Energy storage behavior of side chain-engineered Si-bridged redox-active donor–acceptor conjugated polymers operated in organic electrolytes

Abstract

Structurally modulated donor–acceptor (D–A) moieties are key constituents in designing redox-active conjugated polymers (RACPs), which serve as promising pseudocapacitive electrode materials due to their broad potential windows, superior redox behavior, and tunable electronic properties. Herein, we introduce silylethynyl-functionalized benzodithiophene (Si-BDT), a planar donor moiety developed by lateral side-chain engineering, copolymerized with rylene diimide acceptors, naphthalene diimide (NDI) and perylene diimide (PDI), synthesized by the Stille cross-coupling polymerization reaction. The supercapacitor performance of these polymers was assessed in two organic-salt electrolytes, tetrabutylammonium hexafluorophosphate (TBAPF₆) and tetraethylammonium tetrafluoroborate (TEABF₄). The computational analysis explored non-covalent interaction (NCI) between the electrode and electrolyte and confirmed that both electrodes exhibited a stronger interaction with TBAPF₆, thereby facilitating improved charge transfer and electrochemical performance. Both electrodes were operated within a wide potential window of −1.7 to 0.7 V. Si-BDT-PDI outperformed Si-BDT-NDI, achieving a higher specific capacity of 267 C g⁻¹ at 1 A g⁻¹ due to electrode–electrolyte interactions enhanced by several topographic pores. Furthermore, a hybrid supercapacitor integrating a Si-BDT-PDI anode and a porous activated carbon cathode achieved an outstanding energy density of 55 W h kg⁻¹ and a power density of 13 380 W kg⁻¹ across a 2.7 V potential window.

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1 Introduction

Electrochemical energy storage (EES) devices, such as supercapacitors and batteries, are promising because of their unique features, such as rapid charge/discharge, cycling stability of the supercapacitors and the high energy density and long energy storage ability of the batteries, respectively.¹ Unlike electrochemical double-layer capacitors (EDLCs), pseudocapacitors can combine the features of both batteries and capacitors, showing a linear relationship between stored charge and potential due to surface-level faradaic electron-transfer processes.^2,3 Among pseudocapacitive energy storage materials, redox-active conjugated polymers (RACPs) are particularly attractive for EES devices due to their cost-effectiveness and facile solution processability. However, their use as electrode materials remains limited due to their relatively low electrical conductivity and high electrolyte solubility, which can result in reduced energy storage efficiency and the gradual loss of active components. One approach for improving their stability and charge-storage capacity is to incorporate redox-active groups by polymerizing specific reactive monomers or chemically bonding redox-responsive units to the polymer chains. This modification enhances molecular rigidity and entanglement, mitigating dissolution and making RACPs viable candidates for application as supercapacitor electrodes.⁴ Another way to improve the energy storage efficiency of redox-active pseudocapacitive electrodes is to utilize ideal electrolytes that can improve charge transfer between the electrodes and electrolytes.^4,5 RACPs with donor–acceptor (D–A) architectures are appealing materials for energy storage and harvesting due to their unique multiple features.^6–8 First, the “push–pull” D–A architecture can effectively manipulate the bandgap of conjugated polymers by fine-tuning the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs).^9,10 Second, D–A-based polymers extend the operational potential window in type III or IV polymer-based supercapacitors as they can function in positive and negative voltage ranges.^11,12 Third, modifying the side chains of redox-active D–A building blocks influences key properties of RACPs, such as their solubility, linearity, and bandgap reduction.¹³ These properties can be further optimized by selecting suitable redox-active building blocks before copolymerization.^14,15 Historically, p-type polymers such as polypyrrole (PPy), polyaniline (PANI), poly(3,4-ethylenedioxythiophene) (PEDOT), and polythiophenes (PTh) have been widely explored as pseudocapacitive electrodes for energy storage. Additionally, fused heterocycle derivatives, for example, benzo[1,2-b:4,5-b′] dithiophenes (BDT) had emerged as promising donor moieties due to their planar and fused 2-D hybrid structure, resulting in enhanced electrical conductivity and charge transport.^6,16,17 Our previous studies have shown that side-chain engineering at the 4 and 8 positions of BDT improved its planarity, conductivity, and thermal stability while lowering its LUMO energy levels, making it suitable for supercapacitor applications.^5,13,18 Furthermore, BDT possesses an off-axis dipole moment that can be enhanced through intermolecular interactions, contributing to its ambient stability due to its low-lying highest-occupied molecular orbital (HOMO).¹⁹ By introducing triisopropylsilylethynyl (TIPS) side chains at the 4 and 8 positions of benzo[1,2-b:4,5-b′]dithiophene (BDT), we designed 4,8-bis((triisopropylsilyl)ethynyl)benzo[1,2-b:4,5-b′]dithiophene (Si-BDT)-a highly planar building block that exhibits improved solubility, enhanced absorption properties, and well-tuned molecular energy levels.^20,21 The silicon atoms in the bulky side chains facilitate efficient charge transport by reducing phase separation and steric hindrance while promoting strong π–π stacking interactions in the solid state.²² Although the stability of acceptor moieties in D–A RACPs remains a concern for negative charge storage, recent advancements have demonstrated that incorporating suitable redox-active electron-withdrawing building blocks, such as rylene diimides-perylene diimide (PDI) and naphthalene diimide (NDI), benzodithiophenedione (BDD), diketopyrrolopyrrole (DPP), and benzothiadiazole (BT), can significantly enhance stability.^16,23–25 These rylene diimides-a family of polycyclic aromatic compounds comprising rich carbonyl groups-have been widely recognized as effective n-type building blocks for polymers, enhancing electron-transport properties while imparting high thermal and oxidative stability to the polymer matrix.^24,26–28 Additionally, they are promising candidates for energy storage because they achieve faster charging rates, store high-density energy, and expand the operational voltage range in the negative potential ranges.²⁹ Thus, achieving the wide working potential window of rylene diimide-based polymers requires copolymerization with a suitable electron donor (Si-BDT) to design redox-active donor–acceptor (D–A) polymers, as well as the selection of an appropriate electrolyte for electrochemical energy storage applications. However, it is a critical challenge to identify compatible electrolytes for the RACP-based electrodes due to their unfavorable solubility in electrolyte solutions.^30–33

