Microporous Graphite Composites of Tailorable Porosity, Surface Wettability, and Water Permeability for Fuel Cell Bipolar Plates

Abstract

To address the water management problem in proton-exchange membrane fuel cells, porous graphite composites were prepared by incorporating sacrificial pore-forming agents in the blend of graphite and a phenolic resin formed into a flat plate using compression molding. Sucrose was found to be an effective porogen in the porous plate production process. The effects of relative amounts of graphite, polymer, and porogen in the composite were studied. Bipolar plate properties such as the water uptake by wicking and vacuum infusion, gas-breakthrough pressure across the water-infused pores, electrical conductivity, and flexural strength of the plates were measured and correlated with the plate composition. The plate porosity was evaluated by determining the masses of dry and water-infused plates in air and under water. The porosity, ε, showed a linear increase with an increase in the porogen concentration in the range of 0–10%. The permeabilities, K, of water through the graphite plates with different porosity values were calculated by measuring the water flux over a range of pressures. The water permeability increased with oxidation and hydrophilization of the pore surface. Dynamic water contact angle measurements were used to characterize the effect of chemical and thermal treatments on the water wettability of the plates. The gas-breakthrough pressure of the water-infused plates was found to be linearly correlated with the parameter, γcos⁡θ/K/ε, proportional to the capillary pressure of the gas–water interface of surface tension, γ, and contact angle, θ. Porous plates capable of a total water uptake greater than 25 wt %, with the gas-breakthrough pressure higher than 10 psi, through-plane electrical conductivity exceeding 100 S cm–1, and flexural strength exceeding 25 MPa were obtained.