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ISSN : 1225-0112(Print)
ISSN : 2288-4505(Online)
Applied Chemistry for Engineering Vol.32 No.4 pp.403-408
DOI : https://doi.org/10.14478/ace.2021.1050

Fabrication of a Thin and Flexible Polyaniline Electrode for High-performance Planar Supercapacitors

Seon Gyu Son, Seo Jin Kim, Junho Shin*, Taegon Ryu*, Jae-Min Jeong*†, Bong Gill Choi†
Department of Chemical Engineering, Kangwon National University, Samcheok 25913, Republic of Korea
*Resources Utilization Research Center, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon 34132, Republic of Korea
† Corresponding Author: J. M. Jeong: Korea Institute of Geoscience and Mineral Resources (KIGAM), Resources Utilization Research Center, Daejeon 34132, Republic of Korea; B. G. Choi: Kangwon National University, Department of Chemical Engineering, Samcheok 25913, Republic of Korea Tel: J. M. Jeong: +82-42-868-3503; B. G. Choi: +82-33-570-6545 E-mail: J. M. Jeong: jmjeong@kigam.re.kr; B. G. Choi: bgchoi@kangwon.ac.kr
May 24, 2021 ; June 7, 2021 ; June 9, 2021

Abstract


In this study, a thin and flexible planar supercapacitor (PSC) was fabricated by coating polyaniline (PANI) on a screen-printed carbon electrode. Carbon ink was coated onto the flexible polyethylene terephthalate using a screen-printing method; subsequently, a thin film of PANI was coated onto the carbon surface using a dilute polymerization method. A thin flexible PANI electrode in an interdigitated structure was assembled with a polymer gel electrolyte that resulted in planar-shaped supercapacitor (PSC) devices. The as-obtained PANI/PSC was very thin and flexible, exhibiting a high areal capacitance of 409 μF/cm was obtained at a rate of 10 mV/s. This capacitance retains 46% of its original value at 500 mV/s. The flexible PANI/PSC exhibited an excellent capacitance retention of 82% even under bent states of 180° and 100 repetitive bent cycles.


Planar supercapcitor , Polyaniline , Carbon ink , Screen printing

고성능 평면 슈퍼커패시터를 위한 얇고 유연한 폴리아닐린 전극 제작

손 선규, 김 서진, 신 준호*, 류 태공*, 정 재민*†, 최 봉길†
강원대학교 화학공학과
*한국지질자원연구원

초록


본 논문에서는 얇고 유연한 평면 슈퍼 커패시터(PSC)를 스크린 인쇄된 탄소 전극에 폴리아닐린(PANI)을 코팅하여 제 작하였습니다. 스크린 프린팅 방법을 사용하여 유연한 폴리에틸렌 테레프탈레이트에 탄소 잉크를 코팅한 후 희석 중 합 법을 사용하여 탄소 표면에 PANI 박막을 코팅하였습니다. 서로 맞물린 구조의 얇은 유연한 PANI 전극을 폴리머 겔 전해질로 조립하여 평면 모양의 슈퍼 커패시터(PSC) 장치를 만들었습니다. 상기 제조된 PANI/PSC는 매우 얇고 유연 하였으며, 10 mV/s에서 409 μF/cm2의 높은 면적 정전용량을 나타내었습니다. 이 값은 500 mV/s에서 원래 값의 46%로 유지되었습니다. 유연한 PANI/PSC는 180°의 구부러진 상태와 100번째의 반복적인 피로도 테스트에서도 82% 의 높은 정전 용량 유지를 보여주었습니다.


    1. Indtroduction

    The rapid development of portable, wearable, and highly integrated electronic devices has led to increasing global demand for the development of high-energy and high-power energy storage devices and technologies[ 1-3]. Supercapacitors (SCs) have attracted considerable attention as efficient power sources in electrochemical energy storage devices owing to their high power density, fast charge/discharge ability, long cycle life, and safe operation[4,5]. Typically, three types of SCs, such as electric double-layer capacitors, pseudocapacitors, and hybrid capacitors, have been developed, but their conventional structural features of a vertical sandwich structure restrict the size reduction of the microdevices[6,7]. Planar SCs, which is an emerging branch of SCs, can be a good solution for making progress with regard to the miniaturization of electrinic devices because planar SCs are composed of thin-film electrodes with in-plane interdigital structures[8,9]. Planar SCs with interdigitated electrodes and coated electrolytes on the surface can significantly decrease the thickness in the vertical direction, making compact device design possible[10]. The diffusion of electrolyte ions can be improved owing to the large number of open edges provided by the interdigitated electrode and reducing the distance between the electrodes, thereby helping improve the power density and rate capability.

