Thienothiophene and single-wall carbon nanotube-based hybrid materials: design, photophysical properties and the construction of high-performance supercapacitors

Abstract

Supercapacitors are widely accepted to be highly promising for energy storage due to their high capacitance and power density with super-long cycling stability. In addition, flexible and binder-free nanomaterials play a crucial role in supercapacitor devices and systems. Herein, we present thienothiophene (TT) and single-wall carbon nanotube (SWCNT)-based two hybrid materials, possessing triphenylamine (TPA), thiophene (Th) and EDOT moieties, i.e.TT-Th-TPA-SWCNT and TT-EDOT-TPA-SWCNT, as highly efficient supercapacitors with flexible and free-standing properties. The nanohybrids were obtained by noncovalent modifications of SWCNTs without using any binding agents. Their hybrid electrodes displayed remarkable supercapacitor performances and energy storage properties with an excellent power density of 10 000 W kg⁻¹ at 20 A g⁻¹, a maximum energy density of 5.19 ± 0.13 Wh kg⁻¹ at 0.1 A g⁻¹ and a maximum specific capacitance of 158 F g⁻¹ at 1 mV s⁻¹. Regarding the GCD results, 10 000 cycle stability was achieved with a coulombic efficiency of over 95%. These findings highlight the potential of TT and SWCNT-based hybrid materials as advanced electrodes in energy storage applications.

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1. Introduction

Energy has become an important issue in the 21st century due to the rapid depletion of fossil fuels and the increase in environmental pollution caused by their consumption. There is a growing need for high-efficiency renewable energy storage devices that are environmentally sustainable.^1,2 Among the various energy storage and conversion technologies, electrochemical systems such as batteries, fuel cells and electrochemical supercapacitors are considered critical. Electrochemical supercapacitors, also known as supercapacitors, ultracapacitors and electrochemical double-layer capacitors, have attracted particular attention due to their significantly high specific capacitance, long life cycle, high power density, low maintenance, no memory effect and safety capabilities.^3–5 In a typical supercapacitor setup, usually electrodes, electrolytes, separators and current collectors are the key components. Textural and morphological features at the interface between the electrolyte and the material greatly impede capacitance when the bulk of the material is restricted from accessing the electrolyte. Consequently, it is vital to explore a new category of electrode materials to achieve superior performance in supercapacitors.^6,7

Numerous nanomaterials, including carbon nanostructures, have been investigated as potential electrodes for supercapacitors due to their large surface area, high electrical conductivity and stability in electrochemical environments.^8,9 Carbon nanomaterials, such as carbon nanotubes, graphene, activated carbon and carbon nanocages, are the most commonly studied and applied materials for supercapacitor electrodes.¹⁰ Carbon nanotubes (CNTs) are one-dimensional nanostructures that are extensively used and among the most promising candidates for energy storage applications. CNTs have a high surface area, a large surface-to-weight ratio and offer excellent storage capacity. Thanks to sp² hybridization in carbon–carbon (C–C) bonds, CNTs exhibit extraordinary electrical, thermal and mechanical properties.^11,12 They are categorized into two structural types: (i) multi-wall carbon nanotubes (MWCNTs), consisting of multiple concentric graphene cylinders with an interlayer spacing around 0.34 nm, and (ii) single-wall carbon nanotubes (SWCNTs), formed by rolling a single graphene sheet into a seamless cylindrical tube.¹³ The capacitance characteristics of CNT systems are influenced by various factors, including the number of graphene layers, type of electrode material, composition of the electrolyte solution and arrangement of the CNT layer. Pristine CNTs have a relatively low specific capacitance, typically ranging from 2 to 45 F g⁻¹ for SWCNTs and from 3 to 74 F g⁻¹ for MWCNTs.^14,15 Although both SWCNTs and MWCNTs exhibit excellent conductivity, SWCNTs offer greater advantages for supercapacitor applications. This is primarily due to their superior surface area, higher conductivity and better interconnectivity.¹⁶ To improve the properties of SWCNTs, there is an increasing interest in developing cost-effective and industrially applicable modifications. These methods are broadly categorized into two types; (i) covalent (chemical) functionalization and (ii) non-covalent (physical) functionalization of SWCNT sidewalls.^13,17 Non-covalent functionalization is particularly advantageous over covalent methods as it preserves the sp²-to-sp³ hybridization, the intrinsic conjugation and homogeneity of the SWCNT surface, which can otherwise be disrupted by covalent modifications such as halogenation and acylation reactions. Various non-covalent modification methods are used to tailor the properties of these hybrids, including polymer wrapping, π–π interactions, electron pull–push complexes, hydrogen bonding and van der Waals forces using bonding agents rich in π-electrons or heteroatoms.^18,19 By exploiting these noncovalent modification techniques, the properties of SWCNT-conjugated semiconductor hybrids are improved for specific applications in various fields, such as electronics, energy storage, sensors and optoelectronics.^20,21

Thieno[3,2-b]thiophene (TT) has an extended π-conjugation with a rigid structure, which makes it an appropriate π-linker for altering the band gap of organic materials and enhancing the intermolecular interactions in the solid state.^22–26 Its electron-rich properties make TT a promising candidate for the development of conjugated polymers with low band gaps, which are crucial for applications in energy-related fields such as organic solar cells, organic light-emitting diodes, organic field-effect transistors and capacitors.^27–37 Triphenylamine (TPA) is a typical and fundamental member of the triarylamine (TAA) family, where the central nitrogen atom forms bonds with three carbon atoms through sp² hybridization. This results in a C–N bond length of 1.42 Å and a C–N–C bond angle of 120°. The three surrounding phenyl groups form a propeller-like structure with a torsion angle of 41.7° relative to the plane of the three C–N bonds. With an ionization potential of 6.80 eV, TPA is a strong electron donor compared to many other organic and inorganic substances.^38–40 Its nonplanar structure is beneficial for reducing intermolecular aggregation and forming amorphous structures. This quality is valuable for optical applications as it enables creation of reliable thin films with consistent morphology and uniform properties. Additionally, para-alkoxy-substituted TPAs are widely used in designing various materials as the alkoxy groups enhance the electron-donating ability, influence the HOMO and LUMO energy levels and, thus, reduce the band gaps.^40–43

The use of carbon-based nanomaterials is widespread in supercapacitor applications.^44,45 In addition, in recent years, metal oxide composites have enabled the achievement of significantly high energy and high power densities.^46–49 However, in many supercapacitor configurations, the active material is coated onto a current collector surface, during which a polymeric binder is employed to ensure strong adhesion of the active material to the current collector. Commonly used polymers for this purpose include PTFE, PVDF and Nafion.⁵⁰ A major drawback of these binders is their non-electroactive nature and high resistivity, which lead to increased IR losses. In this context, the use of electroactive materials that can self-adhere via secondary interactions, thus eliminating the need for current collectors, offers a dual advantage and is emerging as an important research area.^51–54

In this report, highly conjugated electron rich TT based structures, TT-EDOT3-TPA3 and TT-Th3-TPA3, having EDOT, thiophene (Th) and TPA units were designed and synthesized applying Pd(0) catalyzed Stille and Suzuki coupling reactions. Their photophysical, electrochemical and thermal properties were examined by UV-vis, fluorescence, cyclic voltammetry (CV) and thermal gravimetric analysis (TGA) methods. The electron rich structures, TT-EDOT3-TPA3 and TT-Th3-TPA3, were attached to the surface of single wall carbon nanotubes (SWCNTs) through noncovalent interactions, i.e. van der Waals, π–π and S–π, requiring no binding agent, which resulted in the production of TT and SWCNT based hybrid nanomaterials, TT-Th-TPA-SWCNT and TT-EDOT-TPA-SWCNT, as flexible films. The films were characterized in detail by SEM and AFM analyses and used as flexible and binder-free supercapacitor electrodes for energy storage. This report makes a significant contribution to the potential applications of TT and SWCNT-based hybrid nanomaterials for energy conversion and storage processes.

