Gelatin Hydrogel Electrolytes and Their Application to Electrochemical Supercapacitors N. A. Choudhury, a S. Sampath, b and A. K. Shukla a,c, * ,z a Solid State and Structural Chemistry Unit, and b Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India c Central Electrochemical Research Institute, Karaikudi 630 006, India Gelatin hydrogel electrolytes GHEs with varying NaCl concentrations have been prepared by cross-linking an aqueous solution of gelatin with aqueous glutaraldehyde and characterized by scanning electron microscopy, differential scanning calorimetry, cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic chronopotentiometry. Glass transition tempera- tures for GHEs range between 339.6 and 376.9 K depending on the dopant concentration. Ionic conductivity behavior of GHEs was studied with varying concentrations of gelatin, glutaraldehyde, and NaCl, and found to vary between 10 -3 and 10 -1 S cm -1 . GHEs have a potential window of about 1 V. Undoped and 0.25 N NaCl-doped GHEs follow Arrhenius equations with activation energy values of 1.94 and 1.88  10 -4 eV, respectively. Electrochemical supercapacitors ESs employing these GHEs in con- junction with Black Pearl Carbon electrodes are assembled and studied. Optimal values for capacitance, phase angle, and relax- ation time constant of 81 F g -1 , 75°, and 0.03 s are obtained for 3 N NaCl-doped GHE, respectively. ES with pristine GHE exhibits a cycle life of 4.3 h vs 4.7 h for the ES with 3 N NaCl-doped GHE. © 2007 The Electrochemical Society. DOI: 10.1149/1.2803501 All rights reserved. Manuscript submitted February 5, 2007; revised manuscript received September 10, 2007. Available electronically November 15, 2007. Polymer electrolytes are widely studied materials with applica- tions in electrochemical devices. Although the ionic conductivity behavior of solid polymer electrolytes, such as polyethylene oxide- salt complexes, was reported as early as in 1973 by Wright, the potential of these materials as a new class of solid electrolytes for energy storage applications was envisaged by Armand in 1978. 1-4 Solid polymer electrolytes exhibit ionic conductivity between 10 -8 and 10 -7 S cm -1 that is too low to be significant for devices. Ac- cordingly, efforts have been expended to enhance the ionic conduc- tivity of polymer electrolytes. 1,2 One such approach involves addi- tion of plasticizer, a low-molecular-weight polar solvent, such as ethylene carbonate, to a polymer-salt system to realize polymer gel electrolyte PGE. 5-8 These PGEs are solid, have good adhesive properties, and exhibit high ionic conductivity of about 10 -3 S cm -1 at ambient temperatures. Although such nonaqueous PGEs have a wider potential window of about 4 V as compared to about 1 V for their aqueous counterpart, preparation and handling of the former require a moisture-free environment that is both involved and cost- intensive. Besides, organic solvents used as plasticizers with non- aqueous PGEs are environmentally toxic. 5 Electrochemical supercapacitors ESs are electrochemical power systems with highly reversible charge-storage and delivery capabilities. ESs have properties complementary to secondary bat- teries and find usage in hybrid energy systems for electric vehicles, heavy-load starting assist for diesel locomotives, utility load level- ing, and military and medical applications. 9,10 Depending on the charge-storage mechanism, an ES is classified as an electrical double-layer capacitor EDLC or a pseudocapacitor. Higher energy density of EDLCs, as compared to dielectric capacitors, is primarily due to the large surface area of the electrode materials, usually com- prised of activated carbons, 11,12 aerogel or xerogel carbons, 11,13-15 and carbon nanotubes. 11,16,17 EDLCs have several advantages over secondary batteries, namely, faster charge–discharge, longer cycle life 10 5 cycles, and higher power density. 9 Pseudocapacitors are also called redox capacitors because of the involvement of redox reactions in the charge-storage and delivery processes. Energy stor- age mechanisms in pseudocapacitors involve fast faradaic reactions such as underpotential deposition, intercalation, or redox processes occurring at or near a solid electrode surface at an appropriate potential. 9 Redox processes often occur in conducting polymers 9,18 and metal oxides, 9,19-28 making them attractive materials for pseudocapacitors. ESs employ both aqueous and nonaqueous elec- trolytes in either liquid or solid state; the latter provide the advan- tages of compactness, reliability and freedom from any leakage of liquid. Organic PGEs have been employed in both lithium batteries 5 and EDLCs. 6-8 There have been reports 29,30 on aqueous PGEs based on polyvinyl alcoholPVA–H 3 PO 4 and their use in ESs, but due to their corrosive nature, acidic electrolytes are suitable only for a limited number of electrode materials. Alkaline PGEs, such as polyethylene oxide PEO–KOH–H 2 O 31-33 and polyacrylic PAA–KOH–H 2 O 34,35 have found applications in nickel–metal hy- dride batteries and EDLCs. Recently, a study on aqueous PGEs comprised of aqueous PVA/PAA blend hydrogel electrolytes with acidic, alkaline, and neutral dopants has been reported. 36 Hydrogels are three-dimensional polymeric networks with large quantities of water absorbed in the polymer matrices. The three- dimensional network formation and its insolubility in the parent so- lution are due to the presence of chemical cross-links or physical entanglements. Physiologically responsive hydrogels often show a swelling behavior in response to a changing external environment. Some of the factors responsible for the swelling behavior are pH, ionic strength, temperature, and electromagnetic radiation. Based on the nature of the pendant groups, hydrogels are classified as neutral or ionic, and as affine or phantom networks based on their structural and mechanical features. Depending on the nature of the polymer, hydrogels are classified as homopolymeric or copolymeric hydro- gels. Hydrogels are also classified as amorphous, semicrystalline, hydrogen-bonded structures, supramolecular structures, and hydro- colloidal aggregates based on the physical structure of the networks. 37 Hydrogels are further subdivided as chemical and physi- cal hydrogels. Physical hydrogels differ from chemical hydrogels in the type of cross-linkages, randomness of the network formation, and the effects of these parameters on the rigidity and elastic moduli of the formed networks. Unlike the covalent cross-linking points in chemical hydrogels formed by the reaction between the polymer and a cross-linking reagent, physical hydrogels are formed through as- sociation of several laterally associated polymer helices in extended junction zones, wherein the hydrogel network is stabilized by physi- cal entanglements, electrostatic attractive forces, and hydrogen bonding; physical hydrogels are thermally reversible and can be viewed as viscoelastic solids. 38 Hydrogels comprising synthetic polymers, such as PVA, have high structural integrity and good mechanical properties. These hy- drogels have a large water content absorbed in the polymer matrix that helps fine-tune their ionic conductivity. Such hydrogels, how- * Electrochemical Society Active Member. z E-mail: shukla@sscu.iisc.ernet.in Journal of The Electrochemical Society, 155 1 A74-A81 2008 0013-4651/2007/1551/A74/8/$23.00 © The Electrochemical Society A74 Downloaded 13 Feb 2009 to 210.212.252.226. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp