High Temperature Proton Exchange Membrane Nanocomposites for Fuel Cells
$$ #DE-FC36-01G011086
M. Hickner, F. Wang, Y.S. Kim, B. Pivovar*, T.A. Zawodzinski*,
and J.E. McGrathMaterials Research Institute and Department of Chemistry
Virginia TechBlacksburg, VA 24061
[email protected]*Los Alamos National Lab
Los Alamos, NMMay 9, 2002
Oct. 2001 to Sept. 2003
J. McGrath, D. Baird,J. Dillard, P. Zelenay,
T. Zawodzinski,M. von Spakovsky
“Advanced Materials for PEM-Based Fuel
Cell Material Systems”
U.S. Dept. of Energy, Golden, CO
TimingPI(s)Project TitleFunding Agency
PEM SynthesisJ.E. McGrath
High Performance Materials
Membrane/Electrode AssemblyJ. Dillard, J. McGrath
Membrane Conductivity TransportT. Zawodzinski, LANL/CWRU
P. Zelenay, LANL
Systems Integration and AnalysisM. von SpakovskyPERFORMANCE
Knowledge TransferGM, DuPont, Motorola
Air Products, UTCSolvay, Hydrosize
PMC/Carbon InjectionMolded Bipolar Plates
D. Baird
Commonwealth of Virginia
Blacksburg,Home of Virginia Tech
*Washington, D.C.
Views of Virginia Tech
Currently Used Proton Exchange Membrane (PEM) - Nafion
AdvantageExcellent Proton ConductivityGood Mechanical & Chemical PropertiesLong term stability
Chemical structure of poly(perfluorosulfonic acid) -Nafion
DisadvantageExpensiveHigh methanol permeabilityLoss of membrane performance at elevated temperature (>80oC)
,
CF2 CF CF2
O
CF2
CF CF3
O
CF2
CF2
SO3H
Objectives
• Fundamental investigations to identify viable alternative PEM systems– H2/AIR, 80ºC or higher (preferably 120-
150ºC)– Direct methanol (DMFC) PEM Systems
with reduced permeability
Why Poly(arylene Ether Sulfone)s?
O Ar O S
O
O n
• High thermal stability• Good stability against acid, bases and oxidants• Good mechanical properties• Film-forming, high-performance thermoplastics• Melt processible• Several monomers are commercially available
Comparison of Polymeric -SO3H Group Stability from Post and Monomer Sulfonation Methods
O O S
O
O
SO3H
n
Activated
• Post sulfonation occurs on the most reactive, but least stable, position• High electron density leads to relatively easy desulfonation
O O S
O
OSO3HSO3H
n
Deactivated
• Monomer sulfonation on the deactivated position• Enhanced stability due to low electron density
Advantages of Direct Polymerization
S ClClO
O
NaO3S
SO3Na
SDCDPS
n
High yields from 40MM lb/year precursorsPrecise control of ionic concentration during synthesisWell-defined ion conductor location; morphology controlHigh H+ conductivityEnhanced stability due to deactivated position of -SO3HCompatible with additives for >100ºC studies Very high molecular weight copolymers possible
Wholly Aromatic Random (Statistical) Poly(arylene ethersulfone) / Poly(arylene ether disulfonated sulfone) Copolymers Via Direct Copolymerization (BPSH-x)
O O SO2 co O O SO2
SO3HSO3H
Hydrophobic Hydrophilic
n x1-x
Biphenyl Sulfone: H Form (BPSH)x = molar fraction of disulfonic acid unit, e.g., 30, 40, etc.
