| Summary: | <p>Polymer electrolyte membrane fuel cells (PEMFCs) are promising energy-conversion devices due to their high efficiency and near-zero carbon emission footprint. PEMFCs still require a high mass of precious metal Pt as an electrocatalyst for oxygen reduction reaction (ORR) to ensure sufficient power output efficiency, but the sources and cost of Pt greatly limit its commercial application in fuel cells technology. The development of high-active and low-cost ORR electrocatalysts is therefore of much significance for the commercialization of fuel cells. Compared to Pt, bimetallic (Pt-Co) nanoparticles exhibit superior catalytic performance with less mass of precious metal, but the intrinsic mechanism of this catalytic enhancement has not been fully explained. Strain and composition are two fundamental effects that are highly associated with the catalytic activity of the Pt-based nanoparticles. Thus, being able to measure local composition and strain at an atomic scale within the individual nanoparticle will further our understanding of the operation of heterogeneous catalysts.</p>
<p>In this thesis, the strain analysis and composition quantification are precisely and simultaneously performed on the individual nanoparticle at an atomic scale for four Pt-Co nanoparticle ensembles, to relate composition and size to strain. 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 composition, whether or not an acid de-alloying treatment is applied. This work makes it possible to design the optimum ORR catalysts by tuning strain, composition, and structure of the Pt-based bimetallic nanoparticles.</p>
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