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

Abstract

Water uptake and electro-osmosis are investigated to improve the understanding and aid the modeling of water transport in proton-exchange membrane fuel cells ͑PEMFCs͒ 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 poly͑arylene ether sulfone͒ ͑BPSH͒ chemistries are reported for vapor equilibrated samples below 0°C. The electro-osmotic drag coefficient of BPSH chemistries is found to be ϳ0.4, and that of Nafion is ϳ1. 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.

Impact of channel wall hydrophobicity on through-plane water distribution and flooding behavior in a polymer electrolyte fuel cell

“…5a and 18th minute in Fig. 5b), increasing liquid water mass under the landings moves in two directions; (1) in vertical (through-plane) direction such that from DM-under-the-landing into the flow channel through the DM-channel wall intersection, (2) in lateral (in-plane) direction within the DM such that from DMunder-the-landing to DM-under-the channel location. The water accumulation under the landing and removal of water through the 90 • interface is common for both cases.…”

“…In polymer electrolyte fuel cells (PEFCs), liquid water accumulation, transport and removal are critical, due to the significant impact on fuel cell operation and durability [1][2][3]. Experimental studies have shown that flooding inside a fuel cell may exist in the catalyst layer (CL), diffusion media (DM), flow channels or along the CL|DM interface [4][5][6][7][8][9][10].…”

Water crystallization inside fuel cell membranes probed by X-ray scattering

“…One has reasonably expected that water cannot freeze inside Nafion ® membrane since it is in a highly confined and acidic environment. When the temperature is decreased, the desorption property is driven by the ratio between the saturated vapor pressure over ice outside the membrane and the saturated vapor pressure of super-cooled water inside the membrane: this ratio is smaller than one and decreases upon cooling [9,10].…”

Cold Start of a Polymer-Electrolyte Fuel Cell II. Model Verification Using Parametric Studies

“…At À20 C, and at a relative humidity of 100%, the approach used in the present work (based on the work of Gallagher et al 9 ) predicts a k of 8.2, whereas a relative humidity of 34% corresponds to 3.5. As discussed in Part I of this study, membrane hydration is often described by k, the moles of H 2 O per mole of SO À 3 in the membrane.…”