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Electrochemistry Communications 11 (2009) 2191–2194 Contents lists available at ScienceDirect Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom Preliminary study of alkaline single flowing Zn–O2 battery Junqing Pan a,*, Lizhong Ji a, Yanzhi Sun a, Pingyu Wan a, Jie Che...

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Preliminary study of alkaline single flowing Zn–O 2 battery Maohong Fan

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Electrochemistry Communications 11 (2009) 2191–2194

Contents lists available at ScienceDirect

Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom

Preliminary study of alkaline single flowing Zn–O2 battery Junqing Pan a,*, Lizhong Ji a, Yanzhi Sun a, Pingyu Wan a, Jie Cheng b, Yusheng Yang a,b, Maohong Fan c,* a

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China Research Institute of Chemical Defence, Beijing 100083, China c School of Energy Resources, University of Wyoming, Laramie, WY 82071, USA b

a r t i c l e

i n f o

Article history: Received 18 August 2009 Received in revised form 23 September 2009 Accepted 23 September 2009 Available online 26 September 2009 Keywords: Single flow battery Electrodeposited zinc electrode Composite oxygen electrode Alkaline battery

a b s t r a c t A new single flow alkaline battery, Zn–K2[Zn(OH)]4–O2 battery, in which electrodeposited zinc is employed as an negative electrode and the oxygen in atmosphere as an high-capacity positive electrode active material is developed. The working process of the battery only depends on the circulation of a single electrolyte solution with assistance of a single pump and no cationic membrane is needed. The newly designed dual catalytical layers of composite oxygen electrode employs nano-structured Ni(OH)2 and the electrolytic manganese dioxide doped with NaBiO3 as two types of novel highly-efficient catalysts for oxygen evolution and reduction process, respectively. Cell (grade 1000 mAh) test results show that high efficiency is achieved with an average coulombic efficiency of 97.4% and an energy efficiency of 72.2% in 150 cycles. Ó 2009 Published by Elsevier B.V.

1. Introduction With the large-scale development of renewable energy resources, such as solar, wind and tide, there is an increasing need to develop an effective energy storage technology in order to achieve power supply stability of these renewable energies and adjust electric power consumption during peak and off-peak periods in large cities. Due to its small space coverage and high energy storage density, the redox flow battery is regarded as one of the most efficient energy storage forms appropriate for large-scale development in the future [1]. Since the first application of the Fe–Cr flow battery by Thaller in 1974 [2], researchers have developed many types of redox flow battery systems by changing the oxidation/reduction couples, such as V5+/V2+, Zn–Br2 and Cr 2þ =CrO2 4 [3–5]. However, these flow battery systems need ion-exchange membranes to separate the anolyte and catholyte. The use of expensive membranes and two electrolyte cycling systems not only decreases the power efficiency but also increases the manufacture cost of the batteries. In order to overcome the two shortcomings mentioned above, people have developed deposition type single flow battery systems. Based on the traditional lead-acid battery, a novel single flow battery system was proposed by Pletcher et al. [6–8]. However, its energy efficiency was only in the range of 55–65% [9]. Alkaline Zn–NiOOH and acidic Cu–PbO2 single flow batteries have been tested to improve energy efficiency reported in * Corresponding authors. Tel./fax: +86 10 64449332. E-mail addresses: [email protected] (J. Pan), [email protected] (M. Fan). 1388-2481/$ - see front matter Ó 2009 Published by Elsevier B.V. doi:10.1016/j.elecom.2009.09.028

succession [10,11]. Recently, it has been discovered that these three kinds of single flow battery systems are not promising due to the limitation of their power storage capacities associated with positive electrode material (PbO2 or NiOOH) [12]. Therefore, exploring new single flow battery system is needed. This research is focused on developing a novel Zn–O2 single flow battery in which deposited zinc electrode is used as negative electrode, new type of oxygen composite electrode with dual-functional layer as a high-capacity positive electrode, flowing KOH– K2[Zn(OH)4] solution stored in a tank and circulated by a pump as an electrolyte. The battery is designed to integrate the advantages of Zn–NiOOH single flow battery with those of traditional Zn– air battery. The structure of the Zn–O2 single flow battery is shown in Fig. 1. 2. Experimental First, 800 mg nano-structured b-Ni(OH)2 [13],200 mg pure expanded graphite and 50 mg PTFE emulsion (wt.60%) were mixed adequately using magnetic stirring for 30 min. The paste was then roll-pressed into an 80 lm thick electrode catalyst film (marked as A in Fig. 1) for oxygen evolution. Then a desired amount of electrolytic manganese dioxide (EMD) powder (Xiangtan Electrochemical Technology Co., Ltd.) containing 5% (wt.) NaBiO3 [14] as an oxygen reducing agent, expanded graphite and activated carbon as electric conductors, and 60% (wt.) PTFE emulsion as an adhesive were thoroughly mixed in ratios of 50:20:15:15, respectively. The resulting product was mixed with acetone to obtain an adhesive product, followed by ultrasonic dispersing for 30 min and drying at 80 °C

