Physics

More Power for Tomorrow's Supercapacitors

As supercapacitors emerge to become an important feature in the world of battery devices, researchers at CIRIMAT¹ in Toulouse and at Drexel University in Philadelphia have made a breakthrough discovery that invalidates years of preconceptions.

Supercapacitors, also known as ultracapacitors, are rechargeable high-power energy storage devices.  They are currently used in personal electronics from mobile phones to cameras, as well as in hybrid electric and fuel-cell vehicles. They have the ability to store at least one million times more energy per unit mass than standard capacitors. They also have an indefinite lifespan as they can be charged and discharged an unlimited number of times.  Until now, their usefulness was limited by their maximum capacitance–the measure of the amount of electric charge that can be stored.

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© P. Dumas

Preparation of supercapacitor electrode active material: TiC-CDC powder after synthesis (far left) and after film processing (left). Such electrodes are used to assemble supercapacitor prototype cells at the CIRIMAT lab (below).

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Unlike traditional batteries, which store electricity chemically, supercapacitors store electricity electrostatically by physically separating positive and negative charges along two electrical conductors: electrolyte ions and high surface area electrodes (highly porous materials such as carbon). This results in an enhancement of the material's ability to hold charges. However, it was previously thought that reducing pore size below one nanometer would halt a material's storage capacity because ions could not penetrate.

It is this very notion that the French-American team has turned on its head by proving that smaller, lighter, and more powerful supercapacitors² could be developed by using  pores smaller than one nanometer. For the first time, they were able to tailor pore sizes in a range of 0.6 to 2.25 nanometers, by using carbide-derived carbon (CDC) as model compounds. Surprisingly, they found that carbon with 0.6-nanometer pores increased the amount of electrical charge by about 50%, an effect  probably due to the deformation of the solvation shell of water molecules surrounding the ion. Potential applications of this new discovery are endless, and a whole new generation of supercapacitors could revolutionize electronic devices.

Marion Girault-Rime