Through-plane permeability

One of the main parameters that would improve the overall performance of a fuel cell is better mass transport of reactants through the diffusion layer toward the active catalyst zones. In order to quantify and characterize how well the gas mass transport is in a specific DL material and design, it is important to measure the in-plane and through-plane permeabilities. Most of the published permeability results report the viscous permeability... 

Although in-plane permeability is critical in order to understand in detail the transport mechanisms of fluids inside diffusion layers, it has not been as commonly used (and measured) as through-plane permeability. The following are a few examples of how in-plane permeability can be determined... 

Through-plane permeability is usually one of the most common parameters given by manufacturers for carbon fiber papers and carbon cloths, even though it is often not specified as through-plane permeability. It is important to note that commercial instruments, such as permeameters and Gurley method instruments, are used in the fuel cell industry to measure this permeability [197,218]. 

Experimental apparatus to measure through-plane permeability. (Reprinted from V. Gurau... 

Other methods to study the through-plane permeabilities were presented by Chang et al. [183] and Williams et al. [90]. However, these methods only determined the viscous permeability coefficient with Darcy s law and did not take into account the inertial component of the permeability. 

Williams et al. [90] reported that the through-plane permeability was 0.8-3.1 X 10 m for baer carbon paper and that the addition of microporous layers decreased this value by approximately two orders of magnitude. Prasanna et al. [91] reported a permeability of 1-8 x 10 m for carbon paper, which decreased significantly as the Teflon loading was increased. 

Product Name Thickness (mm) Weight (g/m ) Bulk Density (g/cm ) Porosity (%) Tensile Strength (MPa) Stiffness (Taber) Through-Plane Air Permeability (sec/100 cc) In-Plane Air Permeability (sec/100 cc) Compressibility (%) Through- Plane Resistivity (mohm cm ) In-Plane Resistivity (mohm cm) Comments ... 

Product Thickness Weight Bulk Density Porosity Tensile Strength Stiffness Through-Plane Air Permeability In-Plane Air Permeability Compressibility Through- plane Resistivity In-Plane Resistivity... 

Gostick et al. [212] designed an apparatus in which the in-plane permeability was measured as a function of the DL thickness with different compression pressures (see Figure 4.25). The DL specimen was compressed between plates, which had spacers of different thicknesses in order to control the total thickness in each test. The sample was located between two grooves or channels, one of which corresponded to the inlet of the air and the other to the air outlet. Therefore, the air had to flow in plane through the sample in order to... 

Feser et al. [214] used a radial flow apparatus to determine the viscous in-plane permeability of differenf DLs af various levels of compression (see Figure 4.26). A stack of round-shaped samples, wifh each layer of material separated with a brass shim, was placed inside two plates. Thicker shim stock was also used in order to control the total thickness of the stack of samples. Compressed air entered fhe apparafus fhrough the upper plate and was forced through the samples in the in-plane direction. After this, the air left the system and flowed through a pressure gage and a rotameter in order to measure the pressure drop and the air flow rate. The whole apparatus was compressed using a hydraulic press for each compression pressure, 10 different flow rates were used. 

Chang et al. [183] presented a similar design in which two discs (with orifices in the middle) were used to compress the sample material. Pressurized air (without any moisture) was then passed through the orifices of fhe discs toward the sample DL, which then flowed peripherally to the atmosphere. The two discs were compressed in order to see how the permeability of the DL changed as a function of the clamping pressure. The permeability coefficient was solved using Darcy s law thus, only the viscous in-plane permeability was taken into account. Other, similar techniques can be found in the literature [215-217]. 

In order to determine the viscous and inert through-plane gas permeabilities of diffusion layers at varied compression pressures, Gostick et al. [212] designed a simple method in which a circular specimen was sandwiched between two plates that have orifices in the middle, aligned with the location of the material. Pressurized air entered the upper plate, flowed through the DL, and exited the lower plate. The pressure drop between the inlet and the outlet was recorded for at least ten different flow rates for each sample. The inert and viscous permeabilities were then determined by fitting the Forchheimer equation to the pressure drop versus flow rate data as explained earlier. 

Park, Lee, and Popov [136] used a similar technique to determine the liquid permeation in different diffusion layers. Feser, Prasad, and Advani [214] used the same method explained in Section 4.4.S.2 to measure the liquid in-plane permeability of DLs. When water was used, flow was forced from a pressurized tank (0-200 kPa) through the apparatus (and the sample), and the outlet water was then collected with a graduated cylinder. 

J. T. Gostick, M. W. Fowler, M. D. Pritzker, M. A. loannidis, and L. M. Behra. In-plane and through-plane gas permeability of carbon fiber electrode backing layers. Journal of Power Sources 162 (2006) 228-238. 

Permeability In-plane Through-plane In-plane/Through-plane... 

A instead of a linear increase in solids, we actually start to remove solids under these conditions. In addition to solids, we also measure through-plane conductivity, permeability, and numerous other characteristics as a function of the position in the gradient. The correlation of these characteristics to the formulation morphology is the goal of these experiments. 

Product Thickness name (mm) Electrical resistivity (mf2 cm) Through plane In plane Thermal conductivity (W/mK) Through In plane plane (room (room In plane temp) temp) (100 °C) Gas permeability (ml mm/ (cm -hr-mmAq) Porosity (%) Bulk density (g/cm ) Surface roughness (pm) Coefficient of thermal expansion (in plane) (25-100 °C) (xl0- /°C) Flexural strength (MPa) Flexural modulus (GPa) Tensile strength (kgf/cm)... 

For GDL carbon materials, Gaston et al. [66] showed using MIP on hand-made woven fabrics that a relationship exists between the amount of macropores and the through-plane gas permeability. They also showed that woven materials can exhibit high in-plane permeability, which means also a low pressure drop. For a non-woven carbon paper, Hiramitsu et al. [67] showed that even small amounts of hydrophobized carbon black introduced in the GDL reduce flooding significantly. 

T.A.L (2011) Effect of weave tightness and stmcture on the in-plane and through-plane air permeability of woven carbon fibers for gas diffusion layers. /. 

For composite parts which are relatively thin and saturated through the thickness, one can reasonably neglect fluid-flow in the thickness direction, particularly when the transverse permeability is very much smaller than the in-plane permeability. For this case, one can derive the fluid mass conservation equation in general three-dimensional form and then specialise it to... 

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