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Invented More Than 250 Years Ago – in 1745 to Be Cover frt.qxp:CVR FEATURE TEMP 8/5/08 15:06 Page 14 electrodes and some electrolyte, making it something of a hybrid between a capacitor and a battery. The most popular is the double layer capacitor, which stores the energy in the double layer formed near the carbon electrode surface. The amount of energy a conventional capacitor can hold is measured in microfarads – even nano or picofarads. In contrast, supercapacitors typically store farads, thousands of times more. Large commercial ones can store as much as 5000F. The energy density of today’s supercapacitors ranges from around 1 to 10Wh/kg, much higher than a standard capacitor but still only about a tenth of an NiMH battery. But as we shall see, this could change radically. One of the main benefits of supercapacitors is that they can be charged extremely quickly, in about 10s. This means large devices can be used for regenerative braking on vehicles. Hundreds of supercapacitors can be connected in series to provide the energy storage needed. Several transportation systems worldwide are now using them in such a way, for example in Shanghai and Mannheim. Charging ahead Will supercapacitors nvented more than 250 years ago – in 1745 to be Another of the supercapacitor’s advantages over Iexact – the humble capacitor has been a batteries is that it can be recharged and discharged enable an energy mainstay of electrical and electronic effectively an unlimited number of times. There is engineering, almost wherever energy needs to be little wear and tear induced by cycling and age does storage revolution? stored. Billions of circuits have been built containing not affect them, with the result that a supercapacitor them. But a device that could transform the world? only deteriorates to about 80% of peak performance, By David Boothroyd. Hardly. even after 10 years. However, if the potential of a new generation of Even though today’s supercapacitors have some supercapacitors turns out to be as great as proponents claim, the world really will become a different place. Such devices could transform the entire nature of electrical storage and make today’s most advanced batteries look positively primitive. And it could happen at both ends of the power storage spectrum: tiny units for consumer electronic products; and Centre: massive ones for the automotive industry. Both Zenn’s Clifford:“I decided the markets might – possibly – be in for a true revolution. world needed a 180° turn from Supercapacitors have been with us for several petroleum and ... saw the decades, occupying relatively niche markets. Whilst a fundamental issue wasn’t drive standard capacitor consists of conductive foils and a systems, but energy storage.” dry separator, the supercapacitor uses special 14 www.newelectronics.co.uk 13 May 2008 Cover frt.qxp:CVR FEATURE TEMP 8/5/08 15:07 Page 15 COVER STORY Supercapacitors clear advantages over batteries, they cannot match suitable only for inner urban transport applications. them for electrical storage density: a typical “I decided the world needed a 180° turn from supercapacitor has only about 5 to 10% of the density petroleum and we saw the fundamental issue wasn’t of the best batteries. But two developments taking drive systems, but energy storage,” Clifford says. “We place could change that radically. think a complete paradigm shift in energy storage is One centres on Texan company EEStor. It is reticent, needed and that is what EEStor is proposing.” but is known to be developing an electrical energy The two companies signed a technology agreement storage unit (EESU) based on a ceramic supercapacitor in 2004 and are working closely together. The target is with a barium-titanate dielectric. This, it says, achieves the cityZENN, an all electric car scheduled for launch extremely high specific energy – the amount of energy in Europe by autumn 2009 (earlier than in North in a given unit of mass. America, where certification takes longer). If it It claims its system has a specific energy of about happens, it will revolutionise the automotive industry 280Wh/kg – compared with around 120 for lithium- at a stroke because its performance will dwarf ion and 32 for lead gel batteries – and that it will anything achieved so far: a top speed of 80mph, a outperform lithium ion batteries in terms of price, range of 250 miles, a recharge time as little as five charge time and safety. minutes, a competitive purchase price and operating By modifying the composition of the barium- costs one tenth of a comparable ordinary car. And of titanate powders, a thousand fold increase in course, zero emissions and noise! supercapacitor voltage can be achieved, it says, The first vehicles to feature EEStor’s supercapacitor Above: giving 1200V to 3500V and possibly higher. may even appear this year, Clifford says “EEStor has Zenn says its electric vehicles Many are sceptical, but EEStor has attracted some committed publicly to