Recently, the demand for flexible, yet, good performing batteries and super capacitors is increasing together with an increase in foldable and stretchable screens and sensors due to the increase of flexible wearables. The super capacitor can be charged faster than secondary batteries, has great charging and discharging efficiency, and has characteristics that secondary batteries do not have. Hence, it has been receiving attention from scientific and industrial groups that produce high-power electrical devices such as electric cars and pacemakers. 

In following this trend, Professor Ko Min-jae, Department of Chemical Engineering, and Lee Hae-won's, Department of Chemistry, joint research team developed a "flexible super capacitor" using a two-dimensional non-material MXene complex compound.

To develop high-performance super capacitors, the development of electrodes with a large surface area is very important. MXene, which is a combination of titanium, carbon and nitrogen, is considered suitable as an electrode material for the flexible super capacitor. However, MXene is criticised for its two-dimensional planar structure, a non-continuous manner, which results in a large difference in electrical conductivity when placed horizontally and vertically. The electrical resistance level showed a difference of about 100 times between horizontal and vertical. This results in a large difference in the electronic transmission speed inside the electrodes during the discharge process, causing the performance of the super capacitor to be slow. 

To address this problem, the joint research team developed a new method to compose the materials. By growing carbon nanotubes on the inner surface of MXene, which consists of multi-stacked structures, it forms a 3-dimensional porous nano-structure that uniquely interconnects the inner part of a MXene structure.
This composed MXene-Carbon nanotube complex has a large surface area per volume with low internal resistance giving it excellent stability. When this compound was used as an electrode for a flexible super capacitor, the electrical amount charged improved by 62% compared to when MXene was used alone. In addition, it performed 4 times better when carbon nanotubes were used. 

The joint research team puts significance in both improving the energy storage performance of the flexible super capacitor and suggesting a new research direction for the improvement of the high-performing energy material. 

Professor Ko Min-jae said, "This study demonstrates the potential of the MXene-Carbon nanotube to be used not only for flexible electronic devices but also for intracellular implantable devices such as pace makers." He added, "The more important thing is that we have opened up the possibility of wider usage of the MXene complex compound by overcoming the performance difference problem."

This study was conducted by the National Research Foundation on fundamental-source technology development for climate change response projects and mid-level research on science and engineering projects. The research result was selected for the cover page and was published in the 「Advanced Functional Materials」 journal under the title "In Situ Grown MWCNTs/MXenes Nanocomposites on Carbon Cloth for High-Performance Flexible Supercapacitors."