Micro-Computed Tomography Reconstruction and Analysis of the Porous Transport Layer in Polymer Electrolyte Membrane Fuel Cells
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A procedure is presented to analyze select geometric and effective properties of the porous transport layer (PTL) of the polymer electrolyte membrane fuel cell (PEMFC) in com- pressed and uncompressed states using micro-computed X-ray tomography (Micro CT). A method of compression using a novel device design was employed to mimic the non-homogeneous compression conditions found in functioning fuel cells. The process also features open source image processing and CFD analysis through the use of software packages Fiji and OpenFOAM (proprietary software is also used such as Matlab). Tomographic images of a PTL sample in different compressive states are first analyzed by measuring local porosity values in the through-plane and both in- plane directions. The objective of this study was to develop a method for imaging the PTL structure to show directionality within its properties using relatively inexpensive and non-destructional means. Three different PTL types were tested, one without any additives, one with Polytetrafluoroethylene (PTFE) and one with PTFE and a microporous layer (MPL). Non-homogeneous porosity was shown to exist with the highest and least variable porosity values obtained from the in-plane direction that was in-line with the direction of fibres. Porosity values compared well with values obtained from the literature. The profile of the PTL with MPL added was unattainable using this procedure as the resolution of the Micro CT was too low to resolve its pore space. The next stage involved the effective properties analysis which included effective electronic conductivity and effective diffusivity. It was found that the through-plane values for the effective electronic conductivity study were higher than expected. The ratio between through-plane and in-plane was found to be much higher than expected from literature. Lack of sufficient resolution of fibre contacts has been shown to play a role in this discrepancy. These contact problems were shown not too affect measurements of diffusivity in the pore phase. The in-plane direction parallel to the direction of fibres was found to have the highest values of effective transport properties. Effective diffusivity ratios of between 0.1 and 0.37 were found to be reasonable with the limited experimental evidence found in literature. The it was found that the Bruggeman relation for calculating diffusivity and percolation theory by Tomadakis and Sotirchos over predicted the values for diffusion within the PTL and it is suggested that these theories are not suitable for predicting diffusivity for this material.