Hierarchical nanostructure enabled high performance supercapacitor for energy storage
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As one of the next generation energy-storage devices, supercapacitors play an important role in future vehicles and microelectronic devices because of their stable cycling life, fast charging–discharging rate, and intrinsically ultra-high power density. The performance of the device depends intimately on the properties and structures of their materials. Conjugated polymers have been shown as a promising electrode material because of the reversible oxidation-reduction activity which enables a high energy density. Also, the structural design provides a promising approach to enhance energy density without sacrificing power density because of the regular nanostructures. In this thesis, three novel, facile, and scalable approaches are presented to control the geometries and structures of the graphene inter-linked aligned PANI nanofibers to achieve optimal supercapacitive performance. By carefully selecting the synthesis approaches and parameters, PANI nanofibers with uniform alignment and narrow size distribution were realized. In the first work, PANI nanowire-pillared graphene was fabricated in the bulk solution by a template-free method with melamine initialized nucleation. PANI nanowire morphology was readily controlled by tailoring the amount of melamine. The PANI nanowires prepared at optimized conditions were vertically aligned on graphene sheets, and showed uniform diameter, length, and inter-nanowire spacing. The nanowire diameter was as small as ∼20 nm. The resultant specific capacitance of PANI nanowire–pillared graphene electrode reached a maximum of 625 F/g. In the second work, vertically aligned 3D network electrodes consisting of PANI array/graphene-film/ PANI array were assembled via a multistep nanowire growth. The acidic doping in the synthesis process played a critical role in the morphologies, structures, and electrochemical performance of 3D network. The 2M HClO4 doped 3D electrode showed optimal geometry and performance. The electrochemical impedance results indicated that the charge transfer resistance was ignorable, and the ion diffusion resistance was 0.22 Ω due to well-defined nanostructures. In order to further decrease the charge transfer resistance, an all electrochemical deposited GO-linked multiple PANI forests 3D electrode was prepared in the third work. When compared to the graphene paper used in the second work, the electrochemical deposited monolayer GO sheets were as thin as 0.88 nm. These sheets served as the transition nodes for the neighboring nanowire arrays after in-situ reduction during the further growth of PANI. The PANI nanofibers were highly oriented with diameters of around 20–30 nm. This carefully defined and sophisticate structure produced an optimal electrochemical performance because of the optimized ion diffusion and charge transferring. The energy density of as-produced supercapacitors was as high as 137 Wh/Kg while the power density was 1980 W/Kg in aqueous electrolyte. In summary, this dissertation points out the simple pathways to tailor electrode architecture for supercapacitors with both high energy density and high power density. Also, a theoretical model has been established to quantify the influences of various factors on the supercapacitor performance in both aqueous and organic electrolytes. Thus, it provides both experimental and theoretical approaches for constructing high energy density and high power density supercapacitors.