nickel batteries

Electrochimica Acta 59 (2012) 64–68

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Lead ion and tetrabutylammonium bromide as inhibitors of the growth of spongy zinc in single flow zinc/nickel batteries Yuehua Wen a,∗ , Tian Wang b , Jie Cheng a , Junqing Pan b , Gaoping Cao a , Yusheng Yang a,b a b

Research Institute of Chemical Defence, Beijing 100191, China College of Science, Beijing University of Chemical Technology, Beijing 100029, China

a r t i c l e

i n f o

Article history: Received 19 September 2011 Received in revised form 10 October 2011 Accepted 12 October 2011 Available online 9 November 2011 Keywords: Electrolyte additive Spongy inhibition Lead ion Tetrabutylammonium bromide Single flow Zn–Ni battery

a b s t r a c t Lead ion and tetrabutylammonium bromide (TBAB) have been testified as inhibitors of spongy zinc electro-growth from flowing alkaline zincate solutions. To assess the efficacy of the two additives, current–time technique using potentiostatic electro-deposition, scanning electron microscopy and cycling test were undertaken. The results show that the growth of spongy zinc in flowing alkaline zincate solutions can be effectively inhibited by the addition of 10−4 M Pb(II) or TBAB at the cathodic potential ( = −100 mV), respectively. But, the dual addition of 10−4 M Pb(II) and 5 × 10−5 M TBAB produces more effective suppression on spongy zinc growth. Obvious synergistic effect of Pb(II) and TBAB on the inhibition of spongy zinc deposits is found. From the charge/discharge cycling tests of the single flow Zn–Ni test cells, it is shown that the rechargeability of Zn anode is highly improved by the mixed introduction of 10−4 M Pb(II) and 5 × 10−5 M TBAB. © 2011 Elsevier Ltd. All rights reserved.

1. Introduction Secondary zinc-based batteries are usually limited in their performance and widespread commercialization by poor cycling characteristics of zinc anode [1]. The short cycle life of zinc anode mainly arises from dendritic growth and shape change with the zinc electrode during charging. So far, many attempts including the use of additives in either the electrode or the electrolyte have been undertaken [2–4]. Although the cycle life and properties of zinc electrodes are improved to some extent, the problem still remains elusive [5–7]. In 2007, Cheng et al. proposed a novel redox flow battery system, namely, single flow zinc–nickel battery [8]. Compared with the conventional solid zinc electrodes, the reaction of zinc electrodes turns into the deposition and dissolution process of zinc. Thereby, this kind of zinc electrode can be called ‘deposited-zinc electrode’. During the charge/discharge cycle of this system, the concentration polarization is substantially reduced due to the use of flowing electrolytes. The zinc dendrite growth might be prevented effectively. The cycling performance of zinc electrodes is improved considerably. Nevertheless, spongy zinc deposits are easy to form from flowing alkaline zincate solutions during the charge. Thus, the performance of batteries is influenced to a significant degree. The use of additives in stationary electrolytes to inhibit the dendrite growth has been widely studied. However, few studies on the

∗ Corresponding author. E-mail address: wen [email protected] (Y. Wen). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.10.042

use of additives are combined with the charge–discharge performance of zinc-based secondary batteries. Additives such as Bi2 O3 [9,10], Pb3 O4 [11] and quaternary ammonium compounds [12] have the beneficial effect on the performance of zinc electrodes. Recently, it was found by us that some inorganic additives such as lead, Na2 WO4 , etc. were also effective to suppress the formation of spongy zinc deposits from flowing electrolyte [13]. This kind of additives with favorable chemical stability could be alternatives of electrolyte additives in alkaline single flow zinc–nickel batteries. In this present work, the effects of lead ions and tetrabutylammonium bromide (TBAB) as electrolyte additives on the morphology, spongy growth and electrochemical behavior of deposited zinc electrodes in flowing electrolytes were studied, and their synergistic effect was also discussed. The logical aim of the study is to optimize the concentration of the two additives so that the deposited zinc electrode has good electrochemical behavior, minimum spongy growth with a view to fabricate an alkaline single flow zinc–nickel system. Subsequently, the battery performance of this system with the two electrolyte additives is evaluated. 2. Experimental All solutions used were freshly prepared using analytical grade chemicals and deionized water. A 8 M KOH solution containing 0.7 M ZnO and 20 g L−1 LiOH was used as the blank solution. The additives such as lead ions or tetrabutylammonium bromide (TBAB) with different concentrations were added into this solution. The current–time measurements were undertaken by a three-electrode

