Capacitive property studies of inexpensive SILAR synthesized polyaniline thin films for supercapacitor application
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The polyaniline (PANI) is an eco-friendly conductive polymer which has been considered for diverse applications. The partially oxidized phase of the PANI is useful for the charge storage application. Here, a unique nanograin/nanofiber structured PANI was grown on inexpensive stainless steel (SS) current collector by the simple oxidative polymerization process and its charge storage properties were systematically investigated. For that, the inexpensive successive ionic layer adsorption reaction method was used to grow a uniform nanostructured PANI on the SS conductor. This evolution of the nanostructure was studied with the Field emission scanning electron microscope. Furthermore, the as-prepared PANI was confirmed by the X-ray diffraction and the Fourier-transform infrared spectroscopy. In the half cell electrochemical testing, the prepared PANI exhibited a maximum specific capacitance of 710 F g−1 with a specific discharge capacity of 119 mAh−1 at 0.2 mA cm−2 in 1 M H2SO4 for the supercapacitor application. Also, by using the power-law relation it was observed that, in a charging and a discharging current, initially a contribution of the diffusive faradaic reactions is more as compared with the surface capacitive non-faradaic reactions.
KeywordsPolyaniline (PANI) Supercapacitor SILAR Stainless steel Power-law Charge storage
Evolution in the electrical systems for a wide spectrum of application in recent years have been increased the demand for electrical energy consumption [1, 2]. The sophisticated energy storage units with desired energy-power output for targeted electrical systems has been the main goal in front of the research community. In the global market, batteries, supercapacitors (SCs), hybrid energy storage systems have been providing the desired requirements of electrical systems. Among several energy storage systems, the Li-ion batteries are still dominating in the market for different applications, from the medical devices to the hybrid vehicles as the main central electrical energy storage and supplying system (EES) unit [3, 4, 5]. However, SCs with low initial capital costs, low operation-maintenance costs, with easy and efficient operation, high power density have been considered as the best option for main backup EES unit [6, 7, 8].
The materials generally used in the SC’s stores electrical energy either in the form of columbic (electric double layer) and faradaic (redox reaction) charge transfer process or the combination of both, which influences its power-energy output [9, 10]. The SCs having more electric double layer transitions can deliver more electric power density due to the fast charge transfer rate of adsorbed ions on the electrode surface. Whereas, the SCs having more redox transitions can deliver low power density due to poor charge transfer rate [11, 12]. More surface adsorption reaction in the charge transfer process increases the operational life cycle of SC, which generates the poor output energy. However, volume expansion and phase transition during the redox reaction leads to a decrease in the operational lifecycle of SCs . This has been encouraging to study the different choices of materials to minimize these possible drawbacks for the efficient SC power-energy output.
PANI is low cost, chemically stable and good electrical conductive polymer with tunable electrochemical properties. Due to its environmental friendly nature, it is used in several applications such as gas sensor , anticorrosive coatings , OLED , conductive adhesive, antistatic textile, electro-rheological (ER), capacitor, solar cell, electromagnetic shield interference . The electrical energy storage property of PANI involves the fast redox reaction due to doping and de-doping of cation from the electrolyte, which makes it a promising candidate for the SC application. Here, the degree of protonation decides the conductivity of PANI. Fully oxidized state or reduced state of the PANI may not be electron-conducting but half oxidized state is conductive .
The PANI has been synthesized using chemical oxidative polymerization, chemical bath deposition, non-emulsion, electrochemical, interfacial polymerization method [18, 19, 20, 21, 22, 23]. These methods have serious drawbacks such as the polymerization of aniline monomer which generates unnecessary precipitations causing wastage of material. Also, in the electro-polymerization the desired electrochemical setup is necessary. On the other side, a simple, inexpensive successive ionic layer adsorption reaction (SILAR) method has been used for the synthesis of PANI to overcome these drawbacks [24, 25, 26]. Also, using the SILAR method it can be possible to synthesize a large area thin films with uniform surface morphology in which thickness and composition can be easily controlled by easy preparative parameters such as a number of cycles and process of immersion [27, 28]. Previously PANI thin films have been synthesized using SILAR method, Kulkarni et al.  prepared a fused nanorods on stainless steel and glass substrate, Parez et al.  prepared a fiber like porous structure on Whatman filter paper. Arejola et al.  prepared on polyethylene board, Patil et al.  used different surfactants to prepare different nanostructures on SS substrate, Chougale et al.  prepared interconnected nano-fibrous structure on a glass substrate. However, more studies related to the capacitive properties of PANI prepared by SILAR method are needed which is imperative for the SC application. The inexpensive, simple preparation method for PANI and the study of its electrochemical properties may provide a very good insight of its possible application.
In this study, the PANI films were synthesized on SS substrate by SILAR method. A number of SILAR cycles was optimized to get the films thickness of PANI on SS substrate. The filed emmision scanning electron microscope (FE-SEM) analysis was performed in order to understand the development of nanostructure on the substrate. The optimized PANI film was subjected to X-ray diffraction (XRD) and fourier-transform infrared (FT-IR) spectroscopy studies for further confirmation. Finally, the electrochemical properties of PANI films were tested by the three electrode measurement setup in order to understand the capacitive origin and to measure the specific capacitance for the SC application.
