The effect of catalyst ink formulations and slot-die coating parameters on PEMFC GDE fabrication

Slot-die coating is a promising method for mass-producing membrane electrode assemblies for polymer electrolyte membrane fuel cells. Precise control of slot-die coating parameters and catalyst ink variables is essential for achieving defect-free catalyst layers. This study investigated catalyst ink rheology and coating windows of typical ink formulations and extremes. The influence of the dispersing solvent ratio, ionomer-to-carbon ratio, and Pt weight percentage on catalyst ink rheology and coating behaviour was examined. Inks with a 75%-water/25%-n-propanol dispersing solvent ratio exhibited shear-thinning and good coatability, while those with high water content (90%-water/10%-n-propanol) displayed Newtonian flow, leading to poor substrate wetting and coating. Increasing the ionomer-to-carbon ratio reduced the ink's viscosity, while increasing the Pt weight percentage increased the ink’s viscosity. While appearing defect-free, x-ray fluorescence and optical microscopy analyses revealed that the coated catalyst layers often displayed nonuniform Pt loadings and cracks, especially with increasing catalyst layer thickness. The nonuniform Pt loadings were attributed to variations in gas diffusion layer roughness, catalyst layer cracks, and limitations of the slot-die coater. Initial membrane electrode assembly performance tests underscored the importance of material selection, the ionomer overlayer, and hot-pressing to enhance performance.