Silicon chip boosts battery power

The EU-funded IONACES project studied how supercapacitors — Electrochemical double layer capacitors or EDLCs — work and devised a process to integrate them onto silicon chips as microsupercapacitors for use in multiple applications.

‘The battery of an electric car is autonomous according to how long it can drive. But if, for instance, if it needs to accelerate, it needs a power boost for a few seconds and this is what a supercapacitor can do,’ explains IONACES project coordinator Patrice Simon, professor of materials science at the Paul Sabatier University in Toulouse, France.

While a standard lithium battery takes 2–3 hours to recharge, a supercap — as supercapacitors are also known — can deliver all its energy within 10 seconds and can fully recharge within a few seconds or up to one minute. ‘This is a significant energy boost,’ says Professor Simon.

The team found that the excellent performance, not properly understood until now, is due to reversible ion absorption in tiny pores less than one nanometre in size in the porous carbon electrodes.

Supercapacitors store the charge by accumulating positive and negative ions in the positive and negative porous carbon powders acting as electrodes. ‘You stick ions onto the carbon surface and you remove ions from the carbon surface to charge and discharge,’ Professor Simon explains.

These holes extend the surface area to include the pores’ wall surface — they can be greater by a factor of 1000 compared to a smooth carbon surface, dramatically increasing the amount of charge stored.

Pore size also matters. ‘We could reduce the discharge and charge time of the supercaps just by playing with the size of the pores,’ Professor Simon says. ‘Decreasing the pore size below one nanometre dramatically increases the number of ions that can be absorbed.’

In the past, researchers were able to reduce pore size to under two nanometres but could not design carbon materials with specific sizes of pores. The IONACES team achieved this goal by using a different method of preparing carbons — using titanium carbide grains or micropowders 10 microns in diameter in a chlorine atmosphere. They removed the titanium which left the porous carbon.

With this process, ‘if you control the chlorination temperature, then you can control the pore size of the carbon very precisely,’ notes Professor Simon.

Designing the microsupercapacitors to fit on a silicon chip took four years of team effort. ‘We deposited a titanium carbide layer onto the chip using spattering techniques and then did the cholorination,’ Professor Simon explains.

‘The trick was to keep mechanical integrity of the carbon film on the chip after chlorination. This was finally achieved using partial chlorination which does not increase the thickness of the layer on the chip.’

A spin-off of this research is that the process can be adapted to remove sodium and chlorine from sea water to make it drinkable. This has been patented by the project team for use in desalination plants.