Resistivity of Mesopore Confined Solvent Free Ionic Liquid Determined By Electrochemical Impedance Spectroscopy

Abstract

Supercapacitors offer higher power delivery and near infinite cycle stability compared to Li-ion batteries, but they lag behind in terms of energy density. Research into improving the energy density of supercapacitors has recently been focused on the rational design of nanopore structure to balance highly energy dense micropores (<2 nm) with higher conductivity mesopores (2 – 50 nm). Replacing aqueous electrolytes with organic electrolytes such as ionic liquids (ILs) can also dramatically increase energy density by enabling voltages higher than 1.23 V. ILs can have electrochemical stability windows as high as 3 – 6 V. Experiments with single conical nanopores have shown drastic increases in resistivity for imidazolium based ILs in very small pores, but a return to bulk resistivity for pores above 20 nm in diameter. Here, we report a 5-fold increase in resistivity of 1-ethyl,3-methylimidazolium tetrafluoroborate (EMIM-BF4) IL confined in carbon nanopores of diameter 24.4 ± 4.6 nm, well into the mesopore regime. Resistivity was determined through the use of detailed analysis of electrochemical impedance spectroscopy measurements of working supercapacitors of varying pore length. By utilizing highly ordered carbon electrodes based on the anodized aluminum oxide template growth method, this analysis offers insight into the effects of mesopore confinement on ionic liquid resistivity. These results will allow for improved rational design of supercapacitors to account for the effect of pore diameter on electrolyte resistivity and overall device performance. DISTRIBUTION STATEMENT A. Approved for public release; Distribution is unlimited 412TW-PA-20358