Organic-inorganic Hybrid Proton Conductive Membranes Doped With Phosphonic Acid Group-modified Inorganic Fillers

The key issue of proton exchange membrane(PEM) is the enhancement of proton conduction property. Incorporating phosphonic acids into PEM was proved to be an effective way to improve proton conductiviy. However, it is essential to develop a convenient and efficient method to immobilize phosphonic acids. Therefore, we chose inorganic nanoparticles as carriers, which were combined with phosphonic acids through covalent bonds, and embedded into polymer matrix to prepare organic-inorganic hybrid membranes. The good proton conduction property of phosphonic acids and its synergisitic effect with inorganic fillers is expected to improve the proton conductivity of hybrid membranes.Sulfonated poly(ether ether ketone)/phosphonic acid-functionalized titania nanohybrid membranes were prepared with TiCl4 as inorganic precursor and amino trimethylene phosphonic acid(ATMP) as modifier. Phosphonic acid-functionalized titania nanoparticles with a uniform size of ~50 nm were formed and dispersed homogeneously in SPEEK. Accordingly, the nanohybrid membranes display remarkably enhanced proton conduction property due to the incorporation of additional sites for proton transport and the formation of well-connected channels by bridging the hydrophilic domains in SPEEK matrix. The nanohybrid membrane with 6 wt. % of phosphonic acid-functionalized titania nanoparticles exhibits the highest proton conductivity of 0.334 S cm-1 at 65 oC and 100% RH, which is 63.7% higher than that of pristine SPEEK membrane. Furthermore, the as-prepared nanohybrid membranes also show elevated thermal and mechanical stabilities as well as decreased methanol permeability.Graphene oxide(GO) was synthesized using Hummers method, and coated with polydopamine through self-polymerization of dopamine under low basic condition to form DGO, which was then modified with alendronic acid through Micheal addtion reaction and form PGO. PGO was then incorporated into Nafion matrix to fabricate Nafion/PGO hybrid membranes. The incorporation of PGO improve the water adsorption and retention ability of hybrid membranes, while phosphonic acid groups distributed on GO sheets with a high density introduce new pathways for proton conduction, thus improved the proton conductivity of hybride membranes under both saturated humidity and low humidities. The hybrid membrane with 2 wt.% PGO exhibits a proton conductivity of 0.277 S cm-1 at 110 oC and 100 RH%, and 0.0441 S cm-1 at 80 oC and 40 RH%, respectively 2.2 and 7.6 times of the pristine Nafion membrane.Octahedral crystal MIL-101(Cr) with a uniform size of ~500 nm was synthesized through hydrothermal reaction. Phosphotungstic acid(HPW) was then formed in the pores of MIL-101(Cr) via ship-in-bottle approach with Na2HPO4 and Na2WO4·2H2O as building blocks. MIL-101(Cr) loaded with HPW(HPW@MIL101) was incorporated into SPEEK to prepare SPEEK/HPW@MIL101 hybrid membranes. The loss of HPW was solved because the size of HPW is larger than that of the windows of MIL-101(Cr). The excellent proton conduction ability of HPW and the hydrogen bond networks formed on the surface of MIL-101(Cr) help to form novel pathways within the pores of MIL-101(Cr) as well as at the interface region of HPW@MIL101 and SPEEK, thus enchancing the proton conduction property of hybrid membranes even under low humidity. The proton conductivity of hybrid membrane with 9 wt.% HPW@MIL101 was 0.00651 S cm-1, about 6 times higher than that of pristine SPEEK membrane.