Hybrid ordered mesoporous carbons doped with tungsten trioxide as supports for Pt electrocatalysts for methanol oxidation reaction

摘要

The electrocatalytic activity towards the methanol oxidation reaction (MOR) of three series of Pt electro-catalysts supported onto the surface of hybrid nanostructures composed of ordered mesoporous carbons (OMCs) and tungsten trioxide (WO_3) is investigated. OMC nanostructures are obtained by hard-template method, through the carbonization of sucrose into the mesopores of the SBA-15 silica and subsequent removal of the template. Hybrid OMC-WO_3 supports are synthesized by impregnation of phosphotungstic acid (PWA) onto OMCs, followed by thermal decomposition in N_2 atmosphere at three different temperatures (400, 500 and 600℃, respectively). Pt/OMC-WO_3 electrocatalysts are prepared by the wetness impregnation technique using a solution of chloroplatinic acid hexahydrated in acetone. Hybrid OMC-WO_3 supports and Pt/OMC-WO_3 electrocatalysts are characterized from the structural-morphological viewpoint by means of X-ray diffraction, X-ray photoelectron spectroscopy with energy-dispersive spectroscopy, scanning and transmission electron microscopies. Cyclic voltammetry, CO stripping and chronoamperometry measurements are used for the electrochemical characterization of the electrocatalysts towards MOR: the electrocatalytic activity is dependent both on the nature of the supporting materials and on the particle size and structure of WO_3. Catalysts containing smaller WO_3 crystallites, in fact, demonstrate superior mass-specific activity towards MOR, compared to those possessing larger WO_3 particles or amorphous WO_3. Finally, the electrochemical behaviour of the best performing electrocatalyst, the Pt/OMC-WO_3-500, is compared to that of its counterpart with no WO_3 (i.e., Pt/OMC) in a single 5 cm~2 DMFC cell: the obtained slightly enhanced electrocatalytic activity is attributed to the mutual effect of the hybrid support and the metal nanoparticles. The presence of WO_3 not only promotes the mass-specific activity of the supported electrocatalysts, but also decreases the ohmic resistance.