For the ultrahigh efficiency of supercapacitor applications, the base materials have to possess high porosity, very high surface area, excellent mechanical properties and fast electron transfer. Due its three-dimensional architectures, graphene-based hydrogels fulfil these fundamental properties
[40]. The preparation method has a great influence on its final output. Chen et al.
[41] reported the preparation of graphene-based hydrogels with nickel hydroxide and obtained a capacitance as high as 3138.5 F. This value is very sufficient to store energy and is equivalent to the theoretical capacitance. This implies that hybrid hydrogels using graphene are excellent materials for supercapacitor applications.
Utilizing 3D architectural graphene structures for the production of supercapacitors using various mechanisms with different composite materials provides excellent performance in terms of specific capacitance value. Graphene-based hydrogels received tremendous attention in recent times for such applications. Flexibility, high energy density, and superior capacitance with acceptable charge-discharge capability demonstrate a promising performance.
3.2.2. Carbon Nanotubes
Carbon-based nanomaterials possess excellent and exceptional mechanical strength, ultrahigh electrical conductivity, high specific surface area and sole hierarchical structures
[42]. Due to the above-mentioned facts, carbon-based nanomaterials are excellent candidates for supercapacitor applications. Carbon-nanotubes (CNTs) are carbon nanomaterials with no exception for such applications. Recent publications indicated that supercapacitors based on CNT is growing.
Figure 3 shows the growing trends of the use of CNTs hydrogels for the production of supercapacitors. Carbon nanotubes’ (CNTs) electrodes have been fabricated using polyvinylidene fluoride (PVDF) and polyvinyl alcohol (PVA) as a gel electrolyte, which help to obtain a capacitance value of 173 F/g
[43].
Figure 3. The increasing trends on the use of carbon nanotubes in the production of supercapacitors searched on the topic of “carbon nanotube-based hydrogels for supercapacitor applications” (as obtained from ScienceDirect databases (accessed on 12 December 2022). The insert representing the hydrogels is formed by CNTs.
The preparation of supercapacitors using CNTs also includes the formation of different composites using different polymers.
3.3. Conductive Polymer-Based Hydrogels
Hydrogels based on conductive polymers are materials that contain conductive polymers within a cross-linked network structure of the electrically insulative polymer gel
[44] and conductive network hydrogels
[45]. The electrically conducting polymer hydrogels can be produced using various polymerization techniques depending on the types of monomers, and the conducting polymer hydrogels are further classified depending on the polymer type. This section discusses the use of different conducting polymers for the preparation of supercapacitors.
3.3.1. PAMPS-Based Hydrogels
Poly (2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) and ammonium molybdate [(NH
4)
2MoO
4] (Mo) composite demonstrated an effective supercapactive performance when it is in a redox-mediated environment
[46]. Conducting polymers are always combined with other compounds to form complete and compensated properties such as the polyelectrolyte additions; for example, the zwitterionic silica sulfobetaine
[47]. This helps to enhance the capacitive performance. The synergic effect with the electrolyte helps to double the capacitance from the pure PAMPS. The use of PAMPS as electrode and electrolyte materials for the preparation of high performance supercapacitor was patented in JUSTIA patents
[48]. The work was performed with comprising molybdate (VI) salts dispersed in a hydrogel matrix and claims 360–550 F/g at a current density in a range of 1–10 A/g, which is relatively very high when compared with other similar hydrogel-based electrodes. By combining PAMPS with other polymeric materials such as polypyrrole
[49], it was possible to produce a hydrogel electrode with a high capacitance value of 698.8 F/g at 5 mV s
−1. The work was performed with carbon fibers, which leads to the use of a PAMPS-based hydrogel for the production of high-performance supercapacitors for the use in smart clothing applications. There are several possibilities to form composites with PAMPS so that the mechanical and electrochemical properties are improved for supercapacitor applications.
Extensive research has been conducted to uncover the flexible energy storage for wearable electronics. Furthermore, the low cost preparation
[50], which is stable at different temperatures
[51], is important for wearable applications. PAMPS with a various composition will be the perfect candidate for such applications.
