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Physico-chemical properties of NaOH and KOH electrolytes are studied as possible state of charge indicators for the alkaline zinc-air redox flow battery. In order to predict the available energy of the battery and to prevent degradation as the result of unsuitable operating conditions, its state of charge has to be known. The state of charge is directly related to the composition of the negative electrolyte. A battery charging operation is simulated by electrodepositing zinc on a rotating cylinder electrode from NaOH electrolyte, while the simulation of a discharging operation is accomplished by dissolving ZnO in KOH electrolyte. The refractive index, conductivity, and density of NaOH- and KOH-electrolytes as well as the rest potential of the zinc electrode in these media are measured for different zincate and alkali concentrations at several temperatures. The electrolyte density and electrolyte conductivity are both linearly correlated with the zincate concentration. A linear correlation is found also between the rest potential and the logarithmic concentration term in the Nernst equation for the zinc half-cell reaction. These indicators permit a simple and straightforward monitoring of the state of charge of alkaline zinc-air redox flow batteries.

Rechargeable Zn/MnO2 alkaline batteries are a promising technology for grid storage applications since they are safe, low cost, and considered environmentally friendly. Here, a commercial ceramic sodium ion conductor which is impervious to zincate [Zn(OH)42−], a contributor to MnO2 cathode failure, is evaluated as the battery separator. As received, the ionic conductivity of this separator was measured with electrochemical impedance spectroscopy to be 3.5 mS cm−1, while its thickness is 1.0 mm, resulting in large total membrane resistance of 25.3 Ω. Reducing the thickness of the ceramic to 0.5 mm provided for a decreased resistance of 9.8 Ω. Crossover experiments conducted using inductively coupled plasma - mass spectrometry measurements failed to measure any Zn(OH)42− transport indicating a diffusion coefficient that is at least two orders of magnitude less than that for the commercial cellophane and Celgard separators. For 5% DOD at a C/5 rate, the cycle lifetime was increased by over 22% using the 0.5 mm thick ceramic separator compared to traditional Celgard and cellophane separators. Scanning electron microscopy/energy dispersive X-ray spectroscopy and X-ray diffraction characterization of cycled electrodes showed limited amounts of zinc species on the cathode utilizing the ceramic separator, consistent with its prevention of Zn(OH)42− transport.

The hydrogen evolution reaction is known as a parasitic reaction during the zinc electrodeposition from alkaline zincate solutions and is thus responsible for current efficiency losses during the electrolysis. Besides that, the rising hydrogen bubbles may cause an extra convection within a diffusion layer, which leads to an enhanced mass transport of zincate ions to an electrode surface. In this work, the mentioned phenomena were studied experimentally in a flow through electrolyzer and the obtained data were subsequently evaluated by mathematical models. The results prove the indisputable influence of the rising hydrogen bubbles on the additional mixing of the diffusion layer, which partially compensates the drop of the current efficiency of the zinc deposition at higher current flows. Moreover, the results show that the current density ratio (i.e., the ratio of an overall current density to a zinc limiting current density) is not suitable for the description of the zinc deposition, because the hydrogen evolution current density is always involved in the overall current density.

Porous zinc electrodes are elaborated, assembled in Zn/Ni 3–4 Ah prismatic batteries and monitored upon charge/discharge cycling (150 cycles at C/3 rate followed by 150 cycles at 1C rate). Migration of zinc within the thickness of the zinc electrodes is revealed by X-Ray radiography, scanning electron microscopy, and X-EDS elemental cartography. Electrodes using either zinc oxide plus calcium hydroxide or directly calcium zincate as active material show similar behavior: neither dendrite formation nor morphological shape changes are observed after 300 charge/discharge cycles. Upon cycling, zinc accumulates at the core of the porous electrode, in the vicinity of the primary current collector and not at the surface, therefore avoiding any risk of dendrite formation. This suggest that the electrode operation is controlled by a core-shell mechanism, with a core of active zinc in the vicinity of the primary current collector, trapped in an electrode shell which is created by the outer portion of the electrode (active material, conductive additives, binder …). However, changing from C/3 to 1C rate after 150 cycles significantly improves the capacity of batteries using calcium zincate; this is correlated to its initial atomic homogeneity associated with higher porosity and an optimal electronic percolation.

