Diffusion layer capillary pressure

As stated earlier, CEP and CC are the most common materials used in the PEM and direct liquid fuel cell due fo fheir nature, it is critical to understand how their porosity, pore size distribution, and capillary flow (and pressures) affecf fhe cell s overall performance. In addition to these properties, pressure drop measurements between the inlet and outlet streams of fuel cells are widely used as an indication of the liquid and gas transport within different diffusion layers. In fhis section, we will discuss the main methods used to measure and determine these properties that play such an important role in the improvement of bofh gas and liquid transport mechanisms. 

One issue wifh fhis mefhod is fhaf, for larger pores shielded by smaller ones, the corresponding pressure necessary for the liquid to intrude them corresponds to the entry pressure for fhe smaller pores thus, the volume for the larger pores is incorrectly attributed to smaller ones. In addition, the assumption that the contact angle of fhe nonwetting liquid is the same on all solid surfaces is nof completely correct because diffusion layers with different treatments have pores with different wetting properties. For example, two pores of fhe same size may have different PTFE content and the entry pressure necessary for fhe liquid to penetrate them will be different for each pore, thus affecting the overall results and the calculated capillary pressures [196]. 

The determination of the capillary pressure of a diffusion layer is critical, not only to have a better understanding of the mass transport mechanisms inside DLs but also to improve their design. In addition, the accuracy of mafhemafical models can be increased with the use of experimental data obtained through reliable techniques. Both Gostick et al. [196] and Kumbur et al. [199] described and used the MSP method in detail to determine the capillary pressures of differenf carbon fiber paper and carbon cloth DLs as a function of the nonwetting phase saturation. Please refer to the previous subsection and these publications for more information regarding how the capillary pressures were determined. 

Nguyen et al. [205] designed a volume displacement technique that was used to measure the capillary pressures for both hydrophobic and hydrophilic materials. One requirement for this method is that the sample material must have enough pore volume to be able to measure the respective displaced volume. Basically, while the sample is filled wifh water and then drained, the volume of water displaced is recorded. In order for the water to be drained from fhe material, it is vital to keep the liquid pressure higher than the gas pressure (i.e., pressure difference is key). Once the sample is saturated, the liquid pressure can be reduced slightly in order for the water to drain. From these tests, plots of capillary pressure versus water saturation corresponding to both imbibitions and drainages can be determined. A similar method was presented by Koido, Furusawa, and Moriyama [206], except they studied only the liquid water imbibition with different diffusion layers. 

J. T. Gostick, M. W. Fowler, M. A. loannidis, et al. Capillary pressure and hydrophilic porosity in gas diffusion layers for polymer electrolyte fuel cells. Journal of Power Sources 156 (2006) 375-387. 

J. D. Fairweather, P. Cheung, J. St-Pierre, and D. T. Schwartz. A microfluidic approach for measuring capillary pressure in PEMFC gas diffusion layers. Electrochemistry Communications 9 (2007) 2340-2345. 

Figure 8. Effective permeability as a function of capillary pressure for the different two-phase models for the gas-diffusion layer. The lines correspond to the models of (a) Berning and Djilali, (b) You and Liu and Mazumder and Cole, (c) Wang et al., (d) Weber and Newman, (e) Natarajan and Nguyen,and (f) Nam and Kaviany. ...

Divisek et al. presented a similar two-phase, two-dimensional model of DMFC. Two-phase flow and capillary effects in backing layers were considered using a quantitatively different but qualitatively similar function of capillary pressure vs liquid saturation. In practice, this capillary pressure function must be experimentally obtained for realistic DMFC backing materials in a methanol solution. Note that methanol in the anode solution significantly alters the interfacial tension characteristics. In addition, Divisek et al. developed detailed, multistep reaction models for both ORR and methanol oxidation as well as used the Stefan—Maxwell formulation for gas diffusion. Murgia et al. described a one-dimensional, two-phase, multicomponent steady-state model based on phenomenological transport equations for the catalyst layer, diffusion layer, and polymer membrane for a liquid-feed DMFC. 

The direct measurement of the various important parameters of foam films (thickness, capillary pressure, contact angles, etc.) makes it possible to derive information about the thermodynamic and kinetic properties of films (disjoining pressure isotherms, potential of the diffuse electric layer, molecular characteristics of foam bilayer, such as binding energy of molecules, linear tension, etc.). Along with it certain techniques employed to reveal foam film structure, being of particular importance for black foam films, are also considered here. These are FT-IR Spectroscopy, Fluorescence Recovery after Photobleaching (FRAP), X-ray reflectivity, measurement of the lateral electrical conductivity, measurement of foam film permeability, etc. 

