Study of phase stability of SrTi0.3Fe0.7O3 perovskite in reducing atmosphere: effect of microstructure

LUCÍA TOSCANI; PETER HOLTAPPELS; MARIANO SANTAYA; DANIEL DRASBAEK; HORACIO TROIANI; LAURA BAQUÉ; KARIN VELS HANSEN; LILIANA MOGNI

Conferencia; 22nd Internacional Conference on Solid State Ionics SSI-22; 2019

Solid  oxide  fuel  cell  (SOFC)  anodes  composed  of  electronically  conducting  perovskites  are alternatives to Ni-YSZ anodes that can provide improved tolerance to redox cycling, fuel impurities, and hydrocarbon fuels. SrTi0.3Fe0.7O3 (STF) is gaining relevance because it yields a power density of 1.1 W cm2 at 850 ºC in humidified hydrogen, approaching the values achieved in state-of-the-art Ni-YSZ anodes. However, it is well known that the increases of surface area of electrodes are the clue to reduce electrode polarization resistance, thus being possible to obtain higher power densities at lower temperature. However, it is not clear that in the case of perovskite based anodes this could reduce the stability of the oxide in the fuel atmosphere. In this work, we studied the effect of particle size  in  either,  the  anode  performance  and  the  thermodynamic  stability  under  the  different  fuel conditions and temperatures. SrTi0.3Fe0.7O3-δ is both prepared via solid state reaction (STF-SSR) and also by an alternative sol-gel route (STF-SG). The sintering temperature is reduced dramatically with the sol-gel method, this fact inducing a higher porosity and a much smaller grain size. As particle size is reduced the stability under fuel condition is also diminished. STF-SG shows Fe nanoparticles ex-solution at 750°C in a wet 10%H2 atmosphere, while no ex-solution is seen for STF-SSR even under more aggressive reducing conditions (850°C in wet 30%H2). Reduction and re-oxidation cycles are studied for both samples by using combined techniques such as XRD, SEM, TEM and RAMAN. It is seen that STF-SG recovers its original state almost immediately after the reducing atmosphere is changed  to  oxidizing  conditions  at  750°C.  This  suggests  that  the  ex-solution  process  is  easily reversible  in  STF  samples.Anode  performances  were  evaluated  by  Impedance  Spectroscopy  as  a function  of  temperature  and  fuel  conditions  by  using  electrolyte  supported  symmetrical  cells. Previously,  the  reactivity  between  LSGM  electrolyte  and  the  LDC  protective  layer  with  STF  was analyzed for both preparation methods, because also the microstructure play a role in the chemical compatibility between materials.