petro based fuel tank = energy container
electrical battery = energy container
which one is a problem when contained energy is expended?
.
Both
Yes, last time I checked, they had about the same energy density as lithium batteries, but the problem is their "effective series resistance," which at least was pretty high, I haven't checked in recent years.
ESR made it difficult to get energy out of the supercap quickly, and you paid a high price in efficiency for charging and discharging.
The good thing about them is that they charge much faster than lithium batteries.
I think some Bluetooth earpieces use supercaps instead of batteries, but I'm not sure.