Comparative Electrocatalytic Performance of Single-Walled and

Article pubs.acs.org/JPCC

Comparative Electrocatalytic Performance of Single-Walled and Multiwalled Carbon Nanotubes for Zinc Bromine Redox Flow Batteries Y. Munaiah,† S. Suresh,† S. Dheenadayalan,† Vijayamohanan K. Pillai,*,†,‡ and P. Ragupathy*,†,‡ †

Electrochemical Power Sources Division and ‡CSIR-Network Institute of Solar Energy (CSIR-NISE), Central Electrochemical Research Institute, Karaikudi 630 006, India ABSTRACT: Carbon nanotubes (CNTs) have been employed as electrode materials in rechargeable zinc bromine redox flow batteries (ZBB) owing to their high electrocatalytic activity, remarkable electrical conductivity, and excellent mechanical strength with high Young’s modulus. The electrocatalytic effect of single-walled carbon nanotube (SWCNT) and multiwalled carbon nanotube (MWCNT) electrodes for the 2Br−/Br2 redox couple has been investigated for zinc bromine redox flow battery application. The anodic peak current density of SWCNT electrode is found to be about 16 mA cm−2, which is almost 50% higher than that of MWCNT, indicating the enhanced electrocatalytic effect of SWCNT perhaps due to a large amount of basal planes. The peak separation between the anodic and cathodic process at SWCNT and MWCNT electrodes is 201 and 126 mV, respectively, demonstrating the quasireversible nature of the 2Br−/Br2 redox reaction. Moreover, the peak separation for the MWCNT electrode is 37% less compared to that on the SWCNT electrode, revealing better reversibility. FTIR, Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) have been used to further investigate the composition and morphological changes of CNT before and after cycling. Zinc bromine redox flow cell made with CNT-anchored carbon felt (CF) as bromine electrode exhibits improved electrochemical performance in terms of efficiency and durability. Particularly, SWCNT-modified electrode possesses 98% energy efficiency retention even after 200 cycles of charge−discharge process, offering great promise as high-performance electrodes for zinc bromine redox flow battery.

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cm−2. In recent years, several new chemistries have also been developed and demonstrated for redox flow batteries3,9−11 Among the various flow batteries, the zinc bromine system (ZBB) has become more appealing as one of the promising energy storage devices due to its good energy density, high cell voltage, high degree of reversibility, use of low-cost and abundant materials, and eco-friendliness5,11,12 The basic electrochemical cell reaction involves in ZBB is at anode:

INTRODUCTION

Electrochemical energy storage (EES) has received much attention in portable electronics, automobiles, and renewable energy systems owing to their low pollution, long life, ease of material availability, flexibility, modularity, and ease of maintenance. Among the various EES investigated over the years, sodium−sulfur battery (SSB)1 and redox flow batteries (RFBs) are well-recognized storage devices for large-scale energy storage and electrical vehicle applications.1,2 Moreover, RFBs are particularly attractive for large-scale storage of electricity especially from renewable energy sources such as wind and solar.3−5 The most important advantage of RFB is the decoupling of energy capacity and power density which is not possible with the other existing conventional batteries utilizing solid active materials.6 Typically, the energy density of RFB mainly depends on the volume of the electrolyte, while battery power is based on the size of the cell stack.7 Based on the redox couple utilized for energy storage, RFBs are referred as iron−chromium, vanadium, bromine polysulfide, and zinc bromine redox flow batteries. Further, Huskinson et al.8 have demonstrated a metalfree organic−inorganic redox flow battery in which quinone/ hydroquinone couple with the 2Br−/Br2 redox couple offering a peak galvanic power density exceeding 0.6 W cm−2 at 1.3 A © 2014 American Chemical Society

