Solid electrolytic capacitor with highly stable conducting polymer as a counter electrode

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Synthetic Metals 102 (1999) 973-974

Solid electrolytic capacitor with highly stable conducting polymer as a counter electrode Yasuo Kudoh, Kenji Akami, Yasue Matsuya Advanced Materials Research Laboratory, Matsushita Research Institute Tokyo, Inc. 3-10-l Higashimita, Tama-ku, Kawasaki 214-8501, Japan Abstract We developed a new type of tantalum solid electrolytic capacitor using poly(3,4-ethylenedioxythiophene) (PEDOT) as a counter electrode. The polymer counter electrode was formed on a sintered and anodized tantalum substrate by chemical polymerization. The capacitor was characterized by excellent frequency characteristics and superior durability, due to which it functioned without any deterioration for more than 1000 h at 125C in air and at 85C/85%RH. Keywords: Polythiophene

and derivative; Transition-metal-catalyzed

1. Introduction A tantalum solid electrolytic capacitor (Ta/MnO,capacitor), using manganese dioxide (MnO?) as a counter electrode, has been widely employed because of its high capacitance per unit volume. However, the capacitor has inferior frequency characteristics due to the relatively low conductivity of MnO,, e.g., on the order of 10.’ S cm-‘. Considerable progress in electronic devices during the past decade has created a strong need for overcoming the shortcomings of the Ta/MnO,capacitor. In response to this need, some tantalum solid electrolytic capacitors employing polypyrrole and polyaniline as the counter electrodes were proposed [l-3]. Recently, poly(3,4ethylenedioxythiophene) (PEDOT) has been attracting much attention because of its excellent environmental stability [4]. We developed another new type of tantalum solid electrolytic capacitor (Ta/PEDOT capacitor) with PEDOT chemically prepared in an aqueous medium. In this paper we describe the characteristics of the PEDOT used and the Ta/PEDOT capacitor. 2. Experimental PEDOT for the evaluation of conductivity and environmental stability was prepared in an aqueous medium containing 3,4ethylenedioxythiophene (EDOT), Fe?(SO,),, sodium alkylnaphthalenesulfonate (NaANS) andp-nitrophenol (pNPh). NaANS and pNPh were used as the emulsifier and the additive for the improvement of environmental stability, respectively. The evaluation techniques of the conductivity and environmental stability are similar to those discussed in a previous paper [5]. A sintered and porous tantalum substrate of 1.6 X 3.3 X 6.3 (mm) with a tantalum wire as an anode lead was used in this study. A dielectric layer was formed on the substrate by anodization at 35 V and about 90°C in 0.5 vol.% of phosphoric acid solution. The capacitance after the anodization measured in 5.2 M of sulfuric acid solution was about 220 ILF at 120 Hz. A PEDOT layer was

reaction; Conductivity;

Environmental

stability; Capacitor

prepared by consecutively soaking the tantalum substrate in an EDOT emulsion containing NaANS and then an aqueous oxidant solution containing Fe,(SO,), and pNPh. Subsequently, the Ta/PEDOT capacitor was completed by connecting a cathode lead in a manner similar to that reported previously [6], followed by encapsulating with an epoxy resin. The characterization methods of the resultant capacitor have been reported elsewhere [7]. 3. Results and discussion The initial conductivity of the PEDOT prepared in this work was about 30 S cm-‘. Fig. 1 shows the environmental stabilities of the PEDOT and the most stable polypyrrole (PPy) chemically polymerized by us [5]. While the moisture stability of the PEDOT was comparable to that of the PPy, the PEDOT was far superior to the PPy in thermal stability. Alkylnaphthalenesulfonic ions (ANS) seem to be the predominant dopants in both polymers because they were polymerized in a similar way. Therefore, the superior environmental stability of the PEDOT can be explained as follows: thermal dedoping is prevented due to the bulky molecular structure of ANS; the oxidation reaction on a PEDOT matrix is inhibited because the 13positions of the thiophene ring are blocked by an ethylenedioxy group. Fig. 2 displays the frequency characteristics of the TaiPEDOT capacitor and the currently used Ta/MnO, capacitor. The newly developed capacitor showed greatly improved performance in the high-frequency range. The use of the PEDOT with high conductivity as the counter electrode is responsible for the excellent frequency characteristics [S]. Moreover, in the TaiPEDOT capacitor, the capacitance attainment ratio, which is defined as the ratio of the capacitance observed after the formation of the PEDOT layer to that after the anodization, was more than 90%. This favorable result is most likely due to the addition of NaANS which is a surfactant. Fig. 3 shows the results of two kinds of shelf-life tests. It is clear

0379-6779/99/$ see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98)010121

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that the TaiPEDOT capacitor functions without any deterioration for more than 1000 h at 125°C in air and at 8%/85%RH. With regard to the moisture stability, the Ta/PEDOT capacitor was much better than the currently used TaiMnO, capacitor [2]. The excellent durability is attributed to the superior environmental stability of the PEDOT, as can be seen in Fig. 1.

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Fig. 1 Change in conductivity of PEDOT and PPy as a function of aging time at 125°C in air (a) and at 85C / 85%RH (b). Polymerization temperature: 45°C for PEDOT and 25°C for PPy; polymerization time: 20 h for PEDOT and 1 h for PPy; monomer concentration: 0.05 M for PEDOT and 0.375 M for PPy; Fe,(SO,X concentration: 0.05 M for PEDOT and 0.1 M for PPy; NaANS concentration: 0.012 M for PEDOT and 0.03 M for PPy; pNPh concentration: 0.05 M; medium: deionized water.

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Fig. 3 Changes in capacitance, dissipation factor and impedance of TaiPEDOT capacitor as a function of aging time at 125°C in air (0) and at 85C / 85%RH (A). 4. Conclusions The developed TaiPEDOT capacitor showed the advantages of good frequency characteristics and superior durability, as well as high capacitance per unit volume. Therefore, the commercialization of the Ta/PEDOT capacitor will give rise to further wide-ranging applications. Acknowledgment We would like to thank S. Nishiyama and his colleagues in the Capacitor Division of Matsushita Electronic Components Co., Ltd., for helpful discussion and support throughout this study. References

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Fig. 2 Impedance and capacitance-frequency characteristics for Ta/PEDOTcapacitor (solid line) and Ta/MnOz capacitor (dotted line).

[1] M. Ooi, T. Fukami, A. Kobayashi, H. Taniguchi, T. Date, NEC Technical J., 48, 10 (1995) 77. [2] Y, Kudoh, T. Kojima, Denki Kagaku, 64, (1996) 41. [3] S. Sano, H. Endou, Y. Hama, M. Ooue, Autumn Meet. of Elcctrochem. Sot. Jpn. 1997. (1997) 133. [4] G. Heywang, F. Jonas, Adv. Mater., 4, 2 (1992) 116. [5] Y. Kudoh, K. Akami. Y. Matsuya, Synth. Met., in press. [6] Y. Kudoh, S. Tsuchiya, T. Kojima, M. Fukuyama, S. Yoshimura, Synth. Met., 41-43 (1991) 1133. [7] Y. Kudoh, M. Fukuyama, S. Yoshimura, Synth. Met., 66 (1994) 157. [8] K. Nishitani, J. 1. E. E. J., 89-7, (1969) 1333.