No-one wants to be near a bin full of crab shells on a hot day.
But if you ratchet the heat up a bit more – like, a few hundred degrees more – the shells might become valuable battery material.
A study published in ACS Omega has shown that the material in crab shells could be used in sodium-ion batteries: cheaper, but so far less energy dense, than lithium-ion.
If scaled up, crab shells could be added to the plethora of sustainable materials that could be used in biodegradable batteries.
Read more: How do batteries work?
The crab shell chitin has two very useful properties: it’s mostly made of carbon, and it’s hard.
The team of Chinese and Japanese researchers heated waste crab shells to 700°C for two hours in an oxygen-free environment, and then purified the shells with hydrochloric acid, washed and dried the substance overnight at 120 °C.
This yielded “crab carbon’: a source of pure, hard carbon. They mixed it with either an iron sulphide or tin sulphide, turning it into an anode for a sodium-ion battery.
Because the crab carbon is so porous, it has a very large surface area. This makes it good at doing a battery anode’s job: transporting electrons and ions.
The researchers tested both anodes in a battery, and found they lasted for at least 200 charge and discharge cycles.
Both batteries also had capacities on par with other experimental sodium-ion batteries.
“Biomass carbon source has the advantages of low cost, porous fibres, and a large specific surface area, which can greatly improve the conductivity of the composite,” write the researchers in their paper.
“This research provides an efficient route to utilise low-cost waste raw materials to construct high-specific energy sodium-ion batteries.”
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