Applications of polyamidoamine dendrimers in polymer electrolyte membrane fuel cells
Dendrimers are highly branched macromolecules with well-ordered three- dimensional architectures. Polyamidoamine (PAMAM), the most common class of dendrimers, have been widely studied due primarily to the following three features: 1) the interior amine and amide groups that can interact with ionic metal precursors through ligand exchange reactions; 2) the presence of an interior void space in the higher generation dendrimers; and 3) the exterior primary amine groups that permit further functionaliztion. These unique structural features have inspired many potential applications. This dissertation describes two applications of PAMAM dendrimers in polymer electrolyte membrane fuel cells (PEMFCs). First, in an effort to improve the utility of Pt in PEMFCs, PAMAM G4 was used as both a template and a stabilizer to synthesize dendrimer encapsulated Pt nanoparticles (Pt DENs) by photoreduction. These nanoparticles are highly monodisperse, exhibit high specific activity for the oxygen reduction reaction, and are inert to methanol oxidation, showing great potential for application in PEMFCs. Then, a simplified membrane electrode assembly (MEA) has been fabricated by the electrostatic self-assembly between Nafion® and Pt DENs and characterized. Two methods were proposed to increase Pt loading: layer-by-layer self-assembly and immobilization of Pt DENs and carbon powder on carbon fibers. Approximately 80 layers were proposed to reach the required loading using a dipping machine. Immobilization of Pt DENs and carbon powder simultaneously on carbon fibers can easily be achieved by electrochemical coupling, which is promising for replacing the conventional method of electrode fabrication. Secondly, in order to reduce the methanol crossover in direct methanol fuel cells (DMFCs), PAMAM G0 doped Nafion® membranes were prepared. Direct TEM imaging of the Naifon® embedded with nanoparticles demonstrates that PAMAM G0 can penetrate into the bulk of Nafion® through cluster channels to re-organize the distribution of sulfonate clusters by interacting with the sulfonic acid groups in different clusters. The presence of PAMAM G0 in the Nafion® membrane causes reduction of both methanol permeability and proton conductivity, but a very beneficial trade off can be reached when a doping concentration of 10⁻⁴ M PAMAM G0 is used. The fuel cell performance is much improved When Nafion® was treated with 10⁻⁴ M PAMAM G0.