The aim of the present investigation was the study of the Ti(IV)/Ti(III) redox couple electrochemical behavior in the CsCl-CsF(10 wt.%)-K₂TiF₆ melt and an estimation of alkaline earth metal cations (Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺) influence on the charge transfer kinetics for this redox couple.

The electrochemical behavior of titanium in (NaCl–KCl)_equimol.-NaF(10 wt %)–K₂TiF₆ and KCl–KF(10 wt %)–K₂TiF₆ melts with addition of alkaline earth metal cations have been studied in our work [1].

The cyclic voltammetry method was used for electrochemical research. Measurements were carried out at the temperature range of 973-1173 K using a VoltaLab 40 dynamic electrochemical laboratory (VoltaMaster 4, version 6). Voltammetric curves were recorded on a glassy carbon electrode (brand SU-2000) vs. a glassy carbon quasi-reference electrode. The melt container was a glassy carbon crucible, which also served as an auxiliary electrode. The sweep rate (v) varied from 0.1 to 2.0 V s^-1. According to cyclic voltammetry diagnostic criteria the electrochemical redox process Ti(IV) + e^- ↔ Ti(III) was classified as a quasi-reversible at a sweep rate 0.75 V s^-1≤ν≤2.0 V s^-1.

The standard rate constants of charge transfer were calculated based on the Nicholson’s method. The values of k_s increase with increasing temperature. The temperature dependence of k_s is described by the empirical equation: log k_s = (6.39±0.90)–(4600±910)/Т. The activation energy of charge transfer calculated from this equation was found equal to (88.08±20) kJ mol^-1. The values of k_s are lower, and the activation energy is significantly higher than in (NaCl-KCl)_equimol.-NaF(10 wt.%)-K₂TiF₆ and KCl-KF(10 wt.%)-K₂TiF₆ melts [1]. The obtained results are in agreement with the theory of the elementary charge transfer act. According to this theory, the discharge of stronger complexes requires more reorganization energy, and the charge transfer proceeds at a lower rate.

As for the previously studied melts, it was found that an addition of alkaline earth metal cations (Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺) leads to the growth of the standard rate constants of charge transfer up to the certain mole ratio of Me²⁺/Ti(IV), which inversely proportional to the ionic potential of cations. However, a further concentration growth of alkaline earth metal cations caused a decrease of k_s. In our opinion, this is connected with the change of double electrical layer structure or with increasing of melt viscosity due to addition of the alkaline earth metal salts to the initial melt.

The linear dependence of the maximum values of k_s on ionic potential of alkaline earth metal cations was established. The temperature dependences for the k_s in melts containing strongly polarizing cations were obtained. The calculated activation energies of charge transfer in the CsCl-CsF(10 wt.%)-K₂TiF₆ melt with additions of alkaline earth metal cations are considerably less than activation energy for the initial system.

It was determined that values of k_s decreased while the activation energies increased in melts with addition of alkaline earth metal cations in transition from (NaCl-KCl)_equimol.-NaF(10 wt.%)-K₂TiF₆ melt to KCl-KF(10 wt.%)-K₂TiF₆ and CsCl-CsF(10 wt.%)-K₂TiF₆ melts [1].

[1] D.A. Vetrova, S.A. Kuznetsov. 2021. J. Electrochem. Soc., 168 (3): 036517.