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.