Anodically deposited manganese-molybdenum oxide anodes with high selectivity for evolving oxygen in electrolysis of seawater

Manganese-molybdenum oxide electrodes were prepared by anodic deposition on an IrO2-coated titanium substrate at a constant current density of 600Am–2 from baths containing 0.2M MnSO4 and 0–0.1M Na2MoO4 at 90C and pH 0.5. These electrodes were characterised for oxygen evolution in the electrolysis at 1,000Am–2 in 0.5M NaCl solution at 30C and pH 8 or 12. The most active and stable oxygen evolving anode exhibited 100% efficiency for oxygen evolution, and an efficiency of 98.5% for over 1,500 h at pH 12 and of 96.5% for over 2,800 h at pH 8 of continuous electrolysis. X-ray diffraction measurement and XPS analysis indicated that the deposits consist of a nanocrystalline single -MnO2 type phase, and manganese and molybdenum in the deposits are in the Mn4+ and Mo6+ states. The electrochemical studies showed that the manganese-molybdenum oxide electrodes drastically reduced the electrocatalytic activity for chlorine evolution to the undetectable level, resulting in 100% efficiency for oxygen evolution, although the addition of molybdenum slightly increased the oxygen overpotential.     

钛基IrO2+SnO2电极的析氧性能及其在电合成丁二酸中的应用

通过热分解法制备了IrO2+ SnO2/Sb2 O3+ SnO2/Ti、IrO2+ SnO2/Ti、IrO2+ Ta2O5/Sb2O3+ SnO2/Ti电极,通过线性伏安、电化学阻抗、强化寿命测试等研究了钛基涂层电极在1 mol· L-1硫酸溶液中的析氧性能.采用EDX、SEM等考察了电极的表面元素分布和电极强化寿命测...     

Service life of Ti/SnO2–Sb2O5 anodes

The service life of SnO2–Sb2O5 coated anodes prepared by the spray pyrolysis technique using Ti or Ti/IrO2 substrate, was studied under galvanostatic conditions (100mAcm–2 in 1m H2SO4 at 25°C. The results showed that the presence of an IrO2 interlayer between the Ti substrate and the SnO2–Sb2O5 coating (Ti/IrO2/SnO2–Sb2O5 anode) strongly increases the service life of the anode. This beneficial action of the IrO2 interlayer was attributed to its high anodic stability and its isomorphous structure with TiO2 and SnO2. Cyclic voltammetry and steady-state polarization curves showed that the electrochemical behaviour of the Ti/IrO2/SnO2–Sb2O5 electrode lies between the behaviour of the Ti/IrO2 and the Ti/SnO2–Sb2O5 electrodes due to incorporation of IrO2 in the SnO2–Sb2O5 coating during its preparation.     

Development of long-lived high-performance zinc-calcium/nickel oxide cells

The addition of Ca(OH)₂ to the zinc electrode of Zn/KOH/NiOOH cells was investigated in order to determine its effect on the rate of zinc active material redistribution (shape change) and cell cycle-life performance. Cells of equal mass and capacity, and therefore the same specific energy, containing 0, 10, 25, and 40 mol% Ca(OH)₂ in their zinc electrodes were constructed and tested. The Ca(OH)₂ and Zn(OH) ₄ ^2– -supersaturated KOH solution formed a calcium-zincate complex during the discharge half-cycle. The solubility of this complex is less than that of ZnO, and the lower zinc species solubility leads to a slower rate of Zn redistribution, thereby extending the cell cycle life. The best cells tested were those with 25%-Ca(OH)₂ electrodes, which lost capacity at a rate of 0.13%/cycle, compared to 0.47%/cycle in calcium-free control cells constructed in the same manner. Also, zinc active material utilization in the calcium-containing electrodes showed a dramatic improvement, compared to the calcium-free zinc electrodes.     

