Measurement of the direct-current (Faradic) resistance of the electrode-electrolyte interface for commonly used electrode materials

The direct-current (Faradic) resistance is important because it is the highest impedance that an electrode-electrolyte interface can attain. In this study, the Faradic resistance (Rf) of identical pairs of 0.5 cm(2) electrodes of bare and chlorided silver, tin and chlorided tin, nickel-silver, copper, and carbon was measured in contact with 0.9% saline at room temperature. It was found that for positive and negative current flow, the data fit the expression R-f=R(f0)e(-alphai) (with a high coefficient of determination), where R-f0 is the zero-current Faradic resistance and alpha is a constant that describes the manner in which R-f decreases with increasing current (i). It was found that chlorided silver exhibited the lowest R-f0; removing the chloride deposit increased R-f0 by more than sixfold. Likewise, chloriding tin reduced R-f0 by a factor of about 2. Electrolytically cleaning an electrode reduced R-f0. The highest value for R-f0 was for carbon. This paper concludes with a summary of the data for R-f0 scaled to 1 cm2 electrode area for the electrode materials measured in the present study and data from the published literature. (C) 2001 Biomedical Engineering Society.