Zinc electrode stability is one of the major problems that restrict the wide application of secondary zinc-based batteries. In this work, the impacts of flowing electrolyte on the suppression of zinc dendrite are systematically investigated through experimental and numerical investigations. In a static electrolyte condition, the stability of the zinc electrode increases with an increase of the applied current density. In the presence of flowing electrolyte, the lifespan of the zinc electrode at the current density of 10 mA cm−2 increases dramatically from 900 cycles in the static electrolyte to 2580 cycles with a flow rate of 30 rad min−1, and further increases to 4725 cycles when increasing the flow rate to 50 rad min−1. Moreover, by changing the inlet flow direction from the side to the bottom, a more stable electrode with the lifespan of 18000 cycles (1200 h) is achieved. The numerical calculation illustrates that the distribution of zincate ions is more uniform in the presence of the flowing electrolyte, which is the key to the dendrite suppression. Thus, with the implementation of flowing electrolyte, a stable zinc electrode with an ultralong lifespan is demonstrated, which is promising for practical applications.

The internal migration of zinc within the thickness of porous composite negative electrodes for rechargeable zinc batteries causes detrimental shape change of the zinc electrode. It depends on various parameters linked to the electron conduction in the electrode: electronic percolation, conductive additives, primary current collector. This study highlights a simple solution, that drastically enhances the electrochemical performance of thick and porous zinc electrodes, by rethinking their architecture. Using a multiplicity of thin current collectors enables to create a 3D network of electronic percolation and a better repartition of the current lines within the electrode; as a result, the formation of multiple active zinc cores in the electrode volume is better controlled. This strategy allows to improve the management of zinc migration within the electrode structure, leading to minimal densification of each zinc core, and overall enables higher reversibility of zinc oxidation/reduction upon discharge/charge. It finally mitigates the initial drastic capacity fading observed on standard type calcium zincate negative electrode.

The practical utilization of high-rate Zn-based secondary batteries is seriously plagued by the limited cycle stability and undesirable dendrite growth of Zn anodes during repeated charge/discharge process. Herein, we present the design and preparation of Zn–Al–F layered double hydroxides (LHDs) to enable the uniform zincate deposition and inhibit dendrite formation. The dendrite-free deposition of zincate is mainly ascribed to the uniform size distribution, fluorine-intercalated functional interlayer, regulated current density distribution, abundant electrochemical active sites and efficient ion diffusion channel of as-prepared Zn–Al–F LHDs. Furthermore, the potential of Zn–Al–F LHDs is investigated in a Ni–Zn aqueous secondary battery, which renders a high specific capacity of 383.08 mA h g−1 based on the total mass of Zn–Al–F LHDs and excellent cyclic stability with a capacity retention of 89.03% after 500 cycles at 1 A g−1. Moreover, at the current density of 100 A g−1, a peak power density of 168.9 kW kg−1 and a high energy density of 466.08 W h kg−1 have been achieved, which are significantly higher than the previously reported LHDs in Ni–Zn batteries.

Preferred orientation of zinc deposits during charging is shown to significantly improve performance and cycle life in flow-assisted alkaline zinc batteries, which has not been demonstrated earlier. The preferred orientation of zinc deposits was investigated using X-ray diffraction (XRD). Compact zinc is found to have ( 11 2 ¯ 2 ) preferred orientation on brass, which contributes to ∼60% of the texture. The effect of charging current and zincate concentration on morphology was investigated in a rotating hull cell and correlated with anodic efficiency. Compact zinc deposits are found to have a fine-grained, bright finish and the highest anodic efficiency. Electrochemical impedance spectroscopy (EIS) proves that compact zinc corresponds to the minimum in the half-cell resistance. Morphological control using compact zinc could be accomplished using innovations such as pulse charging or enhanced mass-transfer to improve anode performance without affecting the cathode.

