**2. Methodology development**

The systems discussed were synthesized by the methodology, as shown in **Figure 1**, in which steps are described later and using the specified precursors and solvents.

#### **2.1. Materials**

**1. Introduction**

76 Graphene Oxide - Applications and Opportunities

place at the electrode/electrolyte interface [5].

in the number of cycle (charge-discharge) [4].

electropolymerization [10].

applications.

with suitable materials, architecture, and structure [2].

With climate change and environmental concern, new energy sources have been created and various advanced energy storage systems. They simultaneously possess high energy and high power density as well as excellent recyclability, low cost, and friendly to the environment [1]. There are energy storage devices such as lithium-ion batteries often possessing high energy (200 Wh/Kg) but relatively low power density (1 KW/Kg), while traditional electrostatic capacitor has high power (40 KW/Kg) but low energy density (0.03 Wh/Kg) [2]. However, the electrochemical capacitor is a new type of high power and high energy density storage and deliverance device, therefore promising for feeding a variety of equipment operating with energy [3].

The capacitors can be divided into two classes based on charge-storage mechanism: (a) electrical double layer capacitors (EDLCs), where in electrode/electrolyte system, direction arrangement of the electron, or ion at the electrode/electrolyte interface forms electrical double layer [4] and (b) pseudo-capacitors, where the pseudo-capacitance arises from Faradaic reactions taking

From the basic characteristics that determine the development of high-performance electrochemical capacitor (EC) electrodes, the most important are design, manufacture electrodes

The typical electrode materials for EDLC are carbon materials (carbon nanotubes, graphene, activated carbon, etc.) due to their high-specific surface area and excellent conductivity [6], and the conductive polymers (polypyrrole, polyaniline, and polythiophene) are often used for pseudo-capacitors due to their high conductivity and large storage capacity [7]. However, the specific capacitance of carbon materials is commonly far less than that of conductive polymers, and the storage capacity of conductive polymers gradually decreases with the increase

In order to alleviate the inherent drawback of single materials, researchers have combined carbon materials and conductive polymers to obtain hybrid or composite materials with both

There are two methods for the preparation of hybrid capacitors: chemical methods in which an oxidizing agent is used and in-situ polymerization occurs, e.g., in situ chemical polymerization of graphene with polyaniline [9], forming the hybrid composite graphene/polyaniline (**Figure 1**). Another method is the electrochemical synthesis of conductive polymeric nanocomposites in which nanomaterials are dispersed in a monomer solution and formed by

Being Au an excellent conductive material, it can be used to improve further the electrical properties of the GO/PANI hybrid material, incorporating it into the polymer matrix, as gold nanoparticles (NpAu), expecting the nanoscale dimension to potentiate the overall hybrid material properties. With all the above ideas in mind, the next section shows some results about the synthesis of graphene oxide/polyaniline/Au nanoparticles hybrid material suitable to be used for energy

high-specific capacitance and good cycle life called hybrid supercapacitors [8].

All reactives were analytic degree: natural graphite powder, sulfuric acid 98% (H<sup>2</sup> SO<sup>4</sup> ), hydrochloric acid (HCl), acetone, ethanol 98%, sodium nitrate (NaNO<sup>3</sup> ), and potassium permanganate (KMnO<sup>4</sup> ) from Meyer. Aniline monomer and gold (III) chloride trihydrate (HAuCl<sup>4</sup> ·H<sup>2</sup> O) and polytetrafluoroethylene (PTFE) were purchased from Sigma Aldrich. The hydrogen peroxide (30% H<sup>2</sup> O2 ) was purchased from Reasol, and ammonium persulfate (APS) was from Golden Bell.

#### **2.2. Synthesis of graphene oxide sheets**

Graphite oxide (GO) was synthesized by a modified Hummer's method [11]. Graphite (10 g), NaNO3 (5 g), and concentrated H<sup>2</sup> SO<sup>4</sup> (230 ml) were mixed and stirred at 0°C in a 2000 ml reaction flask, which was immersed in an ice bath. Then, KMnO<sup>4</sup> (30 g) was added gradually over the stirring mixture, the temperature was controlled below 35°C, and the whole mixture was stirred for 2 h. After 30-min rest, the temperature of the mixture was raised to 98°C, and 460 ml of de-ionized water was slowly added to the suspension during 40 min.

After 30 min, the mixture was diluted by 1.4 l of de-ionized water and treated with (25 ml) H2 O2 30% to reduce residual permanganate to soluble manganese ions until the gas evolution ceased. The resulting suspension was washed with HCl 1 M and de-ionized water until the filtrate became neutral and remaining impurities were removed. The product, graphite oxide, was exfoliated in an ultrasonic bath (2 h) to form graphene oxide (GO) sheets.

