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Electrochemistry Communications 9 (2007) 2639–2642

Preliminary study of single flow zinc–nickel battery

Jie Cheng a,b,*, Li Zhang a,b, Yu-Sheng Yang a,b,*, Yue-Hua Wen b, Gao-Ping Cao b,Xin-Dong Wang a

a Metallurgical and Ecological Engineering School, University of Science and Technology Beijing, Beijing 100083, Chinab Research Institute of Chemical Defence, Beijing 100083, China

Received 18 July 2007; received in revised form 16 August 2007; accepted 17 August 2007Available online 23 August 2007

Abstract

A novel redox flow battery–single flow Zn/NiOOH battery is proposed. The electrolyte of this battery for both negative electrode andpositive electrode is high concentration solutions of ZnO in aqueous KOH, the negative electrode is inert metal such as nickel foil, andthe positive electrode is nickel oxide for secondary alkaline batteries. Typically, there is no requirement for a membrane in the battery.Ni(OH)2 is oxidized to NiOOH at positive electrode and the zincate ions is reduced to zinc and electroplated onto the negative electrodeduring charge. The reverse occurs during discharge. Results obtained with a small laboratory cell show that high efficiencies can beachieved with an average coulombic efficiency of 96% and energy efficiency of 86% over 1000 cycles. High performance obtained indi-cates that the single flow zinc/nickel battery is a promising battery.� 2007 Elsevier B.V. All rights reserved.

Keywords: Redox flow battery; Zinc–nickel battery; Secondary alkaline battery; Zinc anode

Redox flow batteries are electrochemical energy storagedevices that utilize the oxidation and reduction of one ortwo soluble redox couples for charging and discharging.They differ from conventional batteries in that the energy-bearing chemicals used in the redox flow batteries are notstored inside the battery container, but are stored in aseparate liquid reservoir, and the liquid was pumped to cellin which the electrolyte is separated by a ion-conductiveseparator for both charging and discharging.

One of the most important features of these batteries isthat the power and energy capacity of the system can beseparated [1]. The power of the system is determined bythe number of cells in the stack and the size of the elec-trodes, whereas the energy storage capacity is determined

1388-2481/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.elecom.2007.08.016

* Corresponding authors. Address: West Building of 35# of HuayuanbeiRoad, Beijing 100083, China. Tel.: +86 10 66705840; fax: +86 1066748574.

E-mail addresses: chengjie_chj@sohu.com (J. Cheng), yangyush@public. bta.net.cn (Y.-S. Yang).

by the concentration and volume of the electrolyte. Sothey are very suitable for storing large quantities ofenergy.

Since the redox flow cell concept was first proposed byThaller [2] in 1974, many types of redox flow batteries havebeen fabricated and developed. In particular, at least threesystems have been successfully developed: an all-vanadiumsystem (Pinnacle VRB Ltd.), a polysulphide–bromine sys-tem (Regenesys Technologies Ltd.) and a zinc–brominesystem (ZBB Energy Ltd.). However, there still exist somedifficulties to be solved such as expensive ion-exchangemembrane and a net volumetric transfer of water acrossthe membranes [3]. A novel redox flow battery system,lead-acid battery system, which uses a single electrolyte,was proposed by professor Pletcher [4–7] in 2004. This sys-tem differs from all reported flow batteries in that the elec-trolyte is a single solution of lead (II) in aqueousmethanesulfonic acid, lead dioxide and lead was depositedrespectively on positive and negative electrode at charge,then back to the single solution of lead (II) at discharge.

2640 J. Cheng et al. / Electrochemistry Communications 9 (2007) 2639–2642

Hence, no separators are required, eliminating those prob-lems associated with a membrane. And then the design ofstacks is remarkably simplified. This reduces the cost ofthe batteries significantly.

Traditional alkaline zinc–nickel accumulators have highpractical discharge voltages; their theoretical electromotiveforce is above 1.70 V and practical specific energy is about85 Wh/kg. The nominal voltage is 1.6 V per cell and thebattery holds an almost constant voltage during most ofthe discharge period and exhibits voltage stability at differ-ent discharge rates. In addition, raw materials of the zinc/nickel battery are in rich and environmental friendly.

The traditional alkaline zinc–nickel accumulator system,however, can hardly complete more than 300–500 cyclesduring deep-discharge cycles before failing, and so, hasnot penetrated commercial markets. The failure is usuallycaused by the negative electrode, which suffers from the fol-lowing two main phenomena [8,9]. Shape change of thenegative electrode associated with non-uniform current dis-tribution in zinc electrodes, and Zn dendrite resulting fromconcentration polarization.