3.3.2. Polyaniline (PAni) Hydrogel
Polyaniline (PAni), an intrinsically conductive polymer, is an intensively studied polymer that is usually produced using oxidative polymerization means
[52]. Analine monomers are the sources of PAni. Usually the conductivity of PAni varies based on the dopant variety
[53]. By polymerization, PAni-based hydrogels combined with a nonconducting hydrogel matrix has been reported elsewhere
[54] for the production of supercapacitor materials with a sufficient electrochemical performance. Conductive polymer-based hydrogels have a unique advantage of being hydrogels and conductive at the same time. Being flexible, biocompatible, hydrophilic and biodegradable, its electroconductivity makes polymer hydrogels perfect candidates for supercapacitor application, which combines with the lightweight and highly integrability of fabrics identified as a promising class of materials for wearable applications. Like other materials, PAni-based hydrogels are most often characterized with its electrochemical properties.
Typically, PAni are combined with other polymers to obtain better electrochemical properties. For instance, PAni has been combined with polyvinyl alcohol (PVA)
[55] and has obtained a 237 mF/cm
2 at current density of 0.5 mA/cm
2 with excellent charge-discharge cycles. However, there is a difficulty in making a good combination of PVA and PAni; the authors employed a special crosslinking agent to obtain good results. It is also possible to produce supercapacitors with excellent mechanical properties, electrical properties and improved performance in electrochemical properties with PVA-PAni-graphene-based hydrogel composites
[56].
Another work performed by Dou, P., et al.,
[57] combined PAni with amino trimethylene phosphonic acid and achieved an ultrahigh specific capacitance of the specific capacitance over 420 F/g. Amino trimethylene phosphonic acid acts as a dopant and gelator, which helps PAni to increase the porosity and hence the surface area, which is the fundamental quantity that plays to increase the cyclic voltammetry or capacitance of the substance. Actually, the electroconductivity due to the doping surface area because of the gelation addition has been improved by the addition of amino trimethylene phosphonic acid. The use of the hydrogel based on PAni can act as an electrolyte by combining with carboxymethylcellulose and polyacrylamide solution
[58], which indicates that PAni-based hydrogels are not only used as electrodes but also electrolytes in supercapacitor production. The final output for the supercapacitor is when it is served as an energy storage for some applications. The production of the wearable sensors were reported using PAni-based hydrogels as supercapacitors
[59]. The authors in this work claimed that the production of the solid electroless in the preparation of supercapacitors with a sandwich structure obtained a real capacitance of 635 mF/cm
2 with hydrochloric acid as a dopant to increase the electrical conductivity. Therefore, supercapacitors produced from PAni-based hydrogels are feasible for the production of wearable electronics.
3.3.3. Poly(3,4-ethylenedioxythiophene) Polystyrene Sulfonate-Based Hydrogel
Poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) is a well-researched intrinsically-conductive polymer due to its high electromechanical properties
[12]. PEDOT:PSS has received attention not only due its electrochemical properties but also its water dispersibility, ease of production, environmental stability and easy availability
[60]. Similarly, PEDOT:PSS is water dispersible, as the responsible polystyrene sulfonate that makes PEDOT water dispersible; it decreases the electrical conductivity. Doping with ionic solution will help to move the hydrophilic PSS towards the surface after the water dispersion occurred
[12]. This property of PEDOT:PSS makes the polymer a perfect candidate for the formation of electroconductive hydrogels by combining other polymeric compounds. For instance, the PEDOT:PSS-based hydrogel with the AlCl
3-induced cross-linking was prepared using fast gelation principle and obtained a capacitance of 158 F/g at a scan rate of 50 mV/s with 84.9% capacitance retention after 2000 cycles. Ethylene glycol has been used an ionic dopant element to shift the PSS part of the conductive polymer.
The conductive hydrogel-based PEDOT:PSS has received considerable attention for the production of flexible electrodes due several reasons such as flexibility, eco-friendliness, low production cost and excellent mechanical strength
[61][62]. These properties make PEDOT:PSS-based hydrogels to be selected for the applications of wearable electronics. For instance, self-healable and stretchable hydrogels
[63], fiber hybrid-based with excellent volumetric capacitance
[61], and conductive hydrogels
[64] have been investigated for the use of energy harvesting for wearable electronics. Generally, supercapacitors for the energy harvesting-based electrically conducting PEDOT:PSS polymer have been given recent attention for various applications such as wearable electronics due to their light weight and electrochemical performance properties. Sometimes, polyvinyl alcohol (PVA) has been used as electrolyte to increase the supercapacitor performance
[65].
