Development of polyaniline nanostructures using bio-soft-template approaches and graphene-based composites for supercapacitor applications
MD Moniruzzaman SK
Date of Issue2015
School of Mechanical and Aerospace Engineering
The focus of the current work is to develop a supercapacitor that has energy density which is comparable with high energy batteries, but with the advantages of supercapacitors such as high power density and long rechargeable cycle life. The selected approach was to enhance both the electrical double-layer capacitance and pseudo-capacitance. To achieve these, electrodes based on a graphene/polyaniline composite system that has the appropriate morphology, microstructure and composition to provide the optimum properties were developed. The 2D graphene sheets have the tendency to aggregate or stack which reduces the available surface area and limits the ion transport during electrochemical process. This makes it difficult for the stacked graphene assembly to achieve optimum device performance. Thus, we report a strategy to solve these problems by transforming the aggregated graphene sheet into an open assembly structure by introducing a spacer (p-phenylenediamine (PPD)) between the layers by covalent functionalization. Compared to stacked graphene sheet, the modified graphene sheet (GPPD) is capable of delivering a much higher specific capacitance and energy density of 232.96 F/g and 32.38 Wh/kg (almost 2-fold), respectively at the current density of 500 mA/g in aqueous 1 M H2SO4 solution. The retention of capacitance of this electrode was found to be 92.92% after 1000 charge-discharge cycles. Polyaniline (PANI) nanostructures (viz. nanotubes and nanofibers) networks with different morphology were synthesized by chemical oxidative polymerization method using the bio-molecules like vitamin C and heparin. We report the discovery of an unprecedented behavior of vitamin C which forms a rod-like assembly through hydrogen-bonding in water, which produced PANI nanotubes upon the addition of aniline monomer. The tubular growth of PANI at the nanometer scale can be controlled by the variation of molar ratio of vitamin C to aniline. The polymerization rate became slower at higher molar ratio, whereas at lower molar ratio of 0.25 (i.e. [Vitamin C]/[Aniline] = 0.25), uniform nanotubes (PANIV-0.25) were observed. The outer diameter of the nanotube was in the range of 80 - 120 nm. Surprisingly, no polymerization was observed at an equal molar ratio of vitamin C to aniline. Besides this, with heparin template, uniform nanofibers were also synthesized when the weight ratio of heparin to aniline was 0.25 (PANIH-0.25). The uniform nanofibers obtained had average diameters of between 80 - 110 nm. No uniform PANI nanofibers were formed at other weight ratios. We have further studied the novelty of PANI nanotubes and nanofibers as nanostructured electrode materials for supercapacitor applications. The PANIV-0.25 nanotube based electrodes showed higher capacitance and energy density values of 619.76 F/g and 86.14 Wh/kg at 500 mA/g current density in aqueous 1 M H2SO4 solution, respectively, with a cyclic stability of 76.74% capacitance retention after 1000 cycles. On the other hand, the PANI nanofiber electrode (PANIH-0.25) yielded a higher specific capacitance and energy density values of 732.18 F/g and 101.77 Wh/kg, respectively, where the retention of capacitance was 72.28% after 1000 cycles at the same current density. The observed capacitances were also elucidated and justified based on theoretical considerations, which showed the good agreement between the observed and theoretical values. The graphene/PANI nanostructures (nanotubes and nanofibers) based composites were then fabricated and evaluated. The graphene nanosheet/PANI nanotube composites were fabricated by in situ chemical oxidative polymerization of aniline using vitamin C as a template. The G20PNT80 composite showed the best current-voltage response and the maximum specific capacitance was found to be 671.79 F/g at 500 mA/g current density in aqueous 1 M H2SO4 solution. The composite presented an excellent cycle life with 89.33% specific capacitance retention after 1000 cycles. The specific energy density was calculated to be 93.38 Wh/kg. Next, graphene/PANI nanofibers composites were synthesized using a novel in situ chemical oxidative polymerization of aniline using heparin as a template.The novel G25PNF75 composite showed a high specific capacitance of 690.68 F/g and an excellent energy density of 96 Wh/kg at a discharge current density of 500 mA/g. When the current density was reduced to 250 mA/g, the composite showed specific capacitance and energy density values of 890.79 F/g and 123.81 Wh/kg, respectively. This energy density is comparable to that of high energy batteries. Moreover, the composite exhibited excellent cycle life with 88.78% specific capacitance retained after 1000 cycles. The significantly improved specific capacitance is due to the synergistic effect in the composite. The excellent cyclic stability over the entire cycle life can be attributed to the good mechanical stability of the composite electrode. Thus, this composite with such a high specific capacitance is a very promising electrode material for supercapacitor applications.