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.