Water transport in gas diffusion layers

The water transport in the gas diffusion layer (GDL) is very important, as the region is at the intersection of the water and oxygen flows. Additionally, water tends to accumulate under lands and deteriorates the oxygen supply to these areas. Thus, it would be desirable to have a GDL structure, which smoothly guides liquid water to the separator channels from the MPL surface and from the regions under the lands. 

The authors investigated the effect of anisotropic fiber directions of the GDL and conducted numerical simulations of the water flow in porous media with different wettability gradients in the media. The major results of the research in these two investigations are presented in this section. 

Permeability (as fiber direction) Permeability (transverse direction) Fiber diameter Maximum pore size 

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Pasaogullari, U. and Wang, C.Y, Liquid water transport in gas diffusion layer of polymer electrolyte fuel cells, J. Electrochem. Soc., 151, A399, 2004. 

Sinha, P.K. and Wang, C.-Y. (2007) Pore-network modeling of liquid water transport in gas diffusion layer of a polymer electrolyte fuel cell. Bectrochim. Acta, 52, 7936 7945. 

Rebai, M., and Prat, M., 2009, Scale effect and two-phase flow in a thin hydrophobic porous layer. Application to water transport in gas diffusion layers of proton exchange membrane fuel cells , J. Power Sources, 192 (2) pp. 534. 

Luo, G., Ji, Y, Wang, C., 2010, Modeling liquid water transport in gas diffusion layers by topologically equivalent pore network , Electrochim. Acta, 55 (19) pp. 5332. Medici, E. F., and Allen, J. S., 2010, The effects of morphological and wetting properties of porous transport layers on water movement in PEM fuel cells , J. Electrochem. Soc., 157 (10) pp. B1505. 

Hao, L. Cheng, R Lattice Boltzmann simulations of water transport in gas diffusion layer of a polymer electrolyte membrane fuel cell. J. Power Sources 195 (2010), pp. 3870-3881. 

V. Gurau, M. J. Bluemle, E. S. De Castro, et al. Characterization of transport properties in gas diffusion layers for proton exchange membrane fuel cells. 1. Wettability (internal contact angle to water and surface energy of GDL fibers). Journal of Power Sources 160 (2006) 1156-1162. 

Liquid water arrives in the CCL via transport through the PEM or it is generated in the electrochemical reaction. Invariably, PEECs require a medium that is highly effective in transforming liquid water into water vapor otherwise, liquid water will clog pores and channels in gas diffusion layers and flow fields that are needed for the gaseous supply of reactants. 

Polymer electrolyte fuel cell (PEFC) is considered as one of the most promising power sources for futurist s hydrogen economy. As shown in Fig. 1, operation of a Nation-based PEFC is dictated by transport processes and electrochemical reactions at cat-alyst/polymer electrolyte interfaces and transport processes in the polymer electrolyte membrane (PEM), in the catalyst layers consisting of precious metal (Pt or Ru) catalysts on porous carbon support and polymer electrolyte clusters, in gas diffusion layers (GDLs), and in flow channels. Specifically, oxidants, fuel, and reaction products flow in channels of millimeter scale and diffuse in GDL with a structure of micrometer scale. Nation, a sulfonic acid tetrafluorethy-lene copolymer and the most commonly used polymer electrolyte, consists of nanoscale hydrophobic domains and proton conducting hydrophilic domains with a scale of 2-5 nm. The diffusivities of the reactants (02, H2, and methanol) and reaction products (water and C02) in Nation and proton conductivity of Nation strongly depend on the nanostructures and their responses to the presence of water. Polymer electrolyte clusters in the catalyst layers also play a critical... 

One can suppose that the increase of channel width results in a sag of gas diffusion layer above a channel and, as a consequent, in impairment of electric contact efficiency between gas diffusion and catalytic layers. The increase of width of ribs results in aggravated limitations on reagent transport through gas diffusion layer to electrocatalytic layer, in air electrode zone furthermore complicated by counter water flow. 

J., and Banhart, J. (2011) Investigation of 3D water transport paths in gas diffusion layers by combined in situ synchrotron X-ray radiography and tomography. Electrochem. Commun., 13, 1001-1004. 

