D. Marrero-López

Disruption of extended defects in solid oxide fuel cell anodes for methane oxidation

Point defects largely govern the electrochemical properties of oxides: at low defect concentrations, conductivity increases with concentration; however, at higher concentrations, defect–defect interactions start to dominate. Thus, in searching for electrochemically active materials for fuel cell anodes, high defect concentration is generally avoided. Here we describe an oxide anode formed from lanthanum-substituted strontium titanate (La-SrTiO3) in which we control the oxygen stoichiometry in order to break down the extended defect intergrowth regions and create phases with considerable disordered oxygen defects. We substitute Ti in these phases with Ga and Mn to induce redox activity and allow more flexible coordination. The material demonstrates impressive fuel cell performance using wet hydrogen at 950 °C. It is also important for fuel cell technology to achieve efficient electrode operation with different hydrocarbon fuels, although such fuels are more demanding than pure hydrogen. The best anode materials to date—Ni-YSZ (yttria-stabilized zirconia) cermets—suffer some disadvantages related to low tolerance to sulphur, carbon build-up when using hydrocarbon fuels (though device modifications and lower temperature operation can avoid this) and volume instability on redox cycling. Our anode material is very active for methane oxidation at high temperatures, with open circuit voltages in excess of 1.2 V. The materials design concept that we use here could lead to devices that enable more-efficient energy extraction from fossil fuels and carbon-neutral fuels.

Symmetric and reversible solid oxide fuel cells

In the past few years a novel concept of Solid Oxide Fuel Cells (SOFC) has been developed: the symmetrical Solid Oxide Fuel Cells (SSOFC). In this configuration, the same electrode material is used as anode and cathode. Several state-of-the-art SOFC electrode materials have been tested and optimised to operate in such a novel configuration, as is the case of the lanthanum chromites traditionally used as interconnect materials, lanthanum chromium manganites and strontium titanates generally used as anodes and lanthanum manganites typically used as cathodes. Fuel cell performances of approximately 800 mW cm−2 and ∼500 mW cm−2 under H2 and CH4 atmospheres have been obtained using some of these materials in symmetric configuration. Moreover, promising performances have also been reported for interconnect-based materials under city gas containing the impurity H2S. An interesting feature of the compositions evaluated is their potential use as symmetrical electrodes for Solid Oxide Electrolysis Cells (SOEC) with efficiencies in the range of the conventional SOEC systems, i.e. LSM-YSZ/YSZ/Ni-YSZ. Hence, they may be considered all-in-one Symmetric and Reversible SOFCs (SR-SOFCs) with significant advantages compared to traditional configurations, regarding both fabrication and maintenance/operation. In this review, we provide an insight into the most common materials tested as symmetrical electrodes for SOFCs and SOECs, electrolytes employed, configurations tested, new fabrication processes and procedures for microstructural engineering.

New crystal structure and characterization of lanthanum tungstate “La6WO12” prepared by freeze-drying synthesis

Lanthanum tungstates with a La/W atomic ratio between 6 and 4.8 have been synthesized as polycrystalline materials using the freeze-drying wet-chemical precursor method. Our results show that a single phase material is obtained when the La/W ratio is between 5.3 and 5.7 (T = 1500 degrees C). Outside this compositional range, segregation of either La(2)O(3) (La/W > or = 5.8) or La(6)W(2)O(15) (La/W < or = 5.2) are found. We have solved the crystal structure for the composition with a La/W nominal atomic ratio of 5.6 by combining powder X-ray and powder neutron diffraction techniques. This structure substantially differs from that previously reported for Ln(6)WO(12) (Ln = Y, Ho). The main differences between the two structure types are the crystal symmetry, the different coordination environment of the cations and the formula unit. The formula unit can be written as La(6.63)W(1.17)O(13.43) (Z = 4; calculated density = 6.395 g/cm(3)), well in accordance with the diffraction techniques, He-pycnometry and electron probe microanalysis. These materials can be described as a face centred cubic structure with space group F43m. Lattice parameters vary between 11.173 and 11.188 A, depending on composition. Dense ceramic materials are obtained at 1400 degrees C, and microanalyses measurements indicate that no significant tungsten evaporation occurs compared to the nominal values. Compositions with La(2)O(3) segregation show similar conductivity values as the single phase ones, but those containing segregation of W-rich phases show a considerable drop in conductivity with increasing content of the secondary phase.

Microstructural optimisation of materials for SOFC applications using PMMA microspheres

A novel and facile route to control the porosity of materials for Solid Oxide Fuel Cells (SOFCs) applications has been developed through the simple combination of oxide powders, polyvinyl alcohol and poly(methyl methacrylate) (PMMA) microspheres. This method allows the microstructure to be optimised improving the performances of SOFC electrode materials.

