Molten chloride salts for high-temperature thermal energy storage: Continuous electrolytic salt purification with two Mg-electrodes and alternating voltage for corrosion control

Abstract

Molten chloride salts such as MgCl2/KCl/NaCl are promising thermal energy storage (TES) materials and heat transfer fluids (HTF) in next generation concentrated solar power (CSP) plants with elevated operation temperatures (>700 °C) due to their high thermal stability and low material costs. However, they have strong corrosivity against metallic structural materials at high temperatures which can be related to the presence of hydrolysis products such as MgOHCl. In an electrolytic purification method reported in previous work, a W-cathode and Mg-anode were used to reduce the concentration of these impurities, thereby effectively purifying the molten MgCl2/KCl/NaCl salt. However, the W-cathode passivation due to production of MgO on the surface limited its cathode effectiveness. In this work, an improved electrolytic purification method is presented to avoid the electrode passivation by using two identical Mg-electrodes and alternating voltage (AV, i.e., switching the voltage direction applied periodically). A continuous electrolytic salt purification in the AV-mode is successfully performed on the molten MgCl2/KCl/NaCl salt in the 100 g-scale. Cyclic voltammetry (CV) on the molten salt shows that the purification can effectively reduce the concentration of the main corrosive impurity MgOH+Cl−. Potentiodynamic polarization (PDP) measurements on a commercial alloy (Incoloy 800 H) immersed in the molten salt indicate significantly reduced corrosion rates (i.e., reduced salt corrosivity) compared to a non-purified salt. The continuity of the improved method allows for a long-term effective purification without the risk of passivation and deactivation of electrodes, thereby showing great potential in corrosion control of molten chloride salt TES system.