Synthesis And Performance Of Self-assembling Nafion Methanol-blocking Membrane With PDDA Modified Metal Particles

Fabrication of methanol blocking membrane (MBM) via electrostatic self-assembly technology was put forward in order to suppress methanol crossover and maintain high proton conductivity of proton exchange membrane. The feasibilities and advantages of fuel cell with self-assembled MBM were discussed theoretically. Accordingly, charged platinum and palladium nano-particles modified with PDDA ionmers were synthesized in the study. The charged particles were electrostaticly self-assembled on Nafion membrane surface and a novel self-assembled methanol blocking membrane (SA-MBM) for direct methanol fuel cell (DMFC) was fabricated. In the SA-MBM, all Pt and Pd particles contact closely with -SO3^- of Nafion membrane. So it is reasonable to consider that all particles are located in the exit and entrance of methanol channels and has favorable methanol blocking efficiency. Moreover, the particles are too large to enter Nafion membrane, which enable us to maintain high proton conductivity. At the same time, some conclusions had been summarized as following:(1) The obtained charged platinum and palladium particles modified with PDDA, PDDA contact Pt and Pd particles with carbon element. The modified Pt and Pd particles contained positive charge in all pH value range and were stabled with electrostatic and steric effect of PDDA. The particle size of Pt and Pd had obvious correlation with concentration of metal precursor and PDDA, pH value of the solution, and redox of reducer. The particle size of Pt and Pd would minish when content and pH value of the PDDA increase, concentration of metal precursor decrease, and redox of reducer increase; The obtained charged Pt and Pd particles modified with PDDA were sized with 2.1～5.8 nm,2.5～6.2nm in average respectively, by controling reaction condition.(2) All platinum and palladium particles modified with PDDA particles are located in the exit and entrance of methanol channels and without entering Nafion membrane. And the distribution of these particles can be controlled from denseness to sparseness by adjusting the concentration of NaCl solution and dipping time of Nafion(?) membrane in colloidal solution.(3) It is found the proton exchange membrane self-assembled with metal particles have the lower methanol crossover. When the diameter increased, the methanol-crossover and relative selectivity of the membrane increased. When increase the particles size than 4.5 nm had little influence on the methanol-crossover, and their methanol diffusion coefficient were almost 5.20～5.40*10^-7cm²/s, relative selectivity were 3.95 and 4.27 for Pt and Pd particles, respectively. The proton conductivity of the proton exchange membrane also was influence by the particles size. The proton conductivity decreased with little increase of the particles size, and the proton conductivity of self-assembled membrane with Pt paticles are little lower than the Pd particles'. All the self-assembled membranes have higher OCV and performance than original Nafion membrane. When the particles size were 5.8nm and 6.2nm for Pt particles and Pd particles, they would have the highest OCV, 0.745V and 0.757V, respecvtively, due to the reciprocal of the methanol permeation current density and membrane area resistance.methanol diffusion coefficient were almost 5.20～5.40*10^-7cm2/s. The Current-Voltage (I-V) curves show that the self-assembled PEMs have better output than that of the pure Nafion membranes. And their output relate with relative selectivity, the higher relative selectivity, the more output.