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1.5

(A) Potential (V)

1.4

d c

1.3

b 1.2

a a, 25 b, 45 c, 60 d, 75

1.1

1.0 0

100

200

300

400

500

Time (h) 2.0

(B)

3. Results and discussion Platinum and perovskite based oxides are widely studied catalysts for the reduction process of oxygen electrodes [15]. Recently, Takeo Ohsaka has found that the inexpensive MnO2 has the similar catalytic mechanism as Platinum does [16]. However, MnO2 can easily change into Mn3O4 during catalytic process, limiting the application of MnO2. The present paper uses catalytic MnO2 doped with nano-NaBiO3 for oxygen reduction to prevent the formation of electrochemically inert Mn3O4 [14,17]. Fig. 2A shows that under the catalysis of manganese dioxide doped with sodium bismuthate

b c d e f

1.9

Potential (V)

Fig. 1. Schematic diagram of single flow Zn–O2 battery.

in vacuum, and roll-pressed into a 100 lm thick oxygen reducing membrane (marked as B in Fig. 1). Then 0.2 g EMD, 0.2 g expanded graphite, 0.2 g activated carbon and 0.45 g 60% (wt.) PTFE emulsion were completely mixed and 0.40 g NH4NO3 as pore-forming agent was added. The resulting product was mixed with acetone adequately and dried at 80 °C under vacuum before being roll-pressed into 100 lm thick membrane. The NH4NO3 in the membrane was decomposed at 220 °C so as to obtain the oxygen-permeable membrane (marked as C in Fig. 1). The obtained membranes A, B, C together with nickel foam (D) were pressed at 12 MPa in accordance with the order of A–D–B–C into dual-functional membrane of oxygen composite electrode. Highly pure copper (wt.99.95%) is used as negative electrode. The size of positive electrode and negative electrode was 4.2  5.2  0.20 cm3. The electrochemical tests of positive electrode and negative electrode were carried out in experimental cells using three electrodes. The oxygen composite electrode or pure copper electrode was used as working electrode, a pure nickel plate was used as a counter electrode, a Zn(Hg)/ZnO one as a reference electrode with 7 M KOH-0.7M LiOH-0.7M ZnO being the electrolyte. The experimental battery was made up of highly pure copper used as a negative electrode and oxygen composite electrode of dual-functional layer as a positive electrode, 500 ml of 7M KOH0.7M LiOH-0.7M ZnO as an electrolyte solution. The theoretical capacity of this battery can be calculated by Faraday’s law: C = 26.8  2  n(Zn2+) = 26.8  2  0.5  0.7 = 18.76 Ah = 18760 mAh. The electrolyte was circulated between the cell and the tank by a pump at the rate of 450 ml h1. The galvanostatic charge–discharge test was carried out using a LAND CT 2001A battery test system (Jinnuo Corp., China) at 400 mA (20 mA cm2) for 2.5 h and the charge capacity is 1000 mAh (400 mA  2.5 h).

a

1.8

1.7

a, 20 b, 30 c, 40 d, 50 e, 60 f, 70

1.6

1.5 0

2

4

6

8

10

12

14

16

18

20

Time (h) Fig. 2. The discharge (A)/charge (B) curves of composite oxygen electrode single flow Zn–O2 battery.

