Tetrahydrofuran-functionalized multi-walled carbon nanotubes as effective support for Pt and PtSn electrocatalysts of fuel cells
Introduction
Carbon nanotubes (CNTs) are extensively studied as catalyst supports owing to the unique electrical and structural properties of CNTs that improve the electrochemical activity of Pt-based electrocatalysts in low temperature fuel cells [1], [2], [3], [4], [5], [6], [7], [8], [9]. However, CNTs are chemically inert and thus needs to be functionalized to ensure uniform distribution and deposition of Pt and Pt alloy nanoparticles on CNTs. The most common functionalization method is to chemically treat CNTs in HNO3 or H2SO4/HNO3 acid solutions to generate functional groups or defects on the external walls of CNTs for the deposition of catalyst nanoparticles [4], [10], [11], [12]. The acid treatments modify the surface structure of CNTs and could achieve a high dispersion of catalyst. However, the acid treatment inevitably introduces surface defects and causes structural damage to CNTs. Other functionalization methods, such as treatment of CNTs with surfactants, polymers and other capping agents, are also reported [13], [14], [15], [16], [17]. In the case of functionalization by polymer, surfactants, etc, the process is generally tedious and in most cases, additional heat treatment step is needed to get rid of the non-conducting polymer and surfactants attached to the Pt or Pt alloy nanoparticles.
In this study we report a simple and novel method to functionalize the multi-walled carbon nanotubes (MWCNTs) using tetrahydrofuran (THF) solvent. To demonstrate the effectiveness of the method, we selected the syntheses of Pt and binary PtSn nanoparticles on THF-functionalized MWCNTs due to their importance for the electrooxidation reactions of methanol and ethanol in low temperature fuel cells [18], [19]. THF is an oxygen-containing heterocycle with five-membered rings. The electronegativity difference between carbon and oxygen makes the C–O bond moderately polar. Thus THF is a polar, aprotic solvent and has a sterically accessible oxygen atom. In chloroplantic acid solution THF could be protonated. Thus, PtCl62− and Sn4+ ions would be attracted to the sterically accessible oxygen atoms in THF by electrostatic self-assembly. On the other hand there may be a σ–π attractive force between the π bonds of CNTs and the σ bonds of cyclopentanes of THF, similar to the observed π–π stacking [20]. Such interaction also makes the MWCNTs easily dispersible. After treated in H2 at 300 °C for 2 h, Pt and PtSn nanoparticles on the MWCNTs could be formed. Fig. 1 shows the scheme of the synthesis of Pt and PtSn electrocatalysts on MWCNTs via the self-assembly and H2 reduction using THF as the functionalization and anchoring agents. The results indicate that well-dispersed Pt and PtSn nanoparticles can be directly deposited onto the THF-functionalized MWCNTs without any prior chemical oxidation treatments and the as-prepared Pt/MWCNT and PtSn/MWCNT catalysts show a high activity and stability for the ethanol oxidation in acid solutions. The advantages of the THF-functionalized CNT catalyst support are its simplicity and effectiveness in the deposition of highly dispersed Pt and Pt-based electrocatalyts on CNTs. The method could also be applied for the synthesis of other noble metal electrocatalysts on CNTs.
Section snippets
Synthesis of Pt and PtSn catalysts
MWCNTs (Shenzhen Nano, China) were used as the catalyst support without treatments. THF (Sigma–Aldrich) was used as the functionalization and anchoring agent for the MWCNTs and Pt and Sn precursors (H2PtCl6·6H2O and SnCl4·5H2O, Sigma–Aldrich). Pt and Sn precursors with certain atomic ratios were dissolved in THF, followed by addition of MWCNTs. After ultrasonic treatment for 30 min, the suspension was heated at 60 °C under stirring to evaporate the excess solvent till thick slurry was obtained.
TEM, XRD and XPS analysis
Fig. 2 is the TEM micrographs and histograms of Pt and PtSn nanoparticles on MWCNTs. TEM results indicate that the PtSn catalysts investigated here have similar mean particle sizes with homogeneous dispersion and there is no significant change in the particle size with different Sn content in PtSn catalysts synthesized (Fig. 2A). The average particle size of Pt and PtSn nanoparticles is approximately 4 nm with symmetric distribution (Fig. 2B). As shown previously, without functionalization or
Conclusions
In this paper, we demonstrated successfully a simple and new synthesis method to deposit Pt and PtSn nanoparticles on MWCNTs with high dispersion and uniform distribution by self-assembly and H2 reduction method using THF solvent as functionalization and anchoring agent without any prior chemical treatment of the MWCNTs. XRD and TEM results show that the Pt/MWCNTs and PtSn/MWCNTs have the same Pt fcc structure and the particle size is not affected by the Pt/Sn atomic ratios. However, the
Acknowledgement
The project is supported by the Agency for Science, Technology and Research (A*Star), Singapore under SERC Grant No. 072 134 0054.
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