I01A-1033: Mass Transfer in Microporous Layers for Polymer ...
Transcript of I01A-1033: Mass Transfer in Microporous Layers for Polymer ...
Department of Hydrogen Energy Systems, Kyushu University1
Department of Mechanical Engineering, Kyushu University2
744 Motooka, Nishi-ku, Fukuoka 819-0395, JAPAN
Hironori NAKAJIMA1, 2
Shintaro IWASAKI1
Tatsumi KITAHARA1, 2
I01A-1033:
Mass Transfer in Microporous Layers for
Polymer Electrolyte Fuel Cells Analyzed
with Pore Network Modeling240th ECS Meeting (October 10-14, 2021)
2
H2 2H+ + 2e-
1/2O2 + 2H+ + 2e- H2O
Concentration overpotential
by oxygen transport
limitation (voltage drop)
Flooding by excess liquid
product water
Water management by the
gas diffusion layer
(microporous layer)
Background
Polymer Electrolyte Fuel Cells
3Background
50 μm
Substrate
MPL
60
μm
16
0 μm
Microporous Layer (MPL)
MPL has been known to improve
water management characteristics by
mitigating flooding and dehydration.*
Optimized MPL structure and wettabilityVapor-liquid two phase transport model verified by experiment
Gas diffusion layer coated with MPL
5 μm
T. Kitahara, T. Konomi, and H. Nakajima, J. Power Sources, 195, 2202-2211
(2010).
*Z. Qi, A. Kaufman, J. Power Sources 109 (2002) 38-46
Background
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Pore Network Model (PNM)*
Modeling porous media by pores and throats.
Transport is modeled by convection and diffusion.
Pore diameter distribution by cross-sectional
observation of the MPL (FIB-SEM)
Vapor-liquid two phase transport in the MPL
*Gostick, J. T., Ioannidis, M. A., Fowler, M. W., Pritzker, M. D. J. Power Sources, 173(1), 277-290 (2007).
Construction of PNM of hydrophobic MPL(4)
⑥Pore diameter distribution
(Weibull distribution)
⑤Fitting
Validation
k = 𝑄
𝐴∆𝑃
R = ℎ
𝐷
d = 𝑅dry
𝑅wet
①Preparation of GDL with in-house hydrophobic MPL
(FIB scanning electron microscope)
Flow of research
④PNM3D-Porous Structure Analysis
FIB-SEM②FIB-SEM
(4) D. CHEN, K. HARANO, Y. MONDE, H. NAKAJIMA, T. KITAHARA, K. ITO, The Proceedings of the National
Symposium on Power and Energy Systems, C114, The Japan Society of Mechanical Engineers (2018)
Permeance : k
Oxygen diffusion resistance : R
③Mass transfer
measurements
Relative oxygen diffusion resistance : d
5
①Preparation of GDL with hydrophobic MPL
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2. Apply to a GDL substrate1. Slurry production
Drying : 130℃ 5 min
Firing : 350℃ 30 min
3. Drying and firing of GDL
①Pore structure changes with distilled water amount added
(Control of pore diameter distribution)
②Adjust MPL thickness by the number of applications
Carbon black 14-15 wt%
PTFE 4-5 wt%
Distilled water 80-82 wt%
Surfactant 1 wt%
Carbon black (80 wt%)
PTFE (20 wt%)
Stir:2000 rpm 60 min Number of applications
:2 - 4 times
Coating thickness
:10 - 20 μm
Slurry Doctor-blade coating Drying and firing
In-house MPL
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Maximum pore size measurement with wetting liquid
d
cos
PPP
θd
4cos4
max 0
( γ = 0.015 (N/m) )
Galwick(5):Non-volatile wetting
liquid with contact
angle of θ ≈ 0°
(5) Galwick: Porous Materials Inc.
Wetting liquid
Measurement of bubble point pressure
Sample A Sample B Sample C
dmax(µm) 2 5 15
CB (wt%) 80 80 80
PTFE (wt%) 20 20 20
MPL thickness (µm) 60 60 60
Substrate thickness (µm)
(Toray, TGP-H-030)100 100 100
Table1. Specifications of the MPL samples
60
μm
5 μm
50 μm
In-house MPL
Illustration of GDL1
60
μm
①Preparation of GDL with hydrophobic MPL
Wetting liquid
impregnated under
vacuum beforhand
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②FIB-SEM
(6) Amira 6.3: Thermo Fisher Scientific Inc.
