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In these ways, wood power has helped us stay warm, cook food, and extend our day lengths for a long time. Even with the extraction of depletable resources like coal and petroleum and the development of power-generating technologies, hundreds of millions of homes worldwide still rely on burning wood as a central power source.2
Branching out from this conventional combustion, a team of researchers are now working on a novel way to incorporate wood into the power grid. Instead of igniting it, these nanoscientists are engineering it into a tin-anode sodium-ion battery, both improving upon current designs and expanding timber's provision of power.3
The innovation of their model relates to the economics and mechanics of energy storage itself. For the battery's charge carriers, the researchers decided to use sodium ions because they are relatively affordable and abundant. The team then wanted to use tin as the battery's anode—a part that conducts electrical currents—because it has a high capacity for storing energy over time with sodium.
With these commitments, however, the researchers had to address particular drawbacks of the materials. Specifically, with each cycle of being charged and discharged, tin tends to expand and contract greatly in volume. This dynamism leaves tin batteries with rigid structures increasingly brittle with several cycles.
To resolve the issue, the team introduced natural wood fibers into the battery's assembly. Wood from yellow pine trees was shaved into nanowidth slivers and coated with a thin film of tin. Because of the wood fibers' strength and flexibility, the shock of the tin's expanding and contracting was absorbed, and the battery had a much longer life. Indeed, the researchers achieved an initial capacity of 339 mAh/g for 400 cycles without the prototype cracking or falling apart, a record performance that represents "a significant improvement over other reported tin nanostructures."4
The mesoporous wood also acts as an optimal electrolyte reservoir that allows ions to flow from the fibers' outer surfaces to inner ones. According to Liangbing Hu, one of the investigators, "Wood fibers that make up a tree once held mineral-rich water, and so are ideal for storing liquid electrolytes, making them not only the base but an active part of the battery."5 The wood fibers thus improve the kinetics of ion transfer in addition to handling the mechanical stress of it.
Moreover, the low cost of the materials makes the design a good candidate for wind and solar power grids. As these renewable methods generate power intermittently—when the breezes are blowing or the sun is shining—their power grids need the capacity to store enough charge to compensate for when night falls and the winds still.
For these reasons, wood and tin are compelling construction materials for the large, inexpensive grids these alternative powers require.6 To offset potential deforestation, designers of another experimental battery even used lignin, an arboreal compound and waste product of paper processing, as a cathode, thereby repurposing the effluent for a useful device.7
This creativity in energy research is encouraging, and I think that the rustic turn in materials science presents us with a thought-provoking juxtaposition. Wood fibers that evolved over millennia for the benefit of trees are now being inserted into cutting-edge nanotechnologies.
Rather than being instances of biomimicry, to me, these inventions represent a grafting of one of the world's oldest systems to a few of the most new. As long as the raw timber and metals for these devices are sourced sustainably, this idea is one I'd like to see take root.
Image credit: Photo of wood fiber a broken tree branch from Flickr's Rich Anderson.
1. Roebroeks, W. & Villa, P. (2011). On the earliest evidence for habitual use of fire in Europe. PNAS, 108, 5209-5214 PMID: 21402905
2. Rehfuess, E. "Fuel for life: household energy and health." World Health Organization. (2006).
3. Zhu, H., Jia, Z., Chen, Y., Weadock, N., Wan, J., Vaaland, O., Han, X., Li, T. & Hu, L. (2013). Tin anode for sodium-ion batteries using natural wood fiber as a mechanical buffer and electrolyte reservoir. Nano Letters PMID: 23718129
4. Ibid.
5. Heil, M. "A Battery Made of Wood?" University of Maryland Press Release. June 19, 2013
6. The Economist Online. "Wooden Batteries." The Economist. March 22, 2012.
7. Milczarek, G. & Inganäs, O. (2012). Renewable cathode materials from biopolymer/conjugated polymer interpenetrating networks. Science , 335, 1468-1471 PMID: 22442478