As before, we use an asymmetric two-electrode cell with oversized negative electrode and ran cell voltage sweeps with the scan rate controlled by the total cell voltage. The cell exhibits pure double-layer capacitance limited by the capacitance of the positive AC electrode at cell voltages below +0.2 V (Fig. ). Above this voltage, I -is oxidized at the positive AC electrode, leading to a significantly enhanced current (in line with electrochemical data of the in situ Raman measurements). The X-ray transmission of the electrolyte-filled positive AC electrode was simultaneously recorded by a photodiode placed behind the in situ cell, and quantifies the material flux in and out of the positive AC electrode (Fig. ). The significant increase of the Xray attenuation (details in Ref. ) indicates the accumulation of large amounts of iodide, iodine, and polyiodides within the AC pores. The relative scattering intensity as a function of time follows the applied voltage and resulting current (Fig. ). During positive polarization, the scattering intensity in the SAXS regime at q < 5 nm -1 and in the WAXS regime has a clear maximum at the peak voltage.