Flow-reactor synthesis of octahedral Pt-Ni nanoparticles modified with Au and their evaluation as oxygen reduction reaction catalysts.

Developing oxygen reduction reaction (ORR) catalysts with both high catalytic activity and durability is essential for the commercialization of polymer electrolyte fuel cells (PEFCs). One promising strategy to simultaneously enhance the catalytic activity and durability of Pt-based catalysts is surface modification with Au, which has led to substantial durability improvements in various catalysts such as bulk electrodes, nanowires, and core-shell structures. In this study, octahedral Pt-Ni nanoparticles (oct-Pt-Ni NPs), which are known to exhibit exceptionally high ORR activity, were synthesized and modified with Au using a continuous flow reactor to investigate the effects of Au modification on catalytic activity and durability. The flow synthesis enabled uniform Au deposition on individual nanoparticles owing to the rapid mixing and homogeneous contact between the Au precursor and Pt-Ni nanoparticles. Electrochemical measurements revealed that Au modification enhanced the specific activity (SA) by up to 1.5 times, while the mass activity (MA) remained nearly unchanged owing to the decrease in electrochemical surface area of Pt. The activity enhancement suggests that Au atoms promote the catalytic activity of the neighboring Au-free Pt-Ni sites as previously reported. In contrast, the MA of Au-modified oct-Pt-Ni NPs rapidly decreased within several hundred potential cycles along with the decrease in the SA, indicating that Au atoms on the Pt-Ni nanoparticles could not effectively suppress Ni leaching or morphological transformation. These results suggest that the beneficial effect of Au modification on durability is limited for shape-sensitive catalysts with numerous vulnerable edges and corners such as oct-Pt-Ni nanoparticles, unlike the spherical or nanowire catalysts with smooth or well-faceted surfaces.