Molecular Level Study on Oxygen Transport Mechanism Based on Three-Phase Interfacial Features in Catalyst Layer of Proton Exchange Membrane Fuel Cells

Abstract

The oxygen transport resistance in the catalyst layer of a fuel cell is a crucial factor in determining the cell efficiency at low Pt and high current densities. A microscopic model of the three-phase interface based on intermolecular interactions is developed. The oxygen transfer resistances at different parts of the three-phase interface are directly obtained by counting the actual number of oxygen molecules during oxygen directional diffusion at the gas–liquid, ionomer bulk, and solid–liquid interfaces. The non-uniform water film at the gas–liquid interface, the difference in the hydrophilic-hydrophobic transport velocity in the oxygen diffusion channel inside the ionomer, and the non-uniform distribution of the ionomer concentration owing to the Pt-friendly and C-sparing properties of the ionomers at the solid–liquid interface are identified. Oxygen passing through water clusters at the two interfaces will have a “dissolution-separation” interaction with water, resulting in slow oxygen transfer and increased resistance. An invalid oxygen transport mechanism exist at the solid–liquid interface, where oxygen pass directly to the carbon surface by avoiding Pt. The solid–liquid interface resistance is larger than the gas–liquid interface resistance, and the sum of the two interfacial resistances is 5–10 times greater than the internal resistance of the ionomer.

Keywords: three-phase interface, oxygen permeation, resistance, ionomers