- Email: [email protected]
Aqueous based dual-electrolyte rechargeable Pb–Zn battery with a 2.8 V operating voltage
Journal of Energy Storage 29 (2020) 101305
Contents lists available at ScienceDirect
Journal of Energy Storage journal homepage: www.elsevier.com/locate/est
Aqueous based dual-electrolyte rechargeable Pb–Zn battery with a 2.8 V operating voltage
T
Haoran Wu , Szu-Jia Liu, Keryn Lian ⁎
Department of Materials Science and Engineering, University of Toronto, Toronto, Ontario M5S 3E4, Canada
ARTICLE INFO
ABSTRACT
Keywords: PbO2-Zn battery High voltage Dual-electrolyte Aqueous based Rechargeable
A novel rechargeable PbO2-Zn battery system using Zn as negative and PbO2 as positive electrodes in a KOHH2SO4 dual-electrolyte is developed. Using a bi-polar ion exchange membrane that effectively prevents the H+ and OH− from neutralization, the positive and negative electrodes operate in the respective electrolytes with different pH. The Zn electrode provides a low potential of −1.3 V in KOH solution and the PbO2 has a high potential of +1.5 V in H2SO4 solution. This resulted in a cell operating voltage of 2.8 V in an aqueous electrolyte. The system also demonstrated a high rate performance and good rechargeability. This simple, low-cost system not only overcomes the thermodynamic limit of water decomposition and achieves a high voltage aqueous based battery, but also provides a new approach in design of future electrochemical devices.
1. Introduction Aqueous based energy storage devices are generally safer, nonflammable and less expensive compared to their organic based counterparts [1,2]. The operating voltage of these devices, however, is limited by the water decomposition potential window [3,4]. As a result, the energy and power densities of aqueous based devices are relatively low. To extend the operating voltage window of aqueous based energy storage devices, Frackowiak et al. introduced a dual-electrolyte configuration for aqueous based supercapacitors [5,6]. These devices were constructed in a 3-compartment cell with a buffer solution in between acid and alkaline. In other studies, an ion exchange membrane (IEM) was applied to suppress the acid-base neutralization by blocking the cross-diffusion of H+ and OH− [1,6,7], so that the pH value of each chamber remained constant. This concept was demonstrated on activated carbon (AC) symmetrical supercapacitor systems with high cell voltages of 1.8 V [1] and 2.1 V [6]. In a further study, asymmetrical electrodes and dual-electrolyte were integrated to extend the cell voltage. In a battery-capacitor hybrid device with dual-electrolyte, both positive and negative electrodes achieved their equilibrium potentials in their respective electrolytes, leading to a high operating voltage of 2.4 V [7]. If these dual-electrolyte configuration approaches can be applied in other energy storage systems such as lead-acid and alkaline batteries, their voltage may also be significantly enhanced.
⁎
Zn and Pb based battery electrodes are very common in conventional batteries [8]. These batteries are widely used in our daily life for their low-cost and good performance [9,10]. Although Pb is relatively toxic, it is highly recycled [11]. Alkaline and lead acid batteries have cell voltages of 1.5 V and 2.1 V, respectively. Both are higher than that of the water decomposition voltage, due to the high overpotentials of Zn [12] and PbO2 [13] can lower the hydrogen and oxygen evolution, respectively. However, since Zn only operates in alkaline [14] and Pb only works in acid [15], these distinct battery chemistries have never been considered within a single system. In this work, for the first time, we integrated a Zn negative electrode and a PbO2 positive electrode in a dual-electrolyte system to form a battery. In this system, both electrodes can operate in their preferred stable operating environments, leading to a 2.8 V operating voltage in an aqueous based system. 2.Experimental 2.1. Electrodes A thin Ti foil (McMaster-Carr, 127 um thick, Canada) was used as current collector for the Pb based positive electrode. The Ti foil was ultrasonically cleaned in DI water. The Ti surface was cleaned again by isopropyl alcohol (IPA) before electrode coating. A precursor solution was prepared of dissolving 2 g SnCl4 (Alfa Aesar, 98%, US), 0.2 g of
Corresponding author. E-mail addresses: [email protected] (H. Wu), [email protected] (K. Lian).
https://doi.org/10.1016/j.est.2020.101305 Received 26 September 2019; Received in revised form 23 January 2020; Accepted 17 February 2020 2352-152X/ © 2020 Elsevier Ltd. All rights reserved.
