Facile synthesis of nanostructured polyaniline in ionic liquids for high solubility and enhanced electrochemical properties
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As one of the most investigated conducting polymers, polyaniline (PANI) is considered to be of practical use in many applications. In this study, two new ionic liquids, 1-methylimidazolium hydrogen sulfate ([Hmim]HSO4) and 1-methyl-3-n-butylimidazopersulfate ([C4mim]2S2O8), which were synthesized from 1-methylimidazole ([Hmim]), were used as solvent and dopant, oxidizer, respectively, for in situ polymerization of aniline. Because of the application of the unique structure of ionic liquid, we obtained the ionic liquid–doped polyaniline (IL-PANI) with high solubility (25 mg mL−1 in dimethyl sulfoxide (DMSO)). And by adjusting the ratio of [C4mim]2S2O8 to aniline monomer, the preferred PANI nanofibers could be controlled to form a three-dimensional porous structure. It was found that the ion/electron transport channels could be formed inside the 3D structure. Thus, the redox reactions could occur both at the surface and inside the PANI electrode. Electrochemical characterization showed that the fabricated PANI electrode exhibited a specific capacitance of 489 F g−1 at a current density of 0.5 A g−1. Also, the capacity retention rate reached up to 81% after 4000 cycles investigated at 2 A g−1. In addition, a high-energy density of 80.2 Wh kg−1 was measured when [Hmim]HSO4 was used as an electrolyte. Thus, the present work suggested a new strategy for fabricating high-performance PANI electrode for supercapacitor applications.
KeywordsIonic liquid Conducting polymer Energy density Electrochemical stability
Conducting polymers (CPs) are polymers having highly conjugated polymeric chains with properties such as electrical, magnetic, and optical properties like metallic regime. They are the materials that have attracted attention of many researchers in the field of energy storage , catalyst , sensors , membranes , corrosion protection [5, 6, 7], giant magnetoresistance , etc. owing to their unique and adjustable properties. They have semiconducting characteristics that can be tuned by the process called “doping.” Doping involves oxidizing or reducing the conducting polymers that increases the charge carriers in the polymer; thus, its electrical property is modified. Many conducting polymers, such as polypyrrole (PPy) , polyaniline (PANI) , and poly(3,4-ethylenedioxythiophene) (PEDOT) , have been studied for their outstanding electrochemical performances. Among them, PANI has attracted extensive attention than other conducting polymers due to its highly adjustable electronic and electrochemical properties, ease of synthesis, environmental friendliness, low toxicity, and cost [12, 13], as well as good redox reversibility . Although PANI has been considered as one of the most promising conducting polymers, the poor solubility and difficulty to process PANI remain a major challenge for mass-production. In this study, ionic liquids are used for the synthesis of PANI, in order to improve its solubility.
Ionic liquids (ILs) are the organic salts with low melting point (below room temperature). They have been extensively studied due to their low volatility, good thermal stability at elevated temperature, ionic conductivity, and controlled hydrophilicity . Ionic liquids are becoming more popular in the field of electrochemistry as it provides excellent electrochemical stability over wide range of potential due to absence of water [16, 17, 18, 19, 20]. They have been used in various applications, such as the extraction of common organic compounds , increasing the solubility of organic compounds , as well as applied as electrolytes in electropolymerization . The use of ionic liquids has been reported for variety of electrochemical devices [24, 25, 26]; however, their investigation in electrochemical synthesis of conducting polymer is limited. Ionic liquids facilitate in enhancing the conductivity and solubility of conducting polymer by forming hydrogen bond with the polymer chains [27, 28]. However, aprotic ionic liquid is a poor medium for the electropolymerization of aniline, as compared with the aqueous solution system ; so we select 1-methylimidazole ([Hmim]) as a starting material to synthesize ionic liquid with high proton activity .
