Monitoring of membrane failure due to pinhole formation
Viktor Hacker, Eva Wallnöfer
Department of Chemical Engineering and
Environmental Technology, TU GRAZ, Austria
Peter Prenninger
AVL List GmbH, Austria
Trondheim, June 23rd,2009
Professor Horst Cerjak, 19.12.20052
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Overview
1. Introduction degradation of PEMFC membranesinfluence of operating conditions
2. In-situ Degradation Studiesoperating conditionscharacterization methods
3. Conclusions
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Degradation of PEMFC Membranes –impacts of degradation
membrane decomposition (release of HF, SO2, CO2, CO, C-F-compounds)membrane thinninghigher gas permeabilityplatinum particle deposition in the membraneperformance loss of the MEAdecrease of life time
Professor Horst Cerjak, 19.12.20054
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Degradation of PEMFC Membranes –causes
thermal degradation
mechanical degradation
chemical degradation
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Degradation of PEMFC Membranes –influencing factors
o material properties
o cell assembling
o operating conditions
membrane thicknessgas pressuretemperaturegas humiditycell potential
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In Situ Degradation Studies
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Operating Conditions5 to 1 fuel cells (in series), 25 cm2 each operated under the same conditions up to 1300 h
permanent operating 24h/day, 7 days/weekinterruptions only for electrochemical characterizationsevery one or two weeks, one cell was removed (SEM analysis)
gas flow:H2: λ = 1.5 (at OCV: 300 ml min-1)Air: λ = 2.2 (at OCV: 300 ml min-1)
MEAs:pt loading: A: 0.4 mg cm-2, C: 0.6 mg cm-2
membrane: bilayer membrane, reinforced with PTFE, thickness: 35 µmactivation of the MEAs ( 6 h at 0.4 and 0.6 V)
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Characterisation Methods
performance (UI)cell potential (CP)membrane resistance (MR)fluoride emission rate (FER)pinhole detection (PD)membrane thickness and condition (SEM)
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0
2
4
6
8
10
12
14
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50Potential / V
Hyd
roge
n D
iffus
ion
Cur
rent
Den
sity
/ m
A c
m-2
0
2
4
6
8
10
12
14
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50Potential / V
Hyd
roge
n D
iffus
ion
Cur
rent
Den
sity
/ m
A c
m-2
0
2
4
6
8
10
12
14
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50Potential / V
Hyd
roge
n D
iffus
ion
Cur
rent
Den
sity
/ m
A c
m-2
0
2
4
6
8
10
12
14
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50Potential / V
Hyd
roge
n D
iffus
ion
Cur
rent
Den
sity
/ m
A c
m-2
0
2
4
6
8
10
12
14
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50Potential / V
Hyd
roge
n D
iffus
ion
Cur
rent
Den
sity
/ m
A cm
-2
0
2
4
6
8
10
12
14
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50Potential / V
Hyd
roge
n D
iffus
ion
Cur
rent
Den
sity
/ m
A cm
-2
Hydrogen Diffusionanode: H2 flow / cathode: N2 flowstandard conditions H2 diffuses through the membrane and gets oxidisedwith an increasing potentialthe hydrogen diffusion current is limited by diffusion to < 5 mA cm-2
if there is a pinhole, the current increases with increasing potential
Po te n tia l / V
0 h 110 h 280 h 450 h 620 h 790 h 960 h 1130 h
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0
10
20
30
40
50
60
70
80
90
0 100 200 300 400 500 600 700 800 900 1000 1100time / h
hydr
ogen
diff
usio
n cu
rrent
den
sity
be
twee
n 0.
2 am
d 0.
