Proton conductivity at the controlled hydrophilic and hydrophobic surfaces of mesoporous aluminum organophosphonates

Abstract

Aluminum organophosphonate (AOP)-type mesoporous materials can be prepared using amphiphilic organic compounds in which an aluminophosphate (AlPO)-based inorganic unit and a designable organic linker are distributed alternately in non-silica-based inorganic–organic hybrid frameworks around supramolecular-mediated mesopores. In general, the resultant AlPO-based frameworks are amorphous and have potential as proton conductive surfaces due to the presence of abundant free phosphoric acid (P–OH) groups and water (H₂O) molecules coordinated to the tetrahedral AlO₄ units in combination with the smooth transportation of protons inside the mesopores. In this study, a series of AOP-type mesoporous materials was prepared using a polymeric triblock copolymer (e.g., Pluronic P123, EO₂₀PO₇₀EO₂₀) to reveal the derived proton conductivity at AlPO-based surfaces with and without methylene (–CH₂–), ethylene (–C₂H₄–) and phenylene (–C₆H₄–) groups. The networking of H₂O molecules was restricted by the presence of strongly hydrophobic organic linkers, even under high-humidity conditions (95% RH). This was a key factor to change the proton conductive mechanism from the Vehicle mechanism at low temperature to the Grotthuss mechanism at higher temperature, with a highest proton conductivity of >10⁻³ S cm⁻¹, comparable to that observed for hydrophilic AlPO-based frameworks. The activation energy was negatively proportional to the size of the organic linker due to the decrease in the number of hydrogen bonds formed/broken during the proton conduction. These insights are quite unique for controlling the proton/water transport rate and the mechanism by designing the organic linker of AOP-type mesoporous materials.