Understanding the Role of the Micro-Porous Layer on Fuel Cell Performance Using a Non-Isothermal, Two-Phase Model

Water management is a critical factor in improving fuel cell performance at high current densities [1]. Under dry conditions, keeping the ionomer phase in polymer electrolyte membrane (PEM) and catalyst layers (CLs) sufficiently hydrated is critical to maintaining high protonic conductivity and reducing the ohimc losses. When the cell is operating at high relative humidity, removing the excessive water generated in the electrodes is of importance in order to avoid liquid water accumulation and achieve high performance. An efficient membrane electrode assembly (MEA) design therefore requires an extensive understanding of water management in fuel cells. The use of a micro-porous layer (MPL) in fuel cells is known to improve water and heat management resulting in a better performance. Experimental evidence has shown that remarkable fuel cell performance improvements are still possible by modifying the MPL composition [2] and micro-structure, for instance, by milling holes [3]. In situ heat and water flux measurements conducted by Thomas et al. [4] showed that by inserting an MPL between CL and gas diffusion layer (GDL), the temperature in the electrodes increased by at least 1° C at high current density. The warmer electrode therefore, results in a higher evaporation which also facilitates the diffusive transport of water vapour by creating a higher concentration gradient. Based on experimental observations from the ex situ diffusive vapour flux and liquid permeation flux measurements as well as in situ electrochemical performance with varying MPLs, Owejan et al. [5] hypothesized the role of MPL is to prevent the condensed water in the GDL from forming a liquid film at the GDL/CL interface and creating an in-plane diffusive path for reactant. In their study, the thermal conductivity in porous layers demonstrated a significant impact on improving the performance by creating a higher temperature driven diffusive flux. In order to understand the effects of MPL on capillary-driven flow and phase change induced flow, a multi-dimensional, non-isothermal, two-phase numerical model is developed in OpenFCST [6]. The porous media transport properties for two-phase flow are estimated using a micro-scale mathematical pore size distribution model [1] which is capable of accounting for the layer mixed wettabilities and micro-structure. Experimental validation of two-phase flow models is rarely performed even though it is of importance. In this study, the electrochemical performance of an MEA with a SGL 24BA and SGL 24BC is measured in our laboratory at varying operating conditions and also predicted using the numerical two-phase model. Membrane water transport, water fluxes in liquid and vapour form at cathode boundary, and the phase change induced flow are analyzed for studies with and without an MPL. The performance analysis under hot/dry condition indicates that adding an MPL in the dry condition results in an excessive protonic transport loss in the membrane, especially at high current density. Under hot/wet condition, the additional heat preserved on the electrodes by adding the MPL leads to a substantial reduction in water accumulation. Simulation results at the cold/wet condition highlight that adding an MPL not only leads to a decrease in water accumulation in the electrode but also creates an in-plane diffusion pathway for gas transport in the cathode. The increase in temperature in the electrode also results in a decrease in relative humidity, especially at the anode. This leads to a higher membrane water content gradient between the fully saturated cathode and the anode which results in a higher back diffusion. A parametric study of MPL thermal conductivity suggests that the excessive water in the cathode can be removed as water vapour by decreasing the MPL thermal conductivity under fully humidified conditions. However, an extremely low MPL thermal conductivity can lead to a significant deterioration of performance even at high relative humidity due to the membrane dehydration. The paramteric study highlights the optimal MPL conducivity for our cell. An optimal MPL design requires a comprehensive water balance between membrane hydration and sufficient electrode water evaporation. References: [1] A. Z. Weber et al.,J. Electrochem. Soc., 2004, 151, A1715–A1727. [2] P. G. Stampino et al., Catalysis Today, 2009, 147, S30–S35. [3] R. Alink et al., Journal of Power Sources, 2013, 233, 358–368. [4] A. Thomas et al., International Journal of Hydrogen Energy, 2014, 39, 2649–2658. [5] J. P. Owejan et al., J. Electrochem. Soc., 2010, 157, B1456–B1464. [6] M. Secanell et al., ECS Transactions, 2014, 64, 655–680. Figure 1

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