Development of durable anion-exchange membranes using polyfluorene–based electrolytes and pore-filling method for direct formate fuel cells

Highly durable ether-linkage-free polyfluorene–based electrolytes with different ion exchange capacities (IECs) were synthesized, followed by the preparation and evaluation of cast and pore-filling (PF) membranes as anion exchange membranes (AEMs). The performance of these membranes was tested in direct formate fuel cells (DFFCs). The membrane properties, including formate permeability, were investigated to clarify the relationship between membrane performance and the performance of DFFCs. The formate permeability of the membrane was drastically affected by the existence of OH− anion, as OH− ions induce larger membrane swelling. In addition, the PF method was more effective in reducing the formate permeation than the cast membranes. The PF membrane using the polyelectrolyte with a low IEC value showed 20-fold lower formate permeability compared to that of the original cast membrane, leading to a higher improvement in open-circuit voltage (OCV), from 0.55 to 0.95 V in DFFCs. For the first time, a quantitative correlation between the formate permeability of AEMs and OCV values in DFFCs was established. From this correlation, formate permeation less than 0.03 mol L−1 h−1 is necessary to keep high OCV value. Using the developed membrane, we also demonstrated the high stability of DFFC at 80 °C for 5 days.