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A long-life hybrid zinc flow battery achieved by dual redox couples at cathode
Nano Energy 63 (2019) 103822
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A long-life hybrid zinc flow battery achieved by dual redox couples at cathode
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Yuanhui Chenga, Ningyuan Zhanga, Qiuli Wanga, Yinjian Guoa, Shuo Taob, Zhijian Liaoa, Peng Jianga, Zhonghua Xianga,* a
Beijing Advanced Innovation Center for Soft Matter Science and Engineering, State Key Laboratory of Organic-Inorganic Composites, College of Chemical Engineering, College of Energy, Beijing University of Chemical Technology, Beijing, 100029, PR China b School of Chemistry and Chemical Engineering, Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology, Liaocheng University, Liaocheng, Shandong, 252059, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Zinc accumulation Oxygen redox reaction Energy storage Zinc nickel flow battery Zinc air flow battery
Zinc nickel flow battery is one of the most promising energy storage technologies for intermittently renewable solar and wind power. However, unpaired coulombic efficiency of nickel hydroxide cathode and zinc anode causes zinc accumulation in practical operation, which shortens the cycle life and impedes the commercialization of the battery. Here, we induce an additional O2/OH− redox couple integrated with NiOOH/Ni(OH)2 as cathode energy storage materials. Benefit from the new reaction mechanism of the hybrid cathode, deposited zinc upon charge can be fully consumed during discharge, which completely avoids zinc accumulation issue. The new designed battery vigorously operates for more than 1100 h with negligible performance degradation, while the energy efficiency of pristine zinc-nickel flow battery dramatically reduces after 440 h. More importantly, the specific capacity of the cathode synchronously increases by 2.5 times and thereby the energy density. This system dexterously integrates dual redox couples into one hybrid unit at cathode complementing advantages of one another, demonstrating a broader method to solve the problem of unpaired coulombic efficiency in zinc based flow batteries.
1. Introduction Flow batteries are considered as one of the most promising large scale energy storage technologies to increase the utilization of intermittent renewable power from wind and solar owning to the inherent merits of low maintenance cost, high safety, independence of power and capacity and long cycle life [1–3]. Among various flow battery technologies, zinc-based flow batteries composed of a zinc/zinc ion redox couple in acid or alkaline electrolytes and another redox couple, display a high output voltage, high theoretical energy density, fast kinetics, abundance and recyclability of zinc compounds and extremely low cost, and have such warranted special attention [4–8]. Beyond pioneer zinc halogen flow battery [9–11], zinc nickel flow battery (ZNFB) has been widely studied original from the membrane-less structure, which eliminates the common problems of electrolyte crossover and the utilization of high cost membranes in traditional flow batteries [12,13]. Importantly, all original chemicals or materials are abundant, cheap and nontoxic, endowing this technology a broad application prospect in large scale energy storage. The working
*
mechanism of a ZNFB is similar to both a battery and a fuel cell as shown in Scheme 1a. As described in Equation (1), at cathode, Ni(OH)2 is oxidized to NiOOH during charge and reverse reaction occurs upon discharge, like a battery. Meanwhile, zincate ions in the continuously flowing electrolyte will be reduced to metallic zinc at anode during charge, while metallic zinc can be oxidized to zincate ions into the circulated electrolyte upon discharge, like a fuel cell (Equation (2)). Pristine zinc nickel flow battery: Cathode: 2Ni(OH)2 + 2OH− ↔ 2NiOOH + 2H2O + 2e− Ec0 = 0.49 V versus SHE (1) Anode: Zn(OH)42− + 2e− ↔ Zn + 4OH− Ea0 = −1.215 V versus SHE (2) Zinc dendrite was formerly regarded as the primary cycle life limited causes in zinc-based batteries, however it becomes less serious in zinc-based flow batteries [14]. The flowing electrolyte can prevent zinc dendrite formation issue, which can improve battery performance (especially for durability) compared to traditional zinc nickel battery.
Corresponding author. E-mail address: [email protected] (Z. Xiang).
https://doi.org/10.1016/j.nanoen.2019.06.018 Received 22 March 2019; Received in revised form 15 May 2019; Accepted 8 June 2019 Available online 13 June 2019 2211-2855/ © 2019 Elsevier Ltd. All rights reserved.
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Scheme 1. Schematic representation of a) a pristine zinc nickel flow battery and b) accompanying zinc accumulation process.
charge voltage will turn the side reaction, i.e., OER, at cathode into a valid reaction for energy storage (Scheme 2a), which also increases the specific capacity of the cathode and thereby the energy density. As a proof of concept, the hybrid zinc flow battery (HZFB) delivers excellent long cycle life more than 1100 h without performance degradation, while the energy efficiency of pristine zinc-nickel flow battery reduces by a third. The specific capacity of the cathode also easily improves 2.5 times, which can be further enhanced by raising the charge times. Extendedly, our results show an important approach for zinc-based flow battery to achieve desirable battery performance.
Besides, recent progress shows zinc dendrite can be further suppressed via various approaches, like controlling the transport of zincate ions [15,16], adding additional inorganic and organic agents [17,18] and redistributing the current field [19,20]. Therefore, a 25 kWh ZNFB has been demonstrated to evaluate its reliability [21]. Unfortunately, deposited zinc upon charge cannot be fully consumed during discharge as displayed in Scheme 1b. The amount of accumulated zinc at anode is visible after periodic charge-discharge cycles and spans cathode and anode, provoking a short circuit under practically operating condition. Therefore, the unavoidable zinc accumulation becomes one of the most severe issues shortening the cycle life of the ZNFB under practical situation. Zinc accumulation is caused by the unpaired coulombic efficiency of the anode and the cathode. The coulombic efficiency, defined by the ratio of discharge capacity to charge capacity, of the anode is higher than that of the cathode, suggesting that zinc deposited during charge cannot be fully consumed on discharge as found by our previous study [22] and other research [23]. Thermodynamically, the main competing reactions are hydrogen evolution and zinc corrosion at anode, which are generally inferior to the side reaction, i.e., oxygen evolution, at cathode. This is because that the cathode redox couple (NiOOH/Ni (OH)2) has a sluggish kinetic needing large overpotential and simultaneously possesses a similar oxidized potential to that of the oxygen evolution in aqueous alkaline solution. Although some approaches were proposed to solve zinc accumulation issue by raising the side reactions (namely, hydrogen evolution and zinc corrosion) at anode to match with that of the cathode, it sacrifices the coulombic efficiency of the battery [22]. Therefore, it is still a big challenge to eliminate zinc accumulation without loss of coulombic efficiency of the battery. Herein, we proposed a new concept to eliminate zinc accumulation by in situ coupling another O2/OH− redox couple as shown in Scheme 2 and Equations (3)–(5). Hybrid zinc flow battery:
2. Material and methods 2.1. Chemical and material All chemical and material are directly used without further purification. Zinc oxide was supplied by Shanghai Macklin. KOH was supplied by Shanghai Aladdin Bio-Chem Technology. Carbon fiber paper was supplied by TORAY. The PTFE dispersion was supplied by Shanghai Aladdin Bio-Chem Technology. Carbon black and Pt/C (20 wt %) catalyst were purchased from Shanghai Alfa Aesar. Nafion is provided by ALDRICH. Ltd. A sintered nickel hydroxide electrode with a rated capacity of 20mAh cm−2 was supplied by Jiangsu High star Battery Manufacturing. The metallic nickel sheet was purchased from Shanghai Jin Chang Alloy Co. Ltd. 2.2. Preparation of hybrid cathode The hybrid cathode was consisted by a sintered nickel hydroxide electrode and a Pt/C coated gas diffusion layer (GDL). Carbon black and PTFE at 1: 1 ratio were added into the appropriate amount of ethanol and sonicated for 15 min to prepare a gas diffusion layer suspension. The mixture of carbon black and PTFE suspension was sprayed on the monolithic carbon fiber paper at a loading of 4 mg per square centimeter using an ultrasonic spray coater. Then, GDL was heated at 250 °C for 30 min and 350 °C for 30 min with a heating speed of 5 °C min−1 under Ar atmosphere. After that, a suspension containing commercial 20 wt % Pt/C and nafion solution with a mass ratios of 9:1 in ethanol was sprayed onto the GDL with 0.5 mg Pt loading forming the Pt/C coated gas diffusion layer (GDL) for oxygen reduction as shown in Scheme 2.
