| Direct ethanol fuel cell (DEFC) has attached much attention for their potential applications as clean and mobile power sources because ethanol has several important advantages:it is non-toxic liquid, easy to store and transport; it can be produced in large quantities from renewable sources (e.g. sugar, starch and cellulose); and it has a high energy density (8.01kW·h·kg-1). However, the key to improve energy conversion efficiency of DEFC is electrocatalysts. But the most important issues is quite low electrocatalytic activity of catalysts for ethanol oxidation and difficult C-C bond breakage. A big challenge in the development of DEFC technology is to explore highly active catalysts for breaking the C-C bond to accomplish ethanol complete oxidation to CO2releasing12electrons. For the commercialization of DEFC electrocatalysts with high catalytic activity and low cost are needed. This work mainly enhance the activity of the precious metal platinum, give full play the synergy between the platinum and other metals (such as Sn, Rh), improve the surface properties of the catalysts and optimize composition and preparation method of binary PtRh/C catalyst in order to synthesize high performance anode catalysts for direct ethanol fuel cell. The main research contents and corresponding results are integrated into the following two parts.(1) Ethanol oxidation on nano-cubic Pt modified by tin has been synthesized and was used to investigate the role of this adatom in the oxidation mechanism. The onset potential of ethanol oxidation was significantly shifted negatively which can be forward about300mV when the coverage of Sn (θSn) is up to0.9. The electrtochemical in situ FTIR results demonstrated the amount of CO2increased then decreased with θSn increased and it was maximuned when θSn was0.38. Furthermore, acetic acid can be observed at very low potential (-0.05V) after modifying Tin adatom, and the amount of acetic acid increased with θSn increased. That is, tin deposited on platinum surfaces has a double effect on the ethanol oxidation mechanism. First, it facilitates the oxidation of CO coming from the cleavage of the C-C bond in ethanol by a bifunctiontal mechanism. Second, the PtSn catalyzes the oxidation of ethanol to acetic acid. This means that the main product in the oxidation of ethanol for the PtSn system should be acetic acid unless the platinum surface structure has some special sites able to break the C-C bond.(2) Cubic PtRh alloys supported on graphene (GN) with different atomic ratio of Pt and Rh (PtxRhy/GN) were directly synthesized using the modified polyol method with Br" for the shape-directing agents. The process need not add those surface agents such as PVP that occupies the active sites of electrocatalysts and is difficult to remove. For the directly prepared cubic PtxRhy/GN, it can be directly applied in various catalytic systems without further treatment, and the interaction force between nanoparticles and carbon support is stronger compared to their mechanical mixture of catalyst and carbon support. Besides, the π sites (sp2-hybridized carbon) on the GN act as anchoring centers for nanoparticles, thus strengthening the nanoparticle-support interaction. The X ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM) were used to characterize structure and morphology of these electrocatalysts. The results showed that they were composed of homogeneous cubic PtRh alloys. Traditional electrochemical methods, such as cyclic voltammetry and chronoamperometry, were used to investigate electrocatalytic properties of PtxRhy/GN towards ethanol electrooxidation. It can be seen that PtxRhy/GN in all atomic ratio exhibited high catalytic activity, and the most active one has the composition with the Pt:Rh=9:1atomic ratio. Electrochemical in situ FTIR spectroscopy was used to evaluate the cleavage of C-C bond in ethanol at room temperature in acidic solutions, the results illustrated that Rh in alloy can promote the split of C-C bond in ethanol, and the alloy catalyst with the atomic ratio of Pt:Rh=1:1showed an obviously better performance for the C-C bond breaking in ethanol and higher selectivity for tthe enhanced activity he ethanol complete oxidation to CO2than alloys with other ratio of Pt and Rh. We demonstrated that CO adsorption ability on rhodium is stronger than that on platinum, and Rh promotes the cleavage of the C-C bond and improve the selectivity of CO2. The investigation indicates that of PtxRhy/GN electrocatalysts is due to the specific shape of alloys and the synergistic effect of two metal elements as well as graphene support, and combination of traditional electrochemical method and in situ FTIR spectroscopic technology is effective approach to evaluate electrocatalytic performance for ethanol oxidation. |
PDF Full Text Request