Polymer versus Cation of Gel Polymer Electrolytes in the Charge

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Materials and Interfaces

Polymer versus cation of gel polymer electrolytes in the charge storage of asymmetric supercapacitors Bhupender Pal, Amina Yasin, Ria Kunwar, Shengyuan Yang, Mashitah Mohd Yusoff, and Rajan Jose Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03902 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 20, 2018

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Industrial & Engineering Chemistry Research

Polymer versus cation of gel polymer electrolytes in the charge storage of asymmetric supercapacitors Bhupender Pala, Amina Yasina, Ria Kunwara, Shengyuan Yangb, Mashitah Mohd Yusoffa and Rajan Josea* a

b

Nanostructured Renewable Energy Materials Laboratory, Faculty of Industrial Sciences and Technology, Universiti Malaysia Pahang, 26300 Kuantan, Pahang, Malaysia

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, International Joint Laboratory for Advanced Fiber and Low-dimension Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, P. R. China

*Corresponding Author: [email protected] Abstract The gel polymer electrolytes (GPEs) are promising candidates for highly efficient flexible electrochemical energy storage devices as they reduce leakage and size of the device as well as improving versatility with varied choice of solvents, polymers, and ions. However, the electrochemical mechanisms governing the supercapacitive charge storage using varied choice of polymers and cations (PVA, PEG, PEO-based Na+ and K+) are not systematically evaluated. In this work, the role of GPEs on the charge storage mechanism of a flexible solid-state asymmetric supercapacitor fabricated using porous carbon as cathode and SnO2-TiO2 composite flower as anode with various GPEs, viz. poly (vinyl alcohol), poly (ethylene oxide), poly (ethylene glycol)-NaOH and KOH, is reported. The composite electrode greatly improves the ions transportation and the GPEs provide interconnected ion transport channels. As-fabricated porous carbon//GPE//composite electrode flexible asymmetric supercapacitor displays an increased specific capacitance (CS up to ~42.3 F g-1) compare to aqueous electrolytes (up to ~14.1 F g-1). Among the studied GPEs, poly (ethylene oxide)-NaOH-based GPE showed higher CS than (poly

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(vinyl alcohol)-NaOH and poly (ethylene glycol)-NaOH) as the former offered a high cation response under charge/discharge process. Keywords: Energy storage materials; Electrode-electrolyte interaction; Battery-supercapacitor hybrids; Electrochemical double layer capacitors; Energy density 1. Introduction There is a growing demand for flexible, lightweight, and highly efficient energy storage devices due to an extensive increase in flexible and portable wearable electronics1–4 in our daily lives.5–7 Electrochemical capacitors such as supercapacitors are one of the most promising candidates for flexible energy storage devices because of their fast charge/discharge capabilities, long cycling life, wide working temperature ranges, and diversity to be developed in various architectures.8,9 However, their low energy density has emerged as a major challenge for future development. Over the past decade, a significant amount of research has been carried out to develop thin, flexible, and lightweight supercapacitors.10,11 For this purpose, one of the critical parameters is finding proper electroactive materials with good mechanical properties and integrating them into special device configurations. Three kinds of electrode materials were widely studied in the past decades: (i) conductive polymers12–14 such as polyaniline (PANI), polypyrrole (PPy), polythiophene (PTP)7, and conducting polymer coating on cellulose etc. (ii) carbon-based materials such as activated carbon, carbon nanotubes, and graphene, etc.15–17 and (iii) transition metal oxides/hydroxides such as V2O5, MnO2, SnO2, TiO2, RuO2, Ni(OH)2, Co(OH)2, etc.18–20 The titanium dioxide (TiO2)21,22 and tin oxide (SnO2)23,24 are an attractive energy storage materials due to their excellent electrochemical properties, low cost, environmental friendly and easy availability.25–27 These electroactive materials have their own


