Understanding Water Role in Lyotropic Liquid Crystalline Electrolytes for Supercapacitors PDF

Journal of Molecular Liquids 394 (2024) 123705 Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq Understanding the role of water in the lyotropic liquid crystalline mesophase of high-performance flexible supercapacitor electrolyte...

Journal of Molecular Liquids 394 (2024) 123705 Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq Understanding the role of water in the lyotropic liquid crystalline mesophase of high-performance flexible supercapacitor electrolytes using a rheological approach Mert Umut Özkaynak a, b, *, Banu Kocaaga c, Koray Bahadır Dönmez a, Selin Dağlar a, Yurdanur Türker a, Nilgün Karatepe d, F. Seniha Güner a, c, Ömer Dag e, f a Sabanci University Nanotechnology Research and Application Center (SUNUM), Istanbul 34956, Turkey Department of Materials Science and Engineering, Istanbul Technical University, Istanbul 34469, Turkey c Department of Chemical Engineering, Istanbul Technical University, Istanbul 34469, Turkey d Energy Institute, Renewable Energy Division, Istanbul Technical University, Istanbul 34469, Turkey e Department of Chemistry, Bilkent University, Ankara 06800, Turkey f UNAM—National Nanotechnology Research Center and Institute of Materials Science and Nanotechnology, Bilkent University, Ankara 06800, Turkey b A R T I C L E I N F O A B S T R A C T Keywords: Gel electrolyte Lyotropic liquid crystalline mesophase Flexible supercapacitor The effect of water on the structure, properties, and flexibility of lyotropic liquid crystalline (LLC) C12E23-LiClH2O gel electrolytes was explored. Structural techniques, such as X-ray Diffraction (XRD), Polarized Optical Microscopy (POM), and five dynamic measurements, were employed to examine the rheological properties of the LLC mesophase across various water contents. These analyses provided quantitative insights into the influence of water content and LiCl concentration on gel strength, gelation point, and structural recovery. The threedimensional network of the gel encapsulates Li+ and Cl− ions within hydrophilic domains, showing significant performance in supercapacitor applications. The observed increase in storage modules with decreasing water content is attributed to variations in the quantity and average size of junction points owing to system entan­ glement. These research findings highlight that excess water molecules, which break down micellar connections, are responsible for the weakening of the gel. Conversely, at low water concentrations, the micellar domains entangle, displaying viscoelastic behavior akin to that of a transitory polymer network. 1. Introduction Wearable technology, including smartwatches, health trackers, and electronic textiles, has catalyzed the evolution of flexible wearable gadgets. Moreover, the latest progress in wearable and implantable health devices has led to the creation of energy storage systems that harness body-generated energy from actions like breathing, perspira­ tion, and motion. Supercapacitors are considered ideal for wearable applications owing to their durability, high power output, and adapt­ ability to miniaturization. Supercapacitors are known for their high power density, long cycle lifetime, and fast charge/discharge rates compared with regular batte­ ries. They are also ideal for various applications such as hybrid electric vehicles and energy backup systems. In current technology, it is necessary to provide a flexible structure to supercapacitor components to achieve energy storage systems compatible with flexible consumer electronics. The development of supercapacitors with increased energy storage capabilities, improved mechanical flexibility, and self-healing properties remains a challenge. The electrochemical performance of supercapacitors may degrade because of their poor mechanical properties, which limit their applica­ tion in flexible devices. Reliable flexibility, shape adaptability, moldability, and self-healing properties play a critical role in the per­ formance and multifunctionality of flexible electronic devices [4–6]. The flexibility and self-healing properties endow supercapacitors with a variety of shapes, light weight, and strong deformation abilities and extend their life [7–10]. An electrolyte is an essential part of a supercapacitor and supplies ionic conduction between the anode and the cathode. The current flows through the electrolyte between the two electrodes, charging or * Corresponding author at: Sabanci University Nanotechnology Research and Application Center (SUNUM), Istanbul 34956, Turkey. E-mail address: [email protected] (M.U. Özkaynak). https://doi.org/10.1016/j.molliq.2023.123705 Received 22 September 2023; Received in revised form 28 November 2023; Accepted 29 November 2023 Available online 4 December 2023 0167-7322/© 2023 Elsevier B.V. All rights reserved. M.U. Özkaynak et al. Journal of Molecular Liquids 394 (2024) 123705 discharging the supercapacitor. Generally, electrolytes are classified according to their ionic conductivity, working-temperature range, sta­ bility, operating voltage range, and safety. The development of highly conducting electrolytes is important for optimizing the perfor­ mance of supercapacitors. However, in recent years, it has been shown that rheological properties have a significant effect on super­ capacitor performance [13–15]. Various acids, bases, and salts have been used as ion sources for electrolytes. H3PO4, KCl, KOH, and H2SO4 have been widely used as ion sources for aqueous electrolytes. Tertiary butyl-ammonium salts are commonly used as the organic electrolytes. Aqueous and nonaqueous electrolytes of LiCl, LiClO4, LiOH, Li2SO4, and LiPF6 salts have excellent compatibility with various porous electrodes owing to the small size of Li ions, expanding their application in capacitors and bat­ teries. However, liquid electrolytes have limitations for the fabri­ cation of flexible supercapacitors. Because the conductivity of solid (ceramics) electrolytes is insufficient, and there are leakage or vapor­ ization problems in liquid forms, gel electrolytes are advantageous. Most liquid electrolytes are toxic and corrosive, exhibit low reliability, and require expensive packaging for the fabrication of flexible electro­ chemical capacitors. Electrolytes produced in gel matrix are essential in this context because of their high ionic conductivities, wide electro­ chemical potential windows, good mechanical strength, and excellent cycling stability. Various lithium salts and acids have been previously shown to form lyotropic liquid crystalline (LLC) mesophases (gels) [17–20] with oligo (ethylene oxide)-type nonionic surfactants and have already been employed in different energy-storage systems with good performance [19–22]. An LLC mesophase comprises two main components: an amphiphile (surfactant) and solvent (water and aqueous salt solutions). LLC mesophases of aqueous alkali metal salts have superior physical properties such as good thermal stability, high ionic conductivity, and nonvolatility [19,21,22]. Moreover, oligo (ethylene oxide)-type surfac­ tants have self-healing properties because they exhibit shear-thinning properties owing to the dynamic nature of hydrogen bonds, which al­ lows the network to provide viscous flow and autonomously revert to solid-like behavior when the stress is removed. Rheological measurements are useful for investigating the gelation behavior. Viscosity and dynamic modulus are directly related to the physical and mechanical properties of the gel during the manufacturing and operating processes. To the best of our knowledge, there are no previously reported rheological investigations of salt–surfactant gel structures made from oligo (ethylene oxide) surfactants. Dag et al. demonstrated that LiCl salts could form highly ordered mesostructured liquid crystalline gels [17,18]. According to these studies, overconcentrated aqueous LiCl solutions can form highly or­ dered mesostructured liquid crystalline gels with nonionic surfactants. The aqueous solution acts as a solvent in the assembly of 10-lauryl ether (C12H25(OCH2CH2)10OH, abbreviated as C12E10), in which a highly concentrated electrolyte solution can be considered as a molten salt. A strong ion–dipole interaction exists between the salt species and water molecules, in addition to a network of hydrogen bonds in the meso­ phase; the salt-water couple collaboratively acts as a solvent to form LLC mesophases [20–23]. The liquid crystalline mesophase of the LiClC12E10-H2O mixture and any other salt-surfactant mesophase have not yet been implemented in an energy storage system. Most researchers have focused on using inorganic and/or organic additives to improve the performance of gelated electrolytes. In this study, 23-lauryl ether (C12E23, C12H25(OCH2CH2)23OH), LiCl, and H2O were combined to prepare novel gel structures to overcome the disad­ vantages of conventional gel structures, thereby improving the physical and electrochemical properties and optimizing the overall performance of the gel electrolytes. Rheological measurements of the gel-electrolyte formulations containing different proportions of water were per­ formed to optimize the mechanical properties. In this context, strain, frequency sweep, temperature sweep, and creep recovery analyses were conducted. In addition, the shear thinning and self-healing properties were examined. The electrochemical performance of the gel electrolyte prepared by combining 23-lauryl ether, LiCl, and H2O was evaluated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge–discharge (GCD) tests. A promising gel composition for carbon-based supercapacitors was revealed by electro­ chemical and rheological characterization. 