RedOx study of anode supported solid oxide fuel cell

The common technology for solid oxide fuel cells (SOFC) is based on a cermet (ceramic-metal composite) anode of nickel with yttria stabilized zirconia (YSZ), often used as the supporting structure. One of the main limitations of this technology is the tolerance of the anode towards reduction and oxidation (“RedOx”) cycles. In this study, two techniques are used to quantify the anode expansion after a RedOx cycle of the nickel at different temperatures. The first method considers the anode expansion above the electrolyte fracture limit by measuring the crack width in the electrolyte layer. In the second method, the anode porosity is measured using scanning electron microscopy (SEM) image quantification. The same measurement techniques are used to quantify anode expansion after consecutive RedOx cycles at constant temperature. The quantification technique is then applied to cells tested in real stack conditions. The cell corners can undergo several RedOx cycles depending on stack design and fuel utilization. The study of such zones allows estimating the number of cycles that the anode experienced locally.

Redox study of anode supported solid oxide fuel cell

Antonin Faes, Aïcha Hessler-Wyser, Jan Van Herle

The common technology for solid oxide fuel cells (SOFC) is based on a cermet anode of nickel with yttrium stabilized zirconia (YSZ), often used as the supporting structure. One of the main limitations of this anode technology is the redox tolerance. In this study, two techniques are used to quantify the anode expansion after one redox cycle at different temperatures. First, the increase of porosity is measured using scanning electron microscopy (SEM) image quantification. The second method considers the anode expansion above the electrolyte fracture limit by measuring the crack width in the electrolyte layer. An influence of redox cycling temperature on anode volume expansion was already observed by Ettler et al. but not fully understood. Here a model based on nickel oxidation mechanisms is proposed to explain the relation between redox temperature and volume expansion. The same methods are used to quantify anode expansion after consecutive redox cycles at constant temperature. In particular, small cracks caused at low oxidation temperature (600°C) were closed again after a subsequent reduction. The quantifying technique is then applied to cells tested in real stack conditions. The cell corners can undergo redox cycles depending on stack design and fuel utilization. The study of this location can give information on the number of cycles experienced by the anode support. Moreover, cracks in the electrolyte show infiltration of NiO that close the openings. Important infiltration can produce a short circuit between anode and cathode. The presence of strontium chromate on the infiltrated NiO was observed.

2008

Methods for quantification of anode support microstructure

Antonin Faes, Aïcha Hessler-Wyser, Christian Monachon, Jan Van Herle

Degradation observed on long term stack tests may be explained in part by microstructural changes of the electrodes. Particularly at the anode side, the coarsening of nickel particles will affect its performances. Increasing the metal particle size will first diminishes the Three-Phase Boundary (TPB) where fuel oxidation takes place, and secondly decrease the electronic conductivity [1]. In order to quantify the changes in anode microstructure, SEM images analysis is used. Some techniques can be time consuming therefore sample preparation, image treatment and analysis have been optimized to be more efficient. The application of different SEM sample preparation and analysis methods on HTceramix anode supports each show advantages and drawbacks.. One quick method consists in polishing the anode without infiltration and analyzing images manually [2]. A second method requires more elaborate preparation, with image analysis carried out using a Mathematica code. These methods are applied on as sintered and aged (in- stack test) cells based on two different HTceramix anode support structures. 1. Simwonis, D., Tietz, F., and Stoever, D., 2000, Nickel coarsening in annealed Ni/8YSZ anode substrates for solid oxide fuel cells: Solid State Ionics, v. 132, p. 241-251. 2. Monachon, C., Hessler-Wyser, A., Faes, A., Van herle, J., and Tagliaferri, E., 2007, A quick method for characterizing Nickel-Yttria stabilized Zirconia cermet microstructure by scanning electron microscopy: to be submitted.

2007

Quantitative study of Ni-zirconia anode micro- and nanostructure related to SOFC stack degradation

Antonin Faes, Aïcha Hessler-Wyser, Jan Van Herle

As the performances of Solid Oxide Fuel Cells (SOFC) get attractive, long term degradation becomes the main issue for this technology. Therefore it is essential to localise the origin of degradation and to understand its mechanisms in order to find solutions and to improve SOFC durability. Microstructural evolution of anode supported solid oxide fuel cells (SOFC) during medium-term stack testing has been characterized by scanning electron microscopy (SEM). A new technique is presented using low acceleration voltage SEM imaging to separate the three anode phases (nickel, yttria-stabilized zirconia and porosity). Microstructural quantification is obtained using a software code that yields phase proportions, particle sizes, particle size distribution and a direct measure of triple phase boundary (TPB) density (μm/μm3). In addition, an anode degradation model is developed. It describes the gradual degradation of the anode due to particle sintering and the concomitant loss of the three-phase-boundary. Fundamental, operational and structural parameters of the anode can be used to estimate the three-phase-boundary length evolution with time from the degradation rate. The combination of experimental results and modeling allows separating the degradation due to sintering of nickel particles from total stack degradation. Anode degradation occurs principally during the first 500 operating hours. For medium term stack operation (more than 1000 h), anode degradation is responsible for 18 % to 46 % of the total degradation depending on stack technology.

2008