From ink rheology to electrochemical performance: Ionomer-controlled IrOx microstructure in PEM water electrolyzer anode catalyst layers

In proton exchange membrane water electrolyzer (PEMWE) anode catalyst layers (ACLs), optimization has traditionally focused on electrochemical performance while overlooking how ionomer-specific chemistry and loading jointly constrain decal transfer (DT) processability, leaving fabrication and performance considerations largely decoupled. Here, ionomer loading (15, 20, and 25 wt% of total catalyst ink solids) is varied systematically for a short-side-chain ionomer (Aquivion, EW 790) and a long-side-chain ionomer (Nafion, EW 1100) in IrO x -based ACLs. Aquivion exhibited stronger aggregation and a distinct shear-thinning response at high ionomer loading, whereas Nafion showed more gradual rheological evolution. Complete DT requires 20 wt% ionomer for Aquivion and 25 wt% for Nafion, whereas peak performance occurs at 15 and 20 wt%, respectively, delivering cell voltages of approximately 1.82 V (Aquivion) and 1.80 V (Nafion) at 2 A cm −2 . This ionomer-chemistry dependence arises from intrinsic differences in thin-film-forming behavior that shift the balance among proton conduction, electronic percolation, and active-site accessibility across different loading thresholds for each ionomer type. As a result, no universal ionomer loading can satisfy both fabrication and performance requirements. High-ionomer-loaded ACLs deliver superior high-current density performance by improving wettability and suppressing residual losses, a trade-off against low-current density penalties that is most pronounced for Aquivion. These findings demonstrate that fabrication viability and electrochemical performance should be optimized independently and that catalyst-ink formulation requires ionomer-specific consideration.