Growth and cycling of polyaniline electrode in a deep eutectic solvent: A new electrolyte for supercapacitor applications

Highlights

• Polyaniline electrodes were electrodeposited from an ionic liquid.

• Polyaniline was cycled in a non-aqueous and non-acidic electrolyte.

• High capacitance performance of polyaniline (833 F g⁻¹) was achieved in Ethaline.

• The capacitance retention of polyaniline was 87% after 10,000 cycles in Ethaline.

Abstract

Polyaniline electrode on a graphite substrate was produced and evaluated for use in an ionic liquid electrolyte for supercapacitor applications. The aniline monomers were added in Ethaline deep eutectic solvent electrolyte. The deep eutectic solvent is a type of ionic liquid consisting of only ions without water. Aniline was polymerized electrochemically onto graphite current collector from a non-aqueous and non-acidic Ethaline electrolyte. Polyaniline film obtained in Ethaline had homogenous agglomerated nanoparticles. Growth of polyaniline was not autocatalytic reaction in ionic liquid media and the rate of its growth slowed down upon increasing time. Electropolymerized polyaniline was transferred in two different electrolytes (acetonitrile containing LiCIO₄ solution and monomer-free Ethaline) to determine the electrochemical properties of the polyaniline coated graphite electrodes by cyclic voltammetry and galvanostatic charge-discharge techniques. The capacitance of the modified electrode retains 87% of its initial value after 10,000 cycles in Ethaline electrolyte because the polyaniline electrode was probably not effected from swelling and shrinking in an anhydrous ionic liquid for long cycles. The electrode deposited from Ethaline ionic liquid medium and cycled in pure Ethaline electrolyte had a specific capacitance of 833 F g⁻¹ at 20 mV s⁻¹ scan rate. The polyaniline coated graphite electrode electrodeposited using one-step polymerization in Ethaline media is a promising electrode for high-performance supercapacitors that can be used in non-aqueous electrolytes.

Introduction

The depletion of fossil fuels could cause a reduction of energy resources. Global warming and increasing environmental pollution are the results of the use fossil fuels. Therefore, the consumption of fossil fuels threaten people and the globe [1]. Great efforts have been made to research renewable and sustainable energy sources such as wind energy and solar energy. Energy obtained from renewable sources is required to be stored for use in demand. Inexpensive energy storage devices with high surface coverage (porosity) [2], high power density, long cycle stability and fast charge-discharge times are commonly studied [3]. Supercapacitors are energy storage devices for which these requirements are satisfied [4]. Supercapacitors are promising energy storage systems for medical devices, electric vehicles, wearable electronics and foldable electronics [5]. Two basic types of supercapacitors are pseudocapacitors and electrical double-layer capacitors (EDLC) [6]. In EDLC, ions are stored at the interface of carbon-based electrodes. In pseudocapacitors, metal oxides/hydroxides and conductive polymers can react with electrolytes by the Faradaic reaction [7]. Carbon materials with high surface area for electrode materials usage in EDLC are popular due to their high capacitance and electrochemical stabilities. Metal oxides/hydroxide (such as oxide and hydroxide forms of Co [8], Ru [9], Mn [10], Cu [11] and Ni [12]) and conductive polymers (containing polyprrole [13], polyaniline [14] and poly(3,4-ethylenedioxythiophene) [15]) are used as electrode materials in pseudocapacitors [16]. Conductive polymers have been extensively investigated for different redox states for electrochemical energy storage systems due to their long cycle stability and high theoretical specific capacities [17].

Conductive polymers could exhibit a supercapacitive response since they can rapidly have reduction and oxidation reactions [18]. This could allow the supercapacitors electrodes to have higher specific energy and power values [19]. Conductive polymers synthesized by electrochemical methods are of great importance in cases where tailoring of a surface is required, such as thickness and surface coverage [20]. Conducting polymers which have the advantages of being suitable electrode material for supercapacitors are inexpensive and environmentally friendly.

Among the conductive polymers, polyaniline is one of the most promising polymers for electrode materials of supercapacitors because of its high chemical stability, high theoretical capacitance, ease of synthesis and relatively low cost [21]. The electrochemical stability of polyaniline depends on its chemical and physical properties [22]. Fig. 1 displays the polymerization process of aniline in acid medium. In the first step of the polymerization process, aniline monomers in the acidic solution form cation radicals. These cation radicals then bind to form aniline dimers. These dimers could form oligomers of aniline. Nucleation and growth of the polymer chain are then continue to precipitate polyaniline [23]. In this study, polymerization of aniline was carried out in non-aqueous and non-acidic media.

Electrodes and electrolytes are the most important components of electrochemical energy storage devices [24]. The performance, power density and stability of the supercapacitors are directly related to the choice of electrode and electrolyte. The capacitance in the supercapacitors depends on the surface area of the electrode and electrolyte materials. As it is important to develop electrodes with high surface area to store more charge [25], advancements on the fabrication of energy storage material is essential. Here, a new electrolyte environment was studied to electrodeposit polyaniline electrode.

Temperature coefficient and conductivity are important factors to be considered for the selection of cycling electrolyte [26]. Electrolytes should be economical, provide a large potential window, and have high electrochemical stability, low volatility and low toxicity. It is also important that the selected electrolyte does not cause corrosion of electrodes and current collectors in energy storage devices. Aqueous and organic electrolytes are commonly used in supercapacitors [27]. When high potentials are applied to aqueous electrolytes, it limits cell voltage due to water decomposition and offers a narrower potential range. As potential window can reach higher voltages in organic and non-aqueous electrolytes [28], in this study polyaniline was cycled in an ionic liquid and non-aqueous organic electrolyte. Ionic liquids are purely salts consisting of anions and cations with a melting point below 100 °C. Room temperature ionic liquids are liquid at room temperature and free of any solvents [29]. Ionic liquids have been studied for supercapacitors [30] because these electrolytes have low volatility and high potential window.

In this study, a graphite electrode was used as a working electrode and polyaniline was electrodeposited from aniline solution prepared in Ethaline ionic liquid medium. The polyaniline film deposited from Ethaline medium was transferred to Ethaline and acetonitrile containing LiCIO₄ electrolytes to examine the electrochemical performance of modified electrodes in non-aqueous and non-acidic media.