Vanadium has been widely used in making high strength steels for construction industries due to its properties such as toughness and fatigue resistance, superior energy storage. While previous studies with respect to the redox mechanisms of vanadium in alkaline solutions are relatively limited due to the high negative potentials of the electro-reduction reactions of vanadium compounds at most electrode surfaces. So the redox mechanisms are still not fully understood and the reduction product is yet to be clearly identified.
In this regard, the electrochemical behavior of vanadate ions in alkali hydroxide solutions is to be systemically studied using cyclic voltammetry method. Researchers with Institute of Process Engineering (IPE) studied the redox behavior of vanadate ions in alkaline solutions with NaOH concentrationranging from 0.1 M to 1 M.and proposed possible redox mechanisms.
In the electrochemical experiments all regents including NaOH, NaVO3, KOH, KVO3, LiOH, and LiVO3 were of p. a. grade, and were used as received without further purifications. All solutions were prepared using Millipore Milli-Q water (18 MΩcm). Before each experiment, the test solutions were degassed with ultra-pure nitrogen and then maintained under a nitrogen atmosphere.
In this work, the concentration of OH- was controlled to be in the range of 0.1–1 M, corresponding to pH 13 to 14. Thus, vanadate ion, VO43-, was considered to be the only active vanadium specie in electrochemical reduction. In the chemical titration analysis, most of regents were of analytical grade.
Experiments were conducted in a standard three-compartment electrochemical cell with reactions controlled using an electrochemical workstation (Solartron Modulab). A glassy carbon electrode with an apparent area of 0.203 cm2 was used as the working electrode. A large-area Pt plate (10 mm×10 mm×0.2 mm) was served as the counter electrode.
The electrochemical reactions of vanadate ion were investigated using glassy carbon working electrode in alkaline solutions. The cyclic voltammetry results revealed that vanadate ions were irreversibly reduced to V(OH)2 via a three-electron transfer reaction, and the first electron transfer reaction was the rate-determining step. The oxidation processes involved the stepwise oxidation of V(OH)2 to V(OH)3, and V(OH)3 to vanadate ions.
The reduction kinetics was discussed and the diffusion coefficient of vanadate ion was calculated to be 1.40×10-9m2/s. Chemical and XPS analysis results suggested that due to the instability of the direct reduction product V(OH)2, the obtained deposits after reducing electrolysis were mixed valence (+3/+4) vanadium compounds with an empirical compound formula of VO1.87·1.38H2O. X-ray diffraction and TGA/DSC analysis results further confirmed that the collected reducing electrolysis products were amorphous hydrated vanadium compounds.
The paper was published in Electrochimica Acta.
