Thiol Enhanced Stability Of Au^Pt Nano-particles Onto Multiwalled Carbon Nanotubes

Platinum supported by high surface carbon are generally used as both the anode and cathode catalysts in proton exchange membrane fuel cell (PEMFC) in order to increase the dispersion and utility of Pt. However, the performance of the Pt/C catalyst are very difficult to maintain during extended operation under the caustic environment (high water content, low pH (<1), high temperature (50–90?C), high oxidative potential(0.6–1.2 V), and high oxygen concentration) due to the following reasons: (1) Pt particles show a strong tendency to shift and agglomerate since their high specific surface energy and week interaction between the metal and the support; (2) Pt dissolution; (3) loss of platinum with corrosion of the carbon support; (4) Pt surface poisoned. The durability of PEMFC has been recently recognized as one of the most important issues to be addressed before the commercialization of the PEMFC. So developing high activity and long lifetime catalysts are an urgent needed, which is a first determining factor for the commercialization of the PEMFC.In order to overcome the poor stability of the commercial Pt/C catalyst, Au^Pt/MWCNTs-SH catalyst was designed to improved the stability of the PEMFC catalyst by the strong interaction between thiol and metal particles and the modified Pt nanoparticles with gold (Au) clusters.Au^Pt/SH-MWCNTs was prepared by the following steps: (1) Surface thiolation of MWCNTs; (2) Anchoring Au^Pt nano-particles onto the surface of MWCNTs-SH by successive reduction of HAuCl4 and H2PtCl6 precursors with glycol; (3) Heat treating Au^Pt/SH-MWCNTs in H2 atmosphere under 150℃for 1h. The results indicated that Au^Pt/SH-MWCNTs is not only good at activity but also at stability.The catalyst was aged under potential cycling between -0.18 and 1.0 volts in over 1500 cycles to assess the stability. TEM method was also used to characterize the catalyst sizes before and after aging test. There were little changes in the activity and electrochemical surface area (ECSA) of Au^Pt/MWCNTs-SH over the course of cycling, in contrast to sizable losses observed with Johnson Matthey Pt/C (JM-Pt/C) catalyst under the same conditions. TEM analysis also revealed that there was insignificant agglomeration of Au^Pt nano-particles after the potential cycling experiments, while serious agglomeration of Pt nano-particles was found in JM-Pt/C catalyst. The above results suggest that the obtained Au^Pt/MWCNTs-SH catalyst own higher electrochemical stability.Finally, the reasons of thiolated carbon nanotubes and Au contributing to the stability of Au^Pt/SH-MWCNTs catalyst was studied by density functional theory (DFT) method. The results showed: S can not anchor Pt stronger than C, but S can enhance the interaction and adsorption of C near S with Pt, electron density distribution on the surface of support is changed as well, which results in the change of Pt cluster configuration. Thiol can weaken Pt-Pt interaction and thus decrease the agglomeration of Pt cluster; The migration of Pt on the surface of S-SWNTs can also be effectively inhibited by the stronger interaction of Pt with the carbon atoms near S; Compared to the pure SWNTs, the agglomeration and deformation activity energy of Pt cluster on the surface of S-SWNTs is lessened because of the weak Pt-Pt interaction. When the Au cluster exists in Pt cluster, the d-band center of the Pt cluster shifts toward the negative and cohesive energy and M-M bond energy decreases. Contrast to the pure Pt, the Au^Pt have higher coordinate number, lower surface energy, more positive oxidation potential.