Journal of Power Sources 163 (2006) 357–363 Effect of carbon loading in microporous layer on PEM fuel cell performance Sehkyu Park, Jong-Won Lee, Branko N. Popov ∗ Center for Electrochemical Engineering, Department of Chemical Engineering, University of South Carolina, Columbia, SC 29208, USA Received 10 July 2006; received in revised form 18 August 2006; accepted 15 September 2006 Available online 27 October 2006 Abstract The effect of carbon loading in the microporous layer on the PEM fuel cell performance was investigated using mercury porosimetry, electro- chemical polarization techniques and electrochemical impedance spectroscopy. When air was used as an oxidant, a maximum fuel cell performance was obtained for a carbon loading of 0.5 mg cm -2 . The ac-impedance properties were analyzed based on the thin film/flooded-agglomerate dynamics in the catalyst layer and the oxygen diffusion kinetics in the gas diffusion layer. The ac-impedance study indicated that the optimized microporous layer results in an effective water management (i.e. a balancing of water saturations in the catalyst layer and the gas diffusion layer), thereby improving the oxygen diffusion kinetics. © 2006 Elsevier B.V. All rights reserved. Keywords: Proton exchange membrane fuel cell; Gas diffusion layer; Microporous layer; Carbon loading; Water management 1. Introduction Proton exchange membrane fuel cells (PEMFCs) are promis- ing alternative power sources for automotive and stationary applications. The PEMFC should operate at a relatively high current density in order to achieve the highest power output. A gas diffusion layer (GDL) sandwiched between the catalyst layer and the flow field permits gas transport towards the cata- lyst layer from the flow field. The GDL is usually treated with hydrophobic agents to allow an effective removal of liquid water produced at the cathode catalyst [1,2]. The GDL typically has a dual-layer structure, as illustrated in Fig. 1 [3]. The first layer is a carbon-fiber cloth or paper which serves as a current collector and as a physical support for the electrode. The thinner microporous layer (MPL) consists of carbon black powder and a hydrophobic agent. It has been reported [4–8] that the MPL increases the catalyst utilization and the overall fuel cell performance depending on its structure. For instance, Lin and Nguyen [8] tested three different types of commercially available GDLs: bare SGL SIGRACET and Toray TGPH carbon-fiber papers and SGL SIGRACET carbon- ∗ Corresponding author. Tel.: +1 803 777 7314; fax: +1 803 777 8265. E-mail address: popov@engr.sc.edu (B.N. Popov). fiber papers coated with the MPL. They showed that higher fuel cell performance was obtained using the MPL-coated carbon- fiber paper when compared with the bare papers even at lower air stoichiometry. The authors supposed that the MPL facilitates the back diffusion of water from the cathode through the membrane to the anode. Extensive experimental works on the MPL have been per- formed by many researchers to investigate the effects of carbon powder type [9,10], PTFE content [11,12] and pore-forming agents [13,14] on the PEM fuel cell performance. Passalacqua et al. [10] prepared the MPLs by using different types of carbon blacks in order to clarify the effect of porous carbon structure on the fuel cell performance. They showed that the MPL pre- pared with Shawinigan acetylene black led to a better fuel cell performance when compared to the MPLs made using Asbury graphite 850, Mogul L and Vulcan XC-72. The observed perfor- mance improvement was attributed to higher pore volume and smaller pore size of acetylene black which facilitates the gas dif- fusion and also reduces the amount of water accumulation inside the MPL. Giorgi et al. [12] investigated how the PTFE content in the microporous layer affects the fuel cell performance. They suggested that 10 wt% PTFE as a binder in the microporous layer was required to avoid water flooding and to improve the gas transport. Kong et al. [13] made an attempt to modify the porous structure of the MPL by employing Li 2 CO 3 as a pore- 0378-7753/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2006.09.020