www.advenergymat.de FULL PAPER 1700835 (1 of 10) © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim The Space Confinement Approach Using Hollow Graphitic Spheres to Unveil Activity and Stability of Pt-Co Nanocatalysts for PEMFC Enrico Pizzutilo, Johannes Knossalla, Simon Geiger, Jan-Philipp Grote, George Polymeros, Claudio Baldizzone, Stefano Mezzavilla, Marc Ledendecker, Andrea Mingers, Serhiy Cherevko, Ferdi Schüth,* and Karl J. J. Mayrhofer* DOI: 10.1002/aenm.201700835 1. Introduction Global climate changes and the high dependence on conventional energy sources based on fossil fuels demand a transition toward renewable energy sources. [1] We are already witnessing a gradual shift toward efficient harvesting of energy from wind and sun. One of the main challenges is to provide reliable energy conversion and storage systems to overcome the mismatch between supply and demand, an inherent problem of intermittent renewable energy. In this context, proton exchange membrane fuel cells (PEMFC) are promising and effi- cient energy conversion devices for mobile and stationary applications. [1a,2] However, in order to constitute a valid alternative, PEMFCs require efficient catalysts for the anodic hydrogen oxidation reaction (HOR) as well as for the cathodic oxygen reduction reaction (ORR). [3] Despite many studies on non-noble materials, [4] platinum remains the most active catalyst and is therefore con- sidered as the state-of-the-art electrocatalyst for the sluggish ORR. [1a,5] Hence, current research efforts are directed toward minimization of its use in PEMFCs. Common approaches to reduce the high catalyst costs, which limit in practice the broad commercialization of PEMFCs, consist in (i) producing finely dispersed Pt nanoparticles, thus enhancing the electrochemical surface area (ECSA) [6] and/or in (ii) alloying Pt with less noble 3d-transition metals (i.e., Fe, Co, Ni, Cu, Cr, Mn), [7] thus, simul- taneously reducing costs and enhancing the intrinsic ORR activity. [8] Such improvement in performance of Pt-based alloys (Pt-M) can be ascribed to an interplay of ensemble, ligand, and strain effects (also generally known as alloying effects). [9] Indeed, the binding energies of the intermediates (OOH, OH, etc.) are modified, often resulting in an increased activity (reduction of ORR overpotential). [10] Generally, under operation conditions the outer surface layer of Pt-alloy catalysts is composed of pure Pt, since the less noble metal is being leached out under the typically acidic conditions. [11] Various synthesis methods for bime- tallic alloys have been exploited to achieve a pure outermost The performance of polymer electrolyte fuel cells is strongly correlated to the electrocatalytic activity and stability. In particular, the latter is the result of an interplay between different degradation mechanisms. The essential high stability, demanded for real applications, requires the synthesis of advanced electrocatalysts that withstand the harsh operation conditions. In the first part of this study, the synthesis of oxygen reduction electrocata- lysts consisting of Pt-Co (i.e., Pt 5 Co, Pt 3 Co, and PtCo) alloyed nanoparticles encapsulated in the mesoporous shell of hollow graphitic spheres (HGS) is reported. The mass activities of the activated catalysts depend on the initial alloy composition and an activity increase on the order of two to threefold, compared to pure Pt@HGS, is achieved. The key point of the second part is the investigation of the degradation of PtCo@HGS (showing the highest activity). Thanks to pore confinement, the impact of dissolution/dealloying and carbon corrosion can be studied without the interplay of other degra- dation mechanisms that would induce a substantial change in the particle size distribution. Therefore, impact of the upper potential limit and the scan rates on the dealloying and electrochemical surface area evolution can be examined in detail. Hollow Graphitic Spheres E. Pizzutilo, S. Geiger, Dr. J. P. Grote, G. Polymeros, Dr. C. Baldizzone, Dr. S. Mezzavilla, Dr. M. Ledendecker, A. Mingers, Dr. S. Cherevko, Prof. K. J. J. Mayrhofer Department of Interface Chemistry and Surface Engineering Max-Planck-Institut für Eisenforschung GmbH Max-Planck-Strasse 1, 40237 Düsseldorf, Germany E-mail: mayrhofer@mpie.de J. Knossalla, Prof. F. Schüth Department of Heterogeneous Catalysis Max-Planck-Institut für Kohlenforschung Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany E-mail: schueth@kofo.mpg.de Dr. S. Cherevko, Prof. K. J. J. Mayrhofer Forschungszentrum Jülich GmbH Helmholtz Institute Erlangen-Nürnberg for Renewable Energy (IEK-11) Egerlandstr. 3, 91058 Erlangen, Germany Prof. K. J. J. Mayrhofer Department of Chemical and Biological Engineering Friedrich-Alexander-Universität Erlangen-Nürnberg Egerlandstr. 3, 91058 Erlangen, Germany Adv. Energy Mater. 2017, 1700835