Anisotropic elastic strain in Pt-Co catalyst nanoparticles measured by correlated atomic resolution imaging and spectroscopy

Abstract Catalysts for the oxygen reduction reaction are crucial to the performance of fuel cell applications. Pt-based alloys have been shown to exhibit superior catalytic activities compared to pure Pt catalysts. Ligand and strain are two fundamental effects that have been proposed to explain the mechanistic origin of catalytic enhancement. It has been suggested that compressive strain leads to a shift of the d-band centre leading to improved reaction kinetics. Herein, we precisely quantify and correlate composition and strain variations from the same nanoparticle at an atomic scale within alloyed and dealloyed Pt-Co oxygen reduction reaction catalyst systems. Unlike the previously assumed effects of dealloying, we find that no compressive strain on the Pt-rich outer shell is imposed by the alloy core. Dilation strain is found to be distributed throughout the individual nanoparticles, with the radial dilation strain being much larger than the circumferential dilation strain at the surface. Remarkably, all the Pt-Co stoichiometries studied have a relatively invariant surface lattice parameter, which is smaller than that for pure Pt but larger than that predicted using the measured local surface composition, whether or not an acid de-alloying treatment is applied. These findings provide crucial insights to enable a full understanding of the origins of enhanced catalytic performance.