Enhanced proton conductivity assisted by sodium ions in the proton conductive hybrid membranes

Development of new proton exchange membranes for fuel-cells draw considerable attentions. A family of proton conductive membranes based on poly(sodium 4-styrene sulfonate) (PSSNa) and polyvinylidene fluoride (PVDF) combining with various additives oxalic acid (OX), phosphate (PA) and phosphomolybdic acid H3PMo12O40·XH2O (PMo12) have been fabricated through a slurry casting approach, and are defined as PSSNa-PVDF-OX, PSSNa-PVDF-PMo12 and PSSNa-PVDF-PA respectively, whose proton conductivities have been systematically investigated. Among them, PSSNa-PVDF-OX shows the superior proton conductivity, compared to PSSNa-PVDF-PA and PSSNa-PVDF-PMo12, and the best proton conductive value can reach up to 5.0 × 10−3 S cm−1 at 60 °C and 95% relative humidity, proving the good compatibility of OX with PSSNa and PVDF, giving birth to an effective proton transport channel. In order to demonstrate the role of Na+, poly(4-styrene sulfonic acid) (PSSA) is employed to replace PSSNa, generating another series of proton conductive membranes PSSA-PVDF-OX. The proton conductivity of PSSNa-PVDF-OX is greater than those of PSSA-PVDF-OX, which is attributed to the strong interactions between Na+ and water molecules, accelerating proton movement. Summarily, the enhanced conductivity is attributed to the generated efficient conduction pathways constructed from Na+ ions, water molecules, abundant -COOH from OX and -SO3- groups from PSSNa as well as rich F groups from PVDF.