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pH and Alkali Cation Effects on the Pt Cyclic Voltammogram Explained Using Density Functional Theory

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journal contribution
posted on 14.01.2016, 00:00 by Ian T. McCrum, Michael J. Janik
Platinum electrode cyclic voltammograms show features at low potentials which correspond to adsorption/desorption processes on Pt(111), Pt(100), and Pt(110) facets that have traditionally been ascribed to hydrogen adsorption. The 100 and 110 associated features exhibit a dependence on pH beyond the expected Nernstian shift. Herein we use density functional theory (DFT) to explain these shifts. We examine the specific adsorption of hydrogen, hydroxide, water, and potassium onto the low index facets of platinum, Pt(111), Pt(100), and Pt(110). In support of a growing body of evidence, we show that the low potential features which correspond to adsorption/desorption on Pt(100) and Pt(110) contain contributions from the competitive or coadsorption of hydroxide. This allows us to simulate cyclic voltammograms for Pt(100) and Pt(110), as well as Pt(111), which match experimentally measured cyclic voltammograms in a pH = 0 electrolyte. Furthermore, we find that potassium cations can specifically adsorb to all three low index facets of platinum, weakening the binding of hydroxide. As potassium-specific adsorption becomes more favorable with increasing pH, this allows us to explain the measured pH dependence of these features and to simulate cyclic voltammograms for the three low index facets of platinum which match experiment in a pH = 14 electrolyte. This has significant implications in catalysis for hydrogen oxidation/evolution, as well as for any electrocatalytic reaction which involves adsorbed hydroxide.