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Journal of Power Sources 329 (2016) 247e254

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Journal of Power Sources

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A highly permeable and enhanced surface area carbon-cloth electrodefor vanadium redox flow batteries

X.L. Zhou, T.S. Zhao*, Y.K. Zeng, L. An, L. WeiDepartment 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 high performance carbon cloth electrode is developed for flow batteries.� The developed electrode offers good transport properties and high surface area.� The battery with the carbon cloth electrode offers high performance.

a r t i c l e i n f o

Article history:Received 20 June 2016Received in revised form21 July 2016Accepted 19 August 2016Available online 26 August 2016

Keywords:Flow batteryCarbon clothEffective conductivityPermeabilitySurface area

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

http://dx.doi.org/10.1016/j.jpowsour.2016.08.0850378-7753/© 2016 Elsevier B.V. All rights reserved.

a b s t r a c t

In this work, a high-performance porous electrode, made of KOH-activated carbon-cloth, is developed forvanadium redox flow batteries (VRFBs). The macro-scale porous structure in the carbon cloth formed byweaving the carbon fibers in an ordered manner offers a low tortuosity (~1.1) and a broad pore distri-bution from 5 mm to 100 mm, rendering the electrode a high hydraulic permeability and high effectiveionic conductivity, which are beneficial for the electrolyte flow and ion transport through the porouselectrode. The use of KOH activation method to create nano-scale pores on the carbon-fiber surfacesleads to a significant increase in the surface area for redox reactions from 2.39 m2 g�1 to 15.4 m2 g�1. Thebattery assembled with the present electrode delivers an energy efficiency of 80.1% and an electrolyteutilization of 74.6% at a current density of 400 mA cm�2, as opposed to an electrolyte utilization of 61.1%achieved by using a conventional carbon-paper electrode. Such a high performance is mainly attributedto the combination of the excellent mass/ion transport properties and the high surface area rendered bythe present electrode. It is suggested that the KOH-activated carbon-cloth electrode is a promisingcandidate in redox flow batteries.

© 2016 Elsevier B.V. All rights reserved.

1. Introduction

Effective usage of the electricity harvested from the renewableenergy sources that are intermittent in nature requires thedeployment of reliable, efficient, and cost-effective large-scale en-ergy storage systems [1e3]. Among large-scale energy storagesystems, redox flow batteries (RFBs) have received extensive at-tentions because of its high efficiency, excellent scalability, flexibleoperation as well as long lifetime [4e6]. In particular, vanadiumredox flow batteries (VRFBs), which employ the same element asredox active species in two half-cells and thus avoid the cross-contamination issue, have been identified as one of the mostpromising energy-storage systems [7e10].

Although the VRFB possesses attractive advantages, however, itswidespread adoption is still significantly hindered by the highcapital cost, resulting from the precious redox-active materials andlow-performance power pack. For a flow battery systemwith givenredox couples like the VRFB, maximizing the power density(operating current density) to reduce the stack size and increasingelectrolyte utilization (charge-discharge depth) to enable effectiveusage of electrolyte should be the most direct and simple strategyto cut down the capital cost. Simultaneously maximizing these twoparameters relies on minimizing the battery energy losses thatcome from activation loss, ohmic loss and concentration loss,respectively. In this sense, design over electrode structure andsurface properties to manage the electrochemical reactions andelectron/ion/species transport is critically important to achieve theoptimal cell performance [11e14].

The first barrier that needs to be removed is the reaction kinetic

Fig. 1. SEM image of carbon cloth electrode.

X.L. Zhou et al. / Journal of Power Sources 329 (2016) 247e254248

issue. Conventional electrode materials, such as carbon felt andcarbon paper, usually suffered from serious kinetic issue associatedwith poor catalytic activity for redox reactions and low specificsurface area for the access of the active species, limiting the batteryto be operated at current densities lower than 100 mA cm�2

[15e17]. To address this issue, various approaches have been pro-posed to increase the surface area and the catalytic activity. One ofthe common strategies is to thermally or chemically treat theelectrode; hence the amount of functional groups on the surface isdramatically increased and thus the catalytic activity is corre-spondingly enhanced. Improvement on reaction kinetics was alsofound after introducing metal/metal oxide catalysts onto the elec-trode surfaces [18e24]. Similarly, the high surface area carbonmaterials, such as carbon nanotube, graphene, and nanofiber, werealso coated onto the electrode surfaces, aiming at increasing thesurface area and catalytic activity [25e31]. Recently, our groupdemonstrated [29] a simple method to increase the surface area ofcarbon paper electrode by using KOH treatment to form nanoporeson the carbon fiber surfaces and 10 times improvement in surfacearea was found.

