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Applied Energy

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Seawater as an alternative to deionized water for electrolyte preparations invanadium redox flow batteries

L. Wei, L. Zeng, M.C. Wu, X.Z. Fan, 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

• Seawater as alternative solvent to DI water for VRFB electrolyte preparation.

• The solubility of VOSO4 in seawater increases by ∼12% compared with DI water.

• The battery achieves a higher coulombic efficiency and capacity retention.

• The cost of seawater electrolyte is cheaper by 3.6% than DI water electrolyte.

A R T I C L E I N F O

Keywords:VRFBImpuritySeawaterElectrolyte

A B S T R A C T

The vanadium redox flow battery (VRFB) has been identified as one of the most promising candidates for large-scale energy storage systems. Nevertheless, the high capital cost, particularly the cost of electrolytes, has hin-dered its broad market penetration. In this work, we use abundant seawater to replace deionized water for VRFBelectrolyte preparation. Testing results show that both the coulombic efficiency and capacity retention rate in thecase of using seawater electrolytes are higher than using the deionized-water electrolytes, although the batteryvoltage efficiency slightly decreases due to the lower mass transport diffusivity of V3+ in seawater. Moreover,the solubility of vanadium sulfate in seawater increases by ∼12%. The overall cost of seawater-preparedelectrolytes is cheaper by 3.6% than that of the deionized water prepared electrolytes. The results indicate thatseawater offers the potential to replace deionized water as a new avenue for VRFB electrolyte preparations.

1. Introduction

In the past decades, with the depletion of fossil fuels, renewableenergy such as solar and wind has been attracted more and more at-tention [1–6]. However, the unavailability (e.g., sunlight in the night)and unpredictability (e.g., wind power), affect the electricity outputand have largely restricted their applications [7,8]. As a potential so-lution to address this issue, the development of large-scale energy sto-rage systems such as redox flow batteries is in urgent demand [9,10].Up to now, various types of flow batteries have been put forward, in-cluding metal-air systems [11–14], the lithium-based, organic-in-organic redox flow batteries [15–17]. Among these numerous systems,vanadium redox flow battery (VRFB) proposed by Skyllas-Kazacosgroup is considered as one of the most promising energy storage sys-tems owing to its attractive features such as long cycle life, excellentelectrochemical reversibility, and eliminated crossover contaminationeffect by employing the same element in both electrolytes [18–22].

Although significant progress of VRFBs has been made over pastyears of research, this technology has yet to meet the stringent costrequirements for more widespread commercialization in a broaderrange of energy-storage applications [23]. To reduce the capital cost ofthe system, one approach is to develop high-performance electrodes,elevating the operating power densities without sacrificing energy ef-ficiency. Herein, the enhanced power density is tantamount to de-creasing the size of the power stack for the given power requirement,thus reducing the quantity of component materials such as bipolarplates, electrodes and membranes [24–26]. For example, Aaron et al.equipped the VRFB with a zero-gap flow field architecture and carbonpaper electrodes, and a peak power density of 557mW cm−2 at a stateof charge (SOC) of 60% was obtained [24]; Elgammarl et al. demon-strated very high performance attainable in a VRFB configuration,achieving a peak power density> 2500mW cm−2 [27]. In addition tothese works above, many other efforts have been taken to reduce po-larizations of VRFBs through improving the activity of the electrodes,

https://doi.org/10.1016/j.apenergy.2019.113344Received 17 November 2018; Received in revised form 7 April 2019; Accepted 15 May 2019

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

Applied Energy 251 (2019) 113344

0306-2619/ © 2019 Elsevier Ltd. All rights reserved.

T

such as etching the graphite felt surface to increase the oxygen func-tional groups and depositing nanostructured electrocatalysts on theelectrode surface to increase the active sites for redox reactions[28–43].

All of these researches mentioned above are imperative and caneffectively reduce the stack cost to some extent. However, it is worthnoting that another more substantial portion contributing to the capitalcost is the electrolyte. In VRFBs, vanadium electrolytes are prepared byusing VOSO4 or V2O5 as a starting material; they are dissolved intowater with sulfuric acid or hydrochloric acid as supporting electrolyte.During the battery operation, the negative electrolyte contains V2+,V3+, H+, HSO4

−, SO42− and H2O while the positive side contains

VO2+, VO2+, H+, HSO4

−, SO42− and H2O, as schematically shown in

Fig. 1. Up to now, previous researches with regard to electrolytesmainly focused on the following aspects: (1) synthesizing routines andreduction methods from V2O5, (2) solubility and stability investigationof vanadium ions, (3) electrochemical performance of VRFB electro-lytes [44–46].

