**4. Electrochemical properties of the hybrid materials**

### **4.1. Electrochemical capacitors**

Electrochemical impedance spectroscopy (EIS) analysis was then carried out to investigate the electrochemical behavior of the materials synthesized. Hybrid coatings were tested first in carbon cloth acting as a substrate (blank). A typical Nyquist plot consisted of a semicircle in the high-frequency region and a linear part in the low-frequency area is formed [22]. Alternatively, the Bode plot presents the total impedance and phase angle as a function of frequency.

The diameter of the semicircle correlates with the interfacial charge-transfer resistance, usually representing the electrochemical reaction on the electrode (Faradaic resistance) [23]. All Synthesis and Characterization of Reduced Graphene Oxide/Polyaniline/Au Nanoparticles… http://dx.doi.org/10.5772/intechopen.77385 85

**Figure 8.** Impedance plots (a) Nyquist and (b) Bode for carbon cloth material support, and each synthesized material; GO, PANI, GO/PANI, GO/PANI/NpAu.

hybrid materials show very small diameter semicircle demonstrating the good electrical conductivity of the three-element composite, with a semi-straight line at mid to low frequencies, associated to capacitive behavior (mass transfer control) and lower charge-transfer resistance.

The Nyquist and Bode plots obtained are presented in **Figure 8a** and **b**, respectively, and for comparison purposes, the carbon cloth presents the highest overall or total impedance value above 1 kohm·cm<sup>2</sup> at the low frequency limit (0.01 Hz). The GO system shows a lower impedance value around 4500 kohms·cm<sup>2</sup> , both showing an inverse frequency behavior at intermediate frequencies corresponding to double layer capacitance. For the PANI system, the impedance values diminished in all the frequency bandwidth considered around 45 ohms·cm2 , reflecting the conductive properties of the polymer material.

**Figure 7** shows the Raman spectra of graphite, GO, and GO/PANI composite. Graphite exhibits two peaks: a *D* band at 1363 cm−1 corresponding to defects or edge areas and a *G* band at 1577 cm−1

shifts to a higher wavenumber (1589 cm−1) and widens as a result of a loss of interaction between the adjacent layers. The band of GO/PANI at 1358 cm−1 is more intense than that of GO due to the intercalation of oxygen-containing functional groups with covalent bonding in the GO layer [13, 14].

Electrochemical impedance spectroscopy (EIS) analysis was then carried out to investigate the electrochemical behavior of the materials synthesized. Hybrid coatings were tested first in carbon cloth acting as a substrate (blank). A typical Nyquist plot consisted of a semicircle in the high-frequency region and a linear part in the low-frequency area is formed [22]. Alternatively,

The diameter of the semicircle correlates with the interfacial charge-transfer resistance, usually representing the electrochemical reaction on the electrode (Faradaic resistance) [23]. All

the Bode plot presents the total impedance and phase angle as a function of frequency.

**4. Electrochemical properties of the hybrid materials**

**Figure 7.** Raman spectra of graphite, GO, and GO/PANI composite.


related to the vibration of *sp*<sup>2</sup>

84 Graphene Oxide - Applications and Opportunities

**4.1. Electrochemical capacitors**

The GO/PANI/AuNp hybrid exhibits the semicircle smaller than GO/PANI and PANI, suggesting better conductivity and lower charge transfer resistance. A straight sloping line in the lower frequency represents the diffusion resistance (Warburg impedance, W), which reflects the diffusion or mass transfer of redox species in the electrolyte, and a steeper line usually indicates faster ion diffusion.

Based on these electrochemical analyses, the enhanced capacitive behavior was due to the synergistic effect between graphene oxide and PANI, besides the high conductivity of the AuNp. In addition, the small nanometer size can exhibit enhanced electrode/electrolyte interface areas, providing high electro-active regions and short diffusion lengths. This is also true for the GO/PANI and GO/PANI/AuNp hybrid materials showing a further decrease of around 10 ohms·cm2 , presenting low impedance behavior all around [22].

