Blackening in yttria stabilized zirconia due to cathodic processes at solid platinum electrodes

Abstract

The DC cathodic voltage current density (V-J) characteristics at the contact between a solid disc Pt-13% Rh electrode and yttria stabilized zirconia (YZr) electrolyte were investigated to evaluate the conditions under which blackening occurred in the anionic conductor. Polycrystalline and single crystal samples have been studied using oxygen-argon mixtures between 10⁰ and 10⁻³ atm oxygen and at temperatures in the range 800 to 1450°C. It is shown that the rate determining step of the overall cathodic reaction, O₂(g)+2Vö+4e′ar2O ^x_o under low field, ohmic conditions is the first electronation step, but true activation polarization at higher fields is masked by mass transfer limitations. The limiting current density in the polycrystalline material was directly proportional to the oxygen partial pressure$\left( {J_L \propto P_{O_2 } } \right)$ whereas at low temperatures, below 1000°C,$J_L \propto P_{O_2 }^{\tfrac{1}{2}}$ in the single crystal.

The former behaviour is attributed to a flux limit in the ambient oxygen gas, whereas the$J_L \propto P_{O_2 }^{\tfrac{1}{2}}$ regime is believed to be a consequence of molecular dissociation, coupled with surface exchange, onto the mobile interfacial layer. It is further shown that blackening is a consequence of thermionic emission of electrons across the cathodic interface into anion vacancy traps and the cathodicV-J characteristics of the blackened material saturate to obey aJ∞V ² law above about 2 V, indicative of a space charge limited current. A tentative model is proposed.