Managing the Pore Morphologies of Microporous Layers for Polymer Electrolyte Fuel Cells with a Solvent-Free Coating Technique

Abstract

We report a solvent-free coating process for microporous layers (MPLs) in polymer electrolyte fuel cells to eliminate solvents, dispersing agents, and thickening agents of a conventional wet-coating process. In this environmentally friendly process, the MPLs were fabricated by depositing composite powder consisting of carbon and poly(tetrafluoroethylene) directly onto a fibrous carbon substrate using an electrostatic screen printer. This enables the fabrication of sufficiently sized MPLs for the cells of first-generation Toyota MIRAI fuel-cell vehicles. Microscopic observation of the surface and the cross sections of dry-coated MPLs revealed smooth defect-free surfaces. Static water contact angle measurements revealed the hydrophobicity of dry-coated MPLs to be equivalent to that of wet-coated MPLs. Moreover, the pore morphologies of dry-coated MPLs could be managed by controlling the particle arrangement by electrostatic screen printing. The particle and pore morphologies of dry-coated MPLs were characterized by mercury intrusion porosimetry and synchrotron X-ray computed nanotomography. From electrochemical characterizations for dry-coated MPLs, a porosity-graded MPL exhibits excellent flooding tolerance under operating conditions below the dew point, corresponding to 120% relative humidity. A multilayer obtained with the dry-coating process requires a single annealing process and has the necessary gradient features, thus potentially reducing the environmental impact of manufacturing next-generation fuel-cell vehicles.