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merits and drawbacks. The carbon based materials are very stable, but their specific capacitances are relatively low due to their electronic double-layer energy storage mechanism. The transition metal oxides possess high theoretical capacitance,28–30 whereas their electrical conductivity is poor. The conductive polymer such as PANI has good electrical conductivity but suffer from limited cycling stability. The intrinsic limitations of these electrode materials are disadvantageous to develop high performance, flexible, all-solid-state supercapacitors. To overcome these limitations, exploring novel electrode material combinations is urgently needed. The conventional supercapacitors using liquid-state electrolytes fail to meet the energy storage requirement of emerging portable and wearable electronics, due to their bulkiness and risk of electrolyte leakage.31,32 Therefore, much research effort has been devoted to developing supercapacitors using solid-state electrolytes. In comparison to liquid-state electrolytes (i.e. aqueous and organic electrolytes), solid-state electrolytes (i.e. gel polymer electrolytes and dry solid polymer electrolytes, etc.) have the advantages of stabilized form and also can eliminate the risk of electrolyte leakage.33,34 Among the solid-state electrolytes, hydrogel polymer electrolytes are constructed by a combination of polymeric cross-linked network and aqueous solution, they have excellent ionic conductivity, low cost and low fire hazard.35 Recently, flexible all-solid-sate supercapacitors have been designed using solid electrolytes such as polymer electrolytes (PVA/H2SO4 and PBI/H3PO4) and solid-state clay/ionic liquids.36,37 Chodankar et al.38 successfully made a PVA-LiClO4 hydrogel polymer electrolyte with an ionic conductivity of 48 mS/cm. So far, various electric double layer supercapacitors made of solid-state hydrogel polymer electrolytes have been reported in literature. Batisse et al.39 developed an electric double layer supercapacitor by using activated carbon as active electrode material and PVA-Na2SO4 hydrogel as electrolyte and reported an energy density of 13 Wh/Kg and a power density of 106



W/Kg at the current of 0.2 A.g-1. Xu et al.40 used PVA-H2SO4 hydrogel polymer electrolyte and graphene hydrogel film electrodes and reported an energy density of 6.5 Wh/Kg and a power density of 287.4 W/Kg at the current density of 1 A. g-1. The specific capacitance and the working potential window associated with contribution of different polymers and cations are scarcely studied. Such information, however, is important to get a better understanding of the electrolyte/electrode interface. The charges involved in the formation of the double layer plus the charges involved in pseudocapacitive reactions at the electrolyte/electrode interface are likely very different for the cations and polymers. Therefore, the first aim of the present work is to prepare cheap and eco-friendly asymmetric supercapacitor electrode materials and secondly to understand the electrochemical behavior of the different polymers and cations in gel polymer electrolytes for porous carbon//SnO2-TiO2 composite flowers based asymmetric supercapacitor. 2. Experimental Section 2.1. Reagents All the chemicals used in the present work were analytical reagent (AR) grade. Chemicals used in this study include: Polyvinyl alcohol (PVA), Polyethylene glycol (PEG), Polyethylene oxide (PEO), Sodium hydroxide (NaOH), Potassium hydroxide (KOH), Lithium hydroxide (LiOH), Titanium (IV) n-butoxide [Ti(OBu)4], Tin (II) chloride dehydrate (SnCl2.2H2O), Ethanol (CH3CH2OH), Dimethylformamide [(CH3)2NC(O)H], Acetic acid (CH3COOH), and Polyvinyl acetate (C4H6O2)n were obtained from Sigma-Aldrich and used as received. All electrolyte solutions were prepared in de-ionized water (Milli-Q ultrapure water purification system, Millipore Co.). 2.2. Preparation of Porous Carbon and SnO2/TiO2 Composite Flowers


