lable at ScienceDirect

Journal of Power Sources 325 (2016) 329e336

Contents lists avai

Journal of Power Sources

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

A high-performance dual-scale porous electrode for vanadium redoxflow batteries

X.L. Zhou, Y.K. Zeng, X.B. Zhu, L. Wei, T.S. Zhao*

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

h i g h l i g h t s

� A dual-scale porous electrode is developed for flow batteries.� The developed electrode offers high specific surface area.� The battery with the dual-scale electrode offers high performance.

a r t i c l e i n f o

Article history:Received 20 April 2016Received in revised form5 June 2016Accepted 9 June 2016

Keywords:Dual-scaleFlow batteryElectrode

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

http://dx.doi.org/10.1016/j.jpowsour.2016.06.0480378-7753/© 2016 Elsevier B.V. All rights reserved.

a b s t r a c t

In this work, we present a simple and cost-effective method to form a dual-scale porous electrode byKOH activation of the fibers of carbon papers. The large pores (~10 mm), formed between carbon fibers,serve as the macroscopic pathways for high electrolyte flow rates, while the small pores (~5 nm), formedon carbon fiber surfaces, act as active sites for rapid electrochemical reactions. It is shown that theBrunauer-Emmett-Teller specific surface area of the carbon paper is increased by a factor of 16 whilemaintaining the same hydraulic permeability as that of the original carbon paper electrode. We thenapply the dual-scale electrode to a vanadium redox flow battery (VRFB) and demonstrate an energyefficiency ranging from 82% to 88% at current densities of 200e400 mA cm�2, which is record breaking asthe highest performance of VRFB in the open literature.

© 2016 Elsevier B.V. All rights reserved.

1. Introduction

Electricity generated from intermittent renewable energysourcesdsuch as solar and wind need to be harnessed in order tosupplant the grid for cheap, efficient and reliable energy deploy-ment. To do this, large-scale energy storage technologies areindispensable for tackling the issues of intermittent supply andvariable demand [1e3]. Among the selection of energy storagetechnologies, redox flow batteries (RFBs) offer unique characteris-tics, including excellent scalability, high efficiency, long cycle lifeand are generally safe [4e6]. In particular, the vanadium redox flowbattery (VRFB) has been recognized as one of the most promisingtechnologies, primarily because it uses the same element in bothhalf-cells, which avoids the issue associated with cross-contamination between the two half-cell electrolytes [7e10].

Despite its attractive merits, however, VRFB technology has not

made significant progress in market penetration, owing to its highcost [11e14]. The capital cost, expressed in terms of the unit cost ofpower (US$/kW) or the unit cost of energy capacity (US$/kWh), isinarguably the largest contributor of high expenditure by sector.The total capital cost of a VRFB system consists primarily of theelectrolyte cost and stack cost, which can be expressed as [12]:

CtotzCs þ CezUs

.�t$I$Veff

�þ Ue

.�Veff $UE

�(1)

where Ctot, Cs and Ce are the total cost, stack cost and electrolyte cost($ per kWh), respectively; Us is the unit cost of the stack includingelectrodes, membranes and bipolar plates ($ per m2); Ue is the unitcost of the electrolyte including redox elements and supportingsalts/acids ($ per Ah); UE is the electrolyte utilization; I is the cur-rent density (A m�2); t is the designed discharge duration (h); andVeff is the effective discharge cell voltage (V). The effective dischargecell voltage can be estimated by Ref. [12]:

X.L. Zhou et al. / Journal of Power Sources 325 (2016) 329e336330

Veff ¼ Vrev � hact � hohm � hcon (2)

where Vrev is the reversible voltage; hact, hohm, and hcon are theactivation, ohmic and concentration losses, respectively. Accordingto eqns. (1) and (2), a reduction in the capital cost for RFBs withgiven redox couples like VRFBs can only be accomplished bysimultaneously maximizing the operating current density andminimizing the voltage losses. Namely, improving the rate capa-bility and efficiency of the VRFB through cost-effective approachesis critical for commercialization.

