Degradation of some ceria electrolytes under hydrogen contact nearby anode in solid oxide fuel cells (SOFCs)

Abstract

This work is concerned with thermodynamic analysis of the stability of some ceria electro-lytes under contact with hydrogen gas nearby anode in fuel cells. It was considered the following types of ceria-electrolytes: pure ceria, strontium-doped ceria, calcium-doped ceria and calcium-bismuth-doped ceria. The equilibrium Log (pH 2 O/pH2) vs. T diagrams were constructed for x = 0.1 and 0.01, where x is the fraction of initial ceria converted to Ce 2 O 3 (proportional to the ratio between activities of Ce 3+ and Ce 4+ in the ceria electrolyte, which is proportional to the fraction of electronic conduction in the electrolyte at a given temperature). The predictions of the diagrams are as follows: (a) Ce 1.9 Ca 0.1 Bi 0.8 O 5.1 and Ce 0.9 Sr 0.1 O 1.9 are less stable than pure ceria for the whole temperature range (from 0 to 1000 °C); (b) Ce 0.9 Ca 0.1 O 1.9 is more stable than pure ceria below about 650 °C for x = 0.1 and below about 400 °C for x = 0.01; (c) at each temperature in the considered range the pressure ratio pH 2 O(g)/pH 2 (g) has to be higher than thermodynamically predicted in order to keep CeO 2 stable in the electrolyte contacting hydrogen gas. Thermodynamic predictions are entirely capable of explaining experimental data published on the subject (irreversible cell degradation in the case of SrO-doped ceria; weight loss from doped-ceria electrolyte above 700 °C; oxygen gas release during sintering of ceria).