Ordered Mesoporous Carbon Composite Films Containing Cobalt Oxide and Vanadia for Electrochemical Applications

Mesoporous carbon composite thin films (<500 nm) containing cobalt and vanadium oxide were synthesized by triconstituent self-assembly using Pluronic F127 as template, phenol-formaldehyde oligomer (resol) as carbonizable precursor, and cobalt (or vanadyl) acetylacetonate (acac) for the metal source. The ordered mesostructure of the composite films was confirmed by both X-ray diffraction (XRD) and transmission electron microscopy (TEM). During pyrolysis at 800 degrees C to carbonize the film, the d-spacing decreases significantly because of uniaxial contraction; however addition of Co/V content leads to a decrease in the contraction from approximately 68% to 50%, which indicates the Co/V mechanically strengthens the framework. The decrease in contraction also leads to an increase in the average pore size by as much as 60%. Nanoparticles are found to be dispersed within the continuous carbon framework from both high resolution (HR)-TEM and scanning transmission electron microscopy (STEM); small sub-2 nm particles are observed in all cases for V containing films, while particles greater than 10 nm are found at high Co contents. In addition to these structural changes, the electrical conductivity of the mesoporous carbon film can be increased from 22 S/cm to approximately 40 S/cm by adding 10 wt % of either Co(acac)3 or VO(acac)(2) in the precursor solution. The conductivity decreases as the organometallic content is further increased, but still remains quite conductive (19.6 S/cm for 50% VO(acac)(2)). The addition of the transition metals also greatly enhances the electrochemical performance because of their pseudocapacitance. Even after 500 cycles, the composite films maintain a specific capacitance as high as 113 F.g(-1) (for Co) and 159 F.g(-1) (for V) in comparison to the neat carbon, which is initially approximately 22 F.g(-1). These materials exhibit favorable electrochemical properties for potential energy storage applications such as insertion batteries and supercapacitors.