Wettable TiB2 Cathode for Aluminum Electrolysis: A Review

Titanium diboride (TiB2) is considered a promising material for wettable cathodes in aluminum electrolysis. The demand for wettable cathodes is associated with the development of inert anode technologies to eliminate CO2 emissions caused by the conventional aluminum reduction process. Titanium diboride has been given special attention due to its superior properties, such as high wettability, good electrical conductivity, wear resistance, and excellent chemical stability. In this paper, we discuss different synthesis techniques used for the preparation of TiB2 cathode material. The main methods are sintering, electrodeposition, and plasma spraying. Electrodeposition is considered to be the most reliable low-cost method for TiB2 preparation. The vertical anode–cathode distance can be reduced by introducing wetted TiB2 cathodes, through which specific energy consumption can be reduced significantly. For a longer lifetime, the TiB2 cathodes should be resistant to electrolyte penetration. Further research should be conducted to understand the electrochemical behavior of TiB2 in low-temperature electrolytes.


Introduction
The Hall-Heroult process has been the main technology for aluminum production for more than 130 years. In this process, a consumable carbon anode and carbon cathode (covered with liquid aluminum) are used with a cryolite-based electrolyte. The following reaction takes place where the alumina raw material, dissolved in the electrolyte, is reduced to aluminum The Hall-Heroult process is associated with high energy consumption and high amounts of carbon dioxide and greenhouse gas emissions. About 1.5 t CO 2 /t of Al is emitted during the electrowinning of aluminum, of which 0.2 t CO 2 /t of Al comes from perfluorocarbon gas emissions [1], arising from the anode effect and consumption of prebaked carbon anode. In addition to 1.5 t CO 2 /t, about 0.6 t CO 2 /t of Al (1) 3C (s) + 2Al 2 O 3(diss) → 4Al (l) + 3CO 2(g) corresponds to the preparation of prebaked carbon anodes. Specific energy consumption is 12500 − 16,000 kWh/ t of Al for the electrochemical decomposition of alumina compared to the theoretical value of 6330 kWh/ t of Al [2] required to supply the enthalpy for the reaction. The surplus energy is lost in the form of heat.
Replacing consumable carbon anodes with inert anodes can eliminate CO 2 emissions from the electrolysis process. The following reaction takes place by using inert anodes: However, the usage of inert anodes for aluminum production requires many changes in cell design and operating conditions. The inert anodes at low temperatures (around 800 °C) would be less prone to corrosion and thermal shocks. As the chemical energy in carbon is absent from the inert anode system, a cell with inert anodes would have a cell voltage of more than 1 V higher than the one with (2) 2Al 2 O 3(diss) → 4Al (l) + 3O 2(g) carbon anodes if no other changes were made to the process design. This would increase the specific energy consumption [3]. Using a vertical electrodes cell (VEC) allows a reduction of the cell voltage by minimizing the anode-cathode distance (ACD) and thus resistive losses. Using VEC with inert anodes can reduce the specific energy consumption by up to 30% [4].
VEC requires a cathode with high wettability towards Al to minimize the ACD to maintain low cell voltage. In the Hall-Heroult process, the cathode is covered with about 20 cm aluminum liquid, which serves as a functional cathode. As the electric current passes through the cell, it interacts with the magnetic field in the potroom, electromagnetic forces lead to the molten aluminum flow and waves in the metal-electrolyte interface, resulting in a risk of short-circuiting between the aluminum and the anode. To avoid short-circulating, a minimum of 4 to 6 cm ACD is maintained in the cell. For efficient cathodic deposition, it is desirable to maintain a molten aluminum cathode in the vertical anode-cathode system, so in the absence of a liquid metal pool, a wettable inert cathode, which is entirely wetted by liquid aluminum, is required. A stable 0.3-cm-thick layer of liquid aluminum on a wettable cathode can keep the cathode inert and protected [5].
Titanium diboride (TiB 2 ) is considered the most suitable cathode material due to its excellent wettability with molten Al, the low wear rate of 0.25 mm/year in aluminum, resistance towards oxidation at elevated temperatures, high wear resistance and hardness, resistance to corrosion from cryolite melts, and superior electrical conductivity of 9 − 15 × 10 5 S/cm [6,7]. A wettable TiB 2 can reduce the ohmic voltage drop, as this cathode material can work at low ACD, eventually decreasing the specific energy consumption [8]. The two main backdrops of using the TiB 2 are the high production cost and the thermal shocks caused due to the strong covalent bonds between the B and B atoms and the ionic bond between the Ti and B atoms [9]. In this review, we discuss the synthesis methods used in the fabrication of TiB 2 cathodes, properties of TiB 2 such as wettability and corrosion behavior, and finally, the environmental and economic impact of using TiB 2 cathodes.

