High-strength, anti-freezing and recyclable soy protein isolate/poly(vinyl alcohol) gels empowered by a deep eutectic solvent-augmented Hofmeister effect

Abstract

Highly tough hydrogels formed via the Hofmeister effect exhibit broad applications in flexible electronics. However, high concentrations of salting-out salts inevitably induce phase separation of the solvent and polymer chains, thereby affecting the mechanical properties and conductivity of the hydrogels. Herein, we introduced a deep eutectic solvent (DES) to suppress the strong salting-out effect of trisodium citrate (Na₃Cit), thereby preventing phase separation in the soy protein isolate (SPI)/poly(vinyl alcohol) (PVA) mixture. This strategy enabled the formation of a homogeneous SPI/PVA/DES/Na₃Cit composite solution, which was subsequently processed into a gel electrolyte (denoted as SPDNH) through a freeze–thaw cycle. The optimized organohydrogel exhibited a tensile strength of 1.65 ± 0.03 MPa and an elongation at break of 518.8 ± 7.49%, along with an ionic conductivity of 2.01 ± 0.04 S m⁻¹. Moreover, the incorporation of DES imparted remarkable freeze resistance to the SPDNH, with a freezing point as low as −33 °C. A flexible all-solid-state supercapacitor was fabricated using the SPDNH electrolyte and activated carbon electrodes. The device delivered a high areal capacitance of 125.61 mF cm⁻² and demonstrated stable cycling performance at room temperature, retaining 80.20% of its initial capacitance after 1000 charge–discharge cycles. Furthermore, the supercapacitor maintained reliable operation under various bending conditions and at low temperatures (−20 °C), highlighting its potential for use in flexible and low-temperature energy storage applications.

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1. Introduction

The rapid advancement of cutting-edge technologies, particularly in wearable electronics, soft robotics, and bioelectronic interfaces, is driving the demand for flexible electronic devices with increasingly sophisticated performance.^1,2 The core of such devices lies in soft, conductive functional materials capable of conformal adhesion to the human body or complex, dynamic surfaces. Among various candidates, hydrogels have garnered significant attention as a premier material class for constructing next-generation flexible electronics.^3–8 This preference stems from their unique intrinsic properties: a three-dimensional crosslinked polymer network, high water content, excellent biocompatibility, and versatile tunability.

Despite their great promise, conventional hydrogel materials are plagued by a fundamental limitation: poor mechanical robustness.^9,10 Typically, these hydrogels exhibit high brittleness, low strength, and easy fracturability due to their structurally heterogeneous and often inefficient energy-dissipation mechanisms. When subjected to repeated mechanical loading – such as stretching, bending, or twisting – their polymer networks are prone to irreversible damage at the microscopic level. This structural degradation directly leads to the rupture of conductive pathways, resulting in functional failure during operation.^11,12 Such inherent mechanical fragility would not only accelerate performance degradation but would also severely shorten the operational lifespan of hydrogel-based devices, thereby posing a critical barrier to their practical and reliable application in real-world scenarios.^13,14

To address this limitation, strategies based on the Hofmeister effect have been employed to toughen and strengthen hydrogels.^15–20 This ion-specific effect can promote polymer chain dehydration and aggregation, enhancing intermolecular interactions and thereby improving mechanical strength. Nonetheless, a significant drawback accompanies this approach: the strong salting-out effect frequently induces macroscopic phase separation within the polymer network. This results in a heterogeneous, porous structure that compromises the material's stretchability and toughness.¹⁷ Consequently, while strength may increase, the hydrogel often suffers from reduced elongation and, critically, a decline in electrical conductivity due to the disrupted conductive network. To address this issue, Zhou et al. proposed a molecular bridging strategy by introducing urea, which contains both hydrogen bond donors and acceptors to improve the concentration of ZnSO₄ in PVA solution. This approach facilitates the reformation of hydrogen bonds between PVA and water – disrupted by the strong water polarization induced by SO₄²⁻ – through robust intermolecular hydrogen bonding, thereby significantly enhancing the ZnSO₄ loading capacity. In this study the authors ingeniously utilized urea as a dual hydrogen bond donor–acceptor to increase the salt content in the system, providing valuable insights for optimizing salting-out salt concentrations in hydrogels.²¹

The introduction of urea essentially represents a modification of the conventional hydrogel system. To fundamentally create an ideal environment that concurrently accommodates high salt concentration, high strength, and high conductivity, we turned our attention to deep eutectic solvents (DESs), which similarly function as both hydrogen-bond donors (HBDs) and hydrogen-bond acceptors (HBAs). Deep eutectic solvents (DESs) are a class of low-melting mixtures formed through intermolecular interactions between HBDs and HBAs, exhibiting melting points significantly lower than those of their individual components. These solvents possess notable characteristics such as low vapor pressure, excellent thermal stability, tunable properties, and high biocompatibility, making them widely applicable in green chemistry, material synthesis, and biomass processing. Compared to conventional ionic liquids, DESs offer advantages including simple preparation, low cost and the elimination of complex purification steps, thus garnering significant attention in the development of sustainable materials.^22–25 Recently, DES-based polymer gels, known as eutectogels, have emerged as a promising class of soft materials for flexible electronics. Eutectogels combine the advantages of DESs (e.g., non-volatility, wide liquid range, and high ionic conductivity) with the flexibility and processability of polymer networks. Compared to conventional hydrogels and organohydrogels, eutectogels exhibit superior environmental stability, tunable mechanical properties, and enhanced ionic transport, making them ideal candidates for applications in energy storage, sensors, and wearable devices. The properties of DESs can be precisely tailored by adjusting the composition and type of component.²⁶ This combination is particularly suitable for preparing high-performance composite hydrogel materials, offering an ideal platform for developing novel functional hydrogels.

