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Fabrication of solid aluminum electrolytic capacitors utilizing conductive polyaniline solutions
Synthetic Metals 162 (2012) 368–374
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Fabrication of solid aluminum electrolytic capacitors utilizing conductive polyaniline solutions Ye Song ∗ , Longfei Jiang, Weixing Qi, Chao Lu, Xufei Zhu Key Laboratory of Soft Chemistry and Functional Materials of Education Ministry, Nanjing University of Science and Technology, Nanjing 210094, China
a r t i c l e
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Article history: Received 8 September 2011 Received in revised form 19 December 2011 Accepted 23 December 2011 Available online 21 January 2012 Keywords: Aluminum electrolytic capacitor Solid capacitor Conductive polyaniline solution Low impedance Long life
a b s t r a c t To overcome the risk of electrolyte leakage and the shortcoming of higher impedance at high frequencies for the conventional aluminum electrolytic capacitors impregnated with electrolyte solutions, solid aluminum electrolytic capacitors employing conducting polyaniline (PANI) as a counter electrode were developed. The dodecylbenzenesulfonic acid (DBSA) doped PANI/chloroform solution, camphorsulfonic acid (CSA)–DBSA co-doped PANI/chloroform solution and DBSA doped PANI/toluene solution were synthesized by direct and inverse emulsion polymerization pathway. The capacitors using the solution of CSA–DBSA co-doped PANI/chloroform showed the best electrical properties. The performances of the PANI capacitors can be remarkably improved after secondary doping with m-cresol. The as-fabricated solid capacitors have very low impedances at high frequencies and excellent temperature characteristics as well as long shelf-life. The results can be ascribed to high conductivity and superior thermal stability for the CSA–DBSA co-doped PANI. Our strategy of using conductive PANI solution to fabricate the solid capacitors is not only compatible with the existing fabrication procedure for the conventional capacitor but also suitable for commercial mass production of low cost PANI. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, aluminum electrolytic capacitors are widely employed in various electronic devices because of their high capacitance per unit volume and low cost [1]. Generally, a conventional aluminum electrolytic capacitor consists of an anode aluminum foil with a very thin dielectric film (anodic alumina) and a cathode aluminum foil as well as a paper separator interposed between them. They are wound together to form a capacitor element. To increase capacitor plate area or capacitance, the anode aluminum foil has a highly etched surface by an etching process. The dielectric film grown on the surface of the anode aluminum foil is electrically connected with the cathode foil through an electrolyte soaked in the separator. However, the use of the electrolyte leads to two disadvantages, i.e., the risk of electrolyte leakage and the higher equivalent series resistance (ESR) of the capacitors due to lower conductivity of the electrolyte as an ionic conductor. Moreover, as capacitors are used in higher and higher frequency applications, the lower ESR or impedance of a capacitor at high frequencies is becoming more and more important to the industry. Hence, a solid electrolytic capacitor employing conducting polymer as a solid electronic conductor instead of the liquid
∗ Corresponding author. Tel.: +86 25 84315949; fax: +86 25 84276082. E-mail address: soong [email protected] (Y. Song). 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.12.022
electrolyte was proposed. This kind of solid electrolytic capacitor not only eliminates the risk of electrolyte leakage but also has a much lower ESR or impedance. Various conducting polymers such as polyaniline (PANI), polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT) have been introduced as counter electrodes in solid electrolytic capacitors [2–5]. Among these conducting polymers, PEDOT has recently attracted much attention because of its high conductivity and excellent stability [6,7]. Generally, the PEDOT counter electrode was preferentially formed by in situ polymerization of EDOT in the presence of a chemical oxidant to penetrate the porous structure of the separator and the highly structured surface of the anodic film. To have better control of the reaction stoichiometry and obtain more uniform PEDOT film coated on the surface of the anode foil, impregnation is performed by dipping the elements into the mixed solution of the monomer and oxidant. A major obstacle to this process is limited life of the reactant solution. Even at very low conversion rates, PEDOT particles precipitate from the reaction solution and may clog the separator pores during impregnation. Furthermore, a tedious post-treatment process is needed to remove the residue of polymerization reaction. On the other hand, PANI is also one of the most studied conducting polymers because of its advantages such as low cost monomer, easy preparation, good environmental stability and thermal stability, and controllable electrical conductivity [8]. Although the conductivity of PANI is usually lower than that of PEDOT, it is
Y. Song et al. / Synthetic Metals 162 (2012) 368–374
one of the most promising candidates as a processible conducting polymer. Especially, PANI obtained by the emulsion polymerization has good solubility, good solution processability, reasonably high molecular weight, and good electrical conductivity [9]. From a practical point of view, the use of conductive PANI solutions to form the PANI film as the capacitor counter electrode has two advantages over the fabrication method of in situ polymerization. The first is that the ex situ fabrication of conductive PANI can avoid the difficulty in removing all byproducts of in situ polymerization and their potential damage to anodic films. The second is that the fabrication procedure for the solid capacitor via conductive PANI solutions is much more compatible with the existing fabrication procedure for the conventional wet electrolytic capacitor. In this work, three PANI solutions prepared by various approaches were employed to obtain the counter electrode of conducting PANI. The solid aluminum electrolytic capacitors with the wound-type construction were fabricated with these PANI solutions and their fabrication processes were described in detail. The influences of the PANI solutions and secondary doping process on the electrical performances of the capacitors were examined. The frequency characteristics, temperature characteristics and hightemperature durability for the capacitors were reported. 2. Experimental 2.1. Synthesis of conductive PANI solution 2.1.1. Synthesis of dodecylbenzenesulfonic acid (DBSA) doped PANI/chloroform solution In a typical polymerization experiment, a solution of 5 mL of aniline and 8.2 g of DBSA and 150 mL of deionized water was prepared in a 250 mL round bottomed flask and kept under mechanical stirring. Then, 6 mL of n-butanol was added slowly to the milky white solution to form a transparent miniemulsion. Subsequently, 50 mL of 1 M ammonium persulfate solution was added dropwise to the miniemulsion over a period of 30 min. The reaction mixture was kept at 20 ◦ C under mechanical stirring for 4 h. In the end, 100 mL of chloroform was added to the resultant suspension and the mixture was shaken for a few minutes and then kept undisturbed for 12 h. After phase separation, the organic phase containing PANI, which settled down, was separated using a separation funnel. The extraction process was repeated on the resulting organic phase three more times using 100 mL of deionized water to remove all byproducts and excess dopant. The as-prepared DBSA doped PANI/chloroform solution is designated as Solution A. It should be mentioned that the preparation procedure of the PANI-DBSA solution was simplified and the amount of organic solvent used was reduced compared with the conventional synthesis method [10]. 