A quantitative study of shape change in zinc secondary electrodes

Zinc electrode shape changes resulting from cell cycling have been studied. Experiments have been performed using zinc-silver oxide secondary cells of an industrial type. The results indicate that the extent of shape change is directly affected by the rate of discharge and charge, and the ZnO content of the electrolyte. The zinc electrode shape change can be mitigated by adding to the edge sections of the separator system a layer of non-woven fabric treated with Fe(OH)₂, The extent of shape change can also be reduced by increasing the separator thickness at the plate periphery. The orientation of the zinc electrode with respect to the earth's gravitational field (horizontally or vertically) on cycling, and the preparation methods for the zinc electrode (electrodeposition or slurry paste) have no apparent effect on the extent of zinc electrode shape change resulting from cell cycling.     

Study of the performance of secondary alkaline pasted zinc electrodes

Calcium zincate was prepared by a chemical coprecipitation method and characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The electrochemical performance of pasted zinc electrodes with bismuth and calcium additives was investigated by the charge–discharge method. The addition of metallic bismuth powder improves the discharge performance of zinc electrodes due to the formation of an electronic conduction matrix. The calcium-containing zinc electrodes showed higher discharge capacity, less shape change and longer cycle lifetime. Moreover, zinc electrodes using calcium zincate as active material show better electrochemical performance than those with the physical mixture of zinc oxide and calcium hydroxide.     

Non-linear electric analogues of the current distribution in porous electrodes

Porous battery electrodes can, with respect to their volumetric current distribution, be regarded as electrical networks. Linear, time-independent networks again can be treated by analytical methods. In some practical cases, however, deviations have to be considered: non-linear overvoltage functions, changing conductivities. Current distribution in such non-linear and time-dependent systems can be evaluated either by numerical computor calculations, or by the application of corresponding electrical analogues. The latter way is fairly simple and will be treated here.The observed overvoltage functions can be generated by semiconducting diodes. Changing conductivities have been generated by adjustable resistors. Application of special automatic devices, e.g. diaphragms with closing pores, seems possible but has not been effected so far. Voltage and current scales have to be adapted to the characteristics of the electronic components.In general, we can state, that in some practical electrodes the real overvoltage functions may change the current distribution markedly. Particular shoulders in the distribution curves are observed, which ameliorate the electrode utilization. Introduction of measured ionic conductivity changes certainly influences the current distribution but results in deteriorations of the predicted electrode characteristics.

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Zusammenfassung Poröse Batterie-Elektroden können bezüglich ihrer volumetrischen Stromverteilung als elektrische Netzwerke angesehen werden. Lineare, von der Zeit unabhängige Netzwerke hinwiederum können mit analytischen Verfahren theoretisch behandelt werden. In einigen technischen Fällen müssen aber Abweichungen davon berücksichtigt werden: nichtlineare Überspannungs-Funktionen, zeitabhängige Leitfähigkeit. Die Stromverteilung in solchen nichtlinearen und zeitabhängigen Systemen kann entweder numerisch mit einem Rechner oder durch die Untersuchung entsprechender elektrischen Analogmodelle ermittelt werden. Der letztere Weg ist verhältnismässig einfach und wird hier behandelt.Gemessene Überspannungs-Funktionen können z.B. durch Halbleiter-Dioden nachgebildet werden. Änderungen der Leitfähigkeit sind durch nachgeregelte Widerstände berücksichtigt worden. Dazu automatische Bauelemente zu verwenden, z.B. Diaphragmen mit Poren, die sich schliessen, erscheint möglich, ist aber noch nicht erfolgreich durchgeführt worden. Die Maßstäbe für Spannung und Stromstärke müssen den Eigenschaften der verwendeten elektronischen Bauelemente angepaßt werden.Als allgemeingültiges Ergebnis kann man festhalten, dass in technischen Elektroden die tatsächlichen Überspannungs-Funktionen die Stromverteilung merklich beeinflussen können. Absätze oder Schultern in den Verteilungs-Funktionen werden beobachtet, welche die Elektroden-Ausnutzung verbessern. Auch die Berücksichtigung der Änderungen des Ionen-Widerstandes hat Einfluss auf die Stromverteilung, führt aber zu einer Verschlechterung der vorherzusagenden Elektroden-Eigenschaften.

