Functional hydrogel-based supercapacitors for wearable bioelectronic devices

Lili Jiang *^a and Xiong Lu *^b
aKey Laboratory of Fluid and Power Machinery of Ministry of Education, School of Materials Science and Engineering, Xihua University, Chengdu 610039, China. E-mail: qvinjiang@163.com
^bKey Lab of Advanced Technologies of Materials, Ministry of Education, School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu, Sichuan 610031, China. E-mail: luxiong2006@163.com

First published on 17th August 2021

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1. Introduction

The synergistic development of advanced materials/electronics and manufacturing over the decades has accelerated the progress in bioelectronic devices, which have rapidly expanded and are widely employed both as wearable accessories and in vivo implants for healthcare monitoring and clinical therapy. Researchers established that bioelectronic devices have significant clinical applications, such as neuron and cardiac stimulation, which can effectively alleviate the symptoms of epilepsy, Alzheimer's disease, Parkinson's disease, depression, and multiple other medical issues pertaining to aberrant neural behavior, as well as cerebrovascular diseases and infections.^1,2 The positive effects of electrical stimulation in clinical medicine have inspired the development of combined bioelectronic energy storage/supply devices which are currently used widely in clinical tests, auxiliary treatment technologies, and human healthcare equipment.

However, several challenges still remain for developing next-generation bioelectronics owing to the intrinsic differences between biological tissues and electronics in the constantly emerging human–machine interfaces, such as the mismatch in the mechanical properties and charge carriers between man-made devices and human physiology. The human body comprises a broad range of soft and high water-containing tissues and organs; in contrast, the majority of currently used bioelectronic devices rely on rigid and dry electronic components. Specifically, electrons serve as carriers for charge transport in conventional rigid and dry electronic components, whereas in biological systems, the same process relies on ions. Furthermore, biological tissues are capable of enduring high dynamic and mechanical stress; for example, the skin, muscles, and peripheral nerves can experience 30% of tensile strain and displacement during exercise in conventional postures. The heart and blood vessels experience sustained periodic mechanical deformation during cardiovascular activity.^3,4 Thus, in order to effectively adapt to the human physiological and musculoskeletal environment, the next-generation smart, flexible, stretchable, implantable, and portable electronics (such as soft tactile sensors in electronic skins, motion sensors, nervous sensors, electrophysiology sensors, and feedback stimulators) require further research.

To ensure the stable performance and durability of such advanced electronics, novel energy storage devices need to be studied and developed. Supercapacitors (SCs), which are increasingly used in bioelectronic applications, have been studied for decades as advanced energy storage devices. To fulfill the specific energy demands of the aforementioned external and in vivo portable electronics, functional supercapacitors (FSCs) that can be stretched, compressed, bent, twisted, and deformed into arbitrary shapes provide a promising alternative. Therefore, advanced functional materials for use as electrodes and electrolytes in SCs are required to accelerate the investigation and fabrication of FSCs.

Hydrogels with sophisticated structures comprising crosslinked networks of polymer chains with interstitial spaces filled with solvent solution are regarded as the most promising candidates for electrodes/electrolytes in wearable and implantable devices.^5,6 Apart from their intrinsic soft/wet properties that improve the interface between the electronic devices and the human body, the versatile chemical properties of hydrogels provide the possibility of introducing more specific functions, such as conductivity, long-term adhesiveness, self-healing, and toughness. Thus, by using hydrogels as components, the tissue affinity of electronic devices can be further improved, bridging the gap between electronic components and biological systems. Notably, multifunctional hydrogels have been previously used to fabricate FSCs for powering portable electronics.^7–11

This review summarizes the recent progress in hydrogel-based SCs with stretchable, conductive, adhesive, self-healing, and increased-toughness characteristics. Functional hydrogel designs for electrodes/electrolytes in hydrogel-based FSCs will be discussed with an emphasis on the configuration, design principles, and electrochemical and mechanical properties of the materials (Fig. 1). Additionally, the prospects and challenges for the development of hydrogel-based FSCs for wearable and implantable electronic devices will be highlighted and discussed, with the aim of inspiring further research and development in the field of bioelectronics.

2. Structural characteristics and energy storage principles of SCs

Supercapacitors (SCs) have attracted notable scientific interest in the past decades, stemming from their potential for clean energy, easy assembly, and high performance. These advantageous characteristics of SCs have also contributed to the rapid growth of low-power electronics, including wearable and portable electronic devices.^15,16Fig. 2a schematically illustrates two types of energy storage mechanisms of supercapacitors. (i) Electrochemical double layer capacitance (EDLC), in which an electrical double layer is immediately formed on the surface of the electrodes due to electrostatic attraction (Fig. 2b). Given their maximum effective surface space and minimal charge separation distances, EDLCs exhibit higher energy densities than conventional capacitors; however, the electrochemical capacitance of SCs based on EDLC is not good enough.^17,18 (ii) Pseudocapacitance, which is based on the reversible faradaic reaction on the surface of the electrically active materials, including two main kinds of reactions. The first one is the reversible redox chemical reactions combined with dynamic equilibrium oxidation and reduction reactions on the electroactive surface of the materials during the electrochemical energy storage process (Fig. 2c).¹⁹ The other one is the reversible intercalation and exfoliation processes between the electrodes and ions in the electrolyte (Fig. 2d).^20–22 Given their fast reversible faradaic reactions, the electrochemical capacitance of SCs is usually higher than that of EDLC, but energy densities and stability are lower. Along with the development of SCs, more and more studies have used both EDLC and pseudocapacitance simultaneously to further improve the electrochemical performance of SCs, which is named as hybrid SCs.

On one hand, the performance of SCs is primarily dependent on the electrode materials and the interactions between the electrode and the electrolyte. The electrochemical properties, conductivity, and specific surface areas of the electrode materials, as well as the ion conductivity of the electrolyte, are all key factors that determine the electrochemical performance of SCs. Numerous attempts have been accordingly made to improve the electrochemical performance of SCs through these aspects.^23–28 On the other hand, considering the structural characteristics of SCs, achieving advanced functionalities on FSCs (such as flexibility, wearability, and textile features) depends on the specific characteristics of the electrode materials. Meanwhile, replacing the aqueous electrolytes in conventional SCs is also significant in preparing FSCs for wearable and portable electronics.

3. Functional hydrogel electrodes for supercapacitors

Since their invention in 1894, hydrogels, which comprise a three-dimensional network of crosslinked polymer (natural or synthetic) chains infiltrated with high water content, have attracted extensive attention.^29,30 Owing to the inherent crosslinks and structural integrity of the hydrogel network, hydrogels can swell and absorb a large amount of water (>90%) without dissolving. More importantly, hydrogels simultaneously exhibit solid and liquid behavior: their solid behavior enables them to maintain a certain shape and volume under certain conditions, while their liquid behavior is reflected by the diffusion and penetration of the solvent within their structure. The soft and wet nature of the hydrogels has rendered them a promising candidate in tissue engineering, biomedicine, and bioelectronics.³¹ In addition, the introduction of nanofillers and the formation of a network comprising polymer/monomer crosslinks in an aqueous medium endow the hydrogel with versatile properties and functionalities.^32,33 Thus, a variety of approaches toward the rational design and synthesis of functional hydrogels is encouraged, to accommodate the increasing demand of FSCs for portable/wearable bioelectronics.

According to the EDLC and pseudocapacitance principles discussed above (Fig. 2), the materials composing the electrode components in SCs are required to possess both conductivity and energy storage capabilities (e.g., carbon-based materials, nanometal compounds, and conductive polymers). Many approaches have been designed to fabricate electrodes for SCs. The crosslinked polymer chains within the hydrogels could be exploited as carbon sources to obtain 3D interconnected nano/microstructure electrodes for supercapacitors; however, this approach severely compromises the mechanical properties and wet nature of hydrogels. Hence, several studies have focused on improving the electrical conductivity while maintaining the wet nature of hydrogels, by utilizing conductive materials as the backbone chains or compositing electronically conductive fillers. Herein, for the convenience of discussion, two main approaches for achieving SC electrodes are considered: hydrogel-derived 3D porous electrodes and conductive hydrogels.

3.1 Hydrogel-derived 3D porous electrodes for SCs

Based on the EDLC storage mechanism, porous structures with high surface areas can enhance the adsorption of ions at the electrode/electrolyte interface and improve the electrochemical performance of SCs.^34–38 Porous carbon electrodes, as the earliest used electrode materials for SCs, have attracted significant attention because of their high surface area and superior electrochemical stability. The carbonization and activation of biomass-derived carbon precursors is one of the most popular methods for fabricating porous carbon electrodes.^39–41 However, the stacking of carbon particles increases the contact resistance because of the discontinuous internal structure, thereby decreasing the electrochemical performance of SCs during the charging and discharging processes. To address this issue, 3D templates have been highlighted as the most effective way for obtaining a uniform and consistent 3D porous structure.^42–44 Hydrogels exhibit 3D interconnected nano/microstructures with water molecules trapped within the hydrogel matrix, enabling them to achieve an interconnected porous structure with a high specific surface area via an easy high-temperature treatment.^45–47 However, the improved conductivity (originating from the interconnected structure) and increased specific surface area of the hydrogel only lead to a limited improvement of the electrochemical performance. The incorporation of suitable pseudocapacitive materials into the hydrogel network could further increase the energy storage performance.^48–50 Fortunately, functional materials or inorganic nanomaterial precursors with pseudocapacitance behavior can be easily and uniformly mixed within the hydrogel matrix.^51–53 Although these studies have verified the potential electrochemical performance improvement attained by using 3D porous hydrogel-derived carbon electrodes, the 3D porous structure formed via a high-temperature treatment may cause severe decrease in their mechanical properties and thus hinder their use in FSCs.

