Construction of Hydrogel Electrolytes With Wide Potential Windows Based on Dimethyl Sulfone‐Mediated Interface Regulation Mechanisms

ABSTRACT

Water‐based electrolytes with wide electrochemical stable windows (ESW) are crucial for high‐energy‐density supercapacitors. Herein, an interfacial engineering strategy to broaden the ESW of water‐based electrolytes is proposed. It is found that although introducing dimethyl sulfone (MSM) to aqueous electrolytes reduces water activity through hydrogen bonding, at high salt concentrations, MSM enters the ionic solvation layer and weakens its hydrogen bonding, which results in an ESW approximately 0.2 V higher than at low salt concentrations. Further mechanistic studies have revealed that the barrier layer formed by the enrichment of MSM at the electrode/electrolyte interface increases the overpotential of the water splitting reaction. Simultaneously, MSM contributes significantly to the high ESW by binding to and stabilizing the active intermediates generated during the water splitting process. Subsequently, a scalable solvent exchange‐ion cross‐linking phase separation strategy is developed. The synergistic effects of NaClO₄, MSM, water, and the hydrogel framework enable an ESW of 5.2 V. The supercapacitor assembled with laser‐induced graphene (LIG) electrodes exhibits an operating voltage up to 2.4 V, with a capacitance retention rate of 98% after 10 000 charge/discharge cycles. In addition, the device demonstrates excellent adaptability at various folding angles and across a wide temperature range of −40°C to 45°C.

The introduced MSM forms hydrogen bonds with H₂O, thereby reducing the water content. At the same time, MSM can accumulate at the electrode/electrolyte interface, and this accumulated MSM can stabilize the intermediates of the hydrogen/oxygen evolution reactions, thereby suppressing these reactions and broadening the electrochemical window.

1. Introduction

High operating voltage is crucial for enhancing the energy density of supercapacitors [1, 2, 3, 4]. The narrow thermodynamic stability window of aqueous electrolytes severely limits the operating voltage of devices, despite their advantages of high ionic conductivity, low cost, and safety and environmental friendliness. Therefore, developing advanced design strategies for aqueous electrolytes with a wide electrochemical stable window (ESW) is of paramount importance.

Currently, most design strategies for aqueous electrolytes with high ESW properties rely on strong coordination between water molecules and metal cations, which significantly reduces the activity of water. Among these, the “salt‐in‐water” electrolyte [5, 6, 7] has garnered significant attention for constructing safe high‐voltage devices by substantially reducing free water, thereby successfully broadening the ESW beyond 3.0 V [8, 9, 10, 11]. However, high salt concentrations compromise the high ionic conductivity advantage of aqueous electrolytes. Moreover, the tendency for salt crystallization in high‐concentration or even saturated electrolytes at low temperatures inevitably limits the applicable temperature range. Additionally, by introducing organic solvents and polymers with high dielectric constants and low viscosities as co‐solvents, such as acetonitrile [12, 13, 14, 15] (AN), dimethyl sulfoxide [16, 17, 18, 19] (DMSO), and N,N‐dimethylformamide [20, 21, 22, 23] (DMF), etc., the activity of free water is restricted, enabling the preparation of water/organic solvent electrolytes with a high ESW. The participation of co‐solvents simultaneously enhances their low‐temperature performance. The core mechanism of these strategies generally focuses on reducing the bulk water activity by regulating the solvation structure of water or hydrogen bonding interactions between water molecules [24, 25]. This directly and significantly decreases the number of water molecules that can diffuse to the electrode surface, thereby cutting off the supply of reactants for the hydrogen evolution reaction (HER) at its source. Consequently, the potential window is effectively expanded toward negative potentials. However, the electrolyte even with a salt concentration as high as 63 m, the ESW remains below 3.3 V [26], which suggests that simply reducing the bulk water activity is insufficient to further enhance the ESW. Compared with traditional cosolvents, the primary advantage of MSM is its ability to form a dynamically potential‐regulated enrichment layer at the electrode interface and to transform this unique interfacial behavior into an effective strategy for broadening the ESW; that is, stability is achieved by regulating the adsorption behavior of MSM at the interface, rather than relying solely on its bulk properties.

In fact, high‐voltage aqueous electrolytes that suppress the oxygen evolution reaction (OER) are more advantageous for supercapacitors. This is because symmetric supercapacitors primarily utilize the positive potential region within the ESW. Suppressing the OER prevents oxygen evolution and interfacial corrosion on the anode surface at high voltages [27], thereby facilitating the voltage, energy density, and cycling stability of aqueous supercapacitors [28]. However, the primary strategies reported involve adding protective layers to restrict ion diffusion [2, 29]; yet these approaches are often ineffective in suppressing the oxygen evolution reaction [30]. The main reason for this is that the oxygen evolution reaction is a complex four‐electron surface catalytic process that depends on the conversion of adsorbed intermediates on the electrode surface [31]. Clearly, the strategies based on regulating bulk water activity cannot effectively intervene in the adsorption and reaction pathways of these intermediates on electrode surfaces. Therefore, developing strategies to precisely engineer the electrode/electrolyte interface to simultaneously suppress both HER and OER is crucial for broadening the ESW, particularly the oxygen evolution potential.

As a proof of concept, a design strategy for aqueous electrolytes based on interfacial engineering regulation is proposed. By introducing dimethyl sulfone (MSM) as an additive into the aqueous electrolyte, the activity of bulk water is reduced through the hydrogen bonding interaction of two hydrogen bond acceptors (S═O) of MSM with water. Simultaneously, the specific adsorption of MSM on the graphene electrode surface reconfigures the interfacial double‐layer structure, inhibiting the interaction of water at the electrode. Furthermore, MSM can bind with various adsorbed intermediates form water‐splitting, simultaneously broadening the ESW on both the hydrogen evolution and oxygen evolution sides. High concentration of NaClO₄ contributes little to broadening the ESW but significantly enhances the temperature adaptability of the electrolyte. Interestingly, MSM exhibits excellent water solubility, enabling one‐pot preparation of hydrogel electrolyte precursors. Taking advantage, a scalable solvent exchange‐ion cross‐linking phase separation strategy was proposed to fabricate hydrogel electrolyte membranes. The synergistic effects of NaClO₄, MSM, water, and hydrogel framework enable the ESW of the hydrogen electrolyte membrane to reach 5.2 V. Supercapacitors based on hydrogel electrolyte membranes and laser‐induced graphene (LIG) operate at voltages up to 2.4 V. At a current density of 0.4 mA cm⁻², the devices exhibit excellent capacitance (8.31 mF cm⁻²) and energy density (4.73 µWh cm⁻²), maintaining over 98% capacitance retention after 10 000 charge/discharge cycles. These devices also demonstrate superior integration performance, stability at various folding angles, and adaptability to temperatures ranging from −40°C to 45°C.

