Investigation On Modified Nafion Proton Exchange Membrane For PEMFC

Proton exchange membrane (PEM), which is the critical component in a proton exchange membrane fuel cell (PEMFC), transfers protons from the anode side to the cathode and separate them apart. The prevailing PEM used at present is perfluorosulfonic membranes like the Nafion series produced by Dupont company. Except for the good mechanical strength, high conductivity at fully hydrated state and long durability, there are some disadvantages of the Nafion membranes,like high permeation of the fuel especially when small organic moleculars are used, that causes mixed potential at the cathod side and thus reduces the performance of the PEMFC. Besides, the high proton conductivity of the Nafion membranes relies on the water uptaken in the membrane. At elevated temperature, the proton conductivity drops drastically as the water uptaken decrease. That limits the usage of the membrane, typically at 80 oC.In order to solve the problem listed above, Nafion/SiO₂ composite membrane was prepared by sol-gel method on the basis of Nafion pure membrane. The Nafion/SiO₂ composite membrane has higher water uptake ratio and stonger water retention capacity, and effectively reduces the fuel (methanol and dimethyl ether) permeation through the membrane. The water uptake ratio increased from 29.0 % of the Nafion pure membrane to 33.1 % of Nafion/SiO₂ (11.5 mass %) composite membrane. At 80 oC in dimethyl ether solution, the limiting diffusion current density droped with the increase of SiO₂. Compared with Nafion pure membrane, the limiting diffusion current density of the composite membrane with SiO₂ composited at 3.6 mass %,8.0 mass %,11.5 mass % reduced 31.1 %, 49.7 %, 57.5 %, respectively.However, the Nafion/SiO₂ composite reduced the proton conductivity of PEM. In order to compensate for the conductivity loss, sulfonated Nafion/SiO₂ composite membrane was prepared. The test indicates that the proton conductivity of sulfonated Nafion/SiO₂ composite membrane increased with varied sulfonation time compared with Nafion/SiO₂ composite membrane, even higher than Nafion pure membrane. For dimethyl ether as fuel, the polarization curves of the MEA with different membranes showed that MEA with 8 hours sulfonated Nafion/SiO₂ composite membrane has the best performance of 30.0 mW cm-2, while the MEA with Nafion/SiO₂ composite membrane the second of 22.7 mW cm-2 increased 46.5 % and 10.7 % respectively compared to the MEA with pure Nafion membrane of 20.5 mW/cm2.Meanwhile, Nafion/SiO₂-P₂O₅ composite membrane was also prepared by sol-gel method on the basis of Nafion pure membrane. The Nafion/SiO₂-P₂O₅ composite membrane has a higher water uptake ratio of 33.2 % compared with the Nafion pure membrane of 29.0 %, while the fuel permeation in the composite membrane decreased moderately. The proton conductivity of the Nafion pure membrane and the composite membrane are 0.0907 S cm-1 and 0.0827 S cm-1 at 80 oC。The performance of the MEA with pure membrane and composite membrane showed that the peak power density of the MEA with Nafion/SiO₂-P₂O₅ composite membrane increased 54.3 % compared to the MEA with pure Nafion membrane, from 31.6 mW cm-2 to 20.5 mW cm-2, that may attribute to the interface change between the catalyst and the the composite membrane.