Effect of Sulfonation on Conductivity
Conductivity is both a strong function of
sulfonation and water content
0.00
0.04
0.08
0.12
0.16
0.20
10 30 50 70
BPSH Sulfonation / mole %
Con
duct
ivity
/ S
cm-1
30°C in liquid water
Influence of Sulfonation Degree on Water Uptake of Polymer Membranes
30 40 50 60 70 80 900
20
40
60
80
100
120
140
10 20 30 40 50 60 70
Wat
er U
ptak
e, w
t%
0
20
40
60
80
100
120
140
160
180
200
PEEK (b)
H20
Sor
bed
% Disulfonation
BPSH (a)
Degree of Sulfonation %
(a) F. Wang, M. Hickner, Y.S. Kim, T. Zawodzinski and J.E. McGrath, “Synthesis and Characterization of Sulfonated Poly(arylene ether sulfone) Random (Statistical) Copolymers Via Direct Polymerization: Candidates for New Proton Exchange Membranes,” Journal of Membrane Science, 197 (2002), 231-242.
(b) Kaliaguire, S. et al. J. Memb. Sci., 173, (2002).
AFM Phase Images of BPSH Membrane
BPSH(20)
BPSH(50) BPSH(60)
Percolation Threshold
• Scale: 700 nm• Phase angle: 30 degree
BPSH(0)
BPSH(40)
BPSH and NafionTM
1 1 µµmm 700 nm700 nm
Phase Image of BPSHPhase Image of BPSH--40 40 Phase Image of Nafion 117 Phase Image of Nafion 117
Membrane Electrode Assembly
O2H+e-
H2Oe-
150 µm
H2O
H2O
H2OH2O
H2O
Platinum (3-5nm)
PEM
CathodeElectrode
AnodeElectrode
H2
Pt supportedon carbon withpolymer matrix
5 µm
Carbon Black (0.72µm)
Direct Painting Fabrication of Membrane Electrode Assembly
Ink1) Catalyst
Platinum or Pt/Ruthenium black (nanocrystalline metal)
2) Polymer Dispersion2) Polymer DispersionBPSH polymer (acid form)50/50 water/IPA solvent vehicle
Electrode “ink”
Cast Electrode(by painting)
Membrane
M. Wilson thin film electrodeUS #5211984
Catalyst ink painted directly onto dried acid form membrane at 60°CPolymer : Catalyst weight ratio is ~ 1:7 (50:50 volume ratio)Catalyst loading (mg Pt/cm2) is determined by amount of ink applied to active area
Used primarily for DMFC MEA fabrication with high catalyst loadings
Hot-Press Fabrication of Membrane Electrode Assembly
Step 1: Painting of catalyst inkonto a release substrate (Teflon or Kapton)
Step 2: Hot-pressing of electrodesonto membrane at 200°C and 200 psifor 5 min.
Electrode “ink”
Membrane
Cast Electrodes
Cast Electrode(by painting)
Used primarily for H2/Air MEA fabrication with low catalyst loadings
H2/Air Fuel Cell Performance
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
0 200 400 600
Current (mA/cm2)
Volta
ge (V
)
BPSH-40 4 milNafion 117
80°C cell temperature30 psig An/Cath
Anode: 10 mg/cm2 Pt/Ru blackCathode: 6 mg/cm2 Pt black
HFR0.11 Ω⋅cm2
0.17 Ω⋅cm2
note: BPSH-40 membrane has BPS anode and Nafion cathode
BPSH-30 H2/Air Fuel Cell Life Test:Membrane is stable ≥ 800 hours
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 200 400 600 800
time (hours)
curr
ent d
ensi
ty (A
/cm
2 )
Conditions:0.5 V cell voltage 80°C cell temp.full humidificationAnode:
hydrogen 30 psig0.2 mg Pt/cm2
carbon supportedcatalyst
Cathode:air 30 psig0.4 mg Pt/cm2
carbon supportedcatalyst
note: no change in cell resistance over the entire testindicates no change in membrane – minor performance drop most likely a result of electrode degradation
Direct Methanol Fuel Cell Performance
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 50 100 150 200 250
Current (mA/cm2)
Volta
ge (V
)
BPSH-40 4 milNafion 117
Conditions:0.5 M CH3OHunhumidified air0 psig80°C cell temp.
