Abstract
Electrochemical impedance spectroscopy (EIS) technique is used for characterization of single cell symmetric capacitors having different mass loadings of activated carbon (AC). Relevant values of charge storage capacitance (CT) and internal resistance (ESR) were evaluated by the single frequency and multi-frequency analyses of measured impedance spectra. Curve fittings were based on the non-ideal R-C model that takes into account the parasitic inductance, contributions from electrode materials/contacts and the effects of AC porosity. Higher CT and lower ESR values were obtained not only for the cell with higher mass of AC, but also using the single vs. multi-frequency approach. Lower CT and higher values of ESR that are generally obtained using the multi-frequency method and curve fittings should be related to the not ideal capacitive response of porous AC material and too high frequency chosen in applying the single frequency analysis.
Highlights
Electrochemical capacitors/supercapacitors are high power energy storage devices composed basically from two electrodes built-up from an active material put on current collectors and immersed in a liquid ionic conductor 1−8
The present results show that for here characterized activated carbon (AC)//AC cells, the recommended f = 1 kHz is too high for proper electrical series resistance (ESR) evaluations
Single frequency CT and ESR values were determined on the basis of ideal C−R model, using impedance magnitude values measured at low and high frequencies, respectively
Summary
Electrochemical capacitors/supercapacitors are high power energy storage devices composed basically from two electrodes built-up from an active material put on current collectors and immersed in a liquid ionic conductor (electrolyte) 1−8. In applying of either single frequency or multi-frequency EIS method for characterization of capacitive devices, ESR and CT values are usually obtained using the one time (lumped) constant R−C model 1,2,6,28,37 for a capacitive device, having the impedance/frequency response defined as: Z( ) = ESR + 1/i CT (3). Impedance spectra defined by eq (5) is composed of four characteristic frequency regions and described by six impedance parameters (L, RHF, RRC, CRC, Rs and Cs) that all can be determined by the curve fitting procedure. In these conditions, CT = Cs and ESR = RHF + RRC + Rs( ). More details of corrections and statistical criteria for reasonable fits of impedance spectra are described elsewhere 51
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