Study of the Redox Behavior of SOFC’s Anode Functional Layers

Abstract

The cermet Ni-YSZ is the most common fuel electrode used for solid oxide fuel cells[i]. Under polarization, the Nickel phase remains metallic because of the reducing atmosphere. However, troubleshooting like system emergency shutdown may cause the Ni phase to be completely/partially oxidized, resulting in a large bulk volume change (volume expansion >65%)[ii]. Microstructure failures can then occur in the Ni-YSZ matrix. Redox cycles of the Nickel could also degrade the microstructure of the Ni-YSZ anode (for instance, coarsening of the Ni phase yielding loss of Ni percolation), and thus the performance of the cell[iii]. All these phenomena lead to an increase of the polarization resistance of the anode. The target of the OxiGEN project is to develop the next-generation all ceramic Solid Oxide Fuel Cell stack and hot box solution for small stationary applications[iv]. One of the objectives of the project is the improvement of the redox tolerance of the anode functional layer, in order to prevent nickel coarsening. A large experimental matrix with various combinations of parameters (NiO / YSZ ratio, porosity and electrolyte particle size) was established to investigate innovative anode functional layers with better redox resistance and improved electrochemical performance compared to a reference anode prepared with the same NiO powder and a proprietary YSZ based composition. Thin ceramic-polymer tapes (<100µm) have been manufactured by a tape-casting process developed by EIfER following the industrial requirements provided by Saint-Gobain. The microstructural characterization performed after sintering show homogeneous anode functional layers with a thickness below 30 µm and a good repartition between the ceramic phases and the porosity for all samples prepared in this study (see Figure 1). The influence of the modification of the microstructure resulting from the variation of the electrolyte particle size properties was investigated by SINTEF on the electrical properties of the reduced anodes using Van der Paw method. An increase of the total electrical conductivity from 100 to 900 S.cm-1 was observed between the reference and innovative layers, respectively. Specific redox cycle protocols were defined to observe the modification of the microstructure of single layers and complete cells after redox cycles simulating specific emergency shutdown (see figure 2). They are composed of two main steps. The first one corresponds to an oxidation phase where the temperature is decreased while the supply of hydrogen is stopped (controlled emergency shutdown). An air leakage was also simulated by introducing a small quantity of air into the fuel chamber. The second phase represents the restart of the system with an increase of the temperature and the reintroduction of hydrogen to the fuel side. The intermediate stage was also optimized in order to emphasize the effect of the oxidation phase on the fuel electrode. The influence of the integration of advanced layers on the microstructure and redox behavior of layers and complete cells will be discussed. “This project has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement No 779537 . This Joint Undertaking receives support from the European Union’s Horizon 2020 research and innovation programme and Hydrogen Europe and Hydrogen Europe Research”. [i] S.P.S. Shaikh, A. Muchtar, M.R. Somalu, Ren. Sus. Energy Reviews 51 (2015) 1-8. [ii] S. Toros, Ceram. Int. 42 (2016) 8915-8924. [iii] T. Shimura, Z. Jiao, S. Hara, N. Shikazono, J. Power Sources 267 (2014) 58-68. [iv] http://oxigen-fch-project.eu/ Figure 1