Nevertheless, no virtual standard and specifications are aiming forthe rational use of raw materials for electrolyte preparation. An elec-trolyte of high purity seems always to be preferred to avoid any possibleadverse effect on battery performance. It is worth noting that the priceof the reagent (vanadium salt, vanadium oxide, supporting acid, water,etc.) will also substantially increase with the purity level. For example,as the primary resource for electrolyte preparation, a considerableamount of V2O5 is obtained from petroleum residue and spent industrialcatalysts. Various impurities exist, and in order to raise the reagentpurity to a level of more than 99%, the processing cost is inevitablyincreased. Some previous researchers have noticed this issue: to ex-amine the effect of impurities on the battery performance, Burchcompared electrolytes with VOSO4 of different purities and found thatSi was the main impurity that covered the electrode surface and de-creased the battery performance [47]. Park et al. investigated im-purities including Li+, Na+, K+ and they found that Li+ seriously de-graded the VRFB performance even at a relatively low concentration[48]. These works are innovative and mainly focused on the possiblemetal ions introduced to electrolyte induced by impurities in vanadiummetal source, while no reports examined the fair use of water. It isworth mentioning that even in a high concentration vanadium elec-trolyte (Cvanadium > 1.7M), the main component is water (wt.%>60%). In the classic form of filtrating water to the high puritylevel, membrane separation, gaining acceptance as the most effectiveand economical water treatment method, is widely used as support fortraditional water treatment. This process is typically time- and energy-

consuming since high pressure should always be kept to force the wateracross the small pores of the membranes. Therefore, deionized water isexpensive and not conducive to reduce the capital cost of VRFBs.However, in a majority of previous cost analysis works, water ex-penditures are ignored, seemly that water cost does not need to beconsidered in VRFBs. Besides, many types of research in VRFBs usedcostly deionized water to prepare electrolytes [49,50]. On these occa-sions, the water cost should be considered, and its usage remains to beinvestigated.

The VRFBs have been demonstrated to have a wide range of po-tential applications in a distributed generation network on sea islands,including the 200 kW system on King Island, 400 kW system on SumbaIsland, 100 kW system on KIER/Juju Island, 200 kW system on IslandPellworm, 10 kW system on Dalian Snake Island, etc. [51]. In the nearfuture, more island VRFB projects will be constructed to improve thetransmission and distribution systems performance by compensating forelectrical anomalies and disturbances such as voltage sag, unstablevoltage, and presence of subsynchronous resonance. The use of highpurity water in electrolyte production will undoubtedly increase theVRFB capital cost, particularly the transportation expenses of heavy andlarge tanks of vanadium-contained electrolytes. One economically ac-tive approach to address this issue is to employ free seawater near theislands to replace the high-priced deionized water for VRFB electrolytepreparation while keeping the battery performance at the same level.On this occasion, the system cost will decrease, and it would be of greatsignificance for the widespread commercialization of VRFBs. However,the critical issue is that some impurities exist in seawater. The usagecould potentially introduce impurities during electrolyte preparation,and whether their existence affects the battery performance remainsunknown.

In this work, a preliminary study of employing abundant seawaterfor VRFB electrolyte preparation is conducted. It is found that the tinyamounts of impurities in seawater will slightly decrease the voltageefficiency of the battery by decreasing the diffusion coefficient of V3+

in the electrolyte. However, both coulombic efficiency and capacityretention rate increase in the seawater sample. More importantly, thesolubility of vanadium sulfate for the seawater-prepared electrolyteswill increase by about 12% compared to the deionized water, whichwill enhance the energy density and further reduce the correspondingunit cost.