2. Experimental

2.1. Materials

n-Butyllithium (2.5 M in hexane, Across), tributyltin chloride (96%, Sigma-Aldrich), tetrakis(triphenylphosphine)palladium(0) (Pd(PPh₄)₃) (99%, Sigma-Aldrich), hydroquinone 99% (Across) and anhydrous sodium sulfate (99%, Sigma-Aldrich) were obtained from commercial sources and used without further purification unless otherwise mentioned. Dry THF and toluene were obtained via distillation over sodium/benzophenone. Dimethylformamide (HPLC grade) was stored over activated molecular sieves (4 Å). Dichloromethane (Aldrich) was used as received.

2.2. Instrumentation and characterization

All electrochemical experiments were performed using a PARSTAT MC1000 potentiostat galvanostat from Amatek, USA, with a two-electrode split cell setup. Electrodes, each 10 mm in diameter, were cut with a punch and weighed. The split cell was then assembled, using cellulose acetate as a separator and H₃PO₄ solution (85%) as an electrolyte. The working electrode was initially placed into the cell, then, the electrolyte-soaked separator was placed on top of it. The counter electrode, identical to the working electrode, was added, and the cell was sealed. Testing began after the open circuit potential of the cell was stabilized at a constant value. The employed test methods included cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS).

Before starting the CV measurements, the cell was allowed to stabilize at an open circuit potential for 24 hours. The prepared cell underwent 200 cycles between 0 and 1 V to activate the electrodes and facilitate the penetration of ions from the electrolyte onto the electrode surfaces. The CV measurements were conducted at scan rates of 1000, 750, 500, 250, 100, 75, 50, 25, 10, 5 and 1 mV s⁻¹, each comprising three cycles. The second cycle of each measurement is illustrated in the figures. The specific capacitance values reported in the text represent the average of three repeated measurements. Repeated CV cycles were used to calculate the standard deviations. The resulting standard deviation data are presented as error bars along with the mean specific capacitance values. EIS measurements were performed at 10 mV amplitude across a frequency range of 100 000 Hz to 0.1 Hz. The Kramers–Kronig method was used to verify the suitability of the obtained data for fitting with an equivalent circuit model.

In eqn (1), the term m(WE) (g) signifies the mass of the working electrode in contact with the electrolyte. The parameter k represents the scan rate employed in the cyclic voltammetry (CV) measurements. V₁ and V₂ refer to the minimum and maximum voltages, respectively, utilized in the range for determining C_CV.^55,56

Galvanostatic charge–discharge (GCD) tests were performed over a voltage range of 0.0 to 1.0 V at various current densities, including 0.05, 0.10, 0.50, 1.00, 2.00, 2.50, 5.00, 10.00, 15.00 and 20.00 A g⁻¹. The gravimetric specific capacitance (C_GCD, F g⁻¹), specific energy density (E, Wh kg⁻¹) and power density (P, W kg⁻¹) of the supercapacitor were calculated from the galvanostatic charge–discharge (GCD) profiles using the following equations.^57,58

In eqn (2)–(4), I (A) represents the applied current magnitude during the GCD test, Δt (s) denotes the discharge time obtained during the test, m (g) is the total mass of the two electrodes used in the preparation of the cell and ΔV (V) refers to the voltage window (V₂ − V₁), in which the charge–discharge process was conducted.

2.3. Synthesis

2.3.1. Synthesis of 1-(thiophen-2-yl)-2-(thiophen-3-ylthio)ethan-1-one (3). To a solution of 3-bromothiophene 1 (1.0 g, 6.134 mmol) in dry diethyl ether (50 mL) was added n-butyllithium (2.9 mL, 7.36 mmol) dropwise at −78 °C, under a nitrogen atmosphere. After the reaction was stirred for 45 min, elemental sulfur (S₈) (0.21 g, 6.44 mmol) was added and the mixture was further stirred for 45 min. Then, 2-bromo-1-(thiophen-2-yl)ethan-1-one 2 (1.32 g, 6.44 mmol) was introduced in stages into the mixture. After adding the α-haloketone, the temperature was brought to room temperature. Stirring was continued overnight and the reaction was quenched with water. The solution was extracted with dichloromethane and the organic layer was washed with NaHCO₃ (10%) and water. The organic layer was dried over Na₂SO₄, filtered and the solvent was evaporated under reduced pressure. The crude product was purified by flash column chromatography eluting with a mixture of n-hexane : CH₂Cl₂ (3 : 1) to give the title compound 3 (1.13 g, 74%). ¹H NMR (500 MHz, CDCl₃) δ 7.65 (d, J = 4.4 Hz, 2H), 7.30–7.28 (m, 2H), 7.10 (t, J = 4.4 Hz, 1H), 7.06 (dd, J = 4.1, 2.2 Hz, 1H), 4.05 (s, 2H).

2.3.2. Synthesis of 3-(thiophen-2-yl)thieno[3,2-b]thiophene (4). To a solution of polyphosphoric acid (PPA) (4.0 g, 41.61 mmol) in chlorobenzene (7 mL) at 135 °C was added 3 (1.0 g, 4.16 mmol), dissolved in chlorobenzene (6 mL), dropwise. The reaction was heated at this temperature for 7 h, after which the mixture was extracted with CH₂Cl₂, sodium bicarbonate solution and water. The organic layer was dried over Na₂SO₄, filtered and the solvent was evaporated under reduced pressure. The residue was purified by column chromatography eluting with n-hexane to obtain the title compound 4 (0.72 g, 78%). ¹H NMR (500 MHz, CDCl₃) δ 7.53 (d, J = 1.6 Hz, 1H), 7.49–7.47 (m, 2H), 7.33 (dd, J = 5.2, 1.3 Hz, 2H), 7.18 (dd, J = 5.2, 3.5 Hz, 1H).

2.3.3. Synthesis of 2,5-dibromo-3-(5-bromothiophen-2-yl)thieno[3,2-b]thiophene (5). To a solution of 4 (0.4 g, 1.799 mmol), dissolved in DMF (5 mL), was added NBS (1.06 g, 8.94 mmol) at −10 °C in the dark. After the reaction was stirred for 12 h at the same temperature, the mixture was poured into water (50 mL). The precipitate was filtered and purified by column chromatography eluting with n-hexane to obtain 2,5-dibromo-3-(5-bromothiophen-2-yl)thieno[3,2-b]thiophene 5 (0.67 g, 81%). ¹H NMR (500 MHz, CDCl₃) δ 7.29 (d, J = 4.0 Hz, 1H), 7.23 (s, 1H), 7.14 (d, J = 3.9 Hz, 1H).