Changes in electrodeposited zinc morphology were experimentally investigated in a flowing alkaline electrolyte. Deposition of zinc on flat sheets as well as on new mesh-type reduced-area current collectors was studied. Zincate concentration in the electrolyte, flow velocity, and current density was varied over a wide range. The results show that the ratio between the effective current density and the limiting current density (current density ratio) which is directly related to the zincate concentration at the electrode surface determines the zinc morphology. Zinc morphology is mossy and porous when the current density ratio is below 0.4, whereas it is a mixture of mossy and crystalline structures when it is between 0.4 and 0.9. Only when the current density ratio is above 0.9, does zinc deposition become just crystalline and compact. This finding holds for both flat sheet and mesh-type reduced-area current collectors.

The effects of the presence of additives like lead and tungstate ions in flowing alkaline zincate solutions on suppressing spongy zinc electrogrowth are examined. The results show that the two additives with optimal concentrations in flowing electrolytes can suppress spongy zinc initiation and propagation. And, the two additives can bring about more uniform and compact deposits and, thereby, reduce spongy zinc growth. The influence of lead and tungstate ions on the zinc deposition/dissolution is evaluated by cyclic voltammetry. It also shows that the addition of the two additives is largely a blocking action, and the co-deposition of lead and zinc ions may occur. The performance of the zinc-air flow battery with zinc regeneration electrolysis is determined. It shows that by the addition of 0.6 M Na 2 WO 4 or 10 −4 M to 10 −3 M lead, compact or mixed compact–spongy zinc deposits are created and the favorable charge/discharge performance of the battery is achieved with an energy efficiency of approximately 60%.

Zinc deposition from alkaline zincate solution in single flow zinc/nickel battery has been investigated. The effect of different substrates such as copper, cadmium and lead were examined by using cyclic voltammetry and cathodic polarization technique. It was found that the cadmium substrate is better than the others. Zinc deposition was carried out by using galvanostatic technique, and the deposits were examined by SEM. The results demonstrated that there is no zinc dendrite on the cadmium substrate in flowing electrolyte. Coulombic and voltage efficiencies of 98 and 88%, respectively, are obtained in a small laboratory cell.

In this work, zinc from the spent Zn–MnO 2 batteries is recycled. Ionic zinc is recovered from acidic or alkaline solutions using a galvanostatic technique. An optimum current density between 10.0 and 25.0 mA cm −2 was obtained for recovery of ionic zinc from acidic solutions. The optimum current density is equal to 15.0 mA cm −2 for zinc electrodeposition from alkaline solution. The charge efficiency is 80.0% and decreases with increase of current density. In acidic solutions, hydrogen adsorption catalyse Zn 2+ electrodeposition. In alkaline solutions, a parallel reaction of the hydrogen evolution inhibits zincate electrodeposition.

The effectiveness of bismuth ion and tetrabutylammonium bromide (TBAB) as deposit-enhancing additives of zinc electrode has been investigated by means of current–time technique, potentiodynamic polarization and scanning electron microscopy. The experimental results showed that the dendritic growth of zinc in alkaline zincate solution was inhibited by the addition of Bi3+ and TBAB, respectively. However, at a higher cathodic overpotential TBAB produced little effective suppression on zinc dendrites. Obvious synergistic effect of Bi3+ and TBAB on the dendrite inhibition of zinc electrode was also found.

The properties of calcium zincate as negative electrode materials for secondary batteries were examined by powder microelectrode, cyclic voltammetry, charge–discharge cycle measurements and X-ray diffraction (XRD) analysis. The results show that the cycleability of calcium zincate is obviously superior to that of ZnO and that of the mixture of ZnO and Ca(OH)2 (the molar ratio of Zn:Ca ¼ 2:1). Calcium zincate forms zinc metal during the charging and exhibits an initial discharge capacity 230 mAh g � 1 . With the discharge cut-off voltage of 1.0 V, the discharge capacity of the experimental Zn/NiOOH cell does not decay much during 500 cycles, exhibiting good prospect for practical use. # 2001 Elsevier Science B.V. All rights reserved.