Cathode: 2Ni(OH)2 + 2OH− ↔ 2NiOOH + 2H2O + 2e− Ec0 = 0.49 V versus SHE (3) Cathode: 4OH− ↔ O2↑+ 2H2O+ 4e− Ec0 = 0.401 V versus SHE
(4)
Anode: Zn(OH)42− + 2e− ↔ Zn + 4OH− Ea0 = −1.215 V versus SHE (5) Besides the typical reactions in ZNFB, the additional ORR at cathode and unexploited zinc deposits in HZFB constructed a zinc air flow battery (ZAFB) at the end of the discharge process of pristine ZNFB and continuously output power until zinc deposits is totally consumed (Scheme 2b-c). Moreover, considering the charged NiOOH at cathode in pristine ZNFB is also an highly efficient electrocatalyst for oxygen evolution reaction (OER), further raising the charge time or cut off
2.3. Electrochemical measurements The electrochemical measurements were conducted by CHI760e electrochemistry workstation in a typical three-electrode system at room temperature. For the evaluation of the hybrid cathode 2
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Scheme 2. Schematic representation of a hybrid zinc flow battery. a) Charge process, b) discharge process and c) the corresponding surface change of the anode and elimination process of zinc accumulation.
configuration as presented in Scheme 1 and Scheme 2. A sintered nickel hydroxide electrode or a sintered nickel hydroxide electrode coupled with a Pt/C coated GDL was used as the cathode for ZNFB and HZFB, respectively. A polished nickel sheet acted as the anode with the same effective size of the cathode (4 cm2). The electrolyte was composed of 8 M KOH and 0.5 M ZnO, and continuously pumped to the cell. We tested the battery performance through land instrument at constant current charge-discharge process to featured charge capacity and cutoff voltage. The anode potential and cathode potential were also in situ monitored using a self-designed voltage inspecting instrument [12]. The accumulated zinc at anode was investigated by optical camera and scanning electron microscopic instrument (SEM; Hitachi S4700; Hitachi Scientific Instruments Ltd., Japan), using a 20 kV accelerating voltage. Shimadzu AA7000 atomic absorption spectrometer were used to study the zinc accumulation on the anode surface at the end of the first, second and third cycles. Anodes after discharge process were washed with deionized water and dry in a vacuum desiccator before test. All measurements were conducted at room temperature.
performance, the nickel hydroxide electrode was ground into powder, mixed with 20% wt Pt/C catalyst in a ratio of 1: 1, added with proper amount of Nafion and ethanol, and then dropped onto the rotating disk electrode as the working electrode, while metallic nickel sheet acted as the counter electrode and Hg/HgO electrode as the reference electrode (0.098 V vs. standard hydrogen electrode). The electrolyte was 8.0 M KOH and 0.5 M ZnO, the same with battery, which were purged with high purity oxygen or nitrogen gases for at least 30 min before each measurement began. The working electrode was activated by cycling the potential between −0.3 V and 0.6 V at a sweep rate of 100 mV s−1 for 40 cycles. Cyclic voltammetry (CV) was recorded at various sweep rates of 1, 5, 10, 25, 50 and 100 mV s−1. We also conducted a comparative test of the cathode performance under nitrogen conditions at a sweep rate of 10 mV s−1. For the anode performance test, metallic nickel sheet was used as the working electrode, sintered nickelhydroxide as the counter electrode, Hg/HgO electrode as the reference electrode. The electrolyte was also 8.0 M KOH and 0.5 M ZnO, which was purged with high purity nitrogen gas for at least 30 min before each measurement began. CV was performed by cycling the potential between −0.9 and −1.7 V at various sweep rates of 1, 5, 10, 25, 50 and 100 mV s−1. Linear scanning voltammetry (LSV) for pure Ni(OH)2, IrO2 and Pt/C towards OER was measured in 8.0 M KOH and 0.5 M ZnO solution at a scan rate of 5 mV s−1, while LSV curves for pure Ni(OH)2 and Pt/C towards ORR was measured in 1.0 M KOH at a scan rate of 5 mV s−1.
3. Results and discussion To realize this HZFB, we first investigated the electrochemical reactions of the hybrid cathode and anode in a typical three-electrode system in alkaline solution (Fig. 1 and Figs. S1–S3). The hybrid cathode is a commercial nickel hydroxide integrated with Pt/C electrode, while the anode is a metallic nickel sheet. Fig. 1a clearly shows four distinct current peaks at cathode in the presence of O2 atmosphere in the cyclic voltammetry (CV) curves, indicating these dual cathode redox can synchronously react forming hybrid cathodes and batteries. O1 current
2.4. Battery performance measurements The batteries in this paper were assembled in a self-designed battery 3
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Fig. 1. Electrochemical performance of the hybrid cathode. Cyclic voltammetry curves of the hybrid cathode a) in O2 and N2 atmosphere at the scan rate of 10 mV s−1, the dependence of b) anodic peak current and c) cathodic peak current to the scan rate from 1 mV s−1 to 100 mV s−1.