2. Materials and Methods

2.1. Materials

Ammonium persulfate (APS) and N, N'‐Methylenebisacrylamide (MBAA) are purchased from Tianjin Comio Chemical Reagents Co. Acrylamide (AM) is obtained from Tianjin Hynes Biochemical Technology Co. Sodium alginate (SA) is purchased form Tianjin Hynes Biochemical Technology Co. Dimethyl sulfone (MSM), sodium perchlorate (NaClO₄) and calcium chloride (CaCl₂) are obtained from Tianjin Fengchuan Chemical Reagent Technology Co. Polyimide films (PI film) are afford by DuPont, Inc. Deionized water was used for all experiments.

2.2. Preparation of Electrodes

The polyimide film (PI film) was first ultrasonically cleaned with ethanol for 30 min, then rinsed with deionized water three times and dried. Subsequently, it was fixed on a steel substrate and dried at room temperature for 5 min. Then, lase direct writing was performed using a wavelength of 450 nm and a power of 3.5 W to prepare interdigitated graphene electrodes (with a finger spacing of 50 µm). After cleaning and drying, the interdigitated graphene electrodes (LIG) were obtained.

2.3. Preparation of Hydrogel Electrolyte Membrane

First, 3.0 g of AM powder was dissolved in 20 mL of deionized water, and stirred in a water bath at 45°C for 1 h. Subsequently, 1.885–11.31 g of MSM was added and stirred for another 1 h. Finally, 0.4 g of SA was added and continued to stir for 5 h. The solution was allowed to stand for 1 h to eliminate bubbles (M‐PS). It was then uniformly poured onto a dry glass plate and pushed at a constant speed using a casting rod to form a thin membrane. After exposure to air for a few seconds, the membrane was promptly transferred into an aqueous solution with 0.2 m CaCl₂ and various concentration of NaClO₄. The prepared hydrogel membranes spontaneously detached from the glass plate to obtain the hydrogel electrolyte membrane (M‐PS/Ca), as shown in Figure S1. For comparison, the control hydrogel contained no MSM, had a NaClO₄ concentration of 2 m, and followed identical procedures throughout, designated as PS/Ca.

2.4. Assembly of Flexible Supercapacitors

The prepared M‐PS/Ca was cut into 10 × 10 × 1 mm pieces, and the water on the surface was absorbed by filter paper. The M‐PS/Ca was covered onto the interdigital part of the LIG electrode to assemble a flexible supercapacitor (FSC). The assembled device was encapsulated with cling film to ensure the stability of the device during testing. To guarantee good contact between the electrodes and the electrolyte, the encapsulated FSC was lightly compressed for 12 h using a universal compressor. Conductive silver paste was applied to the electrode pin area as a current collector to achieve a more stable current output.

2.5. Measurement

FTIR spectra in the wave number range of 400–4000 cm⁻¹ were recorded on an infrared spectrophotometer (Nicolet 6700, Thermal Scientific, USA). Laser Raman spectroscopy (Raman Spectra) was used to analyze molecular structure and identify the composition of substances. The tensile testing of the hydrogel membrane was conducted using a tensile testing machine (LLY‐06E, China Co., Ltd.). During the test, the samples were cut into a size of 1 cm × 3 cm, and tested with a tensile speed of 50 mm min⁻¹. A scanning electron microscope (Gemini SEM 500, Germany) was used to characterize the microstructure of the hydrogel membrane. The porosity of the hydrogel membrane was determined using the mercury intrusion method (MIP) (AutoPore IV 9520, USA). The ¹H nuclear magnetic resonance (¹H NMR) spectra were analyzed on an Avance 400 MHz instrument (Bruker, USA). Liquid samples were tested using a coaxial NMR tube, with the inner tube filled with chloroform deuterated (CDCl₃) as a field‐frequency lock agent and the outer tube filled with the samples. The contact angles of various solutions on the electrode materials were tested using a contact angle tester (DSA 25) manufactured by Krüss GmbH, Germany. The droplet volume in the tests was 2.5 µL and the results were averaged from three measurements. The electrochemical tests were carried out using an electrochemical workstation (PGSTAT 302N, Switzerland). Conductivity meter (Leici Technology Co Ltd.) was used to determine of ionic conductivity of aqueous solutions. The conductivity of hydrogels is determined through electrochemical impedance spectroscopy calculations at open‐circuit voltage, using the following equation:

$$\sigma =\frac{\mathrm{L}}{{\mathrm{R}}_{\mathrm{b}}\times \mathrm{S}}\times 1000$$

where, L (cm) is the thickness of the hydrogel electrolyte, R _b (Ω) is the volume resistance, and S is the contact area between the electrode and the electrolyte (cm²).

The area specific capacitance CA (mF cm⁻²) of the supercapacitor was calculated from the GCD curves at different current densities using the following equation:

$$\mathrm{CA}=\frac{\mathrm{I}\times \mathrm{\Delta }t}{\mathrm{A}\times \mathrm{\Delta }\mathrm{V}}$$

where I (mA) is the discharge current, Δ t (s) is the discharge time, A (cm²) is the effective area of the electrode, and Δ V (V) is the operating voltage (determined by the voltage of the discharge curve, excluding IR drop). The areal energy density (EA) and power density (PA) were calculated using the following equations:

$$\mathrm{EA}=\frac{1}{2}{\mathrm{C}}_{\mathrm{A}}\mathrm{\Delta }{\mathrm{V}}^2$$

$$\mathrm{PA}=\frac{{\mathrm{E}}_{\mathrm{A}}}{\mathrm{t}}$$

where E _A (mWh cm⁻²) is the surface energy density, C _A (mF cm⁻²) is the surface specific capacitance, Δ V (V) is the operating voltage, P _A (mW cm⁻²) is the surface power density, and t (h) is the discharge time.

