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

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.

Razmatrajući literature o elektrohemijskoj impedansnoj spektroskopiji (EIS) ustanovljeno je da za izračunavanje vrednosti kapaciteta dvojnog sloja (Cdl) iz konstantnog faznog elementa (CPE) postoje dve jednačine, jedna u kojoj figuriše otpor elektrolita (Rs) i druga u kojoj ovaj parametar ne figuriše. Izračunavanjem vrednosti Cdl za definisane parametre (Ydl, , Rs, Rct), u ovom radu je pokazano da se dobijaju drugačiji rezultati za Cdl primenom ovih jednačina. Takođe je konstatovano da je neprihvatljivo da oba parametra (Rct i Rs) na isti način zavise od vremenske konstante, odn. od parametra . Ispitivanjem reakcije izdvajanja vodonika na Ni mrežici u rastvoru 1 M KOH na 25 oC korišćenjem impedansnih merenja, pokazano je da su zavisnosti Cdl vs. E, dobijene korišćenjem pomenutih jednačina, različite. Da bi se izbegao ovaj problem predloženo je da se koristi jednačina u kojoj ne figuriše otpor elektrolita (Rs), jer uvodjenje Rs nema korektan fizički smisao.