Bode impedance diagrams (**Figure 8**) demonstrate that the synthesized materials PANI, GO/ PANI, and GO/PANI/AuNp present good conductivity properties with low impedance values. Also, the capacitive behavior was observed with the carbon cloth (blank) and the GO material. This analysis reveals that the good electrical conductivity and ion diffusion behavior resulted in the electrochemical performance of GO/PANI/AuNp of the three element hybrid material [23].

**Figure 9.** Cyclic voltammetry results of (a) carbon cloth different rates and (b) carbon cloth at 2 and 5 mV/s showing a trapezoidal form.

Another phenomenon that reinforces the theoretical explanation [24] is the reduction of the total impedance about two to three orders of magnitude with respect to the carbon cloth (blank) as can be seen in the Bode impedance diagram (**Figure 8b**), indicating the greatly increased ionic conductivity (high frequency) of the system.

The specific capacitance of the electrodes can be calculated according to the following equa-

**Figure 10.** Cyclic voltammetry results of (a) PANI showing redox peaks to transition of PANI, from leucoemeraldine/ emeraldine and emeraldine/pernigraniline form and (b) carbon cloth, PANI, GO/PANI, and GO/PANI/AuNp.

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

where I is the current, m is the mass of reactive material, and V is the potential scan rate.

The system presents potential to be used as capacitors with obtained values around 100– 160 F/g. Au nanoparticles improve the capacitance behavior in GO/PANI/AuNp hybrid mate-

Another interesting possibility for the hybrid material synthesized may have promising applications as conductive material such as electrodes for fuel cells, in combination with metallic substrates. In the following examples, metal substrate includes copper and stainless

For stainless steel substrate with the four synthesized hybrid component as coatings, immersed

two time constant behavior can be seen. The coatings response and dielectric properties are ascribed to higher frequencies (kHz), whereas at mid to lower frequencies (Hz–mHz), the response is associated with the oxide layer or bare metal substrate coating interface [28]. For the system tested, Nyquist plots present small diameter semicircle followed by a straight line with different slopes, corresponding to mass transfer diffusion process (**Figure 11a**). Bode plots (**Figure 11b**) present at the highest frequency: in the PANI coated stainless steel the highest impedance (70 ohms·cm2), followed by GO, GO/PANI, PANI and the lowest for GO/

solution, the Nyquist and Bode impedance plots are presented in **Figure 11**. The

mV (1)

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

87

tion (1) from CV curves:

rial for energy application.

**4.2. Fuel cell electrodes**

SO<sup>4</sup>

PANI/AuNp (10 ohms·cm2

).

steel [27].

in the H<sup>2</sup>

C = \_\_\_I

Cyclic voltammetry is a tool used to evaluate the faradaic and nonfaradaic processes as well as the capacitive properties and mechanisms of reaction at the electrode interface (**Figure 9**). The electrochemical response of the carbon cloth, of current as a function of potential for different sweep rates, is shown (**Figure 9a**), obtaining the best rate at 5 mV/s, where a more trapezoidal shape was obtained (**Figure 9b**).

**Figure 10a** presents the cyclic voltammetry of carbon cloth coated with polyaniline (PANI) showing oxidation peaks for PANI around 0.5 and 0.96 V, while reduction peaks are observed at −0.2 and 0.2 V. The separation between peaks is more than 0.2 V; therefore, the reaction is quasi-reversible and attributed to transition of PANI, from emeraldine to the pernigraniline form [25]. The incorporation of PANI as a cover demonstrates the increased capacitance from the shape obtained.

In **Figure 10b**, the cyclic voltammogram for the different components applied to the carbon cloth supporting material (blank) in a (H<sup>2</sup> SO<sup>4</sup> ) 1 M solution is presented. The graphs show when incorporating the PANI to the GO matrix the current increases about 63 mA/cm2 as the maximum peak in the anodic current (ia), while an increase in 100 mA/cm2 is observed as the maximum peak, for the whole hybrid system GO/PANI/AuNp. The subsequent incorporation of the different components to the hybrid system increases the total area of the trapezoid shape.