2.1.2. Synthesis of camphorsulfonic acid (CSA)–DBSA co-doped PANI/chloroform solution The polymerization process was the same as the above method except that 8.2 g of DBSA was replaced by the mixture of 7.8 g DBSA and 10.5 g CAS. The as-prepared CSA–DBSA co-doped PANI/chloroform solution is designated as Solution B. 2.1.3. Synthesis of DBSA doped PANI/toluene solution A modified polymerization procedure similar to Shreepathi and Holze [11] was used. In a typical polymerization experiment, 0.625 g of benzoyl peroxide was added to a 250 mL round bottomed flask containing 105 mL of toluene and kept under mechanical stirring. 10 mL of 2-propanol and 26.3 g of DBSA were added to the above mixture followed by the addition of 3 mL of aniline. Then 22 mL of water was added to this clear solution to form an inverted emulsion. The reaction mixture gradually turned green in ca. 2.5 h and stirring was continued for 28 h at 0 ◦ C. At the end of the reaction,
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100 mL of acetone was added to the resultant suspension and kept undisturbed for 12 h. The organic phase containing PANI, which settled down, was separated using a separation funnel. Treatment with acetone (100 mL) and separation were repeated three times to remove all byproducts and excess dopant. The as-prepared DBSA doped PANI/toluene solution is designated as Solution C. 2.2. Fabrication of solid aluminum electrolytic capacitors Capacitor elements (kindly provided by KJ (H.K.) Electronic Co. Ltd.) with rated voltage/capacitance of 25 V/470 F, 10 mm × 10 mm (diameter × height) were used as the examples in this study. To reduce leakage current of the capacitors, a DC voltage of 28 V was applied to the capacitor elements impregnated with aging electrolytes which were solutions of ammonium salts in ethylene glycol until the leakage current remained unchanged. Then the capacitor elements were dried at 80 ◦ C under 100 mbar of pressure. Subsequently, the dried elements were immersed into the above PANI solutions under reduced pressure and then dried at 100 ◦ C for 30 min. Generally, the process of immersion in PANI solutions and drying was repeated for four times to form an intact layer of conductive PANI on the surface of anodic films. Afterward, the elements with PANI were immersed into m-cresol under reduced pressure and then dried at 100 ◦ C under 100 mbar of pressure. Finally, the elements were sealed in an aluminum can by the same encapsulation techniques as conventional aluminum electrolytic capacitors. Solid aluminum electrolytic capacitors were completed ultimately after a DC voltage of 28 V was applied again for 60 min at 100 ◦ C to further reduce the leakage current. 2.3. Measurement The capacitance and dissipation factor of the capacitors were measured at a frequency of 100 Hz and their impedances measured at a frequency of 100 kHz by means of a LCR meter (hp-4284A). The leakage currents of the capacitors were measured by a leakage current tester (TH2685B). The temperature characteristics of the capacitors were obtained in a MINI-SUBZERO environmental chamber (MC-810, TABAI ESPEC Co.). The endurance test of the capacitors was performed at 125 ◦ C for 1000 h. 3. Results and discussion 3.1. Selection of aging electrolytes It is well known that the most vital part of an electrolytic capacitor is the anodic oxide film which acts as the dielectric. Nevertheless, the damage to the anodic film such as cracks is inevitable during the manufacturing process of a capacitor element due to the action of stress. Thus, it is necessary to use an aging electrolyte to repair the anodic film by anodization process at an applied voltage before the capacitor element is impregnated with a conductive PANI solution. The aging electrolytes commonly used are some solutions of ammonium salts dissolved in ethylene glycol. The electrolytes employed in this work were composed of ammonium adipate (4 wt%) and ammonium phosphate (1 wt%). As described above, the aging electrolytes would be eliminated after repairing the anodic film. Considering the requirement for ease of removal of the electrolytes, H2 O/ethylene glycol mixed solvent was used to decrease high boiling point of ethylene glycol. To investigate the effect of the water content on the ability of the electrolytes to heal the anodic film, the content of ammonium salts was kept fixed at 5 wt% while varying the ratio of H2 O to ethylene glycol in the mixed solvent. Fig. 1 shows the change of leakage current with time when the capacitor elements impregnated with the aging electrolytes were polarized at a constant voltage of 28 V. Similar trends
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Y. Song et al. / Synthetic Metals 162 (2012) 368–374 Table 1 Comparison of the electrical characteristics for the capacitors impregnated with three PANI solutions.
Fig. 1. Change of leakage currents of the capacitors with time while polarized in aging electrolytes with different water content.
can be observed in these decay curves of leakage currents, i.e., the leakage currents are initially appreciable but drop fairly rapidly towards steady values. Moreover, the steady values of leakage currents increase with increasing water content. This indicates that the increase of the ratio of H2 O to ethylene glycol is unfavorable for repairing anodic films. That is, high water content causes the final leakage current of the capacitor to be high. This experimental finding is in agreement with previous studies [12,13]. It can be explained by the electrokinetic properties of the anodic film and the separator paper [13]. If the relative zeta potentials of the anodic film and the separator paper in electrolytes were negative and positive, respectively, the tendency of the anodic film was to stay on the aluminum anode when voltage was applied. On the contrary, if both the film and the separator paper had negative potentials, the separator paper would plate out on the anode, and thus interfered with the normal functioning of the anode. This led to corrosion, resulting in the increase of the final leakage current. It has been found that the separator paper has a positive while the film has a negative potential in the glycol-rich solutions, whereas both the film and the separator paper have negative potentials in the water solution [13]. Therefore, the higher the content of ethylene glycol, the lower the leakage currents. In this work, we chose a mixed solvent containing 40 wt% water as the solvent of the aging electrolytes considering the easy removal of the electrolytes in subsequent processing. 3.2. Effect of PANI solutions In wound-type aluminum electrolytic capacitors, porous paper separators inserted between the anode and cathode foils act as liquid impregnation by capillary and diffusion effects. When a capacitor element is impregnated with conventional electrolytes, small molecule solute together with solvent can both penetrate into the inner part of the element. In contrast, for the PANI impregnation solutions, PANI macromolecules cannot easily follow small solvent molecules to penetrate into the inner part of the element because the interaction between PANI macromolecules and the textile fibers of the separator paper (also a polymer) is usually stronger than that between PANI macromolecule and small solvent molecules. Thus, PANI cannot easily coat all anodic film surfaces, especially the inside of the element, by capillary and diffusion effects. In other words, a PANI counter electrode of the capacitor cannot be formed uniformly and thus potential capacitance of the capacitor cannot be achieved. Therefore, PANI impregnation solutions play a key role in the present approach to the fabrication of solid aluminum electrolytic capacitors. In order to select PANI solutions suitable for impregnation, we prepared three PANI