Electrode design is essential to achieving a high performanceand efficient VRFB primarily because it contributes to the systempolarization through not only the ohmic polarization, but alsothrough activation polarization and concentration polarization.Therefore, ideal porous electrodes for high-power VRFBs shouldpossess the following properties: i) high surface activity and spe-cific surface area for electrochemical reactions to minimize theactivation loss; ii) large hydraulic permeability for efficient elec-trolyte transportation to minimize the concentration loss andpumping work; and iii) excellent electronic conductivity to mini-mize the ohmic loss. The most commonly used materials for VRFBelectrodes are carbon fiber-based materials (graphite felts, carbonfelts, and carbon papers) for their high electronic conductivity, highhydraulic permeability, chemical and electrochemical inertness fora wide variety of chemicals and a wide range of potentials [15e20].However, issues associated with the use of these materialsincluding low specific surface area and poor electrochemical ac-tivity limit the VRFB to be operated at a low current density(~50 mA cm�2) [21e23]. Extensive efforts have been devoted toimproving the surface catalytic activities towards the redox re-actions. One strategy is to introduce functional groups onto thecarbon fiber surfaces to create abundant active sites and increasinghydrophilicity. It was found that the oxygen functional groupsintroduced by simple acid or thermal treatment can effectivelycatalyze the vanadium redox reactions [24e27]. Additionally, metaland metal oxide materials have been investigated as efficient cat-alysts for VRFBs. However, many of them are sensitive to parasiticreactions (hydrogen evolution) or are rare metals [10]. Recently,researchers from Pacific Northwest National Laboratory (PNNL)reported several types of catalysts of low cost and high catalyticactivity [21,22], including Bi metal, Nb2O5. VRFBs with these cata-lysts deliver significantly improved rate capability and cyclability.By incorporating Bi nanoparticles onto the solid surfaces of anegative porous electrode, the VRFB can be operated at160 mA cm�2 with an energy efficiency of 79%.

Although some progress has been made in improving the sur-face catalytic activity of carbon-fiber based electrodes, low specificsurface area is still a central issue for developing high-power VRFBsand has yet to be fully-resolved due to its conflict with the transportproperties of the electrode [2]. In the conventional carbon fiber-based electrodes, the pores are several tens of micrometers indiameter and are typically formed by the voids between the fibers.These micron-scale pores can construct efficient electrolyte trans-port pathways, offering high hydraulic permeability and thus lowconcentration loss and pump work. However, the specific surfacearea is extremely low (0.1e3 m2 g�1) due to the large pore sizes[28]. The most straightforward strategy to increase the surface areawould be to use carbon fibers with smaller dimeters to form theelectrode for a reduction of pore sizes. Unfortunately, this methodwill also induce a significant reduction in hydraulic permeability,leading to increased pumpwork and concentration polarization [1].Hence, to increase the specific surface area without sacrificing toomuch permeability, the common strategy is to coat high surfacearea materials (such as carbon blacks, carbon nanotubes and

graphenes) onto the carbon fiber surfaces [29e34]. Although thisapproach increases the surface area of the electrode, it still suffersfrom several issues. For instance, dip-coating is typically adopted tocoat carbon surfaces; the major disadvantage associated with thismethod is the fact that attachment between the high-surface-areamaterials and the carbon fiber surfaces relies on weak Van derWaals forces. Flowing electrolyte can easily destroy or disrupt theattachment, causing a severe problem in durability [35]. Severalgroups have reported to synthesize carbon nanotubes (CNTs), car-bon nanofibers (CNFs) or graphene directly on the carbon fibersurfaces via chemical vapor deposition (CVD) processes, aiming toconnect the high surface area material with carbon fiber surfacesvia covalent bonds [35e38]. However, issues associated with theCVD process including the high price of metal precursors, andcomplex and tedious preparation hinder its mass productionsignificantly. Furthermore, the issue of hydraulic permeabilityreduction and therefore large concentration loss still stands duringbattery operation, as previously reported [30]. The ideal solutionwould be to compose a low-cost porous electrode structure withhigh surface area that does not sacrifice permeability.