Suitable electrolytes for supercapacitors can help to enhance their energy density through their ability to work in high-potential windows.³⁴ The solvents containing organic salts are suitable for this purpose. Tetrabutylammonium hexafluorophosphate (TBAPF₆) and tetraethylammonium tetrafluoroborate (TEABF₄) are the most commonly used salts among organic electrolytes for supercapacitor applications.^35,36 The hexafluorophosphate (PF₆⁻) anion in TBAPF₆ is more chemically stable than the tetrafluoroborate (BF₄⁻) anion in TEABF₄ because PF₆⁻ is less prone to hydrolysis and decomposition than BF₄⁻.³⁷ TBAPF₆ in acetonitrile is ideal for a wide range of potential applications owing to its non-coordinating nature, stability, ease of purification, and solubility.³⁸ Although these prior studies support the principle that the energy storage behavior of pseudocapacitive electrodes is affected by the electrolytes, a comprehensive investigation into the energy storage behavior of redox-active electrodes affected by the interfacial interaction between the electrodes and specific organic electrolytes has not yet been conducted. Molecular-level behavior at the electrode–electrolyte interface is influenced by various factors, such as electrode surface chemistry, electrolyte composition, and ion dynamics, making it difficult to generalize or predict outcomes across different systems.³⁹ A wide variety of electrolytes, each with unique electrochemical properties, is available, and each electrolyte requires specific consideration regarding its compatibility with different redox-active materials, which adds another layer of complexity, even though much of the focus in the field has been on optimizing electrode materials. This research gap is particularly evident in the case of RACPs, where understanding the interplay between electrolyte selection and polymer architecture is crucial for maximizing their performance. Therefore, it is necessary to address this overlooked aspect by systematically investigating the impact of electrolyte choice and interfacial interactions on the energy storage behavior of engineered RACPs in supercapacitor applications.

In this study, we explored the energy storage behavior of two side-chain-engineered RACPs (Si-BDT-NDI and Si-BDT-PDI) operated in two representative electrolytes with organic salts (TBAPF₆ and TEABF₄). Synthesizing molecules with NDI/PDI molecular architectures featuring electron-deficient units can create efficient electron donor–acceptor-based RACPs. The impact of the interfacial interactions between the polymer electrodes and organic electrolytes (TBAPF₆ and TEABF₄) was experimentally studied and theoretically supported by density functional theory calculations and computational studies. According to the results, Si-BDT-PDI operated in the TBAPF₆ electrolyte showed superior energy storage behavior, offering a specific capacity value of 267 C g⁻¹ at a current density of 1 A g⁻¹ over a wide operating potential window of −1.7 V to 0.7 V due to its ambipolar properties and the use of a non-aqueous organic electrolytic environment. The dominance of the rylene diimide-based electron-rich moiety in the D–A polymer, with prominent redox peaks, makes it a suitable anodic candidate for supercapacitor applications. An asymmetric hybrid supercapacitor (AHSC) device fabricated using a porous activated carbon cathode delivered a high energy density of 55 W h kg⁻¹ and a robust stability of ∼88% over 10 000 cycles at a current density of 5 A g⁻¹. The strategy developed in this work to enhance the energy storage behavior of RACPs through side-chain-engineering and operating in organic electrolytes can further be utilized to explore effective polymer electrodes for supercapacitor applications.

2 Experimental

2.1. Synthesis of monomers and polymers and their characterization

Scheme 1 outlines the synthetic pathways for designing the two D–A copolymers, Si-BDT-NDI and Si-BDT-PDI. The detailed synthetic procedures for the Si-BDT donor unit and the two electron-accepting monomers, M1,⁴⁰M2,⁴¹ and M3,¹¹ as well as the copolymers, are provided in the ESI.† All monomers (M1–M3) were synthesized using previously reported methods. Structural characterization of the intermediates, monomers (M2 and M3), and copolymers was conducted using nuclear magnetic resonance (NMR) spectroscopy (ESI S1–S5†). The two D–A copolymers, Si-BDT-NDI and Si-BDT-PDI, were synthesized via Stille cross-coupling polymerization in a deoxygenated sealed microwave vial, using monomers M1 and M2 or M3 in chlorobenzene at 130 °C for 24 h. The polymerization reaction was catalyzed by Pd₂(dba)₃, with P(o-tolyl)₃, CuI, and Pd₂(dba)₃ serving as the ligand, salt, and catalyst, respectively. After polymerization, both polymers were purified by Soxhlet extraction using methanol, acetone, hexane, dichloromethane, and chloroform, each for 24 h. The chloroform-soluble fraction was then concentrated and reprecipitated in methanol to yield the desired copolymers. Gel permeation chromatography (GPC) was used to determine the molecular weights (M_w) of the Si-BDT-NDI and Si-BDT-PDI copolymers (ESI Fig. S6†). Their number-average molecular weights (M_n) were 36.2 kDa and 38.1 kDa, with polydispersity indices (PDI) of 1.98 and 1.44, respectively. It is clearly seen from GPC that the resulting polymers showed good M_w with a narrow PDI, which is a good sign to achieve efficient energy storage devices with high stability. Both copolymers exhibited good thermal stability, with thermal decomposition temperatures (T_d) corresponding to 5% weight loss observed at around 430 °C and 435 °C for Si-BDT-NDI and Si-BDT-PDI, respectively, as depicted in ESI Fig. S7.† These T_d values are sufficiently high for the good electrochemical performance of the copolymers. The Fourier transform infrared spectroscopy (FT-IR) spectra of Si-BDT-NDI and Si-BDT-PDI polymers in ESI Fig. S8† show distinct features in the carbonyl and aromatic stretching regions. FT-IR spectra of the two polymers showed stretching frequencies such as sp² C–H stretching at 2924 (aromatic C–H), 2856 (aliphatic C–H), 1670 (C=O), 1245 (C–N), and 815 cm⁻¹ in the fingerprint zone. Additional peaks in the range of 1600–1400 cm⁻¹ are attributed to aromatic C=C stretching. The overall spectral patterns confirm the presence of characteristic NDI and PDI units in the respective polymers. From NMR, GPC, TGA, and FT-IR spectroscopy, it has been confirmed that the copolymers were successfully synthesized.

