Methodology of the PEM FC Gas Diffusion Layer Permeability Determination and Its Description Related to the Fuel Cell Flow Field Design
Authors: Bouzek Karel , Prokop Martin , Drakselova Monika 
Published: July 2022

Hydrogen fuel cells represent an important part of the new energy scheme for low emission society. Development of the more efficient, durable and energy dense systems requires precise information on the numerous aspects, especially on the local distribution of physio-chemical quantities inside the fuel cells stacks. It is extremely difficult to obtain such information experimentally. Therefore, the mathematical modelling represents a feasible alternative. It offers an efficient and versatile way for the acquisition of required information. Accuracy of mathematical model, however, is primarily determined by the accuracy and reliability of the experimental data available. In this particular case, permeability of gas diffusion layer for reactants and products is of interest. Permeability values in porous environment are available experimentally. It is, however, important to keep in mind that permeability value is strongly dependent on the material porosity and thus on a degree of its compression inside the stack. Therefore, permeability value is varying with position over the electrode. Suitable approach, making possible to describe smoothly variation of the gas diffusion layer permeability with degree of its compression, is desirable. Traditional approach is to use corresponding semi-empirical equations. It is a purpose of this work to verify applicability of the Kozeny-Carman equation for this particular case and thus to fill in this gap in the current knowledge.
Prior the experimental validation of the Kozeny-Carman equation accuracy, methodology of obtaining sufficiently accurate permeability data needs to be developed. In the first step of this process, permeability cells were designed, characterized by the homogeneous gas flow distribution in the channel, used for positioning of the gas diffusion layer sample, to be characterized. A special attention was paid to the absence of any turbulence domains close to the pressure reading ports, which could cause potential error in the observed pressure values. The figure provided documents results of the cell geometry optimization. Mathematical model calculated gas stream lines shown document fulfilling above stated conditions and thus ensuring reliability of the experimental data obtained. The cell lid determined the channel height itself. A series of lids was produced, allowing study of gas diffusion layers permeability at the different degrees of compression, i.e. porosities. The channel depth accuracy was required to be within tolerance of 2 mm. Pressure drop on the channel was determined by varying the streaming gas flow rate. Hydrogen and nitrogen (simulating air) were used as a test gasses. Permeability of gas diffusion layers were determined based on pressure drops using Darcy’s law. This set of data allowed comparing experimental data with Kozeny-Carman equation behavior. The agreement obtained was sufficient to justify the usage of this type of semi-empirical equation for the flow field mathematical modelling. Corresponding conclusion was in the last step of the study verified by means of the model serpentine flow field attached at the gas diffusion layer at the different degrees of compression. Experimental data obtained have shown very good agreement with the model calculations. It can thus be concluded, that Kozeny-Carman provides sufficient approximation of the gas diffusion permeability values to be used in the mathematical models for design of fuel cells and fuel cells stacks for geometry optimization.
The work was supported from European Regional Development Fund-Project "Fuel Cells with Low Platinum Content" (No. CZ.02.1.01/0.0/0.0/16_025/0007414).

Figure 1