The current distribution and shape change of zinc electrodes in secondary silver-zinc cells

The current and potential distributions of zinc electrodes in secondary zinc-silver oxide cells during cycling were studied using a sectioned electrode technique. The shape change occurring in zinc electrodes resulting from cell cycling was examined. The zinc electrodes used for this study were of a conventional type of design and were fabricated by a slurry paste method. The positive electrodes used were sectioned silver oxide electrodes, each of which comprised nine sections of sintered silver plates. The results indicate that, by adding a layer of non-woven fabric treated with Fe₂O₃ to the edge sections of the electrode separator system, the uniformity of the current and potential distributions on the zinc electrodes during cell cycling can be improved, and the zinc electrode shape change resulting from cell cycling can be effectively suppressed.     

Electrocatalytic Reduction of Oxygen on a Pd Ad-Layer Modified Au(111) Electrode in Alkaline Solution

Oxygen reduction was studied on palladium, cadmium and zinc ad-atom modified single crystal Au(111) electrodes. The electrodes were modified by underpotential deposition process and their activity towards oxygen reduction was studied in alkaline media by voltammetry. The reduction peaks obtained were compared with those of bare Au(111), Pd disc and bulk deposited Cd electrodes. Enhanced catalytic activity of the Au(111) electrode in the presence of Pd, Cd and Zn ad-layer can be attributed to a change in surface charge and energy by ad-layer formation. In oxygen saturated medium a well defined sharp reduction peak was observed at ?0.12 V for 1/5 ML Pd ad atom modified Au(111) electrode while it was positioned at ?0.18 V on a Pd disk electrode. The best shift in reduction peak potential was obtained with 2/5 ML Pd ad atom modified Au(111) electrode with similar current density of Pd disc electrode.     

Surface activation of manganese oxide electrode for oxygen evolution from seawater

Utilizing the fact that the equilibrium potential of oxygen evolution is lower than that of chlorine evolution, oxygen evolution in seawater electrolysis was enhanced by decreasing the polarization potential under galvanostatic conditions through increasing the effective surface area of manganese oxide electrodes. Electrodes were prepared by a thermal decomposition method. IrO2-coated titanium (IrO2/Ti electrode) was used as the substrate on which manganese oxide was coated (MnOX/IrO2/Ti electrode). Subsequently, oxide mixtures of manganese and zinc were coated (MnOX–ZnO/MnOX/IrO2/Ti electrode). The effective surface area of the MnOX–ZnO/MnOX/IrO2/Ti electrodes was increased by selective dissolution of zinc (leaching) into hot 6M KOH. The oxygen evolution efficiency of the MnOX/IrO2/Ti electrode was 68–70%. Leaching of zinc from the MnOX–ZnO/MnOX/IrO2/Ti electrodes with 25mol% or less zinc led to a significant increase in the oxygen evolution efficiency. The maximum efficiency attained was 86% after leaching of zinc from the MnOX–25mol%ZnO/MnOX/IrO2/Ti electrode. However, large amounts of zinc addition, such as 40mol% or more are detrimental because of a decrease in the oxygen evolution efficiency. This is due to the formation of a double oxide, ZnMnO3, which is hardly dissolved in hot 6M KOH.     

Zinc deposition and passivated hydrogen evolution in highly acidic sulphate electrolytes: Depassivation by nickel impurities

In highly acidic sulphate electrolytes used for zinc electrowinning, the steady-state behaviour and the impedance of zinc electrodes have been analyzed under potentiostatic control. It is shown that zinc deposition takes place under conditions where hydrogen evolution remains passivated on the deposit surface. A depassivation of hydrogen evolution leading to the deposit corrosion can be provoked by an electrode excursion near the corrosion potential. The presence of nickel impurities facilitates this depassivation process even though the electrode potential remains cathodic. The depassivation becomes easier with the existence of a scratch on the substrate surface, because of bubbles retained on this defect.     

