Cross-Linked Phosphoric Acid Doped PBI Membranes and Their Performance on Operation As Automotive Range Extender

One of the promising application areas for high temperature proton exchange membrane technology is as a range extender in a battery electric vehicle. Unlike low temperature PEM fuel cells dependent on perfluorosulfonic acid type membranes, HT-PEM fuel cells need no humidification, the conductivity of the membrane having much lower dependence on degree of hydration, and their use therefore eliminates water management issues. The operation conditions of a fuel cell for range extender application differ from those when the PEMFC stack is used for the power train. Thus the stack is subjected less to transients and operates in semi-continuous mode, but with stop/start events corresponding to the normal start up and shut down of vehicle operation.

HT-PEM membranes are based upon polybenzimidazole (PBI) polymers that are doped with (generally) phosphoric acid. This acid "doping" may be achieved by immersion of a polymer membrane in acid, or by methods involving the preparation of the PBI polymer in polyphosphoric acid followed by membrane casting and a controlled hydrolysis step, or the dissolution of PBI in a polyphosphoric acid/phosphoric acid followed by membrane casting, in a thermoreversible sol-gel transition. The last two approaches lead to very high acid contents, easily exceeding 20 molecules H₃PO₄/PBI repeat unit, and the resulting membrane is soft with poor mechanical properties. Polymer cross-linking is a recognized means for improving membrane strength. To date, in the case of PBI, acid doping has been carried out on the cross-linked membrane, resulting in low phosphoric acid uptake. We have developed an approach that allows the covalent cross-linking of PBI by introduction of a molecular cross-linking species at the polymer dissolution stage, following by membrane casting and thermal curing to give the cross-linked membrane. Since this approach provides mechanical stabilization to an acid-swollen membrane containing a high content of acid, it is a means leading to highly proton conducting membranes of satisfactory mechanical properties. A range of synthesis parameters including the amount of cross-linking, temperature for thermal curing etc., have been examined and, most recently, we have provided experimental evidence for cross-linking with the help of X-ray photoelectron and nuclear magnetic resonance spectroscopies.

Membrane electrode assemblies (MEAs) have been developed using the novel cross-linked acid doped PBI membranes, and fuel cell test protocols relevant to range extender application, based upon load cycling at 160 °C, have been applied. The results demonstrate that the novel MEAs comprising cross-linked PBI have lower voltage decay on load cycling than is observed using commercial MEAs, reaching >1000 h operation without the voltage dipping below the end-of-life criterion.  

The research leading to these results has received funding from the European Community’s Seventh Framework Programme for the Fuel Cells and Hydrogen Joint Undertaking under grant agreement ARTEMIS no. 303482.