NMR boost: Supercapacitors

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  • Published: Jul 1, 2015
  • Author: David Bradley
  • Channels: NMR Knowledge Base
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Nuclear magnetic resonance spectroscopy has allowed researchers in France, the UK and the US to visualize the activity of individual ions  within battery-like devices known as supercapacitors, which could lead to performance-boosting developments for high-power applications.

Nuclear magnetic resonance spectroscopy has allowed researchers in France, the UK and the US to visualize the activity of individual ions within battery-like devices known as supercapacitors, which could lead to performance-boosting developments for high-power applications, such as providing rapid charging for short bursts of acceleration in electric cars, where they would work alongside batteries to boost the vehicle's performance.

The team used a combination of phosphorus-31 and fluorine-19 in situ NMR spectroscopy with tetraethylphosphonium tetrafluoroborate salt dissolved in acetonitrile as their electrolyte and electrochemical quartz crystal microbalance (EQCM) with a sensitivity of a millionth of a gram to observe the movements of ions within a microporous carbon supercapacitor electrode. The team discovered that while charging, different processes are at work in the two identical pieces of their electrode depending on whether positively or negatively polarised, which contrasts with earlier computer simulations. "For positive polarization charging proceeds by exchange of the cations for anions, whereas for negative polarization, cation adsorption dominates," the team reports in the journal Nature Materials. They point out that the EQCM data corroborate the interpretation of the NMR and indicate that adsorbed ions are only partially solvated. "These results provide new molecular level insight, with the methodology offering exciting possibilities for the study of pore/ion size, desolvation and other effects on charge storage in supercapacitors," the team says.

On balance

"Supercapacitors perform a similar function to batteries but at a much higher power - they charge and discharge very quickly," explains chemist John Griffin. "They’re much better at absorbing charge than batteries, but since they have much lower density, they hold far less of that charge, so they're not yet a viable alternative for many applications. Being able to see what's going on inside these devices will help us to control their properties, which could help to make them smaller and cheaper, and that might make them a high-power alternative to batteries."

Currently, they are used in regenerative braking systems in trains and buses, elevators and cranes, systems that capture the energy of braking that would otherwise be wasted. They are also used in the camera flash of a mobile phones and sometimes as complementary technology to batteries in order to boost performance.

A supercapacitor resembles a conventional battery in many ways but charging and discharging process do not involve chemical reactions, rather electrolyte ions simply "adhere" to the surfaces of the electrodes during charging. On discharge, these ions hop back into the bulk electrolyte. These two processes can occur at a much faster rate than the chemical reactions that occur in a battery. "To increase the area for ions to stick to, we fill the carbon electrode with tiny holes, like a carbon sponge," explains Griffin. "But it's hard to know what the ions are doing inside the holes within the electrode - we don't know exactly what happens when they interact with the surface." The NMR and EQCM provide the researchers with a precise picture of what happens inside a supercapacitor while it charges. In a battery, there are two different electrode materials, in a supercapacitor the electrodes are the same and one might expect them to behave identically, but, says Griffin, the charge storage process in real devices turns out to be far more complicated than we had thought. In the negative electrode, the sticking process they expected to see occurs, but on the positive electrode there is an ion exchange, with negative ions being attracted and positive ions being repelled away from the surface. The EQCM also showed that solvent molecules are accompanying the ions into the electrode as it charges.

Ion counting

"We can now accurately count the number of ions involved in the charge storage process and see in detail exactly how the energy is stored," explains Griffin. "In the future we can look at how changing the size of the holes in the electrode and the ion properties changes the charging mechanism. This way, we can tailor the properties of both components to maximise the amount of energy that is stored."

"We would like to apply the approach we have developed to a wider range of electrolytes and electrode materials. In this way, we can build up a more detailed and comprehensive understanding of ion behaviour in supercapacitors, and determine what effects factors such as electrolyte concentration, viscosity, relative ion/pore sizes and long term cycling of the device have on the charging mechanism and hence energy storage properties," Griffin told SpectroscopyNOW. "Ultimately, this will lead to a more general understanding of the properties that are important when designing supercapacitors and ultimately help us to optimise these important devices."

Related Links

Nature Mater 2015, online: "In situ NMR and electrochemical quartz crystal microbalance techniques reveal the structure of the electrical double layer in supercapacitors"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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