We report the preparation, oxygen reduction reaction (ORR) electrocatalytic activity, and structural transformation of Pt–Ni nanowires (NWs) during potential cycles in the presence and absence of Pt–Ni nanoparticles (NPs). The ORR activity of NWs increases over 25000 potential cycles in the presence of NPs, involving the structural transformation of NWs to branched nanostructures assisted by Ostwald ripening of NPs. This structural transformation is coupled with the surface electronic structural change, as confirmed by in situ X-ray absorption spectroscopy and carbon monoxide stripping voltammetry, leading to catalytic activity improvement and Pt dissolution suppression. Although a similar structural transformation was also observed even in the absence of NPs, greater amounts of Pt were dissolved during potential cycles. These results indicate that the structural transformation is intrinsic to Pt-based NWs but the structural transformation of NWs assisted by Ostwald ripening of NPs is beneficial to suppress the Pt dissolution. The concept of the structural optimization of nanostructured catalysts assisted by Ostwald ripening of NPs under potential cycles will guide us to develop highly active and durable Pt-based electrocatalysts and phase-engineered nanomaterials.

Polymer electrolyte fuel cells (PEFCs) are known as sustainable devices and can produce the electricity with no emission of CO2 gas. The oxygen reduction reaction (ORR) occurs at the cathodes in PEFCs and platinum nanoparticles (NPs) are generally used as an ORR catalyst. Pt on the surface of the catalysts can be oxidatively dissolved under catalytic conditions and then their ORR activity decreases.[1, 2] Because alloying and nanostructuring are known to improve the tolerance of the catalyst to the oxidative dissolution[3], we have studied Pt-Ni nanowires (NWs) as an ORR catalyst.[4] NWs can be expected to show stronger interactions between the Pt surfaces and carbon supports than NPs.[5, 6] However, there is a lack of detailed researches on the durability of NWs. In this research, we synthesized Pt-Ni NWs, conducted an accelerating durability test (ADT) for the ORR activity and investigated the effect of the ADT on the electronic states of NWs by ex situ X-ray photoelectron spectroscopy (XPS) and in situ Pt L3-edge X-ray absorption spectroscopy (XAS). We also characterized NWs by scanning transmission microscopy (STEM), inductively coupled plasma mass spectrometry (ICP-MS) and powder X-ray diffraction (pXRD). Cyclic and linear sweep voltammograms of NWs showed that the mass activity of the NWs was higher than that of NPs. Interestingly, the mass activity of the NWs increases after the ADT whereas the activity of typical NPs decreases.[7] XPS peaks of the NW catalyst in the Pt 4f region were shifted to the higher binding energy after the ADT, compared to those before the ADT. This result suggests that the d-band center of the surface Pt of the NWs were downshifted after the ADT.[8] In situ Pt L3-edge XAS revealed that the potential dependence of the intensity of the NW catalyst in the white line region after the ADT is smaller than that of before the ADT. This result suggests that the formation of Pt oxide was suppressed after the ADT,[1] leading to the improvement of the mass activity. References [1] K. Nagasawa, et al., J. Am. Chem. Soc., 137, 12856 (2015). [2] H. Nejc, et al., Acc. Chem. Res., 49, 2015 (2016). [3] L. Robner et al., ACS Catal., 9, 2018 (2019). [4] L. Xhikun, et al., RSC Adv., 10, 6287 (2020). [5] I. Stephens, et al., Science, 354, 1378 (2016). [6] M. Li, et al., Science, 354, 1414 (2016). [7] G. Mingxing, et al., ACS Catal., 9, 4488 (2019). [8] M. Wakisaka, et al., J. Phys. Chem. B, 110, 23489 (2006). Acknowledgements This work was supported by NEDO.

Oxygen reduction reaction (ORR) is a key reaction in future energy generation devices such as polymer electrolyte fuel cells (PEFCs) and metal–air batteries. To drive the ORR in PEFCs, Pt–M alloy nanoparticles (M = Ni) on carbon supports have been used as catalysts. Under electrocatalytic conditions, these electrocatalysts are known to degrade involving particle detachment, agglomeration and metal dissolution.[1] These phenomena can be suppressed by increase metal/support interactions: using heteroatom-doped carbon supports[2] and/or nanostructured Pt–M alloy catalysts.[3] Herein, we report synthesis, activity and durability of Pt–Ni nanowires (NWs) for the ORR in acidic media. The Pt–Ni NWs showed higher ORR electrocatalytic activity than Pt/C. The Pt–Ni NWs showed almost no activity loss even after 50,000 potential cycles in acidic media. Physicochemical measurements including in situ Pt L3-edge X-ray absorption spectroscopy [4] revealed that the Pt surface oxide formation under potential control was suppressed after potential cycles, relative to the initial state. This finding suggests that potential cycles could induce strong metal/support interactions, resulting in the formation of highly durable NW catalysts. References. [1] N. Hodnik, G. Dehm, K.J.J. Mayrhofer, Acc. Chem. Res., 49, 2015–2022 (2016). [2] M. Kato, K. Ogura, S. Nakagawa, S. Tokuda, K. Takahashi, T. Nakamura, I. Yagi, ACS Omega, 3, 9052–9059 (2018). [3] J. Li et al., Science, 354, 1414–1417 (2016). [4] K. Nagasawa et al., J. Am. Chem. Soc., 137, 12856 (2015). Acknowledgements. This work was supported by NEDO.

Carbon-supported Pt-M (M=Ni or Co) nanoparticles (NPs) have been intensively studied as oxygen reduction reaction (ORR) catalysts for the cathode of polymer electrolyte fuel cells. Under electrocatalytic conditions, these catalysts are known to degrade involving particle detachment, agglomeration and metal dissolution. To develop highly durable ORR electrocatalysts, these phenomena can be suppressed by strong metal/support interactions[1,2] or even utilized as self-healing or sacrificial processes. We present the ORR activity, durability and structural transformation of carbon-supported Pt-Ni nanowires (NWs) in the presence and absence of NPs.[3] In the presence of NPs, NPs were dissolved, and NWs were converted to form branched nanostructures during potential cycles. Because the similar structural transformation of NWs occurred even in the absence of NPs, the structural transformation is an intrinsic phenomenon during potential cycles. Interestingly, the ORR activity increased and the Pt dissolution was suppressed during potential cycles in the co-presence of Pt-Ni NWs and NPs. The electronic and surface structures are also changed before and after potential cycles and will be discussed based on physicochemical measurements including in situ Pt L 3-edge X-ray absorption spectroscopy. This work was supported by NEDO. References [1] M. Kato, K. Ogura, S. Nakagawa, S. Tokuda, K. Takahashi, T. Nakamura, I. Yagi, ACS Omega, 3, 9052–9059 (2018). [2] M. Kato, R. Nakahoshiba, K. Ogura, S. Tokuda, S. Yasuda, K. Higashi, T. Uruga, Y. Uemura, I. Yagi, ACS Appl. Energy Mater., 3, 6768-6774 (2020). [3] M. Kato, Y. Iguchi, T. Li, Y. Kato, Y. Zhuang, K. Higashi, T. Uruga, T. Saida, K. Miyabayashi, I. Yagi, ACS Catal., 12, 259-264 (2022).