Defect Chemistry, Electrical Properties, and Evaluation of New Oxides Sr₂CoNb_1−xTi_xO_6−δ (0≤x≤1) as Cathode Materials for Solid Oxide Fuel Cells

Abstract

The perovskite series Sr CoNb Ti O (0≤x≤1) was investigated in the full compositional range to assess its potential as cathode material for solid oxide fuel cell (SOFC). The variation of transport properties and thus, the area specific resistances (ASR) are explained by a detailed investigation of the defect chemistry. Increasing the titanium content from x=0-1 produces both oxidation of Co to Co (from 0 up to 40 %) and oxygen vacancies (from 6.0 to 5.7 oxygen atom/formula unit), although each charge compensation mechanism predominates in different compositional ranges. Neutron diffraction reveals that samples with high Ti-contents lose a significant amount of oxygen upon heating above 600 K. Oxygen is partially recovered upon cooling as the oxygen release and uptake show noticeably different kinetics. The complex defect chemistry of these compounds, together with the compositional changes upon heating/cooling cycles and atmospheres, produce a complicated behavior of electrical conductivity. Cathodes containing Sr CoTiO display low ASR values, 0,13 Ω cm at 973 K, comparable to those of the best compounds reported so far, being a very promising cathode material for SOFC.

Novel Perovskite Materials for Thermal Water Splitting at Moderate Temperature

“…In recent papers, we have studied the structure and microstructure as well as the performance as SOFC cathodes of materials with compositions Sr 2 CoNb 1− x Ti x O 6− δ (0≤ x ≤1; δ =number of oxygen vacancies), which fulfil all mentioned requirements. Along the whole series, the transport properties and catalytic activity vary with the titanium content.…”

“…Despite this simple average structure, this family of materials presents a complex microstructure . Some of these oxides display performances as SOFC cathodes that put them among the best materials for this application . Good electrochemical properties are related to proper catalytic activity (associated with cobalt) and to high electric conductivity, both electronic and ionic.…”

“…The former is owing to metals in mixed oxidation states, whereas the latter is associated with mobile oxygen vacancies. Both the metal oxidation states and the concentration of anionic vacancies may vary reversibly on heating/cooling under different atmospheres (i.e., p O 2 ) . Thus, samples with x =0.70 and 1.00 treated under Ar and pure oxygen have been studied by NPD.…”

“…The two oxides present some degree of anionic vacancies even after treatment in pure oxygen, with oxygen sub‐stoichiometries of approximately 2 and 4 % for x =0.70 and 1.00, respectively. As regards the results of the experiments under Ar, compounds of the Sr 2 CoNb 1− x Ti x O 6− δ family seem to lose oxygen easily; upon treatment at low oxygen partial pressure in Ar flow ( p O 2 ≈10 −5 atm) the concentration of oxygen vacancies increases up to 11 % for x =0.70 and 14 % for x =1.00. As expected, the oxygen deficiency of samples prepared in air ( p O 2 ≈0.20 atm) is intermediate between those of the two extreme conditions (Table S1 in the Supporting Information).…”

“…In addition, the material's microstructure may play a relevant role in reduction/oxidation because oxygen vacancies can be accommodated in the domain walls . Neither Ti 4+ nor Nb 5+ experience any oxidation/reduction processes; the couple Co 4+ /Co 3+ is solely responsible for the redox behavior of these materials …”