Decoupling Solvent Effects: Perfluorinated Sulfonic Acid Structure and Catalyst Layer Stability in PEM Water Electrolysis

Solvent differences in perfluorinated sulfonic acid ionomer (PFSI) dispersions confound comparisons of catalyst-layer (CL) binders for PEM water electrolysis. We use a propylene-glycol (PG) single-solvent platform to isolate effects of ionomer structure, i.e., side-chain architecture and equivalent weight (EW), across five PFSIs (Nafion D2021, Nafion D2020, Aquivion D98-25BS, Aquivion D72-25BS, and 3M E-22397B) as hydrogen evolution reaction-CL binders. PG collapses ionomer aggregates from microns to sub-100 nm and reduces catalyst–ionomer agglomerates without a systematic EW trend, indicating that performance differences originate during film formation. PG makes CLs more hydrophobic (higher θR), yet aerophobicity (θAir) still increases with decreasing EW, consistent with higher sulfonic-acid density. Under PG, current density at 1.9 V rises with decreasing EW, with thicker short-side chain/mid-side chain (SSC/MSC) films (≥140 nm) and lower ohmic overpotentials than long-side-chain (LSC) Nafion. Durability under Accelerated Stress Test-2 (AST-2; 0.3/3.0 A cm−2, 48 h) splits by class: LSC Nafion shows the largest degradation slopes, whereas SSC/MSC are lower; transmission electron microscope images show Pt growth with Nafion D2020-PG but minimal growth with Aquivion D98-25BS-PG. With solvent effects removed, ionomer structure governs CL performance and stability; SSC (EW ≈ 980) is recommended, and MSC maximizes current with moderate stability.