Electronic Design

Carbon Foam Whips Up Greater Power Density For Supercapacitors

Applications that require pulsed power pose a challenge when they use a battery as their power source. Delivering energy in short, high-current pulses is harder than supplying the same amount of energy over longer intervals. Battery size needs to be greater to deliver these pulses. Over time, the pulsed current drain can degrade battery performance. Some of the strain on the battery can be reduced by using it in conjunction with very high-value capacitors known as supercapacitors. But their usefulness in pulsed-power applications may be limited by their equivalent series resistance (ESR).

A new type of supercapacitor developed by Cooper Electronic Technologies, St. Louis, Mo., achieves significant reductions in ESR over existing capacitors, enabling it to support designs that other devices cannot. The PowerStor Aerogel capacitor, also known as an electrochemical double-layer capacitor (EDLC) or an ultracapacitor, contains electrodes formed from a material called carbon aerogel foam. This foam features high electrical conductivity and controlled ultra-fine pore size, which produces a highly usable surface area for double-layer formation. Low leakage current is said to be another benefit of the new electrode material.

This carbon aerogel foam produces lower ESR than other carbon materials used in supercapacitors. For example, the ESR for a 10-F aerogel capacitor measures just 15 mΩ. According to the company, the ESR for competitive parts would be around 200 mΩ.

The aerogel capacitor's construction is similar to that of Li-ion batteries and aluminum electrolytic capacitors. A porous separator is placed between two electrodes, creating a sandwich that is wound in "jellyroll" fashion and housed in an aluminum or steel can. The can is then filled with electrolyte and sealed. Leads attached to electrodes protrude from the can (see the figure).

Performance, though, is unlike a battery in several respects. The supercapacitor electrostatically stores the charge in the electrochemical double layer formed on the carbon surface of the electrodes. This charge, then, can be released extremely fast. As a result, it can generate specific power up to 4000 W/kg, which may be 10 times greater than a Li-ion battery.

In portable applications—where the battery charges up a supercapacitor—such a high power density would allow the use of a smaller battery. The supercapacitor itself is small, with sizes ranging from that of an AA cell to approximately a third of an AAA cell.

Another effect is the extension of battery life. Secondary batteries can withstand thousands of cycles of charge and discharge, and they may suffer a decrease in capacity with each use. Yet the capacitor can undergo hundreds of thousands of cycles with no degradation in performance.

In addition to providing pulsed power, the aerogel capacitor can furnish hold-up power, delivering currents in the milliamp range for hours or even days. Equipment such as hard-disk drives and remote sensors could benefit from this capability.

Compared to standard aluminum electrolytic types, aerogel capacitors offer 2000 times the volumetric capacitance. Naturally, there's a price to be paid for increased capacitance—the aerogel (like other supercapacitors) specifies a lower operating voltage than standard capacitors.

The devices are currently rated for a maximum operating voltage of 2.75 V. Higher-voltage parts are in the works, and the company expects to introduce a 5.5-V version by the end of the year. Also, surface-mount and other packaging options are now in development.

Pricing for the Aerogel capacitor starts at $1.84 each in 10,000-piece quantities. For more information, contact Bruce Jeppesen, product marketing manager for Cooper Electronic Technologies, at (636) 527-1449; [email protected]; or see www.cooperet.com.

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