Proton conduction in alkali metal ion-exchanged porous ionic crystals

Proton conduction in alkali metal ion-exchanged porous ionic crystals A₂[Cr₃O(OOCH)₆(etpy)₃]₂[α-SiW₁₂O₄₀]·nH₂O [I-A⁺] (A = Li, Na, K, Cs, etpy = 4-ethylpyridine) is investigated. Single crystal and powder X-ray diffraction measurements show that I-A⁺ possesses analogous one-dimensional channels where alkali metal ions (A⁺) and water of crystallization exist. Impedance spectroscopy and water diffusion measurements of I-A⁺ show that proton conductivities are low (10^-7-10^-6 S cm^-1) under low relative humidity (RH), and protons mostly migrate as H₃O⁺ with H₂O as vehicles (vehicle mechanism). The proton conductivity of I-A⁺ increases with the increase in RH and is largely dependent on the types of alkali metal ions. I-Li⁺ shows a high proton conductivity of 1.9 × 10^-3 S cm^-1 (323 K) and a low activation energy of 0.23 eV under RH 95%. Under high RH, alkali metal ions with high ionic potentials (e.g., Li⁺) form a dense and extensive hydrogen-bonding network of water molecules with mobile protons at the periphery, which leads to high proton conductivities and low activation energies via rearrangement of the hydrogen-bonding network (Grotthuss mechanism).