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The recent development toward a suitable and safe supercapacitor has been achieved by developing various polymer electrolytes. Different polymer electrolytes have one common purpose to achieve high ionic conductivity, low crystallinity and hence high energy density/power density when used as the electrolyte in supercapacitor. This section reviews the significant research findings on various polymer electrolytes for supercapacitor applications. The incorporation of boron-containing segments in the polymer matrix appeared as a very attractive approach. Boron atom acts as an acidic center site due to an empty p-orbital and it supports the salt dissociation by interacting with the anion of electrolyte. Recently Jin et al. [36] prepared the polymer electrolyte (GPE) by incorporating the boron-containing segments in a rapid and easy one-step polymerization process assisted with UV light. The prepared GPE system was fully amorphous as confirmed from XRD. The highest conductivity 5.13 mScm^-1 was exhibited by Boron-containing GPE (B-GPE) at 25 °C and activation energy of 2.09 kJ/mol. The tensile stress for B-GPE was 1.30 MPa and the maximum strain was 62.8%. The B-GPE based all-solid-state supercapacitor shows the potential window of 3.2 V. The B-GPE based solid-state SC exhibits the specific capacitance of about 34.35 F/g (at 1 A/g). Figure 3.6 shows the SC performance at various temperatures and specific capacitance increases from 21 to 74 F/g for temperature 0 to 80 °C (with voltage window 3.2 V). The energy density (54.20 Wh/kg) and power density (0.79 kW/kg) were higher (Figure 3.6b, c). The SC cell demonstrates capacity retention of 91.2% after 5000 cycles (Figure 3.6d).

Figure 3.6 (a) CV curves of the all-solid-state supercapacitor with B-GPE under various temperatures from 0 °C to 80 °C at a scan rate of 50 mV s^-1. (b) Ragone plots of the all-solid-state supercapacitor with B-GPE and conventional supercapacitor with 1 M LiClO₄/EMIMBF₄. (c) Ragone plots of the all-solid-state supercapacitor with B-GPE and others from previous articles for comparison (inset: photograph of a LED light powered by the all-solid-state supercapacitor with B-GPE). (d) The cycling performance of the all-solid-state supercapacitor with B-GPE and conventional supercapacitor with 1 M LiClO₄/EMIMBF₄ at a current density of 1 A g^-1. [Reprinted with permission from Ref. [36], © Elsevier 2011].

Another important strategy that is being focused on in research is the tuning of the electrode and electrolyte material. So to enhance the electrochemical performance of the SC cell, a novel approach was proposed by Du et al. [37]. They reported the fabrication of SC using poly(3,4-ethylenedioxythiophene/carbon paper (PEDOT/CP) as an electrode and gel polymer electrolyte [(1-butyl-3-methylimidazoletetrafluoroborate)/polyvinyl alcohol/ sulfuric acid (IL/PVA/H₂SO₄)]. The highest specific capacitance was 86.81 F/g at 1 mA/cm² and capacity retention of 71.61 after 1000 cycles. The energy density values are also high and are 176.90 Wh/kg and power density is 21.27 kW/kg. The strong crosslinking points are generated by the Freezing–Thawing (F/T) method and PVA hydrogels are prepared. The number of F/T cycles is crucial. The specific capacitance increased with the increase of F/T cycles (upto F/T 3) and is 53.73 F/g and then decreases. The increase of F/T cycles increases the number of H-bonds in polymer gel and a 3D crosslinking network is formed that allows easier access to ion migration (Figure 3.7c) [38].

Recently Alexandre et al. [39] reported the preparation of highly adhesive/sticky PIL/IL gel polymer electrolytes for application in solid-state supercapacitor. The GPE comprises poly(ionic liquid) consisting of poly(1-vinyl-3-propylimidazolium bis(fluorosulfonyl) imide) (poly(VPIFSI)) and a commercial ionic liquid: 1-ethyl-3-methyl imidazolium bis(- fluorosulfonyl)imide (EMIFSI). The prepared SC cell MWCNT||PIL/IL-GPE||MWCNT was examined in the voltage window of 2.5 V and the CV curve depicts EDLC nature. The specific capacitance was 9.6 F/g and coulombic efficiency was 94% (Figure 3.8b). The energy density and power density for the cell vary from 8.8 Wh kg^-1 and 268Wkg^-1 to 4.6Wh kg^-1 and 3732Wkg^-1, respectively.

