Ionic liquid dispersed highly conducting polymer electrolyte for supercapacitor application: Current scenario and prospects “ICSEM 2021”

Preparation of the solid polymer electrolyte film

There are several methods employed by researchers to prepare good quality polymer electrolyte films. These include solution casting methods, hot pressing, co-polymerization, screen printing, spin coating and spray printing, etc.^9,23 The solution-casting method has been used in the majority of the research publications that we have reviewed, so we will be describing that. It is one of the most efficient methods in which 50–300 μm thick polymer films were obtained. A suitable solvent (organic/inorganic) was used to dissolve the host polymer and dopant salt separately and then mixed to obtain a homogeneous viscous liquid for casting of films. Usually, additives such as nanofillers and plasticizers were also added to the mixture to improve mechanical as well as electrical properties of the electrolyte films. The mixture was then casted over the glass or polypropylene dishes and the solvent was allowed to evaporate slowly by placing it in the oven. The solution casted films can also be air dried till a mechanically stable solid polymer electrolyte (SPE) film was produced. Finally, the obtained SPE films were conserved in a vacuum desiccator in order to prevent atmospheric contamination and moisture.

Electrode materials for supercapacitors

Carbon black, carbide-derived carbon, activated carbon, carbon nanofibers, carbon nanotubes, carbon aerogels, porous carbon, graphene quantum dots, onion-like carbon, graphene and carbon foams, and other carbon-based compounds have been used as electrode materials in supercapacitors due to their outstanding electrical conductivity, large surface area, controlled pore size distribution and light weight.^24,25

Because of its attractive properties, activated carbon (AC) was used as a common electrode material in EDLCs. The principal advantages of using AC are its low cost, large specific surface area (1,000–2,500 m² g⁻¹) and high porosity.^26,27 However, the pore size of <2 nm circumscribes the accessibility of charge carriers into the AC’s micropores. Since it serves as an obstacle for diffusion of large ions into the pores of small size,^26,28 the mesoporous carbon nanotubes (CNTs) of pore size: 2 × 50 nm have been employed by some researchers to enhance ion absorption characteristics through its unique entangled network onto the larger pores of carbon. ²⁹ CNTs also have excellent properties such as excellent electrical properties, superior mechanical stability, low mass density, high dimensional ratios, improved chemical stability and high charge-discharge capability.^30–32

Another electrode material is Graphene, which is a one atom thick, light-weight two-dimensional material. Charge carriers in Graphene travel thousands of interatomic distances without being scattered. It has been long-established as one of the most attractive and favorable electrode material because of its commendable conductivity, compatibility and larger surface area compared to other materials.^33,34 Various methods can be employed for the synthesis of graphene which comprise of micromechanical cleavage, bottom-up synthesis, chemical vapour deposition, electrochemical methods, epitaxial growth, reduction of graphene oxide and liquid phase exfoliation.^35–37 Among the aforementioned methods, reduction of graphene oxide produces a product of low-to-medium quality but with good yield.^38–40 In this review, we will be discussing the fabrication of electrodes based on graphene oxide, single walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) for EDLCs since they are widely popular.

Fabrication of EDLC cell using graphene oxide as the electrodes

A pure graphite rod extracted from a dry cell was usually chosen as source material for the synthesis of graphene oxide (GO). GO was extracted from graphite rod by applying electrochemical exfoliation method (shown in Figure 2a). A platinum wire and the graphite rod were dipped in the dilute sulfuric acid at a distance of 0.5 ± 0.1 cm. When a potential of 10 V DC was applied, the process of exfoliation begins and was continued for 4 h. Finally, to eliminate any impurities, the dispersion was washed and filtered several times. The prepared GO powder (shown in Figure 2b) was then dried at 80°C for 24 h. ⁴¹ Few of the supercapacitors with their performance are presented in Table 1.

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Figure 2.

(a) Graphical schematic exfoliation of graphene oxide from graphite⁵, (b) Prepared GO powder: graphene oxide⁹, (c) Laboratory-scale sandwiched EDLC⁹.

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Table 1.

Supercapacitors performance and details reported in author’s own laboratory.

S/no.	Electrolyte	Electrode	Conductivity	Capacitance (CV)	Capacitance (imp)	Energy density	Power density	Cycling	References
1	PVdF-HFP/EMIM DCA	MWCNT	3.42 × 10−3 S/cm	21 F/g at 100 mV/s	22 F/g at 10 mHz	_____	_____	_____	24
2	PVA/PMI-TFSI	SWCNT	7.72 × 10−6 S/cm	24 F/g at 50 mV/s	28 F/g at 10 mHz	0.31 Wh kg−1	0.26 kW kg−1	_____	25
3	PVP/NH4I/PC	Graphene oxide	8.08 × 10−5 S/cm	27.6 F/g at 50 mV/s	26.12 F/g	_____	_____	_____	41

After this, an EDLC (shown in Figure 2c) was fabricated using GO as the active material. GO was coated onto a graphite sheet acting as a current collector in the configuration: Electrode (GO)/polymer electrolyte film/electrode (GO). To prepare the EDLC cell, approximately 1 mg of GO was coated over 1 × 1 cm² of a graphite sheet with some binder followed by vacuum drying. The capacitance of the cell was then determined for maximum conducting polymer electrolyte film which was sandwiched between two GO-based electrodes.

Fabrication of EDLC cell using SWCNT and MWCNT as the electrodes

Single Walled Carbon Nanotube (SWCNT) and poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) as binder were taken in appropriate proportions, ²⁵ and thoroughly mixed in mortar and pestle using acetone to make a homogeneous paste to be used in the construction of electrodes. It is then coated on flexible high-density 0.25 mm thick graphite sheets and left overnight at room temperature before being used as electrodes. To fabricate a symmetrical EDLC, two electrodes were loaded over each other and pressed gently with the polymer electrolyte film inserted between them. EDLC fabricated in this way was then placed in a sample holder for electrochemical measurements.

For electrode fabrication with MWCNT, a fixed amount of MWCNTs, poly (vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) (w/w) as binder and acetylene black [70:20:10] were mixed using pestle and mortar. For easy grinding, the slurry was prepared using acetone and was then coated on the graphite sheets with dimension 1 × 1 cm². The prepared electrodes were kept overnight in a vacuum-dried oven at 100°C before the fabrication of the EDLC cell. Finally, efficient symmetric EDLCs were fabricated by sandwiching solid polymer electrolyte (SPE) film between two MWCNT electrodes coated over 1 × 1 cm² graphite sheet.

