Journal of Power Sources 437 (2019) 226918

Available online 24 July 20190378-7753/© 2019 Elsevier B.V. All rights reserved.

Investigation of an aqueous rechargeable battery consisting of manganese tin redox chemistries for energy storage

L. Wei, H.R. Jiang, Y.X. Ren, M.C. Wu, J.B. Xu, T.S. Zhao *

HKUST Energy Institute, Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China

H I G H L I G H T S

� A novel battery using Mn and Sn redox materials is presented. � The battery exhibits an open circuit voltage of 1.7 V. � Coulombic efficiency reaches 98.8% at current density of 30 mA cm� 2. � Energy efficiency can be kept above 91.5% at 10 mA cm� 2 over 100 cycles.

A R T I C L E I N F O

Keywords: Mn Sn battery Mn redox couple Energy efficiency Energy storage

A B S T R A C T

In this work, a novel aqueous battery consisting of manganese in (Mn Sn) redox chemistries is proposed, where Mn redox reactions occur in the positive electrode and Sn deposition/stripping reactions occur in the negative electrode. The battery exhibits an open circuit voltage of 1.7 V, while a coulombic efficiency of higher than 98.8% is obtained at an operating current density of 30 mA cm� 2. Cyclic tests show that the energy efficiency can be kept above 91.5% with no observable decay at a current density of 10 mA cm� 2. In addition, due to the high hydrogen overpotential of Sn metal, the parasitic hydrogen evolution reaction can be largely avoided. With decent battery performance, the Mn Sn system offers a promising solution for future energy storage applications.

1. Introduction

Unlike traditional fossil energy such as petroleum and natural gas, renewable energies such as solar and wind power are reproducible and clean. They have attracted more and more attention and been widely recognized as an alternative energy source to resolve fossil fuel deple-tion and environmental pollution [1–5]. However, the random and intermittent nature of the output from these renewable sources limits their widespread applications, since the introduction of more than 20% intermittent energy from renewable sources without any storage facil-ities could destabilize the voltage and frequency of the grid [6]. Energy storage technology, such as batteries, is a practical approach to address this issue. To date, various battery technologies have been proposed, including lithium-ion, lead-acid, redox-flow, and liquid-metal batteries [7–12]. Although some of them have been successfully used on specific occasions such as the electric vehicle industry, consumer electronics, and energy power supply, a majority of their applications in energy storage for renewable energies are still hindered by technical, economic,

and safety barriers [13,14]. Therefore, searching for new electro-chemical redox species with inherent safety, low cost, and high stability is still desirable at the current stage.

Over the past few decades, manganese-based batteries have been attracted remarkable attention due to rich abundance and low cost. Notably, the electrolytic manganese oxide is one of the most widely used electrode materials, over 230,000 metric tons per year, in non- rechargeable primary batteries [15]. Nevertheless, state-of-the-art rechargeable alkaline Zn Mn batteries suffer from critical issues such as poor rechargeability and low rate capability, restricting its penetra-tion into the present energy storage market [16]. The rich chemistry of manganese species allows it to exist in various states, including Mn7þ, Mn4þ, Mn3þ, Mn2þ, and Mn, offering possibilities for the discovery of other manganese based systems for secondary batteries [17]. Recently, some researchers put their attention into the Mn redox reactions in an acid environment. Zhang et al. reported a manganese zinc battery by using methane sulfonic acid (MSA) as the solvent [18]. Xue et al. and Park et al. proposed manganese vanadium batteries to replace the

* Corresponding author. E-mail address: metzhao@ust.hk (T.S. Zhao).

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https://doi.org/10.1016/j.jpowsour.2019.226918 Received 17 June 2019; Received in revised form 17 July 2019; Accepted 20 July 2019

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high-priced vanadium salt in the positive side in the all-vanadium bat-teries [19–21]. However, although MSA has been widely employed as supporting acid for batteries [22–24], zinc dissolution in acid and rapid crossover rate of vanadium ions through the membrane caused severe self-discharge and short life cycles during these battery operations. Recently, Cui et al. proposed a novel Mn H battery [25]; which exhibited a discharge voltage of �1.3 V with a high rate capability of 100 mA cm� 2

and a rather long lifetime without observable performance decay. Nevertheless, the transport and storage of H2 gas, as well as the utili-zation of expensive noble Pt metal as a catalyst for H2 reduction and oxidation reaction, collectively limit the practical application of Mn H chemistry to some extent.

