Water Balance in PEM Fuel Cells

Abstract

Fuel cells based on polymer electrolyte membranes (PEMs) are attractive power sources because they are efficient, non-polluting, and do not rely on non-renewable fossil fuels. Water management is a critical design issue for these fuel cells because the PEM must be maintained at the proper water content to remain ionically conducting without flooding the electrodes. Furthermore, portable PEM power systems should operate at water balance. That is, water losses from the cell should be balanced by the rate of water production from the fuel cell reaction. A portable system that operates at water balance does not require an external supply of water. The rate of water production depends on the cell’s electrochemical characteristics. The rate of water loss depends on the flow rates of reactants and products, transport of water and fuel across the PEM, and the stack operating temperature. This paper presents the basic design relationships that govern water balance in a PEM fuel cell. Specific calculations are presented based on data from hydrogen/air and direct methanol fuel cells currently under development for portable power systems. We will show how the water balance operating point depends on the cell operating parameters and show the sensitivity to off-design conditions.