PANI solutions
C/F
tan ı
Z/
Solution A Solution B Solution C
210.8 297.7 145.3
0.075 0.033 0.492
0.102 0.031 1.261
solutions for the fabrication of the solid capacitors (see Section 2). Solution A and Solution B were both prepared by the miniemulsion polymerization-extraction approach but with different dopants in polymerizations. The PANI was doped with DBSA in Solution A, whereas it was co-doped with CSA and DBSA in Solution B. Conversely, Solution C was prepared by an inverse emulsion polymerization route and PANI was dissolved into toluene instead of chloroform. All of these PANI solutions exhibited dark green, indicating that the dissolved PANI was in the conducting emeraldine salt state. The solid capacitors were fabricated by utilizing the three PANI impregnation solutions. It was found that the PANI counter electrode is hard to be wholly formed by dipping the elements into the PANI solutions only once. Generally, impregnation and drying process needed to be performed for three or four times, a continuous PANI layer can be formed successfully. The electrical characteristics of the capacitors fabricated with three PANI solutions by four times impregnation and drying is listed in Table 1. As can be seen, the capacitance of the capacitors impregnated with Solution B is the largest. In other words, the surface area of PANI counter electrode is the largest. This result indicates that the PANI solution can reach readily the inside of the capacitor elements while Solution B is impregnated. This is likely because Solution B containing the co-doped PANI has more proper surface tension under the cooperative action of both CSA and DBSA dopants compared with Solution A containing the PANI doped with only DBSA. It is well known that DBSA is a typical anionic surfactant. Compared with CSA molecule, DBSA contains a long non-polar alkyl group which is compatible with non-polar or weakly polar organic solvents (Fig. 2). Therefore, it is not surprising that PANI-DBSA can be soluble in toluene (nonpolar solvent) whereas PANI-CSA cannot [14]. Correspondingly, the molecular interaction between PANI-CSA and chloroform (polar solvent) should be greater than that between PANI-DBSA and chloroform. As a result, PANI macromolecules co-doped with CSA and DBSA can more easily follow small solvent molecules to penetrate into the inner part of the element than those doped with only DBSA. In addition, it has been reported that treatment with some coupling agents can enlarge the contact area between the anodic films and conducting polymer by improving interfacial adhesion, leading to enhanced performance of the capacitors [7,15]. It is believed that DBSA also have similar effects. As for Solution C, the relatively bad performance of capacitors can probably be related to toluene solvent used in the solution. Due to the non-polar character of toluene, the interaction between PANI macromolecules and toluene molecules is weaker than that between PANI macromolecules and chloroform molecules. Hence, PANI macromolecules are easily blocked by the textile fibers of the separator paper as discussed above. A counter electrode of PANI coated on the surface of anodic films cannot be formed wholly, which leads to lower capacitance, higher dissipation and impedance. This result indicates that the polarity of solvent used in PANI solutions is a key factor influencing PANI impregnation process. Because the performance of the capacitors impregnated with Solution B was the best, it was used as the PANI impregnation solution in the following investigations. Table 2 lists the electrical performances of the capacitors impregnated with Solution B after
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Fig. 2. Chemical structures of (a) DBSA and (b) CSA.
Fig. 3. Photographs of (a, c) separator paper and (b, d) anode aluminum foil after (a, b) first dipping and (c, d) fourth dipping.
each cycle of dipping in PANI solutions and drying. As expected, the electrical performances of the capacitors are improved with dipping times. As can be seen from Table 2, the characteristics of the capacitors impregnated with Solution B for four times and five times are almost the same. This suggests that an intact layer of conductive PANI on the surface of anodic films can be formed by a four-times impregnation process. This can be apparently observed while unpacking the capacitor elements. Fig. 3 shows photographs of the attachment of PANI onto the separator paper and anode aluminum foil after the first impregnation and the fourth impregnation of Solution B, respectively. Clearly, a uniform layer of green PANI is coated on both the separator paper and anode aluminum foil after four times impregnation. Thus, the best characteristics of the capacitors can be obtained.
3.3. Effect of secondary doping Earlier studies revealed that the electrical behavior and structural order of doped PANI can be varied by varying the solvent from which the film is cast. For instance, films of PANI-CSA have a conductivity of ca. 0.1 S cm−1 when cast from chloroform, whereas those films with the same composition have a conductivity of ca. 200 S cm−1 when cast from m-cresol [16,17]. This phenomenon is known as secondary doping which was first introduced by MacDiarmid and Epstein [17,18]. In order to improve the conductivity of PANI, the capacitor samples from Table 1 were further treated by a secondary doping process, i.e., the capacitor elements were impregnated with m-cresol and then dried at 100 ◦ C under reduced pressure. Table 3 presents the characteristics of the capacitors after secondary doping with m-cresol. It is obvious that the electrical performances of the capacitors impregnated with three PANI solutions
Table 2 Characteristics of the capacitors with different dipping times. Dipping times
C/F
tan ı
Z/
1 2 3 4 5
130.4 201.5 263.6 297.4 297.7
0.207 0.094 0.043 0.034 0.033
0.116 0.098 0.063 0.031 0.031
Table 3 Characteristics of the capacitors after secondary doping with m-cresol. PANI solutions
C/F
tan ı
Z/
Solution A Solution B Solution C
245.6 325.9 177.6
0.046 0.023 0.145
0.085 0.021 0.131
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Y. Song et al. / Synthetic Metals 162 (2012) 368–374
Fig. 4. Leakage current–time plot for the PANI capacitor.
are all improved through secondary doping process as compared with the data in Table 1. MacDiarmid and Epstein [17] demonstrated that the interaction of PANI with m-cresol could cause a conformational transition of the polymer chain from a “compact coil” to an “expanded coil”. When the PANI chains become more expanded coil-like, the polaron band becomes more delocalized, the carrier mobility and hence the intrachain conductivity increase. In addition, the higher crystallinity caused by more expanded coil conformation results in a higher interchain conductivity [18]. It is the enhancement of PANI conductivity after secondary doping that leads to the lower dissipation and impedance of the capacitors. Moreover, m-cresol can force the PANI coated on the separator paper or anode aluminum foil to spread out into a more uniform film during secondary doping because m-cresol as a polar organic solvent is able to redissolve PANI to some extent. This leads to an improved capacitance of the capacitors due to an increased area of PANI overlayer on the surface of anode aluminum foil.