In this sense, building the new Mn-based battery with high rate capability and long lifetime is still worthwhile. To achieve such a goal, selecting suitable negative electrode with high hydrogen overpotential and fast kinetics is critical. Tin metal, which has been demonstrated to have low toxicity and widely used in the food industry to fabricate tin cans, possesses high hydrogen overvoltage and fast kinetics, offering a potential of about � 0.13 V vs. standard hydrogen electrode (SHE) [26]. Hence, tin may be one of the ideal options as negative electrode mate-rials. Herein, we demonstrate a novel manganese tin (Mn Sn) battery chemistry in acidic conditions by employing a dilute H2SO4 solution as the supporting electrolyte. The detailed working principle of the battery is illustrated in Fig. 1, and the process of electrochemical reactions can be depicted as follows:

Negative electrode: Sn2þ þ 2e� ⇌Sn ðE0 ¼ � 0:13 V vs: SHE�

(1)

Positive electrode: Mn2þ � e� ⇌Mn3þ ðE0 ¼ 1:51 V vs: SHE�

(2)

Overall reaction: 2Mn2þ þ Sn2þ⇌2Mn3þ þ Sn (3)

During the charge process, metallic tin is electrodeposited onto the negative electrode from the electrolyte, while Mn2þ is oxidized to form Mn3þ at the positive electrode. Hþ migrates through the membrane to form a complete electric circuit. A reverse reaction takes place at the corresponding electrode surface during the discharge process. As dis-closed by the experimental results, the present battery demonstrates coulombic efficiencies ranging from 98.2% to 98.8% when the current density increases from 10 to 30 mA cm� 2. Besides, the energy efficiency of the battery maintains stable above 91.5% during the cycle tests under 10 mA cm� 2. The experimental results demonstrate the Mn Sn chemistry

offers a promising solution for future energy storage applications.

2. Experimental

2.1. Preparation of the lab-made Mn Sn battery

The detailed structure of the lab-made Mn Sn battery is illustrated in Fig. S1. Both positive and negative sides of the battery consisted of the transparent acrylic frames where the electrode and electrolytes were placed. The electrode used in this work was carbon felt (Liaoning Jingu Carbon Material Co., Ltd.) with a compressed thickness of 6 mm and an active area of 0.785 cm2 with a porosity higher than 95%. The carbon felt is widely used as electrode for batteries [24], to enhance the elec-trochemical activity and hydrophilicity, the electrode was treated at 480 �C for 6 h in a muffle furnace under ambient air before use. As shown in Fig. S2, the thermally treated carbon felt becomes very hy-drophilic and can be soaked into deionized water immediately while the pristine sample float on the surface of the water. The negative and positive electrolytes were formed by dissolving the stannous and man-ganese sulfate salts into dilute sulfuric acid solutions, respectively. The chemicals used in this work were all commercially available from Sigma Aldrich and used as received. 0.4 mL negative electrolyte (0.3 M SnSO4 þ 2.8 M H2SO4) and 0.4 mL positive electrolyte (0.5 M MnSO4 þ 2.8 M H2SO4) were added into each side of the battery as the initial electrolyte. A polybenzimidazole (PBI) membrane with a thickness of 30 μm pro-vided by Yick-Vic was used to separate positive and negative electrolytes as well as to conduct ions during battery operation. Before applied in the battery, the as-received PBI membrane was pretreated in 3 M H2SO4 for 7 days to increase the ion conductivity. Viton gaskets were placed adjacent to both sides of the membrane to prevent the electrolyte leakage. Soft graphite plate together with the stainless plate was used as endplate in each side to collect electric current during battery operation.

