A capacity for change
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Energy Sector Analysis
Supercapacitors have found a niche in energy efficiency, but reaching a wider market will depend on materials innovation—and lower cost.
A capacity for change By Philip Ball Feature Editor Yury Gogotsi
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wners of a Toyota Prius C have a right to feel smug. With fuel consumption of up to 53 miles per gallon, it’s rated as the most fuel-efficient compact hybrid car on the market. Part of the credit must go to the 20 or so little cells that supply spurts of power for functions such as electric windows and heating. These clever devices, called electrochemical capacitors or supercapacitors (SCs), also capture kinetic energy during braking, storing it electrically and then releasing it when needed, for example to restart the engine when it switches off as the car idles at a red light. Energy-storing SCs are becoming ubiquitous in electrical and hybrid transportation, particularly in public trains, trams, and buses (see “Supercapacitors take charge in Germany” in the September 2012 issue of MRS Bulletin, p. 802). Shanghai now has an extensive public bus network that uses SCs in regenerative braking to lower fuel consumption, and they are found in buses and trams throughout Europe. Whereas batteries take hours to charge, SCs need only a few seconds, so buses using them as the sole power source can recharge as they wait at stops. In diesel buses and trucks, SC-based regenerative braking can cut fuel costs by around 30% annually, with a corresponding reduction in carbon-dioxide emissions. Because they supply short (up to 30-second) bursts of energy, SCs are also well suited to applications such as cordless power tools and heavy-construction machinery. “In the mid-2000s, the use of supercapacitors for opening doors of the A380 Jumbo jet was the first demonstration of the maturity of the technology,” said Patrice Simon, a materials scientist at the Université Paul Sabatier in Toulouse, France. It showed that these devices could perform safely and reliably in a demanding situation. But SCs are still relatively expensive, partly because the limited demand for them precludes economies of large-scale production. The market could expand substantially if their performance can be improved and their manufacture cheapened. That will depend on clever materials engineering. At face value, SCs don’t compare favorably with batteries: state-of-the-art SCs have energy densities (energy capacity per unit mass) of around 4–5 watt-hours per kg, whereas lithium-ion batteries achieve over 120 Wh kg–1. But that’s not really the point. Whereas batteries charge and discharge through slowly
unfolding electrochemical reactions, SCs can grab and dump all their stored charge in an instant. “The main markets are wherever one needs to deliver current bursts or to consume current surges,” said Donald Sadoway, a materials chemist at the Massachusetts Institute of Technology. So they are complementary to, not competitive with, batteries: SCs for power, batteries for stamina. Like familiar electrical capacitors, SCs store energy
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