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Advanced rechargeable zinc-air battery with parameter optimization
Applied Energy 225 (2018) 848–856
Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Advanced rechargeable zinc-air battery with parameter optimization a,⁎
b
a
a
a
Keliang Wang , Pucheng Pei , Yichun Wang , Cheng Liao , Wei Wang , Shangwei Huang a b
T b
School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China State Key Lab. of Automotive Safety and Energy, Tsinghua University, Beijing 100084, China
H I GH L IG H T S
zinc-air battery with a compacted structure was optimally designed. • Rechargeable and structural performance of air electrode was characterized. • Electrochemical performance of the battery was improved by electrolyte management. • Cycling • Oxygen bubbles movement was controlled by electromagnetic coupling.
A R T I C LE I N FO
A B S T R A C T
Keywords: Cycle life Dendrite growth Structure optimization Electrolyte management Magnetic field Rechargeable zinc-air battery
Zinc-air batteries will be a promising candidate for storage energy and power supply due to their high specific energy, environmental compatibility, and economic availability. However, the problem of cycle life of rechargeable zinc-air battery remains unresolved mainly because of dendrite growth of electrodeposited zinc and performance degradation of air electrode. Here we show that rechargeable zinc-air battery with an optimized structure can stably run at large current densities, where air electrode is connected to the charging electrode through a stainless steel frame, and the effective area of charging electrode is larger than that of zinc electrode by way of a trapezoidal structure. This battery structure can control morphological change of zinc electrode and monitor dendrite growth without increasing the battery volume. The results demonstrate that the charge-discharge efficiency of rechargeable zinc-air battery can be improved by nickel foam as gas diffusion layer of air electrode, calcium oxide additive to the electrolyte, or a permanent magnet in parallel with the electrode. The lifetime of rechargeable zinc-air battery can be extended by electrolyte flow or battery structure optimization. These findings will be available for other metal-air batteries and electrolytic metal industry.
1. Introduction Zinc-air batteries have attracted more attention as one of energy storage devices in application of electric grid [1], green energy [2] and power supply [3] due to high-energy-density and non-pollution advantage. Nevertheless, the cycling performance of rechargeable zinc-air battery is unsatisfactory [4], where morphological change of electrodeposited zinc would shorten cycle life of the battery [5], and sluggish kinetics of oxygen redox reaction could decrease energy efficiency of the battery [6]. Energy efficiency of rechargeable zinc-air battery is mostly related to catalytic activation. Study of oxygen redox catalysts is mainly focused on precious metals and their alloys [7], carbon nanostructure materials [8], transition metal oxides [9], and inorganic/organic compound materials [10]. Moreover, the combination of transition metal oxide and carbon nanostructure can be available for improving the
⁎
Corresponding author. E-mail address: [email protected] (K. Wang).
https://doi.org/10.1016/j.apenergy.2018.05.071 Received 12 December 2017; Received in revised form 4 May 2018; Accepted 20 May 2018 0306-2619/ © 2018 Elsevier Ltd. All rights reserved.
comprehensive performance of oxygen redox reaction [11]. Unfortunately, rechargeable zinc-air battery with bifunctional catalysts [12,13] was generally applied at small currents in order to guard against material decomposition and catalyst loss. To meet fast-charge demand, a tri-electrode configuration was developed for rechargeable zinc-air battery [14], namely the charging and discharging proceed was separately employed with single electrode, and zinc electrode was located between air electrode and the charging electrode. Although this structure can avoid impairment of oxygen bubbles on the catalytic layer of air electrode, it would increase the battery volume. In addition, morphological change of zinc electrode would be more severe at large current densities, which can lead to short circuit of rechargeable zincair battery. To inhibit dendrite growth of electrodeposited zinc, many studies have been made in terms of electrolyte additives [15], electrode surfactant [16], and metal alloy [17], but these techniques would contaminate and even reduce active material of zinc electrode. Other
Applied Energy 225 (2018) 848–856
K. Wang et al.
Nomenclature Symbol C D E f F I j L p
P R S v z Zi Zr λ ϕ ρ μ
Unit and value concentration of zinc ion in the electrolyte, mol L−1 diffusivity of zinc ion in the electrolyte, m2 s−1 potential, V driving force, N Faraday constant, 96,485 C mol−1 local current density, A m−2 charging/discharging current density, mA cm−2 zinc electrode length, m pressure, Pa
power device resistance device zinc ion reaction rate, mol L−1 s−1 flow velocity, mL s−1 valence of zinc imaginary part of electronic impedance, +2 real part of electronic impedance mobility of the charged species electrolyte potential, V electrolyte density, kg m−3 dynamic viscosity, N s m−2
Additionally, performance degradation of air electrode becomes more prominent at high current densities, restricting cycle life of rechargeable zinc-air battery. Oxygen has paramagnetic property due to two unpaired electrons in the oxygen molecule, and thus the magnetic field can be applied to promote oxygen transfer. Wang et al. [22] stated that the catalytic activity of oxygen reduction reaction was enhanced under the condition of internal-external magnetic fields. Shi et al. [23] present that the magnetic field was used for improving discharging performance of zinc-air fuel cells. However, the effect of magnetic field on the charging performance of rechargeable zinc-air battery was rarely studied. What’s more, oxygen bubble is easily adhered to the electrode surface, influencing stability of charging proceed. In this work, rechargeable zinc-air battery with a compacted structure is proposed on the basis of bifunctional catalyst and tri-electrode configuration, where the charging electrode is touched with air electrode through the stainless steel framework, so the battery can be
researchers have done much works in battery management in order to control morphological change of zinc electrode. Gavrilović-Wohlmuther et al. [18] analyzed morphology pattern of the zinc deposits at different electrolyte temperatures and various electrolyte flow velocities, demonstrating that electrolyte flow was an effective measure to inhibit dendritic morphology. Hwang et al. [19] developed a rechargeable zinc-air battery using commercial polypropylene membrane coated with polymerized ionic liquid as a separator, which can allow anionic transfer through the separator and minimize the migration of zincate ions to the cathode surface. Pichler et al. [20] employed pulse charging as suppressing dendrite growth of electrodeposited zinc, improving the cycling performance of rechargeable zinc-air battery. Increase of ion diffusion time is beneficial to control morphological change of electrodeposited zinc [21], whereas inefficient electrodeposition, increase of internal resistance and electrolyte pressure fluctuation would be accompanied by the above measures.
