The detected amount of solid I2 produced in the carbon nanopores during voltage cycling is quite remarkable, with a specific capacity reaching 200 mAh gC -1 . To gauge the maximum amount of iodine possible to be electrodeposited in the positive AC electrode, neither the negative AC electrode capacitance nor total I -in the system must limit iodine electrodeposition. This is realized with a large, about 100 times oversized AC counter electrode and a large electrolyte volume (details in Methods, cyclic voltammetry in Supplementary Fig. ).