Giant {Mo₁₃₂} polyoxometalate isolated with diverse organic cations: a systematic proton conductivity study

The development of efficient and stable proton conductors is a pivotal area of research due to their transformative potential in alternative energy technologies. Recently, there has been a surge of interest in synthesizing proton conductors based on polyoxometalate (POM) materials, attributed to their highly negatively charged and oxygen-rich surfaces. In this study, we report on a highly water-soluble giant POM, (NH₄)₄₂[Mo₁₃₂O₃₇₂(CH₃COO)₃₀(H₂O)₇₂]·ca.300H₂O·ca.10CH₃COONH₄ (designated as {Mo₁₃₂}), which was rendered insoluble in water by exchanging its ammonium cations with larger organic cations, specifically histidinium, pyridinium, bipyridinium, and methyl viologen, resulting in His-Mo132, Py-Mo132, Bpy-Mo132 and MV-Mo132, respectively. These ion-exchanged compounds were thoroughly characterized through comprehensive spectral analyses, elemental analyses and microscopic studies. The substitution with organic cations containing nitrogen centres not only rendered {Mo₁₃₂} insoluble, but also increased the number of proton hopping sites, thereby enhancing proton transport. Consequently, His-Mo132, Py-Mo132, Bpy-Mo132 and MV-Mo132 demonstrated impressive proton conductivity. Among these, Py-Mo132 stood out with a proton conductivity of 1.07 × 10^-2 S cm^-1 under 98% relative humidity at 80 °C. All four compounds exhibited proton conduction predominantly via the Grotthuss mechanism. Furthermore, stability assessments of these Mo₁₃₂-based proton conductors were conducted under operational conditions to evaluate their performance in practical applications.