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
Electrochemical double layer capacitors exhibit constant phase element (CPE) behavior in both their cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) response. Recently, Allagui et. al [Sci. Rep., vol. 7, September, p. 46828, 2017.] presented equations for finding CPE parameters from CV measurements. EIS data for porous electrodes exhibiting CPE behavior can be fit using the de Levie model. In this work, several ordered carbon electrodes of varying pore length were prepared by chemical vapor deposition (CVD) of carbon nanotubes (CNTs) within anodized aluminum oxide (AAO) templates. These electrodes were used to create supercapacitors with neat EMIM-BF4 ionic liquid electrolyte. CPE parameters Q and α were found for each device from both CV and EIS, and they are in good agreement between the two methods. However, the values of resistance, another fitting parameter in both models, vary drastically. The physical origin of these resistances is discussed. Additionally, the capacitance of the devices was calculated from both methods, using the Brug formula in the case of EIS. These capacitances disagree greatly, except for the thickest AAO templates (the longest pores). The devices in agreement also showed the lowest resistance values from CV. It is not apparent whether the agreement in capacitance is related to the pore dimensions or the extracted resistance values. {DISTRIBUTION STATEMENT A. Approved for public release; Distribution is unlimited 412TW-PA-20496]
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