Heike Störmer

Model anodes and anode models for understanding the mechanism of hydrogen oxidation in solid oxide fuel cells

This article presents a literature review and new results on experimental and theoretical investigations of the electrochemistry of solid oxide fuel cell (SOFC) model anodes, focusing on the nickel/yttria-stabilized zirconia (Ni/YSZ) materials system with operation under H(2)/H(2)O atmospheres. Micropatterned model anodes were used for electrochemical characterization under well-defined operating conditions. Structural and chemical integrity was confirmed by ex situ pre-test and post-test microstructural and chemical analysis. Elementary kinetic models of reaction and transport processes were used to assess reaction pathways and rate-determining steps. The comparison of experimental and simulated electrochemical behaviors of pattern anodes shows quantitative agreement over a wide range of operating conditions (p(H(2)) = 8×10(2) - 9×10(4) Pa, p(H(2)O) = 2×10(1) - 6×10(4) Pa, T = 400-800 °C). Previously published experimental data on model anodes show a strong scatter in electrochemical performance. Furthermore, model anodes exhibit a pronounced dynamics on multiple time scales which is not reproduced in state-of-the-art models and which is also not observed in technical cermet anodes. Potential origin of these effects as well as consequences for further steps in model anode and anode model studies are discussed.

Degradation and Relaxation Effects of Ni Patterned Anodes in H2 – H2O Atmosphere

Patterned Ni anodes on Y 2 O 3 -stabilized ZrO 2 (YSZ) represent a promising approach to determine the kinetics of electrochemical reactions in solid oxide fuel cells. Contrary to technical Ni/YSZ cermet anodes, the reaction zone for the hydrogen oxidation has the potential to be well defined. This study is focused on the reproducibility of electrochemical characterization results of patterned Ni anodes, with a parameter variation of the partial pressures of H 2 and H 2 O, temperature, and polarization voltage. Considerable (electro)chemical relaxation and degradation processes with time constants in the order of some hours were found and are discussed: (i) an initial decrease in the line specific resistance (LSR) during the first 20-25 h of temperature exposure (T = 800°C) attributed to a restructuring process in the Ni thin film, (ii) reversible changes in LSR upon variations of the partial pressure of H 2 and H 2 O in correlation to the initial gas composition, and (iii) rapid reversible changes in LSR upon anodic and cathodic polarization voltages followed by a slow relaxation. The corresponding preconditions for reliable measurement series were deduced and yield the data set of the LSR values. Furthermore, a detailed comparison of the obtained LSR values with literature data is given.

EELS Investigations of Different Niobium Oxide Phases

Electron energy loss spectra in conjunction with near-edge fine structures of purely stoichiometric niobium monoxide (NbO) and niobium pentoxide (Nb2O5) reference materials were recorded. The structures of the niobium oxide reference materials were checked by selected area electron diffraction to ensure a proper assignment of the fine structures. NbO and Nb2O5 show clearly different energy loss near-edge fine structures of the Nb-M4,5 and -M2,3 edges and of the O-K edge, reflecting the specific local environments of the ionized atoms. To distinguish the two oxides in a quantitative manner, the intensities under the Nb-M4,5 as well as Nb-M2,3 edges and the O-K edge were measured and their ratios calculated. k-factors were also derived from these measurements.

Fast mapping of the cobalt-valence state in Ba0.5Sr0.5Co0.8Fe0.2O3-d by electron energy loss spectroscopy.

A fast method for determination of the Co-valence state by electron energy loss spectroscopy in a transmission electron microscope is presented. We suggest the distance between the Co-L3 and Co-L2 white-lines as a reliable property for the determination of Co-valence states between 2+ and 3+. The determination of the Co-L2,3 white-line distance can be automated and is therefore well suited for the evaluation of large data sets that are collected for line scans and mappings. Data with a low signal-to-noise due to short acquisition times can be processed by applying principal component analysis. The new technique was applied to study the Co-valence state of Ba0.5Sr0.5Co0.8Fe0.2O3-d (BSCF), which is hampered by the superposition of the Ba-M4,5 white-lines on the Co-L2,3 white-lines. The Co-valence state of the cubic BSCF phase was determined to be 2.2+ (±0.2) after annealing for 100 h at 650°C, compared to an increased valence state of 2.8+ (±0.2) for the hexagonal phase. These results support models that correlate the instability of the cubic BSCF phase with an increased Co-valence state at temperatures below 840°C.

Dopant-Site Determination in Y- and Sc-Doped (Ba0.5Sr0.5)(Co0.8Fe0.2)O3-δ by Atom Location by Channeling Enhanced Microanalysis and the Role of Dopant Site on Secondary Phase Formation.

(Ba0.5Sr0.5)(Co0.8Fe0.2)O3-δ (BSCF) is a promising material with mixed ionic and electronic conductivity which is considered for oxygen separation membranes. Selective improvement of material properties, e.g. oxygen diffusivity or suppression of secondary phase formation, can be achieved by B-site doping. This study is concerned with the formation of Co-oxide precipitates in undoped BSCF at typical homogenization temperatures of 1,000°C, which act as undesirable nucleation sites for other secondary phases in the application-relevant temperature range. Y-doping successfully suppresses Co-oxide formation, whereas only minor improvements are achieved by Sc-doping. To understand the reason for the different behavior of Y and Sc, the lattice sites of dopant cations in BSCF were experimentally determined in this work. Energy-dispersive X-ray spectroscopy in a transmission electron microscope was applied to locate dopant sites exploiting the atom location by channeling enhanced microanalysis technique. It is shown that Sc exclusively occupies B-cation sites, whereas Y is detected on A- and B-cation sites in Y-doped BSCF, although solely B-site doping was intended. A model is presented for the suppression of Co-oxide formation in Y-doped BSCF based on Y double-site occupancy.

