A model for solid oxide fuel cell (SOFC) stack degradation

Solid electrolytes are increasingly being used in active electrochemical devices such as fuel cells and batteries. Several fuel cells or batteries are connected in series to form a stack or a battery pack. The present manuscript examines the phenomenon of degradation in such devices, whose origin lies in the very basics of local thermodynamic equilibrium and transport. The specific example of solid oxide fuel cells (SOFC) is addressed here. If a single cell (or a few cells) in a stack exhibits higher resistance than the rest of the cells, stack failure often initiates at such a cell. The cell is then driven by, and exhibits lower voltage than the rest of the cells, and often even a negative voltage. The objective of this paper is to present a model for stack degradation when one of the cells exhibits a negative voltage. The existence of low level electronic conduction through the electrolyte plays central role in degradation. It is shown that if a cell exhibits a negative voltage, the oxygen chemical potential within the electrolyte can exceed that in the oxidant, and/or can drop below that in the fuel. This can lead to high internal oxygen partial pressure resulting in electrode cracking and delamination, and/or very low oxygen partial pressure leading to local electrolyte decomposition. Both situations can lead to cell and stack degradation.