3.2 Conductive hydrogel electrodes for SCs

According to the general structural characteristics (Fig. 2) and energy storage mechanisms of SCs, the electrochemical reactions occurring at the electrode–electrolyte interface are critical factors that determine the electrochemical performance of SCs. Apart from the contribution of the active materials, both the electron conductivity of the electrodes and the ion conductivity of the electrolyte are significant. The ionic conductivity of conventional hydrogels manifests through the contained aqueous medium, making hydrogels a popular solid state electrolyte; however, when applied as electrodes for SCs, their inherent ionic conductivity is not enough to guarantee proper electrochemical performance of SCs.⁵⁴ To alleviate this complication, extensive efforts have been made to improve the conductivity of hydrogels by employing conductive monomers or combing conductive nanofillers. In these studies, conductive polymers/nanofillers enable electron transfer; parallelly, ions are still conducted through the aqueous region within the hydrogels. The synergistic action of these mechanisms significantly improves the overall conductivity of hydrogels and satisfies the requirements for electrodes. The following three types of conductive hydrogel electrodes for SCs are reviewed and discussed, i.e., graphene-based hydrogels, conductive polymer-based hydrogels, and hydrogels with conductive fillers.

4. Hydrogel electrolytes for supercapacitors

Aqueous electrolytes, such as salt, acid, and alkaline solutions, are still the most commonly used option for conventional SCs;^104,105 however, they fail to meet safety restrictions, owing to the leakage of toxic components into the human body when dynamically deformed or implanted. Thus, solid electrolytes (such as hydrogel-based ones) are rapidly being developed to fulfill the dual role of electrolytes and separators in FSCs and provide a wide operational window.^105–108 Generally, hydrogels are polymer networks which are swollen due to the large amounts of absorbed water, which endows them with high ionic conductivity and stability against electrolyte leakage. More importantly, their mechanical characteristics render them crucial for attaining highly flexible and stretchable devices.

Heretofore, hydrogel electrolytes, such as PVA,^109–114 poly(acrylic acid) (PAA),^115–117 and natural biopolymer electrolytes are the mostly applied solid-state electrolytes in various FSCs.^118–121 PVA hydrogels exhibit high structural integrity, good mechanical properties, and easy gelation, and the large water content absorbed in their polymer matrix enhances their ionic conductivity. As such, they are widely used in both flexible^122–125 and stretchable SCs.^126,127 Polyacrylamide (PAAm) hydrogel was also employed as a hydrogel electrolyte,¹²⁸ which was formed by mixing covalently crosslinked PAAm with Al³⁺ ionically crosslinked alginate (Fig. 8). The hydrogels used in the as-assembled SC played multiple roles; apart from functioning as electrolytes and highly deformable and reliable separators, they also secured the electrodes from external mechanical impacts, owing to their high mechanical strength and fracture toughness allowing them to sustain various severe deformations, including dynamic squeezing, folding, compression, and twisting. The guluronic acid units in the alginate chains formed ionic crosslinks with the dissociated Al³⁺ ions, which served as a sacrificial yet reversible auxiliary network and effectively dissipated energy upon stress loading through the rupture of physical bonds and recovered its bonding upon unloading. The as-prepared electrolytes successfully maintained the output of the device after various deformations, which is crucial for the stable performance of the flexible SCs.

The majority of natural polymers, such as gelatin,^118,129,130 lignin^121,131 and cellulose,^132,133 are highly suitable as solid electrolytes and separators for biocompatible FSCs,^106,134 because they are inherently hydrophilic due to the presence of abundant hydrophilic pendant groups that increase water retention in the polymer matrix and thereby enhance ionic conductivity. Park et al.¹³¹ prepared a eco-friendly lignin-based hydrogel electrolyte for free-standing, flexible SCs. The crosslinked networks of the lignin-based hydrogel electrolyte presented both high ionic conductivity (10.4 mS cm⁻¹ at room temperature) and mechanical integrity (532% swelling underwater). A lignin/polyacrylonitrile nanofiber electrode was prepared using an electrospinning process. The as-prepared reproduceable flexible device demonstrated remarkable properties: a capacitance of 129.2 F g⁻¹; a capacitance retention of 95% over 10000 cycles; an energy density of 4.49 W h kg⁻¹; and a power density of 2.63 kW kg⁻¹. Liu et al.¹³⁵ reported a hybrid double-crosslinked lignin hydrogel electrolyte produced by the post-formation of a lignin hydrophobic aggregation via the simple treatment of a single chemically crosslinked lignin hydrogel using a H₂SO₄ solution. The as-prepared hydrogel exhibited significant improvement in terms of mechanical strength (compressive stress at fracture point = 4.74 MPa), along with excellent shape recovery properties and high ionic conductivity (0.08 S cm⁻¹). In addition, the good compactivity, biodegradability, natural abundance, and sustainability of these natural biopolymers further facilitate their application in wearable and implantable devices.^136,137

Recently, along with the appearance of smart FSCs, self-healable electrolytes have also been investigated. Zhang et al.¹³⁸ reported a ferric-enhanced dual physical crosslinking self-healing polyelectrolyte for supercapacitors. The gel electrolyte was copolymerized in an aqueous ferric nitrate solution, using acrylic acid as the hydrophilic monomer, stearyl methacrylate as the hydrophobic monomer, and cetyltrimethylammonium bromide as the cationic surfactant (which contained a sulfuric acid solution). This hydrogel could contain 1 mol L⁻¹ H₂SO₄ while exhibiting high self-healing efficiency and good mechanical properties, namely, an extensibility of >2400% and a stress of >230 kPa, as well as excellent ionic conductivity (>30 mS cm⁻¹). Wang et al.¹³⁹ produced a 3D network-structured acrylic acid-based hydrogel electrolyte using glycol as the crosslinking agent. The as-prepared hydrogel electrolyte possessed strong water absorption and elongation, along with advanced tensile strength, stability, and self-healing properties. Its strain resistance could reach up to 861.8% under a stress of 225 kPa, while also exhibiting an electrical conductivity of 16.7 mS cm⁻¹. After being assembled with active carbon electrodes, the supercapacitor showed a high energy density of 21.3 W h kg⁻¹ at a power density of 350 W kg⁻¹. After 40 cycles of the break/healing processes, its specific capacitance retention was maintained at 68%. It should be noted, however, that self-healing is a passive process requiring the meticulous manual alignment of the electrodes and electrolytes. On certain occasions, external stimuli are also required to ensure a more effective healing. The mechanical strength is susceptible to severe deterioration under such conditions, thus increasing the risks of performance decay and device failure. This challenge requires additional research so as to enable the widespread application of the hydrogels discussed above in smart FSCs.

Hydrogel electrolytes are characterized by limited thermal stability because of the nature of the polymer monomer and the aqueous electrolyte contained within the hydrogels. Temperature-resilient hydrogel electrolytes have recently attracted significant research attention, since flexible energy storage devices, including FSCs, need to maintain their functionality in harsh environments, and especially in severely cold and hot regions. Hence, hydrogel electrolytes with stable performance over wide temperature ranges are highly sought after for all-solid-state FSCs, with numerous studies being conducted to thoroughly investigate them.^{123,137,140–145} Lu et al.¹⁴⁶ prepared a thermally stable and freeze-resistant montmorillonite/PVA hydrogel electrolyte that maintained its operability over the temperature range of −50 to 90 °C, and exhibited high ionic conductivities of 0.17 × 10⁻⁴ and 0.76 × 10⁻⁴ S cm⁻¹, respectively. Remarkably, their flexible supercapacitor also delivered notable capacitance under the same temperatures, 91% of which was retained after 10000 cycles. The realization of all-temperature self-healing hydrogel electrolytes could accelerate the technological advancements needed to overcome the existing limitations in wearable/flexible supercapacitors.

5. Multifunctional hydrogel-based all-in-one wearable supercapacitors

All the studies discussed above centered on the realization of all-in-one hydrogel-based FSCs. These FSCs can be modified into appropriate shapes and size and easily adjusted to adapt to human body/tissue environment, or to other intelligent electronics, under certain stress and strain conditions. Moreover, a suitable hydrogel electrode–electrolyte recombination can generate new ion transport channels at the molecular level, a feature with promising prospects in wearable and implantable bioelectronics.

5.1 FSCs by direct deposition of electrodes on hydrogel electrolyte

The most preferred strategy for fabricating all-in-one FSCs relies on the deposition or in situ growth of conductive polymers^142,147,148/active materials^149,150 as electrodes on both the upper and lower sides of a multifunctional hydrogel electrolyte. For example, Wei et al.^151,152 produced an all-in-one supercapacitor by growing PANi polymers on high-strength PVA hydrogels (Fig. 9). Their all-in-one FSC achieved superior electrochemical properties; specifically, a maximum volumetric capacitance of 2220 mF cm⁻³ at an aniline concentration of 0.5 mol L⁻¹, which remained at 90% after 7000 cycles, along with the energy and areal power densities of 42 and 160 μW cm⁻², respectively.

