Support-Free Connected Nanoparticle Catalysts Synthesized via Platinum Shell Method for Enhanced Oxygen Reduction Performance

The improvement of the activity and durability of Pt-based electrocatalysts for oxygen reduction reaction (ORR) is essential to realize the widespread uses of polymer electrolyte fuel cells (PEFCs). Our group has developed support-free connected Pt-alloy catalysts, which consist of nanonetwork by the connection of nanoparticles with high electrical conductivity, thus eliminating the need for carbon support. The connected Pt 1 –Fe 1 and Pt 3 –Co 1 catalysts with chemically ordered structures show an ORR specific activity about ten times higher than a commercial Pt/C catalyst. These catalysts with carbon-support-free and chemically ordered structures improve the durability against start/stop and load cycles. 1–3 However, these connected catalysts are prepared by the annealing method using high temperature annealing to form partial fusion of nanoparticles, resulting in thicker nanonetwork and decreased electrochemical surface area (ECSA). This study reports a new one-pot synthesis method (Pt shell method) using a two-step polyol process at low temperature (80–100 °C) to form connected nanoparticles. As illustrated in Figure 1a, first, metal nanoparticles are formed on spherical silica template (metal NPs/SiO 2 ) by polyol process, and then the Pt atomic shell is formed on metal NPs/SiO 2 by controlled polyol process, resulting in a connected metal@Pt nanonetwork (connected metal@Pt NPs/SiO 2 ). Finally, by removal of silica template using an alkaline treatment, a support-free, connected metal@Pt core-shell nanoparticle catalyst with a porous hollow capsule structure is obtained, as shown in Figure 1b–d. In this synthesis method, nanoparticles are connected by Pt shell layers, which allow the process to be conducted at low temperatures. Therefore, the connected Pd@Pt nanoparticle catalysts synthesized by the Pt shell method showed 3–4 times higher ECSA than those synthesized by the conventional annealing method. Furthermore, by controlling the thickness of the Pt shell layers on the surface, the connected Pd@Pt nanoparticle with ca. 3.5 Pt-atomic layers on the surface achieved higher ORR mass and specific activities. The developed catalyst also showed high durability against load cycle in 0.1 M HClO 4 electrolyte solution at 60 °C. After 10,000 load cycles, the connected nanoparticle structure remained, showing the high durability of the nanonetwork connected by the Pt shell layers. In this way, this study has demonstrated a new Pt shell method and shown that a support-free connected nanoparticle catalyst is useful as a PEFC cathode catalyst. The Pt shell method can form stable connected structures with metal nanoparticles other than Pd, thus it is expected that further structural optimization, including other metal species, will lead to the realization of advanced ORR catalysts. Acknowledgement: This presentation is based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO), Japan. References: H. Kuroki, T. Yamaguchi et al. , [1] Energy Environ. Sci. , 8, 3545–3549 (2015). [2] ACS Appl. Nano Mater. , 3, 9912–9923 (2020). [3] ACS Appl. Nano Mater. , 8, 3323–3332 (2025). [4] Adv. Sci , 12, 2408614 (2025). Figure 1