A Broad Stability Investigation of Nb-Doped SrCoO_2.5+δas a Reversible Oxygen Electrode for Intermediate-Temperature Solid Oxide Fuel Cells

Abstract

The present work reports a systematic study on the structural, thermal, electrical and electrochemical stability of SrCo1−xNbxO2.5+δ series as a potential reversible oxygen-electrode for intermediate-temperature solid oxide fuel cells. The identified best composition is x = 0.10, which exhibits a stable pseudo primitive cubic structure at <700°C and a reversible oxygen redox reaction at 350°C. The conductivity of this material is p-type and also exhibits a peak at 350°C, implying that the electron hole conduction is closely associated with the oxygen nonstoichiometry. Electrochemical impedance spectroscopy analysis indicates a low polarization resistance rate-limited by a slower surface O2 dissociation step. Overall, the material is thermally stable and oxygen redox reversible below 700°C, above which a catalytically less active brownmillerite SrCoO2.5 is formed.

A new defect chemistry model for Nb-doped SrCoO2.5+δ: The role of oxygen interstitials and delocalized-to-localized electron holes

“…At elevated temperatures, e.g. >300 o C, these oxygen-ordered superlattice structures tend to transform into a single-phase oxygen-disordered perovskite pseudo-cubic unit cell as revealed by our early high-temperature XRD examination 35 .…”

“…where, N A is the Avogadro constant, 6.02×10 23 (mol -1 ); a is the lattice constant of a pseudo-cubic SCN (with 10mol%Nb doping) is 3.90 Å 35 . According to prior studies, Co-3d orbital and O-2p orbital tend to hybridize strongly in SCO systems 21,32 , producing a broad conduction band.…”

“…No higher than 700 o C was pursued in this study because of the concerns over the stability of SCN10 above 700 o C according to our previous study 35 as well as another study 25 . In the meanwhile, P O2 was varied only in the range of 0.001-1 atm in a total of 12 steps at each temperature.…”

Promotion on electrochemical performance of Ba‐deficient Ba1 − xBi0.05Co0.8Nb0.15O3 − δcathode for intermediate temperature solid oxide fuel cells

“…The reduction of cobalt valance state as the temperature was also observed in the Ta-doped SrCoO 3 by Wang 57 , where the average cobalt valance state decreased from 3.25 at 300°C to 3.00 at 600°C. This also happened for Nb-doped SrCoO 2.5 + δ 58 .…”

“…From Figure B, it was clear that the conductivities increased with increasing oxygen pressure at all testing temperatures, revealing the p‐type electron hole ( h • ) conduction. The formation of lattice oxygen and oxidation of cobalt to electron holes may be explained through the oxygen‐ingestion on the oxygen vacancy, via the following equilibrium and its constant K ep : …”