High Temperature Solid Oxide Electrolysis – Technology and Modeling

In the global quest to renounce from fossil fuels, a large demand for the renewable production of hydrogen via water electrolysis exists. In this context, the solid oxide electrolyzer (SOE) is an interesting technology due to its high efficiency resulting from elevated operating temperatures of up to 900 °C. Physical modeling plays a vital role in the development of SOEs, as it lowers experimental costs and provides insight where measurements reach limits. A main challenge for modeling SOEs is the multitude of physical effects, occurring and interacting on various spatial and temporal scales. This requires assumptions and simplifications, particularly when increasing scope and dimensions of a model. In this review, we discuss the different approaches currently available in literature

Kinetic Studies on Ni-YSZ Composite Electrodes

AC and DC techniques were applied to investigate the electrochemical reaction kinetics of porous composite Ni/8-mol% yttria-stabilized zirconia (Ni/8YSZ) solid oxide cell (SOC) electrodes using a novel pseudo-3-electrode cell geometry. From OCV impedance spectra an activation energy Ea of 1.13 eV, pre-factor γan of 3.7∙105∙T, hydrogen and steam partial pressure dependencies a and b respectively of -0.07 and 0.22 were determined. DC current density vs. overpotential curves compared with those predicted using the determined kinetic parameters. Apparent Butler-Volmer charge transfer coefficients α were determined from the current density vs. overpotential curves. Values ranging from 0.57 at 650 °C to 0.64 at 850 °C were determined from the anodic branch and 0.85 to 0.81 from the cathodic branch in the same range, with higher fitting accuracy in the anodic branch. The lower fitting accuracy of the cathodic branch and the need for different α values for each branch suggests that a simple BV model of the measured electrode kinetics is insufficient and/or different reaction mechanisms might be occurring in anodic vs cathodic polarization.</jats:p

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 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