The bi-dispersed porous structure is one of the common struc-tures in porous media and is issued in the evaporators of heat pipes.It is composed of clusters (at macro-level), which are agglomeratedby small particles (at micro-level) [39,40]. This unique structurefeatures higher permeability than the mono-dispersed porousstructure that is composed of macro-level clusters as well as highersurface area due to the micro-level particles [40]. Inspired from theconcept of a bi-dispersed porous structure, we form a dual-scaleporous electrode (DPCE) that has a bimodal distribution of poresizes by selecting carbon papers (CPs) that have large primary pores(~10 mm) as starting materials and employing KOH activationmethod to etch the carbon fiber surfaces to form the desired smallsecondary pores (~5 nm), as shown in Fig. 1. With this uniqueporous structure, large primary pores serve as the macroscopicpathways for electrolyte flow; small secondary pores that are well-connected to primary pores provide ample active sites for elec-trochemical reactions. The expectation is that the surface area isdramatically increased without compromising on the permeabilitybecause of presence of the secondary pores. Our results match thisexpectation and we show that the specific surface area of the novelelectrode structure can reach as high as 42.7 m2 g�1, which is oneorder of magnitude higher than conventional carbon electrodes(2.6 m2 g�1). It is demonstrated that the VRFB with the presentelectrodes can stably operate at 400 mA cm�2 with an energy ef-ficiency over 80%. To the best of our knowledge, it represents one ofthe highest VRFB performances in the open literature. In addition,the preparation method is both straightforward and cost-effectivemaking it particularly appropriate for mass production. This dual-scale porous electrode represents a significant step toward theadvancement of VRFBs, and in principle, other RFBs as well.

2. Experimental

2.1. Electrode fabrication

Pristine CP (SGL 10 AA), provided by Union Chemical IndustrialCo., Ltd., was selected as the starting material. The pristine carbonpaper (CP-p) was first heat treated in a tube furnace at 500 �C in airfor 5 h [15]. In order to create nanopores on the carbon fiber sur-faces, the oxidized CP was thoroughly mixed with KOH slurries atdifferent carbon paper/KOH weight ratios of 1:0.5, 1:1 and 1:2. Theobtained mixture was then heat treated (heating ramp rate of3 �C min�1) in a horizontal furnace under a nitrogen flow at 800 �Cfor 1.5 h to form the DPCE. The resulting DPCE was rinsed severaltimes with 6 M HCl to remove inorganic salts, and subsequently

Fig. 1. The schematic of the conventional carbon-fiber based electrode and the proposed dual-scale porous carbon electrode.

X.L. Zhou et al. / Journal of Power Sources 325 (2016) 329e336 331

rinsed with distilled water until the establishment of a neutral pH.Finally, the DPCE was dried in an oven at 90 �C for 3 h.

2.2. Characterization

The material morphology was acquired by scanning electronmicroscope (SEM, JEOL-6700F and JSM-6300) under an accelera-tion voltage of 20 kV. Transmission electron microscopy (TEM)images were obtained by operating a high-resolution JEOL 2010FTEM system with a LaB6 lament at 200 kV. The samples weredispersed in ethanol, sonicated and dripped onto the carbon-coated Cu grids. Surface elemental composition was characterizedwith X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD, UK).N2 adsorption/desorption measurement was conducted with a gasanalyzer (ASAP2420, Micromeritics) after degassing at 120 �C for3 h to investigate the pore structure of the materials. The Brunauer-Emmett-Teller (BET) and Barrerr-Joyner-Halenda (BJH) methodwere adopted to calculate the specific surface areas and aperturedistribution of electrodes.

2.3. Electrochemical measurements

Cyclic voltammetry tests were conducted using a typical three-electrode cell. Carbon paper was used as the working electrodewith a geometric size of 0.1 cm2, and was connected to a platinumwire with a reference electrode (Saturated calomel electrode), andcounter electrode (Pt mesh) in 0.2 M VOSO4 in 3 M H2SO4 solution(Sigma-Aldrich, 98%) [15]. A single flowcell with an active electrodearea of 4 cm2 was assembled for charge/discharge tests. The in-house designed zero-gap flow battery with a serpentine flowfield was used in this study. Two layers of carbon paper were usedwith the compression ratio of 60% and Nafion 212 was adopted asthe membrane material. Electrolytes of 50 ml were fed into thecompartments using the acrylic flow channels and were circulatedto and from the reservoirs at 15 ml s�1 using a 2-channel peristalticpump (WT-600-2J, Longerpump, China). Measurements wereconducted with 50 ml 1 M V (IV) þ 3 M H2SO4 solution as thepositive electrolyte and 50 ml 1 M V (III) þ 3 M H2SO4 solution asthe negative electrolyte. Charge-discharge and cycling tests were

conducted using an ArbinBT 2000 at a constant current densityranging from 200 to 500 mA cm�2.

