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Journal of Power Sources

journal homepage: www.elsevier.com/locate/jpowsour

An aqueous manganese-copper battery for large-scale energy storageapplications

L. Wei, L. Zeng, M.C. Wu, H.R. Jiang, T.S. Zhao∗

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

H I G H L I G H T S

• Investigate the performance of a novel MneCu battery.

• The battery achieves a significantly low active material cost of $37 kWh−1.

• Coulombic efficiency reaches 94% at current density higher than 20mA cm−2.

• Energy efficiency maintains ∼79% with no decay at 10mA cm−2 over 100 cycles.

A R T I C L E I N F O

Keywords:Mn-Cu batteryCelgard separatorLow costEnergy storage

A B S T R A C T

This work reports on a new aqueous battery consisting of copper and manganese redox chemistries in an acidenvironment. The battery achieves a relatively low material cost due to ubiquitous availability and inexpensiveprice of copper and manganese salts. It exhibits an equilibrium potential of ∼1.1 V, and a coulombic efficiencyof higher than 94% is obtained at an operating current of 20mA cm−2. Cyclic tests confirm that the energyefficiency maintains ∼79% with no observable decay at 10mA cm−2 over 100 cycles. Possessing other ad-vantages such as ease of scalability and capable of using an inexpensive separator, the battery offers a promisingsolution for large-scale energy storage applications.

1. Introduction

Large-scale energy storage (also called grid energy storage) is acollection of methods used to store electrical energy on a large scalewithin an electrical power grid. Electrical energy is stored during timeswhen production exceeds consumption and returned to the grid whenproduction falls below consumption. The increasing deployment of re-newable energy especially from intermittent power plants such as windpower, tidal power, solar power has driven the development of large-scale energy storage systems to bridge the supply shortfall in recentyears. Until now, various technologies have been developed, includingmethods such as compressed air, flywheel, pumped hydro, batteries,hydrogen production, etc. [1–4]. Among these methods, the batterytechnology capitalizing on two redox chemistries to perform the con-version between chemical and electrical energy has been mostly de-ployed in our daily life [5–8]. In the past decades, more than a hundredtypes of organic-inorganic batteries based on the consideration of nu-merous active species have been proposed [9–12]. Although few of

them have been utilized for specific occasions, a majority of their ap-plications for large-scale energy storage are still hindered by technicaland economic barriers [13]. Therefore, searching for novel electro-chemical redox species with low cost and high stability is still highlydesirable at the current stage.

Over the preceding years, the manganese redox couple has attractedremarkable attention. Electrolytic manganese oxide is one of the mostwidely used cathode materials in alkaline primary batteries, lithiummanganese primary batteries, and supercapacitors. Possessing the ad-vantages of rich abundance, low production cost, environmentalfriendliness, high electrode potential, good electrochemical perfor-mance over a wide temperature range, its usage has exceeded 230,000metric tons per annum with an annual growth rate more than 9.6%[14]. Presently, most of the attention has been paid to alkaline man-ganese-based secondary batteries, but they generally suffer from criticalissues such as poor rechargeable and cycle performance [15–19]. Onthe other hand, relatively much less attention has been given to themanganese redox couple in the acid environment. Dong et al. developed

https://doi.org/10.1016/j.jpowsour.2019.03.085Received 21 October 2018; Received in revised form 15 March 2019; Accepted 21 March 2019

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

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0378-7753/ © 2019 Elsevier B.V. All rights reserved.

T

an MneTi battery by using the soluble Mn and Ti as redox species [20].The MneTi battery can suppress generation of precipitation of MnO2

and can be charged and discharged well by containing a titanium ion inthe positive electrode electrolyte. Its coulombic efficiency (CE) couldreach 97.8% when an anion membrane was used in the battery. Xueet al. demonstrated an MneV battery to replace the positive side in all-vanadium redox battery [21]. They investigated the electrochemicalbehavior of the Mn redox couple on different carbon substrates and avoltage efficiency (VE) of 90% could be achieved in the charge-dis-charge battery test. Recently, Chen et al. proposed an MneH chemistry[22]. Their assembled battery exhibited a discharge voltage of ∼1.3 Vand a rather long lifetime without observable decay. They discoveredthat the MnO2 deposition dissolution reaction was a highly reversibleprocess, addressing the rechargeability and stability issues in conven-tional Mn electrode. Moreover, a highly reversible hydrogen electrodeas the anode by utilizing Pt-catalysed HER/HOR reactions was em-ployed to overcome the poor rechargeability of the conventional anodesin the work. The fast kinetics of the reactions contributed to the highrate capability of 100mA cm−2 in the MneH battery test. Althoughthese systems are great inventions and can promote the application ofmanganese redox pair in secondary batteries for energy storage market,they all have some intrinsic issues. For example, the hydrolysis reactionof titanium salt in the aqueous medium is a challenge during long-termoperation. High crossover rates of vanadium ions cause a low cou-lombic efficiency and high capacity decay rate during battery opera-tion, leading to a short lifetime for the MneV battery [23]. Moreover,storage and transport of H2 gas, as well as utilization of noble metal Ptas electrocatalysts, also seriously limits the practical application ofMneH battery.

