Three-electrode Swagelok-type cells with a Ag/AgCl reference electrode were assembled for cyclic voltammetry (Supplementary Fig. ) and to test the maximum amount of iodine that can be electrodeposited in the positive AC electrode (potentiostatic charge/discharge, Fig. ). We used an AC working electrode (diameter = 3 mm) with a mass of 0.3 mg, an AC counter-electrode (diameter = 10 mm) with a mass of 30 mg and about 250 µl electrolyte (three Whatman separators with 10 mm in diameter). This ensures that neither the total amount of I -in the cell nor the double-layer capacitance of the counter-electrode is limiting the I2 uptake in the AC WE pores.