Fig. 1. Structure characteristics, (a) SEM image and X-ray spectrum of air electrode I, (b) SEM image and X-ray spectrum of air electrode II. 849
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centrifugation (DM0636, Dragon Laboratory Instruments Limited), washed with ethanol, then finally dried at 100 °C. Air electrode I was employed as the stratified structure, where the fabricated catalyst was pressed onto one side of nickel foam, and polytetrafluoroethylene was pasted onto another side of nickel foam (Roll squeezer, Kaifeng Crystal Power Lithium Equipment Co. Ltd.). Air electrode II was fabricated by means of the catalyst pressed onto nickel foam, improving hydrophobicity of the nickel foam. Ni was employed as the catalyst of oxygen evolution reaction, and nickel mesh was welded to stainless steel frame as the charging electrode. Morphological structure of electrode was examined by scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy (Sirion 200). Electrochemical kinetics of air electrode was conducted by means of electrochemical workstation (CHI852D, Shanghai Chenghua Instrument Co. Ltd.), where cyclic voltammetry of air electrode was tested at scan rate of 0.05 v s−1. Rechargeable zinc-air battery with a compacted structure was designed, where air electrode was indirectly in contact with the charging electrode through stainless steel frame, the distance between the
operated at large current densities without increasing the battery volume. A non-insulating structure of air electrode and a trapezoidal framework of electrolyte reservoir are used to improve the cycling performance of rechargeable zinc-air battery. The efficiency of electrodeposited zinc is increased by the additive of calcium oxide to the electrolyte. Moreover, a permanent magnet can be conducive to propelling oxygen bubbles out of rechargeable zinc-air battery, improving energy efficiency of the battery. 2. Experimental Oxygen reduction catalyst was synthesized by Fe(NO3)3⋅9H2O, acetylene black, urea and deionized water under conditions of stirring, centrifugation and heat, where Fe(NO3)3⋅9H2O, acetylene black and urea were added to deionized water. The resulting solution was stirred evenly by way of electronic mixer (OS20-S, Dragon Laboratory Instruments Limited), and then it was heated at 100 °C by means of vacuum drying oven (DZF 6050, Shanghai Jing Hong Laboratory Instrument Co., Ltd.) for 6 h. The product was collected by
Fig. 2. Electrochemical characteristics of air electrode, (a) Nyquist plots of air electrode I, II during discharge, (b) cyclic voltammetry curves of air electrode I, II. 850
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Fig. 3. Rechargeable zinc-air battery, (a) structure view of the battery, (b) charge and discharge polarization curves of the battery with air electrode I, II, (c) cycling performance of the battery with air electrode I, II, (d) microstructure of the catalytic layer air electrode I, II before/after the cycling experiment of 35 h. 851
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Electrochemical experiments on air electrode were carried out, including electronic impedance and cyclic voltammetry. Fig. 2 shows that air electrode II is superior to air electrode I in the impedance and catalytic activation. The main reason is that electrolyte permeation would induce pore deformation of polytetrafluoroethylene, leading to performance degradation of air electrode I. More importantly, nickel foam as gas diffusion layer could simplify electrode structure and reduce the internal resistance, improving the performance of air electrode II. To profoundly delve into air electrode performance, rechargeable zinc-air battery with an equivalent two-electrode configuration was developed, where air electrode and the charging electrode were located at the same side of zinc electrode, and air electrode was connected to the charging electrode through a stainless steel frame, as shown in Fig. 3(a). The charge-discharge polarization of rechargeable zinc-air battery with air electrode I, II was illustrated in Fig. 3(b), demonstrating that the performance of rechargeable zinc-air battery with air electrode II is better than that of the battery with air electrode I mainly due to conductivity and pore rigidity. Moreover, the performance attenuation of air electrode I (Fig. 3(c)) is more serious than that of air electrode II, and the structure of air electrode II is more stable than that of air electrode I (Fig. 3(d)). Therefore, structural stability of air electrode is of great importance for cycle life of rechargeable zinc-air battery. Therefore, the cycling performance of rechargeable zinc-air battery with air electrode II would be discussed in the following.
charging electrode and zinc electrode was 5 mm, and the active area of the electrode was 9 cm2. The electrolyte was prepared by 7 mol L−1 potassium hydroxide and 0.6 mol L−1 zinc oxide dissolved in deionized water, and the electrolyte was driven by the peristaltic pump (BT600LKZ15, Changzhou Pre Fluid Technology Co. Ltd.). Cycling performance of the battery was carried out by means of battery testing instrument (CT-4004-10V100A-NTFA, Shenzhen Neware Technology Limited) under different conditions of battery structure optimization, electrolyte management and magnetoelectric coupling. Electrolyte reservoir structure of rechargeable zinc-air battery was designed to be a trapezoidal framework, which can increase the active area of the charging electrode and decrease zinc electrode surface. Rechargeable zinc-air battery was placed vertically at the temperature of 298 K and the atmospheric pressure, and a magnet of NdFeB N50 (50 mm × 50 mm × 5 mm) was located at the external side of the battery for controlling oxygen bubbles movement. Oxygen needed for rechargeable zinc-air battery was directly breathed from the surrounding air. 3. Results and discussion 3.1. Structure and performance of air electrode Air electrode was composed of the catalytic layer, the current collector and gas diffusion layer, as shown in Fig. 1, where the diffusion layer of air electrode I was made of polytetrafluoroethylene (PTFE) in consideration of hydrophobic property, and that of air electrode II was nickel foam in view of electrode structure. Nickel foam of air electrode I possessed a good hydrophilic performance, easily leading to air electrode flooding, while the nickel foam of air electrode II was processed by rolling technique, possessing hydrophobic property. Therefore, a hydrophobic layer of PTFE was used for solving the flooding problem of air electrode I. Moreover, chemical components of the catalytic layer are almost the same, but the carbon and ferric contents are different. Carbon content of air electrode II is higher than that of air electrode I, and carbon support material is beneficial for improving the conductive performance of air electrode. The additive of polytetrafluoroethylene to air electrode I is available for retarding air electrode flooding, but pore deformation of polytetrafluoroethylene would strictly influence oxygen intake. Hydrophobic nickel foam of air electrode II can also work as gas diffusion layer as well as the current collector. Cycle life of rechargeable zinc-air battery is mainly dependent upon shape change of zinc electrode and air electrode failure.