2.6. Computational Details

Molecular dynamics simulations of the solvation structures for various electrolytes were performed using the Forcite module in Material Studio. All molecular and ions were simulated using the COMPASS II configuration. Simulations were conducted in an NVT system at 298.0 K with a time step of 1 femtosecond. The defect‐containing graphene (Gr) was used as the model for the electrode. Density functional theory (DFT) calculations were performed to determine the binding energies between molecules/ions and MSM, the adsorption energies of MSM on the Gr surface, and the interaction energies between MSM and various intermediates at the Gr/intermediate interface during the HER and OER processes. Molecular electrostatic potentials were computed using the Dmol3 module within DFT.

2.7. Statistical Analysis

2.7.1. Pre‐Processing of Data

All quantitative experiments were repeated three times (n = 3). Data were presented as mean ± standard deviation (SD). Outliers were identified and excluded using the Grubbs test (p < 0.05). All statistical analyses were performed using Origin software.

2.7.2. Data Presentation

Data were presented as mean ± SD.

2.7.3. Sample Size (n) for Each Statistical Analysis

All experiments were conducted in triplicate (n = 3).

2.7.4. Statistical Methods Used to Assess Significant Differences With Sufficient Details

All tests were two‐sided, with a significance level of α = 0.05 (p < 0.05).

2.7.5. Software Used for Statistical Analysis

All statistical analyses were performed using Origin and Microsoft Excel.

3. Structure and Discussion

To investigate the interactions among MSM, NaClO₄, and water, mixed solutions at different ratios were characterized using nuclear magnetic resonance (NMR). As shown in Figure S2, the peak for hydrogen in water appears at 4.8 ppm. As MSM concentration increased, the ¹H chemical shift in water shifted slightly toward higher fields, while the ¹H peak in ─CH₃ of MSM shifted toward lower fields. The shifts can be attributed to the formation of hydrogen bonds between H atoms in water and O atoms in MSM, disrupting intermolecular hydrogen bonds in water. Concurrently, the hydrogen in ─CH₃ of MSM exhibited a de‐shielding effect on oxygen in water, causing the ¹H peak in ─CH₃ to shift toward lower fields. With the introduction of 2 m NaClO₄, the solvation further disrupted hydrogen bonds between water molecules, causing a more pronounced shift of the ¹H signal in H₂O toward higher fields (Figure 1a). This appeared to suggest that MSM and NaClO₄ exhibited a synergistic effect in disrupting hydrogen bonds within water molecules, thereby contributing to suppress the undesirable HER and OER. This result differed significantly from previously reported findings in the DMSO system. [32] The Raman spectrum of the mixed solution is shown in Figure 1b. The broad characteristic peak of water at 2900–3700 cm⁻¹ has been reported to be resolvable into regions corresponding to strong, moderate, and weak hydrogen bonds, respectively. [33, 34, 35] After adding MSM, the strong H‐bond peak diminished while the proportions of other H‐bonds (i.e., moderate and weak H‐bonds) increased, indicating that hydrogen bonds formed between MSM and water, while the strong hydrogen bonds between water were disrupted. The ESW of MSM electrolytes at different concentrations was tested using a three‐electrode system to investigate the effect of MSM concentration on the ESW. As shown in Figure 1c, in an aqueous solution containing only 2 m NaClO₄ to enhance conductivity, the ESW ranged from −1.1 to 1.1 V. Adding 0.5 m MSM to this solution shifted the ESW to −1.7 to 1.6 V. This indicated that low concentrations of MSM effectively broadened the ESW, which was a previously unreported phenomenon. As the MSM concentration increased, the ESW for a 1 m MSM solution became −2.0 to 1.7 V. When the MSM concentration reached 2 m, the HER potential and OER potential reached their maximum values, and the ESW expanded to −2.4 to 1.8 V (4.2 V). Further increasing the MSM concentration to 3 m caused the ESW slightly decreased to −2.2 to 1.7 V, indicating that a 2 m MSM provided the optimal effect for broadening the ESW.

FIGURE 1.

(a) ¹H NMR spectra of different concentrations of MSM (fixed NaClO₄ of 2 m) solutions; (b) Raman spectra of H₂O and different concentrations of MSM (fixed NaClO₄ of 2 m) solutions; (c) LSV curves of different concentrations of MSM (fixed NaClO₄ of 2 m) solutions; (d) ¹H NMR spectra of different concentrations of NaClO₄ (fixed at 2 m MSM) solutions; (e) Raman spectra of different concentrations of NaClO₄ (fixed at 2 m MSM) solutions; (f) LSV curves of different concentrations of NaClO₄ (fixed at 2 m MSM) solutions.

