Elsevier

Electrochimica Acta

Volume 55, Issue 15, 1 June 2010, Pages 4412-4420
Electrochimica Acta

Aging of electrochemical double layer capacitors with acetonitrile-based electrolyte at elevated voltages

https://doi.org/10.1016/j.electacta.2010.02.064Get rights and content

Abstract

Laboratory-scale electrochemical capacitor cells with bound activated carbon electrodes and acetonitrile-based electrolyte were aged at various elevated constant cell voltages between 2.75 V and 4.0 V. During the constant voltage tests, the cell capacitance as well as the capacitance and resistance of each electrode was determined. Following each aging experiment, the cells were analyzed by means of electrochemical impedance spectroscopy, and the individual electrodes were characterized by gas adsorption and X-ray photoelectron spectroscopy. At cell voltages above 3.0 V, the positive electrode ages much faster than the negative. Both the capacitance loss and resistance increase of the cell could be totally attributed to the positive electrode. At cell voltages above 3.5 V also the negative electrode aged significantly. X-ray photoelectron spectroscopy indicated the presence of degradation products on the electrode surface with a much thicker layer on the positive electrode. Simultaneously, a significant decrease in electrode porosity could be detected by gas adsorption.

Introduction

Electrochemical double layer capacitors (EDLCs) are a particular class of electrochemical capacitors in which the charge storage ideally occurs entirely via electrostatic forces at the electrode/electrolyte interface [1], [2]. Compared to redox processes commonly encountered in the bulk of battery electrodes, the electrochemical double layer can be both charged and discharged efficiently at high rates and with high reversibility. EDLCs are therefore characterized by higher specific power than galvanic systems due to the restriction of charge storage to the electrode surface. However, the specific energy of EDLCs tends to be lower than that of batteries by roughly one order of magnitude [3], [4]. An increase in the specific energy of EDLCs would represent a major development in performance improvement and in widening the application field of these devices.

A possible strategy to improve both the energy and power density of EDLCs is to increase their operating voltage due to the scaling of these properties with the square of the cell voltage [2], [5]. However, a pronounced increase in the aging rate has been found for EDLCs above their nominal voltage of typically 2.5–2.7 V [6], [7], [8], [9].

There have been a number of studies concerned with the reasons behind the aging of EDLCs. These investigations have dealt with the most commonly employed EDLC systems consisting of activated carbon electrodes in electrolyte solutions of quaternary ammonium salts in acetonitrile (AN) or in propylene carbonate (PC).

In order to account for aging phenomena at elevated voltages, deviations from the idealized capacitive double layer charging mechanism, where the rigid double layer (Helmholtz layer) is represented by a parallel plate capacitor, must be considered. Although ionic charge transfer in the form of ion insertion into the electrode bulk has been proposed as a possible aging mechanism [10], [11], it appears as though this effect is minor for microporous electrodes compared to the effects of electronic charge transfer on aging [12], [13]. The electrolyte decomposition associated with electron transfer may result in gaseous, solid or soluble degradation products. It has been suggested that the rate of degradation is enhanced by an increase in the number of oxygen-containing functional groups of the activated carbon in both PC-based [14] and AN-based [15], [16] electrolytes.

In Et4NBF4/PC, the main gaseous degradation products at elevated cell voltages from 2.6 V up to 4 V have been identified as propene, CO2, ethene, CO and H2 at the negative electrode as well as CO2 and CO at the positive electrode [17], [18]. Gas evolution has been shown to result in a significant pressure increase [19] and may lead to a loss of electrode cohesion [18] and a loss of ions from the electrolyte [6]. On the other hand, it has also been demonstrated that solid electrolyte degradation products in Et4NBF4/PC are already formed at 2.3 V (60 °C) and may adhere to the electrode surface in the form of solid deposits [6], [18], [20], which could result in an electronic insulation of the electrode and a loss of porosity. In fact, through in situ monitoring of the pressure evolution in EDLCs, Hahn et al. [19] found that most irreversible charge loss must be associated with degradation processes other than gas evolution.

In Et4NBF4/AN, the main gaseous decomposition product at 2.5 V and 70 °C has been found to be CO2 [21], presumably from the reaction of acetonitrile with trace water [22], and ethene due to primarily thermal degradation of the cation Et4N+ [21]. Kötz et al. [23] found that the amount of irreversible charge contribution to degradation processes other than gas evolution was even higher in the AN-based electrolyte compared to the PC-based electrolyte. Indeed, both Azaïs et al. [15], [16] and Zhu et al. [24], [25] measured a marked decrease of the electrode specific surface areas due to aging at voltages between 2.3 V and 2.8 V, in particular for the positive electrode, and attributed this to the blockage of pores by solid electrolyte degradation products generated from the AN-based electrolyte. In addition, changes in the chemical composition of the electrodes were found via X-ray photoelectron spectroscopy (XPS) [16], [25] and nuclear magnetic resonance (NMR) [16], and attributed to either the solid degradation products or the functionalization of the electrode surface. Recently, Ruch et al. [26] compared the aging of activated carbon electrodes in acetonitrile as well as in propylene carbonate based electrolytes at 3.5 V and found the aging rate for the single electrodes to depend critically on the solvent, although degradation products could be identified on the electrodes in either electrolyte.

