Abstract
Considering literature of electrochemical impedance spectroscopy (EIS) it was discovered that for calculation of the value of double-layer capacitance (Cdl) from the constant phase element (CPE) two equations, with or without solution resistance (Rs) could be used. After calculation of Cdl for defined values of CPE constant Ydl, CPE exponent , Rs and charge transfer resistance (Rct), it was confirmed in this work that different results for Cdl were obtained by including Rs in the calculation of Cdl. It was also stated that "it is quite difficult to understand how both parameters (Rct and Rs) could be expressed by the same time constant, i.e. the same parameter ." By investigating the hydrogen evolution reaction (HER) at Ni mesh 40 electrode in the solution of 1 M KOH at 25 oC using EIS measurements, it was shown that different Cdl vs. E plots were obtained using these two equations for Cdl calculation. A simple solution to avoid this problem with detailed explanation, the use of equation without Rs , has been suggested in this work.
Highlights
The Nyquist plot, representing the dependence of imaginary vs. real component of impedance, Z’’ vs. Z’, is most frequently used in the interpretation of impedance measurements in electrochemical experiments
Depression of the semi-circle is a common characteristic for almost all electrochemical impedance (EIS) measurements, being ascribed to the presence of non-homogeneity of the double layer capacitance (Cdl), expressed as a constant phase element (CPE), with its impedance defined as
Assuming that Langmuir adsorption isotherm applies, the Tafel slopes of -30, -40 and -120 mV dec-1 are expected at low surface coverages by Hads when the Tafel, Heyrovsky and Volmer reactions (equations (8), (9) and (10)), respectively, are the rate-determining steps (RDSs) in the mechanism, while at the saturation coverage the Tafel slope of -120 mV dec-1 is normally observed for both Heyrovsky and Volmer reactions as the RDS
Summary
The Nyquist plot, representing the dependence of imaginary vs. real component of impedance (for different frequencies), Z’’ vs. Z’, is most frequently used in the interpretation of impedance measurements in electrochemical experiments. Depression of the semi-circle is a common characteristic for almost all electrochemical impedance (EIS) measurements, being ascribed to the presence of non-homogeneity of the double layer capacitance (Cdl), expressed as a constant phase element (CPE), with its impedance defined as (1). As Brug et al [30] stated in their work, “as Cole and Cole have pointed out [28], the special property of this process should be the frequency independence of the ratio of maximum energy stored to the energy dissipated per cycle In this concept there would be no reason for the solution resistance to influence the CPE and the parameter Q is a property of the electrical double-layer only, i.e. Q- 1 f(Rs), contrary to the case of the distribution model”. The question arises which one of these two equations is correct?
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