Non-Stationary Effects of Current-Load on Electrochemical Behavior of La0.8Sr0.2MnO3- δ in Solid Oxide Electrolysis Cell

Abstract

High-temperature solid oxide water electrolysis is a highly efficient and carbon-free technology employed for energy conversion, offering numerous advantages over traditional low-temperature water electrolysis methods. These benefits include fast electrode kinetics without the need for Pt group metals and the potential to operate in reversible regimes (fuel cell/steam electrolysis), attributed to the elevated operating temperature (i.e., 600-800 °C). However, the high operating temperature imposes stringent material requirements.While recent attention has been directed towards developing and characterizing new materials for both electrodes and electrolytes, a comprehensive understanding of the underlying reaction mechanisms remains incomplete, hindering the further advancement of the solid oxide cells.Despite the emergence of more advanced materials in recent years, the lanthanum-strontium-manganite La1-xSrxMnO3-δ (LSM) remains to be widely used for an oxygen electrode construction. This preference is due to its cost-effectiveness, high electrical conductivity, thermal and chemical stability, and compatibility with various solid electrolytes. Although LSM has been known for several decades, the reaction mechanism occurring on this oxygen electrode in a broad range of electrode potentials is inadequately described, with discrepancies arising among different authors. These inconsistencies are often linked to the evolving understanding of the non-stationary properties of the LSM lattice – the inner defect chemistry of LSM.Contemporary studies increasingly adopt the concept of ionic conductivity of LSM enabled by cathodic polarization to describe the oxygen reduction reaction. This shift from exclusive electron to ionic-electron conductivity enhances the performance of the LSM electrode, potentially causing discrepancies in the literature as experimental data may not align with theoretical descriptions. Conversely, for the oxygen evolution reaction during anodic polarization, a reversed reaction mechanism is considered without accounting for ionic conductivity of LSM under either stationary or non-stationary conditions. This approach may oversimplify the defect chemistry of LSM, particularly during transitions between operational regimes (i.e., fuel cell and electrolyser regimes). However, such changes are difficult to describe only by experimental approach. Here a combination with mathematical modelling seems to be promising approach allowing to gain deeper understanding.The primary objective of this study is to further investigate the impact of LSM defect chemistry on electrode performance, proposing a reaction scheme during both operating regimes and, crucially, during transitions between them.To address this goal, a series of experiments were conducted under both current-less and current-load conditions using laboratory button single cells. Various gas compositions and operating temperatures were used to determine whether LSM becomes a mixed ionic-electron conductor. Potentiostatic or potentiodynamic methods were used to clarify how this altered property influences individual operating regimes. The results indicate that cathodic polarization leads to a partial reduction of Mn4+ to Mn2+ in the crystalline lattice of LSM, creating oxygen vacancies through which oxide ions can be transported. Additionally, these lattice changes, though temporary, impact the oxygen evolution reaction region under certain conditions. A mathematical model incorporating parallel reaction pathways for both operating regimes supports these experimental results, predicting a cathodic potential at which ionic conductivity becomes viable and identifying conditions where partial reduction of Mn ions is no longer possible.This work contributes to understanding solid oxide cells, providing a solid foundation for more complex mathematical modeling of electrolysis cells. It underscores the necessity to fully comprehend the electrochemical behavior of electrode materials for the continued development of this technology.This work was supported by the Technology Agency of the Czech Republic under project no. TK04030143 (Advanced reversible system for hydrogen production on base of the high temperature solid oxide cell).