In situ Raman data were measured during cell voltage sweep between -0.25 V and +0.55 V at a scan rate of 0.32 mVs -1 (Fig. ). At negative voltages, the cell shows pure double-layer capacitance, with a current limited by the capacitance of the positive electrode. Upon positive polarization, I -is oxidized at the positive electrode with a significantly enhanced current, which is limited by the capacitive current of the oversized negative electrode. The negative current of virtually the same magnitude during negative polarization above 0 V indicates a high coulombic efficiency of the reversible Faradaic iodide/iodine reaction.