André Weber

Materials and technologies for SOFC-components

Solid oxide fuel cells are a forward looking technology for a highly efficient, environmental friendly power generation. A SOFC is a multilayer structure consisting of ceramic and metallic materials with different electrical transport properties. All components have to show a well adjusted thermal expansion behavior, chemical compatibility of material interfaces and chemical stability in the prevailing atmospheres. The performance of SOFC single cells is not only determined by intrinsic material properties. There is a significant influence due to the fabrication technology respectively the microstructure at the electrode/electrolyte-interfaces. The performance of cells can only be improved by the application of elevated materials using appropriate technologies.

Evaluation and Modeling of the Cell Resistance in Anode-Supported Solid Oxide Fuel Cells

The impedance of anode-supported single cells [Ni/8 yttria-stabilized zirconia (YSZ) anode; La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ cathode; 8YSZ electrolyte; area 1 cm 2 ] was characterized in a broad measuring range of temperature and air/fuel gas composition. The data has been analyzed by calculating the distribution function of relaxation times (DRTs). DRT computations enabled us to separate five different loss mechanisms occurring inside the cathode and anode without the need of an equivalent circuit. Two processes exhibit a systematic dependency on changes in the oxygen partial pressure of the cathode gas and thus can be attributed to diffusional and electrochemical losses on the cathode side. The remaining three processes are very sensitive to changes in the fuel gas but are not affected by variations of the cathode gas. These resistances are classified as a gas diffusion polarization within the anode-substrate and as an electro-oxidation reaction at the triple-phase boundary, respectively.

Oxidation of H2, CO and methane in SOFCs with Ni/YSZ-cermet anodes

Abstract The operation of solid oxide fuel cells with the use of different types of carbon-based fuels (i.e. natural gas, coal gas, etc.) became one of the main topics of SOFC research within the last years. Fuel gases like methane can be steam-reformed or partially oxidized within the SOFC stack. Usually a large amount of steam or air (steam to carbon ratio>2) has to be applied to avoid carbon deposition resulting in a degradation of the anode and a failure of the stack. The influence of the steam to carbon ratio on the performance of planar state of the art SOFC single cells with conventional nickel/yttria-stabilized zirconia cermet anodes has been investigated using CO/H2 mixtures as well as CH4/H2O mixtures as fuel gas. The cells were characterized by dc techniques and impedance spectroscopy under realistic working conditions. In the case of CO/H2 mixtures, a decrease in performance was observed at high CO portions (>90%), whereas the cell performance increased with decreasing S/C ratio using methane as the fuel. In addition, the stability of single cells was investigated. A stable operation using dry methane as fuel (S/C=0) was possible for up to 1000 h without serious degradation.

Oxygen reduction mechanism at porous La1−xSrxCoO3−d cathodes/La0.8Sr0.2Ga0.8Mg0.2O2.8 electrolyte interface for solid oxide fuel cells

The oxygen reduction mechanism was investigated at the porous La1−xSrxCoO3−d cathode/La0.8Sr0.2Ga0.8Mg0.2O2.8 electrolyte interface (x=0.2, 0.3, 0.4). The polarization resistance, measured from the impedance spectra, was compared in the samples of La1−xSrxCoO3−d as functions of x, temperatures, and applied DC voltages. The polarization resistance decreased with an increase of x values in La1−xSrxCoO3−d and with the applied cathodic voltage. The polarization resistance of the higher Sr-concentration in La1−xSrxCoO3−d showed the lower dependence on cathodic overpotential. The values of the activation energy of the interface conductivity (inverse of the polarization resistance) were similar for all La1−xSrxCoO3−d samples (127–143 kJ mol−1) at zero applied voltage (E=0 V). However, under cathodic polarization, the activation energy decreased as the applied voltage became more negative, which indicates a change of the reaction mechanism under cathodic polarization. Under cathodic polarization, oxide ion diffusion in the bulk La1−xSrxCoO3−d can be one of the main factors determining the reaction rates.

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.

