Insights into thermal annealing of highly-active PtCu3/C Oxygen Reduction Reaction electrocatalyst: An in-situ heating transmission Electron microscopy study

Thermal annealing processes for supported Pt-based nanoparticles are usually developed based on iterative empirical findings resulting from ex-situ characterization of pre- and post-annealed samples. Such an approach, however, offers limited insight into processes occurring during the heating step. In this work, we first exemplify typical findings that are accessible by ex-situ investigation using typical conventional techniques such as transmission electron microscopy (TEM), X-ray diffraction (XRD), and thin film - rotating disc electrode (TF-RDE). As a model system we select a well-researched Pt-Cu alloy which, as demonstrated, offers exciting new insights into the dynamics occurring during heat treatment on the nano-to-atomic scale. This dynamics can be viewed by upgrading the ex-situ findings with a high resolution TEM imaging in combination with carefully designed in-situ heating protocol. This way one can directly observe the particle growth mechanisms during heat treatment. Such direct observations, in turn, provide new understanding of morphology-performance correlations in alloys. For example, it is shown that the enhanced activity of the present PtCu3/C electrocatalyst is due to Cu enrichment during heat treatment. This enrichment, however, is only possible due to the presence of relatively large excess CuO needle-like particles left over from the previous double passivation galvanic displacement step. Very importantly, we further show that the mechanism of Cu enrichment at elevated temperatures involves migration of Cu single atoms via the carbon support. At moderate temperatures (up to 500 degrees C), other effects have also been observed such as reshaping into a sphere-like shape as well as ordering of the crystal lattice which could not occur without enrichment of the initial Pt-Cu nanoparticles with Cu. In that region, Cu enrichment is also responsible for the initial growth of PtCu nanoparticles. By contrast, upon heating till 800 degrees C, the growth is mainly due to coalescence. Ostwald ripening, on the other hand, does not seem to play a significant role in the increase in the nanoparticle size. The new general insights can be readily extended to various other similar alloy systems.