  A simplified differential reactor model that embodies the essential physics
controlling PEM fuel cell (PEM-FC) dynamics is presented. A remarkable analogy
exists between water management in the differential PEM-FC and energy balance
in the classical exothermic stirred tank reactor. Water, the reaction product
in the PEM-FC autocatalytically accelerates the reaction rate by enhancing
proton transport through the PEM. Established analyses of heat autocatalyticity
in a CSTR are modified to present water management autocatalyticity in a
stirred tank reactor PEM-FC.
