Download Electrochemical Potential Windows of Supercapacitor Electrolytes

AMTC Letters
Vol. 2 (2010)
© 2010 Japan Fine Ceramics Center
Electrochemical Potential Windows of Supercapacitor Electrolytes
from First-Principles Calculations
Hiroyuki Maeshima1,Craig A. J. Fisher2, Akihide Kuwabara2, Hiroki Moriwake2
1
2
Panasonic Electronic Device Co., Ltd, Kadoma, 571-8506, Japan
Nanostructures Research Laboratory, Japan Fine Ceramics Center, Nagoya, 456-8587, Japan
Electrochemical potential windows of several organic liquid electrolytes for
supercapacitors calculated using ab initio molecular orbital theory are reported. A
supercapacitor (also known as an ultracapacitor or electric double-layer capacitor,
EDLC) is an energy storage device with high power density compared with secondary
batteries (e.g., Li ion batteries). EDLCs can charge/discharge large currents in a short
time, making them promising candidates for power storage devices in fully electric
vehicles (FEVs) and hybrid electric vehicles (HEVs) [1].
The chemical stability of the electrolyte against anodic and cathodic reactions
is one of the most important factors controlling the performance of EDLCs, because it
determines the maximum operational voltage. The electrolyte’s stability is typically
evaluated by measuring its electrochemical potential window. This is defined as the
potential difference across the electrolyte when redox reactions between the electrolyte
and electrode surfaces start to occur.
Previous attempts to estimate the potential windows of electrolytes have been
based on molecular orbital theory. For example, oxidation potentials of organic and
inorganic anions in lithium ion battery electrolytes have been estimated using
semi-empirical methods [2,3] and ab inito methods [4,5]. One shortcoming of these
earlier studies, however, was that they assumed reduction and oxidation potentials were
determinable from the respective species in isolation. In other words, neither
cation-anion interactions nor solute-solvent interactions were taken into account.
In this study, four types of models are used to investigate the effect of
intermolecular interactions in EDLC electrolytes: (1) a single-ion-in-vacuo model, (2) a
single-ion-in-solvent model, (3) an ion-pair-in-vacuo model, and (4) an
ion-pair-in-solvent model. For all calculations, the HF/6-31+G(d,p) level of theory was
used. Solute ion interactions were treated by considering a number of cation-anion pair
confirmations, and solute-solvent interactions were introduced by applying the
isodensity polarizable continuum model (IPCM) [6].
For comparison, electrochemical potential windows of electrolytic solutions
were measured from current-voltage curves by cyclic voltammetry, where the potential
window is defined as the potential region in which no appreciable faradaic current flows.
The reproducibility of the measured electrochemical potential windows was typical of
such experiments (measurement error less than 0.1 V).
Figure 1 shows the results for the four different models by comparing the
theoretical values with experimental values determined by cyclic voltammetry [7]. The
ion-pair-in-solvent model can be seen to quantitatively reproduce the experimental
electrochemical potential windows with high accuracy. This demonstrates that in actual
electrolytes intermolecular interactions, particularly cation-anion and solute-solvent,
play an important role in determining electrochemical potential windows.
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AMTC Letters
Vol. 2 (2010)
© 2010 Japan Fine Ceramics Center
(a)
(b)
(c)
(d)
FIG. 1. Comparison of electrochemical potential windows of seven electrolytes evaluated
by cyclic voltammetry (Experimental) and HF/6-31+G(d,p) calculations using four types
of models (Theoretical): (a) single-ion-in-vacuo model, (b) single-ion-in-solvent model,
(c) ion-pair-in-vacuo model, and (d) ion-pair-in-solvent model. Circles are for BF4--based
electrolytes and squares the PF6--based electrolyte.
References
[1] T. R. Jow, US. DOE. Rep., 39 (1999)
[2] H. Yilmaz, E. Yurtsever and L. Toppare, J. Electroanal. Chem., 261 (1989) 105.
[3] F. Kita et al., J. Power Soc., 68 (1997) 307.
[4] M. Ue, A. Murakami, and S. Nakamura, J. Electrochem. Soc., 149 (2002) A1572.
[5] M. Yoshimura et al., Diamond Relat. Mater., 11 (2002) 67.
[6] J. B. Foresman et al., J. Phys. Chem., 100 (1996) 16098.
[7] H. Maeshima, H. Moriwake, A. Kuwabara and C. A. J. Fisher, J. Electrochem. Soc.,
(2010) in press.
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