Direct Observation of Space Charge Layers Inside Fuel Cell Electrolytes
Solid Oxide Fuel Cells (SOFCs) have gained attention for their potential to provide clean energy, due to their high efficiency and low carbon emissions. These fuel cells use yttria-stabilized cubic zirconia (YSZ) and other oxygen ion conductors as solid electrolytes. A long-standing challenge has been the significant drop in ionic conductivity at the interfaces between numerous crystal grains, known as grain boundaries, within the material.
This drop in conductivity has been thought to be due to space charge layers forming in the nanometer-scale region near the grain boundaries. However, directly observing these layers has proven extremely difficult, leaving fundamental questions about their existence unresolved.
In this study, researchers including Assistant Professor Satoko Toyama, Lecturer Takehito Seki, Project Associate Professor Bin Feng, Specially Appointed Research Professor Yuichi Ikuhara, and Director and Professor Naoya Shibata, utilized advanced electron microscopy techniques to directly demonstrate the presence of space charge layers at the grain boundaries of YSZ.
Their work included examining local electric fields at multiple grain boundaries with different crystal orientations, ultimately identifying some boundaries where space charge layers were absent.
By also conducting atomic structure observations, they found that the presence of space charge layers is strongly linked to the orientation and atomic structure of the grain boundaries. These findings suggest that controlling the structure of grain boundaries could help to eliminate space charge layers and reduce resistance to ion conduction.
The research marks a significant step toward understanding the factors that contribute to ion conduction resistance in battery materials, potentially paving the way for new methods to improve their performance.
The development was part of the "SHIBATA Ultra-atomic Resolution Electron Microscopy" project, supported by the Japan Science and Technology Agency (JST) under the Strategic Basic Research Program ERATO. This project aims to create a new "ultra" atomic resolution electron microscopy technique, surpassing conventional atomic resolution to allow simultaneous observation of atomic structures and electromagnetic field distributions over a wide temperature range. Such advances are expected to enable a direct examination of the origins of materials and biological functions.
Research Report:Direct observation of space-charge-induced electric fields at oxide grain boundaries