Segregation of nickel/iron bimetallic particles from lanthanum doped strontium titanates to improve sulfur stability of solid oxide fuel cell anodes

Perovskite derived Ni catalysts offer the remarkable benefit of regeneration after catalyst poisoning or Ni particle growth through the reversible segregation of Ni from the perovskite-type oxide host. Although this property allows for repeated catalyst regeneration, improving Ni catalyst stability towards sulfur poisoning by H2S is highly critical in solid oxide fuel cells. In this work Mn, Mo, Cr and Fe were combined with Ni at the B-site of La0.3Sr0.55TiO3±δ to explore possible benefits of segregation of two transition metals towards sulfur tolerance. Catalytic activity tests towards the water gas shift reaction were carried out to evaluate the effect of the additional metal on the catalytic activity and sulfur stability of the Ni catalyst. The addition of Fe to the Ni perovskite catalyst was found to increase sulfur tolerance. The simultaneous segregation of Fe and Ni from La0.3Sr0.55Ti0.95-xNi0.05FexO3±δ (x ≤ 0.05) was investigated by temperature programmed reduction, X-ray diffraction and X-ray absorption spectroscopy and catalytic tests after multiple redox cycles. It is shown that catalytic properties of the active phase were affected likely by the segregation of Ni/Fe alloy particles and that the reversible segregation of Ni persisted, while it was limited in the case of Fe under the same conditions

Aerosol spray synthesis of powder perovskite-type oxides

The physicochemical properties and the performance of perovskite‐type oxides are highly dependent on the applied preparation method and correlated synthesis conditions. Spray synthesis methods are available for a wide range of perovskite‐type oxides for use in various applications, including catalysis. Besides enabling a cost‐effective/economic production of materials with a high throughput, reproducibility, and homogeneity, spray methods can offer materials with unusual structural properties that conventional synthesis methods are not able to confer. Taking catalytic applications of perovskite‐type oxides into consideration, various spray synthesis methods are reviewed with the aim to provide a compendium on characteristics of synthesis methods, structural properties, and performance of materials

LST-CGO anodes : deconvolution of impedance spectra and relationship with composition and microstructure

B0607Ni-free anodes in SOFC and SOEC that can withstand severe conditions and provide high performances, which is of great practical and scientific interest. Lanthanum doped strontium titanates (LST) are among the most interesting alternatives to state of the are Ni-YSZ due to their excellent redox stability [1] low reactivity with other fuel cell components [2] and good electronic conductivity. Insufficient ionic conductivity of LST is only one of few drawbacks of this class of materials and can be mitigated through fabrication and optimization of composite electrodes containing LST and good ionic conductor (e.g. CGO or YSZ). In this study the electrochemical performance of ceramic (Ni-free) SOFC anodes, consisting of A-site deficient La0.2Sr0.7TiO3-d (LST) perovskite and Gd0.1Ce0.9O1.95-d (CGO), was thoroughly investigated. Microstructures and compositions were systematically modified to gain information about the microstructural impact on electrochemical performance. The highest impedances are measured at low frequencies, which in contrast to the literature cannot be linked with gas concentration impedance. The low frequency process (ca. ~1 Hz) was attributed to the chemical capacitance. The EIS and advanced microstructure quantification methodology point out that the chemical capacitance correlates inversely with the available surface area of CGO. The influence of CGO surface reactions, such as hydrogen adsorption, which represent the kinetic limitation for the dominant anode process at 1 Hz region and for the associated chemical capacitance, is discussed. The impact of 30 isothermal redox cycles on degradation and the anode performance is presented

Exsolution and integration of nanosized SMART catalysts for next generation SOFC anodes

B1104La-doped strontium titanate (LST) materials are widely recognized among other alternative anodes as good electronic conductors with high tolerance to redox cycles, but with insufficient catalytic activity. However, doping of LST with quasi-stable metal ions (e.g. Ni, Co) allows a selective exsolution of these metals from the bulk onto the materials surface and thus increasing the catalytic activity. Previously we have demonstrated our SMART material concept with selfregeneration effect, in which nano-sized nickel catalyst is repeatedly exsolved from and incorporated back into the La0.2.Sr0.7Ti0.95Ni0.05O3-d (LSTN) perovskite host structure. Nickel nanoparticles are exsolved from LST at SOFC anode conditions and nickel is reincorporated at high pO2, during a redox cycle. This turns redox cycles - the weakness of conventional Ni/YSZ anodes - into an advantage and regenerates the material. The authors present recent advances of the SMART material catalysts based on LSTN. We demonstrate that upon harsh heat treatment (T = 1200°C) depending on the location and site at least three types of nickel particles being generated LSTN: a) fine particles with presumably high catalytic activity (dp 150 nm), generated on the facets of grains are reversibly incorporated into the LSTN host matrix (Fig. 1), while those large ones located at the grain boundaries underwent an oxidation to NiO. Temperature programmed reduction has proven unchanged REDOX reversibility of LSTN materials upon 9 redox cycles a temperature of 900°C, suggesting catalytic reversibility

Synthesis and performance of A-site deficient lanthanum-doped strontium titanate by nanoparticle based spray pyrolysis