The functional hydrogel electrolytes determine the multifunction of the all-in-one SCs. Sun et al.¹⁵³ designed a similar all-in-one supercapacitor with additional functions, such as self-healing, fatigue resistance, and self-recovery, by utilizing a PANi-decorated supramolecular PVA/poly(N-hydroxyethyl acrylamide) (PVA/PHEA) hydrogel electrolyte (HGE). The PANI electrode was also polymerized and decorated onto the surfaces of PVA/PHEA HGE. The multiple hydrogen bonds at the PVA–PHEA interface, as well as the ionic interactions between the PANi molecules, not only enabled the excellent mechanical properties (tensile strength of 1.07 MPa and tearing energy of 2492 J m⁻²) of the HGE to manifest, but also facilitated the electron/ion transfer, enhancing the electrochemical performance of the as-prepared all-in-one FSC (98 mF cm⁻² at a current density of 0.2 mA cm⁻²).

Recently, all-in-one FSCs with anti-freezing properties was also prepared using the strategy of deposition electrodes on an anti-freezing electrolyte. Qu et al.¹⁵⁴ prepared an anti-freeze and highly stretchable supercapacitor (AF-SSC) via the one-step in situ growth of PANi polymers onto a hydrogel electrolyte, which originated from crosslinked PAM networks soaked in an ethylene glycol/water/H₂SO₄ solution. The as-prepared AF-SSC demonstrated a high mechanical stretchability of 200% at −30 °C and sustained 100 consecutive stretch test cycles without significant capacitance loss (73.1% capacitance retention with a 20-fold increase of the current density). After 100000 cycles at −30 °C, the capacitance retention was 91.7%.

In the above studies, however, strong acids were commonly used to pre-treat the hydrogel electrolytes, which are evidently far from skin-friendly, and therefore unsafe for wearable electronics and bioelectronics. In this context, on one hand, biocompatible acids or neutral salt solutions were used to replace strong acids to pre-treat hydrogel electrolytes.^155–157 On the other hand, certain biodegradable or natural polymer-based all-in-one supercapacitors have also attracted scientific interest. Zhong et al.¹⁵⁸ successfully developed an all-in-one flexible, autonomous, and self-healable supercapacitor based on the biodegradable natural polymers of gelatin and cellulose nanocrystals. Initially, rGO and PANi polymers were in situ polymerized and deposited onto both sides of the tannic acid-treated gelatin methacrylate and cellulose hydrogels to form the as-prepared all-in-one supercapacitor, which attained excellent conductivity (5.67 Ω), as well as good electrochemical performance (specific capacitance, energy density, and power density of 1861 mF cm⁻³, 20.65 mW cm⁻³, and 595.59 mW h cm⁻³, respectively), efficient self-healing ability (the conductivity of the as-prepared SC increased to 6.38 Ω after one healing process), and high mechanical properties. Accordingly, there is considerable room for improvement in terms of energy and power density.

Although the electrochemical performance and biocompatibility of the all-in-one FSCs is significantly improved, their mechanical properties, especially stretchability, are rather limited, due to the difference in the mechanical properties of the non-hydrogel electrode and hydrogel electrolyte. Hence, FSCs assembled using both hydrogel electrode and electrolytes have been considered.

5.2 Sandwich-like hydrogel-based all-in-one FSCs

Sandwich-like all-in-one SCs could be fabricated by assembling hydrogel-based electrodes/electrolytes. For instance, Guo et al.¹⁵⁹ achieved a highly stretchable, compressible, and all-in-one hydrogel-based SC (Fig. 10). This supercapacitor is composed of reversible and deformable polyacrylamide/sodium alginate dual-network hydrogel electrodes with CNTs and PEDOT:PSS as the active materials, while using the same hydrogel electrolyte with a salt/redox couple (Na₂SO₄ and potassium hexacyanoferrate [K₃Fe(CN)₆]/[K₄Fe(CN)₆]). The conductivity of the hydrogel electrode, which was improved by the CNT and PEDOT:PSS components, reached 0.35 Ω. The utilization of the same hydrogel matrix for both the electrode and the electrolyte induced the formation of strong hydrogen bonds, thereby improving the self-adhesion features and preventing structural misalignments or delamination during severe mechanical deformations. Thus, the as-prepared supercapacitor could be deformed into arbitrary shapes under various stress–strain conditions, without compromising capacitance (128 mF cm⁻² at 1 mA cm⁻²), energy density (3.6 μW h cm⁻²), long cycle life (over 5000 cycles), and stable energy output. As such, the feasibility of fabricating both hydrogel electrodes and electrolytes in order to obtain all-in-one hydrogel-based supercapacitors was confirmed. Through the proper selection and design of multifunctional hydrogel electrolytes/electrodes, as discussed above, FSCs with the inherited multifunctionalities of hydrogels can be achieved and potentially satisfy the specific demands of wearable or implantable bioelectronics.

6. Challenges and future prospects

In this report, we discussed the up-to-date advancements in multifunctional hydrogels for supercapacitors, focusing on the importance and challenges involved in endowing supercapacitors with multifunctional properties for wearable electronics. The development of soft supercapacitors that can be deformed into arbitrary shapes and adapt to random musculoskeletal deformations in the human body is still a considerable challenge. The few operational multifunctional hydrogel-based SCs that have been reported to date remain largely limited in terms of accommodating external wearable electronics. The development of hydrogel SCs satisfying the requirements for next-generation smart wearable or implantable bioelectronics, such as tactile sensors, implantable nervous sensors, electrophysiology sensors, and feedback stimulators, remains an arduous task, owing to the complicated physiological environment of the human body.

The electrochemical performance and advantageous characteristics of FSCs, including their area, volume, mass capacitance, cycle stability, rate capability, energy density, and power density are usually determined by their behavior under normal, bent, twisted, or compressed shape conditions. The majority of hydrogel-based SCs that have been developed to date possess multifunctionality features, although sacrificing their electrochemical performance to some extent. More importantly, the stable energy output of these devices can be hardly guaranteed under dynamic deformation conditions; furthermore, a majority of studies to date have tested their electrochemical performance under static conditions. Therefore, ensuring the high electrochemical performance of hydrogel-based FSCs, especially in terms of energy/power density, while preserving their multifunctionality features under physiological conditions, is probably the most significant challenge. Herein, we discussed possible future directions for the development of FSCs for wearable and implantable bioelectronics.

6.1 Hydrogel-based multifunctional supercapacitors

One of the primary aspects of modern supercapacitor development is to accommodate the power needs of the soft bioelectronic devices used to monitor human health conditions. To successfully adapt to this unique physiological environment, hydrogel-based supercapacitors with the ability to be stretched, compressed, bent, twisted, and deformed into arbitrary shapes must be further studied and developed. In particular, self-adhesive properties are also a major requirement. Self-adhesion reduces the interfacial resistance between the electrode and the electrolyte, while further stabilizing the power output of the supercapacitors under tensile, compressive, or twisted stress conditions.^160–162 Self-adhesiveness is an essential property enabling the stable, accurate, and simultaneous sensing of multiple stimuli by allowing the direct and robust adhesion of bioelectronics to human skin or other tissues. In a dynamic/random motion environment, flexible, stretchable, tough, and self-adhesive bioelectronics are highly desired to ensure the conformal contact between the device and the biological tissue, a factor which critically affects their performance. Hence, hydrogel-based flexible supercapacitors with self-adhesive properties are highly desirable for matching these advanced bioelectronics.

6.2 Implantable or biodegradable in vivo hydrogel-based FSCs

Implantable or biodegradable hydrogel-based FSCs are completely or partially embedded into the human or animal body and remain inside to match or power other bioelectronics and complete special tasks. The development of implantable hydrogel-based FSCs is an arduous task that must address multiple issues, such as the aspects of biocompatibility and biostability. For the long-term implantation of hydrogel FSCs, power output stability is another important factor that needs to be considered. At the same time, a number of important hydrogel properties, such as mechanical strength and conductivity, are significantly compromised by the swelling of the hydrogels in the water-rich human physiological solutions. Thus, materials with superior biocompatibility and short-term biostability must be designed to obtain functional hydrogel-based FSCs. To achieve power output stability, certain techniques that rely on harvesting energy from movement, chemical reactions, and ion-related changes in the human body also provide inspiration for the design of sophisticated implantable hydrogel-based FSCs.^163–165 Furthermore, some types of SCs consisting of environmentally and biologically degradable byproducts can completely dissolve in vivo after performing their special functions.¹³⁶ Therefore, biodegradable hydrogels need to be designed for functional biodegradable hydrogel-based FSCs.

6.3 Environmentally tolerant hydrogel-based FSCs

Environmental tolerance is of great significance for hydrogel-based FSCs to adapt to wearable electronics, especially for applications in harsh environments. Conventional supercapacitors can operate in the temperature range from −40 to 200 °C.^166,167 For hydrogel-based FSCs, the temperature tolerance of the devices is mainly dependent on the temperature tolerance of the hydrogel electrodes and hydrogel electrolytes. Thus, improving the temperature tolerance and stability of hydrogels without compromising their other functionalities also constitutes a considerable challenge in designing hydrogel-based FSCs with excellent durability and adaptability to harsh and complex environments.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was financially supported by the Key-Area Research and Development Program of Guang Dong Province (2019B010941002), Fundamental Research Funds for the Central Universities (2682020ZT79), and Young Scientific and Technological Innovation Research Team Funds of Sichuan Province (20CXTD0106).