2. Material and methods 2.1. Material LiCl, N-methyl-2-pyrrolidone (NMP), polyvinylidene fluoride (PVDF), and carbon black were purchased from Sigma-Aldrich. C12E23 (23-lauryl ether, C12H25(OCH2CH2)23OH) was provided by JK Scientific and used as received with purities of 99 % and above. Activated carbon was synthesized from olive waste through a series of controlled pyrolysis and activation processes, involving heating the precursor material under inert conditions followed by chemical activa­ tion. A supplementary document provides information on the prepara­ tion of an activated carbon, with the porous textural properties of the activated carbon described in Table S1. Furthermore, the synthesized activated carbon was utilized as a high-performance supercapacitor electrode, demonstrating its exceptional electrochemical properties and potential for energy storage applications. Elaborate electrode prepara­ tion details are available in the Electrochemical Characterization section. 2.2. Preparation of electrolytes LiCl (0.106 g) was dissolved in different amounts (0.875, 1.000, 1.125, and 1.250 mL) of deionized (Milli-Q) water and stirred for 5 min to achieve homogeneity. Then, 0.5 g C12E23 was added to each solution. Finally, the mixture was maintained at 80 ◦ C for 1 h. The gel samples were denoted as LCx(H2O), where x is the amount of water (Table 1). 2.3. Structural characterization X-ray diffraction (XRD) patterns were recorded on a Rigaku Miniflex diffractometer with a high-power Cu Kα source operated at 30 kV/15 mA. Room-temperature measurements were performed by spreading the samples on glass slides. The polarizing optical microscope (POM) images were captured using a ZEISS Axio Scope A1 in transmittance mode. 2.4. Rheological characterization The rheological properties of the samples were investigated using an Anton Paar MCR 301 rheometer (Anton Paar, Graz, Austria) with a 25 mm diameter plate and plate (PP) or cone and plate (CP) geometry and a Peltier temperature hood (H-PTD 200; ±0.1 ◦ C). Viscotherm VT2 con­ trols the temperature of the sample. The gap between the probe and samples was recorded at 1.3 mm distance in all measurements. The samples were heated to 80 ◦ C and allowed to melt completely for 5 min before being placed on a rheometer plate. During all the measurements, the samples were tightly sealed. All analyses were conducted in repli­ cates. All the experimental parameters are listed in Table 2. Table 1 Formulation of gel structures. 2 Sample LiCl (g) LE (g) H2O (mL) LC0.875(H2O) LC1.000(H2O) LC1.125(H2O) LC1.250(H2O) 0.106 0.106 0.106 0.106 0.5 0.5 0.5 0.5 0.875 1.000 1.125 1.250 M.U. Özkaynak et al. Journal of Molecular Liquids 394 (2024) 123705 Table 2 Experimental setup for rheological characterization. Experiment Measured parameters Temperature (◦ C) Stress (Pa) Strain (%) Frequency (rad⋅s− 1) Time (min) Strain sweep Frequency sweep Time sweep Temperature sweep Thixotropy Creep-recovery Stress relaxation G’, G’’, tan δ, LVE, gel point G’, G’’, tan δ, complex viscosity G’, G’’, tan δ G’, G’’, tan δ, gel point G’, G’’, tan δ Strain G/G0, relaxation time (τ) 25 25 25 − 10 - +60 25 25 − 10, 25, 45, 60 – 1 1 0.1–1000 1 1 1 1, 1000 – 1 1 1–100 1 1 1 – – – – 30 – 12 15 16.6 – 10, 25 – 2.4.1. Dynamic oscillatory measurements Strain sweep tests were performed to determine the following: (i) linear viscoelastic range (LVE) or critical strain (ƔL), (ii) G’LVE (elastic modulus at the critical strain), (iii) flow point, which is the strain (stress) caused by internal structure failure to the point, where the material flows (Ɣf) (G’=G’’), and (iv) the cross point of the elastic and viscous moduli (Gf). Frequency sweep tests were performed to determine in the frequency range the slopes of the complex viscosity, elastic modulus (G’), viscous modulus (G’’), and damping factor (tan δ) of the samples within the LVE region. treatment and maintained until the solution was completely dispersed. The dispersion was then dropped onto carbon foam and left at70◦ C overnight to remove excess NMP on the electrode. A 1.2 cm diameter cellulose acetate filter membrane was soaked in the gel for 30 s, and the filter membrane was immediately assembled between identical elec­ trodes to ensure no water loss. A symmetrical supercapacitor cell was prepared from two 1.1-cm-diameter active-carbon-coated carbon foam electrodes. After the cell was closed, the electrolyte was activated via a 50-cycle cyclic voltammetry (CV) scan. This study was conducted from − 1 to 1 V at a scan rate of 25 mV.s− 1. After the cell reached the zero open-circuit potential (OCV), potentiostatic EIS measurements were performed at an amplitude of 10 mV in the 0.1 Hz to 100,000 Hz range. CV and galva­ nostatic charge–discharge (GCD) tests were carried out in the 0–0.5 V range. Ethics Statement: This study did not involve any human or animal subjects, human data, or tissues. 2.4.2. Time and temperature dependence The effects of time and temperature on the elastic and loss moduli were determined within the range of the LVE. The heating/cooling rate for temperature sweep analysis was 1 ◦ C⋅min− 1. During the temperature sweep analysis, the cryostat was filled with an antifreeze solution to avoid freezing Peltier pipes. 3. Result and discussion 2.4.3. Thixotropic oscillation strain test The thixotropic oscillation strain test was performed at a tempera­ ture of 25 ◦ C and frequency of 1 rad⋅s− 1 to evaluate the capacity of the sample for rapid self-healing. A low strain (1 %) step was maintained for one minute at the beginning of the six-cycle test, followed by a high strain (1000 %) step at the end of each cycle. By combining 23-lauryl ether (C12E23), LiCl, and H2O, novel gel structures were created to overcome the limitations of conventional gels and to improve the physical and electrochemical properties of gel electrolytes. The mechanical properties of the gel-electrolyte formula­ tions were optimized through rheological measurements, allowing adjustment of the water proportions. As a result, the novel gel electrolyte exhibited remarkable enhancements in physical and electrochemical performance, surpassing the capabilities of traditional gel structures. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge–discharge (GCD) tests demonstrated the superior electrochemical performance of the gel electrolyte composed of 23-lauryl ether, LiCl, and H2O. Furthermore, the gel exhibits intriguing shear-thinning and self-healing properties, indicating its potential for various applications in advanced electrochemical systems. 2.4.4. Creep-recovery and stress relaxation analysis Creep recovery and stress relaxation are standard tests used to explore the transient viscoelastic behavior of many materials. Both tests were used to study time-dependent rheology in the linear viscoelastic region, and both applied mechanical models (e.g., Kelvin–Voigt) for creep behavior. For creep recovery assays, two constant shear stresses (25 Pa or 10 Pa) were applied for 300 s and then released. The strain was allowed to recover for 600 s, and measured as a function of time during the creep and recovery phases. Dynamic rheological testing revealed that the applied shear stress (either 25 Pa or 10 Pa) was within the linear viscoelastic limit. The stress relaxation modulus was recorded over time to investigate the temperature dependence of the characteristic relaxation time (τ) of the samples LC1.000(H2O) LC1.125(H2O) and LC1.250(H2O) at − 10, 25, 45, and 60 ◦ C. The samples were held at the set temperature for 15 min, 1 % strain was applied, and the stress relaxation curves were obtained. 3.1. Structural characterization The X-ray diffraction (XRD) patterns of the liquid crystalline meso­ phase with a 5.63 mol ratio of LiCl:C12E23 are shown in Fig. 1. The sequence of the XRD patterns of the liquid crystalline gel was recorded to demonstrate that no structural changes occurred during the water evaporation; a mesophase, prepared using 1.125 g water, lost water from ca. 25 down to 4–5 H2O/LiCl for several hours under ambient XRD conditions. The fresh sample may have approximately 25 water mole­ cules per LiCl and display weaker and relatively broader diffraction lines; however, with aging and water loss, the diffraction lines become sharper and well-resolved and can be indexed to a cubic system. There may be some shift in the diffraction lines with water evaporation, but this is minimal between the 10 and 30 min samples, with almost no change in the diffraction angles. The lines are observed at 0.95, 1.52, ◦ 1.90, 2.03, 2.25, and 3.00 , 2Θ, and are indexed to the (1 1 0), (2 1 0), (2 2 0), (3 0 0), (3 1 1), and (4 2 0) planes of the cubic structure with a unit cell parameter of 13.15 nm. It was difficult to determine the water content during the XRD measurements; however, it was kept constant in all rheological measurements by sealing the samples. The LiCl/C12E23 2.5. Electrochemical characterization Electrochemical experiments were performed using a PARSTAT MC 1000 electrochemical workstation. A typical split cell (two-electrode measurement system) comprising an LC1.25(H2O) electrolyte and two identical active carbon-coated current collectors as the working and counter electrodes was used to determine the electrochemical properties of the electrolyte at room temperature. In a typical experiment, 35 mg of activated carbon, 5 mg of Poly­ vinylidene fluoride (PVDF), and 5 mg of carbon black were slowly dispersed in 1.5 mL of N-methyl-2 pyrrolidone (NMP) by ultrasonic 3 M.U. Özkaynak et al. Journal of Molecular Liquids 394 (2024) 123705 Fig. 1. 