Chronopotentiometry at platinum electrode in KF-NaF-AlF3-Al2O3 melt*

Some features of the mechanism of the anode process on platinum in KF–NaF–AlF3–Al2O3 melt at 750–780 °C depending on the of anodic current density (0.5 mA/cm2 to 2.0 A/cm2) and anodic pulse duration have been studied using chronopotentiometry method. In curves of change in the platinum anode potential a small peak at current densities of 10–30 mA/cm2 and a clear peak at current densities of 0.5–2.0 A/cm2 are recorded when the current is cut on. Analysis of dependencies of the transition time on the current density indicates that the first peak in curve is associated with the formation of an oxide compound on the platinum surface, and the second one is related to hindering the diffusion for delivery of electroactive particles to its surface.

At the present time research is directed to the development of physicochemical bases of technology for production of aluminum by electrolysis of KF-NaF-AlF 3 -Al 2 O 3 melt at 750-850 °C using non-consumable anodes conducting in the world [1][2][3]. In order to avoid a rapid corrosion of the anodes it is necessary to select both its formulation and a method of manufacturing, and optimal electrolysis modes. The latter implies the establishment of regularities of kinetics of oxygen release at the anode using electrochemical methods of analysis. Platinum can serve as an experimental model of a non-consumable anode.
Earlier [4] a comparative analysis of experimental and modeling stationary polarization curves for platinum in KF-NaF-AlF 3 -Al 2 O 3 melt at 725-800 °C has been conducted.
It has been shown that the primary charge transfer to form O ads particles adsorbed on platinum is accompanied by DOI: 10.15826/chimtech.2015.2.3.020 № 3 | 2015 Chimica Techno Acta their desorption by physical or electrochemical mechanism. A more complete description of the mechanism features of the process under study can be achieved as the result of using the complex of electrochemical analysis methods, including non-stationary ones.
The present work is devoted to the experimental study of non-stationary anode process on platinum by chronopotentiometry in KF-NaF-AlF 3 -Al 2 O 3 melt at 750-780 °C.

Experiment
Experiments were performed in alundum crucible [4], which was placed in a quartz cell purged with argon or oxygen. The auxiliary nichrome electrode was placed in a porous alundum tube that serves as a diaphragm. As the working electrode platinum purity of 99.9926 % (JSC "Revda Non-Ferrous Metals Processing Works") was used. As the reference electrode a gas carbon electrode [5] in the equilibrium mixture of CO and CO 2 served. Measurements were carried out in the melt (wt. %): 39.6KF-7.1NaF-47.3AlF 3 -6.0Al 2 O 3 at temperature 750-780 °C.
Chronopotentiograms were obtained at anodic current densities from 0.05 mA/cm 2 to 2.0 A/ cm 2 with a sampling frequency up to 750 kHz using PG-STAT AutoLab 320N and software NOVA 1.10 (Eco Chemie, the Netherlands). To calculate the anode overvoltage using the procedure «I-Interrupt» ohmic resistance in the section of polarizing circuit included in the measuring circuit were determined. Figure 1 shows typical curves of change in the potential of the platinum anode obtained on platinum in KF-NaF-AlF 3 -Al 2 O 3 melt at 750-780 °C when cutting on and cutting off the anode current. A peak in cut on curve is observed at low current densities (up to ≈30 mA/cm 2 ). Increasing the anode current density from 0.5 to 30 mA/cm 2 results in shifting of a peak potential from ≈ 0.44 to 0.58 V in respect of the carbon electrode and reducing the transition time from 1.1 to 0.005 s. In our view, electrochemical platinum oxidation accompanied by filling its surface by oxidation product can act as the process in said section. Despite the instability of platinum oxide at temperatures above 560 °C [6] electrochemical formation of platinum oxide was observed in a cryolite-alumina melt at 1000 °C [7]. The diffusive character of the dependence of the transition time on the current density at that may be caused by limiting oxygen delivery to the platinum through the oxidized layer (or the growth rate of the oxide film).

Results and discussion
Increasing the length of the anode current pulses of value up to ≈30 mA/cm 2 (Fig.  1a), does not give rise to the second peak in the cut on curve associated with hindering the diffusion for delivery of electroactive oxygen-containing Al-O-F component from the melt content to the layer near the anode. Its expression is observed for the system under study with an increase in the magnitude of anode current density from 0.5 A/cm 2 (Fig. 1b), at that the transition times are from 2 ms to 10-15 s. Analysis of curves of change in the platinum potential under current interruption points indicates a long period of term relaxation of the platinum potential as after the anode current density pulse of value of 30 mA/cm 2 (30-35 s), so after pulses of value of 0.5-2.0 A/cm 2 (10-15 s).
Data from curves of change in the platinum potential for small (1, 2) and high (3) anodic current density values were built in the coordinates it 1/2 to i (Fig. 2). Enhancement of it 1/2 (1) with increasing i for small values of anode current densities indicates the electrode process (platinum oxidation) followed by slow desorption of products [8], whereas at high anodic current densities constancy of it 1/2 value (3) is observed, which points at slow diffusion of electroactive particles to the electrode surface. The value of the diffusion coefficient of the electroactive particles for high anode current densities at 750-780 °C estimated from the Sanda equation [8] amounted to 0.7-1.5 × 10 -5 cm 2 /s. The presence of the electrochemical oxidation of platinum is supported by the following: -the estimated value of the electric double layer capacity (C d ) in the initial section of curves of change in the platinum potential (times to ≈ 5 × 10 -4 s) when the current is cut on according to expression C d = i/(dη/dt) [1] was 400-560 uF/ cm 2 , which is characteristic of the formation of several oxide compound layers; -dependence of it 3/2 on i (Fig.2, 2) is linear at low current densities, that points at the growth of the oxide film resulting from the electrochemical process [7].

Conclusion
Nonstationary anodic process on platinum in KF-NaF-AlF 3 -Al 2 O 3 melt at 750-780 °C depending on the duration and magnitude of anode current density (from 0.5 mA/cm 2 to 2.0 A/cm 2 ) have been studied using chronopotentiometry method.
It is shown that when the anodic current densities are from 0.5 A/cm 2 the process under study is limited by diffusion of the electroactive particles to the surface of the platinum anode, while at low current densities preceding stage is recorded.
Analysis of dependencies of it 1/2 on i from curves of changes of the potential platinum when anode current is cut on according to equations describing the slow diffusion of electroactive particles and the growth of the oxide film on the electrode surface indicates the electrochemical oxidation of platinum. For high current densities values of the diffusion coefficient of electroactive particles to the platinum anode and the capacity of the electrical double layer were estimated, which amounted to, respectively, 0.7-1.5 × 10 -5 cm 2 /s and 400-560 uF/cm 2 .
The new obtained data can be used to specify the scheme of the process un-der study and create a theoretical model enabling estimation of the effect of the parameters of the anodic process on platinum and other oxygen-releasing electrodes in KF-NaF-AlF 3 -Al 2 O 3 melt and other oxide-fluoride melts in a wide temperature range.