Operando probing and adjusting of the complicated electrode process of multivalent metals at extreme temperature

Significance

High-temperature electrochemistry has significant advantages and many applications in a variety of areas (metallurgy, energy, etc.), not only benefiting from the high-temperature characteristic, which can lower the “threshold” of the reaction thermodynamically and increase the reaction rate kinetically, but also arising from the broad employing of inexpensive inorganic salt electrolyte. However, the in situ and in-depth analysis on HTE is very difficult due to the high temperature itself, strong corrosion, and a complicated reaction cell. Here, we developed a multidimensional instrument that can monitor and adjust the electrode process in a high-temperature and complex operating condition, which will promote the deep understanding on HTE and lay the foundation for real-time and rational process optimization.

Abstract

Nearly half of the elements in the periodic table are extracted, refined, or plated using electrodeposition in high-temperature melts. However, operando observations and tuning of the electrodeposition process during realistic electrolysis operations are extremely difficult due to severe reaction conditions and complicated electrolytic cell, which makes the improvement of the process very blind and inefficient. Here, we developed a multipurpose operando high-temperature electrochemical instrument that combines operando Raman microspectroscopy analysis, optical microscopy imaging, and a tunable magnetic field. Subsequently, the electrodeposition of Ti—which is a typical polyvalent metal and generally shows a very complex electrode process—was used to verify the stability of the instrument. The complex multistep cathodic process of Ti in the molten salt at 823 K was systematically analyzed by a multidimensional operando analysis strategy involving multiple experimental studies, theoretical calculations, etc. The regulatory effect and its corresponding scale-span mechanism of the magnetic field on the electrodeposition process of Ti were also elucidated, which would be inaccessible with existing experimental techniques and is significant for the real-time and rational optimization of the process. Overall, this work established a powerful and universal methodology for in-depth analysis of high-temperature electrochemistry.

The extraction, separation, purification, and plating of many elements, roughly half of the elements in the periodic table, were performed via a high-temperature electrodeposition technology (HTE, temperature generally ranges from 373 to 1,873 K) in industry, because of the intrinsic physicochemical properties (such as metal activity, melting point, etc.) of those elements, and in consideration of the economics and environmental effect of the technology (Fig. 1A) (1–7). However, the electrodeposition processes occurring at the electrode interface and surface are very complex and unstable—especially the electrodeposition of polyvalent metals, which directly triggers a series of engineering problems. For instance, the current efficiency of deposition is highly restricted by the infamous shuttle effect and the disproportionated reaction of variable-valence ions (5, 8–10). Another typical example is that the product purity is limited by the side reactions of impurity ions and the loose morphology of deposition products (11–16). To address these major challenges, understanding the dynamic electrodeposition process in a realistic operating high-temperature cell is therefore critical for HTE.

Fig. 1.

Schematics of the multipurpose operando analysis and tuning the instrument for high-temperature electrochemistry. (A) The elements extracted, refined, or plated using the high-temperature electrochemical method. (B) Schematic of the facility that enables the in situ Raman probe, optical microscope images, and electromagnetic tuning of the cathodic process for multivalent metals in molten salt electrolytes. (C) Schematic diagram of the regulation of the magnetic field for an electrode process.

However, HTE has more severe reaction conditions than room-temperature electrochemistry, e.g., high temperature, and corrosion. It also has more complicated electrolytic cells such as refractory and corrosion-resistant shells, which seriously limit the application of the operando characterization techniques in HTE. To date, the major approach for analysis of the electrodeposition process in high-temperature melts is electroanalytical methods (e.g., cyclic voltammetry, chronopotentiometry) coupled with ex situ characterization techniques. This traditional strategy cannot provide key insights into the real-time dynamic electrode process and exacerbates the knowledge gap between the HTE and state-of-the-art technologies. Consequently, some researchers focus on the development of operando characterization techniques for HTE (5, 17–21).

