Chromium-Related Degradation of Thin-Film Electrolyte Solid Oxide Fuel Cell Stacks

Solid oxide fuel cell short stacks for dezentralized stationary power delivery have been operated with special thin-film electrolyte anode-supported cells. After operation times of up to 3,000h the cells were post-test characterized. The cells have besides very thin electrolyte films (≤ 3µm) either LSCF or LSC cathodes (LSCF: La-Sr-Co-Fe oxide, LSC: La-Sr-Co oxide). Typical operation parameters were temperatures between 600 and 750°C, a current density of 500mA/cm², hydrogen with 20% water vapor and a fuel utilization of approx. 40%. Typical chromium evaporation protection layers like wet powder sprayed manganese oxide and cathode contact layers (a perovskite based on La-Mn-Co-Cu oxide) were used.

Microstructural post-test characterization reveals in the case of using a 1µm 8YSZ electrolyte prepared via sol-gel route and on top a sputtered diffusion barrier layer composed of GDC, the existence of SrCrO₄ either on top of the LSCF cathode as expected or, unexpectedly the formation of the spinel phase at the borderline cathode / barrier (electrolyte) layer. In case of using a 3µm thin screen printed 8YSZ electrolyte and an LSC cathode, the chromite phase has not been detected at the cathode / barrier border.

SEM and TEM post-test analysis of the cells show in case of the sol-gel electrolyte and sputtered GDC also the formation of micro-pores within the GDC layer. Theoretical calculations reveal that the chosen current density may lead to either preferred oxygen reduction or preferred chromium reduction depending on the real local current density within the cathode / barrier (electrolyte) interface.