As of 2018 it's tested:
> By using small single layer cells charged to 1.5 volts for two to five minutes and then run demonstration devices, including a small fan.
> By using a three-cell series stack that is able to be rapidly charged to five volts and operate an LED.
https://www.surrey.ac.uk/mediacentre/press/2018/alternative-...
(Link first posted here by BugsJustFindMe. Also says "if these values of capacitance can be achieved in production, it could potentially see supercapacitors achieving energy densities of up to 180whr/kg - greater than lithium ion batteries.")