2.3.4. Synthesis of 4-methoxy-N-(4-methoxyphenyl)-N-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)aniline (7). To a solution of 6 (1.0 g, 2.6 mmol) in dry THF (50 mL) was added n-BuLi (2.1 mL, 2.5 M, 5.2 mmol) dropwise at −78 °C, under a nitrogen atmosphere. After the reaction was stirred for 45 min, 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolan (1.6 mL, 7.8 mmol) was introduced in stages into the mixture. The reaction was stirred for 12 h at room temperature and then quenched with water. The mixture was extracted with CH₂Cl₂, sodium bicarbonate solution, brine and water. The organic layer was dried over Na₂SO₄, filtered and the solvent was evaporated under reduced pressure. The crude product was purified by flash column chromatography eluting with a mixture of n-hexane : CH₂Cl₂ (7 : 1) to obtain the title compound 7 (0.83 g, 74%). ¹H NMR (500 MHz, CDCl3) δ 7.61 (d, J = 8.3 Hz, 2H), 7.08 (d, J = 8.8 Hz, 4H), 6.88 (d, J = 8.4 Hz, 2H), 6.84 (d, J = 8.8 Hz, 4H), 3.81 (s, 6H), 1.33 (s, 12H).

2.3.5. Synthesis of 4-methoxy-N-(4-methoxyphenyl)-N-(4-(thiophen-2-yl)phenyl)aniline (9). To a mixture of 7 (0.78 g, 1.81 mmol) and 8 (0.27 g, 1.65 mmol), dissolved in THF (25 ml) and degassed for 45 min with N₂, was added K₂CO₃ (5 mL, 2 M) and Pd(PPh₃)₄ (0.165 mmol). The mixture was then saturated with N₂, and the sealed reaction flask was stirred at 80 °C for 48 h, after which the crude product was filtered through Celite, extracted with sodium carbonate and dichloromethane. The organic layer was dried over sodium sulfate, filtered and the solvent was evaporated under reduced pressure. The crude product was purified by flash column chromatography eluting with a mixture of n-hexane : CH₂Cl₂ (8 : 1) to give the title compound 9 (0.44 g, 68%) as a pale-yellow oil. ¹H NMR (500 MHz, CDCl₃) δ 7.42 (d, J = 8.7 Hz, 2H), 7.21–7.18 (m, 2H), 7.09 (d, J = 8.9 Hz, 4H), 7.05 (dd, J = 5.0, 3.7 Hz, 1H), 6.94 (d, J = 8.7 Hz, 2H), 6.85 (d, J = 9.0 Hz, 4H), 3.82 (s, 6H).

2.3.6. Synthesis of 4-methoxy-N-(4-methoxyphenyl)-N-(4-(5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)thiophen-2-yl)phenyl)aniline (10). To a solution of 9 (0.4 g, 1.034 mmol) in dry THF (50 mL) was added 2.5 M n-BuLi (0.62 mL, 1,55 mmol) dropwise at −78 °C, under a nitrogen atmosphere. After the reaction was stirred for 45 min, 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolan (1.6 mL, 3.09 mmol) was introduced portion wise into the mixture. The reaction was stirred for 12 h at room temperature then quenched with water. The mixture was extracted with CH₂Cl₂, sodium bicarbonate solution, brine and water. The organic layer was dried over Na₂SO₄, filtered and the solvent was evaporated under reduced pressure. The crude product was purified by flash column chromatography eluting with a mixture of n-hexane : CH₂Cl₂ (7 : 1) to obtain the title compound 10 (0.40 gr, 76%). ¹H NMR (500 MHz, CDCl₃) δ 7.47 (d, J = 8.8 Hz, 2H), 7.10 (dd, J = 17.3, 8.7 Hz, 4H), 6.92–6.82 (m, 8H), 3.83 (s, 6H), 3.81 (s, 12H).

2.3.7. Synthesis of 2-(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (12). A solution of 11 (1.0 g, 7 mmol) in dry THF (25 mL) at −78 °C under a nitrogen atmosphere was treated with a solution of n-BuLi (3.1 mL, 2.5 M, 7.7 mmol). The temperature of the mixture was maintained at 0 °C for 30 min and then cooled to −78 °C to introduce a solution of 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2.9 mL, 14.2 mmol) in THF (5 mL). The reaction was stirred for 18 h at room temperature, then quenched with a solution of sodium chloride. The product was extracted with diethyl ether, washed with water and dried over sodium sulfate. The product 12 was obtained by precipitation in hexane as a white solid (1.47 g, 77%). ¹H NMR (500 MHz, CDCl₃) δ 6.63 (s, 1H), 4.33–4.28 (m, 2H), 4.21–4.15 (m, 2H), 1.34 (s, 12H).

2.3.8. Synthesis of 4-(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-N,N-bis(4-methoxy phenyl)aniline (13). To a mixture of 6 (0.56 g, 0.15 mmol) and 12 (0.45 g, 0.17 mmol), dissolved in THF (25 mL) and degassed for 45 min with N₂, was added K₂CO₃ (5 mL, 2 M) and Pd(PPh₃)₄ (0.015 mmol). The mixture was then saturated with N₂, and the sealed reaction flask was stirred at 80 °C for 48 h, after which the crude product was filtered through Celite, extracted with sodium carbonate and dichloromethane, dried over sodium sulfate, filtered and the solvent was evaporated under reduced pressure. The crude product was purified by flash column chromatography eluting with a mixture of n-hexane : CH₂Cl₂ (4 : 1) and then, recrystallized from methanol to give the titled compound 13 (0.49 g, 73%) as a light brown powder. ¹H NMR (500 MHz, CDCl₃) δ 7.59 (d, J = 8.7 Hz, 2H), 7.25 (d, J = 7.5 Hz, 3H), 7.12 (d, J = 8.4 Hz, 4H), 7.06 (s, 2H), 7.02 (t, J = 7.3 Hz, 2H), 6.26 (s, 1H), 4.32–4.28 (m, 2H), 4.25–4.21 (m, 2H), 3.81 (s, 6H).

2.3.9. Synthesis of 4-methoxy-N-(4-methoxyphenyl)-N-(4-(7-(tributylstannyl)-2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)phenyl)aniline (14). To a solution of 13 (0.35 g, 0.79 mmol) in dry THF (30 mL) was added n-BuLi (0.7 mL, 1.74 mmol) dropwise at −78 °C, under a nitrogen atmosphere. After the reaction was stirred for 1 h, tributyltin chloride (0.55 mL, 2.0 mmol) was introduced dropwise, after which the temperature was brought to room temperature and the stirring was continued overnight. The reaction was then quenched with water and the mixture was extracted with dichloromethane and washed with brine and water. The organic layer was dried over Na₂SO₄, filtered and the solvent was evaporated under reduced pressure. Compound 14 was obtained as a brown liquid after removal of the solvent and used without further purification for the next step.

