Abstract
Reduction-stable mixed ionic and electronic conductors such as Sr(Ti,Fe)O3-δ (STF) are promising materials for application in anodes of solid oxide fuel cells. The defect chemistry of STF and its properties as solid oxide fuel cell (SOFC) cathode have been studied thoroughly, while mechanistic investigations of its electrochemical properties as SOFC anode material are still scarce. In this study, thin film model electrodes of STF with 30% and 70% Fe content were investigated in H2+H2O atmosphere by electrochemical impedance spectroscopy. Lithographically patterned thin film Pt current collectors were applied on top or beneath the STF thin films to compensate for the low electronic conductivity under reducing conditions. Oxygen exchange resistances, electronic and ionic conductivities and chemical capacitances were quantified and discussed in a defect chemical model. Increasing Fe content increases the electro-catalytic activity of the STF surface as well as the electronic and ionic conductivity. Current collectors on top also increase the electrochemical activity due to a highly active Pt-atmosphere-STF triple phase boundary. Furthermore, the electrochemical activity depends decisively on the H2:H2O mixing ratio and the polarization. Fe0 nanoparticles may evolve on the surface in hydrogen rich atmospheres and increase the hydrogen adsorption rate.
Highlights
Acceptor-doped mixed ionic and electronic conductors are a promising class of materials for application in solid oxide fuel cell (SOFC) electrodes and they are widely investigated as SOFC cathodes due to their low polarization resistance.[1,2,3]
The usually ill-defined geometry of porous electrodes makes the analysis of specific materials parameters such as electronic and ionic conductivity or oxygen exchange activity very challenging
The circuit model proposes that electrochemically driven oxygen exchange is only possible within a characteristic distance (LC) from the current collectors and this could be visualized by 18O isotope exchange.[26]
Summary
Acceptor-doped mixed ionic and electronic conductors are a promising class of materials for application in solid oxide fuel cell (SOFC) electrodes and they are widely investigated as SOFC cathodes due to their low polarization resistance.[1,2,3] Some of these materials are chemically stable and mixed conducting in humidified hydrogen atmosphere,[4] which makes them applicable in SOFC anodes. Thorough studies of the mechanisms of electrochemical oxygen exchange in reducing atmospheres are still largely missing, for STF, but for most reduction stable mixed conductors, except for (Gd or Sm) doped ceria[19,20,21,22,23,24] and a study on (La,Sr)FeO3-δ.25 The typically low electronic conductivity of acceptor doped oxides in reducing atmosphere requires current collectors added to the thin film electrodes.
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