2. Experimental

2.1. Electrolyte preparation for battery tests

The electrolyte solutions used in this work were prepared by usingVOSO4 (ZhongTian Chemical Ltd., China) as starting chemicals. Firstly,VOSO4 and H2SO4 were proportionally dissolved into deionized water(Synergy® Water Purification System) and seawater (obtained from thecoast near HKUST), respectively to get two electrolyte samples (1MVOSO4 and 3M H2SO4). Herein, the seawater was filtered and then setaside the whole day before use to separate the residual particles sincethese carbonates will consume and decrease the concentration of H+.These prepared solutions were then put into an electrolytic cell toconvert the VO2+ to V3+ and VO2

+ in the negative and positivecompartment reaching ca. 100% SOC. Subsequently, 20mL of theelectrolyte from the negative compartment and the same volume ofVOSO4 contained solution were used as initial electrolytes for batterytests.

2.2. Electrochemical studies

Cyclic voltammetry (CV) and electrochemical impedance spectra(EIS) tests were performed on an electrochemical workstation (Autolab,PGSTAT 30). A conventional three-electrode cell was set up with agraphite electrode (0.785 cm2) as the working electrode, a saturated

Fig. 1. Schematic of the electrolyte composition in vanadium redox flow bat-teries based on the H2SO4 electrolyte.

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calomel electrode (SCE) as the reference electrode and a platinum meshas the counter electrode. The CV curves were obtained from −0.8 V to1.4 V (vs. SCE) in the electrolytes containing 0.1M VOSO4+ 3MH2SO4. Meanwhile, EIS tests were performed by applying a perturba-tion voltage of 5mV over the frequency ranging from 10−1 to 105 Hz.The electrode potential was fixed at −0.5 V and 0.9 V for negative andpositive redox reactions, respectively.

The battery tests were conducted using a zero-gap serpentine flow-field structured battery, which was described in our previous work[52]. Commercially available graphite felt (SGL company, GFA series)with an uncompressed thickness of 2mm (active area of 4.7 cm2) wasselected as an electrode. To enhance the electrochemical activity andhydrophilicity, the electrode was oxidized in air at 500 °C in a mufflefurnace for 4 h with the heating rate of 5 °Cmin−1. Nafion 212 (Du-pont) was employed as the membrane. The electrolyte was circulated ata flow rate of 0.6mL s−1 with peristatic pump (Longer pump, WT600-2 J). Before tests, high-purity N2 gas was bubbled to exhaust the air inthe tanks. All measurements were conducted at room temperature andthe batteries were evaluated by an Arbin BT2000 battery test system.

2.3. Solubility measurements

To test the solubility of vanadium electrolyte, a supersaturatedelectrolyte was prepared as follows. Firstly, 7 g VOSO4 was added to a10mL solution containing 3M H2SO4 to form a supersaturated solution.Secondly, the solution was stirred for half an hour and then stabilizedfor 30 days to make the solution reach an equilibrium state. Thirdly,0.5 mL upper electrolyte was taken out by a pipette and then dilutedinto 1 L water. At last, the samples were tested by an inductively cou-pled plasma mass spectrometry (ICP) machine to confirm its vanadiumcontent, schematically shown in Fig. 2. The tested vanadium contentwas converted to the vanadium concentration via the following Eq. (1).C is the saturation concentration of VOSO4 (mol L−1); CICP is ICP con-centration of vanadium (mg L−1); D is the extent of dilution; M is re-lative atomic mass of vanadium (50.94 gmol−1):

=C C D1000 M

ICP(1)

3. Results and discussion

3.1. Component difference and solubility

In general, the deionized water just consists of pure water withoutany impurities, while the ratios of solutes in the seawater differ

dramatically. It contains a variety of dissolved ions such as sodium,chloride, magnesium, sulfate and calcium. On average, the salinity is∼3.5% for the seawater on the earth, and every kilogram of seawaterhas approximately 31–38 g of dissolved salts (predominating by Na+

and Cl− ions) [53]. As detected by the chloride titrate paper (seeFig. 3), the deionized water contains no detectable Cl− ions while theCl− content in seawater diluted by 4 times reaches ∼4281 ppm,equaling to 0.48M of Cl− in seawater. We then evaluated the solubilityof VOSO4 in both samples. It is found that the limited stability of VOSO4

in the seawater reaches 2.15M, exhibiting a value of∼12% higher thanthat of the deionized water (1.92M). The main reason for the solubilityimprovement is ascribed to the formation of a soluble neutral speciesVO2Cl(H2O)2 due to the existence of Cl− in seawater, which is ex-plained in detail by previous work of Pacific Northwest National La-boratory [54,55]. The improvement is tantamount to enhancing theenergy density of the system, as well as decreasing the size of

Fig. 2. Schematic of the solubility test process.

