Electrochemical reduction mechanism of several oxides of refractory metals in FClNaKmelts

3.1 Dissolution characteristics

Figure 1 showed the dissolution characteristics of ZrO₂ and HfO₂ in a NaCl-KCl-NaF melts. It could be seen from Figure 1 that ZrO₂ and HfO₂ were substantially unchanged in the NaCl-KCl-NaF melts at 700^∘C for 3 h. The XRD results of the upper melts at 700^∘C were NaCl, KCl, NaF and HfO₂. No new material formation indicated that ZrO₂ and HfO₂ did not react chemically in the NaCl-KCl-NaF melts.

Figure 1

Dissolved quantity of ZrO₂ HfO₂ changes over time at 700^∘C (a) ZrO₂ (b) HfO₂

At 700^∘C, Figure 2 was an XRD and laser Raman spectroscopy analysis of MoO₃ in a NaCl-KCl-NaF melts. It couldbe seen from Figure 2(b), molybdenumoxy groups were present at 340 cm⁻¹, 375 cm⁻¹, 472 cm⁻¹, 665 cm⁻¹, 810 cm⁻¹, 844 cm⁻¹, 978 cm⁻¹, 992 cm⁻¹, and 1022 cm⁻¹. The stretching vibration peak was made of MoO₃ or Na₂Mo₂O₇, which was consistent with the results of references [21, 22, 23, 24]. It could be seen from Figure 2(a) that there were four substances of NaCl, KCl, NaF and Na₂Mo₂O₇ in the melts of NaCl-KCl-NaF-MoO₃, but no MoO₃ existed. Combined with the thermodynamic analysis of the melts, the standard Gibbs free energy ΔG^θ < 0 about the reaction of NaCl, NaF and MoO₃ at 700^∘C. The reaction could be spontaneously carried out, and Na₂Mo₂O₇ was formed. Therefore, the stretching vibration peak of the molybdenumoxy group was Na₂Mo₂O₇. The dissolution of MoO₃ in the NaCl-KCl-NaF melts was mainly chemical dissolution.

Figure 2

Solubility characteristics of MoO₃ in NaCl-KCl-NaF melts at 700^∘C (a) ΔG^θ and XRD (b) Raman spectrum

3.2 Electrochemical experiment and electrodeposition producted analysis

Figure 3 was a cyclic voltammetry curve of ZrO₂, HfO₂ and MoO₃ in NaCl-KCl-NaF melts at 700^∘C. Figure 3 (1) was a cyclic voltammetry curve of NaCl-KCl-NaF melts. When the scanning range was −2 V~0.3 V, an oxidation peak appears at −1.65 V during forward scanning, which corresponded to the oxidation process of sodium. As the potential changed, an increased in current occurs at 0.6 V, which corresponded to the precipitation of gas. In the reverse scan, the reduction peak that occurredat −2.0 V was the sodium reduction process. Sodium reduction peak at −2.0 V during reverse scan. Figure 3(2) showed the cyclic voltammetry curve of ZrO₂ in NaCl-KCl-NaF melts. A pair of redox peaks a’/a (−0.82 V/−0.96 V) appeared which corresponding to the redox process of zirconium. Figure 3(3) showed the cyclic voltammetry curve of HfO₂ in NaCl-KCl-NaF melt. A pair of redox peaks b’/b (−0.76V/−1.14V) appeared which corresponding to the redox process of hydrazine. Figure 3(4) showed the cyclic voltammetry curve of MoO₃ in NaCl-KCl-NaF melts. Three pairs of redox peaks c’/c (−0.17 V/−0.55 V), d’/d (−0.40 V/−0.91 V) and e’/e (−0.99 V/−1.27 V) appeared in the −2 V~0.7 V scan range, corresponded to a multi-step redox process of molybdenum.

Figure 3

Typical cyclic voltammogram curves in NaCl-KCl-NaF melt (scan rate: 300 mV/s)

Table 1 showed the relationship between theoretical electrolytic potentials E^θ and T of ZrO₂ and HfO₂ at 700^∘C calculated by HSC 6.0 thermodynamics software. The more positive theoretical electrolysis potential occurred, the more reduction reaction happened. The ionic state of Hf was Hf⁴⁺ and Hf²⁺. The Hf stepwise reactions were Hf⁴⁺/Hf²⁺ (−4.80V) and Hf²⁺/Hf (−0.05V), respectively. The potential of Hf⁴⁺ reduced to Hf (−2.43V) was more positive than the potential of Hf⁴⁺ reduced to Hf²⁺ (−4.80V), indicating that Hf⁴⁺ was reduced by a one-step reduction process, that consistent with the cyclic voltammetry results of Hf showed in Figure 3(1). The ionic state of Zr was Zr⁴⁺, and the reduction of Zr⁴⁺ to zirconium metal was a one-step reaction process, which was consistent with the cyclic voltammetry curve of Zr shown in Figure 3(2).

