Poly(phenylene sulfide)-graphite composites for bipolar plates with preferred morphological orientation

Bipolar plates in phosphoric acid fuel cells require inertness to phosphoric acid as well as excellent electrical, thermal, and mechanical properties. For this application, we prepared poly(phenylene sulfide) (PPS)-graphite composites with random or ordered graphite orientations by compression and extrusion-compression processes, respectively. Due to current limitations of extruding graphite-filled polymers, only moderately high graphite concentrations were used (up to 40 wt%). The compressed composites contained graphite sheets in a planar orientation (parallel to the pressing direction) and exhibited highly anisotropic electrical and thermal conductivity, with much higher in-plane than through-plane components. In contrast, composites that were extruded prior to compression exhibited randomly oriented graphite due to shearing forces during extrusion and therefore displayed isotropic properties. Thus, their through-plane electrical and thermal conductivity was superior to those of the ordered composite, while the in-plane properties were inferior. Notably, the internal graphitic structure affected the electrical conductivity more than the thermal conductivity. The randomly oriented composite also exhibited superior flexural strength, although the thermal stability of the two composites was almost equal. This study offers insights into the structure-property relationship of PPS-graphite composites as well as the effect of the orientation of conductive two-dimensional fillers on anisotropic properties.