Proton Exchange Membrane Fuel Cells (PEM FCs)

• Polymer electrolyte membranes capable of functioning at temperatures up to or above 120ºC and low relative humidity

• (below 10%) are needed for fuel cell applications

• PEMs should exhibit high ionic conductivities (σ �0.1 S·cm-1)

Hydrogen, Fuel Cells & Infrastructure Technologies Program Multi-Year Research, Development and Demonstration Plan, US Department of Energy, 2007

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Current technology PEM FCs

Nafion®

• High ionic conductivityσ ~ 0.2 S·cm-1

But• Must be hydrated, which creates

water management problems at operating temperatures

Polybenzimidazole, PBI

• High ionic conductivity

But• Difficult to process; high

acid doping is necessary

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Poly(ethylenimine): Fuel cell material

PEI / Phosphoric acid systems can be used as proton conductors.1,2

1. R. Tanaka, H. Yamamoto, S. Kawamura, T. Iwase, Electrochimica Acta, 1995 40 (13-14) 2421-2424 2. M. F. Daniel, B. Desbat, F. Cruege, O. Trinquet and J. C. Lassegues. Solid State Ionics, 1988 28-30, 637-6413. F. Yepez Castillo and D. T. Glatzhofer. “Proton Conducting Polymer Electrolyte Membranes Based on Linear Poly(ethylenimine)”, 2005 Abstracts, 61th Southwest and 57th Southeast Regional Meeting of the ACS

We have shown3 that operating fuel cells can be made with cross-linked poly(ethylenimine) (xPEI·HCl) MEAs

25ºC0.7 P:N0.62 degree of cross-linking

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How do we make the membranes?• xPEI·HCl / H3PO4 films were made by cross-linking

them with malonaldehyde, generated in situ.

• Polymer solutions were cast onto silicon rubber substrates and allow to sit for three days. Then they were dried at 45ºC for three more days.

Cross-linked Poly(ethylenimine) hydrochloride systems (xPEI·HCl)

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D. T. Glatzhofer, M. J. Erickson, R. Frech, F. Yepez and J. E. Furneaux , Solid State Ionics 2005, 176, 2861-2865 .

xLPEI·HCl / H3PO4 system: Ionic ConductivityxPEI·HCl / H3PO4 :

Ionic conductivity

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1. Y.-L. Ma, J. S. Wainright, M. H. Litt, and R. F. Savinell. J. Electrochem. Soc. 2004, 151,1, A8-A162. S. York, R. Frech, A. Snow, D. Glatzhofer, Electrochim. Acta 2001, 46, 15333. D. T. Glatzhofer, M. J. Erickson, R. Frech, F. Yepez and J. E. Furneaux. Solid State Ionics, 2005, 176, 2861-28654. M. J. Erickson, Ph.D. Dissertation, The University of Oklahoma, 2004.

Sample T(ºC)Relative Humidity

(%)

σ (S·cm-1)

Refs.

Nafion® 103 100 5.4 x 10-4 3

H3PO4 (85 wt.%) 150 0 5.68 x 10-1 4

PBI/H3PO4 (6.3 P per r.u.)

150 5 4.7 x 10-3 1

PBI/H3PO4 (6.3 P per r.u)

150 30 5.9 x 10-2 1

xPEI·HCl/H3PO4 (4.2 P:N)

150 0 1.0 x 10-2 2

The target conductivity for polymer membranes for PEM-FC’s is 0.1 S·cm-1 at an operation temperature range of 100-150ºC and low relative humidity (below 10%).

xPEI·HCl / H3PO4 : Ionic conductivity

Varying the phosphoric acid concentration

• Similarly, by changing the acid content we obtain changes in the conductivity curves

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xPEI·HCl / H3PO4 : Ionic conductivity

Varying the degree of cross-linking

• Changing the degree of cross-linking changes the shape of the ionic conductivity curve.

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• By changing the degree of cross-linking phosphoric acid content we obtained a membrane with an ionic conductivity around or above 0.1 S·cm-1 over a wide range of relative humidities above 90°C

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xPEI·HCl / H3PO4 : Ionic conductivity

1 x 10-1

S·cm-1

Conclusions

• A better characterization of the conduction mechanism in these systems is needed in order to further improve the properties of the material

• Fuel cell experiments are underway to observe the performance of these systems at high temperatures

• LPEI-based membranes show promising features for fuel cell applications

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