2.2. Electrochemical measurements and material characterizations

Cyclic voltammetry (CV) and electrochemical impedance spectros-copy (EIS) tests were carried out in a conventional three-electrode sys-tem connect with an electrochemical workstation (EG&G Princeton, model 2273). The glassy carbon, platinum mesh, and saturated calomel electrode (SCE) with a KCl salt bridge were used as the working, counter, and reference electrodes, respectively. The electrolytes used for CV and EIS tests were as same as the one used for the battery test mentioned above. CV tests were performed under different scan rates ranging from 10 to 50 mV s� 1. The battery tests were evaluated in an Arbin BT2000 instrument controlled by a MITS PRO software. During operation, the battery was charged to a constant capacity of 3.75 A h L� 1 and then discharged to a cut-off voltage of 1.3 V under different testing current densities. The volume of negative electrolyte and positive electrolytes are both 0.4 mL and the charge capacity is 3 mAh. Thus, the volumetric capacity of the charge process (3.75 A h L� 1) is the capacity versus the total volume of electrolyte. The ohmic resistance of the battery was tested by a battery internal resistance tester (DME-20, Nanjing Daming Co. Ltd.). All the electrochemical tests were conducted at room tem-perature around 23 �C.

The morphologies of the electrodes were analyzed by scanning electron microscope (SEM, JEOL 7100F) and the element contents were determined by energy dispersive X-ray spectroscopy (EDX) mapping. The crystal phase and composition of Sn product were analyzed by X-ray diffraction (XRD) on a Philips high-resolution X-ray diffraction system (model PW 1825). X-ray photoelectron spectroscopy (XPS) was per-formed using a Physical Electronics PHI 5600 multi-technique system equipped with an Al monochromatic X-ray source at a power of 350 W. The Sn sample was measured after ion milling with argon gas with the depth of approximately 20 nm to eliminate the signal information from the surface of metal oxide before the XPS test. Fig. 1. Schematic of the Mn Sn system during the charge process.

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3. Results and discussion

3.1. Electrochemical process analysis

Fig. 2 shows the CV curves of the Sn2þ/Sn and Mn3þ/Mn2þ redox reactions on a glassy carbon working electrode. To depict the results more clearly, the electrode potential described in the figures has been changed from SCE to SHE, and the current density is normalized to the geometrical area of the glassy carbon electrode. In Fig. 2a, two pairs of redox reaction peaks are observed. The cathodic peak at � 0.30 V cor-responds to the Sn2þ to Sn reduction reaction and the anodic peak at � 0.16 V during the reverse scan reflects the Sn to Sn2þ oxidation pro-cess. In Fig. 2b, only a single anodic peak was observed at the potential of 1.58 V, corresponding to the oxidation process of Mn2þ to Mn3þ, while the curve becomes complex during the cathodically reverse scan. A broad merged peak consisting of two peaks reflecting Mn3þ to Mn2þ

and MnO2 to Mn2þ, ranging from 0.89 to 1.49 V, is observed in the CV profile. The formation of MnO2 is attributed to the chemical dispro-portionation reaction of Mn3þ in acidic aqueous solution [19,20], which

can be described as equation (4):

2Mn3þ þ 2H2O⇌Mn2þ þ MnO2þ4Hþ (4)

The reason for the merging of two peaks has been disclosed by pre-vious research [20]. Xue et al. investigated the electrochemical behav-iors of Mn redox reactions in sulfuric acid with different concentrations on carbon felt electrode. Their results exhibit that the peak separation of Mn3þ to Mn2þ and MnO2 to Mn2þ can be observed with a H2SO4 con-centration of 1 M. When further increasing the concentration of H2SO4, the peak potential of MnO2 to Mn2þ reduction reaction becomes increasingly positive, and its separation from Mn3þ to Mn2þ becomes not apparent. The CV curves at different scan rates of Sn and Mn redox reactions ranging from 10 to 50 mV s� 1 are shown in Fig. 2c and d. As summarized in Fig. 2e and f, the Sn2þ to Sn reduction reaction and Mn2þ

to Mn3þ oxidation reaction follow linear trends with the square root of the scan rates, indicating the diffusion-controlled process of both re-actions [26–28]. Moreover, it is noteworthy that the peak current den-sity of Mn-based redox reaction is significantly lower than that of the Sn-based redox reaction, indicating the positive Mn reaction has a

Fig. 2. Cyclic voltammograms showing that (a) Sn2þ/Sn and (b) Mn3þ/Mn2þ redox reactions, respectively, at a scan rate of 50 mV s� 1. Cyclic voltammograms of (c) the Sn2þ/Sn redox couple and (d) Mn3þ/Mn2þ redox couple at different scan rates. (e) The peak current densities of Sn2þ to Sn reduction reaction and (f) Mn2þ to Mn3þ oxidation reaction versus the square root of scan rates.