Scheme 1 Synthesis of the Si-BDT-NDI and Si-BDT-PDI copolymers. Stille polymerization conditions: P(o-tolyl)₃, CuI, Pd₂(dba)₃, and chlorobenzene at 130 °C for 48 h.

2.2. Computational methods

To explore the electronic properties and conductivity of Si-BDT-rylene-based electrodes, specifically Si-BDT-NDI and Si-BDT-PDI, and their interactions with electrolytes, TEABF₄ and TBAPF₆, density functional theory⁴² (DFT) calculations were carried out using the 6-31G(d,p)^43,44 basis set and the B3LYP⁴⁵ functional with empirical dispersion (GD3), developed by Grimme et al.⁴⁶ The calculations were performed using acetonitrile as the solvent and the polarized continuum model (PCM) for implicit solvation. The electrode–electrolyte systems were prepared by positioning an ion pair of TEA⁺/BF₄⁻ or TBA⁺/PF6⁻ above the Si-BDT-rylene-based electrode surface for geometry optimization in their ground state. To assess the impact of side-chain modifications in the donor Si-BDT unit on the capacitance and reduce computational complexity, shorter ethyl and longer propyl groups were used to substitute the original branched alkyl substituents in the polymer structures. Normal-mode vibrational frequencies were computed using Hessian analysis of the optimized geometries, confirming that the resulting optimized geometries corresponded to local minima characterized by positive vibrational frequencies. Additional frontier molecular orbital (FMO) analysis and interaction energy calculations were performed using the Gaussian⁴⁷ 16 software. Time-dependent density functional theory (TD-DFT) calculations were carried out using the same level of theory as that used in the Gaussian⁴⁷ software. Natural transition orbital⁴⁸ (NTO) analysis was performed using the Multiwfn 3.8 program.⁴⁹ To investigate the non-covalent interactions between the electrode and electrolyte systems, we employed Bader's atoms in molecules⁵⁰ (AIM) theory along with the non-covalent interaction (NCI) analysis framework introduced by Johnson et al.,⁵¹ which was conducted using Multiwfn 3.8 (ref. 48) and visualized using VMD 1.9.1.⁵²

3 Results and discussion

3.1. Structural and morphological studies

Powder X-ray diffraction (XRD) measurements were conducted to examine the molecular structural arrangements and microstructures of the two-conducting polymers (Fig. 1a). Both Si-BDT-NDI and Si-BDT-PDI electrodes exhibited well-defined first-order (100) peaks at 2θ values of ≈3.75° and 5.3°, corresponding to out-of-plane lamellar stacking with inter-chain distances of 23.53 Å and 16.65 Å, respectively. The second-order (200) peaks appeared at 2θ values of ≈5.5° and 12.0°, corresponding to a d-spacing of 16.65 Å and 7.36 Å for Si-BDT-NDI and Si-BDT-PDI, respectively. Additionally, broad (010) peaks were observed for both polymers at 2θ values of approximately 25.0° (Si-BDT-NDI) and 15.95° (Si-BDT-PDI), corresponding to π–π stacking distances of 3.55 Å and 5.54 Å, respectively. These peaks reflect the separation between polymer chains, which are spaced apart by flexible side chains. Both copolymers displayed in-plane (IP) stacking distances of a few angstroms due to their dominant edge-on-oriented crystallinity.⁵³ Notably, Si-BDT-PDI exhibited a larger IP stacking distance compared to Si-BDT-NDI, suggesting a more loosely packed microstructure. This structural feature likely contributes to a higher density of electrochemically active sites within the electrode, facilitating stronger electrode–electrolyte interactions and making it more favorable for charge storage applications.⁵⁴ The X-ray photoelectron spectroscopy (XPS) analysis provides insights into the surface composition and chemical environment of the Si-BDT-PDI polymer. The wide-scan spectrum (Fig. 1b) confirms the presence of C, N, O, Si, and S elements, consistent with the designed polymer structure. The deconvoluted spectra of individual elements are provided in ESI Fig. S9.† The surface morphologies of the Si-BDT-NDI and Si-BDT-PDI electrodes, shown in Fig. 1c and d, were examined using field-emission scanning electron microscopy (FESEM). Both polymers displayed agglomerated, stacked structures with several pores randomly distributed across their surface.⁵⁵ The XRD results showed that both polymer sheets were separated by a few nanometers of π–π stacking distances.

Fig. 1 (a) X-ray diffraction (XRD) patterns of Si-BDT-NDI and Si-BDT-PDI, (b) survey scan from XPS spectra of the Si-BDT-PDI polymer and FE-SEM images of (c) Si-BDT-NDI and (d) Si-BDT-PDI.

Compared to Si-BDT-NDI, which exhibited less pronounced stacking of polymer sheets, Si-BDT-PDI exhibited a higher density of stacked structures. This enhanced stacking may be attributed to its higher number-average molecular weight (M_n), as determined by GPC measurements. These sheets maintained their two-dimensional structure, and the irregular nanopores within the interconnected stacking of Si-BDT-PDI likely facilitated electrolyte interactions with the electrode surface during electrochemical processes.

3.2. Molecular theoretical calculation

As demonstrated through theoretical studies, incorporating Si-functionalized groups into the BDT unit significantly enhances π-electron delocalization and promotes greater molecular planarization (Fig. 2a and c). This structural modification reduces the HOMO–LUMO gap in Si-BDT (Fig. 2b and d), indicating a lower energy barrier for charge transfer. Consequently, the Si-BDT unit exhibited improved charge mobility and higher conductivity than the unmodified BDT, which enhanced its charge storage and transport capabilities. The experimental results used to determine the electrical conductivity (σ_e) are presented in ESI Fig. S10.† The σ_e values, calculated using eqn (S5),† are found to be 1.54 × 10⁻⁶ S cm⁻¹ for BDT and 6.4 × 10⁻⁴ S cm⁻¹ for Si-BDT, which complement the theoretical analysis.

