The use of voltage probes in impedance spectroscopy
In electrochemical impedance spectroscopy, a reference electrode is often used for the control of the potential of a working electrode by means of a potentiostat in a 3-electrode configuration. However, the use of a reference electrode can be a source of measurement artifacts. Such errors may arise even when an ideal measuring system can be assumed. The reasons are coupling between the different electrodes and stray or leakage currents in combination with the impedance of the reference electrode, which spoils the measured potential in AC mode. The influence of the measuring system in arrangements with one or more voltage probes is analyzed based on equivalent circuits for the whole setup. It is shown how capacitive coupling and non-ideal measuring instruments can introduce low frequency inductance artifacts, additional semicircles in the complex plane plot and so on. Experimental precautions and special amplifiers with active shields are described to reduce the errors. It is demonstrated on a theoretical level and with experimental data that the use of active shielding can decrease such artifacts. The method has been applied in AC 4-probe measurements for the determination of the conductivity of solid electrolytes.