References

1. T. Dvir, B. P. Timko, M. D. Brigham, S. R. Naik, S. S. Karajanagi, O. Levy, H. Jin, K. K. Parker, R. Langer and D. S. Kohane, Nanowired three-dimensional cardiac patches, Nat. Nanotechnol., 2011, 6, 720–725 CrossRef CAS .
2. J. W. Jeong, G. Shin, S. I. Park, K. J. Yu, L. Xu and J. A. Rogers, Soft materials in neuroengineering for hard problems in neuroscience, Neuron, 2015, 86, 175–186 CrossRef CAS PubMed .
3. S. P. Lacour, G. Courtine and J. Guck, Materials and technologies for soft implantable neuroprostheses, Nat. Rev. Mater., 2016, 1, 1–14 Search PubMed .
4. J. Rivnay, H. Wang, L. Fenno, K. Deisseroth and G. G. Malliaras, Next-generation probes, particles, and proteins for neural interfacing, Sci. Adv., 2017, 3, e1601649 CrossRef PubMed .
5. H. R. Lim, H. S. Kim, R. Qazi, Y. T. Kwon, J. W. Jeong and W. H. Yeo, Advanced soft materials, sensor integrations, and applications of wearable flexible hybrid electronics in healthcare, energy, and environment, Adv. Mater., 2020, 32, 1901924 CrossRef CAS .
6. H. Yuk, B. Lu and X. Zhao, Hydrogel bioelectronics, Chem. Soc. Rev., 2019, 48, 1642–1667 RSC .
7. Y. Zhao, Y. Alsaid, B. Yao, Y. Zhang, B. Zhang, N. Bhuskute, S. Wu and X. He, Wood-Inspired Morphologically Tunable Aligned Hydrogel for High-Performance Flexible All-Solid-State Supercapacitors, Adv. Funct. Mater., 2020, 30, 1909133 CrossRef CAS .
8. H. Yu, N. Rouelle, A. Qiu, J.-A. Oh, D. M. Kempaiah, J. D. Whittle, M. Aakyiir, W. Xing and J. Ma, Hydrogen bonding-reinforced hydrogel electrolyte for flexible, robust, and all-in-one supercapacitor with excellent low-temperature tolerance, ACS Appl. Mater. Interfaces, 2020, 12, 37977–37985 CrossRef CAS .
9. Y. Zou, C. Chen, Y. Sun, S. Gan, L. Dong, J. Zhao and J. Rong, Flexible, All-Hydrogel Supercapacitor with Self-Healing Ability, Chem. Eng. J., 2021, 128616 CrossRef CAS .
10. H. Huang, L. Han, X. Fu, Y. Wang, Z. Yang, L. Pan and M. Xu, A Powder Self-Healable Hydrogel Electrolyte for Flexible Hybrid Supercapacitors with High Energy Density and Sustainability, Small, 2021, 17, 2006807 CrossRef CAS PubMed .
11. T.-C. Liu, S. Sutarsis, X.-Y. Zhong, W.-C. Lin, S.-H. Chou, N. Kirana, P.-Y. Huang, Y.-C. Lo, J.-K. Chang and P.-W. Wu, An interfacial wetting water based hydrogel electrolyte for high-voltage flexible quasi solid-state supercapacitors, Energy Storage Mater., 2021, 38, 489–498 CrossRef .
12. T. An, Y. Ling, S. Gong, B. Zhu, Y. Zhao, D. Dong, L. W. Yap, Y. Wang and W. Cheng, A Wearable Second Skin-Like Multifunctional Supercapacitor with Vertical Gold Nanowires and Electrochromic Polyaniline, Adv. Mater. Technol., 2019, 4, 1800473 CrossRef .
13. L. Wang, C. Zhang, X. Jiao and Z. Yuan, Polypyrrole-based hybrid nanostructures grown on textile for wearable supercapacitors, Nano Res., 2019, 12, 1129–1137 CrossRef CAS .
14. Y. Zhao, T. Mei, L. Ye, Y. Li, L. Wang, Y. Zhang, P. Chen, X. Sun, C. Wang and H. Peng, Injectable fiber batteries for all-region power supply in vivo, J. Mater. Chem. A, 2021, 9, 1463–1470 RSC .
15. L. Zhang, X. Hu, Z. Wang, F. Sun and D. G. Dorrell, A review of supercapacitor modeling, estimation, and applications: A control/management perspective, Renewable Sustainable Energy Rev., 2018, 81, 1868–1878 CrossRef .
16. Z. Yu, L. Tetard, L. Zhai and J. Thomas, Supercapacitor electrode materials: nanostructures from 0 to 3 dimensions, Energy Environ. Sci., 2015, 8, 702–730 RSC .
17. M. Salanne, B. Rotenberg, K. Naoi, K. Kaneko, P.-L. Taberna, C. P. Grey, B. Dunn and P. Simon, Efficient storage mechanisms for building better supercapacitors, Nat. Energy, 2016, 1, 1–10 Search PubMed .
18. L. Xie, F. Su, L. Xie, X. Guo, Z. Wang, Q. Kong, G. Sun, A. Ahmad, X. Li and Z. Yi, Effect of pore structure and doping species on charge storage mechanisms in porous carbon-based supercapacitors, Mater. Chem. Front., 2020, 4, 2610–2634 RSC .
19. Y. Shao, M. F. El-Kady, J. Sun, Y. Li, Q. Zhang, M. Zhu, H. Wang, B. Dunn and R. B. Kaner, Design and mechanisms of asymmetric supercapacitors, Chem. Rev., 2018, 118, 9233–9280 CrossRef CAS PubMed .
20. W. Raza, F. Ali, N. Raza, Y. Luo, K. Kim, J. Yang, S. Kumar, A. Mehmood and E. E. Kwon, Recent advancements in supercapacitor technology, Nano Energy, 2018, 52, 441–473 CrossRef CAS .
21. L. Jiang, D. Luo, Q. Zhang, S. Ma, G. Wan, X. Lu and Z. Ren, Electrochemical Performance of Free-Standing and Flexible Graphene and TiO2 Composites with Different Conductive Polymers as Electrodes for Supercapacitors, Chem. – Eur. J., 2019, 25, 7903–7911 CrossRef CAS .
22. L. L. Zhang and X. Zhao, Carbon-based materials as supercapacitor electrodes, Chem. Soc. Rev., 2009, 38, 2520–2531 RSC .
23. L. M. Da Silva, R. Cesar, C. M. Moreira, J. H. Santos, L. G. De Souza, B. M. Pires, R. Vicentini, W. Nunes and H. Zanin, Reviewing the fundamentals of supercapacitors and the difficulties involving the analysis of the electrochemical findings obtained for porous electrode materials, Energy Storage Mater., 2020, 27, 555–590 CrossRef .
24. Z. Chen, X. Wang, B. Xue, W. Li, Z. Ding, X. Yang, J. Qiu and Z. Wang, Rice husk-based hierarchical porous carbon for high performance supercapacitors: The structure-performance relationship, Carbon, 2020, 161, 432–444 CrossRef CAS .
25. T. Wang, H. C. Chen, F. Yu, X. Zhao and H. Wang, Boosting the cycling stability of transition metal compounds-based supercapacitors, Energy Storage Mater., 2019, 16, 545–573 CrossRef .
26. R. Liu, A. Zhou, X. Zhang, J. Mu, H. Che, Y. Wang, T. Wang, Z. Zhang and Z. Kou, Fundamentals, Advances and Challenges of Transition Metal Compounds-based Supercapacitors, Chem. Eng. J., 2021, 128611 CrossRef CAS .
27. T. Nguyen and M. d. F. Montemor, Metal Oxide and Hydroxide-Based Aqueous Supercapacitors: From Charge Storage Mechanisms and Functional Electrode Engineering to Need-Tailored Devices, Adv. Sci., 2019, 6, 1801797 CrossRef .
28. P. Naskar, A. Maiti, P. Chakraborty, D. Kundu, B. Biswas and A. Banerjee, Chemical supercapacitors: a review focusing on metallic compounds and conducting polymers, J. Mater. Chem. A, 2021, 9, 1970–2017 RSC .
29. C. Xie, X. Wang, H. He, Y. Ding and X. Lu, Mussel-Inspired Hydrogels for Self-Adhesive Bioelectronics, Adv. Funct. Mater., 2020, 1909954,  DOI:10.1002/adfm.201909954 .
30. K. Y. Lee and D. J. Mooney, Hydrogels for tissue engineering, Chem. Rev., 2001, 101, 1869–1880 CrossRef CAS .
31. D. Seliktar, Designing cell-compatible hydrogels for biomedical applications, Science, 2012, 336, 1124–1128 CrossRef CAS .
32. Y. Guo, J. Bae, F. Zhao and G. Yu, Functional hydrogels for next-generation batteries and supercapacitors, Trends Chem., 2019, 1, 335–348 CrossRef CAS .
33. Y. Gu, J. Zhao and J. A. Johnson, A (macro) molecular-level understanding of polymer network topology, Trends Chem., 2019, 1, 318–334 CrossRef CAS .
34. L. Hao, X. Li and L. Zhi, Carbonaceous electrode materials for supercapacitors, Adv. Mater., 2013, 25, 3899–3904 CrossRef CAS .
35. L. Zhang, D. Shi, T. Liu, M. Jaroniec and J. Yu, Nickel-based materials for supercapacitors, Mater. Today, 2019, 25, 35–65 CrossRef CAS .
36. Y. Zhang, L. Li, H. Su, W. Huang and X. Dong, Binary metal oxide: advanced energy storage materials in supercapacitors, J. Mater. Chem. A, 2015, 3, 43–59 RSC .
