The polarization impedance of common electrode metals operated at low current density

Abstract

The objective of this study was to characterize the polarization impedance (resistance and capacitance) of several common metal/0.9% saline interfaces operated at low-current density and to thereby provide a useful reference for those wishing to calculate the impedance of such electrodes. The series-equivalent resistance (R) and capacitive reactance (X_c) of stainless steel, platinum, silver, MP35N, palladium, aluminum, rhodium and copper electrodes, all having a surface areas S=0.005 cm² and all in contact with 0.9% saline, were measured as a function of frequency (100 Hz to 20 kHz) at low-current density (0.025 mA/cm²). For all the metals tested, both R and X_c decreased with increasing frequency and the relationships were linear on a log-log plot. That is, R and X_c exhibited power-law behavior (R=A/f^α and X_c=B/f^β). However, it was not generally true that A=B and α=β=0.5 as stated in the Warburg low-current density model. Furthermore, the Fricke constant phase model in which α=β and ϕ=0.5πβ was found not to be applicable in general. In particular, the constraint that α=β was a good approximation for most of the metals tested in this study, but the constraint that ϕ=0.5πβ did not hold in general. Although the Warburg low-current density model provides a useful conceptual tool, it is not the most accurate representation of the electrode-electrolyte interface. The Fricke constant phase model is a better representation of electrode behavior, but it also may not be valid in general. We have found that a better representation is provided by the general power-law model R=A/f^α and X_c=B/f^β, where A, B, α, and β depend on the species of both the metal and electrolyte and A and B depend, in addition, on electrode area. Using this model and the data presented in this study, the impedance of an electrode-electrolyte interface operated at low-current density may be calculated as

$$Z = (0.005/S)\sqrt {(Af^{ - \alpha } )^2 + (Bf^{ - \beta } )^2 } ,$$

where S is the surface area of the electrode in cm².