Application of functionalized CNT-polymer composite electrolytes for enhanced charge storage in "all solid-state supercapacitors"

Abstract

The ability of specifically functionalized carbon nanotubes to enhance proton transport in Nafion and polybenzimidazole membranes leading to improvement in the specific capacitance of an all solid-state supercapacitor is demonstrated. Cyclic voltammetry experiments reveal a 25% improvement (185 and 150 F per gram of RuO₂ for composite and Nafion membranes respectively) in capacitance by a meager 0.05 wt% addition of sulfonated MWCNTs in Nafion membranes. On the other hand, an addition of 1% phosphonated MWCNTs results in ~60% improvement in polybenzimidazole (PBI) based composites (from 160 to 260 F g^-1). Further, composite membranes based on functionalized MWCNTs show increased cycle life which is attributed to the presence of electrostatically linked network structures due to functional moieties on the side walls of carbon nanotubes that increases the interfacial charge density and integrity of the membrane. The equivalent series resistance for the PBI and PBI phosphonated MWCNT (PBpNT) membranes is 470 and 89 mΩ respectively suggesting improved proton conductivity with the composite membrane. Charge discharge measurements reveal a capacitance value of 500 F g^-1 for PBpNT membrane based supercapacitors even after 1000 cycles of operation. Use of such nanocomposite membranes is expected to dramatically improve the life time as well as performance of supercapacitors which in turn would facilitate deployment in different applications such as hybrid electric vehicles.