In this study, we selected choline chloride/glycerol (ChCl/Gly) as the DES system based on the following considerations: (i) ChCl and Gly are both biocompatible and cost-effective, aligning with the principles of green chemistry; (ii) the ChCl/Gly system exhibits strong hydrogen-bonding capability, facilitating the dissolution of SPI and PVA while suppressing phase separation induced by high salt concentrations; (iii) Gly, as a polyol, contributes to excellent anti-freezing performance due to its ability to form strong hydrogen bonds with water, thereby depressing the freezing point of the system; and (iv) the moderate viscosity of ChCl/Gly would ensure good processability during gel preparation. So, we proposed a novel method utilizing DESs to increase the incorporation of Na₃Cit within the system, thereby directly achieving the salting-out effect within the hydrogel matrix to fabricate conductive hydrogels.

Soy protein isolate (SPI) is a natural plant protein derived from renewable resources. Its molecular chain is rich in active functional groups such as amino groups, carboxyl groups, and hydroxyl groups, which provide abundant hydrogen bond interaction sites and endow the material with excellent biocompatibility and biodegradability, conforming to the concepts of green chemistry and sustainable development. However, hydrogels constructed solely with SPI as the matrix often encounter the challenge of insufficient mechanical properties, making it difficult for them to meet the requirements of materials strength and toughness for flexible electronic devices. Poly(vinyl alcohol) (PVA), as a synthetic polymer material, has abundant hydroxyl groups distributed on its long macromolecular chain, which not only form strong intramolecular and intermolecular hydrogen bonds but also endow PVA with good water solubility, film-forming ability, and excellent mechanical properties. By combining SPI and PVA to construct a dual-network hydrogel, the two are linked through the amide groups and carboxyl groups of SPI and the hydroxyl groups of PVA to form abundant intermolecular hydrogen bonds, creating a dense and stable three-dimensional network structure. This dual-network design fully exploits the excellent tensile strength and energy dissipation ability provided by the flexible long chains of PVA, as well as the high strength and structural stability contributed by the rigid chain segments of SPI, achieving synergistic enhancement of mechanical properties. Compared with a single polymer system, SPI/PVA dual-network hydrogels could overcome the insufficient strength when SPI is used alone for gelation. While maintaining good biocompatibility, they provide an ideal material platform for the subsequent construction of high-strength, high-conductivity water gel electrolytes through a DES-enhanced salting-out effect.

Finally, we employed SPI and PVA as the matrix, ChCl/Gly as the solvent, and sodium citrate (Na₃Cit) as the salting-out salt to fabricate an SPI/PVA organohydrogel with high strength, toughness, and conductivity. Through the synergy of salting-out and solvent effects, a high-strength, tough, and conductive SPI/PVA/DES/Na₃Cit (SPDNH) hydrogel was successfully prepared, exhibiting a tensile strength of 1.65 ± 0.03 MPa, an elongation at break of 518.84 ± 7.49%, and a conductivity of 2.01 ± 0.04 S m⁻¹. The DES played a dual role in the hydrogel network: it mitigated the strong precipitation effect of Na₃Cit and simultaneously enhanced the ionic conductivity of the hydrogel. Benefiting from these superior properties, a flexible all-solid-state supercapacitor was assembled using SPDNH as the electrolyte and activated carbon as the electrode. The device demonstrated remarkable specific capacitance (125.61 mF cm⁻² at room temperature) and excellent cycling stability (80.20% capacitance retention after 1000 charge–discharge cycles).

2. Materials and methods

2.1. Materials

SPI was purchased from Linyi Shansong Bio­logical Products Co. Ltd (China). PVA (degree of polymerization was 1700, degree of alcoholysis was 99%) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd (Shanghai, China). ChCl were obtained from Shanghai Macklin Biochemical Technology Co. Ltd (Shanghai, China). Gly and Na₃Cit were provided by Tianjin Zhiyuan Chemical Reagent Co. Ltd. All chemicals were used without further purification. Deionized water was used throughout the experiment.

2.1.1. Preparation of hydrogels. Hydrogels were prepared using the freeze-thawing method. The base formulation involved dissolving SPI and PVA in the solvent (DES composed of ChCl and Gly, together with pure deionized water) at 95 °C for 1 hour. For some variants, Na₃Cit was added with further stirring. The homogeneous mixtures were then cast into a 1 mm-thick mold and subjected to a freezing cycle at −20 °C for 12 hours, followed by thawing at room temperature for 1 hour. Specifically: (1) SPI/PVA/DES/Na₃Cit hydrogel (SPDNH) included all components; (2) SPI/PVA/DES hydrogel (SPD) without the presence of Na₃Cit; (3) SPI/PVA/Na₃Cit hydrogel (SPNH) used water instead of DES with the presence of Na₃Cit; and (4) SPI/PVA hydrogel (SP) prepared merely with deionized water without salt and DES (Table 1).

Table 1 Composition of the prepared hydrogels

Sample	SPI (g)	PVA (g)	ChCl (g)	Gly (g)	Na3Cit (g)	H2O (g)
SPH	3	6	0	0	0	43
SPNH	3	6	0	0	1.0	43
SPDH	3	6	5	10	0	30
SPDNH	3	6	5	10	1.0	30

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2.1.2. Preparation of flexible all-solid-state supercapacitors. Activated carbon (AC) was used as the electrode and SPDNH as the electrolyte to obtain the flexible all-solid-state supercapacitor. AC, acetylene black (CB) and polyvinylidene difluoride (PVDF) were mixed in a 10 mL reagent bottle at a mass ratio of 8 : 1 : 1, to which 3 mL of anhydrous ethanol was then added and dispersed by ultrasonic treatment in an ultrasonic cleaner for 30 min. The resulting mixed liquid was then dried in a vacuum drying oven for 12 h, with the temperature set at 80 °C to ensure that it was completely dried. Subsequently, the resulting electrode material was uniformly coated on the nickel foam mesh. Finally, the nickel mesh coated with the electrode material was extruded under the extrusion pressure of 10 MPa to obtain the working electrode. The flexible all-solid-state supercapacitor was obtained by sandwiching the SPDNH hydrogel electrolyte (1 mm in length, 1 mm in width, and 1 mm in thickness) between the two working electrodes.