3.2. Electrolyte management According to Faraday’s first law, the theoretical quantity of electrodeposited metal is mainly determined by the electric quantity. However, non-faraday effects always arise like hydrogen evolution reaction or heat release under the condition of large currents, reducing electrolytic efficiency. Fig. 4 shows that a charge-discharge cycle of rechargeable zinc-air battery with the prepared electrolyte cannot be achieved in the case of 3 h charging and 2 h discharging, while the battery can be in stable operation by adding calcium oxide (CaO) additive (wt. 0.05%) to the electrolyte. The existence of metal particles can provide nucleus for electrodeposited zinc, improving ion conductivity and the efficiency of electrodeposited zinc. Fig. 5(a) shows that cycle life of rechargeable zinc-air battery can be extended by means of electrolyte flow, inhibiting dendrite growth of electrodeposited zinc. The cycling performance of zinc electrode is relatively stable at the initial stage, but morphological change of zinc electrode becomes more obvious at the lower flow rate, leading to
Fig. 4. Improving the cycling performance of zinc-air battery with calcium additive to the electrolyte at the same flow rate of 5 mL s−1. 852
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Fig. 5. Extending cycle life of rechargeable zinc-air battery by way of electrolyte flow, (a) the cycling performance of the battery at different flow rates of 1 mL s−1 and 5 mL s−1, (b) Nyquist plots of the battery at flow rates of 1 mL s−1 and 5 mL s−1, (c) the effect of flow rate on ion concentration distribution of the electrolyte and local current density of zinc electrode, (d) morphological change of zinc electrode at different flow rates (1 mL s−1, 25 h and 5 mL s−1, 40 h).
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(v·∇C −∇ ·(D∇C + zλFC ∇ϕ) = S ). Moreover, the model of flow electrolyte employs Navier-Stokes equation (ρ (v·∇v ) = −∇p + μ∇2 v + f ), assuming that the electrolyte flow is incompressible and Newtonian. The effect of electrolyte flow on ionic concentration was analyzed by means of COMSOL Multiphysics software, demonstrating that the larger the flow rate is, the more uniform ion concentration distribution is, reducing ion concentration gradient between electrochemical reaction zone and the bulk electrolyte and increasing local current density at the electrode surface (Fig. 5(c)). The morphology of electrodeposited zinc would be influenced by means of electrolyte flow, and dendrite growth of zinc electrodeposits was suppressed at the high flow rate, as shown in Fig. 5(d). It is worth noting that electrolyte flow needs to be matched with the electrochemical reaction rate, timely supplying ion consumption and reducing the uneven distribution of ion concentration. Moreover, electrolyte flow would influence the distribution of the electric
dendrite growth of electrodeposited zinc and short circuit of the battery. Fig. 5(b) shows that electrolyte flow can reduce ohmic resistance of rechargeable zinc-air battery during charging, this because oxygen bubbles of the electrochemical reaction can be quickly carried away from the battery by the flowing electrolyte. However, the flowing electrolyte would increase the resistance of the battery during discharging mainly due to the reactant of zinc oxide dissolved in the electrolyte. In addition, electrolyte flow can also influence ion migration in terms of the flow field perpendicular to the electric field. The flow velocity at the cross section of rechargeable zinc-air battery is not uniform because of the friction between the electrolyte and the electrode, and electrolyte flow inside the boundary layer becomes unstable as the flow goes deeper. Ion transport in the electrolyte is mainly caused by electric field, concentration gradient, and electrolyte hydrodynamics [24], establishing the material balance equation
Fig. 6. Extending cycle life of rechargeable zinc-air battery by means of structure optimization, (a) schematic view of the battery structure, (b) monitoring dendrite growth of electrodeposited zinc based on the battery structure, (c) the cycling performance comparison of the battery before/after structure optimization at the same current density (50 mA cm−2) and flow rate (5 mL s−1). 854
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field, changing the electrodepositing direction.
can monitor dendrite growth of electrodeposited zinc, protecting air electrode from dendrite puncture. Fig. 6(b) shows that rechargeable zinc-air battery can work at the current density of 100 mA cm−2 even though the battery is short-circuited. The reason is that the battery may be immediately switched from the charging mode to the discharging mode when dendritic morphology of electrodeposited zinc is in contact with the charging electrode, reducing the negative effect of dendrite growth on the lifetime of the battery. To extend cycle life of the battery, a trapezoidal framework between the charging electrode and zinc electrode was proposed for retarding dendrite growth. The result shows that rechargeable zinc-air battery with the framework can stably run for 100 h without performance degradation at the current density of 50 mA cm−2 (Fig. 6(c)), whereas the battery with a square structure can
3.3. Structure optimization The short lifetime of rechargeable zinc-air battery is mainly dependent upon shape change of zinc electrode and air electrode failure. Here, air electrode was capsulated with a layer of stainless steel for resisting pressure fluctuation of the electrolyte, and the charging electrode was placed between air electrode and zinc electrode, as shown in Fig. 6(a). Compared to rechargeable zinc-air battery with the traditionally three-electrode configuration [25], this structure is relatively compacted, and the battery can work at high current densities, meeting the demand of fast charge. More importantly, the charging electrode
Fig. 7. Improving the performance of zinc-air battery, (a) schematic view of rechargeable zinc-air battery with magnet, (b) the effect of the magnetic field on the cycling performance of zinc-air battery at the same flow rate of 1 mL s−1. 855
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flow and the magnetic field.
run for no more than 40 h. It is partly because of the uneven distribution of ion concentration with the square structure, resulting in dendrite growth at the boundaries. In addition, the area of the charging electrode is larger than that of zinc electrode, which is conducive to uniform distribution of zinc deposits. The charging performance of rechargeable zinc-air battery is mostly subjected to the sluggish kinetics of oxygen evolution reaction, and oxygen bubbles would be easily adhered to the electrode surface, blocking the following progress [26]. Energy efficiency of rechargeable zinc-air battery is also related to quick removal of oxygen bubbles at the electrode surface as well as catalytic activation. Although electrolyte flow can take oxygen bubbles away from the electrode surface, it would add external pressure to the electrolyte, thus accelerating the failure of air electrode. A permanent magnet was located at one side of zinc-air battery, as illustrated in Fig. 7(a). Ionic current can be formed in the rising process of oxygen bubbles, leading to oxygen bubbles driven by the Lorentz force as well as the buoyancy, and thus oxygen bubbles moving in one direction. Therefore, oxygen bubbles during charging can be expelled out of the battery by means of the magnet, decreasing the charging voltage. Moreover, the magnetic field is also beneficial to the discharging performance of zinc-air battery mainly because of two unpaired electrons of oxygen molecular structure, accelerating oxygen transfer into the reaction zone in the magnetic field [27]. Fig. 7(b) shows that energy efficiency of the battery can be increased by 10% at the current density of 50 mA cm−2 in the magnetic field. Therefore, the magnetic field is an effective solution to improve energy efficiency of rechargeable zinc-air battery. To expel oxygen bubbles out of rechargeable zinc-air battery during charging, electrolyte flow was driven by means of a peristaltic pump, where the flowing rate of the electrolyte is mainly related to charging/ discharging current of the battery, but the high velocity of the electrolyte would damage air electrode structure. The charge-discharge voltage gap of rechargeable zinc-air battery was about 1 V at the current density of 50 mA cm−2, and energy efficiency of the battery can be as high as 52.4%. However, the efficiency of battery system would be decreased to 25.4% in consideration of electrolyte circulation. In view of the above mentioned, directional movement of oxygen bubbles can be accelerated by means of the magnetic field, which can turn down the pump speed and thus reduce power dissipation of the pump.