TiB 2 Ceramic Materials
TiB 2 has poor sinterability, making it complex to fabricate components with dense and large sizes. One of the contributing factors for the poor sinterability of TiB 2 is the presence of TiO 2 and B 2 O 3 oxide layers on the surface, causing difficulties in the densification of TiB 2 samples. TiB 2 can withstand high temperatures as both covalent (B-B) and ionic (Ti-B) bonds exist. The densification of the TiB 2 is limited by its low self-diffusion coefficient. TiB 2 material with a relative density of more than 95% can be fabricated through hot pressing and pressureless sintering or a cold press followed by high-temperature sintering [5].
Kang and Kim [10] were successful in performing pressureless sintering of TiB 2 powder with an average particle size of 0.9 µm at two different temperatures (1800 and 1900 °C) for 2 h to obtain TiB 2 . As a sintering aid, 0.5 wt% of Cr and Fe were added to the TiB 2 powder, followed by 30 min of spex-milling. TEM examination showed that the Ti-Cr-Fe phase existed at a triple junction. Additions of Cr and Fe were found to enhance the densification of the samples. The specimen sintered at 1800 °C possessed better mechanical properties compared to the specimen sintered at 1900 °C. The relative densities of sintered TiB 2 at 1800 °C and 1900 °C were 97.6% and 98.8%, respectively. However, the specimen sintered at 1900 °C has a grain size larger than 50 µm, which leads to preferential grain growth on the surface. Similar observations were found by Jensen et al. [11], where TiB 2 sample obtained after hot-pressed at sintering temperature of 1800 °C had signs of preferential grain growth. The anisotropy of TiB 2 leads to preferential grain growth, where the anisotropy is due to the difference in thermal expansion and isothermal compressibility across the a axis and c axis [11]. The preferential grain growth at the TiB 2 surface can be avoided by sintering below 1800 °C.
Heidari et al. [12] used Ti and Fe with a wt ratio of 7:3 additives in a pressureless sintering process to perform the sintering at a temperature lower than 1700 °C. Initially, the material was sintered at 1150 °C, followed by ball milling for 1 h. The TiB 2 (90 wt%) and Ti 7 Fe 3 (10 wt%) were ballmilled for 10, 30, 60, 120, and 240 min to find the effect of milling on the properties of TiB 2 composite. The milled mixture was sintered under Ar-5%H 2 atmosphere at 1650 °C for 1 h. The specimen made from the 10 min milling had unevenly distributed additives with irregular shapes visible on its surface. The specimen made from the samples milled for 30 min had a uniform microstructure with fine particle size. The relative density of the specimen was 91%. The specimen showed superior electrical conductivity, better wettability by molten Al, and was crack-free. With 24-h exposure to Al, Al penetrated the TiB 2 , but the specimen's geometry remained stable. Although the metallic additives reduce the sintering temperatures significantly and eliminate the preferential grain growth, the additives react with the molten aluminum and dissolve, which leads to secondary phase formation at TiB 2 grain boundaries [13,14]. This could lead to crack formation and uneven thermal expansion. The hot press process is expensive, while the cold press requires high energy consumption, making these processes unfavorable to produce TiB 2 specimens.
Balci et al. [15] consolidated TiB 2 powder (particle size 0.536 µm, purity ≥ 98.4%) using field-assisted sintering technology/spark plasma sintering (FAST-SPS) process at reduced temperatures around 1500 °C. The TiB 2 used in this process was synthesized from a self-propagating hightemperature synthesis process using a TiO 2 -B 2 O 3 -Mg mixture. Figure 1a shows that the relative density of the TiB 2 prepared from FAST-SPS is influenced by the holding time and pressure applied during sintering. A maximum relative density of 96.7% was obtained when the applied pressure was 60 MPa with a holding time of 15 min. No preferential grain growth was observed on the samples, while the TiB 2 grain size ranged between 2.2 and 6 µm. From Fig. 1b, the TG analysis reveals that the oxidation layer on TiB 2 starts forming after 800 °C.