Preparation of oxygen evolving electrodes with long service life under extreme conditions

Among the numerous base metals tested for DSA® type electrodes (e.g., titanium and its alloys, zirconium, niobium etc.), tantalum is a potentially excellent substrate owing to its good electrical conductivity and corrosion resistance, and the favourable dielectric properties of its oxide. Nevertheless, a DSA® type electrode fabricated on a tantalum substrate would be very expensive due to the high cost of the metal. To prepare an anode combining the excellent properties of tantalum at reasonable price, a new material has been developed in our laboratory. This consists of a common base metal (e.g., Cu) covered with a thin tantalum coating. This tantalum layer was obtained by molten salt electroplating in a LiF–NaF–K2TaF7 melt at 800°C. Thus, an anode of the type Metal/Ta/Ta2O5–IrO2 with a surface load of 22gm-2 IrO2, submitted to the severe test conditions used in this work, exhibits a standardized lifetime tenfold greater than one made with ASTM grade 4 titanium base metal. Thus, this type of electrode might be advantageously employed as an oxygen evolution anode in acidic solutions.     

The anodic characteristics of modified Mn oxide electrode: Ti/RuOx/MnOx

The Ti-supported Mn oxide electrode was modified by introducing a Ru oxide film as an intermediate layer into the Ti/Mn oxide interface, and its anodic characteristics were examined in aqueous solutions of 0.5 M H₂SO₄, 1 M KOH and 5 M NaCl. The intermediate thin film of the Ru oxide served effectively as a good conductor to improve the electric characteristics of the Ti-supported Mn oxide electrode and hence the modified electrode exhibited the excellent anodic characteristics similar to those of the Pt-based Mn oxide. From the kinetic considerations, it was proved that the anodic evolution of oxygen takes place on the surface of Mn oxide. The Mn oxide was found to be virtually active electrocatalyst for electrolytic chlorine evolution as well as electrolytic oxygen evolution. As a result of the evaluation of the catalytic activity, it was considered that the Mn oxide is one of the most active material of all, except for some DSA-type electrodes such as Ru and Ir oxides, for the anodic evolution of oxygen. In conclusion, the presented results suggest that the modified Mn oxide electrode has a promising character for the practical use in water electrolysis.     

Effect of preparation procedure of Iro<Subscript>2</Subscript>–Nb<Subscript>2</Subscript>o<Subscript>5</Subscript> anodes on surface and electrocatalytic properties

The influence of the electrode manufacturing procedure on surface and electrocatalytic properties for oxygen and ozone evolution at electrodes of nominal composition Ti/[IrO₂–Nb₂O₅] (45:55 mol%) was investigated. Thermal decomposition at 450 °C (1 h, air stream) was adopted as standard procedure. Metal support pretreatment, solvent mixture, method of applying the precursor mixture and calcination procedure were all investigated. X-ray diffraction, scanning electronic microscopy, voltammetric and differential capacity analysis show the use of HCl 1:1 as solvent and applying the mixture by brush led to fragile rugged/porous oxide coatings. However, for the same conditions, but controlled calcination (heating/cooling rates), the coating becomes more compact. Using isopropanol as solvent results in a more homogeneous coating, presenting the lowest morphology factor. Kinetic investigation shows the rugged/porous coating presents the lowest Tafel slopes and the highest global electrocatalytic activity for OER. The more compact the coating the lower the electrochemically active surface area and the global OER activity. Ozone efficiency depends on the electrochemically active area while support pretreatment strongly influences the lifetime of the electrode. Application of a Pt interlayer between the oxide and Ti base improves the service life.     

