The interfacial capacitance of Rh(111) in HCl solutions

The impedance spectra of Rh(111) in aqueous HCl solutions have been measured as function of potential. In the double layer region, just as for other platinum group metals, the equivalent circuit of the interface is a circuit of a Rad–W–Cad element combination parallely connected to a Cdl capacitance; whereas at the H+/Cl− adsorbate exchange peak an additional parallel Rad,2–Cad,2 branch appears in the equivalent circuit. The (Rad–W–Cad) and the Rad,2–Cad,2 branches were demonstrated to be due to chloride and hydrogen adsorption, respectively. Due to the same anion appearing in the charging of Cad and Cdl the two branches are coupled to each other.