The electrochemical reduction mechanism of Fe₃O₄ in NaCl-CaCl₂ melts

2.1 Materials and reagents

Pure NaCl (99.95%), CaCl₂ (99.95%), Fe₃O₄ (99.95%), AgCl (99.5%) were dried in a vacuum drying oven at 260 °C for 24 h. High-purity graphite rods (Ф15 × 100 mm) were polished with 400, 1000, and 2000 mesh sandpaper in sequence, and the surface of the graphite was repeatedly washed with deionized water and absolute ethanol. The graphite rods were placed in a vacuum drying oven at 120 °C for 48 h. An Ag/Ag⁺ reference electrode was used for electrochemical experiments. NaCl, CaCl₂, and AgCl with a molar ratio of 48: 48: 4 are filled into a aluminum silicate tube protected by Ar (99.999%). Subsequently, Ag wire (Ф0.5 mm, 99% purity) was placed in the tube after salt melting, and the outer part of the tube was connected with nickel wire.

Two working electrodes are used for electrochemical testing. (1) Metal hole electrode (MCE-Fe₃O₄): The electrochemical test working electrode uses a double-hole molybdenum bar that can be filled with Fe₃O₄. (Purity > 99.9%, the thickness is 0.5 mm, the width is 2 mm, and the length is 50 mm) (Rong et al. 2014). In order to avoid Fe₃O₄ falling off after being immersed in melts, it was sintered in a resistance furnace at 800 °C for 300 min to prepare an MCE electrode. (2) Fe Coated electrode (FCE- Fe₃O₄): Repeatedly immersing the stainless steel wire (Ф0.5 mm) into the ethanol suspension containing Fe₃O₄ powder to prepare an FCE electrode (3 g Fe₃O₄ placed in 5 mL ethanol ultrasonic vibration 15 min) (Tang et al. 2013; Yin et al. 2011).

The steel mold with a diameter of 15 mm was adopted for constant cell voltage electrolysis stainless, and presses Fe₃O₄ powder into a porous cylindrical sheet (8Mpa, 14.8 mm in diameter, 1.4 mm in thickness, 0.8 g in mass, 38% in apparent pore). As shown in Figure 1. It was sintered in a resistance furnace at 800 °C for 300 min, and the test piece was wrapped with 400 mesh stainless steel mesh and connected with a stainless steel iron rod (Ф5 mm). An alumina crucible containing 200 g of equimolar NaCl-CaCl₂ (Ф55 mm × 100 mm, purity 99.5%) was placed in a resistance furnace at a heating rate of 5 °C/min. After the temperature in the furnace was raised to 300 °C and kept for 12 h to remove residual moisture, the temperature was then raised to 800 °C and kept for 2 h to prepare for the experiment. The nickel plate cathode and the graphite rod anode constitute a pre-electrolysis loop, and 2.6 V electrolysis is applied for 12 h to remove some trace water, Zn, Mn and other elements. The above experiments were conducted under an atmosphere protected by Ar (> 99.999%).

Figure 1:

(a) Metal molybdenum strip after warp cutting; (b) MCE-Fe₃O₄ working electrode and cathode before sintering.

2.2 Experimental methods

The CHI660E electrochemical workstation was used for experimental testing. Experimental electrode: working electrodes FCE-Fe₃O₄ and MCE- Fe₃O₄ (WE immersion depth of 2–3 mm), counter electrode graphite (CE immersion depth of 30–35 mm), reference electrode Ag/Ag⁺ (RE immersion depth of 3–4 mm). Cyclic voltammetry (CV) and square wave voltammetry (SWV) experiments were carried out to study the reduction mechanism of Fe₃O₄. The GWINSTEK PSM-3004 DC power supply was used for constant compliance voltage electrolysis, the compliance voltage is controlled between 0.3–3.0 V, the whole experiment process is carried out in an atmosphere filled with argon after vacuuming. When the reduction reaction is completed, the electrode is pumped away from the melts and cooled with the furnace under the protection of argon. The electrolytic product was cleaned in distilled water for 60 min with ultrasonic waves, then dried in a drying cabinet at 110 °C for 120 min. After drying, it was characterized.