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more sluggish kinetic. To further verify this hypothesis, EIS tests at the electrode potentials of � 0.21 V and 1.54 V with an excitation signal of 5 mV in the frequency ranging from 100 kHz to 10 mHz were conducted. As shown in Figs. S3a and S3b, the charge transfer resistance of Mn3þ/Mn2þ is much larger than that of the Sn2þ/Sn redox reaction, confirming the sluggish kinetics of Mn3þ/Mn2þ. It should be noted that although the sluggish kinetics of Mn3þ/Mn2þ may bring a relatively higher activation loss during the battery operation, the three-dimensional porous carbon felt electrode, instead of the

two-dimensional planar electrode, can offer more surface area for the Mn3þ/Mn2þ redox reactions to minimize the polarization loss during battery operation [29]. Moreover, some metal oxide catalysts such as CeO2 are verified to have a catalytic effect towards the Mn3þ/Mn2þ

redox reactions [30], which will also be helpful to improve its kinetics in future optimizations.

It is well known that gassing behavior during the charge process is a severe issue that limits the performance of aqueous rechargeable battery systems. Particularly, hydrogen evolution reaction causes irreversible

Fig. 3. Electrochemical performance of the Mn Sn cells. (a) Galvanostatic charge/discharge profiles at 10–30 mA cm� 2, and (b) corresponding coulombic efficiency (CE), voltage efficiency (VE). Energy efficiency (EE) of Mn Sn cells under different operating current densities within 22 cycles of rate capability tests. (d) Galva-nostatic charge/discharge profiles of the 6th and 22nd cycle at 10 mA cm� 2. (e) Galvanostatic charge/discharge profiles of the 10th and 18th cycle at 20 mA cm� 2.

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deterioration or even failure in Fe-Cr [31], all iron [32], Zinc-Ce [23], and all-copper [33] batteries. However, gas-producing side reaction peaks were not observed throughout the whole scan range (from � 0.46 V to 1.74 V vs. SHE), which is wider than the battery operating voltage range. This can be attributed to the high hydrogen evolution overpotential of tin metal [26].

3.2. Performance of the Mn Sn battery and charge storage mechanism analysis

The electrochemical performance of the Mn Sn battery that utilizes the Mn3þ/Mn2þ and Sn2þ/Sn redox couples in H2SO4 electrolyte is characterized by a lab-made cell, as described in Fig. 3. It is worth mentioning that PBI membranes instead of Nafion membranes are used for demonstration in the battery test. Although Nafion is widely used as a proton conducting polymer membrane for other aqueous battery and fuel cell systems due to good Hþ conductivity and stability [34–36], its high price would increase the system cost and limit the commercial application to some extent [37–39]. By contrast, the PBI membranes are demonstrated to be more cost-effective and have a much higher ionic selectivity in a multiple-ion system [40,41]. Fig. 3a shows the galva-nostatic voltage profiles under different operating current densities of 10–30 mA cm� 2, where well-defined flat charge-discharge plateaus are observed. The efficiencies of the battery are summarized in Fig. 3b. The coulombic efficiency increases from 98.2% to 98.8% with the current density increased from 10 to 30 mA cm� 2. The reasons for coulombic efficiency that is lower than 100% are mainly ascribed to two factors: 1) Although the PBI membrane shows a high ion selectivity and can diminish the Mn2þ and Sn2þ crossover to a considerable extent, it is not entirely impermeable to these redox-active ions. 2) Even though the MnO2 generated by disproportionation reaction (4) can be discharged back to Mn2þ via a combination of chemical and electrochemical rou-tines [42], the transformation is still incomplete since the MnO2 parti-cles cannot release all the capacity at the end of the discharge process. Nevertheless, the coulombic efficiency gradually increases with the enhanced current density due to the reduced time for soluble ion’s crossover [43]. With regard to the voltage efficiency, it reaches 94.4% at 10 mA cm� 2 and decreases to 87.0% as the current density increases to 30 mA cm� 2. It is understandable because all battery polarization losses, including activation loss, ohmic loss, and concentration loss increase with operating current density [44]. In addition, it is found that energy efficiency decreases with increased current density, which has the same trend of voltage efficiency, revealing that voltage efficiency is the dominating factor in determining the energy conversion efficiency of the system. The energy efficiency attains 86.0% when the current density increases to 30 mA cm� 2, which is higher than the previously reported Mn-based batteries at the same operating current density [18,20,25]. Another interesting finding is that during the whole cell performance test, especially at the end of the charge process, no apparent hydrogen evolution and oxygen evolution reactions are observed. Recall that the transparent acrylic electrolyte tank is used in the battery setup, if bub-bles are formed during battery operation, they will grow and attach onto the electrode surface, which can be easily captured by the reflection of the liquid-gas interface [42]. However, in the experiments, no bubbles were observed, which is consistent with the CV results.