Journal of Energy Storage 29 (2020) 101305
H. Wu, et al.
Sb2O3 (Alfa Aesar, 99%, US) and 1.3 ml of concentrated HCl (Caledon, 36.5–38%, Canada) into 20 ml IPA. The Ti substrate was dipped into the precursor solution and dried at 120 °C for 15 min. The dip coating and drying process was repeated for 5 times and then calcinated in a muffle furnace (MTI corp., KSL-1100X-S, US) at 550 °C for 1 h to create a thin SnO2-Sb2O3 for the following electrodeposition of PbO2 [16,17]. The SnO2-Sb2O3 coated Ti foil was placed to an electroplating bath comprised of 1 M HNO3 (Caledon, 68–70%, Canada) and 0.5 M Pb (NO3)2 (Alfa Aesar, 99%, US) to deposit PbO2 [18,19]. The electrodeposition was conducted in a pulse mode: ton = 2s, toff = 2 s, at a current density of 1 mA cm−2 for 750 cycles or 3000 s, so that a black PbO2 coating was deposited on the substrate. The positive electrode had an apparent area of 1 cm2 exposed to the electrolyte with an effective loading of 2 mg cm−2 PbO2, leading to an expected capacity of 0.05 mAh cm−2. A thin Zn foil (Alibaba, 25 μm thick, China) was used as the negative electrode, which was ultrasonically cleaned in IPA and DI water prior to its application. The negative electrode had an effective loading of 18 mg cm−2 Zn and with an apparent electrode area of 1 cm−2. The theoretical capacity of the Zn electrode is 14.8 mAh cm−2 [20]. Since the Zn foil was used as both current collector and electrode, the cell was designed such that the Zn had much higher capacity than that of the PbO2. Accordingly, the cell capacity was predominated by PbO2 electrode.
3. Results and discussion 3.1. PbO2-Zn battery concept Prior to electrochemical characterizations, the pH value in each chamber was monitored over time to ensure the stability of the dualelectrolyte in the cell (Fig. 1b). The pH values in the acid and alkaline chambers were −1 and 14.5, resulting in a 15.5 pH difference between the two-side of the bi-polar IEM. The pH values of the two chambers were tracked for a 17 days period and appeared to be constant throughout the time (Fig. 1b). This suggests that the bi-polar IEM can effectively block the diffusion of proton and hydroxide ions, so that no neutralization occurred. The individual electrode performance was first investigated separately in a 3-electrode cell, where Zn electrode was in alkaline (3 M KOH) and PbO2 electrode in acid (5 M H2SO4) electrolyte. The CV characterizations were conducted in their respective redox active potential window at a scan rate of 10 mVs−1. Both electrodes showed typical battery CV profiles with pronounced redox reactions peaks (Fig. 2) [19,22]. The reaction of Zn in strong alkaline media is expressed by reaction 1 [23]:
Zn + 4OH
(1)
Zn(OH)4 2 + 2e
The reaction of PbO2 in strong H2SO4 is shown in reaction 2 [24]:
PbO2 + 4H+ + SO4 2 + 2e 2.2. Cell setup
(2)
PbSO4 + 2H2 O
When combining the two half-cell CV profiles in a single plot (Fig. 2), a maximum potential window between the two electrodes is expected to be around 2.V. This can potentially lead to a new full-cell battery chemistry in reaction 3:
The individual electrode characterizations of PbO2 and Zn were conducted in a 3-electrode cell using either 5 M H2SO4 (Caledon, 95–98%, Canada) for PbO2 or 3 M KOH (Alfa Aesar, 85%, US) for Zn. An Ag/AgCl electrode was used as reference in acid and a Hg/HgO reference was used in alkaline solutions. All potentials were converted to against the standard hydrogen electrode (SHE) for clear comparison. A 2-electrode dual-electrolyte cell setup illustrated in Fig. 1a was assembled with PbO2 electrode in a flask containing 5 M H2SO4 and Zn electrode in another with 3 M KOH. The two flasks were separated by a bi-polar IEM comprised of a layer of commercial cationic IEM (Fumasep, FKB-PK-130, US) and a layer of anionic IEM (Fumasep, FAA-3PK-75, US). The cationic IEM was facing the KOH compartment while the anionic IEM was facing the H2SO4. The diffusion of H+ and OH− were blocked but the transportation of K+, SO42− and water was allowed. Therefore, the two compartments were forming a full electrochemical cell while the pH value within each chamber can be maintained separately. The bi-polar IEMs can also function as a buffer layer between the H2SO4 and KOH electrolytes without a glass fiber separator [1].
