Oxidative Chemical Vapor Deposition (oCVD) Synthesis of Ultrathin Polythiophene Films for Enhanced Charge Storage in Nanostructured Devices
Utilizing conducting polymers as the active material for supercapacitors is a novel approach which has recently shown much promise1-3. Conducting polymers have been shown to have high specific capacity, high energy density, and promising stability. Furthermore, they are less costly than oxide-based materials and have the ability to be p and n- doped. Moreover, conducting polymers enhance charge capacity through Faradaic redox reactions. As stated by Simon and Gogotsi, the integration of ultrathin pseudocapacitive materials within porous carbon nanostructures could result in significant improvement in energy density and power density that can deliver both battery-like (high capacity) and capacitor-like (high rate) behavior in a single electrode4.
Conducting polymers are sometimes deposited via liquid techniques. However the high porosity and tortuosity of nanostructured devices pose many challenges for integrating ultrathin coatings. Utilizing oCVD allows one to bypass the challenges associated with liquid methods and effectively integrate ultrathin polymer conformally inside nanostructured devices with high porosity and tortuosity. oCVD is a solvent free method where the reagents for oxidative polymerization, the monomer and oxidizer, are heated to a vapor that can easily penetrate into the mesoscale pore and polymerize. One advantage of oCVD is the ability to integrate intractable and insoluble conducting polymers such as unsubstituted polythiophene, which is a challenging conjugated polymer to work with as chains longer than eight repeat units are practically insoluble and processing of this polymer is very difficult. By flowing vapors of thiophene and antimony pentachloride (a strong oxidizer), a solid thin film polymer can be polymerized on a substrate. This step-growth mechanism is believed to follow an oxidative mechanism that allows the dehydro-coupling of heterocyclic rings. Simultaneously, the deposited polymer is doped to its conductive form with counter-ions like chloride that are contributed by the oxidant. By understanding the oCVD polythiophene synthesis parameters (reagent flow rates, reactor pressure, substrate temperature, saturation pressure), we were able to control the polymer conjugation length and the film electrical conductivity.
We attribute the charge capacity enhancement to come from both additional faradaic contributions and double layer capacitance of the polythiophene rather than solely from an electrical double layer – the bare TiO2 nanostructure had negligible measurable capacitance while the coated TiO2 nanostructure had significant charge capacity (Figure 1). For coatings on activated carbon, at a polymer-to-activated carbon mass ratio of ~1.5, specific capacitance reaches a maximum value ~50% higher than that for 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).