Due to the accuracy, purity, oxide film, adsorption of atoms and molecules of the electrode surface, it has a great influence on the electrode potential. The electrode potential can be changed up to 1 V. Therefore, in the electrochemical test, the electrode surface is activated by the method of large scan speed and multi-turn scanning. Figure 2 (a) is the open circuit potential diagram (OCP-t) when the MCE electrode is immersed in the NaCl-CaCl₂ melts interface and flows into Ar gas. When the working electrode contacts the melts interface, the open circuit potential stabilizes. When argon was blown in, there was a slight fluctuation, and then the open circuit potential slowly decreased and finally stabilized at about −0.077 V. We can judge the depth of immersion into the liquid surface through this phenomenon. Figure 2 (b) potential time diagram is a polarization experiment to test the reversibility of the MCE electrode. After the external conductor is discharged by short circuit for 1 s, use an external power supply to charge for 1 s. The working electrode reacted quickly and returned to the open circuit potential value within 3 s. After the open circuit potential is stabilized, use the AC impedance method to measure the total resistance of the melts. Experimental parameters (range and frequency: 105–10 Hz, 121, amplitude 0.025 V, applied voltage is the open circuit potential value). Figure 2 (c) is the Nyquist diagram of the AC impedance under the NaCl-CaCl₂ melts. The impedance at the high frequency end is 1.41 Ω, which can be used as positive feedback compensation in electrochemical experiments.

Figure 2:

Electrochemical performance test of MCE electrode in NaCl-CaCl₂ melts (a) Open circuit potential diagram (OCP-t) (b) MCE electrode reversibility test (c) AC impedance Nyquist diagram.

Figure 3:

SWV at different frequencies (1–50 Hz) pure salt and filling Fe₃O₄ in NaCl-CaCl₂ melts at 800 °C, WE: Mo, RE: Ag/Ag⁺, CE: graphite rod.

2.3 Characterization test

The product and morphology were analyzed by scanning electron microscope JEM-2800F, energy spectrum, X-ray diffraction spectrum (XRD, X-ray 6000 with Cu Kα1 radiation λ = 1.5405 Å, scanning speed 10°/min).

3.1 Square wave voltammetry (SWV) of MCE-Fe₃O₄

Square wave voltammetry is highly sensitive and can effectively suppress the problem of capacitive background currents that occur in cyclic voltammetry at high scan rates. By judging the reversibility of the reaction, the number of transferred electrons of the reaction can be calculated (Weng et al. 2017). A square wave voltammetry test was carried out in a NaCl-CaCl₂ melts at 800 °C with MCE electrodes filled with 1 mg Fe₃O₄ and without filled Fe₃O₄ as working electrodes. The test parameters are as follows: step potential 1 mV, amplitude 30 mV, scanning frequency 1–50 Hz. The results are shown in Figure 3. The reduction peaks R1 and R2 appear near −0.77 V and −1.11 V. As the scanning frequency increases, the peak potentials of the reduction peaks R1 and R2 remain basically unchanged.

Figure 4 (a) is the peak shape fitting of the SWV curve at f = 20 Hz, and W_1/2 is obtained. The reversibility of the reaction is judged by the relationship i ∼ f^1/2 and the change of the peak potential with the frequency in Figure 4 (b).The current density of the reduction peak R1 has a linear relationship with the square root of the frequency, and the number of electron transfers of the reduction peak R1 is calculated by Eq. (1) (Li et al. 2018; Y.K. Wu, Chen, and Wang 2020), n ≈ 1.85. It shows that the peak R1 corresponds to the reduction of 2 electrons obtained by Fe₃O₄ to FeO, and the peak R2 is the reaction of transferring 2 electrons from FeO to Fe. The reaction equation is shown in 2–3.

R (8.314 J·mol⁻¹ K⁻¹) is molar gas constant, T (K) is temperature, n is electron transfer number, F (96485C·mol⁻¹) is Faraday constant.

Figure 4:

(a) Peak shape fitting graph of square wave voltammetry curve (f = 20 Hz) (b) The relationship between the peak current densities of R1 and R2 the square root of frequency.

The anode reaction of graphite is as follows:

The O²⁻ removed from the cathode is discharged at the anode to produce CO₂ (Liu et al. 2018). Part of CO₂ is released, and a part of CO₂ is dissolved into NaCl-CaCl₂ melt to form CO₃²⁻, which combines with Ca²⁺ in melt to produce CaCO₃.