The rich chemistry of manganese allows it to exist in diverse statesand provides opportunities for the discovery of novel manganese bat-tery systems. In this work, we propose and demonstrate a manganese-copper (MneCu) battery chemistry in acidic conditions by employing adilute H2SO4 as the supporting electrolyte. The battery operates basedon the redox reaction between soluble Mn3+/Mn2+ as the positiveelectrode and Cu2+/Cu as the negative electrode. To the best of ourknowledge, this battery has not been studied in previous research. It isworthy of note that both active materials (CuSO4 and MnSO4) andsupporting electrolyte (H2SO4) are fairly cheap and have ubiquitousavailability, which is of significant priority for large-scale energy sto-rage applications.

The detailed working principle of the battery is illustrated in Fig. 1.Taken charge process as an example, Cu2+ is reduced to metal Cu at thenegative electrode, while at the positive electrode, Mn2+ is oxidized toform Mn3+. Cations migrate through the membrane to constitute acomplete electric circuit. During the discharge process, a reverse pro-cess occurs. The overall cell operation can be described as follows:

At the positive electrode:

− ↔+ − +Mn e Mn2 3

At the negative electrode:

+ ↔+ −Cu 2e Cu2

Overall reaction:

+ ↔ ++ + +2Mn Cu 2Mn Cu2 2 3

2. Experimental

2.1. Battery setup assembly

Figure S1 illustrates the components of the MneCu battery. Itconsisted of two PMMA frames, in which the positive and negativeelectrodes were placed. Both electrodes were composed of commer-cially available graphite felt (SGL company, GFA series, the porosity of95%, specific area of 0.8m2 g−1, the active area of 0.785 cm2,

uncompressed thickness of 9mm) [24]. They were treated at 500 °C for4 h in a muffle furnace (Nabertherm N11/H) under ambient air beforeuse. There are mainly two possible benefits for the heat treatment ofgraphite felt. On the one hand, it would increase the electrochemicalactivity and hydrophilicity [25]. On the other hand, it will lead to amore uniform distribution of solid metal deposition compared to theoriginal electrode [26]. The electrodes were separated by a Nafion 212membrane (Dupont, USA) without any pretreatment. The Viton gasketswere placed between two sides to prevent electrolyte leakage. Two softgraphite plates were used as endplates to collect electric current. Toavoid the cross-contamination of Cu2+ and Mn2+ across the membrane,0.3 mL solution prepared by dissolving 0.8 M CuSO4 + 0.8 M MnSO4 +0.8 M H2SO4 was added in both positive and negative electrodes as theinitial electrolyte [27,28]. The chemicals used in this work were allcommercially available from Sigma Aldrich and used as received. Theassembled battery was charged to a constant capacity of 8.3 Ah L−1 (5mAh) to suppress possible side reactions and then discharged to a cut-off voltage of 0.3 V under various current densities.

2.2. Electrochemical measurements and material characterizations

The electrochemical performance of the Cu2+/Cu and Mn3+/Mn2+

was investigated by cyclic voltammetry (CV) via a typical three-elec-trode system. A glassy carbon electrode (diameter: 5 mm) was em-ployed as the working electrode, while a saturated calomel electrode(SCE) assembled in a KCl salt bridge, and a platinum mesh were em-ployed as reference and counter electrode, respectively. The electrolytefor CV tests is the same as the one used for battery tests. To make theresults more clearly, the electrode potential described in the figures hasbeen changed from the saturated calomel electrode (SCE) to the stan-dard hydrogen electrode (SHE). The CV and EIS curves were obtainedusing a potentiostat (EG&G Princeton, model 2273). The battery per-formance was evaluated in a battery test system (BT2000, ArbinInstrument, Inc.). The ohmic resistance was tested by a battery internalresistance tester (DME-20, Nanjing Daming). All the electrochemicaltests were conducted at room temperature around 23 °C.

The electrode morphologies were analyzed by scanning electron

Fig. 1. Schematic of the battery during the charge process.

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microscope (JEOL 6700F), and the elemental contents were determinedby an energy dispersive X-ray spectroscopy mapping. The contact an-gles of the graphite felt electrodes are measured by an automated op-tical tensiometer (Attension Theta, Biolin Scientific).

3. Results and discussion

3.1. Electrochemical process analysis

The cyclic voltammetry of positive Mn3+/Mn2+ redox reaction wascarried out between 0.74 and 1.74 V (vs. SHE) at 5mV s−1. As shown inFig. 2a, a single anodic peak was observed at the potential of 1.45 V,corresponding to the oxidation of Mn2+ to Mn3+. Two peaks wereobserved when reversely scanned toward a cathodic direction with apotential of 1.34 and 0.95 V, which relates to the processes of Mn3+ toMn2+ and MnO2 to Mn2+ [21,29], respectively. The formation of MnO2

is mainly due to the disproportionation reaction of Mn3+, described as:

+ ↔ + ++ + +2Mn 2H O Mn MnO 4H32

22 (1)

It is worth noting that the disproportionation reaction is reversible.The equilibrium will move towards left or right when the concentra-tions of Mn3+, Mn2+ and H+ change.

