Rainer Küngas

Synthesis and Oxygen Storage Capacity of Two‐Dimensional Ceria Nanocrystals

Shape-controlled synthesis of inorganic nanomaterials has received great attention due to their unique shapedependent properties and their various applications in catalysis, electronics, magnetics, optics, and biomedicine. Among these nanomaterials, ultrathin twodimensional (2D) anisotropic nanomaterials are especially attractive due to their high surface-to-volume ratio and potential quantum size effects. A variety of approaches have been developed to prepare such nanomaterials. Typical methods include vapor deposition, templated synthesis, electrochemical deposition, sol–gel processing, and solvothermal/hydrothermal treatments. Solution-phase chemical synthesis has proven particularly effective in controlling the size and morphology of the nanomaterials. Ceria has been widely used in catalysis, optics, sensors, and solid oxide fuel cells. Due to its high oxygen storage capacity (OSC), which originates from easy conversion between CeO2 and CeO2 x, ceria has found its primary utilization in catalysis as an oxygen carrier. Ceria nanomaterials with various morphologies, mainly polyhedra, have been reported. Recently, 1D ceria nanostructures, such as nanowires, have also been reported. However, with the exception of one report on the preparation of nanosheets, well-controlled 2D ceria nanomaterials have not been explored and the comparison of the OSC properties between 3D and 2D structures has not been possible. On the other hand, the different properties of the (100), (110), and (111) ceria facets has been debated. There is no consensus on whether crystallographic orientation or particle size affects reactivities. Therefore, high-quality ceria nanocrystals selectively exposing different low Miller-index surfaces, are crucial to enabling experiments that resolve the controversy. Here we report a simple, robust solution-phase synthesis of ultrathin ceria nanoplates in the presence of mineralizers. The morphology of nanoplates can be easily controlled by changing reaction parameters, such as precursor ratio, reaction time, etc. In addition, we also prepare ceria nanomaterials in various 3D morphologies by hydrothermal and combustion methods. The OSC of our 2D ceria materials have been tested and compared to the OSC of their 3D counterparts. In brief, the synthesis of ceria nanoplates involves the thermal decomposition of cerium acetate at 320–330 8C in the presence of oleic acid and oleylamine as stabilizers and employs sodium diphosphate or sodium oleate as mineralizers. Transmission electron microscopy (TEM) images of ceria nanoplates are shown in Figure 1. Square ceria nanoplates (S-nanoplates, Figure 1a) with an edge length of 11.9 nm (s= 7%), are synthesized with sodium diphosphate as the mineralizer while elongated ceria nanoplates (Lnanoplates, Figure 1e) with a length of 151.6 nm (s= 9%) and a width of 14.3 nm (s= 12%), are produced with sodium oleate as the mineralizer. The nanoplates in both samples have a thickness of about 2 nm. As shown in Figure 1c and g, the stacks of nanoplates confirm that the sample consists of 2D plates rather than 3D cubes or rods. S-nanoplates readily form the demonstrated stacking arrays as seen in drop-cast TEM samples. L-nanoplates only form stacks by a selfassembly at a liquid–liquid (e.g. hexane–ethylene glycol) interface. The S-nanoplates also self-assemble to a ceria nanosheet at a hexane–acetonitrile interface, as shown in Figure 3a. High-resolution TEM (HRTEM) images of both nanoplates (Figures 1d,h and S1c in the Supporting Information) reveal an interplanar distance of 0.27 nm, consistent with the (200) lattice spacing of the ceria crystal. The fast Fourier transform (FFT) patterns confirm the {100} textures of ceria nanoplates. Plates (e.g. square plates) could be enclosed by either six (100) facets or a combination of two (100) facets and four (110) facets. As illustrated in Figure S1, our HRTEM images and simulations of HRTEM images suggest that our ceria nanoplates are enclosed by six (100) [*] D. Y. Wang, Y. J. Kang, Prof. C. B. Murray Department of Chemistry University of Pennsylvania, Philadelphia, PA 19104 (USA) E-mail: cbmurray@sas.upenn.edu

A direct carbon fuel cell with a molten antimony anode

The direct utilization of carbonaceous fuels is examined in a solid oxide fuel cell (SOFC) with a molten Sb anode at 973 K. It is demonstrated that the anode operates by oxidation of metallic Sb at the electrolyte interface, with the resulting Sb2O3 being reduced by the fuel in a separate step. Although the Nernst Potential for the Sb-Sb2O3 mixture is only 0.75 V, the electrode resistance associated with molten Sb is very low, approximately 0.06 Ωcm2, so that power densities greater than 350 mW cm−2 were achieved with an electrolyte-supported cell made from Sc-stabilized zirconia (ScSZ). Temperature programmed reaction measurements of Sb2O3 with sugar char, rice starch, carbon black, and graphite showed that the Sb2O3 is readily reduced by a range of carbonaceous solids at typical SOFC operating conditions. Finally, stable operation with a power density of 300 mW cm−2 at a potential of 0.5 V is demonstrated for operation on sugar char.

Modeling Impedance Response of SOFC Cathodes Prepared by Infiltration

A mathematical model has been developed to understand the performance of electrodes prepared by infiltration of La 0.8 Sr 0.2 FeO 3 (LSF) and La 0.8 Sr 0.2 MnO 3 (LSM) into yttria-stabilized zirconia (YSZ). The model calculates the resistances for the case where perovskite-coated, YSZ fins extend from the electrolyte. Two rate-limiting cases are considered: oxygen ion diffusion through the perovskite film or reactive adsorption of O 2 at the perovskite surface. Adsorption is treated as a reaction between gas-phase O 2 and oxygen vacancies, using equilibrium data. With the exception of the sticking probability, all parameters in the model are experimentally determined. Resistances and capacitances are calculated for LSF-YSZ and there is good agreement with experimental values at 973 K, assuming adsorption is rate limiting, with a sticking probability between 10 -3 and 10 -4 on vacancy sites. According to the model, perovskite ionic conductivity does not limit performance so long as it is above ~10 -7 S/cm. However, the structure of the YSZ scaffold, the ionic conductivity of the scaffold, and the slope of the perovskite redox isotherm significantly impact electrode impedance. Finally, it is shown that characteristic frequencies of the electrode cannot be used to distinguish when diffusion or adsorption is rate-limiting.