Fig. 5. Impedance–frequency plots of the two types of capacitors.
at defect sites of anodic films due to local Joule heating caused by the local high currents through the defects. It has been proposed that the conductivity degradation of PANI after a high temperature treatment was mainly due to the segregation of the dopants
3.4. Capacitor characteristics It was found that the capacitors after secondary doping process exhibited appreciable leakage currents, although the anodic oxide films of the elements were repaired by the aging electrolytes in advance. This behavior may be attributed to the mechanical disruption of the anodic films during the processes of drying the aging electrolytes and PANI solutions or the chemical damage during secondary doping process due to the acidic character of m-cresol [19]. To reduce the leakage currents of the electrolytic capacitors, a common approach is using the aging electrolytes to repair the anodic oxide films coated with PANI by the anodization process. In the aging electrolytes, however, not only anodic film defects can be restored by anodic oxidation of aluminum but also the PANI coated on the anodic films can be overoxidized at the same time due to a higher applied voltage. The overoxidized PANI was of very low conductivity [20], leading to the deterioration of capacitor performance. An alternative approach to lower leakage currents is to render PANI counter electrode nonconductive to block the conducting paths between PANI and aluminum electrodes. This can be achieved by applying a certain voltage slightly higher than rated voltage of the capacitor. Fig. 4 shows the leakage current of the capacitor as a function of time while subjected to an applied voltage of 28 V at 90 ◦ C. It is seen that the leakage current decays rapidly with time at the beginning and then decreases slowly to a minimum value. It is believed that the decrease of leakage current can be ascribed to a conductivity change from conducting to insulating for PANI located
Fig. 6. Temperature characteristics of the two types of capacitors: (a) capacitance and (b) dissipation factor.
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the conductivity of PANI increases slightly with temperature in the temperature range of −50 to 125 ◦ C [25]. In fact, this result can be understood when considering the increase of its capacitance with temperature, because the increase of the capacitance can lead to the increase of dissipation factor [13]. Finally, the PANI solid capacitors have excellent hightemperature durability. Fig. 7 illustrates the initial characteristics and their changes for the capacitors when stored on shelf at 125 ◦ C. Compared with the conventional wet electrolytic capacitors, the changes in the capacitance, dissipation factor and impedance for the PANI capacitors are very small during shelf-life tests of 1000 h. The limited lifetime for the conventional wet electrolytic capacitors is due to the time-dependent evaporation of the electrolyte through the seal. The performances of the PANI solid capacitors deteriorate with time at elevated temperatures as a consequence of the decrease of PANI conductivity during high-temperature durability test as mentioned above. The thermal gravimetric analysis indicated that the CSA–DBSA co-doped PANI has better thermal stability than the DBSA doped PANI (not shown). This is in agreement with the excellent high-temperature durability for the PANI solid capacitors fabricated using Solution B. 4. Conclusions
Fig. 7. Changes of capacitance, dissipation factor and impedance with time at 125 ◦ C for the PANI capacitors.
from PANI backbones, the cross-linking of the PANI backbones, and evaporation and degradation of the dopants [21,22]. The as-prepared solid capacitors have a very low impedance in the high-frequency range owing to the relatively high conductivity of PANI. For comparison, a conventional wet electrolytic capacitor was also fabricated using the same capacitor element impregnated with a previously reported organic electrolyte which was a gammabutyrolactone solution of 1.5 M 1-methyl-3-ethylimidazolium hydrogen maleate [23]. Fig. 5 shows the frequency dependence of impedance for the two types of capacitors. The as-prepared solid capacitors possess a fairly high self-resonance frequency and very low impedances at high frequencies relative to the wet capacitors. Their impedances at high frequencies are approximately one order of magnitude lower than those of the wet capacitors. The low impedances at high frequencies for the PANI solid capacitors can meet the growing high-frequency requirements of a vast range of new electronic apparatus. Fig. 6 displays the temperature dependence of capacitance and dissipation factor for the two types of capacitors. It is seen that the capacitance and dissipation factor are highly temperature dependent for the wet capacitors, especially at low temperatures. Because the conductivity of the electrolyte solution as an ionic conductor falls off rapidly with decreasing temperature [23], the dissipation factor of the wet capacitors becomes larger at low temperatures. In addition, the remarkable decrease of capacitance at low temperatures can be attributed to the reduction of the effective contact area between anodic films and the electrolyte solution owing to higher electrolyte viscosity at low temperatures [24]. In contrast, the capacitance and dissipation factor for the PANI solid capacitors are roughly temperature independent. The weak temperature dependence of capacitance may originate from the weak influence of the temperature on the effective contact area between two solid films of anodic oxide and PANI. It can be found that the dissipation factor of the PANI capacitor increases slightly with increasing temperature (Fig. 6b). This is an unexpected result, as it was known that
The solid aluminum electrolytic capacitors whose capacitor element was a wound-type construction were fabricated successfully utilizing conductive PANI solutions. To reduce the leakage current of the capacitors, it is necessary to use aging electrolytes to repair the anodic film before the capacitor element is impregnated with a conductive PANI solution. The lower water content in the aging electrolyte solutions can lead to the lower leakage current of the capacitors. The solution of CSA–DBSA co-doped PANI/chloroform appeared to be the best impregnation effect. The secondary doping with m-cresol can distinctly improve the characteristics of the solid PANI capacitors. The as-prepared solid capacitors exhibit very low impedances at high frequencies and excellent temperature characteristics as well as high-temperature durability. The formation of the capacitor counter electrode using conducting polymer solution rather than in situ polymerization is not only compatible with the existing fabrication procedure for the conventional capacitor but also suitable for commercial mass production of low cost PANI. Acknowledgements This work was supported financially by the National Natural Science Foundation of China (Grant No. 51077072 and No. 61171043) and NUST research funding (Grant No. 2010GJPY048). References [1] A. Nishino, J. Power Sources 60 (1996) 137–147. [2] H. Yamamoto, M. Oshima, M. Fukuda, I. Isa, K. Yoshino, J. Power Sources 60 (1996) 173–177. [3] H. Yamamoto, M. Oshima, T. Hosaka, I. Isa, Synth. Met. 104 (1999) 33–38. [4] L.H.M. Krings, E.E. Havinga, J.J.T.M. Donkers, F.T.A. Vork, Synth. Met. 54 (1993) 453–460. [5] Y. Kudoh, K. Akami, Y. Matsuya, Synth. Met. 102 (1999) 973–974. [6] G. Heywang, F. Jonas, Adv. Mater. 4 (1992) 116–118. [7] K. Nogami, K. Sakamoto, T. Hayakawa, M. Kakimoto, J. Power Sources 166 (2007) 584–589. [8] E.T. Kang, K.G. Neoh, K.L. Tan, Polyaniline, Prog. Polym. Sci. 23 (1998) 277–324. [9] S. Palaniappan, A. John, Prog. Polym. Sci. 33 (2008) 732–758. [10] Y. Cao, A.J. Heeger, Synth. Met. 52 (1992) 193–200. [11] S. Shreepathi, R. Holze, Chem. Mater. 17 (2005) 4078–4085. [12] K.L. Wang, R.F. Chang, J. Power Sources 162 (2006) 1455–1459. [13] P. Robinson, J. Burnham, Trans. Electrochem. Soc. 83 (1943) 125–142. [14] Y. Cao, P. Smith, A.J. Heeger, Synth. Met. 55–57 (1993) 3514–3519. [15] K.S. Jang, B. Moon, E.J. Oh, H. Lee, J. Power Sources 124 (2003) 338–342. [16] Y. Xia, J.M. Wiesinger, A.G. MacDiarmid, A.J. Epstein, Chem. Mater. 7 (1995) 443–445.
