Al-Doped SrMoO3 Perovskites as Promising Anode Materials in Solid Oxide Fuel Cells

Two perovskite materials with SrMo1−xAlxO3−δ (x = 0.1, 0.2) compositions have been synthesized by reduction from the corresponding scheelite phases, with SrMo1−xAlxO4−δ stoichiometry; the pertinent characterization shows that the defective perovskites can be used as anode materials in solid oxide fuel cells, providing maximum output power densities of 633 mW/cm2 for x = 0.2. To correlate structure and properties, a neutron powder diffraction investigation was carried out for both perovskite and scheelite phases. Both perovskites are cubic, defined in the Pm-3m space group, displaying a random distribution of Mo and Al cations over the 1a sites of the structure. The introduction of Al at Mo positions produced conspicuous amounts of oxygen vacancies in the perovskite, detected by neutrons. This is essential to induce ionic diffusion, providing a mixed ionic and electronic conduction (MIEC), since in MIEC electrodes, charge carriers are combined in one single phase and the ionic conductivity can be one order of magnitude higher than in a conventional material. The thermal expansion coefficients of the reduced and oxidized samples demonstrated that these materials perfectly match with the La0.8Sr0.2Ga0.83Mg0.17O3−δ electrolyte, La0.4Ce0.6O2−δ buffer layer and other components of the cell. Scanning electron microscopy after the test in a real solid oxide fuel cell showed a very dense electrolyte and porous electrodes, essential requirements for this type of fuel. SrMo1−xAlxO3−δ perovskites are, thus, a good replacement of conventional biphasic cermet anodes in solid oxide fuel cells

El hidrógeno como fuente de energía: diseño de nuevas perovskitas como cátodos y ánodos en SOFC

Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física de Materiales. Fecha de lectura: 23-10-201

Low temperature phase transitions of the SrMo1-xMxO3-δ (M = Mg and Ga) perovskites

SrMoMgO and SrMoGaO (x = 0.1 and 0.2) compounds have been demonstrated to possess excellent properties as anode materials in SOFC. Additionally, these perovskite oxides, cubic at room temperature, exhibit low-temperature phase transitions that have been characterized by neutron powder diffraction (NPD) at 298, 200 and 2 K. Two phase transitions have been identified in both families: one to a tetragonal structure with I4/mcm space group and the second one to an orthorhombic Imma phase. The first one was observed near 270 K by an inflection in the magnetic susceptibility; the second one occurs between 200 and 2 K. The sequence involves a progressive increase of the (Mo,Mg)O or (Mo,Ga)O octahedra rotation, throughout one (I4/mcm) or two (Imma) of the pseudocubic axes. The magnetic properties suggest a Pauli paramagnetism for all the compounds.We thank the financial support of the Spanish Ministry of Industry and Competitiveness to the project MAT2013-41099-R

New families of Mn+-doped SrCo1-xMxO3-δ perovskites performing as cathodes in solid-oxide fuel cells

We have investigated the effect of M = Ti and V doping on the crystal structure and on the thermal, electrical, and electrochemical properties of SrCoMO (x = 0.03 and 0.05) perovskite oxides performing as cathodes in solid oxide fuel cells (SOFC). The characterization of these materials included x-ray (XRD) and neutron powder diffraction (NPD) measurements. The introduction of Ti and V replacing Co in SrCoO leads to the stabilization of a tetragonal perovskite superstructure at room temperature with a = a, c = 2a (a≈ 3.9 Å) defined in the P4/mmm space group, containing two inequivalent Co positions. Flattened and elongated (Co,M)O octahedra alternate along the c axis sharing corners in a three-dimensional array (3C-like structure). The thermal expansion coefficients of SrCoTiO and SrCoVO have been measured between 25 and 850 °C. The electrical conductivity at the SOFCs working temperatures (650-850 °C) seems to be sufficient to yield a good performance in IT-SOFC; the polarization resistance in symmetrical cells is as low as 0.016 Ω cm at 850 °C for M = Ti. In single test cells these materials generated a maximum power of 824 mW/cm at 850 °C with pure H as a fuel. This good performance make these systems promising candidates as cathode materials in SOFC.We thank the financial support of the Spanish Ministry of Science and Innovation to the project MAT2013-41099-R

Novel Mg-Doped SrMoO3 Perovskites Designed as Anode Materials for Solid Oxide Fuel Cells