The rate capability of the battery was tested for 4 cycles at each operating current density. To enable the battery to reach a steady state during tests, it was activated at 10 mA cm� 2 for extra 2 cycles to reach a steady state before rate capability tests. The corresponding charge- discharge profiles are recorded in Fig. S4. As summarized in Fig. 3c, the coulombic efficiency of the battery is 93.5% in the first cycle and gradually reaches 97.3% in the third cycle. In addition, the capacity under all operating currents can maintain about 3.7 A h L� 1, demon-strating a remarkable capability. Particularly, to assess the battery’s stability, the operating current density is swiftly changed from 30 mA cm� 2 to 20 mA cm� 2 and 10 mA cm� 2 at the 15th and 19th cycle,

respectively. It is noteworthy that both the discharge capacity and en-ergy efficiency of the battery are fully recovered. The voltage profiles at 10 mA cm� 2 (the 6th and 22nd cycle) and 20 mA cm� 2 (the 10th and 18th cycle) before and after switch are recorded and shown in Fig. 3d and e. Results show that the curves exhibit a high overlap ratio, further indicating the electrochemical and chemical robustness of the battery components during operation [45].

To further demonstrate the long-term suitability and stability of the battery, the cycling tests were conducted at 10 mA cm� 2 for 100 cycles. The corresponding charge-discharge profiles are recorded in Fig. S5. As shown in Fig. 4a, the coulombic efficiency and energy efficiency keep above 98.0% and 91.5% during cycle tests. The achieved volumetric discharge capacity reaches about 3.7 A h L� 1 without significant decay. These results demonstrate the stability of the battery over cycling tests. The previously reported Mn Zn and Mn H batteries suffer from self- discharge issues induced by zinc dissolution in sulfuric acid and H2 management. Herein, to evaluate the self-discharge performance, the battery is first charged to 3.75 A h L� 1 and then rest for 100 h. The open circuit voltage of the battery is measured to be 1.70 V and can maintain up to 1.64 V at the end of the test (Fig. 4b), suggesting the low self- discharge rate of the system. The polarization test in Fig. 4c shows the peak power density of the Mn Sn battery can reach 228 mW cm� 2, which is similar or even higher than that of the aqueous batteries under the same structure [46]. The ohmic loss can be calculated from the IV curve and compared with the total battery polarization losses [47,48], which is measured to be 2.3 Ω cm2 by the battery tester. As summarized in Fig. 4d, the battery polarization losses increase from 33 to 783 mV, while the ohmic loss increases from 23 to 575 mV with the current density increased from 10 to 250 mA cm� 2. Therefore, it is estimated that 70%–78% of the voltage loss can be attributed to the ohmic resis-tance at the current density under 250 mA cm� 2. The ohmic resistance mainly results from the ion transport through the membrane and porous electrode as well as electron transport in the porous electrode. To decrease the ohmic loss of the battery, further optimization of battery components such as battery structures, electrode parameters, and elec-trolyte conductivity to minimize the battery ohmic loss will play an essential role for future performance improvement [49]. Practically, it is demonstrated that only one single Mn Sn cell is capable of powering the mini motor fan, as shown in Fig. S6 and the supporting video.

3.3. Characterizations of the porous electrodes after charge and discharge

In this study, porous carbon felt is employed as both negative and positive electrodes. Since the specific surface area of the three- dimensional carbon felt is much higher than that of the flat plate elec-trode, the activation loss can be effectively minimized as the local cur-rent density decreases. To gain a better understanding of the charge and discharge process inside the porous electrodes, ex-situ SEM, EDX, and XPS characterization methods are further conducted.