Supercapacitors with PVA-salt complex as electrolyte

In EDLCs, PVA complexed with salts and acids has been commonly used as an electrolyte material but only small conductivity values have been reported. ⁴² Therefore, choice of salt, additive and their respective concentrations is highly crucial. Recently, Tripathi et al. ⁴³ added PVA and KI to CH₂O₂ to optimize conductivity of polymer electrolyte. The optimized electrolyte [PVA + KI + FA (4.0 M)] was introduced between the two carbon electrodes based on biomass of banyan leaves. The resulting supercapacitor gave quite a commendable performance with energy and power densities of 20 Wh kg⁻¹ and 1.57 kW kg⁻¹, respectively, and specific capacitance of 315.4 mFcm⁻² and 147.2 Fg⁻¹.

Many other PVA-salt complexes have been utilized in the fabrication of supercapacitors and the impact of salt and its concentrations on the ionic conductivity of polymer electrolytes were widely explored. Guan et al. ⁴⁴ choose magnesium triflate (Mg (CF₃SO₃)₂ or Mg (Tf)₂) since these salts are cheap, abundantly available and could be handled easily. ¹³ Specifically, Mg²⁺ ions have good ability to coordinate with the lone pair of oxygen atoms while CF₃SO₃⁻ anion has delocalized negative charge over the sulfonate group. Triflate groups act as good leaving groups as the negative charge present on triflate is well stabilized by resonance. They employed the use of drop casting method for preparing Mg²⁺ doped SPE with PVA and TEDGME. On adding 10% TEDGME the ionic conductivity was found to be 1.43 × 10⁻⁹ Scm⁻¹ and on adding 30% TEDGME it was increased to 3.10 × 10⁻⁵ Scm⁻¹.

Addition of nanofillers to PVA-salt complex

The addition of nanofillers such as TiO₂, SiO₂, Al₂O₃, BaTiO₃ and MMT (montmorillonite-layered silicates) offers a reliable approach for improving the mechanical strength as well as the ionic conductivity of solid polymer electrolytes.^23,45 This approach has been widely used in literature and we will be discussing it in detail. Lim et al. used nano-sized TiO₂ as additive to enhance conductivity of PVA-LiClO₄ based polymer electrolytes. PVA-LiClO₄ had been explored by these researchers earlier but a very low conductivity was obtained. ⁴⁶ In their previous work, Lim and co-workers had added antimony trioxide (Sb₂O₃) as a filler ⁴⁷ to obtain composite polymer electrolyte, but the ionic conductivity recorded was very low and not appropriate for the application in electrochemical devices. In order to overcome this, Sb₂O₃ was replaced with TiO₂. ⁴⁸ The polymer electrolyte having 8 wt% of TiO₂ exhibited the highest ionic conductivity of 1.30 × 10⁻⁴ S cm⁻¹ at room temperature. The integration of TiO₂ improved the flexibility of polymer electrolyte and the EDLC cell fabricated using these electrolytes exhibited specific capacitance of 7.3–12.5 Fg⁻¹. The EDLC also showed enhanced electrochemical stability over 1,000 cycles of the charging/discharging process and a coulombic efficiency of approximately 90% over 1,000 cycles.

Other than this, Sunitha et al. ⁴⁹ prepared PVA-based nanocomposite solid polymer electrolyte (NCSPE) films where they incorporated numerous fillers like barium titanate (BaTiO₃), MMT, strontium titanate (SrTiO₃) and aluminum oxide (Al₂O₃) with particle size less than 100 nm to PVA-LiClO₄ matrix. These samples were denoted as:

A—10 wt%LiClO₄ + 70 wt%PVA + 10 wt%MMT + 10 wt%BaTiO₃

B—10 wt%LiClO₄ + 56 wt%PVA + 10 wt%MMT + 12 wt% (BaTiO₃+ SrTiO₃)

C—10 wt%LiClO₄ + 50 wt%PVA +16 wt%MMT + 8 wt% (BaTiO₃ + SrTiO₃ + Al₂O₃)

The addition of these nanofillers to PVA increased the DC electrical conductivity of the SPE films as shown in Table 2. The results from XRD and DSC show the enhanced amorphous nature of the samples with nanofillers incorporated into the polymer matrix. It was also found that ionic conductivity of the electrolyte film increases non-linearly with electric field.

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Table 2.

Details of supercapacitors reported from the author's laboratory. ⁴⁹

Samples	Ionic Conductivity of Electrolyte σo (Scm−1)	Activation energy Ea (eV)	Ionic Conductivity of Electrolyte (Scm−1) at 313 K and 100 Vcm−1
A	1.30 × 10−07	0.00176	9.127 × 10−06
B	5.81 × 10−07	0.00097	1.288 × 10−05
C	5.20 × 10−05	0.00062	8.344 × 10−04

Beenarani et al. ⁵⁰ modified PVA-H₃PO₄ polymer electrolyte by adding carbon black (CB) and KCl into the system. The OH groups present in PVA participate in H- bonding and help in incorporation of inorganic nanoparticles into the polymer matrix, leading to the formation of the polymer composite. ⁵¹ Here, MWCNT were used as an active base material for electrode preparation since it is highly electrically conductive (10⁴–10⁵ Scm ⁻¹ ), and has a large specific surface area (1,240–2,200 m² g ⁻¹ ). ⁵² Thus, a less expensive, novel, environmentally friendly and good performance PVA-H₃PO₄-KCl-CB nanocomposite was prepared. Initially, the ionic conductivity was improved by the addition of acid salt, ⁵⁰ which caused enhancement of charge carrier concentration and mobility in the polymer electrolyte. When nanofillers were added, the resultant electrolyte, PVA + 0.75 wt% KCl + 0.3 wt% CB, exhibited a specific capacitance of 102 Fg⁻¹ and was considered as the more favorable polymer nanocomposite (PNC). The EDLC performance using this PNC membrane reached a maximum energy density of 14.16 Wh kg⁻¹ (at 1 Ag⁻¹) and showed energy storage capability for 0–1 V with a power density of 1.38 kW kg⁻¹ (at 5 Ag⁻¹). The PNC membrane also displayed capacitance retention of 91.16% over 3,000 cycles, but an enhanced self-discharge of 60.63% were observed after 1,000 min.

PVA based alkaline polymer electrolytes

Alkaline polymer electrolytes are novel materials gaining popularity in supercapacitor applications; particularly the polyvinyl alcohol (PVA) based alkaline polymer electrolytes which, due to their high ionic conductivity, are valuable. ⁵³ A typical electrolyte that can be utilized is alkaline KOH solution. Studies have revealed that an alkaline PEO–KOH based polymer electrolyte displays an ionic conductivity of around 10⁻³ S cm⁻¹ at room temperature. An alkaline-blended polymer electrolyte based on PEO–PVA–KOH for use in Ni–MH ⁵⁴ and primary Zn–air ⁵⁵ batteries was reported by Yang and Lin. They also fabricated^56,57 a PVA–KOH polymer electrolyte for use in Ni–MH and Zn–air batteries. Agel et al.^58,59 developed an alkaline anion exchange membrane for fuel cells by grafting quaternary amines on epichlorohydrin polymer to give ionic conductivities of about 10⁻² Scm⁻¹ and with an anionic transport number (t⁻) greater than 0.9. Following these developments, it was believed that such alkaline polymer electrolytes could also be used in supercapacitors. Yang et al. ⁶⁰ was the first to suggest the use of alkaline PVA–KOH polymer electrolytes in EDLCs and they also examined the fabrication and distinctive capacitance properties of EDLCs. The ionic conductivity of the order of 10⁻² S cm⁻¹, which is quite high, was attained. The experimental data showed that EDLCs using PVA-based SPEs were quite stable and had outstanding cycle-life properties.