The CV curve of the Cu half-cell reaction was recorded between−0.06 V and 0.64 V (vs. SHE) at 5mV s−1 (Fig. 2b). Compared to thepositive voltammetry involved with two reduction peaks, the electro-chemical behavior of negative Cu2+/Cu redox reaction appears rela-tively simpler. It is shown that only a single cathodic peak at 0.16 Vcorresponding to Cu2+ reduction reaction and an anodic peak at 0.37 Vreflecting Cu to Cu2+ oxidation occurs during the reverse scan. More-over, it is noted that the current of Mn redox reactions is several factors

lower than the Cu redox reactions, seemingly the positive Mn redoxreaction has a much more sluggish kinetic. To verify this hypothesis,EIS tests with an excitation signal of 5mV in the frequency rangingfrom 100 kHz to 1mHz at the electrode potential of 1.39 and 0.29 V (vs.SHE) were performed, respectively. As shown in Fig. 2c and d, thecharge transfer resistance of the Mn3+/Mn2+ is much larger than thatof the Cu2+/Cu redox reaction, confirming that the Cu2+/Cu redoxcouple presents much better kinetics than that of Mn3+/Mn2+ redoxcouple, which is consistent with the CV results. These results indicatethat future catalysis efforts to improve the battery performance shouldbe focused more on Mn redox couple in the positive electrode ratherthan the Cu redox couple in the negative electrode.

3.2. Performance of the MneCu battery and charge storage mechanismanalysis

To demonstrate the practical application of these two redox peaks, alab-made battery was assembled, and its performance was furtherevaluated by typical charge-discharge measurements. The open circuitvoltage of the battery is measured to be ∼1.1 V. As seen in Figure S2. Itmaintained above 1.034 V after 192 h (8 days) of the test, suggestingthe stable operation of this system. The average charge voltage platform(Fig. 3a) is 1.12 V at 5mA cm−2, increasing by 30 and 70mV when thecurrent density is increased to 10 and 20mA cm−2, respectively. Whilethe discharge platform exhibits minor differences with a voltage plateauof ∼1 V when the current density increases. Fig. 3b summarizes theefficiencies at different current densities. The coulombic efficiency ofthe battery attains 92.8% at 5mA cm−2, slightly increasing to 93.4%and 94.0% at 10 and 20mA cm−2, respectively. The inefficiencies ofcoulombic efficiency are mainly ascribed to two aspects: 1) crossover of

Fig. 2. Cyclic voltammograms for Mn3+/Mn2+ (a) and Cu2+/Cu (b) obtained at a scan rate of 5mV s−1; Electrochemical impedance spectroscopy for Mn3+/Mn2+

(c) and Cu2+/Cu (d) obtained with an excitation signal of 5mV in the frequency range from 100 kHz to 1mHz at the electrode potential of 1.39 and 0.29 V (vs. SHE).

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soluble active species across the membrane. The pristine Nafion mem-brane will exchange the counterions from protons to multivalent ions(Cu2+, Mn2+) due to the latter's high affinity. These exchanged mul-tivalent ions will also contribute to the ion conduction across the Nafionmembrane during battery operation; 2) incomplete transformation ofMnO2 during the discharge process. Although the equilibrium of reac-tion (1) moves towards the left to form soluble Mn3+ during the dis-charge process, the produced MnO2 particles could not release all thecapacity at the end of the discharge process. Nevertheless, the cou-lombic efficiency gradually increases as the current density increases,due to the reduced time for soluble ions' crossover [30], while thevoltage efficiency exhibits an opposite tendency. It reaches 88.7% at5mA cm−2 and decreases to 81.1% with current density increased to20mA cm−2 because of the enlarged polarizations with an increase ofoperating current.

Before further discussion of the battery performance, it is importantto understand the charge storage mechanism since it will be helpful forfuture design and optimization to improve the battery performance. It isinteresting to note that only one discharge plateau is observed if werealize that there exist two cathodic peaks in the CV result (Fig. 2a).One possible explanation for this phenomenon is that the dischargecapacity is mainly contributed by the reaction of Mn3+ to Mn2+ ratherthan MnO2 to Mn2+ because the electrode potential of the latter reac-tion is as low as 0.95 V, which is much lower than the discharge voltageplateau. To verify this hypothesis, CV tests at different scan rates areconducted, and the results are shown in Figure S3. The battery dis-charge process involves the Mn reduction reaction (cathodic peaks inFigure S3a) and Cu oxidation reaction (anodic peaks in Figure S3b). It isrevealed that the peak potential of MnO2 to Mn2+ increases by 128mVwhen the scan rate increases from 3 to 7mV s−1, while that of Mn3+ toMn2+ reduction reaction keeps almost the same within different scanrates. Therefore, if it is the reduction of MnO2 to Mn2+ that contributesto the discharge capacity, the discharge voltage plateau will show a

significant reduction when the current density increases due to thesevere polarization of MnO2 to Mn2+ reaction. However, the minordecrease in the discharge voltage plateau negates this assumption andvalidates that it is the Mn3+ to Mn2+ rather than MnO2 to Mn2+ thatdominates the discharge process on the positive electrode.

Moreover, it is found that the coulombic efficiency of the batterycan reach as high as 94%, indicating the produced MnO2 can release itscapacity during discharge. This is possible because the concentration ofMn2+ and H+ increase in the positive electrolyte tank as the dischargeprocess proceeds, and the equilibrium of reaction (1) will move towardsthe left to form soluble Mn3+ consequently, which can be electro-chemically converted to Mn2+ during discharge. In this regard, it isbelieved that the MnO2 generated by disproportionation reaction (1)can be discharged back to Mn2+ via a combination of chemical andelectrochemical routines.

