Published: September 01, 2011 r2011 American Chemical Society 2396 dx.doi.org/10.1021/jz201065t | J. Phys. Chem. Lett. 2011, 2, 2396–2401 LETTER pubs.acs.org/JPCL Capacitive Energy Storage from À50 to 100 °C Using an Ionic Liquid Electrolyte Rongying Lin, †,‡ Pierre-Louis Taberna, † S  ebastien Fantini, ‡ Volker Presser, § Carlos R. P  erez, § Franc -ois Malbosc, ‡ Nalin L. Rupesinghe, || Kenneth B. K. Teo, || Yury Gogotsi, § and Patrice Simon* ,† † Universit  e Paul Sabatier, Toulouse Cedex, France; ‡ SOLVIONIC Company, Toulouse, France; § Department of Materials Science and Engineering & A.J. Drexel Nanotechnology Institute, Drexel University, Philadelphia, PA 19104, United States ) AIXTRON, Cambridge, U.K. b S Supporting Information E lectrochemical energy storage (EES) devices are at the center of attention to address one of today’s major technological and societal challenges: sustainable energy solutions. EES devices enable operation of hybrid and electric vehicles and broad imple- mentation of renewable energy sources (solar, wind, and tidal power). Over the past decade, EES systems have seen tremendous improvements in performance; however, all current technologies are limited to a rather narrow range of operation temperatures. EES systems are traditionally divided into energy devices (batteries) and power devices (electrochemical capacitors: ECs). There have been several breakthroughs and significant improve- ments in the battery area during the past years, mainly associated with the development of Li-ion batteries. 1 However, the devel- opment of high voltage cathodes for batteries with extended operational range from low (below À20 °C) to high (beyond 70 °C) temperatures is still hampered by electrolyte stability. 1À3 While batteries are characterized by charge/discharge times ranging from minutes to hours, ECs are high-power devices (∼10 kW/kg) with medium energy density (∼5 Wh/kg), which can be fully charged or discharged in a few seconds. 4 While not limited to that application, ECs complement or replace batteries wherever high power densities are needed for short times (e.g., starting a car or truck engine). 5 ECs can be divided into two groups: electrochemical double layer capacitors (EDLCs) and pseudocapacitors, 6 with the latter benefiting from a combination of electrostatic and Faradic (surface) processes involved in the charge storage mechanism. 7,8 EDLCs, also called supercapacitors or ultracapacitors, store energy electrostatically through ion adsorption from an electro- lyte on the surface of charged high surface area carbon electrodes, by charging the so-called double layer capacitance. 6,9 This charge storage mechanism explains the key features of EDLCs: out- standing cycle life (no redox reaction), high power density (fast surface storage), and identical charging and discharging rates (reversible ion ad-/desorption). As is true for the entirety of EES technologies, ECs have seen great improvements in perfor- mance, which were facilitated by recent discoveries such as the ion desolvation and capacitance increase in subnanometer pores, 12 the evidence of the influence of the nanostructure of oxides on the pseudocapacitive behavior 10 and the measure- ment of ion fluxes in micropores with in situ electrochemical techniques. 11 With a large number of carbon nanomaterials already under investigation, the discovery of graphene has stipulated tremen- dous research effort, and Miller’s group reported high power performance for thin graphene layers explained by the high Received: August 5, 2011 Accepted: September 1, 2011 ABSTRACT: Relying on redox reactions, most batteries are limited in their ability to operate at very low or very high temperatures. While performance of electrochemical capacitors is less dependent on the temperature, present-day devices still cannot cover the entire range needed for automotive and electronics applications under a variety of environmental conditions. We show that the right combination of the exohedral nanostructured carbon (nanotubes and onions) electrode and a eutectic mixture of ionic liquids can dramatically extend the temperature range of electrical energy storage, thus defying the conventional wisdom that ionic liquids can only be used as electrolytes above room temperature. We demonstrate electrical double layer capacitors able to operate from À50 to 100 °C over a wide voltage window (up to 3.7 V) and at very high charge/discharge rates of up to 20 V/s. SECTION: Energy Conversion and Storage