Effect of catalyst layer mesoscopic pore-morphology on cold start process of PEM fuel cells
Ahmed Mohmed DAFALLA1, Fangming JIANG2()
1. Laboratory of Advanced Energy Systems, Guangdong Key Laboratory of New and Renewable Energy Research and Development, CAS Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences (CAS), Guangzhou 510640, China; University of Chinese Academy of Sciences, Beijing 100049, China 2. Laboratory of Advanced Energy Systems, Guangdong Key Laboratory of New and Renewable Energy Research and Development, CAS Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences (CAS), Guangzhou 510640, China
Water transport is of paramount importance to the cold start of proton exchange membrane fuel cells (PEMFCs). Analysis of water transport in cathode catalyst layer (CCL) during cold start reveals the distinct characteristics from the normal temperature operation. This work studies the effect of CCL mesoscopic pore-morphology on PEMFC cold start. The CCL mesoscale morphology is characterized by two tortuosity factors of the ionomer network and pore structure, respectively. The simulation results demonstrate that the mesoscale morphology of CCL has a significant influence on the performance of PEMFC cold start. It was found that cold-starting of a cell with a CCL of less tortuous mesoscale morphology can succeed, whereas starting up a cell with a CCL of more tortuous mesoscale morphology may fail. The CCL of less tortuous pore structure reduces the water back diffusion resistance from the CCL to proton exchange membrane (PEM), thus enhancing the water storage in PEM, while reducing the tortuosity in ionomer network of CCL is found to enhance the water transport in and the water removal from CCL. For the sake of better cold start performance, novel preparation methods, which can create catalyst layers of larger size primary pores and less tortuous pore structure and ionomer network, are desirable.
Both the ionomer network and pore structure in CLs are less tortuous
Case 2
2.5
2.5
Both the ionomer network and pore structure in CLs are tortuous
Case 3
2.5
1.5
The ionomer network is tortuous, while the pore structure in CLs is less tortuous
Case 4
1.5
2.5
The ionomer network is less tortuous, while the pore structure in CLs is tortuous
Tab.1
Source term
Gas channels
GDLs
Catalyst layers
Membrane
0
–
0
(for reactants)
0
0
0
–
–
0
0
0
–
–
Tab.2
Description
Value
Porosity of GDL
0.6
Porosity of CL (initial)
0.53
Volume fraction of ionomer in CL
0.15
Density of gas mixture/(kg?m-3)
1
Heat capacity of membrane, CL, GDL, bipolar plate/(kJ?m-3?K-1)
1650, 3300, 568, 1580
Heat conductivity of membrane, CL, GDL, bipolar plate/(W?m-1?K-1)
0.95, 1, 1, 20
Heat capacity of ice, frost/ (kJ·m-3·K-1)
3369.6
Heat conductivity of ice, frost/(W?m-1?K-1)
2.4
Latent heat of desublimation/(J·mol-1)
5.1 × 104
Permeability of CL/m2
1 × 10-13
Electronic conductivity of GDL, CL, bipolar plate/(S?m-1)
300, 300, 1 × 107
Equivalent weight of ionomer/(kg·mol-1)
1.1
Density of dry membrane/(kg·m-3)
1980
H2/H2O diffusivity in anode/(m2·s-1)
8.67 × 10-5, 8.67 × 10-5
O2/H2O diffusivity in cathode/(m2·s-1)
1.53 × 10-5, 1.79 × 10-5
Tab.3
Fig.4
Fig.5
Description
Value
Cell height, length/mm
2, 600
Land shoulder width/mm
1
Anode, cathode GDL thickness/mm
300, 300
Anode/cathode CL thickness/mm
10, 10
Membrane thickness/mm
30
channel depth, width/mm
1, 1
Initial water content
6.2
Startup temperature/K
253.15
Current density/(mA?cm-2)
100
Anode/cathode stoichiometry
2
Anode/cathode inlet gas temperature/K
253.15
Anode/cathode pressure “absolute”/Pa
1.01 × 105
Tab.4
Fig.6
Fig.7
Fig.8
Fig.9
Fig.10
A
Side surface area of the bipolar plate/m2
a
Water activity
cp
Specific heat capacity/(J·kg−1·K−1)
C
Species concentration/(mol·m−3)
D
Diffusivity/(m2·s−1)
EW
Equivalent weight of dry membrane/(kg·mol−1)
F
Faraday’s constant/(C·mol−1)
h
Latent heat/(J·mol−1)
i
Exchange current density/(A·m−2)
j
Transfer current density/(A·m−3)
K
Permeability/m2
M
Molecular weight of gas
n
Bruggeman factor
nd
Electroosmatic drag coefficient/(H2O/H+ )
p
Pressure/Pa
Water desublimation rate/(mol·m−3·s−1)
R
Universal gas constant/(J·mol−1·K−1)
rCCL
Water transport resistance in CCL/(s·m−1)
rMEM
Water transport resistance in membrane/(s·m−1)
S
Source term
s
Ice fraction
t
Time/s
T
Temperature/K
Uo
Equilibrium cell potential/V
u
Superficial fluid velocity/(m·s−1)
V
Cell voltage/V
x, y, z
Cartesian coordinates
Greek symbols
δ
Thickness/m
ε
Porosity
η
Activation overpotential/V
κ
Ionic conductivity/(S·m−1)
κD
Diffusional conductivity/(S·m−1)
λ
Water content/(mol H2O/mol )
ρ
Density/(kg·m−3)
σ
Electronic conductivity/(S·m−1)
ϕ
Electric potential/V
μ
Viscosity/(Pa·s)
Electrolyte potential
Electron potential
τ
Tortuosity factor
ψ
Water back diffusion flux/(mol·m−2·s−1)
Subscripts/Superscripts
e
Electrolyte
eff
Effective
g
Vapor phase
gs
Vapor-solid phase transition
Kn
Knudsen
m
ionomer phase
MEM
Membrane
Nor
Normal
0
Initial
s
Ice or solid phase
sat
Saturated
solid
Solid phase
v
Species index
w
Component (hydrogen, oxygen, or water)
-
Average
→
Vector
1
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