For example, optical imaging can monitor the bubbles generated on the surface of graphite anode in the Hall-Héroult process (17), but the currently HTE optical methods cannot offer the real-time microscale morphology and structure information of electrodes during electrolysis and image complex three-dimensional (3D) structures. To tackle this issue, our group developed an HTE facility that permits in situ X-ray computer microtomography to collect quantitative 3D images of electrodes during electrolysis (4D) (5). This facility can study the evolution mechanism and even optimize the depositional morphology, but it does not capture the reduction pathway of active ions, which is vital to the electrodeposition process. Raman spectroscopy is an excellent approach to study the structure of ions and it has been employed in room-temperature electrochemical systems to elucidate the reduction pathway of ions (22–24). However, only a relatively small number of in situ Raman studies on HTE systems have been published (18–21). In addition, no in situ Raman experiments have yet monitored the complex electrodeposition process of variable-valence ions in high-temperature melts. Thus, operando and versatile characterization methods that can probe the entire reduction process of active ions and monitor the evolution behavior of the morphology of deposits are essential for improving our in-depth and systematical understanding of HTE.

Here, we report a multipurpose and universal operando experimental instrument combining operando Raman microspectroscopy, optical microscope imaging, and a tunable magnetic field, which possesses the ability to carry out electrochemical experiments at a high temperature of ~1,273 K. In the previous studies, instruments were usually combined in pairs, the scale and scope of the research were limited, and it was difficult to clearly explain the whole electrochemical deposition process, including the ions conversion, metal deposition, etc., and the influence of the external field on this process. Moreover, most of the studies were conducted at room temperature. The combination of these four in-situ methods at high temperatures not only can track the preconversion and nucleation growth process in the electrochemical reaction process in real time, but also can realize in situ regulation and mechanism exploration. To evaluate the feasibility of the proposed apparatus, we chose the electrodeposition experiment of titanium, which is a typical polyvalent metal and generally shows a very complex dynamic electrode process, e.g., multistep reduction, shuttle effect, and disproportionated reaction (25–27). Taking advantage of the multipurpose instrument, we simultaneously revealed a reduction pathway that was confirmed by the Raman peaks of anions, and directly monitored the dynamic evolution of the morphology of Ti deposit by the real-time optical microscope imaging, which vastly enriches our understanding on HTE. Subsequently, the effects of the magnetic field, including the intensity and direction, on the Ti electrodeposition process operating at high temperature were also evaluated via accurate tuning of the magnetic source surrounding the instrument. A multidimensional operando analysis strategy including multiple experimental studies and theoretical calculation was then built to detail the impact of the magnetic field on the electrodeposition process of Ti. The results lay the foundation for real-time and rational process optimization. This work provided a pioneering demonstration of the multidimensional operando methodology for HTE, which helps to inspire researchers to establish more analysis techniques.

Results and Discussion

Multipurpose Operando Instrument for High-Temperature Electrochemistry.

Fig. 1B shows a schematic of the home-made instrument. This system permits real-time capture of the reduction process and images the deposition growth process while probing the regulatory effect under current loads in a high-temperature molten salt. It consists of Raman spectroscopy, optical microscopy, and a magnetic field. The instrument was fabricated with some detachable components such as a cell lid with an optical window and a removable electrode system. It has a cell body with a high-precision electric heating table as well as a mechanical arm that can provide a tunable magnetic field (SI Appendix, Fig. S1A). The electric heating table could offer a high-temperature working condition of ≤1,273 K and a high precision of about 2 K. This meets the requirements of almost all high-temperature molten salt electrochemical experiments. In addition, the high-temperature cell was designed with a gas path and a water pipe. This design permits differing atmospheric conditions and cools the cell body.

Another advantage of the proposed instrument is that the electrode system is adjustable without breaking the atmospheric environment even at high-temperature operating conditions. We could therefore not only control the reaction area of the electrodes in situ but could also adjust the distance between the electrode and melt level to increase the intensity of the Raman spectra—this makes the instrument more efficient and practical. The instrument was fixed on a sample table of the Raman facility or optical microscope system, and the laser could then pass through the optical window to detect the interface Raman spectral information between the electrode and the melt or the surface morphology of the electrode (SI Appendix, Fig. S1 C and D). There are very limited reports on the Raman spectral information of high-temperature molten salts, especially polyvalent metal complex ions (28, 29). Thus, we used ab initio molecular dynamics (AIMD) simulations to analyze and assign the detected Raman peaks. Detailed procedures are discussed in the Materials and Methods.