A number of PEO/PVA based alkaline thin-film electrolytes (PEO–KOH–H₂O along with PVA-KOH-H₂O) have been reported in literature.^61,62 In one of the early investigations Zhang et al. ⁶² found that PVA–KOH–H₂O had the maximum conductivity when the polymer electrolyte contained 4–6 M KOH. Furthermore, the reliance of supercapacitors on the operating temperature is of considerable practical importance in specific applications, particularly for their use in electric vehicles. Thus, it is imperative and crucial that electrochemical capacitors should be functional over wide temperatures (e.g. from −20 to 40°C) in diverse environmental settings.

Later, Yuan et al. ⁶³ fabricated a hybrid supercapacitor based on NiO and activated carbon as positive and negative electrodes respectively and the alkaline polymer gel electrolyte chosen was PVA–KOH–H₂O containing 5 M KOH. The results demonstrated that capacitive performance of the hybrid supercapacitor improved when temperatures were raised from 20 to 40°C. This is because when the temperature rose, the conductivity of the alkaline polymer gel electrolyte increased, and the charge-transfer resistance decreased. Also, much better contact at the interface between the electroactive materials and the electrolyte was established at higher operating temperature. The maximum values obtained of the specific capacitance and energy density of the hybrid supercapacitor were 73.4 Fg⁻¹and 26.1 Wh kg⁻¹ respectively at the current density of 0.1 Ag⁻¹ and the operating temperature of 40°C. If developed and enhanced further, this supercapacitor had the potential to be used in electrical vehicles, etc.

Following this, Zhang et al. ⁵⁷ added TiO₂ ceramic fillers to this matrix. It was found that the domains of amorphous region increased on incorporation of TiO₂ filler into the PVA polymer matrix. This electrolyte showed excellent electrochemical properties. The room temperature (20°C) ionic conductivity was found to be 0.102–0.171 Scm⁻¹ for the films containing 50% water content and the electrochemical stability window at the metal-electrolyte interface was of ±1.2 V for stainless steel electrodes. The specific capacitance of PVA-KOH-TiO₂-H₂O alkaline solid polymer electrolyte (ASPE) based supercapacitor was 44 Fg⁻¹ for the first charge discharge cycle and decreased to 33 Fg⁻¹ after 1,000 cycles. This ASPE base supercapacitor demonstrated excellent properties evaluated using Galvanostatic charging and discharging, cyclic voltammetry and AC impedance spectroscopy.

Polymer blends and blends doped with ionic salt

A pattern has been followed for the creation of degradable synthetic and natural polymers as well as for synthetic/natural polymer blends.^64,65 Among the numerous ways used to improve conductivity, polymer blending requires least effort while exhibiting substantially higher versatility. Researchers can utilize the blending strategy of at least two polymers so that the qualities of individual polymers can be improved. There are a few examples of this methodology, e.g. joining different polymer frameworks. For example, biopolymers along with different compounds possessing polar properties were utilized in a few industrial applications so as to strengthen the mechanical properties and to make it consistent with dopant species. ⁶⁶ This technique is frequently utilized as it can enhance conductivity, upgrade mechanical quality and the physical properties of the compositional system can also be easily controlled. It appears to be an appealing way in empowering novel polymeric materials to have attractive customized properties at a reasonable cost. K-carrageenan–chitosan, ⁶⁷ PVC–poly (methyl methacrylate) (PMMA), ⁶⁸ PVC–PEO, ⁶⁹ PVA–chitosan,^70,71 PVA-polyacrylonitrile (PAN), ⁸⁹ PVA-PMA ¹⁰¹ polymer blends have already been studied. Polymer blends consisting of chitosan and carrageenan have been also tested for the development of electrical double layer capacitors.

While EDLCs are most common there have been little developments in the area of redox supercapacitors too. Rudge et al. proposed three potential strategies, ⁷² with which the conducting polymers could be utilized in redox supercapacitors. According to the first strategy the two electrodes containing indistinguishable and symmetrical p-dopable conducting polymers are utilized and called Type I, while in Type II, two distinctive p-doped conducting polymers are utilized. In third strategy i.e. Type III, both n-and p-doped conducting polymers could be used. While liquid electrolytes like acetonitrile ACN –Et₄NBF₄, ACN –Bu₄NPF₆, ⁷³ ACN –LiClO₄, ⁷⁴ acetonitrile ACN – Me₄NCF₃SO₃, PC –Et₄NBF₄ and propylene carbonate (PC)–LiClO₄ etc. ⁷⁵ are the basis of the majority of studies on the redox supercapacitors they are not that efficient. To improve the supercapacitor, solid electrolytes are needed but only a limited number of solid state redox supercapacitors based on polymeric/gel electrolytes have been reported, namely polyethylene oxide (PEO) –PC –Et₄NBF₄, polyethylene glycol (PEG) –Et₄NBF₄, ⁷⁶ PEO –PEG –LiCF₃SO₃,^77,78 PMMA –PC –ethylene carbonate (EC) –LiClO₄, ⁷⁹ etc. Solid state redox supercapacitors require a lot of improvements in their performance as they are in the initial phase of their development. Hashmi et al. ⁸⁰ studied - Type I solid state redox supercapacitors which were based on poly (3-methyl thiophene) (pMeT) and polypyrrole (pPy) conducting polymer electrodes and polymeric gel electrolytes made up of polyvinyl alcohol (PVA)–H₃PO₄ blend. Polypyrrole based PVA–H₃PO₄ and pMeT-PVA–H₃PO₄ Type I supercapacitors showed 10 mF cm⁻² and 2.5 mF cm⁻² of capacitance and coulombic efficiency of 90 and 82% respectively. The charge/discharge characteristics were observed in the former supercapacitor with a discharge capacitance of 8.0–15.0 mF cm⁻². The specific capacitance of the same order was observed, and the working voltage range was restricted between 0.5 and 1.0 V.