2.3.10. Synthesis of TT-Th3-TPA3. To a mixture of 5 (0.055 g, 0.12 mmol) and 10 (0.2 g, 0.39 mmol), dissolved in THF (25 mL) degassed for 45 min with N₂, was added K₂CO₃ (5 mL, 2 M) and Pd(PPh₃)₄ (0.012 mmol). The mixture was saturated with N₂ and the sealed reaction flask was stirred at 80 °C for 48 h, after which the crude product was filtered through Celite, extracted with sodium carbonate and dichloromethane, dried over sodium sulfate, filtered and the solvent was evaporated under reduced pressure. The crude product was purified by flash column chromatography eluting with a mixture of n-hexane : CH₂Cl₂ (2 : 1) and recrystallization from n-hexane to give the title compound TT-Th3-TPA3 (0.106 g, 64%) as an orange solid. ¹H NMR (600 MHz, thf-d₈) δ 7.50 (s, 1H), 7.45–7.42 (m, 4H), 7.40 (d, J = 8.7 Hz, 2H), 7.25 (dd, J = 5.6, 3.8 Hz, 2H), 7.22 (d, J = 3.7 Hz, 1H), 7.18 (d, J = 2.6 Hz, 2H), 7.07–7.01 (m, 10H), 6.89–6.83 (m, 15H), 3.76 (s, 12H), 3.75 (s, 6H).

2.3.11. Synthesis of TT-EDOT3-TPA3. To a mixture of 5 (0.055 g, 12 mmol) and 14 (0.35 g, 48 mmol), dissolved in toluene (30 mL) and degassed for 1 h with N₂, was added Pd (PPh₃)₄ (0.015 mmol). The mixture was saturated with N₂ and the sealed reaction flask was stirred at 80 °C for 48 h, after which the crude product was filtered through Celite, extracted with sodium carbonate and dichloromethane, dried over sodium sulfate, filtered and the solvent was evaporated under reduced pressure. The crude product was purified by flash column chromatography eluting with a mixture of n-hexane : CH₂Cl₂ (1 : 2) and recrystallization from hexane to give the titled compound TT-EDOT3-TPA3 (0.08 g, 73%) as a dark orange powder. ¹H NMR (500 MHz, CDCl₃) δ 7.60–7.51 (m, 7H), 7.10–7.05 (m, 13H), 6.92 (t, J = 7.9 Hz, 8H), 6.84 (d, J = 6.2 Hz, 11H), 4.42 (s, 2H), 4.33 (m, 8H), 4.24 (s, 2H), 3.81 (s, 18H).

2.3.12. Synthesis of hybrid TT-Th-TPA-SWCNT. To a round-bottom flask were added CoMoCat SWCNTs (Southwest NanoTechnologies, 10 mg), TT-Th3-TPA3 (50 mg) (ratio of TT derivative to nanotube: 5, w/w) and dry THF (100 mL). The resulting mixture was sonicated for 30 min in a low-power sonic bath and then stirred vigorously for 7 days at room temperature. To remove unattached TT derivatives, the mixture was filtered through a 0.2-μm Teflon membrane and washed with excess THF (Sartorious, PTFE; pore size, 0.2 μm). The recovered black hybrid film was dried under vacuum for 24 h.

2.3.13. Synthesis of hybrid TT-EDOT-TPA-SWCNT. The hybrid TT-EDOT-TPA-SWCNT was synthesized applying the same method used for the synthesis of hybrid TT-Th-TPA-SWCNT.

3. Result and discussion

3.1. Design and synthesis

The TT derivatives (4, 5) and 4,4′-dimethoxytriphenylamine (6) were synthesized following our previous reports.^{41,42,58–60} Initially, the synthesis of the core unit, 3-(thiophen-2-yl)thieno[3,2-b]thiophene 4, was conducted starting from 3-bromothiophene (1). The monoketone, 3, was constructed in a one-pot three step reaction in 74% yield; (i) lithiation of 3-bromothiophene (1) with n-butyllithium at −78 °C, (ii) additions of elemental sulfur and (iii) α-haloketone (2). Its ring closure reaction was performed in the presence of polyphosphoric acid (PPA) in refluxing chlorobenzene to give 4 in 78%. The three brominated TT 5 was obtained through bromination of 4 using NBS at −10 °C in DMF in 81% yield. The boronated triphenylamine (TPA) 7 was constructed in a one-pot two-step reaction in 74% yield by lithiation of 4,4′-dimethoxytriphenylamine (6) with n-butyllithium at −78 °C and then addition of 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane. Its Suzuki coupling reaction with 2-bromothiophene (8) produced the thiophene extended TPA, 9, in a yield of 68%. Its boronated derivative 10 was constructed treating 9 with n-butyllithium and then the addition of 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane in 76%. The EDOT based compounds were obtained using similar procedures. EDOT (11) was boronated by lithiation with n-butyllithium and the addition of 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane to give the compound 12 in a yield of 77%, and its Suzuki coupling reaction with 6 produced the EDOT extended TPA (13) in a yield of 73%. It was stannylated by lithiation with n-butyllithium and then the addition of tributyltin chloride in 80% yield. The Suzuki coupling reaction of three-brominated TT 5 with 10 produced the first target compound TT-Th3-TPA3 in 64% yield, and the second one, TT-EDOT3-TPA3, was obtained by Stille coupling reaction of 5 and 14 in a yield of 73% (Scheme 1). These coupling reactions are well-established and were successfully applied in our previous studies with high reproducibility and yield.^61–64 The hybrids TT-Th-TPA-SWCNT and TT-EDOT-TPA-SWCNT were obtained as flexible, easily bendable and smooth black nanohybrid films (Fig. S1, ESI†) through stirring of TT-Th3-TPA3 and TT-EDOT3-TPA3 with SWCNT, respectively, without using any binding agents in THF at room temperature for 7 days (Scheme 2).^65–67 The films were clarified by AFM and SEM.

Scheme 1 Synthesis of the TT and TPA monomers.

Scheme 2 Synthesis of the hybrids TT-Th-TPA-SWCNT and TT-EDOT-TPA-SWCNT.

3.2. Photophysical properties

Photophysical properties (UV-vis absorption and emission spectra) of the compounds were investigated in THF solution at room temperature (Table 1 and Fig. 1a). The maximum π–π* absorption wavelengths (λ_max) of TT-EDOT3-TPA3 and TT-Th3-TPA3 were measured to be 416 and 431 nm, respectively. From the absorption spectra, the onset maximums of 526 and 531 nm were observed for TT-EDOT3-TPA3 and TT-Th3-TPA3, respectively. Then, the optical band gaps were determined to be 2.33 (TT-EDOT3-TPA3) and 2.36 eV (TT-Th3-TPA3). The fluorescence measurements in THF displayed emission maxima of 619 (TT-EDOT3-TPA3) and 616 nm (TT-Th3-TPA3) (Table 1 and Fig. 1b), demonstrating mega Stokes shifts of 203 and 185 nm, respectively. Both molecules exhibit a Stokes shift higher than 100 nm, indicating that they have a strong intramolecular charge transfer character,^27,68 which could be due to the gaining of more electron donating features, providing more efficient intramolecular charge-transfer (ICT) between TT and TPA groups. Moreover, mega Stokes shifts of the compounds indicated a fast relaxation from the excited state to the ground state. Thus, due to ICT, the materials are considered to be optically applicable.^69,70 In particular, TT-EDOT3-TPA3 exhibits a larger Stokes shift (203 nm) compared to TT-Th3-TPA3 (185 nm) indicating a more effective ICT.

Table 1 Photophysical data of TT-EDOT3-TPA3 and TT-Th3-TPA3

Compounds	λ abs,max (nm)	λ onset (nm)	λ fl,max (nm)	Stokes shift (nm)	E g,opt (eV)
a Absorption and emission maximum in THF. b Onset of the absorption in THF. c E g,opt = hc/λonset.
TT-EDOT3-TPA3	416	526	619	203	2.33
TT-Th3-TPA3	431	531	616	185	2.36

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Fig. 1 (a) UV-vis absorption spectra and (b) emission spectra of TT-EDOT3-TPA3 and TT-Th3-TPA3 in THF.

