We present a study on the electrochemical behavior of magnesium hydroxide (MgOH+) reduction on a tungsten (W) cathode in molten chloride salt (MgCl2-KCl-NaCl) across the temperature range of 475-525℃. MgOH+, which forms within the salt upon exposure to moisture, is a leading cause of corrosion. Corrosion is a major barrier to deployment of chloride salts across a number of applications, including concentrating solar power plants and nuclear power plants. While pre-purification protocols have been developed to ensure MgOH+ is removed from molten chloride salts prior to deployment, MgOH+ forms in-situ during operation of chloride-salt based plants. Thus, methods for continuous purification during plant operation are needed. Continuous electrochemical purification via electrolysis using a Mg anode and W cathode has been proposed, but little has been done to assess scalability. Here, we assess fundamental properties of electrochemical removal of MgOH+ to enable future scale up of this method.

Research on molten chloride salts as a high-temperature (500-800 ℃) heat transfer fluid has been reinvigorated in recent years. Its abundance, low cost, and good thermohydraulic properties have drawn attention from the nuclear and concentrated solar power industry. Specifically, a ternary MgKNa chloride salt (45.98/38.91/15.11 wt.%) has been considered a prime candidate for commercial application. However, economic risk remains due to high corrosion of containment alloys. The rate of corrosion in ternary MgKNa chloride salt has been primarily attributed to the impurity MgOH+, stemming from the reaction of MgCl2 with moisture. Extensive research has been conducted to purify chloride salts, but little attention has been given to on-line purification strategies once the salt is molten and circulating in the plant. One proposed strategy is continuous electrochemical purification via electrolysis using a Mg anode and W cathode. This strategy has been successfully demonstrated at the laboratory scale. Work done in our group indicate significant opportunity for the electrochemical process to be scaled up commercially as an electrochemical purification unit. Here, we present a study on the electrochemical behavior of MgOH+ reduction on a W cathode in molten MgCl2-KCl-NaCl across the temperature range of 475-525 ℃. The kinetic parameters from this study aid in modeling and scaling up an electrochemical purification cell at industrially relevant scales.