Jan 19, 2009
CNTs tuned to provide electrocatalyst support
Carbon nanotubes (CNTs) are widely studied as a support material for platinum (Pt) or Pt-based alloy electrocatalysts in fuel cells due to their high surface area, excellent electronic conductivity and high chemical stability. However, the deposition, distribution and size of Pt nanoparticles supported on CNTs strongly depend on the surface functionalization of CNTs. Furthermore, the electrocatalytic activity of Pt is also significantly affected by the nature of their interaction with CNTs and intrinsic properties of CNTs.
Since pristine CNTs are chemically inert, it is necessary to activate the graphitic surface of CNTs. Traditionally, CNTs are functionalized by harsh oxidative processes, such as refluxing in a concentrated mixture acid of HNO3 and H2SO4, thus generating functional groups on CNTs. However, this method generates a large number of defects and reduces the electrical conductivity and corrosion resistance of CNTs.
Researchers at Nanyang Technological University, Singapore, are developing milder methods of functionalizing the surface of CNTs for use as electrocatalyst support in fuel cells. In a recent study, which was published in Nanotechnology, the group demonstrated a novel and highly effective polyelectrolyte functionalization method for CNT using a positively charged polyelectrolyte, poly(diallyldimethylammonium chloride) (PDDA), which could homogeneously wrap/functionalize CNT surface in aqueous solution.
The strong adsorption of the positively charged PDDA on CNTs is due to the π-π interaction between PDDA and the basal plane of graphite of CNTs. This non-covalent polyelectrolyte functionalization not only leads to high density and homogeneous surface functional groups on CNT, but also preserves the intrinsic properties of CNTs without damaging their perfect surface structure. Consequently, the positively charged PDDA around CNT could electrostaticly interact with the negatively charged Pt precursors and nanoparticles, and the uniform distribution of surface functional groups leads to the well dispersed distribution of Pt nanoparticles on CNT, as shown in the figure.
The uniformly distributed Pt nanoparticles PDDA-functionalized CNTs show much higher electrochemically active surface area and the electrocatalytic activity for methanol oxidation than Pt electrocatalysts supported on the harsh acid-oxidized CNTs and the commercial E-TEK Pt/C electrocatalysts.
About the author
Dr Xin Wang is an assistant professor in the school of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore. He is currently working on the nanoelectrocatalyst development for fuel-cell applications.