#### **2.3. Synthesis of polyaniline nanofibers and in situ polymerization GO/PANI**

Polyaniline was synthesized by chemical polymerization using ammonium persulfate (APS) as oxidant and HCl as doping agent. The aniline and APS mole ratio employed was 1:1, dissolved in 100 ml HCl 2 M separately, and put into an ice bath. The two solutions were mixed rapidly at 20°C temperature and put in ultrasonic bath for 3 h. The obtained green mixture was filtered and washed with ethanol and de-ionized water. The final product was put into a vacuum oven at 60°C for 4 h.

The polyaniline/graphene oxide (GO/PANI) composite was prepared by the same polymerization method with the presence of graphene oxide. In this case, the aniline was fixed at 40% wt. with 60% wt. graphene oxide in the acidic solution of HCl 2 M, according to the method reported and mentioned before [11].

## **2.4. Synthesis of Au nanoparticles (AuNp)**

In a flask with 10 ml of 1 mM HAuCl<sup>4</sup> brought to boil with vigorous stirring, rapid addition of 1 ml of 38.8 mM sodium citrate to the vortex of the solution was added and resulted in a color change from yellow to burgundy. The heating at the same temperature and stirring was continued for an additional 15 min, and the resulting colloidal particles solution was stored at 4°C.

model operating at 15 kV, at 1, 5, 10, and 15 kX magnifications. Raman spectroscopy was a WITec alpha 300 AR, laser source: 532 nm (green), power: 15.6 mW, optical objective: 100×,

Synthesis and Characterization of Reduced Graphene Oxide/Polyaniline/Au Nanoparticles…

http://dx.doi.org/10.5772/intechopen.77385

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As was mentioned in the methodology description, to obtain GO sheets, graphite was previously oxidized by a modified Hummer's method [11] consisting in the chemical oxidation of the structure through the use of concentrated sulfuric acid, potassium permanganate, and sodium nitrate. After oxidation, this was followed by ultrasonic bath, to break Van der Waals forces to separate the sheets and to obtain GO (**Figure 2a**). The incorporation of oxygen into the graphite crystalline network was corroborated, determining the carbon/oxygen ratio

FTIR spectroscopy was used to elucidate the covalent grafting and to confirm the change in functional groups during each step. **Figure 3** represents the FTIR spectra of GO, PANI, GO/PANI, and GO/PANI/AuNp. The GO shows absorption bands at 3200 and 1734 cm−1, which correspond to O─H, C═O in COOH [12]. It can be also observed that there are bands around 1605 and 1376, which are due to the intercalated water and deformation vibrations

For pure PANI prominent attributed absorption peaks are seen at 1630 and 1394 cm−1, belonging to C═C stretching deformation of quinoid and C═N stretching of secondary aromatic amine, revealing the presence of emeraldine salt state in PANI [14]. The bands at 1184 and 805 cm−1 correspond to ─C─N stretching vibration and out of plane bending vibrations of C─H in the benzene ring. Around 3281 cm−1, it was observed an absorption band for N─H

The FTIR spectrum of the GO/PANI composite was identical to that of PANI, which confirmed that the GO surface was wrapped by PANI [14]. There is no peak observed at 3200 and 1734 cm−1 (─OH and C═O vibrations, respectively), indicating the reduction of GO took

integration time 5 s, eight accumulations.

**Figure 2.** (a) GO sheets separation and (b) elemental analysis.

through SEM elemental X-ray analysis (**Figure 2b**).

of C─O in C─OH and C─O─C functional groups [13].

**3.2. Elemental analysis of the GO**

**3.3. FTIR spectroscopy**

stretching of the amine group.

## **2.5. Electrodes and electrochemical measurement**

To prepare the working electrode samples, GO, PANI or GO/PANI, and PTFE were mixed (90:10, w/w) and dispersed in ethanol. For the system GO/PANI, AuNp, and PTFE, they were mixed (72:20:8%), respectively, and were also dispersed by sonication in ethanol. Carbon cloth, stainless steel, and copper electrode (1 cm2 area) were coated with the mixture and dried at room temperature for 12 h.

The electrochemical experiments were performed in a three electrode cell arrangement. A graphite rod was used as a counter electrode, and the potentials were measured with respect to a Ag/AgCl standard electrode (saturated with KCl). The electrochemical impedance measurements were carried out by applying an AC voltage of 10 mV amplitude in the 10 kHz–0.01 Hz frequency range. Cyclic voltammetry measurements were carried out in 1 M H<sup>2</sup> SO<sup>4</sup> solution at different scan rates of 2–100 mV/s in a voltage range of −0.4 to 1.2 V. Electrochemical Gill AC Instruments analyzer ACM serial 1039 were used throughout the experiments.