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The porous carbon was prepared by our previously reported method41. Briefly, the palm kernel shell was washed and dried at 100 °C, then pyrolyzed for 4 h at 500 °C. The collected sample was activated with 6M KOH solution, stirred and dried at 100 °C. Finally, the dried sample was annealed at 500 °C for 4 h, and then again washed with 1M HCl and de-ionized water to obtain the neutral pH. The SnO2-TiO2 composite flowers were also prepared by our previously reported method42. Briefly, the polymeric solution for electrospinning was prepared by mixing ethanol, dimethylformamide (DMF), polyvinyl acetate (PVAc)), tin (II) chloride dehydrate, titanium (IV) n-butoxide, and glacial acetic acid. The equal molar ratio (5mM) of titanium (IV) n-butoxide and tin (II) chloride dehydrates were added in a solution of dimethylformamide (25 ml), acetic acid (4 ml), and ethanol (8 ml) followed by the addition of polyvinyl acetate (4 g) in it. Then the solution was stirred for 24 h before electrospinning. The electrospinning parameters were set as: applied voltage 24 kV, 40-45% relative humidity, injection rate 12 mL/h, and 16 cm distance between the collector and tip of the needle. Lastly, the sample was collected and annealed at 550 °C for 3 h to remove the polymeric components and to allow the growth and nucleation of SnO2TiO2 composite flowers. The schematic representation of preparation of porous carbon and SnO2TiO2 composite flowers have been shown in scheme 1 (a-b). 2.3. Materials Characterizations The crystal structure of the material was studied by powder X-ray diffraction (XRD) using a Rigaku Miniflex II X-ray diffractometer employing Cu Kα radiation (λ = 1.5406 Å). The morphology and microstructure of the materials were studied by field-emission scanning electron microscopy (7800F, FE-SEM, JEOL, USA). X-ray photoelectron spectroscopy (XPS; PHI Quantera II, Physical Electronics) analyses were carried out to determine the composition of the as prepared sample.



2.4. Preparation of Electrolytes We have prepared seven different gel polymer electrolytes (PVA-NaOH, PVA-KOH, PVALiOH, PEG-NaOH, PEG-KOH, PEO-NaOH, and PEO-KOH). All the gel polymer electrolytes were prepared as follows: 4 g PVA was mixed with 40 mL deionized water and then 4 g NaOH was added in it. The whole mixture was heated at ~95 °C under vigorous stirring until the solution become clear. Then the sample was cool down and used for electrochemical measurements. Same the procedure was followed for all type of gel polymer electrolytes only by changing the polymers and corresponding salts. 2.5. Electrochemical Analyses The electrodes were fabricated on nickel foam substrates. Firstly, the nickel foam was cleaned with acetone, 1 M HCl for 15 minutes and subsequently washing in ethanol and water for 10 minutes. The working electrode was prepared by mixing as-prepared samples with polyvinylidene fluoride and carbon black in the ratio 80:10:10. For better homogeneity, few drops of N-methyl-2-pyrrolidinone was added in the above mixture and stirred for 24 h. The asprepared slurry was then pasted on a nickel foam substrate (area ~1 cm2) and dried in an oven at 60 °C for 24 h. The mass-loading of the active material was ~2.5 mg cm-2. The dried electrode was then pressed using a pelletizer at a pressure of 5 ton. The electrochemical studies were performed by cyclic voltammetry (CV), Galvanostatic charge-discharge cycling (GCD), and electrochemical impedance spectroscopy (EIS). The electrochemical studies were performed at room temperature using a potentiostat-galvanostat (PGSTAT M101, Metrohm Autolab B.V., The Netherlands) employing NOVA 1.9 software. 2.6. Fabrication and Testing of Asymmetric Supercapacitor


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Seven sets of asymmetric supercapacitors were fabricated, with the porous carbon as cathode and SnO2-TiO2 composite flowers as anode. A glass microfiber filter (fioroni) was used as the separator and seven different gel polymers (PVA-NaOH, PVA-KOH, PVA-LiOH, PEG-NaOH, PEG-KOH, PEO-NaOH, and PEO-KOH) as the electrolytes. The asymmetric supercapacitors were constructed in a cell configuration by combining the anode and cathode in the coin-celltype casing, separated by a glassy fiber separator and gel polymer electrolytes. The schematic representation of fabrication of asymmetric supercapacitor is shown in scheme 2. The electrochemical characteristics of the asymmetric supercapacitor were analyzed in a twoelectrode configuration at room temperature using the potentiostat-galvanostat (PGSTAT M101). 3. Results and Discussion The effect of components of GPEs (polymers and cations) in the supercapacitive charge storage described in this article has been analyzed in an asymmetric supercapacitor configuration employing two previously reported electrodes, viz. porous carbon from palm kernel shell43 as negative electrode and a flower-shaped 3D nanostructure composite44 in the SnO2-TiO2 system as the positive electrode. A detailed structural, morphological, and compositional characterization of the as prepared materials are described in our previous publications41,42. Briefly, the XRD patterns of SnO2-TiO2 composite flowers in fig. 1 (a) show diffraction peaks corresponding to the rutile phases of the its components. The peaks located at ~27, ~35, ~38, ~40, ~53, ~64 and ~69° corresponded to the (110), (101), (200), (111), (211), (310) and (311) planes of the rutile phase of SnO2 and TiO2. The XRD pattern for porous carbon showed two broad diffraction peaks at ~26° and ~42° corresponding to the (002) and (101) plane of carbon, respectively as shown in fig. 1(b). Figure 2 (a-c) shows the morphology of the electrodes; porous carbon in fig. 2 (a) and SnO2-TiO2 composite flowers in fig. 2 (b and c). The flower-like SnO2-