Acknowledgements This work was supported by National Natural Science Foundation of China (No. 21706013, No. 21676158, No. 21376138) and The National Key Research and Development Program of China (No. 2016YFB0101208, No. 2016YFB0101305). References [1] Aneke M, Wang M. Energy storage technologies and real life applications–a state of the art review. Appl Energy 2016;179:350–77. [2] Gu P, Zheng M, Zhao Q, et al. Rechargeable zinc-air batteries: a promising way to green energy. J Mater Chem A 2017;5(17):7651–66. [3] Sherman SB, Cano ZP, Fowler M, et al. Range-extending zinc-air battery for electric vehicle. AIMS Energy 2018;6(1):121–45. [4] Pei P, Wang K, Ma Z. Technologies for extending zinc-air battery’s cyclelife: a review. Appl Energy 2014;128:315–24. [5] Jiang HR, Wu MC, Ren YX, et al. Towards a uniform distribution of zinc in the negative electrode for zinc bromine flow batteries. Appl Energy 2018;213:366–74. [6] Chen YY, Wang HY. Polyelectrolyte microparticles for enhancing anode performance in an air-cathode μ-Liter microbial fuel cell. Appl Energy 2015;160:965–72. [7] He DS, He D, Wang J, et al. Ultrathin icosahedral Pt-enriched nanocage with excellent oxygen reduction reaction activity. J Am Chem Soc 2016;138(5):1494–7. [8] Guo D, Shibuya R, Akiba C, et al. Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts. Science 2016;351(6271):361–5. [9] Hong WT, Risch M, Stoerzinger KA, et al. Toward the rational design of non-precious transition metal oxides for oxygen electrocatalysis. Energy Environ Sci 2015;8(5):1404–27. [10] Mani P, Sheelam A, Das S, et al. Cobalt-based coordination polymer for oxygen reduction reaction. ACS Omega 2018;3(4):3830–4. [11] Xu N, Qiao J, Zhang X, et al. Morphology controlled La2O3/Co3O4/MnO2-CNTs hybrid nanocomposites with durable bi-functional air electrode in high-performance zinc-air energy storage. Appl Energy 2016;175:495–504. [12] Han X, Wu X, Zhong C, et al. NiCo2S4 nanocrystals anchored on nitrogen-doped carbon nanotubes as a highly efficient bifunctional electrocatalyst for rechargeable zinc-air batteries. Nano Energy 2017;31:541–50. [13] Zhang J, Zhao Z, Xia Z, et al. A metal-free bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions. Nat Nanotech 2015;10(5):444–52. [14] Li Y, Gong M, Liang Y, et al. Advanced zinc-air batteries based on high-performance hybrid electrocatalysts. Nat Commun 2013;4:1805. [15] Banik SJ, Akolkar R. Suppressing dendritic growth during alkaline zinc electrodeposition using polyethylenimine additive. Electrochim Acta 2015;179:475–81. [16] Sun KEK, Hoang TKA, Doan TNL, et al. Suppression of dendrite formation and corrosion on zinc anode of secondary aqueous batteries. ACS Appl Mater Interfaces 2017;9(11):9681–7. [17] Wei X, Desai D, Yadav GG, et al. Impact of anode substrates on electrodeposited zinc over cycling in zinc-anode rechargeable alkaline batteries. Electrochim Acta 2016;212:603–13. [18] Gavrilović-Wohlmuther A, Laskos A, Zelger C, et al. Effects of electrolyte concentration, temperature, flow velocity and current density on Zn deposit morphology. J Power Energy Eng 2015;9:1019–28. [19] Hwang HJ, Chi WS, Kwon O, et al. Selective ion transporting polymerized ionic liquid membrane separator for enhancing cycle stability and durability in secondary zinc-air battery systems. ACS Appl Mater Interfaces 2016;8(39):26298–308. [20] Pichler B, Weinberger S, Reščec L, et al. Bifunctional electrode performance for zinc-air flow cells with pulse charging. Electrochim Acta 2017;251:488–97. [21] Wang K, Pei P, Ma Z, et al. Morphology control of zinc regeneration for zinc-air fuel cell and battery. J Power Sources 2014;271:65–75. [22] Wang L, Yang H, Yang J, et al. The effect of the internal magnetism of ferromagnetic catalysts on their catalytic activity toward oxygen reduction reaction under an external magnetic field. Ionics 2016;22(11):2195–202. [23] Shi J, Xu H, Lu L, et al. Study of magnetic field to promote oxygen transfer and its application in zinc-air fuel cells. Electrochim Acta 2013;90:44–52. [24] Guyer JE, Boettinger WJ, Warren JA, McFadden GB. Phase field modeling of electrochemistry. II. Kinetics. Phys Rev E Stat Nonlin Soft Matter Phys 2004;69(2):021604. [25] Ma H, Wang B, Fan Y, et al. Development and characterization of an electrically rechargeable zinc-air battery stack. Energies 2014;7(10):6549–57. [26] Wang K, Pei P, Ma Z, et al. Growth of oxygen bubbles during recharge process in zinc-air battery. J Power Sources 2015;296:40–5. [27] Monzon LMA, Coey JMD. Magnetic fields in electrochemistry: The Lorentz force. A mini-review. Electrochem Commun 2014;42:38–41.
4. Conclusions Cycling performance of rechargeable zinc-air battery was investigated by means of electrochemical experiments and microstructure examination. The related solutions to extending cycle life of the battery were also put forward under different conditions of air electrode, electrolyte management and structure optimization. These findings can promote rechargeable zinc-air battery in application of energy storage. The conclusions in detail are as follows: (1) Non-insulating structure of air electrode was developed, namely nickel foam was employed as gas diffusion layer of air electrode, which can reduce the internal resistance of air electrode and improve the discharging performance of rechargeable zinc-air battery. (2) Rechargeable zinc-air battery with an equivalent two-electrode configuration was developed, and a trapezoidal framework was designed for electrolyte reservoir. The optimized battery structure will be conducive to suppressing dendrite growth, meeting the charge-discharge demand of large current densities. (3) Charging efficiency of rechargeable zinc-air battery can be enhanced by way of calcium oxide additive to the electrolyte, and the cycling performance of the battery can be improved by electrolyte
856
Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Advanced rechargeable zinc-air battery with parameter optimization a,⁎
b
a
a
a
Keliang Wang , Pucheng Pei , Yichun Wang , Cheng Liao , Wei Wang , Shangwei Huang a b
T b
School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China State Key Lab. of Automotive Safety and Energy, Tsinghua University, Beijing 100084, China
H I GH L IG H T S
zinc-air battery with a compacted structure was optimally designed. • Rechargeable and structural performance of air electrode was characterized. • Electrochemical performance of the battery was improved by electrolyte management. • Cycling • Oxygen bubbles movement was controlled by electromagnetic coupling.
