Preparation and Characterization of the Cobalt Doped Polyaniline / MWCNT Nanocomposites for Supercapacitor Application

Transition metal ion doped poly-aniline (PANI)/ multiwall carbon nanotube (MWCNT) composite was successfully prepared by in-situ oxidative polymerization of aniline monomer for supercapacitor application. The Co-doped PANI/MWCNTs composite showed improved capacitive behavior and lower electrical resistance compared to HCl doped PANI (DPANI)/MWCNT composite. By using carbon nano tube with conducting polymer offer exceptional power andhigh performance by supplying high surface area, high conductivity and high retention of mechanical strength under charge-discharge cycle. All the materials were characterized by Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM). The electrochemical capacitive performance of the asymmetric supercapacitor was tested by cyclic voltammetry (CV), impedance spectroscopy (EIS) and cyclic charge-discharge (CCD) test in a three-electrode system. The highest specific capacitance value (576 F/g) was obtained at a scan rate of 2 mV/s in 1 (M) KOH electrolyte for Co-PANI/MWCNTs composite.


Introduction
In recent years, supercapacitors have attracted great attention due to their application in the field of charge storage devices.A typical supercapacitor which is employed in a light weight vehicle or a telecommunications device requires to be made of a material characterized by moderately high energy density, high power density, small size, low initial cost and prolonged life cycle.Supercapacitor forms the bridge between batteries and conventional capacitors with covering several orders of magnitude both in energy (moderately) and in power density (high).Supercapacitors are of two types: electric double layer and pseudocapacitor capacitor and they differ from each other in terms of mechanism followed for charge storage.The first one charge store by forming electrical double layer on electrode/electrolyte interfacewith electrostatic attractionbut latter one charge store completely different mechanismwith redox reaction i.e. faradic reaction process (Gupta et al., 2006).For the preparation of supercapacitor electrode material, we have used multiwall carbon nanotubes.It is more advantageous to use carbon nanotubes than using other carbonaceous materials since carbon nanotubes tend to entangle with each other to from a durable and porous material (Chunsheng, Ning et al., 2007).Polyaniline (PANI) is used as conducting polymer since it has good environmental stability; it is relatively less expensive, exhibits high conduction with protonic acid (Chiang & Macdiarmid, 1986).It consist of two units namely, benzoid and quinonid.Each tetramer consists of two amine and two imine nitrogen atoms which can be doped to obtain the conducting polymer emerilidine salt-PANI.When protonic acid was added to the insulatingemeraldine base (EB) form of PANI, it can be converted to the conducting formandresulting in an increase of conductivity by more than 10 fold in magnitude (Zengin et al., 2002).It can also bind with carbon nano tube via Pi-Pi stacking or electron acceptance from carbon nanotube.Many researchers have investigated transition metal doped conducting polymer and have found higher conductivity with increase in doping level of transition metal.IrCl 3 polypyrrole gives enhanced capacitance for electrochemical energy storage device (Chang & Chien, 2000), RuCl 3 doped polypyrrole gives higher capacitance in NaNO 3 electrolyte solution (Ben et al., 2004), MnCl 2 doped Polyaniline was also used for electrochemical supercapacitor application and gives 474 F/g with highest doping level (Patil et al., 2011), Using transition metal (Co 2+ ), charge accumulation property increasing with increasing doping level due to HSAB principle, hard acids (border line Co 2+ ) prefer to bind to hard bases (PANI).Another reason to take Cobalt (II) has vacant d-orbital to bind up with nitrogen lone pair & Cobalt cation can follow the two step redox mechanism with PANI where as first metal ionoxidized the polymer unit and get reduced, this reduced metal ion again bind with imine nitrogen from quinoide ring of PANI and get oxidized.This phenomena leads to the formation of radical cation segments and therefore corresponding metal ions (Celly et al., 2009;Li et al., 2009).
Our aim is to develop the super capacitor performance and increase the cyclic stability of supercapacitor electrode material.For this purpose we have used carbon nano tube which has high surface area, low resistance, high electron donor and acceptor properties and high stability indicates that it can be used to improve the supercapacitive performances and stability of the supercapacitor electrode materials.
Subsequently, 60 mg of MWCNT was added and sonicated for 30 minutes at room temperature.In another beaker, 2 ml of aniline monomer in 50 ml of 1 (M) HCl was taken and 4 wt % of CoCl 2 (99.99% Aldrich) was added by stirring to achieve a well dispersed suspension.The solutions contained in both the beakers were mixed with each other and sonicated for 10 minutes.Subsequently, 60 ml of HCl containing 2.04 g of ammonium persulphate (APS) (Lobachemiepvt.Ltd, India) was added drop by drop to the above solution and sonicated for another 10 minutes.The reaction mixture was maintained at 1~5 o C for 24 hrs.The resulting precipitate was filtered and washed with distilled water and ethanol several times and dried at 100 o C for 12 hrs.MWCNT/PANI also has been prepared by above procedure without adding metal salt.

