Energy storage nanomaterial revealed

Researchers at Drexel University in the US and Dalian University of Technology in China have chemically engineered a new, electrically conductive nanomaterial that is flexible enough to fold, but strong enough to support many times its own weight.

This flexible new material, which researchers have identified as a conductive polymer nanocomposite, is the latest discovery to be made from examining the family of layered carbide materials just a few atoms thick called MXenes. This group of two-dimensional materials was first discovered by Drexel’s Department of Materials Science and Engineering in 2011.

“The uniqueness of MXenes comes from the fact that their surface is full of functional groups, such as hydroxyl, leading to a tight bonding between the MXene flakes and polymer molecules, while preserving the metallic conductivity of nanometer-thin carbide layers,” said Drexel College of Engineering professor Yury Gogotsi, who has led the research along with Dalian School of Chemical Engineering vice dean Jieshan Qiu.

“This leads to a nanocomposite with a unique combination of properties.”

We can eventually eliminate metal current collectors and make lighter and thinner supercapacitors

Drexel doctoral student Chang Ren

One of the most successful ways developed to help MXenes express their array of abilities is through a process called intercalation, which involves adding various chemical compounds in a liquid form. This allows the molecules to settle between the layers of the MXene and, in doing so, alter its physical and chemical properties.

To produce the new flexible conductive polymer nanocomposite, the researchers intercalated the titanium carbide MXene with polyvinyl alcohol (PVA), a polymer widely used as the paper adhesive known as school glue. They also intercalated with a polymer called PDDA (polydiallyldimethylammonium chloride), which is commonly used as a coagulant in water purification systems.

“We have shown that the volumetric capacitance of an MXene-polymer nanocomposite can be much higher compared to conventional carbon-based electrodes or even graphene,” said Chang Ren, Gogotsi’s doctoral student at Drexel.

“When mixing MXene with PVA containing some electrolyte salt, the polymer plays the role of electrolyte, but it also improves the capacitance because it slightly enlarges the interlayer space between MXene flakes, allowing ions to penetrate deep into the electrode; ions also stay trapped near the MXene flakes by the polymer. With these conductive electrodes and no liquid electrolyte, we can eventually eliminate metal current collectors and make lighter and thinner supercapacitors.”

The testing also revealed hydrophilic properties of the nanocomposite, which means that it could have uses in water treatment systems, such as membrane for water purification or desalinisation, because it remains stable in water without breaking up or dissolving.

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