In summary, it appears as though the electrochemical modification of activated carbon electrodes either through the deposition of solid electrolyte degradation products or surface functionalization represents the most important aging pathway for EDLCs at elevated voltages in both PC-based [18] and AN-based [15], [16], [24], [25] electrolytes. While the above findings provide important insights into the various electrochemical processes which occur during aging of EDLCs, a direct correlation of the physicochemical changes undergone by the single electrodes with their actual electrochemical performances in these electrolytes as a function of increased cell voltage has not yet been given.

In the present work, the aging behavior of activated carbon electrodes in 1 M solutions of Et4NBF4 in acetonitrile was investigated systematically as a function of the cell voltage. The loss of electrochemical performance was quantified on an individual electrode basis and correlated with structural and chemical changes of the aged single electrodes using cyclic voltammetry, nitrogen adsorption and XPS. The results highlight the limitations of current EDLC systems based on acetonitrile with respect to cell voltage and the polarity of the single electrodes.

Section snippets

Materials and electrochemical cell assembly

The activated carbon investigated in the present work was YP17 (Kuraray Chemical, Japan). Free-standing carbon sheets were produced from a slurry consisting of 25 wt% YP17 in a 1:1 mixture by mass of isopropanol and distilled water. Under constant stirring, a suspension of 35.5 wt% poly(tetrafluoroethylene) (PTFE) in water (TE 3554-N, DuPont) was added until an equivalent of 10 wt% PTFE was obtained with respect to YP17. In order to achieve precipitation of PTFE, acetone was added to the

Aging at constant voltage

The capacitance loss over time at various cell voltages is summarized in Fig. 2 for the full cells in 1 M Et4NBF4 in acetonitrile. The initial full cell capacitance was 25 F/g, with a rapid loss of capacitance occurring for cell voltages of 3.5 V and above. For the experiments performed at 2.75 V and 3.0 V, the capacitance loss is moderate and amounts to 20% after 500 h.

From the evolution of the single electrode capacitances (Fig. 3) a dramatic dependence of the aging rate on the electrode polarity

Conclusions

In the present study concerning the performance loss of EDLCs, the aging mechanisms of bound activated carbon electrodes (YP17/PTFE) were investigated for increasing constant cell voltages in 1 M Et4NBF4 in acetonitrile.

The aging of symmetric full cells at voltages ≥3.25 V was found to be entirely dominated by the aging of the positive electrode. The enhanced aging rate led to a significant loss of capacitance and an increase in resistance for the positive electrode and the full cell.

The aged

Acknowledgements

Financial support by PSI and the Swiss National Science Fond (SNSF, Project no. 200021-117607) is gratefully acknowledged. We thank Anastasia Savouchkina for contributing to the XPS measurements.

References (39)

  • R. Kötz et al.

    Electrochim. Acta

    (2000)
  • A. Burke

    Electrochim. Acta

    (2007)
  • A. Burke

    J. Power Sources

    (2000)
  • R. Kötz et al.

    J. Power Sources

    (2006)
  • O. Bohlen et al.

    J. Power Sources

    (2007)
  • P.W. Ruch et al.

    Electrochim. Acta

    (2007)
  • P.W. Ruch et al.

    Electrochim. Acta

    (2009)
  • T. Morimoto et al.

    J. Power Sources

    (1996)
  • P. Azaïs et al.

    J. Power Sources

    (2007)
  • M. Hahn et al.

    Electrochem. Commun.

    (2005)
  • M. Hahn et al.

    Electrochim. Acta

    (2006)
  • F.P. Campana et al.

    Electrochem. Commun.

    (2005)
  • P. Kurzweil et al.

    J. Power Sources

    (2008)
  • R. Kötz et al.

    Electrochem. Commun.

    (2008)
  • M. Zhu et al.

    Carbon

    (2008)
  • P.W. Ruch et al.

    Electrochim. Acta.

    (2010)
  • P.W. Ruch et al.

    J. Electroanal. Chem.

    (2009)
  • H. Keiser et al.

    Electrochim. Acta

    (1976)
  • H. Estrade-Szwarckopf et al.

    Synth. Met.

    (1988)
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    Present address: IBM Zurich Research Laboratory, CH-8803 Rüschlikon, Switzerland.

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