FT-IR Spectroscopy
FT-IR was studied by NEXUS 870 FTIR (Thermo Nicolet) instrument.In dry condition samples were prepared by mixing KBr and samples with weight ratio of 1:10 andpellet were made to perform this test.

Morphology
FESEM analysis was investigated on gold coated samples in Carl Zeiss-SUPRATM 40 FESEM instrument.HRTEM and EDX observations were performed on JEOL 2100 HRTEM by dispersing the sample by sonication in acetone.A drop of the well dispersed mixture was taken in a copper mesh type grid and fully dried in air for TEM analysis.

Electrode Preparation and Electrochemical Analysis
Cyclic voltammetry (CV), Electrochemical Impedance Spectroscopy (EIS), constant current charging/discharging (CCD) measurementsof the materials were carried out on GAMRY reference 3000 instrument using a three electrode system where platinum and standard calomel electrode (SCE) were used as counter and reference electrode.For working electrode preparation, 2 mg sample was dispersed in 200 μlnafion binder (1wt % ethanol solution) by 30 min sonication (Saswat et. al., 2011).This sample solution has been cast on glassy carbon electrode with a diameter of 3 mm and dried fully over vacuum before electrochemical test. 1 (M) KOH solution was used as electrolyte solution for electrochemical measurement.CV were performed with potential window -8 V to +8 V vs SCE.

FT-IR Measurement
The bond vibration frequency of cobalt doped poly-aniline with MWCNT was characterized by FT-IR and the plot is shown in figure 1.The FT-IR spectrum of 1560 cm -1 and 1490 cm -1 are characteristic peak of the stretching vibration of the N=Q=N ring and N-B-N respectively (where B refers to the benzenoic-type rings and Q refers to the quinonic-type rings (Tanga et al., 2009).The characteristic peak appeared at 1300 cm -1 corresponds to C-N bond vibration.The characteristic peaks at 1109 and 800 cm -1 are attributed to the characteristic of B-NH-Q or B-NH-B bonds, and out-of-plane bending vibration of C-H on the 1, 4-disubstituted aromatic ring.Figure 1 shows that band appeared at 1300 cm -1 assigned to C-N stretching vibration shifts to 1236 cm −1 when Co 2+ is added in the reaction system, which indicates that the Co 2+ can interact with the nitrogen atoms in t PANI, esp to the vaca is shown in   with doped PANI harge accumulation property increases.This type of work will help developing the better supercapacitor having high capacitance value and enhanced cyclic charging discharging capabilities.

Conclusion
Cobalt doped polyaniline/MWCNT composites were prepared successfully by the in situ polymerization process.The composite exhibits greater electrical conductivity because of transition metal doping.The FT-IR and Raman spectroscopy reveal that MWCNTs were coated with cobalt doped polyaniline.Morphological study (FESEM and TEM) also confirmed multiple doping sites of the Co 2+ in polymer chain due to rough surface of the composite.The area under cyclic voltammetry and charge-discharge test indicate higher charge accumulation on surface of the composite which generates higher specific capacitance.The more research is required in this particular field specially by changing the doping method and the element in order to develop the better supercapacitor.

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