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The oxygen partial pressure in solid oxide electrolysis cells with two layer electrolytes

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.