Proton conductivity of mesoporous aluminum organophosphonate enhanced by the affinity of an integral organic linker to water molecules

Abstract

The precise design of aluminophosphate (AlPO)-based frameworks is quite promising for improving the smooth transport of protons at surfaces with abundant sites, such as free phosphoric acid (P–OH) groups and water (H₂O) molecules attached to tetrahedral AlO₄ units. Our latest study has demonstrated that enhancing surface hydrophobicity by embedding bulky aromatic groups, such as a phenylene (–C₆H₄– or –Ph–) linker, is essential for improving the stability of aluminum organophosphonate (AOP)-based frameworks. However, such strong hydrophobicity did not seem suitable for amplifying the proton conduction because the length of proton hopping was increased by the presence of bulky organic linkers. In this study, we suggest rational guidelines to aid the design of molecular structures with enhanced proton conductivity inside surfactant-assisted mesopores, especially at the surfaces of AOP-based frameworks. Two AOP-type materials containing a biphenyl (–C₆H₄–C₆H₄– or –BP–) group, with increased hydrophobicity, and a hydroquinonyl (–C₆H₂(OH)₂– or –HQ–) group, which facilitates the proton conduction pathway, were prepared as AOP–BP and AOP–HQ, respectively, using Pluronic P123 (EO₂₀PO₇₀EO₂₀). Consequently, the AOP–HQ-type mesoporous material showed a superprotonic conductivity of 1.31 × 10⁻² S cm⁻¹ under 95% RH at 90 °C. The HQ linker, which provides abundant and additional –OH groups over the hydrophobic –Ph– linker, promotes the formation of a continuous network of H₂O molecules over the entire surface, leading to efficient proton conduction through the Grotthuss mechanism.