Measurement of Oxygen Transport Resistance in PEM Fuel Cells by Limiting Current Methods

Limiting current measurements in a polymer electrolyte membrane (PEM) fuel cell are used to separate the oxygen-transport resistance into individual component parts. By varying the thicknesses of the diffusion medium (DM) and the microporous layer in different cell builds, the total transport resistance is separated into contributions from flow channels, DM, microporous layer, and all other sources. By varying the pressure, the transport resistance is separated into a pressure-dependent component (intermolecular gas diffusion) and a pressure-independent component (Knudsen diffusion or transport through ionomer/liquid water layers). In addition to oxygen diffusion in an anisotropic gas diffusion layer, the analysis accounts for coupled convective diffusion and reactant depletion in the flow channels. The present work is limited to conditions when no condensation occurs inside the cell. The analysis is applied to a large body of limiting current data collected on Toray diffusion media, both plain and treated with poly(tetrafluoroethylene) (PTFE), with and without a microporous layer. Effective diffusion coefficients obtained from these methods for plain Toray papers compare reasonably well with independent ex situ measurements of water vapor diffusion through the same materials.