5.63 LiCl:C12E23:1.125 g H2O sample: aging-time-dependent XRD patterns (inset: plot of d-spacing versus hkl relation for cubic phase). molar ratio (5.63) was considered low for the salt-surfactant mesophase of such a large surfactant. For instance, LiCl-C12E10-H2O (C12E10 is C12H25(OCH2CH2)10OH) mesophases are 2D/3D hexagonal, even at much higher salt concentrations [17,18], and a bicontinuous cubic phase is usually observed at high surfactant concentrations in liquidcrystalline mesophases [24,25]. However, C12E23 has a much longer polar (23 ethylene oxide units) head group than the short nonpolar alkyl chain (only 12C, C12H25 units), making it more hydrophilic and forming spherical micellar domains that pack into cubic micellar mesophases. Dark POM images between the crossed polarizers also confirmed the formation of a cubic mesophase (with the help of XRD data) at all water contents. structure, mechanical properties, and flexibility of the gel structures. Although there is no chemical reaction between the gelling agent and the ion source, viscosity and elasticity changes can occur owing to electrostatic and hydrogen-bonding interactions. Molecular interactions (intermolecular or intramolecular) among gel species affect the dynamic properties of the final structure. 3.2.1. Strain sweep analysis Strain sweep analyses are commonly used to differentiate between a linear viscoelastic region with practically constant G’ and G“ (small deformation), and nonlinear areas with decreasing G’ and G ” and increasing strain (large deformation) [29,27]. As shown in Fig. S1, all formulations exhibited solid-like behavior in the linear viscoelastic re­ gion, where G’ was higher than G’’. G’ remained constant until the strain reached a critical point (edge of the LVE region), at which point G’ began to decrease sharply. In contrast, immediately after the crossover point (flow point; G’=G’’, G’f, Ɣf), G’’ is higher than G’, and the samples exhibit a liquid-like structure. The results showed that the strain amplitude at the flow point (Ɣf; G’=G’’) was significantly higher for LC0⋅875H2O (3.18 %), indicating greater stability under strain (Table 3). Whereas the flow points of LC1⋅125H2O and LC1⋅250H2O (1.22 %) were smaller, LC2⋅000H2O had the lowest flow point (0.289 %), reflecting a very weak structure. The strength of gel-electrolyte formulations can be evaluated by measuring their storage modulus at the critical point (G’LVE) and flow point (Gf) as well as their G’ values at the crossover point. When the water content decreased, the observed increase in the storage modulus suggested strong interactions between the molecules, resulting in a more rigid network structure. The LC0.875(H2O) formulation exhibited the highest G’LVE (113.00 Pa) and Gf (54.400 Pa) values, indicating a high degree of intermolecular interactions and entanglement [30–33]. However, as the water content increased, both G’LVE and Gf decreased, leading to weaker gel-electrolyte structures. An effective supercapacitor should have high power density, quick charge/discharge rate, extended service life, and a wide variety of temperature applications. To resist various mechanical deformations, such as stretching, bending, and compression, supercapacitor gel- 3.2. Rheological characterization Oscillatory rheology measurements are widely used to investigate the viscoelastic behavior of systems. It is widely recognized that the frequency dependence of the storage and loss moduli of a system can differentiate between solids and liquids. Dynamic rheological tests within the linear viscoelastic range yielded three parameters: G’and G’’ and the loss factor (tan δ = G’’/G’). The G’ value quantifies the elastic behavior of a sample by determining the deformation energy absorbed during shear. On the other hand, G’’ measures the deformation energy consumed and subsequently lost by the sample during shear. Numerous variables, including concentration, temperature, degree of dispersion, dissolution, and electrical and ionic charges, affect the rheological characteristics of the structures [26–28]. The performance of the newly formed supercapacitor structures is highly dependent on the supercapacitor architecture developed using a combination of 23-lauryl ether, LiCI salt, and water. Excellent me­ chanical flexibility, shape adaptability, moldability, and self-healing properties play critical roles and high ionic conductivity in the perfor­ mance and multifunctionality of flexible supercapacitors [5,9]. The construction of flexible and robust electrolyte–electrode interfaces with excellent mechanical properties and charge stability for highperformance electrolytes is vital. Therefore, detailed rheological anal­ ysis was performed to understand the relationship between the 4 M.U. Özkaynak et al. Journal of Molecular Liquids 394 (2024) 123705 distinct rheological properties for each gel, and featuring a frequencydependent pattern. According to the results, with an increase in salt concentration from LC1.250(H2O) to LC0.875(H2O), the storage modulus, G′ becomes higher, while the dependency of frequency decreased. Because G’ and G” are slightly frequency-dependent, and G’ is more elevated than G” without any crossover for the LC0.875(H2O), LC1.000(H2O), and LC1.250(H2O) formulations, the gels have interconnected gel-like network structures with a typical solid-like and predominantly elastic behavior. Such behaviour is observed for other polysaccharide solutions without cross-links or strong inter-chain associative interactions such as car­ boxymethyl cellulose gels in NaOH solution. This is consistent with the suggested cubic structure. However, with increasing water content (LC1.250(H2O) formulation), the gel exhibited liquid-like behavior (G’ G’’) in the high-ω region (Fig. 2a). For the lowest water concentration (LC0.875 (H2O) formulation), G’0 (storage modulus at 0.1 rad⋅s− 1) is 2.58 orders of magnitude higher than G’’0 (the loss modulus at 0.1 rad⋅s− 1) [31,37]. However, in the presence of more water, the ratio (G’0/G’’0) decreased to 0.71 (Table 3). The frequency dependence of G’ was described using the power law [38,39] (Eq. (S1), Table S2). Comprehensive details are provided in the supplementary information section. The phase angle (tan δ) can provide deeper insight into this phe­ nomenon. Fig. 2a shows the changes in the damping factor (G“/G’) of the samples depending on their water content. Rheological measure­ ments showed that tan δ decreased significantly when the water con­ centration was reduced to 0.875 g, resulting in a stronger structure (Figs. 2a and S3). This phenomenon also suggests that hydrogen bonding among the gel species is essential for the development of gel structures. In aqueous salt solutions, water molecules solvate ions and Table 3 The strain level at the flow point (Ɣf), elastic modulus at critical strain G’LVE and flow point G’f, damping factor (tan δ), temperature-induced gel point, and re­ covery (RS) of the elastic modulus after high strain of the gels. Code Ɣf (%) G’LVE (Pa) G’f (Pa) G’0(f) / G’’0(f) tan δ Gel point (◦ C) RS% LC0.875 3.18 113.00 54.400 2.58 0.237 54.22 50.76 1.97 100.00 45.500 1.18 0.394 52.73 100 1.22 97.80 37.700 1.17 0.424 50.10 100 1.22 82.00 28.700 0.71 0.518 47.77 100 1.22 4.17 4.231 – – – – 160.00 22.000 – – – – (H2O) LC1.000 (H2O) LC1.125 (H2O) LC1.250 (H2O) LC1.500 (H2O) LC2.000 0.289 (H2O) electrolytes must have a significant deformation capacity. There­ fore, because the G’f values of the LC1.500(H2O) and LC2.000(H2O) formu­ lations were 4.231 Pa and 22.000 Pa (Table 3), respectively, indicating a weaker structure, these formulations were not considered for further investigation. 3.2.2. Frequency sweep analysis The results of the frequency sweep analysis can be used to categorize gel solutions as dilute, concentrated (entanglement network systems), weak, or strong. Fig. S2 displays the changes in both G’ and G“ moduli with varying water content and salt concentrations, showing Fig. 2. Oscillatory analysis of structures. (a) damping factor at various angular frequencies, (b) complex viscosity analysis, and (c) damping factor analysis related to the time sweep. (d) Damping factor analysis related to temperature sweep. (e) Visual observations further support temperature-induced gelation; at 25 ◦ C, they were able to hold their weight in the inverted vial at a particular angle, and at 60 ◦ C, all the samples exhibited flowable characteristics. 