TiO2 composite had an average size of ~2-5 μm (fig. 2b); a high resolution image is in fig. 2 (c). The flower is formed because of the large amount of precursor is enclosed in a tiny polymeric template and increase in activation energy for nucleation.45 The Sn and Ti content in the composite flower determined using EDX is ~10 and 11 at.%, respectively (fig. 2d). Wide scan XPS spectra of SnO2-TiO2 composite flowers confirm that the sample is composed of Sn, Ti, and O as shown in fig. 3 (a); the high resolution spectra of Sn 3d, Ti 2p, and O 1s are in fig. 3 (b-d). The Sn is present in Sn (IV) state as the high resolution spectrum for Sn 3d shows two high intense peaks (~486 eV for 3d5/2 and ~494 eV for 3d3/2) as shown in Fig. 3 (b). The high resolution spectrum for Ti 2p is shown in fig. 3 (c), it shows two major peaks positioned at ~457 eV for 2p3/2 and small peak at ~463 eV for 2p1/2, signifying Ti4+ oxidation state of TiO2. Figure 3 (d) shows the deconvoluted XPS spectra for O 1s and it shows two types of contributions for oxygen, one is corresponding to Ti-O-Ti and other is corresponding Sn-O-Sn. The electrochemical energy storage performance of porous carbon//SnO2-TiO2 composite flower was investigated by two-electrode configurations in various gel polymer electrolytes. Figure 4 (a-c) summarizes the cycle voltammetry (CV), discharge cycling, and electrochemical impedance spectroscopy (EIS) data of two-electrode system configuration of porous carbon//SnO2-TiO2 electrodes in gel polymer electrolytes (PVA-NaOH, PVA-KOH, and PVALiOH). The compared CV graph of porous carbon//SnO2-TiO2 electrodes in PVA-LiOH, PVANaOH, and PVA-KOH is shown in fig. 1 (a) in the potential window rage from 0 to 1.8 V and CV at various scan rates from 1 mV s-1 to 50 mV s-1 are shown in fig. S1 (a-c). Markedly, the CV curves showing well-defined redox peaks in the potential range of 0.9-1.8 V which displays the characteristics of pseudocapacitors. The efficient diffusion of hydroxyl ions into the electrode clearly reveals the electrochemical processes. PVA-NaOH based gel electrolyte shows higher


