AlF₃-Modified Carbon Anodes for Aluminum Electrolysis: Oxidation Resistance and Microstructural Evolution

Abstract

The aluminum electrolysis industry faces significant challenges due to the high consumption and environmental impact of carbon anodes, which are prone to oxidation in high-temperature and strongly oxidizing environments. This study innovatively introduces aluminum fluoride (AlF₃) as an additive to enhance the oxidation resistance of carbon anodes for aluminum electrolysis. By systematically exploring microstructural evolution through SEM, XRD, Raman spectroscopy, and permeability analyses, it reveals that AlF₃ inserts fluorine atoms into carbon interlayers, forming F-C bonds that reduce interlayer spacing while promoting graphitization. Simultaneously, AlF₃-derived α-Al₂O₃ particles densify the anode and make it more compact, reaching the optimum when 7 wt.% AlF₃ is doped. The bulk density of the carbon anode increased to 2.08 g/cm³, porosity decreased to 0.315, and air permeability reached a minimum of 2.3 nPm. In addition, the fluorine intercalation reduces the electrical resistance to 2.12 Ω via conductive F-C clusters. The demonstrated efficacy of AlF₃ additives in enhancing the oxidation resistance and conductivity of carbon anodes suggests strong potential for industrial adoption, particularly in optimizing anode composition to reduce energy consumption.

1. Introduction

As a basic industry with high energy and resource consumption, the core process of the aluminum electrolysis industry is to produce metallic aluminum through the electrolysis of aluminum oxide (Al₂O₃) [1,2,3,4]. In this process, the carbon anode, as the core component of the electrolyzer, assumes the dual functions of conducting electricity and participating in electrochemical reactions [5,6,7]. However, carbon anodes are susceptible to oxidation reactions at high temperatures (about 960 °C) and strong oxidizing environments (O₂, CO₂, etc.) during the electrolysis process, leading to the overconsumption of anodes [8,9,10]. This not only significantly increases the production cost (the cost of carbon anodes accounts for 10–15% of the total cost of aluminum electrolysis) but also releases greenhouse gases, such as CO₂, and aggravates the burden on the environment [11,12,13]. Therefore, how to improve the oxidation resistance performance of carbon anodes has become a key issue that needs to be solved in the aluminum electrolysis industry.

In recent years, academia and industry have conducted a lot of research around the improvement of the oxidation resistance performance of carbon anodes, mainly focusing on the three aspects of material optimization, additive development, and process innovation. Among them, improving the microstructure and reactivity of carbon anodes by introducing additives is a hot topic in current research [14,15,16]. Additives mainly prevent the oxidative damage of carbon anodes in a strong oxidizing environment by reducing porosity or forming a protective film on the surface of the carbon anode, enhancing the chemical stability of the carbon anode [17,18]. For example, the addition of biochar significantly enhanced the coking value, quality index (QI) content, density, viscosity, and softening point of bio pitch but had less effect on surface tension and molecular weight distribution. When the biochar addition was 9.0 wt.%, the modified bio pitch showed the best overall performance and might replace coal tar pitch (CTP) as a binder, as well as reduce the total consumption in the anode formulation and improve the oxidation resistance property of the carbon anode [19]. Aluminum powder enhanced the oxidation resistance property by enhancing the coking property of carbon anodes, increasing the degree of graphitization and reducing the porosity. The effect may originate from the catalytic effect of aluminum volume expansion on pitch pyrolysis during calcination [20]. Ren [21] et al. found that the addition of aluminum powder significantly reduced the CO₂ reactivity of carbon anodes at 970 °C. With the addition of 0.8 wt.% aluminum powder, the degree of anodic oxidation and the shedding rate were reduced by 32.8% and 70%, respectively, compared with the anode without additives, and the total consumption rate was reduced to 39.28 mg·cm⁻²·h⁻¹.

