An electrochemical autoclave cell to investigate water electrolysis in molten salt electrolytes at elevated temperatures and pressures

A flexible autoclave-based electrolysis cell setup has been developed capable of performing electrochemical experiments, such as hydrogen evolution reactions (HER), oxygen evolution reactions, and electrochemical CO 2 reductions, across a wide range of temperatures (from room temperature to 350 °C) and pressures (long-term tested up to 50 bar). The setup was employed to investigate HER in molten electrolytes at medium temperatures and sealed conditions, utilizing dihydrogen phosphates mixture (KH 2 PO 4 and NaH 2 PO 4 ) and pure phosphoric acid as proton conducting electrolytes. Compared to pure KH 2 PO 4 , a specific dihydrogen phosphate mixture lowers the melting point to 230 °C and enhances proton conductivity by 20% at 270 °C. In the molten mixed salt, tungsten and molybdenum electrodes exhibited promising HER activity at medium temperatures, surpassing the platinum electrode. The observed differences in HER performance among Pt, W, and Mo electrodes are primarily attributed to mass transport limitations at the platinum electrode in the highly viscous molten salt electrolyte, rather than to superior intrinsic catalytic activity of tungsten or molybdenum. As expected, in the less viscous phosphoric acid, platinum showed the best HER performance. This autoclave setup enables medium-temperature electrochemical studies such as HER and offers potential for future applications in oxygen evolution and CO 2 reduction. • A flexible (T, p) and safe autoclave-based electrolysis cell setup was developed. • Eutectic mixtures of molten dihydrogen phosphates show high electrical conductivity. • Pt, Mo, and W show high hydrogen evolution activity at medium temperatures.