A research team from The University of Tokyo has succeeded for the first time in directly observing the space charge layers^*1) inside the solid electrolyte of a fuel cell.

Solid Oxide Fuel Cells (SOFCs)^*2) are expected to be a source of clean energy because of their low carbon dioxide emissions and high-power generation efficiency. SOFCs use yttria-stabilized cubic zirconia (YSZ)^*3) and other oxygen ion conductors as solid electrolytes. However, the drastic drop in ionic conductivity at the interfaces between the countless crystal grains (grain boundaries) inside the material is an issue, and it has long been proposed that the cause of this drop is the space charge layers distributed in nanometer range near the grain boundaries. It was extremely difficult to observe these layers directly, and the fundamental question of whether the space charge layers really exist at grain boundaries remained unanswered.

In this study, Assistant Professor Satoko Toyama, Lecturer Takehito Seki, Project Associate Professor Bin Feng, Specially Appointed Research Professor Yuichi Ikuhara, and Director and Professor Naoya Shibata of the Institute of Engineering Innovation, Graduate School of Engineering, The University of Tokyo, have succeeded in directly demonstrating the existence of space charge layers at the grain boundaries of YSZ by observing local electric fields using advanced electron microscope. Furthermore, similar observations were made at several grain boundaries with different crystal orientations (the way the atoms are arranged in the crystal structure), and the researchers succeeded in finding grain boundaries where space charge layers do not exist. By combining this with atomic structure observations using electron microscopy, they revealed that the space charge layers are strongly correlated with the crystal orientation and atomic structure of the grain boundaries. The researchers found that by controlling the structure of the grain boundaries, it is possible to eliminate the space charge layer and reduce the resistance to ion conduction at the grain boundaries.

This research represents a major step toward elucidating the cause of ion conduction resistance at grain boundaries in battery materials, and promises to lead to the establishment of new guidelines for improving the performance of battery materials in the future.

The results of this development were achieved as part of the research project "SHIBATA Ultra-atomic Resolution Electron Microscopy" supported by the Japan Science and Technology Agency (JST) under the Strategic Basic Research Program ERATO. In this project, JST aims to develop a new measurement technique that can be called an "ultra" atomic resolution electron microscopy that goes beyond conventional atomic resolution electron microscopy, allowing simultaneous observation of atomic-scale structures and electromagnetic field distributions in the temperature range from extremely low to high temperatures. This will enable direct "observation" of the origins of materials and biological functions.

https://www.jst.go.jp/erato/en/research_area/ongoing/jpmjer2202_en.html

Notes
(*1) Space charge layer
A layer where the electric field of atoms appears due to uneven charge distribution at grain boundaries, which are the interfaces between crystals.

(*2) Solid Oxide Fuel Cells (SOFCs)
Fuel cells where solid ceramic ion conductors such as YSZ are used as electrolytes to generate electricity through the reaction of hydrogen and oxygen. They are used in various fields, from household fuel cells (one example of a product name is ENE-FARM) to large-scale power generation.

(*3) Yttria stabilized cubic zirconia (YSZ)
Zirconia whose cubic structure has been stabilized by adding yttrium (chemical symbol: Y). This substance is used as a solid electrolyte due to its high oxygen ion conductivity and chemical and thermal stability.