Another benefit of the instrument is that the intensity, direction, and angle of the magnetic field are adjustable via a universal mechanism device with multiple degrees of freedom (SI Appendix, Fig. S1B). Fig. 1C shows the sketch of the regulatory effect of the magnetic field on the electrode process. We also calibrated the magnetic field information surrounding the reaction cell by establishing the relationship between the spatial location of the magnetic field source and the intensity, direction, and angle of the magnetic field (SI Appendix, Fig. S3 A–C). In general, the magnetic field could significantly influence the electrode process (30–32). There are some literature reports on the regulatory effect of magnetic field on high-temperature molten salt electrolysis, but the mechanism, especially the multiple-scale mechanism from atomic scale to macroscopic scale, is still unclear. Thus, the regulatory effect of magnetic field is quite blinded and inefficient. This multidimensional operando instrument was used to study the abovementioned multiple-scale mechanism in depth.

Operando Probing and Real-Time Imaging of the Cathodic Process of Ti in Molten Salt.

To verify the capability of the instrument, we next explicitly identified the species formed on the interface between the working electrode (WE) and molten salt during electrochemical reduction by in situ Raman spectroscopy. Fig. 2A shows that there were three characteristic bands at 287, 303, and 903 cm⁻¹ when the temperature increased from 298 to 823 K. To further confirm the structure of complex ions in molten salt, Gaussian 09W software was used to calculate the vibrational frequencies of all the potential ions (TiF₆²⁻, TiF₄O²⁻, and TiF₂O₂²⁻) (Fig. 2B). The three bands correspond to the TiF₄O²⁻ characteristic bands. The bands at 287 cm⁻¹ and 303 cm⁻¹ are attributed to the Ti-O shearing vibration, and the band at 903 cm⁻¹ is attributed to the symmetric stretching mode of Ti-O stretching vibration (Fig. 2 A, Inset). Note that the presence of oxygen may be caused by exposure to air during sample transfer. The oxygen is easy to form molecular bonds with metals, so Ti-O bonds are easily detected in the Raman test. Based on our many attempts, we found the oxygen really difficult to avoid. We still work hard on this research direction and hope to make some breakthroughs in our future work.

Fig. 2.

In situ Raman spectra of the interface between the Pt electrode and the molten salt. (A) In situ Raman spectra of the interface between the Pt electrode and the molten salt in the heating process and Ti-O vibration mode (Inset). (B) Theoretical Raman spectra of TiF₆²⁻, TiF₄O²⁻, and TiF₂O₂²⁻ by Gaussian 09W software and the Raman spectra of the interface between the Pt electrode and the molten salt at 823 K. (C) Cyclic voltammograms of the Pt electrode in molten NaF-KF-LiF-K₂TiF₆ at 823 K. (D) Relationship of i_pc and v^1/2. (E) Potential-dependent Raman spectra of the interface between the Pt electrode and the molten salt during electrolysis from 0 V to −2 V (versus Pt). (F) Evolution of the frequency of I₁ and I₂ versus potential.

Electrochemical tests were next performed to further understand the reduction process of TiF₄O²⁻ ions. Fig. 2C shows the cyclic voltammogram (CV) curves of the molten salt at a scan rate of 20 and 50 mV s⁻¹. There were two pairs of characteristically reversible peaks in the range of 0.6 to −2.5 V (versus Pt), which means that the tetravalent Ti (TiF₄O²⁻) is reduced to titanium through a two-step process. And the oxidation peak at about 0.5 V (versus Pt) may be attributed to the oxidation of platinum wire. Moreover, the current density of the reduction peak and the square root of the scan rate (SI Appendix, Table S1) were statistically analyzed and showed a good linear relationship (Fig. 2D), which indicates that the electroreduction is controlled by the diffusion of Ti ions.

