The rice husk is the outer covering of a rice kernel that protects it from attack by insects and bacteria. Because it is removed during the harvesting of rice, over 100 million tons of rice husks are produced annually worldwide. Although efforts have been made to recycle rice husks for other uses, these have been limited to low-value agricultural applications. Farmers grow 422 million metric tons of rice per year in roughly 75 countries around the world.
The silica-rich husks left over when rice is harvested are normally turned into fertiliser additives, but there is a better more environmental solution. The abundant waste product could be converted into electrodes with the power to extend the lifetime of the next generation of batteries.
The lithium ion batteries in today’s electronics usually contain electrodes made of graphite, but efforts are ongoing to switch them for silicon electrodes, which can hold 10 times more charge. This is expected to speed the development of more sophisticated portable electronics and better electric cars.
The downside of silicon electrodes is that they degrade even faster than those made of graphite each time the battery is charged and drained, shortening its lifetime. This “capacity fade” is caused by the electrodes drastically swelling and shrinking as the lithium ions circulate between the electrodes, causing them to fracture.
Silicon converted from the silica in rice husks may be able to resist this volume change, says Jang Wook Choi at the Korea Advanced Institute of Science and Technology in Daejeon, South Korea. The tiny holes in the husk that allow the rice kernel to breathe should also mean that any derived silicon would also be porous. Its holes could provide places for the ions to reside on the electrodes during charging and discharging, preventing the volume from significantly changing.
To find out, Choi’s team chemically converted the rice husk silica – or silicon dioxide – into pure silicon and then fashioned battery electrodes out of the material. By treating the husks with acid and heating them, the researchers extracted 99.92 percent pure silica. It showed no capacity fade even after 200 charge-drain cycles. A synthetic silicon electrode used for comparison had a higher initial charge capacity but faded badly: it began performing worse than the rice-husk electrode after 10 to 15 cycles.
It is still too early to say whether the rice husk electrodes can be made commercially competitive – particularly since turning silica into silicon is a high-temperature, costly business. “However, most silicon is produced from similar thermal treatment processes so we should be able to find reasonable competitiveness,” says Choi.
What’s more, artificially nanostructured silicon electrodes have been demonstrated in the lab. Choi’s team is confident that its natural alternative is better as it was developed “via years of natural evolution”, they write.
Source: New Scientist, Asian Scientist,
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