(a) Schematic diagram of the cell for fabrication of carbon nanotubes (b) Scanning electron microscope image showing that the graphite electrodes are almost entirely converted to carbon nanotubes in the new process
A team at Cambridge University is investigating whether nanotubes made with the new method could be used to improve the energy density of lithium-ion batteries for electric vehicles.
Currently carbon nanotubes cost about $1,000(£610)/kg.
Usually existing materials generally have the edge in terms of cost and function. Total worldwide production of carbon nanotubes is only around 1,300 tonnes a year.
Carbon nanotubes are produced using a hot, carbon-rich vapor in the presence of expensive catalysts. It has poor throughput, yield and selectivity.
A new electrochemical method could readily be scaled up (by using a multi-electrode cell with planar graphite electrodes) to produce more than 600kg of carbon nanotubes per day at a projected cost of around $10(£6.10)/kg.
Electrolytic Production of Carbon Nanotubes (2 pages)
A new method for low-cost, high-yield production of carbon nanotubes has been developed. The technique can also produce carbon nanotubes filled with metals such as tin. It is envisaged that the electrochemical method could readily be scaled up (by using a multi-electrode cell with planar graphite electrodes) to produce more than 600 kg/day of carbon nanotubes, far more than is possible with current commercial methods.
It has also been shown that doping the lithium chloride electrolyte with tin chloride can produce carbon nanotubes containing relatively large quantities of tin. When these tin-filled carbon nanotubes are used to make the anode in a lithium ion battery, experiments have shown that the battery capacity after many cycles of charge and recharge, is significantly greater than that of a standard Lithium-ion battery with a standard carbon nanotube anode.
A team headed by Prof Derek Fray, director of the Department of Materials Science and Metallurgy at Cambridge, has been experimenting for some years with a new technique to produce nanotubes directly from readily available graphite using an electrolytic process.
A power supply is connected to the electrodes so that one of the electrodes is at a cathodic potential relative to the other electrode. The polarity of the voltages on the electrodes is periodically reversed ensuring that each electrode is almost entirely consumed to produce carbon nanotubes with around 80 per cent efficiency.
The process appears to be a couple of thousand times faster than the chemical vapour deposition methods, and because we’re working with condensed phases, the volumes are so much less
Another advantage of the new method is that it is relatively easily to dope the nanotubes with elements such as tin and silicon.
A pilot plant for producing carbon nanotubes with the electrolytic method is currently planned for a location in south Wales with the help of industrial partner Morgan Advanced Materials and Technology.
They can increase the capacity of the anode in lithium ion batteries by incorporating tin and silicon into carbon nanotubes.
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