On porosity-overvoltage correlation for carbon anodes in cryolite-alumina melts

The anodic overvoltage in aluminium electrolysis was studied by steady-state current/voltage measurements with carbon anodes of different porosities: vitreous carbon, which is a non-porous material, spectral graphite, which contains micropores and industrial baked carbon of 8–24% porosity (P₁) due to pores larger than 7.5μm. Replacement of vitreous carbon with baked carbon was accompanied by a drastic decrease in overvoltage. For a given overvoltage, baked carbon yields a linear increase of cd with increasing P₁. It was established that the larger the P₁ parameter, the narrower is the range of Tafel behaviour, the lower are the Tafel slopes and the higher is the exchange current. These findings were interpreted in terms of reaction and diffusion overvoltage; their contribution depends directly on the porosity P₁. The higher P₁ for baked carbon, the larger is the diffusion overvoltage due to the diffusion into the anode pores.     

Double layer capacity of graphite in cryolite-alumina melts and surface area changes by electrolyte consumption of graphite and baked carbon

The double layer (d.l.) capacity of pyrolytic graphite in cryolite-alumina melts at 1010°C was found to exhibit a minimum of 20F cm^–2 at 0·9 V positive to the aluminium electrode. The d.l. capacity attained a plateau of 60F cm^–2 at 1·1–1·4 V, while it rose steeply at potentials below 0·7 V. During electrolytic consumption involving CO₂ evolution the d.l. capacity of pyrolytic graphite remained unaffected, while that of baked carbon was changed, reflecting changes in surface area. At low current densities (cds) the surface area increased substantially and the surface was noticeably roughened, while the opposite behaviour was observed at above 2.5 A cm^–2.     

Activity of anodic oxide films on metal and cermet anodes in cryolite-alumina melts

The activity of anodic oxide films on nickel, iron and copper metal anodes in cryolite-alumina melts was calculated using values of the reversible potential obtained from a polarization scan of the corresponding metal electrode. The best results were obtained with prepolarized electrodes. The anodic oxide layer formed on nickel becomes thick and dense and remains adherent during the period of prepolarization. Similar activity calculations were made for selected nickel-, iron- and copper-cermet compositions containing either a MnZn ferrite or a nickel ferrite ceramic phase. Large activities were observed for a NiO type corrosion product on both the nickel and nickelcermet electrodes. The results suggest that a dense surface layer containing a NiO phase is formed on nickel-cermet electrodes. This layer may help lower corrosion by minimizing electrolyte penetration of the anode surface.     

Synthesis and testing of uniform and gradient materials for inert anodes used in the electrolysis of cryolite-alumina melts

Investigations concerning the development of materials that are thermostable in highly reactive electrolytes have been performed. Five types of anodes differing in the technology of production and in the composition of the working layer have been synthesized. The greatest stability in tests performed in cryolite-alumina melts in an electrolysis cell working at a temperature of 960 ± 2°C, a current density 0.8 A/cm², and an electrolysis duration of 72 h is shown up by uniform and gradient metal-ceramic anodes with variable concentrations of the metallic and oxide phases prepared by hot pressing. These samples represent virtually porousless (in the second variant, layered) compositions whose working surface or the outer layer increases the resistance to the action of both atomic oxygen and electrochemical corrosion during electrolysis.     

On the anode effect in cryolite-alumina melts—I

The behaviour of graphite anodes in cryolite-alumina melts prior to and during the occurrence of the anode effect (AE) was studied by potential-sweep and galvanostatic measurements. In melts with low alumina contents a potential jump was observed, corresponding to the beginning of co-discharge of fluoride ions. The AE appears to be preceded by a depletion of oxygen-containing ions followed by codeposition of fluorine, which then provokes the AE. The i/V curves during the AE have a complicated shape with maxima at 45 and 95 V. Sparks were seen in the ranges below 45 V and from 90 V until an arc was lit at 140 V. Anodes maintained at 20–45 B vecame polished.     

