In situ X-ray diffraction studies of PrNi0.5Co0.5O3-δ — Ce0.9Y0.1O2-δ sintered by microwave radiation.
Praseodymium nickelates-cobaltites and their nanocomposites are promising materials for intermediate temperature solid oxide fuel cells (IT SOFC) cathodes and oxygen separation membranes, since they are stable to carbonization. Moreover, they have high oxygen mobility. However, their performance under conditions close to operating ones (at high temperatures and large oxygen chemical potential gradient) has not yet been properly studied. This work aims at elucidating specificity of their oxygen mobility and structure dynamics by in situ studies of the crystal structure relaxation.
PrNi0.5Co0.5O3-δ (PNC), Ce0.9Y0.1O2-δ (YDC), Ce0.65Pr0.25Y0.1O2-δ (YPDC) and Pr6O11 powders were synthesized by Pechini technique. PNC–YDC nanocomposite was obtained via ultrasonic dispersion. PNC and PNC–YDC samples were sintered by microwave radiation at 1000 and 930°C, respectively, while other samples - by conventional sintering at 1100°C. Samples were characterized by XRD and TEM. The oxygen mobility was estimated by analysis of the cell volume relaxation (CVR) after abrupt change of the oxygen partial pressure using synchrotron radiation.
PNC was orthorhombic perovskite (P) with Pr6O11 admixture. Due to Pr cations transfer into the YDC phase, PNC–YDC composite contains P and two fluorite (F) phases (Ce0.65Pr0.25Y0.1O2-δ+PrO2-δ). YPDC cell parameter irreversible increases while heating in He due to the oxygen loss. PrO2 δ oxide loses oxygen with the structure changing from the monoclinic to the cubic (and then trigonal) one.
For Pr6O11 both kchem and Dchem can be calculated accurately and are 5.0·10-4 cm/s and 5.0·10-7 cm2/s at 500°C. For YPDC, the process is affected by the surface exchange with -2.75·10-4 cm/s, so Dchem is estimated to be ~10-7 cm2/s at 600 °C. This suggests that the oxygen transport in the bulk and surface of PNC – YDC nanocomposite is determined by F domains. Thus, at 600°C for PNC–YDC sample, kchem and Dchem were estimated to be 2.0·10-5 cm/s and ~10 7 cm2/s, respectively.
CVR data are in full agreement with the temperature programmed isotope exchange of oxygen data described in our earlier works. Oxygen surface exchange constant and tracer diffusion coefficient are 1.6·10-7 cm/s and ~10-9 cm2/s at 600°C, respectively. These values are close to those of LSFC perovskite. Hence, PNC – YDC nanocomposite with fast oxygen diffusion and exchange determined by Y-Pr-Ce-O domains and developed P+F interface in both equilibrium and conditions under oxygen chemical potential gradient are promising materials for IT SOFC and oxygen separation membranes, while sintering by microwave radiation is favorable for sintering its layers to required mechanical strength.