peak and R1 current peak are corresponding to Ni(OH)2→NiOOH and reverse reaction, like in traditional ZNFB, respectively. Additionally, as it is well known that NiOOH is an effective OER catalyst. Further raising the potential gives rise to OER and contributes to the O2 current peak. To further demonstrate the catalytic activity of NiOOH towards OER, we measured LSV curves, together with commercial IrO2 and Pt/C as benchmark catalyst as shown in Fig. S1a. NiOOH displays higher activity than IrO2 and Pt/C towards OER. R2 current peak is related to ORR catalyzed by Pt/C, which is further confirmed by the CV curve in the absence of O2 atmosphere and LSV curves of pure Ni(OH)2 and Pt/C towards ORR in Fig. S1b. This additional O2/OH− redox couple may induce the performance improvement of a ZNFB. To further investigate the behaviour of the hybrid cathode, we recorded CV curves at various scan rates of 1, 5, 10, 25, 50 and 100 mV s−1 in the potential range of −0.3–0.6 V. As shown in Fig. S2, the anodic peak current (O1) and cathodic peak current (R1) both become larger with the increasing scan rate. Their peak currents and the square-root of scan rates are in linear relation (Fig. 1b and c), indicating these reactions are controlled by the diffusion limited mechanism [19]. The zinc deposition and dissolution behaviour was also measured at various scan rates in the same electrolyte using metallic nickel sheet as the working electrode, nickel hydroxide electrode as the counter electrode. The peak current of zinc deposition and dissolution also increases along the enhanced scan rate from 1 to 100 mV s−1 owing to the tardy ionic transport (Fig. S3a). Apart from that, the deposition peak potential shifts from −1.51 to −1.58 V, and the dissolution peak potential changes from −1.33 to −1.25 V. There is a square-root dependence between peak current and scan rate in both reduction and oxidation process (Figs. S3b and S3c), further suggesting the slow diffusion of zincate ions in the interface of anode and electrolyte. Videlicet, we successfully induce an additional redox O2/OH− coupled with NiOOH/Ni(OH)2 as cathode energy storage materials. As both anodic and cathodic processes are limited by the slow ions transport, inducing flowing electrolyte battery system with a combination of ZAFBs and ZNFBs will significantly reduce the polarization and improve the electrochemical performance. Basing on above electrochemical reaction mechanisms, we dexterously integrated dual redox couples into one hybrid unit at cathode and fabricated a HZFB in a self-designed configuration. The battery was firstly charged at 20 mA cm−2 to featured capacity and discharged at 10 mA cm−2 to the cut-off voltage of 0.9 V as shown in Fig. 2. At low charge capacity of 10 mAh cm−2 (Fig. 2a), the battery only displays a typical charge characteristic of a ZNFB with mid-charge voltage of 1.86 V without visible gas bubbles. However, due to the unpaired
coulombic efficiencies of cathode and anode, deposited zinc upon charge process is not totally exhausted in the first discharge process with voltage platform of around 1.66 V. A second narrow discharge voltage platform around 1.0–1.2 V appears, like the discharge process of a ZAFB by reducing oxygen from air to exhaust extra zinc at anode. The new reaction mechanism of the cathode can thoroughly solve zinc accumulation, which is the major cycle life determined issues in ZNFB under practical operation. When the charge capacity is higher than the rated capacity of 20 mAh cm−2, a second platform of charge voltage presents above 1.95 V corresponding to the OER with abundant visible gas bubbles at cathode. Meanwhile, the second discharge voltage platform is longer with the increasing charge capacity, determined by deposited zinc at second charge process in ZAFB and extra deposited zinc or accumulated zinc after the first discharge process in ZNFB. The corresponding cathode potential also displays two charge platform and two discharge platform with the similar tendency with the cell voltage as preconceived (Fig. 2b), while the anode potentials just show one potential platform for charge process and one potential platform for discharge process as known in other typical zinc-based flow batteries (Fig. 2c) [5,22]. These three couples of charge-discharge platforms integrate into the HZFB constituted by a ZNFB and a ZAFB as demonstrated in Scheme 2. This HZFB was also identified with an oxygen switch in Fig. 2d and e. Although charging the battery to 50 mAh cm−2, the battery cannot express the second discharge voltage platform without oxygen supply. When we switch on the oxygen supply, the second discharge voltage emerges, further exhibiting the second discharge process consumes oxygen to output power, like a ZAFB. It can also be seen from Fig. 2f that HZFB has a similar coulombic efficiency around 98% at various charge capacity from 10 mAh cm−2 to 50 mAh cm−2. Unfortunately, the voltage efficiency decreases from 87% at 10 mAh cm−2 to 78% at 50 mAh cm−2 owing to the larger charge-discharge voltage gap contributed by the sluggish kinetics of ORR and OER. As a result, the energy efficiency of HZFB reduces from 86% at 10 mAh cm−2 to 77% at 50 mAh cm−2. Simultaneously, the specific capacity of cathode in HZFB easily reaches 2.5 times as high as that in ZNFB, which can be further improved by controlling the charge time. We also tested discharge performance of the HZFB at different current densities from 5 to 20 mA cm−2. As shown in Fig. S4a, two platforms of discharge voltages are around 1.70 and 1.17 V at 5 mA cm−2, 1.67 and 1.10 V at 10 mA cm−2, and 1.66 V and 1.0 at 15 mA cm−2, 1.64 and 0.94 V at 20 mA cm−2, indicating HZFB has lower discharge voltage in both first and second discharge state at higher current density. In-situ cathode and anode potential monitoring 4
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Fig. 2. Performance of hybrid zinc flow battery. a) Cell voltage, b) cathode potential and c) anode potential profiles of a HZFB charged at 20 mA cm−2 to featured capacity and discharged at 10 mA cm−2 to 0.9 V, d) schematic illustration of air switch and e) the corresponding charge-discharge voltage curve in the presence and absence of air atmosphere, and f) coulombic efficiency, voltage efficiency and energy efficiency of the HZFB at various charged specific capacity.
OH− coupled with NiOOH/Ni(OH)2, HZFB has wonderful cycling stability vigorously operating for more than 1100 h with ignored performance decay, a coulombic efficiency of nearly 100% and a mean energy efficiency of around 78% (Fig. 3f and g). The anode potential emerges a sudden drop at the end of the discharge process indicating zinc was totally consumed without accumulation (Fig. 3h). Fig. 3i and j also present that there is no visible zinc in the gap between anode and cathode, and the surface of the anode at the end of discharge process. The morphology of anode was also studied in conventional ZNFB and the proposed hybrid system after one, two and three cycles by SEM and corresponding accumulated zinc amount was calculated by inductively coupled plasma optical emission spectrometry (ICP). In conventional ZNFB, zinc accumulation occurs from the first cycle with an amount of 3.69 mg (Fig. 4a and Fig. 4d). With increasing charge-discharge cycles, the amount of zinc accumulation increases with a value of 9.98 mg for two cycles and 21.30 mg for three cycles (Fig. 4b,e and Fig. 4c,f). On the contrary, there is no visible zinc on the surface of anode after one, two and three cycles in proposed hybrid zinc flow battery (Fig. 4g-l), which is similar with the surface of the original anode before cycle (Fig. S5). ICP result show only 0.05 mg zinc exists on the surface of anode after three cycles, which is negligible. These data clearly indicate that inducing an additional redox O2/OH to ZNFB not only improves its specific capacity by 2.5 times, but also eliminates zinc accumulation problem and enables it ultra-long cycle life for more than 1100 h without performance degradation.
