Electrochemical Potential Windows of Supercapacitor Electrolytes from First-Principles Calculations

Hiroyuki Maeshima1,Craig A. J. Fisher2, Akihide Kuwabara2, Hiroki Moriwake2

1Panasonic Electronic Device Co., Ltd, Kadoma, 571-8506, Japan

2Nanostructures 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

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.

(a) (b)

(c) (d)

FIG. 1. Comparison of electrochemical potential windows of seven electrolytes evaluatedby cyclic voltammetry (Experimental) and HF/6-31+G(d,p) calculations using four typesof 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--basedelectrolytes and squares the PF6

--based electrolyte.

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AMTC Letters Vol. 2 (2010)

© 2010 Japan Fine Ceramics Center