Advanced Fuel Cell Membranes Based on Heteropolyacids

John A. Turner and F. J. John PernHydrogen and Electricity, Systems and Infrastructure Group

National Renewable Energy LaboratoryGolden, CO 80401-3393

Andrew M. Herring* and Steven F. Dec*** Department of Chemical Engineering and

** Department of Chemistry and GeochemistryColorado School of MinesGolden, CO 80401-1887

May 18, 2006 Project ID #FC4

This presentation does not contain any proprietary or confidential information

NREL/PR-560-39972Presented at the 2006 DOE Hydrogen, Fuel Cells & Infrastructure Technologies Program Review in Washington, D.C., May 16-19, 2006

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Disclaimer and Government License

This work has been authored by Midwest Research Institute (MRI) under Contract No. DE-AC36-99GO10337 with the U.S. Department of Energy (the “DOE”). The United States Government (the “Government”) retains and the publisher, by accepting the work for publication, acknowledges that the Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for Government purposes.

Neither MRI, the DOE, the Government, nor any other agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe any privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not constitute or imply its endorsement, recommendation, or favoring by the Government or any agency thereof. The views and opinions of the authors and/or presenters expressed herein do not necessarily state or reflect those of MRI, the DOE, the Government, or any agency thereof.

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OverviewTimeline

• Project start date: FY 2005• Project end date: tbd• Percent complete: tbd

Budget• Total project funding

– DOE share: $300k• Funding received in FY05:

– $150K (0.3 FTE)• Funding for FY06:

– $150K (0.3 FTE)

Targets• Low humidity operation (25%

RH).• High conductivity ~0.1 S/cm• Cost $40/m2

Barriers• Barriers addressed

O. Stack materials and manufacturing costs

P. Durability

Partner/Subcontract• Colorado School of Mines

– Prof. Andrew M. Herring– Dr. Steven F. Dec

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Objectives• Develop the methodology for the fabrication of 3D cross-

linked, hydrocarbon-based membranes using immobilized heteropolyacids (HPAs) as the proton conducting moiety.– Conductivity ~0.1 S/cm at 120ºC and <1.5 kPa H2O

• Develop immobilization technology based on covalent attachment of HPAs to oxide nano-particles.

• Acquire an improved understanding of HPAs and their salts made by custom synthesis. – HPAs make up a class of inorganic proton conductors that exhibit

high proton conductivity at low humidity (below 25% RH) and at elevated temperatures (well above 100°C).

• Conduct relevant characterizations of the membranes to better understand their structural, chemical, and thermal properties/stability and proton conductivity.

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HPAs: High H+ Conductivity, High Thermal Stability; Vast Structural Diversity; Known Redox Catalysts

Keggin[SiW12O40]4-

H4SiW12O40-26H2O (W12-STA)

Lacunary(allows easy attachment

points)[SiW11O39]8-

H8SiW11O39-26H2O (W11-STA)

Dawson[P2W18O62]6-

H6P2W18O62-xH2O

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Strategies for Immobilizing HPAsA. Binding Approaches:

1. Covalent bonding to oxide nano-particles insitu, which can bond covalently to, or embed physically in, a polymeric matrix

2. Direct embedding in a polymeric matrix 3. Covalent bonding directly to a polymeric matrix (CSM/3M collaboration,

poster #FCP-6)

B. Modification of Lacunary HPAs:1. By bonding with functional silanes that can then be cross-linked or

polymerized

C. Fabrication Approaches:1. Sol gel method2. Immobilized via silylation

onto supporting particles3. Simple blending

D. Polymeric Matrix:1. Organic2. Inorganic 3. Organic-inorganic hybrid

Ref. 14: “Heteropoly and Isopoly Oxometalates,”by M. T. Pope, Springer-Verlag, New York, 1983,

Chap. 7, Fig. 7.8, p. 126.

