Cyclic Voltammetry for Monitoring Corrosive Impurities in Molten Chlorides for Thermal Energy Storage

Abstract

Molten chlorides are promising alternative thermal energy storage (TES) materials to be applied in concentrating solar power (CSP) plants. Their high thermal stability makes them appropriate candidates to replace the commercial TES materials in CSP, nitrate salts (stable up to 550 °C), when higher operating temperatures (e.g., 700 °C) are required for higher efficiency of energy conversion. However, the application of molten chlorides at higher temperatures causes increased corrosiveness of containers and structural materials. In this work, cyclic voltammetry (CV) is applied to measure the concentration of corrosive oxide/hydroxide impurities in the molten chlorides in-situ, in order to assist the corrosion control. Before the electrochemical experiments, pre-electrolysis (PE) was performed on 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 (AC) measurements on the simultaneously collected salt samples, the parameter for the calculation of the concentration of MgOH+ from the peak current density was determined under the conditions 500 °C and sweep rate of 200 mV/s. By adding NaOH pellets into the molten salts, a significant change of a peak was observed in the cyclic voltammograms due to the concentration change of the most stable hydroxide species MgOH+. Moreover, when the determined parameter was used in the calculation of the concentrations of MgOH+ from the peak current densities, the calculated concentrations of MgOH+ compare well with those from AC.