Electrochemical measurement of corrosive impurities in molten chlorides for thermal energy storage

Abstract

Molten chlorides are promising alternative thermal energy storage (TES) materials to be applied in concentrated solar power (CSP) plants owing to their higher thermal stability (stable at >800 °C) than the commercial TES materials – nitrate salt mixtures (decomposed at ∼550 °C). Higher operating temperatures of TES can increase efficiencies of thermal into electrical energy conversion for CSP power plants, but cause additional challenges, particularly increase corrosiveness of metal containers and structural materials. Corrosion rates significantly depend on concentration of corrosive hydroxide impurity in the molten chlorides. This paper presents an electrochemical method based on cyclic voltammetry (CV) to in-situ measure the concentration of the hydroxide impurity in molten MgCl2/KCl/NaCl (60/20/20 mol%) salts at 500–700 °C. Before each CV experiment, the concentration of the hydroxide impurity in the molten salts was measured via acid consumption (AC) measurements on the simultaneously collected salt sample. The results of CV and AC experiments show that the peak current densities obtained from CV are proportional to the concentrations of the hydroxide impurity obtained from AC in the studied system. The slopes of peak current densities vs. concentrations of the hydroxide impurity are determined, which compare well with literature data. This electrochemical method for monitoring the corrosive hydroxide impurity is expected to assist the corrosion control on containers and structural materials in the molten chlorides.