Apart from improving reaction kinetic, attentions should also bepaid to managing the ion/mass transport through the porouselectrodes, which are also critically important to the battery per-formance, especially at high current densities. The ion transportthrough the porous electrodes has a large contribution to thewholecell resistance while the mass transport through electrode willdetermine the concentration loss as well as the electrolyte utili-zation. Recently, the high current density operation has beenconsidered to be an effective approach to reduce the stack size andthereby the cost. However, the electrolyte utilization is relativelylow at high current density region. In our previous work, althoughthe battery can work at high current densities (>200 mA cm�2)with a high energy efficiency (80%), the electrolyte utilization isoften as low as 60%, which means the precious redox-active ma-terials are not well utilized and results in an increased capital cost.Therefore, in order to further improve the battery performance(efficiency and electrolyte utilization), more attentions should bepaid to the ion/mass transport properties of the electrode, whichdetermines the ohmic and concentration losses.

In general, the reaction kinetics of an electrode mainly dependson surface morphology and surface activity while the ion/masstransport properties should be related to the macro-scale porousstructure [32]. For the ion transport through the porous media,effective electrolyte conductivity was often used to characterize theion conduction capability of a specific porous structure saturatedwith given electrolytes. It is known that the porosity and tortuosityare two key parameters that influence the effective electrolyteconductivity since porosity determines the effective transport areaand tortuosity determines the length of the transport pathway.Typically, the porosity of the electrode of flow battery is around 80%(after compression) and cannot exceed 85% due to the electronicconductivity and surface area concerns. Therefore, the only effec-tive way to increase the effective electrolyte conductivity should betuning the tortuosity of the electrode. For the electrode made ofcarbon fibers, the tortuosity should be determined by the fiberarrangement pattern. Normally, the ordered arrangement patternwill produce a low tortuosity, and vice versa. Currently, the mostwidely used electrode is carbon paper/felt electrodes, which areformed from randomly laced carbon fibers. In this regard, a porouselectrode with ordered fiber arrangement should be promoted tobe used in VRFB systems. For flow batteries like VRFBs, the domi-nant mass-transport mechanism in the porous electrode should beconvection as the intrinsic diffusivity of the metal ions and theoperating temperature are relatively low [8]. Hence, the hydraulicpermeability is the key parameter to evaluate the mass transport

properties. It is known that hydraulic permeability of the porousstructure is closely related to the tortuosity, porosity, and poresizes. In this sense, with a lower tortuosity and larger pore sizes, thepermeability should be larger, and vice versa. Therefore, a porousstructure has large pore sizes and lower tortuosity should be suit-able for the VRFB applications.

With given fiber diameter and porosity, the pore size distribu-tion and arrangement pattern of the carbon fibers should be themain contributor to tortuosity and hydraulic permeability [32e34].As shown in Fig. 1, carbon cloth, which was previously used as gasdiffusion layer in fuel cells and made by weaving the carbon fiber,has a relatively more ordered fiber arrangement pattern and abroad pore distribution from 5 to 100 mm [32] and thus lowertortuosity and higher permeability, as compared with carbon paperunder the same fiber diameter and porosity [33,34], which mayresult in excellent transport properties. On the other hand, KOHtreatment is found to be a simple method to increase the surfacearea of carbon paper electrode by forming nanopores on thecarbon-fiber surfaces, leading to a significant improvement in re-action kinetics. In this work, we propose to develop a KOH-activated carbon-cloth electrode, which can combine the attri-butes of high surface area and excellent transport properties, forVRFBs. It is found that the low tortuosity combined with a broadpore distribution from 5 to 100 mm renders carbon cloth electrodedesirable ion and mass transport properties (effective ionic con-ductivity and hydraulic permeability), which is particularly suitablefor VRFB applications.We then apply the KOH-activationmethod totreat the carbon cloth electrode, increasing the surface area to ashigh as 15.4 m2 g�1, which is six times higher than the pristine one.Finally, a VRFB assembled with as-fabricated electrodes was tested,demonstrating a high energy efficiency of 80.1% and electrolyteutilization of 74.6% at a current density of 400 mA cm�2. The highperformance of the present VRFB should be attributed to theexcellent ion and mass transport properties as well as the highsurface area.

