Annika Utz

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.

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.

Elementary Kinetic Numerical Simulation of Electrochemical CO Oxidation on Ni/YSZ Pattern Anodes

Results of a numerical simulation analysis of recent experimental data obtained in a comprehensive study of electrochemical CO oxidation on well-defined Ni/YSZ patterned model anodes [Utz et al. J. Power Sources, doi:10.1016/ j.powsour. 20.10.056 (2010)] are presented. A computational model representing the coupled behavior of heterogeneous chemistry and electrochemistry in terms of elementary reactions was developed, which allows for a quantitative description of the complete experimental data set, which covers a wide range of CO/CO2 gas compositions (4.0·102 Pa ≤ pCO ≤ 5.1·104 Pa and 9.5·102 Pa ≤ pCO2 ≤ 9.2·104 Pa) and operating temperatures (973 K ≤ T ≤ 1073 K). In the framework of the presented model a direct mechanistic interpretation of the experimentally observed electrochemical characteristics is obtained.