pH-Tunable High Performance PEDOT:PSS Aluminum Solid

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pH-Tunable High Performance PEDOT:PSS Aluminum Solid Electrolytic Capacitors Toshiki Wakabayashi, Masato Katsunuma, Kazuki Kudo, and Hidenori Okuzaki ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00210 • Publication Date (Web): 01 May 2018 Downloaded from http://pubs.acs.org on May 4, 2018

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ACS Applied Energy Materials

pH-Tunable High Performance PEDOT:PSS Aluminum Solid Electrolytic Capacitors Toshiki Wakabayashi1,2, Masato Katsunuma2, Kazuki Kudo2, Hidenori Okuzaki2* 1

Nippon Chemi-Con Co., 185-1 Marunouchi, Yabuki-machi, Nishishirakawa-gun, Fukushima 969-0235, Japan

2

Graduate Faculty of Interdisciplinary Research, University of Yamanashi, 4-4-37 Takeda, Kofu, Yamanashi 400-8510, Japan

*

Corresponding author: [email protected]

Abstract The pH effect of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) water dispersion on colloidal particle size (D50), zeta potential (ζ), and electrical conductivity was investigated. An increase in the pH of the PEDOT:PSS water dispersion from 2 to 11 increased the D50 from 10 to 100 nm owing to the aggregation of the colloidal particles and decreased the electrical conductivity from 750 to 62 S cm-1 by dedoping, respectively, while the ζ remained almost constant at ca. −50 mV. Furthermore, aluminum solid electrolytic capacitors were fabricated using PEDOT:PSS as a cathode material. It was found that the electrical characteristics of the PEDOT:PSS aluminum solid electrolytic capacitors were optimized at pH 3, where D50 and electrical conductivity played an important role for low equivalent series resistance (ESR) and high capacitance (Cap). Furthermore, the ESR decreased and Cap increased by repeating the fabrication process, where the Cap usable rate reached as high as 92% owing to the increase in the surface coverage of the etched aluminum foil with the PEDOT:PSS. Keywords: PEDOT:PSS, aluminum solid electrolytic capacitor, pH, particle size, zeta potential, electrical conductivity

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1. Introduction Conductive polymers are widely applied to antistatic coatings, batteries, capacitors, organic LEDs, and organic solar cells.1,2 Especially, polypyrrole and polythiophene derivatives have been industrialized for more than 20 years as cathode materials for aluminum solid electrolytic capacitors.3,4 Meanwhile, with the advancement of high functionality and the modularization of semiconductor devices, information terminals have shifted to use smaller, lighter, and lower-cost devices, and capacitors are pressed with similar demands. Unlike redox capacitors using conductive polymers and supercapacitors based on electric double layers, aluminum solid electrolytic capacitors store electric charges at the aluminum oxide as a dielectric where the conductive polymer is used as a cathode material. In general, high frequency response is required for charge and discharge in the aluminum solid electrolytic capacitors incorporated in digital equipment such as PCs and game machines operating at high frequencies. In general, aluminum solid electrolytic capacitors were fabricated by “in-situ polymerization” using the chemical oxidative polymerization of monomers by sequential dipping in reservoirs containing monomer and oxidant, and finally heating for completion of the oxidative polymerization in a wound element of the capacitor. Therefore, material efficiency in this case was poor because most of the monomers and oxidants had to be discarded owing to deterioration and contamination during the fabrication process.2 Poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonic acid) (PEDOT:PSS), available in the form of water dispersion of colloidal particles, is a typical conductive polymer with high conductivity and transparency as well as excellent stability.5-8 By using the PEDOT:PSS water dispersion, capacitors can be made with the necessary amount at the necessary place, when necessary, which contributes to the simplification of the fabrication process, improvement of the materials efficiency, and the reduction of the environment load.9,10 Furthermore, aluminum solid electrolytic capacitors using PEDOT:PSS have attracted considerable attention not only from the fundamental viewpoint of organic-inorganic solid interface formation for energy storage, but also from the practical applications for next generation hybrid vehicles (HV) and electric vehicles (EV)11 because of their high thermal stability, low ESR, and high breakdown voltage compared to other conducting polymers. However, the PEDOT:PSS water dispersion is a strong acid with pH < 2 and may react with aluminum. Indeed, it was found that the PEDOT:PSS water dispersion partially dissolved aluminum and formed gels on the surface of the aluminum foil. Furthermore, although the structure and electrical conductivity of the PEDOT:PSS strongly depend on the pH,12 the influence of pH on the electrical characteristics of PEDOT:PSS aluminum solid electrolytic


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capacitors has not been investigated. In this study, we systematically investigated the relationship between pH, electrical conductivity, colloidal particle size, and zeta potential of the PEDOT:PSS water dispersion, and fabricated wound elements of aluminum solid electrolytic capacitors using PEDOT:PSS as a cathode material to clarify the effects of the parameters on the ESR and Cap. The most important result of this study is that the performance of PEDOT:PSS aluminum solid electrolytic capacitor can be controlled by the pH of the PEDOT:PSS water dispersion. In addition, we have succeeded in improving the Cap usable rate by repeating the fabrication process, which may also give an important impact to the field of PEDOT:PSS aluminum solid electrolytic capacitors as energy storage devices.

