Effect of Graphite Type on the Electrical, Thermal, Mechanical, and Gas‐Barrier Properties of Ultrathin Composite Bipolar Plates

This study systematically investigates the influence of graphite type, particle shape, and particle size on the electrical, thermal, mechanical, and gas-barrier properties of ultrathin (0.3 mm) carbon-polymer composite bipolar plates fabricated via the Powder-to-Roll process (P2R). Variation in particle size reveals that smaller particles yield the highest through-plane conductivity and the lowest gas transmission, while intermediate particle sizes deliver the best balance of in-plane conductivity and mechanical strength, and larger particles enhance the overall thermal conductivity of the composite. The incorporation of spherical particles increases the through-plane thermal conductivity to 3.15 W m-1 K-1. Natural graphite increases mechanical stability and reduces gas transmission, but also decreases both electrical and thermal conductivities compared to synthetic graphite. The addition of 10 wt. % expanded graphite in a ternary filler system markedly increases electrical conductivity in both the in-plane and through-plane directions, resulting in a through-plane resistance of 9.6 mΩ cm2 that meets the United States Department of Energy targets, while simultaneously lowering gas permeability without compromising thermal or mechanical performance. These findings demonstrate that a tailored selection and combination of graphite fillers can harmonize the competing requirements for conductivity, strength, and gas-barrier performance in ultrathin composite bipolar plates.