Surface enrichment of ionomer in fuel cell catalyst layer prepared using slot-die coating method

The surface enrichment process of the ionomer during the drying of catalyst ink film produced by the slot-die coating method and its effect on fuel cell performance were investigated, wherein the solvent composition of the ink was considered. The slot-die-coated catalyst layer (CL) surface was observed using helium ion microscopy, conductive atomic force microscopy, and X-ray photoelectron spectroscopy, and ionomer enrichment was observed on the slot-die-coated CL surface. Furthermore, spontaneous adsorption of the ionomer at the air–solution interface was observed in the water-rich dispersion by measuring the surface tension of water–alcohol mixed solvents with and without an ionomer. Therefore, two ionomer enrichment mechanisms were proposed based on the alcohol content of the ink: an evaporation-dominated mechanism for alcohol-rich ink and an adsorption-dominated mechanism for water-rich ink. The effect of surface ionomer enrichment on proton conduction depends on the alcohol content of the catalyst ink. In addition, ionomer enrichment is conducive to the reduction of the oxygen transport resistance because of the reduction in the ionomer distributed inside the CL. These findings provide novel insights into the structural evolution and engineering of the CL.