    In the past decade, many active materials, such as transition metal oxides (NiO, Fe2O3, and MnO2) or conductive polymers [polyaniline (PANI) and polypyrrole (PPy)], have been extensively explored as active electrode materials for improving the energy densities of planar SCs[11-13]. Among the wide variety of active electrode materials with pseudocapacitive properties, PANI has been considered to be one of the suitable active electrode materials owing to its number of advantages, for examples, high theoretical capacity and conductivity, fast reversible electrochemical behavior, low cost, and good environmental friendly[14-16]. Despite the aforementioned advantages, PANI has limited capacity and poor cycling stability owing to its compactness and volume changes during repetitive charging/discharging processes[15]. Several attempts have been made to improve its performance. For example, composite PANI with other materials (e.g., metal oxides, carbon- based materials, and metal-organic framework materials) has been considered a common method. The synergies of these materials with PANI can help improve the capacitance, rate capability, and cycle stability[ 17-19]. In addition, nanostructured PANI (e.g., nanotubes, nanofibers, and nanowires) is another approach to improve performance [20-22]. Several methods have been developed for synthesizing nanostructured PANIs, such as dilution polymerization, stepwise electrochemical deposition, electrospinning, seeding, and template synthesis. While this strategy can effectively improve the electrochemical performance of PANI-based electrode materials, the use of hazardous materials, harsh experimental conditions, complex multi-step processes, and expensive or time-consuming systems limit their practical applications.

    In this study, we designed and successfully fabricated a one-step diluted polymerization of PANI on a flexible carbon electrode. The flexible carbon electrode was prepared using the screen-printing method. This process enabled us to produce highly flexible planar SCs (PSCs) using a high-speed, cost-effective, and simple step technique. The electrochemical results showed that the flexible PANI (f-PANI) PSCs exhibited a high areal capacitance of 409 μF/cm2 at 10 mV/s, an impressive high-rate performance of 46% capacitance retention in the range 10~500 mV/s, and a high capacitance retention of 82% even under bent states of 180° and 100 repetitive bent cycles,

    2. Experimental

    2.1. Preparation of f-PANI PSCs

    Prior to PANI synthesis, a flexible carbon electrode was prepared via screen printing on a polyethylene terephthalate (PET) substrate using carbon ink (FTU-16, Asahi Chemical) as an electrical circuit. The resulting electrode was annealed at 200 ℃ for 2 h. PANI was synthesized on the surface of a carbon electrode via dilute polymerization. The PET film was immersed in 1 M perchloric acid (70%, JUNSEI), 6 mM ammonium sulfate (98%, Sigma-Aldrich), 30 mM aniline (99%, Sigma-Aldrich), and deionized (DI) water. The f-PANI PSC film was obtained after several washes with ethanol and deionized water. A mixture of H2SO4 (6 g) and PVA (6 g) dissolved in DI water (60 mL) was forcibly stirred at 84 ℃ until it was transparent. The PANI electrode was then immersed in the H2SO4-PVA solution for 10 min. Electrodes coated with a thin layer of PVA solution were dried at room temperature in a fume hood for 4 h. Before electrochemical measurements, the generated f-PANI PSCs were sealed with a heat-resistant tape.

    2.2. Characterization

    The morphology was observed using an injection electron microscope (FE-SEM; Magellan 400, FEI Company) and an optical microscope (OM; BX53MTRF-S, OLYMPUS) were used. High-performance liquid chromatography (Dionex Ultimate 3000) was used to measure the chemical state of PANI. The PANI chemical structures were examined using Fourier transform infrared (FT-IR) spectroscopy (FT-IR 4600, JASCO Company).

