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Transport Phenomena in Fuel Cells
Chao-Yang Wang
Electrochemical Engine Center (ECEC), andDepartment of Mechanical and Nuclear Engineering
The Pennsylvania State UniversityUniversity Park, PA 16802
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• Background and Societal Impact– FC is enabling technology for H2 economy– Automotive application for energy
independence– Micro/portable power for tether-free
electronics– FC interfaces with nanotech, IT, and bio-tech– FC is highly interdisciplinary system
requiring breakthroughs not only from materials science but also transport phenomena communities.
– FC physics is fascinating– Ability to control transport of reactants and
products is critical to FC performance, durability, and cost
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• Technical Principles– H2/air polymer electrolyte fuel cells
(PEFC)• H2 + O2 produces H2O• Operation temperatures: -40oC to 80oC• A large range of length and time scales• Power density reaches ICE
– Water transport and removal• Water transport thru ionomeric membranes• Liquid H2O transport through nano-pore
catalyst layer (CL), microporous layer (MPL), and gas diffusion layer (GDL)
• Impact of surface wettability (e.g. hydrophobic GDL)
• Two-phase flow in micro- and mini-channels, flow mal-distribution
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• Technical Principles (PEFC)– Coupled water and heat transport
• H2O vapor saturation pressure exponentially increase with temperature
• Evaporation/condensation – the heat pipe effect– Transient Behaviors
• Critical for PEFC startup, load change, gas purge during shutdown, and materials durability
• Time constants of membrane hydration/dehydration, membrane water diffusion, gas transport, GDL flooding and dewetting
– Subzero startup• Ice formation and melting in CL• Water transport properties at subfreezing T
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• State-of-the-Art (PEFC)
Water transport on CL
Ge & Wang, Electrochim. Acta, 2007
Capillary network theory Pore network modeling 2-phase LB simulation
In-situ imaging and quantification of liq. H2O saturation in GDL:• neutron radiography• NMR• X-ray microtomography
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• State-of-the-Art (PEFC)– Two-phase flow in channels and channel flooding
• Voltage loss due to channel flooding amounts to 120 mV and negates catalyst improvement (4x activity ⇒ 45 mV gain).• Fluctuation sets up voltage cycling at high potentials ⇒ Pt area loss + carbon corrosion
I=0.2 A/cm2
70/70/80oC150 kPa
0.4
0.5
0.6
0.7
0.8
0.9
1
0 20 40 60 80 100 120 140 160 180
Time (min)
E (
V)
0
0.3
0.6
0.9
1.2
1.5
1.8
∆ P (
kP
a)
ξ = 4 ξ = 3.5 ξ = 3 ξ = 2.5 ξ = 2
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Channel walls
(θ<45°)
GDL surface
(θ~110°)
Channel walls
(θ<45°)
GDL surface
(θ~110°)
Channel walls
(θ<45°)
GDL surface
(θ~110°)
Channel walls
(θ<45°)
GDL surface
(θ~110°)
Channel walls
(θ<45°)
GDL surface
(θ~110°)
Channel walls
(θ<45°)
GDL surface
(θ~110°)
Channel walls
(θ<45°)
GDL surface
(θ~110°)
Channel walls
(θ<45°)
GDL surface
(θ~110°)
(a) corner flow w/ droplets on GDL
(b) annular film flow w/ droplets on GDL
(c) slug flow w/ channel clogging
Gas flow
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• State-of-the-Art (PEFC)– Transients
Electro-osmoticDrag
Back-diffusion in membrane
sD OHm
mdiff 101.0~2
2
−= δτsEOD µτ 1~
Membrane hydration/ dehydration
sm 10010~ −τ
ANODEANODE
CATHODECATHODE
MEMBRANEMEMBRANEElectroosmoticDrag= d
InF
Back Diffusion Flux= w
CDδ
∆
ProductionHH22OO
H2/H2O
Air/H2O y
T i m e ( s )
Cel
lvo
ltage
(V)
0 2 4 6 80
0 . 1
0 . 2
0 . 3
0 . 4
0 . 5
0 . 6
0 . 7
0 . 8
S t e p c h a n g e i n c u r r e n t d e n s i t y f r o m 0 . 1 t o 0 . 8 A / c m 2
S t e p c h a n g e i n c u r r e n t d e n s i t y f r o m 0 . 1 t o 0 . 6 A / c m 2
S t e p c h a n g e i n c u r r e n t d e n s i t y f r o m 0 . 1 t o 0 . 8 A / c m 2
S t e p c h a n g e i n c u r r e n t d e n s i t y f r o m 0 . 1 t o 1 . 0 A / c m 2
R H a / c = 5 0 / 0 %
R H a / c = 1 0 0 / 1 0 0 %
R H a / c = 5 0 / 0 %
R H a / c = 5 0 / 0 %
Temporary power loss upon load increase
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• State-of-the-Art (PEFC)– Cold Start
Gaspurge studies
Cold startup cycling
Advanced in-situ diagnostics: imaging ice formation, measuring ice
in CL, etc.
