A Nonhumidified Fuel Cell Using Fluorohydrogenate Ionic Liquid - Polymer Composite Membrane: Advantages of the Use of Polyimide Membrane Supporting Material

Operation at intermediate temperature (> 100 °C) without humidification offers several benefits for PEFCs such as reduction of expensive noble catalyst and simplicity of the unit. In the previous study, we reported a non-humidified fuel cell using a composite membrane of C₂C₁pyrr(FH)_1.7F (C₂C₁pyrr = N-ethyl-N-methylpyrrolidinium) ionic liquid and HEMA (2-hydroxyethylmethacrylate) polymer with polytetrafluoroethylene (PTFE) as a membrane supporting material [1]. However, the single cell performance largely decreased when the operation temperature was elevated from 80 °C to 120 °C. The reason for this deterioration was revealed by the cross-sectional SEM images of the membrane electrode assemblies, showing that the composite membrane was softened and the composite electrolyte penetrated into gas diffusion electrodes (GDEs) to partially plug the gas channels in the gas diffusion layer.

     In this study, to improve the mechanical properties of the membrane at elevated temperature, polyimide (PI) was used as a membrane supporting material instead of PTFE due to its higher mechanical strength and thermal deformation temperature. C₂C₁pyrr(FH)_1.7F and HEMA monomer were mixed in the molar ratio of 8:2 and 9:1. Small amount of AIBN (2,2'-Azobis(isobutyronitrile)) was added as a radical initiator. PI (thickness: 16 and 30 μm, I.S.T) used as a porous supporting material was immersed in the mixed solution under vacuum for 1 h. Then, polymerization was conducted at 70 °C in Ar-filled oven for 12 h. Single cell tests were conducted under nonhumidified conditions at 25-120 °C. The GDEs with 1.0 mg Pt cm^-2 were used for both the anode and cathode.

     Fig. 1 shows i-V and i-p curves for a single cell using C₂C₁pyrr(FH)_1.7F-HEMA (8:2) composite membrane with 16 μm PI supporting material operated at 25-120 °C under nonhumidified conditions. Contrary to the previous result in which PTFE was used as a supporting material, the single cell performance increases with the elevation of temperature. The maximum power density of 35 mW cm^-2 is observed at 120 °C. Fig. 2 shows cross-sectional SEM images of the membrane electrode assembly (MEA) for a single cell using C₂C₁pyrr(FH)_1.7F-HEMA (8:2) composite membrane with 16 μm PI supporting material; (a) before single cell test, and (b) after single cell test at 25-120 °C. Yellow arrow indicates the membrane thickness. Comparing (a) with (b), there is no significant difference on membrane thickness, which means the previous problem of the penetration of composite electrolyte into the gas diffusion layer has been solved. Thus, the deterioration of cell performance does not occur even at over 80 °C.

Reference:

[1] P. Kiatkittikul, T. Nohira, R. Hagiwara, J. Power Sources, 2012, 220, 10.　

Acknowledgment: PI membrane supporting materials were provided from I.S.T Corporation.