     

On the distribution of current generation in porous gas electrodes

In the active layer of porous gas electrodes, the spatial distribution of energy generation is determined by several interacting factors, e.g. pore statistics, distribution of active sites, and a set of correlated transport equations. After a short introduction to the problem, it is shown that the transport phenomena can, in this case, be treated in a very simplified manner. In particular, the specific electron resistance can be neglected. Restriction of gas supply can be described by a formalistic gas resistance _g. Thus, the interaction of the different transport parameters can be treated by considering purely electrical models. The relative magnitudes of the different parameters, in the case under study, are of such an order that finally it is only necessary to consider two of them: the specific ionic resistivity of the porous electrode filled partly with liquid electrolyte, and a special parameterp which describes the overvoltage in the region between gaseous phase and electrolyte. As a result, the spatial distribution of current generation can be indicated in the form of analytical expressions and diagrams. One also obtains values of the penetration depth of current generation which do not disagree with practical experience.     

Potential distribution in flow-through porous electrodes under limiting current conditions

This paper presents an experimental study of metal— solution potential distributions in flow- through porous electrodes of fixed beds of spherical conducting particles.The distributions are determined when the electrodes are working at the limiting diffusion current. They are shown to be in agreement with the corresponding theoretical distributions.     

The anodic behaviour of porous zinc electrodes

The passivation of horizontal and vertical porous zinc (unamalgamated) electrodes in aqueous KOH solutions has been studied. The results are interpreted in terms of the formation of a soluble anodic product which decomposes within the electrode forming an insoluble deposit. The ohmic resistance of the insoluble anodic product provides the major component of the electrode polarization and determines the useful duration of discharge of the electrode.     

Positioning the reference electrode in proton exchange membrane fuel cells: calculations of primary and secondary current distribution

The primary and secondary current distribution study indicates the geometry of a thin electrolyte in a proton exchange membrane (PEM) fuel cell has a direct relation to the measured electrode polarization, thus making the positioning of the reference electrode and ohmic compensation critical. The different kinetic overpotentials on the electrodes can also affect the potential distribution and therefore affect the measurement accuracy. The measurement error can be significant for the fuel cell system with different kinetic overpotentials and with electrode misalignment. The measurement error for both hydrogen and direct methanol fuel cells (DMFC) has been analyzed over the current density region with no mass transfer effects. By using two reference electrodes, the measurement error can be substantially decreased for both anode and cathode measurement in a direct methanol fuel cell, and for the cathode measurement in a hydrogen/air fuel cell.     

Theoretical and experimental studies of current distribution in gas-evolving electrochemical reactors with parallel-plate electrodes

The current density distributions for a gas evolving electrochemical reactor with parallel-plate electrodes at different total currents were measured and calculated. The current density profiles were determined using a segmented electrode method. The numerically computed curves, according to a one-dimensional model, and the experimental measurements show good agreement. Furthermore, a comparison between several mathematical models is made.     

Modelling and calculation of the current density distribution evolution at vertical gas-evolving electrodes

During industrial electrolysis, for hydrogen, dichloride or aluminium production, there is bubbles creation at one or two electrodes which imply a great hydrodynamic acceleration but also a quite important electrical field disturbance. This disturbance can lead to the modification of the local current density and to anode effects for example. There is few works concerning the local modelling of coupled electro active species transport and electrochemical processes in a biphasic electrolyte. There are also few local experimental measurements in term of chemical composition, temperature or current density which would allow the numerical calculations validation. Nevertheless, effects like the anode effect, particularly expensive on the point of the process efficiency, should need a better understanding. Nowadays, the respective roles of the local temperature increases, the electro active specie composition or the transport properties modification due to bubbles are not known.The goal of the present work is the modelling and the numerical simulation of the vertical electrode configuration for a biphasic electrolysis process. Bubbles presence is supposed to modify the electrical properties, and then the electro active species diffusive transport and the current density. Bubbles are also motion sources for the electrolysis cell flow, and then hydrodynamic properties are strongly coupled with species transport and electrical field. The present work shows hydrodynamic and electrical properties in a laboratory scale electrolysis cell with a vertical electrode. The numerical algorithm used was the finite volume used in the computational fluid dynamic software Fluent^®.     

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.     

The electrochemistry of metal oxide additives in pasted zinc electrodes

The effect of small additions (2 w/o–4 w/o) of HgO, Tl₂O₃, PbO, CdO, In₂O₃, In(OH)₃, In and Ga₂O₃ to pasted zinc oxide electrodes has been evaluated using chronopotentiometry, cyclic voltammetry, polarization measurements and scanning electron microscopy. Additives such as PbO, In(OH)₃ and In increase the density and the polarizability of the deposit whereas additives such as Ga₂O₃ and CdO yield finely divided deposits of low polarizability. The results are attributed to a substrate effect on zinc deposition. The beneficial effect of some additives (eg a mixture of 0.5 w/o In(OH)₃ and 1 w/o PbO) is attributed to an improvement in the secondary current distribution due to an increase in electrode polarizability.     

