Novel High-Temperature Alkaline Water Electrolysis Using Molten KOH–H₂O System

To improve water electrolysis efficiency, we investigated the hydrogen and oxygen evolution reactions (HER and OER) at a Ni electrode in a molten KOH–H₂O system (85:15 wt%, 65:35 mol%) at a high temperature (150 °C). The overpotentials for the HER and OER at 500 mA cm⁻² were significantly reduced, by 261 mV and 75 mV, respectively, in the novel system relative to those obtained in a conventional aqueous solution system (30 wt% KOH, 80 °C). The decreases in polarization were attributed predominantly to kinetic effects, including a change in the rate-determining steps, on the HER and OER with the change of electrolytes.