The energy formula

A capacitor stores E = ½CV², with C in farads and V in volts giving joules; divide by 3,600 for watt hours. Carbon is usually rated by specific capacitance measured on one electrode. In a symmetric two electrode cell the two electrodes act in series, so the cell's capacitance is the specific capacitance times the total carbon mass divided by 4, which is the convention this model uses.

With 140 F/g and 5 g per electrode, the cell is about 350 F. At 2.7 V that is roughly 1,300 J, or about 35 Wh per kilogram of carbon. Packaged devices land several times lower because the case, collectors, separator and electrolyte add weight; the hemp nanosheet device in the University of Alberta study reached about 12 Wh/kg.

What the hemp studies found

Researchers at the University of Alberta turned hemp bast fiber into interconnected carbon nanosheets 10 to 30 nm thick with surface areas up to about 2,300 m²/g, reporting about 106 to 144 F/g in an ionic liquid from 0 to 100 °C, even at very fast charge rates. A later study made KOH activated carbons from both hemp hurd and bast and found hurd the better precursor, reaching about 160 F/g.

This model treats capacitance as a fixed number. Real carbons lose some capacitance at high charge rates and low temperature, and the ion motion shown here is slowed far down so you can see it.