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What is a supercapacitor?

In the last few years, supercapacitors have been heralded as the next Lithium-ion battery.

Following their commercial deployment in 1991, the ultralight Li-ion battery revolutionized the energy and electronics industries, making possible small, portable devices and companies like Apple, Samsung, and Tesla. Today, they are the most common battery in electric vehicles (EVs) and grid-scale battery storage.

<strong>Typical construction of a supercapacitor:</strong> (1) power source, (2) collector, (3) polarized electrode, (4) Helmholtz double layer, (5) electrolyte having positive and negative ions, (6) separator.

Recently, supercapacitors started making the news when Tesla bought out Maxwell Technologies, a leading supercapacitor and energy storage firm. But the technology actually dates back to the 1950s.

Céline Merlet  CNRS researcher at Université Paul Sabatier in Toulouse and speaker at ISC 2021  simulates potential supercapacitor materials at the ionic level, bringing better understanding of their energy storage potential. Here, she discusses what exactly supercapacitors are, how they function, and where they could be headed. 

What is a supercapacitor? 

Like batteries, supercapacitors are energy storage devices. However, batteries store and release their energy through chemical reactions, while supercapacitors use static electric charge.

“They’re called supercapacitors — ‘super’ — just because they can store more energy than capacitors. Normal capacitors use two metal plates separated by an insulator,” Dr. Merlet says. The insulator keeps separate the positive and negative ions accumulating on the plates.

“Basically, you have no reactions. It’s just mobile charges,” Dr. Merlet says. Because of this, capacitors can charge and discharge quicker than Li-ion batteries, which are bottlenecked by the time it takes to convert chemical reactions to electric charge.

<strong>Supercapacitors are often called ultracapacitors,</strong> as is the case with this product from Skeleton Technologies.

This makes them ideal for application in camera flashes, communication systems, and computer memory backup systems.

However, they have one significant disadvantage. They store far less energy by weight than Li-ion batteries do. To provide a significant amount of energy, they must be exceptionally large, making them impractical in technologies such as electric cars.

Research to overcome this drawback eventually led to supercapacitors. 

“When you use porous carbon [as the electrode material], the porousness creates a lot of surface area. The charge that can be stored is correlated to the surface area, so supercapacitors can store a lot of energy,” Dr. Merlet says — approximately 10 to 100 times that of a capacitor.

Supercapacitors also last much longer than Li-ion batteries. During operation, chemical reactions cause batteries to heat up, expand, and contract, eroding them over time. They average 500 to 10,000 cycles, while supercapacitors average between 100,000 to 100,000,000 cycles.

Supercapacitor limitations and beyond

Research into applications for supercapacitors is still young, but companies like Toyota and Lamborghini have successfully used them as a supplement to Li-ion batteries in hybrid cars.

The supercapacitors rapidly absorb the energy created during braking — with much greater efficiency than the state-of-the-art Li-ion battery could — and store it for later use in acceleration, reducing the battery’s workload.

And while experimentalists are testing them elsewhere — in spacecrafts, satellites, and wind turbines, for example — their uses to date remain limited. While they improved on the capacitor, supercapacitors too fall short of Li-ion batteries in how long and how much energy they can store (approximately 1/4th that of a Li-ion battery by weight).

These energy densities, related to their capacitances, can be changed based on the electrode material (often some form of carbon) and its structure, explains Dr. Merlet. “[To improve supercapacitors], you have to go all the way to the small-scale,” and look at how the ions flow through the carbon electrodes.

“Depending on how small the pores [of the material] are, how they’re arranged in space, and the size of the particles, for example, you don't get the same [energy storage]. Actually knowing what the maximum capacitance is or what the maximum energy you can store is, is difficult.” 

To better predict these, she built a mesoscopic model that simulates the ions flowing in supercapacitors. “One of the ideas behind this mesoscopic model is to say that if we can predict faster the energy that can be stored in some of the carbon materials, maybe we will come up with a carbon structure that allows us to store more energy.”

“The mesoscopic model will also let us screen a lot of [alternative] materials,” Dr. Merlet says. “Maybe we’ve already reached the maximum [energy storage of capacitors] or maybe the materials you find, you actually can’t make them right now. But there are advances all the time in what you can make.”

In addition to this, her models, and machine-learning based approaches, will also advance knowledge of disordered matter. 

“Carbons are very disordered,” Dr. Merlet says. “In general, characterizing or knowing the exact structure of crystals is easier. Many available techniques are more adapted to understanding crystals than disordered matter.” 

<strong>Crystals, like this sodium chloride structure</strong> are actually quite orderly systems.

“Part of what I'm doing is to try to get a more accurate representation of these pores — this network of pores — and carbon atoms, locally, in space. Because this impacts a lot of properties. And it’s interesting not only for energy storage. These carbons are used in many things, like decontamination, desalination, and gas storage. And in all these cases, the size of the pores and the way they are connected is important, but this is still not very well understood.” 

Better understanding of these carbons, and more comprehensive material screenings, could help experts determine new uses for supercapacitors. Although they’ve already begun revolutionizing hybrid technology, it has been as a supplement, rather than a replacement, to Li-ion batteries. The potential is there for them to do the same in renewable and grid energy storage, and research into new and improved materials will be essential.

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