37. S. Langlois and F. Coeuret, Flow-through and flow-by porous electrodes of nickel foam. I. Material characterization, J. Appl. Electrochem., 1989, 19, 43–50 CrossRef CAS .
38. L. Jiang, Z. Ren, S. Chen, Q. Zhang, X. Lu, H. Zhang and G. Wan, Bio-derived three-dimensional hierarchical carbon-graphene-TiO₂ as electrode for supercapacitors, Sci. Rep., 2018, 8, 1–9 Search PubMed .
39. H. Lu and X. Zhao, Biomass-derived carbon electrode materials for supercapacitors Sustain, Energy Fuels, 2017, 1, 1265–1281 CAS .
40. Y. Sun, R. B. Sills, X. Hu, Z. W. Seh, X. Xiao, H. Xu, W. Luo, H. Jin, Y. Xin and T. Li, A bamboo-inspired nanostructure design for flexible, foldable, and twistable energy storage devices, Nano Lett., 2015, 15, 3899–3906 CrossRef CAS .
41. D. Kang, Q. Liu, J. Gu, Y. Su, W. Zhang and D. Zhang, “Egg-box”-assisted fabrication of porous carbon with small mesopores for high-rate electric double layer capacitors, ACS Nano, 2015, 9, 11225–11233 CrossRef CAS .
42. X. L. Wu, T. Wen, H. L. Guo, S. Yang, X. Wang and A.-W. Xu, Biomass-derived sponge-like carbonaceous hydrogels and aerogels for supercapacitors, ACS Nano, 2013, 7, 3589–3597 CrossRef CAS .
43. W. Qian, J. Zhu, Y. Zhang, X. Wu and F. Yan, Condiment-derived 3D architecture porous carbon for electrochemical supercapacitors, Small, 2015, 11, 4959–4969 CrossRef CAS PubMed .
44. X. Liu, C. Ma, J. Li, B. Zielinska, R. J. Kalenczuk, X. Chen, P. K. Chu, T. Tang and E. Mijowska, Biomass-derived robust three-dimensional porous carbon for high volumetric performance supercapacitors, J. Power Sources, 2019, 412, 1–9 CrossRef CAS .
45. Y. Guo, J. Bae, Z. Fang, P. Li, F. Zhao and G. Yu, Hydrogels and hydrogel-derived materials for energy and water sustainability, Chem. Rev., 2020, 120, 7642–7707 CrossRef CAS PubMed .
46. J. Wei, C. Ding, P. Zhang, H. Ding, X. Niu, Y. Ma, C. Li, Y. Wang and H. Xiong, Robust negative electrode materials derived from carbon dots and porous hydrogels for high-performance hybrid supercapacitors, Adv. Mater., 2019, 31, 1806197 Search PubMed .
47. J. Ding, Z. Li, K. Cui, S. Boyer, D. Karpuzov and D. Mitlin, Heteroatom enhanced sodium ion capacity and rate capability in a hydrogel derived carbon give record performance in a hybrid ion capacitor, Nano Energy, 2016, 23, 129–137 CrossRef CAS .
48. M. Zhi, C. Xiang, J. Li, M. Li and N. Wu, Nanostructured carbon–metal oxide composite electrodes for supercapacitors: a review, Nanoscale, 2013, 5, 72–88 RSC .
49. G. Wang, L. Zhang and J. Zhang, A review of electrode materials for electrochemical supercapacitors, Chem. Soc. Rev., 2012, 41, 797–828 RSC .
50. A. González, E. Goikolea, J. A. Barrena and R. Mysyk, Review on supercapacitors: Technologies and materials, Renewable Sustainable Energy Rev., 2016, 58, 1189–1206 CrossRef .
51. B. Li, Y. Cheng, L. Dong, Y. Wang, J. Chen, C. Huang, D. Wei, Y. Feng, D. Jia and Y. Zhou, Nitrogen doped and hierarchically porous carbons derived from chitosan hydrogel via rapid microwave carbonization for high-performance supercapacitors, Carbon, 2017, 122, 592–603 CrossRef CAS .
52. J. S. Wei, C. Ding, P. Zhang, H. Ding, X. Q. Niu, Y. Y. Ma, C. Li, Y. G. Wang and H. M. Xiong, Robust Negative Electrode Materials Derived from Carbon Dots and Porous Hydrogels for High-Performance Hybrid Supercapacitors, Adv. Mater., 2019, 31, 1806197 Search PubMed .
53. B. You, F. Kang, P. Yin and Q. Zhang, Hydrogel-derived heteroatom-doped porous carbon networks for supercapacitor and electrocatalytic oxygen reduction, Carbon, 2016, 103, 9–15 CrossRef CAS .
54. X. L. Wu and A. W. Xu, Carbonaceous hydrogels and aerogels for supercapacitors, J. Mater. Chem. A, 2014, 2, 4852–4864 RSC .
55. L. Zhang and G. Shi, Preparation of highly conductive graphene hydrogels for fabricating supercapacitors with high rate capability, J. Phys. Chem. C, 2011, 115, 17206–17212 CrossRef CAS .
56. P. Chen, J. Yang, S. Li, Z. Wang, T. Xiao, Y. Qian and S. Yu, Hydrothermal synthesis of macroscopic nitrogen-doped graphene hydrogels for ultrafast supercapacitor, Nano Energy, 2013, 2, 249–256 CrossRef CAS .
57. Y. Xu, Z. Lin, X. Huang, Y. Liu, Y. Huang and X. Duan, Flexible solid-state supercapacitors based on three-dimensional graphene hydrogel films, ACS Nano, 2013, 7, 4042–4049 CrossRef CAS .
58. Y. Xu, Z. Lin, X. Huang, Y. Wang, Y. Huang and X. Duan, Functionalized graphene hydrogel-based high-performance supercapacitors, Adv. Mater., 2013, 25, 5779–5784 CrossRef CAS .
59. P. Li, Z. Jin, L. Peng, F. Zhao, D. Xiao, Y. Jin and G. Yu, Stretchable all-gel-state fiber-shaped supercapacitors enabled by macromolecularly interconnected 3D graphene/nanostructured conductive polymer hydrogels, Adv. Mater., 2018, 30, 1800124 CrossRef .
60. C. W. Lien, B. Vedhanarayanan, J. H. Chen, J. Y. Lin, H. H. Tsai, L. D. Shao and T. W. Lin, Optimization of acetonitrile/water content in hybrid deep eutectic solvent for graphene/MoS2 hydrogel-based supercapacitors, Chem. Eng. J., 2021, 405, 126706 CrossRef CAS .
61. J. Kang, M. Hwang, K.-D. Seong, L. Lyu, D. Ko and Y. Piao, Three-dimensional nanocomposite of graphene/MWCNT hydrogel grafted with Ni–Co hydroxide nanorods as high-performance electrode for asymmetric supercapacitor, Electrochim. Acta, 2020, 346, 136258 CrossRef CAS .
62. A. Khazaeli, G. Godbille Cardona and D. P. Barz, A Novel Flexible Hybrid Battery–Supercapacitor Based on a Self-Assembled Vanadium-Graphene Hydrogel, Adv. Funct. Mater., 2020, 30, 1910738 CrossRef CAS .
63. D. Wu and W. Zhong, A new strategy for anchoring a functionalized graphene hydrogel in a carbon cloth network to support a lignosulfonate/polyaniline hydrogel as an integrated electrode for flexible high areal-capacitance supercapacitors, J. Mater. Chem. A, 2019, 7, 5819–5830 RSC .
64. Y. Zou, Z. Zhang, W. Zhong and W. Yang, Hydrothermal direct synthesis of polyaniline, graphene/polyaniline and N-doped graphene/polyaniline hydrogels for high performance flexible supercapacitors, J. Mater. Chem. A, 2018, 6, 9245–9256 RSC .
65. M. Ates, M. ElKady and R. B. Kaner, Three-dimensional design and fabrication of reduced graphene oxide/polyaniline composite hydrogel electrodes for high performance electrochemical supercapacitors, Nanotechnology, 2018, 29, 175402 CrossRef PubMed .
66. K. Wang, X. Zhang, C. Li, H. Zhang, X. Sun, N. Xu and Y. Ma, Flexible solid-state supercapacitors based on a conducting polymer hydrogel with enhanced electrochemical performance, J. Mater. Chem. A, 2014, 2, 19726–19732 RSC .
67. Y. Shi, L. Pan, B. Liu, Y. Wang, Y. Cui, Z. Bao and G. Yu, Nanostructured conductive polypyrrole hydrogels as high-performance, flexible supercapacitor electrodes, J. Mater. Chem. A, 2014, 2, 6086–6091 RSC .
68. S. Ghosh and O. Inganäs, Conducting polymer hydrogels as 3D electrodes: applications for supercapacitors, Adv. Mater., 1999, 11, 1214–1218 CrossRef CAS .
69. I. Shown, A. Ganguly, L. C. Chen and K. H. Chen, Conducting polymer-based flexible supercapacitor, Energy Sci. Eng., 2015, 3, 2–26 CrossRef CAS .
70. V. Augustyn, P. Simon and B. Dunn, Pseudocapacitive oxide materials for high-rate electrochemical energy storage, Energy Environ. Sci., 2014, 7, 1597–1614 RSC .
71. J. Zhang and X. Zhao, Conducting polymers directly coated on reduced graphene oxide sheets as high-performance supercapacitor electrodes, J. Phys. Chem. C, 2012, 116, 5420–5426 CrossRef CAS .
