Comprehensive investigation of the solvent-ionomer interactions on the fabrication of the catalyst layer for proton exchange membrane fuel cells

A comprehensive understanding of the interactions between ionomers and solvents is crucial for optimizing catalyst ink formulations in industrial coating processes, such as slot-die coating. This study systematically examines interactions between solvents and ionomers and their effect on the rheology and evolved catalyst layer microstructure of the pseudo-catalyst ink. Using pseudo-catalyst inks omits expensive platinum group metals while maintaining similar rheological properties and structuration behaviors that can be translated to actual catalyst inks. Various ionomer types were evaluated in water/1-propanol solvent systems, including Aquivion D72, Aquivion D79 (short side-chain ionomers with shorter perfluoroether segments connecting the sulfonic acid group to the backbone), Nafion D2020, and Nafion D2021 (long side-chain ionomers with longer perfluoroether segments in their molecular structure), using rheological and Fluorine-19 Nuclear Magnetic Resonance measurements. The findings revealed that ionomers with long and short side-chain lengths show distinctly different inter-aggregation behavior in the same solvent system, greatly affecting their viscosity. Dispersions containing the short side chain (SSC) ionomer exhibited higher viscosities, which were more apparent in high n-propyl alcohol (NPA) solvent compositions, suggesting stronger NPA-ionomer interactions. In contrast, the long-side chain (LSC) ionomer dispersions showed increased relative viscosity with increasing proportion of water in the solvent composition. In addition, the impact of solvent-modulated ionomer structures on crack formation was investigated. The results demonstrated that SSC-based inks in NPA-rich dispersions produced crack-free layers but showed cracking in water-rich systems. In contrast, LSC-based inks displayed the opposite behavior. This study highlights the importance of the ionomer's dispersion-phase morphologies in catalyst layer fabrication. • SSC ionomers aggregate more in NPA; LSC ionomers aggregate in water • SSC dispersions show higher viscosity in NPA; LSC is higher in water systems • SSC pseudo-CLs are crack-free in NPA-rich but crack in water-rich; LSC shows the opposite