Hydrogen consuming anodes for energy saving in sodium sulphate electrolysis

Gas diffusion electrodes, made of PTFE-bonded carbon with precious metal catalysts, were investigated as hydrogen consuming anodes in sodium sulphate electrolysis. The catalysts used were platinum and palladium and mixtures of both metals, prepared by two different methods. Various metal meshes were used as current collector. The electrodes performed well in pure sulphuric acid (5–15 wt-%) and in mixtures with sodium sulphate (10 wt-%) at temperatures of 30 to 70°C and current densities up to 5 kA/m². In long-term experiments, at a current density of 2.6 kA/m², the electrodes were stable over three months. The electrodes were characterised by stationary current density/potential curves and by galvanostatic current interruption measurements.     

Consumption of graphite anodes in chlorine manufacture by brine electrolysis

The anodic behaviour of carbon and graphite electrodes in aqueous electrolytes is described and the important reactions leading to the consumption of graphite during chlorine manufacture by brine electrolysis are discussed in detail. The present state of knowledge concerning the effects of chlorine cell electrolysis conditions on graphite consumption is also reviewed. A new investigation of the influence of graphite manufacturing variables and properties on consumption is also described. The variables studied were coke particle size, degree of pitch impregnation and the final heat treatment at the graphitization stage. Properties considered included electrical resistivity, apparent density, average crystallite size and unpaired electron spin density. The resistance of the graphite to anodic oxidation was found to increase with increasing coke particle size, increasing pitch impregnation and increasing final heat treatment temperature (up to 3000°C). Unpaired electron spin density was the property found best to correlate with this resistance.     

The mechanism of the chlorine evolution on different types of graphite anodes during the electrolysis of an acidic NaCl solution

The behaviour of the electrode and the formation of chlorine were studied practically for porous graphite used in the industrial electrolysis of an aqueous NaCl solution only.In this investigation this was done for Acheson graphite electrodes and for highly orientated pyrolytic graphite electrodes of which the edge plane or the basal plane served as electrode surface. Voltammograms were determined by continuous scanning with different rates of the potential between a minimum and maximum value. From the experimental results it follows that two different surface compounds, the lp oxygen and the hp oxygen are formed on all the graphite electrodes investigated. In the stationary state lp oxygen is present at low potential and the hp oxygen at high potentials. The formation and the removal of each compound depends on the electrolytic conditions, especially on the maximum potential of the sweep. The conditions of the sweep determine whether these oxides are really present on the electrode surface during the anodic or the cathodic sweep. Chlorine is formed according to the Volmer-Heyrovsky mechanism on all the three types of electrodes. The current density, the transfer coefficient and the activation energy for the edge plane electrode and the Acheson graphite electrode are of the same order of magnitude, while for the basal plane electrode they deviate strongly.     

The occurrence of perchlorate during drinking water electrolysis using BDD anodes

Electrochemical studies were carried out to estimate the risks of perchlorate formation in drinking water disinfected by direct electrolysis. Boron Doped Diamond (BDD) anodes were used in laboratory and commercially available cells at 20 °C. The current density was changed between 50 and 500 A m⁻². For comparison, other anode materials such as platinum and mixed oxide were also tested. It was found that BDD anodes have a thousandfold higher perchlorate formation potential compared with the other electrode materials that were tested. In long-term discontinuous experiments all the chloride finally reacted to form perchlorate. The same result was obtained when probable oxychlorine intermediates (OCl⁻, ClO₂⁻, ClO₃⁻) were electrolysed in synthetic waters in the ppm range of concentrations. The tendency to form perchlorate was confirmed when the flow rate of drinking water was varied between 100 and 300 L h⁻¹ and the temperature increased to 30 °C. In a continuous flow mode of operation a higher chloride concentration in the water resulted in a lower perchlorate formation. This can be explained by reaction competition of species near and on the anode surface for experiments both with synthetic and local drinking waters. It is concluded that the use of electrodes producing highly reactive species must be more carefully controlled in hygienically and environmentally oriented applications.     

隔膜法电解异常情况及处理措施

针对广东某氯碱厂隔膜法烧碱生产的运行情况,对发生的异常情况进行总结,列举出可能的原因,并提出相应的处理措施.     