5 M.U. Özkaynak et al. Journal of Molecular Liquids 394 (2024) 123705 form significantly stronger interactions than water molecules in media. At high salt concentrations (low water content), micelles hold each other more tightly than at high water content, producing a typical viscoelastic behavior similar to that of transient polymer networks, [42–44] which appears to involve physical interactions (particularly hydrogen bonding) rather than chemical bonding. The system could move slowly, even under a small shear force, and glide along one another while displaying partial or complete disentanglement. This is because the water molecules enter the mesostructure. The increase in stiffness with decreasing water content can be explained by the entan­ glement of micelles and aggregation established in the mesophase. structure. Recently, anti freezing techniques involving the incorporation of LiBr or LiCl have been utilized in wearable applications. However, very high salt concentrations or post-procedural methods have been employed to prepare them [15,52,53]. The introduction of water and salt species in a confined space (hydrophilic domains of the mesophase) also reduces the freezing point of the aqueous salt solutions and salts. This phenomenon is known as confinement effect (CE). CE stabilizes salt-surfactant mesophases at very low temperatures [25,54]. 3.2.6. Self-healing property Owing to significant advancements in self-healing materials inspired by living organisms over the past ten years, endowing devices with selfhealing abilities has become a promising way to significantly enhance device durability and functionality. The healing mechanism is based on the reversible dynamics of weak bonds, including covalent and non­ covalent bonds. A supercapacitor with self-healing capability can repair cracks, breakages, or mechanical damage once they occur, to recover the original performance or minimize the loss of device prop­ erties. The ability of a structure to exhibit self-healing behavior is owing to its shear-thinning properties, which allow it to flow under shear stress and self-recover once the stress is removed. To evaluate the capacity of the mesostructures for rapid self-healing, a six-cycle oscillation strain test, was performed at 25 ◦ C and 1 rad.s− 1, also known as a thixotropic test. The analysis was initiated at a low strain (1 %) for 1 min, followed by a high strain (1000 %) step for 1 min to complete one cycle. Under low-strain conditions, G’ was greater than G’’ for all formulations (Fig. 3). This implies that the mesostructural network remains intact under a small oscillatory strain. Subsequently, when the formulations were subjected to a considerable amplitude of shear strain, G’ decreased drastically by approximately 5000 times, and the structures started to behave as viscous-like structures (G’’>G’), indicating rupture of the system. Subsequently, the mesostructures were subjected to small amplitude strain in the following cycle. The recovery rate of the structures after the 6th cycle was measured using Eq. (1), [ ′ ] G11 RS (%) = 100 − x 100 (1) G′1 3.2.3. Shear thinning property The shear thinning properties of the formulations were examined by frequency sweep tests in the oscillatory mode using complex viscosity curves (Fig. 2b). All gels displayed a typical unique shear-thinning behavior, which imparted self-healing and remolding properties to the gels, as the complex viscosity decreased with increasing shear rate [43,45]. The shear-thinning behavior of the samples can be attributed to the alignment of the micellar domains with flow [43,46]. The shearthinning property also implies that the gel formulations can dynami­ cally adapt to and withstand deformation. Viscosity reached its maximum value as the water content decreased. At the lowest water content, the viscosity began to increase considerably, indicating a transition from a dilute to semi-dilute regime when the assembled mi­ celles became sufficiently long to entangle with each other [47,48]. 3.2.4. Time sweep Time-sweep analyses were conducted at 1% strain and 1 rad.s− 1 at 25 ◦ C for 30 min, and the storage (G’) and loss (G’’) moduli were monitored for all formulations. It is readily apparent from the damping factor (tan δ) curves as a function of time that all formulations were almost stable for 30 min (Fig. 2c). A slight increase in tan δ of the LC1.250 (H2O) and LC1.125(H2O) formulations indicated a small amount of struc­ tural weakening. 3.2.5. Temperature sweep To conduct environmental monitoring associated with temperature changes, temperature sweep assessments were carried out over a wide temperature range from − 10 to + 60 ◦ C. It is known that a super­ capacitor structure can lose its ionic conductivity if it is prone to freezing when exposed to temperatures below zero, limiting its use in extremely cold climates. The G’ and G“ values versus temperature are shown in Fig. S4. At low temperatures, the storage modulus (G’) of all gels was much greater than the loss modulus (G”). The formulations maintained their gel-like behavior up to approximately 50 ◦ C. (Fig. 2d,e and Table 3). Although G’ tended to decrease with increasing temperature, G“ began to increase. This can be explained by the significantly reduced association strength of hydrogen bonding at high temperatures, which resulted in the formulation having a liquid-like character. How­ ever, at low temperatures, the association strength of the hydrogen bonding was significant. The lifetime is such that the interactions among the gel species behave like weak crosslinks between the chains. As shown in Table 3, the temperature-induced gelation point (crossover point of G’ and G’’; gel point, ◦ C) decreased with increasing water content of the formulations, similar to strain-induced gelation (Ɣf) (Fig. 2d, Fig. S1). This can be explained by the production of more ion–dipole (salt-water) and hydrogen bonding interactions in highly concentrated electrolyte solutions. Furthermore, with increasing salt concentration, the jammed or more compact packing of the micelles in the mesophase provides outstanding resistance to large strains owing to the multiple hydrogen bonds that form in the coronal chains, as confirmed by strain sweep tests. It can be speculated that in the case of high salt concentrations (LC0.875(H2O) and LC1.000(H2O))at high tem­ peratures, intermicelle bridges through coronal hydrogen bonds remained, hindering the progression of the liquid character of the where RS (%) represents the storage modulus ratio before and after the cycling. G’11 and G’1 are the average storage moduli at the 11th and 1st steps, respectively, and according to the results, the gel-like character (G’>G’’) was instantaneously recovered for all water concentrations. The G’ and G’’ values recovered to their initial values for the LC1.000 (H2O), LC1.125(H2O), and LC1.250(H2O) samples (Fig. 3; RS% values in Table 3). The LC0.875(H2O) sample recovered 50.76 % of the starting G’. All the results indicate an excellent healing capacity for the LC1.000(H2O), LC1.125(H2O), and LC1.250(H2O) supercapacitor structures, which is a benefit of the dynamic reversible hydrogen bonding in the structures (Fig. 3b, c and d). Hydrogen bonding between the ethylene oxide group and water molecules provides the structures with excellent selfhealing properties [58,59]. The LC0.875(H2O) sample, which showed the weakest self-healing properties, was eliminated, and further rheological tests were performed for formulations LC1.000(H2O),LC1.125(H2O), and LC1.250(H2O), which exhibited excellent electrical conductivity. 3.2.7. Creep recovery Creep is a time and temperature-dependent phenomenon. Creeprecovery tests were performed to determine the long-term durability and reliability of the materials [60–64]. The system is exposed to two different constant shear stresses (creep) (25 Pa or 10 Pa) for 300 s, and time-dependent deformation of the mesostructures is evaluated. Then after the stress is released, the deformation recovery is recorded for an additional 600 s. The creep curves comprised (i) instantaneous elastic deformation, 6 M.U. Özkaynak et al. Journal of Molecular Liquids 394 (2024) 123705 Fig. 3. Rapid self-healing determination of (a) LC0.875(H2O), (b) LC1.000(H2O), (c) LC1.125(H2O), and (d) LC1.250(H2O) formulations by oscillation strain steps (1 % strain to 1000 % strain and back to 1 % strain, each step for 1 min). (ii) delayed elastic deformation, and (ii) viscous (Newtonian) flow ele­ ments of the strain. Following the release of the shear stress, the instantaneous part was first recovered, followed by the retardation element, and finally the viscous strain [65,66]. The creep-recovery data (measured response) are expressed as the creep compliance (J(t)), which is defined as the ratio of the measured strain to the applied shear stress to the material according to Eq. (2): [60,67]. J(t) = γ(t)/σ deformation (spring of Maxwell), followed by gradual deformation (Kelvin-Voigt element) towards an asymptote as time progressed (Fig. S5). During the recovery phase (300 < t < 900 s), after removing the applied shear stress, we recorded the maximum deformation (Jmax), followed by a sharp reduction in creep compliance owing to the irre­ versible deformation behavior. Furthermore, reducing the shear stress value from 25 Pa to 10 Pa resulted in reduced compliance and strain (Table 4). The data indicate that the LC1.000(H2O) formulation has the highest creep recovery (RC%) values, indicating greater elasticity than the other formulations, which is consistent with the G’ data from the frequency sweep analysis. Additionally, decreasing the water concen­ tration increased the robustness and flexibility of the formation, as suggested by the data. The creep curves also suggest that the formulation with the highest water content (LC1.250(H2O)) displays more extensive permanent viscous deformation as the viscoelastic response increases. This formulation had the lowest RC% (6.6 % at 25 Pa and 9.1 % at 10 Pa) (Table 4). Our findings indicate that reducing the water concentration, strain, and creep compliance leads to improved stiffness of the material, which is consistent with the results of oscillatory frequency sweep and stress relaxation tests. (2) where γ(t) is the shear deformation (strain), and σ is the applied constant shear stress. Creep and recovery tests were performed using the four-parameter Burger’s model, which consists of a Maxwell element and Kelvin–Voigt element combined in series (Fig. S5) [68,60]. The Maxwell element adds instantaneous compliance (spring) and zero shear viscosity to control permanent deformation [69–71]. The final recovery percentage of the entire system (RC%) is calculated from Jmax and Jmin, according to the following equation, which was proposed by Dolz, Hernandez, and Delegido (Eq. (3)) [72,43,73,74]. RC(%) = Jmax − Jmin × 100 Jmax (3) 3.2.8. Zero-shear viscosity Creep-recovery tests are crucial for obtaining zero-shear viscosity and describing material behavior and internal structure systems [69,76,77]. Therefore, zero-shear viscosity (viscosity of a material when it is effective at rest, η0) was determined at the end of the creep phase (Eq. (4)). Here, Jmax is the maximum deformation corresponding to the compli­ ance value once the stress is removed, and Jmin is the compliance for the longest time (Fig. S5). Fig. 4a shows the effect of the salt concentration on the creep (0–300 s) and recovery (300–900 s) curves of the gel structures. High compliance (J(t)) indicates that a weaker J(t) reflects a more robust material structure [73,75]. As shown in Fig. 4a, we observed slow changes in creep compliance over very long periods, indicating that the secondary interactions of the mesostructures can break or reorganize during deformation. During the creep phase of all formulations, we initially observed an elastic η0 = τk tanβ (4) where τk is the shear stress, and tanβ is the slope of the creep curve. The 7 M.U. Özkaynak et al. Journal of Molecular Liquids 394 (2024) 123705 Fig. 4. (a) Creep curves of LC1.000(H2O), LC1.125(H2O), and LC1.250(H2O). Stress relaxation test of (b) LC1.000(H2O), (c) LC1.125(H2O), (d) LC1.250(H2O) formulations. This suggests that because of the reversible hydrogen bonding and su­ perior self-healing ability of the LC1.000(H2O), LC1.125(H2O), and LC1.250 (H2O) formulations, all samples were likely to restructure their meso­ structure through dynamic hydrogen bonding and electrostatic in­ teractions. The temperature dependence of the relaxation behavior varied for the sample with the highest water content (LC1.250(H2O)), which showed a much shorter relaxation time than the other formula­ tions at all temperatures, indicating a better dynamic exchange (Fig. 4c). In contrast, the formulation with the lowest water content (LC1.000(H2O)) showed a significant decrease in network mobility when the tempera­ ture was lowered to − 10 ◦ C. Interestingly, all formulations exhibited faster relaxation at higher temperatures, with significant stress release observed at 45 ◦ C and 60 ◦ C, as the structures could flow around these temperatures (Fig. 2d). The relaxation process is mainly controlled by thermo-dissociated hydrogen bonding in the mesophase, as described in the temperature sweep section. The sample with the highest water content (LC1.250(H2O)) showed rapid stress relaxation to zero within a few seconds at 45 and 60 ◦ C because of its gel point of 47 ◦ C. At tem­ peratures above the gel point, the characteristic relaxation times (τ) of all formulations become equal, indicating no temperature dependence on the stress relaxation behavior. This can be attributed to the low en­ ergy barrier above the gel point and the heating to this temperature, facilitating rapid bond exchange. It is proposed that the observed relaxation time of 1 s is determined by the interval network dynamics. Due to the associative exchange mechanism of hydrogen bonding, the relationship between τ and temperature for the LC1.000(H2O), LC1.125 (H2O), and LC1.250(H2O) samples can be described by Maxwell relation (Eq. (5), and Arrhenius relationship (Eq. (6)). Table 4 Maximum deformation corresponding to the compliance value Jmax,25 at 25 Pa, Creep recovery percentage at 25 (RC25 %) and 10 (RC10%) Pa, Zero shear vis­ cosities at 25 and 10 Pa, and Activation Energies of the formulations. Code LC1.000 (H2O) LC1.125 (H2O) LC1.250 Jmax,25 (1/ Pa) RC25 % RC10% 0.00115 79.3 – 85.854 – 63.82 0.000162 14.09 27.78 18.745 34.381 47.37 0.0000423 6.60 9.09 2.635 5.682 41.40 η0 (at 25 η0 (at 10 Pa) Pa) (Pa.s) (Pa.s) Ea (Kj. mol− 1) (H2O) shear strain–time curve (Fig. S6) showed a constant slope that depended only on τk. The results showed that the zero-shear viscosity decreased with increasing applied shear stress (Table 4). 3.2.9. Stress relaxation Stress relaxation experiments were performed to provide insight into the extent of secondary bond exchange and propensity for creep. The stress relaxation modulus at different temperatures (-10, 25, 45, and 60 ◦ C) are recorded over time to investigate the temperature depen­ dence of the characteristic relaxation times (τ) of the samples, namely LC1.000(H2O), LC1.125(H2O), and LC1.250(H2O). The characteristic time (τ) that can highlight the physical effects of the water content is the time required to reach 37 % of the initial stress. The formulations were equilibrated at the set temperature for 15 min, and 1 % strain was applied. The relaxation modulus (G/G0) was normalized and plotted on a logarithmic scale (Fig. 4b,c, and d). The stress relaxation curves for all temperatures exhibited a straightforward relaxation process for the modulus values (Fig. 4 b,c,d). (5) η=G τ ln(τ) = ln(τ0 ) + 8 Ea RT (6) M.U. Özkaynak et al. Journal of Molecular Liquids 394 (2024) 123705 Based on this formulation, the activation energy (Ea) of the formu­ lations can be obtained according to the linear correlation of ln (τ) with 1/T. As shown in Tables S3, S4, and S5, the Ea value of LC1.250(H2O) is equal to 41,40 Kj.mol− 1, which is smaller than those of LC1.000(H2O) (63.82 Kj.mol− 1) and LC1.125(H2O) (47.37 Kj.mol− 1) samples, indicating a dynamic bond exchange in the mesostructure. The high stability of LC1.000(H2O) can be associated with a low water concentration and has a much higher Ea among the formulations. The diffusion-limiting topology at high salt concentrations may explain why Ea increased with increasing salt concentration. Electro­ chemical experiments have shown that a more densely packed structure has a greater tendency to restrict the mobility of molecules and diffusion of ions than low-viscosity systems. This, in turn, prevents bond exchange and leads to an increase in activation energy. The findings of this study offer a promising approach for developing self-healing, anti-freezing, and shear-thinning supercapacitors over a broad temperature range in which they can operate with excellent flexibility. electrolyte exhibited the lowest viscosity, resembling that of a nongelled electrolyte. This finding suggests that at higher molar ratios, the elec­ trolyte becomes more fluid-like, hindering the formation of a stable gel network. In contrast, the LC0.875(H2O) electrolyte exhibited the highest viscosity, which resembled that of a solid-like material. The higher viscosity could be attributed to the increased concentration of gelforming agents within the electrolyte. The high viscosity of the LC0.875 (H2O) electrolyte may impede ion diffusion and hinder overall ionic conductivity, thereby limiting its potential as a gel electrolyte for supercapacitor applications. Considering these results, further in­ vestigations focused on the supercapacitor performance of gel electro­ lytes with LC1.00(H2O), LC1.125(H2O), and LC1.250(H2O). These molar ratios were selected to evaluate the influence of gel-like structure and ionic conductivity on the electrochemical properties of the supercapacitor. All the electrochemical tests were performed at room temperature. A standardized procedure was employed to mitigate the critical issue of water loss in the gel electrolyte during supercapacitor testing. Specif­ ically, the gel electrolyte was promptly sandwiched between the positive and negative electrodes and placed in a split cell configuration. This split-cell design, equipped with o-rings, provided a hermetically sealed environment for the gel electrolyte. By creating a closed-pack system, the split cell effectively prevented gaseous species from entering or escaping the cell. Consequently, the loss of water from the gel electrolyte owing to evaporation was effectively minimized. This meticulous approach to cell assembly and sealing plays a crucial role in maintaining the integrity and stability of the gel electrolyte throughout the 3.3. Electrochemical performance The gel electrolytes prepared in this study were formulated with varying molar ratios (LC0.875(H2O), LC1.00(H2O), LC1.125(H2O), and LC1.250 (H2O)) to investigate their suitability for application in supercapacitors. Initial observations revealed distinct differences in viscosity and gel-like structure formation among different molar ratios. The LC1.250(H2O) Fig. 5. Cyclic voltammograms of (a)LC1.000(H2O), LC1.125(H2O), and