Anion-adsorption-related frequency-dependent double layer capacitance of the platinum-group metals in the double layer region

The interfacial impedance for platinum-group metals in simple binary electrolytes, in their so-called double layer region, is generally assumed to be ideally capacitive. Using single crystalline platinum-group metals with various solutions we demonstrate that this is not necessarily the case when the anion of the solute is specifically adsorbed on the electrode. We attribute the frequency dependence of the interfacial capacitance to the relatively slow exchange of anions between the outer and inner Helmholtz planes.