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Enhancing grain boundary ionic conductivity in mixed ionic-electronic conductors.

Lin Y, Fang S, Su D, Brinkman KS, Chen F - Nat Commun (2015)

Bottom Line: The formation of an emergent phase successfully avoids segregation of the Gd dopant and depletion of oxygen vacancies at the Ce0.8Gd0.2O2-δ-Ce0.8Gd0.2O2-δ grain boundary.This results in superior grain boundary ionic conductivity as demonstrated by the enhanced oxygen permeation flux.This work illustrates the control of mesoscale level transport properties in mixed ionic-electronic conductor composites through processing induced modifications of the grain boundary defect distribution.

View Article: PubMed Central - PubMed

Affiliation: Department of Mechanical Engineering, University of South Carolina, Columbia, South Carolina 29208, USA.

ABSTRACT
Mixed ionic-electronic conductors are widely used in devices for energy conversion and storage. Grain boundaries in these materials have nanoscale spatial dimensions, which can generate substantial resistance to ionic transport due to dopant segregation. Here, we report the concept of targeted phase formation in a Ce0.8Gd0.2O2-δ-CoFe2O4 composite that serves to enhance the grain boundary ionic conductivity. Using transmission electron microscopy and spectroscopy approaches, we probe the grain boundary charge distribution and chemical environments altered by the phase reaction between the two constituents. The formation of an emergent phase successfully avoids segregation of the Gd dopant and depletion of oxygen vacancies at the Ce0.8Gd0.2O2-δ-Ce0.8Gd0.2O2-δ grain boundary. This results in superior grain boundary ionic conductivity as demonstrated by the enhanced oxygen permeation flux. This work illustrates the control of mesoscale level transport properties in mixed ionic-electronic conductor composites through processing induced modifications of the grain boundary defect distribution.

No MeSH data available.


CGO–CGO grain boundaries in CGO–CFO6040 revealed by STEM-EELS.(a) The survey image including the EELS line scan across the CGO–CGO grain boundary (GB) from the CGO–CFO6040 composite; (b) and (c) are EELS line scan signal profiles presenting in two-dimensional and three-dimensional mode, respectively; (d), (e) and (f) are ELNES spectra of Ce M4,5, Gd M4,5 and O K edges extracted at (marked by on) or ∼10 nm away from (marked by off) the CGO–CGO grain boundary core, respectively; (g) profile of the CeM5/M4, Gd/Ce and O/Ce ratio near the CGO–CGO boundary (The solid symbol results are from this work, while the hollow symbol results are from single-phase CGO26. The grain boundary thickness of both samples is ∼4 nm); The proposed oxygen vacancy concentration profiles near the CGO–CGO grain boundary zone of (h) single-phase CGO and (i) CGO–CFO6040 composite. The GB core was highlighted by red and green line in h and i, respectively. Scale bar, 100 nm (a).
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f2: CGO–CGO grain boundaries in CGO–CFO6040 revealed by STEM-EELS.(a) The survey image including the EELS line scan across the CGO–CGO grain boundary (GB) from the CGO–CFO6040 composite; (b) and (c) are EELS line scan signal profiles presenting in two-dimensional and three-dimensional mode, respectively; (d), (e) and (f) are ELNES spectra of Ce M4,5, Gd M4,5 and O K edges extracted at (marked by on) or ∼10 nm away from (marked by off) the CGO–CGO grain boundary core, respectively; (g) profile of the CeM5/M4, Gd/Ce and O/Ce ratio near the CGO–CGO boundary (The solid symbol results are from this work, while the hollow symbol results are from single-phase CGO26. The grain boundary thickness of both samples is ∼4 nm); The proposed oxygen vacancy concentration profiles near the CGO–CGO grain boundary zone of (h) single-phase CGO and (i) CGO–CFO6040 composite. The GB core was highlighted by red and green line in h and i, respectively. Scale bar, 100 nm (a).