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specific capacitance (36.2 F g-1) than other electrolytes. Same trend is also shown in discharge cycling data in fig. 4 (b), PVA-NaOH based gel electrolytes showed higher discharge rate than PVA-KOH. It is reported that when the porous structure of the electrode is well-saturated by the electrolyte, the ions already located on electrode/electrolyte interface are attracted and pushed off from the electrode on a small distance and has efficient charge propagation.47 Therefore, due to bigger hydrated size of Na+ ion (3.59Å) it might remain near to the electrode/electrolyte interface during discharging and then quickly appear at the interface during charging as compare to K + hydrated ion (3.34Å) and shows better performance. The variation of charge with current density is shown in fig. S1 (d). The specific capacitance values for PVA-NaOH, PVA-KOH and PVALiOH was found to be 33.52 F g-1, 11.8 F g-1 and 10.5 F g-1, respectively from discharge curves. Nyquist plot drawn from EIS studies as can be seen in fig. 4 (c) also shows same trends for all three gel polymer electrolytes. The intercept of EIS in high frequency region is a combination of ionic resistance of electrolyte and intrinsic resistance of electrode46. From the fig. 4 (c), we can see that these resistances are very small for PVA-NaOH as compare to other gel electrolytes; therefore, PVA-NaOH shows excellent rate capability of asymmetric supercapacitor. The nearly vertical line in low frequency region implies good capacitive behavior of asymmetric supercapacitor. Figure 5 (a-c) and fig. S2 (a-d) shows the CV, discharge cycling, and EIS graphs of twoelectrode system configuration of porous carbon//SnO2-TiO2 electrodes in PEG-NaOH and PEGKOH at various scan rates. Figure 5 (a) displayed the compared CV curves of porous carbon//SnO2-TiO2 in PEG-NaOH and PEG-KOH in the potential window rage from 0 to 1.7 V and CV graphs at various scan rates from 1 mV s-1 to 50 mV s-1 are shown in fig. S2 (a-b). Noticeably, the CV curves showing well-defined redox peaks in the potential range of 1-1.7 V



which displays the characteristics of pseudocapacitors. The efficient diffusion of hydroxyl ions into the electrode clearly reveals the electrochemical processes. The PEG-KOH-based gel electrolyte shows higher specific capacitance (45.3 F g-1) than PEG-NaOH (15.1 F g-1). PEGKOH shows higher specific capacitance than NaOH due to its high ionic mobility, smaller hydration sphere radius and lower equivalent series resistance. Figure 5 (b) shows the compared discharge rate graphs for PEG-KOH and PEG-NaOH. The discharge curves of asymmetric supercapacitors at various current densities (1-3 A.g-1) in PEG-NaOH and PEG-KOH gel polymer electrolytes is shown in fig. S2 (c-d). The PEG-KOH based gel electrolytes showed higher discharge rate than PEG-NaOH. The specific capacitance values for PEG-KOH and PEG-NaOH was found to be 42.3 F g-1 and 8.8 F g-1, respectively from discharge cycling data. Nyquist plot as shown in fig. 5 (c) also shows same trends for both the gel polymer electrolytes. The intercept of EIS in high frequency region is a combination of ionic resistance of electrolyte and intrinsic resistance of electrode; semicircle in the middle region reflects charge transfer resistance. From the fig. 5 (c), we can see that these resistances are very small for PEG-KOH as compare to PEG-NaOH; therefore, PEG-KOH displays excellent rate capability of asymmetric supercapacitor. In the low frequency region, the nearly vertical line implies good capacitive behavior of asymmetric supercapacitor. The fig. 6 (a-c) and S3 (a-d) display the CV, discharge cycling, and EIS graphs of porous carbon//SnO2-TiO2 electrodes in PEO-NaOH and PEO-KOH at various scan rate (1 to 50 mV.s-1) and at various current densities. The compared CV graph of porous carbon//SnO2-TiO2 electrodes in PEO-NaOH and PEO-KOH is shown in fig. 6 (a). CV curves of the device in the potential range of 0-1.7 V show redox peaks due to the diffusion of OH- ions in the electrodes. The PEO-NaOH based gel polymer electrolyte shows higher specific capacitance (38.2 F g-1)


