Free Ionomer Controls Adsorption in Electrolyzer Inks

The ionomer/catalyst interface in proton-exchange-membrane water-electrolyzers (PEMWEs) anode catalyst layers plays a pivotal role in governing multi-species transport and reaction kinetics. This interface originates from ionomer adsorption onto catalyst particles during ink formation. The inks consist of iridium oxide (IrOx) catalyst particles and PEM ionomer (typically perfluorosulfonic acid, PFSA) dispersed in solvent (water/alcohol mixtures), which are deposited onto a substrate to form the catalyst layer upon solvent evaporation. Understanding how ink formulation controls ionomer adsorption is therefore critical, yet a systematic investigation across industrially relevant formulations (including concentrated inks used in roll-to-roll manufacturing) is missing. Here, we systematically investigate the influence of IrOx concentration, ionomer-to-catalyst (I:C) ratio, and solvent composition on PFSA adsorption using thermal gravimetric analysis. PFSA adsorption is independent of IrOx concentration, being instead determined by free-ionomer concentration and solvent identity, across both dilute and concentrated inks. We further show that I:C ratio is a poor descriptor, with free-ionomer weight percentage providing a more meaningful metric that collapses adsorption data onto a single isotherm. In water-rich solvents, adsorption is primarily limited by electrostatic repulsion, whereas, in alcohol-rich solvents, hydrophobic forces and surfacecoverage effects dominate. Adsorption phenomena also correlate to ink stability (sedimentation, x-ray scattering) and single-cell performance. These results extend to catalyst inks generally, offering practical implications for ink formulation across deposition methods.