Graphite based benzoxazine composites for an application as 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 demand on higher thermal properties, higher electrical conductivity and better mechanical properties of this material. This research aims to study electrical conductivity and mechanical properties of highly filled graphite composites utilizing polybenzoxazine as a matrix. The condition for the compression molding to produce highly filled graphite-polybenzoxazine composites was at temperature of 200oC, and pressure of 15 MPa in a hydraulic hot-press machine for 3 hours to assure a fully cured specimen. The composition of graphite filler was achieved to be in the range of 40 to 80% by weight. The densities of the obtained composites were found to be in a range of 1.19-1.88 g/cm3 as predicted by rule of mixture. The experimental results revealed that at the maximum graphite content of 80wt% or 68vol% filled in the polybenzoxazine, storage modulus at room temperature of the specimen was raised from 5.9 GPa of the neat polybenzoxazine up to about 21.9 GPa in the composites which is about 370% improvement. The glass-transition temperatures (Tg) of the prepared composites were observed to be ranging from 174 to 194oC and the values substantially increased with increasing the graphite contents implying substantial interfacial interaction between the filler and the matrix. Thermal conductivity as high as 10.2 W/mK is achieved for a graphite filled polybenzoxazine at its maximum filler loading. Furthermore, at graphite content of 80wt% in the polybenzoxazine, the composite’s flexural modulus and flexural strength were found to be as high as 17 GPa and 52 MPa, respectively. Water absorption of this filled system was relatively low with the value of about 0.032% at 24 hours. Additionally, electrical conductivity was measured to be 245 S cm-1. Consequently, the data on thermal properties, electrical conductivity and mechanical properties of the graphite filled polybenzoxazine composites indicated the values that highly satisfied the United States Department of Energy (DOE) requirements. Therefore, these graphite filled composites based on polybenzoxazine are highly attractive for bipolar plates in PEMFC applications.