Two new families of proton-conducting polymer membranes for fuel cells exhibit high conductivity and remain stable over a much wider temperature range than conventional membrane materials such as Nafion.
Many electrochemical devices, including fuel cells, require polymer materials that exhibit high ionic conductivity as well chemical and thermal stability. For fuel cell applications, it is especially important that membranes made from these polymers have the ability to absorb water and maintain good mechanical strength over a wide range of operating temperatures.
Electrically conducting polymers (or ionomers) are among the materials best suited for these applications. Ionomer-based materials of most interest for electrochemical applications such as fuel cells are those that selectively pass hydrogen ions (protons).
The most common ion-selective membrane material in current use in fuel cells is a perfluorosulfonate ionomer made by DuPont under the brand name Nafion. Nafion has adequate thermal stability up to approximately 90°C, and the ability to absorb adequate amounts of water. However, the high cost and limited operational temperature range of perfluorosulfonate ionomers in general are significant disadvantages, especially at higher fuel cell operating temperatures
Enerize material sciences has developed and tested two new proprietary families of proton-conducting polymer membranes with performance that exceeds that of Nafion in every respect.
The first set of proton-conducting polymer membrane materials are produced through condensation of polyamides with sulfonated aromatic derivatives of aldehydes in the presence of solvent and acid catalyst (patent pending).
A second family of high-temperature ionomeric materials is comprised of ammonium interpolymer complexes (patent pending). These are produced by the macromolecular reaction of polyamines and polymers with active chloromethyl groups in organic solvent medium.
In both cases, the resulting products have low equivalent weight and high ionic conductivity at room temperature. In addition they exhibit excellent proton function values and insignificant swelling from soaking treatment with water and acid solutions. The products exhibit high mechanical strength and are thermally stable to more than 150°C, well in excess of that for poly-fluorinated compounds presently used in electrochemical membranes and sensors. Thermogravimetric analysis data indicating temperature stability to 250 degrees C is shown above and to the left.
The morphology of polymer electrolytes and fuel cell polymer membrane is dependant upon production methods. Enerize uses advanced analytical tools such as atomic force microscopy (AFM) to monitor the structure and appearance of membranes materials as a function of composition and production method.
AFM images of polymer membranes developed by Enerize team are shown to the left. The images show samples of the same composition with differences in appearance reflecting differences in the way they were produced. Uniformity in morphology is a desirable characteristic because the more uniform morphology is associated with higher conductivity. Thus the method used to produce the material in the top image is preferable to that used for the material in the bottom image.
For fuel cell applications, these new polymers have a number of important advantages, including:
- Operation at a temperatures to 150°C,
- Low resistance (high proton conductivity) under cell operating conditions,
- Long-term chemical and mechanical stability at elevated temperatures in oxidizing and reducing environments,
- Good mechanical strength, preferably with resistance to swelling,
- Low gas cross-over (pinhole free),
- Interfacial compatibility with catalyst layers,
- Low cost,
- Minimal or zero dependence on tightly controlled humidity.
Several members of both of these conducting polymer families developed by Enerize meet the requirements for membrane materials with enhanced physical and electrochemical characteristics for use in fuel cell and bio-sensor applications. Enerize ionomeric materials exhibit superior thermal stability and electrochemical characteristics and can be produced at a significantly lower cost than can the conventional poly-fluorinated materials.