It is well known that high‐concentration salt can broaden the ESW [36, 37] and enhance temperature adaptability [38, 39]. In this study, the influence of NaClO₄ was investigated. As shown in Figure 1d and Figure S3, the ¹H signal in water shifted strongly to the low‐field as the concentration of NaClO₄ increased, which was due to the solvation effect of the salt caused a large number of waters to solvate around the ions, forming compact ion‐solvent clusters that disrupted intensively the hydrogen bonds between water molecules. It was expected that the severely restricted free water activity at high salt concentrations would significantly broaden the ESW. The chemical shift of the ¹H of ─CH₃ in MSM was relatively complex (Figure 1d). As the NaClO₄ concentration increased from 2 m to 4 m, the chemical shift increased, indicating that the hydrogen bonding interaction between water and MSM was dominant at this stage and exhibited a de‐shielding effect. However, when the NaClO₄ concentration further increased, the chemical shift of shifted toward the high‐field region, which may be attributed to two factors. On one hand, the increased salt concentration reduced the water available for hydrogen bonding with MSM due to the solvation, thereby diminishing the de‐shielding effect. On the other hand, MSM may participate in the solvation of Na⁺ at high salt concentrations, due to the strong polarity of the O═S═O group, further weakening the de‐shielding effect. Raman spectroscopy was employed to characterize mixed solutions at different ratios. As shown in Figure 1e, with the MSM concentration was fixed at 2 m, the intensity of the peak of strong hydrogen bonds decreased as the NaClO₄ concentration increased. This indicated that the hydration interaction between salt and water molecules progressively strengthened, which significantly disrupted the hydrogen bonding between water molecules. Additionally, the effects of MSM and NaClO₄ on hydrogen bonding ion water were compared. As shown in Figure S4, at 2 m MSM, the peak corresponding to strong hydrogen bonds were weakened by the interaction between MSM and water, yet the peak intensity remained relatively significant. The introduction of 6 m NaClO₄ further decreased the peak, indicating a significant reduction in free water activity. The above results suggested that at high salt concentrations, solvation and hydrogen bonding of MSM may compete for water molecules, synergistically reducing water activity. The impact of salt concentration on the ESW was investigated. As shown in Figure 1f, at a fixed 2 m MSM, the ESW of the mixed solution gradually increased with rising salt concentration. At 8 m NaClO₄, the ESW reached the maximum of −2.5 to 1.9 V (4.4 V). This result indicated that increasing the concentration of NaClO₄ from 2 to 8 m only broadened the ESW by 0.2 V. Although the introduction of NaClO₄ can significantly reduce the free water activity through synergistic effects, its contribution to widening the ESW appeared less pronounced than anticipated. This finding clearly contradicted most reported strategies for broadening the ESW by lowering water activity.

To further investigate the intrinsic mechanisms underlying the broadening of the ESW, the effects of MSM and NaClO₄ on water structure were explored through theoretical calculations. First, the binding energies between Na⁺, H₂O, and MSM were compared. Figure 2a showed the binding energies for Na⁺─H₂O, Na⁺─MSM, MSM─H₂O, and H₂O─H₂O were 25.98, 13.42, 21.92, and 6.37 kcal mol⁻¹, respectively. This indicated that water exhibited a stronger affinity for binding with Na⁺ and MSM, while Na⁺ also demonstrated a significant tendency to bind with MSM. Additionally, the electrostatic potential distributions of MSM and H₂O were calculated, as shown in Figure S5. MSM and H₂O exhibited a similar molecular structure, with the O═S═O bond in MSM carrying a strong negative charge and the H atom in H₂O bearing a strong positive charge. These opposing charge characteristics enabled them to form a tight bond and indicated a strong tendency for reaction between them. Additionally, it is generally recognized that reducing hydrogen bonds between water molecules reflects a decrease in water activity [24]. Molecular dynamics (MD) simulations were employed to quantify hydrogen bond number in various electrolytes, as shown in Figure S6. In pure water, the hydrogen bond count was 3249. When 2 m MSM was added, the hydrogen bond count decreased by approximately 12.6%, attributed to the formation of hydrogen bonds between water and MSM (138). The number of hydrogen bonds in a 6 m NaClO₄ solution was 59.6% of that in pure water, indicating that high salt concentrations can indeed significantly reduce water activity. This effect can be attributed to the solvation of salts. The 3D visualization of hydrogen bonds in the 2 m MSM and 6 m NaClO₄ electrolyte is shown in Figure 2b, with an enlarged view in Figure 2c. The results indicated that oxygen atoms in MSM form hydrogen bonds with hydrogen atoms in H₂O. Compared to 6 m NaClO₄, the number of hydrogen bonds between water decreased by only 2.5%, while the number of hydrogen bonds between water and MSM decreased by 42.8%, which suggested that although MSM and NaClO₄ did exhibit a synergistic effect in reducing water activity, its significance is not pronounced. A competitive relationship may exist between MSM and NaClO₄, leading to a decrease in the number of hydrogen bond between MSM and water from 138 to 79. Further investigations into the solvation structures of 6 m NaClO₄, 2 m MSM, and 6 m NaClO₄ electrolytes are presented in Figure S7. The radial distribution functions (RDF) and coordination numbers (CN) for both electrolytes (S7a and b) indicated that the coordination number of Na⁺─O (H₂O) changed little upon MSM addition, decreasing from 3.98 to 3.89. However, MSM entered the solvation of Na⁺ (CN: 0.23), indicating its participation in the solvation of Na⁺. This necessarily led to a reduction in hydrogen bonding between MSM and water. Based on the aforementioned theoretical research, it can be reasonably inferred that free water in electrolytes preferentially bonded with Na⁺, while added MSM then formed hydrogen bonds with the remaining free water. These two processes synergistically reduce the bulk water activity. However, in high‐concentration salt solutions, the binding of Na⁺ with MSM resulted in this synergistic effect becoming less pronounced. This may explain why the introduction of high‐concentration salts did not significantly contribute to the ESW.

FIGURE 2.

(a) Binding energy of H₂O to H₂O, MSM to H₂O, Na⁺ to H₂O, Na⁺ to MSM; 3D visualization of hydrogen bonds in a 2 m MSM + 6 m NaClO₄ electrolyte solution (b) Standard view (c) Magnified view; (d) Relative concentrations of MSM and H₂O in a 2 m MSM + 6 m NaClO₄ electrolyte solution; 3D visualization of simulated structures in (e) 2 m MSM + 6 m NaClO₄ electrolyte and (f) 6 m NaClO₄ electrolyte; (g) GCD curves of LIC tested under three‐electrode conditions in 2 m MSM + 6 m NaClO₄ electrolytes at a current density of 0.4 mA cm⁻². IR spectra at different voltages: (h) charging, (i) discharging; (g) Adsorption energy of H₂O and MSM on Gr; (h) Adsorption energy for Gr on 6 m NaClO₄ electrolyte and 2 m MSM + 6 m NaClO₄ electrolyte; (i) Adsorption energy between Gr and MSM after adsorption of the oxygen evolution reaction intermediate.