Engineering of materials for solid oxide fuel cells and other energy and environmental applications

The search for cleaner environmentally friendly power sources has been one of the hot topics in research over the past few years. Solid oxide fuel cells can be considered a promising option for the production of clean energy and one of the simplest routes to improve the efficiency of these devices is the microstructural engineering of component materials. In this work, an insight is provided into several cost-effective procedures of microstructural control of SOFC materials, ranging from an “eye-scale” down to the micrometre scale. The proposed procedures may be considered general and as such they can be used to optimise various materials for energy and environmentally related areas.

A new anode for solid oxide fuel cells with enhanced OCV under methane operation.

A new SOFC anode material based upon oxygen excess perovskite related phases has been synthesised. The material shows better electrochemical performance than other alternative new anodes and comparable performance to the state-of-art of the electrodes, Ni-YSZ cermets, under pure hydrogen. Furthermore, this material shows an enhanced performance under methane operation with high open circuit voltages, i.e. 1.2-1.4 V at 950 degrees C, without using steam excess. The effect of the anode configuration was tested in one and four layer configurations. The optimised electrode polarisation resistances were just 0.12 ohm cm(2) and 0.36 ohm cm(2), at 950 degrees C, in humidified H(2) and humidified CH(4), respectively. Power densities of 0.5 W cm(-2) and 0.35 W cm(-2) were obtained in the same conditions. A very low anodic overpotential of 100 mV at 1 A cm(-2) was obtained under humidified H(2) at 950 degrees C. Samples were tested for two days in reducing and oxidising conditions, alternating heating and cooling processes from 850 degrees C to 950 degrees C, showing stable electrode performance and open circuit voltages. The results show that the substituted strontium titanates are very promising anode materials for SOFC.

Novel microstructural strategies to enhance the electrochemical performance of La0.8Sr0.2MnO3-δ cathodes

Novel strategies based on spray-pyrolysis deposition are proposed to increase the triple-phase boundary (TPB) of La0.8Sr0.2MnO3-δ (LSM) cathodes in contact with yttria-stabilized zirconia (YSZ) electrolyte: (i) nanocrystalline LSM films deposited on as-prepared YSZ surface; (ii) the addition of poly(methyl methacrylate) microspheres as pore formers to further increase the porosity of the film cathodes; and (iii) the deposition of LSM by spray pyrolysis on backbones of Zr0.84Y0.16O1.92 (YSZ), Ce0.9Gd0.1O1.95 (CGO), and Bi1.5Y0.5O3-δ (BYO) previously fixed onto the YSZ. This last method is an alternative to the classical infiltration process with several advantages for large-scale manufacturing of planar solid oxide fuel cells (SOFCs), including easier industrial implementation, shorter preparation time, and low cost. The morphology and electrochemical performance of the electrodes are investigated by scanning electron microscopy and impedance spectroscopy. Very low values of area specific resistance are obtained, ranging from 1.4 Ω·cm(2) for LSM films deposited on as-prepared YSZ surface to 0.06 Ω-cm(2) for LSM deposited onto BYO backbone at a measured temperature of 650 °C. These electrodes exhibit high performance even after annealing at 950 °C, making them potentially suitable for applications in SOFCs at intermediate temperatures.

Is YSZ stable in the presence of Cu

XRD, XPS and electrochemical studies show clear evidence of chemical interaction under oxidising and reducing conditions between YSZ and CuO, both components of a widely studied anode material for high performance solid oxide fuel cells (SOFCs). The aim of this work is to identify and verify the nature of this interaction and find means to prevent its formation during the fabrication process of a fuel cell.

Electrical conductivity of doped and undoped calcium tartrate

The electrical conductivity of polycrystalline samples of calcium tartrate tetrahydrate ([CaC4H4O6 2H2O] 2H2O) in pure form and doped with barium and with strontium were studied in the temperature range (65<T<95°C). According to these results, it seems that two types of conduction exist in these compounds, one at low temperature and the other at high temperature, by the way of extrinsic and intrinsic conduction, respectively. This behavior may be attributed to the rotation of the tartrate ions by thermal energy.

Ti-doped SrFeO3 nanostructured electrodes for symmetric solid oxide fuel cells

Nanostructured electrodes of Sr0.98Fe1−xTixO3−δ are evaluated as both cathode and anode for solid oxide fuel cells. The electrodes are prepared by a low-cost and simple procedure based on spray-pyrolysis deposition on a porous Ce0.8Gd0.2O1.9 (CGO) layer. A homogenous coating layer of electrode catalyst nanoparticles is formed on the CGO backbone surface in a single deposition-firing step. Sr0.98Fe0.8Ti0.2O3−δ (SFT0.2) exhibits high efficiency operating as both cathode and anode with polarization resistance values of 0.1 Ω cm2 in air and 0.07 Ω cm2 in humidified H2 at 700 °C. An electrolyte supported cell with 300 μm thick La0.9Sr0.1Ga0.8Mg0.2O3−δ electrolyte and SFT0.2 symmetric electrodes shows maximum power densities of 700 and 140 mW cm−2 at 800 and 600 °C, respectively.