SOFC anodes: Ni model anodes
Although fundamental studies were already focused on the kinetics of Ni/YSZ cermet anodes in solid oxide fuel cells (SOFC), there is still no consensus with respect to the rate determining steps of electrochemical reactions at the triple phase boundary. Ni anodes with well-defined patterns on Yttria-stabilized Zirconia (YSZ) represent a promising method to determine the reaction kinetics. In contrast to technical anodes, the reaction zone for hydrogen oxidation, i.e. the triple phase boundary between electrolyte, electrode and gas phase is well defined due to the two-dimensional geometry.
The work on patterned Ni anodes was embedded in a collaborative project of four research groups, funded by the German Research Foundation (DFG). Patterned Ni anodes were fabricated on YSZ single-crystal substrates and polycrystalline YSZ substrates at the Laboratory for Electron Microscopy (KIT) employing photolithographic techniques. Different pattern geometries yield three-phase boundary lengths between 1 and 10 m/cm². Detailed electrochemical characterization over a large range of operating conditions was performed at the Institute of Materials for Electrical Engineering (KIT). To detect possible changes of, e.g., triple-phase boundary lengths and morphology of the patterned model anodes upon electrochemical characterization, pre- and post-test microstructural characterization was performed using scanning (SEM) and transmission electron microscopy (TEM). Modeling of elementary kinetic reactions was performed at Interdisciplinary center for Science Computing (University of Heidelberg) and Institute of Technical Thermodynamics (German Aerospace Center, DLR, Stuttgart) allowing the quantitative comparison of various reaction pathways, including surface spillover and bulk-phase charge transfer. Additionally, characterization of surface chemistry by means of temperature-programmed desorption (TPD) was performed at the Institute for Physical Chemistry (University of Heidelberg) to obtain thermodynamic and kinetic data of relevant species.
W. Bessler, M. Vogler, H. Störmer, D. Gerthsen, A. Utz, A. Weber, E. Ivers-Tiffée, Model anodes and anode models for understanding the mechanism of hydrogen oxidation in solid oxide fuel cells, Phys. Chem. Chem. Phys. 12, 13888 (2010), DOI:10.1039/c0cp00541j
A. Utz, H. Störmer, A. Leonide, A. Weber, E. Ivers-Tiffée, Degradation and Relaxation Effects of Ni patterned anodes in H2/H2O Atmosphere, J. Electrochem. Soc. 157, B920 (2010)
A. Utz, H. Störmer, D. Gerthsen, A. Weber, E. Ivers-Tiffée, Microstructure stability studies of Ni patterned anodes for SOFC, Solid State Ionics 192, 565 (2011), http://dx.doi.org/10.1016/j.ssi.2010.05.004