TiB 2 Coatings
TiB 2 coatings are commonly applied on substrates such as graphite, molybdenum, steel, and nickel. Although graphite is preferred as a suitable substrate due to its thermal expansion coefficient (3.8 × 10 −6 K −1 ) has proximity to TiB 2 expansion value (6 × 10 −6 K −1 ). Zou et al. [16] suggested the TiB 2 -SiC coating (via supersonic atmospheric plasma spraying) on a graphite substrate, as the thermal expansion coefficient of the composite layer is much closer to that of graphite, which can minimize the thermal mismatch and reduce the possibility of micro-crack propagation. The adhesiveness between the coating and the substrate is essential for the performance of the coated cathodes. The TiB 2 coating can be achieved mainly using electrodeposition, chemical vapor deposition, and plasma spray technique.
Electrodeposition is considered a cost-efficient and simple method that can be performed at low temperatures around 700 and 1000 °C in molten salts. The electrochemically active precursors dissolved in the molten salts are cathodically deposited on a suitable substrate by applying appropriate potentials or current densities. In TiB 2 electrodeposition, precursors such as KBF 4 (source for B) and K 2 TiF 6 (source for Ti) are dissolved in fluoride or chloride melts and reduced on a substrate.
Wendt et al. [17] describe the cathodic deposition of TiB 2 on carbon electrode in FLiNaK melts (eutectic mixture of LiF, KF, and NaF) containing KBF 4 (2 to 10 mol.%) and K 2 TiF 6 (2 to 4 mol.%) at 700 °C. The concentration ratio C(B)/C(Ti) ranged between 2 and 3. It was noted that the TiB 2 coating was smooth for layers of thickness of less than 0.05 cm, while the coatings became rougher as the thickness exceeded 0.05 cm. A pure TIB 2 layer can be obtained when the electroreduction is performed at low current densities (around 0.1 A/cm 2 ). The current efficiency of the process was temperature-dependent and decreased with an increase in the working temperature. Regardless of different thermal expansion coefficients of carbon and TiB 2 , an adhesive and strong coating was formed, unlike in the case of a copper substrate where the TiB 2 coating cracked upon cooling. According to the electroreduction mechanism, B is reduced initially on the substrate, followed by the deposition of Ti, leading to the intermetallic bond between the B and Ti [18]. FLiNaK containing KBF 4 and K 2 TiF 6 solutes is considered an effective electrolyte due to its good electrochemical window that allows working with active Ti and B ions. It was found that high-purity TiB 2 coatings can be deposited using FLiNaK when the cathode current density is kept under 0.25 A/cm 2 . However, one of the biggest advantages of this electrolyte is being highly corrosive [19].
Similarly, Li and Li [20] used FLiNaK melt containing KBF 4 and K 2 TiF 6 solutes for cathodic deposition of TiB 2 on molybdenum substrate using continuous current plating (CCP) and periodically interrupted current (PIC) techniques. In the case of CCP, the deposition was performed between current densities of 0.1 and 1 A/cm 2 at 700 °C. Below 0.3 A/cm 2 , no TiB 2 was deposited on the substrate, meaning that the overpotential is not high enough for the reduction of Ti and B ions. At current densities between 0.4 and 1.0 A/cm 2 , metallic bright deposits were formed, and the surface morphology of coatings was similar. TiB 2 grain size decreased and the coating thickness increased with an increase in cathodic current density. At 0.5 A/cm 2 , the TiB 2 was adhesive towards the substrate but the TiB 2 layer was not so compact due to the presence of cracks and pores. Meanwhile, TiB 2 layers deposited at 0.5 A/cm 2 using the PIC technique (frequency = 100 Hz, the current time on/ time off = 4/1) contained fine grains and the coating was uniform and compact. Moreover, TiB 2 layers deposited using PIC have lower number of pores with lower diameter. Thus, TiB 2 coating deposited using PIC has superior morphology compared to the one deposited by CCP.
