Biomimetic Synthesis of Noble Metal Nanoparticles and Their Applications as Electro-catalysts in Fuel Cells.

摘要

Today, proton electrolyte membrane fuel cell (PEMFC) and direct methanol fuel cell (DMFC) are attractive power conversion devices that generate fairly low or even no pollution, and considered to be potential to replace conventional fossil fuel based power sources on automobiles. The operation and performance of PEMFC and DMFC depend largely on electro-catalysts positioned between the electrode and the membranes. The most commonly used electro-catalysts for PEMFC and DMFC are Pt-based noble metal nanoparticles, so catalysts share close to 50% of the total cost of the fuel cell. The synthesis of such nanoscale electro-catalysts are commonly limited to harsh conditions (high temperature, high pressure), organic solvent, high amount of stabilizing agent, to achieve the size and morphological control. There is no rational guideline for the selection of stabilizing agent for specific materials, leading to the current "trial and error" approach in selecting stabilizing agent.;This dissertation initially explores a new way to select material-specific stabilizing agents for the synthesis of the noble metal nanoparticles. With the help of phage display (PD) technique, a rational biomimetic approach can be used to select biomolecule (peptide) that specifically recognizes the surface of targeted material (use Pt as a case-study for this dissertation), and the selected peptides can be used as stabilizing agent to synthesize monodispersed Pt nanoparticles with tunable morphologies under mild synthetic conditions (atmospheric room temperature, aqueous solution). With fairly easy processing, the nanoparticles can be used as high surface area cathode electro-catalyst in fuel cells.;With the as-synthesized Pt nanoparticles, bimetallic nanoparticles containing Pt can be prepared for more electro-catalytic applications, such as the oxygen reduction reaction at the cathode of fuel cells, and the oxidation of methanol at the anode in DMFC. The materials synthesized include heterogeneously structured Pt-Pd core-shell nanoparticles, and homogenerously alloyed PtRu nanoparticles. The Pt-Pd core-shell nanoparticles, with Pd shell thickness controlled with atomic-layer precision, show almost 3-fold enhancement in catalytic activity for the ORR as well as better catalytic performance in oxidation of methanol, compared with commercially available catalysts. A specialized electrochemical tool, rotating disk electrode, is used to study the fundamental kinetics and their quantified catalytic activities in ORR. The seed-mediated synthesis of hyperbranched PtRu nanoparticles demonstrates the possibility of low-temperature synthesis of well-alloyed material, and shows the enhanced catalytic activity in methanol oxidation compared with commercial catalysts, with its special formation mechanism studied.