3. Results and discussion

3.1. Electrode design, fabrication and characterization

As shown in Fig. 1, the basic idea is to form the DPCE which haspores in two different scalesdmicron-scale primary pores andnanometer-scale secondary pores. To compose the proposedstructure, we adopted the carbon paper as starting materials whichhave primary pores (~10 mm) and use KOH to etch carbon fibersurfaces to form the secondary pores (~5 nm). It should bementioned that the pristine carbon papers were thermally treatedin air before etching to introduce the oxygen functional groups toincrease the surface activity and hydrophilicity [26]. The mecha-nism of the etching process, known as KOH activation, has beendocumented previously [41]. It is suggested that the activation ofcarbon with KOH proceeds as 6KOH þ C / 2K þ 3H2 þ 2K2CO3,followed by the decomposition of K2CO3 and the reaction of CO2with C. The surface morphology of CP-air and prepared DPCEs withKOH/CP weight ratio of 1:0.5 (DPCE-0.5), 1:1 (DPCE-1) and 1:2(DPCE-2) are depicted in Fig. 2. Fig. 2a showed that the carbon fibersurfaces of CP-air have nearly no pores, which is consistent with theresults reported [18]. In contrast, all of the DPCE samples haverough surfaces with uniform nano-scale pores generated by thewell-dispersed KOH activation agent (Fig. S1). In addition, with theincrease in the KOH/CP weight ratio, the nano-scale pores will beenlarged due to the further reaction between KOH and C.

To investigate the surface structure of the carbon fibers, TEMimages were taken. Fig. 3 revealed a relatively sharp contrast at theedge of each carbon fiber and the depths of the created nano-scalepores are around 20 nm, indicating that the carbon fiber surfacewas successfully etched during activation and did not significantlychange the carbon fiber structure. Since the depth of the nano-scalepores is only 20 nm, the reactant ions can easily diffuse from thebulk electrolytes into the pores for the electrochemical reactionsand the product ions can diffuse back to the bulk electrolytes afterthe reactions, as shown in Fig. 1.

Fig. 2. The scanning electron microscopy images of carbon fiber surfaces of CP-air (a), DPCE-0.5 (b), DPCE-1 (c) and DPCE-2 (d).

Fig. 3. The transmission electron microscopy image of carbon fiber of DPCE-1.

X.L. Zhou et al. / Journal of Power Sources 325 (2016) 329e336332

To test and verify our assumption that the specific surface areacan be increased by this dual-scale porous structure, the N2adsorption/desorption measurements were conducted. Fig. 4showed the N2-absorption and pore-width distribution of pristineCP, air-treated CP and DPCEs. Significantly, the DPCEs have oneorder of magnitude higher specific surface area than the CP-p(2.6 m2 g�1). The specific surface area of DPCE-0.5, DPCE-1 andDPCE-2 is 62.3 m2 g�1, 42.7 m2 g�1 and 26.9 m2 g�1. The specificsurface area decrease with the KOH/CP weight ratio due to thenano-scale pores were enlarged by the further reaction betweenKOH and C, which is consistent with the SEM images. More

importantly, as shown in Fig. 4b, pore sizes of the created pores inDPCEs usually range from 5 to 12 nm, which are beneficial for theelectrochemical reactions due to the reason that pores with poresize of around 5 nm can enable fast diffusion of active species [42].

As previously reported in Ref. [42], the diffusivity of redoxspecies in poreswith diameter of 5 nm is 100 times higher than thatin pores with diameter of 2 nm. If we assume that pores that havediameters larger than 5 nm can provide the effective surface areafor the redox reactions, the effective surface areas of DPCE-0.5,DPCE-1, DPCE-2, CP-a, CP-p should be 12.80 m2 g�1, 14.52 m2 g�1,15.32 m2 g�1, 6.21 m2 g�1 and 1.04 m2 g�1, respectively.