Fig. 2 Optimized ground-state geometries of the (a) BDT and (c) Si-BDT units. Calculated HOMO and LUMO energy levels, and the HOMO–LUMO energy gap (in eV) for the (b) BDT and (d) Si-BDT moieties at the B3LYP-D3/6-31G(d,p) level of theory.

3.3. Electrochemical characterization of the polymers in three electrode systems

Cyclic voltammetry (CV) tests were performed to determine the working potential window and electrochemical performance of both polymeric electrodes operated in two organic electrolytes, TEABF₄ and TBAPF₆. The CV graphs showed prominent oxidation and reduction peaks in both the electrolytes and a decent performance over a wide potential window from −1.7 V to 0.7 V, ascribed to the ambipolar nature of the polymers, as shown in Fig. 3a. Si-BDT-PDI outperformed Si-BDT-NDI in both the electrolytes. The superior electrochemical performance of Si-BDT-PDI as an active electrode material can be attributed to the larger conjugated system and stronger electron affinity due to the PDI moiety.

Fig. 3 (a) CV at 50 mV s⁻¹, (b) GCD curves at 2 A g⁻¹, (c) Nyquist plot of Si-BDT-NDI and Si-BDT-PDI polymeric electrodes in 0.1 M TEABF₄ and TBAPF₆ electrolytes (the inset shows the magnified plot in the high frequency region), (d) CV at variable scan rates (10–250 mV s⁻¹), (e) GCD curves measured at altered current densities, and (f) durability test conducted at 5 A g⁻¹ for the Si-BDT-PDI polymeric electrode in 0.1 M TBAPF₆ electrolyte.

For both polymeric electrodes, a larger area under the curve with broader peaks was observed in the TBAPF₆ electrolyte, indicating greater supercapacitive behavior in this particular electrolytic environment. The galvanostatic charge–discharge (GCD) curves in Fig. 3b exhibit a trend similar to that of the CV results, confirming consistent electrochemical measurements. The specific capacity (C_sp) values of the Si-BDT-NDI and Si-BDT-PDI working electrodes were calculated using the discharge times, as outlined in eqn (1). The Si-BDT-PDI electrode, when tested in 0.1 M TBAPF₆ electrolyte, achieved the highest C_sp of 226 C g⁻¹ at a current density of 2 A g⁻¹. In comparison, the same electrode in TEABF₄ displayed a C_sp of 206 C g⁻¹, suggesting that intermolecular interactions between the electrode and TBAPF₆ provide enhanced energy storage behavior compared to that of the electrode in the TEABF₄ electrolyte. Notably, the Si-BDT-NDI electrode exhibited a greater C_sp value in TBAPF₆ (195 C g⁻¹) than the C_sp value in TEABF₄ (185 C g⁻¹), further demonstrating the superior electrochemical performance of the Si-BDT-PDI electrode. Both polymers exhibited two successive one-electron reduction processes, forming electron polarons, followed by two-electron reduction processes to generate electron bipolarons from their neutral states. This behavior aligns with a plausible redox mechanism for the RACP, as well as with the electrolytes (TBAPF₆ and TEABF₄) proposed and described in the ESI (Fig. S11†). The presence of terminal carbonyl groups in both the NDI and PDI units facilitates redox activity, thereby enhancing the cycling stability of the RACP electrodes.^24,56

The solution resistance (R_s) of the polymeric electrodes in the two electrolytes was determined through electrochemical impedance spectroscopy (EIS) analysis, spanning frequencies from 100 000 to 0.01 Hz, as shown in Fig. 3c. In the high-frequency region, the Si-BDT-PDI electrode exhibited a lower R_s value of 18.5 Ω in TBAPF₆, compared to 24 Ω in TEABF₄, indicating more efficient charge transport in the TBAPF₆ electrolyte.

Additionally, the Si-BDT-NDI electrode showed R_s values of 22 Ω and 33 Ω in the TBAPF₆ and TEABF₄ electrolytes, respectively, further indicating the superior electrochemical behavior of Si-BDT-PDI in TBAPF₆, concurring with the CV and GCD results. Moreover, the impedance curve for Si-BDT-PDI displayed more inclination toward the y-axis (imaginary part) perpendicular to the real impedance (x-axis) in the lower-frequency region for both organic electrolytes, indicating a more favorable charge storage behavior compared to that of Si-BDT-NDI, whose impedance curve was less steep.^56,57

Owing to its superior electrochemical performance, a detailed electrochemical analysis was conducted on the Si-BDT-PDI polymeric electrodes in the TBAPF₆ electrolyte. Fig. 3d presents the CV curves recorded at increasing sweep rates (10 to 250 mV s⁻¹) for the Si-BDT-PDI polymeric electrode in TBAPF₆. Notably, a gradual increase in peak current with increasing scan rate confirms rapid charge-transfer kinetics at the electrode surface, which is characteristic of surface-controlled processes. This behavior suggests that electron transfer dominates the charge storage mechanism rather than diffusion-limited ion transport through the bulk material. The well-defined and symmetric oxidation–reduction peaks, with minimal peak shifts at higher scan rates, further confirm that redox reactions primarily occur at the electrode surface, with minimal bulk diffusion influence. The consistent pseudocapacitive behavior at higher sweep rates indicates that the charge storage process remains efficient and surface-dominated, even as the scan rate increases.^58,59

Fig. 3e shows the GCD results at various current densities, where C_sp is calculated from the obtained discharge times.⁶⁰ A high C_sp value of 267 C g⁻¹ was obtained at a current density of 1 A g⁻¹, with a gradual decrease in the C_sp value to 226, 207, 160, 126, and 96 C g⁻¹ at increasing current densities of 2, 3, 5, 7, and 10 A g⁻¹. Higher current densities require faster electrochemical reactions, which can exceed the rate at which charge is stored or released, resulting in a lower capacity, known as the kinetic constraint.⁶¹ The electrode material exhibited promising pseudocapacitive behavior, with its performance degrading somewhat at higher current densities owing to ion diffusion limitations resulting from less available time for interaction between the electrode and electrolyte.

