Highly Efficient Water Electrolysis Using NaOH–KOH Hydrate Melt (Supporting Information)

To enhance the efficiency of water electrolysis, this study focuses on the NaOH–KOH hydrate melt (NaOH : KOH : H₂O = 9 : 61 : 30 mol%) at temperatures ranging from 100 to 200 °C. We examined the behaviors of hydrogen and oxygen evolution reactions (HER and OER) on a Ni electrode at atmospheric pressure. In the Tafel plots, the upper limit of the Tafel region for both HER and OER expanded with increasing temperature, especially at 200 °C. Additionally, changes in the rate-determining step for HER and OER were observed compared to a 30 wt% KOH aqueous solution at 80 °C. The total overpotential for HER and OER was compared to the value of 1133 mV obtained in the 30 wt% KOH aqueous solution (80 °C, 500 mA cm⁻²). The total overpotentials in the NaOH–KOH hydrate melt at 200 °C and 500, 1000, and 2000 mA cm⁻² were 545, 619, and 714 mV, respectively. The reduction in overpotentials was 52 %, 45 %, and 39 %, respectively. Water electrolysis utilizing the NaOH–KOH hydrate melt shows promising potential for significantly enhancing energy efficiency even at higher current densities relative to traditional alkaline water electrolysis.