Effects of Fuel and Air Impurities on PEM Fuel Cell Performance
New Membranes for PEM Fuel Cells - Energy
22
New Membranes for PEM Fuel Cells Steve Hamrock 3M Fuel Cell Components Program 3M Center 201-1W-28 St Paul MN 55144 USA HTMWG Meeting 5/27/05 This research was supported in part by the U.S. Department of Energy, Cooperative Agreements No. DE-FC36-02AL67621 and DE-FC36-03GO13098. DOE support does not constitute an endorsement by DOE of the views expressed in this presentation.
Transcript of New Membranes for PEM Fuel Cells - Energy
New Membranes for PEM Fuel Cells
Steve Hamrock3M Fuel Cell Components Program
3M Center 201-1W-28St Paul MN 55144 USA
HTMWG Meeting 5/27/05
This research was supported in part by the U.S. Department of Energy, Cooperative Agreements No. DE-FC36-02AL67621 and DE-FC36-03GO13098. DOE support does not constitute an endorsement by DOE of the views expressed in this presentation.
May 27th, 2005
3MFuel Cell Components
The new 3M ionomer has a slightly shorter side chain than standard PFSA Membrane ionomer without the pendant -CF3 group. This allows a higher degree of crystallinity at a given EW.
O(CF2CFO)CF2CF2
CF3
OCF2CF2CF2CF2
SO3H
(CF2CF)n(CF2CF2)m
Standard PFSA
(CF2CF)n(CF2CF2)m
New 3M Polymer
SO3H
May 27th, 2005
3MFuel Cell Components
Monomer synthesisThe 3M monomer is based on electrochemical
fluorination (ECF) of a hydrocarbon starting material.
OSO2
F CF2
CF2CF2
SO2F
O
- CO2
FCO
CF2
CF2CF2
CF2SO2FF3C
COF
CF2
FCO
CF2
CF2CF2
CF2SO2F
FCO
CF2
CF2CF2
CF2SO2FF3C
COF
Na2CO3
F CF2
CF2CF2
SO2F
O
F2C CFO
CF3
ECF
3M monomer
+
May 27th, 2005
3MFuel Cell Components
Mechanical properties of 3M ionomerDynamic Mechanical Analysis shows higher Tg and storage modulus than cast standard PFSA membrane at 1,000 EW
1.E+01
1.E+02
1.E+03
1.E+04
1.E+05
1.E+06
25 50 75 100 125 150 175 200Temp (C)
E' (
PSI)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Tan
Del
ta
Std. PFSA E'
3M Membrane E'
Std. PFSA Tan Delta
3M Membrane Tan Delta
May 27th, 2005
3MFuel Cell Components
Membrane tensile test at 95% RH, vs Temp.
• Tensile tests run in a controlled humidity oven.
• 3M Membrane maintains high modulus up to 75˚C 0
5,000
10,000
15,000
20,000
25,000
30,000
35,000
40,000
20 30 40 50 60 70 80Temp ˚C
PSI
Extruded 50 micron PFSA Down Web
Extruded 50 micron PFSA Cross Web
Cast 3M Membrane (1000EW, 30 microns)
Cast Std. PFSA ( 1,000 EW, 30 Micron)
May 27th, 2005
3MFuel Cell Components
Water Sorption Isotherm for 1000 EW 3M Membrane 30°C
0
2
4
6
8
10
12
14
0 0.2 0.4 0.6 0.8 1
Activity of Water
λ (W
ater
Mol
ecul
es p
er S
ulfo
nate
Gro
up)
3M Membrane ( cast)
Std. PFSA (extruded)Note relatively lower uptake for 3M ionomer
T. Zawodzinski CWRU
May 27th, 2005
3MFuel Cell Components
Conductivity of 1000 EW 3M Membrane 30°C vs. hydration state (λ)
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0 5 10 15 20 25
λ (Water Molecules per Sulfonate Group)
σ (S
/cm
)
Std. PFSA
3M MembraneConductivity data shows sharp increase with water content
T. Zawodzinski CWRU
May 27th, 2005
3MFuel Cell Components
Conductivity vs. temperature for different EW ionomers• AC 4-point probe
measurement, ambient pressure.
