Free-standing carbon electrodes were prepared by mixing 90 mass% of the microporous KOH-activated carbon (MSP-20, Kansai Coke and Chemicals, Supplementary Fig. , Supplementary Table ) with 5 mass% carbon black (C65 from Imerys), and 5 mass% of polytetrafluoroethylene (60% dispersion in water from 3M Chemicals) in ethanol. After stirring at +70°C, the obtained dough was pressed and rolled on a glass plate into a thin sheet. Disc electrodes were punched from the sheet and dried at 120°C, resulting in a final thickness of about 150 μm. Sodium iodide (NaI, 99.5%) was purchased from Alfa-Aeser and dried at 110°C overnight before preparing the aqueous 1.0 M NaI electrolyte with de-ionized water. The pH of the electrolyte was 6.5 and the conductivity 82 mS cm -1 .