Characteristics of the Nafion ionomer-impregnated composite membrane for polymer electrolyte fuel cells Joongpyo Shim * , Heung Yong Ha, Seong-Ahn Hong, In-Hwan Oh Fuel Cell Research Center, Korea Institute of Science and Technology, Seoul 136-791, South Korea Received 19 November 2001; accepted 20 February 2002 Abstract Composite polymer electrolyte membranes were prepared by impregnating Nafion solution into the porous expanded PTFE (ePTFE) films as a substrate and their single cell performance, gas permeability, water flux, and water uptake were investigated. Although the nitrogen permeability of the composite membrane was higher than that of Nafion 112, there was not the serious cross-over of gases to diminish cell performance and it was seen that the cell performance could be improved by reduced thickness of the composite membrane. It was also seen that water uptake and water flux of the composite membrane were dependent on the Nafion loading amount on the substrate and, therefore, the thickness of the membrane. The water uptake as well as the water flux of the composite membrane increased as the Nafion loading amount increased and the increase rate of water uptake with temperature for the composite membranes was found to be larger than Nafion 112. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Polymer electrolyte fuel cell; Composite membrane; Nafion; Water uptake; Water flux 1. Introduction Fuel cells are very promising power sources for electric vehicles (EVs) to replace combustion engines using hydro- carbon fuel in the future [1–3]. The polymer electrolyte fuel cell (PEFC) is a strong candidate for EVs. The advantages of PEFC are a high power output convenience for fuel supply, long lifetime, etc. However, the relatively high cost of manufacture of PEFC stacks has restricted their commercia- lization for EVs [4]. The polymer electrolyte in PEFC plays an important role in cell performance, by increasing the power density (kW/l) based on the total volume of the stack, and therefore, reducing the overall stack manufacturing cost. If cheaper, thinner, and more durable polymer electrolyte can be made, it would be possible to improve the perfor- mance of polymer electrolyte fuel cells easily. Martin and coworkers prepared composite membranes by impregnating ionomer into porous PTFE membranes, to investigate the transport properties of the composite membranes [5,6]. Verbrugge et al. [7] reported the equilibrium and transport characteristics of the composite membranes for a fuel cell application. Unfortunately, however, they did not report any fuel cell performance for the composite membranes. Recently, W. L. Gore & Associates, Inc., and Los Alamos National Laboratories reported that the fuel cell performance was improved by composite membranes with a thickness of 5–20 mm [8]. Although some researchers have reported fuel cell per- formances, ionic conductivities, and mechanical properties of the composite membranes, there were few investigations on the variation of fuel cell performance and water proper- ties of the composite membranes related to the change in Nafion loading. In this work, therefore, composite mem- branes with the various thickness were prepared by impreg- nating Nafion solution as ionomer into the porous ePTFE films. The effect of hot-pressing and humidifying tempera- ture for the composite membrane on the cell performance were examined, and the variation of water uptake and flux along with the Nafion loading amount and, therefore, the membrane thickness were also investigated. 2. Experimental 2.1. Preparation of the composite membranes The ePTFE sheets with a thickness of 13 mm (Tetratex) were used as a substrate for the composite polymer electro- Journal of Power Sources 109 (2002) 412–417 * Corresponding author. Present address: Lawrence Berkeley National Laboratory, Environmental Energy Technologies Division, 1 Cyclotron Road, MS 70-108B, Berkeley, CA 94720, USA. Tel.: þ1-510-486-7257; fax: þ1-510-486-7303. E-mail addresses: jpshim@lbl.gov (J. Shim), oih@kist.re.kr (I.-H. Oh). 0378-7753/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0378-7753(02)00106-4