MICRO/NANO TOMOGRAPHY FOR ANALYSIS OF GAS DIFFUSION
LAYERS OF MICROFUEL CELLS
H. Ostadi1, P. Rama2, Y. Liu2, R. Chen2, X. Zhang3 and K. Jiang1*
School of Mechanical and Manufacturing Engineering, University of Birmingham, Birmingham, UK
2
Depaertment of Aeronautical & Automotive Engineering, Loghborogh University, Loughborogh, UK
3
Depaertment of Engineering, University of Liverpool, Liverpool, UK
E-mail: k.c.jiang@bham.ac.uk
1
Abstract: This paper presents a characterization of a gas diffusion layer (GDL) covered with a microporous layer
(MPL) for polymer electrolyte fuel cell (PEFC) using micro and nano tomography to determine the critical
micro/nanostructural features of the layers, such as porosity, mean pore diameter, structure model index and
degrees of anisotropy. The proposed study was conducted using X-ray microtomography for the GDL and
focused ion beam/scanning electron microscopy (FIB/SEM) nanotomography for the MPL. Furthermore,
permeability of the GDL real structure was obtained through lattice Boltzmann numerical modeling.
Keywords: X-ray microtomography, FIB/SEM nanotomography, lattice Boltzmann numerical modeling
views of a sample over a range of angles and creating
images to map the level of X-ray attenuation in the
views to the density of the sample [4]. Recently, Xray microtomography has been used to quantify liquid
water saturation in GDLs and in determining twophase material parameters with 10µm resolution [5].
However, the results suggest that the technique
requires better resolution for applications to fuel cells.
In addition, the layer does not contain MPL.
The sub-micron porous features of the MPL
requires visualisation with a sub-25 nm resolution.
The most recent X-ray synchrotron facilities cannot
provide a resolution better than 50 nm [6]. The soft
nature of the layer also makes it difficult to prepare
thin samples for such X-ray nanotomography. In order
to visualise the nano-scale structural features of the
porous network of an MPL without extensive sample
preparation, dual-beam focused ion beam/electron
beam tomography (FIB/SEM tomography) has been
applied with a resolution of 15 nm [7].
INTRODUCTION
The current GDL/MPL models rely on the use of
key parameters such as porosity, mean pore radius,
permeability and effective diffusivity. These
parameters of the GDL/MPL have a significant
influence to the flowing property inside the fuel cell
and thus the efficiency of fuel cells. Porosity is used
to determine the capacity of gas and water in
circulation in the GDL/MPL; degrees of anisotropy is
closely related to mechanical strength of the GDL; 3D
pore size distribution reflects the degree of harmony
of the pores in the GDL; mean pore radius is used in
GDL/MPL diffusion characterization; structure model
index is used to describe structural geometry, such as
planar, cylindrical or spherical shapes [1, 2]. In porous
electrode theory, such parameters are usually
estimated based on idealized models or assumed to be
adjustable parameters. Other methods based on
stochastic three-dimensional reconstruction do not
closely reflect nano-structural properties and are
difficult to use in order to predict the results of new
structures for cell improvement [3]. Therefore
extracting those parameters requires rigorous analysis
of the 3D geometry of the real materials. In this study,
the real 3D structures of the GDL/MPL through
micro/nanotomography have been used to obtain the
critical structural features to get a greater
understanding of transportation phenomena occurred
within the fuel cell layers.
X-ray microtomography is a technique which
provides non-invasive 3D visualization and
characterization of both external and internal
structures of objects, e.g. porous gas diffusion layer
(GDL) for fuel cells. It works by acquiring numerous
0-9743611-5-1/PMEMS2009/$20©2009TRF
EXPERIMENTAL
A 300 µm thick GDL supplied by Tehcnical Fibre
Products [8] with a micropoorous layer applied by
Johnson Matthey Fuel Cells [9] was used in this
study. A 2×2 mm sample was imaged using a Skyscan
1072 desktop microtomography system [10], with a
source voltage of 50 kV and a current of 100 µA
without filtering of the X-rays. On average 2 frames
of 2 sec exposure time were acquired at each 0.9°
rotation step. This generated 200 shadow images with
a voxel size of 2 µm in around 60 minutes.
The tomography images were then processed
using CTAN software [10] to reconstruct a 3D digital
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PowerMEMS 2009, Washington DC, USA, December 1-4, 2009
model of the sample, as shown in Fig. 1.