• The lowest EW ionomer tested so far is about 700. The conductivity of this material is about 25-30 mS/cm at 120˚ C, 80˚ C DP.
Proton Conductivity at 80C Dew point
0.01
0.10
1.00
70 80 90 100 110 120 130Temp (˚C)
Con
duct
ivity
, S/c
m3M PFSA 730 EW 3M PFSA 830 EW 3M PFSA 900 EW 3M PFSA 980 EW
May 27th, 2005
3MFuel Cell Components
Membrane chemical stability• Peroxides generated from fuel cell operation are thought
to cause membrane degradation and failure.• The mechanism of this is thought to involve attack of
peroxides or radicals from peroxides at the carboxylate endgoups. Fe accelerates this process.
(CF2)n-CO2H + H2O2 (CF2)n-CO2· + ·OH
(CF2)n-CO2· + ·OH (CF2)n-1-CF2OH + CO2↑
(CF2)n-1-CF2OH (CF2)n-1-COF + HF
(CF2)n-1-COF + H2O (CF2)n-1-CO2H + HF
May 27th, 2005
3MFuel Cell Components
Weight loss and decrease in tear strength during heating in H2O2
• Sample heated to 90˚C for 4 days in 1M H2O2.
• H2O2 changed every day.
• Without Fe, no change observed.
Strength Loss as a function of Peroxide Degradation
50
60
70
80
90
100
110
0 10 20 30 40Weight Loss (%)
% O
rigin
al T
ear S
tren
gth
No Fe(II)10 ppm Fe(II)
1day
4day
3day
2day
1-4 days (no Fe)
May 27th, 2005
3MFuel Cell Components
New process allows ionomer to be made with much lower concentration of carboxylate endgroups.
• Degradation can also be followed by monitoring fluoride levels in peroxide solution.
• Membrane with lower concentration of carboxylate endgroups exhibits much lower rate of degradation.
0
50
100
150
200
250
0 5 10 15 20
Time (days)
mic
ro g
F-/
g po
lym
er/ d
ay
1,000 EW End Group Modified
1,000 EW Control
May 27th, 2005
3MFuel Cell Components
Oxidative stability vs. EW• Samples are
heated to 70˚C in 30% H2O2with 50 ppm Fe.
• H2O2 is changed every 3 days.
• Degradation is followed by monitoring F-content of the solution with an ISE 0
100200300400500600700800900
1000
0 1 2 3 4cycles
F- p
pm/g
mem
bran
e
700 EW
800 EW
900 EW
1000 EW
1100 EW
800 EW Lowend group
May 27th, 2005
3MFuel Cell Components
Fluorinate fragment identified during peroxide testing
• HPLC/MS analysis showed whole side-chain fragment
• No other fragments seen by this method.
*CF2
CF2
CF2CF
O
F2CCF2
F2CCF2
HO3S
n
m
CF2
CF2CF2
SO3HHO2C
Fragment structure indicates cleavage here
May 27th, 2005
3MFuel Cell Components
Additives improve oxidative stability and help mitigate the negative effects of Fe
• High and low levels of additive both with and without added Fe.
• At high additive level, not much difference between w/ and w/o Fe.
• At low additive level, mass loss was much higher for added Fe sample.
Percent Mass Change after 5 Days at 90˚C
-3
-2.5
-2
-1.5
-1
-0.5
0High Additive Low Additive Control
% M
ass
Cha
nge
No added FeAdded Fe
0 %
May 27th, 2005
3MFuel Cell Components
EW vs. performance70˚C H2/air, fully humidified• Lower cell
resistance with lower EW.
• No observed impact on performance at 70˚C, fully humidified.
• MEA’s not optimized for different EW membranes.
