Simultaneous Measurement of Proton Conductivity and Methanol Permeability for the Development of Direct Methanol Fuel Cell Membranes

Much research effort has been directed toward developing alternative polymer membrane chemistries and strategies for direct methanol fuel cells (DMFCs) having high proton conductivity and low methanol permeability (1,2). Fundamentally, membrane and thence fuel cell system design is an optimization effort driven by maximization of proton conductivity while minimizing methanol transport through the membrane. Within certain limits, membrane conductivity (or more basically conductance) can be modified quite easily by, for instance, increasing polymer ion exchange capacity (IEC), decreasing membrane thickness, changing fabrication or post-processing steps or controlling membrane hydration level. While these strategies can increase membrane conductivity, they often also lead to increased methanol crossover (MXO) and its negative attendant effects (lost fuel efficiency, lost activity on the cathode, and the further complication of device operation resulting from the excess heat and water production). Increasing membrane conductivity is the obvious goal of any membrane development program, however, a clear understanding of the cost-benefit relationship to other important transport properties is equally as important. It is usual in the DMFC technical literature for these two important competing properties, i.e. MXO and proton conductivity, to be measured separately, often at different temperatures and presented separately as a function of IEC or water content (e.g. mass or volume percent, or lambda), which presents problems in their comparison since their relevance lies in how they relate to one another. Further, experimental error can result between the separate polymer electrolyte measurements from, e.g. varying (i) ambient conditions, (ii) membrane hydration level, or (iii) sample inhomogeneity from experiment to experiment particularly when candidate membranes are prepared by hand draw-downs, which is a usual method used in screening work. Therefore, the combination of these two measurements into a single simultaneous experiment on a single piece of membrane is of particular interest, making more relevant the extracted fuel cell membrane data. Many research groups rely upon measurements made on membrane electrode assemblies (MEAs) run as fuel cells to give simultaneous MXO and proton conductivity for membrane screening. Although, how well a given membrane performs in an MEA is of ultimate importance in fuel cell development, a number of complications can arise, than can obscure the measurement of underlying membrane properties, for instance, variability in uncompensated hardware resistance, differences in MEA performance level or effectiveness of initial conditioning / break-in (and hence variable MEA hydration level), membrane-catalyst layer interfacial incompatibilities, and MEA fabrication effects (e.g. pressing and/ or heat treatments). In this submission we present our methodology for the simultaneous measurement of membrane conductivity and methanol permeability at controlled temperature. The fixture is, in essence, a modified small volume flow-through diffusion cell with means provided for four point probe in-plane conductivity measurement, and temperature control. Conductivity is measured with electrochemical impedance spectroscopy and diffusion from methanol concentration determination by gas chromatography. This simultaneous measurement method removes ambiguity in the key transport property values and offers considerable cost savings in materials, labor and time. We will apply the method to the study of a series of experimental hydrocarbon polymer fuel cell membranes and discuss polymer composition and hydration level in relation to derived transport properties. Further we will present a graphical means of viewing the resulting data that offers rapid analysis and comparison of candidate membranes. In figure 1 is shown methanol permeability vs. conductivity for a series of experimental sulfonated poly(arylene ether ketone) hydrocarbon membranes of different IEC produced at PolyFuel presoaked to prescribed operating conditions. As the IEC of a polymer is increased, in this example from 1.0 to 1.9 meq / g, the methanol and proton transport characteristics change as well. The Nafion 115 membrane values compare closely with interpolated literature values (3), included for comparison here, has an IEC of roughly 0.9 meq / g and, although having higher conductivity than the set presented here, clearly gives significantly higher methanol permeability, thus demonstrating the MXO advantage typical of hydrocarbon membranes.