Specificity of anodic processes in cyclic voltammetry to the type of carbon used in electrolysis of cryolite-alumina melts

Anodic processes associated with oxidation of carbon anodes used in electrolysis of cryolite-alumina melts, simulating the Hall-Héroult process, were studied by means of cyclic voltammetry in a comparative way at four graphitic carbon materials and at glassy carbon. Conditions were sought that give a current response function characteristic of diffusion-controlled oxidation of the anode by O^2– or oxyfluoride complex anions. Only at glassy carbon anodes are such conditions realized with a linear relation between response current in cyclic voltammetry and Al₂O₃ content in the melt. At the graphitic materials, complex mixed activation and diffusion controlled processes arise that are also relatively irreproducible from one experiment to the next, probably due to irreversible changes of the graphite surfaces. The effects of aluminium metal dissolved in the melt, to simulate practical smelter cell conditions, were also evaluated.     

On the cathodic overvoltage on aluminium in cryolite-alumina melts—I

When a constant cathodic current is applied to an aluminium electrode in a Na₃AlF₆Al₂O₃ melt at 1010°C, the potential decreases gradually and linearly with the square root of time to more negative values. Pronounced potential oscillations occurred at cds above 2A/cm² and the evolution of sodium gas was observed at very high cds. Steady state measurements yielded straight η vs log i plots with a slope of ?0.23V/decade, the overvoltage being ?0.19V at 1A/cm². The overvoltage decreased markedly when the melt was stirred. Potential decay measurements yielded linear log time plots with slopes of 0.01–0.5V/decade. The charge-transfer resistance was determined by double pulse measurements to be 0.0031 ohm cm². Ac impedance measurements gave a similar result. The charge transfer overvoltage accounts for only about two percent of the total overvoltage, the rest is apparently diffusion controlled. The cathodic overvoltage in industrial aluminium cells is of similar magnitude as found in the laboratory investigations.     

Kinetics and mechanism of the Al(iii)/Al electrode reaction in cryolite-alumina melts

Using the relaxation method (RM) with galvanostatic perturbation and electrochemical impedance spectroscopy (EIS), exchange current densities and activation parameters were determined for the electrode reaction on the aluminium electrode in pure cryolite melt and in cryolite-alumina melts with the addition of 2–12 wt % Al₂O₃. In all these melts a three step electrode process was observed, comprising a preceding chemical reaction followed by two charge transfer steps. The exchange current densities for two charge transfer steps were determined as a function of temperature, together with the equilibrium constant of the preceding chemical reaction and its kinetic and diffusion impedance. The third step was found to be independent of diffusion and of the concentration of alumina, whereas the second step showed mixed characteristics. The exchange current densities were of the order of 5–15 A cm^–2.

Abbreviations

List of symbols A electrode area (cm²) - A, B, C coefficients of Equation 12 (V) - C _dl double layer capacitance (F) - c concentration (mol cm^–3) - D _O; D _R diffusion coefficients of the oxidized and reduced ionic species (cm² S^–1) - E _A activation energy (kJ mol^–1) - F Faraday constant (C mol^–1) - j exchange current density (A cm^–2. - K equilibrium constant (dimensionless) - k sum of the forward (k ₁) and backward (k ₂) rate constants (s^–1) - k ₀ standard rate constant (cm s^–1) - L ₁, L ₂ high frequency and low frequency inductances (H) - L _out outer inductance (H) - R _el electrolyte resistance () - n number of electrons (dimensionless) - R molar gas constant (J mol^–1 K^–1) - R _ct charge transfer resistance () - s ₁, s ₂ angular frequencies (s^–1) - T temperature (K) - t time (s) - Z complex impedance () - Z, Z real and imaginary parts of the complex impedance () - Z _w Warburg diffusion impedance ( s^–1/2) Greek symbols _c overall cathodic transfer coefficient (dimensionless) - , coefficients in Equation 12 (s^–1/2) - coefficient in Equation 12 (s^–1) - overpotential (V) - angular frequency (s^–1) - _O, _R Warburg coefficients of the oxidized and reduced ionic species ( s^–1/2)     

Comparison of the behaviour of glassy carbon and some metals for use as nonconsumable anodes in alumina-cryolite melts

