The purpose of this work was to design a heat exchanger for a Polymer Electrolyte Membrane (PEM) fuel cell, which would ensure such a temperature profile along the fuel cell cathode channel resulting in close to 100% relative humidity along the channel without external humidification. To achieve this, 3D numerical simulations of a single PEM fuel cell were performed using commercial Computational Fluid Dynamics (CFD) software (ANSYS Fluent). Based on the simulation results a variable surface area finned heat exchanger was designed which allows for passive fuel cell cooling. The results indicate that it is possible to obtain such temperature and relative humidity conditions inside the fuel cell cathode channel, using a passive heat exchanger with variable surface area. (C) 2012 Elsevier Ltd. All rights reserved.