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Relationship between pore size and reversible and irreversible immobilization of ionic liquid electrolytes in porous carbon under applied electric potential

Publication Type
Journal Name
Applied Physics Letters
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Conference Date

Transport of  electrolytes  in  nanoporous  carbon-based  electrodes  largely defines the function and performance of energy storage devices. Using  molecular dynamics simulation  and quasielastic  neutron scattering,  we investigate the microscopic  dynamics  of a prototypical  ionic liquid   electrolyte,  [emim][Tf2N], under applied electric potential in  carbon  materials with 6.7 nm and 1.5 nm pores. The simulations demonstrate the formation of dense layers of counter-ions near the charged surfaces, which is reversible when the polarity is reversed. In the experiment, the ions immobilized near the surface manifest themselves in the  elastic   scattering  signal. The experimentally observed ion immobilization near the wall is fully reversible as a function of the applied electric potential in the 6.7 nm, but not in the 1.5 nm  nanopores.  In the latter case, remarkably, the first application of the electric potential leads to apparently irreversible immobilization of cations or anions, depending on the polarity, near the  carbon  pore walls. This unexpectedly demonstrates that in  carbon   electrode  materials with the small pores, which are optimal for energy storage applications, the polarity of the electrical potential applied for the first time after the introduction of an  ionic liquid   electrolyte  may define the decoration of the small pore walls with ions for prolonged periods of time and possibly for the lifetime of the electrode.