Double-layer capacitance peaks: Origins, ion dependence, and temperature effects

Differential capacitance (Cdl) is arguably the most important lumped parameter of electrical double layers (EDLs). Two peaks in the Cdl profile have been commonly attributed to the crowding of counterions within the EDL. More recent studies have suggested that the two peaks are primarily caused by orientational polarization of interfacial water molecules. Herein, this recent perspective is extended by considering orientation-dependent adsorption free energy of water and tested at Au(111)-aqueous solution interfaces. Our comparative analysis of the ion dependency of the Cdl profile corroborates the view that the capacitance peaks are caused mainly by the saturation of the orientational polarization of interfacial water molecules. In addition, the temperature dependency of the Cdl profile is consistently interpreted as a consequence of the temperature effects on the orientational polarization of interfacial water.