Alkali metal acid phosphates for use as electrolytes in the electrolysis of water at elevated temperatures

The use of molten salts containing water as electrolytes for the high temperature (approx. 700/sup 0/K) electrolysis of water is being explored. The purpose of this study is to determine the electrolysis behavior of partially neutralized phosphate-based electrolytes, such as MH/sub 5/(PO/sub 4/)/sub 2/, where M represents an alkali metal cation. The main interest is in the characteristics of these molten salts as a function of neutralizing cation, water content, and temperature. The behavior of these molten salts was observed using platinum electrodes while measuring cell potential as a function of time at constant current densities up to 1 A/cm/sup 2/. Cell resistance before and after polarization was monitored in order to correlate electrode conductivity with the degree of dehydration. The most obvious characteristic of these melts is their tendency to continuously dehydrate at elevated temperatures when in an open container. This loss of water increases the potential for any given current and temperature, primarily by increasing the resistance of the electrolyte. Dehydration was most pronounced with sodium- and lithium-based electrolytes. The potassium-based electrolytes have a relatively slow dehydration rate. The lowest cell potentials were obtained for the sodium-based melts at 573 to 623/sup 0/K, where we estimate the cellmore » potential (iR corrected) to increase from about 0.9 V at zero current density to 2.1 V at 1 A/cm/sup 2/. However, any conclusions based on our cell potential measurements are of limited value at this time due to the qualitative nature of our experiments, especially in regard to uncertainties in the water content of the melts.« less