In this study A-site deficient lanthanum doped strontium titanate, which is considered as a promising, redox-stable candidate for full ceramic anodes in Solid Oxide Fuel Cells (SOFCs), was produced by nanoparticle-based spray pyrolysis. In this process cost-effective nitrate salts and titania nanoparticles are used as precursors. LST with a nominal composition of La_0.2Sr_0.7TiO3 showed limited stability at temperatures higher than 1290°C. It was observed that minor contents of secondary phases, which form at elevated temperatures, invoke a drastic loss of the electrical performance. Thermal stability is significantly increased by a slight enrichment of strontium. The phase pure A-site deficient LST shows remarkably higher electrical performance than similar stoichiometric counterparts. Reductive sintering results in a conductivity as high as 600 S cm−1 at 600°C. Furthermore the A-site deficient LST shows a high redox-stability and fast redox-kinetics. With these properties LST is a suitable material for the fabrication of a new generation of Ni-free ceramic SOFC anodes. Secondary phases have a significant influence on the electrical conductivity and redox behaviour

Lanthanum doped strontium titanate - ceria anodes : deconvolution of impedance spectra and relationship with composition and microstructure

Electrochemical performance of ceramic (Ni-free) SOFC anodes based on La0.2Sr0.7TiO3-δ (LST) and Gd0.1Ce0.9O1.95-δ (CGO) is thoroughly investigated. Microstructures and compositions are systematically varied around the percolation thresholds of both phases by modification of phase volume fractions, particle size distributions and firing temperature. Differential impedance spectroscopy was performed while varying gas composition, electrical potential and operating temperature, which allows determining four distinct electrode processes. Significant anode impedances are measured at low frequencies, which in contrast to the literature cannot be linked with gas concentration impedance. The dominant low frequency process (∼1 Hz) is attributed to the chemical capacitance. Combined EIS and microstructure investigations show that the chemical capacitance correlates inversely with the available surface area of CGO, indicating CGO surface reactions as the kinetic limitation for the dominant anode process and for the associated chemical capacitance. In anodes with a fine-grained microstructure this limitation is significantly smaller, which results in an impressive power output as high as 0.34 Wcm−2. The anodes show high redox stability by not only withstanding 30 isothermal redox cycles, but even improving the performance. Hence, compared to conventional Ni-cermet anodes the new LST-CGO material represents an interesting alternative with much improved redox-stability

On the chemical interaction of nanoscale lanthanum doped strontium titanates with common scandium and yttrium stabilized electrolyte materials

Chemical interactions between common electrolyte materials and various La doped strontium titanates (LST), which are redox-stable candidates for SOFCs anodes, were thoroughly investigated. The reactions of nanosized reagents were studied by SEM/EDX microscopy and XRD with subsequent Rietveld refinement. It was found that all A-site deficient LSTs promoted a reaction with Sc and Y stabilized zirconia, whilst stoichiometric LST was chemically stable. Detected structural and microstructural changes were solely assigned to high mobility of Ti. Diffusion of Ti into the zirconia structure promoted formation of tetragonal structures with p42/nmc-type space groups. The results indicate that the reduction of oxygen partial pressure during sintering and application of Sc-containing electrolyte material are successful strategies to hinder or even avoid reactivity

The Rheology of Stabilised Lanthanum Strontium Cobaltite Ferrite Nanopowders in Organic Medium Applicable as Screen Printed SOFC Cathode Layers

The production and optimisation of screen printing (SP) pastes containing La0.58Sr0.42Co0.21Fe0.79O3-δ and La0.61Sr0.41Co0.19Fe0.79O3-δ (LSCF) were investigated. The application of these nanopowders is supposed to improve the cathode's microstructure and increase its mechanical strength. Thirty seven pastes containing LSCF were carried out with variation in binders, dispersants and different particle size distribution. The rheological behaviour of these pastes was investigated. It was found that commercially available dispersant Solsperse 3000 resulted in the best suspension stability, achieving almost 55 times lower viscosity value for pastes containing 20 vol.-%, than pastes without any dispersants. The shear thinning behaviour was found to be favourable for the LSCF cathode deposition. A cathode made from the mixture of nano and submicron powder exhibits a polarisation resistance as low as 0.76 Ω cm2 at 592°C

SMART catalyst based on doped Sr-titanite for advanced SOFC anodes

B1403To increase the durability of SOFC stacks, robust anodes with high catalytic performance and redox tolerance are needed. Among all alternatives, La-doped strontium titanate (LST) materials were recognized to possess good electronic conductivity and high tolerance to redox cycles1,2, but modest electro-catalytic activity, which can be enhanced in conjunction with an appropriate catalyst. Nevertheless, anodes with conventional composite microstructures (e.g. LST with equally sized Ni-phase) are still prone to sulphur poisoning, coking and to coalescence of the Ni phase over time. The authors present recent advances of a SMART material concept with a catalytic and microstructural self-regeneration effect, in which nanosized nickel catalyst is repeatedly exsolved from and incorporated back into the LST perovskite host structure. Ni-nanoparticles are exsolved from LST at low pO2 (i.e. at SOFC anode conditions) and the Ni is re-incorporated hat high pO2. Since titanates are highly tolerant to changes of the oxygen partial pressure, application of controlled redox cycles could therefore lead to the burn-off of harmful sulphides and/or carbon deposits and at the same time the incorporation-exsolution cycles also help circumventing the catalysts coarsening problem. We present the concept in which Ni-doped LST is applied to repetitively exsolute and re-incorporate the Ni catalyst, hence offering a microstructural self-regeneration mechanism