  A PEM fuel cell was specially constructed to determine kinetics under
conditions of well-defined gas phase composition and cell temperature. Steady
state multiplicity was discovered in the autohumidification PEM fuel cell,
resulting from a balance between water production and water removal. Ignition
was observed in the PEM fuel cell for a critical water activity of about 0.1.
Ignition is a consequence of the exponential increase of proton conductivity
with water activity, which creates an autocatalytic feedback between the water
production and the proton conduction. The steady state current in the ignited
state decreases with increasing temperature between 50 to 105 deg C. At
temperatures greater than 70 deg C five steady states were observed in the PEM
fuel cell. The steady state performance has been followed with variable load
resistance and hysteresis loops have been mapped. The dynamics of transitions
between steady states are slow about 10^3 to 10^4 s. These slow dynamics are
suggested to result from a coupling of mechanical and chemical properties of
the membrane electrode assembly due to swelling of the membrane with water
absorption.