When circular microscopic foam films (equilibrium or thinning) are studied it is necessary to know the pressure in the meniscus of the liquid being in contact with the film (see Fig. 2.2 A, B, C). In some cases it is very important to know the precise value of the capillary pressure, for example, in the calculation of low disjoining pressures n and the potential of the diffuse electric layer [Pg.50]

The analysis of the above techniques (Section 3.4.2.2) developed to estimate the conditions under which stable CBF and NBF exist, and reveals the equilibrium character of the transition between them and the particular features of the two types of black films. Furthermore the difference between the techniques of investigation as well as the difference between their intrinsic characteristics proves to be a valuable source of information of these thinnest liquid formations. The transition theory of microscopic films evidences the existence of metastable black films. Due to the deformation of the diffuse electric layer of the CBF, the electrostatic component of disjoining pressure 1 L( appears and when it becomes equal to the capillary pressure plus Ylvw, the film is in equilibrium (in the case of DLVO-forces). As it is shown in Section 3.4.2.3, CBF exhibit several deviations from the DLVO-theory. The experimentally obtained value of ntheoretically calculated. This is valid also for the experimental dependence CeiiCr(r). Systematic divergences from the DLVO-theory are found also for the h(CeiXr) dependence of NaDoS microscopic films at thickness less than 20 nm. 

Another dynamic factor affecting the rate of diffusion transfer, mentioned long ago by Gibbs [9], is the elasticity of the surfactant monolayers which decreases the capillary pressure in small bubbles during their compression and increases it in large bubbles during their expansion. This effect is most pronounced in bubbles whose adsorption layers contain insoluble surfactants. Analysis of the influence of this factor on diffusion transfer has been reported in [486], However, no experimental verification has been performed so far. 

An interesting subcase of the above is the small pore theory, sometimes known as Schmid theory [61. Unlike the preceding large pore theory, in which the assumption Is made that the diffuse layer thickness is small compared to the capillary diameter, here the excess of ions is distributed throughout the entire void volume [24], i.e. it is assumed that the double layers overlap. Hence, the electrical force can act more uniformly across the pore section, as is the case for an external hydraulic pressure. This condition is treated also by FUce and Whitehead [16], for example. The equation for the velocity, when xa 1, becomes... 

The nonwetted portions of the interface comprise small closed gas- or vapor-filled volumes bounded by the substrate and the adhesive. If these volumes are of such size that they contain a sufficiently large number of molecules in the vapor phase to exert nearly normal pressures, these will act against the capillary pressures and diminish the rate of wetting. It is conceivable that diffusion of these molecules into the adhesive layer could be rate-limiting under certain circumstances. 

The flow profile of the EOF has the form of a plug (Fig. 3.4). The flow velocity is identical over the whole capillary diameter, except for the slower moving diffuse layer close to the capillary wall. This homogeneous velocity distribution minimises band broadening and, thus, increases separation efficiency. A radically different situation occurs with the pressure driven flow used in liquid chromatography. Here, the flow profile is parabolic the flow velocities have a large distribution over the column diameter. Analytes in the middle flow considerably faster than analytes... 

The local current density, /, is the forcing term for the system, consuming oxygen, producing heat and water. However, in terms of the mass transport capacity of the gas diffusion layer, the molar and heat fluxes are small and the relaxation times are short. It is, therefore, reasonable to take these transport processes at steady-state and moreover to linearize the transport matrix about the channel values. We assume that the liquid water motion within the hydrophobic GDL is degenerate, that the water does not move unless its volume fraction reaches a percolation threshold, / >0. In particular, we assume that all pores are hydrophobic. In a hydrophobic media, the capillary pressure dominates the gas pressure yielding a liquid water distribution which is a linear function of a liquid water potential. 

A microfluidic approach for measuring capillary pressure in PEMFC gas diffusion layers. Electrochem Comm 9 2340-2345... 

Direct measurement of the capillary pressure characteristics of water-air-gas diffusion layer systems for PEM fuel cells. Electrochem Comm 10 1520-1523... 

The state-of-the-art gas diffusion media are hydrophobized to such an extent that they allow transport of liquid water, an important mechanism at near-saturated conditions, as well as of water vapor and reactant gases. An important role is played by the micro porous layer (MPL). Because of the presence of small hydrophobic pores, a substantial hquid water capillary pressure can be bruit up, enabling a good gradient in the chemical potential of water to drier sections [10]. The optimization of gas diffusion media and the application of the MPL have led to significant improvement of the fuel cell performance at saturated conditions, showing their critical role. 

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