Figure 3.7 Ragone plot of the SSC based on two GPEs (a) and the capacitance retention rate of SSC-IL/ PVA/H₂SO₄ (b) at 5 mA/cm², (c) The structure illustration. [Reproduced with permission from Ref. [37], © MDPI 2020].

Recently, polyelectrolyte (PE) based GPE is being investigated due to their superior electrochemical properties when used in SC application. The high water retention ability of PE provides conductive ion migration channels for ions in electrolyte [40]. So, Yan et al. [41] prepared the PE material by the UV-assisted copolymerization of a novel aprotic monomer N-(2-methacryloyloxy) ethyl-N,N-dimethylpropanammonium bromide (C₃(Br) DMAEMA) and poly (ethylene glycol) methacrylate (PEGMA). The conductivity of the PGPE was 66.8 Scm^-1 at 25 ^oC and the activation energy was 16.09 kJ/mol. The CV curve of the SC cell was almost rectangular and a specific capacitance of 64.92 F/g was observed at 1 A/g and 67.47 F/g at 0.5 A/g. The capacity retention was 84.74% at 0.5 A/g. The flexibility of the device was tested at different deformation rates. The SC cell shows an energy density of 9.34 Wh/kg and power density 2.26 kW/kg. The cyclic stability was also good and capacity retention was 94.63% after 10000 cycles at 2 A/g.

Another important electrolyte category is ‘redox-active electrolytes’ and are prepared by the addition of redox additives like methylene blue (MB), Iodide salts (KI, NaI), hydroquinone (HQ), p-diphenylamine, etc. [42]. Yadav et al. [43] prepared a GPE using 1-butyl-3-methylimidazolium bis(trifluoro-methylsulfonyl)imide (BMITFSI) as IL, and sodium iodide (NaI) as redox additive with poly(vinyledeneflouride-co-hexaflouropropylene) (PVdF-HFP) as host polymer. The specific capacitance was 351 F/g at 5 mV/s for SC cell with redox additive and is higher than redox additive-free cell (128 F/g) (Figure 3.9a). The specific energy was 26.1 Wh/kg and power density of 15 kW/kg (Figure 3.9b). Inset shows the LED glow demonstration for 300 s by connecting 4 cells. The SC cell with redox additive demonstrates superior cyclic stability (5% initial fading) than redox additive-free SC cell (23% initial fading) for 10000 cycles.

Figure 3.8 (a) Cyclic voltammograms obtained at 50 mV/s, (b) Capacitance retention as a function of the number of cycles at the operating voltages of [Reproduced with permission from Ref. [39], © Elsevier 2019].

Figure 3.9 (a) Specific capacitance of Cell#1 and Cell#2 versus charge–discharge cycles measured at constant current density 0.84 A g^-1, and (b) Ragone plots of Cell#1 and Cell#2 (inset shows glow of LED by four cells connected in series). [Reproduced with permission from Ref. [43], © Wiley 2019].

Another crucial approach to increase the electrochemical performance of SC cell is by developing composite materials (iongels) having two networks based on polarity [44]. Here, strongly polar (e.g., PEO, PVA, etc.) network provides superior electrochemical properties while, less polar network (e.g., NBR, natural rubber, PDMS, etc.) leads to enhanced mechanical properties. Based on this, Lu et al. [45] reported the preparation of iongels composite of PEO/NBR by in situ synthesis. The tensile modulus was 0.69 MPa and elongation break about 338% for iongel. The ionic conductivity was 2.4 mS/cm for 60% uptake of IL. Then using PEO/NBR iongels, an SC cell was fabricated using graphene electrodes and tested in the voltage window of 0-2.5 V. The specific capacitance was 2.8 F/g at 1A/g and decreases to 150 F/g at 10 A/g. The SC cell demonstrates good cyclic stability up to 10000 cycles (93.7% capacity retention) and negligible structural degradation as evidenced by XRD (after 10000 cycles). The energy density was very high 181 Wh/kg (comparable to commercial Lithiumion batteries) with a power density of 5.87 kWh/kg [46].