(Zn + 2OH )(pH = 14.5) + (PbO2 + SO4 2 + 2H+) (pH= + PbSO4(pH =
1)
1)
Zn(OH)4 2(pH = 14.5) (3)
Although reaction 3 does not exist in a single electrolyte, a stable dual-electrolyte system, as demonstrated in Fig. 1, can enable this reaction and the expected operating voltage. 3.2. PbO2-Zn battery discharging To verify the hypothesis in Fig. 2, full battery devices were assembled using the dual-electrolyte system as shown in Fig. 1a. The full cell was discharged at a constant current density of 0.05 mA cm−2 and the potential of each electrode was measured simultaneously in Fig. 3a. Such relatively small current density was applied due to a small active material loading of 2 mg cm−2 at the PbO2 positive electrode for proofof-concept purpose. The respective open circuit potentials (OCP) of PbO2 and Zn electrode were 1.5 V and −1.3 V vs. SHE. This resulted in a 2.8 V open circuit voltage (OCV) for the full cell, in agreement with that proposed in Fig. 2. Since the Zn negative electrode had much higher capacity than that of the PbO2 positive electrode, PbO2 was the dominating electrode. Therefore, the capacity of the battery was determined by the utilization of PbO2 electrode (Figs. 3a and b). During charging at 1C in Fig. 3a, the potential of the PbO2 electrode was ca. 1.7 V vs. SHE. During discharging, the electrode potential started at 1.6 V vs. SHE until reaching 90% of the PbO2 electrode capacity, where a sharp drop of potential occurred. On the other hand, the potential of the Zn electrode remained almost constant at −1.3 V vs. SHE at both charging and discharging with a very small 20 mV overpotential difference between charging and discharging (Fig. S1). This 20 mV was negligible compared to the voltage of the full cell. Thus, the cell voltage followed the trend of the PbO2 electrode, which is a typical battery discharging profile (Fig. 3b). These results demonstrated a promising
2.3. Characterizations Electrochemical characterizations including cyclic voltammetry (CV), galvanostatic discharging, electrochemical impedance spectroscopy (EIS) were conducted on a potentiostat (Ivium, CompactStat, Netherlands). An oscilloscope (GW-Instek, GDS-122, Taiwan) was used to track the potential changes of both positive and negative electrodes [7,21] against a Ag/AgCl reference electrode in the H2SO4 chamber (Fig. 1a). The pH values of the electrolytes were tracked by sampling the 5 M H2SO4 and 3 M KOH electrolytes in the 2-electrode dual-electrolyte cells (Fig. 1b), diluting them by 1000 times, and measuring using a pH meter (Oakton, pHTestr30, Canada). The scanning electrode microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) was performed using a (JEOL, 6610LV, Japan) SEM.
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Journal of Energy Storage 29 (2020) 101305
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Fig. 1. (a) Schematic setup of PbO2-Zn battery in dual-electrolyte, (b) pH value of positive and negative electrolytes chambers for a 17-day period.
secondary aqueous based battery system with a high voltage of 2.8 V that has exceeded that of all known aqueous based batteries such as alkaline battery (1.5 V) and lead acid battery (2.1 V). The realization of this high cell voltage can be explained via a pourbaix diagram shown in Fig. 4. Although water decomposition has a voltage of 1.23 V in a single aqueous electrolyte, the actual thermodynamic water decomposition window was extended to 2.14 V (i.e. 1.23 V + 15.5 × 0.059 V) in this dual-electrolyte system (Fig. 4) thanks to the difference of the pH value (Fig. 1b). From a theoretical estimation, the threshold potentials of oxygen evolution reaction (OER) and hydrogen evolutions reaction (HER) were +1.29 V and −0.85 V, respectively [25,26] (Fig. 4). However, the actual measured OCP of the PbO2 electrode was +1.5 V (Fig. 4), a 0.21 V (overpotential) higher than that of the OER. Also, the measured OCP of Zn electrode was −1.3 V (Fig. 4), which was 0.45 V (overpotential) lower than that of the HER (Fig. 3a). Due to the poor kinetics towards OER at PbO2 and HER at Zn [12,13], no significant OER and HER were observed in the cell at 2.8 V. As a result, the total voltage increased from this PbO2-Zn
system with dual-electrolyte is 1.57 V (i.e. 2.8–1.23 V), of which 0.91 V (58%) was attributed to the pH difference across the IEMs and 0.66 V (42%) to the overpotentials. The charge-discharge profiles of the dual-electrolyte full cell are shown in Fig. 5a. The cell operated at 3 different rates from 1 C to 4 C (0.05 to 0.2 mA cm−2), and depicted typical battery discharging profiles with only 3% reduction in capacity from 1C to 4C. The device is capable for high rate performance even with a relatively thick (0.2 mm) bi-polar IEM and a relatively high equivalent series resistance (ESR) of 57 ohm (Fig. S2). However, it is worth noting that the charging at a high rate of 4C resulted in a distortion of the profile, likely due to the growing overpotentials from the limited ion diffusion of the IEMs. A bipolar IEM with lower resistance and thickness may lead to a reduce cell ESR and solid-state devices. The PbO2-Zn cell was further charged and discharged at 1C (0.05 mA cm−2) for 100 cycles shown in Fig. 5b. The charge-discharge profiles almost overlapped with no reduction in capacity over these 100 cycles. Nonetheless, the charge-discharge profile of cycle 80 and cycle 100 showed some spikes of cell voltage, indicating
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H. Wu, et al.
Fig. 2. Cyclic voltammograms of PbO2 in 5 M H2SO4 and Zn electrode in 3 M KOH from individual 3-electrode tests at 10 mVs−1, combined to illustrate the PbO2-Zn battery concept.