charging

2Br− XooooooooooY Br2 + 2e− discharging

E 0 = 1.08 V vs SHE

at cathode: charging

Zn 2 + + 2e− XooooooooooY Zn(metal) discharging

E 0 = 0.76 V vs SHE

net cell reaction: charging

Zn 2 + + 2Br − XooooooooooY Br2 + Zn discharging

E 0 = 1.84 V vs SHE

Received: April 3, 2014 Revised: June 16, 2014 Published: June 16, 2014 14795

dx.doi.org/10.1021/jp503287r | J. Phys. Chem. C 2014, 118, 14795−14804

The Journal of Physical Chemistry C

Article

Scheme 1. Zinc Bromine Redox Flow Cell

of the Zn electrode to minimize the overpotential impacting the efficiencies of ZBB. In this paper, we compare the electrocatalytic activity of MWCNT and SWCNT electrodes on bromine redox reaction. An attempt has been made to understand the reasons for differences in the electrocatalytic effect of CNTs and the role of active sites on CNT in the bromine redox couple. The enhanced electrochemical performance of zinc bromine flow cells made with CNT-based electrodes is explained in detail by the comparing the results of physicochemical and electrochemical characterization.

While charging the metallic zinc deposits on the anode, elemental bromine generates at the cathode. The formed bromine is being stored in the electrolyte in the form of complexes with an addition of quaternary ammonium salt. Zinc and bromide ions are generated in the respective compartments during the discharge process. Typically, carbon-based materials are used as electrodes for RFB due to their wide operating potential window and chemical stability in different electrolytes. Despite the advantages of carbon materials, they suffer from poor kinetic reversibility, limited energy density and lifetime. Though electrodes in RFB do not participate directly in the energy storage process, it plays a crucial role in enhancing the overall performance of the device. The power density of RFB depends on the rate of redox couple at the electrode surface. Hence, the selection of suitable electrode materials including various electrocatalysts such as carbon-based materials,3,13 metals,3,14 and metal oxide15 becomes a key parameter to achieve high-efficiency energy storage systems.13 Therefore, much attention has been paid to develop new electrode materials. In recent years, carbon nanotubes (CNTs), functionalized CNTs, and nitrogen-doped CNTs have appeared as potential electrode materials in RFB technology due to their unique electronic and mechanical properties. For instance, Yuyan Shao et al. have investigated the electrochemical redox behavior of polyhalide ions on CNT electrode16 but the durability of the material and performance of the CNT in real cell have not been explored. MWCNT and graphene oxide hybrid with excellent electrocatalytic redox reversibility toward VO2+/VO2+ redox couple for VRFB has been reported by Han et. al.17 It is demonstrated that functionalized CNTs acts as the best electocatalyst for VO2+/VO2+ in a VRFB.18 Also, nitrogendoped CNT enhances the electrode performance by modifying the electronic properties and generating defect sites in vanadium flow battery.19 Thermally reduced graphene oxide is used as electrode material for VRFB.20 In most of the works, the durability of the material and performance in a real cell16−20 are not explored. We have recently demonstrated zinc bromine flow cells (Scheme 1) made with SWCNT-modified carbon felt and the effect of impurities in SWCNT on bromine electrocatalysis.21 However, the kinetics of the electrode process in 2Br‑/Br2 and comparison of SWCNT and MWCNT have not been explored in detail. Moreover, the Zn/Zn2+ couple reacts faster than 2Br‑/ Br2 in ZBB which leads to reduction in the performance of the battery.3,11 Thus, it becomes very essential to enhance the rate of the reaction of bromine electrode to counterbalance the rate

