Preparation And Characterization Of Functional Graphene/Sulfonated Poly(Phenylene Oxide) Composite Films For Proton Exchange Membranes

Fuel cell has been attracted wide interests with outstanding advantages of zero discharge,eco-friendly property, high energy conversion rate, durable property, high energy density and soon. The proton exchange membrane is core part of the cell, which is a challenge in this field.And now, some series have been industrialized, for example, the Nafion series membrane ofDuPont company, XUS-B204series membrane of Dow company, the Aciplex series membraneof ASAHI company. Nafion is the most successful product, which is popular and widely used allover the world. Even so, the limits were also existed such as high price, methanol crossover andbad heat resistibility. Scholars proposed two research routes: one is the modification of Nafionmembrane, and the other is searching for substitute. On the basis of the preliminary experiment,we choose sulfonated product of poly(phenylene oxide), a kind of cheap engineering plastics, asthe matrix of proton exchange membrane. The composite membrane by incorporating thegraphene nanosheets into the sufonated poly(phenylene oxide) matrix was prepared for the firsttime.There are large structure differences between graphene nanosheets and poly(phenyleneoxide) backbone, without similar groups, which lead to the organic-inorganic mixture difficulties.In this situation, we take several meathods of graphene surface modification to improve thedispersibility of graphene nanosheets in the sulfonated poly(phenylene oxide) matrix, which isbeneficial to enhance the integrated performance of the composite membrane.First, the sulfonated poly(phenylene oxide) was prepared by using chlorosulfonic acid assulfonated reagent, which added to the solution of poly(phenylene oxide) in chloroform dropwiseuntil precipitate was appeared. And then, three kinds of methods were taken to modify thegraphene surface to prevent nanosheets from congregating in composites, its compatibility inpolymer matrix was improved. At last, the membranes can be formed via solution casting.The common polymer polystyrene was used as modified agent because it’s miscible withPPO. Co-sulfonated method was explored that the composite was precipitated under the sonicblend condition at the same time, thus nanosheets congregated was avoided in composites. Theproton exchange composite membrane was poor in the ion exchange capacity, but at90℃theproton conductivity of0.1%GNS/SPPO composite membrane is also arrived at0.0145S·cm-1，which was quantitatively improved comparing with that of pure SPPO membrane at0.00701S·cm-1. The mechanical strength of0.1%GNS/SPPO was increased by32%with the tinyaddition of GNS. 2-acrylamido-2-methyl propane sulfonic acid (AMPS) is a kind of gel monomer withamide bond and sulfonic group on its side chain, which is good for water retention. Sulfonicgroup is the excellent proton carrier, however, PAMPS solid is brittle and very easy to dissolvein water, so that it isn’t suitable as the proton exchange matrix alone. In situ polymerizationtechnique was utilized to modify the graphene with two-dimensional molecular brush structureand incorperate the graphene sheets into SPPO matrix, which was more stable in the preparationprocess than the previous one. The product showed excellent performance, expecially at hightemperature. For example, at90℃, the proton conductivity of1%FGO/SPPO compositemembrane was226%higher than that of SPPO membrane.In order to improve the compatibility of graphene and SPPO, we chose the diazonium saltmethod according to the principle of the dissolution in the similar material structure. Thismethod was used to modify the surface of graphene nanosheets with sulfonic acid groups at lowtemperature, that the3-amino benzene sulfonic acid diazonium salt was coupling reacted withthe active hydrogen groups on the graphene nanosheets. Finally, the sulfonated graphene wasmixed with SPPO in origanic solution and the membrane was formed via subsequent solutioncasting. This method is out of high-temperature operation, so that the hydrophilic groups on thegraphene nanosheets are well preserved. At the same time, without the influence of the crosslinknetwork made by polymerization, the functional graphene was well dispersed in the DMFsolution. The tensile strength of1%SGO/SPPO was13.8MPa, and the proton conductivity at90℃was0.0402S·cm-1, which were increased at different degree with pure membrane.