In Makyta et al. [21], it was found that TiB 2 electrodeposition in cryolite-based melts containing KBF 4 and K 2 TiF 6 components or the one containing B 2 O 3 and TiO 2 was not successful or coherent. The failure corresponds to the thermal deposition of the electrolytes at high temperatures. Meanwhile, a TiB 2 coating with good coherence and adhesion to the substrate was electrodeposited in KF-KCl-KBF 4 -K 2 TiF 6 melts at 800 °C. The electrodeposition was performed on molybdenum and graphite substrates. The TiB 2 layer formed on the graphite was a perpendicularly growing crystalline structure on the substrate. The thickness of the TiB 2 layer increased with an increase in the cathode current density. However, at high current densities, highly porous and irregularly shaped layers are formed. The governing reaction in the formation of the TiB 2 layer is as follows: Electrochemical deposition of TiB 2 on graphite in FLi-NaK melt was performed at 600 °C using a periodically interrupted current technique by Yvenou et al. [22]. The electrodeposition was conducted at two different current densities, − 0.12 and − 0.50 A/cm 2 , for various deposition times (10 to 75 min). As shown in Fig. 2, the coating thickness increases linearly (deposition rate 0.68 µm/min) with time and coincides with the theoretical thickness at j = − 0.12 A/cm 2 . In contrast, the thickness of the layer at j = − 0.50 A/cm 2 grows rapidly (deposition rate 5.8 µm/min) with time. A TiB 2 coating with a denser and preferential crystallographic structure was obtained at j = − 0.12 A/cm 2 . At j = − 0.50 A/cm 2 , a porous layer with numerous microcracks between the coating-substrate interface was observed. It was suggested that a denser and abrasive layer could be obtained at low applied current densities transversal microcracks were obtained, which results in the penetration of molten Al into the coating and culmination of Al.
Ozkalafat et al. [23] electrochemically deposited TiB 2 on nickel substrate in an oxide-type electrolyte containing Ti and B ions. The electrolyte comprises Na 2 B 4 O 7 (94 wt%) and Na 16 Ti 10 O 28 (6 wt%), the source for B and Ti, respectively. The electrodeposition was performed at varying parameters such as current density (0.05-0.150 A/ cm 2 ), temperature (800 − 1000 °C), and deposition time (30 − 360 min) to determine the optimal conditions. XRD methods confirmed the stoichiometry of the TiB 2 layer. The most uniform and thick coating were obtained at a current density of 0.07 A/cm 2 . Figure 3a, b shows the consistent distribution of Ti and B across the layer with a composition of 33 at.% Ti and 67 at.% B. The DZ in Fig. 3a represents the diffusion zone where Ti is dissolved in Ni and the formation of nickel boride. The TiB 2 layer thickness increases with time from 3 to 41 µm between 30 and 240 min, as shown in Fig. 3c. At temperatures above 950 °C, nickel diffusion into the TiB 2 coating was observed. Electrodeposition at 850 °C is recommended because at temperatures below, irregular and thin layers are formed with varying Ti:B ratios.
A novel method was proposed by Huang et al. [24] for the synthesis of TiB 2 cathodes. TiB 2 -TiB/Ti wettable cathode was prepared by boronizing the Ti substrate in Na 2 B 4 O 7 (75 wt%)−K 2 CO 3 (20 wt%)−B 4 C (5 wt%) electrolyte. The thickness of the TiB 2 was 10 µm after 3 h of boriding treatment with an applied current density of 0.2 A/cm 2 at 950 °C. The TiB interlayer between the TiB 2 and Ti acts as a binder. The TiB 2 was adhesive to the substrate. The main advantage of using this technique is that the thermal expansions of Ti, TiB, and TiB 2 are similar, resulting in superior binding force at elevated temperatures.
Kartal and Timur [25] synthesized TiB 2 by boriding the titanium using the "Cathodic Reduction and Thermal Diffusion based boriding (CRTD-Bor)" technique. In the CRTD-Bor method, two main steps are involved in the boriding of titanium substrate. Initially, the atomic borons are electrochemically reduced on the surface of the substrate (cathode). This is followed by the adsorption and diffusion of atomic boron on the surface of titanium, resulting in the formation of intermetallics such as TiB and TiB 2 . The boriding process was performed in an electrolyte with a composition of 90 wt% borax and 10 wt% sodium carbonate. The process was carried out at varying temperatures (900 °C to 1100 °C) and boriding time (15 min to 120 min) with a constant cathodic current density of 0.2 A/cm 2 . Findings suggest that even at low boriding durations (15 min and 30 min), homogeneous thick boride layers containing TiB and TiB 2 phases were formed. SEM cross-sectional micrograph of borided titanium reveals that the top layer was a TiB 2 phase while the intermediate layer between TiB 2 and the titanium substrate was TiB whiskers. The thickness of the TiB 2 layer and the width of TiB whiskers increased with an increase in process temperature. Irrespective of process duration and temperature, the outer TiB 2 layer and TiB whiskers were tightly bonded. The main advantages of this method are the formation of dense and adhesive coating within a short time and being environmentally friendly.
