Effect of Polyhydric Alcohol Addition on Electrodeposition of Fe-Al Alloy in AlCl3-NaCl-KCl-FeCl2 Molten Salts
Authors: Yamamoto Hiroaki , Morishita Masao , Isoya Akihiro 
Published: February 2020

Introduction
The thermoelectric material can directly convert the temperature difference into the electrical energy and is a candidate for clean energy source. Iron-aluminum binary alloy is composed of inexpensive common metals and a fundamental system of the Heusler-type iron-based thermoelectric materials, having a characteristic that the sign of Seebeck coefficient changes based on its composition. The usage of thermoelectric materials made of films has advantage to produce lightweight and high-capacity thermoelectric conversion devices, because the devices usually consist of pairs of p- and n-type materials. Thus, the electrodeposition method, which uses simple apparatus and is low cost film forming method, has been investigated to prepare thermoelectric materials in film. In the previous study, the electrodeposition of Fe-Al binary alloy in AlCl3-NaCl-KCl-FeCl2 quaternary molten salts was reported. Fe-Al alloy films having various compositions were obtained by varying the composition of molten salts, particularly the mole ratio of AlCl3 and FeCl2. In this study, the effects of addition of polyhydric alcohol such as ethylene glycol or glycerin in AlCl3-NaCl-KCl-FeCl2 molten salts were investigated to understand the effect on the composition of Fe-Al electrodeposit.
Experimental
The basic composition of molten salts used in this study was 64.0 mol%AlCl3-26.0 mol%NaCl-10.0 mol%KCl in the vicinity of AlCl3-NaCl-KCl ternary eutectic composition. For the electrodeposition of Fe-Al alloy, FeCl2 was used as Fe ion source, and the mole ratios of AlCl3 and FeCl2 in the molten salts were 40 to 1 - 100 to 1. Ethylene glycol (EG) or glycerin (G) was added in 0 - 5.96 mol% in the molten salts.
Electrolysis was conducted under constant current density of 100 Am-2 for 2.5×105 Cm-2 at a temperature of 423 K using a copper cathode and an iron anode. The preparations of molten salts and electrolyses were carried out in an Ar-gas-filled glove box. After rinsing with acetone and water, and drying, the electrodeposits were analyzed by using X-ray diffraction and an electron probe micro analyzer.
Results
The composition of electrodeposit obtained in the 63.59 mol% AlCl3-25.83 mol% NaCl-9.94 mol% KCl-0.64 mol% FeCl2 molten salts without additive, where the mole ratio of AlCl3 and FeCl2 were 100 to 1, was 99.4 mol% Fe-0.6 mol% Al. On the other hand, that of electrodeposits obtained in the 4.84 mol%EG-added and 4.84 mol%G-added molten salts were 31.20 mol% Fe-68.80 mol% Al and 94.59 mol% Fe-5.41 mol% Al, respectively. It was found that Al content in the electrodeposit was increased by EG or G addition to AlCl3-NaCl-KCl-FeCl2 molten salts, particularly the addition of EG was very effective. Then the relationship between the amount of EG addition and the composition of Fe-Al alloy electrodeposit obtained by the galvanostatic electrolysis at 100 Am-2 was investigated. Al content in the electrodeposit increased with the amount of EG addition in the molten salts, and reached the limit at 4.84 mol% EG. The molten salts got gelatinous at 5.96 mol%EG, and were solidified by the addition of EG more than 7.09 mol%. Thus it was found that the addition of 4.84 mol%EG was effective for increasing Al content in the Fe-Al alloy electrodeposit.
The electrodeposition of Fe-Al alloy was investigated in AlCl3-NaCl-KCl-FeCl2-EG molten salts, where the mole ratio of AlCl3 and FeCl2 are 40 to 1 - 100 to 1 and EG concentration was 4.84 mol%. Al content in the electrodeposit increased as the FeCl2 concentration in the molten salts decreased. Particularly, Al content increased significantly, when the FeCl2 concentrations in the molten salts were lower than 1.2 mol%. Within the limit of experiments, Fe-Al alloy films containing 2.6 - 68.8 mol% Al were obtained by varying the mole ratio of AlCl3 and FeCl2 (FeCl2 concentration) in molten salts. In the molten salts without EG addition, the Al contents in the electrodeposits were less than 1.0 mol% and the FeCl2 concentration was 0.6 mol% or lower where Al content in the electrodeposit increased significantly. It was clarified that EG addition enabled the control of the composition for Fe-Al alloy obtained in the AlCl3-NaCl-KCl-FeCl2 molten salts in which the co-electrodeposition of Al was difficult.
From the cyclic voltammograms measured in the molten salts with and without EG, both cathodic currents derived from the reductions of iron and aluminum ionic species were decreased. Since the electrodeposition potential of Fe is much nobler that that of Al, it was considered that the decrease of Fe electrodeposit due to the EG addition led the relative increase of Al in the electrodeposit.