72. H. Shirakawa, E. J. Louis, A. G. MacDiarmid, C. K. Chiang and A. J. Heeger, Synthesis of electrically conducting organic polymers: halogen derivatives of polyacetylene,(CH)x, J. Am. Chem. Soc., 1977, 578–580 CAS .
73. L. Jiang, X. Lu, C. Xie, G. Wan, H. Zhang and T. Youhong, Flexible, free-standing TiO2–graphene–polypyrrole composite films as electrodes for supercapacitors, J. Phys. Chem. C, 2015, 119, 3903–3910 CrossRef CAS .
74. L. Jiang, Y. Li, D. Luo, Q. Zhang, F. Cai, G. Wan, L. Xiong and Z. Ren, Freestanding RGO–Co₃O₄–PPy composite films as electrodes for supercapacitors, Energy Technol., 2019, 7, 1800606 CrossRef .
75. L. Pan, G. Yu, D. Zhai, H. R. Lee, W. Zhao, N. Liu, H. Wang, B. C.-K. Tee, Y. Shi and Y. Cui, Hierarchical nanostructured conducting polymer hydrogel with high electrochemical activity, Proc. Natl. Acad. Sci. U. S. A., 2012, 109, 9287–9292 CrossRef CAS .
76. R. A. Green, S. Baek, L. A. Poole-Warren and P. J. Martens, Conducting polymer-hydrogels for medical electrode applications, Sci. Technol. Adv. Mater., 2010, 11, 014107 CrossRef PubMed .
77. Y. Wang, Y. Shi, L. Pan, Y. Ding, Y. Zhao, Y. Li, Y. Shi and G. Yu, Dopant-enabled supramolecular approach for controlled synthesis of nanostructured conductive polymer hydrogels, Nano Lett., 2015, 15, 7736–7741 CrossRef CAS PubMed .
78. Z. Chen, J. W. To, C. Wang, Z. Lu, N. Liu, A. Chortos, L. Pan, F. Wei, Y. Cui and Z. Bao, A three-dimensionally interconnected carbon nanotube–conducting polymer hydrogel network for high-performance flexible battery electrodes, Adv. Energy Mater., 2014, 4, 1400207 CrossRef .
79. S. Das, P. Chakraborty, S. Mondal, A. Shit and A. K. Nandi, Enhancement of energy storage and photoresponse properties of folic acid–polyaniline hybrid hydrogel by in situ growth of Ag nanoparticles, ACS Appl. Mater. Interfaces, 2016, 8, 28055–28067 CrossRef CAS .
80. Y. Zhao, B. Zhang, B. Yao, Y. Qiu, Z. Peng, Y. Zhang, Y. Alsaid, I. Frenkel, K. Youssef and Q. Pei, Hierarchically Structured Stretchable Conductive Hydrogels for High-Performance Wearable Strain Sensors and Supercapacitors, Matter, 2020, 3, 1196–1210 CrossRef .
81. L. Li, Y. Shi, L. Pan, Y. Shi and G. Yu, Rational design and applications of conducting polymer hydrogels as electrochemical biosensors, J. Mater. Chem. B, 2015, 3, 2920–2930 RSC .
82. J. Han, H. Wang, Y. Yue, C. Mei, J. Chen, C. Huang, Q. Wu and X. Xu, A self-healable and highly flexible supercapacitor integrated by dynamically cross-linked electro-conductive hydrogels based on nanocellulose-templated carbon nanotubes embedded in a viscoelastic polymer network, Carbon, 2019, 149, 1–18 CrossRef CAS .
83. N. Wen, B. Jiang, X. Wang, Z. Shang, D. Jiang, L. Zhang, C. Sun, Z. Wu, H. Yan and C. Liu, Overview of polyvinyl alcohol nanocomposite hydrogels for electro-skin, actuator, supercapacitor and fuel cell, Chem. Rec., 2020, 20, 773–792 CrossRef CAS PubMed .
84. M. Hua, S. Wu, Y. Ma, Y. Zhao, Z. Chen, I. Frenkel, J. Strzalka, H. Zhou, X. Zhu and X. He, Strong tough hydrogels via the synergy of freeze-casting and salting out, Nature, 2021, 590, 594–599 CrossRef CAS PubMed .
85. J. Ben, Z. Song, X. Liu, W. Lü and X. Li, Fabrication and Electrochemical Performance of PVA/CNT/PANI Flexible Films as Electrodes for Supercapacitors, Nanoscale Res. Lett., 2020, 15, 1–8 CrossRef .
86. Z. Jia, J. Gong, Y. Zeng, J. Ran, J. Liu, K. Wang, C. Xie, X. Lu and J. Wang, Bioinspired Conductive Silk Microfiber Integrated Bioelectronic for Diagnosis and Wound Healing in Diabetes, Adv. Funct. Mater., 2021, 2010461,  DOI:10.1002/adfm.202010461 .
87. Z. Jia, X. Lv, Y. Hou, K. Wang, F. Ren, D. Xu, Q. Wang, K. Fan, C. Xie and X. Lu, Mussel-inspired nanozyme catalyzed conductive and self-setting hydrogel for adhesive and antibacterial bioelectronics, Bioact. Mater., 2021, 6, 2676–2687 CrossRef CAS PubMed .
88. D. Gan, T. Shuai, X. Wang, Z. Huang, F. Ren, L. Fang, K. Wang, C. Xie and X. Lu, Mussel-Inspired Redox-Active and Hydrophilic Conductive Polymer Nanoparticles for Adhesive Hydrogel Bioelectronics, Nano-Micro Lett., 2020, 12, 1–16 CrossRef PubMed .
89. L. Han, K. Liu, M. Wang, K. Wang, L. Fang, H. Chen, J. Zhou and X. Lu, Mussel-inspired adhesive and conductive hydrogel with long-lasting moisture and extreme temperature tolerance, Adv. Funct. Mater., 2018, 28, 1704195 CrossRef .
90. D. Gan, Z. Huang, X. Wang, L. Jiang, C. Wang, M. Zhu, F. Ren, L. Fang, K. Wang and C. Xie, Graphene oxide-templated conductive and redox-active nanosheets incorporated hydrogels for adhesive bioelectronics, Adv. Funct. Mater., 2020, 30, 1907678 CrossRef CAS .
91. L. Han, X. Lu, M. Wang, D. Gan, W. Deng, K. Wang, L. Fang, K. Liu, C. W. Chan and Y. Tang, A mussel-inspired conductive, self-adhesive, and self-healable tough hydrogel as cell stimulators and implantable bioelectronics, Small, 2017, 13, 1601916 CrossRef PubMed .
92. C. Zhang, B. Wu, Y. Zhou, F. Zhou, W. Liu and Z. Wang, Mussel-inspired hydrogels: from design principles to promising applications, Chem. Soc. Rev., 2020, 49, 3605–3637 RSC .
93. E. J. Son, J. H. Kim, K. Kim and C. B. Park, Quinone and its derivatives for energy harvesting and storage materials, J. Mater. Chem. A, 2016, 4, 11179–11202 RSC .
94. C. Han, H. Li, R. Shi, T. Zhang, J. Tong, J. Li and B. Li, Organic quinones towards advanced electrochemical energy storage: recent advances and challenges, J. Mater. Chem. A, 2019, 7, 23378–23415 RSC .
95. Y. Katsuyama, Y. Nakayasu, K. Oizumi, Y. Fujihara, H. Kobayashi and I. Honma, Quinone-Based Redox Supercapacitor Using Highly Conductive Hard Carbon Derived from Oak Wood, Adv. Sustainable Syst., 2019, 3, 1900083 CrossRef CAS .
96. A. Sumboja, C. Y. Foo, X. Wang and P. S. Lee, Large areal mass, flexible and free-standing reduced graphene oxide/manganese dioxide paper for asymmetric supercapacitor device, Adv. Mater., 2013, 25, 2809–2815 CrossRef CAS PubMed .
97. S. Wang, N. Liu, J. Su, L. Li, F. Long, Z. Zou, X. Jiang and Y. Gao, Highly stretchable and self-healable supercapacitor with reduced graphene oxide based fiber springs, ACS Nano, 2017, 11, 2066–2074 CrossRef CAS .
98. J. Chen, Q. Peng, T. Thundat and H. Zeng, Stretchable, injectable, and self-healing conductive hydrogel enabled by multiple hydrogen bonding toward wearable electronics, Chem. Mater., 2019, 31, 4553–4563 CrossRef CAS .
99. K. Ren, Y. Cheng, C. Huang, R. Chen, Z. Wang and J. Wei, Self-healing conductive hydrogels based on alginate, gelatin and polypyrrole serve as a repairable circuit and a mechanical sensor, J. Mater. Chem. B, 2019, 7, 5704–5712 RSC .
100. J. Fan, Z. Shi, M. Lian, H. Li and J. Yin, Mechanically strong graphene oxide/sodium alginate/polyacrylamide nanocomposite hydrogel with improved dye adsorption capacity, J. Mater. Chem. A, 2013, 1, 7433–7443 RSC .
101. Y. Ohm, C. Pan, M. J. Ford, X. Huang, J. Liao and C. Majidi, An electrically conductive silver–polyacrylamide–alginate hydrogel composite for soft electronics, Nat. Electron., 2021, 1–8 Search PubMed .
102. W. Kong, C. Wang, C. Jia, Y. Kuang, G. Pastel, C. Chen, G. Chen, S. He, H. Huang and J. Zhang, Muscle-Inspired Highly Anisotropic, Strong, Ion-Conductive Hydrogels, Adv. Mater., 2018, 30, 1801934 CrossRef PubMed .
