Journal of Power Sources 207 (2012) 91–100 Contents lists available at SciVerse ScienceDirect Journal of Power Sources jo ur nal homep age: www.elsevier.com/locate/jpowsour Effect of microporous layer on MacMullin number of carbon paper gas diffusion layer Michael J. Martínez-Rodríguez a,1 , Tong Cui a , Sirivatch Shimpalee a,∗ , Supapan Seraphin b , Binh Duong b , J.W. Van Zee a a Department of Chemical Engineering, University of South Carolina, Columbia, SC 29208, USA b Department of Materials Science and Engineering, University of Arizona, Tucson, AZ 85721, USA a r t i c l e i n f o Article history: Received 15 November 2011 Received in revised form 19 January 2012 Accepted 21 January 2012 Available online 9 February 2012 Keywords: MacMullin number Effective diffusion Pore size distribution Gas diffusion layer Fuel cells a b s t r a c t The effect of the microporous layer (MPL) and wet proofing on the MacMullin number has been eval- uated for a custom series of Toray TGP-H-060 carbon paper gas diffusion layer (GDL). Complementary characterizations for these GDLs were performed by using scanning electron microscopy (SEM) images, pore size distribution (PSD) and fuel cell performance. The GDLs were customized by the addition of a microporous layer (MPL) and the treatment of, either or both, the substrate and MPL with 10% and 40% hydrophobic agent. SEM images correlated very well with the data shown for PSD. Distinction between the substrate layer and the MPL were clearly shown as two different slopes in the integral distribution and two different peaks in the differential distribution. The MacMullin number increased with increase in wet proofing but decreased with the addition of the MPL. The MacMullin number is a key parameter that contains the missing information for the path length in GDLs, which is generally approximated with the Bruggeman expression. The results provided an overview for the interpretation of the combined effect of the substrate and MPL properties as well as the cell operating conditions. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Development of low-cost gas diffusion layers (GDL) with tai- lored properties is a key factor in the optimization of fuel cell performance. The functionality of the GDL inside the fuel cell relies in maintaining a balance between its mechanical, electrical, thermal and transport properties. A GDL must be able to pro- vide mechanical support to the membrane electrode assembly (MEA), conduct electrons between the catalyst layer (CL) and bipo- lar plates, allow heat removal and maintain uniform temperature, and transport reactant gases to the CL and liquid water away from the CL. As fuel cells become attractive as power sources for automo- tive, stationary and portable applications a large amount of effort has been dedicated on research to understand and improve GDL properties [1]. Research has demonstrated that the addition of a thin layer to the surface of the GDL results in a positive effect in the performance of the fuel cell [2–4]. Therefore, most GDLs for commercial poly- mer electrolyte membrane fuel cell (PEMFC) generally consist of a ∗ Corresponding author. Tel.: +1 803 576 6140; fax: +1 803 777 8265. E-mail address: shimpale@cec.sc.edu (S. Shimpalee). 1 Present address: Savannah River National Laboratory, Aiken, South Carolina 29808, USA. dual-layer carbon-based porous media as shown in Fig. 1. The layer adjacent to the flow field channels is a carbon substrate macro- porous layer. The carbon substrate can be either a carbon cloth or a carbon paper and can be impregnated with a hydrophobic agent, such as polytetrafluoroethylene (PTFE). The layer adjacent to the catalyst layer consists of a thinner microporous layer (MPL) typically made of carbon black powder with a hydrophobic agent as well. The purpose of the microporous layer is to minimize the contact resistance between the macroporous layer and the cata- lyst layer, limit the loss of catalyst to the GDL interior and help to prevent water accumulation within the pore volume of the micro- porous layer thus gases can freely contact the catalyst sites. Due to the importance of the MPL into balancing the transport of reactant gases and the removal of liquid water many recent studies have focused on investigating the properties of the MPL. Accord- ingly, attention has been given to the effect of MPL arrangement in terms of including the MPL in the cathode alone, in both anode and cathode, and without MPL [5], including a double MPL [6], prepa- ration method [7,8], composition [9,10], pore structure [11,12], thickness [13], and wettability, by looking at either hydrophobic [14,15] or hydrophilic [16,17] components. Experimental mea- surements have been performed for GDLs with MPL that include through – plane permeability [18], pressure vs saturation curves [19], and dynamic drainage [20]. Also, significant modeling efforts have been devoted to understanding the interaction of the key 0378-7753/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2012.01.132