Plasma spray is a well-known technique with a high deposition rate that can be used for TiB 2 coating on different types of substrates [26]. For instance, a fine-lamellar structured TiB 2 with a thickness of 800 µm was deposited on a carbon substrate using the atmosphere plasma spray technique (APS) [27]. However, partial oxidation on the surface of TiB 2 was observed. The coating was made of a matrix combined with fully molten particles and agglomerated  [23]. Copyright 2016, Elsevier semi-molten TiB 2 . The coating by APS is resistant to aluminum carbide formation and sodium penetration [27]. Ananthapadmanabhan et al. [28] conducted oxygen analysis and electrical conductivity measurement on TiB 2 layer on alumina substrates using H 2 plasma. The results show that the TiO 2 and B 2 O 3 are formed on the surface, where the oxides are further converted to H 3 BO 3 . The electrical conductivity was 100 times lower compared to TiB 2 produced from the sintering process. The coating's oxidation behavior and electrical conductive were improved when the plasma spray was performed using Ar-H 2 plasma, which means that the environment of plasma spray influences the behavior of the TiB 2 layers.
Yvenou et al. [29] were the first to deposit micrometric TiB 2 particles on graphite substrate using the suspension plasma spray (SPS) technique. The SPS was performed under the Ar atmosphere to minimize the oxidation of the TiB 2 coating during the process. The results suggest that the use of Ar minimized the TiO 2 and B 2 O 3 formation on the layer. The presence of any oxides can enhance the penetration of molten Al into the coating. The SPS TiB 2 coating was cohesive with the graphite substrate. However, it was found that the TiB 2 was porous, and the TiB 2 particles were loosely bonded. The high porosity level is attributed to the TiB 2 particle's low transit time spent in the plasma, resulting in low and uneven melting. It was also found that the Al completely penetrates the coating and reacts with the graphite substrate, thus resulting in weak coherence between the TiB 2 layer and the substrate. A thick TiB 2 coating on a cemented carbide substrate (with 6 wt% Co) was fabricated using direct current magnetron sputtering (DC-MS) by Berger [30]. The deposition was carried out at 6 kW magnetron power, + 50 V substrate bias, and an argon pressure of 3 × 10 −3 mbar. The deposition rate of TiB 2 coating was 0.4 µm/min, with a total thickness of 60 µm for 150 min. Scratch test revealed that the TiB 2 coatings showed excellent cohesion to the substrate.
Chemical vapor deposition (CVD) is one of the most common coating techniques where the reactant gases chemically react in an activated (plasma, heating, laser) environment, which results in the formation of stable compounds on the substrates [31]. TiB 2 coatings can be readily deposited on substrates by CVD using different reagents. One of the most common sets of reagents is TiCl 4 , BCl 3 , and H 2 . Moers [32] was the first to utilize these reagents for TiB 2 coating, and the following reaction (4) takes place, where the reaction can be efficiently performed at a more comprehensive temperature range of 700 − 1400 °C.
The orientation of the substrate and the processing conditions influence the crystallographic structure of the TiB 2 layer [33,34]. Beckloff and Lackey coated TiB 2 layer by (4) TiCl 4(g) + 2 BCl 3(g) + 5H 2(g) → TiB 2(s) + 10HCl (g) CVD technique on graphite substrate using TiCl 4 , BCl 3, and H 2 reagents [35]. The studies suggest that the TiB 2 grain size increased from 0.5 µm to 3 µm with an increase in deposition temperature from 900 °C to 1100 °C. With decreased BCl 3 :TiCl 4 ratio and increased deposition temperatures, the grains were oriented parallel to the substrate. Becht et al. [36] observed that the TiB 2 layer depletion occurs when the deposition is performed at a BCl 3 :TiCl 4 ratio of 8. Pierson and andich suggested that the metallic substrates are not suitable for TiB 2 deposit using CVD, as the metallic substrates can form metallic chlorides, which is undesirable [37].

