A method of impedance spectroscopy for predicting the dynamic behavior of electrochemical system and its application to a high-area carbon electrode

A method for calculating the time-domain response to a periodic signal imposed on a system from impedance spectroscopic data is described. Since impedance is one of the system functions, a current (or voltage) response to voltage (or current) signal imposed on a system can be calculated from the impedance spectrum by performing a Fourier transform of voltage or current signals combined with an operational form of Ohm's law I(omega) = E(omega)/Z(omega) in the frequency domain. To show the applicability of the method to an electrochemical system, a high-area carbon electrode in a nonaqueous electrolyte was selected. A large capacitance characteristic of high-area carbons, 0.52 F per 3 cm(2) of apparent electrode area (140 F per gram based on the carbon sample weight), appeared in the low-frequency region below 1 mHz, while a small capacitance of 25 mu F per 3 cm(2) (a typical value for a double layer) was observed for higher frequencies than 100 Hz. In a midfrequency range of 10(-1) to 10(1) Hz, the electrode behaved like a resistor. Cyclic voltammograms at various sweep rates were calculated from the impedance spectrum. A one-to-one correspondence between calculated and directly measured voltammograms was obtained, and a characteristic feature of a high-area carbon electrode is described. The feasibility of applying the proposed method to materials research for advanced batteries, including supercapacitors, is discussed.