Noncovalent Functionalization of Single Walled Carbon Nanotubes Using Alternate Layer- By-Layer Polyelectrolyte Adsorpti
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Noncovalent Functionalization of Single Walled Carbon Nanotubes Using Alternate LayerBy-Layer Polyelectrolyte Adsorption for Nanocomposite Fuel Cell Electrodes R. B. Dhullipudi, T. A. Dobbins, S.R. Adiddela, Z. Zheng, R. A. Gunasekaran, Y.M. Lvov Louisiana Tech University, Institute for Micromanufacturing, P.O. Box 10137 Ruston, LA 71272 R. Tittsworth Louisiana State University, CAMD, 6980 Jefferson Hwy. Baton Rouge, LA 70806, U.S.A.
ABSTRACT Electrodes which are resistant to chemical poisoning by CO, S, and other fuel impurities are needed to replace Pt in proton exchange membrane (PEM) fuel cells. We have designed composite electrodes comprised of single walled carbon nanotubes (CNTs) within a conducting polymer matrix. A method for solubilizing single-walled carbon nanotubes (CNTs) in aqueous media using polyelectrolyte layer-by-layer (LbL) nanoassembly of polystyrene sulfonate (PSS) and polyallylamine (PAH) at the CNT surface is elucidated. Once soluble, the CNTs were assembled onto planar substrates using alternate LbL nanoassembly to form nanocomposite films. These films will later be tested for their potential as alternative anodes in proton exchange membrane fuel cells. INTRODUCTION Single walled carbon nanotubes (CNTs) are a prime component in the nanotechnology revolution. Emerging applications of CNTs in field emission displays, supercomputing, and hydrogen and methanol oxidation have given these materials much recent attention. However, dispersion of CNTs for the purpose of exploiting their high surface area and ubiquitous properties remains difficult. Van der Waals attraction between CNTs causes them to form bundles—making them difficult to process into device structures. Research is underway to solubilize CNTs in aqueous media1,2. One approach taken has been to noncovalently attach hydrophilic oligomeric and polymeric groups to CNTs. Dispersion of CNTs by noncovalent attachment of polymeric groups at the CNT surface is predicated on the polymers ability to disrupt van der waals forces between the CNTs. For nonionic polymer groups, the mechanism for dispersion is the disruption of nanotube approach to distances which represent the attractive portion of the van der waals potential. For ionic polymer groups, the mechanism for dispersion is an increased surface charge—thus enhancing coulombic repulsion between CNTs. Experimental results have shown that polymers most effective at dispersing CNTs have conjugated π-backbones1. Polymers such as poly (phenylene vinylene) (PmPV), poly(ethylene glycol) (PEG), and poly (vinyl alcohol) (PVA) have been explored as dispersants for CNTs. However, each of these polymers are neutral in charge—thus, the contribution of coulombic repulsions to dispersion is minor. In this paper, we demonstrate a method for noncovalently attaching polyionic molecules onto CNTs using layer-by-layer (LbL) nanoassembly. LbL has been widely used to apply films
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of functional polymers, as well as films of biopolymers and small molecules onto colloidal particles3,4,5,6,7. We
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