Microporous layer for water morphology control in PEMFC Jin Hyun Nam a , Kyu-Jin Lee b , Gi-Suk Hwang c , Charn-Jung Kim b , Massoud Kaviany c, * a School of Mechanical and Automotive Engineering, Kookmin University, Seoul 136-702, Republic of Korea b School of Mechanical and Aerospace Engineering, Seoul National University, Seoul 151-744, Republic of Korea c Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI 48109, USA article info Article history: Received 28 May 2008 Received in revised form 4 December 2008 Available online 11 February 2009 Keywords: Polymer electrolyte membrane fuel cell (PEMFC) Microporous layer (MPL) Water management Catalyst layer (CL) Catalyst effectiveness Gas diffusion layer (GDL) Liquid saturation distribution abstract We have used environmental scanning electron microscope to observe vapor condensation and liquid water morphology and breakthrough in porous layers of polymer electrolyte membrane fuel cell. These suggest presence of large droplets and high liquid saturation at interface of the catalyst layer (CL) and gas diffusion layer (GDL), due to jump in pore size. We develop a model for morphology of liquid phase across multiple porous layers by use of both continuum and breakthrough (percolation) treatments. Using the results of this model we show the liquid morphologies deteriorate the efficiency of electrochemical reac- tions in CL and increase the water saturation in GDL. Then we show that inserting a microporous layer between CL and GDL reduces both the droplet size and liquid saturation and improves the cell performance. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Fuel cells are promising energy conversion devices which di- rectly extract electricity from the chemical energy of fuels without resorting to combustion [1–4]. Polymer electrolyte membrane fuel cells (PEMFC) are regarded as clean and efficient power sources for portable, automobile, and residential applications. The low operat- ing temperature (around 70 °C) renders several advantages to PEMFC, enabling fast start-up and good transient characteristics (which are ideal for short-term, repeated operations). However, the low operating temperature also poses several problems, such as the requirement of costly noble metal catalysts for electrochem- ical reactions and the presence of cumbersome liquid water inside PEMFC. Water is essential for the operation of PEMFC. PEM, the electro- lytes of PEMFC, functions properly when sufficiently hydrated. Thus, fuel and oxidant-gas streams are generally supplied to PEM- FC after adequately humidified to avoid dry-out failure of PEM. However, when there is excessive water in PEMFC, the excess water condenses to form liquid which fills the pores in catalyst layer (CL) and gas diffusion layer (GDL), or even blocks gas channel (GC). This phenomenon is called flooding, and is observed in both the anode and the cathode sides of PEMFC. The cathode flooding is caused by continuous generation of water in the cathode CL, and is believed to significantly limit the oxygen diffusion in GDL (as well as the oxygen reduction reaction in CL). The anode flooding also occurs frequently, due to condensation, especially in the PEM- FC using pure hydrogen fuel, but has less impact on the cell perfor- mance because of the high diffusivity of hydrogen and fast hydrogen oxidation kinetics. On the other hand, the anode water is more difficult to purge and thus presents special challenges for the freeze start-up. These conflicting effects of water in PEMFC are: a higher water content reduces the ohmic loss because of higher ionic (proton) conductivity in PEM, but also increases the activation and concen- tration losses because of lower catalyst activity in CL and a lower mass transport rate in GDL. An operating condition that enhances the performance near the inlet of the GC may deteriorate the per- formance near the outlet. Therefore, the water control in PEMFC has focused on reducing the adverse impacts of liquid water, by devising better flow paths or by optimizing the electrode micro- structures. Serpentine and interdigitated flow patterns are the typ- ical examples of such design modifications to obtain better performances by relieving the channel flooding [5,6]. Likewise, microporous layers (MPL) have been successfully used between CL and GDL to reduce the negative effects of the electrode flooding [7–11]. 0017-9310/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2009.01.002 Abbreviations: C, cathode; CL, catalyst layer; GC, gas channel; GDL, gas diffusion layer; MPL, microporous layer; C–D  , C–D + , CL–GDL interface: CL side () and GDL side (+); C–M  , C–M + , CL–MPL interface: CL side () and MPL side (+); D–CH, GDL– GC interface: GDL side; M–D  , M–D + , MPL–GDL interface: MPL side () and GDL side (+). * Corresponding author. Tel.: +1 734 936 0402; fax: +1 734 647 3170. E-mail address: kaviany@umich.edu (M. Kaviany). International Journal of Heat and Mass Transfer 52 (2009) 2779–2791 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt