Extremely High Proton Mobility in Nanoscopic Titania Hydrates

Abstract

Using protons as the ionic charge carriers in energy conversion and storage devices is an attractive prospect, but designing electrode/electrolyte materials with fast proton mobility at room temperature is still a challenge. Here we report on two novel titania hydrates with high proton mobility: one is one-dimensional H2Ti5O11·H2O material with ~2 µm length and ~150 nm width; the other is zero-dimensional TiO2·0.4H2O material with average dimension ~5 nm. To gain a deeper understanding of the correlation between materials local structure and fast proton transport mechanisms, extended X-ray absorption fine structure, high resolution transmission electron microscopy, and nuclear magnetic resonance pulsed field gradient diffusion measurements were conducted. The H-bonded species are identified with nano-confined water located between the TiO6 octahedra layered structures in H2Ti5O11·H2O and on the surface of TiO6 octahedra in TiO2·0.4H2O. The proton self-diffusion coefficients at room temperature are about 4×10−9 m2 s−1 for both H2Ti5O11·H2O and TiO2·0.4H2O, which exceed that in liquid water at ambient temperature by nearly a factor of two. These values decrease to about 1×10−9 m2 s−1 at −25 oC in both materials. These unusually high diffusion coefficients imply a Grotthuss transport mechanism and suggest attractive possible application in proton batteries or resistive switching. Further, this materials protocol can create new opportunities in designing other proton-based electrodes and electrolytes with nano-confined water as the fast proton carrier in solids.