降低金属阳极电解生产工序电耗的措施

介绍了为降低电解工序的电耗所采取的包括电解厂房设计,合理选用电槽、隔膜、导板,加强电槽管理等措施。     

A transport phenomenon analysis criterion predicting pitting appearance during al anodization in sulphate electrolysis

The conditions of Al anodizing, bath temperatures, current densities, and concentrations of H 2 SO 4 and Al 2 (SO 4 ) 3 in pure H 2 SO 4 , and H 2 SO 4 +Al 2 (SO 4 ) 3 solutions where the growth of porous anodic alumina films is uniform/regular and nonuniform/abnormal with appearance of pitting, were experimentally determined. The equations describing the mass and charge transport phenomena and the composition of the solution at the pore bases during the regular oxide growth were solved. The current densities at which the H 2 SO 4 concentration becomes minimum at low film thicknesses were determined, which coincided with the experimental ones above which pitting appeared. They increased with the bath temperature and the H 2 SO 4 concentration and decreased with the Al 2 (SO 4 ) 3 concentration. A criterion for pitting apearance was thus formulated. It explained the promotion of pitting appearance by the low bath temperatures and H 2 SO 4 concentrations, high current densities, and sulphate additives, which is important for developing methods for producing regularly grown high-quality coatings.     

Activation of passivated titanium anodes from the electrolysis of MnO2

Titanium anodes with and without a MnO₂ precoat were passivated during deposition of electrolytic manganese dioxide and subsequently activated by cathodic reduction. The time dependance of the titanium voltage during passivation and during cathodic reduction was determined by the initial state of the surface. Nonactivated titanium passivated considerably faster than activated titanium with the same thickness of precoat. The application of cathodic currents to passivated anodes caused detachment of the precoat and reduction of the passivating film underneath the precoat. Reduction of the passivating film grown on precoated titanium seems to occur at a much smaller rate than reduction of the film grown on unprecoated titanium.     

金属阳极电解槽的酸性盐水电解

介绍金属阳极电解槽酸性盐水（pH值为3－5）的电解工艺，说明采用该工艺可提高电流效率、减少电解过程的副反应，并可延长金属阳极电解槽的使用寿命，具有较好的经济效益。     

Oxidation of organic pollutants on BDD anodes using modulated current electrolysis

In this paper, a theoretical model is presented for organic pollutants mineralization at high current efficiency (close to 100%) and low energy consumption on boron-doped diamond electrodes. The model is formulated for a perfect mixed electrochemical reactor operated as a batch recirculation system under multiple current steps, in which the applied current is adjusted during the electrolysis to be close to the limiting value. An experimental validation with the anodic oxidation of 3,4,5-trihydroxybenzoic acid is also provided. The results have shown that multiple current steps electrolysis and continuous current control allowed obtaining high oxidation rate and current efficiency.     

Chlorine evolution on graphite anodes in a NaCl-AgCl melt

Chlorine evolution on vitreous carbon and on five different types of graphite materials has been studied in a NaCl-AgCl melt. Steady state cd/potential curves and critical parameters for the anode effect have been obtained.

Galvanostatic single pulses have been used to measure the differential capacitance of the electrodes. The double-pulse method has been used to determine the exchange cd of the charge-transfer reaction, showing this to be a fast reaction.

At low cds the steady state cd/potential curves have slopes corresponding to the theoretical slope RT/2F, showing that a heterogeneous combination reaction is rate-controlling in this range. On vitreous carbon this slope was obtained up to 70–80% of the critical cd for the anode effect. With the graphite materials, changes in slope were found at intermediate and high cds. These changes are attributed to several possible mechanisms, which are discussed.     

电解系统的故障及处理措施

介绍了在离子膜电解槽运行中,通过DCS发现参数异常并及时采取措施从而避免事故的5个事例。     

Electrochemical properties of nanosize Sn-coated graphite anodes in lithium-ion cells

A series of Sn-coated graphite composite materials for lithium-ion batteries were prepared by microencapsulating nanosize Sn particles in graphite. The nanosize Sn particles are homogeneously dispersed in the graphite matrix via electroless chemical reduction. The tin-graphite composite showed a great improvement in lithium storage capacity. Since Sn is an active element to lithium, Sn can react with lithium to form Li_4.4Sn alloys, a reaction accompanied by a dramatic volume increase, whereas the ductile graphite matrix provides a perfect buffer layer to absorb this volume expansion. Therefore, the integrity of the composite electrode is preserved during lithium insertion and extraction. Cyclic voltammetry was employed to identify the reaction process involved in lithium insertion and extraction in the graphite structure, as well as lithium alloying with tin. The tin-graphite composites provide a new type of anode material for lithium-ion batteries with an increased capacity.     

