Electro-Osmosis and Water Uptake in Polymer Electrolytes in Equilibrium with Water Vapor at Low Temperatures

Water uptake and electro-osmosis are investigated to improve the understanding and aid the modeling of water transport in proton-exchange membrane fuel cells (PEMFCs) below 0°C. Measurements of water sorption isotherms show a significant reduction in the water capacity of polymer electrolytes below 0°C. This reduced water content is attributed to the lower vapor pressure of ice compared to supercooled liquid water. At -25°C, 1100 equivalent weight Nafion in equilibrium with vapor over ice has 8 moles of water per sulfonic acid group. Measurements of the electro-osmotic drag coefficient for Nafion and both random and multiblock copolymer sulfonated poly(arylene ether sulfone) (BPSH) chemistries are reported for vapor equilibrated samples below 0°C. The electro-osmotic drag coefficient of BPSH chemistries is found to be ∼0.4, and that of Nafion is ∼1. No significant temperature effect on the drag coefficient is found. The implication of an electro-osmotic drag coefficient less than unity is discussed in terms of proton conduction mechanisms. Simulations of the ohmically limited current below 0°C show that a reduced water uptake below 0°C results in a significant decrease in PEMFC performance.