Oxygen Electroreduction Catalyzed by Palladium Nanoparticles Supported on Nitrogen-Doped Graphene Quantum Dots: Impacts of Nitrogen Dopants

Palladium nanoparticles supported on nitrogen-doped graphene quantum dots (NGQD) were synthesized by hydrothermal coreduction of palladium salts, citric acid, and urea at 160 degrees C for up to 12 h. Transmission electron microscopic studies showed that in the resulting PdNGQD nano composites, small palladium nanoparticles clustered into superstructures of 100 nm and larger. X-ray photoelectron spectroscopic studies showed that the NGQDs contained only p-type pyridinic and pyrrolic nitrogen centers, and although the total concentrations of nitrogen dopants were rather consistent (ca. 10 at. %) among the series of samples, the relative abundance of pyrrolic (pyridinic) nitrogens increased (decreased) with prolonging reaction duration, suggesting thermal conversion of pyridinic nitrogens into pyrrolic ones. The binding energy of the Pd 3d electrons was found to increase accordingly, probably due to enhanced electron withdrawing by the more acidic pyrrolic nitrogens. This suggests apparent interactions between palladium and the nitrogen dopants. Consistent results were obtained in Raman spectroscopic measurements which showed an increase of the D and G band intensity ratio, indicative of an increasingly defective structure of the NGQD. This was consistent with the increasing abundance of pyrrolic centers which provided more structural strains than the six-membered pyridinic heterocycles within the graphitic backbone. Electrochemically, the series of PdNGQDs all showed apparent electrocatalytic activity toward oxygen reduction in alkaline media, and within the context of onset potential and kinetic current density, the sample prepared by 8 h of hydrothermal reaction was found to stand out as the best catalyst among the series, with a top specific activity that was over eight times better than that observed when palladium nanoparticles were supported on undoped GQDs and commercial Pt/C. This might be accounted for by the enhanced electron withdrawing effects of the pyrrolic nitrogen centers that manipulated the electronic interactions between palladium and oxygen intermediates, as compared to oxygenated moieties alone in undoped GQDs.