Mentions: Figure 2 shows the compositional and charge distribution variations for the CGO–CGO grain boundary in the CGO–CFO6040 composite acquired by EELS line scan in the STEM mode. Figure 2g shows a significant difference in the Ce M5/M4, Gd/Ce and O/Ce ratios in the CGO–CGO grain boundary region in CGO–CFO6040 as compared with single-phase CGO26. The Gd/Ce ratio in the CGO–CGO grain boundary core of single-phase CGO is significantly higher than that in the grain interior, demonstrating the accumulation of Gd ions in the grain boundary and concomitant formation of a space charge layer26. The accumulation of Gd ions is closely correlated to the concentration of oxygen vacancies in the space charge layer, which is reflected by the ratio of Ce M5/M4. The Ce M4,5 and Gd M4,5 edges of the CGO–CGO grain boundary in single-phase CGO shifted to higher energy, and the Ce M5/M4 ratio increased 26.3% (0.95→1.20) at the grain boundary core compared with the grain interior40. In contrast, the ELNES spectra of Ce M4,5 edges, Gd M4,5 edges and O K edges at the CGO–CGO grain boundary core in the CGO–CFO6040 composite are all very similar to those from the CGO grain interior (∼10 nm away from core, Fig. 2d–f). There were no obvious edge position shifts or shape changes observed, indicating similar local chemical states, symmetry and atomic environment extending from the grain boundary to the grain interior for the CGO phase inside the CGO–CFO6040 composite4142. Furthermore, the Ce M5/M4 only changed by 3.3% from the grain boundary core to the grain interior (0.92→0.89). This provides additional confirmation that the oxygen vacancy concentration at the CGO–CGO grain boundary core in CGO–CFO6040 is similar to the concentration in the CGO grain interior. Therefore, the depletion of oxygen vacancies inside the space charge layer, observed in single-phase CGO was significantly mitigated and a nearly constant distribution of oxygen vacancy concentration is expected along the CGO–CGO grain boundary inside the CGO–CFO6040 composite. A schematic is shown in Fig. 2h,i to conceptually understand the difference of the space charge effect between single-phase CGO and the modified CGO phase inside the CGO–CFO6040 composite. Figure 2h,i proposes that Gd ion accumulation and the resulting oxygen vacancy depletion in the CGO–CGO grain boundary are avoided in the CGO–CFO6040 composite through the in situ formation of a Gd- and Fe-rich GFCCO phase. Although similar effects were observed inside the CGO–CFO8020 composite, the CGO–CGO grain boundaries in CGO–CFO8020 were much thicker (∼42 nm compared with 4 nm in CGO–CFO6040, shown in Supplementary Fig. 2). The thicker grain boundary provides more resistance for the transport of oxygen ions17.


Enhancing grain boundary ionic conductivity in mixed ionic-electronic conductors.

Lin Y, Fang S, Su D, Brinkman KS, Chen F - Nat Commun (2015)