2.2. Characterization of SPDNH hydrogels

An electronic universal testing machine (SUNS, CMT 6503, China) was used to test the tensile properties. The hydrogel sample was cut into a dumbbell shape (75 mm long and 4 mm wide, and the thickness was measured before the test). To prevent the sample from slipping out during the test, the ends of the hydrogel were wrapped with paper before the test. The tensile speed was 100 mm min⁻¹. Young's modulus was defined as the slope of the hydrogel specimen at 0–5% of the stress–strain curve, and the toughness was defined as the area of integration of the stress–strain curves. Dissipated energy was calculated from the area between the loading and unloading curves. Measurements were repeated five times for all samples.

The micro-morphology of the hydrogels was investigated by field emission scanning electron microscopy (FE-SEM, FEI Nova Nano-SEM 230, USA, accelerating voltage 10 kV). Before observation, the hydrogels were soaked in deionized water and the water was changed every 24 h for 5 consecutive cycles to remove inorganic salts from the hydrogels. Then it was quenched with liquid nitrogen and dehydrated in a freeze-dryer. Finally, the surface of each sample was plated with gold on an ion sputter coater.

The thermal properties of the gels were tested using a differential scanning calorimeter (NETZSCH DSC214, Germany). The hydrogel (5–10 mg) was sealed in an aluminium crucible and equilibrated at −100 °C for 3 min under a nitrogen atmosphere and heated up to 200 °C at a rate of 10 °C min⁻¹.

The adhesivity of the samples was tested using a universal material testing machine. The specification of the tested sample was 20 mm × 20 mm × 1 mm. The gel sample was pasted on the substrate (PVC plastic plate, etc.), and the substrate was mounted on the testing machine. The shear tensile rate was 10 mm min⁻¹.

The electrochemical performance of supercapacitors was tested through the electrochemical workstation CHI660E: cyclic voltammetry (CV) test, constant current charge and discharge (GCD) test and electrochemical impedance (EIS) test. A cuboid hydrogel sample of 10 mm long, 10 mm wide and 1 mm thick was prepared and sandwiched between two foam nickel mesh electrodes. CV tests were performed at different scan rates from 10 to 100 mV s⁻¹ with measuring voltages ranging from 0 to 1 V. The GCD test was performed in the voltage range of 0–1 V, and different current densities of 1–2 mA cm⁻² were tested. In addition, the EIS test was performed at an open-circuit voltage of 5 mV and a frequency range of 0.01–10⁵ Hz. What is more, the specific capacitance of the supercapacitor (C_A, mF cm⁻²) was calculated by the GCD curve from eqn (1).

where I (A) is the charge and discharge current, Δt (s) is the discharge time, ΔV (V) is the voltage window during the test, and A (cm²) is the unilateral area of the supercapacitor, respectively. The conductivity was calculated using eqn (2),where A (cm²) is the cross-sectional area of the hydrogel, d (cm) is the distance between the electrodes, and R (Ω) is the transverse intercept in the EIS diagram, respectively.

3. Results and discussion

3.1. The preparation mechanism of SPDNH

The SPDNH was prepared through the synergy of a solvent effect and salting-out effect. Briefly, SPI and PVA were used as the gel matrix, with DES serving as a binary solvent and Na₃Cit as the salting-out salt. After SPI and PVA were completely dissolved, Na₃Cit was added to the solution. The mixture was frozen for 12 h and then thawed for 1 h to obtain the SPDNH (Fig. S1). SPI and PVA formed a physically crosslinked network via hydrogen bonding between the hydroxyl groups on the PVA chains and the amide groups on the SPI macromolecules.²⁷ As illustrated in Fig. 1, in the DES system, the chloride ions from ChCl and the hydroxyl groups from Gly interacted to form hydrogen bonds, constructing another physically crosslinked network.²⁸ Upon the addition of DES, this network further bonded with the remaining hydroxyl groups on the PVA chains that were not crosslinked with SPI, ultimately forming a three-dimensional (3D) physically crosslinked network. This stable 3D structure acted as a molecular bridge, allowing for an increased loading of Na₃Cit in the system. Meanwhile, Na₃Cit exerted a salting-out effect on both PVA and SPI.^27,29 The salting-out effect strengthened the physical interactions among the internal components of the hydrogel, densified the network structure, and further enhanced the strength and toughness of the hydrogel. Additionally, due to the low freezing point of DES, its incorporation enhanced the freeze-resistance of the hydrogel. Furthermore, the presence of DES contributed to a more compact network structure, thereby improving the thermal stability of the resulting hydrogel.^30,31 In summary, under the combined effects of SPI/PVA/DES/Na₃Cit, an ionic conductive hydrogel with high tensile strength, freeze-resistance, thermal stability, and high conductivity was successfully prepared.

Fig. 1 Schematic diagram showing that DES can increase the allowable concentration of Cit⁻ in two different hydrogel systems.