In this work, we synthesized two new type of ionic liquids using ([Hmim])  as a starting material which can be used as doping agent and in situ oxidizing agent in the polymerization of aniline, respectively. Two ionic liquids, [Hmim]HSO4 and [C4mim]2S2O8, were synthesized and used as dopant and initiator, respectively, regulating the nanofiber structure of PANI. The mechanism follows the interactions of the hydrogen and nitrogen atoms in ionic liquid and the imine structures in PANI chains, which form hydrogen bonds. The addition of the ionic liquid to conducting polymers, not only enhances the ionic transport properties, but also retains the high electronic mobility in the conductive polymers. The chemical structure of PANI was characterized by Fourier transform infrared spectrometer (FTIR) and ultraviolet spectrometer (UV–Vis). The morphology of PANI fibers formed was confirmed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The electrochemical properties of ionic liquid polyaniline (IL-PANI) electrodes were investigated by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and galvanostatic charge–discharge (GCD). IL-PANI showed excellent rate capability and cyclability with specific capacitance as high as 489 F g−1. The results showed that the mixing characteristics of ions and electrons in PANI and ionic liquid enhance the electrochemical performance of the electrode and also improve the solubility and processability of PANI; therefore, the nanoporous PANI film with good performances is obtained facilely due to the high proton activity of [Hmim]HSO4.
Aniline, 1-methylimidazole, and 1-chlorobutane were purchased from BEHRINGER Technology Company. Ammonium persulfate (APS), potassium persulfate (KPS), hydrochloric acid (HCl), and sulfuric acid (H2SO4) were purchased from Tianjin Fuchen Chemical Reagent Factory. Polyvinylidene difluoride (PVDF) and 1-methyl-2-pyrrolidinone (NMP) were purchased from Aladdin Company (Shanghai, China).
2.2 Preparation of IL-PANI
Firstly, 1-methylimidazole sulfate ([Hmim]HSO4) (in Appendix Reference 1) was formulated into a solution with a concentration of 1 M. Then, 0.0015 mol of aniline was completely dissolved in 5 ml of 1 M [Hmim]HSO4, and 0.0005 mol of [C4mim]2S2O8 (in Appendix Reference 2) was mixed into the previous solution at room temperature. After stirring for 10 min, the solution was changed from colorless to dark green obtaining a PANI solution, and the product was dialyzed and freeze-dried to obtain a dark green powder. For comparison, the other parallel tests were performed in the same initiator [C4mim]2S2O8 using different dopants (HCl and H2SO4) to synthesize HCl–PANI and H2SO4–PANI. After that, the products were purified by dialysis (dialysis tube, 1000 MW cutoff, Fisher Scientific) for 2 days. Finally, the products were dried for 48 h by a freeze dryer.
Vector22 FTIR spectrometer was used to measure the infrared spectra of PANI active materials in the range of 400–4000 cm−1 with KBr pellets. A nuclear magnetic resonance (NMR) spectrometer (Bruker, AVANCE III 600 MHz) was used to characterize the chemical structure of ionic liquids at room temperature. The change in absorption wavelength of different PANI samples was tested by ultraviolet spectrometer (UV-3600). The size and morphology of PANI samples were inspected using cold field emission scanning electron microscope (JSM-7500 F). The crystal structure and size of PANI were characterized by X-ray diffraction (Bruker D8 ADVANCE) with Cu Kα radiation (λ = 0.15405 nm).
All electrochemical studies were conducted by an electrochemical analyzer (IVIUM Vertex) at room temperature. For evaluating the electrochemical behavior of the individual electrode in a three-electrode configuration, 1 M HmimHSO4, 1 M H2SO4, and 1 M HCl aqueous solutions were used as electrolytes with a saturated calomel electrode (SCE) and a Pt wire employed as the reference and counter electrode, respectively. In order to make the PANI-C-clothes (PANI-CC) electrode, we mixed thoroughly the PANI active materials, PVDF and super P at a weight ratio of 8:1:1 in NMP, and the obtained slurry was uniformly supported on a carbon cloth by blade coating. Finally, the electrodes were vacuum dried at 60 °C in an oven for 24 h to ensure complete evaporation of the NMP solvent. The loading mass of active materials was controlled at 1–3 mg cm−2.
