Ordering of a Nanoconfined Water Network Around Zinc Ions Induces High Proton Conductivity in Layered Titanate

We demonstrated that the chemical intercalation of Zn2+ions within theinterlayer space of the structure of a disordered layered titanate results in a drasticincrease of the room-temperature bulk proton conductivity from 8.11x10-5Sm-1forthepristineto 3.7x10-2Sm-1forZn-titanate. Because of the crystallographic disorderednature of these compounds, we combined different techniques to establish the structural-transport relationships. The pair distribution function revealed that upon chemicalinsertion of Zn2+, the local lepidocrocite arrangement is maintained, providing a suitablemodel to investigate the effect of chemically intercalated ions on the transport propertiesand dynamics within the interlayer space. Broadband dielectric spectroscopy (50 to 1010Hz) enabled establishing that Zn2+inclusion promotes proton-hopping by self-dissociation of H2O molecules yielding high proton mobility. Using Zn-K edgeextended X-ray absorptionfine structure and chemical analyses (EDX, TGA,1H NMR), Zn2+ions were shown to be stabilized byZnCl2(H2O) complexes within the interlayer space. Such complexes induce an increase of the H-bonding strength as evidenced by1H NMR, yielding a fast proton motion. Molecular dynamics simulations highlighted proton transfer between water molecules fromthe structural interlayer and bonded to Zn2+ions. The increasing interactions between these water molecules favor proton transfer atthe origin of the fast bulk proton conductivity, which was assigned to a Grotthuss-type mechanism taking place at a long-range order.This work provides a better understanding of how ion-water interactions mediated ionic transport and opens perspectives into thedesign of ionic conductors that can be used in energy-storage applications.