Anhydrous Superprotonic Conductivity of a Uranyl-Based MOF from Ambient Temperature to 110 °c

Developing proton-conducting electrolytes that are usable over a wide temperature range (25-150 °C) is highly desirable to enhance the efficiency of fuel cells for on-board automotive applications. Increasing the stability of water above its boiling point is considered one possible method to maintain the hydrogen bond network in composite materials for fast proton conduction. We herein propose an approach to encapsulate LiBr into a negatively charged metal-organic framework of (H3O)[(UO2)4(2-pmb)3(H2O)3]·0.5H2O (1) [2-pmbH3 = 2-(phosphonomethyl)benzoic acid] to enhance the water stability at high temperatures and inhibit the migration of Li ions by Coulombic interactions induced by anionic skeletons. The resulting composite shows a superprotonic conductivity of over 10-2 S cm-1 and a low activation energy of less than 0.4 eV in an anhydrous N2 atmosphere from ambient temperature to 110 °C. Diffusion coefficient tests confirm that protons, rather than lithium ions, are the main contributor to conductivity.