3.3. Electrochemical properties

The energy levels of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of the two compounds were investigated by cyclic voltammetry (CV) (Fig. S2 and Table 2, ESI†). Electrochemical behaviors of TT-EDOT3-TPA3 and TT-Th3-TPA were investigated by cyclic voltammetry (CV) in acetonitrile : dichloromethane (1 : 1) in the presence of tetrabutylammonium hexafluorophosphate (TBAPF₆, 0.1 M) as a supporting electrolyte at a scan rate of 50 mV s⁻¹, using platinum wires as working and counter electrodes, and Ag wire as a reference electrode. Based on the onset potentials, the HOMO energies of TT-EDOT3-TPA3 and TT-Th3-TPA3 were calculated to be −5.33 and −5.38 eV, respectively, using the equation; HOMO = −(E_onset,ox + 4.40) (eV). Then, the LUMO values were calculated using the equation; LUMO = −(E_onset,red + 4.40) (eV). The LUMO energy levels of TT-EDOT3-TPA3 and TT-Th3-TPA3 were found to be −3.42 and −3.31 eV, respectively (Table 2). The electronic band gap of TT-EDOT3-TPA3 (1.91 eV) is lower compared to the electronic band gap of TT-Th3-TPA3 (2.07 eV), as is the case with the optical band gaps. The possible explanation for this is that the EDOT moiety has a higher donor property, leading to faster oxidation and thus a lower HOMO energy and band gap.

Table 2 Electrochemical data of TT-EDOT3-TPA3 and TT-Th3-TPA3

Compounds	E onset,ox (V)	E onset,red (V)	HOMO^a (eV)	LUMO^b (eV)	E elec (eV)
a HOMO = −(Eonset,ox + 4.40) (eV). b LUMO = −(Eonset,red + 4.40) (eV). c E elec = |HOMO − LUMO|.
TT-EDOT3-TPA3	0.93	−0.98	−5.33	−3.42	1.91
TT-Th3-TPA3	0.98	−1.09	−5.38	−3.31	2.07

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3.4. Surface properties

Surface properties of the hybrid materials were investigated by scanning electron microscopy (SEM). Regarding the SEM images, the hybrid TT-EDOT-TPA-SWCNT and TT-Th-TPA-SWCNT demonstrated clear interconnected and tangled bundles at different magnifications (Fig. 2a–d), while the uncoated SWCNT fringes had entanglements with nanometer scaled voids (Fig. S3a, ESI†). On the other hand, after modification of the voids of SWCNT, it became apparent that the carbon fringes slightly clogged with TT, which was considered as an indication for the presence of TTs on the surface of SWCNTs. In addition, while the original uncoated SWCNT is in powder form (Fig. S3b, ESI†), the coated SWCNTs were easily dispersed in THF acquiring an excellent film forming property (Scheme 2), which could be useful for various optoelectronic applications. In order to further clarify the surface modification of SWCNT with the TT derivatives, the TT-Th-TPA-SWCNT and TT-EDOT-TPA-SWCNT hybrids were treated with gold nanoparticle solution (2.4 nM), for an Au–S interaction.^65,66 Clear images of the nanoparticles on the hybrids were demonstrated by scanning electron microscopy (SEM) at 500 nm (Fig. 3a and b), which indicated clear adsorptions of Au by the sulfur atoms of TT-Th3-TPA3 and TT-EDOT3-TPA3 on the SWCNTs.

Fig. 2 SEM images of (a)–(c) TT-Th-TPA-SWCNT and (b)–(d) TT-EDOT-TPA-SWCNT hybrid films at different magnifications.

Fig. 3 SEM images of the hybrid S treated with a gold nanoparticle solution at 500 nm: (a) TT-Th-TPA-SWCNT and (b) TT-EDOT-TPA-SWCNT.

Moreover, further surface properties of the hybrid nanomaterials were investigated by atomic force microscopy (AFM) (Fig. 4a–d). RMS roughness, obtained from the cross section of the AFM images, were determined to be 26 nm for TT-Th-TPA-SWCNT and 24 nm for TT-EDOT-TPA-SWCNT, demonstrating uniform and smooth surface morphologies. Considering our previous studies on flexible film formation between electron donor groups and SWCNT, formation of the flexible film and the characterizations are consistent with the previous studies.^65,66

Fig. 4 AFM images of (a) and (b) TT-Th-TPA-SWCNT and (c) and (d) TT-EDOT-TPA-SWCNT hybrid films at different magnifications.

3.5. Thermal properties

Thermal properties of the compounds were studied with thermal gravimetric analysis (TGA) up to 800 °C at a heating rate of 10 °C min⁻¹ under a N₂ atmosphere (Fig. S4, ESI†). Despite the similar double-step thermal behavior of the compounds, they displayed the first thermal decomposition temperatures (5%) at different points depending on their substituents and extended forms. TT-EDOT3-TPA3 and TT-Th3-TPA3 had thermal decomposition temperatures (T_d, 5%) of 422 and 290 °C, respectively. TT-EDOT3-TPA3 with a longer conjugation and a strong intramolecular interaction (S–O) between EDOT and TT units displayed the highest thermal stability compared to TT-Th3-TPA3. Moreover, with a gradual increase of temperature up to 800 °C, around 58% of TT-EDOT3-TPA3 and 54% of TT-Th3-TPA3 remained ash-free, indicating that the compounds have excellent thermal stability despite their small structures, which is a highly desirable parameter for the preparation of stable and durable electronic materials. Moreover, the modification ratios were determined from the difference between the thermal mass loss of pristine SWCNT and the hybrid materials as 14.5% (TT-Th-TPA-SWCNT) and 13.7% (TT-EDOT-TPA-SWCNT) (Fig. S5 and S6, ESI†).

3.6. Supercapacitor studies

Initially, cyclic voltammetry (CV) measurements at different scan rates to observe the C-rate capabilities of binder-free and freestanding electrodes were conducted to examine the electrochemical behaviors of the electrodes. Thus, although three cycles were obtained at scan rates of 1000, 750, 500, 250, 100, 75, 50, 25, 10, 5 and 1 mV s⁻¹, the second one is presented for each measurement (Fig. 5). Even at a high scan rate of 1000 mV s⁻¹, TT-EDOT-TPA-SWCNT exhibited capacitive behavior and the area under the curves increased with increasing scan rate (Fig. 5a), which demonstrates the potential of the electrode material to exhibit good C rate performances.⁷¹ Analysis of the voltammogram obtained at 100 mV s⁻¹ revealed an oxidation peak at approximately 500 mV and a reduction peak at around 300 mV (Fig. 5b), which was attributed to the presence of EDOT in the structure. The relatively low current response at low scan rates was due to electric double-layer capacitance, while the improved response at higher scan rates with pseudocapacitive behavior resulted from ongoing redox processes at the electrode–electrolyte interface. Despite this, the CV shape remained well preserved even at higher scan rates, demonstrating superior device rate capability.⁷¹ The results indicated that the TT-EDOT-TPA-SWCNT electrode performs well across a wide range of current densities with high specific capacitance.^66,72–74

Fig. 5 CVs for TT-EDOT-TPA-SWCNT obtained at scan rates (a) between 1000 and 100 mV s⁻¹ and (b) between 100 and 1 mV s⁻¹ (electrolyte 85% H₃PO₄, separator: cellulose acetate).

