We have explored the pseudocapacitance of Prussian blue (PB) modified carbon nanotube electrodes.The carbon nanotubes (CNT) are deposited by chemical vapor deposition (CVD) onto a nickel catalyst that had been electroplated onto a metal base electrode. The nanotubes were then characterized by Raman spectroscopy and cyclic voltammetry. Prussian blue was electrochemically deposited onto the CNT electrode from a solution of K₃Fe(CN)₆ by repeated cycling between -0.1 V and 1.0 V verses a Ag/AgCl reference electrode. The ferricyanide redox couple occurs at about 0.5 V while the deposition of PB was seen as a growing redox couple at about 1.0 V see Figure 1.

Optical imaging of the CNT electrode showed a distinct blue-green tint that together with the cyanide stretch in the Raman spectrum gave evidence that PB had been deposited, see Figure 2. This suggests that this electrochemical technique utilizing CV is a viable method to functionalize CNT electrodes. Electrodes thus modified show increased pseudocapacitance compared to the unmodified CNT electrodes. The exact structure and location of the PB bonding is unknown and was the focus of this study. Scanning electron microscopy was utilized to image the electrode surface after each step of the process in order tostudy PB bonding onto the CNT electrode.

Figure 1: A CNT electrode undergoing functionalization in ferricyanide solution. The peaks growing in are thought to be Prussian blue being deposited onto the carbon nanotubes.

Figure 2: Raman spectra of functionalized and unfunctionalized carbon nanotube electrodes.  The G and D bands due to the nanotubes are seen in both spectra at the expected 1587 and 1340 cm^-1, respectively.  For the functionalized electrode, additional peaks in the 2100 cm^-1 range due to cyanide stretching are observed.