Electrochemical Mechanism of Tellurium Reduction in Alkaline Medium

Tingjun Wu; Jiwon Kim; Nosang V Myung

doi:10.3389/fchem.2020.00084

Electrochemical Mechanism of Tellurium Reduction in Alkaline Medium

Front Chem. 2020 Mar 3:8:84. doi: 10.3389/fchem.2020.00084. eCollection 2020.

Authors

Tingjun Wu^{1

2}, Jiwon Kim^{1

3}, Nosang V Myung¹

Affiliations

¹ Department of Chemical and Environmental Engineering, University of California, Riverside, Riverside, CA, United States.
² Laboratory of Advanced Functional Materials, Xiamen Institute of Rare-earth Materials, Haixi Institute, Chinese Academy of Sciences, Xiamen, China.
³ Materials Science and Chemical Engineering Center, Institute for Advanced Engineering, Yongin-si, South Korea.

Abstract

A systematic electrochemical study was conducted to investigate the reduction of tellurium (Te) in alkaline solutions. The effect of various parameters, including tellurite ion concentration, applied potential, and pH was investigated by both linear sweep voltammograms (LSVs) and electrochemical quartz crystal microbalance (EQCM). EQCM was essential to understand the reduction of Te(0) to soluble ${Te}_{2}^{2 -}$ (-I) or Te^2-(-II). The Tafel slopes for two Te reduction reactions [i.e., Te(IV) to Te(0) and Te(0) to Te(-I)] indicated that the electrochemical reduction of Te is strongly dependent on solution pH, whereas it is independent of the concentration of ${TeO}_{3}^{2 -}$ . At relatively weaker alkaline solutions (i.e., pH ≤ 12.5), the discharge of ${Te (OH)}_{3}^{+}$ was determined to be the rate-limiting step during the reduction of Te(IV) to Te(0). For the reduction of Te(0) to Te(-I), the reaction follows a four-step reaction, which consisted of two discharge and two electrochemical reactions. The second discharge reaction was the rate-limiting step when pH ≤12.5 with the Tafel slope of 120 mV/decade. At a higher pH of 14.7, the Tafel slope was shifted to be 40 mV/decade, which indicated that the rate-limiting step was altered to the second electrochemical reaction. Te(0) deposits were found either on the surface of an electrode or in the solution depending on pH due to the different rate-limiting reactions, revealing that pH was a key parameter to dictate the morphology of the Te(0) deposits in alkaline media.

Keywords: EQCM; LSV; Tafel slopes; alkaline bath; electrochemical kinetics; tellurium reduction.