ORIGINAL PAPER The use of in situ X-ray absorption spectroscopy in applied fuel cell research Virginie Croze Æ Frank Ettingshausen Æ Julia Melke Æ Matthias Soehn Æ Dominic Stuermer Æ Christina Roth Received: 13 November 2008 / Accepted: 27 April 2009 / Published online: 13 May 2009 Ó Springer Science+Business Media B.V. 2009 Abstract For a detailed understanding and systematic optimization of fuel cell systems, in situ studies are an indispensable tool, as they provide information on the cat- alyst structure in different operation conditions. X-ray absorption spectroscopy (XAS) is in particular suitable for operando investigations, since it does not require ultra high vacuum conditions or long-range order in the sample. Fur- thermore, it provides in situ information on oxidation state, adsorbed species and catalyst structure, and thus comple- ments ex situ information, e.g. from X-ray diffraction (structure), X-ray photoelectron spectroscopy (oxidation state) and FTIR (adsorbates) nicely. In a spectroelectro- chemistry experiment, XAS can be combined with different electrochemical techniques in order to satisfy different needs and scientific aims. Spectra of both a Pt–Ru anode catalyst and a Pt–Co cathode catalyst were recorded at dif- ferent potentials, while measuring the current-potential characteristics of a single cell. So-called half-cell mea- surements, where the former fuel cell cathode was used with hydrogen as the reference electrode, were performed in water and ethanol to obtain a more detailed mechanistic insight into the ethanol electrooxidation. From a more industrial point of view, different catalysts were tested with a fast potential cycling protocol simulating rapid load changes in a vehicle. Keywords In situ Á Fuel cells Á X-ray absorption spectroscopy Á Spectroelectrochemistry Á Operando 1 Introduction Fuel cells are assumed to become a major component in tomorrow’s energy economy, as they offer the attractive perspective of efficient and clean energy conversion. High and intermediate temperature fuel cells will be used in stationary applications, while low temperature fuel cell systems are expected to provide solutions in the mobile and portable market segment. Among the main obstacles so far preventing the rapid market-introduction of low tempera- ture proton exchange membrane fuel cells (PEMFC) are their high cost and poor durability [1]. Nowadays, platinum is the catalyst material used at both the anode and the cathode side, and, unfortunately, high platinum loadings are needed to speed up the sluggish oxygen reduction reaction (ORR) contributing to the high cost of fuel cell systems [2]. A lot of effort has been spent on replacing Pt by less expensive, non-precious metal alternatives [3–5], but so far largely to no avail. Moreover, although platinum is comparatively stable in the harsh conditions of a PEM- FC, still a not negligible fraction of the platinum is leached out off the electrode and dragged into the membrane [6–9]. There it can re-precipitate and form huge crystals, which are no longer active in catalyzing the electrode reactions and thus lost for the fuel cell performance. Less noble metals, which are often applied as co-catalysts, e.g. Ru in the anode, are expected to show even less stability during long-term operation [10, 11]. V. Croze Á F. Ettingshausen Á D. Stuermer Á C. Roth (&) Renewable Energies, Institute for Materials Science, TU Darmstadt, Petersenstr. 23, 64287 Darmstadt, Germany e-mail: c_roth@tu-darmstadt.de J. Melke Fraunhofer Institute for Solar Energy Systems, Freiburg, Germany M. Soehn Renewable Energies, Institute for Electrical Power Systems, TU Darmstadt, Landgraf-Georg-Str. 4, 64283 Darmstadt, Germany 123 J Appl Electrochem (2010) 40:877–883 DOI 10.1007/s10800-009-9919-x