It was noteworthy that the ESW of 2 m MSM was significantly higher than that of 6 m NaClO₄ (−1.4 – −0.98 V, Figure S8). In comparison, the ESW of 2 m MSM broadened by 1.3 V at the HER side and 0.7 V at the OER side. Generally, the expansion phenomenon at the OER side is closely related to the state of the electrolytic interface [40, 41], which may be attributed to the effects of MSM at the electrode/electrolyte interface. To investigate the role of MSM at the electrode interface, molecular dynamics simulations were conducted to study the concentration distribution of H₂O and MSM at the electrode interface using defect‐containing graphene (Gr) as a model for glassy carbon electrodes [42]. As shown in Figure 2d, H₂O was uniformly distributed both in the bulk phase and at the interface region, whereas MSM showed significant enrichment at the electrode interface. This indicated that the “competitive adsorption” of MSM at the interface may alter the state of the electrode/electrolyte interface. Figure 2e,f exhibits their corresponding 3D visualizations. Compared with the 6 m NaClO₄ electrolyte, the addition of MSM transformed the electrode/electrolyte interface from a relatively simple, sparse structure into a complex, dense, and highly structured composite interface layer. As shown in Figure 2g, the GCD curves of LIC in 2 m MSM and 6 m NaClO₄ electrolytes were measured under a current density of 0.4 mA cm⁻² using a three‐electrode configuration (working electrode: LIC; counter electrode: platinum wire; reference electrode: Ag/AgCl). Infrared spectra were measured at six representative points during the charging and discharging processes (Figure 2h,i). The results show absorption peaks characteristic of S═O and C─S/C─H at various potentials during charging and discharging, indicating that MSM is enriched on the LIC electrode surface. As charging proceeds, positive charges accumulate at the LIC electrode interface, forming a strong electric field. The lone pairs on the O and S atoms of the MSM adsorbed on the electrode surface are stretched by polarization, leading to an increase in the bond constants of S═O and C─S. This causes a significant blue shift in the S═O stretching vibration at 1040 cm⁻¹ and the C─S/C─H vibration at 1270 cm⁻¹ (Figure 2h). The discharge process is exactly the opposite of the charging process; the results show that as the discharge potential decreases, the absorption peaks of S═O and C─S/C─H shift significantly toward the red (Figure 2i). This fundamentally altered the microstructure of the double layer and created a physical barrier that inhibited water molecules from contacting the electrode. The specific adsorption at the interface displaced H₂O, which was supported by the fact that the adsorption energy of MSM on the electrode was 4.88 times greater than that of water (Figure 2j). Specifically, the electrode surface showed a greater thermodynamic affinity for adsorbing MSM than H₂O. This strong adsorption promoted the enrichment of MSM at the interface. Additionally, the adsorption energies of Gr on the two electrolytes (6 _m NaClO₄ electrolyte, 2 m MSM, and 6 m NaClO₄ electrolyte) were calculated (Figure 2k). The results indicated that the electrolyte containing MSM exhibited stronger and more stable adsorption onto the electrode, suggesting that the addition of MSM promoted the formation of a more robust interface. Furthermore, as shown in Figure S9, the double‐layer capacitance (2.21 mF cm⁻², Figure S9c) of the 2 m MSM and 6 m NaClO₄ electrolyte was nearly 54 times that of the pure 6 m NaClO₄ capacitance (0.041 mF cm⁻², Figure S9d). This clearly also demonstrated the strong adsorption effect of MSM in the interfacial double layer.

Since the MSM adsorption layer covered a significant proportion of the active sites on the electrode surface, when water molecules need to adsorb onto graphene to initiate HER/OER, they must first displace or penetrate this layer of MSM molecules. This process is thermodynamically unfavorable and kinetically hindered, thereby effectively increasing the onset overpotential of the reaction and inhibiting its occurrence. The pH detection showed that the 2 m MSM and 6 m NaClO₄ electrolyte displayed alkaline (Figure S10). It has been reported that under alkaline conditions, the first step of the electrochemical hydrogen evolution reaction is the electrochemical adsorption step (Volmer reaction), as shown in Equation 1. Subsequently, either the electrochemical desorption step (Heyrovsky reaction) or the chemical desorption step (Tafel reaction) occurs to achieve hydrogen molecule evolution, as depicted in Equations 2 and 3, respectively, [43, 44].

$${\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}\to {\mathrm{H}}_{\mathrm{ads}}+\mathrm{O}{\mathrm{H}}^{-}$$ (1)

$${\mathrm{H}}_{\mathrm{ads}}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}\to {\mathrm{H}}_2+\mathrm{O}{\mathrm{H}}^{-}$$ (2)

$${\mathrm{H}}_{\mathrm{ads}}+{\mathrm{H}}_{\mathrm{ads}}\to {\mathrm{H}}_2$$ (3)

The oxygen evolution reaction follows a multi‐step proton‐coupled electron transfer process, typically comprising four reaction steps: (1) formation of adsorbed *OH; (2) conversion of adsorbed *OH to *O; (3) reaction of *O with OH⁻ to form *OOH; (4) *OOH reacting with OH⁻ to form oxygen. Each step releases one electron, as shown in Equations (4), (5), (6), (7) [43, 44, 45].

$$\mathrm{O}{\mathrm{H}}^{-}{+}^{\ast }\to \mathrm{O}{\mathrm{H}}^{\ast }+{\mathrm{e}}^{-}$$ (4)

$$\mathrm{O}{\mathrm{H}}^{\ast }+\mathrm{O}{\mathrm{H}}^{-}\to {\mathrm{O}}^{\ast }+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}$$ (5)

$${\mathrm{O}}^{\ast }+\mathrm{O}{\mathrm{H}}^{-}\to \mathrm{OO}{\mathrm{H}}^{\ast }+{\mathrm{e}}^{-}$$ (6)

$$\mathrm{OO}{\mathrm{H}}^{\ast }+\mathrm{O}{\mathrm{H}}^{-}\to {\mathrm{O}}_2{+}^{\ast }+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}$$ (7)

To better understand the contribution of MSM to the ESW, Figure 2l shows the adsorption energies between the MSM and Gr surfaces, as well as the HER/OER evolution reaction intermediates (*H, *OH, *O, and *OOH). The results indicated that MSM exhibited high binding energy with those active intermediates. Even when HER/OER has occurred during the electrochemical reactions and highly reactive HER/OER intermediates had formed, MSM can still anchor these active intermediates onto the electrode surface. The adsorbed MSM stabilized those intermediates and suppressed HER/OER for broadening the ESW. This was likely the primary reason for the expanded ESW in MSM systems. Additionally, the system maintained high ionic conductivity, meeting the requirements for energy storage (Figure S11).