Similar measurements were conducted using an electrochemical cell prepared with the TT-Th-TPA-SWCNT electrodes (Fig. 6). The amount of current delivered by this active material through voltage scanning was not as high as that of the TT-EDOT-TPA-SWCNT electrode. On the other hand, while the area under the voltammograms showed a limited increase with increasing scan rate, the voltammograms exhibited bending trends at the minimum and maximum voltage points. This indicates that the TT-Th-TPA-SWCNT electrode cannot be charged at high current densities comparable to TT-EDOT-TPA-SWCNT.^72,75,76 The TT-Th-TPA-SWCNT electrode exhibited an oxidation peak at approximately 250 mV at a scan rate of 100 mV s⁻¹, and with decreasing scan rate, there was a decreasing trend in the area under the CV curves compared to the measurements conducted at a scan rate of 1 mV s⁻¹. At a scan rate of 1 mV s⁻¹, the shape of the voltammogram became distorted and the electrode did not exhibit capacitive behavior, indicating that, at very low current densities, this electrode active material may not perform effectively.

Fig. 6 CVs for TT-Th-TPA-SWCNT obtained at scan rates (a) between 1000 and 100 mV s⁻¹ and (b) between 100 and 1 mV s⁻¹ (electrolyte 85% H₃PO₄, separator: cellulose acetate).

The specific capacitance data calculated from voltammograms of different scan rates are plotted vs scan rates to compare the specific capacitance performance of the two-active materials with the specific capacitances (C_CV) calculated using eqn (1) (Fig. 7a). While the TT-EDOT-TPA-SWCNT electrode exhibited a C_CV of 153.8 ± 4.2 F g⁻¹ at a scan rate of 1 mV s⁻¹, the TT-Th-TPA-SWCNT electrode did not exhibit capacitive behavior at the same scan rate, thus, no specific capacitance calculation was performed. On the other hand, at a scan rate of 5 mV s⁻¹, TT-EDOT-TPA-SWCNT displayed a C_CV of 125.5 ± 0.5 F g⁻¹, while TT-Th-TPA-SWCNT exhibited a C_CV of 81.6 ± 2.3 F g⁻¹. Furthermore, TT-EDOT-TPA-SWCNT demonstrated a good C-rate performance with increasing scan rate, achieving approximately 36.6 ± 4.25 F g⁻¹C_CV even at the highest scan rate. The supercapacitor cell, prepared using the TT-Th-TPA-SWCNT electrodes, exhibited a weaker capacitive response at high scan rates. These results indicate that TT-EDOT-TPA-SWCNT operates effectively over a wide current density range, whereas TT-Th-TPA-SWCNT delivers performance within a relatively limited C-rate window and capability. Subsequently, EIS measurements were conducted to analyze the resistance behavior offered by the two different cells and compared with the CV results (Fig. 7b). Both cells produced overlapped Nyquist plots, which consisted of two semi-circles and a straight line at approximately 45 degrees. To fit both Nyquist plots, an equivalent circuit model was employed using constant phase elements (CPEs) instead of ideal capacitive semi-circles due to the initial phase angles of the semi-circles in the equivalent circuit. This approach accommodates the characteristics of the electrodes more accurately.⁷⁷ The R_s in the equivalent circuit represents the total resistance originating from the electrolyte and the connections between the supercapacitor device and the test equipment.⁷⁸R_ct-1 and CPE1 represent the charge transfer resistance and the CPE for the electronic transfer processes of the electrode active material. R_Leak is the leakage resistance placed in parallel with CPE2, typically very high and negligible in the circuit. CPE3 represents capacitance, arising from charge transfer processes of the SWCNT electrode. Therefore, a second charge transfer resistance (R_ct-2), representing the resistance of SWCNT to electron transfer and a CPE3, connected in parallel, were included in the equivalent circuit system. R_ct-2 originates from the SWCNT in the flexible and binder-free electrode structure. A Warburg component, connected in series with this resistance, was added to the equivalent circuit to represent mass transfer to signify the diffusion of ions into the porous carbon structure.⁷⁹ This equivalent circuit model was used to accurately fit the impedance data obtained from both Nyquist plots. The resistance values of the two different electrodes are given in Table 3. As expected, the TT-EDOT-TPA-SWCNT electrode exhibited lower R_s, R_ct-1 and R_ct-2 values compared to the TT-Th-TPA-SWCNT electrode. This indicates that a supercapacitor cell, prepared with this electrode, displays a better electron transfer capability, which is consistent with the previously obtained CV results. Although symmetric supercapacitor cells were prepared using the same electrolyte for both electrode active materials, the cell prepared with the TT-EDOT-TPA-SWCNT electrode exhibited a lower R_s compared to the cell prepared with the TT-Th-TPA-SWCNT electrode. As previously mentioned, R_s represents the total resistance originating from the electrolyte and the connections between the potentiostat and the supercapacitor device. This difference reflects the fact that the TT-EDOT-TPA-SWCNT electrode, with its higher electrical conductivity, forms a more electrically conductive interface between the potentiostat and the supercapacitor device.⁸⁰

Fig. 7 (a) Specific capacitance values obtained at different scan rates (1000, 750, 500, 250, 100, 75, 50, 25, 10, 5, and 1 mV s⁻¹) for the TT-EDOT-TPA-SWCNT and TT-Th-TPA-SWCNT electrodes using 85% H₃PO₄ electrolyte and cellulose acetate as the separator. (b) Nyquist plots of the TT-EDOT-TPA-SWCNT and TT-Th-TPA-SWCNT electrodes recorded using 85% H₃PO₄ electrolyte and cellulose acetate as the separator over the frequency range of 100 000 Hz to 0.1 Hz with an applied AC amplitude of 10 mV.

Table 3 The resistance values calculated from the fitting of the experimentally obtained Nyquist curves using the equivalent circuit model shown in Fig. 7b

Materials	EIS			GCD
	R s	R ct-1	R ct-2	E (Wh kg^−1)	P (W kg^−1)
TT-EDOT-TPA-SWCNT	1.6	2.5	5.4	5.19 ± 0.13 (0.1 A g^−1)	50 (0.1 A g^−1)
				2.00 ± 0.12 (20 A g^−1)	10 000 (20 A g^−1)
TT-Th-TPA-SWCNT	5.8	29.9	3.4	0.2.30 ± 0.14 (1 A g^−1)	500 (1 A g^−1)
				0086 ± 0.00058 (10 A g^−1)	5000 (10 A g^−1)

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Due to the conjugation in the thiophene (Th) structure, π orbital overlap occurs along the backbone of the molecule, which promotes the electron transport. Similarly, the electrical conductivity of thiophene structures can be enhanced through chemical modifications. On the other hand, EDOT reduces the effective mass of charge carriers and the HOMO–LUMO gap.⁸¹ In this study, the Th and EDOT structures were functionalized as bridging molecules between TT and TPA with Th in the first hybrid structure and EDOT in the second. Electrochemical characterization of these structures was conducted in a three-electrode electrochemical cell as presented in Section 3.3: electrochemical properties. The results show that TT-EDOT3-TPA3 exhibits a lower electronic band gap (1.91 eV) compared to that of TT-Th3-TPA3 (2.07 eV). This implies that the TT-EDOT-TPA-SWCNT electrode can facilitate electron transfer with lower charge transfer resistance within the same electrolyte. Accordingly, the CV and EIS results, obtained in two-electrode cells, corroborate the band gap values previously observed in the three-electrode electrochemical cell. In addition, electron donor and intramolecular O⋯S interactions of EDOT highly contributed to its capacitive performance as well as its optical properties.^82,83

The performance difference between the two electrodes is not solely due to the lower electronic band gap provided by EDOT. The enhanced interaction of the TT-EDOT-TPA-SWCNT electrode with the aqueous electrolyte further deepens the performance gap between the two active materials. The TT-EDOT-TPA-SWCNT electrode exhibits superior wettability in the H₃PO₄ electrolyte, which is clearly reflected in the R_s values obtained from the EIS results. Similarly, the Warburg impedance observed in the Nyquist plots indicates that ions in the electrolyte can achieve more efficient mass transfer to the electrode surface.

