In search for the origin of irreversible ion immobilization under applied electric potential recently reported for a prototypical room-temperature ionic liquid electrolyte, [emim][Tf2N], confined in 1.5 nm carbon pores, here we compare the microscopic dynamics of cations in [emim][Tf2N] in the 1.5 nm and 6.7 nm carbon pores; in the latter, ion immobilization under applied electric potential is reversible. Using quasielastic neutron scattering, we find that the cation translational diffusivity is reasonably well defined on at least a nanometer length scale in the 6.7 nm pores, but not the 1.5 nm pores. Severely impeded microscopic dynamics of the confined electrolyte may be one of the factors differentiating the 1.5 nm pores from their larger counterparts and contributing to the irreversible immobilization of ions under applied electric potential.

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