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Key Concept and Components in Composite Membrane Fabrication

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Procedure for Fabricating 3D Cross-Linked HPA/SiO2/Functional Silane Sol Gel

Composite & PEM Membrane with PMG TEOS + Functional Silane

+ pH = 1.35 H2O + HPA

Sol Gel Composite

Sol Solution

Host Polymer Dissolved in THF or EtOH

diluted with THF or EtOH

Liquid Mixture

solution cast

As-cast Film

thermal or UV X-linking

Cured PEM

aged (gelled) solution

Bonded STA%, IEC, & Other Analyses/Tests

Curing Agent, X-linker

Silanes: Methacrylate-type: Z-6030Epoxy-type: A-186, A-187

HPAs: H4SiW12O40 (W12-STA)H8SiW11O39 (W11-STA)K8SiW10O36 (KW10-STA)

Polymer: PMG or BSPPO

(3D Network of SiO2/silane/HPA/PMG)

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Formation of SiO2 Nano-Particles in Composite Matrix upon Thermal

Treatments (TEM Analysis)

As grown with 10x diluted PEM-#9D solution

Annealed at 80°C for 20 min

Annealed at 140°C for 5 min

Annealed at 190°C for 2 min

50 nm 50 nm 20 nm

50 nm

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Flexibility of PEM Membranes Fabricated with High HPA Loading

PEM-#9B,C,D Films: W12-STA/(PMG + Cross-Linker) = 174 Wt%

11

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Abs

orba

nce

1000 2000 3000 4000 Wavenumbers (cm-1)

Immobilizing the HPABinding HPA with Z-6030 Silane in Sol Gel Composite

W12-STA Retained

W12-STA/SiO2/Z-6030

As-made

H2O-washed

Z-6030

12-HSiW

SiO2

(W12-STA washed off easily without Z-6030)

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Chemical Stability of Membrane and Composite PEM

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25Exposure Time (h)

Nor

mal

ized

Wei

ght %

Chemical Stability Test of PEM Films in Fenton Reagent

Control (no W12-STA)

PEM-#5

PEM-#6 in H2O

PEM-#6Fenton’s Reagent: 4 ppm Fe2+ + 3% H2O2

at 68oC

PEM-#5: W12-STA at 165 wt%, cured at 145oC/2-ton/5-min PEM-#6: W12-STA at 153 wt%, cured at 145oC/1-ton/5-min

Weight loss due to W12-STA extraction

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0.000 0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

0.020

0.022

0.024

0.026

0.028

0.030

0.032

Abs

orba

nce

1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)

Control (no W12-STA &SiO2)

Control (no W12-STA)

PEM-#7 (W12-STA at 174 wt%)

PEM Mechanical Strength and Flexibility Reduced by Increasing HPA Loading

FTIR-ATR Spectra of Cured Control Blanks and PEM-#7

(H2O)

Film FlexibilityReduced

Low STA wt%

High STA wt%

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H+ Conductivity as a Function of Cell Temperature at 100% RH

0

5

10

15

20

25

30

1 11 21 31 41 51 61 71 81 91 101CV Scan Sequence

H+

Con

duct

ivity

(mS/

cm)

Forward Scan (-0.50V - 0.0V)Return Scan (0.0V - -0.50V)

Proton Conductivity of PEM-#9D, Film #1, Strip#3

Scan Range: -0.50V - +0.50V

Scan Rate (mV/s):

10 50 100

10 50 100

10 50 100

10 50 100

10 50 100 10

27oC/100%RH

80oC/100%RH

70oC/100%RH

60oC/100%RH

40oC/100%RH

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Improving H+ Conductivity with Higher HPA Loading and Better Membrane FabricationTable 1. PEM Compositions vs Proton Condutivity Derived from I-V Curves of CV Scans

