Space charge effect and dopant segregation in acceptor-doped BaZrO3 proton conductors

M. Shirpour1, B. Rahmati2, W. Sigle2, P. A. van Aken2, R. Merkle1, and J. Maier1

1Max Planck Institute for Solid State Research, Stuttgart, Germany,

2Max Planck Institute for Metals Research, Stuttgart, Germany

•• BlockingBlocking effecteffect at at thethe GB and GB and spacespace chargechargemodelmodel::

•• AsAs--preparedprepared samplesample: Spark Plasma Sintered (SPS) at 1600°C / 5min / 50MPa

•• AnnealedAnnealed samplesample: SPS sample annealed at 1700°C / 20 h ρ ≈ 98% ρth

No grain growth during annealing

Secondary phases Inhomogeneous dopant distribution

Symmetry reduction

Positive core charge

Barium loss ...

?BaV ′′

BaV ′′

BaV ′′

BaV ′′ •BaY

BaV ′′BaV ′′

BaV ′′

•BaY

•BaY

•BaY

•BaY

1.0 1.5 2.0 2.5 3.0

-8

-6

-4

-2

0

800 600500 400 300 200 100

0.46eV

0.81eV

T / °C

log(

Tσ /

Scm

-1K

)

1000T-1 / K-1

0.73eV

BulkBulk

PH2O=20 mbar

GBGB--annealedannealedGBGB--asas--preparedprepared

I) High I) High temperaturetemperature annealingannealing II) Dopant II) Dopant segregationsegregation

Depletion and Accumulation of charge carriers

(as-prepared) (annealed)0.2 µ

Bulk

Bulk

TEM image

GB composition by EDXS-TEM

annealed samples:• Same σH+ for bulk, strongly increasedσH+ for GB• Lower space charge potential• Comparable effect for Y- and Sc-doped BaZrO3

as-prepared:slight Y excess at GB

annealing: dopant segregation to GB core and space charge zone⇒ decreased proton depletion

in space charge zone

6at.% Y-doped BaZrO3

1.0 1.5 2.0 2.5 3.0

-8

-6

-4

-2

0

800 600500 400 300 200 100

0.47eV

T / °C

log(σ

T / S

cm-1K

)

1000T-1 /K-1

1.04eV

0.71eVBulkBulk

GBGB--annealedannealed

GBGB--asas--preparedprepared

PH2O=20 mbar

6at.% Sc-doped BaZrO3 annealed:strong Y segregation to GB region

ZrY′

•OH •OH

ZrY′

annealing: dopant segregationto GB core and space charge zone

decreased proton depletion in space charge zone

Sc3+ GB excess ≈ Y3+ GB excess(r = 0.72 Å) (r = 0.90 Å)

electrostatic driving force dominatesover elastic contribution

•• Potential reasons for blocking grain boundaries (Potential reasons for blocking grain boundaries (GBsGBs))

Grain size distribution