Platinum-Nickel Alloy Nanowire Electrocatalysts Transform into Pt-Skin Beads-on-Nanowires Keeping Oxygen Reduction Reaction Activity During Potential Cycling

We synthesized PtNi alloy nanowires (PtNi NWs) at three different temperatures of 433, 494, and 533 K (NW_{433 K}, NW_{494 K}, and NW_{533 K}, respectively) and then investigated their catalytic activity and durability for the oxygen reduction reaction (ORR) in acidic media. Ni contents in the PtNi NWs increase as the synthesis temperatures increase from below 5 at. % for NW_{433 K} up to about 15 at. % for NW_{493 K} and NW_{553 K}. PtNi nanoparticles (PtNi NPs), which are the unconsumed intermediate during the NW growth, also coexist for NW_{433 K} and NW_{494 K} but not for NW_{533 K}. NW_{494 K} and NW_{533 K} show similar initial activity for the ORR but higher than NW_{433 K}, suggesting that higher Ni contents are critical to achieving higher initial ORR activity. Accelerated durability tests (ADTs) show that NW_{493 K} is the most durable, suggesting that the copresence of PtNi NPs is critical to durability. Only NW_{493 K}, with a high Ni content of 15 at. % and coexisting PtNi NPs, gave better results in both cases. Scanning transmission electron microscopy and energy dispersive X-ray spectroscopy of PtNi NWs reveal a structural transformation of NW_{493 K} into Pt-skin beads-on-nanowires, involving the Ostwald ripening of coexisting PtNi NPs. This structural transformation is coupled with changes in surface composition and surface electronic structure, as confirmed by the CO stripping voltammogram and in situ X-ray absorption spectroscopy, resulting in high durability and suppression of Pt and Ni dissolution. Understanding such structural transformation during potential cycling will help us to design and develop highly active and durable Pt-based electrocatalysts.