Electrochemical Capacitors: Ionic Liquid Electrolytes M Mastragostino and F Soavi, University of Bologna, Bologna, Italy & 2009 Elsevier B.V. All rights reserved. Introduction Double-layer carbon supercapacitors (electrochemical double-layer capacitor (EDLCs)) based on two carbon electrodes of high surface area separated by an electrolyte are the most popular electrochemical supercapacitors. The charge process is electrostatic with charge separation at the two electrode–electrolyte interfaces. The EDLCs can thus be modeled with two capacitances in series with what is called equivalent series resistance (ESR). Given that the electrode capacitance depends on the reciprocal of the double-layer thickness, which is on the order of 10 À10 m, and that it is directly related to the carbon surface area, typically of several hundred square meters per gram of carbon, the capacitance of EDLCs is significantly higher than that of the dielectric and electrolytic capacitors. The stored energy of EDLCs is also higher than that of the dielectric and electrolytic capacitors, but it is lower than that of batteries. Indeed, the faradic charge processes in batteries involve all the bulk electrode materials and not just the surface. The maximum energy (E max )ofEDLCis directly related to its capacitance (C sc ) and to the square of the maximum cell voltage (V max ) as in the following equation: E max ¼ 3 8 C sc V 2 max ½1 for a discharge between V max and 1/2V max .Commercial EDLCs featuring the highest specific energy are based on organic electrolytes, such as tetra alkylammonium salts (tetra alkylammonium tetra fluoroborate (Et 4 NBF 4 )) in acetonitrile (ACN) or propylene carbonate (PC), which allow cell voltages of up to 2.5–2.7 V and electrode-specific capacitances r100 F g À1 at room temperature (RT). Although higher specific capacitances (Z150 F g À1 )are feasible in aqueous electrolytes, the narrow electrochemical stability window (ESW) of aqueous electrolytes means that EDLCs operate at V max r1.0 V and, hence, E max is lower. High cell voltage is also important for high maximum power (P max ), which is given by eqn [2]. So despite the fact that organic electrolytes display lower conductivity than aqueous electrolytes, they are beneficial for high P max P max ¼ 1V 2 max 4ESR ½2 Unlikebatteries,EDLCsstorechargeinthedoublelayer at the electrode–electrolyte interface without chemical reactions and physical changes in the electrode materials and, hence, the charge–discharge processes are highly re- versible and fast. This also implies that power and cycle life of EDLCs are significantly higher than that of batteries. Such differences make EDLCs and batteries comple- mentary technologies, both being required for a new sus- tainable-energy economy that fosters energy efficiency upgrade in management and production of electric energy and in transportation. High energy efficiency is particularly important in view of a large exploitation of eolic and solar power plants and of a massive diffusion of hybrid electric vehicles (HEVs). The EDLCs are called upon for those functions that do not require high charge storage capacity, but for which high recharge rate and long cycle life of the storage system are crucial. The EDLCs can be used to enhance electric grid reliability and regulation by buffering the small, rapid (less than a minute), and frequent fluctuations in electric power that can cause shift in grid frequency, thus lowering grid quality and jeopardizing the main power generation plant. Uninterruptible power supply (UPS) systems of commercial and public buildings can also take advantage of EDLCs as they can provide short-time uninterruptible power and can be used for jumpstarting large diesel engine generators in case of grid power losses. In transportation, EDLCs can store energy from regenerative braking with an energy efficiency higher than that of the batteries used in today’s HEVs and they are already being introduced for this purpose in light rail systems. Capture of regenerative energy is also successfully performed by supercapacitors on a large scale in hybrid diesel–electric seaport cranes, where they recover energy otherwise wasted in the fre- quent load rising–lowering cycles. They can also provide and/or assist power train in heavy electric vehicles and HEVs of limited driving range with frequent stop-and-go, such as in demonstrative electric buses operating in Moscow (powered by a hybrid supercapacitor, see below) or in the hybrid city transit service in southern California, where cycle life and safety of energy conversion systems are even more of primary importance. The charge–dis- charge efficiency of EDLCs is even higher than 90%, so that the heat released in each cycle is small and it can be easily dissipated particularly when compared to power batteries. Also note that because they feature equal posi- tive and negative electrode materials, EDLCs are in- trinsically safer than the batteries, in which unpredictable internal short circuits may trigger dangerous, thermal runaway reactions. Furthermore, performance fade is more diagnosable in EDLCs than in batteries and the operating temperature range is wider. 649 . Electrochemical Capacitors: Ionic Liquid Electrolytes M Mastragostino and F Soavi, University of Bologna, Bologna, Italy &. specific capacitances (Z150 F g À1 )are feasible in aqueous electrolytes, the narrow electrochemical stability window (ESW) of aqueous electrolytes means that EDLCs operate at V max r1.0 V and,. carbon supercapacitors (electrochemical double-layer capacitor (EDLCs)) based on two carbon electrodes of high surface area separated by an electrolyte are the most popular electrochemical supercapacitors.