Periodic anti-phase boundaries and crystal superstructures in PtCu3 nanoparticles as fuel cell electrocatalysts

Abstract

To reduce the cost of the electrocatalyst for the oxygen reduction reaction in proton exchange membrane fuel cells, supported metal alloy nanoparticles are commonly used to decrease the amount of the scarce and expensive Pt. Since the structure of the nanoparticles has a crucial influence on the catalytic properties, advanced characterization methods help elucidate these relationships and ultimately enable more rational synthesis. This work focuses on investigating the presence of one particular planar defect, namely periodic anti-phase boundaries in carbon-supported PtCu3 nanoparticles. The studying of this structural phenomenon was approached with several characterization tools. Ex-situ X-ray diffractograms were used in conjunction with computer simulations and Rietveld analyses to reliably determine the modulated unit cell containing anti-phase boundaries, while their temperature-dependent formation and disappearance were observed with high-temperature X-ray powder diffraction. In addition, their presence was also confirmed by electron diffraction and atomically resolved scanning transmission electron microscopy. Furthermore, the average neighborhood of Cu and Pt atoms was confirmed using extended X-ray absorption fine structure. Finally, the electrocatalytic activity for the oxygen reduction reaction for composites with and without anti-phase boundaries was determined using a thin-film rotating disk electrode, and the performance was found to correlate with the degree of alloy ordering but not with the presence of anti-phase boundaries. Overall, this study represents a significant step towards better control over the atomically precise synthesis of advanced functional metallic materials by providing valuable insight into the formation of planar defects in metallic alloy nanoparticles and their impact on the structure-property relationships of electrocatalysts.