Non-monotonic Surface Charging Behavior of Platinum: A Paradigm Change

In electrochemical systems, metal surface charging phenomena dictate the strength of electrostatic interactions between the electrified electrode and ions in solution. These effects are of vital importance for electrochemical reactions in general. Historically, the potential of zero charge (pzc) of the metal has been employed to parametrize the surface charging relation. The structural model of the electrified interface presented in this article goes beyond the oversimplified pzc concept by accounting for the formation of surface oxide and the orientational ordering of interfacial water molecules. The analytical solution of the model reveals a peculiar non-monotonic charging behavior. The Pt surface exhibits a negative effective charge in a low potential region, a positive charge in an intermediate potential region, and a negative charge in a high potential region due to surface oxide dipoles. This non-monotonic behavior is in agreement with a seminal experimental work of Frumkin and Petrii [Electrochim. Acta 1975, 20, 347-359] that had remained hitherto unexplained.