3.2 Cyclic voltammetry (CV) of MCE-Fe₃O₄ and FCE-Fe₃O₄

Figure 5 (a) is the CV of MCE and MCE-Fe₃O₄ electrode with increasing scan rate (0.1–1.0 V·s⁻¹) in NaCl-CaCl₂ melts at 800 °C. The scanning range is −2.4–0.6 V. The CV of the MCE electrode shows an oxidation and reduction peak A’/ A corresponding to Na⁺/Na near −1.9/−2.4 V. In the forward scanning process, the peak current during Na oxidation is lower than that during Na reduction, that is: I_pa < I_pc. This is due to the volatilization of Na in the melts at 800 °C. In the forward scan to around 0.6 V, the peak B’/ B is the redox peak of chlorine gas (Weng et al. 2016). In the range of −2.0–0 V, the background current is less than 0.1 A, there is no obvious redox peak, and it has good electrochemical stability. It can be used as the potential scanning window of MCE-Fe₃O₄. The cyclic voltammetry curve of the MCE- Fe₃O₄ electrode shows that the reduction peak R1, peak R2 and oxidation peak D is appear. It is suggested that the solid Fe₃O₄ is reduced to Fe in two steps at the cathode (Fe₃O₄ → FeO → Fe).

Figure 5:

CVs of different scan rates of (a) MCE- Fe₃O₄ (b) coating FCE-Fe₃O₄ in 800 °C NaCl-CaCl₂ melts.

It can be seen from Figure 5 (a) that the current also increases with the acceleration of the scan rate. When the scan rate increases, the time required to reach the peak potential is shortened, the thickness of the transient diffusion layer becomes thinner, the concentration gradient is larger, the diffusion rate is accelerated, resulting in an increase in peak current. In terms of Fe₃O₄ deoxygenation reduction, the faster scan rate prevents O²⁻ from Fe₃O₄ transferred to the graphite anode in a short time, but remains at the cathode, forming a large concentration gradient and increasing the current. In the reverse scanning process, these O²⁻ participate in the oxidation process of Fe, and a large oxidation peak current appears. When a smaller scan rate of 0.2 V·s⁻¹ is used, the negative potential shift causes the over potential of the reaction to increase, which accelerates the electrochemical reaction rate, the ion concentration gradient at the solid-liquid interface decreases, and the current value decreases.

Figure 5 (b) is the CV of FCE and FCE-Fe₃O₄ electrode with increasing scan rate (0.01–0.05 V·s⁻¹) in NaCl-CaCl₂ melts at 800 °C. The CV curves of the FCE- Fe₃O₄ electrode showed peaks R1 and R2 not found in pure salts near −1.2 V and −1.6 V, respectively, corresponding to oxidation peaks D1 and D2. The reduction process of FCE-Fe₃O₄ at the cathode is similar to the reduction mechanism of the MCE working electrode. The scan rate has little effect on the potential of the peak R1, but the potential of the peak R2 is obviously shifted to the left. This is because the electrochemical reaction rate is slow at a small scan rate, and the ion diffusion rate is relatively fast. With the negative scan of the potential, a reaction with a wider peak shape and a smaller peak current appears. When the scan rate increases, the electrochemical reaction speed is faster. At this time, the ion diffusion rate is small, the reaction peak is sharp, and the peak current is large (Li et al. 2020). The negative shift of peak R2 is also affected by the electrochemical reaction speed of R1. When the reaction speed of R1 is fast, the O²⁻ concentration at the three-phase interface is high, which will hinder the reaction in the second step.

3.3 The effect of voltage on Fe₃O₄ electrolysis products

The reduction mechanism of Fe₃O₄ in the NaCl-CaCl₂ melts was studied, and the Fe₃O₄ sintered at 800 °C was subjected to constant cell voltage electrolysis (0.3–1.0 V). In order to avoid the decomposition of melts, according to the theoretical decomposition pressure, the calculation in Table 1 should be less than 3.2 V. Figure 6 is the XRD pattern of Fe₃O₄ after electrolysis at 600 min in the NaCl-CaCl₂ melts at 800 °C. It can be seen from Figure 5 that after immersing in NaCl-CaCl₂ melts for 600 min, Fe₃O₄ did not undergo a chemical reaction, but when the applied voltage was 0.3–0.4 V, Fe₃O₄ deoxidized and reduced to FeO; after applying 0.5 V for 600 min, it produced little metallic Fe. When the electrolysis voltage is further increased to 0.6 V, the diffraction peak of Fe is more intense, but there is still little FeO in the electrolysis product that has not been reduced. When the electrolysis voltage is 0.8 V, all FeO is reduced to metallic Fe.

