Aging tests of double layer capacitors - cycle vs. hold R. Kötz, D. Weingarth, A. Foelke-Schmitz, M.M. Hantel phone: +41 56 310 20 57, e-mail: ruediger.koetz@psi.ch
Electrochemical Double Layer Capacitors (EDLC) are characterized by high power density and an unsurpassed cycle stability. However, EDLCs suffer from a relatively low energy density, which is at least 10 to 20 times smaller than today's Li-ion batteries. As a consequence many research groups worked on the increase of the energy density by either increasing the voltage window of the capacitor or by increasing the specific capacitance of the active electrode material. In both directions significant progress has been reported in the past. One typical approach towards higher cell voltage of EDLC is the use of an ionic liquid (IL) as electrolyte [1]. IL are so-called room temperature melts, which do not need any solvent, which otherwise limits the electrochemical stability window of the EDLC. Another approach towards higher stability is the utilization of special carbons such as carbon nano tubes (CNT). The high surface area of CNT is mainly provided by the basal plane like walls of the tube, which are assumed to be rather inert [2]. A combination of both, ionic liquids and single walled carbon nanotubes was proposed in [3] and allowed for an EDLC with a cell voltage of 4 V. An attempt to increase EDLC energy density, however, can only be called successful if other parameters like stability and lifetime are not diminished. There are two ways to prove the stability and longevity of the capacitor electrode or capacitor cell. Firstly by cycling the EDLC between the new extended cell voltage and zero and secondly by constant load tests at the increased maximum cell voltage. While in the literature EDLC stability is often demonstrated by cycling experiments lasting over several ten thousand cycles at PSI we prefer the constant voltage hold tests [4].
decrease in capacitance for times above 200 h is significant and leads to the conclusion that the system is not stable under the applied conditions. (a)
(b)
Experimental
Figure 1. Accelerated stability test of an EDLC by (a) constant current cycling and (b) by constant voltage hold.
Electrodes were prepared from a mesoporous carbon MM192 using PTFE as a binder. The electrolyte used was a typical ionic liquid EMIMBF4. During constant voltage hold tests the after every 10 hours the cell was cycled between 2.5 V and 0 V in order to control the capacitance [4]. During cycling at a constant current of mA/g the capacitance could continuously be determined from each cycle.
With the assumption that only the cell voltages above 3.25 V are harmful to the system one can calculate the time the cell was exposed during cycling to cell voltages between 3.25 and 3.75 V. This is about 166 h. Figure 1b clearly shows that a test period of 166 h is not enough to decide whether the system is stable or not. Only after about 300 h severe performance loss is obvious.
Results
From the above results one can conclude that constant voltage hold tests are the more demanding, and less time consuming accelerated aging tests then cycling tests with the same maximum cell voltage.
Aging tests of the capacitor cells using EMIMBF4 and mesoporous carbon was tested by constant voltage hold experiments at a cell voltage of 3.75 V and by cycling experiments between 3.75 V and 0.0 V. The results of the cyclic experiment are reproduced in Figure 1a. The cell appears to be stable over more than 12000 cycles, which was equivalent to a test period of 50 days. The initial increase in capacitance can be ascribed to some kind of a formation period during which the wetting of the pores is completed. The capacitance loss during 12000 cycles is in the order of 10%. The results of the constant load tests are shown in Figure 1b over a time period of 500 hours. While an increase of the capacitance during the first 100 h is in good agreement with the results of the cycling test the
References [1] C. Arbizzani, M. Biso, D. Cericola, M. Lazzari, F. Soavi, M. Mastragostino, J. Power Sources 185, 1575–1579 (2008). [2] A. Izadi-Najafabadi, S. Yasuda, K. Kobashi, T. Yamada, Don N. Futaba, H. Hatori, M. Yumura, S. Iijima, K. Hata, Adv. Mater. 22, E235–E241 (2010). [3] H. Zhanga, G. Cao, Y. Yang, Z. Gu, Carbon 46, 30–34 (2008). [4] P.W. Ruch, D. Cericola, A. Foelske-Schmitz, R. Kötz , A. Wokaun, Electrochim. Acta 55, 4412-4420 (2010).
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