To investigate the hydrogen bonding interactions within different hydrogels, FTIR spectroscopy was performed (Fig. S2). The O–H stretching vibration band of SPH appeared at approximately 3250 cm⁻¹, indicating strong hydrogen bonding between polymer chains and water molecules. Upon the addition of Na₃Cit in the aqueous system (SPNH), the O–H band further red-shifted to a lower wavenumber, confirming that the salting-out effect promoted polymer chain aggregation and enhanced hydrogen bonding interactions. In contrast, for DES-containing samples (SPDH and SPDNH), the O–H stretching vibration band blue-shifted to approximately 3300 cm⁻¹ compared to their aqueous counterparts. This blue shift suggests relatively weaker polymer–polymer hydrogen bonding, which can be attributed to the formation of strong self-associated hydrogen bond networks among DES components (ChCl and Gly) that compete with polymer–polymer interactions. Notably, the addition of Na₃Cit in the DES system (SPDNH) did not cause a further shift of the O–H band, indicating that the DES effectively suppresses the salting-out effect of citrate ions. These results demonstrate that DES plays a dual role in the gel network: it mitigates the strong salting-out effect of Na₃Cit while simultaneously constructing a robust hydrogen-bonded network that contributes to the material's mechanical integrity.

3.2. Mechanical and conductive properties of four types of hydrogel

To examine whether the incorporation of DES could enhance the salt loading capacity and improve gel performance, we performed a straightforward control experiment. In the DES-free system, gradual addition of salt led to visible aggregation when the salt content reached approximately 0.5 wt% of the total mass (Fig. 2a). In contrast, with the presence of DES, the salt loading capacity increased markedly up to 2.0 wt%. We then evaluated the mechanical properties of gels prepared at these two distinct salt loadings. As illustrated in Fig. 2b and c, the SPNH hydrogel (prepared in water with Na₃Cit) exhibited limited mechanical performance, with a tensile strength of only 0.44 MPa and an elongation at break of 168.50%. When pure water was replaced by DES (SPDNH), the resulting hydrogel showed significantly improved mechanical behavior: the tensile strength increased to 1.45 MPa and the elongation at break reached 322.20%. The Young's modulus and toughness of the gel were also substantially improved (Fig. 2d). To visually demonstrate the excellent mechanical properties, a dumbbell-shaped specimen (4 mm width) could easily support a load of 600 g, and a rectangular gel sheet (40 mm × 20 mm) successfully lifted a 2600 g water bottle (Fig. 2e). Furthermore, DES contributed to the improved ionic conductivity: SPNH exhibited a conductivity of 1.15 S m⁻¹, while SPDNH reached 1.73 S m⁻¹ – an increase of 50.43% (Fig. 2f). In summary, the use of DES significantly raised the permissible Na₃Cit content during gel preparation, leading to concurrent improvements in both mechanical and electrochemical performance.

Fig. 2 (a) Comparison chart of whether agglomeration occurred with the same salt addition amount in pure water and DES solvent, respectively; (b) tensile stress–strain curves; (c) tensile strength and elongation at break; (d) Young's modulus and toughness; (e) SPDNH lifting a 600 g and a 2600 g weight; (f) ionic conductivity.

To systematically investigate the effects of the deep eutectic solvent (DES) and salting-out agent (Na₃Cit) on the gel electrolyte properties, we prepared four hydrogel variants: SPDNH, SPDH, SPNH and SPH. As shown in Fig. 3a–c and Fig. S3, the SPH hydrogel – prepared without salt or DES – exhibited relatively poor mechanical performance, with a tensile strength of only 0.33 ± 0.01 MPa, an elongation at break of 261.74 ± 5.22%, a Young's modulus of 0.21 ± 0.01 MPa, a toughness of 0.55 ± 0.03 MJ m⁻³, and an ionic conductivity of 0.54 ± 0.04 S m⁻¹. Remarkably, the introduction of either Na₃Cit or DES led to substantial improvements in mechanical properties. The SPNH hydrogel, containing Na₃Cit, showed a tensile strength of 0.67 ± 0.01 MPa, an elongation at break of 280.99 ± 3.47%, a Young's modulus of 0.18 ± 0.05 MPa, a toughness of 0.89 ± 0.06 MJ m⁻³, and a conductivity of 1.15 ± 0.14 S m⁻¹, respectively. Similarly, the SPDH hydrogel, incorporating DES, displayed enhanced characteristics: a tensile strength of 0.71 ± 0.01 MPa, an elongation at break of 354.01 ± 5.01%, a Young's modulus of 0.125 ± 0.01 MPa, a toughness of 1.37 ± 0.18 MJ m⁻³, and a noticeably higher conductivity of 3.06 ± 0.09 S m⁻¹. Most remarkably, the synergistic combination of DES and Na₃Cit in SPDNH yielded optimal overall performance, achieving a tensile strength of 1.42 ± 0.07 MPa, an elongation at break of 435.43 ± 2.37%, a Young's modulus of 0.46 ± 0.01 MPa, a toughness of 3.36 ± 0.11 MJ m⁻³, and a conductivity of 2.22 ± 0.06 S m⁻¹. It should be noted, however, that the ionic conductivity of SPDNH was slightly lower than that of SPDH, which can be attributed to the reduced amount of free water caused by the salting-out effect of Na₃Cit, thereby limiting ion mobility. This trend suggests that further increasing the salt content would continue to compromise the hydrogel's conductive properties.³²

Fig. 3 Mechanical properties of SPH, SPNH, SPDH and SPDNHs. (a) Stress–strain curves of SPH, SPNH, SPDH and SPDNHs under tension. (b) Tensile strength and elongation at break. (c) Young's modulus and toughness. (d) Compressing stress–strain curves. (e) Loading–unloading cyclic behavior of SPDNH under different strains and (f) loading–unloading tests of the SPDCS hydrogel under different strains (50%, 100%, 200%, 300%, 400%, and 500%), without any rest intervals. (g) Corresponding dissipation energy and dissipation coefficient. (h) Compression-recovery measurements and (i) calculated toughness and energy dissipation coefficient of SPDNH, with continuous compression for 10 times at 50% strain, without any resting time.

