Study Of Pt-Ru Based Anode Catalysts In Direct Methanol Fuel Cells

Direct methanol fuel cells (DMFCs) have been paid plenty of attention as portable power sources and electric devices due to their high energy density and simplicity in structure such as easy storage and supply of fuel, without need of reforming and humidification. However, the DMFCs commercial development is limited by the two main factors: the anode catalyst is easily poisoned by the intermediates of methanol electrooxidation, and the methanol can be crossover from anode to cathode. This work aims to enhance the performance of catalysts for DMFCs, the anodic catalysts were modified as follows: first, carbon support was pretreated at different solutions; and then the PtRu based anodic catalysts were doped by the addition of third metal. Adopting density functional theory (DFT) to study the adsorption of CO on Pt and PtRu, this provided a theory basis for the development anodic catalysts for DMFCs.Multi-walled carbon nanotubes (MWCNTs) are the mecro-pore structure in a three-dimensional reticulation. The results of XRD and TEM tests from PtRu/MWCNTs catalysts indicated that the PtRu metals particles evenly distributed on the MWCNTs, parts of Pt and Ru metals formed alloy. Electrochemical test showed that the performance of PtRu/MWCNTs catalyst for methanol electrooxidation was higher than that of the PtRu/XC-72 catalyst. CO adsorption-oxidation mechanism in pure Pt and the PtRu systems were investigated by DFT calculation. There are three CO adsorption models in pure Pt, namely top adsorption, bridge adsorption, and center adsorption. The activity order for CO desorption on catalyst is: bridge adsorption >center adsorption >top adsorption. It is easy for CO adsorption and oxidation in the PtRu system. So the results showed that the doped Ru in pure Pt promoted greatly adsorption and oxidation of CO, also improved the CO-tolerance ability of the catalyst.The results of Raman spectra, FTIR spectra, and TEM tests on MWCNTs pretreated by mixed acid reflux, KMnO4 oxidation, and H2O2 oxidation method, respectively, showed that the MWCNTs treated by mixed acid reflux method not only were purified, but also modified by the oxygen-containing functional groups in great quantities. In order to predigest the pretreatment technics of MWCNTs, MWCNTs were treated by ultrasonic in mixed acid. The results showed that MWCNTs were processed by mixed acid ultrasonic method to have better functionalization effect, and technics were simple. The influence of ultrasonic frequency on the functionalization of MWCNTs was investigated. The results showed that the tube structure was intact; its surface contained abundant oxygen-containing functional groups, which favored the deposition of the metal Pt and Ru in the catalyst preparing process when ultrasonic frequency of 45 kHz was used. The results of electrochemical test presented that the peak current of PtRu catalysts for methanol electrooxidation, which carbon support was pretreated at 45kHz, was 8 times higher than these of the others. It showed that its performance for methanol electrooxidation is the best.To change metal mole ratio (MPt:MRu:MM) in the PtRuMe(Me=Ce,La,Sn)/ MWCNTs catalysts, ternary alloy catalysts was prepared by impregnation reduction method. The CV result for different doped metal and mole ratio of ternary alloy catalysts showed that the activity of PtRuCe/MWCNTs catalyst (5:4:1 mole ratio), PtRuLa/MWCNTs catalyst (4:4:2 mole ratio), and PtRuSn/MWCNTs catalyst (6:3:1 mole ratio) for methanol electrooxidation, respectively, was higher than that of ternary alloy catalysts with the other mole ratio.To discuss doped metal function for methanol electrooxidation on PtRu catalysts, physical and chemical property of ternary alloy catalysts were investigated. The results from XRD and XPS showed that the doped Ce in the PtRu/MWCNTs catalyst effectively changed the surface characterization of PtRu/MWCNTs catalyst. There was electronic transfer between Ce and Pt atoms. There were abundant oxygen-containing species in CeO2, which could oxidize CO adsorbed on Pt in time, thus to improve the electrochemical oxidation ability of methanol. Electrochemical tests indicated that PtRuCe/MWCNTs catalyst had a higher methanol electrooxidation activity and stronger CO-tolerance ability. The peak current on PtRuCe/MWCNTs catalyst for methanol electrooxidation was about two times as big as that of PtRu/MWCNTs catalyst.The results from XRD and XPS showed that the doped La in the PtRuLa/MWCNTs catalyst behaved a better coordinative effect and provided more activity sites for methanol adsorption. In the meantime, there is electronic interaction between La and Pt, making the CO adsorption intensity on Pt lower. Electrochemical tests indicated that PtRuLa/MWCNTs catalyst had a higher methanol electrooxidation activity and stronger CO-tolerance ability, the peak current of PtRuLa/MWCNTs catalyst for methanol electrooxidation is about 6 times higher than that of PtRu/MWCNTs catalyst. The XPS results for PtRuSn/MWCNTs catalyst indicated that electronic effect between SnO2 and Pt in PtRuSn/MWCNTs catalyst makes CO adsorption force on Pt lower. Electrochemical tests indicated that the peak current of PtRuSn/MWCNTs catalyst for methanol electrooxidation is more about 8 times higher than that of PtRu/MWCNTs catalyst. Namely, the catalytic activity of the PtRuSn/MWCNTs catalyst is higher for methanol electrooxidation than that of the Pt–Ru/MWCNTs catalyst. Its tolerance performance to CO formed as one of the intermediates of methanol electrooxidation is better than that of the Pt–Ru/MWCNTs catalyst.