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

AbstractThe 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 (Xc) 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 Xc decreased with increasing frequency and the relationships were linear on a log-log plot. That is, R and Xc exhibited power-law behavior (R=A/fα and Xc=B/fβ). 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/fα and Xc=B/fβ, 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.