Electrochemical Performance of Solid Oxide Cell Oxygen Electrodes Under Pressurization

Pressurized operation is of interest for solid oxide fuel cells used in combined cycle applications, and for solid oxide electrolysis cells to influence the chemical constitution of the fuel produced.  Here we present results on the effect of oxygen pressure on the electrochemical performance of a number of solid oxide cell (SOC) oxygen electrodes, studied in symmetrical cells by impedance spectroscopy. Electrodes investigated include single-phase (La_0.8Sr_0.2)_0.98MnO_3-δ (LSM), LSM - Zr_0.84Y_0.16O_2-γ (YSZ) composite, LSM-infiltrated YSZ, single-phase La_0.6Sr_0.4Fe_0.8Co_0.2O_3-d (LSCF), and LSCF-Ce_0.8Gd_0.2O_1.95 (GDC) composite. All electrodes exhibited reduced electrode resistance with increasing oxygen pressure, with all but the single-phase LSM showing a resistance decrease of around pO₂^-0.18. The single-phase LSM electrode displayed a pO₂^-0.28 dependence. Two primary features were found in the impedance spectra equivalent model fits. A higher frequency peak around 10³ Hz had around a pO₂^-0.25 dependence and was attributed to charge transfer reaction limitations. Of all the electrodes, only the single-phase LSM sample did not exhibit the higher frequency peak. A lower frequency peak around 10² Hz had around a pO­₂^-0.15 dependence, which was attributed to either ionization of an adsorbed oxygen atom or oxygen ion transport from the electrode material to the electrolyte material. Only the LSM-infiltrated electrode did not display the lower frequency peak. Overall, these results indicated that performance losses due to oxygen electrode resistance can be reduced by operating a SOC at elevated pressure.