A cyclic voltammogram (CV) of HC-PB within the operando XAS cell, in 0.1 M KNO 3 (aq) electrolyte, is shown in Figure . The reduction and oxidation peaks at 0.034 V and 0.22 V are due to the transition of PB to PW (and vice versa). The reduction peak suggests intercalation of K + ion into the HC-PB lattice although, as noted previously, contributions from H + or OH -cannot be excluded (see Scheme 1). The observation of an oxidation peak at 0.22 V, with an identical area and height to the reduction peak, indicates the deintercalation of K + ion as well as the quasi-reversibility of the process. The 186 mV peak separation suggests that the rate of intercalation and deintercalation of the hydrated K + ion in and out of the HC-PB lattice is slow and Nernstian concentration is not maintained throughout these processes. The peaks corresponding to the transition of BG to PB (and vice versa) are at 0.88 V and 0.73 V, respectively, and appear to be quasi-reversible with peak-to-peak separation of 150 mV.