Polymer electrolyte fuel cells (PEFC) have been used as clean power generation systems for households and vehicles because of their high energy conversion efficiency and low emissions. Perfluorosulfonic acid (PFSA) polymers have been used as electrolyte membranes in PEFCs due to their high proton conductivity, excellent chemical stability and superior durability. However, the membranes require water which limits the operating temperature to be below 100 °C leading to a complex system of water management and low effective utilization of heat. So new kinds of electrolyte membranes which can be operated under non-humidified intermediate temperature (120- 200 °C) are required. To achieve this kind of fuel cell, we focused on a mixed electrolyte based on phosphoric acid (PA) and sulfonic acid group containing ionic liquids (ILs) in our previous studies¹. The PA_IL mixed electrolyte not only showed higher thermal stabilities, but also higher electrochemical activities and higher durability of Pt catalyst comparing with pure PA or pure IL. In this study, in order to realize a composite membrane for fuel cell, 3-dimensionally ordered microporous polyimide (3-DOM PI) which has the properties of high porosity, uniform pore structure and high thermal stability was developed as a backbone for composite electrolyte membrane.

Polyamic acid (20 wt.%, JFE chemical Co.) was mixed with silica particles (100 nm, 300 nm and 900 nm in diameters, respectively) to making a slurry. The slurry was applicate on a glass board. Then the slurry membrane was drying for polymerization at 320 °C. At last, the silica was removed by HF aqueous solution (10 wt.%) etching for 12 hours. The 3-DOM PI was washed by distilled water then drying at room temperature for 2 days. The composite membrane was composed by PA_IL mixed electrolyte and 3-DOM PI under vacuum condition for 10 min at 100 °C.

Figure 1 showed the image of 3-DOM PI. From the SEM analysis, it was confirmed that the different uniform pore sizes of 3-DOM PI were successfully fabricated. The composite membranes were evaluated by the thermogravimetric analysis, the ionic conductivities analysis and the fuel cell polarization test under the non-humidified intermediate temperature. The hydrogen crossover was also evaluated by changing the pore size and thickness of composite membranes. The optimized pore size (nanoscale) and thickness (microscale) of composite membranes were discussed to be 100 nm and 25±3 μm, respectively. The maximum current density and power density at 180 °C are 1.8 A cm ^-2 and 450 mW cm ^-2, respectively.

The durability of the composite membranes was observed for more than 280 h with a constant current. From the result in Fig. 2 a), the stability of MEA was not only confirmed by the stable OCV, maximum current density and the stability of cathode Pt catalyst from TEM analysis (Fig. 2 b) and c)). In this study, the PA_IL/3-DOM PI composite membrane for Non-humidified IT-FC was developed.

Acknowledgement

This research was partly supported by “Platform for Technology and Industry” of Tokyo Metropolitan Government.

References

1. Yu, S. Kikuchi, H.P. Peediyakkal, H. Munakata, K. Kanamura. ACS Appl. Mater. & Interfaces2019, 11, 14, 13761-13767