    2.3. Electrochmical measurement

    All electrochemical measurements of cyclic voltammetry (CV), galvanostatic charge and discharge (GCD), and electrochemical impedance spectroscopy were performed using a VersaSTAT 4 (Princeton Applied Research). The electrochemical performance of the electrodes was evaluated using an f-PANI PSC film. The specific capacitance was calculated from the CV curve using the following formula:

    Areal capacitance ( C area ) : C area = 1 υ Δ V A ∫ i ( U ) d U .
    (1)

    where Carea is the specific capacitance, v is the scan rate (V s-1), ΔV is the potential window (V), A is the surface area of the electrode (cm2), and i is the current (A). Based on the GCD curve, the energy (E, Wh/cm2) and power (P, W/cm2) densities were calculated as follows:

    E = ∫ 0 t V I d t 3600
    (2)

    P = E × 3600 Δ t
    (3)

    where I denotes the discharge current density (A/cm2), V is the cell voltage after the iR drop (V), and t is the discharge time (s).

    3. Results and Discussion

    Figure 1 illustrates the layout of flexible PANI PSCs in a three-step procedure: (i) fabrication of carbon electrodes via carbon ink screen-printing, (ii) PANI polymerization onto the carbon surface, and (iii) coating the PVA layer onto the PANI surface. To obtain a flexible carbon electrode, commercial carbon ink was screen-printed onto a PET substrate, followed by annealing at 200 ℃ for 2 h. Thereafter, the dilute polymerization method, which is a fast and simple one-step procedure, allows for the growth of PANI onto carbon electrodes. After coating with the PVA/H2SO4 gel electrolyte, the planar SC was assembled completely and ready for electrochemical testing under mechanical fatigue.

    Material characterization of the as-prepared flexible PANI PSCs was carried out via various analytical measurements. Carbon and PANI were successfully coated via screen printing and dilute polymerization methods. The observation of OM and SEM images before and after coating revealed that carbon and PANI (Figure 2a~f) were well distributed and covered the surface of the flexible PET substrate. SEM images (Figure 2g and h) of the cross-section of carbon and PANI showed a thin stacked coating layer approximately 1 μm thick, indicating that a tightly packed smooth structure was formed during the screen printing and dilute polymerization processes. This indicates that the screen printing and polymerization processes are suitable methods for the fabrication of thin and flexible PANI film electrodes.

    To gain insight into its chemical states, the PANI electrode was investigated using FT-IR spectroscopy (Figure 3). The peaks at 1554, 1469, 1299, 1236, 1120, and 798 cm-1 are the characteristic peaks of PANI in the emeraldine salt form. The peaks located at 1554 and 1469 cm-1 demonstrated the existence of quinoid (Q) and bezenoid rings (B) caused by the N=Q=N and N-B-N stretching vibrations. The peaks at 1299 and 1236 cm-1 were attributed to the C-N-C and C-N stretching of the aromatic amine. The peak at 798 cm-1 is related to the out-of-plane bending vibrations of C-H and C-C in the benzenoid units. According to the discussed results, the PANI sample was successfully introduced into the emeraldine salt form with high electrical conductivity during the synthesis process.

    The electrochemical performance of the f-PANI electrode was investigated by CV measurements using a three-electrode system. The CV experiment was performed from -0.1 to 0.8 V at a scan rate of 25 mV/s (Figure 4a). The f-PANI shows the typical CV redox peaks of PANI, attributed to the reversible faradaic reaction of PANI due to doping/dedoping of dopants. The f-PANI electrode exhibited a significantly higher enclosed area of the CV curves compared to the carbon electrode, indicating a higher pseudocapacitance of the f-PANI electrode. The CV measurements were further performed with increasing scan rates from 10 to 500 mV/s (Figure 4b). According to the scan rates, the peak current increased, and the CV shapes showed redox peaks, indicating the good pseudocapacitive characteristics of the f-PANI electrode. Figure 4c shows the superior long-term cycle life of the f-PANI electrode. Repeating charge/discharge measurement was tested at a high scan rate of 50 mV/s during 5,000 cycles. The f-PANI electrode maintained high capacitance retention of 92% with high Coulombic efficiency of 98%.