at 20 mA cm2 of current density, the discharge voltage of oxygen composite electrode reaches 1.212, 1.273, 1.322, and 1.325 V at temperatures of 25, 45, 60, and 75 °C, respectively. The doped sodium bismuthate could not only improve the electrical activity of manganese dioxide and prevent the formation of inert Mn3O4 [14] but also considerably decrease the decay of oxygen electrode as observed in a more than 500 h of discharge process. The diffusion of oxygen is the rate-determining step at high polarization current. The result shows that higher temperature contributes to the increase of the oxygen diffusion speed and the effective oxygen concentration in the electrode, which leads to the decrease of polarization and increase of discharge potential. Fig. 2B is the anodic charge curve of the oxygen electrode. Results indicate that when temperature increases from room temperature to 70 °C, the oxygen evolution potential decreases gradually from 1.945 to 1.784 V. Generally, the process of oxygen evolution takes place during Ni(OH)2 charge process. When temperature is higher than 50 °C, oxygen evolution becomes the major reaction within Ni(OH)2 electrode [18]. The present study uses nano-structured Ni(OH)2 to accelerate and catalyze the reaction of oxygen evolution at a lower potential range so as to increase the energy efficiency of the battery. In order to find the optimal operation temperature of zinc electrode, Fig. 3 shows the charge–discharge curves of zinc electrode at different temperatures by using three electrodes. The zinc electrode charged for 60 min at 400 mA, and the charge capacity of this cell is 400 mAh (400 mA  1 h). Due to the good reaction reversibility between Zn and ZnðOHÞ2 4 and a stable flow of electrolyte, the

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Capacity Efficiency (%) 0

20

40

80

100

(A)

a, 30 b, 40 c, 50 d, 60 e, 70 f, 80

-0.10 -0.15

Voltage (V)

d

a b c

-0.05

e f

30

40

50

100

b a

1.4 1.2

a b

1.0

st

a,1 th b,5

0.8

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a b

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60

1.6

-0.20 0

40

1.8

f

0.00

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2.0

a b c d e

0.05

Potential (V)

60

Capacity Efficiency (%) 0

0.5

1.0

60

1.5

2.0

2.5

Time (h)

Time (min)

100

Fig. 3. The charge/discharge curves electrodeposition zinc electrode at different temperatures.

95 90

(B)

polarization potential of the zinc electrode only stays in the range of 6–29 mV during charge–discharge process. It can be seen that temperature increase not only improves the polarization but also decreases charge–discharge efficiencies. For example, when temperature was increased from 60 to 80 °C, the current efficiency decreased from 95.2% to 89.2%. This is due to the partial self-discharge of zinc electrode at high temperature in KOH solution. The reaction occurred as follows:

Efficiency (%)

85 80 75 70 65 60 55 50

Capacity Efficiency Energy Efficiency

45

Zn þ 2KOH þ 2H2 O ¼ K2 ZnðOHÞ4 þ H2

ð1Þ

The energy efficiency of the battery is directly proportional to the product of capacity efficiency and potential efficiency. Thus, based on the performance of the two electrodes, 60 °C is chosen as the working temperature. Fig. 4A exhibits the first five charge–discharge cycles curves of the single flow Zn–O2 battery during 150 cycles at 60 °C. The electrolyte was pumped to the battery during charge–discharge process. The flow of the electrolyte increases the transfer speed of the substance near electrode surface, eliminating the concentration polarization and inhibiting the dendrite growth during charging. When discharge is completed, the zinc on the negative electrode is fully dissolved and the negative electrode is restored to the original state [1]. Therefore, the negative electrode is rechargeable. Using oxygen in atmosphere to provide the positive electrode with immense capacity, the electrodeposited zinc negative electrode can be consumed completely during the discharged process. However, it is can be seen from Fig. 4A that the discharge capacity has reached 96.2–97.6% due to reaction (1) involving zinc self-discharge in alkaline electrolyte at 60 °C. The reduction and evolution of oxygen on positive electrode were carried out through two different functional catalytic layers. Theoretically, the service life of oxygen cathode reduction process is very long and mainly determined by the catalyst life span typically longer than 1000–3500 h [19]. The use of nano-structured Ni(OH)2 as catalyst in oxygen positive electrode decreases the oxygen evolution overvoltage, maintaining the actual charge voltage within the range of 1.75–1.86 V at 60 °C. During discharge process, the reactions are as follows:

anode : cathode :

Zn  2e þ 4OH ¼ ZnðOHÞ2 4 1=2O2 þ 2e þ H2 O ¼ 2OH

E ¼ 1:216V

ð2Þ

E ¼ 0:401V

ð3Þ

40 0

20

40

60

80

100

120

140

Cycle Number Fig. 4. The discharge /charge (A) and coulombic efficiency/energy efficiency (B) curves of the single flow Zn–O2 battery in the first 150 cycles.

