On the Atomistic Nature of Capacitance Enhancement Generated by Ionic Liquid Electrolyte Confined in Subnanometer Pores.
The capacitance enhancement experimentally observed in electrodes with complex morphology of random subnanometer wide pores is an intriguing phenomena, yet the mechanisms for such enhancement are not completely understood. Our atomistic molecular dynamics simulations demonstrate that in subnanometer slit-geometry nanopores, a factor of 2 capacitance enhancement (compared to a flat electrode) is possible for the 1-ethyl-3-methylimidazolium (EMIM)-bis(trifluoro-methylsulfonyl)imide (TFSI) ionic liquid electrolyte. This capacitance enhancement is a result of a fast charge separation inside the nanopore due to abrupt expulsion of co-ions from the pore while maintaining an elevated counterion density due to strong screening of electrostatic repulsive interactions by the conductive pore. Importantly, we find that the capacitance enhancement can be very asymmetric. For the negatively charged 7.5 Å wide pore, the integral capacitance is 100% larger than on a flat surface; however, on the positive electrode, almost no enhancement is observed. Detailed analysis of structure and composition of electrolyte inside nanopores shows that the capacitance enhancement and the shape of differential capacitance strongly depend on the details of the ion chemical structure and a delicate balance of ion-surface and ion-ion interactions.
†Department of Materials Science and Engineering, University of Utah, 122 South Central Campus Drive, Salt Lake City, Utah 84112, United States.|‡School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China.
Lidan
L
Xing
†Department of Materials Science and Engineering, University of Utah, 122 South Central Campus Drive, Salt Lake City, Utah 84112, United States.
Jenel
J
Vatamanu
§Electrochemistry Branch, U.S. Army Research Laboratory, 2800 Powder Mill Road, Adelphi, Maryland 20783, United States.
Oleg
O
Borodin
†Department of Materials Science and Engineering, University of Utah, 122 South Central Campus Drive, Salt Lake City, Utah 84112, United States.
Dmitry
D
Bedrov
differential capacitance; electric double layer; electrostatic superscreening; molecular dynamics simulation; nanoporous electrode; room temperature ionic liquid; supercacitor