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EXPERIMENTAL SECTION Materials. Zinc bromide (ZnBr2), perchloric acid (HClO4) single-walled carbon nanotubes (SWCNTs) (density 1.7−1.9 g/cm3), and multiwalled carbon nanotubes (MWCNTs) (density 2.2 g/cm3) were purchased from Sigma-Aldrich and used without further purification. Glassy carbon electrode (GCE) with 3 mm diameter was used for cyclic voltammetry studies. Preparation of CNT-Modified GCE. The CNT-modified GCE was prepared as mentioned in our earlier report;21 in brief, 2.5 mg of CNT was dispersed in 750 μL of doubledistilled water and isopropanol (1:5 ratio) mixture by ultrasonication. Thirty microliters of CNT slurry was dropped onto the polished GCE and dried at ambient condition for 30 m to evaporate the solvent completely. Physical and Electrochemical Characterization. Morphology and structural characterization of CNT were studied by using FTIR (Bruker Tensor 27), laser Raman (Renishaw Invia Raman Spectroscopy and Microscope), scanning electron microscopy (SEM) (Vega-3 TESCA) transmission electron microscopy (TEM), (FEI-Tecanai 20 G2), and X-ray photoelectron spectroscopy (XPS) (Thermo Scientific MULTILAB 2000). The cyclic voltammetry studies were carried out by using potentiostat (Solartron model-1470E) in 1 M HClO4 and 0.05 M ZnBr2 solution using three-electrode configuration. The GCE and CNT-modified GCEs were used as working electrode, while Pt foil and Ag/AgBr were employed as counter and reference electrodes, respectively. Preparation of Electrodes and Electrolytes. The electrodes and cells were made as described previously.21 Typically, a carbon felt (CF) substrate (CereMaterials) (5 cm2) was subjected to ultrasonication in deionized water and dried at 60 °C for 5 h. A 1:1 weight and volume ratio of CNT and dimethylformamide (DMF) were mixed together by ultrasonication for about 10 m. The pretreated CF was immersed in 14796



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CNT−DMF solution and subjected to ultrasonication for 15 m. The CNT-immobilized CF was dried at 100 °C for 24 h and used as positive electrode, and CF was used as negative electrode. An in-house made Teflon flow cell was constructed to study the performance of zinc bromine flow cell. A catholyte solution was prepared by using 3 M zinc bromide aqueous solution and N-methyl-N-ethylmorpholinium bromide (MEP) and N-methyl-N-ethylpyrrolidinium bromide (MEM) (1:1 molar ratio), (US 5188915, US 4491625). An anolyte was obtained by mixing 3 M zinc bromide, 1 M zinc chloride, and quaternary ammonium salt (mixture of MEP and MEM) (US4491625) to enhance the conductivity and reduce the amount of elemental bromine presence in electrolyte. With an aim to understand the eletrocatalytic effect of SWCNTs and MWCNTs on electrochemical behavior of bromine redox reaction, cyclic voltammograms (CV) were recorded for all three electrodes (pristine GCE, SWCNT, and MWCNT modified GCEs) in the range of 0−1.3 V vs Ag/AgBr in the mixture of 0.05 M ZnBr2 and 1 M HClO4 at 20 mV s−1.

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RESULTS AND DISCUSSION Figure 1 shows the superimposed CV curves for comparison of all the electrodes. Substantial changes in the electrochemical

Figure 2. Raman spectra of pristine SWCNT and MWCNT referring to the ID/IG ratio, and their basal and edge plane active sites.