Fig. 3. Discharging profile of PbO2-Zn battery in dual-electrolyte at 1C (a) individual electrode potential tracking vs. time, (b) full cell voltage vs. time.
a slight degradation of bonding between the PbO2 and the substrate. The bonding of PbO2 to the current collector can be further enhanced to prolong the cycle life of the cell. Yet the excellent capacity retention demonstrated the rechargeability of the Pb–Zn battery. The current efficiency was stable with a value of 80% over the entire cycle life testing (Fig. 5c), also showing a stable performance of the Pb–Zn battery upon long time cycling. This is a typical current efficiency of PbO2 electrode, which has a relatively large self discharging at a high state of charge (SOC) [27-29]. No significant ion cross-over was observed across the bi-polar IEMs after cycling, evidenced by: (a) the pH value in both acid and alkaline chambers was also stable over the period of cycle life test; (b) EDX spectrum showed no K at PbO2 electrode nor S at Zn electrode after cycling (Fig. S3). Further, the leakage current density was measured by holding the device at its charging voltage (2.85 V at
1C) for 1 h (Fig. 5d). After the initial drop, the leakage current stabilized at 2 uA cm−2, which was reasonable and can be compensated through a float charging. Moreover, a 8 mg cm−2 higher loading PbO2 electrode was utilized in the PbO2-Zn battery cell, which was charged at a constant 1C rate and discharged from 1 to 4 C (0.2–0.8 mA cm−2) in Fig. S4. The charge-discharge profiles were similar to that in Fig. 5a, confirming its capability for battery applications. 4. Conclusions A new PbO2-Zn battery chemistry leveraging a bi-polar IEM that effectively separates the acid and alkaline electrolytes is demonstrated. Both positive (PbO2) and negative (Zn) electrodes are able to achieve their equilibrium potentials in desired environments, leading to a 2.8 V
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Journal of Energy Storage 29 (2020) 101305
H. Wu, et al.
Fig. 4. Schematic illustration of the voltage improvement of PbO2-Zn battery with measured electrode potentials superimposed on an E-pH diagram.
Fig. 5. Electrochemical results of PbO2-Zn battery in dual-electrolyte (a) at different C rates, (b) cycle life at 1 C, (c) current efficiency of 100 cycles and (d) leakage current density at 2.85 V.
operating voltage. The high cell voltage is attributed to both pH difference of electrolytes and overpotentials of the electrode materials towards water decomposition. The simple and cost-effective PbO2 and Zn electrodes can be integrated into a promising aqueous based secondary battery system with high voltage, good rate capability and cycle life. This work also suggests a new approach in effectively designing energy storage device, which can also be extended to other organic and solid systems.
CRediT authorship contribution statement Haoran Wu: Writing - original draft, Methodology, Investigation, Formal analysis, Writing - review & editing. Szu-Jia Liu: Investigation, Formal analysis. Keryn Lian: Supervision, Project administration, Writing - review & editing.
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Declaration of Competing Interest
[10] C.C.B.M. de Souza, D.C. de Oliveira, J.A.S. Tenório, J. Power Sources 103 (2001) 120–126. [11] P. Van den Bossche, F. Vergels, J. Van Mierlo, J. Matheys, W. Van Autenboer, J. Power Sources 162 (2006) 913–919. [12] L. Felloni, R. Fratesi, E. Quadrini, G. Roventi, J. Appl. Electrochem. 17 (1987) 574–582. [13] G. Zhao, Y. Zhang, Y. Lei, B. Lv, J. Gao, Y. Zhang, D. Li, Environ. Sci. Technol. 44 (2010) 1754–1759. [14] S. Thomas, N. Birbilis, M. Venkatraman, I. Cole, Corrosion, J. Sci. Eng. 68 (2012) 015009-015001-015009-015009. [15] R. Ahuja, A. Blomqvist, P. Larsson, P. Pyykkö, P. Zaleski-Ejgierd, Phys. Rev. Lett. 106 (2011) 018301. [16] X. Yang, R. Zou, F. Huo, D. Cai, D. Xiao, J. Hazard. Mater. 164 (2009) 367–373. [17] H. An, Q. Li, D. Tao, H. Cui, X. Xu, L. Ding, L. Sun, J. Zhai, Appl. Surf. Sci. 258 (2011) 218–224. [18] N. Yu, L. Gao, Electrochem. Commun. 11 (2009) 220–222. [19] N. Yu, L. Gao, S. Zhao, Z. Wang, Electrochim. Acta 54 (2009) 3835–3841. [20] C. Xu, B. Li, H. Du, F. Kang, Angew. Chem. Int. Ed. 51 (2012) 933–935. [21] Q. Tian, K. Lian, Electrochem. Solid-State Lett. 13 (2010) A4–A6. [22] H. Wu, K. Lian, Mater. Chem. Phys. 203 (2017) 346–351. [23] M. Geng, D.O. Northwood, Int. J. Hydrogen Energy 28 (2003) 633–636. [24] P. Ruetschi, J. Power Sources 127 (2004) 33–44. [25] J. Zhao, T. Minegishi, L. Zhang, M. Zhong, M. Nakabayashi, G. Ma, T. Hisatomi, M. Katayama, S. Ikeda, N. Shibata, Angew. Chem. Int. Ed. 53 (2014) 11808–11812. [26] Y. Zhao, S. Chen, B. Sun, D. Su, X. Huang, H. Liu, Y. Yan, K. Sun, G. Wang, Sci. Rep. 5 (2015) 7629. [27] J.W. Stevens, G.P. Corey, Conference Record of the Twenty Fifth IEEE Photovoltaic Specialists Conference-1996, IEEE, 1996, pp. 1485–1488. [28] D. Pletcher, R. Wills, PCCP 6 (2004) 1779–1785. [29] J. Lakeman, J. Power Sources 27 (1989) 155–165.