The traditional hydrogel preparation process is widely criticized for its complexity and the time‐consuming solvent exchange procedures. To fully demonstrate the superiority of MSM, a scalable solvent exchange‐ion cross‐linking phase separation membrane formation strategy utilizing its outstanding hydrophilicity was proposed. The one‐pot method successfully achieved the preparation of M‐PS/Ca, effectively addressing the complexities of traditional hydrogel synthesis and the time‐consuming procedures. The preparation process and mechanism of M‐PS/Ca are illustrated in Figure 3. The synergistic interaction of NaClO₄, MSM, water, and the gel scaffold further reduced the activity of free water in the hydrogel, thereby suppressing HER and OER to broaden its ESW.

FIGURE 3.

Preparation process and mechanism of M‐PS/Ca.

Five hydrogel electrolyte membranes were obtained by adding different concentrations of MSM and 2 m NaClO₄ to the hydrogel electrolyte matrix. Linear sweep tests were conducted in a two‐electrode system. As shown in Figure S12a, the ESW of the blank hydrogel was essentially identical to that in aqueous solution, with a HER potential of −1.1 V and an OER potential of 1.1 V. As the MSM concentration increased, the pattern of ESW variation remained fundamentally consistent with that observed in the aqueous solution. The ESW reached a maximum of approximately −2.7 to 2.3 V at 2 m MSM. In a hydrogel electrolyte membrane prepared with 3 m MSM, excess MSM may form a dense, thick “insulating barrier” on the electrode surface. The barrier impeded the necessary charge transfer reactions occurring at the electrode surface, resulting in a slight reduction in the ESW. As shown in Figure S12b, the ESW of the hydrogel electrolyte membrane increased gradually with rising NaClO₄ concentration (fixed at 2 m MSM). At 6 m NaClO₄, the ESW reached −2.8 to 2.4 V, whereas at 8 m NaClO₄, it reached −2.82 to 2.43 V. Furthermore, it was found that the ESW based on hydrogel electrolyte membranes was significantly broader than that of mixed solutions. This was attributed to the interaction between the hydrogel matrix and water, resulting in lower free water content within the hydrogel compared to that in solution. As shown in Figure 4a, the binding energy between PAM and H₂O (19.46 kcal mol⁻¹) was significantly higher than that between H₂O molecules (6.37 kcal mol⁻¹), indicating that PAM exhibited a stronger tendency to coordinate with H₂O. The electrostatic potential distributions of AM and H₂O (Figure S13) further demonstrated the strong interactions between the hydrogel framework and water molecules. The adsorption and immobilization of water by the hydrogel framework were responsible for the higher ESW of the hydrogel electrolyte membrane compared to the solution. It was important to note that even when the salt concentration was increased to 8 m, the ESW only increased by 0.25 V, indicating that high salt concentration contributed little to the ESW, similar to that in solution. However, it is well known that the freezing‐point depression effect of saltwater can confer excellent low‐temperature performance to a hydrogel electrolyte membrane [38, 46]. As shown in Figure S14, at 6 m NaClO₄, the hydrogel electrolyte membrane exhibited a phase transition at −48°C with a weakened endothermic peak intensity, demonstrating significantly superior performance compared to the phase transition observed at −18°C in the control hydrogel membrane. Additionally, it also demonstrated excellent thermal stability at elevated temperatures. Therefore, despite the limited contribution of high salt concentration to the ESW, 6 m NaClO₄ was ultimately selected as the supporting electrolyte for further research due to its excellent temperature adaptability. At the same time, a comprehensive thermal analysis was performed on the hydrogels before and after the introduction of 2 m MSM and 6 m NaClO₄ (Figure 4b). The results show that the moisture content of M‐PS/Ca is approximately 30% lower than that of PS/Ca. This reduction in moisture content helps lower the free water activity, which provides strong support for broadening the ESW range. As shown in Figure 4c, the ESW of M‐PS/Ca reached 5.2 V. As shown in Figure 4d, PS/Ca exhibited distinct ‐OH stretching and bending vibration peaks around 3000–3700 cm⁻¹ and 1600–1700 cm⁻¹, respectively. After the introduction of MSM and NaClO₄, the hydroxyl peak at 3362 cm⁻¹ in the hydrogel underwent a significant blue shift, indicating that the hydrogen‐bond network in the membrane was weakened due to the hydration of salt and hydrogen bonding interactions between MSM and water. It significantly reduced the activity of free water. The Raman spectrum shown in Figure 4e revealed that the strong hydrogen bond peak of M‐PS/Ca was significantly weakened, while the weak hydrogen bond peak increased. This indicated that the introduction of MSM and NaClO₄ suppressed the hydrogen bonds of water, which was aligned with the trend observed in solution and was consistent with the infrared spectrum data. Additionally, the solvation of the hydrogel electrolyte was calculated via molecular dynamics (MD) simulations, as shown in Figure S15. Compared to the solution, the radial distribution function (RDF) and coordination number (CN) exhibited minimal variation, indicating that the hydrogel electrolyte possessed a solvation structure similar to that in aqueous solution.

FIGURE 4.

(a) Binding energy of H₂O to H₂O, PAM to H₂O; (b) combined thermal analysis of PS/Ca and M‐PS/Ca; (c) LSV of PS/Ca and M‐PS/Ca; (d) infrared spectrogram of PS/Ca and M‐PS/Ca; (e) Raman spectrogram of PS/Ca and M‐PS/Ca; (f) stress–strain curves of PS/Ca and M‐PS/Ca.