To investigate the rate capability of the MneCu cell, it was tested at5, 10 and 20mA cm−2 for 6 cycles at each current density. To enablethe battery to reach a steady state before tests, it was first activated at5mA cm−2 for 4 cycles. As shown in Fig. 3c and d, the energy efficiency(EE) of the battery can reach up to 83.1% at 5mA cm−2 and decrease to78.5% and 76.2% with current density is increased to 10 and20mA cm−2, respectively. The capacity under all operating currentscan maintain up to 7.5 Ah L−1, demonstrating a remarkable rate cap-ability. In addition, to assess the stability of the battery, the operatingcurrent density is swiftly changed from 20 to 10mA cm−2 at the 23rdcycle. It is noted that both the battery energy efficiency and dischargecapacity are fully recovered, indicating the chemical and electro-chemical robustness of the battery components during battery opera-tion.

To further demonstrate the long-term stability and suitability of thebattery, the cycling test of the MneCu battery is conducted at thecurrent density of 10mA cm−2 for 100 cycles. As shown in Figure S4,the coulombic efficiency of the battery is 80.6% in the first cycle and

Fig. 3. (a) Typical charge and discharge curves at 5 (blue), 10 (red) and 20 (black) mA cm−2, respectively. (b) CE and VE of the Mn-Cu battery at different operatingcurrent densities. EE (c) and discharge capacity (d) as a function of cycle number at different current densities. (For interpretation of the references to colour in thisfigure legend, the reader is referred to the Web version of this article.)

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gradually increases to 92.4% in the fifth cycle. During cycle tests, theenergy efficiency keeps around 79% without an observable decay(Fig. 4a). This value is higher than other previously reported Mn-basedbatteries such as MneH [22] and MneV [21] systems and comparableto other aqueous batteries (Copper [31], Zinc [32] and Vanadium [33]based systems). Moreover, the discharge capacity of the batteriesmaintains above 7.5 Ah L−1 at the first 20 cycles and gradually reachesaround 8 Ah L−1 over the last 40 cycles (Fig. 4b). These results furtherdemonstrate the excellent stability of the battery over cycling tests.

3.3. Characterizations of the MneCu cell

To further clarify the energy storage mechanism of the battery, weinvestigated the evolution of both the positive and negative electrodesby SEM and EDS characterizations. As shown in Fig. 5a, the fibers ofgraphite felt are well interconnected. The porous structure of graphitefelt can facilitate the mass transport of the electrolyte during thecharge/discharge process [34,35]. Fig. 5b shows the surface of graphitefiber; which is relatively clean with observable defects induced by theair oxidation at high temperature, providing an appropriate substratefor the deposition of solid products. The contact angle measurements oforiginal and 500 °C thermally treated graphite felts are shown in FigureS5. It can be seen that the original graphite felt is hydrophobic with alarge contact angle of 120°. On the contrary, the water droplet soaks

into the thermally treated graphite felt instantly once it contacts thesurface, indicating the thermally treated graphite felt is very hydro-philic. Compared to the raw electrode, the negative electrode aftercharging to 8.3 Ah L-1 is covered by Cu particles. These particles growonto the surface of fibers and can form micron-sized granular (Fig. 5c).On the contrary, only nano-sized MnO2 particles are uniformly de-posited on the surface of graphite felt after charging in the positiveelectrode (Fig. 5d), which is in well agreement with the previous re-search [22]. The main reason for the disparity of electrode morpholo-gies is ascribed to the different formative process of Cu and MnO2

during the charge process. Specifically, the deposition of Cu particles isan electrochemical process, which can only occur at the two-phase in-terface with the existence of the solid conductive base and a liquidelectrolyte containing Cu2+. Whereas, the MnO2 is formed via a che-mical routine as analyzed in the previous section. Thus, it will bepossibly generated everywhere Mn3+ and H2O coexist, and then ran-domly deposited onto the carbon matrix substrate. The EDS maps of thenegative electrode (Figure S6) and positive electrode (Figure S7) showpronounced peaks of Cu, Mn, and O, respectively, further confirmingthe composition of Cu and MnO2 during the charge process. Fig. 5eshows the morphology of the negative electrode after the dischargeprocess. The large Cu particles in the graphite electrode disappearedafter discharge to 0.3 V and almost returning the graphite felt to itspristine morphology. However, few residual Cu particles with a small

Fig. 4. (a) Energy efficiency of each cycle in the 100-cycle test at 10mA cm−2. (b) Discharge capacity recorded during the corresponding cycle tests.

Fig. 5. SEM images collected for the original graphite felt (a); typical fiber of the thermal-treated graphite felt (b); negative electrode after charge to 8.3 Ah L−1 (c);positive electrode after charge to 8.3 Ah L−1 (d); negative electrode after discharge to 0.3 V (e); and positive electrode after discharge to 0.3 V (f).

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diameter can still be observed on the surface of graphite fiber (FigureS8a) since it is hard to release all the charged electrical capacity whendischarge to 0.3 V. Similar to what happens in the negative electrode, amajority of particles in the positive electrode disappear after discharge(Fig. 5f), leaving small residues on the carbon fiber due to the in-complete dissolution of MnO2 (Figure S8b).

