Kinetic Studies on State of the Art Solid Oxide Cells – A Comparison between Hydrogen/Steam and Reformate Fuels

Abstract

Electrochemical reaction kinetics at the electrodes of Solid Oxide Cells (SOCs) were investigated at 700 °C for two cells with different fuel electrode microstructures as well as on a third cell with a reduced active electrode area. Three fuel mixtures were investigated – hydrogen/steam and reformate fuels hydrogen/carbon-dioxide and hydrogen/methane/steam. It was found that the kinetics at the fuel electrode were exactly the same in both reformates. The hydrogen/steam fuel displayed slightly faster kinetics than the reformate fuels. Furthermore the gas conversion impedance in the hydrogen/steam fuel split into two processes with opposing temperature behavior in the reformate fuels. An 87.5% reduction in active electrode area diminishes the gas conversion impedance in the hydrogen/steam fuel at high fuel flow rates. In both reformates, the second and third lowest frequency processes merged into a single process as the gas conversion was reduced. The SOC with finer electrode microstructure displayed improved kinetics.

(Invited) High Performance Nano-Ceria Electrodes for Solid Oxide Cells

“…The performance of these doped-ceria film electrodes matches that of the best reported nanostructured Ni-based electrodes -R P ~0.01 at 650 °C for Ni-infiltrated LSGM (4,39,40), which is roughly a factor of 20 lower than conventional, stable Ni-SZ composites (7,8). However, the stability of Ni-infiltrated electrodes is poorreports range from an increase in R P by a factor of 10 in 24 h for Ni-infiltrated SZ (41) to a factor of 2 in 500 h for Ni-infiltrated LSGM (39), at 650 °C in pH 2 /pH 2 O 0.97/0.03 atm.…”

“…The conventional nickel and yttria-or scandia-yttria stabilized zirconia composite (Ni-SZ) fuel-electrode provides high reaction rates for the H 2 O/CO 2 reduction and H 2 /CO oxidation reactions, but it is often a major source of long-term performance degradation due to carbon deposition, poisoning of reaction sites, Ni mobility, and other degradation mechanisms (5,6). Furthermore, in state-of-the-art cells, the polarization resistance (R P ) of the Ni-SZ electrode in pH 2 /pH 2 O 0.8/0.2 atm fuel gas is an order of magnitude higher than cobaltite based oxygen-electrodes in air (0.1-0.15 vs 0.015-0.04 Ω cm 2 at 700 °C, and both values are roughly doubled at 650 °C) and is the largest contributor to the total cell resistance (7)(8)(9).…”