indicates the polarization of the battery is basically caused by the cathode (Fig. S4b), and the anode potential does not change substantially at different current densities (Fig. S4c). This is because that the kinetic of zinc deposition/dissolution is much faster than that of NiOOH/Ni(OH)2 and O2/OH− [24]. The discharge current density has a more pronounced effect on the second discharge voltage platform due to the lowest kinetic of O2/OH among the three redox couples. So the voltage efficiency, a parameter reflecting the polarization, reduces from 82% at 5 mA cm−2 to 74% at 20 mA cm−2 (Fig. S4d) owing to the large polarization at high operating current, like all rechargeable battery. In contrast, the coulombic efficiency of the battery increases from 96.0% at 5 mA cm−2 to 100% at 20 mA cm−2 benefiting from the less side reaction at higher current density identified by our previous study [25]. As a result, the energy efficiency just slightly reduces from 77.0% at 5 mA cm−2 to 74.0% at 20 mA cm−2. The cycling stability along with zinc accumulation issue was evaluated on ZNFB and HZFB in the same cell configuration as shown in Fig. 3. The batteries were both firstly charged at 20 mA cm−2 to featured capacity and then discharged to the cut-off voltage of 0.4 V, which is more closely to practically applied conditions in large-scale energy storage. Fig. 3a and b clearly shows ZNFB just steadily run for about 440 h and then exhibits a distinct performance degradation in coulombic efficiency and energy efficiency. After 700 h, the energy efficiency reduces by a third to only 50%. We also in-situ monitored the cell voltage, cathode potential and anode potential to elucidate zinc accumulation problem. Generally the unpaired coulombic efficiency of anode and cathode will cause a sudden potential drop of cathode at the end of the discharge process owing to the seriously concentration polarization. In contrast, extra zinc at anode will display a steady discharge curve as shown in Fig. 3c. Additionally, comparing the cell voltage, cathode potential and anode potential of the 22nd cycle with those of 189th cycle, the discharge time shorten by 15% in the 189th cycle, which also indicates zinc are not fully exhausted at this cycle and bring about zinc accumulation in ZNFB. This is also verified by the photo image of the gaps between anode and cathode in Fig. 3d and SEM image of the anode surface in Fig. 3e. By using additional redox O2/
4. Conclusions In summary, we have introduced the new concept of a hybrid zinc flow battery that coupling an additional redox O2/OH− with NiOOH/Ni (OH)2 as cathode energy storage materials. This hybrid flow battery displays two-set charge/discharge voltage plateaus with a first charge process below 1.95 V (like a ZNFB), a second charge process above 1.95 V (like a ZAFB), a first discharge process around 1.7 V (like a ZNFB) and a second discharge process about 1.0–1.2 V (like a ZAFB). The OER and ORR add additional energy to ZNFB, especially ORR can 5
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Fig. 3. Cycling stability along with zinc accumulation issue evaluation on ZNFB and HZFB. a) Cell voltage and b) related coulombic efficiency, voltage efficiency and energy efficiency of a ZNFB under repeated charge-discharge cycles, c) cell voltage, cathode potential and anode potential in two selected cycles, d) photo image of the gap between anode and cathode and e) of SEM image the anode surface, f) cell voltage and g) related coulombic efficiency, voltage efficiency and energy efficiency of a HZFB under repeated charge-discharge cycles, h) cell voltage, cathode potential and anode potential in two selected cycles, i) photo image of the gap between anode and cathode and k) SEM image of the anode surface.
Acknowledgements
consume all the deposited zinc upon charge and completely solve the problem of zinc accumulation in a traditional ZNFB. Therefore, this new concept can eliminate zinc accumulation issue and increase the cathode specific capacity of conventional ZNFB, thus improving its cycle life and energy density. This work shows an ingenious method to solve the problem of unpaired coulombic efficiency in zinc based flow batteries.
This work was supported by the Natural Science Foundation of China (21606015, 51502012, 21676020); the National Key Research and Development Program of China (2017YFA0206500); the Beijing Natural Science Foundation (17L20060, 2162032); the Fundamental Research Funds for the Central Universities (buctrc201524,
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Fig. 4. The morphology of anode at different stages of cycling. a,d) after one cycle, b,e) after two cycles, c,f) after three cycles in conventional ZNFB, g,j) after one cycle, h,k) after two cycles, i,l) after three cycles in proposed hybrid zinc flow battery.
buctrc201420, buctrc201714); BUCT Fund for Disciplines Construction and Development (XK1502); Young Elite Scientists Sponsorship Program by CAST (2017QNRC001); Talent cultivation of State Key Laboratory of Organic-Inorganic Composites; Open project of State Key Laboratory of Organic–Inorganic Composites (OIC-201801007); Distinguished Scientist Program at BUCT (buctylkxj02) and the ‘‘111” Project of China (B14004).
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Yuanhui Cheng received his PhD in 2015 at Dalian Institute of Chemical Physics, Chinese Academy of Sciences (China), and now he is an Associate Professor at Beijing University of Chemical Technology (BUCT, China). His research interests are in fundamental and applicative aspects of energy materials that related to energy conversion and storage processes such as water splitting, fuel cells, metalair batteries and flow batteries.
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Y. Cheng, et al. Ningyuan Zhang received his Bachelor Degree at Beijing University of Chemical Technology in 2017. He is currently a master under the supervision of Associate Professor Yuanhui Cheng at BUCT. His research is focused on the construction of three-phase reaction interface of zinc based flow battery.
Zhijian Liao received his master degree at Southwest Minzu University in 2016. He is now a PhD student in the supervision of Prof. Zhonghua Xiang at BUCT. His current research interests are focused on the design and synthesis of ORR catalysts for application in fuel cells and metal air batteries.
Qiuli Wang received her Bachelor Degree at Beijing University of Chemical Technology in 2016. She is currently a master under the supervision of Prof.Zhonghua Xiang at BUCT. Her research is focused on the design and function regulation of air electrode for zinc air flow battery.
Peng Jiang received his Bachelor Degree at Beijing University Of Chemical Technology in 2017. He is a master under the supervision of Associate Prof. Yuanhui Cheng at BUCT. His main research is the synthesis of electrocatalysts for fuel cells and metal air batteries.
Yinjian Guo received his Bachelor Degree at Liao Cheng University in 2016. He is currently a master supervised by of Associate Professor Yuanhui Cheng at BUCT. His main research is focused on the synthesis of electrocatalysts and their application in metal-air batteries.
Zhonghua Xiang is a professor and director of the Molecular Energy Materials R&D Center at BUCT. He received his PhD in 2013 at BUCT and was a postdoctoral researcher at Case Western Reserve University (2013–2014). His research interests are focused on the design and synthesis of molecular energy materials, mainly including covalent–organic frameworks for fuel cells and flow battery.
Shuo Tao received his PhD at Dalian Institute of Chemical Physics, Chinese Academy of Sciences (China) in 2016. Now he is a lecturer in College of Chemistry and Chemical Engineering at the Liaocheng University (China). His research interests are focused on synthesis of zeolites and other porous materials and their catalytic applications.
