List of Symbols
2.2. Experimental section 1. Materials
Graphite powder (99.9999%, metals basis), carbon black (50% compressed), acetylene (99.9%) and poly (vinylidene fluoride) powder were purchased from Alfa Aesar. N-Methyl-2- pyrrolidone (~99.5%), Molybdenum (IV) Sulphide (~98%) and Ammonium Persulphate (99%) were purchased from Sisco Research Laboratories. Aniline (Emparta grade, ≥99.0%) and Acetone (Emplura grade, ≥99.0%) were purchased from Merck Millipore and Hydrochloric acid (assay ~35-37%) was purchased from Fisher Scientific. All the chemicals were of analytical reagent (AR) grade with % of purity as indicated, and used as received.
2.2.2. Synthesis of chemically exfoliated graphene and MoS2 nanosheets
The synthesis of graphene was carried out through the solvent exfoliation technique [32], with N-methyl-2-pyrrolidone (NMP) as the solvent. An amount of 50 mg of graphite powder was added to 10 ml of NMP. The mixture was sonicated in an Ultrasonic bath at 33 kHz for 8 hours.
A dark grey dispersion was obtained. The dispersion was allowed to settle overnight, and then centrifuged at 1500 rpm for 30 minutes. The supernatant liquid was collected and used for further experiments. A similar solvent exfoliation method was adopted for the preparation of MoS2 nanosheets [33]. An amount of 50 mg of MoS2 powder (properly grounded) was added to 10 ml of NMP. The mixture was sonicated for 4 hours in an Ultrasonic bath at 33 kHz. The resultant mixture (a light grey dispersion) was left overnight to settle. The dispersion was then centrifuged for 30 minutes at 1500 RPM. The supernatant liquid was used for further experiments. An approximate concentration was determined by measuring the amount of graphite and MoS2 left unexfoliated. The dispersion was allowed to settle over several hours, and the supernatant was further centrifuged. The collective sediment from these processes was then washed with DI water and finally with acetone to remove NMP. The sediment was dried at 40ᵒC to remove acetone. The unexfoliated particles were weighed; the difference between the final and the initial amount was taken as the approximate amount of particles exfoliated in the dispersion. Based on this, the concentrations of graphene and MoS2 in the dispersions were determined.
2.2.3. Synthesis of PANI, PANI-G and PANI-G-MoS2 nanocomposites
The procedure for Polyaniline synthesis as described in the IUPAC technical report was followed with a few modifications [34]. An amount of 3.72 g of aniline monomer was added to a 50 ml of 1M HCl, and stirred for 40 minutes. Another solution was prepared with 9.1 g of ammonium persulfate (APS) in 50 ml of 1M HCl. The APS solution was then added to the aniline solution drop by drop at 0 ᵒC with constant stirring. The solution was a colorless
mixture. Gradually, the color was changed from blue to dark blackish green, indicating the formation of PANI-ES (Polyaniline Emeraldine Salt). The mixture was allowed to polymerize for 24 hours, then filtered and washed with 0.2 M HCl, followed by acetone, and finally with 0.2M HCl. The precipitate of PANI was vacuum dried at 60 ᵒC for 24 hours. The dried sample was collected in a glass petri dish or further studies. PANI nanocomposites were prepared with graphene by in-situ chemical oxidative polymerization in the presence of solvent-exfoliated graphene (Figure 2.1a). A solution was prepared with 0.5 gm of aniline monomers in 50 ml of 1M aqueous HCl and stirred for 15 minutes. To the above solution, x amount of graphene was added from the graphene in NMP dispersion, where x is the weight % of graphene. The concentration of graphene was varied as 0.4, 1 and 2 weight %. A solution of 1.5 gm APS in 40 ml of 1 M HCl was prepared and added dropwise to the above aniline solution. The reaction mixture was stirred at 400 RPM at 0 ᵒC for an hour. Then, the mixture was kept at 0 - 4 °C for 24 hours, to complete the polymerization. The precipitate was filtered and washed with 0.2 M HCl solution, and vacuum dried at 60 ᵒC for 24 hours. For each sample (viz., PANI-G 0.4%, PANI-G 1% and PANI-G 2%) all weight and molar ratios were calculated with respect to the initial amount of the aniline monomer taken.
