Potassium K-edge XANES of HC-PB were recorded at voltages of 0.41 V during charging, and 0.9 V and -0.1 V during discharge (indicated on the CV in Figure , with spectra shown in Figure ). At 0.41 V, the spectrum was comparable with that recorded for PB prior to the addition of electrolyte (Fig S9 ) with notable features at 3610.1 eV (pre-edge), 3612.3 eV (white line shoulder), 3614.3 eV (white line), 3617.7 eV and 3619.8 eV (small doublet peaks after white line), and extended features beyond 3625 eV. Significantly, the XANES spectra of the KNO 3(aq) electrolyte alone is clearly distinguishable, through a more intense pre-edge feature, and a broad single structure centered at 3616.7 eV. The observations show the potential of operando K-edge potassium XANES, as the spectrum of K + within the cathode and electrolyte can be easily differentiated. At 0.9 V (discharge), the spectrum of the cathode is comparable to the electrolyte with residual features associated with PB, showing that almost all K + has been deintercalated from the now oxidized BG structure. The result suggests that K + is acting as a charge carrier under the conditions monitored and can be fully deintercalated from the structure. Lastly, holding at -0.1V resulted in a 3 rd XANES spectrum, which is associated with the fully reduced Fe 2+ Fe II PW structure, with full K + intercalation. The features of the XANES spectrum for the postulated PW structure differ from that of PB, with an enhanced pre-edge feature, a change in intensity ratios of the split main feature, and emergence of another strong shoulder at 3616.3 eV.