Polyvinyl Alcohol Composites as ... - CyberLeninka

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ScienceDirect Energy Procedia 75 (2015) 1869 – 1874

The 7th International Conference on Applied Energy – ICAE2015

Sulfated cellulose/polyvinyl alcohol composites as proton conducting electrolyte for capacitors Boor Singh Lalia, Maitha Alkaabi, Raed Hashaikeh* Department of Materials Science and Engineering, Masdar Institute of Science and Technology Abu Dhabi 54224, United Arab Emirates

Abstract Supercapacitors are attractive devices for energy storage because of their high power density. Electrolytes are important components in supercapacitors because their electrochemical properties directly influence the performance and the internal resistance of the capacitor. In this work, cellulose was modified to be used as an electrolyte. Sulfated cellulose (SC) was prepared by esterification of micro-crystalline cellulose and characterized by elemental analysis, gel permeation chromatography and x-ray diffraction. The sulfated cellulose was dispersed in polyvinyl alcohol (PVA) solution to fabricate a proton conducting electrolyte. The SC/PVA (85:15 % w/w) film activated with 0.25M H2SO4 show high ionic conductivity of 2.5 x 10-1 S/cm and specific capacitance of 96 F/g at 25oC. In the chargedischarge curves, a low internal resistance was observed for SC/PVA electrolyte as compared to the acidic solution electrolyte.

© Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ©2015 2015The The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of Applied Energy Innovation Institute

Keywords: Sulfated cellulose; electric double layer capacitor; cyclic voltammetry; proton conductivity

1. Introduction Electric double layer capacitors (EDLCs) also known as Supercapacitors are source of high-power density and long life cycle for large-scale applications such as uninterrupted power supplies, electric vehicles and hybrid-electric vehicles. The basic structure of EDLCs consists of two activated carbon electrodes and electrolyte. Aqueous (H2SO4 or KOH) and organic electrolyte solutions are commonly used electrolytes in EDLCs. These electrolytes cause leakage problems, expensive sealing and limited miniaturization of size and shape. By replacing the solution electrolytes with polymer/gel electrolyte, leakage problems and flexibility in shape and size of the EDLCs could be improved. Various (acidic or basic) gel polymer electrolytes such as polyvinyl alcohol (PVA)-KOH, poly(ethylene oxide)-KOH, crosslinked poly(acrylate)- KOH, PVA-H2SO4, poly(acrylamide) (PAAM)-H2SO4, PVA-glutaraledehyde-

1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Applied Energy Innovation Institute doi:10.1016/j.egypro.2015.07.167

