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 Müller’s research group. Subsequently, Liu, Müller, 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, Fe 3 [W 72 Fe 30 O 252 (CH 3 COO) 2 (OH) 25 (H 2 O) 103 ]·180H 2 O ( {W 72 Fe 30 } NB ) exhibit a modest proton conductivity of 3.0 × 10 –3 S cm –1 at 80 °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 °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.