This involves two types of capacitive behavior contribution: from the electrochemical double layer capacitance (EDL) produced by GO and pseudo capacitive behavior from the incorporation of PANI. This suggestion is obtained from the two peaks observed in the voltammogram that indicated the existence of faradaic processes. This suggests a good synergism from the components of the matrix as well as good conductive properties [26].

Synthesis and Characterization of Reduced Graphene Oxide/Polyaniline/Au Nanoparticles… http://dx.doi.org/10.5772/intechopen.77385 87

**Figure 10.** Cyclic voltammetry results of (a) PANI showing redox peaks to transition of PANI, from leucoemeraldine/ emeraldine and emeraldine/pernigraniline form and (b) carbon cloth, PANI, GO/PANI, and GO/PANI/AuNp.

The specific capacitance of the electrodes can be calculated according to the following equation (1) from CV curves:

$$\mathbf{C} = \frac{1}{\mathbf{m}V} \tag{1}$$

where I is the current, m is the mass of reactive material, and V is the potential scan rate.

The system presents potential to be used as capacitors with obtained values around 100– 160 F/g. Au nanoparticles improve the capacitance behavior in GO/PANI/AuNp hybrid material for energy application.

#### **4.2. Fuel cell electrodes**

Another phenomenon that reinforces the theoretical explanation [24] is the reduction of the total impedance about two to three orders of magnitude with respect to the carbon cloth (blank) as can be seen in the Bode impedance diagram (**Figure 8b**), indicating the greatly

**Figure 9.** Cyclic voltammetry results of (a) carbon cloth different rates and (b) carbon cloth at 2 and 5 mV/s showing a

Cyclic voltammetry is a tool used to evaluate the faradaic and nonfaradaic processes as well as the capacitive properties and mechanisms of reaction at the electrode interface (**Figure 9**). The electrochemical response of the carbon cloth, of current as a function of potential for different sweep rates, is shown (**Figure 9a**), obtaining the best rate at 5 mV/s, where a more

**Figure 10a** presents the cyclic voltammetry of carbon cloth coated with polyaniline (PANI) showing oxidation peaks for PANI around 0.5 and 0.96 V, while reduction peaks are observed at −0.2 and 0.2 V. The separation between peaks is more than 0.2 V; therefore, the reaction is quasi-reversible and attributed to transition of PANI, from emeraldine to the pernigraniline form [25]. The incorporation of PANI as a cover demonstrates the increased capacitance from

In **Figure 10b**, the cyclic voltammogram for the different components applied to the carbon

maximum peak, for the whole hybrid system GO/PANI/AuNp. The subsequent incorporation of the different components to the hybrid system increases the total area of the trapezoid shape. This involves two types of capacitive behavior contribution: from the electrochemical double layer capacitance (EDL) produced by GO and pseudo capacitive behavior from the incorporation of PANI. This suggestion is obtained from the two peaks observed in the voltammogram that indicated the existence of faradaic processes. This suggests a good synergism from the

) 1 M solution is presented. The graphs show

as the

is observed as the

SO<sup>4</sup>

when incorporating the PANI to the GO matrix the current increases about 63 mA/cm2

maximum peak in the anodic current (ia), while an increase in 100 mA/cm2

components of the matrix as well as good conductive properties [26].

increased ionic conductivity (high frequency) of the system.

trapezoidal shape was obtained (**Figure 9b**).

cloth supporting material (blank) in a (H<sup>2</sup>

the shape obtained.

trapezoidal form.

86 Graphene Oxide - Applications and Opportunities

Another interesting possibility for the hybrid material synthesized may have promising applications as conductive material such as electrodes for fuel cells, in combination with metallic substrates. In the following examples, metal substrate includes copper and stainless steel [27].