103. L. Li, F. Lu, C. Wang, F. Zhang, W. Liang, S. Kuga, Z. Dong, Y. Zhao, Y. Huang and M. Wu, Flexible double-cross-linked cellulose-based hydrogel and aerogel membrane for supercapacitor separator, J. Mater. Chem. A, 2018, 6, 24468–24478 RSC .
104. L. Yu and G. Z. Chen, Ionic liquid-based electrolytes for supercapacitor and supercapattery, Mater. Chem. Front., 2019, 7, 272 CAS .
105. C. Zhong, Y. Deng, W. Hu, J. Qiao, L. Zhang and J. Zhang, A review of electrolyte materials and compositions for electrochemical supercapacitors, Chem. Soc. Rev., 2015, 44, 7484–7539 RSC .
106. N. Choudhury, S. Sampath and A. Shukla, Hydrogel-polymer electrolytes for electrochemical capacitors: an overview, Energy Environ. Sci., 2009, 2, 55–67 RSC .
107. Y. Huang, M. Zhong, F. Shi, X. Liu, T. Zijie, W. Yukun, Y. Huang, H. Hou, X. Xie and Z. Chunyi, A polyacrylamide hydrogel electrolyte enabled intrinsically 1000% stretchable and 50% compressible supercapacitor, Angew. Chem., Int. Ed., 2017, 56, 9141–9145 CrossRef CAS PubMed .
108. L. Han, H. Huang, X. Fu, J. Li, Z. Yang, X. Liu, L. Pan and M. Xu, A flexible, high-voltage and safe zwitterionic natural polymer hydrogel electrolyte for high-energy-density zinc-ion hybrid supercapacitor, Chem. Eng. J., 2020, 392, 123733 CrossRef CAS .
109. X. Shi, Z. S. Wu, J. Qin, S. Zheng, S. Wang, F. Zhou, C. Sun and X. Bao, Graphene-Based Linear Tandem Micro-Supercapacitors with Metal-Free Current Collectors and High-Voltage Output, Adv. Mater., 2017, 29, 1703034 CrossRef PubMed .
110. Z. S. Wu, Z. Liu, K. Parvez, X. Feng and K. Müllen, Ultrathin printable graphene supercapacitors with AC line-filtering performance, Adv. Mater., 2015, 27, 3669–3675 CrossRef CAS PubMed .
111. G. Xiong, P. He, B. Huang, T. Chen, Z. Bo and T. S. Fisher, Graphene nanopetal wire supercapacitors with high energy density and thermal durability, Nano Energy, 2017, 38, 127–136 CrossRef CAS .
112. P. T. Nguyen, J. Jang, Y. Lee, S. T. Choi and J. B. In, Laser-assisted fabrication of flexible monofilament fiber supercapacitors, J. Mater. Chem. A, 2021, 9, 4841–4850 RSC .
113. X. Zhang, Q. Fu, H. Huang, L. Wei and X. Guo, Silver-quantum-dot-modified MoO₃ and MnO₂ paper-like freestanding films for flexible solid-state asymmetric supercapacitors, Small, 2019, 15, 1805235 CrossRef PubMed .
114. J. W. Kim, H. Park, G. Lee, Y. R. Jeong, S. Y. Hong, K. Keum, J. Yoon, M. S. Kim and J. S. Ha, Paper-like, thin, foldable, and self-healable electronics based on PVA/CNC nanocomposite film, Adv. Funct. Mater., 2019, 29, 1905968 CrossRef CAS .
115. C. Fang and D. Zhang, A large areal capacitance structural supercapacitor with a 3D rGO@ MnO₂ foam electrode and polyacrylic acid–Portland cement–KOH electrolyte, J. Mater. Chem. A, 2020, 8, 12586–12593 RSC .
116. E. Cevik and A. Bozkurt, Redox active polymer metal chelates for use in flexible symmetrical supercapacitors: Cobalt-containing poly(acrylic acid) polymer electrolytes, J. Energy Chem., 2021, 55, 145–153 CrossRef .
117. Y. Huang, M. Zhong, Y. Huang, M. Zhu, Z. Pei, Z. Wang, Q. Xue, X. Xie and C. Zhi, A self-healable and highly stretchable supercapacitor based on a dual crosslinked polyelectrolyte, Nat. Commun., 2015, 6, 1–8 Search PubMed .
118. J. Liu, H. Song, Z. Wang, J. Zhang, J. Zhang and X. Ba, Stretchable, self-healable, and reprocessable chemical cross-linked ionogels electrolytes based on gelatin for flexible supercapacitors, J. Mater. Sci., 2020, 55, 3991–4004 CrossRef CAS .
119. M. Kadir, Non-faradaic-based supercapacitor fabricated with fish skin gelatin biopolymer electrolyte, Ionics, 2021, 1–11 Search PubMed .
120. L. Yang, L. Song, Y. Feng, M. Cao, P. Zhang, X.-F. Zhang and J. Yao, Zinc ion trapping in a cellulose hydrogel as a solid electrolyte for a safe and flexible supercapacitor, J. Mater. Chem. A, 2020, 8, 12314–12318 RSC .
121. Z. Peng, Y. Zou, S. Xu, W. Zhong and W. Yang, High-performance biomass-based flexible solid-state supercapacitor constructed of pressure-sensitive lignin-based and cellulose hydrogels, ACS Appl. Mater. Interfaces, 2018, 10, 22190–22200 CrossRef CAS PubMed .
122. L. Jiang, X. Mei, D. Gan, S. Song, X. Lei, F. Cai, Y. Wang, Q. Zhang, X. Lu and Z. Ren, Hybrid transition-metal oxide and nitride@N-doped reduced graphene oxide electrodes for high-performance, flexible, and all-solid-state supercapacitors, Chem. – Eur. J., 2021, 27, 5761–5768 CrossRef CAS PubMed .
123. Z. Liu, J. Zhang, J. Liu, Y. Long, L. Fang, Q. Wang and T. Liu, Highly compressible and superior low temperature tolerant supercapacitors based on dual chemically crosslinked PVA hydrogel electrolytes, J. Mater. Chem. A, 2020, 8, 6219–6228 RSC .
124. H. Peng, Y. Lv, G. Wei, J. Zhou, X. Gao, K. Sun, G. Ma and Z. Lei, A flexible and self-healing hydrogel electrolyte for smart supercapacitor, J. Power Sources, 2019, 431, 210–219 CrossRef CAS .
125. S. Alipoori, S. Mazinani, S. H. Aboutalebi and F. Sharif, Review of PVA-based gel polymer electrolytes in flexible solid-state supercapacitors: Opportunities and challenges, J. Energy Storage, 2020, 27, 101072 CrossRef .
126. Y. Wang, Y. Ding, X. Guo and G. Yu, Conductive polymers for stretchable supercapacitors, Nano Res., 2019, 1–10 Search PubMed .
127. C. Zhao, C. Wang, Z. Yue, K. Shu and G. G. Wallace, Intrinsically stretchable supercapacitors composed of polypyrrole electrodes and highly stretchable gel electrolyte, ACS Appl. Mater. Interfaces, 2013, 5, 9008–9014 CrossRef CAS PubMed .
128. Z. Liu, G. Liang, Y. Zhan, H. Li, Z. Wang, L. Ma, Y. Wang, X. Niu and C. Zhi, A soft yet device-level dynamically super-tough supercapacitor enabled by an energy-dissipative dual-crosslinked hydrogel electrolyte, Nano Energy, 2019, 58, 732–742 CrossRef CAS .
129. N. A. Choudhury, S. Sampath and A. Shukla, Gelatin hydrogel electrolytes and their application to electrochemical supercapacitors, J. Electrochem. Soc., 2007, 155, A74 CrossRef .
130. T. G. Yun, J.-S. Jang, J. Y. Cheong and I.-D. Kim, Organism epidermis/plant-root inspired ultra-stable supercapacitor for large-scale wearable energy storage applications, Nano Energy, 2021, 82, 105776 CrossRef CAS .
131. J. H. Park, H. H. Rana, J. Y. Lee and H. S. Park, Renewable flexible supercapacitors based on all-lignin-based hydrogel electrolytes and nanofiber electrodes, J. Mater. Chem. A, 2019, 7, 16962–16968 RSC .
132. L. Yang, L. Song, Y. Feng, M. Cao, P. Zhang, X. Zhang and J. Yao, Zinc ion trapping in a cellulose hydrogel as a solid electrolyte for a safe and flexible supercapacitor, J. Mater. Chem. A, 2020, 8, 12314–12318 RSC .
133. X. Li, L. Yuan, R. Liu, H. He, J. Hao, Y. Lu, Y. Wang, G. Liang, G. Yuan and Z. Guo, Engineering Textile Electrode and Bacterial Cellulose Nanofiber Reinforced Hydrogel Electrolyte to Enable High-Performance Flexible All-Solid-State Supercapacitors, Adv. Energy Mater., 2021, 2003010,  DOI:10.1002/aenm.202003010 .
134. C. Wang, T. Yokota and T. Someya, Natural Biopolymer-Based Biocompatible Conductors for Stretchable Bioelectronics, Chem. Rev., 2021, 121, 2109–2146 CrossRef CAS PubMed .
135. T. Liu, X. Ren, J. Zhang, J. Liu, R. Ou, C. Guo, X. Yu, Q. Wang and Z. Liu, Highly compressible lignin hydrogel electrolytes via double-crosslinked strategy for superior foldable supercapacitors, J. Power Sources, 2020, 449, 227532 CrossRef CAS .