SrMo1−xMxO3−δ (M = Fe and Cr, x = 0.1 and 0.2) oxides have been recently described as excellent anode materials for solid oxide fuel cells at intermediate temperatures (IT-SOFC) with LSGM as the electrolyte. In this work, we have improved their properties by doping with aliovalent Mg ions at the B-site of the parent SrMoO3 perovskite. SrMo1−xMgxO3−δ (x = 0.1, 0.2) oxides have been prepared, characterized and tested as anode materials in single solid-oxide fuel cells, yielding output powers near 900 mW/cm−2 at 850 °C using pure H2 as fuel. We have studied its crystal structure with an “in situ” neutron power diffraction (NPD) experiment at temperatures as high as 800 °C, emulating the working conditions of an SOFC. Adequately high oxygen deficiencies, observed by NPD, together with elevated disk-shaped anisotropic displacement factors suggest a high ionic conductivity at the working temperatures. Furthermore, thermal expansion measurements, chemical compatibility with the LSGM electrolyte, electronic conductivity and reversibility upon cycling in oxidizing-reducing atmospheres have been carried out to find out the correlation between the excellent performance as an anode and the structural features

Nb5+-Doped SrCoO3−δ Perovskites as Potential Cathodes for Solid-Oxide Fuel Cells

SrCoO3−δ outperforms as cathode material in solid-oxide fuel cells (SOFC) when the three-dimensional (3C-type) perovskite structure is stabilized by the inclusion of highly-charged transition-metal ions at the octahedral positions. In a previous work we studied the Nb incorporation at the Co positions in the SrCo1−xNbxO3−δ system, in which the stabilization of a tetragonal P4/mmm perovskite superstructure was described for the x = 0.05 composition. In the present study we extend this investigation to the x = 0.10–0.15 range, also observing the formation of the tetragonal P4/mmm structure instead of the unwanted hexagonal phase corresponding to the 2H polytype. We also investigated the effect of Nb5+ doping on the thermal, electrical, and electrochemical properties of SrCo1−xNbxO3−δ (x = 0.1 and 0.15) perovskite oxides performing as cathodes in SOFC. In comparison with the undoped hexagonal SrCoO3−δ phase, the resulting compounds present high thermal stability and an increase of the electrical conductivity. The single-cell tests for these compositions (x = 0.10 and 0.15) with La0.8Sr0.2Ga0.83Mg0.17O3−δ (LSGM) as electrolyte and SrMo0.8Fe0.2CoO3−δ as anode gave maximum power densities of 693 and 550 mW∙cm−2 at 850 °C respectively, using pure H2 as fuel and air as oxidant

Design of new Ga-doped SrMoO3 perovskites performing as anode materials in SOFC

We have designed and prepared SrMoGaO (x = 0.1 and 0.2) perovskite oxides. Their performance as anode materials in intermediate-temperature solid-oxide fuel cells (IT-SOFC) has been investigated. The characterization of these oxides included X-ray (XRD) and neutron powder diffraction (NPD) for x = 0.1 and 0.2. At room temperature, SrMoGaO perovskites are defined in the Pm-3m space group. The crystal structure is defined as a simple-cubic perovskite unit cell, as observed from NPD data. The electrical conductivity gave maximum values of 268 and 58 Scm at 850 °C for x = 0.1 and x = 0.2, respectively. In single test cells these materials generated output powers near 900 mW/cm at 850 °C using pure H as fuel, and demonstrated substantial performance with CH. Polarization curves and electrochemical impedance spectra (EIS) under open circuit were investigated. An adequate thermal expansion coefficient, an excellent reversibility upon cycling in oxidizing–reducing atmospheres and chemical compatibility with the electrolyte make these oxides perfect candidates for anodes in intermediate-temperature SOFC (IT-SOFCs).We thank the financial support of the Spanish Ministry of Science and Innovation to the project MAT2013-41099-R

Visualization by neutron diffraction of 2D oxygen diffusion in the Sr 0.7Ho0.3CoO3-δ cathode for solid-oxide fuel cells

Sr0.7Ho0.3CoO3-δ oxide has been recently described as an excellent cathode material (1274 mW cm-2 at 850°C with pure H2 as fuel1) for solid oxide fuel cells (SOFCs) with LSGM as electrolyte. In this work, we describe a detailed study of its crystal structure conducted to find out the correlation between the excellent performance as a cathode and the structural features. The tetragonal crystal structure (e.g., I4/mmm) basically contains layers of octahedrally coordinated Co2O6 units alternated with layers of Co1O4 tetrahedra sharing corners. An >in situ> neutron power diffraction (NPD) experiment, between 25 and 800°C, reveals the presence of a high oxygen deficiency affecting O4 oxygen atoms, with large displacement factors that suggest a large lability and mobility. Difference Fourier maps allow the visualization at high temperatures of the 2D diffusion pathways within the tetrahedral layers, where O3 and O4 oxygens participate. The measured thermal expansion coefficient is 16.61 × 10-6 K-1 between 300 and 850°C, exhibiting an excellent chemical compatibility with the electrolyte.We thank the financial support of the Spanish Ministry of Education to the project MAT2013-41099-R, and we are grateful to the Institut Laue-Langevin (ILL) for making all facilities available