Current interest exists in development of nonconsumable anodes for the Hall-Heroult process of aluminium production and also in situ analytical probes for determination of Al₂O₃ content in the cryolite melts used in this process. A comparison of the behaviour of glassy carbon and metals such as tungsten, tungsten carbide, nickel and stainless steel (SS-316) used as anodes in alumina-cryolite melts is investigated by means of electrochemical transient techniques (cyclic voltammetry and chronoamperometry) and Tafel anodic polarization experiments. The results show that only glassy carbon could be used as a successful sensor electrode for an in situ determination of Al₂0₃ in alumina-cryolite melts and that the metals investigated are unresistant to anodic attack in such melts. Consequently, the metals investigated cannot be used as sensor electrodes for in situ electro-analytical determination of alumina in alumina-cryolite melts, nor as anodes in the production of aluminium by the Hall-Heroult process.     

On the anode effect in cryolite-alumina melts—II the initiation of the anode effect

Polarization curves obtained with graphite anodes in cryolite-alumina melts by steady state, potential sweep and galvanostatic measurements exhibited three steps at low alumina concentrations, due to the formation of CO₂ and CF₄, prior to the anode effect (AE), and probably of F₂ after the occurrence of the AE. Formation of CF₄ started at ~2·8 V positive to the aluminum electrode, while the potential and the critical current density (ccd) at which the AE occurred were 3·5 V and 0·11 A/cm² resp. in purified cryolite and 2·7 V and 2·1 A/cm² at 1 wt per cent Al₂O₃. The proportion of CF₄ in the gas reached 40 per cent in purified melts, but decreased sharply on the addition of alumina. At > 2 wt per cent Al₂O₃ the ccd decreased when the external pressure was lowered approximately in proportion to the volume of escaping gas. It is concluded that the AE occurs when the melt adjacent to the anode is depleted with respect to oxygen-containing ions, and the rate of fluoride ion discharge exceeds the rate of CF₄ formation.     

The faradaic impedance of the carbon anode in cryolite-alumina melt

'The anode reaction on pyrolytic graphite, regular graphite and baked carbon in cryolite-alumina melts was studied by impedance measurements within the cd range 0.05–0.7 A cm⁻². The anode process was found to be reaction controlled on pyrolytic graphite at above i_a = 0.25 A cm⁻² and on regular graphite at above i_a = 0.7 A cm⁻². At lower cds a combination of reaction and diffusion control was observed, and for baked carbon this was the case over the entire cd range. The diminishing influence of diffusion control with increasing cd may be caused by suppression of the anode reaction within pres due to blocking by anode gas. The double layer capacity (C_dl) increased considerably with increasing cd on pyrolytic and regular graphites, while on baked carbon it remained uneffected above i_a = 0.1 A cm⁻². The increase in C_dl was attributed to enlargement of the anode surface area during electrolysis. The anodic overvoltage, porosity and reactivity were also investigated. For all tested anodes Tafel slopes of about 0.3 V dec⁻¹ were found. The overpotential increased with decreasing porosity and reactivity.     

Layer disorder in carbon anodes

Inter- and intra-layer disorder in a representative hard carbon derived from epoxy novolac resin at 1000°C have been investigated by comparing both the elastic structure factor S(Q) and radial distribution function obtained from pulsed neutron time-of-flight measurements with calculations from simple hypothetic systems with specific disorder characteristics. The spatial relationship between these building blocks, planar graphene fragments with length scale of 1 nm, is probed in terms of inter-layer alignment and intra-layer connectivity. Coupled with the density of dangling bonds obtained from ESR and residual hydrogen concentration, these simulations provide sufficient information to construct an overall model of the carbon structure. Of special interest is the physical origin of the ubiquitous broad (002)-like reflection frequently interpreted as that from layers with a uniform interlay spacing (d₀₀₂) It is found to be directly related to the misalignment between layers with an average interlayer spacing as that in crystalline graphite (3.35 Å).     