3.4. Scale-up of the MneCu battery

To increase the cell voltage for large-scale application, we devel-oped a bipolar stacking approach to scale up the batteries. The currentcollector of previous single battery acts as the bipolar plate betweencells in the unit. If a higher voltage is needed, the cell numbers can beincreased; while if a larger capacity is needed, the electrolyte volumeand electrode area can be increased. The ease of scalability makes thebattery to be suitably used as large-scale energy storage systems. As anexample, we assembled a unit consisting of three batteries in seriesconnection (see Figure S9). The charge-discharge profiles of the unit arerecorded and the results are shown in Fig. 6a. Similar to the trend of thesingle battery, the charge voltage of the unit increases significantly withthe current increasing from 5 to 20mA cm−2 while the discharge vol-tage shows minor differences with a voltage platform above 3 V. Fig. 6bsummarizes the efficiencies of the unit. The coulombic efficiency of theunit can reach 91.7% at 5mA cm−2, increasing by 0.7% and 1.3% at 10and 20mA cm−2. The voltage efficiency reaches 88.5% at 5mA cm−2

and decreases to 82.2% when the current density reaches 20mA cm−2.Fig. 6c displays the energy efficiency of the unit within 22 cycles underdifferent operating current densities. It is found that the energy effi-ciency can reach 83.0% at 5mA cm−2 and maintains above 75.3%when increasing to 20mA cm−2. The performance of the assembledunit is comparable to the single MneCu battery, suggesting this bipolarstacking method would not sacrifice the battery performance.

Practically, it is demonstrated that two MneCu cells are capable oflighting a red light-emitting diode for hours, as shown in Figure S10.

3.5. Inexpensive Celgard separator

In previous battery tests, the typical Nafion membrane was used toseparate the positive and negative electrolytes, as well as to conduct theH+ to form an entire circle. Although Nafion is widely used as a protonconducting polymer membrane for hydrogen and methanol fuel cellsand its transport properties has been investigated [36,37], one of themain issues is that its relatively high price would increase the systemcost and limit its commercial application. By contrast, Celgard separa-tors were previously demonstrated to be a low-cost separator and havebeen widely used in lithium-based systems such as commercialized li-thium ion and developing lithium-sulfur batteries [38,39]. Herein, weused the typical Celgard separator (CELGARD 2500) to replace Nafionmembrane. The performance was tested, and the results are comparedwith Nafion membrane. As shown in Fig. 7a, the Celgard separatorequipped battery shows significantly higher voltage efficiency under allcurrent densities. Its ohmic resistance is 1.5Ω, obviously lower than theNafion membrane equipped battery (2Ω). Therefore, the increase ofvoltage efficiency is mainly attributed to the reduced ohmic loss by theabsence of PP separators as all other components are the same. How-ever, the coulombic efficiency shows an opposite trend: the coulombicefficiency of the battery using a Celgard separator is 9.9% lower thanthat using a Nafion membrane at 5mA cm−2 and this difference is re-duced to 2.8% when the current density reaches 20mA cm−2. There aredifferent conduction mechanism for Nafion and Celgard. The Celgard isa separator with nanoscale pores, which can be utilized to conduct thesmall ions (such as protons) by the size exclusion effect. There existabundant pores ranging from 10 to 200 nm in Celgard separator (FigureS11), which are several factors larger than the size of hydrophilic ionic

Fig. 6. (a) Typical charge and discharge curves of the unit at 5 (blue), 10 (red) and 20 (black) mA cm−2, respectively. (b) CEs and VEs of the unit at differentoperating current densities. (c) EEs as a function of cycle number at different current densities. (For interpretation of the references to colour in this figure legend, thereader is referred to the Web version of this article.)

Fig. 7. (a) CE and VE of the batteries with Nafion membrane and Celgard separator at different operating current densities. (b) EE of the batteries as a function ofcycle number at different current densities. (c) The discharge capacity of the batteries recorded during the 100-cycle tests at 10mA cm−2.

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clusters (4–5 nm) for ion transport in Nafion membrane [40]. The largerpores allow the Celgard separator to have a better ion conductivity buta lower ion selectivity in this system. However, because of the moreopen structure, a larger portion of soluble Mn3+ in the positive elec-trode will migrate the separator to the negative electrode, which willreact directly with the Cu metal and decrease the coulombic efficiencyduring the battery operation. Fig. 7b summarizes and compares the rateperformance of two batteries with Nafion membrane and Celgard se-parator. It is found that at a low current density of 5mA cm−2, theenergy efficiency of the battery using a PP separator is ∼7% lower thanthat with a Nafion membrane, primarily due to the much lower cou-lombic efficiency. However, this value becomes equal and even slightlyhigher than that of battery with the Nafion membrane when the op-erating current density increases to 10 and 20mA cm−2, mainly as-cribed to enhanced voltage efficiency. Fig. 7c shows the discharge ca-pacity of the battery in 100 cycles at 10mA cm−2. As can be seen, thebattery with a Celgard separator presents a slightly lower dischargecapacity compared with that using a Nafion membrane. Moreover, al-though some fluctuations at the initial stage are observed, it reaches asteady stage after the 50th cycle, indicative of the stability of the as-sembled battery unit.