Ionic conductivity is a key indicator for evaluating hydrogels as supercapacitor electrolytes. The ionic conductivities of hydrogel electrolyte membranes with different MSM and NaClO₄ ratios were tested at room temperature using the AC impedance method. As shown in Figure S16, when the MSM concentration was fixed, the conductivity of the hydrogel electrolyte membrane first increased and then decreased with increasing NaClO₄ concentration. The highest ionic conductivity of 52.56 mS cm⁻¹ was achieved at 6 m NaClO₄. When the NaClO₄ concentration remained constant, the ionic conductivity increased with rising MSM concentration, reaching a peak at 2 m. Thereafter, the ionic conductivity gradually decreased. This was because at low concentrations, as the NaClO₄ concentration increased, the concentration of hydrated ions raised, leading to increased conductivity. However, at high concentrations, the hydrogen bonding interactions between MSM and water, along with the solvation of electrolyte ions, result in a decrease in the activity of free water. Additionally, interactions between electrolyte ions may also lead to reduced conductivity [47]. Subsequently, the mechanical properties of these hydrogel electrolytes were evaluated through tensile testing. As shown in Figure S17a, with the NaClO₄ concentration fixed at 6 m, the tensile properties gradually increased as the MSM concentration increased from 0.5 to 2.0 m. The fracture strength progressively increased from 92.6 to 152.6 KPa, while the elongation at break gradually increased from 113.2% to 157.6%. The increase in mechanical strength may be attributed to multiple hydrogen bonds formed between MSM and polymer chains. When the MSM concentration was fixed at 2.0 m, as shown in Figure S17b, with increasing NaClO₄ concentration, the tensile strength of the hydrogel increased from 102.4 to 152.6 kPa, while the elongation at break gradually increased from 169.7% to 235.8%. However, when the salt concentration was further increased, the mechanical strength slightly decreased again. This indicated that adding salt enhanced the membrane strength, but excessive salt negatively impacted mechanical strength. Figure 4f also shows that the tensile properties of the M‐PS/Ca electrolyte membrane were significantly improved compared to PS/Ca. The optimal mechanical properties were achieved when 2.0 m MSM and 6 m NaClO₄ were incorporated into the hydrogel. Furthermore, the membrane displayed porous structure (Figure S18), which would facilitate ion migration during the charge/discharge cycle. The swelling results (Figure S19) show that M‐PS/Ca exhibited relatively rapid swelling during the first 150 min, primarily corresponding to the penetration and filling of the gel interior by the solution through macropores and channels; after 150 min, the swelling rate decreased significantly, governed by the relaxation rate of the polymer chains, and the final swelling ratio at equilibrium was 11.32%. This directly reflects the extremely high cross‐linking density of the polymer network, which is a key factor in constructing flexible supercapacitors with high safety and long service life. MIP analysis (Table S1) shows that M‐PS/Ca has a high porosity (89.25%) and an average pore diameter of 800.28 nm. Consistent with SEM observations, this indicates that the hydrogel structure provides continuous pathways for ion migration, significantly reducing the physical resistance to ion transport and contributing to enhanced ionic conductivity. In summary, the hydrogel electrolyte membrane with 2.0 m MSM and 6 m NaClO₄ exhibited the highest ionic conductivity and superior mechanical properties, with an ESW of 5.2 V. The membrane was selected for assembling energy storage devices.

The fabrication process of graphene interdigitated electrodes is illustrated in Figure S20. The sheet resistance of LIG films was exceptionally low (Figure S21), only 18.1294 Ω sq⁻¹. In this work, the area of a single LIG microelectrode was 0.9 mm², with detailed dimensions presented in Figure S22. To demonstrate the effective wetting of the electrode interface by the electrolyte solution, the contact angle between the electrolyte solution and the LIG electrode was measured (Figure S23). The contact angle of pure water on the electrode surface exceeded 90° corresponded to the highly hydrophobic nature of the LIG. However, the contact angle decreased to 45.9° for the 2 m MSM and 6 m NaClO₄, demonstrating the excellent wetting performance. This indicated that the electrolyte can rapidly permeate and wet the graphene electrode, which was conductive to achieving efficient ion transport.

A supercapacitor was assembled using M‐PS/Ca as the electrolyte and LIG as the electrodes. The CV curves (Figure 5a) indicated that below 2.4 V, the device exhibited standard rectangular characteristics, whereas above 2.4 V, the CV curve displayed a marked deviation from the rectangular profile. This deviation can be attributed to polarization behavior arising from the simplified device encapsulation process [48], preventing the device from reaching the ideal operating voltage of 2.6 V. Consequently, 2.4 V was identified as the maximum operating voltage. Figure 5b demonstrated that within the scan rates ranging of 5–200 mV s⁻¹, the CV curves of the supercapacitor exhibited standard rectangular shapes within the voltage range of 0–2.4 V, indicating ideal electrochemical double‐layer behavior. Even at a scan rate of 200 mV s⁻¹, the CV curve remained nearly rectangular, which can be attributed to its low charge transfer resistance and excellent ion diffusion properties. As shown in Figure 5c,d, the device exhibited an approximately GCD curves within 0–2.4V at current densities of 0.4–0.8 mA cm⁻². A GCD was selected for testing at a current density of 0.4 mA cm⁻². Similarly, six points were selected on average during the charging and discharging processes to measure the infrared spectra at the corresponding voltages (Figure S24). The results were consistent with those in solution, with MSM exhibiting interfacial enrichment on LIG. In addition, the device assembled using Ls‐M‐Ps/Ca (2 m MSM and 2 m NaClO₄) revealed an operating of 2.2 V from their CV and GCD curves (Figure S25), while the contribution of 6 m NaClO₄ to the ESW was only 0.2 V, which validated the contribution of MSM to the operating voltage range, consistent with the results in solution. The EIS curve (Figure 5e) indicated that the device exhibited low charge transfer resistance, with a line close to the imaginary axis in the low‐frequency region. The contact resistance is 147.6 Ω, further confirming the presence of interfacial adsorption between the non‐conductive MSM and the electrode. The device maintained a capacity retention rate of over 98% after 10 000 charge–discharge cycles (Figure 5f). The specific capacitance reached 8.31 mF cm⁻² at a scan rate of 5 mV s⁻¹, while it maintained 74.9% at a scan rate of 200 mV s⁻¹(Figure 5g). The energy density of this device was 4.73 µWh cm⁻², which was higher than that of other reported micro‐supercapacitors (MSCs), such as GF@3D‐G‐MSC, (≈0.16 µWh cm⁻²) [49], P‐MSCs‐32, (≈0.06 µWh cm⁻²) [50], LIG‐MSCs‐AE, (≈0.05 µWh cm⁻²) [51], MPG‐MSCs(≈0.04 µWh cm⁻²) [52], and rGO/PVA‐H₃PO₄ (≈0.01 µWh cm⁻²) [53, 54] (Figure 5h). In addition, Figure 5i compared the wide ESW with other reported supercapacitors, demonstrating a wide operating voltage of 2.4 V, which represented a significant advantage.

