Revisiting the ionic conductivity of solid oxide electrolytes: a technical review

Abstract

Oxygen-ionic and proton-conducting oxides are an important class of functional materials for their applications as electrolytes in various high-temperature electrochemical cells, including solid oxide fuel cells (SOFCs), solid oxide electrolysis cells (SOECs), proton ceramic fuel cells (PCFCs), and proton ceramic electrolysis cells (PCECs). The electrolyte, as a heart of these cells, must meet a number of stringent requirements, among which its ionic conductivity is one of the most important parameters determining the possible applications in electrochemical devices and operating temperatures. In the present work, we provide a comparative analysis of the ionic conductivity for several classical electrolyte classes, such as stabilized/doped ZrO₂, CeO₂, LaGaO₃, BaCeO₃, BaZrO₃, and Ba(Ce,Zr)O₃. This analysis provides possible windows of conductivity variation and potential (common and specific) factors responsible for conductivity differences for the same materials and measurement conditions. In addition, the ionic conductivity is discussed not only for massive ceramic electrolytes, but also for their thin-film form in SOFCs/PCFCs or SOECs/PCECs. As a result, the transport properties of the state-of-the-art electrolytes have been revised and compared to show which system prevails over the other under different experimental conditions. Therefore, this technical review can be used as a reference source to provide useful information on oxygen-ionic and protonic conductivity for the most popular solid oxide electrolytes.