2. Experimental

2.1. Fabrication and characterization of KOH activated carbon cloth

The pristine carbon cloth (ELAT® hydrophilic plain), provided byFuel Cells Etc, was selected as the starting material. To introduceoxygen containing functional groups, the pristine carbon cloth was

X.L. Zhou et al. / Journal of Power Sources 329 (2016) 247e254 249

thermally treated at 500 �C under the air atmosphere for 6 h. Thenthe thermally-treated carbon cloth was mixed with KOH slurrieswith different carbon/KOH weight ratios of 1:1, 1:1.5 and 1:3. Afterthat, the formed mixture was heat treated in a furnace under anitrogen flow at 800 �C for 1.5 h to form the KOH-activated carboncloth. The resulting activated carbon cloth was washed severaltimes with 6.0 M HCl to remove the inorganic salts. The finalsamples were named as CC-KOH-1, CC-KOH-1.5, and CC-KOH-3according to the KOH/C weight ratios. The carbon paper (SGL 10AA) was treated in the same procedure as a benchmark.

2.2. Characterization

Thematerial morphology was acquired by the scanning electronmicroscope (SEM, JEOL-6700F and JSM-6300) under an accelera-tion voltage of 20 kV. Transmission electron microscopy (TEM)imagewas obtained by operating a high-resolution JEOL 2010F TEMsystem with a LaB6 lament at 200 kV. The samples were dispersedin ethanol, sonicated and dripped onto the holey carbon-coated Cugrids. Surface elemental composition was characterized with X-rayphotoelectron spectroscopy (XPS, Axis Ultra DLD, UK). N2 adsorp-tion/desorption measurement was conducted with a gas analyzer(ASAP2420, Micromeritics) after degassing process at 120 �C for 3 hto investigate the pore structure of the materials. The Brunauer-Emmett-Teller (BET) and Barrerr-Joyner-Halenda (BJH) methodwere adopted to calculate the specific surface area and aperturedistribution of electrodes.

2.3. Electrochemical measurements

A single flow cell with an active electrode area of 4 cm2 wasassembled for charge/discharge tests. The in-house designed zero-gap flow battery with a serpentine flow field was used in this study[35,36]. The electrode material used here is two layers of carboncloths and the membrane material is Nafion 212. Electrolytes of50 mL were fed into the compartments using the acrylic flowchannels and were circulated to and from the reservoirs at15 mL s�1 using a 2-channel peristaltic pump (WT-600-2 J, Lon-gerpump, China). Measurements were conductedwith 50mL 1M V(IV) þ 3 M H2SO4 solution as the positive electrolyte and 50 mL1M V (III)þ 3MH2SO4 solution as the negative electrolyte. Charge-discharge and cycling tests were conducted using an Arbin BT2000at a constant current density ranging from 200 to 500 mA cm�2.The calculation methods of the coulombic efficiency (CE), voltageefficiency (VE), electrolyte utilization and energy efficiency (EE) forthe single charge/discharge cycle are defined as.

CE ¼ tdtc

� 100% (1)

VE ¼ VdVc

� 100% (2)

UE ¼ CrealCtheoretical

� 100% (3)

and

EE ¼ CE� VE (4)

where td donates the time for discharge; tc represents the time forcharge; Vd donates the average voltage during discharge; Vc is theaverage voltage during charge; Creal donates the real dischargecapacity; and Ctheoretical is the theoretical capacity calculated basedon the amount of redox active materials.

3. Results and discussion

3.1. Analysis of ion transport through the porous electrode

For the state-of-the-art flow battery operated at current den-sities over 200 mA cm�2, reducing the cell resistance becomescritically important for improving the cell performance. Apart fromthe ohmic resistance associated with the ion transport through themembrane, the ion transport through the porous electrode isanother main contributor of the ohmic resistance. The effectiveconductivity of ion transport in the porous electrode can beexpressed as [33].

keff ¼ k

NM(5)

where k is the bulk conductivity of the electrolyte, NM is the Mac-Mullin number and depends on the porous structure of the elec-trode [33]. The bulk conductivity of the electrolyte in VRFB systemis only determined by the electrolyte composition, which are usu-ally in the range of 200e400 mS cm�1 [37]. Hence, a smallerMacMullin number is preferred for a higher effective ionic con-ductivity, which affects the overall cell resistance. The MacMullinnumber for a given porous electrode can be expressed as [33]:

NM ¼ t

ε

(6)

whereε is the porosity of the electrode, andt is the tortuosity ofelectrode. For the carbon paper electrode composed of randomlyorientated fibers, the NM can be described by Ref. [33]:

