Mechanisms of ionic transport in membranes for batteries and fuel cells

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.