Distribution of Oxygen Partial Pressure in Multilayer Electrolytes: Explaining Degradation of Solid Oxide Electrolyzer Cells

Abstract

It is found experimentally that solid oxide electrolyzer cells (SOECs) with mixed ionically and electronically conducting oxygen electrodes such as Lanthanum Strontium Cobalt Ferrite (LSCF) show much-reduced electrode fracture and degradation compared to cells with Sr-doped LaMnO3 (LSM): Yttria-stabilized Zirconia (YSZ) electrodes. In the former case, the cells have a Gadolinium doped Ceria (GDC) barrier sandwiched between the electrode and the YSZ electrolyte, i.e., there is a multi-layer electrolyte. In order to investigate how different electrodes and the GDC barrier layer affect degradation phenomena and optimize the design of SOECs to achieve the desired long-term stability, a diffused interface model is proposed to investigate the distribution of oxygen partial pressure in the SOEC multi-layer electrolyte. Influence of operating condition, structures and properties of the electrolyte on the distribution of oxygen partial pressure are studied. The model quantitatively predicts the conditions under which oxygen electrode fracture/delamination occurs, largely determined by the electrode overpotential given by the electrode polarization resistance. In addition, it is found that when a GDC barrier layer is present, there is a maximum in the oxygen pressure at the interface of the YSZ and GDC that can result in degradation or fracture. An analytical estimation on the oxygen partial pressure at the YSZ/GDC interface is obtained and shown to provide good agreement with numerical results. The predictions are also in accord with experimental observations of fracture or void formation near the GDC/YSZ interface. The results indicate that this peak pressure can be reduced by reducing the GDC layer thickness, use of a lower polarization resistance oxygen electrode, and by altering the electrolyte materials in order to affect their transport properties, e.g. reducing the Y doping level of YSZ to reduce the oxygen vacancy diffusivity. Changes in operating conditions, i.e., lowering the current density and increasing operating temperature can also reduce the peak pressure, and should thereby suppress the formation of pores or cracks at the YSZ/GDC interface. Finally, inter-diffusion between YSZ and GDC during the electrolyte firing process can yield a low oxygen ion diffusivity zone at their interface. Our numerical simulation results show that this low oxygen ion diffusivity layer increases the maximum oxygen partial pressure at the GDC/YSZ interface, thereby exacerbating degradation.