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 capacitance and polarization potential of baked carbon anodes in cryolite-alumina melts

Baked carbon anodes with varying apparent densities and baking temperatures were tested in Na₃AlF₆–Al₂O_3(sat) melts at 1010° C. The double-layer capacitance (C _dl) was used as an indicator of the wetted surface area. For unpolarized anodes,C _dl increased with increasing time of immersion and reached a constant level after 1.5–2h. The values decreased with increasing polarization potential in the range 1–1.5 V positive to aluminium. TheC _dl of polished samples increased markedly during electrolysis, particularly at low current densities. No clear correlation was found betweenC _dl and apparent density. Semi-logarithmic plots of potential versus current could be divided into three segments. The lower two were linear, the ranges and slopes being 0.01–0.1 A cm^–2, 0.20–0.44 V per decade and 0.1–0.5 A cm^–2, 0.18–0.24 V per decade, respectively. At higher current densities the curves bent upwards. The current density corresponding to an overpotential of 0.5 V increased slightly with increasing apparent density, whereas the ohmic voltage drop. at constant current density decreased. The current densities were corrected for differences in wetted surface area on the basis of theC _dl data. The change in baking temperature from 970 to 1100°C had no appreciable effect on the overpotential, whereas samples baked at 1250°C showed a somewhat lower overpotential.     

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.     

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.     

Nucleation and growth of anodic oxide films

The kinetics of formation of anodic oxide films can be derived from measurements of current density i_ox, thickness d and electrode capacity C in dependence on the polarization potential ε and time t. Examples of simultaneous and multistep experiments are discussed in connection with advantages of special combinations.From plots of lg i vs lg t, lg C vs lg Q or d vs ε, characteristic growth laws and properties of autocatalytic and self-inhibiting systems can be derived. Deposition layers of NiOOH and FeOOH and passive films on Au, Pt and Hf are described as examples. All systems with rate determining ionic migration or diffusion within the solid phase belong to the group of self-inhibiting systems. Due to the potential drop within the film they form anodically almost homogeneous films, but during reduction island formation is enhanced. Nucleation phenomena and special growth forms can be studied only in systems of crystallizing oxides with a high conductivity, ie negligible potential drop. Monolayer formation and the influence of surface structure can be distinguished only on single crystal planes of noble metals.     

Growth of anodic oxide films on oxygen-containing niobium

The present study is directed at understanding of the influence of oxygen in the metal on anodic film growth on niobium, using sputter-deposited niobium containing from about 0-52 at.% oxygen, with anodizing carried out at high efficiency in phosphoric acid electrolyte. The findings reveal amorphous anodic niobia films, with no significant effect of oxygen on the field strength, transport numbers, mobility of impurity species and capacitance. However, since niobium is partially oxidized due to presence of oxygen in the substrate, less charge is required to form the films, hence reducing the time to reach a particular film thickness and anodizing voltage. Further, the relative thickness of film material formed at the metal/film interface is increased by the incorporation of oxygen species into the films from the substrate, with an associated altered depth of incorporation of phosphorus species into the films.     

Hydrogen evolution reaction on anodic titanium oxide films

Oxide titanium films were prepared using linear potential sweeps up to different limiting potential values between 0 and 60 V in 0.5 M H₂SO₄. The electronic characteristics of these films were investigated through impedance measurements during their formation. The hydrogen evolution reaction (HER) in either HClO₄ or H₂SO₄ solutions in the pH range between 1 and 4 was studied on the different oxide films. The obtained experimental results could be explained assuming that the recombination of adsorbed hydrogen atoms is the rate determining step (rds), and that the Temkin type isotherm applies to the adsorbed intermediates. The log i vs E plot after correction for diffusion control in the bulk gives a slope of 2.3 RT/F.The influence of the physico-chemical properties of the oxide films on the HER has also been studied from the changes in the current—potential profiles. The oxide film thickness changes the HER overpotential, but the slope of the log i vs E relationship remains independent of it.     

贱金属氧化物不溶性阳极的研究进展

介绍了 PbOZ.SnO2.MnO2.CO3O4等贱金属氧化物不溶性阳极的研究和应用状况以及存在的问题,认为有效改善贱金属氧化物阳极的使用性能特别是表面催化性能以及如何搭配各种贱金属氧化物将是其替代贵重金属氧化物阳极的关键.     

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.     

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.     

