I don't want to sound negative, but 6kw (about 8bhp) isn't going to accelerate anthing very fast. If, with two adults and some luggage on board, this thing weighs (say) 500kg (assuming the ampty vehicle is about 320kg), then at 30mph that's almost 90KJ of kinetic enery, or about 15 seconds of acceleration at 6kw even with 100% efficiency and neglecting losses due to friction, drag and so on.
So I very much hope that those capacitors have got the means to provide considerably more than 6kw peak power output, or we have something much like a G-Wiz in performance, albeit it considerably prettier.
The peak output of bettery powered cars is something of an advantage in acceleration, not to mention the abilitiy to store off-peak and intermittent green power sources. Of course that can be done to generate hydrogen too, but the end-to-end thermodynamic efficiency of the hydrogen cycle is atrocious. By the time that losses in electrolysis and compression are taken into account, you'll be lucky to see 30% of that green electricity delivered at the wheels vs perhaps 80% using batteries.
Anyway, not a bad idea as a city run-about and it does get over the range and kerbside charging issues of battery cars (assuming refill stations are available). However, surely nobody is going on a long journey with one of these. Show it a moderately steep hill and, two-up, you are going to be lucky to see 15-20mph. 6kw is enough to lift one of these things just over a metre per second. On a one-in-ten that equates to perhaps 25mph at most two-up. If it's one-in-six then halve that...