Ionic Liquid as Plasticizer and Liquid Conductive Filler for Polypropylene-Based Composite Bipolar Plates

To develop high-performance polypropylene (PP)-based composite bipolar plates (BPs) with enhanced processability and electrical conductivity, this work introduces a halogen-free ionic liquid (IL) containing long alkyl chains in cation and anion, namely trihexyl(tetradecyl)phosphonium di(2-ethylhexyl) phosphate ([P₆₆₆₁₄][DEHP]), as a multifunctional plasticizer. The composite BPs containing binary graphite (Gr) and carbon black (CB) conductive fillers and IL are fabricated by twin-screw extrusion and compression molding. Results show that after adding 4 wt% IL to the 30PP/63Gr/7CB BP, the torque decreases from 14.3 to 7.8 N m, the storage modulus at processing temperature (210°C) lowers from 1894 to 856 MPa, and the electrical conductivity increases by 192%. The added IL enhances melt fluidity, ensuring uniform filler dispersion; at the same time, it provides additional electron pathways boosting conductivity. Moreover, IL also reduces the density and improves the hydrophobicity of the composite BP. The incorporation of the IL-based plasticizer offers a novel approach for designing easily processable and high-performance composite BPs.