Giant Polyoxometalate Acting as a Nanofiller into a Polymer to Fabricate Proton Exchange Membranes Exhibiting Super Proton Conductivity

The crystalline {Mo 72 Fe 30 } and {W 72 Fe 30 } Keplerates, a polyoxometalate (POM) system, were synthesized in Bielefeld, Germany, by Muller's research group. Subsequently, Liu, Muller, and their co-workers demonstrated the formation of nanoblackberries of {Mo 72 Fe 30 } and {W 72 Fe 30 } in respective aqueous solutions. In this work, we have shown that the isolated tungsten blackberries, Fe3[W72Fe30O252(CH3COO)2(OH)25(H2O)103]<middle dot>180H2O ({W 72 Fe 30 } NB ) exhibit a modest proton conductivity of 3.0 x 10-3 S cm-1 at 80 degrees C and RH 98% with an activation energy of 0.34 eV. We have used this POM-based solid electrolyte of moderate proton conductivity as the nanofiller in an organic polymer, poly [2,2 '-(m-phenylene)- 5, 5 '-benzimidazole] (m-PBI) to fabricate the POM-based polymer composite membranes that are doped with phosphoric acid (PA): 2.5% {W 72 Fe 30 }+PA@m-PBI and 5% {W 72 Fe 30 }+PA@m-PBI for their use as proton exchange membranes (PEMs) at a higher temperature (160 degrees C). The PEMs, 2.5% {W 72 Fe 30 }+PA@m-PBI and 5% {W 72 Fe 30 }+PA@m-PBI, exhibit the proton conductivity values of 0.126 S cm-1 and 0.159 S cm-1, respectively, whereas the PA-doped host-organic polymer (PA@m-PBI) itself shows the conductivity value of 0.056 S cm-1, under an identical experimental condition. Thus, we have demonstrated that a giant {W 72 Fe 30 }-type POM compound showing a moderate proton conductivity in its solid state exhibits super proton conductivity upon its fabrication with an organic polymer, resulting in mixed matrix PEMs.