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

Abstract

The crystalline {Mo₇₂Fe₃₀} and {W₇₂Fe₃₀} 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 {Mo₇₂Fe₃₀} and {W₇₂Fe₃₀} in respective aqueous solutions. In this work, we have shown that the isolated tungsten blackberries, Fe₃[W₇₂Fe₃₀O₂₅₂(CH₃COO)₂(OH)₂₅(H₂O)₁₀₃]·180H₂O ({W₇₂Fe₃₀}_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% {W₇₂Fe₃₀}+PA@m-PBI and 5% {W₇₂Fe₃₀}+PA@m-PBI for their use as proton exchange membranes (PEMs) at a higher temperature (160 °C). The PEMs, 2.5% {W₇₂Fe₃₀}+PA@m-PBI and 5% {W₇₂Fe₃₀}+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 {W₇₂Fe₃₀}-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.