Support-Free, Connected Core-Shell Nanoparticle Catalysts Synthesized via a Low-Temperature Process for Advanced Oxygen Reduction Performance

Abstract

Nanostructured Pt-based catalysts have attracted considerable attention for fuel-cell applications. This study introduces a novel one-pot and low-temperature polyol approach for synthesizing support-free, connected nanoparticles with non-Pt metal cores and Pt shells. Unlike conventional heat treatment methods, the developed support-free and Fe-free connected Pd_core@Pt_shell (Pd@Pt) nanoparticle catalyst possesses a stable nanonetwork structure with a high surface area. This approach can precisely control the atomic-level structure of the Pt shell on the Pd core at a low deposition temperature. The optimized Pd@Pt catalyst with a Pt/Pd atomic ratio of 0.8 and a Pt shell thickness of 1.1 nm exhibits a threefold improvement in oxygen reduction reaction (ORR) mass activity compared to that of commercial carbon-supported Pt nanoparticle catalyst (Pt/C). Durability evaluation demonstrated 100% retention of specific activity after 10,000 load cycles, owing to the stable nanonetwork and uniform coverage of the Pt shell. In addition, the support-free, connected core-shell nanoparticle catalyst overcomes the carbon corrosion issues commonly associated with conventional carbon-supported catalysts while simultaneously improving both ORR activity and load cycle durability. These findings highlight the potential of this innovative approach to develop support-free catalysts for polymer electrolyte fuel cells and other energy devices.

Keywords: Pt shell; connected core–shell nanoparticles; oxygen reduction reaction; polymer electrolyte fuel cell; support‐free catalyst.

Scheme 1

Synthesis of connected nanonetwork catalysts with a porous hollow capsule structure. a) Conventional method using SiO₂ coating and high‐temperature annealing treatment, resulting in a connected metal (M) catalyst with a large crystallite size. b) Pt shell method using a low‐temperature process to obtain a connected core–shell catalyst with a small crystallite size.

Figure 1

XRD patterns of the Pd/SiO₂ and Pd@Pt catalysts.

Figure 2

SEM images of a) Pd nanoparticles uniformly deposited on a silica template (Pd/SiO₂ catalyst), b) Pd catalyst obtained by removing the silica template from the Pd/SiO₂ catalyst via alkaline treatment, and c) Pd@Pt catalyst obtained by forming Pt shells on Pd/SiO₂ catalyst followed by removing silica template via alkaline treatment.

Figure 3

TEM and high resolution‐TEM images of a–c) Pd nanoparticles uniformly deposited on a silica template (Pd/SiO₂ catalyst) and d–f) Pd/SiO₂ catalyst after Pt shell formation and silica removal via alkaline treatment (connected Pd@Pt_0.8 catalyst).

Figure 4

a–d) STEM–EDX elemental mapping images and e) STEM–EDX line scan along the arrow of the connected Pd@Pt_0.8 catalyst.

Figure 5

XPS spectra of different connected Pd@Pt catalysts in the Pt 4f region.

Figure 6

a) CV (N₂ saturated atmosphere, scan rate: 20 mV s⁻¹) and b) LSV curves (O₂ saturated atmosphere, scan rate: 20 mV s⁻¹, scan speed: 1600 rpm) of Pd@Pt catalysts with different Pt/Pd ratios and the commercial Pt/C catalyst. c) Mass activity and specific activity as a function of the Pt/Pd atomic ratio.

Figure 7

Load cycle durability test in a 0.1 m HClO₄ aqueous solution at 60 °C. CV (N₂ saturated atmosphere, scan rate: 20 mV s⁻¹) of a) Pd@Pt_0.3 and b) Pd@Pt_0.8 catalysts after different durability cycles. LSV curves (O₂ saturated atmosphere, scan rate: 20 mV s⁻¹, scan speed: 1600 rpm) of c) Pd@Pt_0.3 and d) Pd@Pt_0.8 catalysts after different durability cycles. e) ECSA f) mass activity, and g) specific activity of the Pd@Pt_0.3 and Pd@Pt_0.8 catalysts and the commercial Pt/C catalyst before and after a specific number of load cycles.

Figure 8

a–d) TEM images of the connected Pd@Pt_0.8 catalyst after 10,000 load cycles of ADT.

Figure 9

a–d) STEM–EDX elemental mapping and e) STEM–EDX line scan along the arrow of the connected Pd@Pt_0.8 catalyst after 10,000 load cycles of ADT.