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Electrochimica Acta 89 (2013) 429– 435

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

jou rn al hom epa ge: www.elsev ier .com/ locate /e lec tac ta

lectrochemical activation of graphite felt electrode for VO2+/VO2+

edox couple application

enguang Zhanga,b, Jingyu Xia,c,∗∗, Zhaohua Lia,b, Haipeng Zhoua,b, Le Liua,enghua Wua, Xinping Qiua,b,∗

Lab of Advanced Power Sources, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, ChinaKey Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, ChinaKey Lab of Thermal Management Engineering and Materials, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China

r t i c l e i n f o

rticle history:eceived 11 September 2012eceived in revised form6 November 2012ccepted 17 November 2012vailable online 26 November 2012

eywords:anadium redox flow battery

a b s t r a c t

In this work, the electrochemical activation of graphite felt electrode for vanadium redox flow battery(VRB) was studied. Graphite felt (GF) electrode was oxidized at a range of electrochemical oxidationdegrees in H2SO4 solution. The electrochemical performance of the treaded GF was discussed, and thelaw of the surface properties of GF which changed along with the electrochemical oxidation degree wasproposed. The structure, composition, surface tension and electrochemical properties of the oxidized GF(OGF) were characterized using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy(XPS), contact angle measurements, cyclic voltammetry (CV), and electrochemical impedance spec-troscopy (EIS). The GF oxidized at 560–840 mAh g−1 exhibited the best activity toward VO2+/VO2

+ redox

raphite felt electrodelectrochemical oxidationathode electrolyte

reaction, according with the highest C OH and COOH content (ca. 34%) on its surface. The mechanismsof VO2+/VO2

+ redox reaction on OGF were also discussed. VRB single cell with pristine GF and OGF as theelectrode were test at various charge–discharge current densities, respectively. The columbic efficiency(CE), voltage efficiency (VE) and energy efficiency (EE) of the cell using OGF electrode are much higherthan the cell using pristine GF, suggested that the electro-oxidation method is a promising technologyfor the activation of GF electrode.

. Introduction

Compared with other energy storage technologies, the vana-ium redox flow battery (VRB) has unique advantages for

arge-scale application and long cycle life [1–4]. In recent years,ts research and development has attracted considerable attention.owever, electrolyte stability, membrane selectivity and electrodectivity still limit the improvement of VRB performance. To a largextent, these factors have hampered the commercialization pro-ess of VRB. Recently, significant progress have been achieved foresearch of electrolyte [5,6] and membrane [7–10] for VRB. The

tudies of electrode material for VRB mainly focus on carbon basedaterials, including carbon paper [11], carbon fiber [12], carbon

anotube [13–15], graphene [14,16,17] and so on. GF has been

∗ Corresponding author at: Key Lab of Organic Optoelectronics and Molecularngineering, Department of Chemistry, Tsinghua University, Beijing 100084, China.el.: +86 10 62794234; fax: +86 10 62794234.∗∗ Corresponding author at: Lab of Advanced Power Sources, Graduate School athenzhen, Tsinghua University, Shenzhen 518055, China. Tel.: +86 755 26036436;ax: +86 755 26036436.

E-mail addresses: xijingyu@gmail.com (J. Xi), qiuxp@tsinghua.edu.cn (X. Qiu).

013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2012.11.072

© 2012 Elsevier Ltd. All rights reserved.

widely used as the electrode material of VRB due to good stabilityand high specific surface area, but the poor electrochemical activ-ity is still one of the defects that limit voltage efficiency and powerdensity of VRB. Hence it is important to develop the modificationmethods of the electrode to enhance the electrochemical proper-ties. A variety of modification methods of GF electrode have beeninvestigated, including electrodeposition of metals [18,19], ther-mal activation [20] and acid treatment [12,21,22]. Electrochemicaloxidation is an effective technique for surface treatment of carbonbased materials. This is because it can introduce numerous types ofoxygen-containing surface functional groups and increase the sur-face roughness; meantime, the technique is also relatively simple tocontrol and can carried out under mild conditions [23]. Nonethe-less, electrochemical oxidation of GF electrode for VRB has beenrarely reported. Li et al. only reported that the activity of OGF wasimproved compared with untreated GF [24].