Anode: 10 mg/cm2 Pt/Ru blackCathode: 6 mg/cm2 Pt black
HFR0.11 Ω⋅cm2
0.17 Ω⋅cm2
DMFC - Fuel CrossoverWhy We Are Interested
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 100 200 300 400 500
Current Density (mA/cm2)
Volta
ge (V
)
BPSH-40Nafion 117
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
0 0.1 0.2 0.3 0.4 0.5
Current Density (A/cm2)
Cro
ssov
er C
urre
nt (A
/cm
2 ) BPSH-40Nafion 117
1M CH3OH60°C
BPSH polymers give similar performance to N117with much lower methanol crossover
Objectives
• Explore the conductivity behavior of Nafion and BPSH membranes at elevated temperature (>100oC) in fully humidified conditions
• Investigate the influence of acidification treatment on electrochemical properties
• Establish the optimum sulfonation level of BPSH for elevated temperature operation of fuel cell
• Investigate the effect of HPA (phosphotungstic acid and zirconium hydrophosphate) incorporated systemson proton conductivity and water management for the use in elevated temperature fuel cell applications
Schematic Setup of Sealed-off Cell
Limit conditionsTemperature: 140oC Relative Humidity:100%
Equilibrium conditions: Placed sample in humidity oven at least 4 hrs at less than 100 oC and 2 hrs at above 100 oC at 100% relative humidity.
Total test hours:~24 hrs.
Sample Preparation and Measurement
AcidificationMethod 1: 1.5M H2SO4, 30°C, 24 hrs,
• deionized H2O 30°C, 24hrs.
Method 2: 0.5M H2SO4, boil, 2 hrs.• boiled deionized H2O, 2hrs.
• All films were stored in deionized H2O at least 2 days before any test
MeasurementHumidity chamber (ESPEC-SH240)Stainless steel sealed-off cell
Proton Conductivity of BPSH as a Function of Temperature (70-140oC)
Temperature (oC)
60 80 100 120 140 160
Prot
on c
ondu
ctiv
ity (S
/cm
)
0.01
0.1
BPSH-35-Method 2BPSH-35-Method 1
II
I
III(b)
I : 1st heating; II: 1st cooling; III: 2nd heating
Temperature (oC)
60 80 100 120 140 160
Prot
on c
ondu
ctiv
ity (S
/cm
)
0.01
0.1
1: BPSH-40-Method 22: BPSH-40-Method 13: BPSH-30-Method 24: BPSH-30-Method 1
1 23
4
(a)
Proton Conductivity of NafionTM as a Function of Temperature (70-140oC)
Temperature (oC)
60 80 100 120 140 160
Prot
on C
ondu
ctiv
ity (S
/cm
)
0.01
0.1
1: Nafion 1135-Method 22: Nafion 1135-Method 1
21
Temperature (oC)
60 80 100 120 140 160
Prot
on C
ondu
ctiv
ity (S
/cm
)
0.01
0.1
1: Nafion 1135-Method 22: Nafion 1135-Method 13: Nafion 1135-continuous heating rate (1oC/min)
21
3
9.7Ref. 4
13.4Ref. 5
9.4Ref. 3
7.8Ref. 2
7.0Ref. 1
13.13
5.4-5.81 and 2
Ea (kJ/mol)Nafion
Ref. 1 Alberti, J. Memb. Sci. 73 (2001)Ref. 2 Kopitzke, J. Electrochem. Soc. 1677 (2000)Ref. 3 Yeo, J. Electrochem. Soc. 533 (1983)Ref. 4 Lufrano, Solid State Ionics 47 (2001)Ref. 5 Halim, Electrochim. Acta. 1303 (1994)
BPSH-40 TM-AFM Phase ImagesSample treatmentSample treatment: drying at 100: drying at 100ooC for 12 hours then samples were C for 12 hours then samples were allowed equilibrate by exposure to 50% relative humidity at 30allowed equilibrate by exposure to 50% relative humidity at 30ooC for 2 C for 2 hours, then imaged immediately in relative humidity of about 40%hours, then imaged immediately in relative humidity of about 40%. Scan . Scan size: 500nm; Zsize: 500nm; Z--range: 10range: 10oo
after high temperature after high temperature exposure (140exposure (140ooC max.)
after after Method 2Method 2after after Method 1Method 1C max.)