    A symmetric all-solid-state flexible PSC device with a planar structure was fabricated by coating a 1 M H2SO4-PVA gel electrolyte onto the interdigitated f-PANI electrodes. As expected, f-PANI/PSC exhibited stable CV curves in the scan rate range of 10~500 mV/s (Figure 5a). Similar results were observed when f-PANI/PSC was tested by GCD measurements at 1 mA/cm2 (Figure 5b). The high specific areal capacitance of f-PANI/PSC based on the device area reached 409 μF/cm2 at 10 mV/s (Figure 5c). More importantly, f-PANI/PSC maintained 46% of its initial capacitance at a high scan rate of 500 mV/s, thereby indicating an excellent rate capability of f-PANI/PSC.

    To further demonstrate the mechanical robustness of f-PANI/PSC, CV measurements were performed under mechanically normal and bent conditions. The all-solid-state f-PANI/PSC exhibited excellent mechanical resistance. The CV measurements of f-PANI/PSC were performed at different bending angles in the range of 0~180°, resulting in a high capacitance retention of over 85%, even at a completely folded angle of 180° (Figure 6a). The fatigue test was also performed on f-PANI/ PSC through CV measurements after repetitive bending cycles (Figure 6b). The device exhibited remarkable flexibility and performance stability with a high capacitance retention of 82% after 100 bending cycles. Figure 6c shows the Ragone plots (E vs. P) of our f-PANI/PSC based on the device area. The f-PANI/PSC delivered a high energy density of 0.11 mWh/cm2 at a power density of 9.0 mW/cm2, which is comparable to and higher than that reported in the literature[23-29].

    4. Conclusion

    Flexible PANI electrodes for planar supercapacitors were fabricated by the one-step diluted polymerization of PANI on screen-printed carbon electrodes. The deposited PANI exhibited a conductive emeraldine salt form that was protonated and transformed from the meraldine-based form. The resulting f-PANI electrode had a high areal capacitance of 0.9 mF/cm2 at 10 mV/s. A flexible, thin, all-solid-state supercapacitor device was fabricated by assembling two f-PANI electrodes. The as-obtained f-PANI/SC exhibited a high areal capacitance of 409 μF/cm2 at 10 mV/s. A remarkable capacitance retention of 46% is observed as the scan rate increases from 10 to 500 mV/s. The electrochemical performance was maintained even in a mechanically bent state and after the fatigue test. Moreover, f-PANI/PSC exhibits high energy and power densities. The thin and flexible f-PANI electrodes offer great promise for improving the electrochemical performance of flexible electrochemical devices.

    Acknowledgements

    This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2021R1A2C1009926). This Research has been performed a by the Industrial Strategic Technology Development Program-Strategic core material Independent technology development project (20010193, Development of Electrode Fabrication Technology using Pitch for MCDI System Applied to Lithium Recovery from Low Grade Brine and Waste Solution and Manufacturing Technology of Lithium Compound for anode material of Lithium secondary batteries) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).

    Figures

    ACE-32-4-403_F1.gif
    Schematic illustration for the fabrication of f-PANI/PSCs.
    ACE-32-4-403_F2.gif
    Optical images of (a) carbon-coated and (b) PANI-deposited electrodes. SEM images of carbon electrodes (c) before and (d) after screen-printing of carbon inks. SEM images of PANI electrodes (e) before and (f) after diluted polymerization of PANI. SEM images of cross-section of (g) carbon and (h) PANI electrodes.
    ACE-32-4-403_F3.gif
    FT-IR spectra of f-PANI electrode (inset: schematic of emeraldine salt foam of PANI).
    ACE-32-4-403_F4.gif
    (a) CV curves of carbon and f-PANI electrodes measured at a scan rate of 10 mV/s. (b) CV curves of f-PANI electrode measured at a scan rate range from 10 to 500 mV/s. and (c) Cycling performance of f-PANI electrode measured at 50 mV/s.
    ACE-32-4-403_F5.gif
    (a) CV curves of f-PANI/PSC device. (b) GCD curves of f-PANI/PSC measured at a current density of 1 mA/cm2. and (c) Rate capability of f-PANI/PSC.
    ACE-32-4-403_F6.gif
    Capacitance retention values of f-PANI/PSC obtained (a) at different bending angles (inset: photographs of f-PANI/PSC under mechanically normal, and bent states) and (b) after bending cycles (inset: photographs of a fatigue test). and (c) Ragone plot of f-PANI/PSC.

    Tables

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