The anodic characteristics of manganese dioxide electrodes prepared by thermal decomposition of manganese nitrate

Manganese dioxide electrodes were prepared by a thermal decomposition of manganese nitrate solution on a titanium or a platinum substrate, and their anodic characteristics were investigated mainly in 1N H₂SO₄ and 1N KOH. The platinum-supported manganese dioxide electrode shows the good anodic characteristics with a relatively low overvoltage for oxygen evolution while the use of the titanium-supported one as an anode needs further modification to reduce its high resistivity resulting from the thick film of titanium dioxide in spite of good adhesion of the oxide film with the titanium substrate.The primary water or hydroxide ion discharge step is rate-controlling in the anodic evolution of oxygen on the platinum-supported manganese dioxide electrode in both acidic and alkaline solutions. The oxygen overvoltage is raised and the mechanical strength of the catalytic oxide on the platinum substrate is weakened by the anodic polarization in acidic solution. These phenomena are explicable on the basis of an increase in the oxygen content in the oxide. In conclusion, the presented results suggest that the manganese dioxide film is a practical usable material for the anodic evolution of oxygen as well as chlorine, especially in alkaline solutions.     

Electrochemical degradation of phenol in aqueous solution on bismuth doped lead dioxide: a comparison of the activities of various electrode formulations

This paper describes the development of electrochemical processes for the oxidative degradation of toxic organic chemicals in waste waters. Doped bismuth lead dioxide anodes have been tested by the kinetic study of phenol anodic oxidation in aqueous solution. The main products during oxidative degradation of phenol are 1,4- benzoquinone, maleic acid and carbon dioxide. Several deposits of Bi2O5–PbO2 on Ti/(IrO2–Ta2O5) substrates have been prepared by anodic oxidation of Pb2+ and Bi3+ in aqueous solutions containing perchloric acid to increase the solubility of bismuth. To study the effect of perchlorate ions, the efficiency of the PbO2 deposit prepared from lead nitrate in an aqueous solution (pure PbO2) was compared with that of a deposit prepared from perchloric acid solution (perchlorate doped PbO2). Although the phenol is oxidized at the same rate on the two deposits, the charge corresponding to the total elimination of 1,4-benzoquinone is three times higher for perchlorate doped PbO2 than for pure PbO2. Phenol degradation is more efficiently carried out on a PbO2 anode doped with perchlorate and with bismuth than on the same electrode doped only with perchlorate. Among the electrodes tested in this work, the pure PbO2 anode is the most efficient for phenol degradation. It is assumed that certain active sites on the anode occupied by perchlorate ions do not participate in the transfer of oxygen atoms and that for the PbO2 electrode doped with bismuth, oxygen evolution is favoured to the detriment of oxygen atom transfer.     

Electrocatalysis of oxygen evolution/reductionon LaNiO3 prepared by a novel malic acid-aided method

A kinetic study of the electroformation and electroreduction of oxygen in KOH solutions has been carried out on a LaNiO3 electrode obtained through a malic acid precursor route. The new electrocatalyst was found to show greatly enhanced activity for both oxygen evolution and reduction. The apparent electrocatalytic activity of this electrode for oxygen reduction was more than 10 times higher than those reported for similar electrodes obtained by other methods. It was, however, observed to be less active electrocatalytically for oxygen evolution as compared to the active LaNiO3 electrode obtained by the Bockris–Otagawa coprecipitation method. The electrochemical reaction order with respect to OH– concentration was found to be approximately 1 in the case of oxygen evolution while that for oxygen reduction, was approximately –1. The Tafel slope values for the reactions were 2.3RT/F and 2×2.3RT/3F, respectively.     

Hydrogen production during zinc deposition from alkaline zincate solutions

The determination of the amount of hydrogen produced during the electrodeposition of zinc from alkaline zincate solutions was carried out using the rotating ring-disc electrode (RRDE) technique. The experimental conditions for which the RRDE technique offers reliable results are discussed. Hydrogen production during zinc deposition was studied for a range of cathodic (disc) current densities (20–500 A m^–2) and electrolyte compositions (1–7 M KOH, 0.01–0.2 M zincate). It was found that an increasing amount of hydrogen was formed with increasing (disc) current density and decreasing KOH and zincate concentration. The impact of hydrogen formation during the charging process on nickel oxide/zinc secondary battery performance is expected to be small. It is concluded that in battery electrolytes (8 M KOH, 1 M zincate) hydrogen is formed chiefly by corrosion of the zinc electrode rather than by electrochemical formation during the electrochemical reduction of zinc.     