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than PEO-KOH (17.3 F g-1). In the case of ions with same charge, the smaller ionic radius has stronger polarization in a polar solvent which leads to the stronger ionic solvation. Afterward, the more ions will attract the surrounding solvent molecules and greater the radius of hydrated ions, hence, Na+ ion has bigger hydrated ion size (3.59Å) than K+ (3.34Å) ion.47 It is also reported that when the porous structure of the electrode is well-saturated by the electrolyte, the ions already located on electrode/electrolyte interface are attracted and pushed off from the electrode on a small distance and has efficient charge propagation.47 Therefore, due to bigger hydrated size of Na+ ion it might remain near to the electrode/electrolyte interface during discharging and then quickly appear at the interface during charging as compare to K+ hydrated ion. Figure 6 (b) shows the compared discharge rate graphs of PEO-KOH and PEO-NaOH. The discharge curves of asymmetric supercapacitors at various current densities (1-3 A.g-1) in PEO-NaOH and PEOKOH gel polymer electrolytes is shown in fig. S3 (c-d). The PEO-NaOH based gel electrolytes showed higher discharge rate than PEO-KOH. The specific capacitance values for PEO-NaOH and PEO-KOH is found to be 35.9 F g-1 and 14.7 F g-1, respectively from discharge cycling data which is matching with the CV data. Nyquist plot drawn from EIS studies as can be seen in fig. 6 (c) also shows same trends for both the gel polymer electrolytes. The intercept of EIS in high frequency region is a combination of ionic resistance of electrolyte and intrinsic resistance of electrode; semicircle in the middle region reflects charge transfer resistance. From the fig. 6 (c), we can see that these resistances are very small for PEO-NaOH as compare to PEO-KOH; therefore, PEO-NaOH displays excellent rate capability of asymmetric supercapacitor. The electrochemical properties of gel polymer electrolytes (PVA-NaOH, PEO-NaOH, and PEG-NaOH) are also compared with aqueous electrolyte (AQU-NaOH). The CV and discharge cycling graphs for AQU-NaOH at various scan rate (1-50 mV.s-1) and various current



densities (1-3 A g-1) are shown in fig. S4 (a-d). Figure 7 (a-c) displayed the comparative data of CV, discharge cycling, and EIS of two-electrode system configuration of porous carbon//SnO2TiO2 electrodes in various gel polymer electrolytes and aqueous electrolyte with NaOH salt (PVA-NaOH, PEO-NaOH, PEG-NaOH and AQU-NaOH). The CV curves in fig. 7 (a) shows redox peaks during the forward and backward runs corresponding to reaction. The prominent redox peaks can be observed at around 0.9 to 1.7 V which can be attributed to the pseudocapacitive behavior of asymmetric supercapacitor. The gel polymer electrolytes (PVANaOH, PEO-NaOH, PEG-NaOH) based asymmetric supercapacitor showed higher specific capacitance values than aqueous electrolyte (AQU-NaOH), which may be due to interconnected ion transport channels provided by gel polymer electrolytes.48 The PEO-NaOH based gel polymer electrolyte shows higher specific capacitance than PVA-NaOH and PEG-NaOH, which may be due to the quicker ions diffusion rate, less ion pairing, and more adequate electrode/electrolyte interfacial contact. Figure 7 (b) shows the compared discharge rate graphs of PEO-NaOH, PVA-NaOH, PEG-NaOH, and AQU-NaOH. The PEO-NaOH based gel electrolytes showed higher discharge rate and specific capacitance than other gel polymer electrolytes, which also matching with CV data. Due to the same trends in CV and discharge cycling data clearly indicates that choice of polymer with particular salt also plays very important role in charge storage of asymmetric supercapacitor. Nyquist plot drawn from EIS studies as can be seen in fig. 7 (c) also shows same trends for all gel polymer and aqueous electrolytes. The intercept of EIS in high frequency region is a combination of ionic resistance of electrolyte and intrinsic resistance of electrode; semicircle in the middle region reflects charge transfer resistance. From the fig. 7 (c), we can see that these resistances are very small for PEO-