We further used XPS to characterize the surface properties ofthe CP-air and DPCEs, as shown in Fig. 5. The XPS results showedthat the O/C ratio of CP decreased after the KOH activation, due tothe fact that KOH can remove oxygen functional groups at hightemperatures, as expected. In addition, the percentage of differentoxygen containing functional groups is also altered. It is apparentthat the air treated CP has a much higher number of CeOH groupswhile post KOH treatment increased C]O group content, sinceCeOH groups can be partially converted to C]O groups during theactivation process, as summarized in Table 1.

3.2. Electrochemical performance

The electrochemical activities of CP-air and DPCEs were inves-tigated in a typical three-electrode system. As depicted in Fig. 6, thepeak currents of each redox reaction are mainly governed by thediffusion of the vanadium ions to the electrode surface [43] andthus no obvious difference was found between the different elec-trodes, indicating that the effect of surface area on the electro-chemical behavior cannot be well-reflected by the peak currents.Hence, we use the potential separation for the reduction andoxidation peaks to characterize the surface activity here. The CVcurve of the CP-air in V(IV) electrolyte showed relatively largeseparation for the reduction and oxidation peaks for the V(II)/V(III)and V(IV)/V(V) redox reactions, as shown in Fig. 6a. In contrast, the

Fig. 4. The nitrogen sorption isotherm plot (a) and the corresponding pore size distribution plot (b) of the CP-p, CP-air and DPCEs.

Fig. 5. O1s XPS spectra of CP-air (a), DPCE-0.5 (b), DPCE-1 (c) and DPCE-2 (d). The dark cyan line, blue line and orange line represent the fitted peaks of HeOeH, CeO and C]O. (Forinterpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 1Atomic fractions of C, O of CP-air, DPCE-0.5, DPCE-1 and DPCE-2.

Components CP-air DPCE-0.5 DPCE-1 DPCE-2

O 1s (%) 6.54 4.18 3.06 2.15C¼O (%) 1.21 1.78 1.27 0.96CeOH (%) 3.94 1.29 1.13 0.61C 1s (%) 93.46 94.79 95.82 96.88

X.L. Zhou et al. / Journal of Power Sources 325 (2016) 329e336 333

DPCE-0.5 had much faster kinetics than the CP-air, as evidenced bya smaller voltage separation, which can be explained by the cata-lytic effect of C]O functional groups, which is consistent with [33].The C]O functional groups may increase the density of states andcause the carbon to behave more like a metal, facilitating theadsorption of the positive charged ions [33,44]. In addition, theelectrochemical activity of DPCE decreased with the increase inKOH/CP weight ratio due to fewer C]O functional groups, asexpected.

To further investigate the electrochemical performance of theprepared electrodes, we conducted a symmetrical half-cell flowbattery test to test the positive and negative sides of a VRFB sepa-rately [43]. Fig. 6b and c compare the polarization curves of CP-airand DPCEs in the negative symmetric cell (0.5 M V(II)/0.5 M V(III))and positive symmetric cell (0.5 M V(IV)/0.5 M V(V)). It is shownthat all the DPCE samples exhibit lower overpotential than the air-treated CP. With an increase in the KOH/C weight ratio, the per-formance of DPCEs in the negative symmetric cell increases initiallyand then decreases. It can be explained that the effective surfacearea increases with the weight ratio but the number of functionalgroups decrease and thus the surface electrochemical activity de-creases. DPCE-1 and DPCE-2 have much larger effective surfaceareas than that of DPCE-0.5. For the performance of DPCEs in thepositive symmetric cell, the performance of DPCEs increases withthe increased amount of KOH loadings, which is a different trendwith the situation in the negative symmetric cell. While the func-tional groups have a large influence on the electrochemical activity

Fig. 6. a) CV in a scan rate of 10 mV s�1 for CP-air, DPCE-0.5, DPCE-1 and DPCE-2 in 0.2 M V(V) þ 3 M H2SO4 solution; potentiostatic measurements of air-treated CP DPCE-0.5, DPCE-1 and DPCE-2 with the cell active area of 4 cm�2 in the negative (b) and positive symmetrical cells (c).

