Influence of Heat Treatment Temperature of Carbon Fiber Felt Substrate on Polyaniline Electrosynthesis and Its Properties
Authors: Poli Anne Karoline dos Santos , Gama Adriana Medeiros , Baldan Maurício Ribeiro , Gonçalves Emerson Sarmento 
Published: February 2020

The composites formed by conductive polymers and carbon fiber felts present unique and promising properties to applications in supercapacitors and batteries. The polyaniline was obtained through electrosynthesis using three voltammetric cycles in potential from -0.5V up to +1.05V, sweep rate 25mVs-1, in aniline solution 0.1molL-1 and H2SO4 0.5 molL-1, on the felts annealed at four temperatures: 1400K, 1600K, 2000K and 2300K. The morphological, structural and electrochemical characterizations of felts and composites were performed by Scanning Electron Microscopy, X-Ray Diffraction, Raman Spectroscopy and Electrochemical Impedance Spectroscopy. The grooves on bare felts are related to the acidic functions from their precursor material, polyacrylonitrile, which are removed in the form of organic volatiles of according with the increase in the heat treatment temperature applied to the felts (Figure 1). This structural evolution can be observed with x-ray diffraction (Figure 2), which shows the evolution of an amorphous felt in 1400K to a crystalline structural organization in 2300K, and by Raman Spectroscopy (Figure 3)with the decrease of the I
D1
/I
G and I
D3
/I
G ratios as a function of the thermal treatment temperature increase and with the increase in the average crystallite diameter L
a (Equation 3-Table 2). In addition to the average diameter of the crystallite, it was possible to evaluate the lamellar stacking height (L
002) and the interlamellar distance (d
002) (Equation 1,2-Table 1), and to determine that the decrease in the incidence of functional groups is inversely proportional to the organization of the crystallographic structure, providing the formation of four different substracts for the growth of polyaniline. In relation to the electrochemical behavior, the felt treated in 1400K presents low electrical resistance and the charge overcome this barrier migrating easily to the double layer (Table 3).The presence of heteroatoms increases the capacitance by attracting a large amount of charge to the double layer. The amount of charge attracted to felts annealed at 1600K is lower to 1400K, because the loss of functional groups leads to a decrease in wettability and provides increased resistance of the double layer. Such behavior extends up to 2000K. The difficulty of the double layer formation continues to exist in felts of 2300K, however, due to the great penetration of the charge, electric internal saturation occurs favoring the formation of the outer double layer. The morphological evolution and the amount of polyaniline formed as a function of the heat treatment temperature applied to the felts are verified by obtaining the composite polyaniline@carbon fiber felts (Figure 5). The formation of polyaniline on the felt treated at 1400K occurs superficially covering it partially. This phenomenon repeats to 1600K but with a greater intensity. 1600K and 2000K show a more uniform growth, being possible to observe to 1600K a second mechanism of growth, of polyaniline on polyaniline, causing the clogging of the pores. This mechanism seems more evident in fibers treated at higher temperatures, especially at 2300K. This is verified through the x-ray diffraction (Figure 6). The felts treated in 1400K have many peaks in several regions, which may be associated with the growth of polyaniline on an excessively oxygenated and nitrogenated fiber by functional groups. This fact can be evidenced by the higher I
D3
/I
G ratio. In 1600K a monocrystalline structure is observed, where the growth of polyaniline is oriented by the felt and predominant on plane 2θ=9,2°, being the orientation more incident for the growth of polyaniline in all the felts. For 2000K and 2300K, the polyaniline orientation appears in more planes than the felt treated in low temperature. In this temperatures the heteroatom are almost completely eliminated and can be confirmed by decrease of the I
D3
/I
G ratio. By Raman Spectroscopy it was possible to determine the contributions of quinoid and benzenoid species and of polaron and bipolaron defects (Equation 4-5). Through the relations between them it was possible to determine the degree of oxidation of polyaniline, close to ½, corresponding to emeraldine and allowing the formation of mobile charge. The amount of these charges in the polyaniline duplicates with the heat treatment from 1400K to 2300K (Table 4). This data can be confirmed by electrochemical impedance spectroscopy (Table 5) where the internal and external double-layer resistances decrease in the temperature range cited. In relation to the treated felts in 1600K, there is an increase of capacitance through nonlinear least squares adjustments (NLLS) used on Nyquist Plot (Figure 9) which can also be confirmed by the smallest ratio of bipolarons per polaron by Raman spectroscopy.

Figure 1