Reduced graphene oxide has received extraordinary attention due to its many special properties, such as good conductivity, mechanical resistance, high accessible surface area (2630 m2 g-1), and good flexibility. These properties makes it the best candidate for flexible electrode materials. Nevertheless, according the literature, the facile agglomeration of graphene, due to the strong Π-Π interactions between their individual sheets, decreases the performance of graphene based materials . In this context, the carbon fibers have been shown to be an ideal support for reduced graphene oxide (rGO). The carbon fiber (CF) offers the advantage for using as electrodes because its 3D structure decreases the packaging of the graphene sheets. In addition, it is not necessary any substrates or binders to prepare the binary composite rGO/CF. Thus, the use of CF is a plausible strategy to maximize the energy density of supercapacitors due to its high surface area as well as its good thermal and chemical stabilities. On the other hand, Polyaniline (PAni) has been considered as a promising pseudocapacitive material due to its high theoretical specific capacitance, low cost, high conductivity, and easy synthesis. In this work were proposed a combination of both double layer capacitance (binary composite rGO/CF) and ternary composite of PAni/rGO/CF associating the PAni pseudocapacitance to enhance this electrode capacitive response. The rGO/CF and PAni/rGO/CF samples were evaluated electrochemically, aiming their performance as electrodes in supercapacitors.
CF samples were produced from polyacrylonitrile (PAN) precursor at heat treatment temperature of 1000 ºC using temperature steps of 330 ºC/h under inert atmosphere of nitrogen, reaching the maximum during 30 min up to its cooling down to room temperature. CF were cut in size of 1 cm2 and previously weighed. The rGO electrodeposition on CF was performed under potentiostatic mode, at a fixed potential of -1.25 V vs. Ag/AgCl/KCl(sat) for 15 min in a 25 mL of the graphene oxide (3mg mL-1) + 0.1 mol L-1 LiClO4 aqueous solution. In sequence, the PAni was electrodeposited on rGO/CF binary composite under galvanostatic mode with current density of 5 mA cm2 for 10 min in 0.5 mol L-1 H2SO4 aqueous with 0.1 mol L-1 aniline. The obtained ternary composite PAni/rGO/CF was washed in acid solution and vacuum dried. The morphological and structural analysis were obtained by Scanning Electronic Microscopy with field emission gun (SEM-FEG) and Raman spectroscopy, respectively. The electrode electrochemical characterizations were studied by cyclic voltammetry (CV) and charge/discharge tests. These measurements were performed in 1.0 mol L-1 H2SO4solution using a conventional three electrode cell.
Results and discussion
The SEM-FEG images showed aggregates with irregular and flakes-like shape and high particle density on CF substrate, which is a characteristic of the rGO. After electrochemical deposition, an uniform PAni coating was observed on binary rGO/CF composite, as shown in Figure 1 (b). Their Raman spectrum showed the characteristic C-Ṅ+ stretching vibration bands of the radical cations observed between 1319 cm-1 and 1340 cm-1 related to PAni emeraldine salt. The ternary composite CV curves presented a quasi rectangular shape with apparent redox peaks characteristic of emeraldine salt, indicating that the charge storage occurs by two process, the electric double layer and pseudocapacitive reactions. The charge-discharge profile of the PAni/rGO/CF electrodes, carried out at a current density of 1 mA cm-1, showed linear charge/discharge profile which indicates that capacitance is mainly due to electric double layer formation. The curves of the PAni/rGO/CF ternary composite showed a long time of the charge/discharge, improving the capacitance value, probably attributed to the electrode high surface area associated with the PAni pseudocapacitive behavior.
The PAni/rGO/CF composite electrodes were synthesized by electrochemical polymerization. The ternary composite presented a better performance in terms of specific capacitance and electrochemical reversibility than those of the conventional PAni films. The results showed that this ternary composite has a great potential as new promising electrode for applications in supercapacitors.
This work was supported by FAPESP 2016/13393-9, CAPES 086/2013 e CNPq.