X.L. Zhou et al. / Journal of Power Sources 325 (2016) 329e336334

toward the V(IV)/V(V) reactions under the low vanadium concen-tration, as shown in Fig. 6, it has been shown that the kinetics ofV(IV)/V(V) reaction has relatively low dependence on carbon sur-face for high vanadium concentration solutions [43]. Hence, in thepractical case with high vanadium concentration, the influence ofcarbon surface on the kinetics of V(IV)/V(V) reaction is quite limitedand the effective surface area becomes the dominant factor. Hence,

Fig. 7. (a) Charge-discharge curves of VRFBs with CP-air, DPCE-0.5, DPCE-1 and DPCE-2 at ththe current densities of 200e500 mA cm�2; (c) the energy efficiency and columbic efficiencthe discharge capacity of VRFBs with different electrodes at the current densities of 200e5during 120 cycles; (f) the discharge capacity of VRFBs with DPCE-1 during 120 cycles.

with the KOH/C ratio increase, the effective surface area increasesand thus the kinetics of V(IV)/V(V) reaction increases.

3.3. VRFB charge-discharge and cycling performance

Fig. 7a shows the charge-discharge curves of the CP-air andDPCE samples between 0.8 and 1.65 V at a current rate of

e current density of 200 mA cm�2; (b) charge-discharge curves of VRFBs with DPCE-1 aty of VRFBs with different electrodes at the current densities of 200e500 mA cm�2; (d)00 mA cm�2; (e) the energy efficiency and columbic efficiency of VRFBs with DPCE-1

X.L. Zhou et al. / Journal of Power Sources 325 (2016) 329e336 335

200 mA cm�2. The CP-air in this work can deliver an energy effi-ciency of 73% at the current density of 200 mA cm�2, which isconsistent with the value reported by other groups [27]. Ascompared to the CP-air, all the DPCE samples showed remarkablydecreased overpotential in both charge and discharge processes.The DPCE-1 exhibited the best performance among the sampleprepared, primary because of the trade-off between the effectivesurface area and the surface properties, which is consistent withthe symmetrical battery test. It should be mentioned that thepristine CP shows even poorer performance than that of the CP-air,as shown in Fig. S2. Fig. 7b shows the rate capability of the VRFBswith DPCE-1, showing that the battery can operate at ultra-highcurrent densities (400 mA cm�2 or 500 mA cm�2) with relativelow overpotential. Fig. 7c and d summarize the coulombic effi-ciency, voltage efficiency, energy efficiency and discharge capacityof VRFBs with different electrodes at current densities ranging from200 mA cm�2 to 500 mA cm�2. It can be clearly seen that VRFBswith DPCE-1 can deliver an energy efficiency ranging from 82% to88% at current densities of 200e400 mA cm�2, which is by far thehighest efficiency ever reported in the open literature [45e47],confirming the efficiency of the porous structure of the designedDPCEs, as shown in Fig. 8. Even at the current density of500 mA cm�2, the battery’s efficiency reaches a relatively highenergy efficiency of 76%. In addition, Fig. 7e and f demonstrate theperformance of VRFBs with DOCE-1 at the current density of400 mA cm�2, showing no obvious degradation during the 120cycles.

4. Conclusion

In summary, we present a simple and cost-effective method toform a dual-scale porous electrode by KOH activation of the fibersof carbon papers. The large pores (~10 mm) formed between carbonfibers serve as the macroscopic pathways for high electrolyte flowrates while the small pores (~5 nm) formed in carbon fibers provideactive sites for fast electrochemical reactions. It is shown that theBrunauer- Emmett-Teller specific surface area of the carbon paperis increased by a factor of 16 while maintaining the same hydraulicpermeability as that of the original carbon paper electrode.We thenapply the dual-scale electrode to a vanadium redox flow battery(VRFB) and demonstrate an energy efficiency ranging from 82% to88% at current densities of 200e400 mA cm�2. To the best of our

Fig. 8. The comparison of the performance of VRFBs with DPCE-0.5/1/2 and the VRFBperformance in the open literature.

knowledge, it represents one of the highest VRFB performances inthe open literature. This dual-scale porous electrode represents asignificant step toward the advancement of VRFBs, and in principleother flow batteries as well. Further optimization of pore size andpore shape of the dual scale porous electrode will help to furtherimprove the battery performance in the future.