Weight Ratio

PEM ID HPA Host Polymer X-Linker HPA/(PMG +

X-Linker) 80oC/100%RH 100oC/46%RH 120oC/23%RH1

1 HSiW12Ox BSPPO No 0.56 0.15

2 HSiW12Ox PMG Yes 0.81 6.9

3 HSiW11Ox PMG Yes 1.09 6.4, 10.46 2.41 0.85

4 KSiW10Ox PMG Yes 1.05 7.56, 13.3 1.61 0.25

5 HSiW12Ox PMG Yes 1.50 8.8

6 HSiW12Ox PMG Yes 1.54 15.57

7 HSiW12Ox PMG Yes 1.74 14.55 2.1

8 HSiW12Ox PMG Yes 1.74 19.17 3.81

9B HSiW12Ox PMG Yes 1.74 22.289C HSiW12Ox PMG Yes 1.74 21.159D HSiW12Ox PMG Yes 1.74 25.45 [28.25 at 70oC/100%RH]

Nafion 112 SO3H 149.9 98.99 49.25

1 Values of the proton conductivity at 120oC/23%RH are with large uncertainty because of rapidly lost linearity on I-V curves

Components Best Proton Conductivity (mS/cm)

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High H+ Diffusion Coefficients for Composite Membrane

0.0E+00

5.0E-06

1.0E-05

1.5E-05

2.0E-05

2.5E-05

3.0E-05

20 40 60 80 100 120Temperature (oC)

H+ D

iffus

ion

Coe

ffici

ent (

cm2 /s

)

(8.8 mS/cm at 80oC/100%RH)

Nafion

PEM-#5

Proton Diffusion Data from PFG-NMR Measurements

(W12-STA at 150 wt%)

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-1.50

-1.25

-1.00

-0.75

-0.50

-0.25

0.00

25 50 75 100 125 150 175 200 225Temperature (oC)

Hea

t Flo

w (W

/g)

DSC Thermograms of W12-STA, SiO2, and Two Sol Gel Composites

W12-STA

SiO2 Sol Gel

TEOS-0303A (Z-6030)

TEOS-0304 (A-186)

60

65

70

75

80

85

90

95

100

25 75 125 175 225Temperature (oC)

Wei

ght (

%)

-0.35

-0.25

-0.15

-0.05

0.05

0.15

0.25

Hea

t Flo

w (W

/g)

TGA and DSC Thermograms for PEM-20050823 Membrane

TGA

DSC22oC

80oC

145oC

145oC

22oC80oC

TGA and DSC of as-cast, 80oC-pressed, and 145oC-cured PEM

W12-STA/SiO2/Z-6030W12-STA/SiO2/A-186

PEM-#6: W12-STA loading level at 153 wt%/ (PMG+ X-linker)

Moisture Retaining Capability of W12-STA and Sol Gel

Composites (DSC Analysis)

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Summary of AccomplishmentsPEM Fabrication and Performance

• We have shown the ability to retain HPAs into a polymer-composite matrix of our design.

• Properties of HPA-based composite PEMs:– high chemical stability (Fenton’s reagent test)– good thermal stability (with highly reactive W12-STA)– good mechanical flexibility– effective binding of silicotungstic acids (Wn-STA) with select functional

silanes (n = 10, 11, 12)– high Wn-STA loading [HPA/(PMG + X-Linker) > 150 wt%] – moderate proton conductivity (25 mS/cm at 80oC/100%RH)

• Clear progress towards meeting the DOE targets

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Achieving Fundamental GoalsFuture Work

• To continue to improve/modify/optimize the current PEM composite formulation, fabrication, and processing conditions– to enhance PEM’s thermal stability in the 90-120oC range– to improve mechanical strength and flexibility– to reduce membrane thickness and improve film uniformity

• To continue to develop immobilization strategies for various HPAs, custom-synthesized at CSM, that show high proton diffusion coefficients and thermal stability.

• To understand the binding mechanism of HPA with functional silanes and SiO2 nano-particles in the polymer matrix.

• To understand the proton conduction mechanism in the 3D cross-linked composite membranes in order to further improve proton conductivity at low humidity and elevated temperatures.

