Fluid Flow in the Gas Diffusion Layer Using Computational Fluid Dynamics and Microscopy Techniques

Abstract

The utilization of a proton exchange membrane fuel cell (PEMFC) as an energy provider using hydrogen as a fuel has increased drastically. The reasons are the current limitation of fossil fuel-based devices, pollutants-free, high efficiency, and zero-carbon emission. The life-cycle assessment results of this type of fuel cell have also indicated the lowest contribution to global warming, human health, and resource scarcity in comparison to other types of fuel cells. In this regard, improving the performance of PEMFC is of importance.As an important component of the PEMFC, the gas diffusion layer (GDL) transports the gas reactants to the catalyst layer with the least electrical resistance. The GDL is often made of carbon fibers and should have a surface with good electrical contact and hydrophobic properties to facilitate the water removal. Remaining water in the GDL will result in difficulties during the cold-start and enhances the degradation of this layer. It is believed that the water removal of the GDL has a direct relationship with the capillary pressure, which is strongly linked to the GDL’s microstructure and wettability. However, further investigations can be done once a comprehensive simulation model is developed to monitor the changes in the GDL liquid removal by the effective parameters.In this regard, the Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM) has been used to perform three-dimensional cross-section imaging of the GDL. The GDL samples are provided by Freudenberg in which the Microporous layer (MPL) is impregnated into the GDL. The average porosity of this sample is in the range of 75% while having the in-plane gas permeability of 2.4 under a compressive stress of 1 MPa. The utilized resin for FIB-SEM imaging is a mixture of Epoxy embedding medium (diglycidyl ether of bisphenol – A) with two different hardeners DDSA (2-Dodecenylsuccinic anhydride) and MNA (Methylnadic anhydride), which will be mixed with DPM – 30 [2,4,6 – Tris(dimethylaminomethyl)phenol] as the accelerator, all provided by Sigma Aldrich. After the preparation of the resin, 0.4 gr of Cobalt (II) acetylacetonate nanoparticles (supported by Sigma Aldrich) were added to 7.14 ml of the resin to improve the contrast of the images. The samples were impregnated with the resin under vacuum to be pressurized (3 MPa) for 20 minutes, and afterward, heated in an oven at 60℃ for 12 hours. The surfaces for analyses were cut by a diamond wire, polished by abrasive plates down to 0.1 and gold coated (20nm).FIB-SEM acquisition consisted of the polishing of cross-sections with a focused ion beam (LMIS Ga+ source) at 30 keV and 1 nA (Zeiss Crossbeam 540), followed by imaging with an electron beam at acceleration voltages of 1.0, 3.0 kV. Milling and imaging were performed at a working distance of 5.2 mm and stage tilt of 54°, i.e., the coincidence point of the electron and ion beams.Once the three-dimensional cross-sections of the GDL are obtained, the segmentation and reconstruction will be made to provide the needed geometry for fluid flow simulation and to calculate the microstructural properties. Fig. 1 shows the generated structure of the GDL after the segmentation and reconstruction of the cross-section images. The geometry will be then used to characterize the effective parameters of the thermal/water management of the PEMFC. This study can also be a valid reference for future computational fluid dynamic analyses in the GDL using the numerical modeling with the conservative equations or the Lattice Boltzmann modeling (LBM) with the kinetic and particle distribution equations. Keywords : Proton exchange membrane fuel cell (PEMFC); Gas Diffusion Layer (GDL); Focused Ion Beam- Scanning Electron Microscopy (FIB-SEM); Three-dimensional simulation; Lattice Boltzmann method (LBM) Figure 1