2. Experimental 2.1 Fabrication of PEDOT:PSS aluminum solid electrolytic capacitors To prepare the PEDOT:PSS water dispersions (solid component 1 wt%) of different pH values, the PEDOT:PSS water dispersion (Clevios PH1000, Heraeus) was neutralized by 1 M aqueous sodium hydroxide (Kanto Kagaku). The aluminum solid electrolytic capacitors were wound elements (diameter 5.5 mm, height 3 mm) manufactured by Nippon Chemi-Con (Figure 1), in which the anode area of the aluminum foil (90 mm long and 2.8 mm wide) was 2.52 cm2 and rated voltage and capacitance were 25 V and 47 µF, respectively. In general, the wound element is practically used for actual aluminum solid electrolytic capacitors, rather than the flat element, in order to enhance Cap. The wound elements were immersed in the PEDOT:PSS water dispersions of the respective pH values and dried at 120 °C for 5 min. Then, the elements were immersed in ethylene glycol (EG, Kanto Chemical) and dried at 200 °C for 5 min to fabricate PEDOT:PSS aluminum solid electrolytic capacitors.

2.2 Measurements At each pH, the median diameter (D50) of the PEDOT:PSS colloidal particles was measured using a dynamic light scattering photometer (Nanotrac UPA-UT151, MicrotracBEL). The zeta potential of the PEDOT:PSS colloidal particles (ζ) and aluminum oxide (ζAl2O3) was measured with a zeta potential analyzer (DelsaNano C, Beckman Coulter). The PEDOT:PSS film was prepared by adding EG (5 wt%) to the PEDOT:PSS water dispersion, followed by casting on a slide glass and drying at 200 °C. The film thickness of the PEDOT:PSS cast film was measured with a stylus profilometer (D-100, KLA-Tencor), and the electrical conductivity was measured by the normal four-probe method using a Loresta-GP (MCP-T610, Mitsubishi



Chemical Analytech). The aggregation state of the PEDOT:PSS colloidal particles was observed using an atomic force microscope (AFM) (SPM-9600, Shimadzu) with a tapping mode. The ultraviolet-visible-near infrared (UV-vis-NIR) absorption spectra of thin PEDOT:PSS films (thickness: 140-220 nm) spin-coated on the glass were measured using a spectrophotometer (V-670, JASCO). Although the rated voltage of the wound element, corresponding to the guarantee voltage against DC voltage, is 25 V, evaluation of the aluminum solid electrolytic capacitors was generally performed at AC 0.5−1 V according to the Japanese Industrial Standards (JIS C 5101-26:2012). Therefore, the AC impedance analysis was carried out at 1 V with a chemical impedance meter (3532-80, HIOKI) in the frequency range of 10 Hz to 1 MHz, where the Cap at 120 Hz and ESR at 120 Hz and 1 MHz were evaluated to characterize the PEDOT:PSS aluminum solid electrolytic capacitors. The morphology and surface coverage of the aluminum foil and the mapping image of sulfur were evaluated with an electron probe micro analyzer (EPMA) (JXA-8200, JEOL). The galvanostatic charge/discharge curves and cyclic voltammograms were measured in a voltage range of 0−1 V at different currents (0.5−10 µA) and scan rates (10−100 mV s-1), respectively, with electrochemical impedance system (1255WB, Solartron) and software (CorrWare, Solartron).