None. Acknowledgements We appreciate the financial support from NSERC Canada (Discovery RGPIN-2016-06219, CREATE 397899-11, and Discovery Accelerator Supplements 493032). H. Wu would like to thank NSERC PGS-D and CGS-D scholarships. Jessica Liu would like to thank UTEA summer research award. References [1] X. Wang, R.S. Chandrabose, Z. Jian, Z. Xing, X. Ji, J. Electrochem. Soc. 163 (2016) A1853–A1858. [2] F. Wang, S. Xiao, Y. Hou, C. Hu, L. Liu, Y. Wu, RSC Adv. 3 (2013) 13059–13084. [3] F. Béguin, V. Presser, A. Balducci, E. Frackowiak, Adv. Mater. 26 (2014) 2219–2251. [4] K. Fic, M. Meller, E. Frackowiak, Electrochim. Acta 128 (2014) 210–217. [5] K. Fic, M. Meller, E. Frackowiak, J. Electrochem. Soc. 162 (2015) A5140–A5147. [6] K. Fic, M. Meller, J. Menzel, E. Frackowiak, Electrochim. Acta 206 (2016) 496–503. [7] H. Wu, K. Lian, J. Power Sources 378 (2018) 209–215. [8] K. Divya, J. Østergaard, Electric Power Syst. Res. 79 (2009) 511–520. [9] Y. Chang, X. Mao, Y. Zhao, S. Feng, H. Chen, D. Finlow, J. Power Sources 191 (2009) 176–183.
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Contents lists available at ScienceDirect
Journal of Energy Storage journal homepage: www.elsevier.com/locate/est
Aqueous based dual-electrolyte rechargeable Pb–Zn battery with a 2.8 V operating voltage
T
Haoran Wu , Szu-Jia Liu, Keryn Lian ⁎
Department of Materials Science and Engineering, University of Toronto, Toronto, Ontario M5S 3E4, Canada
ARTICLE INFO
ABSTRACT
Keywords: PbO2-Zn battery High voltage Dual-electrolyte Aqueous based Rechargeable
A novel rechargeable PbO2-Zn battery system using Zn as negative and PbO2 as positive electrodes in a KOHH2SO4 dual-electrolyte is developed. Using a bi-polar ion exchange membrane that effectively prevents the H+ and OH− from neutralization, the positive and negative electrodes operate in the respective electrolytes with different pH. The Zn electrode provides a low potential of −1.3 V in KOH solution and the PbO2 has a high potential of +1.5 V in H2SO4 solution. This resulted in a cell operating voltage of 2.8 V in an aqueous electrolyte. The system also demonstrated a high rate performance and good rechargeability. This simple, low-cost system not only overcomes the thermodynamic limit of water decomposition and achieves a high voltage aqueous based battery, but also provides a new approach in design of future electrochemical devices.
1. Introduction Aqueous based energy storage devices are generally safer, nonflammable and less expensive compared to their organic based counterparts [1,2]. The operating voltage of these devices, however, is limited by the water decomposition potential window [3,4]. As a result, the energy and power densities of aqueous based devices are relatively low. To extend the operating voltage window of aqueous based energy storage devices, Frackowiak et al. introduced a dual-electrolyte configuration for aqueous based supercapacitors [5,6]. These devices were constructed in a 3-compartment cell with a buffer solution in between acid and alkaline. In other studies, an ion exchange membrane (IEM) was applied to suppress the acid-base neutralization by blocking the cross-diffusion of H+ and OH− [1,6,7], so that the pH value of each chamber remained constant. This concept was demonstrated on activated carbon (AC) symmetrical supercapacitor systems with high cell voltages of 1.8 V [1] and 2.1 V [6]. In a further study, asymmetrical electrodes and dual-electrolyte were integrated to extend the cell voltage. In a battery-capacitor hybrid device with dual-electrolyte, both positive and negative electrodes achieved their equilibrium potentials in their respective electrolytes, leading to a high operating voltage of 2.4 V [7]. If these dual-electrolyte configuration approaches can be applied in other energy storage systems such as lead-acid and alkaline batteries, their voltage may also be significantly enhanced.
⁎
Zn and Pb based battery electrodes are very common in conventional batteries [8]. These batteries are widely used in our daily life for their low-cost and good performance [9,10]. Although Pb is relatively toxic, it is highly recycled [11]. Alkaline and lead acid batteries have cell voltages of 1.5 V and 2.1 V, respectively. Both are higher than that of the water decomposition voltage, due to the high overpotentials of Zn [12] and PbO2 [13] can lower the hydrogen and oxygen evolution, respectively. However, since Zn only operates in alkaline [14] and Pb only works in acid [15], these distinct battery chemistries have never been considered within a single system. In this work, for the first time, we integrated a Zn negative electrode and a PbO2 positive electrode in a dual-electrolyte system to form a battery. In this system, both electrodes can operate in their preferred stable operating environments, leading to a 2.8 V operating voltage in an aqueous based system. 2.Experimental 2.1. Electrodes A thin Ti foil (McMaster-Carr, 127 um thick, Canada) was used as current collector for the Pb based positive electrode. The Ti foil was ultrasonically cleaned in DI water. The Ti surface was cleaned again by isopropyl alcohol (IPA) before electrode coating. A precursor solution was prepared of dissolving 2 g SnCl4 (Alfa Aesar, 98%, US), 0.2 g of
Corresponding author. E-mail addresses: [email protected] (H. Wu), [email protected] (K. Lian).
https://doi.org/10.1016/j.est.2020.101305 Received 26 September 2019; Received in revised form 23 January 2020; Accepted 17 February 2020 2352-152X/ © 2020 Elsevier Ltd. All rights reserved.