As an important component of the electrolyte system for aluminum electrolysis (usually forming a eutectic system with cryolite Na₃AlF₆), the concentration of aluminum fluoride (AlF₃) directly affects the melting point, viscosity, and ion mobility of the electrolyte and the oxygen partial pressure of the electrolyte melt, thereby indirectly inhibiting the surface oxidation reaction of the carbon anode [22]. However, most of the existing studies focus on the effect of AlF₃ on the physicochemical properties of the electrolyte, while the effects of AlF₃ as an additive on the intrinsic oxidation resistance performance of the carbon anode still lacks systematic exploration. It is noteworthy that the evaporation loss of AlF₃ will also aggravate the fluorination reaction on the carbon anode surface, resulting in the formation of CF₄ and C₂F₆ [23]. This phenomenon indicates that changes in the content of AlF₃ may further affect the oxidation behavior of carbon anodes by changing the reaction conditions at the anode–electrolyte interface [24,25]. However, the current studies on the quantitative relationship between AlF₃ content and the oxidation resistance performance of carbon anodes are still insufficient, especially since the synergistic mechanism during long-term electrolysis has not been clarified.

This study systematically investigates the role of AlF₃ additives in enhancing the oxidation resistance of carbon anodes for aluminum electrolysis. By incorporating varying AlF₃ concentrations (1–7 wt.%), we examine its influence on the microstructural evolution through comprehensive material characterization.

2. Results and Discussion

2.1. Effect of AlF₃ on Micromorphology of Carbon Anodes

Figure 1 shows the microscopic morphology of carbon anodes doped with varying contents (1 wt.%, 2 wt.%, 5 wt.%, and 7 wt.%) of AlF₃, respectively. As can be seen from Figure 1a–h, compared with the morphology of the carbon anode without doping AlF₃, the layered structure of the carbon anode is more obvious with the addition of AlF₃, and some residues appear on the surface of the carbon anode. When the AlF₃ content was 2 wt.%, cracks appeared on the surface of the carbon anode and the bifurcation increased, which may be due to the difference in the coefficient of thermal expansion [26,27]. When the AlF₃ content is increased to 5 wt.%, it can be seen that the number of carbon anode layers is organized and increased, and the surface becomes smoother. Where some localized enrichment can also be initially seen on the surface of the carbon matrix at lower magnification, as in Figure 1e,g, Figure 1i shows the swept energy spectrum analysis from the sample surface of Figure 1f, which shows that the surface contains the elements Al, O, C, and F. Among them, Al and O are distributed on the lamellar surface, which may be due to the oxidation reaction of AlF₃ with carbon or oxygen in the air at high temperatures, and the resulting Al₂O₃ particles will adhere to the surface. The chemical equation is given in Equations (1) and (2).

C + AlF₃ → CFx + Al₂O₃

(1)

AlF₃ + O₂ → Al₂O₃ + F₂

(2)

The fluorine (F) and aluminum (Al) elements are also observed between the layers from Figure 1i. This means that AlF₃ has entered the interlayer of the carbon anode. When the AlF₃ content reaches 7 wt.%, as shown in the red circle in Figure 1h, clusters begin to appear in the interlayer.

When AlF₃ is added to carbon anodes, AlF₃ can interact with the layered structure of carbon, especially at high temperatures, where fluoride ions in AlF₃ may interact with the interlayer forces of the carbon material to insert into the carbon layers [28,29]. When a large number of F atom enters the intercalation layer and aggregates, clusters form between the layers, as shown in the red circle in Figure 2. Figure 2 also shows the single-layer ball-and-stick model of F inserted into the carbon layer. All valence bonds of the C atom in the model are involved in bonding, and there are no lone electrons or unsaturated bonds that are not involved in bonding [30,31], which improves its chemical stability and thus is favorable for the improvement of anti-oxidation properties.

