Effect of Electrolysis Temperature on the Preparation of FeO by Molten Salt Electrolysis

Abstract

FeO is a low-price material with high charge storage capacity, biocompatibility and other characteristics. It has been applied in the fields of catalysts, capacitors, electrodes and composite materials. However, the current method of preparing FeO needs to control the temperature and reducing atmosphere, which increase the production difficulty and cost. In this experiment, the molten salt electrolysis method was used to prepare FeO by using the NaCl-KCl molten salt system and Fe₂O₃ and Al₂O₃ as raw materials, and the influence of temperature on the preparation process was explored. The results showed that the electrolysis process of Fe₂O₃ to FeO is mainly divided into the following three stages: the electric double-layer charging process, Fe₂O₃ to Fe₃O₄ process and Fe₃O₄ to FeO process. The increase in temperature can improve the reaction speed and strengthen the electrolysis effect. The higher the temperature, the less Fe₃O₄ and more FeO in the sample. Through analysis, it was found that the increase in temperature will affect the theoretical voltage of the electrolytic reaction in thermodynamics, resulting in the increase in the overall potential provided by the power supply. In terms of kinetics, the increase in temperature will affect the viscosity of molten salt, so that O²⁻ transport has better kinetic conditions.

1. Introduction

FeO is a black transition metal oxide. It has the characteristics of high charge storage capacity [1], biocompatibility [2], non-toxicity [3], and low price, and has attracted the interest of researchers in metallurgy and energy fields. FeO is mainly used to make supercapacitors and electrocatalytic electrodes due to its high volumetric specific capacities and wide operating voltage range [4,5,6]. In recent studies, nano-sized FeO particles were coated on capacitor electrodes and battery electrodes in the form of surface coatings for performance improvement [7,8]. Nano-sized FeO also has applications in biomedicine and as an antibacterial agent [9,10]. Therefore, FeO is a developmental potential material. Among many works on FeO, the FeO-Al₂O₃ system has been identified as having a high application value. The Al/FeO-Al₂O₃/p-Si diodes prepared by the sol-gel coating method have excellent photoreaction performance [11]. WU used FeO, Al₂O₃, Fe₂O₃ as raw materials to prepare water-based nanofluids using the ultrasonic dispersion method, and the product can be used as cooling liquid [12]. In terms of material preparation, FeAl₂O₄ [13,14] prepared with FeO and Al₂O₃ as raw materials has good prospects in the fields of infrared radiation materials [15] and catalysts [16]. Therefore, it is necessary to further explore the potential of the FeO-Al₂O₃ system.

Due to the fact that FeO has thermal instability, the preparation of FeO must be carried out under conditions in which it can exist stably. This requires a weak reducing atmosphere or reducing agent and a relatively low temperature [17]. These conditions hinder the application of FeO and increase the cost. In order to solve these problems, this experiment adopted the green metallurgical process of molten salt electro deoxidation [18], and used Fe₂O₃ and Al₂O₃ as raw materials to prepare FeO. Compared with the traditional preparation method, the electro deoxidation method used Fe₂O₃ as the Fe source, which is common in steel-making solid waste. This can not only greatly reduce the preparation cost, but also facilitate the recycling of secondary resources. The preparation process does not require C and reducing gas, and no harmful gas was discharged, which can reduce energy consumption and environmental pollution. If the theoretical preparation method is to be used in practical industrial applications, energy consumption and cost are the most important factors, and the electrolysis temperature is the most important factor affecting the energy consumption in the electrolysis process. In order to make the design of electrolysis parameters more suitable for practical application, the influence of temperature on electrolysis effect should be explored to minimize energy consumption on the premise of achieving a certain electrolysis effect.

2. Experimental

2.1. Thermodynamic Analysis

The NaCl-KCl molten salt system [19] was used in this experiment because it is cheap and will not react with raw materials. Fe₂O₃ can be deoxidized to FeO under a certain electrolytic voltage, so as to facilitate the subsequent combination of FeO and Al₂O₃ to FeAl₂O₄. In order to determine the experimental conditions and reaction sequence, Factsage 7.3 was used for thermodynamic calculation. The main reactions involved in the experiment are as follows [20].

Al₂O₃ (s) = 2Al (s) + 2/3O₂ (g)

(1)

Fe₂O₃ (s) = 2/3Fe₃O₄ (s) + 1/6O₂ (g)

(2)

Fe₃O₄ (s) = 3FeO (s) + 1/2O₂ (g)

(3)

FeO (s) = Fe (s) + 1/2O₂ (g)

(4)

NaCl (s) = Na (s) + 1/2Cl₂ (g)

(5)

KCl (s) = K (s) + 1/2Cl₂ (g)

(6)

The standard theoretical decomposition voltage E^Θ was calculated using Formula (7).

ΔG^Θ = −nFE^Θ

(7)

where, ΔG^Θ is the standard Gibbs free energy, (kJ·mol⁻¹); E^Θ is the theoretical decomposition voltage in the standard state, (V); F is the Faraday constant, (96,485 C·mol⁻¹); and n is the number of electrons gained or lost in the reaction equation.