2.1 Chemicals and materials
All chemicals used for the synthesis were analytical grade. Aniline monomer, H2SO4, ammonium persulphate ((NH)4)2S2O8, were purchased from S.D. fine chemicals, India. All the solutions were prepared in double distilled water.
2.2 Synthesis of PANI films
In the typical procedure of single SILAR cycle, the pretreated SS substrate was dipped in the chemical bath A for 10 s for adsorption of an aniline monomer on its surface. Then, this substrate was taken out and rinsed in double distilled bath B for 5 s to remove loosely bound adsorbed aniline monomer. Furthermore, the substrate with the adsorbed aniline monomer was taken out from bath B and dipped into the chemical bath C for 15 s. In this step, the oxidation of adsorbed aniline monomer takes place due to the presence of an ammonium persulphate to form a thin layer of PANI on to the substrate surface. Then, the substrate coated with the PANI thin film was taken out from the bath C and rinsed in the double distilled bath D for 5 s to remove loosely bound species. The nanostructured PANI were prepared by repeating 20, 25, 30 and 35 SILAR cycles designated as P-1, P-2, P-3 and P-4 respectively.
2.3 Preparation of electrode for electrochemical testing
3 Result and discussion
The surface capacitive contribution in CV curve of the P-3 at 50 mV s−1 is shown in the inset of Fig. 5d[ii]. It estimated 48.7% of surface capacitive current contribution (shaded area) in the total current (solid line) ascribing the pseudocapacitive electrochemical property of the PANI.
The stability of the P-3 was studied for 1000 CV cycles at a scan rate of 100 mV s−1. The estimated specific capacitance values with the number of CV cycles are shown in Fig. 6c. The amount of 52.5% capacity retention is observed after 1000 cycles. This electrochemical degradation of the P-3 may be associated with the mechanical stress induced during the charging and discharging process leads to the dissolution of PANI in acidic media [71, 72]. In further analysis, the EIS of P-3 was performed after 1st and 1000th cycle at a frequency range of 106–100 Hz with an amplitude of 5 mV; respective Nyquist plots are shown in Fig. 6d with the fitted equivalent circuit diagram is shown in the inset. The curve is semicircular in the high frequency region and inclined at the low frequency region. The intercept of the curve on real impedance (Z′) represents a combined resistance of the electrode material and contact resistance at the interface between the electrode material-current collector termed as equivalent series resistance (ESR) or solution resistance (RS). The diameter of the curve semicircle at high frequency ascribed to the charge transfer resistance (RCT) at the interface of the electrode material and the electrolyte. The slope of 45° portion of the curve represents a Warburg resistant (ZW) associated with the frequency dependent ion diffusion in the vicinity of the electrode surface. CPE-1 defines the pseudocapacitance of active material. RL and CPE-2 are the voltage dependent charge transfer components placed parallel in the circuit . The RS value of both 1st and 1000th curves are nearly 1 Ω as shown in the inset of Fig. 6d. However, the RCT value of 1st and 1000th curves are 323 and 526 Ω respectively. The increase in the RCT value after the 1000th cycle is attributed to the decrease in the conductivity of PANI due to the deprotonation over long tome immersion [73, 74]. In previous studies, Hui et al. prepared the PANI electrode by the pressing composition mixture of PANI nanofiber powder, acetylene black and poly(tetrafluoroethylene) onto stainless steel mesh which exhibited RS values of 1.63 Ω . Li et al.  prepared the PANI electrode by drop casting prepared PANI on the glassy carbon which exhibited the RS values in the range of 5–8 Ω. In another study, Li et al.  prepared the PANI electrode by pressing the composition mixture of prepared PANI powder onto the Ni-mesh which exhibited the RS values between of 5–7 Ω. Poli et al. prepared the PANI on a carbon fiber felt which exhibited the RS values of 1.2–1.8 Ω . Here, the PANI exhibited RS value of 1 Ω indicates formation of good electrical contact between SS and PANI. However, the large value of RCT can be ascribed to the major contribution of the diffusive intercalation charge transfer process in PANI as depicted in above electrochemical studies.
In conclusion, a simple, inexpensive SILAR method is successfully employed for the synthesis of unique mixed nanograin/nanofiber structured PANI on the inexpensive SS current collector. The FE-SEM along with XRD, FT-IR studies confirmed the formation of porous compact nanograin/nanofiber nanostructured PANI. The PANI prepared at 30 SILAR cycles exhibited a maximum specific capacitance of 710 F g−1 at a current density of 0.2 mA cm−2 with specific discharge capacity of 119 mAh g−1 in 1 M H2SO4. The electrochemical studies revealed that the total current contribution during the charging and discharging process in the PANI are initiated by the faradaic diffusion process due to the intercalation of electrolyte ions in the unique nanograin/nanofiber structure and later are dominated by the surface capacitive non-faradaic process after reaching the oxidation–reduction potential. This simple, room temperature synthesis method is very useful for the fabrication of porous PANI electrodes for pseudocapacitive energy storage application.
The authors are grateful to Basic Science Research Program for financial assistance through National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2018R1A6A1A03024962).
Compliance with ethical standards
Conflict of interest
The authors declare that they have no competing interests.
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