Specific Electrical Conductivity in Molten Potassium Dihydrogen Phosphate KH2PO4 Electrolyte at ~300 °C

The conductivity of pure molten KH₂PO₄  salt and two mixtures with more and less water (in the KH₂PO₄–H₂O and KH₂PO₄–KPO₃ systems) were measured at temperatures of 265–320 °C and under their own water vapor pressures. Molten KH₂PO₄ is a promising electrolyte for an intermediate temperature pressurized water electrolyzer because of its high conductivity of ~0.30 Ohm^-1 cm^-1 at 300 °C. The conductivity data are given as polynomial functions of temperature and composition.

The KH₂PO₄ (KDP) salt is one of the most used nonlinear crystals for laser radiation conversion required in laser fusion systems [1], and recently also the application of the melt as the electrolyte for high temperature water electrolysis was suggested [2]. The purpose of the present work was to investigate the level of specific electrical conductivity of the melt, of which there was no previous data available. Also it was an objective to determine analytical expressions for the conductivity versus temperature and water content. Such information is essential in the optimization of the high temperature water electrolysis application of the electrolyte.  Upon heating, the salt melts and becomes a useful electrolyte [2]. During the melting process, the following reaction (1) is known to take place:

2 KH₂PO₄  <-->  K₂H₂P₂O₇    +  H₂O (1)

whereby the melt loses water during evaporation and the melting point drops considerably, from the true value ~280 °C perhaps down to ~262 °C. In an enclosed ampoule, the water vapor pressure over the molten salt is considerable, around 8 bars at 300 ºC, according to our recent results, obtained by Raman spectroscopy [3].

The obtained data are reproduced in Fig. 1 as points along 3 curves, obtained by plotting the logarithm of the conductivity σ (ordinate scale) as a function of reciprocal absolute Kelvin temperature T (against 1000/T):

Log₁₀σ = A - B x 1000/T   (2)

As expected, the curves are approximately linear; i. e. the σ data can be reproduced by (2). The conductivity (2) decreases with addition of KPO₃ (or the loss of water in open cells) and increases with temperature in the range between ~180 and ~300 °C. The studied compositions and temperature ranges were limited by the crystallization points and the vapor pressure (note the risk of explosion) that depended markedly on the temperature [3]. During standing, the viscosity and surface tension increased significantly, judging from visual observations when shaking the melts (dissolution of SiO₂?).

[1]     W. Cai and A. Katrusiak, “Structure of high-pressure phase KH₂PO₄”, Dalton Trans., vol. 42, pp. 863-866, 2013.

[2]     C. B. Prag, “Intermediate Temperature Steam Electrolysis with Phosphate-Based Electrolytes.” Ph.D. thesis, supervisors: N. J. Bjerrum, E. Christensen, Li Qingfeng, I. M. Petrushina, Kgs. Lyngby: DTU Energy, 1- 154, 2014.

[3]     R. W. Berg, A.V. Nikiforov, I. M. Petrushina and N. J.Bjerrum, “Determination of water vapor pressure over and demonstration of water electrolysis in a molten potassium dihydrogen phosphate KH₂PO₄ at ~300 °C by means of Raman spectroscopy”,  EEST2015, extended abstract, 16-22 August 2015, Vancouver, Canada.

Fig. 1. Conductivity results as explained in the text. Insert shows cell of fused quartz glass, sealed under vacuum. (A) Pressure equilibration tube, (B) Cell compartment, (C)  Capillary tube for conduction and (D) Electrode feed throughs adapted from standard halogen lamps.