3. Results and Discussion 3.1. Effect of pH on structure and electrical properties of PEDOT:PSS Figure 2 shows pH dependence of colloidal particle size (D50), zeta potential (ζ), and electrical conductivity of the PEDOT:PSS water dispersion neutralized with sodium hydroxide. It was found that the D50 was 11 nm at pH 3, which was in close agreement with the literature value,13,14 since PEDOT is a 6−18-mer oligomer corresponding to a molecular chain length of 2.3−7 nm.15 Actually, the AFM image at pH 3 (inset) shows the particles with diameters of 10−30 nm, which indicates that the PEDOT forms a primary particle with a size close to the molecular size. An increase in pH causes aggregation of colloidal particles, which arises from the fact that the AFM image at pH 11 (inset) shows the primary particles aggregate to form large secondary particles. On the other hand, the ζ value of the PEDOT:PSS colloidal particles was found to be about −50 mV, irrespective of pH, because many sulfonic acid groups of PSS were present on the surface of the colloidal particles.16 The electrical conductivity of the PEDOT:PSS film measured by the four-probe method was as high as 750 S cm-1 at pH 2 and rapidly decreased as the pH increased to 62 S cm-1 at pH 11. To investigate the effect of pH on the electronic structure of the PEDOT in detail, UV-vis-NIR spectra were


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measured and the results are shown in Figure 3. At pH 2, the large absorption in the NIR region derived from bipolaron transition (free carrier tail) suggests a highly doped state.17-19 With increasing the pH, the absorption in the NIR region decreased but the absorption near 900 nm derived from the polaron transition increased.20,21 This demonstrates a dedoping of PEDOT from the bipolaron to polaron state by neutralization.12

3.2. Effect of pH on electrical characteristics of PEDOT:PSS aluminum solid electrolytic capacitors Figure 4 shows the frequency characteristics of the ESR, Cap, and phase angle for aluminum solid electrolytic capacitors fabricated using the PEDOT:PSS water dispersion with different pH values as the cathode material. It was found that both the ESR and Cap decreased against the frequency, where the increase in the pH increased the ESR and decreased the Cap over almost the entire frequency range. On the other hand, an ideal capacitor should have a phase angle of −90°,22 where the phase angle at 120 Hz is a “factor of merit” to evaluate the performance of the capacitors.23 The phase angle at 120 Hz increased with decreasing the pH and reached −84.6° at pH 3, which was nearly identical to those of commercial aluminum solid electrolytic capacitors.23 Generally, the ESR of the cathode material in the aluminum solid electrolytic capacitor is determined using equation (1), which is composed of the sum of the dielectric loss component (tan δ) of aluminum oxide (dielectrics), resistance of the conductive polymer in the etching pit (Ri), and sum of the conductive polymer in the surface layer of the etching pit and the terminal resistance of the external electrode (Rs), as shown in the inset of Figure 4.24 ESR = tan δ/(ω Cap) + Ri + Rs Ri = Ri1 + Ri2 + Ri3 + ･･･ Rs = Rs1 + Rs2

(1)

Here, Ri is a constant in the low-frequency region, but it decreases in proportion to the half power of the frequency (f = ω/2π) in the high-frequency region. In addition, since the current flows only in the vicinity of the surface of the external electrode (El) at a high frequency of 1 MHz, the sum of the resistance (Rs = Rs1 + Rs2) of the conductive polymer in the surface layer of the etching pit (Rs1) and the resistance of the external electrode (Rs2) is dominant. In a capacitor using the same external electrode material, the resistance of the conductive polymer (Rs1) can be taken into account. On the other hand, at a low frequency of 120 Hz, where the current can flow to the inside of the etching pit, the total resistance of the conductive polymer from the inside of the etching pit to the surface layer is given by Ri + Rs.24 ACS Paragon Plus 5 Environment


In order to clarify the role of the PEDOT:PSS in the electrical properties of aluminum solid electrolytic capacitors, the influence of the electrical conductivity, colloidal particle size (D50), and zeta potential of aluminum oxide (ζAl2O3) exerted on the ESR and Cap was investigated in detail. As seen in Figure 5(a), the ESR at 120 Hz and 1 MHz sharply decreased as the conductivity increased. On the other hand, the fact that the Cap at 120 Hz saturates at conductivities higher than 200 S cm-1 indicates that the current efficiently flows into and out of the etching pit. Upon focusing on the colloidal particle size, the ESR and Cap are almost constant at D50 < 65 nm, while the ESR sharply increases and Cap decreases at D50 = 100 nm (Figure 5(b)) probably due to that the agglomerates of PEDOT:PSS are hard to enter the etching pit of aluminum oxide. In addition, since the ESR at 1 MHz corresponds to the resistance of the surface layer of the etching pit and the external electrode, the increase in the ESR is considered a result of the lowering of the electrical conductivity rather than the diameter of the colloidal particles. From the viewpoint of the zeta potential of aluminum oxide, electrostatic attraction with PEDOT:PSS colloidal particles (ζ = −50 mV) is expected to occur at ζAl2O3 > 0. As shown in Figure 2, ζAl2O3 shows an isoelectric point around pH 9. At pH < 9 protonation of the –OH groups (–O+H2) on the aluminum oxide surface results in a positive zeta potential (ζAl2O3 > 0), while at pH > 9, dissociation of the –OH groups (–O−) leads to a negative zeta potential (ζAl2O3 < 0).25 However, it was found that the ESR and Cap are almost constant at ζAl2O3 > 0, which demonstrates that the electrostatic attraction between the colloidal particles and aluminum oxide had little effect on the improvement of the electrical characteristics of the capacitor (Figure 5(c)). By contrast, both the increase in Cap and decrease in ESR at ζAl2O3 < 0 may not ascribed to electrostatic repulsion but to the large colloidal particles (D50 = 100 nm) that find it difficult to enter the etching pit. From these experimental results, the optimum pH of the PEDOT:PSS water dispersion is found to be 3, where high electrical conductivity and small colloidal particle size play a predominant role in the electrical characteristics of the PEDOT:PSS aluminum solid electrolytic capacitors.