NM ¼ ε�3:8 (7)

With regard to the carbon cloth electrode that is composed ofparallel fibers, the NM can be described by the Bruggeman equation[33]:

NM ¼ ε�1:5 (8)

With a given porosity of 80%, from Eqs. (5)e(8), the tortuosity ofcarbon paper and carbon cloth can be calculated to be 1.8 and 1.1and MacMullin numbers of carbon paper and cloth should be 2.3and 1.4. With the MacMullin numbers and the bulk electrolyteconductivity (assumed to be 307 mS cm�1 [37]), the area specificion transport resistance through the porous electrodes can becalculated by:

R ¼ NM,lk

(9)

where k is the bulk electrolyte conductivity (307 mS cm�1), l is theelectrode thickness. The Fig. 2 shows the comparison of predictedarea specific ion transport resistances through a carbon paper andcarbon cloth. It is seen that the carbon cloth electrode offers muchlower ohmic resistance with the same porosity and the difference isaround 120mU cm2 at the thickness of 400 mmwhen the porosity is80%, indicating that the carbon cloth is more preferred for VRFBapplications in terms of the effective ionic conductivity.

3.2. Analysis of mass transport through the porous electrode

For a typical flow battery, the electrolyte containing activespecies will be pumped to the porous electrode where electro-chemical reaction occurs to store or release electricity. Due to thereason that the concentration and intrinsic diffusivity of vanadiumions is relatively low and the operating temperature is limited to

Fig. 2. Comparison of ion transport resistance through the carbon paper and carboncloth electrode.

Table 1Geometric parameters and material properties.

Width of the channel 3 mmHeight of the channel 3 mmWidth of the electrode 40 mmLength of the electrode 50 mmThickness of the electrode 1 mmPermeability of carbon cloth 69.4 � 10�12 m2 [34]Permeability of carbon paper 37.4 � 10�12 m2 [34]

X.L. Zhou et al. / Journal of Power Sources 329 (2016) 247e254250

lower than 40 �C, convection should be the dominant mechanismfor mass transport. Hence, the flow distribution in the porouselectrode is critically important to achieve a minimized concen-tration loss. To investigate the mass transport property of the car-bon cloth electrode, we perform a 3-D numerical simulation tostudy the flow distribution of the battery with a porous carbonpaper/cloth electrode [38]. The computational domain is showed inFig. 3a, where a typical serpentine flow field is adopted. With theincompressible assumption and negligible volume change of elec-trolyte, the continuity equation of electrolyte applicable to theentire domain is given as follow:

Fig. 3. (a) The schematic of the computational domain. (b) The mean velocity in the carbocarbon paper.

V, v!¼ 0 (10)

The momentum equations in flow channels and the porouselectrode are NaviereStokes equations and Brinkmann equationrespectively:

r½ð v!,VÞ v!� ¼ V,h� pIþ mðV v!þ ðV v!ÞÞT

i(11)

rε

�ð v!,VÞ v!

ε

�¼ V,

h� pIþ m

�V v!þ ðV v!ÞT

�i� mK

v! (12)

wherer is the electrolyte density, p is the pressure, I is the identitymatrix,m is the electrolyte viscosity, ε represents the electrodeporosity and K denotes the electrode permeability. Continuousvelocities and pressures across the interface between channels andthe porous electrode are set. The material properties and structuralparameters are summarized in Table 1. Fig. 3b and c compare themean velocity and pressure drop of the battery with carbon paperand carbon cloth electrode. It is shown that the mean velocity incarbon cloth electrode is much higher than that in carbon paperelectrode and the pressure drop of the battery with carbon cloth issmaller than that of battery with carbon paper, indicating that the

n cloth and carbon paper. (c) The pressure drop of the battery with carbon cloth and

X.L. Zhou et al. / Journal of Power Sources 329 (2016) 247e254 251

mass transport property of carbon cloth electrode is better thanthat of carbon paper electrode. The excellent transport propertiesshould be attributed to the high hydraulic permeability of carboncloth caused by the large pore sizes and low tortuosity.

Fig. 5. The transmission electron microscopy image of carbon fiber of CC-KOH-1.5.

Table 2BET surface area of different electrodes.

