When we think of batteries, we typically think of materials like lithium or nickel. But what if we told you that a battery made from trees is on its way to the market?
At first glance, the idea may seem counterintuitive. After all, wood is an insulator, not a conductor, right? But as with many things in science, there's more to this than meets the eye.
The battery in question is the result of years of research and development by a team of scientists at Linköping University in Sweden, led by Professors Magnus Berggren and Xavier Crispin. They are using lignin, a gluey substance that makes up about 25% of a tree's structure, as a raw material. The rest of the tree is made up of cellulose and hemicellulose. But before you imagine lumberjacks chopping down entire forests, know that the scientists are actually recovering the lignin from the paper milling industry. Paper mills use only the cellulose part of the tree and typically burn lignin as part of a waste slurry known as black liquor.
So, how does lignin become a battery material? Lignin is an aromatic heteropolymer, which means its structure contains small rings of benzene. These benzene rings contain specific types of electrons, called Pi electrons, which, under the right electrochemical conditions, can be liberated without destroying the benzene ring itself. Moving electrons is basically what a battery does, so it looks like we've got something potentially interesting going on here.
But lignin is not an electrical conductor, it's an electrical insulator, so those Pi electrons don't naturally want to go anywhere at all. The solution was discovered in 2012 by another Linköping professor called Olle Inganäs, who found a way of bringing electrons to and from the benzene ring by creating a nanocomposite material of conducting polymers. Down at that mind-blowingly minuscule nanoscale, the polymer chains act like molecular wires which can then be embedded into the lignin to facilitate the movement of electrons into the benzene rings to store a charge.
To make this happen, the researchers utilized dry ball milling. This step crushes carbon together with the lignin, which causes the carbon to mix with the lignin to become a nanocomposite material enabling the electrons to travel along the carbon molecular pathways to reach the aromatic electroactive component of the lignin. On the other side of the cell is a zinc electrode, and the whole thing is contained in an electrolyte made of a super-concentrated solution of potassium polyacrylate.
The resulting battery has several advantages. For one, it avoids problematic components like lithium, cobalt, or nickel in the battery chemistry. Additionally, the volume of paper being produced today is so vast that, according to Professor Crispin's research team, even if all the batteries in the world were made with waste lignin from paper mills, it would only use a fraction of the available lignin.
While the wood-based battery is not yet available for purchase, the potential for sustainable and cost-effective energy storage is exciting. And it goes to show that sometimes the most unexpected materials can hold the key to innovation.
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