Nafion 1135 TM-AFM Phase Images
Sample treatmentSample treatment: drying at 100: drying at 100ooC for 12 hours then samples were C for 12 hours then samples were allowed equilibrate by exposure to 50% relative humidity at 30allowed equilibrate by exposure to 50% relative humidity at 30ooC for 2 C for 2 hours, then imaged immediately in relative humidity of about 40%hours, then imaged immediately in relative humidity of about 40%. Scan . Scan size: 500nm; Zsize: 500nm; Z--range: 10range: 10oo
after high temperature after high temperature exposure (140exposure (140ooC max.)
after after Method 2Method 2after after Method 1Method 1 C max.)
BPSH-35 TM-AFM Phase Images
Sample treatmentSample treatment: drying at 100: drying at 100ooC for 12 hours then samples were C for 12 hours then samples were allowed equilibrate by exposure to 50% relative humidity at 30allowed equilibrate by exposure to 50% relative humidity at 30ooC for 2 C for 2 hours, then imaged immediately in relative humidity of about 40%hours, then imaged immediately in relative humidity of about 40%. Scan . Scan size: 500nm; Zsize: 500nm; Z--range: 10range: 10oo
after high temperature after high temperature exposure (140exposure (140ooC max.)
after after Method 2Method 2after after Method 1Method 1C max.)
Influence of Aging Temperature on Electrochemical Stability
Aging procedureAging procedure: Samples treated by : Samples treated by Method 2Method 2 were placed in fully humidified were placed in fully humidified condition at a given aging temperature. After a certain time beicondition at a given aging temperature. After a certain time being, samples were ng, samples were taken and treated by taken and treated by Method 1Method 1 in order to remove any contaminants during in order to remove any contaminants during aging procedure. Then measured the proton conductivity in liquiaging procedure. Then measured the proton conductivity in liquid Hd H22O at 30O at 30ooC. C.
80oC
Time (hour)
0 20 40 60 80 200250
Prot
on C
ondu
ctiv
ity (S
/cm
)
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Nafion 1135-Method 2 BPSH40-Method 2 BPSH-35-Method 2
Prot
on C
ondu
ctiv
ity (S
/cm
)
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
120oC
Time (hour)
0 20 40 60 80 100Pr
oton
Con
duct
ivity
(S/c
m)
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Nafion 1135-Method 2 BPSH40-Method 2 BPSH-35-Method 2
00
100oC
Time (hour)
0 20 40 60 80 1
Nafion 1135-Method 2 BPSH40-Method 2 BPSH-35-Method 2
Water Absorption Change in Terms of Treatment Conditions
N(H2O)/N(SO3H)
0 20 40 60 80 100
Con
duct
ivity
(S/c
m)
0.00
0.02
0.04
0.06
0.08
0.10
0.12
Proton conductivity versus water Proton conductivity versus water content for content for NafionNafionTMTM 117 Membranes at 117 Membranes at 3030ooC, indicating data for membranes C, indicating data for membranes prepre--swollen in glycerol.swollen in glycerol.