TiB 2 Composite Cathodes
Composite TiB 2 ceramic materials are of great interest as the pure TiB 2 materials are brittle and are difficult to machine due to their mechanical instability. The addition of other ceramic materials such as TiC, AlN, ZrB 2 , and ZrC to TiB 2 can enhance its mechanical properties. Namini et al. [38] studied the influence of SiC addition (15, 20, 25, 30 vol.%) on the mechanical properties of TiB 2 fabricated using vacuum hot pressing technique at 1850 °C for 2 h by applying 20 MPa. Figure 4 shows the influence of SiC vol.% in TiB 2 ceramic composite on relative density and porosity. The studies reveal that composite TiB 2 (70 vol.%) − SiC (30 vol.%) was dense with no porosity. Zhao et al. [39] reported that adding Ni up to 5 wt% could enhance the fracture toughness and hardness of the TiB 2 -SiC composite. Moreover, the Ni prevents the anisotropic growth of TiB 2 when prepared by reactive hot pressing.
Wang et al. [40] compared the creep behavior of graphite and TiB 2 -graphite (30 wt% TiB 2 , 50 wt% C, and 20 wt% binding agents) composite cathodes after specimens were subjected to the electrolysis process in Na 3 AlF 6 − KF (5 wt%) − LiF (5 wt%) − Al 2 O 3 (8 wt%) at 960 °C with cathode current density 0.5 A/cm 2 . It was observed that the TiB 2 -C composite had a lower creep strain (0.2%) and fewer microcracks compared to graphite. The composite was denser, less porous, and was entirely wetted by Al, which would prevent the electrolyte penetration. Fei et al. [41] also observed the superior relative density and flexural strength in TiB 2 /C composite compared to pure graphite. An increase of TiB 2 content up to 70 wt% can further improve the wettability of the material and prevent the penetration of Na and bath. However, when the TiB 2 content exceeds 70 wt%, TiO 2 oxide layer forms on the surface of the composite [42]. The electrical resistance of the TiB 2 /C composite decreases from 31.2 µΩ to 23.8 µΩ when the TiB 2 concentration increases from 30 to 60 wt% [43]. The electrical resistance decreases with a decrease in the TiB 2 particle size, which could be due to the material's low porosity.

Wettability, Interaction, and Corrosion Behavior of TiB 2
The wettability of TiB 2 by molten Al is dependent on the purity and relative density of the TiB 2 ceramic and temperature during the interaction. The most common method used to study wettability is the sessile drop technique. While looking for a cathode material for a VEC for aluminum electrolysis, the wettability of TiB 2 is excellent (having a sessile drop contact angle ≈ 0°). The dihedral angle equilibrium (5) governs the liquid-phase morphology in the grain boundaries of the solid-phase interface where γ B is the surface energy of the grain boundaries, γ SL is the surface energy of the solid-liquid interphase, and γ SL is the contact angle between the liquid phase and the grain boundary. Figure 5 shows surface forces acting at a point where the liquid phase meets the grain boundary of the solid phase. When γ SL is greater than 0.5γ B , then the equilibrium will establish a θ greater than zero. When θ = 120°, γ B is equal to γ SL . However, when γ SL is less than 0.5γ B , there is no value for θ that goes with Eq. (5), thus no equilibrium is established and liquid will penetrate along with the grain boundaries of solid [44].
Heidari et al. [45] studied the wettability and interaction behavior of porous TiB 2 ceramic (prepared from pressureless sintering [12]) with liquid Al. During the wettability test, for the first 9 min, no visible wetting of Al on TiB 2 was observed at 870 °C. With an increase in temperature to 940 °C (after 22 min), there was a visible wetting with a contact angle of 85°. After 50 min of contact, the contact angle of 6° was measured. The molten Al penetrated the pores of TiB 2 , and the additives started dissolving in the Al (see Fig. 6a). Three zones were observed at the Al penetration area (Fig. 6b). The first zone contains Al, the second has TiAl 3 phase, and the third zone has TiAl 3 and Fe 4 Al 13 phases, while the fourth zone is free of Al (see Fig. 6c).