The variation of current density and electrode potential with electrode resistance and its role in cell design

The potential drop along parallel electrodes of finite resistance has been calculated (a) assuming a pure resistive representation of the cell and (b) by considering the polarization characteristics of an irreversible electrode reaction. It was found that a much more uniform potential (and also current) distribution was obtained with the current feeders at opposite ends of the electrodes rather than at the same ends. With this configuration it is advantageous to have electrodes of equal resistance. Equations are given that enable the potential and current density distribution to be easily calculated. A factor I_tot/I_max was derived which is a measure of the effective utilization of the electrode area. It may be used for calculating the required electrode area with non-uniform cd distribution.     

Preliminary comparative studies of zinc and zinc oxide electrodes on corrosion reaction and reversible reaction for zinc/air fuel cells

Even though Zn/air energy system is considered to be a promising power energy source, it has been limited to be applied for an electrically rechargeable system basically due to the problem of the irreversible reaction and the corrosion reaction. In this paper a novel attempt has been made to compare the behavior of zinc electrode with a zinc oxide electrode and a modified zinc oxide electrode containing zinc oxide and lead oxide. The hydrogen overpotential is favorable in the zinc electrode, and the modified zinc oxide electrode shows the improved properties showing the more negative potential than the case of the zinc oxide electrode. Investigations of cyclic voltammogram reveal that the pure zinc electrode is irreversible, while both the zinc oxide and the modified zinc oxide electrodes are reversible. However, as far as dendrite formation is concerned there is no marked improvement in case of the zinc oxide and the modified zinc oxide electrodes.     

Repartition du transfert electrochimique dans les electrodes de puissance

In order to interpret the disparity between performances obtained from fuel-cell electrodes and the orders of magnitude predicted on the basis of current-density/potential curves of simple elements of the electrode surface, a theory of the distribution of potential in power electrodes is presented.

The theory is based on the non-equipotential character of the constituent surface elements of an electrode of whatever geometry, and postulates the constancy of the current-density/potential relation defining local transfer in given chemical and physical conditions.

An application of the theory is presented for the case of sintered electrodes immersed in an electrolyte from which the electrochemically active compound is derived; this application shows the limitation of possibilities of electrochemical transfer to a very thin frontal layer of the electrode in previous designs of fuel-cell electrodes.

Two important consequences follow: the possible design of fuel-cell electrodes in which the electro-active material is carried to the active surface by injection of its solution through a sintered structure; and the lack of validity of conclusions as to the relative importance of the factors limiting electrode performance, based on the analysis of current-density/potential curves, unless the existence of a distribution of potential is taken into account.     

A modelling approach to the optimizaton of lead-acid battery electrodes

A resistive grid model was used to study the current and ohmic overpotential distributions along the surface of lead-acid battery electrodes. Analyses were made under two different regimes: the initial behaviour at high current densities and the response with time at low current densities. At high discharge currents the theoretical results show that the geometry of the electrodes and the position of the lug play the most important role in controlling the magnitude of ohmic losses. The best geometry is a square grid with the lug positioned at the upper centre of the electrode. At low discharge currents the model was used to follow the current distribution along the electrode surface as a function of time. In this last study the appearance, for long discharge times, of short-circuited concentration microcells localized in certain regions of the electrode surface was noted. The other regions of the electrode supply the external discharge current and the excess current necessary to charge the internal microcell.     

Comparative electrochemical studies of zinc chromate and zinc phosphate as corrosion inhibitors for zinc

The anticorrosive performance of two inhibitive pigments, zinc chromate and zinc phosphate, was compared using electrochemical impedance spectroscopy (EIS) and the scanning vibrating electrode technique (SVET) in pigment extracts in 0.1 M NaCl. It was observed that zinc was protected from corrosion in both extracts. In tests using hot dip galvanised steel painted with an epoxy primer incorporating the pigments, the SVET detected the anodic and cathodic distribution along the scribes, although no significant differences were observed among the various primers. On the contrary, EIS was able to distinguish processes occurring on the metal surface exposed by the scribe in different samples. For primers with anticorrosive pigment, a time constant at high frequencies was attributed to a layer of protective nature, probably formed by metal ions from the substrate and inhibitive ions leached from the anticorrosive pigments.     

