Pt-based Heterogeneous Nanomaterials As Electrocatalysts For Direct Methanol Fuel Cells

Direct methanol fuel cell(DMFC)is a type of the proton exchange membrane fuel cells(PEMFCs),which directly uses aqueous methanol or methanol steam as the sources of fuel and oxygen or air as oxidant.The main problems for DMFCs mainly focus on these two points:Methanol fuel will diffuse through the proton exchange membrane from the anode to the cathode,producing a mixed potential to reduce the output efficiency of the battery.At the same time,the cost of proton exchange membrane is too high for the practical application.On the other hand,the intermediate product from the methanol oxidation will be adsorbed on the catalyst surface,occupying the active sites of catalyst,and quickly deactivating the catalyst.In view of the existing problems in the DMFCs,we prepare selected catalyst by structure control of the composite nanomaterials.Owing to strong interaction among the different components,the composite nanomaterials not only have enhanced intrinsic properties,but also show many other excellent characteristics.On the basis of the deep understanding of the mechanism for methanol oxidation and oxygen reduction reduction,we make full use of the lattice strain effect and electronic coupling effect in composite nanomaterials to tune the catalytic performance.The composite nanomaterials with controlled structure we synthesized not only show higher catalytic activity,but also have the selectivity for methanol oxidation or oxygen reduction in DMFCs.Moreover,the using of the selective catalyst for DMFCs can reduce or even get rid of the dependence on the proton exchange membrane.The results obtained are as follows:1 Bimetallic nanodendrites of Au-Pt nanocomposite:The fabrication of bimetallic Au-noble metal nanoparticles with dendritic morphologies involves the synthesis of multiply twinned Au seeds and the subsequent reduction of other noble metal precursors.The bimetallic Au-Pt nanodendrites display superior catalytic activity towards methanol oxidation due to the electronic interaction between the Au core and Pt branches in dendritic bimetallic particles and the abundant atomic steps,edges,and corner atoms in the Pt branches.2 A universal approach to the synthesis of nanodendrites of noble metals:The metal acetylacetonate precursors are reduced into metal nanoparticles,while oleylamine is simultaneously to protect the nanoparticles.The competition between particle aggregation and oleylamide passivation results in the formation of a large number of nanodendrites.In particular,in comparison with commercial PtRu/C catalysts,the alloy PtRuOs nanodendrites exhibit superior catalytic activity toward methanol oxidation.3 The selective anode catalyst for direct methanol fuel cell:To make use of the electronic coupling effect among different components,we designed and fabricated heterogeneously structured Au-Ag2S-Pt nanocomposites.The electron density on Pt surface increases due to the electron transfer from other domains to Pt domain in the nanocomposites,which weakens the adsorption of intermediate products during the oxidation of methanol,and then improve their catalytic activity for the same reaction.4 The selective cathode catalyst for direct methanol fuel cell:We designed and synthesized cage-bell structured Pt-M heterogeneous nanostructures as selective electrocatalysts for the cathode reaction(oxygen reduction reaction)of DMFCs.The discontinuous shell layer can selectively permit the pass through of small molecules and block the ones,making the cage-bell Pt-M nanostructures has a good methanol tolerance.We therefore present a good example to resolve the methanol crossover by constructing a geometric design of nanostructures.5 The activation of supported-platinum electrocatalysts:We report an effective approach for the removal of capping agents from the surface of supported-platinum nanoparticles.This strategy involves the protonation of the surfactants by refluxing the supported-platinum particles in acetic acid and their subsequent removal at a high potential by an electrochemical treatment.We will demonstrate that this combined activation process is essential for the high performance of platinum particles in catalyzing reactions in DMFCs,including methanol oxidation reaction and oxygen reduction reaction.6 Selective catalysts for direct methanol fuel cell:The heterogeneous Au-Ag2S-Pt nanocomposites show good resistance to CO poisoning due to the electronic coupling effect among their different components,and then they could be used as selective catalysts for anodic methanol oxidation.The cage-bell structured Pt-M heterogeneous nanostructures,which exhibit good methanol tolerance performance,can be adopted as selective catalysts for cathode oxygen reduction.We constructed a membraneless model for DMFCs based on these two kinds of selective catalysts and its performance was evaluated.7 Effect of electronic coupling on the electrocatalytic performance of platinum metal:We prepare core-shell Ag-Pt,Au-Pt,and hollow Pt nanoparticles with comparable sizes and morphologies,and tailor the electron density around the Pt atoms via the differences in electronegativities between the core and shell components.Specifically,core-shell Ag-Pt nanoparticles are active for the methanol oxidation reaction due to the presence of electron donation from Ag to Pt,while core-shell Au-Pt nanoparticles exhibit superior activity for the oxygen reduction reaction due to the electron withdrawing effect from Pt by Au.