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Mother Nature's nanotechnology: Oak leaf turned into functioning battery

Oak leaf

(NaturalNews) An oak leaf has been turned into a key component of a high-storage, long-lasting rechargeable sodium battery, in a study conducted by researchers from the University of Maryland and the National Center for Nanoscience and Technology in Beijing, and published in the journal ACS Applied Materials & Interfaces.

"We have tried other natural materials, such as wood fiber, to make a battery," researcher Liangbing Hu said. "A leaf is designed by nature to store energy for later use, and using leaves in this way could make large-scale storage environmentally friendly."

The concept of making batteries out of organic materials is not a new one; children have been making batteries out of lemons and potatoes for decades. But unlike those batteries, the new oak-leaf-containing battery is rechargeable.

Batteries of the future?

For all their advantages, the lithium-ion rechargeable batteries that have become industry standard have problems with stability and are prone to catch fire or explode. They are also expensive to produce.

In theory, sodium should be an ideal substitute. It is highly reactive, like lithium, and is much more abundant and therefore far cheaper. It is also less prone to explosions.

"The fascinating thing is that Na [sodium] chemistry is much richer and has more variety than that of Li [lithium]," Kisuk Kang, Professor of Materials Science and Engineering at Seoul National University, said about another study regarding the chemistry of batteries. "This makes us believe that there will be unexplored Na battery electrodes out there that can far excel the current Li batteries."

Sodium batteries have suffered from two key problems, so far: low energy-storage density, and an inability to endure over many charge/discharge cycles. Researchers have hoped that finding the right sodium storage material for an anode could solve these problems. Graphite, which is used for lithium-ion batteries, cannot be used with sodium. Graphene, which is more compatible, is considered too expensive.

That's why researchers have started experimenting with storing sodium in various biomass materials, including peat moss, melon skin and banana peels. But all of these have required extensive processing — until now.

Scientists just grabbed a leaf from the ground

The scientists started with just an ordinary oak leaf.

"Leaves are so abundant. All we had to do was pick one up off the ground here on campus," researcher Hongbian Li said.

They then baked the leaf at 1,000C (1,832F) for an hour in an oxygen-deprived environment. This burned away all organic components of the leaf except for the underlying carbon structure. The leaf was then immersed in hydrogen chloride for six hours, to remove all remaining inorganic impurities. At the end they had a carbonized leaf, with a smooth top made of nanostructured carbon and still studded with pores underneath. These pores were then filled with sodium electrolyte, with the dense, flat top of the leaf being perfect for collecting electrical currents.

"The natural shape of a leaf already matches a battery's needs: a low surface area, which decreases defects; a lot of small structures packed closely together, which maximizes space; and internal structures of the right size and shape to be used with sodium electrolyte," said visiting student Fei Shen, who was involved with the project.

The process is much faster and cheaper than other biomass sodium anodes attempted. Best of all, it delivers great results. When used as an anode in a sodium battery (using coin batteries with sodium plates as the counter/reference electrodes), the leaf anode was able to store an impressive 360 mAh per gram of mass. After 200 cycles, it was still holding 90 percent of its initial charge, a far superior durability to any prior sodium battery.

Surprisingly, even when the battery was down to 75 percent of its initial charge, its charge efficiency remained high. Researchers believe that this is because of the leaf's low surface area.

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