DOI: 10.1002/cctc.201100343 Oxygen Electroreduction Activity and X-Ray Photoelectron Spectroscopy of Platinum and Early Transition Metal Alloys Ifan E. L. Stephens, [a] Alexander S. Bondarenko, [a, b] Lone Bech, [a] and Ib Chorkendorff* [a] Introduction Today, state of the art polymer electrolyte membrane fuel cells (PEMFCs) use Pt catalysts at their electrodes. The high cost of these electrocatalysts is one of the greatest impediments to the widespread use of emission-free vehicles powered by PEMFCs. [1] Therefore, the Pt loading needs to be reduced sig- nificantly at the cathode, at which the oxygen reduction reac- tion (ORR) takes place. Arguably, the most viable route toward reducing the Pt loading in PEMFCs would be to increase the activity of Pt by alloying it with other metals. [1a, b, 2] Recent estimates suggest that the commercialisation of PEMFCs could be viable if Pt alloy catalysts exhibited a four- to eightfold increase in activity over pure Pt. [1a, b] Alloys of Pt and late transition metals, such as PtNi x , PtCo x , PtFe x and dealloyed PtCu x , have been tested ex- tensively for the ORR in PEMFCs. [2, 3] These catalysts exhibit sig- nificant improvements in activity over pure Pt. However, the activity tends to degrade, over long periods of time, especially at higher (more anodic) potentials. The alloy catalyst degrada- tion occurs as a result of the segregation of the solute compo- nent (e.g., Ni, Co, Fe or Cu) towards the surface, which causes it to dissolve into the electrolyte. [2–4] The susceptibility of these compounds to dealloying is perhaps unsurprising, given that their thermodynamic stability is negligible. [5] In our previous study, we discovered two new catalysts for oxygen reduction, Pt 3 Y and Pt 3 Sc, as the outcome of a theoreti- cal screening study. [5] These catalysts were tested experimen- tally in bulk polycrystalline form and exhibited the highest ac- tivity to date for surfaces prepared in this manner. [6] Only single-crystal Pt 3 Ni(111) showed a slightly higher ORR activity. [6] Not only do Pt 3 Y and Pt 3 Sc show promising activity, but also their exceptionally negative heat of formation may stabilise them against corrosion in PEMFCs, despite the very low disso- lution potentials of Y and Sc. [7] Such high thermodynamic sta- bility is characteristic of bimetallic alloys consisting of both early and late transition metals. [5, 8] This can be understood on the basis that the bonding states of the d band of these alloys are full whereas the antibonding states are empty. Jong Yoo et al. recently tested a range of different sputter- deposited thin films of Pt x Y 100x (64  x  91) as ORR catalysts. Their reported ORR activity for Pt 76 Y 24 ,(  28 mA cm 2 at 0.9 V) is the highest recorded for any metal surface. Moreover, they demonstrated that the catalyst was stable if cycled 3000 times between 0.6 and 1.1 V. [9] Their results confirm that this new alloy system holds promise for PEMFC applications. In the present study, we broaden the scope of our previous investigation of alloys of Pt and early transition metals. [5] We report the oxygen reduction activities of extended surfaces of polycrystalline Pt 2 Y, Pt 5 Y, Pt 3 Hf and Pt 3 Zr and compare them with those of Pt, Pt 3 Sc and Pt 3 Y. Our electrochemical measure- [a] Dr. I. E. L. Stephens, Dr. A. S. Bondarenko, Dr. L. Bech, Prof. I. Chorkendorff Center for Individual Nanoparticle Functionality (CINF) Technical University of Denmark (DTU) Building 312 Kongens Lyngby DK 2800 (Denmark) Fax: (+ 45) 45932399 E-mail : ibchork@fysik.dtu.dk [b] Dr. A. S. Bondarenko Center for Electrochemical Sciences Ruhr-Universität Bochum Universitätsstrasse 150 NC 4/73 44780 Bochum (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.201100343. The oxygen reduction reaction (ORR) was studied experimen- tally on sputter-cleaned, polycrystalline electrodes of Pt 3 Y, Pt 5 Y, Pt 2 Y, Pt 3 Sc, Pt 3 Hf, Pt 3 Zr and Pt under conditions relevant for low-temperature fuel cells. The surfaces were characterised in situ by means of electrochemical methods and ex situ under ultrahigh vacuum conditions. The ORR activity was established in an electrochemical cell containing 0.1 m HClO 4 by use of a rotating ring disk electrode assembly. The surface composition was characterised before and after the electrochemical meas- urements by using angle-resolved X-ray photoelectron spec- troscopy. The ORR activity of the electrodes increased in the following order: Pt 2 Y  Pt 3 Zr  Pt  < Pt 3 Hf < Pt 3 Sc ! Pt 5 Y < Pt 3 Y. At 23 8C, the most active catalyst, Pt 3 Y, exhibited a six- to nine- fold improvement in activity over Pt in the potential range 0.9–0.87 V with respect to a reversible hydrogen electrode. Over the same potential range at 60 8C, Pt 3 Y exhibited a four- to fivefold improvement in activity over Pt. The angle-resolved X-ray photoelectron spectroscopy analyses show that Pt 3 Y and Pt 5 Y formed a Pt overlayer under ORR conditions. In contrast, the surfaces of Pt 3 Hf and Pt 3 Zr comprised a mixture of Pt and HfO x or ZrO x , respectively, which explained their poor performance. ChemCatChem 0000, 00, 1 – 10  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim &1& These are not the final page numbers! ÞÞ