Based on full consideration about characteristics of thezinc/nickel battery and single flow lead/acid battery, weproposed a single flow zinc/nickel battery (see Fig. 1) inthis paper. In this battery, nickel oxides are employed aspositive electrodes and inert metal current collectors areemployed as negative electrodes. Concentrated solutionsof ZnO obtained by dissolving ZnO in a KOH or NaOHaqueous medium serve as the electrolyte, which is circu-lated from an external storage tank by a pump to passthrough the cell.

When the battery is charged, Ni(OH)2 is oxidized toNiOOH at the positive electrode and the zincate ions isreduced to zinc and electroplated onto the inert negativesubstrate. When the battery is discharged, the reverse pro-cess occurs, forming highly soluble products in the alkalineelectrolyte at the zinc electrode. The electrode reactions areas follows:

Fig. 1. Schematic diagram of single flow zinc–nickel cell.

Positive electrode: 2NiOOHþ 2H2Oþ 2e� ¼ 2NiðOHÞ2 þ 2OH�

E0 ¼ 0:490 V

Negative electrode: Znþ 4OH� ¼ ZnðOHÞ2�4 þ 2e�

E0 ¼ �1:215 V

Overall cell reaction: Znþ 2KOHþ 2H2Oþ 2NiOOH

¼ 2NiðOHÞ2 þK2ZnðOHÞ4 V ¼ 1:705 V

In the process of charge/discharge cycles, the thicknessof the diffusion layer is substantially reduced, minimizingthe concentration polarization due to the flow of the elec-trolyte, thus, the zinc dendrite growth might be preventedeffectively when the cell is charged. Simultaneously, theflowing electrolyte makes the current distribution uniform,eliminating the shape change of the zinc electrode due to nostratification occurring in the electrolyte.

In this paper, a small laboratory cell was fabricated andemployed to test the performance of the single flow electro-lyte Zn/nickel cell. Concentrated solutions of ZnO in aKOH aqueous medium were used as the electrolyte to dem-onstrate the principle of this novel flow battery.

1. Experimental

Reagent grade KOH and ZnO were used and the solu-tion of 1 M ZnO in 10 M KOH medium was preparedusing deionized water.

A diagram of the small laboratory cell, used for the con-stant current charge–discharge tests of a small single flowelectrolyte battery, is shown in Fig. 2. This cell consistsof three main parts, the positive and negative electrodesand the concentrated alkaline electrolyte which is pumpedby a magnetic drive pump. Two sheets of sintered nickelhydroxide electrode (0.7 mm in thickness) as the positiveelectrode were placed on both side, and a 0.1mm thicknickel foil which is the negative electrode were placedbetween the positive electrodes. In the process of charge/discharge cycles, an Hg/HgO electrode as reference was sit-uated between the two electrodes to monitor the variationof potentials of positive and negative electrodes. The cellcompartment was filled with 20 mL of 1 M ZnO in 10 MKOH medium as the electrolyte. Before test, the sinterednickel hydroxide electrodes with an area capacity of

Fig. 2. The diagram of the small laboratory cell. (1) Perspex end plate, (2)rubber gasket, (3) flow passage: thickness 0.5 cm, (4) work electrode:2 · 2 cm cadmium-plated nickel plate, (5) counter electrode: 2 · 2 cmsintered nickel electrode and (6) location hole of the reference electrode.The flow rate of electrolyte is 19.5 cm/s.

J. Cheng et al. / Electrochemistry Communications 9 (2007) 2639–2642 2641

25 mAh/cm2 were pre-activated. The size of the positiveand negative electrodes is 2.0 cm · 2.0 cm.

The galvanostatic charge–discharge tests were performedwith a battery test system CT2000A (Jinnuo Wuhan Corp.,China) at room temperature. In the charge–discharge cycles,the cell was charged at a current density of 10 mA/cm2 untilthe capacity reached 20 mAh/cm2 (based on the specificcapacity of the positive electrode used) and discharged atthe same current density down to 0.8 V cut-off. The flowrate of electrolyte is 19.5 cm/s.