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A long-life hybrid zinc flow battery achieved by dual redox couples at cathode
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Yuanhui Chenga, Ningyuan Zhanga, Qiuli Wanga, Yinjian Guoa, Shuo Taob, Zhijian Liaoa, Peng Jianga, Zhonghua Xianga,* a
Beijing Advanced Innovation Center for Soft Matter Science and Engineering, State Key Laboratory of Organic-Inorganic Composites, College of Chemical Engineering, College of Energy, Beijing University of Chemical Technology, Beijing, 100029, PR China b School of Chemistry and Chemical Engineering, Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology, Liaocheng University, Liaocheng, Shandong, 252059, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Zinc accumulation Oxygen redox reaction Energy storage Zinc nickel flow battery Zinc air flow battery
Zinc nickel flow battery is one of the most promising energy storage technologies for intermittently renewable solar and wind power. However, unpaired coulombic efficiency of nickel hydroxide cathode and zinc anode causes zinc accumulation in practical operation, which shortens the cycle life and impedes the commercialization of the battery. Here, we induce an additional O2/OH− redox couple integrated with NiOOH/Ni(OH)2 as cathode energy storage materials. Benefit from the new reaction mechanism of the hybrid cathode, deposited zinc upon charge can be fully consumed during discharge, which completely avoids zinc accumulation issue. The new designed battery vigorously operates for more than 1100 h with negligible performance degradation, while the energy efficiency of pristine zinc-nickel flow battery dramatically reduces after 440 h. More importantly, the specific capacity of the cathode synchronously increases by 2.5 times and thereby the energy density. This system dexterously integrates dual redox couples into one hybrid unit at cathode complementing advantages of one another, demonstrating a broader method to solve the problem of unpaired coulombic efficiency in zinc based flow batteries.
1. Introduction Flow batteries are considered as one of the most promising large scale energy storage technologies to increase the utilization of intermittent renewable power from wind and solar owning to the inherent merits of low maintenance cost, high safety, independence of power and capacity and long cycle life [1–3]. Among various flow battery technologies, zinc-based flow batteries composed of a zinc/zinc ion redox couple in acid or alkaline electrolytes and another redox couple, display a high output voltage, high theoretical energy density, fast kinetics, abundance and recyclability of zinc compounds and extremely low cost, and have such warranted special attention [4–8]. Beyond pioneer zinc halogen flow battery [9–11], zinc nickel flow battery (ZNFB) has been widely studied original from the membrane-less structure, which eliminates the common problems of electrolyte crossover and the utilization of high cost membranes in traditional flow batteries [12,13]. Importantly, all original chemicals or materials are abundant, cheap and nontoxic, endowing this technology a broad application prospect in large scale energy storage. The working
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mechanism of a ZNFB is similar to both a battery and a fuel cell as shown in Scheme 1a. As described in Equation (1), at cathode, Ni(OH)2 is oxidized to NiOOH during charge and reverse reaction occurs upon discharge, like a battery. Meanwhile, zincate ions in the continuously flowing electrolyte will be reduced to metallic zinc at anode during charge, while metallic zinc can be oxidized to zincate ions into the circulated electrolyte upon discharge, like a fuel cell (Equation (2)). Pristine zinc nickel flow battery: Cathode: 2Ni(OH)2 + 2OH− ↔ 2NiOOH + 2H2O + 2e− Ec0 = 0.49 V versus SHE (1) Anode: Zn(OH)42− + 2e− ↔ Zn + 4OH− Ea0 = −1.215 V versus SHE (2) Zinc dendrite was formerly regarded as the primary cycle life limited causes in zinc-based batteries, however it becomes less serious in zinc-based flow batteries [14]. The flowing electrolyte can prevent zinc dendrite formation issue, which can improve battery performance (especially for durability) compared to traditional zinc nickel battery.
Corresponding author. E-mail address: [email protected] (Z. Xiang).
https://doi.org/10.1016/j.nanoen.2019.06.018 Received 22 March 2019; Received in revised form 15 May 2019; Accepted 8 June 2019 Available online 13 June 2019 2211-2855/ © 2019 Elsevier Ltd. All rights reserved.
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Scheme 1. Schematic representation of a) a pristine zinc nickel flow battery and b) accompanying zinc accumulation process.
charge voltage will turn the side reaction, i.e., OER, at cathode into a valid reaction for energy storage (Scheme 2a), which also increases the specific capacity of the cathode and thereby the energy density. As a proof of concept, the hybrid zinc flow battery (HZFB) delivers excellent long cycle life more than 1100 h without performance degradation, while the energy efficiency of pristine zinc-nickel flow battery reduces by a third. The specific capacity of the cathode also easily improves 2.5 times, which can be further enhanced by raising the charge times. Extendedly, our results show an important approach for zinc-based flow battery to achieve desirable battery performance.
Besides, recent progress shows zinc dendrite can be further suppressed via various approaches, like controlling the transport of zincate ions [15,16], adding additional inorganic and organic agents [17,18] and redistributing the current field [19,20]. Therefore, a 25 kWh ZNFB has been demonstrated to evaluate its reliability [21]. Unfortunately, deposited zinc upon charge cannot be fully consumed during discharge as displayed in Scheme 1b. The amount of accumulated zinc at anode is visible after periodic charge-discharge cycles and spans cathode and anode, provoking a short circuit under practically operating condition. Therefore, the unavoidable zinc accumulation becomes one of the most severe issues shortening the cycle life of the ZNFB under practical situation. Zinc accumulation is caused by the unpaired coulombic efficiency of the anode and the cathode. The coulombic efficiency, defined by the ratio of discharge capacity to charge capacity, of the anode is higher than that of the cathode, suggesting that zinc deposited during charge cannot be fully consumed on discharge as found by our previous study [22] and other research [23]. Thermodynamically, the main competing reactions are hydrogen evolution and zinc corrosion at anode, which are generally inferior to the side reaction, i.e., oxygen evolution, at cathode. This is because that the cathode redox couple (NiOOH/Ni (OH)2) has a sluggish kinetic needing large overpotential and simultaneously possesses a similar oxidized potential to that of the oxygen evolution in aqueous alkaline solution. Although some approaches were proposed to solve zinc accumulation issue by raising the side reactions (namely, hydrogen evolution and zinc corrosion) at anode to match with that of the cathode, it sacrifices the coulombic efficiency of the battery [22]. Therefore, it is still a big challenge to eliminate zinc accumulation without loss of coulombic efficiency of the battery. Herein, we proposed a new concept to eliminate zinc accumulation by in situ coupling another O2/OH− redox couple as shown in Scheme 2 and Equations (3)–(5). Hybrid zinc flow battery:
2. Material and methods 2.1. Chemical and material All chemical and material are directly used without further purification. Zinc oxide was supplied by Shanghai Macklin. KOH was supplied by Shanghai Aladdin Bio-Chem Technology. Carbon fiber paper was supplied by TORAY. The PTFE dispersion was supplied by Shanghai Aladdin Bio-Chem Technology. Carbon black and Pt/C (20 wt %) catalyst were purchased from Shanghai Alfa Aesar. Nafion is provided by ALDRICH. Ltd. A sintered nickel hydroxide electrode with a rated capacity of 20mAh cm−2 was supplied by Jiangsu High star Battery Manufacturing. The metallic nickel sheet was purchased from Shanghai Jin Chang Alloy Co. Ltd. 2.2. Preparation of hybrid cathode The hybrid cathode was consisted by a sintered nickel hydroxide electrode and a Pt/C coated gas diffusion layer (GDL). Carbon black and PTFE at 1: 1 ratio were added into the appropriate amount of ethanol and sonicated for 15 min to prepare a gas diffusion layer suspension. The mixture of carbon black and PTFE suspension was sprayed on the monolithic carbon fiber paper at a loading of 4 mg per square centimeter using an ultrasonic spray coater. Then, GDL was heated at 250 °C for 30 min and 350 °C for 30 min with a heating speed of 5 °C min−1 under Ar atmosphere. After that, a suspension containing commercial 20 wt % Pt/C and nafion solution with a mass ratios of 9:1 in ethanol was sprayed onto the GDL with 0.5 mg Pt loading forming the Pt/C coated gas diffusion layer (GDL) for oxygen reduction as shown in Scheme 2.
