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

Abstract

Palladium nanoparticles supported on nitrogen-doped graphene quantum dots (NGQD) were synthesized by hydrothermal coreduction of palladium salts, citric acid, and urea at 160 °C for up to 12 h. Transmission electron microscopic studies showed that in the resulting PdNGQD nanocomposites, 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.

Oxygen Reduction Reaction and Hydrogen Evolution Reaction Catalyzed by Pd–Ru Nanoparticles Encapsulated in Porous Carbon Nanosheets

“…The two peaks can be further deconvoluted into two pairs of doublets, and the peaks at the lower binding energies of 336.3 eV and 341.5 eV can be assigned to metallic Pd [36], while the peaks at higher binding energies of 337.6 eV and 343.1 eV are ascribed to the Pd (II) species [43,44]. This indicates the formation of palladium (II) oxides, which agree well with the Pd-carbon substrate hybrid systems in several previous reports [36,45,46]. Notably, compared with Pd 100 /CNs, the binding energy for the Pd3d 3/2 electrons in Pd 50 Ru 50 /CNs shifted positively slightly, with a value of 0.3 eV.…”

Ultrasmall Palladium Nanoclusters Encapsulated in Porous Carbon Nanosheets for Oxygen Electroreduction in Alkaline Media

“…In the left panel, deconvolution of the C 1s spectra yields three components, sp 2 carbon (284.5 eV), C in CÀO (285.2 eV) and C in C=O/COOH (288.5 eV). [35] This suggests the formation of palladium oxide in the samples. The pair at lower energies (335.6 and 340.9 eV) are consistent with those of metallic Pd, [40] whereas those at somewhat higher energies (336.9 and 342.2 eV) could be assigned to Pd(II) species.…”

“…Recently, Zhao and coworkers prepared well defined thiolate-capped palladium clusters with the average molecular formula of Pd [13][14][15][16][17] (SR) [18][19][20][21][22] , and such clusters demonstrated apparent ORR activity and enhanced stability with or without ligand in alkaline solutions. [34][35] In another study, [36] Barman and coworkers prepared nanocomposites based on Pd nanoparticles-embedded porous graphitic carbon nitride which exhibited apparent ORR activity and robust stability. [33] Additionally, palladium clusters are prone to aggregation, dissolution, or decomposition during the electrocatalytic process.…”

Peptide‐FlgA3‐Based Gold Palladium Bimetallic Nanoparticles That Catalyze the Oxygen Reduction Reaction in Alkaline Solution

“…Au and Pd would be an excellent binary alloy as Pd interacts strongly with oxygen and Au does not . Compared to compounds that include only Au or Pd, AuPd alloys have shown an enhanced electrocatalytic activity toward ORR . The ORR activity of AuPd bimetallic alloys is determined largely by their structural features, which include size, morphology, composition, and surface atomic distribution, hence efforts have been devoted to the manipulation of these features.…”

“…The most widely employed catalysts for the ORR are Pt-and/or Pt-based nanomaterials, [28][29][30][31] however,t he high price and limitede arth abundance of Pt as well as the low stability of such catalysts hinders the widespread commercialization of fuel cells significantly.A ua nd Pd would be an excellent binarya lloy as Pd interacts strongly with oxygen and Au does not. [32] Compared to compounds that include only Au [33,34] or Pd, [35,36] AuPd alloys have shown an enhanced electrocatalytic activity toward ORR. [1] The ORR activity of AuPd bimetallic alloys is determined largely by their structural features, which include size, morphology, composition, and surface atomicd istribution, hence efforts have been devotedt ot he manipulation of these features.…”