Fig. 3. The chloride content in deionized water (a) and 4 times diluted seawater(b) detected by the chloride titrate paper.

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electrolyte tanks under same designed capacity.

3.2. The CV and EIS tests

Fig. 4a shows the CV results of vanadium redox reactions in dif-ferent electrolytes. The regions of redox reactions and their peaks canbe observed and marked in the figure. For both samples, the peakcurrents and potentials of VO2

+/VO2+ and VO2+/V3+ keep almost thesame with a minor difference (Tables 1 and 2), indicating that the ex-isting impurities in seawater have negligible impacts on kinetics andreversibility of positive vanadium redox reactions. Only in the processof V3+ to V2+ reduction reaction (Table 3), the seawater sample(10.23 mA) exhibits an observable lower peak value of ∼4% than thatof the deionized water (10.65 mA). To investigate the reason for thedifference, EIS tests with an excitation signal of 5mV under −0.5 V (vs.SCE) were recorded. The Nyquist plots are fitted with the equivalentcircuit as presented in Fig. 4b. In the equivalent circuit, Rs stands forthe resistance composed of solution resistance, electrode resistance andthe contact resistance. Rct represents the charge transfer resistanceacross the electrode/solution interface. Qm is the constant-phase ele-ment which represents the electric double layer capacitance of elec-trode/solution interface, and Qt is the constant-phase element whichrepresents the diffusion capacitance attributed by the diffusion processof V3+ and V2+ ions [56]. Table 4 specifies the parameters resultingfrom fitting the impedance plots with the equivalent circuit model. It isrevealed that the charge transfer resistance of both samples shows verysimilar value, reflecting these impurities exhibit ignorable effects onreaction kinetics. However, it is worth noting that the low-frequencyregion reflecting the ionic process exhibits different tendencies. For theelectrolyte prepared with seawater, it can be seen that Y0,1 and Y0,2 areboth slightly lower than that of the pristine electrolyte, indicating alessening of the electric double-layer capacitance of the electrode/electrolyte interface and the diffusion capacitance of ions. The resultsuggests that the introduction of seawater could hamper the absorptionand diffusion of V3+ ions to some extent, which is consistent with theCV result and previous work conducted by Park et al. [48].

3.3. Battery performance

Fig. 5a to 5d display the charging/discharging curves of a VRFBwith different electrolytes under the current density of 50, 100, 150 and200mA cm−2 for both samples. As seen from figures, it is found thatthere exists a small gap between charge curves. The gap slightly in-creases from 20 to 40mV when the operating current density increasesfrom 50 to 200mA cm−2, while both samples exhibit very similar dis-charge voltage plateaus. Fig. 5e summarizes the voltage and coulombicefficiency of the battery. The voltage efficiency of the battery withdeionized water is slightly higher (0.5%–1.5%) than that with theseawater sample under the same testing conditions. The main reasonascribed to the difference is due to the decrease of V3+ coefficient asanalyzed in the last section. This mass transport issue will become moresevere with the current density increasing, leading to a decrease inbattery voltage efficiency. However, it is found that the trend of cou-lombic efficiency shows the opposite direction. The coulombic effi-ciency of seawater sample reaches 96.2% and 98.5% at 50mA cm−2

Fig. 4. (a) CV curves of the graphite electrode in different electrolytes at a scan rate of 50mV s−1; AC impedance spectroscopy of graphite electrode in differentelectrolytes at (b) −0.5 V vs. SCE.

Table 1Peak currents and potentials of the VO2

+/VO2+ redox couple.

Samples Peak current (mA) Peak potential (mV)

Ipa Ipc Epa Epc

Deionized water 12.43 −7.01 1061 745Seawater 12.51 −7.11 1057 748

Table 2Peak currents and potentials of the VO2+/V3+ redox couple.

Samples Peak current (mA) Peak potential (mV)

Ipa Ipc Epa Epc

Deionized water 2.85 −2.91 465 351Seawater 2.61 −2.81 464 350

Table 3Peak current and potentials of the V3+/V2+ redox couple.

