Page 1
X-Ray Photoelectron Investigation of Phosphotungstic Acid as a Proton-
Conducting Medium in Solid Polymer Electrolytes
Clovis A. LinkousStephen L. Rhoden
Florida Solar Energy Center University of Central Florida
Cocoa, Florida, USAE-mail: [email protected]
Kirk ScammonAdvanced Materials Processing
and Analysis Center
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Tungsten trioxide, WO3Tungsten trioxide, WO3
• Melting point: 1473 K
• Insoluble in mineral acids
• Soluble in alkali
WO3 + 2NaOH Na2WO4 + H2O
• Formation of phosphotungstic acid, PTA:
H3PO4 + 12WO3 H3PW12O40-xH2O
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OutlineOutline
• PEM fuel cell function
• Conductivity in polymer electrolytes
• Effect of PTA on sulfonic acid polymer membrane conductivity
• XPS observation of W chemical shifts
• Relating chemical shift data to hydration environment
• Thermogravimetry
• Conclusion
• Future work
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Polymer ElectrolyteMembrane
H+
H+
H+
H+
Membrane Electrode Assembly (MEA)/ Catalyst Coated Membrane (CCM)
e- e- e- e- e- e- e-
Gas Diffusion Layer
ANODE
H2 2H+ + 2e-
Catalyst Layer
Polymer Electrolyte Membrane Fuel Cell
CATHODE
O2 + 4H+ + 4e- 2H2O
Flow Field
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Typical Current-Voltage curve for a PEM fuel cell
Typical Current-Voltage curve for a PEM fuel cell
Current density (A/cm2)
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Cel
l vol
tage
(V
)
0.5
0.6
0.7
0.8
0.9
1.0
Kineticcontrol
Ohmiccontrol
Masstransportcontrol
li
i
F
RTiRi
F
RTEV 1lnln0
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Membrane Electrode Assembly-the heart of a PEM fuel cell
Membrane Electrode Assembly-the heart of a PEM fuel cell
sprayed catalyst ink layer
Polymer membranes with catalyst ink
Mem
bra
ne
Mem
bra
ne
Mem
bra
ne
CatalyzedMembrane
MembraneElectrode AssemblyMembrane
+ 2 GDLsSpray
Catalyst
MEA inCell Hardware
+ Flow Fields& Hardware Set
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Base Polymer of interest: PEEK and Base Polymer of interest: PEEK and PEKKPEKK
C
O
O( )x ( )y[ ]
poly(aryletherketone)
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Previous work on NafionPrevious work on Nafion 112 and PTA 112 and PTA compositecomposite
Comparing Four Electrode Conductivity of NTPA to Nafion®
120 oC, 500 sccm H2, 230 kPaComparing Four Electrode Conductivity of NTPA to Nafion®
120 oC, 500 sccm H2, 230 kPa
1
10
100
1000
0 10 20 30 40 50 60 70 80 90 100 110
Relative Humidity (%RH)
Co
nd
uc
tivi
ty (
mS
/cm
)
Baseline NTPA Wet up at 70% RH 3-16-04
Nafion® 112 Wet up at 70% RH 1-8-04
1
10
100
1000
0 10 20 30 40 50 60 70 80 90 100 110
Relative Humidity (%RH)
Co
nd
uc
tivi
ty (
mS
/cm
)
Baseline NTPA Wet up at 70% RH 3-16-04
Nafion® 112 Wet up at 70% RH 1-8-04
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Solid Acid Additives for Membrane Modification
10 Å
Keggin structure
Phosphotungstic acid (PTA)
12WO3*H3PO4*24H2O
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Conductivity vs RH for SPEEK/PTA composite membrane
Conductivity vs RH for SPEEK/PTA composite membrane
0.000001
0.00001
0.0001
0.001
0.01
0.1
1
0 20 40 60 80 100 120
RH (%)
Co
nd
uct
ivit
y (S
/cm
)
Test I
Test II
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Effect of Cs+ treatment on PTA/SPEEK composites
Effect of Cs+ treatment on PTA/SPEEK composites
SPEEK-PTA Composites at 80C
0.000001
0.00001
0.0001
0.001
0.01
0.1
1
0 20 40 60 80 100 120RH (%)
Cond
uct
ivity
(S/c
m)
SPEEK+Cs-PTA
SPEEK+PTA
SPEEK
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Representative W4f spectraRepresentative W4f spectra
0
5000
10000
15000
20000
25000
30000
35000
40000
30313233343536373839404142434445
Binding Energy eV
Na2WO4-2H20
Recrystallized PTA
WO3 Fisher
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Summary of W4f7/2 dataSummary of W4f7/2 dataSample Binding Energy (eV)
WO3 35.1PTA + Cs2CO3 35.4PTA/Cs+/H2SO4 35.6SPEEK/PTA 35.7PTA + CsCl 35.8Na3PTA 35.9PTA-6H2O 36.0PTA-EtOH/DMF 36.2PTA (anhydrous) 36.2PTA – 24H2O (recrystallized) 37.8, 36.0Na2WO4-2H2O 36.7Cs2WO4 (anhydrous) 37.3, 35.3PTA--24H2O (commercial A) 37.5PTA--24H2O (commercial B) 37.6 Cs2WO4-2H2O 37.7
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Weight loss thermogram for PTA-24H2O
Weight loss thermogram for PTA-24H2O
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Weight loss thermogram for PTA-6H2O
Weight loss thermogram for PTA-6H2O
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Weight loss thermogram for PTA treated with Cs2CO3
Weight loss thermogram for PTA treated with Cs2CO3
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Summary of W4f7/2 dataSummary of W4f7/2 dataSample Binding Energy (eV)
WO3 35.1PTA + Cs2CO3 35.4PTA/Cs+/H2SO4 35.6SPEEK/PTA 35.7PTA + CsCl 35.8Na3PTA 35.9PTA-6H2O 36.0PTA-EtOH/DMF 36.2PTA (anhydrous) 36.2PTA – 24H2O (recrystallized) 37.8, 36.0Na2WO4-2H2O 36.7Cs2WO4 (anhydrous) 37.3, 35.3PTA-24H2O (commercial A) 37.5PTA-24H2O (commercial B) 37.6 Cs2WO4-2H2O 37.7
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ConclusionConclusion
• Cs+ exchange improves conductivity of SPEEK/PTA composite membranes.
• W chemical shift related to O-bonding geometry (octahedral vs tetrahedral) and waters of hydration.
• Cs+ treatment lowers PTA water content and its enthalpy of hydration.
• Cs+ functions by destabilizing waters of hydration, rendering them more mobile and better able to conduct protons.
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Future WorkFuture Work
• XRD of PTA vs water content
• IR analysis of retained hydrogen stretching frequencies
• Obtaining conductivity vs RH curves at temperatures on either side of the TGA water transition.
• Fabricating Pt//SPEEK/PTA//Pt membrane electrode assemblies and deriving fuel cell current voltage curve