Gas Diffusivity of PEM Fuel Cells Gas Diffusion Layer: Experimental Measurement and Modeling
Authors: Mohammadi Amin , Bahrami Majid , McCague Claire , Alvar Esmaeil Navaei , Pascal Kim 
Published: December 2024

The rate of gas transport through the PEMFC gas diffusion layers (GDL) is a function of GDL morphology, such as thickness, porosity, fibre diameters, tortuosity, etc.. Numerical methods can capture the intricacies of the GDL pore structure. However, the results obtained are specific to the analyzed structure and cannot be generalized to other GDLs due to inherent structural variations among different GDL types. By using a simplified representation of the complex pore structure, analytical methods offer a systematic approach to isolating the impact of each GDL structural property on GDL diffusivity. This understanding can be leveraged to adjust GDL design levers for an optimal GDL design. One of the key GDL structural properties is tortuosity which has a crucial impact on the transport properties of porous media. The literature [1,2,3], however, is unclear on how to relate tortuosity to gas diffusivity, and confusion arises when the two parameters, tortuosity, and tortuosity factor (defined as tortuosity squared) are used interchangeably. Besides tortuosity, varying cross-section areas along the path of gas transport affect the diffusivity of porous media. This parameter is referred to as the constrictivity factor [4].
In this study, an analytical method is employed to demonstrate the relationship between diffusivity and tortuosity, resolving ambiguity on this matter. Additionally, a model is proposed to determine the constrictivity factor of GDLs. With the constrictivity factor established, the relative diffusivity of the GDL can be estimated through a closed-form analytical solution. The developed model is validated with data collected using our symmetrical modified Loschmidt cell. The gas diffusivity of several GDL samples has been measured, including TGP-060 with different PTFE contents (5, 10, and 20 wt%) which had relative diffusivities of 0.31±0.02, 0.29±0.02 and 0.26±0.01. In addition, experimental data available in the literature on TGP-060, TGP-090 and TGP-120 are also used for the validation of the developed model. The results show that the model estimates diffusivity of the sample with a maximum relative difference of 15%.
[1] Flückiger, Reto, Stefan A. Freunberger, Denis Kramer, Alexander Wokaun, Günther G. Scherer, and Felix N. Büchi. "Anisotropic, effective diffusivity of porous gas diffusion layer materials for PEFC." Electrochimica Acta 54, no. 2 (2008): 551-559.
[2] Inoue, Gen, Takashi Yoshimoto, Yosuke Matsukuma, and Masaki Minemoto. "Development of simulated gas diffusion layer of polymer electrolyte fuel cells and evaluation of its structure." Journal of Power Sources 175, no. 1 (2008): 145-158.
[3] Tjaden, Bernhard, Dan JL Brett, and Paul R. Shearing. "Tortuosity in electrochemical devices: a review of calculation approaches." International Materials Reviews 63, no. 2 (2018): 47-67.
[4] Van Brakel, Jaap, and P. M. Heertjes. "Analysis of diffusion in macroporous media in terms of a porosity, a tortuosity and a constrictivity factor." International Journal of Heat and Mass Transfer 17, no. 9 (1974): 1093-1103.

Figure 1