Cross-linked Proton Exchange Membranes For Direct Methanol Fuel Cells: Preparation And Performance Studies

Direct methanol fuel cells (DMFCs), a kind of proton exchange membrane fuelcells (PEMFCs), which utilize methanol as a fuel generate electricity without the useof any reforming unit, have made a vigorous evolution among the fuel cells becausethey combine high efficiency with low operating temperatures. Proton exchangemembrane (PEM) is a key component of PEMFCs, and the performance of PEMaffects the progress in the applications of PEMFCs. The basic requirements of PEMswere listed as followed:1) High proton conductivity.2) Low fuel permeability.3)Strong thermal and mechanical stabilities.4) Good processability.5) Reasonablecost. As a representative of the most successful PEM materials, Nafion developed byDuPont possesses high ionic conductivity and excellent chemical stability. Due tothe excellent performances of Nafion, DMFCs based on Nafion have beenon their way of commercialization. However, there are still many non-ignorabletechnical limitations which are difficult to overcome that hinder their way ofpopularization. Among the limitations, the high cost,high methanol permeabilityand low proton conductivity over80oC are the principle ones. Thus, with adjustableproton conductivity and excellent mechanical stabilities, sulfonated aromatic mainchain polymers begin to receive growing interests and explore upsurge have been setoff.Among the candidates, SPEEKs have attracted much attention due to their lowcost and appropriate chemical and thermal stabilities. However, like other sulfonatedaromatic main chain polymer, the SPEEKs become more swollen and losemechanical strength with the increasing number of sulfonic acid groups which is thebasic requirement to achieve high proton conductivity. The large dimensional variations and fuel crossover render the membranes unsuitable for the applicationsin DMFCs. Thus, it is nessassery to banlance the performances of the membranesusing different methods.In this work, we aim to choose proper cross-linkers to improve the performanceof the membranes. There are several promising ways, such as covalernt-ionicallycross-linking, covalent cross-linking and layer-by-layer. We use different methods tomake cross-linked membranes. Firstly, we have synthesized a novel bisphenolmonomer with pendant amino groups. Then,1H NMR was used to confirm thestructure of the bisphenol monomer. Because of the activity of amino group, it couldeasily cross-link with other reagents, meanwhile it could also be used as acrosslinker. All the works are described in details as followed:We prepared a series of poly(ether ether ketone) with pendant amino groups(Am-PEEK). In chapter2, we synthesized a water swollen C-SPEEK as the basicmembrane, using Am-PEEK as the crosslinker to prepare covalent-ionicallycross-linked membranes. The results demonstrated that, all the cross-linkedmembranes had excellent thermal stability and mechanical stability, meanwhile thedimensional stability and methanol residence of the membranes were also improved.In particular, C-SPEEK/Am-PEEK-5had a similar proton conductivity as thepristine membrane at80oC, which comfirmed that the ionical bond could slip athigh temperature. The proton conductivity of other cross-linked membranes wasdecreased as the increasing of crosslinker contents.In chapter3, in order to improve the performace while maintain a goodconductivity, we prepared layer-by-layer membranes. Lowly sulfonated SPAEK-Cwith medium ionic conten was selected as the support material, and Am-PEEK wasused as a polycation, by incorporating a highly sulfonated SPAEK-C, we canproduce multilayered membranes with low methanol permeability and high protonconductivity. Then, we prepared15-bilayer cross-linked PEC by heating treatment.FT-IR and SEM indicated that the membranes were successfully modified. After test,the multilayed membranes showed low methanol permeability and high protonconductivity. The15-bilayer cross-linked membrane showed a much lower methanol permeability because of the cross-linking network.In the previous works, we utilized base-acid interaction to prepare cross-linking.Thus, in chapter4, we synthesized sulfonated poly(ether ether ketone) bearingpendant amino groups. Amino group was reactive and could react with severalgroups to creat cross-linking network, avoiding to loss sulfonated acid groups. Wechose four different kinds of cross-linkers to make cross-linked membranes. Bycomparing the results, we studied the effects of main structure of crosslinker on theperforamances of the membranes. We found that TMBP with rigid main chainstructure was the proper crosslinker, because the Am-SPEEK-TMBP showed thebest performances.In the last chapter, we synthesized a benzimidazole timer (rigid-BI) ascrosslinker. Then, we prepared Am-SPEEK-BI-x membranes with different BIcontents. The solubility test and FT-IR were used to confirm that the corss-linkingreaction was completed. The water uptake and swelling ratio were dramaticallyreduced, and the thermal and mechanical stability were also improved. Theselectivity of the cross-linking membranes was also improved, which was higherthan that of Nafion117.