Despite the Al penetration, no significant cracks or change in the geometry of TiB 2 was observed. Xi et al. [14] studied the wetting and interaction between dense TiB 2 ceramic (relative density of 98.7%) with molten Al between 700 and 1400 °C. Molten Al thoroughly wetted the TiB 2 at temperatures above 1000 °C. Al penetrated TiB 2 up to 250 µm at 1400 °C, Al x T, Al 4 C 3 , and Al 2 O 3 particles were found between the Al-TiB 2 interface, despite that TiB 2 grains remain attached. Raj and Skyllas-Kazacos examined the wettability of sintered TiB 2 cathodes through the electrolysis process in sodium cryolite melts (Al 2 O 3 unsaturated and saturated). TiB 2 cathode showed an excellent wetting property in the unsaturated melt [46]. However, TiB 2 was poorly wetted in the saturated melt due to the TiO 2 and B 2 O 3 formation and accumulation on the surface, interfering with Al deposition. Moreover, electrolyte penetration in TiB 2 cathode results in uneven wetting in saturated melts. TiO 2 and B 2 O 3 are readily soluble in the unsaturated melt; this could be why the oxide phases were not detected on TiB 2 cathodes tested in unsaturated melts. The wettability of the TiB 2 /C composite for the aluminum is time-dependent, as it requires time to remove impurities from the surface of the composite. The contact angle from the molten aluminum on TiB 2 /C composite reaches 0° after 90 min at 1000 °C [47].
The molten Al penetrates readily into porous TiB 2 , having a relative density of 90%, while the penetration by Al was ten times slower than the TiB 2 with a relative density of 96% [48]. Weirauch et al. [49] studied the wettability of TiB 2 (on different substrates) with molten Al at a constant temperature of 1025 °C. TiB 2 with 99.7% relative density and more than 99.8% purity has an initial contact angle of Penetration of liquid Al in TiB 2 reduces the flexural strength, hardness, and Young's modulus, resulting in the change of fracture mode from transgranular to intergranular [50]. The surface roughness (0.155 µm − 0.455 µm) of TiB 2 does not influence the wettability of aluminum [51].
Devyatkin and Kaptay investigated the wettability of TiB 2 coating electrodeposited on nickel and carbon substrates from Na 3 AlF 6 −Al 4 B 2 O 9 −CaTiO 3 melt [52]. The thickness of the TiB 2 layer was 20 µm with a deposition rate of 50 µm/h, irrespective of the substrate material. At 1000 °C, after a 4-min contact, the Al and TiB 2 coating (carbon substrate) contact angle was 30°, and the aluminum started penetrating the layer. In the case of TiB 2 coating on nickel substrate, the molten Al had a 0° contact angel, meaning the TiB 2 coating was thoroughly wetted. The solubility of TiB 2 in molten aluminum was estimated to be 6 × 10 -3 wt% after 10 h of exposure.
Kontrik et al. [53] investigated the corrosion behavior of TiB 2 (purity ≥ 98%, contains Ni as a sintering aid) in On the other hand, the TiB 2 coating (thickness 10 µm) on Mo substrate prepared in FLiNaK was tested for corrosion behavior in molten aluminum at 720 °C for 168 h [54]. No trace of corrosion on the TiB 2 layer was observed after the corrosion tests (wear rate 0 mm/year). Moreover, the TiB 2 was thoroughly wetted with molten aluminum.

Industrial Trials Using TiB 2 Cathodes
TiB 2 cathodes have shown some promising results during industrial trials conducted by Chinese researchers and have been well described in the review [5]. Ren et al. [55] performed industrial trials on carbon cathode blocks coated with TiB 2 /C compound layer (using vibration molding process) in a 300 kA aluminum reduction cell at Yichuan aluminum smelter plant. It was estimated that the voltage drop was up to 50 mV less than the conventional cells. Thus, saving energy up to 0.4 kWh/kg Al and improving the current efficiency by 1-2.5%. Titanium concentration in primary aluminum produced was around 0.0025 wt%. Authors suggest that the TiB 2 -based coating cathode blocks could prolong cell life, improve current efficiency, and save energy. Ban et al. [56] reported that the TiB 2 /C composite cathode coating solidified under ambient temperature performed stably in a 300 kA aluminum reduction cell and the average voltage drop was lessened up to 10.3 mV. The current efficiency of the cell was increased by 0.81%. The expected life of the TiB 2 /C composite cathode coating could be about 30 months. Tabereaux et al. [57] tested mushroom-shaped TiB 2 /graphite composite cathodes in a 70 kA aluminum reduction cell for four to five months at the Kaiser Mead smelter. They reported that when the cathode material was intact, the energy consumption of the cell was 8% lower than a conventional cell. However, with time, a breakage in TiB 2 /graphite cathode elements was observed causing cathode lining erosion. TiB 2 -based coatings have shown excellent wear resistance when tested in cells with current loads ranging from 100 to 300 kA. Cathode wear monitoring data showed that the cathode erosion was reduced to less than 4 mm/year while using TiB 2 -based coatings [57]. Feng et al. [58] tested TiB 2 coating cathodes in 1.35 kA drained cathode reduction cells, where the cathode had an inclination angle of 10°. After 100 h of electrolysis, the cell was still performing steadily and the current efficiency was 86% which was approaching the current efficiency of a conventional cell. The TiB 2 coatings showed no damage and had a low dissolution speed of about 1.0 g/h.m 2 in the electrolyte. TiB 2 /C cathodes were used in 92 kA drained cathode reduction cell of Comalco Aluminium Ltd. The lowest energy consumption was reported to be 12.8 kWh/kg Al [59]. However, no significant research was conducted on cathode drained cells and has failed to reach the expectation and industrial adoption [60].

