Electrolytic method could fulfil carbon nanotube potential

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.

The unique electrical and mechanical properties of carbon nanotubes mean they could have wide-reaching applications for a number of industries. However, costs are still prohibitive — currently in the region of $1,000(£610)/kg — meaning that, on balance, existing materials generally have the edge in terms of cost and function. Indeed, total worldwide production of carbon nanotubes is only around 1,300 tonnes a year.

One of the reasons behind this is the method used to produce the material, which essentially involves growing nanotubes from a hot, carbon-rich vapour in the presence of expensive catalysts. It has poor throughput, yield and selectivity.

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,’ Fray told The Engineer.

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 600kg of carbon nanotubes per day at a projected cost of around $10(£6.10)/kg.

Another advantage of the new method is that it is relatively easily to dope the nanotubes with elements such as tin and silicon.

It is known that incorporating tin or silicon in the anode of lithium-ion batteries — widely used in current electric vehicles — increases the amount of lithium, and thus charge, the battery can store.

However, the challenge has been to incorporate this without increasing the volume and weight of the battery — and tin-filled nanotubes may be the answer.

‘We’ve done some trials with the National Research Council in Canada, using 50 per cent graphite and 50 per cent our material for the anode and we increased the [charge] capacity by around 50 per cent and there was no fade after the first 100 [discharge-charge] cycles, it was just a constant line, which is a great improvement,’ Fray said.

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.