2.2. Effect of AlF₃ on Crystalline Microstructure of Carbon Anodes

Figure 3 shows the XRD diffraction patterns of carbon anodes doped with varying AlF₃ contents (1 wt.%, 2 wt.%, 5 wt.%, and 7 wt.%). As shown in Figure 3a, the presence of Al₂O₃ in the carbon anode is confirmed by the appearance of characteristic peaks representing the crystalline phase. Al₂O₃ mainly exists in the form of α-Al₂O₃ and γ-Al₂O₃, where α-Al₂O₃ is the most stable high-temperature phase, which is usually formed at high temperatures. Its main diffraction peaks are located at 35.1°, 43.3°, 57.4°, and 66.5°, corresponding to the crystal planes (104), (110), (113), and (116), respectively. γ-Al₂O₃ is a common amorphous or metastable form, which is more stable at low temperatures. Its diffraction peaks are generally around 17°, 32.2°, 45.9°, and 67.5°, corresponding to the crystal planes (111), (220), (311), and (400), respectively.

The microcrystalline parameters of the carbon anode were calculated using Bragg’s equation, as shown in

Table 1. Microcrystalline parameters of carbon anode doped with varying AlF₃ contents.

AlF3 Content (wt.%)	2θ (°)	FWHM	d002 (nm)	LC (nm)	N
0	25.914	2.983	0.3435	2.7017	7.8645
1	25.776	3.2993	0.3453	2.4425	7.07287
2	25.769	3.3239	0.3454	2.4244	7.0185
5	25.650	1.057	0.3470	7.6223	21.9655
7	25.815	0.960	0.3448	8.395	24.3459

. As the AlF₃ content increases, the layer spacing (d₀₀₂) increases and then decreases. At 5 wt.% AlF₃ doping, the layer spacing reaches 0.3470 nm. Beyond this threshold, when the AlF₃ content exceeds 5 wt.%, the crystallite size (L_C) increases significantly. This is attributed to the higher AlF₃ content, which enhances the intercalation density of F atoms within the carbon anode. This intercalation induces the structural reorganization or expansion of the layered structure, subsequently influencing the grain size of the carbon material [28]. At 7 wt.% AlF₃ doping, the number of layers rises to 24.3459, while the layer spacing decreases to 0.3448 nm. At this doping level, the full width at half maximum (FWHM) of the diffraction peak reaches its minimum value. Additionally, as shown in Figure 3a, the XRD diffraction peaks exhibit increased intensity and sharper profiles, suggesting that higher AlF₃ content improves the atomic arrangement within the carbon anode and reduces the presence of amorphous or disordered phases. This indicates enhanced crystalline ordering of the carbon anode at this concentration [32].

Raman spectroscopy analysis is shown in Figure 3b. It reveals that the I_D/I_G ratio initially increased and then decreased with varying AlF₃ doping content. At 1 wt.% AlF₃ doping, the I_D/I_G ratio reached 1.10, which may be attributed to the intercalation of a small amount of AlF₃ disrupting the ordering of the carbon crystal lattice. This disruption likely impedes the transformation of amorphous carbon into a graphite structure. However, the I_D/I_G ratio decreased to 0.95 at 7 wt.% AlF₃ doping content. This reduction could be linked to the incorporation of fluorine (F) into the carbon layers. The intercalation effect of AlF₃ may enhance the stability of the crystal structure during graphitization, promoting the formation of larger graphite size particles at high temperatures within the carbon anode [30,33].

2.3. Effect of AlF₃ on Oxidation Resistance Properties of Carbon Anodes

Figure 4 illustrates the bulk density and air permeability of carbon anodes doped with varying AlF₃ contents (1 wt.%, 2 wt.%, 5 wt.%, and 7 wt.%). The slight decrease in bulk density observed at lower AlF₃ concentrations can be attributed to the fact that small amounts of AlF₃ may react with the carbon, causing volume expansion or contraction. Furthermore, the bonding between AlF₃ and the carbon may not fully fill the voids, resulting in an increase in the number of localized pores. At high temperatures, AlF₃ is also prone to react with moisture, oxides, or other elements in the carbon, releasing gases (e.g., fluorine gas or other volatiles). The release of these gases can cause an increase in the pores within the carbon anode, thereby reducing its bulk density.