In light of the correspondence between the CV and the Raman spectral features, Raman spectra could offer information about the dominant species at each potential. Fig. 2E shows that potential-dependent Raman spectra range from 0 V to −2 V (versus Pt). The dominant band at 903 cm⁻¹ (I₁) was attributed to the Ti-O stretching vibration as the Raman spectra did not give any explicit spectroscopic for the formation of new ions. Whereas, as the potential decreased to −1.2 V (versus Pt), the band at 903 cm⁻¹ dropped rapidly, another obvious Raman band around 687 cm⁻¹ (I₂) appeared and its intensity increased with the potential further decrease. Combined with the reduction peak on the CV curves at −1.2 V (versus Pt), we conclude that TiF₄O²⁻ was converted to new ions. According to the calculation results (SI Appendix, Fig. S4), we confirm that the band around 687 cm⁻¹ belongs to the Ti-F stretching vibration of TiF₅²⁻, which shows the structure and vibration of Ti ions (TiF₄²⁻, TiF₅²⁻, and TiF₄O²⁻; see Movie S1). When the potential was further reduced to −2.0 V (versus Pt), the red shift of the band around 730 cm⁻¹ appeared slightly, but no new bands appeared. This finding indicates that the TiF₅²⁻ ion is not reduced to lower value ions but is rather directly reduced to metallic titanium. In other words, the reduction of TiF₅²⁻ ion to metallic titanium occurs through one-step three-electron transfer in the LiF–NaF–KF eutectic melt, and no intermediate valence ions are formed during the reduction process. It is consistent with the electroreduction mechanism of Ti(III) ions using cyclic voltammetry in previous research (33). However, the intermediate species is TiF₅²⁻ rather than TiF₆³⁻, as predicted in the previous literature (33), which is significant to optimize the cathodic process in terms of the theoretical calculation. To further analyze the Raman results, the quantitative data in Fig. 2E were collected as shown in Fig. 2F. The Raman spectra are obviously different at the first and second stages. Compared to the single dominant band of Ti-O stretching vibration at the first stage, there are two dominant bands (Ti-O and Ti-F) for stretching vibrations at the second stage. In addition, the two bands exhibit a redshift of 30 to 40 wavenumbers, indicating that the presence of the two bonds are longer than original bond length [I₁ at a potential of 0 V (versus Pt) and I₂ at a potential of −1.2 V (versus Pt)].

To further reveal the complete cathodic process of Ti in molten salt, we next used in situ high-resolution optical microscopy to study the evolution process of Ti morphology and structure on the WE during electrolysis. Fig. 3A shows the time sequenced optical microscope images of a Ti dendrite forming and growing on the WE at −1.8 V (versus Pt). The end of the dendrite was always attached partially to the solid–liquid–gas three-phase interface (3PI) and grew continuously along the molten salt surface, as shown in Fig. 3B. The results indicate that the 3PI is the active site and that it initiates the growth of a titanium dendrite. This may be related to the nonuniform distribution of the electric field and Ti ion concentration close to the 3PI. To prove our conjecture, the electrolytic process was activated for 30 min and then analyzed by finite element numerical simulation (Fig. 3C). The performance difference between the 3PI and other positions on the cathode was dictated by the electrolyte potential, current density, and Ti concentration. The most negative potential and the maximum current density were near the 3PI, which means that the polarization is the strongest here. Thus, Ti was preferentially deposited here. Moreover, the dynamic evolution of electrolyte potential (SI Appendix, Fig. S5), current density (SI Appendix, Fig. S6), and Ti concentration (SI Appendix, Fig. S7) shows that the potential near the 3PI was more negative with increasing time. In other words, the Ti deposition near the 3PI always presents obvious thermodynamic and kinetic preference.

Fig. 3.

Real-time imaging of the cathodic process of Ti in molten salt. (A) In situ optical imaging of Ti dendrite growth. (B) Schematic diagram of nucleation and growth of Ti. (C) Simulation results of the electrolyte potential, current density, and electrolytic concentrations in an electrolytic cell. (D) Schematic of the cathodic process of Ti.

The in situ Raman spectroscopy analysis and actual observations demonstrate a complete cathodic process for titanium electrolysis for the first time. A schematic of the complete cathodic process is shown in Fig. 3D. First, TiF₄O₂⁻ is reduced to TiF₅²⁻ with O²⁻ released. The TiF₅²⁻ close to the WE is then further reduced to titanium, and F⁻ is released. F⁻ and O²⁻ migrate to the anode, and are oxidized to F₂ and O₂ bubbles (Movie S2). Finally, the titanium atoms accumulated near the WE nucleate, grow, and preferentially deposit at the 3PI.

Operando Tuning of Ti Electrodeposition by Magnetic Field.

