No abstract

The successful deployment of advanced energy-conversion systems depends critically on our understanding of the fundamental interactions of the key adsorbed intermediates (hydrogen *H and hydroxyl *OH) at electrified metal-aqueous electrolyte interfaces. Herein, the effect of alkali metal cations (Li+, Na+, K+ and Cs+) on the non-Nernstian pH shift of the step-related voltammetric peak of the Pt(553) electrode is investigated over a wide pH window (1 to 13) by means of experimental and computational methods. Our results show that the co-adsorbed alkali cations along the step weaken the OH adsorption at the step sites, causing a positive shift of the potential of the step-related peak on Pt(553). Density functional theory calculations explain our observations on the identity and concentration of alkali cations on the non-Nernstian pH shift, and demonstrate that cation-hydroxyl co-adsorption causes the apparent pH dependence of “hydrogen” adsorption in the step sites of platinum electrodes.

The ubiquity of aqueous solutions in contact with charged surfaces and the realization that the molecular-level details of water-surface interactions often determine interfacial functions and properties relevant in many natural processes have led to intensive research. Even so, many open questions remain regarding the molecular picture of the interfacial organization and preferential alignment of water molecules, as well as the structure of water molecules and ion distributions at different charged interfaces. While water, solutes and charge are present in each of these systems, the substrate can range from living tissues to metals. This diversity in substrates has led to different communities considering each of these types of aqueous interface. In this Review, by considering water in contact with metals, oxides and biomembranes, we show the essential similarity of these disparate systems. While in each case the classical mean-field theories can explain many macroscopic and mesoscopic observations, it soon becomes apparent that such theories fail to explain phenomena for which molecular properties are relevant, such as interfacial chemical conversion. We highlight the current knowledge and limitations in our understanding and end with a view towards future opportunities in the field.

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.