Z
Y
X
Fig. 2: FIB/SEM and
nd sample configuration. After
milling a 25µm×25µm cube and depositing
deposit
a 100nm
protective layer of Pt, 100 SEM images of the MPL
side walls were acquired repetitively after milling thin
slices of 14nm thickness each. The sectioning
direction was perpendicular to the FIB column.
Fig. 1: A 3D reconstruction of a GDL carbon paper of
1500×2000×300 µm (X,Y,Z) using desktop X
X-ray
microtomography with 2 µm
m resolution.
For the 10 µm thick MPL, a dual beam focused
ion beam/scanning electron microscopy (FIB/SEM)
system has been used as a nanotomography tool to
obtain the 3D structure of a 10 µm3 sample. The
process involves milling away a thin slice (15 nm) on
the side-wall
wall of a trench using focused ion beam (FIB)
and recording an SEM image of the new surface, then
repeatedly milling and imaging to produce a stack of
SEM images. The surface is coated by 100 nm-thick
layer of platinum prior to side-wall
wall milling in order to
protect the soft surface from ion bombardment and to
reduce the re-deposition
deposition effect through FIB
FIB-induced
decomposition of the precursor gases [11].
Each slice of the sample is milled-of
off using a Ga+
ion beam at 30 kV and 50 pA for 120 sec and a dwell
time of 1 µsec with an overlap parameter of 50%
using an FEI Dual Beam Strata 235 system. 100 sl
slices
with a total thickness of 1.5 µm were removed and
SEM images of the slices with pixel size
ze of 8nm were
taken.
The edge of the milled wall on the slides were
used as a fiducial mark to align the images. The 3D
reconstructions of the layer were carried out and the
key nanostructural properties were studied
studied.
Fig. 2 shows the FIB/SEM and the ssample
configuration. In Fig. 3a, pores on the side wall,
deposited platinum on the MPL surface and the
fiducial mark are shown. A 3D reconstruction of the
MPL with pores in light grey solid phas
phase in dark grey
are presented in Fig. 3b.
a
b
Fig. 3: a) An SEM image of the trench side wall
polished through low
w current 50 pA ion beam. A 25×
25
25µm area with a 100nm thick Pt layer was deposited
on the surface. The fiducial mark is used for image
alignment; b) a 1.5×5×1.5µm3 3D reconstructed
image of the MPL with 8×15×8
×8 nm3 voxel size. Dark
and light grey colours show solid and pore networks
respectively.
RESULTS AND DISCUSSIONS
Key structural features
With an accurate digital representation of the 3D
structure, it is possible to determine the critical
features of the assembly. The porosity of the GDL
plays a vital role in fuel cell performance [1]. The
porosity obtained from X-ray
ray microtomography
micro
thresholded image was 80%. The corresponding local
loca
464
porosity value for the MPL attained from FIB/SEM
nanotomographic 3D reconstruction was 40%.
The local effective diffusivity of a gas
(Deff,H2,Deff,O2,Deff,H2O) is a function of the porosity and
level of liquid water saturation in GDL. Its value can
be estimated using the Tomadakis and Sotirchos [12]
approach where :
0.11/0.89. 1 (1)
It is assumed that 25% of the porosity of the
GDL is compromised due to compaction upon cell
assembly ( 61%) , 50% of the porous network of
the GDL is saturated with liquid water, and the
diffusivities of hydrogen, oxygen and water vapour
are 114 mm2/s, 34.5mm2/s and 30.3mm2/s
respectively, the local effective diffusivity of the
gasses can be obtained as 18.21mm2/s, 5.51mm2/s and
4.84mm2/s. Corresponding local effective diffusivities
values with the same assumptions for MPL are
4.72mm2/s,1.43mm2/s and 1.26mm2/s respectively.
The average fiber diameter based on X-ray
microtomography is obtained as 8 µm and the
FIB/SEM nanotomography reveals an average
diameter of 165nm for the MPL.
According to the kinetic theory of gases [1] the
Knudsen diffusion coefficient in MPL is a linear
function of the mean pore radius:
8 ⁄!"
(2)
where r, R,T and M are mean pore radius, gas
constant, temperature in Kelvin and molar mass.
Based on 3D digital image of GDL/MPL an average
size of 35µm and 137nm were obtained for GDL and
MPL mean pore diameter. Therefore the Knudson
diffusion coefficient of the MPL is 0.5% of that of
GDL.
