Cristian Savaniu

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.

Structural origins of the differing grain conductivity values in BaZr0.9Y0.1O2.95 and indication of novel approach to counter defect association

Proton conducting oxides such as BaCe0.9Y0.1O3−δ have considerable promise for intermediate temperature fuel cells. Unfortunately these tend to be unstable, e.g. to attack by carbonation. Previous work has highlighted the possibility of utilising barium zirconate to provide a chemically stable electrolyte; however such materials are difficult to sinter yielding very high overall resistances. Whilst this sintering problem is soluble, there are still very significant questions about the intrinsic grain conductivity, which varies by orders of magnitude for different reports. Here we demonstrate that there are two variants of BaZr0.9Y0.1O2.95, both with the cubic perovskite structure. The α-form exhibits a slightly smaller unit cell and much lower protonic conductivity than the β-form. The α-form is observed in better equilibrated samples and neutron diffraction demonstrates that this difference originates in a small degree of cross substitution of the Y atom onto the A-sites for the β-form, suggesting a novel approach to enhance ionic conductivity by reducing defect association through A-site substitution.

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.

Investigation of proton conducting BaZr0.9Y0.1O2.95 : BaCe0.9Y0.1O2.95 core–shell structures

Barium zirconate shows better chemical and mechanical stability than the corresponding cerate, but lower protonic conductivity largely due to resistive grain boundary contributions. In this work we attempted to reduce the grain boundary resistance of BaZr0.9Y0.1O2.95 starting from a core–shell modification of its grain with a thin film of BaCe0.9Y0.1O2.95. The resultant core–shell material was characterised by X-ray powder diffraction (XRD), thermal gravimetric analysis (TGA) and scanning electron microscopy (SEM). Impedance spectroscopy performed on the material in wet 5% H2/Ar atmosphere showed a significant reduction in terms of both bulk and grain boundary resistances of the initial barium zirconate when modified with a thin film of barium cerate. As by thin films modification the total conductivity of the sample increases by about one order of magnitude, this surface modification concept appears to be a promising way to control the grain boundary properties for a given material.

Modified strontium titanates: from defect chemistry to SOFC anodes

Modified strontium titanates have received much attention recently for their potential as anode material in solid oxide fuel cells (SOFC). Their inherent redox stability and superior tolerance to sulphur poisoning and coking as compared to Ni based cermet anodes could improve durability of SOFC systems dramatically. Various substitution strategies can be deployed to optimise materials properties in these strontium titanates, such as electronic conductivity, electrocatalytic activity, chemical stability and sinterability, and thus mechanical strength. Substitution strategies not only cover choice and amount of substituent, but also perovskite defect chemistry, distinguishing between A-site deficiency (A1−xBO3) and cation-stoichiometry (ABO3+δ). Literature suggests distinct differences in the materials properties between the latter two compositional approaches. After discussing the defect chemistry of modified strontium titanates, this paper reviews three different A-site deficient donor (La, Y, Nb) substituted strontium titanates for their electrical behaviour and fuel cell performance. Promising performances in both electrolyte as well as anode supported cell designs have been obtained, when using hydrogen as fuel. Performances are retained after numerous redox cycles. Long term stability in sulphur and carbon containing fuels still needs to be explored in greater detail.

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.

Preparation via a solution method of La0.2Sr0.25Ca0.45TiO3 and its characterization for anode supported solid oxide fuel cells

La0.2Sr0.25Ca0.45TiO3 is a carefully selected composition to provide optimal ceramic and electrical characteristics for use as an anode support in solid oxide fuel cells. In this study we focus on the process optimization and characterization of A-site deficient perovskite, La0.2Sr0.25Ca0.45TiO3 (LSCTA-), powders prepared via a solution method to be integrated into the SOFC anode supports. A Pechini method has been applied to successfully produce single phase perovskite at 900 °C. Processing conditions have been modified to yield a powder that displays a similar sintering profile to commercial yttria stabilised zirconia. The conductivity behavior of porous bodies under redox has been investigated showing a 2 stage process in both oxidation and reduction cycling that exhibits strong reversibility. For the reduction process, addition of impregnated ceria reduces the onset delay period and increases the apparent rate constant, k values, by 30–50% for both stages. The addition of ceria had less influence on the oxidation kinetics, although the conductivity values of both oxidised and reduced porous bodies were enhanced.

