Supercapacitor performance varies with temperature
Like a hydrogen storage tank, it would need to be spherical, insulated, and temperature controlled, to ensure that it worked as expected thru the full range of -40 to +40 degrees Celsius for automotive. But I'd like to see such a thing work as stated under any conditions. It would be interesting. Of course, whatever powerplant you fancy, be it fuel cell, battery, supercapacitor, or good 'ol internal combustion, they will all use less energy to get you places if the car weighs only a few hundred pounds, instead of the typical 3000 pounds of today's cars.