Batteries have been in the news a lot recently, in part thanks to the work of Tesla, Inc. who have been grabbing headlines for various reasons over the last few years, including shooting a battery powered car into space (I’m sure they had a good reason, but space junk is already a problem…). More practically Tesla have also activated the world’s largest lithium-ion battery in Australia, which is capable of providing 100-megawatts of power. These activities, alongside government led initiatives like the ‘Faraday challenge’, may be seen as confirmation that batteries are the future of energy storage, but a question still remains about whether these devices really are the best fit for every application.

The widespread implementation of efficient lithium-ion batteries is just one outcome of the enormous amount of research is currently taking place to develop energy storage systems. However, although it seems certain that batteries will play a key role in our future energy landscape, there are a number of other viable electrochemical energy storage technologies available; Supercapacitors (also known as ultracapacitors) are one such example. Although the market for supercapacitors is still small compared to that for batteries, it has been predicted that over the coming years it will dramatically increase. This is in part due to their increasing application in electric vehicles (e.g. Capabuses in Shanghai), electronics (e.g. Zap and Go) and smart-grid applications.

People may be more familiar with a traditional parallel plate capacitor, found in many electronic devices, than a supercapacitor. A parallel plate capacitor is (commonly) constructed from two metallic plates separated by a dielectric material (e.g. select ceramics or glasses). When a potential difference is applied across the plates, an electric field is generated and positive and negative charges build up on opposite electrodes, storing energy. Supercapacitors have a similar form, but are (usually) constructed from two high surface area electrodes separated by a liquid electrolyte. The voltage range for these devices is lower than for other types of capacitor, due to electrolyte breakdown at high voltage, but they can store a lot more charge which results in higher energy densities.

Supercapacitors broadly fit into two categories, pseudocapacitors and electrochemical double-layer capacitors (EDLCs). EDLCs store energy just by accumulating electrostatic charges on the electrode surfaces (positive charges on one, negative the other) which are bathed in an electrolyte; no electrons pass through the solid (electrode) – liquid (electrolyte) interface, so the energy storage is purely non-Faradaic. In reality the ions in EDLCs don’t just sit at the surface, but instead fit within the complex pore structure of the electrodes. A relatively recent development has shown that charges can, in fact, be stored in pores that are smaller than the solvated ions, meaning the ions must partially lose their solvation shell – previously thought to be a high energy process.

Pseudocapacitors work more like traditional batteries, utilising redox (Faradaic) processes at the electrode/electrolyte interface e.g. redox reactions of metal oxides. Nonetheless, because these reactions are fast interfacial processes, the pseudocapacitor retains the ability to charge and discharge rapidly.

Through a combination of these mechanisms supercapacitors are able to achieve high power densities (>10 kW kg-1), as they can charge/discharge in seconds; they can also have long cycle lifetimes. Crucially these are areas where battery performance is currently weak. However, unlike batteries, supercapacitors can only achieve low energy densities so can only provide for high-power applications over short periods of time.

To date, the low cost and high surface area of activated carbon has made it the most popular choice for supercapacitor electrodes. This type of amorphous carbon is produced by carbonising organic precursors (cellulose, sucrose, coconut shells) then physically ‘activating’ this it by oxidising it at high temperatures (e.g. with KOH) to increase its surface area and pore volume. In organic electrolytes these materials can provide ~ 300 F g-1, despite their low electrical conductivity, packing density and inter-particle interconnectivity. Other than activated carbons, there is an increasing interest in using nanomaterials in supercapacitors, in-part due to their great potential for extreme electroactive surface area.

So the question is, do supercapacitors really have a widespread commercial future? In short, yes! One of the greatest challenges within energy storage at the moment is the transition to electric vehicles, a problem that is particularly pressing as it has been announced that the sale of new petrol and diesel cars will be banned in the UK as soon as 2040. Unfortunately, a major hurdle for the widespread acceptance of electric vehicles is that people are unwilling to stop after a couple of hundred miles or so to charge their battery for several hours. This is just one of the areas where supercapacitors can offer a unique solution – they can charge in a matter of seconds (unlike batteries). Although supercapacitors can’t store enough energy for long trips, meaning they won’t be able to totally replace batteries, they can perfectly compliment them. Together hybrid supercapacitor/battery systems can combine the benefits of both: charging quickly, maybe at traffic lights or stop signs, and powering long distance travel. In this way supercapacitors may become some of the most important and widely implemented devices for the future.

TSM