Mechanisms of Ionic Transport in Membranes for Batteries and Fuel Cells
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Mechanisms of Ionic Transport in Membranes for Batteries and Fuel Cells. J. Woods Halley, Lingling Jia and Sean Bowman School of Physics and Astronomy University of Minnesota Minneapolis, MN 55455
ABSTRACT Ionic transport in electrolyte membranes limits performance in both battery and fuel cell membranes. The problems have been well known for years, sometimes decades, but empirical progress in solving them has been slow. The focus here is on studies to improve understanding of transport mechanisms, which despite extensive study, remain in dispute in several important cases. For lithium transport in polymer membranes, I will review simulation work by ourselves and others, and contend that the original qualitative picture by Ratner and coworkers is confirmed in many respects by recent work. It means, however, that the fundamental difficulty is that the transport is controlled by torsion forces in the hydrocarbon backbone which are extremely difficult to manipulate experimentally. Turning to possibly promising additives, I review recent work on proton and lithium transport in ionic liquids, on which promising experimental results have been reported. The data, both from simulation and experiment, indicate nontrivial collective effects in the transport properties which need to be sorted out to control these systems. In the case of proton transport, we report results suggesting that high mobilities occur in acid-ionic mixtures with a common anion in mixtures near phase separation.
INTRODUCTION Ion transport through the membranes of both polymer electrolyte membrane fuel cells and lithium polymer batteries have been limiting the development of both of these promising energy technologies for a decade or more. Empirical efforts to find more effective conducting membranes have had limited success. Theoretical and modeling efforts have clarified the mechanism of conduction in the case of lithium polymer membranes but the insights obtained have not yet resulted in the development of membranes of the needed higher conductivities. This is partly because the mechanism depends on the very fundamental torsion forces in the hydrocarbon chains of the polymer and these are not subject to modification simply by manipulation of the details of the hydrocarbon chemistry. The mechanism of proton conductivity in existing proton exchange membranes (PEMs)is still poorly understood. However it is known to depend essentially on the presence of water in the membrane and this limits the performance of existing membranes above 100 C.
There have been several reports [1] suggesting that ionic liquid (IL) additives might improve the performance of both lithium polymer [2]-[5] and PEM fuel cell membranes [6]. Studies of ionic liquids themselves are, of course, not new, and there is a large literature describing their physical and chemical properties. However, less has been done to understand and control the transport properties of the mixtures of ionic liquids with acids and lithium salts with the same anion which we discuss here, mot
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