As far back as 2011, Elon Musk expressed the view that not batteries but supercapacitors would become the power source for future emobility applications.
While apparently not having commented subsequently and led mass battery production with the development of gigafactories, that prediction increasingly looks as though it may yet come to pass with the technology having advanced significantly in the years since he himself was working on it.
Lithium-ion batteries are the main storage type in the energy sector, used in a wide range of applications including electric vehicles (EVs) and renewables integration. While the technology has been advancing, the key considerations for EV drivers are the relatively slow charging ability, limited range and short cycle life that these batteries provide.
Supercapacitors offer the potential of overcoming all these issues with faster charging, no longer than refuelling a standard internal combustion engine vehicle, and much greater cycling capability and hence longer lifetime, as much as double Li-ion.
With their rapid charge and discharge capability, they also have application alongside renewables for network management applications that need this profile, such as voltage stabilisation.
The key challenge with supercapacitors however, is their energy density or capacity, which limits their use in EVs currently without the storage unit becoming impractically large.
But the prospect of overcoming this is in sight with new materials such as graphene – a two-dimensional material based on single atom-thick layers of carbon, which conducts electricity and heat with great efficiency.
The European Commission’s Graphene Flagship formed in 2013 is aimed to take graphene from the lab to society within a decade. Issues that the Flagship is addressing in the energy field include the production of graphene to address the requirement for high quality materials in large quantities, its integration into energy devices while preserving its properties and device performance improvements using scalable methods suitable for commercialisation.
Among the early achievements has been the development of a spray deposition tool capable of scaling up the fabrication of graphene electrodes and which can produce supercapacitors with very high power densities. While much of the focus to date has been on flexible supercapacitors for mobile and wearable applications, the technology also holds promise for automotive storage among others.
Other groups are investigating the use of other materials, including those other than carbon, in supercapacitors. New results from a project at the Universities of Bristol and Surrey developing “novel polymer materials” for supercapacitors suggest these could achieve energy densities surpassing that of Li-ion batteries. The technology has been evolved from that developed for soft contact lenses and in the next step, the British company behind it, Superdielectrics Ltd, is looking to build a research and low volume production centre.
Supercapacitors for EVs
Currently the use of supercapacitors for powering transportation – and fast growing expertise in their manufacture – seems to be occurring mainly in China, where a growing number of buses, now well into the hundreds, are coming into operation with the technology. Charging takes place at stops when passengers are getting on or off and a 10 second full charge is reported to give a travel distance of about 5km.
They also are starting to be introduced by the automotive manufacturers but for other applications. For example, General Motors was the first to introduce the technology in 2015 for start-stop to provide the burst of power to restart the engine following a stop without drawing on and wearing the regular battery. Mercedes is another manufacturer using the technology for start-stop as well as for regenerative braking.
From this discussion, it is clear that the mainstream use of supercapacitors for EVs is still some way off, excepting as illustrated in China for their potential for use in buses or delivery vehicles in cities, which are able to recharge easily short distances apart. Nevertheless, they already can form an important component of a hybrid system in vehicles where their particular benefits can be drawn upon.