The anodic behaviour of porous zinc electrodes. II. The effects of specific surface area of the zinc compact material

A study has been made of the galvanostatic polarization behaviour of porous zinc electrodes as a function of electrode surface area. Correlations have been established between the electrode polarization parameters and surface areas as measured by BET and methylene blue adsorption methods.     

Surface alloy/GC电催化剂的制备与结构表征

以玻碳为载体 ,通过电化学方法在其表面沉积催化物质研制表面合金纳米电催化剂 (Surfacealloy/GC)。运用电化学循环伏安 (CV)和扫描电镜 (SEM)等技术进行结构和性能表征。结果表明 ,在Surfacealloy/GC上氢的析出电位负移至 - 0 40V(SCE) ,与Pt电极相比负移约 15 0mV ,具有较高的电还原应用价值。另外 ,Surfacealloy/GC的氧化电位正移至 0 64V ,其稳定性明显高于电有机合成中常用的Pb、Sb、Cu等金属电极。SEM研究结果表明 ,所研制的Surfacealloy/GC电催化剂是一种由粒度约为 70nm的纳米颗粒合金组成的薄膜     

The pore texture of zinc electrodes characterized by impedance measurements

Impedance measurements have been applied to the characterization of the pore texture of zinc electrodes used in electrochemical batteries. It is shown that these electrodes are equivalent to cylindrical pore electrodes. The electrode parameters (radius of pore, pore depth and surface density of pores) have been determined from the values of the electrode capacitance, the electrolyte resistance inside the pores and the electrode porosity.     

ELECTROOSMOTIC HELICAL FLOW PRODUCED BY COMBINED USE OF LONGITUDINAL AND TRANSVERSAL ELECTRIC FIELDS IN A RECTANGULAR MICROCHANNEL

We theoretically devise and simulate a microelectrode system that produces electroosmotic helical flow in a straight rectangular microchannel. In addition to a pair of primary electrodes that generate a longitudinal electric field, sets of secondary electrodes are installed to produce a transversal electric field. The secondary electrode system consists of point electrodes embedded along two edges of the bottom surface of the channel. The transversal electric field developed across the bottom surface causes the electroosmotic motion of fluid at the bottom of the channel along the transversal direction. As a combined effect of the primary electrode system that produces unidirectional longitudinal flow along the channel and the secondary electrode system that produces transversal flow across the bottom surface, the flow inside a rectangular microchannel follows a helical pattern. After simulating the electroosmotic helical flows developed in microchannels we analyze the mixing of sample liquid in such flow fields by calculating the trajectories of fluid particles.     

ELECTROOSMOTIC HELICAL FLOW PRODUCED BY COMBINED USE OF LONGITUDINAL AND TRANSVERSAL ELECTRIC FIELDS IN A RECTANGULAR MICROCHANNEL

We theoretically devise and simulate a microelectrode system that produces electroosmotic helical flow in a straight rectangular microchannel. In addition to a pair of primary electrodes that generate a longitudinal electric field, sets of secondary electrodes are installed to produce a transversal electric field. The secondary electrode system consists of point electrodes embedded along two edges of the bottom surface of the channel. The transversal electric field developed across the bottom surface causes the electroosmotic motion of fluid at the bottom of the channel along the transversal direction. As a combined effect of the primary electrode system that produces unidirectional longitudinal flow along the channel and the secondary electrode system that produces transversal flow across the bottom surface, the flow inside a rectangular microchannel follows a helical pattern. After simulating the electroosmotic helical flows developed in microchannels we analyze the mixing of sample liquid in such flow fields by calculating the trajectories of fluid particles.     

Effect of the addition of NH4F on anodic behaviors of DSA-type electrodes in H2SO4-(NH4)2SO4 solutions

The effect of NH₄F addition on the high-anodic behaviors of various DSA-type electrodes was investigated in a mixture of sulfuric acid and ammonium sulfate. The pronounced effect of NH₄ addition was observed on DSA-type Ti/RuO₂, Ti/IrO₂?TiO₂ electrodes, but not observed on the other electrodes studied. The close relationship was found between the increment of the electrode potential caused by the NH₄ addition and the quantity of F^? adsorbed on the electrode surface. The effect of NH₄F addition was considered to be resulted from the adsorption of F^? on the electrode surface, and the presence of adsorbed F^? was directly proved.     

