  The design and operation of a differential Polymer Electrolyte Membrane (PEM)
fuel cell is described. The fuel cell design is based on coupled Stirred Tank
Reactors (STR); the gas phase in each reactor compartment was well mixed. The
characteristic times for reactant flow, gas phase diffusion and reaction were
chosen so that the gas compositions at both the anode and cathode are uniform.
The STR PEM fuel cell is one-dimensional; the only spatial gradients are
transverse to the membrane. The STR PEM fuel cell was employed to examine fuel
cell start- up, and its dynamic responses to changes in load, temperature and
reactant flow rates. Multiple time scales in systems response are found to
correspond to water absorption by the membrane, water transport through the
membrane and stress-related mechanical changes of the membrane.

Chemical Physics