**2.5 Electrode and device characterization**

*Science, Technology and Advanced Application of Supercapacitors*

**2.3 Chemical oxidation polymerization**

*Oxygen plasma functionalization set-up for CNT fibers.*

**Figure 2.**

**2.4 Electrode and device fabrication**

The oxidation of aniline in an acidic aqueous medium using ammonium peroxydisulfate (APS) as an oxidant is widely used and reported in the literature [45, 46]. Emeraldine salt (green color) is the form of PANI obtained during this process [32, 47]. PANI can exist in three oxidation states: leucoemeraldine (fully reduced), emeraldine (partially oxidized) and pernigraniline (fully oxidized) [32, 45–47]. At a pH of less than 2.5, the oxidative polymerization of aniline is a chain reaction [48]. The growth of the chains proceeds by the addition of the monomeric aniline molecules to the active chain ends. The chain growth is terminated after at least one of the reactants in the polymerization runs out. If there is an excess of the APS (oxidant), the resulting polymer remains in the pernigraniline form [49], especially at molar ratios of APS to aniline of over 1.5. If the rate of APS to aniline is equal to 1.25 [50] or aniline is in excess, pernigraniline is reduced to emeraldine at the end of the reaction while aniline is oxidized at the same time to emeraldine [48, 51]. We, therefore, ensured in all our tests that we had excess aniline to promote emeraldine growth, the most thermally and environmentally stable form of PANI [52–54]. The oxygen plasma functionalized CNT (OPFCNT) fibers were cut into 7.5 cm portions and affixed to copper tapes with fast drying silver paint (TedPella Inc.). The copper tapes served as the leads used to connect the devices for electrochemical testing. These electrodes were then placed into 10 ml beakers and put into an ice bath. Aniline monomer dissolved in 1 mol/L HCl and the ammonium persulphate (APS) solution also dissolved in 1 mol/L HCl were then put in the various beakers with fibers at different ratios of aniline to APS. The amount of PANI formed on the fibers was controlled by the ratio of aniline to APS used as well as the time the solution was allowed to polymerize. The fibers were taken out after the polymerization time and rinsed in a beaker with deionized water to wash off the excess PANI.

Fiber supercapacitors were created using poly (vinyl alcohol) and sulfuric acid

(PVA-H2SO4), as well as polyvinylidene fluoride-co-hexafluoropropylene and 1-ethyl-3-ethylimidazolium (PVDF-EMIMBF4) gel electrolytes. The PVA-H2SO4 was made with 10 ml DI water, 2 ml H2SO4 and 1 g PVA. The PVDF-EMIMBF4 gel electrolyte was prepared with 15 ml acetone, 1.5 g PVDF, and 3 ml EMIMBF4. The PVA-H2SO4 was operated at a 1 V window, while the PVDF-EMIMBF4 was operated at a 3.2 V window. The larger voltage window the PVDF-EMIMBF4 allowed enabled

**42**

Electrochemical measurements were carried out with an electrochemical workstation (Gamry, Interface 1000) at room temperature. The electrochemical characteristics of the electrodes and devices were evaluated by cyclic voltammetry at various scan rates, galvanostatic charge-discharge tests, and electrochemical impedance spectroscopy measurements from 106 to 10<sup>−</sup><sup>1</sup> Hz using sinusoidal voltage amplitude of 10 mV at the open circuit potential. In a three-electrode configuration test, Ag/AgCl was used as the reference electrode, platinum served as the counter electrode and the experiments were run in 1 M Na2SO4.

The capacitance (*C*) of the electrodes and fiber supercapacitors was calculated from the galvanostatic discharge curves at different current densities by using the equation: *C=IΔt/ΔV.* The gravimetric capacitance (*Cm*) and areal capacitance (*CA*) were calculated by the following formula: *Cm = C/m* and *CA = C/A,* respectively. The gravimetric energy density (*Em*) and power density (*Pm*) were calculated by the expressions: *Em = 1/2(Cm(ΔV)2)/3.6* and *Pm = 3600Em/t*.*Pm* <sup>=</sup> <sup>3600</sup>\_\_\_ *Em <sup>t</sup>* The areal energy density (*EA*) and power density (*PA*) were calculated by the expressions: *EA = 1/2(CA(ΔV)2)/3.6 EV* <sup>=</sup> \_1 2 <sup>∗</sup> *Cv* (*V*)2 \_\_\_\_\_\_\_ 3.6 and *PA = 3600EA***/t***, PV* <sup>=</sup> <sup>3600</sup>\_\_\_ *EV <sup>t</sup>* where *I* is the discharge current, *t* is the discharge time, *ΔV* is the operating voltage window, *m* and *A* refer to the mass and volume of the device, respectively [40, 55].

Scanning electron microscopy (SEM) (FEI XL30, 5 kV) and Raman spectroscopy (Renishaw inVia, 514 nm Ar-ion laser with a laser spot of ~1μm<sup>2</sup> ) were used to characterize the CNT-PANI. The masses of the fibers were taken on a Sartorius SE2 ultra-microbalance. X-ray photoelectron spectroscopy (XPS) data were obtained using a VG Thermo-Scientific MultiLab 3000 ultra-high vacuum surface analysis system, with ~10<sup>−</sup><sup>9</sup> Torr base pressure using an Al Kα source of 1486.6 eV excitation energy. The high-resolution scans for carbon and low-resolution survey scans were taken for each sample on at least two different locations.
