Mass/electron transports in a passive cathode of a proton exchange membrane (PEM) fuel cell have been studied numerically. The porous cathode in contact with a perforated current collector breathes fresh air through an array of orifices. Diffusions of reactant species in the porous cathodes are described by the Stefan-Maxwell equation. Electrochemical reaction on the surfaces of the porous cathode is depicted via the Butler-Volmer equation. Gas flow in the air-breathing porous cathodes is governed by isotropic linear resistance model with constant porosity and permeability. The electron/ion transports in the catalyst/electrolyte are dealt with the charge conservations based on the Ohm's law. A finite-element method is employed to solve the above-coupled equations. The effect of overpotential on the fluid flow, mass transport and electrochemistry is examined. Detailed electrochemical/mass characteristics such as flow velocities, species mass fraction, species flux and current density distributions are presented. They can provide a solid basis for optimizing the geometry of the PEM fuel cell stack running with a passive mode. (c) 2006 Elsevier B.V All rights reserved.