Theoretical Analysis of Critical Conditions for Crack Formation and Propagation, and Optimal Operation of SOECs

Abstract

The formation and propagation of cracks in SOECs originates from an anomalous change in the oxygen chemical potential, μ_(O_2 ), at the oxygen electrode – electrolyte interface. This work offers a theoretical analysis that considers both electrochemistry and elasticity to study the effect of pertinent parameters on the electrochemically driven crack formation, crack growth kinetics, and their dependence on μ_(O_2 ) distributions. Our analysis shows that an electronic insulating and ionic conductive interface between oxygen electrode and electrolyte is in favor of suppressing crack formation and propagation in an SOEC. There exists a critical pressure inside the crack for a YSZ electrolyte, which is 5,878 atm when the diameter of a crack is 1 μm. If the cell voltage is lower than 1.44 V, our analysis shows that no crack propagation is expected under the conditions selected. Conversely, operating the cell at a higher voltage results in a higher pressurization rate, leading to the formation of cracks. The optimal operation between constant current density or constant voltage is discussed by studying the dependence of μ_(O_2 ) on materials transport properties and steam concentration. Results from this work can be used to remedy crack problems and improve performance and durability.

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“…The degradation of ohmic resistance is more significant in ESC, while the degradation of polarization resistance is more critical in CSC. 48,49 Degradation mechanisms include Ni migration, 50 coarsening, 51 poisoning, 52 impurities, 53 crack, 54 and so on. The design and fabrication of SOEC have been improved over the last decade.…”

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“…33 Modeling and experimental studies have shown that the oxygen chemical potential profile through the YSZ electrolyte can be substantially changed by leakage current, which can be created with a ceria dopant in YSZ. [36][37][38] Thus, the oxygen partial pressure near the oxygen electrode interface can be effectively reduced by increasing the electronic conduction in the electrolyte. Therefore, the electronic conductivity or leakage current plays a critical role in determining the tendency of chemo-mechanical failure of SOECs with oxygen electrode delamination.…”

“…The oxygen partial pressure at the electrode/electrolyte interface depends on the ionic and electronic conductivity of the interface; that is, partial pressure depends on the selection of oxygen electrode materials. 27,38 Electrode/electrolyte delamination issues have also been reported in SOFCs under negative voltage conditions, in which ionic and electronic currents flow in the same direction (analogous to the EC mode with respect to the relative direction of ionic and electronic currents). 35,39,40 A cell may be subjected to negative voltage in the FC mode when it has a relatively higher cell resistance than others; that is, when a cell imbalance exists in an SOFC stack.…”