3.6. Active material cost and challenges of the MneCu battery

The cost has been identified as one key factor in determiningwidespread deployment of the large-scale energy storage systems. TheUS Department of Energy (DOE) has set a system capital cost target of$150 per kWh by 2023, and an even lower target of $100 is needed tomatch the existing grid-level technologies of pumped hydro and com-pressed air [41]. Thus, a lower active material cost is needed because itcan no doubt leave more space for the selection of other components(electrodes, membrane, bipolar plates, etc.) to match the DOE's target.To evaluate the active material costs of the systems, the followingequation is used for detailed calculation.

∑=C 3600EF

Q Mni

i i

i

Where C is the active material cost per kilowatt-hour ($ kWh−1); Qis the cost of the active materials per kilogram ($ kg−1); M is the mo-lecular mass of the active material (g mol−1); E is the equilibrium cellvoltage; F is the Faraday's constant (96485 Cmol−1). The prices of theV, Mn and Cu based salts are referred from public websites and researchpapers [13,42,43]. The bulk prices of CuSO4 and MnSO4 resolve around$2.6 and $3.6 per kilogram, respectively, leading to the total raw ma-terial cost of about $37.0 kWh−1 for the present system. As shown inFig. 8, this value is significantly lower than some state-of-the-art sys-tems such as all vanadium battery ($116.4). Moreover, it is also cheaperthan some existing Mn-based systems such as MneV and MneH systems

due to the exclusion of high-priced vanadium and noble metal catalysts.Additionally, our previous results demonstrate that inexpensive cellcomponent materials such as Celgard separators are practical in MneCubattery, rendering this system more attractive for large-scale energystorage.

The MneCu battery has some unique features and advantages overthe previously reported metal battery systems. First, the battery consistsof only carbon substrates without any expensive noble metal catalyst.The fabrication of the cell is rather simple without any complex pre-paration of the conventional MnO2 electrode. Second, both the copperand manganese salts are inexpensive and environmentally friendly dueto their ubiquitous availability and low toxicity. Third, the system canuse inexpensive separators to substitute the Nafion membranes withoutsacrificing energy efficiency when the operating current density ishigher than 10mA cm−2. Overall, the utilization of low-cost raw ma-terials such as MnSO4 and CuSO4 salt, carbon felt and inexpensive se-parators in the battery could make it an inexpensive and promisingfuture metal battery system.

However, despite the battery has some unique features and ad-vantages over the previously reported battery systems, there are stillsome challenges remaining in the present system. DOE has set thesystem efficiency higher than 80% for the long-term requirement. Tomatch this requirement as well as improving the power density of thesystem, further work to decrease the kinetic polarization at the elec-trolyte/electrode interface, ohmic loss of electrolyte, and electrode andconcentration polarization from mass transport limitations are in need.Specifically, the battery's polarization resistance is still relatively highat present (shown in Figure S12a). Take the discharge process as anexample (Figure S12b), the current peak power density of the battery is160mW cm−2, which is similar to the aqueous redox battery at an earlystage [44]. Nevertheless, this value is obviously lower than state-of-the-art flow batteries with optimized structure and electrodes [45,46]. Themain reason for the high polarization resistance is the ohmic resistance.Particularly, due to the ionic conductivity of the aqueous H2SO4 elec-trolyte is magnitude lower than the electronic conductivity of the gra-phite electrode [47], an increase of the electrode thickness (equal to thewidth of the electrolyte) will significantly increase the ohmic loss of thebattery. At present, the battery has an ohmic resistance as high as 2Ω(1.57Ω cm2), much higher than the current state-of-the-art flow batteryequipped with thin electrodes and flow fields (less than 1Ω cm2) [48].The iR-free polarization curve during the discharge process is shown inFigure S12c; it is found that the ohmic loss accounts for ∼90% of thewhole battery polarization loss at the region of current density lowerthan 200mA cm−2 and the power density can improve 63% by sub-tracting the ohmic loss. To decrease the ohmic loss and concentrationpolarization, further optimization of electrode and battery structureswith low ohmic loss and simultaneously enhanced mass transport ofactive species will also be beneficial to the battery performance[49–51]. However, the reduction of electrode thickness will decreasethe active sites for the redox reactions. To address this issue, efficientelectrocatalysts such as CeO2 for Mn3+/Mn2+ can be applied for futureperformance improvement [52]. In addition to this, multiscale-pore-network structured electrodes to simultaneously improve the specificsurface area and activity for the redox reactions are also in need forfuture electrode design of the battery [53,54]. Another issue needs to beaddressed is the crossover induced by the large pores of the separator.Although the battery with Celgard separator showed a comparable ef-ficiency to that with Nafion membrane at high operating current, itsstability and achieved discharge capacity are still lower than that withNafion membrane during cycle tests. One effective approach to solvethis problem is to decrease the pore sizes of the separator by surfacemodification methods [55,56]. Gel polymer electrolyte with adjustableporosity can also be referred for separator design of the MneCu che-mistries [57]. Furthermore, the selection of other types of separatorssuch as hydrocarbon separators can also be considered in the futurestudy. The formation and conversion of the MnO2 particles and theirFig. 8. Active material costs of different battery systems.

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effects on the charge and discharge process also need to be investigatedto improve the utilization of Mn redox species [58]. It is thus believedthat optimizing electrodes, membranes and battery structures will fur-ther improve the efficiency, power density and cycling stability of thesystem.