PVA-H₃PO₄ polymer electrolyte could also act as an electrolyte separator. PVA-H₃PO₄ electrolytes have also been reported by Pettyweeks and Polak ⁸¹ and Hashmi et al.^78,82 Hashim et al. ⁸³ used modified PVA-H₃PO₄ electrolyte by adding cellulose (filter paper) which boosted the mechanical strength of the electrolyte. Owing to the fibrous structure of the electrolyte a link between the PVA-H₃PO₄ polymer electrolyte domains could be established. These linkages offer extra paths for the proton motion and increases the current during charging/discharging. The H⁺ ionic species produced by the phosphoric acid (H₃PO₄) are entrapped in PVA matrix and in the filter paper pores. The EDLC formed by sandwiching the electrolyte between two carbon electrodes exhibited good charge/discharge characteristics with 30 Fg⁻¹ of capacitance and 99% coulombic efficiency.

Using the same blend electrolyte, Akbulut et al. ⁸⁴ fabricated and characterized three dimensional (3D) nanostructured supercapacitor microelectrode arrays on a graphite sheet as substrate containing MnO₂ and carbon nanotubes (MnO₂/CNTs) without any additives or binders. At the scan rate of 1 mVs⁻¹, a very high capacitance value of 1.4 F was observed for the solid-state supercapacitor cell which corresponds to a specific power value of 73.9 W kg⁻¹ and specific energy value of 115.2 Wh kg⁻¹. The continuous current charge/discharge experiments at 1 mA current yielded capacitance of 520 mF providing specific energy value of nearly 44 Wh kg⁻¹, while at a discharge current of 20 mA specific power of 38 W kg⁻¹ was observed. Excellent cycle life with >89% capacitance retention at 20 mA discharge current after 3,000 cycles was also observed.

Kumar et al. ⁸⁵ later provided the first ever report on biodegradable polymer blend electrolyte based on PVA and polystyrene sulfonic acid (PSA) for use in supercapacitors. The electrolyte films displayed good ionic conductivity and strength. The ionic conductivity increased with the increase of polystyrene sulfonic acid content in the blend. The blend film with a PVA: PSA composition of 3:7 exhibited maximum conductivity of 2 × 10⁻² S cm⁻¹. A high capacitive nature and absence of relaxation processes was indicated by dielectric studies. The carbon–carbon supercapacitor developed using this electrolyte displayed good double layer capacitive features and presented 40 F g⁻¹ of capacitance with a scan rate of 5 mV s⁻¹. The supercapacitor was found well stable during charge/discharge cycling and exhibited high coulombic efficiency in the range of 98–99%.

Until now we only saw simple polymer blends. With time, researchers started complexing the blends with salts to further enhance the properties. This started when Kadir et al. ⁸⁶ experimented with PVA-Chitosan (CS) polymer blend which is one of the most explored blends. They prepared a modified polymer blend electrolyte which consisted of (11 wt% PVA-7 wt-% CS–12 wt-% NH₄NO₃–70 wt-% EC. This plasticized electrolyte exhibited a conductivity value of 1.63 × 10⁻³ S cm⁻¹, a breakdown voltage at ∼1.70 V, and could perform for 100 cycles at current densities of 0.095 and 0.381 mA cm⁻². The EDLC fabricated were found to have capacitance of 27.1 and 16.7 F g⁻¹ for the 0.095 and 0.381 mA cm⁻² applied current densities respectively.

Similar work was done when dextran polymer, a polymer released by surplus sucrose solution, was used by Aziz et al. ⁸⁷ Dextran structure can be made by the breaking of 1,6-α-d-glucopyranosidic bonds. Also, it is a non-toxic and biodegradable polymer. ⁸⁸ It was initially observed that there were two main functional groups, -OH and glycosidic bond, present in the backbone of dextran that contained the lone pairs of electrons which are important for the ionic conduction.^89,90 Therefore, PVA: dextran-based proton conducting solid polymer blend electrolytes (SPBEs) doped with different amounts of ammonium iodide (NH₄I) were fabricated. When 20 wt% salts was added, high DC conductivity of 2.08 × 10⁻⁵ scm⁻¹ was obtained. It was discovered by Linear Sweep Voltammetry (LSV) that the electrochemical stability of the SPE blend sample is 1.3 V that made it a good candidate for use in supercapacitors. The constancy in specific capacitance (C_sp) was observed from the first cycle to 100th cycle with an average of 4.2 Fg⁻¹.

Saadiah et al. ⁹⁰ incorporated PVA into carboxymethyl cellulose (CMC) that can affect the polymer’s structure as well as enhance the stability along with the ion conduction properties because of formation of hydrogen bonds. Hence, it should be mentioned that CMC has significant potential which can be explored predominantly in polymer electrolytes. CMC consists of a hydrophobic backbone and numerous hydrophilic carboxyl groups and hence it shows water-soluble characteristics. ⁹¹ Although CMC has conductivity of only ∼10⁻⁸ Scm⁻¹ at 303 K, ⁹² CMC-PVA based blend polymer electrolytes doped with NH₄Br were effectively prepared. ⁹³ The highest dc conductivity of 3.21 × 10⁻⁴ S cm⁻¹ at ambient temperature (303 K) and lowest T_g value was recorded for the sample which contained 20 wt% NH₄Br. The breakdown potential for this electrolyte was observed as 1.55 V. For 2 mV s⁻¹ of scan rate, specific capacitance obtained was ∼17 F g⁻¹. The EDLC fabricated was charged-discharged up to 1,600 cycles and at room temperature the average values of energy density, specific capacitance and power density were calculated to be ∼1.19 Wh kg⁻¹, ∼11 F g⁻¹ and ∼31.36 W kg⁻¹ respectively. The cycling efficiency remained constant at ∼88% for up to 1600th cycles, which is indicative of the fact that a new vision is possible for the application of this polymer blend-based CMC-PVA in energy storage devices.

When considering the proton conducting PVA-based blend polymer membranes like starch-PVA, PVA-poly (vinylacetate), PVA-poly (acrylonitrile), PVA-poly (ethylene glycol), PVA-poly (glycerol suberate) (PGS) polyester, PVA-poly (vinylpyrrolidone),^94–99 with an ionic salt like NH₄SCN, there were a couple reports available. In particular, the PVA: PAM polymer blend was prepared via solution casting method. ¹⁰⁰ PVA-PAM based hybrid membranes with electrolyte material of cesium salts of heteropolyacids were also reported by Halen et al. ¹⁰¹ for direct methanol fuel cell (DMFC) applications. For supercapacitors, Swaminathan et al. ¹⁰² explored solid blend polymer electrolyte based on PVA: PAM: NH₄SCN. In their study, they used an ultrasound-assisted method for the preparation of PVA: PAM: NH₄SCN blend membrane and observed that 20 wt% of salt-added film had lower T_g, better electrical conductivity (1.66 × 10⁻⁴ S cm⁻¹), dielectric nature, smooth surface and specific capacitance of 1230 mF g⁻¹.

