| Carbon nanomaterials have been investigated as electrode materials for high rate electrochemical capacitors (ECs). Multi-walled carbon nanotube (MWCNT) was found to be a good candidate and was further modified with additional pseudocapacitive polyoxometalates (POMs). A "layer-by-layer" (LBL) deposition technique was used to add POM active materials to CNT. MWCNTs were successfully coated with two different POMs, (SiMo12O40 -4 (SiMo12) and PMo12O40-3 (PMo12)). Even with merely a "single-layer" of POM, the modified MWCNTs exhibited more than 2X increase in capacitance compared with that of bare nanotubes. To further improve their electrochemical performances, the deposition sequence of the POM layers was adjusted to form "alternate layer" coating to modify MWCNT. Two coating sequences were developed with: Combination 1 (bottom layer PMo12 + top layer SiMo12) and Combination 2 (bottom layer SiMo12 + top layer PMo12). A synergistic effect on the capacitance and kinetics was observed for both combinations. X-ray Photoelectron Spectroscopy (XPS) and Scanning Electron Microscopy (SEM) also proved the successful coating of POMs on MWCNTs. The potential-pH relationship provided important insights in terms of the deposition mechanism and suggested that the bottom layer close to the electrode substrate was the dominating layer in alternate layer coated MWCNT electrodes. |
