Water vapor pressure over molten KH₂PO₄ and demonstration of water electrolysis at ∼300 °C

Highlights: • The vapor pressure over molten KH₂PO₄ was measured by Raman spectroscopy to be about 8 bars at ∼300 °C. • Raman spectroscopy shows that molten KH₂PO₄ under its own vapor pressure contains much dissolved water. • It is demonstrated spectroscopically that water electrolysis is possible in KH₂PO₄ electrolyte forming H₂ and O₂ at 300 °C. • Molten KH₂PO₄ is a possible electrolyte for water electrolysis. - Abstract: A new potentially high-efficiency electrolyte for water electrolysis: molten monobasic potassium phosphate, KH₂PO₄ or KDP has been investigated at temperatures ∼275–325 °C. At these temperatures, KH₂PO₄ was found to dissociate into H₂O gas in equilibrium with a melt mixture of KH₂PO₄−K₂H₂P₂O₇−KPO₃−H₂O. The water vapor pressure above the melt, when contained in a closed ampoule, was determined quantitatively vs. temperature by use of Raman spectroscopy with methane or hydrogen gas as an internal calibration standard, using newly established relative ratios of Raman scattering cross sections of water and methane or hydrogen to be 0.40 ± 0.02 or 1.2 ± 0.03. At equilibrium the vapor pressure was much lower than the vapor pressure above liquid water at the same temperature. Electrolysis was realized by passing current through closed ampoules (vacuum sealed quartz glass electrolysis cells with platinum electrodes and the electrolyte melt). The formation of mixtures of hydrogen and oxygen gases as well as the water vapor was detected by Raman spectroscopy. In this way it was demonstrated that water is present in the new electrolyte: molten KH₂PO₄ can be split by electrolysis via the reaction 2H₂O → 2H₂ + O₂ at temperatures ∼275–325 °C. At these temperatures, before the start of the electrolysis, the KH₂PO₄ melt gives off H₂O gas that pressurizes the cell according to the following dissociations: 2KH₂PO₄ ↔ K₂H₂P₂O₇ + H₂O ↔ 2KPO₃ + 2H₂O. The spectra show however that the water by virtue of hydrogen-bonding has a high affinity for remaining in the melt. The formed hydrogen and oxygen gasses were detected by means of the characteristic Raman gas-phase spectra.