Study of Deuterium Isotope Separation By Anion Exchange Membranes Water Electrolysis

1. Introduction Hydrogen energy is one of solutions for decarbonized society. Water electrolysis is promising as hydrogen production. Currently, water electrolysis with a solid polymer electrolyte membrane has been developed. The electrolysis using a cation-exchange membrane (PEM) has been widely used, while one using an anion-exchange membrane (AEM) is studied as a next-generation method with nonprecious catalysts. Hydrogen isotope separation is applied as a new application of water electrolysis. The nuclear fusion reactor ITER, which uses deuterium and tritium as fuel, will be operated around 2035 when the isotope separation/enrichment technology will be required. We have studied deuterium (D) separation by both PEM- and AEM-type. The separation ability of PEM electrolysis remarkably depended on the ratio of water supplied to the current density (stoichiometric ratio: λ) [1]. Interestingly, AEM one was better than PEM [2]. In this study, AEM electrolysis was investigated at various λ in order to improve the separation performance. 2. Experimental The electrolyte was 0.1 M KOH with 10 at% D. The D concentration was adjusted by D2O. The electrolyte was supplied from the lower part of only the anode side. The ready-made AEM (A201, Tokuyama) was employed as electrolyte. Pt catalyst was used as the anode catalyst and CuCoOx was used as the cathode one. The electrolysis was conducted at constant current densities or water feed at room temperature. The hydrogen gas was analyzed by a quadrupole mass spectrometer to determine the mass number (m = 2 (H2), 3 (HD), 4 (D2)), while the solution discharged from the anode was investigated by FTIR using the attenuated total reflection (ATR) method with a diamond prism. 3. Results and discussion Figure 1 shows the D content in hydrogen gas evolving from the cathode. In the figure, αgas indicates the separation factor, which is the ratio of protium (H) atomic concentration of the gas to the supplied water. The value of αgas decreased with decreasing λ. The present result suggests H dilution in the gas as well as PEM-type. It should be noted that AEM one could be electrolyzed and keep draining the electrolyte from the anode even at λ ≤ 4, while PEM-type discharged the almost all volume of the electrolyte from the cathode side [1]. This might be explained by the mass transportation by the diffusion and electroosmosis whose flows direction depend on the membrane type. In our presentation, the other results such as D concentration in electrolyte and pH variation depending on λ will be introduced. We will discuss the D separation mechanism by AEM electrolysis from the viewpoints of the water distribution and the mass transportation in the membrane. References K. Harada et al., Int. J. Hydrog. Energy, 45 (2020) 31389-31395. .H. Matsushima et al., Int. J. Electrochemical Society, 166 (10) F566-F568 (2019) Figure 1