In a proton exchange membrane (PEM) fuel cell, oxygen is transported from air›ow channels, through a gas dižusion layer (GDL) and then into a cathode catalyst layer (CCL), where an oxygen reduction reaction occurs. A CCL is a thin coating with a porosity of 30-60% and pore size distribution from several nanometers to 100 nm, permitting the dižusion of reactant oxygen into the CCL and the transport of product water vapor out of the CCL. še oxygen reduction reaction then can take place through the whole depth of the CCL. še rate of oxygen dižusion signiœcantly ažects the uniformity of oxygen reduction reaction through the whole CCL, the CCL lifetime, and the power output of the PEM fuel cell (Shen et al., 2011). In this context, the knowledge of the ežective gas dižusion coe¥cients of the GDL and CCL are crucial for accurately predicting the performance and optimizing the design of PEM fuel cells.