Theoretical consideration of Solid Oxide Electrolyzer Cell with zirconia-based electrolyte operated under extreme polarization or with low supply of feedstock chemicals

• Assumption on pure ionic conductivity in YSZ questioned.
• Proposed model for mixed conducting YSZ.
• New mode of efficiency loss proposed for SOEC.
• Proposed designed condition for safe operation of SOEC.

We report on the consequences of operating Solid Oxide Electrolyzer Cell (SOEC) under an extreme electrical polarization or with a low supply of feedstock chemicals. Under such conditions, subsequent switching from dominantly ionic conductivity to mixed conductivity is lowering the cells’ Faradic efficiency. We propose a theoretical model, able to accurately predict polarization curve, Faradic efficiency and synthetic fuel production for Zirconia electrolyte-based (8YSZ) cells. To the best of our knowledge this is the first model of SOEC constructed without the assumption of pure ionic conductivity in YSZ. Experimental results from literature were used to extrapolate, fit and validated 8YSZ properties for oxygen partial pressure below 10−20 atm. We report a simple fuel map, suggesting design conditions for SOEC, securing the operation with near 100% Faradic efficiency. This consideration suggests strong evidence against a common practice of using Faraday's law to estimate electrolysis yield. Moreover, we speculate that surge of potential difference at high current densities may be due to a switch to the mixed conduction mode, rather than only the concentration loss.