Could supercapacitors be the energy storage systems of the future?
New technology could eventually make traditional lithium batteries obsolete, says Steven Cutts
As things stand, the internal combustion engine looks like a doomed device. The UK government has put plans in place to ban the sale of new petrol and diesel cars by the year 2030, and those plans are critically dependent on the availability of one chemical: lithium.
Lithium ion batteries offer the potential to store energy at much higher levels than the more traditional lead acid batteries that served our car engines in the past. It is this improvement in battery technology more than anything else that has enabled us to believe that an all-electric future might happen.
There are, however, significant problems with this approach. The total amount of lithium in the world is limited, and on current trends we will need to increase lithium production by a factor of five by the year 2050.
Part of the problem here is the nature of electricity itself. It’s a pretty ephemeral product, and scientists everywhere have long struggled to bottle the stuff and store it for a rainy day. It’s true that an electrochemical battery can be recharged by an electric current, and then the energy stored in a different form.
Later on, we can convert the energy back into an electrical current by shifting the chemicals inside the battery down an internal energy gradient. Unfortunately, there exists a limit to the number of times that an electrochemical battery can be charged and recharged and, beyond a certain point, the batteries in your car will have to be replaced.
The advent of the electric car has created a new kind of congestion at the petrol station, with electrical recharging being a much more time-consuming and lethargic process than refuelling a petrol or diesel vehicle.
As a raw material, lithium comes with other issues. Only a few countries have significant lithium reserves, with a high proportion of global lithium deposits being found in Australia and South America. A Chinese group has just announced a plan to extract lithium from a region in Bolivia.
The process of extraction is far from easy, and actually releases carbon dioxide into the Earth’s atmosphere on a massive scale. Since one of the main motives for building lithium ion batteries is the reduction in global CO2 emissions, these figures present the green movement with something of a contradiction.
At the same time, we should not forget that a number of people are now exploring new methods of extraction that might solve at least some of the problems involved. Despite extensive investment in this field, it’s not clear that there will ever be enough raw materials in the world to manufacture batteries for every would-be motorist on the planet. In practice, such a shortage would manifest itself in an increase in the cost of supply, with lithium ion batteries simply becoming too expensive for most people to afford. Oil-producing countries would be unlikely to complain. Countries in which lithium ore is readily available would be delighted.
But what if we could come up with a different kind of storage? A form of storage where the electrical charge itself is contained within a box? A box that could achieve full recharge in the space of two to three minutes, and one that could gain and then release that same electrical charge repeatedly and without damage or decay?
An energy storage system of this kind would be a real game changer, and the inventors of such a system would make a lot of money very quickly. Whilst some politicians, green activists and innovators fret about the failure of BritishVolt to build a battery gigafactory in England, others are looking at an entirely new technology: the supercapacitor.
Capacitors have been around for well over a century; you may well remember learning about them in high school physics. Anybody who has dabbled in the world of hobby electronics will be more than familiar with their role, as well as their limitations.
A capacitor is a device that can store electrical charge, but the maximum amount of charge that can be stored is really quite limited. As such, it may seem absurd to suggest that we could use them to replace batteries.
Capacitors attempt to expose a very large surface area of material to another area, with the two being separated by an insulating material. As soon as they are connected to an outside voltage, an electrical charge builds up on both plates. One of the advantages of storing charge like this is that the charge can then be released back into an electrical circuit very abruptly. The larger the cross-sectional area of the two plates, the larger the stored charge.
Perhaps inevitably this would involve capacitors being very large components indeed, but the manufacturers of capacitors have long solved this problem by rolling the two plates up (similarly to a toilet roll) and wrapping them in a colourful package. This approach also explains why almost all capacitors are cylindrical in shape.
Even with the process of rolling a supercapacitor together accounted for, the total amount of energy stored in a traditional capacitor is really quite small, and if you try to draw on a capacitor in a moment of need, it will only provide you with electrical current for a very brief interval.
Some researchers are already suggesting that a graphene-based supercapacitor might soon be able to out-store the very best batteries on the market today
Then, in 2004, the situation began to change, as two Russian-born researchers were able to produce a new material that had, to that point, only been described theoretically: graphene. Working at the University of Manchester, Andre Geim and Konstantin Novoselov published a series of papers that would win them the Nobel Prize in 2010, in which they described the new material.