Durability and thermal cycling of Ni/YSZ cermet anodes for solid oxide fuel cells

The long-term properties of Ni/yttria stabilized zirconia (YSZ) cermet anodes for solid oxide fuel cells were evaluated experimentally. A total of 13 anodes of three types based on two commercial NiO powders were examined. The durability was evaluated at temperatures of 850 C, 1000 C and 1050 C over 1300 to 2000h at an anodic d.c. load of 300mA cm^–2 in hydrogen with 1 to 3% water. The anode-related polarization resistance, R _P, was measured by impedance spectroscopy and found to be in the range of 0.05 to 0.7 cm². After an initial stabilization period of up to 300h, R _P varied linearly with time within the experimental uncertainty. At 1050 C no degradation was observed. At 1000 C a degradation rate of 10 m cm² per 1000 h was found. The degradation rate was possibly higher at 850 C. A single anode was exposed to nine thermal cycles from 1000 to below 100 C at 100 C h^–1. An increase in R _P of about 30m cm² was observed over the first two cycles. For the following thermal cycles R _P was stable within the experimental uncertainty.     

Reversible reactions within anodic oxide films on titanium electrodes

Evidence is presented for a reversible reaction occurring within the bulk of the surface oxide film on Ti electrodes at negative potentials.

High frequency impedance measurements give results characteristic of a bulk phenomenon and clearly distinguish the behaviour of filmed Ti electrodes from that of some other “valve” metals at low potentials.

The potential/pH dependences of the impedance minima are discussed in terms of various models for the reversible formation of a hydrated oxide in the bulk of the oxide film.     

Stability of anodic tantalum oxide films in NaOH solutions

The dissolution behaviour of the anodic oxide films formed on tantalum was investigated in NaOH solutions of different concentrations. In solutions of [OH^–]<3.0 M, no pronounced dissolution could be detected. However, in concentrated alkali solutions, [OH]>3 M, the dissolution occurred to different extents depending on the alkali concentration. As observed from capacitance and potential measurements, the barrier film on Ta was found to be defective owing to the incorporation of OH^– ions in the film. The nature of the barrier film and its dependence on formation rate, alkali concentration and solution temperature were investigated. Complex plane analysis illustrates the behaviour of the barrier film of different thickness, i.e. formation voltages, in NaOH solutions. The barrier film cannot be considered accurately as a perfect dielectric.     

Nucleation and growth of anodic oxide films on bismuth—II

The rate of nucleation of the anodic oxide film on bismuth in 0.1 M NaOH is limited by electrochemical reaction at the uncovered metal surface or surface diffusion to the periphery of a spreading oxide patch, not by incorporation into the growing lattice. Anodic oxide layers grown on bismuth consist of a thin barrier layer overlaid by a much thicker cracked and porous layer. Discrepancies between previous workers are thus resolved. The growth kinetics of the barrier layer are satisfactory described by the simple model of high field ion transport. Galvanostatic and potentiostatic techniques, and microscopic examination of the metal surface, including scanning electron microscopy, were employed.     

Luminescence,breakdown and colouring of anodic oxide films on aluminium

El and Pl, breakdown and self-colouring phenomena of anodic oxide films on aluminium, which have been found to be closely related with each other, were reviewed according to newly proposed classifications. Thus the hitherto unsolved problems which had caused confusion in complete understanding of the art are almost solved. Firstly galvanostatic anodic behaviours of Al were classified. Secondly it was shown that el is caused either by (1) selected impurities (activators) (Mn, Eu etc) in Al, (2) excited carboxylate ions in the film formed in aliphatic carboxylic acids, or (3) by, ‘flaws’ in the case of el in inorganic electrolytes, which is not the el in the true sense. Thirdly pl is to be seen in aliphatic carboxylic acid films only and the luminescent centres are the same with those of el in the acid. Aromatic acid films show neither el nor pl since the acid can not enter into the films. Further, reference was made to one type of solid-type led (Al—Al₂O₃-SnO₂) constructed. Fourthly, breakdowns have been classified into: (1) electrochemical breakdown including porous film formation, carboxylate breakdown (pitting) and chloride breakdown, and (2) ‘electric breakdown during anodizing’ or ‘electrolytic breakdown’ which is a complicated mixture of electrochemical and electric breakdown at/or through oxide—electrolyte interface, and which includes ‘burning’ (luminescent ‘flame’ propagation process) and sparking (scintillation and arcing). Finally self-colouring of ‘pure’ Al (except colouring by alloying elements or impurities included) has been classified into: (1) colouring by decomposition or cyclisation of excited carboxylate ions in the film (post-el) higher electric field attained across the film. Thick, coloured films show no more el nor pl. (2) colouring by existence of Al⁰ particles in the interstitial positions of the film by high electric field assisted oxide growth in the case of the so-called ‘hard anodizing’, locally grown coloured spots in inorganic acid, eg sulphuric acid, and of so-called integral colour anodizing in sulphonated aromatic dibasic acid (or higher aliphatic carboxylic acid) plus a small amount of sulphuric acid, typically Kalcolor, Duranodic 300 or Permalux processes for architectural use. These films show neither el nor pl.     

