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.