The device uses metallic nanoparticles to coat the cellulose fibres in the paper, creating supercapacitor electrodes with high energy and power densities through a simple layer-by-layer coating technique,
By implanting conductive and charge storage materials in the paper, the technique creates large surface areas that function as current collectors and nanoparticle reservoirs for the electrodes. Testing shows that devices fabricated with the technique can be folded thousands of times without reducing the conductivity.
“This type of flexible energy storage device could provide unique opportunities for connectivity among wearable and internet of things devices,” said Seung Woo Lee, an assistant professor in the Woodruff School of Mechanical Engineering at the Georgia Institute of Technology. “We also have an opportunity to combine this supercapacitor with energy-harvesting devices that could power biomedical sensors, consumer and military electronics, and similar applications.”
Lee and collaborator Jinhan Cho from the Department of Chemical and Biological Engineering at Korea University began by dipping paper samples into a beaker of solution containing an amine surfactant material designed to bind the gold nanoparticles to the paper. Next they dipped the paper into a solution containing gold nanoparticles. Because the fibrs are porous, the surfactants and nanoparticles enter the fibres and become strongly attached, creating the coating.
By repeating the dipping steps, the researchers created a conductive paper on which they added alternating layers of metal oxide energy storage materials such as manganese oxide. The layer-by-layer approach helped minimize the contact resistance between neighboring metal and/or metal oxide nanonparticles. Using the simple process done at room temperatures, the layers can be built up to provide the desired electrical properties.
“It’s basically a very simple process,” said Lee. “The layer-by-layer process, which we did in alternating beakers, provides a good conformal coating on the cellulose fibres. We can fold the resulting metallized paper and otherwise flex it without damage to the conductivity.”
Though the research involved small samples of paper, the solution-based technique can be scaled up using larger tanks or even a spray-on technique. “There should be no limitation on the size of the samples that we could produce,” said Lee. “We just need to establish the optimal layer thickness that provides good conductivity while minimizing the use of the nanoparticles to optimize the tradeoff between cost and performance.”
Next: Power figures and next steps
The researchers demonstrated that their self-assembly technique improves several aspects of the paper supercapacitor, including its areal performance, an important factor for measuring flexible energy-storage electrodes. The maximum power and energy density of the metallic paper-based supercapacitors are estimated to be 15.1mW/cm2 and 267.3 μWh/cm2, respectively, substantially outperforming conventional paper or textile supercapacitors.
The next steps will include testing the technique on flexible fabrics, and developing flexible batteries that could work with the supercapacitors. The researchers used gold nanoparticles because they are easy to work with, but plan to test less expensive metals such as silver and copper to reduce the cost. “We have nanoscale control over the coating applied to the paper,” said Lee. “If we increase the number of layers, the performance continues to increase, and it’s all based on ordinary paper.”
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