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.

Patterned Ni anodes on normalY2normalO3 -stabilized ZrO2 (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 normalH2 and normalH2O , 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 normalH2 and normalH2O 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.

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Carbon monoxide (CO) is a major component in typical feed gases for solid oxide fuel cells (SOFC). This paper presents a combined modeling and experimental analysis of electrochemical CO oxidation on Ni/YSZ patterned model anodes. A computational model representing the coupled behavior of heterogeneous chemistry and electrochemistry in terms of elementary reactions is developed, which allows for a quantitative description of electrochemical impedance spectra and current-voltage behavior. Excellent agreement between model and experiment was achieved for the complete experimental data set, which covers a wide range of CO/CO 2 /N 2 gas compositions (4.0 × 10 2 Pa ≤ pCO ≤ 5.1 × 10 4 Pa and 9.5 × 10 2 Pa ≤ pCO 2 ≤ 9.2 × 10 4 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.

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Patterned Ni anodes are a common method to determine elementary kinetic reaction mechanism at the anode of Solid Oxide Fuel Cells. Their well defined Triple-Phase-Boundary (TPB) of electrode, electrolyte and gas phase, where the electrochemical oxidation takes place, is easily accessible. However, the direct transfer of knowledge gained with patterned Ni anodes to technical Ni/8YSZ cermet anodes is doubtful. For the first time a comparison of the electrochemical performance of both anode concepts is performed by means of (i) transmission line model calculations and (ii) performance simulations using a 3D FEM microstructure model. The good agreement observed between the performance of patterned Ni anodes and Ni/8YSZ cermet anodes indicates similar reaction mechanisms at the TPB of both anode concepts. Especially in H2-H2O atmosphere, the agreement of (i) absolute ASR values and (ii) parameter dependencies is very good and shows the applicability of patterned Ni anodes for the investigation of the electrochemistry of technical Ni/8YSZ cermet anodes.

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