Supercapacitors explained – the future of energy storage?

With Tesla’s battery day event not too far off, and their acquisition of Maxwell Technologies last year, I thought it was worth taking a closer look at supercapacitors. Some believe that supercapacitors might be integrated into future EVs. But, what exactly is a supercapacitor? And what makes them so different from batteries? Are they really the future of energy storage?

Before we get into the nitty gritty of whether supercapacitors can really change energy storage all on their own, it’s worth taking a look at what they are and how they’re different from something like a lithium ion battery.

Both batteries and capacitors are a method of storing energy, but Lithium-Ion batteries rely on chemical reactions to store and release their energy. It’s made up of a positive and negative side, which are called the cathode and anode. Those two sides are submerged in a liquid electrolyte and are separated by a micro perforated separator, which only allows ions to pass through. When the battery charges and discharges, the ions flow back and forth between the cathode and anode. During this process the battery is heating up, expanding and contracting. These reactions degrade the battery over time, giving batteries a limited lifespan. One benefit of battery technology is a very high specific energy, or energy density, so it can store a lot of energy for later use.

But capacitors are different, they don’t rely on chemical play in order to function. Instead, they store potential energy electrostatically. Capacitors use a dielectric, or insulator, between their plates to separate the collection of positive and negative charges building on each plate. It’s this separation that allows the device to store energy and quickly release it¹. It’s basically capturing static electricity. One benefit of this is that a 3V capacitor now will still be a 3V capacitor in 15-20 years time². While a battery may lose voltage capacity over time and use. And unlike a battery, a capacitor has a much higher power throughput, so it can charge and discharge in a fraction of the time … but they have a very low specific energy. It’s good for very small bursts of power.

And that’s where supercapacitors enter the scene. They start to bridge the gap between a battery and capacitor. The concept of a “supercapacitor” is not a new thing. In fact, in 1957 the first supercapacitor device was created by General Electric, but there aren’t any known commercial applications. In 1966, Standard Oil accidentally discovered the double-layer capacitor when working on fuel cells, but it wasn’t until the late 1970’s that the Japanese company, NEC, began commercially offering the first “supercapacitor” for computer memory backup.³ ⁴ In fact, while we commonly refer to many products as “supercapacitors” or “ultracapacitors,” those two terms are used interchangeably and really depends on what the company producing it wants to call it. For the most part it’s really just a trademark thing.

In the 1990’s, products such as ECOND’s PSCap — a starter for diesel trains — began hitting the market and pushing the boundaries of energy storage and capacitor applications. Companies like Maxwell Technologies, Murata and Tecate generally dominate the supercapacitor field. But recent developments in graphene-based capacitors are once again nurturing the growth of supercapacitor efficiency and application … but I’ll talk more about that a bit later.

First we need to talk about how a supercapacitor works? And how it’s different than a regular capacitor? Because it’s kind of cool … it’s starting to venture towards a battery’s design and use an electrolyte on either side of an insulator. When current is applied ions build up on either side of the insulator and create a double-layer of charge.⁵

What makes a supercapacitor truly superior to a normal capacitor, or even a battery, is the distance between the metal plates. In a normal capacitor the distance is around 10-100 microns (a micron is one-thousandth of a millimeter). But in a supercapacitor that distance is narrowed to one-thousandth of a micron, and that smaller distance leads to a larger electric field — i.e. more energy storage⁶. Not too mention, the carbon coated plates on supercapacitors increase the available surface area for storage capacity by up to 100,000 times. That’s a lot more energy available for use than a normal capacitor.

So, what are these power hungry little titans used for?

We’re really just at the beginning of supercapacitor applications. But, in general, they’ve been found to have the biggest potential for application in hybrid-transportation (cue the Tesla/Maxwell speculation here). Toyota, Peugeot-Citroen, Mazda and even Lamborghini have all released models of vehicles that use some combination of supercapacitors and conventional Li-Ion batteries⁷. Believe it or not, even though Tesla invested $200 million in the purchase of Maxwell Technologies, Elon Musk has said his focus is not on expanding the use and development of Maxwell’s supercapacitors for Tesla vehicles but instead in their battery manufacturing technology⁸. However cars like Toyota’s Hybrid-R concept car and Lamborghini’s high powered Sian⁹ are using supercapacitors for a very specific role: Power regeneration systems during deceleration.

In other words, when cars are slowing, the energy generated from that action is stored by supercapacitors onboard and later used for acceleration — saving batteries for less strenuous actions than acceleration and deceleration.¹⁰ It’s taking advantage of a supercapacitors superior power throughput.

A fantastic example of how effective supercapacitors can be is seen in Switzerland where a fleet of buses will be exposed to charging stations at a variety of stops along their route. Just 15 seconds can top the energy charge off and only a few minutes would suffice for a full charge. With frequent top offs it makes up for the lack of energy density and storage. And because supercapacitors draw a lower current over a period of a few minutes at a time, this puts less stress on the grid.¹¹

However, supercapacitors still can’t compete with Li-ion batteries when it comes to that high specific energy and longterm energy storage. But despite that some companies are making progress on projects that are poised to make supercapacitors more universally applicable.

You may have watched my recent videos on graphene and carbon nanotubes. Well, those materials actually play a role in the future of supercapacitors. Companies like NAWA Technologies and Skeleton Technologies have taken supercapacitors to the next level by incorporating graphene into the coating of the metal plates. They’ve taken this and expanded the conventional use of supercaps into markets like components for e-motorcycles, spacecrafts and wave energy technology.¹²¹³¹⁴

Graphene provides the next generation of supercapacitors with an interesting array of improvements. In particular, graphene offers substantially more surface area, giving supercapacitors even more capacity for energy storage. But in addition to that, graphene is ultralight, has unique elasticity and is incredibly strong¹⁵.

In fact, NAWA Technologies, Skeleton and other supercapacitor/battery companies have already found major applications for their graphene-based supercapacitors. Skeleton’s products can be found helping to power major tram-systems in big European hubs like Warsaw and Mannheim.¹⁶ But it’s not just trams and urban transportation that Skeleton has found use for. They’re working with the European Space Agency on a potential approach for sudden power usage on satellites and spacecraft.¹⁷ As well as developing an ultracapacitor module for use in wind turbines to help manage the blade pitch control.¹⁸

NAWA’s “NAWA Racer”, an e-motorcycle that packs a serious punch, is showing the world what ultracap-Li-ion hybrid power systems can do in smaller vehicles by making this bike incredibly efficient. While the bike itself isn’t being rolled out for commercialized sale, what it’s doing is proving to other companies interested in the potential of supercapacitor/Li-ion setups, that the technology is groundbreaking.¹⁹ The bottleneck in absorbing all of the regenerative breaking power is the battery, which has a very slow charge rate. The supercapacitor combo allows it to recoup 80-90% of the energy from braking and then immediately reuse that for acceleration.

But wait … there’s more! Eaton’s supercapacitors are built to pair with battery systems, Dongxu Optoelectronics fast charging laptop battery system, Earthdas supercap/battery mixture for e-bikes and motorcycles and ZapGo’s e-scooters are all examples of small power management companies that are experimenting with the application of supercapacitors in their technology.²⁰²¹

With all these examples, there doesn’t seem to be any mass movement towards the replacement of batteries with supercapacitors in everyday technology. So, why is that?

Well, the short answer is that supercapacitors, while superior to traditional capacitors in their ability to store and release energy, are still not able to replace the function of conventional Li-Ion batteries. Mainly because Li-ion batteries pack a punch that supercapacitors can’t, in the form of specific energy, or energy density (Li-ion ~250Wh/kg vs. Ultracaps ~20Wh/kg). But even companies that focus on supercapacitor technology, like Skeleton Technologies, admit that a hybridization of Li-ion and supercapacitor driven power systems could propel electric technologies into the next era²². And the reality is that most of the applications we see today, are some sort of combination between the two.

When a supercapacitor is placed in parallel with a traditional battery, we see drastic changes in the battery lifespan. The process essentially works like this: the presence of a supercapacitor in parallel with a regular battery basically just reduces the work load and intensity level that the battery has to endure. Giving the battery more longevity. In some research it’s shown that it can extend the life of a battery by up to four times.²³²⁴ So why aren’t we seeing this in all EVs? Well, battery technology in EVs is currently good enough, and getting better, for how we’re using it. The expense for going down this path may not be worth the investment.

For a technology that is nearly 65 years old, supercapacitors have yet to really find their place in electric technology. But it seems that, in unison with Li-ion batteries and with graphene being applied more and more commonly, supercapacitors are slowly building themselves an important role in hybrid-electric technology. And that’s something that’s easy to overlook — the importance of streamlining and improving efficiency of current technologies by adding, not replacing, a component to the mix. Supercapacitors could play that role, making Lithium-ion batteries, which have high energy density, more useful over longer and longer periods of time.

The potential is there for supercapacitors to take charge of the energy-storage game and redefine what energy storage really is. We still have a ways to go, but it will be very interesting to see how they’re applied, not just to electric cars, buses and other transportation, but to renewable energy and the grid as well.