proportions with respect to SSABET of CMP-BT was ca. 79%. Using non-local density functional theory (NLDFT) method, the pore size distribution (PSD) profiles of CMP- BT were obtained, revealing a narrow PSD having a dominant pore width at approximately 0.7 nm (Figure 5.14b), consistent with the median micropore width of 0.69 nm obtained from the Horvath–
Kawazoe model. The N2 sorption isotherm and PSD results further confirm the predominant microporous structures of CMP-BT. This N2 isotherm profile is in good agreement with nano-size pores in CMP-BT observed from the high-resolution transmission electron microscopy (HR-TEM) result (Figure 5.15).
Figure 5.14. Porous network properties of CMP-BT. (a) N2 isotherms for CMP-BT. The closed and open symbol mean the N2 adsorption and desorption, respectively. (b) PSD curves calculated from NLDFT for CMP-BT.
Figure 5.15. HR-TEM result showing nano-size pores of CMP-BT.
5.2.6. Electrochemical analysis of CMP films
The semiconducting property and high surface area of the TPA-based CMPs are suitable for application in supercapacitors. Hence, the energy storage properties of the TPA-based CMPs were investigated by measuring CV in a three-electrode configuration. The TPA-based CMPs show clear redox peaks at 1.3–1.5 V vs. Ag/AgCl, corresponding to the faradic reactions of the TPA moiety in CMPs. The redox peaks are typical characteristics of pseudocapacitor driven by electrochemical reaction (Figure 5.16a). As shown in Figure 5.16b, the energy storage process of the TPA-based CMPs can be rationalized by the electrochemical reaction between the triphenylamine moiety in the TPA- based CMPs and electrolytes (Bu4NPF6).28,29 Moreover, the color change of the TPA-based CMPs in the redox process, called electrochromism, is an additional character of the pseudocapacitor, resulting from the optoelectronic properties of triphenylamine moiety (Figure 5.16c).30-33
Figure 5.16c shows specific gravimetric capacitances of the TPA-based CMPs with respect to scan rate, which is calculated from the CV curves (Figure 5.17). The energy-storage performances of the TPA-based CMPs depend strongly on the degree of electronic coupling between the TPA and adjacent units. The gravimetric and areal specific capacitances are summarized in Table 5.2 and 5.3.
Among the TPA-based CMPs, CMP-BT exhibited the highest charge-storage performance (250 F/g at 10 mV/s), followed by CMP-TPA (90 F/g at 10 mV/s) and CMP-DTT (50 F/g at 10 mV/s). In a high scan rate of 200 mV/s,CMP-BT retains the specific capacitance by 75%, 65% for CMP-TPA, and 54%
explanations. i) The strong electronic coupling (3100 cm−1) between the triphenylamine and the BT moiety enhances the ICT property of CMP-BT.34 ii) The monomer structure of CMP-BT exhibits the most polarized structure resulting from its high electronic coupling (HTPA|BT), as shown in the electrostatic potential (ESP) mapping result (Figure 5.16d). This polarized structure provides better electrochemical and electrostatic interactions between the active materials and electrolytes. iii) The high porous structure of CMP-BT delivers efficient ion channels for the mass transfer of electrolytes.8,22 iv) Swelling-shrinking occurs during charge-storage processes, and this effect should be suppressed to maintain the specific capacitance at a high scan rate. The rigid 3D network structure of CMP-BT can effectively suppress the swelling-shrinking effects, resulting in the high rate capability.7,35
Figure 5.16. Energy storage performance and analysis of the TPA-based CMPs. (a) CV results of the TPA-based CMPs at 100 mV/s. (b) Charge-storage process and (c) reversible electrochromic property of CMP-BT. (c) Specific capacitances of the TPA-based CMPs as a function of scan rate, which are calculated by the CV results. (d) ESP results of the TPA-based monomers. Indicated values are dipole moment.
Figure 5.17. CV results of the TPA-based CMPs at different scan rate. (a–c) CV curves of gravimetric current density versus potential. (d–f) CV curves of areal current density versus potential.
Table 5.2. Gravimetric capacitances of the TPA-based CMPs at different scan rates.
Materials
Gravimetric capacitance (F/g) Rate
capability 10 mV/s 30 mV/s 50 mV/s 100 mV/s 200 mV/s (%)
CMP-TPA 91.4
(83.5 ± 6.1)
82.7 (75.5 ± 5.6)
75.4 (68.8 ± 5.1)
69.4 (63.4 ± 4.7)
62.0
(56.6 ± 4.2) 67.8
CMP-DTT 49.0
(43.4 ± 4.7)
42.1 (37.3 ± 4.1)
35.9 (31.6 ± 3.8)
30.9 (27.2 ± 3.3)
26.5
(23.3 ± 2.8) 54.1
CMP-BT 251
(238 ± 11)
204 (194 ± 9)
198 (188 ± 9)
193 (183 ± 9)
187
(178 ± 8) 74.8
Table 5.3. Area capacitances of the TPA-based CMPs at different scan rates.
