Electrochemically Synthesized CDC for Supercapacitors and Performance Ionic Liquid Electrolytes

Carbide-Derived carbon (CDC) is a new type of porous carbon material with high purity, narrow distribution of pore sizes, and significant specific surface area. Due to these unique properties, CDC was found to be very useful for gas storage, flow sensors, and as electrode material for electrochemical energy storage devices such as supercapacitors. Currently, the most reliable method to synthesize CDC is to remove the metal or metalloid elements selectively from binary or ternary carbide precursors. This is achieved by chlorine gas treatment (chlorination) at a temperature of 200 ^oC or higher. At elevated temperatures, metal / metalloid elements can be oxidized to be volatile metal chlorides and purged off by Ar gas steam. Thus, the left-over carbon is metal free and sp² or sp³ hybridized. So far, this method of etching has been very dominant but one downside is that the Cl₂ gas is very toxic and the process itself is relatively expensive. On the other hand, alternative oxidation methods to prepare CDC at milder conditions have been rarely explored. In this work, conversions using electrochemical methods are reported that oxidize vanadium carbides (VC and V₂C) in aqueous solutions at room temperature to prepare CDC thin film as electrode materials of supercapacitors. It was found that VC and V₂C can both be oxidized at a potential of about 0.5 V vs. Ag/AgCl or higher in neutral, acidic, or basic solutions. X-ray diffraction results indicate that about 33% of the vanadium on the surface layer of the V₂C film was removed and the vanadium is readily observed in the electrolyte solutions by ICP-MS. The so-produced CDC thin film electrode (ca. 2.0 - 2.6 μ thick) has a porous morphology and bears specific double layer capacitance values as high as 0.026 F/cm² (or 130 F/cm³) with some dependence on the oxidation potential, time, and electrolyte solutions. The so-produced CDC thin films showed high double layer capacitance as well when ionic liquid electrolytes are used. The performance depends on the structures of the cations and the anions of ionic liquids.