Solid polymer electrolytes for supercapacitors

SAMUIandP.SIVARAMAN, Naval Materials Research Laboratory, India 

Abstract This chapter reviews state-of-the-art development in the field of sohd polymer electrolytes for supercapacitors and discusses their specific features. The main themes discussed are based on individual series of sohd polymer electrolytes such as ion exchange resins, polyethers, polymer gels and nanocomposites, followed by supercapacitor types, electrode types and supercapacitors reported under various kinds of sohd electrolytes. This is followed by information on hybrid supercapacitors and current research activities and finally a conclnsion. 

Key words sohd polymer electrolytes, supercapacitors, ionic condnctivity, electrode materials, specific capacitance, internal resistance, cycle life. 

It is pertinent to discuss the various kinds of solid electrolytes already developed and their properties. This will enable us to understand their usefulness in supercapacitor applications. 

Work on ion-conducting polymers started after it was reported during 1951 that salts can interact with PEO chains (Rg. 11.1) and the properties of polymer salt solutions were studied during the 1960s. Ionic conductivity in a PEO-alkaline metal ion complex was first reported by Wright in 1975. As the concentration of lithium salt was increased in the PEO, a general reduction in both the conductivity and number of lithium transfers was observed. The reduction was attributed to the motion of the polymer chains, responsible for ion mobility, being restricted and also formation of ion pairs. In turn, it lowered the number of free lithium ions available for conduction. Initial work was carried out by Armand as he realized that this 

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Samui, A. B., and P. Sivaraman. 2010. Solid polymer electrolytes for supercapacitors. In Polymer Electrolytes Fundamentals and Applications, C. Sequeira and D. Santos (Eds), 431—470, Cambridge, UK Woodhead Publishing Limited. 

Electrical and electronic industries Electrodes, sensors, gas separation membrane, fuel cell membrane, solid polymer electrolytes for batteries and supercapacitors... 

Yang, L., J. Hu, G. Lei, and H. Liu. 2014. Ionic liquid-gelled polyvinylidene fluoride/ polyvinyl acetate polymer electrolyte for solid supercapacitor. Chemical Engineering Journal 258 320-326. 

Pandey, G. P., A. C. Rastogi, and C. R. Westgate. 2013. Polyacrylonitrile and 1-ethyl-3-methyliniidazolium thiocyanate based gel polymer electrolyte for solid-state supercapacitors with graphene electrodes. Electrochemical Capacitors 50 145-151. 

Ion conducting solid polymer electrolytes, such as those used in battery and fuel cell membranes, have been explored for use in supercapacitors [153,159,200,201]. While these electrolytes are environmentally benign and do not leak, conductivities are typically much lower than liquid or gel electrolyte systems, especially at subambient temperatures (important for military and space applications). Nevertheless, capacitance in supercapacitors prepared with solid polymer electrolytes has been reported to be as good as or better than the same devices constructed using liquid electrolytes. Nafion [200], polyethylene oxide [153], and polyvinyl alcohol [153] are the polymers of choice for this application. 

Al-air fuel cells, Zn-Mn02 and Al-Mn02 cells, were assembled with anodes, cathodes and alkaline solid polymer electrolyte membranes. The electrochemical cells showed excellent cell power density and high electrode utilization. Therefore, these PVA-based solid polymer electrolyte membranes have great advantages in the applications for all-solid-state alkaline fuel cells. Some other potential applications include small electrochemical devices, such as supercapacitors and 3C electronic products. 

The most simple and efficient approach is based on gelation which is a simple method that allow a good compromise between the retention of the IL and its fluidity inside the polymeric network. These so called ion gels are simpler than solid polymer electrolytes and exhibit improved conductivities. In fact ion gels hold both the processability and mechanical properties of polymers, but with added physico-chemical properties and were primary developed as replacements for current solid-state polyelectrolytes in energy devices, such as dye-sensitized solar cells, supercapacitors, lithium ion batteries, and fuel cells. (Fernicola et al., 2006 Galinski et al., 2006 Le Bideau et al., 2011 Lu et al., 2002 Mazille et al., 2005 Stephan, 2006)... 

Lee et al. reported on a solid-state redox supercapacitor made using acrylonitrile butadiene rubber (NBR)-KCI as the solid polymer electrolyte, and chemically deposited PPY as the conducting polymer electrodes on both surfaces of an NBR film. ° The optimal conditions for the preparation of the PPY/NBR electrode were confirmed to be functions of the uptake of the pyrrole monomer into the NBR matrix and the immersion time in... 

Alkaline polymer gel electrolytes are commonly used in all- solid-state alkaline rechargeable batteries and supercapacitors. The typically used electrode materials for supercapacitors... 

Typically, the solid-state electrolyte can function as both a substrate and separator in the fabrication of a flexible supercapacitor by two steps. For instance, a freestanding PVA/ H3PO4 film was obtained by first casting the gel electrolyte on a glass slide and then peeled off from the glass. It had been further sandwiched between two composite electrodes that were prepared from conducting polymers and nanoparticles to produce the flexible supercapacitor (Fig. 9.3G) (Liu et al., 2010,2013b). 

FIGURE 2.81 Schematic diagrams of (a) dry solid-state polymer electrolyte (e.g., PEO/Li+), (b) gel polymer electrolyte, and (c) polyelectrolyte. (Zhong, C. et al. 2015. A review of electrolyte materials and compositions for electrochemical supercapacitors. Chemical Society Reviews 44 7484-7539. Reproduced by permission of The Royal Society of Chemistry.)... 

Latoszynska, A. A., G. Z. Zukowska, I. A. Rutkowska et al. 2014. Non-aqueous gel polymer electrolyte with phosphoric acid ester and its application for quasi solid-state supercapacitors. Journal of Power Sources 274 1147-1154. 

Hashmi and Upadhyaya compared the electrochemical properties of the electrochemically synthesized MnO /PPy composite electrodes, fabricated with different electrolytes, namely polymer electrolyte film (polyvinyl alcohol [PVA]-HjPO aqueous blend), aprotic liquid electrolyte (LiClO -propylene carbonate [PC]), and polymeric gel electrolyte (poly methyl methacrylate [PMMA]-ethylene carbonate [EC]-PC-NaClO ) [60]. The cell with aqueous PVA-H PO showed non-capacitive behavior owing to some reversible chemical reaction of MnO with water, while the MnO / PPy composite was found to be a suitable electrode material for redox supercapacitors with aprotic (non-aqueous) electrolytes. The solid-state supercapacitor based on the MnO /PPy composite electrodes with gel... 

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