Solid Alkaline Fuel Cells (SAFCs) using Anion Exchange Membranes (AEMs) attracts increasing attention for next-generation fuel cell technology. SAFCs have many advantages over proton exchange membrane fuel cell such as variable choice of metal electrode catalyst and fuels. However, the commercialization of SAFCs is limited due to the lack of high-performance AEMs. Commonly, AEMs have low ionic conductivity and poor durability in alkaline medium. In this study, we developed a novel AEM comprises of wholly aromatic, anthracene-based, backbone as a chemically durable membrane. Commonly, such kind of polymers exhibits very low solubility in common organic solvent due toπ-πstacking. Thus the processability into a membrane is very difficult.

Our unique molecular design is employing bulky Thermal Leaving Group (TLG) on the precursor polymer allowing this polymer to be cast as a film due to high solubility (>300 mg/ml). Then, TLG on the membrane film was removed by performing retro Diels-Alder reaction^1,2 in the solid state to obtain a wholly aromatic membrane (figure 1 (a)).

The precursor polymer was obtained by Suzuki-Miyaura coupling of the monomers. The methyl group of the resulting polymer was converted to benzyl bromide and finally to quaternary trimethylammonium bromide to afford the target precursor polymer (TPP). The polymer structure and molecular weight were analyzed by ¹H-NMR and GPC. Ion exchange capacity (IEC) was determined by titration. The in-plane ionic conductivity of the membrane is evaluated by AC impedance method. Stability test in hot alkaline solution and Fenton solution was performed to check the chemical durability of the membrane.

Molecular weight and IEC of synthesized TPP are 13,000 g/mol and IEC 2.49, respectively. Target polymer (TP) membrane was then obtained by just heating TPP membrane and removing TLG. The best heating temperature was determined to be 180 ^oC by TGA-MS analysis. IEC of the membrane has increased to 3.16 after thermal conversion. Despite higher IEC value, water uptake of TP membrane was lower than TPP membrane probably due to the strong intermolecular interaction of TP membrane. Therefore, TP membrane keeps higher IEC even after water uptake and this may result in high anion conductivity.

Figure 1 (b) shows the ionic conductivity of TP membrane before and after the durability test. The membrane kept initial HCO₃^- conductivity even after exposed to a very harsh alkaline solution (8M NaOH for 21 days at 80 ^oC) and Fenton solution (H₂O₂ 3 wt%, FeSO₄ 3 ppm at 60 ^oC for 8 hours). This is a solid proof that the TP membrane has both high durability and high conductivity, meeting for SAFC application.

These findings suggest that the use of TLG on polymer backbone allow development of new design concepts for highly-durable and highly conductive AEMs.

References:

1. Uemura, T., Mamada, M., Kumaki, D. & Tokito, S. Synthesis of Semiconducting Polymers through Soluble Precursor Polymers with Thermally Removable Groups and Their Application to Organic Transistors. ACS Macro Lett. 2, 830–833 (2013).
2. Ando, S., Facchetti, A. & Marks, T. J. Synthesis and characterization of solution-processable core-cyanated perylene-3,4;9,10-bis(dicarboximide) derivatives. Lett. 12, 4852–5 (2010).

Figure 1. (a) A new concept of preparing highly durable and highly conductive AEMs (b) HCO3 conductivity of TP membrane