Recent experiments showed, that in contrast to traditional opinion, if the cathode was protected by anode limitation the capacity fade of Rechargeable Alkaline Manganese Dioxide Zinc (RAM™) cells was not caused by the EMD cathode, but by the gelled zinc anode. The key is the electrolyte. The cathode competition for the electrolyte and the increasing requirement of chemically formed ZnO for more electrolyte caused an electrolyte deficiency at the front face of the anode and finally caused precipitation of zincate and passivation of zinc. The crust is a mixed material of precipitation and passivation products. The low solubility ZnO is formed by decomposition of electrochemically generated zincate ions [Zn(OH)4]2− and also by recombination of zinc with oxygen during overcharge. The progressively thickened “crust” at the front face of the anode increases the resistance, then finally causes the cell to fade. The crusting is a redistribution of active material and electrolyte between the front and rear of the cylindrical gelled zinc anode. More electrolyte and proper charging can delay such a “crusting” phenomenon.

A cylindrical alkaline cell is modeled as a binary electrolyte system by assuming the direct electrochemical formation of ZnO in the anode. Justifications for replacing the dissolution–precipitation mechanism are provided. Compared to the original model, the binary electrolyte model has a more understandable model formulation, more consistent physical property data, and greater flexibility in certain instances. The binary electrolyte model predicts a longer cell life and higher operating voltage than the ternary electrolyte model for the test case discharge rate. There are no numerical difficulties associated with the zincate ion in the binary electrolyte model, because this species is not considered. The characteristics and advantages of the simplified anode behavior are discussed. An application of the binary electrolyte model is included.

Low initial cell voltage predictions from a mathematical model of a Zn/MnO2 cylindrical alkaline cell are corrected by increasing the cathode specific interfacial area. A simplified model of the cathode region is used to estimate the initial polarization loss when the cathode specific interfacial area is changed, for a particular AA-size design and galvanostatic discharge rate. The model expression for the cathode specific interfacial area is reformulated to match the literature values for this quantity more closely. A new formulation is accepted as the revision, and the discharge time improvement is substantial with respect to the base case when simulations using the complete model are performed. The physical effects of raising the cathode specific interfacial area are analyzed. The increased discharge time leads to a localized region in the cathode that is totally depleted of zincate ion. This creates numerical problems in the cell voltage calculation.

A mathematical model of the Zn/MnO2 cylindrical alkaline cell is revised by using a more appropriate expression for the equilibrium zincate ion concentration. The new expression models the equilibrium behavior of solid zinc oxide in alkaline electrolyte, whereas the previous expression models the solid zinc dihydroxide equilibrium. The new expression results in the prediction of lower and more uniform zincate ion concentration profiles. The influence of the equilibrium zincate ion concentration is analyzed through the precipitation rate of zinc oxide. The zincate ion concentration profiles for the revised case are responsible for the simulated improvement in cell performance. Adverse cell voltage fluctuations near the end of discharging are determined to be caused by a localized region where the zincate ion concentration is zero. These fluctuations are considered numerical discrepancies and are not physically realistic. Numerical remedies that successfully remove the cell voltage fluctuations are provided, but they are employed by weakening the assumption of concentrated ternary electrolyte theory.

Citrate, tartrate and gluconate are examined for their influence as additives to 3 M NaOH electrolyte for use with zinc. Solubility, turbidity and conductivity measurements are presented. Electrochemical oxidation and redeposition of zinc in the medium is investigated using cyclic voltammetry. Products of electrochemical reaction are analysed by IR and Raman spectroscopy. Tartrate and gluconate appear to produce micelles in the zincate medium whereas citrate appears to stabilize a polymeric structure in solution. Hull cell patterns of electroplating of zinc from zincate in presence of additives and morphology of the coatings by SEM pictures are also discussed.

The effects of different ionic types of perfluorosurfactants (including hydrocarbon chain surfactant CTAB) on the electrochemistry behavior of zinc have been examined using voltammetric and potentiostatic polarization techniques. The results show that these surfactants can be used as substitute of mercury for decreasing the corrosion in zinc batteries. It is also shown that the deposition of zinc from zincate electrolyte in the presence of these surfactants can be improved to some extent. The morphology of the electrodeposited zinc shows that these surfactants can bring about more uniform and compact deposits and, thereby, reduce dendritic growth. FC-170C and CTAB are found to be the most effective inhibitors.

