Ellen Ivers-Tiffée

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.

Deconvolution of electrochemical impedance spectra for the identification of electrode reaction mechanisms in solid oxide fuel cells

The polarization processes occurring at the electrode–electrolyte interfaces of solid oxide fuel cells (SOFC) were investigated by electrochemical impedance spectra measured at single cells under realistic operating conditions. The approach presented is based on distributions of relaxation times which are the basic quantity of interest in electrochemical impedance data analysis. A deconvolution method was developed and implemented that yields these characteristic distribution patterns directly from the impedance spectra. In contrast to nonlinear least squares curve fit of equivalent circuit models, no a priori circuit choice has to be made. Even more importantly, the excellent resolving capacity allows the untangling of the impedance contributions of up to three physically distinct processes within one frequency decade. With the method, processes with the highest polarization losses can be identified and targeted to improve cell performance. Based on the distributions, a general strategy for the identification of the reaction mechanisms is given. The evaluation of the distributions in terms of peak parameters is illustrated by a physical model for oxygen reduction at the SOFC cathode–electrolyte interface. The method is expected to find many applications in electrochemistry beyond the field of solid oxide fuel cell development.

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.

High temperature oxygen sensors based on doped SrTiO3

Abstract The sensor properties of semiconducting strontium titanate can be modified by dopants. On the one hand, the ambiguous dependence of the electrical conductivity on the oxygen partial pressure can be changed by a small lanthanum (donor) concentration to an unambiguous one over a wide oxygen partial pressure range. Polycrystalline Sr 0.995 La 0.005 TiO 3 and/or thick films have response times of few milliseconds, caused by a surface type mechanism of the conductivity. The bulk type mechanism is very slow and is responsible for the long-term behaviour. On the other hand, the temperature dependence of strontium titanate can almost be suppressed for a small oxygen partial pressure range by a high iron (acceptor) content. On the basis of SrTi 0.65 Fe 0.35 O 3 a fast thick film oxygen sensor is proposed, which can be applied to control of lean-burn engines.

Principles of solid state oxygen sensors for lean combustion gas control

Abstract This paper gives an overview about oxygen sensors for automotive applications to control the air–fuel ratio in order to reduce emissions and fuel consumption. The three-way catalyst system (TWC) using the potentiometric sensors based on zirconia represents the most effective system for the emission control at this time. New control strategies with linear lambda control at λ =1, for direct injection engines and other lean burn engines operating with excess air ( λ >1) need alternative sensor concepts. Hence, current limiting electrochemical pumping cells (amperometric sensors) based on zirconia have been developed for engine control applications. As a future option we present also new research works in resistive type oxygen sensors based on semiconducting metal oxides.

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.

SOFC Modeling and Parameter Identification by Means of Impedance Spectroscopy

A zero-dimensional stationary model for the I-U characteristics of anode-supported solid oxide fuel cells (SOFC) is presented. The different kinds of electrode polarization resistances are separated from experimental impedance data by means of a detailed equivalent circuit model specified for anode-supported cells. This has the big advantage that partial pressure and temperature dependency of electrode exchange current densities could be determined by a fit of semi-empirical power law model equations. For the first time, the exponents a and b for the pH2- and pH2O-dependency of the anodic exchange current density are obtained independently. Equally, the exponent m for the pO2-dependency of the cathodic exchange current density is derived. The anodic and cathodic gas diffusion polarization is calculated without the estimation of parameters such as tortuosity and porosity. Our approach is advantageous to separate anodic and cathodic activation and diffusion polarization and precisely predicts I-U characteristics for a wide operating range.

Investigation of barium strontium titanate thick films for tunable phase shifters

Abstract The influence of the microstructure of Ba0.6Sr0.4TiO3 (BST) bulk ceramics and thick films on the dielectric properties have been studied. Thick films have been prepared by screen printing technique on Al2O3 substrates. The powder has been prepared by using the common mixed oxide technique. In comparison to dense bulk ceramics, the permittivity of thick films is approximately 10 times less. The effect of temperature on the permittivity and the tunability (change of the dielectric constant with applied voltage) has also been investigated at low frequency (1 kHz). At microwave frequencies, BST thick films have been characterized by measuring coplanar waveguides (CPW) at room temperature and utilizing an enhanced quasi-static CPW model for multilayer dielectric substrates.

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.