However, the nonlinear charge–discharge curves and retention of the plateau at high current densities indicate that the polymeric electrode can still perform well in fast cycling applications.⁶²Fig. 3f shows a GCD cycling stability comparison between the two better-performing polymeric electrodes, Si-BDT-PDI and Si-BDT-NDI, in a TBAPF₆ environment at a high current density of 5 A g⁻¹. A capacity retention of ∼82% was obtained for the PDI moiety after 10 000 cycles, signifying its exceptional electrochemical stability compared to that of the NDI moiety (71%), which was attributed to its greater conjugation and superior redox-active n-type building block. This balance between high capacity at low currents and good rate capability at higher currents makes such pseudocapacitive materials attractive for supercapacitors and other energy storage devices. The superior electrochemical performance and stability of Si-BDT-PDI resulted due to a larger conjugated system, stronger electron affinity, and better π-stacking, compared to those of Si-BDT-NDI.

The power law approach is commonly employed to investigate the kinetic characteristics derived from cyclic voltammetry (CV) measurements at different potentials. The CV curves of Si-BDT-PDI ranging from 10 to 250 mV s⁻¹ (Fig. 3d) depict the involvement of the faradaic charge storage mechanism due to the redox activity of the electrode material. The power-exponent b generally ranges from 0.5 to 1, which is considered to be an important index to estimate the charge storage kinetics. Here the calculated b values (ESI Fig. S13a†) using eqn (S6)† for various fitted curves are approximately 0.53 in the cathodic region and 0.6 in the anodic region, signifying dominant diffusive behavior during the redox reactions. Additionally, the Trasatti method has been employed to further quantify the contributions from capacitive and diffusion-controlled charge storage processes, as illustrated in ESI Fig. S13b and c.†⁵ ESI Fig. S13c† highlights that the polymeric material exhibits a diffusive contribution of approximately 91% calculated in the cathodic region, indicative of fast ionic transport and surface redox activity. The remaining 9% is attributed to capacitive behavior, such as due to surface ion and electron adsorption, particularly evident at a high scan rate of 200 mV s⁻¹.

Experimental electrochemical analysis reveals that TBAPF₆ facilitates stronger electrode–electrolyte interactions than TEABF₄, contributing to improved supercapacitive performance. This enhancement is attributed in part to the higher ionic conductivity of TBAPF₆ (2.3 × 10⁻² S cm⁻¹), calculated using eqn (S9)† and shown in ESI Fig. S14.† Additionally, contact angle measurements performed on a hydrophobic PDMS surface (used due to complete absorption by carbon cloth) show a lower contact angle of ∼40° for TBAPF₆ compared to ∼57.5° for TEABF₄ (ESI Fig. S15†), indicating superior wettability and interfacial adhesion.

3.4. Computational insights into electrode–electrolyte interactions and electronic properties

The above experimental observations are further supported by computational analyses. To understand the electronic properties, the HOMO and LUMO energy levels, as well as the HOMO–LUMO gaps (ΔE_g), affected by the intermolecular interaction of the electrodes and electrolytes, were analyzed, as shown in Fig. 4. The HOMO energy levels for all systems remained almost identical at approximately −5.60 eV for both Si-BDT-NDI and Si-BDT-PDI in both TEABF₄ and TBAPF₆ electrolytes. However, a notable difference was observed in the LUMO energy levels. In TEABF₄, Si-BDT-NDI shows a more negative LUMO energy (−3.32 eV) compared to the −3.35 eV for Si-BDT-PDI (Fig. 4a and b). Both polymers exhibit further lowering of the LUMO energy levels in TBAPF₆, with the LUMO energy level of Si-BDT-NDI at −3.45 eV and that of Si-BDT-PDI at −3.51 eV (Fig. 4c and d). The lower LUMO values suggest enhanced electronic stabilization, facilitating easier electron transfer and improved interactions with the electrolyte, which are crucial for optimizing charge mobility.

Fig. 4 Frontier molecular orbital analysis of the interactions between (a) Si-BDT-NDI and (b) Si-BDT-PDI with the electrolyte TEABF₄, and (c) Si-BDT-NDI and (d) Si-BDT-PDI with the electrolyte TBAPF₆ at the B3LYP-D3/6-31G(d,p) level of theory.

The HOMO–LUMO energy gap (ΔE_g) slightly decreases from 2.28 eV for Si-BDT-NDI in TEABF₄ to 2.15 eV in TBAPF₆ and from 2.25 eV for Si-BDT-PDI in TEABF₄ to 2.12 eV in TBAPF₆ (Fig. 4 and Table S1†). This reduction in ΔE_g further highlights the enhanced charge transport and electronic properties in the presence of TBAPF₆, which provides a more favorable environment for electronic interactions. These findings emphasize the importance of electrolyte selection; TBAPF₆ provides an environment for more effective intermolecular interactions with the electrodes, resulting in enhanced charge mobility, which ultimately improves the supercapacitor performance. The electrode–electrolyte interaction energies (ΔE) can be calculated based on geometry-optimized structures at the 6-31G(d,p) level of theory. The interaction energies were derived using eqn (1).

ΔE = E_complex − (E_electrode + E_electrolyte) (1)

where E_complex, E_electrode, and E_electrolyte are the energies of the complex, electrode, and electrolyte, respectively.

Density functional theory (DFT) calculations showed that TBAPF₆ results in a lower energy gap and reduced LUMO energy levels for the polymeric electrodes compared to TEABF₄, suggesting improved charge transport properties.

A comparison of interaction energies across electrolytes (Table 1) revealed that Si-BDT-NDI interacted more strongly with TBAPF₆ (−27.67 kcal mol⁻¹) than with TEABF₄ (−20.98 kcal mol⁻¹), with a difference of approximately 6.69 kcal mol⁻¹. Similarly, Si-BDT-PDI exhibited stronger interactions with TBAPF₆ (−25.81 kcal mol⁻¹) than with TEABF₄ (−20.75 kcal mol⁻¹), with a difference of around 5.06 kcal mol⁻¹.

