Development of Anode for Electrolysis of (CH3)4NF·4HF Room-Temperature Molten Fluoride

cation in the melt, and hence it will be a new process that can replace the Simons process, in which trimethylamine, (CH3)3N, dissolved in anhydrous HF is electrolyzed using the Ni anode [2, 3, 4]. However, a previous study revealed that the Ni anode is unfavorable in electrolysis for a long time in the (CH3)4NF 4HF only melt, because an insulating film mainly composed of NiF2 is formed on the anode during electrolysis [5]. To develop a new electrolytic process using the (CH3)4NF·mHF melt in an industrial scale, it is important to improve the Ni sheet anode as that covered with the film having a higher electric conductivity and a lower overvoltage for the fluoride ion discharge reaction. It has been reported that the nickel based composite containing nickel oxide with the plural oxidation states gave a high electronic conductivity to the film and decreased the anode overvoltage in the (CH3)4NF·mHF melt [6]. LiNiO2 and LaNiO3 are considered to be useful film materials that are stable in the (CH3)4NF·mHF melt. The LiNiO2 and the LaNiO3 coated Ni sheet anodes were prepared by a sol-gel coating method. The sol solution was prepared from i-C3H7OLi (or La(CH3COO)3·1.5H2O), polyvinlylpyrrolidone (PVP, Mw = 55000), CH3COOH, and i-C3H7OH. The sol solution was dip-coated on a Ni sheet, and was converted to a gel film by heating at 200oC for 20 minutes. Dip-coating and heat-treatment at 200oC were alternately performed with several times. After all coating processes, the gel film was heat-treated at 700 or 750oC for 2 hours in air. Fig. 1 shows the X-ray diffraction patterns of the samples. The XRD analysis revealed that LiNiO2 and LaNiO3 formed on the Ni substrate with oxides such as NiO and/or La2O3. Fig. 2 shows the variations of the potential of the Ni sheet anode, the LiNiO2 coated Ni sheet anode, and the LaNiO3 coated Ni sheet anode with lapse of time during electrolysis at 20 mA cm in the (CH3)4NF·4HF melt. The potential on Ni anode rose up to 10 V for only 1 hour, whereas those on the LiNiO2 and the LaNiO3 coated Ni sheet anodes were kept at 6.54 and 5.04 V for 100 hours, respectively. These results indicate that LiNiO2 and LaNiO3 may give the electric conductivity to the Ni sheet anode during electrolysis. The compositions of the evolved gas at the LiNiO2 and LaNiO3 coated Ni sheet anodes electrolyzed at 20 mA cm for 100 hours in the (CH3)4NF·4HF melt are shown in Table 1. The anode gas was composed of CF4, NF3, C2F6, CHF3, C2HF5, (CF3)3N, (CF2H)2NCF3, and (CF3)2NCF2H. The main constituents in the anode gas were CF4 and (CF3)3N. The maximum ratio of (CF3)3N obtained was 25.4% when electrolysis using the sol-gel LiNiO2 coated Ni sheet anode prepared by the procedure of dip-coating with five times was carried out at 20 mA cm in the (CH3)4NF·4HF melt for 100 hours. These results suggest that the electrolytic production of (CF3)3N from (CH3)4NF·4HF melt using the LiNiO2 and LaNiO3 coated Ni sheet anodes is a useful process because the electrolytic conductivity of these oxide films on the anode is kept higher during electrolysis.