Perovskite-type oxides: study of the transition from bulk to surface properties by correlating their structures with the SOFC electrodes performance.

L. MOGNI

Conferencia; 22nd Int. Conf. Solid State Ionics (SSI-22); 2019

Charla InvitadaMaterials with ABO3-perovskite structure dominate the fieldof air electrodes for Solid Oxide Fuel and Electrolyzer Cells (SOFC-SOEC) by bringingthe needed high electronic conductivity. However, to extend the O2-reductionreaction (ORR) beyond the TPB, it has been proposed new compositions andstructures to provide high ionic conductivity in addition to the electroniccontribution (MIEC). Whereas the electronic conductivity is dominated by the natureand mobility of electronic defects (or ), the ionic conductivity depends of O-point defects (, or ). In this sense, perovkite related structures such as doubled perovskites(AA?B2O6) or Ruddlesden-Popper phases (A2BO4,A3B2O7, A4B3O10,An+1BnO3n+1) called the attention because theselayered structures help to accommodate larger amounts of O-vacancies or extra O-interstitials.However, higher concentration of both, electronics and O-lattice defects, do notguarantee better transport properties since mobility and the low energypreferential pathway also play an important role in the final conductivity. Alongthis work, we will discuss specific results obtained for La0.4Sr0.6Co1-xFexO3-δand La0.5Ba0.5CoO3-δ, perovskites; LaBaCo2O6-δ,GdBaCo2O6-δ and (La,Pr)BaCo2O6-δ  double perovskites, and Nd2NiO4+δ  Ruddlesden Popper phases.We used high temperature thermodynamic data of O non-stoichiometry as afunction of temperature (T) and O2-partial pressure (pO2)in order to study the defect structure of the perovskites related oxides. Wecombined this information with the electrical conductivity determination, as afunction of T and pO2, to elucidate the true effect of the crystalstructure on the transport properties, by excluding the charge carrier?sconcentration effect. We supported our conclusions by including in-situ structural information bycomplementary X-ray and neutron diffraction techniques.We studied also the polarization resistance of differentperovskite-related porous cathodes, as a function of T and pO2 forthe understanding of the ORR mechanism; and as a function of time to analyzethe origin of degradation processes. We combined EIS data with microstructuralinformation of real porous electrodes obtained by 3D SEM tomography, and O-nonstoichiometry from thermodynamic data. Thus, the ORR can be analyzed in termsof the O-exchange coefficient (k) andthe oxygen diffusion coefficient (D);and the effect of crystal structure could be evaluated independently ofmicrostructure and O-defect effects. Once the kinetic parameters wereidentified, it was possible to detect the main contributions to polarizationlosses and re-design the electrodes to decrease these losses. Also, from thetime evolution of these parameters, it is possible to identify degradationmechanisms and propose new strategies to reduce it.  From the previous facts, we concluded thatlocal changes of composition, electronic state and structure near to surfaceimpact directly in electrode performance. Finally, we will discuss the strategyof surface electrode decoration with nano-particles (NPs) to specifically thetune properties of selected perovskites to enhance their electrocatalytic poweror mitigate the degradation phenomena.