Samples Peak current (mA) Peak potential (mV)

Ipa Ipc Epa Epc

Deionized water 6.20 −10.65 −431 −748Seawater 6.38 −10.23 −434 −751

Table 4Parameters resulting from fitting the impedance plots with the equivalent cir-cuit model.

Samples Rs(Ω) Rct(Ω) Qm[CPE] Qt[CPE]

Y0,1 n1 Y0,2 n2

Deionized water 0.86 0.39 3.76× 10−2 0.83 1.12 0.68Seawater 0.95 0.4 2.94× 10−2 0.88 0.65 0.62

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and 200mA cm−2, which is 1.2% and 0.4% than that of the deionizedwater, respectively. The possible reason for the coulombic efficiencyenhancement in seawater is due to the chloride ions in the seawaterwhich will form complexes with vanadium ions [54]. The complexedvanadium ion has larger ion radium, leading to the decrease of redoxspecies across the membrane. This trend is good in good agreementwith the previously reported mixed acid systems which introduce HClinto the H2SO4 electrolyte [54]. Additionally, the rate capabilities of thesamples were performed as increasing operating current density from50 to 200mA cm−2 (Fig. 5f). At the low current density of 50mA cm−2,the energy efficiency of the seawater sample reaches 92.8%, exhibitinga value of about 0.2% higher than the deionized water sample due to anenhanced coulombic efficiency. The corresponding value decreases to86.2% at 200mA cm−2, which is 1.2% lower than that with the deio-nized water sample as a result of decreased voltage efficiency. To assessthe cycling stability of the batteries, the operating current density from200 to 100mA cm−2 at the seventeenth cycle is swiftly changed andnoted that the battery performance of both samples can be fully

recovered, indicating the superior stability of the battery prepared withboth electrolytes.

The battery’s stability is also a very critical factor to determine thebattery performance. To further identify the stability and suitability ofthe batteries with different electrolytes, cycling tests of the assembledVRFBs were conducted at a typical current density of 100mA cm−2 for100 cycles. As depicted in Fig. 6a and 6b, the efficiencies of bothsamples maintained stable during the whole tests. The coulombic effi-ciency of the seawater is ∼0.5% higher than the deionized watersample. On the other hand, its energy efficiency exhibits ∼0.3% lowerdue to an inferior voltage efficiency.

Although the Nafion shows a high conductivity and stability duringVRFB operation, its low selectivity allows a portion of vanadium ions aswell as water to transport crossover. The crossover will cause the im-balanced vanadium active species in the positive and negative sides,thus leading to a capacity decay issue during battery operation. Fig. 6cdisplays the corresponding discharge capacity within 100 cycles; theinitial capacity of the seawater is 0.574 Ah, which is slightly lower than

Fig. 5. Electrochemical performance of VRFBs employing different electrolytes: (a) charge and discharge curve at (a) 50, (b) 100, (c) 150 and (d) 200mA cm−2; (e)Coulombic efficiency and voltage efficiency at different current densities; (f) Energy efficiency as a function of cycle number at different current densities.

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the deionized water (0.58 Ah). However, it surpasses at the fifth cycleand retains a capacity of 0.423 Ah at the 100th cycle, exhibiting anaverage capacity decay rate of 0.15% per cycle, which is slightly lowerthan the deionized water (0.413 Ah at the 100th cycle, an average ca-pacity decay rate of 0.17% per cycle), equating to a higher capacityretention rate. The lower capacity fade over cycling as well as theabove-mentioned higher coulombic efficiency is ascribed to the lowernet vanadium ion transport across the membrane in the chloride-basedsystem. It is worthy to note that the higher capacity retention rate willreduce the maintenance fees to balance the positive and negative so-lutions during the VRFB operation [57–59]. These results further de-monstrate the potential of inexpensive seawater as an alternative sol-vent for the preparation of the VRFB electrolyte.

