Electrocatalytic Activity and Interconnectivity of Pt Nanoparticles on Multiwalled Carbon Nanotubes for Fuel Cells

Platinum nanoparticles (NPs) have been successfully assembled on noncovalently poly(diallyidimethylammonium chloride)-functionalized multiwalled carbon nanotubes (MWCNTs) via microwave-assisted polyol reduction and seed-mediated growth methods. Pt NPs were uniformly deposited on MWCNT support with Pt loading of 10-93 wt %. The results show that the increase in the Pt NP loading on MWCNTs leads to a positive shift of the peak potential for the reduction of Pt-OHad, a negative shift of the peak potential for oxidation of adsorbed CO, an increase in the peak current for the methanol oxidation reaction, and a positive shift for the half-wave potential of the oxygen reduction reaction in acid solutions, even though the size of the Pt NPs also increased with the Pt loading. The increase of the electrocatalytic activity of Pt/MWCNTs shows a characteristic S-shaped profile as a function of the Pt NP loading. To explain the S-shaped dependence of the activity of Pt/MWCNTs on Pt loading, a new concept of interconnectivity of Pt NPs was introduced. Interconnectivity is defined as the ratio of the total number of interconnections between particles divided by the total number of particles involved. The results indicate a close correlation between the electrocatalytic activities of Pt/MWCNTs catalysts and the interconnectivity of Pt NPs on MWCNTs. The electrocatalytic activity of the Pt/MWCNTs was found to first increase linearly and then level off with the interconnectivity of Pt NPs on MWCNTs. The optimum interconnectivity of Pt NPs on MWCNTs is similar to 3, which corresponds to a Pt loading of 50 wt % in the Pt/MWCNTs. The reason for the enhanced catalysis with increased interconnectivity of Pt NPs is considered to be associated with a significant increase in the number of grain boundaries, which are considered to contain the active sites for the fuel cell reactions.