Abstract
The blocking of ion transport at interfaces strongly limits the performance of electrochemical nanodevices for energy applications. The barrier is believed to arise from space-charge regions generated by mobile ions by analogy to semiconductor junctions. Here we show that something different is at play by studying ion transport in a bicrystal of yttria (9% mol) stabilized zirconia (YSZ), an emblematic oxide ion conductor. Aberration-corrected scanning transmission electron microscopy (STEM) provides structure and composition at atomic resolution, with the sensitivity to directly reveal the oxygen ion profile. We find that Y segregates to the grain boundary at Zr sites, together with a depletion of oxygen that is confined to a small length scale of around 0.5 nm. Contrary to the main thesis of the space-charge model, there exists no evidence of a long-range O vacancy depletion layer. Combining ion transport measurements across a single grain boundary by nanoscale electrochemical strain microscopy (ESM), broadband dielectric spectroscopy measurements, and density functional calculations, we show that grain-boundary-induced electronic states act as acceptors, resulting in a negatively charged core. Besides the possible effect of the modified chemical bonding, this negative charge gives rise to an additional barrier for ion transport at the grain boundary.
Highlights
Grain boundaries made use of dielectric spectroscopy on ceramic samples[12,14,15,16] but these experiments do not probe transport barriers at the atomic scale
Instead we find that structural oxygen vacancies are present within ~1 nm at the grain boundary and their positive charge is compensated by negatively charged acceptor states localized at the grain boundary plane
The chemical composition obtained from these electron energy-loss spectroscopy (EELS) maps exhibits large deviations from the bulk on the grain boundary dislocation cores, as typically observed at grain boundaries[24,25,26,27,28]
Summary
YSZ bicrystals with 9% mol yttria content and a symmetrical 33° [001] tilt grain boundary were acquired from MaTeck GmbH. STEM-EELS measurements were carried out in an aberration corrected Nion UltraSTEM200 operated at 200 kV and equipped with a Gatan Enfinium spectrometer and in a Nion UltraSTEM100 operated at 100 kV, equipped with a 5th order aberration corrector and a Gatan Enfina EEL spectrometer. This electron microscope can routinely produce a sub-Ångström electron beam with a full width at half maximum around or below the 0.08 nm range. The STEM specimen thickness was kept in the 0.2– 0.3 inelastic mean free paths (25–30 nm) range in most cases on occasion thicker samples were used to evaluate the role of electron beam broadening. The planewave cutoff for all calculations is 400 eV, and energies were estimated with gamma point calculations
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