In this work, we investigate the distribution of phosphoric acid in gas diffusion electrodes (GDE). The study combines imaging techniques (micro-computed tomography or µCT, FIB-SEM) with modeling (Pore Network Modeling or PNM). To validate the application of PNM, phosphoric acid was injected into different GDEs and the invasion pattern was investigated. The samples were mounted in a sample holder emulating a rib and channel structure, and they were inspected using µCT. The reconstructed 3D image stacks were subsequently segmented into void-, carbon fiber-, MPL-, and CL-voxels1, and the segmented images were then reconstructed into a 3D pore network2. This pore network contains the real shapes of the pores and throats, and therefore the flow characteristics are still representative of the real sample3. The quality of the extracted network is solely limited by the quality of the µCT images. Invasion percolation simulations were performed using the open source package OpenPNM to predict the distribution of phosphoric acid inside GDEs, showing the effect of network parameters on the invasion pattern.
We found that the experimentally observed invasion patterns agree very well with the pore network simulation and that invasion percolation is a valid approach to investigate phosphoric acid distributions in GDEs4. The presence of an MPL restricts the intrusion of phosphoric acid into the carbon fiber substrate and significantly reduces the saturation. Figure 1 shows the simulated phosphoric acid invasion pattern into a densely woven carbon fiber layer. The pathway is tortuous and exhibits capillary fingering, and the acid is creeping alongside the bottom of the rib of the sample holder until it emerges into the channel. These observations are all consistent with experiments.
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2. Hinebaugh, J. & Bazylak, A. Pore Network Modeling to Study the Effects of Common Assumptions in GDL Liquid Water Invasion Studies. ESFuelCell 201291466 (2012).
3. Gostick, J. et al. OpenPNM: A Pore Network Modeling Package. Comput. Sci. Eng. 18,60–74 (2016).
4. Chevalier, S. et al. Role of the microporous layer in the redistribution of phosphoric acid in high temperature PEM fuel cell gas diffusion electrodes. Electrochimica acta, 212, 187-194. (2016).