4. Conclusions

In summary, we investigated an aqueous battery by combiningabundant and cost-effective Mn3+/Mn2+ and Cu2+/Cu redox chemis-tries in a lab-made cell configuration. Experimental results revealedthat both redox couples exhibited a quasi-reversible behavior in sulfuricacid media and an equilibrium cell voltage of 1.1 V was obtained. Thisnewly developed battery was capable of delivering a high energy effi-ciency of 79% without structure optimization and no obvious perfor-mance decay was detected over 100 cycle tests at 10mA cm−2, in-dicating its remarkable rate capability and stable cycle performance.Furthermore, it was demonstrated that stable battery performancecould still be achieved by employing inexpensive Celgard separatorsand it could be scaled up via a bipolar stacking approach. More im-portantly, the cost of active materials in the battery was significantlylowered due to their ubiquitous availability and inexpensive price,especially compared to typical aqueous batteries such as vanadiumbattery and other Mn-based batteries (e.g., MneH and MneV systems).With the superior rate performance, stable cyclability and the otheraforementioned advantages, the MneCu system developed in this workoffers great potential to be used for large-scale and low-cost energystorage applications.

Acknowledgments

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

Appendix A. Supplementary data

Supplementary data related to this article can be found at https://doi.org/10.1016/j.jpowsour.2019.03.085.

References

[1] P. Leung, X. Li, C. Ponce de León, L. Berlouis, C.T.J. Low, F.C. Walsh, RSC Adv. 2(2012) 10125–10156.

[2] B. Dunn, H. Kamath, J.M. Tarascon, Science 334 (2011) 928–935.[3] Y. Li, Y. Feng, X. Sun, Y. He, Angew. Chem. 129 (2017) 5828–5831.[4] L. An, R. Chen, Appl. Therm. Eng. 124 (2017) 232–240.[5] Q. Xu, Y. Ji, L. Qin, P. Leung, F. Qiao, Y. Li, H. Su, J. Energy Storage 16 (2018)

108–115.[6] Q. Xu, F. Zhang, L. Xu, P. Leung, C. Yang, H. Li, Renew. Sustain. Energy Rev. 67

(2017) 574–580.[7] Y. Li, X. Sun, Y. Feng, ChemSusChem 10 (2017) 2135–2139.[8] W. Lu, X. Li, H. Zhang, Phys. Chem. Chem. Phys. 20 (2018) 23–35.[9] M. Skyllas-Kazacos, M.H. Chakrabarti, S.A. Hajimolana, F.S. Mjalli, M. Saleem, J.

Electrochem. Soc. 158 (2011) R55–R79.[10] M. Park, J. Ryu, W. Wang, J. Cho, Nat. Rev. Mater. 2 (2016) 16080.[11] L. Wei, M. Wu, T. Zhao, Y. Zeng, Y. Ren, Appl. Energy 215 (2018) 98–105.[12] Y. Li, Y. Feng, X. Sun, ACS Sustain. Chem. Eng. 6 (2018) 12827–12834.[13] A. Crawford, V. Viswanathan, D. Stephenson, W. Wang, E. Thomsen, D. Reed, B. Li,

P. Balducci, M. Kintner-Meyer, V. Sprenkle, J. Power Sources 293 (2015) 388–399.[14] A. Biswal, B.C. Tripathy, K. Sanjay, T. Subbaiah, M. Minakshi, RSC Adv. 5 (2015)

58255–58283.[15] K. Zhang, X. Han, Z. Hu, X. Zhang, Z. Tao, J. Chen, Chem. Soc. Rev. 44 (2015)

699–728.[16] C. Spanos, D.E. Turney, V. Fthenakis, Renew. Sustain. Energy Rev. 43 (2015)

478–494.[17] N. Zhang, F. Cheng, J. Liu, L. Wang, X. Long, X. Liu, F. Li, J. Chen, Nat. Commun. 8

(2017) 405.[18] H. Pan, Y. Shao, P. Yan, Y. Cheng, K.S. Han, Z. Nie, C. Wang, J. Yang, X. Li,

P. Bhattacharya, Nat. Energy 1 (2016) 16039.[19] Q. Wu, Z. Pan, L. An, Renew. Sustain. Energy Rev. 89 (2018) 168–183.[20] Y. DONG, T.Shigematsu, T. Kumamoto, M. Kubata. Redox flow battery. U.S. Patent

App. 20120045680.[21] F.-Q. Xue, Y.-L. Wang, W.-H. Wang, X.-D. Wang, Electrochim. Acta 53 (2008)

6636–6642.[22] W. Chen, G. Li, A. Pei, Y. Li, L. Liao, H. Wang, J. Wan, Z. Liang, G. Chen, H. Zhang,

Nat. Energy 3 (2018) 428.[23] Y. Zhou, L. Yu, J. Wang, L. Liu, F. Liang, J. Xi, RSC Adv. 7 (2017) 19425–19433.[24] https://www.sglgroup.com/cms/_common/downloads/products/product-groups/

gs/tds/felt/SIGRACELL_TDS_BF_00.pdf.[25] T. Liu, X. Li, H. Zhang, J. Chen, J. Energy Chem. 27 (2018) 1292–1303.[26] H. Jiang, M. Wu, Y. Ren, W. Shyy, T. Zhao, Appl. Energy 213 (2018) 366–374.[27] Y. Zhang, L. Liu, J. Xi, Z. Wu, X. Qiu, Appl. Energy 204 (2017) 373–381.[28] P. Leung, M. Mohamed, A. Shah, Q. Xu, M. Conde-Duran, J. Power Sources 274