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2005 Reviewers’ Comments“One of the few new, alternative ideas for membranes in the whole DOE

program”• Issues:

– …needs to present conductivity values for membranes with “fixed” HPAs…• Done

– HPA approach is sound as a demonstration but water solubility must be addressed…

• Excellent progress has been made in this regard– Nafion doped in HPAs has been shown to be feasible…the PI is in need of new

insight.• Not part of our project, those figures were for introduction to HPAs only• Our project is focused on developing a composite hydrocarbon membrane using

HPAs as the proton conducting moiety that will meet the DOE targets for operation at low RH and higher temperatures

• Future:– Need durability studies in actual operating fuel cell conditions and …thermal

and RH cycling…gas crossover measurements• PEMs of 3D cross-linked PMG matrix were not available yet at the time• These subjects will be investigated for HPA-based PEMs this summer

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Presentations and Publications1. A. M. Herring, J. A. Turner, S. F. Dec, M. A. Sweikart, J. L. Malers, F. Meng, F. J. Pern, J. Horan, and D. Vernon; "The

Use of Heteropoly Acids in Composite Membranes for Elevated Temperature PEM Fuel Cell Operation; Lessons Learnt from Three Different Approaches," 2004 Joint International Meeting, W1 - Fourth International Symposium on Proton Conducting Membrane Fuel Cells, 206th ECS Meeting, October 3-8, 2004, Honolulu, HI.

2. F. J. Pern, J. A. Turner, and A. M. Herring; "Hybrid Proton-Carrier Polymer Composites for High-Temperature FCPEM Applications," in Nanostructured Materials in Alternative Energy Devices, edited by Erik M. Kelder, Edson Roberto Leite, Jean-Marie Tarascon, and Yet-Ming Chiang (Mater. Res. Soc. Symp. Proc. 822, Warrendale, PA , 2004), pp. S.8.6.1 –S.8.6.6.

3. A. M. Herring, J. A. Turner, S. F. Dec, M. A. Sweikart, J. Malers, F. Meng, J. Pern, J. Horan, and D. Vernon; “The Use Of Heteropoly Acids In Composite Membranes for Elevated Temperature PEM Fuel Cell Operation; Lessons Learnt from Three Different Approaches.” Fuel Cell Seminar, 2004.

4. F. J. Pern, J. A. Turner, Fanqin Meng, and A. M. Herring; “Sol-Gel SiO2-Polymer Hybrid Heteropoly Acid-Based Proton Exchange Membranes,” MRS 2005 Fall Meeting, Energy and The Environment Symposium, Session A: The Hydrogen Cycle—Generation, Storage, and Fuel Cells. In press.

5. F. J. Pern, J. A. Turner, and A. M. Herring; “Hybrid Proton Exchange Membranes Based on Heteropoly-Acid and Sulfonic-Acid Proton Conductors,” ECS 2006, Abstract (accepted for oral presentation)

6. J. L. Horan , J. Turner , A. M. Herring, and S. Dec; “Structure and Dynamics of Non-Commercial Heteropoly Acids for Fuel Cell Applications,” ECS 2006, Abstract

7. N. V. Aieta, M. Kuo, F. Meng, J. Turner, and A. M. Herring; “The Use of Heteropolyacids as Additives for Low Humidity Operation of Nafion Membranes for PEM Fuel Cell Applications,” ECS 2006, Abstract

8. A. M. Herring, R. J. Stanis, J. Ferrell III, M. Kuo, J. Turner, and M. Samaroo; “The Use of Heteropoly Acids as Electrocatalysts for the Oxygen Reduction Reaction in PEM Fuel Cells,” ECS 2006, Abstract

9. R. J. Stanis, A. M. Herring, M. Kuo, and J. Turner; “Increased CO Tolerance of Pt Electrodes by Addition of Adsorbed Heteropoly Acids and Salts in PEM Fuel Cell Anode Catalysts,” ECS 2006, Abstract