A R T I C LE I N FO
A B S T R A C T
Keywords: Cycle life Dendrite growth Structure optimization Electrolyte management Magnetic field Rechargeable zinc-air battery
Zinc-air batteries will be a promising candidate for storage energy and power supply due to their high specific energy, environmental compatibility, and economic availability. However, the problem of cycle life of rechargeable zinc-air battery remains unresolved mainly because of dendrite growth of electrodeposited zinc and performance degradation of air electrode. Here we show that rechargeable zinc-air battery with an optimized structure can stably run at large current densities, where air electrode is connected to the charging electrode through a stainless steel frame, and the effective area of charging electrode is larger than that of zinc electrode by way of a trapezoidal structure. This battery structure can control morphological change of zinc electrode and monitor dendrite growth without increasing the battery volume. The results demonstrate that the charge-discharge efficiency of rechargeable zinc-air battery can be improved by nickel foam as gas diffusion layer of air electrode, calcium oxide additive to the electrolyte, or a permanent magnet in parallel with the electrode. The lifetime of rechargeable zinc-air battery can be extended by electrolyte flow or battery structure optimization. These findings will be available for other metal-air batteries and electrolytic metal industry.
1. Introduction Zinc-air batteries have attracted more attention as one of energy storage devices in application of electric grid [1], green energy [2] and power supply [3] due to high-energy-density and non-pollution advantage. Nevertheless, the cycling performance of rechargeable zinc-air battery is unsatisfactory [4], where morphological change of electrodeposited zinc would shorten cycle life of the battery [5], and sluggish kinetics of oxygen redox reaction could decrease energy efficiency of the battery [6]. Energy efficiency of rechargeable zinc-air battery is mostly related to catalytic activation. Study of oxygen redox catalysts is mainly focused on precious metals and their alloys [7], carbon nanostructure materials [8], transition metal oxides [9], and inorganic/organic compound materials [10]. Moreover, the combination of transition metal oxide and carbon nanostructure can be available for improving the
⁎
Corresponding author. E-mail address: [email protected] (K. Wang).
https://doi.org/10.1016/j.apenergy.2018.05.071 Received 12 December 2017; Received in revised form 4 May 2018; Accepted 20 May 2018 0306-2619/ © 2018 Elsevier Ltd. All rights reserved.
comprehensive performance of oxygen redox reaction [11]. Unfortunately, rechargeable zinc-air battery with bifunctional catalysts [12,13] was generally applied at small currents in order to guard against material decomposition and catalyst loss. To meet fast-charge demand, a tri-electrode configuration was developed for rechargeable zinc-air battery [14], namely the charging and discharging proceed was separately employed with single electrode, and zinc electrode was located between air electrode and the charging electrode. Although this structure can avoid impairment of oxygen bubbles on the catalytic layer of air electrode, it would increase the battery volume. In addition, morphological change of zinc electrode would be more severe at large current densities, which can lead to short circuit of rechargeable zincair battery. To inhibit dendrite growth of electrodeposited zinc, many studies have been made in terms of electrolyte additives [15], electrode surfactant [16], and metal alloy [17], but these techniques would contaminate and even reduce active material of zinc electrode. Other
Applied Energy 225 (2018) 848–856
K. Wang et al.
Nomenclature Symbol C D E f F I j L p
P R S v z Zi Zr λ ϕ ρ μ
Unit and value concentration of zinc ion in the electrolyte, mol L−1 diffusivity of zinc ion in the electrolyte, m2 s−1 potential, V driving force, N Faraday constant, 96,485 C mol−1 local current density, A m−2 charging/discharging current density, mA cm−2 zinc electrode length, m pressure, Pa
power device resistance device zinc ion reaction rate, mol L−1 s−1 flow velocity, mL s−1 valence of zinc imaginary part of electronic impedance, +2 real part of electronic impedance mobility of the charged species electrolyte potential, V electrolyte density, kg m−3 dynamic viscosity, N s m−2
Additionally, performance degradation of air electrode becomes more prominent at high current densities, restricting cycle life of rechargeable zinc-air battery. Oxygen has paramagnetic property due to two unpaired electrons in the oxygen molecule, and thus the magnetic field can be applied to promote oxygen transfer. Wang et al. [22] stated that the catalytic activity of oxygen reduction reaction was enhanced under the condition of internal-external magnetic fields. Shi et al. [23] present that the magnetic field was used for improving discharging performance of zinc-air fuel cells. However, the effect of magnetic field on the charging performance of rechargeable zinc-air battery was rarely studied. What’s more, oxygen bubble is easily adhered to the electrode surface, influencing stability of charging proceed. In this work, rechargeable zinc-air battery with a compacted structure is proposed on the basis of bifunctional catalyst and tri-electrode configuration, where the charging electrode is touched with air electrode through the stainless steel framework, so the battery can be
researchers have done much works in battery management in order to control morphological change of zinc electrode. Gavrilović-Wohlmuther et al. [18] analyzed morphology pattern of the zinc deposits at different electrolyte temperatures and various electrolyte flow velocities, demonstrating that electrolyte flow was an effective measure to inhibit dendritic morphology. Hwang et al. [19] developed a rechargeable zinc-air battery using commercial polypropylene membrane coated with polymerized ionic liquid as a separator, which can allow anionic transfer through the separator and minimize the migration of zincate ions to the cathode surface. Pichler et al. [20] employed pulse charging as suppressing dendrite growth of electrodeposited zinc, improving the cycling performance of rechargeable zinc-air battery. Increase of ion diffusion time is beneficial to control morphological change of electrodeposited zinc [21], whereas inefficient electrodeposition, increase of internal resistance and electrolyte pressure fluctuation would be accompanied by the above measures.