Optimized zinc electrode for the rechargeable zinc–air battery

For the development of a long-lived, electrically rechargeable zinc–air battery the structure and wettability of pasted zinc electrodes were optimized. Pasted zinc electrodes containing 1 to 10% cellulose but having almost the same nominal capacities were prepared and tested in zinc/oxygen cells. The effect of discharge rate on cell voltage and delivered capacity, as well as the maximum power, were measured. Furthermore, cell charge–discharge behaviour and cycle life were examined. After different times of operation, the porosity and the pore size distribution of the pasted zinc electrodes were measured by means of mercury porosimetry. The cycle life and peak power drain capability of the Zn/oxygen battery could be substantially improved by the addition of 10wt% cellulose to the pasted zinc electrode.     

Evaluation of the influence of impurities on the oxygen sensitivity of monocrystalline antimony electrodes

Measurements are presented which show that the oxygen sensitivity of ultra pure (6N) monocrystalline antimony electrodes presents an improved reproducibility and constancy as compared with monocrystalline electrodes containing impurity inclusions in the electrode surface. The former electrodes also present a higher potential stability. The time to reach a pseudo-stable potential level (drift <0.6 mV/h) is also faster for this type of electrodes. The improved characteristics of the ultra pure electrodes are thought to be due to a uniform distribution of the anodic and the cathodic areas over the electrode surfaces. On the electrodes with impurity inclusions exposed in the surfaces, a high degree of local corrosion was noted. The oxygen sensitivity of the ultra pure electrodes, dE/d(log p_O2), was found to be 15.3 ± 0.7 and 15.7 ± 0.8 mV, 5 and 25 h after immersion of the electrodes in the electrolyte respectively.     

Experimental studies and model validation for the optimization of electrodes configuration in a molten salt electrorefiner

Molten salt electrorefining is a high-temperature electrochemical process for treating the spent metallic fuel from fast reactors and is aimed at the separation of U and Pu from fission products. Potential and current distribution analysis was carried out by experimental studies as well as modelling using COMSOL Multiphysics for various electrode configurations. A 2D/2D axisymmetric geometry model was used to evaluate the potential and current distributions. The effect of the following parameters was evaluated: (1) configuration having two pairs of electrodes in parallel/staggered arrangement; (2) centre-to-centre distance between the electrodes; (3) solid cathode with and without insulation at the bottom; (4) Cd cathode and (5) cathode surface area. The resistance of the electrorefiner was calculated using COMSOL model for various electrode configurations and compared with that obtained experimentally. There was very good agreement between the experimental values and the simulation results. The computed cell resistance was also validated with published data. A sensitivity analysis was also performed to determine the parameter that more significantly influences the cell resistance. The two parameters that were varied were the electrolyte conductivity and the cell voltage. It is observed that of the two parameters the cell resistance is most sensitive to the electrolyte conductivity and there is no change in cell resistance with cell voltage.     

Characterisation of platinised Ti mesh electrodes using electrochemical methods: methanol oxidation in sodium hydroxide solutions

Characterisation of a platinised Ti mesh electrode, prepared by electrodeposition of Pt, by methanol oxidation has been studied over a range of NaOH and methanol concentrations using cyclic voltammetry (CV), quasi-steady-state polarisation, and electrochemical impedance spectroscopy (EIS). The activity of methanol oxidation in aqueous alkaline systems increased with pH or OH species coverage on the electrode surface. A reaction order of close to 0.5 was obtained for both NaOH and methanol in NaOH solutions indicating that adsorption of methanol and OH⁻ on the platinised electrode follows Termkin isotherm. In the high potential region, a poisoning effect was observed at methanol/[OH⁻] concentration ratio greater than 1, which could have arisen from an excess of methanol at the electrode surface and/or depletion of OH⁻ at the electrode surface. The methanol oxidation behaviour conforms with that on Pt electrodes suggesting a promising new method of preparing electrodes for fuel cells.     

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.