Structure and Energetics of Nanoparticles and Ionomer Films in Fuel Cell Catalyst Layers

Improving the durability and utilization efficiency of the platinum-on-carbon (Pt/C) catalyst is of vital importance to the commercialization of the polymer electrolyte membrane fuel cell (PEMFC). This body of work provides molecular level insights to aid the fulfillment of this goal. Chapter 1 describes the use of molecular dynamics (MD) simulation in an effort to understand the Pt/C degradation issue from the nano-adhesion point of view. The roles of catalyst nanoparticle size, shape, Pt/C surface oxidation and the extent of ionomer film hydration are investigated to study their effects on nano-particle adhesion. It is found that the adhesion force strengthens as the Pt size goes up. Nanoparticle of tetrahedral shape exhibits relatively stronger connection with the carbon. The hydroxylated surface enhances nano-adhesion and epoxidized surface diminishes the adhesion. The presence of ionomer film strengthens the adhesion. Chapter 2 uses MD simulations to investigate the microstructure of the catalyst layer, which is essential information needed for increasing the catalyst utilization rate. The ionomer film thickness, hydration level, surface oxidation of Pt/C, presence of Pt or PtO catalysts are key variables studied for their effects on the catalyst layer microstructure and transport properties. It is concluded that the oxidation of the carbon surface and the presence of Pt or PtO catalyst drastically influence the ionomer film configuration and the water distribution on the surface. The thickness of the ionomer film is directly related with its ability of retaining water. Chapter 3 describes experimental work exploring the effect of radiation damage on the microscopic characterization of the catalyst layer of the PEMFCs. It also provides information on the feasibility of in-situ nano-adhesion measurements inside the SEM. It is found that the radiation damage of the catalyst sample usually starts from the interface of Pt/C and primarily occurs in the form of mass loss accompanied by atomic displacement and edge curl. The results indicate the low reliability of the in-situ nano-adhesion measurement. All three chapters serve to expand the fundamental understanding of the microstructure of the catalyst layer, which contribute to the development of a more durable, less expensive and better performing PEMFC.