The internal morphology and micro-structure of the hydrogels were characterized using scanning electron microscopy (SEM) (Fig. S4). The SPH hydrogel displayed a relatively dense structure with no clearly observable pores, which can be attributed to the compact network formed between SPI and PVA. Interestingly, the individual introduction of either Na₃Cit or DES led to the gradual development of porous structures on the gel surface. When both components were incorporated together, the formation of additional physical cross-linking sites increased the cross-linking density of the network, yielding a more compact yet mechanically robust architecture. Comparative analysis confirmed that the synergistic interaction between DES and Na₃Cit produced hydrogels with superior tensile properties. To further evaluate mechanical performance, compression tests were conducted on all four hydrogels (Fig. 3d). Under 70% compressive strain, SPH exhibited the lowest stress resistance (52.29 kPa), while SPNH and SPDH showed improved stress values of 136.60 kPa and 363.80 kPa, respectively. SPDNH demonstrated exceptional compressing strength (714.68 kPa), unequivocally verifying the mechanical enhancement achieved through the cooperative effect of DES and Na₃Cit. It is particularly noteworthy that physically cross-linked hydrogels with high mechanical strength typically suffer from compromised ionic conductivity.³³ In this study, the SPDNH simultaneously achieves outstanding mechanical properties and high ionic conductivity (2.22 ± 0.06 S m⁻¹)—a combination that has been seldom reported in previous work^34,35 (Fig. S5). This integrated performance makes the SPDNH particularly suitable for applications demanding both mechanical robustness and efficient ion transport.

Furthermore, we prepared samples with varying Na₃Cit contents and observed distinct mechanical trends to systematically investigate the effect of Na₃Cit concentration on the mechanical properties of the hydrogels (Fig. S6). Initially, both the tensile strength and elongation at break increased progressively with the increasing Na₃Cit content. The optimal performance was achieved with the addition of 1.8 wt% of Na₃Cit, at which the SPDNH exhibited a tensile strength of 1.65 ± 0.03 MPa and an elongation at break of 518.84 ± 7.49%. However, as illustrated in Fig. S6, further increases in Na₃Cit content resulted in a gradual decrease in elongation at break, whereas the Young's modulus continued to rise. This behavior can be explained by the enhanced salting-out effect at elevated salt concentrations, which promotes the formation of a more compact network structure while concurrently reducing the water content within the hydrogel system.³⁶ The reduction in water content within the hydrogel matrix compromises its molecular flexibility, resulting in decreased elongation at break and a consequent reduction in toughness. Furthermore, the diminishing availability of water – which serves as the primary ion-transporting medium – also accounts for the continuous decline in ionic conductivity observed with the increasing salt concentration (Fig. S6). This inverse correlation between salt content and electrical performance underscores the delicate trade-off involved in optimizing such multifunctional hydrogel materials.

In modern applications, hydrogels are increasingly expected to simultaneously exhibit superior mechanical properties and excellent electrochemical performance. Based on our comprehensive experimental results, we selected the SPDNH formulation with 1.8% wt of Na₃Cit for supercapacitor assembly, as it achieved an optimal balance between mechanical strength and electrochemical function. This specific hydrogel demonstrated a tensile strength of 1.65 ± 0.03 MPa, an elongation at break of 518.84 ± 7.49%, a Young's modulus of 0.449 ± 0.02 MPa, a toughness of 4.56 ± 0.21 MJ m⁻³, and an ionic conductivity of 2.01 ± 0.04 S m⁻¹, respectively.

Energy dissipation capacity is a critical performance metric for hydrogel-based flexible electronics. To evaluate this property in SPDNHs, we performed systematic loading–unloading tests to elucidate their energy dissipation mechanisms. During tensile deformation, the hydrogel network undergoes progressive rupture of short-chain connections at localized points, while longer polymer chains facilitate effective stress redistribution, thereby inducing further breakage of additional short chains. As shown in Fig. 3e and f, as the applied strain increases from 50% to 400%, an increasing number of physical crosslinks are disrupted, leading to significantly enhanced energy dissipation. Quantitative analysis (Fig. 3g and Fig. S7) indicates that the dissipated energy per unit volume increases markedly from 0.02 MJ m⁻³ at 50% strain to 1.26 MJ m⁻³ at 400% strain. Over the same strain range, the energy dissipation coefficient increases from 36.75% to 76.39%, underscoring the material's exceptional energy dissipation capability under tensile loading. Similarly, cyclic compression tests conducted at 50% strain (Fig. 3h and i) revealed a compressing toughness of 4.024 MJ m⁻³, with 1.136 MJ m⁻³ of energy dissipated per cycle. Especially, the hysteresis loops exhibited nearly perfect overlap over nine consecutive compression cycles, demonstrating outstanding mechanical stability and intrinsic self-recovery behavior. These superior energy dissipation and recovery characteristics make SPDNH a highly promising candidate for applications requiring durable and resilient energy-absorbing materials.

Environmental humidity is a critical factor affecting the practical stability of hydrogels, as it directly modulates the structural integrity and mechanical properties of materials by altering the water content within hydrogels and the intermolecular interactions between polymer chains. Therefore, environmental stability is one of the core indicators for evaluating the practical performance of hydrogels. To this end, we systematically tested the environmental stability of the SPDNH hydrogel, and the relevant results are presented in Fig. S8.