Fig. 1. Structure characteristics, (a) SEM image and X-ray spectrum of air electrode I, (b) SEM image and X-ray spectrum of air electrode II. 849
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centrifugation (DM0636, Dragon Laboratory Instruments Limited), washed with ethanol, then finally dried at 100 °C. Air electrode I was employed as the stratified structure, where the fabricated catalyst was pressed onto one side of nickel foam, and polytetrafluoroethylene was pasted onto another side of nickel foam (Roll squeezer, Kaifeng Crystal Power Lithium Equipment Co. Ltd.). Air electrode II was fabricated by means of the catalyst pressed onto nickel foam, improving hydrophobicity of the nickel foam. Ni was employed as the catalyst of oxygen evolution reaction, and nickel mesh was welded to stainless steel frame as the charging electrode. Morphological structure of electrode was examined by scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy (Sirion 200). Electrochemical kinetics of air electrode was conducted by means of electrochemical workstation (CHI852D, Shanghai Chenghua Instrument Co. Ltd.), where cyclic voltammetry of air electrode was tested at scan rate of 0.05 v s−1. Rechargeable zinc-air battery with a compacted structure was designed, where air electrode was indirectly in contact with the charging electrode through stainless steel frame, the distance between the
operated at large current densities without increasing the battery volume. A non-insulating structure of air electrode and a trapezoidal framework of electrolyte reservoir are used to improve the cycling performance of rechargeable zinc-air battery. The efficiency of electrodeposited zinc is increased by the additive of calcium oxide to the electrolyte. Moreover, a permanent magnet can be conducive to propelling oxygen bubbles out of rechargeable zinc-air battery, improving energy efficiency of the battery. 2. Experimental Oxygen reduction catalyst was synthesized by Fe(NO3)3⋅9H2O, acetylene black, urea and deionized water under conditions of stirring, centrifugation and heat, where Fe(NO3)3⋅9H2O, acetylene black and urea were added to deionized water. The resulting solution was stirred evenly by way of electronic mixer (OS20-S, Dragon Laboratory Instruments Limited), and then it was heated at 100 °C by means of vacuum drying oven (DZF 6050, Shanghai Jing Hong Laboratory Instrument Co., Ltd.) for 6 h. The product was collected by
Fig. 2. Electrochemical characteristics of air electrode, (a) Nyquist plots of air electrode I, II during discharge, (b) cyclic voltammetry curves of air electrode I, II. 850
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Fig. 3. Rechargeable zinc-air battery, (a) structure view of the battery, (b) charge and discharge polarization curves of the battery with air electrode I, II, (c) cycling performance of the battery with air electrode I, II, (d) microstructure of the catalytic layer air electrode I, II before/after the cycling experiment of 35 h. 851
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Electrochemical experiments on air electrode were carried out, including electronic impedance and cyclic voltammetry. Fig. 2 shows that air electrode II is superior to air electrode I in the impedance and catalytic activation. The main reason is that electrolyte permeation would induce pore deformation of polytetrafluoroethylene, leading to performance degradation of air electrode I. More importantly, nickel foam as gas diffusion layer could simplify electrode structure and reduce the internal resistance, improving the performance of air electrode II. To profoundly delve into air electrode performance, rechargeable zinc-air battery with an equivalent two-electrode configuration was developed, where air electrode and the charging electrode were located at the same side of zinc electrode, and air electrode was connected to the charging electrode through a stainless steel frame, as shown in Fig. 3(a). The charge-discharge polarization of rechargeable zinc-air battery with air electrode I, II was illustrated in Fig. 3(b), demonstrating that the performance of rechargeable zinc-air battery with air electrode II is better than that of the battery with air electrode I mainly due to conductivity and pore rigidity. Moreover, the performance attenuation of air electrode I (Fig. 3(c)) is more serious than that of air electrode II, and the structure of air electrode II is more stable than that of air electrode I (Fig. 3(d)). Therefore, structural stability of air electrode is of great importance for cycle life of rechargeable zinc-air battery. Therefore, the cycling performance of rechargeable zinc-air battery with air electrode II would be discussed in the following.
charging electrode and zinc electrode was 5 mm, and the active area of the electrode was 9 cm2. The electrolyte was prepared by 7 mol L−1 potassium hydroxide and 0.6 mol L−1 zinc oxide dissolved in deionized water, and the electrolyte was driven by the peristaltic pump (BT600LKZ15, Changzhou Pre Fluid Technology Co. Ltd.). Cycling performance of the battery was carried out by means of battery testing instrument (CT-4004-10V100A-NTFA, Shenzhen Neware Technology Limited) under different conditions of battery structure optimization, electrolyte management and magnetoelectric coupling. Electrolyte reservoir structure of rechargeable zinc-air battery was designed to be a trapezoidal framework, which can increase the active area of the charging electrode and decrease zinc electrode surface. Rechargeable zinc-air battery was placed vertically at the temperature of 298 K and the atmospheric pressure, and a magnet of NdFeB N50 (50 mm × 50 mm × 5 mm) was located at the external side of the battery for controlling oxygen bubbles movement. Oxygen needed for rechargeable zinc-air battery was directly breathed from the surrounding air. 3. Results and discussion 3.1. Structure and performance of air electrode Air electrode was composed of the catalytic layer, the current collector and gas diffusion layer, as shown in Fig. 1, where the diffusion layer of air electrode I was made of polytetrafluoroethylene (PTFE) in consideration of hydrophobic property, and that of air electrode II was nickel foam in view of electrode structure. Nickel foam of air electrode I possessed a good hydrophilic performance, easily leading to air electrode flooding, while the nickel foam of air electrode II was processed by rolling technique, possessing hydrophobic property. Therefore, a hydrophobic layer of PTFE was used for solving the flooding problem of air electrode I. Moreover, chemical components of the catalytic layer are almost the same, but the carbon and ferric contents are different. Carbon content of air electrode II is higher than that of air electrode I, and carbon support material is beneficial for improving the conductive performance of air electrode. The additive of polytetrafluoroethylene to air electrode I is available for retarding air electrode flooding, but pore deformation of polytetrafluoroethylene would strictly influence oxygen intake. Hydrophobic nickel foam of air electrode II can also work as gas diffusion layer as well as the current collector. Cycle life of rechargeable zinc-air battery is mainly dependent upon shape change of zinc electrode and air electrode failure.
