Characterisation of Ionic Liquid doped Polybenzimidazole Membranes for Application in High-Temperature Proton Exchange Membrane Fuel Cells

Abstract: In this study four membranes were tested and evaluated against each other for use in high-temperature proton exchange membrane fuel cell (HT-PEMFC) applications. A phosphoric acid (PA) doped polybenzimidazole (PBI) membrane, was used as a reference as the current state-of-the-art. The charge carrier in this membrane is the PA, and there are advantages in switching from PA to ionic liquid (IL) as the charge carriers in HT-PEMFC, such as to reduce leaching and lost performance over time. Two of the membranes investigated in this study were PBI mixed with the commercial ILs [dema][TfO] and [HEIm][TFSI]. The last membrane was a mixture of PBI and a newly synthesized IL by Chalmers University of Technology, [FA][TFSI]. Conductivity measurements, polarisation curves and electrochemical impedance spectroscopy (EIS) were done to characterise and evaluate the membranes. The study shows that these IL-PBI membranes have a long way to go to be a commercially competitive technique compared to PA-PBI membranes. In the report it can be seen that all the IL-PBI membranes show very poor performance without the addition of PA. Even with the addition of PA, the performance of the IL-PBI membranes cannot compare to PA-PBI membranes, being a factor 10 lower in the conductivity test and polarisation curves. In the EIS the membrane that got a conductivity closest to PA-PBI was [FA][TFSI]-PBI that had a conductivity that was 70% of the conductivity of PA-PBI. The lower performance for the membranes in the polarisation measurements compared to the EIS can be due to several factors such as changed membrane resistance with applied load or the influence of other cell resistances. Tests where the membranes were doped with the corresponding IL in the electrodes instead of PA also showed poor performance. Future studies in the field of IL-PBI membranes are needed for the technology to become viable as an alternative to PA-PBI.