Native ligand carbonization renders common platinum nanoparticles highly durable for electrocatalytic oxygen reduction: annealing temperature matters

Abstract

Monodisperse nanoparticles have attracted great attention in fundamental and applied research. Current protocols for the synthesis of monodisperse nanoparticles typically involve hydrocarbon molecules as surface-capping ligands. Utilizing platinum (Pt)-based nanoparticles for the oxygen reduction reaction (ORR), however, hydrocarbon ligands must be removed in order to expose the surface sites. Here, highly active and durable Pt catalysts are realized without removing the ligands; instead, the native surface ligands are directly converted into uniform, bilayered graphitic shells conformally coated on individual Pt nanoparticles by simple thermal annealing. Strikingly, the annealing temperature is found to be a critical factor dictating the ORR performance of Pt catalysts. Pt nanoparticles thermally treated at 500 oC show a very poor ORR activity, whereas those annealed at 700 oC become highly active along with exceptional stability. In-depth characterization reveals that thermal treatment from 500 to 700 oC triggers the subtle structural reconstruction of carbon shells through graphitization, gradually opening up the porosity without affecting the carbon shell thickness. Additionally, the ligand-derived graphitic shells can effectively prevent Pt nanoparticles from detachment, agglomeration, and dissolution while largely maintaining the accessibility of surface sites. As a result, such graphitic-shell-coated Pt catalysts can exhibit superior long-term stability, largely retaining the activity after 20,000 accelerated durability test cycles in a membrane electrode assembly. Moreover, this ligand carbonization strategy is amenable to Pt-based alloy nanoparticles and can further be extended to modify commercial Pt/C catalysts with substantially enhanced durability. Our work shows that boosting the ORR performance of common Pt nanoparticles is certainly possible by harnessing the native surface ligands, thus offering a robust approach of designing highly durable catalysts for proton-exchange membrane fuel cells.