Figure 6:

XRD pattern of the electrolysis product of Fe₃O₄ electrolyzed at constant cell voltage at 800 °C of 600 min in NaCl-CaCl₂ melts.

Cyclic voltammetry was used to analyze the behavior of the oxide but a graphite rod was used as the counter electrode during electrolysis. However, science the same graphite rod was used as the counter electrode during cyclic voltammetry, the behavior of the oxide should be consistent with the electrolysis process.

Figure 7 (a) is an SEM image of 0.3 V electrolysis for 600 min. From the figure, it can be seen that FeO has obvious particle boundaries, and the particle size is between 10 and 25 μm. Figure 7 (b–d) are SEM images of electrolysis for 600 min after applying 1.0–3.0 V cell voltage. When the electrolytic voltage of 1.0 V is applied, the size of the Fe particles is uniform, about 5 μm. With the increase of the electrolytic voltage to 1.5 V, the growth rate of reduced metal Fe is greater than the nucleation rate. It is Fe particle of about 10 μm. When an electrolytic voltage of 3.0 V is applied, it appears as more fine Fe particles with a diameter of about 1 μm, because the nucleation rate of reduced Fe is greater than the growth rate when a higher electrolytic voltage is applied. From the above analysis, it can be seen that the choice of electrolysis voltage can effectively control the generation and morphology of the product, and change the particle size and continuity of the Fe (Tang et al. 2016).

Figure 7:

SEM images of Fe₃O₄ sample electrolyzed by constant cell voltage in NaCl-CaCl₂ melts at 800 °C for 600 min(a) 0.3 V (b) 1.0 V (c) 1.5 V (d) 3.0 V (inset: EDS test data).

The electrolysis of Fe₃O₄ samples with constant cell voltage under a two-electrode further illustrates the results obtained by electrochemical tests. With the gradual increase of voltage, the reduction potential of Fe₃O₄ → FeO is first reached. The oxygen at the vertex of the regular octahedron in Fe₃O₄ is ionized and diffused to the interface of Fe₃O₄| NaCl-CaCl₂ (Chen and Fray 2004). When the cell voltage increases to the reduction potential 0.5 V of FeO → Fe, metallic Fe begins to be produced. It is concluded that as the voltage continues to increase, the reduction driving force increases, and the reduction speed will also accelerate, which is consistent with the analysis results in CVs.

3.4 The effect of time on Fe₃O₄ electrolysis products

The process of Fe₃O₄ electro-deoxidation reduction was explained, electrolysis experiments were carried out at 2.0 V cell voltage for different times, and the results are shown in Figure 8. Ca₂Fe₂O₅ appeared when the applied voltage was 2.0 V for different times of electrolysis for 10–600 min. Due to the influence of Ca²⁺ that Fe₃O₄ first formed Ca₂Fe₂O₅. As the applied voltage reaches the reduction potential of Fe, a part of FeO and Fe will appear. In the short 10 min electrolysis process, due to the short electrolysis time, there are only diffraction peaks with little Fe. As the electrolysis time is extended to 30 and 60 min, the amount of metallic Fe generated increases, and the peaks intensity of Ca₂Fe₂O₅ and FeO decreased slightly; after 120 min of electrolysis, the diffraction peak of FeO disappeared, only Fe and little Ca₂Fe₂O₅ remained. Ca₂Fe₂O₅ gradually reduced and the diffraction peak of metallic Fe increased as time went.

Figure 8:

XRD pattern of Fe₃O₄ sample electrolyzed at 2.0 V with constant cell voltage in NaCl-CaCl₂ melts at 800 °C for 10–300 min (a) Scan range:10–90° (b) Scan range:30–50°.