The SPDNH hydrogel exhibits excellent environmental stability within a relative humidity (RH) range of 30% to 100%: during the 30-day long-term test, the quality change rate of the gel remained stable within the range of 90% to 110% under different humidity conditions, with no obvious dehydration-induced embrittlement or excessive swelling observed. This indicates that its three-dimensional crosslinked network can effectively confine water molecules and resist quality fluctuations caused by changes in environmental humidity, reflecting good structural weather resistance. Humidity only exerts a mild regulatory effect on the mechanical properties: as the humidity increases from 30% RH to 100% RH, the tensile strength of the gel slightly decreases from 1.53 ± 0.04 MPa to 1.38 ± 0.05 MPa, and the Young's modulus decreases from 0.50 ± 0.01 MPa to 0.40 ± 0.01 MPa. Meanwhile, the elongation at break increases from 495 ± 6.49% to 593.5 ± 10.45%, and the toughness increases from 4.38 ± 0.06 MJ m⁻³ to 4.75 ± 0.08 MJ m⁻³. This change originates from the fact that more water molecules infiltrate the polymer network under high-humidity conditions, moderately weakening the intermolecular hydrogen bonding interactions and endowing polymer chains with greater mobility, thereby significantly improving the ductility and energy dissipation capacity of the material while slightly reducing its strength and modulus. Overall, the SPDNH hydrogel still maintains a tensile strength of >1.4 MPa and an ultra-high elongation at break of >500% within a wide humidity range, without mechanical performance degradation or structural failure, confirming that it can stably retain the comprehensive mechanical characteristics of “high strength and high toughness” and providing a solid guarantee for the long-term reliable service of flexible electronic devices in complex humidity scenarios.

3.3. Thermoplasticity and freeze resistance of SPDNH

During preparation, the gel exhibited excellent thermoplastic behavior. As illustrated in Fig. 4a, after being stored for two days, the gel was cut into small pieces and transferred into a glass vial. Heating in a 95 °C water bath for 15 minutes liquefied the initially solid gel. The resulting liquid could be poured into a mold of desired shape and subjected to another freeze–thaw cycle, enabling the formation of customized geometries. In this experiment, we performed 10 cycles of heating–cooling processes and compared its mechanical and electrochemical properties at the beginning and after 10 cycles of thermoplastic treatment. This cutting–rehealing–remolding process could be repeated multiple times, facilitating the fabrication of gels with diverse and complex architectures. Such recyclability not only broadens the potential applications of the material but also aligns with the principles of green and sustainable development. The recycled gel retained substantial mechanical robustness (Fig. 4b–d), with a tensile strength of 1.45 ± 0.01 MPa, a fracture elongation of 495.94 ± 13.72%, a Young's modulus of 0.252 ± 0.04 MPa, and a toughness of 3.51 ± 0.06 MJ m⁻³. In addition, the ionic conductivity of the recycled gel was measured as 1.62 ± 0.01 S m⁻¹ (Fig. 4e). The slight reduction in conductivity is mainly attributed to water loss that occurred during the thermal recycling procedure.

Fig. 4 (a) Schematic diagram of the thermoplastic property of SPDNH, (b) tensile stress–strain curves, (c) tensile strength and elongation at break, (d) toughness and Young's modulus, and (e) ionic conductivity.

As shown in Fig. 5a, the freezing point of the SPDNH hydrogel reaches −33 °C after DES incorporation, confirming the effective depression of the freezing point by the introduced DES. Meanwhile, the SPDNH exhibits excellent flexibility under torsion and bending even at −20 °C (Fig. 5b). Its mechanical and electrical properties were systematically evaluated under this low-temperature condition. Compared to its room-temperature performance, SPDNH maintained favorable mechanical properties at −20 °C, achieving a tensile strength of 1.49 ± 0.02 MPa, a fracture elongation of 405.66 ± 2.28%, a Young's modulus of 0.46 ± 0.01 MPa, and a toughness of 3.31 ± 0.12 MJ m⁻³ (Fig. 5c and d). As indicated in Fig. 5e, Young's modulus of the gel at low temperature is slightly higher than that at room temperature (1.0 ± 0.07 MPa), which can be attributed to the enhanced hydrogen bonding within the gel network at reduced temperatures. Furthermore, the ionic conductivity of SPDNH at −20 °C was measured as 1.71 ± 0.01 S m⁻¹ (Fig. 5f). The observed decrease in ionic conductivity is mainly due to the lowered ion mobility resulting from reduced thermal energy.³⁷

Fig. 5 (a) DSC curve of SPDNH; (b) schematic of twisting and bending of SPDNH at −20 °C; (c) tensile stress–strain curves, (d) tensile strength and elongation at break, (e) toughness and Young's modulus, and (f) ionic conductivity.

To comprehensively evaluate the superiority of the SPDNH organic hydrogel prepared in this work, we systematically compared its key performance indicators with the representative DES-based gel systems reported in recent literature (Table S1). From the perspective of mechanical properties, the tensile strength of SPDNH reached 1.68 MPa, significantly superior to most of the reported DES-based gel systems; in terms of elongation at break, 518.8% of SPDNH was lower than that of some super-stretching systems, but it still remained at a relatively high level among high-strength hydrogels, achieving a good balance between strength and toughness. In terms of ionic conductivity, SPDNH reached 2.01 S m⁻¹, superior to the vast majority of comparison systems, which was attributed to the synergistic effect of the ionic components and free water in the DES. In terms of freeze resistance, the working temperature of −33 °C of SPDNH was lower than that of some ultra-low-temperature systems, but it was sufficient to meet the requirements of most practical applications. Overall, SPDNH achieved an excellent balance among mechanical strength, conductive performance, and freeze resistance, demonstrating great potential as a high-performance flexible supercapacitor electrolyte.

