Oxygen Reduction Reaction Activity of Platinum Nanosheets Derived from Layered Platinic Acid
Authors: Tokura Yuki , Toma Shino , Takimoto Daisuke , Sugimoto Wataru 
Published: February 2024

Platinum nanoparticles are mainly utilized as the cathode catalyst for polymer electrolyte fuel cells. However, the utilization of platinum for the oxygen reduction reaction (ORR) is insufficient and one must tackle the trade-off between high surface area and high stability. We have previously shown that metallic nanosheets such as ruthenium nanosheets exhibit high electrocatalytic activity and stability owing to the two-dimensional structure with an atomic scale thickness.1, 2 Platinum nanosheets, if available, should also have the potential to increase the utilization of the atoms and durability. While a few studies have reported the synthesis of platinum nanosheets,3 the thickness of the nanosheets is more than 1 nm, and the utilization of platinum has so far been lower than that of the Pt nanoparticles.

In this study, we report double-layered platinum nanosheets derived from layered platinic acid.4 Layered platinic acid was synthesized through a solid-state-reaction and subsequent acid treatment. From the layered platinic acid, platinum oxide nanosheets were obtained as a colloid via chemical exfoliation. The thickness of the platinum oxide nanosheet was ~0.9 nm, consistent with the thickness of a single PtO6 octahedron. The platinum oxide nanosheets on silicon wafer were topotactically reduced to platinum nanosheets by mild thermal treatment in hydrogen. The thickness of the platinum nanosheet was ~0.5 nm, corresponding to a two atomic layer platinum nanosheet.

Carbon supported platinum nanosheets were prepared by the reduction of platinum oxide nanosheets supported on carbon. The electrochemically active surface area (ECSA) of the carbon supported platinum nanosheets was 100 to 120 m2 (g-Pt)-1, which was considerably larger than that of 3 nm platinum nanoparticle (80 m2 (g-Pt)-1). In addition, the platinum nanosheet showed 1.7 times higher mass activity towards the oxygen reduction reaction compared to that of 3 nm platinum nanoparticles.

Various methods for the topotactic reduction of platinum oxide nanosheets on carbon was studied. In addition to the mild thermal treatment in hydrogen gas, electrochemical reduction was also conducted. The ECSA of the platinum nanosheet prepared by electrochemical reduction at constant potential was 100 to 120 m2 (g-Pt)-1, which was almost same as the platinum nanosheet reduced by the mild thermal treatment in hydrogen. The specific activity was 1.2 to 1.3 times higher than that of the platinum nanosheet reduced by the mild thermal treatment in hydrogen. Therefore, the platinum nanosheet reduced by electrochemical reduction showed high mass activity for ORR.

Acknowledgement

This work was supported as part of the “Polymer Electrolyte Fuel Cell Program” and the FC-Platform projects from the New Energy and Industrial Technology Development Organization (NEDO) of Japan (20001201-0, 22101136-0).

References

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