In situ X-ray diffraction analysis of electrochemical Dy–Ni alloying in molten LiCl–KCl

In situ energy-dispersive X-ray diffraction was employed to investigate electrochemical Dy–Ni alloying in molten LiCl–KCl at 723 K. This enabled the determination of the crystal structures and orientations of the products formed during molten salt electrolysis, while also eliminating the effects of the post-electrolysis processes (such as cooling and washing) that are necessary in ex situ measurements. The diffraction peak intensities of DyNi2 generally increased during potentiostatic electrolysis at 0.50 V vs. Li+/Li, whereas the Ni diffraction peak intensities gradually decreased. Furthermore, the intensity of the X-ray fluorescence peak of Dy gradually increased. Thus, DyNi2 could be directly formed from Ni via electrolysis, as no diffraction peaks corresponding to materials other than Ni and DyNi2 were observed. Moreover, (022)-oriented DyNi2 grew preferentially on the (200)-oriented Ni substrate.”

In situ energy-dispersive X-ray diffraction (EDXRD) is an effective technique for phase identification in high-temperature molten salts owing to the relatively short intervals between each measurement (i.e., a few minutes), without sample cooling or washing. Previously, in situ EDXRD has been used to understand the electrochemical reduction of TiO2 [27], NiTiO3 [28], and SiO2 [29] in molten CaCl2, in addition to that of UO2 [30] in molten LiCl–KCl. In a recent study, based on in situ EDXRD of the electrochemical reduction of SiO2 in molten CaCl2 [29], the Ca2SiO4 phase was reported as an intermediate product existing only inside the electrode and not at its edge. Prior to this study, it was impossible to distinguish between Ca–Si–O compounds formed during electrochemical reduction and those formed by exposure to air during the post-electrolysis process. Additionally, electrochemical reduction of SiO2 progressed via Ca2SiO4 when O2− ion diffusion was slow. Thus, in situ EDXRD can reveal the intermediate products associated with reactions within high-temperature molten salts.”

2. Experimental

Fig. 1(a) shows a schematic diagram of the in situ experimental apparatus. Prior to the experiment, 45.0 g of LiCl–KCl (>99 %, Fujifilm Wako Pure Chemical, Osaka, Japan) was placed in a graphite crucible (oval cylinder-shaped, outer diameter (o.d.): 21 × 47 mm, height: 100 mm, 99 %, Toyo Tanso, Osaka, Japan) set inside a glove box under an Ar atmosphere. Pre-electrolysis was conducted to remove any residual water in the LiCl–KCl using an Al cathode and a graphite crucible anode inside the glove box with an electronic furnace for >12 h at 723 K. After pre-electrolysis, the crucible containing the salt was cooled and placed in a closed container inside the glove box to prevent moisture absorption. The study was conducted at the BL28B2 beamline of SPring-8 in Hyogo, Japan. A vertical electric furnace with four windows was designed to enable the penetration of white X-rays, and the electric furnace was mounted on the transition stage. Electrolysis was conducted in a Pyrex® vessel at 723 K under a dry Ar atmosphere. To avoid heat damage to the X-ray detector, the windows were covered with Al foil. The temperature was controlled using a thermostat connected to a chromel–alumel thermocouple, with an alumina tube sheath installed at the bottom of the Pyrex® vessel outside the graphite crucible.

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Fig. 1. (a) Schematic diagram of in situ Dy–Ni alloying and de-alloying in molten LiCl–KCl at 723 K. (b) Schematic diagram of the Ni electrode and beam path. Top: side view (xz plane) and bottom: front view (yz plane).

Fig. 2(a) shows the current–time curve during electrolysis. After ∼ 40 min, the cathodic current density reaches approximately −10 mA cm−2, and the magnitude of the current density gradually decreases until the electrolysis is complete. Similar behavior was observed in a previous study [22], although the fluctuations in this study are slightly larger. These fluctuations are likely due to contamination caused by moisture from the air during the study. The total charge is 246 C, which is 2.24-fold larger than the theoretical charge required to alloy the entire Ni plate with Dy to form DyNi2.

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Fig. 2. (a) Current density–time curve. (b) EDXRD color map of intensity evolution during electrolysis as a function of time at z = 1.88, 1.94, and 2.00 mm for Dy–Ni alloying in molten LiCl–KCl at 723 K. (c) Selected raw EDXRD data at different times at z = 2.00 mm and the background spectrum of the experimental setup. (*) Peaks due to X-ray fluorescence.

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