Figure 1 shows that the order of reaction was (2) > (3) > (4) > (1) > (5) > (6) above 600 °C. The electrolytic voltage should be controlled within the range that meets the deoxidization of iron oxide and is less than the electrolytic voltage of Al₂O₃, NaCl and KCl. However, this theoretical voltage range cannot achieve the expected electrolysis effect. The complete electro deoxidation process needs to undergo several processes, including O²⁻ diffusion on the cathode surface after dissociation from oxide, O²⁻ transformation to the anode in molten salt, O²⁻ change into oxygen atoms on the anode surface and generation of CO₂ [16]. Each step will affect the electrolysis process. The NaCl-KCl molten salt system used in this experiment had a relatively weak ability to transmit O²⁻. It prevented O²⁻ which fell out of the cathode from reaching the anode graphite rod in time. Therefore, it was necessary to increase the electrolytic driving force and adjust the experimental voltage to 2.3 V.

2.2. Experimental Process

The cathode raw materials were analytically pure (99.5%) Fe₂O₃ and Al₂O₃ powders (Macklin Group Co., Ltd., China, analytical reagent). Fe₂O₃ and Al₂O₃ with a mass ratio of 3:2 was weighed, the two powders were mixed evenly, place into an agate ball milling tank, an agate ball and 6 mL alcohol were added, and then the ball was milled for 6 h to achieve a small and uniform particle size. The powder dried for 12 h and was poured into the sample pressing mold, pressed on the sample pressing machine with a pressure of 6 MPa for 3 min, and then removed. The pellets were sintered for 5 h at 800 °C to ensure they had a certain strength and density. The sintered pellet was the cathode of electrolysis (10 × 3 × 1 mm³). High-purity graphite pellet (100 × 15 × 5 mm³) was used as the electrolytic anode. Weighed NaCl and KCl with a molar ratio of 1:1 was mixed well, and poured into a graphite crucible (the total mass of NaCl and KCl was 180 g). The graphite crucible was placed into the tubular furnace, the furnace temperature was raised to the experimental temperature under argon atmosphere and maintained for 1 h to completely melt the molten salt. Then, the assembled cathode and anode were placed into molten salt, and a voltage of 2.3 V for 4 h was applied. The whole process was carried out in argon atmosphere to prevent the electrolytic device and sample from being oxidized. Argon atmosphere can also prevent oxygen in the air from dissolving into molten salt. After electrolysis, the cathode was rinsed with distilled water in argon atmosphere to remove the salt adhered to its surface, then the cathode pellet was placed into a beaker and 50 mL of alcohol was added, washed in an ultrasonic cleaner for 10 h, and then dried. The schematic diagram of the electrolysis process is shown in Figure 2. The prepared samples were characterized using a D/MAX2500PC diffractometer and Zeiss Gemini 300 electron microscope, and the phase differences and microstructure differences caused by different electrolysis temperatures were analyzed.

3. Results and Discussion

Figure 3 shows the XRD pattern of the product after electrolysis at 2.3 V at different temperatures for 4 h. The products of the cathode pellet at 800 °C were Fe₃O₄, FeO and Al₂O₃ phases, of which Fe₃O₄ was the main phase. This shows that the transformation degree of Fe₃O₄→FeO was not considerable at this temperature, and there was still a considerable amount of Fe₃O₄ in the sample. When the electrolysis temperature was low, the viscosity of molten salt electrolyte was high, the fluidity was poor, the conductivity was also low, and the kinetic conditions of O²⁻ transmission were relatively worse. According to the thermodynamic calculation in Section 2.1, it can be found that with the decrease in temperature, the theoretical potential of the reduction reaction of iron oxide increases. Therefore, under the condition of constant voltage, the reaction at lower temperature had less over potential, and the electrolysis rate was slower, which affected the diffusion of O²⁻ from the cathode to molten salt. The Fe₃O₄ content of the cathode product at 850 °C decreased sharply, and the FeO content increased and became the main phase in the sample. After electrolysis at 900 °C, the main product of the cathode was still FeO, the FeO peak was stronger, and the content of Fe₃O₄ decreased gradually. The main product of electrolysis at 950 °C was FeO. Fe₃O₄ basically disappeared. Because at a high temperature the theoretical electrolytic voltage of iron oxide was low, the over potential of the reaction was larger and the reaction faster. At the same time, it can also be clearly found in the experiment that the viscosity of electrolyte decreased, so the resistance of ion movement was smaller and the movement speed faster, which weakened the concentration polarization and was conducive to the progress of the reaction, making it quickly convert to FeO under the specified conditions. Some researchers found the same phenomenon by electrochemical means. The increase in temperature will reduce the activation energy of the iron oxide electro deoxidation reaction, which will increase the reaction rate [21].

