Lanthanum Strontium Manganite Oxygen Electrode Reaction Mechanism in High-Temperature Solid Oxide Water Electrolysis
Authors: Carda Michal , Budac Daniel , Paidar Martin , Bouzek Karel 
Published: July 2022

High-temperature solid oxide water electrolysis cells have become the center of attention of the research community owing to several advantages over traditional water electrolysis technologies. Among others, solid oxide water electrolysis represents highly efficient technology with the possibility to operate in a reversible regime (fuel cell/steam electrolysis). Despite the fast reaction kinetics due to the high operating temperature (600-900 °C). A more detailed understanding of the reaction mechanism is of critical importance to further develop this technology.
Lanthanum-strontium-manganite La1-xSrxMnO3-δ is still considered a viable material for an oxygen electrode due to its high electrical conductivity, good chemical stability, and thermomechanical compatibility with various solid electrolytes. However, its negligible ionic conductivity is frequently reported in the literature. Although, La1-xSrxMnO3-δ is utilized as an oxygen electrode for several decades and its properties are reported in the literature, the reaction mechanism occurring on this oxygen electrode is insufficiently described, with multiple discrepancies occurring between the individual authors. The open literature almost exclusively considers the oxygen reduction reaction. Here La1-xSrxMnO3-δ is described exclusively as a pure electron conductor. In the case of the oxygen evolution reaction, only the mechanism reversed to the oxygen reduction is considered, which is clearly an oversimplification.
Recent articles tend to connect the defect chemistry of La1-xSrxMnO3-δ and electrochemistry to disapprove the generally accepted description of reaction mechanisms, owing to the rather low agreement between theory and experimental data. In the framework of this study, a series of experiments was conducted, both under current-less as well as current-load conditions using laboratory button cells. Oxygen electrodes based on various mixtures of La0.8Sr0.2MnO3-δ-ZrO2:Y2O3 were tested to prove that La1-xSrxMnO3-δ may become a mixed ionic-electron conductor under specific conditions.
The obtained data showed that conditions, such as oxygen partial pressure, temperature and, in particular, the polarization history, could result in a partial reduction of Mn4+ to Mn2+ in the crystal lattice of La1-xSrxMnO3-δ. The change of valence is of a great importance because it creates oxygen vacancies through which oxide ions can be transported. Therefore, under such conditions La1-xSrxMnO3-δ becomes a partial mixed ionic-electron conductor, significantly increasing the number of active reaction sites in the three-dimensional structure of the electrode. The most recent articles reported this behavior; however, only during the oxygen reduction reaction owing to the most suitable conditions for the Mn oxidation state transition. Within the framework of this study, the aim is to expand this concept also to the conditions of the oxygen evolution reaction, which has been completely omitted in the literature. The presented results significantly contribute to the understanding of solid oxide cells and represent a solid base for further modelling of the electrolysis cell, and optimization of its construction as well as operational conditions.
This work was supported by the Technology Agency of the Czech Republic under project no. TK04030143.