However, combined with XRD analysis, it can be seen from Figure 4a that with the increase of AlF₃ doping content, the bulk density of the carbon anode is improved due to the formation of stable α-Al₂O₃ [34,35]. In addition, AlF₃ itself has a high density (2.5–3.0 g/cm³), which can fill the voids inside the carbon anode through intercalation, thereby improving the density of the carbon anode. Compared with the density of an ordinary carbon anode, which is 1.5–1.8 g/cm³, when the carbon anode contains 7 wt.% AlF₃, the bulk density reaches 2.08 g/cm³ and the porosity is 0.315.

According to the permeability test results, when the content of AlF₃ is 7 wt.%, the permeability reaches the minimum value of 2.3 nPm, as shown in Figure 4b. This is due to the fact that the higher AlF₃ content leads to the formation of a large number of stable α-Al₂O₃ phase particles, which improves the surface densification of the carbon anode and reduces the air permeability. In addition, the high content of AlF₃ forms a saturated F-C structure in the intercalation layer [36], which improves the internal stability of the carbon anode and enhances its oxidation resistance.

2.4. Effect of AlF₃ on Resistance Properties of Carbon Anodes

Figure 5a shows the average resistance values of carbon anodes doped with varying AlF₃ contents (1 wt.%, 2 wt.%, 5 wt.%, and 7 wt.%). With the increase in AlF₃ content, the average resistance value of the carbon anode increases and then decreases. When the content of AlF₃ is 1 wt.%, the average resistance value reaches the maximum value (2.65 Ω). This is due to the fact that AlF₃ dispersion in the carbon anode affects the arrangement and binding of carbon, thus increasing the barrier for the carbon anode current to pass through, leading to a rise in resistance. AlF₃ in the carbon anode also does not interact strongly enough with C and may not be able to sufficiently form a conductive path. And there are more disordered carbon regions or defects in the carbon anode, resulting in limited electron conduction and high resistance.

At an AlF₃ content of 7 wt.%, the average resistance decreases to 2.12 Ω. This reduction is attributed to the high AlF₃ concentration, which significantly enhances the sintering and bonding of carbon particles, fostering closer interparticle connections and forming a more efficient conductive pathway [37]. As illustrated in Figure 5b, the increased density of the carbon anode reduces pore space, shortening the effective path length for current flow and consequently lowering resistance [13]. Furthermore, the elevated AlF₃ content ensures a relatively uniform distribution within the carbon anode, improving ion diffusion. The resulting F-C structure intercalates between carbon layers [38], forming clusters that serve as conductive bridges. This process reduces defects in the carbon material, enhances the ordering of the crystal lattice, and ultimately improves overall conductivity.

3. Materials and Methods

3.1. Experimental Materials and Preparation

Calcined coke (MC Zhenjiang Anode Solutions Co., Ltd., Zhenjiang, China) as the raw material and coal pitch (MC Zhenjiang Anode Solutions Co., Ltd., Zhenjiang, China) as the binder were used to prepare the carbon anode through the steps of batching, mixing and kneading, molding, and sintering. The ratio of calcined coke was coarse coke/medium coke/fine coke/powder = 26:31:9:34. The particle sizes of coarse coke, medium coke, fine coke, and powder were 3–6 mm, 1–3 mm, 0.074–1 mm, and less than 0.074 mm, respectively. The amount of powdered coal pitch added through a 60-mesh (0.25 mm) sieve was 15.3 wt.% of the total raw material. Different contents (1 wt.%, 2 wt.%, 5 wt.%, and 7 wt.%) of AlF₃ were taken as additives for mixing and kneading with the raw material.