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Fabrication of solid aluminum electrolytic capacitors utilizing conductive polyaniline solutions Ye Song ∗ , Longfei Jiang, Weixing Qi, Chao Lu, Xufei Zhu Key Laboratory of Soft Chemistry and Functional Materials of Education Ministry, Nanjing University of Science and Technology, Nanjing 210094, China
a r t i c l e
i n f o
Article history: Received 8 September 2011 Received in revised form 19 December 2011 Accepted 23 December 2011 Available online 21 January 2012 Keywords: Aluminum electrolytic capacitor Solid capacitor Conductive polyaniline solution Low impedance Long life
a b s t r a c t To overcome the risk of electrolyte leakage and the shortcoming of higher impedance at high frequencies for the conventional aluminum electrolytic capacitors impregnated with electrolyte solutions, solid aluminum electrolytic capacitors employing conducting polyaniline (PANI) as a counter electrode were developed. The dodecylbenzenesulfonic acid (DBSA) doped PANI/chloroform solution, camphorsulfonic acid (CSA)–DBSA co-doped PANI/chloroform solution and DBSA doped PANI/toluene solution were synthesized by direct and inverse emulsion polymerization pathway. The capacitors using the solution of CSA–DBSA co-doped PANI/chloroform showed the best electrical properties. The performances of the PANI capacitors can be remarkably improved after secondary doping with m-cresol. The as-fabricated solid capacitors have very low impedances at high frequencies and excellent temperature characteristics as well as long shelf-life. The results can be ascribed to high conductivity and superior thermal stability for the CSA–DBSA co-doped PANI. Our strategy of using conductive PANI solution to fabricate the solid capacitors is not only compatible with the existing fabrication procedure for the conventional capacitor but also suitable for commercial mass production of low cost PANI. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, aluminum electrolytic capacitors are widely employed in various electronic devices because of their high capacitance per unit volume and low cost [1]. Generally, a conventional aluminum electrolytic capacitor consists of an anode aluminum foil with a very thin dielectric film (anodic alumina) and a cathode aluminum foil as well as a paper separator interposed between them. They are wound together to form a capacitor element. To increase capacitor plate area or capacitance, the anode aluminum foil has a highly etched surface by an etching process. The dielectric film grown on the surface of the anode aluminum foil is electrically connected with the cathode foil through an electrolyte soaked in the separator. However, the use of the electrolyte leads to two disadvantages, i.e., the risk of electrolyte leakage and the higher equivalent series resistance (ESR) of the capacitors due to lower conductivity of the electrolyte as an ionic conductor. Moreover, as capacitors are used in higher and higher frequency applications, the lower ESR or impedance of a capacitor at high frequencies is becoming more and more important to the industry. Hence, a solid electrolytic capacitor employing conducting polymer as a solid electronic conductor instead of the liquid
∗ Corresponding author. Tel.: +86 25 84315949; fax: +86 25 84276082. E-mail address: soong [email protected] (Y. Song). 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.12.022
electrolyte was proposed. This kind of solid electrolytic capacitor not only eliminates the risk of electrolyte leakage but also has a much lower ESR or impedance. Various conducting polymers such as polyaniline (PANI), polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT) have been introduced as counter electrodes in solid electrolytic capacitors [2–5]. Among these conducting polymers, PEDOT has recently attracted much attention because of its high conductivity and excellent stability [6,7]. Generally, the PEDOT counter electrode was preferentially formed by in situ polymerization of EDOT in the presence of a chemical oxidant to penetrate the porous structure of the separator and the highly structured surface of the anodic film. To have better control of the reaction stoichiometry and obtain more uniform PEDOT film coated on the surface of the anode foil, impregnation is performed by dipping the elements into the mixed solution of the monomer and oxidant. A major obstacle to this process is limited life of the reactant solution. Even at very low conversion rates, PEDOT particles precipitate from the reaction solution and may clog the separator pores during impregnation. Furthermore, a tedious post-treatment process is needed to remove the residue of polymerization reaction. On the other hand, PANI is also one of the most studied conducting polymers because of its advantages such as low cost monomer, easy preparation, good environmental stability and thermal stability, and controllable electrical conductivity [8]. Although the conductivity of PANI is usually lower than that of PEDOT, it is
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one of the most promising candidates as a processible conducting polymer. Especially, PANI obtained by the emulsion polymerization has good solubility, good solution processability, reasonably high molecular weight, and good electrical conductivity [9]. From a practical point of view, the use of conductive PANI solutions to form the PANI film as the capacitor counter electrode has two advantages over the fabrication method of in situ polymerization. The first is that the ex situ fabrication of conductive PANI can avoid the difficulty in removing all byproducts of in situ polymerization and their potential damage to anodic films. The second is that the fabrication procedure for the solid capacitor via conductive PANI solutions is much more compatible with the existing fabrication procedure for the conventional wet electrolytic capacitor. In this work, three PANI solutions prepared by various approaches were employed to obtain the counter electrode of conducting PANI. The solid aluminum electrolytic capacitors with the wound-type construction were fabricated with these PANI solutions and their fabrication processes were described in detail. The influences of the PANI solutions and secondary doping process on the electrical performances of the capacitors were examined. The frequency characteristics, temperature characteristics and hightemperature durability for the capacitors were reported. 2. Experimental 2.1. Synthesis of conductive PANI solution 2.1.1. Synthesis of dodecylbenzenesulfonic acid (DBSA) doped PANI/chloroform solution In a typical polymerization experiment, a solution of 5 mL of aniline and 8.2 g of DBSA and 150 mL of deionized water was prepared in a 250 mL round bottomed flask and kept under mechanical stirring. Then, 6 mL of n-butanol was added slowly to the milky white solution to form a transparent miniemulsion. Subsequently, 50 mL of 1 M ammonium persulfate solution was added dropwise to the miniemulsion over a period of 30 min. The reaction mixture was kept at 20 ◦ C under mechanical stirring for 4 h. In the end, 100 mL of chloroform was added to the resultant suspension and the mixture was shaken for a few minutes and then kept undisturbed for 12 h. After phase separation, the organic phase containing PANI, which settled down, was separated using a separation funnel. The extraction process was repeated on the resulting organic phase three more times using 100 mL of deionized water to remove all byproducts and excess dopant. The as-prepared DBSA doped PANI/chloroform solution is designated as Solution A. It should be mentioned that the preparation procedure of the PANI-DBSA solution was simplified and the amount of organic solvent used was reduced compared with the conventional synthesis method [10]. 2.1.2. Synthesis of camphorsulfonic acid (CSA)–DBSA co-doped PANI/chloroform solution The polymerization process was the same as the above method except that 8.2 g of DBSA was replaced by the mixture of 7.8 g DBSA and 10.5 g CAS. The as-prepared CSA–DBSA co-doped PANI/chloroform solution is designated as Solution B. 2.1.3. Synthesis of DBSA doped PANI/toluene solution A modified polymerization procedure similar to Shreepathi and Holze [11] was used. In a typical polymerization experiment, 0.625 g of benzoyl peroxide was added to a 250 mL round bottomed flask containing 105 mL of toluene and kept under mechanical stirring. 10 mL of 2-propanol and 26.3 g of DBSA were added to the above mixture followed by the addition of 3 mL of aniline. Then 22 mL of water was added to this clear solution to form an inverted emulsion. The reaction mixture gradually turned green in ca. 2.5 h and stirring was continued for 28 h at 0 ◦ C. At the end of the reaction,
