Grain-Boundary Structures Associated with Ionic Transport in Gd-Doped Ceria Nanostructured Electrolyte

Abstract

High resistive ionic transport at the grain boundary is one of the most decisive factors for designing sophisticated nanostructured electrolytes as oxide ceramics. Here, the grain boundary structures are investigated for nanostructured electrolytes, ceria (CeO 2 ), Gd-doped ceria (GDC), and Li-doped GDC (Li-GDC), by means of element-specific positron annihilation spectroscopy. The angstrom-scale spaces with negative charge are identified in addition to the grain-to-grain contacts essentially identical to the bulk and void-like open spaces created by lattice mismatching. The grain-boundary structures associated with oxygen ionic transport are demonstrated on the basis of space charge theory for the grain-boundary conductivity.

Influence of Lithium on the Sintering Behavior and Electrical Properties of Ce0.8Gd0.2O1.9 for Intermediate‐Temperature Solid Oxide Fuel Cells

“…If alien atoms fill them, the decrease of the number of vacancies is the cause of the grain boundary blocking effect. It has been confirmed by Sato through positron annihilation spectroscopy that the grain boundary is constituted mainly by negatively charged voids adjacent to segregated cations in the vicinity . The negatively charged voids cause the formation of a spatial charge layer in which the positively charged oxygen vacancies are depleted, which thus give a smaller contribution to the grain boundary conductivity.…”

“…The grain boundary contribution is visible for LiGDC samples in the temperature range of 300–500 °C and the bulk contribution can be seen only at 300 °C. This phenomenon is attributed to the effect on spectra of the inductances that were generated within the experimental apparatus . The grain boundary conductivity at 300 °C is one order of magnitude lower than that of the bulk (Table ).…”

“…If ceria is doped with gadolinium, the grain boundary width increases because of the segregation of Gd 3+ , which causes the disappearance of the grain‐to‐grain contact and thus enlarges voidlike open spaces . The addition of Li into GDC as a sintering aid causes the segregation of Li + into the grain boundary and narrows its width by shrinking the voidlike open spaces.…”

“…This phenomenoni s attributed to the effect on spectra of the inductances that were generated within the experimental apparatus. [25] The grain boundary conductivitya t3 00 8Ci so ne order of magnitude lower than that of the bulk (Table 3). Theg rain boundary conductivityd ecreases with lithium content and increases with sintering temperature.T he high resistancea ssociated with al ow grain boundary conductivityi mplies that the grain boundaryp lays an important role to limit the oxygeni onic transport and the total ionic conductivity.G rain boundaries act as barriers to oxygen ion conductivity.…”

“…It has been confirmed by Sato through positron annihilation spectroscopy that the grain boundary is constituted mainly by negatively chargedv oids adjacent to segregated cations in the vicinity. [25,34] Then egatively charged voids cause the formation of as patialc harge layeri nw hich the positively chargedo xygen vacancies are depleted, which thus give as mallerc ontribution to the grain boundaryc onductivity. Theg rain boundary conductivityf or LiGDC samples sintered at 1250 and 1500 8Ca sf unctiono ft emperature in the range 300-500 8Ci s illustrated in Figures 11 and 12.…”