Complex Capacitance Scaling in Ionic Liquids-Filled Nanopores

Abstract

Recent experiments have shown that the capacitance of subnanometer pores increases anomalously as the pore width decreases, thereby opening a new avenue for developing supercapacitors with enhanced energy density. However, this behavior is still subject to some controversy since its physical origins are not well understood. Using atomistic simulations, we show that the capacitance of slit-shaped nanopores in contact with room-temperature ionic liquids exhibits a U-shaped scaling behavior in pores with widths from 0.75 to 1.26 nm. The left branch of the capacitance scaling curve directly corresponds to the anomalous capacitance increase and thus reproduces the experimental observations. The right branch of the curve indirectly agrees with experimental findings that so far have received little attention. The overall U-shaped scaling behavior provides insights on the origins of the difficulty in experimentally observing the pore-width-dependent capacitance. We establish a theoretical framework for understanding the capacitance of electrical double layers in nanopores and provide mechanistic details into the origins of the observed scaling behavior. The framework highlights the critical role of "ion solvation" in controlling pore capacitance and the importance of choosing anion/cation couples carefully for optimal energy storage in a given pore system.

The volumetric capacitance of microporous carbons in organic electrolyte

“…in pores below 1 nm (due to ions desolvation) is used as criteria for the improvement of the gravimetric capacitance [9][10][11][12][13][14][15][16][17].…”

Highly confined ions store charge more efficiently in supercapacitors

“…Although it was observed from experiments and some simulations that the best performances were obtained for CDCs with an average pore size matching the size of the ions [24][25][26][27][28] , in a recent theoretical study, Kondrat et al 29 have shown that the best average pore size depends on the operating voltage. Nevertheless their model was based on a description of the porous carbon based on a distribution of slit-like pores; the present results show that it would be necessary to inject a more detailed adsorption model to fully address this point.…”

Defect‐Engineered Graphene for High‐Energy‐ and High‐Power‐Density Supercapacitor Devices

“…Previously, it was observed that the best performance of SCs can be realized when the average micropore size in nanostructured bulk electrodes (e.g., carbide-derived carbon) matches the size of the ions in the electrolyte [8][9][10][11][12][13][14][15] . It is expected that such a resonant effect is true even for defect-induced pores in quasi-two dimensional FLG substrates ( Figure 1a).…”