Journal of Membrane Science 374 (2011) 49–58 Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci Interpretation of direct methanol fuel cell electrolyte properties using non-traditional length-scale parameters Yu Seung Kim a,∗ , Dae Sik Kim a , Michael D. Guiver b,c , Bryan S. Pivovar d a Sensors & Electrochemical Devices Group, Los Alamos National Laboratory, Los Alamos, NM 87545, USA b Institute for Chemical Process and Environmental Technology, National Research Council, Ottawa, Ontario K1A 0R6, Canada c WCU Department of Energy Engineering, Hanyang University, 17 Haendang-dong, Seoungdong-gu, Seoul 133-791, South Korea d Hydrogen Technologies & System Center, National Renewable Energy Laboratory, Golden, CO 80401, USA article info Article history: Received 17 November 2010 Accepted 4 March 2011 Available online 11 March 2011 Keywords: Direct methanol fuel cell Conductivity Methanol permeability Selectivity Nafion ® Polymer electrolyte membrane abstract Numerous sulfonated polymer electrolyte membranes (PEMs) have been developed for direct methanol fuel cells (DMFCs) during the last decade. An analysis for DMFC PEMs obtained from the literature data and structural information is presented based on non-traditional length scale parameters. The analysis pre- sented highlights specific differences in chemical composition between PEMs including perfluorinated sulfonic acids, hydrocarbon-based and polymers having specific interactions. Differences in cross-linked, homopolymer-like, random and multi-block polymer architectures are also discussed. The analysis pre- sented gives important insight into molecular design aspects of sulfonated PEMs for DMFCs. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Direct methanol fuel cells (DMFCs) are promising energy con- version devices for portable power sources. One of the major research thrusts for DMFC development is the investigation of novel high performance polymer electrolyte membranes (PEMs) having a combination of high proton conductivity and low methanol perme- ability [1]. Extensive efforts have been made in different directions aimed at PEMs with desirable properties for DMFC applications in the last decade [2]. Most literature data reported key properties of PEMs benchmarked against Nafion ® or a control hydrocarbon (HC)-based PEM. These reports generally focus on a single fam- ily of PEMs with very few reports that compare properties across a wide range of DMFC PEMs [3,4]. This work goes beyond prior reports by focusing on systematic analysis across the spectrum of PEMs. Unlike PEMs used in H 2 /air fuel cells, where a single key prop- erty (i.e. proton conductivity) has been the primary emphasis, PEMs for DMFCs have also been investigated with a strong emphasis on methanol permeability. Selectivity, which is defined as the ratio of proton conductivity to methanol permeability, has been sug- gested as a single key property of PEMs for DMFCs to compare ∗ Corresponding author. Tel.: +1 505 667 5782; fax: +1 505 665 4292. E-mail address: yskim@lanl.gov (Y.S. Kim). one polymer versus another [5]. A limitation of selectivity as a general gauge of DMFC PEMs is the requirement of a minimum con- ductivity necessary for effective operation of the DMFC, regardless of how low the methanol permeability is [2]. This minimum con- ductivity is related to the fact that the PEM itself has a minimum attainable thickness to achieve low cell resistance. Selectivity also fails to account for issues of mechanical/chemical robustness or the ability to be fabricated into high performance MEAs. There- fore, selectivity serves only as a screening tool for potential DMFC PEMs. Nonetheless, comparing properties of a wide range of PEMs is extremely beneficial since (i) it helps to better the understanding of structure–property relationships of PEMs, leading to acceler- ated development of advanced PEMs and (ii) it provides accurate information on the current state-of-the-art PEMs for DMFC devel- opers, facilitating faster comparative investigation and ultimately, commercialization. Recently, we reported a new length scale parameter, per- cent conducting volume (PCV) to better analyze and represent electrochemical properties of PEMs [6]. This proposed parame- ter arose from the belief that the use of mass-based parameters (most commonly used for these comparisons) are of limited value in evaluating transport properties across the wide range of PEMs, because PEMs may have significantly different densi- ties and transport properties that occur over length scales more appropriately represented by volume measurements. One of the biggest advantages of using PCV is that this parameter removes 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.03.004