From the results of Figure 2, it showed that the dissolution of MoO₃ in the NaCl-KCl-NaF meltswas chemically dissolved, and the product was Na₂Mo₂O₇. The reaction of Na₂Mo₂O₇ from Mo⁶⁺ to metallic molybdenum was the formula (1)~(6). Figure 4 was a graph showing the relationship between E^θ and T in reactions (1) to (6) at 500 to 1000 ^∘C. At 700 ^∘C, the potentials of Mo⁶⁺/Mo⁴⁺ corresponding reactions (1) and (2) are −0.62V and −1.68V. The theoretical electrolysis potential of reaction (1) was more positive than reaction (2), so reaction (1) was more priority. And Mo2O72−was decomposed into MoO42−and Mo⁴⁺. The theoretical electrolysis potential of the one-step reduction (Mo⁶⁺/Mo) corresponding to reaction (3) was −1.27 V, which was more negative than the theoretical electrolysis potential of Mo⁶⁺/Mo⁴⁺ corresponding to reaction (1), and the process of reduction of Mo⁶⁺ to metallic molybdenum was a stepwise reaction. The reaction of MoO²₄⁻ reduced to metallic molybdenum and MoO₂ was (4) and (5). The theoretical electrolytic potential of reaction (5) was −2.12 V, which was more negative than −1.68V of reaction (4). Therefore, MoO42−was first reduced to metallic molybdenum. The equilibrium electrode potential of the reaction (6) was Mo⁴⁺/Mo(−1.06V). As shown in Figure 3 (3), the results of cyclic voltammetry of Mo showed that the reduction process of molybdenum was Mo2O72−→MoO42−/Mo4+→Mo.

Figure 4

Relationship between the theoretical electrolysis potential Eθ and T of MoO₃ at 500~1000^∘C

Cathodic reaction:

Figure 5(a) were cyclic voltammetry curves at different sweep speeds. The peak current gradually increased with the scanning speed increases,which was because the higher the scanning speed, the higher the electrochemical reaction speeds. The relationship between i_pc and v^1/2 was derived from the data in Figure 5(a). And i_pc had a linear relationship with v^1/2, and the reduction peak potential E_pc did not change with the change of the scanning speed. As could be seen from Figure 5(a), i_pa/i_pc > 1. Therefore, the electrode reaction corresponding to the reduction peak on the cyclic voltammetry curve was a reversible reaction and the product was insoluble, that was, the reduction process of the cathode was completed under diffusion control. The relationship between the reversible reaction E_pc and n satisfied the equation:

Figure 5

Cyclic voltammetry curve of NaCl-KCl-Na-ZrO₂ at different scanning speeds (a) Cyclic voltammetry curve (b) i_pc~ v^1/2 & E_pc ~v^1/2 (rate scan: 300-500mV/s, vs. Pt)

Where E_pc (V) is the peak potential and E_pc/2 (V)is the half-peak potential; n is the electron transfer number and F (96485 C·mol⁻¹) is a Faraday’s constant; R (8.314 J·mol⁻¹·K⁻¹) is the molar gas constant, T(K) is the temperature.

The average value of the number n of reaction electrons corresponding to the reduction peak in Figure 5(a) was calculated to be about 4. Therefore, it could be concluded that the Zr⁴⁺ ion in the meltswasreduced to elemental zirconium in one step, so the cathode reaction on the CV curve was: Zr⁴⁺ + 4e=Zr.

4h constant potential electrolysis was carried out at −1.15V to obtain a material having a metallic luster on the surface, and the energy spectrum analysis product was mainly zirconium. It was indicated that the reduction peak appearing at a potential of −0.935V corresponded to the precipitation process of Zr.