Sr3Ca1-xZnxZr0.5Ta1.5O8.75: a study of the influence of the B-site dopant nature upon protonic conduction

Abstract The new perovskite oxide Sr 3 Ca 1+ x Zr (1− y )− x /2 Ta (1+ y )− x /2 O 8.5−[(5 x −2 y )/4] belongs to the class of complex perovskites of general formula A 3 (B′,B″) 3 O 9− δ . The specific composition Sr 3 CaZr 0.5 Ta 1.5 O 8.75 has been already reported as a promising proton conducting material in a wet hydrogen atmosphere. The extent of vacancy formation and hence degree of hydration in these systems is quite extensive for a perovskite lattice. This seems related to the presence of the polarisable Ca 2+ cation on the B site. In this study, we have replaced Ca with varying amounts of Zn. The extent of hydration is found to rapidly decrease with Zn substitution, as does the extent of proton conductivity. As very little water uptake is observed for compositions with more than 25% of Ca 2+ replaced by Zn 2+ , this effect is not simply statistical. Instead, it may be supposed that extended interactions due to the polarisable Ca 2+ ions facilitate the water uptake. It is proposed that mixed oxides such as Sr 3 CaZr 0.5 Ta 1.5 O 8.75+ y H 2 O are effectively underdoped in terms of protonic conductivity at low temperatures in wet atmospheres.

Sol–gel preparation and characterisation of dense proton conducting Sr3CaZr0.5Ta1.5O8.75 thin films

Abstract Dense proton conducting Sr3CaZr0.5Ta1.5O8.75 films (∼1.25 μm, with grain size in the 200–400 nm range) were deposited, using the sol–gel method, on Al2O3–8%Y2O3-stabilised ZrO2 plates. The obtained gels were characterised by differential and thermogravimetric thermal analysis (DTA–TGA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results of a study of the structural and electrical properties of Sr3CaZr0.5Ta1.5O8.75 films deposited on the mentioned substrates are presented herein. The structural data for the gels and films were compared with those obtained for the same material prepared by solid state synthesis. Electrical properties of the sandwich-type structure were investigated by AC impedance conductivity measurements at different temperatures, in both dry and wet 5% H2/Ar atmospheres. A careful analysis of the impedance spectra for this complex structure was performed, using a model with a series of five electrical circuits, having resistance and capacitance coupled in parallel. The specific responses observed in the impedance spectra were assigned to the corresponding substrate and layer contributions. A significant improvement, by an order of magnitude, in the electrode response was observed in the presence of the interleaving Sr3CaZr0.5Ta1.5O8.75 proton conducting layer, between the electrode and electrolyte. This enhancement is lost at temperatures above that at which the Sr3CaZr0.5Ta1.5O8.75 dehydrates and its protonic conductivity diminishes. Considering the structural and electrical characterisation results, these Sr3CaZr0.5Ta1.5O8.75 sol–gel derived films have a potential use for proton conducting electrolyte or intermediate layer in fuel cells.

New Materials for Improved Durability and Robustness in Solid Oxide Fuel Cell

This chapter provides an overview of the considerations that must be made regarding new materials development for improved durability and robustness in solid oxide fuel cells (SOFCs). A number of recent development concepts are outlined for the core cell materials of anode, electrolyte, and cathode, in particular new catalytic approaches such as catalyst impregnation and exsolution on the anode to improve redox and fuel flexibility and reduced temperature cathodes. Some of the challenges of scaling up into larger stacks are also discussed. Here the interactions of cell materials with stack materials, in particular the interconnect, are summarized, such as chromium poisoning and cell to interconnect electrical contact, both of which feature prominently in SOFC stack lifetime issues. Barriers to new materials development are outlined along with the potential for accelerated testing.