Kinetics of oxidation and reduction of Ni/YSZ cermets

A cyclic reduction and oxidation of Ni/YSZ-cermet anodes for Solid Oxide Fuel Cells (SOFC) resulted in an increase of the polarization resistance. Therefore, investigations concerning kinetics of oxidation/reduction and the impact of redox cycles on the mi-crostructure of Ni/YSZ bulk ceramics were made. The reaction process of the basic system Ni/NiO was compared with cermet bulk samples and the influence of NiO and YSZ particle sizes and sintering temperatures on kinetics and microstructure was studied using thermo-gravimetry and dilatometry. The investigations on bulk ceramics indicated that no length change occurred during reduction, whereas reoxidation led to an increase in the length of the samples which strongly depended on the microstructure. It was shown that bulk samples sintered at temperatures below 1300 °C can withstand redox cycles much better than those sintered at higher temperatures. Furthermore, it was found that by decreasing the NiO particle size and using a NiO/YSZ particle size ratio of aproximately 3:2, a smaller length increase after reoxidation was achieved. An increase of the polarization resistance could be ascribed to the formation of cracks within the bulk sample which interrupt current paths and therefore reduce the amount of the active triple phase boundary.

Impedance Study of Alternative ( La , Sr ) FeO3 − δ and ( La , Sr ) ( Co , Fe ) O3 − δ MIEC Cathode Compositions

In this study, two mixed electronic ionic conducting (MIEC) cathode materials, La 0.68 Sr 0.3 FeO 3-δ and La 0.68 Sr 0.3 Co 0.2 Fe 0.8 O 3-δ , are characterized by electrochemical impedance spectroscopy. The chemical surface and diffusion coefficients of both cathode materials are obtained directly from impedance measurements performed on full anode-supported solid oxide fuel cells. This is done by a combined distribution of relaxation times and an equivalent circuit impedance analysis method, which allows a high resolved identification and deconvolution of each single polarization mechanisms contributing to the overall loss of the cell.

Time-Dependent Electrode Performance Changes in Intermediate Temperature Solid Oxide Fuel Cells

This study gives evidence that the time-dependent performance changes in anode supported cells for intermediate-temperature solid oxide fuel cells is essentially influenced by the mixed ionic-electronic conducting (MIEC) cathode. The impedance spectra recorded during 700 h of operation at 750°C were interpreted using an appropriate equivalent circuit model by (i) a distribution of relaxation time analysis followed by (ii) a complex nonlinear least squares fit. Four electrode polarization processes were separated by selective experimental parameters. The cathodic part, initially the smallest, is only discovered among the anodic contributions by a change in fuel gas composition from H 2 -H 2 O to CO-CO 2 and increases by 310% (15 mΩ cm 2 at 11 h, 62 mΩ cm 2 at 700 h). A Sr (and Co) depletion of the MIEC cathode composition La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ possibly caused this degradation. The anodic polarization has a proportion of 92% at the start and decreases to 73% (168 mΩ cm 2 at 11 h, 173 mΩ cm 2 at 700 h). The anode charge-transfer reaction initially causes 60% of the total polarization losses and 50% after 700 h. This is assigned to a change in the triple phase boundary and/or a degradation in ionic conductivity in the anode functional layer. The gas diffusion polarization remains constant at 58 mΩ cm 2 .

Electrode Reaction of La1 − x Sr x CoO3 − d Cathodes on La0.8Sr0.2Ga0.8Mg0.2 O 3 − y Electrolyte in Solid Oxide Fuel Cells

The electrode reaction mechanism was investigated at the porous La 1-x Sr x CoO 3-d (LSC)/La 0.8 Sr 0.2 Ga 0.8 Mg 0.2 O 3 (LSGM) interface (x = 0.2, 0.3, 0.4). Direct current (dc) polarization curves showed a linear relationship between the overpotential and the current density at the lower cathodic polarization. The interface conductivity was adopted to examine the activity for oxygen reduction at the LSC/LSGM interface, which was measured both from the de polarization curves and the alternating current impedance, The interface conductivity increased with Sr concentration in LSC and cathodic applied voltage. The higher Sr concentration of LSC showed the lower oxygen partial pressure dependence of the interface conductivity. The activation energy for the interface conductivity was almost the same for different Sr concentration of LSC (137-142 kJ mol -1 ). A similar reaction mechanism is assumed for the examined LSC. The interface conductivity has a relation with oxide ion diffusion in LSC, which can be the main factor to determine the cathode reaction rates under cathodic polarization.

Electrochemical Analysis of Reformate-Fuelled Anode Supported SOFC

By applying these methods in the present study, we achieved to separate the polarization mechanisms for anode supported single cells under reformate operation. It is, for the first time, unambiguously demonstrated that (a) only hydrogen is directly oxidized at the anode and (b) an additional, newly identified polarization process occurs at low frequencies, which is most probably linked to the coupling of gas transport and the heterogeneous reforming chemistry within the anode substrate [2].