Previous studies have reported that the magnetic field has a significant effect on the electrodeposition process (31). Our analysis also found a similar phenomenon. But compared with previous studies, our multiscale in situ analysis method more intuitively reflects the influence of the magnetic field on the electrochemical reduction process. The CV curves (SI Appendix, Fig. S3D) under different magnetic field directions showed that the peak current varied with the magnetic field direction. The peak current was the highest when the magnetic induction line direction was perpendicular to the radial direction of the WE. It was about 43.8% higher than that without the magnetic field, perhaps because of the motion direction of charged particles. A detailed mechanism is discussed in the next section. We selected the magnetic field (B = 80 mT) perpendicular to the radial left of the WE as the experimental magnetic field. The resulting CV curves are shown in Fig. 4A. The initial potential of the reduction current under a magnetic field positively shifted compared to that without a magnetic field, which indicates that the magnetic field has a positive effect on the polarizing impact. The peak current of the first reduction reaction was decreased slightly under a magnetic field, but the second peak current was increased obviously, indicating the conversion of tetravalent Ti (TiF₄O²⁻) to titanium was promoted. The approach for analysis in the previous study (31) is the traditional electroanalytical methods [cyclic voltammetry, electrochemical impedance spectroscopy (EIS)] coupled with ex situ characterization technique (scanning electron microscope) at room temperature. As for the transformation of ions in the electrodeposition process, the author makes the judgment based on theory and experience. In contrast, excepting the electrochemical measurements, the ion transition process at high temperatures was also detected by in situ Raman using our multipurpose operando instrument.

Fig. 4.

Operando tuning of Ti electrodeposition by a magnetic field. (A) Cyclic voltammograms of the Pt electrode in a molten NaF-KF-LiF-K₂TiF₆ sample with (green) or without (yellow) a magnetic field at 823 K. Scan rate: 20 mV s⁻¹. (B) Potential-dependent Raman spectra of the interface between the Pt electrode and the molten salt during electrolysis under a magnetic field. (C) Comparison of I₂/I₁ versus potential with or without a magnetic field. (D) Evolution of the value of I_{1(M on)} − I_{1(M off)} and I_{2(M on)} − I_{2(M off)} versus potential. (E) Nyquist plots of EIS data of the Pt electrode in molten salt with (B = 80 mT) or without a magnetic field at an open circuit. Inset: equivalent circuit model. (F) Schematic diagram of thickness reduction of the electric double layer. Here, j is the current flow, δ_d is the thickness of the electric double layer without a magnetic field, U represents the tangent flow induced by the magnetic field, and δ_h is the thickness of the hydrodynamic boundary layer induced by the tangent flow (U). δ_d-B is the thickness of the electric double layer under a magnetic field.

We also analyzed the Raman spectra of active species during electrolysis under a magnetic field (B = 80 mT). Fig. 4B shows the potential-dependent Raman spectra from 0 V to −2 V (versus Pt) with a magnetic field. There was no change in the Raman spectra in the range of 0 to −1 V (versus Pt), and I₂ bond appeared when the potential decreased to −1.2 V (versus Pt), which is consistent with the findings obtained from the CV curves. Moreover, the intensity of I₂ increased significantly when the potential decreased below −1.4 V (versus Pt). The intensity of the Raman band can reflect the corresponding concentration of species. Therefore, the intensity ratio of the characteristic bands at 687 and 903 cm⁻¹ (namely I₂/I₁) can be used as an index to reflect the concentration of tetravalent (TiF₄O²⁻) and trivalent (TiF₅²⁻) titanium. The value of I₂/I₁ increases gradually along with negative potential excursion, with or without a magnetic field—this confirms the conversion of TiF₄O²⁻ to TiF₅²⁻ ions (Fig. 4C). Moreover, the value of I₂/I₁ with a magnetic field is less than that without a magnetic field in the range of −1.2 V to −1.4 V (versus Pt). It is higher than the value without a magnetic field in the range of −1.6 V to −2 V (versus Pt). The finding shows that the influence of the magnetic field on the two reduction steps is different, which will be discussed in the following section. The I₁ and I₂ also exhibited a red shift as the potential was shifted negatively under a magnetic field (SI Appendix, Fig. S8). However, the regulatory effect of magnetic field on TiF₄O²⁻ and TiF₅²⁻ is different (Fig. 4D). The stretching vibration of Ti-O in the TiF₄O²⁻ exhibited a red shift at all potentials under a magnetic field, and the frequency of redshift increased with negative shift of potential. This in turn indicates that the Ti-O bond in TiF₄O²⁻ becomes longer due to the introduction of a magnetic field. Significantly, the influence of the magnetic field on TiF₅²⁻ could be obviously divided into two stages. The stretching vibration of Ti-F in the TiF₅²⁻ exhibited a blue shift at −1.2 V and −1.4 V (versus Pt). The stretching vibration of Ti-F in the TiF₅²⁻ exhibited a blue shift when the potential was more negative than −1.4 V (versus Pt). This in turn indicates that the influence of the magnetic field on the Ti-F bond in TiF₅²⁻ is related to the potential. Overall, the regulatory effect of the magnetic field to the two reduction steps and the active ions stability is different, and it is affected by the reduction potential. The multifunctional in situ experiment methods and multi-dimensional analysis strategy developed here systematically detailed the reduction behavior of typical multivalent ions for the first time including the complex regulatory effect of the magnetic field.