In solid polymer electrolytes (SPE) dielectric constant of the nanofiller also plays an important role in the enhancement of the ion transport parameters and hence the storage capacity of SC cell. So, to examine this Das et al. [47] investigated the effect of TiO₂ (dielectric constant: 80) and ZnO (dielectric constant: 8.5) on polymer matrix of PVDF–HFP incorporating 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF₄) as IL. The ionic conductivity of the prepared solid polymer electrolytes was 1.68 × 10⁻² S/cm (nanofiller free), 2.57 × 10⁻² S/cm (with ZnO) and 3.75 × 10⁻² S/cm (with TiO₂) at 303 K. The electrochemical stability window of solid polymer electrolytes was 4.57 V (nanofiller free), 5.55 V (with ZnO) and 5.98 V (with TiO₂).

The ionic conductivity and voltage stability window were superior for TiO₂ nanofiller based SPE. The specific capacitance for the SC cell [EDLC-I: nanofiller free, EDLC-II: with ZnO, EDLC-III: with TiO₂] using AC as electrode obtained from impedance spectroscopy was 103.5 F/g (nanofiller free), 134.6 F/g (with ZnO) and 206.4 F/g (with TiO₂). While specific capacitance for the SC cell from CV was 79.07 (nanofiller free), 93.07 (with ZnO) and 192.17 F/g (with TiO₂) at 25 mV/s. The increase in specific capacitance for TiO₂ was attributed to a high dielectric constant which supports salt dissociation. The value of specific capacitance for the SC cell from GCD was 104 F/g (nanofiller free), 131 F/g (with ZnO) and 239 F/g (with TiO₂) at 1 A/g (Figure 3.10a, b). The TiO₂ based SC cell shows an energy density of 33.19 Wh/ Kg and a power density of 1.17 kW/Kg. Also, all cells demonstrate a coulombic efficiency of 100 % after 2000 cycles.

Figure 3.10 (a) Comparison of charge–discharge curves at current for EDLC I, EDLC II, and EDLC III at a current density of 1 Ag⁻¹. (b) Variation of the specific capacitance of EDLC I, EDLC II, and EDLC III cells at different constant current densities. (c) Specific capacitance of EDLC I, EDLC II, and EDLC III cells at current density of 2 Ag⁻¹ shown as a function of charge–discharge cycles. (d) Two EDLC III cells in series to light up a yellow LED. [Reproduced with permission from Ref. [47], © Wiley 2020].

Another report from the same group examined the SC performance based on poly(vinylidene fluoride-co-hexafluoropropylene) polymer matrix, 1-propyl-3-methyleimidazolium bis(trifluromethylesulfonyl)-imide as ionic liquid with lithium bis (trifluoromethanesulfonyl)imide salt and plasticizer mixture (ethylene carbonate: propylene carbonate in the ratio 1:1) [32]. The highest specific capacitance from CV for cell 3 was 124.1 F/g at a scan rate of 10 mV/s with an energy density of 23.07 Wh/kg and power density of 0.5333 kW/ kg. Another report investigated the effect of cationic size and viscosity, dielectric constant of the ionic liquids on the electrochemical performance of SC cell [48]. Three polymer electrolytes were prepared: (i) BDMIMBF4-P(VdF-HFP) (BDMIMGPE-1), (ii) BMIMBF4-P(VdF-HFP) (BMIMGPE-2) and (iii) EMIMBF4-P(VdF-HFP) (EMIMGPE-3). The highest ionic conductivity was observed for the EMIMGPE-3 electrolyte and is 12.76 mS/cm which is attributed to the smaller cation size, high dielectric constant and low viscosity.

The value of specific capacitance as estimated from CV was 32.66 F/g (Cell-1), 49.1 F/g (cell-2), and 63.47 F/g (cell 3) at 10 mV/s. The highest specific capacitance for cell 3 is in correlation with ion conductivity results. The cell-3 demonstrates 74% capacity retention after 4000 cycles and 100% coulombic efficiency after 8000 cycles. The energy density and power density also superior to cell-3 (Figure 3.11).