Y. Wen et al. / Electrochimica Acta 59 (2012) 64–68

configuration using a polished Ni sheet (with an area 1 cm2 ) as the working electrode, while Hg/HgO (6 M KOH) as the reference electrode, and a large Ni gauze as the counter electrode, respectively. The working electrode was insulated using epoxy resin except the surface to be tested. Before each experiment, the electrode was polished with emery paper (1000 grade), degreased with acetone and rinsed with deionized water. Current–time curves were acquired using an electrochemical working station (Solartron 1280z) at a constant cathodic overpotential ( = −0.1 V). The surface morphology of deposits was assessed by a scanning electron microscope (Cambridge, S-360). The measurements were performed under the magnetic stirring of electrolytes and at room temperature of 25 ◦ C. The charge/discharge cycling tests were conducted using a battery test system CT2000A (Jinnuo Wuhan Corp., China) on test cells of NiOOH|8MKOH + 0.7 M ZnO + 20 g L−1 LiOH with/without additives|Zn to explore the performance of the proposed electrolytes. This cell consisted of three main parts, the positive and negative electrodes and the concentrated alkaline electrolyte which was pumped by a magnetic drive pump. Two sheets of folded sintered nickel electrodes and one sintered nickel electrode (7 cm × 7 cm, 0.31 mm thick) were placed on both sides of a nickelplated perforated steel strip (0.1 mm thick) which was the negative electrode substrate. It guaranteed that the capacity of the positive electrode exceeded that of the negative electrode, so as not to limit the cycling of the battery. The cell compartment was filled with the studied electrolytes. Before test, the sintered nickel hydroxide electrodes with an area capacity of 25 mAh cm−2 were pre-activated. The size of the positive and negative electrodes is 7.0 cm × 7.0 cm. In the charge–discharge cycles, the cell was charged at a current density of 20 mA cm−2 until the capacity reached 20 mAh cm−2 and discharged at the same current density down to 1.2 V cut-off. The flow rate of electrolyte is 19.5 cm s−1 . Temperature throughout this study was about 25 ◦ C.

65

3. Results and discussion

the rise in the cathodic deposition current of zinc in the flowing blank solution is large (from about 75 to 226 mA). It indicates that even in flowing solutions, the deposition of zinc is still controlled by mass transport polarization to a large degree which may result in the formation of loose and more “open-structured” deposits. Visual observation did find heavy spongy zinc deposits formed on the electrode surface. The addition of TBAB in the electrolyte produces flatter current–time responses. With the concentration of TBAB increasing, the rise of the deposition current during test decreases. At the onset of zinc deposition, a sharp reduction in current is observed, which may be caused by the formation of crystal cores. The higher the concentration of TBAB, the more obvious is this phenomenon. Nearly flat current–time profiles are observed with the TBAB additive of 10−4 M and higher. When the content of TBAB in the electrolyte is 10−4 M, the current–time profile is not only almost flat but also the cathodic deposition current is closed to the initial current in the blank solution. This indicates that the formation of spongy zinc can be effectively inhibited and the charge efficiency of the deposited zinc electrode will not be decreased significantly by the addition of 10−4 M TBAB. Fig. 2 depicts the current–time profiles with different concentrations of the lead additive. Similar to the case of TBAB addition, the efficacy of adding lead ions on the spongy zinc inhibition is raised with increasing lead concentration. As low a concentration as of 5 × 10−5 M, the flattening of cathodic current is obvious. The current changes from 66 to 134 mA (compared with that of the blank solution 75–226 mA) during the 60 min test. When the lead concentration is increased to 5 × 10−4 M, the cathodic current is inhibited to a low value, i.e., ∼40 mA. It exhibits a much lower current value than the initial current in the blank solution before the onset of spongy zinc growth. Thus, the optimal concentration of lead ions should be 10−4 M even though the current–time curve is not completely flat. In addition, compared with the organic additive TBAB, a significant reduction in current at the onset of zinc deposition is not observed in the electrolyte with the lead additive. It may be attributed to the different effect mechanisms of additives.

3.1. Effects of Pb(II) and TBAB on the spongy growth of zinc

3.2. Synergistic effect of Pb(II) and TBAB

The current–time curves of zinc in the flowing alkaline zincate solutions with and without TBAB at a cathodic overpotential  = −100 mV are shown in Fig. 1. Since any rise in cathodic current during a potentiostatic electrodeposition is assumed to be due to an increase in true electrode surface area, the variation in current versus time at constant potential reflect sensitively the change in surface morphology (roughness). It can be seen from Fig. 1 that

