Axel C. Müller

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.

Development of a multilayer anode for solid oxide fuel cells

Abstract In order to develop a high performance multilayer Ni/YSZ-cermet anode with improved long term stability, different NiO/YSZ-composites varying in NiO content and NiO/YSZ particle size ratio have been investigated with respect to their porosity and the coefficient of thermal expansion (CTE) in the oxidized and reduced state. The CTE values showed a dependence on particle sizes of the different NiO and YSZ qualities. The anode/electrolyte interface was realized by screen printing a thin anode (

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.

Engineering of SOFC Anode/Electrolyte Interface

Cofiring of anode/electrolyte leads to improved mechanical adherence and lowers polarization losses at the anode. However, caused by different shrinkage of anode and electrolyte the compound is bending or breaking if it is under weight. Sintering experiments revealed that these mechanical stresses can be reduced by using an anode pattern consisting of a large number of small sized individual areas instead of a continuous layer. A simple static model was used to describe the shrinkage due to sintering by effective constant Youngs moduli and pseudo linear thermal expansion coefficients (TEC). By the use of FEM different anode pattern geometries were analyzed with respect to curvature radius and mechanical stress in the electrolyte during cofiring. It turned out that anodes with a hexagonal pattern like honeycombs are preferable as they show small mechanical stress with high coverage at the same time.

Characterization of Multilayer Anodes for SOFC

SOFC anodes have to combine various tasks. In anode supported single cells a thick anode substrate is used for current collecting and gas distribution whereas a thin functional layer adjacent to the electrolyte is the electrochemically active part of the anode. This functional anode layer is cofired together with the thin film electrolyte to obtain an enhanced interface with low polarisation losses. This multilayer structure was transferred to an electrolyte supported single cell. The electrochemical active Ni/8YSZ anode layer was screen printed onto a 8YSZ electrolyte green tape and subsequently cofired at 1350 °. Mechanical stresses during cofiring due to shrinkage mismatch of anode and electrolyte were avoided by changing the geometry of the anode layer from a continuous layer to a large number of small sized individual areas. Simulations by finite element modeling indicated that a hexagonal pattern similar to honeycombs is preferable. The second layer which adjoins to the fuel gas channels and which is responsible for current collecting and gas distribution was later on screen printed on top and sintered together with the cathode. Single cells with a multilayer anode and different functional layers were electrochemically characterised under realistic operation conditions. The performance and reduction/oxidation stability of this type of anode was investigated. The electrochemically active layer showed only small degradation during redox cycling and long term operation at high fuel utilisation. In contradiction to single layer anodes Nickel agglomeration was not observed in the functional layer.