We report measurements of the conductivity of a series of mixtures of ionic liquids and acids. We find in particular that mixtures of 1-ethyl-3-methylimidazolium (EMI, normalC6normalH11normalN2 )–bis(trifluoromethylsulfonyl)imide [TFSI, (CnormalF3SnormalO2)2N ] with the acid H–TFSI have promising conduction properties for applications to fuel cell membranes. However, these mixtures have a peculiar dependence of conductivity on acid concentration. We present a series of model calculations at coarse-grained and molecular dynamics levels which suggest that the observed nonmonotonic dependence of conductivity on acid concentration arises because of proton clustering in the mixtures and possibly because of phase separation at the higher acid concentrations.

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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.