Mixed addition of lead ions and TBAB was undertaken for further investigations under the higher speed of magnetic stirring. As shown in Fig. 3, the individual addition of 10−4 M lead ions or TBAB produces flatter current–time responses, but the rise of the cathodic current is still obvious, especially for the addition of TBAB. The almost flattened current–time curve depicts that synergistic effect occurs by the dual addition of lead and TBAB with

0.00

0.00

2

-0.04

-0.08

3

-0.08

-0.12

1 0: blank -5 1: 5× 10 M TBAB -4 2: 10 M TBAB -3 3: 10 M TBAB

-0.16 -0.20 -0.24 0

1000

2000

Currnet/A

Current/A

-0.04

3

2

-0.12

1

-0.16

0: blank -5 1: 5 ×10 M Pb(II) -4 2: 10 M Pb(II) -4 3: 5 ×10 M Pb(II)

-0.20

0 3000

Time/sec Fig. 1. Current–time profile at a constant cathodic overpotential (−0.1 V) for electroplating Zn in a blank electrolyte (0.7 M ZnO + 8 M KOH + 20 g L−1 LiOH) with tetrabutylammonium bromide (TBAB) as an additive under magnetic stirring.

-0.24 0

1000

2000

0 3000

4000

Time/sec Fig. 2. Current–time profile at a constant cathodic overpotential (−0.1 V) for electroplating Zn in a blank electrolyte (0.7 M ZnO + 8 M KOH + 20 g L−1 LiOH) with lead ion as an additive under magnetic stirring.

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Y. Wen et al. / Electrochimica Acta 59 (2012) 64–68

Table 1 Cathodic current characteristics for deposits from electrolytes studied under an equal-charge (338 C) regime at the cathodic overpotential  = −100 mV. Solution composition

Time (min)

Initial current (mA)

Final current (mA)

R = (FA − IA )/(FB − IB )

Blank solution 10−4 M Pb(II) 10−4 M TBAB 10−4 M Pb(II) + 5 × 10−4 M TBAB 10−4 M Pb(II) + 10−4 M TBAB

35 54 51 60 59

90.2 75.5 70.8 80.8 73.1

234 136.7 158.5 114.2 127.9

1.0 0.42 0.61 0.23 0.38

R, deposit ratio; FA , final current with additive; IA , initial current with additive; FB , final current in blank solution; and IB , initial current in blank solution.

0.00 -0.05

Current / A

-0.10

4 3 2 1

-0.15 -0.20

0: blank -4 1: 10 M TBAB -4 2: 10 M Pb(II) -4 -4 3: 10 M Pb(II)+10 M TBAB -4 -5 4: 10 M Pb(II)+5×10 M TBAB

-0.25 -0.30

0

-0.35 0

1000

2000

3000

4000

Time / Sec Fig. 3. Current–time profile at a constant cathodic overpotential (−0.1 V) for electroplating Zn in a blank electrolyte (0.7 M ZnO + 8 M KOH + 20 g L−1 LiOH) with lead ions and TBAB as additives under magnetic stirring.

the optimal concentration of 10−4 M. It suggests that the ability of spongy zinc inhibition becomes stronger by mixed addition of lead ions and TBAB. However, the inhibition effect is so large that the cathodic deposition current is reduced dramatically compared to the initial current in the blank solution. The charging efficiency of the deposited zinc electrode will be low. When the concentration of TBAB is decreased to 5 × 10−5 M, a more flattened current–time curve can be observed. Moreover, the cathodic deposition current is similar to the initial current in the blank solution. This shows that the growth of spongy zinc is greatly strained by joint addition of Pb(II) and TBAB in the electrolyte. Also, the charging efficiency of the deposited zinc electrode would be hardly influenced. The current transients and deposit ratio taken from the current–time trials for a definite charge (338 C) are listed in Table 1. The electro-deposition time and final current values are obtained for a defined charge of 338 C (the charge delivered from a blank solution containing 10−4 M Pb(II) and 5 × 10−5 M TBAB over the 60 min electrodeposition). The data from these tests indicate that both additives exhibit superior performance to the blank solution. It can be found that the deposit ratio for the dual addition of Pb(II)

