Dynamics of autohumidifed PEM fuel cell operation

Start-up and dynamic responses to changes in load are reported for a dry feed, autohumidified polymer electrolyte membrane fuel cell (PEMFC). The fuel cell current can be ignited by water injection; criteria are derived for the required water injection as functions of fuel cell size, temperature, and reactant feed rates. Steady-state multiplicity is observed where the fuel cell current can be either high ("ignited") or low (" extinguished") with the same flow rates, temperature, pressure, and load. Different steady-state voltages were obtained for the same current depending on the operational history. The steady-state multiplicity results from a positive feedback between the membrane resistance and water production. Membrane water content is determined from the balance of water produced and water removed by convection. Current ignition occurs when the water produced is greater than its removal, hydrating the membrane; current extinction occurs when the water produced is less than its removal, which dehydrates the membrane. Through the use of mass balances and constitutive equations, design criteria are obtained for sizing and operating autohumidified PEMFCs.