Ionic and electronic transport across interfaces in thin electrolyte film, anode supported solid oxide fuel cells.

摘要

In transport studies in oxygen ion conductors, oxygen chemical potential (muO2 ) has been usually assumed to be equilibrated across gas/solid electrolyte interfaces. However, since the interfaces exhibit different properties from the bulk, they must have their own ionic and electronic properties. In this study, Pt reference electrodes were embedded within the electrolyte (gadolinia-doped ceria; GDC) in an anode-supported solid oxide fuel cell to measure the electrochemical potential of electrons (ϕ) through the bulk electrolyte and its interfaces under fuel cell operating condition. Based on local equilibrium assumption, which leads to relations between electrochemical potentials of charged species and chemical potential of neutral species, the corresponding muO 2 was estimated. When the GDC is protected by a thin layer of a predominantly ionic conductor from reducing atmosphere, the muO 2 varied monotonically through the GDC layer, exhibiting a relatively small change across the cathode interface region. By contrast, when the GDC was exposed to hydrogen, it was significantly reduced, resulting in higher electron concentration. The corresponding muO 2 was small through the GDC layer, exhibiting an abrupt change across the cathode interface region. This difference in the muO 2 variation depending upon the relative electronic conduction in the electrolyte resulted in a large difference in the cathode overpotential.;The direction of ionic/electronic current and the corresponding internal muO2 through the electrolyte can have a profound effect on its stability. If cell imbalance exists in a series-connected fuel cell stack, a "bad" cell characterized by a higher resistance can be operated under a negative voltage. To investigate the SOFC stack failure by simulating abnormal behavior in a single cell test, yttira stabilized zirconia (YSZ) electrolyte cells were tested with an applied DC bias. When operating under a negative voltage, rapid degradation occurred characterized by increased cell resistance. Microscopic examination revealed that delamination occurred along the anode interface. This was attributed to the high internal muO2 formed just under the anode interface in the electrolyte. However, when 8 mol % ceria was doped in the YSZ electrolyte, no delamination occurred since the increased electronic conduction released the internal muO2 . Thus, the electronic conduction in the electrolyte plays an important role in its thermodynamic stability of the electrolyte.