3.2. Electrolyte management According to Faraday’s first law, the theoretical quantity of electrodeposited metal is mainly determined by the electric quantity. However, non-faraday effects always arise like hydrogen evolution reaction or heat release under the condition of large currents, reducing electrolytic efficiency. Fig. 4 shows that a charge-discharge cycle of rechargeable zinc-air battery with the prepared electrolyte cannot be achieved in the case of 3 h charging and 2 h discharging, while the battery can be in stable operation by adding calcium oxide (CaO) additive (wt. 0.05%) to the electrolyte. The existence of metal particles can provide nucleus for electrodeposited zinc, improving ion conductivity and the efficiency of electrodeposited zinc. Fig. 5(a) shows that cycle life of rechargeable zinc-air battery can be extended by means of electrolyte flow, inhibiting dendrite growth of electrodeposited zinc. The cycling performance of zinc electrode is relatively stable at the initial stage, but morphological change of zinc electrode becomes more obvious at the lower flow rate, leading to
Fig. 4. Improving the cycling performance of zinc-air battery with calcium additive to the electrolyte at the same flow rate of 5 mL s−1. 852
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Fig. 5. Extending cycle life of rechargeable zinc-air battery by way of electrolyte flow, (a) the cycling performance of the battery at different flow rates of 1 mL s−1 and 5 mL s−1, (b) Nyquist plots of the battery at flow rates of 1 mL s−1 and 5 mL s−1, (c) the effect of flow rate on ion concentration distribution of the electrolyte and local current density of zinc electrode, (d) morphological change of zinc electrode at different flow rates (1 mL s−1, 25 h and 5 mL s−1, 40 h).
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(v·∇C −∇ ·(D∇C + zλFC ∇ϕ) = S ). Moreover, the model of flow electrolyte employs Navier-Stokes equation (ρ (v·∇v ) = −∇p + μ∇2 v + f ), assuming that the electrolyte flow is incompressible and Newtonian. The effect of electrolyte flow on ionic concentration was analyzed by means of COMSOL Multiphysics software, demonstrating that the larger the flow rate is, the more uniform ion concentration distribution is, reducing ion concentration gradient between electrochemical reaction zone and the bulk electrolyte and increasing local current density at the electrode surface (Fig. 5(c)). The morphology of electrodeposited zinc would be influenced by means of electrolyte flow, and dendrite growth of zinc electrodeposits was suppressed at the high flow rate, as shown in Fig. 5(d). It is worth noting that electrolyte flow needs to be matched with the electrochemical reaction rate, timely supplying ion consumption and reducing the uneven distribution of ion concentration. Moreover, electrolyte flow would influence the distribution of the electric
dendrite growth of electrodeposited zinc and short circuit of the battery. Fig. 5(b) shows that electrolyte flow can reduce ohmic resistance of rechargeable zinc-air battery during charging, this because oxygen bubbles of the electrochemical reaction can be quickly carried away from the battery by the flowing electrolyte. However, the flowing electrolyte would increase the resistance of the battery during discharging mainly due to the reactant of zinc oxide dissolved in the electrolyte. In addition, electrolyte flow can also influence ion migration in terms of the flow field perpendicular to the electric field. The flow velocity at the cross section of rechargeable zinc-air battery is not uniform because of the friction between the electrolyte and the electrode, and electrolyte flow inside the boundary layer becomes unstable as the flow goes deeper. Ion transport in the electrolyte is mainly caused by electric field, concentration gradient, and electrolyte hydrodynamics [24], establishing the material balance equation
Fig. 6. Extending cycle life of rechargeable zinc-air battery by means of structure optimization, (a) schematic view of the battery structure, (b) monitoring dendrite growth of electrodeposited zinc based on the battery structure, (c) the cycling performance comparison of the battery before/after structure optimization at the same current density (50 mA cm−2) and flow rate (5 mL s−1). 854
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field, changing the electrodepositing direction.
can monitor dendrite growth of electrodeposited zinc, protecting air electrode from dendrite puncture. Fig. 6(b) shows that rechargeable zinc-air battery can work at the current density of 100 mA cm−2 even though the battery is short-circuited. The reason is that the battery may be immediately switched from the charging mode to the discharging mode when dendritic morphology of electrodeposited zinc is in contact with the charging electrode, reducing the negative effect of dendrite growth on the lifetime of the battery. To extend cycle life of the battery, a trapezoidal framework between the charging electrode and zinc electrode was proposed for retarding dendrite growth. The result shows that rechargeable zinc-air battery with the framework can stably run for 100 h without performance degradation at the current density of 50 mA cm−2 (Fig. 6(c)), whereas the battery with a square structure can
3.3. Structure optimization The short lifetime of rechargeable zinc-air battery is mainly dependent upon shape change of zinc electrode and air electrode failure. Here, air electrode was capsulated with a layer of stainless steel for resisting pressure fluctuation of the electrolyte, and the charging electrode was placed between air electrode and zinc electrode, as shown in Fig. 6(a). Compared to rechargeable zinc-air battery with the traditionally three-electrode configuration [25], this structure is relatively compacted, and the battery can work at high current densities, meeting the demand of fast charge. More importantly, the charging electrode
Fig. 7. Improving the performance of zinc-air battery, (a) schematic view of rechargeable zinc-air battery with magnet, (b) the effect of the magnetic field on the cycling performance of zinc-air battery at the same flow rate of 1 mL s−1. 855
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flow and the magnetic field.
run for no more than 40 h. It is partly because of the uneven distribution of ion concentration with the square structure, resulting in dendrite growth at the boundaries. In addition, the area of the charging electrode is larger than that of zinc electrode, which is conducive to uniform distribution of zinc deposits. The charging performance of rechargeable zinc-air battery is mostly subjected to the sluggish kinetics of oxygen evolution reaction, and oxygen bubbles would be easily adhered to the electrode surface, blocking the following progress [26]. Energy efficiency of rechargeable zinc-air battery is also related to quick removal of oxygen bubbles at the electrode surface as well as catalytic activation. Although electrolyte flow can take oxygen bubbles away from the electrode surface, it would add external pressure to the electrolyte, thus accelerating the failure of air electrode. A permanent magnet was located at one side of zinc-air battery, as illustrated in Fig. 7(a). Ionic current can be formed in the rising process of oxygen bubbles, leading to oxygen bubbles driven by the Lorentz force as well as the buoyancy, and thus oxygen bubbles moving in one direction. Therefore, oxygen bubbles during charging can be expelled out of the battery by means of the magnet, decreasing the charging voltage. Moreover, the magnetic field is also beneficial to the discharging performance of zinc-air battery mainly because of two unpaired electrons of oxygen molecular structure, accelerating oxygen transfer into the reaction zone in the magnetic field [27]. Fig. 7(b) shows that energy efficiency of the battery can be increased by 10% at the current density of 50 mA cm−2 in the magnetic field. Therefore, the magnetic field is an effective solution to improve energy efficiency of rechargeable zinc-air battery. To expel oxygen bubbles out of rechargeable zinc-air battery during charging, electrolyte flow was driven by means of a peristaltic pump, where the flowing rate of the electrolyte is mainly related to charging/ discharging current of the battery, but the high velocity of the electrolyte would damage air electrode structure. The charge-discharge voltage gap of rechargeable zinc-air battery was about 1 V at the current density of 50 mA cm−2, and energy efficiency of the battery can be as high as 52.4%. However, the efficiency of battery system would be decreased to 25.4% in consideration of electrolyte circulation. In view of the above mentioned, directional movement of oxygen bubbles can be accelerated by means of the magnetic field, which can turn down the pump speed and thus reduce power dissipation of the pump.
