Investigation of temperature-driven water transport in polymer electrolyte fuel cell: Thermo-osmosis in membranes

The objective of this work is to investigate water transport in polymer electrolyte fuel cell membranes subjected to a temperature gradient. Direct thermo-osmotic experiments on different polymer membrane types (non-reinforced Nafion®, and reinforced Gore-Select® and Flemion® membranes) were performed using an in-house fuel cell fixture with a parallel flow field and pure membrane sheet without catalyst layers. Thermo-osmotic flow was observed in all membranes, and the water flow direction in the membrane was determined to always flow from the cold to hot side, as anticipated for a small pore hydrophilic porous medium. The water flux was found to be proportional to temperature gradient, and increase with average membrane temperature. The dependency of the thermo-osmotic diffusivity on average temperature showed predictable Arrhenius-type behavior. True interfacial temperatures of the membrane were estimated using a two-dimensional thermal model, and empirical relations for the thermo-osmotic diffusivity for the membrane types tested were developed. These can be of use to design engineers concerned about achieving optimal water balance during steady and transient operation.