The objective of this article is to investigate the performance of an air-breathing proton exchange membrane (PEM) fuel cell operating with hydrogen fed at the anode and air supplied by natural convection at the cathode. Considering a dual-cell cartridge configuration with a common anode flow chamber, a comprehensive two-dimensional, non-isothermal, multi-component numerical model is developed to simulate the mass transport and electrochemical phenomena governing the cell operation. Systematic parametric studies are presented to investigate the effects of operating conditions, cell orientation and cell geometry on the performance. Temperature and species distributions are also studied to assist the understanding of the single cell performance for different conditions. It is shown that the cell orientation affects the local current density distribution along the cell and the average current density, particularly at lower cell voltages. The cell performance is shown to improve with increase of temperature, anode flow rate, anode pressure and anode relative humidity. (c) 2007 Elsevier Ltd. All rights reserved.