Journal of Energy Storage 29 (2020) 101305
H. Wu, et al.
Sb2O3 (Alfa Aesar, 99%, US) and 1.3 ml of concentrated HCl (Caledon, 36.5–38%, Canada) into 20 ml IPA. The Ti substrate was dipped into the precursor solution and dried at 120 °C for 15 min. The dip coating and drying process was repeated for 5 times and then calcinated in a muffle furnace (MTI corp., KSL-1100X-S, US) at 550 °C for 1 h to create a thin SnO2-Sb2O3 for the following electrodeposition of PbO2 [16,17]. The SnO2-Sb2O3 coated Ti foil was placed to an electroplating bath comprised of 1 M HNO3 (Caledon, 68–70%, Canada) and 0.5 M Pb (NO3)2 (Alfa Aesar, 99%, US) to deposit PbO2 [18,19]. The electrodeposition was conducted in a pulse mode: ton = 2s, toff = 2 s, at a current density of 1 mA cm−2 for 750 cycles or 3000 s, so that a black PbO2 coating was deposited on the substrate. The positive electrode had an apparent area of 1 cm2 exposed to the electrolyte with an effective loading of 2 mg cm−2 PbO2, leading to an expected capacity of 0.05 mAh cm−2. A thin Zn foil (Alibaba, 25 μm thick, China) was used as the negative electrode, which was ultrasonically cleaned in IPA and DI water prior to its application. The negative electrode had an effective loading of 18 mg cm−2 Zn and with an apparent electrode area of 1 cm−2. The theoretical capacity of the Zn electrode is 14.8 mAh cm−2 [20]. Since the Zn foil was used as both current collector and electrode, the cell was designed such that the Zn had much higher capacity than that of the PbO2. Accordingly, the cell capacity was predominated by PbO2 electrode.
3. Results and discussion 3.1. PbO2-Zn battery concept Prior to electrochemical characterizations, the pH value in each chamber was monitored over time to ensure the stability of the dualelectrolyte in the cell (Fig. 1b). The pH values in the acid and alkaline chambers were −1 and 14.5, resulting in a 15.5 pH difference between the two-side of the bi-polar IEM. The pH values of the two chambers were tracked for a 17 days period and appeared to be constant throughout the time (Fig. 1b). This suggests that the bi-polar IEM can effectively block the diffusion of proton and hydroxide ions, so that no neutralization occurred. The individual electrode performance was first investigated separately in a 3-electrode cell, where Zn electrode was in alkaline (3 M KOH) and PbO2 electrode in acid (5 M H2SO4) electrolyte. The CV characterizations were conducted in their respective redox active potential window at a scan rate of 10 mVs−1. Both electrodes showed typical battery CV profiles with pronounced redox reactions peaks (Fig. 2) [19,22]. The reaction of Zn in strong alkaline media is expressed by reaction 1 [23]:
Zn + 4OH
(1)
Zn(OH)4 2 + 2e
The reaction of PbO2 in strong H2SO4 is shown in reaction 2 [24]:
PbO2 + 4H+ + SO4 2 + 2e 2.2. Cell setup
(2)
PbSO4 + 2H2 O
When combining the two half-cell CV profiles in a single plot (Fig. 2), a maximum potential window between the two electrodes is expected to be around 2.V. This can potentially lead to a new full-cell battery chemistry in reaction 3:
The individual electrode characterizations of PbO2 and Zn were conducted in a 3-electrode cell using either 5 M H2SO4 (Caledon, 95–98%, Canada) for PbO2 or 3 M KOH (Alfa Aesar, 85%, US) for Zn. An Ag/AgCl electrode was used as reference in acid and a Hg/HgO reference was used in alkaline solutions. All potentials were converted to against the standard hydrogen electrode (SHE) for clear comparison. A 2-electrode dual-electrolyte cell setup illustrated in Fig. 1a was assembled with PbO2 electrode in a flask containing 5 M H2SO4 and Zn electrode in another with 3 M KOH. The two flasks were separated by a bi-polar IEM comprised of a layer of commercial cationic IEM (Fumasep, FKB-PK-130, US) and a layer of anionic IEM (Fumasep, FAA-3PK-75, US). The cationic IEM was facing the KOH compartment while the anionic IEM was facing the H2SO4. The diffusion of H+ and OH− were blocked but the transportation of K+, SO42− and water was allowed. Therefore, the two compartments were forming a full electrochemical cell while the pH value within each chamber can be maintained separately. The bi-polar IEMs can also function as a buffer layer between the H2SO4 and KOH electrolytes without a glass fiber separator [1].
