Unravelling the Proton Conduction Mechanism from Room Temperature to 553 K in a 3D Inorganic Coordination Framework

The preparation of proton-conducting materials that are functional and stable at intermediate temperatures (393–573 K) is a focal point of fuel cell development. The purely inorganic material, HNd­(IO3)4, which possesses a dense 3D framework structure, can reach a maximum of 4.6 × 10–4 S·cm–1 at 353 K and 95% relative humidity and exhibit a high conductivity of 8.0 × 10–5 S·cm–1 from 373 to 553 K under the flow of wet N2. HNd­(IO3)4 exhibits a variety of improvements including high thermal stability, low solubility in water, and resistance to reducing atmosphere. The proton conductivity in such a wide temperature range originates from the intrinsic liberated protons in the structure and the resulting 1D hydrogen-bonding network confirmed by bond valence sum calculation and solid-state NMR analysis. Moreover, two different activation energies are observed in different temperature regions (0.23 eV below 373 K and 0.026 eV from 373 to 553 K), indicating that two types of proton motion are responsible for proton diffusion, as further domenstrated by temperature-dependent open-circuit voltage hysteresis in a tested fuel cell assembly as well as variable-temperature and double quantum filtered solid-state NMR measurements.