The calcined coke and coal pitch were dried and mixed at 120 °C for 30 min to form a mixture. A total of 65 g of the mixture was weighed and placed into a mold preheated to 130 °C. The mixture was then molded into green anodes with dimensions Φ50 × 20 mm using a hydraulic press. The green anode was placed in a crucible, which was buried in the metallurgical coke((MC Zhenjiang Anode Solutions Co., Ltd., Zhenjiang, China) to ensure that the carbon anode was completely isolated from the atmosphere. The sintering temperature and keeping time were carried out according to Figure 6. As shown in the figure, the temperature was rapidly raised to 200 °C and then slowly rose to 400 °C at a heating rate of 1 °C/min. After 30 min of heat preservation, the temperature was heated to 800 °C at 2 °C/min. After 15 min of heat preservation, the temperature was heated to 1100 °C for 30 min.

3.2. Characterization Method

The micromorphology of the carbon anodes was observed with a field emission scanning electron microscope (NovaNanoSEM450, FEI, Portland, OR, USA), the air permeability of the carbon anode samples was characterized by an air permeability tester (TD-ISPC-27, Beijing Tongde Entrepreneurship Technology Co., Ltd., Beijing, China), and their phase structure and crystallinity were analyzed using an X-ray diffractometer (D8 Advance, BRUKER-AXS, Karlsruhe, Germany). The layer spacing, layer stacking height, and number of stacked layers of carbon anodes were calculated by Bragg’s Equations (3), (4), and (5), respectively [39,40,41].

$$d_{002}={\displaystyle \frac{\lambda }{2\sin {\theta }_{002}}}$$

(3)

$$L_C={\displaystyle \frac{0.90\lambda }{{\beta }_{002}\cos {\theta }_{002}}}$$

(4)

$$N={\displaystyle \frac{L_C}{d_{002}}}$$

(5)

where λ is the X-ray wavelength (0.15418 nm); θ₀₀₂ denotes the diffraction angle (°) corresponding to the maximum diffraction peak of the (002) crystalline surface and d₀₀₂ characterizes the spacing of the laminae (nm); L_C is the height of the lamellae stacking (nm); and N denotes the number of stacked layers.

The internal structure of carbon anodes was analyzed by a laser microscopy Raman spectrometer (Thermo Fisher DXR, Scientific, Waltham, MA, USA), typically using I_D/I_G to characterize the degree of graphitization of the carbon material. And the samples were treated in a vacuum drying oven (DZF-6020, Shanghai Jiecheng Experimental Instrument Co., Ltd., Shanghai, China); then, the bulk density and porosity of the carbon anodes were determined by the Archimedes drainage method, in which the bulk density was calculated according to Equation (6) and the porosity was calculated according to Equation (7).

$$\rho ={\displaystyle \frac{m_1}{m_3-m_2}}\times {\rho }_{{\mathrm{H}}_2\mathrm{O}}$$

(6)

$$\Phi ={\displaystyle \frac{m_3-m_1}{m_3-m_2}}$$

(7)

where m₁ represents the dry weight when the mass is weighed, m₂ represents the suspended weight after submerging the sample in water for 30 min, and m₃ refers to the wet weight weighed directly after removing it from the water.

The electrical resistance of carbon anodes was tested by using a multimeter. The resistance of the specimens was tested five times, and the average value was taken as the average resistance of the specimens.

4. Conclusions

This study demonstrates that AlF₃ as an additive enhances the oxidation resistance of carbon anodes by improving their microstructure, bulk density, and crystallinity. At 7 wt.% AlF₃ doping, optimal performance of the AlF₃-modified carbon anode was achieved, including increased bulk density (2.08 g/cm³), reduced porosity (0.315), and minimized air permeability (2.3 nPm). The intercalation of fluorine into carbon layers promoted graphitization, stabilized the crystal structure, and lowered electrical resistance (2.12 Ω) by forming conductive pathways. This innovative approach exhibits remarkable potential for industrial implementation due to its demonstrated ability to significantly improve oxidation resistance and electrochemical performance. Particularly noteworthy is its capacity to optimize anode composition, leading to substantial reductions in energy consumption. These advantages collectively present an environmentally favorable solution for mitigating the ecological footprint of aluminum electrolysis processes.