Simultaneous determination of chlorine and oxygen evolving at RuO2/Ti and RuO2–TiO2/Ti anodes by differential electrochemical mass spectroscopy

Chlorine and oxygen evolving at RuO2/Ti and RuO2–TiO2/Ti anodes have been simultaneously determined at electrode potentials from 1.0 to about 2V (vs Ag/AgCl) by differential electrochemical mass spectroscopy (DEMS). On the RuO2/Ti anodes, the threshold electrode potential for oxygen evolution increased with a decrease in RuO2 loading, while the chlorine evolution potential was unchanged. Low RuO2 loading anodes gave a high chlorine evolution ratio under various constant electrolysis potentials. On the RuO2–TiO2/Ti anodes, the threshold electrode potential for oxygen evolution increased with an increase in the TiO2 content more remarkably than that for chlorine evolution. High TiO2 content anodes gave a high chlorine evolution ratio at various constant electrolysis potentials. The combination of RuO2 and TiO2 exhibits a remarkable effect with respect to the enhancement of chlorine evolution selectivity.     

IrO2-RUO2/Ti电极处理含氰含银电镀废水

采用热氧化分解法制备了IrO2-RuO2/Ti电极,用X射线衍射(xRD)和扫描电镜(SEM)以及X射线能谱仪(EDS)对其进行表征,并考察其对脉冲电解含氰含银电镀废水中银离子的还原和氰化物分解的影响.结果表明:用热氧化分解法制备的IrO2-RuO2/Ti电极表面具有"泥裂"现象,其主要成分为IrO2,RuO2和TiO2.以IrO2-RuO2/Ti为阳极,不锈钢圆筒为阴极,在脉冲电压0.5 V,脉冲频率1200 Hz,占空比50%,曝气量1.0Lmin,电解液循环流速100mL/min和pH值10～11的条件下,常温电解4.0 h,发现IrO2-RuO2/Ti电极性能优于石墨电极、PbO2/Ti电极和不锈钢电极,在其作用下,废水中银离子的回收率达到99.66%,氰分解率达到91.63%.     

Silver–carbon electrocatalyst for air cathodes in alkaline fuel cells

A method for producing electrocatalysts containing silver supported on different carbons was developed. The catalysts were investigated in air (oxygen) diffusion electrodes in alkaline electrolyte (7 M KOH). Depending on the carbon support used, up to a threefold improvement in electrode performance was achieved compared with the activity of the uncatalysed carbon in this media. At ambient temperature and atmospheric pressure, a current density of 150 mA/cm^–2 was obtained at electrode potential 1.2 V vs zinc (0.75 vs HE). A correlation between electro catalytic activity and wetted surface area of the electrocatalysts was found.     

Experimental optimization of alkaline zinc-silver oxide primary cell with respect to the zinc electrode preparation and composition

The optimization of parameters controlling the fabrication of zinc electrodes, by the slurry paste method, has been carried out. The parameters varied were active material composition, applied compression and electrode thickness. The optimum values obtained by a factorial experimental programme were: a zinc material that contains 2–4 wt% HgO, 0·5–2% PVA, and 94–97·5% ZnO; a compression of 500–1500 psi applied to the zinc electrode; and an electrode thickness of 0·265-0·35 mm.     

Effect of an Nb2O5 nanolayer coating on ZnO electrodes in dye-sensitized solar cells

Nanostructured porous zinc oxide electrodes for use in dye-sensitized solar cells (DSSCs) were coated with thin niobium oxide layers by using sol–gel transformation of niobium pentaethoxide in air. Coating solutions were prepared by mixing niobium pentaethoxide and ethanol. A dip-coating technique was adopted at a low withdrawal speed of 100 μm s⁻¹. The coated electrodes were then heat-treated at temperatures between 400 and 600 °C. The presence of niobium in the coated electrodes was confirmed by X-ray photoelectron spectroscopy. As expected, the niobium oxide layers worked as an energy barrier between the ZnO electrode and electrolyte. Open-circuit voltage (V_OC) of the cells using the coated electrodes was then enhanced up to 0.768 V, which was attributable to the suppression of the recombination of photogenerated electrons with oxidized species in electrolytes. An additional benefit of the coating was that grain growth of ZnO particles in the electrodes was hindered and short-circuit photocurrent density (J_SC) was kept relatively high due to large amounts of adsorbed dye. An overall light-to-electricity conversion efficiency was increased to a maximum of 5.19%, indicating that the proper coating technique was the key for improving the performance of ZnO-based DSSCs.     

