4. C HAPTER 2. E LECTRONIC C OUPLING E NGINEERING FOR R EALIZING V ECTORIAL
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5
Electronic Coupling Engineering of Conjugated Microporous Polymers for Energy Storage Applications
This chapter includes the published contents:
Deok-Ho Roh†, HyeonOh Shin†, Hyun-Tak Kim†, and Tae-Hyuk Kwon* ACS Appl. Mater.
Interfaces. 2021, 13, 61598-61609. DOI: 10.1021/acsami.1c13755. Reproduced with permission of Copyright © 2021 American Chemical Society.
5.1.INTRODUCTION
Organic materials for energy storage have received significant attention over several decades owing to their naturally abundant elements, easy recycling, inexpensive, and low pollution.1-4 Among the organic materials, CMPs have shown potential as energy-storage materials due to their semiconducting properties, three-dimensional porous structures, intrinsic high surface areas, and nano- size pores.5,6 These CMP materials can store energy via electrostatic and electrochemical interactions with ions in electrolytes. Electrostatic interactions are directly connected to nano- and micro-structure properties of active materials, such as pore size and surface area.7 To enhance the electrostatic interactions, the physical structures of CMPs were controlled by molecular units and synthesis conditions.8-10 On the other hand, electrochemical reactions in CMPs occur at redox-active molecular units, such as amine, carbonyl, nitrile, azo, and imine groups.1,11,12 Energy-storage performance via electrochemical interactions is mainly associated with the number of redox-active sites, redox activity and porosity of active organic materials. Therefore, one way to enhance the energy density of active
organic materials is to increase the redox-active sites and modify their molecular units.2,4,13 However, molecular engineering strategy of active organic materials for increasing redox activity was underexplored, even though it is important to maximize their energy-storage performance.
Herein, we controlled electronic coupling in monomers of CMPs to improve energy storage performances. Three triphenylamine (TPA)-based monomers with donor–π–donor configurations (M- TPA, M-DTT, and M-BT) were designed, and their electronic coupling of the monomers was modulated by introducing different electron-donating or electron-withdrawing π-spacers (Figure 5.1a). Based on these monomers, three TPA-based CMPs (CMP-TPA, CMP-DTT, and CMP-BT) were synthesized via oxidative carbon–carbon (C–C) coupling reaction (Figure 5.1b). The polymerization yields, surface areas, and electrochemical properties of the TPA-based CMPs depend strongly on the electronic coupling values of the monomers. In particular, the strong electronic coupling between the TPA and the adjacent units improves the redox activity of CMPs, resulting in CMP-BT exhibiting a higher specific capacitance (250 F/g) than those of other CMPs. To enhance energy storage performance further, we developed layer-by-layer (LBL) structured active electrodes composed of CMP-BT and carbon nanotubes (CNTs). These hybrid electrodes exhibit microporosity and microporosity required in a high- performance supercapacitor. Furthermore, an asymmetric supercapacitor comprising the CMP-based electrode as the positive electrode and a carbon-based electrode14 as the negative electrode demonstrates a high energy-storage performance.
Figure 5.1. Chemical structures of the TPA-based monomers and CMPs. (a) Schematic of electronic coupling in the TPA-based monomers. (b) Synthetic scheme of the TPA-based CMPs, which were synthesized by previously reported ultrasonic spray synthesis method.15
5.2.RESULTS AND DISCUSSION
5.2.1. Monomer Design Strategy and Electronic Coupling
Because the para-position hydrogen atoms of TPA can react with an oxidative agent (FeCl3), we designed three TPA-based monomers, which form a network polymer structure through oxidative coupling reaction (Figure 5.1). Different π-spacer units were introduced into the monomers to modulate the electronic coupling (HTPA|π) between the TPA and π-spacer, and the electronic coupling (HTPA|TPA) between the TPA units; dithienothiophene (DTT) for M-DTT and benzothiadiazole (BT) for M-BT, but M-TPA had no π-bridge units as a control material. Figure 5.2 shows the synthesis scheme of the monomers, and detailed procedures are summarized in section 2.2.2.
Figure 5.2. Synthetic scheme of the TPA-based monomers. The synthesis of intermediate and final compounds were synthesized by previous reported procedures.16-18
The electronic values of HTPA|π and HTPA|TPA were evaluated by measuring the electronic absorption spectra of the one-electron oxidized-state TPA-based monomers. In the oxidized state, the electronic absorption peaks appear at 750–2000 nm (Figure 5.3). These peaks correspond to IVCT transitions between molecular units in the monomers (Figure 5.3b). DFT calculations were performed to assign these IVCT peaks (Figure 5.3c). As shown in Figure 5.3b and 5.3c, the NIR absorption bands at 1000–2000 nm (IVCT 1) and 750–1000 nm (IVCT 2) are the IVCT transitions between the TPA units, and between the TPA and π-spacer, respectively. The electronic coupling between the molecular units was calculated by following the Mulliken–Hush equation:23
HD|A = j4.2×10–4 · εmax · Δv1/2 · Eabs ®d (5.1) where HD|A (cm−1) is the electronic coupling between D and A units, Eabs (cm−1) is the energy of IVCT transition maximum, ∆v1/2 (cm−1) is FWHM of the fitted curve, εmax (M−1 cm−1) is the molar extinction
coefficient of transition band, and d (Å) is distance between D and A. The distance values were estimated by DFT calculation. The distance, HTPA|π, and HTPA|TPA values are summarized in Table 5.1.
M-BT exhibits a high HTPA|BT value of 3100 cm−1 owing to the strong electron-withdrawing group (BT), resulting in efficient ICT. On the other hand, HTPA|TPA of 700 cm−1 in M-BT is a lower than that of M- TPA (HTPA|TPA = 2100 cm−1) because of a long distance (Å) between the TPA units in M-BT. M-DTT exhibits a lower HTPA|DTT of 2100 cm−1 than the corresponding HTPA|BT of 3100 cm−1 because the DTT core unit is an electron-donating group, and it could not lead to efficient ICT in M-DTT.
Table 5.1. Electronic coupling values and distance between the molecular units.
M-TPA M-DTT M-BT
dTPA|π (Å) - 9.0 7.2
dTPA|TPA (Å) 9.9 17 14
HTPA|π (cm−1) - 620 700
HTPA|TPA (cm−1) 2100 2100 3100
Figure 5.3. Evaluation electronic coupling of the TPA-based monomers. (a) Electronic absorption spectra changes of the TPA-based monomers. Monomer solutions are 50 µM in CH3CN/CHCl3, and the ratio is 19:1 (v/v). Oxidizing agent solution of Cu(ClO4)2 is 10 mM in CH3CN. (b) Electronic absorption spectra of the TPA-based monomers in oxidized state. (c) DFT calculated electronic absorption spectra
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.