Table 1 Computationally calculated electrode–electrolyte interaction energies

Sl. no.	Electrode–electrolyte system	Interaction energy (I. E.) (kcal mol^−1)	Interaction energy (I. E.) (eV)
1	Si-BDT-NDI-TEABF4	−20.98	−0.91
2	Si-BDT-NDI-TBAPF6	−27.67	−1.20
3	Si-BDT-PDI-TEABF4	−20.75	−0.90
4	Si-BDT-PDI-TBAPF6	−25.81	−1.12

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These results indicate that TBAPF₆ interacts more strongly with both the polymeric electrodes than TEABF₄. Notably, Si-BDT-NDI showed a more pronounced increase in the interaction strength (∼6.69 kcal mol⁻¹) than Si-BDT-PDI (∼5.06 kcal mol⁻¹) (Table 1). Although the interaction energy of Si-BDT-PDI with TBAPF₆ was slightly less negative than that of Si-BDT-NDI, this does not diminish the significant advantages provided by the PDI group. The reduced HOMO–LUMO gap, enhanced charge delocalization, improved ion accessibility, and structural flexibility of Si-BDT-PDI promotes more efficient electrolyte ion penetration and charge accumulation, ultimately contributing to higher capacity and better overall supercapacitor performance.

The reduced density gradient (RDG) iso-surface plots (Fig. 5) provide insights into the nature and strength of non-covalent interactions between Si-BDT-based electrodes (Si-BDT-NDI or Si-BDT-PDI) and the electrolyte (TEABF₄ or TBAPF₆). The blue regions on the RDG iso-surfaces correspond to strong, attractive interactions, such as ion-dipole interactions between the ion pairs (TEA⁺/BF₄⁻ or TBA⁺/PF₆⁻) and the polar regions of the electrode surfaces, which facilitate charge transfer. The green regions indicate weak van der Waals interactions, primarily arising from dispersion interactions between the aromatic Si-BDT-NDI/PDI backbone and the alkyl groups of TEA⁺/TBA⁺, which stabilize the electrode–electrolyte interface.

Fig. 5 Non-covalent interaction (NCI) plots and reduced density gradient, s(r), versus electron density, ρ(r), multiplied by the sign of the second Hessian eigenvalue (λ₂) iso-surfaces for (a) Si-BDT-NDI & TEABPF₄, (b) Si-BDT-PDI & TEABF₄, (c) Si-BDT-NDI & TBAPF₆, and (d) Si-BDT-PDI & TBAPF₆. In the NCI plots, the red regions at the center of the rings indicate steric repulsion, while the green and light brown regions represent van der Waals and dispersion interactions. The hydrogen atoms are omitted for clarity. The reduced density gradient iso-surfaces are colored on a blue-green-red scale based on the signs of the λ₂ values and ρ(r) ranging from −0.05 to 0.05 a. u. Blue corresponds to strong, attractive (ionic) interactions, green indicates weak van der Waals and dispersion interactions, and red indicates strong repulsive interactions (such as steric clash). The plots were generated using cuboid grids with step sizes of 0.01 and 0.02 a. u. for ρ(r) and s(r), respectively.

In contrast, the red regions highlight the steric repulsion due to the bulkiness of the ionic species of the electrolyte or the extended conjugated framework of the Si-BDT-rylene-based electrodes. The green and light-brown regions in the adjacent non-covalent interaction (NCI) plots further confirm the existence of dominant van der Waals and dispersion interactions (Fig. 5) in these electrode–electrolyte systems. In particular, π–cation and π–anion interactions play a crucial role in stabilizing these systems, where the TEA⁺ or TBA⁺ cations interact with the π-conjugated Si-BDT framework, while the BF₄⁻ or PF₆⁻ anions contribute to additional electrostatic stabilization, as evident in the RDG plots.

Although all studied systems exhibit a negligible difference in their RDG iso-surface plots, the NCI analysis of the Si-BDT-PDI and-TBAPF₆ system reveals the key stabilizing interactions that significantly contribute to its electrochemical performance, particularly in supercapacitor applications. In the Si-BDT-PDI and-TBAPF₆ system, the ion–dipole interaction arises between the extended π-conjugation of the aromatic core of PDI and bulky TBA⁺/PF₆⁻ ion-pair, promoting more localized non-covalent interactions with the electrolyte. Additionally, the dispersion interactions between the π-electron clouds of the PDI facilitate π–cation and π–anion interactions, reducing steric hindrance and strengthening molecular stabilization, leading to enhanced charge transfer and electrochemical properties, thus making it outperform the rest of the electrode–electrolyte systems. Atoms-in-molecule (AIM) analysis in Fig. 6 further validates these findings.

Fig. 6 Atoms-in-molecule (AIM) analysis of (a) Si-BDT-NDI & TEABF₄, (b) Si-BDT-PDI & TEABF₄, (c) Si-BDT-NDI & TBAPF₆, and (d) Si-BDT-PDI & TBAPF₆, respectively. The orange and yellow dots represent bond critical points (BCPs), indicating bonding interactions. The green dots represent cage critical points (CCPs), highlighting the electron density confinement in the Si-BDT-NDI/PDI π-system and its interactions with the TEABF₄/TBAPF₆ electrolyte. The bond paths are shown as orange lines, showing the interaction pathways between atoms or critical points.

