Use of Dendrimers during the Synthesis of Pt-Ru Electrocatalysts for PEM Fuel Cells: Effects on the Physical and Electrochemical Properties

Abstract

In this work, Pt-Ru catalysts were synthesized by a novel methodology which includes the use as encapsulating molecules of dendrimers of different generation: zero (DN-0), one (DN-1), two (DN-2), and three (DN-3). Synthesized catalysts were heattreated at 350• C, and the effects of this treatment was established from the physical (X-ray dispersive energy (XDE) and X-ray diffraction (XRD)) and electrochemical characterization (cyclic voltammetry and chronoamperometry). Results showed that the heat-treatment benefits the catalytic properties of synthesized materials in terms of CO and methanol electrochemical oxidation. The curves for CO stripping were more defined for heat-treated catalysts, and methanol oxidation current densities were higher for these materials. These changes are principally explained from the removal of organic residues remaining on the surface of the Pt-Ru nanoparticles after the synthesis procedure. After the activation of the catalysts by heating at 350• C, the real importance of the use of these encapsulating molecules and the effect of the generation of the dendrimer become visible. From these results, it can be concluded that synthesized catalysts are good catalytic anodes for direct methanol fuel cells (DMFCs).

Effect of the Dendrimer Generation Used in the Synthesis of Pt-Ru Nanoparticles Supported on Carbon Nanofibers on the Catalytic Activity towards Methanol Oxidation

“…This effect was previously attributed to the low encapsulating capacity of the zero generation dendrimer, in contrast to the three-generation one [45]. Nevertheless, it must be taken into account that these values were calculated from the Pt (220) reflection, which is affected by the Ru oxides contribution, a fact observed in the catalysts DN-2, DN-1 TT, DN-2 TT and DN-3 TT (see Figure 2 in [45]). The metal surface areas were determined by means of the equation SA (m 2 ·g −1 ) = 6 × 103/ρd [63], where d is the average crystallite size (nm) and ρ is the alloy density.…”

“…Table 1 reports the results obtained for the composition, particle/crystallite sizes, metal surface and electrochemical surface areas for the synthesized catalysts, which were determined in [45] and repeated in this work considering their importance to understand the electrochemical activity of the catalysts. The XRD patterns exhibited the typical face-centered cubic structure of platinum located at 40 • , 47 • , 68 • and 82 • , corresponding to the (111), (200), (220) and (311) crystalline reflections.…”

“…The studied catalysts were synthesized and physically characterized previouslty [45]. Table 1 reports the results obtained for the composition, particle/crystallite sizes, metal surface and electrochemical surface areas for the synthesized catalysts, which were determined in [45] and repeated in this work considering their importance to understand the electrochemical activity of the catalysts.…”

“…The XRD patterns exhibited the typical face-centered cubic structure of platinum located at 40 • , 47 • , 68 • and 82 • , corresponding to the (111), (200), (220) and (311) crystalline reflections. Besides, an additional peak was observed toward 28.5 • , especially in the case of the heat treated catalysts, which was assigned to the presence of Ru oxides formed after this treatment [45]. For the calculation of the crystallite size, the dimensions of the peak (220) of platinum has been used, bearing in mind there is no influence of the other crystalline plane reflections on this peak, allowing the establishment of a clear and accurate baseline.…”

“…These results suggest that the high number of branches, especially present in the dendrimers DN-2 and DN-3 decrease the crystallite sizes and increase the metal surface areas, due to a favored dispersion of the nanoparticles on the carbon nanofibers. a This data was calculated from the Pt (220) reflection, although a meaning error associated to the Ru oxides peaks overlaying on this peak is considered [45]; b This data is reported in [64].…”