Changes in SOFC Cathode Crystallographic Structure Induced by Oxygen Deficiency in Cathode Room

Tuesday, 25 July 2017: 15:20
Grand Ballroom West (The Diplomat Beach Resort)
I. Kivi (Institute of Chemistry, University of Tartu), J. Aruväli (Institute of Ecology and Earth Sciences), G. Nurk, and E. Lust (Institute of Chemistry, University of Tartu)
Stationary solid oxide fuel cell systems (SOFC) are highly efficient chemical energy conversion systems generating electricity and high quality heat. SOFC devices should be usable during over 40 000 hours and at least 100 thermal cycles. In this study, the changes of unit cell of cathode materials as a function of current density, pO2 and temperature were analyzed as a risk factor to increase the degradation rate of cathode electrode, during the cell working time.

Reversible changes in the lattice parameters of SOFC cathode were observed depending on the temperature (T) and oxygen partial pressure (pO2) applied. The cathode potential, T and pO2 noticeably influences the unit cell volume, thus, the oxygen stoichiometry and concentration of vacancies.

The electrode structure (porosity) has strong influence to the change kinetics of crystallographic parameters of cathode at in-situ element operating conditions. By polarizing the cathode electrode cathodically, the activity of electrode increase and the molecular oxygen concentration near the electrode surface decrease. Decrease of oxygen concentration in the porous cathode matrix causes the expansion of lattice parameter of cathode material. The electrode material (LSC and GSC) lose oxygen, Co4+ reduces to Co3+ and because of that unit cell expands.

Only slight influence of oxygen partial pressure and potential to the crystallographic parameters of LSM (La0.6Sr0.4MnO3δ) was registered. At the same time for LSC (La0.6Sr0.4CoO3δ) and GSC (Gd0.85Sr0,15CoO3δ) the crystallographic unit cell volume was a function of electrode polarization as well as the oxygen partial pressure. There is no remarkable vacancy formation detectable with HT-XRD in LSM cathode lattice caused by electrode polarizations applied.

Thermochemical crystallographic expansivity study of LSM, LSC and GSC cathodes with in-situ HT-XRD method gives valuable experimental data about the crystallographic changes caused by changes in operating parameters like cell potential, temperature, and oxygen partial pressure in cathode compartment and therefore helps to understand thermomechanical and degradation behavior as well as redox behavior of B site element (Co) as a function of different conditions applied for SOFC cathode.

Figure 1. Selected segment from XRD pattern for LSCO and GSCO cathode at electrode potentials 0 and -0.9 V vs Pt/Pt/O2, respectively.