Defect Chemistry, Electrical Properties, and Evaluation of New Oxides Sr2 CoNb1-x Tix O6-δ (0≤x≤1) as Cathode Materials for Solid Oxide Fuel Cells

Superior Performance as Cathode Material for Intermediate-Temperature Solid Oxide Fuel Cells of the Ruddlesden-Popper n = 2 Member Eu2SrCo0.50Fe1.50O7-δ with Low Cobalt Content

The effects of the contents of iron and cobalt on the crystal structure, oxygen content, thermal expansion coefficient, and electrical-electrochemical properties of materials Eu₂SrCo_xFe_2-xO_7-δ (x = 0.50 and 1.00) are reported. These oxides are well-ordered new members of the Ruddlesden-Popper series (Eu,Sr)_n+1(Co,Fe)_nO_3n+1 system with n = 2 as determined by selected area electron diffraction and high-resolution transmission electron microscopy and X-ray diffraction studies. The two materials are semiconductors of p-type, with much higher total conductivity under working conditions for the low cobalt compound, Eu₂SrCo_0.50Fe_1.50O_7-δ. Composite cathodes prepared with this oxide present much lower area-specific resistance values (0.08 Ω·cm² at 973 K in air) than composites containing Eu₂SrCo_1.00Fe_1.00O_7-δ (1.15 Ω·cm²). This significant difference is related to the much higher total conductivity and a sufficiently high content of oxygen vacancies in the Fe-rich phase. The excellent electrochemical performance of Eu₂SrCo_0.50Fe_1.50O_7-δ with low cobalt content, which shows one of the lowest area-specific resistance reported so far for a Ruddlesden-Popper oxide, makes it a good candidate for application as a cathode material for solid oxide fuel cells at intermediate temperatures in real devices.

The Effects of Sr Content on the Performance of Nd1-xSrxCoO3-δ Air-Electrode Materials for Intermediate Temperature Solid Oxide Fuel Cells under Operational Conditions

The potential of the perovskite system Nd_1-xSr_xCoO_3-δ (x = 1/3 and 2/3) as cathode material for solid oxide fuel cells (SOFCs) has been investigated via detailed structural, electrical, and electrochemical characterization. The average structure of x = 1/3 is orthorhombic with a complex microstructure consisting of intergrown, adjacent, perpendicularly oriented domains. This orthorhombic symmetry remains throughout the temperature range 373-1073 K, as observed by neutron powder diffraction. A higher Sr content of x = 2/3 leads to stabilization of the cubic perovskite with a homogeneous microstructure and with a higher oxygen vacancy content and cobalt oxidation state than the orthorhombic phase at SOFC operation temperature. Both materials are p-type electronic conductors with high total conductivities of 690 and 1675 S·cm^-1 at 473 K in air for x = 1/3 and 2/3, respectively. Under working conditions, both compounds exhibit similar electronic conductivities, since x = 2/3 loses more oxygen on heating than x = 1/3, associated with a greater loss of p-type charger carriers. However, composite cathodes prepared with Nd_1/3Sr_2/3CoO_3-δ and Ce_0.8Gd_0.2O_2-δ present lower ASR values (0.10 Ω·cm² at 973 K in air) than composites prepared with Nd_2/3Sr_1/3CoO_3-δ and Ce_0.8Gd_0.2O_2-δ (0.34 Ω·cm²). The high activity for the oxygen electrochemical reaction at intermediate temperatures is likely attributable to a large disordered oxygen-vacancy concentration, resulting in a very promising SOFC cathode for real devices.

Novel Perovskite Materials for Thermal Water Splitting at Moderate Temperature

Materials with the formula Sr₂ CoNb_1-x Ti_x O_6-δ (x=1.00, 0.70; δ=number of oxygen vacancies) present a cubic perovskite-like structure. They are easily and reversibly reduced in N₂ or Ar and re-oxidized in air upon heating. Oxidation by water (wet N₂ ), involving splitting of water at a temperature as low as 700 °C, produces hydrogen. Both compounds displayed outstanding H₂ production in the first thermochemical cycle, the Sr₂ CoNb_0.30 Ti_0.70 O_6-δ material retaining its outstanding performance upon cycling, whereas the hydrogen yield of the x=1 oxide showed a continuous decay. The retention of the materials' ability to promote water splitting correlated with their structural, chemical, and redox reversibility upon cycling. On reduction/oxidation, Co ions reversibly changed their oxidation state to compensate the release/recovery of oxygen in both compounds. However, in Sr₂ CoTiO_6-δ , two phases with different oxygen contents segregated, whereas in Sr₂ CoNb_0.30 Ti_0.70 O_6-δ this effect was not evident. Therefore, this latter material displayed a hydrogen production as high as 410 μmol H 2  g^-1 _perovskite after eight thermochemical cycles at 700 °C, which is among the highest ever reported, making this perovskite a promising candidate for thermosolar water splitting in real devices.