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NaOH as compare other gel polymer electrolytes. Therefore, PEO-NaOH displays excellent rate capability of asymmetric supercapacitor. The electrochemical properties of gel polymer electrolytes (PEG-KOH, PEO-KOH, and PVA-KOH) are also compared with aqueous electrolytes (AQU-KOH). Figure 8 (a-c) displayed the comparative data of CV, discharge cycling, and EIS of two-electrode system configuration of porous carbon//SnO2-TiO2 electrodes in various gel polymer electrolytes and aqueous electrolyte with KOH salt (PEG-KOH, PEO-KOH, PVA-KOH and AQU-KOH). The CV curves in fig. 8 (a) shows prominent redox peaks at around 1 to 1.7 V which can be attributed to the pseudocapacitive behavior of asymmetric supercapacitor. The gel polymer electrolytes (PEGKOH, PEO-KOH, and PVA-KOH) based asymmetric supercapacitor showed higher specific capacitance values than aqueous electrolyte (AQU-KOH), which may be due to interconnected ion transport channels provided by gel polymer electrolytes.48 The PEG-KOH based gel polymer electrolyte shows higher specific capacitance than PEO-KOH and PVA-KOH, which may be due to the quicker ions diffusion rate, less ion pairing, and more adequate electrode/electrolyte interfacial contact. Figure 8 (b) shows the compared discharge rate graphs of PEG-KOH, PEOKOH, PVA-KOH, and AQU-KOH. The PEG-KOH based gel electrolytes showed higher discharge rate and specific capacitance than other gel polymer electrolytes, which also matching with CV data. Due to the same trends in CV and discharge cycling data clearly indicates that choice of polymer with particular salt also plays very important role in charge storage of asymmetric supercapacitor. Nyquist plot drawn from EIS studies as can be seen in fig. 8 (c) also shows same trends for all gel polymer and aqueous electrolytes. The intercept of EIS in high frequency region is a combination of ionic resistance of electrolyte and intrinsic resistance of electrode; semicircle in the middle region reflects charge transfer resistance. From the fig. 8 (c),



we can see that these resistances are very small for PEG-KOH as compare other gel polymer electrolytes. Therefore, PEG-KOH displays excellent rate capability of asymmetric supercapacitor. 4. Conclusions In the present work, we have shown that the contribution of polymers and cations of gel polymer electrolytes contribute positively to the charge storability in asymmetric supercapacitors. Among the studied polymers systems using KOH, the PEG-KOH shows the highest charge storage capacity compared to other polymer-salt combination like PEO-KOH, PVA-KOH and aqueous KOH electrolyte which may be due to the quicker ions diffusion rate, less ion pairing, more adequate electrode/electrolyte interfacial contact, and lower equivalent series resistance. The lower series resistance could be attributed to the higher electrical conductivity of PEG as well as its relatively lower molecular weight compared to the others, which improved the rheological properties of the resulting GPE. The gel polymer electrolytes based asymmetric supercapacitor studied here showed higher specific capacitance values than aqueous electrolyte, which may be due to interconnected ion transport channels provided by gel polymer electrolytes. The correct combination of polymer and cation can help in the improvement of overall performance of the energy storage devices. Acknowledgements Bhupender Pal acknowledges the Research & Innovation Department of Universiti Malaysia Pahang (http://ump.edu.my) for award of Postdoctoral Fellowship. This project is funded under Flagship Strategic Leap 3 (RDU 172201) of Universiti Malaysia Pahang and International Joint Laboratory for Advanced Fiber and Low-dimension Materials, Donghua University, Shanghai.


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Figure Captions Fig. 1. Powder X-ray diffraction patterns of (a) SnO2/TiO2 composite flowers and (b) porous carbon. Fig. 2. Field-emission scanning electron microscopic (FE-SEM) images of (a) porous carbon and (b-c) SnO2/TiO2 composite flowers at various resolution; (d) energy-dispersive X-ray spectroscopic data of SnO2/TIO2 composite flowers. Fig. 3. X-ray photoelectron spectroscopic (XPS) spectra of SnO2/ TiO2 composite flowers (a) wide scan. XPS core level spectra of (b) Sn 3d, (c) Ti 2p and (d) O 1s, respectively. Scheme 1: Schematic diagram of preparation of (a) porous carbon, (b) SnO2/TiO2 nanoflowers and their asymmetric supercapacitor fabrication. Scheme 2: Schematic diagram of fabrication of asymmetric supercapacitor. Fig. 4. (a) Cycle voltammetry (CV) data of asymmetric supercapacitors in different gel polymer electrolytes (PVA-KOH, LiOH and NaOH) at a scan rate of 50 mV s−1. (b) Discharge curves of asymmetric supercapacitors at a current density of 1 A g−1 in various gel polymer electrolytes. (c) Nyquist plot drawn from EIS studies. Fig. 5. (a) CV data of asymmetric supercapacitors in gel polymer electrolytes (PEG-KOH and NaOH) at a scan rate of 50 mV s−1. (b) Discharge curves of asymmetric supercapacitors at a current density of 1 A g−1 in gel polymer electrolytes (PEG-KOH and NaOH). (c) Nyquist plot drawn from EIS studies. Fig. 6. (a) CV data of asymmetric supercapacitors in gel polymer electrolytes (PEO-KOH and NaOH) at a scan rate of 50 mV s−1. (b) Discharge curves of asymmetric supercapacitors at a current density of 1 A g−1 in gel polymer electrolytes (PEO-KOH and NaOH). (c) Nyquist plot drawn from EIS studies. Fig. 7. (a) CV data of asymmetric supercapacitors in gel polymer electrolytes (PVA, PEO, PEG, AQU-NaOH) at a scan rate of 50 mV s−1. (b) Discharge curves of asymmetric supercapacitors at a current density of 1 A g−1 in gel polymer electrolytes (PVA, PEO, PEG, AQU-NaOH). (c) Nyquist plot drawn from EIS studies. Fig. 8. (a) CV data of asymmetric supercapacitors in gel polymer electrolytes (PVA, PEO, PEG, AQU-KOH) at a scan rate of 50 mV s−1. (b) Discharge curves of asymmetric supercapacitors at a current density of 1 A g−1 in gel polymer electrolytes (PVA, PEO, PEG, AQU-KOH). (c) Nyquist plot drawn from EIS studies.