Materials
Area capacitance (mF/cm2)
10 mV/s 30 mV/s 50 mV/s 100 mV/s 200 mV/s
CMP-TPA 4.75
(4.34 ± 0.32)
4.30 (3.93 ± 0.29)
3.92 (3.58 ± 0.26)
3.61 (3.30 ± 0.24)
3.22 (2.94 ± 0.22)
CMP-DTT 3.09
(2.74 ± 0.30)
2.65 (2.35 ± 0.26)
2.26 (1.99 ± 0.24)
1.95 (1.71 ± 0.21)
1.67 (1.47 ± 0.18)
CMP-BT 16.8
(15.9 ± 0.7)
13.6 (13.0 ± 0.6)
13.2 (12.6 ± 0.6)
12.9 (12.3 ± 0.6)
12.6 (11.9 ± 0.5)
5.2.7. The LBL structured CMP/CNT supercapacitor
The main weakness of CMP-BT is its inferior cycle stability owing to its poor electric conductivity (<2 μS/cm2), which is a typical limitation of the CMP-based materials. Moreover, CMP- BT only possesses nano-size pores, which is also a limation in ion diffusion. To address these issues, we synthesized the LBL structured supercapacitor composed of CMP-BT and CNTs using the ultrasonic spray deposition method15, as shown in Figure 5.18a. SEM image of Figure 5.18b shows hierarchical porous morphology of CMP-BT/CNT, resulting from the microporosity of CMP-BT and the macroporosity of the CNTs. The LBL structured CMP-BT/CNT supercapacitor exhibits a higher surface area of 770 m2/g than CMP-BT of 540 m2/g, allowing efficient electrostatic and electrochemical interactions between the active materials and electrolytes. As shown in the inset of Figure 5.18c, the LBL structured CMP-BT/CNT supercapacitor shows a rectangular-shaped I–V curve with a peak at 1.3–
1.6 V corresponding to the Faradaic reaction process. This result indicates that the LBL structured CMP- BT/CNT supercapacitor is a hybrid supercapacitor possessing both EDLC and pseudocapacitive behaviors. Furthermore, the LBL structured CMP-BT/CNT supercapacitor exhibits a higher energy storage performance (450 F/g at 100 mV/s) than that of CMP-BT (Figure 5.18c) because of its better surface areas and conductivity (Figure 5.18d). In addition, the robust cycle stability of the LBL structured CMP-BT/CNT is confirmed by 95% capacitance retention after twenty thousand galvanostatic charge–discharge (GCD) cycles (Figure 5.18e).
Figure 5.18. The LBL structured CMP/CNT supercapacitor. (a) Synthesis of the LBL structured
CMP-BT/CNT supercapacitor by ultrasonic spray deposition method.15 (b) Electron microscopy image of the LBL structured CMP-BT/CNT supercapacitor. (c) Gravimetric specific capacitances of the LBL structured CMP-BT/CNT supercapacitor and CMP-BT in a SC configuration at 100 mV/s. Inset:
corresponding CV results. (d) EIS results of the LBL structured CMP-BT/CNT supercapacitor and CMP-BT. (e) Stability profile of the LBL structured CMP-BT/CNT supercapacitor at 100 mV/cm2. Inset:
Initial and final results of the LBL structured CMP-BT/CNT supercapacitor.
5.2.8. Asymmetric Supercapacitor: CMP-BT/CNT//RGO/CNT
To improve the charge storage performance of the TPA-based CMPs further, an asymmetric supercapacitor (ASC) was assembled using the LBL structured CMP-BT/CNT electrode and the similar LBL structured RGO/CNT electrode14, as described in Figure 5.19a. Potential windows of the LBL structured RGO/CNT and the LBL structured CMP-BT/CNT are 0–1.0 and 0.9–1.7 V, respectively, as shown in Figure 5.19b. The working potential of the CMP-based ASC was optimized by measuring CV and GCD (Figure 5.19c and 5.19d). The optimized working potential of the CMP-based ASCs is 0–1.4 V. As shown in Figure 5.19e, the CMP-based ASC exhibits both EDLC and pseudocapacitor properties and retains the CV curves when a scan rate increases from 10 to 200 mV/s. The energy storage performances of the CMP-based ASC were evaluated by GCD results (Figure 5.19f), and their values are summarized in Figure 5.19g. The CMP-based ASC exhibits a high specific capacitance of 450 F/g at 1.0 A/g and maintains its capacitance by 72% when the current density increases up to 20 A/g. Furthermore, the CMP-based ASC shows over 95% high Columbic efficiencies at all current densities (1–20 A/g). Robust cycle stability of the CMP-based ASC is confirmed by ~95% retention efficiency after ten thousand cycles at 10 A/g, as shown in Figure 5.19h. Owing to a broad potential window and high specific capacitances, the CMP-based ASC can achieve higher energy and power densities (130 Wh/kg and 14000 W/kg) than other supercapacitors, as shown in Ragone plots of Figure 5.20.11-14,36-42
Figure 5.19. Energy storage performance of the CMP-based ASC. (a) Device structure of the CMP- based ASC. The membrane is omitted out in the figure. (b) CV results of the negative and positive electrodes of the CMP-based ASC at 100 mV/s. (c and d) Potential window optimization of the CMP- based ASC. (e) CV and (f) GCD results of the CMP-based ASC. (g) Energy storage performances of the CMP-based ASC evaluated by the GCD results. Their coulombic efficiencies are denoted in paraentheses in the panel. (h) Stability profile of the CMP-based ASC. Current density in GCD was 10 A/g. Inset: Initial and final results of the CMP-based ASC.
Figure 5.20. Ragone plots of the CMP-based ASC and other supercapacitors using organic materials. Filled and open symbols indicate asymmetric and symmetric supercapacitors, respectively.
5.3.CONCLUSION
Three TPA-based CMPs were synthesized for energy storage applications. Their electronic couplings were controlled by introducing donor or acceptor core units. Changing the electronic coupling of the monomers in CMPs significantly affects the reaction activity, surface area, and energy storage performances. All the TPA-based CMPs have microporous structure and pseudocapacitive behavior, where CMP-BT exhibits the highest specific capacitance because of its high surface area and polarized structure driven by strong electronic coupling between the TPA and BT units. Furthermore, an LBL hybrid active electrode (CMP-BT/CNT) shows a high energy storage performance of 446 F/g at 100 mV/s in an SC. Furthermore, a CMP-based ASC achieves a high energy density (94–130 Wh/kg).
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