Durability tests & degradation analysis
Multiphase modeling of cold-start
ValidationPredicted Product Water (mg/cm2)
Exp
erim
enta
lPro
duct
Wat
er(m
g/cm
2 )
0.2 0.4 0.6 0.8 1.0 1.20.0
0.2
0.4
0.6
0.8
1.0
1.2
Eq. Purge, 30 µ m membrane, 40 mA, -20° CEq. Purge, 60 µ m membrane, 40 mA, -20° CEq. Purge, 30 µ m membrane, 100 mA, -20° CEq. Purge, 60 µ m membrane, 100 mA, -20° CDry Purge, 30 µ m membrane, 100 mA, -20° CDry Purge, 30 µ m membrane, 40 mA, -20° CEq. Purge, 30 µ m membrane, 40 mA, -30° CDry Purge, 30 µ m membrane, 100 mA, -30° C
+20%
-20%
200 µ m200 µ m200 µ m
Gore-Select Membrane
-20oC -30oC
Sub-zero
Gore-Select Membrane
-20oC -30oC
Sub-zero
Interfacial delamination after 110 cycles of cold-start at 500 mA/cm2
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• Barriers (PEFC)– Liq. H2O transport in CL
• Mixed wettability: hydrophilic/hydrophobic pores• Role of evaporation driven by heat generation• Removal of liq. H2O from CL
– Liq. H2O transport thru GDL• Impact of surface wettability and structure• What is structural building block of GDL?• Pore-level physics of water transport• In-situ measurements of liquid saturation in CL,
MPL and GDL w/ spatial resolution ~1 µm.– Micron-resolution sensing
• Current, humidity, temperature in-plane distributions between channel and land
• In-situ diagnostics of FC stacks using MEMS sensors in a bandage form
– How to maintain intricate water balance btw anode and cathode
Feunberger et al., J Electrochem. Soc. 2006
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• Technical Principles– Direct oxidation fuel cells (DOFC)
• Oxidation of liquid fuel (methanol, ethanol, DME, etc.) at anode
• High energy density and suited for consumer electronics applications
– Methanol & water crossover thru membrane
Conventional DMFC system
New DMFC system using low-α MEA
MeO
H c
ross
over
cu
rren
t den
sity
H2O crossover coeff. α
Wang & Liu (2006)
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• State-of-the-Art (DOFC)– Direct methanol fuel cells (DMFC)
• Power density ~ 120 mW/cm2 @ 0.4V• MeOH feed concentration 70-100% directly into anode• Fuel efficiency >80%• Total Pt loading ~ 3.5 mg/cm2
• 3,000 hr durability with <20% power fade• All the above together
– Direct ethanol fuel cells (DEFC)• Power density is half of DMFC @ 0.4V• Electrolyte and catalyst are under active pursuit• Limited to lower operating temperatures• Durability studies are in infancy
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• Barriers (DOFC)– Direct methanol fuel cells (DMFC)
• Fundamental understanding of water, methanol and heat transport in DMFC
• How to manipulate surface wettability and porous structures to control water crossover from the anode to cathode
• More compact, more efficient cell design– Direct ethanol fuel cells
• Complete oxidation of ethanol (with C-C bond)• Water management• Pt-free catalysts• Electrolytes: proton or alkaline anion type?• Durability of DEFC materials
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• Technical Principles– Solid oxide fuel cells (SOFC)
• SOFC R&D directions: lower operating temperatures, and internal reforming of HC fuel, especially CH4
• O2- ion conductivity is low. • Larger temperature distribution in through-
plane direction • Water is needed for reforming, WGS and
avoiding carbon deposition, but water is produced from H2 electro-oxidation.