In present work, the activation of electrochemically oxidizedGF (OGF) for VO2+/VO2

+ redox reaction was studied in detail.The surface morphologies, wettability, oxygen-containing surface

functional groups and electrochemical properties of OGF showedregular changes with the degree of electrochemical oxidation.These results are helpful to understand the electrochemical oxi-dation mechanism of GF, and this study also provides effective

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heoretical guides for optimizing technological parameters in VRBlectrode activation.

. Experimental

.1. Materials

Samples of 5 mm thick PAN-based GF was supplied by Shanghaiijie Limited Co., China. VOSO4·4H2O (>99%) was purchased fromhenyang Haizhongtian Fine Chemical factory and concentrated2SO4 (98 wt%) was obtained from Guangzhou Donghong Chemicalactory.

.2. Samples preparation

PAN-based GF was cut into a size of 1.5 cm × 1.5 cm before elec-rochemical oxidation, and then all the samples were thoroughlyashed with distilled water and dried at 70 ◦C for 48 h. The GF plateas used as the anode, and graphite plate was used as the cathode.

lectrochemical oxidation was carried out in 1 M H2SO4 solution.he GF electrode (1.5 cm × 1.5 cm) used as anode was produced byressing a piece of GF between two PVC sheets, one of the PVCheets had a hole of 1.0 cm × 1.0 cm, the GF connected with elec-rolyte through the hole. Another side of the GF was connected with

graphite sheet current collector. GF samples were electro-oxide at current density of 100 mA cm−2 for different oxidation times. Theesulted OGF was taken out and washed thoroughly with distilledater, then dried at 70 ◦C for 48 h.

.3. Characterizations

For electrochemical characterizations, a three-electrode cellas used with the resulted OGF as the working electrode, a sat-rated calomel electrode (SCE) as the reference electrode, andraphite plate as the counter electrode. The cyclic voltammetryCV) characterization were carried out in 0.015 M VOSO4 + 3 M2SO4 between 0.4 V and 1.4 V (vs. SCE) with a scan rate of

mV s−1. The electrochemical impedance spectroscopy (EIS) mea-urements were performed under open circuit potential (OCP)n 1.5 M VOSO4 + 3 M H2SO4 with an excitation signal of 5 mV inhe frequency range of 100 kHz and 10 mHz. The electrochemicalxperiments were obtained by the PARSTAT 2273 electrochemicalorkstation (Princeton Applied Research).

The surface morphology was characterized by scanning electronicroscopy (SEM) on a Hitachi S-4800 instrument. The samplesere analyzed by X-ray photoelectron spectroscopy (XPS) using a

HI5300 XPS, and spectra were analyzed using Spectrum softwareXPS PEAK 41). The wettability of samples was analyzed by contactngle tester (JC2000D3, Shanghai Zhongchen in digital technologyimited Co.).

.4. Single cell test

To perform a VRB single cell test, the OGF samples with a sizef 5 cm × 5 cm were also prepared by the electro-oxidation methods described in Section 2.2.

The VRB used in the charge–discharge tests was fabricated byandwiching the Nafion 117 ion exchange membrane (6 cm × 6 cm)etween two pieces of GF (5 cm × 5 cm) and then clampinghe sandwich between two graphite polar plates. The area ofhe electrode for the reaction was 25 cm2. At the beginning of

harge–discharge cycles, 60 mL of 2 M VO2+ in 3 M H2SO4 solutionas pumped into the cathode side and 60 mL of 2 M V3+ in 3 M2SO4 solution was pumped into the anode side, respectively. Tovoid the corrosion of the GF and graphite polar plates, the cell was

Acta 89 (2013) 429– 435

charged to 2400 mAh with a corresponding redox couples utiliza-tion of 75%. The low voltage limit for discharge was controlled to0.8 V. The VRB were galvanostatically charged and discharged withdifferent current densities by battery testing system (CT-3008W-5V3A-164, Neware Technology Limited Co.).

3. Results and discussion

3.1. Cyclic voltammetry behavior

To characterize the electrochemical activity of OGF forVO2+/VO2

+, CVs of OGF in 3 M H2SO4 and in 0.015 M VOSO4 + 3 MH2SO4 were measured, respectively. The results were shown inFig. 1(a) and (b). As can be seen from Fig. 1(a), CVs of OGF withoutVO2+/VO2

+ only showed oxidation current between 1.1 and 1.4 V(vs. SCE). The oxidation current can be attributed to the oxygenevolution.

Compared with Fig. 1(a) and (b) not only showed oxidationcurrent between 1.1 and 1.4 V, but also there were anodic peakO and reduction peak R at 0.9–1.0 V and 0.7–0.8 V, respectively.Therefore, the anodic peak O and reduction peak R correspondedto VO2+/VO2

+ redox reaction. In addition, Fig. 1(b) showed thatthe untreated sample had the largest peak potential separation(�Ep) and the lowest oxidation and reduction peak current den-sity, suggesting that the untreated sample exhibited the worstelectrochemical activity and complete irreversibility for VO2+/VO2

+

redox reaction. This indicated that the electrochemical activity ofOGF enhanced significantly for VO2+/VO2

+ couple, which can beattributed to the formation of oxygen-containing groups on the sur-face of OGF. The oxygen-containing groups provided active sites forVO2+/VO2

+ redox reactions. Furthermore, as seen in Fig. 1(b), redoxpeak potentials and reduction peak current densities changed littlewith the oxidation degree, but the oxidation peak current densityexhibited significant change with the electrochemical oxidationdegree. It can be seen from Fig. 1(c) that the oxidation peak currentdensity changed with the electrochemical oxidation degree.

Fig. 1(c) shows the oxidation peak current densities of threeparallel samples with each oxidation degree. As Fig. 1(c) canbe seen, the oxidation peak current densities of OGF with thesame oxidation degree showed some reproducibility. The oxida-tion peak current densities of VO2+ to VO2

+ increased with theoxidation degree from 0 to 560 mAh g−1. The oxidation peak cur-rent density changed little between 560 mAh g−1 and 840 mAh g−1.However, the oxidation peak current density decreased when theoxidation degree continually increased after 840 mAh g−1. Theserules may be due to the concentration of oxygen-containing func-tional groups increasing with increasing oxidation degree from 0to 560 mAh g−1. The oxygen-containing surface functional groupscan provide more active sites for the oxidation reaction of VO2+

to VO2+. The concentration of oxygen-containing surface func-

tional groups acting as active sites reached saturation at oxidationdegree of 560–840 mAh g−1. When the oxidation degree continu-ally increased, the content of oxygen-containing groups decreasedowing to further oxidation to CO2.

3.2. Electrochemical impedance spectroscopy studies

We carried out EIS measurement to further understand theelectrode structure and charge transfer properties on the surfaceof resulting OGF electrode at various electrochemical oxidationdegrees.

Fig. 2 displayed the Nyquist plots of GF and OGF electrode in1.5 M VOSO4 + 3 M H2SO4 solution at the open circuit potential.All the Nyquist plots included a semicircle part at high fre-quency and a linear part at low frequency, suggesting that the

W. Zhang et al. / Electrochimica

Fig. 1. CV curves on OGF electrodes at current density of 100 mA cm−2 for differentd −1

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2+ +

egrees in 3 M H2SO4 (a), 0.015 M VOSO4 +3 M H2SO4 (b), scan rate = 1 mV s ;(c)xidation peak current densities of three parallel OGF samples with each oxidationegree.

lectrochemical process was mix-controlled by charge transfernd diffusion steps. The high frequency semi-arc arose from theharge transfer reaction at the electrolyte/electrode interface. Theadius of the semi-arc reflected the charge transfer resistance.he low frequency linear part can be attributed to the diffusionrocesses associated with the diffusion of VO2+/VO2

+ in the 3D

ore channel of the electrode [12,25].

