Modelling of the agglomeration of Ni-particles in anodes of solid oxide fuel cells

The degradation of anodes of solid oxide fuel cells (SOFC), which consist of a porous metal - solid electrolyte material is described by a two particle model. The model is based on two main assumptions. Firstly, the difference in metal particle diameter is the driving force for the observed coarsening of the larger metal particle during long term annealing. Secondly, surface diffusion of metal atoms on the particle surface is the dominant diffusion mechanism. Additionally, a function was introduced which considers the limited space for the growth of the nickel particles in the cermet material. The found analytical function for the growth kinetics was compared to experimental results for the growth of nickel particles in a nickel - yttria stabilised zirconia (YSZ) anode annealed at 1000 degreesC up to 4000 h. The model describes the time dependence of the observed particle radii in an adequate way. The resultant surface diffusion coefficients for Ni are lower than results found in literature. Possible explanations are discussed. However, the result shows that the proposed mechanism - surface diffusion of nickel atoms - is fast enough to explain the found amount of Ni agglomeration in SOFC anodes and is therefore considered to be the dominant mechanism. (C) 2001 Kluwer Academic Publishers

Nickel coarsening in annealed Ni/8YSZ anode substrates for solid oxide fuel cells

In order to study the nickel coarsening in porous Ni/8YSZ anode cermets of solid oxide fuel cells (SOFC), a series of exposure tests was carried out with anode substrates used in SOFC development at the Research Centre Julich. The changes in electrical conductivity as well as in the microstructure of the material were investigated. The microstructure of the cermets was characterized by digital on-line image analysis and microstructural parameters were determined for the metallic and the ceramic phase as well as for porosity. A decrease of 33% of the initial electrical conductivity was measured after exposing the cermet in Ar/4% H-2/3% H2O at 1000 degrees C for 4000 h, which is linked to the agglomeration of the metallic particles of this material. The durability of anode cermets for operations of up to 40 000 h was estimated on the basis of the measured grain coarsening and the decrease of conductivity. The microstructural parameters - like volume fraction, particle and pore size - were used to model the electrical conductivity by theoretical microstructure-property relationships. (C) 2000 Elsevier Science B.V. All rights reserved

Evaluation of commercial nickel oxide powders for components in solid oxide fuel cells

Various commercial nickel oxides were examined with respect to their relevant powder properties for application as anodes or anode substrates in solid oxide fuel cells (SOFC). The powders were characterized regarding their morphology, grain size distribution, BET surface and sintering behavior. Furthermore, anode substrates were produced from these powders together with yttria-stabilized zirconia (YSZ) powder. The sintering behavior, gas permeability and electrical conductivity of these components were determined and compared with the current standard material used for SOFC development at Research Centre Julich. A comparison of the powder and component properties indicate a significant influence of the powder properties on the components produced. (C) 2000 Elsevier Science Ltd. All rights reserved