Reversible high temperature cells for power generation and hydrogen production using mixed ionic electronic conducting solid electrolytes

Abstract

High temperature solid oxide cells can be used both as fuel cells (SOFC) for power generation and as electrolyzer cells (SOEC) for hydrogen generation. Such cells are called reversible cells. They are typically made using solid electrolytes which exhibit purely ionic conduction and negligible electronic conduction. In this work reversible cells made of mixed ionic electronic conducting (MIEC) materials with significant electronic conduction as electrolyte are investigated. It is shown that MIEC cells can be designed to operate as efficiently as those made of a purely ionic conducting electrolyte (e.g. YSZ). Thus, in SOFC mode, suitably designed MIEC electrolyte cells consume fuel at the same rate, deliver the same power and release the same amount of joule heat as purely ion conducting electrolyte cells. Similarly, in SOEC mode, MIEC electrolyte cells consume water vapor at the same rate, generate hydrogen at the same rate, consume the same electrical power and release the same joule heat as purely ionic conducting electrolyte cells. Experimental results are presented on SOEC stacks made of YSZ and MIEC electrolyte cells. The MIEC electrolyte cells exhibit more stable operation compared to purely ion conducting electrolyte (YSZ) cells. The improved durability of MIEC electrolyte cells is attributed to the smoothening of the chemical potential of oxygen, μO2(x), variation in the electrolyte. The analysis also shows that highly active electrodes should lower the tendency for degradation even with a purely ion conducting electrolyte cell.