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100 mL of acetone was added to the resultant suspension and kept undisturbed for 12 h. The organic phase containing PANI, which settled down, was separated using a separation funnel. Treatment with acetone (100 mL) and separation were repeated three times to remove all byproducts and excess dopant. The as-prepared DBSA doped PANI/toluene solution is designated as Solution C. 2.2. Fabrication of solid aluminum electrolytic capacitors Capacitor elements (kindly provided by KJ (H.K.) Electronic Co. Ltd.) with rated voltage/capacitance of 25 V/470 F, 10 mm × 10 mm (diameter × height) were used as the examples in this study. To reduce leakage current of the capacitors, a DC voltage of 28 V was applied to the capacitor elements impregnated with aging electrolytes which were solutions of ammonium salts in ethylene glycol until the leakage current remained unchanged. Then the capacitor elements were dried at 80 ◦ C under 100 mbar of pressure. Subsequently, the dried elements were immersed into the above PANI solutions under reduced pressure and then dried at 100 ◦ C for 30 min. Generally, the process of immersion in PANI solutions and drying was repeated for four times to form an intact layer of conductive PANI on the surface of anodic films. Afterward, the elements with PANI were immersed into m-cresol under reduced pressure and then dried at 100 ◦ C under 100 mbar of pressure. Finally, the elements were sealed in an aluminum can by the same encapsulation techniques as conventional aluminum electrolytic capacitors. Solid aluminum electrolytic capacitors were completed ultimately after a DC voltage of 28 V was applied again for 60 min at 100 ◦ C to further reduce the leakage current. 2.3. Measurement The capacitance and dissipation factor of the capacitors were measured at a frequency of 100 Hz and their impedances measured at a frequency of 100 kHz by means of a LCR meter (hp-4284A). The leakage currents of the capacitors were measured by a leakage current tester (TH2685B). The temperature characteristics of the capacitors were obtained in a MINI-SUBZERO environmental chamber (MC-810, TABAI ESPEC Co.). The endurance test of the capacitors was performed at 125 ◦ C for 1000 h. 3. Results and discussion 3.1. Selection of aging electrolytes It is well known that the most vital part of an electrolytic capacitor is the anodic oxide film which acts as the dielectric. Nevertheless, the damage to the anodic film such as cracks is inevitable during the manufacturing process of a capacitor element due to the action of stress. Thus, it is necessary to use an aging electrolyte to repair the anodic film by anodization process at an applied voltage before the capacitor element is impregnated with a conductive PANI solution. The aging electrolytes commonly used are some solutions of ammonium salts dissolved in ethylene glycol. The electrolytes employed in this work were composed of ammonium adipate (4 wt%) and ammonium phosphate (1 wt%). As described above, the aging electrolytes would be eliminated after repairing the anodic film. Considering the requirement for ease of removal of the electrolytes, H2 O/ethylene glycol mixed solvent was used to decrease high boiling point of ethylene glycol. To investigate the effect of the water content on the ability of the electrolytes to heal the anodic film, the content of ammonium salts was kept fixed at 5 wt% while varying the ratio of H2 O to ethylene glycol in the mixed solvent. Fig. 1 shows the change of leakage current with time when the capacitor elements impregnated with the aging electrolytes were polarized at a constant voltage of 28 V. Similar trends
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Y. Song et al. / Synthetic Metals 162 (2012) 368–374 Table 1 Comparison of the electrical characteristics for the capacitors impregnated with three PANI solutions.
Fig. 1. Change of leakage currents of the capacitors with time while polarized in aging electrolytes with different water content.
can be observed in these decay curves of leakage currents, i.e., the leakage currents are initially appreciable but drop fairly rapidly towards steady values. Moreover, the steady values of leakage currents increase with increasing water content. This indicates that the increase of the ratio of H2 O to ethylene glycol is unfavorable for repairing anodic films. That is, high water content causes the final leakage current of the capacitor to be high. This experimental finding is in agreement with previous studies [12,13]. It can be explained by the electrokinetic properties of the anodic film and the separator paper [13]. If the relative zeta potentials of the anodic film and the separator paper in electrolytes were negative and positive, respectively, the tendency of the anodic film was to stay on the aluminum anode when voltage was applied. On the contrary, if both the film and the separator paper had negative potentials, the separator paper would plate out on the anode, and thus interfered with the normal functioning of the anode. This led to corrosion, resulting in the increase of the final leakage current. It has been found that the separator paper has a positive while the film has a negative potential in the glycol-rich solutions, whereas both the film and the separator paper have negative potentials in the water solution [13]. Therefore, the higher the content of ethylene glycol, the lower the leakage currents. In this work, we chose a mixed solvent containing 40 wt% water as the solvent of the aging electrolytes considering the easy removal of the electrolytes in subsequent processing. 3.2. Effect of PANI solutions In wound-type aluminum electrolytic capacitors, porous paper separators inserted between the anode and cathode foils act as liquid impregnation by capillary and diffusion effects. When a capacitor element is impregnated with conventional electrolytes, small molecule solute together with solvent can both penetrate into the inner part of the element. In contrast, for the PANI impregnation solutions, PANI macromolecules cannot easily follow small solvent molecules to penetrate into the inner part of the element because the interaction between PANI macromolecules and the textile fibers of the separator paper (also a polymer) is usually stronger than that between PANI macromolecule and small solvent molecules. Thus, PANI cannot easily coat all anodic film surfaces, especially the inside of the element, by capillary and diffusion effects. In other words, a PANI counter electrode of the capacitor cannot be formed uniformly and thus potential capacitance of the capacitor cannot be achieved. Therefore, PANI impregnation solutions play a key role in the present approach to the fabrication of solid aluminum electrolytic capacitors. In order to select PANI solutions suitable for impregnation, we prepared three PANI
PANI solutions
C/F
tan ı
Z/
Solution A Solution B Solution C
210.8 297.7 145.3
0.075 0.033 0.492
0.102 0.031 1.261
