Anionic multiblock copolymer membrane based on vinyl addition polymerization of norbornenes: Applications in anion-exchange membrane fuel cells

Stable, non-hydrolysable, hydroxide-conducing polymers are of interest for electrochemical devices such as fuel cells, electrolyzers and flow-batteries. In this study, a series of tetrablock copolymers containing an all-hydrocarbon backbone were synthesized. The synthesis is based on vinyl addition polymerization of norbornene using (eta(3)-allyl)Pd((Pr3P)-Pr-i)Cl as the catalyst and lithium tetrakis(pentafluorophenyl)-borate center dot(2.5Et(2)O) (Li[FABA]) as the activator. The tetrablock polymers were cast into membranes with an ion-exchange capacity (IEC) between 1.55 and 2.60 milliequivalents per gram (meq/g). The number of bound and unbound water molecules per ion pair was measured and correlated with conductivity. The membrane with the highest IEC (2.60 meq/g) had 10.6 unbound (i.e. free) and 17.9 bound water molecules per ion pair within the polymer. The presence of excess unbound water has led to flooding of the ion conducive channels and low hydroxide ion conductivity. The optimal anion conductivity was found with about 6.7 unbound and 11.9 bound water molecules per ion pair. Excellent hydroxide conductivity (> 120 mS/cm at 80 degrees C) was obtained at an ion-exchange capacity of 1.88 meq/g. The results show that hydroxide mobility is aided by phase segregation within the copolymer. The ion conducing polymer was stable in 1 M NaOH solution at 80 degrees C, showing essentially no loss in ion conductivity in 1200 h. Thermogravimetric analysis showed that the membrane backbone was stable up to 400 degrees C, consistent with previous polynorbornene-based materials. The peak power density of a H-2/O-2, hydroxide conducing fuel cell containing one of these membranes was 542.57 mW/cm(2) at 0.43 V and current density of 1.26 A/cm(2) at 60 degrees C.