Alkaline fuel cell membranes from xylylene block ionenes

A new class of anion-exchange membranes, based on xylylene ionene segmented block copolymers, has been fabricated and characterized for possible use in alkaline fuel cells. The ionene materials were composed of non-polar blocks which, in principle, conferred mechanical strength and water swelling control to the membrane, and polar blocks with quaternary dibenzyl dimethyl ammonium groups located on the polymer backbone which served as anion-exchange sites. The best non-polar block material, in terms of its thermal properties and solubility in normal membrane casting solvents, contained 4,4′-(9-fluorenylidene)-diphenol (FDP) and sulfonyldiphenol (SDP) moieties. Membrane conductivity and water uptake were found to be independent of the FDP/SDP ratio in the non-polar blocks, but correlated well with membrane ion-exchange capacity (IEC) up to an IEC of 2.34 mmol/g (where the room temperature OH− ion conductivity and water swelling were 0.05 S/cm and 180%, respectively). At a higher IEC, the ionic conductivity decreased due to excessive membrane swelling which diluted the polymerʹs quaternary ammonium charged groups. The mechanical properties of ionene segmented block copolymers films with FDP/SDP non-polar blocks improved with increasing non-polar block length, e.g., a stress at break of 16.8 MPa and 5.0% elongation at break for membranes with a non-polar block containing 12 repeat FDP/SDP units. To improve membrane properties, ionene segmented block copolymers were embedded into a porous polypropylene support with a porosity of 0.50. The resulting composite films were flexible, their conductivity was half that of the neat ionene, and there was no loss of conductivity after a 7 day immersion in 1 M NaOH at 60 °C.