Evaluation of the Effects of Substrate Heat Treatment and Monomer/Acid Ratio Synthesis Parameters on the Performance of PANI/CFF Electrodes

The association of capacitive charging of the double-layer and a faradic redox reaction is desirable on carbon fiber felt (CFF) when oxygen functional groups or other materials such as conducting polymers are present on its surface enhancing its capacitive properties. In this work, a systematic study of the composites formed by carbon fiber felt felt produced at two different heat treatment temperatures (HTT) of 1000 and 2000 ºC and polyaniline (PANI) was performed upon two approaches: influence of heat treatment temperatures of substrate CFF and monomer/acid ratio. CFF samples were produced from polyacrylonitrile (PAN) precursor at HTT of 1000 and 2000 ºC, with step temperature of 330 ºC h-1 under nitrogen atmosphere. The maximum level temperature was kept for 30 min and then, cooled down to room temperature. The CFF samples were cut in 1 cm2 with thickness about 2 mm. Afterwards, the PANI was electrodeposited on CFF under galvanostatic mode with current density of 5 mA cm-2, using 1:1 and 2:1 H2SO4/aniline mol L-1 aqueous solution at deposition time of the 1350 seconds, for both proportions acid/monomer. The obtained PANI/CFF binary composite was washed in acid solution and vacuum dried. The morphological and structural analyses were investigated by Field emission gun-scanning (FEG-SEM), Surface Area Analysis (BET – Brunauer, Emmett and Teller), Thermogravimetric analysis (TG) and Fourier-Transform Infrared Spectroscopy (FT-IR). Electrochemical responses were analyzed by galvanostatic charge/discharge, electrochemical impedance spectroscopy, and cyclic voltammetry. The electrochemical analyses are recorded in a three electrodes system using Ag/AgCl electrode as reference electrode and a platinum sheet as counter-electrode in deaired solution with N2 using a potentiostat PGSTAT 302 with FRA module (Metrohm – AUTOLAB). The charge/discharge (CD) tests are obtained by applying constant current I of ±1.0, 0.75 and 0.5 mA (Ecut-off from -0.1 to 0.78 V vs. Ag/AgCl). Cyclic voltammograms (CV) are recorded in previously defined potential at sweep rate range from 10, 25, 50, 75 and 100 mV s−1. The electrochemical impedance spectroscopy (EIS) measurements are performed at open circuit potential (OCP) with ± 10 mV of the potential amplitude in the frequency range from10-3 to 105 Hz in 1.0 mol L-1 H2SO4. As can be seen, the FFC 2000 2:1 sample with 1350 seconds of synthesis presents higher specific capacitance as well as higher coulombic efficiency compared to FFC 2000 1:1 sample also with 1350 seconds of synthesis, with values at around 80 F/g and 90% respectively. On the other hand, the CFF 1000 1:1 sample with 1350 seconds of synthesis present higher values (33 F/g and 100%) than the sample CFF 1000 2:1 with 1350 seconds of synthesis (26 F/g and 100%). In addition, the increase in the intensity of the Faradaic peaks that were provided by the greater presence of the monomer synthesized on the surface of the substrate. The analogous way the excess of monomer may favor the formation of polymer on polymer which will intensify, also the redox peaks and will also favor the formation of reticles providing that form para- (cross) bonds in addition to ortho- (linear) bonds. Therefore, it is suggested that in comparison to the FFC 2000 samples we have a greater favor of para- in the polymeric chains, for the FFC 1000 samples it is suggested that it has formed a greater number of ortho-type bonds, as can be seen in the FT-IR data. By FEG-SEM it is possible to verify that in CFF 2000 2:1 1350 seconds there is a smaller formation of the polymer film with agglomerates by the entire structure of the felt comparing with the microscopy of the sample CFF 2000 1:1 1350 seconds. Also, a smaller surface area and/or a smaller amount of pores available as noted in the N2 adsorption isotherm but it is similarly observed that polymer chains in overlapping layers, which also suggests the formation of para- providing lower values of specific capacitance and coulombic efficiency. Note that the N2 adsorption isotherm suggests that the CFF 2000 1:1 sample has greater surface area and/or greater amount of pores available on its surface compared to fiber under the same conditions, which contribute to the greater accumulation of charge and coloumbic efficiency on storage devices.