Panasonic and ELNA

Literature Review
2.1 Overview of supercapacitors
In 1853, Helmholtz showed that electrical double-layer (DL) could be used to
store charge based on the fact that charged electrodes immersed into electrolyte repel the
co-ions and attract counterions in the interface [1]. The first patent on porous carbon
electrodes in sulfuric acid electrolyte to store electrical energy was described in 1957 by
Becker for General Electric [2]. It was believed that high specific capacitances (around 5-
50 µF cm-2) could be obtained for carbons with high surface area in molten salts or
aqueous electrolytes [3]. NEC developed aqueous electrolyte/activated carbon capacitors
as “supercapacitors” in 1971 by licensing the technology of SOHIO [4]. In the following
two decades, Panasonic and ELNA marketed the first generation of EDLCs which were
mainly used for low current applications such as memory backup devices due to
relatively high internal resistance [4,5]. Supercapacitors with low internal resistance were
developed, which were used for the U.S. military applications with the Pinnacle Research
Institute (PRI) in 1982 [6].
Inspired by the work on conventional electrochemical battery, researchers studied
another kind of supercapacitors (pseudopcapcitors) working in a different mechanism.
Pseudocapacitors store energy through electrosorption, redox reactions, and intercalation
processes rather than an electrostatical double-layer charging process in the EDLCs [7-9].
By deposition onto conductive metallic substrates, conducting polymers and transitionalmetal oxides (ruthenium oxide, nickel oxide, iridium dioxide) exhibited a higher energy
density than EDLCs. 10
In 2007, Department of Energy assessed the technologies on electrical storage and
specifically showed the increasing potential of electrochemical capacitors in hybrid
electric vehicle (HEV), portable devices and residential application [10]. Worldwide sales
of supercapacitor reached $400 million in 2010 and it is estimated to be $3.0 billion by
2016 according to Nanomarkets [11]. A number of companies including Ness, Panosonic,
Maxwell, EPCOS and ESMA have successfully marketed commercial ECs. Although
being able to deliver higher power per unit mass (Ragone plot, Figure 2-1), E