Internal Precipitation of Molecular Oxygen and Electromechanical Failure of Zirconia Solid Electrolytes

Abstract

The phenomenon of electromechanical failure of zirconia has been examined. It is shown that spatial variation of electronic conductivity in a predominantly oxygen ion conductor can lead to internal precipitation of molecular oxygen and its pressurization. Internal pressures high enough to crack zirconia can develop if the electronic conductivity is higher on the oxygen exit side. The calculations demonstrate that the kinetics of pressurization are very sensitive to the relative magnitudes of the electronic and the ionic conductivities. Oxygen was electrolyzed at 800°C through zirconia solid electrolytes with lanthanum strontium manganate (LSM) electrodes under an applied dc potential of 1.0 V. Spalling of the solid electrolytes was observed. Composite disk electrolytes were fabricated such that part of the disk contained Ti02 as a dopant which was added to enhance the electronic conductivity. The electrolytes exhibited severe cracking and pitting when the TiOz-doped region was on the oxygen exit side. No degradation occurred when the TiOzdoped region was on the oxygen entry side. Implications regarding the use of mixed conductors as dense electrodes in electrochemical systems are discussed. [

Active Cathodes for Super-High Power Density Solid Oxide Fuel Cells Through Space Charge Effects

“…In many cases, this inconsistency often does not lead to significant problems. However, as will be discussed in this report, and has been discussed to some extent in earlier manuscripts, there are many situations in which the neglect of electronic current, however small, leads to substantial difficulties in analysis -and thus also in the interpretation of experimental results [14][15][16].…”

“…We will first consider here the case wherein the oxygen partial pressures at the two electrodes are identical so that . Then, the rate of loss of input work into heat at electrode I is given by 18 For membranes with spatially dependent transport properties (but without the incorporation of interfacial effects), the transient case was previously examined in [15,16]. …”

Modeling Degradation in Solid Oxide Electrolysis Cells

“…Nevertheless, the general approach necessary to solve the relevant differential equations has been previously described for a couple of cases. 7,38,41 The general conclusion is that the greater the overall electronic conductivity, the faster are the kinetics of achieving the steady state. 41 However, as described here, by a suitable choice of transport parameters, it is possible to achieve the final steady state which is subcritical, that is, it does not lead to degradation.…”

“…The incubation time depends in a complicated way on a number of transport parameters. 38,41 However, critical condition may be reached long before the steady state is reached, leading to failure. Thus, from the standpoint of long-term stability, it is preferable to introduce some electronic conductivity so that steady state corresponding to subcritical condition is reached and high pressures are not generated.…”

“…The kinetics of this growth process, however, depends on the net flux of oxygen depositing in the crack, which depends upon the current and the various transport parameters and the operating conditions, as described in earlier work on related topics. 38,41 Also, if some interdiffusion occurs between the electrolyte and the oxygen electrode, it may alter local transport properties. Under such conditions, it is possible that the initiation of delamination may occur inside the electrolyte some distance from the interface.…”