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H2SO4, PVA-perchloric acid and poly(acrylate)-KCl have been proposed for use in EDLCs [1-7]. In these gel polymer electrolytes, polymer acts as a polymer matrix and does not contribute to the conduction of ions inside the polymer matrix. Proton exchange membranes such as Nafion, sulfonated-poly(ether ether ketone) (S-PEEK), sulfonated-poly(fluorenyl ether nitrile oxynaphthalate) (SPFENO), sulfonatedpolypropylene used in fuel cells have been proposed as an electrolyte for EDLCs[8-11]. Moreover, acidic cellulose-chitin hybrid gel electrolyte has been proposed for EDLCs[12].With this motivation, we have prepared sulfated cellulose (SC) and used it for the development of SC-PVA-H2SO4 polymer electrolyte for EDLC applications. Cellulose belongs to a family of natural polysaccharide and abundant organic material produced in biosphere [13]. Cellulose is a sustainable, biodegradable and low cost polymer. Due to these promising properties, it could be used to develop electrolytes for EDLCs. In this work, we propose a sulfated cellulose-PVA film soaked with 0.25M H2SO4 solution as a quasisolid electrolyte for EDLCs. Sulfated cellulose, a proton conductor, could be prepared by controlled acid hydrolysis of microcrystalline cellulose in H2SO4 solution. Electrochemical characteristics of the EDLC assembled with activated carbon electrodes and SC-PVA film soaked in 0.25MH2SO4 was investigated and compared with the aqueous 0.25M H2SO4 solution. 2. Experimental Microcrystalline Cellulose (MCC) (Avicel-PH101), PVA (Mw=130,000 Aldrich) and Sulfuric Acid (95-97%, reagent grade, Scharlau), poly(vinylidenefluoride) (PVdF, Kynar Flex), activated carbon (Forest Group Co. Ltd., China) and Super P carbon (TIMCAL) were used as received. 2.1 Synthesis of sulfated cellulose 2. 10 gm of MCC powder was mixed in 100 ml 70% H2SO4 using Varian Dissolution System (VK7010) at 35°C for 30 minutes at 250 rpm agitation. After mixing, 900 ml water used to dilute the solution and centrifuge at 4700 rpm for 10 minutes using Allegra™ 25R Centrifuge. The upper layer was collected and dialyzed in tab water for three days until the pH is 6-7. The same was dialyzed again with distilled water to remove all the cations until the pH is 3-4. After dialysis the yield was calculated by drying sample using a freeze dryer. The yield is calculated based on the solid product weight after hydrolysis and drying compared to the starting weight. 2.2 Preparation of SC/PVA polymer electrolyte 3. Polymer electrolyte containing SC and PVA were prepared by solution casting method. A weight ratio of 85: 15 % (w/w) of SC and PVA respectively was dissolved in double distilled deionized water (Millipore Milli-Q) to form a homogeneous viscous solution and pour in PTFE dish. The water was allowed to evaporate at room temperature to form a semi-transparent film. The above composition is opted because 15 % PVA is the minimum amount of PVA to form a film. The SC/PVA films were soaked in 0.25 M H2SO4 solution and used for electrochemical measurements. 2.3 Fabrication of EDLC A symmetric EDLC was fabricated by placing the SC/PVA electrolyte soaked in 0.25M H 2SO4 between activated carbon electrodes and used for electrochemical measurements. Activated carbon electrodes were prepared by mixing activated carbon (80 wt%), PVdF binder (10 wt%) and Super P carbon (10 wt%). The electrode paste was coated on stainless steel current collector (area=1.76 cm2) and dried at 80oC for 12 hours. 2.4 Measurements 2.4.1 Elemental analysis Elemental analysis of SC was performed on Flash 2000 series CHN/O analyzer (Thermo fisher Scientific). Three samples were analyzed and average out to obtain %C, %H, %N and %S. The instrument was calibrated using 2,5-(bis(5-tert-butyl-2-benzo-oxazol-2-yl) thiophene) (BBOT) before analyzing the SC sample.


2.4.2 Gel permeation chromatography (GPC) The degree of polymerization of SC was measured using Viscotek® GPCmax VE2001 (gel permeation chromatography) with TDA305 (triple detection array). 2.4.3 X-ray diffraction (XRD) X-ray diffractograms of the cellulose powder (starting material) and SC were obtained using a X-ray diffractometer (PANalytical, Empyrean) operated at 45 kV and 40 mA with Ni1.5056 Å) radiations in 5 – 70 theta range. A diffractogram of the silicon substrate is obtained and used to correct the diffractograms of the samples measured. 2.4.4 Electrochemical analysis The conductivity and pH measurement of the solution electrolytes were performed on Accumet XL-50 (Fisher Scientific) dual channel pH/conductivity meter. The conductivity, cyclic voltammetry and chargedischarge measurements of the EDLC were performed on Autolab-PGSTAT302N potentiostat/galvanostat. 3. Results and discussion The hydrolysis of cellulose in sulfuric acid involves the scission of glucosidic bonds and esterification of the hydroxyl groups [14]. The reaction mechanism is shown in Scheme 1. The acid hydrolysis with

70% H2SO4 completely hydrolyzed the cellulose and forms high and low molecular weight sulfated cellulose and glucose. The low molecular weight sulfated cellulose and glucose are soluble in water and form the upper layer (after centrifugation), whereas high molecular weight cellulose insoluble in water and forms a bottom layer. The low molecular weight sulfated cellulose has a high degree of sulfation confirmed by the elemental analysis (C=35.4%, H=5.8%, N=0.8, and S=3.8%) and used for fabrication of polymer electrolyte. X-ray diffractograms of the MCC and SC are shown in Figure 1. MCC is highly crystalline in nature and show characteristic peaks of cellulose at 2T = 14.7o, 16.4o and 22.6o [15]. After acidic hydrolysis and esterification of MCC, addition of water leads to the formation of SC and a more stable Cellulose II (mostly amorphous phase). It is well known fact that regeneration after acid hydrolysis favors the formation thermodynamically stable cellulose II [16]. The

