Synthesis, Characterization And Proton/Methanol Transport Mechanism Of Methanol-blocking Proton Exchange Membrane For Direct Methanol Fuel Cell (DMFC)

The direct methanol fuel cell (DMFC) is a promising power source candidate forportable electronic devices and electric vehicles due to its high energy efficiency andenvironmental compatibility. Its fuel, methanol, which is liquid at room temperature,can be easily and safely stored and transported. In addition, the DMFC system issimple in design and can be operated without fuel reforming. So far, theperfluorosulfonic acid (PFSA) membrane (e.g. Nafion by Dupont) has been usedwidely as proton exchange membrane (PEM) for the DMFC, owing to its excellentstability both in chemical and thermal environment, as well as its high protonconductivity when contains enough water. However the main disadvantage of PFSAmembrane is methanol crossover, and over 40% of methanol could across themembrane at most. Methanol crossover causes loss of fuel, reduced cathode voltageand cell performance. Such disadvantage is also one of the major problems in DMFCapplication.In order to improve the performance of the existing PEM, it's important tounderstand the methanol crossover mechanism and the effects of PEM properties onthe methanol transport in PEM. Therefore, the thesis mainly concentrated onmodifying the PFSA membrane to lower its methanol permeability, trying to designpartial fluorizated PEM candidate, as well as investigating the structure, themethanol-blocking performance, methanol and proton transport mechanism of thePEM. The main results of the thesis are as follows:1.The effects of organic silica bearing different organic groups on the performancesof the Nafion/organic silica composite membranes.A series of Nafion/organic silica hybrid membranes has been prepared by usingorganic silane coupling agents (SCA) bearing different hydrophilic functional groupsWith approximate silica content, the composite membrane modified by SCA bearingaliphatic amino groups (SILCPM3) was found to exhibit extremely low protonconductivity and methanol permeability compared with other composite membranes. Moreover, by controlling the electronegativity of the group connected to the aminogroup, the methanol permeability and proton conductivity of the compositemembrane could also be adjusted. E.g. the composite membrane modified by SCAbearing urea group was found to exhibit 89% lower methanol permeability, 49%lower proton conductivity, as well as improved cell performance compared withNafion 117 membrane. The results showed that, in order to decrease methanolcrossover in the films, we should modify the PFSA membrane aiming at themethanol transfer path(the connected ion clusters formed by terminal -SO₃Hgroups). Thus the additive which has interaction with sulfonic group could alsoaffect the formation of ion clusters, and restricts the methanol permeability as well.However, proton transportation also involves in the connected clusters, the effect ofadditive on sulfonic group may also depress the proton conductivity, and worsen thecell performance of the composite membranes.2. The study of Nafion/cross-linked PVP semi-interpenetrating polymer network(IPN) membrane.From the study of Nafion/organic silica composite membranes, it can be seen thatthe transfer of proton and methanol decreases simultaneously under modifying. Thissuggests that protons and methanol have similar molecular transport mechanisms insulfonic acid containing PEMs, which makes it difficult to improve selectivity forthe DMFC application. Interpenetrating polymer networks (IPNs) consists of twoindependent cross-linked polymer, the IPNs may obtain high selectivity in theory byadjusting the structure, the molar ratio, and the cross-linking degree of the twopolymers.The thesis chose 4, 4'-diazostilbene-2, 2'-disulfonic acid disodium salt (DAS)as cross-linking agent to crosslink poly(vinyl pyrrolidone) (PVP) in commercialNafion117 membranes under the irradiation of ultraviolet. The semi-IPNNafion/PVP membranes exhibited decreasing methanol permeability with theincreasing of the content of PVP. However, with appropriate amount of PVP, thesemi-IPN membrane showed even higher proton conductivity than Naifonl 17(～30% higher). It was also found that the decomposition temperature of the fluorinated etherside-chain of semi-IPN membranes was 60℃higher than that of Nafion117, whichmeans that the semi-IPN membranes had better thermal stability than pristineNafion1 17. From the characterization of wide angle XRD and 1 H solid-state NMRspectra, it can be concluded that the introduction of cross-linked PVP networkrestricted the free squirm of Nafion side-chain, which influenced the hydration ofterminal sulfonic acid group and weakened the connection of ion clusters, thusdepressed the methanol crossover. Howerver, the extra sulfonic acid groups suppliedby crosslinking agent DAS as well as the acid-base interaction produced by aminogroup and sulfonic acid group strengthened the hopping transfer of proton, thus thesemi-IPN membranes exhibited improved proton conductivity with appropriateamount of PVP. Furthermore, the unique Grotthuss transfer mechanism of protoncompared with methanol moleculer should be taken into account for designing andsynthesizing more suitable proton exchange membrane candidates for DMFC.3. The preliminary study of PVDF/AMPS interpenetrating polymer networkmembrane.Aiming at reducing the cost of PEM, and the further exploration of new IPNmembranes with good performance, a novel partial fluorizated PEM candidate wasalso been prepared. Poly (vinylidene fluoride) (PVDF) was chosen as polymermatrix, 2-acryamido-2- methyl-1-propane sulfonic acid (AMPS) was chosen asionomer. AMPS was cross-linked by different cross-linking agents (e.g.vinyltriethoxysilane (VTES), divinylbenzene (DVB), andN,N'-Methylenebisacrylamide (NMBA)) to form PVDF/ AMPS IPN membranes.Through the pre-polymerization of AMPS, selecting proper cross-linking agent, aswell as optimizing molar ratio, reaction time and temperature, the performance of thePVDF/AMPS IPN membranes had been optimized. Compared with Naifonl17, thenew membrane exhibited～2/3 lower methanol permeability but similar protonconductivity, and～1.5 time higher proton selectivity. It was also found that thecross-linking degree, the micro structure and the surface morphology of the IPN membranes was more important to proton selectivity than the amount of ionomer.The thesis also tried to do some surface modification of PVDF/AMPS IPNmembrane via photoinitiated graft polymerisation with AMPS. The results showedthat, after surface modification, the proton selectivity of the new membrane canreach 3 times of that of Naifon117.