2. Results and discussion

The charge–discharge curve of the single flow zinc–nickel battery and the potential change of the positiveand negative electrodes in charge–discharge process areshown in Fig. 3. The cell voltage changed drastically atthe start of charge and at the end period of discharge. Thisindicated that the polarization of the cell was large, whichwas mainly attributed to the polarization of the negativeelectrode (see Fig. 3b). Comparatively, the polarization ofthe positive electrode is minor (see Fig. 3c). In unsaturatedzincate solutions, the redox reaction occurring at the nega-tive electrode is the deposition and dissolution of zinc [10].Overall, the reaction of the negative electrode is a twophase transformation process, and the voltage vs. timecurve obtained during the galvanostatic charge–dischargeexhibits a platform. As seen from Fig. 3b, the drasticchange in the zinc electrode potential at the start of chargeand at the end period of discharge showed that most of zincwas dissolved into the electrolyte during discharge and wasre-deposited onto the substrate during charge. This can beconfirmed by the following phenomenon. In the process ofcharge–discharge, it is observed that the nickel foil as thenegative substrate alternately exhibited light and shade incolour at the switch of charge and the end of discharge.

Fig. 3. Charge–discharge curve of the single flow zinc–nickel battery. (a)Charge–discharge curve, (b) potential of the negative electrode and (c)potential of the positive electrode. The flow rate of electrolyte is 19.5 cm/s.

To the zinc electrode, it can be seen that the charge pla-teau was between �1.34 V and �1.35 V and the fluctuationof the potential was significant by magnifying the plateauarea of the charge–discharge curve in Fig. 3b. This illumi-nated that in the process of zinc deposition the behavior ofelectrode was still controlled alternately by the nucleationand growth. In the case of the nucleation control, thepotential of the zinc electrode dropped, indicating anincrease in the polarization. The electrode potential riseback when the electrode process was controlled by thegrowth of zinc crystal, indicating a decrease in the polariza-tion. The discharge potential plateau of the negative elec-trode was around �1.36 V, which was lower than thecharge potential, possibly due to the fact that the chargereaction of the zinc electrode is the anions reduction onthe surface of the negative electrode.

The change of coulombic efficiency and energy efficiencyduring charge /discharge cycles of the single flow zinc–nickel battery was presented in Fig. 4. It can be found thatthe coulombic efficiency and the energy efficiency basicallyshowed no deterioration over 1000 charge–discharge cycles,obtaining an average coulombic efficiency of 96% and anaverage energy efficiency of 86%. The cycling performanceof the battery highly improved in contrast to the traditionalzinc/nickel battery [9]. This is directly associated with thosemeans taken, such as employing the flow electrolyte andunsaturated zincate solution etc.

Some problems were also discovered during the cycle lifeexperiments of the single flow zinc–nickel battery. Forexample, the fall-off phenomenon of active species takesplace on the positive electrode surface for a long cycle ofcharge/discharge. In the process of the zinc dissolution,some residual zinc still remains on the negative substratein the latter cycles, resulting in protuberances producedwhen the zinc is re-deposited. In addition, there is anotherproblem associated with the equilibrium between thecapacity of the positive electrode and that of the negativeelectrode.

According to the capacity calculation, it is known thatonly 15% of the zinc in the electrolyte is consumed at theend of charge. This indicates that on one hand the areacapacity of the positive electrode needs to be enhanced fur-ther; on the other hand the amount of zinc in the electrolyteis rather excessive compared with that of the positive active

Fig. 4. Coulombic efficiency and energy efficiency of the single flow zinc–nickel battery. (a) Coulombic efficiency and (b) energy efficiency during1000 cycles. The flow rate of electrolyte is 19.5 cm/s.

2642 J. Cheng et al. / Electrochemistry Communications 9 (2007) 2639–2642

species. Nevertheless, it is confirmed by tests that the singleflow zinc/nickel battery system is feasible. This single flowbattery not only has those basic virtues of traditional redoxflow batteries such as the all vanadium flow battery, butalso can be characterized by the following features:

1. The use of alkaline electrolytes permits the use of inex-pensive available materials in construction and can solvethe problem easily about the corrosion resistance of elec-trode materials;

2. There is only one circulation of the electrolyte whicheliminated the cross-contamination of solutions, simpli-fying the battery system;

3. It is not necessary to use expensive ion exchangemembranes;

4. The battery materials are inexpensive except for themetal nickel.

3. Conclusions

A novel single electrolyte flow zinc/nickel battery whichemploys the nickel hydroxides as the positive electrode, theinert metals as the negative electrode substrate and concen-trated solutions of ZnO+KOH as the electrolyte was

reported. It is demonstrated by preliminary studies thatthe zinc deposited on the negative electrode is likely to bedissolved entirely during charge/discharge, obtaining pref-erable cycling performance of the battery. Average cou-lombic and energy efficiencies of up to 96% and 86%respectively are achieved over 1000 cycles in a small labo-ratory cell. Therefore, the single flow zinc/nickel batteryis an attractive system being worthy to make great effortsto investigate it deeply further.

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