50% higher than that of MWCNT electrode, illustrating the best electrocalatalytic effect of SWCNT toward 2Br−/Br2 redox reaction. This behavior is mainly attributed to large available basal plane in SWCNT as confirmed by Raman analysis (see later). It is well documented that more active sites (edge and basal plane) are responsible for electrocatalytic effect CNT and graphene.22−24 For instance, Li et al. have demonstrated the effect of electrocatalytic activity of MWCNT in vanadium redox couple for vanadium redox flow battery.18 Moreover, the nonFaradaic current for SWCNT electrode is significantly larger than that of MWCNT possibly due to the high surface area of SWCNT. The peak separations between the anodic and cathodic peak potentials are 126 and 201 mV on the MWCNT and SWCNT, respectively, at a scan rate of 20 mV s−1. The peak separation of MWCNT electrode is 75 mV less compared to that of the SWCNT electrode, indicating the high reversibility of MWCNT electrodes for 2Br−/Br2. In spite of the higher degree of reversibility, the peak current density is lower than that of SWCNT. The significant reduction in peak separation is possibly attributed to the existence of a higher amount of edge plane active sites in MWCNT22,23,25 (Figure 2). The electrocatalytic activity and reversibility of reaction are the key factors in envisioning the performance of any redox flow battery. The onset potentials for the oxidation of bromide ions are 920 and 816 mV on MWCNT and SWCNT electrodes, respectively, demonstrating the electrocatalytic effect of CNTs compared to that of pristine GCE. Interestingly, the onset oxidation potential of SWCNT-modified electrode is negatively shifted by 104 mV less than that of the MWCNT electrode, indicating the ease of oxidation of bromide ions at the SWCNT electrode as seen in Figure 1. The lower oxidation potential of bromide ion is more beneficial with respect to energy storage efficiency as it connotes a lower charge voltage for ZBB. The presence of edge planes and basal planes is conformed by using Raman spectroscopy. Figure 2 shows the comparison of Raman spectra of SWCNT and MWCNT. ID/IG ratios of SWCNT and MWCNT are 0.043 and 1.16, respectively, which clearly indicates the large amount of basal planes and edge

Figure 1. Cyclic voltammograms of CNT-modified electrodes and GCE at 20 mV s−1 in 0.05 M ZnBr2 containing 1 M HClO4. Note that star marks indicate the open circuit potential of the electrode vs Ag/ AgBr reference electrode.

Table 1. Cyclic Voltammetry Data Obtained from Figure 1 electrode type

Epa (V)

MWCNT SWCNT

1.084 1.117

Epc (V)

peak separation (V)

E1/2 (V) ((Epa + Epc/2)

Ipa/Ipc

0.958 0.916

0.126 0.201

1.021 1.0166

0.802 0.830

response are evident from the variation of current density and peak potential. The electrochemical redox behavior of 2Br−/Br2 at GCE electrode is very sluggish; however, CNT-modified GCE exhibits the well-defined redox behavior as seen in Figure 1. The anodic and cathodic peak potentials of SWCNT- and MWCNT-modified GCE are observed at 1117 and 1084 mV and 958 and 916 mV, respectively, demonstrating the oxidation of bromide ions and reduction of bromine. The electrochemical parameters such as peak potentials (Epa, Epc), ratio of anodic and cathodic peak current densities (Ipa/Ipc), and peak separations obtained from CV are listed in Table 1. The anodic peak current density of SWCNT-modified electrode is 14797



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Figure 4. Cyclic voltmogram of bromine redox process of bromine on SWCNT-modified (a) and MWCNT-modified (b) GCE in electrolyte with cycle number. No change in CV of bromine redox couple is strong evidence for durability of CNT in bromine environment.

Figure 3. Cyclic voltmogram of SWCNT-modified (a) and MWCNTmodified (b) electrode at different scan rates in the electrolyte. Increase in the peak separation and asymmetry peak shape reveals the redox reaction is quasireversible and bromide ion is weakly adsorbed on the surface of the CNTs. Inset shows peak current vs scan rate plot for bromine redox process on SWCNT- and MWCNT-modified electrode. Peak separation vs scan rate plot, which is strong evidence for the quasireversible nature of the bromine redox reaction on CNT electrode (c).

Figure 5. Tafel plot of bromide ion oxidation at CNTs electrodes in the electrolyte.

