Abstract
Doped perovskite oxide have been widely investigated in recent years as proton conducting solid electrolyte for a variety of electrochemical devices such as fuel cells, hydrogen sensors, electrolyzer and hydrogen pumps [1] [2]. The excellent chemical stability of BaZr0.8Y0.2O3-δ (BZY) makes it one of the most promising electrolyte materials for protonic fuel cells [3]. However, the conductivity of BZY, reported by many groups varies widely, introducing major challenges for its implementation in real devices. Theoretical studies of doped barium zirconate, suggested that this material may exhibits high proton conductivity minimizing the grain boundary contribution to the transport properties [4]. The effect in epitaxial BZY thin films have been recently investigated confirming the theoretical predictions [5] and showing a protonic conductivity even higher than oxygen ion conductors in the low-to-intermediate temperature range (300-600°C). These results have stimulated further investigations of the properties in BZY epitaxial thin films especially concerning the interplay between structural properties and transport properties [6]. Surprisingly, very high values of conductivity, at intermediate temperature (550oC-600oC) have been reported by Foglietti et al. [7] in thin film of BZY grown on (110) NdGaO3 (NGO), with a lattice mismatch between film and substrate very large: about 10%. These results suggest that heavily strained interfaces may represent a key parameter for tailoring defect densities in thin epitaxial films thus enhancing their conductivity. Here we report nanoscale evidence of higher electrochemical activity at the interface by Electrochemical Strain Microscopy (ESM), both in-plane and cross-section geometry. The ESM study, together with a scanning transmission electron microscopy (STEM) analysis, show a clear correlation between the conductivity of our perovskite thin films and the defective interfaces [7] [8]. We demonstrate that the microscopic origin of such giant proton conductivity is the strongly defective interface between our BZY film and the substrateYttrium-doped barium zirconate (BZY) thin films recently showed surprising electric transport properties. Experimental investigations conducted mainly by electrochemical impedance spectroscopy suggested that a consistent part of this BZY conductivity is of protonic nature. These results have stimulated further investigations by local unconventional techniques. Here, we use electrochemical strain microscopy (ESM) to detect electrochemical activity in BZY films with nanoscale resolution. ESM in a novel cross-sectional measuring setup allows the direct visualization of the interfacial activity. The local electrochemical investigation is compared with the structural studies performed by state of art scanning transmission electron microscopy (STEM). The ESM and STEM results show a clear correlation between the conductivity and the interface structural defects. We propose a physical model based on a misfit dislocation network that introduces a novel 2D transport phenomenon, whose fingerprint is the low activation energy measured.