3.3. Performance improvement of PEDOT:PSS aluminum solid electrolytic capacitors by repetitive fabrication However, the Cap of the PEDOT:PSS aluminum solid electrolytic capacitor is generally small in comparison with that of the capacitor using an electrolytic solution (15 wt% aqueous ammonium adipate solution) as the cathode material (broken lines in Figure 6). Actually, the Cap usable rate (P), defined as the Cap ratio of the capacitor using PEDOT:PSS to that using the electrolyte solution as the cathode material, was 63% at 120 Hz if the PEDOT:PSS water


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dispersion was immersed only once at pH 3. This is probably because few PEDOT:PSS colloidal particles are brought into the etching pit, or even if it is brought in, it cannot be drawn out as current. Therefore, we tracked the characteristics of the capacitor by repeating the immersion and drying processes for the PEDOT:PSS water dispersion and EG (Figure 1(c)). As shown in Figure 6, as the number of repetitions (n) increased, the ESR decreased but Cap and phase angle increased over almost the entire frequency range. In particular, at n ≥ 4, the P and phase angle at 120 Hz reached higher than 90% and −86°, respectively, indicating that the etching pit was sufficiently filled with the PEDOT:PSS colloidal particles and low ESR reduced the energy loss. Furthermore, the ESR at 1 MHz was lower than that for the electrolyte at n ≥ 2, which demonstrated that the PEDOT:PSS adequately filled the surface layer of the etching pit. To clarify the mechanism in more detail, the coated state of PEDOT:PSS on the surface of the etched aluminum foil was evaluated using an EPMA and mapping images of sulfur contained in the PEDOT:PSS are displayed in Figure 7. At n = 1, sulfur was obviously present in small amounts near the center of the aluminum foil, and the surface coverage (SC) calculated from the image analysis was 60%. It is considered that the PEDOT:PSS water dispersion seeps into the etched aluminum foil from the top and bottom of the wound element, but could not diffuse to the vicinity of the center. On the other hand, the SC increased upon repeating the fabrication and reached 99% at n = 6, where the sulfur was found in the vicinity of the center of the aluminum foil and the surface was almost completely covered. It is seen from Figure 8 that the ESR at 120 Hz and 1 MHz decreased as the SC increased, which is probably due to the enhancement of the contact between PEDOT:PSS inside the etching pit and the surface layer in the etched aluminum foil. It is noted that the P increased almost linearly with the SC, reaching 92% at SC = 99% (n = 6). Interestingly, the theoretical curve (broken line), for which P was assumed to be proportional to SC, was almost consistent with the obtained experimental results. This means that in the area where the etched aluminum foil surface is covered by PEDOT:PSS, the colloidal particles can sufficiently enter the etching pit in the underlayer. In fact, SEM and EPMA observations on the cross section of the etched aluminum foil (n = 6) reveal that the sulfur is uniformly distributed not only in the surface layer but also in the etched area (inset of Figure 8). Thus, the results allow us to conclude that the electrical characteristics of the PEDOT:PSS aluminum solid electrolytic capacitors are remarkably improved by repeating the fabrication, where the SC and P at n = 6 reached 99% and 92%, respectively.