It can be seen from Fig. 2 that the untreated GF electrodead the largest semi-arc radius, indicating that untreated GF elec-rode had the largest charge transfer resistance for VO2+/VO2

+.

Acta 89 (2013) 429– 435 431

The magnitude of high frequency semi-arc decreased with theincreasing oxidation degree from 0 mAh g−1 to 560 mAh g−1. How-ever, the semi-arc increased when the oxidation degree continuedto grow to 1120 mAh g−1. It showed that electrochemical oxidationof GF electrode can efficiently reduce the charge transfer resistancefor VO2+/VO2

+, the charge transfer resistance decreased with oxida-tion degree from 0 mAh g−1 to 560 mAh g−1, and the charge transferresistance increased owing to further oxidation. The rules were inagreement with the changes of CV in Fig. 1(b).

3.3. XPS analysis

The increased in the activity of the OGF for VO2+/VO2+ should be

related to the surface state of OGF. In order to characterize the sur-face states of the samples, XPS measurements were made, whichcan provide direct information of functional groups. Fig. 3 showedthe XPS spectra of C 1s for OGF at various oxidation degrees. Thecontents of surface groups of the samples can be obtained basedon Fig. 3 by measuring the relative peak areas and the results arelisted in Table 1. As shown in Fig. 3, the carbon in C 1s XPS hadseveral electronic states. On the basis of references, the main peakat 284.7 eV was attributed to graphitized carbon, the other fourpeaks were attributed to defects on the GF structure (285.1 eV)[12], C OH (286.3 eV) [17], COO, C O (286.8 eV) [26], and COOH(288.9 eV) [27]. On the basis of the structural unit of carbon fiberand graphite fiber (Fig. 4) [28], the defective structure of GF shouldbe attributed to the part of defective carbon. The edge unsaturatedcarbon and defective carbon were high-activity area, which wereeasily oxidized.

As shown in Fig. 3, there was C OH peak (286.3 eV) for OGFcompared with untreated samples, confirming the effectiveness ofelectrochemical activation. As listed in Table 1, the content of defec-tive carbon decreased from 34% to 14% for the OGF at 20 mAh g−1,and the C OH content increased to 18% compared with untreatedsample. There was no apparent change for the other functionalgroups content. So, the C OH played a key role on improving elec-trochemical activity for the OGF at 20 mAh g−1. The content ofdefective carbon continually decreased and C OH, COOH increasedwith increasing oxidation degree from 20 mAh g−1 to 560 mAh g−1.However, the content of C OH decreased and COOH changed littlefor 1120 mAh g−1 compared with 560 mAh g−1.

As can be seen from Table 1, the total content of C OH andCOOH increased with the oxidation degree from 0 to 560 mAh g−1,

and when the oxidation degree increased from 560 mAh g−1 to1120 mAh g−1, the total content decreased. The trends were con-sistent with electrochemical activity of OGF electrodes (Fig. 1). Thetrends were in accordance with the analysis of CV and EIS. The elec-trochemical activity of OGF for VO2+/VO2

+ increased with oxidationdegree from 0 mAh g−1 to 560 mAh g−1, which can be attributedto the increase of C OH and COOH content. The activity for1120 mAh g−1 decreased compared with 560 mAh g−1, which mightbe attributed to the decrease of C OH content. The VO2+/VO2

+

redox reactions in Eq. (1) indicated that an oxygen atom partici-pated in the charge and discharge reaction process, which may bethe rate of determining step in the overall electrochemical reaction,so, the C OH and COOH functional groups on the electrode sur-face can behave as active sites for the oxidation reaction of VO2+/toVO2

+, catalyzing the vanadium species reactions.