Electrode BET surface area (m2 g�1)

Pristine carbon cloth 2.39Air-treated carbon cloth 3.56CC-KOH-1 10.25CC-KOH-1.5 15.38CC-KOH-3 23.17

3.3. Fabrication and characterization of KOH activated carbon cloth

Although the carbon cloth electrode has advantages over carbonpaper electrode in terms of ion andmass transport properties, it hasnot beenwidely utilized as the electrode of VRFBs thus far primarilydue to its low specific surface area. In order to achieve a high per-formance electrode based on carbon cloth, we develop a high-surface-area carbon cloth electrode through using the KOH acti-vation method to create nanopores onto the fiber surfaces, whichcan significantly increase the surface area. The mechanism of theetching process, known as KOH activation, has been reported pre-viously [39]. The activation of carbon by KOH proceeds as6KOH þ C/2Kþ3H2þ2K2CO3, followed by the decomposition ofK2CO3 and the reaction of CO2 with C. To confirm the successfulcreation of nanopores on the fiber surfaces, the SEM and TEM im-ages are taken, in addition to the BET test. Fig. 4 shows the SEMimage of the fiber surfaces of air-treated carbon cloth and KOH-activated carbon cloth with different KOH/C weight ratios. Fig. 4ashows that the air-treated carbon cloth has much smoother sur-faces. With regard to the KOH-activated carbon cloth, nanoporesare uniformly distributed on the fiber surfaces, as seen in Fig. 4bed.With the increased KOH/C weight ratio, these pores will beenlarged due to the further reaction between KOH and C.

To further investigate structure of these nanopores, we con-ducted the TEM. A sharp contrast at the fiber edge can be observed,as shown in Fig. 5. In addition, the depth of the nanopores is around25 nm, inferring that the carbon fiber surface was successfullyetched during the KOH activation and did not significantly changethe carbon fiber structure.

In order to confirm the increased specific surface area with KOHactivation, we conduct the N2 adsorption/desorption

Fig. 4. The scanning electron microscopy images of carbon fiber surfaces of air-treated carbon cloth (a), CC-KOH-1 (b), CC-KOH-1.5 (c) and CC-KOH-3 (d).

Table 3Atomic fractions of C, O of different electrodes.

Components Air-treated carbon cloth CC-KOH-1 CC-KOH-1.5 CC-KOH-3

O 1s (%) 3.48 5.46 6.12 4.16C¼O (%) 1.00 2.88 3.13 1.93CeOH (%) 1.10 1.80 2.17 1.60C 1s (%) 96.36 94.27 93.27 95.35

X.L. Zhou et al. / Journal of Power Sources 329 (2016) 247e254252

measurements. Table 2summarizes the BET surface areas ofdifferent electrodes. With the increase of KOH/C ratio, the specificsurface area increases as more nanopores are generated with thefurther reaction of KOHwith carbon.When the KOH/C ratio reaches3:1, the specific surface area can reach 24m2 g�1. This trend is quitedifferent from carbon paper in our previous work. It can beexplained by that carbon paper involves a lot of carbon binders,which have a different properties (such as graphitization degree)with the carbon fiber. In summary, these results confirm the suc-cessful creation of nanopores on the carbon fiber surface of carboncloth by KOH activation.

Surface properties of different electrodes were characterizedwith X-ray photoelectron spectroscopy. Fig. 6 depicts the O1s XPSspectra of air-treated carbon cloth, CC-KOH-1, CC-KOH-1.5 and CC-KOH-3. It is found that the fiber surfaces of KOH activated carboncloths possesses more C]O groups than that of air-treated carboncloth, which can be explained by that the KOH can oxidize theCeOH groups to C]O groups. Further increased KOH/Cweight ratiowill reduce the amount oxygen containing functional groups due topart of C]O and CeOH groups will be oxidized to CO2, as sum-marized in Table 3.

3.4. VRFB charge-discharge and cycling performance

The charge-discharge curves of batteries assembledwith the air-treated carbon cloths and KOH-activated carbon cloths at a currentdensity of 200 mA cm�2 are shown in Fig. 7a. As compared to thebattery with air-treated carbon cloths, the present batteries withKOH-activated carbon cloths show remarkably decreased over-potential in both charge and discharge process. The improvedperformance should be attributed to the oxygen functional groupsgenerated and the increased surface area in the KOH activation