Acknowledgements This work was supported by National Natural Science Foundation of China (No. 21706013, No. 21676158, No. 21376138) and The National Key Research and Development Program of China (No. 2016YFB0101208, No. 2016YFB0101305). References [1] Aneke M, Wang M. Energy storage technologies and real life applications–a state of the art review. Appl Energy 2016;179:350–77. [2] Gu P, Zheng M, Zhao Q, et al. Rechargeable zinc-air batteries: a promising way to green energy. J Mater Chem A 2017;5(17):7651–66. [3] Sherman SB, Cano ZP, Fowler M, et al. Range-extending zinc-air battery for electric vehicle. AIMS Energy 2018;6(1):121–45. [4] Pei P, Wang K, Ma Z. Technologies for extending zinc-air battery’s cyclelife: a review. Appl Energy 2014;128:315–24. [5] Jiang HR, Wu MC, Ren YX, et al. Towards a uniform distribution of zinc in the negative electrode for zinc bromine flow batteries. Appl Energy 2018;213:366–74. [6] Chen YY, Wang HY. Polyelectrolyte microparticles for enhancing anode performance in an air-cathode μ-Liter microbial fuel cell. Appl Energy 2015;160:965–72. [7] He DS, He D, Wang J, et al. Ultrathin icosahedral Pt-enriched nanocage with excellent oxygen reduction reaction activity. J Am Chem Soc 2016;138(5):1494–7. [8] Guo D, Shibuya R, Akiba C, et al. Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts. Science 2016;351(6271):361–5. [9] Hong WT, Risch M, Stoerzinger KA, et al. Toward the rational design of non-precious transition metal oxides for oxygen electrocatalysis. Energy Environ Sci 2015;8(5):1404–27. [10] Mani P, Sheelam A, Das S, et al. Cobalt-based coordination polymer for oxygen reduction reaction. ACS Omega 2018;3(4):3830–4. [11] Xu N, Qiao J, Zhang X, et al. Morphology controlled La2O3/Co3O4/MnO2-CNTs hybrid nanocomposites with durable bi-functional air electrode in high-performance zinc-air energy storage. Appl Energy 2016;175:495–504. [12] Han X, Wu X, Zhong C, et al. NiCo2S4 nanocrystals anchored on nitrogen-doped carbon nanotubes as a highly efficient bifunctional electrocatalyst for rechargeable zinc-air batteries. Nano Energy 2017;31:541–50. [13] Zhang J, Zhao Z, Xia Z, et al. A metal-free bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions. Nat Nanotech 2015;10(5):444–52. [14] Li Y, Gong M, Liang Y, et al. Advanced zinc-air batteries based on high-performance hybrid electrocatalysts. Nat Commun 2013;4:1805. [15] Banik SJ, Akolkar R. Suppressing dendritic growth during alkaline zinc electrodeposition using polyethylenimine additive. Electrochim Acta 2015;179:475–81. [16] Sun KEK, Hoang TKA, Doan TNL, et al. Suppression of dendrite formation and corrosion on zinc anode of secondary aqueous batteries. ACS Appl Mater Interfaces 2017;9(11):9681–7. [17] Wei X, Desai D, Yadav GG, et al. Impact of anode substrates on electrodeposited zinc over cycling in zinc-anode rechargeable alkaline batteries. Electrochim Acta 2016;212:603–13. [18] Gavrilović-Wohlmuther A, Laskos A, Zelger C, et al. Effects of electrolyte concentration, temperature, flow velocity and current density on Zn deposit morphology. J Power Energy Eng 2015;9:1019–28. [19] Hwang HJ, Chi WS, Kwon O, et al. Selective ion transporting polymerized ionic liquid membrane separator for enhancing cycle stability and durability in secondary zinc-air battery systems. ACS Appl Mater Interfaces 2016;8(39):26298–308. [20] Pichler B, Weinberger S, Reščec L, et al. Bifunctional electrode performance for zinc-air flow cells with pulse charging. Electrochim Acta 2017;251:488–97. [21] Wang K, Pei P, Ma Z, et al. Morphology control of zinc regeneration for zinc-air fuel cell and battery. J Power Sources 2014;271:65–75. [22] Wang L, Yang H, Yang J, et al. The effect of the internal magnetism of ferromagnetic catalysts on their catalytic activity toward oxygen reduction reaction under an external magnetic field. Ionics 2016;22(11):2195–202. [23] Shi J, Xu H, Lu L, et al. Study of magnetic field to promote oxygen transfer and its application in zinc-air fuel cells. Electrochim Acta 2013;90:44–52. [24] Guyer JE, Boettinger WJ, Warren JA, McFadden GB. Phase field modeling of electrochemistry. II. Kinetics. Phys Rev E Stat Nonlin Soft Matter Phys 2004;69(2):021604. [25] Ma H, Wang B, Fan Y, et al. Development and characterization of an electrically rechargeable zinc-air battery stack. Energies 2014;7(10):6549–57. [26] Wang K, Pei P, Ma Z, et al. Growth of oxygen bubbles during recharge process in zinc-air battery. J Power Sources 2015;296:40–5. [27] Monzon LMA, Coey JMD. Magnetic fields in electrochemistry: The Lorentz force. A mini-review. Electrochem Commun 2014;42:38–41.
4. Conclusions Cycling performance of rechargeable zinc-air battery was investigated by means of electrochemical experiments and microstructure examination. The related solutions to extending cycle life of the battery were also put forward under different conditions of air electrode, electrolyte management and structure optimization. These findings can promote rechargeable zinc-air battery in application of energy storage. The conclusions in detail are as follows: (1) Non-insulating structure of air electrode was developed, namely nickel foam was employed as gas diffusion layer of air electrode, which can reduce the internal resistance of air electrode and improve the discharging performance of rechargeable zinc-air battery. (2) Rechargeable zinc-air battery with an equivalent two-electrode configuration was developed, and a trapezoidal framework was designed for electrolyte reservoir. The optimized battery structure will be conducive to suppressing dendrite growth, meeting the charge-discharge demand of large current densities. (3) Charging efficiency of rechargeable zinc-air battery can be enhanced by way of calcium oxide additive to the electrolyte, and the cycling performance of the battery can be improved by electrolyte
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