Enhancement of the electrochemical capacitors of porous carbon from cross-linked polymers through heteroatoms and cobalt incorporation

Abstract

This Ph.D. work is based on the synthesis and characterization of activated carbon from cross-linked polymers and the enhancement of its electrochemical performance through heteroatoms and cobalt incorporation. These materials were investigated as electrodes for supercapacitor applications and were synthesized through hydrothermal method followed by one-step chemical activation process using a tubular furnace. Different techniques were performed to characterize the synthesized porous carbons including X-ray diffraction (XRD), Raman spectroscopy, N2 adsorption-desorption, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The electrochemical performances were explored in three and two-electrodes setup in 2.5 M KNO3 aqueous electrolyte, using cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS). Many strategies have been used to attain the aim of this study which is to enhance the electrochemical performance of the activated carbon from cross-linked polyvinyl alcohol/polyvinyl pyrrolidone (AC-PVA/PVP). The first strategy focused on the incorporation of reduced graphene oxide (rGO) which was synthesized by modified Hummer’s method, into the cross-linked PVA/PVP to synthesize a composite material. The goal of this approach was to enhance the conductivity, the specific surface area and the stability of the PVA/PVP structure. In fact, the incorporation of rGO into the activated carbon has the capacity to demonstrate a good charge storage ability. The second strategy involved the improvement of activated carbon from the cross-linked polymers by co-doping with nitrogen (N) and phosphorous (P). The N, P co-doped activated carbon from PVA/PVP was obtained by one-step pyrolysis using one precursor dopant diammonium hydrogen phosphate (AP) and denoted as AC-PVA/PVP/AP. A high specific surface area of 2656 m2 g-1 was achieved, corresponding to 1.08 cm3 g-1 total pore volume. The incorporation of heteroatoms (N and P) provided pseudocapacitance and more active sites to the carbon matrix leading to the improvement of the electrode material performance. The last approach was the doping of the activated carbon AC/PVA/PVP by the transition metal cobalt which is a pseudocapacitive material. The cobalt was incorporated in the cross-linked PVA/PVP by hydrothermal process using cobalt nitrate as a source of cobalt. Afterwards, one-step activation was utilized to synthesize the doped activated carbon labelled AC-PVA/PVP/Co. The optimized doped sample have led to enhancement on the performance with a specific capacitance of 280 F g-1 higher than the pristine AC/PVA/PVP (170 F g-1). The increase in the electrochemical performance is related to the fast-Faradic redox reaction, great conductivity, change in surface chemistry and the stability brought by the cobalt -doping. Three symmetric supercapacitors devices using the optimized electrode materials: AC-PVA/PVP/rGO//AC-PVA/PVP/rGO; AC-PVA/PVP/AP-0.5//AC-PVA/PVP/AP-0.5 and AC-PVA/PVP/Co-1//AC-PVA/PVP/Co-1 were fabricated within cell potential of 1.6 V in 2.5 M KNO3. The constructed supercapacitors displayed a specific energy and power of (19.5 Wh kg-1; 400 W kg-1), (27.2 Wh kg-1; 400 W kg-1) and (32.0 Wh kg-1; 401 W kg-1) for AC-PVA/PVP/rGO//AC-PVA/PVP/rGO, AC-PVA/PVP/AP-0.5//AC-PVA/PVP/AP-0.5 and AC-PVA/PVP/Co-1//AC-PVA/PVP/Co-1, at 0.5 A g-1. The cobalt-based device demonstrates a superior electrochemical performance owing to the enhanced redox reaction. Thus, this research demonstrated a simple way to synthesize mono and co-doped activated carbon from cross-linked polymers as novel electrode materials for high-performing supercapacitors.