Figure 7(a) showed the cyclic voltammetry curves of NaCl-KCl-NaF-HfO₂ at different sweep speeds, and Figure 7(b) showed the i_pc and v^1/2 curves obtained from the data in Figure 7(a). It could be seen from Figure 7(b) that i_pc had a linear relationship with v^1/2, and the reduction peak potential E_pc did not change with the change of the scanning speed. i_pa/i_pc > 1 was showed in Figure 7(a), therefore, the electrode reaction corresponding to the reduction peak of the cyclic voltammetry curve was reversible and the product was insoluble, that was, the reduction process of the cathode was completed under diffusion control. The calculated value of the number n of reaction electrons corresponding to the reduction peak in Figure 7(a) was about 4. Therefore, the reduction process of HfO₂ in NaCl-KCl-NaF meltswas a one-step electronic reaction, Hf ⁴⁺+4e = Hf.

Figure 6

EDS of Zr electrodeposited layer

Figure 7

Cyclic voltammetry curve of NaCl-KCl-NaF-HfO₂ at different scanning speeds (a) Cyclic voltammetry curve (b) i_pc~ v^1/2 & E_pc ~ v^1/2 curve (rate scan: 300~600 mV/s, vs. Pt)

The potentiostatic electrolysis was carried out under the conditions of −1.3 V and 4 h, and the surface energy spectrum of the electrolysis product was analyzed. The results were shown in Figure 8. From the energy spectrum, the main substance of the deposited layer was Hf.

Figure 8

EDS and XRD of Hf electrodeposited layer

Figure 9 showed the cyclic voltammetry curve of NaCl-KCl-Na-MoO₃ as the scanning speed changes. As could be seen from Figure 9(a), as the scanning speed increased, the peak current density gradually increased because the electrochemical reaction rate was affected by the scanning speed. Using the data in Figure 9(a), the relationship between i_pc~ v^1/2 and E_pc ~ v^1/2 was obtained, as shown in Figure 9(b). The i_pc of peak c was linear with the change of v^1/2, and the potential of peak c’/c moved to the positive and negative directions of the potential with the increase of scanning speed. The scanning speed increased, and the peak c’ gradually disappeared and changed towards irreversible. Therefore, the number of electrons lost could be calculated from the irreversible reaction. As the scanning speed v increased, the E_pa and E_pc of the peak d’/d moved to the positive and negative directions respectively, and i_pc and v^1/2 did not have a linear relationship. Therefore, the peak d was a quasi-reversible reaction process. The potential E_pc of the peak e moved in the negative direction of the potential with the increase of the scanning speed v, i_pc was linear with v^1/2, and there was no corresponding obvious oxidation peak, so the peak e was an irreversible reaction process.

Figure 9

Cyclic voltammetry curves of different scanning speeds of NaCl-KCl-Na-MoO₃ (a) Cyclic voltammetry curve (b) i_pc~ v^1/2 & E_pc ~ v^1/2 curve (rate scan: 200-500mV/s, vs Pt)

According to the values of current density i_p and potential E_pc corresponding to each scanning rate peak c in Figure 9(a), a log i_p vs. E_pc was obtained in Figure 10.

Figure 10

lnv vs. E_p

As the scanning speed increased, the cathode peak potential E_pc gradually shifts to the negative direction and had a linear relationship with the sweep speed lnv, as shown in Figure 10. For the irreversible process E_p~lnv satisfied the following formula [25, 26]:

Where α is transfer coefficient, k⁰ is standard rate constant of the reaction, n is electron transfer number involved in the rate-determining step, v is san rate, E^0′ is formal potential, Epc is the reduction peak potential, E_pa is the oxidation peak potential (T=973K, R=8.314 J·mol⁻¹·K⁻¹, F=96485 C·mol⁻¹)

According to the above equation and the slope of the linear relationship of E_p ~lnv in Figure 10, the symmetry factor α was calculated as 0.2685. n was calculated as 2.23 ≈ 2. Therefore, the peak c corresponds to the 2 electron reduction process. It was proved that the reaction (1) corresponds to a 2-electron process in which Mo²O²₇⁻ was reduced to MoO₂. From the nonlinear relationship of the peak d in Figure 9(b), the number of electrons to be lost could not be calculated by the formula. However, molybdenum in MoO₂ was tetravalent, and therefore, the reaction corresponding to peak d was a 4 electron reduction process of Mo^{4+ →} Mo.

Using graphite as anode and low carbon steel as cathode, the temperature was set to 700^∘C, the molar ratio of melts system was NaCl: KCl: NaF = 1:1: 0.86, MoO₃ (mass fraction is 20%), and the mixture was evenly mixed. In pure corundum, the current density was 100mA·cm⁻² and the electrodeposition time was 60 min. After deposition, the surface of the substrate was loose and porous. From the XRD results, it was found that metal molybdenum was formed after electrolysis.

Figure 11

XRD of molybdenum electrodeposited layer