In addition to analyzing the influence of magnetic field at the molecular scale, we further revealed the regulatory effect of magnetic field at the micro-/nanoscale. Therefore, the electric double layer was analyzed by EIS measurements. Fig. 4E shows the Nyquist plots of the measured impedance of the electric double layer on the WE surface in molten salt with and without a magnetic field (B = 80 mT). Obviously, the lines were close to the perpendicular line, which shows that the electric double layer between the Pt WE surface and the molten salt is similar to an ideal electrode situation, regardless of whether or not a magnetic field is present. The electrical model of an ideal polarized electrode is composed of a resistor (R) and a capacitor (C) in series. Furthermore, the equivalent circuit model (Fig. 4 E, Inset) was built with ZView software, where R₁ is the charge-transfer resistor, R_e is the electrolyte resistor, CPE₁ is the high-frequency double-layer capacitance, and CPE₂ is the low-frequency double-layer capacitance related to the diffusion. The parameter values of the model are shown in SI Appendix, Table S2. The total resistance R_e under a magnetic field was extremely close to that without a magnetic field at high frequency, thus revealing that the magnetic field has no impact on the structure of the molten salt. Furthermore, β1, the exponent value of the electric double-layer impedance CPE₁, was close to 1, suggesting that the Pt electrode surface is smooth (34); consequently, we could reasonably simplify the electric double layer as a pure capacitor. The parallel-plate capacitor formula is (35)

$$C_d=\frac{{\epsilon }_0{\epsilon }_{r}A}{d},$$ [1]

where ε₀ is the dielectric constant of free space, ε_r is the relative permittivity of the electrolyte, d is the distance between positive and negative charge centers (effective thickness of the electric double layer), and A is the electrode surface area. When a magnetic field was introduced, the CPE₁ value increased from 1.23 × 10⁵ to 1.50 × 10⁵. Eq. 1 shows that the thickness of the electric double layer between Pt electrode and molten salt becomes thinner due to the magnetic field. The conversion for the thickness of the electric double layer between the WE surface and molten salt is shown in Fig. 4F. The introduction of magnetic field produced a tangential flow with an initial speed of U₀ (36), thus reducing the thickness of the electric double layer to δ_d-B. As a result, a thinner electric double layer results in an enhanced diffusion of active species, which in turn promotes the deposition of Ti on the cathode. In addition, the Bode diagram (SI Appendix, Fig. S9) shows that the resistance under a magnetic field was lower than that without a magnetic field at medium and low frequencies. This finding provides new evidence for the influence of magnetic field on the electric double layer.

Mechanism of the Magnetic Field on the Electrodeposition Process of Ti.

The influence and mechanism of the magnetic field on the electrochemical process at room temperature were comprehensively reviewed (37). However, as mentioned above, due to the severe reaction conditions and complicated electrolytic cells, there was a lack of research on the effect of magnetic field on HTE. As a result, the influence and mechanism of the magnetic field on HTE was still unclear, further research will be needed. In fact, the magnetic field has a multiscale and multidimensional influence on the electrochemical system, such as the Lorentz force, the Kelvin force, the Maxwell stress effect, and the spin selective effect (32). This makes mechanistic research difficult. The spin selective effect mainly occurs in ferromagnetic atoms, and thus, this work did not analyze it, but we did focus on other effects. Fig. 5A shows the force analysis of Ti ions (TiF₅²⁻) in molten salt under a magnetic field. The anion (TiF₄O²⁻ or TiF₅²⁻) moving on the current arc received a Lorentz force perpendicular to the electrode surface when the external magnetic field was perpendicular to the plane between the current direction and the electrode radial direction. There was also an electric force on the cathode and a coulomb force in the connection direction with the cation (Li⁺, Na⁺, or K⁺) adsorbed on the cathode surface. This reveals that the distance between a Ti ion (TiF₄O²⁻ or TiF₅²⁻) and the cathode surface is decreased upon introduction of a magnetic field. As a result, the resistance of active species transferred in the molten salt is reduced, and the cathodic process is promoted. This is consistent with the EIS measurements (Fig. 4E). The effect is not limited to microscopic molecules. The magnetic field has a stress effect on the polarizable droplets, i.e., the Maxwell stress effect. Fig. 5B shows that the stress could cause a transverse or longitudinal deformation of the shape of molten salt droplets. The interfacial tension, contact angle, wettability, and adhesion between molten salt and electrode surface are also affected, which is similar to the phenomenon of magnetite nanoparticles in a water-based ferrofluid with a magnetic field (38). Therefore, the shape of the ionic cloud near the electrode and the electric double layer can be affected under a magnetic field.