It is worth noting that the diffraction peak of FeO disappeared from 120 to 300 min, and only a small amount of Ca₂Fe₂O₅ and most of Fe existed. This point is satisfied by the thermodynamic data, such as Eqs. (4) and (5). The thermodynamic data the ΔG^Θ of Ca₂Fe₂O₅ decomposing into FeO is greater than that of FeO decomposing into Fe at 800 °C. According to the theoretical decomposition voltage corresponding to Table 1, the theoretical decomposition voltage of FeO reduction to Fe is −1.01 V, which is just the theoretical decomposition voltage of Ca₂Fe₂O₅ reduction to FeO is −1.13 V, so there is no FeO after 120 min.

The formation of Ca₂Fe₂O₅ is an extremely easy process. Its generation is not due to the chemical reaction between CaO and Fe₃O₄ produced by the hydrolysis of CaCl₂, but when the voltage is energized, Ca²⁺ moves to the cathode and adsorbs on the surface of Fe₃O₄. All the reactions involved are shown in Eqs. (6–8):

Figure 9 is the SEM image and EDS data of the Fe₃O₄ substrate at a constant cell voltage of 2 V in melts of NaCl: CaCl₂ = 0.48: 0.52 at 800 °C for 10–300 min. The particles gradually become finer with time. The O contenting the electrolysis products keeps decreasing and the Fe content keeps increasing.

Figure 9:

SEM images of Fe₃O₄ sample electrolyzed by constant cell voltage of 2 V in NaCl-CaCl₂ melts at 800 °C (a) 10 min (b) 30 min (c) 60 min (d) 120 min (e) 180 min (f) 300 min.

Within the first 60 min of electrolysis time, interconnected Ca₂Fe₂O₅ is mainly formed, and its structure becomes denser and the porosity decreases. According to the de-oxidation of TiO₂, CaTiO₃ is easily formed during the electrolysis process, which has a certain effect on the effective migration of ions (Dolganov et al. 2020). In the early stage of Fe₃O₄ de-oxidation, Ca₂Fe₂O₅ is easily formed and has an effect on the migration of ions, the Ca content in this period of time is higher and the O content decreases less (Li et al. 2010). During the electrolysis to 120–180 min, the interconnected Ca₂Fe₂O₅ is gradually deoxidized and decomposed into granular metallic Fe with a certain porosity, which accelerates the mass transfer and electron conduction of O²⁻ from the inside of the solid cathode to the molten salt. When the electrolysis time is 300 min, due to the removal of oxygen, the reduced Fe atoms gradually nucleate and grow, forming metallic Fe with a particle size of 5–10 μm.

2Figure 10 is a line graph of the changes of Fe, O and Ca content with the electrolysis time. From the figure, the O content gradually decreases with the increase of the electrolysis time. During the first 60 min of electrolysis, Ca₂Fe₂O₅ is mainly formed with low Fe content. At 60–120 min of electrolysis, Fe content increases rapidly and in the stage of rapid deoxygenation and reduction. After 120 min of electrolysis, the Fe content increases slowly, because the O²⁻ ions generated through the rapid de-oxidation stage are too late to migrate and gather near the cathode, which hinders the progress of electrolytic de-oxidation.

Figure 10:

EDS of Fe₃O₄ sample electrolyzed by constant cell voltage in NaCl-CaCl₂ melts at 800°C for 300 min.

3.5 Current efficiency

With the increase of voltage, the current first reaches the maximum due to the electron accumulation. Fe₃O₄ can be reduced at 1 ∼ 3 V. When Fe₃O₄ begins to reduce, the outermost electrons reach saturation, and active ions appear at the three-phase interface, and O²⁻ migrates to anode discharge. After electrolysis at 1 V for 38 min, the applied energy is the same as the energy required for the activation ion migration, and the current approaches equilibrium. The reaction rate is the highest in the first 7 min of 2 V electrolysis. With the increase of time, the reaction proceeds to the inside, the reaction resistance increases gradually, and the current decreases slowly. At 3 V, the nucleation, growth and decomposition of Ca₂Fe₂O₅ hinder the migration of O²⁻ and slow down the reaction rate. Finally, the current tends to be stable. Metal iron can be obtained by electrolysis for 8 h at a constant cell voltage of 1.0 V. The electrolytic efficiency reaches 96.4% and the electrolytic energy consumption is 3.4 kW·h·kg⁻¹.

Figure 11:

I-t plots of electrolysis under 3 V voltages for 8 h in 800 °C NaCl-CaCl₂ melts.