Guar gum (GG) as a binder was for the first time used by Sumana et al. ¹⁰³ They developed a SPE which provided constant backing to ions towards the electrode and retained the ions in its tubular structure during charging. An optimized ratio of LiClO₄ and GG/PVA were used to form a blend polymer electrolyte film for the supercapacitor. Movement of Li⁺ ions were made possible by the channel created by the microcannular network created between electrode material and electrolyte. These microcannular structures assisted in overcoming the loss of energy density caused by sudden discharge in the conventional systems, as evident during charge/discharge studies. Also, a decrease in resistance was noticed from the AC impedance studies. This conveyed the importance of jelly GG in both the regions. The specific capacitance at 5 mVs⁻¹ observed was 542 Fg⁻¹ with time constant of 0.4 s and 99% Coulombic efficiency with consistent cyclic pattern during galvanostatic charge/discharge studies.

Polymer blends doped with nanofillers

Other than being added to the polymer and salt matrix like mentioned previously, nanofillers can also be added directly to the polymer matrix. Graphene and its allotropes have sparked significant interest in the forefront of materials research during the last two decades due to its exceptional characteristics and enormous application possibilities. Graphene is capable of replacing the metal conductors in electronic applications because of its extraordinary electrical properties in comparison to other carbon based nanofillers.^104,105 A practical course for the graphene synthesis is the exfoliation of graphite into its derivatives like graphene oxide (GO). However, GO forms a stacked structure owing to its high aspect ratio when compared with other carbon nanofillers, and Van der Waal forces of interactions which tend to accumulate clusters in polymer. In order to remove this concern, some chemical or surface moderation is the necessity, where hydrophobic or hydrophilic groups are grafted to avoid agglomeration. ¹⁰⁶ This method is a scalable process and it enables the production of GO having sp² and sp³ hybridized carbons with carbonyl and carboxyl groups at the edges and hydroxyl and epoxy functional groups on its basal plane.^107–109 The presence of oxygen containing functional groups makes GO hydrophilic which is easily dispersible in water. Due to hydrogen bonding interactions GO readily exfoliates in protic solvents and water.^110,111 Therefore, polymer/GO composites processing is better than polymer/graphene composites due to presence of oxygen-containing functional groups (epoxy, carbonyl, hydroxyl) on the surface of GO providing compatibility between polymers and GO resulting in the fabrication of number of high-performance polymer/GO composites.^112–115

Recently, the dielectric properties of pristine GO exhibiting high dielectric constant (∼10⁶) at 1 KH with a low dielectric loss were reported by Kumar et al. ¹¹⁶ GO in various other studies^117–120 were found to be exceptional filler for developing low dielectric loss and high dielectric constant polymer composites for energy storage applications. The composites of PVDF which were combined with PVA modified reduced graphene oxide (PVA-RGO) found to have higher values of dielectric constant and low dielectric loss in comparison to PVDF/RGO composites. ¹²¹ However, uniform dispersion of GO in the polymer matrix is necessary to get superior characteristics of GO. Deshmukh et al. developed GO reinforced novel polymer composites containing poly (4-styrenesulfonic acid) (PSSA) and PVA blend matrix by means of colloidal processing technique. Just by loading 3 wt% GO in PSSA/PVA the dielectric constant increased from 82.67 to 297.91 at 50 Hz and 150°C temperature, although a slight increase in dielectric loss was observed from 1.56 to 2.64. These investigations gave new knowledge to manufacture adaptable, high-k dielectric composite as a promising material for energy storage applications. ¹²²

GO displayed extremely low electrical conductivity that was attributed to the presence of functional moieties. So, in order to overcome this problem, the GO can be reduced or modified to enhance the (π-π) sp² network in comparison to pristine GO. ¹²³ Therefore, Yang et al. reported that 14% nanofiller addition to reduced GO-PVA nanocomposite found to have electrical conductivity of 5.92 S m⁻¹. GO was also added to the optically active polymer, Chitosan which despite having a lot of advantages like biodegradability, removal of heavy metals toxicity, antimicrobial applications, bears disadvantages like low mechanical properties and processability that limits its applications. A silane functionalized agent like 3-aminopropyltriethoxysilane (APTS) was also developed which lead to covalent bonding between GO and polymer matrix. It is also well known that carboxylate-modified GO exhibit strong interfacial interactions, ¹²⁴ and Tris-hydroxymethyl amino-methane (TRIS) which forms amine and amide bonds with the edges of GO sheets, leads to homogeneous dispersion and better characteristics. Therefore, Shekhar et al. ¹²⁵ prepared various GO-polymer loadings by functionalizing GO using the above mentioned reagents, 0.7% GO-PVA-CS, neat PVA-chitosan, GO-APTS-PVA-CS, GO-TRIS-PVA-CS and GO-COOH-PVA-CS. Silane-treated PVA-CS displayed the highest mechanical strength due to its strong bonding through amino groups and less misalignment of nanoparticles in the polymer matrix. The presence of reinforcing agents in GO play an important role in morphological and thermal properties. Thermal stability was found to be improved for all the developed nanocomposites due to the presence of Van der Waal forces of attraction and hydrogen bonding among polymer matrix and nanofiller.

Other nanofillers were also used. Singh et al. ¹²⁶ loaded SiO₂ nano-particles and observed its effect on dielectric relaxation behavior and electrical transport properties of the PVA-PVP-NaI based nanocomposite polymer blend electrolytes (NCPBEs). The frequency dependent AC conductivity with different concentrations of SiO₂ in electrolytes follows the Jonscher power law (JPL) and it was also noticed that mobility varied with the concentration of SiO₂. Sample with 0.6 wt% of SiO₂ has showed optimum DC conductivity (σ_dc ∼ 5.07 × 10⁻⁶ Scm⁻¹) and minimum relaxation time.

PVA based hybrid polymer electrolytes

Hybrid polymer electrolytes (HPEs) show great potential from the preparation perspective because they utilize the blending method. Hybrid polymers are also ideal as a host polymer since they have more complexation sites to promote better ionic conduction. The structural and electrical properties of HPEs could also be modified to make it more useful in various applications. The physical properties of HPEs, such as crystallinity and thermal properties and mechanical strength can be tailored easily through the blending method ¹²⁷ providing an exceptional combination of physical properties due to its dependent characteristics.

Hashim et al. ¹²⁸ developed a highly conducting hybrid solid polymer electrolyte (HSPE) in which a host polymer, polyvinyl alcohol and ionic dopant, phosphoric acid are taken. Due to its porous nature, Whatman filter paper was used which helped in increasing the electrical conductivity by supporting the electrolyte system. The sample which consisted of 70% loading of the acid displayed a conductivity of 1.04 × 10⁻⁴ S cm⁻¹. Specific capacitance of the EDLC fabricated was found to be 34 F g⁻¹ and when the cell was charged-discharged at 10 mA a very less internal resistance is seen which is 1 Ω and 85–97% efficiency is comes out.