FIGURE 5.

(a) CV curves of the device at 100 mV s⁻¹ scan rate for different potential windows; (b) CV curves of the device at 5–200 mV s⁻¹ for scan rate; (c) GCD curves of the device at 0.4 mA cm⁻¹ current density for different voltages; (d) GCD curves of the device at 0.4–0.8 mA cm⁻¹ current density; (e) Device's impedance plot; (f) cyclic stability curve of the device at 10 000 charge/discharge cycles; (g) area capacitance of the device versus scan rate; (h) comparison of the Ragone plot of the device with the area‐specific energy density and power density of other reported supercapacitors; and (i) comparison of the voltage window of the present study with that of previous literature.

High flexibility and mechanical stability are fundamental key requirements for supercapacitors. As shown in Figure S26a–c, the CV, GCD, and EIS curves of the supercapacitor at different bending angles, especially at 180°, were essentially consistent with its initial state, demonstrating the device exhibited excellent electrochemical stability under various severe deformation conditions. Figure 6a,b presents the CV and GCD curves for a single device and two devices connected in series, respectively. When two devices were connected in series, the operating voltage can be increased to 4.0 V, and their discharge cycles were close to those of a single device. The Voltage was below the theoretical value, likely due to the simplified device assembly and electrode edge polarization at a relatively lower voltage. Figure 6c shows the GCD curves for a single device and two devices connected in parallel. Their operating voltage remained essentially identical, while the discharge time extended to nearly twice that of a single device. These results demonstrated that supercapacitors using M‐PS/Ca as the electrolyte exhibited excellent series‐parallel capacitance characteristics. Two devices connected in series can readily illuminate a red light‐emitting diode (Figure 6d).

FIGURE 6.

(a) and (b) CV curves and GCD curves for a single device and two devices connected in series; (c) GCD curves for a single device and two devices connected in parallel; (d) Optical photograph of two devices connected in series lighting a small light bulb.

Temperature adaptability is a crucial parameter for evaluating hydrogel electrolyte membrane performance. Since water evaporates at high temperatures and freezes at low temperatures, temperature fluctuations can severely compromise device stability, thereby affecting its lifespan under extreme weather conditions. As shown in Figure 7a, after exposing both hydrogels to −40°C and −20°C for one day, PS/Ca underwent freezing and hardening at low temperatures due to its high content of free water. In contrast, within the same temperature range, M‐PS/Ca exhibited no significant changes, maintaining transparency and excellent flexibility. Furthermore, water in PS/Ca completely evaporated after 1 h at 45°C, causing it to become extremely brittle and prone to fracture under minimal external force. In contrast, M‐PS/Ca exhibited no morphological changes at 45°C. This resilience stemmed from MSM forming strong hydrogen bonds with H₂O, which restricted water movement. This mechanism lowered the freezing point and saturated vapor pressure, enabling it to withstand to both lower and higher temperatures. In addition, as shown in Figure S27, ignition tests on the M‐PS/Ca electrolyte membrane demonstrated that the hydrogel electrolyte was completely non‐flammable, which significantly enhanced safety.

FIGURE 7.

(a) PS/Ca and M‐PS/Ca were placed at temperatures of −40°C, −20°C, 25°C, and 45°C respectively Optical photographs after one day; (b) changes in ionic conductivity of the M‐PS/Ca from −40°C to 45°C; (c) CV curves in different temperature ranges; (d) GCD curves in different temperature ranges.

To investigate the wide‐temperature adaptability of M‐PS/Ca electrolyte supercapacitors, CV, GCD, and EIS tests were conducted within the temperature range of −40°C–45°C, and the ionic conductivity of the hydrogel was also calculated at each temperature. As shown in Figure 7b, the ionic conductivity at 45°C decreased to 31.53 mS cm⁻¹. This may be attributed to the increased thermal motion of water at elevated temperatures, which partially disrupted the hydrogen bonds between MSM and water. Water inevitably evaporated lead to a decrease in conductivity. However, the reduction remained sufficient to ensure the device maintained excellent electrochemical performance. As shown in Figure 7c,d, the attenuation of CV and GCD curves was less than 5% at both low and high temperatures, which may be attributed to slight deformation of the hydrogel electrolyte membrane. The simplified assembly inevitably led to a reduced contact area at the electrode‐electrolyte interface, which may also contribute to the decline in device performance. The results confirmed that M‐PS/Ca exhibited excellent temperature adaptability.

4. Conclusion

In summary, a water‐based electrolyte based on interfacial engineering strategies was proposed. It was demonstrated that the competitive adsorption of MSM at the electrolytic interface and the stabilization of active intermediates were the primary reasons for the broadening of ESW, particularly the elevation of the OER potential. Although the synergistic effect of the hydrogen bonding of MSM and the solvation effect of salt collectively reduced the bulk water activity, competition between these effects led to a negligible contribution of high salt concentrations to the ESW, while significantly enhanced the temperature adaptability. A scalable strategy was proposed for preparing hydrogel electrolyte membranes via solvent exchange‐ion cross‐linking phase separation. The synergistic interactions among the hydrogel framework, MSM, NaClO₄, and water enhanced the ESW, mechanical strength, and conductivity. The M‐PS/Ca displayed an ESW of 5.2 V and excellent temperature adaptability. Supercapacitors assembled with the M‐PS/Ca and LIG exhibited an operating voltage of 2.4 V, a specific capacitance of 8.31 mF cm⁻² and a high energy density of 4.73 µWh cm⁻². After 10 000 charge–discharge cycles, the capacitance retention remained above 98%. The device exhibited outstanding cycle stability, good integration performance, favorable mechanical stability, and exceptional adaptability across a temperature range of −40°C–45°C. This study pioneered an innovative approach to broadening the ESW of aqueous electrolytes through interfacial engineering, paving a new path for the construction of high‐performance supercapacitors.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: smll73825‐sup‐0001‐SuppMat.docx.

Data Availability Statement

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