Cathode: 2Ni(OH)2 + 2OH− ↔ 2NiOOH + 2H2O + 2e− Ec0 = 0.49 V versus SHE (3) Cathode: 4OH− ↔ O2↑+ 2H2O+ 4e− Ec0 = 0.401 V versus SHE
(4)
Anode: Zn(OH)42− + 2e− ↔ Zn + 4OH− Ea0 = −1.215 V versus SHE (5) Besides the typical reactions in ZNFB, the additional ORR at cathode and unexploited zinc deposits in HZFB constructed a zinc air flow battery (ZAFB) at the end of the discharge process of pristine ZNFB and continuously output power until zinc deposits is totally consumed (Scheme 2b-c). Moreover, considering the charged NiOOH at cathode in pristine ZNFB is also an highly efficient electrocatalyst for oxygen evolution reaction (OER), further raising the charge time or cut off
2.3. Electrochemical measurements The electrochemical measurements were conducted by CHI760e electrochemistry workstation in a typical three-electrode system at room temperature. For the evaluation of the hybrid cathode 2
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Scheme 2. Schematic representation of a hybrid zinc flow battery. a) Charge process, b) discharge process and c) the corresponding surface change of the anode and elimination process of zinc accumulation.
configuration as presented in Scheme 1 and Scheme 2. A sintered nickel hydroxide electrode or a sintered nickel hydroxide electrode coupled with a Pt/C coated GDL was used as the cathode for ZNFB and HZFB, respectively. A polished nickel sheet acted as the anode with the same effective size of the cathode (4 cm2). The electrolyte was composed of 8 M KOH and 0.5 M ZnO, and continuously pumped to the cell. We tested the battery performance through land instrument at constant current charge-discharge process to featured charge capacity and cutoff voltage. The anode potential and cathode potential were also in situ monitored using a self-designed voltage inspecting instrument [12]. The accumulated zinc at anode was investigated by optical camera and scanning electron microscopic instrument (SEM; Hitachi S4700; Hitachi Scientific Instruments Ltd., Japan), using a 20 kV accelerating voltage. Shimadzu AA7000 atomic absorption spectrometer were used to study the zinc accumulation on the anode surface at the end of the first, second and third cycles. Anodes after discharge process were washed with deionized water and dry in a vacuum desiccator before test. All measurements were conducted at room temperature.
performance, the nickel hydroxide electrode was ground into powder, mixed with 20% wt Pt/C catalyst in a ratio of 1: 1, added with proper amount of Nafion and ethanol, and then dropped onto the rotating disk electrode as the working electrode, while metallic nickel sheet acted as the counter electrode and Hg/HgO electrode as the reference electrode (0.098 V vs. standard hydrogen electrode). The electrolyte was 8.0 M KOH and 0.5 M ZnO, the same with battery, which were purged with high purity oxygen or nitrogen gases for at least 30 min before each measurement began. The working electrode was activated by cycling the potential between −0.3 V and 0.6 V at a sweep rate of 100 mV s−1 for 40 cycles. Cyclic voltammetry (CV) was recorded at various sweep rates of 1, 5, 10, 25, 50 and 100 mV s−1. We also conducted a comparative test of the cathode performance under nitrogen conditions at a sweep rate of 10 mV s−1. For the anode performance test, metallic nickel sheet was used as the working electrode, sintered nickelhydroxide as the counter electrode, Hg/HgO electrode as the reference electrode. The electrolyte was also 8.0 M KOH and 0.5 M ZnO, which was purged with high purity nitrogen gas for at least 30 min before each measurement began. CV was performed by cycling the potential between −0.9 and −1.7 V at various sweep rates of 1, 5, 10, 25, 50 and 100 mV s−1. Linear scanning voltammetry (LSV) for pure Ni(OH)2, IrO2 and Pt/C towards OER was measured in 8.0 M KOH and 0.5 M ZnO solution at a scan rate of 5 mV s−1, while LSV curves for pure Ni(OH)2 and Pt/C towards ORR was measured in 1.0 M KOH at a scan rate of 5 mV s−1.
3. Results and discussion To realize this HZFB, we first investigated the electrochemical reactions of the hybrid cathode and anode in a typical three-electrode system in alkaline solution (Fig. 1 and Figs. S1–S3). The hybrid cathode is a commercial nickel hydroxide integrated with Pt/C electrode, while the anode is a metallic nickel sheet. Fig. 1a clearly shows four distinct current peaks at cathode in the presence of O2 atmosphere in the cyclic voltammetry (CV) curves, indicating these dual cathode redox can synchronously react forming hybrid cathodes and batteries. O1 current
2.4. Battery performance measurements The batteries in this paper were assembled in a self-designed battery 3
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Fig. 1. Electrochemical performance of the hybrid cathode. Cyclic voltammetry curves of the hybrid cathode a) in O2 and N2 atmosphere at the scan rate of 10 mV s−1, the dependence of b) anodic peak current and c) cathodic peak current to the scan rate from 1 mV s−1 to 100 mV s−1.