For stainless steel substrate with the four synthesized hybrid component as coatings, immersed in the H<sup>2</sup> SO<sup>4</sup> solution, the Nyquist and Bode impedance plots are presented in **Figure 11**. The two time constant behavior can be seen. The coatings response and dielectric properties are ascribed to higher frequencies (kHz), whereas at mid to lower frequencies (Hz–mHz), the response is associated with the oxide layer or bare metal substrate coating interface [28]. For the system tested, Nyquist plots present small diameter semicircle followed by a straight line with different slopes, corresponding to mass transfer diffusion process (**Figure 11a**). Bode plots (**Figure 11b**) present at the highest frequency: in the PANI coated stainless steel the highest impedance (70 ohms·cm2), followed by GO, GO/PANI, PANI and the lowest for GO/ PANI/AuNp (10 ohms·cm2 ).

Both metal coating systems present good conductive properties for the hybrid synthesized coating system. Lower resistance, good ionic conductivity, and capacitive properties, which present GO/PANI/AuNp compound, make it attractive for different technological develop-

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

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89

It was possible to synthesize the hybrid material as evidenced by the characterization techniques. Electrochemical analysis demonstrates the capacitive redox behavior of PANI, but graphene, which is a material with large surface area (the property that increases the capacitance) when combined with the polyaniline conducting polymer, acts as a good material for electrical conduction rather than good capacitive behavior. The introduction of the AuNp in the network further increases the electric conduction. The trapezoidal shape of the voltammetric curves indicates maximum peaks with a capacitance of 160 F/g. This quantity indicates that the current is delivered very fast, which is not convenient for a capacitor. These results conclude that it is not a very good material for energy storage; however, it has promising applications as conductive material such as electrodes for fuel cells. Good electrochemical properties were obtained for metal elec-

trodes coated with the hybrid systems, suggesting good energy electrode applications.

thank CONACyT for the student grants received during this work.

The authors confirm that this content has no conflict of interest.

Autónoma del Estado de Morelos, Cuernavaca, Morelos, Mexico 2 Departamento de Química, UAM-Iztapalapa, Mexico, DF, Mexico

\*Address all correspondence to: juch25@uaem.mx

, Elsa Pereyra-Laguna<sup>1</sup>

, Miguel A. García-Sánchez<sup>2</sup>

1 Centro de Investigación en Ingeniería y Cciencias Aplicadas-(IICBA), Universidad

, César García-Pérez<sup>1</sup>

,

\*

and Jorge Uruchurtu-Chavarín<sup>1</sup>

The authors wish to thank SEP-PROMEP for the support provided to both the Academic Body "Desarrollo y Análisis de Materiales Avanzados" (UAEMOR-CA-43) and the Academic Network "Diseño Nanoscópico y Textural de Materiales Avanzados." Finally, the authors

ments and applications in the energy sector.

**5. Summary**

**Acknowledgements**

**Conflict of interest**

**Author details**

Carmina Menchaca-Campos<sup>1</sup>

Miriam Flores-Domínguez<sup>1</sup>

**Figure 11.** Impedance for stainless steel and different coating components immersed in H<sup>2</sup> SO<sup>4</sup> 1 M solution (a) Nyquist and (b) Bode plots.

**Figure 12.** Impedance for copper and different coating components immersed in H<sup>2</sup> SO<sup>4</sup> 1 M solution (a) Nyquist and (b) Bode plots.

In a similar way as before, the coating systems were applied, but this time in a copper metal substrate. A similar response was obtained (**Figure 12**), where the Nyquist plots show a semicircle with lower impedance resistance values as compared with stainless steel samples; followed by a straight line with different slopes associated to the coating protection and mass transport (**Figure 12a**) [29].

The high frequency impedance obtained was for the GO, followed by Cu, PANI, GO/ PANI, and the lowest impedance value (15 ohms·cm<sup>2</sup> ) for the GO/PANI/AuNp coating sample. The highest impedance modulus (low frequency) seen is for the bare copper sample (650 ohms·cm2 ) lowering its values for PANI, GO, and GO/PANI, and the smallest value registered is for the coating GO/PANI/AuNp system (**Figure 12b**). The mass transfer is reflected in the impedance modules obtained [30].

Both metal coating systems present good conductive properties for the hybrid synthesized coating system. Lower resistance, good ionic conductivity, and capacitive properties, which present GO/PANI/AuNp compound, make it attractive for different technological developments and applications in the energy sector.