VO2++H2O − e− Charge�

DischargeVO2

+ + 2H+ (1)

The mechanism with the participation of C OH for VO /VO2redox reaction had been proposed by Sun and Skyllas-Kazacos [20],which indicated that the existence of C OH on electrode surfaceaccelerated electron transfer reaction and made oxygen transfer

432 W. Zhang et al. / Electrochimica Acta 89 (2013) 429– 435

Fig. 2. Nyquist plot of untreated GF electrode (a) and OGF electrodes with different oxidation degrees (b) in 1.5 M VOSO4 + 3 M H2SO4.

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ig. 3. XPS C 1s curve fitting spectra of OGF versus the degree of electrochemicalxidation at current density of 100 mA cm−2.

rocess easier than directly from H2O, therefore, catalyzing theO2+/VO2

+ reaction. The C OH group provided H+ for the trans-ort of VO2+ ions from the solution to electrode surface on the firsttep of the overall mechanism, as shown in Eq. (2).

OH + VO2+ � R O V O+ + H+ (2)

However, the COOH group provided H+ easier than C OHroup, indicating that COOH group probably played an importantole on catalyzing the VO2+/VO2

+ reaction, which just explained

hat the activity for 1120 mAh g−1 was better than 20 mAh g−1.lthough the total content of C OH and COOH for 20 mAh g−1 was

arger than 1120 mAh g−1, the content of COOH for 1120 mAh g−1

as larger. Therefore, the proposed mechanism of OGF surface

able 1he contents of various functional groups on the surface of OGF at current density of 100

Oxidation degree(mAh g−1)

Graphitized carbon284.7 eV

Defective carbon285.1 eV

0 29% 34%

20 30% 14%

560 39% 2%

1120 35% 13%

Fig. 4. Structural unit of carbon fiber and graphite fiber.

oxygen-containing groups toward VO2+/VO2+ reaction can be

shown in Fig. 5.

3.4. Morphology characterizations

The surface state of OGF changed, which meant that the surfacestructure of OGF might have some changes. The surface morphologyof OGF under the scanning electron microscope was characterizedand was presented in Fig. 6. As shown, there was little differenceat a magnification of 3000. However, there was noticeable differ-ence at a magnification of 20,000. It can be seen that the surfaceof untreated sample is smooth and clean. The surface of OGF at20 mAh g−1 appeared a small amount of dotted material, while thesurface of OGF at 560 mAh g−1 had a lot of flaky material. Thismay be due to the product of graphite corrosion forming on the

surface of OGF during electro-oxidation, and the corrosion prod-uct increased with increasing oxidation degrees. This indicatedthat the rough degree and defect sites increased with oxidationdegrees. These were helpful to increase specific area to a great

mA cm−2 for different degrees.

C OH286.3 eV

C O286.8 eV

COOH288.9 eV

Total content( C OH, COOH)

0 26% 11% 11%18% 27% 12% 30%19% 25% 15% 34%14% 23% 15% 29%

W. Zhang et al. / Electrochimica Acta 89 (2013) 429– 435 433

Fig. 5. The mechanism of OGF toward VO2+/VO2+ redox reaction by OH and COOH groups.

Fig. 6. SEM images of untreated GF (a), and OGF at current density of 100 mA cm−2 for different oxidation degrees (b) 20 mAh g−1, (c) 560 mAh g−1, (d) 1120 mAh g−1 (Scalebar: left column, 10 �m; right column, 2 �m).

434 W. Zhang et al. / Electrochimica Acta 89 (2013) 429– 435

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reduced the polarization resistance during the charge–dischargeprocess.