LC1.250(H2O) electrolytes, (b) LC1.125(H2O) electrolyte at different scan rates in the voltage window of 0–0.5 V, and (c) electrochemical impedance spectra of the LC1.000(H2O), LC1.125(H2O), and LC1.250(H2O) electrolytes in the frequency range of 0.1–100.000 Hz. 9 M.U. Özkaynak et al. Journal of Molecular Liquids 394 (2024) 123705 supercapacitor testing, ensuring reliable and consistent results. To further confirm the roles of water content and electrical con­ ductivity, the supercapacitor performances of the LC1.00(H2O), LC1.125 (H2O), and LC1.250(H2O) electrolytes were investigated using a twoelectrode split cell (Fig. 5a). The CV curves of the LC1.00(H2O), LC1.125 − 1 were (H2O), and LC1.250(H2O) electrolytes at a scan rate of 50 mV.s measured between symmetrical active carbon electrodes. Although each electrolyte formulation exhibited well-shaped, balanced, and rectan­ gular CV curves, the highest specific capacitance was obtained for the LC1.250(H2O) electrolyte. This can be explained by the fact that LC1.250 + (H2O) has a relatively higher water content with more free ions (Li and Cl-), and its mobility is not restricted by the surfactant domains accu­ mulated by electrostatic interactions, resulting in improved capacitance. The CV curves were compatible with the rheological behavior of the materials. Rheological behavior reveals how dominant the aggregation becomes with decreasing water content (aggregation slows down and/or limits the free ion mobility). As shown in Fig. 5b, the CV curves for electrolyte formulation exhibit a symmetric parallelogram shape at the applied scan rates, demonstrating ideal capacitive behavior. EIS measurements were also performed over a frequency range of 100.000–0.1 Hz with a 10 mV amplitude to characterize the response of electrolyte systems to alternative currents. As shown in Fig. 5c, all Nyquist plots exhibit a semicircle in the high-frequency range and a straight line in the lower-frequency region. Among the LC1.000(H2O), LC1.125(H2O), and LC1.250(H2O) electrolytes, the Nyquist plot of the LC1.250 (H2O) electrolyte shows a smaller ohmic resistance, exhibiting a lower solution resistance in the high-frequency region than the other electro­ lytes, which directly indicates an increase in ion mobility. Compared with the LC1.000(H2O) and LC1.125(H2O) electrolytes, the LC1.250(H2O) electrolyte showed a lower Warburg impedance, corresponding to faster diffusion of ions into the electrode. Gel electrolytes prepared with varying molar ratios exhibit distinct viscosities and gel-like structures. The LC1.250(H2O) electrolyte exhibited the highest ionic conductivity, but lacked normal gel-like properties. Thus, the LC1.125(H2O) electrolyte was chosen for galvanostatic charge and discharge tests. This molar ratio represents the lowest concentration of the gelling agent while still providing sufficient material to form a stable three-dimensional gel structure with high ionic conductivity. To evaluate the performance of the LC1.125 (H2O) electrolyte further, galvanostatic charge–discharge (GCD) measurements were performed at different current densities. Fig. 6a shows the GCD curves of the LC1.125 (H2O) electrolyte in a symmetric supercapacitor cell. The GCD curves revealed a nearly isoscale-triangular shape with a small ohmic drop, which illustrates that the LC1.125(H2O) electrolyte has ideal capacitive performance and good charge reversibility, which is consistent with the CV curves. Moreover, a linear discharge potential on-time behavior was observed, suggesting that no central Faradaic process occurred. The specific capacitances, calculated from the charge–discharge curves based on the GCD curves at various current densities, are also illustrated in Fig. 6a. As expected, at 0.25 A.g− 1 current density, the cell exhibits the highest specific capacitance, 316F.g− 1. Even at a high current den­ sity of up to 1 A g− 1, the specific capacitance of the electrode covered with LC1.125(H2O) was approximately 80 %. Fig. 6b shows the cycling stability of the LC1.125 (H2O) electrolyte in the two-electrode system at 0.5 A.g− 1 for 1500 cycles. The specific capacity remained almost constant during testing. The electrolyte was stable for the remaining 500 cycles, although a small drop occurred in capacitance after 1000 cycles. The corresponding coulombic efficiency was approximately 80 % after almost 1000 cycles, which was attributed to its excellent stability and high conductivity. The LC1.125(H2O) gel structure exhibits good cycling stability and coulombic efficiency. The XRD patterns displayed diffraction lines originating from a cubic phase with a unit cell parameter a of 13.2 nm for the liquid crystalline mesophase with a LiCl: C12E23 mole ratio of 5.63. The increase in the line intensity over 30 min indicates some water loss and improved orienta­ tion within the stable structure. In the strain-sweep analysis, The LC0.875 (H2O) formulation exhibited the highest strain amplitude at the flow point (3.18 %), whereas the LC1.125(H2O), LC1.250(H2O), and LC2.000(H2O) formulations exhibited lower flow points (1.22 % and 0.289 %, respectively), indicating weaker structures. The LC0.875(H2O) formulation had the highest storage modulus at the critical point (G’LVE) of 113.00 Pa and a flow point (Gf) of 54.400 Pa, suggesting extensive intermolecular interactions and a stiffer network structure. In the frequency sweep analysis, the LC0.875(H2O), LC1.000(H2O), and LC1.250(H2O) formulations displayed solid-like behavior, with a storage modulus (G’) higher than the loss modulus (G’’). The LC1.250(H2O) formulation exhibited liquidlike behavior at low frequencies and solid-like behavior at high fre­ quencies. The storage modulus ratio at 0.1 rad.s− 1 (G’0) to loss modulus at 0.1 rad.s− 1 (G’’0) decreased with increasing water content. In the shear-thinning analysis, the gel formulations exhibited decreased com­ plex viscosity with increasing shear rate, indicating a shear-thinning behavior. The LC0.875(H2O), LC1.000(H2O), and LC1.250(H2O) formulations showed maximum viscosity values at lower water content, indicating a transition from a dilute to a semi-dilute regime. During self-healing evaluation, the LC0.875(H2O), LC1.000(H2O), and LC1.250(H2O) formulations demonstrated the ability to recover 100 % of their initial storage moduli after rupture. The LC1.000(H2O) formulation exhibited exhibited a slightly lower recovery rate of 50.76 %. The zero-shear viscosity decreased with increasing applied shear stress, as observed in the creep recovery tests. In the stress relaxation analysis, the LC1.250(H2O) formulation exhibited shorter relaxation times, indicating better dynamic exchange among other systems. The activation energy (Ea) values were smaller for the LC1.250(H2O) formulation (41.40 Kj.mol− 1) compared to the LC1.000(H2O) (63.82 Kj.mol− 1) and LC1.125(H2O) (47.37 Kj.mol− 1) formulations, Fig. 6. (a) Galvanostatic charge–discharge curves of the LC1.125(H2O) electrolyte at different current densities (inset is specific capacitance versus current density) and (b) variation of coulombic efficiency (%) and capacity retention (%) depending on the number of cycles. 10 Journal of Molecular Liquids 394 (2024) 123705 M.U. Özkaynak et al. indicating a dynamic bond exchange in the mesostructures. The LC1.250 (H2O) formulation exhibits the highest specific capacitance, whereas the LC1.000(H2O) formulation exhibits a high specific capacitance of 316F.g− 1 at 0.25 A.g− 1. The LC1.250(H2O) electrolyte demonstrated good cycling stability, maintaining an almost constant specific capacity over 1500 cycles with a coulombic efficiency of approximately 80 %. 4. Conclusion Different gel formulations with a constant LiCl to C12E23 mole ratio (5.63:1) and different water concentrations were studied from me­ chanical and electrochemical points of view for flexible supercapacitor applications. Although the liquid crystalline mesophase loses water as soon as thin films are prepared, it exhibits a strong diffraction line over time at all water concentrations. Both XRD and POM data confirmed the formation of a cubic mesophase at all water concentrations. Rheological analysis provided quantitative information on how the gel strength and gelation rate changed with the water content, LiCl concentration, and temperature. In addition, the present work systematically illustrates the reorganizing behavior (self-healing) of a gel structure prepared for the first time by combining water/23-lauryl ether with LiCl salt. The designed gel structure encapsulates Li+ and Cl- ions by forming a threedimensional network in the hydrophilic domains, and exhibits signifi­ cant performance in supercapacitor applications. Funding This work was financially supported by Sabanci University Nano­ technology Research and Application Center (SUNUM) and Istanbul Technical University (ITU) Department of Materials Science and Engi­ neering. The funders had no role in the study design, data collection and analysis, decision to publish, or manuscript preparation. CRediT authorship contribution statement Mert Umut Özkaynak:. Banu Kocaaga: Data curation, Formal analysis, Investigation, Visualization, Writing – original draft, Writing – review & editing. Koray Bahadır Dönmez:. Selin Dağlar: Investiga­ tion, Validation. Yurdanur Türker: Conceptualization, Investigation, Methodology, Writing – original draft, Writing – review & editing. Nilgün Karatepe:. F. Seniha Güner:. Ömer Dağ: Methodology, Writing – original draft, Writing – review & editing. