Ionic liquids for polymer based supercapacitors

The interest in supercapacitors has increased enormously in recent years mainly for applications in electric vehicle in which the supercapacitors operating in parallel with batteries and fuel cells are expected to provide power peaks during acceleration, especially with fuel cells which have lower performance power than batteries. Given such application, the focus of interest is on supercapacitors with relatively high specific energy. We have recently developed an hybrid supercapacitor of high specific energy and power based on activated carbon (AC) as negative electrode and poly(3-methylthiophene) (pMeT) as positive, operating between 3 V and 1.5 V in organic liquid electrolyte (propylene carbonate, PC – Et4NBF4), which outperforms even on a preindustrial scale the double layer activated carbon supercapacitors on the market [1]. To further improve the energy performance of such hybrid supercapacitor, we are pursuing the strategy of increasing its maximum operating voltage and, with this aim, we are evaluating the viability of ionic liquids as electrolytes. Indeed ionic liquids are known for their wide electrochemical stability window and chemical stability at high temperature. Ionic liquids display chemical-physical properties which depend on the cation and anion chemistry. The imidazolium cation dislays a high cathodic stability and several imidazolium based ionic liquids, generally with ethyl and methyl in 1 and 3 position, with different anions (such as BF4_, PF6_, (CF3SO2)2N_, CF3SO3_, ...) which in turn differently affect the limit of the anodic window, the melting point and hence the conductivity of the ionic liquids, have been prepared and characterized [2]. A few studies have also been devoted to their application in AC double-layer supercapacitors [3] and in polymer-based supercapacitors [4, 5], but the results of the last configuration are related to polymer electrodes too thin to be of practical interest. Here we report and discuss the results of the study on the 1-buthyl-3-methyl-1-H-imidazolium (BMIM+) X_ (X= BF4_, PF6_) ionic liquids carried out in view of their application in AC//pMeT hybrid supercapacitor. Given that a high electrolyte conductivity is important for high performance supercapacitors, conductivity data from room temperature up to high temperatures of interest for electric vehicle applications, especially for those involving fuel cells, are presented. Also the results of a voltammetric study on the AC and pMeT electrodes sized for a practical supercapacitor, with data of electrochemical stability window of BMIMBF4 and BMIMPF6, and of capacity and capacitance of AC and pMeT electrodes are reported and discussed. All these data are also compared with those obtained with the conventional PC-Et4NBF4 electrolyte.