solutions for the fabrication of the solid capacitors (see Section 2). Solution A and Solution B were both prepared by the miniemulsion polymerization-extraction approach but with different dopants in polymerizations. The PANI was doped with DBSA in Solution A, whereas it was co-doped with CSA and DBSA in Solution B. Conversely, Solution C was prepared by an inverse emulsion polymerization route and PANI was dissolved into toluene instead of chloroform. All of these PANI solutions exhibited dark green, indicating that the dissolved PANI was in the conducting emeraldine salt state. The solid capacitors were fabricated by utilizing the three PANI impregnation solutions. It was found that the PANI counter electrode is hard to be wholly formed by dipping the elements into the PANI solutions only once. Generally, impregnation and drying process needed to be performed for three or four times, a continuous PANI layer can be formed successfully. The electrical characteristics of the capacitors fabricated with three PANI solutions by four times impregnation and drying is listed in Table 1. As can be seen, the capacitance of the capacitors impregnated with Solution B is the largest. In other words, the surface area of PANI counter electrode is the largest. This result indicates that the PANI solution can reach readily the inside of the capacitor elements while Solution B is impregnated. This is likely because Solution B containing the co-doped PANI has more proper surface tension under the cooperative action of both CSA and DBSA dopants compared with Solution A containing the PANI doped with only DBSA. It is well known that DBSA is a typical anionic surfactant. Compared with CSA molecule, DBSA contains a long non-polar alkyl group which is compatible with non-polar or weakly polar organic solvents (Fig. 2). Therefore, it is not surprising that PANI-DBSA can be soluble in toluene (nonpolar solvent) whereas PANI-CSA cannot [14]. Correspondingly, the molecular interaction between PANI-CSA and chloroform (polar solvent) should be greater than that between PANI-DBSA and chloroform. As a result, PANI macromolecules co-doped with CSA and DBSA can more easily follow small solvent molecules to penetrate into the inner part of the element than those doped with only DBSA. In addition, it has been reported that treatment with some coupling agents can enlarge the contact area between the anodic films and conducting polymer by improving interfacial adhesion, leading to enhanced performance of the capacitors [7,15]. It is believed that DBSA also have similar effects. As for Solution C, the relatively bad performance of capacitors can probably be related to toluene solvent used in the solution. Due to the non-polar character of toluene, the interaction between PANI macromolecules and toluene molecules is weaker than that between PANI macromolecules and chloroform molecules. Hence, PANI macromolecules are easily blocked by the textile fibers of the separator paper as discussed above. A counter electrode of PANI coated on the surface of anodic films cannot be formed wholly, which leads to lower capacitance, higher dissipation and impedance. This result indicates that the polarity of solvent used in PANI solutions is a key factor influencing PANI impregnation process. Because the performance of the capacitors impregnated with Solution B was the best, it was used as the PANI impregnation solution in the following investigations. Table 2 lists the electrical performances of the capacitors impregnated with Solution B after
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Fig. 2. Chemical structures of (a) DBSA and (b) CSA.
Fig. 3. Photographs of (a, c) separator paper and (b, d) anode aluminum foil after (a, b) first dipping and (c, d) fourth dipping.
each cycle of dipping in PANI solutions and drying. As expected, the electrical performances of the capacitors are improved with dipping times. As can be seen from Table 2, the characteristics of the capacitors impregnated with Solution B for four times and five times are almost the same. This suggests that an intact layer of conductive PANI on the surface of anodic films can be formed by a four-times impregnation process. This can be apparently observed while unpacking the capacitor elements. Fig. 3 shows photographs of the attachment of PANI onto the separator paper and anode aluminum foil after the first impregnation and the fourth impregnation of Solution B, respectively. Clearly, a uniform layer of green PANI is coated on both the separator paper and anode aluminum foil after four times impregnation. Thus, the best characteristics of the capacitors can be obtained.
3.3. Effect of secondary doping Earlier studies revealed that the electrical behavior and structural order of doped PANI can be varied by varying the solvent from which the film is cast. For instance, films of PANI-CSA have a conductivity of ca. 0.1 S cm−1 when cast from chloroform, whereas those films with the same composition have a conductivity of ca. 200 S cm−1 when cast from m-cresol [16,17]. This phenomenon is known as secondary doping which was first introduced by MacDiarmid and Epstein [17,18]. In order to improve the conductivity of PANI, the capacitor samples from Table 1 were further treated by a secondary doping process, i.e., the capacitor elements were impregnated with m-cresol and then dried at 100 ◦ C under reduced pressure. Table 3 presents the characteristics of the capacitors after secondary doping with m-cresol. It is obvious that the electrical performances of the capacitors impregnated with three PANI solutions
Table 2 Characteristics of the capacitors with different dipping times. Dipping times
C/F
tan ı
Z/
1 2 3 4 5
130.4 201.5 263.6 297.4 297.7
0.207 0.094 0.043 0.034 0.033
0.116 0.098 0.063 0.031 0.031
Table 3 Characteristics of the capacitors after secondary doping with m-cresol. PANI solutions
C/F
tan ı
Z/
Solution A Solution B Solution C
245.6 325.9 177.6
0.046 0.023 0.145
0.085 0.021 0.131
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Fig. 4. Leakage current–time plot for the PANI capacitor.
are all improved through secondary doping process as compared with the data in Table 1. MacDiarmid and Epstein [17] demonstrated that the interaction of PANI with m-cresol could cause a conformational transition of the polymer chain from a “compact coil” to an “expanded coil”. When the PANI chains become more expanded coil-like, the polaron band becomes more delocalized, the carrier mobility and hence the intrachain conductivity increase. In addition, the higher crystallinity caused by more expanded coil conformation results in a higher interchain conductivity [18]. It is the enhancement of PANI conductivity after secondary doping that leads to the lower dissipation and impedance of the capacitors. Moreover, m-cresol can force the PANI coated on the separator paper or anode aluminum foil to spread out into a more uniform film during secondary doping because m-cresol as a polar organic solvent is able to redissolve PANI to some extent. This leads to an improved capacitance of the capacitors due to an increased area of PANI overlayer on the surface of anode aluminum foil.
Fig. 5. Impedance–frequency plots of the two types of capacitors.
at defect sites of anodic films due to local Joule heating caused by the local high currents through the defects. It has been proposed that the conductivity degradation of PANI after a high temperature treatment was mainly due to the segregation of the dopants
3.4. Capacitor characteristics It was found that the capacitors after secondary doping process exhibited appreciable leakage currents, although the anodic oxide films of the elements were repaired by the aging electrolytes in advance. This behavior may be attributed to the mechanical disruption of the anodic films during the processes of drying the aging electrolytes and PANI solutions or the chemical damage during secondary doping process due to the acidic character of m-cresol [19]. To reduce the leakage currents of the electrolytic capacitors, a common approach is using the aging electrolytes to repair the anodic oxide films coated with PANI by the anodization process. In the aging electrolytes, however, not only anodic film defects can be restored by anodic oxidation of aluminum but also the PANI coated on the anodic films can be overoxidized at the same time due to a higher applied voltage. The overoxidized PANI was of very low conductivity [20], leading to the deterioration of capacitor performance. An alternative approach to lower leakage currents is to render PANI counter electrode nonconductive to block the conducting paths between PANI and aluminum electrodes. This can be achieved by applying a certain voltage slightly higher than rated voltage of the capacitor. Fig. 4 shows the leakage current of the capacitor as a function of time while subjected to an applied voltage of 28 V at 90 ◦ C. It is seen that the leakage current decays rapidly with time at the beginning and then decreases slowly to a minimum value. It is believed that the decrease of leakage current can be ascribed to a conductivity change from conducting to insulating for PANI located
Fig. 6. Temperature characteristics of the two types of capacitors: (a) capacitance and (b) dissipation factor.