The same group developed yet another powerful and cost-effective strategy ¹²⁹ for creating an improved supercapacitor incorporated with PVA as Hybrid Solid Polymer Electrolyte (HSPE) and a multi-walled carbon nanotube. The electrolyte showed high electrical conductivity of 6.42 × 10⁻⁴ Scm⁻¹ and 1.84 × 10⁻⁴ Scm⁻¹ with a gap of 7 days at very low resistance. The fabricated supercapacitor was found to have a very good lifetime of more than 1,000 cycles and efficiency of 90% under operating potential window of 0.8 volts. The specific capacitance, power density and energy were calculated to be 33.3 Fg⁻¹, 8.3 Jg⁻¹s⁻¹ and 16.7 Jg⁻¹ respectively.

Zulkifli et al. ¹³⁰ in their report showed that the conductivity and stability of a CMC-PVA blend structure were improved by using a blending technique in comparison with a single host polymer. They devised hybrid polymer electrolytes consisting of a CMC-PVA blend doped with various amounts of LiNO₃ using the solution casting technique. The optimum ionic conductivity they got was 3.54 × 10⁻³ S cm⁻¹ at 20 wt% concentration of LiNO₃ at which the polymer electrolyte film showed the lowest percentage of crystallinity. The stability window of the highest conducting sample was found out to be 1.43 V. From GCD profile it could be seen that the prepared EDLC was stable up to the 5000th cycle with average specific capacitance of ∼100 Fg⁻¹. These improvements were attributed to the number of free mobile ions which could diffuse easily even in the solid state to promote better conduction. The addition of ionic dopants such as Li⁺, Na⁺, Mg²⁺, and H⁺ can provide more ionic conducting species. Since polymer blends could offer a greater number of complexation sites, these charge-carrying species are free to migrate in large numbers without restriction, resulting in an increase in ionic conductivity. The highest ionic conductivity was obtained for the sample with 20 wt% LiNO₃ due to the enhancement of amorphous properties that was driven by the interaction at the active complexation sites and the surface morphology of the system. Based on the results, CMC–PVA doped with LiNO₃ portrayed well to be used as a potential polymer host in energy storage applications.

Adding ionic liquid as additive

Ionic liquids are molten salts comprising of ions having number of attractive features like high thermal and electrochemical stability, ion content, non-flammability, non-volatility, large potential window with high energy density, low viscosity^131–133 as well as environment friendly nature. ¹³⁴ Addition of ionic liquid as dopant (Figure 3) is a promising approach for improving the ionic conductivity of the polymer electrolyte.^20,22 Ionic liquids are used in supercapacitors since there is good compatibility between the ionic liquid and polymer matrix which is essential to obtain higher ionic conductivity value and excellent mechanical properties.

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Figure 3.

Schematically shown the doping effect of low viscosity IL into the matrix of polymer electrolyte (a) Polymer electrolyte doped with a potassium salt (b) Polymer electrolyte after the addition of ionic liquid to the salt doped electrolyte. ¹³⁵

Kang et al. ¹³⁶ developed solid-state flexible supercapacitors by means of carbon nanotubes, regular office papers and ionic-liquid-based composite gel electrolytes. Synthesis of gel electrolytes was done by mixing of ionic liquids with fumed silica nano powder and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][NTf₂]) in PVA. The supercapacitor prepared, exhibited high energy density and power performance, with a specific capacitance of 135 F g⁻¹ at 2 A g⁻¹. The energy density and power obtained for the supercapacitors were 41 Wh kg⁻¹ and 16.4 kW kg⁻¹ respectively. The energy density, specific capacitance and power of this supercapacitor observed to change by less than 5% over 100 bending cycles with a radius of 4.5 mm. Their research conveyed that the performance of ionic liquid doped supercapacitors and silica-based gel electrolytes were better in comparison to the more frequently used H₂SO₄/PVA-based electrolytes. Thus, the use of gel electrolyte-based supercapacitors containing CNT/paper has great potential for flexible energy storage applications because of their high-performance and low-cost.

Another group, Liew et al. published a series of research papers where they selected ammonium acetate as doping salt and BmImBr, ¹³³ BmImCl, ¹³⁷ BmImI ¹³⁸ and BmImTf ¹³⁹ ionic liquids to obtain polymer electrolyte. Ammonium acetate (CH₃COONH₄) was preferred because of its plasticizing effect. Previously, the PVA-CH₃COONH₄ polymer system had been extensively studied by Hirankumar et al.^140–142 but the ionic conductivity was too low due to which it could not receive appreciation for application in electrochemical devices. Liew et al. in their first paper used Poly (vinyl alcohol) (PVA)/ammonium acetate (CH₃COONH₄)/1-butyl-3-methylimidazolium chloride (BmImCl) to obtain polymer electrolytes using solution casting method. Temperature played a role in increasing ionic conductivity and that happened because of the fast-molecular vibration modes generated in the polymer membrane upon heating. Different compositions of the ionic liquid were taken in different samples. Ionic conductivity of 7.31–0.01 mS cm⁻¹ at 120°C was displayed by the polymer electrolyte which contained 50 wt% of BmImCl. Upon impregnation of the ionic liquid, the T_g of polymer electrolytes was lowered down to sub-ambient value while the capacitive nature of EDLC was substantially increased. The fabricated EDLC could be charged up to 4.8 V and gave the specific capacitance of 28.36 Fg⁻¹. In addition to the improvement in the interfacial contact between electrode and electrolyte, the electrochemical property of supercapacitors was increased, and the cell demonstrated power density of 19.79 W kg⁻¹ and energy density of 2.39 Wh kg⁻¹. The synthesized polymer electrolyte was a promising candidate as more than 90% of the Coulombic efficiency was retained over 500 charging and discharging cycles.

By using solution cast technique Liew and coworkers also prepared PVA/CH₃COONH₄/BmImBr protons conducting polymer electrolytes. With 60 wt% of BmImBr obtained a maximum ionic conductivity of (9.29 ± 0.01) mScm⁻¹ at ambient temperature. This amorphous conducting polymer electrolyte exhibited lower values for glass transition temperature (T_g), crystalline melting temperature (T_m) and crystallization temperature (T_c) than the ionic liquid-free polymer electrolytes. Use of ionic liquid improved electrochemical potential window as well as thermal stability of polymer electrolyte. The EDLC fabricated possessed high specific capacitance of 21.89 Fg⁻¹ which was way higher than that of a supercapacitor fabricated using ionic liquid free polymer electrolytes. In the case of PVA doped with CH₃COONH₄ as salt and BmImI as the ionic liquid, the results exhibited VTF (Vogel Tamman Fulcher) relationship. The electrochemical potential window was improved from 3.3 V to 3.8 V. This framework was appropriate for EDLC due to its high energy and power densities. The specific capacitance of EDLC utilizing ionic liquid-included polymer electrolyte is higher than that of the EDLC involving polymer electrolytes without ionic liquid and the Coulombic efficiency was observed to be >80% all through 500 charge and discharge cycles.