and TBAB(R = 0.23–0.38) is much lower than that for the individual addition of 10−4 M Pb(II) and TBAB (R = 0.42–0.61). There exists a trade-off between the charging time and the inhibition of spongy zinc growth. As can be seen, for the dual addition of 10−4 M Pb(II) and 5 × 10−5 M TBAB, the deposit ratio is not only the lowest but also the initial current is closed to that for the blank solution. Thus, the dual addition of 10−4 M Pb(II) and 5 × 10−5 M TBAB would produce the excellent performance for the deposited zinc anode. The electrodeposited zinc surfaces at the cathodic overpotential  = −100 mV under magnetic stirring of electrolytes were examined by SEM. As shown in Fig. 4, an additive-free solution produces granular and spongy-like zinc deposits (Fig. 4a). The addition of additives in the solution causes the deposition morphologies of zinc to be completely different. The addition of 10−4 M Pb(II) brings about a less open, relatively compact deposit with some pits and hollows(see Fig. 4b). The addition of 10−4 M Pb(II) and 5 × 10−5 M TBAB produces a dense, smooth and compact deposit indicative of almost complete inhibition of spongy zinc growth (see Fig. 4c). 3.3. The influences of additives on the charge–diacharge performance of zinc electrode Fig. 5 presents the charge–discharge efficiency versus cycle number of the test cells using either blank solution or that with the addition of 10−4 M Pb(II)and with the mixed addition of 10−4 M Pb(II) and 5 × 10−5 M TBAB tested at 20 mA cm−2 . From the profiles, cells using blank solution exhibit low coulombic and energy efficiencies and a sharp drop when the cycle number of the cell is beyond 34. With the addition of 10−4 M Pb(II), the cycling stability of the cell is enhanced to a great extent. Substantially improved stability is demonstrated by cells with the mixed addition of 10−4 M Pb(II) and 5 × 10−5 M TBAB. Further, with the addition of 10−4 M Pb(II), there are two small drops on the cell efficiency and then it recovered gradually to steady performance. With the mixed addition of Pb(II) and TBAB, on the first cycle, the cell has a relatively low charge–discharge efficiency. Subsequently, the charge–discharge efficiency of the cell progressively increases and then it shows very steady performance starting from the fifth cycle.

Fig. 4. Zinc electrodeposit at  = −100 mV from the blank solution (a) and the alkaline solution containing 10−4 M Pb(II) (b) and the alkaline solution containing 10−4 M Pb(II) and 5 × 10−5 M TBAB (c).

Efficiency / %

Y. Wen et al. / Electrochimica Acta 59 (2012) 64–68

100 90 80 70 60 50 40 30 20 10 0

Current efficiency Energy efficiency

(a) 0

67

10

20

30

40

50

100 90 80 70 60 50 40 30 20 10 0

Current efficiency Energy efficiency

(b) 0

Efficiency / %

Efficiency / %

Cycle number

10

20 30 Cycle number

40

50

100 90 80 70 60 50 40 30 20 10 0

Current efficiency Energy efficiency

(c) 0

10

20

30

40

50

Cycle number

Fig. 5. The charge–discharge efficiency vs. cycle number of test cells using (a) blank solution, (b) blank + 10−4 M Pb(II) and (c) blank + 10−4 M Pb(II) + 5 × 10−5 M TBAB; tested at 20 mA cm−2 .

1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0

b

1

23

1- 10-4 MPb (II) 2- blank solution 3- 10-4 MPb (II) + 5x 10-5 MTBAB

0

10

20

30

40

50

60

Voltage / V

Voltage / V

a

1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0

1

3

2

1- blank

solution -4 2 -10 MPb (II) -5 -4 3 - 10 MPb (II) + 5×10 MTBA B

0

10

Time / min

20

30

40

50

60

Time / min

Fig. 6. The 25th discharge curves (a) and the 40th discharge curves (b) of the tested cells.

Fig. 6 shows the comparison of the 25th and 40th discharge curves of test cells. It can be seen that on the 25th cycle, few differences on the discharge curves are observed except that with the mixed addition of Pb(II) and TBAB the discharge voltage is a little bit higher than the other two curves. However, on the 40th cycle, cells using additives exhibit higher discharge voltage than that of the blank electrolyte, especially for the dual addition of Pb(II) and TBAB. The synergistic effect of Pb(II) and TBAB is further demonstrated.

 = −100 mV. The growth of spongy zinc is more greatly suppressed by the synergistic effect of lead and TBAB. A smooth and compact zinc deposit is obtained from the flowing electrolyte containing Pb(II) and TBAB. Highly improved rechargeability is achieved during charge–discharge cycling of cells by the mixed addition of 10−4 M Pb(II) and 5 × 10−5 M TBAB into the blank solution. The synergistic effect of Pb(II) and TBAB is further demonstrated.

4. Conclusions

This work was financed by the National Basic Research Program (973 Program) of China (2010CB227201).

The effectiveness of lead ions and TBAB as inhibitors of spongy zinc growth from flowing alkaline zincate solutions has been examined. Further, the stability of the additives to prolonged cycling of single flow zinc–nickel cells is also evaluated. Minor addition of lead ions or TBAB in the flowing alkaline zincate solution can inhibit the growth of spongy zinc at the cathodic overpotential

Acknowledgement

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