Effects Of Phosphotungstic Acid On Carbon Supported Pt Electrode Methanol Electrocatalytic-Oxidation

The fuel cell has high efficiency in power generation and no pollution; especiallydirect methanol fuel cell has become a research focus in recent years due to the lowprice of methanol, and easy storage, and carry. The most important factor affectingthe performance of direct methanol fuel cell is the proton exchange membrane and theanode catalyst; for proton exchange membrane, the main problem is the high rate formethanol to pass through the proton exchange membrane and high cost; for the anodecatalyst, the main problem is low activity of methanol electro-oxidation catalyst andsusceptible to the poison of intermediate products. Heteropoly acid is acage-structured compound formed by the oxygenated heteropolyanion and with thebalance of hydrogen, and with a very strong redox capacity, and can help improve theredox reactions of electrons and protons transmission environment. Heteropoly acid‘sunique nature of the proton conductivity and catalytic attracts much attention ofresearch in the preparation of the direct methanol fuel cell materials. To solve aboveproblem, the direct methanol fuel cell material properties （proton exchange membraneand the anode catalyst） were optimized in this research, utilizing the unique structureof phosphotungstic acid.Following work has been done in this thesis:（1）DuPont’s Nafion membrane has high proton conductivity, chemical andmechanical stability, and is widely used in direct methanol fuel cell. However Nafionmembrane has high rate of methanol transmittance, so modification of the Nafionmembrane to reduce the methanol through membrane is a hot research topic: thesurface of the membrane can be simply modified with proton conductive materialswith low transmittance rate of methanol. Nafion surface modification is completed byfirstly modified by a layer of chitosan film, and then immersing in a certainconcentration of phosphotungstic acid solution; the chitosan and phosphotungstic acidcan react to form the CS/PWA compound to make a composite film of Nafion/CS/the PWA. Compared with Nafion, Nafion/the CS/PWA composite membrane canlower methanol transmittance rate by about40%, while the proton conductivity doesnot change significantly. The modified Nafion MEA in single-cell performance issignificantly better than the unmodified Nafion membrane.（2）The behavior of phosphotungstic acid on the activated carbon was studied.phosphotungstic acid adsorbed on carbon materials is dispersed very evenly; thethermal decomposition of phosphotungstic acid adsorbed on carbon support can beused to prepare uniformly dispersed and WO₃/C complex with different content ofWO₃, which can be used as the carrier to prepare Pt-WO₃/C catalyst. Pt-WO₃/C Ptnanoparticles are uniformly dispersed on activated carbon, the average particle size ofabout2.5nm, and the distribution range is very narrow; the catalyst activity of Pt-WO₃/C catalyst is almost double of Pt/C and Pt/C （E-TEK） catalyst, and the stability issignificantly better than the latter two, which indicates that the introduction ofWO₃greatly improved the activity and stability of electro-catalytic oxidation ofmethanol. The high Co catalysis of the WO₃comes mainly from the hydrogenspillover effect: the formation of hydrogen tungsten bronze （HxWO₃） by overflowin hydrogen from Pt to tungsten oxide can help to remove the methanol oxidationintermediates, and improves the Pt utilization by providing the proton channel, tosolve the problem of the loss of catalytic activity due to falling of Pt into microporousholes of the carbon carrier and no contact with the electrolyte.