In order to understand the nature of the electrode reaction, CVs were recorded for both SWCNT and MWCNT electrodes at different scan rates. Figure 3 shows the CV of SWCNT (Figure 3a) and MWVNT (Figure 3b) at different scan rates and the inset shows linear variation of peak current density with the square root of scan rate; however, it is not proportional to the square root of scan rate but increases linearly with scan rate.

planes available on CNT having a significant impact on the peak current density and peak separation, respectively, as revealed in the CV (Figure 1). 14798



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Table 2. Kinetic Parameters for the Bromide Redox Reaction on CNT-Modified Electrodesa diffusion coeff (D) (cm2/s) electrode type SWCNT MWCNT

2

exchange current density (J0) (mA/cm ) −2

2.20 × 10 1.88 × 10−2

rate const (k0) (cm/s)

anodic transfer coeff (α)

−6

1.13 × 10 2.43 × 10−6

0.96 0.96

Br

−

Br2 −5

5.21 × 10 1.82 × 10−5

3.46 × 10−5 2.42 × 10−5

a

Exchange current density and transfer coefficient values are calculated from Tafel plots, and diffusion coefficient values are calculated from Randles−Sevcik equation. Rate constant values are calculated through the Nicholson method.

Figure 4a,b shows the CVs of SWCNT and MWCNT recorded at different cycles, respectively. Peak current density of both CNT-based electrodes after the 50th cycle remains the same as observed in the first cycle, indicating the stability and high durability of CNT in ZnBr2 electrolyte. However, there is substantial change in the nonfaradic region (0.4−0.5 V) at the SWCNT electrode and increases as increasing the cycle numbers (Figure 4a). This behavior is mainly attributed to the redox reaction of formed functional groups during CV29 in addition to the high surface area of SWCNT. Figure 5 shows the Tafel plots for anodic polarization of bromide oxidation at both MWCNT and SWCNT electrodes in aqueous ZnBr2 solution. Exchange current densities (i0) of MWCNT and SWCNT electrodes are calculated to be 1.876 × 10−2 and 2.20 × 10−2 mA cm−2, respectively (Table 2). Exchange current density not only depends up on the material but also the concentration of the electrolyte. Sergio Ferro et al. have studied the kinetics of 2Br−/Br2 redox couple on polycrystalline platinum30 and boron-doped diamond31 with i0 values of 17.9 and 0.141 × 10−6 mA cm−2, respectively. The exchange current density of 2Br−/Br2 redox couple on CNT electrode is significantly larger than on the boron-doped diamond electrode, indicating the enhanced rate of the reaction. However, this value is lower than that of Pt due to the inherent nature of Pt and electrolyte concentration. Since the exchange current density depends on concentration, rate constant is used as measurement for rate of the reaction. The Nicholson method has been widely used to calculate the rate constant for quasireversible reaction.32 The Nicholson equation is given below. Ψ = Λπ 1/2 =

(D0 /DR )1/2 K 0 (πD0 -ϑ)1/2

In the above equation, Ψ is a dimensionless rate parameter, Λ is equivalent conductivity of the solution. D0 and DR are the diffusion coefficients of the oxidizing and reducing species, respectively, and K0 is the rate constant of the reaction. - is a factor of RT/nF and ϑ is the scan rate. The ratio of the diffusion coefficients of Br‑ and Br2 is estimated to be unity. The k0 values for SWCNT and MWCNT electrodes are 1.131 × 10−3 and 2.433 × 10−3 cm s−1, respectively. The rate constant values do not vary much, indicating that the rate of the reaction is the same on both the electrodes. In spite of similar rate constants for both the electrodes, the peak current density is higher for SWCNT. This can be attributed to the availability of large basal planes for the reaction on SWCNT. Figure 6a,b shows the IR spectra of SWCNT and MWCNT before and after the 50 cycles of CV. A sharp peak observed at 3430 cm−1 corresponds to O−H stretching. The peaks at 2964 and 1263 cm−1 are responsible for C−H and −CH3 stretching frequencies, respectively. The O−H and C−O stretching are evident from the peaks at 1630 and 1109 cm−1, respectively.

Figure 6. IR spectra of SWCNT (a) and MWCNT (b) before and after 50 cycles of CV. The changes in the spectra are evidence for the formation of oxygen functionalities.

