A three-dimensional model is developed to study the species-electrochemical characteristics in a free-breathing cathode of a planar fuel cell. A perforated current collector is attached to the porous cathode that breathes the fresh air through an array of circular holes and routes out the electric current through its solid counterpart. The effects of breathing-hole arrangements (in-line and staggered) and open-area ratios (0.8% < lambda < 73.5%) of the current collector on the electrochemical performance are examined. The Brinkman extension to the Darcy's law is employed to describe the gas-mixture flow in the porous cathode. Electrochemical reaction on the three-phase boundary of the porous matrices is illustrated via the Butler-Volmer correlation. The multispecies diffusive transports in the porous cathode are depicted using the Stefan-Maxwell equation. The electron/ion transports in the porous matrices are dealt with by Ohm's law. The coupled equations are solved by a finite-volume-based computational fluid dynamics technique. Detailed distributions of species-electrochemical characteristics such as flow velocities, mass fractions, mass fluxes, potentials, and current densities are presented. Results show that the breathing holes arranged in a staggered manner perform better than those in arranged in-line. In addition, the optimal open- area ratio of the current collector for the best electrochemical performance is about 30%. (c) 2006 The Electrochemical Society.