CGO–CGO grain boundaries in CGO–CFO6040 revealed by STEM-EELS.(a) The survey image including the EELS line scan across the CGO–CGO grain boundary (GB) from the CGO–CFO6040 composite; (b) and (c) are EELS line scan signal profiles presenting in two-dimensional and three-dimensional mode, respectively; (d), (e) and (f) are ELNES spectra of Ce M4,5, Gd M4,5 and O K edges extracted at (marked by on) or ∼10 nm away from (marked by off) the CGO–CGO grain boundary core, respectively; (g) profile of the CeM5/M4, Gd/Ce and O/Ce ratio near the CGO–CGO boundary (The solid symbol results are from this work, while the hollow symbol results are from single-phase CGO26. The grain boundary thickness of both samples is ∼4 nm); The proposed oxygen vacancy concentration profiles near the CGO–CGO grain boundary zone of (h) single-phase CGO and (i) CGO–CFO6040 composite. The GB core was highlighted by red and green line in h and i, respectively. Scale bar, 100 nm (a).
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f2: CGO–CGO grain boundaries in CGO–CFO6040 revealed by STEM-EELS.(a) The survey image including the EELS line scan across the CGO–CGO grain boundary (GB) from the CGO–CFO6040 composite; (b) and (c) are EELS line scan signal profiles presenting in two-dimensional and three-dimensional mode, respectively; (d), (e) and (f) are ELNES spectra of Ce M4,5, Gd M4,5 and O K edges extracted at (marked by on) or ∼10 nm away from (marked by off) the CGO–CGO grain boundary core, respectively; (g) profile of the CeM5/M4, Gd/Ce and O/Ce ratio near the CGO–CGO boundary (The solid symbol results are from this work, while the hollow symbol results are from single-phase CGO26. The grain boundary thickness of both samples is ∼4 nm); The proposed oxygen vacancy concentration profiles near the CGO–CGO grain boundary zone of (h) single-phase CGO and (i) CGO–CFO6040 composite. The GB core was highlighted by red and green line in h and i, respectively. Scale bar, 100 nm (a).
Mentions: Figure 2 shows the compositional and charge distribution variations for the CGO–CGO grain boundary in the CGO–CFO6040 composite acquired by EELS line scan in the STEM mode. Figure 2g shows a significant difference in the Ce M5/M4, Gd/Ce and O/Ce ratios in the CGO–CGO grain boundary region in CGO–CFO6040 as compared with single-phase CGO26. The Gd/Ce ratio in the CGO–CGO grain boundary core of single-phase CGO is significantly higher than that in the grain interior, demonstrating the accumulation of Gd ions in the grain boundary and concomitant formation of a space charge layer26. The accumulation of Gd ions is closely correlated to the concentration of oxygen vacancies in the space charge layer, which is reflected by the ratio of Ce M5/M4. The Ce M4,5 and Gd M4,5 edges of the CGO–CGO grain boundary in single-phase CGO shifted to higher energy, and the Ce M5/M4 ratio increased 26.3% (0.95→1.20) at the grain boundary core compared with the grain interior40. In contrast, the ELNES spectra of Ce M4,5 edges, Gd M4,5 edges and O K edges at the CGO–CGO grain boundary core in the CGO–CFO6040 composite are all very similar to those from the CGO grain interior (∼10 nm away from core, Fig. 2d–f). There were no obvious edge position shifts or shape changes observed, indicating similar local chemical states, symmetry and atomic environment extending from the grain boundary to the grain interior for the CGO phase inside the CGO–CFO6040 composite4142. Furthermore, the Ce M5/M4 only changed by 3.3% from the grain boundary core to the grain interior (0.92→0.89). This provides additional confirmation that the oxygen vacancy concentration at the CGO–CGO grain boundary core in CGO–CFO6040 is similar to the concentration in the CGO grain interior. Therefore, the depletion of oxygen vacancies inside the space charge layer, observed in single-phase CGO was significantly mitigated and a nearly constant distribution of oxygen vacancy concentration is expected along the CGO–CGO grain boundary inside the CGO–CFO6040 composite. A schematic is shown in Fig. 2h,i to conceptually understand the difference of the space charge effect between single-phase CGO and the modified CGO phase inside the CGO–CFO6040 composite. Figure 2h,i proposes that Gd ion accumulation and the resulting oxygen vacancy depletion in the CGO–CGO grain boundary are avoided in the CGO–CFO6040 composite through the in situ formation of a Gd- and Fe-rich GFCCO phase. Although similar effects were observed inside the CGO–CFO8020 composite, the CGO–CGO grain boundaries in CGO–CFO8020 were much thicker (∼42 nm compared with 4 nm in CGO–CFO6040, shown in Supplementary Fig. 2). The thicker grain boundary provides more resistance for the transport of oxygen ions17.

Bottom Line: The formation of an emergent phase successfully avoids segregation of the Gd dopant and depletion of oxygen vacancies at the Ce0.8Gd0.2O2-δ-Ce0.8Gd0.2O2-δ grain boundary.This results in superior grain boundary ionic conductivity as demonstrated by the enhanced oxygen permeation flux.This work illustrates the control of mesoscale level transport properties in mixed ionic-electronic conductor composites through processing induced modifications of the grain boundary defect distribution.

View Article: PubMed Central - PubMed

Affiliation: Department of Mechanical Engineering, University of South Carolina, Columbia, South Carolina 29208, USA.

ABSTRACT
Mixed ionic-electronic conductors are widely used in devices for energy conversion and storage. Grain boundaries in these materials have nanoscale spatial dimensions, which can generate substantial resistance to ionic transport due to dopant segregation. Here, we report the concept of targeted phase formation in a Ce0.8Gd0.2O2-δ-CoFe2O4 composite that serves to enhance the grain boundary ionic conductivity. Using transmission electron microscopy and spectroscopy approaches, we probe the grain boundary charge distribution and chemical environments altered by the phase reaction between the two constituents. The formation of an emergent phase successfully avoids segregation of the Gd dopant and depletion of oxygen vacancies at the Ce0.8Gd0.2O2-δ-Ce0.8Gd0.2O2-δ grain boundary. This results in superior grain boundary ionic conductivity as demonstrated by the enhanced oxygen permeation flux. This work illustrates the control of mesoscale level transport properties in mixed ionic-electronic conductor composites through processing induced modifications of the grain boundary defect distribution.

No MeSH data available.