Kumamoto University develops novel fuel cell electrolyte from clay: Nanosheet extraction and precise layering

A research group led by Assistant Professor Kazuto Hatakeyama and Professor Shintaro Ida from the Institute of Industrial Nanomaterials (IINa) at Kumamoto University has successfully developed a novel solid electrolyte for fuel cells by extracting nanosheets from natural clay minerals and precisely layering them. Their research was published in Journal of Materials Chemistry A.

Fuel cells using inorganic materials as solid electrolytes have been commercialized for residential power applications. However, their operating temperatures remain high at over 800℃, while fuel cells mounted in commercially available vehicles primarily use solid polymer electrolytes that operate steadily at 80-90℃. Next-generation fuel cell vehicles require operation at temperatures above 100℃, but solid polymer electrolytes have issues, including greater hydrogen crossover (a phenomenon where hydrogen leaks out, reducing power generation efficiency) during medium-temperature operation than ceramic electrolytes of similar thickness, and environmental concerns due to fluorine use. The search for new materials that can solve all these issues simultaneously is a challenge for next-generation fuel cells.

In this study, the research group successfully created laminated films of inorganic nanosheets by extracting only monolayer nanosheets from montmorillonite, a natural clay mineral, and precisely layering them. The fabricated membrane uses no binders or other materials for mechanical strength reinforcement and consists of 100% inorganic materials with silicon, aluminum, and magnesium as the main components. By stacking only monolayer nanosheets into a membrane, the group succeeded in creating a membrane that is flexible despite being made of inorganic materials. The membrane also has high formability and can be easily fabricated with controlled thickness, shape, and area using methods such as drop-drying, suction filtration, and spin coating. Clay deposits such as montmorillonite, the raw material, exist worldwide and are available domestically in Japan, potentially keeping raw material costs low.

The inorganic nanosheet laminated membrane developed by the group showed proton conductivity across a wide temperature range from -20 to 140℃, exhibiting 0.0087 S/cm of proton conductivity at 140℃ (100% relative humidity). It simultaneously demonstrated hydrogen gas barrier properties more than 100 times superior to those of the polymer proton conductor Nafion and confirmed stability for over two weeks under hydrogen and oxygen exposure.

These results demonstrate that the inorganic nanosheet laminated membrane satisfies the proton conductivity, hydrogen gas barrier properties, and chemical stability required for solid electrolyte membranes. In fact, performance evaluation of a fuel cell using the inorganic nanosheet laminated membrane as the solid electrolyte revealed an output density of 264 mW/cm² at 90℃ (100% relative humidity). This was made possible by the flexibility, high gas barrier properties, and chemical stability of the inorganic nanosheet laminated membrane, allowing incorporation into the fuel cell without electrode short circuits or hydrogen leaks despite being only 2.5 micrometers thick. Furthermore, this fuel cell was confirmed to operate across a wide temperature range from -10 to 140℃.

Fuel cell operation below freezing or in the medium-temperature range above 100℃ depends not only on electrolyte performance but also on electrodes and operating conditions as important control factors, making current output insufficient. The research group is continuing research aimed at improving output through further membrane structure improvements.

Journal Information
Publication: Journal of Materials Chemistry A
Title: Low-temperature fuel cells using proton-conducting silicate solid electrolyte
DOI: 10.1039/D5TA02486B