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.2016.06.048.

References

[1] W. Wang, Q. Luo, B. Li, X. Wei, L. Li, Z. Yang, Adv. Funct. Mater. 23 (2013)970e986.

[2] A.Z. Weber, M.M. Mench, J.P. Meyers, P.N. Ross, J.T. Gostick, Q. Liu, J. Appl.Electrochem 41 (2011) 1137e1164.

[3] X. Zhou, T. Zhao, L. An, L. Wei, C. Zhang, Electrochim. Acta 153 (2015)492e498.

[4] K. Lin, Q. Chen, M.R. Gerhardt, L. Tong, S.B. Kim, L. Eisenach, A.W. Valle,D. Hardee, R.G. Gordon, M.J. Aziz, M.P. Marshak, Science 349 (2015)1529e1532.

[5] B.R. Chalamala, T. Soundappan, G.R. Fisher, M.R. Anstey, V.V. Viswanathan,M.L. Perry, Proc. IEEE 102 (2014) 976e999.

[6] M.L. Perry, A.Z. Weber, J. Electrochem. Soc. 163 (2016) A5064eA5067.[7] H. Zhang, H. Zhang, F. Zhang, X. Li, Y. Li, I. Vankelecom, Energy Environ. Sci. 6

(2013) 776e781.[8] X. Zhou, T. Zhao, L. An, Y. Zeng, X. Yan, Appl. Energy 158 (2015) 157e166.[9] X. Li, H. Zhang, Z. Mai, H. Zhang, I. Vankelecom, Energy Environ. Sci. 4 (2011)

1147e1160.[10] M. Park, J. Ryu, Y. Kim, J. Cho, Energy Environ. Sci. 7 (2014) 3727e3735.[11] Y. Zeng, T. Zhao, L. An, X. Zhou, L. Wei, J. Power Sources 300 (2015) 438e443.[12] K. Gong, X. Ma, K.M. Conforti, K.J. Kuttler, J.B. Grunewald, K.L. Yeager,

M.Z. Bazant, S. Gu, Y. Yan, Energy Environ. Sci. 8 (2015) 2941e2945.[13] L. Wei, T. Zhao, L. Zeng, X. Zhou, Y. Zeng, Energy Technol. (2016), http://

dx.doi.org/10.1002/ente.201600016.[14] V. Viswanathan, A. Crawford, D. Stephenson, S. Kim, W. Wang, B. Li, G. Coffey,

E. Thomsen, G. Graff, P. Balducci, J. Power Sources 247 (2014) 1040e1051.[15] M. Park, Y. Jung, J. Kim, H.I. Lee, J. Cho, Nano Lett. 13 (2013) 4833e4839.[16] C. Yao, H. Zhang, T. Liu, X. Li, Z. Liu, J. Power Sources 218 (2012) 455e461.[17] A.M. Pezeshki, J.T. Clement, G.M. Veith, T.A. Zawodzinski, M.M. Mench,

J. Power Sources 294 (2015) 333e338.[18] Q. Liu, G. Grim, A. Papandrew, A. Turhan, T.A. Zawodzinski, M.M. Mench,

J. Electrochem. Soc. 159 (2012) A1246eA1252.[19] Q. Liu, A. Turhan, T.A. Zawodzinski, M.M. Mench, Chem. Commun. 49 (2013)

6292e6294.[20] C. Sun, F.M. Delnick, L. Baggetto, G.M. Veith, T.A. Zawodzinski, J. Power Sources

248 (2014) 560e564.[21] B. Li, M. Gu, Z. Nie, Y. Shao, Q. Luo, X. Wei, X. Li, J. Xiao, C. Wang, V. Sprenkle,

Nano Lett. 13 (2013) 1330e1335.[22] B. Li, M. Gu, Z. Nie, X. Wei, C. Wang, V. Sprenkle, W. Wang, Nano Lett. 14