As shown in Fig. 5a, the fibers of carbon felt interconnect well with each other forming a conductive network with pores to facilitate the mass transport of the electrolyte during the charge and discharge pro-cess [50]. Fig. 5b exhibits the enlarged image of the carbon fiber surface, from which can be seen it is clean and smooth, providing an appropriate surface for the deposition of metal products. Compared to the pristine electrode, the negative electrode at the end of the charge process is covered by the metallic Sn products (Fig. 5c). A portion of the Sn metal forms the rock-like particles, while others cover the fiber surface with a film-like layer (Fig. 5d). The morphology of the positive electrode at the end of the charge process is exhibited in Fig. 5e and f, which shows only a rough MnO2 layer on the fiber surface. It is worthy to note that although film-like products appear on both sides of the electrodes, the formative routine of Sn and MnO2 is different. The former one is an electroplating process (electrochemical routine), which can only occur at the two-phase interface with the existence of the liquid electrolyte containing Sn2þ and the conductive carbon surface. Notably, Jiang et al.

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confirmed that the oxygenous functional groups induced by the thermal treatment of carbon felt play an active role to facilitate the uniform distribution of metal products [51]. Thus, the film-like Sn layer can be overserved on the fiber surface of the thermally-treated carbon felt. By contrast, the MnO2 particle is formed via a chemical routine as dispro-portionation reaction (4) mentioned above. It can be possibly generated in the space where H2O and Mn3þ coexist, and randomly deposits onto the electrode surface. The EDX maps of the corresponding SEM images of Fig. 5d and f are recorded and shown in Figs. S7 and S8, respectively. Pronounced peaks of Sn, Mn and O elements can be observed in the negative and positive electrodes, respectively, confirming the existence of Sn and MnO2 after the charge process. Fig. 5g and h shows the morphology of the negative electrode at the end of the discharge pro-cess. It is found that both rock-like Sn particles and film-like Sn layers disappeared with only small residual particles existing on the fiber surface. Similarly, the rough MnO2 layer also disappeared but left small residuals on the electrode surface (Fig. 5i and j). The residual particles left on the fiber surface at the end of the discharge process as the battery was unable to release all the charged electrical capacities towards a higher depth of discharge, particularly at current densities higher than 10 mA cm� 2. This can be primarily due to the increased cell polarization, leading to cell voltage lower than the default voltage limit of 1.3 V.

To confirm the chemical compositions of the charged product on the negative electrode, XRD measurements of pristine and negative elec-trodes were conducted to verify the product on the negative electrode, and the result is shown in Fig. 6a. It can be seen that the characteristic peaks of the product obtained are consistent with those of β-tin (Sn: PDF#04-0673), indicating that metallic tin is deposited onto the nega-tive electrode during the charge process. To further confirm the oxida-tion state of Sn and Mn present in the electrodes, XPS analysis was carried out. As shown in Fig. 6b, a distinctive peak arisen at around 484.8 eV is considered to stem from the electron binding energy of tin metal. Besides, a trace amount of tin oxide (peak at 486.4 eV) is also

detected due to the formation of a protective oxide layer at the surface of Sn metal as exposure of the sample in the air before and during tests. A similar phenomenon can be observed in XPS spectra of other electro-plating Sn researches [52]. Moreover, the samples of the depth of 0, 10, and 20 nm has been shown in Fig. S9. It is evident that the Sn peaks become stronger while the SnO2 becomes weaker with the depth going, confirming the Sn is oxidized from the air. The spectra of Mn species is shown in Fig. 6c. The Mn 2P3/2 and 2P1/2 peaks are centered at 642.1 and 653.8 eV, respectively, corresponding to a spin energy separation of 11.7 eV, which is consistent with previous researches reported data for Mn 2p3/2 and Mn 2p1/2 of MnO2, confirming the presence of MnO2 in the positive electrode [53,54].

3.4. Future optimizations

To estimate the active material costs of the systems, the following equation (5) is used for detailed calculation [42]:

C ¼3600EF

X

i

QiMi

ni(5)

Where C is the active material cost per kilowatt-hour ($ kWh� 1); Q is the cost of the active materials per kilogram ($ kg� 1); M is the molecular mass of the active material (g mol� 1); n is the number of electrons involved in the redox reaction; E is the equilibrium cell voltage; F is the Faraday’s constant (96,485 C mol� 1). The bulk prices of high-purity (>99%) SnSO4 and MnSO4 resolve around 13.0 $/kg and 2.2 $/kg ac-cording to a recent quotation (Shenzhen Hengdiyuan Co., Ltd.) [55], respectively. The total raw material cost of about $37.0 kW h� 1 for the present system. It is noteworthy that the cost of active species is cheaper than some existing energy storage systems such as all-vanadium redox flow batteries ($116.4 kW h� 1). Meanwhile, the price is also lower than some Mn-based batteries such as Mn V and Mn H systems, due to the exclusion of high-priced vanadium and noble metal catalysts.

