Researchers fabricate flexible supercapacitors on paper

Wearable devices are powered by flexible electronics that use plastic or metal foil electrodes. However, plastics have poor adhesion and metal foils make the devices bulky. Paper is a promising alternative, as it is flexible, porous, light and thin. Paper also has randomly distributed fibres that provide a large surface area for depositing active electrode material, making it suitable for energy storage. However, paper-based supercapacitors use multiple stacked sheets for storing energy, which can cause mechanical issues. Now, researchers have fabricated multi-layer electrodes vertically integrated within a single sheet of paper — the resulting supercapacitor enhances energy storage.

The paper-based supercapacitors previously developed by researchers store electric charge and energy by stacking multiple sheets, acting as positive and negative electrodes and separators. However, such an arrangement increases device size and resistance. It also tends to form creases, peel off, and slip over, which could further deteriorate device performance.

To address these issues, researchers from Chung-Ang University, led by Professor Suk Tai Chang and Associate Professor Inho Nam, fabricated a structure comprising multi-layer electrodes vertically integrated within a single sheet of paper. The novel design overcomes the problems associated with stacked sheets while retaining the inherent advantages of a paper-based substrate. The research findings were published in the Chemical Engineering Journal.

Of the fabrication process, Nam said a water-repellent paraffin wax layer was first printed and heated on both sides of a filter paper. This formed a water-friendly channel surrounded by a wax barrier within the paper. Then the paper was successively dipped in gold nanoparticle and gold enhancement solutions, which penetrated the channel via capillary action, resulting in a gold electrode in the middle of the paper. Similar electrodes were then fabricated on top and bottom surfaces on the paper to obtain a multi-layer electrode platform.

The researchers completed the supercapacitor design by depositing manganese oxide — an active electrode material — on the gold-paper electrode, which was then immersed in a polyvinyl alcohol-sodium sulfate gel electrolyte solution. After the gel had solidified, they characterised the manganese dioxide-gold-paper electrodes using various electrochemical measurement techniques, such as cyclic voltammetry, galvanostatic charge and discharge, and electrochemical impedance spectroscopy.

The supercapacitor design showed a low chemical resistance, high foldability and good mechanical strength. Manganese dioxide enhanced its active surface area, which further boosted the electrochemical performance.

The supercapacitor also demonstrated high energy storage with maximum areal energy and power densities of 13.73 μW-hr-cm-2 and 1.6 mW-cm-2, respectively. It retained its storage capacity even after undergoing 6000 charge–discharge cycles. The multi-layer electrode supercapacitor platform is a super-dense energy storage device that uses the two-dimensional paper surface as a three-dimensional scaffold. It can also be used in parallel as well as serial integrated circuit configurations without modifying external wires and circuits.

“Our proposed platform circumvents most fabrication challenges related to two-dimensional energy storage sheets. We believe that the findings of our study will guide the future fabrication of paper-based electronics with more multi-layered electrodes,” Chang said.

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