show an increasing I5 -to I3 -band intensity ratio with increasing cell voltage. Hence, relative to I3 -, I5 -forms at accelerated rates at higher cell voltages. To check whether I2 had formed (next to I3 -and I5 -), we soaked charged (positively polarized) and washed AC electrodes for several minutes in either pure water or 1 M NaI and recorded ex situ UV-vis spectra (Fig. , details in the Methods). While pure water would leave I2 confined to the AC nanopores, it should be dissolved in the 1 M NaI solution via I + I ⇌ I . Indeed, the 1 M NaI solution turned brownish, while pure water remained colorless (Supplementary Fig. ). The UV-vis spectra in Fig. confirm a large quantity of I3 -in the 1 M NaI solution and hence the presence of confined I2 in the charged positive AC electrode . As an independent proof, we detected iodine after immersing the charged positive AC electrode for several minutes in ethanol, which directly dissolves I2 (Supplementary Fig. ) .