Professor Jinhan Cho’s research team develops a supercapacitor device using traditional Hanji.

High expectations of providing a high power, large capacity flexible/wearable device platform

Professor Jinhan Cho’s research team of the Department of Chemical and Biological Engineering at KU College of Engineering and Professor Seung Woo Lee’s research team of Georgia Tech used readily available Hanji (Korean traditional paper) to develop a high performance supercapacitor device that can instantaneously produce a high output.

* Supercapacitor: An energy storage device with an increased capacitance compared to a conventional capacitor (electrical condenser). Although its energy density (charge) is less than that of a secondary battery, it can instantaneously produce high power (5 times that of a lithium battery).

The surface area of textile materials such as paper and cotton is very large, and because it is light and flexible, this makes it easier to process. When this is applied to electrical or electronic devices, flexible or wearable devices that require wide electrode surface areas can be manufactured. Because textile materials have a high level of insulation, they have been used in attempts to introduce carbon-based electroconductive materials such as carbon nanotubes. However, their relatively lower conductance compared to metals and their low energy density have continued to be the most significant limitations in increasing the power and energy density of energy storage devices.

The joint research team developed a new molecule ligand layer-by-layer assembly method, and by uniformly and densely coating the surface of a textile material with nano-sized metal and metallic oxide material particles, they were able to succeed in manufacturing a flexible paper supercapacitor device that uses metallic paper electrodes as current collectors.

* Layer-by-layer assembly (LbL): Based on the strong affinity (electrostatic interaction, hydrogen bonding, covalent bonding, etc.) between materials, this refers to a bottom-up nanofabrication technique where each material layer is layered in a crisscross manner to fabricate a thin-film functional composite.

The fabricated paper electrode had no changes in its mechanical and structural characteristics, which are inherent to textile materials, and showed the same level of conductivity as metal. The paper supercapacitor device fabricated based on this paper electrode showed a high capacitance value and output value from the wide surface area of its porous structure. The molecule ligand-mediated layer-by-layer assembly method developed in this research was used, and allowed the research team to become the first to implement an energy device on a textile material. The distance between the particles was minimized to greatly reduce the electrode’s internal resistance, and in turn, a high output and capacitance value was achieved.

* Ligand: This refers to a molecule or ion that surrounds the central atom of a complex compound by coordinate bonding.

* Molecule Ligand-mediated LbL: An assembly method that utilizes the affinity (binding power) difference between materials. If a thin film is fabricated by means of a molecule ligand with a higher affinity than the ligand attached to the surface of a particle, the ligand attached to the particle surface falls off and a new molecule ligand is replaced in that space.

Korea University Professor Jinhan Cho commented on the significance of this research: “This research is the first known case of directly coating a textile material with inorganic particles to fabricate a compatible electrode for electric and electronic devices. This could be applied to many forms of devices, and it was easy to increase the surface area because the entire process was conducted in a solution. Going forward, I am hopeful that it will provide a new platform for the flexible/wearable electric and electronic device markets.”

This research was conducted with the support of the Ministry of Science & ICT and the National Research Foundation of Korea Fundamental Research Project (Individual Research), and was published in the September 14 edition of the international scientific journal, Nature Communications.

- Title of Article: Flexible Supercapacitor Electrodes Based on Real Metal-like Cellulose Papers

- Author Information: Professor Jinhan Cho (corresponding author, Korea University), Professor Seung Woo Lee (corresponding author, Georgia Tech. USA), Yongmin Ko, Ph.D. (primary author, Korea University)

[ Terminology ]

1. Nature Communications

ㅇ World renowned scientific journal in the field of natural science with a citation index of 12.124.

2. Supercapacitor

ㅇ An energy storage device with an improved capacitance value compared to a conventional capacitor. It can compose an electrode either by increasing the energy accumulating surface area of the electrode or by inserting an oxidation-reduction reaction inducing material.

3. Layer-by-layer assembly, LbL

ㅇ Based on the strong affinity (electrostatic interaction, hydrogen bonding, covalent bonding, etc.) between materials, this refers to a bottom-up nanofabrication technique where each material layer is layered in a crisscross manner to fabricate a thin-film functional composite.

4. Ligand

ㅇ A common name for a molecule or ion that surrounds the central atom of a complex compound by coordinate bonding.

5. Molecule Ligand-mediated LbL

ㅇ A layer-by-layer assembly method that utilizes the affinity (binding power) difference between materials. In other words, by fabricating a thin film using a molecule ligand with a higher affinity compared to the ligand attached to the surface of a particle, the ligands attached to the particle surface fall off and are replaced with new molecule ligands.

[ Figure Description ]

Figure 1. The distance between particles according to the electrode fabrication methods

When fabricating an electrode using metallic nanoparticles, the distance between the particles greatly influences the electric/electrochemical characteristics of the final particle. For electrode fabrication methods that use simple assembly methods such as blending (left) or the electrostatic interaction-based layer-by-layer assembly method in water (center), the distance between the particles is long because of the insulating organic ligand and polymer linker of the particle surface. Consequently, movement of the electron becomes limited. However, in the ligand-mediated LbL method (right), the insulating polymer ligand is removed from its place and is replaced by a molecule ligand, and because a composite that minimizes the distance between the particles is maintained, high conductance can be achieved.

Figure 2. Textile substrate-based electrode manufacturing

Through this electrode fabrication method, metal conductivity can be achieved on textile materials such as large areal paper (Korean traditional paper Hanji) (left) or cotton (right). Notably, because electrodes fabricated with this method can manifest the inherent mechanical characteristics of textiles, these electrical characteristics can be maintained even under various stressful conditions.