Degradation of gas diffusion layers through repetitive freezing Yongtaek Lee a,1 , Bosung Kim a , Yongchan Kim a,⇑ , Xianguo Li b a Department of Mechanical Engineering, Korea University, Seoul 136-713, Republic of Korea b Department of Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 article info Article history: Received 5 May 2011 Received in revised form 5 July 2011 Accepted 9 July 2011 Available online 3 August 2011 Keywords: Gas diffusion layer Microporous layer Freezing Contact angle Porosimetry Permeability abstract This work investigates the degradation of an individual gas diffusion layer (GDL) by repeated freezing cycles. The pore size distribution, gas permeability, surface structure, and contact angle on the surface of the GDL were measured in four different types of GDL: SGL paper with a microporous layer (MPL); SGL paper with 5 wt% of polytetrafluoroethylene (PTFE) loading; Toray paper without PTFE loading; and Toray paper with 20 wt% of PTFE loading. After repeated freezing cycles, the porosity of the GDL with- out PTFE was reduced by 27.2% due to the volumetric expansion of the GDL. The peak of the log differ- ential intrusion moved toward a smaller pore diameter slightly because of the repeated freezing process. The crack of the MPL increased in its width and length after repeated freezing cycles. The through-plane gas permeability of the GDL with the MPL doubled after repeated freezing cycles due to the growth of the crack in the MPL, but was very small for the GDLs with Toray paper. Besides, the GDLs with PTFE loading showed a relatively larger decrease in the contact angle on the surface than the GDL without PTFE loading due to the separation of PTFE from the carbon fiber during the repeated freezing process. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Proton exchange membrane fuel cells (PEMFCs) have been spot- lighted as prospective renewable energy sources because of several advantages: high efficiency, no pollution, and promptness in recharging. Currently, the major issues regarding PEMFCs are reli- ability and stable performance achievement at various extreme operating conditions. Since water is indispensable to the PEMFC, the freezing of water in very cold conditions could cause some major problems. Numerous studies have been conducted on the characteristics of PEMFCs after repetitive freezing and cold starting [1–12]. Cho et al. [1] found significant performance degradation after four freezing cycles. Lee et al. [2] examined the degradation of a PEMFC for up to 47 freezing cycles by using two different kinds of gas diffusion layer (GDL) and expressed the effects of the micro- porous layer (MPL) in freezing conditions. A few investigations have revealed that the degradation caused by freezing can be min- imized by a purging process [3–5]. The performance degradation during freezing cycles was also studied by investigating the oper- ating characteristics of each component that composes the PEMFC stack [6–9]. Kim and Mench [6] and Kim et al. [7] observed the physical degradation of a catalyst layer using scanning electron microscopy (SEM). They revealed that damage in the catalyst layer cannot be completely prevented irrespective of whether or not the catalyst layer has virgin cracks or is supported by gas diffusion media. Yan et al. [8] compared SEM images for the components (membrane, catalyst layer, and GDL) under three operating conditions: (a) new components, (b) PEMFC operating at room temperature, and (c) PEMFC operating at 15 °C. As shown in Fig. 1, the water that remains in the membrane, catalyst layer, GDL, and gas-flow channel causes mechanical damage to each component and interface due to volumetric expansion during the freezing process [8]. The GDL plays an important role in the trans- port of water, and a large amount of water remains in the pores of the GDL after the operation ceases. Therefore, the influence of ice formation on the degradation of GDLs deserves to be investigated comprehensively from the viewpoint of the durability and the development of control systems for PEMFCs. The GDL constitutes a porous medium in which reactant gas diffuses from the flow-channel to the catalyst layer and simulta- neously water is transported from the catalyst layer to the flow- channel. It is essential to know the morphological characteristics, such as the porosity and pore-size distribution, and the mass transport phenomena of the GDL to analyze the materials moving through porous networks in the GDL. Cindrella et al. [13] reviewed the literature concerning the porosity and permeability of GDLs [14–21]. Williams et al. [14] measured the porosity of one in-house and five commercial GDLs and developed a relationship with respect to the permeability and limiting current. Wang et al. [15] 0306-2619/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2011.07.011 ⇑ Corresponding author. Address: Department of Mechanical Engineering, Korea University, Anam-dong, Seongbuk-gu, Seoul 136-713, Republic of Korea. Tel.: +82 2 3290 3366; fax: +82 2 921 5439. E-mail address: yongckim@korea.ac.kr (Y. Kim). 1 Present address: Department of Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1. Applied Energy 88 (2011) 5111–5119 Contents lists available at ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy