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Graphene supercapacitors: could they be the future of electric cars?

The world is hurtling headlong down the road of electrifying transport, but we still haven’t found a way of storing energy that can rival the existing fossil fuel-based systems of powering cars. Rohit Jaggi examines whether graphene could provide the breakthrough

Home life Graphene supercapacitors: could they be the future of electric cars?
Graphene supercapacitors: could they be the future of electric cars?

The benefits of electric cars are convincing; they are non-polluting at the point of use, quieter and cheaper to run than their internal combustion-based equivalents. They also use a form of power that can be generated from a wide variety of sources and for which a distribution infrastructure is well established and covers much of the globe.

Almost all the factors limiting a shift away from internal combustion engines are being dealt with, including some of the more esoteric. The engine-driven pumps needed for power-steering systems, for example, have been rendered unnecessary by the development of electric assistance for steering.

But one big problem remains. We still haven’t developed a means of storage for electricity that gives sufficient non-stop range to cope with an undeveloped network of charging stations while at the same time meeting drivers’ expectations based on their experience with petrol or diesel cars and filling stations. What’s needed is a quick and easy way to recharge batteries that, again, does not require drivers to accept too much loss of utility and convenience compared with traditional vehicles powered by internal combustion engines. So far, electric cars work in the city, but are much less popular outside urban areas.

Graphene supercapacitors: could they be the future of electric cars?

The problem with batteries
Despite a rapid pace of innovation and improvement in technology, batteries are still not particularly energy dense – a measure of the amount of energy stored per unit of mass – as well as being slow to charge. Their use as a power source is not helped by the fact that fossil fuel is highly energy dense, nor by the global networks of filling stations built up over more than a century that make refilling cars with gasoline, diesel or compressed natural gas both quick and easy.

Solve the capacity and charging constraints and suddenly electric vehicles are the obvious answer to many transport problems, from pollution to noise to cost.

Batteries are not the only medium for storing electricity. There are also capacitors, which can be charged more quickly and release their energy much faster than batteries, but are less energy dense.

Then there are supercapacitors, which have the quick charge and release qualities of capacitors, with greater energy density. Even so, for a given performance they have only about 7 per cent of the energy density of lithium-ion batteries, measured in watt hours per kilogram (Wh/kg) – and those kilos are important in vehicles that have to use power to haul around weight.

Hail, graphene
This is where graphene, discovered by scientists in 2004, comes in. Graphene is another form of pure carbon – along with graphite and diamonds – in layers just one atom thick. It is so new that commercial-scale production techniques have only recently been devised.

Carbon is one of the most common elements in nature and all life – on earth, at least – is carbon-based. This means its use is both sustainable and potentially ecologically friendly, depending on the process for producing the form in which it is used.

Graphene, at an atomic level, is tightly bonded. This makes it possibly the strongest material ever discovered, but the way its carbon atoms are arranged means it is also elastic. It is also both the lightest material by volume and the best conductor of electricity. 

While the physics sounds impossibly complex, the science is actually simple. The regular hexagonal honeycomb of graphene’s atoms makes it strong but stretchy, while the open and flat atomic structure means it lets electrons through easily, which explains its properties as an efficient conductor of electricity. In this it is almost as efficient as superconductors, but they often have to be cooled with water or some other medium, while graphene is effective at room temperature.

More research needed
All of which suggests that when used for the electrodes of supercapacitors, the conductive features of this two-dimensional material would allow it to improve on the activated carbon used in capacitors by a big margin. It would shed a lot of weight – thus improving the important Wh/kg figures and making supercapacitors capable of both being charged rapidly and holding a large amount of energy.

But it’s not that easy.

Even using graphene, or graphene and another material such as carbon nanotubes, another form of the element with a cylindrical structure, supercapacitors trail far behind Li-ion batteries in energy density – by about four-fifths, in fact.

Research into the use of graphene is still at a very early stage, though, and that gap is likely to be closed. Already the power density – the ability to deliver lots of electricity in short bursts – of graphene supercapacitors can rival that of Li-ion batteries.

Great expectations
The potential payoff in automotive uses is large enough, but even beyond that there is almost no end of applications for graphene batteries or graphene supercapacitors in electronics and mobile devices. More and more companies, many of them international giants such as South Korea’s Samsung, are hard at work on research that will result in rapid innovation and development. Graphene-based supercapacitors can be flexible, which opens up revolutionary possibilities in wearable tech. Furthermore, graphene can be used as a component in Li-ion batteries to improve their resistance to degradation with each charge.

More excitingly for the future of transport, graphene supercapacitors can also be made in irregular or complex shapes, offering the opportunity to fit them into otherwise unusable spaces in vehicles. Or even, given the material’s immense strength, engineering them into the actual structure of vehicles themselves.

There are even more potential benefits. If supercapacitors in cars can be charged rapidly with large amounts of electricity and then rapidly discharged, then another application could be to use them as power sources for individual homes, for example to smooth spikes in renewable power generation. Japanese auto company Nissan’s Power to Home system already uses its electric Leaf car, which has Li-ion batteries, in such a way, but a large-capacity graphene supercapacitor would do the job so much better.

Research into graphene supercapacitors is still at an early stage and many of the encouraging results are based on small-scale laboratory trials. For such supercapacitors to be of use as a power source for cars, the technologies have to be scalable. Even if they cannot deliver all that has been promised, however, it seems likely that graphene supercapacitors will play a transformative role in the future of electric transport.

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