Electrochemical Method for Monitoring Corrosive Impurities in Molten MgCl2/KCl/NaCl Salts for Thermal Energy Storage

Abstract

Molten chloride salt mixtures are promising thermal energy storage (TES) materials to be applied in concentrating solar power (CSP) plants, due to their physicochemical and thermal properties. Their high thermal stability makes them appropriate candidates to replace the commercial TES materials in CSP, nitrate salt mixtures (stable up to 550 °C), when higher operating temperatures are required (up to 700 °C) for higher efficiency of thermal to electrical energy conversion. However, the application of molten chloride salts at higher temperatures causes additional challenges, particularly increased corrosiveness of containers and structural materials. It is well known that the corrosion rates significantly depend on the concentration of oxide/hydroxide impurities in the molten chloride salts. In this work, an electrochemical analysis system based on cyclic voltammetry (CV) is being developed to measure the concentration of these impurities in-situ, in order to assist the corrosion control on the molten salts. Before the electrochemical experiments, pre-electrolysis (PE) was performed on the studied molten MgCl2/KCl/NaCl (60/20/20 mole%) salts to remove most of the impurities including oxide/hydroxide. By fitting the CV data from PE with those from the acid consumption measurements on the simultaneously collected salt samples, the constant for the calculation of the concentration of MgOH+ from the current density under the conditions 500 °C and sweep rate of 200 mV/s was determined. After PE, by adding NaOH pellets into the molten salts, a significant change of a peak in the cathodic potential range of the cyclic voltammogram was observed. This is considered to be caused by the change of the concentration of the stable hydroxide species (MgOH+) according to literature. When the determined constant based on the CV data from PE experiments was used in the calculation of the concentrations of MgOH+ from the current density of the peak for the reduction reaction of MgOH+, the concentrations of MgOH+ determined from titration compare well with those from the CV experiments.