Polyphosphazene-Based H2/O2 Fuel Cells

S.N. Lvov, X.Y. Zhou, E. Chalkova, and J.A. Weston The Energy Institute & Department of Energy and Geo-Environmental Engineering

H.R. Allcock, M.A. Hofmann, C.M. Ambler, and A.E. Maher Department of Chemistry

The Pennsylvania State University University Park, PA 16802 Objective

• The ultimate goal of the project is to develop a high temperature fuel cell using a new class of proton exchange membranes based on polyphosphazenes.

• We are exploring polyphosphazenes because of the thermo-oxidative and reductive stability of the - backbone, and because of the ability of this system to permit large or subtle changes to be made in the side group structure in order to optimize membrane properties. Poly[(aryloxy)phosphazenes]

R

• Mechanically stable • Easily fabricated

O • Good film forming characteristics

NP • High chemical and thermo-oxidative

O n stability • Resistant to free radical skeletal cleavage

R Approach

• Polymer design, synthesis, molecular characterization, and membrane property optimization • Preparation of Membrane Electrode Assemblies (MEAs) using different techniques • Electrochemical characterization and evaluation of the membranes and MEAs over a wide range of temperature • Construction of a bench-scale prototype fuel cell and determination of MEA performance over a wide range of temperature Program Timeline

Synthetic Investigations

Membrane Development

Membrane Optimization

Development of Membrane Characterization Methods

Preliminary Membrane Characterization

MEA Development

Single Cell Testing

Short Stack Testing

1999 2000 2001 2002 2003 2004 2005 2006 Accomplishments - Synthesis

• A method for the synthesis of the sulfonimide

containing side group, HOC6H4SO2NNaSO2CF3, for incorporation into phosphazene has been developed. • This side group was used to prepare a sulfonimide- functionalized phosphazene polymer with an - exchange capacity of ~1.0 meq/g, conductivity of ~0.06 S cm-1, and swelling of ~40%. • The membrane properties may be improved through a number of optimization methods, including cross-linking and blending techniques. Accomplishments – Membrane Properties at Ambient Conditions

Polymer Cross- IEC Swelling Conductivity membrane/ linking (meq g-1) (%) (S cm-1) method Radiation (Mrad)

N/A 0.91 30 0.100 Nafion 117

Sulfonated 20 1.07 38 0.035 Phosphazene Polymer Phosphonated 40 1.35 14 0.053 Phosphazene Polymer Sulfonimide 40 0.99 42 0.058 Phosphazene Polymer Accomplishments – Conductivity of Membranes at High Temperatures

100 100

125°C 100°C 80°C 60°C 40°C -1 -1 22.5°C 10-1 10-1 cm cm / S / S

Nafion 117 (PSU)

suphonated POP 10-2 10-2 phosphonated POP Conductivity Conductivity

sulphonimide POP

10-3 10-3 0.0024 0.0026 0.0028 0.0030 0.0032 0.0034 0.0036 T-1 / K-1 Accomplishments – Fuel Cell Test

• A proton conductive sulfonimide polyphosphazene-

based MEA was developed and tested in a H2/O2 fuel cell. The catalyst loading was 0.33 mg cm-2 for both the anode and the cathode.

• At 22°C the limiting current density of the

sulfonimide polyphosphazene-based H2/O2 fuel cell was 1.12 A cm-2 and the peak power densities was 0.36 W cm-2. Accomplishments – Fuel Cell Test

1.0 0.5

0.9

0.8 0.4 -2

0.7

0.6 0.3 / W cm

0.5 voltage / V

l 0.4 0.2 density r e Cel 0.3

0.2 Cell voltage 0.1 Pow

0.1 Power density

0.0 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Current density / A cm-2

Polarization and power density curves for the sulfonimide polyphosphazene based H2/O2 fuel cell at 22°C. Conditions: H2 and O2 pressures are 2 and 3 bar, respectively; there was no humidification of either the anode and cathode. Conclusions

• Sulfonimide polyphosphazene proton conducting membranes have been synthesized and tested at ambient conditions • A Sulfonimide polyphosphazene-based MEA has been prepared and evaluated in a H2/O2 fuel cell • The limiting current density of the sulfonimide polyphosphazene-based H2/O2 fuel cell was 1.12 A cm-2 at 22oC and the peak power densities was 0.36 W cm-2 at 22oC. • The performance results are comparable to those obtained for a home made Nafion 117 MEA. Future Studies

• Improvement of the polyphosphazene membranes by side group modifications • Further improvement of the membrane characteristics through blend formation with a variety of polymers, including PVDF • Optimization of polyphosphazene MEA preparation techniques • Membrane characterization tests at elevated temperature • Fuel cell tests at elevated temperatures Acknowledgements

This work was funded by the United States Department of Energy under contracts:

• DE-PS02-98EE504493 • DE-FC36-01GO11085