Direct Thermal Annealing Synthesis of Ordered Pt Alloy Nanoparticles Coated with a Thin N-Doped Carbon Shell for the Oxygen Reduction Reaction

Abstract

Ordered Pt alloy electrocatalysts supported on carbon nanomaterials have attracted widespread attention, especially for the oxygen reduction reaction (ORR), due to the catalytic performance derived from their unique electronic and geometric structures. However, it is still urgent to fabricate uniform and structurally ordered Pt alloy electrocatalysts based on simple methods. Herein, a two-step direct annealing method was applied to synthesize uniform and ordered PtFe alloy nanoparticles loaded on single-wall carbon nanohorns (SWCNHs) under the protection of a thin N-doped carbon (NC) shell, which was in situ generated from the polymerization and pyrolysis of a small organic ligand, namely, aniline, during the first annealing treatment. After the second annealing treatment in a H2 atmosphere for 9 h, the obtained sample, denoted as PtFe@NC/SWCNHs(H2-9h), exhibited uniform and ordered PtFe nanoparticles with a face-centered tetragonal (fct) structure (ordered degree: >80%, mean size: ∼5.2 nm) on the graphitic SWCNH support. Without removing the NC shell, the PtFe@NC/SWCNHs(H2-9h) sample showed mass activity (1.53 A/mgPt at 0.9 V) and specific activity (3.61 mA/cm2 at 0.9 V) toward the ORR due to the enhanced electronic interaction derived from the ordered fct-PtFe structure. Importantly, it still retained high catalytic activity after a long-term stability test, mainly owing to the ordered fct-PtFe structure and the protection of the NC shell, which provides strong resistance toward the Fe leaching and nanoparticle aggregation, respectively. The presented strategy is generalized to fabricate different ordered PtM or Pt3M (M = Fe/Co) alloy electrocatalysts.