Fig. 4. (a) Cycling retention in coulombic efficiency, energy efficiency, and volumetric capacity of the Mn Sn battery. (b) Open circuit voltage of the battery during the 100-h test. (c) Polarization curve obtained during the discharge process. (d) The total battery voltage losses, ohmic loss and the ratio of ohmic loss to total battery voltage losses under the current density of 10–250 mA cm� 2 in the polarization test.

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In 2013, the US Department of Energy (DOE) developed a cost and performance target of storage technology development for the grid. The cycle life should be more than 5000. The system capital cost should be under $150/kWh while the energy efficiency should be over 80%. Compared to the DOE’s target, this work is a preliminary investigation

exhibiting the potential to be used as a future energy storage system. In the future, advanced optimizations can be used to decrease the kinetic polarization at the electrolyte/electrode interface, ohmic loss of elec-trolyte, and electrode and concentration polarization from mass trans-port limitations. For example, the present battery exhibits a high ohmic

Fig. 5. SEM images of the (a–b) thermally treated carbon felt; (c–d) negative electrode after charge; (e–f) positive electrode after charge; (g–h) negative electrode after discharge; (i–j) positive electrode after discharge.

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resistance of 2.3 Ω cm2; which is estimated that 70%–78% of the voltage loss can be attributed to the ohmic resistance at the current density under 250 mA cm� 2. To decrease the ohmic loss, thinner electrode with flow-by structures of flow batteries can be inferred. However, the reduction of electrode thickness will simultaneously decrease the active sites for the redox reactions. To address this issue, efficient electro-catalysts such as CeO2 for Mn3þ/Mn2þ can be employed for future performance improvement [30]. Recently, Li et al. proposed etching method to create secondary pores on the carbon fiber surface and found that the effective active sites for redox reactions can be largely increased, providing another direction for future electrode design [27]. Another issue that needs to be solved is the crossover of active species through the membrane. Although the PBI membrane exhibits relatively good stability during battery operation, the cycle life of the battery needs to be further prolonged. Selection of other membranes (such as hydrocarbon membranes) and modification methods (Gel deposition) are helpful to achieve this goal [56].

Meanwhile, to further employ Mn redox couple for redox flow bat-teries, the disproportionation reaction has to be inhibited. It is accept-able for a static battery since most of the MnO2 could release its capacity back to electricity. However, this disproportionation reaction is a chemical reaction, and MnO2 particles can be generated in the place where Mn3þ and H2O coexists. The generated particles have the risk of blocking the inlet or outlet of the flow battery during long-term opera-tion, leading to a safety hazard. To inhibit the disproportionation re-action, the effect of additives such as TiOSO4 and MSA supporting electrolyte on the formation and particle size of MnO2 needs to be investigated in future research [21,57].

4. Conclusions

In summary, a Mn Sn battery in which stannous sulfate negative electrode was separated from the manganese sulfate positive electrode. The present battery exhibited an open circuit voltage of 1.7 V. Due to the

good reversibility of Sn deposition/stripping and manganese redox re-action on the carbon surface, the coulombic efficiency of present battery ranged from 98.2% to 98.8% with current density increasing from 10 to 30 mA cm� 2. The energy efficiency of the battery reached 86.0% at 30 mA cm� 2. The stability of the present battery was also confirmed by the cycling test. The results showed that the energy efficiency of the battery maintained above 91.5% without observable decay at 10 mA cm� 2. With a decent rate and cycle performance, it is envisioned that the Mn Sn battery possesses the potential for future energy storage applications.

Acknowledgments

The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. T23-601/17-R).

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.226918.

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Fig. 6. (a) XRD results of negative electrode after charge and pristine electrode, providing evidence for the formation of metallic tin in the negative electrode. XPS results, confirming the composition of Sn (b) and MnO2 (c) in the negative and positive electrode, respectively.

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