(Zn + 2OH )(pH = 14.5) + (PbO2 + SO4 2 + 2H+) (pH= + PbSO4(pH =
1)
1)
Zn(OH)4 2(pH = 14.5) (3)
Although reaction 3 does not exist in a single electrolyte, a stable dual-electrolyte system, as demonstrated in Fig. 1, can enable this reaction and the expected operating voltage. 3.2. PbO2-Zn battery discharging To verify the hypothesis in Fig. 2, full battery devices were assembled using the dual-electrolyte system as shown in Fig. 1a. The full cell was discharged at a constant current density of 0.05 mA cm−2 and the potential of each electrode was measured simultaneously in Fig. 3a. Such relatively small current density was applied due to a small active material loading of 2 mg cm−2 at the PbO2 positive electrode for proofof-concept purpose. The respective open circuit potentials (OCP) of PbO2 and Zn electrode were 1.5 V and −1.3 V vs. SHE. This resulted in a 2.8 V open circuit voltage (OCV) for the full cell, in agreement with that proposed in Fig. 2. Since the Zn negative electrode had much higher capacity than that of the PbO2 positive electrode, PbO2 was the dominating electrode. Therefore, the capacity of the battery was determined by the utilization of PbO2 electrode (Figs. 3a and b). During charging at 1C in Fig. 3a, the potential of the PbO2 electrode was ca. 1.7 V vs. SHE. During discharging, the electrode potential started at 1.6 V vs. SHE until reaching 90% of the PbO2 electrode capacity, where a sharp drop of potential occurred. On the other hand, the potential of the Zn electrode remained almost constant at −1.3 V vs. SHE at both charging and discharging with a very small 20 mV overpotential difference between charging and discharging (Fig. S1). This 20 mV was negligible compared to the voltage of the full cell. Thus, the cell voltage followed the trend of the PbO2 electrode, which is a typical battery discharging profile (Fig. 3b). These results demonstrated a promising
2.3. Characterizations Electrochemical characterizations including cyclic voltammetry (CV), galvanostatic discharging, electrochemical impedance spectroscopy (EIS) were conducted on a potentiostat (Ivium, CompactStat, Netherlands). An oscilloscope (GW-Instek, GDS-122, Taiwan) was used to track the potential changes of both positive and negative electrodes [7,21] against a Ag/AgCl reference electrode in the H2SO4 chamber (Fig. 1a). The pH values of the electrolytes were tracked by sampling the 5 M H2SO4 and 3 M KOH electrolytes in the 2-electrode dual-electrolyte cells (Fig. 1b), diluting them by 1000 times, and measuring using a pH meter (Oakton, pHTestr30, Canada). The scanning electrode microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) was performed using a (JEOL, 6610LV, Japan) SEM.
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Fig. 1. (a) Schematic setup of PbO2-Zn battery in dual-electrolyte, (b) pH value of positive and negative electrolytes chambers for a 17-day period.
secondary aqueous based battery system with a high voltage of 2.8 V that has exceeded that of all known aqueous based batteries such as alkaline battery (1.5 V) and lead acid battery (2.1 V). The realization of this high cell voltage can be explained via a pourbaix diagram shown in Fig. 4. Although water decomposition has a voltage of 1.23 V in a single aqueous electrolyte, the actual thermodynamic water decomposition window was extended to 2.14 V (i.e. 1.23 V + 15.5 × 0.059 V) in this dual-electrolyte system (Fig. 4) thanks to the difference of the pH value (Fig. 1b). From a theoretical estimation, the threshold potentials of oxygen evolution reaction (OER) and hydrogen evolutions reaction (HER) were +1.29 V and −0.85 V, respectively [25,26] (Fig. 4). However, the actual measured OCP of the PbO2 electrode was +1.5 V (Fig. 4), a 0.21 V (overpotential) higher than that of the OER. Also, the measured OCP of Zn electrode was −1.3 V (Fig. 4), which was 0.45 V (overpotential) lower than that of the HER (Fig. 3a). Due to the poor kinetics towards OER at PbO2 and HER at Zn [12,13], no significant OER and HER were observed in the cell at 2.8 V. As a result, the total voltage increased from this PbO2-Zn
system with dual-electrolyte is 1.57 V (i.e. 2.8–1.23 V), of which 0.91 V (58%) was attributed to the pH difference across the IEMs and 0.66 V (42%) to the overpotentials. The charge-discharge profiles of the dual-electrolyte full cell are shown in Fig. 5a. The cell operated at 3 different rates from 1 C to 4 C (0.05 to 0.2 mA cm−2), and depicted typical battery discharging profiles with only 3% reduction in capacity from 1C to 4C. The device is capable for high rate performance even with a relatively thick (0.2 mm) bi-polar IEM and a relatively high equivalent series resistance (ESR) of 57 ohm (Fig. S2). However, it is worth noting that the charging at a high rate of 4C resulted in a distortion of the profile, likely due to the growing overpotentials from the limited ion diffusion of the IEMs. A bipolar IEM with lower resistance and thickness may lead to a reduce cell ESR and solid-state devices. The PbO2-Zn cell was further charged and discharged at 1C (0.05 mA cm−2) for 100 cycles shown in Fig. 5b. The charge-discharge profiles almost overlapped with no reduction in capacity over these 100 cycles. Nonetheless, the charge-discharge profile of cycle 80 and cycle 100 showed some spikes of cell voltage, indicating
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Fig. 2. Cyclic voltammograms of PbO2 in 5 M H2SO4 and Zn electrode in 3 M KOH from individual 3-electrode tests at 10 mVs−1, combined to illustrate the PbO2-Zn battery concept.
