The Solvent Effect on Ionomer Self-Assembly: From Molecular Morphology to Catalyst Layer Performance

The catalyst-bound ionomer layer (CBIL) in catalyst slurries consists of surface-adsorbed ionomers and solvent-diffused hydronium ions, where ionomers exhibit solvation diameters spanning tens to hundreds of nanometers with distinct phase-separated structures. This work elucidates how solvent composition and polarity regulate ionomer solvation-induced phase separation, thereby governing the CBIL architecture on catalyst aggregates, slurry network heterogeneity, bimodal pore distribution in coatings, and electrode polarization performance. Key findings reveal that water-rich solvents reduce ionomer exclusion volumes and induce morphological collapse, accompanied by decreased side-chain offset, indicating constrained -SO₃H group extension. These changes yield a denser CBIL structure with highly negative surface zeta potentials (<-70 mV), weakening interaggregate network strength. The concurrent main-chain collapse and side-chain curling lower surface sulfur content in catalyst layers, mitigating Pt poisoning risks from the -SO₃H groups. Notably, elevated CBIL zeta potentials enhance interaggregate repulsion, effectively suppressing particle overaggregation in coatings and significantly improving the polarization performance of proton exchange membrane fuel cells (PEMFCs).