In contrast to conventional ceramic
ionic conductors relying on
bulk ionic transport, making use of interfaces such as grain boundary
and surface may provide various new possibilities to develop novel
ionic conductors. Here we demonstrate that nanograined structures
of yttria-doped zirconia (YSZ), of which the bulk property involves
negligible proton solubility or conductivity, are endowed with appreciable
proton conductivity via interfacial hydrated layers. A combination
of nanopowder synthesis and ultra high-pressure compaction (4 GPa)
at room temperature enables us to fabricate the nanograined specimens.
The material thus prepared can retain an appreciable amount of protons
and water within the grain-boundary or “internal surface”,
resulting in a hierarchical structure of hydroxyl groups and water
molecules with different thermal stability and thereby mobility. The
physicochemical properties of those protonic species have been investigated
by means of in situ FT-IR, 1H MAS NMR, and thermal desorption
spectroscopy. At lower temperatures, proton conductivity prevails
over normally observed oxide ion conductivity, which is facilitated
by interplay of those protonic species at the interfaces. The present
study provides a new prospect for developing proton-conducting materials
which are based on “surface protonics” of nanograined
oxides.
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