Abstract

Ba_0.9Co_0.7Fe_0.2Nb_0.1O_3 − δ (BCFN) perovskite material was synthesized and evaluated as cathode for La_0.8Sr_0.2Ga_0.83Mg_0.17O_3 − δ (LSGM) electrolyte supported intermediate temperature solid oxide fuel cells (IT-SOFCs). X-ray diffraction results showed that BCFN was chemically compatible with the LSGM electrolyte. Maximum power densities of 0.36, 0.57, 0.80 and 1.1 W/cm² were obtained for LSGM electrolyte supported cells with BCFN as cathode and Ni-GDC as anode operated at 650, 700, 750 and 800 °C, respectively. Further, the cell performance was stable under a constant current of 0.6 A/cm² for over 204 h at 750 °C.

Highlights

? Ba_0.9Co_0.7Fe_0.2Nb_0.1O_3 − δ (BCFN) was synthesized and evaluated as cathode for IT-SOFCs. ? BCFN shows a very low cathode polarization resistance of 0.04 and 0.02 Ωcm² at 750 and 800 °C. ? Cell power of 0.80 and 1.1 W/cm²was achieved at 750 and 800 °C for LSGM-electrolyte supported cells. ? Using BCFN as cathode, cell performance was stable under 0.6 A/cm² for over 204 h at 750 °C.

Abstract

The fabrication and electrochemical properties of graded La_0.7Sr_0.3CuO_3−δ–Sm_0.2Ce_0.8O_2−δ (LSCu–SDC) composite cathodes were investigated in this paper. The phase composition, microstructure and electrochemical properties of the electrodes were characterized using X-ray diffraction (XRD), electron microscopy, electrochemical impedance spectroscopy (EIS) and cathodic polarization examinations. The results showed that the triple-layer graded cathode had super electrochemical performance comparing with the monolayer cathode. The graded LSCu–SDC cathode showed a polarization resistance of 0.094 Ωcm², a value much lower than the monolayer LSCu cathode of 0.234 Ωcm² at 800 °C in air. The current density of the graded cathode was 0.341 A cm⁻², more than double higher than monolayer LSCu of 0.146 A cm⁻² at an overpotential of 30 mV. The improved electrochemical performance could be attributed to the improved physical and chemical compatibility of the cathode layers in graded compositions with SDC electrolyte as well as the enlargement of triple-phase boundary for oxygen reduction.

Abstract

The SrSc_0.2Co_0.8O_3−δ (SSC) perovskite was investigated as a cathode material for low temperature solid-oxide fuel cell. The material showed an almost linear thermal expansion from room temperature to 1000 °C in air with the average thermal expansion coefficient of only 16.9 × 10⁻⁶ K⁻¹. The Sc-doping made the absence of Co⁴⁺ in SSC, which resulted in not only dramatically reduced thermal expansion coefficient but also extremely high oxygen vacancies concentrations in the lattice at low temperature. The area specific polarization resistance was 0.206 Ω cm² for SSC at 550 °C, which is about 52% lower than the value of a Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ-based cathode. A peak power density as high as 564 mW cm⁻² was obtained at 500 °C based on a 20 μm thick Sm_0.2Ce_0.8O_1.9 electrolyte by adopting SSC cathode.

Abstract

La and Sc co-doped SrTiO₃ was synthesized via solid state reaction. The oxygen ion migration energy was investigated by first-principles calculations in SrBO₃ systems (B = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Ga, Ge, As, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Cd, In, Sn and Sb) with perovskite structure. Structure with Sc showed the lowest oxygen migration energy, and thus Sc was selected as B-site dopant with the primary aim to improve the ionic conductivity of SrTiO₃-based anode materials. With increasing Sc-doping amount, the electrical conductivity of La_0.3Sr_0.7Sc_xTi_1−xO_3−δ decreased in 25–1000 °C, while the ionic conductivity increased significantly between 500 and 1000 °C. The ionic conductivity for La_0.3Sr_0.7Sc_0.10Ti_0.90O_3−δ was 1 × 10⁻² S cm⁻¹ and increased about 230% compared with La_0.3Sr_0.7TiO_3−δ at 800 °C and under oxygen partial pressure of 10⁻¹⁹ atm. Sc-doping increased the oxygen vacancy concentration and decreased the oxygen migration energy, thus facilitating the conduction process of oxygen ions in La and Sc co-doped SrTiO₃. The possible charge compensation mechanism of Sc-doped La_0.3Sr_0.7TiO_3−δ can be described as (x = x₁ + x₂).