5 V
/ mA
cm
-2
Hydrogen Diffusion - Results
the formation of a pinhole can not be forecasted end of membrane lifetime:time interval, at which the hydrogen diffusion current density is in the range between 4 and 5 mA cm-2
standardlow humiditylow temperaturehigh pressure45 mA90 mA135 mA405 mA90 mA, low humidity
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Cell Performancethe cumulated performance of the stack was investigated; H2: λ = 1.5 / Air: λ = 2.2no gas pressure; T: 70 °Cpolarisation curves: changes with operating time
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0.0 0.2 0.4 0.6 0.8 1.0 1.2Current Density / A cm-2
Pot
entia
l / V
standardPo ten tia l / V
0 h 110 h 280 h 450 h 620 h 790 h 960 h 1130 h
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0.0 0.2 0.4 0.6 0.8 1.0 1.2Current Density / A cm-2
Pot
entia
l / V
low humidity (40 % rH)
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0.0 0.2 0.4 0.6 0.8 1.0 1.2Current Density / A cm-2
Pot
entia
l / V
low temperature (40 °C)
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0.0 0.2 0.4 0.6 0.8 1.0 1.2Current Density / A cm-2
Pot
entia
l / V
45 mA cm-2
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0.0 0.2 0.4 0.6 0.8 1.0 1.2Current Density / A cm-2
Pot
entia
l / V
135 mA cm-2
Professor Horst Cerjak, 19.12.200512
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Cell Performance - Results
maximum power density changes with operating time
the performance loss is related to the membrane degradation, but also influenced by the electrode degradation
the formation of the first pinhole is not exactly related to a certain loss of performance
standardlow humiditylow temperaturehigh pressure45 mA90 mA135 mA405 mA90 mA, low humidity
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OCV decreases with the operating time (non-linear)
Cell Performance - Results
OCV
normalised OCV (to 0.95 V)
the formation of the first pinhole is related to a loss of 30 - 70 mV (mostly ~ 50 mV) of the initial OCV of the “stack”because of the increasing gas diffusion through the degradated membrane
a certain influence of the electrode degradation may exist
standardlow humiditylow temperaturehigh pressure45 mA90 mA135 mA405 mA90 mA, low humidity
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FER in the anode and cathode exhaust water was nearly the same, even though the degradation of the anode side was higher (→ SEM investigations)
FER was slightly decreasing with operating time
Fluoride Emission Rate - Results
0.000
0.002
0.004
0.006
0.008
0.010
0 100 200 300 400 500 600 700 800
Time / h
FER
/ m
g F
- cm-2
h-1
Anode Standard
Cathode Standard
Professor Horst Cerjak, 19.12.200515
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Fluoride Emission Rate - Results
accumulation of the total anode and cathode FER:
the formation of the first pinhole is related to a certain cumulated FER:
standardlow humiditylow temperaturehigh pressure45 mA90 mA135 mA405 mA90 mA, low humidity
Professor Horst Cerjak, 19.12.200516
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the resistance does not correlate clearly to
operating conditions
operating time
pinhole formation
membrane thickness (SEM)
a slight increase of the resistance could be observed at higher degradation/ long operating time in most cases
Membrane Resistance - Results
the resistance is influenced by
the loss of proton conducting, hydrophilic functional groups
the structural changes of the hydrophobic phase
the thinning of the membrane
change of the resistance with the operating time
standardlow humiditylow temperaturehigh pressure45 mA90 mA135 mA405 mA90 mA, low humidity
5.0E-03
6.0E-03
7.0E-03
8.0E-03
9.0E-03
1.0E-02
1.1E-02
1.2E-02
0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300time / h
mem
bran
e re
sist
ance
/ Ω
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Scanning Electron Microscopy - Results
membrane thickness the thinning of the
membrane is not clearly related to operating conditions
membrane thinning occurs under moderate and harsh conditions
pinhole formation is not related to a significant membrane thinning
5
10
15
20
25
30
35
0 100 200 300 400 500 600 700 800 900 1000 1100time
mem
bran
e th
ickn
ess
/ µm
standardlow humiditylow temperaturehigh pressure45 mA90 mA135 mA405 mA
Professor Horst Cerjak, 19.12.200518
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Conclusions a low cell current and a low gas humidity accelerate membrane degradationa lower temperature and higher cell currents slow membrane degradationthe current density influences the impact of other parameters (e.g. gas humidity)the end of lifetime of the MEA was indicated by the detection of the first pinhole (at moderate performance losses)an exceed of a certain value of cumulated FER could indicate theformation of pinholes a drop of OCV below a certain value could indicate the formation of pinholesmembrane thinning at the anode side occurred at long-time operation even under moderate operating conditionsthe membrane resistance did not correlate clearly to the membrane degradation, the formation of pinholes and the membrane thinning
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future research activities
evaluate and understand the complex interactions of fuel cell operating conditions, membrane parameters and membrane lifetimeobserve membrane and electrode degradation separately find a (simple) analysis instrument to observe membrane ageing during fuel cell operation
thank you for your kind attention!
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