Environmental and Economic Impact
The importance of inert anodes for aluminum electrolysis is well known. A new cell design with vertically placed electrodes enables energy savings and eliminates CO 2 emissions from the electrolysis. Such a cell design should include inert anodes and wettable cathodes. TiB 2 material has been considered a suitable cathode material due to its excellent wettability towards aluminum and good resistance towards electrolyte penetration, enabling a molten aluminum cathode surface. The TiB 2 also works at low ACDs, reducing cell potential and eventually reducing specific energy consumption. Using VEC can reduce the operating cost by up to 6% compared to the conventional cell [61]. The following table includes the energy savings from using wettable cathodes. RUSAL replaced graphite cathodes with wettable TiB 2 cathodes in conventional Hall-Heroult cells, which enabled reduction in specific energy consumption by up to 1.5 kWh/ kg Al. Norgate et al. [64] mentioned that the specific energy consumption required for aluminum could be reduced up to 30% if conventional aluminum reduction cells are replaced by VEC equipped with wettable TiB 2 cathodes and inert anodes (Table 1).

Conclusion
TiB 2 wettable cathodes have been studied for several decades due to their attractive properties and good wettability with molten aluminum, which is suitable for replacing non-wetted Replacing conventional cathodes by carbon cathode coated with TiB 2 layer in a cell with 14 kWh/kg Al 0.4 kWh/kg Al Industrial cell (300 kA), with a voltage drop up to 0.05 V (improving current efficiency by 2%) [63] Replacing conventional cell (with specific energy consumption 15 kWh/kg Al) with VEC (with TiB 2 cathodes and inert anodes) 4 kWh/kg Al Can reduce global warming potential by 30% [64] Replacing conventional cathodes by carbon cathode coated with TiB 2 layer in a drained cathode cell with 14.3 kWh/kg Al 2.3 kWh/kg Al Reduces the greenhouse gas emission by 2.3 t CO 2 e/t aluminum [65] cathodes and becoming an integral part of VEC with inert anodes. TiB 2 cathodes can be synthesized using sintering, electrodeposition, plasma spray, CVD, etc. Using sintering/ hot press techniques, the processing cost of TiB 2 is not on par with the manufacturing of cathodes, making it economically unsuitable for fabricating the cathodes. Although, TiB 2 deposition is an economically attractive process; however, there are a few obstacles, such as finding a suitable substrate (with a thermal expansion coefficient closer to TiB 2 ), finding an environmentally friendly electrolyte that does not release harmful byproducts, achieving fully dense and non-porous deposit, and finally, a coating resistant to electrolyte penetration. According to the literature, the electrodeposition should be conducted at low cathode current densities and low deposition temperatures to avoid microcracks and obtain non-porous coating. Moreover, the TiB 2 layer should have good wettability towards aluminum, which helps to increase cathode lifespan. The TiB 2 surface should be free of oxide layers as these layers act as barriers between molten aluminum and TiB 2 , further influencing the wettability. It is preferred to use lowtemperature electrolytes for a longer cell lifetime.
Funding Open access funding provided by NTNU Norwegian University of Science and Technology (incl St. Olavs Hospital -Trondheim University Hospital).

Conflict of interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.
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