Study of anodic oxide films formed on solid-state sintered SiC-ceramic at high anodic potentials

The present work studies the formation, chemical composition, and structure of an oxide layer formed on the technical solid-state sintered ceramic (EKasic®D) in a strong alkaline solution (1 M NaOH at pH 14) at high anodic potentials (30 V vs. 3 M Ag/AgCl). The observed formation of oxide films on SiC in alkaline solution is in contradiction to the thermodynamic laws (Pourbaix-diagram). The film thickness was determined by SEM/EDX measurements using the specific thin film analysis tool “AZtec” (Oxford Instruments) as well as the transmission electron microscopy. The thickness of the oxide film formed at 30 V amounts to 30 nm that corresponds with a field strength of E = 10 MV cm^?1, which corresponds with the formation according to the high-field mechanism. The chemical composition was studied by EDX-analysis in a transmission electron microscope as well as by X-ray photoelectron spectroscopy (XPS). The oxide layer is completely amorphous and consists of non-stoichiometric SiO_x and SiO_xC_y. The layer is assumed as graded with a higher amount of SiO_x in the outermost regions and an increased amount of SiO_xC_y in the inner region of the passive layer. Additionally, the passive layer is doped by a small amount of aluminum originating from a sinter additive used in the manufacture of the SiC ceramic and completely incorporated into the SiC grains.     

Sealing effects of anodic oxide films formed on Mg-Al alloys

Mg alloys were anodized in alkaline NaOH solutions with various additives as a non-chromate method. Specimen AZ91 was anodized at a potential that produced a strong surface dissolution reaction and generated a large amount of Mg(OH)₂. The effect of sealing after anodizing was investigated, focusing on the effects of sealing time, temperature and solution conditions. The current density decreased with increasing A1(OH)₃ concentration in 1 M NaOH solution during anodizing; sparking occurred at potentials above 80 V. The best corrosion resistance with anodizing in 1 M NaOH solution occurred at a potential of 4 V, which caused the strongest active dissolution reaction. The sealing effect improved with increasing time and temperature, and corrosion resistance was proportional to the relative ratio of Mg(OH)₂. If the oxygen thickness observed by EDX equaled the film thickness, the film formed at 4 V in 1 M NaOH was 10–15 Μm thickness. The optimum corrosion resistance in sealing at various solutions after anodizing was 1M-NaOH solution.     

Photoluminescence and molecular orbital theory in anodic oxide films formed on aluminium

We use molecular orbital theory (MOT) to explain the existence of one or two excitation maxima in photoluminescence excitation spectra of Al₂O₃ formed on Al in different aqueous electrolytes (H₂SO₄, H₃PO₄, H₂CrO₄, Na₂CO₃, H₂C₂O₄ and HCOOH). All oxides were formed at the same temperature and current density. Comparing experimental and calculated values of the maximum of the excitation wavelength λ_max we concluded: (a) that structural change in the formed oxides occurred; (b) that differences in λ_max are in connection with different types of incorporated anions or anion fragments.     

Electron-transfer reactions at semiconducting anodic niobium oxide films

Anodic niobium oxide in a n-type semiconductor due to an oxygen deficit. Electron donor densities and the dependence of excitation energies upon donor density were determined from measurements of space-charge capacitances as a function of electrode potential. Rates of Fe(III) reduction and of Fe(II) oxidation were measured at oxide films of different donor densities as functions of electrode potential and pH value. The results indicate electronic states in the space-charge layer below the oxide surface to participate in the electron-transfer reactions. Experimental polarisation curves are compared to curves calculated theoretically. From the shape of the polarisation curve flat-band potentials of the oxide are estimated. The observed polarisation curves are compatible with λ = 1·2 eV for the reorganisation energy of the Fe(CN)^3?₆/Fe(CN)^4?₆ redox couple.