Study Of Ruthenium-Promoted Pt Electrocatalyst: Chemical State And Stability Of Ruthenium

PtRu electrocatalyst is so far the most effective anode catalyst for direct methanol fuel cell (DMFC). However, the exact state of the active ruthenium component that takes effect in promoting the catalytic performance of Pt is still a point of discussion. A comprehensive investigation of the actual chemical state and the corresponding promotional effect of ruthenium would be of great significance for improving the electrocatalytic activity of Pt catalyst as well as the performance of DMFC. In the present dissertation, a series of Pt-(RuOxHy)m electrocatalysts with RuOxHy as the exclusive ruthenium species were carefully prepared. The influence of catalyst composition (atomic Ru/Pt ratio) and pretreatment potential range on the promotional effect of RuOxHy was systematically investigated, and further, various approaches were studied in depth in a hope to efficiently stabilize the RuOxHy species. The results obtained are as follows:1) Multi-walled carbon nanotube supported Pt-(RuOxHy)m electrocatalysts (m is atomic Ru/Pt ratio), in which amorphous hydrous ruthenium oxide (RuOxHy) is the exclusive Ru-containing species, were prepared and comprehensively characterized by X-ray diffraction, X-ray photoelectron spectroscopy, temperature programmed reduction, thermogravimetric analysis and transmission electron microscopy techniques. Before the electrochemical measurement, the electrode catalyst was pretreated in H2SO4 by cyclic voltammetric scanning over two different potential ranges:narrow (-0.20-0.46 V vs. SCE) and extended (-0.20～0.96 V vs. SCE) potential ranges. The results show that RuOxHy can effectively promote the oxidative removal of CO on Pt surface, but also lead to a loss of electrochemically active surface area of Pt. When the electrode catalysts were pretreated in the narrow potential range, Pt-(RuOxHy)0.10/MWCNTs exhibited the highest activity for methanol oxidation among all the investigated samples, with both mass-specific activity (MSA) and intrinsic activity (IA) one order of magnitude higher than those of the reference Pt/MWCNT catalyst, which further verified the significant promotion effect of RuOxHy. After being subjected to electrochemical pretreatment over the extended potential range, however, the activity for methanol oxidation of Pt-(RuOxHy)m/MWCNTs decreased remarkably relative to their counterparts pretreated in narrow potential range duing to the dissolution of RuOxHy. The effects of preparation conditions on the electrochemical behavior of Pt-RuOxHy catalyst and the stability of RuOxHy were also studied. 2) It was found that the doping of transition metal oxides (WOm and MoOm) could effectively improve the stability of RuOxHy in Pt-(RuOxHy)0.10/MWCNTs. The dissolution of RuOxHy in Pt-(RuOxHy)0.10/MWCNT catalyst could be reduced from 70% to ca.15% by the addition of WOm or MoOm. Furthermore, the resultant Pt-RuOxHy-WOm/MWCNT and Pt-RuOxHy-MoOm/MWCNT catalysts showed enhanced CO tolerance, and also more than double in IA value of Pt for methanol electro-oxidation, as compared with Pt-RuOxHy/MWCNT catalyst.3) The effects of the carbon support properties on Pt-RuOxHy were also investigated. The stability of RuOxHy in Pt-RuOxHy catalysts supported on XC-72 was significantly higher than that in the catalysts supported on MWCNTs. The introduction of WOm or MoOm into Pt-RuOxHy/MWCNTs could induce an enhanced stability of RuOxHy, while it was not applicable for Pt-RuOxHy/XC catalyst. These results showed that the effects of WOm and MoOm on the the stability of RuOxHy were closely related with the type of carbon support.