Karin Vels Hansen

H 2 ­ H 2 O ­ Ni ­ YSZ Electrode Performance Effect of Segregation to the Interface

The electrochemical properties of the interface between Ni and yttria-stabilized zirconia (YSZ) in the H 2 -H 2 O-Ni-YSZ electrode have been investigated in order to obtain knowledge on the effect of segregation of impurities to the electrode-electrolyte interface on performance. Even for a relatively pure nickel electrode, a film of impurities was found at the contact between Ni and YSZ and on the surface of YSZ. Impurity ridges were also found at the three-phase boundary. Impurities in these locations all have the potential of impeding processes and reactions at the electrode-electrolyte interface. The pure electrodes showed a lower resistance compared to most electrodes discussed in the literature.

A framework for automatic segmentation in three dimensions of microstructural tomography data.

Routine use of quantitative three dimensional analysis of material microstructure by in particular, focused ion beam (FIB) serial sectioning is generally restricted by the time consuming task of manually delineating structures within each image slice or the quality of manual and automatic segmentation schemes. We present here a framework for performing automatic segmentation of complex microstructures using a level set method. The technique is based on numerical approximations to partial differential equations to evolve a 3D surface to capture the phase boundaries. Vector fields derived from the experimentally acquired data are used as the driving forces. The framework performs the segmentation in 3D rather than on a slice by slice basis. It naturally supplies sub-voxel precision of segmented surfaces and allows constraints on the surface curvature to enforce a smooth surface in the segmentation. Two applications of the framework are illustrated using solid oxide cell materials as examples.

A Critical Review of Models of the H2/H2O/Ni/SZ Electrode Kinetics

Various models of the H2/H2O/Ni/SZ (SZ = stabilized zirconia) electrode kinetics have been presented in the literature in order to explain the reported experimental data. However, there has been a strong tendency of using a limited set of data to “verify” a given model, disregarding other data sets, which do not fit the model. We have inspected some models in the literature, and problems (e.g. no quantitative model has explained the large variation in reported values of apparent activation energy of the electrode kinetics) as well as strengths of the models are discussed. We point out experimental findings that a useful model must be able to explain such as difference in sensitivity to poisoning by H2S due to differences in the detailed composition of the SZ and large change in apparent activation energy by change in cermet preparation. Finally, we will point out some elements, which seem important for any realistic and useful mathematical model of the H2/H2O/Ni/SZ electrode.

Improved controlled atmosphere high temperature scanning probe microscope

To locally access electrochemical active surfaces and interfaces in operando at the sub-micron scale at high temperatures in a reactive gas atmosphere is of great importance to understand the basic mechanisms in new functional materials, for instance, for energy technologies, such as solid oxide fuel cells and electrolyzer cells. Here, we report on advanced improvements of our original controlled atmosphere high temperature scanning probe microscope, CAHT-SPM. The new microscope can employ a broad range of the scanning probe techniques including tapping mode, scanning tunneling microscopy, scanning tunneling spectroscopy, conductive atomic force microscopy, and Kelvin probe force microscopy. The temperature of the sample can be as high as 850 °C. Both reducing and oxidizing gases such as oxygen, hydrogen, and nitrogen can be added in the sample chamber and the oxygen partial pressure (pO2) is monitored by an oxygen sensor. We present here some examples of its capabilities demonstrated by high temperature topography with simultaneously ac electrical conductance measurements during atmosphere changes, electrochemical impedance spectroscopy at various temperatures, and measurements of the surface potential. The improved CAHT-SPM, therefore, holds a great potential for local sub-micron analysis of high-temperature and gas induced changes of a wide range of materials.

Geometrical characterization of interconnected phase networks in three dimensions.

In electrochemical devices such as fuel cells or batteries the microstructure is a determining factor for the performance of the device. To be able to optimize the microstructure it is important to be able to quantitatively measure key structural parameters, such that systematic studies can be made. We present several general methods for quantitative characterization of network structures without prior assumptions of shape or application. The characterization is performed by extracting distributions of values rather than single value descriptions, thus allowing more detailed comparisons between samples to be made. The methods characterize tortuosity, path diameters, the novel dead ends property and a particle shape independent alternative to a particle size distribution. The parameters are calculated by the computation of arrival time maps by the fast marching method. The methods are applied to the analysis of each of the three phases in a solid oxide fuel cell sample.

Dynamic behavior of impurities and native components in model LSM microelectrodes on YSZ

Strontium-doped lanthanum manganite is a widely used cathode material in solid oxide fuel cells. Segregation phenomena can have a critical impact on performance and durability, especially when they cause active interfaces to degrade. The segregation behavior in polarized and non-polarized strontium-doped lanthanum manganite ((La0.75Sr0.25)0.95MnO3) microelectrodes with a diameter of 100 μm and a thickness of ∼500 nm on an yttria-stabilized zirconia electrolyte were analyzed post-mortem after ∼200 h at temperatures up to 850 °C. Time-of-flight secondary ion mass spectrometry was used to study the dynamic behavior of the native components (La, Sr, Mn) and selected impurities (Si, K, Na) both laterally and in-depth. Manganese was found to be especially mobile and showed both segregation onto the electrolyte as a result of temperature and polarization and dissolution into the electrolyte below the microelectrodes. All native components showed a complex in-depth dynamic behavior, and a nanoscale in-depth analysis of the electrode–electrolyte interface revealed the formation of a well-defined lanthanum zirconate layer. The selected impurities segregated to the electrolyte and microelectrode surfaces and Na- and K-rich layers formed at different depths.

High temperature conductance mapping for correlation of electrical properties with micron-sized chemical and microstructural features

High temperature AC conductance mapping is a scanning probe technique for resolving local electrical properties in microscopic areas. It is especially suited for detecting poorly conducting phases and for ionically conducting materials such as those used in solid oxide electrochemical cells. Secondary silicate phases formed at the edge of lanthanum strontium manganite microelectrodes are used as an example for correlation of chemical, microstructural and electrical properties with a spatial resolution of 1-2µm to demonstrate the technique. The measurements are performed in situ in a controlled atmosphere high temperature scanning probe microscope at 650°C in air.

Controlled atmosphere high-temperature scanning probe microscopy (CAHT-SPM)

Abstract High temperature scanning probe microscopy at in operando conditions at temperatures up to 850°C in reducing and oxidizing atmospheres is a relatively new technique. The unique controlled atmosphere high temperature scanning probe microscopes are used for studying electrical and electrochemical properties of metal and metal oxide surfaces on a submicron scale. Localized probing techniques such as AC conductance mapping, local impedance spectroscopy and Kelvin probe force microscopy are used for elucidating solid oxide cell degradation mechanisms and surface reactions. The ability to combine electrical information with chemical and microstructural information from other techniques on a submicron scale gives a more complete and detailed picture of the surface properties.

Dynamic and Impure Perovskite Structured Metal Oxide Surfaces

Surfaces of LSF and LSCF perovskite model electrodes were investigated using a variety of analytical methods on flat model electrodes that were prepared as either pellets or as thin films on top of YSZ pellets in other to throw more light on the widely discussed segregation of layers and particles on the electrode surfaces. An experimental test of the suggestion that the segregation might happen in the vacuum in the analysis equipment gave a negative result. Formation of particles containing significant amounts of S and Cr from segregation of the trace impurities in the acquired powders were observed, and lead us to a new hypothesis about the differences between flat model electrodes and technical nano-sized composite electrodes.