The columbic efficiency (CE), voltage efficiency (VE) and energyefficiency (EE) of the VRB with GF and OGF at various current

Fig. 7. Contact angle measurement of untreated GF (a) an

xtent and improve surface energy. However, the surface of OGFt 1120 mAh g−1 showed cracking striation in the axial direction,hich may reduce the electronic conductivity of OGF as electrodeaterials.

.5. Contact angle characterizations

To evaluate the effect of GF surface state and structure onhe wettability of GF, the contact angle of water on GF surfaceas measured and was presented in Fig. 7. As shown, the initial

ontact angle of untreated GF was 131◦, and the contact anglehanged little with time. This indicated that the surface energyf untreated GF was very low and the wettability was very poor,hich was in accordance with the smooth surface morphology ofntreated felt (Fig. 6a) and the low peak current densities. The ini-ial contact angle of OGF at 560 mAh g−1 was 118◦, and the waterroplet completely disappeared within five seconds, confirminghe OGF surface energy greatly increased and wettability signifi-antly improved.

.6. VRB single cell test

In order to characterize the performance of OGF used as

lectrode in actual VRB, the VRB single cell with untreated GF5 cm × 5 cm) and OGF (5 cm × 5 cm) as electrodes were fabri-ated, respectively. The oxidation degree of OGF electrodes were60 mAh g−1.

ig. 8. Charge–discharge curves for the VRB single cell with GF and OGF as elec-rodes at various current densities. The oxidation degree of OGF electrodes were60 mAh g−1.

F (b) at current density of 100 mA cm−2 for 560 mAh g−1.

The charge–discharge curves of single cell at various currentdensities were shown in Fig. 8. By comparing the charge–dischargecurves of VRB with GF and OGF, it can be seen that the VRB withOGF had a lower charge voltage plateau and a higher discharge volt-age plateau than the VRB with GF under the same current density,thereby the voltage efficiency of the VRB with OGF was higher thanthe VRB with GF, suggesting the electrochemical oxidation of GF

Fig. 9. Coulombic efficiency (CE), voltage efficiency (VE), and energy efficiency (EE)of the VRB with GF and OGF at various current densities. The oxidation degree ofOGF electrodes were 560 mAh g−1.

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ensities were shown in Fig. 9. As Fig. 9 can be seen, for all currentensities, the voltage efficiency of the VRB with OGF was 4% higherhan the VRB with GF, and the energy efficiency of the VRB withGF was 5% higher than the VRB with GF. The CE, VE and EE of

he VRB with GF and the VRB with OGF showed the same treadith current density of charge–discharge, indicating that the

urface oxidation state of OGF electrode was stable in the differentharging/discharging conditions of the battery.

. Conclusions

The electrochemical activation of GF at a range of oxidationegrees has been discussed in details. The optimal electrochemi-al activity of OGF for VO2+/VO2

+ was obtained at oxidation degreef 560–840 mAh g−1. The improvement of the OGF activity can bettributed to the formation of the oxygen-containing groups onGF surface, which provided active sites for VO2+/VO2

+ redox reac-ions. The contact angle measurement showed that the wettabilityf OGF had significant improvement compared with untreatedample, which can be attributed to the hydrophilic of oxygen-ontaining functional surface groups and surface corrosion of OGF.he improved activity of OGF led to a significant improvementn VRB resistance values. For all current densities, the VRB withGF showed higher CE, VE, and EE than that of GF. The voltagend energy efficiency of the VRB with OGF were 4–5% higher thanhe VRB with GF. Therefore, electro-oxidation of GF for VRB was aromising technology for the activation of GF electrode.

cknowledgements

This work was supported by the National Natural Scienceoundation of China (20973099), Shenzhen Science Fundor Distinguished Young Scholars (JC201104210149A) andhe Basic Research Program of Shenzhen (JC201005310703A,C201005310712A, CXB201005250040A). J. Xi thanks Prof. Sili Renor contact angle analysis.

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