Liquid water transport

The research group led by Dr. Djilali at the University of Victoria has developed an ex situ experimental technique using fluorescent microscopy to study the liquid water transport mechanisms inside diffusion layers and on their surfaces [239-243]. The diffusion layer is usually placed between two plates (the top plate may or may not have a channel) the liquid water, which is pumped through a syringe pump, flows from the bottom plate through the DL. Fluorescein dye is added to the water for detection with the microscope. 

X. G. Yang, R Y. Zhang, A. L. Lubawy, and C. Y. Wang. Visualization of liquid water transport in a PEFC. Elechtrochemical and Solid State Letters 7 (2004) A408-A411. 

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. 

A. Bazylak, J. Heinrich, N. Djilali, and D. Sinton. Liquid water transport between graphite paper and a solid surface. Journal of Power Sources 185 (2008) 1147-1153. 

In 2000 and 2001, fuel-cell models were produced by the dozens. These models were typically more complex and focused on such effects as two-phase flow ° where liquid-water transport was incorporated. The work of Wang and co-workers was at the forefront of those models treating two-phase flow comprehensively. The liquid-water flow was shown to be important in describing the overall transport in fuel cells. Other models in this time frame focused on multidimensional, transient, and more microscopic effects.The microscopic effects again focused on using an agglomerate approach in the fuel cell as well as how to model the membrane appropriately. 

The first major CFD models were those by Liu and co-workers " at the University of Miami. They are nonisothermal and the first multidimensional models. They allowed for a more in-depth study of the effects along the channels than the models described above. While the original model by Gurau et al. did not include liquid-water transport, it did have a variable water content in the membrane. To study... 

The more rigorous approach to liquid water transport is a true two-phase model in which the two phases travel at different velocities. At the same time, the interfacial tension effect and GDL wettability, essential for successful PEEC operation, are fully accounted for. The work of Wang et al., Nguyen et ai 69 71 You and Liu, Mazumder and Cole, Bern-ing and Djilali, and Pasaogullari and Wang falls into this category. These two-phase models are reviewed in section 3.7. 

To summarize, to properly model liquid water transport and ensuing flooding effect on cell performance, one must consider four submodels (1) a model of catalytic surface coverage by liquid water inside the catalyst layer, (2) a model of liquid water transport through hydrophobic microporous layer and GDL, (3) an interfacial droplet model at the GDL surface, and last (4) a two-phase flow model in the gas channel. Both experimental and theoretical works, in academia and industry alike, are ongoing to build models for the four key steps of water generation, transport, and removal from a PEFC. 

Figure 33. Visualization of liquid water transport in an operating transparent PEFC (45 /rm membrane with EW < 1000 GDL, Toray paper TGPH 090 with 20 wt % PTFE loading with a microporous layer).

A transparent PEM fuel cell with a single straight channel was designed by Ma et al.11 to study liquid water transport in the cathode channel (this study is also mentioned in Section 2.5). The pressure drop between the inlet and outlet of the channel on the cathode side was used as a diagnostic signal to monitor liquid water accumulation and removal. The proper gas velocities for different currents were determined according to the pressure drop curves. 

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... 

In this chapter, the development of a mesoscopic modeling formalism is presented in order to gain fundamental insight into the structure-wettability influence on the underlying liquid water transport and interfacial dynamics in the PEFC CL and GDL. 

The mesoscopic modeling approach consists of a stochastic reconstruction method for the generation of the CL and GDL microstructures, and a two-phase lattice Boltzmann method for studying liquid water transport and flooding phenomena in the reconstructed microstructures. 

The multi-faceted functionality of a GDL includes reactant distribution, liquid water transport, electron transport, heat conduction and mechanical support to the membrane-electrode-assembly. 

In this study, two numerical experiments are designed for investigating liquid water transport and two-phase dynamics through the... 

The computational approach couples the two-phase LB model for the liquid water transport and the DNS model for the species and charge transport for the CL.25-27,68 The two-phase simulation using the LB model is designed based on the ex-situ, steady-state flow experiment for porous media, detailed earlier in the section 4.3, in order to obtain the liquid water distributions within the CL microstructure for different saturation levels resulting from the dynamic interactions between the two phases and the underlying pore morphology. The details of the simulation setup are provided in our work.27,61 62 Once steady state is achieved, 3-D liquid water distributions can be obtained within the CL, as shown in Fig. 13. From the liquid water distributions within the CL structure, the information about the catalytic site coverage effect can be extracted directly. 

Figure 26 exhibits the polarization curves in terms of the cathode overpotential variation with current density for the CL27 obtained from the 3-D, single-phase DNS model prediction,25,27 the experimental observation25,27 and the liquid water transport corrected 1-D macrohomogeneous model.27 The polarization curve refers to the cathode overpotential vs. current density curve in the... 

Diffusion media (DM) are prone to flooding with liquid water. Although the DM is an essential component of PEFCs that enable distribution of species and collection of current and heat, little was known about capillary transport in DMs until recently. In Chapters 7 Gostick et al. provide a description of liquid water transport in porous DM due to capillarity and describe experimental techniques used to characterize DM properties. 

FIGURE 12.11 Comparison of liquid water transport for two 50-cm single-cell PEM fuel cells using commercial graphite composite bipolar plates (a) surface modified and (b) as received (0.1 A/cm, 1.5/2.0 Hj-air stoichiometry, 100% RH). 

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

Bazylak, A. et al., Effect of compression on liquid water transport and microstructure of PEMFC gas diffusion layers, J. Power Sources, 163, 784, 2007. 

Possess a sufficient amount of hydrophilic pores to allow liquid water transport through the GDL and to maintain the electrolyte membrane in contact with liquid water. 

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