After the tests, Djilali s group used mathematical assumptions and equations to correlate the intensity of the dye in the image with the depth in the gas diffusion layer. With this method they were able to study the effect of compression on diffusion layers and how fhaf affects water transport. Water removal in a flow charmel has also been probed with this technique and it was observed that, with a dry DL slug, formation and flooding in the FF channels followed the appearance and detachment of water droplets from the DL. Even though this is an ex situ technique, it provides important insight into water transport mechanisms with different DLs and locations. 

C. Xu, T. S. Zhao, and Y. L. He. Effect of cathode gas diffusion layer on water transport and cell performance in direct methanol fuel cells. Journal of Power Sources 171 (2007) 268-274. 

S. Litster, D. Sinton, and N. Djilali. Ex situ visualization of liquid water transport in PEM fuel cell gas diffusion layers. Journal of Power Sources 154 (2006) 95-105. 

Transport properties of hydrated PFSA membranes strongly depend on nanophase-segregated morphology, water content, and state of water. In an operational fuel cell, these characteristics are indirectly determined by the humidity level of the reactant streams and Faradaic current densities generated in electrodes, as well as the transport properhes of catalyst layers, gas diffusion layers, and flow... 

As can be seen in the different boundary conditions, the main effects of having ribs are electronic conductivity and transport of oxygen and water, especially in the liquid phase. In terms of electronic conductivity, the diffusion media are mainly carbon, a material that is fairly conductive. However, for very hydro-phobic or porous gas-diffusion layers that have a small volume fraction of carbon, electronic conductivity can become important. Because the electrons leave the fuel cell through the ribs, hot spots can develop with large gradients in electron flux density next to the channel. " Furthermore, if the conductivity of the gas-diffusion layer becomes too small, a... 

Overall, the rib effects are important when examining the water and local current distributions in a fuel cell. They also clearly show that diffusion media are necessary from a transport perspective. The effect of flooding of the gas-diffusion layer and water transport is more dominant than the oxygen and electron transport. These effects all result in non-uniform reaction-rate distributions with higher current densities across from the channels. Such analysis can lead to optimized flow fields as well as... 

Diffusion medium properties for the PEFC system were most recently reviewed by Mathias et al. The primary purpose of a diffusion medium or gas diffusion layer (GDL) is to provide lateral current collection from the catalyst layer to the current collecting lands as well as uniform gas distribution to the catalyst layer through diffusion. It must also facilitate the transport of water out of the catalyst layer. The latter function is usually fulfilled by adding a coating of hydrophobic polymer such as poly(tet-rafluoroethylene) (PTFE) to the GDL. The hydrophobic polymer allows the excess water in the cathode catalyst layer to be expelled from the cell by gas flow in the channels, thereby alleviating flooding. It is known that the electric conductivity of GDL is... 

Water content affects many processes within a fuel cell and must be properly managed. Proton conductivity within the polymer electrolyte typically decreases dramatically with decreasing water content (especially for perfhiorinated membranes such as Nation ), while excessive liquid water in the catalyst layers (CLs) and gas diffusion layers (GDLs) results in flooding, which inhibits reactant access to the catalyst sites. Water management is complicated by several types of water transport, such as production of water from the cathode reaction, evaporation, and condensation at each electrode, osmotic drag of water molecules from anode to cathode by... 

A typical PEFC, shown schematically in Fig. 1, consists of the anode and cathode compartments, separated by a proton conducting polymeric membrane. The anode and cathode sides each comprises of gas channel, gas diffusion layer (GDL) and catalyst layer (CL). Despite tremendous recent progress in enhancing the overall cell performance, a pivotal performance/durability limitation in PEFCs centers on liquid water transport and resulting flooding in the constituent components.1,2 Liquid water blocks the porous pathways in the CL and GDL thus causing hindered oxygen transport to the... 

The catalyst layer and gas diffusion layer play a crucial role in the overall PEFC performance due to the transport limitation in the presence of liquid water and flooding phenomena. The... 

Proton exchange membrane (PEM) fuel cells are the primary choice for transportation systems, but they can also be useful for stationary power production or local hydrogen production. Most of the challenges of PEM fuel cell commercialization center around cost and materials performance in an integrated system. Some specific issues are the cost of catalyst materials, electrolyte performance, i.e., transport rates, and water collection in the gas diffusion layer (GDL). 

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