At a planar platinum surface, triiodide (I3 -) forms by electrochemically oxidizing iodide (I -) to iodine (I2), followed by comproportionation of I -and I2 . In aqueous hybrid supercapacitors and iodine batteries, the reaction occurs in the nanopores of positively polarized carbon electrodes . Confinement in such electrodes may change rates of individual reaction steps and hence the stability of I2 and I3 -. While the polyiodides I3 - and I5 -are generally accepted to form during I -oxidation, described mechanisms are ambiguous . The rather slow self-discharge of iodine-based electrochemical energy storage devices is currently attributed to immobile I3 -and I5 -confined in the narrow carbon pores of below 1 nm diameter . However, based on the known reaction mechanism on planar platinum electrodes and the use of battery electrodes with physically impregnated I2 , alternative electrodeposition of solid I2 in carbon nanopores ought to be considered.