Calculation of a pure double-layer capacitance from a constant phase element in the impedance measurements

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.