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Otto -

I've heard of them. Just googled and found they have a relationship with Zenn electric vehicles :

EEStor - Wikipedia, the free encyclopedia

Skepticism
EEStor's claims for the EESU exceed the energy storage capacity of any capacitor currently sold, by orders of magnitude. Many in the industry have expressed great skepticism regarding the claims. Jim Miller, a capacitor expert who visited EEStor to evaluate the technology for potential investors, stated he was very skeptical because of current leakage typically seen at high voltages and because there should be microfractures from temperature changes. He stated "I'm surprised that Kleiner has put money into it."[24] Andrew Burke, another expert who visited EEStor stated the consensus among experts was that the dielectric constant could not stay as high as claimed at the voltage levels claimed (i.e. energy density could not be that high). He stated EEStor would not provide any data to contradict the experts.[25]

Despite going into talks with numerous clients, including GM, Lockheed Martin, LightEV, Venture Vehicles, Zenn, and a number of others, no EESU prototypes have been publicly independently tested or acknowledged by anyone outside the company.

Even if the technology were to be proved sound, doubts remain regarding its potential safety and real-world applicability. High energy density, an order of magnitude higher than best commercially available supercapacitors and 2-3 times higher than that of lithium-ion batteries, combined with the mechanism of action that depends on maintaining potential differences of thousands of volts across micrometer distances, suggest that EESU capacitors could be prone to catastrophic meltdowns; in other words, external damage to the unit (say, in a traffic accident), that results in an internal short could instantaneously release all stored energy (52 kwh in a fully charged standard unit - the equivalent of 45 kg of TNT), evaporating the unit and causing severe damage to everything in its proximity.

CarloSW2