(589c) Layered Carbon Nanostructures for Fuel Cell Electrocatalyst Applications
Carbon nanoparticles (platelets, spheres, chains, and tubes) have recently received much attention for their potential use as high surface area supports for fuel cell electrocatalysts, battery electrodes, as well as conductive filler for electrostatic discharge and electromagnetic shield applications. This presentation demonstrates how non-covalent interactions between the various types of carbon nanoparticles (size, shape, and extent of graphitization) may be tuned and combined with layer-by-layer (LBL) assembly to control supra-molecular structure of carbon nanoparticle-based coatings. The strong polyelectrolyte acrylamide/β-methacryl-oxyethyl-trimethyl-ammonium copolymer was used as the cationic binder. Gold was used as a model substrate. Zeta potential, ζ, measurements as a function of pH and alcohol type and content were employed to assess the relative role of electrostatic and non-electrostatic interactions. Parameter ζ for the carbon nanoparticles is found to vary in the range of -60 mV to +10 mV, in a manner inconsistent with the Debye-Huckel approximation. Such behavior, combined with substantial shifts in the iso-electric point (IEP), suggests specific adsorption of alcohol on the carbon surfaces. X-ray analysis (EDS) is combined with electron flight Monte-Carlo simulations to obtain adsorbed layer thicknesses while field-emission scanning electron microscopy (FE-SEM) is employed to examine surface morphology. Whereas ζ is found to have a substantial influence on the kinetics of the deposition process, equilibrated thicknesses are more strongly dependent on the type of nanoparticle employed. In every case, nanoparticles were found to self-assemble onto gold substrates via two distinct yet overlapping modes. The first mode is characterized by logarithmic weight uptake with respect to the number of deposition cycles and clustering of graphite nanoparticles on the gold surface. The second mode results from adsorption of nanoparticles onto previously deposited carbon and is characterized by true LBL assembly (i.e., linear weight uptake with respect to the number of deposition cycles). Complete surface coverage proceeds via the growth and coalescence of individual clusters or islands of carbon nanoparticles. Equilibrated bi-layer thicknesses range from uniform coatings with an average thickness comparable to the particle diameter (19 nm in the case of amorphous carbon nanospheres and 21 nm in the case of graphite platelets) to highly non-uniform layers (63 nm for amorphous porous spheres impregnated with Pt catalyst). Contact resistance measurements for all coatings were obtained at 200 psi contact pressure using a custom-built press with gold coated platens connected to a galvanostat and employing Toray gas diffusion media (GDM) as the compliant element. Contact resistances as low as 5 mΩ-cm2 were obtained for LBL-assembled coatings. Finally, fuel cell test data is presented to demonstrate the feasibility of the LBL approach in electrocatalysis applications.