136. W. Tian, Y. Li, J. Zhou, T. Wang, R. Zhang, J. Cao, M. Luo, N. Li, N. Zhang and H. Gong, Implantable and Biodegradable Micro-Supercapacitor Based on a Superassembled Three-Dimensional Network Zn@ PPy Hybrid Electrode, ACS Appl. Mater. Interfaces, 2021, 13, 8285–8293 CrossRef CAS PubMed .
137. M. Chen, J. Chen, W. Zhou, X. Han, Y. Yao and C. Wong, Realizing an All-Round Hydrogel Electrolyte toward Environmentally Adaptive Dendrite-Free Aqueous Zn–MnO2 Batteries, Adv. Mater., 2021, 33, 2007559 CrossRef CAS PubMed .
138. Y. Lin, H. Zhang, H. Liao, Y. Zhao and K. Li, A physically crosslinked, self-healing hydrogel electrolyte for nano-wire PANI flexible supercapacitors, Chem. Eng. J., 2019, 367, 139–148 CrossRef CAS .
139. J. Zhao, J. Gong, G. Wang, K. Zhu, K. Ye, J. Yan and D. Cao, A self-healing hydrogel electrolyte for flexible solid-state supercapacitors, Chem. Eng. J., 2020, 401, 125456 CrossRef CAS .
140. J. Wang, F. Liu, F. Tao and Q. Pan, Rationally designed self-healing hydrogel electrolyte toward a smart and sustainable supercapacitor, ACS Appl. Mater. Interfaces, 2017, 9, 27745–27753 CrossRef CAS PubMed .
141. F. Tao, L. Qin, Z. Wang and Q. Pan, Self-healable and cold-resistant supercapacitor based on a multifunctional hydrogel electrolyte, ACS Appl. Mater. Interfaces, 2017, 9, 15541–15548 CrossRef CAS PubMed .
142. H. Yu, N. Rouelle, A. Qiu, J. A. Oh, D. M. Kempaiah, J. D. Whittle, M. Aakyiir, W. Xing and J. Ma, Hydrogen bonding-reinforced hydrogel electrolyte for flexible, robust, and all-in-one supercapacitor with excellent low-temperature tolerance, ACS Appl. Mater. Interfaces, 2020, 12, 37977–37985 CrossRef CAS PubMed .
143. P. Yang, C. Feng, Y. Liu, T. Cheng, X. Yang, H. Liu, K. Liu and H. J. Fan, Thermal Self-Protection of Zinc-Ion Batteries Enabled by Smart Hygroscopic Hydrogel Electrolytes, Adv. Energy Mater., 2020, 10, 2002898 CrossRef CAS .
144. J. Xu, R. Jin, X. Ren and G. Gao, A wide temperature-tolerant hydrogel electrolyte mediated by phosphoric acid towards flexible supercapacitors, Chem. Eng. J., 2021, 413, 127446 CrossRef CAS .
145. J. Yang, Z. Xu, J. Wang, L. Gai, X. Ji, H. Jiang and L. Liu, Antifreezing Zwitterionic Hydrogel Electrolyte with High Conductivity of 12.6 mS cm⁻¹ at −40 °C through Hydrated Lithium Ion Hopping Migration, Adv. Funct. Mater., 2021, 2009438 CrossRef .
146. C. Lu and X. Chen, All-temperature flexible supercapacitors enabled by antifreezing and thermally stable hydrogel electrolyte, Nano Lett., 2020, 20, 1907–1914 CrossRef CAS PubMed .
147. B. Yin, S. Zhang, K. Ke and Z. Wang, Advanced deformable all-in-one hydrogel supercapacitor based on conducting polymer: Toward integrated mechanical and capacitive performance, J. Alloys Compd., 2019, 805, 1044–1051 CrossRef CAS .
148. L. Guo, W. Ma, Y. Wang, X. Song, J. Ma, X. Han, X. Tao, L. Guo, H. Fan and Z. Liu, A chemically crosslinked hydrogel electrolyte based all-in-one flexible supercapacitor with superior performance, J. Alloys Compd., 2020, 843, 155895 CrossRef CAS .
149. Y. Guo, K. Zheng and P. Wan, A flexible stretchable hydrogel electrolyte for healable all-in-one configured supercapacitors, Small, 2018, 14, 1704497 CrossRef PubMed .
150. M. Hu, J. Wang, J. Liu, J. Zhang, X. Ma and Y. Huang, An intrinsically compressible and stretchable all-in-one configured supercapacitor, Chem. Commun., 2018, 54, 6200–6203 RSC .
151. K. Wang, X. Zhang, C. Li, X. Sun, Q. Meng, Y. Ma and Z. Wei, Chemically crosslinked hydrogel film leads to integrated flexible supercapacitors with superior performance, Adv. Mater., 2015, 27, 7451–7457 CrossRef CAS PubMed .
152. K. Wang, X. Zhang, X. Sun and Y. Ma, Conducting polymer hydrogel materials for high-performance flexible solid-state supercapacitors, Sci. China Mater., 2016, 59, 412–420 CrossRef CAS .
153. J. Yang, X. Yu, X. Sun, Q. Kang, L. Zhu, G. Qin, A. Zhou, G. Sun and Q. Chen, Polyaniline-decorated supramolecular hydrogel with tough, fatigue-resistant, and self-healable performances for all-in-one flexible supercapacitors, ACS Appl. Mater. Interfaces, 2020, 12, 9736–9745 CrossRef CAS PubMed .
154. X. Jin, L. Song, H. Yang, C. Dai, Y. Xiao, X. Zhang, Y. Han, C. Bai, B. Lu and Q. Liu, Stretchable Supercapacitor at −30 °C, Energy Environ. Sci., 2021, 14, 3075–3085 RSC .
155. Q. Ding, X. Xu, Y. Yue, C. Mei, C. Huang, S. Jiang, Q. Wu and J. Han, Nanocellulose-mediated electroconductive self-healing hydrogels with high strength, plasticity, viscoelasticity, stretchability, and biocompatibility toward multifunctional applications, ACS Appl. Mater. Interfaces, 2018, 10, 27987–28002 CrossRef CAS PubMed .
156. A. Pullanchiyodan, L. Manjakkal, S. Dervin, D. Shakthivel and R. Dahiya, Metal coated conductive fabrics with graphite electrodes and biocompatible gel electrolyte for wearable supercapacitors, Adv. Mater. Technol., 2020, 5, 1901107 CrossRef CAS .
157. T. Gao, Z. Zhou, J. Yu, D. Cao, G. Wang, B. Ding and Y. Li, All-in-one compact architecture toward wearable all-solid-state, high-volumetric-energy-density supercapacitors, ACS Appl. Mater. Interfaces, 2018, 10, 23834–23841 CrossRef CAS PubMed .
158. H. H. Hsu, Y. Liu, Y. Wang, B. Li, G. Luo, M. Xing and W. Zhong, Mussel-Inspired Autonomously Self-Healable All-in-One Supercapacitor with Biocompatible Hydrogel, ACS Sustainable Chem. Eng., 2020, 8, 6935–6948 CrossRef CAS .
159. J. Zeng, L. Dong, W. Sha, L. Wei and X. Guo, Highly stretchable, compressible and arbitrarily deformable all-hydrogel soft supercapacitors, Chem. Eng. J., 2020, 383, 123098 CrossRef CAS .
160. J. C. Yang, J. Mun, S. Y. Kwon, S. Park, Z. Bao and S. Park, Electronic Skin: Electronic Skin: Recent Progress and Future Prospects for Skin-Attachable Devices for Health Monitoring, Robotics, and Prosthetics (Adv. Mater. 48/2019), Adv. Mater., 2019, 31, 1970337 CrossRef .
161. Y. Lee, J. Myoung, S. Cho, J. Park, J. Kim, H. Lee, Y. Lee, S. Lee, C. Baig and H. Ko, Bioinspired Gradient Conductivity and Stiffness for Ultrasensitive Electronic Skins, ACS nano, 2020, 15, 1795–1804 CrossRef PubMed .
162. Q. Hua, J. Sun, H. Liu, R. Bao, R. Yu, J. Zhai, C. Pan and Z. L. Wang, Skin-inspired highly stretchable and conformable matrix networks for multifunctional sensing, Nat. Commun., 2018, 9, 1–11 CrossRef CAS PubMed .
163. M. Gao, P. Wang, L. Jiang, B. Wang, Y. Yao, S. Liu, D. Chu, W. Cheng and Y. Lu, Power generation for wearable systems, Energy Environ. Sci., 2021, 14, 2114–2157 RSC .
164. Z. Liu, H. Li, B. Shi, Y. Fan, Z. L. Wang and Z. Li, Wearable and implantable triboelectric nanogenerators, Adv. Funct. Mater., 2019, 29, 1808820 CrossRef .
165. W. Liu, Z. Wang, G. Wang, G. Liu, J. Chen, X. Pu, Y. Xi, X. Wang, H. Guo and C. Hu, Integrated charge excitation triboelectric nanogenerator, Nat. Commun., 2019, 10, 1–9 CrossRef PubMed .
166. X. Zang, R. Zhang, Z. Zhen, W. Lai, C. Yang, F. Kang and H. Zhu, Flexible, temperature-tolerant supercapacitor based on hybrid carbon film electrodes, Nano Energy, 2017, 40, 224–232 CrossRef CAS .
167. Q. Rong, W. Lei, J. Huang and M. Liu, Low temperature tolerant organohydrogel electrolytes for flexible solid-state supercapacitors, Adv. Energy Mater., 2018, 8, 1801967 CrossRef .

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