EFFECT OF PEM FUEL CELLS GAS DIFFUSION LAYER COMPRESSION ON IN-PLANE WATER PERCOLATION

Effects of gas diffusion layer (GDL) compression on inplane water percolation was studied with an ex-situ approach. Toray carbon paper was compressed at four different compressions and water percolation in stable displacement regime was visualized with a CCD camera. Images were analyzed to obtain water percolation evolution. Liquid water wetted area was normalized with GDL sample size and evaluated as a function of time during the injection. It was observed that the normalized wetted area increase faster in time as the GDL compression increases. This is due to smaller pore size at higher GDL compression which requires larger surface area to accommodate the same amount of injected water. In addition, liquid water percolation pressure was measured with a pressure transducer during experiments. Results suggested that the percolation pressure is higher for GDLs with greater compressions. Liquid water pressure increased at the beginning of experiments for GDLs with greater compressions and increased monotonically afterwards. As the GDL compression increase GDL pore size decrease which results in greater capillary pressure in the porous media. Therefore, liquid water pressure requires a greater pressure to percolate within the GDL at higher compressions. ∗contributed equally †Address all correspondence to this author, mehdi.mortazavi@wne.edu INTRODUCTION Proton exchange membrane (PEM) fuel cells are considered as a pollutant free energy system which also benefit from high efficiency and high volumetric power density. During the operation of a PEM fuel cell, hydrogen fuel is converted into useful power with water and heat as the two byproducts. While some portion of the produced water can be useful for the operation of the fuel cell as it hydrates the membrane, any excess amount of water within the fuel cell should be carefully removed from inside the cell. This is because the excess water can, for instance, fill open pores of the porous layers and block transport of reactant gases to the electrodes. The porous layers in PEM fuel cells include the gas diffusion layer (GDL) and the microporous layer (MPL). The membrane which transports protons from anode to cathode is also a porous layer. These layers are different in thicknesses and pore sizes. The GDL is the thickest porous layer in a PEM fuel cell with an average thickness between around 100 to 300 μm and an average pore size around 30 μm [1,2]. The GDL is used in PEM fuel cells in order to provide a uniform transport of reactants to the catalyst layer, to remove excess water from the membrane by providing pathways of water to the gas channel, to mechanically protect the membrane as a fragile thin layer, and to provide electrical conductivity between the electrodes and the current collectors. GDL flooding occurs when the produced 1 Copyright c © 2019 by ASME water and/or the condensed water from humidified reactants fills the open pores of the GDL and prevents a uniform distribution of reactants across electrodes. This eventually lowers the performance of the cell and should be carefully avoided [3–5]. Furthermore, the accumulation of excess water in GDL can lead to non-homogeneous current density [6], ineffective heat removal, and membrane swelling [7]. In addition, the excess water may also result in the delamination of fuel cell components during the thermal cycling associated with freeze/thaw processes [8, 9]. An appropriate design of the GDL has been reported to have a significant contribution on proper water balance within the cell [10]. It has been reported that the accumulated liquid water within the GDL emerges from its surface at some preferential locations [11]. Depending on the water production rate and gas flow rate within the flow channel this emerged water droplet can be removed from the surface of the GDL by different mechanisms [12]. For water production rate greater than water removal rate a water lens may form within the gas channel. This lens can grow in size until it completely clogs the flow channel and causes channel flooding. Channel flooding, similar to GDL flooding, has been reported to lower the performance of the cell [5, 6, 13]. Emergence of liquid water in PEM fuel cell flow channels causes liquid-gas two-phase flow within the flow channels which has been investigated extensively [14–18]. PEM fuel cells are prone to flooding at high current density and/or low flow stoichiometry [19]. Water transport within and on the surface of the GDL has been extensively investigated in literature [2,11,20–40]. Different techniques such as direct optical visualization [19,41–43], neutron imaging [44–46], and nuclear magnetic resonance imaging [47, 48] have been employed. Litster et al. [28] used a novel fluorescence microscopy technique to visualize the transport of liquid water within the PEM fuel cell porous media and suggested that water transport is dominated by fingering and channel. This was in contrast to the earlier model that proposed water transports by converging capillary [49]. The nonuniform water distribution in GDL was also reported by Tsushima et al. [48] based on magnetic resonance imaging and by Satija et al. [50] who employed neutron imaging in their study. Drainage and phase diagram in the porous structure of the PEM fuel cell is investigated by Medici and Allen [51]. The drainage phenomenon refers to the displacement of a wetting fluid by the injection of a nonwetting fluid. In GDLs, for instance, liquid water is the nonwetting fluid while air in GDL pores is the wetting fluid which is displaced as liquid water percolates. In general, three types of fluid flow may occur when drainage occurs in a porous media. These three types of fluid flow depend on the injection rate of the nonwetting fluid as well as the viscosity ratio of two fluids. These three types of fluid flow are (i) viscous fingering, (ii) capillary fingering, and (iii) stable displacement. Viscous fingering occurs when the injected fluid has lower viscosity than the displaced fluid. In this type of drainage mechanism the injected fluid permeates irregularly through the porous media and causes multiple conduits or fingers. Capillary fingering and stable displacement occur when the injected fluid has a viscosity greater than the viscosity of the displaced fluid. However, capillary fingering occurs for low injection flow rates while stable displacement is characterized at higher injection rates. In capillary fingering, the injected fluid generates irregular conduits within the porous media while in stable displacement the injected fluid permeates evenly through the porous media. Due to even permeation of the injected fluid, no fingers form in the stable displacement flow mechanism. These three types of flow were presented on a phase diagram proposed by Lenormand [52, 53]. This diagram is known as the Drainage Phase Diagram and is shown in Figure 1. in Table 1 indicate that viscous effects are more dominant than inertia effects. Fig. 3 shows the range of water flow rates in the current study overlaid on the drainage phase diagram.