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Transport in Proton Exchange Membranes for Fuel Cell Applications

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Transport in Proton Exchange Membranes for Fuel Cell Applications ( transport-proton-exchange-membranes-fuel-cell-applications )

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materials Article Transport in Proton Exchange Membranes for Fuel Cell Applications—A Systematic Non-Equilibrium Approach Angie L. Rangel-Cárdenas and Ger J. M Koper * Chemical Engineering, Delft University of Technology, 2629 HZ Delft, The Netherlands; A.L.RangelCardenas@tudelft.nl * Correspondence:G.J.M.Koper@tudelft.nl;Tel.:+31-15-278-8218 Academic Editor: Haolin Tang Received: 20 March 2017; Accepted: 19 May 2017; Published: 25 May 2017 Abstract: We hypothesize that the properties of proton-exchange membranes for fuel cell applications cannot be described unambiguously unless interface effects are taken into account. In order to prove this, we first develop a thermodynamically consistent description of the transport properties in the membranes, both for a homogeneous membrane and for a homogeneous membrane with two surface layers in contact with the electrodes or holder material. For each subsystem, homogeneous membrane, and the two surface layers, we limit ourselves to four parameters as the system as a whole is considered to be isothermal. We subsequently analyze the experimental results on some standard membranes that have appeared in the literature and analyze these using the two different descriptions. This analysis yields relatively well-defined values for the homogeneous membrane parameters and estimates for those of the surface layers and hence supports our hypothesis. As demonstrated, the method used here allows for a critical evaluation of the literature values. Moreover, it allows optimization of stacked transport systems such as proton-exchange membrane fuel cell units where interfacial layers, such as that between the catalyst and membrane, are taken into account systematically. Keywords: non-equilibrium; interfacial effects; transport properties; coupling effects; transport coefficient matrix; PEM fuel cell; proton conductivity; water permeability; hydrogen permeability; diffusivity; electro-osmotic drag PACS: 82.47.-a 1. Introduction Proton-conducting, polymer electrolyte membranes (PEM), play an important role in fuel cell applications as they serve not only the function of separation between the anode and cathode sides, but also act as a solid electrolyte allowing the transport of charge. The most common material used for these applications is NafionTM (DuPont, Wilmington, DE, USA), which consists of a tetrafluoroethylene (TFE) backbone and perfluoroalkyl ether (PFA) side chains terminated in sulfonic acid groups [1,2]. The combination of the hydrophobicity of the backbone with the hydrophilicity of the sulfonic acid functional group in one macromolecule confers NafionTM the properties necessary for this application. Given the evident importance of PEM membranes, a plethora of studies on their different properties has been done over the last few decades. However, it proves difficult to reach a consensus on their meaning as research aimed at understanding the underlying phenomena describing the behavior of membrane properties is performed in many different ways and under different conditions which often do not match the reality of a membrane in an operating fuel cell. Within this framework, and from Materials 2017, 10, 576; doi:10.3390/ma10060576 www.mdpi.com/journal/materials

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