The atoms-in-molecule (AIM) analysis provides a detailed insight into the electron density distribution and bonding interactions within the electrode–electrolyte systems, complementing the findings from the NCI analysis. Bond critical points (BCPs), represented by orange and yellow dots, indicate key non-covalent interactions such as π–cation and π–anion (ionic or van der Waals) interactions between the electrolyte ions (TEA⁺/BF₄⁻ or TBA⁺/PF₆⁻) and the aromatic π-system of the Si-BDT-rylene-based electrodes (Si-BDT-NDI or Si-BDT-PDI). These interactions, highlighted by bond paths (orange lines) connecting the BCPs further validate the electrostatic and van der Waals interactions within the hydrophobic regions of the Si-BDT backbone, particularly between the bulky alkyl groups of TEA⁺ or TBA⁺ and the π-conjugated framework of the electrode, thereby stabilizing the electrode–electrolyte interface and enhancing structural integrity and charge transfer. Cage critical points (CCPs), denoted by green dots, emerge in regions of spatial confinement, particularly within the complex electrode–electrolyte arrangements. These points indicate areas of enclosed electron density, reflecting steric and electrostatic influences that dictate the molecular organization of the electrolyte (TEABF₄ or TBAPF₆) around the electrode surface or Si-BDT framework.

The AIM analysis further supports the superior stability of the Si-BDT-PDI & TBAPF₆ system compared to other studied systems, as evidenced by a denser network of non-covalent interactions compared to TEABF₄. The stronger π-ion interactions and enhanced charge transfer efficiency observed in this system contribute to its higher electrochemical stability and improved supercapacitor performance, aligning with the NCI analysis findings.

Based on the natural transition orbital (NTO) analysis, TBAPF₆ appears to interact more efficiently with the Si-BDT-PDI electrode (ESI Fig. S16†), as it allows for delocalization of both holes and electrons over the entire electrode structure.

In contrast, Si-BDT-NDI-based electrodes show weaker interaction energies due to reduced interaction strength and higher HOMO–LUMO gaps due to the smaller, less π-conjugated NDI moiety compared to PDI. The non-planar geometry of NDI, characterized by a greater torsional angle between the core and its substituents, diminishes π-conjugation. Thus, the reduced π-electron density leads to weaker π–cation interactions with TBA⁺ and less effective π–anion stabilization by PF₆⁻, ultimately limiting charge transport efficiency. Similarly, systems incorporating TEABF₄ as the electrolyte (Si-BDT-NDI & TEABF₄ and Si-BDT-PDI & TEABF₄) display weaker overall interactions due to the less effective charge stabilization by the BF₄⁻ anion and the smaller TEA⁺ cation. These computational findings establish a theoretical basis for the superior electrochemical performance of Si-BDT-PDI & TBAPF₆ highlighting the role of π-conjugation and ion–pair interactions in enhancing electronic properties and optimizing electrode–electrolyte interfaces for next-generation energy storage applications.

3.5. Asymmetric hybrid supercapacitor device application using Si-BDT-PDI as the anode

Asymmetric hybrid supercapacitors (AHSCs), which integrate two different charge-storage mechanisms, that is, electrical double-layer capacitance (EDLC) and batteries, have the promising advantage of operating over a wide potential window without compromising the stability of the device by providing a high energy density.⁶³ To explore its practical applications, an AHSC device was fabricated using Si-BDT-PDI as the anode, given its superior performance in the negative potential window, and petroleum-coke-derived porous activated carbon (PAC) as the cathode in a non-aqueous TBAPF₆ electrolyte.⁶⁴ The details of the three-electrode electrochemical performance of the PAC in the same 0.1 M TBAPF₆ electrolyte in the cathodic region are presented in the ESI (ESI Fig. S18†).

The CV curves (Fig. 7a) measured at a scan rate of 50 mV s⁻¹ clearly show that the Si-BDT-PDI polymeric electrode exhibits a battery-type nature in the anodic region, whereas PAC behaves as a proper capacitor with no oxidation or reduction peaks owing to its non-faradaic charge storage properties. This asymmetric configuration helps enhance the overall operating potential window of the constructed device to 2.7 V, attributed to the 0 to −1.7 V for the anodic polymer and 0 to 1 V for the PAC in the cathodic region (without considering the overlap region on the positive side). Furthermore, CV and GCD (Fig. 7b and c) of the device were performed in different windows to confirm the optimal working potential range. The CV curves obtained in three different potential ranges (0–2.4, 2.7, and 3 V) exhibit the same prominent peaks during cycling. To further confirm the performance, GCD was performed at a current density of 1 A g⁻¹ in the same potential ranges of 0–2.4, 2.7, and 3 V, showing specific capacities of 23, 73, and 70 C g⁻¹, respectively. The higher C_sp and lower IR drop observed in a potential window of 2.7 V, compared to those observed in the other tested ranges, make it a suitable range for energy storage devices. Therefore, we studied the electrochemical behavior of the AHSC device in the potential window of 2.7 V. Fig. 7d shows the CV curves for the Si-BDT-PDI//PAC AHSC device across a range of scan rates, from 10 to 150 mV s⁻¹. Clear oxidation and reduction peaks are visible at a slower scan rate of 10 mV s⁻¹. Notably, even at higher scan rates of up to 150 mV s⁻¹, the CV curves show minimal distortion in the peaks, with a slight left shift in the reduction and a right shift during the oxidation process. The constant increase in the area under the curve indicates that the redox processes remained stable under these faster cycling conditions, with enhanced energy storage behavior. This stable performance, even at higher scan rates, was attributed to the strong redox-active nature of the PDI unit in the Si-BDT-PDI anode, which ensured that the charge/discharge processes occurred efficiently, even at increased speeds. Further electrochemical analyses were performed using GCD measurements, and the results are shown in Fig. 7e. The C_sp values of the AHSC device were determined at various current densities: 146 C g⁻¹ at 1 A g⁻¹, decreasing to 60 C g⁻¹ at 5 A g⁻¹.

Fig. 7 (a) CV of PAC and Si-BDT-PDI at 50 mV s⁻¹, (b) CV and (c) GCD at different cut-off potentials, (d) CV at variable scan rates (10–150 mV s⁻¹), (e) GCD curves across various current densities of 1–5 A g⁻¹, and (f) Ragone plot of the Si-BDT-PDI//PAC AHSC device.