(7) D. CHEN, K. HARANO, Y. MONDE, H. NAKAJIMA, T. KITAHARA, K. ITO, The Proceedings of the National Symposium
on Power and Energy Systems, C114, The Japan Society of Mechanical Engineers (2018)
Cross-sectional observation with FIB-SEM (FIB scanning electron microscope)
0
200
400
600
800
1000
0 1 2 3 4 5F
requ
en
cy
Pore diameter(µm)
sample B(max 5)
Approximate with Weibull distribution
and use for PNM
FIB-SEM cross-section
observationBinarization of cross-
sectional imagesAnalyzing 3D structure
Pore diameter distribution by watershed analysis
method
Amira
Watershed
Flow of porous Structure Analysis
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𝑘 =𝑄
𝐴 × ∆𝑃
k : Air permeance (µm/(Pa・s))
Q: Flow rate (cm3/s)
A : Air permeable area (=19.6 cm2)
ΔP : Gauge Pressure (=1.23 kPa)
1. Air permeation measurement
2. Oxygen diffusion measurement
Permeance
A
CQJ 44
)(
)(ln
)()(
43
21
4321
CC
CC
CCCCC
J
CR
Oxygen diffusion resistance
J: Oxygen flux
𝐶: Oxygen concentration
R: Oxygen diffusion
resistance (s/m)
③Mass transfer measurements
Schematic of the air permeability measurement
Experimental apparatus of oxygen diffusivity
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Oxygen diffusivity measurementAnalysis of the concentration boundary layer
Air
N2
Sample
Multilayered GDL measurements for
diffusion resistance of the concentration boundary layer
(In analogy with temperature boundary layer)
Boundary layer
③Mass transfer measurements
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Diffusion resistance of the concentration boundary layer
RTorayH030= 16.9𝑛 + 26.5
R = 𝑅Subn + 𝑅BL
RBL = 26.5 s/m (intercept)
RBL
RBL:R of boundary layer
RSub
①Air: 2.8 m/s
N2: 2.8 m/s
② ③Air
N2
Air
N2
43.4
60.377.2
R = 16.9n + 26.5
0
10
20
30
40
1 2 3
R (
s/m
)
Numbers of Substrate
R of Toray H030
RBL
③Mass transfer measurements
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Diffusion resistance of the microporous layer
Substrate(TGP-H-030)
MPL
60
µm
16
0µ
m
Substrate(TGP-H-030)1
00
µm MPL
60
µm
Boundary layer
RMPL = 14.8 s/mMPL:
dmax = 5 µm
Porosity = 0.37
RMPL = RGDL - Rsub
③Mass transfer measurements
(5) Galwick:Porous Materials Inc.
Galwick (5) : Non-volatile wetting liquid with a contact angle of θ ≈ 0 °Penetrating into both hydrophilic and hydrophobic pores
① Install GDL impregnated
with the wetting liquid into
the apparatus
② Discharge the wetting
liquid with increasing air
pressure
③ Gas chromatography
analysis under
atmospheric pressure.
𝑑 =𝑅dry
𝑅wet
Liquid saturation
with pressure
control
(intrusion volume
fraction)
Schematic of oxygen diffusivity test with GDL impregnated with the wetting liquid
Fig.5 Wetting liquid
3. Relative oxygen diffusion resistance with wetting liquid
③Mass transfer measurements
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14
O2 diffusion resistance under wet condition
𝑃in >4𝛾cos𝜃
𝑑𝑃in <
4𝛾cos𝜃
𝑑
MPL saturated with the wetting liquid
Pressure difference, Pin
The wetting liquid is discharged to a
saturation level depending on Pin
and capillary pressure of each throat
Vacant throats (gas pathway)
Pin
Wetting liquid
(Galwick)
θ ≈ 0°
③Mass transfer measurements
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O2 diffusion resistance under wet condition
Larger pressure gives larger r - Wetting liquid is discharged
(Smaller liquid saturation, larger effective O2 diffusivity)
PNM reproduces the tendency of the effective O2 diffusivity
Comparison of 𝑟 =𝑅dry
𝑅wet(0 ≤ r ≤ 1) between the PNM and experiment
Sample B (dmax = 5 µm)
Results and discussion
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Comparison with the Leverett function
PNM appears to be more reliable for the MPL
analysis than the Leverett approach proposed for fluid flow
through packed soil beds
Sample B (dmax = 5 µm)
Leverett function : Pc = γ cosθ𝜀
𝑘
0.5J(sl)
Results and discussion
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Air and liquid water relative permeability
Empirical fits for the relative permeabilities exhibit high-order power-law
dependence, indicating a wide pore diameter distribution
Asymmetric behavior of the permeability of liquid water and air suggests a
difference in their percolation structures in the pore network.
Sample B (dmax = 5 µm)
0.0
0.2
0.4
0.6
0.8
1.0
0.0 0.2 0.4 0.6 0.8 1.0
Re
lative
pe
rme
ab
ility
Liquid water saturation
Water
Air
kr,water=s^2.5
kr,air=(1-s)^9
Results and discussion
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Air permeability and oxygen diffusion resistances for the dry and wet
conditions of the MPL agree well between the PNM and experiment.
PNM is validated and useful to model oxygen and water transports in MPLs
with employing wettability (contact angle) of water in the pores.
Numerical models with the PNM for optimized designs of MPLs are feasible.
Conclusion