Moreover, the peak separation of both the electrodes (Figure 3c) is greater than 59 mV, indicating the quasi-reversible reaction of the 2Br−/Br2 redox process. Upon increasing the scan rate, anodic peak and cathodic peak positions shift toward more positive and more negative, respectively. The shift in the cathodic peak is higher than that of the anodic process. These observations are mainly attributed to quasireversible reaction and weakly adsorbed bromide ions over the surface of the CNT electrode.26 The diffusion coefficient values are calculated to be 5.21 × 10−5 and 1.82 × 10−5 cm2 s−1 for both SWCNT and MWCNT electrodes, respectively. These values are consistent with the reported literature values.27,28 With an aim to assess the stability of the CNT in ZnBr2 solution, 50 cycles of CVs were recorded in the electrolyte. 14799



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Figure 7. TEM imges SWCNT (a,b) and MWCNT (c,d) before (a,c) and after (b,d) cyclic voltammetry. The images are shown that CNT are debundled and well separated from each other after CV.

The peaks between 600 and 670 cm−1 clearly confirm the existence of C−Br bond, corresponding to the formation of C− Br bond stretching. The IR spectrum of MWCNT given in Figure 6b shows a substantial change after 50 cycles. The change of the chemical entity of CNT during cycling does not reflect any changes on the CV data, demonstrating the high durability and stability of the CNT electrodes. Figure 7 depicts the TEM images of SWCNT before (a) and after (b) 50 cycles of CV. There are several changes in the morphology of SWCNT, such as the diameter of CNT becomes wider after 50 cycles of CV. The morphology of SWCNT is changed during the course of the reaction and forms the nanoribbon kind of structure (as marked in Figure 7b). The morphological changes are attributed to oxygen functional groups on CNTs. Interestingly, the morphological changes did not cause any change in the electrochemical performance of SWCNT electrode. Figure 7 shows the TEM images of MWCNT before (c) and after (d) 50 cycles of CV. There are not much changes in the morphology of the MWCNT which is reflected in CV studies, but they are crumpled into individual tubes which can possibly be attributed to the formation of oxygen functional groups.33 This is possibly due to the concentric rings of CNT in MWCNT which holds the morphology even after the formation of functional groups. With an aim to understand the effect of bromination on both SWCNT and MWCNT, XPS spectra were recorded for CNT before and after cycling. The typical wide range XPS spectrum of SWCNT is shown in Figure 8a. The spectrum contains two well-defined peaks at 284.9 and 533.2 eV corresponding to C 1s and O 1s electrons, respectively. The deconvoluted C 1s spectra of pristine SWCNT and after cycling are shown in Figure 8, b and c, respectively. The C 1s spectrum can be

composed into five components as reported elsewhere.34−36 The peaks centered at 284.7 and 285.2 eV are the characteristic of sp2 and sp3 carbon atoms, respectively. The existence of the covalently bonded oxygen atom with carbon (ethers, alcohols, and phenols) and carbonyl atoms (quinone groups) is evident from the peaks at 286.6 and 287.2 eV, respectively. The peak at 290.1 eV is possibly attributed to the shake-up satellite (π → π*) process. On the other hand, there is a concomitant change in the C 1s spectrum of SWCNT after cycling, indicating the change in the chemical entity of SWCNT as shown in Figure 8c. The reduction in the intensity of 284.9 eV corresponding to sp2 is possibly attributed to the formation of functional groups. The peak at 286.4 eV corresponding to carbon covalently bonded to oxygen and bromine reveals the possibility of bromine intercalation into the SWCNT. Moreover, the peak at 285.2 eV disappeared after cycling, indicating the bromine functionalization on SWCNT. Further, the peak at 287.2 eV ensures the existence of the aliphatic carbon atom covalently bonded to single bromine.34 The peak at 288.3 eV is responsible for the carbonyl carbon in carboxylic functional groups. The peak at 290.8 eV is probably due to shake-up (π → π*) process.34,35 As seen in Figure 8a, the relative intensity of the O 1s peak increases after CV, indicating the increased oxygen functional groups on SWCNT as further supported by IR and CV data. In order to further understand the insights of bromine interaction with SWCNT, Br 3d spectra were deconvoluted into five different peaks (see Figure 8d). More importantly, the peaks at 70.2 eV (Br 3d5/2) and 71.4 (Br 3d3/2) are strong evidence for the C−Br bond formation.36 The peak at 68.9 eV is due to bromide ion (Br−), in this case from the electrolyte (ZnBr2). The peaks at 75.2 and 76 eV are mainly attributed to 14800