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data availability All relevant data supporting the findings of this study are available within the paper and its Supporting Information files. Additional data related to this study were obtained from the corresponding authors. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi. org/10.1016/j.molliq.2023.123705. References R. Jamil, D. S. Silvester, Ionic liquid gel polymer electrolytes for flexible supercapacitors: challenges and prospects, Curr. Opin. Electrochem. 35. (2022) Elsevier B.V.. doi: 10.1016/j.coelec.2022.101046. S. Mathela, et al., Ionic liquid dispersed highly conducting polymer electrolyte for supercapacitor application: current scenario and prospects “ICSEM 2021”, High 11 Perform. Polym. 34 (6) (2022) 652–672, https://doi.org/10.1177/ 09540083221099432. R. Singh, J. Tharion, S. Murugan, A. Kumar, ITO-free solution-processed flexible electrochromic devices based on PEDOT:PSS as transparent conducting electrode, ACS Appl. Mater. Interf. 9(23) (2017) 19427–19435. American Chemical Society. doi: 10.1021/acsami.6b09476. S. Alipoori, S. Mazinani, S. H. Aboutalebi, F. Sharif, Review of PVA-based gel polymer electrolytes in flexible solid-state supercapacitors: Opportunities and challenges, J. Energy Storage 27. Elsevier Ltd, Feb. 01, 2020. doi: 10.1016/j. est.2019.101072. X. Peng, L. Peng, C. Wu, Y. Xie, Two dimensional nanomaterials for flexible supercapacitors, Chem. Soc. Rev. vol. 43, no. 10. Royal Society of Chemistry, pp. 3303–3323, May 21, 2014. doi: 10.1039/c3cs60407a. K.L. Zhou, H. Wang, J.T. Jiu, J.B. Liu, H. Yan, K. Suganuma, Polyaniline films with modified nanostructure for bifunctional flexible multicolor electrochromic and supercapacitor applications, Chem. Eng. J. 345 (2018) 290–299, https://doi.org/ 10.1016/j.cej.2018.03.175. L. Zang, et al., Design and fabrication of an all-solid-state polymer supercapacitor with highly mechanical flexibility based on polypyrrole hydrogel, ACS Appl. Mater. Interfaces 9 (39) (2017) 33941–33947, https://doi.org/10.1021/acsami.7b10321. L. Wen, F. Li, H.M. Cheng, Carbon nanotubes and graphene for flexible electrochemical energy storage: from materials to devices, Adv. Mater. 28 (22) (2016) 4306–4337, https://doi.org/10.1002/adma.201504225. C. Lu, X. Chen, Latest advances in flexible symmetric supercapacitors: from material engineering to wearable applications, Acc. Chem. Res. 53 (8) (2020) 1468–1477, https://doi.org/10.1021/acs.accounts.0c00205. H. Dai, et al., Polymer gel electrolytes for flexible supercapacitors: Recent progress, challenges, and perspectives, Energy Storage Mater. 34 (2021) 320–355, https:// doi.org/10.1016/j.ensm.2020.09.018. T. S. Bhat, P. S. Patil, R. B. Rakhi, Recent trends in electrolytes for supercapacitors, J. Energy Storage 50. Elsevier Ltd, Jun. 01, 2022. doi: 10.1016/j.est.2022.104222. G. Kaur, S.C. Sivasubramanian, A. Dalvi, Solid-state supercapacitors using ionic liquid dispersed Li+-NASICONs as electrolytes, Electrochim. Acta 434 (2022), 141311, https://doi.org/10.1016/j.electacta.2022.141311. J. Chen, D. Shi, Z. Yang, W. Dong, M. Chen, A solvent-exchange strategy to develop stiff and tough hydrogel electrolytes for flexible and stable supercapacitor, J. Power Sources 532 (2022), https://doi.org/10.1016/j.jpowsour.2022.231326. P. Lestari Handayani, T. Kim, Y. Hwa Song, J. Seo Park, S. Jae Yang, U. Hyeok Choi, Tailoring molecular interaction in heteronetwork polymer electrolytes for stretchable, high-voltage fiber supercapacitors, Chem. Eng. J. (2022), 139432, https://doi.org/10.1016/j.cej.2022.139432. M. Taghavikish, S. Subianto, Y. Gu, X. Sun, X.S. Zhao, N.R. Choudhury, A poly (ionic liquid) gel electrolyte for efficient all solid electrochemical double-layer capacitor, Sci. Rep. 8 (1) (2018), https://doi.org/10.1038/s41598-018-29028-y. M. M. Amaral, R. Venâncio, A. C. Peterlevitz, H. Zanin, Recent advances on quasisolid-state electrolytes for supercapacitors, J. Energy Chem. 67. Elsevier B.V., pp. 697–717, Apr. 01, 2022. doi: 10.1016/j.jechem.2021.11.010. C. Albayrak, A. Cihaner, Ö. Dag, A new, highly conductive, lithium salt/nonionic surfactant, lyotropic liquid-crystalline mesophase and its application, Chem. A Eur. J. 18 (14) (2012) 4190–4194, https://doi.org/10.1002/chem.201103705. E. Yılmaz Topuzlu, H.I. Okur, B. Ulgut, Ö. Dag, Role of water in the lyotropic liquid crystalline mesophase of lithium salts and non-ionic surfactants, Langmuir 37 (49) (2021) 14443–14453, https://doi.org/10.1021/acs.langmuir.1c02411. F.M. Balci, S. Balci, C. Kocabas, Ö. Dag, Lyotropic liquid-crystalline mesophase of lithium triflate-nonionic surfactant as gel electrolyte for graphene optical modulator, J. Phys. Chem. C 121 (21) (2017) 11194–11200, https://doi.org/ 10.1021/acs.jpcc.7b03622. E. Tunkara, C. Albayrak, E.O. Polat, C. Kocabas, Ö. Dag, Highly proton conductive phosphoric acid-nonionic surfactant lyotropic liquid crystalline mesophases and application in graphene optical modulators, ACS Nano 8 (10) (2014) 11007–11012, https://doi.org/10.1021/nn505199q. M.U. Özkaynak, Y. Türker, K.B. Dönmez, N. Karatepe, F.S. Güner, Highperformance supercapacitor electrolytes based on high-mole-ratio phosphoric acid/lauryl ether surfactant liquid crystalline gel, Int. J. Energy Res. (2022), https://doi.org/10.1002/er.8126. C.B. Uzundal, F. Mert Balci, B. Ulgut, Ö. Dag, Lyotropic liquid crystalline mesophase of sulfuric acid-nonionic surfactant stabilizes lead(II) oxide in sulfuric acid concentrations relevant to lead acid batteries, ACS Omega 2 (7) (2017) 3785–3791, https://doi.org/10.1021/acsomega.7b00833. C. Xu, T. Mandal, R.G. Larson, D. Wang, H. Wang, A molecular dynamics simulation of the structure of sodium lauryl ether sulfate and poly(vinyl alcohol) at the air/water interface, Colloids Surf. A Physicochem. Eng. Asp 563 (2019) 84–94, https://doi.org/10.1016/j.colsurfa.2018.11.060. C. Albayrak, N. Özkan, Ö. Dag, Origin of lyotropic liquid crystalline mesophase formation and liquid crystalline to mesostructured solid transformation in the metal nitrate salt-surfactant systems, Langmuir 27 (3) (2011) 870–873, https:// doi.org/10.1021/la1035932. P. Sakya, J.M. Seddon, R.H. Templer, R.J. Mirkin, G.J.T. Tiddy, Micellar cubic phases and their structural relationships: the nonionic surfactant system C 12 EO 12 /water, Langmuir 13 (1997) 3706–3714. https://pubs.acs.org/sharingg uidelines. A.F. Sepulveda, M. Kumpgdee-Vollrath, M.K.K.D. Franco, F. Yokaichiya, D.R. de Araujo, Supramolecular structure organization and rheological properties modulate the performance of hyaluronic acid-loaded thermosensitive hydrogels as drug-delivery systems, J. Colloid Interface Sci. 630 (2023) 328–340, https://doi. org/10.1016/j.jcis.2022.10.064. M.U. Özkaynak et al. Journal of Molecular Liquids 394 (2024) 123705 Z. Pang, H. Deeth, N. Bansal, Effect of polysaccharides with different ionic charge on the rheological, microstructural and textural properties of acid milk gels, Food Res. Int. 72 (2015) 62–73, https://doi.org/10.1016/j.foodres.2015.02.009. P. Kujawa, A. Audibert-Hayet, J. Selb, F. Candau, Effect of ionic strength on the rheological properties of multisticker associative polyelectrolytes, Macromolecules 39 (1) (2006) 384–392, https://doi.org/10.1021/ma051312v. Z. Abbasian Chaleshtari, H. Salimi-Kenari, R. Foudazi, Glassy and compressed nanoemulsions stabilized with sodium dodecyl sulfate in the presence of poly (ethylene glycol)-diacrylate, Soft Matter 19 (31) (2023) 5989–6004, https://doi. org/10.1039/D3SM00349C. L. R. B. Santos, C. V Santilli, S. H. Pulcinelli, Sol±gel transition in SnO 2 colloidal suspensions: viscoelastic properties. W. Cai, T. Hu, Q. Huang, A polysaccharide from Lignosus rhinocerotis sclerotia: self-healing properties and the effect of temperature on its rheological behavior, Carbohydr. Polym. 267 (2021), https://doi.org/10.1016/j.carbpol.2021.118223. M. D. Sacks, R.-S. Sheu, Rheological properties of silica sol-gel materials, 1987. S. Razmkhah, S.M.A. Razavi, M.A. Mohammadifar, Dilute solution, flow behavior, thixotropy and viscoelastic characterization of cress seed (Lepidium sativum) gum fractions, Food Hydrocoll. 63 (2017) 404–413, https://doi.org/10.1016/j. foodhyd.2016.09.030. Y. Zou, et al., Flexible, all-hydrogel supercapacitor with self-healing ability, Chem. Eng. J. 418 (2021), https://doi.org/10.1016/j.cej.2021.128616. I.J.M. Corzo, J.H.L. da Fonsêca, M.A. d’Ávila, Influence of carboxymethyl cellulose solutions on rheological properties of Laponite dispersions, Rheol. Acta (2023), https://doi.org/10.1007/s00397-023-01401-3. X. Zhao, D. Li, L. Jun Wang, Y. Wang, Rheological properties and microstructure of a novel starch-based emulsion gel produced by one-step emulsion gelation: Effect of oil content, Carbohydr. Polym. 281 (2022), https://doi.org/10.1016/j. carbpol.2021.119061. C.G. Lopez, W. Richtering, Oscillatory rheology of carboxymethyl cellulose gels: influence of concentration and pH, Carbohydr. Polym. 267 (2021), https://doi. org/10.1016/j.carbpol.2021.118117. A. Saxena, M. Kaloti, H.B. Bohidar, Rheological properties of binary and ternary protein-polysaccharide co-hydrogels and comparative release kinetics of salbutamol sulphate from their matrices, Int. J. Biol. Macromol. 