Water transport channels

Figures 5e and 5f show OCT images of two flax seeds (GMF) 5 minutes after being soaked in water. Multiple grouping of water transport channels with formation of lens-shaped water-bearing structures can be clearly distinguished.

Fig. 5 a, b - 2 different seeds of long-fibred flax (control group) c, d - 2 seeds of the control group after 5 minutes in water e, f- 2 seeds of GMF group after 5 minutes in water. Arrows indicate lens-shaped water-bearing structures. Water transport channels are below them. 

Fig. 7.20 Schematic of the liquid water transport with and without water transport channel (WTC) [119] (Reprinted by permission of the publisher)...

As the lateral transport of water in a GDL is faster than the vertical transport, creating vertical water transport channels (WTCs) can optimize the transport properties in the GDL. Here again, experimental data show the appHcability of laser techniques, where tailored WTGs significantly enhance the water transport in the vertical direction by the use of capillary forces [33]. 

The preparation methods of gas diffusion electrodes, short GDE, have strong influence on the properties of the electrode. Within the manufacturing process, for example, the formation of electron conducting pathways through the electrode, the formation of pores to get a high gas distribution, and the effective connection of the electrolyte with proton and product water transport channels can get controlled. All this parameters are very important to get a good three phase boundary zone inside the catalyst layer and with this a working MEA [3]. 

Fig. 19.11 Artificial channels using pillar[n]atenes. (a) Proton channels using a pillar[5]arene dimer, (c) Water transport channels using (b) a pillar[5]arene with expanded hydrazine subunits, (d) Chirality-selective transport channels for transporting amino acids using a peptide-appended pillar[6]arene (a Reproduced with permission from Ref. [48]. Copyright 2011 WUey-VCH Verlag GmbH Co. KGaA. c Reproduced with permission from Ref [49]. Copyright 2012 American Chemical Society, d Reproduced with permission from Ref [50]. Copyright 2013 American Chemical Society)...

Awatani T, Midorikawa H, Kojima N, Ye J, Marcott C. Morphology of water transport channels and hydrophobic clusters in Nation from high spatial resolution AFM-IR spectroscopy and imaging. Electrochemistry Communications 2013 30 5. 

Modulation of CL channels is expected to alter paracellular drug transport across the cornea and the conjunctiva by modulating either net water transport or intracellular levels of Ca2+. In general, epithelia with CL channels secrete CL along... 

Secretory epithelia control transport of water and solutes from the subluminal compartment (blood) into the lumen or body exterior. At present, there is no single unifying model for transepithelial fluid or water transport. In some epithelia, transcellular routes of fluid transport via water channels may predominate [88a], However, in other types of epithelia, such as the cervical-vaginal epithelia, transport of fluids usually occurs via the paracellular route [1, 14], In the latter, movement of fluid can be driven by three main mechanisms (Figure 15.1C) ... 

An example of a transparent PEMFC was presented by Spemjak, Prasad, and Advani [87], who used a 10 cm transparent fuel cell to investigate different cathode DL materials (with and without MPLs) influence on water management. The FF channels had a single-path serpentine design with rectangular channel cross sections 1 mm deep and 0.8 mm wide. In these researchers study, the analyzed images corresponded to those in the lower section of the cathode s active area (closest to the outlet) because most of the water droplets were observed in this area away from the inlet. To observe how different DLs affected the water transport in the anode, this side was also visualized (see Section 4.3.3.2). 

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. 

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. 

The simple water charmel models can explain the ionomer peak and the small-angle upturn in the scattering data of fhe unoriented samples as well as of the oriented films. Interestingly, the helical structure of backbone segments is responsible for fhe sfabilify of fhe long cylindrical charmels. The self-diffusion behavior of wafer and protons in Nation is well described by the water channel model. The existence of parallel wide channels af high wafer uptake favors large hydrodynamic confributions to electro-osmotic water transport and hydraulic permeation. 

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... 

As shown in Figure 16b, the 2-D rib models deal with how the existence of a solid rib affects fuel-cell performance. They do not examine the along-the-channel effects discussed above. Instead, the relevant dimensions deal with the physical reality that the gas channeFdiffusion media interfaces are not continuous. Instead, the ribs of the flow-channel plates break them. These 2-D models focus on the cathode side of the fuel-cell sandwich because oxygen and water transport there have a much more significant impact on performance. This is in contrast to the along-the-channel models that show that the underhumidification of and water transport to the anode are more important than those for the cathode. 

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... 

Figure 18. Profile of the net water transport coefficient through the membrane in the midchannel cross-section of a 36-channel fuel cell at 1/ceii = 0.65V and /avg = 0.91 A/cm. ...

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. 

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