The ternary nanocomposite of PANI, graphene and MoS2 was prepared in a similar manner as described above (Figure 2.1b). In a solution of 0.5 gm aniline and 50 ml of 1M aqueous HCl, equal amount of graphene (2 weight %) and MoS2 (2 weight %) were added from their respective dispersion in NMP. This mixture was stirred for 15 minutes at room temperature. It was then sonicated at 33 kHz for 45 minutes. The mixture was then cooled down to 0 ᵒC. A solution of 1.5 gm of APS in 40 ml of 1M HCl has prepared. The APS solution was then added drop by drop to the above mixture at constant stirring. The reaction mixture was stirred at 400 RPM at 0ᵒC for an hour, and then kept for 24 hours to complete the polymerization. Following this, the mixture was filtered and the precipitate was washed with 0.2M HCl solution, acetone and again with 0.2M HCl solution. The washed sample was vacuum dried at 60 ᵒC for 24 hours.
The dried dark green powder was collected in a plain petri dish for the further study.
Figure. 2.1. Schematic diagram of the experimental method: (a) binary nanocomposites of PANI and graphene, (b) ternary nanocomposite of PANI, graphene and MoS2. Dispersion of MoS2 and graphene nanosheets followed by an in situ polymerization of PANI.
2.2.4. Materials characterization and measurements
The Micro Raman spectra of the nanocomposites were recorded using Horiba Jobin Vyon, Model LabRam HR with an excitation wavelength of 514 nm. The FTIR spectra were recorded using Shimadzu model no IR Affinity-1, with the dried KBr in the range of 400-4000 cm-1, after palletization of the solid powder samples. The UV-Visible absorption spectra were collected via Shimadzu, UV-2600 230V EN, in the wavelength range, 200 - 800 nm. The crystalline structures of the nanocomposites were evaluated using X-ray diffraction (XRD) analysis. The structural patterns were carried out in a rotating anode high power XRD (Rigaku, model TX-III), operated at 50 kV, 180 mA, with radiation of Cu kα, with an angle ranging from 10° to 70°. Following this, the crystallographic planes were analyzed via xpert high score plus software. The morphological features of the polymer nanocomposites were studied by a field emission scanning electron microscope (JEOL, JSM-7610F FESEM), at an accelerating voltage of 15 kV and the dimensions of the nanocomposites were analyzed with the help of ImageJ software. The transmission electron microscopy (TEM) performance was carried out on a JEOL, JEM 2100 at the acceleration voltage of 200 kV. The thermogravimetric analysis (TGA) of the nanocomposites was carried out by using TGA/DTG (Netzsch, Model STA449F3A00), at a heating rate of 10 °C min-1 under an inert atmosphere by purging argon gas at 20 mL min-1. The electrochemical measurements were carried out in three electrode and
two electrode setup using the Autolab electrochemical work station of Metrohm (PGSTAT204), with 1M H2SO4 at room temperature. The working electrodes (WE) were fabricated by mixing of active materials in the 80:15:5 ratios. Typically, 5.1 mg of the nanocomposite material, 0.9 mg of acetylene black (as a negative electrode material) and 0.3 mg of PVDF (as a binding agent), were put into a mortar, and a ‘paste’ was prepared by mixing with adequate amount of NMP. The paste was coated on a stainless steel woven mesh and pressed under 20 MPa pressure to ensure an evenly dispersed system to have adequate electrical properties. The detailed description of the electrochemical measurements has been given as follows:
Cyclic Voltammetry (CV) results were analyzed at various scan rates from 5 to 100 mV sec-1. The electrochemical impedance spectroscopy (EIS) measurements were analyzed by applying an AC voltage at 10 mV in the frequency range of 0.1Hz to 100 kHz. The galvanostatic charge/discharge (GCD) measurements were also recorded at various current densities. The value of the specific capacitance (Cs) was calculated from cyclic voltammetry and GCD curves as given by following equations [35]:
Cs from CV curves Cs= ∫i dv
m*ν*ΔV (2.1) Cs from GCD curves
Cs = I*∆t
m*∆V (2.2) According to the calculated value of Cs, energy density (E) and power density (P) have been analyzed according to the following equations[36].
E = 1
2 * 3.6*Cs*∆V2 (Wh kg-1) (2.3) P =E
t*3600 (W kg-1) (2.4) where, total integrated absolute value of CV curve is∫ idv, v is the scan rate (mV s-1), I is the discharge current (A), the discharge time is ∆t (s), ∆V is the potential window (volt, V), m is the mass of the active electrode materials (g), Cs is the specific capacitance (F g-1) and t is the discharging time (s).