If batteries are the marathon runners of the energy storage world, then supercapacitors are sprinters: great for short bursts of power but not so great long-term. Now engineers from Georgia Tech and Korea University have developed a new supercapacitor that's designed to store more energy for longer – and it's made out of ordinary paper, meaning it's flexible enough to power wearable electronics.

Batteries have high energy density but low power density, meaning they're great at storing energy for long periods of time but they can only release power in a relative trickle. Supercapacitors meanwhile have the opposite problem: they can blast out far more power at once, but their reservoirs can never be quite as full in the first place. For their new design, the researchers wanted to develop a supercapacitor with the right balance between energy density and power density.

To this end, the team developed a relatively easy process to make just such a device. First, a piece of paper is dipped into a solution that contains an amine surfactant material, before it's then dipped into a solution full of gold nanoparticles. The surfactant helps the gold enter the fibers in the paper and stick there.

Next, the scientists use the same method to add layers of metal oxide materials, including manganese oxide. In the end, the gold layers conduct electricity and the metal oxide layers store it, making for a superconductor that not only has high energy and power densities, but can be folded and scrunched without losing those abilities.

"It's basically a very simple process," says Seung Woo Lee, co-author of the study. "The layer-by-layer process, which we did in alternating beakers, provides a good conformal coating on the cellulose fibers. We can fold the resulting metallized paper and otherwise flex it without damage to the conductivity. We have nanoscale control over the coating applied to the paper. If we increase the number of layers, the performance continues to increase. And it's all based on ordinary paper."

The supercapacitor starts with regular paper (left), which is dipped in different solutions to metalize it (right), where gold nanoparticles line the insides of fibers in the paper Ko et al., Nature Communications

The metallic-paper supercapacitor has a power density of 15.1 mW/cm2 and an energy density of 267.3 uW/cm2, which the researchers say makes it the highest-performing textile supercapacitor yet developed. The technique to make it can be scaled up and possibly sprayed onto larger sheets of paper.

"There should be no limitation on the size of the samples that we could produce," says 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 up, the team plans to try using fabric as the base material, and eventually develop batteries using the same process. The gold nanoparticles also have to go, since they're too expensive for wider use, so the researchers plan to experiment with copper or silver nanoparticles to produce the same effect.

"This type of flexible energy storage device could provide unique opportunities for connectivity among wearable and internet of things devices," says Lee. "We could support an evolution of the most advanced portable electronics. 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."

The research was published in the journal Nature Communications.

Source: Georgia Tech