Carbonizing a MOF with added salts transforms it into a nano-diatom, much like a dragon egg turns into a fire-born dragon after fire treatment in Game of Thrones. Image: Jingwei Hou.
Carbonizing a MOF with added salts transforms it into a nano-diatom, much like a dragon egg turns into a fire-born dragon after fire treatment in Game of Thrones. Image: Jingwei Hou.

Researchers at Queen Mary University of London and the University of Cambridge in the UK and the Max Planck Institute for Solid State Research in Germany have discovered how a pinch of salt can be used to drastically improve the performance of batteries. They found that adding salt to the inside of a supermolecular sponge and then baking it at a high temperature transformed the sponge into an intricate carbon-based structure.

Surprisingly, the salt reacted with the sponge in special ways, turning it from a homogeneous mass into an intricate structure with fibers, struts, pillars and webs. This kind of three-dimensional hierarchically organised carbon structure has proven very difficult to grow in a laboratory, but could prove crucial for providing unimpeded ion transport to active sites in a battery.

In the study, the researchers found that the use of these carbon-based materials in lithium-ion batteries not only allows the batteries to be charged-up rapidly, but also increases their capacity. The researchers report their findings in a paper in the Journal of the American Chemical Society.

Due to the intricate architecture of the structures, the researchers termed them 'nano-diatoms', and believe they could be used for energy storage and conversion applications, such as electrocatalysts for hydrogen production.

"This metamorphosis only happens when we heat the compounds to 800°C and was as unexpected as hatching fire-born dragons instead of getting baked eggs in the Game of Thrones," said lead author and project leader Stoyan Smoukov from Queen Mary's School of Engineering and Materials Science. "It is very satisfying that after the initial surprise, we have also discovered how to control the transformations with chemical composition."

Carbon-based materials such as graphene and carbon nanotubes are highly versatile, used in catalysis and electronics because of their conductivity and chemical and thermal stability. Three-dimensional carbon-based nanostructures with multiple levels of hierarchy not only retain useful physical properties like good electronic conductivity, but also can have other unique properties. These include improved wettability (to facilitate ion infiltration), high strength per unit weight and directional pathways for fluid transport.

It has, however, proved very challenging to make carbon-based multilevel hierarchical structures, particularly via simple chemical routes. Yet such routes would be useful if these structures are to be made in large quantities for industry.

The supermolecular sponge used in the study is also known as a metal organic framework (MOF). MOFs are attractive, molecularly designed porous materials with many promising applications such as gas storage and separation. Their retention of a high surface area after carbonization – or baking at a high temperature – makes them interesting as electrode materials for batteries. So far, however, carbonizing MOFs has resulted in the production of a dense carbon foam. By adding salts to these MOF sponges and carbonizing them, the researchers produced a series of carbon-based materials with multiple levels of hierarchy.

"This work pushes the use of the MOFs to a new level," said co-author Vasant Kumar from the University of Cambridge. "The strategy for structuring carbon materials could be important not only in energy storage but also in energy conversion, and sensing."

"Potentially, we could design nano-diatoms with desired structures and active sites incorporated in the carbon as there are thousands of MOFs and salts for us to select," said lead author Tiesheng Wang from the University of Cambridge.

This story is adapted from material from Queen Mary University of London, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.