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: calink@fsec.ucf.edu

Kirk ScammonAdvanced Materials Processing

and Analysis Center

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

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

     

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

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

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

Base Polymer of interest: PEEK and Base Polymer of interest: PEEK and PEKKPEKK

C

O

O( )x ( )y[ ]

poly(aryletherketone)

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

Solid Acid Additives for Membrane Modification

10 Å

Keggin structure

Phosphotungstic acid (PTA)

12WO3*H3PO4*24H2O

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

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

Representative W4f spectraRepresentative W4f spectra

0

5000

10000

15000

20000

25000

30000

35000

40000

30313233343536373839404142434445

Binding Energy eV

Na2WO4-2H20

Recrystallized PTA

WO3 Fisher

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

Weight loss thermogram for PTA-24H2O

Weight loss thermogram for PTA-24H2O

Weight loss thermogram for PTA-6H2O

Weight loss thermogram for PTA-6H2O

Weight loss thermogram for PTA treated with Cs2CO3

Weight loss thermogram for PTA treated with Cs2CO3

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

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.

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