Vapor pressure and specific electrical conductivity in the H2O–LiH2PO4–LiPO3 system—a novel electrolyte for water electrolysis at elevated temperature

Abstract

Lithium dihydrogen phosphate, LiH2PO4 (LDP) was studied for water electrolysis at elevated temperatures. It is shown that pure LDP, melting at ~224 °C and with correct DSC and X-ray diffraction diagram, in presence of sufficient humidity is stable up to 360 °C. Both solid and liquid LDP is structurally disordered and super-protonic conducting with conductivities reaching 0.48 S * cm -1 , when determined in sealed conductivity cells. The vapor pressure above LDP, pure and mixed with LiPO3 or H2O, was determined by means of Raman spectroscopy based on N2 as internal reference gas. Pressures up to ~20 bar at 350 °C were found and conductivities are given for solid and liquid states. Water splitting by electrolysis, 2H2O  2H2 + O2, was demonstrated by Raman spectroscopy at ~225°C and ~4.2 bar with Pt/W electrodes, thus showing that molten LDP has a significant potential for elevated temperature water electrolysis with high efficiency.