(2013) 158e165.[23] Y. Shao, Y. Cheng, W. Duan, W. Wang, Y. Lin, Y. Wang, J. Liu, ACS Catal. 5

(2015) 7288e7298.[24] B. Sun, M. Skyllas-Kazacos, Electrochim. Acta 37 (1992) 2459e2465.[25] L. Yue, W. Li, F. Sun, L. Zhao, L. Xing, Carbon 48 (2010) 3079e3090.[26] B. Sun, M. Skyllas-Kazacos, Electrochim. Acta 37 (1992) 1253e1260.[27] A.M. Pezeshki, J.T. Clement, G.M. Veith, T.A. Zawodzinski, M.M. Mench,

J. Power Sources 294 (2015) 333e338.[28] H. Zhou, H. Zhang, P. Zhao, B. Yi, Electrochim. Acta 51 (2006) 6304e6312.[29] M. Ulaganathan, A. Jain, V. Aravindan, S. Jayaraman, W.C. Ling, T.M. Lim,

M.P. Srinivasan, Q. Yan, S. Madhavi, J. Power Sources 274 (2015) 846e850.[30] J. Ryu, M. Park, J. Cho, J. Electrochem. Soc. 163 (2016) A5144eA5149.[31] W. Li, J. Liu, C. Yan, Electrochim. Acta 79 (2012) 102e108.[32] Z. Gonz�alez, P. �Alvarez, C. Blanco, S. Vega-Díaz, F. Trist�an-L�opez,

L.P. Rajukumar, R. Cruz-Silva, A.L. Elías, M. Terrones, R. Men�endez, Sustain.Energy Technol. Assess. 9 (2015) 105e110.

[33] W. Li, J. Liu, C. Yan, Carbon 55 (2013) 313e320.[34] Z. Gonz�alez, C. Botas, P. �Alvarez, S. Rold�an, C. Blanco, R. Santamaría, M. Granda,

R. Men�endez, Carbon 50 (2012) 828e834.[35] V. Yarlagadda, G. Lin, P.Y. Chong, T. Van Nguyen, J. Electrochem. Soc. 163

(2016) A5134eA5143.

X.L. Zhou et al. / Journal of Power Sources 325 (2016) 329e336336

[36] W. Li, Z. Zhang, Y. Tang, H. Bian, T. Ng, W. Zhang, C. Lee, Adv. Sci. (2015),http://dx.doi.org/10.1002/advs.201500276.

[37] S. Wang, X. Zhao, T. Cochell, A. Manthiram, J. Phys. Chem. Lett. 3 (2012)2164e2167.

[38] R. Huang, C. Sun, T. Tseng, W. Chao, K. Hsueh, F. Shieu, J. Electrochem. Soc. 159(2012) A1579eA1586.

[39] B. Yu, P. Cheng, Int. J. Heat. Mass Transf. 45 (2002) 2983e2993.[40] Z. Chen, P. Cheng, T. Zhao, Int. Commun. Heat. Mass Transf. 27 (2000)

293e302.[41] E. Raymundo-Pinero, P. Azais, T. Cacciaguerra, D. Cazorla-Amor�os, A. Linares-

Solano, F. B�eguin, Carbon 43 (2005) 786e795.

[42] C. Wang, X. Li, X. Xi, W. Zhou, Q. Lai, H. Zhang, Nano Energy 21 (March 2016)217e227.

[43] N. Pour, D.G. Kwabi, T. Carney, R.M. Darling, M.L. Perry, Y. Shao-Horn, J. Phys.Chem. C 119 (2015) 5311e5318.

[44] P. Chen, R.L. McCreery, Anal. Chem. 68 (1996) 3958e3965.[45] Q. Zheng, F. Xing, X. Li, T. Liu, Q. Lai, G. Ning, H. Zhang, J. Power Sources 266

(2014) 145e149.[46] D. Reed, E. Thomsen, W. Wang, Z. Nie, B. Li, X. Wei, B. Koeppel, V. Sprenkle,

J. Power Sources 285 (2015) 425e430.[47] D. Reed, E. Thomsen, B. Li, W. Wang, Z. Nie, B. Koeppel, V. Sprenkle, J. Power

Sources 306 (2016) 24e31.