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Figure 8. XPS spectra of SWCNT before and after 50 cycles of CV (a), survey spectra of SWCNT before and after CV (b), C 1s spectra of pristine SWCNT (c), and C 1s spectra of SWCNT after 50 cycles of CV. (d) Br 3d spectra of SWCNT after 50 cycle of CV. The spectral data is in good agreement with FTIR and CV data.

oxybromate derivatives such as BrO3− and BrO5− as reported earlier.34,36,37 Figure 9a shows the full range XPS spectra of MWCNT before and after 50 cycles, indicating that there are not much changes in the spectra as further supported by CV and TEM analysis. However, there are little changes in the C 1s spectrum after cycling (Figure 9b,c). The peaks at 284.9 and 285.8 eV have their characteristic features as unveiled in SWCNT. It is observed that the intensity of the peak (284.9 eV) slightly decreases after CV while the peak at 285.8 eV almost disappeared in SWCNT, indicating the different mechanism of bromine interaction with SWCNT and MWCNT. Further, the peak at 286.9 eV increased after CV, demonstrating the formation of functional groups as confirmed by IR data. The peak at 290.7 eV is possibly due to shake-up (π → π*) process. Figure 9d shows the Br 3d spectrum, and the intensity of the 3d spectrum is much less compared to SWCNT. This may be possibly attributed to the lower surface area of MWCNT offering minimum active sites for bromine functionalization. Scanning electron microscope was used to investigate the morphology of the CNT-modified CF. Consequently, a smooth surface of pristine CF of 8 μm dimensions is seen in Figure 10a, while SWCNT and MWCNT anchored on the surface of CF as shown in Figure 10, b and c, respectively. The density of the

SWCNT on the surface is significantly large compared to that of MWCNT, indicating the better anchoring nature of SWCNT on the surface of CF. In order to evaluate the performance of zinc bromine redox flow cell, electrodes made with CNT were subjected to charge− discharge cycles at controlled current density. Flow cells were initially charged at a current density of 20 mA cm−2 for 30 m and then discharged to a cutoff voltage of 1 V. Typical charge− discharge curves of zinc bromine flow cell are shown in Figure 11a. The average charging plateau of CF, SWCNT, and MWCNT electrodes appear at 1.98, 1.89, and 1.82 V, respectively, while discharge plateaus are at 1.28, 1.48, and 1.51 V, respectively. Very interestingly, the charging voltage drops to 1.82 and 1.89 V from 1.98 V on replacing the CF with SWCNT and MWCNT electrodes, respectively. Accordingly, the discharge voltage increases in both SWCNT and MWCNT electrodes. The increased discharge plateau is mainly attributed to the decreased overpotential of the positive electrode. The charge−discharge voltage gap is found to be 410 and 320 mV for MWCNT and SWCNT electrodes, respectively, while pristine CF offers a maximum voltage gap of about 650 mV as indicated in Figure 11a. The high surface area of SWCNT (higher amount of basal planes) has reduced the onset potential of bromine oxidation, reflecting the increase in the voltaic 14801



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Figure 9. XPS spectra of MWCNT before and after 50 cycles of CV. (a) Survey spectra of MWCNT before and after CV. (b) C 1s spectra of pristine MWCNT. (c) C 1s spectra of MWCNT after 50 cycles of CV. (d) Br 3d spectra of MWCNT after 50 cycles of CV.