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the conductivity of PANI increases slightly with temperature in the temperature range of −50 to 125 ◦ C [25]. In fact, this result can be understood when considering the increase of its capacitance with temperature, because the increase of the capacitance can lead to the increase of dissipation factor [13]. Finally, the PANI solid capacitors have excellent hightemperature durability. Fig. 7 illustrates the initial characteristics and their changes for the capacitors when stored on shelf at 125 ◦ C. Compared with the conventional wet electrolytic capacitors, the changes in the capacitance, dissipation factor and impedance for the PANI capacitors are very small during shelf-life tests of 1000 h. The limited lifetime for the conventional wet electrolytic capacitors is due to the time-dependent evaporation of the electrolyte through the seal. The performances of the PANI solid capacitors deteriorate with time at elevated temperatures as a consequence of the decrease of PANI conductivity during high-temperature durability test as mentioned above. The thermal gravimetric analysis indicated that the CSA–DBSA co-doped PANI has better thermal stability than the DBSA doped PANI (not shown). This is in agreement with the excellent high-temperature durability for the PANI solid capacitors fabricated using Solution B. 4. Conclusions
Fig. 7. Changes of capacitance, dissipation factor and impedance with time at 125 ◦ C for the PANI capacitors.
from PANI backbones, the cross-linking of the PANI backbones, and evaporation and degradation of the dopants [21,22]. The as-prepared solid capacitors have a very low impedance in the high-frequency range owing to the relatively high conductivity of PANI. For comparison, a conventional wet electrolytic capacitor was also fabricated using the same capacitor element impregnated with a previously reported organic electrolyte which was a gammabutyrolactone solution of 1.5 M 1-methyl-3-ethylimidazolium hydrogen maleate [23]. Fig. 5 shows the frequency dependence of impedance for the two types of capacitors. The as-prepared solid capacitors possess a fairly high self-resonance frequency and very low impedances at high frequencies relative to the wet capacitors. Their impedances at high frequencies are approximately one order of magnitude lower than those of the wet capacitors. The low impedances at high frequencies for the PANI solid capacitors can meet the growing high-frequency requirements of a vast range of new electronic apparatus. Fig. 6 displays the temperature dependence of capacitance and dissipation factor for the two types of capacitors. It is seen that the capacitance and dissipation factor are highly temperature dependent for the wet capacitors, especially at low temperatures. Because the conductivity of the electrolyte solution as an ionic conductor falls off rapidly with decreasing temperature [23], the dissipation factor of the wet capacitors becomes larger at low temperatures. In addition, the remarkable decrease of capacitance at low temperatures can be attributed to the reduction of the effective contact area between anodic films and the electrolyte solution owing to higher electrolyte viscosity at low temperatures [24]. In contrast, the capacitance and dissipation factor for the PANI solid capacitors are roughly temperature independent. The weak temperature dependence of capacitance may originate from the weak influence of the temperature on the effective contact area between two solid films of anodic oxide and PANI. It can be found that the dissipation factor of the PANI capacitor increases slightly with increasing temperature (Fig. 6b). This is an unexpected result, as it was known that
The solid aluminum electrolytic capacitors whose capacitor element was a wound-type construction were fabricated successfully utilizing conductive PANI solutions. To reduce the leakage current of the capacitors, it is necessary to use aging electrolytes to repair the anodic film before the capacitor element is impregnated with a conductive PANI solution. The lower water content in the aging electrolyte solutions can lead to the lower leakage current of the capacitors. The solution of CSA–DBSA co-doped PANI/chloroform appeared to be the best impregnation effect. The secondary doping with m-cresol can distinctly improve the characteristics of the solid PANI capacitors. The as-prepared solid capacitors exhibit very low impedances at high frequencies and excellent temperature characteristics as well as high-temperature durability. The formation of the capacitor counter electrode using conducting polymer solution rather than in situ polymerization is not only compatible with the existing fabrication procedure for the conventional capacitor but also suitable for commercial mass production of low cost PANI. Acknowledgements This work was supported financially by the National Natural Science Foundation of China (Grant No. 51077072 and No. 61171043) and NUST research funding (Grant No. 2010GJPY048). References [1] A. Nishino, J. Power Sources 60 (1996) 137–147. [2] H. Yamamoto, M. Oshima, M. Fukuda, I. Isa, K. Yoshino, J. Power Sources 60 (1996) 173–177. [3] H. Yamamoto, M. Oshima, T. Hosaka, I. Isa, Synth. Met. 104 (1999) 33–38. [4] L.H.M. Krings, E.E. Havinga, J.J.T.M. Donkers, F.T.A. Vork, Synth. Met. 54 (1993) 453–460. [5] Y. Kudoh, K. Akami, Y. Matsuya, Synth. Met. 102 (1999) 973–974. [6] G. Heywang, F. Jonas, Adv. Mater. 4 (1992) 116–118. [7] K. Nogami, K. Sakamoto, T. Hayakawa, M. Kakimoto, J. Power Sources 166 (2007) 584–589. [8] E.T. Kang, K.G. Neoh, K.L. Tan, Polyaniline, Prog. Polym. Sci. 23 (1998) 277–324. [9] S. Palaniappan, A. John, Prog. Polym. Sci. 33 (2008) 732–758. [10] Y. Cao, A.J. Heeger, Synth. Met. 52 (1992) 193–200. [11] S. Shreepathi, R. Holze, Chem. Mater. 17 (2005) 4078–4085. [12] K.L. Wang, R.F. Chang, J. Power Sources 162 (2006) 1455–1459. [13] P. Robinson, J. Burnham, Trans. Electrochem. Soc. 83 (1943) 125–142. [14] Y. Cao, P. Smith, A.J. Heeger, Synth. Met. 55–57 (1993) 3514–3519. [15] K.S. Jang, B. Moon, E.J. Oh, H. Lee, J. Power Sources 124 (2003) 338–342. [16] Y. Xia, J.M. Wiesinger, A.G. MacDiarmid, A.J. Epstein, Chem. Mater. 7 (1995) 443–445.
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