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.
Highlights
Proton-conducting, polymer electrolyte membranes (PEM), play an important role in fuel cell applications as they serve the function of separation between the anode and cathode sides, and 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]
We present a transport coefficient matrix method (TCM) that allows a systematic approach to the problem and a literature survey of the properties considered meaningful for PEM fuel cell systems
Summary
Proton-conducting, polymer electrolyte membranes (PEM), play an important role in fuel cell applications as they serve the function of separation between the anode and cathode sides, and act as a solid electrolyte allowing the transport of charge. Kreuer et al [2] did impressive work at using this approach to describe membranes for fuel cell applications; they provided an extensive survey on methods from simulation to experimentation and transport mechanisms as understood to the date The formalism they propose is not entirely consistent as the number of independent driving forces has to equal the number of independent fluxes, which is not the case in their approach [2]. The transport coefficients defined above depend on the temperature and on the water content, apart from more material dependencies such as the porosity of the ionomer One generally assumes these to be uniform across the membrane volume of the ionomer whereas it is generally known not to be the case [2]. In the following we shall analyze the literature information on a standard ionomer membrane both on the bulk and on the interfacial coefficient values
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