48 (2) (2011) 263–270, https://doi.org/10.1016/j.ijbiomac.2010.11.008. P.N. Dave, P.M. Macwan, B. Kamaliya, Synthesis and rheological investigations of gum-ghatti-cl-poly(NIPA- co -AA)-graphene oxide based hydrogels, Mater Adv. (2023), https://doi.org/10.1039/d3ma00092c. D. Daniloski, N.A. McCarthy, I. Gazi, T. Vasiljevic, Rheological and structural properties of acid-induced milk gels as a function of β-casein phenotype, Food Hydrocoll. 131 (2022), https://doi.org/10.1016/j.foodhyd.2022.107846. S.K. Kundu, M. Yoshida, M. Shibayama, Effect of salt content on the rheological properties of hydrogel based on oligomeric electrolyte, J. Phys. Chem. B 114 (4) (2010) 1541–1547, https://doi.org/10.1021/jp906312f. A.L. Kwiatkowski, V.S. Molchanov, A.S. Orekhov, A.L. Vasiliev, O.E. Philippova, Impact of salt co- and counterions on rheological properties and structure of wormlike micellar solutions, J. Phys. Chem. B 120 (49) (2016) 2547–12556, https://doi.org/10.1021/acs.jpcb.6b09817. C.A. Dreiss, Wormlike micelles: Where do we stand? Recent developments, linear rheology and scattering techniques, Soft Matter 3 (8) (2007) 956–970, https://doi. org/10.1039/b705775j. Z. Wu, et al., Ultrasensitive, stretchable, and fast-response temperature sensors based on hydrogel films for wearable applications, ACS Appl. Mater. Interfaces 13 (18) (2021) 21854–21864, https://doi.org/10.1021/acsami.1c05291. P. Alaee, M. Kamkar, M. Arjmand, Fumed silica-based suspensions for shear thickening applications: a full-scale rheological study, Langmuir 38 (16) (2022) 5006–5019, https://doi.org/10.1021/acs.langmuir.2c00591. R. Kotsilkova, S. Tabakova, Exploring effects of graphene and carbon nanotubes on rheology and flow instability for designing printable polymer nanocomposites, Nanomaterials 13(5) (2023), doi: 10.3390/nano13050835. A.L. Kwiatkowski, V.S. Molchanov, A.S. Orekhov, A.L. Vasiliev, O.E. Philippova, Impact of salt co- and counterions on rheological properties and structure of wormlike micellar solutions, J. Phys. Chem. B 120 (49) (2016) 12547–12556, https://doi.org/10.1021/acs.jpcb.6b09817. X.J. Luo, et al., Gas channeling control with an in-situ smart surfactant gel during water-alternating-CO2 enhanced oil recovery, Pet. Sci. (2023), https://doi.org/ 10.1016/j.petsci.2023.03.003. S. Wu, et al., One-pot synthesis of anti-freezing carrageenan/polyacrylamide double-network hydrogel electrolyte for low-temperature flexible supercapacitors, Chem. Eng. J. 435 (2022), https://doi.org/10.1016/j.cej.2022.135057. A. Shabbir, H. Goldansaz, O. Hassager, E. Van Ruymbeke, N.J. Alvarez, Effect of hydrogen bonding on linear and nonlinear rheology of entangled polymer melts, Macromolecules 48 (16) (2015) 5988–5996, https://doi.org/10.1021/acs. macromol.5b00757. Y.K. Lee, S.K. Jung, Y.H. Chang, Rheological properties of a neutral polysaccharide extracted from maca (Lepidium meyenii Walp.) roots with prebiotic and antiinflammatory activities, Int. J. Biol. Macromol. 152 (2020) 757–765, https://doi. org/10.1016/j.ijbiomac.2020.02.307. S. Liu, H. Zhang, W. Yu, Simultaneously improved strength and toughness in κ-carrageenan/polyacrylamide double network hydrogel via synergistic interaction, Carbohydr. Polym. 230 (2020), https://doi.org/10.1016/j. carbpol.2019.115596. S. Liu, L. Li, Ultrastretchable and self-healing double-network hydrogel for 3D printing and strain sensor, ACS Appl. Mater. Interfaces 9 (31) (2017) 26429–26437, https://doi.org/10.1021/acsami.7b07445. M. Basu, P.A. Hassan, S.B. Shelar, Modulation of surfactant self-assembly in deep eutectic solvents and its relevance to drug delivery-a review, J. Mol. Liq. 375 (2023), 121301, https://doi.org/10.1016/j.molliq.2023.121301. X.W. Yun, B. Tang, Z.Y. Xiong, X.G. Wang, Understanding self-assembly, colloidal behavior and rheological properties of graphene derivatives for high-performance supercapacitor fabrication, Chin. J. Polym. Sci. (English Ed.) 38 (5) (2020) 423–434, https://doi.org/10.1007/s10118-020-2411-0. E.A. Appel, M.W. Tibbitt, M.J. Webber, B.A. Mattix, O. Veiseh, R. Langer, Selfassembled hydrogels utilizing polymer-nanoparticle interactions, Nat. Commun. 6 (2015), https://doi.org/10.1038/ncomms7295. L. Yu, J.R. Stokes, G.E. Yakubov, Viscoelastic behaviour of rapid and slow selfhealing hydrogels formed by densely branched arabinoxylans from Plantago ovata seed mucilage, Carbohydr. Polym. 269 (2021), https://doi.org/10.1016/j. carbpol.2021.118318. M.A. Morini, P.V. Messina, P.C. Schulz, The interaction of electrolytes with nonionic surfactant micelles, Colloid Polym. Sci. 283 (11) (2005) 1206–1218, https:// doi.org/10.1007/s00396-005-1312-7. Z. Ye, H. Lu, G. Chai, C. Wu, J. Chen, L. Lv, Glycerol-modified poly(vinyl alcohol)/ poly(ethylene glycol) self-healing hydrogel for artificial cartilage, Polym. Int. 72 (1) (2023) 27–38, https://doi.org/10.1002/pi.6444. T.C. Brito-Oliveira, I.C.F. Moraes, S.C. Pinho, O.H. Campanella, Modeling creep/ recovery behavior of cold-set gels using different approaches, Food Hydrocoll. 123 (2022), https://doi.org/10.1016/j.foodhyd.2021.107183. E. Krempl, F. Khan, Rate (time)-dependent deformation behavior: an overview of some properties of metals and solid polymers, Int. J. Plast (2003) 1069–1095, https://doi.org/10.1016/S0749-6419(03)00002-0. C.F. Buitrago, et al., Creep attenuation in glassy polymer nanocomposites with variable polymer-nanoparticle interactions, Soft Matter 16 (38) (2020) 8912–8924, https://doi.org/10.1039/d0sm01247e. Ł. Pejkowski, J. Seyda, K. Nowicki, D. Mrozik, Mechanical performance of nonreinforced, carbon fiber reinforced and glass bubbles reinforced 3D printed PA12 polyamide, Polym. Test. 118 (2023), https://doi.org/10.1016/j. polymertesting.2022.107891. N. Mandlekar, B. Rana, P. Maurya, B.S. Butola, M. Joshi, Long-term prediction of creep and stress-relaxation behaviour in synthetic fabrics using the timetemperature superposition principle, Fibers Polym. 24 (6) (Jun. 2023) 2195–2207, https://doi.org/10.1007/s12221-023-00181-0. R. I. Baron et al., Physical hydrogels of oxidized polysaccharides and poly(vinyl alcohol) forwound dressing applications, Materials 12(9) (2019) doi: 10.3390/ ma12091569. O. Starkova, Z. Zhang, H. Zhang, effect of temperature, moisture and strain rate on limits of linear viscoelastic behaviour of polyamide 66 filled with Tio2 nanoparticles, Mater. Sci. Eng. A 498 (1–2) (2008) 242–247. Y.L. Xu, S.B. Xiong, Y.B. Li, S.M. Zhao, Study on creep properties of indica rice gel, J. Food Eng. 86 (1) (2008) 10–16, https://doi.org/10.1016/j. jfoodeng.2007.09.002. A. Kurt, A. Cengiz, T. Kahyaoglu, The effect of gum tragacanth on the rheological properties of salep based ice cream mix, Carbohydr. Polym. 143 (2016) 116–123, https://doi.org/10.1016/j.carbpol.2016.02.018. A. Sharma, G.R. Rongmei Naga, P. Kumar, S. Raha, Development of an empirical relationship between non-recoverable creep compliance & zero shear viscosity for wide-ranging stiffness of asphalt binders, Constr. Build. Mater. 326 (2022) Apr, https://doi.org/10.1016/j.conbuildmat.2022.126764. R. Moučka, M. Sedlačík, Z. Pátíková, Fractional viscoelastic models of porcine skin and its gelatin-based surrogates, Mech. Mater. 177 (2023), https://doi.org/ 10.1016/j.mechmat.2023.104559. Z. Tian, J. He, Y. Wang, W. Zhao, L. Duan, Quantitative insights into rheology of self-crosslinking dialdehyde carboxymethyl cellulose/collagen solution: a comprehensive experimental and modelling investigation, Mech. Mater. 170 (2022), https://doi.org/10.1016/j.mechmat.2022.104356. X. Liu, J. Xu, Y. Li, H. Zhao, S. Guo, Mechanism of the glucono-δ-lactone induced soymilk gelation: enthalpy and entropy transformation in the cross-linking of protein molecules, Food Res. Int. 169 (2023), https://doi.org/10.1016/j. foodres.2023.112868. M. Dolz, M.J. Hernández, J. Delegido, Creep and recovery experimental investigation of low oil content food emulsions, Food Hydrocoll. 22 (3) (2008) 421–427, https://doi.org/10.1016/j.foodhyd.2006.12.011. M.T. Ghannam, Creep-recovery experimental investigation of crude oil-polymer emulsions, J. Appl. Polym. Sci. 92 (1) (2004) 226–237, https://doi.org/10.1002/ app.13645. N. Sozer, Rheological properties of rice pasta dough supplemented with proteins and gums, Food Hydrocoll. 23 (3) (2009) 849–855, https://doi.org/10.1016/j. foodhyd.2008.03.016. M. Cofelice, M.C. Messia, E. Marconi, F. Cuomo, F. Lopez, Effect of the xanthan gum on the rheological properties of alginate hydrogels, Food Hydrocoll. 142 (2023), https://doi.org/10.1016/j.foodhyd.2023.108768. C. Hermida-Merino, M. Pérez-Rodríguez, M.M. Piñeiro, M.J. Pastoriza-Gallego, Evidence of viscoplastic behavior of exfoliated graphite nanofluids, Soft Matter 12 (8) (2016) 2264–2275, https://doi.org/10.1039/c5sm02932e. 12 M.U. Özkaynak et al. Journal of Molecular Liquids 394 (2024) 123705 J.J. Cash, T. Kubo, D.J. Dobbins, B.S. Sumerlin, Maximizing the symbiosis of static and dynamic bonds in self-healing boronic ester networks, Polym. Chem. 9 (15) (2018) 2011–2020, https://doi.org/10.1039/c8py00123e. X. Zhang, S. Wang, Z. Jiang, Y. Li, X. Jing, Boronic ester based vitrimers with enhanced stability via internal boron-nitrogen coordination, J. Am. Chem. Soc. 142 (52) (2020) 21852–21860, https://doi.org/10.1021/jacs.0c10244. Y. Chen, Z. Tang, X. Zhang, Y. Liu, S. Wu, B. Guo, Covalently cross-linked elastomers with self-healing and malleable abilities enabled by boronic ester bonds, ACS Appl. Mater. Interfaces 10 (28) (2018) 24224–24231, https://doi.org/ 10.1021/acsami.8b09863. 13