Investigation of electrodeposition behavior for Nd(III) in [P2225][TFSA] ionic liquid by EQCM methods with elevated temperatures

- The electrodeposition behavior of Nd(III) in [P2225][TFSA] ionic liquid was investigated by EQCM.
- The electrodeposition of Nd(0) metal in [P2225][TFSA] ionic liquid was demonstrated from the CPE/EQCM.
- The EDX and XPS analyses of the electrodeposits were identified as a greater part of metallic neodymium.

The electrodeposition behavior of Nd(III) in an ionic liquid, triethyl-pentyl-phosphonium bis(trifluoromethyl-sulfonyl) amide ([P2225][TFSA]), was investigated on an electrochemical quartz crystal microbalance (EQCM) at elevated temperatures. Cyclic voltammetry with EQCM measurements of Nd(III) in [P2225][TFSA] at 363, 373 and 378 K were performed. A clear cathodic peak with increasing mass and decreasing ηρ (where η is the viscosity and ρ is the density) was observed at −2.79 V. Considering the results of our previous investigations, this peak indicated the reduction of Nd(III) to Nd(0) as [Nd(III)(TFSA)5]2− + 3e− → Nd(0) + 5[TFSA]−. In addition, the apparent molar mass, Mapp, between −2.49 and −2.94 V calculated from the mass change was 140.4 g mol−1, which was close to the theoretical value for the electrodeposition reaction of Nd(III)/Nd(0), 144.2 g mol−1.Moreover, controlled potential electrolysis with EQCM (CPE/EQCM) measurements at −3.20 V was performed for 0.05 and 0.10 M Nd(III) in [P2225][TFSA] at 373 K. The calculated Mapp values in 0.05 and 0.10 M Nd(III) in the initial stage of CPE/EQCM analysis were 147.3 and 148.8 g mol−1, respectively, which were close to the theoretical value for reduction of Nd(III) to Nd(0), 144.2 g mol−1; thus, electrodeposition of Nd(0) metal was also proved by the CPE/EQCM analysis. Furthermore, energy-dispersive X-ray and X-ray photoelectron spectroscopy analyses of the blackish electrodeposits obtained by the CPE/EQCM measurements were identified as primarily metallic neodymium because the Nd 3d5/2 spectrum was detected within 980.5-981.5 eV.