3.4. Flexible all-solid-state supercapacitors based on SPDNH hydrogels

To assess the applicability of SPDNH hydrogels in flexible energy storage devices, we fabricated an all-solid-state supercapacitor using activated carbon electrodes and SPDNH hydrogel as the electrolyte (Fig. S9 and S10). The electrochemical performance was systematically evaluated using an electrochemical workstation. The assembled device showed a bulk resistance of 13.0 Ω, as derived from the Nyquist plot (Fig. 6c). Cyclic voltammetry (CV) tests conducted at scan rates ranging from 5 to 100 mV s⁻¹ within a voltage window of 0–1.0 V displayed well-defined quasi-rectangular shapes (Fig. 6a), suggesting ideal electric double-layer capacitive behavior. Notably, the CV curves retained their symmetrical form across all scan rates (Fig. 6b), reflecting excellent electrochemical reversibility and stability. Galvanostatic charge–discharge (GCD) measurements exhibited nearly symmetrical triangular profiles under various current densities (Fig. 6b), further confirming highly reversible charge storage characteristics typical of double-layer capacitance. Based on the GCD results, the areal specific capacitance was calculated to be 125.61 mF cm⁻² at 1 mA cm⁻², while the Coulombic efficiency increased from 84.46% at 1 mA cm⁻² to 89.04% at 2 mA cm⁻² (Fig. 6d). In addition, the device achieved a high power density of 499.99 µW cm⁻² and an energy density of 17.45 µWh cm⁻² at 1 mA cm⁻² (Fig. 6e). Long-term cycling performance tests further revealed excellent durability, with 80.20% capacitance retention after 1000 consecutive charge–discharge cycles at 10 mA cm⁻² (Fig. 6f), underscoring the robustness of the SPDNH-based supercapacitor for flexible electronics applications.

Fig. 6 Flexible all-solid-state supercapacitor based on SPDNH: (a) CV curves, (b) GCD curves, (c) EIS plot, (d) area-specific capacitance and Coulombic efficiency, (e) power density and energy density, and (f) area-specific capacitance retention of the supercapacitor after 1000 charge/discharge cycles (the inset showing the GCD curves at the 1st and 1000th charge/discharge cycles).

For practical deployment in flexible electronics, supercapacitors must preserve both electrochemical performance and operational reliability under mechanical deformation. As shown in Fig. 7a, the electrochemical impedance spectroscopy (EIS) curves recorded under various bending angles nearly overlap with that of the initial flat state, indicating highly stable capacitive behavior even under deformation. The cyclic voltammetry (CV) curves (Fig. 7b) remained almost identical to those of the undeformed device at mild bending angles. Notably, the device retained 86.34% of its initial capacitance even when twisted to 180° (Fig. 7c), confirming reliable operation under extreme mechanical strain. To further assess temperature adaptability, we systematically evaluated the SPDNH-based supercapacitor across a broad temperature range. To further systematically evaluate the long-term operational reliability of this flexible supercapacitor under repeated mechanical deformation, we conducted 1000 consecutive bending cycle tests on the device and compared the electrochemical performance changes before and after the cycles. The relevant results are shown in Fig. S11. The test results indicate that after 1000 bending cycle tests, the cyclic voltammetry (CV) curve of the device remained highly consistent with the initial un-bent state, and always presented typical quasi-rectangular characteristics, without significant curve deformation or response current attenuation. This shows that even after long-term repeated mechanical bending, the double-layer capacitance storage mechanism of the device did not change, and the internal charge transfer and storage process maintained excellent electrochemical reversibility. At the same time, the constant current charge–discharge (GCD) curve after the bending cycle also maintained a highly symmetrical isosceles triangular shape, with no significant difference in charging and discharging duration from the initial state, and no significant voltage drop (IR drop). This proved that 1000 consecutive bending cycles did not cause the detachment of electrode active substances, the peeling of gel electrolyte from the electrode interface, or other irreversible structural damages.

Fig. 7 Supercapacitors at different bending angles: (a) CV curves (100 mV s⁻¹), (b) GCD curves (2 mA cm⁻²), and (c) area-specific capacitance and capacitance retention with respect to bending angles. (d) CV curves (100 mV s⁻¹), (e) GCD curves (2 mA cm⁻²), and (f) area-specific capacitance and capacitance retention with respect to temperature.

The CV curves obtained at 100 mV s⁻¹ (Fig. 7d) indicated improved electrochemical performance with rising temperature, as reflected by the progressively larger integrated areas of the CV curves. Galvanostatic charge–discharge (GCD) profiles (Fig. 7e) consistently exhibited the characteristic symmetrical triangular shape throughout the tested temperature range (−15 °C to 80 °C). Quantitative analysis (Fig. 7f) showed that the areal specific capacitance increased from 92.87 mF cm⁻² at −15 °C to 121 mF cm⁻² at 80 °C, corresponding to a capacitance retention improvement from 89.76% to 116.95% over the same span. These results collectively underscore the device's exceptional stability across a wide temperature range while fully maintaining its electrochemical integrity.

To further contextualize the performance of the SPDNH-based supercapacitor, we compared its key electrochemical metrics with those of recently reported flexible supercapacitors (Table S2). The SPDNH device delivers an areal capacitance of 125.61 mF cm⁻², which is competitive with or superior to many previously reported values. Notably, it could operate over a remarkably wide temperature range (from −20 to 80 °C), a feature not simultaneously achieved in most referenced works. Additionally, the device exhibited a high ionic conductivity of 2.01 S m⁻¹ and an energy density of 17.45 µWh cm⁻², alongside excellent cycling stability (80.20% after 1000 cycles). This comprehensive performance profile underscores the advantage of our DES-enhanced Hofmeister strategy in achieving multifunctional integration – balancing mechanical robustness, ionic transport, and environmental adaptability – making it a promising candidate for next-generation flexible energy storage devices.

