Phase-Changeable Polyoxometalate-Based Acid–Base Adduct for High-Temperature Proton Conduction

Most reported proton-conducting materials use water molecules as media for proton transport, but their low operating temperatures (below the boiling point of water) and humidity-dependent conductivity limit their extensive applications. This motivated researchers to explore high-temperature proton-conducting materials employing nonaqueous media. The acid–base adduct HPW-SA formed by Keggin-type phosphotungstic acid (H3PW12O40, abbreviated as HPW) and sulfamic acid (SA) shows high proton conductivity in the high-temperature region (160–190 °C), owing to its proton mobility remarkably enhanced by its solid–liquid transition. Inspired by this, we encapsulated the HPW-SA adduct into the cages of MIL-101 by the postsynthetic method to prepare the composite HPW-SA@MIL-101, aiming at taking full advantage of the proton conduction feature of the HPW-SA adduct while avoiding the problems of flowing away and corrosion caused by the independent use of HPW-SA adduct. As expected, HPW-SA@MIL-101 exhibits high proton conductivity (0.85 × 10–3–1.1 × 10–3 S cm–1) in the range of 160–190 °C. Its proton transfer activation energy (Ea) is 0.58 eV in the range of 160–190 °C, which indicates that proton migration is dominated by the diffusion effect following a vehicle mechanism. In addition, the proton conductivity of HPW-SA@MIL-101 did not decrease after six cycles of heating and cooling tests, showing good repeatability and stability.