Reforming reaction (endothermic) CH4 + H2O -> CO + 3H2 (steam reforming) CO + H2O -> CO2 + H2 (water gas shift)
CH4+H2O
H2O
H2
CH4
SOFC Anode
Steam reforming; WGSendothermic
Electro-oxidation
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• State-of-the-Art (SOFC)– CFD-based modeling of species, heat and charge transport on
cell level – Micro-models of transport phenomena in porous electrodes – Limited experimental characterization, especially distribution
data of current, species and temperature – Few studies on water and heat transport in the anode with
direct feed of HC fuel• Water is reactant of reforming reaction but product of
electrochemical reaction• Heat is consumed in reforming reaction but produced from
electrochemical reaction• Heat/water sink and source co-exist in HC fueled SOFC
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• Barriers (SOFC)– Multi-component transport models accounting for
competitive reactions in anode– Measurements of current, species and temperature
distributions, especially water vapor and temperature distributions in the anode
– Effective water and heat management– Understand and develop electrode microstructure-
performance relationship
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• Recommendations1. Novel concepts of water management for automotive fuel cells2. Understanding pore-level physics of water transport3. Multiscale models to predict electrode flooding and mass transport loss4. In-situ characterization techniques with micron-resolution5. Dynamic behavior of polymer electrolyte fuel cells6. Subzero startup of polymer electrolyte fuel cells7. Basic understanding of water, methanol and heat transport in direct
methanol fuel cells8. More compact, efficient DMFC designs9. Transport phenomena in direct ethanol fuel cells10. Models and experimental data of water and heat transport in SOFC 11. Understanding electrode microstructure-performance relationship
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• References1. C.Y. Wang, Chemical Reviews, 104, 4727 (2004)2. K.D. Kreuer et al., Chemical Reviews, 104, 4637 (2004).3. J.H. Nam and M. Kaviany, Int J Heat Mass Trans., 46, 4595 (2003)4. U. Pasaogullari and C.Y. Wang, J Electrochem Soc., 151, A399 (2004)5. S.A. Freunberger et al., J Electrochem Soc, 153, A1258 (2006)6. X.G. Yang et al., Electrochem & Solid-State Lett., 7, A408 (2004)7. S. Ge and C.Y. Wang, Electrochim. Acta, 52, 4825 (2007)8. Y. Wang and C.Y. Wang, Electrochim. Acta, 50, 1307 (2005).9. P.K. Sinha et al., J Mater. Chem., in press, 2007.10. G.Q. Lu et al., Electrochem & Solid-State Lett, 8, A1 (2005)11. C.Y. Wang and F.Q. Liu, Chapter 10 in 2006 Small Fuel Cell Symp., Knowledge
Foundation, 2006.12. Gurau et al., AIChE J, 44, 2410 (1998)13. S. Um et al., J Electrochem Soc, 147, 4485 (2000)14. A.V. Virkar et al., Solid State Ionics, 131, 189 (2000)15. T. Thampan et al., J Electrochem Soc., 147, 3242 (2000)