peak and R1 current peak are corresponding to Ni(OH)2→NiOOH and reverse reaction, like in traditional ZNFB, respectively. Additionally, as it is well known that NiOOH is an effective OER catalyst. Further raising the potential gives rise to OER and contributes to the O2 current peak. To further demonstrate the catalytic activity of NiOOH towards OER, we measured LSV curves, together with commercial IrO2 and Pt/C as benchmark catalyst as shown in Fig. S1a. NiOOH displays higher activity than IrO2 and Pt/C towards OER. R2 current peak is related to ORR catalyzed by Pt/C, which is further confirmed by the CV curve in the absence of O2 atmosphere and LSV curves of pure Ni(OH)2 and Pt/C towards ORR in Fig. S1b. This additional O2/OH− redox couple may induce the performance improvement of a ZNFB. To further investigate the behaviour of the hybrid cathode, we recorded CV curves at various scan rates of 1, 5, 10, 25, 50 and 100 mV s−1 in the potential range of −0.3–0.6 V. As shown in Fig. S2, the anodic peak current (O1) and cathodic peak current (R1) both become larger with the increasing scan rate. Their peak currents and the square-root of scan rates are in linear relation (Fig. 1b and c), indicating these reactions are controlled by the diffusion limited mechanism [19]. The zinc deposition and dissolution behaviour was also measured at various scan rates in the same electrolyte using metallic nickel sheet as the working electrode, nickel hydroxide electrode as the counter electrode. The peak current of zinc deposition and dissolution also increases along the enhanced scan rate from 1 to 100 mV s−1 owing to the tardy ionic transport (Fig. S3a). Apart from that, the deposition peak potential shifts from −1.51 to −1.58 V, and the dissolution peak potential changes from −1.33 to −1.25 V. There is a square-root dependence between peak current and scan rate in both reduction and oxidation process (Figs. S3b and S3c), further suggesting the slow diffusion of zincate ions in the interface of anode and electrolyte. Videlicet, we successfully induce an additional redox O2/OH− coupled with NiOOH/Ni(OH)2 as cathode energy storage materials. As both anodic and cathodic processes are limited by the slow ions transport, inducing flowing electrolyte battery system with a combination of ZAFBs and ZNFBs will significantly reduce the polarization and improve the electrochemical performance. Basing on above electrochemical reaction mechanisms, we dexterously integrated dual redox couples into one hybrid unit at cathode and fabricated a HZFB in a self-designed configuration. The battery was firstly charged at 20 mA cm−2 to featured capacity and discharged at 10 mA cm−2 to the cut-off voltage of 0.9 V as shown in Fig. 2. At low charge capacity of 10 mAh cm−2 (Fig. 2a), the battery only displays a typical charge characteristic of a ZNFB with mid-charge voltage of 1.86 V without visible gas bubbles. However, due to the unpaired
coulombic efficiencies of cathode and anode, deposited zinc upon charge process is not totally exhausted in the first discharge process with voltage platform of around 1.66 V. A second narrow discharge voltage platform around 1.0–1.2 V appears, like the discharge process of a ZAFB by reducing oxygen from air to exhaust extra zinc at anode. The new reaction mechanism of the cathode can thoroughly solve zinc accumulation, which is the major cycle life determined issues in ZNFB under practical operation. When the charge capacity is higher than the rated capacity of 20 mAh cm−2, a second platform of charge voltage presents above 1.95 V corresponding to the OER with abundant visible gas bubbles at cathode. Meanwhile, the second discharge voltage platform is longer with the increasing charge capacity, determined by deposited zinc at second charge process in ZAFB and extra deposited zinc or accumulated zinc after the first discharge process in ZNFB. The corresponding cathode potential also displays two charge platform and two discharge platform with the similar tendency with the cell voltage as preconceived (Fig. 2b), while the anode potentials just show one potential platform for charge process and one potential platform for discharge process as known in other typical zinc-based flow batteries (Fig. 2c) [5,22]. These three couples of charge-discharge platforms integrate into the HZFB constituted by a ZNFB and a ZAFB as demonstrated in Scheme 2. This HZFB was also identified with an oxygen switch in Fig. 2d and e. Although charging the battery to 50 mAh cm−2, the battery cannot express the second discharge voltage platform without oxygen supply. When we switch on the oxygen supply, the second discharge voltage emerges, further exhibiting the second discharge process consumes oxygen to output power, like a ZAFB. It can also be seen from Fig. 2f that HZFB has a similar coulombic efficiency around 98% at various charge capacity from 10 mAh cm−2 to 50 mAh cm−2. Unfortunately, the voltage efficiency decreases from 87% at 10 mAh cm−2 to 78% at 50 mAh cm−2 owing to the larger charge-discharge voltage gap contributed by the sluggish kinetics of ORR and OER. As a result, the energy efficiency of HZFB reduces from 86% at 10 mAh cm−2 to 77% at 50 mAh cm−2. Simultaneously, the specific capacity of cathode in HZFB easily reaches 2.5 times as high as that in ZNFB, which can be further improved by controlling the charge time. We also tested discharge performance of the HZFB at different current densities from 5 to 20 mA cm−2. As shown in Fig. S4a, two platforms of discharge voltages are around 1.70 and 1.17 V at 5 mA cm−2, 1.67 and 1.10 V at 10 mA cm−2, and 1.66 V and 1.0 at 15 mA cm−2, 1.64 and 0.94 V at 20 mA cm−2, indicating HZFB has lower discharge voltage in both first and second discharge state at higher current density. In-situ cathode and anode potential monitoring 4
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Fig. 2. Performance of hybrid zinc flow battery. a) Cell voltage, b) cathode potential and c) anode potential profiles of a HZFB charged at 20 mA cm−2 to featured capacity and discharged at 10 mA cm−2 to 0.9 V, d) schematic illustration of air switch and e) the corresponding charge-discharge voltage curve in the presence and absence of air atmosphere, and f) coulombic efficiency, voltage efficiency and energy efficiency of the HZFB at various charged specific capacity.
OH− coupled with NiOOH/Ni(OH)2, HZFB has wonderful cycling stability vigorously operating for more than 1100 h with ignored performance decay, a coulombic efficiency of nearly 100% and a mean energy efficiency of around 78% (Fig. 3f and g). The anode potential emerges a sudden drop at the end of the discharge process indicating zinc was totally consumed without accumulation (Fig. 3h). Fig. 3i and j also present that there is no visible zinc in the gap between anode and cathode, and the surface of the anode at the end of discharge process. The morphology of anode was also studied in conventional ZNFB and the proposed hybrid system after one, two and three cycles by SEM and corresponding accumulated zinc amount was calculated by inductively coupled plasma optical emission spectrometry (ICP). In conventional ZNFB, zinc accumulation occurs from the first cycle with an amount of 3.69 mg (Fig. 4a and Fig. 4d). With increasing charge-discharge cycles, the amount of zinc accumulation increases with a value of 9.98 mg for two cycles and 21.30 mg for three cycles (Fig. 4b,e and Fig. 4c,f). On the contrary, there is no visible zinc on the surface of anode after one, two and three cycles in proposed hybrid zinc flow battery (Fig. 4g-l), which is similar with the surface of the original anode before cycle (Fig. S5). ICP result show only 0.05 mg zinc exists on the surface of anode after three cycles, which is negligible. These data clearly indicate that inducing an additional redox O2/OH to ZNFB not only improves its specific capacity by 2.5 times, but also eliminates zinc accumulation problem and enables it ultra-long cycle life for more than 1100 h without performance degradation.