Figure 3.11 Ragone plots (gravimetric specific energy versus specific power) for all EDL cells. [Reproduced with permission from Ref. [48], © Elsevier 2018].

Recently, Choi et al. [49] reported a novel strategy to achieve long-term cyclic stability and high energy density. The authors used the nanofiber cellulose incorporated nanomesh graphene-carbon nanotube hybrid buckypaper electrodes and ionic liquid-based solid polymer electrolyte. The SC cell using cPT-200 polymer electrolyte demonstrates areal capacitance of 291 mF cm^-2 at a current density of 0.75 mA cm^-2 and capacity retention of about 96.3% after 50000 cycles at 7.5 mA/cm². Even after bending the SC cell, the capacity retention was 98.4% after 50000 cycles. The enhanced performance was attributed to the high ionic conductivity of polymer electrolyte (3.0 mS/cm) and high electrical conductivity of nanofiber cellulose incorporated nanomesh graphene-CNT hybrid buckypaper (540 S/cm). The SC cell exhibits gravimetric energy density: 33.6 Wh kg^-1 and volumetric energy density: 6.68 mWh cm^-3.

Recently Jin et al. [50] reported the fabrication of SC cell using novel quasi-solid-state polymer electrolyte (QPE) of porous acrylate rubber/tetraethylammonium tetrafluoroborate-acetonitrile (pACM/Et₄NBF₄-AN) and nitrogen-doped porous graphene (NPG) film-supported vertically aligned polyaniline nanocones (NPG@PANI). The specific capacitance for NPG@PANI-2C electrode cell was 259.5 mF cm^-2 (330.2 F/g; 51.9Fcm^-3) at 1mAcm^-2 (Figure 3.12a). The anode NPG@PDAA-3C was chosen and demonstrates specific capacitance of about 254.5 mF cm^-2 (294.4 F/g; 50.9 F cm^-2) at 1mAcm^-2 (Figure 3.12b). Then an asymmetric SC device was fabricated using NPG@ PANI cathode, NPG@PDAA anode and pACM/Et₄NBF₄-AN as polymer electrolyte (Figure 3.12c). The specific capacitance of asymmetric device was 6.2 F cm^-3 (124.7mF cm^-2; 72.1 F/g) at 0.5 mAcm^-2 with 88.7% capacity retention after 10000 cycles. The energy density was 6.18mWh cm^-3 (123.5 mWh cm^-2; 71.4 Wh/kg) with power density of 0.033Wcm^-3 (0.668mWcm^-2; 0.386kW/kg).

Figure 3.12 (a) CV curves at 10 mV s^-1 and (b) GCD curves at 1mA cm^-2 of NPG@PANI-2C and NPG@ PDAA-3C in three-electrode mode. (c) Schematic diagram of as-assembled NPG@PANI//QPE//NPG@PDAA oAFSC. [Reproduced with permission from Ref. [50], © Elsevier 2019].

To increase the energy density of the SC, various efforts have been done to tune the voltage window of the electrolytes by the addition of IL and plasticizers [51]. In continuation of this, Kang et al. [52] reported the preparation of solid electrolyte comprising poly(ethylene glycol) behenyl ether methacrylate-gpoly((2-acetoacetoxy)ethyl methacrylate) (PEGBEM-g-PAEMA) graft copolymer by one-pot free-radical polymerization process for application in bendable SC. The ionic conductivity of the PE was 1.23 × 10^-3 S/cm and the increase was attributed to high polarity and amorphous nature. Two SC cells were fabricated using PEGBEM-g-PAEMA (Cell-1) and PVA/H₃PO₄ (Cell-2) as a solid electrolyte and activated carbon as an electrode. The PEGBEM-g-PAEMA based SC demonstrated the specific capacitance of about 55.5 F/g at 1.0 A/g (for Cell-2: 40.8 F/g at 1.0 A/g) with a power density of 900 and corresponding energy density of 25 Wh/kg. It is important to note that even after bending with an angle of 135^o, the performance was good. Table 3.4 summarizes some reported polymer electrolytes, their ionic conductivity and the electrochemical performance of the cell using them. Table 3.5 shows some patents on the supercapacitor device using different separators.

