Electrolyte related parameters of coulometric solid state devices

Abstract

Coulometric solid electrolyte sensors and measuring systems are gaining in importance for a variety of applications. One of their main advantages is their relatively precise function based on Faraday's law enabling calibration free performance over long time spans. By using devices with yttria-stabilized zirconia (YSZ) as solid electrolyte for the measurement of very small amounts of analytes like oxygen or gases reacting with oxygen ions, deviations of the measured cell current from the Faraday's law become visible. In this work, these deviations were measured at tubular YSZ cells with 6mm outer diameter, 1mm electrolyte thickness and 60mm length with the Hebb–Wagner method, using ultra clean nitrogen at different temperatures and oxygen partial pressures. At constant potentials of the measuring electrode between −850 and −350mV vs. Pt/air reference the deviation of the cell current from the Faraday's law was found to be caused completely by the hole conductivity of YSZ within the possible precision of measurement. At constant potentials below −1400mV the deviation is significantly lower than predicted by the values for electron conductivity already published for YSZ by Park and Blumenthal in 1989. Beside these static deviations at constant electrode potentials the coulometric cells show a dynamic non-Faraday charge transfer after potential steps of −50mV. This charge transfer, depending on temperature, start potential and prehistory, ranges between 0.45 and 310mAs. It is caused, inter alia, by oxygen release from the electrolyte and the porous environment of the electrode. Since the values of this charge transfer outnumber the targeted precision of coulometric measurements of traces gases by several orders of magnitude these measurements have to be carried out at constant electrode potentials.