Surface and Grain Boundary Effect in the Local Ionic Transport of (La,Sr)(Co,Fe)O3 Observed By in Situ Scanning Probe Microscopy
At a Glance
Section titled āAt a Glanceā| Metadata | Details |
|---|---|
| Publication Date | 2020-11-23 |
| Journal | ECS Meeting Abstracts |
| Authors | Hung-Sen Kang, Orbel Barkhordarian, Min Hwan Lee |
| Institutions | University of California, Merced |
Abstract
Section titled āAbstractāIonic transport in solid oxides lays the fundamentals of solid-state electrochemical energy and sensor devices. The transport is understood as a thermally activated hopping of point defects (i.e. oxygen vacancies), and its kinetics is highly dependent upon the bonding state of the ionic species with their surrounding lattice environment. While the heterogeneous ionic transport in polycrystalline oxides has been studied for decades, the exact role of interfaces and surfaces in the transport is still an open question. In this presentation, we report our recent effort to directly observe the ionic transport in (La,Sr)(Co,Fe)O 3 (LSCF), the most widely used cathode material for intermediate temperature solid oxide fuel cells, by the use of Kelvin probe force microscopy (KPFM) and conductive atomic force microscopy (CAFM). By providing a positive bias to the tip-LSCF contact in a CAFM mode, oxygen vacancies are generated via oxygen evolution reaction and the amount of newly formed oxygen vacancies are quantified by measuring the total charge transferred during the bias application. Subsequently, the evolution of charge distribution (and thus the distribution of oxygen vacancies) are observed by KPFM at various temperatures. The resulting KPFM maps were analyzed to gauge the heterogeneous ionic transport, in particular, around the grain boundary (GB) areas, and the effective ionic diffusivity within each grain and across GBs were to quantified. From this practice, a direct observation of charge transport was made to confirm the ionic diffusion across GBs is indeed much slower than those within each GB, and the effective resistance in ionic transport through GB was quantified. In addition, 3D KPFM maps were also acquired by an in situ etching with a diamond-coated tip, providing the charge distribution map with depth. From this, it was directly observed that the ionic transport along the surface (~5 nm) is significantly higher than the transport through the interior of a grain. In combination with a simplified Monte Carlo simulation, the ionic diffusivity and its activation energy were quantified for both surface and bulk diffusion, respectively. The authors acknowledge the support by U.S. National Science Foundation CAREER Award (DMR 1753383).