The charging process of alkaline zinc secondary batteries is accompanied by the formation of hydrogen. Both the current and the coulombic efficiency, and in part the energy efficiency, are dictated by the production of hydrogen. Gasometric methods are used to determine the rates of hydrogen evolution and a potentiodynamic polarization method is employed to study the behaviour of zinc in alkaline solutions. It is found that an increase in ZnO concentration and a decrease in KOH concentration lower the rate of hydrogen evolution.

The cycle-life performance of the Zn/AgO cell is limited by high rates of Zn active material redistribution (shape change) and Zn dendrite growth. Reduced-zinc-solubility electrolytes, prepared by adding F−, CO32−, BO33− and PO43− salts to aqueous KOH solutions, were used in 5 A h Zn/AgO cells to determine their effect on cell lifetimes. Ca(OH)2 additions to the Zn electrode were also evaluated in cells of the same capacity and mass as the calcium-free cells. It was found that all of the anion additives to KOH electrolytes resulted in lower cell capacities and shorter lifetimes, which could be attributed the formation of soluble Ag-containing salts and subsequent degradation of the Ag electrode performance. The calcium-containing Zn electrode was found to significantly improve the performance of the Zn/AgO cell by promoting higher capacities, especially over the initial 20 cycles. The higher capacities could be attributed to the formation of a calcium zincate complex, crystals of which were found in the cycled Zn electrodes.

This paper describes experimental work relating to the development of long cycle life NiZn cells. In all cases, these cells were constructed with a large excess of electrolyte such that, at the oxidized state, the greater part of the zinc was dissolved as zincate ions. Two cell types were investigated: cells with flowing electrolyte and cells with stationary electrolyte. The second type required a combination of additives in solution and pulsed current during charge in order to obtain good cycle life.

A novel method for the separation of zincate ions in alkaline media has been developed. This development uses a precipitation reaction of Mn(OH)2 and application on a microporous separator (Celgard ...

Because of internal shorting due to zinc (Zn) dendrite formation at the Zn electrode, nickel-zinc (NiZn) secondary cell cycle life is somewhat less than the NiCd cell cycle life. To solve this problem, a new kind of separator was developed which consisted of nylon non-woven cloth coated with poly(vinyl alcohol) (PVA) containing boric acid. This separator has a high ionic conductivity, but is resistant to zincate ion penetration. A large number of additives, effective for Zn dendrite suppression, was also tested. From them, bismuth oxide (Bi 2 O 3 ) was selected and added, together with calcium hydroxide (Ca(OH) 2 ), to the Zn electrode. A charging method, applying an intermittent anodic pulse, was found to be effective to reduce the Zn electrode shape change and prolong the cycle life of this cell. By using these materials and investigating a cell construction containing an electrolytic solution and hydrogen gas absorber, sizes AA, C, and D sealed NiZn cells were developed whose charging and discharging cycle life achieved over 500 cycles, for a size C cell.

The effect of small additions (2–10 wt.%) of Bi2O3 to zinc electrodes has been studied using chronopotentiometry, cyclic voltammetry, and polarization studies. Methods were devised for preparing electrode samples for scanning electron microscopy so that the bismuth deposits could be clearly observed. In addition, studies were carried out into zinc deposition on bismuth single-crystals in zincate electrolytes, in an effort to elucidate the interaction of zinc with bismuth. Investigations were also carried out on silver electrodes in bismuth oxide-saturated zincate electrolytes. The chronopotentiometry results indicate that the Bi2O3 additive is essentially quantitatively reduced to bismuth prior to deposition of zinc. The reduced Bi2O3 forms a bismuth matrix in the electrode and consists of needle-like deposits. The nucleation overvoltage for zinc deposition on bismuth is ≈ 20 mV. The deposits have an hexagonal morphology. The beneficial effect of bismuth appears to accrue from the conductive bismuth matrix and the maintenance of an active deposit.