(2015) 651–658.[29] A. Zolfaghari, F. Ataherian, M. Ghaemi, A. Gholami, Electrochim. Acta 52 (2007)

2806–2814.[30] H. Jiang, W. Shyy, Y. Ren, R. Zhang, T. Zhao, Appl. Energy 233 (2019) 544–553.[31] P. Leung, J. Palma, E. Garcia-Quismondo, L. Sanz, M. Mohamed, M. Anderson, J.

Power Sources 310 (2016) 1–11.[32] M. Wu, T. Zhao, L. Wei, H. Jiang, R. Zhang, J. Power Sources 384 (2018) 232–239.[33] Z. He, Y. Jiang, Y. Wei, C. Zhao, F. Jiang, L. Li, H. Zhou, W. Meng, L. Wang, L. Dai,

Electrochim. Acta 259 (2018) 122–130.[34] L.F. Castañeda, F.C. Walsh, J.L. Nava, C.P. de León, Electrochim. Acta 258 (2017)

1115–1139.[35] Z. He, Y. Jiang, W. Meng, F. Jiang, H. Zhou, Y. Li, J. Zhu, L. Wang, L. Dai, Appl.

Surf. Sci. 423 (2017) 111–118.[36] K. Kreuer, J. Membr. Sci. 185 (2001) 29–39.[37] J.-P. Melchior, T. Bräuniger, A. Wohlfarth, G. Portale, K.-D. Kreuer,

Macromolecules 48 (2015) 8534–8545.[38] R. Lashari, M. Zhao, Q. Zheng, H. Gong, X. Song, Energy & Fuels 32 (2018)

10016–10023.[39] X.L. Zhou, T.S. Zhao, L. An, L. Wei, C. Zhang, Electrochim. Acta 153 (2015)

492–498.[40] J.-D. Kim, L.-J. Ghil, M.-S. Jun, J.-K. Choi, H.-J. Chang, Y.-C. Kim, H.-W. Rhee, J.

Electrochem. Soc. 161 (2014) F724–F728.[41] K. Gong, X.Y. Ma, K.M. Conforti, K.J. Kuttler, J.B. Grunewald, K.L. Yeager,

M.Z. Bazant, S. Gu, Y.S. Yan, Energy Environ. Sci. 8 (2015) 2941–2945.[42] Y. Zeng, T. Zhao, X. Zhou, L. Wei, Y. Ren, J. Power Sources 346 (2017) 97–102.[43] https://alphachemicals.com/.[44] E. Kjeang, R. Michel, D.A. Harrington, N. Djilali, D. Sinton, J. Am. Chem. Soc. 130

(2008) 4000–4006.[45] D.S. Aaron, Q. Liu, Z. Tang, G.M. Grim, A.B. Papandrew, A. Turhan,

T.A. Zawodzinski, M.M. Mench, J. Power Sources 206 (2012) 450–453.[46] Q. Zheng, F. Xing, X.F. Li, T. Liu, Q.Z. Lai, G.L. Ning, H.M. Zhang, J. Power Sources

277 (2015) 104–109.[47] A.A. Shah, R. Tangirala, R. Singh, R.G.A. Wills, F.C. Walsh, J. Electrochem. Soc. 158

(2011) A671.[48] Y. Zeng, X. Zhou, L. An, L. Wei, T. Zhao, J. Power Sources 324 (2016) 738–744.[49] O. Nibel, S.M. Taylor, A. Pătru, E. Fabbri, L. Gubler, T.J. Schmidt, J. Electrochem.

Soc. 164 (2017) A1608–A1615.[50] L. An, C. Jung, Appl. Energy 205 (2017) 1270–1282.[51] Y. Zeng, F. Li, F. Lu, X. Zhou, Y. Yuan, X. Cao, B. Xiang, Appl. Energy 238 (2019)

435–441.[52] M.A. Rodrigues, A.C. Catto, E. Longo, E. Nossol, R.C. Lima, J. Rare Earths 36 (2018)

1074–1083.[53] Q. Wu, X. Zhang, Y. Lv, L. Lin, Y. Liu, X. Zhou, J. Mater. Chem. 6 (2018)

20347–20355.[54] R. Wang, Y. Li, Y. He, J. Mater. Chem. (2019), https://doi.org/10.1039/

C1039TA00807A.[55] J. Xi, Z. Wu, X. Qiu, L. Chen, J. Power Sources 166 (2007) 531–536.[56] Y. Ren, M. Liu, T. Zhao, L. Zeng, M. Wu, J. Power Sources 342 (2017) 9–16.[57] Y. Zhu, S. Xiao, M. Li, Z. Chang, F. Wang, J. Gao, Y. Wu, J. Power Sources 288

(2015) 368–375.[58] H.J. Lee, S. Park, H. Kim, J. Electrochem. Soc. 165 (2018) A952–A956.

L. Wei, et al. Journal of Power Sources 423 (2019) 203–210

210