Chemical and electrochemical oxidation of heterogeneous carbon anodes

At the electrode electrolyte interface, three reactions have been found to occur during the electrochemical oxidation of carbon while electrowinning aluminium. The three reactions are: electrochemical evolution of carbon monoxide, which is dependent on the structure of the carbon; electrochemical evolution of carbon dioxide; and thirdly a chemical reàction between carbon dioxide and carbon which occurs in the pore structure immediately behind the electrode interface. The first and last mentioned of these are very strongly dependent on the carbon structure real surface area and porosity of constituent carbons leading to differential reactivity in the normal heterogeneous anodes.     

Specific anion and cation capacitance in porous carbon blacks

Porous carbon black was modified to introduce oxygen and nitrogen surface functionality and characterized using wet titration methods, elemental analysis, XPS, TEM, thermal analysis, adsorption of nitrogen and carbon dioxide. Then the electrochemical capacitance was measured for cations and anions in 1 M H₂SO₄. The results were compared to those obtained on activated carbons. The modified carbon black samples have four times higher adsorption of anions than cations. It is hypothesized that the graphitic microstructure of carbon blacks with structural defects is responsible for intercalation of anions in-between the graphene layers which take place at potentials higher than 0.75 V vs. Ag/AgCl and at moderately low current loads. This process is reversible and the deintercalation occurs at approximately 0.4 V vs. Ag/AgCl during the cathodic reduction. Introduction of nitrogen have generally a detrimental effect on the anion adsorption capacitance due to a structural defects blockage. At high current loads this phenomenon of enhanced anion adsorption capacitance becomes less pronounced due to the kinetic limitations. For amorphous activated carbons enhanced anion electrosorption at low current loads is governed by proton assistance while at high current loads larger cation capacitance is due to the pseudocapacitive interactions of protons with nitrogen and oxygen functional groups.     

敞盖环式焙烧炉焙烧铝用阴极炭块的工艺技术探讨

利用敞盖环式焙烧炉,通过优化焙烧升温曲线,规范填充料、装炉等工艺技术操作,成功焙烧出高质量铝用阴极炭块.     

木质素衍生炭在碱金属离子电池负极中的研究进展

碱金属离子在商品化石墨负极材料的嵌入/脱出过程中会发生较大的体积膨胀,导致容量衰减快、倍率性能差等问题。木质素衍生炭材料具有原料丰富、经济、制备工艺简单及结构可控等优点,作为碱金属离子电池负极表现出较高的容量、较好的倍率性能和循环稳定性。木质素衍生炭材料在过去十多年中取得了一些研究进展。基于此,简要介绍了碱金属离子电池碳材料负极的储能机理及特点,系统综述了木质素衍生炭材料在碱金属离子电池负极材料中的最新研究进展,重点总结了其合成策略、结构特征、储存机理以及其电化学性能等,指出了层间距调控、碳层排序和表面功能化与电化学性能之间的构效关系。此外,拓展概述了木质素衍生炭材料的发展前景和面临的挑战,为木质素衍生炭材料的下一步研究和开发提供参考。     

木质素衍生炭在碱金属离子电池负极中的研究进展

碱金属离子在商品化石墨负极材料的嵌入/脱出过程中会发生较大的体积膨胀,导致容量衰减快、倍率性能差等问题。木质素衍生炭材料具有原料丰富、经济、制备工艺简单及结构可控等优点,作为碱金属离子电池负极表现出较高的容量、较好的倍率性能和循环稳定性。木质素衍生炭材料在过去十多年中取得了一些研究进展。基于此,简要介绍了碱金属离子电池碳材料负极的储能机理及特点,系统综述了木质素衍生炭材料在碱金属离子电池负极材料中的最新研究进展,重点总结了其合成策略、结构特征、储存机理以及其电化学性能等,指出了层间距调控、碳层排序和表面功能化与电化学性能之间的构效关系。此外,拓展概述了木质素衍生炭材料的发展前景和面临的挑战,为木质素衍生炭材料的下一步研究和开发提供参考。     

Enhanced ablation of small anodes in a carbon nanotube arc plasma

The ablation rate of a graphite anode is investigated as a function of anode diameter for a carbon nanotube arc plasma. It is found that anomalously high ablation occurs for small anode diameters. This result is explained by the formation of a positive anode sheath. The increased ablation rate due to this positive anode sheath could imply greater production rate for carbon nanotubes.