Graphene Quantum Dots-Supported Palladium Nanoparticles\nfor Efficient Electrocatalytic Reduction of Oxygen in Alkaline Media

Abstract

Graphene quantum dots (GQDs)-supported\npalladium nanoparticles\nwere synthesized by thermolytic reduction of PdCl₂ in 1,2-propanediol\nat 80 °C in the presence of GQDs and then were subject to hydrothermal\ntreatment at an elevated temperature within the range of 140 to 200\n°C. Transmission electron microscopic measurements showed a raspberry-like\nmorphology for the samples before and after hydrothermal treatment\nat temperatures ≤160 °C, where nanoparticles of ca. 8\nnm in diameter formed large aggregates in the range of 50 to 100 nm\nin diameter, and at higher hydrothermal temperatures (180 and 200\n°C), chain-like nanostructures were formed instead. X-ray photoelectron\nand Raman spectroscopic measurements revealed that the GQD structural\ndefects were readily removed by hydrothermal treatments, and the defect\nconcentrations exhibited a clear diminishment with increasing hydrothermal\ntemperature, as indicated by the loss of oxygenated carbons in XPS\nand a drop in the D to G band ratio in Raman measurements. Voltammetric\nstudies showed apparent electrocatalytic activity toward oxygen reduction,\nwith a volcano-shaped variation of the activity with GQD defect concentration,\nand the peak activity was observed for the sample prepared at 180\n°C with a mass activity of 23.9 A/g_Pd and specific\nactivity of 1.08 A/m² at +0.9 V vs RHE. This peak activity\nis attributed to optimal interactions between Pd and GQD where the\nGQD defects promoted charge transfer from metal to GQDs and hence\nweakened interactions with oxygenated intermediates, leading to enhanced\nORR activity. The corresponding defect concentration was higher than\nthat identified with the platinum counterparts due to the stronger\naffinity of oxygen to palladium.