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Skeleton Technologies introduces SkelCap Series ultracapacitors with very high power and energy densities; carbide-derived carbons

Skeleton Technologies (earlier post), has introduced its new SkelCap series ultracapacitor family with devices ranging from 2.47 kW to 12.53 kW. SkelCap cells offer power densities of more than 40 kW/kg and energy densities of up to 10 Wh/kg: about 4x the gravimetric power density and 2x the gravimetric energy density of competitive products. Volumetrically, SkelCap cells offer power densities of around 5x and energy densities of around 2x other products.

As examples, notes Skeleton Technologies’ CEO Taavi Madiberk, Skeleton’s 320 F cell has a power density of 67.7 kW/L, compared to Maxwell’s 350 F device with 11 kW/L. The Skelcap 3500 F device has an energy density of 14.1 Wh/L, compared to the Maxwell Technologies 3000 F device at 7.3 Wh/L.

The basis behind the new generation of ultracapacitor devices is Skeleton’s patented carbide-derived carbon (CDC) SkeletonC material, which enables the modification of pore size and structure at the single nanometer level. The surface of the Skeleton C carbon particles contains larger pores than the inside of the particles—allowing increased access to nanopores by the liquids, in turn raising the energy density. The low internal resistance of the nanostructured material raises the power density. CDC has very high volumetric capacitance of 100 F cm3.

Carbides are compounds composed of carbon and a less electronegative element (e.g., silicon carbide, SiC). Carbide-derived carbons are formed by extracting the non-carbon atoms from the carbide structural network; the nanostructure and properties of the resulting CDC thus significantly depend on the precursor carbide.

There are several methods to extract the non-carbon atoms from carbide, Skeleton notes, the most wide-spread of which is a chemical extraction with chlorine at high temperature. The theoretical yield of carbon from different carbides may range from ~6% (wt.) in the case of molybdenum carbide to almost 30% (wt.) for silicon carbide.

Process parameters such as reaction temperature and rate of the gas flow can control the chlorination reaction of carbide and its products. A general feature is that the higher the chlorination temperature, the higher the structural order and the lower the porosity of the resulting carbon material.

Even small changes in carbide chlorination temperature can affect the average pore size and pore-size distribution of resulting carbon, Skeleton says, with reproducible results. This makes the carbide-derived carbons very attractive to the majority of adsorption based applications such as adsorbents, electrode materials in energy storage devices, molecular sieves for separation/purification processes and so on, the company says.

Several examples of SkeletonC materials are:

  • Silicon carbide derived SkeletonC is highly nanoporous carbon, which has high apparent density and noticeably narrow pore size distribution.

  • Molybdenum carbide derived SkeletonC may be fine-tuned from microporous to mesoporous carbon and is therefore useful as a substrate for catalysts or as the adsorbent for selective adsorption.

  • Aluminum carbide derived SkeletonC may have a nanostructure from amorphous to graphitic nanobarrels. Formation of nanobarrels can be promoted with catalytic d-metal compounds dispersed into the carbide.

Skeleton has specially designed a novel process of manufacturing roll press electrodes from carbide-derived carbon composites which give exceptionally low internal resistance and power density.

SkeletonC-based electrodes are prepared from the mixture of ~90% (wt.) of carbide-derived porous carbon and ~10 % (wt.) PTFE binder. The carbon films are rolled stepwise into the electrodes with a desired thickness in the range of 50–100 μm. After drying, the raw electrode sheets are coated from one side with a thin aluminium layer using the plasma activated physical vapor deposition method.

The ultracapacitors are made from asymmetrically coupled CDC electrodes. Specifically, the anion-attracting electrodes are made from highly nanoporous SkeletonC and the cation-attracting electrodes are made from SkeletonC having improved transport porosity.

The resulting package of positive and negative electrodes is placed in a prismatic aluminium can, sealed and dried prior to the adding of the electrolyte: ~1.2 M triethylmethylammonium tetrafluoroborate in anhydrous acetonitrile (H2O <0.003 %). Skeleton selected Et3MeNBF4 salt because of the use of highly nanoporous carbon materials, which according to studies prefer the smaller Et3MeN+ cations, but not the Et4N+ ions commonly used in non-aqueous ultracapacitors.

