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

The crystalline <b>{Mo</b>_<b>72</b><b>Fe</b>_<b>30</b><b>}</b> and <b>{W</b>_<b>72</b><b>Fe</b>_<b>30</b><b>}</b> 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 <b>{Mo</b>_<b>72</b><b>Fe</b>_<b>30</b><b>}</b> and <b>{W</b>_<b>72</b><b>Fe</b>_<b>30</b><b>}</b> in respective aqueous solutions. In this work, we have shown that the isolated tungsten blackberries, Fe₃[W₇₂Fe₃₀O₂₅₂(CH₃COO)₂(OH)₂₅(H₂O)₁₀₃]·180H₂O (<b>{W</b>_<b>72</b><b>Fe</b>_<b>30</b><b>}</b>_<b>NB</b>) 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): <b>2</b>.<b>5% {W</b>_<b>72</b><b>Fe</b>_<b>30</b><b>}+PA@m-PBI</b> and <b>5% {W</b>_<b>72</b><b>Fe</b>_<b>30</b><b>}+PA@m-PBI</b> for their use as proton exchange membranes (PEMs) at a higher temperature (160 °C). The PEMs, <b>2</b>.<b>5% {W</b>_<b>72</b><b>Fe</b>_<b>30</b><b>}+PA@m-PBI</b> and <b>5% {W</b>_<b>72</b><b>Fe</b>_<b>30</b><b>}+PA@m-PBI</b>, 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 (<b>PA@m-PBI</b>) itself shows the conductivity value of 0.056 S cm^-1, under an identical experimental condition. Thus, we have demonstrated that a giant <b>{W</b>_<b>72</b><b>Fe</b>_<b>30</b><b>}</b>-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.