Substituted phosphonium cation based electrolytes for nonaqueous electrical double-layer capacitors
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Tetrakis(diethylamino)phosphonium tetrafluoroborate (TDENPBF4), tetrakis (diethylamino)phosphonium hexafluorophosphate (TDENPPF6), and tetrakis (dimethylamino)phosphonium tetrafluoroborate (TDMNPBF4) in acetonitrile (AN) have been studied as electrical double-layer capacitor electrolytes in a two-electrode test cell using titanium carbide derived carbon, C(TiC), as an electrode material. Electrochemical characteristics for C(TiC)j1 M TDENPBF4 þ AN, C(TiC)j1 M TDENPPF6 þ AN, and C(TiC)j1 M TDMNPBF4 þ AN interfaces have been obtained by cyclic voltammetry, constant current charging/discharging, and electrochemical impedance spectroscopy. High-capacitance (85 F/g) and gravimetric power (269 kW/kg) have been achieved at cell voltage 3.2 V. Data obtained have been compared with results published previously.
I. INTRODUCTION
Electrical double-layer capacitors (EDLC)1–11 have proven to be necessary for optimizing energy use in many consumer goods. Previous studies have shown that power density of EDLC is mainly determined by electrolyte properties in the microporous electrode material and porous separator matrix used.4–6 The strong influence of molar volume and composition of cation has been observed for microporous electrode material, with average pore diameter in the same order as the totally or partially desolvated cations, i.e., tetraethylammonium and triethylmethylammonium ions.2–10 This article considers characteristics for titanium carbide derived carbon [C(TiC)] in 1 M electrolyte, prepared using tetrakis(diethylamino)phosphonium tetrafluoroborate (TDENPBF4), tetrakis(diethylamino)phosphonium hexafluorophosphate (TDENPPF6), and tetrakis(dimethylamino)phosphonium tetrafluoroborate (TDMNPBF4) in acetonitrile (AN), obtained by the cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and constant current charging/discharging (CC) methods. Comparing the electrode–electrolyte interfaces instead of bulk electrolytes or electrode materials has been also suggested on the basis of the latest studies presented in literature,3–11 where the need for systematic study of different electrode–electrolyte interfaces has been emphasized, because it is currently impossible to predict the combination of salt, solvent, and electrode material showing high capacitance, energy and power density, and short charging/discharging times at the moment being. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0185 J. Mater. Res., Vol. 25, No. 8, Aug 2010
TDENPBF4, TDENPPF6, and TDMNPBF412,13 as the surface inactive electrolytes have been selected to study the influence of organic cation and anion (BF4 or PF6) compositions, molar volume, polarizability, effective dipole moment, and possible Gibbs adsorption differences (caused by variation of so-called “squeezing out” effect from AN solvent) on the EDLC single-cell characteristics. The methyl or ethyl substituted phosponium cations have gained our interest because they have superior thermal stability, spherical shape, and, because of t
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