Core-shell electrocatalysts are of interest for the oxygen reduction reaction in proton exchange membrane fuel cells as enhancements in activity have been reported compared to the traditionally-used Pt/C electrocatalysts. To fully understand the origin of the activity of these samples, and how the structure changes in the electrochemical environment, full in situ structural characterisation is required. The aim of this work was to develop and apply in situ synchrotron based X-ray characterisation techniques (EXAFS, WAXS, SAXS and ASAXS) to study a set of well-defined Pd-core, Pt-shell electrocatalysts with 0.5, 1, 2 and 4 monolayers (ML) of Pt, with the primary motivation of determining the advantages and limitations of each technique when applied to materials of this nature.<br/><br/>Measurements were conducted in and ex situ to study the effect of Pt-shell thickness on the structure of the nanoparticles and the response to applied electrode potential. Catalyst electrodes subjected to accelerated ageing tests were also characterised to investigate the mechanisms of degradation responsible for an observed loss in electrochemical surface area, with the dominant mechanism found being dissolution for the core-shell catalysts with ? 2 ML shell and coalescence (Ostwald ripening) for the 4 ML.<br/><br/>EXAFS was found to be the most useful of the four techniques owing to the detailed structural information obtainable and the relative ease of data collection. The SAXS data provided information not obtained from the EXAFS, and combined with electrochemical measurements, enabled determination of the mechanisms of degradation. WAXS provided complementary information to the EXAFS. Attempts to fit the ASAXS data with a simplistic model were unsuccessful, thus, without the application of more complex theoretical models, little additional information could be obtained from this technique
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