Development of a novel metal hydride–air secondary battery

A laboratory metal hydride/air cell was evaluated. Charging was via a bifunctional air gas-diffusion electrode. Mixed nickel and cobalt oxides, supported on carbon black and activated carbon, were used as catalysts in this electrode. At 30mAcm–2 in 6m KOH, the air electrode potentials were –0.2V (oxygen reduction) and +0.65V (oxygen evolution) vs Hg/HgO. The laboratory cell was cycled for 50 cycles at the C/2 rate (10mAcm–2). The average discharge/charge voltages of the cell were 0.65 and 1.6V, respectively. The initial capacity of the metal hydride electrode decreased by about 15% after 50 cycles.     

Effect of Ag on the microstructure and electrical properties of ZnO

Various amounts of silver particles, 0.08–7.7 mol%, are mixed with zinc oxide powder and subsequently co-fired at 800–1200 °C. The effects of Ag addition on the microstructural evolution and electrical properties of ZnO are investigated. A small Ag doping amount (<0.76 mol%) promotes the grain growth of ZnO; however, a reversed trend in grain growth is observed for a relatively larger Ag addition (>3.8 mol%). It is evident that a tiny amount of Ag (0.08 mol%) may dissolve into the ZnO lattice. High-resolution TEM observations give direct evidences on the segregation of Ag solutes at the ZnO grain boundaries. The grain boundary resistance of ZnO increases 35-fold with the presence of Ag solute segregates. The Ag-doped ZnO system exhibits a nonlinear electric current–voltage characteristic, confirming the presence of an electrostatic barrier at the grain boundaries. The barrier is approximately 2 V for a single grain boundary.     

Oxygen evolution on semiconducting oxides

The oxygen evolution reaction is of particular interest to secondary metal air batteries and water electrolysis plants. However, most of the earlier work has been on precious metals and there are no guidelines for the choice of semiconducting oxides as oxygen evolving electrodes. In this study, the role of the metal/metal oxide or the lower metal oxide/higher metal oxide couple in determining the minimum voltage required for the evolution of oxygen is emphasized, together with other essential requirements such as electrical resistivity, electrode microstructure, corrosion resistance and catalytic properties. A survey of various metal oxides based on the above criterion suggested that NiCo₂O₄ is of particular interest and Teflon bonded electrodes based on this material gave over 13000 A/m² at 1.63 V vs dhe, 70°C, 5 N KOH.     

Kinetics and mechanism of the formation of doped skeletal copper catalysts: the effect of zincate compared to undoped and chromate-doped systems

Addition of zincate to the leach liquor for the preparation of skeletal copper increases the copper surface area; however it does not stabilize the structure against rearrangement. The leaching kinetics have been studied using a rotating disc electrode (RDE) at 269–293 K in 2–8 M NaOH and 0.0005–0.1 M Na₂ZnO₂. Zincate ions precipitate as zinc oxide, due to the local consumption of hydroxide ions near the leach front as the aluminium dissolves. This oxide hinders the aluminium dissolution, slowing the leaching rate. It also hinders copper dissolution/redeposition and prevents copper diffusion, thus reducing the structural rearrangement significantly, and causing the formation of a much finer copper structure with increased surface area. The zinc oxide redissolves as the leach front passes, releasing the copper to rearrange once more, thereby allowing the surface area to decrease with time. The activation energy for leaching was found to be 84 ± 6 kJ mol^–1.