Cellulose nanocrystals–blended zirconia/polysulfone composite separator for alkaline electrolyzer at low electrolyte contents

Alkaline water electrolysis (AWE) is a mature technology for producing hydrogen using variable renewable sources. AWE typically uses a concentrated electrolyte and a porous separator between the two electrodes to deliver ionic conductivity and to separate the released gases. The porous separator typically requires highly concentrated electrolytes (25~30 wt%) to provide high ionic conductivity. However, the circulation of the concentrated electrolyte in the electrolyzer block causes loss of Faraday efficiency and corrosion. Herein, we show that a cellulose nanocrystals (CNCs)-blended Zirconia/Polysulfone composite porous separator exhibits both low area resistance of 0.18 Ω cm² and low hydrogen permeability of 4.7 × 10⁻¹² mol bar⁻¹ s⁻¹ cm⁻¹ at low electrolyte contents (10 wt% KOH solution). Meanwhile, a commercial Zirfon® separator exhibited poor performances of the high area resistance of 0.71 Ω cm² and high hydrogen permeability of 305 × 10⁻¹² mol bar⁻¹ s⁻¹ cm⁻¹ under the same condition. The cell comprising the optimized composite separator displayed a remarkable capability of 1.83 V at 600 mA cm⁻² with 10 wt% KOH solution for 300 h in a stable mode. Hydrophilic cellulose nanocrystals were successfully incorporated into the hydrophobic polymer network, resulting in lowering the area resistance and gas permeability of the separator. These results demonstrate that AWE equipped with (CNCs)-blended Zirconia/Polysulfone composite porous separators can achieve high performance using low concentration electrolytes, contributing to lifetime.