3 Results and discussion
3.1 Structure and morphology
In the present investigation, the acidic ionic liquid [Hmim]HSO4 was used as a dopant and the oxidized ionic liquid [C4mim]2S2O8 as an oxidizing agent [34, 35]. This is because [C4mim]2S2O8 (comparing to ammonium persulfate, [NH4]2S2O8) slows down the reaction, which can control the size and morphology of nanoparticles. This is due to its high solubility in water, and the formed IL-PANI chain has a small surface tension which is difficult to break down, and the phenomenon of agglomeration in water is not obvious. Additionally, the imidazolium ions present in the oxidized ionic liquids cater to the formation mechanism of PANI.
The facile synthesis route of our IL-PANI provides a detailed method for synthesizing porous fibers. The resulting nanostructure is more effective than electrochemical sensing device as it provides large effective surface areas due to its porosity. Besides, the interconnected network provides accessible channel for easy transport of electron and ions. This is the result of electron rearrangement in PANI chain during synthesis process. Moreover, the interaction between ionic liquid and PANI results in hydrogen bond that acts as a cross-linking agent, which virtually reduces the dissociation in ionic liquids by reducing the inter-layer spacing between PANI chains. This decreases the path for ion and electron to transport and thus increases the conductivity.
3.2 Electrochemical property
PANI has its inherent morphology and its electrochemical properties are greatly affected by the doping and oxidation levels. In order to study the electrochemical performance of IL-PANI, we performed several experiments. The experiments involved to study the effect of different doping acids on electrochemical performance of PANI. Further, performance of produced IL-PANI with molar ratio was investigated. The three-electrode system was used for electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and galvanostatic charge–discharge (GCD) measurements.
Figure 8b shows the CV curves of IL-PANI-1-0.34 with varying scan rates. The electrodes are stable in [Hmim]HSO4 aqueous solution within the employed potential range (− 0.2 to 0.9 V), and the peaks from Faradic current (current from redox reaction of electrodes, normally accompanied with peaks on both oxidation and reduction curves) were observed. The plots of current density versus the different scan rates for the highest oxidation peaks are given in the inset of Fig. 8b. It is shown that the oxidation and reduction peak currents increase linearly with scan rates, which mean that transport of ions in electrode is mainly due to the redox process occurring at the electrode surface. This indicates that redox process was found to be confined to the electrode surface, which is due to the small dimensions of the polymeric nanostructure. However, Fig. 7 illustrates that the redox reaction occurs in the electrode because of the hydrophilicity of the ionic liquid, which reduces the self-aggregation of the nanofibers and allows the electrolyte ions to diffuse into the electrode material.
The specific capacitances of the different PANI samples at a current density of 0.5 A g−1 are shown in the inset of Fig. 9a and b. The specific capacitance of IL-PANI-1-0.34 was calculated as high as 489 F g−1 at 0.5 A g−1 (specific capacitance measured at the electrochemical window of 0.1–0.8 V is 530 F g−1). The specific capacitances of HCl–PANI and H2SO4–PANI in Fig. 9a are 174 F g−1 and 201 F g−1, respectively. While the specific capacitances of IL-PANI-1-0.5 and IL-PANI-1-0.25 in Fig. 9b are 239 F g−1 and 404 F g−1, respectively. The ionic liquid in IL-PANI has good compatibility with water, reduces the agglomeration of nanoparticles, and the effective control of the functional oxidized ionic liquid on the morphology which is beneficial to the ordered nanostructure of IL-PANI, which enhances the transport of electrons and ions.
Specific capacitances of the electrodes at different current densities are illustrated in Fig. 9c and d, which clearly showed that IL-PANI has a higher discharge capacity than HCl–PANI and H2SO4–PANI. Notably, IL-PANI displayed excellent cyclability at various current densities. This indicates that electrode possesses a relatively higher energy and power density. However, the specific capacitance decreases with the increase in charge–discharge current density, which is attributed to the fact that the redox reaction rate and the charge diffusion cannot match the rapid increase of the current densities .
In this study, we successfully synthesized IL-PANI by in situ polymerization. The introduction of ionic liquid resolved the following issue: (i) improved the solubility of PANI assisted by the polarity of the ionic liquid, (ii) the use of functional oxidizing ionic liquids improved the nanofiber formation of PANI and controlled the reaction rate, and (iii) enhanced the electrochemical performance (energy and power densities, cycling stability) of the ionic liquid–doped PANI.
The authors received support from Auburn University Internal Grant Program (AU-IGP).
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