Abstract

A systematic study and evaluation were performed on the effect of scandium doping at the B site of Pr_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (PSCF) on key material properties as cathode for intermediate temperature solid oxide fuel cells (IT-SOFC). The doped products Pr_0.6Sr_0.4(Co_0.2Fe_0.8)_(1−x)Sc_xO_3−δ (PSCFS_x, x=0.0–0.2) retained perovskite structure confirmed by X-ray diffraction, and their particles were smaller than the non-doped materials as evidenced by TEM. The electrical conductivity (EC) of PSCFS_x decreased with increasing Sc³⁺ content, but EC values were still larger than 100 S cm⁻¹ in temperature range of 300–800 °C as x ≤ 0.1. The thermal expansion coefficients (TEC) of PSCFS_x were observed to generally decrease with increasing x especially at lower temperature range of 50–600 °C. In addition, the AC impedance revealed better electrochemical performance of PSCFS_x cathode as x ≤ 0.1 than that of the undoped sample PSCF. Therefore, PSCFS_x (x ≤ 0.1) shows some potential as cathode electrode for IT-SOFC. The function of Sc³⁺ dopant was tentatively elucidated and discussed.

Research highlights

? Scandium doped Pr_0.6Sr_0.4Co_0.2Fe_0.8O_3–δ was evaluated as cathode of IT'SOFC. ? The doped products Pr_0.6Sr_0.4(Co_0.2Fe_0.8)_(1–x)Sc_xO_3–δ (PSCFS_x) showed smaller particles. ? The conductivity and thermal expansion coefficients of PSCFS_x (x = 0.0∼0.2) decreased as raising x. ? PSCFS_x (x ≤ 0.1) exhibited better cathode performance than PSCF and PSCFS_x (x > 0.1) did. ? The function of Sc³⁺ dopant was tentatively elucidated and discussed.

Abstract

Temperature dependent X-ray and neutron powder diffraction have been exploited to revisit the structural evolution of SrCoO_2.5 brownmillerite-like phase from room temperature to 1200 °C, in complement with DTA–TG, conductivity and thermal expansion measurements. Three different polymorphs have been identified: the orthorhombic “O” brownmillerite phase between room temperature and 653 °C, the hexagonal “H” phase between 653 °C and 920 °C, and the cubic perovskite “C” above 920 °C, which is transformed again, by cooling, into the “H” phase at 774 °C. The metastable “O” brownmillerite phase can only be obtained by quenching in liquid N₂ from 1000 °C. The refinement of the crystal and magnetic structures of SrCoO_2.5 from NPD data collected “in situ” as a function of temperature in an air atmosphere led to define structural details of the very distinct coordination polyhedra present in the different polymorphs, which have been correlated with the transport properties. The conductivity shows little dependence up to 250 °C, and it abruptly increases above this temperature, which is concomitant with a contraction of the Co2–O3 bonds of the tetrahedral CoO₄ units of the “O” brownmillerite structure at 350 °C, suggesting the presence of an insulator-to-metal transition in this structural polymorph. The dramatic reduction of conductivity above 500 °C is connected with the transformation to the “H” polymorph, with a complete oxygen sublattice and a face-sharing octahedral framework with a poor 1D electronic conduction. Further heating above 900 °C boosts again the conductivity when the sample is entering the cubic perovskite region, characterized by a 3D vertex-sharing network of CoO₆ octahedra. The total conductivity displays a maximum value of 150 S cm⁻¹ at 900 °C and it increases during the cooling run, thus displaying a characteristic metallic behaviour for the “C” phase. This polymorph presents conductivity values substantially above the required 100 S cm⁻¹ in solid oxide fuel cells, suggesting that it is a good candidate for cathode materials in energy conversion devices.

Graphical abstract

Three very distinct polymorphs of overall stoichiometry SrCoO_2.5 are identified in the temperature range 25–1200 °C, starting from the “O” brownmillerite phase previously obtained by quenching in liquid N₂ from 1000 °C. A metal–insulator transition is detected in the “O” phase, correlated to subtle structural changes. A drop of conductivity associated to the “H” hexagonal polymorph is followed by a huge increase of σ when entering the cubic “C” phase.