3.3.4. Polypyrrole (PPy)-Based Hydrogels
Polypyrrole (PPy) is a similar kind of intrinsically conducting polymer with excellent electrical properties. However, since it is a п-electron conjugated polymer type, it is brittle and limits its practical use, especially for flexible electronics
[66]. Therefore, the common way of preparing the PPy-based hydrogels is making composites to compensate its poor mechanical properties. For instance, Das, D. and S. Kurungot,
[67] reported the gelation initiated by the cross-linking of the dopant 5,10,15,20-tetrakis (4-sulfonatophenyl)-21H,23H-porphine manganese (III) chloride (MnTSPP) anion in the PPy chains, that is able to produce flexible supercapacitors that can be further used in the production of wearable clothing for various health-related applications. Another means to overcome such a poor mechanical property is wrapping PPy onto graphene-based hydrogels
[68]. The hydrogel based on PPy-graphene achieved high performance supercapacitors with large specific capacitance, good rate capability and cycle stability. This is because graphene is a 3D architecture with high surface area, which helped to improve the poor mechanical property of PPy. When PPy is presented in a hydrogel form, poor mechanical properties can be improved. For instance, the solid-state high performance and flexible supercapacitor was prepared using the PPy hydrogel
[69]. The high mechanical property with excellent elongation and flexibility makes easier use of PPy-based hydrogels for the production of wearable clothing.
The environmentally friendly process, low manufacturing cost, flexibility, lightweightdness and ultrahigh porous structure are required and preferred for the manufacturing of energy storage supercapacitors. This issues are currently realized by the combination of bio-based materials suing the lignosulfonate/polypyrrole (Lig/PPy) hydrogel (LP54)
[70]. However, electromechanical performances are weak, excepting some enhancement mechanisms that are carried out. Sometimes, without the additions of any cross-linkers, it was possible to create supercapactive PPy-based hydrogels by controlling its morphology
[71].
3.4. Cellulose-Based Hydrogels for Supercapacitor Applications
The major commercial materials for the production of supercapacitors mainly depend on carbon and carbon-based compounds in the activated carbon form because of their high surface area due to its porous structure
[72]. Most recently, cellulose-based hydrogels have also been paid some attention
[73]. Even though the base material is cellulose (bamboo), the basic principle behind it is changing the cellulose into activated carbon
[74]. Recent advances in the field of hydrogel preparation for supercapacitor applications have been focused on the use of cellulose-based hydrogels due to their outstanding properties.
The contribution of cellulose-based materials for the sustainable environment is very important. Attributed to this fact, the lignin-based biodegradable material based on electroconductive hydrogels is among them
[75]. Lignin has demonstrated a high specific capacitance of 298.6 F/g at 10 mV/s) and excellent rate performance, bringing a high energy density of 13.7 Wh k/g and outstanding capacitance retention of 89.9% after 10
4 cycles
[76]. The report in Ref.
[77] demonstrated an excellent electrochemical performance when other materials make composites with cellulose. Converting the cellulose materials into carbon materials is an important step to obtain the best conductivity
[78] and hence apply as an electrode in the supercapacitor device fabrication.
3.5. Other Hydrogels for Supercapacitor Production
There are also other hydrogels that have been used for the production of supercapacitors and in the assembly of wearable clothes. In this section, some other hydrogel materials that have been used in the production of supercapacitors for wearable clothes are discussed.
3.5.1. Chitosan-Based Hydrogels
Chitosan is a biocompatible, biodegradable and hydrophilic natural cationic copolymer
[79]. Chitosan is a unique bio-polymer, and it is possible to obtain gels out of it using different means such as the physical/chemical reticulation process (bond formation) and gels without any crosslinkers (such as the acetylation process)
[80], which helps to prepare hydrogels with various functions. Even though the best applications for chitosan are potentially the engineering scaffolds that obtain tissue repair achievements, nowadays, the use of chitosan as an active electrode for the preparation of a supercapacitor is also obtained as an equivalent attention. For instance, the authors in Ref.
