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

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 model reformate fuels-hydrogen/carbon-dioxide and hydrogen/methane/steam. It was found that the electrode kinetics at the fuel electrode were exactly the same in both reformates. The hydrogen/steam fuel displayed 5-7% faster kinetics than the reformate fuels. 19% faster kinetics were recorded for the cell with a finer microstructure. The measured gas conversion impedance was compared with models in literature for both the 16- and the 2 cm2 cells. The continuously stirred tank reactor (CSTR) AC model approximated the overpotential of the smaller cells (2 cm2) with greater accuracy in the current density range 0-0.5 A/cm2 while the plug flow reactor (PFR) model although derived for the case of zero DC bias predicted the 16 cm2 cell ASR better than the zero bias CSTR model. Furthermore, the gas conversion impedance in the hydrogen/steam fuel split into two processes with opposing temperature behavior in the reformate fuels. By using a 87.5% smaller active electrode area the gas conversion impedance was diminished in the hydrogen/steam fuel at (the same absolute) 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.