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average molecular weight of the SC, extracted from the top layer, was found to be 2,135 measured using GPC. The variation of conductivity and pH of the solution electrolyte with different weight percent of SC is shown in Figure 2. For low concentration of SC, conductivity increases and pH decreases and becomes saturated at high concentrations of SC. The decrease in pH value and increase in ionic conductivity with addition of SC show that it provides protons after dissolution in water. This property of SC attracted us to use it for fabrication of proton conducting polymer electrolyte for EDLCs. The SC/PVA film soaked in 0.25 M H2SO4 shows a high value of conductivity of 2.5 x 10-1 S/cm at room temperature, which -1 is the same order of magnitude, i.e. 1.0 x 10 S/cm obtained for 0.25M H2SO4 solution. Figure 3a shows the cyclic voltammograms for activated carbon electrode using SC/PVA-0.25M H2SO4 between -0.5 and 0.5 V potential ranges at scan rates of 5, 10, 20 and 50 mV/sec. The CV shows an ideal rectangular shape,

and no redox peaks corresponds to Faradic current observed in the scanned potential range. The current (I) response increases with the increase in sweep rate (dV/dt), according to I=CdV/dt, and even at a high scan rate of 50 mV/sec votlammogram shows a characteristic behavior of the capacitor. For comparison, the cyclic voltammograms of activated carbon electrodes with SC/PVA-0.25M H2SO4 and 0.25M H2SO4 is shown in Figure 3b. The EDLC with SC/PVA electrolyte shows a similar behavior. Specific capacitance of the electrodes in the polymer electrolyte and 0.25M H 2SO4 solution were found to be 76 F/g and 68 F/g respectively. The reproducibility of results was confirmed by repeating the same experiment several times. The other studies using PAAM-H2SO4 and sulfonated polypropylene –KOH shows similar results


[4, 11], polymer/hydrogels electrolyte reveals good capacitive performance as compared to the solution electrolytes. Figure 4a shows the galvanostatic charge – discharge curve for the EDLCs with SC/PVA-0.25M H2SO4 polymer electrolyte and 0.25M H2SO4 solution at 10 mA/cm2 current density. Almost linear charge – discharge behavior has been observed for both the polymer and solution electrolyte which confirm a low resistance electrolyte suitable for EDLC. The supercapacitor with SC/PVA-0.25 M H2SO4 exhibit a smaller iR drop compared with the supercapacitor with solution electrolyte. This is supposed to be due to proton conducting SC used for preparation of polymer electrolyte. SC/PVA membrane could help to reduce the electrolyte depletion in the charged state of EDLC and helps to reduce the equivalent series resistance (ESR) of the cell. Figure 4b shows the variation of discharge capacitance of supercapacitor with SC/PVA-0.25M H2SO4 polymer electrolyte at different current densities viz. 1, 2, 5 and 10 mA/cm2. The specific capacitance values at 1, 2, 5 and 10 mA/cm2 are found to be 96, 89, 75 and 64 F/g respectively. These values of specific capacitance are comparable to the reported results [4-6], however, in some cases its value is lower. This is due to the low concentration of sulfuric acid used in this study. Usually, the cell capacitance increases with the molar concentration of the electrolyte and show a maximum value and then decline at very high concentrations of the electrolyte due to high viscosity and ion aggregations which effects the fast transportation of ions between the electrode and electrolyte. This will be further investigated in the extension of this work. 4. Conclusions Proton conducting sulfated cellulose (SC) has been used to prepare polymer electrolytes for electric double layer capacitors (EDLCs). SC/PVA-0.25M H2SO4 electrolyte showed a high conductivity value of 2.5 x 10-1 S/cm at room temperature comparable to 0.25M H2SO4 solution. Cyclic voltammograms and galvanostatic charge – discharge tests using SC/PVA-0.25M H2SO4 showed a similar capacitive behavior as that of 0.25 M H2SO4 solution. Higher specific capacitance and lower iR drop for EDLC with SC/PVA0.25M H2SO4 has been observed compared to their liquid counterpart 0.25M H 2SO4. These electrochemical results indicate the applicability of a sustainable polymer - namely cellulose in commercial EDLCs.

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