We describe studies of performance decay and recovery in polymer electrolyte fuel cells in response in step changes in the level of CO in the anode feed stream. Following a step from 0 to 100 ppm CO in the hydrogen anode feed, completion of current decay at constant cell voltage of 0.6 V in a 50 cm~2 cell operating at 80 deg C with anode loading of 0.2 mg Pt cm~(-2) took 300 seconds. The observed current decay is controlled by the rate of surface dosing, determined primarily, if not exclusively, by inlet flow of CO, and the non-linear dependence of anodic HOR rate on (1- theta _(CO)). Performance recovery in response to a pure hydrogen purge at 80 deg C was found to be sluggish, with full recovery taking as long as 1000 seconds. The rate of recovery upon exposure to a hydrogen purge is a function of temperature and is apparently controlled by the rate of thermal desorption of CO. Cell performance decay was found much faster (1-10 sec) or a step from 10 ppm to 200 ppm inlet CO. The recovery time was very slow (> 1500 sec) on return from 200 ppm to 10 ppm inlet. Air bleed into the anode significantly slows down decay times and shorten recovery times in operation with transient levels of CO.
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