Fig. 3. Discharging profile of PbO2-Zn battery in dual-electrolyte at 1C (a) individual electrode potential tracking vs. time, (b) full cell voltage vs. time.
a slight degradation of bonding between the PbO2 and the substrate. The bonding of PbO2 to the current collector can be further enhanced to prolong the cycle life of the cell. Yet the excellent capacity retention demonstrated the rechargeability of the Pb–Zn battery. The current efficiency was stable with a value of 80% over the entire cycle life testing (Fig. 5c), also showing a stable performance of the Pb–Zn battery upon long time cycling. This is a typical current efficiency of PbO2 electrode, which has a relatively large self discharging at a high state of charge (SOC) [27-29]. No significant ion cross-over was observed across the bi-polar IEMs after cycling, evidenced by: (a) the pH value in both acid and alkaline chambers was also stable over the period of cycle life test; (b) EDX spectrum showed no K at PbO2 electrode nor S at Zn electrode after cycling (Fig. S3). Further, the leakage current density was measured by holding the device at its charging voltage (2.85 V at
1C) for 1 h (Fig. 5d). After the initial drop, the leakage current stabilized at 2 uA cm−2, which was reasonable and can be compensated through a float charging. Moreover, a 8 mg cm−2 higher loading PbO2 electrode was utilized in the PbO2-Zn battery cell, which was charged at a constant 1C rate and discharged from 1 to 4 C (0.2–0.8 mA cm−2) in Fig. S4. The charge-discharge profiles were similar to that in Fig. 5a, confirming its capability for battery applications. 4. Conclusions A new PbO2-Zn battery chemistry leveraging a bi-polar IEM that effectively separates the acid and alkaline electrolytes is demonstrated. Both positive (PbO2) and negative (Zn) electrodes are able to achieve their equilibrium potentials in desired environments, leading to a 2.8 V
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Fig. 4. Schematic illustration of the voltage improvement of PbO2-Zn battery with measured electrode potentials superimposed on an E-pH diagram.
Fig. 5. Electrochemical results of PbO2-Zn battery in dual-electrolyte (a) at different C rates, (b) cycle life at 1 C, (c) current efficiency of 100 cycles and (d) leakage current density at 2.85 V.
operating voltage. The high cell voltage is attributed to both pH difference of electrolytes and overpotentials of the electrode materials towards water decomposition. The simple and cost-effective PbO2 and Zn electrodes can be integrated into a promising aqueous based secondary battery system with high voltage, good rate capability and cycle life. This work also suggests a new approach in effectively designing energy storage device, which can also be extended to other organic and solid systems.
CRediT authorship contribution statement Haoran Wu: Writing - original draft, Methodology, Investigation, Formal analysis, Writing - review & editing. Szu-Jia Liu: Investigation, Formal analysis. Keryn Lian: Supervision, Project administration, Writing - review & editing.
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Declaration of Competing Interest
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None. Acknowledgements We appreciate the financial support from NSERC Canada (Discovery RGPIN-2016-06219, CREATE 397899-11, and Discovery Accelerator Supplements 493032). H. Wu would like to thank NSERC PGS-D and CGS-D scholarships. Jessica Liu would like to thank UTEA summer research award. References [1] X. Wang, R.S. Chandrabose, Z. Jian, Z. Xing, X. Ji, J. Electrochem. Soc. 163 (2016) A1853–A1858. [2] F. Wang, S. Xiao, Y. Hou, C. Hu, L. Liu, Y. Wu, RSC Adv. 3 (2013) 13059–13084. [3] F. Béguin, V. Presser, A. Balducci, E. Frackowiak, Adv. Mater. 26 (2014) 2219–2251. [4] K. Fic, M. Meller, E. Frackowiak, Electrochim. Acta 128 (2014) 210–217. [5] K. Fic, M. Meller, E. Frackowiak, J. Electrochem. Soc. 162 (2015) A5140–A5147. [6] K. Fic, M. Meller, J. Menzel, E. Frackowiak, Electrochim. Acta 206 (2016) 496–503. [7] H. Wu, K. Lian, J. Power Sources 378 (2018) 209–215. [8] K. Divya, J. Østergaard, Electric Power Syst. Res. 79 (2009) 511–520. [9] Y. Chang, X. Mao, Y. Zhao, S. Feng, H. Chen, D. Finlow, J. Power Sources 191 (2009) 176–183.
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