Water Transport Across a Polymer Electrolyte Membrane under Thermal Gradients

Abstract

A fundamental experimental and numerical study of the water transport across a perfluorosulfonic acid membrane under a temperature gradient is presented. The water transport phenomenon was experimentally investigated through water flux measurement and neutron radiography. The experimental observations found that water is transported in the direction from the high temperature side to the low temperature side, when both sides of the membrane are sufficiently humidified, and suggest that the transport mechanism is concentration gradient driven. The neutron radiography measurements detected the presence of water content gradient across the membrane and higher water content is seen at a larger thermal gradient. A numerical model was developed to investigate the experimental results. Water transport predictions agreed qualitatively but more accurate material and transport property characterizations are needed for further improvement.High current density operation is desirable in automotive and portable applications of polymer electrolyte fuel cells ͑PEFCs͒ in particular to minimize the size and weight of the system balance of the plant. However, the resulting heat generation due to high overpotentials, predominantly in the cathode catalyst layer, and the Joule heating, especially in the electrolyte, cause a temperature gradient across the polymer electrolyte membrane ͑PEM͒ as well as in gas diffusion media ͑GDM͒. An experimental measurement of the temperature difference of nearly 2 K between the membrane electrode assembly ͑MEA͒ and the bipolar plates with 1.0 K differential across the MEA at current density of 0.6 A/cm 2 has been reported. 1 The nonisothermal nature of the PEFC contributes to a localized dry-out of the PEM, which leads to a deterioration of the performance even when the inlets are sufficiently humidified. On the other hand, during low temperature operation, such as during a start-up, the temperature gradient can also induce unwanted condensation of water in the GDM, a condition known as flooding, which impedes the transport of reacting species. Similar temperature differential across the GDM can also occur during the shutdown process. Thus, establishing a proper water and thermal management is essential for an optimized performance of PEFCs and has been an intensely studied subject over the years; therefore, the present paper aims to understand the description of water transport across a nonisothermal membrane.Water transport across the PEM is known to occur by several driving modes, which include electric potential, water concentration gradients, and hydraulic pressure gradients. In addition to the aforementioned transport modes, water transport in the through-plane direction induced by the presence of a thermal gradient across the PEM has been known to exist for some time but attention toward the phenomenon in PEFCs has been mostly nonexistent in the literature until recently. The few investigators who studied this phenomenon, however, arrived at various conclusions on the mechanism of the transport...

Numerical Validation of Water Transport in a Polymer Electrolyte Membrane

“…Their study found the importance of heat and water management inside the cell under high current density operations. Fu et al (10) also supported their experimental results with a one-dimensional, two-phase, non-isothermal model. Their numerical analysis revealed the capabilities and limitations of the numerical prediction of the through-plane water distribution inside the MEA.…”

Study of coupled heat and water transfer in proton exchange membrane fuel cells by the way of internal measurements

“…50% to 70% of the energy exiting the cell as heat, recent works have therefore examined the impact of the temperature field on water transport in the fuel cells MEA [1][2][3][4][5][6][7][8][9][10]. In 2002, Djilali and Lu [1] focused on the modeling of non-isothermal and non-isobaric effects (including Knudsen diffusion and Soret effects).…”

“…Eikerling [4] also showed that at a current density of 1 A.cm -2 , the evaporation rate at the electrode is sufficient to remove all water produced at the cathode in vapor phase. To visualize this phenomenon, Hickner et al [5,6], Kim and Mench [7,8] and Fu et al [9], used neutron radiography highlighted the importance of evaporation at high current densities. The temperature influence on the water is significant, on its state but also on its transport.…”

Investigation of dissimilar metal welds by energy-resolved neutron imaging

“…laboratory X-rays) cannot be implemented. Neutron imaging with thermal and cold neutrons provides unique contrast and is often used to study internal structure and the presence of defects in various metal samples (Kockelmann et al, 2007;Bilheux et al, 2009;Kardjilov et al, 2012;Lehmann et al, 2009), as well as the distribution of hydrogen-containing substances within metals (Grosse et al, 2013;Griesche et al, 2015;Tremsin, Morgano et al, 2015), water in proton exchange membrane fuel cells (Fu et al, 2011;Stahl et al, 2015), coolant in heat pipes (Cimbala et al, 2004), diesel in fuel injectors (Lehmann et al, 2015) and many others. At the same time, neutron diffraction is widely used for studies of crystallographic properties as the wavelengths of thermal and cold neutrons are comparable to lattice parameters (several ISSN 1600-5767 å ngströ ms) (Vogel, 2000;Santisteban et al, 2001Santisteban et al, , 2002Hutchings et al, 2005).…”