00.10.20.30.40.50.60.70.80.9
1
0 0.2 0.4 0.6 0.8 1 1.2 1.4A/cm2
Volts
0.045
0.065
0.085
0.105
0.125
0.145
0.165
mO
hms-
cm2
700 EW
800 EW
900 EW
1000 EW
May 27th, 2005
3MFuel Cell Components
EW vs. performance constant current at 0.5 A/cm2, 90˚C H2/air, 28% RH
• Performance increases with lower EW under hot, dry conditions.
• MEA’s not optimized for different EW membranes.
V at 0.5A/cm2
0.400
0.420
0.440
0.460
0.480
0.500
0.520
0.540
3M 1,000 EW 3M 900 EW 3M 800 EW 3M 700EW
Volts
V at 0.5A/cm2
May 27th, 2005
3MFuel Cell Components
Normalized lifetime under accelerated testing –Lifetime defined as when OCV drops below 800mV
• Test run at 90˚C, 12% RH.
• Lifetime defined as when OCV drops below 800mV.
• No difference between 700 to 1,000 EW in this test.
Normalized lifetime 5 samples each
0
20
40
60
80
100
120
140
160
180
200
3M 1,000 EW 3M 900 EW 3M 800 EW 3M 700EW
Nor
mal
ized
Life
time
May 27th, 2005
3MFuel Cell Components
Fluoride Release under accelerated testing conditions (90 ˚C, 12% RH). F generation rate of MEA’s with new 3M membrane is about 100 times less than that of 50 micron extruded standard PFSA membrane under these conditions.
0.1
1.0
10.0
100.0
1000.0
3M Membrane Standard PFSA MembraneFluo
ride
Ion
Rele
ase
Rat
e (n
gram
s/m
in)
Anode Cathode
near detectibility limit
May 27th, 2005
3MFuel Cell Components
MEA Accelerated Lifetime testing90º C, 28% RH, Load cycled from 0 to 0.5 A/cm2 , 50 cm2 cell, 0.4/0.4 Pt/Pt, 7/0 PSIG. Life defined as when OCV drops below 800mV
Accelerated lifetime test 90/60/60
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 1000 2000 3000 4000 5000 6000Hours
V
3M Membrane3M Membrane3M Membrane3M Membrane3M Membrane3M MembraneCast Standards PFSA Membrane
Two cells still running
May 27th, 2005
3MFuel Cell Components
MEA Accelerated Lifetime testing90º C, 28% RH, Load cycled from 0 to 0.5 A/cm2 , 50 cm2 cell, 0.4/0.4 Pt/Pt, 7/0 PSIG. Life defined as when OCV drops below 800mV
4289
235
0
1000
2000
3000
4000
5000
6000
New 3M Membrane (Mix of 800 and 1,000EW samples)
Standard PFSA Membrane
Hou
rsNew 3M membrane shows >15 X increase in lifetime over 50 micron extruded standard PFSA membrane .
May 27th, 2005
3MFuel Cell Components
Performance during accelerated durability testing
SEF (M2/M2)
H2 Crossover (mA/cm2)
Short Resistance (OHM-CM2)
Testing Time
178.3 4.2 >500 0
102.9 3.4 >500 2800
78.1 17.0 >500 3500
Accelerated durability testing was stopped periodically and sample was tested at 70˚C 100%RH. No increase in crossover or shorting was detected before 3500 hours.
00.10.20.30.40.50.60.70.80.9
1
0 0.2 0.4 0.6 0.8 1 1.2
Current Density
Volta
ge
0.060.070.080.090.10.110.120.130.14
mO
hms-
cm2
StartAfter 2800 hoursAfter 3500 hoursResistance Start Resistance 2800 hoursResistance 3500 hours
May 27th, 2005
3MFuel Cell Components
Summary
• A new perfluorinated sulfonic acid ionomer (PFSA) has been developed at 3M. This polymer has:
• High Tg (125ºC for 1,000 EW) • High modulus, tear and puncture resistance both dry and
under use conditions.• Lower EW membranes have increased conductivity,
providing MEA’s with better performance under hotter/drier conditions.
• Excellent oxidative stability for EW’s from 700 to 1,100.• Low fluoride content in the water coming from the cell.• Longer MEA life in accelerated testing.
