Polymer electrolyte membrane water electrolysis (PEMWE) is a promising technology for converting renewable electricity into hydrogen, owing to its stable operation under fluctuating power input. However, both the current density achievable at practical full‑cell voltages and the long‑term durability still do not meet societal and industrial expectations. While catalyst development is commonly used to overcome these limitations, catalyst ink composition may also play a significant role, as ink optimization is widely used to improve fuel cell performance. Nevertheless, its effects in PEMWE remain insufficiently understood. In this study, we varied catalyst ink ratios and evaluated their impact on current density and full‑cell voltage characteristics.

Both cathode and anode inks consisted of catalyst, ionomer, water, and alcohol. The cathode catalyst was 20 Pt-wt% Pt/C, and the anode catalyst was amorphous IrO₂; Nafion (20 wt% in water and 1‑propanol) served as the ionomer. We first examined the influence of alcohol species, that is, ethanol versus 1‑propanol. Although the current densities at 1.9 V and 80°C were similar (2.2–2.4 A/cm²), ethanol yielded slightly higher values, likely due to the higher porosity of catalyst layers formed by its faster evaporation during drying.

Next, the catalyst‑to‑ionomer ratio was varied. For the cathode, the carbon‑to‑ionomer ratio in Pt/C inks was adjusted from 1:0.45 to 1:1.5, while for the anode, the IrO₂‑to‑ionomer ratio ranged from 1:0.1 to 1:1.5. At 1.8 V and 80°C, the resulting current densities spanned 1.6–1.85 A/ cm² for Pt/C and 0.8–1.85 A/cm² for IrO₂. Constant‑current operation at 2.0 A/cm² for several hours revealed an optimal cathode ratio of 1:1 for current density stability, whereas the optimal anode ratio was less distinct. The optimal current density was obtained with 1:0.22 (or 0.1) for the anode ratio, whereas the differences were small for the cathode ratio. The broader current density variation observed for the anode likely reflects the stronger rate limitation imposed by the oxygen evolution reaction relative to proton transport through the electrolyte. Current stability appears to correlate with the mechanical robustness of the cathode catalyst layer.

These findings demonstrate that, in addition to catalyst development, the physicochemical environment of the catalyst layer plays a critical role in determining PEMWE performance.