Composite separators comprising ceramic filler and thermoplastic polymer are widely used in alkaline electrolyzers due to their stability in harsh conditions. The ceramic filler should possess high chemical stability and strong interaction with the polymer binder. TiO2 is a good filler candidate due to its hydrophilicity and potential capabilities. However, the impact of TiO2 size on composite separator properties has yet to be studied. Herein, we investigated the size effect of various TiO2 nanoparticles (18, 40, and 100 nm) on the performance of the titania/polysulfone-based separators. Smaller TiO2 nanoparticles increased bubble point pressure (BPP) and decreased H2 permeability. Meanwhile, the ohmic resistance decreased with increasing TiO2 size. The interaction between TiO2 nanoparticles and polysulfone increased with the smaller sizes of TiO2 nanoparticles. Robust interactions increased resistance by reducing the pore size distribution. We selected 100TiO2/PSU (445 ± 5
m thickness) as a suitable separator due to its low ohmic resistance (0.15 Ω cm2) and high BPP (3.0 ± 0.2 bar). The cell operated stably, maintaining a cell voltage of 2.10–2.12 V for 1000 h at 2.0 A cm−2 at 80 °C. This study will help select the optimal size of the inorganic filler of the composite separator in alkaline water electrolyzers.