Table 3.4 Reported polymer electrolytes and fabricated supercapacitor performance.

Polymer electrolyte	Conductivity	Specific capacitance	Capacity retention	Energy density	Power density	Ref.
Boron-containing GPE	5.13 mS/cm	34.35 F/g (at 1 A/g) (RT) 74 F/g (80 °C)	91.2 % (after 5000 cy.)	54.20 Wh/kg	0.79 kW/kg	[36]
IL/PVA/H2SO4	-	86.81 F/g at 1 mA/cm2	71.61 % (after 1000 cy.)	176.90 Wh/kg	21.27 kW/kg	[37]
PIL/IL-GPE	-	9.6 F/g	-	8.8 Wh/kg and 4.6 Wh/kg	268 W/kg and 3732 W/kg	[39]
(C3(Br) DMAEMA)-PEGMA	66.8 S/cmat 25 °C	64.92 F/g at 1 A/g and 67.47 F/g at 0.5 A/g	84.74 %	9.34 Wh/kg	2.26 kW/kg	[41]
BMITFSI-NaI-(PVdF-HFP)	-	351 F/g at 5 mV/s	95 % (after 10000 cy.)	26.1 Wh/kg	15 kW/kg	[43]
PEO/NBR	2.4 mS/cm	150 F/g at 10 A/g	93.7 % (after 10000 cy.)	181 Wh/kg	5.87 kW/kg	[45]
PVDF-HFP-EMIMBF4	1.68 × 10-2S/cm (nanofiller free)	103.5 F/g	100 %	-	-	[47]
PVDF-HFP-(EMIMBF4)-ZnO	2.57 × 10-2S/cm (with ZnO)	134.6 F/g	100 %	-	-
PVDF-HFP-(EMIMBF4)-TiO2	3.75 × 10-2S/cm (with TiO2)	206.4 F/g	100 %	33.19 Wh/Kg	1.17 kW/kg
EMIMBF4-P(VdF-HFP)	12.76 mS/cm	63.47 F/g at 10 mV/s.	74 % (after 4000 cy.)	18 Wh/kg	1.2 kW/kg	[48]
Nanofiber Cellulose-Incorporated Nanomesh Graphene	3.0 mS/cm	291 mF cm-2 at 0.75 mA cm-2	96.3 % (after 50000 cy.)	33.6 Wh/kg, 6.68 mWh cm-3		[49]
pACM/Et4NBF4-AN		6.2 F cm-3(124.7mF cm-2;72.1 F/g) at 0.5 mAcm-2	88.7 % (after 10000 cy.)	6.18mWh cm-3 (123.5 mWh cm-2; 71.4Wh/kg)	0.033Wc-3 (0.668 mWcm-2; 0.386 kWkg-1	[50]
PEGBEM-g-PAEMA	1.23 × 10-3S/cm	55.5 F/g at 1.0 A/g	-	25 Wh/kg	900 kW/kg	[52]
chitosan (CS), starch, glycerol, LiClO4	3.7 × 10-4 S/cm	133(10 mV/s)	-	50 Wh/kg	8000 W/kg	[53]
pDADMATFSI	5 × 10-4 S/cm (25 oC) 3 × 10-3 S/cm (60 °C)	100 F/g (1mA cm2)	—	32 Wh/kg (20 °C) 42 Wh/ kg(60 °C)	-	[54]
PVA-H2SO4-HQ	29.3 mS/cm	491.3 F/g (0.5 A/g)	82.9 %	18.7 Wh/kg	245 W/kg	[55]
PVA-H2SO4-MB	29.6 mS/cm	563.7 F/g (0.5 A/g)	81.8 %	-	-
(PVA)/CH3COONH4/BmImCl	7.31 mS/cm	27.76 F/g	-	2.39 Wh/kg	19.79 W/kg	[56]
PMMA - C4BO8Li or LiBOB -EC/PC	3.27 S/cm m (298 K) 7.46 mS/cm (303 K)	685 mF g-1	-	-	-	[57]
PVdF-HFP- Mg(CF3SO3)2	2.16 × 10-4 S/cm	106 F/g	-	23 Wh/kg	-	[58]
PVA/BmImCl, BmImBr, BmImI	(5.74 ± 0.01) mS/cm (BmImCl)	19.42 F/g	-	1.77 Wh/kg	37.83 kW/kg	[59]
(9.29 ± 0.01) mS/cm (BmImBr)	21.82 F/g	-	2.19 Wh/kg	41.27 kW/kg
(9.63 ± 0.01) mS/cm (BmImI).	52.78 F/g	-	6.92 Wh/kg	50.25 kW/kg
PILTFSI/PYR14TFSI (IL-b-PE1)	0.5 mS/cm	110 F/g	-	35 Wh/kg	250 W/kg	[60]
PILTFSI/PYR14FSI (IL-b-PE2)	2.1 mS/cm	150 F/g	-	36 Wh/kg	230 W/kg
PVDF-HFP/EMimTFSI ϸ LiTFS	4.5 mS/cm	108 F/g	-	15 Wh/kg	213 W/kg	[61]
(poly(VA-co-AN))-1-ethyl-3-methylimidazolium (IL)/LiBF4	2 × 10-4 S/cm at RT and 7 × 10-3 S/cm at 100 °C	80 F/g(1 A/g)	99 % (after 1000 cy)	61 Wh/kg	500 W/kg	[62]
PVA-H2SO4-P-benzenediol	-	474.29 F/g	91 % (after 3000 cy)	11.31 Wh/kg	-	[63]

