Electrochemical capacitors (ECs, also sometimes denoted as “supercapacitors” or “ultracapacitors”) [1,2] are energy-storage devices that bridge the performance gap between the high energy density provided by batteries and the high power density (but very limited energy density) derived from dielectric capacitors. Commercially available electrochemical capacitors exhibit gravimetric energy density up to 8.5 Wh.kg^-1 and usable power density up to 9.0 kW.kg^-1. In the field of electrochemical capacitors there is often confusion between the electrical parameters of a full device and the electrochemical properties of the individual electrodes that comprise the cell [3]. The focus of this communication is to describe the distinctions between these various devices and their constituents, starting with a comparison of dielectric capacitors versus electrochemical capacitors, followed by discussion of other electrochemical energy storage devices with regard to their electrical properties.

The electrochemical behavior of common electrode materials used in ECs and related devices will be discussed in terms of capacitive, pseudocapacitive [3] and Faradic charge-storage mechanisms, as well as recommended methods with which such electrodes should be characterized. The distinctions between carbon-based capacitive electrodes [4] that are commonly found in commercial ECs, and pseudocapacitive electrodes such as RuO₂ [5,6], or MnO₂[7,8], that have the electrochemical signature of a capacitive electrode but express different charge-storage mechanisms, will be highlighted. Finally, the important distinctions between high-power battery-type electrodes and pseudocapacitive electrodes will be described. New emerging concepts such as extrinsic pseudocapacitance [9] or intercalation pseudocapacitance [10] will be discussed at the light of recent results in the field.

[1] Conway BE. Electrochemical Capacitors: Scientific Fundamentals and Technology Applications. New-York:Kluwer Academic/Plenum Publishers;1999.

[2] Béguin F, Frackowiak E. Supercapacitors: Materials, Systems, and Applications, Weinheim, Germany Wiley-VCH Verlag GmbH & Co.; 2013.

[3] Brousse T, Bélanger D, Long JW. To be or not to be pseudocapacitive? J Electrochem Soc 2015;62(5): A5185-9.

[4] Simon P, Gogotsi Y. Materials for electrochemical capacitors. Nature Materials 2008; 7: 845-854.

[5] Ardizzone S, Fregonara G, Trasatti S. “Inner” and “outer” active surface of RuO₂ electrodes. Electrochim Acta 1990;35(1):263-7.

[6] Zheng JP, Cygan PJ, Jow TR, Hydrous ruthenium oxide as an electrode material for electrochemical capacitors. J Electrochem Soc 1995;142(8):2699-703.

[7] Lee HY, Goodenough JB., Supercapacitor Behavior with KCl Electrolyte. J Solid State Chem 1999;144(1):220-3.

[8] Toupin M, Brousse T, Bélanger D. Charge storage mechanism of MnO₂ electrode used in aqueous electrochemical capacitor. Chem Mater 2004;16:3184-90.

[9] Augustyn V, Simon P, Dunn B. Pseudocapacitive oxide materials for high-rate electrochemical energy storage. Energy Environ Sci.2014;7:1597-1614.

[10] Simon P, Gogotsi Y, Dunn B., Where do batteries end and supercapacitors begin? Science 2014;343 :1210-1.