UBC Theses and Dissertations
Influences of growth conditions and porosity on polypyrrole supercapacitor electrode performance Lam, Joanna Wing Yu
Supercapacitors are electrical energy storage devices that offer high power density and high energy density. The current energy density of supercapacitors is, however, not sufficient to meet the requirements of many applications. By using polypyrrole (PPy) as an alternate electrode material, the energy density of supercapacitors can be increased. Approaches for simplifying and speeding the production of PPy electrodes are investigated, as are means of increasing power density. The tradeoffs in performance of PPy are investigated when electrochemical deposition conditions – current density and temperature -- are modified to reduce costs. Although the surface morphology changes according to deposition conditions, PPy’s performance in capacitance and charging time is not greatly affected. The best electrode performance is obtained using electrodeposition conditions in which a current density of 0.125 mA/cm² is used and the temperature is held at -30°C. Higher temperatures and faster deposition rates can lead to more voluminous films which are lower in density, volumetric and specific capacitances. Further work is needed to investigate the impact of growth conditions on cycle and shelf life. To decrease the charging time of PPy the hypothesis is that additional porosity will help by creating channels of high ionic mobility. The porosity is achieved by polymerizing onto carbonized polyacrylonitrile nanofibres (NF). PPy-coated NF samples with a density of 1.2 g/cm³ exhibit similar volumetric (160 F/cm³) and specific capacitances (130 F/g) similar to that of pure PPy. The use of NF can increase the apparent ionic conductivities of PPy, allowing NF/PPy samples to charge just as quickly as pure polypyrrole electrodes that are four times less capacitive. However, based on the current model, the advantages of increasing porosity should be more dramatic, suggesting that other mechanisms such as uncompensated resistances and ion depletion may also influence charging time. As such, further work on NF/PPy is needed to determine and hopefully to mitigate the effects of such mechanisms.
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