X-Ray Photoelectron Investigation of
Phosphotungstic Acid as a Proton-Conducting
Medium in Solid Polymer Electrolytes
Clovis A. Linkous
Stephen L. Rhoden
Florida Solar Energy Center
Kirk Scammon
Advanced Materials Processing
and Analysis Center
University of Central Florida
Cocoa, Florida, USA
E-mail: calink@fsec.ucf.edu
Tungsten 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
Outline
• 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
Polymer Electrolyte Membrane Fuel Cell
Membrane Electrode Assembly (MEA)/
Catalyst Coated Membrane (CCM)
e-
e-
e-
e-
e-
e-
e-
H+
H+
ANODE
H2 → 2H+ + 2e-
CATHODE
O2 + 4H+ + 4e- → 2H2O
H+
H+
Flow Field
Catalyst Layer
Polymer
Electrolyte
Membrane
Gas Diffusion
Layer
Typical Current-Voltage curve
for a PEM fuel cell
Cell voltage (V)
1.0
V = E0 −
0.9
0.8
RT
RT ⎛
i
ln i − RΩ i +
ln⎜⎜1 −
F
F ⎝ il
Kinetic
control
Mass
transport
control
Ohmic
control
0.7
⎞
⎟⎟
⎠
0.6
0.5
0.0
0.2
0.4
0.6
0.8
Current density (A/cm2)
1.0
1.2
Membrane Electrode Assemblythe heart of a PEM fuel cell
Polymer membranes
with catalyst ink
sprayed catalyst ink layer
Catalyst
+ 2 GDLs
Membrane
Spray
Membrane
Catalyzed
Membrane
Membrane
Membrane
Membrane
Electrode
Assembly
MEA in
Cell Hardware
+ Flow Fields
& Hardware Set
Base Polymer of interest: PEEK
and PEKK
O
[( C
)x ( O
poly(aryletherketone)
)y ]
Previous work on Nafion® 112 and
PTA composite
Comparing Four Electrode Conductivity of NTPA to Nafion®
120 oC, 500 sccm H2, 230 kPa
1000
Conductivity (mS/cm)
Baseline NTPA Wet up at 70% RH 3-16-04
Nafion® 112 Wet up at 70% RH 1-8-04
100
10
1
0
10
20
30
40
50
60
70
Relative Humidity (%RH)
80
90
100
110
Solid Acid Additives for Membrane Modification
Keggin structure
Phosphotungstic acid (PTA)
12WO3*H3PO4*24H2O
10 Å
Conductivity vs RH for
SPEEK/PTA composite
membrane
Conductivity (S/cm)
1
0.1
0.01
0.001
Test I
Test II
0.0001
0.00001
0.000001
0
20
40
60
RH (%)
80
100
120
Effect of Cs+ treatment on
PTA/SPEEK composites
SPEEK-PTA Composites at 80C
1
SPEEK+Cs-PTA
Conductivity (S/cm)
0.1
SPEEK+PTA
SPEEK
0.01
0.001
0.0001
0.00001
0.000001
0
20
40
60
RH (%)
80
100
120
Representative W4f spectra
40000
35000
30000
Na2WO4-2H20
Recrystallized PTA
WO3 Fisher
25000
20000
15000
10000
5000
0
45
44
43
42
41
40
39
38
37
36
Binding Energy eV
35
34
33
32
31
30
Summary of W4f7/2 data
Sample
WO3
PTA + Cs2CO3
PTA/Cs+/H2SO4
SPEEK/PTA
PTA + CsCl
Na3PTA
PTA-6H2O
PTA-EtOH/DMF
PTA (anhydrous)
PTA – 24H2O (recrystallized)
Na2WO4-2H2O
Cs2WO4 (anhydrous)
PTA--24H2O (commercial A)
PTA--24H2O (commercial B)
Cs2WO4-2H2O
Binding Energy (eV)
35.1
35.4
35.6
35.7
35.8
35.9
36.0
36.2
36.2
37.8, 36.0
36.7
37.3, 35.3
37.5
37.6
37.7
Weight loss thermogram for
PTA-24H2O
Weight loss thermogram for
PTA-6H2O
Weight loss thermogram for
PTA treated with Cs2CO3
Summary of W4f7/2 data
Sample
WO3
PTA + Cs2CO3
PTA/Cs+/H2SO4
SPEEK/PTA
PTA + CsCl
Na3PTA
PTA-6H2O
PTA-EtOH/DMF
PTA (anhydrous)
PTA – 24H2O (recrystallized)
Na2WO4-2H2O
Cs2WO4 (anhydrous)
PTA-24H2O (commercial A)
PTA-24H2O (commercial B)
Cs2WO4-2H2O
Binding Energy (eV)
35.1
35.4
35.6
35.7
35.8
35.9
36.0
36.2
36.2
37.8, 36.0
36.7
37.3, 35.3
37.5
37.6
37.7
Conclusion
• 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.
Future 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