|Mechanism of microporous layer for PEMFC explainedPublished on September 2010 by Catherine Lepiller, PhD, for Pragma Industries
After years of scientific debate due to contradictory results, the beneficial effect of the microporous layer (MPL), a thin porous layer made of carbon nanoparticles mixed with a hydrophobic agent that is usually inserted between the Pt/C catalyst layer (CL) and the carbon-based gas diffusion layer (GDL) in low temperature fuel cells, appears close to full elucidation now. Both experimental studies [1,2] and fuel cell models [3,4] have recently demonstrated that one main function of the microporous layer is to reduce the number of injection sites for liquid water from the catalyst layer to the gas diffusion layer, which in turn reduces the overall saturation. Less liquid water in the GDL means enhanced gas transport to and from the reaction sites at the CL, and leads to the improved mass transfer noticeable especially under high current conditions when adding an MPL to the fuel cell sandwich.
First, by simultaneous measurement of the capillary pressure and liquid water saturation, Gostick et al.  have demonstrated a drastic reduction of the saturation for commercial GDLs in the presence of an MPL, i.e., from ca. 25% to ca. 5%, and concluded that most pores in the GDL are rendered inaccessible to water by the smaller pore sizes of the MPL; hence much lower water saturation is reached.
Second, searchers at the Seoul National University  have used a quite original approach called “similarity model experiment”, through which the liquid water transport in hydrophobic GDLs was indirectly investigated: the drainage process was analyzed by replacing the porous layer with transparent hydrogel spheres initially saturated with liquid water, and then slowly injecting a dyed non-miscible fluid having equal density for visual inspection of the internal transport process. By controlling the dimensionless hydraulic parameters in the setup to be close to those existing in actual GDLs, they were able to show that the liquid water drainage process is indeed a capillary-driven process governed by an invasion-percolation mechanism with dendrite-like penetration (also called capillary fingering). Morphological similarities between the non-wetting fluid distribution and predictions by a pore-network model were evidenced as well.
These experimental findings are highly consistent with a simulation work performed at the French University of Toulouse , which strongly suggests using pore network models rather than continuum models for PEMFCs. In a strict point of view, continuum models are not valid for fuel cell materials because the pore size (~50 µm) and thickness (170-400 µm) of a GDL are separated by less than a decade. Therefore, the condition for applying the continuum approach to a porous media, i.e., having an elementary volume of much smaller size than the porous domain (at least 10 times) for computation, is not met in a GDL. These length scale effects can explain both the inability of continuum models in predicting fuel cell performances accurately and the liquid-vapor phase distribution being in the form of a fractal capillary fingering regime [2,3]. The computational study of the water invasion process as a function of the injection boundary condition has confirmed that liquid water injection through discrete points is much more favourable to gas transport than injection through the total surface since the resulting saturation will be lower everywhere along the GDL thickness. As observed experimentally, such condition of one-point injection is achieved in the presence of an MPL . Surface injection is instead related to a fuel cell configuration without MPL leading to the formation of large droplets and high water saturation level at the CL surface due to a large gap size .
Finally, the Korean team in  has further clarified the previous points and the exact role of the MPL . They have developed a model for morphology control of liquid phase across multiple porous layers and proposed two roles of MPL in this aspect, in strong agreement with other selected papers [1,2,3]: the MPL inserted between the CL and the GDL reduces the number and the size of interfacial droplets formed on CL surface. Both effects reduce the saturation level and the overall liquid breakthrough toward the GDL. Smaller pores in MPL (vs. GDL) at the catalyst interface prevent water droplets from growing very large and clogging the CL surface. This mechanism seems to be the way the MPL effectively alters the liquid water distribution in the fuel cell for better management!
 J.T. Gostick, M.A. Ioannidis, M.W. Fowler, M.D. Pritzker, Electrochem. Commun., 11 (2009), 576.
Pictures courtesy of the Seoul National University.
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