During an EIS measurement, this is measured as an increase in impedance. The depletion of species in front of electrodes is well understood and described by the Cottrell equation.ĭue to the lack of species in front of the electrode, fewer species are converted and less current flows while the same potential is applied. At low frequencies oxidizing or reducing potentials are held long enough that depletion of the species in front of the electrode becomes relevant. If a free diffusing species is converted at the electrode, this behavior isn’t observed. Up to here, the expected EIS would be a semi-circle just as for the simplified Randles circuit. The current can go through the interface of the working electrode by capacitive current caused by the electrochemical double layer, represented by the double-layer capacity C dl, or by Faraday current caused by an electrochemical reaction, which requires an electron transfer and so needs to go through the charge transfer resistance R ct. All current needs to go through the solution. As mentioned before, the solution resistance R sol is the serial resistor. The Randles circuit ( Figure 6.7) is quite close to an electrochemical experiment. It was observed that some effects occur in EIS that can’t be modeled with classic electronic components, so new components were introduced.
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