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Figures

Fig. 1. Powder X-ray diffraction patterns of (a) SnO2/TiO2 composite flowers and (b) porous carbon.



Fig. 2. Field-emission scanning electron microscopic (FE-SEM) images of (a) porous carbon and (b-c) SnO2/TiO2 composite flowers at various resolution; (d) energy-dispersive X-ray spectroscopic data of SnO2/TIO2 composite flowers.


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Fig. 3. X-ray photoelectron spectroscopic (XPS) spectra of SnO2/ TiO2 composite flowers (a) wide scan. XPS core level spectra of (b) Sn 3d, (c) Ti 2p and (d) O 1s, respectively.



Scheme 1: Schematic diagram of preparation of (a) porous carbon, (b) SnO2/TiO2 nanoflowers and their asymmetric supercapacitor fabrication.

Scheme 2: Schematic diagram of fabrication of asymmetric supercapacitor.


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Fig. 4. (a) Cycle voltammetry (CV) data of asymmetric supercapacitors in different gel polymer electrolytes (PVA-KOH, LiOH and NaOH) at a scan rate of 50 mV s−1. (b) Discharge curves of asymmetric supercapacitors at a current density of 1 A g−1 in various gel polymer electrolytes. (c) Nyquist plot drawn from EIS studies.



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Fig. 5. (a) CV data of asymmetric supercapacitors in gel polymer electrolytes (PEG-KOH and NaOH) at a scan rate of 50 mV s−1. (b) Discharge curves of asymmetric supercapacitors at a current density of 1 A g−1 in gel polymer electrolytes (PEG-KOH and NaOH). (c) Nyquist plot drawn from EIS studies.


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Fig. 6. (a) CV data of asymmetric supercapacitors in gel polymer electrolytes (PEO-KOH and NaOH) at a scan rate of 50 mV s−1. (b) Discharge curves of asymmetric supercapacitors at a current density of 1 A g−1 in gel polymer electrolytes (PEO-KOH and NaOH). (c) Nyquist plot drawn from EIS studies.



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Fig. 7. (a) CV data of asymmetric supercapacitors in gel polymer electrolytes (PVA, PEO, PEG, AQU-NaOH) at a scan rate of 50 mV s−1. (b) Discharge curves of asymmetric supercapacitors at a current density of 1 A g−1 in gel polymer electrolytes (PVA, PEO, PEG, AQU-NaOH). (c) Nyquist plot drawn from EIS studies.


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Fig. 8. (a) CV data of asymmetric supercapacitors in gel polymer electrolytes (PVA, PEO, PEG, AQU-KOH) at a scan rate of 50 mV s−1. (b) Discharge curves of asymmetric supercapacitors at a current density of 1 A g−1 in gel polymer electrolytes (PVA, PEO, PEG, AQU-KOH). (c) Nyquist plot drawn from EIS studies.



Graphical Abstract


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Polymer versus Cation of Gel Polymer Electrolytes in the Charge

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