These results demonstrate excellent capacity retention at low current densities and a gradual decrease at higher current densities. This is expected because of the reduced interaction time between the electrode and electrolyte at higher rates. The GCD curves clearly show two distinct charge–discharge platforms aligned with the observed redox peaks in the CV analysis, further confirming the battery-like behavior of the Si-BDT-PDI anode and the capacitor-like behavior of the activated carbon cathode. The Ragone plot shown in Fig. 7f illustrates the tradeoff between the energy density (E_d) and power density (P_d) for the Si-BDT-PDI//AC AHSC device, which was calculated using eqn (S3) and (S4)† to assess its overall performance.

The hybrid device demonstrates a remarkable energy density of 55 W h kg⁻¹ at a power density of 2712 W kg⁻¹. This high energy density is primarily attributed to the broad operational voltage window of 2.7 V, facilitated by the redox-active polymeric electrode in the organic electrolyte and the synergistic effect of the hybrid supercapacitor configuration. At a higher current density of 5 A g⁻¹, the device exhibited a peak power density of 13 380 W kg⁻¹, with an energy density of 22.3 W h kg⁻¹, further highlighting its excellent performance across a wide range of power demands. Herein, the side-chain engineering of the D–A polymer, combined with the use of optimized organic electrolytes, enables stable operation over a wide potential window of 2.7 V, resulting in significantly enhanced E_d. A comparison of E_dversus P_d values with those of previously reported polymeric electrodes for asymmetric supercapacitors is presented in Fig. 7f.^5,13,65–68

The cycling durability test of the AHSC device was conducted at a current density of 5 A g⁻¹, and the results are presented in Fig. 8a. An excellent capacity retention of ∼88% was obtained even after 10 000 cycles, with a drop in specific capacity from an initial 60.0 to 52.8 C g⁻¹. Incorporating the highly stable PAC as the cathode contributed largely to the stability of the constructed device and its efficient performance over a wide potential window of 2.7 V. A coulombic efficiency greater than 95% also indicates the efficiency of the device in the long run.

Fig. 8 (a) Cycling stability test conducted at 5 A g⁻¹, (b) EIS after the 1^st and 10 000^th cycle, and (c) C-rate performance of the Si-BDT-PDI//PAC AHSC device.

The Nyquist plots obtained from the EIS study after the 1^st and 10 000^th cycles, shown in Fig. 8b, support the decrease in capacity. The plots reflect the device's electronic resistance (R_s) change in the high-frequency region from 10.4 to 22.2 Ω, with the larger semi-circle suggesting greater resistance between the electrode and electrolyte after several cycles. An evident deviation from the y-axis in the low-frequency region after 10 000 cycles (red line) suggests sluggish ion diffusion. To understand the stability of the charge-storage mechanism, the C-rate performance was studied (Fig. 8c), which confirmed the excellent rate capability of the AHSC device. The GCD performance for 10 continuous cycles at a particular current density was analyzed by switching the current from high to low (2 A g⁻¹ to 1 A g⁻¹) and then again from low to high (1 A g⁻¹ to 5 A g⁻¹) for 70 cycles, showing only a negligible deviation; the device retained a specific capacity value of ∼99%.⁶⁴

These electrochemical results highlight the excellent stability, reversibility, and energy storage capability of the Si-BDT-PDI//PAC AHSC device. The device shows promising performance with a high energy density and stable cycling behavior, even at elevated scan rates and current densities, making it a suitable candidate for practical energy storage applications.

4 Conclusions

In this study, the supercapacitor performances of two planar donor–acceptor (D–A) redox-active conjugated polymers (RACPs), Si-BDT-NDI and Si-BDT-PDI, operated in two different electrolytes with organic salts, TBAPF₆ and TEABF₄, were extensively characterized by various electrochemical analyses. Si-BDT-PDI exhibited an outstanding specific capacity of 267 C g⁻¹ at 1 A g⁻¹. This superior performance is attributed to the incorporation of the acceptor PDI, which enhances conjugation, reduces the bandgap, and promotes greater electron delocalization. Additionally, the extended operating potential range in the negative region, leading to broad reduction peaks, results from the strong electron-withdrawing moieties in the polymeric backbone, making them highly suitable as anodic materials. Computational calculations, including electrode–electrolyte interaction energy calculations, AIM and non-covalent interaction (NCI) plots, further confirmed that both electrodes interact more favorably with TBAPF₆, which facilitates improved charge transfer and enhanced electrochemical performance. Furthermore, an asymmetric hybrid supercapacitor (AHSC) device was constructed using a Si-BDT-PDI anode and a PAC cathode. The device was operated over a broad potential window of 2.7 V and delivered a high energy density of 55 W h kg⁻¹ and a power density of 2712 W kg⁻¹. Notably, the device exhibited excellent cycling stability, retaining ∼88% of its initial capacity over 10 000 galvanostatic charge–discharge (GCD) cycles while maintaining a coulombic efficiency of >95% due to the presence of an electric double-layer capacitance (EDLC)-type cathode. These comprehensive analyses underscore the significance of optimizing electrode–electrolyte interactions, which play a crucial role in designing hybrid energy-storage devices capable of operating over wide potential ranges for achieving high energy and power densities.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Author contributions

Conceptualization, investigation, methodology, and formal analysis: S. K. Pati. Synthesis and data curation: S. K. Pati and D. Patra. Formal analysis, software: S. Muduli, S. Oh, and B. J. Kim. Writing – original draft: S. K. Pati, S. Muduli, and D. Patra. Validation, visualization, writing – review & editing: S. K. Pati, D. Patra, S. Mishra and S. Park. Funding acquisition: S. Mishra and S. Park. Supervision: D. Patra, S. Mishra, and S. Park.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This study was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean Government (MSIT) (RS-2024-00335216) and the Ministry of Trade, Industry, and Energy (MOTIE, Korea) (20024254). S. M. acknowledges the financial support from the UGC, New Delhi. S. M. acknowledges the support from the SERB, Govt. of India (CRG/2022/004088 and SR/FST/CSII-026/2013), and CSIR, Govt of India (01(2987)/19/EMRII). This study used the resources of the Paramshakti supercomputing facility of the IIT, Kharagpur, established under the National Supercomputing Mission of the Government of India and supported by the CDAC, Pune.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta02268a

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