© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 wileyonlinelibrary.com www.MaterialsViews.com www.advenergymat.de FULL PAPER Vasiliki Papaefthimiou, Maxim Shishkin, Dimitris K. Niakolas, Michalis Athanasiou, Yeuk Ting Law, Rosa Arrigo, Detre Teschner, Michael Hävecker, Axel Knop-Gericke, Robert Schlögl, Tom Ziegler, Stylianos G. Neophytides, and Spyridon Zafeiratos* On the Active Surface State of Nickel-Ceria Solid Oxide Fuel Cell Anodes During Methane Electrooxidation 1. Introduction Fuel cells are devices that directly convert fuels into electricity. Solid oxide fuel cells (SOFCs) are among the most attractive fuel cell types because of their high effi- ciency and fuel flexibility. Hydrogen or hydrocarbon fuels are electrooxidized at the anode surface by mobile oxygen ions from the electrolyte, while the electrons produced flow to the external circuit per- forming work on their way to the cathode. Since hydrogen is mainly produced by the reforming of natural gas (containing up to 80% methane), there is consider- able interest in methane-fuelled SOFCs, so as to increase their overall efficiency. [1] The main drawback of this so-called “direct methane” operation is that the commonly used anodes based on yttria- stabilized zirconia (YSZ) are susceptible to carbon deposition. Incorporation of ceria (CeO 2 ), particularly gadolinium-doped ceria (GDC), limits carbon deposition and improves the overall cell performance. [2–4] Solid oxide fuel cells (SOFCs) have grown in recognition as a viable tech- nology able to convert chemical energy directly into electricity, with higher efficiencies than conventional thermal engines. Direct feeding of the SOFCs anode with hydrocarbons from fossil or renewable sources, appears more attractive compared to the use of hydrogen as a fuel. The addition of mixed oxide-ion/electron conductors, like gadolinium-doped ceria (GDC), to commonly used nickel-based anodes is a well–known strategy that signifi- cantly enhances the performance of the SOFCs. Here we provide in situ experimental evidence of the active surface oxidation state and composi- tion of Ni/GDC anodes during methane electroxidation using realistic solid oxide electrode assemblies. Ambient pressure X-ray photoelectron and near edge X-ray absorption fine structure spectroscopies (APPES and NEXAFS respectively) combined with on line electrical and gas phase measurements, were used to directly associate the surface state and the electrocatalytic performance of Ni/GDC anodes working at intermediate temperatures (700°C). A reduced anode surface (Ce 3+ and Ni), with an optimum Ni to Ce surface composition, were found to be the most favorable configuration for maximum cell currents. Experimental results are rationalized on the basis of first principles calculations, proposing a detailed mechanism of the cell function. DOI: 10.1002/aenm.201200727 Dr. S. Zafeiratos LMSPC-UMR 7515 CNRS-ECPM-Université de Strasbourg 25, rue Becquerel F 67087 Strasbourg Cedex 2, France E-mail: spiros.zafeiratos@unistra.fr Dr. V. Papaefthimiou, Y. T. Law LMSPC-UMR 7515 CNRS-ECPM-Université de Strasbourg 25, rue Becquerel F 67087 Strasbourg Cedex 2, France Dr. M. Shishkin, Prof. T. Ziegler Department of Chemistry University of Calgary University Drive 2500 Calgary, Alberta T2N 1N4, Canada Dr. D. K. Niakolas, M. Athanasiou, Dr. S. G. Neophytides FORTH/ICE-HT, GR-26504 Rion (Patras), Greece Dr. R. Arrigo, Dr. D. Teschner, Dr. M. Hävecker, Dr. A. Knop-Gericke, Prof. R. Schlögl Fritz-Haber-Institut der MPG Faradayweg 4-6, D-14195 Berlin (Dahlem), Germany Dr. M. Hävecker Helmholtz-Zentrum Berlin/ BESSY II Albert-Einstein-Str. 15, 12489 Berlin, Germany Adv. Energy Mater. 2013, DOI: 10.1002/aenm.201200727