Fig. 6. O1s XPS spectra of air-treated carbon cloth (a),

process. The C]O bonds were found to play an important role inimproving the reaction activity [29], especially for V(II)/V(III) redoxreactions. In addition to the influence of C]O functional groups,the increased surface area is also responsible for the improvedperformance since amount of active sites for redox reactions areincreased. The KOH-activated carbon clothwith KOH/Cweight ratioof 1.5:1 exhibits the best performance, which agrees well with theelectrochemical impedance spectroscopy (EIS) result in Fig. S1. Thiscan be explained by that the surface area are increased withincreased KOH/C ratio but the catalytic activity will be decreased asa result of the decreased amount of the oxygen functional groups. Itshould be noted that the pristine carbon cloth shows an even lowerperformance than the air-treated carbon cloth, as shown in Fig. 7a.Fig. 7b depicts the charge-discharge curves of the battery with CC-KOH-1.5 at the current densities ranging from 200 mA cm�2 to500 mA cm�2. It is seen that the CC-KOH-1.5 enables the batterywork at high current density region with relatively low charge anddischarge overpotentials. Fig. 7c and d summarize the columbicefficiency (CE), voltage efficiency (VE), energy efficiency (EE) andelectrolyte utilization (UE) of VRFBs with different electrodes atcurrent densities ranging from 200 to 500mA cm�2. Clearly, the CC-KOH-1.5 exhibits an excellent electrochemical performance interms of VE, EE and UE and can maintain an energy efficiency of

CC-KOH-1 (b), CC-KOH-1.5 (c) and CC-KOH-3 (d).

Fig. 7. (a) The charge-discharge curves of VRFBs with air-treated carbon cloth, CC-KOH-1, CC-KOH-1.5 and CC-KOH-3. (b) The charge-discharge curves of VRFBs with CC-KOH-1.5 atthe current densities of 200e500 mA cm�2. (c) The energy efficiency of VRFBs with different electrodes at the current densities of 200e500 mA cm�2. (d) The discharge capacity ofVRFBs different electrodes at the current densities of 200e500 mA cm�2.

X.L. Zhou et al. / Journal of Power Sources 329 (2016) 247e254 253

80.1% even at the current density of 400 mA cm�2, which is amongthe highest performance in the open literature. It should bementioned that the electrolyte utilization of CC-KOH-1.5 can be as

Fig. 8. (a) The comparison of the performance of VRFB with activated carbon cloth and activof VRFB with CC-KOH-1.5 during 200 cycles.

high as 90.8% at the current density of 200 mA cm�2 and canmaintain at a high value of 74.6% even at the current density of400 mA cm�2, demonstrating the excellent mass transport

ated carbon paper. (b) The columbic efficiency, voltage efficiency and energy efficiency

X.L. Zhou et al. / Journal of Power Sources 329 (2016) 247e254254

properties of the present electrode.To further confirm the excellent transport properties, we

compare the charge-discharge performance of KOH-activated car-bon cloth and KOH-activated carbon paper, as shown in Fig. 8a. In aVRFB, the mass transport issue normally becomes sever at the endof the charge or discharge process as the inlet concentrationwill bevery low at that time. It can be clearly seen that the carbon clothelectrode has a much smaller overpotential at the end of charge/discharge process and also enables the deeper charge/dischargedepth, namely higher electrolyte utilization, which is of greatimportance to the VRFB system that surfers from high electrolytecost. The charge/discharge depth is increased about 26.7% with theuse of carbon cloth, demonstrating an excellent transport proper-ties, which is consistent with our simulation results. In addition,Fig. 8b shows the cycling performance of VRFBs with CC-KOH-1.5,showing that no obvious degradation after 200 cycles.

4. Conclusion

In this work, a high-performance porous electrode, made ofKOH-activated carbon-cloth, is developed for vanadium redox flowbatteries (VRFBs). The micro-scale porous structure in the carboncloth offers low tortuosity (~1.1) and a broad pore distribution from5 to 100 mm, rendering it a high hydraulic permeability and higheffective ionic conductivity, which are beneficial for the electrolyteflow and ion transport through the porous electrode. The use ofKOH activation method to create nano-scale pores on the surfacesof the carbon fibers leads to a significant increase in the surface areafrom 2.39 m2 g�1 to 15.4 m2 g�1. The battery assembled with thepresent KOH activated carbon-cloth electrode delivers an energyefficiency of 80.1% and an electrolyte utilization of 74.6% at a cur-rent density of 400 mA cm�2. These results suggest that the KOH-activated carbon-cloth electrode is a promising candidate inredox flow batteries. Further optimization of the electrode micro-structure, including fiber orientation, arrangement, and fiberdiameter, will help the performance improvement of vanadiumredox flow batteries and in principle other flow batteries as well.

Acknowledgements

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. 623313).

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.jpowsour.2016.08.085

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