  A number of degradation mechanisms have been observed during the long-term
operation of solid oxide electrolysis cells (SOEC). Using an electrolyte charge
carrier transport model, we quantify the oxygen potentials across the
electrolyte and thereby provide insights into these degradation mechanisms. Our
model describes the transport of charge carriers in the electrolyte when the
oxygen partial pressure is extremely low by accounting for the spatial
variation of the concentration of oxygen vacancies in the electrolyte.
Moreover, we identify four quantities that characterize the distribution of
oxygen partial pressure in the electrolyte, which are directly related to the
degradation mechanisms in the electrolyte as well: the two oxygen partial
pressures at the interfaces of the electrodes and the electrolyte, the oxygen
partial pressure at the interface of YSZ/GDC, and the position of the abrupt
change in oxygen potential near the p-n junction that develops in YSZ when one
side of the cell is exposed to fuel (low oxygen potential, n-type conduction)
and the other side is exposed to oxidant (high oxygen potential, p-type
conduction). We give analytical estimates for all of these quantities. These
analytical expressions provide guidance on the parameters that need to be
controlled to suppress the degradation observed in the electrolyte. In
addition, the effects of operating conditions, particularly current density and
operating temperature, on degradation are discussed.
