Charge Transport across Single Grain Boundaries in Oxide Electrolytes

Xu, Xin

doi:https://doi.org/10.21985/n2-22dn-k594

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Charge Transport across Single Grain Boundaries in Oxide Electrolytes

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Transport along and across the grain boundaries of solid-state electrolytes has implications for a broad range of materials and in an equally broad range of technologies. Over the past 2-3 decades, a substantial body of literature has been developed to explain grain boundary transport properties within the context of space charge theory. This theory holds that the grain boundaries in ionic materials are inherently charged due to the difference in energetics between creating point defects in the bulk and at their interface surfaces. While generally effective at predicting material properties, two aspects of this approach have remained unsatisfactory. The first is the assumption that every grain boundary is approximately the same, with the same level of grain boundary charge imbalance (which generates a space charge potential of given magnitude), and the second is the difficulty in fundamentally predicting why a charge imbalance occurs. In this study we employ electron holography to study several individual/isolated grain boundaries in lightly doped, high-purity ceria. We find a remarkable variation in the electric field perturbation from one grain boundary to the next, suggesting orders of magnitude differences in the transport properties. Similarly, 3 orders of difference are observed in grain boundary conductivity by impedance measurement on single grain boundaries in ceria fibers. Using atom probe tomography (APT) and secondary ion mass spectroscopy, we are able to identify the chemical nature of essentially every atom in the grain boundary region. Here we find that trace impurities, < 25 ppm Si and Al, are concentrated at the grain boundary core, and are the apparent cause of the interfacial charge. These tetra and trivalent species, located within interstitial sites, generate a positive charge at the interface that is balanced by a depletion of oxygen vacancies in the neighboring space charge zone, in a manner analogous to conventional space charge theory. The APT studies further reveal that the dopant element, Sm, is enhanced in the grain boundaries, which has the effect of screening the impurity charge. Our definitive demonstration of the origins of the space charge effect in ceria provides clear guidance on how to tune interfacial charge transport at will. Our work further clarifies why introduction of alkaline earth dopants or simply increasing the concentration of conventional rare earth element dopants so dramatically increases the grain boundary ionic conductivity.

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