Development of highly thermally conductive graphite/graphene filled polybenzoxazine for bipolar plates in fuel cells

Abstract

Development of a suitable and efficient bipolar plate material for polymer electrolyte membrane fuel cell (PEMFC) is scientifically and technically important due to the critical required on higher thermal properties, higher electrical conductivity and better mechanical properties of this material. Therefore, this research emphasizes on the development of highly filled graphite/graphene filled polybenzoxazine composites and investigates thermal conductivity, electrical conductivity and mechanical properties of the obtained composites. Graphite at aggregate size of 240 μm was used in this research. The overall carbon filler content was fixed at 83wt% and varying composition of graphene from 0-10wt% at an expense of the graphite. The experimental results revealed that storage modulus at 25oC of the composites filled with 7.5wt% of graphene content (75.5wt% of graphite) filled in the polybenzoxazine, raised from 5.9 GPa of the neat polybenzoxazine up to about 16.9 GPa in the composites which is about 166% improvement. The glass-transition temperatures (Tg) of our composites were observed to be in the range of 200 to 209oC. The Tg of the composites was found to increases with an increasing graphene contents due to good interfacial interaction between the filler and the matrix which can hinder the polymer chain movement. Moreover, thermal conductivity of the composite filled with 7.5wt% of graphene loading was found to be as high as 14.5 W/mK. This phenomenon suggested that the aggregate graphite provided beneficial in producing a small number of interfaces, thus resulting in a high thermal conductivity in overall. In addition, an incorporation of a small amount of graphene in the composite enhanced the interaction between the graphite by bridging the gaps between them. The flexural modulus and flexural strength of the composites filled with 7.5wt% of graphene were found to be as high as 16.8 GPa and 55 MPa, respectively. Water absorption of graphite/graphene filled polybenzoxazine composite was relatively low with the value of only about 0.04% at 24 hours of immersion. Additionally, electrical conductivity of the composite was measured to be 323 S/cm at 7.5wt% of graphene loading. From the results, the values of thermal conductivity, electrical conductivity, flexural modulus, flexural strength as well as water absoprtoin of graphite/graphene filled polybenzoxazine composites satisfied the use as bipolar plate according to the United States Department of Energy (DOE) requirements. Therefore, these graphite/graphene filled composites based on polybenzoxazine are highly attractive for bipolar plates in PEMFC applications.