3.4. Cost of the electrolyte

The VRFB electrolyte cost can be estimated by the Eq. (2) describedin the previous work [60]:

∑=C 3600EF

Q Mni

i i

i (2)

where C is the active material cost per kilowatt-hour ($ kWh−1); Q isthe cost of the active materials per kilogram ($ kg−1); M is the mole-cular mass of the active material (g mol−1); E is the equilibrium cellvoltage; F is the Faraday’s constant (96485 Cmol−1). The detailedcomponent cost data is referred from previous open literature andvarious websites. The prices of V2O5, H2SO4 and distilled water (batteryuse level) are 24 [60,61], 0.3 [62] and 0.2 [62] $/kg. Among thecomponents of electrolytes, the V2O5 undoubtedly accounts for a largepercentage of the overall electrolyte cost due to the high price of va-nadium-contained materials. Taken the typically commercialized va-nadium electrolyte as an example (see Fig. 7). The cost of V2O5 anddeionized water is $116.4 and $4.5 kWh−1, accounting for 94.5% and3.6% of total electrolyte cost ($123.2 kWh−1), respectively. While thecase of seawater preparation can cut out the water cost since seawater isfree. In addition to this, another aspect that should be considered is thedelivery expense. It is known that the VRFBs are perceived as importantbuffer devices to improve the transmission and distribution systemsperformance by compensating for electrical anomalies and dis-turbances. These distributed energy resources are usually located inremote areas such as Sea Islands and outer suburbs. The transportationexpenses of heavy and large tanks of liquid vanadium-contained elec-trolytes are also very high due to the upward tendency of oil price. Ifelectrolyte can be prepared using nearby water sources, it is beneficialto decrease considerable transport fees in connection with electrolytes,which will further reduce the capital cost of the system.

3.5. Discussion

Due to variations of standards and environment, the impurities in

seawater differ from area to area. The amount and categories of che-mical substances slightly change depending on geological and climaticconditions. Additionally, the entry of industrial, agricultural, residentialwaste and invasive organisms will also lead to a change of impurities insome regions of oceans. However, the results here hint us that em-ploying materials with lower purity material to prepare VRFB electro-lyte is feasible and can be recognized as an effective way to reduce thecapital cost of the VRFB system.

At present, the vanadium price reached 8 year high today [63].Unluckily, the price of the vanadium price is determined by the mineralrarity, refine difficulty and other market factors. As a result, it is hard toreduce in a short time. The trend of the growing price will no doubtincrease the capital cost of the VRFB system, restricting its furthercommercial penetration in the future. To address this issue, on the onehand, we need to reduce the quantity of the components in the stack,this part is determined by the battery polarizations related to reactionkinetics, battery ohmic losses, and mass transport issues. Substantialcost reduction can be achieved by future optimization via improvingthe battery operating power density. On the other hand, it is wellknown that the main components including electrolytes, membranes,and auxiliary facilities can be recyclable, more work involving on ra-tional selection and application of these materials to cut down the VRFBcost should be considered in future research.

4. Conclusions

In summary, seawater was explored as an alternative solvent forelectrolyte preparation for VRFBs. The solubility of the vanadium sul-fate, electrochemical performance of vanadium reactions, and batteryperformance in seawater samples are tested and compared with the one

Fig. 6. Cycling tests of VRFBs using different electrolytes. (a) Coulombic efficiency, (b) energy efficiency and (c) discharge capacities of VRFBs at a current density of100mA cm−2.

Fig. 7. Capital cost breakdown of the base case for the VRFB electrolyte con-taining 1.7MV and 3M H2SO4.

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in the deionized water sample. It is found that the solubility of vana-dium sulfate in seawater can reach 2.15M, which is 12% higher thanthat of deionized water due to the existence of Cl−. However, the masstransport diffusivity of V3+ in seawater slightly decreased, leading to adecrease in battery voltage efficiency, while the coulombic efficiencyand capacity retention rate exhibits an opposite tendency. The cou-lombic efficiency of seawater sample reaches 96.2% and 98.5% at50mA cm−2 and 200mA cm−2, which is 1.2% and 0.4% than that ofthe deionized water, respectively. The average capacity decay rate ofseawater sample was 0.15% per cycle, also lower than the deionizedwater (0.17% per cycle) during the cycle tests. Moreover, the overallcost of seawater-prepared electrolyte decreased by 3.6% than that ofthe deionized water prepared electrolyte. With the advantages of im-proved energy density, higher capacity retention rate, and decreasedcost, VRFB technology with seawater electrolyte is of great potential toaccelerate the market penetration of the renewable energy sources tothe electrical grid.

Acknowledgements

The work described in this paper was supported by the grant fromResearch Grants Council and the Innovation and TechnologyCommission of the Hong Kong Special Administrative Region, China(Project No. T23-601/17-R and Project No. ITS/177/17FP).

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