Temperature Gradient Induced Water Transport in Polymer Electrolyte Fuel Cells

Abstract

The objective of this work is to investigate water transport of polymer electrolyte fuel cell (PEFC) materials subjected to a temperature gradient. Direct thermo-osmotic experiments on different polymer membrane types (non-reinforced Nafion ® , reinforced A, and reinforced B membrane) were performed using an in-house fuel cell fixture with a parallel flow field. Thermoosmotic flow was observed in all membranes, and the water flow direction (in the membrane only) was determined to always flow from the cold side to the hot side. The water flux was found to be proportional to temperature gradient and increased with average temperature. The dependency of the thermo-osmotic diffusivity on average temperature showed predictable Arrhenius-type behavior. For the membrane electrode assembly (MEA), net flow was determined to be from the hot to cold side due to a phase-change effect, which will be discussed in detail in a later publication. Interface temperatures of membrane were estimated using 2-dimensional thermal model, leading to empirical relations of thermo-osmotic diffusivity across the membrane.

Investigation of Temperature-Driven Water Transport in Polymer Electrolyte Fuel Cell: Phase-Change-Induced Flow

“…where D TO is thermo-osmotic diffusivity in the membrane and is a function of temperature. 26,27 3. Energy conservation in the porous media…”

“…Kim and Mench 26,27 observed that thermo-osmotic flux of water in three different commercial perfluorinated PEMs flows from the cold to hot side of the membrane and that reinforcement did not affect the flow direction. The net flux, however, was altered by the reinforcement structure.…”

“…Temperature-driven flow during fuel cell operation, as well as at shutdown, can be important, but is still not completely understood. Kim and Mench 26,27 observed net water flux reversal in membranes when hydrophilic DM were replaced with hydrophobic DM. With fully saturated hydrophilic DM and membranes, the net flow was thermo-osmosis through the membrane from the cold to the hot side.…”

“…Vapor diffusion always results in flow from higher saturation pressure ͑high tempera-ture͒ to lower saturation pressure ͑low temperature͒, but thermoosmotic water transport in a perfluorinated membrane flows from the cold to the hot side. [25][26][27][28] Thus, the temperature-gradient-induced water transport in the fuel cell should be considered to be the net flux of phase-changeinduced flow in porous media and thermo-osmosis in the membrane. We also expect that the phase-change-induced flow would be more dominant with increasing temperature because the saturation pressure is a highly nonlinear function of temperature.…”

“…Thermo-osmotic water flux is from the cold to the hot side, the opposite direction to mass diffusion. Thermoosmotic water flux was estimated by empirical relations, 26,27 and is 0.00024 and 0.00041 kg/m 2 s for Nafion 112 and R-PEM membranes, respectively.…”