Carbon nanotube supported Pt electrodes for methanol oxidation: A comparison between multi- and single-walled carbon nanotubes

This work presents a detailed comparison between multi-walled (MWNT) and single-walled carbon nanotubes (SWNT) in an effort to understand which can be the better candidate of a future supporting carbon material for electrocatalyst in direct methanol fuel cells (DMFC). Pt particles were deposited via electrodeposition on MWNT/Nafion and SWNT/Nafion electrodes to investigate effects of the carbon materials on the physical and electrochemical properties of Pt catalyst. The crystalloid structure, texture (surface area, pore size distribution, and macroscopic morphology), and surface functional groups for MWNT and SWNT were studied using XRD, BET, SEM and XPS techniques. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed to characterize the electrochemically accessible surface area and charge transfer resistances of the MWNT/Nafion and SWNT/Nafion electrodes. CO stripping voltammograms showed that the onset and peak potentials on Pt-SWNT/Nafion were significantly lower that those on the Pt-MWNT/Nafion catalyst, revealing a higher tolerance to CO poisoning of Pt in Pt-SWNT/Nafion. In methanol electrooxidation reaction, Pt-SWNT/Nafion catalyst was characterized by a significantly higher current density, lower onset potentials and lower charge transfer resistances using CV and EIS analysis. Therefore, SWNT presents many advantages over MWNT and would emerge as an interesting supporting carbon material for fuel cell electrocatalysts. The enhanced electrocatalytic properties were discussed based on the higher utilization and activation of Pt metal on SWNT/Nafion electrode. The remarkable benefits from SWNT were further explained by its higher electrochemically accessible area and easier charge transfer at the electrode/electrolyte interface due to SWNT's sound graphitic crystallinity, richness in oxygen-containing surface functional groups and highly mesoporous 3D structure.