Ref. Ref. T. ZawodzinskiT. Zawodzinski, Advances in , Advances in Electrochemical Science and Electrochemical Science and Engineering, Engineering, WileyWiley--VCH, p264VCH, p264
NANA>1000c148BPSH-60
19
39
32
24
Method 1
29
74
51
43
Test Aa
36c
170c
73
NA
Test Bb
19Nafion 1135
58BPSH-40
38BPSH-35
31BPSH-30
Method 2
treatment
Water Absorption (%) Water Absorption (%)
a Test A: high temperature conductivity (70-140oC, 24 hr)
b Test B: Aging (120oC, 60hr)c Mechanically unstable at wet condition
HPA Composite Membranes
1 1 µµmm 1 1 µµmm
Phase Image of Phase Image of PhosphotunsticPhosphotunstic acid acid incorporated system
Phase Image of Zirconium Phase Image of Zirconium hydrogen phosphate/ hydrogen phosphate/ BPSHBPSH--40 composite incorporated system 40 composite
Heteropolyacid (H3PW12O40)
P
OW
Conductive material (0.17 S/cm)
Crystalline form (H3PW12O40 29H2O)
Soluble in water or polar solvent
Stable at temperatures higher than 100 oC
Structure of Keggin anion (PW12O403-)
B.B. Bardin, S.V. et. al. J. Phys. Chem. B, 102, (1998) 10817.
Cross-Sectional Morphology ofNanocomposite Membranes by Scanning Electron Microscope
1 µm 200 nm 200 nm 200 nm
HPA/PBPSH-0 HPA/PBPSH-20 HPA/PBPSH-40 HPA/PBPSH-60
* HPA concentration= 30 wt.%
Stress-Strain Properties and Modulus by In-Situ DMA
Strain (%)
0 10 20 30 40 50 60 70 80 180 200
Stre
ss (1
07 N/m
2 )
0
2
4
6
8
10
1
2
34
5
0
400
800
1200
1600
2000
N1135 BPS-40 BPS-4030% HPA
Stor
age
Mod
ulus
/ M
Pa
DryWet
1: PBPSH-402: HPA 30 %3: HPA 45 % 4: HPA 60 %5: Nafion 117
stress-strain dynamic modulus
dry: 25C ambient humiditywet: 30C liquid water
In Situ Synthesis of Composite Membranes Containing Zirconium Hydrophosphate
Heteropolyacid (HPA)
ZrOCl2 + 2H3PO4 Zr(HPO4)2 • H2O + 2HClMediumAqueous
80°C, 6hr
• Process conducted on water swollen acid form membranes (e.g., BPSH-40) first swollen for 1 hour in boiling water
• Composite membranes washed with deionized water for 4 hours to remove excess H3PO4 and HCl
• Wt% HPA confirmed by TGA
Proton Conductivity of Phospho TungsticAcid/BPSH-30 at Elevated Temperature
Temperature (oC)
60 80 100 120 140 160
Prot
on c
ondu
ctiv
ity (S
/cm
)
0.01
0.1
1: BPSH-30-Method 22: BPSH-30-Method 13: HPA/BPSH-30 (3:7)
1
2
3(a)
Time (hour)
0 10 20 30 40 50 60Pr
oton
con
duct
ivity
(S/c
m)
0.01
0.1
Conductivity of HPA/BPSH-30 at 120oC
(b)
Proton conductivity as a function of aging at 120oC
• Proton conductivity as a function of temperature
Stress-Strain Curves of ZrP/BPSH-40 Composite Membranes
Strain (%)
0 50 100 150 200
Stre
ss (k
gf/c
m2 )
0
200
400
600
800
1000
1: BPSH-40 (2.2%)2: ZrP 17.2% (2.3%)3: ZrP 35.5% (2.0%)4: Nafion 117 (1.4%)
1
2
3
4
Strain (%)
0 50 100 150 200
Stre
ss (k
gf/c
m2 )
0
200
400
600
800
1000
1: BPSH-40 (21.8%)2: Zrp 17.2% (16.7%)3: ZrP 35.5% (16.1%)4: Nafion (15.9%)
123
4
Dry Conditions Wet Conditions Parenthesis: water content in weight percentTest conditions: 25oC, 40% RH, Crosshead speed: 5mm/min, Number of specimen: 5
Influence of ZrP on Water Uptake and Proton Conductivity