Transition metal-doped rare earth vanadates: a regenerable catalytic material for SOFC anodes

The physical and electrochemical properties of cerium vandates in which a portion of the cerium cations have been substituted with transition metals (Ce1−xTMxVO4−0.5x, TM = Ni, Co, Cu) were investigated and their suitability for use in solid oxide fuel cell (SOFC) anodes was assessed. Similar to other transition metal doped perovskites, the metals were found to move out of and into the oxide lattice in response to exposure to reducing and oxidizing conditions at elevated temperatures. This process produces nanoparticle metal catalysts that decorate the surface of the conductive cerium vanadate. Solid oxide fuel cells (SOFC) with Ce1−xTMxVO3–YSZ composite anodes exhibited high electrochemical activity. It was also demonstrated that doping with the alkaline earth ions, Ca2+ and Sr2+ enhances the electronic conductivity of the vanadate and Ce0.7Sr0.1Ni0.2VO3–YSZ composite SOFC anodes were found to have both high electrochemical activity and unusually high redox stability.

Doped-Ceria Diffusion Barriers Prepared by Infiltration for Solid Oxide Fuel Cells

The most commonly used material for cathodes in solid oxide fuel cells is a composite of Sr-doped LaMnO3 LSM and yttriastabilized zirconia YSZ, with the LSM in the composite providing electronic conductivity and catalytic activity for oxygen reduction. The addition of YSZ to the electrode provides ionic conductivity to increase the length of the three-phase boundary by providing ionconducting channels from the electrolyte into the electrode. 1 In most cases, LSM‐YSZ composites are prepared by sintering a mixture of LSM and YSZ powders onto the YSZ electrolyte. Relatively high temperatures 1300 K 2 are required to sinter the YSZ particles in the electrode to the electrolyte. Significantly improved performance can be achieved by replacing LSM with mixed conducting perovskites, such as Sr-doped LaCoO3 LSCo, 3-7 LaFeO3 LSF, 8-12

Effect of the Ionic Conductivity of the Electrolyte in Composite SOFC Cathodes

Solid oxide fuel cell (SOFC) cathodes were prepared by infiltration of 35 wt % La 0.8 Sr 0.2 FeO 3 (LSF) into porous scaffolds of three, zirconia-based electrolytes in order to determine the effect of the ionic conductivity of the electrolyte material on cathode impedances. The electrolyte scaffolds were 10 mol % Sc 2 O 3 -stabilized zirconia (ScSZ), 8 mol % Y 2 O 3 -stabilized zirconia (YSZ), and 3 mol % Y 2 O 3 - 20 mol % Al 2 O 3 -doped zirconia (YAZ), prepared by tape casting with graphite pore formers. Each electrolyte scaffold was 65% porous, with identical pore structures as determined by scanning electron microscopy (SEM). Both symmetric cells and fuel cells were prepared and tested between 873 and 1073 K, using LSF composites that had been calcined to 1123 or 1373 K. Literature values for the electrolyte conductivities were confirmed using the ohmic losses from the impedance spectra. The electrode impedances decreased with increasing electrolyte conductivity, with the dependence being between to the power of 0.5 and 1.0, depending on the operating temperature and LSF calcination temperature.

Direct in situ probe of electrochemical processes in operating fuel cells.

The function of systems and devices in many technologically important applications depends on dynamic processes in complex environments not accessible by structure and property characterization tools. Fuel cells represent an example in which interactions occur under extreme conditions: high pressure, high temperature, in reactive gas environments. Here, scanning surface potential microscopy is used to quantify local potential at electrode/electrolyte interfaces in operating solid oxide fuel cells at 600 °C. Two types of fuel cells are compared to demonstrate two mechanisms of ionic transport at interfaces. Lanthanum strontium ferrite-yttria-stabilized zirconia (LSF-YSZ) and lanthanum strontium manganite-yttria-stabilized zirconia (LSM-YSZ) cross-sectional electrode assemblies were measured to compare mixed ionic electronic conducting and electronic conducting mechanisms. Direct observation of the active zones in these devices yields characteristic length scales and estimates of activation barrier changes.

Systematic Studies of the Cathode-Electrolyte Interface in SOFC Cathodes Prepared by Infiltration

In this study, the effect of the morphology and ionic conductivity of the electrolyte material in SOFC composite cathodes is systematically studied. The specific surface area of prous yttria-stabilized zirconia (YSZ) scaffolds was varied by almost two orders of magnitude using different pore formers and surface treatment with hydrofluoric acid (HF). The effect of ionic conductivity on the performance of SOFC cathodes was studied for electrodes prepared by infiltration of 35 wt % LSF into 65% porous scandiastabilized zirconia (ScSZ), YSZ, or yttria-alumina co-stabilized zirconia (YAZ) scaffolds of identical microstructure cathodes. Disciplines Biochemical and Biomolecular Engineering | Chemical Engineering | Engineering Comments Suggested Citation: R. Kungas, J.M. Vohs and R.J. Gorte. (2011). Systematic Studies of the Cathode-Electrolyte Interface in SOFC Cathodes Prepared by Infiltration. ESC Transactions, 35(1) 2085-2095).