We further fabricated supercapacitors using thermally recycled SPDNH and systematically evaluated their electrochemical performance. Electrochemical impedance spectroscopy revealed a bulk resistance of 18 Ω for the recycled hydrogel supercapacitor (Fig. 8a). Cyclic voltammetry tests conducted at scan rates ranging from 5 to 100 mV s⁻¹ exhibited well-maintained symmetrical and quasi-rectangular CV curves (Fig. 8b), suggesting that the fundamental electrochemical behavior was preserved after recycling. Galvanostatic charge–discharge curves displayed nearly symmetrical triangular shapes across current densities of 1–2 mA cm⁻² (Fig. 8c), further confirming the retention of electrochemical reversibility in the recycled device. Systematic analysis indicated that while the areal specific capacitance and energy density decreased with increasing current density, both the Coulombic efficiency and power density showed gradual improvement (Fig. 8d and e). Notably, at a current density of 2 mA cm⁻², the recycled supercapacitor retained 91.84% of its initial areal specific capacitance (Fig. 8f), demonstrating outstanding performance retention. These results collectively verify that supercapacitors assembled from recycled SPDNH maintain excellent electrochemical properties, underscoring the practical recyclability and potential sustainability of our proposed material system for advanced energy storage applications.

Fig. 8 Electrochemical performance of supercapacitors prepared from thermoplastic recycled SPDNH: (a) AC impedance spectrum curves, (b) cyclic voltammetry curves, (c) GCD curves, (d) specific capacitance and Coulomb efficiency, (e) energy density and power density, and (f) comparison of the specific capacitance retention rate of the initial gel at different current densities (the illustration shows a comparison of the GCD curves of the two at a current density of 1 mA cm⁻²).

4. Conclusions

In this work, we developed a straightforward method that employs a deep eutectic solvent (DES) to enable high salt loading during gel preparation. This strategy directly induces the salting-out effect within the gel matrix, thereby simplifying the fabrication process and allowing the production of SPDNH gels with high strength, toughness, freezing resistance, and recyclability. The DES system imparts remarkable freezing resistance to the gel, with the freezing point as low as −33 °C, while the salting-out effect from Na₃Cit endows the hydrogel with robust mechanical properties, achieving a tensile strength of 1.65 ± 0.03 MPa and an elongation at break of 518.84 ± 7.49%. A flexible all-solid-state supercapacitor assembled with the SPDNH gel as the electrolyte and activated carbon as the electrode delivers a high areal capacitance of 125.61 mF cm⁻² and exhibits good cycling stability at room temperature, retaining 80.20% of its initial capacitance after 1000 charge–discharge cycles. Moreover, the device functions reliably across a broad temperature range from −20 to 80 °C and under various bending conditions. Supercapacitors fabricated with recycled SPDNH gel also maintain high performance. Overall, this study presents a novel approach for preparing fully physically crosslinked hydrogels with outstanding mechanical properties and ionic conductivity by synergistically utilizing the Hofmeister effect and solvent engineering. We anticipate that this work will offer new perspectives for the development of ion-conductive hydrogel electrolytes.

Author contributions

Guoquan Zhang wrote the original draft, performed data curation, methodology, investigation, and validation. Yongchang Chen performed investigation, data curation and methodology. Gao Xiao wrote review & edited the original draft, and performed supervision, and funding acquisition. Xiancai Jiang wrote reviewed & edited the original draft, and performed resources, supervision, funding acquisition, and conceptualization.

Conflicts of interest

The authors declare no conflicts of interest.

Data availability

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

Supplementary information (SI): Fig. S1. The preparation process of SPDNH; Fig. S2. FTIR spectra of four different hydrogels; Fig. S3. Four different gels: (a) EIS plots and (b) conductivity; Fig. S4. SEM images of the (a) SPH hydrogel, (b) SPNH hydrogel, (c) SPDH hydrogel, and (d) SPDNH hydrogel; Fig. S5. Comparison of tensile strength and ionic conductivity with those reported in the literature; Table. S1. Comparison of tensile strength and ionic conductivity with those reported in the literature; Fig. S6. Four different Na₃Cit addition amounts (the percentage of Na₃Cit in the system mass fraction): (a) stress–strain curves of hydrogels under tension. (b) Tensile strength and elongation at break. (c) Young's modulus and toughness. (e) EIS plots and (g) conductivity; Fig. S7. Total energy and dissipated energy of SPDNH gel under different strains; Fig. S8. (a) The quality changes of SPDNH 1.7.15 at 30%RH, 70%RH, and 100%RH over a period of 30 days. (b) Stress–strain curves, (c) tensile strength and elongation at break, (d) Young's modulus and toughness of SPDNH after being placed at 30%RH, 70%RH, and 100%RH for 30 days; Fig. S9. Schematic diagram of the composition of the flexible all-solid-state supercapacitor; Fig. S10. (a) Composition of supercapacitors; gel, nickel mesh coated with electrode materials, and insulating sheet; (b) supercapacitor after planar plastic packaging; (c) side view diagram of supercapacitor; Fig. S11. Supercapacitor after being bent 1000 times: (a) CV curves (100 mV s⁻¹), (b) GCD curves (2 mA cm⁻²), at different temperatures: (c) area-specific capacitance and capacitance retention; Table S2. Comparison of electrochemical performance data of carbon materials. See DOI: https://doi.org/10.1039/d6nj00043f.

Acknowledgements

This work is supported by the Natural Science Foundation of Fujian Province (2024J01282).

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