Niobium as new material for electrolyte capacitors with nanoscale dielectric oxide layers

New niobium electrolyte capacitors were produced on the base of newly developed capacitorgrade niobium metal powder. To gain a comprehensive understanding of the reactions taking place during the fabrication process which comprises several anodic oxidation and thermal treatment steps, the influence of heat treatments on the electrical and structural properties of the nanoscale dielectric niobium pentoxide layers was investigated. Capacitance measurements at fixed frequency were used to examine the electrical properties of the dielectric oxide layers directly after oxidation steps and heat treatments. Thermogravimetric analysis was applied to characterize the behavior of oxidized niobium anodes during the thermal annealing processes. Scanning electron and high-resolution transmission electron microscopy was performed in order to determine the thickness and microstructure of the nanoscale dielectric niobium pentoxide layers. In the following, the first results of these characterizations will be presented.

Grain-size dependence of the deterioration of oxygen transport for pure and 3 mol% Zr-doped Ba0.5Sr0.5Co0.8Fe0.2O3−δ induced by thermal annealing

In this study, the influence of long-term annealing at intermediate temperatures on oxygen transport of Ba0.5Sr0.5Co0.8Fe0.2O3 d (BSCF) and 3 mol% Zr-doped BSCF (BSCF-Z3) ceramics with different grain sizes was studied by means of in situ electrical conductivity relaxation (ECR) measurements. Ceramics with different grain sizes in the range of 3–80 mm were obtained by varying the temperature and dwell time during sintering. For both compositions, the apparent values of the chemical diffusion coefficient Dchem and surface exchange coefficient kchem extracted from the data of ECR measurements are found to decrease with the time of annealing. The strongly correlated decreases in Dchem and kchem, being greater in magnitude at a lower grain size and temperature, are observed significantly more pronounced for BSCF than for BSCF-Z3. The results from microstructural analysis provide a clear rationale for the observations from ECR. High-resolution transmission electron microscopy (TEM) images recorded before and after annealing under pure oxygen at 700 C for 13 d show excessive formation of hexagonal and plate-like lamellar precipitates at the grain boundaries and in the interior of grains of BSCF ceramics during the annealing process, whilst secondary phase formation is restricted solely to the grain boundary regions in BSCF-Z3. The possible importance of grain boundaries in determining the oxygen surface exchange kinetics is emphasized.

Electrooxidation of Reformate Gases at Model Anodes

This paper summarizes the experimental and modeling results concerning the electrooxidation of hydrogen and carbon monoxide, the main oxidizable compounds in reformates, at patterned nickel anodes on polycrystalline yttria stabilized zirconia electrolytes. The line specific resistance of the three phase boundary was evaluated within a wide range of gas compositions and temperature. The investigations showed that microstructural stability, impurities, accelerated degradation and reversible dynamic changes are key issues which have to be considered. Elementary kinetic models, parameterized with literature data, temperature-programmed desorption and reaction and quantum chemical calculation results were in excellent agreement with the experimental data. For the first time it could be shown that the line specific resistance values evaluated by means of patterned anodes are applicable in homogenized and space resolved models for cermet anodes.

Effect of Yttrium (Y) and Zirconium (Zr) Doping on the Thermodynamical Stability of the Cubic Ba0.5Sr0.5Co0.8Fe0.2O3-δ Phase

Mixed ionic-electronic conducting (MIEC) ceramic materials are currently considered as membranes for oxygen production due to their oxygen-ion and electronic conductivity. Among other MIECs Ba0.5Sr0.5Co0.8Fe0.2O3-δ (denoted BSCF) possesses superior oxygen permeation properties due to its high oxygen non-stoichiometry. However, the inherent instability of the cubic phase during long-time operation in the temperature range of 700 800 °C is a major drawback. Below 825 °C, a gradual decline in oxygen permeation over time is associated with partial decomposition of the cubic phase into a hexagonal phase [1,2]. The formation of the hexagonal phase can be correlated with the increase of the oxygen concentration at lower temperatures, which requires an increase of the average valence state for the multivalent Co-cations [3]. TEM studies also revealed additional secondary phases in BSCF with low crystal symmetry and plate-like morphology. These plate-like phases consist of lamellar stackings of the cubic, hexagonal and Ban+1ConO3n+3(Co8O8) (n 2) phase [3]. Moreover, the Co-cation in BSCF has a high tendency to diffuse out of the cubic lattice forming CoO precipitates which might act as nuclei for the lamellar phases. To counteract the cation valence change and, hence, decomposition of the cubic phase B-site doping with monovalent transition metals was proposed. Recent studies have shown beneficial effects for doping with Y or Zr [4,5]. However, the impact regarding the microstructure is not well understood yet. Therefore, an extensive SEM and TEM study was performed on Yand Zr-doped BSCF which includes the variation of numerous parameters like sintering temperature, annealing temperature and dopant concentration. To verify that the dopant is incorporated on the desired B-type lattice site, the technique of atom location by channeling enhanced microanalysis was used [6].

TEM Study of SiCN Glasses; Polymer Architecture versus Ceramic Microstructure

Polymer-derived SiCN glasses with tailored polymer architectures were characterized by TEM and EF-SAED upon pyrolysis at 1000 °C and subsequent annealing. Main emphasis of this work was to verify as to whether the intrinsic amorphous structure of polymer-derived bulk materials exhibits different structural features upon pyrolysis, depending on the functionalities of the pre-ceramic polymer, and whether such characteristic features are maintained upon high-temperature anneal.