Abstract
The oxygen reduction reaction (ORR) is catalyzed at the cathode in polymer electrolyte fuel cells (PEFCs). Since the ORR is sluggish and the electrocatalytic activity of the ORR catalyst determines the overall performance of PEFCs, highly active and durable ORR electrocatalysts have been intensively studied. The combination of alloying and nanostructuring of Pt is one of the promising approaches to develop such ORR electrocatalysts. Among the Pt-based shape-controlled electrocatalysts, Pt or Pt–M alloy nanowires (M = Ni and Co) are known to exhibit high ORR activity in the half cell [1,2]. Since nanowires have larger contact areas and therefore potentially higher adhesion to the carbon support than conventional nanoparticles, Pt-based nanowires should have higher durability. However, details of the effect of durability testing (potential cycles) on the structure of nanowires remain unclear.In this work, we report synthesis of PtNi alloy nanowires at different temperatures under different atmospheres, and their structural transformation after potential cycles. PtNi nanowires were prepared at 433 K under Ar and air [3]. The difference in atmosphere affects the presence or absence of PtNi nanoparticles with PtNi nanowires: both PtNi nanowires and nanoparticles are produced under the inert condition of Ar, whereas PtNi nanowires are exclusively produced under the oxidative condition of air. PtNi nanowires with and without PtNi nanoparticles were converted into branched nanostructures after durability tests. PtNi nanowires were also prepared at different temperatures of 433, 493 and 553 K under Ar [4]. The Ni content in the PtNi nanowires increases from <5 at.% up to about 15 at.% with increasing synthesis temperature. The number of PtNi nanoparticles also decreases with increasing synthesis temperature. The PtNi nanowires in the co-presence of PtNi nanoparticles prepared at 493 K gave highly durable beads-on-nanowires with the Pt skin via Ostwald ripening of coexisting PtNi nanoparticles. This structural transformation is coupled with changes in the surface electronic structure, confirmed by in situ X-ray absorption spectroscopy. Understanding such a structural transformation will help us to design and develop highly durable Pt alloy nanostructured electrocatalysts for the ORR.References.[1] M. Li et al., Science, 354, 1414 (2016).[2] Z. Zhao et al., Adv. Mater.,31, 1808115 (2019).[3] M. Kato, Y. Iguchi, T. Li, Y. Kato, Y. Zhuang, K. Higashi, T. Uruga, T. Saida, K. Miyabayashi, I. Yagi, ACS Catal., 12, 259 (2022).[4] Y. Zhuang, Y. Iguchi, T. Li, Y. Kato, M. Kato, Y. A. Hutapea, A. Hayashi, T. Watanabe, I. Yagi, ACS Catal., 14, 1750 (2024).
Published Version
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