Figure 4 shows the recorded I-t curve corresponding to different electrolysis temperatures. The curve can be divided into three stages of A, B and C. Stage A is the electric double layer charging process. After the power is turned on, the electrode will complete the polarization charging in a short time, and the current will rise to the maximum value quickly. When the current reaches the maximum value, it means that the electric double layer charging is completed. At this time, the three-phase boundary is formed, and subsequent electric deoxidation can be carried out. Stage B is the process of electro deoxidation of Fe₂O₃ to Fe₃O₄, which represents the gradual entry of molten salt into the cathode pellet and the formation of a new three-phase boundary constantly advancing towards the interior of the pellet at the deoxidized Fe₃O₄ and undeoxidized Fe₂O₃. At this stage, with the transition from Fe₂O₃ to Fe₃O₄, the current gradually decreased and tended to be stable. The current entered a short plateau period, and Fe₂O₃ was completely deoxidized to Fe₃O₄. Stage C represents the process of electro deoxidation of Fe₃O₄ to FeO, and the current in this stage is smaller than that in stage B. As the reaction occurred inside the cathode pellet, the three-phase boundary was also deeper, which led to the resistance of the pellet becoming larger, so the overpotential provided by the power supply was smaller, the driving force of the reaction was smaller, and the current further dropped [22]. Combined with the duration and current of stages B and C, it was more difficult for Fe₃O₄ to transform into FeO than Fe₂O₃ to Fe₃O₄. As the rate curve in electrochemical reaction, the slope of I-t curve in each stage can reflect the reaction speed in this stage to a certain extent, and can be used to compare the reaction kinetics under different conditions.

Table 1. Absolute value of slope after linear fitting of stage A and B.

Temperature	Stage A Slope	Stage B Slope
800 °C	1.234	5.051
850 °C	1.603	5.722
900 °C	4.552	7.764
950 °C	4.992	8.031

shows the absolute value of the current slope obtained by the linear fitting of A and B stages. The fitting was carried out using Origin 2021. Comparing the I-t curves at different temperatures, when the electrolysis temperatures were different, its initial currents were different, and the slope of current change with time was also different. The higher the temperature was, the higher the electric double-layer charging speed and current peak after charging were. The shorter time required for stage B, the better kinetic conditions of the reaction. XRD and I-t curves both show that the electrolysis temperature had a significant effect on the electrolysis reaction rate. As the temperature of molten salt decreases, the theoretical decomposition voltage required for raw oxide will increase. If the voltage is constant, the driving force of the electrolytic reaction will decrease at this time, which is bound to affect the reaction rate dynamically. On the other hand, a higher electrolysis temperature can shorten the time required to complete stage A and B, so in the case of limited time, the time left for stage C will be longer, and the amount of FeO obtained by deoxidation will be greater.

Figure 5 and Figure 6 show the SEM and energy spectrum surface scan images of the samples after electrolysis for 4 h at different temperatures. The large block particles were Al₂O₃, and the small particles were the products of Fe₂O₃ deoxidization, which adhere to Al₂O₃ particles to form nuclei and mature. With the increase in the electrolysis temperature, the particles of deoxidized Fe₂O₃ become increasingly smaller. According to the XRD results, this phenomenon was due to the increasing amounts of FeO obtained by deoxidation. The electrolysis temperature of 950 °C was suitable for this experiment.

Combined with the XRD, I-t curve and SEM results, the influence mechanism of temperature on the preparation process of FeO can be obtained. In thermodynamics, the rise in temperature leads to the decrease in the theoretical electrolytic voltage and the rise in the experimental overpotential, which increases the speed of the reaction. In dynamics, as the temperature increases, the average energy of all molecules also increases. The energy required for iron oxide molecules from the normal to active state decreases, and the number of activated molecules participating in the reaction increases, so the reaction proceeds faster, as shown in Figure 7. In addition, the decrease in the molten salt viscosity caused by increasing temperature also provides better kinetic conditions for ion transport.

4. Conclusions

In this paper, Fe₂O₃ and Al₂O₃ were used as raw materials to prepare FeO by molten salt electro deoxidation, and the influence of temperature on the preparation process was explored. At the electrolysis temperature of 800 °C, the components of the sample were Fe₃O₄, FeO and Al₂O₃, in which Fe₃O₄ was the main phase. As the electrolysis temperature continued to rise, the main item of the sample was FeO, and the amount of Fe₃O₄ continued to decrease, while the amount of FeO continued to increase. From the corresponding I-t curve, the increase in the electrolysis temperature can accelerate the electric double layer charging process and electrolysis speed, and the overall current of the electrolysis process is also stronger. SEM images also show a matching phenomenon, in which the higher the temperature, the more FeO is present in smaller particles. These phenomena show that the increase in the electrolysis temperature improved the effect of iron oxide electro deoxidation, and increased the number of activated iron oxide molecules. According to thermodynamic calculation, the theoretical voltage of electro deoxidation of iron oxide will decrease with the increase in temperature, so the higher the temperature, the greater the overpotential of the electrolytic reaction and the stronger the electrolytic driving force. On the other hand, the higher the temperature of molten salt, the better the kinetic conditions of ion migration and the faster the reaction. When the electrolysis temperature reached 950 °C, only FeO and Al₂O₃ were present in the sample, thereby achieving the electrolysis target.