BmImTf was yet another ionic liquid selected by Liew and co-workers because of its appealing cations and high delocalization of triflate anions. When the ionic liquid was added to the polymer matrix, the potential stability window and the conductivity were considerably enhanced. The Tg decreased significantly, but the ionic conductivity of polymer electrolytes was discovered to rise with temperature. The specific capacitance of the created EDLC was improved from 0.14 F g⁻¹ to 2.02 F g⁻¹. Specific discharge density of 3.35 F g⁻¹ was measured in the charge–discharge experiment. The created EDLC could deliver power densities and energy of 18.37 kW kg⁻¹ and 0.17 Wh kg⁻¹ respectively.

Jang et al. ¹⁴³ gave an account of a functionalized activated carbon (AC) supercapacitor which consisted of PVA/H₃PO₄/ionic liquid (1-ethyl-3-methylimidazolium tetrafluoroborate, [EMIM]BF₄) as redox-mediated gel polymer electrolyte. Different composition of [EMIM]BF₄ (0, 25, 50, 75, 100 wt%) in PVA/H₃PO₄ gel polymer electrolyte was taken and analyzed. There was an increase in the specific capacitance because of the improved ionic conductivity, but extra pseudo-reaction is observed to happen in the electrode/electrolyte interfaces. Using PVA/H₃PO₄/[EMIM]BF₄ (50%) as electrolyte, the specific capacitance of the supercapacitor was improved up to three folds and showcased a high value of specific capacitance, 271 Fg⁻¹ at 0.5 Ag⁻¹ discharge current, which is a lot higher than PVA/H₃PO₄ (103 Fg⁻¹) based supercapacitor. This supercapacitor also showed increased power density and energy of 23.88 kW kg⁻¹ and 54.3 Wh kg⁻¹. Also, the supercapacitor indicated similar specific capacitance and that retained even after 3,000 patterns of charge/discharge.

Other than the widely used CH₃COONH₄ salt, there are other salts like sodium salts that can be used with ionic liquids. These days sodium is much cheaper, easily available and its reserves are abundant than lithium which makes it better choice for energy storage applications. However, due to large size sodium ions suffer from sluggish kinetics. This can be compensated by the addition of various additives like ionic liquids. Based on this Farah et al., ¹⁴⁴ developed SPE using PVA host polymer, NaTf salt and ionic liquid 1-butyl-3-methylimidazolium bromide (BmImBr) by solution casting method. The prepared SPE was sandwiched in between carbon-based electrodes to fabricate EDLC. The results show that by addition of 50 wt% BmImBr ionic liquid in PVA with PVA: NaTf weight ratio of 60:40 ionic conductivity is enhanced from 4.87 × 10⁻⁶ Scm⁻¹ to 2.31 × 10⁻³ Scm⁻¹ at room temperature. It was also found to be thermally stable up to at least 236°C and provided a maximum specific capacitance of 16.32 F g⁻¹ at 3 mVs⁻¹. The long-term cycling performance of the supercapacitor over 1,000 cycles was conducted by repeating the charge/discharge test at a current density of 200 mAg⁻¹. Initially, the specific capacitance was stable up to 350 cycles. With further charge/discharge cycling, a slow decrease was observed and after 1,000 cycles, the capacitive retention dropped by 30%. The supercapacitor also exhibited coulombic efficiency that remained at 77–87% over 1,000 cycles. The supercapacitor cell remained stable until the 150th cycle and then there was a slight drop until the 1000th cycle and the efficiency reached to the minimum value of 81%. This conveyed that the electrode and electrolyte were in close contact with each other.

Siyahjani et al. ²⁴ created a solid gel polymer electrolyte including a low viscosity ionic liquid, i.e. 1-propyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide (PMI-TFSI, viscosity 38 cP at 20°C) and PVA utilizing solution cast technique. They blended PMI-TFSI with PVA which gave extra ions for charge transport and diminished the crystalline framework of PVA and introduced exceptional improvement in electrical conductivity (Figure 4). The X-Ray diffraction pattern, DSC curves and optical micrographs of PVA and PVA: 250 wt% IL containing gel polymer electrolyte (GPE) films are shown in Figures 5–7 respectively. The EDLC fabricated utilizing SWCNT as symmetric cathodes and PVA: 250 wt% PMI-TFSI as electrolyte displayed a capacitance of around 28 F g⁻¹.

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Figure 4.

The ionic conductivity as a function of composition for PVA: x wt% IL (x = 0, 40, 80, 120, 170, 250, 300) GPE films. ²⁴

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Figure 5.

X-ray diffraction pattern of PVA and PVA: 250 wt% IL containing GPE films. ²⁴

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Figure 6.

DSC curves of (a) pure PVA and (b) PVA: 250 wt% ionic liquid-incorporated gel polymer electrolyte films. ²⁴

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Figure 7.

Optical micrographs of (a) pure PVA and (b) PVA: 250 wt% ionic liquid incorporated gel polymer electrolyte films. ²⁴

A nonexclusive way to construct all-solid-state supercapacitors was introduced by Hong et al. ¹⁴⁵ in their research. They did that by incorporating PVA based solvent exchangeable layers and super-flat vertically aligned single-walled carbon nanotubes (VA-SWNTs). The permeable PVA film acts like a sponge retaining the ionic liquid electrolytes (H₃PO₄ watery electrolyte or 1-butyl-3-methylimidazolium chloride (BMIMCl including 15% water)). Their reconfigurable solid-state adaptable capacitor demonstrated good electrochemical properties and incredible cycling strength. The subsequent ionic liquid electrolyte-based supercapacitor displayed stable behavior for 0–2 V, yields high capacitance, good long-term stability and energy, and power performance. This one-of-a-kind element of reconfigurable electrolyte and nanostructured electrode showed high energy densities and power, as well as noteworthy stability over 10,000 charge/discharge cycles. The incorporation of reconfigurable electrode/electrolyte frameworks permitted the manufacture of exceptionally lightweight, adaptable, all-solid-state, and elite supercapacitors for application in the convenient, wearable electronic and auxiliary energy devices for use as a feature of a wing or fuselage of communication satellites, unmanned aeronautical vehicles, airplanes, submarines, and others.

Another work was done by taking ammonium acetate as dopant salt. In the work done by Rag et al., ¹⁴⁶ 1-ethyl-3-methylimidazolium ethyl sulphate ([EMIM][EtSO₄]) was chosen as ionic liquid and a combination of 1-ethyl-3- methylimidazolium ethyl sulphate. PVA and ammonium acetate were used as base materials for preparation of electrolyte. The conductivity was found to be 6.56 × 10⁻⁴ S cm⁻¹ which is appropriate for application in supercapacitors. The capacitance obtained of the supercapacitor was 138 F g⁻¹ and the Coulombic efficiency was found out to be 98%.

