The species-electrochemical transport phenomena in a free-breathing cathode of a printed-circuit-board (PCB)-based fuel cell have been studied numerically. A perforated current collector is attached to the porous cathode that breathes the fresh air through an array of circular holes. Parametric studies include the breathing-hole diameter (d) and the cathode-layer thickness (delta). Gas flow in the porous cathode is governed by the Darcy equation with constant porosity and permeability. Electrochemical reaction on the surfaces of the porous matrices is depicted via the Butler-Volmer equation. The multi-species diffusive transports in the porous cathode are described using the Stefan-Maxwell equation. The charge transports in the porous matrices are dealt with the Ohm's law. The coupled equations are solved by a finite-element-based CFD, technique. Results show that the mean flow of the gaseous mixture directs outward from the porous cathode to ambient. In addition, the species diffusion dominates the mass transports in the free-breathing cathode. Furthermore, the enhancement of species diffusion compromises the increase of Ohmic resistance shows the optimal breathing-hole diameter around d = 2.1 mm that provides the best electrochemical performance. (c) 2006 Elsevier Ltd. All rights reserved.