ZrP content (wt%)
0 20 40 60 80 100
Wat
er u
ptak
e (%
)
0
10
20
30
40
50
60 ZrP/BPSH-40ZrP/BPSH-35ZrP/BPSH-30
N(H2O)/N(ZrP) ~ 3
ZrP content (wt%)
0 10 20 30 40Pr
oton
con
duct
ivity
(S/c
m)
0.00
0.02
0.04
0.06
0.08
0.10
0.12
ZrP/BPSH-40ZrP/BPSH-35ZrP/BPSH-30
Water uptake at 30oC Proton conductivity at 30oC in liquid water
Effect of Zirconium Hydrogen Phosphate Incorporation on Proton Conductivity at Elevated
Temperature
Temperature (oC)
60 80 100 120 140 160
Prot
on C
ondu
ctiv
ity (S
/cm
)
0.01
0.1
1: BPSH-35-Method 22: ZrP/BPSH-35 (ZrP wt%=30.9)
1
2
Temperature (oC)
60 80 100 120 140 160
Prot
on C
ondu
ctiv
ity (S
/cm
)0.01
0.1
1: BPSH-40-Method 22: ZrP/BPSH-40 (ZrP wt%=14.6)3: ZrP/BPSH-40 (ZrP wt%=35.7)
1 2
3
Proton conductivity of BPSH-35 and ZrP/BPSH-35 as a function of temperature
Proton conductivity of BPSH-40 and ZrP/BPSH-40 as a function of temperature
Trade-off for HPA Incorporation for Elevated Temperature Fuel Cell Operation
⇑⇑Thermal Resistance
⇑⇑Dimensional Stability
GoodPoor for high degree of disulfonation
HPA RetentionStability
⇓⇑ (strength) ⇓ (elongation)
Mechanical Properties
⇑⇑Morphological Stability
⇓⇑Conductivity
Zirconium hydrogen phosphate HPA
Phosphotungstic acid HPA
Ref. Y.S. Kim et.al., J.Memb.Sci., submitted (2002)Y.S.Kim et.al., 8th International symposium for polymer electrolyte (2002)
Summary• Proton conductivity for BPSH was considerably dependent
on the acidification temperature while that of Nafion 1135 remained constant.
• BPSH-40 and Nafion-1135 showed maximum conductivity at the temperature around 120oC. TM-AFM results indicated that the conductivity decrease at high temperature was due to the excessive water absorption and and subsequent morphological instability.
• Aging test in fully humidified condition above 120oC showed that the conductivity of Nafion 1135 decreased significantly after 60 hours, while BPSH-35 showed a slight increase in proton conductivity presumably due to the positive morphological change.
Summary• Phosphoric tungstic HPA incorporated
sulfonated BPSPPO composites had a strong hydrogen bonding interaction between tungstic oxide and sulfonic acid resulting in not only enhanced dimensional stability but also improved proton conductivity at the temperature range of 70-140oC.
Zirconium hydro phosphate HPA/BPSH composites showed enhanced dimensional stability with acceptable proton conductivity above 100oC, compared to pure BPSH copolymer.
McGrath Research Group – June 2001
From L to R: Dr. Y.S. Kim, Rachael Hopp (SURP,* UW-Stevens Point), Brian Einsla, Kerry O’Connor, Melinda Hill (SURP, Bloomsburg Univ.), Jennifer Quinn, Jennifer Leeson (SURP, West Virginia Univ.),Jason Rolland, Natalie Arnett (SURP, Grambling St. Univ.), W. David Polk, Prof. James E. McGrath,Curtis Cleveland (SURP, Hampton Univ.), Dr. V. Bhanu, Kent Wiles, Dr. Charles Tchatchoua, William Harrison, Prof. Allan Shultz, Michael Hickner, Xinyu Li, Dr. Feng Wang*SURP: Summer Undergraduate Research Program
