Hydronium Ions Stabilized in a Titanate-Layered Structure with High Ionic Conductivity: Application to Aqueous Proton Batteries

Proton chemistry is a fascinating field with both fundamental and applied aspects. The development of solid-state proton conductors relying on abundant elements could help bring these two aspects. In this scope, we synthesized a disordered structure which, as revealed by the real-space refinement of the pair distribution function, has been identified to be the trititanate arrangement. The layered structure is stabilized by the presence of hydronium ions and water molecules located in the interlayer space. This compound displays a high ionic conductivity of 4·10–2 S/m with an activation energy of 0.24 eV, assigned to H+ mobility as shown by broadband dielectric spectroscopy. Proton mobility was further evidenced by solid-state proton nuclear magnetic resonance. Density functional theory calculations revealed that proton transfer occurs both within the interlayer space and with terminal oxide of the titanate framework through a Grotthuss-based mechanism rationalizing the high conductivity measured experimentally. Finally, we investigated the electrochemical properties with respect to the proton as a charge carrier using proton-free (KCl) and proton-donor (buffer acetic acid) electrolytes. The results showed that the structure can reversibly intercalate protons at a very high rate opening existing perspectives in the development of negative electrode materials for aqueous proton batteries. Overall, this study helps better understand the proton transfer mechanism occurring in a confined layered-type structure.