3.4. Galvanostatic charge/discharge curves and cyclic voltammograms of PEDOT:PSS aluminum solid electrolytic capacitor To evaluate the performance of the PEDOT:PSS aluminum solid electrolytic capacitors (n = 6) in more detail, gasvanostatic charge/discharge curves and cyclic voltammograms were measured at different currents and scan rates. As shown in Figure 9, the galvanostatic charge/discharge curves show typical triangle shapes in a current range of 0.5−10 µA without notable voltage drop (IR drop) at the beginning of each discharge curve, indicative of the excellent capacitors with low ESR. It is also found that the capacitance calculated by dividing the constant current by voltage drop rate was almost constant ca. 51 µF regardless of the current, demonstrating the excellent rate performance. On the other hand, the cyclic voltammograms show rectangular shapes, exhibiting a typical capacitor, at scan rates ranging from 10 to 100 mV s-1 (Figure 10). The capacitance calculated from the slope of charge-voltage plot was nearly constant ca. 51 µF in the experimental scan rates, which is the same as the values obtained by the galvanostatic charge/discharge measurements. Using the weight of the wound element (160 mg), energy density of the PEDOT:PSS aluminum solid electrolytic capacitor was calculated to be 0.16 Wh kg-1, which is comparable to other aluminum solid electrolytic capacitors.26

4. Conclusion An increase in the pH of PEDOT:PSS water dispersion increased the colloidal particles but decreased the electrical conductivity owing to the dedoping, while the zeta potential of the colloidal particles was less dependent on the pH. It turned out that the electrical characteristics of the PEDOT:PSS aluminum solid electrolytic capacitors were optimized at pH 3, and that the colloidal particle size and electrical conductivity played an important role in achieving a low-ESR and high-Cap. Furthermore, the electrical characteristics of the PEDOT:PSS aluminum solid electrolytic capacitors were remarkably improved by repeating the fabrication because the contact between PEDOT:PSS inside the etching pit and the surface layer on the aluminum foil was enhanced. Thus, we have succeeded in fabricating the high performance PEDOT:PSS aluminum solid electrolytic capacitors with SC and P as high as 99% and 92%, respectively. In order to improve the capacitance, it is necessary to increase the surface area of the etched aluminum foil and/or to decrease the thickness of the aluminum oxide dielectric layer, which results in finer etching pits. Since the PEDOT:PSS colloidal particles are difficult to enter the finer etching pits of the aluminum foil, repetitive fabrication is essential to achieve high P and low ESR values for the practical application to the semiconductor devices


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and information terminals. Therefore, soluble and highly conductive polymer will be expected as a novel cathode material of the aluminum solid electrolytic capacitors.



Figure 1 (a) Schematic diagram of wound element of aluminum solid electrolytic capacitor, (b) cross sectional SEM images of etched aluminum foil, and (c) fabrication process of PEDOT:PSS aluminum solid electrolytic capacitors by immersing and drying for PEDOT:PSS water dispersion and EG.


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Figure 2 pH dependence of colloidal particle size (D50), zeta potential (ζ), and electrical conductivity of PEDOT:PSS (filled symbols) and Al2O3 (open symbols). Inset: AFM images of the PEDOT:PSS particles at pH 3 and 11.



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Wavelength (nm) Figure 3 UV-vis-NIR spectra of the PEDOT:PSS water dispersion of different pH values. Inset: Chemical structures of PEDOT in polaron and bipolaron states.


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Figure 5 Dependence of (a) electrical conductivity and (b) D50 of the PEDOT:PSS and (c) ζAl2O3 on ESR at 120 Hz and 1 MHz and Cap at 120 Hz of the PEDOT:PSS aluminum solid electrolytic capacitors.


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Frequency (Hz) Figure 6 Changes in frequency dependence of ESR, Cap, and phase angle of the PEDOT:PSS aluminum solid electrolytic capacitors fabricated using the PEDOT:PSS water dispersion at pH 3 on number of repetitions (n). The broken lines represent the results of aluminum electrolytic capacitors using an electrolyte solution (15 wt% aqueous ammonium adipate solution).



Figure 7 EPMA mapping images and surface coverage (SC) of sulfur on the etched aluminum foil of PEDOT:PSS aluminum solid electrolytic capacitors with different numbers of repetitions (n).


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Figure 8 Dependence of ESR at 120 Hz and 1 MHz and usable rate of Cap at 120 Hz (P) of the PEDOT:PSS aluminum solid electrolytic capacitors on SC. Inset: Cross sectional SEM (left) and EPMA (right) images of the etched aluminum foil at n = 6.



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Current (µA) Figure 9 (a) Galvanostatic charge/discharge curves measured at different currents (0.5−10 µA) and (b) relation between current and Cap of the PEDOT:PSS aluminum solid electrolytic capacitor (n = 6).


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Scan rate (mV/s) Figure 10 (a) Cyclic voltammograms measured at different scan rates (10−100 mV s-1) and (b) relation between scan rate and Cap of the PEDOT:PSS aluminum solid electrolytic capacitor (n = 6).



Table of Contents


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in

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