In-situ electrochemical atomic force microscopy study of aging of magnetron sputtered Pt-Co nanoalloy thin films during accelerated degradation test Ivan Khalakhan a, *, Mykhailo Vorokhta a , Michal Václavu  a , B  retislav Šmíd b , Jaroslava Lavková a,c , Iva Matolínová a , Roman Fiala a , Nataliya Tsud a , Tomáš Skála a , Vladimír Matolín a a Charles University in Prague, Faculty of Mathematics and Physics, Department of Surface and Plasma Science, V Holešovi9 ckách 2, 18000 Prague 8, Czech Republic b National Institute for Materials Science, Fuel Cell Materials Group, Battery Materials Unit, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan c ICB—Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR 6303 CNRS-Université de Bourgogne, 9 Av. A. Savary, BP 47870, F-21078 Dijon Cedex, France A R T I C L E I N F O Article history: Received 13 January 2016 Received in revised form 6 June 2016 Accepted 6 June 2016 Available online 7 June 2016 Keywords: electrochemical AFM thin film Pt-Co cyclic voltammetry oxygen reduction reaction A B S T R A C T A Pt-Co nanoalloy thin film catalyst was prepared by using simultaneous magnetron sputtering of Pt and Co. The catalyst was characterized during accelerated degradation test using in-situ electrochemical atomic force microscopy complemented with ex-situ techniques such as energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy and synchrotron radiation photoelectron spectroscopy. The combined results gave the full step-by-step picture of the catalyst behavior during the aging test. ã 2016 Elsevier Ltd. All rights reserved. 1. Introduction The proton exchange membrane fuel cell (PEMFC) is one of the most promising and challenging subjects of research because it is expected to become a major source of clean energy [1]. To date, the major obstacles of PEMFC commercialization are the slow kinetics of oxygen reduction reaction (ORR), as well as high cost of Pt, which still remains the only suitable catalyst for commercial exploitation. Therefore, it is critical to overcome these limitations by reducing Pt loading and increasing reaction kinetics. Over the past years, studies have focused on alloying platinum with different transition metals in attempts to promote the activity of traditionally employed pure platinum catalyst. It has been shown that alloying with Co, Ni, Fe can improve catalytic activity and stability of pure Pt toward ORR and, hence, reduce the catalyst cost [2–7]. Pt 3 Co was found to be one of the beneficial candidates [3,8,9]. However, general consensus concerning applicability of Pt-Co alloys as efficient PEMFC cathode electrocatalysts has not been achieved. Contradictory conclusions have been published, mainly due to low stability of the most widely used PtCo/C electrocatalysts [10–12]. The high activity of such bimetallic catalyst was explained mainly by electronic effects (the d-band shift of Pt-Co relative to that of pure Pt leading to an increase of ORR activity due to lower O binding energy [8]) and/or geometrical modifications (alloying causes a lattice contraction, leading to a Pt-Pt distance more favorable for the dissociative adsorption of oxygen [13]). In contrast, however, Nikkuni et al. reported a decrease of the ORR activity of Pt-Co/C catalysts submitted to electrochemical aging tests depending on stepping potential conditions [12]. Overall, the process of electrochemical dissolution of Co from a Pt-Co alloy surface can lead to the formation of a thin layer of Pt covering bulk alloy, so-called Pt core-shell or Pt skin alloy catalysts. This process has attracted a great attention of researchers intrigued by its high complexity and attracted by the potential impact on new catalyst tailoring. Despite intensive work in this field, the search for a cheap, efficient and stable bimetallic catalyst for ORR has not been completed. So far, a large number of studies reported Pt-Co alloy nanoparticles prepared by wet techniques. Although the number of publications devoted to PtCo materials obtained by physical deposition methods is relatively small [14–17], it has been * Corresponding author. E-mail address: khalakhan@gmail.com (I. Khalakhan). http://dx.doi.org/10.1016/j.electacta.2016.06.035 0013-4686/ã 2016 Elsevier Ltd. All rights reserved. Electrochimica Acta 211 (2016) 52–58 Contents lists available at ScienceDirect Electrochimica Acta journal homepa ge: www.elsev ier.com/locate/electacta