(70b) Ultra-Thin Polythiophene within Nanostructured Electrode with Enhanced Charge Storage Capacity
Unsubstituated polythiophene (PTh) was synthesized using a unique oxidative chemical vapor deposition (oCVD) approach. The deposited polymer was found to be conductive with measured conductivity as high as 70 S/cm, which is an indication of a low degree of defects. The deposited PTh film showed strong electrochromic activity as evidenced by the color change from dark blue to bright red upon charge-discharge in organic electrolyte solutions. The charge storage capacity was measured in a three electrode setup using 1 M tetraethylammonium tetrafluoroborate in acetonitrile and values up to 70 F/g were obtained. Polymer films of thickness above 800 nm were found to be unstable and degraded upon charge-discharge as evidenced by the formation of micro-cracks and loss of capacity. However, thinner films with thickness down to 500 nm were stable upon charge-discharge. The observed charge trapping in the film at high scan rates ( >200 mV/s) disappeared when the film thickness was reduced further to sub-200 nm. The electrochemically active film was also successfully deposited within mesoporous electrodes. To investigate the influence of nanostructure on electrochemical charge storage capacity of the polymer, the parameters influencing oCVD were carefully adjusted to achieve conformal coating within the nanostructure of anodized aluminum oxide with straight pores of ~200 nm in diameter and mesoporous TiO2 consisting of random 10-25 nm pore channels within a sintered network of TiO2 nanoparticlesThe polymer deposited within the 3D TiO2 network and assembled on a conductive electrode was characterized in a three electrode setup for charge storage capacity. The charge storage capacity of the bare, uncoated titania electrode matrix was measured to be around 1 F/g, while the coated sample showed a capacitance as high as 250 F/g. Additionally, the specific capacitance of the polymer coating within the nanostructured electrodes was compared with that deposited on planar electrodes as blanket films. Interestingly, we have realized a further increase in the specific capacitance when the polymer was integrated as an ultrathin layer within the TiO2 electrode compared to the blanket film. The enhanced capacitance was found to be not related to any mass transfer limitations within the different polymer films. We attribute the enhanced charge storage capacity to a nanoconfinement effect of the polymer within the nanopores. The enhanced capacitance, which is concomitant with the increase in the peak height in the cyclic voltammogram, suggests that the charge stored per unit of monomer exceeds that observed on the planar electrode. This observation led us to fabricate electrodes by using conventional activated carbon as the matrix layer. For coatings on activated carbon, a polymer-to-activated carbon mass ratio of ~1.5 was found to deliver a maximum specific capacitance that is ~50% higher than that of bare activated carbon, 145 vs. 92 F/g. This capacitance translates to over a 250% increase in volumetric capacitance since the volume contribution of the ultrathin polymer coating is negligible (120 vs. 47 F/cm3).