Characteristics of Capacitors Based on Ionic Liquids: From Dielectric Polymers to Redox-Active Adsorbed Species

Abstract

The development of novel devices for energy recuperation and storage systems is of paramount importance for the realization of an energetically sustainable society. Capacitors play a huge role in mentioned complexes, being assumed as versatile devices for power applications. Ionic liquids (IL) have been tabbed as “designer electrolytes”, thus offering a multitude of unique and advantageous properties, such as negligible vapour pressure, high electrochemical and temperature stability, good ionic conductivity and the ability to modify the salt for novel applications. The presentation reviews the recent developments regarding the use of ILs as electrolytes for capacitors, starting from the formation of the electrical double layer (EDL) and ending up with some unique applications of ILs. Capacitor technology is fundamentally based on the formation of the EDL and the storage of charge within the EDL. It is thus important to understand the probable mechanism of charge storage and the formation of the EDL in ILs at different electrode surfaces. Experiments with highly oriented pyrolytic graphite, carbide derived carbons and bismuth electrodes in ILs have shown that the systems are indeed ideally polarizable [1], thus ideally suited for capacitor applications. It has been shown that the value of double layer capacitance in ILs is of the same order of magnitude as that for organic electrolyte based systems and the capacitance-potential curve of carbon electrodes shares the same principal V-shape as those seen for aqueous and organic electrolyte solutions. Although many theoretical studies have been published about the subject, the agreement between experimental and theoretical results is limited. One of the most widely studied IL applications as electrolytes are AC/AC supercapacitors (SC). SCs based on ILs have been shown to possess higher energy density, owing to their wider electrochemical window, but lower power density due to higher viscosity and lower ionic conductivity than more conventional organic electrolyte based SCs[2]. Although extremely wide electrochemical stability has been suggested for many ILs, this has not been realized in SCs, allowing only slightly higher voltage than commercial SCs. This has been explained by IL decomposition at extreme potentials confirmed by both infrared and photoelectron spectroscopy studies[1,3]. Because of this, SCs based on ILs are not competitive in the mainstream, however, serving many alternative applications in high or low temperature systems that require higher security with regards to failure. In order to increase the energy density of SCs based on ILs, the addition of redox species or redox-active organic compounds has been explored. The specific adsorption and subsequent oxidation of halide species has been tested for both bismuth and carbon electrodes. It has been shown that redox and surface active ions can significantly increase the gravimetric capacitance of the system with the drawback of limiting the applied potential range . Small additions of organic solvents to IL has been shown to increase the power density of SCs with negligible effect on the voltage limit and gravimetric capacitance, while the addition of specifically adsorbing organic compounds to ILs has been observed to produce systems with properties similar to those for dielectric capacitors [5]. The electrochemical breakdown of ILs can also be advantageous with regards to specific capacitor applications. It has been shown that some nitrile group-containing anions have the tendency to electropolymerize at high positive potentials, forming thick dielectric layers on the electrode surface [6]. This process has been applied in order to produce dielectric capacitors based porous carbon electrodes with high surface area and has been shown to possess extremely high energy density compared to that of traditional electrolytic capacitors based on electro-oxidized aluminium and tantalum electrodes [6]. Aknowledgements This work was supported by the Estonian Ministry of Education and Research (projects no. IUT 20-13, PUT55 and PUT1107), and Estonian Centres of Excellence (projects no. TK117T and TK141).