Potential distribution model for rechargeable sulphur electrodes in sodium-sulphur cells

The efficiency of laboratory type sodium-sulphur cells, especially the utilization of reactants, is critically dependent upon the design of the sulphur electrode. The satisfactory rechargeability of a new design, based on a graduated electronic resistance inside the carbon fibre matrix is analysed in terms of two models: (a) predominant influence of potential distribution, (b) predominant influence of free flow in open spaces. Experimental results in favour of model (a) are discussed. The potential distribution of the new design is computed using appropriate assumptions for the one-phase region and compared with that in the absence of a graduated electronic resistance. It appears that the improved rechargeability can be explained by the different potential distribution.     

Computation of primary current distribution and cell resistance at model electrodes with circular perforations

A model of a gas-evolving production-type cell with a circularly perforated anode is described. A unit of the model was composed of a disk cathode, a separator and a ring anode in turn. These were located in a cylindrical cell filled with solution. Primary current and potential distributions in the unit cell were computed by solving the Laplace equation in cylindrical coordinates by the finite element method. Geometric parameters determining the distributions were primarily the interelectrode distance and the percentage open area. Current distribution in the open part was larger than that in a rectangular cell with the same geometric parameters because of the cylindrically concentrated supply of the current from the inner part of the ring and the back side of the anode. The unit cell resistance was evaluated as a function of the geometric parameters. It exhibited a linear variation of the interelectrode distance and the square of the percentage open area. There was, however, a slight dependence of the percentage open area on the unit cell resistance and hence it is concluded that circularly perforated electrodes provide higher performance than louvre-type electrodes.     

The cycling behaviour of zinc electrodes in a nickel oxide-zinc accumulator

The behaviour of porous zinc electrodes was investigated by monitoring the potential distribution along the surface of the electrode during charging and discharging in a nickel oxide-zinc battery. Impedance measurements of the zinc electrode were taken as a function of state-of-charge and of the cycle number. The composition of the electrode was varied with HgO and PbO additives. Shape change was observed for all electrodes. The observed potential distribution has led to the tentative explanation that the shape change phenomenon is caused by diffusion of zincate along the surface.     

Cyclic voltammetric study of zinc and zinc oxide electrodes in 5.3 M KOH

The behaviour of zinc and zinc oxide in 5.3 M KOH in the presence of alkaline earth oxides, SnO, Ni(OH)₂ and Co(OH)₂ was examined by cyclic voltammetry. The influence of the alkaline earth oxides was compared with additives of established effects (Bi₂O₃, LiOH, Na₂CO₃ and CdO). The alkaline earth oxide each exhibits a distinct behaviour towards zincate. Whereas, a single process of interaction with zincate was shown by CaO; two modes of reaction were obtained with SrO and BaO. Solid solution formation was noticed with BeO and MgO. The other additives forming solid solution with ZnO were CdO, SnO. The ionic sizes of Ni(OH)₂ and Co(OH)₂ allow solid solution formation with Zn(OH)₂. Both Bi₂O₃ and Na₂CO₃ enter into complexation with zincate. LiOH forms two distinct zincates, of which one is an oxo zincate leaching the `hydroxyl' functionality. Cyclic voltammetry revealed the deposition of the oxide/hydroxide additives as metal prior to the onset of zinc deposition and the potential range for this additive metal deposition is almost the same for different additives (SnO, CdO, Ni(OH)₂). The beneficial action of these additives to zinc alkaline cells is associated with a substrate effect. The implication of this electrocatalytic deposition of metals on a zinc oxide electrode is also discussed.     

A numerical method of calculating secondary current distributions in electrochemical cells

A numerical method is proposed based on the analogy between the potential distribution in an electrolytic solution and the temperature distribution in a heat-conducting medium. Thus the equation of non-steady-state heat conduction which contains a hypothetical temperaturev(x, y, t) is solved numerically with appropriate boundary conditions. In the steady state the distribution ofv(x, y, t) corresponds to the distribution of potentialφ _s (x,y) which satisfies Laplace's equation. The method is useful not only for conventional electrochemical cells but also for complicated systems such as a bipolar electrode for which boundary conditions provide neither the potential nor the current density at the electrode surface.