[81] prepared chitosan-based hydrogels in combination with Li
+/Ag
+ and achieved an aerial capacity of 1 mF cm
−2 at a 1.8 mA cm
−2 current density, which can survive for more than 10,000 cycles without losing its capacitive performance. Efficient supercapacitors can be produced using chitosan hydrogels
[82]
As stated above, the chitosan-based hydrogels can be formed either by the use of external crosslinkers or without using such crosslinkers. Yang, H. et al.,
[83] reported the crosslinking of chitosan using HCl and obtained a capacitance of 45.9 F g
−1, energy density of 5.2 Wh kg
−1 and power density of 226.6 W kg
−1, and realized the solid-state supercapacitor using a phase separation means. The electrolyte uptake and conductivity of the transparent film was super high. In addition to crosslinking with other biopolymers, the modification of the chitosan to form hydrogels was also achieved
[84]. Yang, H. et al., prepared a modified carboxylated chitosan synthesized through a free radical graft copolymerization of acrylamide monomers and chemical crosslinking principles to obtain higher ionic conductivity, and produced an electric double layer capacitor (EDLC) with excellent performance.
For wearable applications, flexible supercapacitors are required. Flexible supercapacitors with a lightweight and simple film structure have been reported elsewhere
[85]. The flexible supercapacitors were prepared by using a blend of the chitosan/graphene@ (manganese carbonate nanoparticles) MnCO
3 polymer hydrogel and obtained a specific capacitance value of 312.5 F g
−1. In addition, the PVA/quaternary ammonium–chitosan hydrogel electrolyte was prepared for detection supercapacitors with brilliant performance
[86], which can be attached to the wearable clothes for sensing applications. The chitosan-based oxygen-doped activated carbon/graphene composite for flexible supercapacitors was also reported
[87]. This kind of supercapacitor for portable energy storage purposes can be integrated with wearable clothes for energy sources. Overall, the chitosan-based hydrogels are gaining recent attention for the making of flexible, portable and high-geared supercapacitors. These kinds of supercapacitors have the capability to be integrated with wearable clothing for smart textile applications. The other reason why chitosan-based hydrogels are gaining attention is due to its biodegradability, biocompatibility, eco-friendly nature and easy accessibility from natural resources such as plants, animals and other natural resources
[88]. Therefore, it is highly recommendable to invest in and utilize chitosan-based hydrogels for supercapacitor production and other similar purposes.
3.5.2. Sodium Alginate
Hydrogels with a electrically conductive nature are being highly explored in the fabrication of flexible and portable energy storage devices due to their excellent electrochemical properties. Sodium alginate hydrogels provide such an excellent electrochemical conductivity when doped with conductive polymers such as polypyrrole
[89], reduced graphene oxide (rGO)
[90] and Ag nanoparticles
[91], which exhibited an excellent electrochemical performance and are able to prepare high performance supercapacitors. It was possible to obtain a high capacitance, excellent rate capability and good cycling ability with the sodium alginate-based hydrogels. Furthermore, flexible supercapacitors using chitosan/sodium alginate composite hydrogels has been reported
[92], which are likely to be used for wearable clothing applications. Sometimes, the binder may not be required, and using the physical crosslinking can help in obtaining highly conductive sodium alginate hydrogels
[93]. Strain sensors with flexible supercapacitor production approach was claimed in Ref.
[94]. Such kinds of supercapacitors are highly stretchable and self-healable, which can be an excellent potential for wearable clothing. All the other information guided to understand that highly conductive sodium alginate hydrogels are capable of producing highly geared supercapacitors for wearable clothing applications. Composites of sodium alginate and rGO are able to provide a capacitance of 753 F. g
−1 at 1 A. g
−1 with a good rate capability and cycling stability up to 5000 cycles
[90].
3.5.3. MXene-Based Hydrogels
MXene is a group of two-dimensional inorganic substance with high electron density, which can be predicted to be metal except for its hydrophilic properties due to the hydroxyl- or oxygen-terminated surfaces
[95]. Its high conductivity might be used where a high electrochemical property is required. MXene-based hydrogels have been used in the preparation of flexible supercapacitors and possess excellent electrochemical performance
[96].
To increase the conductivity of the hydrogel, MXene has been combined with graphene as a gelation element and obtained a superhigh gravimetric and volumetric capacitance
[97]. For wearable clothing applications, high conductivity is not sufficient, rather flexibility and light delightedness are equally important. Flexible supercapacitors based on MXene-based hydrogels composites were made possible
[98].