In Situ XAS Study of Pr0.6Sr0.4CoO3-d for IT-SOFC Cathode Application

F. F. MUÑOZ; ACUÑA, L. M.; FUENTES, R. O.

Cracovia

Conferencia; 17th International Conference on X-ray Absorption Fine Structure; 2018

Jagiellonian University in Kraków

Cobaltites arehigh-temperature mixed ionic and electronic conductors that have been studiedfor long time as possible cathode materials for solid oxide fuel cell (SOFC)application. The high working temperature (900-1000ºC) of SOFCs requires theuse of expensive materials as current collectors and interconnectors andreduces the life span of the device. So, there is a great interest indeveloping new materials to reduce the working temperature to intermediatetemperatures (IT-SOFC) in the range of 500 to 700 ºC. In this work, we presentan in situ XAS study on Pr0.6Sr0.4CoO3-d(PSC) nanopowders, focused on the speciation of Co atoms under differentenvironment conditions of temperature and O2(g) concentration inview of their possible application as cathode in IT-SOFCs.            PSC was synthesized via thegel-combustion method using nitrates of Pr, Sr and Co and glycine as complexingagent and fuel. PSC nanopowders were obtained after the ashes were calcined at700 ºC for 5 h in air. In situ XASstudies were performed at the D04B-XAFS-1 beamline of the Laboratório Nacionalde Luz Síncrotron (LNLS), Brazil. The Co K- Sr K- and Pr L3-edges,were measured in transmission mode, from 20 to 700 ºC in controlled atmospherewith different O2(g) concentrations. WinXas 3.1, FEFF8.2 and FEFFITsoftware were used for data reduction and analysis of the XANES and EXAFS regions.The calibration curve and the linear combination fitting (LCF) were the methodsused to calculate the average oxidation state and speciation of Co atoms. In situ XPD analysis was performed atthe D10B-XPD beamline of the LNLS, under identical experimental conditions.            Rietveld refinements were carriedout assuming the Pmm space group in the wholetemperature range achieving good quality of fits though slight profiledistortions suggest a possible mixture of phases. The average crystallite size(D≈30 nm) was calculated using Scherrer?s formula and the (110) reflection.XANES study showed that Pr atoms are in +3 oxidation state, independently ofthe temperature or O2(g) concentration. On the other hand, Co atoms showedto be very sensitive to temperature and O2 content exhibitingvariations in the relative amount of Co +2, +3 and +4 oxidation states. Evenso, the average oxidation state of Co atoms was invariant, close to +3.2. Thisis so because of the electronic compensation reaction that creates pairs of Co2+-Co4+from 2Co3+. So, on one hand, Co2+ cations expand the unitcell and, on the other, Co4+ cations are small enough to let O2-anions diffuse through the lattice. For comparison, PSC micropowders (D>300nm) exhibited almost the same average oxidation state as PSC nanopowders, butthe relative amounts of Co species were invariant under different conditions.In addition, the amount of Co2+ and Co4+ weresignificantly lowers than those exhibited by PSC nanopowders.            This study shows that the electroniccompensation reaction would play an important role in the electrochemicalproperties of PSC materials. This mechanism would explain in part the enhancedelectrochemical performance of PSC nanopowders when used as IT-SOFC cathode,along with the higher specific surface area, due both to the local expansiongenerated by Co2+ cations and the smaller ionic radius of Co4+that would favor the diffusion of O2- anions.  AcknowledgementsANPCyT, Argentina (PICT 2013 Nº 1587 Y PICT2016 Nº 1921). This work was supported by the LNLS underproposal XAFS1-16206