To improve the durability of polymer electrolyte membrane fuel cells (PEMFCs) for auto mobile applications the effects of a procedure involving hydrogen and the supply of air during the start up process on the long term stability of these cells were investigated through measurements of the polarization curves and through the use of electrochemical impedance spectroscopy (EIS) cyclic voltammetry (CV) and linear sweep voltammetry (LSV) with repeating start up/shutdown cycles Two gas supply cases were compared The first involved a concurrent supply of hydrogen and air In the second hydrogen was supplied prior to air to a single cell for the start up process The latter case exhibited slower decay in terms of both the cell performance and the electrochemical active surface area It also showed less of an increase in the charge transfer resistance during repetitive start up/ shutdown cycles On the other hand the open circuit voltage and hydrogen crossover current density were nearly constant during the start up/shutdown cycles in both cases These results suggest that supplying hydrogen prior to air can effectively mitigate performance degradation by retarding degradation of the electrodes rather than the membrane (C) 2010 Professor T Nejat Veziroglu Published by Elsevier Ltd All rights reserved