Polymer blends with ionic liquid as additive

Ionic liquids can also be added to a blend of two or more polymers. Recently, an innovative and simple approach was introduced by Yadav et al., ¹⁴⁷ to obtain supercapacitors with enhanced performance by modifying the electrolyte. For comparative research, GPEs with symmetrical electrodes of graphene nanoplatelets (GNPs), multiwalled carbon nanotubes (MWCNTs) and biomass-derived activated charcoal (AC) were used to fabricate quasi-solid-state supercapacitors. Activated carbon-based electrodes show higher capacitive responses over other carbon-electrodes due to their appropriate porous texture with hierarchical porosity. Here, an ionic liquid (IL)- blended redox-active gel polymer electrolyte (GPE) based on polymer blend of poly (vinyl alcohol) (PVA) and poly (vinyl pyrrolidone) (PVP) was presented for supercapacitor applications. The Gel Polymer Electrolyte, comprising ionic liquid 1-ethyl-3-methylimidazolium hydrogen-sulphate (EMIHSO₄), added with hydroquinone (HQ), immobilized in PVA/PVP. Hydroquinone (HQ) is a derivative of the quinone family, whose redox activity plays a significant role in the enhancement of capacity in energy storage devices.^148,149 The redox reaction of hydroquinone involves the transfer of two electrons and two protons. Due to the redox reaction to quinone, hydroquinone enhances the electrolyte’s ionic conductivity and specific capacitance of the device.

The better electrochemical, thermal and mechanical properties and optimum ionic conductivity of ∼9.3 m Scm⁻¹ (Figure 8) at room temperature of this IL-based GPE film make it attractive as a substitute of not only liquid electrolytes but also an excellent replacement of GPEs like PVA-H₂SO₄, etc. The redox processes, owing to reversible HQ⇌Q reactions at the interfaces, as observed in CV responses in the form of redox peaks and plateau regions in GCD patterns, are responsible for the substantial improvement in charge/energy storage capacity as compared the devices without redox-active electrolyte. The comparative study (Figure 9) indicated the superior performance of AC-electrodes with redox active GPE in terms of specific capacitance (∼485 F g⁻¹) and specific energy (∼24.3 Wh kg⁻¹) with respect to MWCNTs (26 F g⁻¹ and 1.3 Wh kg⁻¹) and GNPs (106 F g⁻¹ and 5.3 Wh kg⁻¹). A further comparison indicated the better performance of supercapacitor with IL-based GPE, and AC electrodes as compared to the device with H₂SO₄.based GPE (Specific capacitance ∼218 F g⁻¹ and specific energy ∼4.9 Wh kg⁻¹).

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Figure 8.

(a) Variation of ionic conductivity of GPE as a function of HQ contents (inset shows photograph of GPE solutions containing (i) 0.3 g, and (ii) 0.4 g of HQ), (b) temperature dependent ionic conductivity of GPE-1 (EMIHSO₄-based GPEs without redox additive) and GPE-2 (EMIHSO₄-based GPEs with redox additive) (inset shows lnσT^1/2 versus 1/(T-To) plot), (c) LSV patterns of GPEs in anodic (black) and cathodic (red) regions to estimate electrochemical stability windows, and (d) TGA curves of GPE-1 and GPE-2. ¹⁴⁷

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Figure 9.

(a) Comparative cyclic voltammetric curves for AC-based supercapacitor cells Cell#5 (with GPE-1) and Cell#6 (with GPE-2) recorded at scan rate of 10 m V s-1, (b) CV curves of the supercapacitor Cell#6 (with GPE-2 containing HQ) at different scan rates, (c) cathodic and anodic peak currents versus square root of scan rates for Cell#6, and (d) CV curves of a typical AC-based Cell#6 at different voltage ranges. ¹⁴⁷

The vast difference in the porous nature of the different carbon materials, used in the present studies, is responsible for substantial difference in performance characteristics of the cells. The Activated Carbon-based capacitor shows optimum performance characteristics due to the synergistic effect of hierarchical porous nature of activated carbon and redox active behaviour of the IL based gel polymer electrolyte. Effect of redox activity at the interfaces has also been observed on the cyclic performance of the EDLC. The optimized device with AC-electrodes offered ∼100% Coulombic efficiency and almost stable capacitance up to 5,000 charge-discharge cycles with ∼24% initial fading. In the solid polymer electrolyte film which was developed via solution cast technique using 1-ethyl-3-methylimidazolium tricyanomethanide incorporated in PVP and NH₄I complex, the conductivity was found to be increased to 4.0*10⁻⁴ S/cm when an IL ranged from 0 to 4 wt% in the complex. The supercapacitor developed from the film showed a specific capacitance of 15.8 F/g at scan rate 20 mV/s in a voltage range 0–1 V. ¹⁵⁰ The various supercapacitors fabricated with electrodes/electrolytes are incorporated in Table 3.

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Table 3.

Supercapacitors based on polymer electrolytes.

S/no.	Electrolyte	Electrode	Conductivity	Capacitance (CV)	Capacitance (Imp)	Energy density	Power density	Cycling	References
1	PVA/KI/formic acid	Activated carbon	10−1 S/cm	253.3 F/g at 1 mV/s	222.6 F/g at 1 mHz	20 Wh kg−1	1.57 kW kg−1	_____	43
2	PVA/KCl/carbon black	MWCNT	13.65 × 10−3 S/cm	102 F/g	_____	14.16 Wh kg−1	_____	_____	50
3	PVA/PSA	Activated carbon	2 ×10−2 S/cm	40 F/g at 5 mV/s	_____	_____	_____	_____	85
4	PVA/Gaur gum/LiClO4	Activated carbon	9.6 × 10−3 S/cm	542 F/g at 5 mV/s	_____	2854 Wh kg−1	3780 W kg−1	_____
5	PVA/CH3COONH4/BmImBr	Activated carbon/carbon black/carbon nanotube	9.29 ± 0.01 S/cm	21.89 F/g	21.63 F/g	2.18 Wh kg−1	41.27 kW kg−1	70–89% after 500 cycles	133
6	PVA/CH3COONH4/BmImCl	Activated carbon	(7.31 ± 0.01) mS/cm	28.36 F/g	27.76 F/g	2.39 Wh kg−1	19.79 W kg−1	98% after 500 cycles	137
7	PVA/CH3COONH4/BmImI	Activated carbon	9.63 mS/cm	52.78 F/g	53.38 F/g	6.92 Wh kg−1	50.25 kWkg−1	80% after 500 cycles	138
8	PVA/CH3COONH4/BmImTf	Activated carbon/carbon black/carbon nanotube	(2.37 ± 0.02) mS/cm	2.02 F/g	_____	0.17 Wh kg−1	18.37 kW kg−1	____	139
9	PVA/CH3COONH4/[EMIM][EtSO4]	Reduced graphene oxide	6.56 × 10−4 S/cm	138 F/g at 10 mV/s	_____	0.27 Wh g−1	32.3 W g−1	_____	146