Constant Phase Element Analysis of Ordered Carbon Electrodes Via Electrochemical Impedance Spectroscopy and Cyclic Voltammetry

Abstract

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]