Figure 10. SEM images of CF (a), SWCNT (b), and MWCNT (c) modified CF. Arrow marks show the CNT anchored on CF.

efficiency compared to the MWCNT. The significant reduction in voltage drop in CNT-based electrodes clearly emphasizes the electrocatalytic activity of both SWCNT and MWCNT, reflecting directly on the voltaic efficiency. As a result, the voltaic efficiency (VE) has remarkably increased due to high reversibility and enhanced electrocatalytic effect of CNTs as reflected in CV data (Figure 1). For instance, VE of about 82.9% and 78.3% could be reached in the case of SWCNT and MWCNT electrodes, respectively, while CF offers only 64.6%. Energy efficiencies (EE) of SWCNT- and MWCNT-modified and pristine CF electrodes are calculated to be 70%, 66.3%, and 57.1%, respectively. It can be seen that the efficiencies of CNT-

based electrodes are substantially larger than that of pristine CF (Figure 11b). However, the Coulombic efficiencies (CE) of all three electrodes remain almost the same due to the nature of the electrode and the electrolyte used. Based on the experimental results, the SWCNT electrode has been extended for repeated charge−discharge cycling as shown in Figure 11c. Apparently, there is no significant change in the overall efficiencies, indicating the stability and structural integrity of the SWCNT. Figure 12a shows the charge−discharge curves recorded at different current densities to investigate the rate capability, and the inset shows VE and CE of the SWCNT electrode. Very 14802



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Figure 12. (a) Charge−discharge curves recorded at various current densities (20−50 mA cm−2). Inset shows VE and CE of SWCNT electrodes. (b) EE at various current densities.

redox flow cell has been compared in 0.05 M ZnBr2 aqueous solution to reflect interesting electrocatalytic effects in the order of SWCNT > MWCNT > GCE. In particular, SWCNT and MWCNT electrodes exhibit a remarkable electrochemical behavior in terms of peak current density and peak separation perhaps due to a large amount of basal planes and edge planes, respectively, which have direct impact on the zinc bromine redox flow battery performance. The zinc bromine cells fabricated with CNT electrodes offer high energy efficiency compared to that on other graphite and carbon fiber based electrodes and, more significantly, superior rate capability and reversibility during charge−discharge cycling, indicating excellent structural stability and durability. All electrochemical characterization data indicate that functionalized SWCNT can be promising electrode materials in redox flow batteries.

Figure 11. (a) Charge−discharge curves of pristine CF, MWCNTdecorated CF, and SWCN-decorated CF as positive electrode recorded at 20 mA cm−2. (b) Efficiency profile of various electrodes. (c) Cycle life test for the SWCNT electrode in ZBB cell.

interestingly, the same discharge time under the fixed condition for various current densities (20−50 mA cm−2) indicates that the maximum storage capacity can be realized even at high current density. These results clearly demonstrate the enhanced rate performance of a ZBB cell fabricated with SWCNT electrodes. When the current density increases, the VE gradually decreases, while CE remains the same. EE of SWCNT electrodes decreases from 70% to 56.3% on raising the current density from 20 to 50 mA cm−2 as seen in Figure 12b. This moderate change in EE further confirms the rate capability of SWCNT in ZBB flow cell.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +91 04565 241 500. *E-mail: [email protected]. Tel.: +91 04565 241 361. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y.M. and P.R. thank UGC, CSIR-TAPSUN (NWP 56), and CSIR-CECRI (MLP 0052) for providing financial support, respectively.

CONCLUSIONS The electrocatalytic activity of SWCNT and MWCNT electrodes toward the 2Br−/Br2 redox reaction for zinc bromine 14803



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