commercialisation of its may be charged overnight impressive partners. In January, Lockheed Martin technology in 2008. The LSVs we are building today are using a standard plug. But it signed a deal to use EEStor supercapacitors for ‘EEStor ready’ and they will be the first to have the also envisions ‘charging military and homeland security applications. It is also EEStor systems. ZENN will not be building the highway stations’, where the job could backed by Kleiner Perkins, a venture vehicle, but will work with global automotive partners – be done in five minutes. capital company whose track that has always been our intention,” he adds. “With record includes Google, EEStor storage and our drive system technology, we will Below: Amazon and Sun. be an enabler.” ZENN will supply the complete electric Whilst Zenn’s current LSV EEStor’s closest partner is drive system, including the motor controller and has a top speed of 25mph the Zenn Motor Company of associated drive electronics. and restricted range, the Toronto, a supplier of low Also available will be a ZENNergy drive train forthcoming cityZENN will speed electric vehicles powered by EEStor. A key target market here is reach 80mph and have a (LSVs), founded by its ceo Ian ‘retrofitting’ standardised fleets, such as London cabs range of 250miles – if Clifford in 2001. LSVs have a – installed bases of vehicles, operating in controlled EEstor’s supercapacitor top speed of around 25mph environments. The aim is to enable them to convert to technology lives up to and a restricted range, all electric drive. Ultimately, Clifford envisages the expectations. development of conversion centres, where consumers could take their car and have it converted. For recharging, there are several possibilities. The simplest is to do it by plugging into a standard household socket, which will achieve a charge in about four hours, certainly overnight. But www.newelectronics.co.uk 13May 2008 15 Cover frt.qxp:CVR FEATURE TEMP 8/5/08 15:08 Page 16 COVER STORY Supercapacitors storage capacity could be increased significantly. This is exactly what can be achieved by coating the supercapacitor’s electrodes with millions of nanotubes, each 100,000 times as long as they are wide. The vertically aligned nanotubes in MIT’s supercapacitor have a regular shape and are only several atomic diameters in width. The result is a more effective surface area, providing increased storage capacity. It is done by growing a microscopic ‘forest’ of the vertically aligned nanotubes on a silica substrate by use of a thin catalyst layer and a chemical vapour deposition process. The silica is coated by a nanometre thick layer of a catalyst such as iron. When the material is heated in a vacuum, the catalyst breaks into tiny droplets. A hydrocarbon gas is then dedicated charging centres may reduce this to less passed over the substrate and the catalyst ‘grabs’ than five minutes. More announcements are due soon carbon atoms. A long nanotube then self assembles – ZENN’s agreement with EEStor calls for regular and pushes upward from the catalyst droplet. milestones to be met. “We have demonstrated that nanotube forests can Transportation is obviously a huge market, but by be grown on a conductive substrate,” says Joel no means the only one for EEStor’s technology. Grid Schindall, Professor of Product Design at MIT’s load levelling and consumer electronics are two other Electrical Engineering and Computer Science potentially massive areas. Storage for wind and solar department. “Compared to activated carbon, this is power are other possibles. And Clifford says that, somewhat like a paintbrush, as opposed to a sponge. The forest’s surface area should exceed the effective surface area of activated carbon devices by at least a “We have demonstrated that nanotube factor of five and we anticipate being able to operate at forests can be grown on a conductive higher voltages, due to the nanotube’s inert chemistry. “We have grown the required nanotube array and are now testing it in small battery cells, with reasonable substrate.” Prof Joel Schindall, MIT performance – currently, about the same as existing activated carbon systems. But we are still looking for unlike batteries, raw materials are no problem. ways to increase the density of the nanotubes and, by “Think of the impact of hundreds of millions of doing that, exceed today’s devices.” electric cars. Take lithium: there are about 17million At Cambridge University’s Electronics Power and tons available on the planet, most of it in three areas Energy Conversion (EPEC) Group, Professor Gehan that are relatively unstable. EEStor’s technology uses Amaratunga heads a team that has developed barium-titanate and there is more than 2billion tons nanoscale supercapacitors made from multiwalled of it. So, critically, it is a technology that, once carbon tubes roughly 70nm wide, grown vertically from commercialised, is scaleable to a global level.” nickel catalyst dots on niobium films.
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