Based on Eq. (2) and Eq. (3), the electromotive force of the battery is 1.617 V, and the actual discharge voltage is 1.2–1.4 V. The charging process is presented as follows:

anode :

2NiðOHÞ2  2e þ 2OH ¼ 2NiOOH þ H2 O;

2NiOOH þ H2 O ¼ 2NiðOHÞ2 þ 1=2O2 ð> 50  CÞ overall reaction of anode :

2OH  2e ¼ 1=2O2 þ H2 O

cathode : ZnðOHÞ2 4 þ 2e ¼ Zn þ H2 O

ð4Þ ð5Þ

Results in Fig. 4B indicate that the efficiencies of the Zn–O2 single flow battery do not decrease significantly at 20 mA cm2 current density during 150 cycles. The test results show that the capacity efficiency reaches 94.1–98.5% (941.1–985.2 mAh of discharge capacity). Based on the analyses of LANDdt software, the energy efficiency could be as high as 69.2–72.4% during charge– discharge cycles. When we extended the charge time to 5 and 8 h, the average capacity efficiency were 96.2% and 93.7%, respectively. It was also found in the experiment that a small quantity of zinc powder drops in the electrolytic cell. The fall of this electrode active substance accounts for the loss of battery’s capacity efficiency. The further study would focus on the improvement in the deposition efficiency of zinc electrode and the utilization rate of electrolyte.

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4. Conclusion A new type of inexpensive high-capacity single flow battery for energy storage has been developed. Through newly designed catalytic dual-layer of composite oxygen electrode which employs nano-structured Ni(OH)2 and the EMD doped with NaBiO3 as two types of novel effective catalysts for oxygen evolution and reduction process, respectively, the problem of oxygen electrode polarization during charge–discharge could potentially be solved and the overall Zn–O2 single flow battery energy efficiency could be enhanced. The dendritic crystallization and deformation problems could be overcome through electrolyte circulation. The built battery provides a discharge voltage of 1.32 V, with an average coulombic efficiency of 97.4% and an energy efficiency of 72.2%. Acknowledgement This work is supported by Beijing Nova Program (No. 2008B17), National Natural Science Foundation of China (No. 50804050), and the Fund of CRE for New Faculty (No. CRE-B-2008109). References [1] L. Zhang, J. Cheng, Y.S. Yang, Y.H. Wen, X.D. Wang, G.P. Cao, J. Power Sources 179 (2008) 381.

[2] L.H. Thaller, USP 3996064, 1974. [3] T. Yamamura, Y. Shiokawa, H. Yamana, H. Moriyama, Electrochim. Acta 48 (2002) 43. [4] G. Oriji, Y. Katayama, T. Miura, Electrochim. Acta 49 (2004) 3091. [5] J.Q. Pan, Y.Z. Sun, P.Y. Wan, Z.H. Wang, X.G. Liu, Chem. Commun. (2005) 3340. [6] A. Hazza, D. Pletcher, R. Wills, Phys. Chem. Chem. Phys. 6 (2004) 1773. [7] D. Pletcher, R. Wills, J. Power Sources 149 (2005) 96. [8] X.H. Li, D. Pletcher, Frank C. Walsh, Electrochim. Acta 54 (2009) 4688. [9] H.Y. Peng, H.Y. Chen, W.S. Li, S.J. Hu, H. Li, J.M. Nan, Z.H. Xu, J. Power Sources 168 (2007) 105. [10] J. Cheng, L. Zhang, Y.S. Yang, Y.H. Wen, G.P. Cao, X.D. Wang, Electrochem. Commun. 9 (2007) 2639. [11] J.Q. Pan, Y.Z. Sun, J. Cheng, Y.H. Wen, Y.S. Yang, P.Y. Wan, Electrochem. Commun. 10 (2008) 1226. [12] J.Q. Pan, L.Z. Ji., Y.Z. Sun, J. Cheng, Y.S. Yang, P.Y. Wan, CNP 20090076983.6, 2009. [13] J.Q. Pan, Y.Z. Sun, Z.H. Wang, P.Y. Wan, Y.S. Yang, M. Fan, J. Power Sources 188 (2009) 308. [14] J.Q. Pan, Y.Z. Sun, P.Y. Wan, Z.H. Wang, X.G. Liu, Electrochim. Acta 51 (2006) 3118. [15] N.L. Wu, W.R. Liu, S.J. Su, Electrochim. Acta 48 (2003) 1567. [16] T. Ohsaka, L. Mao, K. Arihara, T. Sotomura, Electrochem. Commun. 6 (2004) 273. [17] J.Q. Pan, Y.Z. Sun, Z.H. Wang, P.Y. Wan, Y.S. Yang, X.G. Liu, M. Fan, J. Alloys Compd. 470 (2009) 75. [18] J. Fan, Y.F. Yang, P. Yu, W.H. Chen, H.X. Shao, J. Power Sources 171 (2007) 981. [19] ....