In the subsequent process, GCD measurements were performed at different current densities and the charge–discharge characteristics of the electrodes were investigated. The electrodes were charged and discharged at current densities of 20, 15, 10, 5, 2.5, 2, 1, 0.5, 0.1 and 0.05 A g⁻¹. The specific capacitance (C_GCD), energy density (E) and power density (P) values of the electrodes were calculated at different current densities using the GCD curves and eqn (2)–(4), respectively. The cells underwent 10 charge–discharge cycles at each current density and the curves obtained at the 5th cycle for each current density are presented (Fig. 8a and b).

Fig. 8 GCD curves for the TT-EDOT-TPA-SWCNT electrode at current densities of (a) 20, 15, 10, 5, 2.5, 2, and 1 A g⁻¹ and (b) 0.5, 0.1, and 0.05 A g⁻¹ (electrolyte 85% H₃PO₄, separator: cellulose acetate).

A decrease of charge–discharge time was observed with an increase of current density, yet the TT-EDOT-TPA-SWCNT electrode provided consistent charge–discharge curves at all current densities. Except for the 0.05 A g⁻¹ current density, all charge–discharge tests exhibited Coulombic efficiencies above 95%. The lower efficiency at 0.05 A g⁻¹ was attributed to the difficulty in charging the cell at such a low current density. Nevertheless, the electrode was able to charge and discharge even at this very low current density of 0.05 A g⁻¹. Additionally, the obtained charge–discharge (GCD) curves are highly consistent with the CV results. A bend representing the oxidation peak around 400 mV occurred during charging, while a bend representing the reduction peak around 300 mV was observed in the GCD curves during discharging.

The GCD curves are generally triangular, symmetrical and exhibit charge–discharge behavior over a wide range of current densities, indicating that the TT-EDOT-TPA-SWCNT electrode operates with high coulombic efficiency and demonstrates a good C-rate capability. To compare the GCD performances of the TT-EDOT-TPA-SWCNT and TT-Th-TPA-SWCNT electrodes, the discharge curves obtained at a current density of 1 A g⁻¹ are given in Fig. 9a. TT-Th-TPA-SWCNT exhibited a lower Coulombic efficiency compared to the TT-EDOT-TPA-SWCNT electrode. Additionally, the discharge curve indicated that the C_GCD of TT-Th-TPA-SWCNT is not as high as that of the TT-EDOT-TPA-SWCNT electrode. The C_GCD data calculated from the charge–discharge curves of both electrodes are shown in Fig. 9b. Using 10 repeated GCD measurements, the mean C_GCD and standard deviations were calculated for each current density. The calculated standard deviations are presented as error bars along with the mean C_GCD values. The TT-EDOT-TPA-SWCNT electrode provided a C_GCD of approximately 149.7 ± 3.7 F g⁻¹ at a current density of 0.1 A g⁻¹. On the other hand, this current density was insufficient to charge the TT-Th-TPA-SWCNT electrode and no specific capacitance was obtained at this current density. The C_GCD values at a current density of 0.5 A g⁻¹ for the TT-EDOT-TPA-SWCNT and TT-Th-TPA-SWCNT electrodes were calculated to be 109.2 ± 1.4 F g⁻¹ and 66.3 ± 3.9 F g⁻¹, respectively. These results exhibit a high correlation with the results obtained from CV measurements. Similarly, the TT-Th-TPA-SWCNT electrode did not exhibit charge–discharge behavior at current densities of 15 and 20 A g⁻¹, which highlighted the synergistic effect of the components within the TT-EDOT-TPA-SWCNT structure, demonstrating its high performance and good C-rate capability. Subsequently, a 10 000-cycle GCD test was conducted at a current density of 2 A g⁻¹ to examine the stability of the two electrode active materials. The capacity retention data obtained as a function of cycle number are given in Fig. 10. The 10 000-cycle stability graphs show that the TT-EDOT-TPA-SWCNT electrode exhibited approximately a 10% increase in capacity within the first 500 cycles, maintaining nearly the same specific capacity for the remainder of the test. As expected, the TT-Th-TPA-SWCNT electrode initially provided a slightly lower specific capacitance compared to the TT-EDOT-TPA-SWCNT electrode and displayed a decreasing capacity trend with increasing cycle numbers. These results demonstrated that the TT-EDOT-TPA-SWCNT electrode offers an excellent cycling stability, i.e. over 95% Coulombic efficiency, a high specific capacitance, a good C-rate capability and high energy and power densities. The specific capacitance (F g⁻¹), power density (W kg⁻¹), energy density (Wh kg⁻¹) and capacity retention (%) data, obtained with the symmetric supercapacitor cell of TT-EDOT-TPA-SWCNT electrodes, are presented comparatively in Table 4.

Fig. 9 (a) GCD curves for the TT-EDOT-TPA-SWCNT and TT-Th-TPA-SWCNT electrodes at a current density of 1 A g⁻¹. (b) Specific capacitance values calculated from GCD curves for the TT-EDOT-TPA-SWCNT and TT-Th-TPA-SWCNT electrodes at different current densities (electrolyte 85% H₃PO₄, separator: cellulose acetate).

Fig. 10 Cycling stability graph for the TT-EDOT-TPA-SWCNT and TT-Th-TPA-SWCNT electrodes at a current density of 2 A g⁻¹ (in both supercapacitor cells, the electrolyte is 85 wt% H₃PO₄ and the separator is cellulose acetate).