Oxygen evolution anodes composed of anodically deposited Mn-Mo-Fe oxides for seawater electrolysis

Preparation of anodes for oxygen evolution in seawater electrolysis was carried out. Manganese-molybdenum double oxides, Mn_1−xMo_xO_2+x, prepared by anodic deposition from MnSO₄-Na₂MoO₄ solutions showed the 100% oxygen evolution efficiency at a current density of 1000 A m⁻² in 0.5 M NaCl at 30 °C and pH 12, but an increase in solution temperature resulted in dissolution of the oxides as molybdate and permanganate ions. In order to increase the stability of the electrodes at higher temperatures the addition of iron to the manganese-molybdenum oxides was performed by anodic deposition in MnSO₄-Na₂MoO₄-FeNH₄(SO₄)₂ solutions. The electrodes thus prepared showed the 100% oxygen evolution efficiency at 1000 A m⁻² in 0.5 M NaCl at 30-90 °C, when proper amounts of molybdenum and iron were contained. The iron addition also enhanced the oxygen evolution efficiency. The electrodes were not composed of oxide mixtures but triple oxides, Mn_1−x−yMo_xFe_yO_2+x−0.5y, consisting of Mn⁴⁺, Mo⁶⁺ and Fe³⁺. The formation of the triple oxides seemed responsible for enhancement of both oxygen evolution efficiency and stability.     

Study on graphite surface modification by non-electrode plasma electrolysis

A novel non-electrode plasma electrolysis technique was developed in the paper in order to rapidly prepare a coating on graphite. The coating was mainly composed of SiO₂ with the thickness of ~ 7?µm. A small amount of SiC was also detected in the coating. The coating exhibited superior oxidation resistance at 1000?°C in air, evidenced by the visible polishing marks, smooth surface and the much decreased roughness from ~ 120?µm to ~ 50?µm. The simulation of thermal effects was conducted in order to establish the formation mechanism of the coating on graphite. We believe that the novel non-electrode plasma electrolysis technique will find a wide range of applications in preparing high-performance graphite and its composites.     

The role of impregnants in determining the behaviour of graphite anodes

The lifetime of graphite anodes in sulphuric acid media can be increased by a factor of 3.2 by multiple impregnation with coal tar pitch. For single impregnation, linseed oil yielded the largest improvement. The results were interpreted on the premise that properties such as viscosity, melting point and coking value determine the effectiveness of the impregnant. The proper selection of the impregnant and procedure of impregnation is dependent on the electrolysis temperature.     

铝电解钠对阴极渗透膨胀的影响及解决方法

分析了在铝电解槽中,钠对阴极的渗透膨胀过程。介绍了改善钠对阴极渗透膨胀的两种方法:提高阴极炭块石墨含量或石墨化程度和使用TiB2涂层及复合材料,并对目前国内这两种方法的研究和应用现状进行总结。     

Thermal cycling induced capacity enhancement of graphite anodes in lithium-ion cells

Graphite electrodes were electrochemically cycled in Li-ion cells at 50 and 60 °C in order to determine the changes in their surface properties in comparison to the electrodes tested at 25 °C. A 17% drop in planar capacity occurred during the first cycle at 60 °C compared to a 40% at 25 °C and reduced the amount of damage that occurred to graphite due to a rapidly formed solid electrolyte interphase (SEI). During the following cycles, a planar capacity of 3.11 ± 0.12 mAh cm⁻² was attained at 60 °C rather than 0.53 ± 0.03 mAh cm⁻² at 25 °C. The SEI layer formed at 60 °C predominantly consisted of Li₂CO₃ and was devoid of residual LiClO₄ detected at 25 °C. At 25 °C, the diffusion coefficient of Li⁺ (D_Li⁺) was calculated as 1.07 × 10⁻⁸ cm² s⁻¹, whereas at 60 °C, D_Li⁺ increased to 3.25 × 10⁻⁸ cm² s⁻¹. A pre-treatment conducted at 60 °C enhanced the cyclic performance of graphite subsequently cycled at 25 °C; a Li₂CO₃-enriched SEI, generated during the 60 °C pre-treatment, covered the graphite surface uniformly and resulted in a 28% increase in battery capacity at 25 °C.     

Anodic oxidation of copper cyanide on graphite anodes in alkaline solution

The anodic oxidation of copper cyanide has been studied using a graphite rotating disc with reference to cyanide concentration (0.05–4.00 M), CN:Cu mole ratio (3–12), temperature (25–60 °C) and hydroxide concentration (0.01–0.25 M). Copper had a significant catalytic effect on cyanide oxidation. In the low polarization region (about 0.4 V vs SCE or less), cuprous cyanide is oxidized to cupric cyanide complexes which further react to form cyanate. At a CN:Cu ratio of 3 and [OH^–] = 0.25 M, the Tafel slope was about 0.12 V decade^–1. Cu(CN)₃ ^2– was discharged on the electrode and the reaction order with respect to the predicted concentration of Cu(CN)₃ ^2– is one. With increasing CN:Cu mole ratio and decreasing pH, the dominant discharged species shifted to Cu(CN)₄ ^3–. Under these conditions, two Tafel slopes were observed with the first one being 0.060 V decade^–1 and the second one 0.17–0.20 V decade^–1. In the high polarization region (about 0.4 V vs SCE or more), cuprous cyanide complexes were oxidized to copper oxide and cyanate. Possible reaction mechanism was discussed.