Researchers from the Northeastern University in Boston have now discovered that a possible source for cheap, sustainable batteries can be obtained from none other than tannin, which is known to be naturally abundant and comes from tree bark. While tannin is commonly found in tea and wine, it has shown great promise for use in a low-cost yet high-performance battery that could find many applications in the future.
Its abundance is the primary reason why researchers are now considering the use of tannin for a new type of battery. For reference, a tree bark is around 15 percent tannin based on weight, which is as abundant as they are widespread. Tannin has also been found to have high levels of phenol -- in fact, they have the highest level of phenol out of any polymer found in living organisms -- which is crucial as it can serve as the primary charge storage mechanism of a tannin-based battery. All of this makes tannin an excellent choice for a cheap and metal-free battery.
The research on the use of tannin was led by Hongli Zhu from the Northeastern University in Boston, along with Alolika Mukhopadhyay, a Ph.D. student who served as the lead author, and co-authors from the National Renewable Energy Laboratory in Golden, Colorado. Their findings have been published in a paper titled, "Heavy Metal-Free Tannin from Bark for Sustainable Energy Storage," which appeared in a recent issue of Nano Letters.
According to Zhu, tannin is a perfect candidate for a new type of battery cathode just because of how much of it is available in nature. "The greatest benefit of using a renewable polymer tannin is that Nature produces a huge amount of [it], which can be extracted from underused bark with minimal cost and efforts," Zhu explained. "The redox-active phenolic hydroxyl groups of tannins are more than 5000 times higher than lignin, which was previously considered to be the most promising biopolymer for electrochemical energy storage. Due to tannin's significant low molecular weight and extremely high phenolic hydroxyl content, the interpenetrating network of tannins and polypyrrole shows an outstanding electrochemical performance. We think tannin is the new champion of naturally occurring redox-active biopolymers."
Through the course of their study, the researchers were able to test various types of tannin, such as the one called "ellagitannin" that comes from the bark of a chestnut tree. They say that it can be used to create cathodes for a battery that show a high level of performance, which includes a large capacitance and high energy density. This could represent a big leap from current standards that use lithium metal as the primary material.
While current standards for making batteries do exist, and experts have achieved varying degrees of success with them, there is a huge potential upside for the use of a renewable source like tannin. And it's not the only plant-based material that shows great promise as an alternative battery material.
One such material is called lignin, a biopolymer that has also been extensively studied by researchers and can be found inside the cell walls of plants. If scientists could figure out a way to change them in a way that would allow for improved stability and safer performance, perhaps the world would be that much closer to a new low-cost, natural energy resource.
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