Abstract
Composite materials have been developed for use in supercapacitors utilising both electrochemical double-layer capacitance and pseudocapacitance in the same electrode. These combined a high surface area carbon with a conducting polymer (which produces a large specific capacitance), resulting in a device which produces both a high power and energy density with increased stability of the specific capacitance. A variety of carbon materials were chosen for this research including multiwalled carbon nanotubes (MWNT), vapour grown carbon fibres (VGCF) and Super P (Li), each with unique properties and morphologies. The carbons were combined with a polyoxometalate and conducting polymer to fabricate a polypyrrole (PPy) impregnated composite material or a caesium-anchored polyoxometalate fabricating a dispersed polyoxometalate carbon composite. The polymer was chosen for its conductive properties and cost compared to conductive metal oxides. The polyoxometalate acted as a dopant, increasing the conducting properties of the polymer matrix. The dispersed polyoxometalate composite material used caesium to anchor the polyoxometalate, making that insoluble. The electrodes were characterised using electron microscopy, inductively coupled plasma-mass spectroscopy and CHN analysis. The PPy composite created a coating over the carbon tubes/fibres/powder, while the dispersed polyoxometalate formed particulates which were seen either attached to the carbon or as a physical mixture. Elemental analysis showed that the carbons each interacted with the POM differently, in some cases barely interacting chemically at all. Electrochemical testing was used to study the performance of the electrodes when placed in test cells simulating a supercapacitor. Cyclic voltammetry, galvanostatic cycling and electrochemical impedance spectroscopy were used to study the electrodes. The dispersed electrodes continually showed the highest specific capacitances during testing (~ 78 F/g for both MWNTs and Super P Li at 10 A/g), with the composites containing MWNTs providing the best results (impregnated - 40 F/g and dispersed - 80 F/g at 10 A/g). The results produced using Super P (Li) were only slightly lower relative to MWNTs, indicating these materials are strong contenders for composite electrodes in supercapacitors at a fraction of the cost of those incorporating MWNTs.