indicates the polarization of the battery is basically caused by the cathode (Fig. S4b), and the anode potential does not change substantially at different current densities (Fig. S4c). This is because that the kinetic of zinc deposition/dissolution is much faster than that of NiOOH/Ni(OH)2 and O2/OH− [24]. The discharge current density has a more pronounced effect on the second discharge voltage platform due to the lowest kinetic of O2/OH among the three redox couples. So the voltage efficiency, a parameter reflecting the polarization, reduces from 82% at 5 mA cm−2 to 74% at 20 mA cm−2 (Fig. S4d) owing to the large polarization at high operating current, like all rechargeable battery. In contrast, the coulombic efficiency of the battery increases from 96.0% at 5 mA cm−2 to 100% at 20 mA cm−2 benefiting from the less side reaction at higher current density identified by our previous study [25]. As a result, the energy efficiency just slightly reduces from 77.0% at 5 mA cm−2 to 74.0% at 20 mA cm−2. The cycling stability along with zinc accumulation issue was evaluated on ZNFB and HZFB in the same cell configuration as shown in Fig. 3. The batteries were both firstly charged at 20 mA cm−2 to featured capacity and then discharged to the cut-off voltage of 0.4 V, which is more closely to practically applied conditions in large-scale energy storage. Fig. 3a and b clearly shows ZNFB just steadily run for about 440 h and then exhibits a distinct performance degradation in coulombic efficiency and energy efficiency. After 700 h, the energy efficiency reduces by a third to only 50%. We also in-situ monitored the cell voltage, cathode potential and anode potential to elucidate zinc accumulation problem. Generally the unpaired coulombic efficiency of anode and cathode will cause a sudden potential drop of cathode at the end of the discharge process owing to the seriously concentration polarization. In contrast, extra zinc at anode will display a steady discharge curve as shown in Fig. 3c. Additionally, comparing the cell voltage, cathode potential and anode potential of the 22nd cycle with those of 189th cycle, the discharge time shorten by 15% in the 189th cycle, which also indicates zinc are not fully exhausted at this cycle and bring about zinc accumulation in ZNFB. This is also verified by the photo image of the gaps between anode and cathode in Fig. 3d and SEM image of the anode surface in Fig. 3e. By using additional redox O2/
4. Conclusions In summary, we have introduced the new concept of a hybrid zinc flow battery that coupling an additional redox O2/OH− with NiOOH/Ni (OH)2 as cathode energy storage materials. This hybrid flow battery displays two-set charge/discharge voltage plateaus with a first charge process below 1.95 V (like a ZNFB), a second charge process above 1.95 V (like a ZAFB), a first discharge process around 1.7 V (like a ZNFB) and a second discharge process about 1.0–1.2 V (like a ZAFB). The OER and ORR add additional energy to ZNFB, especially ORR can 5
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Fig. 3. Cycling stability along with zinc accumulation issue evaluation on ZNFB and HZFB. a) Cell voltage and b) related coulombic efficiency, voltage efficiency and energy efficiency of a ZNFB under repeated charge-discharge cycles, c) cell voltage, cathode potential and anode potential in two selected cycles, d) photo image of the gap between anode and cathode and e) of SEM image the anode surface, f) cell voltage and g) related coulombic efficiency, voltage efficiency and energy efficiency of a HZFB under repeated charge-discharge cycles, h) cell voltage, cathode potential and anode potential in two selected cycles, i) photo image of the gap between anode and cathode and k) SEM image of the anode surface.
Acknowledgements
consume all the deposited zinc upon charge and completely solve the problem of zinc accumulation in a traditional ZNFB. Therefore, this new concept can eliminate zinc accumulation issue and increase the cathode specific capacity of conventional ZNFB, thus improving its cycle life and energy density. This work shows an ingenious method to solve the problem of unpaired coulombic efficiency in zinc based flow batteries.
This work was supported by the Natural Science Foundation of China (21606015, 51502012, 21676020); the National Key Research and Development Program of China (2017YFA0206500); the Beijing Natural Science Foundation (17L20060, 2162032); the Fundamental Research Funds for the Central Universities (buctrc201524,
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Fig. 4. The morphology of anode at different stages of cycling. a,d) after one cycle, b,e) after two cycles, c,f) after three cycles in conventional ZNFB, g,j) after one cycle, h,k) after two cycles, i,l) after three cycles in proposed hybrid zinc flow battery.
buctrc201420, buctrc201714); BUCT Fund for Disciplines Construction and Development (XK1502); Young Elite Scientists Sponsorship Program by CAST (2017QNRC001); Talent cultivation of State Key Laboratory of Organic-Inorganic Composites; Open project of State Key Laboratory of Organic–Inorganic Composites (OIC-201801007); Distinguished Scientist Program at BUCT (buctylkxj02) and the ‘‘111” Project of China (B14004).
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Yuanhui Cheng received his PhD in 2015 at Dalian Institute of Chemical Physics, Chinese Academy of Sciences (China), and now he is an Associate Professor at Beijing University of Chemical Technology (BUCT, China). His research interests are in fundamental and applicative aspects of energy materials that related to energy conversion and storage processes such as water splitting, fuel cells, metalair batteries and flow batteries.
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Nano Energy 63 (2019) 103822
Y. Cheng, et al. Ningyuan Zhang received his Bachelor Degree at Beijing University of Chemical Technology in 2017. He is currently a master under the supervision of Associate Professor Yuanhui Cheng at BUCT. His research is focused on the construction of three-phase reaction interface of zinc based flow battery.
Zhijian Liao received his master degree at Southwest Minzu University in 2016. He is now a PhD student in the supervision of Prof. Zhonghua Xiang at BUCT. His current research interests are focused on the design and synthesis of ORR catalysts for application in fuel cells and metal air batteries.
Qiuli Wang received her Bachelor Degree at Beijing University of Chemical Technology in 2016. She is currently a master under the supervision of Prof.Zhonghua Xiang at BUCT. Her research is focused on the design and function regulation of air electrode for zinc air flow battery.
Peng Jiang received his Bachelor Degree at Beijing University Of Chemical Technology in 2017. He is a master under the supervision of Associate Prof. Yuanhui Cheng at BUCT. His main research is the synthesis of electrocatalysts for fuel cells and metal air batteries.
Yinjian Guo received his Bachelor Degree at Liao Cheng University in 2016. He is currently a master supervised by of Associate Professor Yuanhui Cheng at BUCT. His main research is focused on the synthesis of electrocatalysts and their application in metal-air batteries.
Zhonghua Xiang is a professor and director of the Molecular Energy Materials R&D Center at BUCT. He received his PhD in 2013 at BUCT and was a postdoctoral researcher at Case Western Reserve University (2013–2014). His research interests are focused on the design and synthesis of molecular energy materials, mainly including covalent–organic frameworks for fuel cells and flow battery.
Shuo Tao received his PhD at Dalian Institute of Chemical Physics, Chinese Academy of Sciences (China) in 2016. Now he is a lecturer in College of Chemistry and Chemical Engineering at the Liaocheng University (China). His research interests are focused on synthesis of zeolites and other porous materials and their catalytic applications.
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