Molecular level study on oxygen transport mechanism based on three-phase interfacial features in catalyst layer of proton exchange membrane fuel cells

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.