Paving the way to nanoionics: atomic origin of barriers for ionic transport through interfaces

FRECHERO M. A.; ROCCI, M.; SANCHEZ-SANTOLINO, G.; KUMAR, AMIT; SALAFRANCA, JUAN; SCHMIDT,R.; DIAZ GUILLEN, M.; DURA, O.; RIVERA-CALZADA, A.; MISHRA, R.; JESSE, STEPHEN; PANTELIDES,S.; KALININ, SERGEI; VARELA, M.; PENNYCOOK, STEVE; SANTAMARIA,J.; LEON,C.

Scientific Reports

Nature Group

Año: 2015 p. 1 - 9

2045-2322

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 grainboundary.