Fig. 5.

Mechanism of the regulatory effect of magnetic field on the electrodeposition process of Ti. (A) Force analysis of Ti ions in a molten slat on the Pt electrode under a magnetic field. (B) Change in the shape of a polarizable molten salt droplet induced by the Maxwell stress effect under a magnetic field. (C) Gibbs free energy profiles of Ti species on a Pt electrode. (D) PDOS of TiF₄O²⁻ with or without magnetic field. (E) PDOS of TiF₅²⁻ with or without a magnetic field. (F) Band lengths of Ti-O and Ti-F in TiF₄O²⁻ with or without a magnetic field. (G) Band lengths of Ti-F in TiF₅²⁻ with or without a magnetic field.

We further used first-principles (DFT) calculation calculations to explore the regulatory mechanism of magnetic field at the atomic scale. Fig. 5C shows the Gibbs free energy of the reduction pathway of active ions on the Pt electrode. The Gibbs free energy of TiF₄O²⁻ to Ti under a magnetic field was significantly lower than that without a magnetic field, which indicates that the electroreduction process of Ti is promoted in thermodynamics due to the introduction of a magnetic field. This finding is consistent with our CV data (Fig. 4A). As shown by the projected density of states (PDOS, Fig. 5 D and E), the overlap of Ti-3d and F-2p, O-2p orbitals increased with a magnetic field, thus forming a stronger 3d-2p hybrid orbital. As a result, the charge transfer at the interface was enhanced, and this could reduce the energy barrier for electrochemical reaction (39) to promote the reduction reaction.

Moreover, we calculated the bond lengths and bond angles of TiF₄O²⁻ and TiF₅²⁻ with or without a magnetic field. The Ti-O bond of TiF₄O²⁻ was longer because of the magnetic field (Fig. 5F), which indicates that the Ti-O bond is easier to break. The finding is consistent with the redshift of I₁ and I₂ measured by in situ Raman spectroscopy (Fig. 4D). Moreover, the average length of Ti-F bonds of TiF₅²⁻ increased upon introduction of a magnetic field (Fig. 5G), thus indicating that the magnetic field promotes the electroreduction of Ti. Notably, the applied magnetic field has no obvious influence on the bond angles of two Ti ions (SI Appendix, Tables S3 and S4). This discussion suggests a mechanism by which the magnetic field regulates Ti electrodeposition: the magnetic field not only changes the microstructure of TiF₄O²⁻ and TiF₅²⁻ but also promotes electron transfer and enhances species diffusion. The above mechanistic analysis can help guide future work on regulating electrochemical systems using magnetic fields.

Conclusions

Herein, we have successfully developed a versatile high-temperature electrochemical instrument that enables the operando monitoring and controlling the complex electrodeposition process of polyvalent metals in real time and real space from an experimental viewpoint by exploiting. This system exploits Raman microspectroscopy analysis, optical microscope imaging, and a tunable magnetic field. The feasibility of the proposed instrument and the multidimensional analysis strategy based on this instrument was demonstrated by performing electrodeposition experiments of Ti in a high-temperature halide melt. This instrument offered direct experimental insights into the complex multistep reduction process of high-valence Ti ions as well as the nucleation and growth of deposited Ti atom. We also saw how the magnetic field affects the cathodic process of Ti. In addition, a scale-span mechanism of the magnetic field on the electrodeposition process of Ti was explored. This insight can help optimize Ti electrodeposition. The experimental, theoretical, and computational strategies shown here can be widely employed in other high-temperature electrochemical systems.

Materials and Methods

All materials and methods, including electrochemical measurements and in situ X-ray tomography, are described in detail in SI Appendix.

Supplementary Material

Appendix 01 (PDF)

Movie S1.

The vibration modes of Ti-F and Ti-O in TiF₄^2-, TiF₅^2- and TiF₄O^2-

Movie S2.

Bubbles generated on the anode during electrolysis

Data, Materials, and Software Availability

All study data are included in the article and/or supporting information.