Table 3.5 Reported polymer electrolytes and fabricated supercapacitor performance.

Patent application number	Year	Invention
US6356432B1 United States	2002	Supercapacitor having a non-aqueous electrolyte and two carbon electrodes each containing a binder and an electrochemically active material constituted by active carbon having a Specific Surface area greater than about 2000 m/g.
1263/MUM/2004 A	2006	Polyaniline thin films synthesized by electrochemical anodization at constant potentials. The electrochemical capacitor was formed with H2SO4 solution. The specific supercapacitance of 650 F/g and interfacial capacitance of 0.14F/cm2 were obtained.
US20070076349A1 United States	2007	Supercapacitors having organosilicon electrolytes, high surface area/porous electrodes, and optionally organosilicon separators.
US7226702B2 United States	2007	Solid electrolyte made of an interpenetrating network type solid polymer comprised of two compatible phases: a crosslinked polymer for mechanical strength and chemical stability, and an ionic conducting phase.
US20100259866A1 United States	2010	Fabrication of a supercapacitor by constructing a mat of conducting fibers, binding the mat with an electrolytic resin, and forming a laminate of the electrodes spaced by an insulating spacer.
EP 2 880 667 B1	2014	Structural supercapacitors, more specifically to structural supercapacitors that may replace structural components based on composite materials.
CN105006377A China	2015	A composite electrolyte taking an azo substance as an additive and a preparation method thereof. The composite electrolyte is composed of a blank electrolyte and an electrolyte additive, wherein the blank electrolyte is a KOH solution, and the electrolyte additive is an azo substance.
WO2014011294A2 WIPO (PCT)	2015	Mechanically flexible and optically transparent thin-film solid-state supercapacitors are fabricated by assembling nano-engineered carbon electrodes in porous templates. The nanostructured electrode morphology and conformal electrolyte packaging provide enough energy and power density for electronic devices in addition to possessing excellent mechanical flexibility and optical transparency.
US20170271094A1 United States	2016	Polymer supercapacitor fabricated by loading a flexible electrode plate of a high surface area material with metal oxide particles, then encasing the electrode plate in a coating of a polymer electrolyte.
207701 (India)	2017	Fabrication and demonstration of high-performance electrochemical redox supercapacitors, which employ conducting polymers such as polyaniline (PANI) as the active material.
US 10 , 199 , 180 B2	2019	Fabric supercapacitors disclosed herein exhibit great flexibility.
US 10 , 269 , 504 B2	2019	A supercapacitor or electrochemical capacitor includes spaced-apart electrodes which are separated from each other by a separator made of electrically insulating material. Each electrode is formed of carbonaceous material and capable of being impregnated with a liquid electrolyte.