Anion conducting multiblock copolymer membranes with partial fluorination and long head-group tethers
Anion conductive polymers are of value in anion exchange membrane (AEM) fuel cells. A series anion conductive polymers composed of partially fluorinated multiblock copoly(arylene ether)s (mPEs) with long head-group tethers were synthesized via polycondensation and Friedel–Crafts reaction. The relationship between chemical structure, membrane morphology and physical properties was explored by varying the length of the hydrophilic and hydrophobic blocks and the number of tethers per hydrophilic repeat unit in the synthesized multiblock copolymers. Efficient, ion-conductive nano-channels were formed by using nanophase-separation of the multiblock copolymer to improve channel formation and ionic conductivity without inducing excess water uptake (WU). The hydrophobicity of the partially fluorinated backbone further reduced the WU. Doubling the number of head-groups resulted in more than doubling the hydroxide ion conductivity. From the study of the number of freezable and bound water molecules per head-group, it was found that bound water played a dominant role in ion transport, while excess unbound water led to higher unproductive WU. Multiblock copolymer AEMs with high hydroxide conductivity, up to 119 mS cm−1 were obtained. Polymers with an attractive OH− conductivity of 94 mS cm−1 at 80 °C, and low WU of 26.7% with a modest ion exchange capacity were also obtained. The membranes showed excellent alkaline stability due to the use of the long head-group tether structure and partial fluorination. Less than 1.5% conductivity loss was observed after soaking the membrane in 1 M NaOH solution at 60 °C for over 1000 h. Membranes with higher ionic conductivity showed lower oxygen diffusivity and permeability.