The anisotropy in porous media properties was
found to be the most important factor of mechanical
strength [13]. Isotropy (or anisotropy) is a measure of
3D symmetry or the presence (or absence) of
preferential alignment of structures along a particular
directional axis. Degrees of anisotropy calculated in
this way vary from 0 (fully isotropic) to 1 (fully
anisotropic). Structure model index indicates the
relative prevalence of rods and plates in a 3D
structure. An ideal plate, cylinder and sphere have
values of 0, 3 and 4 respectively [10].
The two parameters above have been calculated
directly from the voxel representation through CTAN
software. Degrees of anisotropy were found to be 0.9
and 0.65 for GDL and MPL respectively, which
means that both structures are highly anisotropic at
nano scale. Structure model index shows that the GDL
can be modeled as rods since the index is 2.85.
Lattice Boltzmann simulation
A lattice Boltzmann model has been applied
directly to the current work [14]. The model operates
by tracking the streaming and collision of a number of
fictitious particles in a lattice in terms of particle
distribution functions.
The particle distribution function $% &, ( defines
the mass of a particle at location x at time t and
moving with velocity )% in the direction i:
*+ &,,
/
2
- )% . .$% &, ( 1$% &, ( $% &, (3
(3)
*,
0
2
where $% &, ( is the equilibrium distribution function
which is the value of $% &, ( under an equilibrium
state and λ is a relational parameter which controls the
2
rate at which $% &, ( approaches$% &, (.
A sample 3D image of W×L×H =100×300×100
3
µm generated from X-ray microtomography is
uploaded to the lattice Boltzmann model as a binary
data file, with 0 representing pore space and 1
representing solid space. The flow direction is along
L. Each binary digit represents a cubic voxel in the 3D
image and the characteristic length of each voxel is
equal to the fixed resolution of the X-ray tomography
images, denoted dx. The LB model considers the
particle distribution functions at each voxel in turn
and determines a set of nineteen velocities in the 3D
spatial domain for the node of each voxel. The
nineteen velocities considered are as follows:
stagnation at the origin (0,0,0)/dt, two velocities in the
x direction (±dx,0,0)/dt, two in the y direction
(0,±dx,0)/dt, four in the x-y plane (±dx,±dx,0)/dt, four
in the y-z plane (±dx,0,±dx)/dt and four in the y-z
plane (0,±dx,±dx)/dt. This scheme is commonly
known as the D3Q19 scheme. The single-phase model
assumes that the pores of the GLD are infiltrated by
air. Using the detailed velocity field, it is possible to
calculate the three components of the permeability
tensor for the imaged sample using Darcys law;.
672
672
672
455 ∆:⁄;8 ; 45= ∆:⁄;> ; 45? ∆:⁄;@ (4)
8
8
8
where ρ is the density of air, µ is the kinetic viscosity
of air, qi is the average velocity in the direction i, ∆P
is the pressure applied in the principal flow direction
and Li is the overall sample length in the direction i.
The average velocity is directly related to the velocity
field as determined by the LB model.
Because the absolute permeability represents the
linear dependence of gas flow rate on pressure
gradient, it must be ensured that the flow rate in the
simulations is also in this linear range. As such, the
pressure difference applied to the region is set to 20
Pa. The simulation is carried out on a dual-core 2.01
GHz workstation with 3.25 GB of RAM. A singlephase simulation for the region takes 500 minutes.
465
The through plane permeability has been found as
2.0×10-6 mm2.
[7]
CONCLUSIONS
[8]
The study was conducted using reconstruction of
3D images of a GDL and MPL through X-ray
microtomography and FIB/SEM nanotomography
with 2 µm and 15 nm resolution respectively. Both
images have been used to determine the critical
structural parameters of the layers, such as porosity,
mean pore diameter, effective diffusivity, Knudson
diffusion coefficient, structure model index and
degrees of anisotropy which have significant impact
on the fuel cell performance.
In addition, a successfully tested lattice
Boltzmann modeling was used to simulate the gas
velocity field to predict the through-plane
permeability of the real GDL structure.
[9]
[10]
[11]
[12]
[13]
ACKNOWLEDGMENTS
This research was supported by the UK
Technology Strategy Board (TSB Project No.:
TP/6/S/K3032H). We acknowledge industrial partners
AVL List GmbH, Intelligent Energy Ltd., Johnson
Matthey Plc., Saati Group Inc. and Technical Fibre
Products Ltd. for their support of this work.
[14]
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