DOI: 10.1002/celc.201300107 Position of Cu Atoms at the Pt(111) Electrode Surfaces Controls Electrosorption of (H)SO 4 (2)À from H 2 SO 4 Electrolytes Jakub Tymoczko, [a, b] Wolfgang Schuhmann, [a, b] and Aliaksandr S. Bandarenka* [a] 1. Introduction Adsorption of electrolyte components involving charge trans- fer through the electrode/electrolyte interface is one of the most frequently studied electrochemical phenomena. [1–5] Un- derstanding and control over electrosorption processes are of growing significance in physical chemistry and electrochemis- try due to a variety of reasons. Nowadays, those are mainly re- lated to challenges in the development and optimization of electrochemical energy conversion devices, [6–9] sensors [10] and advanced electroanalytical methods. [11–18] All these relatively di- verse research areas have similar requirements. For instance, rates of electrocatalytic reactions in fuel cells or electrolyzers are essentially governed by a balance between adsorption en- ergies for reaction intermediates and electrolyte components at the electrode surface. [19–23] However, to be able to optimize existing or develop new electrocatalysts, binding energies for the adsorbed intermediates often have to be adjusted by a pre- dictable tailoring of the electronic structure of a given sur- face. [8, 9, 24] On the other hand, in many cases, specific electro- sorption of electrolyte components in itself can be used as a sensitive means to probe various surface properties. [25–29] Therefore, deep understanding on how to predictably change the surface adsorption properties and how to accurately assess them are the key to design new functional materials and de- velop new physico-chemical methodologies. In electrochemical surface science, and particularly in elec- trocatalysis, experiments using single-crystal electrodes provide a wealth of key information about how the structure and com- position of electrode surfaces relate to their adsorption charac- teristics. [30–33] Among these systems, the interface between Pt(111) single-crystal surfaces and aqueous acidic electrolytes containing adsorbing/desorbing (bi)sulfate anions is one of few relatively well-understood model objects where new concepts and methodologies in electrode surface modification and char- acterization can be initially tested. [34] Additionally, electrocata- lytic reactions in many important energy conversion devices, such as fuel cells, are significantly influenced by competitive adsorption of (bi)sulfates (for instance, the proton-conducting Nafion or polybenzimidazole membranes in polymer electro- lyte membrane fuel cells are often pretreated in H 2 SO 4 solu- tion). Consequently, better understanding of what determines the adsorption properties of Pt electrodes and how these properties can be predictably tailored are of fundamental and practical importance. In this work, we demonstrate that monolayer and sub-mon- olayer amounts of metallic Cu located at different positions rel- ative to the topmost surface layer of Pt(111) electrode controls the adsorption of (bi)sulfate anions. In these systems, Cu has been selectively placed as pseudomorphic overlayers, surface Selective positioning of monolayer amounts of foreign atoms at the surface and subsurface regions of metal electrodes is a promising way to fine-tune the properties of the electrode/ electrolyte interface. The latter is critical as it largely governs the adsorption of electrolyte components and reaction inter- mediates and, therefore, controls many key electrocatalytic processes. Using model Pt(111) single-crystal electrodes, we demonstrate how the relative position of Cu atoms at the sur- face drastically changes the adsorption energies for (bi)sulfate anions. Our measurements involve pseudomorphic overlayers of Cu on Pt(111) as well as Cu–Pt(111) surface and sub-surface alloys, where Cu atoms were located either in the first or in the second atomic layers of Pt, respectively. In the case of Cu– Pt(111) surface alloys, specific adsorption of the anions starts earlier compared to the unmodified Pt(111) surface. In contrast, placing Cu atoms into the second atomic layer weakens the binding between the surface and the anions. Surprisingly, Cu pseudomorphic overlayers do not reveal any specific adsorp- tion of (bi)sulfates (within the region of the overlayer stability). Taking into account that electrified interfaces between Pt(111) electrodes and sulfate-containing electrolytes often play the role of benchmark systems in fundamental physico-chemical and, particularly, electrocatalytic studies, our findings demon- strate a promising and relatively easy route of tuning the prop- erties of these interfaces. [a] J. Tymoczko, Prof. W. Schuhmann, Dr. A. S. Bandarenka Center for Electrochemical Sciences (CES) Ruhr-Universität Bochum Universitätsstr. 150 44780 Bochum (Germany) E-mail : aliaksandr.bandarenka@rub.de [b] J. Tymoczko, Prof. W. Schuhmann Analytische Chemie, Elektroanalytik & Sensorik Ruhr-Universität Bochum Universitätsstr. 150 44780 Bochum (Germany)  2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemElectroChem 0000, 00,1–8 &1& These are not the final page numbers! ÞÞ CHEMELECTROCHEM FULL PAPERS