Skeleton Technologies is firstly aimed at the high-end defense and space sector where the extra performance gives our customers a significant advantage over the competition. Starting with rocket launchers and ending with electromagnetic weapons, SkelCap series’ solid aluminium casing can withstand the harshest environments.

Our current customers such as the European Space Agency serve as a good validation for our products before taking on the automotive, wind-turbine and electricity-grid markets where ultracapacitors recuperate breaking energy or provide peak and back-up power. However the company's long term goal is to use our technological advantage to bring down the cost of ultracapacitors 8x times from current conventional ultracapacitor manufacturers to less than 1,50 dollars per kW.

—Taavi Madiberk

The Skelcap family will be introduced at Eurosatory—the largest defense exhibition in Europe—held in Paris from 11–15 June.



At less than $1.50/Kw, these ultra caps could be ideal for energy (deceleration-braking) maximized recovery. Could be used in vehicles with simple stop-start system or combined with battery packs used in HEVs, PHEVs and BEVs.


I wonder if Red Bull is using these already for their F1 cars? I found an article in the June issue of Racecar Engineering that showed RB were using supercaps rather than batteries (they even had pics of how the two packs were integrated into the floor tray over the rear axle).

I'd post a link to it, but I don't believe you can get to it without subscribing and the "e-zine" doesn't let you cut and paste. But for those of you who are geeky and interested in racing, great magazine.

It would be interesting to know if they are using something like these Skelcap supercaps or some older Maxwell units.


Yes could be a better solution for HEV car than a battery, recover more energy and last much longer for similar weight and volume


@DaveD, if RB used Skelcap Alonso wouldn´t be in the lead anymore :). So at the moment they don´t. Taavi Madiberk, CEO, Skeleton Technologies



ROFL!!! I will agree with that! I admit that he's a great driver....but I still don't have to like him.

Thanks for the laugh, and the feedback! I love to see when people like yourself drop by and give us some info.

good luck, and thanks for the advances. I've always been a supercap fan so it's great to see you guys making such great progress. PLEASE tell me you'll hit your price targets one day!


the "e-zine" doesn't let you cut and paste.
Turn off Javascript and you can usually do whatever you want.

At even $12/kW, 100 kW of instantaneous power would only cost $1200.  That's enough to accelerate a car at a very good pace through about 50 kph, or brake a 1500 kg vehicle travelling at 50 kph at about 1/2 g.  This performance would allow a radically downsized engine in a hybrid, with transient response discarded in favor of emissions and efficiency.  The result would be a very drivable, fun car with high fuel economy:  a better vehicle in all respects.



But to offer full assistance to 120 km/h (for merging onto the motorway from low speed), you'd need at least 4x that, costing close to $ 5000.

In terms of energy density: 40 kg of these supercaps would offer 400 Wh of usable energy, roughly the same as the Prius NiMH battery, which is kept between 40% and 80% SoC.

Supercaps could technically replace the battery in a hybrid, but economically.... not yet. Getting awfully close though.


Wouldn't a combo high power density ultra caps and high energy density batteries be a better solution? The high power ultra caps could be recharged with: 1) braking/deceleration energy; 2) on-board batteries; or 3) on-board genset; 4) domestic or public charging points.

With maximized ultra caps use, batteries would last much longer. The electonic control system would be more complicated but should not be a major challenge.

to offer full assistance to 120 km/h (for merging onto the motorway from low speed), you'd need at least 4x that
Speaking from experience, 100 kW is quite adequate to that task so long as it is supplied for long enough.  One could also expect the ICE to be running at full power under such conditions, for perhaps 35 kW.

BTW, Anne, the kinetic energy of a 1500 kg vehicle moving at 120 kph is only about 230 Wh.  If you have a 36 kW engine running for 10 seconds to assist, that's 100 Wh leaving 130 Wh to take from storage.


Ah, I see your point.  To get 230 Wh also gives 920 kW of power capacity, and at $12/kW this is about $11,000.  The 8x cost reduction is certainly needed to make the ultracap-buffered hybrid car economic.

Calvin Brock

Even small changes in carbide chlorination temperature can affect the average pore size and pore-size distribution of resulting carbon, Skeleton says, with reproducible results.
acne treatment

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