Interface description of Milli-Q water cells: Temperature dependence of the CPE parameters

Wereport on the temperature dependence of the Milli-Qwater cell response, limited by surgical steel electrodes, to an external periodic excitation. Using the Electrochemical Impedance Spectroscopy technique (EIS), the spectra of the real and imaginary parts of the electrical impedance of the cell are experimentally obtained. Important deviations from the theoretical predictions of the Poisson–Nernst–Planck model, assuming blocking electrodes, are observed. Our experimental results indicate that the properties of the electrodes play an important role in the low frequency part of the spectra. In the absence of a simple physically based theoretical model, we fit our data utilizing three equivalent electric circuits containing a constant phase element (CPE) charged to describe the properties of the electrodes. From our analysis it follows that a “good” equivalent electric circuit for the analysis of the response of an electrolytic cell to an external periodic voltage is formed by a parallel of a bulk capacitance and resistance of ionic origin. The CPE is then charged to mimic the interface effects. From a mathematical point of view this is equivalent to assume that the phenomenological parameters describing the non-ideal blocking properties of the electrode, in the Chang-Jaffe, Ohmic or Langmuir models, are frequency dependent.