Preparation of thermostable electroconductive composite plates from expanded graphite and polyimide

Electroconductive composite plates as potential bipolar plates for polymer electrolyte membrane fuel cells (PEMFCs) were prepared using expanded graphite (EG) as carbon filler and polyimide (PI) as binder by a hot compression molding method. The thermal behaviors of PI were investigated by means of DSC, DMA and TG analysis techniques. The results reveal that PI and its composites after curing are highly stable with negligible weight loss till 200 °C. A thermal aging test for the EG/PI composite plates was further conducted at 250 °C for 50 h in air atmosphere. Only slight decrease in bulk density, electrical conductivity and flexural strength was found. The effects of molding temperature and EG expansion ratio as well as PI content on various properties of the EG/PI composite plates were discussed. Molding at 220 °C proves suitable for the composite plates to attain satisfied electrical conductivity and flexural strength. Large EG expansion ratio benefits both the electrical conductivity and flexural strength. Increasing PI content can help to reduce the bulk density and H2 permeation rate and enhance the flexural strength and shore hardness, but does harm to the electrical conductivity. EG/PI composite plates with high thermal stability qualifying for application to high temperature PEMFCs can be acquired by adjusting the PI content to 40–55 wt.% of the composite.

► Polyimide (PI) was adopted as a thermostable binder for preparing expanded graphite (EG) based composite plates. ► A thermal aging test for the EG/PI composite plates was made. ► PI content in the range of 40–55 wt.% is applicable to prepare qualified EG/PI composite plates with high thermal stability. ► Large EG expansion ratio benefits both the electrical conductivity and flexural strength of the composite plate. ► The EG/PI composite plates are potential bipolar plates for high-temperature polymer electrolyte membrane fuel cells.