Three-Dimensional Reconstruction of Porous LSCF Cathodes
D. Gostovic,
*
,z
J. R. Smith,
*
D. P. Kundinger, K. S. Jones,
**
and
E. D. Wachsman
**
University of Florida–U.S. Department of Energy High Temperature Electrochemistry Center,
University of Florida, Gainesville, Florida 32611-6400, USA
In this initial study the electrochemically active region of a La
0.8
Sr
0.2
Co
0.2
Fe
0.8
O
3-
LSCF cathode was reconstructed in three
dimensions using a focused ion beam/scanning electron microscope. The reconstructed volume totaled 1065 m
3
from the free air
surface to the dense yttria-stabilized zirconia electrolyte interface. Various microstructural properties were measured, including
overall porosity, closed porosity, graded porosity, surface area, tortuosity, triple-phase boundary length, and pore size. Electro-
chemical impedance spectroscopy data was correlated to microstructure.
© 2007 The Electrochemical Society. DOI: 10.1149/1.2794672 All rights reserved.
Manuscript submitted October 31, 2006; revised manuscript received September 10, 2007.
Available electronically October 15, 2007.
Solid oxide fuel cells SOFCs are efficient, environmentally
friendly, and fuel-flexible electrochemical devices for the generation
of electrical power and heat.
1
They consist of three basic layers:
cathode, electrolyte, and anode. The cathode is a porous, conductive
catalyst for the reduction of O
2
and for the oxidation of fuel. Be-
tween the cathode and anode is the dense electrolyte. The circuit is
completed via cathode and anode contacts to an external load.
The basic chemical formula for the cathodic reduction reaction is
1
2
O
2
+V
o
··
+ 2e =O
o
x
1
Current SOFC performance is limited by cathode polarization,
which increases with decreasing operational temperatures.
2,3
Cath-
ode microstructure and morphology have a strong effect on this
polarization.
2-4
In this initial study a dual-beam focused ion
beam/scanning electron microscope FIB/SEM was utilized to re-
construct an actual three-dimensional 3D model of a
La
0.8
Sr
0.2
Co
0.2
Fe
0.8
O
3-
LSCF cathode and its interface with a
dense yttrium-stabilized zirconia YSZ electrolyte. This high-
resolution, 3D technique advances the understanding of the cathode
microstructure’s effect on performance. The identification of critical
microstructural properties such as surface area, tortuosity, and inter-
facial porosity may be correlated with the ionic, electronic, and cata-
lytic processes for a better fundamental understanding of electro-
chemical performance. With this tool, SOFC material and
microstructural design can be more effective in reducing cathodic
polarization at lower operational temperatures.
The semiconductor industry has used the FIB since the 1980s to
deposit, etch, micromachine, and image specimens during different
stages of circuit processing.
5,6
This technology was brought forward
to reconstruct 3D, geometrically complex submicrometer
structures.
7-11
With the advent of 3D modeling software, nanoto-
mography utilizing the dual-beam FIB/SEM technique was used to
quantify nanoceramic suspended powders.
10-12
This technique was
applied to SOFC cermet anodes to quantify microstructural proper-
ties such as porosity, triple-phase-boundary TPB length, and de-
gree of anisotropy via tortuosity.
13
Such a technique has never be-
fore been applied to reconstruct a cathode and the cathode/
electrolyte interface.
Experimental
Square LSCF symmetric cell cathodes 8 8 mm were screen
printed using premixed LSCF ink NexTech Materials, Inc. on both
sides of a 100 m thick polycrystalline YSZ electrolyte Marketech
International, Inc. using common techniques.
14
After low-
temperature drying to eliminate the organic vehicle, three samples
were sintered at 850, 950, and 1100°C, respectively, for 1 h. The
resulting porous cathode films were approximately 20 m thick.
Impedance measurements were collected in air, with platinum mesh
pressure contacts using a Solartron 1260 frequency-response ana-
lyzer under a potentiostatic modulation of 50 mV. The frequency
range was 10 MHz to 10 mHz with ten points per decade.
The automated sectioning and imaging was conducted with a FEI
Strata DB 235 FIB/SEM dual-beam system. A schematic of the
dual-beam orientation is shown in Fig. 1. The ion and electron
pole pieces are oriented at 52°. The system has an in situ liquid
* Electrochemical Society Student Member.
** Electrochemical Society Active Member.
z
E-mail: gostovic@ufl.edu
Figure 1. a As-cracked LSCF/YSZ sample cross section; b front milling
with FIB; c, d angled milling to create fiduciary mark in x-y plane; e
platinum deposition to protect fiduciary mark; f zero tilt view of area to be
milled/reconstructed, z axis points into plane of page; g schematic of ion
and electron beams incident upon ROI which consists of a platinum protec-
tion layer, porous cathode, and dense electrolyte; and h representative FEG-
SEM image of actual reconstructed LSCF cathode with platinum protection
layer, and fiduciary ridgeline present.
Electrochemical and Solid-State Letters, 10 12 B214-B217 2007
1099-0062/2007/1012/B214/4/$20.00 © The Electrochemical Society
B214
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