Graphene is, in effect, another allotrope (structural form) of carbon. As a teenager, you may have learnt that carbon can manifest itself as a diamond or as graphite or as a piece of coal. Geim and Novoselov came up with configuration number four: a two-dimensional layer of carbon atoms that resembles a mesh. This material was found to have exquisite properties; some of them previously unimagined.
Think about the constraints of our current electric vehicles. Tesla – the market leader – produces a car that can drive at a maximum of around 375 miles on one charge. This is a reasonably good performance, and it should be added that Tesla has done more than any other company to popularise the idea of electric motoring. On the downside, if we attempted to recharge the batteries in such a car using a conventional 120-volt electrical socket, it would take somewhere between 20 and 40 hours to complete that process.
Ordinarily, a capacitor cannot store as much electrical energy as an electrochemical battery, and would not be considered a viable alternative to battery technology by the transport industry. However, the advent of the supercapacitor has given us a product that can hold between 10 and 100 times as much energy as their traditional equivalent, in part at least because of the introduction of new materials.
It has long been understood that one gram of activated carbon can have the equivalent surface area of a tennis court. However, it is now understood that graphene has even more surface area than activated carbon.
In fact, graphene is one of the new materials that is being studied at length with a view to producing supercapacitors that have the ability to rival the very best of current battery technology. Graphene is a form of inorganic carbon with remarkable properties, including the ability to build as sheets that are one atom thick. This means that a sheet of graphene is now about as thin as any material can realistically be and that if we roll up two sheets of graphene in close proximity, we can achieve a cross-sectional area that would be unthinkable with almost any other material.
The ability to store electrical energy on demand and in very large amounts will also make the intermittent nature of renewables far less of a barrier to modern energy supply
The result is a supercapacitor with a fantastic amount of electrical charge onboard, and there are some researchers who are already suggesting that a graphene-based supercapacitor might soon be able to out-store the very best batteries on the market today. At the same time, such a supercapacitor could be fully recharged in the space of two to three minutes. They can be manufactured from carbon – one of the most abundant substances on the planet – and the absolute number of capacitors we could produce would be unlikely to be constrained by our access to raw materials.
Thus far, supercapacitors made of graphene have been held back by very high manufacturing costs, but most authorities believe that this problem can be overcome within the next decade (and possibly much sooner than that).
During the 20th century, it became clear those countries that (by sheer geological accident) had access to oil would also be very wealthy. As things stand, it looks like those countries with access to lithium or various other vital mineral ores will soon be able to charge pretty much anything they like for their assets.
In contrast, a future where large-scale energy storage has been achieved using a system of supercapacitors would put a stop to such extreme movements of wealth, and indeed the relative impoverishment of energy-poor countries. The ability to store electrical energy on demand and in very large amounts will also make the intermittent nature of renewables far less of a barrier to modern energy supply.
While more research is needed, supercapacitors are already appearing in large-scale engineering projects. Some tram systems have already incorporated supercapacitors into their onboard systems.
Most trams drive beneath an electrical overhead pick-up cable. In effect, the overhead cable is the energy supply to the tram. However, some systems have been able to avoid the expense of continuous pick-up cables by storing the energy onboard the tram using supercapacitors. Every so often, the tram drives beneath another pick-up point – usually at a tram stop – and recharges the capacitors before moving on.
It has also been suggested that a supercapacitor in a smartphone could be fully recharged within two minutes of being connected to the mains and – unlike a more traditional battery – would be unlikely to malfunction over time with multiple recharges (though in their current state, supercapacitors are not able to hold nearly as much energy as a traditional battery, making them impractical).
Most supercapacitors would fail to compete with batteries because of their low energy density, but with the advent of the graphene-based supercapacitor things may one day change. How far and how fast this sort of work progresses will determine how quickly we can transition to an all-electric future.
It may also have a profound effect on the market value of lithium. Part of the argument for new nuclear reactors in the UK is that nuclear is capable of continuous power output, whereas renewables are, by their very nature, unpredictable.
The ability to store electrical charge in affordable devices on a very large scale could be the final nail in the coffin for fission nuclear power.
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