Charge storage and transport mechanisms in printable supercapacitors
Wee, Grace Tsyh Ying
Date of Issue2010
School of Materials Science and Engineering
Energy Research Institute @ NTU
Supercapacitors (SCs) exhibiting 20 to 200 times greater capacitance and energy density (5 Wh kg-1, 80-100 F g-1) than conventional capacitors and delivering higher power density ( 10 kW kg-1) than batteries, represent an unique class of technology with niche applications amongst other energy storage devices. While conventional SCs suffer from its bulky architecture comprising of metallic current collectors and amorphous carbon, incorporation of the carbon nanotubes (CNTs) network as a single-layer current collector and active material has been explored, which leads to a light-weight and printable charge storage device that offers a “printed power solution” that can be potentially integrated with printed electronics applications. This thesis reports on the concept of introducing printable SCs by first exploring the use of CNTs as sole electrode materials that enables the replacement of metal current collectors. Due to the inter-tube resistance of the CNT network, the power density of this first generation of CNT-based SC is limited. To circumvent this issue, silver nanoparticles (AgNPs) with high conductivity were decorated onto the functionalized CNTs with the purpose of improving the power density of CNT electrodes. The enhancement observed with AgNP decoration is highly size-dependent and is related to the improved intertube contact resistance, electroactive surface considerations, as well as the participation of Ag in a faradaic reaction induced pseudocapacitance . Besides power density, the energy density of the CNT-based electrodes was further improved by incorporating transition metal oxide nanostructured materials with pseudocapacitive characteristics into the CNT/carbon-based materials. The role of these nanomaterials in charge transport, electrical double layer formation, pseudocapacitive properties, charge retention, and rapid charging and discharging has been investigated through various materials and electrochemical characterization techniques such as field-emission scanning electron microscopy, X-ray diffraction, cyclic voltammetry, galvanostatic charge-discharge, electrochemical impedance spectroscopy, and X-ray photoelectron spectroscopy. These studies provide strong support to the theory that morphology, surface area, electrode-electrolyte interfacial properties affected by process parameters, electrode and electrolyte treatment are the determining factors which impact charge storage and transport mechanisms in printable SCs. A systematic study on the impact of electrolyte on SC performance was carried out; including correlations with electrolyte pH, and performance comparison between aqueous and organic electrolytes. In order to realize a completely printable SC, a new class of printable electrolyte, known as polyelectrolyte was also evaluated. The effect of the ionic conductivity on the polyelectrolyte film towards improved device performance, and also limiting factors which degrade device performance were investigated through frequency dependent device characteristics at different levels of relative humidity (RH). The cycle stability of the printable SCs was examined by employing galvanostatic charge-discharge test, and the reasons for capacitance degradation after cycling were investigated. Lastly, printable SC was integrated with an energy harnessing (organic photovoltaic) device demonstrating and confirming its viability in applications ranging from printable electronics to innovative energy harnessing and storage applications.