Table 4 The performance indicators obtained with the supercapacitor cell prepared using the TT-EDOT-TPA-SWCNT electrode compared with data obtained in some other studies in the literature

Electrode	Specific capacitance (F g^−1)	Power density (W kg^−1)	Energy density (Wh kg^−1)	Capacity retention (%)	Ref.
Ti/MoS2	133 (1 A g^−1)	530	11	93% (1000 cyc./1 A g^−1)	90
Porous 3D interconnected carbon framework	65 (0.2 A g^−1)	400	12	100% (3000 cyc./1 A g^−1)	91
Mxene/graphdiyne nanotubes	337.4 (2 A g^−1)	750	19.7	88.2% (10 000 cyc./8 A g^−1)	92
Paper/activated carbon	165 (5 mA cm^−2)	28.22	6.01	100% (10 000 cyc./5 mA cm^−2)	93
PANI/CNF	234 (1 A g^−1)	500	32	90% (1000 cyc./1 A g^−1)	94
Ni foams/PPy	38 (0.2 A g^−1)	6200	14	82% (2000 cyc./1 A g^−1)	95
CNTs/nitrogen-doped carbon polyhedral hybrids	135 (5 mV s^−1)	250	12	100% (20 000 cyc.)	96
Graphene/MnO2/CNTs	61 (0.1 A g^−1)	106	8.9	95% (1000 cyc./4 A g^−1)	97
TT-EDOT-TPA-SWCNT	149.7 ± 3.7 (1 A g^−1)	10 000 (@20 A g^−1)	5.19 ± 0.13 (@0.1 A g^−1)	110% (10 000/2 A g^−1)	This work
		50 W kg^−1 @0.1 A g^−1)	2.00 ± 0.12 (@20 A g^−1)

---

After the charge–discharge cycling tests, the surface morphology of the TT-EDOT-TPA-SWCNT and TT-Th-TPA-SWCNT electrodes was re-examined using SEM analysis (Fig. 11a–d). The SEM images did not show any substantial changes in the measurements before and after the cycles of GCD tests for both hybrid electrodes. Similar interconnected and tangled bundles are clearly visible before and after 10 000 cycles, indicating high stability of the hybrid electrodes during charge–discharge cycles. On the other hand, when examining the SEM image of the TT-EDOT-TPA-SWCNT electrode before GCD, aggregation formations belonging to SWCNT were observed. However, when the morphology of the electrode was examined after 10 000 GCD cycles, it was noticed that the aggregated structures disappeared, the morphology became more porous and the TT-EDOT3-TPA3 molecules are embedded into the CNT framework. SEM images obtained prior to the GCD tests revealed that the TT-EDOT-TPA-SWCNT electrode exhibits a more integrated structure with SWCNT nanofibers forming large bundles that appear interconnected and cohesive, although the nanotube network is randomly entangled and shows a relatively disordered distribution. After 10 000 GCD cycles, however, partial debonding of the SWCNT bundles was observed, resulting in a more homogeneous distribution and a morphology resembling a three-dimensional mesh. This indicates that the morphology of the electrode changes upon wetting with the acidic media or, alternatively, the electrochemical process itself may trigger such a morphological transformation on the surface, providing a larger surface area for electrochemical interaction.^84,85 On the other hand, the absence of any capacity loss after 10 000 cycles suggests that the π–π interactions within the electrode structure remain intact and that the electrode retains its structural integrity without degradation. A similar situation is not present in the SEM images of the TT-Th-TPA-SWCNT electrodes recorded after 10 000 cycles of GCD. This suggests that the increase in porosity of the TT-EDOT-TPA-SWCNT electrode and a better adhesion between the TT-EDOT3-TPA3 molecules with the SWCNT framework after use provides a better electrode–electrolyte interface, which may lead to a decrease in electron transfer resistance. To validate this, the Nyquist plot of the cell, subjected to 30 potentiostatic and 90 galvanostatic charge–discharge cycles, was compared with that of the freshly prepared cell. The Nyquist plots obtained from the freshly prepared cell and the cell aged for 120 cycles using the TT-EDOT-TPA-SWCNT electrodes are presented in Fig. S5 (ESI†). Although there was no significant change in the R_s, with the increasing number of cycles, the R_ct of the electrode showed a notable decrease. This result is in agreement with the 10 000-cycle GCD tests performed with the cell prepared using the TT-EDOT-TPA-SWCNT electrodes and explains why the cell performance improves during the first few hundred usage cycles. Furthermore, the electrochemical performance of TT-EDOT-TPA-SWCNT competed with and outperformed many SWCNT based materials as well as other CNT-based materials (Table S1, ESI†).^86–89

Fig. 11 SEM image of the TT-EDOT-TPA-SWCNT electrode (a) before 10 000 cycles of GCD testing and (b) after 10 000 cycles of GCD testing. SEM image of the TT-Th-TPA-SWCNT electrode (c) before 10 000 cycles of GCD testing and (d) after 10 000 cycles of GCD testing (in both supercapacitor cells, the electrolyte is 85 wt% H₃PO₄ and the separator is cellulose acetate).

4. Conclusion

In conclusion, we designed and synthesized highly conjugated thienothiophene (TT) based compounds, TT-EDOT3-TPA3 and TT-Th3-TPA3, possessing EDOT, thiophene (Th) and triphenylamine (TPA) units. Their electronic and optical properties were investigated by spectroscopic methods of UV-vis and fluorescence, and cyclic voltammetry (CV). The compounds were attached onto the single wall carbon nanotube (SWCNT) by noncovalent interactions, i.e. van der Waals, π–π and S–π, without using any binding agents, which gave novel TT and SWCNT based hybrid nanomaterials, TT-EDOT-TPA-SWCNT and TT-Th-TPA-SWCNT, as flexible and smooth films. The hybrid films were characterized by SEM, which showed interconnected and tangled bundles, and AFM analysis, which revealed cross-sectional roughness. TT-EDOT-TPA-SWCNT and TT-Th-TPA-SWCNT were utilized as flexible and free-standing electrodes by CV, galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS) methods. The hybrid electrode materials demonstrated superior supercapacitor and energy storage performances such as a high specific capacitance of 158 F g⁻¹ at 1 mV s⁻¹, an excellent power density of 10 000 W kg⁻¹ at 20 A g⁻¹, a maximum energy density of 5.32 Wh kg⁻¹ at 0.1 A g⁻¹ and a superior charge and discharge reversibility with cycling stability up to 10 000 cycles. The main strategy of this study is to convert SWCNTs into electrodes through non-covalent functionalization with electroactive molecules (TT-EDOT-TPA and TT-TPA-Th) without the use of binders or current collectors. This approach not only simplifies the electrode fabrication process but also enhances the electrochemical performance by eliminating inactive and resistive components. Although TT-EDOT-TPA-SWCNT electrodes demonstrate excellent stability, high specific capacitance and superior power density, their energy density remains relatively limited. To overcome this limitation, two key strategies are proposed for future work. First, the use of organic electrolytes and salts could widen the operating voltage window, directly increasing the energy density of the device. Second, treating these freestanding and binder-free electrodes as current collectors and decorating their surfaces with metal oxides could significantly enhance their faradaic contribution. In particular, using the decorated TT-EDOT-TPA-SWCNT as the positive electrode in asymmetric supercapacitor configurations may enable the development of devices with ultra-high energy and power densities to make them highly suitable for next-generation energy storage applications. Considering the free-standing supercapacitor electrodes available in the literature, the performance of TT-EDOT-TPA-SWCNT outperformed and competed with those of many hybrid SWCNT based materials available in the literature.^98–102 Considering these remarkable achievements, we believe that TT and SWCNT based nanomaterials will have great potential for electronic applications for clean and renewable energy-storage protocols.

Conflicts of interest

There are no conflicts to declare.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and its ESI.† Raw data that support the findings of this study are available from the corresponding author, upon reasonable request.

Acknowledgements

We thank Istanbul Technical University (ITU), TGA-2023-45124, TGA-2023-44077, TGA-2023-47044, TDA-2024-45680, TYLB-2023-45051, PTA-2024-45861 and FHD-2024-45962 numbered ITU BAP projects, 122Z568 and 124Z353 numbered TUBITAK 1001 Projects, and Unsped Global Lojistik, Turkey, for financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5tc01766a

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