Preparation And Properties Of Pt Based Anode Catalyst Improved By Ruthenium/manganese Oxide For Direct Methanol Fuel Cell

Direct methanol fuel cells (DMFC) have attracted more and more attention due to their advantages, such as higher power density, zero or low exhaust, and quickly startup at low temperature, etc. Catalysts, as a key component of DMFC, have received much attention not only from academia but also from industries. Among the numerous catalysts, supported platinum is the most prevailing catalyst for the electrocatalytic oxidation of methanol in DMFC. However, before the commercially application of direct methanol fuel cells anode platinum catalysts, there are still many major problems needing to be solved. There are to improve the catalytic efficiency of noble metal, to enhance the carbon monoxide tolerance ability and to promote the durability of catalysts. This work focuses on the R&D of novel anode catalysts for DMFC by using ruthenium dioxide and manganese dioxide to improve the activity and the stability.Pt/CNTs, PtRu/CNTs, and Pt/RuO2/CNTs were firstly prepared and compared with the commercial PtRu/C catalyst. The results show that the electrocatalytic activity for the catalysts is in the order of Pt/RuO2/CNTs> PtRu/CNTs> PtRu/C> Pt/CNTs. And the stability for the catalysts is Pt/CNTs> PtRu/CNTs> Pt/RuO2/CNTs> PtRu/C. It was found that the presence of Ru, no matter in alloy or in oxide, can improve the CO electrooxidation ability. RuO2 in PtRu binary catalyst, which situates between CNTs and Pt particles, can exert its CO electrooxidation ability best. Another role of RuO2 in PtRu binary catalyst is to promote Pt nanoparticles to disperse uniformly on CNTs to obtain larger EAS, resulting in higher activity of methanol electrocatalytic oxidation. Compared to carbon black, CNTs is more stable and excellence as the supporting material for DMFC catalysts.Because Ru is a kind of noble metal and unstable in acidic electrolyte, manganese was designed to replace it. Pt/MnO2/CNTs and PtRu/MnO2/CNTs catalysts were synthesized by successively loading hydrous MnO2 and Pt (or PtRu alloy) nanoparticles on CNTs and were used as anodic catalysts for DMFCs. The existence of MnO2 on the surface of CNTs effectively increases the proton conductivity of the catalyst, which then could remarkably improve the performance of the catalyst in methanol electro-oxidation. As a result, Pt/MnO2/CNTs show higher electrochemical active surface area and better methanol electro-oxidation activity, compared with Pt/CNTs. When PtRu alloy nanoparticles were deposited on the surface of MnO2/CNTs instead of Pt, the PtRu/MnO2/CNT catalyst shows not only excellent electro-oxidation activity to methanol with forward anodic peak current density of 901 A/gPt but also good CO oxidation ability with lower preadsorbed CO oxidation onset potential (0.33 V vs Ag/AgCl) and peak potential (0.49 V vs Ag/AgCl) at room temperature.A green and facile synthesized method of Pt, RuO2 and MnO2 nanocomposites catalyst was proposed based on PtRu/MnO2/CNTs. RuO2-MnO2 complex supported by CNTs was firstly synthesized by the oxidation-reduction precipitation of RuCl3 and KMnO4 with a green and facile method in one step. Then Pt was loaded onto the obtained RuO2-MnO2/CNTs to fabricate a novel anodic catalyst Pt/RuO2-MnO2/CNTs for DMFCs. Pt nanoparticles were found uniformly dispersed on the surface of CNTs with the average diameter of about 2.0 nm. The activities of methanol and CO electrocatalytic oxidation were analyzed, and the reaction mechanism of methanol electro-oxidation on Pt/RuO2-MnO2/CNTs catalyst was discussed. The MnO2 in the catalysts improves the proton conductivity and electrochemical active surface area (EAS) for the catalysts. RuO2 improves the CO oxidation activity and Pt dispersion. CNTs provide effectively electron channels. Thus the Pt/RuO2-MnO2/CNTs catalyst has high utilization of the noble metal Pt, high CO oxidation ability and excellent methanol electro-oxidation activity, being an outstanding anode catalyst for DMFC.The improvement of the stability for PtRu catalysts was designed by covering with manganese dioxide. The PtRu/C and PtRu/CNTs coated with manganese dioxide (denoted as MnO2/PtRu/C and MnO2/PtRu/CNTs, respectively) were facile prepared and used as the catalysts for CH3OH electrooxidation. The as prepared catalysts showed much higher stability for electrocatalytic oxidation of methanol. After 1000 potential cycles,65% activity was still remained for MnO2/PtRu/C, while only 41% activity was left for PtRu/C; 83% activity was still remained for MnO2/PtRu/CNTs catalyst, while only 59% activity was left for PtRu/CNTs. It was found that MnO2 in MnO2/PtRu/C and MnO2/PtRu/CNTs prevented the dissolution of PtRu particles as well as the corrosion of the CNTs supports, resulting in the improvement of the stability and activity.